Characterization Techniques for Perovskite Solar Cell Materials 9781839161407, 183916140X

Citation preview

CHARACTERIZATION TECHNIQUES FOR PEROVSKITE SOLAR CELL MATERIALS

CHARACTERIZATION TECHNIQUES FOR PEROVSKITE SOLAR CELL MATERIALS Edited by

MEYSAM PAZOKI

˚ ngstro¨m Laboratory, Solid State Physics, Department of Engineering Sciences, A Uppsala University, Uppsala, Sweden

ANDERS HAGFELDT

Laboratory of Photomolecular Science, E´cole Polytechnique Fe´de´rale de Lausanne, Lausanne, Switzerland

TOMAS EDVINSSON

˚ ngstro¨m Laboratory, Solid State Physics, Department of Engineering Sciences, A Uppsala University, Uppsala, Sweden

List of contributors Antonio Abate

Helmholtz-Zentrum Berlin for Materials and Energy, Berlin, Germany

Mojtaba Abdi-Jalebi Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge, United Kingdom ˚ ngstro¨m Laboratory, Uppsala University, Uppsala, Gerrit Boschloo Department of Chemistry, A Sweden Aniela Czudek Young Investigator Group Hybrid Materials Formation and Scaling, HelmholtzZentrum Berlin for Materials and Energy, Berlin, Germany; Faculty of Physics, Warsaw University of Technology, Warsaw, Poland ˚ ngstro¨m Laboratory, Tomas Edvinsson Department of Engineering Sciences, Solid State Physics, A Uppsala University, Uppsala, Sweden Somayeh Gholipour Department of Physics, Nanophysics Research Laboratory, University of Tehran, Tehran, Iran; Department of Physics, Faculty of Basic Sciences, University of Mazandaran, Babolsar, Iran Dibyajyoti Ghosh Theoretical Division and Center for Nonlinear Studies, Los Alamos National Laboratory, Los Alamos, NM, United States Anders Hagfeldt Institute of Chemical Sciences Engineering, Laboratory of Photomolecular Science, E´cole Polytechnique Fe´de´rale de Lausanne, Lausanne, Switzerland M. Ibrahim Dar Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, E´cole Polytechnique Fe´de´rale de Lausanne, Lausanne, Switzerland T. Jesper Jacobsson Department of Chemistry, Uppsala University, Uppsala, Sweden Jesu´s Jime´nez-Lo´pez Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Tarragona, Spain ˚ ngstro¨m Laboratory, Uppsala University, Erik M.J. Johansson Department of Chemistry, A Uppsala, Sweden Hui-Seon Kim Laboratory of Photomolecular Science, E´cole Polytechnique Fe´de´rale de Lausanne, Lausanne, Switzerland ˚ ngstro¨m Laboratory, Uppsala University, Uppsala, Jolla Kullgren Department of Chemistry, A Sweden Gabriel J. Man Department of Physics and Astronomy, Molecular and Condensed Matter Physics, Uppsala University, Uppsala, Sweden Marı´a Me´ndez Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Tarragona, Spain Nu´ria F. Montcada Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Tarragona, Spain Emilio Palomares Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Tarragona, Spain

ix

x

List of contributors

˚ ngstro¨m Laboratory, Meysam Pazoki Department of Engineering Sciences, Solid State Physics, A Uppsala University, Uppsala, Sweden Bertrand Philippe Department of Physics and Astronomy, Molecular and Condensed Matter Physics, Uppsala University, Uppsala, Sweden Nga Phung

Helmholtz-Zentrum Berlin for Materials and Energy, Berlin, Germany

˚ ngstro¨m Mohammad Ziaur Rahman Department of Engineering Sciences, Solid State Physics, A Laboratory, Uppsala University, Uppsala, Sweden Ha˚kan Rensmo Department of Physics and Astronomy, Molecular and Condensed Matter Physics, Uppsala University, Uppsala, Sweden Aditya Sadhanala Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge, United Kingdom Majid Safdari Solibro Research AB, Uppsala, Sweden; Division of Applied Physical Chemistry, Department of Chemistry, KTH Royal Institute of Technology, Stockholm, Sweden Michael Saliba Technical University of Darmstadt, Department of Materials Science Optoelectronics, Darmstadt, Germany; IEK-5 Photovoltaik, Forschungszentrum Ju¨lich GmbH, Ju¨lich, Germany Wolfgang Tress Laboratory of Photomolecular Science, E´cole Polytechnique Fe´de´rale de Lausanne, Lausanne, Switzerland Eva L. Unger Young Investigator Group Hybrid Materials Formation and Scaling, HelmholtzZentrum Berlin for Materials and Energy, Berlin, Germany; Department of Chemistry and NanoLund, Lund University, Lund, Sweden Matthew J. Wolf Wenxing Yang Sweden

Department of Physics, University of Bath, Bath, United Kingdom ˚ ngstro¨m Laboratory, Uppsala University, Uppsala, Department of Chemistry, A

Preface Interaction between light and matter is the basis of life on earth through photosynthesis, but can also be harvested as electrical energy by converting sunlight into electricity via the photovoltaic effect in solar cells. The photovoltaic effect was famously first reported for selenium by Becquerel in 1839 while the first solar cell patent was filed by Anthony H. Lamb in 1932 as a “photoelectric device” and approved 1935 and materials are exemplified with “cuprous oxide, selenium, tellurium, selenides, tellurides and the like”, where selenides and tellurides are common thin film technologies today. The first practical solar cell was based on Si, however, reported 1954 by Bell laboratory researchers with 4.5% power conversion efficiency (PCE) and subsequently increased to 6% the same year. The Si solar cell demonstration was preceded by Russell Shoemaker Ohls discovery of the pn-junction 1939 and patenting of the Si solar cell as a light-sensitive electric device in 1941. Heterostructured Si solar cells presently show above 26% record PCE while thin film technologies with CdTe and CIGS have champion PCEs of 22.1% and 23.4%, respectively, on par with and even surpassing the record for multicrystalline silicon solar cells (22.3%). Solar cells based on III-Vs and tandem approaches have very low market share due to the higher cost-to-power ratio but show a record 29% PCE for single junction solar cells without solar concentration and 44.4% and 47.1% PCE for triple and four-junctions devices with concentrated solar light. With the dramatic lowering of price of silicon-based solar cell technologies, together with sufficiently high efficiency in modules and long stability, it is a favorable photovoltaic technology. Due to the variation in solar flux in the world and the high energy demand in producing the silicon, however, it is still not cheap enough to compete with fossil fuels globally. Here, thin film technologies based on CIGS, CdTe, and the newest member in the field, metal halide perovskite solar cells (PSC) have a large potential. There are several compelling reasons to look beyond silicon technology such as lower material consumption, lower energy consumption in the fabrication as well as new possibilities that arise for materials with tunable band gaps aiming for color matched architectural integrations and fabrication of high efficient thin film tandem devices. Lead halide perovskites (LHP) were first implemented as a light absorbing pigment in dyesensitized solar cell with a liquid electrolyte by Miyasaka and coworkers in 2009 with 3.8% PCE with, at the time, quite low attention. This situation changed drastically in 2012 with incorporation of LHPs in solid state devices based on the dye-sensitized solar cell architecture and PCEs around 10% by Snaith, Park and Gra¨tzel, Miyasaka and coworkers in two separate publications, starting a dramatic increase in research interest world-wide. From their exceptional absorption and charge conductivity properties, together with the defect tolerance and low temperature processing, an exceptionally fast development ended up in more than 23% PCE in the year of 2018 for small area (,1 cm2) solar cells and a record PCE of 25.2% in 2019.

xi

xii

Preface

Apart from the ability to be used in a single junction thin film solar cells, the band gap tunability also make them highly interesting for tandem applications together with conventional solar cell technologies or in all-perovskite approaches. The LHP, as with the larger class of general perovskites, show a plethora of intriguing phenomena such as ferroelectricity, photovoltaic polarizable domains, ferroelasticity, photoinduced giant dielectric constants, photoinduced Stark effects, switchable photovoltaic effects and anomalous hysteresis in the current voltage curve. These hybrid materials can also be used in light emitting and sensing applications where the tunability and combination of organic and inorganic material introduce soft modes and properties that can be difficult to attain with fully organic or inorganic semiconducting materials. Characterizing the properties of the materials and understanding the processes in the materials heterojunctions and devices is important not only to understand the origin of the quite remarkable photovoltaic effect and other phenomena in these materials, but also to further improve the materials for targeted applications. This book is a compilation of chapters aiming to give a thorough introduction to some of the most important material and device properties with focus on the characterization techniques that can be advantageously applied to study the material in the LHPs. In Chapter 1, the fundamental principle of light absorption, band gap tuning and compositional exchange are described together with their general characterization. Chapters 2 and 3 describe and review X-ray diffraction, Raman spectroscopy and absorption and photoluminescence spectroscopy in metal halide perovskites. Current voltage characterization and hysteresis effects are presented in Chapter 4. Chapters 5 and 6 describe X-ray photoelectron spectroscopies and time resolved photoinduced optical spectroscopy for metal halide perovskites. Photovoltage/photocurrent transient techniques, temperature dependent characterizations, and methods and results related to the stability of the materials and devices are presented in Chapters 7 9 respectively. In Chapter 10, density functional theory calculations on metal halide perovskite solar cell materials are described. Organic inorganic Metal Halide Perovskite Tandem Devices, spectral matching and measurements related to this are reported in Chapter 11 and the book ends with some concluding remarks in Chapter 12. In view of that the book encompasses both characterization techniques and material properties, with specific emphasis when the techniques should be directly adopted or altered for the hybrid organic inorganic material, we hope that it would be valuable for both scientists entering the field as well as for more experienced perovskite researchers in both academia and industry.

Meysam Pazoki1, Anders Hagfeldt2 and Tomas Edvinsson1 ˚ ngstro¨m Laboratory, Uppsala University, Solid State Physics, Department of Engineering Sciences, A 2 ´ Uppsala, Sweden Laboratory of Photomolecular Science, Ecole Polytechnique Fe´de´rale de Lausanne, Lausanne, Switzerland 1

C H A P T E R

1 Bandgap tuning and compositional exchange for lead halide perovskite materials Somayeh Gholipour1,2 and Michael Saliba3,4 1

Department of Physics, Nanophysics Research Laboratory, University of Tehran, Tehran, Iran 2 Department of Physics, Faculty of Basic Sciences, University of Mazandaran, Babolsar, Iran 3 Technical University of Darmstadt, Department of Materials Science - Optoelectronics, Darmstadt, Germany 4IEK-5 Photovoltaik, Forschungszentrum Ju¨lich GmbH, Ju¨lich, Germany

Goldschmidt tolerance factor determines the suitability of cation to be compatible with the 3D structure of ABX3 perovskite, which empirically was found to be between 0.8 and 1 for photoactive “black phase” perovskites.

Characterization Techniques for Perovskite Solar Cell Materials DOI: https://doi.org/10.1016/B978-0-12-814727-6.00001-3

1

Copyright © 2020 Elsevier Inc. All rights reserved.

2

1. Bandgap tuning and compositional exchange for lead halide perovskite materials

1.1 Introduction PSCs consisted of materials with ABX3 structures (see Fig. 1.1B), where A 5 cesium (Cs), methylammonium (MA), or formamidinium (FA); B 5 tin (Sn) or lead (Pb); X 5 chlorine (Cl), bromine (Br), or iodine (I). They have improved considerably in recent years starting with PCEs from 3.8% [1] in 2009 to 24.2% [2] in 2019. Engineering of material properties and specifically the bandgap of perovskite materials for solar cell applications was conducted recently by changing the material composition. The aim of the current chapter is to present the recent efforts and available techniques for such a compositional engineering, by considering the impacts and consequences on solar cell performance. These perovskites can be processed by a myriad of techniques (including inexpensive solution processing). They have a tunable band gap from about 1.2 to 3.0 eV by interchanging the above mentioned cations [3,4], metals [5,6], or halides [7]. This is illustrated schematically in Fig. 1.1A, where colloidal CsPbX3 (X 5 Cl, Br, I) solutions under UV light have a photoluminescence (PL) peak (Fig. 1.1C) ranging from 1.77 (CsPbI3) to 3.06 eV (CsPbCl3). The intermediate band gaps can be reached by mixing the respective halides [8]. In

FIGURE 1.1 Band gap tunability for ABX3 structures ranging from 1.15 to 3.06 eV. (A) Colloidal library of CsPbX3

(X 5 Cl, Br, I) solutions under UV light. Reproduced with permission [8]. Copyright 2019, American Chemical Society. (B) Crystal structure of a generic ABX3 perovskite. (C) Representative PL spectra of CsPbX3 extended towards MA (Sn/Pb)I3 perovskites. The PL peaks for the colloidal CsPbCl3 and CsPbI3 are at 405 nm (3.06 eV) and 700 nm (1.77 eV), respectively [8]. For the Sn/Pb metal, the peaks for MAPbI3 and MASnPbI3 are at 780 nm (1.59 eV) and 960 nm (1.29 eV), respectively [11]. Due to the band gap anomaly of these compounds, the most red-shifted peak can be found for MASn0.8Pb0.2I3 at 1080 nm (1.15 eV) [11]. Reproduced with permission L. Protesescu, S. Yakunin, M.I. Bodnarchuk, F. Krieg, R. Caputo, C.H. Hendon, R.X. Yang, A. Walsh, M.V. Kovalenko, Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X 5 Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut, Nano Lett. 15 (2015) 3692
3696. Available from: https://doi.org/10.1021/nl5048779. Copyright 2019, American Chemical Society. For the purpose of this chapter, the shape of these PL spectra is simulated using Gaussian functions to resemble the colloidal PL data illustrating the versatile shifting of the PL position from CsPbCl3 (3.06 eV) to MASn0.8Pb0.2I3 (1.15 eV).

Characterization Techniques for Perovskite Solar Cell Materials

1.1 Introduction

3

addition, by using Sn/Pb metal mixtures, band gaps from 1.59 (MAPbI3) to 1.29 eV (MASnI3) become available. Interestingly, the mixed metals do not follow a linear Vegard trend [5,6,9,10] and the most red-shifted band gap is at 1.15 eV for an intermediary MASn0.8Pb0.2I3 [11]. Moreover, germanium is the second closest element to lead in the periodic table with oxidation state 2 1 , which has a lower cationic size, lower electronegativity and lower toxicity [12]. Bandgap tuning of germanium based perovskites is pursued by utilizing different monovalent cations [13], halides [14] and mixtures of halides and cations [15] leading to bandgaps of 1.3
3.7 eV. Non-cubic germanium perovskite compounds show nonlinear optical properties as reported in ref. [13] and [16]. As an alternative, Bi31 has been the most investigated trivalent metal cation for lead exchange in halide perovskite solar cell applications so far. Antimony is a neighbor of Bi in the periodic table with oxidation state 3 1 and a smaller cationic radius. In similarity to bismuth, the antimony cation tends to form layered perovskites with the formula A2Sb3X9. Thus, trivalent double perovskites including Cs2Bi3I9, MA2Bi3I9 and Cs3Sb2I9 lead to large band gap of 2.0 eV [17], 2.1 eV [17], 1.96 eV [18], respectively. The large band gap range has made perovskites attractive for various applications outside of photovoltaics, ranging from lasing [19], light-emitting devices [20], sensing [21], photodetectors [22,23] as well as X-ray and particle-detection [24
27]. This also enables various options for photovoltaic applications. Some of the simplest and most accessible lead-based perovskites, e.g. MAPbI3 [28,29], can already achieve band gaps between 1.55 and 1.62 eV, which are nearly ideal for single-junction photovoltaic (PV) applications [30]. Even targeting directly the Shockley-Queisser optimum for singlejunctions at 1.42 eV is an option by using a small amount of Sn in Sn/Pb metal mixtures [30]. Increasing the amount of Sn further results in band gaps of  1.2 eV which is close to silicon. Larger band gaps of 1.7 eV can also be achieved by using halide Br/I mixtures, enabling tandem solar cells with silicon. Even perovskite on perovskite tandem solar cells become accessible [31
36]. According to the Shockley-Queisser limit and the design of the single-junction solar cell, the maximum achievable efficiency is inherently limited to 31% due to thermalization and transmission losses [30]. Thus, a tandem design that consists of multiple absorber cells stacked in a horizontal fashion can be realized to absorb a larger portion of the solar spectrum and consequentially yield higher PCEs (see Chapter 12). The same band gap can be achieved through different compositions by varying the ratios of the cations, metals, or anions. For example, increasing the Br content blue-shifts the band gap towards 1.7 eV, which is highly desired for perovskite/silicon tandems. However, this imposes new challenges: for example, using an equimolar amount of mixed halides (Br and I) may lead to phase segregation with distinct Br and I domains occurs in single cation MA-perovskites under full sun illumination as reported by Hoke et al. [37]. Such dephasing is problematic for long-term stable materials, since results in the formation of more iodine rich phases acting as recombination centers [38]. This effect can then be counteracted through perovskites with multiple cations [39]. More generally, depending on the precise composition, complex ion dynamics influence the film characterization and have a distinct effect on devices such as hysteresis. Thus, careful compositional engineering is a key aspect in perovskite research [40]. Adding more and more components towards increasingly complex perovskites also affects the film quality including the role of grain boundaries. This is particularly

Characterization Techniques for Perovskite Solar Cell Materials

4

1. Bandgap tuning and compositional exchange for lead halide perovskite materials

important for planar films that do not have a protective, mesoporous buffer layer which provides more relaxed processing conditions during deposition. Moreover, high efficiency planar PSCs need particularly fine-tuned charge extraction layers to ensure good contact between the perovskite and the charge extraction layer [41]. In the following sections, we review mixed composition perovskites, the impacts of the composition on the device stability and performance and finally a full picture of different cation/anion/metal replacements in perovskite solar cell materials together with conclusions would be mentioned at the end. A special attention is devoted to structural changes as a consequence of different compositions that are representing the state of the art devices with highest reported PCEs and/or possessing the most stable performance in the perovskite solar cell family.

1.2 Organic/inorganic ion mixing Since the pure perovskite single cation/anion compounds come with numerous disadvantages for PV applications, the mixed cations and halides is an important theme. For example, there are only relatively few reports with MAPbI3 with .20% efficiency [42]. Seok et al. have reported a superoxide colloidal solution route for preparing a Lanthanum (La)
doped BaSnO3 (LBSO) electrode under relatively mild conditions (below 300  C). The PSCs fabricated with methylammonium lead iodide (MAPbI3) show a steady-state PCE of 21.2% [29]. Furthermore, as an encapsulating agent, octyl ammonium (OA) produced individual MAPbI3 grains in full armor without the formation of layered structures, which achieved a stabilized PCE of 20.1% [43]. In addition, MA is volatile and sensitive to moisture, heat, light and thus a long-term risk factor for stability [44,45]. The band gap of MAPbI3 is at 1.55 eV which is not optimized for single-junction solar cells [28]. On the other hand, FA-based perovskites are more stable with a more red-shifted band gap [46,47]. However, FA brings its own set of challenges, as it is not phase-stable at room temperature exhibiting a photoinactive, “yellow phase” [3,48]. Especially, when Br is increased, FA-compounds are not phase stable as a photoactive “black” perovskite. The “yellow phase gap” was closed by McMeekin et al. using double-cation CsFA mixtures following prior work by Park, Zhu, and Yi [46,49,50]. The resulting materials were also observed to suppress halide segregation [39]. On the other hand, FAPbI3 and CsPbI3 are not stable in the cubic (or pseudo cubic) α-phase at room temperature. CsPbI3 and FAPbI3 both form the undesirable yellow phase under ambient condition while the mixture of Cs and FA forms the desired black perovskite as shown in Fig. 1.2. The δ phases of FAPbI3 and CsPbI3 differ significantly in their atomistic structure. In the case of the FAPbI3, the δ-phase consists of 1D pillars made of face sharing PbI6 octahedra. These pillars, aligned along the crystallographic c direction, are separated by domains containing only FA. The δ
CsPbI3 crystal is also made of 1D PbI3 pillars surrounded by the cation, Cs1 in this case, but these pillars consist of stacked and shifted edge sharing PbI6 octahedra. Moreover, the compositional space of Pb-based mixed MA/FA and mixed Br/I perovskites has been mapped experimentally [38]. Such studies reveal that FAPbBr3 has a cubic or pseudo cubic structure already at room temperature with a band gap of 2.3 eV, which

Characterization Techniques for Perovskite Solar Cell Materials

1.2 Organic/inorganic ion mixing

5

FIGURE 1.2 CsPbI3 and FAPbI3 both form the undesirable yellow phase under ambient condition while the mixture forms the desired black perovskite. Reproduced with permission C. Yi, J. Luo, S. Meloni, A. Boziki, N. AshariAstani, C. Gra¨tzel, et al., Entropic stabilization of mixed A-cation ABX3 metal halide perovskites for high performance perovskite solar cells, Energy Environ. Sci. 9 (2016) 656
662. Available from: https://doi.org/10.1039/C5EE03255E. Copyright 2019, Royal Society of Chemistry.

could be interesting for multijunction or light emitting device (LED) applications. FA is preferred to MA because of its higher thermal stability and beneficially redshifted band gap. However, small amounts of MA (up to 20%) stabilize the perovskite structure preventing the transition into the yellow polymorph [3,51,52]. Introducing Br allows for tuning of the band gap (see Fig. 1.1C), which is favorable for tandem applications, and also suppresses the yellow-phase formation for FA-perovskites [39,48]. The precise composition has a large impact on the final device performance with the best PCE at 21% for FA2/ 3MA1/3Pb(Br1/3I2/3)3 [38,48,53,54]. As mentioned by Vaynzof et al., fractional and quite likely unintentional deviations in precursor stoichiometry have a profound effect on the properties, performance and stability of perovskite photovoltaic devices [54]. For the MAPbBrxI(3-x)-series, simulations indicate that a large window in the compositional mixture may be thermodynamically unstable at room temperature with respect to phase separation [55]. Therefore, it has become an important design principle to mix cations and halides to achieve perovskite compounds combining the advantages of the constituents while avoiding their drawbacks.

1.2.1 Perovskite “black-phase” stability: role of cations The success of the double-cation MAFA mixtures demonstrates that a small amount of MA already induces a preferable crystallization of FA perovskite into its photoactive black phase resulting in a more thermally and structurally stable composition than pure MA or FA compounds [3,48]. The same holds true for CsFA [46,49] mixtures that showed even increased suppression of halide segregation enabling intermediate band gaps for tandem applications [39]. Following this approach, CsMAFA triple cation perovskites were synthesized to improve crystal quality [56]. The triple cation direction was guided by the choice

Characterization Techniques for Perovskite Solar Cell Materials

6

1. Bandgap tuning and compositional exchange for lead halide perovskite materials

FIGURE 1.3 Tolerance factor for different cations and reproducibility of triple cation perovskites. (A) Tolerance factor of APbI3 perovskite with A being Na, K, Rb (too small); Cs, MA, FA (established); and IA, EA, GA (too large). The inset images depict the cation structures. Empirically, perovskites with a tolerance factor between 0.8 and 1.0 (dashed lines) show a photoactive black phase (solid circles) as opposed to non-photoactive phases (open circles). Rb is very close to this limit making it a candidate for integration into the perovskite lattice using a multi-cation approach. The inset shows CsPbI3 (first row) and RbPbI3 (second row) at 28 oC, 380 oC, and % % 460 oC. Irreversible melting for both compounds occurs at 460 oC. RbPbI3 never shows a black phase [62], and (B) % % PCE comparison, using box plots alongside the corresponding data points, between 40 MAFA double-cation and 98 CsMAFA triple-cation perovskite devices. The standard deviation, a metric for reproducibility, improved from 16.37 6 1.49% for MAFA to 19.20 6 0.91% for CsMAFA. 20 independent devices showed efficiencies larger than 20%. Reproduced with permission [70] J.-P. Correa-Baena, M. Saliba, T. Buonassisi, M. Gra¨tzel, A. Abate, W. Tress, et al., Promises and challenges of perovskite solar cells, Science. 358 (2017) 739
744. Available from: https://doi.org/10.1126/science.aam6323. Copyright 2019, Science.

of cations Cs, MA, and FA, which were reported to have a photoactive black phase perovskite [57
61]. Recently, an unlikely cation was successfully used: the seemingly too small Rb that does not have a photoactive black phase when used as a single-cation RbPbI3 compound [62]. Only the single-cation Cs, MA, and FA are “established perovskites” with a black phase and reports of high efficiency PSCs. The alkali metals are particularly attractive because of their inherent oxidation-stability. This is illustrated further in Fig. 1.3A (inset) where only CsPbI3 but not RbPbI3 has a black phase showing that Cs and Rb are indeed the demarcation line between photoactive and photoinactive perovskites. As mentioned, the double-cation perovskites showed suppressed halide segregation. On the other hand, the currently highest performances are achieved with complex perovskite mixtures containing multiple cations (from Rb, Cs, MA, FA), metals (Pb, Sn), or halides (Br, I). For example, a PCE of 21.6% (stabilized) was reached using a multi-cation mixture of Rb, Cs, MA, and FA; and mixed Br and I halides [31,32,63]. This alludes to a broader theme: the phase stabilization and suppressed halide segregation of perovskites through multi-cation engineering [62]. Thus, a more ideal perovskite compound ought to avoid Br because of the “blue penalty” and MA for stability reasons. This leaves iodine as the preferred halide and the

Characterization Techniques for Perovskite Solar Cell Materials

1.2 Organic/inorganic ion mixing

7

thermally more stable Rb, Cs, and FA as the preferred cations rendering RbCsFAPbI3 perovskites particularly relevant. An optimized RbCsFAPbI3 perovskite (without MA and Br) with a bandgap of 1.53 eV, close to the single-junction optimum, with high short-circuit currents of 25.06 mA cm22 and a high PCE of 20.44% (stabilized at 20.35%) for planar PSCs. The perovskites do not require any heating beyond 100  C rendering them compatible with perovskite/silicon tandem solar cells or flexible solar cells, which are among the most attractive pathways towards commercialization. More generally, using only inorganic additives is a compositional design strategy which can be used to phase stabilize the thermally relatively stable FA perovskites and intermediate bandgaps as well, which is highly attractive for tandem applications. Especially the prospect of a perovskite/silicon tandem has inspired much progress in the field. However, silicon solar cells are stable over many decades and therefore any unstable perovskite component, such as MA, must be avoided if PSCs were to be combined with silicon [45]. The suitability for a cation to be compatible with a high-performance, “black phase” APbI3 3D-perovskite can be assessed with the empirical measure for lattice distortion through the Goldschmidt tolerance factor [64,65]: rA 1 rI t 5 pffiffiffi 2ðrPb 1 rI Þ

(1.1)

where r are the respective ionic radii. A tolerance factor of 0.9 2 1 is compatible with the ideal cubic perovskite structure, for a tolerance factor between 0.7 and 0.9, the A ion is too small, or the B ion is too large, for a cubic structure. This instead results in an orthorhombic, rhombohedral, or tetragonal structure. For a large A cation, t gets larger than one, which results in layered perovskite structures [66,67], and a wide range of stoichiometries and superstructures are known, like for example Ruddlesden 2 Popper, Aurivillius, and Dion 2 Jacobson phases [68]. Empirically, “black phase” 3D perovskites tend to form, when 0.8 , t , 1.0 is fulfilled [64]. This is illustrated in Fig. 1.3A where only Cs, MA, and FA fall within the “established perovskites”. Almost all elemental cations are too small (see Na, K, Rb) or too large (see imidazolium (IA), ethylamine (EA), and guanidinium (GA)) for consideration. Perovskites at the edge of the tolerance factor requirement, such as FAPbI3 (t B 1) and CsPbI3 (t B 0.8), have a distorted lattice resulting in the presence of an additional yellow phase at room temperature. MAPbI3 (t B 0.9) does not have a yellow phase. To lower the effective cationic radius of FA-perovskites, the smaller MA cation was added giving rise to MAFA perovskites. This resulted in more stable black phase perovskites at room temperature triggering a remarkable success story with all currently published world records using MAFA mixtures [4,69]. Interestingly, despite the high performances, the XRD data of MAFA still shows detrimental yellow phase impurities [56]. This is shown in Fig. 1.4A, where an XRD pattern of a double-cation MAFA perovskite is depicted. It shows that the photoinactive “yellow phase” impurities are present. Hence, addition of small amounts of Cs to MAFA compositions was posited to reduce the yellow phase impurities (because of bettermatched t values). Importantly, the CsMAFA black phase already forms at room temperature without the need for additional annealing steps. This becomes particularly noteworthy when analyzing the XRD data of films without annealing. For MAFA,

Characterization Techniques for Perovskite Solar Cell Materials

8

1. Bandgap tuning and compositional exchange for lead halide perovskite materials

FIGURE 1.4 Organic
inorganic cation incorporation into the perovskite structure. (A) XRD of a mixed MA/ FA perovskite and that with the incorporation of Cs. The yellow phase impurities, at a measured angle of diffraction (2θ) of 11.6 , in the MAFA spectrum disappear upon addition of Cs and (B) Statistics for 40 MAFA and 98 CsMAFA perovskite photovoltaic devices. All device parameters i.e., open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and power conversion efficiency (PCE) as well as the standard deviation (a metric of the reproducibility) are improved upon addition of Cs. Twenty different devices have a PCE of more than 20%. Reproduced with permission M. Saliba, T. Matsui, J.-Y. Seo, K. Domanski, J.-P. Correa-Baena, M.K. Nazeeruddin, et al., Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency, Energy Environ. Sci. 9 (2016) 1989
1997. Available from: https://doi.org/10.1039/C5EE03874J. Copyright 2019, Royal Society of Chemistry.

multiple crystalline precursor states exist. On the other hand, for CsMAFA, a clear perovskite peak emerges at room temperature. Thus, the perovskite crystallization starts with the photoactive black phase. This is highlighted by the fact that fully roomtemperature processed triple cation perovskites can achieve 18% results whereas unannealed MAFA perovskites are barely functional [71]. The random starting conditions of MAFA are consistent with its sensitivity to temperature, ambient environment such as temperature and solvent vapors. These “hidden variables” are likely one main reason why so many groups struggled to reproduce results despite using seemingly the same procedures and protocols. CsMAFA perovskites increased the performance baseline significantly. Most importantly, the often-overlooked parameter of reproducibility was improved as evidenced by many follow-up works using this and similar composition [56,72,73]. One could only pinpoint these “hidden variables” through large batch sizes and meticulous recording of all results including “bad” ones. Therefore, it is noted that providing statistically relevant data is of utmost relevance for the future direction of the field (see Fig. 1.3B). With the triple-cation perovskites (see Fig. 1.4B), the stabilized PCEs of 21.1% and improved reproducibility was obtained [56]. Using CsMAFA was guided by the certainty that each single-cation perovskite, i.e., CsPbI3, MAPbI3, and FAPbI3, exhibits a black-phase. Therefore, mixing Cs, MA, FA was likely to result in a black phase perovskite as well. This approach resulted in compounds with novel and unexpected properties. Therefore, exploring even more complex perovskites is the next logical step. However, there are no other cations reported to form a single-cation black phase PSCs. Thus, the triple cation perovskite was the most complex compositions that could be devised. However, the seemingly too small Rb could be integrated using a multi-cation approach despite RbPbI3 not forming a black-phase single-cation perovskite. Using a new

Characterization Techniques for Perovskite Solar Cell Materials

1.2 Organic/inorganic ion mixing

9

cation doubled up the possible perovskites compositions providing RbFA, RbMAFA, RbCsFA, RbCsMAFA perovskites as a new avenue for prospective high-class materials [62]. Indeed, all of the above compounds showed high PCEs reaching 20%. Especially, the quadruple cation perovskite RbCsMAFA showed excellent conversion efficiency at 21.6% (stabilized) with an open-circuit voltage at 1.24 V at a band gap of 1.62 eV resulting in a loss-of potential (the difference between the band gap and open-circuit voltage) of 390 mV, one of the lowest ever reported for any solar cell material. The addition of Rb suppresses the yellow phase even further. Also, X-ray photoelectron spectroscopy (XPS) studies reveal that the presence of Cs aids Rb integration into the perovskite lattice [74]. Recent publications on Rb-containing perovskites confirm the results and also report suppressed halide segregation using multi-cation perovskites [31,63,75]. The quadruple-cation RbCsMAFA perovskite see Fig. 1.5A yielded the best performance (i.e., with a stabilized PCE of 21.6%). The mentioned low loss in potential indicates that it is a nearly recombination-free material. Thus, a solar cell made from this material could be operated as an LED, even at ambient conditions: see Fig. 1.5B. It has also demonstrated (see inset to Fig. 1.5A) that polymer-coated multi-cation PSCs can be operated at elevated temperatures (more than 85  C) to achieve full illumination and load for 500 hours (i.e., exceeding industrial requirements). As shown in recent works [62,76,77], polymeric hole transporting materials (HTMs) can prevent detrimental metal electrode migration at elevated temperatures.

FIGURE 1.5 (A) Current density
voltage curve for the best-performing Rb-containing solar cell (i.e., a RbCsMAFA device), with a stabilized PCE of 21.6%. Inset: Thermal stability of a polymer-coated perovskite solar cell that has an efficiency of more than 17%. This device was aged for 500 hours at 85  C, under continuous illumination and maximum power point tracking conditions, in nitrogen atmosphere (red curve). The cell retained 95% of its initial performance, as indicated by the dashed gray line, which is normalized (norm) to the aged result [62], and (B) Photograph of a RbCsMAFA solar cell (mounted in a custom-made device holder) operated as an LED, with temperature and ambient-gas control. A bright emission at 1.63 eV is visible even under ambient room-light conditions. Reproduced with permission M. Saliba, T. Matsui, K. Domanski, J.-Y. Seo, A. Ummadisingu, S.M. Zakeeruddin, et al., Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance, Science 354 (2016) 206. Available from: https://doi.org/10.1126/science.aah5557. Copyright 2019, Science.

Characterization Techniques for Perovskite Solar Cell Materials

10

1. Bandgap tuning and compositional exchange for lead halide perovskite materials

The multi-cation approach is currently demonstrated using Rb, Cs, MA, and FA. Analogously to Rb, a doubling of available compounds could be achieved by identifying an additional cation. Currently, there are no more cations that have been demonstrated for high-efficiency PSCs beyond 20% efficiency. Looking at the tolerance factor in Fig. 1.3A, there are 2 directions: selecting even smaller cations than Rb or larger cation than FA. These could be alkali metals smaller than Rb, e.g. Na, K or cations larger than FA, e.g. IA, EA, and GA (see Fig. 1.6A for the structures). Thus far, K has been shown to integrate into

FIGURE 1.6 Matrix of the potential ion combinations for a library of ABX3 perovskites including: Cs1, ammo-

nium, NH41(M), hydroxylammonium NH3OH1 (HA), hydrazinium NH2NH31 (DA), methylammonium CH3NH31 (MA), formamide NH3COH1 (FM), formamidinium CH(NH2)21 (FA), ethylammonium CH3CH2NH31(EA), dimethylamine NH2(CH3)21 (DEA) and guanidine amine C(NH2)31 (GA) for the A site; Ge, Sn and Pb group-IV metalloids (MIV) for the B site; Cl, Br and I halogens (XVII) for the X site. Each column of the matrix corresponds to one class of the nine compounds with fixed A cation. The red and the gray squares indicate the more (selected) and the less (abandoned) promising ion combination to prepare ABX3 perovskites for solar cells application, respectively. The selection takes into account: stability, band gap, effective electron-hole masses (me* mh*), exciton binding energy and defect tolerance. Lead-free combinations are highlighted in yellow in the last row. Reproduced with permission [88] D. Yang, J. Lv, X. Zhao, Q. Xu, Y. Fu, Y. Zhan, et al., Functionality-directed screening of Pb-free hybrid organic
inorganic perovskites with desired intrinsic photovoltaic functionalities, Chem. Mater. 29 (2017) 524
538. Available from: https://doi.org/10.1021/acs.chemmater.6b03221. Copyright 2019, American Chemical Society.

Characterization Techniques for Perovskite Solar Cell Materials

1.4 Perovskite compositions in devices

11

a Cs-based perovskite, which however has a suboptimal blue-shifted band gap towards 2 eV [78]. Sodium has been investigated as well [79]. On the other side, there are larger molecules. Adding these to 3D perovskites (where the metal-halide octahedra are interconnected) frequently results in 2D/3D perovskites where the metal layers are separated by the larger cations [80,81]. These compounds are promising but are still trailing in terms of overall performances. In terms of multication approach that maintains a 3D structure, slightly larger molecules than FA would be obvious candidates, e.g. IA, EA, and GA. Thus so far, only EA and GA have been used for PSCs [82,83]. However, only small amounts were tolerated without exacerbating the optoelectronic film properties. IA is an interesting case because it appears to have a similar cationic radius to FA [84]. However, the molecule is planar and rigid which could be the reason it has not been successfully integrated into a 3D perovskite lattice. This also highlights that the cationic radii for molecule need further revision before molecules can be compared to each other in the way elements are [85
87].

1.3 Ion library Fig. 1.6 shows that PV perovskites can be prepared using a large library of A, B and X ions. Mixed formulations comprising two or more ions for each position have been demonstrated with superior photovoltaic performances as compared to the pure formulation [48]. Currently, the majority of reports focuses on Pb and Sn for the metal position, as well as Br and I for the halide position. Among the organic cations, FA is the largest to still show a black phase with the most red-shifted band gap compared to MA or Cs. Additionally, FA is thermally more stable than MA. Consequently, FA is the majority cation in almost all recent high-performance results [5,62,69]. Remarkably, the perovskites so far explored are only a small fraction of a large library of potential compositions as can be seen in Fig. 1.6. Further advances in tailoring material properties are expected from exploring pure and mixed combinations [88].

1.4 Perovskite compositions in devices The highest certified PCE, so far is at 24.2%, albeit this result is listed as “not stabilized” [2]. One of the higher stabilized PCEs to date is at 21.6%. This was achieved using a perovskite with multiple cations (Rb, Cs, MA, and/or FA) and halides (Br and I) [62]. However, a simpler system would be preferable in terms of more facile preparation and characterizations. For example, purely inorganic perovskite would be a simpler compound and potentially more stable than perovskites with organic components. Unfortunately, hardly any elemental cation is large enough to maintain a perovskite structure (see below for the tolerance factor discussion). Cs is already among the largest, stably monovalent, and nonradioactive cations. Effectively, this renders CsPbI3 as one of the few options for inorganic perovskites with an attractive band gap of B1.7 eV that is suitable for tandems with silicon (but suboptimal for single junctions). However, not only black phase of CsPbI3 occurs at higher temperatures but also, permanent stabilization at room temperature is still

Characterization Techniques for Perovskite Solar Cell Materials

12

1. Bandgap tuning and compositional exchange for lead halide perovskite materials

challenging; albeit, progress was made by adding quantum dots [89]. The instability of single-cation perovskites is not specific to Cs and all currently used single cation perovskites (that produced highly efficient PSCs) suffer from phase, temperature or humidity instabilities as well as lacking reproducibility. As mentioned, mixing the different cations was a key insight for enabling reproducible, high quality films that are less sensitive to “hidden” processing parameters (such as environmental temperature, solvent vapors), e.g. the triple cation perovskites resulted in a stabilized efficiency of 21.1% and were adopted by various groups since [56,72,90,91]. Very recently, RbFA and RbMAFA were reinvestigated confirming the great potential of multi-cation engineering [75,92,93]. Generally, the concept of more and more complex, multicomponent perovskite was successfully applied to various band gaps, e.g. CsFA and MAFA with Sn/Pb and/or Br/I mixtures [32,63,94,95]. The approach of mixing may be extended even further, e.g. by using smaller cations than Cs or larger ones than FA [80,82,96].

1.5 Band gap engineering strategy In addition to impressive efficiencies and low cost fabrication properties of metal halide perovskites, an attractive feature of these classes of materials is the facile band-gap tunability. Thus, appropriate compositional substitutions, resulting in an array of translucent colors [38], less toxicity [97], even perovskite as a charge selective layer [98], and using wide band-gap perovskites together with another solar cell in a tandem stack [34]. Moreover, by band gap tuning of perovskites different materials with promising properties can be obtained, for example, a benefit of raising the band gap with Cs is that the same band gap can be achieved with a lower concentration of Br. This increases photostability as higher Br contents lead to more rapid halide segregation under illumination [37,99].

1.6 Lead replacement An attractive feature of metal halide perovskites is that key material properties can be tailored by engineering the ionic composition of the ABX3 lattice (Fig. 1.7A). Therefore, it remains an attractive pursuit to identify alternative divalent metal species that are capable of tuning the material properties of the perovskite and preserving its excellent optoelectronic properties without exacerbating the stability or toxicity of the inherent material. As is indicated by Fig. 1.7B, many of the alkaline earth and transition metals can achieve a stable divalent oxidation state and are compatible with solution-processing, which makes them also suitable candidate species for generating new mixed-metal perovskite compositions. As depicted in Fig. 1.7C, to build the so-called inverted device architecture, the perovskite solution was spin-coated on an indium tin oxide (ITO) substrate coated with a PEDOT: PSS layer and crystallized by drying and annealing the film. This was followed by sequentially spin-coating and heat-treating a layer of the n-type fullerene derivative, phenyl-C61-butyric acid methyl ester (PCBM), and depositing either Ca/Al or bathocuproine (BCP)/Ag as the interlayer/electrode material pair.

Characterization Techniques for Perovskite Solar Cell Materials

1.6 Lead replacement

13

FIGURE 1.7 (A) Schematic of the ABX3 perovskite crystal structure, (B) Elements that are used as ions in the mixed-metal perovskite materials, and (C) Mixed-metal perovskite materials are integrated into an inverted device architecture using a one-step method where methylammonium iodide (MAI), lead acetate (Pb(OAc)2), and a second divalent metal salt (B0 (OAc)2 or B0 I2) are dissolved in DMF, the solution is spin-coated onto PEDOT:PSS, and the perovskite is crystallized by drying and annealing the film. Reproduced with permission [100] M.T. Klug, A. Osherov, A.A. Haghighirad, S.D. Stranks, P.R. Brown, S. Bai, et al., Tailoring metal halide perovskites through metal substitution: influence on photovoltaic and material properties, Energy Environ. Sci. 10 (2017) 236
246. Available from: https://doi.org/10.1039/C6EE03201J. Copyright 2019, Royal Society of Chemistry.

Thus, to investigate the opportunities available with alternative mixed-metal perovskite compositions Pb has been replaced with a second divalent metal species to form methylammonium mixed-metal triiodide films, denoted here as MA(Pb:B0)I3 where B0 5 {Co, Cu, Fe, Mg, Mn, Ni, Sn, Sr, and Zn}. Snaith et al. have shown that MAPbI3 is most tolerant to Co, Cu, Sn, and Zn, and device performance can often be improved upon modest levels of replacement, with a 63Pb:1Co molar ratio yielding a champion performance of 17.2% in optimized solar cells. This performance improvement arises from the ability of Co to tune the Fermi level and valence band edge (VBE) of the perovskite in a manner that is decoupled from the band gap and shift the material into a more favorable energetic alignment with PEDOT: PSS. Furthermore, crystallographic analysis reveals that Co21 is able to occupy the B-site of the perovskite lattice and can mediate the transition of the crystal phase from cubic to tetragonal at room temperature, giving strong evidence that successful B-site substitution of the perovskite lattice can occur with species outside the Group IV elements [100]. Toxicity, especially for the Pb-compounds, is a major concern for commercial application. One strategy to avoid this problem is replacing Pb with Sn (or Ge) [97]. However, the optoelectronic properties of Sn (and also Ge) films are compromised due to the chemical oxidation- instability. In addition, less toxic compounds, similar to perovskites, have been proposed, e.g. Cs2AgBiBr6 [101] or A3Bi2I9 with A 5 Cs or MA [102,103].

Characterization Techniques for Perovskite Solar Cell Materials

14

1. Bandgap tuning and compositional exchange for lead halide perovskite materials

In addition, theoretical results show that a number of stable compounds with bandgaps close to 2.0
3.2 eV can be formed which contain a lanthanide metal (B0: Eu, Dy, Tm, and Yb) instead of lead in MA(Pb:B0)I3 compounds. Thus, the lanthanide based compounds may be implemented either as charge selective layer or solar absorbers in tandem applications [98]. At the moment, the photovoltaic performances of these “perovskite-like” materials are still low compared to ABX3 perovskites [102]. Moreover, there are only relatively few toxicity studies on PSCs. Studies by Benmessaoud et al. [104]. and Babayigit et al. [105] show that Pb-perovskites are toxic upon direct exposure to cell cultures and organisms. In addition, other studies reveal that the organic salt may be as toxic as Pb [106]. A similar discussion occurred for the highly toxic cadmium that can be found in commercially available CdTe solar cells. However, different from CdTe, the lead salts within PSCs are highly soluble in water and thus may contaminate the groundwater harming public health. Thus, appropriate procedures are needed based on a thorough risk assessment including encapsulation strategies and strict regulations [105,107] that can be implemented for PSCs using comprehensive encapsulation and recycling schemes [108
110]. At this stage, more research is needed that includes the fields of toxicology and policymaking.

1.7 Anion exchange Thiocyanic (SCN) is a pseudohalide, which has a similar ionic radius to I2 and chemical behavior resembling halides. Moreover, the interaction between Pb21 and SCN2 is much stronger than the one in between Pb21 and I2, which offers the PSCs based on CH3NH3PbI3-x(SCN)x active layer resistance to moisture [111
114]. Thus, PSCs consist of FTO/TiO2/CH3NH3PbI3-x(SCN)x/Spiro-OMeTAD/Au structure has been studied and for convenience of presentation, these structures are abbreviated as “the reference films/ PSCs” in which no any alkali thiocyanate additive is included, “NaSCN-films/PSCs” and “KSCN-films/PSCs” in which the NaSCN and KSCN additives are respectively added into the Pb(SCN)2 precursor solution to fabricate the PSCs. As shown by Zhang et al., the as-prepared NaSCN-films and KSCN-films do have large grain size and homogenous morphology, resulting in enhanced photovoltaic performance and stability in humid air circumstance. While the PCE of the reference PSCs is 12.73%, the NaSCN-PSCs and KSCN-PSCs at the optimized concentrations of 2.5% NaSCN and 3.5% KSCN, respectively, show the PCE of 16.59% and 15.62%. More importantly, the NaSCN/KSCN-PSCs show an improved long-term stability. Upon exposure to humid air for 45 days without encapsulation, the PCE of the KSCN-PSCs and NaSCN-PSCs can respectively retain 97% and 93% of the initial values, but the PCE of the reference PSCs can maintain 87% [115]. As reported by Sheng et al., the 5-ammonium valeric acid iodide (5-AVAI) templating helps to produce high-quality mixed-cation perovskite crystals in stable mesoscopic perovskite solar cells (MPSCs). By the incorporation of MABF4 into the (5-AVA)0.034MA0.966PbI3 perovskite precursor solution, high-performance MPSCs up to 15.5%, with high Voc of 0.97 V were obtained. The mixed cation/mixed halide (5-AVA)0.034MA0.966PbI2.95(BF4)0.05 perovskites could alleviate carrier recombination and enhance charge carrier extraction at the perovskite

Characterization Techniques for Perovskite Solar Cell Materials

References

15

and charge collection layer interfaces. In addition, a simple one-step solution processing strategy has been introduced to fabricate a new mixed cation/mixed halide perovskite [116]. In addition, Chen et al. have reported the interface engineering via ion exchange reaction using molecular ion PF62 and the application of anion exchange reaction to form compact, large-grain, and pinhole-free MAPbI32xBrx thin films, which resulting in improvement of PCE and enhancement of device stability as well. Iodide ion in FA0.88Cs0.12PbI3 is partially exchanged with PF62 to introduce thin FA0.88Cs0.12PbI32x(PF6)x layer between perovskite and spiro-OMeTAD HTM. Hence, carrier life time is increased and defect density is decreased by the introduction of an interlayer having PF62 ion, which eventually leads to higher FF and Voc. In addition, current
voltage hysteresis is reduced, and stability is much improved [117].

1.8 Conclusions In summary, the multi-cation approach described above yielded some of the most stable and efficient solar cell devices so far. For the future applications, the strategy of creating more compounds by adding cations can be explored further using even elements or larger molecular cations. Multi-cation perovskites are a promising direction for achieving robust and reproducible solar cells, i.e., which are less prone to phase, temperature, and humidity instabilities, as compared to single-cation perovskites. In addition, it has been demonstrated that polymer-coated PSCs can withstand stress tests that are harsher than industrial norms (heating at 85  C, under full illumination and load conditions, for 500 hours), which is a crucial step toward the industrialization of perovskite materials, however achieving a fully stable compound in commercial scales is still lacking. Thus, multi-cation approach will include an assessment of viability toward upscaling, as well as further stability testing (e.g., cycling of temperature, humidity, and sealing).

References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050
6051. Available from: https://doi.org/10.1021/ ja809598r. [2] Best Research-Cell Efficiency Chart, Photovoltaic Research, NREL https://www.nrel.gov/pv/cell-efficiency. html (accessed Jul 7, 2019). [3] N. Pellet, P. Gao, G. Gregori, T.-Y. Yang, M.K. Nazeeruddin, J. Maier, et al., Mixed-organic-cation perovskite photovoltaics for enhanced solar-light harvesting, Angew. Chem. Int. Ed. 53 (2014) 3151
3157. Available from: https://doi.org/10.1002/anie.201309361. [4] W.S. Yang, J.H. Noh, N.J. Jeon, Y.C. Kim, S. Ryu, J. Seo, et al., Solar cells. High-performance photovoltaic perovskite layers fabricated through intramolecular exchange, Science 348 (2015) 1234
1237. Available from: https://doi.org/10.1126/science.aaa9272. [5] Y. Ogomi, A. Morita, S. Tsukamoto, T. Saitho, N. Fujikawa, Q. Shen, et al., CH3NH3SnxPb(1
x)I3 perovskite solar cells covering up to 1060 nm, J. Phys. Chem. Lett. 5 (2014) 1004
1011. Available from: https://doi.org/ 10.1021/jz5002117.

Characterization Techniques for Perovskite Solar Cell Materials

16

1. Bandgap tuning and compositional exchange for lead halide perovskite materials

[6] C.C. Stoumpos, C.D. Malliakas, M.G. Kanatzidis, Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties, Inorg. Chem. 52 (2013) 9019
9038. Available from: https://doi.org/10.1021/ic401215x. [7] J.H. Noh, S.H. Im, J.H. Heo, T.N. Mandal, S.I. Seok, Chemical management for colorful, efficient, and stable inorganic
organic hybrid nanostructured solar cells, Nano Lett. 13 (2013) 1764
1769. Available from: https://doi.org/10.1021/nl400349b. [8] L. Protesescu, S. Yakunin, M.I. Bodnarchuk, F. Krieg, R. Caputo, C.H. Hendon, et al., Nanocrystals of cesium lead halide perovskites (CsPbX3, X 5 Cl, Br, and I): novel optoelectronic materials showing bright emission with wide color gamut, Nano Lett. 15 (2015) 3692
3696. Available from: https://doi.org/10.1021/nl5048779. [9] L. Vegard, Die Konstitution der Mischkristalle und die Raumfu¨llung der Atome, Z. Fu¨r Phys. 5 (1921) 17
26. Available from: https://doi.org/10.1007/BF01349680. [10] F. Hao, C.C. Stoumpos, R.P.H. Chang, M.G. Kanatzidis, Anomalous band gap behavior in mixed Sn and Pb perovskites enables broadening of absorption spectrum in solar cells, J. Am. Chem. Soc. 136 (2014) 8094
8099. Available from: https://doi.org/10.1021/ja5033259. [11] B. Zhao, M. Abdi-Jalebi, M. Tabachnyk, H. Glass, V.S. Kamboj, W. Nie, et al., High open-circuit voltages in tin-rich low-bandgap perovskite-based planar heterojunction photovoltaics, Adv. Mater. 29 (2017) 1604744. Available from: https://doi.org/10.1002/adma.201604744. [12] R.H. Kretsinger, V.N. Uversky, E.A. Permyakov (Eds.), Encyclopedia of Metalloproteins, Springer-Verlag, New York, 2013. Available from: www.springer.com/de/book/9781461415329. accessed January 6, 2019. [13] C.C. Stoumpos, L. Frazer, D.J. Clark, Y.S. Kim, S.H. Rhim, A.J. Freeman, et al., Hybrid germanium iodide perovskite semiconductors: active lone pairs, structural distortions, direct and indirect energy gaps, and strong nonlinear optical properties, J. Am. Chem. Soc. 137 (2015) 6804
6819. Available from: https://doi. org/10.1021/jacs.5b01025. [14] P.-P. Sun, Q.-S. Li, L.-N. Yang, Z.-S. Li, Theoretical insights into a potential lead-free hybrid perovskite: substituting Pb2 1 with Ge2 1 , Nanoscale 8 (2016) 1503
1512. Available from: https://doi.org/10.1039/ C5NR05337D. [15] M.-G. Ju, J. Dai, L. Ma, X.C. Zeng, Lead-Free Mixed Tin and Germanium Perovskites for Photovoltaic Application, J. Am. Chem. Soc. 139 (2017) 8038
8043. Available from: https://doi.org/10.1021/ jacs.7b04219. [16] L.-C. Tang, Y.-C. Chang, J.-Y. Huang, M.-H. Lee, C.-S. Chang, First Principles Calculations of Linear and Second-Order Optical Responses in Rhombohedrally Distorted Perovskite Ternary Halides, CsGeX3 (X 5 Cl, Br, and I), Jpn. J. Appl. Phys. 48 (2009) 112402. Available from: https://doi.org/10.1143/JJAP.48.112402. [17] M. Pazoki, M.B. Johansson, H. Zhu, P. Broqvist, T. Edvinsson, G. Boschloo, et al., Bismuth iodide perovskite materials for solar cell applications: electronic structure, optical transitions, and directional charge transport, J. Phys. Chem. C. 120 (2016) 29039
29046. Available from: https://doi.org/10.1021/acs.jpcc.6b11745. [18] B. Saparov, F. Hong, J.-P. Sun, H.-S. Duan, W. Meng, S. Cameron, et al., Thin-film preparation and characterization of Cs3Sb2I9: a lead-free layered perovskite semiconductor, Chem. Mater. 27 (2015) 5622
5632. Available from: https://doi.org/10.1021/acs.chemmater.5b01989. [19] G. Xing, N. Mathews, S.S. Lim, N. Yantara, X. Liu, D. Sabba, et al., Low-temperature solution-processed wavelength-tunable perovskites for lasing, Nat. Mater. 13 (2014) 476
480. Available from: https://doi.org/ 10.1038/nmat3911. [20] Z.-K. Tan, R.S. Moghaddam, M.L. Lai, P. Docampo, R. Higler, F. Deschler, et al., Bright light-emitting diodes based on organometal halide perovskite, Nat. Nanotechnol. 9 (2014) 687
692. Available from: https://doi. org/10.1038/nnano.2014.149. [21] H.-H. Fang, S. Adjokatse, H. Wei, J. Yang, G.R. Blake, J. Huang, et al., Ultrahigh sensitivity of methylammonium lead tribromide perovskite single crystals to environmental gases, Sci. Adv. 2 (2016) e1600534. Available from: https://doi.org/10.1126/sciadv.1600534. [22] L. Dou, Y. (Micheal) Yang, J. You, Z. Hong, W.-H. Chang, G. Li, et al., Solution-processed hybrid perovskite photodetectors with high detectivity, Nat. Commun. 5 (2014) 5404. Available from: https://doi.org/10.1038/ ncomms6404. [23] Y. Fang, Q. Dong, Y. Shao, Y. Yuan, J. Huang, Highly narrowband perovskite single-crystal photodetectors enabled by surface-charge recombination, Nat. Photonics 9 (2015) 679
686. Available from: https://doi.org/ 10.1038/nphoton.2015.156.

Characterization Techniques for Perovskite Solar Cell Materials

References

17

[24] S. Yakunin, M. Sytnyk, D. Kriegner, S. Shrestha, M. Richter, G.J. Matt, et al., Detection of X-ray photons by solution-processed lead halide perovskites, Nat. Photonics 9 (2015) 444
449. Available from: https://doi. org/10.1038/nphoton.2015.82. [25] S. Yakunin, D.N. Dirin, Y. Shynkarenko, V. Morad, I. Cherniukh, O. Nazarenko, et al., Detection of gamma photons using solution-grown single crystals of hybrid lead halide perovskites, Nat. Photonics. 10 (2016) 585
589. Available from: https://doi.org/10.1038/nphoton.2016.139. [26] H. Wei, Y. Fang, P. Mulligan, W. Chuirazzi, H.-H. Fang, C. Wang, et al., Sensitive X-ray detectors made of methylammonium lead tribromide perovskite single crystals, Nat. Photonics 10 (2016) 333
339. Available from: https://doi.org/10.1038/nphoton.2016.41. [27] Q. Dong, Y. Fang, Y. Shao, P. Mulligan, J. Qiu, L. Cao, et al., Electron-hole diffusion lengths . 175 μm in solution-grown CH3NH3PbI3 single crystals, Science 347 (2015) 967
970. Available from: https://doi.org/ 10.1126/science.aaa5760. [28] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Efficient hybrid solar cells based on mesosuperstructured organometal halide perovskites, Science 338 (2012) 643
647. Available from: https://doi. org/10.1126/science.1228604. [29] S.S. Shin, E.J. Yeom, W.S. Yang, S. Hur, M.G. Kim, J. Im, et al., Colloidally prepared La-doped BaSnO3 electrodes for efficient, photostable perovskite solar cells, Science 356 (2017) 167
171. Available from: https:// doi.org/10.1126/science.aam6620. [30] W. Shockley, H.J. Queisser, Detailed balance limit of efficiency of p-n junction solar cells, J. Appl. Phys. 32 (1961) 510
519. Available from: https://doi.org/10.1063/1.1736034. [31] D. Zhao, Y. Yu, C. Wang, W. Liao, N. Shrestha, C.R. Grice, et al., Low-bandgap mixed tin
lead iodide perovskite absorbers with long carrier lifetimes for all-perovskite tandem solar cells, Nat. Energy 2 (2017) 17018. Available from: https://doi.org/10.1038/nenergy.2017.18. [32] G.E. Eperon, T. Leijtens, K.A. Bush, R. Prasanna, T. Green, J.T.-W. Wang, et al., Perovskite-perovskite tandem photovoltaics with optimized bandgaps, Science (2016) aaf9717. Available from: https://doi.org/10.1126/science.aaf9717. [33] D. Forga´cs, L. Gil-Escrig, D. Pe´rez-Del-Rey, C. Momblona, J. Werner, B. Niesen, et al., Efficient monolithic perovskite/perovskite tandem solar cells, Adv. Energy Mater. 7 (2017) 1602121. Available from: https://doi. org/10.1002/aenm.201602121. [34] J.H. Heo, S.H. Im, CH3NH3PbBr3
CH3NH3PbI3 perovskite
perovskite tandem solar cells with exceeding 2.2 V open circuit voltage, Adv. Mater. 28 (2016) 5121
5125. Available from: https://doi.org/10.1002/ adma.201501629. [35] F. Jiang, T. Liu, B. Luo, J. Tong, F. Qin, S. Xiong, et al., A two-terminal perovskite/perovskite tandem solar cell, J. Mater. Chem. A. 4 (2016) 1208
1213. Available from: https://doi.org/10.1039/ C5TA08744A. [36] M. Anaya, J.P. Correa-Baena, G. Lozano, M. Saliba, P. Anguita, B. Roose, et al., Optical analysis of CH3NH3SnxPb1 2 xI3 absorbers: a roadmap for perovskite-on-perovskite tandem solar cells, J. Mater. Chem. A. 4 (2016) 11214
11221. Available from: https://doi.org/10.1039/C6TA04840D. [37] E.T. Hoke, D.J. Slotcavage, E.R. Dohner, A.R. Bowring, H.I. Karunadasa, M.D. McGehee, Reversible photoinduced trap formation in mixed-halide hybrid perovskites for photovoltaics, Chem. Sci. 6 (2015) 613
617. Available from: https://doi.org/10.1039/C4SC03141E. [38] T.J. Jacobsson, J.-P. Correa-Baena, M. Pazoki, M. Saliba, K. Schenk, M. Gra¨tzel, et al., Exploration of the compositional space for mixed lead halogen perovskites for high efficiency solar cells, Energy Environ. Sci. 9 (2016) 1706
1724. Available from: https://doi.org/10.1039/C6EE00030D. [39] D.P. McMeekin, G. Sadoughi, W. Rehman, G.E. Eperon, M. Saliba, M.T. Ho¨rantner, et al., A mixed-cation lead mixed-halide perovskite absorber for tandem solar cells, Science. 351 (2016) 151
155. Available from: https://doi.org/10.1126/science.aad5845. [40] E.L. Unger, E.T. Hoke, C.D. Bailie, W.H. Nguyen, A.R. Bowring, T. Heumu¨ller, et al., Hysteresis and transient behavior in current
voltage measurements of hybrid-perovskite absorber solar cells, Energy Environ. Sci. 7 (2014) 3690
3698. Available from: https://doi.org/10.1039/C4EE02465F. [41] J.P.C. Baena, L. Steier, W. Tress, M. Saliba, S. Neutzner, T. Matsui, et al., Highly efficient planar perovskite solar cells through band alignment engineering, Energy Environ. Sci. 8 (2015) 2928
2934. Available from: https://doi.org/10.1039/C5EE02608C.

Characterization Techniques for Perovskite Solar Cell Materials

18

1. Bandgap tuning and compositional exchange for lead halide perovskite materials

[42] C. Rolda´n-Carmona, P. Gratia, I. Zimmermann, G. Grancini, P. Gao, M. Graetzel, et al., High efficiency methylammonium lead triiodide perovskite solar cells: the relevance of non-stoichiometric precursors, Energy Environ. Sci. 8 (2015) 3550
3556. Available from: https://doi.org/10.1039/C5EE02555A. [43] M. Jung, T.J. Shin, J. Seo, G. Kim, S.I. Seok, Structural features and their functions in surfactant-armoured methylammonium lead iodide perovskites for highly efficient and stable solar cells, Energy Environ. Sci. 11 (2018) 2188
2197. Available from: https://doi.org/10.1039/C8EE00995C. [44] B. Conings, J. Drijkoningen, N. Gauquelin, A. Babayigit, J. D’Haen, L. D’Olieslaeger, et al., Intrinsic thermal instability of methylammonium lead trihalidetrihalide perovskite, Adv. Energy Mater. 5 (2015) 1500477. Available from: https://doi.org/10.1002/aenm.201500477. [45] S.-H. Turren-Cruz, A. Hagfeldt, M. Saliba, Methylammonium-free, high-performance and stable perovskite solar cells on a planar architecture, Science (2018) eaat3583. Available from: https://doi.org/10.1126/science. aat3583. [46] J.-W. Lee, D.-H. Kim, H.-S. Kim, S.-W. Seo, S.M. Cho, N.-G. Park, Formamidinium and cesium hybridization for photo- and moisture-stable perovskite solar cell, Adv. Energy Mater. 5 (2015) 1501310. Available from: https://doi.org/10.1002/aenm.201501310. [47] T.M. Koh, K. Fu, Y. Fang, S. Chen, T.C. Sum, N. Mathews, et al., Formamidinium-containing metal-halide: an alternative material for near-IR absorption perovskite solar cells, J. Phys. Chem. C. 118 (2014) 16458
16462. Available from: https://doi.org/10.1021/jp411112k. [48] N.J. Jeon, J.H. Noh, W.S. Yang, Y.C. Kim, S. Ryu, J. Seo, et al., Compositional engineering of perovskite materials for high-performance solar cells, Nature 517 (2015) 476
480. Available from: https://doi.org/10.1038/ nature14133. [49] C. Yi, J. Luo, S. Meloni, A. Boziki, N. Ashari-Astani, C. Gra¨tzel, et al., Entropic stabilization of mixed Acation ABX3 metal halide perovskites for high performance perovskite solar cells, Energy Environ. Sci. 9 (2016) 656
662. Available from: https://doi.org/10.1039/C5EE03255E. [50] Z. Li, M. Yang, J.-S. Park, S.-H. Wei, J.J. Berry, K. Zhu, Stabilizing Perovskite Structures by Tuning Tolerance Factor: Formation of Formamidinium and Cesium Lead Iodide Solid-State Alloys, Chem. Mater. 28 (2016) 284
292. Available from: https://doi.org/10.1021/acs.chemmater.5b04107. [51] C. Quarti, E. Mosconi, J.M. Ball, V. D’Innocenzo, C. Tao, S. Pathak, et al., Structural and optical properties of methylammonium lead iodide across the tetragonal to cubic phase transition: implications for perovskite solar cells, Energy Environ. Sci. 9 (2016) 155
163. Available from: https://doi.org/10.1039/ C5EE02925B. [52] A. Maaej, M. Bahri, Y. Abid, N. Jaidane, Z.B. Lakhdar, A. Lautie´, Raman study of low temperature phase transitions in the cubic perovskite CH3NH3PbCI3, Phase Transit. 64 (1998) 179
190. Available from: https://doi.org/10.1080/01411599808207997. [53] D. Bi, W. Tress, M.I. Dar, P. Gao, J. Luo, C. Renevier, et al., Efficient luminescent solar cells based on tailored mixed-cation perovskites, Sci. Adv. 2 (2016) e1501170. Available from: https://doi.org/10.1126/sciadv.1501170. [54] P. Fassl, V. Lami, A. Bausch, Z. Wang, M.T. Klug, H.J. Snaith, et al., Fractional deviations in precursor stoichiometry dictate the properties, performance and stability of perovskite photovoltaic devices, Energy Environ. Sci. (2018). Available from: https://doi.org/10.1039/C8EE01136B. [55] F. Brivio, C. Caetano, A. Walsh, Thermodynamic origin of photoinstability photoinstability in the CH3NH3Pb(I1
xBrx)3 hybrid halide perovskite alloy, J. Phys. Chem. Lett. 7 (2016) 1083
1087. Available from: https://doi.org/10.1021/acs.jpclett.6b00226. [56] M. Saliba, T. Matsui, J.-Y. Seo, K. Domanski, J.-P. Correa-Baena, M.K. Nazeeruddin, et al., Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency, Energy Environ. Sci. 9 (2016) 1989
1997. Available from: https://doi.org/10.1039/C5EE03874J. [57] G.S. Johnson, XLVI.—on ammonium triiodide, J. Chem. Soc. Trans. 33 (1878) 397
401. Available from: https://doi.org/10.1039/CT8783300397. [58] F.D. Chattaway, XI.—ammonium perhaloids, J. Chem. Soc. Trans. 107 (1915) 105
108. Available from: https://doi.org/10.1039/CT9150700105. [59] F.D. Chattaway, G. Hoyle, LXXVIII.—perhalides of quaternary ammonium salts, J. Chem. Soc. Trans. 123 (1923) 654
662. Available from: https://doi.org/10.1039/CT9232300654. [60] R.M. Bozorth, L. Pauling, The crystal structures of cesium tri-iodide and cesium dibromo-iodide, J. Am. Chem. Soc. 47 (1925) 1561
1571. Available from: https://doi.org/10.1021/ja01683a009.

Characterization Techniques for Perovskite Solar Cell Materials

References

19

[61] D.B. Mitzi, K. Liang, Synthesis, resistivity, and thermal properties of the cubic perovskite NH2CH 5 NH2SnI3 and related systems, J. Solid State Chem. 134 (1997) 376
381. Available from: https://doi.org/10.1006/jssc.1997.7593. [62] M. Saliba, T. Matsui, K. Domanski, J.-Y. Seo, A. Ummadisingu, S.M. Zakeeruddin, et al., Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance, Science 354 (2016) 206. Available from: https://doi.org/10.1126/science.aah5557. [63] T. Duong, Y. Wu, H. Shen, J. Peng, X. Fu, D. Jacobs, et al., Rubidium multication perovskite with optimized bandgap for perovskite-silicon tandem with over 26% efficiency, Adv. Energy Mater. 7 (2017) 1700228. Available from: https://doi.org/10.1002/aenm.201700228. [64] V.M. Goldschmidt, Die Gesetze der Krystallochemie, Naturwissenschaften. 14 (1926) 477
485. Available from: https://doi.org/10.1007/BF01507527. [65] T.J. Jacobsson, M. Pazoki, A. Hagfeldt, T. Edvinsson, Goldschmidt’s rules and strontium replacement in lead halogen perovskite solar cells: theory and preliminary experiments on CH3NH3SrI3, J. Phys. Chem. C. 119 (2015) 25673
25683. Available from: https://doi.org/10.1021/acs.jpcc.5b06436. [66] D.B. Mitzi, S. Wang, C.A. Feild, C.A. Chess, A.M. Guloy, Conducting layered organic-inorganic halides containing ,110 . -oriented perovskite sheets, Science 267 (1995) 1473
1476. Available from: https://doi.org/ 10.1126/science.267.5203.1473. [67] J. Calabrese, N.L. Jones, R.L. Harlow, N. Herron, D.L. Thorn, Y. Wang, Preparation and characterization of layered lead halide compounds, J. Am. Chem. Soc. 113 (1991) 2328
2330. Available from: https://doi.org/ 10.1021/ja00006a076. [68] H.G. Kim, O.S. Becker, J.S. Jang, S.M. Ji, P.H. Borse, J.S. Lee, A generic method of visible light sensitization for perovskite-related layered oxides: substitution effect of lead, J. Solid State Chem. 179 (2006) 1214
1218. Available from: https://doi.org/10.1016/j.jssc.2006.01.024. [69] D. Bi, C. Yi, J. Luo, J.-D. De´coppet, F. Zhang, S.M. Zakeeruddin, et al., Polymer-templated nucleation and crystal growth of perovskite films for solar cells with efficiency greater than 21%, Nat. Energy 1 (2016) 16142. Available from: https://doi.org/10.1038/nenergy.2016.142. [70] J.-P. Correa-Baena, M. Saliba, T. Buonassisi, M. Gra¨tzel, A. Abate, W. Tress, et al., Promises and challenges of perovskite solar cells, Science 358 (2017) 739
744. Available from: https://doi.org/10.1126/science. aam6323. [71] T. Matsui, J.-Y. Seo, M. Saliba, S.M. Zakeeruddin, M. Gra¨tzel, Room-temperature formation of highly crystalline multication perovskites for efficient, low-cost solar cells, Adv. Mater. 29 (2017) 1606258. Available from: https://doi.org/10.1002/adma.201606258. [72] B. Conings, A. Babayigit, M.T. Klug, S. Bai, N. Gauquelin, N. Sakai, et al., Protocol for planar heterojunction solar cells with high efficiency based on hybrid lead halide perovskite families, Adv. Mater. 28 (2016) 10701
10709. Available from: https://doi.org/10.1002/adma.201603747. [73] H. Tan, A. Jain, O. Voznyy, X. Lan, F.P.G. de Arquer, J.Z. Fan, et al., Efficient and stable solution-processed planar perovskite solar cells via contact passivation, Science (2017) eaai9081. Available from: https://doi. org/10.1126/science.aai9081. [74] B. Philippe, M. Saliba, J.-P. Correa-Baena, U.B. Cappel, S.-H. Turren-Cruz, M. Gra¨tzel, et al., Chemical distribution of multiple cation (Rb 1 , Cs 1 , MA 1 , and FA 1 ) perovskite materials by photoelectron spectroscopy, Chem. Mater. 29 (2017) 3589
3596. Available from: https://doi.org/10.1021/acs.chemmater.7b00126. [75] M. Zhang, J.S. Yun, Q. Ma, J. Zheng, C.F.J. Lau, X. Deng, et al., High-efficiency rubidium-incorporated perovskite solar cells by gas quenching, ACS Energy Lett. 2 (2017) 438
444. Available from: https://doi.org/ 10.1021/acsenergylett.6b00697. [76] K. Domanski, J.-P. Correa-Baena, N. Mine, M.K. Nazeeruddin, A. Abate, M. Saliba, et al., Not all that glitters is gold: metal-migration-induced degradation in perovskite solar cells, ACS Nano. 10 (2016) 6306
6314. Available from: https://doi.org/10.1021/acsnano.6b02613. [77] T. Matsui, I. Petrikyte, T. Malinauskas, K. Domanski, M. Daskeviciene, M. Steponaitis, et al., Additive-free transparent triarylaminetriarylamine-based-based polymeric hole-transport materials for stable perovskite solar cells, ChemSusChem. 9 (2016) 2567
2571. Available from: https://doi.org/10.1002/cssc.201600762. [78] J.K. Nam, S.U. Chai, W. Cha, Y.J. Choi, W. Kim, M.S. Jung, et al., Potassium incorporation for enhanced performance and stability of fully inorganic cesium lead halide perovskite solar cells, Nano Lett. 17 (2017) 2028
2033. Available from: https://doi.org/10.1021/acs.nanolett.7b00050.

Characterization Techniques for Perovskite Solar Cell Materials

20

1. Bandgap tuning and compositional exchange for lead halide perovskite materials

[79] M. Abdi-Jalebi, M.I. Dar, A. Sadhanala, S.P. Senanayak, M. Gra¨tzel, R.H. Friend, Monovalent cation doping of CH3NH3PbI3 for efficient perovskite solar cells, J. Vis. Exp. JoVE. (2017). Available from: https://doi.org/ 10.3791/55307. [80] H. Tsai, W. Nie, J.-C. Blancon, C.C. Stoumpos, R. Asadpour, B. Harutyunyan, et al., High-efficiency twodimensional Ruddlesden
Popper perovskite solar cells, Nature 536 (2016) 312
316. Available from: https:// doi.org/10.1038/nature18306. [81] J.-C. Blancon, H. Tsai, W. Nie, C.C. Stoumpos, L. Pedesseau, C. Katan, et al., Extremely efficient internal exciton dissociation through edge states in layered 2D perovskites, Science 355 (2017) 1288
1292. Available from: https://doi.org/10.1126/science.aal4211. [82] D.M. Nicholas, Z. Huanping, C. Qi, S. Pengyu, L. Zonghao, M. Lei, et al., Guanidinium: a route to enhanced carrier lifetime and open-circuit voltage in hybrid perovskite solar cells, Nano Lett. 16 (2016) 1009
1016. Available from: https://doi.org/10.1021/acs.nanolett.5b04060. [83] J.-H. Im, J. Chung, S.-J. Kim, N.-G. Park, Synthesis, structure, and photovoltaic property of a nanocrystalline 2H perovskite-type novel sensitizer (CH3CH2NH3)PbI3, Nanoscale Res. Lett. 7 (2012) 353. Available from: https://doi.org/10.1186/1556-276X-7-353. [84] G. Kieslich, S. Sun, A.K. Cheetham, Solid-state principles applied to organic
inorganic perovskites: new tricks for an old dog, Chem. Sci. 5 (2014) 4712
4715. Available from: https://doi.org/10.1039/C4SC02211D. [85] G. Kieslich, S. Sun, A.K. Cheetham, An extended tolerance factor approach for organic
inorganic perovskites, Chem. Sci. 6 (2015) 3430
3433. Available from: https://doi.org/10.1039/C5SC00961H. [86] M. Becker, T. Klu¨ner, M. Wark, Formation of hybrid ABX3 perovskite compounds for solar cell application: first-principles calculations of effective ionic radii and determination of tolerance factors, Dalton Trans. 46 (2017) 3500
3509. Available from: https://doi.org/10.1039/C6DT04796C. [87] W. Travis, E.N.K. Glover, H. Bronstein, D.O. Scanlon, R.G. Palgrave, On the application of the tolerance factor to inorganic and hybrid halide perovskites: a revised system, Chem. Sci. 7 (2016) 4548
4556. Available from: https://doi.org/10.1039/C5SC04845A. [88] D. Yang, J. Lv, X. Zhao, Q. Xu, Y. Fu, Y. Zhan, et al., Functionality-directed screening of Pb-free hybrid organic
inorganic perovskites with desired intrinsic photovoltaic functionalities, Chem. Mater. 29 (2017) 524
538. Available from: https://doi.org/10.1021/acs.chemmater.6b03221. [89] A. Swarnkar, A.R. Marshall, E.M. Sanehira, B.D. Chernomordik, D.T. Moore, J.A. Christians, et al., Quantum dot
induced phase stabilization of α
CsPbI3 perovskite for high-efficiency photovoltaics, Science. 354 (2016) 92
95. Available from: https://doi.org/10.1126/science.aag2700. [90] J. Peng, T. Duong, X. Zhou, H. Shen, Y. Wu, H.K. Mulmudi, et al., Efficient indium-doped TiOx electron transport layers for high-performance perovskite solar cells and perovskite-silicon tandems, Adv. Energy Mater. 7 (2017) 1601768. Available from: https://doi.org/10.1002/aenm.201601768. [91] A.R. bin, M. Yusoff, H.P. Kim, X. Li, J. Kim, J. Jang, et al., Ambipolar triple cation perovskite field effect transistors and inverters, Adv. Mater. 29 (2017) 1602940. Available from: https://doi.org/10.1002/adma.201602940. [92] T. Duong, H.K. Mulmudi, H. Shen, Y. Wu, C. Barugkin, Y.O. Mayon, et al., Structural engineering using rubidium iodide as a dopant under excess lead iodide conditions for high efficiency and stable perovskites, Nano Energy 30 (2016) 330
340. Available from: https://doi.org/10.1016/j.nanoen.2016.10.027. [93] Y.H. Park, I. Jeong, S. Bae, H.J. Son, P. Lee, J. Lee, et al., Inorganic rubidium cation as an enhancer for photovoltaic performance and moisture stability of HC(NH2)2PbI3 perovskite solar cells, Adv. Funct. Mater. 27 (2017) 1605988. Available from: https://doi.org/10.1002/adfm.201605988. [94] Z. Yang, A. Rajagopal, C.-C. Chueh, S.B. Jo, B. Liu, T. Zhao, et al., Stable low-bandgap Pb
Sn binary perovskites for tandem solar cells, Adv. Mater. 28 (2016) 8990
8997. Available from: https://doi.org/10.1002/ adma.201602696. [95] Y. Lin, B. Chen, F. Zhao, X. Zheng, Y. Deng, Y. Shao, et al., Matching charge extraction contact for widebandgap perovskite solar cells, Adv. Mater. 29 (2017) 1700607. Available from: https://doi.org/10.1002/ adma.201700607. [96] I.C. Smith, E.T. Hoke, D. Solis-Ibarra, M.D. McGehee, H.I. Karunadasa, A. Layered, Hybrid perovskite solarcell absorber with enhanced moisture stability, Angew. Chem. 126 (2014) 11414
11417. Available from: https://doi.org/10.1002/ange.201406466. [97] H. Hu, B. Dong, W. Zhang, Low-toxic metal halide perovskites: opportunities and future challenges, J. Mater. Chem. A. 5 (2017) 11436
11449. Available from: https://doi.org/10.1039/C7TA00269F.

Characterization Techniques for Perovskite Solar Cell Materials

References

21

[98] M. Pazoki, A. Ro¨ckert, M.J. Wolf, R. Imani, T. Edvinsson, J. Kullgren, Electronic structure of organic
inorganic lanthanide iodide perovskite solar cell materials, J. Mater. Chem. A. 5 (2017) 23131
23138. Available from: https://doi.org/10.1039/C7TA07716E. [99] R.E. Beal, D.J. Slotcavage, T. Leijtens, A.R. Bowring, R.A. Belisle, W.H. Nguyen, et al., Cesium lead halide perovskites with improved stability for tandem solar cells, J. Phys. Chem. Lett. 7 (2016) 746
751. Available from: https://doi.org/10.1021/acs.jpclett.6b00002. [100] M.T. Klug, A. Osherov, A.A. Haghighirad, S.D. Stranks, P.R. Brown, S. Bai, et al., Tailoring metal halide perovskites through metal substitution: influence on photovoltaic and material properties, Energy Environ. Sci. 10 (2017) 236
246. Available from: https://doi.org/10.1039/C6EE03201J. [101] M.R. Filip, S. Hillman, A.A. Haghighirad, H.J. Snaith, F. Giustino, Band gaps of the lead-free halide double perovskites Cs2BiAgCl6 and Cs2BiAgBr6 from theory and experiment, J. Phys. Chem. Lett. 7 (2016) 2579
2585. Available from: https://doi.org/10.1021/acs.jpclett.6b01041. [102] F. Giustino, H.J. Snaith, Toward lead-free perovskite solar cells, ACS Energy Lett. 1 (2016) 1233
1240. Available from: https://doi.org/10.1021/acsenergylett.6b00499. [103] B.-W. Park, B. Philippe, X. Zhang, H. Rensmo, G. Boschloo, E.M.J. Johansson, Bismuth based hybrid perovskites A3Bi2I9 (A: methylammonium or cesium) for solar cell application, Adv. Mater. 27 (2015) 6806
6813. Available from: https://doi.org/10.1002/adma.201501978. [104] IR. Benmessaoud, A.-L. Mahul-Mellier, E. Horva´th, B. Maco, M. Spina, H.A. Lashuel, et al., Health hazards of methylammonium lead iodide based perovskites: cytotoxicity studies, Toxicol. Res. 5 (2016) 407
419. Available from: https://doi.org/10.1039/C5TX00303B. [105] A. Babayigit, A. Ethirajan, M. Muller, B. Conings, Toxicity of organometal halide perovskite solar cells, Nat. Mater. 15 (2016) 247
251. Available from: https://doi.org/10.1038/nmat4572. [106] N. Espinosa, L. Serrano-Luja´n, A. Urbina, F.C. Krebs, Solution and vapour deposited lead perovskite solar cells: ecotoxicity from a life cycle assessment perspective, Sol. Energy Mater. Sol. Cells. 137 (2015) 303
310. Available from: https://doi.org/10.1016/j.solmat.2015.02.013. [107] J. Bohland, K. Smigielski, First Solar’s CdTe module Manufacturing Experience; Environmental, Health and Safety Results, Proc. 28th IEEE Photovoltaic Specialists Conference. (2000) 575
578. Available from: https://doi.org/10.1109/PVSC.2000.915904. [108] A. Binek, M.L. Petrus, N. Huber, H. Bristow, Y. Hu, T. Bein, et al., Recycling perovskite solar cells to avoid lead waste, ACS Appl. Mater. Interfaces. 8 (2016) 12881
12886. Available from: https://doi.org/10.1021/ acsami.6b03767. [109] J.M. Kadro, N. Pellet, F. Giordano, A. Ulianov, O. Mu¨ntener, J. Maier, et al., A. Hagfeldt, proof-of-concept for facile perovskite solar cell recycling, Energy Environ. Sci. 9 (2016) 3172
3179. Available from: https:// doi.org/10.1039/C6EE02013E. [110] Q. Dong, F. Liu, M.K. Wong, H.W. Tam, A.B. Djuriˇsi´c, A. Ng, et al., Encapsulation of perovskite solar cells for high humidity conditions, ChemSusChem. 9 (2016) 2597
2603. Available from: https://doi.org/ 10.1002/cssc.201600868. [111] A. Halder, R. Chulliyil, A.S. Subbiah, T. Khan, S. Chattoraj, A. Chowdhury, et al., Pseudohalide (SCN (-))-doped MAPbI3 perovskites: a few surprises, J. Phys. Chem. Lett. 6 (2015) 3483
3489. Available from: https://doi.org/10.1021/acs.jpclett.5b01327. [112] Q. Jiang, D. Rebollar, J. Gong, E.L. Piacentino, C. Zheng, T. Xu, Pseudohalide-induced moisture tolerance in perovskite CH3NH3Pb(SCN)2I thin films, Angew. Chem. Int. Ed. 54 (2015) 7617
7620. Available from: https://doi.org/10.1002/anie.201503038. [113] Q. Tai, P. You, H. Sang, Z. Liu, C. Hu, H.L.W. Chan, et al., Efficient and stable perovskite solar cells prepared in ambient air irrespective of the humidity, Nat. Commun. 7 (2016) 11105. Available from: https:// doi.org/10.1038/ncomms11105. [114] Z. Xiao, W. Meng, B. Saparov, H.-S. Duan, C. Wang, C. Feng, et al., Photovoltaic properties of twodimensional (CH3NH3)2Pb(SCN)2I2 perovskite: a combined experimental and density functional theory study, J. Phys. Chem. Lett. 7 (2016) 1213
1218. Available from: https://doi.org/10.1021/acs. jpclett.6b00248. [115] Z. Zhang, Y. Zhou, Y. Cai, H. Liu, Q. Qin, X. Lu, et al., Efficient and stable CH3NH3PbI3-x(SCN)x planar perovskite solar cells fabricated in ambient air with low-temperature process, J. Power Sources. 377 (2018) 52
58. Available from: https://doi.org/10.1016/j.jpowsour.2017.11.070.

Characterization Techniques for Perovskite Solar Cell Materials

22

1. Bandgap tuning and compositional exchange for lead halide perovskite materials

[116] Y. Sheng, A. Mei, S. Liu, M. Duan, P. Jiang, C. Tian, et al., Mixed (5-AVA)xMA1 2 xPbI3 2 y(BF4)y perovskites enhance the photovoltaic performance of hole-conductor-free printable mesoscopic solar cells, J. Mater. Chem. A. 6 (2018) 2360
2364. Available from: https://doi.org/10.1039/C7TA09604F. [117] J. Chen, S.-G. Kim, N.-G. Park, FA0.88Cs0.12PbI3 2 x(PF6)x interlayer formed by ion exchange reaction between perovskite and hole transporting layer for improving photovoltaic performance and stability, Adv. Mater. 30 (2018) 1801948. Available from: https://doi.org/10.1002/adma.201801948.

Characterization Techniques for Perovskite Solar Cell Materials

C H A P T E R

2 X-ray diffraction and Raman spectroscopy for lead halide perovskites Mohammad Ziaur Rahman and Tomas Edvinsson

˚ ngstro¨m Laboratory, Department of Engineering Sciences, Solid State Physics, A Uppsala University, Uppsala, Sweden

2.1 Introduction The broad field of perovskites have been attractive for condensed matters physics and material sciences for many years due to the breath of physical properties that can be found in the system. Oxide and fluoride perovskites have been extensively studied over the years since they exhibit a plethora of intriguing photonic and electronic properties, ionic conductivity, multiferroicity, piezoelectricity, superconductivity, and metalinsulator transitions, where their properties can be tuned by the octahedral tilting with inclusion of different metals [1]. Hybrid perovskite solar cells (HPSCs), based on organic-inorganic lead halide materials, have recently emerged as a promising class of materials in photovoltaic technology [2,3]. Following the report of solid state perovskite solar cells with efficiencies up to 10% in 2012 [4], a great surge in world-wide research efforts into perovskite solar cells research can be noticed- evident by more than 4000 publications on HPSCs in 2018. These efforts and the previous experience in the other hybrid organic-inorganic solar cell systems (e.g. the dye-sensitized solar cell) lead to a certified record efficiency surpassing 23% in lab scale devices in 2018 [5]. Other possible application for the hybrid metal halide perovskite materials are hybrid light emitting diodes (HLEDs) or utilization as X-ray scintillators [6]. Perovskites are materials with a crystal structure analogous with CaTiO3, having a general chemical formula ABX3, where A is a large cation, B a smaller sized cation, and X is an anion (Fig. 2.1). In the lead halide perovskites that are investigated for photovoltaics, B is normally Pb21, X is a halide (i.e. I-, Br-, Cl-), while A is a small organic dipolar cation,

Characterization Techniques for Perovskite Solar Cell Materials DOI: https://doi.org/10.1016/B978-0-12-814727-6.00002-5

23

Copyright © 2020 Elsevier Inc. All rights reserved.

24

2. X-ray diffraction and Raman spectroscopy for lead halide perovskites

FIGURE 2.1 (A) Perovskite crystal structure ABX3 for a cubic symmetry and (B) the cubic structure of the MAPbI3 hybrid perovskite [7]. Reproduced with permission from reference T.J. Jacobsson, M. Pazoki, A. Hagfeldt, T. Edvinsson, Goldschmidt’s rules and strontium replacement in lead halogen perovskite solar cells: theory and preliminary experiments on CH3NH3SrI3, J. Phys. Chem. C 119 (2015) 25673
25683. Copyright 2015, American Chemical Society.

such as methylammonium (MA, CH3NH31), formamidinium (FA, HC(NH2)21), or a relatively large inorganic cation such as Cs1 then forming a fully inorganic perovskite. The structure, bandgap and further optoelectronic properties depends on the halide and the cation composition. The iodide compounds, APbI3, have a bandgap of 1.45, 1.60 and 1.73 eV for A 5 FA, MA and Cs, respectively. Utilizing MA as an A-cation and varying the halide, the MAPbX3 materials have bandgaps of 1.6, 2.2 and 3.0 eV for X 5 I, Br and Cl, respectively [6]. Here, an intermediate tunability allows bandgap optimization for tandem solar cells or HLED application with tunable wavelength. For example, the band gap of MAPb(I1-xBrx)3 can be linearly tuned between 1.60 eV and 2.33 eV by increasing the Br content (Fig. 2.2) [8,9]. Recently, a new record efficiency of 23.7% for a perovskite solar cell was reported on the NREL chart for certified cell efficiencies [5]. This value surpasses the record efficiencies reported for many established photovoltaic technologies based on polycrystalline materials, such as CdTe (22.1%), CIGS (22.9%), and multicrystalline Si (22.3%) [5]. All of the competing technologies, however, still have problem in competing with single crystalline Si (26.1%) or Si heterostructured solar cells (26.6%) that have shown a dramatic price drop in the last few years. Notably, the efficiency of the reported HPSC devices were measured for small area lab devices (,1 cm2), while the ability to reach stable high efficiencies with large area cells ( . 1 cm2) remains a grand challenge. For example, MAPbI3 is sensitive to water exposure and high temperatures [10]. Recently, it has been reported that HPSC could sustain good stability up to 1000 h under continuous illumination with a light source equivalent to one sun (i.e. 1000 W m22 at AM1.5 G). However, such high stability was achieved by tailoring the contact materials rather than the perovskite material [11,12]. Presumably, a higher efficiency can also be attained with multi-cation cascades. Nonetheless, it is likely that the perovskites have to go to tandem approaches for being truly competitive with single crystal silicon. Tandem devices made from metal halide perovskites represent a highly tunable system that potentially can be made with very costeffective methods and short energy pay-back time compared to epitaxially grown III-V

Characterization Techniques for Perovskite Solar Cell Materials

2.1 Introduction

25

FIGURE 2.2 (Top) Optical photographs, (A) PL of films, and (B) band gap change with linear bandgap tuning by I-to-Br halogen exchange. (C) Incident-photon-to-current-efficiency (IPCE) for solar cell devices with Br fraction up to 10%. [9]. Adapted with permission from reference B.-w. Park, B. Philippe, S.M. Jain, X. Zhang, T. Edvinsson, H. Rensmo, et al., Chemical engineering of methylammonium lead iodide/bromide perovskites: tuning of opto-electronic properties and photovoltaic performance, J. Mater. Chem. A 3 (2015) 21760
21771. Copyright 2015, Royal Society of Chemistry.

Characterization Techniques for Perovskite Solar Cell Materials

26

2. X-ray diffraction and Raman spectroscopy for lead halide perovskites

semiconductor tandem systems. Toward the design of highly efficient tandem HPSCs, or fully inorganic PSCs, it is crucial to quantify their crystalline structure and symmetries. Because, such understanding is vital to control the structure as well as the fundamental optoelectronic properties for a resilient device structure with a durable performance. In this context, X-ray diffraction and Raman spectroscopy provide useful information of the material properties, such as the crystal structure, symmetries, displacements, defects, micro-crystallinity, orientations, vibrations, phonon lifetimes, and their interdependence on material alterations. These techniques contribute with crucial information that is necessary to find structure-to-property relationships from materials to function.

2.1.1 XRD and Raman spectroscopy Structural and chemical characterization with X-ray and Raman techniques, respectively have proven successful in uncovering a broad range of microstructural, morphological ˚ ngstro¨m molecular chemistry to device-scale alignment. and chemical features from sub-A These techniques become more popular with advancement in instrumentation, X-ray and light source brightness, and detector technologies. These techniques now allow one to have high quality data and quantitative information to obtaining precise description of the molecular structure and microstructure of the materials of interest. This wealth of information combined with electrical characterization and modeling is nowadays commonly been practiced in everything from fundamental investigations to the facilitation of design rules for a rational design of materials and complete devices. The discovery of the possibility to diffract X-rays from crystal and the constructive reflection maximum condition by Max von Laue with the incoming (k) and outgoing wavevectors (k0 ) satisfying R (k- k0 ) 5 2πm where R is the Bravais lattice vector of the crystal and m the Miller index triplet, was monumental for the later development and use of X-ray diffraction. The Laue condition can also be written in the equivalent 0 form ei(k-k )  R 5 1, where Δk 5 (k- k0 ) is referred to as the scattering vector. Utilizing monochromatic X-rays that are elastically scattered, W.H Bragg and W.L Bragg constructed a geometric analog satisfying the Laue condition, where the constructive diffraction could be formulated into the Bragg equation



2dhkl sinθb 5 nλ

(2.1)

Where dhkl is the distance between diffraction planes with Miller index (h k l ), λ is the wavelength of the incoming X-ray, θB is the Bragg angle, and n is a positive integer representing the diffraction order. The Laue constraint for the intensity I(R)- maximum is now naturally fulfilled by the constructive interference for a fixed wavelength and replacing the general angle θ with the angle for which the intensity is at maximum, θB. The geometric situation is illustrated in Fig. 2.3A in a Bragg-Brentano setup where the two extra path lengths of dhkl sinθ for the bottom lattice plane elastic scattering can be seen and XRD diffractograms for cubic Pm-3m and tetragonal I4/mcm MAPbI3 (Fig. 2.3C). Although the maximum intensity at angle θ is instructive to infer the set of distances in the crystalline materials, and thus reflecting the internal structure and symmetry, also the peak shape contains information from the number of crystalline planes to form or to cancel



Characterization Techniques for Perovskite Solar Cell Materials

2.1 Introduction

27

FIGURE 2.3 (A) Schematic illustration of a Bragg-Brentano geometry XRD setup, (B) illustration of a XRD setup with multiple angles for generation of 2D q-range and construction of pole figures [16]. (C) Synchrotron XRD patterns of MAPbI3 for the cubic Pm-3m and the tetragonal I4/mcm structure, and R-symmetry point superlattice peaks [17]. (D) GIWAXS images and structure orientation information of polycrystalline and (E) near single-crystalline (BA)2(MA)3Pb4I13 Ruddlesden
Popper phases [18]. (B) Adapted with permission from reference K. Inaba, S. Kobayashi, K. Uehara, A. Okada, S.L. Reddy, T. Endo, High resolution X-ray diffraction analyses of (La, Sr)MnO3/ZnO/Sapphire(0001) double heteroepitaxial films, Adv. Mater. Phys. Chem. 03 (2013) 72
89. Copyright 2013, Scientific Research. (C) Reproduced with permission from reference P.S. Whitfield, N. Herron, W. E. Guise, K. Page, Y.Q. Cheng, I. Milas, et al., Structures, phase transitions and tricritical behavior of the hybrid perovskite methyl ammonium lead iodide, Sci. Rep. 6 (2016) 35685. Copyright 2016 Nature Publishing Group. (D and E) Reproduced with permission from reference H. Tsai, W. Nie, J.-C. Blancon, C.C. Stoumpos, R. Asadpour, B. Harutyunyan, et al., Highefficiency two-dimensional Ruddlesden
Popper perovskite solar cells, Nature 536 (2016) 312. Copyright 2016 Nature Publishing Group.

the non-constructive interference signal. Peak shape analysis can here be used to extract local disorder, strain, and crystallite size. The ability to separate of distortion and particle size broadening in X-ray patterns, however, as well as coupling effect from local disorder and strain have to be taken into care when performing detailed peak shape analysis [13,14]. When the sample is single crystalline or textured one can quantify the crystal symmetry or crystallite orientations by extending the diffraction analysis to other directions compared to the Bragg angle, either by using a wide-angle detector, repositioning the detector to angular ranges perpendicular to the Bragg angle, or rotate the sample (Fig. 2.3B). In the most general case, one can construct a pole figure to fully characterize the distribution of diffractions from a textured sample with a variation in crystallite orientations by

Characterization Techniques for Perovskite Solar Cell Materials

28

2. X-ray diffraction and Raman spectroscopy for lead halide perovskites

rotating the sample axes while having the corresponding Bragg reflection of interest fixed, successively change the Bragg angle, and add data from an additional perpendicular axis rotations (rocking curve) of the sample. The two first parts of this are illustrated in Fig. 2.3D and E where diffraction pattern from gracing incident wide angle X-ray scattering (GIWAXS) of polycrystalline and near single-crystalline perovskite derived Ruddlesden
Popper (RP) phases are shown. The crystallographic unit cell of RP phases can be seen as an elongation of the dimensions in the perovskite structure, keeping layers of perovskite symmetries (ABX3) in between with the general RP phase composition formula An11BnX3n11 [15]. For more complicated A-site cation compositions in RP phases, they can also be represented as An-1A0 2BnX3n11, where A and A0 (A-prime) represent two different cations and B refers to the Pb, Bi, or Sn metal cations usually incorporated in the hybrid metal halides. The A cations are located in the perovskite layer with a 12-fold cuboctahedral coordination to the anions while the A0 cations have a 9-fold coordination and are located at the interface between the perovskite layer and the intermediate layer before the next perovskite layer. Apart from a general structure determination and peak shape analysis, more details can be extracted from diffraction by analyzing the diffraction intensities or shifts in more detail. First, one should note that X-rays are diffracted from the electron clouds around the nuclei forming the lattice planes where any anisotropic polarization will give an error in the determination of the derived nuclei positions. In these cases, neutron diffraction characterization is advised for a high resolved structure determination. The amplitude of the scattering (within the Fraunhofer condition) from electrons around a nuclei can be described by a phase factor e2iΔk  r(i,j) containing the scattering vector Δk and the pairwise distance between all pairs of point charges r(i,j). It can be formulated into an integral of the contribution from a charge distribution into an atomic form factor [19]. ð ρe ðrÞ e2Δk  r dr (2.2) fatom 5 atom



where ρe(r) is the electron density charge distribution around be determined via the square of the electronic wave function. one can combine the individual atomic form factors N P F5 fatom eiΔkrn where fatom is the integral in Eq. 2.2 and rn

the atom, and can formally For a distribution of atoms, into a structure factor denotes the position of the

n51

n-th atom in the structure. Combining this with the Laue condition and the notation for Miller indices, one could obtain FðhklÞ 5

N X

fatom e½i2πðhxn 1kyn 1lzn Þ

(2.3)

n51

For the simple cubic systems, with N 5 1 and the atoms in the primitive unit cells at (0,0,0), reflections can be seen for all miller index combinations. For the FCC lattice with N 5 2 and cubic sub-symmetry with at the basis (0,0,0) and (1/2, 1/2, 1/2), the structure factor becomes 4fatom when all hkl are even or odd and 0 otherwise. Of interest here is a general form factor for the perovskite structure (ABX3), here exemplified for the cubic structure with space group Pm3m with the A-site cation at (0,0,0), B cation at (1/2, 1/2, 1/2), and 3 X

Characterization Techniques for Perovskite Solar Cell Materials

29

2.1 Introduction

halogens at (0, 1/2, 1/2), (1/2, 0 1/2), (1/2, 1/2, 0) and the possibility that the B-site cation have a small off-center displacement (ΔB) or tilt of the X-B-X axis from two different positions of the halide ion (denoted X and X2 with a shift of ΔX and ΔX2, respective) formulated from Eq. 2.3 as 2

2 6 6 6 6 Fperovskite ðhklÞ 5 fA 1 eiπðh1kÞ 6 6 6 6 fX 4

3

7 7 7 97 8 7 ½  exp i2πlΔ 1 > X > > 2 3> =7 < 7   7   1 5 4 5 ½  1 Δ exp i2πl 1 1 exp iπðh 2 kÞ > > X2 > > ; : 2 fB





3  1 1 ΔB 5 1 exp4i2πl 2 

(2.4)

Hybrid perovskites are considered approximately cubic at high temperature from the thermally averaged structure from the motion of the organic A-site cation, while an off-center shift of the B-site central metal will also introduce a pseudo-cubic structure. A small elongation of the unit cell (e.g. 0.5%) along the c-axis compared to the a-axis will result in a tetragonal structure with c/a 5 1.005, where already a ΔX of 5 picometer (pm) will result in a detectable shift for the corresponding pseudo-cubic reflection of about 0.2 using Cu Kα radiation and Eq. 2.1. A Glazer notation [20] for octahedral tilting is often used to categorize symmetries from different tilt combinations using subscripts of 0, 1 , and
with respect to tilts around the x, y, and z-axes of the underlying cubic perovskite structure Pm3m where a, b, and c represents the magnitudes of the tilts. As an example, a2b1b1 denotes a negative tilt around the x-axis and positive tilts around the y- and z-axes, where the double notation of b1b1 denotes a similar magnitude in the tilts about the y- and z-axes. Glazer found 23 different perovskite structures from the possible tilts [20,21], while more recent analysis of sub-symmetry dependent groups have reduced this to 15 [22]. Taking symmetry breaking from off-center displacements as for ΔB 6¼ 0 in Eq. 2.4, forty (40) additional symmetries are possible [23]. Recalling the temperature induced phase transitions from a tetragonal I4/mcm crystal structure into cubic Pm-3m structure for higher temperatures in Fig. 2.3C, and that the out-of-phase rotations are located at the R-symmetry point (1/2 1/2 1/2) at the Brillouin zone (BZ) boundaries and the in-phase rotations are at the M-symmetry point (1/2 1/2 0), the phase transition lead to new lattice vectors, with modified unit cells where the M- or R-symmetry points fold over to the Γ point in the lower symmetry phase, corresponding to an orthorhombic P4/mbm and tetragonal I4/mcm symmetry for the M-symmetry point and R-symmetry point instability, respectively [17]. The corresponding Glazer notations is then naturally the untilted a0a0a0 for the cubic structure, while the tilting and broken symmetry along the c-axis can be represented by a0a0c2 for the tetragonal crystal structure. Analysis of symmetries and textures on different scales can be obtained by combining GIWAXS and gracing incident small angle X-ray scattering (GISAXS) with a detector placed at longer distances from the sample (Fig. 2.4). Localized spots occur for the diffraction of the textured films, with successive transformation to even semi-circle intensities for films with fully randomly ordered crystallites.

Characterization Techniques for Perovskite Solar Cell Materials

30

2. X-ray diffraction and Raman spectroscopy for lead halide perovskites

FIGURE 2.4 (A) Illustration of how GIWAXRD and GISAXS can be used for texture analysis in thin films depending on the required dimensions where (B) represents randomly oriented arrangements of crystallites with circular diffraction rings, except for the slightly oriented (200) reflection. (C) corresponds to a textured and preferentially oriented films with diffraction patterns as arcs of different intensities and (D) corresponds to a highly oriented films produce more localized diffraction spots/ellipses [24]. Reproduced with permission from reference J. Rivnay, S.C.B. Mannsfeld, C.E. Miller, A. Salleo, M.F. Toney, Quantitative determination of organic semiconductor microstructure from the molecular to device scale, Chem. Rev. 112 (2012) 5488
5519. Copyright 2012, American Chemical Society.

Thermal, light- and chemical induced phase transformations are also important to characterize and relates to mechanical stability during day-night operation, and longtime stability. Certainly, the promises of lead halide perovskites (LHPs) as a solar cell material is undeniable but it is very important that the crystal structure integrity and chemical composition can be retained during extended use. LHP undergoes degradation in contact with water, humidity, light, and temperature [25,26]. The hybrid lead halogen perovskites are water soluble salts where uncapsulated perovskites take on water very

Characterization Techniques for Perovskite Solar Cell Materials

2.1 Introduction

31

easily [27], while light exposure can break the bonds into halogen vacancy-halogen interstitial pairs that enable halogens to migrate [28], or convert any oxygen that might be present into highly reactive superoxide [29]. Particularly, blue and ultraviolet light generate electrons in antibonding N2p-H1 s orbitals with degradation of the MA1 organic cation [30,31]. Moreover, when exposed to heat and light, perovskites react with almost all metals. Because, heat volatilizes halide species and light enhances halogen mobility [26]. Additionally, due to remarkably low fracture energy (below 1 J m22) and high their thermal expansion coefficient, approximately 10 times higher than that of glass substrates or transparent conducting oxide electrodes [32], perovskites build up stress during temperature changes that may lead to delamination or accelerated decomposition [32
34]. Therefore, design of structurally stable perovskites remains a grand challenge to make solar panels with metal halide perovskite that will last for .25 years. With XRD measurements, it is possible to devise design strategies toward a structurally stable perovskites as well as approached to cap the 3D perovskites with 2D perovskites as a chemical effusion blocking layer. We here below discuss few of the recent XRD studies on LHP. Methylammonium lead iodide (MAPbI3), a common LHP, showed reversible phase transition between tetragonal and cubic symmetry at solar cell operating temperatures [35]. For example, MAPbI3 crystallize at room temperature to a wide bandgap hexagonal symmetry (P63mc) and a trigonal (P3m1) black phase at a higher temperature [36]. This structural phase transition adversely influences photovoltaic properties [35]. Therefore, the development of temperature immune perovskites is highly essential. Cation exchange is one of the possible schemes to overcome the phase transition with MAPbI3 [37
40]. This can be done by employing formamidinium lead iodide (FAPbI3) as a mixture with MAPbI3 [40]. In this mixture of MAPbI3 and FAPbI3, the exchange of the organic cation from methylammonium (MA) to formamidinium (FA) results in a material with either a trigonal structure (black color, α phase) or a hexagonal structure (yellow color, δ phase) depending on the synthesis temperature [36,40]. Notably, no significant δ phase transition happened within the solar cell operating temperature. Because, a phase transition from the δ phase to α phase takes place at B125  C, which is way higher than the solar cell operating temperature [37]. Incorporation of a smaller cation (MA) with a high dipole moment stabilizes the trigonal phase of FAPbI3 without any lattice shrinkage or changes in the optical properties due to enhanced I
H hydrogen bonds between the cation and the inorganic cage or to an increase in the Madelung energy of the structure [38
41]. As a result, the δ phase transition can be completely suppressed as is shown in temperature-dependent X-ray diffraction. Fig. 2.5A illustrates the differences between the crystal structure of MAPbI3 and FAPbI3. When using a mixture of MA and FA, the incorporation of MA formed a 3D network of layered octahedra by reducing the number of shared iodide ions from three to one. At room temperature, the octahedra are not perfectly aligned but results in a close to identical twist of every second octahedral layer in the c direction. The unit cell expands with increasing temperature to help aligning the octahedra perfectly along the c axis, yet maintaining the tetragonal structure. To be precise, the tetragonal symmetry collapses and the trigonal structure of α
FAPbI3 is formed if the perovskite structure consists of .80%

Characterization Techniques for Perovskite Solar Cell Materials

32

2. X-ray diffraction and Raman spectroscopy for lead halide perovskites

FIGURE 2.5 (A) Illustration of the crystal structure of PbI2 and MAPbI3 at different temperatures together with the α
FAPbI3 and δ
FAPbI3 phase and (B and C) comparison between the experimental and theoretical XRD patterns [36]. Reproduced with permission from reference A. Binek, F.C. Hanusch, P. Docampo, T. Bein, Stabilization of the trigonal high-temperature phase of formamidinium lead iodide, J. Phys. Chem. Lett. 6 (2015) 1249
1253. Copyright 2015, American Chemical Society.

of FA [36]. This can be observed in X-ray diffraction measurements by a shift of the diffraction pattern to lower angles. The XRD patterns of the synthesized yellow FAPbI3 and stabilized black FAPbI3 and their theoretical patterns are illustrated in Fig. 2.5B and C. Temperature-dependent XRD measurements is widely practiced to investigate the phase transformation between the between different phases of LHP. In XRD patterns, a few peaks disappear when the tetragonal phase transforms into the cubic structure whilst some double peaks merge into single peaks. A peak shift in the XRD with the temperature not only gives information about the phase transformation, but also about thermal expansion coefficients. With this information, one can evaluate how the materials behaves under thermal stress. A contour plot of individual groups of peaks in XRD measurements as a function of the temperature between 35 and 95  C is given in in Fig. 2.6, in which the phase transformation between the tetragonal and cubic phases is clearly seen. At room temperature, the data perfectly agree with a tetragonal structure. However, some peaks disappear and some double peaks merge into single peaks at higher temperatures that matches the cubic structure, as is seen in Fig. 2.6. Noticeably, the phase transformation

Characterization Techniques for Perovskite Solar Cell Materials

FIGURE 2.6 (A) Temperature dependent X-ray diffractograms of some selected diffraction peaks for analysis of the cubic-to-tetragonal phase transition and lattice expansion coefficient. (B) Refined lattice parameters during orthorhombic-to-cubic phase transitions using synchrotron X-ray powder diffraction. The 3D figure show the (200) cubic Bragg peak and the (220)/(004) tetragonal Bragg peaks as a function of temperature. Reproduced with permission from (A) reference T.J. Jacobsson, L.J. Schwan, M. Ottosson, A. Hagfeldt, T. Edvinsson, Determination of thermal expansion coefficients and locating the temperature-induced phase transition in methylammonium lead perovskites using X-ray diffraction, Inorg. Chem. 54 (2015) 10678
10685. Copyright 2015, American Chemical Society. (B) Reference P.S. Whitfield, N. Herron, W.E. Guise, K. Page, Y.Q. Cheng, I. Milas, et al., Structures, phase transitions and tricritical behavior of the hybrid perovskite methyl ammonium lead iodide, Sci. Rep. 6 (2016) 35685. Copyright 2016, Nature Publishing Group.

34

2. X-ray diffraction and Raman spectroscopy for lead halide perovskites

was pinpointed to 54  C within 1  C of uncertainty [32]. This fully reversible process occurs at the temperature swings of an operational solar cell. The phase transition has also been studied with time-of-flight neutron powder diffraction and high resolution synchrotron X-ray powder diffraction [17]. The X-ray data for d6
MAPbI3 (Fig. 2.6B) showed very clearly that the cubic and tetragonal phases coexisted over a wide temperature range (nearly 30 K)- referring that this phase transition was first-order. The orthorhombic-to-tetragonal phase transition was also first-order from Pnma to I4/mcm symmetry with co-existing phases (Fig. 2.6B). The Pnma is not a subgroup of I4/mcm- meaning that a continuous second-order phase transition is not possible. It is because the Pnma to I4/mcm transformation requires modes with two different irreducible representations to condense, and thus has two primary order parameters. These requirements violate one of the Landau conditions for a second-order phase transition to occur [42]. Raman spectroscopy is a useful tool to investigate the lattice vibrations symmetry, the temperature and pressure phase diagrams, the degree of crystallinity, the structural changes in mixed halide perovskites, and the stability of halide perovskites during their operation in solar cells [43
47]. These possibilities also lead to study the vibrational properties, electron
phonon interactions, dielectric screening, heat transport, and elastic properties of MAPbI3 and its related compounds [48
50]. In the Raman Effect, a small portion of light is idealistically scattered close to the incoming monochromatic light, if the polarizability α of the electron cloud is changing during the vibration. The polarizability is formally a tensor but can for polarized light and an isotropic material in a fixed geometry be treated as scalar and defined as the ability to create a dipole µ 5 αE in a given electrical field E. A fundamental evaluation of the polarizability requires a quantum mechanical picture but we will here just briefly show the inelastic scattering process without further details on the intensity of the inelastically scattered light. The polarizability for a small nuclei displacement, z0, in a bond can be Taylor   expanded as α 5 α0 1 @α z 1 . . . where α0 is the polarizability that does not depend on @z 0 the externally varying field and ð@α=@zÞo is the change in polarizability during a displacement in z-direction. Fixing the coordination system and only consider a plane polarized light, the scalar electric field E 5 E0cos(2πν 0t), with ν 0 as the frequency of the light and considering only the time-dependent part of the electric field at a fixed location, denoting the material vibration frequency as ν M and recalling that μ 5 αE, one formulate the dipole contributions as     1 @α 1 @α z0 E0 cos½2πðυ0 1 υM Þt 1 z0 E0 cos½2πðυ0 2 υM Þt (2.5) μ 5 α0 E0 cosð2πυ0 Þ 1 2 @z 0 2 @z 0 The re-radiated field ER at position r can then be formulated from the acceleration of the dipole, via the second time derivative of the dipole using conventional classical electrodynamics and a fixed coordinate system extracting the contribution in z-direction with [51]   q @2 μ=@t2 sinΦz (2.6) ER 5 4πε0 c2 r

Characterization Techniques for Perovskite Solar Cell Materials

2.1 Introduction

35

where q is the charge, ε0 the permittivity of vacuum, c the speed of light. Combining Eqs. 2.5 and 2.6, the first term in Eq. 2.5 will contributes to the elastic (Rayleigh) scattering, the second to the anti-Stokes inelastic scattering and the third to the Stokes inelastic scattering. As the intensity of the scattered light is proportional to the square of the field, this relation and that the light frequencies are extracted as inner derivatives of μ in Eq. 2.5, give the well known λ24 dependence on both the elastic and inelastic scattering intensity. The transitions between energy levels are the extracted as Δν 5 1/λincident-1/λscattered and most commonly reported as a Raman shift in cm21 from the incoming laser frequency (defined as zero). The lattice vibrations of MAPbI3 can categorically be classified into three distinct groups: internal vibrations of the organic MA cations that span the frequencies between 300 and 3200 cm21; libration and spinning modes of MA cations between 60 and 180 cm21; and internal vibrations of the inorganic PbI3 network below 120 cm21 [52]. See Fig. 2.7A and B. The normal modes of MAPbI3 are thus resulting from both vibrations of the PbI3 network and vibrations of the MA cations and their couplings. Study the symmetry of the normal modes using a group analysis in each case show that there are 48 normal modes as vibrations of the PbI3 network (3 acoustic and 45 optical modes), consistent with the number of Pb and I atoms in the unit cell. Whereas for the MA cations, there are 4 spinning modes, 20 normal modes that are combinations of 12 translations and 8 librations modes, and 72 internal vibrations [52,53]. The symmetry representations of vibrations of the PbI3 network is Γ 5 7B1 u 1 6B2 u 1 7B3 u 1 5Ag 1 4B1 g 1 5B2 g 1 4B3 g 1 7Au where the g type (gerade, even) contains inversion symmetry of the phase and the u type (ungerade, odd) a phase change of the corresponding orbitals. The internal vibrations of an isolated MA cation has a symmetry representations of A2 1 5A1 1 6E, with the E modes being doubly degenerate [52]. The modes of the MA cations appear grouped in quadruples of nearly degenerate frequencies—suggesting that the inter-cation coupling is small. The modes of the MA cations can be described as symmetric or asymmetric CH3- and NH3deformation modes, symmetric C
N stretching modes, symmetric or asymmetric C
H and N
H stretching modes, and asymmetric CH3
NH3 rocking modes. For PbI3 network, Ag, B1 g, B2 g, and B3 g symmetries are predicted to be Raman-active (Fig. 2.7C), whereas B1 u, B2 u, and B3 u symmetries are predicted to be IR-active. None of the modes is at the same time Raman- and IR-active revealing a center-of-inversion symmetry. The Au modes are expected to be silent [54]. For MA cations, the molecular intramolecular vibrations can be assigned while their Raman and IR activity shifts from their inclusion and symmetry adoption in the hybrid lead halide perovskites are still under investigation [55]. IR spectroscopic vibrational investigation in the 700
3500 cm21 region on inorganic and hybrid (layered) perovskites provide useful guidelines for the LHPs Raman spectral assignment [49,56]. Hybrid perovskites are intrinsically complex materials where assignment of the Raman spectrum of MAPbI3 are helped by first-principles DFT calculations, explaining of the general role of various types of interactions and local structural disorder for the materials properties. Recently, a combined experimental and theoretical Raman vibrational analysis of MAPbI3 in the low-frequency region was employed to figure out the bands associated to the vibrations of the interacting inorganic/organic constituents

Characterization Techniques for Perovskite Solar Cell Materials

36

2. X-ray diffraction and Raman spectroscopy for lead halide perovskites

FIGURE 2.7 (A) Comparison between the measured (a) and calculated (b) Raman spectra of MAPbI3 in the orthorhombic structure. (B) Schematic representation of the most important Raman-active modes of the PbI3 network in MAPbI3. The symmetry and calculated frequency of each normal mode are indicated at the top of the each panel, and the solid black lines indicate the unit cell [52]. Reproduced with permission from reference M.A. Pe´rezOsorio, Q. Lin, R.T. Phillips, R.L. Milot, L.M. Herz, M.B. Johnston, et al., Raman spectrum of the organic
inorganic halide perovskite CH3NH3PbI3 from first principles and high-resolution low-temperature raman measurements, J. Phys. Chem. C 122 (2018) 21703
21717. Copyright 2018, American Chemical Society. Characterization Techniques for Perovskite Solar Cell Materials

2.2 Resonance Raman spectroscopy of halide substituted hybrid perovskites

37

[52]. The proposed assignment of the low frequency Raman spectrum of orthorhombic MAPbI3 is indicated in Fig. 2.7A and B by the dashed black lines. The measured peak at: 9 cm21 (labeled as “0”) is assigned to Pb
I
Pb rocking modes with Ag symmetry; the peak at 26 cm21 corresponds to Pb
I
Pb bending modes with Ag symmetry (1); the small peak at 32 cm21 originates from Pb
I
Pb bending modes with B2 g symmetry (2); the peaks at 42 and 49 cm21 are assigned to Pb
I
Pb bending modes with Ag (3) and B2 g (4) symmetries, respectively; the peak at 58 cm21 corresponds to librational modes of the MA cations (5); the peak at 85 cm21 is assigned to librational modes of the MA cations as well as to Pb
I stretching modes with B3 g symmetry (6); the peak at 97 cm21 stems from Pb
I stretching modes of Ag symmetry (7), and the small peak at 142 cm21 (8) is assigned to librational modes of the MA cations [52]. The Raman spectrum of the related organohalide MAPbCl3 perovskite shows various bands between 40 and 321 cm21, associated to the librations of the MA organic cations and a band at 483 cm21, assigned to the MA torsional mode [50]. The frequency of the MA torsional modes in the crystal structures may be the result of two opposite effects associated to the interactions between the cation and the inorganic cage. The deformation of the MA molecule shifts the torsional mode toward higher frequencies while the formation of hydrogen bonding interactions shifts this mode toward lower frequencies [55
58], while the physics of the inorganic cage of the structure is more well known [57,58]. The orientational disorder of the organic cations in MAPbI3 results in torsional frequency being spread over a large range of values [59]. Generally, the Raman peaks observed below 50 cm21 in the experiments are internal vibrations of the inorganic PbI3 network, whereas the features at 58 and 142 cm21is internal vibrations of the inorganic network as well as combinations of MA libration and the inorganic network vibrations at higher wavenumbers. Because the Raman peaks P1, P3, P4, P5 (at 26, 42, 49, and 58 cm21) are intense and well separated, they could be used as markers for monitoring of the MAPbI3 structure and stoichiometry. Particularly, the ratios of the intensities P1/P5, P3/P5, and P4/P5 could be used to monitor the PbI2/MAI ratio in the system to measure deviations from the ideal stoichiometry or to monitor degradation [49].

2.2 Resonance Raman spectroscopy of halide substituted hybrid perovskites Halide substitution in metal halide perovskites (MHPs) with the nominal composition CH3NH3PbI2X, where X is I, Br, or Cl, influences the morphology, charge quantum yield, and local interaction with the organic MA cation. Raman spectroscopies combined with theoretical vibration calculations supported the early findings that iodide-chloride perovskites phase separate and have also revealed that halide substitution with halides forming a shorter bond length to Pb can delocalizes the charge to the MA cation; therefore, liberate the vibrational movement of the MA cation that lead to a more adaptive organic phase [60]. Raman spectroscopy with 532 nm laser excitation are performed within the absorption profile of MHP materials and are thus under resonance (electronically exciting) conditions, providing information on vibrations in excited state and can also give clues for the charge separation/transfer mechanisms. Fundamental vibrations in the isolated clusters and the trends in the splitting of the degenerate states when different halogens are included

Characterization Techniques for Perovskite Solar Cell Materials

38

2. X-ray diffraction and Raman spectroscopy for lead halide perovskites

were investigated with low-frequency Raman measurements (down to 10 cm21) and nonperiodic DFT calculations with an emphasis on the ordering of the peaks to determine Raman properties of MHPs [60]. The experimental Raman spectrum for MAPbI3 shows vibration peaks at 40, (54), (63), 71, 94, 108, 135, and 145 cm21, whereas MAPbI2Cl shows corresponding peaks at 40, NA, NA, 71, 97, 110, and a broad peak at 166 cm21 as seen in Fig. 2.8. The NA (not applicable) notation, emphasize that the peaks cannot be resolved and the spectra instead show a shoulder in that area. Besides the strongest Raman peaks at 69
73, 94
97, and 108
110 cm21, peaks also at 40 and 54 cm21 depends on halide composition during synthesis. Periodic DFT calculations provides a suitable model system for single-crystalline materials where the different orientation of the cations must be considered to be periodic. For non-periodic or non-crystalline materials, cluster calculations can instead be informative for the study of local effects in the inorganic octahedron and the behavior of the organic cation. In the present example, an inorganic octahedron unit cluster PbX6 with Oh symmetry is used with two MA-dipole canceling cations. A PbI6 octahedron has 15 internal degrees of freedom (3 N
3, where N is the number of iodine atoms). According to group theoretical representation [61], it can be written as A1 g 1 Eg 1 2T1 u 1 T2 g 1 T2 u, where A1 g, Eg, and T2 g are Raman active, the two T1 u modes are IR active, and T2 u is a silent mode (neither Raman or IR active). PbI6 as a molecular unit within the lattice belongs to the Oh symmetry group. Deviation from this symmetry would result in splitting of the degenerate states, and eventually complete removal of symmetry and 15 different bands. Calculated Raman spectra for PbX6 and (MA)2PbX6 clusters are shown in Fig. 2.8A
G, and experimental data for MAPbI3 and MAPbI2Cl are shown in Fig. 2.8H and I. The calculations reveal three different groups of vibrational modes, a triply degenerate asymmetric X-Y, X-Z, and Y-Z vibrations (mode A), a double-degenerate asymmetric “breathing” (mode B), and non-degenerated symmetric “breathing” (mode C), as well as the MA vibrations (rotation, wagging, MA
MA stretch) shown in Fig. 2.8J. A shift in the Raman peaks in (MA)2PbX6 clusters to higher wavenumbers (energy) compared to those in the PbX6 clusters were observed, caused by the organic cations that extend the motion of X from the Pb21 atom. The calculated Raman spectrum of (MA)2PbCl6 (see Fig. 2.8C) differs somewhat from the other clusters while mode A shifts to lower wavenumbers and mode B appears as two peaks. Notably, all (MA)2PbX6 structures showed Raman activity of the MA groups between 140 and 180 cm21 and Pb-I vibrations at lower wavenumbers. DFT calculations on mixed halide clusters, such as (MA)2PbI5Cl and (MA)2PbI4Cl2 has previously shown that a single Cl substitution does not result in large differences in calculated Raman vibrational spectra compared to the (MA)2PbI6 cluster, while larger changes are found for two introduced Cl atoms. These results carries valuable information for the possibility to identify doped phases. Fig. 2.8F and G show “Raman activities compared to the (MA)2PbI6 cluster, which is assigned to asymmetric vibrations of Pb
I between 40 and 95 cm21. Moreover, there are two small appearances of additional vibration modes, for instance N10 (green color, N
Cl stretch via H at 240 cm21) and D2 (green color, Pb
Cl stretch at 190 cm21) in Fig. 2.8F. The case of (MA)2PbI4Cl2 with double Cl substitution

Characterization Techniques for Perovskite Solar Cell Materials

FIGURE 2.8 (A) Experimental Raman spectra, (B) the three main vibrational modes of the ideal and perturbed inorganic Oh octahedra, and bimethylammonium-installed octahedra DFT-calculated Raman spectra for (C) PbI6 with 2 MAPbI6, (D) PbBr6 with 2 MAPbBr6, and (E) PbCl6 with 2 MAPbCl6, and comparison of 2 MAPbI6 with (F) 2 MAPbI5Br, (G) 2 MAPbI4Br2 (H) 2 MAPbI5Cl, and (I) 2 MAPbI4Cl2. (J, K) Normalized experimental Raman spectra recorded at very low laser intensity (,0.01 mW) [60]. Reproduced with permission from reference B.-w. Park, S.M. Jain, X. Zhang, A. Hagfeldt, G. Boschloo, T. Edvinsson, Resonance Raman and excitation energy dependent charge transfer mechanism in halide-substituted hybrid perovskite solar cells, ACS Nano 9 (2015) 2088
2101. Copyright 2015, American Chemical Society.

40

2. X-ray diffraction and Raman spectroscopy for lead halide perovskites

in the octahedral unit in Fig. 2.8G has rather different vibration modes, viz., mode D4 (violet color, asymmetric Pb
Cl stretch, 75
82 cm21) and highly increased intensity of mode D20 (violet color, asymmetric Pb
Cl stretch at 185 cm21) compared with mode D2 (green colored solid line in Fig. 2.8F). Moreover, in Fig. 2.8G the Raman activity of mode M4 (violet color, at 138
143 cm21) is shown to shift to lower wavenumber compared to mode M3 (green color, Fig. 2.8F), but mode N20 (violet color, at 247 cm21) is shifted to higher wavenumber for mode M20 (green color, Fig. 2.8F)” [60]. Observing the new vibration signatures with relative intensity shifts in the Raman activities of (MA)2PbI4Cl2 compared to the undoped analog, it is thus possible to identify local halide substitution or different phases. For example, both the single- and double-Clsubstituted OMHPs have shown commonly a decreased Raman intensity of modes C3 and C4 (green and violet colors) compared to mode C0 (black color). The DFT-calculated Raman spectra for three vibrational modes and the experimental Raman signal of MAPbI3 (Fig. 2.8A, H, and I) showed three shoulders or peaks in the 70
120 cm21 associated to modes A, B, and C. Additional four peaks (m, d, e, and f) appear between 140 and 400 cm21 are assigned to MA rotation, MA wagging, and symmetric MA
MA stretch [60,62]. Because of the limitation of cluster models to emulate the full symmetry of the crystal, cluster analysis should only be taken as representative of the local modes of the crystalline system, but can as such be used to analyze the details of chloride effects during crystallization [63] or effects from local charge delocalization and transfer [60]. If one analyze the Raman spectrum of MAPbI2Cl in more detail, it can be seen that the mode B as peak “b0 ” shows peak shift to lower intensity. This is correlated with disordered inorganic frameworks where higher Raman activities have been observed previously from the due to increased electron
phonon interaction between the central cationic metal and anionic oxygen [64]. This same effect holds for MAPbI3 and the sub
MAPbI3 crystal in MAPbI2Cl on modes A and C. In the range of 136
150 cm21, that MAPbI3 sample shows 1.3 times higher Raman intensity at 143 cm21 than that of MAPbI2Cl at B145 cm21 as is seen in Fig. 2.8. During resonance excitation at 532, absence of the feature at 143 cm21 nm implies the disappearance of initial and final state polarization for the corresponding Raman rotation/wagging in the MA cation. The difference at 143
145 cm21 between MAPbI3 and MAPbI2Cl samples seems well correlated with the local charge transfer yield [60] and could be part of the origin of the improvements in the mixed halogen systems [65,66]. DFT calculations for OMHPs display the transitions of charges from the HOMO to LUMO (with LUMO 1 1 and LUMO 1 2). For example, the HOMO of 2MAPbI6 showed an I 5p π-bonding orbital while its LUMO, LUMO 1 1, and LUMO 1 2 were decomposed to “Pb(6 s)
I(5p)” σ-antibonding, “Pb(6p)
I(5p)” σ-antibonding, and “Pb (6 s)
I(5p)” σ-antibonding orbitals, respectively [60]. The observed 7 times lower intensity for the 143 cm21 mode in the MAPbI2Cl sample corresponds to a loss of polarization on mode M of internal MAPbI3. A photoexcited state that cannot change polarizability of an MA cation radical or neutral MA molecule can then play an important role as an organic cation charge stabilizer or a neutral dipolar molecule in the cage of the inorganic framework [67]. The understanding of these phenomena lies deep within the origin of the relative intensities in Raman measurements that comes from the losses of the electromagnetic energy in the incident light to an excited vibration level and the change in

Characterization Techniques for Perovskite Solar Cell Materials

2.3 Raman spectroscopy probing bleaching and recrystallization process of CH3NH3PbI3 film

41

polarizability of the electron cloud during the vibration, and thus also the local electron density properties during the light-matter interaction, and subsequent vibrations and delocalization.

2.3 Raman spectroscopy probing bleaching and recrystallization process of CH3NH3PbI3 film Recently, Raman spectroscopy was employed to understand the pyridine-induced bleaching and recrystallization process of CH3NH3PbI3 films, see Fig. 2.9A. The asprepared CH3NH3PbI3 film showed the typical features of the CH3NH3PbI3, with Pb
I bending at 71 cm21, Pb
I stretching at 94 cm21 and the CH3NH31 libration mode at 111 cm21 [68,69], whereas the bleached film showed a substantial decrease in intensity of the inorganic cage features at 50, 71 cm21 and 94 cm21. The results were compliable with a full pyridine induced dissolution of the PbI6 framework and the CH3NH3PbI3 crystalline structure [70]. The disappearance of pyridine peak and stronger peaks for CH3NH3PbI3 with time implies the recrystallization process as well as a reversible intercalation of pyridine in the perovskite framework. The simultaneous appearance of the CH3NH31 libration mode and adsorbed pyridine mode are indicative of the association of pyridine that helps in alignment and recrystallization of CH3NH31 groups into a more ordered phase. Significantly, the relative enhanced intensity of the Raman spectrum of recovered CH3NH3PbI3 film at band at 71 cm21 (Pb
I bending), 94 cm21 (Pb
I stretching) associated 111 cm21 (CH3NH31 libration mode) and the appearance of new band at 145 cm21 associated with ordering of CH3NH31 groups indicates enhanced crystallinity following the recrystallization process [70]. Raman spectra of the intermediate phase (the marked yellow region in Fig. 2.9B) at 34 and 49 cm21, during the pyridine-bleaching and recrystallization process, can be attributed to plumbates (PbI32 and PbI642) [71]. Analyzing the Raman spectra of pyridine in the bleached phase, the bonding information of pyridine can be retrieved while interacting with CH3NH3PbI3 (Fig. 2.9B). A shift of 11 cm21 is found for the symmetric ring breathing mode involving nitrogen. The pure pyridine shows fingerprint vibrations at 991 and 1029 cm21. The 991 cm21 corresponds to the ν(N
C) breathing mode and the 1029 cm21 is due to the hydrogen wagging on the ring. Interaction with CH3NH3PbI3 shifts the ν(N
C) breathing mode of pyridine from 991 to 1002 cm21 but the band at 1029 cm21 remains unperturbed. It means that pyridine likely bonds with CH3NH3PbI3 via the nitrogen group on the pyridine ring. Among the possibilities are bondings within the halogen elimination reaction of pyridine, pyridine bonding to Pb21, or to other positive species such as the A-site cation. DFT calculations was further utilized to investigate the different possible bonding situations, showing that the nitrogen is not fully bonded to a Lewis acid but steric hindered where iodide interaction competed with a pure Lewis pair interaction [70]. Therefore, only partially bonding is prevailed due to a frustrated Lewis pair effect [72,73]. In a frustrated bonding situation, pyridine would bond to iodine via hydrogen from either isolated protons or protons attached to the hydrogen in the CH3NH31 (MA1) ion or to Pb21 exposed surfaces and pyridine
(PbIn)(22n) plumbate complexes [74].

Characterization Techniques for Perovskite Solar Cell Materials

42

2. X-ray diffraction and Raman spectroscopy for lead halide perovskites

FIGURE 2.9 (A) Room temperature optical bleaching of CH3NH3PbI3 perovskite by pyridine vapor. (B) Sequential resonance Raman spectroscopy of the recovery of CH3NH3PbI3 film in ambient air after bleaching with pyridine. The marked yellow region show vibrations characteristic of isolated plumbates in the intermediate phase. (C) The ν(N
C) breathing mode of the pyridine ring unperturbed and bonded and the calculated shift when interacting with possible species in the intermediate bleached phase: (pyr)
IH and (pyr)PbI2. Experimental and theoretical assignments of the symmetric ring breathing and H-wagging modes of pyridine (top) and experimental Raman shifts for pyridine in liquid capillary and when interacting with the perovskite during bleaching. Reproduced with permission from reference S.M. Jain, Z. Qiu, L. Ha¨ggman, M. Mirmohades, M. B. Johansson, T. Edvinsson, et al., Frustrated Lewis pair-mediated recrystallization of CH3NH3PbI3 for improved optoelectronic quality and high voltage planar perovskite solar cells, Energy Environ. Sci. 9 (2016) 3770
3782. Copyright Royal Society of Chemistry 2016.

Characterization Techniques for Perovskite Solar Cell Materials

2.4 Conclusions

43

For a practical application, one should also note that care have to be taken with the intensities used in confocal Raman spectroscopy as always, due to the high concentration of light in the measured spot. For metal halide perovskites, a longer measurements or higher intensity first lead to an improved crystallinity for a poor starting material, followed by a possible temperature phase transition, and finally decomposition into the precursor salts, e.g. PbI2 and MAI in the case of MAPbI3. Another concern is the change in Raman activity and symmetry changes as a perfect cubic Pm-3 m metal halide perovskite is Raman inactive as reported quite early for MAPbCl3 [50] and exemplified more recently while monitoring formation of Bi-based double perovskites (MA)3Bi2I9 [75]. Apart from the normal cancellation of the change in polarizability change from group theory analysis, a recent study also report this from the aspect of center line broadening from local polar fluctuations [76].

2.4 Conclusions X-ray diffraction and Raman spectroscopy provide key structural and vibrational information of LHP materials, containing information on the crystal structure, symmetries, displacements, defects, micro-crystallinity, orientations, thermal expansion, phase transitions, vibrations, effects from doping, and local charge transfer effects and their interdependence on material alterations. XRD techniques are more or less necessary to elucidate the structural peripteries in modern material science where Raman spectroscopy can be used as an important complement that provide vibrations characterization under non-resonant and resonant conditions and aid in determination of structure-toproperty relationships from materials to function. The techniques can provide crystalline ˚ ngstro¨m variations in the structure useful for both and chemical features from sub-A understanding the molecular chemistry during creation and the structure and properties of the resulting LHP crystalline materials. The chapter briefly introduces different commonly used XRD approaches as well as some recent results from their applications. Raman spectroscopy is here a complementing technique that do not only provides information about the chemical composition and phases in crystalline materials, but can also be used for characterization of vibration in amorphous structures, molecular materials, solvents, and gases. With this knowledge, one can extract chemical identities and processes during syntheses as well as details in the crystalline materials via the observed local or extended lattice vibrations, bonding interactions, orientations, symmetries, and phonon modes to complement the structural characterization from XRD. A more detailed analysis of relative intensities in resonance Raman measurements, related to the losses of the electromagnetic energy in the incident light to an excited vibration level and the change in polarizability of the electron cloud during the vibration, additionally allows investigations of the primary light-matter interaction and subsequent charge delocalization.

Acknowledgment We acknowledge financial support from the Swedish Research Council (2015
03814), the Swedish Research Council for sustainable development (2016-00908), and the Swedish Energy Agency (P44648-1).

Characterization Techniques for Perovskite Solar Cell Materials

44

2. X-ray diffraction and Raman spectroscopy for lead halide perovskites

References [1] T.R.J.D. Perovskites, Structure-Property Relationships, John Wiley & Sons, Singapore, 2016. [2] M.A. Green, A. Ho-Baillie, Perovskite solar cells: the birth of a new era in photovoltaics, ACS Energy Lett. 2 (2017) 822
830. [3] M.A. Green, A. Ho-Baillie, H.J. Snaith, The emergence of perovskite solar cells, Nat. Photonics 8 (2014) 506. [4] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Efficient hybrid solar cells based on mesosuperstructured organometal halide perovskites, Science 338 (2012) 643
647. [5] https://www.nrel.gov/pv/assets/images/efficiency-chart.png. [6] Y. Zhao, K. Zhu, Organic
inorganic hybrid lead halide perovskites for optoelectronic and electronic applications, Chem. Soc. Rev. 45 (2016) 655
689. [7] T.J. Jacobsson, M. Pazoki, A. Hagfeldt, T. Edvinsson, Goldschmidt’s rules and strontium replacement in lead halogen perovskite solar cells: theory and preliminary experiments on CH3NH3SrI3, J. Phys. Chem. C 119 (2015) 25673
25683. [8] J.H. Noh, S.H. Im, J.H. Heo, T.N. Mandal, S.I. Seok, Chemical management for colorful, efficient, and stable inorganic-organic hybrid nanostructured solar cells, Nano Lett. 13 (2013) 1764
1769. [9] B.-w. Park, B. Philippe, S.M. Jain, X. Zhang, T. Edvinsson, H. Rensmo, et al., Chemical engineering of methylammonium lead iodide/bromide perovskites: tuning of opto-electronic properties and photovoltaic performance, J. Mater. Chem. A 3 (2015) 21760
21771. [10] T.A. Berhe, W.-N. Su, C.-H. Chen, C.-J. Pan, J.-H. Cheng, H.-M. Chen, et al., Organometal halide perovskite solar cells: degradation and stability, Energy Environ. Sci. 9 (2016) 323
356. [11] A. Mei, X. Li, L. Liu, Z. Ku, T. Liu, Y. Rong, et al., A hole-conductor
free, fully printable mesoscopic perovskite solar cell with high stability, Science 345 (2014) 295
298. [12] W. Chen, Y. Wu, Y. Yue, J. Liu, W. Zhang, X. Yang, et al., Efficient and stable large-area perovskite solar cells with inorganic charge extraction layers, Science 350 (2015) 944
948. [13] B.E. Warren, B.L. Averbach, The separation of cold-work distortion and particle size broadening in X-Ray patterns, J. Appl. Phys. 23 (1952). 497-497. [14] J. Rivnay, R. Noriega, R.J. Kline, A. Salleo, M.F. Toney, Quantitative analysis of lattice disorder and crystallite size in organic semiconductor thin films, Phys. Rev. B 84 (2011) 045203. [15] S.N. Ruddlesden, P. Popper, The compound Sr3Ti2O7 and its structure, Acta Crystallogr. 11 (1958) 54
55. [16] K. Inaba, S. Kobayashi, K. Uehara, A. Okada, S.L. Reddy, T. Endo, High resolution X-ray diffraction analyses of (La, Sr)MnO3/ZnO/Sapphire(0001) double heteroepitaxial films, Adv. Mater. Phys. Chem. 03 (2013) 72
89. [17] P.S. Whitfield, N. Herron, W.E. Guise, K. Page, Y.Q. Cheng, I. Milas, et al., Structures, phase transitions and tricritical behavior of the hybrid perovskite methyl ammonium lead iodide, Sci. Rep. 6 (2016) 35685. [18] H. Tsai, W. Nie, J.-C. Blancon, C.C. Stoumpos, R. Asadpour, B. Harutyunyan, et al., High-efficiency twodimensional Ruddlesden
Popper perovskite solar cells, Nature 536 (2016) 312. [19] R.W. James, The Optical Principles of the Diffraction of X-rays, Cornell University Press, New York, 1965. [20] A.M. Glazer, The classification of tilted octahedra in perovskites, Acta Crystallogr. Sect. B 28 (1972) 3384
3392. [21] A. Glazer, Simple ways of determining perovskite structures, Acta Crystallogr. Sect. A 31 (1975) 756
762. [22] C.J. Howard, H.T. Stokes, Group-theoretical analysis of octahedral tilting in perovskites, Acta Crystallogr. Sect. B 54 (1998) 782
789. [23] C.J. Howard, H.T. Stokes, Structures and phase transitions in perovskites - a group-theoretical approach. This article is dedicated to Helen D. Megaw (1907-2002), in appreciation of her many contributions to the crystallography of inorganic and mineral compounds, including her seminal studies of perovskites. Some of the material was presented by CJH in the Megaw memorial session at the 21st European Crystallography Meeting, Durban, South Africa, August 2003, Acta Crystallogr. Sect. A 61 (2005) 93
111. [24] J. Rivnay, S.C.B. Mannsfeld, C.E. Miller, A. Salleo, M.F. Toney, Quantitative determination of organic semiconductor microstructure from the molecular to device scale, Chem. Rev. 112 (2012) 5488
5519. [25] C.C. Boyd, R. Cheacharoen, T. Leijtens, M.D. McGehee, Understanding degradation mechanisms and improving stability of perovskite photovoltaics, Chem. Rev. 119 (2019) 3418
3451. [26] C.C. Boyd, R. Cheacharoen, K.A. Bush, R. Prasanna, T. Leijtens, M.D. McGehee, Barrier design to prevent metal-induced degradation and improve thermal stability in perovskite solar cells, ACS Energy Lett. 3 (2018) 1772
1778.

Characterization Techniques for Perovskite Solar Cell Materials

References

45

[27] A.M.A. Leguy, Y. Hu, M. Campoy-Quiles, M.I. Alonso, O.J. Weber, P. Azarhoosh, et al., Reversible hydration of CH3NH3PbI3 in films, single crystals, and solar cells, Chem. Mater. 27 (2015) 3397
3407. [28] G.Y. Kim, A. Senocrate, T.-Y. Yang, G. Gregori, M. Gra¨tzel, J. Maier, Large tunable photoeffect on ion conduction in halide perovskites and implications for photodecomposition, Nat. Mater. 17 (2018) 445
449. [29] N. Aristidou, C. Eames, I. Sanchez-Molina, X. Bu, J. Kosco, M.S. Islam, et al., Fast oxygen diffusion and iodide defects mediate oxygen-induced degradation of perovskite solar cells, Nat. Commun. 8 (2017) 15218. [30] N.H. Nickel, F. Lang, V.V. Brus, O. Shargaieva, J. Rappich, Unraveling the light-induced degradation mechanisms of CH3NH3PbI3 perovskite films, Adv. Electron. Mater. 3 (2017) 1700158. [31] F. Lang, O. Shargaieva, V.V. Brus, H.C. Neitzert, J. Rappich, N.H. Nickel, Influence of radiation on the properties and the stability of hybrid perovskites, Adv. Mater. 30 (2018) 1702905. [32] T.J. Jacobsson, L.J. Schwan, M. Ottosson, A. Hagfeldt, T. Edvinsson, Determination of thermal expansion coefficients and locating the temperature-induced phase transition in methylammonium lead perovskites using X-ray diffraction, Inorg. Chem. 54 (2015) 10678
10685. [33] R. Cheacharoen, N. Rolston, D. Harwood, K.A. Bush, R.H. Dauskardt, M.D. McGehee, Design and understanding of encapsulated perovskite solar cells to withstand temperature cycling, Energy Environ. Sci. 11 (2018) 144
150. [34] J. Zhao, Y. Deng, H. Wei, X. Zheng, Z. Yu, Y. Shao, et al., Strained hybrid perovskite thin films and their impact on the intrinsic stability of perovskite solar cells, Sci. Adv. 3 (2017) eaao5616. [35] T. Baikie, Y. Fang, J.M. Kadro, M. Schreyer, F. Wei, S.G. Mhaisalkar, et al., Synthesis and crystal chemistry of the hybrid perovskite (CH3NH3)PbI3 for solid-state sensitised solar cell applications, J. Mater. Chem. A 1 (2013) 5628
5641. [36] A. Binek, F.C. Hanusch, P. Docampo, T. Bein, Stabilization of the trigonal high-temperature phase of formamidinium lead iodide, J. Phys. Chem. Lett. 6 (2015) 1249
1253. [37] C.C. Stoumpos, C.D. Malliakas, M.G. Kanatzidis, Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties, Inorg. Chem. 52 (2013) 9019
9038. [38] M.R. Filip, G.E. Eperon, H.J. Snaith, F. Giustino, Steric engineering of metal-halide perovskites with tunable optical band gaps, Nat. Commun. 5 (2014) 5757. [39] N. Pellet, P. Gao, G. Gregori, T.Y. Yang, M.K. Nazeeruddin, J. Maier, et al., Mixed-organic-cation perovskite photovoltaics for enhanced solar-light harvesting, Angew. Chem. Int. Ed. Engl. 53 (2014) 3151
3157. [40] N.J. Jeon, J.H. Noh, W.S. Yang, Y.C. Kim, S. Ryu, J. Seo, et al., Compositional engineering of perovskite materials for high-performance solar cells, Nature 517 (2015) 476
480. [41] N.J. Jeon, J.H. Noh, Y.C. Kim, W.S. Yang, S. Ryu, S.I. Seok, Solvent engineering for high-performance inorganic
organic hybrid perovskite solar cells, Nat. Mater. 13 (2014) 897. [42] Landau Theory of Phase Transitions, in: Ultracold Quantum Fields, Springer, Netherlands, Dordrecht, 2009, pp. 193
212. [43] M.N. Iliev, M.V. Abrashev, J. Laverdie`re, S. Jandl, M.M. Gospodinov, Y.Q. Wang, et al., Distortiondependent Raman spectra and mode mixing in RMnO3 perovskites(R 5 La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y), Phys. Rev. B 73 (2006). [44] M.N. Iliev, M.V. Abrashev, H.G. Lee, V.N. Popov, Y.Y. Sun, C. Thomsen, et al., Raman spectroscopy of orthorhombic perovskitelike YMnO3 and LaMnO3, Phys. Rev. B 57 (1998) 2872
2877. [45] J. Kim, J. Kwon, M. Kim, J. Do, D. Lee, H. Han, Low-dielectric-constant polyimide aerogel composite films with low water uptake, Polym. J. 48 (2016) 829. [46] Y. Guo, O. Yaffe, D.W. Paley, A.N. Beecher, T.D. Hull, G. Szpak, et al., Interplay between organic cations and inorganic framework and incommensurability in hybrid lead-halide perovskite CH3NH3PbBr3, Phys. Rev. Mater. 1 (2017). [47] G. Abdelmageed, L. Jewell, K. Hellier, L. Seymour, B. Luo, F. Bridges, et al., Mechanisms for light induced degradation in MAPbI3 perovskite thin films and solar cells, Appl. Phys. Lett. 109 (2016). [48] A. Letoublon, S. Paofai, B. Ruffle, P. Bourges, B. Hehlen, T. Michel, et al., Elastic constants, optical phonons, and molecular relaxations in the high temperature plastic phase of the CH3NH3PbBr3 hybrid perovskite, J. Phys. Chem. Lett. 7 (2016) 3776
3784. [49] M. Ledinsky, P. Loper, B. Niesen, J. Holovsky, S.J. Moon, J.H. Yum, et al., Raman spectroscopy of organicinorganic halide perovskites, J. Phys. Chem. Lett. 6 (2015) 401
406.

Characterization Techniques for Perovskite Solar Cell Materials

46

2. X-ray diffraction and Raman spectroscopy for lead halide perovskites

[50] A. Maalej, Y. Abid, A. Kallel, A. Daoud, A. Lautie´, F. Romain, Phase transitions and crystal dynamics in the cubic perovskite CH3NH3PbCl3, Solid State Commun. 103 (1997) 279
284. [51] J.D. Jackson, Classical Electrodynamics, 3rd ed., Wiley, New York, 1999. [52] M.A. Pe´rez-Osorio, Q. Lin, R.T. Phillips, R.L. Milot, L.M. Herz, M.B. Johnston, et al., Raman spectrum of the organic
inorganic halide perovskite CH3NH3PbI3 from first principles and high-resolution low-temperature raman measurements, J. Phys. Chem. C 122 (2018) 21703
21717. [53] D.L. Rousseau, R.P. Bauman, S.P.S. Porto, Normal mode determination in crystals, J. Raman Spectrosc. 10 (1981) 253
290. [54] P.Y. Yu, M. Cardona, Vibrational properties of semiconductors, and electron-phonon interactions, in: P.Y. Yu, M. Cardona (Eds.), Fundamentals of Semiconductors: Physics and Materials Properties, Springer Berlin Heidelberg, Berlin, Heidelberg, 1996, pp. 99
147. [55] R.G. Niemann, A.G. Kontos, D. Palles, E.I. Kamitsos, A. Kaltzoglou, F. Brivio, et al., Halogen effects on ordering and bonding of CH3NH3 1 in CH3NH3PbX3 (X 5 Cl, Br, I) hybrid perovskites: a vibrational spectroscopic study, J. Phys. Chem. C 120 (2016) 2509
2519. [56] J.-X. Cheng, X.S. Xie, Coherent anti-stokes raman scattering microscopy: instrumentation, theory, and applications, J. Phys. Chem. B 108 (2004) 827
840. [57] J. Even, L. Pedesseau, J.-M. Jancu, C. Katan, Importance of spin
orbit coupling in hybrid organic/inorganic perovskites for photovoltaic applications, J. Phys. Chem. Lett. 4 (2013) 2999
3005. [58] D.B. Mitzi, Templating and structural engineering in organic
inorganic perovskites, J. Chem. Soc., Dalton Trans. (2001) 1
12. [59] A. Poglitsch, D. Weber, Dynamic disorder in methylammoniumtrihalogenoplumbates (II) observed by millimeter-wave spectroscopy, J. Chem. Phys. 87 (1987) 6373
6378. [60] B.-w. Park, S.M. Jain, X. Zhang, A. Hagfeldt, G. Boschloo, T. Edvinsson, Resonance Raman and excitation energy dependent charge transfer mechanism in halide-substituted hybrid perovskite solar cells, ACS Nano 9 (2015) 2088
2101. [61] G. Blasse, A.F. Corsmit, Electronic and vibrational spectra of ordered perovskites, J. Solid State Chem. 6 (1973) 513
518. [62] C. Quarti, G. Grancini, E. Mosconi, P. Bruno, J.M. Ball, M.M. Lee, et al., The Raman spectrum of the CH3NH3PbI3 hybrid perovskite: interplay of theory and experiment, J. Phys. Chem. Lett. 5 (2014) 279
284. [63] G. Grancini, S. Marras, M. Prato, C. Giannini, C. Quarti, F. De Angelis, et al., The impact of the crystallization processes on the structural and optical properties of hybrid perovskite films for photovoltaics, J. Phys. Chem. Lett. 5 (2014) 3836
3842. [64] G. Coslovich, B. Huber, W.S. Lee, Y.D. Chuang, Y. Zhu, T. Sasagawa, et al., Ultrafast charge localization in a stripe-phase nickelate, Nat. Commun. 4 (2013) 2643. [65] J.M. Frost, K.T. Butler, F. Brivio, C.H. Hendon, M. van Schilfgaarde, A. Walsh, Atomistic origins of highperformance in hybrid halide perovskite solar cells, Nano Lett. 14 (2014) 2584
2590. [66] T. Umebayashi, K. Asai, T. Kondo, A. Nakao, Electronic structures of lead iodide based low-dimensional crystals, Phys. Rev. B 67 (2003). [67] H.L. Bostrom, J.A. Hill, A.L. Goodwin, Columnar shifts as symmetry-breaking degrees of freedom in molecular perovskites, Phys. Chem. Chem. Phys. 18 (2016) 31881
31894. [68] S.M. Jain, B. Philippe, E.M.J. Johansson, B.-w. Park, H. Rensmo, T. Edvinsson, et al., Vapor phase conversion of PbI2 to CH3NH3PbI3: spectroscopic evidence for formation of an intermediate phase, J. Mater. Chem. A 4 (2016) 2630
2642. [69] R. Gottesman, L. Gouda, B.S. Kalanoor, E. Haltzi, S. Tirosh, E. Rosh-Hodesh, et al., Photoinduced reversible structural transformations in free-standing CH3NH3PbI3 perovskite films, J. Phys. Chem. Lett. 6 (2015) 2332
2338. [70] S.M. Jain, Z. Qiu, L. Ha¨ggman, M. Mirmohades, M.B. Johansson, T. Edvinsson, et al., Frustrated Lewis pairmediated recrystallization of CH3NH3PbI3 for improved optoelectronic quality and high voltage planar perovskite solar cells, Energy Environ. Sci. 9 (2016) 3770
3782. [71] B.R. Vincent, K.N. Robertson, T.S. Cameron, O. Knop, Alkylammonium lead halides. Part 1. Isolated PbI64 2 ions in (CH3NH3)4PbI6•2H2O, Can. J. Chem. 65 (1987) 1042
1046. [72] D.W. Stephan, G. Erker, Frustrated Lewis pair chemistry: development and perspectives, Angew. Chem. Int. Ed. Engl. 54 (2015) 6400
6441.

Characterization Techniques for Perovskite Solar Cell Materials

References

47

[73] D.W. Stephan, G. Erker, Frustrated Lewis pairs: metal-free hydrogen activation and more, Angew. Chem. Int. Ed. Engl. 49 (2010) 46
76. [74] H. Miyamae, H. Toriyama, T. Abe, G. Hihara, M. Nagata, The structure of PbI2-pyridine adducts. I. Lead(II) iodide-pyridine (2/4), catena-di-[mu]-iodo-bis(pyridine)lead(II), [Pb2I4(C5H5N)4], Acta Crystallogr. Sect. C 40 (1984) 1559
1562. [75] S.M. Jain, D. Phuyal, M.L. Davies, M. Li, B. Philippe, C. De Castro, et al., An effective approach of vapour assisted morphological tailoring for reducing metal defect sites in lead-free, (CH3NH3)3Bi2I9 bismuth-based perovskite solar cells for improved performance and long-term stability, Nano Energy 49 (2018) 614
624. [76] O. Yaffe, Y. Guo, L.Z. Tan, D.A. Egger, T. Hull, C.C. Stoumpos, et al., Local polar fluctuations in lead halide perovskite crystals, Phys. Rev. Lett. 118 (2017) 136001.

Characterization Techniques for Perovskite Solar Cell Materials

C H A P T E R

3 Optical absorption and photoluminescence spectroscopy Mojtaba Abdi-Jalebi1, M. Ibrahim Dar2, Aditya Sadhanala1, Erik M.J. Johansson3 and Meysam Pazoki4 1

Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge, United Kingdom 2Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, E´cole Polytechnique Fe´de´rale de Lausanne, Lausanne, Switzerland 3Department of ˚ ngstro¨m laboratory, Uppsala University, Uppsala, Sweden 4Department of Chemistry, A ˚ ngstro¨m laboratory, Uppsala University, Engineering Sciences, Solid State Physics, A Uppsala, Sweden

3.1 Introduction Optical absorption and photoluminescence spectroscopy are important tools for studying semiconductors and electronic devices because they are non-destructive and nonintrusive. In particular, for a semiconductor film within a solar cell device, it is substantially important to measure the bandgap and estimate the absorbed light in order to determine the theoretical limit for the power conversion efficiency and photovoltaic parameters of the subsequent solar cells. In addition, determination and analysis of luminescence in a semiconductor is key to understand the origin of losses in optoelectronic devices.

3.2 Optical absorption spectroscopy Absorption of a photo-absorber semiconductor is an important aspect for its application in an optoelectronic device as it determine the optical bandgap, which is key to calculate the theoretical limit for various parameters of a semiconductor device. Metal-halide perovskite has a direct bandgap and therefore thin film based perovskite is suffice to efficiently absorb the light while in indirect bandgap based semiconductors such silicon another of magnitude thicker films is required to absorb same amount of light. Characterization Techniques for Perovskite Solar Cell Materials DOI: https://doi.org/10.1016/B978-0-12-814727-6.00003-7

49

Copyright © 2020 Elsevier Inc. All rights reserved.

50

3. Optical absorption and photoluminescence spectroscopy

3.3 Steady state UV
Vis
NIR spectroscopy Ultraviolet
visible
near-infrared spectroscopy (UV
Vis
NIR) refers to absorption spectroscopy in the ultraviolet
visible
near-infrared spectral region. Absorption spectroscopy fluorescence/photoluminescence spectroscopy are complementary in nature wherein, the transitions from excited state to ground state results in photoluminescence and the reverse
transition from ground state to excited state due/leads to absorption of photons. The absorption is quantified by the Beer
Lambert law:
 A 5 log10 I=I0 (3.1) where A is absorbance, I0 is the intensity of the incident light, and I is the transmitted intensity (Fig. 3.1). All the parameters are for a given wavelength of light and when we scan all the wavelengths from UV to visible to near IR, a complete absorbance spectra of the perovskite semiconductor can be obtained. Transmittance ‘T’ is given by I/I0 and is expressed in % as %T, when measuring the transmitted light and the absorbance is then given by:   %T A 5 log10ðTÞ 5 log10 (3.2) 100% When measuring reflectance, the spectrophotometer measures I the intensity of light reflected from a sample, with reference to a reference sample reflection I0 which is generally obtained by using a white reflective sample coated with Barium Sulfate (BaSO4). Herein, I/I0 is defined as the reflectance ‘R’ and is usually represented as %R. Due to the crystalline to polycrystalline nature of the perovskite samples in solar cell devices, the perovskite samples do show significant reflection and scattering leading to errors in simple UV
Vis measurement. In this case one should use an integrating sphere based UV
Vis
NIR measurement that accounts for the total scattering and reflection providing the correct contribution from of absorption.

3.3.1 Photothermal deflection spectroscopy (PDS) Photothermal Deflection Spectroscopy (PDS) technique is a highly sensitive absorption measurement technique that can probe absorption coefficients down to 1 cm21 or an absorbance level of down to 1025 [1], which corresponds to a 4
5 orders of magnitude FIGURE 3.1 Schematic of UV
Vis
NIR technique showing incident light intensity of I0 and transmitted intensity I through the sample upon absorption.

Characterization Techniques for Perovskite Solar Cell Materials

51

3.3 Steady state UV
Vis
NIR spectroscopy

FIGURE 3.2 Schematic of (A) transverse and (B) collinear PDS techniques.

dynamic sensitivity range. Such high sensitivity is achieved at ambient temperatures by using the principles of mirage effect. Jackson et al. first demonstrated the PDS technique in 1981 [2]. This technique was originally used for the detection of defect states in inorganic semiconductors like a-Si, GaAs, InGaAsP, etc [3
5]. PDS techniques are mainly of two types: transverse and collinear PDS techniques respectively (Fig. 3.2) [2]. Both systems have a tunable monochromatic excitation source (or heating beam) and a fixed wavelength continuous wave (CW) probe laser source used for probing the absorption in the sample for a given wavelength of excitation. In the collinear PDS system the excitation and the probe laser sources go through the sample and are arranged in such a way that they overlap each other while in the sample. Whereas, in the case of transverse PDS configuration, the excitation source is normal to the plane of the sample and the probe laser is perpendicular to the excitation beam path and parallel to the sample surface grazing it. Furthermore, the excitation beam and the probe beam overlap each other within the Rayleigh range of interaction. The deflection of the probe beam upon creation of a mirage near the sample surface (due to non-radiative relaxation of excited species created upon absorption of light) causes the deflection of the probe laser beam that can be measured using a photodetector. By scanning through different wavelengths, one can measure the absorption spectra of the sample. Standard absorption measurement techniques like UV
Vis
NIR spectrometers that usually measure absorption in the transmission mode, are plagued with errors due to various optical effects like
light scattering, reflection, and interference, which limits the sensitivity. The PDS however is immune to any optical effect because of the working principle of the measurement is on the basis of absorption induced heating effect in the sample, due to non-radiative relaxation of the excited species. It is notable that the UV
Vis
NIR spectrometers with an integrating sphere and a reference has ability to suppress the optical effects errors though in most of the laboratories a simplified set-up is used which suffers from the aforementioned optical effect errors. The measurement range depends on the emission spectrum of the lamp used, for example
an ozone free Xenon lamp would facilitate the PDS setup to measure absorption in the wavelength range of 380
2100 nm. The samples used for the PDS measurements mostly involve thin-films coated on quartz substrates. Fig. 3.3 shows the comparison between UV
Vis
NIR and PDS technique where a CH3NH3PbI3 perovskite thin-film is measured using these two techniques. Optical effects

Characterization Techniques for Perovskite Solar Cell Materials

52

3. Optical absorption and photoluminescence spectroscopy

FIGURE 3.3 Comparison between the UV
Vis
NIR and PDS technique
absorption spectra of a rough CH3NH3PbI3 thin-film. Linear fit (dash line) on the PDS absorption spectra is used to extract the Urbach energy (see Section 2.4.2).

like scattering and reflection affect the UV
Vis
NIR measurements while, the PDS is minimally affected by these effects and hence, is capable of measuring the band-tails down to absorbance of 1025 i.e. five orders of magnitude dynamic range as compared to standard UV
Vis
NIR technique. Considering a 1D formalism
probe laser beam deflection angle (φ) is given by [6]:   s dn dTðz; tÞ (3.3) φ5 n dT dz where, n 5 refractive index of the fluid in which the sample is immersed, gradient, s 5 interaction path length.

dT dz 5 temperature

Rosencwaig and Gersho have given a detailed description of the sample thickness ‘l’, absorption coefficient ‘α’, thermal diffusion coefficients of the sample (μs), and measurement frequencies on the deflection signal [7,8]. There are few important considerations to keep in mind while measuring using the PDS; (1) absorbance is directly proportional to the magnitude of the PDS signal for an optically transparent sample i.e. where the optical penetration depth is more than the sample thickness. (2) PDS signal saturates for thermally thin samples (l , μs) at αl 5 1. (3) PDS signal saturates for thermally thick samples (1l , α , μ1 ) at αμs 5 1. s

3.3.2 Estimation of the bandgap As mentioned earlier, estimation of the bandgap is substantially important in any optical absorption measurements. There are a number of main methods of bandgap calculation using absorption data obtained using UV
Vis
NIR or PDS or any other absorption measurement technique as described below.

Characterization Techniques for Perovskite Solar Cell Materials

3.3 Steady state UV
Vis
NIR spectroscopy

53

3.3.2.1 Simple calculation Band gap energy; E 5

hc λ

(3.4)

where, Planks constant h 5 6.626 3 10234 Js Speed of light c 5 3 3 108 m/s λ is the wavelength of light (the onset in the UV
Vis spectrum) in meters 3.3.2.2 Tauc plots Herein, we first have to extract the absorption coefficient “α” data from the absorbance/transmission data measured from UV
Vis
NIR or PDS or any other absorption measurement technique including the Diffuse Reflectant spectroscopy (DRS) that measures reflection spectra. The absorption coefficient is given by the following equation:
n A hv2Eg α5 (3.5) hv 1

1

(αhv)n 5 An hv 2 A1=n Eg

(3.6)

where, A is absorbance (obtained from transmittance T or reflectance R), hv is the photon energy represented in eV units 5 1240/(incident wavelength in nm) h i 1 1 If you plot a graph of (αhvÞn against hv, then the hv axis intercept (αhvÞn 5 0 , of the linear slope fitted to the linear section of the plot will give the bandgap Eg. Wherein: n 5 12 for direct bandgap semiconductors, n 5 2 for indirect bandgap semiconductors, n 5 3 for semiconductors with direct forbidden transitions, and n 5 4 for semiconductors with indirect forbidden transitions. Most of the photoabsorber films implemented in the highly efficient perovskite devices have direct band gap. It is notable that derivative method is a rudimentary way of obtaining the bandgap. In this method, we use the absorption spectra obtained from various techniques and take the derivative of the spectra and the position where the maximum intensity peak position obtained in the derivative spectra gives the bandgap.

3.3.3 Near band edge trap states The near bandgap defects arise due to various reasons no limited to the
sample processing conditions, crystallinity, structural disorder, defects, doping, etc. These near band edge defects play a major role in affecting the charge transport and other optoelectronic properties that can determine the quality of a semiconductor device. An empirical way of quantifying these near band edge is by estimation of Urbach energy. Using PDS data or any other sensitive absorption measurement technique that can measure sub-bandgap

Characterization Techniques for Perovskite Solar Cell Materials

54

3. Optical absorption and photoluminescence spectroscopy

absorption one can estimate an empirical parameter known as “Urbach energy” (see Fig. 3.3) annotated as ‘EU’ defined as:   E A 5 A0  exp (3.7) EU 1  ½  dE lnðAÞ

EU 5
d

(3.8)

The Urbach energy EU can be calculated from the inverse slope of the linear fit to the Urbach tail (or Urbach front) in the absorption spectra plotted on a natural logarithmic scale (as shown in Fig. 3.3). It has a unit of energy (eV) per decade however, generally only the energy unit (eV) is explicitly specified. Franz Urbach first conceived this in 1953 [9]. Furthermore, temperature dependent absorption measurements done by Knox showed the direct proportionality and correlation between the Urbach energy and thermal energy (kT, where k is Boltzmann constant and T is temperature in Kelvin) [10], leading to a conclusion that transitions below the bandgap are mainly due to phonons [10]. However, Urbach tail can also be caused by other mechanisms, which induce potential fluctuations that could result in local energy level and hence bandgap variation in the semiconductors. Hence, various theoretical models used to describe the Urbach energy rule [11
15]. However, there is no unified theory yet that describes the origin of Urbach energy which, can be best described as an empirical parameter. Similar to PDS there are several other measurement techniques that are capable of measuring absorption in a sensitive way. Many of these techniques depend on the measurement of photocurrent and hence are limited to measuring contribution due to only exciton species that can result in a free charge pair and doped samples that contain a fixed ion and a free opposite charge does not show up in such measurements [16]. However, PDS measures contribution due to all such species as shown in the Fig. 3.4. Herein, the perovskite samples containing tin (Sn) show intrinsic doping and have free charge carriers that cannot be extracted by doing electrical measurements and hence, electrical measurements lead to misleading absorption data that lacks contribution due to doping that is captured accurately using PDS. However, both electrical and PDS show similar conclusions for undoped samples like lead only perovskite CH3NH3PbI3. Therefore, it is important to choose the right sensitive absorption measurement as the findings have significant impact on the analysis obtained about the resultant semiconductor quality and their optoelectronic properties [17
19]. The PDS technique as a powerful and ultra sensitive absorption tool is extensively used to measure the near band edge trap states of various semiconductors. In particular, improving the quality of titanium dioxide as a key semiconductor for the electron transport layer in perovskite solar cell [20] and dye-sensitized solar cell [21,22] architectures and providing an interface with the minimum defect density have been studied using the PDS technique. In Fig. 3.5A and B, the absorption spectra near the band-edge of TiO2 has significantly changed upon two different treatments on the titania mesoporous layer (mp-TiO2), TiCl4 treatment [20] and lithium treatement [23], respectively, where the subgap level as well as the Urbach energy of the titania reduced substantially.

Characterization Techniques for Perovskite Solar Cell Materials

3.3 Steady state UV
Vis
NIR spectroscopy

55

FIGURE 3.4 (A
C) shows the normalized PDS and the photocurrent (EQE) spectra of the CH3NH3PbI3, CH3NH3Pb0.4 Sn0.6I3, and CH3NH3 Pb0.2Sn0.8I3 perovskites respectively. The shaded region indicates the difference between EQE and PDS spectra that signifies the states that do not cause charge generation. The estimate Urbach energies obtained from PDS and EQE based measurements is stated for each sample. Reproduced from B. Zhao, M. Abdi-Jalebi, M. Tabachnyk, H. Glass, V.S. Kamboj, W. Nie, et al., High open circuit voltages in tin-rich low-bandgap perovskites based planar heterojunction photovoltaics, Adv. Mater. 29 (2) (2016) 1604744 [24].

These treatment also enhances the quality of the perovskite capping layer providing a better interface between mp-titania and perovskite (Fig. 3.5C). Furthermore, addition of various modifier to the top surface of mp-TiO2 can indeed passivated the intra bandgap states within TiO2 and provides a superior interface with the perovskite top layer as it is evident from Fig. 3.5D.

Characterization Techniques for Perovskite Solar Cell Materials

FIGURE 3.5 (A) PDS absorbance spectra of mp-TiO2 films, pristine and TiCl4 post-treated. The inset shows the corresponding Urbach energies. (B) Absorbance spectra of pristine (black curve) and Li-treated (green curve) mp-TiO2. (C) Absorbance spectra of FAPbBr3 perovskite deposited on pristine (black curve) and Li-treated (blue curve) mp-TiO2. The inset shows the corresponding Urbach energies. (D) PDS absorption spectra of MAPbI3 films on TiO2 with different modifiers deposited on quartz glass. The inset shows PDS absorption spectra of TiO2 only and its modified analogs. (A) Reproduced from M. Abdi-Jalebi, M.I. Dar, A. Sadhanala, S.P. Senanayak, F. Giordano, S.M. Zakeeruddin, et al., Impact of a mesoporous titania
perovskite interface on the performance of hybrid organic
inorganic perovskite solar cells, J. Phys. Chem. Lett. 7 (16) (2016) 3264
3269, Copyright 2016 American Chemical Society. (C) Reproduced from N. Arora, M.I. Dar, M. Abdi-Jalebi, F. Giordano, N. Pellet, G. Jacopin, et al., Intrinsic and extrinsicstability of formamidinium lead bromide perovskite solar cells yielding high photovoltage, Nano Lett. 16 (11) (2016) 7155
7162, Copyright 2016 American Chemical Society. (D) Reprinted with permission from K.K. Wong, A. Fakharuddin, P. Ehrenreich, T. Deckert, M. Abdi-Jalebi, R.H. Friend, et al., Interface-dependent radiative and nonradiative recombination in perovskite solar cells, J. Phys. Chem. C 122 (20) (2018) 10691
10698 [25], Copyright 2018 American Chemical Society.

3.3 Steady state UV
Vis
NIR spectroscopy

57

3.3.4 Absorption properties of metal-halide perovskites Metal-halide perovskites have in general strong light absorption properties. This is very important for highly efficient solar cell devices, since a strong light absorption enable the use of a thin photoactive layer in the solar cell to harvest the solar light. After light absorption in the thin photoactive layer, the photogenerated charges only needs to travel a short distance to reach the charge extraction layers, which reduces the recombination of the photo-generated charges. In comparison to other solar cell materials, the metal-halide perovskite shows a much stronger light absorption than silicon, and similar light absorption as other thin film solar cell materials, such as CIGS, CdTe and GaAs (see Fig. 3.6) [26]. In addition to the strong light absorption for the metal-halide perovskites, they also show a rather sharp optical absorption edge close to the reported band gap energy of the material (see Fig. 3.7) [27]. By using photo-thermal deflection spectroscopy (PDS) and Fourier transform photocurrent spectroscopy (FTPS), it was observed that the optical absorption edge is sharp and a small Urbach energy of 15 meV reported for CH3NH3PbI3 [28]. In PDS, the sample is immersed in a liquid and the sample is illuminated with different wavelengths of light. When light is absorbed by the sample, the light induced temperature change in the liquid affects the deflection of a laser beam in the liquid, and the change in deflection can be used to detect the light absorption spectrum of the sample. However, in FTPS the photocurrent between two contacts connected to the material is measured at a bias voltage. The results from the two methods (PDS and FTPS) were similar, and the absorption edge was found to be exponential over four orders of magnitude [27]. The small Urbach energy suggests a low degree of structural disorder, and no optically detectable deep band gap states [29]. The sharp absorption edge and small Urbach energy may be connected to the high open-circuit voltage obtained for the CH3NH3PbI3 based solar cell and an empirical trend shows lower voltage losses in solar cell materials with low Urbach energies [27,30].

FIGURE 3.6 (A) The absorption coefficients for a number of solar cell materials. (B) The real and imaginary parts of the dielectric constant for CH3NH3PbI3 at 300 K, as a function of frequency. Reprinted by permission from Springer Nature M.A. Green, A. Ho-Baillie, H.J. Snaith, The emergence of perovskite solar cells, Nat. Photonics 8 (7) (2014) 506
514.

Characterization Techniques for Perovskite Solar Cell Materials

58

3. Optical absorption and photoluminescence spectroscopy

FIGURE 3.7 PDS, Fourier transform photocurrent spectroscopy (FTPS) and 1-RspecularTspecular spectra for CH3NH3PbI3 perovskite. All spectra were measured at room temperature. Adopted with permission from S. De Wolf, J. Holovsky, S.-J. Moon, P. Lo¨per, B. Niesen, M. Ledinsky, et al., Organometallic halide perovskites: sharp optical absorption edge and its relation to photovoltaic performance, J. Phys. Chem. Lett. (2014) In Press, 140305122150008, copy right 2014 American Chemical Society.

3.3.5 Light absorption process in metal-halide perovskites The top valence band in CH3NH3PbI3 is mainly composed of halide p-orbitals mixed with a smaller contribution of lead states, and the conduction band is mainly composed of lead p-orbitals mixed with a smaller amount of halide states [31]. Light absorption near the band gap energy is therefore mainly a transition between the halide p-orbitals (mixed with lead orbitals) in the valence band to lead p-orbitals (mixed with halide orbitals) in the conduction band. The optical transition is therefore mainly including the lead halide part of the material and the organic cations are not involved in light absorption close to the band gap energy. However, by changing the organic cation, the Pb
I octahedrals will be affected, which affects the band gap energy. For example by changing the organic cation from methylammonium (MA) to formamidinium (FA), the crystal structure of the perovskite moves from tetragonal to quasi cubic [32]. This structural change also results in a change of the optical properties with a lower band gap value for the perovskite with formamidinium cations. The structural change s can be explained by different sizes of the organic cations and electrostatic interactions between the cations and negatively charged inorganic Pb
I matrix. The changes in the Pb
I matrix includes changes in bond length, bond angles as well as octahedral tiltings that modifies the electronic structure and therefore changes the band gap value. The changes in the band gap due to differences in Pb
I bond angles from different Pb
I octahedral tilting in the structure was theoretically explained by changes in the Pb
I bonds and the spin-orbit coupling (see Fig. 3.8) [33]. A number of different cations have been inserted in the perovskite structure, which show that the cation affects the light absorption properties [34,35]. One important reason for investigating different cations in the perovskite structure is also to obtain more stable perovskite materials. Therefore, also inorganic cations such as Cesium have been introduced in the perovskite structure to obtain more stable perovskites [36]. Due to the changes in the P
I matrix, the light absorption is also affected, which is important for the solar cell properties.

Characterization Techniques for Perovskite Solar Cell Materials

FIGURE 3.8 (A) Calculated total energy by SR- and SOC-DFT as a function of the α-dihedral angle. Zero energy is set at α 5 0. (B) Band gap (Eg) calculated by SR- and SOC-DFT and the difference between them for CsPbI3. (C) Partial density of states for lead (solid lines) and iodide (dashed lines), for variation of the tilting angle α in CsPbI3. (D) Calculated average effective electron mass (me, dashed line) and hole mass (mh, solid line) calculated by SOC-DFT for the CsPbI3, varying the tilting angle α. Adopted from A. Amat, E. Mosconi, E. Ronca, C. Quarti, P. Umari, M.K. Nazeeruddin, et al., Cation-induced band-gap tuning in organohalide perovskites: interplay of spin-orbit coupling and octahedra tilting, Nano Lett. (2014).

60

3. Optical absorption and photoluminescence spectroscopy

3.3.6 Excitons in metal-halide perovskites For solar cell devices, the excited electron needs to be separated from the positive hole to be able to produce a photocurrent. In a perovskite solar cell device, the perovskite is sandwiched between an electron extraction layer and a hole extraction layer, and understanding of the exciton (bound excited electron and hole pair) and its separation into free charges is essential. In the first perovskite solar cell devices it was not clear if the charge separation occurred at the interfaces with the electron extraction layer (or hole extraction layer) or within the perovskite material itself. Experimental observations of the exciton binding energy and the photoconductivity later on showed that the charge separation can occur directly in the perovskite material at room temperature, giving a photocurrent directly in the perovskite layer [37
39]. By investigating the magnetic field dependence of the light absorption, it was possible to determine the exciton binding energy to be around 16 meV at 4 K, which is comparable to conventional III-V semiconductors with similar band gap energy, and at room temperature the exciton binding energy in the perovskite is even lower and the photo-induced carriers are therefore essential free carriers (Fig. 3.9) [37,38].

3.3.7 Tuning of the light absorption spectrum via chemical modifications in metal-halide perovskite One important property of the metal-halide perovskites is the tunability of the band gap. This is important in solar cells in order to either obtain a band gap close to the optimal for a single junction solar cell, or to obtain a band gap for optimal performance in tandem or multi junction solar cells. The band gap of CH3NH3PbI3 can be tuned towards larger band gap energy by mixing iodide with bromide, see Fig. 3.10 [40]. The pure bromide perovskite, CH3NH3PbBr3, has a band gap energy around 2.3 eV, which is significantly higher compared to the iodide based perovskite with a band gap around 1.6 eV. The shift in band gap energy is obtained due to changes in the valence band structure, where the halide is dominating the electronic structure [41]. In the iodide-based perovskite, the valence band is shifted towards lower binding energies compared to the valence band for the bromide-based perovskite. The mixed iodide and bromide perovskites have band gaps between the pure iodide and bromide perovskites, and by choosing a ratio of iodide and bromide the band gap can be tuned to obtain a specific value, see Fig. 3.10. The light absorption of lead based perovskites can be tuned by changing the halide or the cations, as discussed above. On the other hand, the divalent cation (e.g. lead) may be replaced in the structure by other metal ions. It was early suggested that tin, Sn, can replace lead in the perovskite structure for efficient solar cells [18,19]. The tin based perovskites has a red shifted light absorption edge compared to the lead based perovskites. This makes it possible to extend the absorption range into the near IR-region for perovskites, which for lead perovskites is limited to around 800 nm. Furthermore, other metal ions have been suggested to replace lead, and bismuth halides has shown to be a low toxic alternative to the lead based perovskites for solar cells [42]. The bismuth halides have larger band gap than the lead halide based perovskites, and for Cs3Bi2I9, the band gap is around 2 eV [43]. This material has zero dimensional electronic properties, and the optical properties are not significantly affected by the cations and the band gap is indirect.

Characterization Techniques for Perovskite Solar Cell Materials

3.3 Steady state UV
Vis
NIR spectroscopy

61

FIGURE 3.9

(A) Optical density (Log(1/transmission)) spectra recorded in the presence of a single pulse of the incoming magnetic field. To enable comparison the spectra are offset. (B, C) Ratios of the transmission in magnetic field T(B) to that measured at zero field. (B) The 2 s absorption at lower fields and (C) at higher fields. Reprinted by permission from Springer Nature A. Miyata, A. Mitioglu, P. Plochocka, O. Portugall, J.T.-W. Wang, S.D. Stranks, et al., Direct measurement of the exciton binding energy and effective masses for charge carriers in organicinorganic tri-halide perovskites, Nat. Phys. 11 (7) (2015) 582
587.

Although the band gap is indirect, the light absorption is high, which makes the material interesting for photovoltaic applications. Combinations of bismuth and silver halides have also shown interesting optical and photovoltaic properties, and by combining bismuth and silver halides, double perovskite materials can be formed [44]. Moreover, other structures can form for mixed bismuth and silver halides, with a range of optical properties, interesting for solar cells [45]. In addition to bismuth and silver halides, copper- and germaniumbased perovskites have shown interesting light absorption properties for photovoltaic applications [46]. The light absorption of the metal-halide perovskites therefore in general

Characterization Techniques for Perovskite Solar Cell Materials

62

3. Optical absorption and photoluminescence spectroscopy

FIGURE 3.10 Band gap of perovskite absorber as a function of the nature of halide content. (A) UV
Vis absorption spectra of FTO/bl-TiO2/mp-TiO2/ MAPb (I1
xBrx)3/Au solar cells measured using an integral sphere. (B) Photographs of 3D TiO2/MAPb(I1
xBrx)3 bilayer nanocomposites on FTO glass substrates. (C) A quadratic relationship of the band-gaps of MAPb(I1
xBrx)3 as a function of Br composition (x). Reprinted with permission from J.H. Noh, S.H. Im, J.H. Heo, T.N. Mandal, S.Il. Seok, Chemical management for colorful, efficient, and stable inorganic
organic hybrid nanostructured solar cells, Nano Lett. 13 (4) (2013) 1764
1769, copy right 2013 American chemical Society.

seem to be rather strong, and many new metal-halide perovskite materials interesting for solar cell application with different light absorption spectra can still be expected to be discovered.

3.4 Photoluminescence spectroscopy Emission characteristics of perovskite absorbers provide us with sufficient evidence to understand high performance shown by perovskite optoelectronic devices. The perovskite semiconductors absorb light over a wide range, depending on their bandgaps, generating charge carriers, which relax towards the conduction band minimum and valence band maximum before producing strong emission via radiative recombination (Fig. 3.11). For example, the iodide-based perovskites exhibit a very broad spectral response, i.e. absorb photons of different energies, generating majorly free charge carriers, which

Characterization Techniques for Perovskite Solar Cell Materials

3.4 Photoluminescence spectroscopy

63

FIGURE 3.11 (A) Absorption and emission characteristics of MAPbI3 perovskite, and (B) the calculated band structure of MAPbI3 perovskite. Adopted with permission from J. Even, Pedestrian guide to symmetry properties of the reference cubic structure of 3D all-inorganic and hybrid perovskites, J. Phys. Chem. Lett. (2015) [47], 2015 American Chemical society.

recombine around the top of the valence band and bottom of the conduction band. Such a band-to-band relaxation is radiative in nature, leading to a bright and narrow emission. Depending on the excitation source, one could study either photoluminescence, electroluminescence or cathodoluminescence. As the photovoltaic applications involve the absorption of photons from the sun spectrum, the investigation of photoluminescence features allows us to understand the photo-physical processes occurring within the operational perovskite solar cells.

3.4.1 Processes involved in photoluminescence In a solar cell device, after the photo excitation, the generated charge carriers in conduction band (CB) and valence band (VB) of perovskite build a special charge density profile within the film which depends on the extinction coefficient at each specific wavelength. Depending on the working condition of the device, part of the carriers would be collected at charge selective contacts and part of them would remain stationary at the film to build up a certain Fermi level. The latter is under a balance in between partial charge generation and total recombination rates (R 5 G); At open circuit condition the total generation rate equals the recombination rate. The recombination processes can be classified into the radiative and non-radiative recombination. If the charge carriers are not separated and collected at contacts, after a certain time
called carrier life time
they will recombine; in the case of radiative recombination one electron-hole pair emits a photon instead which can be detected in the PL spectra. In the PL spectroscopy, the film is excited by photons normally with a higher energy than the band gap and the emission spectrum would be recorded. Special care should be taken into account for the stability of the air and humidity sensitive samples, i.e. by measuring the spectrum at inert atmosphere or using protection layers, tuning the right

Characterization Techniques for Perovskite Solar Cell Materials

64

3. Optical absorption and photoluminescence spectroscopy

excitation intensity and time and checking the absorption spectrum of the film before and after the measurement. The intensity of PL peak at band edges is indicative of radiative recombination of charge carriers and thus an evaluation of charge recombination within the film and also the charge collection at contacts can be obtained from that. The former/ later can be evaluated from the time resolved PL measurements of perovskite absorber versus time with/without the presence of charge selective contacts [48]. PL spectrum contains information about the band edge and presence of excitonic peaks [49], presence of trap states, and the degree of electron-phonon interactions [50]. Exciton binding energy can be estimated from the temperature dependent PL spectrum as well [51,52]. These parameters are of crucial importance for characterization and optimization of solar cell devices
see Chapter 6. The schematic of the recorded emission in the PL spectrum are illustrated in Fig. 3.12 and can be due to the direct recombination of electron holes at the band edge (process 2), or before the cooling (process 1), exciton radiative dissociation (process 3), or from the trap states (process 4). Process 5 is indicative of carrier cooling. In the following sections, several examples of PL spectroscopy for perovskite solar cell materials are shown. On the other note, the intra-band gap trap states within the bandgap of the material is one of the main source of the PL quenching via trapping the electron or holes, or making new bands in the PL spectrum related to the energy of the traps [53,54]. The observed traps in the metal-halide perovskite films can also be photo-induced where the effect is reversible and can be heeled in dark or in the presence of oxygen [54]. Density and energy of trap states can be studied by temperature dependent PL spectroscopy [53]. In the following sections, we present examples of PL spectroscopy for studying different photovoltaic processes within the perovskite solar cell device.

3.4.2 Diffusion length and carrier lifetime In the absence of a charge selective layer, the recombination of free electrons and holes is mainly responsible for the time dependent PL spectra. However, in the presence of the charge selective layer, diffusion and collection of the charge carriers towards the contacts interfere and compete with electron-hole recombination and changes the estimated PL decay times. The decay time of the PL peak to 1/e of its initial value is considered as a fair FIGURE 3.12 An illustration of fundamental processes involved in the PL emission of perovskite film (left) and a typical PL spectrum of perovskite MAPbI3 (right).

Characterization Techniques for Perovskite Solar Cell Materials

3.4 Photoluminescence spectroscopy

65

evaluation of the corresponding lifetimes. For example PL decay time of perovskite by itself represents the free electron-hole recombination time or as the so-called electron life time (τrec) within the device. Moreover the PL decay time (τ) in the presence of charge extraction layers has contributions from both charge transport (to extraction layers
τtrans) and charge recombination (τrec) that can be phrased as: 1 1 1 5 1 τ τ trans τ rec

(3.9)

For a photo-absorber to work properly, the collection time should be much faster than the recombination time and otherwise the distinguishing the contributions of two processes would be hard to be evaluated. In the highly efficient devices, charge transport time is significantly faster than recombination time and it is plausible to correlate the PL decay time with the charge transport time. Furthermore, Einstein relation can be used to calculate the corresponding charge diffusion length from the carrier transport time and the film thickness. The carrier lifetime measured in different regimes i.e. different fluences (Fig. 3.13) or temperatures gives fundamental insights about the recombination kinetics in the perovskite thin films.

3.4.3 Photon recycling in metal-halide perovskites The radiative recombination can be reabsorbed in the perovskite film and be implemented for the generation of charge carriers [56]. From the PL spectrum of the perovskite film with different physical distances of excitation and the PL detection probes, Pazos-Quton et al shown that the Beer
lambert’s law of photon absorption expected for the perovskite FIGURE 3.13 Time-resolved PL spectra of MAPbI3 in different fluences. Represented from T. Handa, D.M. Tex, A. Shimazaki, A. Wakamiya, Y.Kanemitsu, Charge injection mechanism at heterointerfaces in CH3NH3PbI3 perovskite solar cells revealed by simultaneous time-resolved photoluminescence and photocurrent measurements, J. Phys. Chem. Lett. (2017) [55]. (Ref DOI: 10.1021/ acs.jpclett.6b02847).

Characterization Techniques for Perovskite Solar Cell Materials

66

3. Optical absorption and photoluminescence spectroscopy

FIGURE 3.14

Experimentally measured emission map for different distances between the (A) excitation and detection probes and (B) the Beer
Lambert prediction. Adopted with permission from L.M. Pazos-Outon, M. Szumilo, R. Lamboll, J.M. Richter, M.Crespo-Quesada, M. Abdi-Jalebi, et al., Photon recycling in lead iodide perovskite solar cells, Science 351 (6280) (2016) 1430
1433.

film is different from the experimentally measured PL spectra (shown in Fig. 3.14) [56]. This phenomenon is considered as a factor that can further supports higher efficiencies; however, the net effect in a 300 nm perovskite film is not significant. The photon recycling effect is the origin of huge reduction in the external photoluminescence quantum yield (PLQE) from the internal yield as reported for metal halide perovskite [57]. This statement was confirmed further via the substantial enhancement in the PLQE of the deposited perovskite thin films on textured substrates to a high value of 57% compare to only 15% in planar film (Fig. 3.15) [57]. This significant increase in PLQE is related to efficient light out-coupling in perovskite films deposited on textured substrates. Therefore, an efficient strategy to enhance the conversion efficiencies in the both solar cells and light emitting diodes (LEDs) is to use textured active layer to maximize the PLQE of the halide perovskite layer in the device.

3.4.4 Exciton binding energy and excitonic peaks Exciton binding energy is an important parameter for photovoltaic performance of the device, indicating how good the charge separation happens in the excited state. With a low exciton binding energy, in the range of thermal energy 25 meV for lead halide perovskites, there would not be any binding between the photo-generated holes and electrons, which ease the selective charge collection at contacts. The most widely used method for determination of exciton binding energy is the fitting of the near band edge absorption spectrum with Elliot’s theory (Fig. 3.16) [52]. Temperature dependent PL quenching can also determine the exciton binding energy as reported in Ref. [51]. Excitonic peaks show up as sharp peaks in the absorption spectrum near the band edge. The perovskite phases that show high photovoltaic performance with a charge separation efficiency close to 1, such as CH3NH3PbI3, show no such peaks in the absorption spectrum. However, MaPbBr3 or the alloyed perovskites with high Br constituents can

Characterization Techniques for Perovskite Solar Cell Materials

3.4 Photoluminescence spectroscopy

67

FIGURE 3.15 (A) Sketch illustrating the improved sunlight in-coupling of textured solar cells. In a planar film, light will leave the film at the second interface. In a textured film, light undergoes about 30 total internal reflections and can therefore travel significantly longer in the active layer. (B) Measured external PLQEs of CH3NH3PbI32xClx films when deposited on a textured (circles) and planar (squares) substrate under CW excitation, together with computed PLQEs for different photon escape probabilities ηesc. While we were estimating the escape probability for the planar film to be 12.7%, we determine an escape probability of about 50% for the textured substrate, yielding an external PLQE of 57%. Adopted from J.M. Richter, M. Abdi-Jalebi, A. Sadhanala, M. Tabachnyk, J.P.H. Rivett, L.M. Pazos-Outo´n, et al., Enhancing photoluminescence yields in lead halide perovskites by photon recycling and light out-coupling, Nat. Commun. 7 (2016) 13941. FIGURE 3.16 Temperature-dependent integrated PL intensity of the CH3NH3PbI3 film under excitation of a 532 nm continuous-wave laser beam. The solid line is the best fit based on the figure adopted from S. Sun, T. Salim, N. Mathews, M. Duchamp, C. Boothroyd, G. Xing, et al., The origin of high efficiency in low-temperature solution processable bilayer organometal halide hybrid solar cells, Energy Environ. Sci. 7 (1) (2014) 399
407, published by The Royal Society of Chemistry.

show the exciton absorption peak in the UV
Vis spectrum [49]. These excitons show the fingerprints at similar wavelengths in the PL spectra as well. Fig. 3.17 compares PL and absorption spectra of two different mixed perovskite phases.

3.4.5 Tunability and stability of PL in alloyed perovskites The wavelength of photoluminescence can be systematically tuned by changing the composition of the perovskite absorber material exhibiting the general formula ABX3. In

Characterization Techniques for Perovskite Solar Cell Materials

FIGURE 3.17 PL spectra of alloyed perovskite thin films with two different compositions. Reproduced with permission from J.T. Jacobsson, J.P. Correa Baena, M. Pazoki, M. Saliba, K. Schenk, M. Gra¨tzel, et al., Exploration of the compositional space for mixed lead halogen perovskites for high efficiency devices, Energy Environ. Sci. 9 (2016) 1706
1724, Royal Society of Chemistry.

3.4 Photoluminescence spectroscopy

69

principle and as clarified in Chapter 1, by changing A-cation, B-cation and/or halide (X) or using a mixture, the desired composition can be formulated. For example, by substituting MA1 with FA1, photoluminescence in the range of 780
850 nm can be obtained from APbI3 (MA1 or/and FA1). Similarly, using a mixture of halides seems to have a larger impact on the bandgap as compared to using a mixed A-cation formulation (Fig. 3.18) [40,49,58]. For example, by changing the iodide-bromide ratio in MAPbI3-xBrx, the emission wavelength can be finely tuned from 550 nm to 780 nm. On the other hand, a complete substitution of MA1 with FA1 from MAPbBr3 exhibits a minimal effect on the bandgap as the λmax of photoluminescence shifts by 10 nm from 560 nm for FAPbBr3 to 550 nm to MAPbBr3 [59]. This kind of flexibility in bandgap tuning by modifying the precursor concentrations in chemical solutions is rather unique for perovskites compared to other thin film solar cell technologies like silicon and CIGS which need expensive techniques with much less flexibility in the obtained bandgap. The state of the art perovskite composition that shows the highest efficiencies as well as the best reported stabilities, composed from a mixed composition, i.e. different halides and monovalent cations within the perovskite lattice. However, under the light illumination, a photo-induced phase segregation has been observed by which i.e. a mixed perovskite composed from I and Br halogens can be decomposed to a iodine reach and bromine reach perovskite regions (Fig. 3.19) [60]. Such phase segregations can be understood by a PL mapping technique in which PL spectrum from different regions of the film can be recorded and therefore fingerprints of iodine- or bromine- rich regions in comparison to the mixed-phase can be detected. This type of phase segregation is reported to be reversible after keeping the film long enough at dark [61]. A similar behavior has been observed from PL mapping data in pure MAPbI3 in which the iodine anions can diffuse towards the grain boundaries under light and come back after keeping in the dark in a process called photo-induced ionic movement [62] that has been suggested by stark spectroscopy

FIGURE 3.18 (A) Tuning the PL of alloyed perovskite thin films via altering the type of halide and monovalent cation and the ratios. (B) Macro images of the perovskite films deposited on glass corresponding to the PL spectra. Adopted from J.T. Jacobsson, J.P. Correa Baena, M. Pazoki, M. Saliba, K. Schenk, M. Gra¨tzel, et al., Exploration of the compositional space for mixed lead halogen perovskites for high efficiency devices, Energy Environ. Sci. 9 (2016) 1706
1724.

Characterization Techniques for Perovskite Solar Cell Materials

70

3. Optical absorption and photoluminescence spectroscopy

FIGURE 3.19 Photo-induced segregation of mixed halide based perovskites. Adopted from E.T. Hoke, D.J. Slotcavage, E.R. Dohner, A.R. Bowring, H.I. Karunadasa, M.D. McGehee, Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics, Chem. Sci. 6 (1) (2015) 613
617; M.C. Brennan, S. Draguta, P.V. Kamat, M. Kuno, Light-induced anion phase segregation in mixed halide perovskites, ACS Energy Lett. (2018) [64,65], Published by The Royal Society of Chemistry (right) and American Chemical Society (left).

as well [63]. Moreover, a pure tetragonal perovskite encounter phase transitions to cubic and orthrombic at different temperatures that accompanied by peak shifts or additional peaks at PL spectrum. The precise tenability of emission wavelength achieved by mixing iodide and bromide generates an enormous amount of interest, primarily for light emission applications, however, the segregation of iodide-rich and bromide-rich perovskite domains under continuous illumination had eluded various application of these mixed-halide compositions (Fig. 3.19) [64]. This photo-induced segregation also imposed restrictions on the content on bromide present in iodide-based perovskite crystal structure. Furthermore, the existence of substantial parasitic non-radiative recombination in perovskite thin films and when interfaced into devices, is one of the main losses in the performance of perovskite-based devices that preventing them to reach their efficiency limit [66]. Recently, a novel approach known as potassium passivation is developed by AbdiJalebi et al. as an effective way to not only inhibit the photo-induced phase segregation and bandgap instability of alloyed perovskite materials but also leads to stabilized high luminesce yield (Fig. 3.20) [67]. In the potassium passivation, by filling the halide vacancies in the perovskite crystal structure via introducing excess halide and decorating the hybrid grains with K1 where the potassium draws out the Br from the perovskite lattice and lock the excess halide in the grain boundaries, the photo-induced segregation of mixed iodide-bromide compositions is completely mitigated [67]. Furthermore, the mechanistic insights gained through a combination of techniques including experiments and theory establish that energetically favorable segregation of iodide dominant domains induces the formation of phase mixture. The main criteria in an optoelectronic device to reach the efficiency limit is to maximize the luminescence quantum yield and inhibit any non-radiative losses in the active layer in particular when it is interfaced with the device electrodes (see Chapter 6 for the detailed description). There have been several approaches to enhance the PLQE of the perovskite films via ligand passivation where the transport properties were effected by adding the organic molecule into the perovskite layer [68]. However, using the potassium passivation

Characterization Techniques for Perovskite Solar Cell Materials

3.4 Photoluminescence spectroscopy

71

FIGURE 3.20 (A) Stabilization of PLQE and (B
D) mitigation of photo-induced instability of perovskites thin films via potassium passivation. (E) Schematic showing the mechanism of the passivation approach. In all the panels, x, represent the fraction potassium added to the precursor solution of perovskites. Adopted from M. AbdiJalebi, Z. Andaji-Garmaroudi, S. Cacovich, C. Stavrakas, B. Philippe, J.M. Richter, etal., Maximizing and stabilizing luminescence from halide perovskites with potassium passivation, Nature 555 (7697) (2018) 497
501.

FIGURE 3.21 Enhanced (A) time-resolved photoluminescence lifetime and (B) external PLQE of the perovskite layer interfaced with device electrodes (n-type: TiO2/perovskite, p-type: perovskite/Spiro OMeTAD and Stack: TiO2/perovskite/Spiro OMeTAD). Adopted from M. Abdi-Jalebi, Z. Andaji-Garmaroudi, S. Cacovich, C. Stavrakas, B. Philippe, J.M. Richter, et al., Maximizing and stabilizing luminescence from halide perovskites with potassium passivation, Nature 555 (7697) (2018) 497
501.

approach, Abdi-Jalebi et al. able to reach significantly enhanced PL lifetime and high PLQE in the device stack (e.g. 15%) without perturbing the charge transport properties of the perovskite films (Fig. 3.21).

Characterization Techniques for Perovskite Solar Cell Materials

72

3. Optical absorption and photoluminescence spectroscopy

3.4.6 Impact of perovskite crystalline quality, fluence and charge extraction layer on PL In addition to the radiative recombination, the charge-carrier dynamics also involves other processes, which are non-radiative in nature, such as, trap-assisted recombination also known as Shockley
Read
Hall (SRH) recombination and higher order recombinations, such as Auger recombination. The SRH recombination can be controlled by improving the crystal quality through minimizing the trap-state density within the perovskite material. These traps or defects can be classified into two major categories; (1) deep-traps, and (2) surface traps. The origin of deep traps is majorly associated with the halide vacancies or interstitial ions, whereas surface traps arise from the unsaturated bonds present at the grain boundaries and/or from the impurity phases interfacing with the absorber layer. Overall, the nature and amount of defects directly influence the kinetics of the charge carrier recombination thus the contribution from the corresponding recombination constants (Fig. 3.22) [69]. The PL fluence alters the charge carrier density, which directly influences the kinetics of recombination. For example, the contribution from Auger recombination becomes significant only under high influences when the charge carrier density exceeds 1017 cm23 (Fig. 3.23).

FIGURE 3.22 (A) Steady-state absorption, (B) time-integrated and (C
D) time-resolved emission dynamics as a function of the excitonic quality of FAPbBr3 films. FA (1) film deposition involves DMF solvent, FA (2) film deposition involves a mixture of DMF and DMSO solvent. Adopted from N. Arora, M.I. Dar, M. Hezam, W. Tress, G. Jacopin, T. Moehl, et al., Photovoltaic and amplified spontaneous emission studies of high-quality formamidinium lead bromide perovskite films, Adv. Funct. Mater. (2016).

Characterization Techniques for Perovskite Solar Cell Materials

3.4 Photoluminescence spectroscopy

73

FIGURE 3.23 Charge carrier dynamics as a function of fluences in MAPbI3 films recorded at 300 K. (A) Fluence-dependent time-resolved PL of the tetragonal phase and (B) charge carrier lifetime (t10) recorded in the tetragonal phase at 300 K decreases with increasing fluence (t10, time at which the maximum PL intensity decreases by a factor of 10). Adopted from M.I. Dar, G. Jacopin, S. Meloni, A. Mattoni, N. Arora, A. Boziki, et al., Origin of unusual bandgap shift and dual emission in organic-inorganic lead halide perovskites. Sci. Adv. 2 (10) (2016).

Under low fluences, the trapping of charges at various defect sites plays a critical role, which can have a noticeable effect on the overall performances of the solar cells and light emitting devices [50,70]. The analytical expressions for kinetic of different recombination processes in the perovskite material are presented in Chapter 6. The recombination of charge carriers occurring within the perovskite absorber layer can be influenced by the presence of selective contacts including electron and hole transporting layers [71]. This is primarily due to a decrease in the initial density of electrons or holes present in the absorber layer when interfaced, respectively, with electron and hole transporting layers. Fig. 3.9A and B displays the effect of HOMO position on the hole extraction property of HTM. By increasing the energy gap between the valence band of perovskite layer and the HOMO of HTM, the extraction of holes from the former layer becomes relatively more rapid as evident from the PL quenching and time-resolved PL studies (Fig. 3.24A and B). Similarly, by improving the interfacial properties between the absorber layer and ETL, the extraction of electrons from the perovskite layer becomes relatively more efficient as shown by steady-state and time-resolved PL studies (Fig. 3.24C and D) [23]. As mentioned in previous Section 4.3.2, time resolved PL in the presence/ absence of charge extraction layers can be implemented to study the transport and diffusion processes in the perovskite film.

3.4.7 Temperature dependent PL in metal halide perovskite In addition to the fluence, various photo-physical processes show a strong dependence on the temperature as the chemical bonding between the constituent atoms or ions gets affected. Consequently, the lattice constant, lattice vibrations, and electron-phonon interactions etc. vary and influence the bandgap and charge-carrier dynamics. Contrary to

Characterization Techniques for Perovskite Solar Cell Materials

74

3. Optical absorption and photoluminescence spectroscopy

FIGURE 3.24 Steady-state and time-resolved photoluminescence studies demonstrating the effect of interfaces on the charge extraction processes across various interfaces. (A and B) Adopted from N. Arora, S. Orlandi, M.I. Dar, S. Aghazada, G.Jacopin, M. Cavazzini, et al., High open-circuit voltage: fabrication of formamidinium lead bromide perovskite solar cells using fluorene-dithiophene derivatives as hole-transporting materials, ACS Energy Lett. (2016); (C and D) N. Arora, M.I. Dar, M. Abdi-Jalebi, F. Giordano, N. Pellet, G. Jacopin, et al., Intrinsic and extrinsic stability of formamidinium lead bromide perovskite solar cells yielding high photovoltage, Nano Lett. 16 (11) (2016) 7155
7162.

conventional semiconductors, the band gap of perovskite materials increases with increasing temperature. Such an unusual behavior cannot be explained using the Varshni model, according to which the bandgap should decrease with increasing temperature due to the dilatation of the lattice [72]. Although the similar phenomenon of lattice expansion occurs in the perovskite materials at higher temperatures, the origin of the unusual bandgap shift can be explained by evoking the antibonding nature of the valence and conduction band orbitals (Fig. 3.25) [50]. Another interesting feature shown by the perovskite emitters at low temperature ( . 120 K) is the dual emission, which is well distinct and well evident in MA-cation based lead halide perovskites. The classical molecular dynamics (CMD) shows that at low temperature, e.g., .150 K for MAPbI3, the organic cation can exhibit two different orientations; (1) perfectly antiparallel (ordered), and (2) random (disordered) alignment. The former is denoted as the ordered orthorhombic domain yielding emission around 750 nm at 15 K, whereas the latter gives rise to the disordered orthorhombic domain yielding emission

Characterization Techniques for Perovskite Solar Cell Materials

75

3.4 Photoluminescence spectroscopy

FIGURE 3.25 (A) Normalized photoluminescence as a function of temperature in MAPbI3 recorded from 15 to 300 K. (B) Central energy of the emission peaks corresponding to MA-ordered (blue color, top plot) and MA-disordered orthorhombic domains (red symbols, bottom plot) and tetragonal phase (red symbols, bottom plot, T . 150 K) of CH3NH3PbI3 as a function of temperature. Adopted from M.I. Dar, G. Jacopin, S. Meloni, A. Mattoni, N. Arora, A. Boziki, et al., Origin of unusual bandgap shift and dual emission in organic-inorganic lead halide perovskites, Sci. Adv. 2 (10) (2016). (A)

(B) 1.8

(d)

(c)

1.75

100 K

(a)

300 K

(b)

Eg (eV)

1.7 1.65 1.6

Ortho ordered Ortho disordered Tetra

1.55

(e) 1.5

6.25

a*

6.3 a (Å)

6.35

6.4

Temperature

FIGURE 3.26 Classical molecular dynamics simulations. (A) Snapshots extracted from the simulations at (a) 100 K and (b) 300 K. Panels (c), (d), and (e) show the configurations of the MA-ordered and MA-disordered orthorhombic and the tetragonal domains, respectively. (B) Bandgap as a function of the pseudo-cubic lattice parameter (a) for the MA-ordered (square symbols) and MA-disordered (circle symbols) orthorhombic systems and the tetragonal system (rhombus symbols). Adopted from M.I. Dar, G. Jacopin, S. Meloni, A. Mattoni, N. Arora, A. Boziki, et al., Origin of unusual bandgap shift and dual emission in organic-inorganic lead halide perovskites, Sci. Adv. 2 (10) (2016).

around 788 nm, which is redshifted when compared to ordered orthorhombic domain (Fig. 3.26). Such a redshift could be explained by evoking the dipole moment of the organic cation, producing a local Stark-like effect, which is stronger in case of ordered phases than in disordered ones. In addition, due to the higher dipole moment of MA1 than FA1, the band gap difference between ordered and disordered systems is B85 meV,

Characterization Techniques for Perovskite Solar Cell Materials

76

3. Optical absorption and photoluminescence spectroscopy

allowing to resolve the emission peaks. In case of an FA-based system, this energy difference turns out to be quite small, i.e., 20 meV, which apparently makes it difficult to resolve the emission features associated with ordered or disordered domains. Interestingly, the energy difference seems to be invariant of the nature of halide anion as substitution of all iodides with bromides from the MAPbI3 system does not influence the band gap difference between ordered and disordered systems, which remained constant, i.e., B85 meV.

References [1] A. Sadhanala, F. Deschler, T.H. Thomas, S.E. Dutton, K.C. Goedel, F.C. Hanusch, et al., Preparation of singlephase films of CH3 NH3 Pb(I 1
x Br x)3 with sharp optical band edges, J. Phys. Chem. Lett. 5 (15) (2014) 2501
2505. [2] W.B. Jackson, N.M. Amer, A.C. Boccara, D. Fournier, Photothermal deflection spectroscopy and detection, Appl. Opt. 20 (8) (1981) 1333. [3] W. Jackson, N. Amer, Direct measurement of gap-state absorption in hydrogenated amorphous silicon by photothermal deflection spectroscopy, Phys. Rev. B 25 (8) (1982) 5559
5562. [4] W.B. Jackson, Optical absorption spectra of surface or interface states in hydrogenated amorphous silicon, Appl. Phys. Lett. 42 (1) (1983) 105. [5] A.S. Kumar, K.L. Narasimhan, R. Rajalakshmi, S.S. Chandvankar, B.M. Arora, Photothermal deflection spectroscopy of heat-treated GaAs, InP, and InGaAsP alloys, Appl. Phys. Lett. 55 (24) (1989) 2512. [6] A.C. Boccara, D. Fournier, J. Badoz, Thermo-optical spectroscopy: detection by the ‘mirage effect’, Appl. Phys. Lett. 36 (2) (1980) 130
132. [7] A. Rosencwaig, A. Gersho, Theory of the photoacoustic effect with solids, J. Appl. Phys. 47 (1) (1976) 64. [8] D.S. Thomas, Photothermal Deflection Spectroscopy: Design, Realisation and Implementation in Organic Semiconductor Research, University of Cambridge, Cambridge, UK, 1999. Vol. Ph.D. Diss. [9] F. Urbach, The long-wavelength edge of photographic sensitivity and of the electronic absorption of solids, Phys. Rev. 92 (5) (1953). 1324
1324. [10] R.S. Knox, Theory of Excitons, Academic Press, New York, 1963. [11] T. Keil, Theory of the Urbach rule, Phys. Rev. 144 (2) (1966) 582
587. [12] J. Dow, D. Redfield, Toward a unified theory of Urbach’s rule and exponential absorption edges, Phys. Rev. B 5 (2) (1972) 594
610. [13] T. Skettrup, Urbach’s rule derived from thermal fluctuations in the band-gap energy, Phys. Rev. B 18 (6) (1978) 2622
2631. [14] S. John, C. Soukoulis, M.H. Cohen, E.N. Economou, Theory of electron band tails and the Urbach opticalabsorption edge, Phys. Rev. Lett. 57 (14) (1986) 1777
1780. [15] S. John, M. Chou, M. Cohen, C. Soukoulis, Density of states for an electron in a correlated gaussian random potential: theory of the Urbach tail, Phys. Rev. B 37 (12) (1988) 6963
6976. [16] W. Rieger, R. Dimitrov, D. Brunner, E. Rohrer, O. Ambacher, M. Stutzmann, Defect-related optical transitions in GaN, Phys. Rev. B 54 (24) (1996) 17596
17602. [17] C.C. Stoumpos, C.D. Malliakas, M.G. Kanatzidis, Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties, Inorg. Chem. 52 (15) (2013) 9019
9038. [18] F. Hao, C.C. Stoumpos, D.H. Cao, R.P.H. Chang, M.G. Kanatzidis, Lead-free solid-state organic
inorganic halide perovskite solar cells, Nat. Photonics 8 (6) (2014) 489
494. [19] N.K. Noel, S.D. Stranks, A. Abate, C. Wehrenfennig, S. Guarnera, A. Haghighirad, et al., Lead-free organicinorganic tin halide perovskites for photovoltaic applications, Energy Environ. Sci. 7 (9) (2014) 3061
3068. [20] M. Abdi-Jalebi, M.I. Dar, A. Sadhanala, S.P. Senanayak, F. Giordano, S.M. Zakeeruddin, et al., Impact of a mesoporous titania
perovskite interface on the performance of hybrid organic
inorganic perovskite solar cells, J. Phys. Chem. Lett. 7 (16) (2016) 3264
3269. [21] M. Abdi-Jalebi, M.R. Mohammadi, D.J. Fray, Double-layer TiO2 electrodes with controlled phase composition and morphology for efficient light management in dye-sensitized solar cells, J. Clust. Sci. 25 (4) (2014) 1029
1045.

Characterization Techniques for Perovskite Solar Cell Materials

References

77

[22] A.K. Chandiran, M. Abdi-jalebi, M.K. Nazeeruddin, M. Gra, Analysis of electron transfer properties of ZnO and TiO2 photoanodes for dye-sensitized solar cells, ACS Nano 8 (3) (2014) 2261
2268. [23] N. Arora, M.I. Dar, M. Abdi-Jalebi, F. Giordano, N. Pellet, G. Jacopin, et al., Intrinsic and extrinsic stability of formamidinium lead bromide perovskite solar cells yielding high photovoltage, Nano. Lett. 16 (11) (2016) 7155
7162. [24] B. Zhao, M. Abdi-Jalebi, M. Tabachnyk, H. Glass, V.S. Kamboj, W. Nie, et al., High open circuit voltages in tin-rich low-bandgap perovskites based planar heterojunction photovoltaics, Adv. Mater. 29 (2) (2016) 1604744. [25] K.K. Wong, A. Fakharuddin, P. Ehrenreich, T. Deckert, M. Abdi-Jalebi, R.H. Friend, et al., Interfacedependent radiative and nonradiative recombination in perovskite solar cells, J. Phys. Chem. C 122 (20) (2018) 10691
10698. [26] M.A. Green, A. Ho-Baillie, H.J. Snaith, The emergence of perovskite solar cells, Nat. Photonics 8 (7) (2014) 506
514. [27] S. De Wolf, J. Holovsky, S.-J. Moon, P. Lo¨per, B. Niesen, M. Ledinsky, et al., Organometallic halide perovskites: sharp optical absorption edge and its relation to photovoltaic performance, J. Phys. Chem. Lett (2014). In Press, 140305122150008. [28] S. De Wolf, J. Holovsky, S.J. Moon, P. Lo¨per, B. Niesen, M. Ledinsky, et al., Organometallic halide perovskites: sharp optical absorption edge and its relation to photovoltaic performance, J. Phys. Chem. Lett. 5 (6) (2014) 1035
1039. [29] M. Abdi-Jalebi, M.I. Dar, A. Sadhanala, S.P. Senanayak, M. Gra¨tzel, R.H. Friend, Monovalent cation doping of CH3NH3PbI3 for efficient perovskite solar cells, J. Vis. Exp. No. 121 (2017) 1
9. [30] M. Abdi-Jalebi, M.I. Dar, A. Sadhanala, S.P. Senanayak, M. Franckeviˇcius, N. Arora, et al., Impact of monovalent cation halide additives on the structural and optoelectronic properties of CH3NH3PbI3 perovskite, Adv. Energy Mater. 6 (10) (2016) 1502472. [31] R. Lindblad, D. Bi, B.W. Park, J. Oscarsson, M. Gorgoi, H. Siegbahn, et al., Electronic structure of TiO2/ CH3NH 3PbI3 perovskite solar cell interfaces, J. Phys. Chem. Lett. (2014). [32] G.E. Eperon, S.D. Stranks, C. Menelaou, M.B. Johnston, L.M. Herz, H.J. Snaith, Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells, Energy Environ. Sci. 7 (3) (2014) 982. [33] A. Amat, E. Mosconi, E. Ronca, C. Quarti, P. Umari, M.K. Nazeeruddin, et al., Cation-induced band-gap tuning in organohalide perovskites: interplay of spin-orbit coupling and octahedra tilting, Nano. Lett. (2014). [34] C.W. Myung, J. Yun, G. Lee, K.S. Kim, A new perspective on the role of A-site cations in perovskite solar cells, Adv. Energy Mater. 8 (14) (2018) 1702898. [35] M. Abdi-Jalebi, M. Pazoki, B. Philippe, M.I. Dar, M. Alsari, A. Sadhanala, et al., Dedoping of lead halide perovskites incorporating monovalent cations, ACS Nano 12 (7) (2018) 7301
7311. [36] M. Saliba, T. Matsui, J.-Y. Seo, K. Domanski, J.-P. Correa-Baena, K. Mohammad, et al., Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency, Energy Environ. Sci. 9 (6) (2016). [37] A. Miyata, A. Mitioglu, P. Plochocka, O. Portugall, J.T.-W. Wang, S.D. Stranks, et al., Direct measurement of the exciton binding energy and effective masses for charge carriers in organic-inorganic tri-halide perovskites, Nat. Phys. 11 (7) (2015) 582
587. [38] Z. Yang, A. Surrente, K. Galkowski, N. Bruyant, D.K. Maude, A.A. Haghighirad, et al., Unraveling the exciton binding energy and the dielectric constant in single-crystal methylammonium lead triiodide perovskite, J. Phys. Chem. Lett. (2017). [39] T. Leijtens, S.D. Stranks, G.E. Eperon, R. Lindblad, E.M.J. Johansson, I.J. McPherson, et al., Electronic properties of meso-superstructured and planar organometal halide perovskite films: charge trapping, photodoping, and carrier mobility, ACS Nano (2014). [40] J.H. Noh, S.H. Im, J.H. Heo, T.N. Mandal, S.Il. Seok, Chemical management for colorful, efficient, and stable inorganic
organic hybrid nanostructured solar cells, Nano. Lett. 13 (4) (2013) 1764
1769. [41] R. Lindblad, N.K. Jena, B. Philippe, J. Oscarsson, D. Bi, A. Lindblad, et al., Electronic structure of CH3NH3PbX3 perovskites: dependence on the halide moiety, J. Phys. Chem. C (2015). [42] B.W. Park, B. Philippe, X. Zhang, H. Rensmo, G. Boschloo, E.M.J. Johansson, Bismuth based hybrid perovskites A3Bi2I9 (A: methylammonium or cesium) for solar cell application, Adv. Mater. (2015).

Characterization Techniques for Perovskite Solar Cell Materials

78

3. Optical absorption and photoluminescence spectroscopy

[43] M. Pazoki, M.B. Johansson, H. Zhu, P. Broqvist, T. Edvinsson, G. Boschloo, et al., Bismuth iodide perovskite materials for solar cell applications: electronic structure, optical transitions, and directional charge transport, J. Phys. Chem. C (2016). [44] F. Giustino, H.J. Snaith, Toward lead-free perovskite solar cells, ACS Energy Lett. (2016). [45] H. Zhu, M. Pan, M.B. Johansson, E.M.J. Johansson, High photon-to-current conversion in solar cells based on light-absorbing silver bismuth iodide, ChemSusChem (2017). [46] Z. Shi, J. Guo, Y. Chen, Q.Q. Li, Y. Pan, H. Zhang, et al., Lead-free organic
inorganic hybrid perovskites for photovoltaic applications: Recent advances and perspectives, Adv. Mater. (2017). [47] J. Even, Pedestrian guide to symmetry properties of the reference cubic structure of 3D all-inorganic and hybrid perovskites, J. Phys. Chem. Lett. (2015). [48] S.D. Stranks, G.E. Eperon, G. Grancini, C. Menelaou, M.J.P. Alcocer, T. Leijtens, et al., Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber, Science 342 (6156) (2013) 341
344. [49] J.T. Jacobsson, J.P. Correa Baena, M. Pazoki, M. Saliba, K. Schenk, M. Gra¨tzel, et al., Exploration of the compositional space for mixed lead halogen perovskites for high efficiency devices, Energy Environ. Sci. 9 (2016) 1706
1724. [50] M.I. Dar, G. Jacopin, S. Meloni, A. Mattoni, N. Arora, A. Boziki, et al., Origin of unusual bandgap shift and dual emission in organic-inorganic lead halide perovskites, Sci. Adv. 2 (10) (2016). [51] S. Sun, T. Salim, N. Mathews, M. Duchamp, C. Boothroyd, G. Xing, et al., The origin of high efficiency in low-temperature solution-processable bilayer organometal halide hybrid solar cells, Energy Environ. Sci. 7 (1) (2014) 399
407. [52] L.M. Herz, Charge carrier dynamics in organic-inorganic metal halide perovskites, Annu. Rev. Phys. Chem. 67 (2016) 65
89. [53] X. Wu, M.T. Trinh, D. Niesner, H. Zhu, Z. Norman, J.S. Owen, et al., Trap states in lead iodide perovskites, J. Am. Chem. Soc. 137 (5) (2015) 2089
2096. [54] S.G. Motti, M. Gandini, A.J. Barker, J.M. Ball, A.R. Srimath Kandada, A. Petrozza, Photoinduced emissive trap states in lead halide perovskite semiconductors, ACS Energy Lett. 1 (4) (2016) 726
730. [55] T. Handa, D.M. Tex, A. Shimazaki, A. Wakamiya, Y. Kanemitsu, Charge injection mechanism at heterointerfaces in CH3NH3PbI3 perovskite solar cells revealed by simultaneous time-resolved photoluminescence and photocurrent measurements, J. Phys. Chem. Lett. (2017). [56] L.M. Pazos-Outon, M. Szumilo, R. Lamboll, J.M. Richter, M. Crespo-Quesada, M. Abdi-Jalebi, et al., Photon recycling in lead iodide perovskite solar cells, Science 351 (6280) (2016) 1430
1433. [57] J.M. Richter, M. Abdi-Jalebi, A. Sadhanala, M. Tabachnyk, J.P.H. Rivett, L.M. Pazos-Outo´n, et al., Enhancing photoluminescence yields in lead halide perovskites by photon recycling and light out-coupling, Nat. Commun. 7 (2016) 13941. [58] P. Brenner, T. Glo¨ckler, D. Rueda-Delgado, T. Abzieher, M. Jakoby, B.S. Richards, et al., Triple cation mixedhalide perovskites for tunable lasers, Opt. Mater. Express (2017). [59] M.I. Dar, A. Hinderhofer, G. Jacopin, V. Belova, N. Arora, S.M. Zakeeruddin, et al., Function follows form: correlation between the growth and local emission of perovskite structures and the performance of solar cells, Adv. Funct. Mater. 27 (26) (2017). [60] S. Draguta, O. Sharia, S.J. Yoon, M.C. Brennan, Y.V. Morozov, J.M. Manser, et al., Rationalizing the lightinduced phase separation of mixed halide organic
inorganic perovskites, Nat. Commun. 8 (1) (2017) 200. [61] D.J. Slotcavage, H.I. Karunadasa, M.D. McGehee, Light-induced phase segregation in halide-perovskite absorbers, ACS Energy Lett. (2016). [62] D.W. DeQuilettes, W. Zhang, V.M. Burlakov, D.J. Graham, T. Leijtens, A. Osherov, et al., Photo-induced halide redistribution in organic
inorganic perovskite films, Nat. Commun. 7 (May) (2016) 11683
11692. [63] M. Pazoki, T.J. Jacobsson, J. Kullgren, E.M.J. Johansson, A. Hagfeldt, G. Boschloo, et al., Photoinduced stark effects and mechanism of ion displacement in perovskite solar cell materials, ACS Nano 11 (3) (2017) 2823
2834. [64] E.T. Hoke, D.J. Slotcavage, E.R. Dohner, A.R. Bowring, H.I. Karunadasa, M.D. McGehee, Reversible photoinduced trap formation in mixed-halide hybrid perovskites for photovoltaics, Chem. Sci. 6 (1) (2015) 613
617.

Characterization Techniques for Perovskite Solar Cell Materials

References

79

[65] M.C. Brennan, S. Draguta, P.V. Kamat, M. Kuno, Light-induced anion phase segregation in mixed halide perovskites, ACS Energy Lett. (2018). [66] S.D. Stranks, Nonradiative losses in metal halide perovskites, ACS Energy Lett. 2 (7) (2017) 1515
1525. [67] M. Abdi-Jalebi, Z. Andaji-Garmaroudi, S. Cacovich, C. Stavrakas, B. Philippe, J.M. Richter, et al., Maximizing and stabilizing luminescence from halide perovskites with potassium passivation, Nature 555 (7697) (2018) 497
501. [68] D.W. deQuilettes, S. Koch, S. Burke, R. Paranji, A.J. Shropshire, M.E. Ziffer, et al., Photoluminescence lifetimes exceeding 8 Ms and quantum yields exceeding 30% in hybrid perovskite thin films by ligand passivation, ACS Energy Lett. (2016) 1
7. [69] N. Arora, M.I. Dar, M. Hezam, W. Tress, G. Jacopin, T. Moehl, et al., Photovoltaic and amplified spontaneous emission studies of high-quality formamidinium lead bromide perovskite films, Adv. Funct. Mater. (2016). [70] S.D. Stranks, V.M. Burlakov, T. Leijtens, J.M. Ball, A. Goriely, H.J. Snaith, Recombination kinetics in organicinorganic perovskites: excitons, free charge, and subgap states, Phys. Rev. Appl. 2 (3) (2014) 034007. [71] N. Arora, S. Orlandi, M.I. Dar, S. Aghazada, G. Jacopin, M. Cavazzini, et al., High open-circuit voltage: fabrication of formamidinium lead bromide perovskite solar cells using fluorene-dithiophene derivatives as holetransporting materials, ACS Energy Lett. (2016). [72] K. Wu, A. Bera, C. Ma, Y. Du, Y. Yang, L. Li, et al., Temperature-dependent excitonic photoluminescence of hybrid organometal halide perovskite films, Phys. Chem. Chem. Phys. 16 (41) (2014) 22476
22481.

Characterization Techniques for Perovskite Solar Cell Materials

C H A P T E R

4 Current-voltage analysis: lessons learned from hysteresis Eva L. Unger1,2, Aniela Czudek1,3, Hui-Seon Kim4 and Wolfgang Tress4 1

Young Investigator Group Hybrid Materials Formation and Scaling, Helmholtz-Zentrum Berlin for Materials and Energy, Berlin, Germany 2Department of Chemistry and NanoLund, Lund University, Lund, Sweden 3Faculty of Physics, Warsaw University of Technology, Warsaw, Poland 4Laboratory of Photomolecular Science, E´cole Polytechnique Fe´de´rale de Lausanne, Lausanne, Switzerland

4.1 “Hysterical” current-voltage behavior of perovskite solar cells Review papers and book chapters have been dedicated to describe the (r)evolution of perovskite solar cells, PSCs [1
4]. The break-through of these novel semiconductors in solar energy conversion is owed to the unprecedented increase in reported power conversion efficiencies, illustrated in Fig. 4.1A. The power conversion efficiency of solar cells is commonly derived from current density-voltage, J
V, curves which can be characterized by the diode equation, including shunt and series resistance, Rsh and Rs:
qðV1IRs Þ  V 1 IR s (4.1) J 5 Jph 2 j0 e nkB T 2 1 2 Rsh where Jph is the photocurrent density, j0 the saturation current density of the diode, q is the elementary charge, n the diode ideality factor and kBT the thermal energy. The most significant metric derived from J
V measurements of a solar cell is its maximum output power density Pmpp, obtained at the maximum power point (MPP). Pmpp is commonly expressed by the open circuit voltage, VOC, short-circuit current density, JSC, and fill factor, FF. The power conversion efficiency, PCE, of a solar cell is defined with respect to the

Characterization Techniques for Perovskite Solar Cell Materials DOI: https://doi.org/10.1016/B978-0-12-814727-6.00004-9

81

Copyright © 2020 Elsevier Inc. All rights reserved.

82

4. Current-voltage analysis: lessons learned from hysteresis

FIGURE 4.1 (A) (R)evolution of reported (X) and certified (O) power conversion efficiencies of perovskite devices of different device types, contrasted with the number of publications with the fraction including the keyword “hysteresis” marked in orange (Search performed on 28th December 2018 on SCOPUS with key words “perovskite” OR “CH3NH3PbI3” AND “solar cells” in the title, abstract or keyword list and “halide” anywhere in the text as well as “hysteresis” anywhere in the text). Since 2015 (indicated in yellow) measures to verify reported power conversion efficiency metrics have been in place. (B) Example of severe discrepancy between forward and reverse scan direction similar to the data shown in reference [9] in current-voltage measurements of PSCs at difference scan rates, often referred to as “hysteresis”, reproduced from Refs. [6,9
11]. This phenomenon is strongly affected by device architecture and measurement conditions and has been speculated to have different origins. Credit: E. Unger; data shown in Figure (B) was presented at the MRS fall meeting 2013, E.T. Hoke E.L. Unger M.D. McGehee Charge Recombination and Transport in Hybrid Perovskite Solar Cells M.R.S. Fall Meet, Boston, 2013.

incident power, which is usually a calibrated reference spectrum such as the AM 1.5 G spectrum, normalized to a power density of 1000 W/m2: PCE 5



Vmpp Jmpp ðV Pmpp 5 W 1000ðmW2 Þ 1000ðm2 Þ

 mA Þ 5 VOC  JSC  FFðV  mA Þ 2

2

1000ðmW2 Þ

(4.2)

4.1.1 Hysteria around hysteresis The evolution of PSCs has been marked by debates regarding the correct assessment of device performance from J
V measurements as these are often strongly affected by scan conditions and exhibit hysteresis between measurements carried out in different scan directions. This can lead to an over- or underestimation of the power conversion efficiency [5
7]. When reporting J
V data for solar cells, experimental details are rarely specified beyond the type of the light source and potentiostat or source-meter unit used for the measurements. Various reports have emphasized the importance of specifying properties of light sources used and how the active area is defined when reporting solar cell device performance metrics [8]. As evident from Fig. 4.1B, PSCs may exhibit strong performance discrepancies as a function of voltage-sweep direction and scan rate. It is hence of great importance to define exact scan conditions, as will be discussed in further detail in the

Characterization Techniques for Perovskite Solar Cell Materials

4.1 “Hysterical” current-voltage behavior of perovskite solar cells

83

following sections. This importance of “hysteresis” in PSC research becomes obvious in keyword searches: almost half of all publications on PSCs comment on hysteresis as illustrated in Fig. 4.1A. Hysteresis in PSCs was commented on first in 2013 [12] and discussed during scientific meetings such as the fall MRS meeting that year, where the data shown in Fig. 4.1B has been presented [11]. J
V hysteresis as such is not a phenomenon unique to PSCs. Cadmium telluride (CdTe) devices have been shown to exhibit hysteresis in capacitance-voltage measurements increasing upon device degradation ascribed to mobile Cui1 ions [13]. High-performance silicon devices with high internal capacitance exhibit J
V hysteresis at fast scan rates [14]. Dye-sensitized solar cells (DSCs) exhibit characteristic capacitive effects, when charge carrier transport in the liquid electrolyte becomes a rate-determining step at fast scan-rates [15]. What is peculiar regarding the J
V hysteresis of PSCs is their slow electronic response that can occur on various different timescales from sub-seconds to hours. The J
V discrepancy can be negligible in certain scan-rate regimes making the absence of hysteresis an uncertain criterion with respect to the appropriateness of J
V measurement conditions [6]. Therefore, verification schemes for reporting device performance data have been formulated, [16,17] often comprising alternative measurements such as maximum power point tracking, [5,6] comparing short-circuit current densities with integrated external quantum efficiency [6] and/or analyzing PSCs at scan rates in both fast and slow time domains [6]. Verification of experimental power conversion efficiency data is now also sometimes required by scientific journals [18] and independent certification of new record efficiency claims have become common as illustrated in Fig. 4.1A.

4.1.2 Scan-rate dependence The controversy around hysteresis in PSCs has highlighted that measurement scan-rate, s, and voltage-sweep direction need to be defined. The scan-rate by itself is not sufficient to unambiguously specify the scan conditions, if the J-V scan is not carried out as a linear sweep. Often, J-V measurements are carried out as consecutive potential-steps, in a staircase voltammetry fashion [19], where voltage step size, ΔV, and a delay time or voltage settling time, td, prior to sampling the current for a defined amount of time, ts, are defined. In this case, the Δt span passing prior to the consecutive voltage step would be the sum of td, ts and potentially an additional waiting time, twait: s5

dV ΔV ΔV 5 5 dt Δt td 1 ts 1 twait

(4.3)

To illustrate the difference, in Fig. 4.2 we show a J-V measurement carried out as a staircase-voltammetry as a function of time elapsed [19]. The scan-rate would in this example equate to 50 mV/15 s 5 3.33 mV/s. From the enlarged current transient response for the voltage step from 0.65 to 0.6 V shown in Fig. 4.2 (right), it is evident that the current overshoots upon changing the voltage followed by an exponential decay of the current. As the td and ts could be arbitrarily defined for each step, the scan rate defined by equation

Characterization Techniques for Perovskite Solar Cell Materials

84

4. Current-voltage analysis: lessons learned from hysteresis

FIGURE 4.2 (left) Time-resolved current density-voltage measurement staircase voltammetry scan in reverse direction (from 1.1 V towards 0 V). The figure on the right shows an enlarged view of the response for the voltage step from 0.65 to 0.6 V to illustrate the overshoot of the current followed by an exponential decay. This figure illustrates the importance of defining the delay and sampling time when JV-measurements are carried out as staircase rather than linear sweep voltammetry. Credit: E. Unger; data shown here previously published in reference M. Christoforo, E. Hoke, M. McGehee, E. Unger, Transient response of organo-metal-halide-metal-halide solar cells analyzed by time-resolved current-voltage measurements, Photonics 2 (2015) 1101
1115. https://doi.org/10.3390/ photonics2041101.

(4a) would not be sufficient for a comparison of measurement conditions between laboratories. For the forward scan of the data shown in Fig. 4.2, the current-response would be the opposite with a negative overshoot followed by an exponential increase. If one were to carry out a J
V measurement with very short delay times, the current would be overestimated during the reverse scan and underestimated for the forward scan due to the transient capacitive response of the device giving rise to the discrepancy between forward and reverse scans—hysteresis—as discussed in detail also by Chen et al. [20] For J
V measurements to be performed under steady-state conditions, the delay time in each voltage step has to be long enough for the transient response to stabilize. A suitable delay time, td, can therefore be estimated by characterizing the transient response of a device and choosing the delay time to be approximately 3 times the longest transient time constant τ long: td . 3 τ long. Not only does the analysis of transients in the J
V response enable the rationalization of the J
V discrepancy between forward and reverse scans and the estimation of a suitable minimum td to carry out J
V measurements under steady-state conditions, but the amplitude and characteristic transient time constants themselves can be used to gain insight into the dynamic device response [19
23]. A more detailed description of photocurrent/photo-voltage transient techniques can be found in Chapter 7 of this book. The time-dependent current density, J(t), can be expressed as the sum of transient components, Jtrans (t), and the steady-state current sustained by the device, JSS:
 t2t n X 2 τ0 i J ðtÞ 5 JSS 1 Jtrans ðtÞ 5 JSS 1 (4.4) Ai e i50

Characterization Techniques for Perovskite Solar Cell Materials

4.1 “Hysterical” current-voltage behavior of perovskite solar cells

85

where Ai are the amplitudes and τ i the time constants of the transient current response. Chen et al. [20] used the depiction and analysis of current-transient during a step-wise change in the applied potential to illustrate how the voltage step-size and total duration of a voltage step influence the result and cause hysteresis. This analysis can also be carried out for every voltage step during a staircase voltammetry scan, from which the transient time constants, τ i, and amplitudes, Ai, can be evaluated as a function of applied potential [19,21]. From the analysis of the transient current response, a minimum suitable td can be estimated in order to perform J
V measurements on a device in quasi steady-state conditions.

4.1.3 Quantification of hysteresis: hysteresis indices In the attempt to quantify the J
V discrepancy, hysteresis indices HI have been introduced. Several definitions have been proposed and used in the literature based on the difference between the reverse, Jrev, and forward, Jfwd, current at a defined voltage [22,23] (Eq. 4.5a and b), the difference between integrated J
V curves (Eq. 4.6) [24] or as the difference between device performance descriptors of fill factors, FF, or power conversion efficiencies, PCE, [25] (Eq. 4.7a and b): HI 0:5Voc 5

Jrev ð0:5VOC Þ 2 Jfwd ð0:5VOC Þ Jrev ð0:5VOC Þ

Jrev ð0:8VOC Þ 2 Jfwd ð0:8VOC Þ Jrev ð0:8VOC Þ ÐV Ð VOC Jrev ðV ÞdV 2 0 OC Jfwd ðVÞdV 0 HI int 5 Ð VOC Jrev ðV ÞdV 0 HI 0:8Voc 5

(4.5b) (4.6)

FFrev 2 FFfwd FFrev

(4.7a)

PCErev 2 PCEfwd PCErev

(4.7b)

HI FF 5 HI PCE 5

(4.5a)

Fig. 4.3 shows J
V curves from reference [6] with HIs calculated using Eqs. (4.6) and (4.7a and b) shown in Fig. 4.3B) for measurments at different delay times. The top axis indicates the corresponding scan rate, calculated using Eq. (4.3) for voltage step sizes of 50 mV. This example highlights that the discrepancy between J
V sweeps in forward and reverse direction may appear “hysteresis-free” both for very short delay times (here 1 ms) and long delay times (5 s) while the absolute device performance may differ greatly. In log-scale, the HI adopts a maximum value for a delay time of 54 ms, meaning that at a scan-rate of approximately 1.5 V/s the J
V discrepancy becomes the largest. For slower scan-rates, hysteresis but also performance decreases whereas higher performance is obtained at fast scan-rates and negligible hysteresis. The peculiarity of an increasing J
V hysteresis with decreasing scan-rate has been coined “anomalous” hysteresis [5]. This is, however, the result of a limit in the capacitive device response at faster scan rates, as discussed in Section 4.2.1.

Characterization Techniques for Perovskite Solar Cell Materials

86

4. Current-voltage analysis: lessons learned from hysteresis

FIGURE 4.3 (A) J
V scans at 50 mV steps, indicated by symbols, in reverse ◄ and forward ¤ direction with different delay times td for a perovskite device exhibiting pronounced hysteresis for intermediate td but none for very short or long td. The data shown here is data published in Ref. [6], re-analyzed to quantify the hysteresis in terms of HI shown in the plot in (B) Comparison of the device performance (right y-axis, red color scale) as a function of delay time td resulting in hysteresis for intermediate td compared here using three different formulas (HIintegral: equation x, HI0.5Voc: equation z, and HI0.8Voc: equation y) as a function of the logarithm of the delay time td. The Gaussian fit to the values of HI0.5Voc is shown in the back to illustrate that this particular device is expected to exhibit maximum hysteresis at td of 54 ms, corresponding to the frequency domain of 18.5 Hz. Credit: Replotted data also shown in reference E.L. Unger, E.T. Hoke, C.D. Bailie, W.H. Nguyen, A.R. Bowring, T. Heumu¨ller, et al., Hysteresis and transient behavior in current-voltage measurements of hybrid-perovskite absorber solar cells Energy Environ. Sci. 7 (2014) 3690
3698, https://doi.org/10.1039/c4ee02465f. Published originally by the Royal Society of Chemistry.

The important take-home message is that just as J
V measurements carried out at a single scan rate do not suffice and “negligible hysteresis” (a low HI) is an insufficient criteria to verify that measurement conditions are appropriately chosen [25]. Conversely, the investigation of J
V response and HI as a function of the delay time (scan rate) can give valuable insight into the dynamic behavior of a device and should hence be characterized in detail to unravel the underlying mechanisms causing hysteresis. Because of the difficulty to validate the steady-state PCEs of PSCs from J
V measurements, commenting on hysteresis and providing J
V data at different scan rates is often required by reviewers and journals when reporting J
V measurement data for PSCs [18].

4.1.4 Pre-conditioning & poling Apart from J
V discrepancies on the measurement time scale, the shape of J
V curves and hysteresis may strongly depend on the conditions the device is subjected to prior to the measurement and on the starting voltage of the J
V scan [6,7,26]. Unger et al. [6] showed that devices exhibiting s-shaped J
V curves after storage in the dark could be driven into more diode-like J
V curves upon light soaking under forward bias. Conversely, devices could be driven towards an s-shape by light soaking under reverse

Characterization Techniques for Perovskite Solar Cell Materials

4.2 Origin of hysteresis

87

bias, indicative of changes in charge carrier extraction barriers at interfacial layers hypothesized to originate from ion redistribution, which has been corroborated by other reports [27]. This implies that the internal conditions of the device change upon storage, lightsoaking and pre-biasing leading to an additional time-dependence of the measured performance occurring at a much slower time-scale than the scan-rate dependent phenomena described in the previous section. Poling effects can indeed be so pronounced that the polarity of PSC devices reverse their selectivity as demonstrated by Xiao et al. [28].

4.2 Origin of hysteresis Early phenomenological reports on hysteresis hypothesized that intrinsic material properties of metal-halide perovskites could cause hysteresis, namely: ferroelectricity [5,12,29], charge carrier trapping/de-trapping [5,6] and ion migration [5
7]. These hypotheses stimulated experimental and theoretical work investigating the fundamental material properties that might give rise to hysteresis in PSCs summarized in recent reviews [30,31].

4.2.1 Capacitive and non-capacitive origin of hysteresis The J
V discrepancy for different scan directions and rates observed in PSCs exhibits trade-marks of capacitive behavior in that the amplitude is scan rate dependent. There have hence been attempts to ascribe discrepancies in J
V curves to capacitive charging or discharging currents, JC, [32] which are a function of the capacitance, C, and voltage step size per time interval according to: JC 5 C

dV ΔV 5C dt Δt

(4.8)

Capacitive charging/discharging currents should be included as additional, scan-rate dependent, terms in Eq. (4.2) and experimental hysteretic J
V curves can be modeled using an equivalent circuit that contains a double diode and double capacitor with extremely large capacitances [33]. Capacitive currents in the nA/cm2 range have been identified for devices operated around 0 V in the dark and measured under intermediate voltage scan rates. There are, however, also non-capacitive effects that contribute to the J
V hysteresis. Capacitive currents due to charge carrier storage alone are too small to explain the sometimes dramatic hysteretic effects at slow scan rates (low frequencies) [34]. Capacitive effects can be distinguished from non-capacitive effects in J
V discrepancy via their frequency dependence [35,36]. As apparent in Fig. 4.3, hysteresis can be found to be negligible also at high scan rates in case the measurement is carried out beyond the device response limit. Non-capacitive effects causing hysteresis are e.g. changes in the charge carrier collection efficiency and recombination currents of electrons and holes due to changes in the built-in potential because of ion re-equilibration. [7,37] The strong coupling between ion redistribution and dynamics of electrons and holes makes the interpretation of transient phenomena in PSCs non-trivial, as a variety of possible mechanisms can give rise to a capacitive

Characterization Techniques for Perovskite Solar Cell Materials

88

4. Current-voltage analysis: lessons learned from hysteresis

response ranging from the dielectric response of the semiconductor, chemical capacitive effects, depletion or accumulation layers and electrode polarization [32] to be discussed in subsequent sections.

4.2.2 The dielectric response of metal-halide perovskites Time- and frequency-resolved techniques are instrumental to unravel characteristic time-scales of transient phenomena causing hysteresis in metal-halide perovskite devices [32]. To distinguish potentially different causes of the transient response of PSCs categorized by the time and frequency domain they are expected to be observed, we summarized literature data [3,38,39] on the frequency dependence of the dielectric constant in Fig. 4.4. Already in 1987, Poglitsch and Weber [41] attributed the frequency and temperature dependence of the complex permittivity of methylammonium lead iodide (MAPbI3) to dynamic disorder of the methylammonium (MA) cation in the material hypothesized to screen free charge carriers and cause ferroelectric domains [42]. MAPbI3 perovskites have been reported to exhibit ferroelectric domains upon poling by piezo-response force microscopy [43,44]. Molecular rotations of the MA cation have been experimentally determined to occur within 3 ps through quasi-elastic neutron scattering (QENS) measurements by Leguy et al. [40] and pump-probe vibrational spectroscopy by Bakulin et al. [45], indicated in Fig. 4.4. Differences in the photo-induced absorption features for metal-halide

FIGURE 4.4 Dielectric constant of MAPbI3 across a wide frequency spectrum reproduced by digitalization of data originally published in references [3,38,39] and data points cited therein. Marked sections highlight domains of different material response contributing to dielectric properties: ionic motion & electrode polarization (purple shading), dipole relaxation (blue shading), lattice vibrations (green shading) and internal vibrations of e.g. methylammonium molecules (yellow shading). Credit: Adapted from digitalized data and replots from references [3,31,38
40].

Characterization Techniques for Perovskite Solar Cell Materials

4.2 Origin of hysteresis

89

perovskites have been rationalized with differences in the dielectric relaxation of the cations [46]. Due to the fast time scale of the response, ferroelectricity due to cation reorientation is unlikely to be the dominant mechanism behind current-voltage hysteresis as these transients with transients observed on time-scales of seconds [40]. In our discussion we will herein predominantly focus on ion migration and the associated variations in electric field distribution and charge carrier traps PSCs, as they are likely to be the main cause of current-voltage hysteresis observed on time scales of seconds to minutes [47].

4.2.3 Ionic defect formation & migration Prior to be considered for solar energy conversion, metal-halides and metal-halide perovskites were investigated as ionic conductors [48,49]. There is an increasing amount of experimental evidence that hysteresis is dominantly related to ionic defects [15,42]. Transients observed in current-voltage measurements are however not directly caused by an ionic current but rather due to changes in interfacial charge carrier extraction efficiencies, [50] electric field distribution within the device, trapping/de-trapping of electronic charge carriers into ionic defects and non-radiative recombination rates [51]. The low-frequency response of the dielectric constant of MAPbI3, shown in Fig. 4.4, exhibits a dramatic increase to about 1000, which becomes even more pronounced upon illumination of the sample as electronic charge carriers are being generated [39]. The magnitude of this low-frequency component and effect of light has been shown to strongly depend on device architecture, as the interface between charge selective contact and perovskite will affect ion accumulation by the ion transmissivity as well as the surface termination may critically affect the defect density at this interface. There are ongoing debates in the literature as to which ionic species would be dominantly causing the transient photocurrent response. The diffusion of both halide (vacancies) [52,53] as well as MA cations [54] have been experimentally verified to occur through measurements on MAPbI3 pellets and the diffusion coefficient, D, was determined to be on the order of 1028
1027 cm2/s [55]. The ion conductivity, σion, of a material depends on the number of defects, ND, as well as the ion mobility μion, with the latter depending on the activation energy, EA, and D for solid state diffusion of that species. ND is primarily affected by the defect formation energy, which has been calculated to be only 0.14 eV in Ref. [56]. Defect densities on the order of 1017
1020 cm23 are predicted for perovskite semiconductors [57] suggesting a constant presence of halide vacancies and interstitials. In addition, halide vacancy/interstitial pairs can form upon photolysis reactions implicating that ND might be increasing under illumination [58,59]. In addition, bonds in the MA cation could be broken by photolysis [60] generating protons that have been hypothesized to be diffusing charged species causing hysteresis in PSCs [61,62]. With an EA on the order of 0.2
0.8 eV and D of 10212 cm2/s, halide vacancy migration is the most likely. Cation diffusion (e.g. MA) is hypothesized to occur at a higher activation energy predicted to exhibit a slower response but cation mobilities of about 1029 cm2/Vs have been reported that allow them to migrate through a 300 nm perovskite absorber layer in seconds to minutes [63]. Cation diffusion has been attributed to slow components in the transient behavior of devices causing reversible performance losses in day/night cycling experiments [47].

Characterization Techniques for Perovskite Solar Cell Materials

90

4. Current-voltage analysis: lessons learned from hysteresis

Ionic defect diffusion in PSCs is hence likely because they thermodynamically are prone to exist or form and the EA for their displacement is relatively low. However as discussed in Chapter 10, most vacancies and interstitials are predicted to cause shallow defect states that should not dramatically increase the non-radiative recombination rate [57,64]. However, they would affect charge carrier transport through trapping-de-trapping and accumulation of ions or ion vacancies at selective contact layers likely causing changes in the charge carrier extraction efficiency [7,54] as discussed in the next section.

4.2.4 Modeling hysteresis The current understanding of the main cause of hysteresis in metal-halide perovskites is thus the following: [7,31,65] Ionic charge carriers redistribute through drift and diffusion throughout the metal-halide perovskite layer e.g. upon application of an external field, and accumulate at the interfaces to contacts screening the electric field in the perovskite layer. A phenomenological picture of how ion redistribution affects charge carrier extraction is illustrated in Fig. 4.5. For a forward scan, negative ions would be preferentially accumulated at the electron selective contact, impeding charge carrier extraction leading to a lower extracted charge carrier density. For a reverse scan and after biasing at more positive voltages, positive ions would accumulate at the electron selective contact, promoting charge carrier extraction and hence leading to a larger net extracted charge carrier density. Different approaches have been applied to model hysteresis ranging from equivalentcircuit models [66] to numerical device simulation [67
70]. The former are mainly applied to describe the frequency-dependent impedance response, and furthermore to model the whole J
V curve [33]. The latter are capable of providing more insights because the J
V

FIGURE 4.5 Simplified band diagram of perovskite solar cell under illumination (quasi Fermi levels dashed, e2 5 electrons, h1 5 holes, CB 5 conduction band edge, VB 5 valence band edge) to illustrate changes in built-in potential and band-bending upon ion accumulation at interfaces that impede charge carrier extraction at selective contacts. Credit: Reprinted with permission from reference W. Tress, Metal halide perovskites as mixed electronic-ionic conductors: challenges and opportunities - from hysteresis to memristivity, J. Phys. Chem. Lett. 8 (2017) 3106
3114, https://doi.org/10.1021/acs.jpclett.7b00975. Copyright 2017 American Chemical Society.

Characterization Techniques for Perovskite Solar Cell Materials

4.3 A window into device operation

91

curve is modeled taking into account several microscopic processes. These are described by rate equations (e.g. for recombination), continuity equation, expressions for drift and diffusion currents, and the role of space charge using Poisson’s equation. This so-called drift-diffusion simulation is a very common approach in modeling semiconductor devices, where electrons and holes are considered mobile, whereas ionized dopant atoms give rise to a localized space charge. To simulate hysteresis, these models have recently been extended to include mobile ionic species that are modeled analogously to other mobile charge carriers but with much lower diffusion constant/mobility [67
69,71,72]. The challenge of such models is the huge amount of input parameters required, many amongst them unknown. Therefore, many parameter configurations are likely to yield similarly good agreement with experiment, especially when performing semi-quantitative comparisons. That is why the different simulation studies reach various claims on conditions that need to be fulfilled to observe hysteresis. e.g. van Reenen et al. [69] claim that hysteresis can only be reproduced assuming mobile ions and a high density of interface traps promoting recombination, a conclusion that is mainly based on the assumption of a very low recombination rate in the bulk of the perovskite. Richardson et al. [51] in contrast, reproduce hysteresis just assuming mobile ionic charges without commenting on interfacial recombination. They are assuming Shockley
Read
Hall recombination with lifetimes in the order of , 1 ns. The ion diffusion coefficient (10212 cm2/s) and the shape and ratedependence of the modeled J
V hysteresis fit well to experimentally determined data. Calado et al. [67] extend their model to simulate transient photocurrent and
voltage responses as well, concluding that interface recombination is required to reproduce hysteresis. They find that mobile ionic charge alone does not necessarily lead to hysteresis, which is not surprising as the discussed case is a J
V curve not limited by charge collection, but recombination only. Neukom et al. [72] clarify what finally matters: A charge collection efficiency that is strongly dependent on the electric field. They express this by a low diffusion length or enhanced surface recombination that lead to a more pronounced hysteresis. These are parameters that govern the efficiency of a (non-hysteretic) solar cell in general. Therefore, they conclude that a low hysteresis is correlated with a high performance, a common observation, which is, however, not to be generalized. A recently published surface polarization model tries to simulate the overshoot in current for a reverse scan (see e.g. Fig. 4.3A) by a release of electronic charge that has been piled up under forward bias. However, this model fails to explain the experiment as it explains the effect with extra non-photogenerated current, allowing for current higher than the maximum photocurrent. This, however, has not been experimentally observed so far. In summary, the simulation studies have reached a level where a semi-quantitative comparison to experimental data is possible, and can therefore serve as a starting point for more quantitative modeling as electronic parameters such as dielectric constant, mobilities and ion densities are characterized more and more thoroughly [68].

4.3 A window into device operation As discussed in previous sections, hysteresis of PSCs varies with measurement conditions and could be caused by different origins. In this section we highlight examples of

Characterization Techniques for Perovskite Solar Cell Materials

92

4. Current-voltage analysis: lessons learned from hysteresis

characteristic differences in the magnitude of hysteresis and the transient devices response as a function of device architecture, perovskite composition and device age.

4.3.1 Device architecture & selective contact layers Device architecture [5,6,73] but particularly also the choice of electron and hole selective contacts [5,74] strongly influence the magnitude of hysteresis observed in PSCs. As discussed in Section 4.2.3, this is interpreted as ion accumulation or defects at the perovskite and charge selective layers critically determining the interfacial charge carrier extraction and recombination dynamics as discussed [67,75]. With respect to device architecture, early work by Kim et al. [73] showed that J
V discrepancy is typically more pronounced in n-i-p devices compared to p-i-n devices. The terminology refers to PSCs device stacks, illustrated in Fig. 4.6B, with “n” and “p” referring to n-type and p-type selective contacts while the perovskite semiconductor is assumed to be intrinsic, indicated by “i”. In Fig. 4.6, we show results for PCEs and HIs calculated according to Eq. 4.5b for forward and reverse scan directions and different scan rates for different architecture types. Again, the highest discrepancy in J
V curves is observed for the n-i-p device with titanium dioxide and spiroMeOTAD selective contacts (TiO2/MAPbI3/spiroMeOTAD). The HI0.8Voc becomes maximal at a td of 200 ms, which indicates limitations in the device response rate, discussed previously in relation to Fig. 4.3. Hysteresis is substantially reduced by replacing

FIGURE 4.6 (A) Effect of device architecture on hysteresis index (HI0.8Voc) and power conversion efficiency (PCE) with respect to the voltage settling time (delay time, td) for n-i-p and p-i-n devices with different contact layers indicated in the figure. Gray, blue and light blue represent td of 100 ms, 200 ms and 500 ms, respectively. Voltage sweep direction for PCE measurement is represented with symbols, ◄ (reverse scan) and ¤ (forward scan). The devices were prepared with planar structure without any mesoporous film. (B and C) illustrate the different device layer stacks for n-i-p and p-i-n devices. Credit: (A) Original data from Hui-Seon Kim; (B) sketches by E. Unger.

Characterization Techniques for Perovskite Solar Cell Materials

4.3 A window into device operation

93

titania (TiO2) with phenyl-C61-butyric-methyl-ester (PCBM) or utilization of a TiO2/ PCBM double layer, as reported elsewhere [76,77]. Fig. 4.6 also includes inverted p-i-n devices, first introduced by Docampo et al. [78], comprising both poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) and nickel oxide (NiOx) as hole-selective contacts. Both show negligible hysteresis as often found for p-i-n devices [79,80]. However, hysteresis may become apparent at lower temperature and negligible hysteresis does not imply the absence of mobile ionic species [67,81]. The results shown in Fig. 4.6, indicate that interfaces formed between the perovskite and charge selective contacts critically determine the magnitude of hysteresis. The nature of the physical and chemical interaction at the interface between perovskite absorber and n- or p-selective contact layer influences the layer adhesion as well as interfacial defects formed. Patel et al. [76] demonstrated for thermally evaporated MAPbI3 layers on TiO2 and PCBM that the electron selective contact layer critically affects the crystallinity of the perovskite layer grown on top, with more crystalline layers grown on PCBM exhibiting substantially lower hysteresis interpreted to be due to fewer lattice imperfections at the interface. As discussed in Section 4.2.4, simulation data indicate that the interfaces to the charge-transport layers play a crucial role on whether hysteresis can be observed. At first glance, this seems to be inconsistent with the ion migration hypothesis. However, energy barriers at the interfaces, changes in the conductivity of the charge transport layers, and changes in the distribution of ionic species acting as trapping sites lead to different shielding and carrier capture effects. Therefore, displaced ions might affect the charge carrier collection efficiency differently. Another important factor is the permeability of a selective contact layer towards certain ionic species. Devices based on organic PCBM [27] and inorganic CuI [82] or CuSCN [74] contacts exhibit negligible J
V hysteresis presumably because ions do not accumulate at these contacts. By which exact mechanism the interface between charge selective contact layer and perovskite absorber causes or suppresses hysteresis needs to be investigated in detail for every device.

4.3.2 Light and temperature dependence Fig. 4.4 illustrates that under illumination, the low-frequency real part of the permittivity is found to increase with intensity. [39] This phenomenon has been observed irrespective of device architecture but exhibits characteristic differences as a function of device type and selective contact layers [73]. Levine et al. [75] investigated the light- and temperature dependence of hysteresis, quantified by the hysteresis index, HIint, according Eq. (4.6), shown as contour plots in Fig. 4.7 for both n-i-p (a-c) and p-i-n (D-f) type devices measured at different scan rates of 50 mV/s (A, D), 300 mV/s (B, E) and 1500 mV/s (C, F). The plots show that hysteresis increases with increasing scan rate, increases with decreasing temperature and decreases with increasing light intensity. The effect of scan-rate and temperature is more complex and exhibits a more pronounced temperature dependence for n-i-p devices compared to p-i-n devices. This indicates that the EA of the underlying mechanism causing hysteresis is very different for the different device types and that the interfaces

Characterization Techniques for Perovskite Solar Cell Materials

94

4. Current-voltage analysis: lessons learned from hysteresis

FIGURE 4.7 Hysteresis index (factor) according to Eq. 4.3 for n-i-p (A
C) and p-i-n (D
F) devices as a function of scan rates (50 mV/s, 300 mV/s, 1500 mV/s), light intensity and temperature. Credit: Reprinted with permission from reference I. Levine, P.K. Nayak, J.T.W. Wang, N. Sakai, S. Van Reenen, T.M. Brenner, et al., Interfacedependent ion migration/accumulation controls hysteresis in MAPbI3 solar cells, J. Phys. Chem. C. 120 (2016) 16399
16411, https://doi.org/10.1021/acs.jpcc.6b04233. Copyright 2016 American Chemical Society.

between charge selective contacts and perovskites critically determine the light and temperature dependence of hysteresis. Pazoki et al. [83] investigated the photon energy dependence of hysteresis demonstrating that blue light excitation gives rise to a much more pronounced J
V hysteresis compared to red light excitation, which they attribute to thermalization assisted ion migration or vacancy generation. Previously, Tress et al. [84] reported distinct differences in inverted hysteresis of mixed perovskite absorber devices between blue and red illumination that was attributed to more charge carriers generated in the recombination region close to the titanium dioxide electron selective contact. Light intensity but also spectral variations may hence strongly affect transient processes in PSCs. Illumination conditions need therefore to be defined. As indicated in the previous section and Fig. 4.7D
F, hysteresis in inverted p-i-n devices might be negligible at room temperature but become pronounced at lower temperature. [85] This is also the case for nip devices (Fig. 4.7A
C) measured at slow scan rates whereas the opposite is observed for a scan rate of 1500 mV/s, indicating that n-i-p devices exhibit a more intricate behavior shown in Fig. 4.7A
C.

Characterization Techniques for Perovskite Solar Cell Materials

4.3 A window into device operation

95

From the temperature dependence of the transient device response assessed by capacitance-frequency measurements [86,87] or chrono-amperometry [52,85,88], an activation energy can be estimated using the Arrhenius relation. By comparison with theoretical predictions of activation energies for the migration of a certain vacancy, discussed in detail in Chapter 10, transient effects have sometime been attributed to the migration of a particular ion or ion vacancy with often ambiguous results [57]. In conclusion, careful studies of transient phenomena as a function of illumination conditions as well as temperature are needed to further unravel the potentially different origins of transient phenomena in metal-halide perovskites. This will require reconciling experimental and theoretical work to identify the role of a specific ionic species in J
V hysteresis.

4.3.3 Perovskite layer morphology and composition Morphology and composition of the perovskite absorber layer have been found to affect device hysteresis. Large, monolithic grained thin films with high coverage are on average found to exhibit lower hysteresis compared to small-grained films with low coverage [22,89,90]. While grain boundaries seem to be less detrimental in metal-halide perovskites compared to other semiconductors, they are far from benign as they can cause charge carrier recombination [91] and impede charge carrier and ionic transport. These results indicate that grain boundaries in the direction of charge carrier transport are detrimental and give rise to more pronounced hysteresis, confirmed by a faster device response for larger monolithic grains demonstrated by intensity modulated photocurrent spectroscopy (IMPS) measurements [90]. On the other hand, vacuum deposited perovskite solar cells can show very low hysteresis despite small grains [92]. Stoichiometric variations within the perovskite absorber layer may cause self-doping effects that lead to changes in the internal field distribution within the perovskite layer [28]. In this respect, excess lead iodide (PbI2), which can either be formed by thermal decomposition [93] or introduced as excess during synthesis [94] has been demonstrated to be beneficial for devices performance. Jacobsson et al. [95] investigated the effect of precursor stoichiometry on performance and hysteresis in regular n-i-p devices. They found that the integral hysteresis HIint but also device performance was lower for stoichiometric or slightly PbI2 deficient samples due to a reduced photocurrent. This was attributed to an inhibition of charge carrier transport across interfaces with excess organic cations. These findings are in agreement with recent reports claiming that larger organic cation additives reduce hysteresis [96]. Lee et al. [97] found a decreasing HIint and increasing device efficiency and photoluminescence yield with increasing PbI2 content generated upon prolonged thermal annealing of the samples at 150  C. In this case, a thin capping layer of PbI2 is formed at grain boundaries, [93] which is interpreted to lead to the passivation of interfacial defects and may hence reduce J
V hysteresis. Compensation of halide vacancies by introducing I2 in the synthesis, was shown to reduce hysteresis and trap states by Yang et al. [98]. These experimental findings indicate that stoichiometry but probably more importantly the distribution of cations, PbI2, and ionic defects within the sample affects ion migration.

Characterization Techniques for Perovskite Solar Cell Materials

96

4. Current-voltage analysis: lessons learned from hysteresis

Recent years have demonstrated compositional engineering to be a fruitful strategy in device optimization, summarized in Chapter 1 [24,99,100]. Jacobsson et al. [24] compared 49 different compositions in a matrix of cation/anion alloys of methylammonium/formamidinium (MAyFA1-y)-lead(Pb)-bromide/iodide(BrxI1-x)3 allowing to establish the relation between device performance metrics (VOC, FF and PCE) and hysteresis (HIint) as a function of perovskite composition (absorption onset) shown in Fig. 4.8 [24]. Regarding the halide composition, there are generalizable trends in these datasets that can be divided into three different regimes. In the regime indicated as “stable” (blue), the average fill factor, FFav, open circuit voltage, VOC,av, and power conversion efficiency, PCEav (values in reverse and forward scan conditions indicated as error bars) exhibit little deviation between scan directions and increase for all alloys up until an absorption onset of ca. 1.7 eV, with a maximum performance of 20% reached for (MA0.33FA0.67)Pb(Br0.17I0.83)3. In this regime of steadily increasing performance, hysteresis is found to be low. With the onset of the photo-induced phase segregation [101] due to halide ion migration (marked red) also hysteresis is found FIGURE 4.8 Comparison of FF, VOC, average PCE and hysteresis index (HI) calculated according to Eq. (4.6) HIint as a function of absorption onset plotted from data provided in [24]. Devices were n-i-p with meso-porous TiO2 and Li-doped spiroMeOTAD as selective contacts. J
V measurements were carried out at a scan rate of 20 mV/s [24]. Credit: Plotted from data sets published in reference J.T. Jacobsson, J.P. Correa Baena, M. Pazoki, M. Saliba, K. Schenk, M. Gra¨tzel, et al., An exploration of the compositional space for mixed lead halogen perovskites for high efficiency devices, SI Energy Environ. Sci. 41 (2016) 1
35, https://doi.org/10.1039/C6EE00030D.

Characterization Techniques for Perovskite Solar Cell Materials

4.3 A window into device operation

97

to increase. This data set therefore provides direct experimental evidence of hysteresis being linked to ion migration besides the argument based on time scales discussed in relation to Fig. 4.4. The data also shows MA-based devices to exhibit more pronounced hysteresis than FA-based devices potentially related to the higher mobility of MA cations as suggested by Haruyama [53]. It is, however, not straight forward to attribute performance and hysteresis discrepancies to compositional variation as changing the ion ratio in the precursors solutions also strongly affect the thin film morphology as noted by Jacobsson et al. [24]. Compositional variation effects on hysteresis should be carefully distinguished from morphological effects in samples due to different formation kinetics from mixed cation/anion precursors has to be carefully distinguished in future studies. As apparent in Fig. 4.8, for some compositions the HI becomes negative, coined “inverted hysteresis” by Tress et al. [84]. In Fig. 4.9A, J
V measurements at different scan rates for regular n-i-p devices with titanium dioxide and spiroMeOTAD electron and hole selective contacts are shown, exhibiting similar scan rate dependence of performance and hysteresis as the data shown in Fig. 4.3. In Fig. 4.9B, J
V scans for the mixed cation/anion absorber in a similar device architecture are shown. Strikingly, the forward scan yields better device performance compared to the reverse scan giving negative values for HIint as shown in Fig. 4.9D, calculated according to Eq. (4.6). The scan at the fastest scan rate of 10 V/s even exhibits a cross-over between the forward and reverse scan with both scans yielding similar power conversion efficiencies. The direct comparison of Fig. 4.9C and D shows that plotting J
V performance as well as HI as a function of scan rate provides an intuitive representation of the difference in dynamic response of the two devices under comparison. Inverted hysteresis has been interpreted to be caused by energetic extraction barriers at the interfaces to selective contacts that could be reduced upon ion accumulation featuring a higher charge carrier collection efficiency for the forward scan. [84] Potential reasons for such a barrier could be energetic misalignment, compositional variations or detrimental band bending due to accumulated ionic charges [70]. Inverted hysteresis was shown to be induced in devices by prolonged light-soaking under reverse bias conditions [102].

4.3.4 Defect engineering, passivation and external ionic species Additives to selectively passivate defects have been introduced to PSCs and are often found to reduce hysteresis. Apart from lower hysteresis observed for perovskite deposited on PCBM in p-i-n devices discussed in Fig. 4.6, the post-deposition of fullerenes was shown to lead to a reduction of defect density by two orders of magnitude [103]. The mechanism by which fullerenes reduce hysteresis has been attributed to the passivation of trap states and impediment of ion transport across grain boundaries. Yoo et al. [104] investigated the effect of different phenylalkylammonium iodide used in post-treating FA0.9Cs0.1PbI2.9Br0.1 perovskite layer after deposition and found a suppression of hysteresis and slight improvement in JSC and VOC attributed to the reduction of trap-mediated interfacial recombination due to defect passivation. Apart from molecular compounds, alkali halide salts of e.g. rubidium (Rb1) and potassium (K1) have been recently proposed to act as defect passivation. Unlike the larger

Characterization Techniques for Perovskite Solar Cell Materials

98

4. Current-voltage analysis: lessons learned from hysteresis

FIGURE 4.9 (A) “Normal” hysteresis in pure MAPbI3 based perovskite solar cells compared to (B) “inverted” hysteresis evolving in mixed cation/anion lead halide perovskite with slight cation deficiency (MA0.2FA1) Pb1.1(Br0.22I1.1)3. (C) Plot of HIint and power conversion efficiency as a function of scan rate for the data shown in (A) and plot of HIint as function of Scan rate for data shown in (B) to illustrate differences between “normal” and “inverted” hysteresis. Credit: Reprinted with permission from reference W. Tress, J.P. Correa Baena, M. Saliba, A. Abate, M. Graetzel, Inverted current
voltage hysteresis in mixed perovskite solar cells: polarization, energy barriers, and defect recombination, Adv. Energy Mater. 6 (2016) 1600396, https://doi.org/10.1002/aenm.201600396. Copyright Wiley.

cesium cation, incorporation of these alkali cations into the “A-site” of the perovskite crystal lattice becomes less likely. Duong et al. [105] demonstrated a higher average power conversion efficiency and a slightly inverted hysteresis for regular n-i-p devices for the multi-cation FA0.75(MA0.6Cs0.4)0.25PbI2Br perovskite with 5% rubidium additive at a scan rate of 50 mV/s. The better device performance and lower hysteresis were attributed to an improved crystallinity of the perovskite absorber layer and longer photoluminescence lifetime both indicating a suppression of ionic defect migration. Alternatively, the addition of rubidium or other ionic additives might change the dynamic response of perovskite devices. Turren-Cruz et al. [106] proposed a faster ion re-equilibration upon addition of rubidium cations as frequency-resolved intensity modulated photocurrent spectroscopy

Characterization Techniques for Perovskite Solar Cell Materials

4.3 A window into device operation

99

(IMPS) measurements exhibited distinct differences in the medium to low frequency range below 100 Hz. Son et al. [107] investigated an alkaline cation additive series and found potassium (K1) to have the most positive effect in reducing hysteresis for mixed cation PSCs as shown in Fig. 4.9. The positive effect of potassium was corroborated by work of Abdi-Jaledi et al. [108] and was attributed to the compensation of halide deficiency and substitution of bromide by iodide upon addition of potassium iodide (KI). The smaller alkaline cation K1 is too small to occupy the A-site in the perovskite crystal structure but was found to occupy interstitial sites suggested from solid state NMR measurements [109]. The increased materials quality was attributed to a combined effect of halide vacancy compensation by halide excess as well as the prevention of Frenkel defects due to K1 blocking interstitial sites [107,108]. Little attention has been paid to the influence of external ionic species on hysteresis that could be introduced e.g. as an additive to the electron or hole-selective layer such as lithium, [110] shown to exhibit a more pronounced hysteresis compared to potassium in Fig. 4.10. Kim et al. [73] analyzed the dependence of J
V hysteresis on additives introduced with the spiroMeOTAD hole conductor and concluded that interfacial polarization effects at the perovskite/spiroMeOTAD interface are strongly affected by the additives tert-butyl pyridine (tBP) and lithium bis(trifluoromethanesulfonyl)imide (LiTSFI). The small Li alkali cation likely diffuses through the perovskite absorber layer and possibly intercalates or accumulates at the metal-oxide contact where it can contribute to changing the electric field distribution and interfacial charge carrier extraction dynamics. Li et al. [111] investigated hydrogen (H1), sodium (Na1) and lithium (Li1) as extrinsic ions in perovskite solar cells demonstrating all these extrinsic ions to clearly affect device performance and hysteresis of PSCs. In conclusion, the particular mechanism through which different additives affect device performance and hysteresis by either truly eliminating hysteresis through compensation for ionic defects, passivating grain boundaries or preventing cross-boundary ion migration or even by causing faster device response need to be carefully discerned and investigated in further studies.

FIGURE 4.10 (A
E) J
V curves of PSCs based on FA0.875Cs0.125Pb(Br0.125I0.875)3 including 10 μmol of LiI, NaI, KI, RbI and CsI measured at a scan rated of 130 mV/s under AM 1.5 G illumination exhibiting negligible hysteresis for KI. Credit: Reproduced with permission from reference D.-Y. Son, S.-G. Kim, J.-Y. Seo, S.-H. Lee, H. Shin, D. Lee, et al., Universal approach toward hysteresis 2 freehysteresis 2 free perovskite solar cell via defect engineering, J. Am. Chem. Soc. (2018), https://doi.org/10.1021/jacs.7b10430jacs.7b10430. Copyright 2018 American Chemical Society.

Characterization Techniques for Perovskite Solar Cell Materials

100

4. Current-voltage analysis: lessons learned from hysteresis

4.3.5 Hysteresis and stability As discussed in Section 4.2, migration and accumulation of ionic vacancies and interstitials are the most likely cause of hysteresis as electric field distribution and collection efficiency are altered. This could have negative implications for the long-term device stability. Particularly n-i-p devices exhibit light-induced performance losses that are shown to partially recover in the dark [112] indicating that changes occur in devices under long-term operation that are complex and need to be analyzed with care and in detail. Leijtens et al. [113] demonstrated bias-induced degradation of MAPbI3 to lead iodide through loss of the MA cation. They found that this process occurs via charged defects. Therefore, they highlight that device stability testing should be carried out under operating conditions of the device under light and bias. Thermal-, photo- and bias-induced degradation, further discussed in Chapter 11 may lead to an increase in local defects or mobile ionic species over the lifetime of a device that could amplify transient phenomena in metal-halide perovskite solar cells. Tress et al. [7] compared the changes in device performance and hysteresis upon aging at open-circuit conditions under constant 1 sun illumination from an LED light source for 6.5 hours. In Fig. 4.11, we show the difference in J
V discrepancy and HI0.8Voc as replots from the data discussed in reference [7]. For slow scan rates, hysteresis is reduced upon aging while it increases for fast scan rates. This example illustrates, that the dynamic response of the device considerably changes upon light soaking and single scan rate comparisons would lead to misleading conclusions. Reversible device performance decrease on the time scale of hours has been attributed to the motion of mobile ionic species [47,87] or light-induced formation of deep-level trap states [114]. However, this reversible behavior is not found in all types of perovskite solar cells and seems to depend on the employed charge transport layers, discussed in Section 4.3.1. J-V measurements being generally easily affected by transient phenomena, device Ð performance over time quantified as the integral lifetime energy yield t LEY 5 0 PCEðtÞdt has been proposed as a more valid figure of merit for PSCs [112,115]. FIGURE 4.11

Average power conversion efficiency (PCEav) with the bars indicating the performance in forward and reverse direction and hysteresis index HI0.8Voc, calculated according to Eq. 4.6 as a function of scan rate for a fresh and LEDilluminated device aged for 6.5 hours. Credit: Data replotted and analyzed from reference W. Tress, N. Marinova, T. Moehl, S.M. Zakeeruddin, M.K. Nazeeruddin, M. Gra¨tzel, J
V hysteresis, slow time component, and aging in CH3NH3PbI3 perovskite solar cells: the role of a compensated electric field, Energy Environ. Sci. 8 (2015) 995
1004, https://doi. org/10.1039/C4EE03664F.

Characterization Techniques for Perovskite Solar Cell Materials

4.4 Conclusion and outlook

101

FIGURE 4.12 Overview of ionic and electronic response of metal-halide perovskites illustrating the interplay between how changes in ion distribution affect electric field and interfacial charge carrier accumulation that give rise to hysteresis. Credit: Reprinted with permission from reference W. Tress, Metal halide perovskites as mixed electronicionic conductors: challenges and opportunities - from hysteresis to memristivity, J. Phys. Chem. Lett. 8 (2017) 3106
3114, https://doi.org/10.1021/acs.jpclett.7b00975. Copyright 2017 American Chemical Society.

Device performance should hence be assessed by performing maximum power point (MPP) tracking over a longer period of time, preferably, using maximum power point tracking algorithms [116,117].

4.4 Conclusion and outlook Device performance metrics derived from conventional J
V measurements might be obscured by transient phenomena and make the definition of the “steady state” power conversion efficiency of solar cells difficult [118]. Hysteresis indices (HIs) as means to quantify the discrepancy between J
V curves are of limited scientific significance if only measured for a limited scan-rate domain [6,25,75]. To avoid ambiguity, maximum power point (MPP) tracking data for a prolonged time should be measured and presented when reporting on the performance of PSCs. Reliable measurement protocols need to be unanimously adopted by the majority of the PSC research community to ensure comparability of device performance metrics obtained in different laboratories [6,17,18,118]. The origin of hysteresis may be related to several transient processes occurring on different time scales. Slow processes associated with the migration of ions changing electric field distribution, charge carrier extraction efficiency and recombination have been identified to play a major role in hysteresis phenomena as discussed in Section 4.2. Fig. 4.12 summarizes and visualizes the complex interplay between a device’s ionic and electronic response further discussed in reference [31]. Further research is needed to discern effects of device architecture, selective contact layers and perovskite absorber composition and morphology as well as the effect of additives and passivants in particular with respect to the long-term operational stability of PSCs.

Characterization Techniques for Perovskite Solar Cell Materials

102

4. Current-voltage analysis: lessons learned from hysteresis

References [1] N.-G. Park, M. Gra¨tzel, T. Miyasaka, Organic-Inorganic Halide Perovskite Photovoltaics, From Fundamentals to Device Architectures, 1st ed., Springer International Publishing Switzerland, 2016. Available from: https://doi.org/10.1007/978-3-319-35114-8. [2] S. Il Seok, M. Gra¨tzel, N.-G. Park, Methodologies toward highly efficient perovskite solar cells, Small 1704177 (2018) 1704177. Available from: https://doi.org/10.1002/smll.201704177. [3] M.A. Green, A. Ho-Baillie, H.J. Snaith, The emergence of perovskite solar cells, Nat. Photonics 8 (2014) 506
514. Available from: https://doi.org/10.1038/nphoton.2014.134. [4] J.-P. Correa-Baena, A. Abate, M. Saliba, W. Tress, T.J. Jacobsson, M. Gra¨tzel, et al., The rapid evolution of highly efficient perovskite solar cells, Energy Environ. Sci. 10 (2017) 710
727. [5] H.J. Snaith, A. Abate, J.M. Ball, G.E. Eperon, T. Leijtens, N.K. Noel, et al., Anomalous hysteresis in perovskite solar cells, J. Phys. Chem. Lett. 5 (2014) 1511
1515. Available from: https://doi.org/10.1021/jz500113x. [6] E.L. Unger, E.T. Hoke, C.D. Bailie, W.H. Nguyen, A.R. Bowring, T. Heumu¨ller, et al., Hysteresis and transient behavior in current-voltage measurements of hybrid-perovskite absorber solar cells, Energy Environ. Sci. 7 (2014) 3690
3698. Available from: https://doi.org/10.1039/c4ee02465f. [7] W. Tress, N. Marinova, T. Moehl, S.M. Zakeeruddin, M.K. Nazeeruddin, M. Gra¨tzel, J
V hysteresis, slow time component, and aging in CH3NH3PbI3 perovskite solar cells: the role of a compensated electric field, Energy Environ. Sci. 8 (2015) 995
1004. Available from: https://doi.org/10.1039/C4EE03664F. [8] H.J. Snaith, How should you measure your excitonic solar cells? Energy Environ. Sci. 5 (2012) 6513
6520. Available from: https://doi.org/10.1039/c2ee03429h. [9] D.A. Egger, E. Edri, D. Cahen, G. Hodes, Perovskite Solar Cells: Do We Know What We Do Not Know? J. Phys. Chem. Lett. 6 (2015) 279
282. Available from: https://doi.org/10.1021/jz502726b. [10] T.M. Brenner, D.A. Egger, L. Kronik, G. Hodes, D. Cahen, Hybrid organic - inorganic perovskites: low-cost semiconductors with intriguing charge-transport properties, Nat. Rev. Mater. 1 (2016) 15007. Available from: https://doi.org/10.1038/natrevmats.2015.7. [11] E.T. Hoke, E.L. Unger, M.D. McGehee, Charge recombination and transport in hybrid perovskite solar cells, M.R.S. Fall Meet Bost 2013 (2013). [12] A. Dualeh, T. Moehl, N. Te´treault, J. Teuscher, P. Gao, M.K. Nazeeruddin, et al., Impedance spectroscopic analysis of lead iodide perovskite-sensitized solid-state solar cells, ACS Nano 8 (2014) 362
373. Available from: https://doi.org/10.1021/nn404323g. [13] D.S. Albin, J.A. del Cueto, Correlations of capacitance-voltage hysteresis with thin-film CdTe solar cell performance during accelerated lifetime testing, 2010 IEEE Int. Reliab. Phys. Symp., IEEE (2010) 318
322. Available from: https://doi.org/10.1109/IRPS.2010.5488811. [14] M. Jankovec, M. Topiˇc, M. Herman, Optimisation of the I
V measurement scan time through dynamic modelling of solar cells, IET Renew. Power Gener. 7 (2013) 63
70. Available from: https://doi.org/10.1049/ iet-rpg.2012.0020. [15] L. Contreras, J. Idı´goras, A. Todinova, M. Salado, S. Kazim, S. Ahmad, et al., Specific cation interactions as the cause of slow dynamics and hysteresis in dye and perovskite solar cells: a small-perturbation study, Phys. Chem. Chem. Phys. 18 (2016) 31033
31042. Available from: https://doi.org/10.1039/ c6cp05851e. [16] J.A. Christians, J.S. Manser, P.V. Kamat, Best practices in perovskite solar cell efficiency measurements. Avoiding the error of making bad cells look good, J. Phys. Chem. Lett. 6 (2015) 852
857. Available from: https://doi.org/10.1021/acs.jpclett.5b00289. [17] E. Zimmermann, K.K. Wong, M. Mu¨ller, H. Hu, P. Ehrenreich, M. Kohlsta¨dt, et al., Characterization of perovskite solar cells: towards a reliable measurement protocol, APL Mater. 4 (2016). Available from: https://doi.org/10.1063/1.4960759. [18] Nature, Reporting Checklist For Solar Cell Manuscripts, 2015 (2015) 2015
2016. [19] M. Christoforo, E. Hoke, M. McGehee, E. Unger, Transient response of organoorgano-metal-halide-metalhalide solar cells analyzed by time-resolved current-voltage measurements, Photonics 2 (2015) 1101
1115. Available from: https://doi.org/10.3390/photonics2041101. [20] B. Chen, M. Yang, X. Zheng, C. Wu, W. Li, Y. Yan, et al., Impact of capacitive effect and ion migration on the hysteretic behavior of perovskite solar cells, J. Phys. Chem. Lett. 6 (2015) 4693
4700. Available from: https://doi.org/10.1021/acs.jpclett.5b02229.

Characterization Techniques for Perovskite Solar Cell Materials

References

103

[21] L.K. Ono, S.R. Raga, S. Wang, Y. Kato, Y. Qi, Temperature-dependent hysteresis effects in perovskite-based solar cells, J. Mater. Chem. A. 3 (2015) 9074
9080. Available from: https://doi.org/10.1039/c4ta04969a. [22] H.S. Kim, N.-G. Park, Parameters affecting I-V hysteresis of CH3NH3PbI3 perovskite solar cells: effects of perovskite crystal size and mesoporous TiO2 layer, J. Phys. Chem. Lett. (2014). Available from: https://doi. org/10.1021/jz501392m. 140811175830000. [23] R.S. Sanchez, V. Gonzalez-Pedro, J.W. Lee, N.G. Park, Y.S. Kang, I. Mora-Sero, et al., Slow dynamic processes in lead halide perovskite solar cells. Characteristic times and hysteresis, J. Phys. Chem. Lett. 5 (2014) 2357
2363. Available from: https://doi.org/10.1021/jz5011187. [24] J.T. Jacobsson, J.P. Correa Baena, M. Pazoki, M. Saliba, K. Schenk, M. Gra¨tzel, et al., An exploration of the compositional space for mixed lead halogen perovskites for high efficiency devices - SI, Energy Environ. Sci. 41 (2016) 1
35. Available from: https://doi.org/10.1039/C6EE00030D. [25] S.N. Habisreutinger, N.K. Noel, H.J. Snaith, Hysteresis index: a figure without merit for quantifying hysteresis in perovskite solar, ACS Energy Lett. 3 (2018) 2472
2476. Available from: https://doi.org/10.1021/ acsenergylett.8b01627. [26] J. Wei, Y. Zhao, H. Li, G. Li, J. Pan, D. Xu, et al., Hysteresis analysis based on the ferroelectric effect in hybrid perovskite solar cells, J. Phys. Chem. Lett. 5 (21) (2014) 3937
3945. Available from: https://doi.org/10.1021/jz502111u. [27] M. De Bastiani, G. Dell’Erba, M. Gandini, V. D’Innocenzo, S. Neutzner, A.R.S. Kandada, et al., Ion migration and the role of preconditioning cycles in the stabilization of the J-V characteristics of inverted hybrid perovskite solar cells, Adv. Energy Mater. 6 (2016) 1
9. Available from: https://doi.org/10.1002/aenm.201501453. [28] Z. Xiao, Y. Yuan, Y. Shao, Q. Wang, Q. Dong, C. Bi, et al., Giant switchable photovoltaic effect in organometal trihalide perovskite devices, Nat. Mater. 14 (2015) 193
198. [29] K. Butler, J.M. Frost, A. Walsh, Ferroelectric materials for solar energy conversion: photoferroics revisited, Energy Environ. Sci. (2014). Available from: https://doi.org/10.1039/C4EE03523B. [30] N.K. Elumalai, A. Uddin, Hysteresis in organic-inorganic hybrid perovskite solar cells, Sol. Energy Mater. Sol. Cells. 157 (2016) 476
509. Available from: https://doi.org/10.1016/j.solmat.2016.06.025. [31] W. Tress, Metal halide perovskites as mixed electronic-ionic conductors: challenges and opportunities - from hysteresis to memristivity, J. Phys. Chem. Lett. 8 (2017) 3106
3114. Available from: https://doi.org/10.1021/ acs.jpclett.7b00975. [32] J. Bisquert, G. Garcia-Belmonte, A. Guerrero, Impedance characteristics of hybrid organometal halide perovskite solar cells, in: N.-G. Park, M. Gra¨tzel, T. Miyasaka (Eds.), Org. Halide Perovskite Photovoltaics From Fundam. to Device Archit, Springer International Publishing Switzerland, 2016, pp. 163
199. Available from: https://doi.org/10.1007/978-3-319-35114-8_7. [33] L. Cojocaru, S. Uchida, P.V.V. Jayaweera, S. Kaneko, J. Nakazaki, T. Kubo, et al., Origin of the hysteresis in IV curves for planar structure perovskite solar cells rationalized with a surface boundary induced capacitance model, Chem. Lett. advpub (2015). Available from: https://doi.org/10.1246/cl.150933. [34] O. Almora, C. Aranda, I. Zarazua, A. Guerrero, G. Garcia-Belmonte, Noncapacitive hysteresis in perovskite solar cells at room temperature, ACS Energy Lett (2016) 209
215. Available from: https://doi.org/10.1021/ acsenergylett.6b00116. [35] G. Garcia-Belmonte, J. Bisquert, Distinction between capacitive and noncapacitive hysteretic currents in operation and degradation of perovskite solar cells, ACS Energy Lett. 1 (2016) 683
688. Available from: https:// doi.org/10.1021/acsenergylett.6b00293. [36] O. Almora, I. Zarazua, E. Mas-Marza, I. Mora-Sero, J. Bisquert, G. Garcia-Belmonte, Capacitive dark currents, hysteresis, and electrode polarization in lead halide perovskite solar cells, J. Phys. Chem. Lett. 6 (2015) 1645
1652. Available from: https://doi.org/10.1021/acs.jpclett.5b00480. [37] D.A. Jacobs, H. Shen, F. Pfeffer, J. Peng, T.P. White, F.J. Beck, et al., The Two Faces of Capacitance: New Interpretations for Electrical Impedance Measurements of Perovskite Solar Cells and Their Relation to Hysteresis, 2018. http://arxiv.org/abs/1807.00954. ˇ Svirskas, M. Sanlialp, G. Lackner, et al., Dielectric response: answer [38] I. Anusca, S. Balˇciunas, P. Gemeiner, S. ¯ to many questions in the methylammonium lead halide solar cell absorbers, Adv. Energy Mater. 7 (2017) 1
12. Available from: https://doi.org/10.1002/aenm.201700600. [39] E.J. Juarez-Perez, R.S. Sanchez, L. Badia, G. Garcia-Belmonte, Y.S. Kang, I. Mora-Sero, et al., Photoinduced giant dielectric constant in lead halide perovskite solar cells, J. Phys. Chem. Lett. 5 (2014) 2390
2394. Available from: https://doi.org/10.1021/jz5011169.

Characterization Techniques for Perovskite Solar Cell Materials

104

4. Current-voltage analysis: lessons learned from hysteresis

[40] A.M. Leguy, J.M. Frost, A.P. McMahon, V.G. Sakai, W. Kochelmann, C. Law, et al., The dynamics of methylammonium ions in hybrid organic- inorganic perovskite solar cells, Nat. Commun. 6 (2015) 7124. Available from: https://doi.org/10.1038/ncomms8780. [41] A. Poglitsch, D. Weber, Dynamic disorder in methylammoniumtrihalogenoplumbates (II) observed by millimeter-wave spectroscopy, J. Chem. Phys. 87 (1987) 6373
6378. Available from: https://doi.org/10.1063/ 1.453467. [42] J.M. Frost, K.T. Butler, A. Walsh, Molecular ferroelectric contributions to anomalous hysteresis in hybrid perovskite solar cells, APL Mater. 2 (2014) 081506. [43] H. Ro¨hn, T. Leonhard, M.J. Hoffmann, A. Colsmann, Ferroelectric domains in methylammonium lead iodide perovskite thin-films, En. Env. Sci. (2017). [44] Z. Fan, J. Xiao, K. Sun, L. Chen, Y. Hu, J. Ouyang, et al., Ferroelectricity of CH3NH3PbI3 perovskite, J. Phys. Chem. Lett. 6 (2015) 1155
1161. [45] A.A. Bakulin, O. Selig, H.J. Bakker, Y.L.A. Rezus, C. Mu¨ller, T. Glaser, et al., Real-time observation of organic cation reorientation in methylammonium lead iodide perovskites, J. Phys. Chem. Lett. 6 (2015) 3663
3669. Available from: https://doi.org/10.1021/acs.jpclett.5b01555. [46] M. Pazoki, T.J. Jacobsson, J. Kullgren, E.M.J. Johansson, A. Hagfeldt, G. Boschloo, et al., Photoinduced stark effects and mechanism of ion displacement in perovskite solar cell materials, ACS Nano. 11 (2017) 2823
2834. [47] K. Domanski, B. Roose, T. Matsui, M. Saliba, S.H. Turren-Cruz, J.P. Correa-Baena, et al., Migration of cations induces reversible performance losses over day/night cycling in perovskite solar cells, Energy Environ. Sci. 10 (2017) 604
613. Available from: https://doi.org/10.1039/c6ee03352k. [48] J. Mizusaki, K. Arai, K. Fueki, Ionic conduction of the perovskite-type halides, Solid State Ionics 11 (1983) 203
211. [49] J.F. Verwey, N.G. Westerink, Photoconductivity in lead chloride and lead bromide, Physica 42 (1969) 293
302. Available from: https://doi.org/10.1016/0031-8914(69)90021-4. [50] J.P. Correa Baena, L. Steier, W. Tress, M. Saliba, S. Neutzner, T. Matsui, et al., Highly efficient planar perovskite solar cells through band alignment engineering, Energy Environ. Sci. 8 (2015) 2928
2934. Available from: https://doi.org/10.1039/C5EE02608C. [51] S.A.L. Weber, I.M. Hermes, S.-H. Turren-Cruz, C. Gort, V.W. Bergmann, L. Gilson, et al., How the formation of interfacial charge causes hysteresis in perovskite solar cells, Energy Environ. Sci. (2018). Available from: https://doi.org/10.1039/C8EE01447G. [52] C. Eames, J.M. Frost, P.R.F. Barnes, B.C. O’Regan, A. Walsh, M.S. Islam, Ionic transport in hybrid lead iodide perovskite solar cells, Nat. Commun. 6 (2015) 2
9. Available from: https://doi.org/10.1038/ncomms8497. [53] J. Haruyama, K. Sodeyama, L. Han, Y. Tateyama, First-principles study of ion diffusion in perovskite solar cell sensitizers, J. Am. Chem. Soc. 137 (2015) 10048
10051. Available from: https://doi.org/10.1021/ jacs.5b03615. [54] J.M. Azpiroz, E. Mosconi, J. Bisquert, F. De Angelis, Defect migration in methylammonium lead iodide and its role in perovskite solar cell operation, Energy Environ. Sci. 8 (2015) 2118
2127. Available from: https:// doi.org/10.1039/c5ee01265a. [55] T.Y. Yang, G. Gregori, N. Pellet, M. Gra¨tzel, J. Maier, The significance of ion conduction in a hybrid organicinorganic lead-iodide-based perovskite photosensitizer, Angew. Chem. - Int. Ed. 54 (2015) 7905
7910. Available from: https://doi.org/10.1002/anie.201500014. [56] A. Walsh, D.O. Scanlon, S. Chen, X.G. Gong, S.H. Wei, Self-regulation mechanism for charged point defects in hybrid halide perovskites, Angew. Chem. - Int. Ed. 54 (2015) 1791
1794. Available from: https://doi.org/ 10.1002/anie.201409740. [57] T. Kirchartz, L. Kru¨ckemeier, E.L. Unger, Research update: recombination and open-circuit voltage in leadhalide perovskites, APL Mater. 6 (2018) 100702. Available from: https://doi.org/10.1063/1.5052164. [58] D. Meggiolaro, S.G. Motti, E. Mosconi, A.J. Barker, J. Ball, C. Andrea Riccardo Perini, et al., Iodine chemistry determines the defect tolerance of lead-halide perovskites, Energy Environ. Sci. 11 (2018) 702
713. Available from: https://doi.org/10.1039/c8ee00124c. [59] K.G. Stamplecoskie, J.S. Manser, P.V. Kamat, Dual nature of the excited state in organic
inorganic lead halide perovskites, Energy Environ. Sci. 8 (2015) 208
215. Available from: https://doi.org/10.1039/ C4EE02988G.

Characterization Techniques for Perovskite Solar Cell Materials

References

105

[60] N.H. Nickel, F. Lang, V.V. Brus, O. Shargaieva, J. Rappich, Unraveling the light-induced degradation mechanisms of CH3NH3PbI3 perovskite films, Adv. Electron. Mater. 3 (2017) 1700158. Available from: https://doi.org/10.1002/aelm.201700158. [61] D.A. Egger, L. Kronik, A.M. Rappe, Theory of hydrogen migration in organic-inorganic halide perovskites, Angew. Chem. - Int. Ed. 54 (2015) 12437
12441. Available from: https://doi.org/10.1002/anie.201502544. [62] D.A. Egger, A.M. Rappe, L. Kronik, Hybrid organic-inorganic perovskites on the move, Acc. Chem. Res. 49 (2016) 573
581. Available from: https://doi.org/10.1021/acs.accounts.5b00540. [63] Y. Yuan, J. Chae, Y. Shao, Q. Wang, Z. Xiao, A. Centrone, et al., Photovoltaic switching mechanism in lateral structure hybrid perovskite solar cells, Adv. Energy Mater. 5 (2015) 1
7. Available from: https://doi.org/ 10.1002/aenm.201500615. [64] W.-J. Yin, T. Shi, Y. Yan, Unusual defect physics in CH3NH3PbI3 perovskite solar cell absorber, Appl. Phys. Lett. (2014). Available from: https://doi.org/10.1063/1.4864778. [65] R.A. Belisle, W.H. Nguyen, A.R. Bowring, P. Calado, X. Li, S.J.C. Irvine, et al., Interpretation of inverted photocurrent transients in organic lead halide perovskite solar cells: proof of the field screening by mobile ions and determination of the space charge layer widths, Energy Environ. Sci. 10 (2017) 192
204. Available from: https://doi.org/10.1039/c6ee02914k. [66] A. Pockett, G.E. Eperon, N. Sakai, H.J. Snaith, L.M. Peter, P.J. Cameron, Microseconds, milliseconds and seconds: deconvoluting the dynamic behaviour of planar perovskite solar cells, Phys. Chem. Chem. Phys. 19 (2017) 5959
5970. Available from: https://doi.org/10.1039/c6cp08424a. [67] P. Calado, A.M. Telford, D. Bryant, X. Li, J. Nelson, B.C. O’Regan, et al., Evidence for ion migration in hybrid perovskite solar cells with minimal hysteresis, Nat. Commun. 7 (2016) 1
10. Available from: https://doi. org/10.1038/ncomms13831. [68] G. Richardson, S.E.J. O’Kane, R.G. Niemann, T.A. Peltola, J.M. Foster, P.J. Cameron, et al., Can slow-moving ions explain hysteresis in the current
voltage curves of perovskite solar cells? Energy Environ. Sci. 9 (2016) 1476
1485. Available from: https://doi.org/10.1039/C5EE02740C. [69] S. Van Reenen, M. Kemerink, H.J. Snaith, Modeling anomalous hysteresis in perovskite solar cells, J. Phys. Chem. Lett. 6 (2015) 3808
3814. Available from: https://doi.org/10.1021/acs.jpclett.5b01645. [70] D.A. Jacobs, Y. Wu, H. Shen, C. Barugkin, F.J. Beck, T.P. White, et al., Hysteresis phenomena in perovskite solar cells: the many and varied effects of ionic accumulation, Phys. Chem. Chem. Phys. 19 (2017) 3094
3103. Available from: https://doi.org/10.1039/C6CP06989D. ´ vila, M. Sessolo, H.J. Bolink, et al., Recombination in perovskite [71] T.S. Sherkar, C. Momblona, L. Gil-Escrig, J. A solar cells: significance of grain boundaries, interface traps, and defect ions, ACS Energy Lett. 2 (2017) 1214
1222. Available from: https://doi.org/10.1021/acsenergylett.7b00236. [72] M.T. Neukom, S. Zu¨fle, E. Knapp, M. Makha, R. Hany, B. Ruhstaller, Why perovskite solar cells with high efficiency show small IV-curve hysteresis, Sol. Energy Mater. Sol. Cells. 169 (2017) 159
166. Available from: https://doi.org/10.1016/j.solmat.2017.05.021. [73] H.-S. Kim, I.-H. Jang, N. Ahn, M. Choi, A. Guerrero, J. Bisquert, et al., Control of I
V hysteresis in CH3NH3PbI3 perovskite solar cell, J. Phys. Chem. Lett. 6 (2015) 4633
4639. Available from: https://doi.org/ 10.1021/acs.jpclett.5b02273. [74] N. Arora, M.I. Dar, A. Hinderhofer, N. Pellet, F. Schreiber, S.M. Zakeeruddin, et al., Perovskite solar cells with CuSCN hole extraction layers yield stabilized efficiencies greater than 20%, Science 80 (2017). Available from: https://doi.org/10.1126/science.aam5655. [75] I. Levine, P.K. Nayak, J.T.W. Wang, N. Sakai, S. Van Reenen, T.M. Brenner, et al., Interface-dependent ion migration/accumulation controls hysteresis in MAPbI3 solar cells, J. Phys. Chem. C. 120 (2016) 16399
16411. Available from: https://doi.org/10.1021/acs.jpcc.6b04233. [76] J.B. Patel, J. Wong-Leung, S. Van Reenen, N. Sakai, J.T.W. Wang, E.S. Parrott, et al., Influence of interface morphology on hysteresis in vapor-deposited perovskite solar cells, Adv. Electron. Mater. 3 (2017) 1
6. Available from: https://doi.org/10.1002/aelm.201600470. [77] L. Kegelmann, C.M. Wolff, C. Awino, F. Lang, E.L. Unger, L. Korte, et al., It takes two to tango - double-layer selective contacts in perovskite solar cells for improved device performance and reduced hysteresis, ACS Appl. Mater. Interfaces. 9 (2017) 17245
17255. Available from: https://doi.org/10.1021/acsami.7b00900. [78] P. Docampo, J. Ball, M. Darwich, G. Eperon, H.J. Snaith, Efficient organometallic trihalide perovskite planarheterojunction solar cells on flexible polymer substrates. Nat. Commun. 4 (2013) 2761.

Characterization Techniques for Perovskite Solar Cell Materials

106

4. Current-voltage analysis: lessons learned from hysteresis

[79] J.H. Heo, H.J. Han, D. Kim, T.K. Ahn, S.H. Im, Hysteresis-less inverted CH3NH3PbI3 planar perovskite hybrid solar cells with 18.1% power conversion efficiency, Energy Environ. Sci. 8 (2015) 1602
1608. Available from: https://doi.org/10.1039/C5EE00120J. [80] C.-G. Wu, C.-H. Chiang, Z.-L. Tseng, M.K. Nazeeruddin, A. Hagfeldt, M. Gra¨tzel, High efficiency stable inverted perovskite solar cells without current hysteresis, Energy Environ. Sci. 8 (2015) 2725
2733. [81] L. Bertoluzzi, R. Belisle, K.A. Bush, R. Cheacharoen, M.D. McGehee, B.C. O’Regan, In situ measurement of electric-field screening in hysteresis-free PTAA/FA0.83Cs0.17Pb(I0.83Br0.17)3/C60 perovskite solar cells gives an ion mobility of B3 3 10
7 cm2/(V s), 2 orders of magnitude faster than reported for metal-oxidecontacted perovskite C, J. Am. Chem. Soc. 140 (2018) 12775
12784. [82] G.A. Sepalage, S. Meyer, A. Pascoe, A.D. Scully, F. Huang, U. Bach, et al., Copper(I) iodide as hole-conductor in planar perovskite solar cells: probing the origin of J-V hysteresis, Adv. Funct. Mater. 25 (2015) 5650
5661. Available from: https://doi.org/10.1002/adfm.201502541. [83] M. Pazoki, T.J. Jacobsson, S.H.T. Cruz, M.B. Johansson, R. Imani, J. Kullgren, et al., Photon energy-dependent hysteresis effects in lead halide perovskite materials, J. Phys. Chem. C. (2017). Available from: https://doi. org/10.1021/acs.jpcc.7b06775. [84] W. Tress, J.P. Correa Baena, M. Saliba, A. Abate, M. Graetzel, Inverted current
voltage hysteresis in mixed perovskite solar cells: polarization, energy barriers, and defect recombination, Adv. Energy Mater. 6 (2016) 1600396. Available from: https://doi.org/10.1002/aenm.201600396. [85] D. Bryant, S. Wheeler, B.C. O’Regan, T. Watson, P.R.F. Barnes, D. Worsley, et al., Observable hysteresis at low temperature in “hysteresis free” organic-inorganic lead halide perovskite solar cells, J. Phys. Chem. Lett. 6 (2015) 3190
3194. Available from: https://doi.org/10.1021/acs.jpclett.5b01381. [86] M.H. Futscher, J.M. Lee, T. Wang, A. Fakharuddin, L. Schmidt-Mende, B. Ehrler, Quantification of Ion Migration in CH3NH3PbI3 Perovskite Solar Cells by Transient Capacitance Measurements, 2018. http:// arxiv.org/abs/1801.08519. [87] M. Bag, L.A. Renna, R.Y. Adhikari, S. Karak, F. Liu, P.M. Lahti, et al., Kinetics of ion transport in perovskite active layers and its implications for active layer stability, J. Am. Chem. Soc. 137 (2015) 13130
13137. Available from: https://doi.org/10.1021/jacs.5b08535. [88] H. Yu, H. Lu, F. Xie, S. Zhou, N. Zhao, Native defect-induced hysteresis behavior in organolead iodide perovskite solar cells, Adv. Funct. Mater. 26 (2016) 1411
1419. Available from: https://doi.org/10.1002/adfm.201504997. [89] S. Chen, L. Lei, S. Yang, Y. Liu, Z.-S. Wang, Characterization of perovskite obtained from two-step deposition on mesoporous titania, ACS Appl. Mater. Interf. 7 (2015) 25770
25776. [90] J.P. Correa-Baena, M. Anaya, G. Lozano, W. Tress, K. Domanski, M. Saliba, et al., Unbroken perovskite: interplay of morphology, electro-optical properties, and ionic movement, Adv. Mater. 1 (2016) 5031
5037. Available from: https://doi.org/10.1002/adma.201600624. [91] C.G. Bischak, E.M. Sanehira, J.T. Precht, J.M. Luther, N.S. Ginsberg, Heterogeneous charge carrier dynamics in organic-inorganic hybrid materials: nanoscale lateral and depth-dependent variation of recombination rates in methylammonium lead halide perovskite thin films, Nano Lett. 15 (2015) 4799
4807. Available from: https://doi.org/10.1021/acs.nanolett.5b01917. [92] C. Momblona, L. Gil-Escrig, E. Bandiello, E.M. Hutter, M. Sessolo, K. Lederer, et al., Efficient vacuum deposited p-i-n and n-i-p perovskite solar cells employing doped charge transport layers, Energy Environ. Sci. 9 (2016) 3456
3463. [93] T. Supasai, N. Rujisamphan, K. Ullrich, A. Chemseddine, T. Dittrich, Formation of a passivating CH3NH3PbI3/PbI2 interface during moderate heating of CH3NH3PbI3 layers, Appl. Phys. Lett. 103 (2013) 2
5. Available from: https://doi.org/10.1063/1.4826116. [94] D. Bi, W. Tress, M.I. Dar, P. Gao, J. Luo, C. Renevier, et al., Efficient luminescent solar cells based on tailored mixed-cation perovskites efficient luminescent solar cells based on tailored mixed-cation perovskites, Sci. Adv. 2 (2016) 1
7. Available from: https://doi.org/10.1126/sciadv.1501170. [95] T.J. Jacobsson, J.P. Correa-Baena, E. Halvani Anaraki, B. Philippe, S.D. Stranks, M.E.F. Bouduban, et al., Unreacted PbI2 as a double-edged sword for enhancing the performance of perovskite solar cells, J. Am. Chem. Soc. 138 (2016) 10331
10343. Available from: https://doi.org/10.1021/jacs.6b06320. [96] Y. Cho, A.M. Soufiani, J.S. Yun, J. Kim, D.S. Lee, J. Seidel, et al., Mixed 3D-2D passivation treatment for mixed-cation lead mixed-halide perovskite solar cells for higher efficiency and better stability, Adv. Energy Mater. 1703392 (2018) 1
10. Available from: https://doi.org/10.1002/aenm.201703392.

Characterization Techniques for Perovskite Solar Cell Materials

References

107

[97] J.W. Lee, S.G. Kim, S.H. Bae, D.K. Lee, O. Lin, Y. Yang, et al., The interplay between trap density and hysteresis in planar heterojunction perovskite solar cells, Nano Lett. 17 (2017) 4270
4276. Available from: https://doi.org/10.1021/acs.nanolett.7b01211. [98] W.S. Yang, B.-W. Park, E.H. Jung, N.J. Jeon, Iodide management in formamidinium-lead-halide
based perovskite layers for efficient solar cells, Science 356 (2017) 1376
1379. Available from: https://doi.org/ 10.1126/science.aan2301 (80-. ). [99] N.J. Jeon, J.H. Noh, W.S. Yang, Y.C. Kim, S. Ryu, J. Seo, et al., Compositional engineering of perovskite materials for high-performance solar cells, Nature. 517 (2015) 476
480. Available from: https://doi.org/ 10.1038/nature14133. [100] M. Saliba, T. Matsui, J.Y. Seo, K. Domanski, J.P. Correa-Baena, M.K. Nazeeruddin, et al., Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency, Energy Environ. Sci. 9 (2016) 1989
1997. Available from: https://doi.org/10.1039/c5ee03874j. [101] E.T. Hoke, D.J. Slotcavage, E.R. Dohner, A.R. Bowring, H.I. Karunadasa, M.D. McGehee, Reversible photoinduced trap formation in mixed-halide hybrid perovskites for photovoltaics, Chem. Sci. 6 (2015) 613
617. Available from: https://doi.org/10.1039/c4sc03141e. [102] G.A. Nemnes, C. Besleaga, V. Stancu, D.E. Dogaru, L.N. Leonat, L. Pintilie, et al., Normal and inverted hysteresis in perovskite solar cells, J. Phys. Chem. C. 121 (2017) 11207
11214. Available from: https://doi.org/ 10.1021/acs.jpcc.7b04248. [103] J. Xu, A. Buin, A.H. Ip, W. Li, O. Voznyy, R. Comin, et al., Perovskite-fullerene hybrid materials suppress hysteresis in planar diodes, Nat. Commun. 6 (2015) 1
8. Available from: https://doi.org/10.1038/ ncomms8081. [104] H.S. Yoo, N.G. Park, Post-treatment of perovskite film with phenylalkylammonium iodide for hysteresisless perovskite solar cells, Sol. Energy Mater. Sol. Cells. 179 (2018) 57
65. Available from: https://doi.org/ 10.1016/j.solmat.2018.02.015. [105] T. Duong, Y.L. Wu, H. Shen, J. Peng, X. Fu, D. Jacobs, et al., Rubidium multication perovskite with optimized bandgapbandgap for perovskite-silicon tandem with over 26% efficiency, Adv. Energy Mater. 7 (2017) 1
11. Available from: https://doi.org/10.1002/aenm.201700228. [106] S.H. Turren Cruz, M. Saliba, M.T. Mayer, H. Juarez Santiesteban, X. Mathew, L. Nienhaus, et al., Enhanced charge carrier mobility and lifetime suppress hysteresis and improve efficiency in planar perovskite solar cells, Energy Environ. Sci. 11 (2017) 78
86. Available from: https://doi.org/10.1039/C7EE02901B. [107] D.-Y. Son, S.-G. Kim, J.-Y. Seo, S.-H. Lee, H. Shin, D. Lee, et al., Universal approach toward hysteresis 2 free perovskite solar cell via defect engineering, J. Am. Chem. Soc (2018). Available from: https://doi.org/ 10.1021/jacs.7b10430. jacs.7b10430. [108] M. Abdi-Jalebi, Z. Andaji-Garmaroudi, S. Cacovich, C. Stavrakas, B. Philippe, J.M. Richter, et al., Maximizing and stabilizing luminescence from halide perovskites with potassium passivation, Nat. Publ. Gr. 555 (2018) 497
501. Available from: https://doi.org/10.1038/nature25989. [109] D.J. Kubicki, D. Prochowicz, A. Hofstetter, S.M. Zakeeruddin, M. Gra¨tzel, L. Emsley, Phase segregation in Cs-, Rb- and K-doped mixed-cation (MA)x(FA)1-xPbI3 hybrid perovskites from solid-state NMR, J. Am. Chem. Soc. 139 (2017) 14173
14180. Available from: https://doi.org/10.1021/jacs.7b07223. [110] W.H. Nguyen, C.D. Bailie, E.L. Unger, M.D. Mcgehee, Enhancing the hole-conductivity of spiro-OMeTAD without oxygen or lithium salts by using spiro(TFSI)2 in perovskite and dye- sensitized solar cells, J. Am. Chem. Soc. 136 (2014) 10996
11001. Available from: https://doi.org/10.1021/ja504539w. [111] Z. Li, C. Xiao, Y. Yang, S.P. Harvey, D.H. Kim, J.A. Christians, et al., Extrinsic ion migration in perovskite solar cells, Energy Environ. Sci. 10 (2017) 1234
1242. Available from: https://doi.org/10.1039/ C7EE00358G. [112] M.V. Khenkin, K.M. Anoop, I. Visoly-Fisher, Y. Galagan, F. Di Giacomo, B.R. Patil, et al., Reconsidering figures of merit for performance and stability of perovskite photovoltaics, Energy Environ. Sci. 11 (2018) 739
743. Available from: https://doi.org/10.1039/c7ee02956j. [113] T. Leijtens, E.T. Hoke, G. Grancini, D.J. Slotcavage, G.E. Eperon, J.M. Ball, et al., Mapping electric fieldinduced switchable poling and structural degradation in hybrid lead halide perovskite thin films, Adv. Energy Mater. 5 (2015) 1
11. Available from: https://doi.org/10.1002/aenm.201500962. [114] A. Merdasa, Y. Tian, R. Camacho, A. Dobrovolsky, E. Debroye, E.L. Unger, et al., “Supertrap” at work: extremely efficient nonradiative recombination channels in MAPbI3 perovskites revealed by luminescence

Characterization Techniques for Perovskite Solar Cell Materials

108

[115]

[116] [117] [118]

4. Current-voltage analysis: lessons learned from hysteresis

super-resolution imaging and spectroscopy, ACS Nano. 11 (2017) 5391
5404. Available from: https://doi. org/10.1021/acsnano.6b07407. M. Saliba, M. Stolterfoht, C.M. Wolff, D. Neher, Measuring aging stability of perovskite solar cells measuring aging stability of perovskite solar cells, Joule (2018). Available from: https://doi.org/10.1016/ j.joule.2018.05.005. A.J. Cimaroli, Y. Yu, C. Wang, W. Liao, L. Guan, C.R. Grice, et al., Tracking the maximum power point of hysteretic perovskite solar cells using a predictive algorithm, J. Mater. Chem. C. 5 (2017) 10152
10157. M. Bliss, A. Smith, J. Baker, F. De Rossi, T. Watson, K. Schutt, et al., I-V performance characterisation of perovskite solar cells, Photovolt. Sci. Appl. Technol (2018) 5
8. R.B. Dunbar, B.C. Duck, T. Moriarty, K.F. Anderson, N.W. Duffy, C.J. Fell, et al., How reliable are efficiency measurements of perovskite solar cells? The first inter-comparison, between two accredited and eight nonaccredited laboratories, J. Mater. Chem. A. 5 (2017) 22542
22558. Available from: https://doi.org/10.1039/ c7ta05609e.

Characterization Techniques for Perovskite Solar Cell Materials

C H A P T E R

5 Photoelectron spectroscopy investigations of halide perovskite materials used in solar cells Bertrand Philippe, Gabriel J. Man and Ha˚kan Rensmo Department of Physics and Astronomy, Molecular and Condensed Matter Physics, Uppsala University, Uppsala, Sweden

5.1 Introduction The chemical nature of the class of materials referred to as the halide perovskites (HaPs) was presented in the first chapters of this book. This class of materials includes methylammonium lead tri-iodide (CH3NH3PbI3), the prototypical organic-inorganic halide perovskite, and more complex systems comprised of multiple organic and/or inorganic cations (MA1, FA1, Cs1, Rb1, K1, etc.) and mixture of halides (I2, Br2, Cl2). The peculiarities of these materials include their wide malleability and tunability of their physical properties (e.g. electronic bandgap, thermal and atmospheric stability, etc.) and performance in optoelectronic devices with simple changes in chemical composition. The determination and knowledge of the crystal structure were of great importance for the development and improvement of perovskite materials, and X-ray diffraction (XRD) remains an accessible tool for determining the quality and structural properties of perovskite materials as discussed in Chapter 2. A deeper understanding of the atomic and electronic structure of these materials, especially at their interfaces with other materials utilized in a solar cell, is also an important requirement for further improvement of material properties such as bandgap, composition of the valence band (VB) and conduction band (CB), and energy positions of their edges in relation to the Fermi (energy) level or vacuum (energy) level. The chemistry of a perovskite material is also of importance, especially near the interfaces and chemical differences can be found when comparing the surface, the bulk, or the interface with another component of a solar cell, e.g. hole or electron transporting materials (HTM, ETM) or the metal contact. These differences or

Characterization Techniques for Perovskite Solar Cell Materials DOI: https://doi.org/10.1016/B978-0-12-814727-6.00005-0

109

Copyright © 2020 Elsevier Inc. All rights reserved.

110

5. Photoelectron spectroscopy investigations of halide perovskite materials used in solar cells

inhomogeneity within the material may then have a strong impact on the overall performance of the solar cell. The atomic and electronic structure of a material can be experimentally studied via spectroscopic techniques and this chapter primarily focuses on the use of direct photoemission spectroscopy to probe the occupied electronic states. Photoemission spectroscopy, also referred to as photoelectron spectroscopy (PES), is one of the most effective techniques for investigating the chemical nature of material surfaces and interfaces at an atomic level. The research field of perovskite solar cells has already benefitted, and can further benefit from the unique information provided by PES [1
4]. Crucial information about the electronic structure, such as variations of the valence band maximum (VBM) or the highest occupied molecular orbital (HOMO) depending on the type of materials, as well as the energy alignment between the different constituents of the device that is crucial when designing a whole solar cell device, can also be extracted. The composition of the valence structures, hybridization, work-function, as well as effects induced by the presence of dopants can also be experimentally investigated and in combination with theoretical calculations obtained, for example, using density functional theory (DFT) calculations (see Chapter 10), one can gain atomic level insights in how electronic structure may explain the functionality of optoelectronic devices [5]. The first part of this chapter contains a general description of the principles of photoelectron spectroscopy and some important concepts to understand before interpreting PES spectra. Various developments of the photoemission techniques, e.g. emergence of new opportunities with the development of synchrotron light sources, and the unique information these different techniques can provide in the context of perovskite materials, will be discussed as well. More explicit examples of perovskite material investigations via PES are given in the second part of this chapter; the three main foci are (i) chemical characterization, (ii) electronic structure, and (iii) interfacial energy level alignment.

5.2 Photoelectron spectroscopy X-ray Photoelectron Spectroscopy (XPS) or photoelectron spectroscopy (PES) was developed in the 1950s by Prof. Kai M. Siegbahn and co-workers at Uppsala University in Sweden [6]. The technique was originally called ESCA for Electron Spectroscopy for Chemical Analysis due to the chemical information it provides, and Kai M. Siegbahn was awarded the 1981 Nobel Prize in Physics “for his contribution to the development of highresolution electron spectroscopy” [7]. The basic principles of PES are explained in this section; the content is largely based on reference [8] and on a previous work by the authors [9]. Interested readers are encouraged to refer to the following references for more details on the fundamental aspects of photoelectron spectroscopy [1,2,10,11].

5.2.1 Basic principles The spectroscopy of photoelectrons is based on the exploitation of the photoelectric effect [12,13], in which an electron is ejected (with some probability) when a quantum of

Characterization Techniques for Perovskite Solar Cell Materials

111

5.2 Photoelectron spectroscopy

light (photon) with a suitable energy (hν) is absorbed by the matter. The photo-emitted free electron, also referred to as a photoelectron, carries with it a specific kinetic energy (Ek) that can be experimentally measured. The principle of energy conservation can be used to relate the binding energy to the photon energy and the kinetic energy of the photo-emitted electron as shown in Eq. 5.1. hν 1 Ei ðNÞ 5 Ef ðN 2 1Þ 1 Ek 0

0

hν 5 E b 1 Ek with E b 5 Ef ðN 2 1Þ 2 Ei ðNÞ

(5.1) (5.2)

In Eq. 5.1 hν is the excitation energy, Ei(N) is the total energy of the initial system of N electrons and Ef(N 2 1) is the resulting energy of the final system containing N 2 1 electrons. In a PES experiment, the photon energy is selected, and the kinetic energy Ek is measured. The energy difference (Ef(N 2 1) 2 Ei(N)) can be determined and is defined as the binding energy of the photoelectron (E0 b) as presented in Eq. 5.2. A photoelectron can be ejected from a material only if the energy of the incoming photon hν is larger than its binding energy (E0 b). In the above definition, the binding energy E0 b is referenced to a vacuum level Evac. While in a gaseous phase, Evac near the sample is similar to the vacuum level at infinity, Evac for solid-state sample is more complex. Evac for a solid-state sample is often defined just outside its surface and is a characteristic of that particular sample surface. Partly for this reason, as a convention, the binding energy for solid samples is often referenced to the Fermi level. In practice, the measured kinetic energy is then corrected for a work function (φ) that represents the energy separation between the vacuum level just outside the sample and the Fermi level of the sample. The Eq. 5.2 can thus be rewritten as follows: 0

hν 5 Eb 1 Ek 1 φ with Eb 5 E b 2 φ

(5.3)

Fig. 5.1 shows the essential components of a PES experiment: a radiation source, the sample, and an electron analyzer. The radiation source would ideally be monochromated. The sample is depicted as a solid in Fig. 5.1, but could also be present in a different phase, such as a gas or a liquid. In practice, PES measurements are typically performed in a (ultra-high) vacuum environment, as the mean free path of the photoelectrons needs to be sufficiently large to travel to the electron analyzer. The photoelectrons emitted from the sample are then collected, analyzed and counted through an electron energy analyzer. The final output is a PES spectrum where the photoelectron counts or intensity is plotted as function of the binding energy (Eb) or photoelectron kinetic energy (Ek). An overview/survey spectrum of a mixed composition (in terms of the A-site cation and X-site anion) lead halide perovskite (MA0.15FA0.85Pb(I0.85Br0.15)3) acquired with an incident photon energy of 2100 eV, is presented in Fig. 5.2A. The survey spectrum can typically be separated into two distinct energy regions, although the boundary between the two regions is not absolute. The higher binding energy region contains well-defined photoelectron peaks corresponding to the atomic core levels. The binding energy of these core level peaks is element specific and will also depend on the chemical environment of the element in the material making PES a technique with chemical sensitivity. The study of the core levels is often referred to as core-level spectroscopy, in contrast to the valencelevel spectroscopy where more complex electronic structures found at lower binding

Characterization Techniques for Perovskite Solar Cell Materials

112

5. Photoelectron spectroscopy investigations of halide perovskite materials used in solar cells

FIGURE 5.1 Schematic representation of the three main components of a photoelectron spectroscopy experiment: radiation source, sample, and (photo-)electron analyzer.

energy (0
20 eV, since the bandwidth of most inorganic solids ranges between 10 and 20 eV) are investigated. These structures are characteristic of the valence levels with mixed character. The valence spectrum of the mixed perovskite is shown in Fig. 5.2B while the principles of core-level and valence-level spectroscopy are illustrated in Fig. 5.2C. After a photoemission event, the final excited/ionized state of the electronic system relaxes to its ground state by either emission of an Auger electron or via X-ray fluorescence emission. These two processes are presented in Fig. 5.3. Auger electrons can also be collected by the electron analyzer and may result in extra peaks in the PES spectrum.

5.2.2 Core-level photoelectron spectroscopy The binding energy values of the core level peaks comprising a PES spectrum allow the elements present in the sample to be identified. Each element has a characteristic sequence of core levels that will have specific binding energies and thus can be easily recognized in a spectrum. If we take the example of lead, Pb 4p, 4d, 4f, 5p and 5d, core levels can be found within the 0
700 eV binding energy range presented in Fig. 5.2A. Note that the deeper core Pb 1s would be found with a Eb of 88 keV. In practice, for a given element, the core level either with the highest intensity (which generally results from having the highest relative photo-ionization cross-section), with the best energy resolution and showing the clearest chemical shift is measured. For example, in the case of lead, Pb 4f is often used. The core level peaks of the main elements found in halide perovskite solar cell absorbers are presented in Fig. 5.4 and are respectively classified as cations, metals and halides in (A), (B) and (C). These spectra will also be used as a guide to explain the notion of “chemical shift” and “spin-orbit splitting”. Chemical shift
The motivation to obtain chemical state information has significantly contributed to the successful development of PES. It is based on the observation that for a

Characterization Techniques for Perovskite Solar Cell Materials

5.2 Photoelectron spectroscopy

113

FIGURE 5.2 (A) Overview spectrum, and (B) valence band region spectrum of a mixed perovskite solar cell material MA0.15FA0.85Pb(I0.85Br0.15)3. All of the core level peaks observed in the overview spectrum have been labeled with the element name and the core level. (C) Illustration of the principles of core-level and valence-level spectroscopy. The example of a semiconductor is taken here as there are no states at the Fermi level and spin orbit splitting of the 2p states are not presented here for simplification. (B) Modified from B. Philippe, T.J. Jacobsson, J.-P. Correa-Baena, N.K. Jena, A. Banerjee, S. Chakraborty, et al., Valence level character in a mixed perovskite material and determination of the valence band maximum from photoelectron spectroscopy: variation with photon energy, J. Phys. Chem. C 121 (2017) 26655
26666, https://pubs.acs.org/doi/abs/10.1021/acs.jpcc.7b08948 with permission from the American Chemical Society (ACS). Further permissions related to the material excerpted should be directed to the ACS. (C) Modified from B. Philippe, Insights in Li-Ion Battery Interfaces Through Photoelectron Spectroscopy Depth Profiling, Acta Universitatis Upsaliensis, 2013, Doctoral thesis, comprehensive summary, ISBN 978-91-554-8662-4.

Characterization Techniques for Perovskite Solar Cell Materials

114

5. Photoelectron spectroscopy investigations of halide perovskite materials used in solar cells

FIGURE 5.3 Illustration of two mechanisms by which an excited state of the system can decay to its ground state: Auger electron emission and X-ray fluorescence. Modified from B. Philippe, Insights in Li-Ion Battery Interfaces Through Photoelectron Spectroscopy Depth Profiling, Acta Universitatis Upsaliensis, 2013, Doctoral thesis, comprehensive summary, ISBN 978-91-554-8662-4.

specific element, the peak position can vary, in a range of close to 10 eV, depending on the local chemical (i.e. oxidation states, presence of charge transfer, etc.) environment of the photo-emitting atom. A spectrum of the Bi 4f core level shown in Fig. 5.4B illustrates some basic properties. The concept of electronegativity can sometimes be used to rationalize the direction of the chemical shift. Iodine will withdraw electrons from bismuth; therefore, the bismuth in BiI3 (formally Bi31) exhibits a reduced electron density compared to metallic bismuth. It is thus more energy consuming to eject an electron from Bi31 compared to Bi0 (metallic bismuth) and thus, the binding energy for the Bi 4f peaks for BiI3 is expected to be higher than the one for Bi0. The same reasoning can be applied to explain the peak shift between Sn41 and Sn21 in the Sn 3d core level spectrum presented in Fig. 5.4B. In line with the reasoning presented above, a simple electrostatic theory, called “charge potential model” has been used to interpret chemical shifts and is based on the consideration of the electrical potential surrounding the atom and the modification induced by the charge of neighboring atoms [2,17]. More sophisticated and recent theories take so-called relaxation or “final state” effects (redistribution of the electronic charge during ionization) into account and theoretical tools allow the interpretation and prediction of chemical shifts using for example, the (Z 1 1) approximation [18,19]. Spin-orbit splitting
The annotation of a core level in PES is based on the main atomic quantum number (n 5 1, 2, 3. . .etc) and the angular momentum quantum number (l 5 0, 1, 2, 3 i.e. s, p, d, f respectively) of the state of the electron involved in the photoemission process. The spin-orbit splitting originates from a coupling of the spin and orbital angular momentum that results in a doublet associated with states characterized by the quantum number j (j 5 l 6 s, where s 6 1/2 for electrons/fermions). In a PES experiment such splitting is observed for all levels except s-states (i.e. for l6¼0). An observed core level is then

Characterization Techniques for Perovskite Solar Cell Materials

5.2 Photoelectron spectroscopy

115

FIGURE 5.4 Core level spectra of the main elements present in halide perovskite materials commonly utilized as solar cell light absorbers, grouped according to their position in the ABX3 chemical formula (A) A-site organic/inorganic cation, (B) B-site metal and (C) X-site anion. The N 1s, Rb 3d, Cs 3d, Pb 4f, I 3d and Br 3d spectra were acquired from a quadruple mixed cation (MA1, FA1, Cs1, Rb1) perovskite sample. Modified with permission from B. Philippe, M. Saliba, J.-P. Correa-Baena, U.B. Cappel, S.-H. Turren-Cruz, M. Gra¨tzel, et al., Chemical distribution of multiple cation (Rb1, Cs1, MA1, and FA1) perovskite materials by photoelectron spectroscopy, Chem. Mater. 29 (8) (2017) 3589
3596, Copyright 2017 American Chemical Society [14]. The Cl 2p spectrum was measured from a MAPbI3 perovskite prepared by mixing MAI with PbCl2. Modified with permission from B. Philippe, B.-w. Park, R. Lindblad, J. Oscarsson, S. Ahmadi, E.M.J. Johansson, et al., Chemical and electronic structure characterization of lead halide perovskites and stability behavior under different exposures a photoelectron spectroscopy investigation, Chem. Mater. 27 (2015) 1720
1731 [15], Copyright 2015 American Chemical Society. The Bi 4f core level was measured from a BiI3 film. Nearly all spectra were acquired with a photon energy of 4000 eV. The Sn 3d spectrum was measured from MASnI3 with a photon energy of 1486.6. The K 2p spectrum was measured at 758 eV from a mixed perovskite (Cs, FA, MA)Pb(I0.85Br0.15)3 sample where 5% KI was added into the precursor solution [16].

more precisely identified using the notation nlj. The relative intensity between the two split components is to a first approximation given by the ratio (2j 1 1) based on the number of different spin combinations that can give j (degeneracy of each spin state). Fig. 5.4 contains example of spin-orbit splitting of p, d and f orbitals into respectively p3/2 and p1/2, d5/2 and d3/2, f7/2 and f5/2. The doublet separation is also characteristic of the elements, its redox state and of the core levels considered and can vary from a few meV to several 10s of eV.

Characterization Techniques for Perovskite Solar Cell Materials

116

5. Photoelectron spectroscopy investigations of halide perovskite materials used in solar cells

Quantitative measurements, surface sensitivity and detection limits
The integrated intensity/area of the measured core-level photoelectron peaks can yield quantitative information on the elemental composition at the surface of a sample. The atomic fraction of an element A in a material containing up to i elements is given by Eq. 5.4: IA =SA CA ð%Þ 5 X Ii =Si

(5.4)

i

where IA is the integrated peak intensity of a given core-level of element A and SA is the atomic sensitivity factor for the same core-level. The atomic sensitivity factor can be modeled as:    β 3 2 sin φ 2 1 (5.5) SA 5 σA λA HA 1 1 A 2 2 and quantifies the probabilities for a photoemission event to occur and for the photoelectron to be detected. It depends on H, the spectrometer transmission function, β, a factor which accounts for the anisotropic characteristics of the emission, φ, the angle between the polarization direction of the incoming photon and the direction of the emitted photoelectron, σ, the cross-section which quantifies the probability of photo-ionization of a core level of a specific element (units in barn, 1 barn10
28 m2), and λ, the inelastic mean free path (IMFP). The cross-section of an atomic orbital is a function of the photo-excitation energy, and its value can be outputted from theoretical calculations; cross-section values are reported in several databases [20,21]. The cross-sections of the Pb 4f, I 3d, Br 3d, C 1s, and N 1s core levels as a function of the photo-excitation energy are presented in

FIGURE 5.5 (A) Photoionization cross-sections of the Pb 4f, I 3d, Br 3d, C 1s and N 1s core levels as a function of the irradiating photon energy. These values were extracted from Ref. [22] for the 0 2 1500 eV energy range and are available in Ref. [23] while energies above 1500 eV were based on values from the Scofield database. (B) Dependence of the IMFP of photoelectrons emitted from Pb, I, Br, C, N core levels as a function of their kinetic energy. Modified from B. Philippe, T.J. Jacobsson, J.-P. Correa-Baena, N.K. Jena, A. Banerjee, S. Chakraborty, et al., Valence level character in a mixed perovskite material and determination of the valence band maximum from photoelectron spectroscopy: variation with photon energy, J. Phys. Chem. C 121 (2017) 26655
26666, https://pubs.acs.org/doi/abs/ 10.1021/acs.jpcc.7b08948 with permission from the American Chemical Society (ACS). Further permissions related to the material excerpted should be directed to the ACS. Characterization Techniques for Perovskite Solar Cell Materials

5.2 Photoelectron spectroscopy

117

Fig. 5.5A. In general, photo-ionization cross-sections decrease with increasing photon energy, and high photon flux sources (such as a synchrotron) are needed to perform PES at higher photon energies. The IMFP, also known as the electron escape depth, is a probability-weighted distance of how far a photoelectron with a given kinetic energy will travel in a given solid before scattering in-elastically due to electron
electron and/or electron
phonon interactions. Photoelectron spectroscopy is generally considered to be a surface-sensitive technique, but what gives the technique its surface sensitivity? The simplified Berglund and Spicer three-step model of photoemission consists of (1) optical excitation of an electron, which subsequently (2) travels to, and (3) transmits through the surface [10]. Step 1 is generally not responsible for the surface sensitivity, as the photons typically used for PES (10s to 1000s of eV in energy) absorb over many orders of atomic layers. If we assume the transmission of photoelectrons through the surface is energy-dependent and not dependent on the location where they originated from, then step 2, the propagation of the photoelectron to the surface, determines the surface sensitivity of the technique. Since energy loss due to inelastic scattering results in the loss of information about the origin of the photoelectron, the IMFP essentially dictates the region of the material from which useful, informationcontaining photoelectrons can be collected. The intensity of core level informationcontaining photoelectrons that can be detected decreases as a function of the distance d the photoelectron has to travel to the surface, and follows an exponential decay function (Eq. 5.6): I ðdÞ 5 I0 expð2 d=λ sinðθÞÞ

(5.6)

where I0 is the intensity of the emitted electron at the surface, λ is the IMFP of the photoelectron emitted from a particular core level of an element and θ is the angle between the surface of the sample and the emitted electron trajectory (Fig. 5.6). For example, within a depth of one λ from the free surface, approximately 1 2 e21 B 63% of the photoelectrons traveling normal to the surface of the sample, have experienced no inelastic scattering events, which preserves the information on their electronic level origin and contribute to the intensity at the core-level energy. The in-elastically scattered photoelectrons, if detected, will contribute to the background level of the spectrum. By convention, as exponential functions do not decay to zero, the depth of analysis or probing depth is often considered to be 3λ, as the sum of the information-containing photoelectrons (gray area under the curve of Fig. 5.6) is approximately 95% at 3λ. The determinants of the IMFP for a bound but nearly-free photoelectron are the strengths of the (kinetic energy-dependent) electron
electron and electron
phonon scattering interactions. Therefore, one could expect the IMFP, for a given photoelectron kinetic energy, to vary between materials. Interestingly, the IMFPs for many materials, at a given kinetic energy, have been experimentally found to be similar, resulting in a “universal curve” for IMFP as a function of energy (see Figure 1.9 in [10]). One explanation for this finding is that for the kinetic energies of interest (10s to 1000s of eV), the electrons in a solid can be approximately described by a free-electron gas, and since the mean electron
electron distance is roughly similar for all materials, the IMFP is essentially material-independent [10]. One should note, however, that different experiments and

Characterization Techniques for Perovskite Solar Cell Materials

118

5. Photoelectron spectroscopy investigations of halide perovskite materials used in solar cells

FIGURE 5.6 (A) Plot of the exponential relationship between photoelectron intensity and probing depth. (B) The ratio I(d)/I0 presented as a function of the depth of analysis with θ 5 90 . The area below the curve (in gray) represents the cumulative probability that a photoelectron has originated from a depth ranging between 0 and d, i.e. 63% between 0 and λ, 95% between 0 and 3λ. Modified from B. Philippe, Insights in Li-Ion Battery Interfaces Through Photoelectron Spectroscopy Depth Profiling, Acta Universitatis Upsaliensis, 2013, Doctoral thesis, comprehensive summary, ISBN 978-91-554-8662-4.

calculations have yielded different values for the IMFP, for the same element/compound. Taking aluminum, a typical free-electron gas metal, as an example, the IMFP at 20 eV is ˚ according to one report [10], and is 1.3 A ˚ according to a model (calcuapproximately 8 A lated by the software QUASES, which is based on the model TPP-2 M [24,25]). At a kinetic ˚ as opposed to 14 A ˚ according to QUASES-TPP-2 M. energy of 500 eV, the IMFP is B10 A Differences of an order of magnitude between reported values of the IMFP, for the same compound and the same energy, are not uncommon. Differences in reported IMFP values tends to decrease, as the electron kinetic energy increases. Users who aim to perform absolute quantification (Eq. 5.4) should be mindful of the potential inaccuracies introduced by IMFP variations. It is good practice to calculate the atomic fraction for the element of interest using a range of IMFP values, and then decide whether the interpretation of the data is still valid. Relative quantification (a ratio of the quantity of element A to element B) is more accurate, especially if the IMFPs for the core levels from elements A and B are comparable which is usually the case for XPS performed with higher photon energies as λA and λB may be nearly equal. Examples of relative quantification of halide perovskite materials can be found in Section 5.4.2.1. As an order-of-magnitude estimate, the IMFP for a photoelectron originating from the ˚ , for photoelectrons generated core levels of most materials varies between 5 and 25 A using 1486.6 eV soft X-rays. This photon energy is generated by the Al-Kα X-ray source found on most in-house PES systems. Photoelectron spectroscopy features relatively low detection limits that vary with the sample and the experimental setup (type of electron spectrometer, etc). With XPS performed with conventional in-house systems, most elements are detectable when present at a concentration of about 1 at% to 0.1 at%. This detection limit can decrease to 0.01 at% if heavy (high atomic number) elements are dispersed in a matrix comprised of light

Characterization Techniques for Perovskite Solar Cell Materials

5.3 From UPS to HAXPES: variation of the photon excitation energy

119

element(s) [26]. This low detection limit is of great practical importance when investigating materials doped with few percentages of another material. In summary, quantitative investigations of the chemical composition of materials are possible with PES, the surface sensitivity of PES is due in large part to the scattering length of the photo-emitted electrons, and PES features relatively low detection limits of 100
1022 at%. The high surface sensitivity of PES enables it to complement more bulksensitive techniques such as XRD or Raman spectroscopy.

5.2.3 Valence band photoelectron spectroscopy The valence band spectra can be considered as an experimental visualization or approximation of the occupied valence electronic states of a material, which govern all of the physical (i.e electrical, magnetic, thermal, etc.) properties of solid-state materials. The interpretation of valence band spectra is significantly different and more difficult than the interpretation of core-level spectra; one reason for this is due to the mostly atomic-like characteristics of the core-levels, while the valence band typically contains contributions from multiple elements and orbitals. Progress in interpreting valence band spectra is generally made with the help of electronic structure calculations. In the field of solar cell materials research, the study of valence band spectra is important as the measurements directly yield the energy position of the valence band edge for inorganic or hybrid organic
inorganic materials or HOMO for polymeric and molecular organic materials in relation to the Fermi and/or vacuum level(s), with minimal additional interpretation (mostly due to the appropriate fitting of the band edge feature) required. With this insight the energy level diagrams relevant to the operation of optoelectronic devices can be obtained. The knowledge of the electronic structure properties (i.e. states found at the VB and CB edge) allows then to control some energy level material matching through chemical modification or doping.

5.3 From UPS to HAXPES: variation of the photon excitation energy Photoelectron spectroscopy is an umbrella term that covers a range of sub-techniques that are classified by the photo-excitation energy used. Ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) are generally referred to as “inhouse” techniques; these techniques are commonly utilized in laboratories and complete systems (containing the light sources, electron spectrometer, vacuum chambers and pumps, etc.) to perform these techniques are commercially available. The light source for UPS is typically a helium gas discharge lamp yielding XUV photon energies of 21.2 eV or 40.8 eV (He I and II lines, respectively). Other inert/noble gases such as neon can be used, yielding the photon lines Ne I at 16.6 eV and Ne II at 26.8 eV. The very low photon energies used in UPS, and consequently the low kinetic energies (10s of eV) of the photoelectrons detected, gives rise to some uncertainty in the probing depth. As mentioned previously, the IMFP is determined by the strengths of electron
electron

Characterization Techniques for Perovskite Solar Cell Materials

120

5. Photoelectron spectroscopy investigations of halide perovskite materials used in solar cells

and electron
phonon scattering. Electron
electron scattering is present at all kinetic energies, while electron
phonon scattering primarily plays a role at kinetic energies below B100 eV. For metals and insulators, the degree of electron
phonon coupling is different, hence the “universal” curve diverges for different materials at lower (10s of eV) kinetic energies (see Figure 1.7 in reference [27]). This implies that the depth of analysis is highly material-dependent, and could vary by an order of magnitude. Based on its availability, and independent of some general difficulties in determining its IMPF, UPS has been applied to research on functional materials. The use of UPS has partly been to extract the valence band density of states, the energy position and profile of the valence band edge, and the work function of the materials. Using measurements of the work function and the energy position of the valence band edge, it is possible to construct energy level diagrams, which are crucial for understanding opto-electronic device operation. If an interface between different materials (used as a hole- or electronselective layer, for example) is incrementally formed, and followed by sequential UPS measurements, then the interfacial energy-level/band alignment can be deduced. The insight from such energy-level alignment diagrams is important for understanding whether the interface of interest facilitates or impedes efficient charge extraction or injection. XPS utilizes higher photon energy compared to UPS, and the photon lines used are typically Al-Kα (1486.7 eV) and Mg-Kα (1253.7 eV). The X-ray sources employed in XPS may be combined with a (crystal) monochromator to attenuate/eliminate the satellite photon lines generated by the X-ray sources and achieve higher energy resolution in the measured spectra. XPS is occasionally used to acquire valence band spectra, similar to how UPS is used, but is more frequently used as a core-level spectroscopy technique to investigate the properties of the separate materials including chemical composition (quantitative and qualitative). Other anode materials are also available with higher photon energy including Zr (Lα at 2042.4 eV), Ag (Lα at 2984.3 eV), Ti (Kα at 4510.9 eV), Cr (Kα at 5414.8 eV) [28], Cu (Kα at 8047.8 eV) or Ga (Kα at 9251.7 keV) [29]. The higher photon energy allows exploring higher IMFP in the order of few nanometers as explain earlier. In addition to laboratory tools, the development of synchrotron radiation has spurred the development of spectroscopic techniques where the incident photon energy can be tuned from a few eV to several keV covering both the traditional UPS and XPS energy ranges. PES beamlines offering soft-X-ray (20
1500 eV) energies are sometimes referred to as SOXPES (Soft X-ray Photoelectron Spectroscopy) beamlines, while PES beamlines offering photon energies from 2 up to 15 keV, and nowadays available at most synchrotron facilities are labeled as HAXPES (Hard X-ray Photoelectron Spectroscopy) beamlines. The use of high photon energy leads to a larger photoelectron IMFP as seen in various universal or master curves for IMFP as a function of kinetic energy. Therefore, measurements on buried interfaces as well as more bulk-sensitive information on the chemical composition and electronic structure of materials can be obtained with HAXPES. The aforementioned techniques above are summarized in Table 5.1 together with the main photon sources used, their associated energies and a rough estimation of the probing depth investigated.

Characterization Techniques for Perovskite Solar Cell Materials

121

5.4 PES investigations of halide perovskite materials

TABLE 5.1 Emission lines generated by common laboratory-scale x-ray sources, their photon energies, and estimates of the probing depth offered by UPS, XPS, SOXPES and HAXPES. Technique name

Emission line

Photon energy or energy range

Depth of analysis (  3λ in nm)

UPS

He I or II

21.1 and 40.8 eV

1
5 nm

Ne I or II

16.6 and 26.8 eV

Mg Kα

1253.7 eV

Al Kα

1486.7 eV

SOXPES

Synchrotron radiation

20
1500 eV

1
10 nm

HAXPES

Synchrotron radiation

2000
15,000 eV

. 10 nm

XPS

3
10 nm

The depth of analysis gives some estimated values (see text for details).

To conclude, it is important to keep in mind a few factors when changing the incident photon energy used in PES and going from UPS to HAXPES. First of all, as already mentioned, the IMFP of the emitted electrons increases when their kinetic energy increases, and as a consequence the depth probed is increased. The variation of the analysis depth open the possibility to investigate either the outermost layer or more bulk-like properties and differences can thus be observed in non-homogeneous films. The notion of cross-section, also introduced earlier, is depending on the elements and the orbital considered but also on the photon energy used. As shown in Fig. 5.5A, the cross-section of a given core level of an element generally decreases when the photon energy (hν) increases. The probability for an electron to be ejected without energy loss is indeed becoming smaller as the emitted electrons comes from a deeper layer and has to go through a longer path without scattering. The cross-section evolution as function of the photon energy however evolved at very different rates depending on the element and orbital state. The modification and tunability of the photon energy can then be used also as a tool to either highlight or inhibit some orbitals through a wise choice of the incoming energy [5]. An example will be seen later in this chapter.

5.4 PES investigations of halide perovskite materials As a technique which has been successfully applied over the past few decades to investigate the electronic structure and chemical composition of multiple classes of materials, photoelectron spectroscopy was a natural technique to use for studying halide perovskite materials. However, some precautions must be taken before and during the measurement, to ensure meaningful information on the (a) electronic structure, (b) chemical composition of HaP materials and interfaces with other device-relevant materials such as electron- and hole-selective contacts, and (c) energy level alignment at HaP/buffer layer interfaces, are obtained. The following section summarizes precautions to take regarding binding energy calibration, sample preparation and handling, and radiation damage as well as selected studies where new knowledge was acquired on HaP materials using PES.

Characterization Techniques for Perovskite Solar Cell Materials

122

5. Photoelectron spectroscopy investigations of halide perovskite materials used in solar cells

5.4.1 Precautions 5.4.1.1 Binding energy calibration In an angle-integrated photoelectron spectroscopy experiment, the experimentally measured quantity is the intensity of photoelectrons as a function of the photoelectron kinetic energy. Electronic states in a material, where the photoelectrons originate from, are referenced to the Fermi level, defined as zero eV binding energy. A practical approach to reference the kinetic energy of a detected photoelectron to the binding energy of the electronic level where the photoelectron originated from, is to obtain the kinetic energy of electrons photo-emitted from the Fermi level and treat it as a calibration value. As an aside, the Fermi level exists only in a material in equilibrium; the assumption, during photoemission, is that the electronic system is only perturbed slightly; the number of electrons lost to photoemission, and subsequently replenished, is “small” relative to the overall number of electrons in the material; so the quasi-Fermi level is essentially the Fermi level of the unperturbed system. If a spectrum of the Fermi step is obtained, typically from a metallic sample, a sigmoid function fit will yield the Fermi level. To perform Fermi energy calibration for a sample without a measurable Fermi level (e.g. semiconductor or insulator), an external sample can be utilized. A commonly used external sample is a sheet of gold foil, placed in electrical contact with the same sample holder/stage used for measurements on the actual sample. The Au 4f7/2 peak position is located at a well-known value (84.0 eV) with respect to the Fermi level, so adding 84.0 eV to the kinetic energy of Au 4f7/2 photoelectrons will yield the photoelectron kinetic energy which corresponds to the Fermi level. Internal energy calibration using the actual sample is based on the same principle, but the measurements of reference core levels from the sample itself is utilized. Thin films of HaPs are typically deposited onto a conductive substrate. If the overlying HaP film is discontinuous and/or very thin, photoelectrons from the underlying substrate (e.g. photoelectrons from the Ti 2p core level of a titanium dioxide electron-selective layer or Sn 3d core level of the indium or fluorine-tin-oxide (ITO or FTO electrode)) will be detectable. This procedure is particularly useful when the band offset between the perovskite and an adjacent layer such as the electron transport layer are studied [15,30]. In the case of a decomposed HaP film, the photoelectron peak corresponding to metallic lead (Pb0) has also been used. If the substrate cannot be detected (and an external reference is unavailable), the situation is more complex and exact energy referencing may not be possible. A reference such as “adventitious” carbon, i.e. C
C hydrocarbon bond, which can be detected on all ex situ prepared samples exposed at some point to the environment is then sometimes used. Its binding energy is however not well defined and values generally used are 284.8
285.0 eV. The situation is even more complex with insulating samples [31
33]. Residual charge can appear at the surface of the sample after emission of the photoelectrons. This charging effect results in a broadening of the peaks and the measured binding energy is often shifted towards a higher value. This charging effect can be compensated by the use of a flood gun (low energy electron gun) but it is generally difficult to perfectly compensate for charging. As a consequence, the Fermi levels of the sample and of the spectrometer are different and an internal calibration is the only viable solution.

Characterization Techniques for Perovskite Solar Cell Materials

5.4 PES investigations of halide perovskite materials

123

5.4.1.2 Sample preparation and handling Halide perovskite thin film samples are typically prepared by solution-coating techniques inside gloveboxes filled with a dry, inert gas. Photoelectron spectroscopy measurements are typically performed in high/ultra-high vacuum (B1029 to 10211 Torr). During fabrication and measurement, the thin films are present in environments which are considered to be clean; the potential for uncontrolled environmental exposure exists in the transfer of the sample between the two environments. The environmental instability of HaP has been discussed in the literature, with the typical environmental stressors determined to be water vapor, oxygen, and light. Light soaking (in nitrogen) has been shown to induce iodide migration in MAPI films [34]. Water vapor degrades both MAPI and FAPI films, though the tolerance of each to a given % relative humidity is different [35]. MAPI films degrade substantially after one month of air exposure; storage of the samples at a low vacuum (B1 mTorr) also degrades the samples but at a much lower rate compared to air exposure [36]. As PES is a highly surface sensitive technique, and environmentallyinduced degradation of materials is typically initiated at the surface of a material, protecting the sample against unintended and uncontrolled degradation is essential to ensure the results obtained are meaningful. Typically, there is a time delay (on the order of hours to days) between the fabrication of a sample to the actual measurement. The primary objective of the sample transport and handling procedure is to minimize the risk of sample degradation due to uncontrolled environmental exposure. A preferred approach is to eliminate moisture, oxygen, and light exposure to the greatest extent possible. For ex situ prepared samples one practical method to achieve this objective is to use a double (air) barrier system were the freshly synthesized samples are encapsulated before transportation to the spectrometer system. Prior measurement, the sample is removed in an oxygenfree environment, and the samples should be loaded into a transfer suitcase which can be connected to the load lock of the measurement system. Transporting a sample “quickly” in air between a glovebox and the load lock may be acceptable, depending on the sample. 5.4.1.3 Radiation damage Photoelectron spectroscopy necessarily involves irradiating a sample with photons (ultraviolet, soft/hard X-ray, etc.) to generate the photoelectrons that are collected and measured. Soft X-rays (Al-Kα B1487 eV) were shown to decompose MAPI films after extended exposure; the films were specifically prepared to withstand UHV-only induced decomposition for a week [37]. For many HaP solar cell materials, the corresponding changes to the nitrogen (i.e. N 1s) and lead (i.e. Pb 4f) core level spectra can be utilized as signs of sample degradation/decomposition. The degradation of MAPbI3 into lead diiodide (PbI2) is easily observed via a decrease of the N 1s peak intensity and a shift of the Pb core level peak position. The formation of metallic Pb0 can also be monitored over time in the Pb spectrum [38]. More subtle radiation damages can occur where no obvious changes in the spectra can be observed during the measurement, but the ratio of elements changes as a function of total radiation exposure. Complex HaP formulations, consisting of multiple cations and anions, may display this behavior since a multitude of effects such as ion migration, defect formation, etc., can occur during irradiation. More rigorous preliminary work thus becomes necessary, where each core level spectrum is measured over

Characterization Techniques for Perovskite Solar Cell Materials

124

5. Photoelectron spectroscopy investigations of halide perovskite materials used in solar cells

a longer period of time [39]. Practical approaches to limit radiation damage include tuning the beamline parameters and/or using filters to decrease the photon flux, and periodically changing the irradiated spot on the sample. A PES measurement session typically consists of a short period of experimentation at the beginning, where the usable “lifetime” of a spot for a given sample is assessed semi-quantitatively. The measurement protocol may include acquiring the N 1s and Pb 4f spectra first, performing a series of other measurements, then acquiring the N 1s and Pb 4f spectra again (Fig. 5.7).

5.4.2 Selected results The methodology used to investigate HaP materials has evolved in tandem with the complexity of the materials. The prototypical HaP formulation is MAPbI3, which contains FIGURE 5.7 (A) Valence band spectra of a FAMAPbIBr perovskite undoped and doped either with Cs, Rb, K, or Na and (B) Evolution of the ratio of metallic to oxidized lead, which is used as a semiquantitative proxy for the amount of radiation damage. Reprinted with permission from T.J. Jacobsson, et al., Extending the compositional space of mixed lead halide perovskites by Cs, Rb, K, and Na doping, J. Phys. Chem. C 122 (25) (2018) 13548
13557, Copyright 2018 American Chemical Society.

Characterization Techniques for Perovskite Solar Cell Materials

5.4 PES investigations of halide perovskite materials

125

one cation, one halide, i.e. a total of four elements (carbon, nitrogen, lead, iodine). Current state-of-the-art HaP solar cells feature at least four cations (MA, FA, Cs, K) and two halides (I, Br) [16]. To answer more sophisticated questions about the spatial distribution of the cations, for example, investigations have transitioned from lab-scale PES setups to more sophisticated experimental stations with tunable photon sources (i.e. synchrotron beamline/end-station). 5.4.2.1 Chemical characterization As a standard tool, and possibly the default tool, for chemical characterization, PES has been used since the early days of HaP solar cell development to answer questions about film composition. PES-based investigations contributed substantially to the discussion concerning the role of chlorine and its location within the perovskite film. Attempts were made to fabricate thin films of the mixed halide MAPbI2Cl by mixing MAI with PbCl2 (molar ratio 3:1) [40], but they were unsuccessful. The absence of chlorine at the surface of the materials was clearly demonstrated via PES [29,41,42]. A chlorine 2p spectrum is shown in Fig. 5.4C, and consists of just the background signal. In contrast, later investigations focused on dopants and additives, which were readily detected. Examples include elements such as Rb1, Cs1 or K1 in Fig. 5.4A or Ag [43], Cu or Na [44]. The presence of metallic lead or bismuth in HaP films has also been reported and addressed via PES [29,45,46]. The gradual transformation of PbI2 into MAPbI3 via a growth process (Vapor Assisted Solution Process or VASP) was observed visually, and confirmed with PES. Lead di-iodide was first spin-coated onto a TiO2/FTO substrate, then exposed to a vapor of MAI for an increasing period of time [47]. The formation of the dark, optically-absorbing perovskite phase could be observed as the film changed color from yellow to dark gray. Iodine-tolead (I/Pb) intensity ratios, derived from soft-XPS (hν 5 1486.6 eV) measurements of the iodine and lead core levels, increased from 1.9 (PbI2) to 3.0 (MAPbI3) after 60 minutes of reaction time, as shown in Fig. 5.8B. Sequential measurements of the carbon 1s and nitrogen 1s core levels (Fig. 5.8C) show the growth in the amount of methylammonium, relative to the iodide and lead present in the PbI2. The extent of degradation of a HaP can be uniquely elucidated with PES, due to its tunable surface sensitivity. MAPbI3 decomposes into PbI2 when placed in direct contact with water. This form of bulk and surface degradation can be easily observed, as the sample changes color from dark gray to yellow (PbI2). However, it is not always possible to observe small compositional changes at the surface of the sample with optical techniques. Fig. 5.9 shows the optical evolution of MAPbI3 samples stored in different environments: dry argon and ambient air. Both samples were kept in the dark. After 83 days, both samples looked the same, and appeared to be pristine. XPS measurements (hν 5 1486.6 eV) revealed that the surface of the perovskite stored in air had almost completely degraded to PbI2, as shown by the decrease of the I/Pb intensity ratio from B3 to about B2 (Fig. 5.9B) and the gradual disappearance of the nitrogen 1s photoelectron peak (Fig. 5.9C). The sample kept in the argon environment remained similar to the pristine sample, with intensity ratios at day 83 being essentially the same as those on day 0. PES has been used to correlate the effects of processing changes on the composition of the surface and near-surface regions of HaP films containing multiple cations and halides. Several key figures from an investigation on MA0.15FA0.85Pb(I0.85Br0.15)3 are displayed in

Characterization Techniques for Perovskite Solar Cell Materials

126

5. Photoelectron spectroscopy investigations of halide perovskite materials used in solar cells

FIGURE 5.8 (A) Illustration of the VASP growth mechanism and (B) evolution of the iodine 4d and lead 5d spectra as a function of the reaction time of PbI2 with a vapor of MAI (before vapor exposure, after 10, 30, 45 and 60 minutes of vapor exposure). A picture with the corresponding samples as well as the experimental I/Pb atomic ratios are shown as inset. (C) Evolution of the C 1s and N 1s core levels. Modified from S.M. Jain, B. Philippe, E.M. J. Johansson, B.-w. Park, H. Rensmo, T. Edvinsson, et al., Vapor phase conversion of PbI2 to CH3NH3PbI3: spectroscopic evidence for formation of an intermediate phase, J. Mater. Chem. A 4 (2016) 2630
2642 with permission from The Royal Society of Chemistry.

Fig. 5.10 [48]. Fig. 5.10A shows the nitrogen 1s spectra of the mixed perovskite, FAPbI3, and MAPbBr3; FA1 (CH(NH2)21) and MA1 (CH3-NH21) cations can be differentiated due to the differences in the N 1s peak positions, and both can be seen in the N 1s spectrum of the mixed cation perovskite. Fig. 5.10B shows an energy window which incorporates the shallow core levels of bromide, iodide, and lead, and the valence band. It is convenient to use this spectral region to perform quantification of the material, as the inelastic mean free paths of the photoelectrons originating from the shallow core levels are nearly the same, especially for higher excitation energies such as hν 5 2 and 4 keV. Through the use of three different photon energies, the effective probing depth can be varied between B5 nm and B18 nm (Fig. 5.10D), revealing the effects of varying the PbI2 precursor concentration.

Characterization Techniques for Perovskite Solar Cell Materials

5.4 PES investigations of halide perovskite materials

127

FIGURE 5.9 Evolution of a MAPbI3 sample deposited on a TiO2/FTO substrate after 45 and 83 days of storage in argon or air. The reference point (day 0) was taken directly after preparation of the film. (A) Sample preparation prior storage and picture of the sample after 83 days. (B) Evolution of the I/Pb ratio determined by XPS. (C) Evolution of the N 1s spectra at day 0 (in black) and after 45 and 83 days (in full and dashed line, respectively) after storage in argon (top) and in air (bottom).

Fig. 5.10C shows the experimentally-derived I/Pb ratios for the three samples as well as for a PbI2 film, which is treated as a reference (I/Pb B2 both at the surface and in the “bulk”). In the case of a mixed perovskite, clear differences can be observed when comparing surface and bulk, and the main one is probably the iodine excess observed towards the surface and the relative heterogeneity within the first few nanometers of the film surface. The variation of the amount of PbI2 introduced is significantly modifying the surface as illustrated in the schematic of Fig. 5.10D. Similar investigations have been performed on HaP films where cesium, rubidium [14] or potassium [16] were added; PES was used to track the spatial location of these additives within the films. 5.4.2.2 Electronic structure A valence band spectrum provides much more than just the energy position of the VBM. It contains information on the hybridized atomic levels which contribute to the valence band quasi-continuum of states. The detailed interpretation of a valence band is highly dependent on calculation methods such as DFT that is developed in this book in Chapter 10 DFT calculation on Lead perovskite solar cell Materials. Fig. 5.11 illustrates the

Characterization Techniques for Perovskite Solar Cell Materials

128

5. Photoelectron spectroscopy investigations of halide perovskite materials used in solar cells

FIGURE 5.10

(A) N 1s spectra of a mixed perovskite (MA0.15FA0.85)Pb(I0.85Br0.15)3, FAPbI3 and MAPbBr3 deposited on amorphous SnO2/FTO substrate recorded with a photon energy of 4000 eV (in top, middle and bottom respectively). (B) 80
0 eV binding energy area spectra containing the Br 3d, I 4d, and Pb 5d core levels of the same samples. All the spectra were intensity normalized to the Pb 5d5/2 core level peak. (C) Estimated probing depth and I/Pb ratio calculated from the experimental I 4d and Pb 5d core levels peaks measured with a photon energy of 758, 2100 and 4000 eV for a PbI2 reference and three different mixed perovskite materials prepared in stoichiometric condition (0%) or with 210% or 110% PbI2. (D) Schematic illustration summarizing the main difference observed by PES between three different mixed perovskite materials. Modified with permission from T. J. Jacobsson, J.-P. Correa-Baena, E. Halvani Anaraki, B. Philippe, S.D. Stranks, M.E.F. Bouduban, et al., Unreacted PbI2 as a double-edged sword for enhancing the performance of perovskite solar cells, J. Am. Chem. Soc. 138 (2016) 10331
10343, Copyright 2016 American Chemical Society.

calculated TDOS and PDOS of MAPbI3 and MAPbBr3 that were used as model systems. The results were then used to assign the structures of an experimental valence band of a mixed perovskite. The electronic structure of a material does not only focus on the valence band region but also on the conduction band (unoccupied sates). The unoccupied states of a material can also be experimentally probed using Inverse Photoemission spectroscopy (IPES) [48] or X-ray absorption (XAS) [49]. Resonant Inelastic X-ray Scattering (RIXS) is a more recent and very promising technique where information on both the frontier electronic states can be obtained complementing the detailed picture of the electronic structure of a material.

Characterization Techniques for Perovskite Solar Cell Materials

5.4 PES investigations of halide perovskite materials

129

FIGURE 5.11 (A) Total density of states (TDOS) of MAPbI3 and MAPbBr3 decomposed into partial density of states (PDOS) contributed by lead, iodine/bromine, carbon, and nitrogen. The calculated spectra are decomposed into s- and p-character, in dashed and solid lines, respectively. (B) Experimental valence band recorded with a photon energy of 250 eV and assignments of the main structures observed. Adapted from B. Philippe, T.J. Jacobsson, J.-P. Correa-Baena, N.K. Jena, A. Banerjee, S. Chakraborty, et al., Valence level character in a mixed perovskite material and determination of the valence band maximum from photoelectron spectroscopy: variation with photon energy, J. Phys. Chem. C 121 (2017) 26655
26666, https://pubs.acs.org/doi/abs/10.1021/acs.jpcc.7b08948 with permission from the American Chemical Society (ACS). Further permissions related to the material excerpted should be directed to the ACS.

5.4.2.3 Energy level alignment PES and especially valence band spectroscopy is often used in solar cell research to determine the energy level alignment of the different materials composing a device, i.e. light absorbers, electron/hole transport materials and metal contacts. Optimal energy alignment at the interfaces between the different constituents of a solar cell is crucial to obtain efficient devices. Prior to drawing any energy diagram and as mentioned earlier, a precise energy calibration is crucial. PES spectra are generally presented after calibration to the Fermi level (EF). The position of the valence band maxima (VBM) is then generally determined via linear extrapolation. Such approach is quite accurate when comparing the energy difference between similar materials, nevertheless the difference between the VBM and EF is trickier and the estimated value can be higher than the bandgap (Eg) especially for n-doped materials as illustrated in Fig. 5.12A. In such case, the VBM is better described by focusing on the lower density of states at the actual valence band edge that can be highlighted by using a logarithmic scale [5,50].

Characterization Techniques for Perovskite Solar Cell Materials

130

5. Photoelectron spectroscopy investigations of halide perovskite materials used in solar cells

FIGURE 5.12 (A) Comparison of the UPS spectrum and DFT calculation of the density of states of MAPbBr3, and illustration of the VBM determination when using a linear and a logarithmic intensity scale. (B) The VBM position as a function of six different samples: CH3NH3PbI3 deposited on TiO2 determined by UPS and XPS. (C) Evolution of the valence band spectra (14 2 0 eV binding energy range) of the mixed perovskite materials as a function of the photon energy. (A) Reprinted from J. Endres, D.A. Egger, M. Kulbak, R.A. Kerner, L. Zhao, S.H. Silver, et al., Valence and conduction band densities of states of metal halide perovskites: a combined experimental 2 theoretical study, J. Phys. Chem. Lett. 7 (2016) 2722
2729, https://pubs.acs.org/doi/10.1021/acs.jpclett.6b00946 with permission from the American Chemical Society (ACS). Further permissions related to the material excerpted should be directed to the ACS. (B) Reproduced from E.M. Miller, Y. Zhao, C.C. Mercado, S.K. Saha, J.M. Luther, K. Zhu, et al., Substrate-controlled band positions in CH3NH3PbI3 perovskite films, Phys. Chem. Chem. Phys. 16 (2014) 22122
22130 [51] with permission of the PCCP Owner Societies. (C) Adapted from B. Philippe, T.J. Jacobsson, J.-P. Correa-Baena, N.K. Jena, A. Banerjee, S. Chakraborty, et al., Valence level character in a mixed perovskite material and determination of the valence band maximum from photoelectron spectroscopy: variation with photon energy, J. Phys. Chem. C 121 (2017) 26655
26666, https:// pubs.acs.org/doi/abs/10.1021/acs.jpcc.7b08948 with permission from the American Chemical Society (ACS). Further permissions related to the material excerpted should be directed to the ACS.

Even if the VBM position can be estimated using any PES techniques, UPS has been probably the technique used the most so far as it is an in-house tool, it is quick to obtain a spectra and it gives the possibility to measure the VBM and the work function of a material at the same time. Consequently, band alignment diagram can directly be plotted versus the vacuum level (Evac).

Characterization Techniques for Perovskite Solar Cell Materials

5.4 PES investigations of halide perovskite materials

131

Nevertheless, it is very important to remember that (1) UPS can be much more surface sensitive than XPS or HAXPES (shorter IMFP) and (2) perovskite materials can be quite heterogeneous at their surface as highlighted earlier. The VBM obtained via XPS and HAXPES are thus often more reproducible and consistence compare to the one determined via UPS [51] as illustrated in Fig. 5.12B. The evolution of the valence band spectra of the mixed perovskite materials as a function of the photon energy is illustrated in Fig. 5.12C. The differences in the valence structure observed are largely related to the variation of the cross-section of the different orbitals occurring when changing the photon energy of the incident source. Photon energies higher than 500 eV give a valence band where contributions of the iodine, lead, and bromine, are dominant while the nitrogen and carbon will be more visible when lower photon energies are used. Carbon-containing contamination present at the surface of the samples can thus be enhanced when low photon energies are used. Some examples of band energy alignment performed using HAXPES, XPS and UPS are presented in Figs. 5.13
5.15. The valence band spectra of different perovskite solar cells materials are presented in Fig. 5.13A and compared to the one of a bare TiO2 substrate. A zoom of the valence band edge in Fig. 5.13B illustrates how the VBM was determined and even the absolute difference (EVBM 2 EF) is different depending of the approaches used, i.e. linear extrapolation or actual edge, the relative energy difference between the edge of the MAPbI3 perovskite and the TiO2 remain the same, i.e. B2.1 eV in this work. A similar work was done on a series of eleven MAPb(I1-xBrx)3(Cl)y films investigated by XPS. It was observed that the valence FIGURE 5.13 (A) Valence band spectra of the meso-TiO2 film, MAPbCl3, MAPbI3, and MAPbI32xClx on mesoTiO2, measured by PES with an excitation energy of 4000 eV. The spectra are normalized using the highest peaks for simple comparison. (B) Expanded view of the valence band edge. (C) Schematic drawing of the energy level diagram. Modified with permission from B. Philippe, B.-w. Park, R. Lindblad, J. Oscarsson, S. Ahmadi, E.M.J. Johansson, et al., Chemical and electronic structure characterization of lead halide perovskites and stability behavior under different exposures - a photoelectron spectroscopy investigation, Chem. Mater. 27 (2015) 1720
1731, Philippe, B. et al. Chem. Mater. 2015, 27, 1720 2 1731. Copyright 2015 American Chemical Society.

Characterization Techniques for Perovskite Solar Cell Materials

132

5. Photoelectron spectroscopy investigations of halide perovskite materials used in solar cells

FIGURE 5.14 (A) Pictures of the full set of prepared MAPb(I1-xBrx)3(Cl)y films. (B) Optical band gap and schematic drawing of the energy levels for the series of samples. (C) Valence levels spectra of the corresponding samples measured by XPS (hν 1486.6 eV). A zoom of the valence band edge (13.5 to 22 eV energy range) including a linear extrapolation and an indication of the experimental spectral edge for the MAPbI3 and MAPbBr3 samples is presented in inset. The energy scale in (B) was set versus the Fermi level using the experimental spectral edge (full arrows). Reproduced from B.-w. Park, B. Philippe, S.M. Jain, X. Zhang, T. Edvinsson, H. Rensmo, et al., Chemical engineering of methylammonium lead iodide/bromide perovskites: tuning of optoelectronic properties and photovoltaic performance, J. Mater. Chem. A 3 (2015) 21760
21771 [52] with permission from The Royal Society of Chemistry.

edge was gradually shifting toward higher binding energy when going from MAPbI3Cly to MAPbBr3Cly following the gradual increase of their corresponding band gap (respectively B1.6 and B2.3 eV) and highlighting the strong I 5p or Br 4p character at the valence band edge [52].

Characterization Techniques for Perovskite Solar Cell Materials

5.4 PES investigations of halide perovskite materials

133

FIGURE 5.15 (A) Energy level diagrams and electron injection characteristics of SnO2 and TiO2-based planar PSCs. (B) Conduction band alignment diagram of the perovskite films and the electron selective layers, TiO2 and SnO2 for (FAPbI3)0.85(MAPbBr3)0.15, labeled as ‘mixed’. (C) UPS spectra (He I) of the ALD TiO2 and the mixed perovskite/TiO2 and UPS spectra ALD SnO2 and mixed perovskite/SnO2. Ionization energies (IE) and the difference between the valence band energies of the electron selective layers and the perovskite (ΔE) are shown for all materials. Adapted with permission from J.-P. Correa Baena, L. Steier, W. Tress, M. Saliba, S. Neutzner, T. Matsui, et al., Highly efficient planar perovskite solar cells through band alignment engineering, Energy Environ. Sci 8 (2015) 2928
2934 [53], published by The Royal Society of Chemistry.

Finally, a band alignment resulting from UPS measurements on a mixed perovskite deposited on two different ETM; TiO2 or SnO2, is presented in Fig. 5.15. The authors were able to explain the benefit of SnO2, which indicate a favorable electron path from the perovskite conduction (CB) band to the CB of SnO2 compared to TiO2 that have an energy barrier to go through [53].

Characterization Techniques for Perovskite Solar Cell Materials

134

5. Photoelectron spectroscopy investigations of halide perovskite materials used in solar cells

5.5 Conclusions and outlook Halide perovskites are an interesting class of materials to investigate, both scientifically and technologically, and photoelectron or photoemission spectroscopy (PES) is a powerful technique for probing the chemical composition and electronic structure of the materials. The spectroscopy of photoelectrons exploits the photoelectric effect, and the simplest possible experimental setup involves a radiation source, sample, and photoelectron analyzer. PES is generally considered as a surface-sensitive technique, and the scattering length of photoemitted electrons is a primary determinant of the probing depth. PES can be conceptually divided into two types of spectroscopy: core-level and valence-level spectroscopy, depending on the binding energy region probed. Core-level spectroscopy can be utilized to quantify the elemental composition of the surfaces of materials, with a relatively low detection limit of 100
1022 at%, while valence-level spectroscopy can be used to extract the electronic levels of interest for opto-electronic device operation, and with the aid of electronic structure calculations, differentiate element- and orbital-specific contributions to the valence band. PES is an umbrella term for multiple sub-techniques that differ by the photo-excitation used: UPS (for ultraviolet energies ranging from a few eV to a few 10s of eV), SOXPES (for soft X-ray energies ranging from 20 to 1500 eV), and HAXPES (for hard X-ray energies .2000 eV). Photoelectron spectroscopic measurements have provided a wealth of information on the surface/interface chemical composition and electronic structure of HaP materials, and the energy level alignment at HaP/buffer layer interfaces found in HaP-based solar cells, and selected results have been summarized here. To obtain meaningful results from PES measurements, one must ensure in what way the binding energy scale is calibrated, the HaP samples are prepared and transported in conditions which avoid photo-, oxygen-, and water vapor-induced degradation of the samples, and beam-induced radiation damage is carefully monitored. Guidelines to achieve this are provided in the chapter. PES measurements on halide perovskites could revealed the lack of chlorine incorporation, when attempts were made to fabricate methylammonium lead with both iodine and chloride and the gradual transformation of PbI2 into MAPI, when exposed to vapors of MAI, could be tracked with PES. The surface degradation of MAPI, which is not readily observable with the naked eye and detectable with optical techniques, was revealed by PES. By comparing density-of-states calculations, outputted from techniques such as DFT, to valence band spectra acquired from PES measurements, the contributions of the various orbitals of lead, iodine, and other elements, to the valence electronic structure were identified. Through the use of PES, the energy level alignment of several different halide perovskites with materials typically used as electron-selective contacts in solar cells: titanium dioxide and tin dioxide, was uncovered. Photoelectron spectroscopy is a well-established technique which will continue to be indispensable for the investigation of halide perovskites and other materials.

Acknowledgments We acknowledge financial support from the Swedish Research Council (2014
6019, 2018-04330, 2018-06465), the Swedish Energy Agency (P43549-1, P43294-1), the Swedish Foundation for Strategic Research (15
0130), and the StandUP for Energy program.

Characterization Techniques for Perovskite Solar Cell Materials

References

135

References [1] D. Briggs, M.P. Seah (Eds.), Pratical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, Wiley, 1983. [2] C.R. Brundle, A.D. Baker (Eds.), Electron Spectroscopy: Theory, Techniques and Applications, Academic Press, New York, 1978. [3] C.S. Fadley, X-ray photoelectron spectroscopy: progress and perspectives, J. Electron. Spectrosc. Relat. Phenom. 178
179 (2010) 2
32. [4] B. Philippe, M. Hahlin, K. Edstro¨m, T. Gustafsson, H. Siegbahn, H. Rensmo, Photoelectron spectroscopy for lithium battery interface studies, J. Electrochem. Soc. 163 (2016) A178
A191. [5] B. Philippe, T.J. Jacobsson, J.-P. Correa-Baena, N.K. Jena, A. Banerjee, S. Chakraborty, et al., Valence level character in a mixed perovskite material and determination of the valence band maximum from photoelectron spectroscopy: variation with photon energy, J. Phys. Chem. C 121 (2017) 26655
26666. [6] K. Siegbahn, C. Nordling, A. Fahlman, R. Nordberg, K. Hamrin, J. Hedman, et al., ESCA: Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy, 20 (1967) Nova Acta Regiae Soc. Sci. Upsaliensis Ser. IV. [7] K.M. Siegbahn, Facts, Nobelprize.org. Nobel Media AB 2014. Web. 11 Apr 2018. http://www.nobelprize. org/nobel_prizes/physics/laureates/1981/siegbahn-facts.html, (accessed 11.04.2018). [8] B. Philippe, Insights in Li-Ion Battery Interfaces Through Photoelectron Spectroscopy Depth Profiling, Acta Universitatis Upsaliensis, 2013, Doctoral thesis, comprehensive summary, ISBN 978-91-554-8662-4. [9] E.M.J. Johansson, R. Lindblad, H. Siegbahn, A. Hagfeldt, H. Rensmo, Atomic and electronic structures of interfaces in dye-sensitized, nanostructured solar cells, ChemPhysChem 15 (2014) 1006
1017. [10] S. Hu¨fner, Photoelectron Spectroscopy - Principles and Application, Springer, 2003, ISBN: 978-3-662-09280-4. [11] S. Hu¨fner, Very High Resolution Photoelectron Spectroscopy, Springer, 2007, ISBN: 978-3-540-68133-5. [12] H. Hertz, Ueber einen Einfluss des Ultravioletten Lichtes auf die Electrische Entladung, Ann. Phys. 31 (1887) 983. in German. [13] A. Einstein, Ueber einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt, Ann. Phys. 322 (1905) 132
148. in German. [14] B. Philippe, M. Saliba, J.-P. Correa-Baena, U.B. Cappel, S.-H. Turren-Cruz, M. Gra¨tzel, et al., Chemical distribution of multiple cation (Rb1, Cs1, MA1, and FA1) perovskite materials by photoelectron spectroscopy, Chem. Mater. 29 (8) (2017) 3589
3596. [15] B. Philippe, B.-w. Park, R. Lindblad, J. Oscarsson, S. Ahmadi, E.M.J. Johansson, et al., Chemical and electronic structure characterization of lead halide perovskites and stability behavior under different exposures a photoelectron spectroscopy investigation, Chem. Mater. 27 (2015) 1720
1731. [16] M. Abdi-Jalebi, Z. Andaji-Garmaroudi, S. Cacovich, C. Stavrakas, B. Philippe, J.M. Richter, et al., Maximizing and stabilizing luminescence from halide perovskites with potassium passivation, Nature 555 (2018) 497
501. [17] D.A. Shirley, Advances in Chemical Physics, Vol. 23, Wiley, New York, 1973. [18] A.D. Gandubert, E. Krebs, C. Legens, D. Costa, D. Guillaume, P. Raybaud, Optimal promoter edge decoration of CoMoS catalysts: a combined theoretical and experimental study, Catal. Today 130 (2008) 149
159. [19] D. Costa, P. Marcus, Electronic core levels of hydroxyls at the surface of chromia related to their XPS O 1s signature: A DFT 1 U study, Surf. Sci. 604 (2010) 932
938. [20] J.H. Scofield, Hartree-Slater subshell photoionization cross-sections at 1254 and 1487 eV, J. Electron. Spectrosc. Relat. Phenom. 8 (1976) 129
137. [21] J.J. Yeh, I. Lindau, Atomic subshell photoionoization cross sections and asymmetry parameters: 1 # Z # 103, At. Data Nucl. Data Tables 32 (1985) 1
155. [22] J.J. Yeh, Atomic Calculations of Photoionization Cross Sections and Asymmetry Parameters, Gordon and Breach, Langhorne, PA, 1993. [23] Elettra Sincrotrone Triest, Atomic Calculations of Photoionization Cross Sections and Asymmetry Parameters. https://vuo.elettra.trieste.it/services/elements/WebElements.html, (accessed 01.12.2017). [24] S. Tanuma, C.J. Powell, D.R. Penn, Calculations of electron inelastic mean free paths. V. Data for 14 organic compounds over the 50
2000 eV range, Surf. Interface Anal 21 (3) (1994) 165
176.

Characterization Techniques for Perovskite Solar Cell Materials

136

5. Photoelectron spectroscopy investigations of halide perovskite materials used in solar cells

[25] H. Shinotsuka, S. Tanuma, C.J. Powell, D.R. Penn, Calculations of electron inelastic mean free paths. X. Data for 41 elemental solids over the 50 eV to 200 keV range with the relativistic full Penn algorithm, Surf. Interface Anal. 47 (9) (2015) 871
888. [26] A.G. Shard, Detection limits in XPS for more than 6000 binary systems using Al and Mg Kα X-rays, Surf. Interface Anal. 46 (2014) 175
185. [27] B. Feuerbacher, B. Fitton, R.F. Willis, Introduction, in: B. Feuerbacher, B. Fitton, R.F. Willis (Eds.), Photoemission and the Electronic Properties of Surfaces, John Wiley & Sons, Noordwijk, The Netherlands, 1978. [28] ThermoScientific XPS, Analysis Features X-ray Generation. https://xpssimplified.com/xray_generation.php, (accessed 11.01.2018). [29] Scienta Omicron, HAXPES-lab. http://www.scientaomicron.com/en/products/414/1307, (accessed 11.01.2018). [30] R. Lindblad, N.K. Jena, B. Philippe, J. Oscarsson, D. Bi, A. Lindblad, et al., The electronic structure of CH3NH3PbX3 perovskites; the dependence on the halide moiety, J. Phys. Chem. C 119 (2015) 1818
1825. [31] R.T. Lewis, M.A. Kelly, Binding-energy reference in X-ray photoelectron spectroscopy of insulators, J. Electron Spectrosc. Relat. Phenom. 20 (1980) 105
115. [32] T.L. Barr, in: D. Briggs, M.P. Seah (Eds.), Practical Surface Analysis, Wiley, New York, 1983. [33] T.L. Barr, An XPS study of Si as it occurs in adsorbents, catalysts, and thin films, Appl. Surf. Sci. 15 (1983) 1
35. [34] D.W. deQuilettes, W. Zhang, V.M. Burlakov, D.J. Graham, T. Leijtens, A. Osherov, et al., Photo-induced halide redistribution in organic
inorganic perovskite films, Nat. Commun. 7 (2016) 11683. [35] J.S. Yun, J. Kim, T. Young, R.J. Patterson, D. Kim, J. Seidel, et al., Humidity-induced degradation via grain boundaries of HC(NH2)2PbI3 planar perovskite solar cells, Adv. Funct. Mater. 28 (2018) 1705363. [36] A. Alberti, I. Deretzis, G. Pellegrino, C. Bongiorno, E. Smecca, G. Mannino, et al., Similar structural dynamics for the degradation of CH3NH3PbI3 in air and in vacuum, ChemPhysChem 16 (2015) 3064. [37] K.X. Steirer, P. Schulz, G. Teeter, V. Stevanovic, M. Yang, K. Zhu, et al., Defect tolerance in methylammonium lead triiodide perovskite, ACS Energy Lett. 1 (2016) 360. [38] T.J. Jacobsson, et al., Extending the compositional space of mixed lead halide perovskites by Cs, Rb, K, and Na doping, J. Phys. Chem. C 122 (25) (2018) 13548
13557. [39] U.B. Cappel, S. Svanstro¨m, V. Lanzilotto, F.O.L. Johansson, K. Aitola, B. Philippe, et al., Partially reversible photoinduced chemical changes in a mixed-ion perovskite material for solar cells, ACS Appl. Mater. Interfaces 9 (40) (2017) 34970
34978. [40] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Efficient hybrid solar cells based on mesosuperstructured organometal halide perovskites, Science 338 (2012) 643
647. [41] E.L. Unger, A.R. Bowring, C.J. Tassone, V.L. Pool, A. Gold-Parker, R. Cheacharoen, et al., Chloride in lead chloride-derived organo-metal halides for perovskite-absorber solar cells, Chem. Mater. 26 (24) (2014) 7158
7165. [42] D.E. Starr, G. Sadoughi, E. Handick, R.G. Wilks, J.H. Alsmeier, L. Ko¨hler, et al., Direct observation of an inhomogeneous chlorine distribution in CH3NH3PbI3-xClx layers: surface depletion and interface enrichment, Energy Environ. Sci. 8 (2015) 1609
1615. [43] Q. Chen, L. Chen, F. Ye, T. Zhao, F. Tang, A. Rajagopal, et al., Ag-incorporated organic 2 inorganic perovskite films and planar heterojunction solar cells, Nano. Lett. 17 (5) (2017) 3231
3237. [44] M. Abdi-Jalebi, M.I. Dar, A. Sadhanala, S.P. Senanayak, M. Franckeviˇcius, N. Arora, et al., Impact of monovalent cation halide additives on the structural and optoelectronic properties of CH3NH3PbI3 perovskite, Adv. Energy Mater. 6 (2016) 1502472. [45] R. Lindblad, D. Bi, B.-w. Park, J. Oscarsson, M. Gorgoi, H. Siegbahn, et al., Electronic structure of TiO2/ CH3NH3PbI3 perovskite solar cell interfaces, J. Phys. Chem. Lett. 5 (2014) 648
653. [46] G. Sadoughi, D.E. Starr, E. Handick, S.D. Stranks, M. Gorgoi, R.G. Wilks, et al., Observation and mediation of the presence of metallic lead in organic 2 inorganic perovskite films, ACS Appl. Mater. Interfaces 7 (2015) 13440
13444. [47] S.M. Jain, B. Philippe, E.M.J. Johansson, B.-w. Park, H. Rensmo, T. Edvinsson, et al., Vapor phase conversion of PbI2 to CH3NH3PbI3: spectroscopic evidence for formation of an intermediate phase, J. Mater. Chem. A 4 (2016) 2630
2642. [48] T.J. Jacobsson, J.-P. Correa-Baena, E. Halvani Anaraki, B. Philippe, S.D. Stranks, M.E.F. Bouduban, et al., Unreacted PbI2 as a double-edged sword for enhancing the performance of perovskite solar cells, J. Am. Chem. Soc. 138 (2016) 10331
10343.

Characterization Techniques for Perovskite Solar Cell Materials

References

137

[49] C. Vorwerk, C. Hartmann, C. Cocchi, G. Sadoughi, S.N. Habisreutinger, R. Fe´lix, et al., Exciton-Dominated core-level absorption spectra of hybrid organic-inorganic lead halide perovskites, J. Phys. Chem. Lett. 9 (2018) 1852
1858. [50] J. Endres, D.A. Egger, M. Kulbak, R.A. Kerner, L. Zhao, S.H. Silver, et al., Valence and conduction band densities of states of metal halide perovskites: a combined experimental 2 theoretical study, J. Phys. Chem. Lett. 7 (2016) 2722
2729. [51] E.M. Miller, Y. Zhao, C.C. Mercado, S.K. Saha, J.M. Luther, K. Zhu, et al., Substrate-controlled band positions in CH3NH3PbI3 perovskite films, Phys. Chem. Chem. Phys. 16 (2014) 22122
22130. [52] B.-w. Park, B. Philippe, S.M. Jain, X. Zhang, T. Edvinsson, H. Rensmo, et al., Chemical engineering of methylammonium lead iodide/bromide perovskites: tuning of optoelectronic properties and photovoltaic performance, J. Mater. Chem. A 3 (2015) 21760
21771. [53] J.-P. Correa Baena, L. Steier, W. Tress, M. Saliba, S. Neutzner, T. Matsui, et al., Highly efficient planar perovskite solar cells through band alignment engineering, Energy Environ. Sci 8 (2015) 2928
2934.

Characterization Techniques for Perovskite Solar Cell Materials

C H A P T E R

6 Time resolved photo-induced optical spectroscopy Meysam Pazoki and Tomas Edvinsson

˚ ngstro¨m laboratory, Department of Engineering Sciences, Solid State Physics, A Uppsala University, Uppsala, Sweden

6.1 Introduction The emergence of high efficiency perovskite solar cell materials in solid state devices in 2012 [1,2] inspired a great amount of research by which the efficiency was increased from about 10% in 2012 [1] to more than 23% certified solar-to-electricity power conversion efficiency (PCE) in 2018, increased to 25.2% in 2019 [3]. Behind this fast development are previous experience within dye sensitized- organic-, and thin film solar cells and the unique physical and optical properties in the perovskite solar cell materials with the ability to synthesize high quality polycrystalline materials with varying chemical compositions using a multitude of methods. Addressing the key-role processes of the photon absorption, carrier thermalization, exciton dissociation, charge carrier transport and recombination together with other relevant phenomena such as Stark effect, defect migration and electron-phonon interactions are essential to inspect the underlying physics processes for a certain material and their importance for the photovoltaic performance. Time resolved optical spectroscopy techniques provide analysis of the optical fingerprints of the above-mentioned phenomena and the corresponding time evolutions within the times scales of femtoseconds up to seconds. The principle of photo-induced time resolved optical spectroscopy [4] is to trigger the desired process, i.e. by light excitation of the sample and subsequently investigate the consequences on the absorption or emission spectra of the material. For example in the case of contact free perovskite film, radiative recombination rate of electron and holes can be estimated from time resolved photoluminescence (PL) spectrum by recording the emission of the material—at wavelengths near the band edge—after the photoexcitation.

Characterization Techniques for Perovskite Solar Cell Materials DOI: https://doi.org/10.1016/B978-0-12-814727-6.00006-2

139

Copyright © 2020 Elsevier Inc. All rights reserved.

140

6. Time resolved photo-induced optical spectroscopy

In a perovskite film on a contact material, however, the same measurement will also be dependent on the PL quenching by transport of carriers out of the film and back contact recombination. The time resolved behavior of optical fingerprints of such phenomena can be estimated and analyzed in the terms of charge carrier dynamics as well as on the choice of heterojunction contact material. The fastest processes here cannot be examined by other techniques than electromagnetic (light, X-rays) pump-probe due to the limitations of the electronics speed, while the slowest process have their best evaluation from a combination of photo-induced optical spectroscopy techniques and modulated photo-voltage/photocurrent transient techniques mentioned in Chapter 7. A proper understanding of the photovoltaic processes is achievable through a careful design of the setup, i.e. utilize intensities of the perturbing light that are relevant for solar cell operation, to be in the relevant time scale, and also consider and rationalize the possible interfering phenomena. These techniques are well established for investigation and characterization of solar cell devices especially for the characterization of the dye sensitized solar cells [5], from which the emergence of perovskite solar cell family came to reality [6]. Here, a summary of relevant processes that can be studied by time resolved optical spectroscopy techniques together with an up-to-date presentation of some recent achievements are succinctly reviewed in this chapter with special attention to the prospects and limitations of the techniques for perovskite solar cells. Some case studies for situations corresponding to real device applications are shown with focus on understanding of local field effects and properties important for high performance and stable perovskite solar cells devices. The same methodology applies to the other devices based on organic
inorganic perovskites such as light emitting diodes (LEDs) and sensors, but we will in this chapter mainly focus on the solar cell applications.

6.2 Fundamental processes within the perovskite film Fundamental processes within the perovskite film that influence the performance during the solar cell photovoltaic action as well as a general overview of some commonly used methods are schematically presented in Fig. 6.1. The main processes consist of absorption of the incoming photons (process 1), thermalization of charge carriers, generation of free charge carriers and charge separation (process 2), transport of charges towards the contacts (process 3) and selective collection of the electrons and holes at charge selective contacts (process 4). The overall efficiency of these processes can be formulated into an external quantum efficiency (EQE) expressed in Eq. (6.1).







EQEðλÞ 5 LHEðλÞ Φsep ðλÞ Φtrans ðλÞ Φcoll

(6.1)

where LHE is the light harvesting efficiency, λ is the wavelength, Φsep and Φtrans are the quantum efficiency of the charge separation and charge transport, respectively, and Φcoll is the charge collection efficiency. In the most general case, the charge separation and transport are wavelength dependent but also depend on the time scales of the processes as well as the specific band structure and transport processes active in the material.

Characterization Techniques for Perovskite Solar Cell Materials

6.2 Fundamental processes within the perovskite film

141

FIGURE 6.1 Schematic illustration of commonly used techniques, typical time scales and processes influencing the performance of the perovskite device, including light absorption, carrier thermalization, electron-hole recombination, charge transport and extraction.

Undesired processes, however, such as bonded charge carriers or excitons and recombination of charges (process 5) as well as other processes such as trapping of the charges, defect migration, and band filling effects play important roles in the device photovoltaic behavior. An interplay of all these effects determines the working condition of the device i.e. the competition between the charge generation and recombination and their energy levels determines the charge density profile at certain temperature which is directly related to the quasi Fermi levels and thus the open circuit voltage of the device. The photo-voltage in the solar cell is the difference between the quasi Fermi levels for the electrons and holes under illumination at different electrodes and is lower than the difference in energy between the band edges within the photo-absorber. This loss in energy between the band gap and the resulting photo-voltage (ηloss) can be seen as the energy penalty for withholding the electric field separating the electrons from the holes as well as energy loss coming from un-ideal material properties. A formulation of this loss,

Characterization Techniques for Perovskite Solar Cell Materials

142

6. Time resolved photo-induced optical spectroscopy

(ηsep), was derived by Shockley and Queisser in the 1960s and can by neglecting higher order terms be written as Eq. (6.2) " # 8πðkB TÞ2 n2 Eg Voc 5 Eg 2 ηsep 5 Eg 2 kB T ln αLΦrec (6.2) c 2 h3 jgen where φrec is the ratio between the non-radiative and radiative recombination rates Voc is the open circuit voltage, Eg is the band gap, kB is the Boltzmann’s constant, T is the temperature, c is the speed of light in vacuum, h Planck’s constant, n is the refractive index, jgen is the rate of photon absorption in the AM1.5 spectra, α is the absorption coefficient, Φrec is the ratio between the non-radiative and radiative recombination rates, and L is the minority carrier diffusion length; and should be replaced with the material thickness, d, if d , L. Several of these parameters are material dependent and the loss is then naturally dependent on the choice of photo-absorber and the purity of the material. Typical experimental values of voltage loss for state-of-the-art materials in solar cells are: 0.3 eV in GaAs, 0.36 eV in silicon, 0.4 eV in hybrid perovskites, 0.4 eV in InP, 0.4 eV in CIGS, 0.6 eV in CdTe, 0.61 eV in amorphous silicon, and . 0.6 eV in organic solar cells. The hybrid perovskites show a quite remarkable defect tolerance and display low voltage loss and will be one of the topics addressed in Section 6.2.1.

6.2.1 Processes at open circuit condition At open circuit condition, the electron generation rate (G) is equal to the recombination rate (R) G5R

(6.3)

Therefore, with lower recombination rate at each fixed light intensity, the steady state density of electron and holes at valence band (VB) and conduction band (CB) would be higher leading to a higher photo-voltage. Interface recombination [7], ionic movement dependent recombination [8], effect of polarizability domains [9,10] as well as the density of the sub band gap traps [11] within the film are determining and impactful parameters influencing the average charge density dependent recombination rate in the device. The generation rate depends on the steady state absorption spectrum of the light absorber layer and on atomistic level the its relation to the density of empty states at perovskite CB, the number of available states at top VB and the transition probability and the dipole strength for optical transition. Depending on the wavelength dependency of absorption spectrum, a steady state charge profile (electrons and holes with densities n and p) builds up within the film at open circuit conditions. The relation between the device open circuit voltage and charge carrier densities can be described by Eq. 6.4 [11]:   Nc Nv e:Voc 5 Eg 2 kB TLn (6.4) np where e is the elementary charge, and Nc/v are the total available density of charge carriers. Eq. 6.4 can be seen as a phenomenological version of Eq. 6.2, where the effective

Characterization Techniques for Perovskite Solar Cell Materials

6.2 Fundamental processes within the perovskite film

143

number of charge carriers can be estimated without factorizing the contributing factors from the material properties. The loss of the effective number of charge carriers via recombination could be classified to the interface recombination, bulk recombination and trap assisted recombination, all of which can be classified into radiative and non-radiative recombination with their accumulated ratios expressed as Φrec in Eq. 6.2. The number density, energy states and location of traps within the film are of crucial importance for recombination kinetics for perovskite solar cells. The response by photo-excitation is often dependent on the intensity and history of the film under illumination and should be considered when performing photoinduced absorption spectroscopy as will be discussed later in this chapter. The radiative recombination can be recycled [12] and re-implemented for charge carrier generation, while the non-radiative recombination through the sub band gap states lower the device efficiency from the expected theoretical limit. The latter has been considered as the main bottle neck for further increase of the efficiency at the time for world record 23% perovskite solar cell devices [11] and triggered more research on the modification of crystal growth techniques for obtaining higher quality films. The time evolution of photogenerated carriers n’ can be described by an average recombination rate (k) which is inversely proportional with carrier life time (τ). dn0 1 5 kn0 and kðn0 Þ 5 kradiative 1 k1 1 k2 1 . . . 5 τ ðn0 Þ dt

(6.5)

The recombination rate has contribution from band-to-band radiative recombination (krad) and different non-radiative recombination rates i.e. trap assisted, ionic movement assisted or interface recombination.

6.2.2 Processes at short circuit condition The short circuit current density (Jsc) at each specific wavelength λ is directly proportional to light harvesting efficiency (LHE) and charge collection efficiency (Φcol) expressed in Eq. 6.1 by considering the charge transport within the layers and charge transfer kinetics at interfaces within the scope of charge collection: JscðλÞ ~ LHEðλÞ:Φcol ðλÞ

(6.6)

LHE ðλÞ 5 1 2 102AðλÞ

(6.7)

Φcol ðλÞ 5 1 2 τtrans =τrec

(6.8)

LHE depends on the absorption spectrum of the light absorber (A(λ)) (Eq. 6.7). τtrans and τrec are the corresponding recombination lifetime and transport time of charge carriers. The transport time depends on the carrier mobility that in single crystal MAPbI3 perovskite correlates strongly with electron-phonon interactions [13,14]. The response of the device at short circuit can be affected by history of the film under illumination [15] and can be discussed within framework of trap formation/annihilation, interaction with polarizability domains and photo-induced trap migration that are shortly discussed in Section 6.6. Due to the strong light absorption and charge conduction abilities, the

Characterization Techniques for Perovskite Solar Cell Materials

144

6. Time resolved photo-induced optical spectroscopy

short circuit current densities in world record mixed perovskite solar cells are already close to the maximum theoretical limit [16] with close to 100% internal quantum efficiencies [17].

6.2.3 Devices under working conditions The current voltage (I
V) relation for solar cell devices has the general form of diode equation (Eq. 6.9) in which the total resistance of the device (R) including all the series, ionic movement, transport and shunt resistances, determining the shape of the current voltage curve and therefore affect the fill factor (FF) and consequently also the power conversion efficiency of the device.     eV I 5 I0 exp 21 (6.9) nkB T   1 eV ~ exp (6.10) R nkB T Eqs. (6.9) and (6.10) describe the diode equation and the resistivity where e, kB and n are elementary charge, Boltzmann constant and the diode ideality factor, respectively. The transport resistance within the perovskite film is directly proportional to the mobility, while the shunt resistance depends on electron lifetime and different recombination processes. There are different processes such as trapping of electrons in defects, defect migration, and band filling effects that play roles for the above-mentioned processes under working conditions. Table 6.1 presents the related time scales of important phenomena for perovskite solar cell devices. A proper evaluation of the effects, the relevant time scale and possible interfering phenomena which are necessary to be considered for obtaining TABLE 6.1 Typical time scales for photovoltaic processes in the MAPbI3 based perovskite solar cells. Process

Time scale

Reference

Excitation

Instantaneous

[74]

Exciton dissociation

A few ps

[74]

Band filling

ps-ns

[22]

recombination

ns- a few μs

[36,46,75]

transport

ns-μs

[76]

Charge transfer

tens of ps

[46]

Stark effect

ns-s

[20]

Thermalization

fs-low ps

[77]

Electron-phonon scattering time

fs

[70]

See also Ref. [14] for a detailed discussion.

Characterization Techniques for Perovskite Solar Cell Materials

6.3 Light absorption and charge separation kinetics

145

the relevant information as well as identifying when the time scales of the different processes overlap. A detailed summary about the charge carrier dynamics in perovskite solar cell materials can be found in Ref. [14]. All these processes are intimately interconnected and by changing one part of the device or composition/thickness of the perovskite light absorber, many of these parameters can change. The latter stress the importance of characterization of these phenomena under relevant and well-defined conditions for finding suitable criteria for materials and device optimization. Our focus in this chapter would be to describe the concepts based on the MAPbI3 hybrid perovskite, which also is one of the most thoroughly investigated materials within the perovskite family. The approach would be the same for high efficiency mixed cation and mixed halide perovskite materials.

6.3 Light absorption and charge separation kinetics Incoming photons with energies higher than the band gap are able to excite electrons from the energy states in the valence band (VB) to states in the conduction band (CB), leaving a hole in the valence band. Excited electrons (holes) thermalize and cool down to the lowest unoccupied states in the CB (VB) and liberate the additional energy as phonons to the lattice. For indirect band gaps, which is not the case for the common lead halide perovskite materials, a conservation of energy and momentum, should be fulfilled i.e. a transfer of additional momentum is essential for the electron to be excited and thus implies that the light absorption occurs at a lower rate compared to direct band gap material. For metal halogen based perovskite solar cell materials, a wide range of band gaps from less than 1.2 eV (for CH3NH3Sn0.5Pb0.5I3) [18] to more than 4 eV (for CH3NH3BaI3) [19] have been reported and tuning of the gap by metal, monovalent cation and anion exchange as well as mixed approach is under ongoing investigations (see Chapters 1 and 3 for a detailed description). The band gap and the absorption coefficient are of fundamental importance for device efficiency in the stand-alone or tandem applications, and relates to the device color, transparency, efficiency per mass and the material cost (as stressed in Chapter 11), while the band gap tuning also can be detrimental for other processes resulting in lower power conversion efficiency. In MAPbI3, a halide-to-metal charge transfer is the main process for the VB to CB transition, while a charge transfer from iodide localized electrons to the organic cation is also possible in blue and UV light according to density functional theory calculations [20]. In the latter case, a negatively charged inorganic network and positively charged organic molecule can approach a state of lower charges; in which the material behavior under illumination can change significantly. In particular, this can trigger formation of CH3NH2 and a free H1 which subsequently can couple to I2 and by that diffuse more easily as HI in the material. Different excitation wavelengths (i.e. for MAPbI3, near band edge 760 nm and blue light 420 nm) also result in different thermalization degrees. Recently, excitation wavelength dependence of ionic movement in the perovskite devices by which the degree of thermalization was investigated and revealed that the amount of released phonons using near bandgap light (red light) and light absorbing deeper in the band structure (blue light)

Characterization Techniques for Perovskite Solar Cell Materials

146

6. Time resolved photo-induced optical spectroscopy

are significantly different and thus that thermally assisted ion displacements play an important role for the device current voltage behavior and hysteresis [20,21]. Transient absorption spectrum of the MAPbI3 perovskite show two distinct peaks at 470 nm and near band edge 750 nm. The near band edge peak has been considered as a fingerprint transition to analyze Stark effects [20], Burstein-Mott effects [22], excited state properties [23], excitonic features [24], while the nature of the first band is still less investigated. In some cases the transitions are discussed in terms of two valence band transitions to one CB [22] and the second band as a charge transfer state to higher CB states [25]. Order disorder phases and phase mixtures within the hybrid perovskites have also been analyzed in terms of multiple CB and VB states in Ref. [26]. The generated electron and hole pairs might be attracted by Columbic type forces; into a bounded electron-hole pair, an exciton. Presence of strong excitons can be beneficial for light emitting devices but not favorable for photovoltaic devices in which one desires to separate the excitons into free charge carriers in the bulk or at charge selective contacts. High dielectric constant of the lead perovskite is one of the key properties leading to high dielectric screening of Columbic forces and thus lowers exciton binding energy in 3D perovskites which can be overcame by thermal energy at room temperature (B 25 meV) and is beneficial for high photovoltaic efficiency of the device [27]. The reported values for exciton binding energies for MAPbI3 are in the range of 2
63 meV [28] which is of course dependent on the crystalline phase and thus also dependent on the working temperature of the device. Therefore, the charge separation within the tetragonal and cubic phases of MAPbI3, which are the most relevant phases in the device under working condition, are close to 100%. Typical mixed perovskite films spontaneously form free un-bonded charge carriers in the femtosecond regime (Table 6.1). In polymer and organic solar cells, however the charge separation of the excitons to the free carriers is a challenging task limiting the efficiency of the device. The extended perovskite solar cell family, two-dimensional (2D) and bromine based perovskites [29,30] as well as bismuth perovskites [31] have excitons with higher binding energy which can be beneficial for LED applications but less beneficial for solar cell applications. Beard and co-workers have compared the trap assisted and Auger recombination kinetics in bromine and iodine based lead perovskites through time resolved photoinduced spectroscopy techniques. Different exciton binding energies is responsible for higher recombination rates in bromine based perovskites [32]. A analytical expression for different carrier recombination kinetics in lead perovskite films, Eq. 6.5, has been derived in Ref. [33]. Excitonic fingerprint wavelengths are close to the band edge usually showing up as sharp peaks near the band edge in the absorption spectrum. As an example, in Ref. [34] it has been suggested that Br rich compositions shows strong excitonic peaks in mixed perovskite solar cell family near the band edge which are detectable in the PL spectra. There is no direct and certain method to find excitonic peaks in the UV
vis spectrum at room temperature since many different phenomena have their optical fingerprints near the band edge, while the overall behavior of the peaks in different conditions, temperatures, etc can be studied and fitted within exciton-theories criteria. Exciton binding energy can be evaluated by several approaches i.e. fitting of Elliot’s

Characterization Techniques for Perovskite Solar Cell Materials

6.4 Charge recombination, transfer and transport kinetics

147

theory to the band edge curvature [35], thermally activated PL quenching [36] or thermal broadening of the absorption onset [37]. Pump probe spectroscopy has also been implemented widely to study exciton formation and dissociation within the perovskite solar cells. The determination of the excitons however is not very straightforward, due to the large variation of dielectric constant depending on frequency, temperature, light intensity and the interpretation of near band edge features [38]. Exciton dynamics became more important in the materials which carry higher binding energy excitons such as 2D perovskites and bromide based lead perovskite that are investigated i.e. in Ref. [29,30].

6.4 Charge recombination, transfer and transport kinetics Photoluminescence spectra and the time evolution of the PL peaks have been extensively used to study the charge recombination kinetics within the perovskite solar cell materials family. Since the electron lifetime and transport time within the film are on the orders of a few microseconds and are complicated by ion displacements in some cases, the approaches taken fromother new generation solar cells with longer electronic life times, such as dye sensitized and polymer solar cell technologies, have to be applied with care (see Chapter 7). In general, three types of electron-hole recombination are commonly considered in which the time derivative of the charge density (dn/dt) is the linear, second and third power of the charge density within the film [14,33] expressed in Eqs. 6.11 and 6.12. dn 5 2 k3 n3 2 k2 n2 2 k1 n dt

(6.11)

R 5 k3 n2 1 k2 n 1 k1

(6.12)

They can be ascribed to trap assisted (linear term), free charge carrier (second order) and Auger (third order) recombination kinetics [14,39] in which just the free charge recombination is radiative and there is no strong signature from radiative trap assisted recombination in perovskites yet. R is total recombination rate. Different charge recombination kinetics via time and spatially resolved micro-photoluminescence spectroscopy of large and small grain MAPbI3 perovskite films have been explored by Mohite et al. in Ref. [6]. The nature of traps within MAPbI3 have been evaluated by density functional theory calculations [39,40] and experimental approaches [41]. The results show the lack of deep traps within the bandgap, where the processes from recombination to deep traps seem to play no dominant role in the photovoltaic performance for high efficiency devices based on MAPbI3 and mixed perovskites. Defect densities of about 109
1014 have been reported for the MAPbI3 perovskite [39]. Although density of traps is quite low within the single crystal lead halide perovskite materials [42], the trap density in polycrystalline materials and perovskites based on other metals can be substantially higher and could

Characterization Techniques for Perovskite Solar Cell Materials

148

6. Time resolved photo-induced optical spectroscopy

dominate the recombination kinetics in such cases. On the other hand, trap assisted recombination depends on the light intensity, in higher light intensities, traps are already filled with charge carriers and show less impact on the charge carrier recombination during the perovskite solar cell device operation [43]. Very recently, Abdi-Jalebi et al. showed that the presence of monovalent cation additives can reduce the near band edge trap density and increase the lifetime of the carriers [44]. In the absence of a charge-selective layer, the recombination of free electrons and holes is mainly responsible for the time dependent PL spectra in a thin film. However, in the presence of a charge selective layer, diffusion and collection of the charge carriers towards the contacts compete with electron-hole recombination [6]. The decay time of the Pl peak to 1/e of its initial value is a fair evaluation of the corresponding lifetimes from which one can estimate the diffusion length of carriers within the film (see Chapter 3 for a detailed description and examples). However, other effects have recently been shown to interfere within the observed spectra where special care has to be taken when interpreting PL data (the fingerprint wavelength for the near band edge transition is around 780 nm for MAPbI3 and mixed perovskites with optical bandgap 1.6 eV). A phase segregation within the mixed perovskite solar cells can occur, especially when high amount of halogens with significantly different lead-halogen bond lengths are used, which can affect the time evolution of the PL spectra. This can for example occur when the amount of bromide is not negligible within a mixed halide perovskite material. The time dependent decay of the main perovskite PL spectrum can then not exclusively be assigned to only the electron-hole recombination anymore as the change in the spectra also can be inferred and ascribed to a phase segregation process [34]. Here, the main mixed perovskite film can be segregated into lead bromine perovskite (finger print wavelength 550 nm bandgap 2.1 eV) and lead iodine perovskite (finger print wavelength close to 780 nm) rich regions where another PL peak arise simultaneously at the expense of a decrease of the main phase PL peak [45]. A comprehensive study about temperature dependence of mono-, bi-molecular as well Auger recombination analyzed via transient PL measurements at different excitation fluencies as well as pump-probe spectroscopy and fitting the data with Eq. 6.6, is reported in Ref. [28]. Charge transfer kinetics for the MAPbI3 films with different scaffolds sandwiched by the typical TiO2 and spiroOMeTAD charge selective layers has been investigated by ultrafast transient absorption spectroscopy [46]. Xing et al. have estimated the diffusion lengths of electron and holes within the MAPbI3 perovskite by PL decay measurements at ns time scale. The decay time of PL peak in the presence of charge selective layers has been compared to the case of an absent selective layer and considering the thickness of the film, a diffusion length of about 100 nm was concluded [23]. Moser et al. reported that the charge separation at both electron and hole selective layers occur close to simultaneously within the timescale of femto- to picosecond time scale [46]. An imbalanced charge transfer results in accumulation of charges in contacts and suppressing the photovoltaic performance of the perovskite solar cell in the case of implementation of improper charge selective layers. De-doping of the perovskite absorber can help to achieve a better balance between the electron and hole transport within the film and thus the device PCE [44].

Characterization Techniques for Perovskite Solar Cell Materials

6.5 Stark effects, defects and defect migration in perovskite solar cells

149

6.5 Stark effects, defects and defect migration in perovskite solar cells 6.5.1 Stark effects Photo-induced near band edge optical absorption changes contain important information about the local electric field and thus the local material properties and give insights into how different processes occur within the device after excitation. One near band edge effect is the band filling effect (Burstein-Moss) [22] that bleach the photo-induced absorption spectrum where pre-filled states by electrons and holes can widen the observable optical band gap. These effects are dependent on the light intensity and can remain on the femtosecond to nanosecond time scales and thus overlap with Stark effect features on the GHz to THz frequency scales. Charge accumulation and recombination together with band shifts within the perovskite film can be investigated by kinetics of band filling effects [22]. The optical Stark effect can be defined as the spectral change caused by the presence of an electric field and can be photo-induced - or triggered by ab externally applied electric fields. The effect can be analyzed in terms of the small frequency shift Δν of a selected optical transition due to the electric field E, which is related to the change in the dipole moment between ground-state and excited-state Δμ and the change in polarizability Δα [20,47]. 1 h 3 Δν 5 E:Δμ 2 E:Δα:E 2

(6.13)

Here h is Planck’s constant, Δν the frequency shift and Δα the change in polarizability. The resulting experimentally measured absorption change ΔA is a function of electric field E. For the details of the quantum mechanical aspects of the Stark effect see the supporting information of Pazoki et al. [20]. The criteria for observing the optical fingerprints of the Stark effect can be distinguished according to Eq. 6.13: in the case that E is a small electric field and thus be considered a linear perturbation. Based on the symmetries of the system Hamiltonian, the near band edge absorption changes can be expanded in a power series with respect to the electric field i.e. in terms of first order Stark effect (linear in E) and second order Stark effect (second power of the electric field) that can be experimentally measured and analyzed via: ΔA 3 h 5 2

dA 1 dA E:μ 2 E:Δα:E 1 . . . dv 2 dv

(6.14)

The first observation of a Stark effect in perovskite solar cells was reported by De Angelis and co-workers [48] in 2014. The photo-induced absorption spectroscopy (PIA) [20] spectra of the perovskite solar cell materials have been shown to have Stark effect characteristics. In photo-induced absorption spectroscopy the perturbation of the system under illumination can be done by small or large amplitude pulsed lights and the material absorption changes in presence and absence of perturbation can be measured and finally a delta absorption spectrum can be extracted [20]. Fig. 6.2 shows a schematics of PIA

Characterization Techniques for Perovskite Solar Cell Materials

FIGURE 6.2 (A) A schematic of PIA setup for Stark spectroscopy of perovskite solar cell materials. Blue (light gray in print version) and red (dark gray in print version) perturbation lights implemented to probe the Stark effects and the photo induced optical changes recorded by the Si detector. (B) A typical frequency dependent Stark spectrum measured with different photon energy excitations [blue (light gray in print version) and red (dark gray in print version)] from a mixed lead halide perovskite device indicating the extension of the Stark effect to the time scales of several seconds. Figure adapted from Ref. [20] M. Pazoki, T.J. Jacobsson, J. Kullgren, E.M.J. Johansson, A. Hagfeldt, G. Boschloo, et al., Photoinduced stark effects and mechanism of ion displacement in perovskite solar cell materials, ACS Nano 11 (2017) 2823
2834. https://doi.org/10.1021/acsnano.6b07916. Copyright (2017) American Chemical Society.

6.5 Stark effects, defects and defect migration in perovskite solar cells

151

spectroscopy together with typical frequency dependent Stark measurements of the perovskite solar cells. In the PIA setup, the accuracy and sensitivity of the detector are high, making the absorption change measurements of up to 1027 feasible. The latter is a superior characteristic of PIA compared to conventional transient absorption spectroscopy (TAS) spectrum in laser spectroscopy. High sensitivity of PIA Stark spectrum can detect unique near band edge fingerprints of the perovskite material and is thus able to be implemented for the studies of phase segregation processes during light illumination of mixed anion perovskite films. In Fig. 6.3, the Stark effect with blue light (470 nm) and red light (630 nm) excitation is shown for lead halide perovskites with different A-cation dipole strength (Cs 1 -FA 1 -MA 1 ). An A-site cation with a stronger dipole is seen to charge compensate a locally created field better than a cation with lower dipole, and red light excitation show a frequency dependence in contrast to the blue light excitation [20]. The effect is also apparent in delayed Voc decay in high efficient mixed cation hybrid perovskite devices (PCE . 19%) when illuminated with blue light. The picture emerging is that blue light provides sufficient excess energy after thermalization to overcome the iodide displacement activation energy illustrated in Fig. 6.3 (F). On the other hand, the perturbed electric fields can also be applied externally and not through the photo induced electric fields. In this case, the same setup of PIA can be used

FIGURE 6.3 (A) Cation dependent Stark shift, (B) and (C) first (1H), second (2H), and third harmonics (3H) changes in DA at intermediate frequencies (1 kHz) and high frequencies (10kHz) for MAPbI3 and FAPbI3, respectively. (D) The frequency dependent photoinduced Stark effect with red (630 nm) and blue light (470 nm) excitation for MAPbI3, (E) Blue and red excitation dependence on the VOC decay in a 19% efficient mixed cation hybrid perovskite device, and (F) illustration of the maximum available thermalization energy under red and blue light excitation and the LO-phonon activation in the perovskite structure. Figure adapted from Ref. [20] M. Pazoki, T.J. Jacobsson, J. Kullgren, E.M.J. Johansson, A. Hagfeldt, G. Boschloo, et al., Photoinduced stark effects and mechanism of ion displacement in perovskite solar cell materials, ACS Nano 11 (2017) 28232834. https://doi.org/10.1021/ acsnano.6b07916. Copyright (2017) American Chemical Society.

Characterization Techniques for Perovskite Solar Cell Materials

152

6. Time resolved photo-induced optical spectroscopy

without perturbing excitations and instead by using applied electric field on the sample, which then transforms the technique into electro-adsorption spectroscopy [20]. Investigated second harmonic electro-reflectance [48] spectra of MAPbI3, and FAPbI3 have been reported by Wu et al. [49].

6.5.2 Dielectric relaxation Perovskite solar cell materials are soft in the terms of possible structural changes during photovoltaic action of the device and include energetically favorable photo-induced structural changes [50], such as ionic movement [51], trap formation [45], annihilation [52], reorientation of dipolar cations, as well as interfacial changes [53]. These photoinduced changes can be both limiting the performance but also beneficial for the photovoltaic operation of a device in a larger perspective i.e. tilting of the metal halogen octahedra in inter-atomistic scales can cause optical absorption changes [54]. Reorientation of the dipolar cations can modify the band gap [55] and charge carrier mobility [55], and ionic movement affect the distribution of the ionic species in the material and thus the local charge recombination [8] and even device stability which is reflected in the current voltage hysteresis of the device. These structural changes often cause changes in local electric fields, which can be studied by Stark spectroscopy during the photovoltaic operation of the device. So far, the Stark effects in MAPbI3 have mainly been used for analysis within the framework of interfacial charges in dye-sensitized solar cells [49] and the local electric fields due to the structural changes such as ionic movement and tilting of the octahedral [20]. These structural changes can be counted as dielectric response of the material. The latter is interestingly depending on the dipole moment of the monovalent cation as recently reported [20]. Higher rotational freedom and dipolar moment of the monovalent cation can screen the local electric fields after the photoexcitation and thus resulting in a lower-intensity Stark effect. The phenomenonis related to the instantaneous dielectric response of the monovalent cation and the local distortions within the PbI6 octahedra in the perovskite films.

6.5.3 Relevance to defects Defect formation, ion migration and photo induced defect migration have been investigated widely and are related to current voltage hysteresis effects, optimization of the photovoltaic efficiency, stability and the fundamental photo-physical processes within the perovskite solar cell materials and devices [21]. Depending on the formation energy and energy state of the trap, they can have optical fingerprints in the PL or absorption spectrum or even indirectly affect other relevant processes such as photo-induced Stark effects outlined above and charge recombination. Light illumination can cause defects [45] and reversibly healed in the dark and thus form a situation with dynamically formed and healed defects; their presence and density were analyzed from PL spectrum in Ref. [45]. On the other hand, De Angelis and coworkers reported the annihilation of defects after illumination which was studied by illumination-time dependent PL kinetics near the band edge [52]. The latter depended

Characterization Techniques for Perovskite Solar Cell Materials

6.6 Electron-phonon interactions and polarons in CH3NH3PbI3 perovskites

153

strongly on the availability of oxygen for the film during the illumination. The defects play important role in the photovoltaic action of the device, as they dictate the maximum voltage and efficiency of the perovskite devices [56], can migrate within the film and cause phenomena such as current-voltage hysteresis and further can accumulate at interfaces and cause degradation processes within the perovskite and at the interfaces [57]. Proper encapsulation of the device [58] or presence of very thin intermediate stable layers such as two-dimensional perovskites [59] turns out to be beneficial for achieving a good device stability. As defects migration play important roles within the device, more investigations, are needed to fully rationalize and understand how the defect migration affects the device operation and stability under working conditions which encourages further characterization of their optical fingerprints by photo-induced absorption spectroscopy studies. Photoinduced absorption changes in the perovskite solar cell materials can give information on local field changes and has recently been used to study thermalization effects, dipolar response, ion-, and defect migration [20,21,60].

6.5.4 Comparison with other solar cell technologies Stark spectroscopy is not limited to perovskite solar cells and have been used for characterization of dye sensitized [49,61], and organic solar cells [62] as well as quantum confined systems [63,64]. The time constants and the origins of the spectral shifts in different material classes, however, are very different making a full comparison in-between fields challenging. In dye sensitized solar cells the processes involved are rather widely explored: photo-injected electrons within the individual nanoparticles together with surrounding counter ions from the electrolyte within the Helmholtz layer are able to produce perpendicular electric fields relative to the dye. This particular configuration makes it possible to study the Helmholtz layer capacitance and dye coverage [61] within time scales of micro to milliseconds. Electron-hole lifetime here is a limit since by the electron hole recombination the photo-induced electric field would disappear. In perovskite solar cells this time-constants are extended from ns to several seconds and is intimately suggested to be related to both octahedral tilting and ion migration [20]. Therefore, together with Stark features, many other interfering phenomena can occur simultaneously and contribute to the photo-induced absorption spectrum. In organic solar cells Stark spectroscopy has been used to characterize the interfacial electric fields resulting from the charge separation at interfaces via ultra-fast time resolved optical spectroscopy in the fs regime [62]. As described in the article Stark realities [47], the Stark spectroscopy is a promising tool not fully explored so far for the perovskite solar cell materials and worth further investigations.

6.6 Electron-phonon interactions and polarons in CH3NH3PbI3 perovskites Thermalization: Immediately after the light absorption, the generated charge carriers (here called hot carriers) cool down to the CB and VB edges by transferring the excess energy to the lattice through electron (hole)-phonon interactions (process 2 in Fig. 6.1). The relevant time scale is sub or a few ps for many material systems. Slow hot-hole cooling in

Characterization Techniques for Perovskite Solar Cell Materials

154

6. Time resolved photo-induced optical spectroscopy

lead iodide perovskite films has been experimentally measured [65] by time resolved techniques and demonstrated by DFT calculations as well [66]. The phrase ‘slow’ for cooling processes here still relates to sub ps scales, but are referred to as slow since carrier cooling in perovskite films are slower than the ones found in silicon and CIGS films, but also come quite naturally from the heavier elements included in the lead-halide perovskites. Photo-induced absorption spectroscopy has been used for determining the carrier cooling time constants for the planar MAPbI3 films [65] which is one order of magnitude slower than carrier cooling in GaAs films. Recording the time resolved transient absorption spectrum or PL spectra under different temperatures (Fig. 6.4) the energy distribution of hot-electronic states can give information about the carrier cooling dynamics. Frost et al. have showed that the thermalization kinetics for above band-gap photoexcitation likely includes an energy transfer to large polaronic states through the electron (hole)-phonon interactions [67]. The thermalization cooling rate has been estimated by fitting the data with Fro¨hlisch model and determined to be 78 meVps21 for MAPbI3 films [67]. These cooling kinetics are importantly relevant for hot-carrier solar cell applications that are theoretically able of passing the Schockley-Queisser limit [68] as expressed in Eq. 6.2 within special photovoltaic regimes. The published data about the carrier cooling, so far, show an advantage of perovskite solar cells in comparison to other thin film solar cell technologies in terms of the carrier cooling, by having an order of magnitude slower cooling time and could be worth exploring further. Impacts on mobility and polarons: The Drude model [69] has been widely used for describing transport phenomena in perovskite solar cells [14]. Based on the Drude model, the static conductivity (σ) of the carriers can be described by σ5

ne2 τ m

(6.15)

FIGURE 6.4 (A) Hot carrier cooling measured by time resolved transient absorption spectroscopy from MAPbI3 films. Spectral absorption changes during the time gives information about the energetic distribution of carriers and cooling dynamics. Temperature dependent PL decay of (B) FAPbI3 and (C) MAPbI3 perovskites. The PL line broadening analysis determines the electron-phonon interaction within the films. (A) Reproduced from Ref. [65] Y. Yang, D.P. Ostrowski, R.M. France, K. Zhu, J. van de Lagemaat, J.M. Luther, et al., Observation of a hot-phonon bottleneck in lead-iodide perovskites. Nat. Photonics. 10 (2015) 54
59. https://doi.org/10.1038/nphoton.2015.213, with permission from permission from Springer Nature. (B and C) Reproduced from Ref. [70] A.D. Wright, C. Verdi, R.L. Milot, G.E. Eperon, M.A. Pe´rez-Osorio, H.J. Snaith, et al. Electron
phonon coupling in hybrid lead halide perovskites, Nat. Commun. 7 (2016) 11775. https://doi.org/10.1038/ncomms11755, under the Creative Commons 4.0 license.

Characterization Techniques for Perovskite Solar Cell Materials

6.7 Summary and outlook

155

where m* is the effective mass, e the elementary charge, and τ the carrier scattering lifetime. Carrier scattering can be categorized into defect scattering, phonon scattering, or carrier-carrier scatterings or other system dependent factors depending on the physical state of the material and perturbation. Scattering of already-cooled-down charge carriers with phonons is considered as a limiting temperature dependent factor which limits the carrier mobility and charge transport within the perovskite films; temperature dependence of the photoluminescence (PL) peak intensity possess deterministic information for the electron-phonon interactions and its effect on the carrier motilities [70]. A few fs scattering time has been measured for the scattering of electrons via lattice phonons by Terahertz conductivity measurement [71]. The estimated charge carrier mobilities for perovskite solar cell materials lie in the range of 10 cm2V21s21, [14] in comparison to CIGS and amorphous Si solar cells that typically have the corresponding values of about 0.5 and 1 cm2V21s21 respectively [72,73]. The interaction of electrons with polaron based lattice distortions can play an important role in photovoltaic action of perovskite solar cells [67,74]. This is partly reflected by the effects on the electronic current, where defects and defect migration have been reported in the framework of device hysteresis i.e. in ionic movement dependent electron hole recombination [8] and current decay kinetics [15].

6.7 Summary and outlook The principles of photo-induced time resolved optical spectroscopy have been briefly reviewed with respect to the characteristic time scales of the processes occurring in hybrid perovskite solar cells. The spectral response during and after the incoming photon energies have been transformed into charge carriers, can give important information on the fundamental processes and thus on identification of the beneficial and detrimental processes for the photovoltaic performance. The time scales of the main fundamental processes within perovskite solar cell were outlined as well as an up-to-date presentation of the present understanding of the processes as characterized with these techniques. The processes varies from femtosecond to several seconds and many are strongly dependent on the chemical composition and crystal quality of the material. Phenomena such as local phase segregation, photo-induced trap formation, ion migration, and illumination history dependent carrier dynamics are more unique for perovskite solar cells and need specific attention for relevant device characterization. Special attention is given to photo-induced absorption spectroscopy used as a Stark spectroscopy where the absorption changes from the change in the local electric fields are analyzed. Studies on carrier cooling dynamics and electron-phonon interactions can further shed lights on the fundamental aspects as well as accessible mobilities and PCEs for practical device applications. Analysis of defect formation and dynamics under illumination and the impact on the device performance and stability are discussed as well which are necessary for future device optimization towards the commercialization of perovskites. The approach as well as more conventional ultra-fast spectroscopies can reveal many relevant

Characterization Techniques for Perovskite Solar Cell Materials

156

6. Time resolved photo-induced optical spectroscopy

underlying physical processes occurring in the system, their fingerprints and how they can be detected, and bears promise also for future improved understanding as well as widening of possible applications.

References [1] H.-S. Kim, C.R. Lee, J.H. Im, K.B. Lee, T. Moehl, A. Marchioro, et al., Lead Iodide perovskite sensitized allsolid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%, Sci. Rep. 2 (2012) 591
597. Available from: https://doi.org/10.1038/srep00591. [2] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Efficient hybrid solar cells based on mesosuperstructured organometal halide perovskites, Science 338 (2012) 643
647. Available from: https://doi. org/10.1126/science.1228604 (80-.). [3] National Renewable Energy Laboratory (NREL), Nationl Cent. Photovoltaics. https://www.nrel.gov/pv/ cell-efficiency.html, 2019. [4] A. Balocchi, T. Amand, X. Marie, Time-resolved optical spectroscopy, in: N. Balkan, A. Patane (Eds.), Semicond. Res. Springer Ser. InMaterials Sci, 150, Springer, Berlin, 2012, p. 223. Available from: https://doi. org/10.1007/978-3-642-23351-7. [5] M. Pazoki, U.B. Cappel, E.M.J. Johansson, A. Hagfeldt, G. Boschloo, Characterization techniques for dyesensitized solar cells, Energy Environ. Sci. 10 (2017) 672
709. Available from: https://doi.org/10.1039/ C6EE02732F. [6] W. Nie, et al., High-efficiency solution-processed perovskite solar cells with millimeter-scale grains, Science 347 (2015) 522
525. Available from: https://doi.org/10.1126/science.aaa0472. [7] P. Calado, A.M. Telford, D. Bryant, X. Li, J. Nelson, B.C. O’Regan, et al., Evidence for ion migration in hybrid perovskite solar cells with minimal hysteresis, Nat. Commun. 7 (2016) 13831. Available from: https://doi. org/10.1038/ncomms13831. [8] A. Pockett, G. Eperon, N. Sakai, H. Snaith, L.M. Peter, P.J. Cameron, Microseconds, milliseconds and seconds: deconvoluting the dynamic behaviour of planar perovskite solar cells, PCCP 19 (2017) 5959
5970. Available from: https://doi.org/10.1039/C6CP08424A. [9] S. Meloni, T. Moehl, W. Tress, M. Franckeviˇcius, M. Saliba, Y.H. Lee, et al., Ionic polarization-induced current
voltage hysteresis in CH3NH3PbX3 perovskite solar cells, Nat. Commun. 7 (2016) 10334. Available from: https://doi.org/10.1038/ncomms10334. [10] J.M. Frost, K.T. Butler, A. Walsh, Molecular ferroelectric contributions to anomalous hysteresis in hybrid perovskite solar cells, APL Mater. 2 (2014) 081506. Available from: https://doi.org/10.1063/1.4890246. [11] W. Tress, Perovskite solar cells on the way to their radiative efficiency limit
insights into a success story of high open-circuit voltage and low recombination, Adv. Energy Mater. 7 (2017). Available from: https://doi. org/10.1002/aenm.201602358. [12] M. Crespo-quesada, M. Abdi-jalebi, H.J. Beeson, M. Vru, Photon recycling in lead iodide perovskite solar cells, Science 351 (2016) 1430
1434. [13] A.H. Hill, K.E. Smyser, C.L. Kennedy, E.S. Massaro, E.M. Grumstrup, Screened charge carrier transport in methylammonium lead iodide perovskite thin films, J. Phys. Chem. Lett. 8 (2017) 948
953. Available from: https://doi.org/10.1021/acs.jpclett.7b00046. [14] L.M. Herz, Charge carrier dynamics in organic-inorganic metal halide perovskites, Annu. Rev. Phys. Chem. 67 (2016) 65
89, doi:10.1146/annurev-physchem-040215-112222. [15] C. Eames, J.M. Frost, P.R.F. Barnes, B.C. O’Regan, A. Walsh, M.S. Islam, Ionic transport in hybrid lead iodide perovskite solar cells, Nat. Commun. 6 (2015) 7497. Available from: https://doi.org/10.1038/ncomms8497. [16] E.L. Unger, L. Kegelmann, K. Suchan, D. So¨rell, L. Korte, S. Albrecht, Roadmap and roadblocks for the band gap tunability of metal halide perovskites, J. Mater. Chem. A. 5 (2017) 11401
11409. Available from: https:// doi.org/10.1039/C7TA00404D. [17] Q. Lin, A. Armin, R.C.R. Nagiri, P.L. Burn, P. Meredith, Electro-optics of perovskite solar cells, Nat. Photonics. 9 (2015) 106
112. Available from: https://doi.org/10.1038/nphoton.2014.284.

Characterization Techniques for Perovskite Solar Cell Materials

References

157

[18] F. Hao, C.C. Stoumpos, R.P.H. Chang, M.G. Kanatzidis, Anomalous band gap behavior in mixed Sn and Pb perovskites enables broadening of absorption spectrum in solar cells, J. Am. Chem. Soc 136 (2014) 8094
8099. Available from: https://doi.org/10.1021/ja5033259. [19] A. Kumar, K.R. Balasubramaniam, J. Kangsabanik, Vikram, A. Alam, Crystal structure, stability, and optoelectronic properties of the organic-inorganic wide-band-gap perovskite CH3NH3BaI3: candidate for transparent conductor applications, Phys. Rev. B. 94 (2016) 1
5. Available from: https://doi.org/10.1103/ PhysRevB.94.180105. [20] M. Pazoki, T.J. Jacobsson, J. Kullgren, E.M.J. Johansson, A. Hagfeldt, G. Boschloo, et al., Photoinduced stark effects and mechanism of ion displacement in perovskite solar cell materials, ACS Nano 11 (2017) 2823
2834. Available from: https://doi.org/10.1021/acsnano.6b07916. [21] M. Pazoki, T.J. Jacobsson, S.H.T. Cruz, M.B. Johansson, R. Imani, J. Kullgren, et al., Photon energy-dependent hysteresis effects in lead halide perovskite materials, J. Phys. Chem. C. (2017). Available from: https://doi. org/10.1021/acs.jpcc.7b06775. acs.jpcc.7b06775. [22] J.S. Manser, P.V. Kamat, Band filling with free charge carriers in organometal halide perovskites, Nat. Photonics. 8 (2014) 737
743. Available from: https://doi.org/10.1038/nphoton.2014.171. [23] G. Xing, N. Mathews, S.S. Lim, Y.M. Lam, S. Mhaisalkar, T.C. Sum, Long-range balanced electron- and holetransport lengths in organic-inorganic CH3NH3PbI3, Science (2013) 498
500 (80-.). [24] G. Grancini, A.R. Srimath Kandada, J.M. Frost, A.J. Barker, M. De. Bastiani, M. Gandini, et al., Role of microstructure in the electron
hole interaction of hybrid lead halide perovskites, Nat. Photonics 9 (2015) 695
701. Available from: https://doi.org/10.1038/nphoton.2015.151. [25] K.G. Stamplecoskie, J.S. Manser, P.V. Kamat, Dual nature of the excited state in organic
inorganic lead halide perovskites, Energy Environ. Sci. 8 (2015) 208
215. Available from: https://doi.org/10.1039/ C4EE02988G. [26] B. Park, X. Zhang, E.M.J. Johansson, A. Hagfeldt, G. Boschloo, S. Il Seok, et al., Analysis of crystalline phases and integration modelling of charge quenching yields in hybrid lead halide perovskite solar cell materials, Nano Energy 40 (2017) 596
606. Available from: https://doi.org/10.1016/j.nanoen.2017.08.055. [27] M.A. Green, A. Ho-Baillie, H.J. Snaith, The emergence of perovskite solar cells, Nat. Photonics. 8 (2014) 506
514. Available from: https://doi.org/10.1038/nphoton.2014.134. [28] R.L. Milot, G.E. Eperon, H.J. Snaith, M.B. Johnston, L.M. Herz, Temperature-dependent charge-carrier dynamics in CH3NH3PbI3 perovskite thin films, Adv. Funct. Mater. 25 (2015) 6218
6227. Available from: https://doi.org/10.1002/adfm.201502340. [29] K. Abdel-Baki, F. Boitier, H. Diab, G. Lanty, K. Jemli, F. Le´de´e, et al., Exciton dynamics and non-linearities in two-dimensional hybrid organic perovskites, J. Appl. Phys. 119 (2016). Available from: https://doi.org/ 10.1063/1.4941345. [30] J. Aneesh, A. Swarnkar, V. Kumar Ravi, R. Sharma, A. Nag, K.V. Adarsh, Ultrafast exciton dynamics in colloidal CsPbBr3 perovskite nanocrystals: biexciton effect and auger recombination, J. Phys. Chem. C. 121 (2017) 4734
4739. Available from: https://doi.org/10.1021/acs.jpcc.7b00762. [31] M. Pazoki, M.B. Johansson, H. Zhu, P. Broqvist, T. Edvinsson, G. Boschloo, et al., Bismuth iodide perovskite materials for solar cell applications: electronic structure, optical transitions and directional charge transport, J. Phys. Chem. C (120AD) 29039
29046. [32] Y. Yang, M. Yang, Z. Li, R. Crisp, K. Zhu, M.C. Beard, Comparison of recombination dynamics in CH3NH3PbBr3 and CH3NH3PbI3 perovskite films: influence of exciton binding energy, J. Phys. Chem. Lett. 6 (2015) 4688. Available from: https://doi.org/10.1021/acs.jpclett.5b02290. [33] S.D. Stranks, V.M. Burlakov, T. Leijtens, J.M. Ball, A. Goriely, H.J. Snaith, Recombination kinetics in organicinorganic perovskites: excitons, free charge, and subgap states, Phys. Rev. Appl. 2 (2014) 0034007. Available from: https://doi.org/10.1103/PhysRevApplied.2.034007. [34] J.T. Jacobsson, J.P. Correa Baena, M. Pazoki, M. Saliba, K. Schenk, M. Gra¨tzel, et al., Exploration of the compositional space for mixed lead halogen perovskites for high efficiency devices, Energy Environ. Sci. 9 (2016) 1706
1724. Available from: https://doi.org/10.1039/C6EE00030D. [35] J. Even, L. Pedesseau, C. Katan, Analysis of multivalley and multibandgap absorption and enhancement of free carriers related to exciton screening in hybrid perovskites, J. Phys. Chem. C 118 (2014) 11566
11572. Available from: https://doi.org/10.1021/jp503337a.

Characterization Techniques for Perovskite Solar Cell Materials

158

6. Time resolved photo-induced optical spectroscopy

[36] T.J. Savenije, C.S. Ponseca, L. Kunneman, M. Abdellah, K. Zheng, Y. Tian, et al., Thermally activated exciton dissociation and recombination control the carrier dynamics in organometal halide perovskite, J. Phys. Chem. Lett. 5 (2014) 2189
2194. Available from: https://doi.org/10.1021/jz500858a. [37] V. D’Innocenzo, G. Grancini, M.J.P. Alcocer, A.R.S. Kandada, S.D. Stranks, M.M. Lee, et al., Excitons versus free charges in organo-lead tri-halide perovskites, Nat. Commun. 5 (2014) 1
6. Available from: https://doi. org/10.1038/ncomms4586. [38] M.B. Price, J. Butkus, T.C. Jellicoe, A. Sadhanala, A. Briane, J.E. Halpert, et al., Hot-carrier cooling and photoinduced refractive index changes in organic
inorganic lead halide perovskites, Nat. Commun. 6 (2015) 8420. Available from: https://doi.org/10.1038/ncomms9420. [39] J.M. Ball, A. Petrozza, Defects in perovskite-halides and their effects in solar cells, Nat. Energy. 1 (2016) 16149. Available from: https://doi.org/10.1038/nenergy.2016.149. [40] M.L. Agiorgousis, Y. Sun, H. Zeng, S. Zhang, Strong covalency-induced recombination centers in perovskite solar cell material CH3NH3PbI3, J. Am. Chem. Soc. (2014). Available from: https://doi.org/10.1021/ ja5079305. [41] S. De Wolf, J. Holovsky, S.J. Moon, P. Lo¨per, B. Niesen, M. Ledinsky, et al., Organometallic halide perovskites: sharp optical absorption edge and its relation to photovoltaic performance, J. Phys. Chem. Lett. 5 (2014) 1035
1039. Available from: https://doi.org/10.1021/jz500279b. [42] D. Shi, V. Adinolfi, R. Comin, M. Yuan, E. Alarousu, A. Buin, et al., Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals, Science 347 (2015) 519
522. Available from: https://doi.org/10.1126/science.aaa2725 (80-.). [43] A. Baumann, S. Va¨th, P. Rieder, M.C. Heiber, K. Tvingstedt, V. Dyakonov, Identification of trap states in perovskite solar cells, J. Phys. Chem. Lett. 6 (2015) 2350
2354. Available from: https://doi.org/10.1021/acs. jpclett.5b00953. [44] M. Abdi-Jalebi, M. Pazoki, B. Philippe, M.I. Dar, M. Alsari, A. Sadhanala, et al., Dedoping of lead halide perovskites incorporating monovalent cations, ACS Nano (2018). Available from: https://doi.org/10.1021/ acsnano.8b03586. acsnano.8b03586. [45] E.T. Hoke, D.J. Slotcavage, E.R. Dohner, A.R. Bowring, H.I. Karunadasa, M.D. McGehee, Reversible photoinduced trap formation in mixed-halide hybrid perovskites for photovoltaics, Chem. Sci. 6 (2015) 613
617. Available from: https://doi.org/10.1039/C4SC03141E. [46] A. Marchioro, J. Teuscher, D. Friedrich, M. Kunst, R. van de Krol, T. Moehl, et al., Unravelling the mechanism of photoinduced charge transfer processes in lead iodide perovskite solar cells, Nat. Photonics. 8 (2014) 250
255. Available from: https://doi.org/10.1038/nphoton.2013.374. [47] S.G. Boxer, Stark realities, J. Phys. Chem. B. 113 (2009) 2972
2983. [48] V. Roiati, E. Mosconi, A. Listorti, S. Colella, G. Gigli, F. De Angelis, et al., Stark effect in perovskite/TiO2 solar cells: evidence of local interfacial order, Nano Lett. 14 (2014) 2168
2174. Available from: https://doi. org/10.1021/nl500544c. [49] G. Boschloo, A. Hagfeldt, Photoinduced absorption spectroscopy of dye-sensitized nanostructured TiO2, Chem. Phys. Lett. 370 (2003) 381
386. Available from: https://doi.org/10.1016/S0009-2614(03)00029-0. [50] R. Gottesman, L. Gouda, B.S. Kalanoor, E. Haltzi, S. Tirosh, E. Rosh-Hodesh, et al., Photo-induced reversible structural transformations in free-standing CH3NH3PbI3 perovskite films, J. Phys. Chem. Lett. 6 (2015) 2332
2338. Available from: https://doi.org/10.1021/acs.jpclett.5b00994. [51] D.W. deQuilettes, W. Zhang, V.M. Burlakov, D.J. Graham, T. Leijtens, A. Osherov, et al., Photo-induced halide redistribution in organic
inorganic perovskite films, Nat. Commun. 7 (2016) 11683
11692. Available from: https://doi.org/10.1038/ncomms11683. [52] E. Mosconi, D. Meggiolaro, H.J. Snaith, S.D. Stranks, F. De Angelis, Light-induced annihilation of Frenkel defects in organo-lead halide perovskites, Energy Environ. Sci. 9 (2016) 3180
3187. Available from: https:// doi.org/10.1039/C6EE01504B. [53] B. Wu, K. Fu, N. Yantara, G. Xing, S. Sun, T.C. Sum, et al., Charge accumulation and hysteresis in perovskite-based solar cells: an electro-optical analysis, Adv. Energy Mater. 5 (2015) 1
8. Available from: https://doi.org/10.1002/aenm.201500829. [54] Q. Dong, Y. Fang, Y. Shao, P. Mulligan, J. Qiu, L. Cao, et al., Electron-hole diffusion lengths . 175 μm in solution-grown CH3NH3PbI3 single crystals, Science 347 (2015) 967
970. Available from: https://doi.org/ 10.1126/science.aaa5760 (80-.).

Characterization Techniques for Perovskite Solar Cell Materials

References

159

[55] C. Quarti, E. Mosconi, F. De Angelis, Interplay of orientational order and electronic structure in methylammonium lead iodide: implications for solar cell operation, Chem. Mater. 26 (2014) 6557
6570. [56] J.-P. Correa-Baena, M. Saliba, T. Buonassisi, M. Gra¨tzel, A. Abate, W. Tress, et al., Promises and challenges of perovskite solar cells, Science 358 (2017) 739
744. Available from: https://doi.org/10.1126/science.aam6323 (80-.). [57] T. Leijtens, G.E. Eperon, N.K. Noel, S.N. Habisreutinger, A. Petrozza, H.J. Snaith, Stability of metal halide perovskite solar cells, Adv. Energy Mater. 5 (2015) 1500963. Available from: https://doi.org/10.1002/aenm.201500963. [58] K.A. Bush, A.F. Palmstrom, Z.J. Yu, M. Boccard, J.P.M. Rongrong Cheacharoen, D.P. McMeekin, et al., 23.6%Efficient monolithic perovskite/silicon tandem solar cells with improved stability, Nat. Energy. 2 (2017) 17009. Available from: https://doi.org/10.1038/nenergy.2017.9. [59] I. Zimmermann, E. Mosconi, X. Lee, D. Martineau, S. Narbey, F. Oswald, et al., One-year stable perovskite solar cells by 2D/3D interface engineering, Nat. Commun. 8 (2017) 15684. Available from: https://doi.org/ 10.1038/ncomms15684. [60] M. Pazoki, M.J. Wolf, T. Edvinsson, J. Kullgren, Vacancy dipole interactions and the correlation with monovalent cation dependent ion movement in lead halide perovskite solar cell materials, Nano Energy. 38 (2017) 537
543. Available from: https://doi.org/10.1016/j.nanoen.2017.06.024. [61] M. Pazoki, G. Boschloo, A. Hagfeldt, Stark effect in D35 sensitized mesoporous TiO2: influence of dye coverage, electrolyte composition and additives, Electrochem. Acta. 179 (2014) 174
178. [62] S. Gelinas, A. Rao, A. Kumar, S.L. Smith, A.W. Chin, J. Clark, et al., Ultrafast long-range charge separation in organic semiconductor photovoltaic diodes, Science 343 (2014) 512
516. Available from: https://doi.org/ 10.1126/science.1246249 (80-.). [63] D. a B. Miller, D.S. Chemla, T.C. Damen, a C. Gossard, W. Wiegmann, T.H. Wood, et al., Band-edge electroabsorption in quantum well structures: the quantum-confined stark effect, Phys. Rev. Lett. 53 (1984) 2173
2176. Available from: https://doi.org/10.1103/PhysRevLett.53.2173. [64] T.J. Jacobsson, T. Edvinsson, Quantum confined stark effects in ZnO quantum dots investigated with photoelectrochemical methods, J. Phys. Chem. C 118 (2014) 12061
12072. Available from: https://doi.org/ 10.1021/jp503098q. [65] Y. Yang, D.P. Ostrowski, R.M. France, K. Zhu, J. van de Lagemaat, J.M. Luther, et al., Observation of a hotphonon bottleneck in lead-iodide perovskites, Nat. Photonics. 10 (2015) 54
59. Available from: https://doi. org/10.1038/nphoton.2015.213. [66] H. Kawai, G. Giorgi, A. Marini, K. Yamashita, The mechanism of slow hot-hole cooling in lead-iodide perovskite: first-principles calculation on carrier lifetime from electron-phonon interaction, Nano Lett. 15 (2015) 3103
3108. Available from: https://doi.org/10.1021/acs.nanolett.5b00109. [67] J.M. Frost, L.D. Whalley, A. Walsh, Slow cooling of hot polarons in halide perovskite solar cells (2017) 1
7. Available from: http://arxiv.org/abs/1708.04158. [68] W. Shockley, H.J. Queisser, Detailed balance limit of efficiency of p-n junction solar cells, J. Appl. Phys. 32 (1961) 510
519. [69] D.N. Mermin, Neil W. Ashcroft, Solid State Physics (1976) 179
180. [70] A.D. Wright, C. Verdi, R.L. Milot, G.E. Eperon, M.A. Pe´rez-Osorio, H.J. Snaith, et al., Electron
phonon coupling in hybrid lead halide perovskites, Nat. Commun. 7 (2016) 11775. Available from: https://doi.org/ 10.1038/ncomms11755. [71] M. Karakus, S.A. Jensen, F. D’Angelo, D. Turchinovich, M. Bonn, E. Ca´novas, Phonon-electron scattering limits free charge mobility in methylammonium lead iodide perovskites, J. Phys. Chem. Lett. 6 (2015) 4991
4996. Available from: https://doi.org/10.1021/acs.jpclett.5b02485. [72] E.A. Schiff, Carrier drift-mobilities and solar cell models for amorphous and nanocrystalline silicon, MRS Proc 1153 (2009). Available from: https://doi.org/10.1557/PROC-1153-A15-01. 1153-A15-01. [73] S.A. Dinca, E.A. Schiff, W.N. Shafarman, B. Egaas, R. Noufi, D.L. Young, Electron drift-mobility measurements in polycrystalline CuIn 1 2 x Ga x Se 2 solar cells 103901 (2012) 30
32. [74] A.J. Neukirch, W. Nie, J.C. Blancon, K. Appavoo, H. Tsai, M.Y. Sfeir, et al., Polaron stabilization by cooperative lattice distortion and cation rotations in hybrid perovskite materials, Nano Lett. 16 (2016) 3809
3816. Available from: https://doi.org/10.1021/acs.nanolett.6b01218. [75] C.S. Ponseca, T.J. Savenije, M. Abdellah, K. Zheng, A. Yartsev, T. Pascher, et al., Organometal halide perovskite solar cell materials rationalized: ultrafast charge generation, high and microsecond-long balanced

Characterization Techniques for Perovskite Solar Cell Materials

160

6. Time resolved photo-induced optical spectroscopy

mobilities, and slow recombination, J. Am. Chem. Soc. 136 (2014) 5189
5192. Available from: https://doi. org/10.1021/ja412583t. [76] M.B. Johnston, L.M. Herz, Hybrid perovskites for photovoltaics: charge-carrier recombination, diffusion, and radiative efficiencies, Acc. Chem. Res. 49 (2016) 146
154. Available from: https://doi.org/10.1021/acs. accounts.5b00411. [77] Z. Guo, J.S. Manser, Y. Wan, P.V. Kamat, L. Huang, Spatial and temporal imaging of long-range charge transport in perovskite thin films by ultrafast microscopy, Nat. Commun. 6 (2015) 7471. Available from: https://doi.org/10.1038/ncomms8471.

Further reading J.M. Richter, F. Branchi, F.V. de, A. Camargo, B. Zhao, R.H. Friend, et al., Ultrafast carrier thermalization in lead iodide perovskite probed with two-dimensional electronic spectroscopy, Nat. Commun. 8 (2017) 1
7. https:// doi.org/10.1038/s41467-017-00546-z.

C H A P T E R

7 Photovoltage/photocurrent transient techniques Emilio Palomares1, Nu´ria F. Montcada1, Marı´a Me´ndez1, Jesu´s Jime´nez-Lo´pez1, Wenxing Yang2 and Gerrit Boschloo2 1

Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science ˚ ngstro¨m Laboratory, and Technology, Tarragona, Spain 2Department of Chemistry, A Uppsala University, Uppsala, Sweden

7.1 Introduction Characterization of photovoltaic devices by means of photovoltage and photocurrent transients is relatively easy to perform and gives very useful information on several important properties of the devices, such as the charge carrier lifetime. In these techniques the device is perturbed by a modulated light and the photovoltage/photocurrent is recorded in the time or frequency domain, from which important information about the device can be obtained. An advantage is that such measurements can be done on complete devices, under conditions comparable to practical operating conditions. This is especially important for solar cell technologies that display light-intensity-dependent properties, such as dye-sensitized solar cells (DSCs) and also perovskite solar cells (PSCs). As mentioned in previous chapters, light soaking and/or application of a voltage bias, can strongly affect the performance of the PSCs [1
3]. The precise reason for this is still under intensive debate, but it is generally recognized that ion movement in the perovskite material plays an important role in this respect. In DSCs photocurrent and photovoltage transients display a nonlinear response of electron transport and recombination as a function of light intensity [4
6]. Therefore, these processes are best studied using small-modulation techniques, in which the modulation is superimposed on a base light intensity. In the small modulation regime charge carrier concentrations, and occupation of trap states can be considered to be constant during the measurement, which strongly simplifies the analysis. In this regime, the amplitude of the response should scale linearly with the modulation amplitude, while time constants found

Characterization Techniques for Perovskite Solar Cell Materials DOI: https://doi.org/10.1016/B978-0-12-814727-6.00007-4

161

Copyright © 2020 Elsevier Inc. All rights reserved.

162

7. Photovoltage/photocurrent transient techniques

should be independent of the modulation amplitude. Analysis of these time constants as a function of light intensity, short-circuit photocurrent density (JSC), or open-circuit potential (VOC) gives useful trends. Similar to DSC, perovskite solar cell is a complex system. Perovskite displays both electronic and ionic conductivity, selective contacts could be either flat or mesoporous, and performance can strongly depend on the working conditions, i.e., it can differ in dark condition or under illumination. Therefore, the application of small-modulation transient photocurrent and photovoltage methods is also very useful for PSC. These can give information on charge transport, accumulation and recombination. Section 7.2 describes the different small modulation methods that have been used to characterize PSCs. Additional useful information from PSCs can be obtained from large modulation transient methods. Here, typically, the illumination is completely switched off, and current or voltage decay is monitored as function of time. These methods are discussed in Section 7.3.

7.2 Small modulation transient techniques 7.2.1 Transient photo-voltage technique (TPV) Transient Photo-Voltage (TPV) is a time-resolved technique employed to study carrier recombination processes in solar cells, including dye-sensitized [5,7] and organic solar cells (OSC) [8
10]. This technique is based on the excitation of the device by a fast and small perturbation of incoming light perturbing the solar cell open-circuit voltage (VOC), which can be directly related to a small perturbation of the quasi-Fermi level [10]. In order to perform a TPV, the solar cell is continuously irradiated using a light source, that promotes a constant and stable VOC. The solar cell is kept at open-circuit conditions, so no current can flow through the contacts and the solar cell is connected to an oscilloscope that can register the changes in voltage over time (Fig. 7.1A). After reaching a stable VOC, the solar cell is excited with an additional short-lived laser pulse that generates a small perturbation of the VOC (Fig. 7.1B). The variation of the VOC (ΔV) is proportional to the photo-generated carriers by the laser pulse. As the solar cell is in open-circuit, as mentioned above, the “extra” photo-generated carriers are forced to recombine, which leads to the registration of the transient to the initial VOC. It is important to notice that TPV differs significantly from the Voc decay technique, which is also used to measure carrier recombination lifetime [11] and will be discussed below. The described TPV technique aims to promote a small perturbation of the Fermi level of the solar cell, while the VOC decay records the full decay from a given light intensity until the VOC decays zero, in other words, all the change in VOC over time is measured. The main difference, in results, between both techniques, is that TPV technique leads to mono-exponential decays and the VOC decay, due to the fitting of more complicated kinetics, results in multi-exponential decays [11]. An example of a VOC decay measured for a typical perovskite solar cell using TPV is shown in Fig. 7.2A. It is important to highlight that, as the light intensity increases, so does the solar cell VOC and therefore faster decays are registered (Fig. 7.2B), and thus, it is possible to compare different decay kinetics at different light bias as explained further below (Fig. 7.3).

Characterization Techniques for Perovskite Solar Cell Materials

7.2 Small modulation transient techniques

163

FIGURE 7.1 (A) Schematic illustration of the transient photo-voltage setup and (B) the representation of the corresponding signals that can be monitored at the oscilloscope.

FIGURE 7.2 (A) A TPV transient at 100 mW/cm2 light bias for a mesoporous
TiO2/methyl ammonium lead iodide/OMeTAD solar cell. The initial VOC (Voc0) is close to 1.072 V and upon the laser pulse the VOC increases to 1.084 V. The ΔV is close to 12 mV (B) normalized TPV transients at different light bias.

Characterization Techniques for Perovskite Solar Cell Materials

164

7. Photovoltage/photocurrent transient techniques

FIGURE 7.3 Transient lifetime at different photo-induced voltages for three different mesoporous
TiO2/ methyl ammonium lead iodide/hole transporting material(HTM) solar cells where Device 1 (circles) use P3HT as HTM, Device 2 (triangles) PCPDTBT and Device 3 (squares) spiroOMeTAD as HTM.

It is possible to correlate the changes in the VOC with the carrier lifetime extracted from TPV measurements by using Eq. 7.1 τ 5 τ0 e2βVoc

(7.1)

where τ (s) is the carrier lifetime and β (s/V) the decay constant. Although the direct comparison of β values can be useful to compare devices; it is important to recall that the carrier recombination kinetics depends on charge density (n). It can be therefore more instructive to plot the data extracted from the TPV measurements versus charge density rather than voltage. Two different techniques can be employed to measure the charge density of a solar cell under the same illumination condition as in TPV measurements. These techniques are the Differential Capacitance (DC) analysis and the Charge Extraction (CE) method, which will be discussed in Sections 7.2.2 and 7.3.2, respectively.

7.2.2 Transient photo-current decay (TPC) and differential capacitance (DC) A useful method to measure the charge density in solar cells, using small-modulation techniques, is the Differential Capacitance analysis [12]. To carry out the DC measurements we must first measure the Transient Photo-Current decays (TPC), which will be used to estimate the charge present at the solar cell. An alternative method to determine charge density is Charge Extraction (CE), which is a large modulation technique that will be discussed later (see Section 7.3.2). An advantage of the DC method over the CE method is that it can be used even in the systems with high recombination rates, where the TPV decay and the CE decay have similar kinetics, which makes a complete extraction of all charges impossible.

Characterization Techniques for Perovskite Solar Cell Materials

7.2 Small modulation transient techniques

165

FIGURE 7.4 (A) Scheme of the setup employed for transient photo-current measurements and (B) the representation of the corresponding running process at the oscilloscope.

However, if charge recombination is slower than CE, results from both DC and CE methods should be similar. TPC decays are the current response of a solar cell that is held in short circuit conditions and is measured under the same conditions as TPV. For this reason, the setup of this experiment is very similar to the one used for TPV (Fig. 7.1), with the exception that the device is held at short-circuit, it is connected to a small resistor as shown in Fig. 7.4. The laser pulse generates a perturbation in the device current that is measured on the oscilloscope as a voltage drop over the resistor that can easily be converted into a transient current by applying Ohm’s law. This transient current is measured and integrated over time to calculate the amount of photo-generated charges (Δq) induced by the pulse. This method, however, presents some applicability limitations because it is only valid when charge carrier losses at short-circuit conditions are negligible, in other words, the charge collection process should not be affected by the charge recombination process. Three different experimental criteria must be checked in order to verify the validity of this last statement, before making use of TPC to carry out Differential Capacitance analysis: 1. The solar cell short-circuit current (JSC) dependence with light intensity (LI) must fit to a power law (JSC ~ LIα) where α 5 1 as shown in Fig. 7.5. This result implies that there are no significant charge losses at short circuit. 2. TPC decays must be similar under different light irradiation conditions (see Fig. 7.6), as the generated charges by the laser pulse must be independent on the background light intensity. 3. TPC decays must be faster than TPV decays (see Fig. 7.7) implying that charge collection is faster than charge recombination. If all of these requirements are accomplished, the Differential Capacitance analysis of the solar cell at different given voltages can be calculated by taking Δq from the TPC

Characterization Techniques for Perovskite Solar Cell Materials

166

7. Photovoltage/photocurrent transient techniques

FIGURE 7.5 Current density at different light intensities with its corresponding fitting to a power law for a mesoporous
TiO2/ methylammonium lead iodide/OMeTAD solar cell.

FIGURE 7.6 Normalized TPC decays for comparison under 1 sun-simulated light and dark conditions for a mesoporous TiO2/methylammonium lead iodide/ OMeTAD solar cell.

decays and the amplitude of the small perturbation decay obtained in the TPV measurements as shown in Eq. 7.2. dCðVOC Þ 5

Δq ; ΔVðVocÞ

(7.2)

where Δq is the amount of charges calculated by the integration of the TPC decay (Fig. 7.8A), and ΔV is the voltage amplitude produced by the laser pulse in TPV decays (Fig. 7.8B). The Differential Capacitance (DC) can be defined also as the ability of a device to store the additional charges (Δq), generated by the laser pulse, that creates the correspondent perturbation in the voltage estimated by TPV, under different light irradiation conditions. Hence, differential capacitance can be related with the light intensity or the device VOC. Fig. 7.9A shows an example of a calculated DC versus solar cell VOC.

Characterization Techniques for Perovskite Solar Cell Materials

167

7.2 Small modulation transient techniques

FIGURE 7.7 Normalized TPC and TPV decays for comparison purposes measured at 1 sun-simulated conditions for a mesoporous
TiO2/methylammonium lead iodide/ OMeTAD solar cell.

FIGURE 7.8 Schematic information taken from (A) TPC and (B) TPV transient decays to calculate the differential capacitance.

Finally, the total amount of charges stored at the device can be calculated by the integration of differential capacitance with respect to VOC (Eq. 7.3), obtaining a correlation of the charge density at different potentials as shown in Fig. 7.9B. ð Voc ½dCðVOC ÞdVOC (7.3) qðVOC Þ 5 0

In summary, TPC and the calculation of the differential capacitance, are alternative methods to CE technique, particularly in cases when CE decays appear slower than their corresponding TPV, as TPC is faster than the TPV.

7.2.3 Square-wave modulation for photovoltage and photocurrent transients (SW-PVT and SW-PCT) Square-wave (SW) modulated excitation techniques have been developed for fast and convenient analysis of charge transport and recombination in DSC [13,14]. Their main

Characterization Techniques for Perovskite Solar Cell Materials

168

7. Photovoltage/photocurrent transient techniques

FIGURE 7.9 (A) The Differential Capacitance obtained from a combination of TPC and TPV at different light intensities versus the mesoporous
TiO2/methyl ammonium lead iodide/OMeTAD solar cell VOC. (B) Charge carrier density for this device obtained from the integration of the differential capacitance.

advantage compared to the frequency-modulated techniques, intensity-modulated photocurrent and photovoltage techniques (IMPS and IMVS, respectively, see discussion below), which had been developed earlier, is the speed of measurement and the ease of analysis. Whereas in IMPS/IMVS a large number of frequencies is measured, one after the other, in SW a single measurement is sufficient. Even though some averaging is applied, a complete measurement is as fast as a single frequency measurement in IMPS/IMVS. In comparison to the laser-induced transient described above, the advantage of the SW techniques is that it can be performed with low-cost equipment such as LED for illumination and a digital acquisition board for recording transients. For most simple DSC systems only a single time constant is extracted from SW-PCT and SW-PVT, that is identified as the electron transport time for photocurrent transients

Characterization Techniques for Perovskite Solar Cell Materials

7.2 Small modulation transient techniques

169

(if recombination can be neglected), and as the electron lifetime for photovoltage transients. The SW response signal comprises of two step response functions that can be analyzed separately. The rise and fall time constants and the step amplitudes should be nearly equal if the modulation is sufficiently small, it should be typically less than 10% of the base light intensity. Considering photocurrent transients recorded at short-circuit, in DSCs the photocurrent response time reflects the time it takes for the electron to move through the mesoporous TiO2 to the FTO contact. While diffusing in the mesoporous TiO2, electrons are charge compensated by ions in solution [4]. Only when electrons reach the conducting FTO contact, current is registered in the external circuit. PSC is expected to behave differently from DSC and more like typical solid-state thin film solar cell device, where ionic charge compensation in electrolyte does not play a role. The expected time constants that are determined should essentially be the RC time constant of the system, where R and C are the cell resistance and capacitance, respectively. The cell resistance is in this case due to the series resistance and the recombination resistance, R21 5 Rseries21 1 Rrecomb21. For photovoltage transients, normally recorded under open circuit conditions, the kinetics found reflect recombination processes. In case of DSC this is the recombination of electrons in TiO2 with acceptors in the electrolyte, while in PSC it is that of electrons and holes. The time constant that is found can again be considered to be the RC time constant of the solar cell, but in the open-circuit case, recombination is the only discharge process for the capacitor, i.e., R 5 Rrecomb. In an early work on PSCs, the device structure was similar to that of a solid-state DSC and the resulting trends in small modulation SW photocurrent/voltage transients were found to be quite similar, as shown in Fig. 7.10. With increasing the base light intensity, carrier lifetimes as well as transport times were found to decrease. Interestingly, the degree of perovskite coverage into the mesoporous structure has a large effect on the time constants found: very short carrier lifetime and slow transport was found for the lowest coverage, while optimum perovskite coverage resulted in carrier lifetimes that were two orders of magnitude higher at the same VOC and carrier transport times one order of

FIGURE 7.10 Carrier lifetime and transport times of Au/spiroOMeTAD/MAPbI3/meso
TiO2/dense
TiO2/ FTO solar cells with different amount of MAPbI3 prepared by a 2-step method. The concentration of the PbI2 spincoating solution (shown in the figures) determines this amount. Reproduced from Ref. [15] D. Bi, A.M. El-Zohry, A. Hagfeldt, G. Boschloo, Unraveling the effect of PbI2 concentration on charge recombination kinetics in perovskite solar cells, ACS Photonics 2 (5) (2015) 589
594. https://doi.org/10.1021/ph500255t.

Characterization Techniques for Perovskite Solar Cell Materials

170

7. Photovoltage/photocurrent transient techniques

magnitude shorter [15]. The results for transport time can be understood by assuming that the perovskite has a better electron conductivity than the mesoporous
TiO2. Separate measurements of carrier mobilities and conductivities shows that this is indeed the case. It is interesting that the carrier transport seems to follow a similar trend as in DSC, suggesting that in PSC with mesoporous TiO2 contact charge compensation of the electrons in the mesoporous TiO2 takes place, either by ions or by holes in the perovskite layer. The situation changes if the mesoporous TiO2 is removed or replaced by an inert mesoporous material, as was done in the seminal paper by Snaith and co-workers [16]. Interestingly, carrier transport was found to be faster in absence of mesoporous TiO2, demonstrating that there is good electron conductivity in perovskite, see Fig. 7.11. Furthermore, there was no longer any light-intensity dependence found, suggesting that the transport of electrons in the mesoporous TiO2 is responsible for the light-dependent carrier transport properties. In a more detailed analysis of the small-modulation transients for perovskite solar cells, we found recently that the transient response was nearly always biphasic. An example of this behavior is shown for voltage transients on a mixed-ion perovskite solar cells in Fig. 7.12. Using rapid modulation (2 kHz in Fig. 7.12A), rise and decay follow strictly monoexponential functions. However, using the slow modulation (e.g., 2 Hz in Fig. 7.12B), 60% of the rise/ decay amplitude is from the fast process, and 40% from the slow process. This ratio was found to be independent of the applied bias light intensity in the experiment of Fig. 7.12. In Fig. 7.12C the time constants are plotted as function of open circuit potential. The fast process clearly becomes faster with light intensity and VOC, whereas the slow process seems to be independent of light intensity or VOC. The fast process should be related to the recombination of “free” charge carriers, while the slow one (t2, about 100 ms) to carriers is probably linked to ionic motion in the PSC. At the highest light intensities the fast time constant appears to become constant, but it was found that this could be attributed to limitations in the instrument response.

FIGURE 7.11 Carrier transport times determined by SW-PCT of perovskite-sensitized TiO2 cells (black circles) and Al2O3 cells (red crosses). Inset shows normalized photocurrent transients for these cells set to generate 5 mA/cm2 photocurrent from the background light bias. Reproduced from Ref. [16] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites, Science 338 (6107) (2012) 643
647. https://doi.org/10.1126/science.1228604.

Characterization Techniques for Perovskite Solar Cell Materials

7.2 Small modulation transient techniques

171

FIGURE 7.12 SW-PVT transients under open circuit conditions for a perovskite solar cell with the composition FTO/TiO2(dense)/TiO2(meso)/ (FAPbI3)0.85(MAPbBr3)0.15/spiroMeOTAD/Au, using a square wave light modulation of 2 kHz (A), or 2 Hz (B) at the same light intensity. (C) Time constants obtained from fitting VOC rise/decay as function of open-circuit potential.

Characterization Techniques for Perovskite Solar Cell Materials

172

7. Photovoltage/photocurrent transient techniques

7.2.4 Intensity-modulated photocurrent and photovoltage (IMPS and IMVS) Intensity-modulated photocurrent and photovoltage techniques (IMPS and IMVS, respectively) have been used to analyzed semiconductor/electrolyte interfaces [17] and DSCs [18]. LED or CW laser light sources whose intensity is sinusoidally modulated by 1
10% at a certain frequency are used to excite the solar cell. The resulting signal from the photoelectrochemical cell or solar cell is analyzed by a lock-in amplifier or frequency response analyzer tuned at the frequency of the modulation, giving the amplitude of the modulated signal as well as its phase shift with respect to the excitation. In a typical experiment a wide range of frequencies (logarithmically distributed) is tested, i.e., 105 to 1021 Hz. For IMPS the solar cell is held under potentiostatic (constant voltage) conditions, typically under short-circuit conditions (V 5 0 V). For IMVS the solar cell is held under galvanostatic (constant current) conditions, typically under open-circuit conditions (V 5 VOC). The resulting IMPS/IMVS spectra are subsequently analyzed using an advanced mathematical model, similar to those applied in electrochemical impedance spectroscopy (EIS). When the response is relative simple, such as a single semicircle, the time constant τ can directly be extracted from the frequency (f ) where the maximum (or minimum) occurs in the imaginary part of the response, using τ 5 (2πf )21. IMPS/IMVS measurement have been performed on perovskite solar cells by several research groups [19
21]. The IMPS spectrum depends on device configuration, as is shown in Fig. 7.13. In a PSC device with mesoporous TiO2 electrode, two semicircles were found by Guillen et al. when plotting the imaginary part of the modulated photocurrent against the real part of the modulated photocurrent, both in the same quadrant [19]. In contrast, Pockett et al. found for planar device structures two semicircles in opposite

FIGURE 7.13 IMPS spectra for different PSC device structures (A) with mesoporous TiO2 [19] (B) planar device structure [20].

Characterization Techniques for Perovskite Solar Cell Materials

7.3 Large modulation techniques

173

FIGURE 7.14 IMVS spectra recorded at open-circuit potential at different bias light intensities for a perovskite solar cell with planar device structure [21].

quadrants. Their explanation was that the high frequency process is the RC-time of the device, while the lower frequency process refers to charge transport and recombination. IMVS response is typically a single semicircle with a characteristic radial frequency at its maximum (minimum) [19,20]. IMVS provides a convenient way to determine the recombination lifetime, without the need of fitting data to a complex equivalent circuit as is done in case the related technique of electrochemical impedance spectroscopy. In recent work, however, up to 3 semicircles could be identified [21], see Fig. 7.14. The high frequency semi-circle is related the geometric capacitance of the PSC and the recombination resistance (τHF 5 Rrecomb Cgeo) [20]. The medium and low frequency semicircles were attributed to recombination coupled to ionic motion, since the temperature dependence of gave activation energies of about 0.6 eV, consistent with computationally predicted values for thermally activated ion movement [22].

7.3 Large modulation techniques 7.3.1 VOC rise and decay A general method to look at charge carrier recombination in photovoltaics devices is to analyze the VOC decay transient after switching off the illumination. For DSCs it was derived that the electron lifetime can be calculated from the slope of the VOC transient [23]:   kT dVOC 21 τn 5 2 (7.4) e dt

Characterization Techniques for Perovskite Solar Cell Materials

174

7. Photovoltage/photocurrent transient techniques

FIGURE 7.15 VOC rise and decay measurements for a planar PSC [21]. Different light intensities were applied as indicated in the figure. Data was plotted using a linear voltage scale (A) and a logarithmic voltage scale (B).

The validity of this relation was proven by comparison with results using IMVS and EIS. The advantage of this method is that the electron lifetime can be determined in wide potential range with one single measurement. This measurement can also be performed without the use of light by means of application of an initial potential before measuring a VOC decay. The VOC decay transient is also a useful method for PSC devices, however, its analysis is more complex. For the carrier lifetime in a thin layer solar cell the following relation was derived [20]:    21 2kT dV OC 21 dVOC d2 VOC 2 : (7.5) τ VOCD 5 2 e dt dt d2 t This equation was derived using an approach similar to that for Eq. 7.4, but with the assumption of two mobile charge carriers with concentrations are higher than the doping density of the material. The second term is usually small and may be omitted. Examples for VOC rise and decay measurements with different light intensities are shown in Fig. 7.15 [21]. Both the rise and the decay were found to be biphasic, with a fast part in the microsecond to millisecond regime and a slow part in the second to minute timescale. The slow part of the VOC decay was almost linear on a semi-logarithmic plot, suggesting that kinetics follow first order kinetics. From analysis of the temperature dependence of the this part an activation energy of 0.58 eV was found [21]. The fast part corresponds to discharge of the geometric capacitance across the recombination resistance, while the slow part is related to the relaxation of the ionic charge (Fig. 7.16). The wavelength of excitation light was found to have a pronounced effect on voltage decay transients, see Fig. 7.16 [24]. Blue light induced much more of the slow tail in the VOC decay than red light excitation. It was proposed that this effect can be ascribed to the fact the blue photons have excess energy with respect to the bandgap of perovskite,

Characterization Techniques for Perovskite Solar Cell Materials

7.3 Large modulation techniques

175

FIGURE 7.16 VOC decay measurements showing the effect of excitation wavelength on a PSC with composition: FTO/TiO2(dense)/TiO2(meso)/(FAPbI3)0.85(MAPbBr3)0.15/spiroMeOTAD/Au. The light on period was 10 s and the photon flux for red and blue light was kept the same. (A similar result was found when power of blue and red light was kept the same). Adapted from Ref. [24] M. Pazoki, T.J. Jacobsson, S.H.T. Cruz, M.B. Johansson, R. Imani, J. Kullgren, et al., Photon energy-dependent hysteresis effects in lead halide perovskite materials, J. Phys. Chem. C 121 (47) (2017) 26180
26187. https://doi.org/10.1021/acs.jpcc.7b06775.

and that the excess energy can lead to increased ion displacement [24]. Details of the effect and its dependency on the composition of the perovskite is not fully understood yet.

7.3.2 Charge extraction (CE) The CE is a quick and easy-to-use measurement, in which the solar cell VOC is stabilized at some point of the IV curve, after the VOC is equilibrated the solar cell is switched to short circuit (V 5 0 V) whilst at the same time the light source is switched off too. When short circuited, there is a current transient due to the solar cell discharge through the contacts. Fig. 7.17A illustrates the CE setup. The solar cell is placed in front of a white LED array where the light can be modulated at different intensities often to match the VOC used to measure the TPV decays. In perovskite solar cells, the VOC stabilization is a key parameter due to the existence of the ion migration process and, frequently, some devices require tens of seconds before the VOC is stabilized. Thereafter, as mentioned above, the LEDs are switched off and the device is short-circuited through a circuit containing a small resistance (typically 50 Ohms). The solar cell is also connected to the oscilloscope to monitor the change in voltage over time (Fig. 7.17B). Using Ohm’s law the voltage decay can be converted to charge by integrating the transient voltage versus time. It is important to highlight that CE extracts all the charge, indiscriminately (ionic, carriers and the geometrical charge), present at the solar cell at a given voltage. Importantly, the charge extraction must be faster than the carrier recombination to avoid charge losses before the cell is short-circuited. Less often used but also a valid method, the CE can be carried out not through light bias but applying a direct voltage to the solar cell, which is known as charge extraction in dark conditions [25].

Characterization Techniques for Perovskite Solar Cell Materials

176

7. Photovoltage/photocurrent transient techniques

FIGURE 7.17 (A) Schematic illustration of the charge extraction setup. (B) Representation of the running process at the oscilloscope.

FIGURE 7.18 Example of a charge extraction plot from a mesoporous
TiO2/methyl ammonium lead iodide/OMeTAD solar cell. Notice that the charge due to the device geometrical capacitance has not been subtracted. This accounts for the linear part of the experimental data between 0 and 0.8 V.

The integration of the current transient (Eq. 7.6) is used to calculate the charge. 1 Q5 R

t5t ð

V ðtÞdt

(7.6)

t50

where Q is the charge, R is the resistance and V(t) is the voltage value measured at a given time. As the CE measurement can be done across different points of the IV curve we can plot the relation between the device VOC and the measured charge density (Fig. 7.18). In perovskite solar cells, in contrast to DSC and OSC, the results from the CE and DC are not identical and, as can be seen in Fig. 7.19, the CE method gives values of charge at

Characterization Techniques for Perovskite Solar Cell Materials

177

7.3 Large modulation techniques

FIGURE

7.19 Measured charge versus solar cell voltage (light bias) using CE and DC for two mesoporous
TiO2/methylammonium lead iodide/OMeTAD solar cells.

FIGURE 7.20 Transient decay for a CE measurement carried out at 100 mW/cm2 sunsimulated light intensity for a mesoporous
TiO2/methylammonium lead iodide/OMeTAD solar cell.

a given solar cell voltage far larger than the DC method. This result was initially addressed by B. O’Regan and E. Palomares [2]. Very recently, E. Palomares and M. Nazeeruddin carried out further work on CE and TPV by using solar cells with different perovskite composition and identical selective contacts [26]. From the analysis in depth of the CE, DC, TPV and TPC of these solar cells, it was concluded that the difference in charge between both techniques, is due to the differences in the feasibility to extract the charges due to the ionic migration process occurring at the perovskite in operando conditions. On the one hand, in CE the time scale of the measurement will allow the ionic reorganization process in the perovskite solar cells (switching from open-circuit voltage to short-circuit conditions) -as it may take hundreds of microseconds for the voltage decay generated after the solar cell short-circuit to reach zero (Fig. 7.20). On the other hand, the quick TPV decay (Fig. 7.2A) promoted by the laser flash does not allow the ions in the perovskite to reorganize fast enough after the pulse and so occurs in the TPC decay.

Characterization Techniques for Perovskite Solar Cell Materials

178

7. Photovoltage/photocurrent transient techniques

Thus, CE experiment probably extracts not only the photo-generated carriers (electrons and holes) but also the charges associated to the ion migration that has taken place when the solar cell has been irradiated until the VOC equilibrates. By no means, this observation implies that the CE can be used to measure the reorganization of the ions within the perovskite solar cell upon switching from open-circuit voltage to short-circuit. However, it certainly highlights the importance to find techniques that can decouple the different charges present at the perovskite solar cells under operando conditions, attending on their nature, either associated to the ionic migration process, the geometrical charge or to the effect of the photo-induced formation of carriers (electrons and holes). Moreover, the different nature of the perovskite composition also affects the ionic reorganization [26], while such effect over the recombination of carriers between the perovskite and the hole transport material is less pronounced.

7.3.3 Current interrupt voltage (CIV) A method similar to the charge extraction method is the current interrupt method, but here instead of the current the voltage is “extracted” from a solar cell device. Boschloo and co-workers developed this method for DSCs in order to obtain information on the fermilevel of the mesoporous TiO2 under short-circuit conditions [27]. O’Regan et al. applied this method to perovskite solar cells for the first time [2]. The measurement sequence is as follows: the device is illuminated under short-circuit conditions. Then, simultaneously, the light is switched off and the device is switched to open-circuit conditions. The charges left in the device will move towards the selective electrodes and develop a potential, which will subsequently decay due to charge recombination. In dye-sensitized solar cells, where charge extraction is rather slow, the maximum potential that is obtained is a rough measure of the quasi-fermi level splitting in the device under SC illumination. An example of CIV for an inverted-structure perovskite solar cell is shown in Fig. 7.21. It looks similar to that reported for the conventional PSC [2]. The maximum voltage is FIGURE 7.21 Voltage transients in the dark for a PSC following current interrupt after 5 s short circuit illumination with different intensities up to 1 sun equivalent. The PSC device has the inverted structure: FTO/NiOx/ (FAPbI3)0.83(MAPbBr3)0.17/ C60/Ag.

Characterization Techniques for Perovskite Solar Cell Materials

References

179

recorded several milliseconds after the switch. The origin of the current interrupt voltage must lie in the photoinduced gradient of ions in the perovskite solar cell that is formed under illumination under short-circuit conditions.

7.4 Conclusions Photovoltage and photocurrent transients are useful characterization methods for perovskite solar cells, which can give information on carrier transport, accumulation and recombination, as well as on ionic motion. To study charge recombination, the smallmodulation photovoltage methods (TPV, SW-TPV, IMVS) as well as the large largemodulation VOC decay are useful. The fast components are related to processes involving free charge carriers and the slow components are associated with ionic movement in the perovskite. The fast time constants reflect the discharge of the geometric capacitor of the cell (electron-selective contact/perovskite/hole-selective contact) with a time constant that several orders of magnitude higher than the radiative recombination in the perovskite layer, which is typically on the nanosecond time scale. The slow time constant process(es) (ms to many seconds timescale) have high activation energies (B0.5 eV). Photocurrent transients are useful to study carrier transport and can be used to determine charge accumulation in the PSC.

References [1] E.L. Unger, E.T. Hoke, C.D. Bailie, W.H. Nguyen, A.R. Bowring, T. Heumuller, et al., Hysteresis and transient behavior in current-voltage measurements of hybrid-perovskite absorber solar cells, Energy Environ. Sci. 7 (11) (2014) 3690
3698. Available from: https://doi.org/10.1039/C4EE02465F. [2] B.C. O’Regan, P.R.F. Barnes, X. Li, C. Law, E. Palomares, J.M. Marin-Beloqui, Optoelectronic studies of methylammonium lead iodide perovskite solar cells with mesoporous TiO2: separation of electronic and chemical charge storage, understanding two recombination lifetimes, and the evolution of band offsets during J
V hysteresis, JACS 137 (15) (2015) 5087
5099. Available from: https://doi.org/10.1021/jacs.5b00761. [3] H.J. Snaith, A. Abate, J.M. Ball, G.E. Eperon, T. Leijtens, N.K. Noel, et al., Anomalous hysteresis in perovskite solar cells, J. Phys. Chem. Lett. 5 (9) (2014) 1511
1515. Available from: https://doi.org/10.1021/jz500113x. [4] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Dye-sensitized solar cells, Chem. Rev. 110 (11) (2010) 6595. [5] P.R.F. Barnes, K. Miettunen, X. Li, A.Y. Anderson, T. Bessho, M. Gratzel, et al., Interpretation of optoelectronic transient and charge extraction measurements in dye-sensitized solar cells, Adv. Mater. 25 (13) (2013) 1881
1922. Available from: https://doi.org/10.1002/adma.201201372. [6] M. Pazoki, U.B. Cappel, E.M.J. Johansson, A. Hagfeldt, G. Boschloo, Characterization techniques for dyesensitized solar cells, Energy Environ. Sci. 10 (3) (2017) 672
709. Available from: https://doi.org/10.1039/ c6ee02732f. [7] A.B. Walker, L.M. Peter, K. Lobato, P.J. Cameron, Analysis of photovoltage decay transients in dye-sensitized solar cells, J. Phys. Chem. B 110 (50) (2006) 25504
25507. Available from: https://doi.org/10.1021/jp064860z. [8] A. Sa´nchez-Dı´az, L. Burtone, M. Riede, E. Palomares, Measurements of efficiency losses in blend and bilayertype zinc phthalocyanine/C60 high-vacuum-processed organic solar cells, J. Phys. Chem. C 116 (2012) 16384
16390. Available from: https://doi.org/10.1021/jp3054422. [9] N.F. Montcada, R. Dominguez, B. Pelado, P. de la Cruz, E. Palomares, F. Langa, High photocurrent in oligothienylenevinylene-based small molecule solar cells with 4.9% solar-to-electrical energy conversion, J. Mater. Chem. A 3 (21) (2015) 11340
11348. Available from: https://doi.org/10.1039/c5ta01632k.

Characterization Techniques for Perovskite Solar Cell Materials

180

7. Photovoltage/photocurrent transient techniques

[10] J.W. Ryan, E. Palomares, Photo-induced charge carrier recombination kinetics in small molecule organic solar cells and the influence of film nanomorphology, Adv. Energy Mater. 7 (10) (2017). Available from: https:// doi.org/10.1002/aenm.201601509. 1601509-n/a. [11] J. Bisquert, A. Zaban, M. Greenshtein, I. Mora-Sero´, Determination of rate constants for charge transfer and the distribution of semiconductor and electrolyte electronic energy levels in dye-sensitized solar cells by open-circuit photovoltage decay method, JACS 126 (41) (2004) 13550
13559. Available from: https://doi. org/10.1021/ja047311k. [12] A. Maurano, C.G. Shuttle, R. Hamilton, A.M. Ballantyne, J. Nelson, W. Zhang, et al., Transient optoelectronic analysis of charge carrier losses in a selenophene/fullerene blend solar cell, J. Phys. Chem. C 115 (13) (2011) 5947
5957. Available from: https://doi.org/10.1021/jp109697w. [13] S. Nakade, T. Kanzaki, Y. Wada, S. Yanagida, Stepped light-induced transient measurements of photocurrent and voltage in dye-sensitized solar cells: application for highly viscous electrolyte systems, Langmuir 21 (2005) 10803
10807. [14] G. Boschloo, L. Ha¨ggman, A. Hagfeldt, Quantification of the effect of 4-tert butylpyridine addition to I-/I3redox electrolytes in dye-sensitized nanostructured TiO2 solar cells, J. Phys. Chem. B 110 (2006) 13144
13150. [15] D. Bi, A.M. El-Zohry, A. Hagfeldt, G. Boschloo, Unraveling the effect of PbI2 concentration on charge recombination kinetics in perovskite solar cells, ACS Photonics 2 (5) (2015) 589
594. Available from: https://doi. org/10.1021/ph500255t. [16] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Efficient hybrid solar cells based on mesosuperstructured organometal halide perovskites, Science 338 (6107) (2012) 643
647. Available from: https:// doi.org/10.1126/science.1228604. [17] L.M. Peter, Dynamic aspects of semiconductor photoelectrochemistry, Chem. Rev. 90 (5) (1990) 753
769. Available from: https://doi.org/10.1021/cr00103a005. [18] A.C. Fisher, L.M. Peter, E.A. Ponomarev, A.B. Walker, K.G.U. Wijayantha, Intensity dependence of the back reaction and transport of electrons in dye-sensitized nanocrystalline TiO2 solar cells, J. Phys. Chem. B 104 (5) (2000) 949
958. [19] E. Guille´n, F.J. Ramos, J.A. Anta, S. Ahmad, Elucidating transport-recombination mechanisms in perovskite solar cells by small-perturbation techniques, J. Phys. Chem. C 118 (40) (2014) 22913
22922. Available from: https://doi.org/10.1021/jp5069076. [20] A. Pockett, G.E. Eperon, T. Peltola, H.J. Snaith, A. Walker, L.M. Peter, et al., Characterization of planar lead halide perovskite solar cells by impedance spectroscopy, open-circuit photovoltage decay, and intensitymodulated photovoltage/photocurrent spectroscopy, J. Phys. Chem. C 119 (7) (2015) 3456
3465. Available from: https://doi.org/10.1021/jp510837q. [21] A. Pockett, G.E. Eperon, N. Sakai, H.J. Snaith, L.M. Peter, P.J. Cameron, Microseconds, milliseconds and seconds: deconvoluting the dynamic behaviour of planar perovskite solar cells, Phys. Chem. Chem. Phys. 19 (8) (2017) 5959
5970. Available from: https://doi.org/10.1039/C6CP08424A. [22] C. Eames, J.M. Frost, P.R.F. Barnes, B.C. O/’Regan, A. Walsh, M.S. Islam, Ionic transport in hybrid lead iodide perovskite solar cells, Nat. Commun. (2015) 6. Available from: https://doi.org/10.1038/ncomms8497. [23] A. Zaban, M. Greenshtein, J. Bisquert, Determination of the electron lifetime in nanocrystalline dye solar cells by open-circuit voltage decay measurements, ChemPhysChem 4 (2003) 859
864. [24] M. Pazoki, T.J. Jacobsson, S.H.T. Cruz, M.B. Johansson, R. Imani, J. Kullgren, et al., Photon energy-dependent hysteresis effects in lead halide perovskite materials, J. Phys. Chem. C 121 (47) (2017) 26180
26187. Available from: https://doi.org/10.1021/acs.jpcc.7b06775. [25] J.W. Ryan, J.M. Marin-Beloqui, J. Albero, E. Palomares, Nongeminate recombination dynamics
device voltage relationship in hybrid PbS quantum dot/C60 solar cells, J. Phys. Chem. C 117 (34) (2013) 17470
17476. Available from: https://doi.org/10.1021/jp4059824. [26] N.F. Montcada, M. Me´ndez, K.T. Cho, M.K. Nazeeruddin, E.J. Palomares, Photo-induced dynamic processes in perovskite solar cells. The influence of perovskite composition in the charge extraction and the carriers recombination, Nanoscale (2018). Available from: https://doi.org/10.1039/c8nr00180d. [27] G. Boschloo, A. Hagfeldt, Activation energy of electron transport in dye-sensitized TiO2 solar cells, J. Phys. Chem. B 109 (2005) 12093
12098.

Characterization Techniques for Perovskite Solar Cell Materials

C H A P T E R

8 Temperature effects in lead halide perovskites T. Jesper Jacobsson Department of Chemistry, Uppsala University, Uppsala, Sweden

8.1 Introduction Thermal motion is an omnipresent background to the web of reality, but also something that can have an extensive impact on the properties of materials. This is especially true for semiconductors where even the defining characteristics, conduction with limits, is highly temperature dependent. Semiconductor devices, like photovoltaic cells, are thus also affected by temperature. The difference between summer and winter, night and day, sunshine and shade, and the tropics and the north, means that the temperature for a solar cell can switch from below 220
C to above 80
C. For space applications, the operational temperature range can vary even more. For traditional solar cell materials, e.g. silicon, CIGS, GaAs, etc. the device performance gradually decrease when the temperature is increased. For lead halide perovskites, which are highly dynamic systems, the situation turns out to be more complex. The amount of thermal motion has a direct impact on the crystal structure of lead halide perovskites and the dynamics of its constituents, which has implications on, for example: defect formation, ionic movement, charge transport properties, dielectric behavior, recombination, and the interface chemistry. Temperature can be used as a variable parameter providing leverage for understanding physics of the system, e.g. activation energies for various processes. At low temperatures, new effects emerge, which give insights into the fundamental physics of the perovskites. Higher temperatures are on the other hand useful for investigating crystallization dynamics as well as degradation, which is a problem that must be supressed. For temperatures there between, where solar cells operate, plenty of physics is going on which impact the properties of the materials and the performance of devices. In this chapter, we look at how temperature play a role for the crystal structure of lead halide perovskites and the binding, motion, and importance of the organic ions. We discuss phase transformations, thermal

Characterization Techniques for Perovskite Solar Cell Materials DOI: https://doi.org/10.1016/B978-0-12-814727-6.00008-6

181

Copyright © 2020 Elsevier Inc. All rights reserved.

182

8. Temperature effects in lead halide perovskites

expansion, optical properties, and stability. Finally, we go through what we known about how these can influence the performance of perovskite solar cell devices.

8.1.1 Crystal structure and phase transitions The most central motive out of which properties emerge is the crystal structure, i.e. the geometrical configuration in 3-dimentional space of the atoms in the compound. This configuration is very much constrained by the available thermal energy and the impact it has on the physical space occupied by the structure’s constituents, the excitation of the excitable, and the energetics of the bonding between neighboring atoms. In a discussion about the impact on temperature on the properties of perovskites, a recapture of the crystal structure is thus a natural starting point. The general perovskite structure has the composition ABC3, and the ideal structure has cubic symmetry and is composed of a backbone of corner sharing BC6-octahedra with cuboctahedra voids occupied by the A-cations (Fig. 8.1AC). Central for the structure is the relative size of the three ions. This can, at least partly, be captured by the concept of tolerance factor, t, as in Eq. 8.1 where rA, rB, and rC, are the ionic radius of the A, B, and C ions respectively [1]. Table 8.1 summarizes the impact the tolerance factors has on the structure.

FIGURE 8.1 (A) Ideal cubic unit cell of MAPbI3 illustrating the octahedral coordination around the lead ions. (B) Illustration of the extended structure and linking of the PbI6 octahedra. (C) The cuboctahedral coordination around the organic ion. (D) the MA ion. (E) Simulated energy landscape of the MA ion in the cubic phase in the form of a polar plot of the orientational energy surface [4]. (F and G) Illustration of the octahedra tilting pattern in the form of the PbI bond angel in the ideal tetragonal and cubic phase [3]. (H and I) Time distribution of simulated bond angels in the room temperature tetragonal phase (H) and the high temperature cubic phase (I) [3]. (E) Reproduced from J.S. Bechtel, R. Seshadri, A. Van der Ven, Energy landscape of molecular motion in cubic methylammonium lead iodide from first-principles, J. Phys. Chem. C 120 (23) (2016) 1240312410, with permission from the American Chemical Society; (FI) Adapted from C. Quarti, E. Mosconi, J.M. Ball, V. D’Innocenzo, C. Tao, S. Pathak, et al., Structural and optical properties of methylammonium lead iodide across the tetragonal to cubic phase transition: implications for perovskite solar cells, Energy Environ. Sci. 9 (1) (2016) 155163, permission from the Royal Society of Chemistry.

Characterization Techniques for Perovskite Solar Cell Materials

183

8.1 Introduction

TABLE 8.1 Tolerance factors for the perovskite structure. Tolerance factor, t

Structure

Comment

,0.7





0.70.9

Tetragonal/Ortorombic/Rhombohedral

A too small or B too large

0.91.0

Cubic

Ideal perovskite structure

.1.0

Various layered structures

A-cation too large

Reproduced from T.J. Jacobsson, et al., J. Phys. Chem. C 119 (46) (2015) 2567325683, with permission from the American Chemical Society.

rA 1 rC t 5 pffiffiffi 2ðrB 1 rC Þ

(8.1)

There is a large class of related perovskite compounds with promising PVcharacteristics. Out of those, methyl ammonium lead iodide, CH3NH3PbI3 (or MAPbI3), is so far the most extensively investigated, and can therefore be seen as a standard perovskite and a model compound. Much of the subsequent discussion therefore use MAPbI3 as a baseline case. Given the ionic radius of Pb21 5 0.132 nm, I2 5 0.206, and CH3NH31 5 0.18 nm [2], MAPbI3 should according to the tolerance factor form a tetragonal structure, which also is the case at room temperature. The room temperature structure of organicinorganic lead halide perovskites is by no mean static, but is a highly dynamic system with atoms having a large amplitude around their equilibrium positions [3]. Core reasons behind this dynamism are the non-spherical symmetry of the organic MA/FA ions (Fig. 8.1D) and their week binding to the inorganic backbone of PbI6-octahedra. The organic ions reside in cuboctahedral voids with a nonuniform energy landscape spanned by the halogen atoms (Fig. 8.1E) [4]. In this landscape, the MA ions interact with dipoledipole interactions, where the NH?I interactions are the strongest [46], and have a number of energetically favorable orientations. The weak interactions means that the energy barriers for turning the MA-ions away from their favorable orientations are small, thus enabling jumping between preferred orientations in the halide cages [5,7]. At room temperature, the time scale for flipping around the MA ions in MAPbI3 is in the range of femtosecond to picoseconds [3,79], where a distinction can be made between a faster wobbling motion around the crystal axis and a slower reorientation of the molecular dipole with respect to the iodide lattice [8]. Defects can complicate the picture even further [10]. Those rotating dipoles implies a mean for screening charges by aligning towards them. This ability is reflected in a high dielectric constant [11,12], and is contributing to the low exciton energy observed [1315], and for why the hybrid perovskites are not excitonic materials at room temperature [13,15]. The ability to reorient around a localized charge may also have an impact on charge transport and it has been suggested that it slow down recombination [16]. Exactly how important the reorientabilty of the organic cation is for device performance is still an open question, especially as purely inorganic perovskite solar cells have been made [17,18], even if with lower efficiencies. To which extent the organic dipoles collectively order themselves into ferroelectric domains, and how this affect the charge charier separation, transport, and recombination

Characterization Techniques for Perovskite Solar Cell Materials

184

8. Temperature effects in lead halide perovskites

has gained a lot of attention [7,1922]. There is a balance between increased entropy by randomness and energetic gain by collective organization, and any collective ordering will decrease while the temperature increases. This balance will also depend on the perovskite composition. Given the current literature, much more investigations are required until we reach a comprehensive understanding and consensus of how it really works. The ionic rotation is highly temperature dependent. At higher temperatures, the probability to overcome the activation energy for flipping increases, which leads to faster rotation. It also decreases the relative energetic contribution from columbic interactions with stationary charges or external fields compared towards the thermal energy. This leads to a gradual decrease in the dielectric constant [11,12,23,24], and the probability of collective ordering. By changing the composition of the perovskite by gradually exchanging the halogen, or the organic ion, the energy landscape in the halogen cage can be tuned, and it has been observed that this can shift the behavior of the rotations, as well as the activation barriers [25]. If the temperature is low, the thermal energy will not be enough to overcome the rotational barriers and the organic ions freeze in position [26], most likely in an ordered fashion [23,27]. For MAPbI3, this results in a phase transformation to a low temperature orthorhombic phase around 2113
C [15,23,24,27], which has a substantially lower dielectric constant [11,12,23,24], a higher excition energy [23], lower mobility [28], and appears to be useless for solar cell applications [29]. At higher temperatures, the thermal energy can be substantially higher than the weak bonding energy between the organic ion and the halide cage, leading to essentially free organic cation rotation. One effect of this is that the time-average shape of the organic ions goes towards spherical symmetry, whereby they also effectively take up a larger volume. In terms of the tolerance factors in Eq. 8.1, this means a larger rA and consequently a larger t, which pushes the structure towards cubic symmetry. For MAPbI3, this leads to a phase transition between a tetragonal to a cubic high temperature phase, which in diffraction measurements is observed around 54
C [27,3032]. Accompanying this transition are changes in the tilt of the lead-halogen octahedrals (Fig. 8.1F and G) which can effects, for example, the band gap [33]. By changing the organic ion and the halogen, the temperature of this phase transition can be shifted, but the general trend towards higher symmetric phases at higher temperatures are general for perovskites, and observed also for completely inorganic perovskites. For MAPbBr3, the tetragonal to cubic phase transition is pushed down to 262
C [24], and for FAPbI3, it is observed at 273
C [32]. Mixed perovskites where part of the MA is replaced by FA and part of the I is replaced with Br can thus give a cubic phase at room temperature [34], and thereby shift the tetragonal to cubic phase transition out of the normal operational window of terrestrial solar cells. Even if the time average shape of the organic ions at higher temperatures are spherical, they still have a non-spherical molecular shape (Fig. 8.1D). If the inorganic lead halide framework dynamically can shift in line with the rotation of the organic ions, the local structure may deviate from the cubic time average seen in for example XRD and Raman measurements. Simulations have shown that the lead halide framework dynamically accommodate the motion on a time scale fast enough for this to be the case [3]. The high temperature cubic structure may thus better be described by a fast switching between different tetragonal local configurations, which average out to a cubic structure [3]. This is

Characterization Techniques for Perovskite Solar Cell Materials

8.1 Introduction

185

illustrated in Fig. 8.1FI with the simulated time distribution of lead-halide bond angels for both the tetragonal and the cubic phase. On the timescale of electronic transitions, the material may thus seldom experience a cubic environment, but rather a shifting distorted tetragonal one. This is perfectly in line with the observation that the phase transition between the tetragonal to the cubic phase of MAPbI3 at B55
C not give more than a small and gradual change in for example absorption and device characteristics [3,29].

8.1.2 Thermal expansion coefficients Phase transitions in perovskites can be investigated by, for example, X-ray diffraction measured as a function of temperature, which has been done both for MAPbI3 [30], and a few other compositions [27]. Polycrystalline MAPbI3 undergoes a phase transformation around 54
C, and single crystals around 55
C, between a tetragonal and a high temperature cubic phase, which is of special interest as it is within the operational temperature window of solar cells. The tetragonal and the cubic phase have similar, but still distinctly different, XRD-patterns as illustrated in Fig. 8.2A where a number of double peaks for the tetragonal phase merge into single peaks for the higher symmetry cubic phase. Experimental data demonstrating the change of the XRD-pattern over the phase transition is given in Fig. 8.2B [30]. While cycling the temperature between room

FIGURE 8.2 (A) Comparison between experimental XRD data and simulated data for both the tetragonal and the cubic phase. The peaks most clearly distinguishing the experimental room temperature structure of MAPbI3 as tetragonal are highlighted in gray. (B) XRD-data as a function of temperature illustrating the phase transformation from the tetragonal to the cubic phase at 54
C for a drop-cast sample of MAPbI3. (C) A graphical illustration of the change in cell parameters with increased temperature for MAPbI3. (D) Primitive unit cell volume of MAPbI3 as a function of temperature. (A, C, and D) Adapted from T.J. Jacobsson, L.J. Schwan, M. Ottosson, A. Hagfeldt, T. Edvinsson, Determination of thermal expansion coefficients and locating the temperature-induced phase transition in methylammonium lead perovskites using X-ray diffraction, Inorg. Chem. 54 (22) (2015) 1067810685; (B) Reproduced from T.J. Jacobsson, W. Tress, J.P. Correa-Baena, T. Edvinsson, A. Hagfeldt, Room temperature as a goldilocks environment for CH3NH3PbI3 perovskite solar cells: the importance of temperature on device performance, J. Phys. Chem. C 120 (21) (2016) 1138211393, both published with permission from the American Chemical Society.

Characterization Techniques for Perovskite Solar Cell Materials

186

8. Temperature effects in lead halide perovskites

temperature to 80
C, this transition is fully reversible, at least during a few cycles [30]. If the transition would be reversible also over the thousands of cycles perovskites in solar cells modules would experience under their lifetimes is still an open question. Something seen in experimental XRD-data (Fig. 8.2B) is a drift in the peak positions towards smaller angels with higher temperature, i.e. an expansion of the lattice [30]. With increased temperature, the tetragonal structure expands along the a-axis and contracts along the c-axis (Fig. 8.2C) and does so in a linear fashion [30]. After the phase transition at 54
C, the a-axis continues to expand, essentially without a discontinuity. This is in line with a gentle and gradual phase transition, which as discussed above is based on the rotational energy of the MA ions in the lead-halogen octahedron, and that the phase transition not requires atomic reorganizations in the form of breaking or formation of covalent or ionic bonds. Given the lattice parameters, the primitive cell volume can be extracted (Fig. 8.2D) which also is found to expand linearly with temperature. With lattice parameters and cell volume as a function of temperature, the thermal expansion coefficients are readily extracted, where the linear thermal expansion coefficient in the length dimension L, αL, is given by Eq. 8.2, and the volumetric expansion coefficient, αV, is given by Eq. 8.3 where V is volume. αL 5

1 @L L @T

(8.2)

αV 5

1 @V V @T

(8.3)

The thermal expansion coefficients are largely independent of temperature between 25
C and 80
C, and were determined to: αa_tet 5 1.32 1024 K21, αc_tet 5 21.06 1024 K21, and αa_cub 5 4.77 1025 K21 [30]. The volumetric thermal expansion coefficient was determined to; αv 5 1.57 1024 K21, and is independent of the perovskite phase. This is a rather high expansion coefficient. It is, for example, more than six times larger than for soda lime glass (αv 5 2.6 1025 K21), four times large than for steel (αvB3.5 1025 K21) and twice that of lead (αv 5 8.7 1025 K21). It is also considerably higher than for other thin film solar cell materials, like CIGS (αv 5 2.7 1025 K21) [35] and CdTe (αv 5 1.4 1025 K21) [36] which are reasonably close to the one of soda lime glass. Thermal expansion coefficients that are significantly different between the absorber layer and the substrate is potentially a problem as it results in mechanical stresses during temperature cycling. The temperature changes occurring under years of daynight cycling could thus pose a problem for the microscopic mechanical stability of perovskite solar cells. The severity of the problem is still an open question. To our knowledge, thermal expansion coefficients has not been extracted for other perovskite compositions than MAPbI3. Until further data is provided, it is reasonable to assume the expansion coefficients to be relatively high for all hybrid perovskites. The phase transition between the tetragonal and the cubic phase appears, at least from a crystallographic perspective, not to be a major problem. It is, however, probably best to avoid if possible, and by partially replacing the methyl ammonium ions with larger ions,

Characterization Techniques for Perovskite Solar Cell Materials

8.1 Introduction

187

e.g. formamidinium, the tolerance factors can be modified so that the phase transition is shifted to lower temperatures giving a preference for the cubic phase already at room temperature [34].

8.1.3 Optical properties Optical absorption is the basis for charge carrier generation, and is thereby one of the most important aspects of a solar cell material. When the temperature decreases, the absorption onset for MAPbI3 has been observed to sharpen and shift to lower energies [3,15,3740]. This is illustrated in Fig. 8.3AE where the optical absorption for MAPbI3

FIGURE 8.3 (A) Absorption for MAPbI3 at selected temperatures. Data is background corrected. (B) A photo of the MAPbI3 film on which absorption was measured. (C) Band gap deduced from absorption data as a function of temperature. The small dip around 260
C is an artifact due to condensation on the measurement chamber. (D) and (E) Photos of the Lincam cell used for temperature dependent UVvis and IV measurements. Reproduced from T.J. Jacobsson, W. Tress, J.P. Correa-Baena, T. Edvinsson, A. Hagfeldt, Room temperature as a goldilocks environment for CH3NH3PbI3 perovskite solar cells: the importance of temperature on device performance, J. Phys. Chem. C 120 (21) (2016) 1138211393, published with permission from the American Chemical Society.

Characterization Techniques for Perovskite Solar Cell Materials

188

8. Temperature effects in lead halide perovskites

was measured between 2190
C and 80
C [29]. The same behavior is reflected in steady state photoluminescence measurements [3739]. A shift in the absorption onset is equivalent to a shift in the band gap, which for MAPbI3 thus decreases with decreased temperature [3,41] (Fig. 8.3B), and while the temperature was changed from 80
C down to 2190
C, the band gap shifted from 1.61 eV down to 1.58 eV [29]. For most common semiconductors, e.g. Si, Ge, InP, InAs, GaAs, etc., the band gap does instead decrease with increased temperature, which can be explained by thermal expansion and changes in the electron-phonon interactions [42]. A possible explanation for the reversed behavior of MAPbI3 lies in the antibonding nature of its valence states [43]. At lower temperatures, the interatomic distances decreases which moves the energy of the antibonding states to higher energies due to increased orbital splitting, and thus decreases the band gap [37]. No changes or discontinuities were observed around the phase transition temperatures; 54
C for the tetragonal to cubic transition, and at 2113
C for the orthorhombic to tetrahedral transition. This behavior is in line with the phase transitions being gentle and gradual events. A change of the band gap of 0.03 eV is from a practical point of view a small change, especially given a temperature difference of almost 300 K. Significant temperature dependent changes in, for example, device performance can thus not be attributed to changes in the optical properties of the perovskite.

8.1.4 Degradation at higher temperature Perovskite solar cells have achieved impressive efficiencies over the last few years. Efficiencies that are high enough for devices to make a real technological impact if the long-term stability also can be controlled. Impressive improvements in stability have been reported over the last few years, i.e. from minutes to a few thousand hours under operational conditions, but further improvements are still needed. The pathways for perovskite degradation are plentiful and involves for example: moisture [44,45], light [46], oxygen [47,48], metal diffusion from contacts [49,50], etc. Increased temperature, which is the focus in this chapter, is also an important trigger for degradation. At elevated temperatures, MAPbI3 decompose into solid PbI2 and methyl ammonium and hydrogen iodine that can disappear in the gas phase (Eq. 8.4) [51]. Another suggested pathway involves decomposition of the methyl ammonium into ammonia and methyl iodine (Eq. 8.5) [52].   (8.4) CH3 NH3 PbI3 ðsÞ-PbI2 ðsÞ 1 CH3 NH2 ðgÞ 1 HI g   (8.5) CH3 NH3 PbI3 ðsÞ-PbI2 ðsÞ 1 NH3 g 1 CH3 IðgÞ The exact temperature when MAPbI3 decomposes is non-trivial to pinpoint. Thermogravimetric analysis (TGA) indicates that MAPbI3 can be stable up to around 200
C [27,51,52], whereas considerably lower temperatures appears to be a problem in most environments. In part, this discrepancy is about timing as only a small part of the material needs to decompose before the device performance is compromised. It is also due

Characterization Techniques for Perovskite Solar Cell Materials

8.1 Introduction

189

to combined effects as increases in temperature can accelerate decomposition along other pathways requiring for example light or moisture [53]. A typical annealing temperature during crystallization of MAPbI3 is 100
C. That is probably not high enough to decompose the perovskite in itself, unless there are other decomposition pathways available effected by the elevated temperature. To make it even more complicated, a certain degradation can under some conditions be beneficial. If the outermost surface layer decomposes during the annealing, a small excess of lead iodine can form which is beneficial for device performance [5456], as long as the other decomposition products are vented out in the gas phase. In terms of thermal degradation of MAPbI3, the weak point appears to be MA ions, and attempts have thus been made to replace them. One approach is to go for all inorganic perovskites with Cs as the cation. Cs based all inorganic solar cells have been made [17,18], indicating that it is possible, and those perovskites are indeed more thermally stable. Efficiencies are so far, however, inferior, but it is an open question whatever or not they can be as good as the hybrid perovskites, or if the dipole moment of the organic ion is necessary for achieving the highest efficiencies. Another alternative is to replace MA with formamidinium, FA [57], or guanidinium [58]. The FA perovskites appears to be more thermally stable [59]. Unfortunately, the cubic (or pseudocubic) α-phase of FAPbI3 easily transform into a yellow polymorph at room temperature which has hexagonal symmetry (P63mc) and is unsuitable for PVapplications [32,57,60,61]. That transformation could be avoided by mixing in some MA or Cs. At the time of writing, an interesting composition is mixed FACs-perovskites [62,63]. FA stabilises the Cs-perovskite, and Cs stabilising the FA perovskite, giving a mixture that has a stable cubic phase at room temperature. Those compositions thereby avoid MA and gain in thermal stability. So far, those perovskites are, however, not the best performing ones. Some of the best composition are at the time of writing Cs and/or Cs-Rb doped FAMA perovskites [64,65] (e.g. Cs0.5FA0.79MA0.16PbBr0.51I2.49) which has demonstrated good stability under illumination at 80
C [64,65]. This indicates that a small amount of MA-ions can be beneficial in terms of crystallization and in stabilising the FA perovskite, and that they potentially not seriously compromise the thermal stability up to 80
C as long as they only are present in a small amount.

8.1.5 Device performance From a solar cell perspective, the most important temperature dependent parameter is device performance. For MAPbI3, a few groups has investigated this both above [29,41,6668] and below [29,6668] room temperature. The details differ between the measurements but the general trends that here will be discussed are the same. Device performance as a function of temperature is in Fig. 8.4A 2 D given for MAPbI3 solar cells based on data from Jacobsson et al. [29], which are representative of the temperature dependent performance measurements so far reported. While increasing the temperature from room temperature up to 80
C, the highest efficiencies where measured at 3035
C. At higher temperatures, η, Voc, and Jsc, all gradually decreased (Fig. 8.4AC), whereas the FF peaked around 60
C (Fig. 8.4D). When the cells returned to room

Characterization Techniques for Perovskite Solar Cell Materials

190

8. Temperature effects in lead halide perovskites

FIGURE 8.4 Device performance as a function of temperature for devices of standard configuration, i.e. FTO/ TiO2/mesoporous TiO2/MAPbI3/Spiro-MeOTAD/Au. Two different cells where used for the measurements; one for temperatures above room temperature, and another one for lower temperatures. (A) Efficiency. (B) Open circuit voltage. (C) Short circuit current. (D) Fill factor. Data has previously been published by T.J. Jacobsson, W. Tress, J. P. Correa-Baena, T. Edvinsson, A. Hagfeldt, Room temperature as a goldilocks environment for CH3NH3PbI3 perovskite solar cells: the importance of temperature on device performance, J. Phys. Chem. C 120 (21) (2016) 1138211393.

temperature, they recovered most of the loss in performance, showing a reasonable degree of reversibility. The decrease in measured efficiency with increased temperature was approximately 0.08% K21. A decrease in efficiency at elevated temperatures is expected based on the behavior of other solar cell materials [69]. The magnitude is, however, larger than observed for silicon, and large enough to be a concern with only 80% of the room temperature performance remaining at 60
C. This is problematic but not a showstopper for outdoor use of perovskite solar cell; especially given the rapid evolution of both device performance and stability, which makes it reasonable to assume further improvements. The open circuit voltage is expected to decrease at higher temperatures as an effect of increased entropy, which reduces the electrochemical energy of electrons and holes in the conduction and valence band, broadens the Fermi-Dirac distribution, and pushes the quasi-Fermi levels further away from the band edges. The data up to 50
C follows this

Characterization Techniques for Perovskite Solar Cell Materials

8.1 Introduction

191

trend (Fig. 8.4A) [29,41], indicating that the Voc change is governed by charge carrier recombination in the perovskite film within that temperature range. At higher temperatures, the Voc decrease accelerates, which was attributed to irreversible degradation [29]. It is assumed that the MA ions trigger a thermal instability [27,5153], which may be the cause for the observed degradation. Other compositions, especially Cs and Cs/Rb doped mixed perovskites (e.g. Cs0.5FA0.79MA0.16PbBr0.51I2.49) has showed higher thermal stability with up to 1000 hour of stability at 80
C [64,65], indicating that thermal stability probably can be accomplished within the operational window of terrestrial photovoltaics. A question of interest is if the perovskite phase, and in particular the phase transformation from tetragonal to cubic seen for MAPbI3 at 54
C, has an effect on the device performance. The downward trend in η, Vov and Jsc while the temperature is increased is uniform. The measurement point at 60
C, just after the phase transition, give somewhat higher values of all four solar cell parameters: η, Vov, Jsc, and FF, as well as a lower hysteresis. This indicates that the cubic phase may be somewhat advantageous, which is in line with that the best performing solar cells at the moment are based on formamidinium rich perovskites [57,7072], which have a cubic symmetry at room temperature [73]. The temperature trend is, however, stronger than this effect and the increase in performance at 60
C, just above the phase transition temperature, is merely a dent in the downward trend. At lower temperatures, the changes in device performance are more dramatic. The Voc increases with decreasing temperature down to 280
C, which continues the trend seen for higher temperatures up to 50
C (Fig. 8.4B). That is in line with the expected behavior for inorganic semiconductor solar cells, and has been observed by different groups [29,66]. At even lower temperatures, the Voc decreases, and at temperatures below 2160
C, it goes to zero. The lower Voc is indicative of a reduction in charge carrier density, i.e. by either an increased recombination rate or by a reduced probability of charge carer generation by exciton dissociation [74]. The FF decreases unevenly down to 2120
C (Fig. 8.4D) where the IV-curve has gone from the characteristic shape of a solar cell response to a straight line. That is indicative of a highly resistive behavior, which for temperatures below 280
C could be attributed to a high series resistance [29]. The current was stable down to 260
C where after it continuously dropped down to almost zero at 2190
C (Fig. 8.4C), consistent with a high series resistance [29]. Also the hysteresis is strongly affected and vanishes completely at 280
C [29], which probably is an effect of restriction of the ionic motion often seen as responsible for the hysteresis [7580]. All those effects appears to be, given proper encapsulation, reversible with respect to temperature [29]. Several groups have reported similar device behavior at decreased temperatures [29,6668]. The best performance was also for the measurements at decreased temperatures found around room temperature [29,66,67], even if the drop in performance down to 280
C was reasonable low from an operational perspective (Fig. 8.4A). This behavior is in contrast to conventional PV-materials e.g. silicon, where a lower temperature is strictly better for the device performance, indicating that other physical mechanisms are in play in the perovskites. The behavior of the Voc and the increased series resistance were found to be unrelated pointing at more than one mechanism for decreasing performance at low temperatures

Characterization Techniques for Perovskite Solar Cell Materials

192

8. Temperature effects in lead halide perovskites

[29]. One possible explanation for the increased resistance is a decrease of thermally activated charge hopping in the Spiro-MeOTAD hole conductor, which is in line with reported decrease in conductivity and hole mobility at lower temperature [81,82]. Other possible mechanisms relate to the perovskite phase. THz spectroscopy indicate that the mobility increases at lower temperature [39], in line with the simple Drude model [83], but the charge transport and recombination at grain boundaries might limit the device performance. Another possibility is the phase transformation from a tetragonal phase to a low temperature orthorhombic phase around 2113
C [15,23,24,27], which correlate reasonably well with the drop in Jsc and η as well as to a reported increase in the recombination constants [39]. A possible microscopic explanation is based on the movement and rotation of the organic dipolar cations, which at the lower temperatures is frozen in place when the thermal energy is not enough to flip them between their energetically preferred orientations in the lead halide cages [26], which possibly limits the current transport trough the perovskite. From a practical perspective. Elevated temperatures decrease cell performance, in line with expectations based on other solar cell technologies. The rate of decrease observed for MAPbI3 is 0.08% K21, which is high, but not catastrophically high. There is also a problem with thermal stability. It is, however, reasonable to assume that both aspects can be improved with compositional engineering of the perovskite and better interface control. At lower temperatures but within the boundaries for normal solar cell usage, performance decreases somewhat but not enough to be a problem. Room temperature thus seems to be something of a Goldilock’s zone for perovskite solar cells. In part because of peculiarities in the perovskite physics, and in part also because room temperature is the temperature where optimization has occurred. At even lower temperatures, perovskite solar cells does not appear to work at all. Why is still an open question, but it means that perovskites made with the present hole-selective layers not will be a good technology for space applications.

References [1] V.M. Goldschmidt, The laws of crystal chemistry, Naturwissenschaften 14 (1926) 477485. [2] N.K. McKinnon, D.C. Reeves, M.H. Akabas, 5-HT3 receptor ion size selectivity is a property of the transmembrane channel, not the cytoplasmic vestibule portals, J. Gen. Physiol. 138 (4) (2011) 453466. [3] C. Quarti, E. Mosconi, J.M. Ball, V. D’Innocenzo, C. Tao, S. Pathak, et al., Structural and optical properties of methylammonium lead iodide across the tetragonal to cubic phase transition: implications for perovskite solar cells, Energy Environ. Sci. 9 (1) (2016) 155163. [4] J.S. Bechtel, R. Seshadri, A. Van der Ven, Energy landscape of molecular motion in cubic methylammonium lead iodide from first-principles, J. Phys. Chem. C 120 (23) (2016) 1240312410. [5] Y. Ren, I.W. Oswald, X. Wang, G.T. McCandless, J.Y. Chan, Orientation of organic cations in hybrid inorganicorganic perovskite CH3NH3PbI3 from subatomic resolution single crystal neutron diffraction structural studies, Cryst. Growth Des. 16 (5) (2016) 29452951. [6] T. Baikie, N.S. Barrow, Y. Fang, P.J. Keenan, P.R. Slater, R.O. Piltz, et al., A combined single crystal neutron/ X-ray diffraction and solid-state nuclear magnetic resonance study of the hybrid perovskites CH3NH3PbX3 (X 5 I, Br and Cl), J. Mater. Chem. A 3 (17) (2015) 92989307. [7] A.M.A. Leguy, J.M. Frost, A.P. McMahon, V.G. Sakai, W. Kochelmann, C.H. Law, et al., The dynamics of methylammonium ions in hybrid organic-inorganic perovskite solar cells, Nat. Commun. 6 (2015).

Characterization Techniques for Perovskite Solar Cell Materials

References

193

[8] A.A. Bakulin, O. Selig, H.J. Bakker, Y.L.A. Rezus, C. Muller, T. Glaser, et al., Real-time observation of organic cation reorientation in methylammonium lead iodide perovskites, J. Phys. Chem. Lett. 6 (18) (2015) 36633669. [9] C. Goehry, G.A. Nemnes, A. Manolescu, Collective behavior of molecular dipoles in CH3NH3PbI3, J. Phys. Chem. C 119 (34) (2015) 1967419680. [10] M. Pazoki, M. Wolf, T. Edvinsson, J. Kullgren, Vacancy dipole interactions and the correlation with monovalent cation dependent ion movement in lead halide perovskite solar cell materials, Nano Energy 38 (2017) 537543. [11] S. Govinda, B.P. Kore, M. Bokdam, P. Mahale, A. Kumar, S. Pal, et al., Behavior of methylammonium dipoles in MAPbX3 (X 5 Br and I), J. Phys. Chem. Lett. 8 (17) (2017) 41134121. ˇ Svirskas, M. Sanlialp, G. Lackner, et al., Dielectric response: answer [12] I. Anusca, S. Balˇciunas, P. Gemeiner, S. ¯ to many questions in the methylammonium lead halide solar cell absorbers, Adv. Energy Mater. 7 (2017) 19. [13] A. Miyata, A. Mitioglu, P. Plochocka, O. Portugall, J.T.-W. Wang, S.D. Stranks, et al., Direct measurement of the exciton binding energy and effective masses for charge carriers in organic-inorganic tri-halide perovskites, Nat. Phys. 11 (7) (2015) 582587. [14] S.A. March, C. Clegg, D.B. Riley, D. Webber, I.G. Hill, K.C. Hall, Simultaneous observation of free and defect-bound excitons in CH3NH3PbI3 using four-wave mixing spectroscopy, Sci. Rep. 6 (2016) 39139. [15] V. D’Innocenzo, G. Grancini, M.J.P. Alcocer, A.R.S. Kandada, S.D. Stranks, M.M. Lee, et al., Excitons versus free charges in organo-lead tri-halide perovskites, Nat. Commun. 5 (2014). [16] C. Motta, F. El-Mellouhi, S. Kais, N. Tabet, F. Alharbi, S. Sanvito, Revealing the role of organic cations in hybrid halide perovskite CH3NH3PbI3, Nat. Commun. 6 (2015). [17] C.Y. Chen, H.Y. Lin, K.M. Chiang, W.L. Tsai, Y.C. Huang, C.S. Tsao, et al., All-vacuum-deposited stoichiometrically balanced inorganic cesium lead halide perovskite solar cells with stabilized efficiency exceeding 11%, Adv. Mater. 29 (12) (2017). [18] M. Kulbak, D. Cahen, G. Hodes, How important is the organic part of lead halide perovskite photovoltaic cells? Efficient CsPbBr3 cells, J. Phys. Chem. Lett. 6 (13) (2015) 24522456. [19] M. Hu, C. Bi, Y. Yuan, Z. Xiao, Q. Dong, Y. Shao, et al., Distinct exciton dissociation behavior of organolead trihalide perovskite and excitonic semiconductors studied in the same system, Small 11 (18) (2015) 21642169. [20] J. Beilsten-Edmands, G.E. Eperon, R.D. Johnson, H.J. Snaith, P.G. Radaelli, Non-ferroelectric nature of the conductance hysteresis in CH3NH3PbI3 perovskite-based photovoltaic devices, Appl. Phys. Lett. 106 (17) (2015). [21] G.A. Sewvandi, K. Kodera, H. Ma, S. Nakanishi, Q. Feng, Antiferroelectric nature of CH3NH3PbI3 2 xCx perovskite and its implication for charge separation in perovskite solar cells, Sci. Rep. 6 (2016) 30680. [22] J.M. Frost, K.T. Butler, A. Walsh, Molecular ferroelectric contributions to anomalous hysteresis in hybrid perovskite solar cells, APL Mater. 2 (8) (2014) 081506. [23] H. Wang, L. Whittaker-Brooks, G.R. Fleming, Exciton and free charge dynamics of methylammonium lead iodide perovskites are different in the tetragonal and orthorhombic phases, J. Phys. Chem. C 119 (34) (2015) 1959019595. [24] N. Onoda-Yamamuro, T. Matsuo, H. Suga, Dielectric study of CH3NH3PbX3 (X 5 Cl, Br, I), J. Phys. Chem. Sol. 53 (7) (1992) 935939. [25] O. Selig, A. Sadhanala, C. Mu¨ller, R. Lovrincic, Z. Chen, Y.L. Rezus, et al., Organic cation rotation and immobilization in pure and mixed methylammonium lead-halide perovskites, J. Am. Chem. Soc. 139 (11) (2017) 40684074. [26] M.C. Ge´lvez-Rueda, D.H. Cao, S. Patwardhan, N. Renaud, C.C. Stoumpos, G.C. Schatz, et al., Effect of cation rotation on charge dynamics in hybrid lead halide perovskites, J. Phys. Chem. C 120 (30) (2016) 1657716585. [27] T. Baikie, Y. Fang, J.M. Kadro, M. Schreyer, F. Wei, S.G. Mhaisalkar, et al., Synthesis and crystal chemistry of the hybrid perovskite (CH3NH3) PbI3 for solid-state sensitised solar cell applications, J. Mater. Chem. A 1 (18) (2013) 56285641. [28] T.J. Savenije, C.S. Ponseca Jr, L. Kunneman, M. Abdellah, K. Zheng, Y. Tian, et al., Thermally activated exciton dissociation and recombination control the carrier dynamics in organometal halide perovskite, J. Phys. Chem. Lett. 5 (13) (2014) 21892194.

Characterization Techniques for Perovskite Solar Cell Materials

194

8. Temperature effects in lead halide perovskites

[29] T.J. Jacobsson, W. Tress, J.P. Correa-Baena, T. Edvinsson, A. Hagfeldt, Room temperature as a goldilocks environment for CH3NH3PbI3 perovskite solar cells: the importance of temperature on device performance, J. Phys. Chem. C 120 (21) (2016) 1138211393. [30] T.J. Jacobsson, L.J. Schwan, M. Ottosson, A. Hagfeldt, T. Edvinsson, Determination of thermal expansion coefficients and locating the temperature-induced phase transition in methylammonium lead perovskites using X-ray diffraction, Inorg. Chem. 54 (22) (2015) 1067810685. [31] A. Poglitsch, D. Weber, Dynamic disorder in methylammoniumtrihalogenoplumbates(Ii) Observed by millimeter-wave spectroscopy, J. Phys. Chem. 87 (11) (1987) 63736378. [32] C.C. Stoumpos, C.D. Malliakas, M.G. Kanatzidis, Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties, Inorg. Chem. 52 (15) (2013) 90199038. [33] M.R. Filip, G.E. Eperon, H.J. Snaith, F. Giustino, Steric engineering of metal-halide perovskites with tunable optical band gaps, Nat. Commun. 5 (2014). [34] T.J. Jacobsson, J.-P. Correa-Baena, M. Pazoki, M. Saliba, K. Schenk, M. Gratzel, et al., Exploration of the compositional space for mixed lead halogen perovskites for high efficiency solar cells, Energ. Environ. Sci. 9 (5) (2016) 17061724. [35] S.R. Kodigala, Cu(In1-xGax)Se2 Based Thin Film Solar Cells, vol. 35, Academic Press, 2011. [36] P. Capper, Properties of Narrow Gap Cadmium-Based Compounds, INSPEC, 1994. [37] B.J. Foley, D.L. Marlowe, K. Sun, W.A. Saidi, L. Scudiero, M.C. Gupta, et al., Temperature dependent energy levels of methylammonium lead iodide perovskite, Appl. Phys. Lett. 106 (24) (2015). [38] Y. Yamada, T. Nakamura, M. Endo, A. Wakamiya, Y. Kanemitsu, Near-band-edge optical responses of solution-processed organic-inorganic hybrid perovskite CH3NH3PbI3 on mesoporous TiO2 electrodes, Appl. Phys. Express 7 (3) (2014). [39] R.L. Milot, G.E. Eperon, H.J. Snaith, M.B. Johnston, L.M. Herz, Temperature-dependent charge-carrier dynamics in CH3NH3PbI3 perovskite thin films, Adv. Funct. Mater. 25 (39) (2015) 62186227. [40] M.N.F. Hoque, N. Islam, Z. Li, G. Ren, K. Zhu, Z. Fan, Ionic and optical properties of methylammonium lead iodide perovskite across the tetragonalcubic structural phase transition, ChemSusChem 9 (18) (2016) 26922698. [41] W.L. Leong, Z.-E. Ooi, D. Sabba, C. Yi, S.M. Zakeeruddin, M. Graetzel, et al., Identifying fundamental limitations in halide perovskite solar cells, Adv. Mater. (2016). n/an/a. [42] Y.P. Varshni, Temperature dependence of the energy gap in semiconductors, Physica 34 (1) (1967) 149154. [43] T. Umebayashi, K. Asai, T. Kondo, A. Nakao, Electronic structures of lead iodide based low-dimensional crystals, Phys. Rev. B 67 (15) (2003) 155405. [44] Q.-D. Dao, R. Tsuji, A. Fujii, M. Ozaki, Study on degradation mechanism of perovskite solar cell and their recovering effects by introducing CH3NH3I layers, Org. Electron. 43 (2017) 229234. [45] M. Shirayama, M. Kato, T. Miyadera, T. Sugita, T. Fujiseki, S. Hara, et al., Degradation mechanism of CH3NH3PbI3 perovskite materials upon exposure to humid air, J. Appl. Phys. 119 (11) (2016) 115501. [46] S.-W. Lee, S. Kim, S. Bae, K. Cho, T. Chung, L.E. Mundt, et al., UV degradation and recovery of perovskite solar cells, Sci. Rep. 6 (2016) 38150. [47] D. Bryant, N. Aristidou, S. Pont, I. Sanchez-Molina, T. Chotchunangatchaval, S. Wheeler, et al., Light and oxygen induced degradation limits the operational stability of methylammonium lead triiodide perovskite solar cells, Energy Environ. Sci. 9 (5) (2016) 16551660. [48] N. Aristidou, I. Sanchez-Molina, T. Chotchuangchutchaval, M. Brown, L. Martinez, T. Rath, et al., The role of oxygen in the degradation of methylammonium lead trihalide perovskite photoactive layers, Angew. Chem. Int. Ed. 54 (28) (2015) 82088212. [49] C. Besleaga, L.E. Abramiuc, V. Stancu, A.G. Tomulescu, M. Sima, L. Trinca, et al., Iodine migration and degradation of perovskite solar cells enhanced by metallic electrodes, J. Phys. Chem. Lett. 7 (24) (2016) 51685175. [50] K. Domanski, J.-P. Correa-Baena, N. Mine, M.K. Nazeeruddin, A. Abate, M. Saliba, et al., Not all that glitters is gold: metal-migration-induced degradation in perovskite solar cells, ACS Nano 10 (6) (2016) 63066314. [51] A. Dualeh, P. Gao, S.I. Seok, M.K. Nazeeruddin, M. Gra¨tzel, Thermal behavior of methylammonium leadtrihalide perovskite photovoltaic light harvesters, Chem. Mater. 26 (21) (2014) 61606164. [52] E.J. Juarez-Perez, Z. Hawash, S.R. Raga, L.K. Ono, Y. Qi, Thermal degradation of CH3NH3PbI3 perovskite into NH3 and CH3I gases observed by coupled thermogravimetrymass spectrometry analysis, Energy Environ. Sci. 9 (11) (2016) 34063410.

Characterization Techniques for Perovskite Solar Cell Materials

References

195

[53] Y. Han, S. Meyer, Y. Dkhissi, K. Weber, J.M. Pringle, U. Bach, et al., Degradation observations of encapsulated planar CH3NH3PbI3 perovskite solar cells at high temperatures and humidity, J. Mater. Chem. A 3 (15) (2015) 81398147. [54] C. Roldan-Carmona, P. Gratia, I. Zimmermann, G. Grancini, P. Gao, M. Graetzel, et al., High efficiency methylammonium lead triiodide perovskite solar cells: the relevance of non-stoichiometric precursors, Energy Environ. Sci. 8 (12) (2015) 35503556. [55] D. Bi, W. Tress, M.I. Dar, P. Gao, J. Luo, C. Renevier, et al., Efficient luminescent solar cells based on tailored mixed-cation perovskites, Sci. Adv. 2 (1) (2016). [56] Y.C. Kim, N.J. Jeon, J.H. Noh, W.S. Yang, J. Seo, J.S. Yun, et al., Beneficial effects of PbI2 incorporated in organo-lead halide perovskite solar cells, Adv. Energy Mater 6 (2016) 4. [57] T.M. Koh, K. Fu, Y. Fang, S. Chen, T.C. Sum, N. Mathews, et al., Formamidinium-containing metal-halide: an alternative material for near-IR absorption perovskite solar cells, J. Phys. Chem. C 118 (30) (2014) 1645816462. [58] A.D. Jodlowski, C. Rolda´n-Carmona, G. Grancini, M. Salado, M. Ralaiarisoa, S. Ahmad, et al., Large guanidinium cation mixed with methylammonium in lead iodide perovskites for 19% efficient solar cells, Nat. Energy 2 (12) (2017) 972. [59] G.E. Eperon, S.D. Stranks, C. Menelaou, M.B. Johnston, L.M. Herz, H.J. Snaith, Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells, Energy Environ. Sci. 7 (3) (2014) 982988. [60] A. Binek, F.C. Hanusch, P. Docampo, T. Bein, Stabilization of the trigonal high-temperature phase of formamidinium lead iodide, J. Phys. Chem. Lett. 6 (7) (2015) 12491253. [61] T.J. Jacobsson, J.-P. Correa-Baena, E. Halvani Anaraki, B. Philippe, S.D. Stranks, M.E.F. Bouduban, et al., Unreacted PbI2 as a double-edged sword for enhancing the performance of perovskite solar cells, J. Am. Chem. Soc. 138 (32) (2016) 1033110343. [62] J.W. Lee, D.H. Kim, H.S. Kim, S.W. Seo, S.M. Cho, N.G. Park, Formamidinium and cesium hybridization for photo- and moisture-stable perovskite solar cell, Adv. Energy Mater. 5 (20) (2015). [63] Z. Li, M. Yang, J.-S. Park, S.-H. Wei, J.J. Berry, K. Zhu, Stabilizing perovskite structures by tuning tolerance factor: formation of formamidinium and cesium lead iodide solid-state alloys, Chem. Mater. 28 (1) (2016) 284292. [64] M. Saliba, T. Matsui, K. Domanski, J.Y. Seo, A. Ummadisingu, S.M. Zakeeruddin, et al., Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance, Science 354 (6309) (2016) 206209. [65] M. Saliba, T. Matsui, J.Y. Seo, K. Domanski, J.P. Correa-Baena, M.K. Nazeeruddin, et al., Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency, Energy Environ. Sci. 9 (6) (2016) 19891997. [66] H. Zhang, X. Qiao, Y. Shen, T. Moehl, S.M. Zakeeruddin, M. Graetzel, et al., Photovoltaic behaviour of lead methylammonium triiodide perovskite solar cells down to 80 K, J. Mater. Chem. A 3 (22) (2015) 1176211767. [67] L. Cojocaru, S. Uchida, Y. Sanehira, V. Gonzalez-Pedro, J. Bisquert, J. Nakazald, et al., Temperature effects on the photovoltaic performance of planar structure perovskite solar cells, Chem. Lett. 44 (11) (2015) 15571559. [68] S. Aharon, A. Dymshits, A. Rotem, L. Etgar, Temperature dependence of hole conductor free formamidinium lead iodide perovskite based solar cells, J. Mater. Chem. A 3 (17) (2015) 91719178. [69] E. Skoplaki, J.A. Palyvos, On the temperature dependence of photovoltaic module electrical performance: a review of efficiency/power correlations, Sol. Energ. 83 (5) (2009) 614624. [70] N. Pellet, P. Gao, G. Gregori, T.-Y. Yang, M.K. Nazeeruddin, J. Maier, et al., Mixed-organic-cation perovskite photovoltaics for enhanced solar-light harvesting, Angew. Chem. Int. Edit. 53 (12) (2014) 31513157. [71] J.-W. Lee, D.-J. Seol, A.-N. Cho, N.-G. Park, High-efficiency perovskite solar cells based on the black polymorph of HC(NH2)(2)PbI3, Adv. Mater. 26 (29) (2014) 49914998. [72] W.S. Yang, J.H. Noh, N.J. Jeon, Y.C. Kim, S. Ryu, J. Seo, et al., High-performance photovoltaic perovskite layers fabricated through intramolecular exchange, Science 348 (6240) (2015) 12341237. [73] M.F. Aygueler, M.D. Weber, B.M.D. Puscher, D.D. Medina, P. Docampo, R.D. Costa, Light-emitting electrochemical cells based on hybrid lead halide perovskite nanoparticles, J. Phys. Chem. C 119 (21) (2015) 1204712054.

Characterization Techniques for Perovskite Solar Cell Materials

196

8. Temperature effects in lead halide perovskites

[74] F. Gao, W. Tress, J. Wang, O. Inganas, Temperature dependence of charge carrier generation in organic photovoltaics, Phys. Rev. Lett. 114 (12) (2015). [75] J.M. Azpiroz, E. Mosconi, J. Bisquert, F. De Angelis, Defect migration in methylammonium lead iodide and its role in perovskite solar cell operation, Energy Environ. Sci. 8 (7) (2015) 21182127. [76] J. Haruyama, K. Sodeyama, L. Han, Y. Tateyama, First-principles study of ion diffusion in perovskite solar cell sensitizers, J. Am. Chem. Soc. 137 (32) (2015) 1004810051. [77] E.L. Unger, E.T. Hoke, C.D. Bailie, W.H. Nguyen, A.R. Bowring, T. Heumueller, et al., Hysteresis and transient behavior in current-voltage measurements of hybrid-perovskite absorber solar cells, Energy Environ. Sci. 7 (11) (2014) 36903698. [78] C. Eames, J.M. Frost, P.R.F. Barnes, B.C. O’Regan, A. Walsh, M.S. Islam, Ionic transport in hybrid lead iodide perovskite solar cells, Nat. Commun. 6 (2015). [79] S. Meloni, T. Moehl, W. Tress, M. Franckevicius, M. Saliba, Y.H. Lee, et al., Ionic polarization-induced current-voltage hysteresis in CH3NH3PbX3 perovskite solar cells, Nat. Commun. 7 (2016). [80] L.K. Ono, S.R. Raga, S. Wang, Y. Kato, Y. Qi, Temperature-dependent hysteresis effects in perovskite-based solar cells, J. Mater. Chem. A 3 (17) (2015) 90749080. [81] D. Poplavskyy, J. Nelson, Nondispersive hole transport in amorphous films of methoxy-spirofluorenearylamine organic compound, J. Appl. Phys. 93 (1) (2003) 341346. [82] A. Dualeh, T. Moehl, M.K. Nazeeruddin, M. Graetzel, Temperature dependence of transport properties of Spiro-MeOTAD as a hole transport material in solid-state dye-sensitized solar cells, ACS Nano 7 (3) (2013) 22922301. [83] M. Pazoki, T.J. Jacobsson, S.H.T. Cruz, M.B. Johansson, R. Imani, J. Kullgren, et al., Photon energy-dependent hysteresis effects in lead halide perovskite materials, J. Phys. Chem. C 121 (47) (2017) 2618026187.

Characterization Techniques for Perovskite Solar Cell Materials

C H A P T E R

9 Stability of materials and complete devices Nga Phung and Antonio Abate Helmholtz-Zentrum Berlin for Materials and Energy, Berlin, Germany

9.1 Introduction Over the last few years, the research on halide perovskite-based solar cells progressed extraordinary fast. The power conversion efficiency (PCE) approached more than 23%, which surpasses the established photovoltaic (PV) technologies such as CIGS or polycrystalline silicon solar cells until this chapter written [1]. However, large-scale industrial application of perovskite solar cells (PSCs) is facing the challenge of long-term stability as the initial efficiency of PSCs rapidly drops in the working conditions, i.e. light, biases, atmospheric exposure, etc. Current technologies with similar efficiencies are commercially viable for large-scale energy production since they are relatively stable for more than 20 years. This chapter will focus on the stability of PSCs in common terrestrial working condition. The first discussion point is stability testing. Testing is important to determine or to predict accurately how the solar cells will operate in real working conditions over time. We will review the established testing schemes that are currently available for PSCs. Throughout the chapter, stability is defined as the ability to keep its initial efficiency in operational conditions. Degradation of the initial performances stems from various stressors which will be discussed in the latter part of the chapter. The stressors can be from the internal factors, such as the intrinsic stability of perovskite material and stemming from other layers within the devices, as well as aroused from external, i.e. if the material is decomposed into non-photovoltaic functional material because of environmental factors such as water and oxygen. We will discuss the perovskite stability as well as the role of contact layers and electrodes within a whole device. We try to isolate the effect of each stressing parameter on each device component. Nonetheless, degradation is often a convolution of several external factors and device components, which creates a complex puzzle to understand the stability of PSCs thoroughly.

Characterization Techniques for Perovskite Solar Cell Materials DOI: https://doi.org/10.1016/B978-0-12-814727-6.00009-8

197

Copyright © 2020 Elsevier Inc. All rights reserved.

198

9. Stability of materials and complete devices

9.2 Stability testing 9.2.1 Conventional testing Severe testing protocols define a PV technology as ready for scaling up from a lab scale to an industrial application. Up to date, the solar energy industry has adopted a detailed testing procedure to determine the performance of a PV panel. The procedure includes an efficiency test under standard testing condition (STC), i.e. cells at 25
C, illuminated by 1000 W/m2 with AM1.5 global solar spectrum. The power conversion efficiency is extracted from a current densityvoltage (JV) scan under STC. To ensure the reliability of solar cells in real working conditions, the devices are subjected to a sequence of testing, i.e. the International Electrotechnical Commission (IEC) standards. IEC standards comprise a damp heat test, i.e. 85
C and 85% relative humidity (RH) for 1000 hours, a thermal cycles test. i.e. 200 temperature cycles between -40
C and 185
C, and an accelerated stress test to examine the robustness of material and the reliability of the module’s design [2]. The test result can be used to indicate the T80 of a module, which describes the time it takes for a panel degrading until 80% of its initial PCE. For established PV technologies, the T80 can be up to 2025 years [3]. This above testing scheme was firstly developed for crystalline silicon PV modules. Since each PV technologies have different characteristics, the testing scheme is modified accordingly to the common detected failures of a certain PV technology. Perovskite community has embraced the knowledge from both inorganic and organic PV (OPV) testing to determine its stability, e.g. to identify its T80 when the device’s efficiency degrades to 80% of its initial value as well as TS80 as the stabilized efficiency which neglects the burn-in period or initial fast decay of efficiency typically seen in OPV. Compared to the lifetime of established inorganic PV technologies, PSCs exhibit a much faster degradation. Although there is no standardized testing for PSCs to date, in the next section, we will review the current testing procedures that have been proposed to report the stability of the PSCs.

9.2.2 Perovskite testing 9.2.2.1 Initial efficiency testing Standard testing conditions making use of JV scans are widely used to measure the initial efficiency of PSCs. However, both power conversion efficiencies (PCE) of forward scan (from short circuit current to forward bias, i.e. from 0 V to VOC) and reverse scan (from forward bias to short circuit, i.e. from VOC to 0 V) need to be reported owing to the well-known hysteresis effects in JV curves reported for the first time by Snaith et al. [4] in 2014. It was observed that not only the efficiencies obtained from forward or reverse scans are different, but also that all the photovoltaic parameters depend on the scan rates and prior conditions such as pre-bias [5], and device’s temperature [6,7]. Although most reported JV scans are done with AM1.5 simulated solar spectrum following STC, it has been shown that the hysteresis is affected by certain light bias, in other words, the device’s behavior is influenced by the intensity and spectrum of the input light [8]. A higher rate of ionic movement had been suggested by Pazoki et al. in devices based on mixed cation

Characterization Techniques for Perovskite Solar Cell Materials

9.2 Stability testing

199

perovskite films by which the high-energy blue photons caused more ionic movement and a larger hysteresis in JV scans compared to the red photons [8]. Hence, it is crucial to provide detailed testing conditions of JV measurement for PSCs and to complete the device JV characterization with a stabilized power output, which is extracted tracking the MPP for few minutes [9]. 9.2.2.2 Laboratory long-term stability testing One common method to determine the device stability is to measure JV scans of PSCs periodically up until the device efficiency drops to the 80% of the initial value [10]. Nonetheless, it is still inadequate to rely only on JV scans to examine the performance of PSCs. It has been reported that the efficiency obtained from periodic JV scans can be higher compared to the efficiency of the same device extracted from the maximum power point (MPP) tracking [11], i.e. stabilised power output [12]. Since MPP represents the real working condition, efficiency obtained from MPP will be more accurate to estimate the energy output of a solar cell in operation. Hence, only data from JV scans gives an estimated picture of device’s performance that is different from the corresponding value in the long-term operational conditions, so information from MPP tracking is crucial to examine the stability of PSCs. To the best of our knowledge, standard MPP tracking algorithm for PSCs has not been established yet at the time of writing, although efforts have been made to find the most suitable algorithm for PSCs [13,14]. In this section, we will take the conventional perturb and observe (P&O) method for our discussion because of its balance between the accuracy and the ease of use [14]. At first, the algorithm applies maximum power point voltage (VMPP) from previous or initial JV scan and measures the power output. Then, it increases/decreases applied voltage and calculates the power difference, if the power output increases, it will continue to increase/decrease applied voltage and the reverse is true. Eventually, the applied voltage fluctuates around VMPP. During a relatively short time (commonly around 25 minutes), assuming that the degradation is negligible, MPP curves of PSCs can reach a quasi-steady state, which can be considered as the stabilized efficiency of the device. In a longer timescale (i.e. several hours), the PSCs usually show a relatively large decay in the output power. The MPP curves can be fitted by a double exponential decay as shown in Fig. 9.1A [15]. The device’s efficiency decreases at a fast rate in the beginning, and then the degradation rate reduces significantly. An initial transient of the power output is not unique for halide perovskite-based devices. The fast decay observed in PSCs is somewhat similar to what has been reported in amorphous silicon (a-Si) solar cells and polymer solar cells which is called ‘burn-in’ period [16]. After this period, the degradation rate is noticeably slower. However, the degradation of PSCs has fundamental differences compared to other PV technologies, though depending on the composition of the perovskite, the degradation rates differ. As has been seen in various perovskite compositions, the efficiency loss due to light soaking can be divided into reversible losses and nonreversible losses or permanent losses. While for a-Si solar cells, the recovery may take place over weeks due to heating in summer [17], in PSCs, the partial recovery can happen overnight. Domanski et al. showed that after operating for few hours in one sun illumination, PSCs (FTO/TiO2/Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3/Spiro-OMETAD/Au) degraded to 90%

Characterization Techniques for Perovskite Solar Cell Materials

200

9. Stability of materials and complete devices

FIGURE 9.1 Perovskite solar cells experience the reversible/nonreversible losses after day-night cycles in N2 atmosphere after UV filtered (A) the device’s MPP curves can be fitted at double exponential decay with two time scales, (B) one device with 4 day-night cycles where efficiency decreases within 5 hours at one sun, (C) distinction from reversible and permanent degradation (black symbols are representative of continuous MPP tracking, and red symbols (gray symbols in print version) represent device’s MPP curve with day/night cycles) [18]. Reproduced with permission. Copyright 2017, The Royal Society of Chemistry.

of initial efficiency, but they partially recovered the efficiency loss after night resting in the dark, as shown in Fig. 9.1 [18] (note that MA here stands for methylammonium, and FA is formamidinium). The authors accounted this for the effect of ions and ion defects migration due to an internal electric filed under the working condition. The process of reversible losses can relate to Debye layers at the interface of perovskite and contact layers that can act as a charge collection barrier [18]. Because the ionic movement is reversible, the efficiency of the devices is degraded but can be recovered when ion vacancies are redistributed in the dark [18]. The dark recovery of PSCs has also been observed in other works [19]. Nonetheless, it has been also reported for PSCs to improve its efficiency over time [20] which might relate to improvement in the contact’s interfaces with the perovskite layer, especially in the case with NiO [21].

Characterization Techniques for Perovskite Solar Cell Materials

9.3 Perovskite stability

201

9.2.2.3 Outdoor testing The real roadblock for any PV technology to emerge from the niche market is an outdoor test in real working conditions. The test is also crucial to estimate the energy yield and its payback time. Even though perovskite modules have been fabricated [22], field test study of PSCs has been lagging with little information. An extensive test was carried out in Italy by Bella et al. in 2016 [23]. They developed fluorinated photopolymer coatings for PSCs as a barrier against moisture, UV filter, and encapsulation against soiling and heavy rain. The devices kept more than 90% of the initial efficiency after 5 weeks of outdoor testing at the temperature ranging from 111 to 132
C with 12 rainy days out of 42 measured days. Even though this is an excellent result and very encouraging, it is worth noticing that established PV technologies report less than 1% of the initial PCE loss in one year outdoor [24]. Hence, improvement is still needed along with the path towards industrialization of perovskite PV technology. The evaluation of the power output of halide perovskite-based devices and modules from outdoor testing needs to take nonreversible and reversible loses into account. It is proposed that instead of defining lifetime as the time for a device reaching 80% of its initial efficiency, the lifetime T800 is defined as the time for the produced energy of PSCs reaching 80% of its initial output energy value per day [25]. The produced energy is calculated and projected every day until it drops below 80% which can account for reversible losses [25]. Therefore, one of the most important steps is to create a standard protocol for stability tests of halide perovskite solar cells including MPP tracking with defined day/night cycles, dark storage lifetime, damp heat test, and outdoor testing. In spite of lacking a standard guideline on testing to date, it is well-known that PSCs are highly sensitive to external stressors, which will be explored in the next section.

9.3 Perovskite stability 9.3.1 Atmospheric water and oxygen stability In the ambient environment, moisture and oxygen are the culprits in most cases for severe damage to the PSCs performances. The endurance against oxygen and moisture is also a concern for other PV technologies because water and O2 can oxidize some components in PV panels [26]. Hence, even widely commercialized PV technologies need proper encapsulation which can be used in PSCs. Efforts have been made to find good encapsulation for PSCs which can preserve the initial efficiency, e.g. in the damp heat test [27]. Nevertheless, the improvement of intrinsic stability of PSCs can partially reduce the strict requirements of encapsulation. Furthermore, PSCs have a huge potential for flexible PV modules and the more stable the devices are, the less difficult (and most likely less expensive) to produce flexible PSCs [28]. Moisture is known to degrade the perovskite layer [29] even with encapsulation [30]. DFT calculation shows that the main component of perovskite solar cell family CH3NH3PbI3 (MAPbI3) is unstable in the presence of water because of highly reactive and water-soluble MA cations [31]. Upon water exposure, MAPbI3 is decomposed into HI,

Characterization Techniques for Perovskite Solar Cell Materials

202

9. Stability of materials and complete devices

CH3NH2 and eventually H2O and PbI2 [32]. The relative humidity values as low as 10% RH are enough to trigger rapid water absorption in perovskite film [33]. When MAPbI3 is in contact with water, it most probably forms light yellow hydrate crystals CH3NH3PbI6 2H2O. As the hydrogen bonds between organic cation with the octahedral structure of PbI42 6 weaken, water can form strong hydrogen bonds with the organic cation forming the hydrate phase [34]. The hydrate phase can decompose into methylamine and hydroiodic acid as shown in Eq. 9.1.



H2 O

CH3 NH3 PbI3 2 CH3 NH2 1 HI 1 PbI2 4HI 1 O2 22I2 1 2H2 O

(9.1)

hν

2HI 2 H2 1 I2 Another proposed route of moisture induced irreversible degradation in halide perovskite is via trap state [35]. Defects, especially at the grain boundaries, can trap the photogenerated charges, which leads to local electric fields. These fields can deprotonate organic cations then decompose into amine products. Also, the electric fields distort the PbI42 6 octahedral structure and allow more water to penetrate inside the perovskite lattice [35]. On the contrary, it has been shown that a small addition of water inside perovskite precursor or exposure to humidity during the crystallization process is beneficial [3638]. Water molecules can promote homogeneous nucleation and preferable crystallinity in the MAPbI3 film during annealing [36] and passivate dangling bonds [39]. Nonetheless, in illumination condition, combining both light and oxygen, PSCs degrade at a very fast rate [40]. Aristidou et al. proposed an oxygen-induced degradation route of MAPbI3 through the deprotonation of MA in which methylamine gas and lead iodide were formed according to the reactions presented in Eq. 9.2 [41]. The photo-excited electrons are accepted by oxygen molecules leading to the formation of superoxide species (O2 2 ) which is energetically favorable in light and is likely to reside in iodide vacation sites. The superoxide anions then deprotonate MA cations and decompose the material [41]. The smaller the crystal size, the faster the oxygen diffusion and perovskite degradation [4042]. CH3 NH3 PbI3 1

1 2 deprotonation 1 1 O2 ! CH3 NH2 1 PbI2 1 I2 1 H2 O 4 2 2

(9.2)

Grain boundaries in MAPbI3 have a large impact on the rate of degradation by oxygen because of their high defect density. Fig. 9.2 shows the degradation due to oxygen which starts from the grain boundary and progresses to consume the whole grain after a period of light and oxygen exposure [43]. Using PbAc2 in the perovskite precursor results in a large grains and compact film which has a higher tolerance against oxygen-induced degradation compared to small grains sol-eng film. The PV performance parameters are reduced significantly even with the presence of only 1% oxygen [43]. The defect-assisted oxygen and light-induced degradation are also observed in mix cations FA0.83Cs0.17Pb(I0.8Br0.2)3, though the degradation rate is lower compared to MAPbI3 [44]. In atmospheric condition, the presence of both moisture and oxygen increases the rate of degradation. The inclusion of water provides higher proton concentration and shifts the

Characterization Techniques for Perovskite Solar Cell Materials

9.3 Perovskite stability

203

FIGURE 9.2 Degradation of perovskite by oxygen, the degradation starts from grain boundaries and eventually consumes the whole grains. SEM images of (A and B) PbAc2 as the lead source in the perovskite precursor and (C and D) sol-eng fabricated films [43]. Reproduced with permission. Copyright 2017, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

equilibrium towards degraded products, i.e. enhances the rate of deprotonation of superoxide species [45]. Despite the detrimental effect of moisture and oxygen mentioned above, it has been documented that it is possible and sometimes even beneficial to process PSCs in the ambient condition (average RH is 5070%) [46,47].

9.3.2 Thermal stability As detailed in the previous chapters, halide perovskites are commonly defined by the formula ABX3. Depending on A, B, and X elements, the desirable photovoltaic active phase (α-phase) exists at different temperatures. The hybrid organic-inorganic perovskite MAPbI3 has the α-phase at room temperature. However, it has low chemical stability against external environmental factors, readily to form MAI and PbI2 [31]. It has been shown that FAPbI3 can have the photoactive black phase at room temperature. However, it is more energetically favorable for the material to be in the unwanted, photo-inactive δ-phase or yellow phase at room temperature [48]. Pure inorganic perovskite CsPbI3, which is more thermally stable, only has a black phase at above 300
C [49]. MA, FA, and Cs are few examples of endless possibilities for the A site in a 3D perovskite structure. Intermixing cation strategy has been used to boost the stability of the material while achieving high efficiency [50,51]. In the range of working temperature from -40
C to 85
C, it was found that a significant change in PV performance of PSCs happens depending on the working temperature [5254] as illustrated in Fig. 9.3. Fig. 9.3 showed MPP curves of an FTO/compact TiO2/ mesoporous TiO2/CsMAFA triple cation perovskite/Spiro-OMETAD/Au cells in different temperatures. At 65
C, the PCE of the cell decayed quickly, and the devices lost 20% its initial efficiency after less than 50 hours of light soaking whereas at 20
C the cell retained its 90% of initial value even after 500 hours at one sun illumination [53]. At higher than

Characterization Techniques for Perovskite Solar Cell Materials

204

9. Stability of materials and complete devices

FIGURE 9.3

Temperature-dependent PV performance of PSCs. (A) MPP track curves at different temperatures: black line at 20
C, blue line at 210
C, red line at 65
C, and green line shown PCE of cells subjected to temperature cycling from 210
C to 65
C [53]. Reproduced with permission. Copyright 2018, Macmillan Publishers Limited, part of Springer Nature.

50
C, irreversible change in the material occurs which is not desirable for PV application [54]. It has also been shown that device’s temperature cycling in real condition (from 40 to 55
C) degrades the MAPbI3 based PSCs faster than keeping constant device’s temperature at 25
C [55]. Methylammonium lead iodide has a lower tolerance to thermal stress in the presence of water and oxygen [56]. Changing the organic cation from methylammonium to formamidinium improves the intrinsic thermal stability of perovskite [57]. The higher stability can be linked to the lower acidity of FA compared to MA or, in other words, FA is less prone to give up electrons to form HI [28]. Inorganic perovskite for example cesium perovskite also shows better thermal stability [58].

9.3.3 Light stability As mentioned above, MAPbI3 degrades under illumination in the presence of oxygen. Different from the mechanism described in the previous section, light induces degradation may take place even in the absence of oxygen [59,60]. Indeed, under illumination, photons excite ‘Pb1. . .Io’ complex that energetically tends to form the reactive I2 and 2Pb1. Subsequently, the unstable lead species can further react with oxygen and water, which diffuse from the external environment into the perovskite film. This process eventually generates lead iodide and methylamine leading to the collapse of the perovskite 3D structure [60]. Instead of photochemical changes, the light-induced degradation of MAPbI3 can be explained by the structural changes [6164]. Under illumination exposure, the hydrogen bond in between the hydrogen and MA1 can be weakened, and hence, MA1 can rotate more freely [64]. The dipole cation can respond to an electric field (if present) or align by

Characterization Techniques for Perovskite Solar Cell Materials

9.3 Perovskite stability

205

interaction among dipoles. This dipole motion may cause a change in the structure of MAPbI3 lattice under light [62]. Under illumination, ionic diffusion is also accelerated in combination with heat. Two effects expand the lattice and activate the diffusion of CH3NH31 causing the material to decompose into PbI2 [61]. Also, ions migration increases the defect concentration in the material [65] which accelerates further the degradation [63]. Due to the MA vacancies, the bond angle of Pb-I-Pb changes in prolonged illumination until the structure disintegrates permanently to lead iodide [63]. To counteract this process, the inclusion of larger cation namely formamidinium can improve the device stability due to a higher activation energy of ion transportation [61]. In the mixed cation/anion halide perovskite, illumination also induces reversible phase segregation first introduced by Hoke et al. [66]. which is thought to be closely related to halide migration in the material leading to I-rich and I-poor domains in the material during light exposure. When the material is allowed to rest in the dark, it can recover, however, not entirely [67].

9.3.4 Electric field stability In the operational condition, solar cells will be held in a certain applied voltage. All ionic species within the perovskite film can potentially migrate in between, i.e. the vacancies of iodide [65,68], in response to the external electric fields. From theoretical calculation, iodide ions and their vacancies have the smallest energy barrier to migrate whereas lead components have the highest energy cost [64,69]. In the perovskite structure, cations can also migrate from the cavity of the octahedral cage of PbI42 6 rather easily, which leads to the lattice distortions and contractions, and eventually degrades the material [70]. This ionic movement can affect the device’s stability. Leijtens et al. showed that the internal electric field caused by defect movement would be expanded throughout the material depth [71]. They proposed an interplay between applied bias and humidity in material degradation mechanism in which methylammonium cation is drifting to the negative electrode in the presence of water and consequently, lead iodide was formed at the positive electrode. Because CH3NH31 accumulated at the positive electrode, it left unstable PbI42 6 octahedral behind, which eventually was decomposed into PbI2. The proposed hypothesis is described by Eq. 9.3. The degradation is likely to stem from weakening the hydrogen bond between the MA1 cation and I2 owing to the polar solvent attack, in this case, water presented in the atmosphere [71]. moisture 1 E 2 filed

2 1 CH3 NH3 PbI3            ) CH3 NH3 1 I 1 PbI2

(9.3)

Another factor, which has a negative effect on the stability of PSCs, is the interaction between ions migrate from perovskite to electrodes [30,7274]. For example, I2 can react with Ag to form AgI which causes metal electrode corrosion [73]. The role of electrodes in long-term stability will be further demonstrated in the next section. Notably, the charge defects (vacancies, interstitials) can accumulate at the interfaces of perovskite and other layers leading to a faster degradation rate due to the external stresses. Systematic studies are still needed to understand the effect of ion migrations on the long-term stability of perovskite solar cells.

Characterization Techniques for Perovskite Solar Cell Materials

206

9. Stability of materials and complete devices

9.3.5 Mechanical stability In solar energy application, the materials used in PV panels need to endure mechanical stress especially for thin film technologies. Perovskite solar cells have been so far fabricated on the flexible substrates and exhibited a good bending angle with high efficiency [75,76]. It has been confirmed that perovskite has a high tolerance towards bending stress through bending cycle tests [77,78]. As for mechanical fracture stress, it is shown that MAPbI3 is brittle and fragile [79] especially in planar structure due to the lack of metal oxide scaffold [80,81]. Also, the hardness of the material is closely related to the grain sizes [81]. Moreover, due to the large difference in the thermal expansion coefficient as well as phase transition within the operational condition (54
C from tetragonal to cubic for MAPbI3), the day-night temperature cycling can induce more mechanical stress [82] to the perovskite module which can lead to permanent damage. So far, there is little understanding about the mechanical stability of PSCs, which will be important in long-term stability and commercial validity of this technology.

9.4 Device and interface stability 9.4.1 Charge selective contacts In the beginning, the development of perovskite solar cells was closely related to dyesensitized solar cells. Hence, mesoporous TiO2 was first adopted as an electron selective layer (ESL) for PSCs [83,84] (Fig. 9.4). However, the presence of the TiO2 in PSCs can result

FIGURE 9.4 PSCs schematic in regular structure with a mesoporous scaffold/planar electron transport layer (n-i-p) and inverted (p-i-n) planar architecture [92]. Reproduced with permission. Copyright 2018, American Chemical Society.

Characterization Techniques for Perovskite Solar Cell Materials

9.4 Device and interface stability

207

in UV degradation [85]. The surface of TiO2 contains many oxygen vacancies, which act as electron donors. These electron donors react with atmospheric oxygen forming O2 anion and Ti41 complex. The UV light-generated holes in TiO2 can recombine with adsorbed oxygen sites leaving free electrons which eventually can recombine with holes in the hole selective layer (HSL) and reduce the charge extraction. Subsequently, TiO2 is left with unfilled, immobile deep electron traps. The deep traps degrade rapidly the performance of PSCs overtime especially the photocurrent after few hours exposing to full sunlight illumination [85]. To overcome this problem, modified TiO2 [19,86] to reduce the density of deep trap states or to use the Al2O3 scaffold and other interlayers to prevent electron transport from ESL [85,87] have been explored so far to improve the stability of the devices. More metal oxides options have been used as ESL for more stable PSCs, in particular, SnO2 based PSCs efficiency has shown excellent progress both in mesoporous structure and planar structure to date [8891]. Notably, devices optimization from Anaraki et al. showed stabilized efficiency near 21% with a SnO2 layer with triple cation CsMAFA perovskite absorber [91]. The SnO2 band edges are well-aligned with those of perovskite which enables efficient electron collection and has been proven to be more stable under illumination compared to the TiO2 analog [90]. Since SnO2 has a larger bandgap than TiO2, SnO2 based devices are not suffered from photo-catalyst effect from near UV light, which leads to the degradation as mentioned above. For inverted p-i-n structure/architecture, as shown in Fig. 9.4, the pioneering works have used phenyl-C61-butyric acid methyl ester (PCBM) as ESL [93,94]. Because PCBM is degraded under the exposure to oxygen and water [95,96], buffer layers have been used to improve the stability of PCBM, e.g. ZnO [97]. Also, various metal oxides have been used in contact layers as barriers to prevent water and oxygen penetration inside the perovskite and also volatile organic components escaping to the outside [98]. Moreover, metal oxides show better thermal stability compared to organic counterparts [99]. Brinkmann et al. used bilayer Al-doped ZnO with SnOx on top of PCBM in an inverted PSCs. Thanks to the presence of metal oxides, the decomposition of MAPbI3 due to heat and water slowed down resulting in a more stable device without encapsulation in ambient air (23
C and 50% RH) for more than 400 hours [98]. Although inorganic charge transport layers are more stable than organic ones, the difference in crystal structures and thermal expansion coefficients of the charge selective layers can create unwanted deep traps at the interfaces and also form cracks in the films during working conditions, i.e. when the temperature changes dramatically from day to night [100]. Currently, high efficient PSCs utilize organic HSL such PTAA, Spiro-OMETAD and their derivatives. However, the solution processed Spiro-OMETAD layer is porous which leads to the penetration of water and oxygen inside the perovskite layer. The hygroscopic nature of Li-TFSI dopant common in Spiro-OMETAD and PTAA also accelerates the moisture degradation of PSCs [101]. Hence, there has been a lot of research effort to find alternative dopant-free HSL such as tantalum doped tungsten oxide (TaWOx) between dopant-free π-conjugate polymer and perovskite [102]. The device retained more than 90% of its initial efficiency under one sun illumination without encapsulation in the N2 environment [102]. Another approach is to modify pristine Spiro-OMETAD with its dicationic salt Spiro-OMETAD(TFSI)2 to improve its conductivity without the use of dopants [103]. Another commonly used HSL is PEDOT: PSS, especially in inverted p-i-n structure.

Characterization Techniques for Perovskite Solar Cell Materials

208

9. Stability of materials and complete devices

However, because of its hygroscopic characteristic, PEDOT: PSS can take up water in the atmosphere and reduces the stability of solar cells [104]. Alternatively, metal oxide especially NiO is used to improve the whole device stack stability, in particular, resistance to heat. Kim et al. showed that Cu-doped-NiO MAPbI3 device could improve the stability of PSCs in ambient air. However, the devices still lost more than 10% of its initial efficiency after 250 hours in the ambient air even without continuous one sun illumination [105]. Silver- doped- NiOx employed MAPbI3 solar cells has shown to be stable at its 80% initial efficiency in 30% RH air up to one month in the dark with periodic JV measurements [106]. Aside from using inorganic contact layers to improve the stability of PSCs, researchers have also been seeking a contact layer free PSCs to improve devices stability. To illustrate, Mei et al. used a mixed cation perovskite from methylammonium (MA) iodide, and 5-ammoniumvaleric acid (5-AVA) in a device without hole selective layer which resulted into a more stable devices at 12.8% efficiency compared to using Spiro-OMETAD for more than 1000 hours exposure to one sun illumination [107]. Nonetheless, to date, the recorded efficiency of PSCs still belongs to complete devices with having both ESL and HSL, for example, a certified 22.1% efficiency for single junction PSCs from Seok’s group [108]. Furthermore, modification of perovskite structure and surface [109] and 2D/3D hybrid structures [10,50] enhance atmospheric stability of the device due to excellent 2D perovskite resistance to moisture [110].

9.4.2 Metal contact The use of top metal contacts is essential for an efficient charge collection. The metal electrodes require a good energy alignment with the contact layers and the perovskite absorber layer for high efficiency. Here, we will discuss the stability of commonly used metals as PSCs contact. Aluminum is used in various PV applications owing to its affordability and availability. Nevertheless, in an ambient environment, water and oxygen can diffuse inside the pores of the aluminum layer. Due to its high reactivity, Al will be oxidized, and a metal oxide layer is formed in the interface with the organic layer. This metal oxide layer is insulated and hence degrade the performance of the solar cells [111]. Silver has high conductivity, and because of its good reflective property, it can also be used as a back reflector in thin-film solar cells [112]. Nonetheless, Ag has been widely reported to be corroded in PSCs application [30,73,85]. Kato et al. observed that silver electrode turned into a yellow film (AgI) after being stored in ambient air for three weeks in a complete device with TiO2/MAPbI3/Spiro-OMETAD/Ag whereas, for samples without perovskite layer, silver had no sign of degradation. It was proposed that in humidity exposure, MAPbI3 was degraded into volatile iodine compounds (MAI and HI) which reacted with silver and caused degradation [30,73]. In the contrary, copper-based devices show better air stability where the copper layer keeps its color when storing in atmospheric condition up to one month [113]. Alternatively, gold is used for state-of-the-art PSCs mostly in the regular n-i-p structure. At the high temperature of 70
C (which is in the normal working condition temperature range of solar devices), gold can diffuse inside the perovskite film through the porous Spiro-OMETAD [74]. This metal migration results in an irreversible

Characterization Techniques for Perovskite Solar Cell Materials

9.5 Conclusion and outlooks

209

photovoltaic performance degradation. It induces shunt paths and recombination centers which reduces FF, JSC, and VOC [74,114]. Another proposed degradation pathway is through the interaction between gold and mobile, highly reactive polyiodide. Under prolonged UV irradiation exposure, MAPbI3 degrades forming polyiodide MAInI2 which react with gold at room temperature. Subsequently, the reaction forms phase segregation of (MA)2Au2I6 and further degrades the cells [115]. The deep traps caused by gold migration are not exclusive in PSCs but also reported in silicon solar cells [116]. To overcome this degradation mechanism, Cr and its oxide, as well as In-dopedSnO2 have been proven to be an effective barrier layer [20,74,117]. Taking into account the metal contact degradation mechanisms, the migration of the mobile iodine anions can be one of the main roots for metal electrode degradation [118]. One way to mitigate the problem is to reduce mobile ions by buffer layer [72] or modified contact layer [119]. Back et al. used amine modified metal oxide to neutralize mobile ionic component of perovskite which resulted in shelf lifetime of the devices was up to 9000 hours (samples were stored in the dark, in the N2 atmosphere and had periodic JV measurements). The amine functional groups interacted with mobile halide ions which then, in turn, reduced the possibility of halide interaction with metal electrodes [119]. Also, efforts have been made to replace the metal electrode completely, mostly using cheap carbon-based electrodes, yet the efficiency is still lacking behind metal contact employed PSCs [120122].

9.5 Conclusion and outlooks This chapter presented a brief overview of the long-term stability of PSCs. Long-term stability is a challenging roadblock that PSCs need to overcome before delivering a product for the market of large-energy production. Because of the material’s complex ionic and crystalline nature, testing the stability of PSCs needs particular considerations. Taking the existing knowledge of both inorganic and organic PV technologies, the research community started to develop new testing protocols designed specifically for the perovskites. To determine the efficiency of PSCs, current density-voltage scans and maximum power point tracking are both crucial. Also, MPP tracking for hundreds to thousand hours is vital to examine the long-term stability of the cells. Thanks to the reversible nature of some degradation mechanisms, it can be important to test stability under reproduced day/night cycling. Stability tests under different stressing conditions are important to isolate different degradation mechanisms. This chapter also provided a broad sketch on the stability of perovskite material as well as other layers comprising PSCs in device’s stack under external stresses, i.e. water, oxygen, electric field. Exploring new perovskite compositions, as well as proper encapsulation and interlayers together with robust charge contact layers, in our beliefs, is the way to achieve stable PSCs. So far, stable devices in one sun illumination for 1000 hours MPP tracking have been reported either in N2 atmosphere or encapsulated devices [20,102,123]. Until this chapter is written, the MPP tracking test for hundred to thousand of hours has been used as unofficial standard for stability testing. With this encouraging shift from only shelf lifetime testing to widely used MPP tracking, we believe that the stability test has become more accurate to determine the reliability of a certain

Characterization Techniques for Perovskite Solar Cell Materials

210

9. Stability of materials and complete devices

devices architecture. This show promising progress in the field especially the photostability and somewhat resistance to heat (as the devices can be heated up during the intense illumination). Therefore, in our opinion, it is possible to achieve stable perovskite based devices for PV application according to 20 years standard in the near future.

References [1] NREL, NREL best research-cell efficiencies, in: National Renewable Energy Laboratory, 2018. [2] W. Herrmann, N. Bogdanski, Outdoor weathering of PV modules—effects of various climates and comparison with accelerated laboratory testing, in: Photovoltaic Specialists Conference (PVSC), 2011 37th IEEE, IEEE, 2011, pp. 002305002311. [3] C.R. Osterwald, T.J. McMahon, History of accelerated and qualification testing of terrestrial photovoltaic modules: a literature review, Prog. Photovoltaics 17 (2009) 1133. [4] H.J. Snaith, A. Abate, J.M. Ball, G.E. Eperon, T. Leijtens, N.K. Noel, et al., Anomalous hysteresis in perovskite solar cells, J. Phys. Chem. Lett. 5 (2014) 15111515. [5] P. Calado, A.M. Telford, D. Bryant, X. Li, J. Nelson, B.C. O’Regan, et al., Evidence for ion migration in hybrid perovskite solar cells with minimal hysteresis, Nat. Commun. 7 (2016) 13831. [6] D. Bryant, S. Wheeler, B.C. O’Regan, T. Watson, P.R. Barnes, D. Worsley, et al., Observable hysteresis at low temperature in “hysteresis free” organicinorganic lead halide perovskite solar cells, J. Phys. Chem. Lett. 6 (2015) 31903194. [7] L.K. Ono, S.R. Raga, S. Wang, Y. Kato, Y. Qi, Temperature-dependent hysteresis effects in perovskite-based solar cells, J. Mater. Chem. A 3 (2015) 90749080. [8] M. Pazoki, T.J. Jacobsson, S.H. Cruz, M.B. Johansson, R. Imani, J. Kullgren, et al., Photon energy-dependent hysteresis effects in lead halide perovskite materials, J. Phys. Chem. C 121 (2017) 2618026187. [9] J.A. Christians, J.S. Manser, P.V. Kamat, Best practices in perovskite solar cell efficiency measurements avoiding the error of making bad cells look good, 6 (2015) 852857 [10] G. Grancini, C. Rolda´n-Carmona, I. Zimmermann, E. Mosconi, X. Lee, D. Martineau, et al., One-year stable perovskite solar cells by 2D/3D interface engineering, Nat. Commun. 8 (2017) 15684. [11] M. Saliba, M. Stolterfoht, C.M. Wolff, D. Neher, A. Abate, Measuring aging stability of perovskite solar cells, Joule 2 (2018) 10191024. [12] J.-P. Correa-Baena, A. Abate, M. Saliba, W. Tress, T.J. Jacobsson, M. Gra¨tzel, et al., The rapid evolution of highly efficient perovskite solar cells, Energy Environ. Sci. 10 (2017) 710727. [13] N. Pellet, F. Giordano, M. Ibrahim Dar, G. Gregori, S.M. Zakeeruddin, J. Maier, et al., Hill climbing hysteresis of perovskite-based solar cells: a maximum power point tracking investigation, Prog. Photovoltaics: Res. Appl. (2017). [14] A.J. Cimaroli, Y. Yu, C. Wang, W. Liao, L. Guan, C.R. Grice, et al., Tracking the maximum power point of hysteretic perovskite solar cells using a predictive algorithm, J. Mater. Chem. C 5 (2017) 1015210157. [15] A. Abate, S. Paek, F. Giordano, J.-P. Correa-Baena, M. Saliba, P. Gao, et al., Silolothiophene-linked triphenylamines as stable hole transporting materials for high efficiency perovskite solar cells, Energy Environ. Sci. 8 (2015) 29462953. [16] C.H. Peters, I. Sachs-Quintana, W.R. Mateker, T. Heumueller, J. Rivnay, R. Noriega, et al., The mechanism of burn-in loss in a high efficiency polymer solar cell, Adv. Mater. 24 (2012) 663668. [17] M. Nikolaeva-Dimitrova, R.P. Kenny, E.D. Dunlop, Controlled conditioning of a-Si: H thin film modules for efficiency prediction, Thin Solid Films 516 (2008) 69026906. [18] K. Domanski, B. Roose, T. Matsui, M. Saliba, S.-H. Turren-Cruz, J.-P. Correa-Baena, et al., Migration of cations induces reversible performance losses over day/night cycling in perovskite solar cells, Energy Environ. Sci. 10 (2017) 604613. [19] H. Tan, A. Jain, O. Voznyy, X. Lan, F.P.G. de Arquer, J.Z. Fan, et al., Efficient and stable solution-processed planar perovskite solar cells via contact passivation, Science 355 (2017) 722726. [20] K.A. Bush, A.F. Palmstrom, J.Y. Zhengshan, M. Boccard, R. Cheacharoen, J.P. Mailoa, et al., 23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability, Nat. Energy 2 (2017) 17009.

Characterization Techniques for Perovskite Solar Cell Materials

References

211

[21] H. Tsai, R. Asadpour, J.-C. Blancon, C.C. Stoumpos, O. Durand, J.W. Strzalka, et al., Light-induced lattice expansion leads to high-efficiency perovskite solar cells, Science 360 (2018) 6770. [22] H. Chen, F. Ye, W. Tang, J. He, M. Yin, Y. Wang, et al., A solvent-and vacuum-free route to large-area perovskite films for efficient solar modules, Nature 550 (2017) 92. [23] F. Bella, G. Griffini, J.-P. Correa-Baena, G. Saracco, M. Gra¨tzel, A. Hagfeldt, et al., Improving efficiency and stability of perovskite solar cells with photocurable fluoropolymers, Science (2016) aah4046. [24] D.C. Jordan, S.R. Kurtz, Photovoltaic degradation rates—an analytical review, Prog. Photovoltaics Res. Appl. 21 (2013) 1229. [25] M.V. Khenkin, K. Anoop, I. Visoly-Fisher, Y. Glagan, F. Di Giacomo, B.R.R. Patil, et al., Reconsidering figures of merit for the performance and stability of perovskite photovoltaics, Energy Environ. Sci. 11 (2018) 739743. [26] V. Sharma, O. Sastry, A. Kumar, B. Bora, S. Chandel, Degradation analysis of a-Si, (HIT) hetro-junction intrinsic thin layer silicon and mC-Si solar photovoltaic technologies under outdoor conditions, Energy 72 (2014) 536546. [27] L. Shi, T.L. Young, J. Kim, Y. Sheng, L. Wang, Y. Chen, et al., Accelerated lifetime testing of organicinorganic perovskite solar cells encapsulated by polyisobutylene, ACS Appl. Mater. Interfaces 9 (2017) 2507325081. [28] T. Leijtens, K. Bush, R. Cheacharoen, R. Beal, A. Bowring, M.D. McGehee, Towards enabling stable lead halide perovskite solar cells; interplay between structural, environmental, and thermal stability, J. Mater. Chem. A 5 (2017) 1148311500. [29] J.H. Noh, S.H. Im, J.H. Heo, T.N. Mandal, S.I. Seok, Chemical management for colorful, efficient, and stable inorganicorganic hybrid nanostructured solar cells, Nano Lett. 13 (2013) 17641769. [30] Y. Han, S. Meyer, Y. Dkhissi, K. Weber, J.M. Pringle, U. Bach, et al., Degradation observations of encapsulated planar CH3NH3PbI3 perovskite solar cells at high temperatures and humidity, J. Mater. Chem. A 3 (2015) 81398147. [31] E. Tenuta, C. Zheng, O. Rubel, Thermodynamic origin of instability in hybrid halide perovskites, Sci. Rep. 6 (2016) 37654. [32] J.M. Frost, K.T. Butler, F. Brivio, C.H. Hendon, M. Van Schilfgaarde, A. Walsh, Atomistic origins of highperformance in hybrid halide perovskite solar cells, Nano Lett. 14 (2014) 25842590. [33] C. Mu¨ller, T. Glaser, M. Plogmeyer, M. Sendner, S. Do¨ring, A.A. Bakulin, et al., Water infiltration in methylammonium lead iodide perovskite: fast and inconspicuous, Chem. Mater. 27 (2015) 78357841. [34] J.A. Christians, P.A. Miranda Herrera, P.V. Kamat, Transformation of the excited state and photovoltaic efficiency of CH3NH3PbI3 perovskite upon controlled exposure to humidified air, JACS 137 (2015) 15301538. [35] N. Ahn, K. Kwak, M.S. Jang, H. Yoon, B.Y. Lee, J.-K. Lee, et al., Trapped charge-driven degradation of perovskite solar cells, Nat. Commun. 7 (2016) 13422. [36] C.-G. Wu, C.-H. Chiang, Z.-L. Tseng, M.K. Nazeeruddin, A. Hagfeldt, M. Gra¨tzel, High efficiency stable inverted perovskite solar cells without current hysteresis, Energy Environ. Sci. 8 (2015) 27252733. [37] J. You, Y. Yang, Z. Hong, T.-B. Song, L. Meng, Y. Liu, et al., Moisture assisted perovskite film growth for high performance solar cells, Appl. Phys. Lett. 105 (2014) 183902. [38] G.E. Eperon, S.N. Habisreutinger, T. Leijtens, B.J. Bruijnaers, J.J. van Franeker, D.W. deQuilettes, et al., The importance of moisture in hybrid lead halide perovskite thin film fabrication, ACS Nano 9 (2015) 93809393. [39] W. Zhou, Y. Zhao, C. Shi, H. Huang, J. Wei, R. Fu, et al., Reversible healing effect of water molecules on fully crystallized metalhalide perovskite film, J. Phys. Chem. C 120 (2016) 47594765. [40] N. Aristidou, I. Sanchez-Molina, T. Chotchuangchutchaval, M. Brown, L. Martinez, T. Rath, et al., The role of oxygen in the degradation of methylammonium lead trihalide perovskite photoactive layers, Angew. Chem. Int. Ed. 54 (2015) 82088212. [41] N. Aristidou, C. Eames, I. Sanchez-Molina, X. Bu, J. Kosco, M.S. Islam, et al., Fast oxygen diffusion and iodide defects mediate oxygen-induced degradation of perovskite solar cells, Nat. Commun. 8 (2017) 15218. [42] D. Bryant, N. Aristidou, S. Pont, I. Sanchez-Molina, T. Chotchunangatchaval, S. Wheeler, et al., Light and oxygen induced degradation limits the operational stability of methylammonium lead triiodide perovskite solar cells, Energy Environ. Sci. 9 (2016) 16551660. [43] Q. Sun, P. Fassl, D. Becker-Koch, A. Bausch, B. Rivkin, S. Bai, et al., Role of microstructure in oxygen induced photodegradation of methylammonium lead triiodide perovskite films, Adv. Energy Mater. 7 (2017) 1700977.

Characterization Techniques for Perovskite Solar Cell Materials

212

9. Stability of materials and complete devices

[44] M. Alsari, A.J. Pearson, J.T.-W. Wang, Z. Wang, A. Montisci, N.C. Greenham, et al., Degradation kinetics of inverted perovskite solar cells, Sci. Rep. 8 (2018) 5977. [45] N. Aristidou, C. Eames, M.S. Islam, S.A. Haque, Insights into the increased degradation rate of CH3NH3PbI3 solar cells in combined water and O2 environments, J. Mater. Chem. A 5 (2017) 2546925475. [46] Q. Tai, P. You, H. Sang, Z. Liu, C. Hu, H.L. Chan, et al., Efficient and stable perovskite solar cells prepared in ambient air irrespective of the humidity, Nat. Commun. 7 (2016) 11105. [47] S.R. Raga, M.-C. Jung, M.V. Lee, M.R. Leyden, Y. Kato, Y. Qi, Influence of air annealing on high efficiency planar structure perovskite solar cells, Chem. Mater. 27 (2015) 15971603. [48] T.M. Koh, K. Fu, Y. Fang, S. Chen, T. Sum, N. Mathews, et al., Formamidinium-containing metal-halide: an alternative material for near-IR absorption perovskite solar cells, J. Phys. Chem. C 118 (2013) 1645816462. [49] C.K. Møller, Crystal structure and photoconductivity of caesium plumbohalides, Nature 182 (1958). 14361436. [50] J.W. Lee, D.H. Kim, H.S. Kim, S.W. Seo, S.M. Cho, N.G. Park, Formamidinium and cesium hybridization for photo-and moisture-stable perovskite solar cell, Adv. Energy Mater. 5 (2015) 1501310. [51] M. Saliba, T. Matsui, J.-Y. Seo, K. Domanski, J.-P. Correa-Baena, M.K. Nazeeruddin, et al., Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency, Energy Environ. Sci. 9 (2016) 19891997. [52] H. Zhang, X. Qiao, Y. Shen, T. Moehl, S.M. Zakeeruddin, M. Gra¨tzel, et al., Photovoltaic behaviour of lead methylammonium triiodide perovskite solar cells down to 80 K, J. Mater. Chem. A 3 (2015) 1176211767. [53] K. Domanski, E.A. Alharbi, A. Hagfeldt, M. Gra¨tzel, W. Tress, Systematic investigation of the impact of operation conditions on the degradation behaviour of perovskite solar cells, Nat. Energy 3 (2018) 6167. [54] T.J. Jacobsson, W. Tress, J.-P. Correa-Baena, T. Edvinsson, A. Hagfeldt, Room temperature as a goldilocks environment for CH3NH3PbI3 perovskite solar cells: the importance of temperature on device performance, J. Phys. Chem. C 120 (2016) 1138211393. [55] J.A. Schwenzer, L. Rakocevic, R. Gehlhaar, T. Abzieher, S. Gharibzadeh, S. Moghadamzadeh, et al., Temperature variation-induced performance decline of perovskite solar cells, ACS Appl. Mater. Interfaces 10 (2018) 1639016399. [56] B. Conings, J. Drijkoningen, N. Gauquelin, A. Babayigit, J. D’Haen, L. D’Olieslaeger, et al., Intrinsic thermal instability of methylammonium lead trihalide perovskite, Adv. Energy Mater. 5 (2015) 1500477. [57] G.E. Eperon, S.D. Stranks, C. Menelaou, M.B. Johnston, L.M. Herz, H.J. Snaith, Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells, Energy Environ. Sci. 7 (2014) 982988. [58] R.E. Beal, D.J. Slotcavage, T. Leijtens, A.R. Bowring, R.A. Belisle, W.H. Nguyen, et al., Cesium lead halide perovskites with improved stability for tandem solar cells, J. Phys. Chem. Lett. 7 (2016) 746751. [59] Y. Wei, P. Audebert, L. Galmiche, J.-S. Lauret, E. Deleporte, Photostability of 2D organic-inorganic hybrid perovskites, Materials 7 (2014) 47894802. [60] K. Jemli, H. Diab, F. Le´de´e, G. Trippe´-Allard, D. Garrot, B. Geffroy, et al., Using low temperature photoluminescence spectroscopy to investigate CH3NH3PbI3 hybrid perovskite degradation, Molecules 21 (2016) 885. [61] M. Bag, L.A. Renna, R.Y. Adhikari, S. Karak, F. Liu, P.M. Lahti, et al., Kinetics of ion transport in perovskite active layers and its implications for active layer stability, J. Am. Chem. Soc 137 (2015) 1313013137. [62] R. Gottesman, L. Gouda, B.S. Kalanoor, E. Haltzi, S. Tirosh, E. Rosh-Hodesh, et al., Photoinduced reversible structural transformations in free-standing CH3NH3PbI3 perovskite films, J. Phys. Chem. Lett. 6 (2015) 23322338. [63] A. Merdasa, M. Bag, Y. Tian, E. Ka¨llman, A. Dobrovolsky, I.G. Scheblykin, Super-resolution luminescence microspectroscopy reveals the mechanism of photoinduced degradation in CH3NH3PbI3 perovskite nanocrystals, J. Phys. Chem. C 120 (2016) 1071110719. [64] M. Pazoki, T.J. Jacobsson, J. Kullgren, E.M. Johansson, A. Hagfeldt, G. Boschloo, et al., Photoinduced stark effects and mechanism of ion displacement in perovskite solar cell materials, ACS Nano 11 (2017) 28232834. [65] J.M. Azpiroz, E. Mosconi, J. Bisquert, F. De Angelis, Defect migration in methylammonium lead iodide and its role in perovskite solar cell operation, Energy Environ. Sci. 8 (2015) 21182127. [66] E.T. Hoke, D.J. Slotcavage, E.R. Dohner, A.R. Bowring, H.I. Karunadasa, M.D. McGehee, Reversible photoinduced trap formation in mixed-halide hybrid perovskites for photovoltaics, Chem. Sci. 6 (2015) 613617.

Characterization Techniques for Perovskite Solar Cell Materials

References

213

[67] G.F. Samu, C. Jana´ky, P.V. Kamat, A victim of halide ion segregation. How light soaking affects solar cell performance of mixed halide lead perovskites, ACS Energy Lett. 2 (2017) 18601861. [68] W. Tress, N. Marinova, T. Moehl, S. Zakeeruddin, M.K. Nazeeruddin, M. Gra¨tzel, Understanding the ratedependent JV hysteresis, slow time component, and aging in CH3NH3PbI3 perovskite solar cells: the role of a compensated electric field, Energy Environ. Sci. 8 (2015) 9951004. [69] C. Eames, J.M. Frost, P.R. Barnes, B.C. O’regan, A. Walsh, M.S. Islam, Ionic transport in hybrid lead iodide perovskite solar cells, Nat. Commun. 6 (2015) 7497. [70] J. Haruyama, K. Sodeyama, L. Han, Y. Tateyama, First-principles study of ion diffusion in perovskite solar cell sensitizers, JACS 137 (2015) 1004810051. [71] T. Leijtens, E.T. Hoke, G. Grancini, D.J. Slotcavage, G.E. Eperon, J.M. Ball, et al., Mapping electric fieldinduced switchable poling and structural degradation in hybrid lead halide perovskite thin films, Adv. Energy Mater. 5 (2015) 1500962. [72] E.M. Sanehira, B.J. Tremolet de Villers, P. Schulz, M.O. Reese, S. Ferrere, K. Zhu, et al., Influence of electrode interfaces on the stability of perovskite solar cells: reduced degradation using MoO x/Al for hole collection, ACS Energy Lett. 1 (2016) 3845. [73] Y. Kato, L.K. Ono, M.V. Lee, S. Wang, S.R. Raga, Y. Qi, Silver iodide formation in methyl ammonium lead iodide perovskite solar cells with silver top electrodes, Adv. Mater. Interfaces 2 (2015) 1500195. [74] K. Domanski, J.-P. Correa-Baena, N. Mine, M.K. Nazeeruddin, A. Abate, M. Saliba, et al., Not all that glitters is gold: metal-migration-induced degradation in perovskite solar cells, ACS Nano 10 (2016) 63066314. [75] P. Docampo, J.M. Ball, M. Darwich, G.E. Eperon, H.J. Snaith, Efficient organometal trihalide perovskite planar-heterojunction solar cells on flexible polymer substrates, Nat. Commun. 4 (2013) 2761. [76] M.H. Kumar, N. Yantara, S. Dharani, M. Graetzel, S. Mhaisalkar, P.P. Boix, et al., Flexible, lowtemperature, solution processed ZnO-based perovskite solid state solar cells, Chem. Commun. 49 (2013) 1108911091. [77] B.J. Kim, D.H. Kim, Y.-Y. Lee, H.-W. Shin, G.S. Han, J.S. Hong, et al., Highly efficient and bending durable perovskite solar cells: toward a wearable power source, Energy Environ. Sci. 8 (2015) 916921. [78] Y. Li, L. Meng, Y.M. Yang, G. Xu, Z. Hong, Q. Chen, et al., High-efficiency robust perovskite solar cells on ultrathin flexible substrates, Nat. Commun. 7 (2016) 10214. [79] N. Rolston, B.L. Watson, C.D. Bailie, M.D. McGehee, J.P. Bastos, R. Gehlhaar, et al., Mechanical integrity of solution-processed perovskite solar cells, Extreme Mechanics Letters 9 (2016) 353358. [80] B.L. Watson, N. Rolston, A.D. Printz, R.H. Dauskardt, Scaffold-reinforced perovskite compound solar cells, Energy Environ. Sci. 10 (2017) 25002508. [81] N. Rolston, A.D. Printz, J.M. Tracy, H.C. Weerasinghe, D. Vak, L.J. Haur, et al., Effect of cation composition on the mechanical stability of perovskite solar cells, Adv. Energy Mater. 8 (2017) 1702116. [82] T.J. Jacobsson, L.J. Schwan, M. Ottosson, A. Hagfeldt, T. Edvinsson, Determination of thermal expansion coefficients and locating the temperature-induced phase transition in methylammonium lead perovskites using x-ray diffraction, Inorg. Chem. 54 (2015) 1067810685. [83] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, JACS 131 (2009) 60506051. [84] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Efficient hybrid solar cells based on mesosuperstructured organometal halide perovskites, Science 338 (2012) 643647. [85] T. Leijtens, G.E. Eperon, S. Pathak, A. Abate, M.M. Lee, H.J. Snaith, Overcoming ultraviolet light instability of sensitized TiO2 with meso-superstructured organometal tri-halide perovskite solar cells, Nat. Commun. 4 (2013) 2885. [86] B. Roose, K.C. Go¨del, S. Pathak, A. Sadhanala, J.P.C. Baena, B.D. Wilts, et al., Enhanced efficiency and stability of perovskite solar cells through Nd-doping of mesostructured TiO2, Adv. Energy Mater. 6 (2016) 1501868. [87] S. Guarnera, A. Abate, W. Zhang, J.M. Foster, G. Richardson, A. Petrozza, et al., Improving the long-term stability of perovskite solar cells with a porous Al2O3 buffer layer, J. Phys. Chem. Lett. 6 (2015) 432437. [88] B. Roose, J.-P.C. Baena, K.C. Go¨del, M. Graetzel, A. Hagfeldt, U. Steiner, et al., Mesoporous SnO2 electron selective contact enables UV-stable perovskite solar cells, Nano Energy 30 (2016) 517522. [89] B. Roose, C.M. Johansen, K. Dupraz, T. Jaouen, P. Aebi, U. Steiner, et al., Ga-doped SnO2 mesoporous contact for UV stable highly efficient perovskite solar cells, J. Mater. Chem. A 6 (2017) 18501857.

Characterization Techniques for Perovskite Solar Cell Materials

214

9. Stability of materials and complete devices

[90] J.P.C. Baena, L. Steier, W. Tress, M. Saliba, S. Neutzner, T. Matsui, et al., Highly efficient planar perovskite solar cells through band alignment engineering, Energy Environ. Sci. 8 (2015) 29282934. [91] E.H. Anaraki, A. Kermanpur, L. Steier, K. Domanski, T. Matsui, W. Tress, et al., Highly efficient and stable planar perovskite solar cells by solution-processed tin oxide, Energy Environ. Sci. 9 (2016) 31283134. [92] M. Saliba, J.-P. Correa-Baena, C.M. Wolff, M. Stolterfoht, N. Phung, S. Albrecht, et al., How to make over 20% efficient perovskite solar cells in regular (nip) and inverted (pin) architectures, Chem. Mater. 30 (2018) 41934201. [93] J.H. Heo, H.J. Han, D. Kim, T.K. Ahn, S.H. Im, Hysteresis-less inverted CH3NH3PbI3 planar perovskite hybrid solar cells with 18.1% power conversion efficiency, Energy Environ. Sci. 8 (2015) 16021608. [94] W. Nie, H. Tsai, R. Asadpour, J.-C. Blancon, A.J. Neukirch, G. Gupta, et al., High-efficiency solutionprocessed perovskite solar cells with millimeter-scale grains, Science 347 (2015) 522525. [95] Q. Bao, X. Liu, S. Braun, M. Fahlman, Oxygen-and water-based degradation in [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) films, Adv. Energy Mater. 4 (2014) 1301272. [96] M. Manceau, S. Chambon, A. Rivaton, J.-L. Gardette, S. Guillerez, N. Lemaıˆtre, Effects of long-term UVvisible light irradiation in the absence of oxygen on P3HT and P3HT: PCBM blend, Sol. Energy Mater. Sol. Cells 94 (2010) 15721577. [97] L.Q. Zhang, X.W. Zhang, Z.G. Yin, Q. Jiang, X. Liu, J.H. Meng, et al., Highly efficient and stable planar heterojunction perovskite solar cells via a low temperature solution process, J. Mater. Chem. A 3 (2015) 1213312138. [98] K.O. Brinkmann, J. Zhao, N. Pourdavoud, T. Becker, T. Hu, S. Olthof, et al., Suppressed decomposition of organometal halide perovskites by impermeable electron-extraction layers in inverted solar cells, Nat. Commun. 8 (2017) 13938. [99] J. You, L. Meng, T.-B. Song, T.-F. Guo, Y.M. Yang, W.-H. Chang, et al., Improved air stability of perovskite solar cells via solution-processed metal oxide transport layers, Nat. Nanotechnol. 11 (2016) 7581. [100] M. Pazoki, T.J. Jacobsson, A. Hagfeldt, G. Boschloo, T. Edvinsson, Effect of metal cation replacement on the electronic structure of metalorganic halide perovskites: replacement of lead with alkaline-earth metals, Phys. Rev. B 93 (2016) 144105. [101] T. Leijtens, T. Giovenzana, S.N. Habisreutinger, J.S. Tinkham, N.K. Noel, B.A. Kamino, et al., Hydrophobic organic hole transporters for improved moisture resistance in metal halide perovskite solar cells, ACS Appl. Mater. Interfaces 8 (2016) 59815989. [102] Y. Hou, X. Du, S. Scheiner, D.P. McMeekin, Z. Wang, N. Li, et al., A generic interface to reduce the efficiency-stability-cost gap of perovskite solar cells, Science 358 (2017) 11921197. [103] W.H. Nguyen, C.D. Bailie, E.L. Unger, M.D. McGehee, Enhancing the hole-conductivity of spiroOMeTAD without oxygen or lithium salts by using spiro (TFSI) 2 in perovskite and dye-sensitized solar cells, JACS 136 (2014) 1099611001. [104] W.-Y. Chen, L.-L. Deng, S.-M. Dai, X. Wang, C.-B. Tian, X.-X. Zhan, et al., Low-cost solution-processed copper iodide as an alternative to PEDOT: PSS hole transport layer for efficient and stable inverted planar heterojunction perovskite solar cells, J. Mater. Chem. A 3 (2015) 1935319359. [105] J.H. Kim, P.W. Liang, S.T. Williams, N. Cho, C.C. Chueh, M.S. Glaz, et al., High-performance and environmentally stable planar heterojunction perovskite solar cells based on a solution-processed copper-doped nickel oxide hole-transporting layer, Adv. Mater. 27 (2015) 695701. [106] Y. Wei, K. Yao, X. Wang, Y. Jiang, X. Liu, N. Zhou, et al., Improving the efficiency and environmental stability of inverted planar perovskite solar cells via silver-doped nickel oxide hole-transporting layer, Appl. Surf. Sci. 427 (2018) 782790. [107] A. Mei, X. Li, L. Liu, Z. Ku, T. Liu, Y. Rong, et al., A hole-conductorfree, fully printable mesoscopic perovskite solar cell with high stability, Science 345 (2014) 295298. [108] W.S. Yang, B.-W. Park, E.H. Jung, N.J. Jeon, Y.C. Kim, D.U. Lee, et al., Iodide management in formamidinium-lead-halidebased perovskite layers for efficient solar cells, Science 356 (2017) 13761379. [109] X. Li, M.I. Dar, C. Yi, J. Luo, M. Tschumi, S.M. Zakeeruddin, et al., Improved performance and stability of perovskite solar cells by crystal crosslinking with alkylphosphonic acid ω-ammonium chlorides, Nat. Chem. 7 (2015) 703711. [110] K.T. Cho, G. Grancini, Y. Lee, E. Oveisi, J. Ryu, O. Almora, et al., Selective growth of layered perovskites for stable and efficient photovoltaics, Energy Environ. Sci. 11 (2018) 952959.

Characterization Techniques for Perovskite Solar Cell Materials

References

215

[111] M. Jørgensen, K. Norrman, S.A. Gevorgyan, T. Tromholt, B. Andreasen, F.C. Krebs, Stability of polymer solar cells, Adv. Mater. 24 (2012) 580612. [112] J. Mu¨ller, B. Rech, J. Springer, M. Vanecek, TCO and light trapping in silicon thin film solar cells, Sol. Energy 77 (2004) 917930. [113] Y. Deng, Q. Dong, C. Bi, Y. Yuan, J. Huang, Air-stable, efficient mixed-cation perovskite solar cells with Cu electrode by scalable fabrication of active layer, Adv. Energy Mater. 6 (2016) 1600372. [114] S. Cacovich, L. Cina´, F. Matteocci, G. Divitini, P. Midgley, A. Di Carlo, et al., Gold and iodine diffusion in large area perovskite solar cells under illumination, Nanoscale 9 (2017) 47004706. [115] N.N. Shlenskaya, N.A. Belich, M. Gra¨tzel, E.A. Goodilin, A. Tarasov, Light-induced reactivity of gold and hybrid perovskite as a new possible degradation mechanism in perovskite solar cells, J. Mater. Chem. A 6 (2017) 17801786. [116] D. Lang, H. Grimmeiss, E. Meijer, M. Jaros, Complex nature of gold-related deep levels in silicon, Phys. Rev. B 22 (1980) 3917. [117] A. Guerrero, J. You, C. Aranda, Y.S. Kang, G. Garcia-Belmonte, H. Zhou, et al., Interfacial degradation of planar lead halide perovskite solar cells, ACS Nano 10 (2015) 218224. [118] C. Besleaga, L.E. Abramiuc, V. Stancu, A.G. Tomulescu, M. Sima, L. Trinca, et al., Iodine migration and degradation of perovskite solar cells enhanced by metallic electrodes, J. Phys. Chem. Lett. 7 (2016) 51685175. [119] H. Back, G. Kim, J. Kim, J. Kong, T.K. Kim, H. Kang, et al., Achieving long-term stable perovskite solar cells via ion neutralization, Energy Environ. Sci. 9 (2016) 12581263. [120] Z. Ku, Y. Rong, M. Xu, T. Liu, H. Han, Full printable processed mesoscopic CH3NH3PbI3/TiO2 heterojunction solar cells with carbon counter electrode, Sci. Rep. 3 (2013) 3132. [121] Y. Yang, J. Xiao, H. Wei, L. Zhu, D. Li, Y. Luo, et al., An all-carbon counter electrode for highly efficient hole-conductor-free organo-metal perovskite solar cells, RSC Adv. 4 (2014) 5282552830. [122] S. Gholipour, J.P. Correa-Baena, K. Domanski, T. Matsui, L. Steier, F. Giordano, et al., Highly efficient and stable perovskite solar cells based on a low-cost carbon cloth, Adv. Energy Mater. 6 (2016) 1601116. [123] S.S. Shin, E.J. Yeom, W.S. Yang, S. Hur, M.G. Kim, J. Im, et al., Colloidally prepared La-doped BaSnO3 electrodes for efficient, photostable perovskite solar cells, Science 356 (2017) 167171.

Characterization Techniques for Perovskite Solar Cell Materials

C H A P T E R

10 Characterizing MAPbI3 with the aid of first principles calculations Matthew J. Wolf1, Dibyajyoti Ghosh2, Jolla Kullgren3 and Meysam Pazoki4 1

Department of Physics, University of Bath, Bath, United Kingdom 2Theoretical Division and Center for Nonlinear Studies, Los Alamos National Laboratory, Los Alamos, NM, United ˚ ngstro¨m Laboratory, Uppsala University, Uppsala, States 3Department of Chemistry
A 4 ˚ ngstro¨m Laboratory, Sweden Department of Engineering Sciences, Solid State Physics, A Uppsala University, Uppsala, Sweden

10.1 Introduction Density functional theory (DFT) has become an indispensable workhorse for theoretical investigations in materials science, primarily due to its highly favorable ratio of computational cost to accuracy. Although originally conceived as a method to compute the ground state energy of solid state systems without having to compute the associated many body wave function, developments over subsequent decades, both theoretical and in terms of computational resources, have led to the application of DFT far beyond its initial remit. For example, modern hybrid density functionals provide calculated chemical reaction enthalpies which approach chemical accuracy, while forces obtained via the HellmannFeynman theorem can be used to find minimum energy structures, or to perform ab initio molecular dynamics simulations in order to include finite temperature effects. Excited states can be accessed using the Kohn
Sham orbitals as a starting point for the application of many body perturbation theory via the GW approximation, or the time dependent extension to DFT. Density functional perturbation theory can be used to obtain phonon spectra, electron
phonon couplings, and dielectric constants. Thus, DFT can provide atomic scale information which not only helps us to understand observed phenomena by providing detailed information which is either difficult or impossible to obtain using experimental techniques, but even predict the properties of materials in silico, thereby guiding experimental research efforts.

Characterization Techniques for Perovskite Solar Cell Materials DOI: https://doi.org/10.1016/B978-0-12-814727-6.00010-4

217

Copyright © 2020 Elsevier Inc. All rights reserved.

218

10. Characterizing MAPbI3 with the aid of first principles calculations

Research on metal-halide perovskites is no exception in this regard, as evidenced by the large number of reviews on the application of DFT (and its extensions) to the elucidation of various aspects of the physics and chemistry of these materials [1
19]. Despite the fact that the best performing perovskite based solar cells utilize materials containing a mix of both cations and anions, in this chapter, we focus on the insights provided by the application of such methods to methylammonium lead iodide (CH3NH3PbI3, or simply MAPbI3). This is motivated by the prototypical nature of the material, and its being to date the most extensively studied metal halide perovskite. We also restrict ourselves to intrinsic properties of the material, with the aim of providing a more in-depth picture of the insights which DFT calculations have given. The remainder of the chapter is organized as follows. We first review the attempts to understand the nature of chemical bonding in MAPbI3, with a particular emphasis on the ways in which the presence of the organic MA molecule augments the standard picture of ionic and covalent bonding in inorganic semiconductors. We then discuss the lattice dynamics where in anharmonic effects are believed to be particularly significant. The well recognized consequences of spin
orbit coupling (SOC) on the electronic structure are then reviewed, along with other characteristics which influence charge carrier generation, transport and recombination. The penultimate section focuses on the nature and role of intrinsic point defects on charge carrier lifetimes and device performance, following which we conclude. Herein, We therefore aim to present an overall picture from the most recent efforts to investigate the halide perovskite solar cell materials based on DFT calculations.

10.2 Structure and bonding The general chemical formula of stoichiometric perovskite materials is ABX3, with structures that can be thought of as a matrix of corner sharing octahedra of X anions, with B cations contained within the octahedra, and A cations in the voids between them. The ‘aristotype’ or ‘ideal’, perovskite structure is cubic, but a number of related lower symmetry structures, or ‘hettotypes’, exist, which can be described primarily in terms of unit cells containing multiple octahedra that are tilted with respect to each other, apically and/or equatorially. MAPbI3 is known to exhibit three hettotypes: orthorhombic at low temperature, tetragonal at intermediate temperatures (including room temperature), and a high temperature cubic structure. Due to the relatively narrow temperature range in which each hettotype is stable, there have been numerous studies that attempt to understand the structural details of the phases and the role of various types of bonding. The simplest model of an ionic material is one of charged hard spheres, bound together by electrostatic interactions. Such a model has been found to be appropriate for the oxide perovskites, with the success of the Goldschmidt tolerance factor in describing the lowest energy structures being perhaps the most well known example of its application; however, there are a number of reasons why this model should be less applicable to hybrid halide perovskites such as MAPbI3 [20]. The electronic bonding in the PbI3 sublattice is partially covalent, as evidenced by partial charge analysis giving values of approximately half of the formal values [21] for Pb and I. The covalency is also evident in the character of the valence band, which consists of

Characterization Techniques for Perovskite Solar Cell Materials

10.2 Structure and bonding

219

antibonding states derived from Pb-6s and I-5p orbitals [22
25]. On the other hand, the near integer charges of the MA and PbI3 units [21], and the negligible contribution of MA orbitals to the regions around the band edges [25,26], indicate that the bond between these components is mainly ionic, although the aspherical nature of the MA molecule necessarily complicates the assignment of an ionic radius to it [27,28]. Both the C and N “ends” of the MA molecule are terminated with three protons, which opens up the possibility of hydrogen bonding between the molecule and the inorganic sublattice. Furthermore, the MA molecule possesses a large dipole moment, which is only slightly diminished in the solid state phase from its value in the vacuum [21]; the presence of this dipole, in combination with the polarisability of the diffuse Pb and I ions suggests that van der Waals (VdW) interactions could also play a significant role. This has lead numerous authors to investigate and to attempt to quantify the contribution of such interactions to the bonding and structural properties of the material, with a particular emphasis on the orientation of the MA molecules and their interactions with the inorganic sublattice. In the orthorhombic phase, the C
N axes of the four MA molecules in the unit cell are aligned approximately along the [101] and ½101 directions, in a head-to-tail, antiferroelectric configuration [29], although calculations have shown that a large number of local minima exist within a few tens of meV [30]; Therefore, one can expect that depending on the thermodynamics of crystal growth and the initial growth conditions one can reach different orientational ordering of MA molecules in orthorhombic phase. Based on a distance criterion, three hydrogen bonds between the protons on the N end of the MA molecule and three I ions have been identified in this phase [29]. By comparing the orthorhombic and cubic structures, with the same MA ordering, the hydrogen bonding has been shown to be much stronger in the orthorhombic phase with its tilted octahedra; [29] indeed, it has been argued that without hydrogen bonding, no octahedral tilting would take place at low temperatures, since octahedral tilting is disfavorable for electronic bonding [31]. The orientation of the MA molecules is not well defined in the higher temperature tetragonal or cubic phases, and hence efforts have been made to explore the potential energy surface associated with MA orientations in these phases and the interactions governing its topography. However, the number of local minima, and the role of hydrogen bonding versus VdW interactions has been something of a controversial point. In the pseudocubic phase, from calculations using a unit cell containing a single formula unit, the existence of local minima corresponding to the orientation of the molecule along h100i, h110i and h111i directions have been reported [23,32]; however, while some authors report the lowest energy orientation to be h100i [23,32,33], others report that it is the h110i [34] and h111i [3,35] orientations that are lowest in energy. A complete scan of the potential energy surface of the cubic phase with respect to rigid body MA rotations of the C
N bond axis; rotations of the two subgroups about this axis; and translation of the molecule (albeit with the PbI3 framework held fixed), found only two minima [36] with orientation of the MA molecule along h100i and h111i (see Fig. 10.1). The role of displacement of the MA molecule by 0.3
0.6 Ǻ (defined with respect to the molecule’s geometric center of mass’ being located at the ideal A site) in stabilizing the structure, due to the improved hydrogen bonding between the N end of the molecule and the I ions was also noted [36]. A similar observation was made in Ref. [33], which also remarked upon the significant distortion of the inorganic cages to the same end.

Characterization Techniques for Perovskite Solar Cell Materials

220

10. Characterizing MAPbI3 with the aid of first principles calculations

FIGURE 10.1 (A) Interpolated energy surfaces for molecular reorientation of CH3NH31 within cubic PbI6 octahedral cages. At each orientation, the energy corresponding to the minimum energy on-axis rotation and minimum-energy translation is plotted. (B) Polar plot of orientational energy surface where the radius is proportional to |E 2 βEmax| of scale from (A) where β 5 1 2 1/1000. (C) Energies for selected rotational pathways along the edges of the asymmetric orientation region which represent rotations within the (001) and ð110Þ lattice planes via [100] - [110] and [001] - [111] - [110] rotations, respectively. Reprinted with permission from J.S. Bechtel, R. Seshadri, A. Van der Ven, Energy landscape of molecular motion in cubic methylammonium lead iodide from firstprinciples, J. Phys. Chem. C 120 (23) (2016) 12403
12410. Copyright 2016 American Chemical Society.

In the calculations on the pseudocubic phase discussed in the previous paragraph, the periodic unit cell was taken to be a single formula unit, and therefore, when periodic boundary conditions are applied, the MA molecule is aligned with all of its periodic images, effectively corresponding to a ferroelectric configuration. Supercells must be used in order to study the interactions between MA molecules, resulting in (static) disorder of their orientations [37]. The lowest energy structure of a 2 3 2 3 2 supercell was found to be one in which all of the MA molecules are aligned in a face-to-face configuration. Significantly, the energy gained by distortion of the inorganic lattice is considerably larger than estimated from the single unit cell calculations, due to the weaker constraints on the structure. Furthermore, the directions of neighboring dipoles are correlated primarily at right angles to one another or antiparallel. Interestingly, and in contradiction to earlier assumptions by other authors [38,39], it was concluded that the electrostatic interaction between the dipoles was not the most significant interaction between them [37]. In the tetragonal phase, the orientation of the MA molecules such that its projection on to the ab plane lies along the h110i direction has been found to be the energetically favorable one [26], in which the C
N bond is at an angle of approximately 6 30 degrees with respect to the ab plane [40]. Due to the lower symmetry of the tetragonal phase with respect to the cubic one, there are two distinct chemical environments corresponding to h001i orientation, resulting in two modes of hydrogen bonding (designated α and β by the authors, and shown in Fig. 10.2), differing primarily in the angle which the C
N axis makes with the ab plane, with the two configurations differing in energy by 45 meV per MA unit [41]. Regarding VdW interactions, a number of studies report that adding a dispersion term to a GGA functional improves the lattice parameters of the orthorhombic phase of MAPbI3 [42,43], as well as the cubic [33,34,44] and tetragonal phases [44]. Additionally, the

Characterization Techniques for Perovskite Solar Cell Materials

10.2 Structure and bonding

221

Hydrogen bonding modes in the tetragonal phase (A) α-interaction mode viewed along [110] (left), viewed along [001] (center), and viewed from an arbitrary axis (right). (B) β-interaction mode viewed along [110] (left), viewed along [001] (center), and viewed from an arbitrary axis (right). Reprinted from J.H. Lee, J.-H. Lee, E.-H. Kong, H.M. Jang, The nature of hydrogen-bonding interaction in the prototypic hybrid halide perovskite, tetragonal CH3NH3PbI3, Sci. Rep. 6 (2016) 21687. Licensed under CC BY 4.0.

FIGURE 10.2

pairwise contributions to the dispersive interactions in the cubic phase has been analyzed; while the I
I contribution was asserted to be the largest contribution in [44], the MA
I was shown to be largest in a subsequent study, with the MA
Pb contribution deemed to be of a similar magnitude to the I
I contribution [33]. On the other hand, it has been shown that a similarly accurate structural description can be obtained using a suitable GGA functional (SCAN) in the orthorhombic phase [45]. A comparison of the energies of snapshots from MD with those of high-level RPA calculations on the same structures also showed that VdW functionals do not give a systematic improvement for the cubic phase [46]. Furthermore, the use of VdW functionals has a negligible effect on the vibrational spectrum [47], and give an incorrect description of the MA dynamics [48]. Therefore, the significance of VdW interactions in MAPbI3 currently remains something of an unresolved question.

Characterization Techniques for Perovskite Solar Cell Materials

222

10. Characterizing MAPbI3 with the aid of first principles calculations

10.3 Phonons, anharmonicity and MA dynamics Harmonic phonon spectra of all three phases have been calculated in a number of studies [49
52]. Detailed studies of the orthorhombic phase show that the frequencies of the modes extend to values upwards of 3000 cm21 (see Fig. 10.3). Modes with frequencies between 300 and 1500 cm21 correspond to the internal vibrations of the MA cations that originate from vibrations of the molecules as a whole (twisting, rocking and stretching of the C
N bond), and at around 3000 cm21, from stretching of the C
H and N
H bonds [52]. In the low-frequency region, the normal modes with frequencies below 60 cm21 are dominated by PbI3 contributions, while modes between 140 and 180 cm21 comprise combinations of librations and translations of the MA cations coupled with vibrations of the PbI3 network [51,53]. The spectrum of tetragonal MAPbI3 at room temperature in the spectral range from 50 to 450 cm21 has also been calculated. Three sharp peaks at 62, 94, and 119 cm21 and one broad feature around 250 cm21, were identified. These peaks were assigned, respectively, to bending of the Pb
I
Pb bonds; stretching of the Pb
I bonds and libration of the MA cations; libration of the MA cations; and torsion of the MA cations [49]. Similar assignments hold for the cubic phase [50].

FIGURE 10.3 (A) Total and partial vibrational density of states (VDOS) of the orthorhombic phase. (B) Detail of total and partial VDOS in the range 0
200 cm21. The color code is as follows: green and blue are for the internal vibrations of MA and PbI3, respectively; yellow and red are for MA spin and libration, respectively; black and gray are for MA and PbI3 translations, respectively; brown is for the rotations of the octahedra. The total VDOS is shown as a thin dashed black line. For interpretation of the references to color in this figure legend, the reader can refer web version of this chapter. Reprinted with permission from M.A. Perez-Osorio, R.L. Milot, M.R. Filip, J.B. Patel, L.M. Herz, M.B. Johnston, F. Giustino, Vibrational properties of the organic
inorganic halide perovskite CH3NH3PbI3 from theory and experiment: factor group analysis, firstprinciples calculations, and lowtemperature infrared spectra, J. Phys. Chem. C 119 (46) (2015) 25703
25718. Copyright 2015 American Chemical Society.

Characterization Techniques for Perovskite Solar Cell Materials

10.3 Phonons, anharmonicity and MA dynamics

223

In addition to the assignment of spectral features, phonon calculations have also been used to investigate the role of vibrational entropy in determining the phase stability. The role of low frequency intermolecular modes in particular, was found to be large in the cubic and tetragonal phases, in comparison to the prototypical oxide perovskite, SrTiO3 [54]. Furthermore, calculations including vibrational entropy in the harmonic approximation have suggested that MAPbI3 is intrinsically thermodynamically metastable in the tetragonal phase and only marginally stable in the orthorhombic phase, with respect to phase separation into MAI and PbI2 [55]. Another point of interest is anharmonic effects, which have been extensively studied. Here, we distinguish between the rotations of the MA molecules, to be discussed in the following paragraph, and the vibrations of the lattice, which we discuss first. For example, in the pseudocubic structure, soft (imaginary frequency) modes are observed at the Brillouin zone boundary points, R and M, which correspond to collective tilting modes of the PbI3 octahedra [50] (see Fig. 10.4). Further studies showed that the structure with no octahedral tilting is actually at a saddle point, with two equivalent, lower symmetry structures either side of it [56,57], suggesting that the cubic phase represents a time and/or spatial average of these lower symmetry structures. In contrast, no soft modes were observed for the orthorhombic or tetragonal structures. Furthermore, anharmonic lattice dynamics based on DFT reveals phonon
phonon interactions that are order of magnitudes stronger than for conventional inorganic semiconductors such as GaAs and CdTe [56], which significantly decreases phonon lifetimes and impedes heat transport through the material [58]. To describe anharmonic effects fully, and MA rotational dynamics in particular, one must go beyond a perturbative approach and utilize molecular dynamics (MD)

FIGURE 10.4 (Left) Phonon band structure and density of the cubic structure of MAPbI3. Note the imaginary modes in the region around the R and M points of the Brillouin zone. (Right) Potential energy surface from frozen-phonon calculations of the imaginary eigenmode present at the R point in the cubic structure. Q represents the normal mode coordinate (phonon amplitude). The solution of a 1D Schro¨dinger equation (states shown as horizontal lines with wave functions in the upper panel) are used to generate a thermalized probability density of states (DOS; lower panel). Reprinted with permission from L.D. Whalley, J.M. Skelton, J.M. Frost, A. Walsh, Phonon anharmonicity, lifetimes, and thermal transport in CH3NH3PbI3 from many-body perturbation theory, Phys. Rev. B 94 (22) (2016) 220301. Copyright 2016 American Physical Society.

Characterization Techniques for Perovskite Solar Cell Materials

224

10. Characterizing MAPbI3 with the aid of first principles calculations

simulations. It should be noted that, in order to be able to describe the octahedral tilting modes and MA correlations correctly, supercell expansions of even numbers must be used [59], although it has also been observed that usage of a 2 3 2 3 2 supercell leads to biased dynamics with respect to larger cells [60,61]. Regarding the rotational dynamics of the MA molecules, notwithstanding the fixed orientations of the C
N bond of the MA molecule in the orthorhombic phase, the methyl group can rotate about the C
N bond axis, while the amine group is held essentially static by the hydrogen bonding to the inorganic cages [53,62]. In the higher temperature phases, barriers for the rotation of the MA as a whole of just a few tenths of an eV have been reported, indicating the existence of both static and dynamic disorder of the MA molecular orientations [22,23,34]. This has been confirmed by MD simulations, in which the MA molecules were observed to rotate freely in the cubic phase, with a re-orientation time of the order of ps within a few tens of degrees around room temperature [61,63,64], and slightly longer in the tetragonal phase [64]. In the tetragonal phase, MD simulations have indicated a spontaneous ordering of the MA molecular orientations, with a relaxation time on the order of tens of picoseconds [65]. MA reorientation has also been posited as the origin of the tetragonal
orthorhombic phase transition [66]. Dynamical correlations between the MA molecules have also been analyzed using MD simulations, in part motivated by speculation of ferroelectric domain formation arising from the large dipole moment of the MA molecule [39,40,67,68]. Due to the coupling between MA molecules and the inorganic frameworks, correlated re-orientations of the MA molecules appear in the tetragonal phase (see Fig. 10.5). With increasing temperature,

Dynamical correlation between molecules in the first four NN shells of a 4 3 4 3 4 supercell at temperatures between 100 and 300 K, expressed by square Pearson correlation coefficient (r2c ) for θiθj, φiθj, and φiφj. The filled symbols correspond to the θiθj correlation in a 6 3 6 3 6 supercell. Experimental phase-transition temperatures are indicated by dashed lines. Reprinted with permission from J. Lahnsteiner, G. Kresse, A. Kumar, D. Sarma, C. Franchini, M. Bokdam, Room-temperature dynamic correlation between methylammonium molecules in leadiodine based perovskites: an ab initio molecular dynamics perspective, Phys. Rev. B 94 (21) (2016) 214114. Copyright 2016 American Physical Society.

FIGURE 10.5

Characterization Techniques for Perovskite Solar Cell Materials

10.4 Electronic band structure and charge carrier dynamics

225

the correlations become smaller, as the MA molecules are less hindered by their interactions with the inorganic frameworks. The conclusion is that the interactions between MA molecules are mediated by the PbI3 framework rather than the electrostatic dipole
dipole interactions [61], in keeping with earlier observations [63]. This is also in keeping with the static analysis reported in Ref. [37].

10.4 Electronic band structure and charge carrier dynamics It is well known that local and semi-local DFT systematically, and often severely, underestimate the band gaps of semi-conducting and insulating materials. Nevertheless, initial studies on MAPbI3 using LDA or GGA functionals reported band gaps which compare well to experimentally measured values [22,23]. The same calculations reported that the conduction bands are derived mainly from Pb-6p atomic orbitals, while the valence band maximum is derived from I-5p and Pb-6s orbitals [22,23,69]. Lead and iodine are sufficiently heavy elements that the effects of spin
orbit coupling (SOC) should be significant, and indeed inclusion of SOC in DFT calculations has a profound effect on the calculated band gap, reducing it by approximately 1 eV, and recovering the expected underestimation of the band gap by (semi-)local functionals. The band gap reduction is due primarily to the strong splitting of the Pb derived conduction bands [24,35,70]. In order to recover a band gap closer to that measured experimentally, hybrid functionals (with a judiciously chosen value for the proportion of exact exchange and/or screening length) [42,71] or GW corrections [70,72
74] must be used, in conjunction with SOC (see Fig. 10.6). “Standard” DFT, neglecting the inclusion of SOC, predicts that MAPbI3 possesses a direct band gap; in the cubic phase, this occurs at the R point in the Brillouin zone [22
25], while in the tetragonal and orthorhombic phases it occurs at the Γ point, due to Brillouin zone folding [75]. However, SOC in conjunction with a breaking of the inversion symmetry due to the MA molecules splits the conduction and valence bands valley into two, two-fold, degenerate valleys, leading to gaps separated slightly in k space [76
81]. This so called Rashba
Dresselhaus splitting has been suggested to lead to reduced radiative recombination rates in the material [82
84]. However, a numerical analysis has suggested that the splitting is so small as to have an essentially negligible impact on radiative recombination rates [85,86], which are similar to other direct gap III
V semiconductors [87]. On the other hand, it has been argued that Auger recombination is particularly high in MAPbI3, due to a coincidental resonance of the bandgap value with the gap between the two lowest conduction bands [88]. The dispersion around the band edges are also affected by SOC (see Fig. 10.6), which influences charge transport; in particular, effective masses are reduced by SOC [35,72] and while highly anisotropic according to LDA calculations [26], are made more isotropic in the tetragonal phase [72] and the orthorhombic [42] phases. SOC renders them significantly non-parabolic, to the extent that in some directions in the Brillouin zone, the valence band is quasi-linear as it approaches its energetic maximum [70]. However, despite the non-parabolicity of the bands, carrier densities are too low under solar irradiation for thermalized carriers to explore the nonparabolic region of the bands [89]. The band structure provides useful information regarding electronic effects in a material, but assumes independence of the electrons. The Bethe-Salpeter equation has been Characterization Techniques for Perovskite Solar Cell Materials

226

10. Characterizing MAPbI3 with the aid of first principles calculations

FIGURE 10.6 (Left and center) Electronic band structures of MAPbI3, without (left) and with (center) the spin
orbit coupling interaction. The origin of the energy scale is taken at the top of the VB. Copyright 2013 American Physical Society. (Right) Quasi-particle self-consistent (QS)GW band structure of the cubic phase. Zero denotes the valence band maximum. Bands are colored according to their orbital character: green depicts I 5p, red depicts Pb 6p, and blue depicts Pb 6s. Points denoted M and R are zone-boundary points close to (12,12,0) and (12,12,12), respectively. The valence band maximum and conduction band minimum are shifted slightly from R as a consequence of the spin
orbit coupling. Valence bands near 22 eV (conduction bands near 1 3 eV) are almost purely green (red) showing that they consist largely of I 5p (Pb 6p) character. Bands nearer the gap are darker as a result of intermixing with other states. Light-gray dashed lines show the corresponding bands calculated using LDA. The dispersionless state near 25 eV corresponds to a molecular level of methylammonium, which in the QSGW calculation is pushed down to 27.7 eV. (Left and Center) Reprinted with permission from J. Even, L. Pedesseau, J.-M. Jancu, C. Katan, Importance of spin
orbit coupling in hybrid organic/inorganic perovskites for photovoltaic applications, J. Phys. Chem. Letters 4 (17) (2013) 2999
3005. Copyright 2013 American Physical Society; (Right) Reprinted with permission from F. Brivio, K.T. Butler, A. Walsh, M. Van Schilfgaarde, Relativistic quasi-particle self-consistent electronic structure of hybrid halide perovskite photovoltaic absorbers, Phys. Rev. B 89 (15) (2014) 155204. Copyright 2014 American Physical Society.

applied to understand the role of excitonic effects in optical generation of charge carriers. Such calculations indicate that the exciton binding energy is of the order of a few tens of meV [90
93], explaining the facile charge separation of electrons and holes. As discussed in the previous sections, while the orientations of the MA molecules are fixed in the orthorhombic phase, in the tetragonal and cubic phases, they are not. Although the electronic states of the MA molecules do not contribute to the band edges, their orientations have a strong influence on the electronic structure [26], and related properties, such as the dielectric function and absorption coefficients [26] and the shift current [94]. The band gaps in these phases have also been noted to depend rather strongly on the orientation of the MA molecules, which in turn can be linked to their influence on the structure of the PbI3 sublattice [34,41]. Molecular dynamics simulations on the cubic phase show that the position of the valence band maximum (VBM) fluctuates by approximately 0.34 eV [63], and by up to 0.16 eV [60,95] in the tetragonal phase with approximately equal contributions from changes in the energetic positions of the conduction band minimum (CBM) and VBM [95]. However, the effect on the exciton binding energy is far less pronounced [93]. Due to the softness of MAPbI3, electron
phonon coupling is believed to be particularly important. Orthorhombic MAPbI3 has been shown to exhibit significant electron
phonon

Characterization Techniques for Perovskite Solar Cell Materials

10.5 Intrinsic point defects

227

FIGURE 10.7 (A) Density of electron
phonon coupling strength associated with polar phonons in MAPbI3. The yellow, blue, and red regions correspond to bending, stretching, and libration
translation modes, respectively. The vertical arrows indicate the energies and couplings of the compact, three-phonon model used in the analysis presented in Ref. [96]. (B)
(D) Schematic ball-andstick representations of the three groups of vibrations appearing in (A). The Pb and I atoms are at the centers and at the corners of the octahedra, respectively; C is in gray, and N is in blue. The H atoms are not shown for clarity. Reprinted with permission from M. Schlipf, S. Ponce, F. Giustino, Carrier lifetimes and polaronic mass enhancement in the hybrid halide perovskite CH3NH3PbI3 from multi-phonon Fro¨hlich coupling, Phys. Rev. Lett. 121 (8) (2018) 086402. Copyright 2018 American Physical Society.

coupling involving three longitudinal optical (LO) phonon modes (see Fig. 10.7), which have been argued to lead to ultrafast intraband electronic relaxation [96]. Recent developments in first principles calculations of these quantities have been leveraged to determine the scattering mechanisms and limits of mobilities in MHPs [96
98], which emphasize the pre-dominance of phonon scattering from polar optical modes involving stretching of the Pb
I bond in the orthorhombic phase [98]. Enhanced scattering is due to the low energy of these modes, leading to large populations, compared to more conventional semiconductors [98]. Recently, it has been argued that higher order terms in the electron
phonon coupling should be taken into account [97,99]. A further possible effect of interactions between charge carriers and polar optical phonons is that of (large) polaron formation. The most well established effect is an increase in the effective mass of a charge carrier due to drag induced by its interaction with the lattice [96,100]. However, it has also been argued that the larger effective mass of the polaron could lead to reduced scattering [101,102]. Polaron formation has also been invoked as an explanation for the low carrier cooling observed in MAPbI3 under high fluence irradiation, based on the possibility that, at higher charge carrier densities, the finite sized polaronic states overlap [103]. Molecular dynamics calculations have lead researchers to conclude that large polaron formation also impedes radiative recombination due to an apparent spatial separation in the electron and hole wave functions [104]. We note that some authors have suggested that charge carriers self trap to form small polarons [105,106]. However, this would lead to a mobility which increases with temperature, due to the thermally activated hopping process by which the charge carriers move, which has not been observed experimentally.

10.5 Intrinsic point defects Perovskite based cells are typically produced via low temperature solution-based methods, which, according to conventional semi-conductor wisdom, should lead to high

Characterization Techniques for Perovskite Solar Cell Materials

228

10. Characterizing MAPbI3 with the aid of first principles calculations

concentrations of point defects. In semiconductors, generally these point defects introduce spatially localized defect states inside the band gap, which trap charges and act as nonradiative recombination centers, thereby shortening carrier lifetimes and ultimately reducing the efficiency of devices. Ionized defects also act as scattering centers for mobile charges, reducing the carrier mobility of the material. Despite the anticipated high defect concentration, halide perovskites exhibit exceptionally long carrier lifetimes and large mean free paths, which is even more remarkable given their relatively low carrier mobilities. Either defects exist in much lower concentrations, or their nature is such that their impact upon lifetimes is significantly less severe, than in conventional semi-conductors. As in other ionic semiconductors, native point defects in MAPbI3 can in principle take the form of vacancies, antisites, interstitials, all of which have been investigated thoroughly with DFT based simulations, and the formation, stability and electronic properties of such defects have been explored widely using computational methods [15,71,107
111]. However, we note that, due to the usage of different level of computational approaches, the defect properties vary quite drastically among different reports. Initial computational studies on defects in MAPbI3 mostly made use of GGA exchange correlation functionals, as they spuriously reproduce the experimental band gap (see discussion and references in section 10.4). In the cubic phase of MAPbI3, it was reported that the defects with calculated low formation energies, namely interstitial MA and I (MAi, Ii), lead and iodide vacancies (VPb, VI) form shallow traps for charge carriers [112]. Furthermore, the deep trap forming defects, which are mostly antisites [i.e. iodine-on-MA (IMA) iodine-on-lead (IPb)] and Pb interstitials (Pbi), exhibit relatively high formation energies. Based on these observations, the authors claimed that the so-called defect tolerance of hybrid perovskites strongly contribute to the long carrier diffusion length of hybrid perovskites. A subsequent study found similar results, but pointed out that defect geometries can significantly change depending on the charge state, which can alter the character of the defect-level; due to significant ionic relaxation, lead dimers form in the presence of negatively charged VI defects, and iodine trimers form in the presence of neutral IMA (see Fig. 10.8) [107]. The significantly covalent nature of MAPbI3 lattices has been argued as the reason for the new chemical bond formation in the defective lattice, leading to the appearance of deep levels inside the band gap capable of trapping charge carriers. Further studies examined the dependence of defect formation energies on chemical potentials, determining that, along with MAi and VPb, the iodide vacancy (VI) is also a stable defect under a wide range of growth conditions, and Ii and Pbi act as deep traps [113,114]. More recently it has been demonstrated that simulations with GGA functionals only produce reliable structures and internal energies of halide perovskites. These functionals fail to provide enough accuracy in describing the band gap and relative positions of band edges, both of which strongly affect the defect chemistry of these materials. To overcome the limitations of GGA functionals, inclusion of spin
orbit coupling and many-body interactions of electrons are necessary. Including these interactions, sufficiently accurate electronic properties have been reported for MAPbI3. Considering these high-level computational approaches, it has been demonstrated that the type and nature of well known point defects significantly depend on the chemical environment [15,115
117]. In this regard, most experimental approaches considered I2 vapor pressure as the controlling

Characterization Techniques for Perovskite Solar Cell Materials

229

10.5 Intrinsic point defects

FIGURE 10.8 Electron defect states associated with an iodide vacancy (VI ) in MAPbI3. The plotted band structure along highsymmetry path Γ to A (1/2, 1/2, 1/2) shows the appearance of a defect state in the gap along with Pb
Pb dimer formation. Reproduced with permission from M.L. Agiorgousis, Y.-Y. Sun, H. Zeng, S. Zhang, Strong covalency-induced recombination centers in perovskite solar cell material CH3NH3PbI3, J. Am. Chem. Soc. 136 (41) (2014) 14570
14575. Copyright 2014 American Chemical Society.

FIGURE 10.9 Charge transition levels of various point defects in MAPbI3, calculated with a semi-local functional (PBE) and screened (HSE06) and unscreened (PBE0) hybrid functionals, with the latter calculations incorporating spin
orbit coupling (SOC). The gray regions indicate the valence and conduction bands. Reprinted with permission from D. Meggiolaro, F. De Angelis, Firstprinciples modeling of defects in lead halide perovskites: best practices and open issues, ACS Energy Lett. 3 (9) (2018) 2206
2222. Copyright 2018 American Chemical Society.

factor. Following that, computational studies also computed the defect formation energy under different environments such as I-rich, medium-I and I-poor [115,118]. In all environmental growth conditions, I with different charge states appear to be the main source of deep transition levels (see Fig. 10.9). Under I-rich conditions, positively charged Ii ðI1 i Þ is stable and its transition to the neutral charge states i.e. ( 1 /0) is placed deep inside the band gap. In other growth conditions, Ii with negative charge (I2 i ) becomes stable. For these defects, the (0/ 2 ) transition level gets placed near to the VBM and can trap holes. The lattice rearranges slightly upon hole trapping in these centers. Further, lead vacancies are the other stable defects in all growth processes. The (0/2 2 ) transition state is deep in the band gap, however these have weak trapping activity due to the low cross section of

Characterization Techniques for Perovskite Solar Cell Materials

230

10. Characterizing MAPbI3 with the aid of first principles calculations

the two-electron trapping process. Apart from these defects, other stable defects with low formation energy such as MA interstitials have their transition level inside the bands and are therefore ineffective in charge trapping. Other defects, positive iodine and negative MA vacancy and positive lead interstitials, form midgap states however their formation energies are comparatively high and consequently they form in low concentrations. Thus, the iodine interstitials are the most potent recombination centers for charge trapping in hybrid perovskites. Another aspect of the influence of point defects is their ability to migrate under operational conditions, and their effects on stability and power conversion efficiencies of halide perovskites have also been explored widely. The defect migration in these materials has been modeled as the hopping of the ions to the nearest defect sites through a minimum energy path. The energetic barrier for this defects in MAPbI3 have been calculated using the nudged elastic band method in a number of publications [119,120]. In Ref. [120]. migration barriers for different defects were calculated and it was found that the iodide vacancy migration barrier is 0.58 eV, which is smaller than the migration barriers for MA and Pb ions. In contrast, in Ref. [119] a much smaller migration barrier of 0.08 eV was found for iodide migration in the same material. However, this study also reports that the MA and Pb migration barriers are larger than that of iodide migration. The consensus is that the lowest barriers are found for iodide migration, followed by MA then Pb, although the activation energies themselves vary between publications.

10.6 Conclusion In this chapter, we have attempted to provide an overview of the insights provided by first principles calculations in the characterization of the prototypical hybrid halide perovskite, MAPbI3. This includes the influence of different types of bonding on determining the structure and phase stability of the material; lattice dynamics, and anharmonic effects and MA rotations; the role of spin
orbit coupling in determining the electronic structure; the scattering mechanisms affecting charge carrier transport; and the electronic and transport properties of defects. Much of the research on structure and bonding has focused on the interactions between the MA cation and the inorganic framework. The orientation and position of the MA molecule within the voids between octahedra are strongly linked to the tilting of the latter, which has been ascribed primarily to hydrogen bonding between the NH3 group and iodide ions. Given the large dipole of the MA cation, and the large polarisible ions comprising the inorganic sublattice, it has been argued that van der Waals interactions are of considerable importance, but this remains somewhat controversial. Regarding lattice dynamics, phonon spectra demonstrate a coupling between the organic and inorganic components at low frequencies. In the cubic structure, imaginary modes at the zone boundaries have been shown to exist, indicating that this structure is at an energetic saddle point, and that the experimentally observed structure is likely to be a dynamical average of lower symmetry structures. Correlations between MA orientations have been shown to be mediated by interactions between the molecules and the inorganic sub-lattice, rather than through direct dipole
dipole interactions.

Characterization Techniques for Perovskite Solar Cell Materials

References

231

Spin
orbit coupling has been shown to have a profound effect on the band structure, both on the magnitude of the band gap, the dispersion around the band edges, and in the form of Rashba
Dresselhaus splitting. Although numerous authors have suggested that the latter feature has a significant effect on the rate of radiative recombination, a quantitative analysis suggests otherwise. Due to the softness of the material, electron
phonon interactions are believed to be particularly strong, although which type of phonon (acoustic vs. polar optical) most strongly limits charge carrier mobility remains a point of controversy. Furthermore, the implications of polaron formation, be they large or small, remains something of an open question. Considering the various point defects that can form in MAPbI3, it has been demonstrated that most of them introduce electronic states either close to the band edges, or within the bands themselves, and therefore do not have a significant influence on charge carrier dynamics. However, depending on the experimental conditions, defects with levels deep in the band gap, such as interstitial iodide, can be introduced, which act as detrimental nonradiative recombination centers. Migration barriers for defects have also been calculated, which show that iodide vacancies in particular are highly mobile. Although we have limited ourselves to studies related to the bulk properties of this material, we note that studies have also been performed on surfaces and interfaces, and on other materials of the metal
halide perovskite type, further emphasizing the continuing role that first principles calculations in improving our understanding of these intriguing materials.

References [1] J. Even, L. Pedesseau, C. Katan, M. Kepenekian, J.-S. Lauret, D. Sapori, et al., Solid-state physics perspective on hybrid perovskite semiconductors, J. Phys. Chem. C 119 (9) (2015) 10161
10177. [2] A. Walsh, Principles of chemical bonding and band gap engineering in hybrid organic
inorganic halide perovskites, J. Phys. Chem. C 119 (11) (2015) 5755
5760. [3] W.J. Yin, J.-H. Yang, J. Kang, Y. Yan, S.-H. Wei, Halide perovskite materials for solar cells: a theoretical review, J. Mater. Chem. A 3 (17) (2015) 8926
8942. [4] J.M. Frost, A. Walsh, Molecular motion and dynamic crystal structures of hybrid halide perovskites, Organic-Inorganic Halide Perovskite Photovoltaics, Springer, 2016, pp. 1
17. [5] J.M. Frost, A. Walsh, What is moving in hybrid halide perovskite solar cells? Acc. Chem. Res. 49 (3) (2016) 528
535. [6] E. Mosconi, T. Etienne, F. De Angelis, First-principles modeling of organohalide thin films and interfaces, Organic-Inorganic Halide Perovskite Photovoltaics, Springer, 2016, pp. 19
52. [7] Y. Yan, W.-J. Yin, T. Shi, W. Meng, C. Feng, Defect physics of CH3NH3PbX3 (X 5 I, Br, Cl) perovskites, Organic-Inorganic Halide Perovskite Photovoltaics, Springer, 2016, pp. 79
105. [8] W.-J. Yin, T. Shi, Y. Yan, Structural, electronic, and optical properties of lead halide perovskites, Unconventional Thin Film Photovoltaics, Royal Society of Chemistry, 2016, pp. 177
201. [9] J. Jankowska, R. Long, O.V. Prezhdo, Quantum dynamics of photogenerated charge carriers in hybrid perovskites: dopants, grain boundaries, electric order, and other realistic aspects, ACS Energy Lett. 2 (7) (2017) 1588
1597. [10] M. Kepenekian, J. Even, Rashba and Dresselhaus couplings in halide perovskites: accomplishments and opportunities for spintronics and spin
orbitronics, J. Phys. Phys. Chem. Lett. 8 (14) (2017) 3362
3370. [11] L.D. Whalley, J.M. Frost, Y.-K. Jung, A. Walsh, Perspective: theory and simulation of hybrid halide perovskites, J. Chem. Phys. 146 (22) (2017) 220901. [12] Z. Xiao, Y. Yan, Progress in theoretical study of metal halide perovskite solar cell materials, Aust. Energy Mater. 7 (22) (2017) 1701136.

Characterization Techniques for Perovskite Solar Cell Materials

232

10. Characterizing MAPbI3 with the aid of first principles calculations

[13] S. Yun, X. Zhou, J. Even, A. Hagfeldt, Theoretical treatment of CH3NH3PbI3 perovskite solar cells, Angew. Chem., Int. Ed. 56 (50) (2017) 15806
15817. [14] M.R. Filip, G. Volonakis, F. Giustino, Hybrid halide perovskites: fundamental theory and materials design, in: Handbook of Materials Modeling: Applications: Current and Emerging Materials, 2018, pp. 1
30. [15] D. Meggiolaro, F. De. Angelis, First-principles modeling of defects in lead halide perovskites: best practices and open issues, ACS Energy Lett. 3 (9) (2018) 2206
2222. [16] J. Shi, S. Yun, First-principles DFT calculations for perovskite solar cells, in: Counter Electrodes for DyeSensitized Perovskite Solar Cells, 2, 2018, pp. 487
509. [17] X. Zhou, J. Jankowska, H. Dong, O.V. Prezhdo, Recent theoretical progress in the development of perovskite photovoltaic materials, J. Energy Chem. 27 (3) (2018) 637
649. [18] J. Even, C. Katan, Ab initio and first principles studies of halide perovskites, in: Halide Perovskites: Photovoltaics, Light Emitting Devices, and Beyond, 2019, pp. 25
53. [19] C.-J. Yu, Advances in modelling and simulation of halide perovskites for solar cell applications, J. Phys. Energy 1 (2) (2019) 022001. [20] W. Travis, E. Glover, H. Bronstein, D. Scanlon, R. Palgrave, On the application of the tolerance factor to inorganic and hybrid halide perovskites: a revised system, Chem. Sci. 7 (7) (2016) 4548
4556. [21] M.E. Madjet, F. El-Mellouhi, M.A. Carignano, G.R. Berdiyorov, Atomic partial charges on CH3NH3PbI3 from first-principles electronic structure calculations, J. Appl. Phys. 119 (16) (2016) 165501. [22] Y. Chang, C. Park, K. Matsuishi, First-principles study of the structural and the electronic properties of the lead-halide-based inorganic-organic perovskites (CH3NH3) PbX3 and CsPbX3 (X 5 Cl, Br, I), J. Korean Phys. Soc. 44 (2004) 889
893. [23] F. Brivio, A.B. Walker, A. Walsh, Structural and electronic properties of hybrid perovskites for highefficiency thin-film photovoltaics from first-principles, APL Mater. 1 (4) (2013) 042111. [24] J. Even, L. Pedesseau, J.-M. Jancu, C. Katan, Importance of spin
orbit coupling in hybrid organic/inorganic perovskites for photovoltaic applications, J. Phys. Chem. Lett. 4 (17) (2013) 2999
3005. [25] G. Giorgi, J.-I. Fujisawa, H. Segawa, K. Yamashita, Cation role in structural and electronic properties of 3D organic
inorganic halide perovskites: a DFT analysis, J. Phys. Chem. C 118 (23) (2014) 12176
12183. [26] A. Filippetti, A. Mattoni, Hybrid perovskites for photovoltaics: insights from first principles, Phys. Rev. B: Condens. Matter Mater. Phys. 89 (12) (2014) 125203. [27] A. Amat, E. Mosconi, E. Ronca, C. Quarti, P. Umari, M.K. Nazeeruddin, et al., Cation-induced band-gap tuning in organohalide perovskites: interplay of spin
orbit coupling and octahedra tilting, Nano Lett. 14 (6) (2014) 3608
3616. [28] M. Becker, T. Klu¨ner, M. Wark, Formation of hybrid ABX3 perovskite compounds for solar cell application: first-principles calculations of effective ionic radii and determination of tolerance factors, Dalton Trans. 46 (11) (2017) 3500
3509. [29] J.-H. Lee, N.C. Bristowe, P.D. Bristowe, A.K. Cheetham, Role of hydrogen-bonding and its interplay with octahedral tilting in CH3NH3PbI3, Chem. Commun. 51 (29) (2015) 6434
6437. [30] A. Filippetti, P. Delugas, M.I. Saba, A. Mattoni, Entropy-suppressed ferroelectricity in hybrid lead-iodide perovskites, J. Phys. Chem. Lett. 6 (24) (2015) 4909
4915. [31] J.-H. Lee, N.C. Bristowe, J.H. Lee, S.-H. Lee, P.D. Bristowe, A.K. Cheetham, et al., Resolving the physical origin of octahedral tilting in halide perovskites, Chem. Mater. 28 (12) (2016) 4259
4266. [32] Y. He, G. Galli, Perovskites for solar thermoelectric applications: a first principle study of CH3NH3AI3 (A 5 Pb and Sn), Chem. Mater. 26 (18) (2014) 5394
5400. [33] J. Li, P. Rinke, Atomic structure of metal-halide perovskites from first principles: the chicken-and-egg paradox of the organic-inorganic interaction, Phys. Rev. B: Condens. Matter Mater. Phys. 94 (4) (2016) 045201. [34] C. Motta, F. El-Mellouhi, S. Kais, N. Tabet, F. Alharbi, S. Sanvito, Revealing the role of organic cations in hybrid halide perovskite, Nat. Commun. 6 (2015) 7026. [35] G. Giorgi, J.-I. Fujisawa, H. Segawa, K. Yamashita, Small photocarrier effective masses featuring ambipolar transport in methylammonium lead iodide perovskite: a density functional analysis, J. Phys. Chem. Lett. 4 (24) (2013) 4213
4216. [36] J.S. Bechtel, R. Seshadri, A. Van der Ven, Energy landscape of molecular motion in cubic methylammonium lead iodide from first-principles, J. Phys. Chem. C 120 (23) (2016) 12403
12410. [37] J. Li, J. Ja¨rvi, P. Rinke, Multiscale model for disordered hybrid perovskites: the concept of organic cation pair modes, Phys. Rev. B: Condens. Matter Mater. Phys. 98 (4) (2018) 045201.

Characterization Techniques for Perovskite Solar Cell Materials

References

233

[38] J.M. Frost, K.T. Butler, F. Brivio, C.H. Hendon, M. Van Schilfgaarde, A. Walsh, Atomistic origins of highperformance in hybrid halide perovskite solar cells, Nano Lett. 14 (5) (2014) 2584
2590. [39] J.M. Frost, K.T. Butler, A. Walsh, Molecular ferroelectric contributions to anomalous hysteresis in hybrid perovskite solar cells, APL Mater. 2 (8) (2014) 081506. [40] C. Quarti, E. Mosconi, F. De Angelis, Interplay of orientational order and electronic structure in methylammonium lead iodide: implications for solar cell operation, Chem. Mater. 26 (22) (2014) 6557
6569. [41] J.H. Lee, J.-H. Lee, E.-H. Kong, H.M. Jang, The nature of hydrogen-bonding interaction in the prototypic hybrid halide perovskite, tetragonal CH3NH3PbI3, Sci. Rep. 6 (2016) 21687. [42] E. Menendez-Proupin, P. Palacios, P. Wahno´n, J. Conesa, Self-consistent relativistic band structure of the CH3NH3PbI3 perovskite, Phys. Rev. B: Condens. Matter Mater. Phys. 90 (4) (2014) 045207. [43] Y. Wang, T. Gould, J.F. Dobson, H. Zhang, H. Yang, X. Yao, et al., Density functional theory analysis of structural and electronic properties of orthorhombic perovskite CH3NH3PbI3, Phys. Chem. Chem. Phys. 16 (4) (2014) 1424
1429. [44] D.A. Egger, L. Kronik, Role of dispersive interactions in determining structural properties of organic
inorganic halide perovskites: insights from first-principles calculations, J. Phys. Chem. Lett. 5 (15) (2014) 2728
2733. [45] J.A. Flores-Livas, D. Tomerini, M. Amsler, A. Boziki, U. Rothlisberger, S. Goedecker, Emergence of hidden phases of methylammonium lead iodide (CH3NH3PbI3) upon compression, Phys. Rev. Mater. 2 (8) (2018) 085201. [46] M. Bokdam, J. Lahnsteiner, B. Ramberger, T. Scha¨fer, G. Kresse, Assessing density functionals using many body theory for hybrid perovskites, Phys. Rev. Lett. 119 (14) (2017) 145501. [47] M.A. Perez-Osorio, A. Champagne, M. Zacharias, G.-M. Rignanese, F. Giustino, Van der Waals interactions and anharmonicity in the lattice vibrations, dielectric constants, effective charges, and infrared spectra of the organic
inorganic halide perovskite CH3NH3PbI3, J. Phys. Chem. C 121 (34) (2017) 18459
18471. [48] J. Lahnsteiner, G. Kresse, J. Heinen, M. Bokdam, Finite-temperature structure of the MAPbI3 perovskite: comparing density functional approximations and force fields to experiment, Phys. Rev. Mater. 2 (7) (2018) 073604. [49] C. Quarti, G. Grancini, E. Mosconi, P. Bruno, J.M. Ball, M.M. Lee, et al., The raman spectrum of the CH3NH3PbI3 hybrid perovskite: interplay of theory and experiment, J. Phys. Chem. Lett. 5 (2) (2014) 279
284. [50] F. Brivio, J.M. Frost, J.M. Skelton, A.J. Jackson, O.J. Weber, M.T. Weller, et al., Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal, and cubic phases of methylammonium lead iodide, Phys. Rev. B: Condens. Matter Mater. Phys. 92 (14) (2015) 144308. [51] M.A. Pe´rez-Osorio, R.L. Milot, M.R. Filip, J.B. Patel, L.M. Herz, M.B. Johnston, et al., Vibrational properties of the organic
inorganic halide perovskite CH3NH3PbI3 from theory and experiment: factor group analysis, first-principles calculations, and low-temperature infrared spectra, J. Phys. Chem. C 119 (46) (2015) 25703
25718. [52] A. Mattoni, A. Filippetti, M. Saba, C. Caddeo, P. Delugas, Temperature evolution of methylammonium trihalide vibrations at the atomic scale, J. Phys. Chem. Lett. 7 (3) (2016) 529
535. [53] M.A. Pe´rez-Osorio, Q. Lin, R.T. Phillips, R.L. Milot, L.M. Herz, M.B. Johnston, et al., Raman spectrum of the organic
inorganic halide perovskite CH3NH3PbI3 from first principles and high-resolution low-temperature raman measurements, J. Phys. Chem. C 122 (38) (2018) 21703
21717. [54] K.T. Butler, K. Svane, G. Kieslich, A.K. Cheetham, A. Walsh, Microscopic origin of entropy-driven polymorphism in hybrid organic-inorganic perovskite materials, Phys. Rev. B: Condens. Matter Mater. Phys. 94 (18) (2016) 180103. [55] Y.-Y. Zhang, S. Chen, P. Xu, H. Xiang, X.-G. Gong, A. Walsh, et al., Intrinsic instability of the hybrid halide perovskite semiconductor CH3NH3PbI3, Chi. Phys. Lett. 35 (3) (2018) 036104. [56] L.D. Whalley, J.M. Skelton, J.M. Frost, A. Walsh, Phonon anharmonicity, lifetimes, and thermal transport in CH3NH3PbI3 from many-body perturbation theory, Phys. Rev. B: Condens. Matter Mater. Phys. 94 (22) (2016) 220301. [57] A.N. Beecher, O.E. Semonin, J.M. Skelton, J.M. Frost, M.W. Terban, H. Zhai, et al., Direct observation of dynamic symmetry breaking above room temperature in methylammonium lead iodide perovskite, ACS Energy Lett. 1 (4) (2016) 880
887.

Characterization Techniques for Perovskite Solar Cell Materials

234

10. Characterizing MAPbI3 with the aid of first principles calculations

[58] M. Wang, S. Lin, Anisotropic and ultralow phonon thermal transport in organic
inorganic hybrid perovskites: atomistic insights into solar cell thermal management and thermoelectric energy conversion efficiency, Adv. Funct. Mater. 26 (29) (2016) 5297
5306. [59] J. Even, M. Carignano, C. Katan, Molecular disorder and translation/rotation coupling in the plastic crystal phase of hybrid perovskites, Nanoscale 8 (12) (2016) 6222
6236. [60] M.A. Carignano, A. Kachmar, J. Hutter, Thermal effects on CH3NH3PbI3 perovskite from ab initio molecular dynamics simulations, J. Phys. Chem. C 119 (17) (2015) 8991
8997. [61] J. Lahnsteiner, G. Kresse, A. Kumar, D. Sarma, C. Franchini, M. Bokdam, Room-temperature dynamic correlation between methylammonium molecules in lead-iodine based perovskites: an ab initio molecular dynamics perspective, Phys. Rev. B: Condens. Matter Mater. Phys. 94 (21) (2016) 214114. [62] J. Li, M. Bouchard, P. Reiss, D. Aldakov, S. Pouget, R. Demadrille, et al., Activation energy of organic cation rotation in CH3NH3PbI3 and CD3NH3PbI3: quasi-elastic neutron scattering measurements and first-principles analysis including nuclear quantum effects, J. Phys. Chem. Lett. 9 (14) (2018) 3969
3977. [63] E. Mosconi, C. Quart, T. Ivanovska, G. Ruani, F. De Angelis, Structural and electronic properties of organohalide lead perovskites: a combined IR-spectroscopy and ab initio molecular dynamics investigation, Phys. Chem. Chem. Phys. 16 (30) (2014) 16137
16144. [64] C. Goehry, G. Nemnes, A. Manolescu, Collective behavior of molecular dipoles in CH3NH3PbI3, J. Phys. Chem. C 119 (34) (2015) 19674
19680. [65] I. Deretzis, B.N. Di Mauro, A. Alberti, G. Pellegrino, E. Smecca, A. La Magna, Spontaneous bidirectional ordering of CH3NH3 1 in lead iodide perovskites at room temperature: the origins of the tetragonal phase, Sci. Rep. 6 (2016) 24443. [66] I. Deretzis, A. La. Magna, Exploring the orthorhombic
tetragonal phase transition in CH3NH3PbI3: the role of atom kinetics, Nanoscale 9 (18) (2017) 5896
5903. [67] A. Stroppa, C. Quarti, F. De Angelis, S. Picozzi, Ferroelectric polarization of CH3NH3PbI3: a detailed study based on density functional theory and symmetry mode analysis, J. Phys. Chem. Lett. 6 (12) (2015) 2223
2231. [68] A.L. Montero-Alejo, E. Mene´ndez-Proupin, P. Palacios, P. Wahno´n, J. Conesa, Ferroelectric domains may lead to two-dimensional confinement of holes, but not of electrons, in CH3NH3PbI3 perovskite, J. Phys. Chem. C 121 (48) (2017) 26698
26705. [69] R. Lindblad, D. Bi, B.-w. Park, J. Oscarsson, M. Gorgoi, H. Siegbahn, et al., Electronic structure of TiO2/ CH3NH3PbI3 perovskite solar cell interfaces, J. Phys. Chem. Lett. 5 (4) (2014) 648
653. [70] F. Brivio, K.T. Butler, A. Walsh, M. Van Schilfgaarde, Relativistic quasi-particle self-consistent electronic structure of hybrid halide perovskite photovoltaic absorbers, Phys. Rev. B: Condens. Matter Mater. Phys. 89 (15) (2014) 155204. [71] M.H. Du, Efficient carrier transport in halide perovskites: theoretical perspectives, J. Mater. Chem. A 2 (24) (2014) 9091
9098. [72] P. Umari, E. Mosconi, F. De Angelis, Relativistic GW calculations on CH3NH3PbI3 and CH3NH3SnI3 perovskites for solar cell applications, Sci. Rep. 4 (2014) 4467. [73] M.R. Filip, F. Giustino, GW quasiparticle band gap of the hybrid organic-inorganic perovskite CH3NH3PbI3: effect of spin-orbit interaction, semicore electrons, and self-consistency, Phys. Rev. B: Condens. Matter Mater. Phys. 90 (24) (2014) 245145. [74] E. Mosconi, P. Umari, F. De Angelis, Electronic and optical properties of MAPb3 perovskites (X 5 I, Br, Cl): a unified DFT and GW theoretical analysis, Phys. Chem. Chem. Phys. 18 (39) (2016) 27158
27164. [75] C. Katan, L. Pedesseau, M. Kepenekian, A. Rolland, J. Even, Interplay of spin
orbit coupling and lattice distortion in metal substituted 3D tri-chloride hybrid perovskites, J. Mater. Chem. A 3 (17) (2015) 9232
9240. [76] J. Even, L. Pedesseau, J.-M. Jancu, C. Katan, DFT and k p modelling of the phase transitions of lead and tin halide perovskites for photovoltaic cells, Phys. Status Sol. (RRL)
Rapid Res. Lett. 8 (1) (2014) 31
35. [77] M. Kepenekian, R. Robles, C. Katan, D. Sapori, L. Pedesseau, J. Even, Rashba and Dresselhaus effects in hybrid organic
inorganic perovskites: from basics to devices, ACS Nano 9 (12) (2015) 11557
11567. [78] T. Etienne, E. Mosconi, F. De Angelis, Dynamical origin of the Rashba effect in organohalide lead perovskites: a key to suppressed carrier re-combination in perovskite solar cells? J. Phys. Chem. Lett. 7 (9) (2016) 1638
1645. [79] E. Mosconi, T. Etienne, F. De Angelis, Rashba band splitting in organohalide lead perovskites: bulk and surface effects, J. Phys. Chem. Lett. 8 (10) (2017) 2247
2252.



Characterization Techniques for Perovskite Solar Cell Materials

References

235

[80] T. Etienne, E. Mosconi, F. De Angelis, Dynamical Rashba band splitting in hybrid perovskites modeled by local electric fields, J. Phys. Chem. C 122 (1) (2018) 124
132. [81] S. McKechnie, J.M. Frost, D. Pashov, P. Azarhoosh, A. Walsh, M. Van Schilfgaarde, Dynamic symmetry breaking and spin splitting in metal halide perovskites, Phys. Rev. B: Condens. Matter Mater. Phys. 98 (8) (2018) 085108. [82] F. Zheng, L.Z. Tan, S. Liu, A.M. Rappe, Rashba spin
orbit coupling enhanced carrier lifetime in CH3NH3PbI3, Nano Lett. 15 (12) (2015) 7794
7800. [83] P. Azarhoosh, S. McKechnie, J.M. Frost, A. Walsh, M. Van Schilfgaarde, Research update: relativistic origin of slow electron-hole recombination in hybrid halide perovskite solar cells, APL Mater. 4 (9) (2016) 091501. [84] A. Filippetti, C. Caddeo, A. Bosin, P.D. Delugas, A. Mattoni, Donuts and spin vortices at the Fermi surfaces of hybrid lead-iodide CH3NH3PbI3 perovskites, J. Phys. Chem. C 123 (11) (2019) 6753
6762. [85] X. Zhang, J.-X. Shen, W. Wang, C.G. Van de Walle, First-principles analysis of radiative recombination in lead-halide perovskites, ACS Energy Lett. 3 (10) (2018) 2329
2334. [86] X. Zhang, J.-X. Shen, C.G. Van de Walle, Three-dimensional spin texture in hybrid perovskites and its impact on optical transitions, J. Phys. Chem. Lett. 9 (11) (2018) 2903
2908. [87] A. Filippetti, P. Delugas, A. Mattoni, Radiative recombination and photo-conversion of methylammonium lead iodide perovskite by first principles: properties of an inorganic semiconductor within a hybrid body, J. Phys. Chem. C 118 (43) (2014) 24843
24853. [88] J.-X. Shen, X. Zhang, S. Das, E. Kioupakis, C.G. Van de Walle, Unexpectedly strong Auger recombination in halide perovskites, Adv. Energy Mater. 8 (30) (2018) 1801027. [89] L.D. Whalley, J.M. Frost, B.J. Morgan, A. Walsh, Impact of non-parabolic electronic band structure on the optical and transport properties of photovoltaic materials, Phys. Rev. B: Condens. Matter Mater. Phys. 99 (8) (2019) 085207. [90] T. Ahmed, T. Salim, Y. Lam, E.E. Chia, J.-X. Zhu, et al., Optical properties of organometallic perovskite: an ab initio study using relativistic GW correction and Bethe-Salpeter equation, EPL (Europhys. Lett.) 108 (6) (2014) 67015. [91] X. Zhu, H. Su, R.A. Marcus, M.E. Michel-Beyerle, Computed and experimental absorption spectra of the perovskite CH3NH3PbI3, J. Phys. Chem. Lett. 5 (17) (2014) 3061
3065. [92] M. Bokdam, T. Sander, A. Stroppa, S. Picozzi, D. Sarma, C. Franchini, et al., Role of polar phonons in the photo excited state of metal halide perovskites, Sci. Rep. 6 (2016) 28618. [93] P. Umari, E. Mosconi, F. De Angelis, Infrared dielectric screening determines the low exciton binding energy of metal-halide perovskites, J. Phys. Chem. Lett. 9 (3) (2018) 620
627. [94] F. Zheng, H. Takenaka, F. Wang, N.Z. Koocher, A.M. Rappe, First-principles calculation of the bulk photovoltaic effect in CH3NH3PbI3 and CH3NH3PbI3
xClx, J. Phys. Chem. Lett. 6 (1) (2014) 31
37. [95] M. Mladenovi´c, N. Vukmirovi´c, Effects of thermal disorder on the electronic structure of halide perovskites: insights from MD simulations, Phys. Chem. Chem. Phys. 20 (40) (2018) 25693
25700. [96] M. Schlipf, S. Pon´ce, F. Giustino, Carrier lifetimes and polaronic mass enhancement in the hybrid halide perovskite CH3NH3PbI3 from multi-phonon Fro¨hlich coupling, Phys. Rev. Lett. 121 (8) (2018) 086402. [97] W.A. Saidi, A. Kachmar, Effects of electron
phonon coupling on electronic properties of methylammonium lead iodide perovskites, J. Phys. Chem. Lett. 9 (2018) 7090
7097. [98] S. Pon´ce, M. Schlipf, F. Giustino, Origin of low carrier mobilities in halide perovskites, ACS Energy Lett. 4 (2) (2019) 456
463. [99] M.Z. Mayers, L.Z. Tan, D.A. Egger, A.M. Rappe, D.R. Reichman, How lattice and charge fluctuations control carrier dynamics in halide perovskites, Nano Lett. 18 (12) (2018) 8041
8046. [100] J.M. Frost, Calculating polaron mobility in halide perovskites, Phys. Rev. B: Condens. Matter Mater. Phys. 96 (19) (2017) 195202. [101] X.-Y. Zhu, V. Podzorov, Charge carriers in hybrid organic
inorganic lead halide perovskites might be protected as large polarons, J. Phys. Chem. Lett. 6 (23) (2015) 4758
4761. [102] F. Zheng, L.-w. Wang, Large polaron formation and its effect on electron transport in hybrid perovskites, Energy & Environ. Sci. 12 (4) (2019) 1219
1230. [103] J.M. Frost, L.D. Whalley, A. Walsh, Slow cooling of hot polarons in halide perovskite solar cells, ACS Energy Lett. 2 (12) (2017) 2647
2652. [104] F. Ambrosio, J. Wiktor, F. De Angelis, A. Pasquarello, Origin of low electron
hole recombination rate in metal halide perovskites, Energy Environ. Sci. 11 (1) (2018) 101
105.

Characterization Techniques for Perovskite Solar Cell Materials

236

10. Characterizing MAPbI3 with the aid of first principles calculations

[105] A.J. Neukirch, W. Nie, J.-C. Blancon, K. Appavoo, H. Tsai, M.Y. Sfeir, et al., Polaron stabilization by cooperative lattice distortion and cation rotations in hybrid perovskite materials, Nano Lett. 16 (6) (2016) 3809
3816. [106] A.J. Neukirch, I.I. Abate, L. Zhou, W. Nie, H. Tsai, L. Pedesseau, et al., Geometry distortion and small polaron binding energy changes with ionic substitution in halide perovskites, J. Phys. Chem. Lett. 9 (24) (2018) 7130
7136. [107] M.L. Agiorgousis, Y.-Y. Sun, H. Zeng, S. Zhang, Strong covalency-induced recombination centers in perovskite solar cell material CH3NH3PbI3, J. Am. Chem. Soc. 136 (41) (2014) 14570
14575. [108] H. Shi, M.-H. Du, Shallow halogen vacancies in halide optoelectronic materials, Phys. Rev. B: Condens. Matter Mater. Phys. 90 (17) (2014) 174103. [109] M.-H. Du, Density functional calculations of native defects in CH3NH3PbI3: effects of spin
orbit coupling and self-interaction error, J. Phys. Chem. Lett. 6 (8) (2015) 1461
1466. [110] A. Walsh, D.O. Scanlon, S. Chen, X. Gong, S.-H. Wei, Self-regulation mechanism for charged point defects in hybrid halide perovskites, Angew. Chem. Int. Ed. 54 (6) (2015) 1791
1794. [111] D. Han, C. Dai, S. Chen, Calculation studies on point defects in perovskite solar cells, J. Semicond. 38 (1) (2017) 011006. [112] W.-J. Yin, T. Shi, Y. Yan, Unusual defect physics in CH3NH3PbI3 perovskite solar cell absorber, Appl. Phys. Lett. 104 (6) (2014) 063903. [113] A. Buin, P. Pietsch, J. Xu, O. Voznyy, A.H. Ip, R. Comin, et al., Materials processing routes to trap-free halide perovskites, Nano Lett. 14 (11) (2014) 6281
6286. [114] A. Buin, R. Comin, J. Xu, A.H. Ip, E.H. Sargent, Halide-dependent electronic structure of organolead perovskite materials, Chem. Mater. 27 (12) (2015) 4405
4412. [115] D. Meggiolaro, E. Mosconi, F. De Angelis, Mechanism of reversible trap passivation by molecular oxygen in lead-halide perovskites, ACS Energy Lett. 2 (12) (2017) 2794
2798. [116] D. Meggiolaro, S.G. Motti, E. Mosconi, A.J. Barker, J. Ball, C.A.R. Perini, et al., Iodine chemistry determines the defect tolerance of lead-halide perovskites, Energy Environ. Sci. 11 (3) (2018) 702
713. [117] F. De Angelis, Modeling materials and processes in hybrid/organic photovoltaics: from dye-sensitized to perovskite solar cells, Acc. Chem. Res. 47 (11) (2014) 3349
3360. [118] D. Meggiolaro, E. Mosconi, F. De Angelis, Modeling the interaction of molecular iodine with MAPbI3: a probe of lead-halide perovskites defect chemistry, ACS Energy Lett. 3 (2) (2018) 447
451. [119] J.M. Azpiroz, E. Mosconi, J. Bisquert, F. De Angelis, Defect migration in methylammonium lead iodide and its role in perovskite solar cell operation, Energy Environ. Sci. 8 (7) (2015) 2118
2127. [120] C. Eames, J.M. Frost, P.R. Barnes, B.C. Oregan, A. Walsh, M.S. Islam, Ionic transport in hybrid lead iodide perovskite solar cells, Nat. Commun. 6 (2015) 7497.

Characterization Techniques for Perovskite Solar Cell Materials

C H A P T E R

11 Organic-inorganic metal halide perovskite tandem devices Majid Safdari1,2 and Anders Hagfeldt3 1

Solibro Research AB, Uppsala, Sweden 2Division of Applied Physical Chemistry, Department of Chemistry, KTH Royal Institute of Technology, Stockholm, Sweden 3Institute of Chemical Sciences Engineering, Laboratory of Photomolecular Science, E´cole Polytechnique Fe´de´rale de Lausanne, Lausanne, Switzerland

11.1 Introduction The urgent need to address the climate change by adopting a low-carbon based energy production is foreseen to be provided in a large part by photovoltaic panels. Hence, advancement in this clean technology is a global matter which demands considerable investment. Several decades of research in photovoltaics (PV) has emerged to optimization and commercialization of several PV technologies. These advancements empowered by recent decay in cost of photovoltaic production and installation, make PV to deliver the cheapest energy production in some parts of the world [1,2]. Silicon-based solar cells technology retains an excellent photovoltaic performance and mature manufacturing, so it is by far the dominant photovoltaic technology. However, it is still not cheap enough to globally compete with fossil fuels. Other thin film technologies have shown promising advancement but they are yet shadowed by silicon PV. The enormous scientific attention allocated to the perovskite solar cells (PSC) concluded in a monumental leap which has positioned this technology as one of the game changers in photovoltaic technology. An efficiency of 24.2% [3] was reported for the PSCs verifying its nomination as the fastest growing photovoltaic technology [4]. This conversion efficiency surpasses that of the multicrystalline silicon solar cells being the dominant solar cell technology which is 22.3% [5]. Moreover, it is comparable to the reported efficiencies of other PV competitors such as copper indium gallium selenide (CIGS) [6,7] cadmium telluride (CdTe) [8], gallium arsenide (GaAs) [9]. Nonetheless, it is a massive challenge for the perovskite technology to proceed from laboratory-scale cells to 25-years stable solar

Characterization Techniques for Perovskite Solar Cell Materials DOI: https://doi.org/10.1016/B978-0-12-814727-6.00011-6

237

Copyright © 2020 Elsevier Inc. All rights reserved.

238

11. Organic-inorganic metal halide perovskite tandem devices

modules which demands the scaling up fabrication procedures and design new effective module system for this technology. The enormous production expense shortening of the current commercialized solar modules corresponded to as low as less than half of the cost for the total solar power utility installation. For solar cells researchers and manufacturers several fixed price items, balance of system cost (BOS), i.e. converter, junction box, interconnections etc., are dictated and are not related to the solar modules. So, increasing the output generated power per module area is a logic perspective to reduce the overall power production cost and overcome the barrier of competing with fossil fuels.

11.2 Multi junction solar cells The mechanism of light absorption in semiconductors goes through the transformation of energy of absorbed light to the excitation of electron/s from valence band to the conduction band. As an essential factor, the energy of this specific light should be equal or higher than the semiconductor bandgap (Eg). The excessive energy of the incident light compare to the bandgap will be wasted through thermalization loss. So for conventional one absorber single junction solar cells the higher energy part of solar spectrum will be wasted through thermal relaxation of the excited electron into the conduction band. To achieve maximum electrical power the band gap energy of the absorber must be optimized. The limits were considered by Shockley and Queisser limit reporting that the maximum conversion efficiency is slightly above 30% for single junction solar cells. To introduce a new game changer is imminent for increasing the theoretical and practical efficiency of solar cells. Alternative PV devices based on multiple junctions have shown great potential to achieve efficiencies as high as 32.8% for double junction [5] and 37.9% for triple junctions III-V multijunctions [5]. The highest efficiency reported for multiple junction devices is 46.0% under concentrated sunlight proving the high capability of these devices [5]. As an example in a tandem, double junction, device, the higher-energy solar photons are absorbed in a high-bandgap material in the so-called top cell. It is transparent for the transmitted lower energy photons of the solar spectra will be absorbed by the bottom cell with a lower bandgap. Hence, the top cell can generate higher voltage than the bottom solar cell. The spectral regions of the solar spectrum being absorbed by the top cell (blue (dark gray in print version)) and the bottom cell (red (light gray in print version)) have been illustrated in Fig. 11.1. Some unique properties of the perovskite solar cells that make them ideal candidates in tandem cells are the high absorption coefficients and the possibility to tune the band gap over a broad range of energies by compositional engineering [10 12]. The potential of PSCs as a top cell in tandem with a silicon cell has been shown by Oxford PV, reporting a certified efficiency of 28% [13] that is higher than the record efficiency of a single crystalline silicon cell of 26.7% [5]. Tandem devices can be fabricated in two main configurations. These device architectures represent different levels of optical and electrical independency for each cell. Four-terminal (4T) device consists of top and bottom cells that are independently connected to the outer circuit through their individual cathode and anode electrodes. In this

Characterization Techniques for Perovskite Solar Cell Materials

239

11.2 Multi junction solar cells

FIGURE 11.1 Different regions of the solar irradiance spectrum being absorbed in a tandem device, blue (dark gray in print version) region by top cell/higher bandgap light absorber and red (light gray in print version) by the bottom cell/lower bandgap absorber.

configuration two devices are mechanically stacked together having the high bandgap material as top cell (see Fig. 11.2A). In another approach, they are not necessarily stacked together and light absorption will be managed by utilizing a light splitter. The light splitter is used to divide the light to two beams at the bandgap of the top cell and steer them toward the two different devices. A 4T device using this beam splitter configuration has shown 28% efficiency in Silicon/ Perovskite cells [14]. The perovskite top cell was fabricated in the configuration of FTO/ compact TiO2 /m TiO2/CH3NH3PbI3/spiroMeOTAD/Au. For the bottom cell the monocrystalline silicon heterolunction (HJ) solar cells have been used [14]. The measurement setup is illustrated in Fig. 11.3. Monolithic tandem cells commonly named as the two terminal (2T) architecture consist of two solar cells connected in series. The compositional layers of the two solar cells are deposited sequentially on one single substrate (Fig. 11.2B). The fabrication process parameters for the layers and their related treatments have to be appropriate and nondestructive to the previously deposited layers. The top and bottom cell are connected by a recombination layer (or so-called tunnel junction), so only two external electrodes are needed for the power extraction. The demands for fabrication of solar cells in this architecture are increasing based on its advantages such as the lower manufacturing cost (monolithic integration and omission of an additional substrate and electrode) and improving the functionality of the devices by reduced optical and resistive losses. However, the technological advancement for fabrication processes of different layers is still challenging. The lower layers must abide the fabrication routes for the upper layers, which is challenging if for example the top layers of perovskite and electrical contacts are solution processed. The solvent addition can have damaging effect on the lower layers.

Characterization Techniques for Perovskite Solar Cell Materials

240

11. Organic-inorganic metal halide perovskite tandem devices

FIGURE 11.2 Schematic diagram of the (A) four terminal (4T), and (B) two terminal (2T) tandem perovsikite tandem device. The bottom cell can be selected as silicon solar cell, CIGS or low bandgap perovskite.

FIGURE 11.3 (A) Schematic configuration and (B) representative photograph of the measurement setup for the optical splitting system. Reprinted from reference H. Uzu, M. Ichikawa, M. Hino, K. Nakano, T. Meguro, J.L. Herna´ndez, et al., High efficiency solar cells combining a perovskite and a silicon heterojunction solar cells via an optical splitting system, Appl. Phys. Lett. 106 (2015) 013506, with the permission of AIP publishing.

In 2T devices, the upper and lower junctions are connected in series meaning the total voltage of the devices is the sum of the two sub cells. The device current is limited to the cell that gives the lowest current, normally the bottom cell (which is lower due to the less light absorption). Current matching must be reached through the whole device which demands careful choice of different layers to obtain the optimal efficiency. In this respect 4T devices have advantages with no limiting demand on current matching since the two top and bottom cells are converting sunlight to electricity separately.

Characterization Techniques for Perovskite Solar Cell Materials

11.2 Multi junction solar cells

241

Methylammonium lead(II) iodide (MAPbI3), the first perovskite material that reached notable device efficiency, has a bandgap of 1.6 eV [15,16]. The bandgap of methylammonium lead based perovskite can easily be adjusted through the alternation of halide composition. MAPb(I1-xBrx)3 shows band gaps within the window of 1.6 eV for the pure iodide to 2.2 eV for pure bromide [17]. A comprehensive discussion for the bandgap tuning was provided in Chapter 1. Given the bandgap tunability, perovskite is an excellent candidate for tandem applications. It can be either used on top of existing photovoltaic technologies such as silicon or thin film solar cells, or be applied as an all-perovskite tandem device. The latter alternative gained attention after the invention of the mixed Sn-Pb perovskite with a small bandgap to be used as a bottom sub-cell. However, the former option has acquired the largest commercial interest and is considered as the most appropriate roadmap for industrialization of the perovskite technology. Many reports have discussed the promising future of multijunction solar cells employing PSCs [18,19]. Several challenges for fabrication of these devices can be foreseen such as choice of proper bandgaps for the top and bottom cell to achieve the most efficient light harvest. Different factors for tandem cell needs to be considered while selecting the compositional varieties for different layers in a commercially reliable product. Overall one can summarize them in 4 major aspects: 1. Maximum transparency of the top cell for the light with energy lower than the band gap of the light absorber 2. Maximum light absorption for the absorber layers and minimum reflection in all of the layers or interfaces 3. To pass the stability standard test and suitable for 25 years application 4. To consist of non-toxic and earth-abundant materials/elements The light absorbing layer is obviously a crucial component but the other layers must be considered to fulfill similar standards. Several alternatives have been applied in tandem devices, which briefly will be discussed.

11.2.1 Transparent conductive contact High transmission and excellent conductivity are two vital factors for a proper TCO, which are normally in contrast and cannot be obtained at the same time. In two terminal tandem devices only one of the contacts (electrode of the top cell) needs to be transparent while in four terminal devices three of them need to be transparent. The fabrication method for TCO layers needs to be compatible with the preparation of the high quality perovskite film. In order to fabricate transparent electrodes different deposition techniques have been used such as sputtering, spin coating or mechanical transfer of Ag nanowires, carbon nanotubes, and graphene networks. High power usage in sputtering arose some concerns about damaging the perovskite layer. Different modifications [20] are, however, under development in order to obtain an appropriate recipe for fabrication of the transparent

Characterization Techniques for Perovskite Solar Cell Materials

242

11. Organic-inorganic metal halide perovskite tandem devices

conductive oxides (TCO), such as indium tin oxide (ITO), aluminum doped zinc oxide (AZO), or indium zinc oxide (IZO). Among these oxides ITO is widely utilized in optoelectronic devices since it has high conductivity and proper transparency. Moreover, it has a proper work function, which can be matched with other neighboring layers. Magnetron sputtering is the dominant procedure in producing the ITO layer. The energy of sputtered particles may damage the perovskite film, and a buffer layer be needed to protect the perovskite film. Another choice is a dielectric/metal/dielectric stack as for instance MoOx(10 nm)/Au (8.5 nm)/MoOx (10 nm) [21]. The top MoOx layer is applied to tune the light dispersion characteristics of the semi-transparent electrode while it does not increase the sheet resistance (the sheet resistances for MoOx/Au and MoOx/Au/MoOx are 12.2 and 11.8 per square, respectively). Drawbacks for this option are firstly that the Au layer shows significant absorption in the infrared range even though it is very thin, and secondly that the iodine in the perovskite can react with the MoOx [22]. Other reports have shown promising results with MoO3/ITO, nanoparticles/ITO, MoO3/Al:ZnO, Ag nanowires or thin metal layers [23 28].

11.2.2 Recombination layer In 2T tandem devices, the recombination layer is used as bridge between upper and lower cells, which should efficiently recombine electrons and holes and maintain minimum loss of voltage and maximum transparency. Fabrication of compatible recombination layers with minimal voltage and optical losses, as well as showing no reactivity towards neighboring layers, is a crucial point in making tandem devices. As a proper choice, ITO has been used in 2T junctions (perovskite/silicon) with 23.6% efficiency [29]. It shows excellent transparency and reasonable conductivity, it possess significant diffusion barrier towards ambient moist which boost the stability of the perovskite layer. In another prominent work [30], a nanocrystalline hydrogenated silicon (nc-Si:H) recombination junction was used as recombination layer to cover the front side of doubleside-textured SHJ bottom cell. In this work, nc-Si:H was more efficient compare to ITO layer due to the its best consistency with the other layers and textured bottom cell. The Spiro-TTB hole transporting layer was aggregated on top of ITO while on nc-Si:H was evenly deployed.

11.3 Perovskite tandem devices The efficiency of perovskite tandem devices has increased even faster than the perovskite single junction efficiency through acquisition of the improvement in perovskite single junction devices as well as benefiting from the existing knowledge of the other PV technologies. Different designs for the perovskite tandem device have been developed. The perovskite/silicon tandem solar cells are the most renowned example, which has shown great potential toward commercialization. The other option mostly studied is to couple the

Characterization Techniques for Perovskite Solar Cell Materials

11.4 Theoretical calculations on the potential of perovskite tandem

243

perovskite with CIGS solar cells. The approach to develop tandem technologies with commercially established PVs, such as silicon and CIGS is considered as the fastest way for perovskite commercialization. However, all perovskite tandem devices has been introduced from about 2016, so several materials research and process development have to be developed to reach the full large potential of these technologies. Below, we will discuss these options in more detail and offer a perspective to the field rather than having detailed discussions on each specific report on these alternatives.

11.4 Theoretical calculations on the potential of perovskite tandem The theoretical approach to tandem device and desire for the best light absorption efficiency through coupling of different light absorbers has started decades ago [31] and are still on progress through the discovery of new materials and their potential application in solar cells. Several complete reports have been dedicated to report the potential of the perovskite tandem devices [18,32,33]. These studies have been used as directing paths to optimize different aspects of the perovskite applications in tandem devices including optical losses, reflective losses, thickness limitations, resistance in the films, etc. The theoretical efficiency of the 2T and 4T devices based on the bandgap combination of top cell and bottom cell have been presented in Fig. 11.4 [34]. The red (light gray in print version) and dark red (dark gray in print version) areas represent where the maximum efficiencies can be obtained through double junction devices. As shown, the 4T tandem devices represent more freedom for the bandgap choices compare to 2T because the current matching is not needed in the prior one.

FIGURE 11.4 The calculated maximum conversion efficiency of the tandem devices based on the choice of bandgap for the top cell and bottom cell in 2T devices (left) and 4T devices (right). This theoretical calculation assumes no absorption losses. Reprinted by permission from nature G.E. Eperon, M.T. Ho¨rantner, H.J. Snaith, Metal halide perovskite tandem and multiple-junction photovoltaics, Nat. Rev. Chem. 1 (2017) 0095.

Characterization Techniques for Perovskite Solar Cell Materials

244

11. Organic-inorganic metal halide perovskite tandem devices

11.5 Perovskite/silicon tandem devices The early examples of perovskite tandem devices used MAPbI3 with a bandgap of 1.6 eV as the top cell. This is, however, as described above not the optimal bandgap for a top cell to be matched with a 1.1 eV band gap bottom cell. Bailie et al. [23] reported on using a silver nanowire semi-transparent electrode for the perovskite top cell. In the 4T approach, this perovskite top cell was coupled with a 11.4% efficiency unfiltered multicrystalline silicon device, which boosts the overall efficiency of the tandem device to 17%. The same perovskite semitransparent top cell was used to boost the efficiency of 17% unfiltered CIGS device to 18.6% tandem efficiency. Although the initial rear devices did not have the highest efficiencies of their kind, the proof-of-concept were successfully presented with higher obtained overall efficiency of the tandem device compared to the individual single cells. Werner et al. [35] reported on a perovskite/silicon tandem device using a rear-side texture in silicon hetro-junction bottom cells to enhance the NIR response. The top cell was a low-temperature planar near infrared (NIR) transparent perovskite solar cells with 0.25 cm2 active area generating a 16.4% conversion efficiency. This efficient top cell made it possible to obtain highly efficient devices in a mechanically stacked 4-terminal perovskite/SHJ tandem configuration, with an efficiency of up to 25.2%. In the same work, monolithic tandem device were presented. The top perovskite cell was fabricated by a two-step process involving thermal evaporation of a PbI2 layer followed by spin coating the solution of methylammonium iodide dissolved in ethanol. The perovskite film (bandgap of 1.55 eV) was obtained after annealing at 120  C [35]. The choice of the selective layers are presented in Fig. 11.5.

FIGURE 11.5 Schematic configuration of different layers in a monolithic tandem solar cell wherein the bottom cell has a rear-side texture. The related SEM cross-section images are shown for more clarity. Reprinted from reference J. Werner, L. Barraud, A. Walter, M. Bra¨uninger, F. Sahli, D. Sacchetto, et al., Efficient near-infrared-transparent perovskite solar cells enabling direct comparison of 4-terminal and monolithic perovskite/silicon tandem cells, ACS Energy Lett. 1 (2016) 474 480, with ACS publications permission.

Characterization Techniques for Perovskite Solar Cell Materials

11.5 Perovskite/silicon tandem devices

245

The fabricated cell yields an initial steady-state efficiency of 20.5%. The same type of top perovskite cell were used for both 4T and 2T configurations, which allows for a fair comparison between them. The authors mentioned that the higher efficiency for 4T compare to 2T devices originates from efficient light trapping and no need for current matching between the cells in the 4T device. The ideal band gap coupling for a double junction cell would be B1.8 eV for the top cell and 1.1 1.2 eV as bottom cell [18,36,37]. Through tuning of the bandgap of the perovskite this ideal bandgap can easily be obtained. Cesium and formamidinium mixed perovskite FA0.83Cs0.17Pb(I0.6Br0.4)3 were reported as an excellent choice to obtain the proper bandgap and at the same time overcome the fundamental issue of methylammonium evaporation [28]. This optimal material has been used in state-of-the-art tandem devices. After inclusion of rubidium as cation in making high crystalline and stable mixed perovskite films [38] this new approach was utilized to make a new breakthrough in tandem devices. A stable and highly crystalline perovskite film with an optimal bandgap of 1.74 eV were fabricated to be employed on top of a silicon bottom cell yielding a 26.4% efficient device [39]. In 2015, a MAPbI3/TiO2 based top cell was used in a 2T architecture on top of a silicon cell to obtain 13.7% efficiency for 1 cm2 devices [40]. An important milestone for the 2T perovskite/silicon tandem device was the report by Bush et al. [29] in Feb 2017 that presented a 23.6% efficient monolithic (2T) with 1 cm2 area. The top perovskite cell was fabricated using a mixed formamidinium/cesium lead halide perovskite [Cs0.17FA0.83Pb (Br0.17I0.83)3] delivering a bandgap of 1.63 eV with enhanced stability compare to the conventional MAPbI3 perovskite. A nickel oxide layer was used as hole transport material to enhance the stability (compared to a PEDOT:PSS layer that was conventionally used in this configuration). Standard stability test was performed on the top cell for which an excellent durability for industrial application was demonstrated. The detailed layer configurations of the single junction perovskite and 2T perovskite/silicon tandem are presented in Fig. 11.6. The semi-transparent top perovskite cell shows 14.5% conversion efficiency with a short circuit current density of 18.7 mA cm22 and 0.98 V open circuit voltage. The relatively lower voltage can be attributed to the difficulty in crystallization of mixed perovskite on a planar surface within this inverted structure. In this 2T tandem device amorphous silicon/crystalline silicon heterojunction design has been used for the bottom cell to obtain high VOC. Calculations suggested that by further widening the bandgap of the perovskite, more efficient absorption management is possible. Another approach for preparation of highly crystalline perovskite was presented by Sahli et al. [30]. Textured crystalline silicon cell were used as bottom cell and on top an inverted structure highly efficient perovskite was fabricated. The conformal perovskite layer was prepared in two steps, co-evaporation of porous lead iodide/cesium bromide and then spin coating of the cation solution (formamidinium iodide (FAI), and formamidinium bromide, (FABr)). The final perovskite absorber layer of CsxFA12xPb(I, Br)3 was annealed at 150  C in ambient air. This two-step method is efficiently addressing the spin coating problem of perovskite precursor on textured bottom cell and avoids the craggy perovskite films. For this reason previously record 23.6% [29] two terminal perovskite/silicon cell was made by spin coating perovskite top cell on a front-side-polished

Characterization Techniques for Perovskite Solar Cell Materials

246

11. Organic-inorganic metal halide perovskite tandem devices

FIGURE 11.6 Schematic layer configuration of (A) perovskite top cell and (B) perovskite/silicon tandem cell. Reprinted by permission from nature, K.A. Bush, A.F. Palmstrom, J.Y. Zhengshan, M. Boccard, R. Cheacharoen, J.P. Mailoa, et al., 23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability, Nat. Energy 2 (2017) 17009.

c-Si cell which has a B1 mA cm22 lower practical photocurrent compare to fully textured design [41,42]. All this modifications were concluded to a fully textured monolithic/SHJ tandem device with current density of 19.5 mA cm22 and certified conversion efficiency of 25.2% [30]. The certified performance of the champion cell and the schematic overview of this solar cell with the optional sequential layer design are presented in Fig. 11.7.

11.6 Perovskite/CIGS tandem devices One of the advantages of CIGS solar cells as bottom cell is the bandgap tunability with limited domain for these solar cells (compare to silicon solar cell). Guchhait et al. [20] employed Ag and MoOx as buffer layers for sputtering ITO in semitransparent perovskite solar cells to be coupled with CIGS bottom cell. Semitransparent cells with Ag/ITO and

Characterization Techniques for Perovskite Solar Cell Materials

11.6 Perovskite/CIGS tandem devices

247

FIGURE 11.7 (A) Schematic design of monolithic tandem device, (B) certified QE, and (C) certified J V data (measured at 100 mV S21 of the champion device with 1.42 cm2 aperture area). Reprinted by permission from Nature Materials, F. Sahli, J. Werner, B.A. Kamino, M. Bra¨uninger, R. Monnard, B. Paviet-Salomon, et al., Fully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiency, Nat. Mater. (2018) 1.

MoOx/ITO electrodes attained a reduced PCE of 15.3% and 13.8%, respectively. The transparency of the solar cells was measured at 54% in the 800 1400 nm region. As shown in Fig. 11.8, the semitransparent perovskite cell was joined with Cu(In, Ga)Se (CIGS) cell in a 4-terminal (4T) tandem configuration and reached the efficiency of 20.7%.

Characterization Techniques for Perovskite Solar Cell Materials

248

11. Organic-inorganic metal halide perovskite tandem devices

FIGURE 11.8

(A) The schematic configuration, (B) SEM cross sectional image of transparent perovskite (triple cation) top cell, and (C) the schematic configuration of the tandem device. Reprinted with permission from reference A. Guchhait, H.A. Dewi, S.W. Leow, H. Wang, G. Han, F.B. Suhaimi, et al., Over 20% efficient CIGS perovskite tandem solar cells, (2017). Copyright 2017 American Chemical Society.

Fu et al. presented an efficient configuration for perovskite solar cells which involves: glass substrate/In2O3:H/PTAA/CH3NH3PbI3/PCBM/ZnO nanoparticles/ZnO:Al/Ni-Al grid. Such a solar cell showed an excellent efficiency of 16.1% with a high VOC of 1.116 V. The fabricated perovskite top cells demonstrate high transmittance of 80.4% in the wavelength range of 800 1200 nm. A 4T tandem device based on this perovskite top cell coupled with Cu(In, Ga)Se2 and CuInSe2 bottom cells reached efficiencies of 22.1% and 20.9%, respectively. This result is impressive considering that the applied perovskite were pure MAPbI3 with a bandgap of 1.56 eV. In late 2017, the same group at EMPA reported an increase in efficiency of 4T perovskite/CIGS tandem device to 22.7%, through improvement of the perovskite film. They fabricated the perovskite film by a compositionally graded approach, which enhanced the functionality of the device by decreasing interface recombination [43]. The 2T perovskite tandem device was challenging to produce in the beginning due to fabrication difficulties for the different successive layers. The first ever report on a 2T perovskite based tandem device was based on MAPbI3 on top of a kesterite Cu2ZnSn(S, Se)4 bottom cell. The attempt was mostly aimed to be a proof-of-concept using aluminum as transparent contact (with 50% transparency), which was successfully reported with a 4.6% efficiency in autumn 2014. In 2015, the same group [44] reported on a successful attempt to increase the efficiency of the 2T monolithic tandem device. A ZnO-free CIGS structure could withstand annealing treatments for several hours at 120  C without any damage, which is necessary for the manufacturing the top layers. For a precise bandgap alignment of the perovskite, the film formation was monitored in a chamber where a vapor-based halide exchange reaction was performed on top of a PbI2-deposited substrate. The fabricated 2T tandem device using an Al contact showed an efficiency of 10.98% and VOC of 1450 mV. The poor overall performance of the device was originated from the elimination of ZnO which migitate the CIGS

Characterization Techniques for Perovskite Solar Cell Materials

11.7 Perovskite-perovskite tandem devices

249

performance. Moreover the high series resistance and poor contact between the layers led to low fill factor of 60%. Yang Yang group reported on a successful utilization of CIGS and perovskite in a 2T tandem device which leads to an efficiency of 22.4% [45]. In this work the roughness of the buffer layer has been modified to maintain excellent contact between the two sub cells. The original top layers of CIGS perovskite cell have been preserved (CdS/iZnO/BZO) and a polished layer of indium tin oxide (ITO) has been used as buffer layer. The polished CIGS device performance were compared with the stand-alone device, representing a slightly lower current originated from ITO absorbance within the region of 400 500 nm. The composition of Cs0.09FA0.77MA0.14Pb(I0.86Br0.14)3 with bandgap of 1.59 eV was used as the top perovskite cell providing 14.83% conversion efficiency and average transmittance of 80% between 770 and 1300 nm. In another pioneering research, Shen et al. [46] reported a huge advancement in efficiency of 4T CIGS/perovskite tandem device resulted in 23.9% overall conversion efficiency. A quadruple-cation perovskite was applied and the molybdenum oxide/indium doped zinc oxide/metal grid (gold) as the transparent front contact. The optimized thin layer of perovskite with composition of Cs0.05Rb0.05FA0.765MA0.135PbI2.55Br0.45 was yielded to 18.1% steady-state conversion efficiency for the transparent top cell. The shadowed CIGS cell were measured at 5.8% efficiency (compare to 16.5% standalone efficiency) leading to overall 23.9% efficiency. An anti-reflection layer of MgF2 was applied to minimization the reflectance and improves the overall light absorption efficiency. The authors claimed that the hysteresis behavior of the applied quadruple perovskite cells was negligible compared to significant “inverted hysteresis” of Cs0.17FA0.83PbI1.8Br1.2 devices.

11.7 Perovskite-perovskite tandem devices An all-perovskite tandem device is an ambitious alternative providing the possibility of utilizing well-integrated low-cost fabrication methods for all absorber layers. The concept was firstly proposed in 2014 through a computational study where the potential of obtaining high open circuit voltage were discussed [47]. Heo et al. reported one of the early efforts on an all-perovskite 2T tandem device by coupling methylammonium lead bromide (bandgap of 2.2 eV) and methylammonium lead iodide (bandgap of 1.6 eV) as top and bottom layer, respectively [48]. The novel concept was clearly demonstrated, but, however, with a modest efficiency of 10.8% due to low light harvesting efficiency of the device based on imperfect bandgap matching. Since the bandgap of lead based perovskite bandgap cannot be lower than 1.48 eV (for FAPbI3), tin based perovskites, due to the similar structural properties with lead perovskites, was a reasonable choice to obtain a low bandgap material employed as a bottom cell. The synthesis and fabrication of these Sn-based perovskites is feasible through similar procedures as lead based perovskites, which also make them proper alternatives to be used as bottom cell. Methylammonium tin iodide had a band gap of 1.3 eV, but when applied in solar cells it has so far shown limited efficiency and low stability [49,50]. Another different feature of tin perovskite is their faster crystal formation compared to lead perovskite, which directly affect the smoothness and quality of the fabricated films.

Characterization Techniques for Perovskite Solar Cell Materials

250

11. Organic-inorganic metal halide perovskite tandem devices

The bandgap tuning for mixed lead-tin perovskites has been presented in 2014 [51]. Such mixtures produced a more stable low bandgap product compared to the extremely airsensitive pure tin perovskite (oxidizes to Sn41). Several reports have presented improvement on the fabrication of high quality Sn-Pb perovskite films using optimized solvent mixtures and anti-solvent treatments [52,53]. The development in the processing of low bandgap Sn-Pb perovskite (1.2 1.3 eV) to higher efficiencies and stabilities comparable to Pb perovskite make them at present to be interesting candidates to replace silicon or CIGS bottom cells in tandem devices. A big challenge for fabrication of all-perovskite 2T devices is to prepare the top layer by solution processing methods. The typical solvents being utilized in the solution preparation normally dissolve the perovskite bottom layer or at least damages the film quality. Several efficient procedures have been discussed before where the optimization of perovskite films is discussed. The buffer layer with limited electrical loss between the perovskite films is crucial for high efficiency. The processing procedure for the buffer layer should be compatible with the chemical stability of the perovskite bottom layer. The conventionally used sputtering deposition method is difficult due to the high energy deposition flux, damaging the perovskite. Hence, several modifications [54] are being developed to achieve an appropriate recipe for fabrication of buffer layers such as indium tin oxide (ITO), aluminum doped zinc oxide (AZO), or indium zinc oxide (IZO). Among these oxides ITO is widely utilized in optoelectronic devices since it has high conductivity and good transparency. Moreover, it has proper a work function, which can be matched with adjacent layers. Fabrication of 4T all perovskite tandem devices is fundamentally easier. Still, the challenge to fabricate the low bandgap bottom cell remains. The efforts to realize these tandem cells have shown a fast trend. A 19.1% 4T-perovskite was obtained by applying a top cell of 1.6 eV MAPbI3 on top of a 1.3 eV FA0.75Cs0.25Pb0.75Sn0.25I3 bottom cell [55]. By optimizing the respective perovskite layer band gaps, using a top cell of 1.8 eV FA0.83Cs0.17Pb (I0.5Br0.5)3 on top of 1.2 eV FA0.75Cs0.25Sn0.5Pb0.5I3, Snaith-McGehee and coworkers obtained a 20.3% efficient 4T device [53]. Continuous modifications on the alternative layers have improved the efficiency to 23%. Zhao et al. [56]. reported on a 1.75 eV bandgap FA0.8Cs0.2Pb-(I0.7Br0.3)3 on top of a 1.25 eV bandgap (FASnI3)0.6(MAPbI3)0.4 bottom cell. The layer engineering is illustrated in Fig. 11.9 showing the transparent electrode of MoOx/indium tin oxide (ITO) with enhanced transmittance applied on the top cell. The development of 2T all-perovskite devices started with the use of a 1.6 eV MAPbI3 bottom cell, which is far higher than the optimal bandgap [48,57]. As previously mentioned, the application of mixed Sn-Pb perovskite layers with 1.2/1.4 eV bandgap has been essential to process devices with high efficiencies. The research interests in these devices have increased substantially leading to improvements in their functionality and reliability. As an example, in a pioneer report an ITO buffer layer was introduced between the combination of 1.2 eV FA0.7Cs0.25Pb0.5Sn0.5I3 and 1.8 eV FA0.83Cs0.17Pb(I0.5Br0.5)3. The fabricated cell showed a conversion efficiency of 17.0% with VOC of 1.66 V. Although the efficiency of these devices has not surpassed that of single junction devices, the future potential triggers intense research interests to address the different challenges.

Characterization Techniques for Perovskite Solar Cell Materials

11.8 Outlook

251

FIGURE 11.9 The engineering of 23% 4T mechanically stacked all-perovskite tandem device. The highly transparent back contact for the top cell resulted in a 7.4% efficient shadowed bottom cell. Reprinted with permission from reference D. Zhao, C. Wang, Z. Song, Y. Yu, C. Chen, X. Zhao, et al., 4-Terminal all-perovskite tandem solar cells achieving power conversion efficiencies exceeding 23%, ACS Energy Lett. (2018). Copyright 2018 American Chemical Society.

In another pioneering work, Forga´cs et al. [58]. employed solution processed Cs0.15FA0.85Pb(I0.3Br0.7)3 [28] with a band gap of 2 eV as top layer, and vacuum-vapor deposited MAPbI3 as rear cell. The efficient bandgap coupling of 2 eV and 1.55 eV results in an overall efficiency of 18.1%. This work also demonstrated great promises for the vapor deposition techniques as a future alternative for production of perovskite films in multilayer devices.

11.8 Outlook In this chapter we focused on two junction tandem devices which open up the question of the possibility of going beyond two junctions. Theoretical calculations show that triple junction perovskite device has a slightly higher Shockley-Queisser efficiency limit (46.9% for triple junction device compared to 46.0% for a double junction) [18], which means that the effort/cost of adding an extra junction could be of less commercial interest. However, it has been suggested that implementation of a double junction perovskite on top of a

Characterization Techniques for Perovskite Solar Cell Materials

252

11. Organic-inorganic metal halide perovskite tandem devices

silicon hetero junction can achieve a practical limit of 35.3% efficiency, which would be higher than that of a double junction perovskite. This stems from the bandgap of silicon of 1.1 eV, for which an efficient and stable perovskite is far from being developed. A greater number of layers in tandem devices compared to single junctions increase the problems with optical and electric losses. Reflections at the layer interfaces and internal surfaces are the major part of optical losses. This can be minimized through for example refractive optimization of the different interfaces and optimization of the thicknesses of the compositional layers. Electronic losses similar to single junction devices are mostly related to charge carriers which are trapped or recombine to the ground state before extraction. The thickness of the different layers should be optimized to have maximum absorption and at the same time not surpass the carrier diffusion length through each material/layer. In order to eliminate these losses some new aspects are foreseen to be implemented in the procedures for the manufacturing of the different layers. As an example, textured structures can be mentioned which is well-known in the preparation of silicon solar cells. Considering the fast progress in a relatively short time, there are big opportunities to overcome the different challenges. The currently reported efficiencies for perovskite tandem devices have surpassed the single junction values and also passed the silicon record efficiency as reported in a press release on December 20th, 2018, by Oxford PV (28%). The anticipated efficiency rise to 30% seems thus achievable in the next few years. The evolution of perovskite solar cells increases the potential in the PV market, and yet its potential to enhance the commercially established technologies through tandem devices is another exciting aspect for perovskite solar cells.

Acknowledgment We acknowledge financial support from the Swedish Research Council Formas (2017-01134).

References [1] B. Obama, The irreversible momentum of clean energy, Science 355 (2017) 126 129. [2] R. Fu, D. Feldman, R. Margolis, K. Ardani, M. Woodhouse, NREL US solar photovoltaic system cost benchmark Q1 2017 report, in: National Renewable Energy Laboratory-Data (NREL-DATA), Golden, CO; National Renewable Energy Laboratory, 2017. [3] https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies-190416.pdf. [4] M.A. Green, A. Ho-Baillie, H.J. Snaith, The emergence of perovskite solar cells, Nature Photonics 8 (2014). nphoton. 2014.2134. [5] M.A. Green, Yoshihiro Hishikawa, Ewan D. Dunlop, Dean H. Levi, J. Hohl-Ebinger, A.W.Y. Ho-Baillie, Solar cell efficiency tables (version 51), Progress inPhotovoltaics 26 (2018) 3 12. [6] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, et al., New world record efficiency for Cu (In, Ga) Se2 thin-film solar cells beyond 20%, Progress inPhotovoltaics 19 (2011) 894 897. [7] P. Reinhard, A. Chirila, P. Blosch, F. Pianezzi, S. Nishiwaki, S. Buechelers, et al., Review of progress toward 20% efficiency flexible CIGS solar cells and manufacturing issues of solar modules, in: Photovoltaic Specialists Conference (PVSC), vol. 2, IEEE 38th, IEEE, 2012, pp. 1 9. [8] X. Wu, High-efficiency polycrystalline CdTe thin-film solar cells, Solar energy 77 (2004) 803 814. [9] E.D. Kosten, J.H. Atwater, J. Parsons, A. Polman, H.A. Atwater, Highly efficient GaAs solar cells by limiting light emission angle, Light: Science Applications 2 (2013) e45. [10] P. Gao, M. Gra¨tzel, M.K. Nazeeruddin, Organohalide lead perovskites for photovoltaic applications, Energy Environmental Science 7 (2014) 2448 2463.

Characterization Techniques for Perovskite Solar Cell Materials

References

253

[11] P. Docampo, T. Bein, A long-term view on perovskite optoelectronics, Accounts ofchemical research 49 (2016) 339 346. [12] L. Yang, A.T. Barrows, D.G. Lidzey, T. Wang, Recent progress and challenges of organometal halide perovskite solar cells, Reports onProgress Physics 79 (2016) 026501. [13] https://www.oxfordpv.com/news/oxford-pv-perovskite-solar-cell-achieves-28-efficiency. [14] H. Uzu, M. Ichikawa, M. Hino, K. Nakano, T. Meguro, J.L. Herna´ndez, et al., High efficiency solar cells combining a perovskite and a silicon heterojunction solar cells via an optical splitting system, Applied Physics Letters 106 (2015) 013506. [15] M. Safdari, A. Fischer, B. Xu, L. Kloo, J.M. Gardner, Structure and function relationships in alkylammonium lead(ii) iodide solar cells, Journal Materials Chemistry A 3 (2015) 9201 9207. [16] T. Baikie, Y. Fang, J.M. Kadro, M. Schreyer, F. Wei, S.G. Mhaisalkar, et al., Synthesis and crystal chemistry of the hybrid perovskite (CH3NH3)PbI3 for solid-state sensitised solar cell applications, Journal Materials Chemistry A 1 (2013) 5628 5641. [17] J.H. Noh, S.H. Im, J.H. Heo, T.N. Mandal, S.I. Seok, Chemical management for colorful, efficient, and stable inorganic organic hybrid nanostructured solar cells, Nano letters 13 (2013) 1764 1769. [18] M.T. Ho¨rantner, T. Leijtens, M.E. Ziffer, G.E. Eperon, M.G. Christoforo, M.D. McGehee, et al., The potential of multijunction perovskite solar cells, ACS Energy Letters 2 (2017) 2506 2513. [19] Niraj N. Lal, D. Yasmina, L. Wei, H. Qicheng, C. Yi-Bing, B. Udo, Perovskite tandem solar cells, Advanced Energy Materials 7 (2017) 1602761. [20] A. Guchhait, H.A. Dewi, S.W. Leow, H. Wang, G. Han, F.B. Suhaimi, et al., Over 20% efficient CIGS perovskite tandem solar cells, (2017). [21] D. Zhao, Y. Yu, C. Wang, W. Liao, N. Shrestha, C.R. Grice, et al., Low-bandgap mixed tin lead iodide perovskite absorbers with long carrier lifetimes for all-perovskite tandem solar cells, Nature Energy 2 (2017) 17018. [22] P. Liu, X. Liu, L. Lyu, H. Xie, H. Zhang, D. Niu, et al., Interfacial electronic structure at the CH3NH3PbI3/ MoOx interface, Applied Physics Letters 106 (2015) 193903. [23] C.D. Bailie, M.G. Christoforo, J.P. Mailoa, A.R. Bowring, E.L. Unger, W.H. Nguyen, et al., Semi-transparent perovskite solar cells for tandems with silicon and CIGS, Energy Environmental Science 8 (2015) 956 963. [24] T. Todorov, T. Gershon, O. Gunawan, C. Sturdevant, S. Guha, Perovskite-kesterite monolithic tandem solar cells with high open-circuit voltage, Applied Physics Letters 105 (2014) 173902. [25] P. Lo¨per, S.-J. Moon, S.M. De Nicolas, B. Niesen, M. Ledinsky, S. Nicolay, et al., Organic inorganic halide perovskite/crystalline silicon four-terminal tandem solar cells, Physical Chemistry Chemical Physics 17 (2015) 1619 1629. [26] L. Kranz, A. Abate, T. Feurer, F. Fu, E. Avancini, J. Lo¨ckinger, et al., High-efficiency polycrystalline thin film tandem solar cells, Thejournal physical chemistry letters 6 (2015) 2676 2681. [27] K.A. Bush, C.D. Bailie, Y. Chen, A.R. Bowring, W. Wang, W. Ma, et al., Thermal and environmental stability of semi-transparent perovskite solar cells for tandems enabled by a solution-processed nanoparticle buffer layer and sputtered ITO electrode, Advanced Materials 28 (2016) 3937 3943. [28] D.P. McMeekin, G. Sadoughi, W. Rehman, G.E. Eperon, M. Saliba, M.T. Ho¨rantner, et al., A mixed-cation lead mixed-halide perovskite absorber for tandem solar cells, Science 351 (2016) 151 155. [29] K.A. Bush, A.F. Palmstrom, J.Y. Zhengshan, M. Boccard, R. Cheacharoen, J.P. Mailoa, et al., 23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability, Nature Energy 2 (2017) 17009. [30] F. Sahli, J. Werner, B.A. Kamino, M. Bra¨uninger, R. Monnard, B. Paviet-Salomon, et al., Fully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiency, Nature materials (2018) 1. [31] A. De Vos, Detailed balance limit of the efficiency of tandem solar cells, Journal Physics D: Applied Physics 13 (1980) 839. [32] M. Filipiˇc, P. Lo¨per, B. Niesen, S. De Wolf, J. Krˇc, C. Ballif, et al., CH3NH3PbI3 perovskite/silicon tandem solar cells: characterization based optical simulations, Optics express 23 (2015) A263 A278. [33] R. Asadpour, R.V. Chavali, M. Ryyan Khan, M.A. Alam, Bifacial Si heterojunction-perovskite organic-inorganic tandem to produce highly efficient (η T*B 33%) solar cell, Applied Physics Letters 106 (2015) 243902. [34] G.E. Eperon, M.T. Ho¨rantner, H.J. Snaith, Metal halide perovskite tandem and multiple-junction photovoltaics, Nature Reviews Chemistry 1 (2017) 0095.

Characterization Techniques for Perovskite Solar Cell Materials

254

11. Organic-inorganic metal halide perovskite tandem devices

[35] J. Werner, L. Barraud, A. Walter, M. Bra¨uninger, F. Sahli, D. Sacchetto, et al., Efficient near-infraredtransparent perovskite solar cells enabling direct comparison of 4-terminal and monolithic perovskite/silicon tandem cells, ACS Energy Letters 1 (2016) 474 480. [36] W. Shockley, H.J. Queisser, Detailed balance limit of efficiency of p-n junction solar cells, Journal applied physics 32 (1961) 510 519. [37] M.H. Futscher, B. Ehrler, Modeling the performance limitations and prospects of perovskite/Si tandem solar cells under realistic operating conditions, ACS Energy Letters 2 (2017) 2089 2095. [38] M. Saliba, T. Matsui, K. Domanski, J.-Y. Seo, A. Ummadisingu, S.M. Zakeeruddin, et al., Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance, Science 354 (2016) 206 209. [39] T. Duong, Y. Wu, H. Shen, J. Peng, X. Fu, D. Jacobs, et al., Rubidium Multicationmultication perovskite with optimized bandgap for perovskite-silicon tandem with over 26% efficiency, Advanced Energy materials 7 (2017). [40] J.P. Mailoa, C.D. Bailie, E.C. Johlin, E.T. Hoke, A.J. Akey, W.H. Nguyen, et al., A 2-terminal perovskite/silicon multijunction solar cell enabled by a silicon tunnel junction, Applied Physics Letters 106 (2015) 121105. [41] R. Santbergen, R. Mishima, T. Meguro, M. Hino, H. Uzu, J. Blanker, et al., Minimizing optical losses in monolithic perovskite/c-Si tandem solar cells with a flat top cell, Optics express 24 (2016) A1288 A1299. [42] B.W. Schneider, N.N. Lal, S. Baker-Finch, T.P. White, Pyramidal surface textures for light trapping and antireflection in perovskite-on-silicon tandem solar cells, Optics express 22 (2014) A1422 A1430. [43] F. Fu, S. Pisoni, T.P. Weiss, T. Feurer, A. Wa¨ckerlin, P. Fuchs, et al., Compositionally graded absorber for efficient and stable near-infrared-transparent perovskite solar cells, Advanced Science 5 (2018) 1700675. [44] T. Todorov, T. Gershon, O. Gunawan, Y.S. Lee, C. Sturdevant, L.Y. Chang, et al., Monolithic perovskite-CIGS tandem solar cells via in situ band gap engineering, Advanced Energy Materials 5 (2015). [45] Q. Han, Y.-T. Hsieh, L. Meng, J.-L. Wu, P. Sun, E.-P. Yao, et al., High-performance perovskite/Cu (In, Ga) Se2 monolithic tandem solar cells, Science 361 (2018) 904 908. [46] H. Shen, J. Peng, D. Jacobs, N. Wu, J. Gong, Y. Wu, et al., Mechanically-stacked perovskite/CIGS tandem solar cells with efficiency of 23.9% and reduced oxygen sensitivity, Energy Environmental Science 11 (2018) 394 406. [47] C.-W. Chen, S.-Y. Hsiao, C.-Y. Chen, H.-W. Kang, Z.-Y. Huang, H.-W. Lin, Optical properties of organometal halide perovskite thin films and general device structure design rules for perovskite single and tandem solar cells, Journal Materials Chemistry A 3 (2015) 9152 9159. [48] J.H. Heo, S.H. Im, CH3NH3PbBr3 CH3NH3PbI3 perovskite perovskite tandem solar cells with exceeding 2.2 V open circuit voltage, Advanced Materials 28 (2016) 5121 5125. [49] N.K. Noel, S.D. Stranks, A. Abate, C. Wehrenfennig, S. Guarnera, A.-A. Haghighirad, et al., Lead-free organic-inorganic tin halide perovskites for photovoltaic applications, Energy Environmental Science 7 (2014) 3061 3068. [50] F. Hao, C.C. Stoumpos, D.H. Cao, R.P.H. Chang, M.G. Kanatzidis, Lead-free solid-state organic-inorganic halide perovskite solar cells, Nat. Photon 8 (2014) 489 494. [51] F. Hao, C.C. Stoumpos, R.P. Chang, M.G. Kanatzidis, Anomalous band gap behavior in mixed Sn and Pb perovskites enables broadening of absorption spectrum in solar cells, Journal American Chemical Society 136 (2014) 8094 8099. [52] W. Liao, D. Zhao, Y. Yu, N. Shrestha, K. Ghimire, C.R. Grice, et al., Fabrication of efficient low-bandgap perovskite solar cells by combining formamidinium tin iodide with methylammonium lead iodide, Journal American Chemical Society 138 (2016) 12360 12363. [53] G.E. Eperon, T. Leijtens, K.A. Bush, R. Prasanna, T. Green, J.T.-W. Wang, et al., Perovskite-perovskite tandem photovoltaics with optimized band gaps, Science 354 (2016) 861 865. [54] A. Guchhait, H.A. Dewi, S.W. Leow, H. Wang, G. Han, F.B. Suhaimi, et al., Over 20% efficient CIGS perovskite tandem solar cells, ACS Energy Letters 2 (2017) 807 812. [55] Z. Yang, A. Rajagopal, C.C. Chueh, S.B. Jo, B. Liu, T. Zhao, et al., Stable low-bandgap Pb Sn binary perovskites for tandem solar cells, Advanced Materials 28 (2016) 8990 8997. [56] D. Zhao, C. Wang, Z. Song, Y. Yu, C. Chen, X. Zhao, et al., 4-Terminal all-perovskite tandem solar cells achieving power conversion efficiencies exceeding 23%, ACS Energy Letters (2018). [57] F. Jiang, T. Liu, B. Luo, J. Tong, F. Qin, S. Xiong, et al., A two-terminal perovskite/perovskite tandem solar cell, Journal Materials Chemistry A 4 (2016) 1208 1213. [58] F. Da´vid, G.E. Lido´n, P.D.R. Daniel, M. Cristina, W. Je´re´mie, N. Bjoern, et al., Efficient monolithic perovskite/perovskite tandem solar cells, Advanced Energy Materials 7 (2017) 1602121.

Characterization Techniques for Perovskite Solar Cell Materials

C H A P T E R

12 Concluding remarks Meysam Pazoki1, Anders Hagfeldt2 and Tomas Edvinsson1 ˚ ngstro¨m laboratory, Uppsala Department of Engineering Sciences, Solid State Physics, A University, Uppsala, Sweden 2Institute of Chemical Sciences Engineering, Laboratory of Photomolecular Science, E´cole Polytechnique Fe´de´rale de Lausanne, Lausanne, Switzerland 1

Introduced as ‘’The next Big thing in Photovoltaics’’, Perovskite Solar cells (PSCs) are among the promising future thin film solar technologies to meet the increasing global demand for carbon neutral energy. Since the introduction of solid state PSCs in 2012, a rapid evolution of research on perovskite solar cell materials and fast rise of power conversion efficiency (PCE) have been seen. The current certified solar-to-electricity PCE is above 25% and is enough for commercialization in terms of power conversion efficiency. Remaining concerns are instead related to long-term stability, up-scaling, and also to possible legislation issues regarding manufacturing and utilization of lead containing compounds. The PCE for PSC has surpassed polycrystalline CdTe, CIGS, and polycrystalline silicon for small research cells but is so far below the record for high quality single crystal silicon (26.1%) and silicon heterostructured solar cells (26.7%). There are several reasons to look beyond silicon technologies in future PV technology with regards to lower material consumption, lower energy consumption in the fabrication process, and the advantages with a material class having tuneable band gaps for either color matched architectural integrations or fabrication of high efficient thin film tandem devices. Here, compositional engineering that involve low and high bandgap perovskite materials and compositions with moisture repelling groups or halide tolerable contact materials and/or using twodimensional intermediate thin layers at interfaces are actively investigated. Fundamental knowledge of both the bulk and the properties at interfaces are here important for the incorporation and compatibility with other materials when applied in tandem heterojunctions. Creation of tandem devices together with already established solar cell technologies, such as silicon or CIGS, is a possible route to early market introduction while long-term stability also here is a key issue if the PSC technologies should be viable. Apart from solar cell applications, applications as hybrid light emitting diodes or as photo- and x-ray detectors are also future possibilities of societal relevance. The plethora of physical properties that can be accommodated in the perovskite structure together with tuning of the

Characterization Techniques for Perovskite Solar Cell Materials DOI: https://doi.org/10.1016/B978-0-12-814727-6.00012-8

255

Copyright © 2020 Elsevier Inc. All rights reserved.

256

12. Concluding remarks

optical band gap in the hybrid perovskites, are likely to be explored also in many neighboring applications in the future. In terms of material engineering for solar cell applications, the goal of even higher PCEs, altered absorber and contact material properties for improved stability, or development of lead free analogs are all relying on proper characterization techniques, or quantification of structure and properties from theoretical calculations. The characterization techniques implemented for perovskite solar cells so far, have been adopted from the parental fields of dye sensitized and thin film solar cell technologies and from characterizations used for conventional oxide- and fluoride perovskites. Not all the techniques can be utilized exactly in the same way because of different properties arising from the presence of organic counter ions, limitations in temperature stability as well as charge accumulation and ionic migration on the nano scale with overlapping time scales. The contributors for the chapters in this book are scientist actively working in the field and give a thorough introduction to some of the most important characterization techniques with their advantages and short comings when applied to perovskite solar cell materials. The hybrid lead halide perovskites shows phenomena such as photo-induced defect creation, photoinduced giant dielectric constant, and hysteresis effects, which requires modification and care when applying specific characterization techniques where the history and state of the material is important when analyzing and comparing the data with other studies. One of the key phenomenon that can affect the perovskite materials is dynamic ionic movement that in some cases can cause local phase separations or different properties in different parts of the thin films and makes their characterization more complicated as stressed in different chapters of this book. This also implies higher complexity in assessing and controlling the device stability both from the absorber material in itself and from possible degradation processes at the contact interfaces. The main physical properties and most commonly applied characterization techniques used for the perovskite solar cells materials and devices are here presented in a pedagogical way with emphasis on their strength and limitations for both fundamental understanding of material properties and for assessing the structure-to-function relationships in the device with the aim to improve the device performance in terms of both efficiency and stability. As known in the field and described in the book, lead halide perovskite solar cell materials have rather soft mechanical properties and the energy barrier for defect migration is rather low (down to a few hundred meV). Therefore, under the device working conditions, the vacancies can migrate easily, accompanied with local structural changes. Furthermore, the movement of ionic species is illumination and temperature dependent, with a resulting accumulation at interfaces and subsequent degradation processes within the film and at the interfaces. Although the density of detrimental defects are not very high in typical lead halide perovskite materials, ionic movement, photo induced structural changes, and defect creation/annihilation under illumination can make the voltage decay or photoluminescence decay more complicated. These phenomena have special fingerprints in the solar cell device behavior. For example, the corresponding time scales of the main processes within the perovskite solar cell is different from other solar cell technologies and dependent on the chemical composition of the perovskite material and the illumination history of the material/device phenomena of Stark effects, Burstein Mott and hysteresis effects can be observed. In comparison with other thin film solar cell

Characterization Techniques for Perovskite Solar Cell Materials

Concluding remarks

257

technologies, more specific evaluation procedures are thus necessary for estimation of power conversion efficiency of perovskite solar cells and involves J V curve characterization at different scan-speeds and power-point tracking to assess and mediate hysteresis effects. Reliable measurement protocols need to be unanimously adopted by the PSC research community to ensure comparability of device performance metrics obtained in different laboratories as emphasized in the book. Apart from more standard methods, Stark spectroscopy is introduced as a powerful tool to study the phenomenon of ionic movement for perovskite solar cells. This includes the response of the perovskite device in photo-induced or voltage current transient techniques implemented for device evaluation and even accompanied with the local structural changes that can be characterized by Raman or X-ray techniques. Protocols for long-term stability tests are also necessary for a robust evaluation of stability properties of different perovskite compositions and adjacent contact materials. Surface sensitive methods such as Raman spectroscopy and X-ray photoemission spectroscopy can provide valuable information about the surface composition of the perovskite device in degradation processes and even more complicated phenomena of crystalline reformations and interactions with contact materials during device behavior. As the progress in materials engineering and device optimization goes on, a better basic understanding of fundamental processes together with development of standard characterization protocols to enable cross-comparison of materials developed in different groups are necessary. The current book summarizes the recent and up-to-date efforts in analysis and characterization of perovskite solar cell devices and materials. We hope that the presentations of material properties and characterization techniques from some of the active groups in the field can be of good use for both new and established researchers, with the aspiration of forwarding the field for future applications.

Characterization Techniques for Perovskite Solar Cell Materials

Index Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively.

A Absorption coefficients, for solar cell materials, 57, 57f Absorption spectroscopy, optical, 49 Alloyed perovskites, tunability and stability of photoluminescence in, 67 71, 69f, 70f, 71f Aluminum doped zinc oxide (AZO), 250 Anion exchange, 14 15 Antimony, 2 3 Atmospheric water and oxygen stability, 201 203, 203f AZO. See Aluminum doped zinc oxide (AZO)

B Band gap, 3 estimation, 52 53 intermediate, 2 3 optical, 49 photoexcitation, 154 range, 3 tunability, 1 2, 2f tuning of germanium, 2 3 Band gap engineering, 12 Beer Lambert law, 50 Berglund and Spicer three-step model of photoemission, 117 Bismuth halides, 60 62 Bragg-Brentano geometry X-ray diffraction setup, 26, 27f

C Cadmium telluride (CdTe) devices, 83 Capacitive effects, 87 Cation exchange, 31 CB. See Conduction band (CB) C C hydrocarbon bond, 122 CE. See Charge extraction (CE) Charge carrier dynamics, 139 140, 145 MAPbI3, 225 227 Charge carrier transport, 230 Charge extraction (CE), 164 165 Charge potential model, 114 Charge recombination, transfer and transport kinetics, 147 148

Charge selective contacts, 206 208, 206f Charge separation kinetics, 145 147 CH3NH3PbI3 perovskites band gap of, 60 electron-phonon interactions and polarons in, 153 155 Raman spectroscopy probing bleaching and recrystallization process of, 41 43, 42f valence band in, 58 CIGS. See Copper indium gallium selenide (CIGS) Collinear photothermal deflection spectroscopy system, 50 51 Conduction band (CB), 58, 109 110, 225 Copper indium gallium selenide (CIGS) solar cells, 246 247 tandem devices, 246 249 Core-level photoelectron spectroscopy, 112 119, 113f, 114f, 115f, 116f, 118f Crystal structure, perovskites, 23 24, 24f, 26 27 CsMAFA perovskites, 7 8, 11 12 CsPbI3, 4, 5f Current-voltage measurement, 89

D

DC. See Differential capacitance (DC) Defect engineering, 97 99 Defect formation, 123 124, 152, 155 156 Defect migration, 144 145, 152 153, 230 Defect passivation, 97 99 Degradation of perovskite, 202, 203f Density functional theory (DFT), 217 Device and interface stability charge selective contacts, 206 208, 206f metal contact, 208 209 Device architecture & selective contact layers, 92 93, 92f DFT. See Density functional theory (DFT) Dielectric relaxation, 152 Dielectric response, of metal-halide perovskites, 88 89, 88f Differential capacitance (DC), 164 167, 168f Diffuse Reflectant spectroscopy (DRS), 53 Dipolar cations, reorientation of, 152

259

260 Double-cation perovskites, 6 8 DRS. See Diffuse Reflectant spectroscopy (DRS) Drude model, 154 155 DSCs. See Dye-sensitized solar cells (DSCs) Dye-sensitized solar cells (DSCs), 83, 161 photocurrent and photovoltage transients, 161 162, 163f

E

EIS. See Electrochemical impedance spectroscopy (EIS) Electric field stability, 205 Electrochemical impedance spectroscopy (EIS), 172 Electron-phonon interactions, CH3NH3PbI3 perovskites, 153 155 Energy conservation, 111 EQE. See External quantum efficiency (EQE) Exciton binding energy, 66, 67f in metal-halide perovskites, 60, 61f External quantum efficiency (EQE), 140

F

FA. See Formamidinium (FA) Fabrication of solar cells, 239 FAPbI3, 4, 5f Fermi energy calibration, 122 Fluoride perovskites, 23 Formamidinium (FA), 4 6, 31 32 Formamidinium lead iodide (FAPbI3), 31 32 Fourier transform photocurrent spectroscopy (FTPS), 57, 58f FTPS. See Fourier transform photocurrent spectroscopy (FTPS)

G Germanium, 2 3 bandgap tuning of, 2 3 Germanium perovskite compounds non-cubic, 2 3 GISAXS. See Gracing incident small angle X-ray scattering (GISAXS) GIWAXRD, 29, 30f GIWAXS. See Gracing incident wide angle X-ray scattering (GIWAXS) Glazer notations, 29 Goldschmidt tolerance factor, 1, 7, 218 Gracing incident small angle X-ray scattering (GISAXS), 29, 30f Gracing incident wide angle X-ray scattering (GIWAXS), 29 of polycrystalline, 27 28, 27f

Index

H Halide perovskites (HaPs), 109 110 materials, photoelectron spectroscopy investigations, 121 133 Halides, bismuth, 60 62 Halide substituted hybrid perovskites, resonance Raman spectroscopy of, 37 41, 39f Halide substitution, in metal halide perovskites (MHPs), 37 HaPs. See Halide perovskites (HaPs) Hard X-ray Photoelectron Spectroscopy (HAXPES), 119 121, 121t HAXPES. See Hard X-ray Photoelectron Spectroscopy (HAXPES) Hellmann-Feynman theorem, 217 Higher-energy solar photons, 238 Highest occupied molecular orbital (HOMO), 110 of hole transporting materials (HTMs), 73 HLEDs. See Hybrid light emitting diodes (HLEDs) Hole transporting materials (HTMs), 9 extraction property of, 73 highest occupied molecular orbital (HOMO) of, 73 HOMO. See Highest occupied molecular orbital (HOMO) HPSCs. See Hybrid perovskite solar cells (HPSCs) HTMs. See Hole transporting materials (HTMs) Hybrid density functionals, 217 Hybrid lead halogen perovskites, 30 31 Hybrid light emitting diodes (HLEDs), 23 24 Hybrid perovskites, 29, 35 37, 255 256 Hybrid perovskite solar cells (HPSCs), 23 24 Hydrogen bonding, 218 219 modes, 220, 221f between the NH3 group, 230 vs. van der Waals (VdW) interactions, 219 Hysteresis non-capacitive effects causing, 87 88 in perovskite solar cells (PSCs), 83, 91 92 and stability, 100 101, 100f, 101f Hysterical current-voltage behavior, of PSCs, 81 87, 82f hysteria around hysteresis, 82 83, 82f pre-conditioning & poling, 86 87 quantification of hysteresis, 85 86, 86f scan-rate dependence, 83 85, 84f

I

IMFP. See Inelastic mean free path (IMFP) IMPS. See Intensity-modulated photocurrent (IMPS) IMVS. See Intensity-modulated photovoltage (IMVS) Incident-photon-to-current-efficiency (IPCE), for solar cell devices, 25f

261

Index

Indium tin oxide (ITO), 241 242, 250 Indium zinc oxide (IZO), 250 Inelastic mean free path (IMFP), 117 118 Intensity-modulated photocurrent (IMPS), 172 173, 172f Intensity-modulated photovoltage (IMVS), 172 173, 173f Inverse photoemission spectroscopy (IPES), 128 Iodide-based perovskites, 62 63 Iodized defects, 227 228 Ionic defect formation & migration, 88f, 89 90 Ionic movement, 142 146, 152 Ionic rotation, 184 Ionic species, external, 97 99 Ion library, 10f, 11 Ion migration, 205 Ion mixing, organic/inorganic, 4 11 IPES. See Inverse Photoemission spectroscopy (IPES) ITO. See Indium tin oxide (ITO) IZO. See Indium zinc oxide (IZO)

K Kohn Sham orbitals, 217

L Lanthanum (La) doped BaSnO3 (LBSO) electrode, 4 Large modulation techniques, 173 179 charge extraction (CE), 175 178, 176f, 177f current interrupt voltage (CIV), 178 179, 178f VOC rise and decay, 173 175, 174f, 175f Laue condition, 26 Lead based perovskites, light absorption of, 60 62 Lead halide perovskites (LHPs), 30 34, 181, 256 257 crystal structure and phase transitions, 182 185, 182f degradation at higher temperature, 188 189 device performance, 189 192, 190f optical properties, 187 188, 187f thermal expansion coefficients, 185 187, 185f tolerance factors, 183t Lead replacement, 12 14, 13f LHE. See Light harvesting efficiency (LHE) LHPs. See Lead halide perovskites (LHPs) Light absorption, 145 147 of lead based perovskites, 60 62 in metal-halide perovskites, 58 59 in semiconductors, 238 spectrum, tuning, 60 62 Light and temperature dependence, 93 95, 94f Light emitting device, 4 5 Light emitting diodes, 66 Light harvesting efficiency (LHE), 140, 143 144 Light illumination, 152 153

Light stability, 204 205 Longitudinal optical (LO) phonon modes, 226 227

M

MA. See Methylammonium (MA) MAFA perovskites, 7 8 MAPbI3. See Methylammonium lead iodide (MAPbI3) Mesoscopic perovskite solar cells (MPSCs), 14 15 Metal contact, 208 209 Metal-halide, photo-induced absorption for, 88 89, 88f Metal-halide perovskites, 49, 218 absorption properties of, 57 chemical modifications in, 60 62 dielectric response of, 88 89, 88f excitons in, 60, 61f ionic and electronic response of, 101f light absorption in, 58 59 photon recycling in, 65 66, 66f, 67f property of, 60 Metal halide perovskites (MHPs), 12 13 absorption of, 37 38 halide substitution in, 37 temperature dependent photoluminescence in, 73 76, 75f Methylammonium (MA), 4 6, 31 32 Methylammonium lead iodide (MAPbI3), 4, 31, 32f, 218 charge carrier dynamics, 225 227 dielectric constant of, 88 89, 88f electronic band structure, 225 227, 226f intrinsic point defects, 227 230, 229f lattice vibrations of, 35 normal modes of, 35 perovskites, 88 89 phonons, anharmonicity and MA dynamics, 222 225 Raman spectrum for, 38 structure and bonding, 218 221, 220f, 221f Methylammonium lead tri-iodide, 109 110 MHPs. See Metal halide perovskites (MHPs) Modern hybrid density functionals, 217 Monolithic tandem solar cells, 239, 244f, 247f MPSCs. See Mesoscopic perovskite solar cells (MPSCs) Multi-cation approach, 10 11 Multi-cation cascades, 24 26 Multi-cation perovskites, 8 9, 15 Multi junction solar cells, 238 242 recombination layer, 242 transparent conductive contact, 241 242

N Nanocrystalline hydrogenated silicon (nc-Si:H) recombination junction, 242

262 Near band edge trap states, 53 56 Near bandgap defects, 53 54 Neutron diffraction, 28 Non-cubic germanium perovskite compounds, 2 3

O

OA. See Octyl ammonium (OA) Octyl ammonium (OA), 4 Ohm’s law, 165 Optical absorption spectroscopy, 49 Optical bandgap, 49 Optical effect errors, 51 52 Optical transition, 58 Organic-inorganic lead halide materials, 23 Organic/inorganic ion mixing, 4 11 Origin of hysteresis, 87 91 capacitive and non-capacitive, 87 88 dielectric response of metal-halide perovskites, 88 89, 88f ionic defect formation & migration, 88f, 89 90 modeling hysteresis, 90 91, 90f Orthorhombic-to-tetragonal phase transition, 34 Oxide perovskites, 23 Oxygen stability, 201 203, 203f

P

PCBM. See Phenyl-C61-butyric-methyl-ester (PCBM) PCE. See Power conversion efficiency (PCE) PDS. See Photothermal deflection spectroscopy (PDS) Perovskite See also specific types of perovskites absorbers, emission characteristics, 62 “black-phase” stability, 5 11, 6f, 8f, 9f crystal structure, 23 24, 24f CsMAFA, 7 8, 11 12 degradation of, 202, 203f double-cation, 6 MAFA, 7 8 non-cubic germanium, 2 3 photovoltaic devices, 4 5, 8f reproducibility of triple cation, 6f triple cation, 8 9 Perovskite film, fundamental processes within, 140 145 device under working conditions, 144 145, 144t at open circuit condition, 142 143 at short circuit condition, 143 144 Perovskite layer, morphology and composition, 95 97, 96f Perovskite-perovskite tandem devices, 249 251 Perovskite stability atmospheric water and oxygen stability, 201 203, 203f electric field stability, 205

Index

light stability, 204 205 mechanical stability, 206 thermal stability, 203 204, 204f Perovskite tandem devices, 242 244 theoretical calculations on potential of, 243, 243f PES. See Photoelectron spectroscopy (PES) Phase transitions, 29, 31, 33f, 34, 43 in perovskites, 185 Phenyl-C61-butyric-methyl-ester (PCBM), 93 Photo-absorber semiconductor, absorption of, 49 Photocurrent transients, 169 Photoelectron spectroscopy (PES), 110 119 components of, 111, 112f core-level, 112 119, 113f, 114f, 115f, 116f, 118f investigations of halide perovskite materials, 121 133 binding energy calibration, 122 chemical characterization, 125 127 electronic structure, 127 128, 129f energy level alignment, 129 133, 130f, 131f, 132f, 133f radiation damage, 123 124, 124f sample preparation and handling, 123 principles, 110 112 spin-orbit splitting, 114 115 valence band, 119 Photoemission spectroscopy, 110 Photo-excitation, 143 band gap, 154 Photo-induced absorption for metal-halide, 88 89, 88f spectrum, 149 Photo-induced absorption spectroscopy (PIA), 149 156 Photo-induced time resolved optical spectroscopy, 139 140 Photoluminescence (PL) spectrum, 139 140 wavelength of, 67 69 Photoluminescence quantum yield (PLQE), 66, 70 71, 71f Photoluminescence spectroscopy, 62 76, 63f diffusion length and carrier lifetime, 64 65, 65f exciton binding energy and excitonic peaks, 66 67, 67f fluence and charge extraction layer on photoluminescence, 72 73, 72f, 73f, 74f impact of perovskite crystalline quality, 72 73, 72f, 73f, 74f photon recycling in metal-halide perovskites, 65 66, 66f, 67f processes involved in photoluminescence, 63 64, 64f temperature dependent photoluminescence, 73 76, 75f

Index

tunability and stability of photoluminescence, 67 71, 69f, 70f, 71f Photon excitation energy, variation of from UPS to HAXPES, 119 121 Photon recycling effect, 66 in metal-halide perovskites, 65 66, 66f, 67f Photothermal deflection spectroscopy (PDS), 50 52, 51f, 52f, 54 55, 55f, 57, 58f absorbance spectra, 54 55, 56f Photo-voltage, in solar cell, 141 142 Photovoltaic devices characterization of, 161 perovskite, 4 5, 8f Photovoltaic perovskites, 10f, 11 PIA. See Photo-induced absorption spectroscopy (PIA) PL. See Photoluminescence (PL) PLQE. See Photoluminescence quantum yield (PLQE) Polarons, in CH3NH3PbI3 perovskites, 153 155 Potassium passivation, 70 Power conversion efficiency (PCE), 1 2, 197 199, 255 256 Pump probe spectroscopy, 146 147 Pyridine, Raman spectra of, 41

Q

QENS. See Quasi-elastic neutron scattering (QENS) Quadruple-cation RbCsMAFA perovskite, 9 Quasi-elastic neutron scattering (QENS), 88 89

R Radiation, synchrotron, 120 Raman effect, 34 Raman spectroscopy, 26 37, 36f probing bleaching and recrystallization process of CH3NH3PbI3 film, 41 43, 42f RbCsMAFA perovskites, 8 9 RbCsMAFA solar cell, 9f Resonance Raman spectroscopy, of halide substituted hybrid perovskites, 37 41, 39f for methylammonium lead iodide (MAPbI3), 38 Resonant Inelastic X-ray Scattering (RIXS), 128 Reversible loss, 199 201, 200f RIXS. See Resonant Inelastic X-ray Scattering (RIXS)

S Semiconductors light absorption in, 238 photo-absorber, absorption of, 49 Shockley-Queisser optimum, 3 Shockley Read Hall (SRH) recombination, 72 Silicon tandem devices, 244 246 Silicon-based solar cells technology, 237 238

263 Small modulation transient techniques, 162 173 differential capacitance (DC), 164 167, 168f intensity-modulated photocurrent and photovoltage (IMPS and IMVS), 172 173 square-wave modulation for photovoltage and photocurrent transients (SW-PVT and SW-PCT), 167 171 transient photo-current decay (TPC), 164 167, 166f, 167f transient photo-voltage (TPV) technique, 162 164, 163f, 164f SOC. See Spin orbit coupling (SOC) Sodium, 10 11 Soft X-ray Photoelectron Spectroscopy (SOXPES), 120, 121t Solar cells See also specific types of solar cell devices, 60 incident-photon-to-current-efficiency (IPCE) for, 25f fabrication of, 239 materials, absorption coefficients, 57, 57f perovskite, 24 photo-voltage in, 141 142 RbCsMAFA, 9f tandem, 24 technologies, 153 Solar photons, higher-energy, 238 SOXPES. See Soft X-ray Photoelectron Spectroscopy (SOXPES) Spectroscopy See specific types of spectroscopy Spin orbit coupling (SOC), 225 Square-wave modulation for photocurrent transients (SW-PCT), 167 171 Square-wave modulation for photovoltage transients (SW-PVT), 167 171 Stability testing conventional testing, 198 perovskite testing initial efficiency testing, 198 199 laboratory long-term stability testing, 199 200, 200f outdoor testing, 201 Standard absorption measurement techniques, 51 52 Stark effects, 139, 149 152, 150f Stark spectroscopy, 153 Steady state UV Vis NIR spectroscopy, 50 62, 50f Stoichiometric perovskite materials, 218 Stressors, 197 Surface-sensitive technique, 117 SW-PCT. See Square-wave modulation for photocurrent transients (SW-PCT) SW-PVT. See Square-wave modulation for photovoltage transients (SW-PVT) Synchrotron radiation, 120

264 T Tandem cells, monolithic, 239 Tandem devices, 24 26 copper indium gallium selenide (CIGS), 246 249 perovskite-perovskite, 249 251 Tandem solar cells, 3, 24 monolithic, 244f, 247f perovskite, 6 7 silicon, 6 7 TCO. See Transparent conductive oxides (TCO) TGA. See Thermogravimetric analysis (TGA) Thermal energy, 184 Thermal expansion coefficients, 30 34, 185 187, 185f Thermalization, 153 154 Thermal motion, 181 Thermal stability, 203 204, 204f Thermogravimetric analysis (TGA), 188 Thiocyanic (SCN), 14 Time- and frequency-resolved techniques, 88 Time resolved optical spectroscopy techniques, 139 Tolerance factor, 6f, 7 Goldschmidt, 7 TPC. See Transient photo-current decay (TPC) TPV technique. See Transient photo-voltage (TPV) technique Transient photo-current decay (TPC), 164 167, 166f, 167f Transient photo-voltage (TPV) technique, 162 164, 163f, 164f Transient response, 83 84 of perovskite solar cells, 88 temperature dependence of devices, 95 Transparent conductive contact, multi junction solar cells, 241 242

Index

Transparent conductive oxides (TCO), 241 242 Triple cation perovskite, 8 9

U Ultraviolet photoelectron spectroscopy (UPS), 119 121, 121t Ultraviolet visible near-infrared (UV Vis NIR) spectroscopy, 50 62, 50f UPS. See Ultraviolet photoelectron spectroscopy (UPS) Urbach energy, 54

V Valence band (VB), 109 110 Valence band edge (VBE), 13 Valence band maxima (VBM), 110, 129 130 Valence band photoelectron spectroscopy, 119 Van der Waals (VdW) interactions, 218 221 vs. hydrogen bonding, 219 Vapor Assisted Solution Process (VASP), 125, 126f VASP. See Vapor Assisted Solution Process (VASP) VB. See Valence band (VB) VBE. See Valence band edge (VBE) VBM. See Valence band maxima (VBM) VDOS. See Vibrational density of states (VDOS) Vibrational density of states (VDOS), 222f

X

XAS. See X-ray absorption (XAS) XPS. See X-ray photoelectron spectroscopy (XPS) X-ray absorption (XAS), 128 X-ray diffraction (XRD), 26 37, 27f, 30f, 33f X-ray photoelectron spectroscopy (XPS), 8 9, 110, 119 120, 121t XRD. See X-ray diffraction (XRD)