Organic Solar Cells 1774690721, 9781774690727

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Organic Solar Cells

ORGANIC SOLAR CELLS

Edited by: Sujata N. Mustapure

ARCLER

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www.arclerpress.com

Organic Solar Cells Sujata N. Mustapure

Arcler Press 224 Shoreacres Road Burlington, ON L7L 2H2 Canada www.arclerpress.com Email: [email protected]

e-book Edition 2023 ISBN: 978-1-77469-647-7 (e-book)

This book contains information obtained from highly regarded resources. Reprinted material sources are indicated and copyright remains with the original owners. Copyright for images and other graphics remains with the original owners as indicated. A Wide variety of references are listed. Reasonable efforts have been made to publish reliable data. Authors or Editors or Publishers are not responsible for the accuracy of the information in the published chapters or consequences of their use. The publisher assumes no responsibility for any damage or grievance to the persons or property arising out of the use of any materials, instructions, methods or thoughts in the book. The authors or editors and the publisher have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission has not been obtained. If any copyright holder has not been acknowledged, please write to us so we may rectify.

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© 2023 Arcler Press ISBN: 978-1-77469-072-7 (Hardcover)

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ABOUT THE EDITOR

Dr. Sujata Nagnath Mustapure (1988) is presently serving as teaching associate, Department of Electrical & Other Energy sources, College of agricultural engineering & Technology, Vasantrao Naik Marathwada Krishi Vidhyapeet, Parbhani, Maharashtra. She obtained her B.Tech (agril. Engg.) in 2010 from Vasantrao Naik Marathwada Krishi Vidhyapeet, Parbhani. Gold medal in M.Tech (Agril. Engg.), 2014 from Dr. Panjabrao Deshmukh Krishi Vidyapeeth, Akola and Ph.D in Renewable energy engineering in 2019 from Maharana Pratap University of Agriculture and Technology, Udaipur. She has also worked as assistant professor in College of Agricultural, Naigoan, Vasantrao Naik Marathwada Krishi Vidhyapeet, Parbhani.

TABLE OF CONTENTS



List of Figures.................................................................................................xi



List of Tables................................................................................................ xvii



List of Abbreviations..................................................................................... xix

Preface................................................................................................... ....xxiii

Chapter 1

Introduction to Organic Solar Cells........................................................... 1 1.1. Introduction......................................................................................... 2 1.2. Characteristics of Organic Solar Cells (SCS)....................................... 11 1.3. The Current Situation......................................................................... 14 1.4. Operational Principles of OSCS......................................................... 15 1.5. Solar Cell Architectures...................................................................... 21 1.6. Characterization of Organic Solar Cells (SCS).................................... 25 References................................................................................................ 28

Chapter 2

Polymeric Materials for Solar Cells.......................................................... 37 2.1. Introduction....................................................................................... 38 2.2. Description of Novel Organic Materials............................................. 42 2.3. Carbon Nanotube/Polymer Nanocomposites..................................... 46 2.4. Fullerene-Containing Polymers for Organic Solar Cells (SCS)............. 48 2.5. Soluble Functionalized Polyanilines................................................... 51 2.6. Charge Transport in Thin Polymer Films............................................. 53 2.7. Organic Solar Cells (SCS) Based on Thin Polymer Films..................... 58 2.8. Polymerizable Methanofullerene as a Buffer Layer Material for Organic Solar Cells (SCS)............................................. 60 References................................................................................................ 66

Chapter 3

Donor Materials for Organic Solar Cells.................................................. 77 3.1. Introduction....................................................................................... 78 3.2. Performance Parameters of SCS (Solar Cells)...................................... 79 3.3. Smdms-Centered Photovoltaics (PVS)................................................ 81 3.4. Oligothiophene-Centered Smdms...................................................... 81 3.5. Oligothiophene-Bdt (Benzodithiophene) Hybrids As SMDMS............ 85 3.6. IDT (Indacenodithiophene)-Centered Smdms..................................... 95 References................................................................................................ 98

Chapter 4

Acceptors Materials for Organic Solar Cells.......................................... 105 4.1. Introduction..................................................................................... 106 4.2. Rylene Diimide-Centered Polymer Acceptors.................................. 109 4.3. Fluorene and Bt-Centered Polymer Acceptors.................................. 120 4.4. CN-Replaced Polymer Acceptors..................................................... 121 4.5. Other Polymer Acceptors Comprising Electron-Removing Units....... 125 4.6. Summary......................................................................................... 127 References.............................................................................................. 129

Chapter 5

Fabrication Techniques for Organic Solar Cells..................................... 139 5.1. Introduction..................................................................................... 140 5.2. The Merger Challenge...................................................................... 142 5.3. Typical Structure of a Device........................................................... 144 5.4. The Sea of Film-Forming Methods.................................................... 144 5.5. Coating and Printing Methods.......................................................... 146 5.6. Patterning and Juxtaposition of the Multilayer Films......................... 158 5.7. Roll-To-Roll (R2R) Methods............................................................. 161 5.8. Other Techniques............................................................................. 162 References.............................................................................................. 163

Chapter 6

Characterization of Organic Solar Cells................................................. 173 6.1. Introduction..................................................................................... 174 6.2. Dark Current-Voltage Characteristics............................................... 175 6.3. Open-Circuit Voltage Versus Light Intensity...................................... 178 6.4. Charge Extraction By Linearly Increasing Voltage (CELIV)................. 180 6.5. Transient Photovoltage (TPV) and Open-Circuit Voltage Decay (OCVD).............................................................................. 181 viii

6.6. Impedance Spectroscopy................................................................. 183 References.............................................................................................. 187 Chapter 7

Applications of Polymer and Graphene Nanocomposites in Solar Photovoltaics............................................................................. 193 7.1. Introduction..................................................................................... 194 7.2. Graphene/Polymer Nanocomposites as Transparent Conductive Electrodes (TCES)........................................................ 194 7.3. Graphene/Polymer Nanocomposites as Active Layers (ALS)............. 200 7.4. Graphene/Polymer Nanocomposites as Interfacial Layers (IFLS)....... 203 References.............................................................................................. 206

Chapter 8

Organic Tandem Solar Cells................................................................... 211 8.1. Introduction..................................................................................... 212 8.2. Polymer Tandem Solar Cell Structure and Operation Mechanism................................................................................... 213 8.3. Development and Current Status of Polymer Tandem Solar Cells (SCS)............................................................... 215 8.4. Polymer Materials for Tandem Solar Cells (SCS)............................... 217 8.5. Tandem Device Engineering and Measurement................................ 217 References.............................................................................................. 223

Index...................................................................................................... 227

ix

LIST OF FIGURES

Figure 1.1. Schematic illustration of a solar cell Figure 1.2. Working arrangement of an organic solar cell Figure 1.3. Schematic diagram of solar energy radiation Figure 1.4. Diagram of bonding-antibonding interactions among the HOMO/LUMO levels of an organic semiconductor Figure 1.5. Chemical structure of organic solar cell acceptor and donor materials Figure 1.6. Diagram outline of an organic solar cell (architecture of an organic photovoltaic device). The negative electrode is indium tin oxide (ITO), aluminum, is a usual transparent electrode, and the substrate is glass. The diagram depicts a bulk heterojunction (BHJ) active layer where the acceptor and donor blend form phaseseparated domains inside the active layer. The structure of the BHJ is crucial to the performance of a solar device Figure 1.7. Numerous solution processible conjugated polymers and a fullerene derivative utilized in organic solar cells. Abbreviations and chemical structures of certain conjugated organic molecules. From left: poly (para-phenylene-vinylene) (PPV); poly(acetylene) (PA); a relieved PPV (MDMO-PPV), poly(3-hexyl thiophene) (P3HT); and a C60 derived in every compound one could identify a sequence of irregular single and double bonds Figure 1.8. The structure of a single-layer and a multilayer organic solar cell Figure 1.9. Flexible and transparent solar cells Figure 1.10. AM1.5G reference solar spectrum Figure 1.11. Band placement of donor and acceptor materials for a heterojunction Figure 1.12. Diagram of the principle of charge separation in a solar cell Figure 1.13. Structure of a bilayer solar cell Figure 1.14. Structure of a bulk heterojunction solar cell Figure 1.15. Structure of a tandem cell Figure 1.16. Working of a solar cell at different biases: (a) large reverse bias; (b) small reverse bias; (c) positive bias, 0 resultant internal fields, and conforming to open circuit condition; (d) positive bias, carrier injection Figure 2.1. Energy diagrams of polymer binary structure: (a) photon absorption; (b) exciton generation; and (c) charge separation procedures xi

Figure 2.2. Structure of potential solid-state stacking patterns of DBA and (DBAB) n-type block copolymers Figure 2.3. The improvement of the near-field near the nanoparticle surface Figure 2.4. Ring inaugural metathesis polymerization of fullerene comprising norbornene monomers Figure 2.5. Ring inaugural metathesis copolymerization of fullerene comprising norbornene monomers with associated fullerene-free compounds Figure 2.6. Homopolymerization of 2-(1-methyl-2-buten-1-yl) aniline 12 Figure 2.7. Dependencies of (a) the electrical conductance and (b) I/T2 on the opposite temperature for films of copolymers (o-toluidine with 2-(1-methyl-2-butene-1-yl) aniline) in diverse molar ratios: (2) 1:3, (3) 1:1, and (4) 3:1 Figure 2.8. (a) An energy level pictorial of the PANI/FCM system; (b) procedure of photon absorption and charge detachment in this structure; (c) multilayer film structure of OSC Figure 2.9. Diagram architecture of an inverted organic solar cell Figure 2.10. The molecular structures of the materials utilized to create the ETL buffer layer of the devices Figure 2.11. Designated CV characteristics of the inverted P3HT/[60]PCBM solar cells formed on bare ITO (reference) and utilizing buffer layers formed from polymerized 17 or 18 Figure 3.1. Histogram displaying the number of scientific publications backing to the subject “OSCs (organic solar cells)” by year. The era was 2010 to 2017 and the codeword was “organic small molecule solar cells.” This search was made through ISI, web of science Figure 3.2. (a) A usual I-V J-V characteristics of the solar cells; (b) standard structural design of BHJ (bulk-heterojunction); and (c) inverted structure Figure 3.3. Oligothiophene-centered SMDMs Figure 3.4. (a) Normalized ultra-violet-vis-near-infrared absorption spectra of the 6 in chloroform solution or as the thin film; (b) photoluminescence spectra of pure 6 and the 6:PC71BM blend films Figure 3.5. Chemical structures of BDT(benzodithiophene) hybrids utilized as SMDMs Figure 3.6. Chemical structures of IDT(Indacenodithiophene)-centered SMDMs Figure 3.7. (a) Photographic picture of the slot-die-covered large area PV modules during the process of the printing process; (b) conforming J-V curves Figure 3.8. (a) Representative J-V curves for enhanced 16: PC71BM and 17:PC71BM large, firm-module devices under the simulated AM 1.5 G irradiation; (b) ultravioletVis absorption spectra of 19 and 18 in dilute CHCL3 (chloroform) solutions

xii

Figure 3.9. (a) Transmission electron microscopy (TEM) pictures of TSA treated 22b:PC71BM and 22a:PC71BM thin films; bar is nearly 100 nanometer; and (b) Enhanced current density-voltage curves for SM-OSCs centered on 23a-c Figure 3.10. (a) Graphic representation of the ternary active layer; (b) levels of the energy of the utilized materials (arrows signify the direction of charge carrier transport and lightning bolt signifies a transfer of energy from 25 to the DIB-SQ Figure 4.1. (a) Illustrative device configuration of OPVs (organic photovoltaic cells); and (b) the molecular structures of PC61BM, P3HT (D1), and PC71BM Figure 4.2. Molecular structures of PDI (perylene diimide)-centered polymer acceptors (1–12) Figure 4.3. Molecular structures of the polymer donors (D2 to D9) Figure 4.4. Molecular structures of NDI (naphthalene diimide)-centered polymer acceptors (13 to 25) Figure 4.5. Molecular structures of the polymer donors (D10 to D12) Figure 4.6. Molecular structures of BT (benzothiadiazole) and fluorene-based polymer acceptors (26 to 29) together with the polymer donor (D13) Figure 4.7. Molecular structures of cyano-replaced polymer acceptors (30 to 38) Figure 4.8. Molecular structures of the polymer donors (D14 to D19) Figure 4.9. Molecular structures of the polymer acceptors comprising electronremoving units (39 to 44) Figure 5.1. A representation of the tandem polymer SC (solar cell) consisting of 8 layers with the electrical connections (left) and the images of actual devices as observed from the front having backside lighting to highlight the color. The three devices displayed are the single junction of P3CT/ZnO having a red color, a tandem cell of P3CTTP/ ZnO, and P3CT/ZnO displaying the brown color (top right), and the single junction of P3CTTP/ZnO having a green color. The film absorption spectra of the active layer are also exhibited with P3CT/ZnO drawn with the solid line and the P3CTTP/ZnO drawn with the broken line (bottom right) Figure 5.2. The Venn diagram briefing the merger challenge where the trouble lies in discovering the conditions that syndicates process, stability, and efficiency into the same device and materials. Good instances are signifying every area and some instances that span 2 of the areas but generally, there has been no news on the considerable way out such that all of the areas can be covered. Examples that merge: efficiency and stability is P3HT-PCBM, stability, and the process is P3CT*ZnO and only the process as MEHPPV-PCBM Figure 5.3. Photographs exhibiting inhomogenous drying with crystallization (top-right), casting (top-left), an instance of a fruitfully cast film exhibiting a good homogeneity (bottom-left), and the poor outcome exhibiting precipitation, drying defects, and picture framing (bottom-right)

xiii

Figure 5.4. Graphic representation of spin-coating (top-left) along with the image of a usual spin-coating function in the glovebox environment (top-right) and the high-speed pictures exhibiting application of the solution of a MEHPPV to the rotating substrate and formation of a film. The pictures were taken at 300 images s1 (below). The image timing (left to right) after the influence of a first drop is t =17ns, 100ns, 137ns, and 180 ms Figure 5.5. An image of an Erichsen Coatmaster 509 MC-I which can be utilized for doctor blading technique (left) and an image displaying doctor blading of a MEHPPV (right) Figure 5.6. Representation of the process of screen-printing (above) and instances of the laboratory screen printer (bottom-left) and the industrial screen printer (bottomright) Figure 5.7. An image of the full R2R Klemm line encompassing the picture. The system includes unwinder, corona treatment, an edge guide, web cleaning, ultraviolet-curing vertical drying ovens, printing, and rewinding (left). A graphic representation of the rotary screen printer (right) Figure 5.8. Graphic representation of the process of inkjet printing utilizing the data pulse train to produces droplets on demand through the pressure transducer (top-left) and the system where droplets are produced endlessly and the pattern formed by deflecting the undesirable droplets far away from the substrate (top-right). Images of the laboratory where an ink-inkjet printer is utilized for printing the polymer SCs Figure 5.9. An image of the pad printer in an open position where normally the gravure is uncovered (left) and the cycle of pad printing (right) Figure 5.10. A representation of the three-layer SC made by 0-D (left) and 1-D (right) techniques of the coating. The translucent electrode is displayed in gray, the metallic electrode in black, and the active layer in red color. The sequence of coating of the layers and positions are displayed (top) and the connected introduction of the aperture loss when linking two cells in series. The electrical connections are displayed as dotted lines Figure 6.1. Relationship between current and voltage at various light intensities Figure 6.2. (a–e) Dark JV-curve simulations for numerous cases; (f) extraction of dark ideality factors with the help of Eqn. (2) Figure 6.3. (a–e) Simulation of the open-circuit voltage relied upon light intensity for numerous cases; (f) Light ideality factors acquired from the results of simulation-an average is used Figure 6.4. Schematic demonstration of a photo-CELIV experiment. The charge carriers are extracted by the linearly increasing voltage and turn to a peak (jmax) in the current. The calculations of charge carrier mobility are done using tmax Figure 6.5. OCVD simulations for all cases in prior figures. The light is switched off at t = 0. The analytic solution is marked by the gray line supposing purely bimolecular recombination and homogeneous charge densities

Figure 6.6. The graph between impedance real and impedance image concerning light changes Figure 7.1. Systematic explanation of sulfonated graphene (SG)/poly (3, 4-ethylenedioxythiophene): Poly (styrene-sulfonate) (PEDOT) nanocomposite along with its synthesis-reaction conditions Figure 7.2. (A) Band structure and diagrammatic depiction of a PSC with the structure glass/indium tin oxide (ITO)/ZnO/P3HT:PCBM/Au/PEDOT:PSS/G; (B) J-V characteristics evaluated from two sides of the PSC using G top electrode and various active layer thicknesses Figure 7.3. (a) Plan of the electrochemical exfoliation of graphite; (b) optical figures of the exfoliation process; (c) diagrammatic portrayal of spray deposition of exfoliated graphene (EG) dispersion over poly(ethylene 2,6-naphthalate) (PEN); (d) J-V properties of the cell under dark (dashed line) and light conditions (solid line) Figure 7.4. (a) Diagrammatic description of the PSC with G anode and structure G/ PEDOT:PEG(PC)/PEDOT:PSS/DBP/C60/BCP/Al; (b) cross-sectional transmission electron microscope (TEM) image (that is on left side) of the device represented in (a), with an energy-dispersive line scan on a figure of the device cross-section (that is on right side); (c) flat-band energy level chart of the PSC; (d) J-V properties of the G-based device (red lines) in contrast with ITO reference cells (blue lines) Figure 7.5. (A) Graphical image of ITO/PEDOT:PSS/G-P3HT:C60/Al PSC; and (B) J-V properties with P3HT:C60 or G-P3HT:C60 being the active layer Figure 7.6. (A) Scheme-wise representation; and (B) J-V curves of ITO/PEDOT:PSS/ P3HT:GQDs/Al device formed on aniline-modified GQDs with various GQDs content Figure 7.7. (A) Diagrammatic view of the synthesis of reduced graphene oxide quantum dots (rGOQDs) and graphene oxide quantum dots (GOQDs), where the edge functional groups are regulated by controlling the thermal reduction time; (B) J-V curves of the PSCs with various kinds of GQDs Figure 7.8. (A) Graphical illustration of ITO/ZnO@G:EC/P3HT:PC61BM/MoO3/Ag device; (B) atomic force microscope (AFM) pictures of ZnO@G:EC nanocomposites with various G contents; (C) J-V curves of PSCs with various nanocomposites Figure 8.1. Conventional polymer tandem cell containing two cells stacked on top of each other Figure 8.2. Energetic illustration of a device Figure 8.3. The absorption spectra and the tandem device structure Figure 8.4. Device system and results of current-voltage

xv

LIST OF TABLES Table 2.1. Parameters of the best inverted solar cells invented on bare ITO and utilizing buffer layers created from polymerized 17 and 18 Table 2.2. CV characteristics of inverted solar cells utilizing different concentrations of 17 Table 2.3. Parameters of inverted PCDTBT/[60]PCBM and P3HT/[60]PCBM organic solar cells containing 17 + FPI buffer layers as a function of 17 concentrations in the pioneer solution Table 3.1. Description of Frontier Levels of Energy, Performance Parameters, and Device Structure of the Oligothiophene-Centered SMDMs (Huang and Huang, 2014; Wang et al., 2016; Zhang et al., 2017) Table 3.2. Description of frontier levels of energy, performance parameters, and device structure of oligothiophene-BDT (benzodithiophene) hybrids Table 3.3. Description of frontier levels of energy, performance parameters, and device structure of the IDT (Indacenodithiophene)-centered SMDMs Table 4.1. PDI (perylene diimide)-centered polymer acceptors Table 5.1. Comparison of techniques of film-forming by coating and printing

LIST OF ABBREVIATIONS

A-D-A

Acceptor-Donor-Acceptor

AIBN

Azoisobutyronitride

ALs

Active Layers

BDT

Benzodithiophene

BDTT

Bithienyl-Benzodithiophene

BHJ

Bulk Heterojunction

BT

Benzothiadiazole

CB

Chlorobenzene

CELIV

Charge Extraction by Linearly Increasing Voltage

CN

Cyano

CPDT

Cyclopenta [2, 1-b: 3, 4-b’] Dithiophene

D-A

Donor-Acceptor

DMF

Dimethylformamide

DPE

Diphenyl Ether

DTCDI

Dithienocoronene Diimide

DTP

5H-Dithieno [3, 2-b: 2’, 3’-d] Pyran

DTT

Dithienothiophene

EC

Ethyl Cellulose

Eg

Energy Gaps

EG

Exfoliated Graphene

EIS

Electro-Chemical Impedance Spectroscopy

EQE

External Quantum Efficiency

ETL

Electron-Transport Buffer Layers

FF

Fill Factor

Fl Fluorine FN

Fowler-Nordheim

GO

Graphene Oxide

GOQDS

Graphene Oxide Quantum Dots

GPS

Global Positioning System

GQDs

Graphene Quantum Dots

GRIM

Grignard Metathesis

HOMO

Highest Occupied Molecular Orbital

IAPP

Institut Fur Angewandte Photophysik

ICL

Interconnecting Layer

IDT

Indacenodithiophene

IFLs

Interfacial Layers

IPCE

Incident Photon to Electron Conversion Efficiency

IPR

Intellectual Property Rights

ITO

Indium Tin Oxide

LBG

Low Bandgap

LUMO

Lowest Unoccupied Molecular Orbital

MBG

Medium Bandgap

MIM

Metal-Insulator-Metal

MIS

Metal-Insulator-Semiconductor

NDI

Naphthalene Diimide

NFM-OSC

Non-Fullerene Small-Molecule Organic Solar Cell

NPs

Nanoparticles

NT

Naphtho [1,2-c:5,6-c’]bis[1,2,5]Thiadiazole

OCVD

Open-Circuit Voltage Decay

ODCB

1,2-Dichlorobenzene

OFET

Organic Field-Effect Transistor

OLED

Organic Light-Emitting Diode

OPV

Organic Photovoltaic

OSMSS

One-Sun Multi-Source Simulator

P3HT

Poly (3-Hexylthiophene)

PBDTT-DPP poly(2,6’-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,4b]dithiophene- alt-5-dibutyloctyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c] pyrrole-1,4-dione) PCBM

Phenyl-C61 Butyric Acid Methyl Ester

PCE

Power Conversion Efficiency

PDI

Perylene Diimide

PEDOT: PSS

Poly (3, 4-Ethylene Dioxythiophene): Poly (Styrene Sulfonate)

PEN

Poly (Ethylene 2, 6-Naphthalate)

PET

Polyethylene Terephthalate

PI Polyimide

PMMA

Poly (Methyl Methacrylate)

PRGO

Polyacrylonitrile-Grafted rGO

PV

Photovoltaic

R2R

Roll-to-Roll

rGO

Reduced Graphene Oxide

RSch

Richardson-Schottky

S Sulfur SCLC

Space Charge Limited Current

SCs

Solar Cells

SG

Sulfonated Graphene

SVA

Solvent Vapor Annealing

TA

Thermal Annealing

TCEs

Transparent Conductive Electrodes

TMO

Transition Metal Oxides

TPV

Transient Photovoltage

V Volume WBG

Wide Bandgap

ZnO

Zinc Oxide

xxi

PREFACE

In the past few years, organic photovoltaics (OPV) has received immense attention owing to their exceptional features, such as low-temperature synthesis, light, and cheap materials, solution processability, and tunable electronic properties. There are numerous solar cell technologies with higher conversion efficiencies. However, organic photovoltaics are famous for their low cost, low toxicity, and minimal environmental impact. Apart from environmental and economic benefits, most of the organic solar cells (SCs) exhibit higher efficiencies which are comparable with the efficiencies of silicon solar cells. They have exhibited conversion efficiencies of more than 13% to date. This book encompasses the fundamentals of organic solar photovoltaics. The detailed content of the book addresses the photovoltaic energy conversion limits and provides a well-explained overview of molecular electronics, which focuses on the working principle, manufacturing, and characterization of polymeric solar cells. Different chapters of the book focus on the electrochemical processes taking place in organic solar cells by offering a detailed explanation of the exciton separation, charge-carrier transport, and electricity generation. The book also focuses on the experimental methodologies for getting a thorough understanding of the key photovoltaic processes in different types of polymeric solar cells. The primary focus of this book is to provide a comprehensive analysis of the fundamental features of organic solar cells. There are eight chapters in the book. The first chapter of the book discusses the fundamentals of solar energy, solar cells, and organic photovoltaics. Moreover, the characteristics of organic solar cells and their working principle are also discussed in the chapter. Chapter 2 mainly focuses on the properties of the materials which are employed in the manufacturing of organic solar cells. Chapter 3 deals with the characteristics and classification of donor materials for organic solar cells. Chapter 4 essential covers the basic details of acceptor materials used in organic solar cells. Chapter 5 specifically deals with different synthesis methods employed for the fabrication of organic solar cells. Detailed analysis of different synthesis techniques including coating, printing, and patterning is offered in the chapter. Chapter 6 mainly discusses the modern techniques used for the characterization of organic solar cells. Chapter 7 deals with the applications of composite materials in solar photovoltaics. The chapter contains a detailed discussion regarding the synthesis of organic solar

cells using graphene/polymer nanocomposites. Finally, Chapter 8 discusses the role of organic photovoltaic materials in the synthesis of multi-junction (tandem) solar cells for enhanced efficiency. This book is primarily intended for graduate students in the fields of engineering, material science, chemistry, and physics. This book will also prove helpful for researchers and scientists working in the area of solar photovoltaics. Industrialists, professionals, and government organizations can also benefit from this book for analyzing the potential of organic solar cells in power generation.

CHAPTER

1

INTRODUCTION TO ORGANIC SOLAR CELLS

CONTENTS 1.1. Introduction......................................................................................... 2 1.2. Characteristics of Organic Solar Cells (SCS)....................................... 11 1.3. The Current Situation......................................................................... 14 1.4. Operational Principles of OSCS......................................................... 15 1.5. Solar Cell Architectures...................................................................... 21 1.6. Characterization of Organic Solar Cells (SCS).................................... 25 References................................................................................................ 28

2

Organic Solar Cells

1.1. INTRODUCTION Polymer solar cells (SCs) have numerous intrinsic advantages, like their flexibility, low material, lightweight, and low manufacturing costs. Lately, polymer tandem SCs have got substantial attention because of their potential to attain higher performance than specific cells. Photovoltaics (PVs) deal with the transformation of sunlight into electrical energy. Classic PV SCs founded on inorganic semiconductors had developed significantly since the initial realization of a silicon solar cell by Fuller, Pearson, and Chapin, in the Bell labs in 1954 (Chapin et al., 1954; Goetzberger et al., 2003). Currently, silicon is still the principal technology on the global market of PV SCs, with power transformation effectiveness approaching 15 to 20% for monocrystalline devices. Although the solar energy industry is greatly subsidized for several years, the costs of silicon solar cell power plants or panels are still not economical with other conventional combustion methods-except for numerous niche products. A method for reducing the manufacturing costs of SCs is to utilize organic materials that could be processed under lowdemanding situations (Figure 1.1).

Figure 1.1: Schematic illustration of a solar cell. Source: https://www.researchgate.net/figure/Working-principle-of-a-solarcell_fig1_322628682.

Organic photovoltaics (OPVs) had been formed for more than 30 years, however, within the previous decade, the research field expanded

Introduction to Organic Solar Cells

3

significantly in momentum (Hoppe et al., 2004; Spanggaard and Krebs, 2004). The quantity of solar energy lighting up Earth’s landmass each year is almost 3000 times the total quantity of annual human energy usage. However, to contest energy from fossil fuels, PV devices must transform sunlight into electricity with a specific measure of efficiency. For polymerfounded OPV cells, which are far less costly to manufacture than siliconbased SCs, scientists had long believed that the reason for high effectiveness rests in the clarity of the polymer/organic cell’s two domains-donor and acceptor (Rowell et al., 2006; Zhao et al., 2017). Organic SCs could be distinguished through the production method, the character of the materials, and device design. The two main production methods could be distinguished as either thermal evaporation or wet processing. Device architectures are bulk heterojunction (BHJ), bilayer heterojunction, and single layer with the diffuse bilayer heterojunction as an intermediary between the bilayer and the BHJ (Kawano et al., 2006; Cheng and Zhan, 2016). While the single layer includes only single active material, the other architectures are founded on respectively two kinds of materials: electron acceptors (A) and electron donors (D). The variance of these architectures lies in the charge production mechanism: single-layer devices needs usually a Scotty barrier at one contact, which permits splitting photo excitations in the barrier field. The DA SCs used the photo persuaded electron transfer to detach the electron from the hole (Sariciftci et al., 1992; Kawano et al., 2006). The photoinduced electron transfer happens from the excited state of the donor (LUMO: lowest unoccupied molecular orbital) to the LUMO of the acceptor, which consequently had to be a good electron acceptor with a tougher electron affinity (Shrotriya et al., 2006; Roncali, 2009). Following charge separation, both the hole and the electron had to stretch the opposite electrodes, the anode and the cathode, respectively. Therefore, a direct current could be delivered to an exterior circuit. As the indication of global warming remains to build up, it is becoming evident that we would have to find methods to generate electricity without the discharge of carbon dioxide and other greenhouse gases. Luckily, we have renewable energy sources that neither finish out nor had any substantial harmful impacts on our environment. Harvesting energy directly from the sunlightconsuming PV technology is being extensively recognized as an important constituent of future global energy generation (Figure 1.2) (Wöhrle and Meissner, 1991; Clarke and Durrant, 2010).

4

Organic Solar Cells

Figure 1.2: Working arrangement of an organic solar cell. Source: https://www.tcichemicals.com/AT/de/c/12802.

1.1.1. Solar Energy The extent of energy that the Earth gets from the sun is huge: 1.75 × 1017 W. For instance, the world energy usage in 2003 amounted to 4.4 × 1020 J, Earth obtains enough energy to achieve the yearly global demand for energy in fewer than an hour. Not entire of that energy touches the Earth’s surface due to scattering and absorption, though, the PV transformation of solar energy remains a significant challenge. State-of-the-art inorganic SCs had the highest power conversion effectiveness near 39%, however, commercially available solar panels, had considerably lower effectiveness of around 15 to 20% (White et al., 2006; Kaltenbrunner et al., 2012). Another method of making SCs is to utilize organic materials, like conjugated polymers. SCs founded on thin polymer films are mainly attractive due to their mechanical flexibility, ease of processing, and potential for low-cost fabrication of huge areas. Moreover, their material properties could be tailored by altering their chemical makeup, causing greater customization than traditional SCs permit. Though substantial progress has been made, the efficiency of transforming solar energy into electrical power got with plastic SCs still doesn’t warrant commercialization: the most effective devices had an efficiency of 4 to 5%

Introduction to Organic Solar Cells

5

(Brabec et al., 2001; Li et al., 2011). To increase the efficiency of plastic SCs it is, thus, vital to understand what limits their performance (Figure 1.3).

Figure 1.3: Schematic diagram of solar energy radiation. Source: https://www.alternative-energy-news.info/technology/solar-power/.

1.1.2. Organic Solar Cells (SCs) Organic materials had the potential to form a long-term technology that is economically feasible for huge-scale power generation founded on environmentally harmless materials with infinite availability. Organic semiconductors are a less expensive alternative to inorganic semiconductors like Si; they could have tremendously high optical absorption coefficients, which provides the possibility for the manufacturing of very thin SCs (Koster et al., 2012). Further attractive features of organic PVs are the potential for thin flexible devices that could be fabricated utilizing high throughput, lowtemperature methods that employ well-established printing methods in a roll-to-roll (R2R) procedure (Shaheen et al., 2001; Koster et al., 2005). This possibility of utilizing flexible plastic substrates in a simply scalable highspeed printing procedure can decrease the balance of structure cost for organic PVs, causing a smaller energetic payback time (Gustafsson et al., 1992; Brabec, 2004). The electronic structure of entire organic semiconductors is founded on conjugated πelectrons. A conjugated organic structure is made of

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an alternation amongst single and double carbon-carbon bonds. Single bonds are identified as σ-bonds and are linked with localized electrons, and double bonds comprise a σ-bond and a π-bond (Schilinsky et al., 2004; Waldauf, 2004). The π-electrons are considerably more mobile than the σ-electrons; they could jump from site to site amongst carbon atoms appreciations to the mutual overlap of π orbitals alongside the conjugation path, which forms the wave functions to delocalize above the conjugated backbone. The π-bands are either vacant (termed the LUMO: lowest unoccupied molecular orbit) or occupied with electrons (called the HOMO: highest occupied molecular orbital). The bandgap of these materials extends from 1 to 4 eV. This π-electron system had all the important electronic features of organic materials: light emission and absorption, charge production, and transport (Shaheen et al., 2001; Hoppe et al., 2003). Since the discovery in 1954 of high conductivity in the peryleneiodine complex, organic semiconductors had been under extreme research (Goetzberger et al., 2003). Potential applications of organic semiconductors occurred when Tang et al. revealed the first OLED in the 1970s (Chapin et al., 1954). With the distinctive properties of organic semiconductors of thinness, flexibility, and simple fabrication procedure, OLEDs had laid a unique industry of ultra-thin and flexible displays. OLEDs had already been accepted in commercial applications like small OLED displays on mobile appliances, and large-area displays like televisions are getting more attention (Riedel et al., 2004; Mihailetchi, 2005). Other than OLEDs, another significant application of organic semiconductors is OSCs (organic solar cells). In contrast to OLEDs, OSCs make utilization of organic semiconductors to engross light and transform it into electrical energy (Brabec et al., 2002a, b). With the inorganic SCs technology moving into cost blockages for huge area applications, the cheap and simple fabrication procedure of OSCs provides a vast potential for huge area applications. Furthermore, OSCs have the distinctive properties of lightweight and flexibility that might also result in novel applications like portable solar panels. The properties of OSCs are very exciting for understanding organic devices. Their basic principles are described in this chapter (Schilinsky et al., 2002; Dyakonov, 2004).

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1.1.3. Overview of Organic Semiconductors Organic semiconductors are carbon-based materials for owning semiconductor features. Atoms inside an organic semiconductor molecule are attached by conjugated p-bonds, while molecules are attached through weak van der Waal’s force, as opposed to the massive covalent structure presented by inorganic semiconductors. The bonding structure provides organic semiconductors its distinctive weight, low sublimation point, and flexibility which permit easy processing (Akamatu et al., 1954; Tang et al., 1987). From the macroscopic point of opinion, the band structure of organic semiconductors could be treated like inorganic semiconductors. The valence band is usually occupied with electrons and the conduction band is usually free of electrons. Inorganic semiconductors, the LUMO and the HOMO are analogs to the conduction band and valence band, respectively. The LUMO and HOMO of organic semiconductors signify the hybridization among antibonding and bonding of the conjugated p-electrons (Bredas et al., 2002; Kymissis, 2009). Organic semiconductors are formed of organic molecules which are formed through a p-conjugated system. Carbon atoms are sp2 hybridized and the sp2 bonds form 3 strong r-bonds with adjacent atoms. The leftover p-orbitals of the C atoms create a delocalized cloud of electrons by the formation of feebler p-bonds. This bond structure makes a quasi-onedimensional structure for the conjugated organic semiconductors. The p-bond structure could have diverse bonding configurations as per the electron wavefunction overlap of adjacent atoms. For instance, in Figure 1.4, we could see two diverse states of the p-bonds, with the antibonding and bonding states corresponding to diverse energy levels (Koster, 2006). The LUMO and HOMO of organic semiconductors raise energy bands that relate to diverse hybridization states of the p-bonds which would consequence in diverse energy levels of an organic semiconductor. When an electron is a trigger from the HOMO to the LUMO of an organic semiconductor, the molecule itself is excited into a higher energy state, in contrast to the real excitation of a free electron from the valence band to the conduction band in inorganic semiconductors (Gregg, 2003).

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Figure 1.4: Diagram of bonding-antibonding interactions among the HOMO/ LUMO levels of an organic semiconductor. Source: https://www.springer.com/gp/book/9781447148227.

The carrier transport procedure in organic semiconductors is also changed from that of inorganic semiconductors. Inorganic semiconductors, thermally stimulated ‘hopping’ of carriers happens to overwhelmed the energy barriers inside the disordered conjugated polymer structure, therefore permitting carrier transport inside the semiconductor (Hu et al., 2002; Sundar et al., 2004). This is extremely different from charge transport in the inorganic semiconductors, which could be described through the movement of free carriers in the conduction band or valence. The hopping transport procedure provides organic semiconductors with relatively low mobility when related to their inorganic complements. Up to*1.5 9 10–3 m2 V–1 s–1 hole mobility is attained for small-molecule organic semiconductors; however, silicon has mobility of up to *4.5 9 10–2 m2 V–1 s–1 (McCulloch et al., 2006; Kietzke, 2007). On the further hand, electron mobility for a few small molecule materials extents *1 9 10–5 m2 V–1 s–1, however, silicon had considerably higher electron mobility of 0.1 m2 V–1s–1. In solar cell applications, the usually studied P3HT: PCBM blend had a hole and electron mobilities of the order of *10–7–10–8 m2 V–1 s–1 in the assorted blend film (Gundlach, 2005; Anthopoulos, 2006). The low mobility, when related to inorganic semiconductors is the main drawback for organic semiconductors, and resulting, diverse devices were proposed to overwhelme this weakness.

1.1.4. Structure of Organic Solar Cell For organic SCs founded on polymer: the magnitude of JSC, VOC, fullerene BHJs, and FF rely on parameters like as: light intensity, temperature, the

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composition of the constituents, thickness of the active layer, the choice of electrodes utilized, as well as the solid-state morphology of the film (Brütting, 2006; Mayer et al., 2007). Their maximization and optimization need clear know-how of the device photocurrent and operation, Jph, production, and its limitations in these devices. The relation between the material parameters and experimental Jph (i.e., bandgap, charge-carrier mobility, relative dielectric constant or molecular energy levels, bandgap) required to be understood and regulated to permit for the additional design of novel materials that could improve the efficiency of this kind of SCs (Coakley and McGehee, 2004; Min, 2010). A first effort to understand the physics besides the organic BHJ SCs was completed by utilizing numerical models and concepts that are well-known for inorganic SCs, like the p-n junction model. To enhance the contract of the classical p-n model through the experimental Jph of an organic BHJ cell, an extended replacement circuit had been presented (Andersson, 2008; Lee et al., 2009). This model substitutes the photoactive layer through an ideal diode and a parallel and a serial resistance, which had a vague physical meaning for an organic cell. Though, different from traditional p-n junction cells with spatially detached p- and n-type areas of doped semiconductors, BHJ cells comprise an intimate combination of two (un-doped) intrinsic semiconductors that are nanoscopically varied and that produce a randomly oriented interface. Furthermore, due to the different transport, recombination processes, and charge generation, in BHJs, the traditional p-n junction model is not appropriate to define the Jph of these SCs (Peumans et al., 2000). An alternate approach is to utilize the MIM (metal-insulator-metal) concept, where a homogenous mixture of two unipolar semiconductors (acceptor/ donor) is termed as a single semiconductor with properties derivative from the two materials. This means that the photoactive layer is defined as a single ‘virtual’ semiconductor supposing that its conduction band is given through the LUMO of the acceptor and its valence band is described by the HOMO of the donor-type material. In PV operation mode, the possible difference accessible in the MIM device, that pushes the photo-produced charge carriers to the collection electrodes, is affected by the difference in the work functions of the metal electrodes (Tang, 1986; Jean-Michel, 2002). As displayed in Figure 1.5, numerous acceptor and donor materials are being reported, however, none of them gets over 3% efficiency but for P3HT/PCBM or PCPDTBT/PCBM (Figures 1.6 and 1.7).

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Figure 1.5: Chemical structure of organic solar cell acceptor and donor materials. Source: http://pubs.sciepub.com/rse/2/3/2/figure/1.

Figure 1.6: Diagram outline of an organic solar cell (architecture of an organic photovoltaic device). The negative electrode is indium tin oxide (ITO), aluminum, is a usual transparent electrode, and the substrate is glass. The diagram depicts a bulk heterojunction (BHJ) active layer where the acceptor and donor blend form phase-separated domains inside the active layer. The structure of the BHJ is crucial to the performance of a solar device. Source: http://www-ssrl.slac.stanford.edu/content/science/ highlight/2011-01-31/effects-thermal-annealing-morphologypolymer%E2%80%93fullereneblends-organic#sthash.iE7FUkF8.dpuf.

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Figure 1.7: Numerous solution processible conjugated polymers and a fullerene derivative utilized in organic solar cells. Abbreviations and chemical structures of certain conjugated organic molecules. From left: poly(para-phenylene-vinylene) (PPV); poly(acetylene) (PA); a relieved PPV (MDMO-PPV), poly(3-hexyl thiophene) (P3HT); and a C60 derived in every compound one could identify a sequence of irregular single and double bonds. Source: https://www.springer.com/gp/book/9781447148227.

1.2. CHARACTERISTICS OF ORGANIC SOLAR CELLS (SCS) 1.2.1. Organic Solar or Photovoltaic (PV) Materials Plastic or organic SCs utilize organic materials (carbon-compound based) frequently in the form of dendrimers, small molecules, and polymers, to transform solar energy into electric energy. These semiconductive organic molecules can absorb light and persuade the transport of electrical charges from the conduction band of the absorber to the conduction band of the acceptor molecule. There are numerous types of OPVs (cells), comprising multilayered and single-layer structured cells. Both kinds are presently used in research and small area applications, and both have their particular disadvantages and advantages (Figure 1.8).

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Figure 1.8: The structure of a single-layer and a multilayer organic solar cell.

1.2.2. Advantages of Flexible Organic Compared to Rigid Conventional Solar Cells (SCs) The most recent progress in molecular engineering had exposed a series of organic cell possible advantages that might eventually outbalance the advantages of silicon-based SCs. Though conventional SCs presently dominate the current market, the case might be quite different in the coming future.

1.2.3. Manufacturing Process and Cost Organic SCs could be easily manufactured as compared to silicon-created cells, and this is because of the molecular nature of the materials utilized. Molecules are easy to work with and could be utilized with thin-film substrates that are 1000 times thinner than silicon cells (order of a rare 100 nanometers). This reality by itself could reduce the production cost considerably. As organic materials are extremely compatible with an extensive range of substrates, they show flexibility in their production techniques. These methods comprise solution procedures (paints or inks), high quantity printing methods, R2R technology, and several more, that permit organic SCs to cover huge thin film surfaces effortlessly and cost-efficiently. All the above techniques had temperature demands and low energy compared to conventional semiconductive cells and could decrease cost by a factor of 10 or 20.

1.2.4. Tailoring Molecular Properties A significant benefit of organic materials utilized in solar cell manufacturing is the capability to modify the molecule properties to fit the application.

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Molecular engineering could alter the bandgap, molecular mass, and capability to produce charges, by modifying, for example, the functional group and length of polymers. Furthermore, new distinctive formulations could be developed with a mixture of inorganic and organic molecules, making it probable to print the organic SCs in any appropriate pattern or color.

1.2.5. Desirable Properties The modification of molecular properties and the flexibility of production techniques described on the previous page allow organic polymer SCs to extant a series of desirable properties. These solar modules are remarkably lighter and more elastic compared to their rigid and heavy counterparts, and therefore less prone to harm and failure. They could exist in numerous movable forms (for example. rolled form) and their flexibility makes installation, storage and transport much more convenient.

1.2.6. Environmental Impact The energy used to produce a solar cell is less than the amount needed conventional inorganic cells. Therefore, energy transformation efficiency doesn’t have to be as high as the conventional cell’s effective. The wide utilization of organic SCs could pay to the enhanced utilization of solar power worldwide and make renewable energy sources approachable to the average consumer.

1.2.7. Multiple Uses and Applications The current situation shows that organic SCs can’t alternate with silicon cells in the energy transformation field. Though their usage seems to be more targeted towards precise applications like recharging surfaces for phones, packages, laptops, and clothes, or to provide the power for small movable devices, like MP3 players and cellphones Other than domestic usage, the latest developments had shown a military application potential for organic solar modules. Research in the Konarka (US) had shown that organic cells could be utilized in soldier tents to produce electricity and provide power to other military devices like global positioning system (GPS) receivers and night vision scopes. This technology is supposed to be exceptionally valuable for challenging missions (Koster et al., 2005).

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1.3. THE CURRENT SITUATION Organic SCs had certain drawbacks comprising their low efficiency (only 5% efficiency compared to the 15% of silicon cells) and short lifetime. However, their several benefits could justify the current international investment and research in emerging novel polymeric materials, novel combinations, and structures to increase efficiency and attain low-cost and large-scale manufacturing within the next years. A commercially feasible organic solar cell creation is the target of the next period (Figure 1.9).

Figure 1.9: Flexible and transparent solar cells. Source: https://onlinelibrary.wiley.com/doi/10.1002/aenm.201701791.

Donor-acceptor (D-A) founded organic SCs are presently showing power transformation effectiveness of more than 3.5%. Enhancing the nanoscale morphology together with the growth of new low bandgap (LBG) materials is predicted to lead to power transformation efficiencies approaching 10%. The flexible, huge area applications of organic SCs might open up novel markets like “textile integration.” Organic SCs in precise and organic semiconductor devices in usual could be incorporated into production lines of packing materials, labels, and so forth. Since there is a strong progress effort for organic electronics incorporation into diverse products globally, the solar powering of a few of these products would be desired. The subsequent generation of microelectronics is pointing to applications of “electronics all over,” and such organic semiconductors would play the main role in these upcoming technologies. Groupings of organic SCs with fuel cells, batteries,

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and so forth, would increase their product incorporation. This inerrability of organic SCs into numerous products would be their technological gain (Günes et al., 2007). The Si solar cell which had high manufacturing procedure cost demonstrates delayed commercialization because of difficulties in overwhelming its manufacturing cost restraint as Si wafer raw material supply scarcity deepens. On the other hand, the conjugated structure polymer/organic material founded organic solar cell is predicted to decrease the manufacturing cost through new processes like as printing process. Thus, the commercialization appears only possible through maximizing the energy transformation efficiency through the growth of new conjugation structure organic materials with a decreased bandgap (Koster, 2005).

1.4. OPERATIONAL PRINCIPLES OF OSCS 1.4.1. Exciton Generation Upon absorption of a photon, an electron in the organic semiconductor is thrilled from the HOMO to the LUMO. This is similar to thrilling an electron from the valence band to the conduction band of the inorganic semiconductors. Though, because of the low dielectric constant and hole wavefunctions and localized electrons in organic semiconductors, tough Coulombic attraction occurs among the electron-hole pair. The resultant bound electron-hole pair is termed an exciton, with a binding energy of 0.1 to 1.4 eV, in contrast to a far lower binding energy of a rare meV in inorganic semiconductors. Therefore, it is a comparatively higher possibility to produce free charge carriers later to absorption of photons in an inorganic semiconductor, for instance, the electron-hole pairs simply dissociate through absorbing thermal energy; however, strongly bound excitons are produced in organic semiconductors (Mihailetchi et al., 2005). The absorption coefficient of the organic materials is usually high at *105 cm–1. Hence, though the width of the active layer of OSCs is restricted through electrical conduction, a few hundred of nanometers of the active layer are dense enough to absorb a suitable amount of light and demonstrate substantial solar cell characteristics (Lenes, 2006). To further increase absorption, mainly in the wavelength range where the essential absorption of the material is feeble, numerous light trapping techniques like gratings, folded cells, and lens concentrators had been recommended (Shrotriya, 2006). A key concern for organic materials is the usually small absorption

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range and the large bandgap, which lead to low absorption effectiveness of photons in the long-wavelength region. With a LUMO-HOMO alteration of 1.1 eV, it is stated that *77% of solar light is absorbed. In comparison, the standard material for OSCs, P3HT, had a bandgap of *1.9 eV, however, most organic materials had band gaps of *2 eV. Figure 1.10 displays the AM1.5G reference solar spectrum. Associated with common materials that don’t absorb in the region [700 nm, it is obvious that a large portion of energy could be harvested in the long-wavelength regions ([700 nm) and the absorption effectiveness could be improved by utilizing LBG materials (Keis et al., 2002).

` Figure 1.10: AM1.5G reference solar spectrum. Source: spectra.

https://www.pveducation.org/pvcdrom/appendices/standard-solar-

1.4.2. Exciton Diffusion and Dissociation With an exciton produced, the following question is how to distinguish the bound electron-hole pair to produce free charges which ultimately leads to electricity generation. An ingenious solution was given by Tang et al. who revealed that utilizing two different organic materials with properly aligned band levels could result in effective SCs. The junction between the two materials is termed the heterojunction. Even later that discovery, the heterojunction had become the base of OSC design. To attain exciton

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dissociation, 2 organic materials with band alignment displayed in Figure 1.11 are placed nearby to each other. The difference between the HOMO of material A and the LUMO of material B had to be lesser than the potential difference between the bound electron-hole pair, for example., the bandgap of any material A or B minus the exciton binding energy (Kuwabara et al., 2008). When an exciton is produced in, for example, material A, it migrates toward the heterojunction. For instance, the potential difference among LUMOB and HOMOA is lesser than that of the energy of the exciton, the transfer of an electron from the exciton to LUMOB is a vigorously favorable procedure. An electron is therefore moved from the exciton to HOMOB, however, a hole remains in HOMOA. As a consequence of this charge transfer procedure, materials A and B have been termed a donor and acceptors correspondingly. The competing procedure of luminescence, which comprises the radiative recombination of excitons, happens at a timescale of *1 ns. In contrast, the charge transfer procedure happens at a much littler timescale of *45 fs, permitting effective exciton dissociation at the heterojunction. After separation, electron-hole pairs create a charge pair termed a geminate pair, which are charges yet Coulombically bound and had to be detached through an internal field (Waldauf, 2006).

Figure 1.11: Band placement of donor and acceptor materials for a heterojunction. Source: https://www.mdpi.com/2079-9292/6/4/75.

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The distance that excitons could diffuse earlier recombination is termed the exciton diffusion length. Usually, exciton diffusion lengths in organic semiconductors are very little, at rare tens of nanometers. Excitons produced at a distance from the heterojunction longer than this length would recombine earlier reaching the heterojunction, consequent in lower exciton dissociation effectiveness. Therefore, the active layers (ALS) had to place thin to confirm that phase separation among the donor and acceptor is inside the exciton dissociation length. Though, a thin active layer consequence in a severe tradeoff of low absorption effectiveness. Therefore, it is significant to have a huge interface area for the exciton dissociation and suitable phase separation to confirm effective exciton dissociation. To attain this, advanced device architectures like the nanostructured ALs and BHJ had been recommended.

1.4.3. Carrier Transport The geminate pairs produced after exciton separation had to travel to electrodes for collection in their lifetimes. The key driving forces for the transference of electrons to the cathode and holes to the anode are diffusion and drift currents. The drift current corresponds to the movement of the carrier beside the potential gradient inside the solar cell. This potential gradient is chiefly determined through the choice of the electrode in a solar cell. Usually, a low work function cathode and high work function anode are utilized, and this difference produces a built-in electric field inside the solar cell that regulates the open-circuit voltage (Voc) of the cell. When an exterior bias is applied, the internal electric field is improved and the drift current fluctuates (Hayakawa, 2007). The carriers drift along the subsequent internal electric field of the solar cell to the particular electrodes for collection. Another process of carrier transport is the diffusion current, which is the diffusion of carriers alongside the carrier absorption gradient inside a solar cell. Since the geminate pairs are produced around the solar cell heterojunction, the concentration of holes and electrons is usually higher around the heterojunction. Carriers, therefore, diffuse along the concentration gradient left from the heterojunction, taking to the diffusion current. The diffusion current mostly dominates when the applied bias alters the internal electric field approximately to zero, however, the drift current leads when the internal electric field is huge. The key constraint of carrier transport is the movement in the active layer. As electron and hole mobilities in organic materials are usually low, the active layer had to be kept comparatively thin to permit carriers to reach the electrodes in their lifetimes. The mobility variance is also a crucial factor

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in defining charge transport characteristics, as a variance of more than a factor of 10 would lead to SCLC (space charge limited current). In short, SCLC rises when one kind of carrier, say electrons (electrons usually had higher mobility in OSC materials) is conveyed much more proficiently to the cathode. As the rate of electrons getting to the cathode is higher than that of holes to the anode, electrons might accumulate in the active layer close to the cathode interface and produces the space charge effect, which alters the charge transport characteristics of the active layer and produces an upper limit for the current productivity of a solar cell. Hence, to attain effective carrier transport in the active layer of a solar cell, electron mobilities and a balanced hole are desired (Brabec et al., 2001; Peng, 2011).

1.4.4. Charge Extraction at Electrodes Later the charge carriers transference to the active electrode/layer interface, they are taken out from the active layer to the electrodes. To attain high effectiveness in charge extraction, the prospective barrier at the active layer/ electrode interfaces had to be decreased (Xie, 2011). Therefore, the work function of the anode is perfectly predicted to match the donor HOMO, however, the work function of the cathode is predicted to match the acceptor LUMO. When these happen, the contacts are termed ohmic contacts, and Voc correlates completely with the difference between the donor HOMO and acceptor LUMO. On the other hand, if the work functions of cathode and anode materials are not close to the acceptor LUMO or donor HOMO, respectively an ohmic contact can’t be formed. In this situation, the carrier extraction behavior is ruled by the MIM model (Parker, 1994; Han et al., 2009). A technique to progress the work function similar to the electrodes is to use different kinds of materials as electrodes. Usually, ITO (indium tin oxide) is utilized as an anode contact since its work function of *4.7 eV matches well with HOMO of the P3HT (Scharber et al., 2006). High work function metals, for example., Au (5.1 eV) could also be utilized as the anode contact. Low work function metals, on the cathode side, like Al (4.2 eV) are usually utilized to match the LUMO of PCBM. Further changing materials to attain work function matching, interlayers could also be inserted among the active layer and the electrodes to well align the active layer HOMO or LUMO and the electrode work function. For example, a very tinny layer of LiF is commonly evaporated on the active layer earlier fabricating the cathode to make an ohmic contact (Brabec, 2002; Mihailetchi, 2003). Solution-processed materials like TiO2 and ZnO had also been revealed to

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efficiently increase electron collection. Transition metal oxides (TMO), on the anode side, like WO3 or MoO3 had been utilized as an interlayer to make ohmic contacts (Kyaw, 2008; Schmidt, 2009). When TMO is utilized as the interlayer, it was validated that even little work function metals like Al could be evaporated on the upper to make an anode in an inverted configuration. Other than altering the work function of the electrodes, enhancing the interface area or roughness of the electrodes could also provide a huge area for more effective charge collection (Tao, 2008; Jiang et al., 2010).

1.4.5. Summary of the Operation The complete operation of SCs is summarized in 4 stages as follows: • • •

Photon absorption taking to exciton generation; Exciton diffusion to an acceptor/donor heterojunction; Exciton separation at a heterojunction to make geminate pairs; and • Carrier extraction and carrier transport at the electrodes. The 4 steps are shown in Figure 1.12.

Figure 1.12: Diagram of the principle of charge separation in a solar cell. Source: https://www.springer.com/gp/book/9781447148227.

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The effectiveness of the 4 steps is represented through gA, gdiff, gdiss, and gC, respectively. The EQE (external quantum efficiency) is defined as (1) The EQE signifies the percentage of photons that are ultimately transformed to charge carriers gathered at the electrodes. Usually, two main factors limit the EQE. (1) Partial absorption of the solar spectrum, rather because of a thin active layer or due to a narrow absorption band, leads to a decrease in gA. (2) Recombination of excitons because of numerous reasons like quenching at metal electrodes and restricted phase separation in an active layer takes to a lower EQE and loss of photogenerated excitons. To decrease these losses, diverse solar cell architectures had been suggested, and they would be discussed in the following section.

1.5. SOLAR CELL ARCHITECTURES 1.5.1. Bilayer Solar Cell The creation of the heterojunction by Tang et al. (1986) was initially demonstrated in the type of a bilayer solar cell. As displayed in Figure 1.13, the usual structure of a bilayer solar cell comprises a hole collection layer, anode, active layer compromised of donor and acceptor, cathode fabricated, and electron collection layer serially. The electron collection layer and hole collection layer are utilized to amend the work function of the electrodes to make ohmic contact. A single, well-defined interface occurs among the donor and acceptor at which excitons detach. Having this structure, the bilayer solar cell is the simplest structure explained by the simple operating principle of the solar cell. A significant disadvantage for bilayer SCs is that the little exciton diffusion length of organic materials restricts the thickness of the acceptor and donor layers. If the acceptor or donor layer is too thick, the excitons produced far away from the heterojunction might recombine earlier to reach the heterojunction. Also, the acceptor and donor layers are restricted to tens of nanometers which leads to feeble absorption. To confirm that excitons are produced near the heterojunction, interference impact had to be considered entirely throughout the design of bilayer SCs. These shared trade-off factors take to low EQE and execute challenges in the design of bilayer OSCs.

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Figure 1.13: Structure of a bilayer solar cell. Source: https://www.researchgate.net/figure/Typical-device-structure-of-bilayer-solar-cell-The-charge-dissociation-only-occurs-at_fig11_270550059.

1.5.2. Bulk Heterojunction (BHJ) Solar Cells (SCs) One of the most significant innovations in the field of OSCs is perhaps the discovery of the BHJ in the mid-1990s (Yu et al., 1995). The BHJ structure is displayed in Figure 1.14. Though thermal co-deposition techniques could be utilized to fabricate a BHJ, the junction is usually formed by merging acceptor and donor materials in a solution, then creating the active layer through the spin coating of a mixed solution on a substrate. The resultant film is an interpenetrating nanoscale network of acceptor and donor materials. The phase separation inside the film is usually 10–20 nm, which is inside the exciton diffusion length of numerous organic semiconductors. Therefore, close unity internal quantum effectiveness had been attained for BHJ solar cell, which means that approximately all photogenerated excitons are detached. Carriers are then transported by percolated pathways inside the active layer toward the particular contacts for collection (Chen et al., 2009; Park et al., 2009). Because of the little nanoscale phase separation in BHJs, a thicker active layer could be fabricated in these cells when matched to bilayer

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SCs. Though, as the spin-coating procedure is intrinsically less controlled than the vapor deposition procedure commonly utilized in bilayer SCs, the performance of BHJ SCs is vulnerable to numerous parameter changes. The effectiveness of SCs is strongly reliant on the morphology of the BHJ and numerous methods like thermal annealing (TA), solvent annealing, and altering polymer functional groups had been studied to enhance the performance of OSCs (Savenije et al., 2005; Li et al., 2007).

Figure 1.14: Structure of a bulk heterojunction solar cell. Source: https://www.researchgate.net/figure/Bulk-heterojunction-organic-solar-cells_fig1_340090481.

Amongst numerous materials, P3HT: PCBM BHJ is the most frequently utilized and well-optimized active layer utilized in OSCs. Using these mixed materials, Schilinsky et al. (2002) revealed a short-circuit current of 8.7 mA/cm2, which was the maximum current at that time. In a year, the effectiveness of P3HT: PCBM SCs was pushed up to 3.5% efficiency, and the P3HT: PCBM came under extreme research. TA of P3HT: PCBM BHJ was found to expand carrier mobility and the basis of the crystallization of P3HT in the BHJ. The impact of the regioregularity of P3HT was studied, with enhancing regioregularity displaying improved performance because of better stacking of P3HT chains (Vanlaeke et al., 2006). Studies like optimization P3HT: PCBM ratio and utilizing additives in solution were also performed to improve the performance. Ultimately, over 5% efficiency was reported by Yang et al. utilizing a solvent annealing approach which enhanced the morphology of the P3HT: PCBM layer. Due to the simple fabrication procedure and consistency of this material, P3HT: PCBM OSC had become a standard solar cell for investigating numerous device mechanisms in SCs (Padinger et al., 2003; Chen et al., 2009).

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1.5.3. Tandem Solar Cells (SCs) To overwhelm the restraint of weak absorption strength and absorption extent of the active layer of OSCs, fabricating SCs in tandem had been suggested. Stacking the SCs in sequence (a 2-terminal structure) would produce large Voc and ALs with diverse absorption regions in the tandem structure that could permit the cell to absorb light over an extensive wavelength range (Hoppe, 2006; Kim et al., 2006). For example, this was revealed in a ZnPc: C60 and P3HT: PCBM tandem solar cell which showed enhanced Voc and extensive absorption range. A thin layer of Ag or Au as the intermediate layer fabricated through thermal evaporation was also effectively utilized in tandem cells (Li et al., 2005; Yao et al., 2008). It had also been revealed that tandem cells could be completely solution-processed, by utilizing polymer: little molecule ALs and titanium oxide/PEDOT: PSS as the interlayer among subcells. Other interlayers like ZnO/PEDOT: PSS were also validated successfully to produce effective tandem SCs (Gilot, 2007). Apart from linking the subcells in series, it had been revealed that linking the cells in parallel (a 3-terminal structure) could also result in improved effectiveness. When the cells are linked in parallel, the short circuit current is preferably the total of the current outputs from the 2 subcells. Sista et al. had revealed a large short-circuit current of 15.1 mA/cm2 and power conversion efficiency (PCE) of 4.8% utilizing a 3-terminal configuration and also displayed that both the common-cathode and common-anode configurations are probable by utilizing TiO2:Cs/Al/Au and PEDOT: PSS/Au/V2O5 as the intermediate layer correspondingly (Figure 1.15) (Sista et al., 2010).

Figure 1.15: Structure of a tandem cell. Source: https://pubs.rsc.org/en/content/articlelanding/2009/ee/b817952b.

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Regardless of the advantages, tandem SCs undergo trouble in matching subcells. As carriers from the bottom and top sub-cells recombine at the interlayer, a good interlayer had to be selected to permit effective recombination and had to be transparent to decrease optical losses. Moreover, current matching among sub-cells had to be attained to avoid charge accumulation on one of the sub-cells which decline inefficiency (Dennler, 2006). During fabrication, it is also significant that newly fabricated layers would not harm the layers below as extra layers are fabricated. These factors give engineering challenges for tandem solar cell design.

1.6. CHARACTERIZATION OF ORGANIC SOLAR CELLS (SCS) 1.6.1. J-V Characteristics OSCs are usually characterized below 1000 W/m2 light of AM 1.5 solar spectrum. The act of a solar cell at different bases is demonstrated in Figure 1.16 (Shrotriya et al., 2006; Cravino et al., 2007). At (i) reverse bias, the applied bias supports the built-in electric field, increasing charge transport and exciton dissociation and consequences in a large photocurrent. Drift current is leading due to the existence of a strong electric field. When (ii) the applied bias is near zero, mainly the built-in field occurs in the device, and the built-in field energies the carriers to the corresponding electrodes for gathering. When the applied bias is enhanced in a positive direction, the positive bias competes with the built-in field. Therefore, the resultant field in the device decreases, the drift current becomes slighter, and the magnitude of the current declines (Janssen, 2007; Berger et al., 2013). Ultimately the field touches a point where (iii) the applied field is equivalent to the built-in field. Round this point, diffusion current leads the current, as the electric field is very slight inside the device. When (iv) the exterior bias is further enhanced, the applied field is greater than the built-in field and the potential gradient in the device is overturned. Since the barrier is currently triangular, carrier injection happens through the tunneling mechanism and has positive current outcomes (Peumans, 2003; Vakhshouri et al., 2013). When the current and applied bias are opposite in direction, power is outputted from the solar cell. The stage where the magnitude of the product of J and V is extreme is the maximum power output stage. A number of

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the parameters that are usually utilized to gauge solar cell performance are described below.

Figure 1.16: Working of a solar cell at different biases: (a) large reverse bias; (b) small reverse bias; (c) positive bias, 0 resultant internal fields, and conforming to open circuit condition; (d) positive bias, carrier injection. Source: https://www.springer.com/gp/book/9781447148227.

Short circuit current (Jsc). The short circuit current is described as the current at which the outside applied voltage is 0. Jsc signifies the number of charge carriers that are produced and ultimately gathered at the electrodes at a short circuit state. Enhanced electrical /optical parameters like high absorption coefficient, smaller phase separation, small bandgap, and high carrier mobility increase Jsc (Fabregat-Santiago et al., 2011). Open circuit voltage (Voc). The open-circuit voltage is described as the voltage at which the current density output is zero. Voc had been stated to be mainly relying on the work function difference of metal contacts. If an ohmic contact is made at the electrodes, Voc is reliant on the HOMO-LUMO difference between the acceptor and the donor (Kim et al., 2007). as:

The fill factor (FF) describes the shape of the J-V curve and is defined

(2)

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where; Jmpp and Vmpp are the current density and the voltage at the stage of maximum output power correspondingly. As displayed above, FF is the ratio between the maximum power output point and the maximum attainable power output, for example, Jsc times Voc. FF signifies the reliance of current output on the internal field of the device and is measured by the shunt resistance and series resistance. For example, low carrier mobility would cause carriers to recombine earlier to reach a heterojunction. In this situation, enhancing the external bias would sweep the carriers, which otherwise might recombine at lesser field strength, to the heterojunction for dissociation, causing a rise in current output. This leads to a strong reliance on current on the applied bias, which is displayed by a lower FF (Vandewal et al., 2008). PCE. Lastly, the PCE represents the effectiveness of the solar cell and can be calculated as follows:

(3)

where; Pin is the input power density.

1.6.2. Incident Photon to Electron Conversion Efficiency (IPCE) J-V characteristics only are not enough to fully characterize a solar cell, as they don’t demonstrate optical factors in detail. To further study the effectiveness of our solar cell at every wavelength, the measure of IPCE (incident photon to electron conversion efficiency) is significant. IPCE signifies the percentage of incident photons that are transformed into carriers that are lastly collected at the electrodes under short circuit circumstances and is equal to EQE. Therefore, the integration of an IPCE spectrum is proportional to the short circuit current. The shape of the IPCE curve is highly reliant on the absorption curve of the active layer. In specific, IPCE is a useful sign when applying methods to increase absorption in particular wavelength positions (for example, plasmonic structures) could be reflected as development in IPCE at the corresponding wavelengths. Other than electrical effects, optical effects like enhanced charge collection are revealed as shifts of a whole IPCE curve as electrical characteristics are mainly wavelength independent.

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CHAPTER

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POLYMERIC MATERIALS FOR SOLAR CELLS

CONTENTS 2.1. Introduction....................................................................................... 38 2.2. Description of Novel Organic Materials............................................. 42 2.3. Carbon Nanotube/Polymer Nanocomposites..................................... 46 2.4. Fullerene-Containing Polymers for Organic Solar Cells (SCS)............. 48 2.5. Soluble Functionalized Polyanilines................................................... 51 2.6. Charge Transport in Thin Polymer Films............................................. 53 2.7. Organic Solar Cells (SCS) Based on Thin Polymer Films..................... 58 2.8. Polymerizable Methanofullerene as a Buffer Layer Material for Organic Solar Cells (SCS)............................................. 60 References................................................................................................ 66

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2.1. INTRODUCTION Organic materials with semiconductor properties had lately become the object of concentrated research meant at developing numerous elements of organic electronics: memory cells, light-emitting diodes, solar cells (SCs), field-effect transistors, and sensors. After the start of conductive polyacetylene, conjugated polymers are deliberated as an alternate for the inorganic semiconductors. In the field of the development of OSCs (organic solar cells), actual progress had been possible since the mid-1990s next to the synthesis of conductive conjugated polymers of the most recent generation utilized for the manufacturing of field-effect transistors and modern lightemitting diodes (Chiang et al., 1977; Neugebauer et al., 2000). Such polymers had outstanding mechanical properties and the capability to process, a range of forms and derivatives. They had a great absorption coefficient in the optical extent, which permits their use in the kind of ultrathin films (around 100 nm thickness). The benefits stated above, as well as the probability of depositing films from solutions at regular pressure onto flexible substrates of a huge area, make it probable to produce an OSC utilizing such comparatively cheap techniques as stamping technique and inkjet printing (Shaheen et al., 2005). Regardless of these positive factors for the usage of polymers, commercialization of the OSC is hindered both by PCE comparatively PCEl (low-power conversion efficiency) of ~6–7% and through the need for protective encapsulation from environmental impacts (Li et al., 2012). Nearly all known kinds of organic photovoltaic (OPV) cells could be divided into two major groups. The first group comprises batteries with a binary system in which the photoactive constituents of the acceptor and donor types are deposited in distinct layers. The second group comprises batteries with a bulk heterojunction (BHJ), in which there is a merely single photoactive layer, which is a mixture of an acceptor and a donor. In polymer SCs, the dynamic layers of the device must be positioned among 2 layers of conducting electrodes, one of which is crystal clear to incident light (Gopinath et al., 2019). Normally, for this drive, a compound consisting of ITO (indium tin oxide) coatings (a combination of indium and tin oxides) applied to a flexible polymer or a glass substrate is utilized. Additionally, the ITO layer is covered with a film of a conductive polymer utilized to transport holes poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT: PSS). This film also assists to smoothen the surface of the ITO and avoiding hunts from surface abnormalities and to improving the effectiveness of hole

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collection due to the better matching of energy levels among the electrode yield and the HOMO (highest occupied molecular orbital) level of the donor polymer level. On the contrary side of the dynamic layer, a metal electrode with a little work function is applied. Generally, this is an aluminum electrode, which could be further improved by applying a tinny layer (~1 nm) of LiF beneath it, which enhances the efficiency of SCs (Markov et al., 2005; Chen et al., 2015). Illumination of such an element through sunlight is carried out from the lateral of a polymer substrate or transparent glass. Radiation is immersed in the composite layer or the working polymer, and excitons (electron-hole pairs) are produced, which then decay into holes and electrons collected on contrary electrodes. These procedures are generally represented utilizing energy diagrams (Figure 2.1). The photovoltaic (PV) impact underlying the operation of the OSC comprises the production of holes in semiconductor materials and current carrier-electron when they are exposed to light. The nature of the comparatively low effective polymer OSCs in contrast with their inorganic analogs lies in the diverse mechanisms of photogeneration of free charge carriers in such a system. When the inorganic semiconductors are lightened by photons with an energy larger than the bandgap, that is, the energy difference between the conduction band and the valence band, free charge carriers (holes and electrons) are produced, which are then detached through the PN junction of the solar cell (Mohajeri and Omidvar, 2015). Inorganic semiconductors, as a consequence of the absorption of photons, electrons from upper occupied molecular orbitals are excited towards lower free molecular orbitals. A significant difference in the mechanisms of photogeneration in organic and inorganic materials is the point that in free inorganic SCs, free charge carriers are made in the main part of the material, however in OSC, as a consequence of their comparatively low dielectric permittivity, such materials are bound through Coulomb interaction electron-hole pairs excitons. To distinct excitons and get free charges, extra dissociation energy of excitons (binding energy) is needed, which for diverse organic semiconductors is 0.2 to 1.0 eV. Generation of charges because of dissociation of excitons could be realized at the edge of two organic semiconductors (acceptor and donor), that is, on a heterojunction (Dittmer et al., 2000; Cai et al., 2010).

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Figure 2.1: Energy diagrams of polymer binary structure: (a) photon absorption; (b) exciton generation; and (c) charge separation procedures. Source: https://www.intechopen.com/books/emerging-solar-energy-materials/ new-organic-polymers-for-solar-cells.

Through shifting the energy levels among the corresponding orbitals, organic materials could work as acceptors or electron donors. At the donoracceptor (D-A) boundary, the procedure of charge transfer happens, which leads to the presence of holes in a material with a little ionization potential (donor) and electrons in a material with a great electron affinity (acceptor). These carriers are still linked by the Coulomb interaction however could be separated through an internal electric field or built-in potential of the SCs, which is formed in association with the alteration in the work function of the two diverse electrodes in the sandwich configuration of organic heterojunctions (Spanggaard and Krebs, 2004; Jørgensen et al., 2008). Holes transfer through the donor material to the electrode with a large work function and electrons by the acceptor layer to the electrode with a little work function. The characteristic distance moved by the exciton throughout its lifetime, that is, the diffusion length lD, inorganic semiconductors, is restricted by a distance of around 10 nm due to low mobility and their short lifetime (Dennler et al., 2005). Observably, only photons close to the heterojunction plane, immersed by the characteristic length lD, give to the photocurrent. Only excitons that seem at distances similar to lD could efficiently move toward the interface, thus ensuring the production of charge carriers. In practice, in the OSC with a binary system, only a slight part, around 0.01 absorbed photons, could contribute to the photocurrent. A flat binary heterostructure comprising two organic materials with moved energy levels for the realization of the procedure of detachment of an exciton into free charges was initially demonstrated in 1986 by Tang (Scharber, 2006). It was presented that a PV effect happens in a two-layer

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D-A system: metal phthalocyanine/perylene compound with reasonably high efficiency. The coefficient of transforming the energy of light into electrical energy was around 1%. A rise in light transformation efficiency of up to 2.5% was attained in SCs founded on fullerene C60 as an acceptor material in a mixture with Zn or Cu phthalocyanines (Krebs et al., 2004; Bertho et al., 2008). A significant step in enhancing the effectiveness of the OSC was the shift to a BHJ, which is got by mixing acceptor and donor materials. The principle of operation of an OSC founded on a BHJ is regulated by the vital property of polymer materials, which comprises the striving for phase detachment at the nanometer level. In the OSC of this kind, the D-A interface, which enters the whole volume of the material, ensures the detachment of excitons, as well as the transport of holes and electrons to the electrodes (Kim et al., 2006, 2007). For the first time, solar batteries founded on volumetric heterojunction got from solutions were stated in 1995. Later then, the number of publications in this area had started to grow exponentially, and the PCE had improved from 1 to 5% (Peet, 2007; Harada et al., 2010). In the initial years, poly [2-methoxy, 5-(20-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV)/ C60 composites were future replaced by a better processed mixture of poly [2-methoxy5-(30,70-dimethyl octyl oxy)-1,4phenylene vinylene] (MDMO-PPV)/[6,6]-phenyl-C61/71-butyric acid methyl ester ([60]PCBM or [70]PCBM). Due to the rather low mobility and large bandgap of PPV-type polymers, the effectiveness at best remained at 3%, and the overall interest in this class of materials vanished (Green et al., 2001; Brabec et al., 2002). Lately, research efforts had attentive on poly(alkyl-thiophenes) and specifically on poly(3-hexyl-thiophene) (P3HT). In 2002, the first inspiring results for P3HT:[60]PCBM SCs at a 1:3 weight ratio were printed. At this time, the short-circuit current density was the biggest ever noticed in an organic solar cell (8.7 mA/cm2) (Schilinsky et al., 2002; Zhang et al., 2012). A combination of P3HT:[60]PCBM was and remains leading in studies of organic SCs. Suppose the material P3HT, which engages photons with a wavelength of fewer than 675 nm (energy of the bandgap Eg ≈ 1.85 eV). Supposing that in the P3HT:[60]PCBM blends the polymer decides the optical gap of the composite, it is probable to calculate both the absorbed power density and the density of the absorbed photons. A distinctive spectrum of light incident on the Earth’s surface is known by the standard

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AM1.5G. This standard describes parameters like an integrated photon flux of 4.31 × 1021 1/s × m and an integrated power density of 1000 W/m2 (100 mW/cm2), distributed over an extensive range of wavelengths (280–4000 nm) needed for the characteristics of SCs. The P3HT layer: [60]PCBM could absorb, at best, 44.3% of the available power and 27% of the available photons. Regardless of this, the real effectiveness value for an organic solar cell founded on P3HT:[60]PCBM doesn’t exceed 5% (Dennler et al., 2009). To further enhance the effectiveness of SCs, it is essential to develop donor polymers that engage light in an even lengthier wavelength region than P3HT, that is, the absorption limit should lie at wavelengths larger than 700 nm. Such polymers must have a bandgap (the difference in the energies of the HOMO and LUMO (lowest unoccupied molecular orbital) of less than 2 eV (Al-Ibrahim et al., 2005). The number of identified donor polymers giving acceptable light transformation efficiency in PV cells is still little. In addition to the synthesis of novel polymers, work is also underway to attain new fullerene compounds for the drive of utilizing them rather than the [60]PCBM in PV cells. In this regard, the goal of our chapter was the development of novel acceptor components founded on modified fullerene donors and С60, founded on soluble derivatives of polyaniline for usage in organic SCs.

2.2. DESCRIPTION OF NOVEL ORGANIC MATERIALS Plastic or organic SCs are striking for solar photoelectric energy transformation applications where lightweight, flexible shape, and low cost (like a large area) are desired. The photoelectric power transformation efficiencies of presently reported polymeric/OPV materials are yet comparatively low (typically less than 10% under AM 1.5 and one Sun intensity), and the three main losses are still severe, for example, the ‘photon loss’ because of mismatch of materials energy gaps vs the sunlight photons energies, the ‘carrier loss’ and the ‘exciton loss’ because of poor solid-state morphologies of prevailing polymeric acceptor/donor binary systems (Bazan, 2007; Matano and Imahori, 2009). Thus, both phase morphologies and molecular frontier orbitals (HOMOs, LUMOs) are required to be engineered to further increase efficiency. In this chapter, our latest efforts on energy gap engineering and frontier orbital and terminal functionalizations of conjugated polymer blocks, and a donor bridge acceptor kind block copolymer method would be reviewed (Polyzos

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et al., 2003; Haraguchi, 2011). For example, a new DBA or donor-bridgeacceptor kind conjugated block copolymer structure had been effectively characterized, synthesized, and SCs founded on the new materials had been preliminarily verified displacing better performance of the block copolymer structure vs the donor/acceptor simple blend structure. Additionally, dyesensitized polymer and inorganic/organic hybrid nanostructured SCs were also examined as dyes absorb more sunlight photon and had extra available gaps and energy levels that could better match the solar spectrum than old SCs (Eo et al., 2012, 2014). Extensive usage of inorganic-founded solar cell technology as an alternate energy source is still restricted due to the high energy consumption or high cost related to the elaborate fabrication procedures involving high vacuum, raised temperature as well as a recent shortage of feedstock materials (Biglova et al., 2015). Polymeric and organic SCs provide many competitive benefits, comprising appropriate fine turning of materials chemical structures, materials durability, energy gaps (Eg), frontier orbitals (HOMOs and LUMOs), and low cost and flexibility for solution-based hugescale industrial fabrications and processing, comprising well-established polymer solution printing methods or an R2R (roll-to-roll) tinny film processing protocol (Miftakhov et al., 2014). Additionally, polymeric, and organic semiconductors display much greater optical absorption coefficients likened to their inorganic counterparts, therefore opening opportunities for the production of very tinny solar films or sheets that could protect a large number of materials (Biglova et al., 2017). The existing best-reported polymer-founded solar cell has a power transformation efficiency of around 8–10% under one Sun at AM 1.5, the cells normally contain a mixture of donor kind polymer with an acceptor (usually fullerene derivatives) (Mehrotra et al., 1997; Nayak et al., 1997). Though fullerenes are not costeffective so far, the morphology (like donor/acceptor phase domain size and ordering) of a physical mixture is not easy to control (Kondratiev et al., 2011). The critical achievement factors for polymeric and organic SCs comprise the improvement of photon capture through energy gap engineering, specifically in the most concentrated sunlight radiations of 1–2 eV, and the development of charge generation and transport through polymer morphology engineering, as it is now obvious that the photo-induced charge detachment is crucially affected by the donor/acceptor domain size, and charge mobility is crucially affected by the polymer morphology (Otero et al., 2012).

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The complete photoelectric power transformation efficiency of a polymeric/organic solar cell is determined by at least the following five critical steps (Tarver et al., 2009): • • •

Exciton generation or photon capture; Exciton dispersion to donor/acceptor interface; Carrier generation or exciton dissociation at the donor/acceptor interface; • Carrier diffusion to specific electrodes; and • Carrier gathering by the respected electrodes. Till yet, none of the five steps had been improved, which accounts for the comparatively low power transformation efficiencies of OPVs (the best stated/announced efficiency is around 10%). Though all these five steps could be and *must be improved through systematic enhancements in materials design, processing, synthesis, and fabrications (Borole et al., 2004, 2006). In the first step of photon capture (also happening in the photosynthesis of natural plants), a basic need is that the materials optical excitation energy gap (Eg) must look at the incident photon energy. In most organic materials, the energy gap defaults to the difference between the LUMO and the HOMO frontier orbital levels (Zade and Bendikov, 2006). For terrestrial solar cell applications, it is appropriate that the energy gaps of the extent of the material a broad range of 1.0 to 2.0 eV. Many extensively utilized conjugated semiconducting polymers utilized in the organic solar cell had energy gaps larger than 2.0 eV. This is because the photon capture for organic PV cells is yet required to be optimized at AM 1.5. This ‘photon loss’ issue is very usual in most of the presently reported OPV materials. One benefit of organic materials is the processability of their gaps and energy levels through molecular design and synthesis. Thus, sufficient opportunity exists for development (Abdrakhmanov et al., 1988; Hadziioannou and Malliaras, 2006). The existing main performance barriers for PSCs could be attributed to the three main losses comprising the photogenerated ‘exciton loss,’ the photogenerated ‘carrier loss,’ and the sunlight ‘photon loss,’ (Schmechel and Von Seggern, 2004). As mentioned previously, the most concentrated sunlight at the surface of the earth is in a photon energy extent of 1.02.0 eV, however, most usual stable conjugated polymers (like PPVs) had optical excitation energy gaps above 2 eV, so the ‘photon loss’ is the first

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performance obstacle that needs to be resolved. Moreover, since sunlight is very comprehensive radiation with photon energy extending from UV to IR, a tandem style consecutively connected and parallel set cell structure with energy gaps steadily sliding from UV to IR along the light propagation direction is projected to yield very high-power transformation efficiency, as this would allow a broad capture of maximum solar photons and at the same time the summation of immersed photovoltage (Salikhov et al., 2007, 2008; Gadiev et al., 2011). Thus, the development of a diversity of sunlight energy matched (1.0–2.0eV), stable, chain-end functional, processable, and cost-effective conjugated polymer blocks and their joined block copolymers are crucial. A high effectiveness photoelectric conversion not only needs efficient photon capture but also needs a donor/acceptor bicontinuous nanophase detached morphology for efficient charge carrier transport and exciton dissociation For example, a nanodomain sized ‘honeycomb” shaped column morphology seems to be able to decrease both the photon and exciton losses simultaneously (Sze and Ng, 2006). The major challenge is the synthetic chemistry that could covalently relate an acceptor conjugated block with a donor conjugated block through a non-conjugated bridge unit. Slight organic donor-bridge-acceptor structures had been comprehensively studied and revealed effective photo-induced charge separations. However, the small organic molecular DBA structures suffer from charge transport harms for solar cell device applications because of the absence of bicontinuous nanophase detached morphology. For this purpose, a (DBAB)n-type conjugated block copolymer structure has been effectively developed and revealed by us previously (Fowler, 1928). Precisely, a PV device comprised of a (DBAB)n-type block copolymer tinny film exhibits a substantial performance enhancement over its conforming D/A blend or donor/ acceptor under identical situations, where the donor is a ROPPV (oxyalkyl derivatized poly-p-phenylenevinylene) conjugated block, the acceptor is a sulfone-alkyl derivatized PPV conjugated block, and the bridge is a flexible and non-conjugated unit. XRD and AFM displayed nano phased wellorganized packing in (DBAB)n block copolymer that was distracted in the D/A blend. We thus attribute the optoelectronic enhancement to the block copolymer inherent molecular self-assembly and nanophase morphology that consequences in the decrease of the carrier and exciton losses (Antoniadis et al., 1993; Biglova et al., 2017). However, one restriction of the (DBAB)n structure is that the molecular size distribution (polydispersity) must be exactly slim to make good long-

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range well-organized packing as displayed in the top scheme of Figure 2.2. If the acceptor blocks or the donor blocks had extensive molecular size distribution, it would be tough to form ordered domain packing and sizematched in long ranges, merely small molecular mass fractions at a small domain might be probable. However, if a DBA-type block copolymer or donor-bridge-acceptor is developed, even if the acceptor blocks or the donor blocks might have extensive molecular size distributions, they would still be capable to pack nicely because of the free space or volume surrounding every block as displayed in the middle scheme of Figure 2.2 (Salikhov et al., 2013a). Moreover, additional acceptor or donor blocks added to the DBA system could further support the self-assembly of the DBA structure, this is because DBA could act as a backbone or surfactant or facilitate or guide the self-assembly of acceptor and donor blocks as shown in the lower scheme of Figure 2.2 (Torosyan et al., 2012; Patra et al., 2014).

Figure 2.2: Structure of potential solid-state stacking patterns of DBA and (DBAB) n-type block copolymers. Source: https://www.sciencedirect.com/science/article/pii/ S1876610214013782.

2.3. CARBON NANOTUBE/POLYMER NANOCOMPOSITES While numerous efforts are presently being followed in the field of donor polymer engineering, the acceptor material also is enhanced. In the photoactive layer, the conducting polymer donates to the photoconduction procedure and works as the exciton generation point, however, carbon nanotubes, a carbon allotrope same as the fullerenes, are also stated to contribute to photoconduction (Freitag et al., 2003).

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The discovery of CNTs in 1991 by Ijima produced great interest in discovering the properties of CNTs and their applications. It was found that the integration of CNTs in the organic conducting polymers could increase the performance of the polymer PV cell. The research constantly enhances the performance (for example, short circuit current, open-circuit voltage, and efficiency) of the CNT conducting polymer merged PV cell. CNTs could be applied to two significant features of OPV devices: a transparent conductive layer to substitute ITO and the active layer as a combination with conductive polymers. When utilized as a composite material in the active layers (ALs), CNTs could improve device efficiency by numerous orders of magnitude. When light is excelled on an OPV, excitons produced in the conducting polymer diffuse by the polymer to reach the CNT = conducting polymer junction. The final works as an exciton dissociation midpoint and the holes and electrons are transported to the electrodes by the conducting polymer and CNTs, respectively. Even though CNTs enhance the characteristics of OPVs, the power conversion effectiveness of the CNT and conducting polymer PV cell is still little related to that of the inorganic PV cell. The cause for this low effectiveness is that most of the excitons produced in the conducting polymer recombine earlier to reaching the CNT = conducting polymer junction because of their small diffusion lengths (Figure 2.3) (Velasco-Santos et al., 2005; Min et al., 2010).

Figure 2.3: The improvement of the near-field near the nanoparticle surface. Source: https://pubmed.ncbi.nlm.nih.gov/20168344/.

Decreasing the recombination rate of holes and electrons could enhance the efficiency of this PV cell, which is accomplished through depositing a very tinny (on the order of a rare 100 nanometers), consistently dispersed

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CNT = conducting polymer photoactive layer. The CNTs utilized should be extremely crystalline and free from metal contaminations to avert the creation of charge traps in the photoactive layer. For the best performance of a PV cell, the CNTs should be consistently dispersed in the polymer, and it had been noticed that the polymer chains cover the walls of CNTs producing heterojunction alongside the wall of CNTs. This is essential for effective charge dissociation at the CNT = polymer junction due to the low exciton diffusion length. Another benefit of well-dispersed CNTs is the creation of percolation routes by CNTs through the composite, which increases electron transportation and decreases the recombination frequency of charges in the active layer. Saini et al. (2009) had presented that the integration of MWCNTs in the P3HT matrix could drastically enhance the morphological, thermal, optical, structural, and electrical properties of P3HT. Furthermore, Petra et al. (2014) confirmed that the existence of MWCNTs in P3HT could influence the dielectric conduct of P3HT polymer, in addition to the rise in melting temperatures and crystallization (Dalmas et al., 2005; Gulotty et al., 2013).

2.4. FULLERENE-CONTAINING POLYMERS FOR ORGANIC SOLAR CELLS (SCS) The insertion of fullerene molecules in the polymer chains as electroactive and photo moieties (the subject of competitive and intense research in the latest years) should lead to producing novel materials with unique electrochemical, photophysical, and structural properties. In the latest years, numerous works that extensively utilize the metathesis strategy to get materials for PV cells had been printed (Wang and Schiff, 2007; Yang et al., 2013). For instance, the synthesis of vinyl-kind polynorbornenes whose structure comprises fragments of [60]PCBM, a conventional electron-withdrawing constituent of the active layer in the OPV cells, was recommended by Eo et al. (2014). PV cells where the fullerene comprising copolymer represented as the n-type semiconductor in the active layer were formed based on these polymers (Figures 2.4 and 2.5) (Kim et al., 2015).

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Figure 2.4: Ring inaugural metathesis polymerization of fullerene comprising norbornene monomers. Source: https://link.springer.com/article/10.1134/S0023158417020021.

Figure 2.5: Ring inaugural metathesis copolymerization of fullerene comprising norbornene monomers with associated fullerene-free compounds. Source: https://www.researchgate.net/publication/266204904_Fullerene_containing_norbornenes_Synthesis_and_ring-opening_metathesis_polymerization.

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Also of interest are numerous works in which FCMs (fullerenecontaining monomers) were subjected to metathesis polymerization utilizing a Grubbs catalyst and the products were verified in SCs (Torosyan et al., 2014). This portion of our work was dedicated to synthesizing copolymers from norbornene-type monomers and new fullerene-containing polymers in the existence of the first-generation Grubbs catalyst [(PCy3)2Cl2RuCHPh]. Probed in the work the fullerene-containing norbornene monomers comprise (Garcia et al., 2007; Marconnet et al., 2011):

2-dihydro-C60-fullerene (endo:exo = 6:1), (1-methoxycarbonyl)-1-[(2bicyclo[2.2.1]hept-5-en-2-yl)ethoxycarbonyl]-1,2-methano)-1 1, (1-chloro1-[(2-bicyclo[2.2.1]hept-5-en-2-yl) ethoxycarbonyl]-1,2-methano)1,2-dihydro-C60-fullerene (endo) 2 [18] and bis[2-[(2S*)-bicyclo[2.2.1] hept-5-en-2-yl]etoxycarbonyl)-1,2-dihydro-C60-fullerene 3 (Salikhov et al., 2018). The ring-opening metathesis polymerization of monomers 1–3 was done in the existence of the first-generation Grubbs catalyst in CH2Cl2 at room temperature in an argon atmosphere. In both circumstances, the precipitation of the polymers and the consumption of the starting norbornenes 1–3 (TLC monitoring) were observed for the first 3 hours (Yang et al., 2013). Synthesized homopolymers 4–6 were noticed to be insoluble in general organic solvents (PhMe, C6H6, CHCl3, EtOAc, and C5H4F), and to swell only partly in dimethyl sulfoxide, thus, it looked impossible to characterize their structures through spectral techniques and to approximate their molecular weights (Dowland et al., 2017; Pierini et al., 2017). Note that the results got don’t contradict the other data accessible in this field. Some works revealed that the integration of C60 fullerene into the polymer, in numerous cases, significantly worsens its solubility, which is because of the creation of intermolecular bonds including C60 fullerene and polynorbornene fragments, as well as because of the limited solubility of fullerene itself (Torosyan et al., 2014; Biglova et al., 2015). One of the probable methods to prepare soluble fullerene-containing polymers is the immersion of fullerene monomers into copolymerization with extremely soluble comonomers. This procedure is accompanied by the “effect of dilution” of stiff C60-containing units because of the reduction in the concentration of fullerene molecules in the polymer chain, which had a favorable impact on the solubility of the last polymer. To reproduce this result, norbornenes 1, 2 and 3 were copolymerized with associated fullerene-free compounds 2-[(bicyclo[2.2.1]hept-5-en-2-yl-carbonyl)

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oxy]ethylmethyl malonate (exo:endo = 6:1) 7, 2-[(2,2-dichloroacetyl)oxy]ethyl bicyclo[2.2.1]hept5-ene-2-carboxylate (exo:endo = 6:1) 8 and bis[2-[(2S*)-bicyclo[2.2.1] hept-5-en-2-yl carbonyl]oxy)ethyl)malonate 9, correspondingly (Zhao et al., 2015). In all circumstances, the metathesis polymerization consequence in the creation of copolymers 10, 11 (CHCl3, dimethyl sulfoxide) soluble in certain organic solvents with good degrees of conversion (Salikhov et al., 2013b; Benduhn et al., 2017).

2.5. SOLUBLE FUNCTIONALIZED POLYANILINES The progress of a novel generation of sensor devices is related mainly to 2 conductive high-molecular compounds, namely, polypyrrole, and PANI, which had been utilized in extremely selective devices for the analysis of mixtures of liquids and gases, the so-called “electronic tongues” and “electronic noses” (Biglova et al., 2015; Salikhov et al., 2018). Biomedical studies of PANI are very promising. It had been revealed that PANI could be utilized as a biocompatible electrode: electrical signals provided to an in-vivo deposited polymer layer boost the acceleration of tissue regeneration (Li et al., 2012; Nuzhdin and Bukhtiyarova, 2015). There is an extensive range of potentially possible and already accessible applications of PANI. Yet, the practical utilization of this material is restricted by several serious issues. The first issue is linked to the synthesis of PANI with reproducible properties. Samples of the polymer could comprise an extensive variety of aniline oxidation products with electrical conductivities that change dozens of times. These products also alter their magnetic and spectral characteristics and could have a fundamentally diverse morphology. Such a vagueness leads to ambiguous consequences and needs a comprehensive investigation of the oxidative polymerization of aniline (Zhang et al., 2006; Singh et al., 2011). The second issue is related to the formation of materials for practical applications. A substantial disadvantage of PANI is that it doesn’t melt and is practically insoluble in conventional organic solvents. So, PANI belongs to the group of non-recyclable materials. Moreover, this polymer is a powder that has no bond to other materials (Xu et al., 2008; Gizdavic‐Nikolaidis et al., 2010). Concerning this, it is clear that the synthesis is a key procedure in the preparation of not only PANI but also PANI-based composites. Regardless of the apparent simplicity, the oxidative polymerization of aniline is a complicated multistage reaction. The conventional procedure for the

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chemical synthesis of PANI comprises the oxidative polymerization of the monomer in an aqueous solution of an inorganic acid (Kazmerski, 2006; Luque and Hegedus, 2011). These situations provide the creation of an unmeltable powder that is insoluble in the bulk of available organic solvents. To eradicate the above disadvantages, PANI could be altered in different ways. An alternate version of the optimization of performance features of the polymer is the functionalization of the first monomer rather than of the target product. In precise, the introduction of o-anisidine and o-toluidine (rather than aniline) into the polymerization procedure leads to the precipitation of the high-molecular compounds solvable in organic solvents (Salikhov and Salikhov, 2015). Further, the homopolymer founded on o-toluidine could be utilized in the design of PV and electrochromic devices. There are instances where the electrochemical polymerization of o-toluidine was done with diverse solutions of acids utilized as electrolytes. In specific, Borole et al. (2004, 2006) utilized sulfamic acid, n-toluene sulfonic acid, sulfuric acid, and sulfosalicylic acid. A relative analysis of the synthesized substances revealed that the extreme electrical conductivity was shown by a polymer soluble in the maximum of conventional solvents, which was remote with the participation of sulfonic acids. In several works, the technique was suggested for the synthesis of high-molecular compounds with good solubility and high electrical conductivity by changing the ratio of comonomers (polymerization and electrochemical). This made it probable to synthesize copolymers founded on o-toluidine and o-anisidine. It was noted that the utmost stable films are made from a copolymer in which the content of pyrrole is more than 50% concerning o-toluidine (Photovoltaics, 2003; Bazunova et al., 2018). We made research recognizing the most effective representatives and increasing the range of electrically conductive high-molecular compounds, mainly utilizing functionalized aniline and researching the physicochemical and electrophysical properties of the target products. Taking into consideration that the electrical conductivity of a highmolecular compound enhances with the elongation of the conjugation chain, we curved to the development of the polymerization procedure of the functionalized imitative of aniline, despite the aniline itself, and to the investigation of the physicochemical and physical properties of the attained products (Sariciftci and Sun, 2005; Sun and O’Neill, 2011). The monomer utilized for the oxidative polymerization was earlier synthesized 2-(1-methyl-2-butene-1-yl)aniline 12 with an alkenyl substituent

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that inhabits the o-position of the aromatic ring and enhances the conjugation chain. The diversity of the molecular morphology, structure, and properties of the oxidation products of aniline is linked with the occurrence of the main reagents, namely, the growing chain in protonated and unprotonated forms and the monomer as well as with the presence of two mechanisms of oxidation: the recombination of cation-radical centers and the chain reaction of electrophilic substitution. The impact of the two reactions relies on the protonation state of the reagents and, subsequently, on the pH of the reaction medium (Günes et al., 2007; Krebs, 2008). The homopolymerization of 12 was passed out through means of its oxidation, which caused the creation of a dark green hasty of polymer 13 in aqueous solutions of acids. The most regularly utilized oxidizing agent is ammonium persulfate. It is assumed that the utilization of ammonium persulfate takes to the creation of a high molecular mass polymer with high electrical conductivity (Service, 2011). The oxidation of aniline was done in an acidic medium with hydrochloric acid at the pH = 0–2 as pert the scheme displayed in (Figure 2.6). Aniline derivative copolymers 14–16 were produced in diverse molar ratios of o-toluidine and 12 (1:3. 1:1. 3:1, correspondingly) according to processes same to those utilized for the synthesis of homopolymer 13. The production of the copolymer was ~80% (Amb et al., 2011a, b).

Figure 2.6: Homopolymerization of 2-(1-methyl-2-buten-1-yl) aniline 12. Source: https://link.springer.com/article/10.1007/s10118-019-2261-9.

2.6. CHARGE TRANSPORT IN THIN POLYMER FILMS The electronic conductivity of organic molecular compounds varies from that of metal and inorganic semiconductors like germanium and silicon. The well-recognized band theory of crystal lattice is a decent base to recognize the conduction mechanism of the crystalline molecular solids and unconjugated

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and conjugated polymers (Yoo et al., 2004; Yamanar et al., 2009). At a similar time, the applicability of the best-elongated chain model to materials with a complex morphology is naturally restricted. Even inside the edges of the idealized model, the inorganic semiconductors and conductors differ significantly from polymers. Further, in polymers, the screening of interfaces among charge carriers is less; electron-hole and electron-electron interactions play a significant role in initiating substantial localization of electron states as matched with inorganic materials (Kim et al., 2007; Li et al., 2012). Lack of macroscopic ordering means insufficiency of the band conduction model to explain electron conductivity of mass polymer materials, though it could be utilized to a limited range when studying the conduction procedure (Huo et al., 2015). In amorphous layers of tinny organic films, the terms “valence band” and “conduction band” are generally replaced by the terms the HOMO and the LUMO, respectively. The states’ density is primarily described quite reasonably by the Gaussian distribution of localized molecular orbitals of the individual molecules (Kroon et al., 2008; Yang et al., 2019). Relying on the size of the obstacle on the interface of an electrode with the polymer film, electric current moving through the sample could be of injection kind, which is, restricted by space charge. In this situation, one of the electrodes should be an ohmic one, that is, it should give more charge carriers in time unit than the sample is capable to transport, not breaching Poisson’s law. Otherwise, charge carrier transference across the interface would be limited by the barrier. The tunneling model of RichardsonSchottky’s (RSch) and Fowler-Nordheim (FN) thermionic emission models are normally utilized to study injection in polymers despite strong electric field (Sun et al., 2014; Sun, 2015). A thermal electron release from hot metal is termed thermionic emission. Electronic release from the metal contact into a vacuum or dielectric conduction band through their thermal transportation by a potential barrier in an electric field is termed Schottky emission. Taking into consideration image forces in the parabolic estimation, it is probable to get the RichardsonSchottky equation for current density (Sun et al., 2007, 2012): (1) where; J is a current density; е is an electron charge; F is a field density; ε0 is the electric constant; k is the Boltzmann constant; А is the Richardson constant; φB is a barrier height; ε is a dielectric permeability of a sample

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and Т is temperature. A significant supposition in the RSch model is that an electron could be taken out from the metal once it acquires adequate heat energy to cross the potential barrier which is made by a superimposition of the image’s forces and external field (Gosztola et al., 1996). As per the quantum theory, the electron wave function inside the dielectric area located among two electrodes is different from 0. Wave function exponentially reduces with a distance into the barrier. If the barrier is very slim, the possibility to pass through the barrier for an electron has a limited value relying on the form and height of the potential barrier. Tunneling (auto-emission) could be observed in the case of a wide barrier if its effective thickness reduces under the impact of a strong electric field. In the FN model image, forces are overlooked, and the tunneling of electrons from metal by a triangle hurdle to free states of the conduction area is deliberated. When the field intensity rises, the width and height of the potential barrier reduce to such a point that a novel physical effect seems and prevails: quantum mechanics tunneling of an electron through the potential barrier. The current produced by the tunnel release assisted by a field is explained by the FN equation. In this situation the current density could be described by the expression (Wu et al., 2015): (2) which is independent of temperature. Now, ħ is Planck’s constant; and meff is the effective mass of a charge carrier in the polymer. Regardless of the disadvantages of both RSch and FN concepts, they had been applied positively to explain injections of a charge carrier in the organic light-emitting diodes (OLED). For instance, the FN model was applied to provide reasonable values for the obstacle height and to take into consideration the independence of the temperature characteristic J(F) in strong fields (Nam et al., 2015). Thermionic emission occurs at high temperatures and comparatively low electric fields. Currently produced by tunnel release takes place at low temperatures and high values of the electric fields. In Biglova et al.’s works (2015), the temperature dependencies of the electrical conductance were calculated for films of diverse polyaniline forms. The measurements were taken out for soluble forms of the altered polyaniline homopolymer, that is, 12, and its copolymers with o-toluidine in diverse molar ratios. The measurements of the temperature of the electrical

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conductance G of the polymer films in the range from 300–450 K revealed that the reliance of G on the temperature T had an exponential character (Chen et al., 2016): (3) In the lnG-1000/T manages, the experimental points, within the limits of error, fall on a straight line (Figure 2.7). The quantity ΔE could be taken as the interval between LUMO and HOMO (an analog of the band gap) in semiconductor polymer films.

Figure 2.7: Dependencies of (a) the electrical conductance and (b) I/T2 on the opposite temperature for films of copolymers (o-toluidine with 2-(1-methyl2-butene-1-yl) aniline) in diverse molar ratios: (2) 1:3, (3) 1:1, and (4) 3:1. Source: https://www.intechopen.com/books/emerging-solar-energy-materials/ new-organic-polymers-for-solar-cells.

From the data accessible in the literature, it follows that the bandgap fluctuates from sample to sample and falls in the energy range from 1.39– 1.66 eV. The reliance of the bandgap on the molar ratio of the copolymers utilized for the preparation of tinny films is a tremendously significant characteristic of their practical application in numerous electronic devices. The polymer compounds studied in this chapter could be utilized for the successive development of electronic devices, same as those found on inorganic Ga1–x AlxAs heterostructures. To know how charge transfer happens through the metal-polymer interface, we calculate the temperature dependencies of the current I moving through the film structure. In the line/T2–1000/T manages, the graphical dependencies, inside the error of measurement, are well estimated

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by straight lines (Figure 2.7) by Eqn. (1). The current density is defined as J = I/S, where S is the cross-sectional area of the film, which remains unaffected during the measurement. Thus, the graphical dependencies could be constructed utilizing the values of the current flowing by the sample, despite the values of current density. According to Eqn. (1), the slopes of the straight-line sections are proportional to the Schottky barrier height ϕB.

The analysis of the dependencies got in this study permits the assumption that the key mechanism of charge carrier transference through the interface among the polymer film and the metal substrate is the Schottky thermionic emission, which regulates carrier transport in the temperature range from 300–450 K. This endorses the assumption that the transfer of charge carriers by the metal-polymer interface happens as a consequence of the abovebarrier transport. In this situation, the barrier height is described by the difference between the electron affinity of the polymer and the work function of the metal. For instance, the calculation according to the outcomes of the electrophysical measurements for film samples of copolymers 15 provides the barrier height of 0.77 eV. Taking into consideration that the work function of aluminum is 4.26 eV and the electron affinity of the polymer falls in the range from 3.5–3.6 eV, we attain the barrier height ranging from 0.76–0.66 eV, that is, we have a value similar to that calculated inside the framework of the Schottky model. As the field addition in Eqn. (1) doesn’t exceed 0.1 eV, it is overlooked. Therefore, the above calculations are additional evidence in favor of the model of above-barrier transport at the metal-polymer interface (Hilal and Han, 2018). The attained values of LUMO and HOMO show that the polyanilines studied in our work could be utilized for the development of novel organic SCs. The short-circuit current of the photo-converter is diligently associated with the difference in the energy between the HOMO of the donor (PANI) and the LUMO of the acceptor. The most suitable acceptor could be signified by a methanofullerene. This difference also defines the open-circuit voltage. Furthermore, the bandgap of the donor decides the maximum wavelength or minimum energy of the absorbed photons. For efficient absorption in the observable part of the solar spectrum, the bandgap should be in the range of 1.4–1.5 eV (Salikhov et al., 2013a, b). Therefore, the poly-2-(1-methyl-2-butene-1-yl)aniline/methanofullerene heterojunction, which is made of newly synthesized compounds, is ideal for manufacturing a laboratory sample of a solar energy photoconverter.

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2.7. ORGANIC SOLAR CELLS (SCS) BASED ON THIN POLYMER FILMS The method of formation of tinny films of polyanilines and fullerenecontaining polymers through vacuum deposition from a Knudsen effusion cell was utilized (Torosyan et al., 2012). The length of the cylindrical cell was 25 millimeters, the internal diameter was 4 millimeters, and the working temperature was diverse within the range 500 to 650 K. Thermal heating of FCMs (fullerene-containing monomers) during deposition takes to their polymerization. Some thin films were made by the spin coating method from a solution of FCMs. All the attained films were uniform in thickness, and their conductivity was around 0.1–1.0 mS/cm. To enhance the conductivity of polyaniline layers, the temperature situations of deposition from the Knudsen cell were selected. The temperature range of 500–550 K showed to be the finest. Besides, protonation of the recently made films in vapors of the hydrochloric acid solution was taken out. For PANI films a conductivity value of 1.0 mS/cm was attained as a result (Wang and Schiff, 2007). The thickness and surface condition of the deposited films were checked on the base of the analysis of AFM images got by a nanoscan 3D. The thickness of photoactive layers was diverse and took on values in the range of 100 to 200 nm. It should be known that a very large thickness of the films leads to exciton recombination and decreases the effectiveness of charge separation. On the contrary, the number of formed excitons and incident photon absorption reduces in overly thin films. The organic solar cell examines samples founded on the D-A polymer systems explained earlier that was formed on a glass substrate with transparent and conductive ITO layers. The resistance of ITO layers was around 10 Ω/m. For an experimental system of the OSC in this research the following organic substances were utilized: conventional fullerene, PANI, and a newly synthesized monomer, monosubstituted methanofullerene derivative (Figure 2.8(a) and (b)). The aluminum films made up of thermo-diffusion deposition in a vacuum were covered as the upper electrode. Figure 2.8(c) presents the system of the OSC in which fullerene-containing polymers and thin films of PANI were utilized as photoactive layers (Yang et al., 2013).

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Figure 2.8: (a) An energy level pictorial of the PANI/FCM system; (b) procedure of photon absorption and charge detachment in this structure; (c) multilayer film structure of OSC. Source: https://cwww.intechopen.com/books/emerging-solar-energy-materials/ new-organic-polymers-for-solar-cells.

The CV characteristics (current-voltage characteristics) of all the ready OSC samples were calculated, and the numerical values of such parameters as short-circuit current, open-circuit voltage, PCE, and filling factor were calculated on their basis. Calculating the CV characteristics of a PV cell is normally done by revealing it to steady-state illumination and a recognized temperature. The sunlight simulator or sun could act as a light source. Approximations of the coefficient of efficiency were founded on standard sun intensity P0 = 1000 W/m2 (AM 1.5 G conditions).

The values of these parameters for the numerous OSC experimental system studied in this work seemed to be FF = 0.6–0.8 (filling factor), Jsc = 0.6–1.8 mA/cm2 (short-circuit current), Voc = 0.6–0.8 V (open-circuit voltage). The highest values of PCE for the inspected organic SCs were around 2%. These values were got for the structures founded on methanofullerene derivatives. Therefore, it was demonstrated that a mixture of PANI with fullerenecontaining polymers is very significant for the formation of OSC on the base of binary D-A systems. The SCs inspected here change from earlier ones that they could be fabricated on the flexible substrates.

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2.8. POLYMERIZABLE METHANOFULLERENE AS A BUFFER LAYER MATERIAL FOR ORGANIC SOLAR CELLS (SCS) In the latest years, new mixtures of semiconductor materials founded on electron-conjugated polymers (p-type materials) and fullerene derivatives (n-type materials) are being aggressively developed all over the globe. It is supposed that the high efficiency of transformation of light in organic SCs could be achieved only by utilizing charge-selective buffer layers (Yang et al., 2013). Normal materials for creating such layers are PEDOT: PSS and several inorganic oxides. Since PEDOT: PSS shows acidic properties, its usage adversely impacts the duration of the operation of SCs. At a similar time, the metal oxides in high oxidation states (WO3, V2O5, and MoO3) display oxidizing properties on the materials of the photoactive layer enabling their breaking. The problem is detected even with comparatively unreactive TiO2 (titanium dioxide) (Kim et al., 2015). The greatest predictions in terms of practical execution had upset configuration organic SCs that don’t contain high active metals and had considerably increased operational steadiness. However, the formation of these devices needs the development of selective ETL (electron-transport buffer layers) founded on semiconductor materials of n-type. We have fabricated overturned SCs which ITO cathode, ETL or fullerene-containing buffer layer, photoactive layer, hole-transporting layer MoO3, and Ag anode (Figures 2.9 and 2.10).

Figure 2.9: Diagram architecture of an inverted organic solar cell. Source: https://www.researchgate.net/figure/a-Schematic-illustration-of-inverted-type-organic-solar-cell-b-energy-level-diagram_fig1_319278556.

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Figure 2.10: The molecular structures of the materials utilized to create the ETL buffer layer of the devices. Source: https://www.intechopen.com/books/emerging-solar-energy-materials/ new-organic-polymers-for-solar-cells.

The photoactive layer of organic SCs was produced on the base of the traditional composites: the acceptor component [70]PCBM or [60]PCBM and poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl2,1,3-benzothiadiazole4,7-diyl-2,5-thiophenediyl] (PCDTBT) or conjugated polymer Р3НТ. In our study, we suggest the usage of previous synthesized (1-methoxycarbonyl)1-[2-(methacryloyloxy)ethyloxycarbonyl]-1,2methane)-1,2-dihydro-C60-fullerene 18 and (1-methoxycarbonyl)-1-[2(acryloyloxy) ethyloxycarbonyl]-1,2-methane)-1,2-dihydro-C60-fullerene 17, comprising in their structure methacrylate fragments and unsaturated acrylate, taking into consideration that buffer layer must fulfill with the number of requirements. First, the forming technique of its film must be reasonably technological and straightforward. Covering the ITO surface with methanofullerene solution in chlorobenzene (CB), as it twisted out, was a pretty simple buffer layer-forming method, which didn’t request such procedures as high-temperature annealing or vacuum thermal evaporation. Second, the formed film should be unaffected by the effect of other solvents. Hence, after placing one on the ITO surface we have had before us a contest of FCM insolubilization. For that cause solid-state radical polymerization has been conducted, which caused the formation of polymethacrylates and fullerene-containing polyacrylates (Torosyan et al., 2014).

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In the first stage, the effect of the temperature of the heating of the buffer layer on the effectiveness of light conversion in solar batteries was calculated on the instance of photoactive materials Р3НТ and PCBM. The CV characteristics of organic SCs were calculated under standard conditions utilizing simulated solar light of AM 1.5 spectrum and intensity of 100 mW/cm2 (calibrated Si diode utilized as reference) and a generalpurpose source meter Keithley 400. The subsequent parameters of the SCs are given in Table 2.1. The attained data reveal the positive influence on the characteristics of solar cell buffer layers formed by polymerization of fullerene derivatives 17 and 18. Mainly exciting were high open-circuit voltages of 637–652 mV attained through using polymerized 18 as a buffer layer. We would like to stress that such high voltages are very exceptional for the P3HT-PCBM SCs (Figure 2.11).

Figure 2.11: Designated CV characteristics of the inverted P3HT/[60]PCBM solar cells formed on bare ITO (reference) and utilizing buffer layers formed from polymerized 17 or 18. Source: https://www.intechopen.com/books/emerging-solar-energy-materials/ new-organic-polymers-for-solar-cells.

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Table 2.1: Parameters of the Best Inverted Solar Cells Invented on Bare ITO and Utilizing Buffer Layers Created from Polymerized 17 and 18 Buffer Layer

Voc (mV) Jsc (mA/cm2)

Т(Polymerization) (°С)*

FF (%)

PCE (%)

–

–

437

7.2

46

1.5

17

120

542

7.5

50

2.0

–

160

526

6.8

47

1.7

18

120

608

7.5

55

2.5

–

160

652

8.2

50

2.7

–

200

528

7.8

38

1.6

*Annealing temperature of the buffer layer material 17 and 18 is given. In the second stage, we studied the effect of buffer layers on PCE and their forming techniques on the substrate surface in the instance of photoactive materials PCDTBT and PCBM. In the latest years, a composite of PCDTBT: PCBM was regularly utilized as an active layer in the standard organic SCs OSC. This is founded on the fact that the absorbance of PCBM is much tougher than that of PCBM and this property is very significant for PV materials. Four kinds of devices had been fabricated: without buffer layer (reference device) and with the concentration of 17 in buffer layer 0.625, 1.25, and 2.5 mg/ml. Their CV characteristic is given in Table 2.2. Table 2.2: CV Characteristics of Inverted Solar Cells Utilizing Different Concentrations of 17 Concentration 1 (mg/ml)

Voc (mV)

Jsc (mA/cm2)

FF (%)

PCE (%)

–

446

8.7

36

1.4

0.625

618

11.1

39

2.7

1.250

587

9.1

39

2.1

2.500

620

8.6

36

1.8

Table 2.2 displays that the PCEs of the devices with ETL are greater than the PCE of the reference device. The data in Table 2.2 also marks a durable increase in open-circuit voltage at the execution of 17, which is also evident, while other characteristics vary. The most possible explanation is that an n-type semiconductor enables photoelectric work function growth, and in

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turn, Voc relies on the work function. A low FF highlights the necessity to conduct an extra optimization for active-layer forming to expand PV cell morphology since FF relies on photoactive film morphology. Authors reported that FF could attain 60–70% for the PCDTBT: PCBM system. With this value of FF, our devices could attain a PCE of 4.5–4.8%. Therefore, the highest performance had been revealed by the device with a minimal concentration of 1. More optimal PCEs are set in the low-value areas of concentration. Correctly, the fewer the concentration of the compound, the fewer the thickness of the formed layer. Further studies on enhancing SCs’ efficiency will be held utilizing the small thickness of the buffer layer. In the third stage, we inspected the influence of the concentration of the buffer layer on the effectiveness of light transformation in solar batteries as the instance of photoactive materials PCDTBT or PCBM. For this, researchers suggested ETL in inverted organic solar cells utilizing a mixture of pyrrolidinofullerene and acrylate derivative of fullerene-17 (Salikhov et al., 2007). The key parameters of the solar cells are given in Table 2.3. Table 2.3: Parameters of Inverted PCDTBT/PCBM and P3HT/PCBM Organic Solar Cells Containing 17 + FPI Buffer Layers as a Function of 17 Concentrations in the Pioneer Solution Photoactive Materials P3HT/[60] PCBM

PCDTBT/[60] PCBM

The Concentration of 17 in the Precursor Solution (mg/mL)*

Voc (mV)

Jsc (mA/cm2) FF (%)

PCE (%)

–

409

6.9

46

1.3

1.25

582

7.4

42

1.8

2.50

591

6.5

43

1.7

5.00

486

7.0

43

1.5

–

585

6.6

42

1.6

0.625

618

11.1

39

2.7

1.25

677

8.3

54

3.0

2.50

707

9.1

46

2.9

5.00

712

7.5

41

2.2

*note that the concentration of FPI in the pioneer solutions was always 25 mol percent concerning the amount of 17.

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The gained results propose that the electron-selective buffer layers founded on the blends of the polymerizable and fullerene derivatives FPI 17 could be successfully utilized for fabricating inverted organic SCs. The power transformation efficiencies for the inverted devices given in this chapter were only 25 to 30% lesser than the parameters of the standard configuration of organic SCs. Though, the latter comprises reactive metal (calcium in our instance) cathode that persuades inherent instability taking to the rapid corrosion of the device parameters even under an inert atmosphere. Inverted devices presented lower open-circuit voltages (around 100 mV) and FFs as related to the standard ones. Seemingly, the electron work function of the fullerene-based buffer layer material is very much high concerning the conduction band (LUMO level) position of the n-type constituent of the photoactive layer ([60]PCBM). Therefore, a Schottky-type obstacle might be made at the interface among the buffer and the photoactive layers. This might be a credible reason for the observed decrease in the open-circuit voltages and block up factors of the inverted devices. To resolve this issue, further research is required with the purpose to design some new fullerene-based buffer-layer materials with lower-electron work functions. The work done in the field of organic SCs presented the advisability of the application of novel organic materials for solar cell development. A mixture of PANI with fullerene-containing polymer was utilized for the creation of OSC on the base of binary D-A systems. The poly-2-(1-methyl2-butene-1-yl)aniline/methanofullerene BHJ, which is made of recently synthesized compounds, is ideal for the manufacturing of SCs (Li et al., 2012). The potential utilization of methacrylate fullerene and polymerizable acrylate derivatives to form a buffer electron discriminating charge transport layer in inverted configuration SCs was validated. Achieved light transformation efficiency shows prospects for further expansion in this research. Optimization of technological conditions of the tinny film fabrication and precise selection of the configuration of the organic materials would provide higher values of OSC effectiveness (Nuzhdin and Bukhtiyarova, 2015).

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CHAPTER

3

DONOR MATERIALS FOR ORGANIC SOLAR CELLS

CONTENTS 3.1. Introduction....................................................................................... 78 3.2. Performance Parameters of SCS (Solar Cells)...................................... 79 3.3. Smdms-Centered Photovoltaics (PVS)................................................ 81 3.4. Oligothiophene-Centered Smdms...................................................... 81 3.5. Oligothiophene-Bdt (Benzodithiophene) Hybrids as SMDMS............ 85 3.6. Idt (Indacenodithiophene)-Centered Smdms...................................... 95 References................................................................................................ 98

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3.1. INTRODUCTION Solar energy is among the most prevailing substitutes for non-renewable fossil fuels. Unluckily, one has remained unable to utilize the full capability of solar energy, to some extent because of engineering challenges and mainly because of the absence of efficient PV (photovoltaic) cells, which transform sunlight into electrical energy. Centered on the utilized solidstate electronic materials system, a PV cell can be split into a crystalline elemental, thin-film (CdTe, CIGS, amorphous Si), organic, or hybrid (Ong and Levitsky, 2010). Currently, the market for PV is dominated (greater than 90%) by the polycrystalline, amorphous, and crystalline inorganic materials centered on solar cells (SCs) (Powell et al., 2012). Regardless of the point that these SCs have outstanding light to electricity transformation ability, their slow manufacturing, high manufacturing cost, low flexibility, sensitivity to impurities, etc., pose substantial challenges to the individual and industrial consumers (Wolden et al., 2011). To master these challenges, OPV (organic photovoltaic) cells appeared as a capable substitute. Organic semiconductors are interconnected materials that generally conduct electricity when an adequate number of alternating double and single bonds are achieved and have conductivity between an insulator and the metal (Okamoto and Brenner, 1964). Organic materials have various benefits over inorganic semiconductors such as broad absorption range and high absorption coefficient, which can be adjusted by chemical functionalization. The simple functionalization of organic semiconductors is the main benefit that surpasses all benefits of the inorganic semiconducting materials. The last era witnessed a marvelous upsurge in the expansion of the OPV cells with a high PCE (power conversion efficiency) utilizing organic donor material. Even though the value is quite lower as compared to their inorganic counterparts, it is adequate for utilization, which is normally set at 10% (Scharber et al., 2006; Green et al., 2017). Centered on the kind of molecular system utilized, OPV cell is divided into 2 classes: SMDSCs (small molecule-donor solar cells) and PDSCs (polymer-donor solar cells). Fascinatingly, the effectiveness of SMDSCs has improved significantly from 0.001% in 1975 to 1% in 1986 to greater than 11.3% in 2017, which is quite near to the highest value attained by any OPV cell (Tang and Albrecht, 1975; Zhao et al., 2017). This unbelievable

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upsurge in performance has been probable because of various advantages provided by the SMDMs, for example, synthetic reproducibility with high purity, synthetic ease, well-defined molecular structure, discrete molecular weight, low cost and weight, appropriateness for large-area applications, mechanical flexibility and high charge carrier mobility (Salleo et al., 2010; Deng et al., 2016). Considering the significance of this quickly developing field, the progress made in current years in the growth of SMDMs for OPV cells is presented herein. Figure 3.1 represents the result produced by Web of Science when looking for the “Organic Small Molecule Solar Cells” from 2010 to 2017. A total of around 553 articles have been produced since the start of the 21st century. Moreover, the citation associated with this topic has grown immensely. Both SMDSCs and PDSCs have been stated in the literature (Tang, 1986; Yang et al., 2017).

3.2. PERFORMANCE PARAMETERS OF SCS (SOLAR CELLS) There are numerous excellent books and articles which record the mechanism and structure of PV devices. The performance parameters needed for characterizing SCs are described here briefly (Gundlach et al., 2005; Anthopoulos et al., 2006). Usually, the OSCs are categorized under the 1000 Wm–2 light with the spectrum corresponding to of sun on the surface of the earth at the incident angle of nearly 48.2° (known as AM 1.5 spectrum) (Shrotriya et al., 2006; Kumar et al., 2015). A usual I-V (current-voltage) curve of the solar cell in dark and under the light are displayed in Figure 3.2(a) together with the structures of typical PV cells (Figure 3.2(b) and (c)). In dark, there is nearly no current till the forward bias for the voltages is larger as compared to the VOC (open circuit voltage). Under the light, the SCs start producing power which is measured with the source meter (Ameri et al., 2009; Roncali, 2009). For categorization of any solar cell, the main parameter is the PCE is the ratio of maximum Pm (electrical power) produced by the device to total Pin (incident optical power) (Eqn. (1)). The pin is generally given by spectral intensity corresponding to the intensity of the sun on Earth at an angle of nearly 48.2° (Monestier et al., 2007; Jun et al., 2013).

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Figure 3.1: Histogram displaying the number of scientific publications backing the subject “OSCs (organic solar cells)” by year. The era was 2010 to 2017 and the codeword was “organic small molecule solar cells.” This search was made through ISI, web of science. Source: https://www.sciencedirect.com/science/article/abs/pii/ S1566119918301551.

(1)

Figure 3.2: (a) A usual I-V J-V characteristics of the solar cells; (b) standard structural design of BHJ (bulk-heterojunction); and (c) inverted structure (Mishra & Bäuerle, 2012). Source: https://pubmed.ncbi.nlm.nih.gov/22344682/.

The term Voc (open circuit voltage) in Eqn. (1) is described as the voltage potential at which the current is 0 and depends on LUMO and HOMO energy level variation of the acceptor (A) and donor (D). Thus, the opencircuit voltage of the solar cell can be amplified either by rising the LUMO level of the acceptor material or decreasing the HOMO level of the donor

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81

(Mazzio and Luscombe, 2014; Khan et al., 2016). The second term in Eqn. (1) Jsc is the maximum produced photocurrent density. With the reduction of the band-gap of material, the value of Jsc upsurges and can be disturbed by the hole and electron transport effectiveness of the vigorous material (Gadisa et al., 2004; Burkhard et al., 2010). The third term is the FF (fill factor) and is described as the ratio of perceived maximum output power to the theoretical output power. Maximum output power is given as Pm (= Im × Vm), and theoretical output power is the product of Voc and Jsc. Thus, the FF can be given by Eqn. (2). (2) The FF recommends how quickly the charges can be eliminated from the cells, and in an ideal situation, the value is one. Several factors can disturb the FF of SCs and they frequently interact in difficult ways (Koster et al., 2005; Vandewal et al., 2009). The Rs (series resistance), Rsh (parallel resistance) are two other significant factors that disturb the FF and performance of SCs. The physical phenomena controlling Jsc and Voc have been documented well. Though, factors distressing the FF are still not understood so clearly (Mishra and Bäuerle, 2012; Bartesaghi, 2015).

3.3. SMDMS-CENTERED PHOTOVOLTAICS (PVS) Various reviews have reported the development made on SMDMs-centered PV cells. Recent articles with reasonable to high PCE are reviewed here (Lin et al., 2012).

3.4. OLIGOTHIOPHENE-CENTERED SMDMS During the survey of the literature, it was observed that the majority of the SMDMs are centered on electron-abundant thiophene units, bracing the significance of heterocyclic core in materials science. Sulfur comprising heterocycles like simple thiophene, non-fused, and fused oligothiophenes, and their byproducts are usually engaged as the electron-donor units (Huang and Huang, 2014; Ostroverkhova, 2016). The BHJ (bulk-heterojunction) solar cell parameters and electronic properties of some currently reported oligothiophene-centered SMDMs are described in Table 3.1. The outcomes specify that structurally easy SMDM molecules are possible contenders as a donor for accomplishing highly efficient organic SCs. Liu et al. (2012) stated an oligo-thiophene (n = 7) centered small-donor 1 (Figure 3.3) made up of

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3-octylthiophene as the dominant donor adorned with 3-ethylrhodanine at the edge. Table 3.1: Description of Frontier Levels of Energy, Performance Parameters, and Device Structure of the Oligothiophene-Centered SMDMs (Huang and Huang, 2014; Wang et al., 2016; Zhang et al., 2017) Entry Device Architecture

HOMO/LUMO Jsc (mA/ PCE (%) (eV) cm2)

VOC (V)

FF (%)

1

ITO/PEDOT:PSS/1:PC61BM/LiF/Al

−5.00/−3.28

13.98

6.10

0.92

47.4

2

ITO/PEDOT:PSS/3:PC71BM/LiF/Al

−5.28/–2.80

11.03

7.14

0.93

69.6

3

ITO/PEDOT:PSS/2:PC71BM/LiF/Al

−5.18/–2.68

10.26

5.89

0.86

66.8

4a

ITO/PEDOT:PSS/4a/Ca/Al

−5.52/–3.23

1.37

0.41

0.62

48.2

4b

ITO/PEDOT:PSS/4b/Ca/Al

−5.24/–3.61

5.32

2.16

0.82

49.2

4c

ITO/PEDOT:PSS/4c/Ca/Al

−5.32/–3.63

4.00

1.69

0.83

50.9

4d

ITO/PEDOT:PSS/4d/Ca/Al

−5.22/–3.69

6.59

2.40

0.78

46.9

4e

ITO/PEDOT:PSS/4e/Ca/Al

−5.16/–3.57

3.63

1.57

0.76

57.0

5

Glass/ITO/MoO3/6:PC71BM/LiF/Al

−5.10/−3.45

12.98

5.04

0.79

49.0

6

Glass/ITO/PEDOT:PSS/5/PrC60MA/Al

−5.12/−3.19

14.59

9.23

0.96

66.0

7a

Glass/ITO/PEDOT:PSS/7a:PC61BM/Al

−5.38/−3.32

7.93

4.87

0.97

64.1

7b

Glass/ITO/PEDOT:PSS/7b:PC61BM/Al

−5.25/−3.25

1.36

0.39

0.90

32.7

7c

Glass/ITO/PEDOT:PSS/7c:PC61BM/Al

−5.31/−3.43

1.50

0.41

1.00

27.5

8a

ITO/PEDOT:PSS/8a:PC71BM/PDINO/Al

−5.38/−3.71

12.88

6.68

0.79

65.8

8b

ITO/PEDOT:PSS/8b:PC71BM/PDINO/Al

−5.25/−3.61

10.08

4.33

0.88

48.9

8c

ITO/PEDOT:PSS/8c:PC71BM/PDINO/Al

−5.15/−3.68

7.21

3.30

0.87

52.5

Abbreviations: PDINO: perylenediimide N-oxide; PEDOT:PSS: poly(3,4ethylenedioxythiophene): polystyrene sulfonate. Ilmi et al. (2018) reported that the electron-removing nature of 3-ethylrhodanine made a strong D-A (donor-acceptor) interaction inside the oligomer, triggering absorption in the visible region having a low bandgap (LBG) of nearly 1.72 eV. Oligomer 1 formed a high-quality thin film with the help of the solution processing method and exhibited broad absorption in the visible region (450 to 750 nm) with λmax = 618 nanometer having a red-shift of almost 110 nanometers as compared to the parent molecule 1 in CHCL3 (chloroform). Utilizing PC61BM as an acceptor and as a donor (Figure 3.3), Ilmi et al. (2018) accomplished a high PCE (6.10%) with Jsc of 13.98 mAcm– 2 and Voc of 0.92 V. An escalation in the number of thiophene units in the central core (n = 8) with an extension of the alkyl chain at termini (Figure 3.3) didn’t affect the PV performance quite much. However, an important enhancement in the PCE and some other parameters were observed upon fluorination of the central thiophenes (Figure 3.3). Both of the SMDs showed

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wide absorption with very good thermal stability; though, the fluorinated analog had a relatively deeper HOMO level of energy (Wang et al., 2016). The fluorination of the central unit encourages enhanced planarity in SMD 3 than the non-fluorinated SMD 2 causing improved carrier mobility and molecular packing. Under improved conditions, an inverted PV cell centered on 3 gave the PCE of 7.14%. Liang et al. (2017) made and developed D2A-D1-A-D2 kind of SMDs (4a–e, Figure 3.3) and studied systematically the influence of quantity of the thiophene units on PCE of the invented device (Table 3.1). They decided that the SMDs having an odd number (1,3,5.) of thiophene units have higher PCE as compared to the SMDs having an even number (2,4,6….) of thiophene units. Zhang et al. (2017) made and developed a new SMD 5 (Figure 3.3) made up of the planar electronremoving 1,3-bis(4-(2-ethylhexyl)-thiophene-2-yl)-5,7-bis(2-ethylhexyl) benzo[1,2-c:4,5-c’]-dithiophene-4,8-dione unit as the fundamental core. After TA (thermal annealing) and SVA (solvent vapor annealing), a PCE of 9.35% was accomplished having high values of Jsc, FF, and Voc (Table 3.1). The high value of open-circuit voltage was accredited to a low HOMO level of energy whereas high FF and Jsc values were accredited to the improved absorption of the blend films, good charge mobilities, and finetuned morphology. These outcomes recommend that an electron-removing central unit having a large planar structure and acceptor-donor-acceptor (A-D-A) configuration is a proficient way to accomplish high-performance PV cells. Hong et al. (2017) made and developed a new SMD 6 (Figure 3.3), having dithieno[3,2-b:2’,3’-d]phosphole oxide as the fundamental core and 3-ethylrhodanine as the termini parted by alkyl terthiophene. The molecule of DTP forms a rare hyper conjugated ring with firm pyramidal geometry, which generally lowers the level of energy of LUMO and thus makes them a strong acceptor of the electron (Ren and Baumgartner, 2012; Park et al., 2015). The absorption spectrum of the SMD 6 in thin-film and solution displayed a wide absorption range (300 to 750 nanometer) with maxima at nearly 516 nanometer, which was 78 nanometer red-shifted in the thin film because of the intermolecular π-π stacking (Figure 3.4). SMD 6 displayed an emission peak at nearly 724 nanometers which were considerably quenched (97%) in the 6:PC71BM blend film. The BHJ device centered on 6 displayed a PCE value of 5.04% with Jsc of 12.98 mAcm–2 and Voc of almost 0.79 V. These favorable values of PCE were accomplished without any post- or pretreatments and recommending that DTP-centered small molecule can also be a favorable candidate for PV application.

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Chen et al. (2017) evaluated BHJ-SCs centered on cis-stilbene without or with a spiro linker, acceptor end groups, and terthiophene arms 7(a–c) (Figure 3.3). They stated that BHJ made up of spiro comprising molecule 7a displayed improved light-harvesting ability as compared to 7b. The device centered on 7a and the PC71BM with DIO (1,8-diiodooctane) additive displayed optimal and uniform domains with improved percolation length in the film causing PCE of 4.87% and a high FF of 64.1%. Along with a comparable line, Wang et al. (2017) made and developed 3 A-π-

Figure 3.3: Oligothiophene-centered SMDMs. Source: https://www.sciencedirect.com/science/article/abs/pii/ S1566119918301551.

Figure 3.4: (a) Normalized ultra-violet-vis-near-infrared absorption spectra of the 6 in chloroform solution or as the thin film; (b) photoluminescence spectra of pure 6 and the 6:PC71BM blend films. Source: https://daneshyari.com/article/preview/6469462.pdf.

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D-π-A SMD 8 with STF (spiro[cyclopenta[1,2-b:5,4-b’]dithiophene4,9’fluorene]) as the fundamental donor core. The SMDs 8(a to c) (Figure 3.3), exhibited broad absorption bands (300 to 850 nm) having high molar absorption coefficients (4.82 × 104–7.56 × 104 M−1 cm−1) and comparatively low HOMO energy levels (–5.15 – –5.38 eV). The enhanced organic SCs centered on these molecules provide power conversion efficiencies of 6.68%, 3.30%, and 4.33% for 8a, b, and c, correspondingly. The higher PCE of 8a-centered organic SCs was accredited to its enhanced absorption ability, balanced, and higher charge mobilities, and better active layer morphology. This was the main and first instance of evolving the A-π-D-π-A kind small molecules with the spiro-fundamental donor core for very high-performance organic SCs applications.

3.5. OLIGOTHIOPHENE-BDT (BENZODITHIOPHENE) HYBRIDS AS SMDMS Furthermore, for oligothiophenes and thiophene-centered SMDMs, various BDT (benzodithiophene) centered hybrid materials have been examined currently (Figure 3.5). Table 3.2 displays the performance of OPV centered on BDT hybrids. When fluorinated the 2,2’-bithiophene unit was substituted by BDT and 3-ethylrhodanine by the cyano functionalities, the consequential SMD 9 (Figure 3.5) produced a PCE of 4.56% in an amalgamation of PC61BM as an acceptor (Wang et al., 2017; Zhou et al., 2012). Fascinatingly, when 3-ethylrhodanine was connected back to the SMD 10 (Figure 3.5), it triggered a sharp upsurge in PCE (6.38%) and a substantial rise in the current density (Jsc = 10.78 mA/cm2). The improved performance was accredited to the superior light absorption of 10 as compared to the 9. Moreover, when an acceptor was transformed to PC71BM, the device centered on 9 gave a considerably low efficiency along with current density (PCE = 2.09%, Jsc = 3.74 mAcm–2) whereas the device centered on 10 produced an enhanced PCE (6.92%) and Jsc (11.40 mAcm–2). All these PV parameters reached the maximum values (Jsc = 12.21 mAcm–2, PCE = 7.38%, Voc = 0.93 V,) by addition of the trimethylsiloxyl terminated PDMs (polydimethylsiloxane) during the process of film-forming. These outcomes display that the acceptor along with some other additives affects the performance of the device. Studies exhibited that the substitution of alkoxy group over BDT by thioether and alkyl also enhances the performance of the device (Kan et al., 2014; Ni et al., 2014). For instance, utilizing oligomer 11 (Figure 3.5), a PCE of about 8.26% was stated, whereas 12 (Figure 3.5) centered

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devices gave the maximum PCE of 9.95%. It is useful to observe that the improved performance couldn’t be accredited to only the structure alteration but also the absorption profile, structure of the active layer, and morphology. The institution of an additional alkylthio-thiophene as the side chains on the BDT unit (13, Figure 3.5) occasioned a PCE of 9.20% without any kind of processing (Cui et al., 2015). Fascinatingly, 13:PC71BM centered device having an active area of nearly 14.4 cm2 produced PCE of 6.68%. A slight upsurge in performance was reported for the devices centered on the donor 14 (9.6%) and 15 (9.3%) (Figure 3.6) bearing more diverged side chains. The 14-centered device also gave a high FF (70%) and PCE (8%) at active layer thicknesses around 400 nm (Sun et al., 2019; Li et al., 2015). To commercialize organic SCs, the effective demonstration of PV devices through printing techniques is necessary to smear them into the process of roll-to-roll (R2R). To validate the influence of the R2R wellsuited method, Heo et al. (2017) utilized small molecule 15 and developed three kinds of devices: • •

As cast films without the treatment; Blend films following the solvent vapor annealing (SVA) treatment; and • Blend films treated with a DPE (diphenyl ether) additive slotdie produced devices with the standard structure of ITO/PEDOT: PSS/15:PC71BM/Ca/Al. The device (1) without the treatment exhibited a PCE of 3.81%. The device (2) displayed an improved PCE of 7.46%, as stated earlier (Sun et al., 2019). Fascinatingly, DPE-treated devices also displayed high PCE (6.56%), because of a substantial increase in Jsc and FF, although there were trivial drops in Voc. Moreover, the authors also accomplished a PCE of 4.80% utilizing large-area (10 cm2) PV modules (Figure 3.4). These outcomes indicate the prospective of SMDMs in the large-scale fabrication of organic SCs through R2R well-suited printing techniques without utilizing a halogenated solvent.

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Badgujar et al. (2016) developed (16 and 17, Figure 3.5) and carried out a relative study of the performance of the device by upsurging the quantity of the BDT units in the SMD core. They discovered that the upsurge in the quantity of BDT units in molecule 17 gives an upsurge to strong intermolecular interactions, which encouraged the anticipated interconnected structure and thus, improved free-charge carrier transport and exciton diffusion as compared to the 16. The device centered on 17 exhibited PCE of 8.56% with a high FF (73%) under the AM 1.5 G irradiation (100 mWcm–2). As this PCE is accomplished without the help of any additive, the outcome is unambiguously significant for the high fabrication and reproducibility of the large-area modules. The device’s performance centered on 17 having an area of nearly 777.5 cm2 exhibited a PCE of 7.45% compared to 4.5% as displayed by 16 (Figure 3.7(a)). Incomparable aspect, Guo et al. (2017) made and fabricated two unsymmetrical BDT core dimers 19 and monomer 18 (Figure 3.5). The dimer was linked by the octamethylene spacer. The device centered on monomer 18 displayed a higher PCE (8.18%) as compared to dimer 19 (7.07%). The lower PCE of the dimer was accused of lowering the Jsc (10.88 mAcm–2), which in turn is associated with the reduced absorption profile (Figure 3.7(b)), low internal quantum effectiveness, and the photoinduced charge relocation in the dimer. In a new very motivating study Deng et al. (2016) joint the customary molecular design with fluorination. They made and fabricated 3 SMDs (20ac, Figure 3.5) with an increasing number of Fl atoms, 20a, b, and c. The molecules designed exhibited optimum morphology, which is significant for charge collection, charge transfer, and recombination suppression causing reduced loss of Jsc, FF, and Voc instantaneously. As an outcome, fluorinated molecules displayed outstanding inverted device performance and an average PCE of 11.08%, and FF 76.0% was accomplished with 20c having 2 atoms of fluorine (Eastham et al., 2017).

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Figure 3.5: Chemical structures of BDT(benzodithiophene) hybrids utilized as SMDMs. Source: https://www.sciencedirect.com/science/article/abs/pii/ S1566119918301551.

With quite a comparable strategy, Eastham et al. (2017) developed 2 new small molecules centered on BDT-DPP2 (diketopyrrolopyrrolebenzodithiophene-diketopyrrolopyrrole) skeleton without and with fluorine (Fl) on the scented side chains 21b and 21a (Figure 3.5). The ternary solar cell with changing ratios of 2 SMDs and PC61BM outcomes in tunable opencircuit voltage (0.833 to 0.944 V) because of the fluorination-induced shift

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in the levels of energy and the electronic alloy created from the mixing of the 2 SMDs. A 15% upsurge in PCE is noted at the optimum ternary SMD ratio, with considerably increased current density (9.18 mAcm–2), because of the amplified optical absorption of a blend. Ilmi et al. (2018) developed and studied the influence of fluorination on PV performances of 2 small D1-A-D2-A-D1 kind skeleton with EHDTP,D1 (2,4-bis(2-ethylhexyl)-4Hdithieno[3,2-b:2’,3’-d] pyrrole and OBDT,D2 (4,8-bis((2-ethylhexyl)oxy) benzo[1,2-b:4,5-b’] dithiophene) as the terminal and fundamental donor, and the benzo [c][1,2,5] thiadiazole (22-a) and 5,6-difluorobenzo[c][1,2,5] thiadiazole (22-b) (Figure 3.5). The synthesized BHJ-SCs with 2-step annealed active layers (ALs) of the 22a and 22b exhibited an overall PCE of 5.46% and 7.91%, respectively. The better performance of the 22b centered device was credited to the promising morphology and the nanoscale interpenetrating network in the 22b: PC71BM active layer (Ji et al., 2017). Table 3.2: Description of Frontier Levels of Energy, Performance Parameters, and Device Structure of Oligothiophene-BDT (Benzodithiophene) Hybrids Entry

Device Architecture

HOMO/ LUMO (eV)

Jsc (mA/ cm2)

PCE (%)

VOC (V)

FF (%)

9

Glass/ITO/PEDOT-PSS/9/LiF/Al

10

ITO/(PEDOT-PSS)/11:PC71BM/ ZnO/Al

−5.04/–3.24

3.74

2.09

0.93

60.1

−5.08/–3.27

12.56

8.26

0.94

70.0

11 12

Glass/ITO/PEDOT-PSS/10/LiF/Al

−5.02/–3.27

12.21

7.38

0.93

65.0

ITO/PEDOT:PSS/13:PC71BM/Ca/Al

−5.18–3.25

13.45

9.20

0.97

70.5

13

ITO/PEDOT:PSS/12:PC71BM/ETL1/Al

−5.07/–3.30

14.45

9.60

0.91

73.0

15

ITO/PEDOT:PSS/17: PC71BM/Ca/ Al

−5.13/–3.37

13.17

8.56

0.89

73.0

16

ITO/PEDOT:PSS/16: PC71BM/Ca/ Al

−5.14/–3.37

13.02

7.18

0.89

62.0

17

ITO/PSS:PEDOT/19:PC71BM/ ETL-1/Al

−5.02/–3.30

10.62

7.07

0.86

72.0

18

ITO/PSS:PEDOT/18:PC71BM/ ETL-1/Al

−4.94/–3.30

12.54

8.18

0.87

74.0

20a

ITO/ZnO/20a/MoOx/Ag

−4.91/–3.20

14.0

8.3

0.93

64.0

20b

ITO/ZnO/20b/MoOx/Ag

−4.98/–3.28

15.3

10.4

0.94

72.0

20c

ITO/ZnO/20c/MoOx/Ag

−5.05/–3.37

15.7

11.3

0.95

76.0

21a

ITO/PEDOT:PSS/21a: PC71BM/ LiF/Al

−5.36/–3.66

8.36

4.15

0.83

59.6

21b

ITO/PEDOT:PSS/21b: PC71BM/ LiF/Al

−5.47/–3.75

7.81

4.26

0.94

57.8

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22a

ITO/PEDOT:PSS/22a: PC71BM/ PFN/Al

−5.55/–3.65

10.52

5.46

0.91

57.0

22b

ITO/PEDOT:PSS/22b: PC71BM/ PFN/Al

−5.66/–3.69

12.23

7.91

0.98

66.0

23a

ITO/PEDOT:PSS/23a: PC71BM/ Ca/Al

−5.05/−2.88

14.31

9.37

0.95

68.9

23b

ITO/PEDOT:PSS/23b: PC71BM/ Ca/Al

−5.06/−2.88

14.92

10.78

0.96

75.3

23c

ITO/PEDOT:PSS/23c: PC71BM/ Ca/Al

−5.06/−2.89

13.85

8.55

0.98

63.1

24

ITO/PEDOT:PSS/24:PC71BM/Ca/Al

−5.06/−3.45

13.33

8.17

0.90

67.0

25

Ternary

−5.38/−3.61

15.44

10.3

0.90

73.8

25

Ternary

−5.38/−3.61

21.4

11.40

0.75

70.0

26

Ternary

−5.29/−3.27

14.52

7.77

0.77

70.3

27

ITO/MoO3/27:IC-C6IDT-IC/Al

−5.29/−3.34

14.25

9.08

0.98

65.0

28

ITO/PEDOT:PSS/29:IDIC/PDINO/ Al

−5.04/−2.70

10.77

5.32

0.76

64.40

29

ITO/PEDOT:PSS/28:IDIC/PDINO/ Al

−5.24/−2.82

15.18

10.11

0.90

73.55

30

ITO/PEDOT:PSS/31:IDIC:PDINO/ Al

−5.28/−3.01

10.51

5.51

0.955

54.89

31

ITO/PEDOT:PSS/30:IDIC:PDINO/ Al

−5.31/−3.03

15.21

9.73

0.977

65.46

Abbreviations: PFN: poly(9,9-bis(3’-(N,N-dimethylamino)propyl); PEDOT:PSS: poly(3,4-ethylenedioxythiophene): polystyrene sulfonate, PDINO: perylenediimide N-oxide.

Figure 3.6: Chemical structures of IDT(Indacenodithiophene)-centered SMDMs. Source: https://www.sciencedirect.com/science/article/abs/pii/ S1566119918301551.

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Figure 3.7: (a) Photographic picture of the slot-die-covered large area PV modules during the process of the printing process; (b) conforming J-V curves. Source: https://pubs.acs.org/doi/10.1021/acsami.7b12420.

This was encouraged by the atoms of fluorine (Fl) on the BT acceptor, which considerably improved the separation of excitons, charge collection efficiency, and the charge transport, and repressed bimolecular reconnected in the BHJ. The noted higher PCE specified 22b is among the best BTcentered donor materials for the small molecular BHJ-SCs. Ji et al. (2017) examined the influence of molecular structure, for instance, halogenation on properties and subsequently on the PV performance. They stated 3 new SMDs (23 a to c, Figure 3.5) integrating BDT fundamental core having different atoms of halogen (chlorine, bromine, and fluorine) and the 3-ethylrhodanine as terminal groups. Because of the existence of fluorine (Fl) atom in the SMD, film made up of 23a: PC71BM exhibited a high extent of crystallinity and a poor phase of the blend film. Substituting fluorine with bromine (23c) and chlorine (23b) triggered decreased crystallinity; though, strong aggregations were efficiently suppressed. The invented device with the Chlorine-bearing SM (23-b) displayed the best PCE value of 10.78% (Figure 3.6), Jsc of 14.92 mAcm–2, a FF of 75.3%, and Voc of 0.96 V. To disclose the structure-property association of the scented side-chain in replaced IDT-centered SMDMs as the function of π-bridge and postannealing circumstances, Liu et al. (2017) stated a molecule 24 (Figure 3.5) which displayed a PCE of 8.17%, Jsc of 13.33 mAcm–2, an FF of 67% and Voc of 0.904 V after SVA (Lu et al., 2015; An et al., 2016). The making of ternary organic SCs has gained much interest because the absorption range deprived of the utilization of complex tandem cell structures. Zhang et al. (2017) developed organic SCs of architecture

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25:PC71BM with a PCE of 9.37%. To fabricate the ternary organic solar cell (Figure 3.9(a)), they utilized the small organic molecule DIB-SQ as the 3rd constituent in the host 25:PC71BM. The invented device displays an incredibly high PCE of 10.3%. This improvement in PCE is because of the increased JSC (15.44 mAcm–2) and FF (73.8%), which was credited to the enhanced photon harvesting of the active layer, improved transfer of energy from 25 to the DIB-SQ (Figure 3.9(b)) and the molecular packing for effective exciton dissociation and the charge transport. Likewise, Huang and hisco-workers (2017), developed another ternary organic solar cell with a similar nematic liquid crystalline SMDM 25 but changed polymer donor such as PTB7-Th: PC71BM (Figure 3.5). The institution of 25 molecules into the binary system enhanced the morphology of blend film, reduced π-π stacking distance, improved domain purity, and enlarged coherence length (Ameri et al., 2013). These changes led to effective charge separation, lower bimolecular recombination, and faster charge transport, causing PCE of 11.40% even with the thick active layer (250 nanometers). Zhu et al. (2017) made and developed a 26 SMD (Figure 3.5) and a binary device centered on it exhibited a PCE of 5.71% with the acceptor PC71BM. Moreover, ternary organic SCs made up by doping 10 wt.% of 26 in the PTB7-Th: PC71BM, enhanced charge mobility, better phase separation, and decreased resistance. This ternary device exhibited a PCE of 7.77% with a JSC 14.52 mAcm–2, a FF of 70.3%, and a Voc 0.77 V. Yang et al. (2017) have made and developed a new WBG (wide bandgap) SMD 27 (Figure 3.5) by integrating a 2-D trialkyl thienyl replaced BDT core., The subsequent material has a bandgap of 2.0 eV with a low-lying HOMO energy level of −5.51 eV. They made the NFM-OSC (non-fullerene smallmolecule organic solar cell) by engaging IDIC as the acceptor (Figure 3.5). The device exhibited 9.08% PCE with a high Voc (0.98 V). This outcome determines that this molecular design of the WBG donor to create the wellmatched D-A pair with an LBG non-fullerene small-molecule acceptor with the delicate morphological control offers great potential to comprehend high-performance NFSM-OSCs. In a similar struggle, Qui et al. (2017) made and developed two WBGs A-D-A SMDs bearing electron-removing ester end group with CN (cyano) group 28 and without CN group 29 and utilized acceptor molecule LA-1

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(Figure 3.5). The existence of the CN group on 28 provides the molecule with stronger absorption, lower-lying HOMO level of energy, and higher chargemobility as compared to SMD 29. A film centered on 28 and 29 displayed λmax at 566 and 521 nanometers which were normally red-shifted by 66 and 55 nanometers compared to a solution. The device based on 28 exhibited a PCE of 10.11% and a high FF (73.55%) compared to molecule 29, which displays a PCE of 5.32%. The outcomes specify that the cyano replacement in 28 generally plays a vital role in enhancing the PV performance of NFSMOSC. Bin et al. (2017) made and developed MBG (medium bandgap) SMD 30 (Figure 3.5) with BDTT (bithienyl-benzodithiophene) as a fundamental donor unit and fluorobenzotriazole as an acceptor unit. They fabricated the control molecule 31 (Figure 3.5) without thiophene-associated side chains on BDT. The device centered on 30 as a donor and IDIC as an acceptor exhibited a better PCE of 9.73% as compared to the device centered on 31 with a similar configuration. The advantage of 30-centered devices was credited to intense absorption, higher hole mobility, low-lying HOMO level of energy, and well-ordered bimodal crystallite packing in blend films (Figures 3.8–3.10).

Figure 3.8: (a) Representative J-V curves for enhanced 16: PC71BM and 17:PC71BM large, firm-module devices under the simulated AM 1.5 G irradiation; (b) ultraviolet-Vis absorption spectra of 19 and 18 in dilute CHCL3 (chloroform) solutions. Source: https://pubs.acs.org/doi/full/10.1021/acs. chemmater.7b00642?src=recsys.

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Figure 3.9: (a) Transmission electron microscopy (TEM) pictures of TSA treated 22b:PC71BM and 22a:PC71BM thin films; bar is nearly 100 nanometer; and (b) Enhanced current density-voltage curves for SM-OSCs centered on 23a-c. Source: https://www.x-mol.com/paper/392615.

Figure 3.10: (a) Graphic representation of the ternary active layer; (b) levels of the energy of the utilized materials (arrows signify the direction of charge carrier transport and lightning bolt signifies a transfer of energy from 25 to the DIB-SQ. Source: https://www.sciencedirect.com/science/article/abs/pii/ S2211285517304573.

Not like the performance parameters Jsc and Voc, optimization of the FF isn’t so evidently understood to envisage high PV performance from the OPV cell. To optimize the FF, Aldrich et al. (2017) made and developed a series of very small molecules by integrating the chalcogen (oxygen, selenium, and sulfur) into the side chains 32 (Figure 3.5). It was observed that SMD having

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a higher atomic number chalcogen exhibits an improved FF value. They discovered a considerably high FF ∼of 8%, which upsurged on moving from oxygen through sulfur to selenium across the entire series of SMDs. This substantial improvement in the FF was because of the amalgamation of more well-ordered morphology and reduced charge recombination in the blend films for high-atomic number chalcogen SMDs. Likewise, Wang et al. (2017a, b) made-up SMD 33 (Figure 3.5), D2-A-D1-A-D2, where the D1 an alkylthienyl replaced the BDT unit, A signifies the tetrazine unit, and D2 is the terthiophene or bithiophene ending donor unit. The outcomes of the PV study displayed that extension of key chain π-conjugation improved device performance (FF = 65.3% and Voc = 1.03 V, with PCE = 6.49%), whereas in side-chain causes larger absorption coefficients, more promising blend morphology, and lower HUMO levels. Wan et al. (2017) made and developed two SMDs 34a and 34b (Figure 3.5) with BDT (benzo[1,2-b:4,5-b’]dithiophene) as the fundamental donor unit and the electron-deficient NT (naphtho[1,2-c:5,6-c’]bis[1,2,5]thiadiazole) group. This prolonged the π-conjugation length of the entire small molecular backbone. The familiarization of sulfur (S) atom inside chains further decreased the LUMO/HOMO energy levels and consequential in the higher Voc (0.93 V) with quite low energy loss (0.57 V) for a device 20. Centered on device engineering and rational material design, the consequential SMOSCs treated with the halogen-free CS2 solvent displayed a high efficiency of around 11.53% with a very small Eloss of 0.57 eV.

3.6. IDT (INDACENODITHIOPHENE)-CENTERED SMDMS Indacenodithiophene (IDT) is another interesting class of fragments normally known for strong PV potential. Table 3.3 describes the performance of devices centered on IDT. Wang et al. (2017b) stated that 2 tetrafluorinated novels SMDs 35 and 36 (Figure 3.6), comprise electron-abundant central core (IDT) and electron-scarce difluorobenzothiadiazole as acceptor units, and the donor end-capping groups having different π-bridge (selenophene and thiophene). The π-bridge and fundamental core units in the SMDs played a vital role in the creation of the nanoscale split-up of blend films. The existence of the electron-abundant selenophene spacer in an SMD 36 has a noticeable influence on the absorption spectrum and the molar extinction coefficients, which display a redshift of nearly 20 nanometers as compared to the SMD 35 (554 nanometers). The device centered on

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selenophene spacer for instance., 36, displayed better PCE (7.31%) with FF 70% compared to the 35 (PCE 5.73% and FF 58%) after annealing because of the augmented morphology and enhanced charge transport. With the comparable strategy of utilizing the electron-abundant selenium, Wang et al. (2017c) have developed 2 mixed selenium and sulfur-based on small molecules 37b and 37a (Figure 3.6), with the sulfur and selenium atoms in diverse positions. Fascinatingly, the BHJ-Organic Solar Cell device centered on molecule 37b exhibited a PCE of 9.3%, which is the maximum efficiency centered on the donor end-capped oligomers. The outcomes validate that the sequence of π-bridge and hardening treatments play vital roles in enhancing crystalline and ordered morphology and improved PCE, and thus provide a beneficial strategy toward highly effective SMD for BHJOrganic SCs. Despite the point that SM-Organic SCs have received substantial attention, the main factor that restricts the performance of the small molecules is the large loss of energy (Eloss 0.6 and 1.0 eV) as compared to inorganic and perovskite SCs (Eloss less than 0.5 eV). In this context, Yang et al. (2017) developed a novel A-D-A kind dimeric squarine with a very small donor molecule 38 (Figure 3.6). The solution of molecule exhibited strong absorption which bounces to the Near-Infrared region (600 to 750 nm) with the maximum molar extinction coefficient of around 2.84 × 105/Mcm at 705 nanometers because of the efficient delocalization of π-electron amongst their essential units. The BHJ device made-up of the LBG (low band-gap) materials (Eg° = 1.49 eV) showed high PCE (7.05%), low Eloss (0.56 eV), and high Voc (0.93 V). The outcome recommends that the A-D-A-structured dimeric molecular approach can be utilized as an efficient way to attain high Voc and low Eloss, thus accomplishing enhanced performance. Table 3.3. Description of Frontier Levels of Energy, Performance Parameters, and Device Structure of the IDT (Indacenodithiophene)-Centered SMDMs Entry

Device Architecture

HOMO/LUMO (eV)

Jsc (mA/ cm2)

PCE (%)

VOC (V)

FF (%)

35

ITO/PEDOT:PSS/35:PC71BM/ PFN/Al

−5.43/−3.65

11.17

5.45

0.85

58.0

36

ITO/PEDOT:PSS/36:PC71BM/ PFN/Al

−5.41/−3.67

12.62

7.16

0.82

69.0

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37a

ITO/ PEDOT:PSS/37a:PC71BM/ PFN/Al

−5.24/−3.19

11.23

6.44

0.85

67.0

37b

ITO/ PEDOT:PSS/37b:PC71BM/ PFN/Al

−5.28/−3.22

14.30

9.26

0.87

72.0

Abbreviations: PFN: poly(9,9-bis(3’-(N,N-dimethylamino)propyl)fluorene2,7-diyl)-alt-(9,9-dioctyl-fluorene-2,7-diyl); PEDOT:PSS: poly(3,4ethylenedioxythiophene): polystyrene sulfonate.

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CHAPTER

4

ACCEPTORS MATERIALS FOR ORGANIC SOLAR CELLS

CONTENTS 4.1. Introduction..................................................................................... 106 4.2. Rylene Diimide-Centered Polymer Acceptors.................................. 109 4.3. Fluorene and Bt-Centered Polymer Acceptors.................................. 120 4.4. CN-Replaced Polymer Acceptors..................................................... 121 4.5. Other Polymer Acceptors Comprising Electron-Removing Units....... 125 4.6. Summary......................................................................................... 127 References.............................................................................................. 129

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4.1. INTRODUCTION This chapter gives a present status summary of the several n-type of polymer acceptors for utilization as the active materials in OPV (organic photovoltaic cells). Generally, the polymer acceptors are split into 4 categories. The first part of this chapter emphasizes the rylene diimide-based polymers, comprising perylene diimide (PDI), dithienocoronene diimide (DTCDI)centered polymers, and naphthalene diimide (NDI). The good stability and high electron mobility of the rylene diimides make them appropriate for utilization as polymer acceptors in OPV cells (Heliatek, 2014). The second part deals with Fl (fluorine) and benzothiadiazole (BT)-based polymers like poly(9,9’-dioctylfluorene-co-benzothiadiazole), and the subsequent part emphases the cyano-replaced polymer acceptors. Poly(3cyano-4-hexylthiophene) and cyano-poly(phenylenevinylene) have been utilized as acceptors in OPV cells and display high electron affinity coming from the electron-removing cyano (CN) groups in the vinylene group of the poly(phenylenevinylene) or thiophene ring of the polythiophene. Lastly, several electron-deficient groups like thiazole, oxadiazole, and diketopyrrolopyrrole have been introduced onto the polymer backbones to tempt n-type features in the polymer. To the first report in 1995 on the allpolymer solar cells (SCs), the best power transformation efficiency attained with these devices to date has been nearly 3.45%. The general inclination in the growth of the n-type polymer acceptors is given in this chapter (Parida et al., 2011; Dou et al., 2013). The drive towards the conservation of energy has fueled rigorous research into the expansion of alternative sources of energy. Solar energy provides the benefits of being clean and renewable, therefore making SCs striking as the prospective alternative source of energy. PV (Photovoltaic) cells centered on the inorganic materials are presently the major commercially utilized devices due to their comparatively high efficiencies (for instance., 15 to 20% for silicon-centered PVs); though, these devices are restricted by the high production cost and associated environmental concerns (Darling and You, 2012; Yue et al., 2012). Thus, OPV (OPV cells), which provide the benefits of comparatively low production cost, flexibility, and easy processing, have gained emphasis despite their comparatively low efficiencies (Boudreault et al., 2010; Liao et al., 2013). The growth of OPV cells has progressed quickly with the combination of novel organic materials, command of the processing condition like annealing, and the utilization of additives, along with the introduction of several device structures like the inverted and tandem structure (Chen et al., 2009; You et al., 2013). Additionally, command of

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107

the morphology of the active layers (ALs) and growth of the purification by eliminating residual catalysts in the conjugated polymers have been considered significant problems to accomplish high-performance, consistent OPV cells. Presently, the highest PCE (power conversion efficiency) of 12% has been reported by Heliatek (2014). Despite the comparatively low PCEs of OPV cells associated with those of the inorganic-centered SCs, the growth of OPV cells is nevertheless quickly centered on the expectation that the several benefits can compensate for the low PCE of OPVs (Chen et al., 2012; Yi and Gong, 2013). OPV cells include an active layer comprising of the organic materials that are squeezed in amongst two electrodes with diverse work functions (e.g., Al, and indium tin oxide (ITO) as cathode and anode, correspondingly), and interfacial (electron/hole transporting) layers can normally be added amongst the active layer and both electrodes (Tang, 1986; Nikiforov et al., 2013). The ALs in OPV cells are generally composed of 2 electron acceptor (A) and electron donor (D) materials for the production of Coulomb-bound electron-hole pair by the photoexcitation of the donor. The dispersed excitons are then alienated into charges of holes and electrons on the surface of D-A, trailed by free charge carrying and collection at the electrodes. The suitable HOMO (highest occupied molecular orbital), LUMO (lowest unoccupied molecular orbital) level of energy of the acceptors and donors, and the low band-gap are significant for high OPV cell performance, along with good film-forming properties, high charge mobility, and strong absorption ability. OPV cells have been made up of bi-layer and BHJ (bulk-heterojunction) SCs conferring to the configuration of an active layer. The bi-layer OPV cells comprising separate acceptor and donor layers were first described in 1986 by Tang (1986); their performance is restricted by a small chargeproducing interfacial area amongst the acceptor and donor layers (Granstrom et al., 1998; Jenekhe and Yi, 2000). The bulk-heterojunction SCs, made by Yu and Heeger et al. can be produced by meek spin-coating of the blended solution of acceptor and donor, and have a diffused network with the large D-A interfacial area (Yu et al., 1995). Bulk-heterojunction SCs have been widely utilized in the production of high-efficiency OPV cells, and several processing methods have been made to accomplish better film morphology of bulk-heterojunction SCs, like thermal annealing (TA) and utilization of the small quantities of additives (Lee et al., 2008). The material system comprising [6,6]-phenyl-C61 butyric acid methyl ester (PC61BM) and poly(3-hexylthiophene) (P3HT, D1) as a respective electron acceptor and a donor is representative of the active layer in the

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OPV cells (Figure 4.1). In current decades, several polymeric and the smallmolecule electron acceptor and donor materials have been combined and developed to accomplish high-efficiency OPV cells, with particular focus on the growth of polymer donors with a prolonged interconnected system for the solution-processable OPV cells. At the current stage, high power conversion efficiencies of nearly 9.2% have been accomplished by utilizing polymeric donor thieno[3,4-b]thiophene/benzodithiophene (PTB7) having the inverted device structure (Zhicai et al., 2012). The expansion of the donor materials for OPV cells has primarily concentrated on the combinations of low-band-gap interconnected materials made of electron-deficient and electron-rich repeating units (e.g., D-A type) for the effective absorption of the solar spectrum. Centered on this synthetic rule of design, several lowband-gap combined polymers have been produced and engaged as donors in the polymer PV cells (Guo et al., 2013). Maximum building blocks for the electron-abundant units are centered on phenylene or thiophene in the fused form or having bridging atoms for amplified planarity of the polymer backbone and subsequently improved PCE and JSC (short circuit current). Instances of electron-abundant units comprise cyclopenta[2,1b:3,4-b’]dithiophene (CPDT), 5H-dithieno[3,2-b:2’,3’-d]pyran (DTP), and dithieno[3,2-b:2’,3’-d]silole (Zhu et al., 2007; Hou et al., 2008). Several electron-scarce units have normally been copolymerized, and instances of the building blocks for electron-scarce units are given below. The growth of high proficiency small-molecule donors has usually been the emphasis in more current studies, and the high-PCE of 8.12% has been accomplished utilizing donor-acceptor (D-A) type oligothiophenes having strong electronremoving dye units at both ends (Dou et al., 2013; Zhou et al., 2013). To improve the PCE, several small-molecule and polymeric donors have been combined and developed.

Figure 4.1: (a) Illustrative device configuration of OPVs (organic photovoltaic cells); and (b) the molecular structures of PC61BM, P3HT (D1), and PC71BM. Source: https://www.mdpi.com/2073-4360/6/2/382.

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Conversely, fullerene derivatives like PC71BM and PC61BM have been broadly utilized as archetypal acceptor materials for attaining high PCEs in OPV cells due to their better electron mobility as the n-type materials, sufficient band-gaps, and better interaction with the donor materials in OPV cells (Sonar et al., 2011; Lin et al., 2012). Currently, non-fullerene acceptor materials centered on strong electron-removing units, which displayed high electron mobility in the OFET (organic field-effect transistor) applications, have been stated and are debated in the research papers (Qu and Tian, 2012; Kozma and Catellani, 2013). Instances comprise rylene imide, vinazene, diketopyrrolopyrrole, and metallophthalocyanins units. PCEs of nearly 3.45% and 4.03% have correspondingly been accomplished for OPV cell devices engaging small-molecule acceptors and polymer acceptors (Zhang et al., 2013; Cheng et al., 2014). Despite their comparatively low efficiencies, polymer acceptors have unique benefits like the high absorption coefficients in the visible spectral region and the easily tunable levels of energy, compared to non-fullerenes and fullerene small-molecule acceptors (Darling, 2009; Facchetti, 2013). Likewise, the idea of combined BCPs (block copolymers) has been currently introduced to combine the acceptor and donor block into a single macromolecular platform and appeared as a favorable class of materials for OPV cells (Bang et al., 2009; Segalman et al., 2009). A large-scale macroscopic phase parting is hindered in the block copolymer because of the covalent connectivity of 2 blocks and the selfassembly of the block copolymers into mesoscale ordered morphologies is perfect for the active layer of the OPV cells. The performance of nearly 3.1% was accomplished at the current stage (Zhang et al., 2009; Sommer et al., 2009, 2010). Herein, the focus is laid on several polymer acceptors for the all-polymer SCs, which have been seldom reported compared to the small-molecule acceptors. Polymer acceptors are generally categorized into 4 classes based on their structures, i.e., fluorene-, and BT-based polymers, rylene diimidebased polymers, CN (cyano)-replaced polymers, and the other polymer acceptors comprising several electron-removing units (Nakabayashi and Mori, 2012; Guo et al., 2013).

4.2. RYLENE DIIMIDE-CENTERED POLYMER ACCEPTORS Besides their good thermal, photochemical, and chemical stability, rylene diimide-centered polymers also display good electron mobility and high

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electron affinity resulting from the electrons accepting imide groups, therefore making the polymers appropriate for utilization in several electronic fields (Zhou et al., 2013; Sommer, 2014). Here in this part, the rylene diimidecentered polymers utilized as acceptors in OPV cells are summarized. These comprise NDI-, DTCDI-, and PDI-based polymer acceptors (Zhan et al., 2007, 2009).

4.2.1. PDI-Centered Polymer Acceptors The electron-removing PDI cores can normally be replaced in the imide or bay position when copolymerized with several electron-abundant units like DTT (dithienothiophene) and DTP to create electron-accepting polymers (Tan et al., 2008). PDI-centered polymers replaced in the bay position might display good solubility due to the long-branched alkyl (CnH2n+1) chain on imide N-atom. Imide-replacement outcomes in polymers comprising the PDI unit in the backbone or the polymers with suspended PDIs. The device performance parameters and photophysical properties of PDI-centered polymer acceptors are given in Table 4.1. Marder and his co-workers first made the polymer acceptors with a bayreplaced PDI unit. Better solubility was accomplished by familiarizing long and branched alkyl (CnH2n+1) chains onto imide N-atom. In the year 2007, they invented a novel combined polymer (PPDI-DTT, 1, Figure 4.2) having alternating PDI and DTT units that displayed high electron mobility of around 1.3 × 10−2 cm2/Vs, outstanding thermal stability (nearly 410°C), and a quite high electron affinity, with the LUMO energy level of nearly –3.9 eV. The weight Mw (average-molecular weight) of 1 was 15000 with a limited polydispersity index of around 1.5 (Liao et al., 2013). All-polymer SCs were produced by utilizing polymer acceptor 1 and the polymer donor of a polythiophene derivative (D2, Figure 4.3). The bulk-heterojunction device displayed an average PCE of 1% with Voc (open circuit voltage) of nearly 0.63 V, an FF (fill factor) of 0.39, and the JSC of 4.2 mAcm–2. Consequently, they altered the polymer structures with the help of adding extra DTT moieties in the backbones of the polymer, occasioning in polymer acceptors 2 and 3 (Figure 4.2) in which the cores of PDI were bay replaced with 2 and 3 DTT units, correspondingly. The highest PCE was accomplished with polymer acceptor 2 having 2 DTT units in polymers repeating unit when utilizing D3 (Figure 4.3) as the donor, primarily due to the high JSC. These devices were adjusted at the blend ratio of almost 3:1 (donor: acceptor, w/w) and displayed a JSC of nearly 5.02 mAcm–2, VOC of 0.69 V, a FF of 0.43, and the PCE of 1.48% under the simulated AM 1.5 light at 100 mWcm–2.

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Currently, Zheng and his co-workers presented a longer alkyl (CnH2n+1) side chain into polymer acceptor 1, occasioning in polymer 4 (Figure 4.2). They produced BHJ SCs with 2 diverse donors centered on combined sidechain separated polythiophene derivatives (PT4TV (D4) and PT4TV-C (D5), Figure 4.3). Despite the structural similarity of donors, D4 gave a better PCE of 0.99% than accomplished with D5 (0.57%). The higher PCE of D4 was primarily accredited to the good FF (above 0.50) which was accredited to the balanced and high hole/electron mobility of D4:4 blend with the quick transfer of the produced carriers. After adding 10% of chloronaphthalene as the solvent, the PCE of D4:4 was improved from 0.99% to 1.17%. More currently, Cheng and his co-workers invented devices with 1 and D6 (PBDTTT-C-T) and exhibited the highest PCE of 3.45% utilizing the binary additives which are usually the best PCE accomplished with allpolymer SCs to date. The non-volatile additive improved the miscibility of acceptor and donor overwhelming accumulation of 1, and 1,8-diiodooctane, increased crystallization and aggregation of D6 occasioned balance charge transport and appropriate phase separation (Zhou et al., 2010, 2011). Hasimoto and his coworkers synthesized various PDI-centered electron acceptors comprising several co-monomer units of carbazole (PC-PDI, 6), DTP (PDTP-PDI, 5), vinylene, fluorene, dibenzosilole, and thiophene as substitutions for DTT unit in the polymer 1 (Figure 4.2). The devices were produced with several donors of a polythiophene derivative D7, DPPcentered low band-gap polymer D8 (Figure 4.3), and D1 for contrast. The performance of the device fluctuated in the range of almost 0.11 to 1.15% centered on the moieties compared to the unit of perylene. For instance, the BHJ SC produced with 5:D7 displayed a PCE of 0.93% under the AM 1.5 (100 mWcm–2) light, which was quite higher than accomplished with 5:D1 cells (0.17%). The reduced efficiency attained with D1 was credited to lower JSC because of the coarse phase separation and rough surface morphology associated with poor miscibility of the D1 and PDI-centered acceptors. Amongst the 6 acceptors, 6 gave the highest PCE of 1.15% with donor D7, utilizing chlorobenzene (CB) solvent in an active layer. By varying the solvent to chloroform/ toluene, the PCE accomplished with D7:6 was enhanced to 2.23%. Imide-replaced PDI-centered polymers were first made by Falzon et al. (2003) for OPV cells in 2003 (Neuteboom et al., 2003). They are made up two alternating polymers (7 and 8, Figure 4.2) comprising PDI and oligo (p-phenylene vinylene) segments connected through saturated

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spacers of flexible unconjugated phenyl or alkyl groups, therefore forming a novel class of D-A polymers. The devices with 8/LiF/Al or ITO/PEDOT: PSS/7 configuration displayed high values of VOC (1.20 V and 0.97 V, correspondingly), while the values of JSC were very low due to rapid geminate recombination. Table 4.1: PDI (Perylene Diimide)-Centered Polymer Acceptors Acceptor

Mobility, μe [cm2V−1s−1]

Mn Mw

VOC [V]

JSC [mA/cm2]

FF

PCE [%]

HOMO/ LUMO (Eg [eV])

1

1.3 × 10−2 b

10,000

0.63

4.2

0.39

1

−5.9/−3.9

15,000

(ITO/PEDOT:PSS/D2:1(1:1)/Al)

–

0.75

3.37 ×10−5 c

8.55

0.52

(2.0) 3.45

−5.9/−3.9 (2.0)

1.48

−5.7/−3.8

0.77

−5.4/−4.0

(ITO/PEDOT:PSS/D6:1(1:1)/Ca/Al) 2

3

4

–

–

–

20,000

0.69

5.02

43,000

(ITO/PEDOT:PSS/D3:2(3:1)/Ca/Al)

15,000

0.69

27,000

(ITO/PEDOT:PSS/D3:3(1:1)/Ca/Al)

–

0.67

2.80

−5.7/−3.8

0.43

0.40

0.51

(1.4) 1.17

(ITO/PEDOT:PSS/D4:4(2:1)/Ca/Al) 0.75

1.60

0.45

−5.7/−3.8 (1.9)

0.57

(ITO/PEDOT:PSS/D5:4(3:1)/Ca/Al) 5

–

6,300

0.66

8,500

(ITO/PEDOT:PSS/D7:5(2:1)/Ca/Al) 0.42

3.05

1.86

0.46

0.53

0.93

−5.49/−3.83 (1.66)

0.41

(ITO/PEDOT:PSS/D8:5(1:1)/Ca/Al) 2.3 × 10−4 b

6,300

0.46

0.76

0.50

8,500

(ITO/PEDOT:PSS/D1:5(2:1)/Ca/Al)

0.17

−5.49/−3.83 (1.67)

Acceptors Materials for Organic Solar Cells 6

1.7 × 10−4 b

12,100

0.70

19,600

(ITO/PEDOT:PSS/D7:6(2:1)/Ca/Al) 0.58

6.35

0.91

0.50

0.55

2.23

113

−5.83/−3.66 (2.17)

0.29

(ITO/PEDOT:PSS/D1:6(2:1)/Ca/Al) 9

10

11

12

8.5 × 10−3 c

5×10−4 d

–

–

7,800

0.6

2.98

19,000

(ITO/D9:9(1:1)/Al) e

6,000 –

0.33

13,600 –

0.51

29,500

0.44

33,900

(ITO/PEDOT: PSS/12/LiF/Al)

0.60

0.39

2.32

−5.75/−3.95 (1.76)

0.46

0.1

(ITO/PEDOT:PSS/D1:10(2:1)/Al) 2.57

0.37

0.49

(ITO/PEDOT: PSS/11/LiF/Al) 1.5

0.25

0.2

(1.93)

Far along, Sharma, and his co-workers developed the interchanging phenylenevinylene and PDI copolymer 9 (Figure 4.2) through Heck coupling for utilization as an acceptor in the BHJ SCs (Mikroyannidis et al., 2009; Liang et al., 2011). Copolymer 9 displayed broad absorption ranging up to around 800 nanometers with the maximum peak at ca. 500 nanometers and the optical band gap of nearly 1.66 eV. The Copolymer’s solubility augmented upon the institution of hexyloxy and tert-butyl side groups with particular Tg (glass transition) and Td (decomposition temperatures) of 72 and 370°C. A PCE of 1.67% was attained by mixing acceptor 9 and the poly(3-phenyl hydrazone thiophene) (PPHT, D9, Figure 4.3) donor. After hardening, the improved PCE (2.32%) was demonstrated by an upsurge in the separation efficiency of exciton; this PCE is amongst the highest stated values accomplished with imide-replaced PDI-centered polymer acceptors.

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Figure 4.2: Molecular structures of PDI (perylene diimide)-centered polymer acceptors (1–12). Source: https://www.mdpi.com/1996-1944/13/9/2148/htm.

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Figure 4.3: Molecular structures of the polymer donors (D2 to D9). Source: https://www.mdpi.com/2073-4360/6/2/382.

Liang et al. (2011) also stated an imide-replaced PDI-centered polymer 10 (Figure 4.2) has the poly (ethylene glycol) spacer. The stretchy spacer occasioned improved solubility, encouraging π-π interactions amongst the cores of perylene. However, the low PCE of 0.1% was attained due to the phase-separation of D1 and 10 with the JSC of nearly 0.6 mAcm–2, VOC of almost 0.33 V, and the FF of 0.46 (Alam and Jenekhe, 2004). An additional method in the growth of imide-replaced PDI-centered polymers includes the connection of PDI to the polymeric scaffold. Zhang and Sommer stated accomplishing PCEs of 0.49% and 0.20% with acceptors 11 and 12, correspondingly, in the single-component devices utilizing the BCPs comprising PDI moieties as the side chains (Figure 4.2).

4.2.2. NDI-Centered Polymer Acceptors In the early studies, NDI-centered small molecules were stated to exhibit comparatively poor characteristics as acceptors in OPV cells compared to the PDI-centered counterparts, accredited to large band-gap, a small fused-ring unit, and slight absorption of previous in the visible region. In the late studies, polymerization of units of NDI was engaged to upsurge the conjugation length and improve the PCE. The device performance

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parameters and photophysical properties of NDI-centered polymer acceptors (13 to 24) are available in the literature. The 1st NDI-centered polymer was the ladder-kind poly (benzimidazobenzophenanthroline ladder) (BBL, 13, Figure 4.4) synthesized through the one-step condensation of tetra-aminobenzene and naphthalene tetracarboxylic acid in the polyphosphoric acid by Jenekhe et al. (2011).

Figure 4.4: Molecular structures of NDI (naphthalene diimide)-centered polymer acceptors (13 to 25). Source: https://pubs.acs.org/doi/10.1021/cm0497069.

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The spin-covered bi-layer bulk-heterojunction cells were produced with the poly (phenylenevinylene) (PPV, D10, Figure 4.5) donor utilizing the device configuration of ITO/D10/13/Al. The projected PCE value of 0.7% was attained utilizing 10 mWcm–2 illuminations. After hardening at 100°C, the PCE increased up to 1.5% (Alam and Jenekhe, 2004).

Figure 4.5: Molecular structures of the polymer donors (D10 to D12). Source: https://www.mdpi.com/2073-4360/6/2/382.

In the year 2011, Fabiano and his co-workers (2011) introduced the all-polymer BHJ SCs made up of NDI-centered polymer acceptor, P (NDI2OD-T2) (14, Figure 4.4) and the polymer donor, D1. The PCE of 0.16% was accomplished using o-dichlorobenzene and CB. Polymer 14 was produced with the help of Stille coupling reaction amongst 5,5’-bis(trimethylstannyl)-2,2’-dithiophene and N,N’-dialkyl-2,6dibromonaphthalene-1,4,5,8-bis(dicarboximide). The small band-gap of polymer 14 (ca. 1.6 eV) occasioned in Ultra-Violet absorption near 850 nanometers, therefore the absorption was corresponding to the range of visible spectral regions in the situation of blend film. A FF value of almost 0.67 was attained for these devices, recommending free carrier generation and compatible charge transfer in the edge of the D1:14 blend. Despite the outstanding charge carrying, the devices engaging o-dichlorobenzene or CB as the solvent displayed the low values of JSC. Blending the acceptor and donor utilizing xylene as the solvent occasioned the PCE of almost 0.62% which was credited to enhanced phase separation of the D1:14, consequential in the two-fold upsurge of the values of JSC. Sirringhaus and his co-workers also utilized polymer 14 as the electron acceptor (Moore et al., 2011). Despite the high electron mobility of 0.8 cm2/Vs), NIR absorption band, and the compatible levels of energy of polymer 14, the PCE of BHJ SCs produced with 14 and D1 utilizing chloroform as the solvent was just 0.21%. The low efficiency was explicated in terms of the rough phase

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separation of D1:14 blends having domains in the range 0.2–1 micrometer and the quick, early geminate recombination of charge population within 200 ps of excitation. In the year 2012, an enhanced PCE of 1.4% as stated by Schubert et al. (2012) utilized the same acceptor and donor materials by varying the solvent to chloronaphthalene and p-xylene. The improved PCE was primarily accredited to the large upsurge of JSC with the utilization of the proper solvent (chloronaphthalene: p-xylene = 50:50). They also made up another NDI-centered polymer acceptor, P (NDI-TCPDTT) (15, Figure 4.4), with an extra CPDT moiety. The PCE of 1.1% and a FF of nearly 0.70 were attained in D1:15 cells utilizing tetralin as the solvent, which gave a higher value of JSC as compared to other solvents like p-xylene, chloroform, and a mixture of chloronaphthalene and p-xylene. Hwang et al. (2012) resented selenophene into 14 rather than thiophene as the structural modification. The freshly invented crystalline copolymer acceptor (PNDIBS, 16, Figure 4.4) displayed high electron mobility of 0.07 cm2/Vs) and the broad visible-NIR absorption band having an optical band gap of almost 1.4 eV. The all-polymer BHJ SCs encompassed polymer 16 as an electron acceptor and D1 as the donor exhibited a PCE of 0.9%. Later, they also made 3 other acceptors; PNDIS (18), PNDIT (17), and PNDISHD (19, Figure 4.4) which have 1 selenophene or thiophene next to the unit of NDI in the iterating unit. The 3 acceptors were mixed with the thiazolothiazole copolymer donor (PSEHTT, D11, Figure 4.5). The NDIthiophene-centered polymer 17 gave the lower PCE (1.3%) as compared to the NDI-selenophene-centered congeners, 18 (2.96%) and 19 (3.26%). Nakabayashi and his co-workers stated the completely conjugated D-A BCPs made up of PNBI (poly (naphthalene bisimide))-centered electronaccepting segments and the regioregular P3HT-centered electron-donating segments P3HT-PNBI-P3HT (20, Figure 4.4) (Fabiano et al., 2011). BCPs were produced utilizing the Yamamoto coupling reaction and quasi-living Grignard metathesis (GRIM) polymerization and had Mw in the range of 21,800 to 26,000. The polymer acceptors displayed a wide absorption in the range (350 to 850 nanometers) and with an optical band-gap of nearly 1.46 eV. Moreover, TA prolonged the light absorption band up to 893 nanometers, which aided to reduce the optical bandgap to almost 1.38 eV. The device D1:20 accomplished a PCE of 1.28% with a JSC of 4.57 mAcm–2, a VOC of nearly 0.56 V, and a FF of 0.50. Absorption of the blend film with the 1:1 (donor: acceptor, w/w) blend ratio displayed wide absorption to almost 950 nm (Moore et al., 2011).

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Luscombe and the co-workers copolymerized fused thiophenes with electron-removing NDIs. The copolymers varied in terms of the number of the thiophene rings infused thiophene systems, occasioning in PNDI3fTh (22), PNDI-2fTh (21), and PNDI-4fTh (23, Figure 4.4). The device produced with D1:23 displayed the highest PCE of 0.13%, which was related to the highest FF (0.55) and JSC (0.57 mA/cm2) among the assessed polymers. The charge mobility values were improved by upsurging the number of fused thiophene moieties inside the NDI-copolymers, occasioning in the improved JSC (Earmme et al., 2013; Yuan et al., 2013). Currently, Cheng et al. (2013) made 3 angular-shaped naphthalene tetracarboxylic diimide polymers 24 (m = 1–3, Figure 4.4) as the acceptors utilizing the reactions of Stille coupling. The best PCE of 0.32% was accomplished with polymer 24 (m = 1) and the D1 donor in BHJ SCs. The angular-shaped NDI-comprising polymers were categorized by the higher VOC (0.94 V) as compared to the linear-shaped NDI-comprising polymers (> 1, and the photocurrent grows linearly along with the light intensity, we get:

(3) where; L depicts the normalized light intensity; and C1 indicates a temperature factor that does not rely upon L. Note that C1 is not constant with the temperature while constant with illumination. There is an increase in open-circuit voltage with light intensity and a decrease of open-circuit voltage with temperature. The slope of open-circuit voltage versus light intensity relies only upon the temperature, and the light ideality factor. The light ideality factor is measured as:

(4) The light ideality factor nidL can additionally count on the light intensity. For example, SRH recombination is more sticking out at low light intensities. Usually, the average is computed to get a single number for ideality. However, it is also intriguing to examine and correlate the ideality factor versus open-circuit voltage (Michels et al., 2016).

(5)

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179

Normally, an ideality factor of 1 is assigned to bimolecular recombination (radiative recombination), while an ideality factor of 2 is associated with prominent SRH recombination (Dongaonkar et al., 2010). Nonetheless, we want to refer that the basis of the idea is a single zero-dimensional device model. In an actual device, the charge carrier distribution differs in energy and space which can impact the ideality factor even though no traps exist. Inorganic SCs, by the reason of Onsager-Braun dissociation of excitons into free carriers, the photocurrent jph can rely upon the voltage. The interpretation of the light ideality factor is perhaps inclined to errors in devices supporting field-dependent charge generation (Yeo et al., 2011). Simulated open-circuit voltages versus light intensity are shown in Figure 6.3 for numerous cases. The light ideality factor is illustrated in Figure 6.3(f) which is computed from the average slope of the Voc versus the light intensity by Eqn. (5). An ideality factor of the base case is exactly one. Besides the case ‘deep traps’ and ‘low shunt resistance’ the ideality factor is about 1. In the case of increasing recombination pre-factor (b), the Voc is reduced, however, the Voc-slope remains constant. For ‘deep traps’ (c), the slope (Voc vs. L) is considerably precipitous turning to an average ideality factor of 1.8. For ‘low shunt resistance’ (d), the Voc drops at a minimum light intensity, and the estimation of an average ideality factor does not require (Seemann et al., 2011).

Figure 6.3: (a–e) Simulation of the open-circuit voltage relied upon light intensity for numerous cases; (f) Light ideality factors acquired from the results of simulation-an average is used. Source: https://www.fluxim.com/measurement-techniques-perovskite-solarcells.

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Hence, it is shown by our simulation outcomes that as we obtained for the case ‘deep traps,’ for the investigation of the significance of the SRHrecombination in a device, the light ideality factor can be helpful. If the effect is not camouflaged by a low shunt resistance, then the examination works only.

6.4. CHARGE EXTRACTION BY LINEARLY INCREASING VOLTAGE (CELIV) For the estimation of the charge carrier mobilities in organic SCs, the popular method is charge extraction by linearly increasing voltage (CELIV). The principle of CELIV is shown in Figure 6.4 diagrammatically. A linearly growing voltage in opposite direction is enforced to the device V(t) = A⋅t, where A indicates the ramp rate. The linearly growing voltage brings a constant displacement of current density jdisp, which is computed as:

(6)

where; S represents device area; Cgeom indicates geometric capacitance; ε0 signifies vacuum permittivity; εr denotes relative dielectric permittivity, and d depicts active layer thickness. If charge carriers exist in the device, then they are withdrawn and cause a spike in the transient current. The mobility of the charge carrier can be calculated following the time of the current peak (tmax). The charges that are obtained by the voltage ramp could be intrinsic (darkCELIV), be inserted by a positive voltage preceding extraction (injectionCELIV), or be achieved by illumination preceding extraction (photoCELIV). Recent performance with metal-insulator-semiconductor (MIS) devices permits distinguishing between extracted holes (MIS-CELIV) and extracted electrons. Hither, the alternate formula is used for the extraction of the charge carrier mobility, and additionally, charge dynamics are different (Yu et al., 1995; Ferlauto et al., 2002). The high-quality dielectric layer is strenuous because of the thin deposition. We have shown MIS-CELIV applying polar tris(8-hydroxyquinoline) aluminum (Alq3).

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Figure 6.4: Schematic demonstration of a photo-CELIV experiment. The charge carriers are extracted by the linearly increasing voltage and turn to a peak (jmax) in the current. The calculations of charge carrier mobility are done using tmax. Source: https://www.researchgate.net/figure/Schematics-of-the-CELIV-experiment-explaining-the-experimental-quantities-used-in-the_fig1_316733335.

6.5. TRANSIENT PHOTOVOLTAGE (TPV) AND OPEN-CIRCUIT VOLTAGE DECAY (OCVD) The external current, under open-circuit conditions, in the solar cell is zero. Thus, charge generation and charge recombination are equal. Methods penetrating the device under an open circuit are usually appropriate to analyze recombination and trapping dynamics. Information concerning shunt resistance and recombination is revealed by the open-circuit voltage decay (OCVD, which is sometimes known as large-signal TPV) measurements. In OCVD measurements, a laser or an LED illuminates the solar cell first to generate charge carriers. After that, the light is switched off and the voltage decay is determined over time (Häusermann et al., 2009; Neukom et al., 2017).

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Outcomes of OCVD simulation of the specified cases are illustrated in Figure 6.5. Commonly in all cases, voltage collapses substantially beyond 50 ms after turning off the light. This is connected to the shunt resistance. There is a most pronounced and apparent effect in the case ‘low shunt resistance’ for the ‘base’ case (see Figure 6.5(d)). The charges move via shunt resistance and consequently deplete the device rather than recombining gradually. The voltage decays more speedily on the decrease of shunt resistance. A shunt resistance of the base case is 160 MΩ, this parallel resistance causes the kink at 50 ms. A logarithmic reliance on time is shown by the voltage decay before 50 ms (Bartesaghi et al., 2015). There is a higher decay rate in the case of deep traps as appears in Figure 6.5(c). When trapped putting off the recombination, charges are immobilized. Therefore, the voltage decay is slower with shallow traps (Diethelm et al., 2020).

Figure 6.5: OCVD simulations for all cases in prior figures. The light is switched off at t = 0. The analytic solution is marked by the gray line supposing purely bimolecular recombination and homogeneous charge densities. Source: https://www.fluxim.com/measurement-techniques-perovskite-solarcells.

In a solar cell, the open-circuit voltage Voc can be expressed to:

(7)

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where; Eg represents the energy of the bandgap; q indicates the unit charge; kB signifies the Boltzmann constant; T represents the temperature; N0 denotes the effective density of states; n is the electron density, and p represents the hole density. By inserting the decay of a homogeneous charge carrier density (dn/dt = −β⋅n² with n = p) into Eqn. (7), we get:

(8) where; β represents the recombination pre-factor; and the n(0) denotes the initial charge carrier density at open-circuit. Concerning Eqn. (8), there is an anticipation of decaying the voltage signal with a logarithmic dependence on time. This is illustrated with gray lines in the plots in Figure 6.5. Parameter β is selected concerning the ‘base’ case. The numerical simulation is only fitted by the analytic solution (Eqn. (8)) at the very beginning. The sense behind this is that the charge is not homogeneously dispersed within the device (Wagner et al., 2012). The densities are higher near the electrodes, and charges flow haltingly into the center of the device, the place of their recombination. For that reason, zero-dimensional models are not appropriate to express the OCVD within p-i-n structured SCs. A similar consideration also employs lifetimes resolved from IMVS or TPV, which are also explained in this manuscript, or to recombination coefficients obtained from CELIV with the OTRACE approach (Dibb et al., 2013; De Castro et al., 2016). No material parameters can be inferred directly from OCVD measurements. However, it can be helpful to achieve parameter extraction by fitting numerical simulations or by contrasting various devices.

6.6. IMPEDANCE SPECTROSCOPY A famous technique for the investigation of SCs is impedance spectroscopy. It is abbreviated as IS or sometimes named EIS (electrochemical impedance spectroscopy). It is also known as admittance spectroscopy (where admittance is considered to be the inverse of impedance). The impedance of the device is calculated at different frequencies by relating a small sinusoidal voltage and determining the current in the frequency domain. Various physical effects in the device can be recognized by using an enormous range of frequencies because of their diverse transient dynamics. For example, traps can exhibit a huge impact in the low-frequency range (Figure 6.6) (Kirov and Radev, 1981; Nie et al., 2015).

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Figure 6.6: The graph between impedance real and impedance image concerning light changes. Source: https://www.researchgate.net/figure/Imaginary-vs-real-impedance-Nyquist-graph-for-anatase-and-TiO-2-C-sample-A1_fig8_264429483.

•

Analyzed Parameters: Trapping dynamics, charge carrier mobility, equivalent circuit. A small sinusoidal voltage V(t) is performed to the solar cell in the impedance spectroscopy as: (9) where; V0 represents the offset voltage; Vamp indicates the voltage amplitude and ω shows the angular frequency 2⋅π⋅f. The system can be assumed to be linear if the voltage amplitude Vamp is low enough. Hence, the current density j(t) is sinusoidal as well. The phase shift and the amplitude of the current are determined. Impedance spectroscopy is applied at numerous frequencies or/ and offset illuminations or/and offset voltages (refers to later section). The complex impedance Z is measured using the transient current signal and the transient voltage as:

(10) where; Y indicates the admittance; N denotes the number of periods; T represents the period 1/f; i signifies the imaginary unit and ω expresses the angular frequency. For the study of the impedance, usually, the conductance ‘G’ and the capacitance ‘C’ are applied versus offset voltage or frequency are measured as:

Characterization of Organic Solar Cells



185

(11)

(12) where; ω denotes the angular frequency; Im() signifies the imaginary part, Re() represents the real part. Often, impedance spectroscopy data is mapped on the Cole-Cole plot. Hither, the imaginary and real part of the impedance Z is drawn in the complex plane for various frequencies. Such simulation results are shown in the additional information. As a matter of choice, the capacitance C is drawn versus the frequency. A key benefit of using impedance spectroscopy is that effects appearing on unusual time scales can be distinguished. For instance, trapping and detrapping appear normally on a protracted-time scale (decreased frequency) in contrast with the movement of free carriers. Most ordinarily impedance spectroscopy data is examined with equivalent circuits. By that, electric circuits are manufactured from inductors, capacitors, resistors, and more electric elements such that the calculated frequency-dependent impedance can be regenerated. The limitation of the equivalent circuits is that the parameters cannot be directly linked with macroscopic material parameters and the outcomes can be obscure (McNeill et al., 2009; Street, 2011). The drift-diffusion equations were solved by Knapp and Ruhstaller with less signal study to simulate impedance spectroscopy data (Hwang et al., 2009). Hither, as simulation input, physical parameters are used that permit direct analysis of the outcomes. The same method is carried out in the software Setfos that is applied in this study (Duffy et al., 2000). The occupation of trap sites can be probed with the measurement of capacitance because of the space charge effects. The capacitance can be increased by the slow traps at low frequencies that are illustrated by numerical simulation (Shuttle et al., 2008). Also, the capacitance can be increased at low frequencies as the result of the slow ionic charges that might be present in SCs (O’Regan et al., 2015). Recombination of charge carriers turns to a decline in capacitance; it can even reach negative. It is also analyzed by Knapp and Ruhstaller that the self-heating of a device can result in a negative capacitance (Lange et al., 2013). A negative capacitance indicates that the phase-shift between current and voltage becomes positive (i.e., current leading voltage) and a positive capacitance exhibits that the phase-shift is positive (i.e., voltage leading current).

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In this study, the impedance simulations are shown under illumination in the SI with a variety of offset-voltage mapped on the Cole-Cole representation. Usually, it is debated that the dimensions of the semicircle in the Cole-Cole plot indicate the recombination in the device. It is concluded from our simulation results that the dimensions of the semi-circle are impacted by many effects in the complex plane. Hence, it is advised to analyze such results attentively (Basham et al., 2014). At low frequency, the real part of the impedance and the inverse of the current slope in the JV-curve have coincided at the same offset voltage. One mostly assesses the DC characteristics, if the probing frequency is small enough. Hence, for consistency control of the impedance measurement, an IV curve can be used. The JV-curve can be reconstructed from low-frequency impedance data without the use of equivalent circuits (Wright et al., 2017).

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20. Lee, H. K. H., Li, Z., Constantinou, I., So, F., Tsang, S. W., & So, S. K., (2014). Batch‐to‐batch variation of polymeric photovoltaic materials: Its origin and impacts on charge carrier transport and device performances. Advanced Energy Materials, 4(16), 1400768. 21. McNeill, C. R., Hwang, I., & Greenham, N. C., (2009). Photocurrent transients in all-polymer solar cells: Trapping and detrapping effects. Journal of Applied Physics, 106(2), 024507. 22. Michels, J. J., Oostra, A. J., & Blom, P. W., (2016). Short-circuit prevention strategies in organic light-emitting diodes and solar cells. Smart Materials and Structures, 25(8), 084015. 23. Neukom, M. T., (2016). Charge Carrier Dynamics of Methylammonium Lead-Iodide Perovskite Solar Cells, 1, 1–22. arXiv preprint arXiv:1611.06425. 24. Neukom, M. T., Züfle, S., & Ruhstaller, B., (2012). Reliable extraction of organic solar cell parameters by combining steady-state and transient techniques. Organic Electronics, 13(12), 2910–2916. 25. Neukom, M. T., Züfle, S., Knapp, E., Makha, M., Hany, R., & Ruhstaller, B., (2017). Why perovskite solar cells with high efficiency show small IV-curve hysteresis. Solar Energy Materials and Solar Cells, 169, 159–166. 26. Neukom, M., Züfle, S., Jenatsch, S., & Ruhstaller, B., (2018). Optoelectronic characterization of third-generation solar cells. Science and Technology of Advanced Materials, 19(1), 291–316. 27. Nie, W., Tsai, H., Asadpour, R., Blancon, J. C., Neukirch, A. J., Gupta, G., & Wang, H. L., (2015). High-efficiency solution-processed perovskite solar cells with millimeter-scale grains. Science, 347(6221), 522–525. 28. O’Regan, B. C., Barnes, P. R., Li, X., Law, C., Palomares, E., & MarinBeloqui, J. M., (2015). 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. Journal of the American Chemical Society, 137(15), 5087– 5099. 29. Schroder, D. K., (1997). Carrier lifetimes in silicon. IEEE Transactions on Electron Devices, 44(1), 160–170.

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CHAPTER

7

APPLICATIONS OF POLYMER AND GRAPHENE NANOCOMPOSITES IN SOLAR PHOTOVOLTAICS

CONTENTS 7.1. Introduction..................................................................................... 194 7.2. Graphene/Polymer Nanocomposites as Transparent Conductive Electrodes (TCES)........................................................ 194 7.3. Graphene/Polymer Nanocomposites as Active Layers (ALS)............. 200 7.4. Graphene/Polymer Nanocomposites as Interfacial Layers (IFLS)....... 203 References.............................................................................................. 206

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7.1. INTRODUCTION This chapter sums up the latest advancements in the improvement of graphene (G)/polymer nanocomposites to the amount of PSC applications. Graphene has appeared as a nanomaterial because of its versatility and unique properties, which are valuable for different device parts like active layers (ALs), transparent conductive electrodes (TCEs), and the interfacial layers (IFLs). Our emphasis will be on the preparation scheme of G/polymer nanocomposites, the photovoltaic (PV) properties of PSCs integrating these nanocomposites, graphene oxide (GO), and reduced graphene oxide (rGO), and the synthesis of G and its derivatives. Even though a large number of reviews about PSCs are presented, but literature reviews about G-based materials for solar-cell applications are insufficient to the best of our knowledge, and not any one of them targets the nanocomposites (Hoppe et al., 2005; Cheng et al., 2009). G-based materials have been utilized in various layers of PSCs, e.g., ALs, TCEs, and IFLs. The the literature on this topic is so vast that only the most related examples concerning polymer/G nanocomposites are discussed.

7.2. GRAPHENE/POLYMER NANOCOMPOSITES AS TRANSPARENT CONDUCTIVE ELECTRODES (TCES) For the replacement of traditional ITO electrodes in PSCs, G, and its derivatives have been applied as TCEs. With the help of flexible polymers like polyethylene terephthalate (PET), being a substrate, a lot of work has been accomplished. In this regard, by spin coating the materials over plasma conducted rGO/PET, the PV device was fabricated to make a hydrophilic surface, and rGO films that were synthesized by thermal annealing (TA) were deposited over PET. On the usage of rGO films with a transmittance of 65% and a thickness of 16 nm, a maximum of Voc of 0.56 V, JSC of 4.39 mA/cm2, and PCE of 0.78% was obtained. Exceptionally, the device achievement could endure up to 1200 bending cycles without losing device performance, whereas the conventional cells covering ITO normally crack and decline upon bending because of the fragile nature of ITO. Quite upgraded performance (e.g., PCE of 3.05%) was acquired by depositing a rGO micromesh attained through a laser-pattering method onto PET, lower sheet resistance (565 Ω/sq) of the rGO micromesh in contrast to pristine rGO, and impute to the superior transparency (59%). More significantly, this PSC has shown good bending stability and a commensurate PCE with

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ITO-based devices (3.82%). The high density of defects of these rGO-based electrodes that hinder device efficiency is its vital shortcoming (Thompson and Fréchet, 2008; Chen et al., 2009). The preparation of TCEs relied on sulfonated graphene (SG)/PEDOT composites processed by in situ polymerization as reported by Xu et al. (2009). SG was arranged from GO in four stages: (i) reduction of GO with NaBH4; (ii) sulfonation with the aryl diazonium salt of sulfanilic acid; (iii) post-reduction with N2H4; and (iv) functionalization with sulfanilic acid, NaNO3, and azoisobutyronitride (AIBN). Afterward, SG was dislodged in water pursued by the inclusion of the monomer EDOT and Fe2(SO4)3. For 48 hours at 50°C, the mixture was mixed and later poured into methanol. The overabundance of EDOT and other impurities were extracted via many washing cycles. Good processability was shown by the composites both in water and organic solvents, high thermal conductivity, superior transparency, and thermal stability. For films having a few nm thicknesses, transmittances greater than 80% in the 400–1800 nm wavelength range, and a conductivity of 0.2 S/cm were noticed. As compared to the conductivity of a commercial PEDOT: PSS product (10−6–10−5 S/cm), this conductivity is much higher. Furthermore, a poly (methyl methacrylate) (PMMA) sheet coated with the mentioned composite was angled inward, even though it still maintained high electrical conductivity (0.18 S/cm) (Figure 7.1).

Figure 7.1: Systematic explanation of sulfonated graphene (SG)/poly (3,4-ethylenedioxythiophene): Poly(styrene-sulfonate) (PEDOT) nanocomposite along with its synthesis-reaction conditions (Xu et al., 2009). Source: https://link.springer.com/article/10.1007/s12274-009-9032-9.

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The preparation of a G-based composite electrode was done by spin coating a blended solution of PEDOT:PSS and surfactant functionalized G, with the altered G sheets evenly dispersed in the PEDOT:PSS matrix. The transparency and conductivity of this TCE were able compared to those of an ITO electrode. More significantly, it can be bent over 1000 cycles with just a 5% growth in its resistance and it has shown high stability (both electrical and mechanical). Still, the existence of the surfactant stabilizer is unwanted from an application perspective. It was found that an aqueous G suspension stabilized by PEDOT:PSS was synthesized by Jo et al. (2011) via the chemical devaluation of GO in the existence of this polymer, without the demand for surfactants. This approach incorporates the fixed backbone of PEDOT and strong π-π interactions within rGO sheets, and intermolecular electrostatic repulsions surrounded in negatively charged PSS bound and the RGO sheets, which transmit colloidal stability to the arising hybrid nanocomposite of rGO/PEDOT. A conductivity of 2.3 kΩ/sq and a transmittance of 80% were exhibited by the film. Additionally, its conductivity was nearly maintained following 100 bending cycles (Green, 1981; Dennler et al., 2009). Recently, in inverted PSCs that were fabricated by spray coating, a 4-layered CVDdoped G employed with PEDOT:PSS was practiced as a cathode. The active layer was made up of phenyl-C71-butyric acid methyl ester (PC70BM) being an acceptor, and poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)alt-(3,3”‘-di(2-octyldodecyl)2,2’;5,’2”;5,”2”‘-quaterthiophen-5,5”‘-diyl)] (PffBT4T-2OD) being a donor. The resulting device demonstrated a PCE of 2.8%, Voc of 0.72 V, FF of 0.37, and Jsc of 10.5 mA cm−2, respectively, not much from the related figures of an ITO-based cell. The vibration-assisted ultrasonic spray coating has prepared G-doped PEDOT:PSS nanocomposites which are a fast, single-step, and scalable process. The resulting films offered an utmost electrical conductivity of 298 S·cm−1, with transparency able to compare to that of ITO-coated glasses, approximate a 10-fold increase in contrast to pristine PEDOT:PSS films. From when the G sheets bridge within the PEDOT:PSS rings through strong π-π interactions that serve as high-mobility channels, this remarkable progress was attributed to the higher carrier concentration and carrier mobility. Moreover, the number of defect sites in PEDOT:PSS is reduced by the π-π interactions. More significantly, the energy level of G-doped PEDOT:PSS films showed superior mechanical properties involving hardness and wear resistance, better stability, and could be tuned (De Kok et al., 2004; Luo et al., 2009). It was developed by layer-by-layer electrophoretic deposition. The resulting device gave Jsc of 0.42 mA·cm−2, a PCE of 0.92%, a Voc of 0.48

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V, and FF of 0.23, respectively, which were greater than those of devices having ZnS- or PPy-modified electrodes. The PV efficiency was improved by the nanocomposite because ZnS nanoparticles (NPs) operated as bridges, PPy served as a hole acceptor and a remarkable sensitizer, while G enacted as a phenomenal conductive transporter and collector. Semitransparent PSCs were fabricated by Liu et al. (2012) that were based on P3HT: phenylC61-butyric acid methyl ester (PC61BM) by a CVD-grown single-layer G film doped with ITO as the bottom electrode and PEDOT:PSS and Au NPs as the top electrode. An increase in conductivity has resulted from the use of the doped G electrode which is approximately 400% as compared to pristine G, and a maximum PCE of 2.7% was obtained on the illumination from the G side, which was associated with the improved transmittance of the G electrode. PSCs with higher efficiency are anticipated to be obtained with the usage of the single-layer G with superior quality and by optimizing the processing conditions (Figure 7.2) (Yun et al., 2011).

Figure 7.2: (A) Band structure and diagrammatic depiction of a PSC with the structure glass/indium tin oxide (ITO)/ZnO/P3HT:PCBM/Au/PEDOT:PSS/G; (B) J-V characteristics evaluated from two sides of the PSC using G top electrode and various active layer thicknesses. Source: https://pubs.acs.org/doi/10.1021/nn204675r.

Another flexible PSC was fabricated by some researchers on polyimide (PI) substrates having multilayer CVD G doped with P3HT:PC61BM as ALs and Au NPs and PEDOT:PSS as top TCE (Taleghani et al., 2009; Walker et al.,

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2011). A maximum PCE of ~3.2% was shown by the device with a structure of G/Au/PEDOT:PSS/P3HT:PCBM/ZnO/Ag/PI, which was reduced merely by around 8% subsequent to 1000 bending cycles, signifying impressive stability and flexibility. It was found, more considerably, that air could not spread out over the fine space within the G layers, therefore giving a great packaging effect to flexible SCs. Multilayer G can serve as an environmental obstruction and shield the PSCs from air pollution, which minimizes the related costs and simplifies the device fabrication (Das et al., 2015). In recent times, G was formed through electrochemical exfoliation of graphite with a method that intercalated sulfate ions. The processed exfoliated graphene (EG) films were sprayed onto flexible poly (ethylene 2,6-naphthalate) (PEN), and after that, the composite was used as an anode in PSCs. The device applying PTB7:PC71BM being the active layer attained a PCE of 4.23%, which was retained after 150 bending cycles. This solution-treated G-based electrode had a low sheet resistance of 0.52 kΩ/sq, and a transparency of 70% and was mechanically sturdy, without altering resistance at various bending angles (Figure 7.3) (You et al., 2013).

Figure 7.3: (a) Plan of the electrochemical exfoliation of graphite; (b) optical figures of the exfoliation process; (c) diagrammatic portrayal of spray deposition of exfoliated graphene (EG) dispersion over poly(ethylene 2,6-naphthalate) (PEN); (d) J-V properties of the cell under dark (dashed line) and light conditions (solid line). Source: https://pubs.acs.org/doi/abs/10.1021/acsami.7b09702.

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Another device was established by An et al. (2014) covering a PMMA/G composite as an anode. The polymer was spin-coated upon highly uniform G formed by CVD, leading to a bilayer composite that was again deposited over a flexible PET substrate. The PMMA made better the adhesion of G upon the substrate, hindering both crack formation and contamination. Because of this novel introduced process, the G sheet resistance was minimized by around 50% at the same transmittance, turning to a device with a structure: PET/PMMA-G/MoO3/PEDOT:PSS/Poly[N-9’-heptadecanyl-2,7-carbazolealt-5,5 (4›,7›-di-2-thienyl-2’,1’,3’-benzothiadiazole)] (PCDTBT): PC70BM/ Ca/Al which showed Jsc of 8.88 mA cm−2, a PCE of 3.3%, a Voc of 0.83 V, and FF of 0.45, approximate of 200% higher efficiency in contrast to the reference cell without G (Figure 7.4).

Figure 7.4: (a) Diagrammatic description of the PSC with G anode and structure G/PEDOT:PEG(PC)/PEDOT:PSS/DBP/C60/BCP/Al; (b) cross-sectional transmission electron microscope (TEM) image (that is on left side) of the device represented in (a), with an energy-dispersive line scan on a figure of the device cross-section (that is on right side); (c) flat-band energy level chart of the PSC; (d) J-V properties of the G-based device (red lines) in contrast with ITO reference cells (blue lines).

Source: https://www.nature.com/articles/srep01581.

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7.3. GRAPHENE/POLYMER NANOCOMPOSITES AS ACTIVE LAYERS (ALS) At first, the structure of OSCs was similar to that of traditional SCs: a single flat semiconductor heterojunction that was composed of a film of acceptor and a thin layer of active polymer (donor), intervening between two electrodes. The field formed in the donor-acceptor (D-A) interface is effective for exciton dissociation within free electron-hole pairs. Because of the reduced interface area, the PCEs of OSCs formed on this structure is very low. In other words, only excitons created nearest to the interface (i.e., just a few nm) can be detached into free-charge carriers. Hence, flat heterojunction OSCs should be thin, which causes low Jsc and poor light absorption (He et al., 2012). Later, most OSCs were developed with undoubtedly higher PCE which was based on the BHJ structure (Wan et al., 2016). In this kind of cell, there is a creation of an interpenetrating network in the active layer having nanoscale phase separation (comprising of a fullerene-derivative acceptor and a polymer donor). The improvement of PCE in BHJ SCs in contrast with those of in bilayer SCs is chiefly because of the more adequate exciton dissociation permitted by the increased charge-carrier collection and the maximized heterojunction interface as a result of the creation of the interpenetrating network. In this view, the 2D structure of G and the vast specific surface area assist the composition of a bicontinuous interpenetrating network of acceptor and donor materials towards the nanometer scale. The first usage of solution-processable G functionalized with poly(3octylthiophene) (P3OT) being a donor and phenyl isocyanate being an acceptor in PSCs was reported by Liu and his coworkers (Chen et al., 2015). The device, having an ITO/ PEDOT:PSS/P3OT:G/LIF/Al architecture, produced the highest PCE of 1.4% toward a G content of 5 wt.% optimized by an annealing process (at 160°C, for 20 min). The same researchers used the same functionalized G in a maximum concentration (10 wt.%) being an acceptor and P3HT being a donor in a very much alike PSC (Lin et al., 2016), with the highest PCE of 1.1%, Jsc of 4.0 mA·cm−2, FF of 0.38, and Voc of 0.72 V, subsequent to an annealing treatment for 10 min at 160°C. The progress of the device performance was interpreted with the consideration of the increase of the exciton dissociation area because of the speedy electron transport via G. Still, annealing at maximum temperatures (i.e., 210°C) prompted a reduction in the PCE (0.57%). Wang et al. (2011) developed a

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similar PSC applying a P3OT/solution-processable G composite being the active layer. The device exhibited a Jsc of 4.6 mA·cm−2, a PCE of 1.14%, FF of 0.37, and Voc of 0.67 V. Other researchers formed a P3HT/solutionprocessable G/functionalized multiwalled carbon nanotubes (f-MWCNTs) nanocomposite, where G served as a percolation path for the electrons and an electron acceptor, P3HT enacted as an electron donor, and the f-MWCNTs gives percolation paths of holes (Sun et al., 2015). The consequent device having the structure: ITO/PEDOT:PSS/P3HT-f-MWCNTs-SPFGraphene/ LiF/Al produced a PCE of 1.05%, Voc of 0.67 V, FF of 0.32, and a Jsc of 4.7 mA·cm−2. However, the PCE of these cells among solution-processable G is pretty low, and could probably be made better by tuning the processing conditions and G content; still, for G-based PSCs, the theoretical studies envision an efficiency exceeding 12%. The CH2OH-terminated regioregular P3HT was grafted by Yu et al. (2010) at the COOH groups of the GO surface using an esterification reaction. It was found that the consequent P3HT-grafted GO sheets were soluble in many organic solvents, which is worthwhile for solution processing. They utilized P3HT-g-GO/C60 being the active layer to construct a PV device having architecture: ITO/PEDOT:PSS/G-P3HT:C60/ Al, depicting a PCE of 0.61%, which is approximately 200% improved in contrast to the device based on P3HT/C60, therefore, the chemical grafting of P3HT upon G can improve light absorption and electron delocalization (Figure 7.5) (Wallace, 1947; Chang and Wu, 2013).

Figure 7.5: (A) Graphical image of ITO/PEDOT:PSS/G-P3HT:C60/Al PSC; and (B) J-V properties with P3HT:C60 or G-P3HT:C60 being the active layer. Source: https://pubs.acs.org/doi/10.1021/nn101671t.

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For opening the bandgap of the graphene, a very persuasive approach is to convert 2D G into 0D quantum dots (GQDs). Additionally, the GQDs are soluble in organic and aqueous solvents and rich in oxygenated groups, hence enabling more functionalization. GQDs having sizes 3–5 nm were utilized as electron-acceptors in a device that has the structure of ITO/PEDOT:PSS/ P3HT:GQDs/Al, they were synthesized using an electrochemical approach, resulting in a PCE of 1.28% and a Voc of 0.67 V. A PCE of 1.14% is obtained from the aniline-modified GQDs that were prepared using a hydrothermal method and utilized as acceptors in PSCs where P3HT was acted as a donor. GQDs that were derived from double-walled carbon nanotubes have also been processed by a solution-based approach and included an active layer of P3HT:PCBM, generating a PCE of 5.24%. The latest method to enhance the efficiency of PSCs is P3HT:PCBM:GQDs ternary composite (Figure 7.6) (Weiss et al., 2012; Wei et al., 2016).

Figure 7.6: (A) Scheme-wise representation; and (B) J-V curves of ITO/ PEDOT:PSS/P3HT:GQDs/Al device formed on aniline-modified GQDs with various GQDs content. Source: https://pubs.acs.org/doi/10.1021/ja2036749.

Reduced graphene oxide quantum dots (rGOQDs) and graphene oxide quantum dots (GOQDS) have been mixed with a PTB7:PC71BM active layer to improve the charge-carrier extraction of PSCs and optical absorptivity, turning to a device having the structure: ITO/PEDOT:PSS/ GOQDs:PTB7:PC71BM/Al. An increase in both FF and Jsc was noticed for rGOQDS, as a result of the oxygenated functional groups on the surface of the QDs and their better conductivity, turning to an utmost PCE of 7.6%. Doping with heteroatoms is another scheme that makes rise the bandgap

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of G (Zheng and Kim, 2015). Therefore, PSCs have been built up by integrating nitrogen-doped rGO into P3HT:PCBM ALs. The maximum PCE attained (4.5%) with this device (i.e., ITO/PEDOT:PSS/N-doped graphene:P3HT:PCBM/Al) was around 40% more compared to that of a device without G. In general, the efficiency of these PSCs with G-based materials is less compared to that of traditional cells combining C60 and its derivatives. Advance research should be conducted to optimize the device fabrication process to develop PSCs with maximum PCE and to regulate the structure and characteristics of G (Figure 7.7) (Nair et al., 2008; DíezPascual et al., 2015).

Figure 7.7: (A) Diagrammatic view of the synthesis of reduced graphene oxide quantum dots (rGOQDs) and graphene oxide quantum dots (GOQDs), where the edge functional groups are regulated by controlling the thermal reduction time; (B) J-V curves of the PSCs with various kinds of GQDs. Source: https://pubmed.ncbi.nlm.nih.gov/23889189/.

7.4. GRAPHENE/POLYMER NANOCOMPOSITES AS INTERFACIAL LAYERS (IFLS) The IFLs that are the electron-transporting layer and hole-transporting layer within the anode/or cathode and active layer respectively, firmly condition the efficiency of PSCs. They are usually utilized for the improvement of the electrical contacts and the enhancement of the charge collection and transport. Moreover, IFLs can enhance radiation distribution and light absorption in the active layer and improve performance stability. Chargecarrier recombination is reduced by IFLs in electrodes by selectively

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permitting the wanted carriers to penetrate and restricting carriers from moving towards the opposite electrodes (Soltani-Kordshuli et al., 2016; La Notte et al., 2018). This is significantly important in BHJ PSCs, where the acceptor and donor semiconductors are distributed randomly in the active layer. Hence, both semiconductors could approach cathode and anode, growing the probability of recombination. IFLs can be developed to attain less contact resistance at the electrodes, bypassing this specification to be satisfied at the active layer, which can be devised without that constraints. The absorption and light distribution in PSCs is improved by IFLs too by minimizing the reflection of light at interfaces (Wu et al., 2008; DiezPascual and Diez-Vicente, 2016). Because of the advantages of the G and its derivatives that they have superior energy-band structure which gives less corrosion for the ITO electrode and efficient charge transport, they have been used in the role of both kinds of layers (Su et al., 2011). As the hole transport layer, a GO/PEDOT:PSS composite was utilized in a PCDTBT:PC71BMbased BHJ PSC, resulting in a PCE of 4.28%, which is more compared to devices using either PEDOT:PSS or GO as hole transport layers (PCEs of 3.57% and 2.77%, respectively). Additionally, increased stability and reproducibility were noticed. The enhanced performance was attributed to the well-matched work function of PEDOT:PSS and GO that minimized series resistance and maximized charge carriers’ mobility. Apart from that GO could effectively limit the electrons because of its high bandgap of ~3.6 eV, resulting in a maximized shunt resistance (Dreyer et al., 2014; CarrascoValenzuela et al., 2017). A PEG-modified Au NPs/GO nanocomposite was developed as the holetransport layer within a device having the structure: ITO/PEDOT:PSS(Au@ PEG-GO)/PBDTTT-CT by Chuang and Chen (2015). The solubility of the nanocomposite was improved by PEG, so it was well diffused in different organic solvents and water. A comparably maximum PCE of 7.26% was obtained. Except for that, when the nanocomposite was arranged at various locations, various spectral-enhancement regions were found to expose several dielectric environments adjoining the NPs, which could be advantageous for progressing the broadband absorption of solar irradiation (Eda and Chhowalla, 2010; Chen et al., 2013). A polyacrylonitrile-grafted rGO (PRGO) nanocomposite was synthesized by Jung et al. (2017) that was enacted being a hole-transport layer in PSCs formed on PTB7-Th. A covalent strategy was used for the fabrication of the nanocomposite based on graft polymerization with styrylfunctionalized GO and acrylonitrile, and in situ radiation-induced reduction.

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It presented good electrical conductivity (0.87 S/cm), homogeneous thinfilm morphology, phenomenal weather stability, and high work function (4.87 eV). For the improvement of PV stability and efficiency, this combo of characteristics makes it appropriate as interfacial material in PSCs (Lima et al., 2016). The resulting device displayed Voc of 0.76 V, FF of 0.64, a PCE of 7.24%, and Jsc of 14.78 mA/cm2, whereas the device based on PEDOT:PSS displayed Voc of 0.78 V, FF of 0.66, PCE of 7.17%, and Jsc of 13.91 mA/cm2. More significantly, superior durability was shown by it (Jo et al., 2011; Pei and Cheng, 2012). Hu et al. (2015) proposed a novel electron-transport layer based on ZnO nanocrystals diffused in a G matrix along with ethyl cellulose (EC) being a stabilizer. The ZnO@G:EC nanocomposites with several G contents displayed an almost smooth morphology, and maintained the initial actual structure of G with maximum conductivity. A PCE of 3.9% was given by the device based on P3HT:PC61BM having the structure: ITO/ZnO@G:EC/ P3HT:PC61BM/MoO3/Ag, which is around 20% maximum than that accompanying bare ZnO nanocrystals. Taking over the active layer by PTB7:PC71BM, the PCE increased to 8.4%. For PSCs, this simple way can give highly conductive electron-transport layers (Figure 7.8) (Xu et al., 2009; Konios et al., 2015).

Figure 7.8: (A) Graphical illustration of ITO/ZnO@G:EC/P3HT:PC61BM/ MoO3/Ag device; (B) atomic force microscope (AFM) pictures of ZnO@G:EC nanocomposites with various G contents; (C) J-V curves of PSCs with various nanocomposites.

Source: https://pubmed.ncbi.nlm.nih.gov/26143932/.

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CHAPTER

8

ORGANIC TANDEM SOLAR CELLS

CONTENTS 8.1. Introduction..................................................................................... 212 8.2. Polymer Tandem Solar Cell Structure and Operation Mechanism................................................................................... 213 8.3. Development and Current Status of Polymer Tandem Solar Cells (SCS)............................................................... 215 8.4. Polymer Materials for Tandem Solar Cells (SCS)............................... 217 8.5. Tandem Device Engineering and Measurement................................ 217 References.............................................................................................. 223

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8.1. INTRODUCTION The manufacturing of solar cells (SCs) with tiny molecules was initiated by the use of the tandem structure in organic SCs through the process of thermal evaporation. Structures with many layers of tiny molecule OPVs and organic light-emitting diodes (OLEDs) are sustained by vacuum evaporation. Through the evaporation of more layers, it can usually be elongated into tandem structures. In 2003, it was revealed by Peumans et al. (2003) that the extension of their photovoltages can be led by an ultrathin evaporated metal between two sub-cells. In Germany, nowadays, Heliotek, and Leo’s group in Institut für Angewandte Photophysik (IAPP) are the professional heads in tiny molecule tandem OPV (You et al., 2013a, b). Today, polymer SCs in comparison to thermal evaporated tiny molecule cells have a more simplified device structure and better performance in single-junction devices, but on account of the lack of better solutionprocessed interconnecting layers (ICLs), they presented the concept of polymer tandem. It has many benefits, such as it is helpful in cost-saving and it has the rapid advancement in active materials and interfacial layers (IFLs) in single-junction polymer SCs. Important research struggles in polymer tandem solar cells have been increased. Now 10.6% power conversion efficiency (PCE) has been reached in a polymer tandem solar cell (Chapin et al., 1954). When the photons of energy more than the bandgap (which controls the photovoltage) consume the hot carries formed by thermalization, a chief loss mechanism for single-junction SCs is noticed, known as photovoltage loss. By utilizing tandem SCs in which two or more single cells consume balancing wavelength ranges that are fixed together, we can reduce the drawbacks of single-junction photovoltaics (PVs). By following this method, we can undoubtedly enhance the use of photon productivity, and also from the usage of materials having different band gaps lower the thermalization losses, and the use of photos can be efficiently increased. While the Shockley-Quiesser drawback of single-junction solar cell is 33.7% at Eg ~1.4 eV, inorganic multijunction approves the tandem SCs with efficiency up to 43.5% (Yu et al., 1995; Shaheen et al., 2001). It was seen by Brabec et al. (2001) that it is probable to attain 15% efficiency polymer tandem SCs in a two-cell tandem arrangement. Within the consumed spectral range (i.e., both cells have a flat EQE from violet to their consumption edge), the expectations of a constant (65%) external quantum efficiency (EQE) for both cells depend on the simulations. Since

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it is valuable in reality to have two sub-cells in the tandem approach with a fewer overlap in consumption, hence the optimization method of the tandem structure can be analyzed. In tandem polymer solar cell design, this can be comprehended by particularly employing PC61BM or PC71BM, which own suggestively diverse extinction coefficients due to their different structure of molecules (Brabec et al., 2001; Peumans et al., 2003). Few outstanding review articles on organic tandem SCs have been issued by Brabec et al. (2001) which delivers an absolute review of the various tandem cell working techniques and device structures. We are primarily engaged in the materials issues of tandem polymer solar cells (both interfacial materials and active layer materials for sub-cell interconnection) in this chapter. The latest advancement and performance measurements in polymer tandem SCs are also mentioned.

8.2. POLYMER TANDEM SOLAR CELL STRUCTURE AND OPERATION MECHANISM A typical polymer tandem cell comprised of two cells stacked on top of each other is shown in Figure 8.1(a) donor-acceptor (D-A) bulk heterojunction (BHJ) solar cell is each individual cell. Figure 8.1(b) is the absorption of wide and low bandgap (LBG) cells and the solar spectrum is also visible in this figure.

Figure 8.1: Conventional polymer tandem cell containing two cells stacked on top of each other. Source: https://www.sciencedirect.com/science/article/pii/ S0079670013000397.

It can cover 60% of the photons from the sun when we stack two complementary cells with a small (Eg=1.4 eV) and large bandgap (Eg=1.9

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eV) polymer. We can connect the two sub-cells either in parallel or in serial in the tandem device by varying the interconnecting scheme. Figure 8.2 represents the energetic illustration of the device where the serial connection is the most widely adopted one (Günes et al., 2007; Thompson and Fréchet, 2008).

Figure 8.2: Energetic illustration of a device. Source: https://onlinelibrary.wiley.com/doi/abs/10.1002/adma.200702337.

We describe the sub-cell aside from Glass/ITO substrates as a rear cell and the sub-cell near to the Glass/ITO substrate side a front cell. The low and wide bandgap (WBG) polymer depends on the sub-cells that are generally utilized as rear and front cells, respectively, for the maximum usage of higher energy photons and enhancement of present balance (Bundgaard and Krebs, 2007). To guarantee the arrangement of the quasi-Fermi level of electrons in the acceptor of the lowest cell with the quasi-Fermi level of holes in the donor of the highest cell (or conversely in an upturned architecture) is the role of the ICL. To put it another way, the recombination of holes approaching from one sub-cell with electrons approaching from the other should be permitted in the intermediate layer, and the summation of the two sub-cells is the open-circuit voltage of the tandem solar cell (Chen et al., 2009; Dennler et al., 2009).

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8.3. DEVELOPMENT AND CURRENT STATUS OF POLYMER TANDEM SOLAR CELLS (SCS) We need good-performance individual cells utilizing polymers with complementary consumption of solar spectrum to build high-productivity tandem polymer SCs. Utilizing two PV cells, the perception of demonstration of the tandem polymer solar cell was initially directed based on the Voc summation in the former years of polymer solar cell development. It was made up of the polymer/tiny molecule hybrid or the same conjugated polymer (PPV derivative or P3HT) (Li et al., 2007). The primary explanation of a semi-transparent version of MEH-PPV depending upon polymer solar cell was given by Yang and Li (2016) and to realize the view, he stacked it straight with another non-transparent solar cell. To realize an optimal tandem cell in that stacked geometry with alike absorption spectra, the chief parameter to tune the relative performance of the two cells is the width of the organic films. In 2006 Hadipour et al. showed the primary polymer tandem solar cell which contains two subcells having two distinct materials (Figure 8.3).

Figure 8.3: The absorption spectra and the tandem device structure. Source: https://pubs.rsc.org/en/content/articlelanding/2011/ee/c0ee00754d.

The efficiency has been enormously improved in contrast with each subcell even though the tandem efficiency is just around 0.57%. The (n-type) TiO2/ (p-type) PEDOT:PSS ICL was used by Li et al. (2007) to connect two higher performance single cells to comprehend a tandem structure. The rear cell is formed with poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C71 butyric acid methyl ester (PC70BM), and the front cell active layer is [6,6] phenyl-C61 butyric acid methyl ester (PCBM) blend, and poly[2,6-(4,4-bis(2-ethylhexyl)-4-Hcyclopenta[2,1-b;3,4-b’] dithiophene)-alt-4,7-(2,1,3benzothiadiazole)] (PCPDTBT). 6.5% efficiency of the tandem solar cell

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was accomplished with the front cell of 3.0% efficiency, and the rear cell of 4.7% efficiency. The LBG polymer PCPDTBT cell exhibited a lower fill factor (FF) and quantum efficiency compared to the P3HT:PCBM cell which narrows down the performance of the tandem device. Currently, a special LBG polymer is designed poly(2,6’-4,8-di(5-ethylhexylthienyl) benzo[1,2-b;3,4b]dithiophene-alt-5-dibutyloctyl-3,6-bis(5-bromothiophen2-yl)pyrrolo[3,4-c]pyrrole-1,4-dione) (PBDTT-DPP) having bandgap approximate to ~1.45eV. This material is specifically produced with confined absorption to prevent the overlap with P3HT. 6–7% efficiency was shown by both inverted and regular structure single-junction devices with the FF around 70% (Coakley and McGehee, 2004). An excellent joint with a P3HT:ICBA cell as the WBG cell is formed by this leading to a tandem efficiency of 8.62% (i.e., certified by NREL). Development in polymer tandem SCs was represented by this novel concept. Figure 8.4 depicts the device structure and the certified results. An additional enhancement in processing and material has turned to an almost 10% PCE efficient tandem solar cell. In the near future, this paves a strong ground for aiming for 15% PCE (Li et al., 2011). In the next sections, the polymer materials will be focused on including the polymers, which could be or have been utilized for tandem polymer SCs, and also the ICLs will be discussed.

Figure 8.4: Device system and results of current-voltage. Source: https://www.nature.com/articles/ncomms2411.

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8.4. POLYMER MATERIALS FOR TANDEM SOLAR CELLS (SCS) The double junction tandem solar cell is the simplest form of a tandem solar cell which comprises a rear cell with a LBG and a front cell with a WBG, or vice-versa. For the achievement of high-performance tandem SCs, the polymer bandgap for each sub-cell must be chosen carefully. Brabec et al. (2001) simulation demonstrated that while assuming both cells have 65% flat quantum efficiency, then to obtain 15% efficiency in a tandem solar cell, the LBG cell donor must have a bandgap of 1.3 eV, and the WBG cell donor must have a bandgap of 1.6 eV. The EQE of the LBG (i.e.,