A comprehensive study of mechanically stacked tandem photovoltaic devices: Materials selection and efficiency analysis using SCAPS

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Energy Conversion and Management 300 (2024) 117904

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A comprehensive study of mechanically stacked tandem photovoltaic devices: Materials selection and efficiency analysis using SCAPS Md. Nazmul Ahsan Dipon a, 1, Miah Abdullah Sahriar a, 1, Saugata Sarker a, 1, Md. Tohidul Islam b, Abrar Rauf a, Mohd. Rakibul Hasan Abed a, Ahsiur Rahman Nirjhar a, Sadika Jannath Tan-Ema a, Kazi Md. Shorowordi a, Saquib Ahmed c, * a b c

Department of Materials and Metallurgical Engineering (MME), Bangladesh University of Engineering and Technology (BUET), East Campus, Dhaka 1000, Bangladesh Department of Materials Design and Innovation, University at Buffalo, Buffalo, NY 14260, United States Department of Mechanical Engineering Technology, SUNY – Buffalo State, 1300 Elmwood Avenue, Buffalo, NY 14222, United States

A R T I C L E I N F O

A B S T R A C T

Keywords: Mechanically stacked four-terminal tandem Numerical simulation Solar cell capacitance simulator Power conversion efficiency MAPbI3 on c-Si

To advance the photovoltaic industry, highly efficient solar modules with reduced manufacturing and installa­ tion costs are needed amidst rapid market growth. To surpass the performance limitations of single-junction solar cells, multijunction configurations have been the focus of rigorous study. The current work showcases a comprehensive investigation into the development and optimization of four terminal tandem solar cell archi­ tectures, with a focus on exploring the most technologically viable impactful, and promising combinations of top cell materials (CdTe, GaAs, MAPbI3, and MASnI3) and bottom cell options (c-Si and CIGS). Through numerical simulations using the Solar Cell Capacitance Simulator SCAPS and meticulous analysis, considering crucial pa­ rameters such as bandgap, charge carrier mobility, and defect densities, this study aims to identify the most promising material combinations for achieving high-efficiency tandem devices. The findings reveal that when cSi is used as the bottom cell absorber, the tandem device as a whole produces a higher power conversion effi­ ciency than when CIGS is used. The top cell options, namely CdTe, GaAs, MAPbI3, and MASnI3 in conjunction with c-Si as the bottom cell, achieve maximum efficiencies of 27.23%, 29.31%, 31.66%, and 15.64% respec­ tively. In contrast, when paired with CIGS as the bottom cell, the efficiencies slightly decrease to 23.76%, 26.43%, 28.45%, and 12.83%. Notably, the investigation involving 25%, 50%, and 75% Br-doped MASnI3 alongside c-Si reveals efficiencies of 16.59%, 14.94%, and 14.28% respectively. In the case of CIGS, the corre­ sponding efficiencies for 25%, 50%, and 75% Br doping are slightly lowered to 13.78%, 12.17%, and 11.43%. The obtained results offer valuable insights that can guide future research endeavors and foster the development of more efficient and commercially viable solar energy conversion technologies in the field of tandem photovoltaics.

1. Introduction The photovoltaic industry has witnessed remarkable expansion in recent years. To advance the industry further, it is crucial to develop devices and modules that offer increased efficiency while reducing manufacturing and installation costs [12]. Currently, single junction c-Si cells dominate the solar market, accounting for over 90 % of global annual production [3]. The PCE of single c-Si cells, on the other hand, reached a saturation point of roughly 26 %. [45]. Additionally, ac­ cording to Shockley and Quessier, in a single-junction solar cell, there

exists a theoretical saturation limit of approximately 33 % for PCE [6]. To surpass this limit, researchers are focusing on the development of multijunction solar cells that utilize materials with different bandgaps. This approach proved to be effective in enhancing solar device perfor­ mance beyond the theoretical limitations. At present, the focus of research is on developing tandem solar cells that are affordable and capable of achieving the highest possible PCE. [17]. Tandem cells consist of top cells with high bandgap (1.5–1.9 eV) and bottom cells with low bandgap (0.9–1.3 eV) to tap into a larger section of the electro­ magnetic spectrum. The tandem architecture enables the top cell to absorb photons with high energy, minimizing thermalization losses and

* Corresponding author. E-mail address: [email protected] (S. Ahmed). 1 Contributed equally to this manuscript. https://doi.org/10.1016/j.enconman.2023.117904 Received 4 August 2023; Received in revised form 6 November 2023; Accepted 17 November 2023 0196-8904/© 2023 Elsevier Ltd. All rights reserved.

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Nomenclature 2T 4T CBM CdTe CIGS c − Si DOS dmati ETL Eg FF GaAs HTL JMP JSC MAPbI3 MASnI3 NA NC ND NV PCE Pin QE S(λ) S0 (λ) VBM VMP VOC αmati (λ) ε ε0

η μe μh ϰ

Two terminal Four terminal Conduction band minimum Cadmium telluride Copper indium gallium selenide Crystalline silicon Density of states Thickness of a layer of a material Electron transport layer Bandgap Fill factor Gallium arsenide Hole transport layer Current density at maximum power Short − circuit current density Methyl ammonium lead iodide Methyl ammonium tin iodide Acceptor density Density of states at conduction band Donor density Density of states at valence band Power conversion efficiency Input power Quantum efficiency Filtered spectrum from top cell Incident A.M 1.5G spectrum Valence band maximam Voltage at maximum power Open − circuit voltage

Absorption coefficient of layer of material at different wavelengths Dielectric constant Efficiency of solar cell Electron mobility Hole mobility Electron affinity

generating high voltage, while the bottom cell captures transmitted lowenergy photons, resulting in improved utilization of the solar spectrum [8]. For double junction devices, the two most popular tandem archi­ tectures are mechanically stacked 4 T tandem and 2 T monolithic structures. In the widely employed 4 T tandem structure, two single solar cells are fabricated independently without any additional pro­ cessing requirements. Afterward, they are arranged in a stacked configuration and linked together via an external circuit [9]. The theoretical PCE limit for the 4 T tandem structure can reach up to 45 % when the bandgaps of the top and bottom cells are 1.73 eV and 0.93 eV, respectively [10]. The fabrication and operation processes for 4 T tan­ dem devices are relatively straightforward due to their simplicity. On the other hand, the 2 T monolithic structure involves connecting single solar cells in series using an interconnection layer, treating the entire device as a single entity [9]. This structure offers advantages such as lower manufacturing costs owing to the use of a single transparent conducting electrode for the entire device and the stacking of cells at relatively mild temperatures. However, a significant limitation of the 2 T monolithic structure is the requirement for matching the current den­ sities between the single junction cells to avoid power loss. This requires thickness values of the absorber layer of both single junction cells to be tuned [7]. This restriction poses challenges in the fabrication of the optimum 2 T monolithic structure compared with the 4 T tandem structure. Consequently, the current research focuses on an extensive

study of 4 T tandem architecture. When choosing the upper and lower cells for tandem setups, the bandgap of each cell plays a pivotal role in optimizing the utilization of the solar spectrum. c-Si is a suitable candidate as a bottom cell absorber material due to its low bandgap of 1.12 eV, low manufacturing cost, nontoxicity, and excellent stability [111213]. Cadmium Indium Gallium diselenide (CIGS is another potential candidate for the bottom cell absorber material, which possesses a narrow bandgap of approximately 1.2 eV. It offers advantages such as simpler and more cost-effective processing as well as excellent stability and durability [141516]. For the top cell, both inorganic and organic materials can be suitable depending on their wide bandgap. Inorganic options, such as CdTe and GaAs, are potential candidates. GaAs, with a direct bandgap of 1.42 eV [17], demonstrates high mobility, high coefficient of absorption, and excellent radiation resistance [18]. However, it is associated with high manufacturing costs [19]. Conversely, CdTe, with a direct bandgap of 1.5 eV and long-term stability, has emerged as the solar industry’s most lucrative commercial thin film technology [2021]. In recent times, solar cells based on a distinctive ABX3 structure, known as organic–inorganic hybrid perovskite solar cells, have made tremendous progress and have drawn a great deal of attention [2223]. Lead-based perovskites, such as MAPbI3, have exhibited appealing characteristics, including extended charge carrier lifetimes and mobil­ ities, as well as substantial absorption coefficients [242526]. Despite these favorable traits, the incorporation of lead into these devices gives 2

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Fig. 1. Contrasting light intensity of A.M 1.5G 1 sunlight and filtered spectrum post-top cell passage utilizing CdTe as absorber layer.

rise to considerable challenges, as it can have detrimental impacts on both human health and the environment [27]. As a potential substitu­ tion, the tin-based perovskite structure, MASnI3, possesses a bandgap of 1.3 eV and stands out as a non-toxic option, offering promising potential [28]. Although Sn-based perovskites may show a propensity for oxida­ tion under environmental conditions, they consistently uphold dependable device performance over a designated range of temperatures [29]. Currently, research endeavors in the field of photovoltaics aim to enhance their performance beyond the threshold limit. Experimental efforts are concentrated on exploring viable material combinations for tandem architecture. This study concerns juxtaposing the various com­ binations (top cell: CdTe, GaAs, MAPbI3, and MASnI3; bottom cell: c-Si and CIGS) against each other in a very comprehensive way, evaluating relativistic performance, and delineating all relevant kinetics, to offer experimentalists an accurate and clear pathway for investigation. The selection of the materials mentioned in this study is based on their strong technological viability and significant impact on the field. With sus­ tainability being a key consideration, these materials offer promising prospects from a manufacturing standpoint, making them highly attractive for the development of next-generation photovoltaic tech­ nologies. Solar cell capacitance simulator (SCAPS) 1D software is used to conduct the simulations. Furthermore, considering the relatively low efficiency of the MASnI3 single cell, the investigation also includes examining the impact of different doping levels (25 %, 50 %, and 75 % Br) on the performance of MASnI3, given that Br substitution of I can lead to higher performance.

comprehensive capabilities for analyzing parameters and characteris­ tics, and has been instrumental in evaluating the performance and ef­ ficiency of the studied tandem structures [3132]. Owing to the absorption coefficient and layer thickness of the top cell, the bottom cell in a tandem arrangement typically gets a small percentage of the ambient sunlight (A.M. 1.5G 1) that the top cell ab­ sorbs [33]. Mechanically stacked 4 T tandem devices can experience optical losses between single junction cells. The SCAPS 1D program does not allow multijunction solar cell modeling. In the tandem configura­ tion, the top and bottom cells were considered to be in an ideal junction with no electrical or optical loss. Utilizing A.M 1.5G 1 sunlight, the simulation is conducted for the top cell, while for the bottom cell, the simulation employs a filtered spectrum. This strategy is widely used for the simulation of tandem structures using SCAPS 1D [3435]. The filtered spectrum is calculated using Equation (1). ∑4 (1) S(λ) = S0 (λ)exp( i=1 − αmati (λ)dmati ) The absorption coefficient, denoted by α(λ), is related to the photon flux Nphoton (λ,x) at position x and the thickness of the layer represented by d. It is noteworthy that the impact of uncertain environmental condi­ tions on the effectiveness and functionality of renewable energy sources, such as wind turbines and solar cells, is a significant aspect to consider [363738]. The present investigation adopts a uniform A.M 1.5G 1 sun­ light irradiation for all devices to assess the PCE and to facilitate a comprehensive evaluation and comparison of their performance under identical irradiation conditions. In the tandem device with a mechanical stacking of 4 T, notable differences can be observed in the open-circuit voltage (Voc), fill factor (FF), and short-circuit current density (Jsc) between the top and bottom cells. To derive the device FF, a specific calculation approach is employed, taking into account various parameters and factors:

2. Methodology To conduct numerical simulations and evaluate the photovoltaic qualities of the tandem configurations, this study makes utilization of the SCAPS software, a widely used and standardized tool created by the Department of Electronic and Informative Systems (ELIS) at the Uni­ versity of Gent in Belgium [30]. SCAPS has gained worldwide recogni­ tion as a widely accepted tool for simulating solar cells, offering

FF =

3

JMp VMp Jsc Voc

(2)

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Fig. 2. Left column is top cells (a) CdTe, (b) GaAs, (c) MAPbI3, (d) MASnI3: Right column is bottom cells (e) c-Si (f) CIGS.

Fig. 3. Overview of the simulation process.

In this context, the current density (JMp) and voltage (VMp) at the maximum power points are specified. The device efficiency (η) is sub­ sequently determined by normalizing it to the input power (Pin), employing a defined approach that takes into account relevant param­ eters and their interplay:

η=

Voc Jsc FF Pin

power appears to decrease. 3. Device structure The current study focuses on the utilization of CIGS and c-Si as bottom cells, while CdTe, GaAs, MAPbI3, and MASnI3 are employed as top cell absorbers. Numerical analyses have been conducted on me­ chanically stacked 4 T tandem architectures for comparative purposes. Fig. 2 illustrates the schematics of the individual cells used in this investigation, along with the thickness values of their respective mate­ rial layers. Four top cells, as previously mentioned, have been examined. In the CdTe system, ZnO, CdTe, and were used as the absorber, buffer layer, and electron transport layer (ETL), respectively. CdSxTe(1-x), formed during the CdTe film growth process in CSS, was also employed [39]. For

(3)

The individual PCEs of both single junction cells are added to attain the overall PCE. The filtered spectrum for different absorber thicknesses was calcu­ lated using Equation (1). CdTe is used as an example here, and Fig. 1 displays the relative intensity of the input A.M. 1.5G spectrum and the reduced light intensity after going through various CdTe top cell absorber layer thicknesses. As the absorber thickness rises, the incident 4

[48] [49] [50 51] 1 1.6 100

1 1.6 25

Bulk CdTe

1 × 109 1 × 1015 0

10 1020 1018 1018

16

1016

1014 1014 1017

Defect location

109 0 1017

[45 46] [47]

[43] [44]

[42]

[41] [39]

31 31 50 60 60 400 400 400 25 450 450 450 80 2 × 10-04 100 100 350 500 500 8500 8500 8500 100 1400 1400 1400 200 1 × 10-04 0 0 0 1× 5× 1× 0 0 2× 0 1× 1× 1× 1× 0 1019 1018 0 0 0 0 1017 0 1020 0 0 0 0

Table 2 SCAPS Input parameters for bulk and interfacial defects of all devices. Defect energy level, above Ev (eV)

Type of defect

Capture cross section for electrons (cm− 2)

Capture cross section for holes (cm− 2 )

2.0 × 10-

0.6

Neutral

1.0 × 10-12

1.0 × 10-12

14

Bulk CIGS

1.0 × 10-

1.0 × 10-15

1.0 × 10-15

Bulk GaAs

5.0 × 10-

1.0 × 10-15

1.0 × 10-15

Bulk MAPbI3 Bulk MASnI3 TiO2 / MAPbI3 interface MAPbI3 / Cu2O interface TiO2 / MASnI3 interface MASnI3 / SpiroOMeTAD interface

5.0 × 10-

1.0 × 10-15

1.0 × 10-15

1.0 × 10-

1.0 × 10-15

1.0 × 10-15

1.0 × 10-

1.0 × 10-19

1.0 × 10-19

1.0 × 10-

1.0 × 10-19

1.0 × 10-19

1.0 × 10-

1.0 × 10-19

1.0 × 10-19

1.0 × 10-

1.0 × 10-19

1.0 × 10-19

16 15 14 16 14

15

14

14

3.9 × 1018 1 × 1018 6 × 1017

1.8 × 1019 1.8 × 1019 2.2 × 1018 7.5 × 017 7.5 × 1017 7 × 1018 7 × 1018 7 × 1018 1.5 × 1019 1.04 × 1019 1.04 × 1020 1.04 × 1021 1.1 × 1019 1 × 1019

Doping concentration in MASnI3

Band gap, Eg (eV)

Electron affinity, χ (eV)

Reference

25 % 50 % 75 %

1.43 1.66 1.75

3.97 3.75 3.67

[52]

Table 4 Comparison between PV parameters of the current study and previous findings for top cells. 2.75 × 1018 ×1018 2 × 1017

2.2 × 1018 2.2 × 1018 2.2 × 1018 7.5 × 1017 7.5 × 1017 4.7 × 1017 4.7 × 1018 4.7 × 1019 2 × 1017 2.8 × 1019 2.8 × 1020 2.8 × 1021 2.02 × 1018 2.8 × 1019

Top Cell: Cell CdTe

10 8.2 10

GaAs

MAPbI3

3.93 4.1 4.2

Result Current study Previous Work % Offset Reference Current study Previous Work % Offset Reference Current study Previous Work % Offset Reference Current study Previous Work % Offset Reference

J-V Characteristics Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

0.93 1.00 7 [39] 0.90 0.87 3.45 [53] 1.09 1.16 6.03 [54 29] 0.79 0.88 10.23 [55 56]

25.64 26.15 1.95

79.27 76.9 3.08

18.85 18.3 3.01

24.93 24.52 1.67

84.86 85.70 0.98

19.73 21.05 6.13

22.32 22.83 2.28

82.52 76.8 6.93

20.14 20.47 1.61

25.36 24.8 2.26

34.92 38 8.11

6.92 6.80 1.76

GaAs, its p-type layer served as the hole transport layer (HTL), followed by an intrinsic GaAs absorber layer. An n-type GaAs layer was then used as the ETL. Regarding the perovskite MAPbI3 cell, TiO2 was employed as the ETL and Cu2O as the HTL as previous studies have shown the highest effi­ ciency with these materials [29]. Similarly, for the MASnI3 device, spiro OMeTAD served as the HTL, and TiO2 was used as the ETL [40].

1.55 1.3 3.26

3.3 3.3 2.42 1.41 1.45 1.42 1.42 1.42 1.2 1.12 1.12 1.12 2.17 3.06

i-ZnO ZnO:Al CdS CdSxTe1-x CdTe p-GaAs i-GaAs n-GaAs CIGS n + Si p Si p + Si Cu2O SpiroOMeTAD MAPbI3 MASnI3 TiO2

4.4 4.4 4.5 4.28 4.28 4.07 4.07 4.07 4 4.05 4.05 4.05 3.2 2.05

MASnI3

Band gap, Eg (eV)

Electron affinity, χ (eV)

Defect density (cm− 2)

Table 3 Critical Parameters for doped MASnI3 devices.

9 9 9 9.4 9.4 12.4 12.4 12.4 13.6 11.9 11.9 11.9 7.11 3

Density of states at conduction band, Nc (cm− 3) Dielectric constant, ε/ε0

Energy Conversion and Management 300 (2024) 117904

Parameter

Table 1 SCAPS input parameters for ETL, HTL, and absorber layer of all devices.

Density of states at valance band, Nv (cm− 3)

Donor density, ND (cm− 3)

Acceptor density, NA (cm− 3)

Electron mobility, μe (cm2/Vs)

Hole mobility, μh (cm2/Vs)

Reference

Md. Nazmul Ahsan Dipon et al.

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4. Simulation setup and validation

Table 5 Comparison between PV parameters of current work and previous findings for bottom cells.

The flow chart illustrated in Fig. 3 outlines the comprehensive simulation setup adopted in this study. Optimal utilization of solar spectrum is ensured by selecting top cells with high bandgap and bottom cells with low bandgap. Proper ETL and HTL are chosen to facilitate effective charge carrier separation, drawing from previous research on single junction devices. The top cells and bottom cells are simulated with A.M 1.5G 1 spectrum. To perform the simulation using SCAPS-1D, a specific set of input parameters must be defined. For this study, the required input parameters were obtained from an extensive range of experimental and numerical investigations. These parameters, which are crucial for the simulation, are outlined in Table 1. Additionally, Table 2 provides a comprehensive list of the simulation input parame­ ters about bulk and interface defects. Table 3 lists the modified pa­ rameters for Br-doped MASnI3 devices. As validation exercises, simulations were conducted separately on single-junction top and bottom cells using the parameters outlined in Table 1. The obtained results were then compared to previous experi­ mental and numerical studies of the same devices. In Fig. 2, a schematic representation displays the thicknesses of all the layers within each solar device. J-V characteristics of the top cells and bottom cells are enlisted in Table 4 and Table 5 respectively. Comparison between the current study and the findings from published works shows high congruence. This confirms the accuracy and reliability of the methodology employed. The top cells are simulated at varying absorber layer thickness and Equation (1) is utilized to calculate the filtered spectrum from top cells at different absorber layer thicknesses. Subsequently, the simulation of bottom cells is conducted based on the derived filtered spectrum. The 4 T tandem efficiency, obtained as the sum of the PCE of both top and bottom cells, is measured across varying top cell absorber thicknesses. Finally, the identification of the top three tandem devices, delivering the highest PCE among the 14 devices considered, concludes the analysis.

Bottom Cell Cell

Result

J-V Characteristics Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

C-Si

Current study

0.76

39

85.38

25.36

0.76 0 [57 58] 0.75 0.74 1.35 [10]

40.04 2.60

85.47 0.11

25.96 2.31

CIGS

Previous Work % Offset Reference Current study Previous Work % Offset Reference

35.63 35.4 0.65

76.43 77.5 1.38

20.54 20.3 1.18

The c-Si bottom cell comprises three layers, each serving specific functions. At the back surface, there is a 20 µm thick layer of heavily implanted p-type Si material, which acts as the back surface field (BSF). Serving as the absorber, the middle layer consists of a moderately doped 500 µm thick p-type Si. Lastly, the top layer incorporates a highly doped n-type Si material, functioning as the emitter. For the CIGS bottom cell, an aluminum-doped ZnO layer serving as ETL and a CdS buffer layer were integrated with the CIGS absorber layer. The 4 T configuration involves four terminal points, with two allo­ cated to the top cell and the remaining two attached to the bottom cell. When sunlight strikes the top cell, it initiates excitation, resulting in the transportation of electrons through the ETL, while the holes are trans­ ported via the HTL. The bottom cell employs a c-Si material that absorbs photons, while specialized layers made of n and p-doped Si serve different purposes, not only facilitating the separation of electrons and holes but also enabling their efficient conduction.

Fig. 4. CdTe on c-Si system (a) Schematic diagram of 4 T tandem architecture (b) Energy band diagrams (c) PCE of top CdTe cell, bottom c-Si cell, and overall tandem device (d) J-V curve of CdTe and c-Si cells (e) QE of CdTe and c-Si cells. 6

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Fig. 5. CdTe on CIGS system (a) Schematic diagram of 4 T tandem architecture (b) Energy band diagrams (c) PCE of top CdTe cell, bottom c-Si cell, and overall Tandem device (d) J-V curve of CdTe and CIGS cells (e) QE of CdTe and CIGS cells.

discontinuities or misalignments between the two layers. The buffer layer facilitates a smooth transition, minimizing energy losses and enhancing the collection efficiency of charge carriers. Fig. 4(b) shows that the absorber layer, the ETL (ZnO: Al), and the buffer layer (CdS) in the CdTe on c-Si tandem device meet the basic conditions for energy band alignment, ensuring optimum collection of charge carriers from the absorber. Fig. 4(c) shows the performance trends of the CdTe cell, c-Si cell, and the total 4 T tandem structure in terms of PCE concerning CdTe absorber thickness. With the increase in thickness of the CdTe absorber, the ef­ ficiency of the CdTe cell also improves. However, the efficiency of the bottom c-Si cell decreases as the absorber thickness of CdTe increases. Because the top cell absorbs the majority of the incident light, a small portion is available for the bottom cell to absorb. The cumulative effi­ ciency of the 4 T tandem cell results from adding the PCE values of both single junction CdTe and c-Si cells. Based on Fig. 1, the maximum PCE of the tandem structure is 27.23 % with a CdTe absorber thickness of 0.3 µm. The bottom cell absorber thickness remains constant at 300 µm throughout the simulations. As the CdTe absorber thickness further in­ creases, the PCE of the 4 T tandem cell decreases slightly and then reaches a point of stability. The J-V curve in Fig. 4(d) presents the current–voltage characteris­ tics of the CdTe and c-Si cells. When comparing the two, the c-Si single junction device exhibits a higher Jsc of 39 mA/cm2 but a lower Voc of 0.76 V in contrast to CdTe, which displays Jsc of 25.64 mA/cm2 and Voc of 0.93 V. The quantum efficiency (QE) of the CdTe and c-Si is displayed in Fig. 4(e). QE provides insight into the efficiency of converting incident photons into measurable output signals. CdTe demonstrates QE values of 90–98 % for photon energies ranging from 1.8 to 3 eV, while c-Si ex­ hibits QE values of 90–98 % for photon energies between 1.25 and 2.60 eV. CdTe absorbs high-energy photons, while c-Si absorbs the lowerenergy photons that pass through from CdTe. As a consequence, this configuration utilizes a wider percentage of the solar spectrum, resulting

Table 6 J-V characteristic parameters and PCE values for CdTe on c-Si and CdTe on CIGS 4 T tandem devices. Cell

Voc (V)

Jsc (mA/ cm2)

FF (%)

PCE (%)

CdTe (0.3 µm) c-Si with filtered spectrum (300 µm) CIGS with filtered spectrum (3 µm) CdTe on c-Si

0.9 0.74

22.54 18.13

77.44 85.16

15.78 11.45

0.73

14.43

75.78

7.98

0.90, 0.74 0.90, 0.73

22.54, 18.13 22.54, 14.43

77.44, 85.16 77.44, 75.78

27.23

CdTe on CIGS

23.76

5. Results and discussion: analysis of tandem 4 T 5.1. CdTe on c-Si and CdTe on CIGS Fig. 4(a) displays the arrangement of a mechanically stacked tandem device that combines CdTe on c-Si. In this setup, CdTe captures sunlight with the A.M 1.5G 1 spectrum, while c-Si receives a filtered spectrum. The energy band alignment diagram depicted in Fig. 4(b) plays a crucial role in facilitating efficient charge separation at the interface between the two layers. To extract current from a solar cell, it is critical to properly segregate and route the photogenerated electron and hole pairs via an external circuit. To achieve this, certain criteria need to be met. For successful electron extraction, the absorber layer’s conduction band minimum (CBM) must be greater than that of the electron transport layer (ETL) and lower than that of the hole transport layer (HTL). Similarly, for efficient hole flow, the absorber layer’s valence band maxima (VBM) must be greater than the ETL’s and lower than the HTL’s. In solar cells, a buffer layer is often utilized between the absorber layer and ETL. It aims to match the energy levels, reducing any potential 7

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Fig. 6. GaAs on c-Si system (a) Schematic diagram of 4 T tandem architecture (b) Energy band diagrams (c) PCE of top GaAs cell, bottom c-Si cell, and overall Tandem device (d) J-V curve of GaAs and c-Si cells (e) QE of GaAs and c-Si cells.

Fig. 7. GaAs on CIGS system (a) Schematic diagram of 4 T tandem architecture (b) Energy band diagrams (c) PCE of top GaAs cell, bottom c-Si cell, and overall Tandem device (d) J-V curve of GaAs and CIGS cells (e) QE of GaAs and CIGS cells.

in increased tandem device PCE. In Fig. 5(a), the configuration of the CdTe on CIGS 4 T tandem structure is depicted. The energy bands of all the layers depicted in Fig. 5

(b) match the required conditions for effective electrons and holes extraction from the absorber. The trends in PCE for the CdTe, CIGS, and the overall tandem cell with respect to the thickness of the CdTe 8

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CdTe on c-Si and CdTe on CIGS systems.

Table 7 J-V characteristic parameters and PCE values for CdTe on c-Si and CdTe on CIGS 4 T tandem devices. Cell

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

GaAs for c-Si cell (0.7 µm) GaAs for CIGS cell (1 µm) c-Si with filtered spectrum (300 µm) CIGS with filtered spectrum (3 µm) GaAs on c-Si

0.92 0.91 0.73

27.18 29.03 13.11

84.22 83.51 85.05

21.13 22.17 8.18

0.71

7.98

74.87

4.26

0.92, 0.73 0.91, 0.71

27.18, 13.11 29.03, 7.98

84.22, 85.05 83.51. 74.87

29.31

GaAs on CIGS

5.2. GaAs on c-Si and GaAs on CIGS Fig. 6(a) showcases the 4 T tandem structure of the GaAs on c-Si device. In the GaAs top cell, the p-GaAs and n-GaAs layers function as HTL and ETL, respectively. The energy band alignment diagram given in Fig. 6(b) displays the CBM and VBM of the absorbers in both GaAs and cSi cells. PCE trends for the GaAs, c-Si, and the whole 4 T tandem device with regard to GaAs thickness are shown in Fig. 6(c). The maximum effi­ ciency achieved by the overall tandem device is 29.31 % when the thickness of GaAs is 700 nm. Even with a deviation from the absorber thickness that provides maximum efficiency, the device would still maintain at least 94 % of that PCE. The J-V curves for the single junction c-Si and GaAs devices are depicted in Fig. 6(d). The Jsc value of the GaAs device is 24.93 mA/cm2, and the Voc value is 0.90 V. The QE of single junction GaAs and c-Si cells are presented in Fig. 6 (e). GaAs demonstrates excellent QE ranging from 90 % to 99 % for photon energies in the range of 2 to 4.1 eV. Fig. 7(a) presents a schematic representation of the GaAs on CIGS system, illustrating the configuration of the tandem structure. The band alignment of the layers in the bottom cell and GaAs, as demonstrated in Fig. 7(b), meet the requirements for effective retrieval of holes and electrons. Fig. 7(c) showcases similar patterns in the PCE curve for the GaAs on CIGS 4 T tandem system. However, the PCE values for the bottom cell are lower compared to those of the c-Si system, resulting in lower overall tandem cell efficiencies. The maximum PCE achieved by the tandem structure is 26.43 % with a GaAs absorber thickness of 1 µm. In contrast to the GaAs on c-Si system, the maximum PCE in the GaAs on CIGS system is lower (26.44 % vs. 29.31 %) and occurs at a higher thickness of GaAs absorber (1 µm vs. 0.7 µm).

26.43

absorber are presented in Fig. 5(c). At a CdTe absorber thickness of 0.3 µm, the overall tandem structure attained its peak PCE of 23.76 %. However, with a rise in the thickness of the CdTe absorber, the PCE values display a slight decline. As anticipated, the PCE of the CdTe on CIGS 4 T tandem device is lower than that of the CdTe on c-Si device due to the lower bandgap and less favorable material properties of the CIGS absorber compared to c-Si. Fig. 5(d) displays the J-V curve for the CIGS device, showing a Jsc value of 35.63 mA/cm2 and a Voc value of 0.75 V. These figures are marginally smaller than those of the c-Si device, contributing to a lower single junction conversion efficiency. The QE of the single junction CdTe and CIGS devices is shown in Fig. 5(e). As mentioned earlier, CdTe ex­ hibits QE values of 90–98 % for photon energies ranging from 1.8 to 3 eV. For the single junction CIGS device, QE reaches 90–98 % for photon energies in the range of 1.45 to 3.20 eV, while for c-Si it is in the range of 1.25 to 2.60 eV. Consequently, the CdTe on CIGS device exhibits slightly lower efficiency compared to the CdTe on c-Si device. Table 6 provides the photovoltaic performance parameters of CdTe under A.M 1.5 G 1 Sun, c-Si under the filtered spectrum, and the 4 T tandem device for the

Fig. 8. MAPbI3 on c-Si system (a) Schematic diagram of 4 T tandem architecture (b) Energy band diagrams (c) PCE of top MAPbI3 cell, bottom c-Si cell, and overall tandem device (d) J-V curve of MAPbI3 and c-Si cells (e) QE of MAPbI3 and c-Si cells. 9

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Fig. 9. MAPbI3 on CIGS system (a) Schematic diagram of 4 T tandem architecture (b) Energy band diagrams (c) PCE of top MAPbI3 cell, bottom CIGS cell and overall tandem device (d) J-V curve of MAPbI3 and CIGS (e) QE of MAPbI3 and CIGS.

MAPbI3 perovskite exhibits a Jsc of 22.32 mA/cm2 and a high Voc of 1.09 V. Fig. 8(e) illustrates the quantum efficiency (QE) of both single cells in relation to photon energy. The QE of MAPbI3 reaches 90–98 % for photon energies in the range of 2.2–3.6 eV. Due to the low bandgap of cSi, the overall QE of the device is excellent for photon energy ranging from 1.25 to 3.6 eV. In Fig. 9(a), the schematic structure of a MAPbI3 on CIGS device is depicted. Fig. 9(b) showcases the band alignment of both MAPbI3 and CIGS, ensuring efficient collection of charge carriers. Fig. 9(c) showcases the PCE curve for the MAPbI3 on CIGS 4 T tandem system. The behavior of the MAPbI3-on-CIGS tandem device is similar to the MAPbI3-on-c-Si system. The maximum efficiency is 28.45 % for a MAPbI3 layer thickness of 0.4 µm. Then the PCE value decreases to 26.78 % when the thickness of the MAPbI3 absorber increases to 2 µm. Fig. 9(d) displays the J-V curve of the distinct cells within the tandem system. The QE vs photon energy curve in Fig. 9(e) indicates that the overall device performs better for photon energies in the range of 1.5–3.6 eV, with a QE of 90–98 %. The photovoltaic parameters of the single MAPbI3 cell, bottom c-Si, and CIGS cell, and overall 4 T tandem structure are summarized in Table 8.

Table 8 J-V characteristic parameters and PCE values for MAPbI3 on c-Si and MAPbI3 on CIGS 4 T tandem devices. Cell

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

MAPbI3 (0.4 µm) c-Si with filtered spectrum (300 µm) CIGS with filtered spectrum (3 µm) MAPbI3 on c-Si

1.1 0.74

23.87 16.46

81.53 85.13

21.31 10.35

0.73

12.99

75.56

7.14

1.1, 0.74 1.1, 0.73

23.87, 16.46 23.87, 12.99

81.53, 85.13 81.53, 75.56

31.66

MAPbI3 on CIGS

28.45

The J-V curves for the single GaAs and CIGS systems are displayed together in Fig. 7(d), with the Jsc and Voc values mentioned earlier. The QE of GaAs and CIGS are depicted in Fig. 7(e). The overall QE of the tandem device exhibits excellent performance for photon energy ranging from 1.5 to 4.1 eV. Detailed JV characteristics of the GaAs, c-Si, and the overall 4 T tandem GaAs on c-Si and GaAs on CIGS systems are listed in Table 7.

5.4. MASnI3 on c-Si and MASnI3 on CIGS

5.3. MAPbI3 on c-Si and MAPbI3 on CIGS

Fig. 10(a) depicts the structure of a non-lead perovskite (MASnI3) on c-Si 4 T tandem device. The band alignment of all the layers in both the MASnI3 and c-Si satisfies the requirements for the effective separation of charge carriers, shown in Fig. 10(b). In Fig. 10(c), the PCE of the MASnI3 perovskite single cell, c-Si cell, and the overall 4 T tandem structure are shown with respect to the thickness of the MASnI3 absorber. The maximum PCE observed is 15.65 % for an MASnI3 absorber thickness of 0.2 µm. However, this PCE drops significantly to only 8.17 % when the thickness of MASnI3 rises to 2 µm. The low PCE of the tandem archi­ tecture is due to the low PCE of the single junction MASnI3 cell, yielding a value of 7.41 %.

Fig. 8(a) showcases a 4 T tandem device consisting of lead perovskite (MAPbI3) on c-Si, as illustrated schematically. The energy band align­ ments of HTL, ETL, and perovskite layers are illustrated in Fig. 8(b). The relationship between the MAPbI3 absorber thickness and the PCE of the MAPbI3 top cell, c-Si bottom cell, and the overall 4 T tandem structure is depicted in Fig. 8(c). The maximum efficiency of 31.66 % is achieved at a MAPbI3 layer thickness of 0.4 µm. As the thickness in­ creases, the PCE slightly decreases from the maximum value. Fig. 8(d) depicts the J-V curve of the single MAPbI3 and c-Si cells. The 10

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Fig. 10. MASnI3 on c-Si system (a) Schematic diagram of 4 T tandem architecture (b) Energy band diagrams (c) PCE of top MASnI3 cell, bottom c-Si cell and overall tandem device (d) J-V curve of MASnI3 and c-Si cells (e) QE of MASnI3 and c-Si cells.

Fig. 11. MASnI3 on CIGS system (a) Schematic diagram of 4 T tandem architecture (b) Energy band diagrams (c) PCE of top MASnI3 cell, bottom CIGS cell and overall tandem device (d) J-V curve of MASnI3 and CIGS cells (e) QE of MASnI3 and CIGS cells.

The J-V curves of MASnI3 and c-Si cells are presented in Fig. 10(d). MASnI3 exhibits a Jsc value of 25.36 mA/cm2 and a Voc value of 0.79 V. The presence of series resistance and a very low FF of only 34.92 % compared to the other devices is reflected in the shape of the J-V graph.

The MASnI3 device shows a relatively lower QE of 90–93 % for photon energies in the range of 2.2–3.5 eV, depicted in Fig. 10(e). Once again, the lower QE of the top cell contributes to the overall lower tandem efficiency. For the low performance of the MASnI3 device, high defect 11

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5.5. Br doped MASnI3 on c-Si and CIGS

Table 9 J-V characteristic parameters and PCE values for MASnI3 on c-Si and MASnI3 on CIGS 4 T tandem devices. Cell

Voc (V)

Jsc (mA/cm2)

FF (%)

MASnI3 (0.2 µm) c-Si with filtered spectrum (300 µm) CIGS with filtered spectrum (3 µm) MASnI3 on c-Si

0.8 0.73

24.72 13.2

37.55 85.06

7.41 8.23

0.72

9.95

75.69

5.42

0.8, 0.73 0.8, 0.72

24.72, 13.2 24.72, 9.95

37.55, 85.06 37.55, 75.69

15.64

MASnI3 on CIGS

The maximum PCE values of MASnI3 on c-Si and MASnI3 on CIGS are quite low compared to the other analyzed tandem devices. To enhance the efficiency of MASnI3 devices, Br was included in the perovskite structure (replacing Sn), in MASnI3. Iodine is frequently utilized because of its advantageous qualities, however, it can also be poisonous and injurious to soil [60]. Br leads to lower series resistance, a higher conduction band edge, a higher FF, and improved Voc [61]. Addition­ ally, Br doping improves energy-level matching with charge transport layers, reduces nonradiative recombination losses, and improves carrier production, diffusion, charge separation, and longevity [62,63]. Consequently, higher efficiency and greater solar cell performance are promised by this swap [60]. Therefore, an effort to find enhanced maximum efficiency of the tandem cell with 25 %, 50 % and 75 % Br doped MASnI3 top cell was investigated. Figs. 12(a), 13(a), and 14(a) depict the schematic structures of tan­ dem devices with different concentrations of bromine (Br) doped MASnI3 and c-Si. The impact of Br doping on the VBM of MASnI3 is negligible, but the CBM of the perovskite is affected [64]. This difference in energy band alignment can be observed in Figs. 12(b), 13(b), and 14 (b). The tandem structures with varying Br doping concentrations show similar patterns in the PCE curves as the thickness of the top cell absorber rises, as shown in Figs. 12(c), 13(c), and 14(c). The maximum efficiencies are found to be 16.59 %, 14.98 %, and 14.24 % at an absorber thickness of 0.2 µm for the 25 %, 50 %, and 75 % Br-doped structures, respectively. However, as the absorber thickness increases, the PCE decreases rapidly. The J-V curves for the single junction Br-doped MASnI3 perovskite cells and the c-Si cell are presented together in Figs. 12(d), 13(d), and 14 (d). The 25 % Br doped single junction exhibits a lower Jsc of 22.32 mA/ cm2 compared to the non-doped MASnI3 device (25.36 mA/cm2), but a higher Voc of 0.9 V compared to 0.79 V. With increasing doping to 50 % and 75 %, the Jsc values decrease to 17.53 mA/cm2 and 20.25 mA/cm2,

PCE (%)

12.83

densities of the MASnI3 absorber layer and low stability are the prime concerning issues [59]. Fig. 11(a) illustrates the schematic architecture of a MASnI3 on CIGS 4 T tandem device. As presented in Fig. 11(b), the energy band align­ ment of all the layers is suitable for the efficient separation of charge carriers across the different layers. Similarly, Fig. 11(c) demonstrates a comparable relationship between the efficiency of MASnI3 top cell, CIGS bottom cell, and the overall 4 T tandem structure. The maximum PCE value of 12.84 % is achieved with an MASnI3 layer thickness of 0.2 µm. However, as the thickness of the layer increases to 2 µm, the PCE value drops significantly to 6.42 %. Fig. 11(d) presents the J-V curves for single junction MASnI3 and CIGS of the MASnI3 on CIGS tandem device, while Fig. 11(e) shows the QE graphs. The QE graphs indicate that the device performs optimally within the photon energy range of 1.45–3.5 eV. Table 9 provides a summary of the photovoltaic parameters for the tandem architecture as well as the individual devices comprising the tandem structure.

Fig. 12. 25 % Br doped MASnI3 on c-Si system (a) Schematic diagram of 4 T tandem architecture (b) Energy band diagrams (c) PCE of top cell, bottom c-Si cell, and overall tandem device (d) J-V curve of 25 % Br doped MASnI3 and c-Si cells (e) QE of 25 % Br doped MASnI3 and c-Si cells. 12

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Fig. 13. 50 % Br doped MASnI3 on c-Si system (a) Schematic diagram of 4 T tandem architecture (b) Energy band diagrams (c) PCE of top cell, bottom c-Si cell, and overall tandem device (d) J-V curve of 50 % Br doped MASnI3 and c-Si cells (e) QE of 50 % Br doped MASnI3 and c-Si cells.

Fig. 14. 75 % Br doped MASnI3 on c-Si system (a) Schematic diagram of 4 T tandem architecture (b) Energy band diagrams (c) PCE of top cell, bottom c-Si cell, and overall tandem device (d) J-V curve of 75 % Br doped MASnI3 and c-Si cells (e) QE of 75 % Br doped MASnI3 and c-Si cells.

respectively. The Voc values slightly decreased to 0.85 V and 0.86 V for the 50 % and 75 % doping levels, respectively. The QE graphs for the single junctions of 25 %, 50 %, and 75 % Br

doped MASnI3 perovskite and c-Si are shown in Figs. 12(e), 13(e), and 14(e). The doping with different concentrations alters the bandgap of MASnI3, resulting in changes in the range of photon energy where the 13

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Energy Conversion and Management 300 (2024) 117904

Fig. 15. 25 % Br doped MASnI3 on CIGS system (a) Schematic diagram of 4 T tandem architecture (b) Energy band diagrams (c) PCE of top cell, bottom CIGS cell and overall tandem device (d) J-V curve of 25 % Br doped MASnI3 and CIGS (e) QE of 25 % Br doped MASnI3 and CIGS.

Fig. 16. 50 % Br doped MASnI3 on c-Si system (a) Schematic diagram of 4 T tandem architecture (b) Energy band diagrams (c) PCE of top cell, bottom CIGS cell and overall tandem device (d) J-V curve of 50 % Br doped MASnI3 and CIGS cells (e) QE of 50 % Br doped MASnI3 and CIGS cells.

devices perform optimally. It can be observed that the QE of 25 % Br doped MASnI3 device reaches 90–95 % for photon energy ranging from 2.2 to 3.5 eV. For the 50 % and 75 % doping levels, the QE reaches 85–90 % for photon energy in the range of 2.5–3.5 eV and 2.65–3.4 eV, respectively. The higher QE for the 25 % Br-doped MASnI3 device in­ dicates its superior performance with a maximum PCE of 16.59 % among all the MASnI3 on c-Si devices.

The 4 T tandem structure with different concentrations (25 %, 50 %, and 75 %) of Br-doped MASnI3 and CIGS is shown schematically in Figs. 15(a), 16(a), and 17(a) respectively. The energy band alignment depicted in Figs. 15(b), 16(b), and 17(b) confirms the optimal collection of charge carriers from the Br-doped perovskite layer. The trend of PCE for the 25 %, 50 %, and 75 % Br-doped MASnI3 on CIGS tandem systems follows a similar pattern to that observed in systems using c-Si. As shown 14

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Fig. 17. 75 % Br doped MASnI3 on CIGS system (a) Schematic diagram of 4 T tandem architecture (b) Energy band diagrams (c) PCE of top cell, bottom CIGS cell and overall tandem device (d) J-V curve of 75 % Br doped MASnI3 and CIGS cells € QE of 75 % Br doped MASnI3 and CIGS cells.

Table 10 J-V characteristic parameters and PCE values for 25 % Br doped MASnI3 on c-Si and 25 % Br doped MASnI3 on CIGS 4 T tandem devices. Cell

Voc (V)

Jsc (mA/cm2)

FF (%)

25 % Br doped MASnI3 (0.2 µm) c-Si with filtered spectrum (300 µm) CIGS with filtered spectrum (3 µm) 25 % Br doped MASnI3 on c-Si

0.93 0.73

20.91 13.2

42.98 85.06

8.36 8.23

0.72

9.95

75.69

5.42

0.93, 0.73 0.93, 0.72

20.91, 13.2 20.91, 9.95

42.98, 85.06 42.98, 75.69

25 % Br doped MASnI3 on CIGS

Table 12 J-V characteristic parameters and PCE values for 75 % Br doped MASnI3 on c-Si and 75 % Br doped MASnI3 on CIGS 4 T tandem devices.

PCE (%)

Cell

Voc (V)

Jsc (mA/cm2)

FF (%)

0.86 0.73

17 13.2

41.24 85.06

6.01 8.23

0.72

9.95

75.69

5.42

16.59

75 % Br doped MASnI3 (0.2 µm) c-Si with filtered spectrum (300 µm) CIGS with filtered spectrum (3 µm) 75 % Br doped MASnI3 on c-Si 75 % Br doped MASnI3 on CIGS

17.00, 13.2 17.00, 9.95

41.24, 85.06 41.24, 75.69

14.24

13.78

0.86, 0.73 0.86, 0.72

Table 11 J-V characteristic parameters and PCE values for 50 % Br doped MASnI3 on c-Si and 50 % Br doped MASnI3 on CIGS 4 T tandem devices. Cell

Voc (V)

Jsc (mA/cm2)

FF (%)

50 % Br doped MASnI3 (0.2 µm) c-Si with filtered spectrum (300 µm) CIGS with filtered spectrum (3 µm) 50 % Br doped MASnI3 on c-Si

0.86 0.73

19.69 13.2

39.71 85.06

6.75 8.23

0.72

9.95

75.69

5.42

0.86, 0.73 0.86, 0.72

19.69, 13.2 19.69, 9.95

39.71, 85.06 39.71, 75.69

14.98

50 % Br doped MASnI3 on CIGS

PCE (%)

11.43

Table 13 Overall tandem device efficiency of different systems utilizing c-Si.

PCE (%)

Top cell

Bottom cell

PCE (%)

CdTe GaAs MAPbI3 MASnI3 25 % Br doped MASnI3 50 % Br doped MASnI3 75 % Br doped MASnI3

c-Si

27.23 29.31 31.66 15.64 16.59 14.98 14.24

The JV and QE graphs of the doped MASnI3 cells and CIGS cells are shown in Figs. 15(d) and (e), 16(d) and (e), and 17(d) and (e). Tables 10, 11 and 12 list the overall efficiency of all Br-doped MASnI3 on c-Si and MASnI3 on CIGS devices.

12.17

in Figs. 15(c), 16(c), and 17(c), the maximum PCE values are 13.78 %, 12.71 %, and 11.43 % respectively, achieved with a top cell absorber layer thickness of 0.2 um. Subsequently, with the increase of the thickness of the absorber, the PCE of the devices decreases significantly. 15

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Energy Conversion and Management 300 (2024) 117904

DOS of the conduction band, the concentration of defects, mobility of charge carriers, and their interplay also significantly impact the final efficiency. Among the tandem systems investigated, the device with MAPbI3 as the top cell demonstrates the highest efficiency, reaching an impressive 31.66 %. Despite its low charge carrier mobility, the top MAPbI3 cell possesses the highest bandgap, favorable DOS of the conduction band, and comparatively lower defect concentration. These specific properties significantly contribute to its superior performance within the tandem structure (Fig. 18). Conversely, the tandem device with the non-lead perovskite MASnI3 as the top cell exhibits the lowest efficiency. This can be attributed to the MASnI3 absorber’s low bandgap, low charge carrier mobility, and high defect concentration. These unfavorable properties of the MASnI3 top cell result in reduced tandem efficiency (Fig. 18). In case of comparing the two non-perovskite devices, GaAs on c-Si device demonstrates a slightly higher efficiency compared to CdTe on cSi (29.31 % vs. 27.23 %). GaAs and CdTe have comparable bandgaps and DOS in the conduction band. Despite GaAs having an order of magnitude higher defect concentration than CdTe, its superior efficiency can be attributed to its high charge carrier mobility among the devices and better optical properties, resulting in better QE as shown in Fig. 4(e) and 6(e). Overall, the efficiency values of the 4 T tandem structure depend on a combination of factors, including the bandgap, DOS in the conduction band, charge carrier mobility, defect concentration, and critical optical properties of the chosen top cell.

Table 14 Overall tandem device efficiency of different systems utilizing CIGS. Top cell

Bottom cell

Efficiency (%)

CdTe GaAs MAPbI3 MASnI3 25 % Br doped MASnI3 50 % Br doped MASnI3 75 % Br doped MASnI3

CIGS

23.76 26.43 28.45 12.83 13.78 12.17 11.43

6. Comparison between all the devices Table 13 and 14 provide an executive summary of the PCE values of the 4 T tandem systems probed in this study. As previously mentioned, the PCE of mechanically stacked tandem cells depends on the selection of both single junction cells, influenced by their inherent optical and electrical properties. This study has explored two of the most techno­ logically viable options c-Si and CIGS as bottom cells. From the infor­ mation presented in Table 2, it can be observed that c-Si has certain advantages, such as a lower bandgap, a higher DOS of the conduction band, and greater mobility of electrons and holes, when compared to CIGS. Consequently, all of the tandem devices utilizing c-Si demonstrate higher PCE values. It is equally relevant to note that the top cell in a tandem structure can critically modulate the device’s overall efficiency. While the bandgap of the top cell absorber is important, other factors such as the

Fig. 18. The relative impacts of (a) bandgap, (b) DOS of conduction band, (c) mobility, and (d) defect concentration on the efficiency of all tandem architectures. 16

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Energy Conversion and Management 300 (2024) 117904

7. Conclusion

the work reported in this paper.

This study presents a comprehensive analysis of mechanically stacked 4 T tandem devices utilizing the most technologically viable and impactful materials as top and bottom cells. The devices investigated include CdTe, GaAs, MAPbI3, and MASnI3 as top cells, while c-Si and CIGS are employed as bottom cells. A comparative examination is con­ ducted to assess the performance of these tandem configurations. The top cell absorber’s thickness is adjusted to maximize PCE. The tandem devices featuring c-Si exhibit higher PCE compared to those with CIGS. This is due to the lower bandgap, higher DOS of the conduction band, and higher mobility of charge carriers of c-Si. The application of different top cell options, such as CdTe, GaAs, MAPbI3, and MASnI3 in combination with c-Si as the bottom cell, yields maximum PCEs of 27.23 %, 29.31 %, 31.66 %, and 15.64 %, respectively. Conversely, when coupled with CIGS as the bottom cell, the PCEs marginally decrease to 23.76 %, 26.43 %, 28.45 %, and 12.83 %. Among the top cell options, MAPbI3 demonstrates the highest bandgap and DOS of the conduction band, leading to the highest PCE of 31.66 % for a MAPbI3 layer thickness of 0.4 μm combined with c-Si. With an efficiency of 27.31 % for a GaAs layer thickness of 0.5 μm, the GaAs device outperforms the CdTe device, which has a performance of 27.23 % for a CdTe layer thickness of 0.3 μm, among the two non-perovskite tandem devices. This is because of the higher mobility of electrons and holes in GaAs. The MASnI3 on CIGS system exhibits the lowest PCE of 12.83 % because of the low bandgap and high concentration of defects in the MASnI3 layer. Additionally, excessive introduction of iodine (I) into the soil can disturb natural ecosystems, leading to adverse effects on soil microorganisms and dis­ rupting essential nutrient cycles required for plant growth. To address the low efficiency of the single junction MASnI3 device and the injurious effect of I to soil, different doping concentrations (25 %, 50 %, and 75 % Br) in MASnI3 devices were investigated. The exploration of 25 %, 50 %, and 75 % Br-doped MASnI3 alongside c-Si reveals PCEs of 16.59 %, 14.94 %, and 14.28 %, respectively. Meanwhile, for CIGS as the bottom cell, the corresponding PCEs for 25 %, 50 %, and 75 % Br doping are of slightly lower value of 13.78 %, 12.17 %, and 11.43 %. Among the doped MASnI3 tandem devices, the 25 % doping MASnI3-on-c-Si configuration achieves the highest PCE of 16.84 %. Finally, the finding of this study concludes by selecting the top three devices, namely, MAPbI3 on c-Si, GaAs on c-Si, and MAPbI3 on CIGS. These choices provide precise and invaluable guidelines for experimentalists to develop highly efficient photovoltaic devices. The findings of this study are expected to serve as a roadmap for the development of nextgeneration tandem photovoltaic technologies.

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CRediT authorship contribution statement Md. Nazmul Ahsan Dipon: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Writing – original draft, Visualization. Miah Abdullah Sahriar: Conceptualization, Methodol­ ogy, Formal analysis, Investigation, Data curation, Writing – original draft, Visualization. Saugata Sarker: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Writing – original draft, Visualization. Md. Tohidul Islam: Conceptualization, Methodology, Visualization. Abrar Rauf: Data curation, Formal analysis. Mohd. Rakibul Hasan Abed: Conceptualization, Formal analysis. Ahsiur Rahman Nirjhar: Methodology, Investigation. Sadika Jannath TanEma: Conceptualization, Methodology. Kazi Md. Shorowordi: Super­ vision, Project administration. Saquib Ahmed: Conceptualization, Su­ pervision, Writing – review & editing, Formal analysis, Project administration. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence 17

Md. Nazmul Ahsan Dipon et al.

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