Numerical modeling to enhance the efficiency of experimentally fabricated Sb₂Se₃-based solar cell

Abstract

Remarkable optical and electrical characteristics make antimony selenide (Sb₂Se₃) a potential absorber layer for heterojunction solar cells. In this work, a novel heterojunction Sb₂Se₃-based thin film solar cell using non-toxic tin sulfide (SnS₂) as buffer layer instead of toxic cadmium sulfide (CdS) is designed utilizing Solar Cell Capacitance Simulator in one-dimension (SCAPS-1D). To validate the simulation model, the results of experimentally fabricated glass/SnO₂:F(FTO)/CdS/Sb₂Se₃/Au solar cell structure with efficiency of 5.17% is reproduced in SCAPS. SnS₂ buffer layer provides better band alignment with the Sb₂Se₃ absorber than CdS buffer layer and improves efficiency. The device performance is optimized by altering thickness, doping concentration, bandgap, defect density, interface defect and capture cross-section for the different layers. The maximum efficiency obtained for the optimized FTO/SnS₂/Sb₂Se₃/Au photovoltaic structure is 12.31% when the Sb₂Se₃ absorber, SnS₂ buffer and FTO layer thickness are optimized at 0.4 µm, 0.03 µm, and 0.1 µm respectively and their doping are optimized at 10¹⁶ cm⁻³, 10¹⁸ cm⁻³ and 10²⁰ cm⁻³ respectively. Addition of tin monosulfide (SnS) as back surface field (BSF) layer boosts the efficiency by decreasing carrier recombination and preventing electrons from reaching back contact due to proper band alignment formed at SnS/Sb₂Se₃ interface. The efficiency of 24.86% with V_OC = 0.94 V, J_SC = 31.98 mA cm⁻², and FF = 83.09% is obtained for the proposed FTO/SnS₂/Sb₂Se₃/SnS/Au photovoltaic structure with SnS BSF layer at thickness of 0.2 µm and doping of 10²⁰ cm⁻³. Moreover, the impacts of operating temperature, parasitic resistance and back contact work function on the performance parameters of the designed solar cell are analyzed. These findings indicate that non-toxic SnS and SnS₂ can be utilized as a promising BSF and buffer layer respectively to produce cost-effective, environmental friendly and extremely efficient Sb₂Se₃-based thin film solar cell.

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1. Introduction

At present, the expeditious exhaustion of fossil fuels because of large-scale industrialization as well as the quick rise in requirement for conventional energy sources has resulted in severe economic and environmental concerns.^1–4 In this respect, solar cell is a promising solution for ecological and sustainable energy generation which can convert sunlight into high-efficient and cost-effective electrical energy.^5–7 The photovoltaic (PV) researchers are introducing different novel concepts of solar-cell architectures and production considering the stability, climate issues, efficacy and cost in terms of industrial production. Thin film solar cells (TFSCs) have achieved widespread attention because of high performance and fast growth among the various types of PV solar cells.^8–10 At present, metallic chalcogenides copper-indium-gallium-selenide (CIGS) and cadmium telluride (CdTe) based TFSCs are considered as promising candidates for electricity generation with efficiency beyond 23%.^11,12 But these absorbers have several disadvantages such as toxicity, high cost and scarcity.^13,14 Conventional kesterite-based absorbers such as copper zinc tin sulphide selenide (CZTSSe) obtained highest efficiency of 12.6% due to problems of controlling the composition and defects during fabrication.¹⁵ Recently, Sb₂Se₃, a binary compound, has displayed superb potential as an absorber layer in TFSC technology.^16–18 It is composed of earth-abundant and less-toxic elements. It has a band gap of 1.1–1.3 eV, long carrier lifetime, a high absorption coefficient over the visible spectrum (>10⁵ cm⁻¹), low melting point (550 °C) and decent carrier mobility (hole mobility up to 42 cm² V⁻¹ s⁻¹ and electron mobility above 16.9 cm² V⁻¹ s⁻¹).^{12,17,19–21} Moreover, benign grain boundary structure of Sb₂Se₃ makes it a potential absorber material.²² An efficiency of 0.66% was obtained by Messina et al. in 2009 using CdS as a buffer layer.²³ Choi et al. and Zhou et al. obtained 3.21% and 2.26% efficiency using TiO₂ buffer layer in 2014.^24,25 In 2017, Chen et al. and Wang et al. achieved 6.5% and 5.93% efficiency with CdS and ZnO buffer layers respectively.^16,18 By employing a vapor-transport method, Wen et al. obtained 7.6% efficiency using CdS buffer layer in 2018.²⁶ In 2019, Li et al. obtained highest efficiency of 9.2% using CdS buffer layer for Sb₂Se₃ based solar cells by junction interface engineering.¹⁹ In 2021, Guo et al. obtained an efficiency of 7.15% by incorporating NiO_x as back surface field (BSF) layer.²⁷ Recently, Tang et al. have obtained efficiency of 8.64% by sputtering method.²⁸ In 2022, Dong et al. obtained an efficiency of 6.7% by optimizing deep defects containing V_Se on the Sb₂Se₃ film.²⁹ The efficiency of Sb₂Se₃-based solar cells still far from the theoretical value of 31% according to Shockley–Queisser limit.³⁰ Different methods namely electrodeposition, spin coating, sputtering, spray pyrolysis, chemical bath deposition and vacuum evaporation have been employed to achieve high quality Sb₂Se₃ film.³¹ Sb₂Se₃ has various applications in memory gadgets, solar cells, photodetectors and batteries.³² To obtain the theoretical efficiency of Sb₂Se₃ based solar cells, various simulation studies have been performed. Basak et al. have obtained efficiency of 12.62% for Sb₂Se₃/CdS heterojunction solar cell structure.³³ An efficiency of 13.20% was obtained by Baig et al. using CNT/Cu₂O/Sb₂Se₃/In₂S₃/ITO photovoltaic architecture.³⁴ Kumari et al. achieved 24% efficiency in Mo/Sb₂Se₃/ZnSe/Ag solar cell structure.¹⁷ Mamta et al. gained 27.84% efficiency in Sb₂Se₃/CdS/ZnO solar cell structure.³⁵ Ahmed et al. obtained 29.35% efficiency for Mo/BaSi₂/Sb₂Se₃/CdS/FTO/Al solar cell architecture.³⁶ Sunny et al. achieved 29.89% efficiency for Al/F:SnO₂(FTO)/CdS/Sb₂Se₃/SnS/Mo solar cell structure.³⁷

An ideal buffer layer should have high electrical conductivity, decent carrier mobility and a proper band alignment with minimum conduction band offset (CBO).³⁸ In experimental Sb₂Se₃ based solar cell, CdS functions as mostly used buffer layer due to its electrical and chemical stability.^12,39,40 The main problem with CdS is its toxicity which causes human and environmental health issues.^41,42 Moreover, large lattice mismatch and diffusion of S and Cd into the Sb₂Se₃ film degrades cell performance.¹⁷ Several works have used other buffer layers such as ZnO,⁴³ TiO₂ (ref. 44) and SnO₂ (ref. 45) instead of CdS layer. The two-dimensional (2D) chalcogenide sulfur (S)-based tin sulfide (SnS₂) with an appropriate bandgap of 1.82–2.88 eV and a large absorption coefficient of ∼10⁵ cm⁻¹ is a promising buffer material for producing high-performance solar cells.^46,47 It is cheap, non-toxic and has high electron mobility values of 50 cm² V⁻¹ s⁻¹.⁴⁸ The wide bandgap of SnS₂ allows loss-free passage of incident light spectrum. Moreover, proper band alignment with absorber layer provides efficient transporting of electrons from absorber layer to the contacts reducing the recombination of holes and electrons and thus boosting the cell efficiency. It can be deposited employing spray pyrolysis, hydrothermal, co-precipitation, chemical vapor deposition (CVD) and chemical bath deposition.^49–52 In this work, SnS₂ is used as a buffer layer for the proposed Sb₂Se₃ based solar cell.

Ineffective carrier transport and collection by the front and back contacts and carrier recombination at the back contact are the main reasons for poor solar cell efficiency. By inserting the back surface field (BSF) layer between the back contact and absorber layer, the solar cell performance can be enhanced by lowering the surface recombination rate (SRV) and delivering easy transportation of holes to the back electrode by minimizing the barrier height of the back electrode.^53,54 Many researchers have added inorganic and organic BSF layers such as NiO_x, Cu₂O, VO_x, CZTA, P3HT, PEDOT:PSS, SpiroMeOTAD.^{12,34,55–58} Due to toxicity of inorganic BSFs and instability of organic BSFs, researchers are looking for non-toxic and stable BSF layer. In previous studies, inorganic tin monosulfide (SnS) is employed as a BSF layer to reduce carrier recombination at BSF/back contact interface.^37,59,60 It has excellent optoelectronic characteristics such as suitable bandgap of 1.1–1.4 eV, high hole mobility, nontoxicity, cheap and large absorption coefficient (>10⁴ cm⁻¹).^61–64 Various techniques such as spray pyrolysis, electron beam evaporation, chemical synthesis, sputtering, atomic layer deposition (ALD), thermal evaporation (TE) and vapor transport deposition (VTD) have been used for deposition of SnS films.^62,65–71 In this work, SnS is proposed as BSF layer for Sb₂Se₃ based TFSC due to its proper band alignment with absorber layer. Though SnS and SnS₂ are used as BSF and buffer layer respectively in previous studies,^37,72 this novel SnO₂: F(FTO)/SnS₂/Sb₂Se₃/SnS/Au dual-heterojunction (DH) structure using combined use of SnS and SnS₂ as BSF and buffer layer respectively in Sb₂Se₃ based solar cell has never been simulated to the best of my knowledge. The reason for high efficiency and light trapping of DH solar cell has been achieved as a result of the longer wavelength photon absorption in the p⁺-SnS BSF layer through a tail-states assisted (TSA) two-steps photon upconversion phenomenon. Most previous studies have explored on single absorber structures, overlooking the promising efficiency enhancements obtained by dual absorber layer architectures. There is also limited research on how such dual-heterojunction structures effect efficiency and carrier transport mechanisms in thin film solar cells. The present study aims to systematically explore a novel thin-film solar cell architecture comprising Sb₂Se₃, SnS₂ and SnS as absorber, buffer and BSF layer respectively to examine how such an architecture can enhance solar cell performance while eliminating toxic element dependency, thereby bestowing to both material sustainability and technological evolution in next-generation solar cells.

In this present work, a numerical modeling is applied employing SCAPS-1D software to compare simulation results with experimental findings of glass/SnO₂:F(FTO)/CdS/Sb₂Se₃/Au solar cell structure by Li et al.⁷³ After that, toxic CdS is replaced with non-toxic SnS₂ buffer layer which boosts device performance. Then investigation is performed by tuning thickness, doping density, bandgap, defect density and capture cross section of different layers to boost efficiency of proposed cell. Afterwards, SnS is inserted as BSF layer and its physical parameters are optimized to enhance device performance. Finally, the influence of back contact work function, temperature and series and shunt resistance on electrical parameters such as open circuit voltage (V_OC), short-circuit current density (J_SC), fill factor (FF) and efficiency (ƞ) of proposed cell is also analyzed. The proposed architecture has the potential to notably enhance the stability and performance of Sb₂Se₃ based photovoltaic by enhancing the carrier collection and decreasing the back surface recombination.

2. Numerical simulation and methodology

2.1 Simulation methodology and device architecture

To analyze the proposed glass/FTO/SnS₂/Sb₂Se₃/SnS/Au photovoltaic, the one-dimensional Solar Cell Capacitance Simulator (SCAPS) software version 3.3.09 is applied for numerical simulations. SCAPS-1D application was invented at the Department of Electronics and Information Systems of the University of Ghent, Belgium. There are several other programs to model and simulate solar cell characteristics depending on the device's input hierarchy, including SILVACO ATLAS, COMSOL, AMPS, AFORS-HET, PECSIM and Wx-AMPS.^74,75 However, most of them require expensive licensing fees and high computational resources, making them rarely available for quick device optimization and design. But SCAPS software has many advantages compared to others including the potential to carry out performance examination across up to seven levels, in depth and batch analyses, and simple findings to comprehend and investigate.⁷⁶ It is the best accurate non-commercial tool with friendly dialog box, straightforward in operation and supports multi-junction solar cells.⁷⁷ Interestingly, the SCAPS-1D is available cost-free and provides results that matches with experimental findings. It is designed to simulate and model solar cells with a heterojunction and homojunction. Thus, providing it a significant edge over the other solar cell simulators. It incorporates complicated simulations of carrier transport, recombination, generation, defects and interface impacts in solar cells which makes it effective and reliable for simulation of photovoltaic devices.⁷⁸ All input files that are provided in SCAPS are user-accessible text files, for example the spectral data. Moreover, it is very fast which other programs may lag significantly and the incorporation of metal contacts, shunt resistance, series resistance, capacitance and frequency response and capacitance and voltage relation make this software robust.⁷⁹ It has been extensively validated across solar cell research community and different material systems, highlighting its predictive correctness and reliability.⁸⁰ Thus, the present research is carried out using the SCAPS-1D simulation tool. It is applied to predict and analyze the optoelectronic parameters of solar cell architectures such as open circuit voltage (V_OC), short-circuit current density (J_SC), fill factor (FF) and efficiency (ƞ) by dealing fundamental equations such as drift-diffusion equation, Poisson's equation and carrier continuity equation.⁸¹ These equations are numerically solved in one dimension by this program.

Fig. 1a depicts the Sb₂Se₃ baseline superstrate glass/SnO₂:F(FTO)/CdS/Sb₂Se₃/Au solar cell structure manufactured experimentally by Li et al. implementing close-space sublimation (CSS) method which provides higher quality of Sb₂Se₃ absorber film deposition and improved device heterojunctions due to independent control of the substrate and source temperatures.⁷³ Fig. 1b illustrates proposed glass/FTO/SnS₂/Sb₂Se₃/SnS/Au photovoltaic architecture for the maximum efficiency that is the objective of this article. A p-type Sb₂Se₃ absorber layer, n-type SnS₂ buffer, and FTO (fluorine-doped tin oxide) window layer compose the proposed solar cell. Moreover, SnS is added as the BSF layer to boost efficiency. FTO and gold (Au) are used as front and back contacts respectively. Table 1 represents material parameters applied in the proposed Sb₂Se₃-based photovoltaic with BSF layer obtained from both simulation and practical works. In order to find equality between experimental and simulation findings, bulk layers and absorber/buffer interface are incorporated with defects. The interfacial defect and contact parameters applied in this work are listed in Tables 2 and 3 respectively. To reproduce experimental results, the BSF layer is not inserted in the initial section of simulation. The simulation study has been performed under AM 1.5 G spectral irradiation (100 mW cm⁻²) at 300 K. Moreover, the absorption coefficient α(λ) for films is obtained using the new modified E_g-sqrt model, which is an updated version of the traditional sqrt (hv − E_g) model. The “Tauc law” demonstrates this relationship as illustrated in eqn (1).

Here, α(λ) represents the optical absorption coefficient as a function of wavelength (λ) or photon energy (hν) and E_g denotes the bandgap. Eqn (2) and (3) relate the model constants α₀ and β₀ to the conventional model constants A and B as illustrated below:

α₀ = A√E_g (2)

Fig. 1 (a). Schematic diagram of the reference glass/FTO/CdS/Sb₂Se₃/Au solar cell. (b) Schematic diagram of the proposed glass/FTO/SnS₂/Sb₂Se₃/SnS/Au solar cell.

Table 1 Material parameters employed for simulating Sb₂Se₃ based thin film solar cell

Parameter	Sb2Se3 (ref. 20)	CdS^20	SnS2 (ref. 5)	FTO^20	SnS^86
Absorption co-efficient (α) (cm^−1)	SCAPS	SCAPS	SCAPS	SCAPS	SCAPS
Thickness (µm)	0.54	0.05	0.05	0.5	0.1
Bandgap (eV)	1.03	2.4	2.24	3.6	1.2
Electron affinity (eV)	4.15	4.0	4.2	4.0	4.2
Dielectric permittivity	14.5	10	10	9	13
CB effective density of states (cm^−3)	8 × 10^17	2.2 × 10^18	2.2 × 10^18	2.2 × 10^18	1.18 × 10^18
VB effective density of states (cm^−3)	1.8 × 10^19	1.8 × 10^19	1.8 × 10^19	1.8 × 10^19	4.76 × 10^18
Electron thermal velocity (cm s^−1)	1 × 10^7	1 × 10^7	1 × 10^7	1 × 10^7	1 × 10^7
Hole thermal velocity (cm^−1)	1 × 10^7	1 × 10^7	1 × 10^7	1 × 10^7	1 × 10^7
Electron mobility (cm^2 V^−1 s^−1)	15	80	50	20	30
Hole mobility (cm^2 V^−1 s^−1)	15	25	50	10	90
Shallow uniform donor density, ND (cm^−3)	0	1 × 10^18	1 × 10^18	1 × 10^17	0
Shallow uniform acceptor density, NA (cm^−3)	1 × 10^13	0	0	0	1 × 10^18
Defect type	Donor	Acceptor	Acceptor	Donor	Donor
Energetic distribution	Single	Single	Single	Single	Single
Capture cross section of electrons (cm^2)	2.3 × 10^−11	1.5 × 10^−12	1.5 × 10^−12	1 × 10^−12	1 × 10^−15
Capture cross section of holes (cm^2)	4.39 × 10^−11	1.5 × 10^−12	1.5 × 10^−12	1 × 10^−14	1 × 10^−15
Defect density, Nt (cm^−3)	1 × 10^13	1 × 10^18	1 × 10^15	1 × 10^16	1 × 10^14

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Table 2 Interface defect parameters used in simulation

Parameters	Sb2Se3/CdS	Sb2Se3/SnS2	Sb2Se3/SnS
Defect type	Neutral	Neutral	Neutral
Energetic distribution	Single	Single	Single
Capture cross section of electrons/holes (cm^2)	1 × 10^−10	1 × 10^−10	1 × 10^−19
Defect density, Nt (cm^−2)	5 × 10^17	5 × 10^17	1 × 10^10

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Table 3 Contact parameters used in simulation

Parameter	Back contact	Front contact
Work function	5.1 eV	4.4 eV
Surface recombination velocity of holes (cm s^−1)	1 × 10^7	1 × 10^7
Surface recombination velocity of electrons (cm s^−1)	1 × 10^7	1 × 10^7

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The material absorption spectra is produced automatically depending on the √E_g model and the input parameters supplied, including A and B in SCAPS-1D.^82,83 The sub-bandgap absorption impact on solar cell performance is automatically accounted by SCAPS software once the optical data are supplied. Conversely, there are limitations of SCAPS. One of its limitations is that it can provide imprecision results if applied to complex structures or long simulation times because it uses a simplified optical model. It can overestimate the incident light intensity that generates charge due to its inability to account automatically reflection losses that is present at intermediate interfaces. Additionally, there is an uncertain illustration for a secondary barrier or n–p (rather than p–n) junction.⁷⁵ Nanostructural characteristics or plasmonic impacts are some vital light-management methods that are not precisely simulated by it.⁸⁴ It is required to incorporate wavelength–dependent complex refractive index (n–k) values for materials in SCAPS to increase the simulation correctness.^80,85 Despite these limitations, SCAPS is still advantageous due to its ability to produce results close to experimental findings and its flexible configurations.

2.2 Methodology for optimizing solar cell performance parameters

At first, the parameters listed in Tables 1–3 are used to produce solar cell performance parameters that matches experimental findings to validate the simulation model in this research. Afterwards, 15 steps are carried out to obtain optimal values based on varied parameters listed in Tables 1–3. The optimized result obtained in each step is used for the reference cell for the next step.

Step 1: except for the thickness of Sb₂Se₃ absorber layer that is varied from 0.1 µm to 1.5 µm, the rest of the parameters are kept constant.

Step 2: the optimum thickness of Sb₂Se₃ from the previous step is used as a constant parameter in this step and the carrier density of this layer is changed from 10¹²–5 × 10¹⁶ cm⁻³.

Step 3: the defect density of Sb₂Se₃ is varied in the range of 10¹⁰–10¹⁵ cm⁻³ to study its impact on optimized cell obtained in the previous step.

Step 4: in this step, the bandgap of Sb₂Se₃ in the range of 1–1.3 eV is investigated to study its impact on the proposed cell performance.

Step 5: after optimizing the Sb₂Se₃ absorber layer, the thickness of SnS₂ buffer layer is varied from 0.01 µm to 0.1 µm to study its impact on device performance.

Step 6: the impact of SnS₂ buffer layer carrier density on the photovoltaic parameters of the optimized cell in the previous step is studied by varying it between 10¹³ cm⁻³ and 10¹⁸ cm⁻³.

Step 7: the defect density of the SnS₂ layer is varied between 10¹⁰ cm⁻³ and 10¹⁸ cm⁻³ while the rest parameters are kept at their optimum values.

Step 8: the effect of FTO window layer thickness is investigated in the range of 0.1–0.6 µm.

Step 9: the carrier concentration of FTO layer is explored in the range of 10¹⁷–10²⁰ cm⁻³ to optimize its value.

Step 10: the impact of Sb₂Se₃/SnS₂ interface defect on performance parameters of the proposed cell is explored in the defect density range of 10⁷–10¹⁸ cm⁻².

Step 11: the capture cross-section of electron and hole of the Sb₂Se₃ layer is varied between 10⁻¹⁹ cm⁻² and 10⁻⁹ cm⁻² while the rest parameters are kept at their optimum values.

Step 12: in this step, the impact of the capture cross-section of electron and hole of the Sb₂Se₃/SnS₂ interface layer is explored to study its impact on performance parameters of optimized cell in the previous step in the range of 10⁻²⁰–10⁻⁸ cm⁻².

Step 13: after optimizing Sb₂Se₃, SnS₂ and FTO layer, the thickness of SnS BSF layer is changed from 0.02 µm to 0.2 µm to optimize its value.

Step 14: the acceptor concentration of SnS layer is investigated in the range of 10¹⁵–10²⁰ cm⁻³ to study its impact on device output.

Step 15: to optimize defect density in SnS BSF layer, defect density is investigated in the range of 10¹⁰–10²⁰ cm⁻³ in this last step.

Finally, the effect of back contact work function (4.5–5.4 eV), temperature (300–400 K), series resistance (0–8 Ω cm²) and shunt resistance (200–1600 Ω cm²) on the optimized cell is explored.

3. Results and analysis

3.1 Validation and comparison of simulation model

The reproduced, experimental and proposed current density–voltage characteristics of the Sb₂Se₃ based photovoltaic are compared, as depicted in Fig. 2. Table 4 displays the obtained performance parameters of simulation results in consideration with experimental outcomes. The device outputs including V_OC of 0.358 V, J_SC of 26.71 mA cm⁻², FF 54.05%, and efficiency of 5.17% are achieved from the simulated J–V characteristics with CdS buffer layer by neglecting series and shunt resistances. On the contrary, the efficiency of 5.17% with V_OC of 0.386 V, J_SC of 24.6 mA cm⁻², and FF 54.5% is found for the experimental Sb₂Se₃ based photovoltaic.⁷³ The approximately same result obtained from the experimental and simulation works validates this simulation model. To limit extreme illumination instability due to the diffusion of Cd²⁺ from the buffer layer and toxicity of CdS buffer layer, non-toxic SnS₂ is used as a buffer layer in this simulation work.⁷³ An efficiency of 5.25% with V_OC of 0.362 V, J_SC of 26.89 mA cm⁻², and FF 53.99% is obtained for the proposed Sb₂Se₃ based photovoltaic with SnS₂ as a buffer layer. The result indicates that SnS₂ can be used as a novel buffer layer for Sb₂Se₃ based reference solar cell instead of toxic CdS buffer layer as it provides better efficiency. As a result, SnS₂ is used as a buffer layer for Sb₂Se₃ based reference photovoltaic in this simulation study.

Fig. 2 Comparison of experimental and simulated J–V curves of Sb₂Se₃-based solar cells.

Table 4 Comparison of electrical parameters of Sb₂Se₃-based photovoltaic

Solar cell	VOC (V)	JSC (mA cm^−2)	FF (%)	ƞ (%)
Experimental^73	0.386	24.6	54.5	5.17
Simulated^20	0.362	27.51	51.95	5.17
Simulated (this work) (CdS buffer layer)	0.358	26.71	54.05	5.17
Simulated (this work) (SnS2 buffer layer)	0.362	26.89	54.02	5.25

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3.2 Impact of Sb₂Se₃ absorber layer thickness

An optimal absorber layer thickness ensures that photo-generated electron–hole pairs reach their respective contacts without recombination. The influence of the Sb₂Se₃ absorber layer thickness is examined by tuning absorber layer thickness from 0.1–1.5 µm, while different layers material parameters are kept unchanged. Solar cell output corresponding to absorber layer thickness variation are shown in Fig. 3. It is observed that J_SC increases from 9.63 mA cm⁻² to 26.88 mA cm⁻² as absorber layer thickness is increased from 0.1–0.6 µm due to increase in electron–hole pair generation. Afterwards it starts decreasing due to recombination and reaches 13.13 mA cm⁻² as absorber layer thickness is enhanced to 1.5 µm. V_OC remains almost constant at 0.395 V as absorber layer thickness is enhanced from 0.1–0.2 µm. Then, it starts decreasing due to recombination and reaches 0.27 V as absorber layer thickness is enhanced to 1.5 µm.^87,88 FF decreases from 69.9–32.1% because of rise in series resistance as absorber layer thickness is increased from 0.1–1.5 µm. The combined impact of FF, V_OC and J_SC cause efficiency to rise from 2.66–5.56% as absorber layer thickness is increased from 0.1–0.4 µm. Then efficiency starts to decline as absorber layer thickness is increased and reaches 1.14% as absorber layer thickness is enhanced to 1.5 µm. A thin absorber layer results in low both J_SC and efficiency due to reduction of absorption capacity of light. Enhancement of absorber layer thickness increases efficiency of the solar cell due to large number of photons being absorbed and rise in carrier generation.¹⁴ Efficiency declines as absorber layer thickness is enhanced above 0.4 µm due to recombination as the path length covered by photo-generated carriers become very large.⁸⁹ So absorber layer thickness is optimized at 0.4 µm which falls within range published in previous works.^12,34

Fig. 3 Impact of Sb₂Se₃ absorber layer thickness on solar cell electrical parameters.

3.3 Influence of Sb₂Se₃ absorber layer carrier density

The effect of the Sb₂Se₃ absorber layer carrier density on device output is examined and simulation outcomes are displayed in Fig. 4. The Sb₂Se₃ absorber layer acceptor density is tuned from 10¹²–5 × 10¹⁶ cm⁻³. V_OC increases from 0.38 V to 0.41 V and FF rises from 58.12–66.36% as absorber layer acceptor concentration is increased in considered range. J_SC increases slightly from 25.43 mA cm⁻² to 25.86 mA cm⁻² as doping density is tuned from 10¹²–10¹⁶ cm⁻³ after which it starts decreasing. An enhanced doping density results in rise in the bulk and interface recombination rate, thereby declining J_SC.⁵⁴ The combined effect of V_OC, J_SC and FF causes efficiency to rise from 5.56–6.31% with increase in acceptor concentration from 10¹²–10¹⁶ cm⁻³. Afterwards, efficiency degrades. Increase in the carrier density creates the built-in potential at the interface which decreases the recombination and rises the efficiency up to a certain limit.⁹⁰ Additional hole recombination centers created at high doping density in the absorber due to increased acceptor impurity increase recombination and declines efficiency.¹⁴ So the optimal absorber layer doping density chosen is 10¹⁶ cm⁻³ which is within doping concentration value used in previous works.^12,37

Fig. 4 Impact of Sb₂Se₃ absorber layer acceptor concentration on device performance.

3.4 Effect of Sb₂Se₃ absorber layer defect density

The defect density of the absorber layer which are inevitable during the fabrication plays a vital role in impacting the performance of solar cells. Defects in the absorber layer results from antisite defects, point defects, crystal imperfections and interstitials. The effect of the Sb₂Se₃ absorber layer defect density is examined and outcomes are depicted in Fig. 5. The Sb₂Se₃ absorber layer defect density is altered from 10¹⁰–10¹⁵ cm⁻³. Both V_OC and J_SC remain almost fixed up to defect density of 10¹² cm⁻³. Thereafter, both decline significantly as defect density is increased in considered range. FF and ƞ remain unaltered as defect density is increased from 10¹⁰–10¹¹ cm⁻³. Then both decrease significantly. It is observed that J_SC, V_OC, FF and ƞ decrease from 27.86 mA cm⁻² to 3.94 mA cm⁻², 0.49 V to 0.126 V, 74.58–33.13%, and 10.14–0.16% respectively as defect density is increased from 10¹⁰–10¹⁵ cm⁻³. Increase in absorber layer defect density causes increase in Shockley Read Hall (SRH) recombination of carriers which reduces the carrier lifetime and diffusion length.^14,91,92 As a result, the production of electron–hole is reduced due to prevention of the carriers from reaching the junctions which degrades the solar cell performance parameters.⁹³ The absorber layer defect density selected is 10¹³ cm⁻³ for maximum efficiency which is in good agreement with research works.^17,53

Fig. 5 Impact of Sb₂Se₃ absorber layer defect density on solar cell output.

3.5 Optimization of Sb₂Se₃ absorber layer bandgap

The effect of the Sb₂Se₃ absorber layer bandgap is examined by altering absorber layer bandgap from 1–1.3 eV. Solar cell output parameter changes corresponding to absorber layer bandgap variation are depicted in Fig. 6. It is observed that J_SC decreases from 26.4 mA cm⁻² to 17.57 mA cm⁻² as bandgap rises from 1 to 1.3 eV. V_OC increases from 0.375 V to 0.64 V with rise in bandgap in considered range due to decline in radiative recombination rate.⁹⁴ Both FF and ƞ rise up to bandgap of 1.15 eV. Afterwards, both degrade with rise in bandgap. The generated rate of carriers is enhanced due to improved absorber/buffer junction which causes efficiency to increase as bandgap rises from 1 to 1.15 eV.⁹⁵ The mismatch between Sb₂Se₃ and SnS₂ bandgap creates recombination centers which increases carriers' recombination with further increase of Sb₂Se₃ bandgap above 1.15 eV and declines efficiency.⁹⁶ So the absorber layer bandgap is optimized at 1.15 eV at which ƞ reaches 6.96%.

Fig. 6 Solar cell characteristics with respect to Sb₂Se₃ absorber layer bandgap.

3.6 Effect of SnS₂ buffer layer thickness

The influence of SnS₂ buffer layer thickness on solar cell output parameters is also explored. The thickness is tuned from 0.01–0.1 µm. Fig. 7 displays the simulation findings obtained. It is observed that both V_OC and J_SC decrease and FF remains almost constant as buffer layer thickness is enhanced in considered range. The photo-generated carriers need to travel longer distance to go to the active region with the rise in the thickness of buffer layer which increases the recombination rate which in turn decreases both J_SC and V_OC.⁹⁷ The efficiency decreases from 7.34–6.6% as buffer layer thickness is varied from 0.01–0.1 µm. Less number of photons reach the Sb₂Se₃ layer which led to reduction of the production of electron–hole pair with increase in the thickness of SnS₂ layer due to the increase in absorption of photons in the buffer layer which declines efficiency.⁹³ When the SnS₂ layer is very thin, the space charge width of the junction is decreased and the pinholes might be created so that Sb₂Se₃ directly connects to the front contact.²⁰ So the optimal buffer layer thickness chosen is 0.03 µm which is also used in published researches.^98,99

Fig. 7 Impact of SnS₂ buffer layer thickness on cell performance parameters.

3.7 Influence of SnS₂ buffer layer donor density

The effect of the SnS₂ buffer layer donor concentration is examined and simulation findings are displayed in Fig. 8. The SnS₂ buffer layer donor density is altered from 10¹³–10¹⁸ cm⁻³. FF declines as donor density is enhanced in considered range. V_OC and J_SC increase from 0.45 V to 0.51 V and 15.36 mA cm⁻² to 22.44 mA cm⁻² respectively which causes ƞ to rise from 5.86–7.14% as buffer layer carrier density is enhanced from 10¹³–10¹⁸ cm⁻³. At low carrier concentration, the depletion region extends out of the buffer layer thickness and the diode behavior of the device is decreased.⁴⁸ The electric field on the Sb₂Se₃/SnS₂ junction and the SnS₂//FTO junction rise which augments the separation of the photo-generated charges and efficiency with rise in buffer layer doping concentration.¹⁰⁰ So the buffer layer donor density is optimized at 10¹⁸ cm⁻³ which is compatible with published work.⁴⁸

Fig. 8 Photovoltaic output as a function of SnS₂ buffer layer donor concentration.

3.8 Effect of SnS₂ buffer layer defect density

To examine the influence of SnS₂ buffer layer defect density on photovoltaic characteristics, the defect density is altered from 10¹⁰–10¹⁸ cm⁻³ and photovoltaic output changes obtained are displayed in Fig. 9. All the electrical parameters remain almost constant up to defect density of 10¹⁷ cm⁻³. Thereafter, all parameters start to degrade drastically. FF starts increasing particularly at 10¹⁷ cm⁻³ because of rise in energy barrier height.^93,101 The efficiency decreases from 7.12–5.87% as defect density is increased from 10¹⁷–10¹⁸ cm⁻³. The enhanced defect density reduces the carriers' diffusion length and life time and as a result, efficiency decreases as photo-generated carriers find it hard to travel.¹⁰² So buffer layer defect density of 10¹⁵ cm⁻³ is chosen as optimum value which matches with previous work.¹²

Fig. 9 Effect of SnS₂ buffer layer defect density on photovoltaic output.

3.9 Effect of FTO window layer thickness

The influence of FTO window layer thickness on solar cell output is also simulated. The thickness is altered from 0.1–0.6 µm. Fig. 10 displays the simulation outcomes obtained. It is observed that both V_OC and J_SC decrease very slightly and FF remains almost constant as window layer thickness is enhanced within 0.1–0.6 µm. The efficiency decreases from 7.16–7.13% as window layer thickness is varied from 0.1–0.6 µm. As FTO layer thickness rises, the absorption of a large part of the light which incidents in the FTO layer declines transmission, which creates an optical loss and as a result degrades efficiency in the solar cell.¹⁰³ So the optimal window layer thickness chosen is 0.1 µm.

Fig. 10 Impact of FTO window layer thickness on cell output parameters.

3.10 Influence of FTO window layer doping concentration

The effect of the FTO window layer donor concentration is examined and simulation results are outlined in Fig. 11. The FTO window layer doping concentration is tuned from 10¹⁷–10²⁰ cm⁻³. V_OC remains constant and both FF and J_SC increase with increase in window layer donor concentration in considered range. Efficiency increases very slightly from 7.16–7.21% as window layer donor concentration is increased from 10¹⁷–10²⁰ cm⁻³. It is concluded that the FTO layer doping density has insignificant effect on the performance of the proposed Sb₂Se₃ based solar cell when compared to the effect of the absorber and buffer layer. The photo-generated carries may be collected by electrodes efficiently with rise of FTO layer donor density and thus increases efficiency.¹⁰⁴ The optimal window layer donor density chosen is 10²⁰ cm⁻³ which is compatible with research work.¹⁰⁵

Fig. 11 Impact of FTO window layer donor density on device performance.

3.11 Effect of Sb₂Se₃/SnS₂ interface defect density

Metallurgical discontinuities, lattice mismatch and symmetry breaking at the interfaces result in interface defects. Interface with low lattice mismatch results in low both interfacial defects and recombination rate and enhances performance parameters. The Sb₂Se₃/SnS₂ interface defect density is altered from 10⁷ cm⁻² to 10¹⁸ cm⁻² to investigate its impact on device output. Simulation findings are depicted in Fig. 12. It is observed that all performance parameters decrease significantly as interface defect density is increased from 10⁷ cm⁻² to 10¹⁰ cm⁻². Afterwards, all performance parameters remain nearly constant. The V_OC, J_SC, FF and efficiency decreases from 0.52 V to 0.5135 V, 24.19 mA cm⁻² to 22.52 mA cm⁻², 63.3–62.36% and 7.96–7.21% respectively as interface defect density is increased from 10⁷–10¹⁸ cm⁻². Enhanced recombination centers at the Sb₂Se₃/SnS₂ interface due to increase in electron trap centers at high density of Sb₂Se₃/SnS₂ interface defects are responsible for the degradation of performance parameters.¹⁰⁶ The optimal Sb₂Se₃/SnS₂ interface defect density selected is 10¹⁰ cm⁻² which is in good agreement with earlier research works.^107,108

Fig. 12 Influence of Sb₂Se₃/SnS₂ interface defect density on cell output.

3.12 Effect of Sb₂Se₃ absorber layer capture cross section

The influence of the Sb₂Se₃ absorber layer capture cross section is investigated by altering the Sb₂Se₃ absorber layer electron and hole capture cross section from 10⁻¹⁹–10⁻⁹ cm⁻². Simulation results are displayed in Fig. 13. It is observed that both V_OC and J_SC remain almost constant as capture cross section is enhanced from 10⁻¹⁹–10⁻¹² cm⁻². Then both decrease as capture cross section is increased in considered range. Both FF and ƞ remain unaltered as capture cross section is enhanced from 10⁻¹⁹–10⁻¹³ cm⁻². Afterwards, both decline as capture cross section is increased in considered range. Efficiency decreases from 10.23–1.24% as capture cross section is enhanced from 10⁻¹⁹–10⁻⁹ cm⁻². Both carrier diffusion length and lifetime is declined because of large lattice mismatch and enhancement of recombination rate as capture cross-section of defect is increased which degrades device performance parameters.^109,110 The optimal Sb₂Se₃ absorber layer capture cross section selected is 10⁻¹⁵ cm⁻² which matches with reported earlier work.¹²

Fig. 13 Effect of Sb₂Se₃ absorber layer capture cross-section on device electrical parameters.

3.13 The influence of Sb₂Se₃/SnS₂ interface capture cross-section

The Sb₂Se₃/SnS₂ interface capture cross section of electron and hole is tuned from 10⁻²⁰–10⁻⁸ cm⁻² to examine its impact on device output and simulation results are depicted in Fig. 14. It is observed that all performance parameters remain almost constant as capture cross section is enhanced from 10⁻²⁰–10⁻¹³ cm⁻². Then all parameters decline as capture cross section is increased in considered range. Efficiency decreases from 12.31–10.18% as capture cross section is enhanced from 10⁻²⁰ cm⁻² to 10⁻⁸ cm⁻². It is evident that interface capture cross-section has less effect on device output compared to absorber capture cross-section of carriers. Enhancement in capture cross-section of electron and hole results in degradation of the Sb₂Se₃/SnS₂ interface quality, enhancement in recombination and as a result, the efficiency declines.^111,112 The optimal Sb₂Se₃/SnS₂ interface capture cross section chosen is 10⁻¹⁹ cm⁻² which is compatible with research work.¹¹¹

Fig. 14 Solar cell characteristics as a function of Sb₂Se₃/SnS₂ interface capture cross-section.

3.14 Impact of SnS BSF layer thickness

The BSF layer allows easy transportation of holes and blocks the electrons to the back contact simultaneously. Thus BSF layer has a major role in solar cell performance. The effect of SnS BSF layer thickness on solar cell characteristics is also explored. The thickness is varied from 0.02–0.2 µm. Device performance changes with respect to SnS thickness variation are displayed in Fig. 15. The acceptor concentration and defect density of BSF layer is maintained fixed at 10¹⁸ cm⁻³ and 10¹⁴ cm⁻³ respectively. It is observed that the thickness of SnS shows significant effect on the solar cell performance parameters. V_OC, J_SC, FF and ƞ increases from 0.7 V to 0.83 V, 26.76 mA cm⁻² to 31.91 mA cm⁻², 80.97–85.96% and 15.12–22.87% as SnS BSF layer thickness is increased from 0.02–0.2 µm. The decrease of recombination rate of carriers with increasing BSF thickness improves both V_OC and J_SC.¹⁰⁵ The generation of carriers is increased with the rise in the thickness of BSF layer with the same amount of incident photon and thus improves efficiency.⁹¹ To minimize production cost and the quantity of material used by the absorber layer, the optimal SnS BSF layer thickness chosen is 0.2 µm which matches with reported study.⁹⁸

Fig. 15 Effect of SnS BSF layer thickness on device characteristics.

3.15 Influence of SnS BSF layer acceptor concentration

The effect of the SnS BSF layer acceptor density is examined and simulation outcomes are represented in Fig. 16. The SnS BSF layer doping concentration is tuned from 10¹⁵–10²⁰ cm⁻³. All electrical parameters except FF enhance with rise in SnS BSF layer carrier density from 10¹⁵–10²⁰ cm⁻³. FF increases from 72.65–85.96% as SnS BSF layer acceptor concentration is enhanced from 10¹⁵–10¹⁸ cm⁻³ due to decrease in series resistance. Afterwards, FF decline due to increase of recombination at high doping. V_OC, J_SC and efficiency increases from 0.66 V to 0.94 V, 26.6 mA cm⁻² to 31.98 mA cm⁻² and 12.74–24.86% as doping density of BSF layer is increased from 10¹⁵–10²⁰ cm⁻³. The large potential created at the SnS/Sb₂Se₃ interface reduces recombination rate of carriers at high carrier density of BSF layer. Ohmic contact is created with the back contact due to closeness of BSF Fermi level with the valence band (VB) at high BSF doping density which supports easy transportation of holes towards back electrode and enhances device performance.¹¹³ The optimal BSF layer acceptor density chosen is 10²⁰ cm⁻³. The result is within range of earlier simulation studies.^37,114

Fig. 16 Impact of SnS BSF layer acceptor density on photovoltaic output.

3.16 Effect of SnS BSF layer defect density

The SnS BSF layer defect density is varied from 10¹⁰–10²⁰ cm⁻³ to examine its impact on device electrical parameters and simulation outcomes are displayed in Fig. 17. The efficiency is nearly fixed till defect density of 10¹⁷ cm⁻³. Afterwards, efficiency starts to degrade. The efficiency declines from 24.86–12.79% as defect density is enhanced from 10¹⁰–10²⁰ cm⁻³ due to increase in recombination. The sudden rise in FF from defect density 10¹⁷–10¹⁹ cm⁻³ is due to increase in energy barrier height.^93,101 The optimal BSF layer defect density selected is 10¹⁴ cm⁻³ which matches with earlier study.¹¹⁴

Fig. 17 Influence of SnS BSF layer defect density on cell output parameters.

3.17 Impact of back contact work function

The back contact work function majorly impacts the efficiency of the solar cell. A high work function back contact is needed for a proper ohmic contact at the absorber or BSF/back contact interface. To explore the influence of back contact on photovoltaic output, back contact work function is tuned from 4.5–5.4 eV and simulation outcomes obtained are represented in Fig. 18. J_SC increases by a small margin as work function is enhanced within 4.5–5.4 eV. Both V_OC and FF enhance prior to work function of 5 eV and thereafter stay almost saturated. The combined increase of V_OC and FF enhances the efficiency till work function of 5 eV and then remain saturated. With rise in work function of the back electrode, the carriers' barrier height at the back surface decreases which enhances device performance.¹⁴ So for maximum efficiency of proposed solar cell, back electrode work function is required to be larger or equal to 5 eV.

Fig. 18 Impact of back contact work function on device performance with BSF layer.

3.18 Impact of temperature

Long-term stability under ambient air condition is a major requirement for the application of photovoltaic device. The influence of the working temperature on solar cell is investigated in a range of 300–400 K and the simulation outcomes are depicted in Fig. 19. J_SC increases slightly due to reduction of the bandgap of the absorber which rises the generation of carrier in solar cell as temperature is increased in considered range.¹¹⁵ The rise of temperature makes thermally generated electrons unstable due to vibration and recombine with the holes before being collected at the electrodes, reducing V_OC linearly. Moreover, the rise in temperature impacts the carrier properties such as mobility and carrier density and as a result, declines the FF of device.¹¹⁶ The combined decrease in V_OC and FF declines efficiency from 24.86–20.37% as temperature is enhanced from 300–400 K.

Fig. 19 Solar cell output variation with changes in temperature with BSF layer.

3.19 Influence of parasitic resistance

Series resistance (R_S) and shunt resistance (R_Sh) have a vital effect on photovoltaic output. The solar cell performs best at maximum R_Sh and minimum R_s values. R_S is the sum of resistance between different layers of solar cells and at the back and front contacts.¹¹⁷ When the current created by sunlight flows through the various channels produced by small R_Sh, efficiency degrades.¹⁴

The Sb₂Se₃ solar cell performance parameters with SnS BSF layer is investigated by varying the R_S from 0 to 8 Ω cm². The obtained results are shown in Fig. 20. It is observed that V_OC remains constant and J_SC decreases slightly with rise of series resistance. J_SC decreases because of rise in internal resistance which impedes the transport of carriers to the contacts.¹¹⁸ FF decreases significantly from 83.09–59.1% with increase of series resistance in considered range. As a result, efficiency decreases from 24.86–17.68% for optimized FTO/SnS₂/Sb₂Se₃/SnS/Au solar cell as R_S is increased from 0 to 8 Ω cm². R_s should be kept as low as possible to achieve high efficiency.

Fig. 20 Effect of series resistance (R_S) on cell output parameters with BSF layer.

The R_Sh is changed from 200 Ω cm² to 1600 Ω cm² to investigate its effect on the cell output with SnS BSF layer and findings are shown in Fig. 21. J_SC is found to be unaltered, V_OC increases slightly and FF rises significantly with increase of shunt resistance in considered range. Efficiency rises from 21.6–24.45% as shunt resistance is increased from 200–1600 Ω cm² for proposed Sb₂Se₃ solar cell with SnS BSF layer mainly due to rise of FF. FF increases with increase in R_Sh because of low recombination rates.¹¹⁹ It is observed that both FF and efficiency rise with increase in the R_Sh till 1200 Ω cm² and almost saturate on further increase in R_Sh. As a result, R_Sh should be high to achieve high efficiency.

Fig. 21 Impact of shunt resistance (R_Sh) on solar cell characteristics with BSF layer.

3.20 Overall cell performance

The influence of inserting highly doped p-type SnS BSF layer of Sb₂Se₃ based solar cell on the current density–voltage (J–V) is depicted in Fig. 22. The electrical parameters of Sb₂Se₃ based solar cell without and with SnS BSF layer are summarized in Table 5. The optimum parameters of different layers of FTO/SnS₂/Sb₂Se₃/SnS/Au solar cell are illustrated in Table 6. The optimized FTO/SnS₂/Sb₂Se₃/Au solar cell structure obtains V_OC, J_SC, FF and ƞ of 0.66 V, 25.59 mA cm⁻², 73.33% and 12.31% respectively. The insertion of SnS BSF layer enhances V_OC, J_SC, FF and ƞ to 0.94 V, 31.98 mA cm⁻², 83.09% and 24.86% respectively. It is noticed that all electrical parameters are enhanced due to addition of SnS BSF layer. The creation of large built-in potential at the Sb₂Se₃/SnS interface rises the value of V_OC.¹²⁰ The efficient separation and collection of carriers due to the addition of BSF layer enhances the value of J_SC.¹²¹ The improvement of FF is due to rise in the maximum power produced by FTO/SnS₂/Sb₂Se₃/SnS/Au solar cell structure due to reduction of recombination rate at back surface due to the insertion of SnS BSF layer.¹²² The efficiency has significantly enhanced due to the insertion of SnS BSF layer because of combined increment of V_OC, J_SC and FF.

Fig. 22 J–V characteristic curve of Sb₂Se₃ based photovoltaic with and without SnS BSF layer.

Table 5 Performance parameters of the proposed FTO/SnS₂/Sb₂Se₃/Au solar cell structure without and with SnS BSF layer

Structure	VOC (V)	JSC (mA cm^−2)	FF (%)	ƞ (%)
FTO/SnS2/Sb2Se3/Au	0.66	25.59	73.33	12.31
FTO/SnS2/Sb2Se3/SnS/Au	0.94	31.98	83.09	24.86

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Table 6 Optimized parameter values of the FTO/SnS₂/Sb₂Se₃/SnS/Au solar cell

Optimized parameter	Value
Sb2Se3 absorber layer thickness	0.4 µm
Sb2Se3 absorber layer acceptor concentration	10^16 cm^−3
Sb2Se3 absorber layer defect density	10^13 cm^−3
Sb2Se3 absorber layer bandgap	1.15 eV
SnS2 buffer layer thickness	0.03 µm
SnS2 buffer layer donor concentration	10^18 cm^−3
SnS2 buffer layer defect density	10^15 cm^−3
FTO window layer thickness	0.1 µm
FTO window layer donor concentration	10^20 cm^−3
Sb2Se3/SnS2 interface layer defect density	10^10 cm^−2
Sb2Se3 absorber layer capture cross-section of electron and hole	10^−15 cm^−2
Sb2Se3/SnS2 interface capture cross-section of electron and hole	10^−19 cm^−2
SnS BSF layer thickness	0.2 µm
SnS BSF layer acceptor concentration	10^20 cm^−3
SnS BSF layer defect density	10^14 cm^−3
Back contact work function	5 eV

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The quantum efficiency (QE) curve for Sb₂Se₃ based solar cell at initial and with optimization with and without SnS BSF layer is displayed in Fig. 23. The strong back surface electric fields at the SnS/Sb₂Se₃ interface due to addition of SnS BSF layer in the simulated cell structure decreases carrier recombination and increases the absorption at the wavelength range from 400 to 1030 nm, which increases the electrical parameters of the designed photovoltaic. This results prove that SnS can be a potential BSF layer for improving the efficiency of Sb₂Se₃-based photovoltaic.

Fig. 23 QE characteristic curve of Sb₂Se₃ based photovoltaic with and without SnS BSF layer.

In this research, a promising efficiency of 24.86% was obtained by incorporating SnS as a BSF layer due to n-SnS₂/p-Sb₂Se₃/p⁺-SnS forming of n–p–p⁺ double heterojunction cell. This double junction cell efficiency improvement can be demonstrated by Tail-States-Assisted (TSA) two-steps photon upconversion process. In this double heterostructure, with the insertion of SnS BSF layer, the sub-bandgap photons may get absorbed notably, specifically in the longer wavelength, which causes an enhancement in solar cell performance. The sub-bandgap photons may be absorbed by the Urbach energy states and these lower energy sub-bandgap photons participate in tail-state-assisted (TSA) two-steps photon upconversion.^107,123,124 These absorbed photons generate additional electron–hole pairs, resulting in a noticeable improvement in cell photocurrent. A photoactive material such as SnS BSF layer can cause this TSA upconversion effectively when it possesses favorable bandgap, high absorption coefficient and adequate doping concentration.^105,125 Thus, it is believed that this is the main reason of the high-efficiency of 24.86% observed in proposed double heterojunction solar cell. The Urbach energy, E₀, of the photoactive material determines the resulting increase in photocurrent and the degree of upconversion. The higher Urbach energy remarkably promotes to boosting the quantum efficiency (QE), specifically in the longer wavelength.^5,126

The energy band diagram of the optimized Sb₂Se₃-based solar cell architecture of FTO/SnS₂/Sb₂Se₃/SnS/Au is displayed in Fig. 24. It is noticed that the energy level of conduction and valence band of SnS₂ buffer layer is lower than the Sb₂Se₃ absorber layer. As a result, electrons can travel easily to the front contact through the Sb₂Se₃/SnS₂ interface due to small conduction band offset (CBO) of −0.05 eV. On the other hand, the valence band offset (VBO) between the SnS₂ buffer layer and Sb₂Se₃ absorber layer is large enough to prevent the flow of holes from Sb₂Se₃ absorber to front contact. For the SnS BSF layer, the energy level of valence and conduction band is higher than the Sb₂Se₃ absorber layer. Moreover, a proper barrier height with a suitable conduction band offset (CBO) at the SnS BSF/Sb₂Se₃ absorber junction hinders electrons from travelling from the absorber to the back electrode which in turn reduces recombination process at the back surface.^14,98 On the contrary, the holes can travel from absorber to the back contact easily due to small VBO of 0.1 eV.

Fig. 24 Energy band diagram of proposed FTO/SnS₂/Sb₂Se₃/SnS/Au solar cell.

3.21 Comparative study

Table 7 compares various results from past experimental or theoretical work on Sb₂Se₃ based solar cells with various buffer layers without and with BSF layer. In comparison to the previous reported values, enhanced ƞ have been achieved by this simulation study.

Table 7 Comparison of proposed solar cell with other reported Sb₂Se₃ based solar cells

Structure	Study type	Photovoltaic parameters				References
		VOC (V)	JSC (mA cm^−2)	FF (%)	ƞ (%)
FTO/CdS/Sb2Se3/CZTA/Au	Experimental	0.421	28.40	57.10	6.84	73
Ag/ITO/ZnO/CdS/Sb2Se3/Mo/SLG	Experimental	0.454	18.4	50.6	4.22	127
Ag/ITO/CdS/Sb2Se3/Mo	Experimental	0.343	16.79	35.3	2.1	128
FTO/CdS/Sb2Se3/Spiro-OMeTAD/Au	Experimental	0.360	29.00	51.50	5.40	55
ITO/CdS/Sb2Se3/Au	Experimental	0.379	27.93	55.6	5.91	129
AZO/i-ZnO/CdS/TiO2/Sb2Se3/Mo	Experimental	0.4	32.58	70.30	9.20	19
ITO/CdS/Sb2Se3/CuSCN/Au	Experimental	0.413	30.51	58.99	7.5	130
ITO/CdS/Sb2Se3/Au	Experimental	0.420	29.90	60.40	7.60	26
Metal contact/ZnO:Al/i-ZnO/CdS/Sb2Se3/Mo	Simulation	0.56	31.79	70.81	12.62	33
ZnO:Al/ZnO/CdS/TiO2/Sb2Se3/MoS2/Mo	Simulation	0.4	32.56	70.3	9.17	131
FTO/Zn(Sn,O)/Sb2Se3/CZTSe/Au	Simulation	0.66	34.66	81.18	18.50	132
Ag/phenyl-C61-butyric acid methyl ester (PCBM)/Sb2Se3/HTL/ITO	Simulation	0.858	34.18	84.04	24.66	133
Ag/ZnSe/Sb2Se3/Mo	Simulation	0.85	34	83.6	24.0	17
Al/ITO/CdS/Sb2Se3/CuS/Ni	Simulation	0.761	37.59	80.97	23.16	12
FTO/SnS2/Sb2Se3/Au	Simulation	0.66	25.59	73.33	12.31	This work
FTO/SnS2/Sb2Se3/SnS/Au	Simulation	0.94	31.98	83.09	24.86	This work

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4. Conclusions

The electrical parameters of Sb₂Se₃-based thin film solar cell with and without SnS BSF layer have been compared in this simulation work using SCAPS-1D. First of all, experimental result is reproduced for FTO/CdS/Sb₂Se₃/Au solar cell structure to validate the numerical model and carry out analysis. Next, non-toxic SnS₂ has been used as a buffer layer instead of the hazardous cadmium sulfide (CdS) for the proposed solar cell which results in enhanced efficiency. Afterwards, to maximize efficiency different parameters such as thickness, doping concentration, bandgap, capture cross section and defect density that effect the electrical output is optimized. The optimal thickness for the Sb₂Se₃ absorber layer, SnS₂ buffer layer and FTO window layer have been chosen to be of 0.4 µm, 0.03 µm and 0.1 µm respectively. The doping density have been optimized to be of 10¹⁶ cm⁻³, 10¹⁸ cm⁻³ and 10²⁰ cm⁻³ respectively. The defect density in the Sb₂Se₃ absorber layer, SnS₂ buffer layer and the interface defect at Sb₂Se₃/SnS₂ interface have been selected to be of 10¹³ cm⁻³, 10¹⁵ cm⁻³ and 10¹⁰ cm⁻² respectively for optimal performance. The capture cross-section in the Sb₂Se₃ absorber layer and at Sb₂Se₃/SnS₂ interface have been optimized to be of 10⁻¹⁵ cm⁻² and 10⁻¹⁹ cm⁻² respectively. The optimized FTO/SnS₂/Sb₂Se₃/Au solar cell structure provides J_SC of 25.59 mA cm⁻², V_OC of 0.66 V, FF of 73.33% and efficiency of 12.31%. The insertion of SnS BSF layer with thickness of 0.2 µm, carrier density of 10²⁰ cm⁻³ and defect density of 10¹⁴ cm⁻³ results in enhanced efficiency of 24.86% including J_SC of 31.98 mA cm⁻², V_OC of 0.94 V and FF of 83.09% for optimized novel FTO/SnS₂/Sb₂Se₃/SnS/Au solar cell structure. The findings of this simulation work provide guidelines for the production of non-toxic, cost-effective and highly efficient Sb₂Se₃-based solar cell.

Conflicts of interest

The author declares that there is no conflict of interest regarding the publication of this paper.

Data availability

The data will be made available on reasonable request.

Acknowledgements

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. The author is grateful to Dr Marc Burgelman at University of Ghent, Belgium for allowing to use the SCAPS-1D simulation package.

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