SnTe as a BSF enhances the performance of Sb₂Se₃ based solar cell: A numerical approach

Abstract

This theoretical investigation’s primary goal is to investigate how the Sb₂Se₃ solar cell’s performance may be improved. Here, SnTe, as an innovative back surface field (BSF) layer, has been added between the rear contact (Mo) and absorber layer (Sb₂Se₃). Above the absorber layer, the structure comprises a thin CdS buffer layer. For each layer of the Al/CdS/Sb₂Se₃/SnTe/Mo structure, the physical characteristics such as the active layer’s thicknesses, carrier concentration, defect density, and rear electrode’s work function are determined. The suggested cell outperformed the solar cell without the SnTe layer, which had an efficiency of 20.33%, with enhanced efficiency and open-circuit voltage (Voc) of 28.25% and 0.86 V, respectively, at 300 K. The above solar cell used 0.15 μm SnTe layer, 0.05 μm CdS, and 2.0 μm Sb₂Se₃ layer. The features of the antimony selenide (Sb₂Se₃) based solar structure is examined using the SCAPS-1D software, which simulates solar cells in one dimension. Investigations have also been done into how working temperatures influence the I–V parameters of the structure.

Sb₂Se₃; BSF; SnTe; Solar Cell; SCAPS-1D.

1. Introduction

With the depletion of non-renewable energy sources, research is being done to harness energy from renewable sources. Solar energy is among the most common renewable and environment-friendly sources currently undergoing rapid research and implementation to fulfill rising global energy demand, owing to its relative abundance. Furthermore, the growing usage of solar energy necessitates advancing innovative and efficient photovoltaic (PV) technologies that are cost-effective to produce and have improved power conversion efficiency (PCE). Antimony selenide (Sb₂Se₃) has lately been recognized as a potential PV absorber layer due to its favorable optoelectronic properties, such as higher absorption coefficient (>10⁵ cm⁻¹) [1], proper direct (1.17 eV), and indirect (1.03 eV) band gap for the absorption of a large amount of solar spectrum [2], good carrier mobility [3] and stable orthorhombic structure made by (Sb₄Se₆)_n ribbons [2,4]. Additionally, antimony and selenium are present in ample amounts on the planet, are low-toxic, and have low cost. In 2009, Nair et al. reported using Sb₂Se₃ as absorber material in PV devices with a conversion efficiency of only 0.66 % [5]. Further, by tuning selenization parameters, Tang et al. obtained an efficiency of 6.06% and V_OC of 0.494 V [6], and Chen et al. achieved an efficiency of 7.04% with grain boundary inversion attained by n-type Cu interstitial doping [7]. In 2021, Guo et al. reported PCE of 7.15% and V_OC of 0.41 V using a 50 nm NiO_x layer as HTL [8]. The highest PCE of 10.7% for Sb₂(S, Se)₃ thin film solar cells was reported by Zhao et al., opening a new path for efficiency gains [9]. Simulation studies have been carried out to guide the improvement of Sb₂Se₃-based solar cells. Li et al. reported an efficiency of 23.18% for Sb₂Se₃-based solar cells with CuO as HTL [10]. Karade et al. reported a PCE of 17.75% and V_OC of 0.603 V for a Sb₂Se₃ photovoltaic cell having SnS as a BSF layer [11]. Furthermore, Sunny et al. reported a PCE of 29.89% by incorporating a 0.05 μm thin SnS as HTL in Al/F:SnO₂ (FTO)/CdS/Sb₂Se₃/SnS/Mo [12]. Cadmium sulfide (CdS) with a bandgap of 2.4 eV is used as a buffer layer to allow incoming photons to penetrate the absorber layer with the least absorption loss [13]. Primary reasons, such as carrier recombination at the rear surface and ineffective carrier transport and gathering via the rear and front contacts, could cause poor photovoltaic performance. As a result, more research into designing a new Sb₂Se₃-based thin-film solar cell (TFSC) is required to obtain optimal photovoltaic outputs.

One of the ways to boost the efficacy of photovoltaic cells is by introducing the BSF layer between the back contact and absorber layer. It has improved cell performance by lowering the surface recombination rate (SRV) [10]. The BSF layer minimizes the barrier height of the rear electrode and the loss of minority carriers at the back-electrode, among other things [14,15]. The primary function of the BSF layer is to restrict photo-induced and prevent the charge carriers from reaching the p-n junction, allowing them to be collected efficiently over the back contact [16]. Here, the tin telluride (SnTe) layer is used as BSF to enhance the carrier transport properties near back contact. SnTe, a semiconductor with a small bandgap (Eg = 0.18 eV) and an extremely high hole concentration, benefits from low resistance and high mobility. Unintentionally doped SnTe often has a carrier concentration of more than 10²⁰ cm⁻³ [17]. In this study, various parameters of Sb₂Se₃ based solar structure are varied to find the photovoltaic device’s performance with the aid of the SCAPS-1D simulation model.

SCAPS (Solar Cell Capacitance Simulator) is a one-dimensional simulation software for solar cells. It is built on the resolution of Poisson and continuity equations for electrons-holes [18], as shown in Eqs. (1), (2), and (3), respectively.

$$\frac{d^2}{{dx}^2}\psi \left(\mathrm{x}\right)=\frac{-q}{{\epsilon }_0{\epsilon }_r}\mathrm{p}\left(\mathrm{x}\right)-\mathrm{n}\left(\mathrm{x}\right)+{\mathrm{N}}_{\mathrm{D}}-{\mathrm{N}}_{\mathrm{A}}+{\rho }_P-{\rho }_n$$ (1)

$$-\frac{1}{q}\frac{d{J}_n}{dx}=(\mathrm{G}-\mathrm{R})$$ (2)

$$\frac{1}{q}\frac{d{J}_p}{dx}=(\mathrm{G}-\mathrm{R})$$ (3)

here Ψ are electrostatic potentials, ε₀ and ε_r are vacuum and relative permittivity, p and n show the concentration of holes and electrons, N_A and N_D are the acceptor density and the donor density, and R and G are recombination rate and generation rate, respectively.

The modeling and numerical simulation of Sb₂Se₃-based TFSC with a heterostructure of Al/CdS/Sb₂Se₃/SnTe/Mo is done. CdS and SnTe were utilized as the buffer layer and BSF, respectively. The overall performance of the device in terms of I–V parameters, i.e., open-circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and PCE, are reported in this work. The proposed innovative device structure’s photovoltaic performance was investigated by adjusting distinct physical parameters of the BSF and absorber layers.

2. Device structure, energy band diagram, and simulation parameters

The pictorial representation of Sb₂Se₃-based solar cell is displayed in Figure 1. Here, Sb₂Se₃ is used as the absorber layer, and CdS and SnTe are taken as a buffer and BSF layer, respectively. Al and Mo are utilized as front and rear electrodes, respectively. The schematic energy band diagram is displayed in Figure 2. It shows the energy bands of the optimized solar cell. The EFn level of SnTe is larger than the Valence Band level, but the EFp level of CdS is smaller than the Conduction Band level. As a result, photo-induced electrons are directed to the n-type CdS buffer layer and are blocked by the BSF layer. Also, the buffer layer blocks photo-induced holes and leads them to the p-type BSF layer. Subsequently, the front and rear contact electrodes can easily collect the absorber layer’s holes and electrons.

Figure 1.

The schematic of CdS/Sb₂Se₃/SnTe based solar structure.

Figure 2.

Energy Band curves of CdS/Sb₂Se₃/SnTe structure.

The Sb₂Se₃-based n-CdS/p-Sb₂Se₃/p-SnTe structure is numerically simulated using SCAPS simulation software by M. Burgelman et al. at the University of Gent, Belgium [19]. The simulation was done under the illumination of AM_1.5 G, 1000 W/m² at 300 K temperature. Here, the radiative recombination coefficient was neglected. All the layers specified have a Gaussian energy distribution for acceptor and donor deformity. For the absorber, buffer, and BSF layer, thermal velocity was set to 10⁷ cm/s. The CdS/Sb₂Se₃ and Sb₂Se₃/SnTe interface defects are neutral, having a density of 10¹⁰ cm⁻². The simulation parameters needed to establish the baseline device efficiency were taken from [10,16,17,20] and are summarized in Table 1.

Table 1.

Physical Parameters of CdS/Sb₂Se₃/SnTe solar cell used in this simulation.

Parameters	p-SnTe	p-Sb2Se3	n-CdS
Thickness (μm)	0.1	Varying	0.05
Band-Gap (eV)	0.18	1.2	2.4
Electron Affinity (eV)	5.1	4.04	4.2
Dielectric Permittivity (relative)	100	18	10
CB effective density of states (cm−3)	1016	2.2. 1018	2.2. 1018
VB effective density of states (cm−3)	1017	1.8. 1019	1.8. 1019
Electron-thermal velocity (cm/s)	107	107	107
Hole-thermal velocity (cm/s)	107	107	107
Electron-mobility (cm2/Vs)	500	15	100
Hole-mobility (cm2/Vs)	2720	5.1	25
Donor density ND (cm−3)	0	0	1018
Acceptor density NA (cm−3)	1018	1013	0
Defect type	Donor	Donor	Neutral
Capture cross-section of electrons and holes (cm2)	10−15	10−15	10−15
Energetic-distribution	Gaussian	Gaussian	Gaussian
Reference for defect energy level, Et	Above EV	Above EV	Above EV
Energy level with respect to a reference (eV)	0.6	0.6	0.6
Characteristic energy (eV)	0.1	0.1	0.1
Nt total (1/cm3)	1. 1014	1. 1013	1. 1014

3. Result and discussion

3.1. Sb₂Se₃ absorber layer’s effects on the functionality of the structure

This section examines the consequences of the Sb₂Se₃ absorber layer’s thickness, defect density, and doping density on the n-CdS/p-Sb₂Se₃/p-SnTe structure. The impact of the varied thickness of Sb₂Se₃ on the device performance can be seen in Figure 3(a). Here, by varying the absorber layer’s thickness from 0.5 to 3.0 μm, all the performance characteristics (Voc, Jsc, FF, and PCE) increased. Voc rises by a factor of 2.3%, Jsc rises by 9.8%, Fill factor (FF) rises by 0.6%, and PCE rises by 13.4% (Supplementary Table ST1). However, 2.0 μm is used as the optimized absorber layer’s thickness. The increase in Jsc is logical because of the generation of more e-h pairs in a thicker absorber layer by absorbing more incident photons. According to Schokley-Queisser Eq. (4), Voc is given by-

$$Voc=\frac{kT}{e}\ln \left[\frac{Jsc}{J_0}+1\right]$$ (4)

Figure 3.

Effect of changing Sb₂Se₃ parameters: (a) thickness, (b) shallow acceptor doping concentration (N_A), and (c) defect density (N_t) on the performance of the n-CdS/p-Sb₂Se₃/p-SnTe structure.

As Voc is a function of photogenerated current (Jsc) and dark saturation current (J₀), recombination increases at higher thickness, which enhances J₀. So, there is a slight increment of 20 mV in Voc with enhancement in Jsc. The increment in FF is due to a decrease in series resistance (because of the high compliance of the material interface). The increase of all factors results in increased efficiency.

Figure 3b shows the response of varying the carrier concentration of the absorber on efficiency and other device parameters. The value of Voc remains constant at 0.86 V when shallow acceptor density (N_A) increases from 10¹³ to 10¹⁷ cm^−3, and it increases to a value of 0.88 V for 10¹⁸ cm⁻³ (Supplementary Table ST2).

The performance of the PV cell in the absorber layer is known to be significantly impacted by imperfections/defects [21]. Defects provide an active site for efficient recombination or trapping centers. In this case, hole (majority carriers) traps act as recombination centers for electrons, and the lifetime of electrons will be less, thus deteriorating the solar cell’s performance [22]. From Figure 3c, we can depict the response of varying defect density (N_t) of the Sb₂Se₃ layer on the photovoltaic parameters. Voc, Jsc, FF and PCE decreased from 0.88 to 0.79 V, 36.59 to 24.04 mA/cm², 86.12 to 79.72 % and 27.87 to 15.15 %, respectively (Supplementary Table ST3).

3.2. Impact of the BSF layer on the functionality of the device

A narrow band gap compound SnTe (0.18 eV) was placed between the rear contact and absorber layer to minimize the barrier height at the back electrode and probable recombination loss in Sb₂Se₃ solar cells, resulting in a reduced rear surface recombination rate at their junction. This material with a low band gap would function as a BSF, bouncing back carriers (electrons) from the Sb₂Se₃/SnTe junction and aiding carrier. SnTe has the advantages of high mobility, low resistance, and high hole concentration.

This section depicts the impact of doping concentration, thickness, and defect density of the SnTe layer on the solar device performance parameters. Figure 4a shows the response to altering the thickness of the SnTe layer on the device parameters. When the SnTe layer’s thickness increased from 0.01 μm to 0.15 μm, Voc increased by a factor of 2.3%, Jsc increased by 3.8%, FF increased by 0.6%, and PCE from 6.4%. At the interface of p⁺-SnTe – p-Sb₂Se₃, the developed electric field restricts the flow of electrons to the back surface by acting as a potential barrier. The bouncing back of electrons results in decreased dark current and increased Jsc. The improvement in Voc is due to reduced recombination. In contrast, for the thickness of SnTe above 0.15 μm, all the photovoltaic parameters remained constant (Supplementary Table ST4). There were no effects on the photovoltaic parameters of the device upon increasing thickness beyond 0.15 μm.

Figure 4.

Effect of changes in BSF parameters on the functionality of n-CdS/p-Sb₂Se₃/p-SnTe solar device: (a) thickness, (b) shallow acceptor doping concentration (N_A), and (c) defect density (N_t).

Figure 4(b) shows the impact of N_A of the SnTe layer on device parameters. With an increase in the doping concentration from 10¹³ to 10¹⁶ cm⁻³, the Voc, Jsc, FF, and PCE remained constant, but at higher doping of 10¹⁸ cm⁻³, there is a slight increase in the photovoltaic parameters of the device (Supplementary Table ST5). The increment in Jsc and PCE is due to electric field generation at the back surface, which leads to the protection of minority carriers from recombination by an electric field.

Figure 4c shows no variation by varying N_t of the SnTe layer from 10¹³ to 10¹⁶ cm⁻³ on the Voc and FF, whereas Jsc and efficiency of the device decreased when the N_t varied from 10¹³ to 10¹⁶ cm⁻³ (supplementary table ST6).

3.3. Influence of back electrode work function on the functionality of the device

In this section, the work function of the back contact (rear) electrode is varied from 4.5 to 5.1 eV to check the overall device performance. Results are shown in Figure 5. Back contacts with a work function below 5.0 eV will not collect the charge carriers effectively, which resulted in lower Voc than 0.8 V. In contrast, when the work function of a rear contact was above 5.0 eV, the device showed Voc of 0.86 V (Supplementary Table ST7). The value of Jsc remains unaffected by the change in the work function of the rear contact. The FF and efficiency of the device decreased for the work function of back contact below 5.0 eV as there is a significant energy level difference, which causes the Schottky junction to occur.

Figure 5.

Influence of back contact metal work functions on the behavior of the n-CdS/p-Sb₂Se₃/p-SnTe structure.

3.4. I–V characteristics of the device with and without a BSF layer

Figure 6 shows the I–V curve with and without the BSF (SnTe) layer at optimized device parameters. Optimized thickness, N_A, and N_t of the Sb₂Se₃ absorber layer were 2.0 μm, 10¹³ cm⁻³, and 10¹² cm⁻³, respectively. For the SnTe layer, the optimized thickness, acceptor density, and defect density were 0.150 μm, 10¹⁸ cm⁻³, and 10¹⁴ cm⁻³, respectively.

	Voc (V)	Jsc (mA/cm2)	FF (%)	PCE (%)
With SnTe	0.86	37.81	86.84	28.25
Without SnTe	0.65	37.90	82.02	20.33

Figure 6.

I–V plot for Sb₂Se₃-based solar cell with and without SnTe at optimized parameters.

3.5. Impact of working temperature on device performance

The operating temperature impacts the performance of the solar cell. The increased working temperatures impact material properties such as bandgap, hole and electron mobility, concentration, and density of states. Simulations were run with cell operating temperatures varying from 290 K to 350 K to explore the influence of increased operating temperatures on the output of the solar device with SnTe as BSF (Supplementary Table ST8), and the results are shown in Figure 7. The increment in the operating temperature per 10 K leads to a very slight increment in Jsc. In contrast, the value of Voc and FF decreases, leading to reduced efficiency.

Figure 7.

Influence of temperature on the I–V parameters of PV device.

4. Conclusion

In this work, the impact of absorber and BSF layer parameters are varied to obtain an optimized solar device. The optimized Sb₂Se₃ absorber layer thickness was 2.0 μm, while the optimized doping and defect density for Sb₂Se₃ was 10¹⁴ and 10¹² cm⁻³, respectively. The thickness of the back surface field (SnTe) affected the output parameters. For BSF, the optimized thickness was 0.15 μm, and the maximum efficiency was achieved at this thickness. A large work function of the rear contact results in more alignments of the back-end energy bands, increasing efficiency. Mo (5.0 eV) is therefore used as the ideal work function for rear contact. Also, with an increase in the operating temperature, device efficiency decreases. After optimization, 28.25 % efficiency with Voc of 0.86 V, Jsc of 37.81 mA/cm², and FF of 86.84 % is attained at 300 K. Our findings could be applied to the production of affordable, highly effective, and stable solar cells.

Declarations

Author contribution statement

Raman Kumari: conceived and designed the experiments; performed the experiments; analyzed and interpreted the data, wrote the paper

Mamta, Rahul Kumar, V.N. Singh: analyzed and interpreted the data, wrote the paper.

Funding statement

This research received no specific grant from public, commercial, or not-for-profit funding agencies.

Data availability statement

Data included in article/supplementary material/referenced in the article.

Declaration of interests’ statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.

Appendix A. Supplementary data

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