Modeling and Simulation of Tin Sulfide (SnS)-Based Solar Cell Using ZnO as Transparent Conductive Oxide (TCO) and NiO as Hole Transport Layer (HTL)

This paper describes the simulation by Solar Cell Capacitance Simulator-1D (SCAPS-1D) software of ZnO/CdS/SnS/NiO/Au solar cells, in which zinc oxide (ZnO) is used as transparent conductive oxide (TCO) and nickel oxide (NiO) is used as a hole transport layer (HTL). The effects of absorber layer (SnS) thickness, carrier concentration, SnS defect density, NiO HTL, ZnO TCO, electron affinity and work function on cell performance have been evaluated. The effect of interface defect density of SnS/CdS on the performance of the heterojunction solar cell is also analysed. As the results indicate, a maximum power conversion efficiency of 26.92% was obtained.


Introduction
Global energy consumption is rising as a result of population growth and a wide use of digital substances in practically every aspect of our lives. The major source of energy is fossil fuels [1] which is not a long-term energy source. The burning of fossil fuels emits a large amount of carbon dioxide [2]. Furthermore, fossil fuels are not a sustainable source. For these reasons, the energy sector is expanding but shifting to sustainable sources of energy as the optimum choice. Solar cells can be a viable substitute for fossil fuels. Many researchers have recently concentrated on highly efficient polymer and perovskite cells [3,4], but these solar cells suffer from instability which limits their long-term application areas. As a result, the thin-film solar cell may be used in place of polymer and perovskite solar cells.
Tin monosulphide (SnS) has been developed as a significant absorbent layer in thinfilm solar cells having various advantages such as a high absorption coefficient that allows SnS to absorb a considerable portion of the spectrum. SnS is an Earth-abundant material which can be manufactured at low cost [5][6][7][8][9]. The energy gap at the absorber and back contact interface has a significant impact on the performance of the cell. This energy gap can be reduced by using a small resistive back contact with higher work function (WF). High WF transition metal oxides (TMOs) such as MoO X [10][11][12] , V 2 O 5 [13], NiO [14] and WO 3 [15] were used as HTLs in several solar cells for performance improvement. NiO is a highly useful HTL within all TMOs due to its wide bandgap (3.5-3.8 eV) and high WF (5 eV). Polycrystalline CdS is used as a window layer, having many features such as high absorption coefficient, electron affinity, low resistivity, high optical transmittance, wide bandgap and good electrical properties that are suitable for solar cell fabrication and applications as well. TCO in a solar cell is usually indium tin oxide (ITO) or fluorine-doped tin oxide (FTO) which are quite expensive, and ITO has stability limitations. ZnO is used as TCO, having a wide bandgap of 3.37 eV, excellent electrical and optical properties, and it is Earth abundant [16,17]. Furthermore, different parameters such as carrier concentration, SnS layer thickness and defects have a considerable impact on the performance of the cell [18][19][20][21].
In this study, a new highly efficient structure of ZnO/CdS/SnS/NiO/Au of SnS-based solar cells is designed. SCAPS 1-D software is used to perform a numerical study of the developed cell. The influence of various parameters of ZnO, SnS and NiO HTL on the performance of the solar cell has been studied. The improved solar cell achieved a maximum efficiency of 26.92%.

Modelling and Simulation
Simulation software of solar cells is designed to solve semiconductor material property equations. SCAPS 1-D software is much more popular than other simulation software due to its ability to simulate up to seven layers of structure [22,23]. SCAPS 1-D simulator is used in the modelling of ZnO/CdS/SnS/NiO/Au heterostructure cells. It includes the following equations for measurements of J-V, C-V, C-F properties and quantum efficiency of the solar cell.
Poisson's equation Electron continuity equation: where ε = dielectric constant, q = charge of the electron, N A = acceptor density, N D = donor density, Ψ = electrostatic potential, Jp = current density due to holes, Jn = current density due to electrons, G op = carrier generation rate, R = total recombination rate, ρ p and ρ n = hole and electron distribution, respectively. The semiconducting material's holes and electron charge carrier characteristics are represented by the drift and diffusion equations [24] given below: where µ p and µ n = holes and electron mobility, respectively, E Fp and E Fn = p-type and n-type fermi level, respectively.

Solar Cell Structure and Material Properties
The schematic structure and energy band diagram of the proposed ZnO/CdS/SnS/ NiO/Au cell are shown In Figure 1a,b with ZnO serving as TCO, CdS as the window layer, SnS as the absorber layer, NiO as HTL and Au as back metal contact. ZnO is an n-type semiconductor used as the substitute of ITO and FTO because of its excellent electrical and optical properties. Figure 1b shows that the conduction band of SnS is smaller than that of CdS and the conduction band offset (CBO) between them is also very low. Thus, electrons can flow easily from SnS to ZnO through CdS. The valence band of NiO is higher than that of SnS and the valence band offset (VBO) between them is smaller. Furthermore, CBO between SnS and NiO is relatively high, stopping electrons from entering the back electrode. For benchmarking purposes, we have used a ZnO/CdS/CdTe/SnS/Ni device structure [25]. The physical parameters used in the modelling of ZnO/CdS/SnS/NiO are stated in Table 1 and interface parameters of SnS/CdS are listed in Table 2.
Micromachines 2022, 13, x FOR PEER REVIEW 3 of 10 electrical and optical properties. Figure 1b shows that the conduction band of SnS is smaller than that of CdS and the conduction band offset (CBO) between them is also very low. Thus, electrons can flow easily from SnS to ZnO through CdS. The valence band of NiO is higher than that of SnS and the valence band offset (VBO) between them is smaller. Furthermore, CBO between SnS and NiO is relatively high, stopping electrons from entering the back electrode. For benchmarking purposes, we have used a ZnO/CdS/CdTe/SnS/Ni device structure [25]. The physical parameters used in the modelling of ZnO/CdS/SnS/NiO are stated in Table 1 and interface parameters of SnS/CdS are listed in Table 2.   [26], CdS [27], SnS [28] and NiO [14] used in the simulation of proposed solar cell.    [26], CdS [27], SnS [28] and NiO [14] used in the simulation of proposed solar cell. Absorption coefficient (cm −1 ) SCAPS 1 × 10 5 1 × 10 5 1 × 10 5 Table 2. Interface parameters used in the simulation of the proposed solar cell.

Results and Discussion
The heterostructure ZnO/CdS/SnS solar cells have V OC , J SC , FF and PCE of 0.4732 V, 33.739 mA/cm 2 , 71.79% and 11.46%, respectively. Figure 2a

Impact of Carrier Concentration and Thickness of NiO and ZnO
The cell performance was studied in terms of carrier concentration and layer thickness. The thickness of NiO and ZnO was varied from 100 to 12,000 nm and carrier concentration was varied from 10 14 to 10 21 cm −3 . The VOC is nearly independent of both carrier concentration and NiO thickness. The JSC varies significantly depending on carrier concentration and thickness. At a NiO carrier concentration of 10 18 cm −3 and thickness of 250 nm, the optimal JSC of 34.20 mA/cm 2 was obtained. The FF and PCE increase with an increase in carrier concentration but remain almost constant with increasing thickness. The series resistance decreases as the carrier concentration increases due to which FF and PCE rise [10]. At a NiO carrier concentration of 10 21 cm −3 and thickness of 250 nm, the maximum PCE of 26.92% is recorded.
The thickness of ZnO does not have any effect on VOC but fluctuates with carrier concentration and achieves its maximum value of 0.9048 V at 10 17 cm −3 . The JSC worked similarly to the VOC and gave an optimal value of JSC 34.20 mA/cm 2 at 10 17 cm −3 carrier concentration. The increase in ZnO carrier concentration induced band bending resulting in a

Impact of Carrier Concentration and Thickness of NiO and ZnO
The cell performance was studied in terms of carrier concentration and layer thickness. The thickness of NiO and ZnO was varied from 100 to 12,000 nm and carrier concentration was varied from 10 14 to 10 21 cm −3 . The V OC is nearly independent of both carrier concentration and NiO thickness. The J SC varies significantly depending on carrier concentration and thickness. At a NiO carrier concentration of 10 18 cm −3 and thickness of 250 nm, the optimal J SC of 34.20 mA/cm 2 was obtained. The FF and PCE increase with an increase in carrier concentration but remain almost constant with increasing thickness. The series resistance decreases as the carrier concentration increases due to which FF and PCE rise [10]. At a NiO carrier concentration of 10 21 cm −3 and thickness of 250 nm, the maximum PCE of 26.92% is recorded.
The thickness of ZnO does not have any effect on V OC but fluctuates with carrier concentration and achieves its maximum value of 0.9048 V at 10 17 cm −3 . The J SC worked similarly to the V OC and gave an optimal value of J SC 34.20 mA/cm 2 at 10 17 cm −3 carrier concentration. The increase in ZnO carrier concentration induced band bending resulting in a small rise in J SC . The FF had a reducing nature with increasing thickness of ZnO up to 10 17 cm −3 because of increasing series resistance. Maximum efficiency of 26.92% is recorded at carrier concentration and thickness of 10 17 cm −3 and 100 nm, respectively.

Impact of SnS/CdS Interface Defect Density
The influences of SnS/CdS interface defect density (IDD) on the performance of the proposed solar cell have been analysed. The SnS/CdS defect interface can increase the series resistance and carrier trapping of the cell. Due to the increase in carrier recombination rate at the interface, V OC decreases with IDD but IDD has no impact on J SC until 10 17 cm 2 . J SC increases from 19.48 to 28.25 mA/cm 2 when the width of SnS layer increases from 200 to 1200 nm. When IDD increases from 10 11 to 10 18 cm 2 , FF decreases rapidly from 84.9% to 54.8% due to an increase in the series resistance of the cell, indicating that high defect density at SnS/CdS is responsible for high series resistance. A significant decrease in PCE was also seen with increasing IDD. An interface defect is one of the reasons for lower PCE. Defects in the interfacial layer density are induced in the cell as a result of structural changes in the materials produced during the fabrication process.

Impact of Defect Density and Thickness of Absorber Layer SnS
The influence of absorber layer defect density and thickness is depicted in Figures 3 and 4, respectively. The defect is caused by displacement and surface defects. Defects serve in carrier recombination, reducing mobility and carrier lifetime. The Shockley-Read-Hall (SRH) process modulates the recombination rate of SnS at higher defect density.
Micromachines 2022, 13, x FOR PEER REVIEW 5 of 10 small rise in JSC. The FF had a reducing nature with increasing thickness of ZnO up to 10 1 cm −3 because of increasing series resistance. Maximum efficiency of 26.92% is recorded a carrier concentration and thickness of 10 17 cm −3 and 100 nm, respectively.

Impact of SnS/CdS Interface Defect Density
The influences of SnS/CdS interface defect density (IDD) on the performance of the proposed solar cell have been analysed. The SnS/CdS defect interface can increase the se ries resistance and carrier trapping of the cell. Due to the increase in carrier recombination rate at the interface, VOC decreases with IDD but IDD has no impact on JSC until 10 17 cm 2 JSC increases from 19.48 to 28.25 mA/cm 2 when the width of SnS layer increases from 200 to 1200 nm. When IDD increases from 10 11 to 10 18 cm 2 , FF decreases rapidly from 84.9% to 54.8% due to an increase in the series resistance of the cell, indicating that high defec density at SnS/CdS is responsible for high series resistance. A significant decrease in PCE was also seen with increasing IDD. An interface defect is one of the reasons for lower PCE Defects in the interfacial layer density are induced in the cell as a result of structura changes in the materials produced during the fabrication process.

Impact of Defect Density and Thickness of Absorber Layer SnS
The influence of absorber layer defect density and thickness is depicted in Figure 3 and Figure 4, respectively. The defect is caused by displacement and surface defects. De fects serve in carrier recombination, reducing mobility and carrier lifetime. The Shockley-Read-Hall (SRH) process modulates the recombination rate of SnS at higher defect den sity. The absorber layer thickness and defect density varied from 100 to 1200 nm and from 10 11 to 10 16 cm −3 , respectively. VOC decreases with an increase in thickness and defect den sity of SnS but there is no effect of defect density on JSC below 10 15 cm −3 due to an increase  in carrier recombination. Due to the high absorption of increased wavelength photons in the layer, JSC increases up to a certain level with an increase in the thickness of SnS. After that, it saturates due to light absorption saturation. PCE is increased with SnS thickness as the JSC increases and saturates at 1200 nm due to the light absorption saturation. The fill factor shows the same nature as VOC. JSC, VOC and FF are all associated with PCE. At defect density of 10 11 cm −3 and thickness of the SnS layer of 1000 nm, the maximum PCE of 26.92% is recorded.  Figure 5 depicts the effect of SnS electron affinity (EA). The maximum performance of the cell was obtained at SnS EA of 4.3 eV. Due to a decrease in FF, PCE falls with EA > 4.3 eV due to a mismatch of energy level between the SnS and CdS layer for electron transport as shown in Figure 5. VOC does not show any significant change with affinity due to insufficient carriers. The effect of back contact metal work function (WF) is shown in Figure 6. As the WF increases, JSC, VOC, FF and PCE also increase up to a certain WF level. This indicates that when WF rises, the majority carrier barrier height reduces. As a result, PCE rises with WF till 5 eV, and after 5 eV, PCE saturates. Thus, WF has a significant influence on the performance of the solar cell. Proper metal contact is necessary to achieve high efficiency. Table 3 shows a comparison of the physical parameters of various The absorber layer thickness and defect density varied from 100 to 1200 nm and from 10 11 to 10 16 cm −3 , respectively. V OC decreases with an increase in thickness and defect density of SnS but there is no effect of defect density on J SC below 10 15 cm −3 due to an increase in carrier recombination. Due to the high absorption of increased wavelength photons in the layer, J SC increases up to a certain level with an increase in the thickness of SnS. After that, it saturates due to light absorption saturation. PCE is increased with SnS thickness as the J SC increases and saturates at 1200 nm due to the light absorption saturation. The fill factor shows the same nature as V OC . J SC , V OC and FF are all associated with PCE. At defect density of 10 11 cm −3 and thickness of the SnS layer of 1000 nm, the maximum PCE of 26.92% is recorded. Figure 5 depicts the effect of SnS electron affinity (EA). The maximum performance of the cell was obtained at SnS EA of 4.3 eV. Due to a decrease in FF, PCE falls with EA > 4.3 eV due to a mismatch of energy level between the SnS and CdS layer for electron transport as shown in Figure 5. V OC does not show any significant change with affinity due to insufficient carriers. The effect of back contact metal work function (WF) is shown in Figure 6. As the WF increases, J SC , V OC , FF and PCE also increase up to a certain WF level. This indicates that when WF rises, the majority carrier barrier height reduces. As a result, PCE rises with WF till 5 eV, and after 5 eV, PCE saturates. Thus, WF has a significant influence on the performance of the solar cell. Proper metal contact is necessary to achieve high efficiency. Table 3 shows a comparison of the physical parameters of various simulated device structures. Our findings seem to have a satisfying consistency with those that have already been published.

Conclusions
The performance of the cell structure ZnO/CdS/SnS/NiO/Au has been investigated using SCAPS 1-D software. It is shown that NiO is a suitable HTL material with the ability to improve the efficiency of SnS-based solar cells. ZnO can be a good replacement for ITO and FTO in the fabrication of low-cost and highly efficient SnS-based solar cells. At ZnO and NiO carrier concentrations of 10 17 and 10 21 cm −3 , respectively, and thicknesses of 100

Conclusions
The performance of the cell structure ZnO/CdS/SnS/NiO/Au has been investigated using SCAPS 1-D software. It is shown that NiO is a suitable HTL material with the ability to improve the efficiency of SnS-based solar cells. ZnO can be a good replacement for ITO and FTO in the fabrication of low-cost and highly efficient SnS-based solar cells. At ZnO and NiO carrier concentrations of 10 17 and 10 21 cm −3 , respectively, and thicknesses of 100 and 250 nm, respectively, the maximum PCE of 26.92% is recorded. The thickness and defect density of the SnS layer were also investigated. As the SnS thickness increases, the PCE of the cell also increases. The maximum PCE of the proposed solar cell is recorded at thickness and defect density of 1000 nm and 10 11 cm −3 . The present work might give an insight into the modelling and development of the high performance of SnS-based solar cells.