Numerical Study and Optimization of CZTS-Based Thin-Film Solar Cell Structure with Different Novel Buffer-Layer Materials Using SCAPS-1D Software

Md. Zamil Sultan^*, Arman Shahriar, Rony Tota, Md. Nuralam Howlader, Hasibul Haque Rodro, Mahfuja Jannat Akhy, Md. Abir Al Rashik

Department of Electrical and Electronic Engineering, Hajee Mohammad Danesh Science and Technology University, Dinajpur, Bangladesh.

DOI: 10.4236/epe.2024.164009                

Abstract

This study explored the performances of CZTS-based thin-film solar cell with three novel buffer layer materials ZnS, CdS, and CdZnS, as well as with variation in thickness of buffer and absorber-layer, doping concentrations of absorber-layer material and operating temperature. Our aims focused to identify the most optimal thin-film solar cell structure that offers high efficiency and lower toxicity which are desirable for sustainable and eco-friendly energy sources globally. SCAPS-1D, widely used software for modeling and simulating solar cells, has been used and solar cell fundamental performance parameters such as open-circuited voltage (), short-circuited current density (), fill-factor() and efficiency() have been optimized in this study. Based on our simulation results, it was found that CZTS solar cell with Cd_0.4Zn_0.6S as buffer-layer offers the most optimal combination of high efficiency and lower toxicity in comparison to other structure investigated in our study. Although the efficiency of Cd_0.4Zn_0.6S, ZnS and CdS are comparable, Cd_0.4Zn_0.6S is preferable to use as buffer-layer for its non-toxic property. In addition, evaluation of performance as a function of buffer-layer thickness for Cd_0.4Zn_0.6S, ZnS and CdS showed that optimum buffer-layer thickness for Cd_0.4Zn_0.6S was in the range from 50 to 150nm while ZnS offered only 50 – 75 nm. Furthermore, the temperature dependence performance parameters evaluation revealed that it is better to operate solar cell at temperature 290K for stable operation with optimum performances. This study would provide valuable insights into design and optimization of nanotechnology-based solar energy technology for minimizing global energy crisis and developing eco-friendly energy sources sustainable and simultaneously.

Keywords

Thin-Film Solar Cell, CZTS, Buffer-Layer, Renewable Energy, Green-House Gases, Efficiency

1. Introduction

Recent year, the greenhouse gases (GHGs) such as carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O) etc. upsurge uninterruptedly in the atmosphere by energy burning and industrial revolution, which is a global warning to everyone in terms of climate change [1] . On the other hand, the reserve of fossil fuels, such as coal, natural gas and oil are limited but the global energy demand increases drastically, therefore continuous fulfilling the global energy demands is a serious issue of concern. Alternately, it is great demand to upsurge potential usage of renewable energy sources for keeping green-clean world as well as for satisfying global high energy requirement simultaneously and sustainably [2] [3] [4] . Renewable solar energy could a vibrant source in this circumstance for energy production by using photovoltaic technology [5] [6] , such as solar cell as it is environmentally eco-friendly as well as has enormous globally. However, the relative higher cost [7] , flexibility and efficiency limitations of solar cells till remain the biggest problems compared to conventional systems. The problems are expected to be overcome as the technology progresses. For single junction photovoltaic cells, the maximum efficiency is limited to be 33.7% according to the Shockley-Queisser (S-Q) calculations [8] . An approach of using multi-junction solar cells has been considered to overcome the limitation of single junction solar cell [9] , however the structure suffers structural complexity. A more humble solution is a thin-film solar cell in terms of cost/watt ratio [10] , light weight [11] and flexible fabrication process [12] . Since the fundamental performance parameters of thin-film photovoltaic cells, such as open circuit voltage ( $V_{oc}$ ), short circuit current density ( $J_{sc}$ ), fill-factor ( $FF$ ) and efficiency ( $\eta$ )can be precisely controlled by fabrication materials as well as structures, there is a major scope of research on this device for obtaining better performances by optimizing thin-film structure, structural parameters and their fabrication materials. Therefore, the aims of this study have focused to improve device performance through evaluating absorber layer thickness, buffer layer thickness, buffer layer materials and working temperature. To obtain this, CZTS based thin-film solar cell structure has been studied with three novel buffer layer materials Cd_0.4Zn_0.6S, ZnS and CdS.

Over the recent years, the technologies and significant efficiency improvement of CuInGaSe₂ (CIGS) thin-film solar cells have been demonstrated in several reports [13] [14] . However, the scarcity and high prices of In, Ga, and the toxicity of In, Cd, and Se eventually could limit the production growth of this type of solar cells. Cu₂ZnSnS₄ (CZTS) has been considered as a potential alternative compound which contains more abundant elements preferable for the realization of low-cost solar cells. Nevertheless, CZTS is a direct band gap material with high absorption coefficient [15] in order of 10⁴ cm⁻¹ and contains less toxic material S which is desirable for environment-friendly high performances optoelectronics applications. Several researchers have given a lot of successful work on this CZTS till now [16] [17] [18] [19] .

Though CZTS has emerged as a viable solar cell technology, buffer layer tuning remains a challenge. The buffer layer aligns the bands between the CZTS and the window layer while also reducing flaws and interfacial strain caused by the window layer [20] . Among different buffer layer materials, cadmium sulfide (CdS) [21] [22] , zinc sulfide (ZnS) [23] [24] and ternary alloy cadmium zinc sulfide (CdZnS) [25] [26] [27] have been attracted more attention to the researchers because of their unique optical and electrical properties suitable for thin-film solar cell structure. CdS has a bandgap of 2.42 eV so that absorbs photons with wavelengths less than 590 nm, therefore covering 24% of the solar spectrum. An environment-friendly material zinc sulfide (ZnS) can also be used as an alternative buffer layer [23] . ZnS has a higher bandgap of 3.5 eV compared to CdS which results in less absorption of low-wavelength photons. It also produces a better interface with CZTS creating a potential barrier to separate the electron-hole pair. The third buffer materials used in this study, Cd_0.4Zn_0.6S [28] is a ternary compound semiconductor alloy in which band gap energy is 2.98 eV which is suitable for maximum solar energy conversion.

Although, CdS, ZnS and CdZnS have promising performance as buffer layer materials for the thin-film solar cells; as far we know no research work has been reported previously on the comparison among these three novel buffer layer materials for the CZTS-based solar cells. For the first time, this research work explored the best buffer material among these three novel CdS, ZnS and CdZnS materials while CZTS being used as absorber layer. Then, the device structural parameters have also been suggested for the high performance photo-voltaic conversion.

Thus, the main objective of this work is to offer a CZTS solar cell with high efficiency by optimizing devices structure with a suitable buffer layer material. To obtain this, CZTS solar cell device models were numerically simulated and analyzed using SCAPS-1D for various thickness of buffer layer and absorber layer, doping concentrations of absorber layer material and operating temperature as well as for three different buffer layers such as CdS, ZnS, and Cd_{0 4}Zn_{0 6}S. Four fundamental solar cell performance parameters, $V_{oc}$ , $J_{sc}$ , $FF$ and $\eta$ were observed and optimized.

The paper consists of four sections. Section 1 starts the paper with brief introduction of the research paper. In section 2 of this paper, methods and methodology such as proposed structure of thin-films solar cell and simulation method will be discussed. Section 3 represents analysis and optimization of $V_{oc}$ , $J_{sc}$ , $FF$ and $\eta$ for various thickness of absorber layer and buffer layer, doping concentration of absorber layer as well as operating temperature for three buffer layer materials CdS, ZnS, and Cd_{0 4}Zn_{0 6}S. Section 4 will summarize the results with brief conclusion.

2. Methodology

The schematic diagram of the proposed thin-film solar cell structure, n-ITO/i-ZnO/n-CdS or ZnS or Cd₀.₄Zn₀.₆S/p-CZTS/p-MoSe₂ has been illustrated in Figure 1. In this study, we consider five layered structure. The topmost layer n-type indium tin oxide (ITO) is an electron transport layer (ETL). Intrinsic ZnO is used for carrier multiplication. The n-type buffer layer and p-type CZTS absorber layer formed a pn-junction and a depletion region has been formed at that junction. Finally, the bottom most highly doped p-type MoSe₂ is used as hole transport layer (HTL). The solar cell is illuminated under 100 mW/cm² with global air mass AM1.5 G solar spectrum at operating temperature 300 K. We have considered ideal conditions for the series (R_s) and shunt (R_sh) resistances for the solar cell structure. The simulation of the proposed structure and investigation of the performance parameters have been done by one of the very popular and more reliable computer simulation tools titled, “Solar Cell Capacitance Simulator’s one-dimensional simulation software (SCAPS-1D)” invented by Burgelman et al at the Department of Electronics and Information Systems, University of Gent, Belgium [29] . The SCAPS-1D provides the opportunity for solar cell researchers to analyze the device structure effectively [30] [31] . It is a very useful tool used to perform electrical characterizations and spectral responses of solar cells. The parameters of thin film solar cells applied to compute our numerical simulations are listed in Table 1 according to the previous studied research works.

Figure 1. Schematic diagram of proposed thin-film solar cell structure, n-ITO/i-ZnO/n-CdS or ZnS or Cd₀.₄Zn₀.₆S/p-CZTS/p-MoSe₂.

Table 1. Numerical values of different parameters used in this study collected from previous reports [24] [28] [32] .

3. Results and Discussions

The proposed thin-film solar cell structure has been simulated by SCAPS-1D software. The schematic energy band diagram of this proposed structure with possible optical transitions have been demonstrated in Figure 2. Three novel buffer layer materials with different optical and electrical properties have been considered in this study. During illumination, the incident photons having energy larger than band gap energy of buffer layer material are possibly absorbed within the layer, while medium and lower energy or longer wavelength photons are mostly absorbed within the depletion region and within the p-CZTS absorber layer, respectively. For example, the photon having energy larger than 2.4 eV can be absorbed in the n-CdS based buffer layer for carriers generation. Intrinsic-ZnO acts as depletion region of charge carrier that has been used for the additional charge carriers generation by the solar energy.

Through these ways, incident solar energy generates electron-hole pairs in solar cell. The electron drifts towards the n- side buffer layer and makes the region negative. Similarly, hole drifts towards p side absorber layer and thereby makes this side positive. Since the electrons have a higher mobility and lifetime than holes, so the diffusion length of electron is kept larger than that of hole. This phenomenon has been assured in the structure by taking the thickness of absorber larger than that of buffer layer. This confirms the greater possibility of carrier collection across the load terminals. Thus, the effects of thicknesses of absorber layer as well as buffer layer on the performance parameters have been evaluated for the proposed structure. The uppermost n-ITO based ETL layer in this structure creates a potential barrier ( $\Delta E_{v1}$ ) to block hole flow while permits electron to be transported towards the front electrode. Similarly, the bottom

Figure 2. Schematic energy band diagram of the proposed thin-film solar cell structure, n-ITO/i-ZnO/n-buffer (CdS or ZnS or Cd₀.₄Zn₀.₆S)/p-CZTS/p-MoSe₂ with possible optical transitions.

most p-MoS₂ based HTL layer creates a potential barrier ( $\Delta E_{C2}$ ) to block electron flow while permits holes to be transported towards back electrode. Thus, the maximum number of electron-hole pair generation may possibly occur by the solar irradiation in the proposed thin-film solar cell structure. In addition, the losses of photo-generated electrons and holes are minimized by using appropriate HTL and ETL layers, respectively in the proposed structure.

3.1. Optimization of V_oc, J_sc, FF and η for the Various Thickness of CZTS Absorber Layer of Thin-Film Solar Cell

The thickness of the absorber layer is one of the most important parameters for increasing the performance of solar cells [33] . In this section, we firstly study the effect of absorber layer thickness of the proposed structure for achieving optimized $V_{oc}$ , $J_{sc}$ , $FF$ and $\eta$ . To demonstrate this, the performance characteristics of the thin film solar cell have been visualized as a function of thickness of absorber layer CZTS with three buffer layer materials as shown in Figure 3. For each simulation, the thickness of CZTS was varied from 100 to 2000 nm with a fixed buffer layer thickness of 50 nm at a temperature of 300 K. A single donor-type defect with a defect density of 1 × 10¹³cm⁻³ was introduced in the CZTS layer. By investigating the curves of $J_{sc}$ , $V_{oc}$ , $FF$ and $\eta$ as a function thickness of absorber layer as shown in Figure 3(a)-(d) respectively, it has been found that all performance parameters except $V_{oc}$ showed similar changing tendency with changing absorber layer thickness in our study. The open circuit voltage shows a different approach.

As can be seen from the Figure 3(b), Figure 3(d) and Figure 3(a), performance parameters $J_{sc}$ , $FF$ and $\eta$ upsurges linearly with rising thickness of CZTS absorber layer in the lower range from nearly 100 to 900 nm while $V_{oc}$ is found to be saturated shown in Figure 3(c). The results can be clarified by the facts that as the thickness of absorber layer increases, the photons absorption of incident light in this region becomes more which leads to excess electron-hole

Figure 3. Dependency of V_oc, J_sc, FF and η parameters on the thickness of CZTS absorber layer of thin-film solar cell.

pair generation, and consequently promotes the short circuit current density in Figure 3(b). The behavior of the $FF$ is likely similar to that of $V_{oc}$ and $J_{sc}$ [34] while that of the efficiency can be illustrated by the behavior of $V_{oc}$ , $J_{sc}$ and $FF$ [35] . Owing to the high-absorption coefficient of CZTS [15] , a few hundred nanometer film is capable of absorbing enough sunlight. Therefore, the increment in $J_{sc}$ , $FF$ and $\eta$ is found more significant at the thickness increment in the lower range from 100 nm to 900 nm of the absorber layer. In addition to the improvement of light absorption, the chance of carrier recombination also increases simultaneously with increasing thickness of absorber layer because the charge carriers have to travel longer distances for diffusion in a thicker absorber layer. Therefore, the rise in thickness above 900 nm leads the nearly saturated photo current density, fill-factor and efficiency. On the other hand, the open circuit voltage remained approximately constant since the increasing thickness of absorber layer results in the increment of carrier diffusion length, the probability of recombination rate of photo-generated carrier and life-time of photo-generated carrier have been affected, the amount of carrier collection at the electrodes decreases and consequently saturation in $V_{oc}$ with increasing thickness. They all increase up to 900 nm and then possesses a slow increase in the efficiency up to almost 1800 nm and then shows a stiff straight line up to 2000 nm with really less comparable efficiency for all three materials.

In addition to the results, the numerical values of $V_{oc}$ , $J_{sc}$ , $FF$ and $\eta$ are comparable for the three buffer layer materials. For examples, the best effi­ciencies for ZnS/CZTS, CdS/CZTS, and Cd₀.₄Zn₀₆S/CZTS structures were found as 26.71% at 0.9 µm, 26.72% at 1.3 µm, and 27.00% at 1.7 µm, respectively as can be seen from Figure 3(a). As can be seen from Table 1, the highest electron mobility of Cd₀.₄Zn₀₆S among the three materials would result the highest photo-voltaic conversion efficiency of the solar cell. For CdS, ZnS, and Cd₀.₄Zn₀₆S buffer layers, the upper limit of the CZTS absorber layer thickness was discovered at 900 nm, 1300 nm, and 1700 nm, respectively. With the Cd₀₄Zn_{0 6}S buffer layer, the greatest efficiency (27.0%) was discovered for the 1700 nm thick CZTS layer. Therefore, we have chosen the optimal thickness of CZTS absorber layer as around 2000 nm for further simulation because of optimized values of performances parameters.

3.2. Optimization of V_oc, J_sc, FF and η for the Various Thickness of Buffer Layer of Thin-Film Solar Cell

In this section, the buffer layer thickness was varied from 50 to 150 nm with a fixed absorber layer thickness of 2000 nm for simulation. A single acceptor-type defect with defect density of 6 × 10¹⁶ cm⁻³ was introduced in the buffer layer. Figure 4 shows the variation of solar cell parameters as a function of buffer layer thickness.

It was noticeably found that all fundamentals parameters of solar cell remain same in their respective scales and independent of thickness of buffer layer in the relatively lower range for the three buffer layer materials. It is observed from Figure 4(c) that no significant variation on $V_{oc}$ was found with increasing thickness of buffer layer for all buffer layer materials used in this study. This result has a good agreement with the previous reported result [36] [37] . As can be seen from Figure 4(b), the short circuit density shows slightly degradation tendency with increasing of buffer layer thickness throughout the whole range for CdS and Cd₀₄Zn₀₆S. It finally becomes lowest at 150 nm. However, the parameter $J_{sc}$ remains nearly independent of buffer layer thickness in case of ZnS material. Thus, although $J_{sc}$ for three buffer layer materials possess approximately

Figure 4. η, J_sc, V_oc and FF of thin-film solar cell as a function of thickness of buffer layer.

same value at lower thickness of buffer layer, it is possible to obtain higher short circuit density even at higher thickness of buffer layer, such as 150 nm by using ZnS instead of CdS and Cd₀₄Zn₀₆S as buffer layer material. These obtained results can be explained by the facts that as the thickness of buffer layer increases, the diffusion length of carriers exceeds, this may lead to the inefficient photo-generated carrier’s separation and collection, thereby may cause reduction in short circuit density [38] . Since the donor density of ZnS is much higher and uniformly distributed than that of CdS and Cd₀₄Zn₀₆S as seen from Table 1, the reduction in $J_{sc}$ was occurred less or negligible in ZnS than either CdS or Cd₀₄Zn₀₆S at higher thickness. The fill factor for CdS and Cd_0.4Zn_0.6S do not show any change with increasing buffer layer thickness. It has also been investigated that although $FF$ and $\eta$ seen from Figure 4(d) and Figure 4(a) respectively remained unchanged with increasing buffer layer thickness in the lower range from 50 nm to about 90 nm for ZnS buffer layer like CdS and Cd₀₄Zn₀₆S, after 90 nm thickness of buffer layer, both parameters of the solar cell for ZnS based buffer layer start decreasing dramatically. The decreasing behavior in $FF$ for ZnS has been seen from around 65 nm which becomes more significant at 150 nm while $\eta$ falls to almost 15% at 150 nm thickness, which is dissimilar to CdS and Cd₀.₄Zn₀₆S. Since CB effective density of states of ZnS is lower than that of CdS and Cd₀₄Zn₀₆S, thereby carrier concentrations and energy distributions of carriers are relatively lower. In addition, as the increasing buffer layer thickness results in longer diffusion length, higher series resistance and consequently increases the probability of carrier recombination rate. By these ways, the carriers separation becomes limited more in ZnS based buffer layer due to those facts at higher thickness of buffer layer [39] and thereby reduction in $FF$ and finally fall in $\eta$ as well. The obtained optimum efficiencies for CdS, ZnS, and Cd₀₄Zn₀₆S buffers are 27.21%, 27.11%, and 27.11%, respectively which is shown in Figure 4(a). Since band gap energy of ZnS is the highest than either CdS or Cd_0.4Zn_0.6S as shown in Table 1, free electron is only generated by solar radiation having photon energy equal or greater than 3.5 eV. On the other hand, photon energy lower than 3.5 eV can also contribute to generate free electron in either CdS or Cd_0.4Zn_0.6S-based buffer layer since band gap energy of either CdS or Cd_0.4Zn_0.6S is lower than 3.5 eV. This is one of the possible reasons for lowering the performance efficiency of ZnS based solar cell. From this study, we clarify that either CdS or Cd_0.4Zn_0.6S is suitable as a buffer layer materials for any thickness of that layer in the range of 50-150 nm while the range is limited to 50-75 nm for ZnS.

3.3. Optimization of V_oc, J_sc, FF and η for the Various Doping Concentration of Absorber Layer Material of Thin-Film Solar Cell

The doping concentration of the absorber layer plays a significant role on the performance of solar cells. In this section, the doping concentration of the absorber layer material of the proposed structure has been evaluated for achieving optimized $V_{oc}$ , $J_{sc}$ , $FF$ and $\eta$ for three buffer layer materials. To demonstrate this, the performance characteristics have been visualized as a function of doping concentration ranging from 1.00 E + 14 cm⁻¹ to 1.00 E + 18 cm⁻¹ of the absorber layer material CZTS with three buffer layer materials as shown in Figure 5. It is clearly found that all fundamentals parameters of solar cell remain almost constant with increasing acceptor doping concentration up to 1.00 E + 18 cm⁻¹ for the two buffer layer materials ZnS and CdS. Solar cell with Cd₀₄Zn₀₆S based buffer layer showed distinct behaviors in $J_{sc}$ , $FF$ $\eta$ in the lower range of doping concentration of absorber layer which made it apart from ZnS and CdS. ZnS and CdS show almost stable $J_{sc}$ values while $J_{sc}$ of Cd_0.4Zn_0.6S tends to decrease as the doping concentration is increased from 1.0 E + 14 cm⁻¹ to 3.33 E + 17 cm⁻¹ as shown in Figure 5(b). After 3.33 E + 17 cm⁻¹, $J_{sc}$ of Cd_0.4Zn_0.6S becomes saturated. Since the electron mobility of Cd_0.4Zn_0.6S is relatively higher than that of ZnS and CdS, it is apparent that the reduction in the width of depletion layer becomes sufficient while doping concentration increases in absorber layer with fixed thickness, thus much more photo-generated carriers rapidly recombine with holes, there is a weaken the carrier collection and finally reduction tendency in $J_{sc}$ [40] . As the acceptor carrier concentration increases, open circuit voltage $V_{oc}$ for all the buffer layer materials tend to maintain an almost stable voltage of 1.10 V which can be seen from Figure 5(c). This can be explained by the facts that with increasing doping concentration at a constant thickness of absorber layer, the carrier collection probability becomes immediately saturated for the fixed collection length, thus additional charge carriers vanish by the recombination. This explanation has good agreement with previous report [41] .

Figure 5. V_oc, J_sc, FF and η of thin-film solar cell as a function of doping concentration of the absorber layer material, CZTS with three buffer layer materials.

As can be seen from Figure 5(d), the $FF$ of Cd_0.4Zn_0.6S based solar cell increases linearly from very low value with increasing doping concentration from 1.00 E + 14 cm⁻¹ to 3.33 E + 17 cm⁻¹, then stays a constant value till the last. However, ZnS and Cds shows almost constant of $FF$ around 87% throughout the variation in doping concentration shown in Figure 5(d). The behavior of the $\eta$ could be described by the whole behavior of the $V_{oc}$ , $J_{sc}$ and $FF$ [35] . Similar shape like $FF$ was found in $\eta$ as shown in Fig. 5(a) for all buffer layer materials. ZnS and Cds shows almost constant of $\eta$ around 27% throughout the variation in doping concentration while Cd_0.4Zn_0.6S shows a uprising state in $\eta$ from 1.00 E + 14 cm⁻¹ but after 3.33 E + 17 cm⁻¹, it becomes a stable value of about 25% from up to 1.00 E + 18 cm⁻¹.

3.4. Evaluation of Temperature Effects on V_oc, J_sc, FF and η of Thin-Film Solar Cell

Operating temperature has a vital impact on the solar cells performances [42] . All of the simulations in this study were run at 300 K, which is the national average for ambient temperature. In this section, the temperature was raised from 290 K to 350 K to account for how the temperature affects $V_{oc}$ , $J_{sc}$ , $FF$ and $\eta$ of CZTS based thin-film solar cells for three buffer layer materials as demonstrated in Figure 6. It should be noted that the specified range of temperature considered in this section is consistence with the range of temperature subsists from winter to summer seasons in Bangladesh. It can be seen from the Fig. 6 that all parameters except short circuit current density have been strongly affected by the change in temperature in our study. It is seen from Figure 6(b) that the value of $J_{sc}$ is found nearly temperature independent but strongly dependent and shows three

Figure 6. Temperature effects on V_oc, J_sc, FF and η of CZTS based thin-film solar cell for three buffer layer materials CdS, ZnS and Cd_0.4Zn_0.6S.

different straight lines for three different buffer layers. The value of $J_{sc}$ can be restricted by the device ohmic losses such as series and shunt resistances, metal contact and recombination losses. The constant current density with increasing temperature apparently indicates that the combined effects of these mentioned parameters might mitigate the variation in $J_{sc}$ in our simulation. The $J_{sc}$ of Cd_0.4Zn_0.6S is the highest, CdS shows the lowest. ZnS being in the middle of them.

It has been noticed that $V_{oc}$ , $FF$ and $\eta$ in Figure 6(c), Figure 6(d) and Figure 6(a) respectively showed degradation tendency with increasing temperature from 290 K to 350 K for three novel buffer layer materials. The reasons of such degradation can be explained by the facts that temperature affects solar cell performance negatively through touching several parameters of materials simultaneously. Increasing temperature shifts the band gap energy of the semiconductors to lower energy [43] and also simultaneously increases the velocity-instability of charged particles [44] , reverse saturation current and resistivity of the materials as well. As a result, the probability of recombination rate of charge carriers before reaching the depletion region promotes and eventually degradation of $V_{oc}$ occurs as seen from Figure 6(c). Furthermore, the behavior of $FF$ is dependent on the combined behavior of $V_{oc}$ and $J_{sc}$ [45] [46] . Constant $J_{sc}$ while decrement in $V_{oc}$ jointly led to a decrease in $FF$ of the device for all buffer layer materials. It is found from Figure 6(d) that the $FF$ of ZnS and CdS shows decreasing in parallel line, while CdS showing the higher between the two. Cd₀₄Zn₀₆S shows little different approach than the other two materials during temperature uprising in the lower range. It slightly increases first from 290 K to 310 K, then starts to decrease gradually like to either ZnS or CdS. The value of $\eta$ reflects the total behavior of $V_{oc}$ , $J_{sc}$ and $FF$ . Constant $J_{sc}$ and decrement in both $V_{oc}$ and $FF$ showed the way to a decrease in the $\eta$ of the device as shown in Figure 6(a). From the simulation, the best efficiencies for CdS/CZTS, ZnS/CZTS, and Cd_0.4Zn_0.6S/CZTS were found as 27.33%, 27.78%, and 27.54%, respectively, at a temperature of 290 K. In addition, it was also noticeably that Cd₀₄Zn₀₆S offers the most efficient short circuit current density while least $FF$ in this section.

3.5. Comparison among Various Photovoltaic Parameters of the Proposed Model with the Related Thin-Film Solar Cell Models Reported Previously

Finally, we have tried to make comparison between our proposed model and the related models of the other researchers reported previously in terms of performance parameters as shown in Table 2.

Our proposed model of thin-film solar cell supports comparatively higher open circuit voltage, short-circuited current density, fill-factor and efficiency successfully with lower toxicity.

Table 2. Comparison among photovoltaic parameters of the solar cell based on CZTS absorber layer.

4. Conclusion

The ecofriendly CZTS based thin-film solar cell structure with three novel buffer layer materials have been studied and simulated by using Solar Cell Capacitance Simulator in one dimensional (SCAPS-1D) software program. The effects of thickness of absorber layer and buffer layer, doping concentration of absorber layer material as well as operating temperature on the solar cell performance have been considered in this study. The four fundamental performance parameters $V_{oc}$ , $J_{sc}$ , $FF$ and $\eta$ of this structure have been investigated and optimized. The investigation of performance parameters for various thicknesses of CZTS absorber layer has clarified that the thickness around 2000 nm is preferable for achieving better efficiency from the proposed solar cell. Although the efficiency of Cd_0.4Zn_0.6S, ZnS and CdS are nearly comparable, Cd_0.4Zn_0.6S is preferable to use as buffer layer for its non-toxic property. In addition, the evaluation of performance as a function of buffer layer thickness for Cd_0.4Zn_0.6S, ZnS and CdS exposed that the optimum buffer layer thickness for Cd_0.4Zn_0.6S was in the range from 50 to 150 nm while ZnS offered only 50–75 nm. Furthermore, it has been explored by evaluating the dependence of $V_{oc}$ , $J_{sc}$ , $FF$ and $\eta$ on doping concentration of absorber layer materials with three buffer materials that dramatic behavior was found in all performance parameter for the Cd_0.4Zn_0.6S based buffer layer with doping concentration of absorber layer. Finally, the studied of the efficiency and other performance parameters on temperature ranging from 290 K to 350 K have been exposed that 290 K is the suitable working temperature of solar cell for stable operation. This research finding would be very helpful for developing nanotechnology based solar energy technology for reducing global energy crisis as well as green-house gas emission simultaneously and sustainably in the world.

Acknowledgement

We are grateful to the Department of Electrical and Electronic Engineering, Hajee Mohammad Danesh Science and Technology University, Dinajpur-5200, Bangladesh for providing necessary technical supports.

Funding

The research works have been conducted by self-funded.

Data Availability

The data of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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