Introduction

The Cu2ZnSnS4 quaternary chalcogenide material has demonstrated excellent potential for photovoltaic applications due to its earth-abundant, non-toxic composition. However, its progress has been hindered by band-tailing effects, which destabilize the conduction and valence bands, limiting the efficiency of solar cells1,2. Specifically, antisite defects such as CuSn, SnCu, ZnSn, and SnZn introduce deep-level defect states within the bandgap, leading to potential fluctuations in the electronic band structure3. These defects cause band tailing, increase charge carrier recombination rates, and significantly limit fabricated devices’ open-circuit voltage (VOC)4,5,6. Although CZTS has been widely studied for photovoltaic applications7, its limitations necessitate exploring alternative quaternary materials with superior properties. Various studies have demonstrated the potential of CZTS-based solar cells through innovative structural and material combinations. For example, a CZTS-based device with ZnTe as a buffer layer achieved an impressive power conversion efficiency (PCE) of 23.47% in the Mo/CZTS/ZnTe/ZnO/ZnO: Al structure8. Similarly, integrating CZTS with a MASnI3 perovskite absorber (FTO/TiO2/MASnI3/CZTS/Au) yielded a JSC of 31.66 mA/cm², VOC of 0.96 V, and an efficiency of 20.28%9. However, band-tailing effects and antisite defects in CZTS continue to limit VOC and overall efficiency, as seen in multi-absorber configurations such as CZTS/InSe, which only reached 16.30% efficiency10. These limitations motivate the search for alternative materials like CSTS, which offer improved defect tolerance and reduced band-tailing. To address these challenges, this study investigates Cu2SrSnS4 (CSTS), a novel alternative where Sr substitutes Zn. The larger ionic radius of Sr reduces band-tailing, which offers more stability and electronic properties. Theoretical calculations indicate that Cu2SrSnS4 (CSTS) exhibits a direct bandgap of approximately 1.79 eV11,12, high light absorption efficiency (with absorption coefficients over 10⁴ cm− 1)13, the abundance of non-toxic elements in its composition, and excellent defect tolerance, making it promising alternative for photovoltaic application.

According to previous reports, CSTS may also be a promising candidate for thin-film solar cells and PEC (photoelectrochemical) water-splitting applications, owing to its advantageous defect properties and superior optical characteristics14,15.

To further explore CSTS potential, we evaluated its performance in heterojunction solar cells, incorporating MoS2 and WS2 as buffer layers, compared to CZTS, which has been integrated with various buffer materials like CdS16, InSe17, and ZnTe18, CSTS paired with MoS2 or WS2 demonstrates a significant efficiency advantage. Showcasing its potential as a high-performance alternative considering realistic defect densities, recombination rates, and electron affinity. These transition metal dichalcogenides are known for their superior charge carrier transport properties, which can significantly enhance device efficiency19,20. Devices with two varied buffer layers, MoS2 and WS2, were analyzed. Under optimized conditions, the ZnO/MoS2/Cu2SrSnS4 and ZnO/WS2/Cu2SrSnS4 devices achieved photo conversion efficiencies exceeding 35.6% and 29.1%, respectively. Both devices exhibited comparable short-circuit current densities (JSC), though the MoS2 buffer layer delivered a higher open-circuit voltage (VOC). Overall, this study highlights Cu2SrSnS4 (CSTS) as a promising absorber material for high-efficiency heterojunction solar cells.

The inclusion of these transition metal dichalcogenides is significant, as they can enhance charge carrier transport and improve overall device efficiency. By comparing the results from both architectures, insights can be gained into the optimal design strategies for enhancing the efficiency of Cu2SrSnS4-based solar cells, potentially paving the way for their practical application in renewable energy technologies. This study explores the potential of CSTS as a high-performance absorber material for thin-film solar cells, addressing the challenges associated with CZTS. To achieve this, we analyze the material properties, evaluate the simulation setup using a solar cell capacitance simulator (SCAPS-1D)21,22, and assess the photovoltaic performance of devices using MoS2 and WS2 buffer layers. The findings are discussed to highlight their implications for optimizing CSTS-based solar cells, culminating in key conclusions and suggestions for future research directions.

Device simulation

The two device architectures, ZnO/MoS2/Cu2SrSnS4 and ZnO/WS2/Cu2SrSnS4 were examined using SCAPS-1D software, with all simulations based on the standard AM 1.5G solar spectrum luminosity. Numerical simulations are essential for optimizing solar cell design, allowing researchers to predict the behavior of solar cells under various conditions before physical fabrication. In this study, the focus was on assessing the photovoltaic performance of Cu2SrSnS4-based solar cells, where MoS2 or WS2 were investigated as potential buffer layers.

Key variables such as absorber layer thickness, acceptor density, donor density, and operating temperature were systematically adjusted to determine their impact on the Cu2SrSnS4-based solar cells performance. All simulations were conducted at a temperature of 300 K under a 1.5 G solar spectrum, representing standard operating conditions, with a series resistance of 5 Ω and a shunt resistance of 300 Ω.

Fig. 1
figure 1

Configuration of ZnO/(MoS2 or WS2)/Cu2SrSnS4 solar cell used in the simulations.

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The device structure is illustrated in Fig. 1, which consists of ZnO as the front reflector layer to enhance light transmission and reflection toward the absorber layer, followed by MoS2 or WS2 as buffer layers to reduce recombination losses and improve charge carriers transport, Cu2SrSnS4 as the absorber layer, and Mo is employed as the back contact to enhance charge extraction and the overall performance of the device. Figure 2 shows the energy band alignment and energy band diagram of the CSTS-based solar cell using various buffer layers.

Fig. 2
figure 2

(a) Energy level alignment and (b) band diagrams of CSTS solar cells with MoS2 and WS2 buffer layer configurations.

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Figure 3 shows the absorption coefficient of CZTS observed (> 104 cm− 1), confirming its excellent absorption properties as an absorber material for high-performance solar cells.

Fig. 3
figure 3

Absorption coefficient of CSTS as a function of photon energy, illustrating its optical properties and light absorption capability across various energy ranges23.

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Table 1 provides the specifications for the device layers used in the ZnO/(MoS2 or WS2)/Cu2SrSnS4 simulations.

Table 1 Material parameters used in the numerical simulation.
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Results and discussion

Figure 4 illustrates the photovoltaic characteristics of the simulated structure as a function of the Cu2SrSnS4 layer thickness, ranging from 1 to 3 μm, and for different buffer layers. The results reveal that both the short-circuit current (JSC) and open-circuit voltage (VOC) increase rapidly before stabilizing. In thicker Cu2SrSnS4 thin films, the active layer captures more photons, thereby generating additional electron-hole pairs, as noted in prior studies30. This observation highlights the importance of absorber layer thickness in improving photon absorption31. Figure 4 indicates that the ideal Cu2SrSnS4 absorber thickness aligns with a significant rise in short-circuit current, identified at an optimum thickness of 3 μm. Increasing the active layer thickness from 1 to 3 μm results in a noticeable rise in short-circuit current density, from 29.98 to 33.6 mA/cm2 and 30.67 to 34.1 mA/cm2 for MoS2 and WS2 buffer layers, respectively. Meanwhile, the open-circuit voltage shows a linear improvement from 1.2 to 1.21 V and 1.3 to 1.34 V for WS2 and MoS2. It is observed that, at the optimal thickness, the open-circuit voltage is nearly identical between the two buffers. This indicates that short-circuit current primarily influences the performance of these solar cells. The simultaneous increase in JSC and VOC enhances both efficiency (η) and fill factor (FF). For the optimum thickness, efficiency (and fill factor) reaches 29.1% and 35.6% (71.19% and 79.62%) for WS2 and MoS2 buffers, respectively. The optimized performance parameters (VOC, JSC, FF, and η) for ZnO/MoS2/Cu2SrSnS4 and ZnO/WS2/Cu2SrSnS4 solar cells are based on values reported in the literature32,33.

Fig. 4
figure 4

Effect of Cu2SrSnS4 thickness on the photovoltaic parameters.

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The generation-recombination (G-R) profile for ZnO/MoS2/Cu2SrSnS4 and ZnO/WS2/Cu2SrSnS4 solar cells, as simulated using SCAPS-1D (Fig. 5), reveals critical insights into charge generation and recombination mechanisms within these multi-layer structures. The SCAPS-1D simulations indicate substantial charge generation at the ZnO/MoS2/Cu2SrSnS4 and ZnO/WS2/Cu2SrSnS4 interfaces, which can be attributed to the high absorption coefficients of MoS2 and WS2 in the visible spectrum34. However, the G-R profile also highlights recombination losses at these interfaces, likely due to lattice mismatches or interface defects at the ZnO/MoS2 and ZnO/WS2 junctions. Such recombination hinders charge carrier transport and reduces both VOC and FF.

Bulk recombination within the Cu2SrSnS4 absorber layer further degrades performance, as intrinsic defects act as trap states, reducing carrier lifetimes and contributing to efficiency loss35. Comparatively, the G-R profile suggests that the ZnO/WS2/Cu2SrSnS4 configuration may experience slightly lower recombination rates, potentially due to better energy alignment at the ZnO/WS2 interface, which could enhance charge extraction and transport. Addressing these recombination pathways through passivation-optimized buffer layers can improve photovoltaic efficiency in the designed solar cell. The significance of SCAPS-1D modeling in identifying these mechanisms emphasizes its role in improving material interfaces for better performance36.

Fig. 5
figure 5

G-R profile of ZnO/(MoS2 or WS2)/Cu2SrSnS4 structure.

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Figure 6 illustrates the effect of varying the doping levels of the Cu2SrSnS4 absorber layer on key photovoltaic parameters. The absorber layer acceptor density values range from 1014 to 1016 cm− 3. Notably, an increase in the doping concentration correlates with a decline in these parameters. This trend indicates that higher levels of doping can adversely affect the efficiency and overall performance of the solar cells. Furthermore, the analysis reveals that solar cells incorporating a MoS2 buffer layer are more sensitive to increased acceptor concentration compared to those utilizing a WS2 buffer layer. These findings align with previous research documented in the scientific literature37, highlighting the importance of buffer layer material in optimizing the performance of doped solar cells. The carrier concentration in the Cu2SrSnS4 absorber layer is influenced by acceptor impurities, as well as any deviations from stoichiometry or defects in the synthesized material38.

Fig. 6
figure 6

Cu2SrSnS4 acceptor density impaction the photovoltaic parameters.

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The variation of the solar cell efficiency as a function of thickness and donor density of the buffer layer is shown as two-dimensional (2D) contour plots in Fig. 7. Both the performance of Cu2SrSnS4-based solar cells and their dependence on the thickness and donor density of MoS2 and WS2 are evident. We conclude that the conversion efficiency of the solar cell with the MoS2 buffer layer is higher than that with the WS2 buffer layer. Additionally, photovoltaic efficiency remains low at low donor densities, regardless of the buffer layer thickness. This is attributed to the reduced contribution of photo-generated carriers in the n-type quasi-neutral regions, as most electron-hole pairs recombine in this zone before they can reach the depletion region39.

Fig. 7
figure 7

Contour plots of efficiency dependence of thickness and donor density of buffer layer.

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The effect of temperature on Cu2SrSnS4 based solar cells with different buffer layers was analyzed by changing the temperature from 290 K to 320 K. The results of this investigation are illustrated in Fig. 8.

As temperature increases, the reduction in bond energy, driven by the higher velocity of charge carriers, leads to a narrowing of the semiconductor band gap. This change results in a higher recombination rate of electrons and holes, causing a drop in open-circuit voltage (VOC)40. The reverse saturation current also rises with temperature, further reducing both VOC and the fill factor (FF)41. The combined decrease in short-circuit current density (JSC), fill factor (FF), and open-circuit voltage (VOC) ultimately lowers the overall efficiency. Consequently, the efficiency of Cu2SrSnS4 based solar cells with MoS2 and WS2 buffer layer decreases as the temperature rises.

Fig. 8
figure 8

Variation of temperature as a function of the photovoltaic parameters.

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Figure 9 presents the simulated J-V curve of the designed solar cell with the configuration ZnO/(MoS2 or WS2)/Cu2SrSnS4. The simulation was conducted using an energy band gap of 1.79 eV, an acceptor density of 1016 cm− 3 for Cu2SrSnS4, and Cu2SrSnS4 layer thickness of 3 μm, all at an operating temperature of 300 K. The results demonstrate that the ZnO/MoS2/Cu2SrSnS4 and ZnO/WS2/Cu2SrSnS4 devices achieved impressive power conversion efficiencies of 35.6% and 29.1%, respectively. These efficiency levels highlight the potential of these configurations for high-performance solar cell applications, making them promising candidates for future research and development in renewable energy technologies42.

Fig. 9
figure 9

J-V curve of the designed structure.

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MoS2 outperforms WS2 as a buffer layer in optoelectronic devices due to its bandgap, higher carrier mobility, and smoother, less defective films. These enhance charge transfer, reduce recombination losses, and improve interface quality. MoS2’s superior mechanical and thermal stability further solidifies its preference for photovoltaic applications43,44.

Table 2 presents a comprehensive summary of the optimal results attained in this investigation, alongside key performance parameters of solar cells from numerical and experimental studies available in the literature for comparison. This table shows that the results we obtained were comparable to the literature’s experimental values. By comparing parameters such as short-circuit current density (JSC), open-circuit voltage (VOC), and overall efficiency (η), it becomes evident that the performance metrics from our study are comparable to or even exceed certain experimental benchmarks. This comparison reinforces the efficacy of SCAPS-1D as a powerful tool for forecasting solar cell performance under realistic conditions, making it valuable for both predictive modeling and the design of next-generation photovoltaic devices. Furthermore, these results provide valuable insights into optimizing material properties and device architectures, facilitating improvements in solar cell technologies45.

Table 2 Comparative analysis of the proposed Cu2SrSnS4 based solar cells against previously reported cells.
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Fig. 10
figure 10

Quantum Efficiency of ZnO/(MoS2 or WS2)/Cu2SrSnS4 solar cells.

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The graph in Fig. 10 illustrates the variation in quantum efficiency for MoS2 and WS2 buffer layers-based structures over a range of wavelengths from 300 nm to 1250 nm. MoS2 exhibits a higher quantum efficiency than WS2 across most of the wavelength spectrum, particularly in the shorter wavelength range. Both materials show an increasing quantum efficiency with longer wavelengths, with MoS2 maintaining superior performance throughout48. This data highlights the potential of MoS2 as a more efficient buffer layer compared to WS2 in photovoltaic applications.

Conclusions

This study highlights the significant potential of Cu2SrSnS4 (CSTS) as an effective absorber material for high-efficiency solar cells, especially when combined with transition metal dichalcogenide (TMD) buffer layers like MoS2 and WS2. SCAPS-1D simulations demonstrate that the MoS2 buffer layer surpasses WS2, attaining a remarkable power conversion efficiency (PCE) of 35.6%, in contrast to 29.1% for WS2-based devices. The improved performance of MoS2 is due to the adjustment of numerous critical factors, such as absorber layer thickness and doping concentration, which greatly affect efficiency. The study indicates that augmenting the thickness of the CSTS absorber layer enhances light absorption and overall device efficiency, however high doping concentrations result in efficiency decline due to increased recombination rates. Additionally, the research of temperature dependence reveals a decline in performance at higher temperatures, underscoring the necessity of integrating temperature-stabilizing mechanisms into forthcoming device designs. The results indicate that CSTS, due to its advantageous defect tolerance and stability, serves as a feasible alternative to traditional materials like CZTS, addressing issues such as open-circuit voltage constraints. The integration of CSTS with MoS2 as a buffer layer presents a viable approach for the development of high-efficiency, stable solar cells, with significant implications for the advancement of photovoltaic technology.