1. Introduction
The rise of the population, urbanization, technological progresses, and the evolving lifestyles are all prophesied to cause a colossal surge in global energy demand in the foreseeable future. Accordingly, it is expected that the energy demand will reach to 30 terawatts (TW) by 2050 [1] . Currently, a large portion of energy demands are satisfied by the utilization of fossil fuels; nevertheless, this limited resource is approaching a state of exhaustion. Furthermore, the widespread use of fossil fuels is resulting in adverse consequences for the environment, hence necessitating a concerted effort to prioritize the advancement of sustainable and renewable energy alternatives [2] . To mitigate climate change and foster a more environmentally sustainable future for next generations, renewable energy sources including solar, wind, hydropower, biomass, and geothermal needs to be exploited in place of fossil fuels [3] . Solar energy has emerged as a highly promising alternative in the ongoing efforts to mitigate greenhouse gas emissions and address the challenges posed by climate change. It is a plentiful and sustainable form of energy that is harnessed by utilizing SCs to directly convert sunlight into electrical energy [4] [5] .
Thin film solar cells have garnered significant attention in the field of photovoltaic technology due to their cost-effectiveness, ease of manufacture and greater efficiency [6] . Amorphous silicon (a-Si), cadmium telluride (CdTe), and copper indium gallium selenide (CIGS) are the three most popular TFSC technologies, with PCE of 9.13%, 20.77%, and 23.03%, respectively [7] . Presently, there is a significant research focus on transition metal dichalcogenide (TMD) materials as promising semiconductors for TFSC. This interest stems from their exceptional energy gap, satisfactory carrier transport properties, and captivating optical absorption characteristics [8] [9] . TMD compounds, specifically MoS2 have garnered significant attention due to its remarkable light absorption coefficient surpassing 105 cm−1, ideal band gap of 1.29 eV, relatively high carrier mobility, eco-friendliness, cost-effectiveness, and abundance in the Earth’s crust [10] . The MoS2-based SC (ITO/MoS2/Au) exhibited experimental PCE of merely 0.7% with a 110 nm absorber layer thickness and 1.8% with a 220 nm absorber layer thickness [11] . With a PCE of 19.62%, the SC configuration comprising ZnO/CdS/MoS2 exhibited exceptional performance, thus highlighting the suitability of MoS2 as a highly effective absorber layer for SCs [12] . The optimized ITO/ZnSe/MoS2 SC exhibited a theoretical maximum efficiency of 19.48%, while the addition of SnS as HTL to the structure (ITO/ZnSe/MoS2/SnS), increased the theoretical maximum efficiency to 21.39%. Valance band offset at the HTL/absorber interface significantly improves PCE of SC [13] . Thus, in order to design a highly efficient MoS2 TFSC, a suitable HTL material with enhanced electrical and optical properties is indispensable. Several inorganic materials, including Cu2O, CuSCN, CuI, and NiO, have shown promising use as HTL in various organic and inorganic solar cells [14] [15] . Among these options, Cu2O is a promising material for fulfilling HTL roles due to its indirect bandgap (2.1 - 2.6 eV), electron affinity (3.4 eV), abundance, environmental advantages, and synthesis practicality [16] . The effective extraction of holes is made possible by the synergy between high hole-mobility and precisely matched band alignment with MoS2.
Therefore, this paper proposed a novel design of a MoS2-based TFSC employing Cu2O as HTL with the structure Al/ZrS2/ZnO/MoS2/Cu2O/Ni. The proposed SC has been designed and examined using SCAPS-1D. The widespread adoption and utilization of SCAPS-1D in the domain of SC numerical simulations can be attributed to its unique combination of open-source nature, versatility, user-friendliness, accuracy, and the active support from its community of users and developers [1] [6] [10] . The numerical calculations revealed that this new design resulted in a substantial improvement in SC performance. This study explores the utilization of Cu2O as HTL to enhance device performance, reaching a maximum efficiency of 26.70% with Voc is 1.089 V, Jsc is 30.33 mA/cm2, FF is 80.85% whereas without HTL the maximum efficiency of 18.87% with Voc is 0.9425 V, Jsc is 25.41 mA/cm2, FF is 78.79%. Additionally, we delve into the impacts of altering the thickness of the absorber layer and HTL, alongside varying the doping concentration and defect density of the absorber layer, the doping concentration and electron affinity of HTL, interface defect density, and temperature. The ultimate goal is to attain enhanced efficiency through careful examination of these factors.
2. Device Architecture and Material Parameters
The SCAPS-1D, Version 3.9 software was utilized for the design and performance analysis of the proposed Al/ZrS2/ZnO/MoS2/Cu2O/Ni SC configuration. The Department of Electronics and Information Systems at the University of Ghent, Belgium, has developed a sophisticated software that meticulously analyzes the electrostatic potential and behavior of free carriers within the solar cell. This software achieves its insights by skillfully incorporating continuity equations and the Poisson equation [17] . Figure 1(a) illustrates the schematic diagram of the hetero-junction structure used in the simulation. The SC comprises a p-MoS2 absorber layer, an n-ZrS2 (Zirconium disulfide) window layer, an n-ZnO electron transport layer (ETL), and a Cu2O HTL. In this design, Al is utilized as the front contact material, while Ni functions as the back contact material, forming a well-structured device for efficient solar energy conversion. ZrS2 used as window layer, a type of TMDCs, is one of the most promising photovoltaic materials due to its high absorption coefficient and variable bandgap energy of 1.2 - 2.2 eV, allowing for successful energy conversion and device engineering [18] . ZnO is used as ETL because to its cost-effectiveness, physical and chemical stability, and non-toxicity. The wide bandgap of ZrS2 (2.0 eV) and ZnO (3.27 eV) play a crucial role in the hetero-junction structure by allowing significant optical throughput. Additionally, the desirable electron affinity of the ZnO (~4.1 eV) ETL enables it to form a suitable junction with MoS2 absorber layer. Furthermore, Cu2O is used as HTL, effectively reducing the recombination loss of photo-generated carriers at the back edge [19] [20] . Figure 1(b) shows the energy band diagram of the reported SC structure. All simulations are conducted at a working temperature of 300 K, with AM 1.5 G light. Leveraging prior research findings, we have integrated experimentally and theoretically derived
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Figure 1. The schematic (a) proposed Al/ZrS2/ZnO/MoS2/Cu2O/Ni structure (b) Energy band.
parameters of different layers to optimize the performance of the recently developed MoS2 SC with Cu2O HTL. The pertinent physical parameters for each layer have been delineated in Table 1 and Table 2.
3. Result and Discussion
3.1. Influence of Absorber Layer Thickness and Doping Concentration on PV Parameters
Figure 2 demonstrated a comprehensive analysis of photovoltaic (PV) parameters for MoS2-based hetero-junction TFSC, considering variations in MoS2 thickness and acceptor concentration. It is evident from Figure 2(a) that the Voc exhibits minor fluctuations in response to changes in thickness. Moreover, a notable increase in Voc is observed when the acceptor concentration is raised from 1017 to 1021 cm−3, resulting in a rise from 0.96 V to 1.20 V [13] .
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Table 1. Material parameters employed in our simulations [2] [10] [18] .
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Table 2. Input parameters of interface defect layers [10] [18] .
Figure 2(b) shows that Jsc increases with thickness and acceptor concentration. However, it is noteworthy that the impact of MoS2 thickness on Jsc is significantly greater, since the increase in thickness is directly related to increased light absorption, which subsequently results in a higher creation of hole-electron pairs. As the acceptor concentration increases, the FF demonstrates an upward trend, whereas the impact of MoS2 thickness on FF is found to be minimal when the acceptor concentration increases, as depicted in Figure 2(c). Finally, the efficiency of the SC exhibits an increase from 20.85% to 24.49% as the thickness of MoS2 is elevated from 0.4 µm to 1 µm, alongside an increase in the acceptor concentration from 1017 to 1021 cm−3, as illustrated in Figure 2(d). For the simulation, a thickness of 1 µm for MoS2 absorber layer and an acceptor concentration of 1019 cm−3 were identified as the optimal values.
3.2. Impact on PV Parameters Due to Defects in MoS2 Absorber Layer
Figure 3 depicted the dependence of PV performance parameters on absorber defect density, which ranges from 1012 to 1016 cm−3. At a defect density 1012 cm−3 the maximum values of Voc Jsc, FF, and efficiency are obtained. As a consequence of Shockley-Read Hall non-radiative recombination, there is a decrease in the minority charge carrier lifetime and an increase in recombination carriers within the absorber layer, leading to a reduction in Voc, FF, and PCE with higher defect density [21] [22] [23] . Nonetheless, it is observed that the Jsc remains relatively stable until the defect density reaches 1015 cm−3. Beyond this point, Jsc begins to decrease, likely attributed to heightened carrier recombination, which reduces the number of available carriers responsible for generating the short-circuit current. Consequently, the optimal value for the defect density of MoS2 is 1014 cm−3.
3.3. Effect of HTL Thickness, Doping Density and Electron Affinity on PV Parameters
Figure 4 illustrated the impact of Cu2O thickness, doping density, and electron affinity on PV parameters. As depicted in Figure 4(a) and Figure 4(b), the variations in Cu2O thickness and doping density do not exhibit a significant effect
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Figure 3. Performance parameters as a function of defect density in MoS2 absorber layer.
on the PV parameters. However, the electron affinity of Cu2O exerts a substantial influence on enhancing the PV parameters [24] . Notably, as the electron affinity increases up to 3.4 eV, all PV parameters exhibit a consistent upward trend, as illustrated in Figure 4(c). The investigation yields the determination of the optimal values for the thickness of the HTL and the doping density, which are found to be 0.4 µm and 1016 cm−3, respectively. Additionally, the electron affinity of the HTL is found to be 3.4 eV, indicating a favorable characteristic for the overall performance of the device.
3.4. Influence of Interfacial Defect Density on PV Parameters
Figure 5(a) and Figure 5(b) provide a depiction of the influence of interfacial defect density variations, spanning from 1010 cm−2 to 1018 cm−2, at the ZnO/MoS2 and MoS2/Cu2O interfaces on PV parameters. As the defect density approaches 1018 cm−2, there is a notable and significant decrease observed in all PV parameters. The presence of trap states at the interface acts as recombination centers, resulting in a reduction in photo-generated carriers and hindering efficient carrier collection. These defects related to interface states significantly diminish the overall performance parameters [1] . The value of 1011 cm−2 for interfacial defect density has been specified for both interfaces in order to reach a balance on PV characteristics.
3.5. Impact of Temperature on PV Parameters
The assessment of PV parameters, namely Voc, Jsc, PCE, and FF, has been conducted to investigate the impact of temperature fluctuations ranging from 275 K to 475 K, as depicted in Figure 6. It has been apparent that enhancing the operating temperature results in a reduction of the Voc. As temperature increases, the band gap of MoS2 decreases and the reverse saturation current is enhanced, both of which lead to a decline in Voc [25] . Conversely, the values of Jsc exhibit a slight increase with elevating operating temperatures, owing to the increase in generation of electron-hole pairs [26] [27] . The proposed SC has an estimated FF of 80.18% at 275 K and 85.48% at 360 K, after which it declines to 81.84% as the temperature goes up. It is also noticed from Figure 6 that PCE of the proposed SC exhibits lower value at high temperature as a consequence of reduced Voc.
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Figure 4. The effect of HTL layer (a) thickness (b) doping density (c) electron affinity on PV parameters.
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Figure 5. The change of output parameters due to the variation of defect density at (a) ZnO/MoS2 and (b) MoS2/Cu2O interface.
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Figure 6. The impact of temperature on PV parameters.
3.6. Influence of HTL on SC Output Characteristics
Figure 7(a) and Figure 7(b) present the J-V characteristics and Quantum Efficiency (QE) curves for the optimized designed MoS2-based TFSC with and without HTL layer. Through numerical simulation, the values of Voc, Jsc, FF, and PCE for the newly designed Al/ZrS2/ZnO/MoS2/Cu2O/Ni hetero-junction SC were estimated to be 1.089 V, 30.33 mA/cm2, 80.85%, and 26.70%, respectively. In comparison, the Al/ZrS2/ZnO/MoS2/Ni SC exhibited Voc, Jsc, FF, and PCE values of 0.9425 V, 25.41 mA/cm2, 78.79%, and 18.87%, respectively. Notably, the presence of Cu2O in the structure significantly increases the calculated SC parameters compared to the structure without Cu2O. The improvement of Voc, Jsc, FF and PCE is due to reduction of dark current for surface recombination [28] [29] [30] . The MoS2-based TFSC without HTL suffers from a high rate of minority carrier recombination, leading to reduced performance parameters, especially a lower Voc value. In contrast to the structure without HTL, as shown in Figure 7(b), the QE with the Cu2O HTL layer is higher.
3.7. Comparative Study
Table 3 presents a comprehensive analysis of MoS2-based TFSC architectures that have been examined in previous research studies. The numerical simulations of the MoS2-based TFSC with Cu2O HTL demonstrated superior solar cell performance compared to previous studies. In this study, the advantage of incorporating Cu2O HTL with MoS2-based TFSCs has been investigated. Cu2O and MoS2 exhibit better energy level alignment which is essential for enhancing the performance of PV parameters.
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Figure 7. The numerically examined (a) JV curves with HTL and without HTL; (b) QE curve with HTL and without HTL of the Proposed SC.
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Table 3. Comparison of proposed optimized SC with the previously reported studies.
4. Conclusion
In this paper, we have presented an investigation of the MoS2-based TFSC structure Al/ZrS2/ZnO/MoS2/Cu2O/Ni. By employing the SCAPS-1D software and conducting numerical calculations, the study demonstrates a significant enhancement in MoS2-based TFSC performance with Cu2O HTL, as compared to MoS2-based TFSC without Cu2O HTL. This study explores the utilization of Cu2O HTL, which has demonstrated notable advancements in key SC parameters, reaching a maximum efficiency of 26.70% with Voc of 1.089 V, Jsc of 30.33 mA/cm2 and FF of 80.85%. HTL foster the effective charge extraction, collection and transport, while also aiding in minimizing charge recombination which in turns significantly enhanced the SC performance. The optimal thickness of the MoS2 absorber is 1 µm with acceptor concentration of 1019 cm−3 and defect density 1014 cm−3. Furthermore, the MoS2/ZnO and Cu2O/MoS2 interface defects were estimated to be 1011 cm−3. In future, density functional theory can be used to explain the optoelectronic properties of MoS2 absorber.
Acknowledgements
The University of Gent’s Prof. Marc Burgelman and his colleagues kindly provided the SCAPS-1D software that is discussed in this article, and the authors are appreciative of their assistance.