The purpose of this study is the efficiency improvement of the Sb2Se3 solar cells. In this study, an experimental Sb2Se3 solar cell (glass/Mo/MoSe2/Sb2Se3/CdS/ZnO/ZnO:Al/Ag) with an efficiency record of 9.2% has been simulated. Absorber/buffer interface engineering plays a significant role in enhancing the efficiency of Sb2Se3 solar cells. To achieve this purpose, two approaches have been considered: first, adding sulfur to Sb2Se3 absorber, and second, using an alternative buffer with a wider bandgap instead of conventional CdS buffer. The effects of various x = Se/(S + Se) ratios of Sb2(S1−xSex)3 absorber layer on the photovoltaic performance were investigated. For Sb2(S,Se)3/CdS solar cell, optimum Se/(S + Se) mole fraction of 0.6 < x < 0.8 leads to improved efficiency. Also, ZnO1−ySy buffer layer was applied to replace the conventional CdS buffer layer of Sb2Se3 solar cells to reduce parasitic absorption and improve the short-circuit current density (Jsc). Bandgap for ZnO1−ySy semiconductor is higher than CdS. So, this leads to improved external quantum efficiency at short wavelengths. Optimization of band alignment through ZnO1−ySy buffer layer reduces interface carrier recombination and improves open-circuit voltage (Voc). The band alignment at the Sb2(S1−xSex)3/ZnO1−ySy interface is optimized by adjusting the selenium-to-sulfur ratio in the absorber layer and sulfur-to-oxygen ratio in the buffer layer. This work reveals that the most suitable interface for Sb2(S1−xSex)3/ZnO1−ySy heterojunction is formed when sulfur mole fractions ranging 0.5–0.6 in the Zn(O,S) buffer layer and 0.1–0.2 in the Sb2(S,Se)3 absorber layer. The results show that the efficiency improves from 9.2% to 15.65%, which represents a 70% improvement compared with the conventional Sb2Se3/CdS solar cell.
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