The effect of CuZn+ZnCu defect complex on Cu2ZnSnS4 thin film solar cell: A density functional theory study

Due to its non-toxic and earth-abundant constituent elements, Cu2ZnSnS4 (CZTS) is a promising material for thin-film solar cells. To date, it is believed that the best-performing CZTS solar cells were fabricated with Cu/(Zn + Sn) = 0.8, Zn/Sn = 1.2 ratios, and efficiency was over 12.6%. Besides, defect also plays an essential role in solar cells' performance. It is known that the most abundant defect complex in this condition is CuZn + ZnCu. This paper shows the first investigation of CuZn + ZnCu defect complex towards CZTS solar cells performance using density functional theory (DFT). The work was carried out using Vienna ab-initio Simulation Package (VASP). First, GGA exchange-correlation functional was applied to perform the first structural relaxation. Then, for better electronic properties, the hybrid functional was used to analyze the density of states calculations with the screening parameter of 0.2 (HSE-06 exchange-correlation functional). The optical properties were also conducted through the HSE-06 exchange-correlation functional. Then, the predicted J-V characteristics were estimated using Spectroscopic Limited Maximum Efficiency (SLME). The result shows that this defect increased the predicted short-circuit current density from 36.593 to 39.392 mA/cm2. As a result, the optimum solar cell efficiency considering SQ Limit achieves 29.68%. Furthermore, this study shows that the defect induced a lower charge carrier effective mass. Therefore, it can be concluded that this defect is one of the reasons for the optimum ratios of Cu/(Sn + Zn) and Zn/Sn being 0.8 and 1.2, respectively.