Alloy engineering in mixed Sn–Ge perovskites for photovoltaic application

Abstract

Recently, mixed Sn–Ge perovskites have been proposed as promising lead-free candidates in the photovoltaic (PV) field. However, as an important component of mixed Sn–Ge perovskites, the all-inorganic Sn-based perovskites have a dominant defect, Sn_I (Sn replacing I, anti-site), which induces a deep level in the band structure and limits the power conversion efficiency. Therefore, it is interesting to discover whether Sn_I and Ge_I (Ge replacing I, anti-site) can still act as non-radiative recombination centers in mixed Sn–Ge perovskites. Using first-principles calculations, we explored the effects of Sn_I and Ge_I in CsGe_1−xSn_xI₃ (x = 0.25, 0.50, and 0.75) on the PV performances. It was shown that both Sn_I and Ge_I are benign only in CsGe_0.5Sn_0.5I₃, and we suggest that CsGe_0.5Sn_0.5I₃ is a promising light-absorbing material with benign defects (Ge_I and Sn_I), strong light absorption, and small carrier effective masses. Interestingly, the concerted action of alloying on both band edges and defect states was found to result in better defect properties in CsGe_0.5Sn_0.5I₃, and this is different from results of previous studies where the modulation of introducing alloys on defects is mainly caused by regulating the band edges alone. Our work not only provides theoretical guidance for the experimental synthesis of mixed Sn–Ge perovskites with superior PV performances, but it also shows the potential of alloy engineering in regulating defect levels in perovskites.