Roles of SnX₂ (X = F, Cl, Br) Additives in Tin-Based Halide Perovskites toward Highly Efficient and Stable Lead-Free Perovskite Solar Cells

Preserving the stability of Sn-based halide perovskites is a primary concern in developing photovoltaic light-absorbing materials for lead-free perovskite solar cells. Whereas the addition of SnX₂ (X = F, Cl, Br) has been demonstrated to improve the photovoltaic performance of Sn halide perovskite solar cells, the mechanistic roles of SnX₂ in the performance enhancement have not yet been studied appropriately. Here we perform a comparative study of CsSnI₃ films and devices and examine how SnX₂ additives affect their stability, and the results are corroborated by first-principles-based theoretical calculations. Unlike the conventional belief that the additives annihilate defects, we find that the additives effectively passivate the surface and stabilize the perovskite phase, promoting the stability of CsSnI₃. Our mechanism suggests that SnBr₂, which shows ca. 100 h of prolonged stability along with a high power conversion efficiency of 4.3%, is the best additive for enhancing the stability of CsSnI₃.