Phase Evolution in Lead-Free Cs-Doped FASnI₃Hybrid Perovskites and Optical Properties

Tin halide perovskites are among the candidates having potential to substitute lead-based perovskites due to their environmentally benign components and potential medical usage. Nevertheless, the air stability remains a challenge due to Sn2+ oxidation. The recent developments have shown that adding SnF2 as an additive and A-site cation replacement by an inorganic element such as Cs+ can improve the film stability. However, exploring the structural change through A-site doping in Sn-based perovskites experimentally requires an in-depth investigation. Here, the phase evolution mechanism from the transformation of CsxFA1-xSnI3 to Cs2SnI6 via Cs2-xFAxSnI6 in the presence of the intermediate phase SnI2-(dmf)x due to Cs substitution in FASnI3 and the substitution's influences on the optical properties were identified and investigated. Introducing a small amount of Cs+ (≤5% CsI) significantly promoted Sn2+ oxidation due to the anharmonic lattice dynamics. Later, at 10% CsI, self-doping was initiated, resulting in the coexistence of Sn2+/Sn4+. However, phase separation of Cs2SnI6 via Cs2-xFAxSnI6 occurred at contents greater than 10% CsI. Notably, the absorption coefficient was amplified along with the increasing Cs content (at 10% CsI, 6 times greater than that of FASnI3). The air stability was also enhanced as a result of Cs substitution. This work demonstrates the structural engineering of A-site cations in order to obtain various material properties for photovoltaic (PV) and non-PV applications.