Phase Evolution in Lead-Free Cs-Doped FASnI<sub>3</sub> Hybrid Perovskites and Optical Properties
Pimsuda Pansa‐Ngat, Hideki Nakajima, Ratchadaporn Supruangnet, Sujin Suwanna, Pasit Pakawatpanurut, Somboon Sahasithiwat, Pongsakorn Kanjanaboos
Abstract
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.