Low-Temperature Fabrication of Phase-Pure α-FAPbI3 Films by Cation Exchange from Two-Dimensional Perovskites for Solar Cell Applications

α-FAPbI3-based perovskite solar cells have recently attracted increasing attention as a result of an ideal bandgap and longer exciton lifetime of FAPbI3 compared to perovskites with other compositions. However, in a traditional fabrication method, the α-FAPbI3 films were usually obtained by a direct phase transition from δ to α phase at a high annealing temperature, leading to low quality with poor crystallinity and numerous defects. The formation and stabilization of phase-pure, material-pure, high-quality α-FAPbI3 films remain challenging. In this work, a FA vapor-assisted cation-exchange pathway from low-dimensional perovskites to three-dimensional α-FAPbI3 was built, through which phase-pure and material-pure α-FAPbI3 films were achieved at 100 °C below the temperature of thermodynamic δ-to-α phase transition (∼150 °C). Through an in-depth study, the cation-exchange pathway was found to have a low reaction barrier directly toward α-FAPbI3 and suppress the formation of δ-FAPbI3, leading to high-quality α-FAPbI3 with high orientation and few trap states at a low annealing temperature. Consequently, small-area devices and large-area modules with as-prepared α-FAPbI3 films were achieved with improved performance, showing great potential for further study and application.