Growing phase-stable α-FAPbI3 thin films through a mixing-vapor-solid reaction

Organic-inorganic hybrid perovskites are photoelectric materials with exceptional optical and electrical properties. Among them, solar cells based on formamidinium lead iodide (FAPbI3) are receiving increasing attention due to their narrow bandgap and superior thermal stability compared to methylammonium lead iodide (MAPbI3). However, pure α-phase FAPbI3 perovskite thin films tend to undergo a phase transition to the non-photoactive δ-phase at room temperature, posing a challenge to their stability. To address this issue, we developed a vapor-solid reaction method to prepare pure α-phase FAPbI3 perovskite thin films using a FAI/FACl mixing vapor. Our results demonstrated that FACl played a crucial role in promoting the growth of FAPbI3 grains, but it also had the potential to induce the formation of the non-photoactive δ-phase. Through optimization, we achieved high-quality, large-area (10 cm × 10 cm) FAPbI3 thin films with large grain size and low trap density. Furthermore, the pure α-phase FAPbI3 perovskite thin films were used to fabricate efficient PSCs, with a champion power conversion efficiency of up to 20.91%. Our method presents a promising solution to the challenge of preparing phase-stable α-FAPbI3, and provides new insights into the instability mechanism of the conventional solution-processed FAPbI3 perovskite.