Homogeneous FACsPbI 3 Films via Sequential Deposition for Efficient and Stable Perovskite Solar Cells

Abstract Despite significant advancements in the power conversion efficiency (PCE) of FAPbI 3 ‐based perovskite solar cells (PSCs), their commercialization remains hindered by stability issues. These challenges arise primarily from the phase transition of the α‐phase to the δ‐phase under operation. Alloying FAPbI 3 with Cs to form FA‐Cs perovskite (FACsPbI 3 ) emerged as a promising approach to enhance phase and thermal stability. In this study, it is demonstrated that adding a Cs source to the PbI 2 solution promotes the formation of a structurally stable α‐phase in the PbI 2 film. This stabilization reduces cation diffusion but leads to Cs accumulation at the surface of the perovskite layer. To address this issue, a δ‐phase perovskite in the PbI 2 film by predepositing the Cs source before PbI 2 deposition is constructed. This approach facilitates the uniform vertical distribution of FA and Cs cations, resulting in a homogeneous perovskite (h‐perovskite) device. The h‐perovskite device achieves a higher PCE of 24.59%, compared to 22.96% for the inhomogeneous perovskite (i‐perovskite) device. Operando GIWAXS measurements reveal that the h‐perovskite exhibits a slower degradation rate than the i‐perovskite during device operation. This difference is attributed to the formation of the δ‐phase and a stronger crystal lattice contraction observed in the i‐perovskite during the operando measurements.