Homogeneous FACsPbI$_3$ Films via Sequential Deposition for Efficient and Stable Perovskite Solar Cells

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. ... mehrThis 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.