Homogeneous FACsPbI3 Films via Sequential Deposition for Efficient and Stable Perovskite Solar Cells

A compositional homogeneous perovskite is achieved by constructing a δ‐phase perovskite in the PbI2 film by predepositing the cesium source before PbI2 deposition and facilitating the uniform vertical distribution of FA and Cs cations. The compositional homogeneous perovskite solar cells achieve a higher champion PCE of 24.59%, compared to 22.96% for the inhomogeneous perovskite device. Abstract Despite significant advancements in the power conversion efficiency (PCE) of FAPbI3‐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 FAPbI3 with Cs to form FA‐Cs perovskite (FACsPbI3) emerged as a promising approach to enhance phase and thermal stability. In this study, it is demonstrated that adding a Cs source to the PbI2 solution promotes the formation of a structurally stable α‐phase in the PbI2 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 PbI2 film by predepositing the Cs source before PbI2 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. A compositional homogeneous perovskite is achieved by constructing a δ-phase perovskite in the PbI 2 film by predepositing the cesium source before PbI 2 deposition and facilitating the uniform vertical distribution of FA and Cs cations. The compositional homogeneous perovskite solar cells achieve a higher champion PCE of 24.59%, compared to 22.96% for the inhomogeneous perovskite device. 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. Advanced Science, Volume 12, Issue 43, November 20, 2025.