However, the photoactive FAPbI3 black phase would easily transform into a non-photoactive yellow δ-FAPbI3 phase at room temperature, especially under humid conditions. The poor phase stability challenges both efficiency and long-term stability of PSCs based on FAPbI3 . It is generally believed that the phase instability of FAPbI3 perovskites originates from its unsuitable tolerant factor. To address this problem, alloying FA + with MA + /Cs + cations or partially substituting I -with Br -ions have been employed to tune the tolerant factor . The resulting mixed-ion FA-based perovskites exhibit improved resistance to phase transition. Among these different alloying approaches, formamidinium-cesium mixed-cation pure iodide (FA1-xCsxPbI3) perovskites are particularly promising, because they avoid the concerns on volatile MA cations and phase segregation induced by mixed halide ions (Br-I) . However, because of the complex crystallization dynamics of formamidinium and cesium, these pure iodide FA-Cs perovskites suffer from poor composition homogeneity and high defects/traps densities . The PSCs based on these films are therefore facing relatively low efficiencies.