| Summary: | In this study, we fabricated and characterized uniform multi-cation perovskite FA<sub>x</sub>MA<sub>1−x</sub>PbI<sub>3</sub> films. We used the dynamic spin-coating method to control the cation ratio of the film by gradually increasing the FA+, which replaced the MA+ in the films. When the FA+ concentration was lower than xFA ~0.415 in the films, the stability of the multi-cation perovskite improved. Above this concentration, the film exhibited δ-phase FAPbI<sub>3</sub> in the FA<sub>x</sub>MA<sub>1−x</sub>PbI<sub>3</sub> films. The formation of δ-phase FAPbI<sub>3</sub> disturbed the homogeneity of the photoluminescence spatial distribution and suppressed the absorption spectral bandwidth with the increasing bandgap. The precise control of the cation ratio of multi-cation perovskite films is necessary to optimize the energy-harvesting performance.
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