| Summary: | Blending perovskite with different cations has been successful in improving performance of perovskite solar cells, but the complex pathway of perovskite crystal formation remains a mystery, hindering its further development. In this paper, the detailed crystallization process of formamidinium lead iodide (FAPbI<sub>3</sub>) perovskite films doped by methylammonium lead bromide (MAPbBr<sub>3</sub>) additive was investigated by in situ grazing incident wide-angle X-ray scattering measurements during both spin coating and annealing. During spin-coating, it was found that the FAPbI<sub>3</sub> perovskite precursor easily formed a mixture of black perovskite phase (α phase) and non-perovskite yellow phase (δ phase) after the addition of MAPbBr<sub>3</sub>, whereas only δ phase formed without MAPbBr<sub>3</sub>. The δ phase gradually converted to α phase during annealing and there was only α phase left in both films with and without MAPbBr<sub>3</sub>. However, the doped films presented high film quality without PbI<sub>2</sub> residue in contrast to the undoped films. These findings imply that the MAPbBr<sub>3</sub> additive can effectively suppress the formation of the unfavorable δ phase and trigger the formation of the optically active α phase even during spin-coating, which enhances the film quality possibly by removing the energy barriers from δ phase to α phase at room temperature. Finally, PSCs based on MAPbBr<sub>3</sub>-doped FAPbI<sub>3</sub> were fabricated with a champion efficiency as high as 19.4% from 14.2% for the PSCs based on undoped FAPbI<sub>3</sub>.
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