Impacts of MAPbBr3 Additive on Crystallization Kinetics of FAPbI3 Perovskite for High Performance Solar Cells

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(3)) perovskite films doped by methylammonium lead bromide (MAPbBr(3)) 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(3) perovskite precursor easily formed a mixture of black perovskite phase (alpha phase) and non-perovskite yellow phase (delta phase) after the addition of MAPbBr(3), whereas only delta phase formed without MAPbBr(3). The delta phase gradually converted to alpha phase during annealing and there was only alpha phase left in both films with and without MAPbBr(3). However, the doped films presented high film quality without PbI2 residue in contrast to the undoped films. These findings imply that the MAPbBr(3) additive can effectively suppress the formation of the unfavorable delta phase and trigger the formation of the optically active alpha phase even during spin-coating, which enhances the film quality possibly by removing the energy barriers from delta phase to alpha phase at room temperature. Finally, PSCs based on MAPbBr(3)-doped FAPbI(3) were fabricated with a champion efficiency as high as 19.4% from 14.2% for the PSCs based on undoped FAPbI(3).