Revealing the crystallization process and realizing uniform 1.8 eV MA-based wide-bandgap mixed-halide perovskites via solution engineering

Wide-bandgap perovskites are recently drawing tremendous attention in the community for high-efficiency all-perovskite tandem solar cells. However, the formamidinium (FA+) and methylammonium (MA+) based wide-bandgap mixed halide perovskites suffered from high density of traps and pin-holes, respectively. Fundamental understanding on the crystallization and film formation processes are keys to overcome those challenges but not yet clearly understood. In this study, an in-situ photoluminescence technique was used to investigate the perovskite crystallization during the thermal annealing process. It is found that the crystallization of a mixed halide perovskite with bromide (Br−) and iodine (I−) ions following the Ostward ripening crystal growth. Interestingly, it is found that the initial nucleation reaction is quickly completed in the first few seconds, however, leaving the small crystals with inhomogeneous composition. The different aggregation affinities of such inhomogeneous small crystals provoke the formation of pin-holes during the thermal annealing process. By engineering the precursor solution to control the nucleation rate, the chemical composition of the small crystals has become homogenous. Uniform pin-hole free high Br−composited wide-bandgap MA0.9Cs0.1Pb(I0.6Br0.4)3 perovskite films with bandgap energy of 1.8 eV have been realized. The corresponding photovoltaic devices have achieved an encouraging device efficiency of 15.1% with superb photostability.