Achieving the high phase purity of CH₃NH₃PbI₃ film by two-step solution processable crystal engineering

To date, it is still a great challenge for highly efficient perovskite devices to realize the high quality perovskite films with high purity, high coverage ratio and good crystallization by two-step scalable solution method. In this study, a series PbI₂ films with tunable micro-architecture of PbI₂ crystals are prepared via solution processable crystal engineering. The perovskite film, prepared by optimized pit spacing in gas pumped PbI₂ film at 1000 Pa, shows the highest film quality, including no residual PbI₂ phase, compact morphology, and improved photoluminescence intensity. A transformation kinetics shows that the pit spacing strongly influences both the mass transfer and the sequential intercalation reaction between CH₃NH₃I and PbI₂ crystals, which ultimately determines the full reaction state of the perovskite film. The perovskite solar cells assembled by the perovskite film show both high power-conversion efficiency and good reproducibility of photovoltaic performance due to the restrained charge recombination arising from the high quality perovskite film.