Growth of Compact CH₃NH₃PbI₃ Thin Films Governed by the Crystallization in PbI₂ Matrix for Efficient Planar...

As a convenient preparation technique, a two-step method, which is normally done by spin-coating CH₃NH₃I onto PbI₂ film followed by a thermal annealing, is generally used to prepare solution-processed CH₃NH₃PbI₃ films for planar perovskite solar cells. Here, we prepare the compact CH₃NH₃PbI₃ thin films by the two-step method at a low temperature (<80 °C) and investigate the effects of PbI₂ crystallization on the structure-property correlation in the CH₃NH₃PbI₃ films. It is found that the importance of the crystallization in PbI₂ matrix lies in governing the transition from the (001) plane of trigonal PbI₂ to the (002) plane of tetragonal CH₃NH₃PbI₃ in the rapid reaction process for atoms to coordinate into perovskite during spin-coating, which actually determines the morphology and the type of vacancy defects in resulting perovskite; a better crystallized PbI₂ film has a much stronger ability to react with CH₃NH₃I solution and produces larger CH₃NH₃PbI₃ grains with a higher crystallinity. The CH₃NH₃PbI₃/TiO₂ planar solar cell derived from a better crystallized PbI₂ film exhibits significantly improved performance and stability as the result of the higher crystallinity inside the perovskite film. Moreover, it is demonstrated that the crystalline PbI₂ film matrix subjected to the annealing after a slow heating process prior to contacting CH₃NH₃I solution is more effective for CH₃NH₃PbI₃ formation than that with a direct annealing history. The results in this paper provide a guide for preparing high-quality CH₃NH₃PbI₃ thin films for efficient perovskite solar cells and CH₃NH₃PbI₃ interfacial films over the layers susceptible to temperature.