Deciphering the Formation Process of 2D to 3D Halide Perovskite Thin Films

The challenge of obtaining high-performing organo-metal halide perovskite (MHP) thin films of various dimensions for different applications persists. For this purpose, a comprehensive understanding of the growth mechanism and crystallization process of 2D to 3D MHP films during thermal annealing, as the crucial step of most MHP film preparations, is required. Here, we investigate the formation mechanism of MHP which composition varies from 2D to 3D. For this systemic description, eight different initial precursor solution systems and two organic cation spacers have been investigated. They have allowed us to unveil the complex relationship between the growth direction of various phases and the direction of solvent removal. Then, we have comprehensively analyzed the internal structure changes of the MHP low-dimension phases and found a phase splitting phenomenon and revealed a partial spacer elimination during thermal annealing. Our original findings reveal various 2D phase transitions from the film surface to the bottom by a sequential process involving the partial elimination of spacer, phase decomposition, and recrystallization. These generalizable results provide valuable insights for future synthesis optimizations and to produce high-performance MHP film-based devices for a wide range of applications.