Challenges in Controlling the Crystallization Pathways and Kinetics for Highly Reproducible Solution-Processing of Metal Halide Perovskites

Successful and reproducible conversion toward metal halide perovskites from the corresponding precursors through solution processing necessitates a comprehensive understanding of the crystallization pathways and kinetics. On the contrary to the straightforward preparation achieved by simply blending precursors in solution, molecular complexations leading to a variety of iodoplumbate species occur at the early stages within solution, each of which dictates the initial stages of crystallization following distinct routes. Here, we first provide an overview of preceding studies on the coordination chemistry governing the formation of the complexes, controlled by excess iodide ions and solvents choices. The environmental factors such as light illumination and thermal annealing to precursor solutions are also responsible for modulations in solution equilibria. Following discussion of the relationship between the molecular structure of the complexes and the final crystal structures and morphologies of perovskites, with specific attention to the role of monovalent cations, we shift our focus to the interaction complexity determining the stability of intermediate complexes, which is key to the fabrication of high-quality perovskite films. Meticulous control of the environmental conditions during every step toward crystallization completion is highlighted for reproducible processing of perovskites. We believe that this Perspective will provide guidance for a general approach to perovskite solution-processing throughout all fabrication stages, emphasizing precautions based on the understanding of the role of factors delicately influencing perovskite crystallization.