Ion-Exchange-Induced 2D–3D Conversion of HMA_1−xFA_xPbI₃Cl Perovskite into a High-Quality MA_1−xFA_xPbI₃ Perovskite

High-quality phase-pure MA_1−xFA_xPbI₃ planar films (MA=methylammonium, FA=formamidinium) with extended absorption and enhanced thermal stability are difficult to deposit by regular simple solution chemistry approaches owing to crystallization competition between the easy-to-crystallize but unwanted δ-FAPbI₃/MAPbI₃ and FA_xMA_1−xPbI₃ requiring rigid crystallization conditions. Here A 2D–3D conversion to transform compact 2D mixed composition HMA_1−xFA_xPbI₃Cl perovskite precursor films into 3D MA_1−xFA_xPbI₃ (x=0.1–0.9) perovskites is presented. The designed Cl/I and H/FA(MA) ion exchange reaction induced fast transformation of compact 2D perovskite film, helping to form the phase-pure and high quality MA_1−xFA_xPbI₃ without δ-FAPbI₃ and MAPbI₃ impurity. In all, we successfully developed a facile one-step method to fabricate high quality phase-pure MA_1−xFA_xPbI₃ (x=0.1–0.9) perovskite films by 2D–3D conversion of HMA_1−xFA_xPbI₃Cl perovskite. This 2D–3D conversion is a promising strategy for lead halide perovskite fabrication.