Ion‐Exchange‐Induced 2D–3D Conversion of HMA 1− x FA x PbI 3 Cl Perovskite into a High‐Quality MA 1− x FA x PbI 3 Perovskite

High‐quality phase‐pure MA 1− x FA x PbI 3 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‐FAPbI 3 /MAPbI 3 and FA x MA 1− x PbI 3 requiring rigid crystallization conditions. Here A 2D–3D conversion to transform compact 2D mixed composition HMA 1− x FA x PbI 3 Cl perovskite precursor films into 3D MA 1− x FA x PbI 3 ( 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− x FA x PbI 3 without δ‐FAPbI 3 and MAPbI 3 impurity. In all, we successfully developed a facile one‐step method to fabricate high quality phase‐pure MA 1− x FA x PbI 3 ( x= 0.1–0.9) perovskite films by 2D–3D conversion of HMA 1− x FA x PbI 3 Cl perovskite. This 2D–3D conversion is a promising strategy for lead halide perovskite fabrication.