Crystalline Intermediates and Their Transformation Kinetics during the Formation of Methylammonium Lead Halide Perovskite Thin Films

The morphology of methylammonium lead halide perovskite thin films significantly affects the performance of opto-electronic devices that comprise them. Using X-ray diffraction studies, we elucidated the mechanisms of thin-film formation to complete the complex picture of structural development of perovskites under an inert atmosphere. The presence of excess methylammonium iodide during perovskite crystallization leads to the formation of layered intermediates and low-dimensional perovskites; these intermediates are correlated with the formation of large and continuous grains in the final films. When the precursors are present in stoichiometric equivalence, initial stages of crystallization instead involve the formation of solvates; the fully crystallized films have poor surface coverage and comprise needle-like structures. The activation energy of crystallization in films with stoichiometric excess methylammonium iodide is higher than that for films comprising stoichiometrically equivalent precursors; the higher energy barrier is consistent with the need to sublime excess methylammonium iodide during film formation. Replacing lead iodide with lead chloride does not qualitatively alter the crystallization process or the final morphology; it lowers the activation energy for crystallization, presumably because sublimation of methylammonium chloride is less energetic than that of methylammonium iodide. Our study suggests the necessity of layered structures and low-dimensional perovskites for the formation of technologically relevant continuous thin films.