Microscopic Mechanisms, Morphology, and Defect Formation in the Thermally Activated Crystallization of Methylammonium Lead Iodide.

Abstract

In this study, we unravel the atomistic mechanisms that govern the crystallization process of methylammonium lead iodide through the application of microsecond time scale molecular dynamics simulations.The findings indicate that methylammonium iodide (MAI) and lead iodide (PbI₂) precursors exhibit a propensity to aggregate into a disordered film, which ultimately undergoes a thermally activated disorder-to-order transformation to achieve crystallization. Notably, the crystal evolution during the annealing process reveals morphological characteristics consistent with the Straski-Krastanov growth mode. The temperature dependence of the crystal growth rate demonstrates an activation energy of 0.37 eV, which may be ascribed to the energy required to dissociate defective Pb-I bonds and facilitate Pb diffusion. Finally, the mechanisms underlying the spontaneous generation of lead vacancies are examined, suggesting a kinetic origin for such optically active defects. In principle, the latter suggests the potential for reducing their concentration through optimization of the growth parameters.