First-Principles Investigation of the Thermal Degradation Mechanisms of Methylammonium Lead Triiodide Perovskite

The poor stability of organometallic halide perovskites, especially CH 3 NH 3 PbI 3 (MAPbI 3 ), in high-temperature environments is one of the challenges retarding its wider applicability. A better understanding of how MAPbI 3 perovskite degrades thermally will allow us to develop strategies to improve its “intrinsic” stability. In this study, we employed first-principles density functional theory to obtain detailed thermal degradation energy landscapes for MAPbI 3 . We focused on studying two reaction pathways: P1, for proton abstraction from methylammonium (MA) to iodine, and P2, for substitution (S N 2) of an iodide anion (I –, as nucleophile) at the carbon atom of MA to yield iodomethane (CH 3 I) and ammonia (NH 3 ). Our calculations provided kinetic data, allowing us to assess the “intrinsic” (in)stability of MAPbI 3 under heat stress and to make sense of the seemingly contradictory experimental results. The I – species is more reactive for reaction P1 but not for P2 . The energy of the reaction along pathway P1 increases monotonically with respect to proton abstraction. The reaction along pathway P2 is a two-step process, with a CH 3 –I–Pb–X intermediate formed in the crystal in conjunction with the release of NH 3 gas. Our results suggest that the method of preparation (e.g., iodine-deficient conditions) and the morphology (e.g., improved crystallinity) of MAPbI 3 could both influence its thermal stability.