Competing Dissolution Pathways and Ligand Passivation-Enhanced Interfacial Stability of Hybrid Perovskites with Liquid Water.

Material instability issues, especially moisture degradation in ambient operating environments, limit the practical application of hybrid perovskite in photovoltaic and light-emitting devices. Very recent experiments demonstrate that ligand passivation can effectively improve the surface moisture tolerance of hybrid perovskites. In this work, the interfacial stability of as-synthesized pristine and alkylammonium-passivated methylammonium lead iodide (MAPbI) with liquid water is systematically investigated using molecular dynamics simulations and reaction kinetics models. Interestingly, the more hydrophilic [PbI] surface is more stable than the less hydrophilic [MAI] surface because of the higher polarity of the former surface. Linear alkylammoniums significantly stabilize the [MAI] surface with highly reduced (by 1-2 orders of magnitude) dissociation rates of both MA and ligands themselves, while branched ligands, surprisingly, lead to higher dissociation rates as the surface coverage increases. Such anomalous behavior is attributed to the aggregation-assisted dissolution of surfactant-like ligands as micelles during the degradation process. Short-chain linear alkylammonium at the full surface coverage is found to be the optimal ligand to stabilize the [MAI] surface. This work not only provides fundamental insights into the ionic dissolution pathways and mechanisms of hybrid perovskites in water but also inspires the design of highly stable hybrid perovskites with ligand passivation layers. The computational framework developed here is also transferrable to the investigation of surface passivation chemistry for weak ionic materials in general.