A Unified Microscopic Picture of Cation and Anion Migration in MAPbI3

Passivating defects and restricting defect mobilities in halide perovskites to increase device lifetimes has become a main field of research. Modeling structure and mobility of point defects is an essential contribution to this endeavor. We employ molecular dynamics, based on neural network potentials trained on density functional theory data, to model ion migration in MAPbI3 triggered by I and MA vacancies or interstitials. Most of these species diffuse rapidly at room temperature, with migration barriers between 0.15 and 0.20 eV. MA interstitials are highly mobile despite their molecular nature, because of a collective reorientation of neighboring MA ions. No evidence of MA vacancy migration is obtained. While the diffusion of I-related defects appreciably depends on their charge state, the diffusion of MA defects does not. These results revise the conventional picture of ion transport in hybrid perovskites and highlight the role of collective molecular motion in enabling fast ionic migration.