A Unified microscopic picture of cation and anion migration in MAPbI$_3$

TL;DR

This study uses neural network-enhanced molecular dynamics to model ion migration in MAPbI$_3$, revealing rapid diffusion of most defects and emphasizing collective molecular motion's role in ionic transport.

Contribution

It provides a unified microscopic understanding of cation and anion migration mechanisms in MAPbI$_3$, challenging previous models.

Findings

Most defects diffuse rapidly at room temperature with barriers of 0.15-0.20 eV.

MA interstitials are highly mobile due to concerted migration involving multiple ions.

Diffusion of I-related defects depends on charge state, unlike MA defects.

Abstract

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 MAPbI$_3$ 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, owing to a concerted migration mechanism involving multiple MA ions. No evidence of MA vacancy migration is obtained. Whereas diffusion of I-related defects appreciably depends on their charge state, 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.