Halogen Vacancy Migration at Surfaces of CsPbBr$_3$ Perovskites: Insights from Density Functional Theory

TL;DR

This study uses density functional theory to analyze bromine vacancy migration at surfaces of CsPbBr3 perovskites, revealing that surfaces facilitate migration and that passivation with alkali halides can suppress this process, impacting material stability.

Contribution

It provides the first detailed comparison of vacancy migration barriers at surfaces versus bulk in CsPbBr3 and explores surface passivation strategies to inhibit vacancy migration.

Findings

Surfaces lower bromine vacancy migration barriers compared to bulk.

Passivation with alkali halide monolayers increases migration barriers.

Migration barriers correlate with lattice mismatch between surface and passivation layer.

Abstract

Migration of halogen vacancies is one of the primary sources of phase segregation and material degradation in lead-halide perovskites. Here we use first principles density functional theory to compare migration energy barriers and paths of bromine vacancies in the bulk and at a (001) surface of cubic CsPbBr$_3$. Our calculations indicate that surfaces might facilitate bromine vacancy migration in these perovskites, due to their soft structure that allows for bond lengths variations larger than in the bulk. We calculate the migration energy for axial-to-axial bromine vacancy migration at the surface to be only half of the value in the bulk. Furthermore, we study the effect of modifying the surface with four different alkali halide monolayers, finding an increase of the migration barrier to almost the bulk value for the NaCl-passivated system. Migration barriers are found to be correlated to the lattice mismatch between the CsPbBr$_3$ surface and the alkali halide monolayer. Our calculations suggest that surfaces might play a significant role in mediating vacancy migration in halide perovskites, a result with relevance for perovskite nanocrystals with large surface-to-volume ratios. Moreover, we propose viable ways for suppressing this undesirable process through passivation with alkali halide salts.