Atomistic insights into the degradation of halide perovskites: a reactive force field molecular dynamics study

TL;DR

This study develops reactive force fields for large-scale molecular dynamics simulations to understand phase instability and defect-driven degradation in inorganic CsPbI3 halide perovskites, revealing mechanisms behind their stability issues.

Contribution

First development of reactive force fields enabling detailed MD simulations of degradation processes in halide perovskites, providing new insights into their phase transitions and defect effects.

Findings

Phase transitions driven by anharmonicity and entropy.

Cs cations destabilize the perovskite structure at low temperatures.

Iodine vacancies promote decomposition to PbI2.

Abstract

Halide perovskites make efficient solar cells due to their exceptional optoelectronic properties, but suffer from several stability issues. The characterization of the degradation processes is challenging because of the limitations in the spatio-temporal resolution in experiments and the absence of efficient computational methods to study the reactive processes. Here, we present the first effort in developing reactive force fields for large scale molecular dynamics simulations of the phase instability and the defect-induced degradation reactions in inorganic CsPbI$_{3}$. We find that the phase transitions are driven by a combination of the anharmonicity of the perovskite lattice with the thermal entropy. At relatively low temperatures, the Cs cations tend to move away from the preferential positions with good contacts with the surrounding metal halide framework, potentially causing its conversion to a non-perovskite phase. Our simulations of defective structures reveal that, although both iodine vacancies and interstitials are very mobile in the perovskite lattice, the vacancies have a detrimental effect on the stability, initiating the decomposition reactions of perovskites to PbI$_{2}$. Our work puts ReaxFF forward as an effective computational framework to study reactive processes in halide perovskites.