Inhomogeneous Defect Distribution in Mixed-Polytype Metal Halide Perovskites

TL;DR

This study investigates how phase inhomogeneity in mixed-polytype CsPbI3 perovskites affects defect distribution and mobility, revealing that face-sharing regions have higher defect concentrations and mobility, impacting device performance.

Contribution

It provides first-principles insights into defect energetics and transport at phase interfaces in mixed-polytype perovskites, highlighting their role in device hysteresis.

Findings

Face-sharing regions have a million-fold higher defect concentration.

Defects are more mobile in face-sharing regions with lower activation energy.

Hexagonal phase inclusions can trap charges, affecting device stability.

Abstract

The competition between corner, edge and face-sharing octahedral networks is a cause of phase inhomogeneity in metal halide perovskite thin-films. Here we probe the charged iodine vacancy distribution and transport at the junction between cubic and hexagonal polytypes of CsPbI$_3$ from first-principles materials modelling. We predict a lower defect formation energy in the face-sharing regions, which correlates with a longer Pb$-$I bond length and causes a million-fold increase in local defect concentration. These defects are predicted to be more mobile in the face-sharing regions with a reduced activation energy for vacancy-mediated diffusion. We conclude that hexagonal phase inclusions or interfaces will act as defect sinks that could trap charges and enhance current-voltage hysteresis in perovskite-based solar cells and electrical devices.