Steric engineering of point defects in lead halide perovskites

TL;DR

This paper investigates how mixing A-site cations in lead halide perovskites influences defect activity and carrier trapping, revealing that steric effects can be engineered to optimize photovoltaic performance.

Contribution

It introduces a first-principles analysis showing cation mixing dramatically affects defect activity and proposes steric engineering as a new method to control carrier trapping.

Findings

Cation mixing increases hole trapping rate at iodine interstitials by eight orders of magnitude.

Defect activity varies widely depending on A-site composition and lattice dynamics.

Steric effects can be used to tune defect properties and improve material performance.

Abstract

Due to their high photovoltaic efficiency and low-cost synthesis, lead halide perovskites have attracted wide interest for application in new solar cell technologies. The most stable and efficient ABX$_3$ perovskite solar cells employ mixed A-site cations, however the impact of cation mixing on carrier trapping and recombination -- key processes that limit photovoltaic performance -- is not fully understood. Here we analyse non-radiative carrier trapping in the mixed A-cation hybrid halide perovskite MA$_{1-x}$Cs$_x$PbI$_3$. By using rigorous first-principles simulations we show that cation mixing leads to a hole trapping rate at the iodine interstitial that is eight orders of magnitude greater than in the single cation system. We demonstrate that the same defect in the same material can display a wide variety of defect activity -- from electrically inactive to recombination centre -- and, in doing so, resolve conflicting reports in the literature. Finally, we propose a new mechanism in which steric effects can be used to determine the rate of carrier trapping; this is achieved by controlling the phase and dynamical response of the lattice through the A-site composition. Our findings elucidate crucial links between chemical composition, defect activity and optoelectronic performance, and suggest a general approach that can help to rationalise the development of new crystalline materials with target defect properties.