Steric engineering of point defects in lead halide perovskites

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 combined with techniques initially developed for organic molecular materials, we show that cation mixing leads to a hole trapping rate at the iodine interstitial that is seven 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 composition 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 steric size of the A-site cations. Our findings elucidate crucial links between chemical composition, defect activity and optoelectronic performance, and present a general approach that can help to rationalise the development of new materials with target defect properties