Steric engineering of point defects in lead halide perovskites

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 ABX3 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 analyze non-radiative carrier trapping in the mixed A-cation hybrid halide perovskite MA1–xCsxPbI3. By using rigorous first-principles simulations, we show that cation mixing leads to a hole trapping rate at the iodine interstitial that is 8 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 center─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 rationalize the development of new crystalline materials with target defect properties.