Steric Engineering of Point Defects in Lead Halide Perovskites

Due to their high photovoltaic efficiency and low-costsynthesis,lead halide perovskites have attracted wide interest for applicationin 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 keyprocesses that limit photovoltaic performance is not fullyunderstood. Here we analyze non-radiative carrier trapping in themixed A-cation hybrid halide perovskite MA(1-x )Cs( x )PbI(3). By using rigorousfirst-principles simulations, we show that cation mixing leads toa hole trapping rate at the iodine interstitial that is 8 orders ofmagnitude greater than in the single-cation system. We demonstratethat the same defect in the same material can display a wide varietyof defect activity from electrically inactive to recombinationcenter and, in doing so, resolve conflicting reports in theliterature. Finally, we propose a new mechanism in which steric effectscan be used to determine the rate of carrier trapping; this is achievedby controlling the phase and dynamical response of the lattice throughthe A-site composition. Our findings elucidate crucial links betweenchemical composition, defect activity, and optoelectronic performanceand suggest a general approach that can help to rationalize the developmentof new crystalline materials with target defect properties.