Defects play an important role in the degradation processes of hybrid halide perovskite absorbers, impeding their application for solar cells. Among all defects, halide anion and organic cation vacancies are ubiquitous, promoting ion diffusion and leading to thin-film decomposition at surfaces and grain boundaries. Here, we employ fluoride to simultaneously passivate both anion and cation vacancies, by taking advantage of the extremely high electronegativity of fluoride. We obtain a power conversion efficiency of 21.46% (and a certified 21.3%-efficient cell) in a device based on the caesium, methylammonium (MA) and formamidinium (FA) triple-cation perovskite (Cs0.05FA0.54MA0.41)Pb(I0.98Br0.02)3 treated with sodium fluoride. The device retains 90% of its original power conversion efficiency after 1,000 h of operation at the maximum power point. With the help of first-principles density functional theory calculations, we argue that the fluoride ions suppress the formation of halide anion and organic cation vacancies, through a unique strengthening of the chemical bonds with the surrounding lead and organic cations.
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This work is supported by the National Natural Science Foundation of China (51722201; 51672008; 91733301), National Key Research and Development Program of China grant no. 2017YFA0206701, the Natural Science Foundation of Beijing, China (grant no. 4182026), the Young Talent Thousand Program, National Key Research and Development Program of China grant no. 2016YFB0700700, the National Natural Science Foundation of China (51673025) and Beijing Municipal Science and Technology Project no. Z181100005118002. S.T. acknowledges funding from the Computational Sciences for Energy Research tenure track programme of Shell, NWO and FOM (project no. 15CST04-2). The authors would like to thank W. Zou and J. Wang (Nanjing Tech University) for the PLQE measurement during the revision process, and Z. Dai for providing the dynamic light scattering measurement.
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