Lead halide perovskites have rapidly emerged as candidate materials for high-performing solar cells, but show serious issues related to long-term stability. Methylammonium (MA) lead perovskites with mixed iodide–bromide compositions, MAPb(I_1–xBr_x)₃, are reported to exhibit improved stability, but the origin of such behavior is not fully understood. Here, we report new insights into the degradation properties of MAPb(I_1–xBr_x)₃ using ab initio simulations and a range of spectroscopic techniques. Absorbance spectroscopy shows that as the Br content increases, the material stability toward oxygen and light increases. Isothermal gravimetric analysis and time-resolved single photon counting show that the amount of oxygen incorporation into perovskite films decreases significantly with increasing Br content. Ab initio simulations indicate that the degradation reaction involving superoxide species is energetically exothermic for pure MAPbI₃ but becomes less favorable with increasing Br content with an endothermic energy for pure MAPbBr₃, suggesting that the degradation of MAPbBr₃ in the presence of oxygen and light is unfavorable. The simulations indicate shorter N–H...Br hydrogen bonds between the MA⁺ cation and Br ions, which would promote greater structural stability upon bromide substitution. Thin-film passivation with iodide salts is shown to enhance the stabilities of mixed-halide perovskite films and solar cell devices. The greater fundamental understanding of mixed iodide–bromide systems gained from this study is important for the future design of stable perovskite solar cells.

中文翻译：

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了解碘化钙钛矿中溴化物替代物增强的稳定性

卤化钙钛矿已经迅速出现，成为高性能太阳能电池的候选材料，但显示出与长期稳定性相关的严重问题。据报道，甲基碘（MA）钙钛矿与碘化物-溴化物混合成分MAPb（I _{1- x} Br _x）₃具有更好的稳定性，但这种行为的起因尚未完全明了。在这里，我们报告了有关MAPb（I _{1– x} Br _x）₃降解性质的新见解。使用从头算模拟和各种光谱技术。吸收光谱表明，随着Br含量的增加，材料对氧和光的稳定性也随之增加。等温重量分析和时间分辨的单光子计数表明，随着Br含量的增加，钙钛矿膜中掺入的氧气量显着减少。从头模拟表明，涉及超氧化物物种降解反应是纯MAPbI大力放热₃但变得不太有利的与吸热能量为纯MAPbBr增加溴含量₃，这表明MAPbBr的降解₃在氧气和光的存在下是不利的。模拟表明，MA ⁺阳离子和Br离子之间的N–H ... Br氢键较短，这将在溴化物取代后促进更大的结构稳定性。碘化物盐薄膜钝化显示增强了混合卤化物钙钛矿薄膜和太阳能电池装置的稳定性。从这项研究中获得对混合碘化物-溴化物系统的更深刻的基础了解对于稳定钙钛矿太阳能电池的未来设计非常重要。