Mixed-cation and mixed-halide lead halide perovskites show great potential for their application in photovoltaics. Many of the high-performance compositions are made of cesium, formamidinium, lead, iodine, and bromine. However, incorporating bromine in iodine-rich compositions and its effects on the thermal stability of the perovskite structure has not been thoroughly studied. In this work, we study how replacing iodine with bromine in the state-of-the-art Cs_0.17FA_0.83PbI₃ perovskite composition leads to different dynamics in the phase transformations as a function of temperature. Through a combination of structural characterization, cathodoluminescence mapping, X-ray photoelectron spectroscopy, and first-principles calculations, we reveal that the incorporation of bromine reduces the thermodynamic phase stability of the films and shifts the products of phase transformations. Our results suggest that bromine-driven vacancy formation during high temperature exposure leads to irreversible transformations into PbI₂, whereas materials with only iodine go through transformations into hexagonal polytypes, such as the 4H-FAPbI₃ phase. This work sheds light on the structural impacts of adding bromine on thermodynamic phase stability and provides new insights into the importance of understanding the complexity of phase transformations and secondary phases in mixed-cation and mixed-halide systems.

中文翻译：

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溴的掺入影响卤化铅钙钛矿的相变和热稳定性


混合阳离子和混合卤化物卤化铅钙钛矿在光伏领域显示出巨大的应用潜力。许多高性能组合物由铯、甲脒、铅、碘和溴制成。然而，在富含碘的组合物中掺入溴及其对钙钛矿结构热稳定性的影响尚未得到彻底研究。在这项工作中，我们研究了在最先进的 Cs _0.17 FA _0.83 PbI ₃钙钛矿组合物中用溴代替碘如何导致不同的相变动力学随温度的变化。通过结构表征、阴极发光图、X射线光电子能谱和第一性原理计算的结合，我们揭示了溴的掺入降低了薄膜的热力学相稳定性并改变了相变产物。我们的结果表明，高温暴露期间溴驱动的空位形成导致不可逆转变为PbI ₂ ，而仅含碘的材料会转变为六方多型体，例如4H-FAPbI ₃相。这项工作揭示了添加溴对热力学相稳定性的结构影响，并为理解混合阳离子和混合卤化物系统中相变和二次相的复杂性的重要性提供了新的见解。