The technique of alloying FA⁺ with Cs⁺ is often used to promote structural stabilization of the desirable α-FAPbI₃ phase in halide perovskite devices. However, the precise mechanisms by which these alloying approaches improve the optoelectronic quality and enhance the stability have remained elusive. In this study, we advance that understanding by investigating the effect of cationic alloying in Cs_xFA_1−xPbI₃ perovskite thin-films and solar-cell devices. Selected-area electron diffraction patterns combined with microwave conductivity measurements reveal that fine Cs⁺ tuning (Cs_0.15FA_0.85PbI₃) leads to a minimization of stacking faults and an increase in the photoconductivity of the perovskite films. Ultra-sensitive external quantum efficiency, kelvin-probe force microscopy and photoluminescence quantum yield measurements demonstrate similar Urbach energy values, comparable surface potential fluctuations and marginal impact on radiative emission yields, respectively, irrespective of Cs content. Despite this, these nanoscopic defects appear to have a detrimental impact on inter-grains’/domains’ carrier transport, as evidenced by conductive-atomic force microscopy and corroborated by drastically reduced solar cell performance. Importantly, encapsulated Cs_0.15FA_0.85PbI₃ devices show robust operational stability retaining 85% of the initial steady-state power conversion efficiency for 1400 hours under continuous 1 sun illumination at 35 °C, in open-circuit conditions. Our findings provide nuance to the famous defect tolerance of halide perovskites while providing solid evidence about the detrimental impact of these subtle structural imperfections on the long-term operational stability.

中文翻译：

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减少纳米结构相杂质可提高铯甲脒金属卤化物钙钛矿的光电性能、器件性能和稳定性

^{FA +}与 Cs ⁺合金化技术通常用于促进卤化物钙钛矿器件中所需 α-FAPbI ₃相的结构稳定。然而，这些合金化方法提高光电质量和增强稳定性的精确机制仍然难以捉摸。在这项研究中，我们通过研究阳离子合金化在 Cs _x FA _{1− x} PbI ₃钙钛矿薄膜和太阳能电池器件中的影响来推进这一理解。选区电子衍射图与微波电导率测量相结合表明，精细的 Cs ⁺调谐 (Cs _0.15 FA _0.85 PbI ₃ ) 可最大限度地减少堆垛层错并提高钙钛矿薄膜的光电导率。超灵敏的外部量子效率、开尔文探针力显微镜和光致发光量子产率测量分别证明了相似的乌尔巴赫能量值、相当的表面电势波动和对辐射发射产率的边际影响，而与 Cs 含量无关。尽管如此，这些纳米级缺陷似乎对晶粒间/域间的载流子传输产生不利影响，传导原子力显微镜证明了这一点，并通过太阳能电池性能的急剧下降证实了这一点。重要的是，封装的 Cs _0.15 FA _0.85 PbI ₃器件表现出强大的运行稳定性，在 35 °C 的连续 1 太阳光照下、开路条件下，在 1400 小时内仍能保持 85% 的初始稳态功率转换效率。我们的研究结果为卤化物钙钛矿著名的缺陷容限提供了细微差别，同时为这些微妙的结构缺陷对长期运行稳定性的有害影响提供了坚实的证据。