The optical and electronic properties of metal halide perovskites provide insight into the operation of solar cells as well as their long-term operational stability. Halide mobility in perovskite films is an important factor influencing solar cell performance. One can visualize halide ion migration through halide exchange between two nanocrystal suspensions or between physically paired films of two different metal halide perovskites. The ability to tune band gap by varying halide ratios (Cl:Br or Br:I) allows the synthesis of mixed halide perovskites with tailored absorption and emission across the entire visible spectrum. Interestingly, mixed halide (e.g., MAPb(Br_0.5I_0.5)₃) films undergo phase segregation to form Br-rich and I-rich sites under steady state illumination. Upon halting illumination, segregated phases mix to restore original mixed halide compositions. Introducing multiple cations (Cs, formamidinium) at the A site or alloying with Cl greatly suppresses halide mobilities. Long-term irradiation of MAPb(Br_0.5I_0.5)₃ films also cause expulsion of iodide leaving behind Br-rich phases. Hole trapping at I-rich sites in MAPb(Br_0.5I_0.5)₃ is considered to be an important step in inducing halide mobility in photoirradiated films. This Account focuses on halide ion migration in nanocrystals and nanostructured films driven by entropy of mixing in dark and phase segregation under light irradiation.

中文翻译：

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钙钛矿纳米晶体和纳米结构中卤离子的迁移

金属卤化物钙钛矿的光学和电子特性为深入了解太阳能电池的运行及其长期运行稳定性提供了见识。钙钛矿薄膜中的卤化物迁移率是影响太阳能电池性能的重要因素。一个人可以通过两个纳米晶体悬浮液之间或两个不同金属卤化物钙钛矿的物理配对膜之间的卤化物交换来观察卤离子的迁移。通过改变卤化物比率（Cl：Br或Br：I）来调节带隙的能力允许合成混合卤化物钙钛矿，并在整个可见光谱范围内具有特定的吸收和发射。有趣的是，混合卤化物（例如MAPb（Br _0.5 I _0.5）₃）薄膜在稳态照明下经历相分离以形成富Br和富I的位点。停止照射后，分离的相混合以恢复原始的混合卤化物组成。在A位点引入多种阳离子（Cs，甲酰胺）或与Cl合金化可大大抑制卤化物的迁移。长期辐照MAPb（Br _0.5 I _0.5）₃膜也会驱逐碘化物，留下富Br相。MAPb（Br _0.5 I _0.5）_3中富I位的空穴陷阱被认为是诱导光辐射膜中卤化物迁移率的重要步骤。该帐户重点研究了在黑暗中混合熵和光照射下的相偏析驱动的纳米晶体和纳米结构薄膜中的卤化物离子迁移。