Hybrid perovskites form an emerging family of exceptional light harvesting compounds. However, the mechanism underpinning their photovoltaic effect is still far from understood, which is impeded by a lack of clarity on their structures. Here we show that iodide ions in the methylammonium lead iodide migrate via interstitial sites at temperatures above 280 K. This coincides with temperature dependent static distortions resulting in pseudocubic local symmetry. Based on bond distance analysis, the migrating and distorted iodines are at lengths consistent with the formation of I2 molecules, suggesting a 2I−→I2+2e− redox couple. The actual formula of this compound is thus (CH3NH3)PbI3−2x(I2)x where x∼0.007 at room temperature. A crucial feature of the tetragonal structure is that the methylammonium ions do not sit centrally in the A-site cavity, but disordered around two off-centre orientations that facilitate the interstitial ion migration via a gate opening mechanism.
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机译:杂化钙钛矿形成了一系列新兴的出色的光收集化合物。然而,支撑其光电效应的机理仍远未弄清,这是由于其结构缺乏清晰度而受到阻碍。在这里,我们发现甲基铵碘化铅中的碘离子在高于280 K的温度下通过间隙位置迁移,这与温度相关的静态畸变相符,从而导致伪立方局部对称。根据键距分析,迁移和变形的碘原子的长度与I2分子的形成一致,表明2I - sup>→I2 + 2e - sup>氧化还原对。因此,该化合物的实际分子式为(CH3NH3)PbI2-3-2x(I2)x,其中x在室温下为0.007。四方结构的一个关键特征是甲基铵离子不在A位腔中居中,而是在两个偏心方向周围无序排列,这有利于间隙离子通过门打开机制迁移。
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