Direct Observation of Dynamic Symmetry Breaking above Room Temperature in Methylammonium Lead Iodide Perovskite
Direct Observation of Dynamic Symmetry Breaking above Room Temperature in Methylammonium Lead Iodide Perovskite
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DOI：
10.1021/acsenergylett.6b00381
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发表时间：
2016-10-01
影响因子：
22
通讯作者：
Billinge, Simon J. L.
中科院分区：
文献类型：
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作者：
Beecheri, Alexander N.;Semonin, Octavi E.;Billinge, Simon J. L.
卤化铅钙钛矿，例如甲基铵三碘化铅（CH 3NH3 PbI 3），在光伏应用中具有出色的光学和电子性能，但目前尚不完全了解这种可溶液加工的材料如何发挥如此好的作用。先前的研究表明，CH3NH3PbI3具有多种形式的静态无序，无论其制备方法如何，鉴于其优异的性能，这是令人惊讶的。使用高能量分辨率的非弹性X射线（HERIX）散射，我们测量声子色散CH3NH3PbI3和发现直接证据的另一种形式的无序单晶：大振幅非谐区边缘旋转不稳定性的PbI6八面体，持续到室温及以上，遗留下来的结构相变发生几十到几百度以下。声子计算表明，这些模式的甲基铵（CH3NH3+）的取向强烈耦合和合作。其结果是一个非中心对称的，瞬时的本地结构，我们观察到在原子对分布函数（PDF）的测量。这种局部对称性破缺是布拉格衍射无法观察到的，但可以解释关键的材料特性，如结构相序，超低的热传输，以及大的少数电荷载流子寿命，尽管载流子迁移率适中。从PDF中，我们估计波动对称破缺域的大小在直径1和3 nm之间。
Lead halide perovskites such as methylammonium lead triiodide (CH3NH3PbI3) have outstanding optical and electronic properties for photovoltaic applications, yet a full understanding of how this solution-processable material works so well is currently missing. Previous research has revealed that CH3NH3PbI3 possesses multiple forms of static disorder regardless of preparation method, which is surprising in light of its excellent performance. Using high energy resolution inelastic X-ray (HERIX) scattering, we measure phonon dispersions in CH3NH3PbI3 and find direct evidence for another form of disorder in single crystals: large-amplitude anharmonic zone edge rotational instabilities of the PbI6 octahedra that persist to room temperature and above, left over from structural phase transitions that take place tens to hundreds of degrees below. Phonon calculations show that the orientations of the methylanunonium (CH3NH3+) couple strongly and cooperatively to these modes. The result is a noncentrosymmetric, instantaneous local structure, which we observe in atomic pair distribution function (PDF) measurements. This local symmetry breaking is unobservable by Bragg diffraction but can explain key material properties such as the structural phase sequence, ultralow thermal transport, and large minority charge carrier lifetimes despite moderate carrier mobility. From the PDF we estimate the size of the fluctuating symmetry broken domains to be between 1 and 3 run in diameter.