Materials Department and Materials Research Laboratory, University of California , Santa Barbara, California 93106, United States.
Department of Chemistry and Biochemistry, University of California , Santa Barbara, California 93106, United States.
J Am Chem Soc. 2017 Nov 22;139(46):16875-16884. doi: 10.1021/jacs.7b09536. Epub 2017 Nov 2.
The role of organic molecular cations in the high-performance perovskite photovoltaic absorbers, methylammonium lead iodide (MAPbI) and formamidinium lead iodide (FAPbI), has been an enigmatic subject of great interest. Beyond aiding in the ease of processing of thin films for photovoltaic devices, there have been suggestions that many of the remarkable properties of the halide perovskites can be attributed to the dipolar nature and the dynamic behavior of these cations. Here, we establish the dynamics of the molecular cations in FAPbI between 4 K and 340 K and the nature of their interaction with the surrounding inorganic cage using a combination of solid state nuclear magnetic resonance and dielectric spectroscopies, neutron scattering, calorimetry, and ab initio calculations. Detailed comparisons with the reported temperature dependence of the dynamics of MAPbI are then carried out which reveal the molecular ions in the two different compounds to exhibit very similar rotation rates (≈8 ps) at room temperature, despite differences in other temperature regimes. For FA, rotation about the N···N axis, which reorients the molecular dipole, is the dominant motion in all phases, with an activation barrier of ≈21 meV in the ambient phase, compared to ≈110 meV for the analogous dipole reorientation of MA. Geometrical frustration of the molecule-cage interaction in FAPbI produces a disordered γ-phase and subsequent glassy freezing at yet lower temperatures. Hydrogen bonds suggested by atom-atom distances from neutron total scattering experiments imply a substantial role for the molecules in directing structure and dictating properties. The temperature dependence of reorientation of the dipolar molecular cations systematically described here can clarify various hypotheses including those of large-polaron charge transport and fugitive electron spin polarization that have been invoked in the context of these unusual materials.
有机分子阳离子在高性能钙钛矿光伏吸收体，即碘化甲基铵（MAPbI）和碘化甲脒（FAPbI）中的作用一直是一个令人费解的研究课题。除了有助于光伏器件的薄膜加工外，还有人提出，卤化物钙钛矿的许多显著特性可以归因于这些阳离子的偶极性质和动态行为。在这里，我们使用固态核磁共振和介电光谱学、中子散射、量热法和从头算计算相结合的方法，确定了 FAPbI 中分子阳离子在 4 K 到 340 K 之间的动力学及其与周围无机笼的相互作用性质。然后，我们与报道的 MAPbI 动力学随温度的变化进行了详细比较，结果表明，尽管在其他温度范围内存在差异，但两种不同化合物中的分子离子在室温下表现出非常相似的旋转速率（≈8 ps）。对于 FA，绕 N···N 轴的旋转，它重新定向分子偶极，是所有相中主要的运动，在环境相中具有≈21 meV 的激活势垒，而对于 MA 的类似偶极重取向，其激活势垒为≈110 meV。FAPbI 中分子-笼相互作用的几何位阻导致无序 γ 相和随后更低温度下的玻璃态冻结。来自中子全散射实验的原子-原子距离表明氢键的存在，暗示了分子在指导结构和决定性质方面的重要作用。这里系统描述的偶极分子阳离子的重取向的温度依赖性可以澄清各种假设，包括在这些异常材料背景下提出的大极化子电荷输运和易失电子自旋极化的假设。
