Ultrafast optical snapshots of hybrid perovskites reveal the origin of multiband electronic transitions
Ultrafast optical snapshots of hybrid perovskites reveal the origin of multiband electronic transitions
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DOI：
10.1103/physrevb.96.195308
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发表时间：
2017-11-28
影响因子：
3.7
通讯作者：
Sfeir, Matthew Y.
中科院分区：
文献类型：
--
作者：
Appavoo, Kannatassen;Nie, Wanyi;Sfeir, Matthew Y.
将混合钙钛矿的复杂电子激发与其复杂的有机-无机晶格结构联系起来，对能量转换和光电技术具有重要意义。在这里，我们详细的多频带，多通道的卤化物混合钙钛矿的电子结构，通过测量的吸收瞬态的毫米级晶粒薄膜，因为它经历了一个热控制可逆的四方正交相变。探测这种混合钙钛矿的几乎单个晶粒，我们观察到未报告的能量分裂（简并提升）的高能量2.6 eV带的tetraxide相，进一步分裂的无序CH 3 NH3+分子的旋转自由度降低时，样品冷却。这种能量分裂急剧增加，在延长的相变共存区域，持续从160至120 K，变得更加明显的斜方晶相。通过跟踪依赖于温度的光学跃迁能量，并使用对称性分析，描述了电子状态的演变，从四方相的正交相，我们分配这种能量分裂的近简并过渡在四方相从R-和M-点导出的状态。重要的是，这些分配解释了动量守恒效应如何导致室温下的四方相的长的热载流子寿命，当混合钙钛矿结构转变为正交相时，由于伽马点处的散射增强，热载流子弛豫更快。
Connecting the complex electronic excitations of hybrid perovskites to their intricate organic-inorganic lattice structure has critical implications for energy conversion and optoelectronic technologies. Here we detail the multiband, multivalley electronic structure of a halide hybrid perovskite by measuring the absorption transients of a millimeter-scale-grain thin film as it undergoes a thermally controlled reversible tetragonal-to-orthogonal phase transition. Probing nearly single grains of this hybrid perovskite, we observe an unreported energy splitting (degeneracy lifting) of the high-energy 2.6 eV band in the tetragonal phase that further splits as the rotational degrees of freedom of the disordered CH3NH3+ molecules are reduced when the sample is cooled. This energy splitting drastically increases during an extended phase-transition coexistence region that persists from 160 to 120 K, becoming more pronounced in the orthorhombic phase. By tracking the temperature-dependent optical transition energies and using symmetry analysis that describes the evolution of electronic states from the tetragonal phase to the orthorhombic phase, we assign this energy splitting to the nearly degenerate transitions in the tetragonal phase from both the R- and M-point-derived states. Importantly, these assignments explain how momentum conservation effects lead to long hot-carrier lifetimes in the room-temperature tetragonal phase, with faster hot-carrier relaxation when the hybrid perovskite structurally transitions to the orthorhombic phase due to enhanced scattering at the Gamma point.