Asynchronous Photoexcited Electronic and Structural Relaxation in Lead-Free Perovskites
Asynchronous Photoexcited Electronic and Structural Relaxation in Lead-Free Perovskites
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无铅钙钛矿中的异步光激发电子和结构弛豫
DOI：
10.1021/jacs.9b04557
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发表时间：
2019-08-21
影响因子：
15
通讯作者：
Zhang, Xiaoyi
中科院分区：
文献类型：
--
作者：
Liu, Cunming;Wang, Yingqi;Zhang, Xiaoyi
具有更稳定晶体结构的空位有序无铅钙钛矿作为解决卤化铅钙钛矿(LHPs)毒性和长期稳定性问题的替代材料，已被广泛研究。同时，这些钙钛矿中紧密堆积的卤化物八面体亚晶格产生的色散能带预计将有利于电荷载流子的迁移率。然而，这些钙钛矿材料出乎意料地遭受着电荷载流子传输不良的困扰。为了解决这个问题，我们利用超快、元素特有的X射线瞬时吸收谱(XTA)直接探测了典型的空位有序无铅钙钛矿(Cs3Bi2Br9)的光激发电子和结构动力学。我们发现，光生空穴迅速自陷于bR中心，同时扭曲了局域晶格结构，很可能形成V-k中心(bR-2(-)二聚体)构型的小极化子。更重要的是，我们发现了一种令人惊讶的长寿命、结构扭曲态，寿命类似于59亩S，类似于比载流子复合慢3个数量级的寿命。这种长寿命的结构变形可能在连续光照下产生瞬变的“背景”，从而影响载流子沿晶格骨架的输运。
Vacancy-ordered lead-free perovskites with more-stable crystalline structures have been intensively explored as the alternatives for resolving the toxic and long-term stability issues of lead halide perovskites (LHPs). The dispersive energy bands produced by the closely packed halide octahedral sublattice in these perovskites are meanwhile anticipated to facility the mobility of charge carriers. However, these perovskites suffer from unexpectedly poor charge carrier transport. To tackle this issue, we have employed the ultrafast, elemental-specific X-ray transient absorption (XTA) spectroscopy to directly probe the photoexcited electronic and structural dynamics of a prototypical vacancy-ordered lead-free perovskite (Cs3Bi2Br9). We have discovered that the photogenerated holes quickly self-trapped at Br centers, simultaneously distorting the local lattice structure, likely forming small polarons in the configuration of V-k center (Br-2(-) dimer). More significantly, we have found a surprisingly long-lived, structural distorted state with a lifetime of, similar to 59 mu s, which is similar to 3 orders of magnitude slower than that of the charge carrier recombination. Such long-lived structural distortion may produce a transient "background" under continuous light illumination, influencing the charge carrier transport along the lattice framework.