Zu Fengshuo, Wolff Christian M, Ralaiarisoa Maryline, Amsalem Patrick, Neher Dieter, Koch Norbert
Institut für Physik & IRIS Adlershof , Humboldt-Universität zu Berlin , 12489 Berlin , Germany.
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH , 12489 Berlin , Germany.
ACS Appl Mater Interfaces. 2019 Jun 19;11(24):21578-21583. doi: 10.1021/acsami.9b05293. Epub 2019 Jun 5.
The tremendous success of metal-halide perovskites, especially in the field of photovoltaics, has triggered a substantial number of studies in understanding their optoelectronic properties. However, consensus regarding the electronic properties of these perovskites is lacking due to a huge scatter in the reported key parameters, such as work function (Φ) and valence band maximum (VBM) values. Here, we demonstrate that the surface photovoltage (SPV) is a key phenomenon occurring at the perovskite surfaces that feature a non-negligible density of surface states, which is more the rule than an exception for most materials under study. With ultraviolet photoelectron spectroscopy (UPS) and Kelvin probe, we evidence that even minute UV photon fluxes (500 times lower than that used in typical UPS experiments) are sufficient to induce SPV and shift the perovskite Φ and VBM by several 100 meV compared to dark. By combining UV and visible light, we establish flat band conditions (i.e., compensate the surface-state-induced surface band bending) at the surface of four important perovskites, and find that all are p-type in the bulk, despite a pronounced n-type surface character in the dark. The present findings highlight that SPV effects must be considered in all surface studies to fully understand perovskites' photophysical properties.
金属卤化物钙钛矿取得了巨大成功，尤其是在光伏领域，这引发了大量关于理解其光电特性的研究。然而，由于所报道的关键参数（如功函数（Φ）和价带最大值（VBM）值）存在巨大差异，对于这些钙钛矿的电子特性尚未达成共识。在此，我们证明表面光电压（SPV）是在钙钛矿表面发生的一种关键现象，其表面态密度不可忽略，对于大多数正在研究的材料而言，这是普遍规律而非个别情况。通过紫外光电子能谱（UPS）和开尔文探针，我们证实即使是极微小的紫外光子通量（比典型UPS实验中使用的通量低500倍）也足以诱导SPV，并使钙钛矿的Φ和VBM相对于黑暗环境移动数百毫电子伏特。通过结合紫外光和可见光，我们在四种重要钙钛矿的表面建立了平带条件（即补偿表面态引起的表面能带弯曲），并发现尽管在黑暗中表面具有明显的n型特征，但所有钙钛矿在体相中均为p型。目前的研究结果表明，在所有表面研究中都必须考虑SPV效应，以全面理解钙钛矿的光物理性质。
