Quintero-Bermudez Rafael, Proppe Andrew H, Mahata Arup, Todorović Petar, Kelley Shana O, De Angelis Filippo, Sargent Edward H
Department of Electrical and Computer Engineering , University of Toronto , 10 King's College Road , Toronto , Ontario M5S 3G4 , Canada.
Department of Chemistry , University of Toronto , 80 St George Street , Toronto , Ontario M5S 3G4 , Canada.
J Am Chem Soc. 2019 Aug 28;141(34):13459-13467. doi: 10.1021/jacs.9b04801. Epub 2019 Aug 14.
Two-dimensional (2D) and quasi-2D perovskite materials have enabled advances in device performance and stability relevant to a number of optoelectronic applications. However, the alignment among the bands of these variably quantum confined materials remains a controversial topic: there exist multiple experimental reports supporting type-I, and also others supporting type-II, band alignment among the reduced-dimensional grains. Here we report a combined computational and experimental study showing that variable ligand concentration on grain surfaces modulates the surface charge density among neighboring quantum wells. Density functional theory calculations and ultraviolet photoelectron spectroscopy reveal that the effective work function of a given quantum well can be varied by modulating the density of ligands at the interface. These induce type-II interfaces in otherwise type-I aligned materials. By treating 2D perovskite films, we find that the effective work function can indeed be shifted down by up to 1 eV. We corroborate the model via a suite of pump-probe transient absorption experiments: these manifest charge transfer consistent with a modulation in band alignment of at least 200 meV among neighboring grains. The findings shed light on perovskite 2D band alignment and explain contrasting behavior of quasi-2D materials in light-emitting diodes (LEDs) and photovoltaics (PV) in the literature, where materials can exhibit either type-I or type-II interfaces depending on the ligand concentration at neighboring surfaces.
二维（2D）和准二维钙钛矿材料推动了与多种光电子应用相关的器件性能和稳定性的进步。然而，这些具有可变量子限域的材料的能带间排列仍然是一个有争议的话题：有多个实验报告支持I型能带排列，也有其他报告支持II型能带排列，即在低维晶粒之间。在此，我们报告了一项结合计算和实验的研究，结果表明晶粒表面可变的配体浓度会调节相邻量子阱之间的表面电荷密度。密度泛函理论计算和紫外光电子能谱表明，给定量子阱的有效功函数可以通过调节界面处配体的密度来改变。这些在原本为I型排列的材料中诱导出II型界面。通过处理二维钙钛矿薄膜，我们发现有效功函数确实可以向下移动多达1 eV。我们通过一系列泵浦 - 探测瞬态吸收实验证实了该模型：这些实验表明电荷转移与相邻晶粒之间至少200 meV的能带排列调制一致。这些发现揭示了钙钛矿二维能带排列情况，并解释了文献中准二维材料在发光二极管（LED）和光伏（PV）中呈现出的对比行为，即材料根据相邻表面的配体浓度可表现出I型或II型界面。
