Center for Organic Photonics and Electronics, School of Chemistry & Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, USA.
Acc Chem Res. 2012 Mar 20;45(3):337-46. doi: 10.1021/ar200119g. Epub 2011 Oct 19.
Transparent metal oxides, in particular, indium tin oxide (ITO), are critical transparent contact materials for applications in next-generation organic electronics, including organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs). Understanding and controlling the surface properties of ITO allows for the molecular engineering of the ITO-organic interface, resulting in fine control of the interfacial chemistries and electronics. In particular, both surface energy matching and work function compatibility at material interfaces can result in marked improvement in OLED and OPV performance. Although there are numerous ways to change the surface properties of ITO, one of the more successful surface modifications is the use of monolayers based on organic molecules with widely variable end functional groups. Phosphonic acids (PAs) are known to bind strongly to metal oxides and form robust monolayers on many different metal oxide materials. They also demonstrate several advantages over other functionalizing moieties such as silanes or carboxylic acids. Most notably, PAs can be stored in ambient conditions without degradation, and the surface modification procedures are typically robust and easy to employ. This Account focuses on our research studying PA binding to ITO, the tunable properties of the resulting surfaces, and subsequent effects on the performance of organic electronic devices. We have used surface characterization techniques such as X-ray photoelectron spectroscopy (XPS) and infrared reflection adsorption spectroscopy (IRRAS) to determine that PAs bind to ITO in a predominantly bidentate fashion (where two of three oxygen atoms from the PA are involved in surface binding). Modification of the functional R-groups on PAs allows us to control and tune the surface energy and work function of the ITO surface. In one study using fluorinated benzyl PAs, we can keep the surface energy of ITO relatively low and constant but tune the surface work function. PA modification of ITO has resulted in materials that are more stable and more compatible with subsequently deposited organic materials, an effective work function that can be tuned by over 1 eV, and energy barriers to hole injection (OLED) or hole-harvesting (OPV) that can be well matched to the frontier orbital energies of the organic active layers, leading to better overall device properties.
透明金属氧化物，特别是铟锡氧化物（ITO），是下一代有机电子学应用中至关重要的透明接触材料，包括有机发光二极管（OLEDs）和有机光伏电池（OPVs）。了解和控制 ITO 的表面性质可以实现 ITO-有机界面的分子工程，从而实现对界面化学和电子的精细控制。特别是，在材料界面处的表面能匹配和功函数兼容性都可以显著提高 OLED 和 OPV 的性能。尽管有许多方法可以改变 ITO 的表面性质，但更成功的表面改性之一是使用基于具有广泛变化的端基官能团的有机分子的单层。已知膦酸（PA）与金属氧化物强烈结合，并在许多不同的金属氧化物材料上形成坚固的单层。与其他官能团（如硅烷或羧酸）相比，它们还具有几个优势。最值得注意的是，PA 可以在环境条件下储存而不会降解，并且表面改性程序通常是坚固且易于实施的。本账目重点介绍我们研究 PA 与 ITO 的结合、由此产生的表面的可调性质以及对有机电子器件性能的后续影响。我们使用表面特性技术，如 X 射线光电子能谱（XPS）和红外反射吸收光谱（IRRAS），确定 PA 以主要的双齿方式与 ITO 结合（其中 PA 的三个氧原子中的两个参与表面结合）。PA 上的 R 基团的修饰使我们能够控制和调谐 ITO 表面的表面能和功函数。在一项使用氟化苄基 PA 的研究中，我们可以保持 ITO 的表面能相对较低且恒定，但调谐表面功函数。ITO 的 PA 改性导致材料更稳定，与随后沉积的有机材料更兼容，有效功函数可调节超过 1 eV，空穴注入（OLED）或空穴收集（OPV）的能垒可以很好地与有机活性层的前沿轨道能量匹配，从而导致更好的整体器件性能。
