Improving the electrode interface in organic electronics
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Abstract
Indium tin oxide (ITO) is the most widely used transparent electrode in organic light-emitting devices and solar cells, and in liquid-crystal displays. The electronic and geometric structure of the interface between ITO and the organic material strongly affects the charge-injection characteristics, and thus largely determines the overall efficiency of these organic electronic devices. Two common issues are the large barrier for charge-carrier injection from ITO into the organic layer, which limits the electrical current, and the lack of compatibility between the hydrophilic surface of the oxide and the hydrophobic organic surface, which leads to poor adhesion between the materials. Researchers have solved these problems by chemically modifying the ITO surface using small-molecule organic adsorbates.1–6 Among various compounds that can selfassemble on OH-terminated surfaces, phosphonic acids (PAs) are especially promising for surface modification of various oxides, including ITO. PAs form robust monolayers without a need for the cross-linking that is common, for example, in silane surface modification.1–6 However, the impact of these selfassembled monolayers on the interfacial electronic and geometric structure is not well characterized. Our group at the Georgia Institute of Technology (Georgia Tech) has developed a theoretical approach to enable investigation of the ITO-PA interface. Our goal was to gain a detailed understanding of the interfacial structure and to provide guidelines for synthetic efforts aimed at lowering the charge-injection barrier via PA deposition.7 Our first step was, to our knowledge, the first theoretical characterization of a model ITO surface using density-functional theory (DFT). Based on the bulk indium oxide structure, a periodic supercell was chosen that includes examples of all structurally inequivalent surface sites. Each oxygen atom located above the top layer of metal atoms was saturated with hydrogen, to model a realistic, OH-terminated surface. The Figure 1. Top view of an OH-terminated indium tin oxide (ITO) surface slab, optimized at the density-functional theory level of calculation. The rectangle indicates the supercell, which is periodically repeated in the calculation. Tin substitutions were randomly distributed over the cationic positions throughout the slab.