Improving the electrode interface in organic electronics

Tag : Organic Acid

Our group at the Georgia Institute of Technology (Georgia Tech) hasdeveloped a theoretical approach to enable investigation of theITO-PA interface. Our goal was to gain a detailed understanding ofthe interfacial structure and to provide guidelines for syntheticefforts aimed at lowering the charge-injection barrier via PAdeposition. 7 Our first step was, to our knowledge, the first theoreticalcharacterization of a model ITO surface using density-functionaltheory (DFT). Based on the bulk indium oxide structure, a periodicsupercell was chosen that includes examples of all structurallyinequivalent surface sites. Each oxygen atom located above the toplayer of metal atoms was saturated with hydrogen, to model arealistic, OH-terminated surface. The ratio of tin to indium atomswas set to 0.14, which lies within the typical doping range,0.1–0.2, found in commercial ITO. The resulting supercell isshown in Figure 1 . Two types of potentially chemically active sites were found onthe ITO surface: metal ions with oxygen coordination numbers lowerthan that of the bulk cationic sites, and surface hydroxyl groups.
Our next step was to investigate the adsorption of a PA monolayeron the ITO surface at the DFT level of calculation. The calculationused the surface slab shown in Figure 1 and a representative PA compound, n -octylphosphonic acid. We characterized more than 20 adsorptionpossibilities. Among the possible adsorption modes illustrated inFigure 2 , (c) and (d) were most frequently obtained as a result ofstructural optimizations. These binding modes are consistent bothwith earlier experimental assessments and with recent x-rayphotoelectron spectra measured by Neal Armstrong's group at theUniversity of Arizona on samples synthesized by Seth Marder's groupat Georgia Tech. 7 Interestingly, in the case of phenyl PAs, for which R in Figure 2 represents a phenyl group, adsorption modes (c) and (d) requirethat the phenyl groups be oriented nearly perpendicular to thesurface.