XPS investigation of electrode/polymer interfaces of relevance to the phenylene vinylene polymer-based LEDs

Abstract

X-ray photoelectron spectroscopy (XPS) was employed for the study of the physicochemical interactions at electrode/polymer interfaces involving metals of high and low work functions and thin films of poly(p-phenylene vinylene) (PPV) and poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV). The changes in the C 1s core-level line shape of the polymers, the evolution of the metal core-level spectra and the changes in chemical compositions at the interfaces involving polymer coated on indium-tin oxide (ITO), sputtered ITO on polymer, and in situ evaporated magnesium (Mg) on polymer were carefully monitored. Indium of the ITO in both the ITO/polymer and polymer/ITO assemblies was found to diffuse towards the polymer at the interface. Oxygen migration was also observed at the ITO/polymer interfaces. The results obtained from in situ analysis by angle-resolved XPS of the interactions between thermally evaporated Mg and PPV and Mg and MEH-PPV indicated the formation of an interfacial layer in each case. The surface diffusion of adsorbed oxygen from the bulk of the conjugated polymer film played a dominant role in the interfacial reactions. The interfacial layer consisted of Mg oxides, Mg clusters, Mg metal and Mg-polymer complexes.