The effect of oxygen plasma treatment of gold electrodes on the molecular orientation of CuPc films

Abstract

Gold (Au) is a commonly used material for many organic electronic device electrodes due to its chemical stability and high work function, which is required for p-type semiconductors. The orientation and morphology of organic semiconductors deposited on such electrodes often depends crucially on the cleanliness of the Au surface. In this study, we examine the effect of a common cleaning method, namely oxygen plasma cleaning, on sputter-deposited polycrystalline Au electrodes and subsequently deposited copper-(II)-phthalocyanine (CuPc) films. Using X-ray diffraction (XRD), atomic force microscopy (AFM) and near-edge X-ray absorption fine structure (NEXAFS) measurements, we show that the metastable Au oxide typically formed during an oxygen plasma cleaning process significantly impacts the molecular orientation and morphology in CuPc thin films, forcing molecules to adopt an upright orientation. We further demonstrate that the typical recumbent orientation of CuPc adopted on clean polycrystalline Au can be achieved by an additional annealing step after oxygen plasma treatment of the Au electrode that causes a decomposition of the gold oxide.