Paradoxical hole injection enhancement by contamination on the indium tin oxide surface

In organic electronics, contact formation at the interface between an electrode and an organic layer has a substantial influence on device performance. Clean substrates are therefore considered prerequisite to obtain high charge injection efficiency. In this study, however, we found that contamination caused by decomposition of a plastic cover film on indium tin oxide (ITO) improved hole injection. Ultraviolet and X-ray photoelectron spectroscopy measurements showed that a contamination layer formed on ITO that had been stored for several months in plastic wrapping, which reduced its work function. As a result, the hole injection barrier between the Fermi level of ITO and the highest occupied molecular orbital level of poly(9-vinylcarbazole) (PVK) was higher than that at the interface between PVK and cleaned ITO. However, in hole-only devices, the contaminated ITO exhibited a higher current density value than the cleaned ITO in the high bias regime. This paradoxical improvement in hole injection could be explained by an insulating buffer model. In this model, the contamination layer functions as an insulating anode buffer layer on the ITO surface. As high bias is applied, the contamination layer results in formation of an energy barrier with a triangular shape. Thus, holes can be injected efficiently from ITO to PVK through tunneling.