Work Function Modification of Indium–Tin Oxide by Surface Plasma Treatments Using Different Gases

We report on the effects of surface treatment with N2, O2, and N2O plasmas on the work function of indium–tin oxide (ITO). UV photoelectron spectroscopy (UPS) showed that the work function on the ITO surface treated with N2O plasma increased more than that on the samples treated with N2 or O2 plasma. X-ray photoelectron spectroscopy (XPS) showed that the intensity of the O–O bonding peak at 532.3 eV markedly increased owing to the adsorption of O- ions on the ITO surface from breaking bonds in N2O gas by the plasma. The dipole layer formed by O- ions on the ITO surface increases the work function of ITO. Accordingly, N2O plasma treatment leads to a reduction of the potential barrier between the Fermi level of ITO and the highest occupied molecular orbital (HOMO) level of an organic layer when ITO is used as an anode for organic light-emitting devices (OLEDs) and related devices. Therefore, N2O plasma treatment enhances the hole-injection properties from the ITO thin film to the organic layer.