Work function modification of indium–tin–oxide used in organic light emitting devices

We report measurements on the modification of the work function (WF) of indium–tin–oxide (ITO) by deposition of Au and Pt overlayers, chosen because of their high WF and inertness. ITO films on glass substrates (resistance∼10 Ω/sq) were cleaned with an ultraviolet (UV)–ozone treatment before entry into an ultrahigh vacuum system. Auger electron spectroscopy was used to determine the surface composition. The change of WF of the surface (±10 mV) was determined by a Kelvin probe technique and correlated with the absolute coverage (in the range 0<θ<2×1016 Au(Pt) cm−2 calibrated by Rutherford backscattering). The topography of the films was determined by atomic force microscopy. The original ITO surface contains grains of about 50–60 nm in size. Initially, low coverages of gold atoms fill the “valleys” between the bumps formed by ITO grains. UV–ozone cleaned surfaces exhibited the highest WF. Surprisingly, in sequential Au deposition experiments, the WF decreased to a minimum at ∼1×1015 Au cm−2 before increasing again and becoming constant at θ>5×1015 Au cm−2. The final WF values were not very different from the non-UV–ozone treated samples. Studies on bulk Au and ITO covered by Au in a single deposition, showed that the low final WF value in the sequential experiments was attributable to adsorption of hydrocarbons from the 1×10−10 Torr vacuum, and that useful increases in WF are attainable on ITO electrodes with gold layers which are thin enough to cause no obvious change in ITO’s transparency (⩽10%). A similar trend was observed for the Pt layer but with less sensitivity to hydrocarbon adsorption.