Interface electronic structures of organic light-emitting diodes with WO3 interlayer: A study by photoelectron spectroscopy

The energy level alignment and chemical reaction at the interface between the hole injection and transport layers in an organic light-emitting diode (OLED) structure has been studied using in-situ X-ray and ultraviolet photoelectron spectroscopy. The hole injection barrier measured by the positions of the highest occupied molecular orbital (HOMO) for N,N'-bis(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB)/indium tin oxide (ITO) was estimated 1.32 eV, while that with a thin WO3 layer inserted between the NPB and ITO was significantly lowered to 0.46 eV. This barrier height reduction is followed by a large work function change which is likely due to the formation of new interface dipole. Upon annealing the WO3 interlayer at 350 degrees C, the reduction of hole injection barrier height largely disappears. This is attributed to a chemical modification occurring in the WO3 such as oxygen vacancy formation. (C) 2009 Elsevier B.V. All rights reserved.