Interface electronic structures of organic light-emitting diodes with WO_3 interlayer: A study by photoelectron spectroscopy

摘要

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-l,l'-biphenyl-4,4'-diamine (NPB)/indium tin oxide (ITO) was estimated 1.32 eV, while that with a thin WO_3 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 WO_3 interlayer at 350 ℃, the reduction of hole injection barrier height largely disappears. This is attributed to a chemical modification occurring in the WO_3 such as oxygen vacancy formation.