Interface electronic structures of organic light-emitting diodes with WO₃ interlayer

Abstract

The energy level alignment and chemical reaction at the interface between the hole injection and transport layers in an OLED structure have 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 $WO_3$ 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.