Enhancement of efficiency of white organic light-emitting diodes with p-type hole injection layer

Abstract

This study elucidates the optoelectronic properties of high-efficiency p–i–n white organic light-emitting diodes (WOLEDs) with molybdenum oxide (MoO x )-doped 4,4′,4″-tris[2-naphthyl(phenyl)amino] triphenylamine (2-TNATA) as a p-doping hole injection layer ( p-HIL). Devices with 10 wt.% MoO x doping in 2-TNATA had electroluminescent (EL) efficiencies of 7.71 cd/A and 4.32 lm/W, respectively, at a driving voltage of 5.65 V and current density of 20 mA/cm 2. The X-ray photoelectron spectroscopy (XPS) and UV–vis–NIR absorption spectra of MoO x -doped 2-TNATA layers revealed that MoO x -doped 2-TNATA p-HIL had an improvement on hole conductivity. Such the improvement is caused by the formation of the charge transfer (CT) complex (MoO x −/2-TNATA +) that is generated by doping MoO x into 2-TNATA, markedly increasing the number of hole carriers, improving the balance of the electrons and holes in the recombination zone. Additionally, annealing improves the morphological stability of the p-HIL and extends the lifetime of the p–i–n WOLED.