The origin of the hole injection improvements at indium tin oxide/molybdenum trioxide/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl- 4,4′-diamine interfaces

We investigated the interfacial electronic structures of indium tin oxide (ITO)/molybdenum trioxide $\left({\text{MoO}}_3\right)/N,N^{\prime }$ -bis(1-naphthyl)-$N,N^{\prime }$ -diphenyl-$1,1^{\prime }$ -biphenyl-$4,4^{\prime }$ -diamine (NPB) using in situ ultraviolet and x-ray photoemission spectroscopy to understand the origin of hole injection improvements in organic light-emitting devices (OLEDs). Inserting a ${\text{MoO}}_3$ layer between ITO and NPB, the hole injection barrier was remarkably reduced. Moreover, a gap state in the band gap of NPB was found which assisted the Ohmic hole injection at the interface. The hole injection barrier lowering and Ohmic injection explain why the OLED in combination with ${\text{MoO}}_3$ showed improved performance.