Current efficiency in organic light-emitting diodes with a hole-injection layer

We have systematically investigated the effect of layer structures on the current efficiency of prototypical hole-injection layer (HIL)/hole-transport layer (HTL)/electron-transport layer (ETL) organic light-emitting diodes based on 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (MTDATA) as the HIL, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB) as the HTL, and tris(8-quinolinolato)aluminum (Alq) as the ETL. With bilayer devices, the current efficiency is limited by exciplex emissions in the case of MTDATA/Alq and quenching of Alq emissions by NPB+ radical cations in NPB/Alq. The improved current efficiency in trilayer MTDATA/NPB/Alq devices can be attributed to a reduction in NPB+ radical cations at the NPB/Alq interface and a strong electric field in the NPB layer.