Hole-injection enhancement by copper phthalocyanine (CuPc) in blue polymer light-emitting diodes

This article reports the study on enhancement of copper phthalocyanine (CuPc) for hole injection in blue polymer light-emitting diodes based on three blue emissive polymeric materials with different electronic structures under the circumstances of including or excluding an additional poly(N-vinylcarbazole) (PVK) layer. A thin CuPc layer may effectively enhance the hole injection from the anode to the emissive polymer layer and results in a dramatic decrease of operating voltage of the device. The thickness of the CuPc layer is optimized. The addition of a PVK layer between CuPc and emissive polymer may prevent the potential problem of the undesired electron flow from the emissive layer to the CuPc layer. The efficiency of CuPc for enhancing hole injection is almost independent of the energy levels of the highest occupied molecular orbitals of the polymers which are contacted with it. It is suggested that the higher affinity between the two organic materials in comparison with that between indium-tin-oxide and an organic material and the resulted rough interface play a dominated role in the enhancement for hole injection.