Improved Performance of Organic Light-Emitting Diodes Using a Metal-Phthalocyanine Hole-Injection Layer

In this paper, we systematically investigated the physical characteristics of the various metal phthalocyanines (MPcs) and the influence of the MPcs hole-injection layer on the electroluminescence performance of indium tin oxide/MPc/naphthylphenylbiphenyl diamine (NPB)/ devices. The characteristics were measured at room temperature with a thickness variation of the MPc layer. The individual highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO) energies of MPcs were derived from the photoelectron emission and the optical absorption measurements. The results showed that the HOMO and LUMO level energies of MPcs are dependent on their central metal atoms. The turn-on voltage for the devices is lowered by inserting MPc layers and remains virtually the same as the MPc layer thickness is adjusted in the range of . In addition, the turn-on voltage decreases significantly with the increase of the HOMO levels of the MPc films, demonstrating that the MPc/NPB interface instead of the ITO/MPc interface plays an important role in the hole injection.