Effect of host materials on eelectrophosphorescence properties of PtOEP-doped organic light-emitting diodes

Abstract

We have studied the effect of host materials on the electrophosphorescence properties by comparing three different host materials such as tris(8-hydroxyquinoline)-aluminum (III) (Alq3), bis(8-hydroxyquinoline)-zinc (II) (Znq2), and 4,4'-N, N'-dicarbazole-biphenyl (CBP) doped with a red-emissive phosphorescent dye, 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum (II) (PtOEP). The EL spectra show a strong red emission (peak at 650 nm) from the triplet excited state of PtOEP and a very weak emission from an electron transport layer of Alq3 and a hole transport layer of N, N'-diphenyl-N, N'-bis(3-methylphenyl)-1,1-biphenyl-4,4'-diamine (TPD). We find that the triplet exciton lifetime and the quantum efficiency decrease in the order of CBP, Alq3, and Znq2 host materials. The results are interpreted as a poor exciton confinement in Alq3, and Znq2 host compared with in CBP. Therefore, it is very important for the triplet-exciton confinement in the emissive layer for obtaining a high efficiency. © 2007 Taylor & Francis Group, LLC.