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.