Organic electrophosphorescent devices with mixed hole transport material as emission layer

We report on the reduction of driving voltage in organic electrophosphorescent devices with mixed hole transport material as emission layer. The emission layer was mixed with N,N′-diphenyl-N,N′-(3-methylphenyl) -[1,1′-biphenyl]-4,4′-diamine (TPD) as a hole-transport material, 4,4′-bis(9-carbazolyl)-biphenyl (CBP) as a host material for a phosphorescent material, tris(2-phenylpyridine) iridium (Ir(ppy)3) as a phosphorescent material and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (bathocuproine: BCP) as an electron-transport material. The emission occurred from Ir(ppy)3 and was independent on the ratio of TPD and CBP. For triple-layered device with the TPD:CBP mixing ratio of 10:90, maximum luminance obtained was 104,000 cd/m2 at V=12 V, J=490 mA/cm2. Maximum efficiency was 20 lm/W at 3.1 mA/cm2.