Phosphorescence of red Os ( fptz ) 2 ( P Ph 2 Me ) 2 doped organic light-emitting devices with n and p hosts

We have applied a stericly-hindered red phosphorescent dopant Os ( fptz ) 2 ( P Ph 2 Me ) 2 [ fptz = 3 -trifluoromethyl-5-(2-pyridyl)-1,2,4-triazole, P Ph 2 Me = phosphine ligand] in p -type 4, 4 ′ - N , N ′ -dicarbazole-biphenyl or n -type bis (2-methyl-8-quinolinolato)( p -phenylphenolato) aluminum host and found that the latter produced higher luminance efficiency at lower doping concentration. We present a model to rationalize this phenomenon in which the n -type host impedes hole transport, which leads to narrower recombination zone near the hole transport layer/emission layer interface than the p -type host, hence, more effective recombination.