Effect of mixed hole transporting host on the mobility, Gaussian density of states and efficiencies of a heterojunction phosphorescent organic light emitting diode

摘要

We present an in-depth study of the hole transport in poly(vinylcarbazole) PVK films blended with small molecule tris(4-carbazoyl-9-ylphenyl) amine (TcTa). Doping TcTa in PVK introduces shallow hole traps when the doping concentration is lower than 20 wt%. It becomes percolative at higher concentrations. The energetic disorder sigma of the blended system reduces from similar to 72 meV at 0 wt% TcTa to similar to 41 meV at 50 wt% TcTa. A correlation between sigma and the film morphologies suggests that the blending of TcTa molecules in the film does not only change the film homogeneity and roughness but also the energetic disorder. In addition to the mobility study, we fabricated a red phosphorescent organic light emitting diode with the same blending system. By doping merely 5 wt% of TcTa into PVK as mixed hole-transporting hosts, the efficiency of the deep red heterojunction phosphorescent organic light emitting diode increased from 2 cd A(-1) to 4 cd A(-1), suggesting that TcTa molecules assist in hole injection.