Phenylbenzimidazole-Based New Bipolar Host Materials for Efficient Phosphorescent Organic Light-Emitting Diodes

Two new bipolar host materials based on a 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) core with the carbazole and diphenylamine groups were designed, synthesized, and applied in phosphorescent organic light-emitting diodes (PhOLEDs). The DFT calculations indicated desirable distribution of HOMO and LUMO densities, suggesting potential for bipolar charge transport. In addition, the electrochemical and phosphorescence studies revealed that neither the LUMO level nor the triplet energies differ significantly from the parent TPBI suggesting that the new materials would be suitable as hosts capable of both electron and hole transport and suitable for harvesting green electrophosphorescence. As a result of broader charge recombination zone resulting from the bipolar properties of new hosts, the devices with a simple architecture achieved significantly better current efficiencies of 48 and 60 cd/A (the power efficiencies of 46 and 70 lm/W), compared with a device using conventional TPBI host (19 cd/A; 21 lm/W) as well as a more complex device utilizing 4,4',4 ''-tris(N-carbazolyl)triphenylamine (TCTA) as an exciton blocking layer.