Carbazole and triazine-based bipolar hosts with extremely low efficiency roll-offs for green PhOLEDs

In this paper, two new bipolar host materials (CZ-p-TRZ and CZ-m-TRZ) which consist of 9-phenylcarbazole as the donor and 2,4,6-triphenyl-1,3,5-triazine as the acceptor were synthesized and characterized. Two compounds exhibit excellent thermal stability and appropriate triplet energy levels (ET). Based on the above advantages, Ir(ppy)3-based green phosphorescent organic light-emitting diodes (PhOLEDs) were fabricated with CZ-p-TRZ and CZ-m-TRZ as host. Both PhOLEDs show good performance with maximum external quantum efficiencies (EQEmax) of 16.83 and 19.40%, respectively. Especially, two devices also exhibit extremely small efficiency roll-off, and at a practical luminance of 1000 cd/m2, only 0.1% drop in efficiency was observed for two devices, which is among the best results ever reported as green bipolar hosts.