A novel bipolar host material based on carbazole and 1,3,5-triazine with an extremely low efficiency roll-off for green PhOLEDs
Abstract: In this contribution, a novel bipolar host material, namely CzTrz, consisting of a phenylcarbazole for hole transport and a diphenyltriazine for electron transport, was synthesized and investigated. Besides, a carbazole-based unipolar host BCz2 was designed as a reference. Both CzTrz and BCz2 display good thermal stability, high triplet energy level (ET) and suitable frontier orbital levels. Green PhOLEDs hosted by CzTrz and BCz2 show turn-on voltage (Von) of 3.5 and 3.8 V, maximum luminance (Lmax) of 30,280 and 35,050 cd m−2, and maximum external quantum efficiency (EQEmax) of 15.49 and 11.84%, respectively. Remarkably, the CzTrz-based device reveals an extremely low efficiency roll-off at the high luminance, for example, 15.35% at 1000 cd m−2 and 13.85% at 5000 cd m−2, corresponding to merely 0.9% and 10.6% roll-off percentage, which is among the best results ever reported as green bipolar hosts.
Keywords: Organic light-emitting diodes (OLEDs) / Host material / Carbazole / 5-Triazine / Low efficiency roll-off
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