Two novel bipolar hosts based on 1,2,4-triazole derivatives for highly efficient red phosphorescent OLEDs showing a small efficiency roll-off

Achieving high efficiency and small efficiency roll-off simultaneously for red phosphorescent organic light-emitting diodes (PhOLEDs) is still a challenge, which is largely related to the host material used in device fabrication. In this wok, we designed and synthesized two novel bipolar host materials, termed 2Cz-TAZ-2Cz and 3Cz-TAZ-3Cz in which 1,2,4-triazole was introduced at the C-2 and C-3 positions of 9-phenylcarbazole, respectively. The materials' photophysical properties were studied in detail.. It was found that 2Cz-TAZ-2Cz and 3Cz-TAZ-3Cz possess excellent thermal stability with thermal decomposition temperature (Td) of 457 and 432 °C, respectively, and that 2Cz-TAZ-2Cz has a better bipolar carrier transport compared to the typical host 4, 4′-bis(9H-carbazole-9-yl)-biphenyl (CBP). Based on the excellent performance of these materials, red phosphorescent OLEDs with 2Cz-TAZ-2Cz and 3Cz-TAZ-3Cz as hosts and bis(1-phenylisoquinoline)(acetylacetonate)iridium (III) (Ir(piq)2acac) as emitter were fabricated. The optimized 2Cz-TAZ-2Cz-based device achieved a high maximum current efficiency and external quantum efficiency (EQE) of 12.4 cd/A and 16.60%, respectively. Moreover, this device also exhibits a small efficiency roll-off, i.e., the EQE is lowered by only 12.0% and 29.4% at a brightness of 1000 cd/m2 and 10,000 cd/m2, respectively, which is superior to CBP-based device, indicating a potential far-reaching application.