Novel bipolar host materials including carbazole and dioxy[2,3-b] pyrazine units for red phosphorescent organic light-emitting diodes

In this work, two novel bipolar host materials, 3,5-bis(9H-carbazol-9-yl) phenyl) benzo [1,4,5,6] dioxy [2,3-b] pyrazine (a-CzDp) and 7-(9-phenyl-9H-[3,9′- bicarbazole]-3′-yl) benzo [1,4,5,6] dioxy [2,3-b] pyrazine (b-CzDp) were designed and synthesized. In these two host materials, carbazole and dioxy [2,3-b] pyrazine act as the hole and electron transport unit, respectively. The two dipolar hosts both exhibit good thermal stability and deep highest occupied molecular orbital (HOMO) energy levels of -5.31 eV and −5.39 eV. The phosphorescent organic light-emitting diodes (PhOLEDs) including a-CzDp and b-CzDp as host and red phosphorescent material bis(1-phenylisoquinoline)(acetylacetonate)iridium(III) (Ir(piq)2acac) as guest in the emitting layer. The corresponding PhOLED devices with a-CzDp and b-CzDp as host exhibit the maximum brightness of 8932 cd m−2 and 12782 cd m−2 at 624 nm, together with the maximum external quantum efficiency (EQE) of 10.32% and 13.82%, respectively. B-CzDp based red PhOLED device shows higher EQE and lower turn-on voltage due to the more balanced hole and electron transport. The results demonstrate the potential of a-CzDp and b-CzDp as efficient host for red PhOLEDs.