Bipolar Host Material for Phosphorescent OLEDs Based on 2,7‐Diazacarbazole as a New Electron‐transporting Unit

The novel electron‐transporting moiety of 2,7‐diazacarbazole was used to construct a bipolar host molecule for phosphorescent organic light‐emitting diodes (PHOLEDs). This moiety was incorporated with typical hole‐transporting carbazole moieties to synthesize a 2,7‐diazacarbazole/carbazole hybrid called 9‐(3,5‐bis(carbazol‐9‐yl)phenyl)–2,7‐diazacarbazole. The hybrid exhibits a high triplet energy above 2.88 eV and very high glass transition temperature of Tg = 148°C. It shows good thermal stability owing to the rigid nature of its symmetrical bipolar structure. The 2,7‐diazacarbazole‐based host material turns on at 3.6 and 3.7 V and approaches external quantum efficiencies above 16.0 and 22.6% in blue and green PHOLEDs, respectively. These results demonstrate that 2,7‐diazacarbazole should be an excellent electron‐transporting unit for bipolar phosphorescent hosts.