Highly efficient phosphorescent organic light-emitting diodes with low turn-on voltages using N-phenylcarbazole/pyrimidine-based bipolar host materials

Herein, three novel bipolar host materials with D-π-A structure, termed 2CzPm, 3CzPm, and 9CzPm, were successfully synthesized by incorporating a pyrimidine moiety as an accepter and N-phenylcarbazole group as a donor through different connection sites. The three hosts exhibited good thermal stability with a glass transition temperature (Tg) of 104, 103, and 102 oC, respectively. They also possessed high enough triplet energy level (ET = 2.58–2.75 eV) to be efficient hosts for red and green phosphorescent organic light-emitting diodes (PhOLEDs) (ETdopants <2.5 eV). Moreover, considerable hole and electron current density in the conventional voltage range were observed, suggesting their good bipolar transport properties. When utilized in green and red PhOLEDs, the devices hosted by 9CzPm exhibited the best electroluminescence (EL) performance with low turn-on voltages (Von) of 2.6 and 2.5 V, and maximum external quantum efficiency (EQEmax) of 21.79% and 25.03%, respectively, indicating that pyrimidine moiety is a promising building block for bipolar host materials.