High-triplet-energy tri-carbazole derivatives as host materials for efficient solution-processed blue phosphorescent devices

A novel series of solution-processible carbazole-based host materials, 3,6-bis(N-carbazolyl)-N-phenylcarbazole (BCC-36), 3,6-bis(3,6-di-tert-butyl-9-carbazolyl)-N-phenylcarbazole (BTCC-36), 2,7-bis(N-carbazolyl)-N-phenylcarbazole (BCC-27), and 2,7-bis(3,6-di-tert-butyl-9-carbazolyl)-N-phenylcarbazole (BTCC-27), is designed and synthesized. Owing to the highly twisted configuration, these hosts exhibit high triplet energy levels (2.90-3.02 eV) and high glass transition temperatures (147-210 degrees C). They also exhibit appropriate HOMO energy levels (-5.21--5.36 eV), resulting in an improved hole-injection property. These novel compounds are employed to fabricate phosphorescent organic lighting-emitting diodes (OLEDs) as the host materials doped with the guests of iridium(III) bis (4,6-difluorophenylpyridinato)-picolinate (FIrpic) and iridium(III) bis(4',6'-difluorophenylpyridinato) tetrakis(1-pyrazolyl)borate (FIr6) by spin coating. The best device performance of FIrpic based blue-emitting devices has a rather low turn-on voltage of 3.9 V, a maximum efficiency of 27.2 cd A(-1) (11.8 lm W-1), and a maximum external quantum efficiency of 14.0%. Moreover, the best device performance of FIr6 based deep-blue-emitting devices exhibits a turn-on voltage of 4.9 V, a maximum efficiency of 11.5 cd A(-1) (4.9 lm W-1), and a maximum external quantum efficiency 6.8%. The performance data are outstanding for solution-processed blue phosphorescent OLEDs.