Benzimidazole/carbazole-based bipolar host materials for highly efficient green phosphorescent OLEDs with negligible efficiency roll-off

Host materials which can restrain the detrimental and intrinsic triplet-triplet annihilation play a critical role in the development of highly efficient and stable phosphorescent organic light-emitting diodes (OLEDs). Two benzimidazole-carbazole hybrid bipolar host materials 9,9'-(4-(1-phenyl-1H-benzo[d]imidazole-2-yl)-1,3-phe-nylene)bis(9H-carbazole) (BCzBI) and 9,9'-(3-(1-phenyl-1H-benzo[d]imidazole-2-yl)pyridine-2,6-diyl)bis(9H- carbazole) (N-BCzBI) via subtly modulating the ratio of donor/acceptor were designed and synthesized. The electron-withdrawing pyridine group was introduced with the expectation to promote electron-injecting/-transporting ability for N-BCzBI. In comparison with BCzBI, N-BCzBI showed high thermostability (397 versus 378 degrees C), obviously lowered LUMO level and almost unchanged triplet energy level and energy bandgap (2.49/ 2.46 eV; 3.45/3.33 eV). Green phosphorescent OLEDs adopting BCzBI/N-BCzBI as host were investigated and exhibited excellent electrophophorescent performances. More interestingly, the N-BCzBI-hosted green phos-phorescent device demonstrated better performance of 24.6% (the maximum EQE), 88.5 cd A-1 (the maximum current efficiency) and 78.5 lm W-1 (the maximum power efficiency). Notably, the efficiencies of N-BCzBI-hosted OLEDs remained to be as high as 24.4% and 24.2% even at high luminance of 1000 and 5000 cd m- 2. This work eloquently indicates that rational regulation on carrier-transporting characteristics of host materials plays an important role in enhancing phosphorescent OLEDs performances.