Constructing diazacarbazole-bicarbazole bipolar hybrids by optimizing the linker group for high efficiency, low roll off electrophosphorescent devices

Abstract A series of novel bipolar host materials, 9-(4-(5 H -pyrrolo [2,3- b :4,5- b ′]dipyridin-5-yl)phenyl)-9′-phenyl-9 H ,9′ H -3,3′-bicarbazole ( NCzpPh2Cz ), 9-(6-(5 H -pyrrolo[2,3- b :4,5- b ′]dipyridin-5-yl)pyridin-3-yl)-9′-phenyl-9 H ,9′ H -3,3′-bicarbazole ( NCzpPy2Cz ) and 9-(6-(5 H -pyrrolo[2,3- b :4,5- b ′]dipyridin-5-yl)pyridin -2-yl)-9′-phenyl-9 H ,9′ H -3,3′-bicarbazole ( NCzmPy2Cz ), employing the donor of 9-phenyl-9 H ,9′ H -3,3′-bicarbazole and the acceptor of 1,5-diazacarbazole, have been designed and synthesized. By adjusting the linkers and linkage topologies, the thermal, photophysical, and electrochemical properties can be fine-tuned. All the three hosts showed excellent thermal stability with high T d (>420 °C) and high T g (>148 °C). Among the three host materials, NCzmPy2Cz using a meta- substituted pyridine as the linker outperformed the other two hosts in blue devices. The device based on NCzmPy2Cz exhibited high efficiencies with a η EQE , max of 20.0 %, η C,max of 42.8 cd/A and η P,max of 39.1 lm/W. More importantly, the efficiency roll-off of NCzmPy2Cz -based device was only 2 % even up to brightness of 1000 cd/m 2 . These results demonstrate that engineering the linker between the donor and acceptor could provide an efficient solution to design bipolar host materials for high efficiency, low roll-off blue PhOLEDs.