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

A series of novel bipolar host materials, 9-(4-(5H-pyrrolo [2,3-b:4,5-b']dipyridin-5-yl)pheny1)-9'-phenyl-9H,9'H-3,3'-bicarbazole (NCzpPh2Cz), 9-(6-(5H-pyrrolo[2,3-b:4,5-b']dipyridin-5-yl)pyridin-3-yl)-9'phenyl-9H,9'H-3,3'-bicarbazole (NCzpPy2Cz) and 9-(6-(5H-pyrrolo[2,3-b:4,5-b']dipyridin-5-yl)pyridin -2-y1)-9'-phenyl-9H,9'H-3,3'-bicarbazole (NCzmPy2Cz), employing the donor of 9-phenyl-9H,9'H-3,3-'bicarbazole and the acceptor of 1,5-diazacarb.azole; have been designed and synthesized. By adjusting the linkers and linkage topologies, the thermal, photophysical, and electrochemical properties can be finetuned. All the three hosts showed excellent thermal stability with high T-d (>420 degrees C) and high T-g (>148 degrees 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 eta(EQE,max) of 20.0 %, eta(C,max) of 42.8 cd/A and eta(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. (C) 2016 Published by Elsevier Ltd.