Highly twisted biphenyl-linked carbazole-benzimidazole hybrid bipolar host materials for efficient PhOLEDs

Three new highly twisted bipolar host materials (PhczNBI, PhczCBI, and PhczDCBI) comprising of a biphenyl core linking a hole-transport 9-phenylcarbazole (donor) and an electron-transport N-phenylbenzimidazole (acceptor) with different topologies were synthesized, characterized, and applied as host materials for full-color phosphorescent OLEDs. The highly twisted conformations imparted through a 2,2'-biphenyl linkage effectively disrupt the p-conjugation and electronic coupling between the donor and the acceptor, thus giving the bipolar hosts promising properties such as bipolar charge transport features (mu(h) approximate to mu(e) approximate to 10(-6) to 10(-4) cm(2) V-1 s(-1)) and high triplet energies (E-T = 2.55-2.71 eV). The PhOLED device gave a maximum external quantum efficiency (eta(ext)) of 19.6% for the PhczNBI host with a yellow phosphor (Bt)(2)Ir(acac). A two-color, single-emitting-layer and all-phosphor WOLED hosted by PhczDCBI achieved high efficiencies (13.9%, 34.9 cd A(-1), and 24.8 lm W-1) and a CRI up to 65.5.