Abstract
Red phosphorescent organic light‐emitting diodes (phOLEDs) can achieve 100% internal quantum efficiency by harvesting both singlet and triplet excitons through spin orbit coupling. To improve the external quantum efficiency (EQE) and device lifetime, many molecular designs of bipolar hosts with appropriate energy band gap and triplet energy were studied in doping system phOLEDs. Two kinds of bipolar hosts were synthesized by combining benzocarbazole hole type donor and diphenyltriazine electron type acceptor to achieve relatively low triplet energy for red phOLEDs. Two hosts with the benzocarbazole and diphenyltriazine linked through para and meta positions of the phenyl unit proved a triplet energy of 2.52 eV and bipolar carrier transport properties. The band gap of the emitters between highest energy occupied molecular orbital (HOMO) and lowest energy occupied molecular orbital (LUMO) is 2.89 eV and 2.83 eV, while the EQE was 21.7% and 20.7% respectively, as a result of doping with red emitter. It was confirmed that life time was 30 times longer than that of general 4,4′‐bis(n‐carbazolyl)‐1,1′‐biphenyl (CBP) host at 5000 cd/m2.
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