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Highly efficient deep-blue-phosphorescent organic light-emitting diodes have been developed using bipolar-type high-triplet-energy host materials with a carbazole core and two phosphine oxide (PPO21) or phosphine sulfide (PPS21) units. A high quantum efficiency of 19.2% with a deep-blue color coordinate of (0.14,0.16) was achieved in the deep-blue device with PPO21 as a host material (see figure).
Jeon et al. (Wed,) studied this question.