Most phosphorescent devices suffer from severe triplet−triplet annihilation (TTA), and the efficiency recording at high luminance is much lower than that at low luminance, making the practicality worse than expected. In this study, a series of donor−acceptor (D−A) molecules consisting of dicyano-imidazole and phenylcarbazole were synthesized and applied to the host materials. For electroluminescence applications, imM-m-Cz-based green-emitting organic light-emitting diodes show a maximum luminance of 1.68 × 10 5 cd m −2 at 11.2 V, which is 20% higher than that of the benchmark host 4,4′-bis(N-carbazolyl)-1,1′-biphenyl at 13.4 V; in addition, the turn-on voltage (V on ) is only 2.3 V. In terms of hole and electron mobility, imM-m-Cz shows one of the most balanced carrier mobilities in the reported literature (electron and hole mobilities of 3.64 × 10 −5 and 4.23 × 10 −5 cm 2 V −1 s −1 , respectively). The balanced carrier mobility can help expand the recombination region and thus reduce the formation of TTA. Furthermore, the high maximum luminance of about 2.80 × 10 5 cd m −2 obtained in an imM-m-Cz-based tandem device demonstrates a sufficiently high current density/luminance, and the peak efficiency achieves an even higher efficiency of 40.6% (1.51 × 10 5 cd A −1 and 98.6 lm W −1 ), which is among the highest reported properties based on imidazole-based host materials. Combining the balanced carrier mobility with the proper device design in this study, the efficiency roll-off under high luminance can be greatly reduced, and the practicality of the resulting phosphorescent device can be significantly improved.

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