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 x 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 x 10(-5) and 4.23 x 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 x 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 x 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.
- HIGH-PERFORMANCE RED
- DELAYED FLUORESCENCE