Towards High Efficiency and Low Roll‐Off Orange Electrophosphorescent Devices by Fine Tuning Singlet and Triplet Energies of Bipolar Hosts Based on Indolocarbazole/1, 3, 5‐Triazine Hybrids

Orange‐emitting phosphorescent organic light‐emitting diodes (PHOLEDs) are drawing more and more attention; however, high‐performance hosts designed for orange PHOLEDs are rare. Here, four indolocarbazole/1, 3, 5‐triazine hybrids are synthesized to optimize the singlet and triplet energies, as well as transporting properties, for ideal orange PHOLEDs. By introducing moieties with different electronegativity, a graded reduction of the singlet and triplet energies is achieved, resulting in minimum injection barrier and minimum energy loss. Besides, the charge transporting abilities are also tuned to be balanced on the basis of the bipolar features of those materials. The optimized orange PHOLED shows a maximum external quantum efficiency (EQE) of 24.5% and a power efficiency of 64 lm W –1 , both of which are among the best values for orange PHOLEDs. What is more, the efficiency roll‐off is extremely small, with an EQE of 24.4% at 1000 cd m –2 and 23.8% at 10 000 cd m –2 , respectively, which is the lowest efficiency roll‐off for orange PHOLEDs to date, resulting in the highest EQE for orange PHOLEDs when the luminance is above 1000 cd m –2 . Besides the balanced charges, the small roll‐off is also attributed to the wide recombination zone resulting from the bipolar features of the hosts.