Organic light-emitting diodes (OLEDs) have been successfully applied as displays and also recognized as a next-generation lighting technology because their several advantages such as self-emission, high luminous efficiency, full-color capability, wide viewing angle, high contrast, low power consumption, low weight, large area manufacture, transparence and flexibility. Now, the red and green phosphorescent OLEDs are qualified for the commercial products. The bottleneck still is the blue OLED. Currently, the high efficiency (100% internal quantum efficiency, and over 20% external quantum efficiency, EQE) blue OLED can be achieved using phosphorescent and thermal activated delay fluorescent (TADF) emitters, which were the crucial factor to determine color purity, efficiency, and lifetime of device. Beside the emitters, the other important factor is the host material.
In this talk, we synthesized several novel carbazole-based, benzimidazole-based, and their combined derivers to be the universal hosts with wide energy bandgap for red, green, blue phosphorescent emitters and also applied for green and blue TADF emitters. These novel host materials could be classified as electron transport, hole transport, and bipolar. The carrier dynamics in OLEDs with these host materials were investigated by probing the position of main recombination zone and optimizing the efficiency performance. With increasing electron mobility of host material, the main carrier recombination zone in emitting layer was moved from the ETL side to the HTL side. Finally, the high EQE of over 30% was achieved in green and blue phosphorescent and TADF OLEDs with our host system. In addition, the operational lifetime was also improved by using our host system, comparing that of OLED with commercial mCP host.