Tuning the Exciton Behavior of TADF-based Assistant Dopants via Distortion of the Central Axis of Donor Moiety

Abstract

OLEDs with an emitting layer consisting of a host medium, TADF assistant dopants, and phosphorescent emitters (end dopants) have recently garnered considerable interest because they can lead to high efficiency even with emitter concentration as small as a fraction of a percent. In these systems, promoting the reverse intersystem crossing (RISC) of TADF assistant dopants has generally been regarded as important in assuring high efficiency and reduced roll-off behavior. However, we here report that TADF system with intermediate RISC may still be all right provided that one can utilize dexter energy transfer (DET) channel between the TADF assistant dopants and the end dopants. To prove this notion, we first showed that the RISC can be critically varied upon distortion of the central axis of the donor moiety in a TADF system. In this way, we prepare a series of benzonitrile (BN) based TADF molecules with different RISC values but with everything else kept almost equivalent. A comparison study done with those TADF assistant dopants reveals that the DET type channel can still play an important role in shortening the overall triplet exciton residence in the EL system, making it possible to achieve the high efficiency as well as alleviation of roll-off behavior. The discussions in the proposed triplet-harvest system is supported by transient exciton dynamics experiments and the quantification of roll-off at the steady-state condition.