The roles of thermally activated delayed fluorescence sensitizers for efficient red fluorescent organic light-emitting diodes with D-A-A type emitters

A thermally activated delayed fluorescence (TADF) sensitizer plays a key role in harvesting 100% excitons and transferring energy in TADF-assisted fluorescent organic light-emitting diodes (TAF-OLEDs). Despite its important roles, there have been few studies investigating how a TADF sensitizer affects the overall device performance and resonance energy transfer (RET) process in TAF-OLEDs. In this study, we investigate the effects of different parameters of sensitizers, i.e., spectra overlap and radiative constant (k(r)) on the device performances and RET process by developing two red emitters (OTPA-BT-CN and POZ-BT-CN) and choosing two TADF materials with similar photoluminescence spectra (4CzIPN and OSTFB). Mechanism studies reveal that k(r) of the sensitizer, rather than the overlap of sensitizer and fluorescent emitter, plays a more significant role in TAF-OLEDs. As a result, the device with higher k(r) of the sensitizer achieves a maximum external quantum efficiency (EQE) of 12.4% with an emission peak of 612 nm, which is higher by 2.0-fold compared with that of a 4CzIPN-based device (6.3%).