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

Abstract

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%).