Stable Thermally Activated Delayed Fluorescence‐Sensitized Red Fluorescent Devices through Physical Suppression of Dexter Energy Transfer

Abstract To date, thermally activated delayed fluorescence‐sensitized fluorescent organic light‐emitting diodes (TSF‐OLEDs) have undergone substantial research to achieve high efficiency and good operational stability in wide color gamut regions. Usually, to achieve a highly efficient TSF device, the Förster resonance energy transfer rate ( k FRET ) should be enhanced, whereas the Dexter energy transfer rate ( k DET ) should be suppressed. Even though highly efficient devices are achieved in all RGB color regions by satisfying the BT2020 requirements, achieving long device lifetimes is still challenging. Herein, a highly stable red‐TSF device is reported by adopting a new Dexter energy transfer suppressive layer (DSL) adjacent to the main emissive layer. Here, the DSL can improve the distance between the excitons generated from the host‐TADF layer and the final dopant (FD) of the TSF device, which allows for suppressing the k DET. Furthermore, the detailed device mechanistic pathways are analyzed by varying the DSL doping concentration with different thicknesses in different positions. Among the fabricated devices, the DSL‐TSF device manifested a longer operational lifetime (LT 95 ) over 370 h at 5000 cd m ‐2 and reduced efficiency roll‐off compared with TSF devices. Such long lifetime and high stability in DSL‐TSF OLEDs are owing to the decreased k DET than TSF devices.