Pure-green OLEDs with CIEy over 0.7 via employing multi-resonance thermally activated delayed fluorescence...

Multiple-resonance thermally activated delayed fluorescence (MR-TADF) materials have emerged as key candidates for next-generation ultra-high-definition (UHD) display organic light-emitting diodes (OLEDs) owing to their narrowband emission and high efficiency. Due to their long delayed fluorescence lifetimes, most MR-TADF devices suffer from severe efficiency roll-off at high brightness. Incorporating sensitizers into the emissive layer has been proven to be the most feasible and effective solution. Nevertheless, many sensitized devices suffer from incomplete energy transfer, which compromises the color purity of electroluminescence. Herein, we propose an ultra-low concentration doping MR-TADF-sensitized fluorescence (MR-TSF) strategy. Three new MR-TADF sensitizers exhibit reverse intersystem crossing rates (k _RISC) of 1.42-1.58 × 10⁵ s^-1, along with radiative decay rates (k _r) exceeding 10⁸ s^-1, endowing them with nearly unity photoluminescence quantum yields (PLQYs). When the doping concentration of the sensitizer is only 3.0 wt%, the external quantum efficiency (EQE) roll-offs of the sensitized devices at 1000 cd m^-2 are reduced from the original values of 49.2%, 40.7%, and 40.7% to 22.5%, 23.2%, and 23.1%, respectively. Meanwhile, the MR-TSF devices have a minimal impact on the intrinsic color purity of the emitters.