Alleviated Efficiency Roll-off in Organic Light-Emitting Diode through Optimizing Peripheral Substituents in Multiple-Resonance Thermally Activated Delayed Fluorescence Material

Multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters have shown great potential for ultrahigh-definition organic light-emitting diode (OLED), owing to their high emission efficiency and distinctive narrowband spectra. However, their electroluminescent performance often suffers from significant efficiency roll-off at high luminance due to the slow reverse intersystem crossing rate. Based on the prototypal MR skeleton, the integration of electron donor does not exhibit long-range charge transfer nature, which cannot achieve significant regulation in luminous properties. Herein, by grafting the electron acceptor, the MR-TADF emitter demonstrates hybridized transition characteristic of short-range and long-range charge transfer, leading to enhanced spin-orbit coupling and redshifted emission without spectral broadening. Consequently, the emitter exhibits pure green narrowband emission, featuring a high quantum yield of 88%. In OLED devices, it achieves a maximum external quantum efficiency (EQE) of 22.2%. Meanwhile, the phosphor sensitized devices show an EQE of 26.3% with minimal efficiency roll-off. Notably, they maintain EQE values at a top-tier level of 25.2%, 20.2%, and 18.0% at high luminance of 1000, 5000, and 10 000 cd m⁻², respectively. This work demonstrates the peripheral substituent's influence on luminous properties and presents a feasible approach to alleviate the efficiency roll-off issues for MR-TADF emitters in OLED.