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

Abstract 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.