Highly Efficient Asymmetric Multiple Resonance Thermally Activated Delayed Fluorescence Emitter with EQE of 32.8 % and Extremely Low Efficiency Roll‐Off

Multiple resonance thermally activated delayed fluorescence (MR‐TADF) emitters show great potentials for high color purity organic light‐emitting diodes (OLEDs). However, the simultaneous realization of high photoluminescence quantum yield (PLQY) and high reverse intersystem crossing rate ( k RISC ) is still a formidable challenge. Herein, a novel asymmetric MR‐TADF emitter (2Cz‐PTZ‐BN) is designed that fully inherits the high PLQY and large k RISC values of the properly selected parent molecules. The resonating extended π‐skeleton with peripheral protection can achieve a high PLQY of 96 % and a fast k RISC of above 1.0×10 5 s −1 , and boost the performance of corresponding pure green devices with an outstanding external quantum efficiency (EQE) of up to 32.8 % without utilizing any sensitizing hosts. Remarkably, the device sufficiently maintains a high EQE exceeding 23 % at a high luminance of 1000 cd m −2 , representing the highest value for reported green MR‐TADF materials at the same luminescence.