Efficient Narrowband\nMultiple Resonance Thermally\nActivated Delayed Fluorescent Emitters with Alleviated Efficiency\nRoll-Off via Steric Shielding and Heavy Atom Effect

Organic light-emitting diodes (OLEDs) based on multiresonance-induced\nthermally activated delayed fluorescent (MR-TADF) materials are revolutionizing\nthe field of organic electroluminescence and are expected to be the\nnext-generation ultrahigh definition display technology. The rapid\nreverse intersystem crossing (RISC) process of the TADF emitter is\ncrucial for achieving high-performance OLEDs with low-efficiency roll-off.\nHere, we present a novel B–N framework-based MR-TADF emitter,\nCz-2PTz-BN, which exhibits a narrow full width at half maxima of 37\nnm in toluene and a high photoluminescence quantum yield (PLQY) of\n93% in a doped film. The introduced sulfur atoms in Cz-2PTz-BN enhance\nits spin–orbital couplings (SOC) and accelerate the RISC process.\nThe corresponding OLED demonstrates the maximum external quantum efficiency\n(EQE<sub>max</sub>) of 28.7% with low roll-off, maintaining EQEs of\n27.2 and 20.8% even at brightnesses of 100 and 1000 cd m<sup>–2</sup> without a sensitizer. These outcomes indicate the promise of this\nmolecular design strategy to obtain OLEDs with ultrahigh color purity\nand efficiency.