Efficient Narrowband Multiple Resonance Thermally Activated Delayed Fluorescent Emitters with Alleviated Efficiency Roll-Off via Steric Shielding and Heavy Atom Effect

Organic light-emitting diodes (OLEDs) based on multiresonance-induced thermally activated delayed fluorescent (MR-TADF) materials are revolutionizing the field of organic electroluminescence and are expected to be the next-generation ultrahigh definition display technology. The rapid reverse intersystem crossing (RISC) process of the TADF emitter is crucial for achieving high-performance OLEDs with low-efficiency roll-off. Here, we present a novel B–N framework-based MR-TADF emitter, Cz-2PTz-BN, which exhibits a narrow full width at half maxima of 37 nm in toluene and a high photoluminescence quantum yield (PLQY) of 93% in a doped film. The introduced sulfur atoms in Cz-2PTz-BN enhance its spin–orbital couplings (SOC) and accelerate the RISC process. The corresponding OLED demonstrates the maximum external quantum efficiency (EQEmax) of 28.7% with low roll-off, maintaining EQEs of 27.2 and 20.8% even at brightnesses of 100 and 1000 cd m–2 without a sensitizer. These outcomes indicate the promise of this molecular design strategy to obtain OLEDs with ultrahigh color purity and efficiency.