Rationally Designed Multi-Resonance Emitters Achieving >42% EQE in Ultra-Green OLEDs

Achieving ultra-high-definition green emitters that meet the ultrahigh definition display standard remains a major challenge in organic electronics. A key limitation arises from the difficulty of reconciling narrow emission with efficient exciton harvesting and high light-outcoupling efficiency. Here, we introduce a molecular design strategy that employs a fluorene bridge to rigidly lock the bay region of a BNCz-based emitter, while a planar electron-rich N-phenyl-carbazol-3-yl group enforces near-parallel alignment with the emissive plane. This architecture stabilizes the rigid core, promotes horizontal dipole orientation, and triggers through-space charge transfer to generate high-lying excited states, accelerating reverse intersystem crossing and enhancing exciton utilization. Steric shielding suppresses aggregation and quenching, maintaining a high photoluminescence quantum yield. The resulting emitter delivers ultragreen emission with Commission Internationale de l'Éclairage coordinates approaching the BT.2020 standard and a maximum external quantum efficiency of 39.6%. When combined with a thermally activated delayed fluorescence sensitizer, the device achieves a record 42.2% external quantum efficiency with suppressed roll-off (25.5% at 1000 cd m^-2). This work establishes a design principle for simultaneously optimizing emission color, exciton harvesting, and light outcoupling in high-resolution organic light-emitting diodes.