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

ABSTRACT 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.