Steric Encapsulation of Multi-Resonance TADF Emitters Enabling Narrowband Deep-Blue OLEDs with High Efficiency and High Brightness at Elevated Doping Levels

• • QNa-BN, with a fully encapsulated MR core, maintains photoluminescence quantum yields exceeding 96% even at elevated doping concentrations, directly addressing aggregation-caused quenching that typically limits efficiency in solid-state devices. • • Sensitizer-free OLEDs based on QNa-BN achieve a maximum external quantum efficiency (EQE_max) of 34.4% with deep-blue emission at 458 nm and a full width at half maximum of only 22 nm, meeting stringent color purity requirements for ultra-high-definition displays. • • The hyperfluorescence architecture further boosts EQE_max to 38.7% while significantly suppressing efficiency roll-off, demonstrating a viable path to high-brightness operation exceeding 20,000 cd m−2 without severe efficiency loss. • • The steric encapsulation strategy preserves fast radiative decay rates (order of 10^8 s−1) and reverse intersystem crossing rates (~10^5 s−1), ensuring efficient exciton utilization and fast delayed fluorescence, critical for reducing efficiency roll-off at high current densities.