Steric Hindrance Engineering of Multi‐Resonance TADF Emitter Suppresses Aggregation Quenching for High‐Efficiency Narrowband OLEDs at High Doping Levels

ABSTRACT Multiple‐resonance thermal‐activated delayed fluorescence (MR‐TADF) materials have demonstrated significant potential for application in organic light‐emitting diodes (OLEDs) due to their reverse intersystem crossing process and narrowband emission characteristics. However, their typically planar and rigid molecular structures frequently result in severe aggregation‐caused quenching (ACQ), thereby limiting efficiency at high doping levels. Herein, a novel MR‐TADF emitter, HTTM‐BNCz, is designed and synthesized by functionalizing the BNCz core with a bulky tris(2,4,6‐trichlorophenyl)methane (HTTM) group at the boron para‑position. This approach to steric‑hindrance engineering can effectively mitigate intermolecular π ‑‐ π stacking while maintaining the localized excited state, yielding narrow full width at half maximum (FWHM = 26 nm in solution) and high photoluminescence quantum yield in doped films. OLEDs employing HTTM‑BNCz demonstrate remarkable resistance to concentration quenching, with maximum external quantum efficiencies ( η ext,max s) remaining high at 24.0%–27.3% over a wide doping range of 3–30 wt.%. Notably, the device achieves a record‑high power efficiency of 100.4 lm W −1 at 30 wt.% doping among non‐sensitized MR‐TADF‐based OLEDs. Furthermore, by employing a hyperfluorescence structure with a TADF sensitizer, the η ext,max is boosted to 30.1% with a narrower FWHM of 29 nm. This work further demonstrates that steric‑hindrance engineering is a powerful strategy to suppress ACQ and enable high‑performance, high‑doping‑level narrowband OLEDs.