Beschreibung:
<jats:title>Abstract</jats:title><jats:p>Multi‐resonance (MR) based organoboron emitters exhibiting high‐efficiency narrowband thermally activated delayed fluorescence (TADF) have become a critical material component for constructing high‐performance organic light‐emitting diodes (OLEDs) with high color purity and high color gamut. However, most of the MR‐TADF devices suffer from severe efficiency roll‐off at high current density due to the relatively large singlet–triplet splitting energy, long excited state lifetime, and slow reverse intersystem crossing rate (<jats:italic>k</jats:italic>
<jats:sub>RISC</jats:sub>). Herein, by utilizing a sulfur atom‐fused donor unit, 7<jats:italic>H</jats:italic>‐benzo[4,5]thieno[2,3‐b]carbazole, an organic boron emitter (BTC‐BNCz) capable of high efficiency and low efficiency roll‐off narrowband TADF emission is developed. Benefitting from the heavy atom effect of the sulfur atom, <jats:italic>k</jats:italic><jats:sub>RISC</jats:sub> up to 1.6 × 10<jats:sup>5</jats:sup> s<jats:sup>−1</jats:sup> is achieved, which could greatly reduce the undesired quenching effect. Consequently, the OLED using BTC‐BNCz as dopant exhibits high external quantum efficiency (EQE) of 27.0% with full width at half maximum (FWHM) of 35 nm. Moreover, by utilizing a TADF material with a high horizontal dipole ratio as assistant host, outstanding device performance is achieved with a maximum EQE of 35.2% and FWHM of 37 nm, featuring slow efficiency roll‐off.</jats:p>