Highly Efficient Green Multi‐Resonance Thermally Activated Delayed Fluorescence Emitters with Suppression of Concentration Quenching

Abstract High‐efficiency multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters with narrowband emission show great potential for organic light‐emitting diodes (OLEDs). However, their inherent planar rigid structures often lead to intractable challenges of spectral broadening, self‐quenching, and low device efficiency at high dopant concentrations. Herein, two steric isomers, BN‐1TPh and BN‐2TPh, are designed by incorporating bulky shielding unit (1,3,5‐triphenylbenzene) at the para‐ position of the B atom in the MR skeleton to hinder intermolecular interactions. They both show enhanced photoluminescence quantum yields (PLQYs) as compared with the model compound BCzBN. The corresponding OLEDs based on BN‐1TPh and BN‐2TPh display the maximum external quantum efficiency (EQE max ) values of up to 30.8% and 30.4% with narrow full width at half maximum (FWHM) bands of 27 and 28 nm, respectively. It is worth noting that even at the high doping ratio of 20%, the EQEs are still maintained 24.8% and 25.7% with almost unchanged emission spectra. These results show that segregating the planar MR‐TADF skeleton with spatial shielding structure can weaken the intermolecular interaction, which is one of the effective ways to resist the aggregation‐caused quenching effect and achieve high‐efficiency concentration‐indispensible MR‐TADF OLEDs.