Large Steric Hindrance Engineering through 4,4′‐Bicarbazole Derivatives Incorporation: Facilitating Efficient Narrowband Multiple Resonance TADF Emitters with High Doping Conditions

Abstract Organic light‐emitting diodes (OLEDs) employing multiple resonance‐thermally activated delayed fluorescence (MR‐TADF) emitters exhibit high exciton utilization efficiency and outstanding color purity. Nevertheless, typical MR‐TADF emitters possessing well‐developed conjugated planarity often encounter significant aggregation‐induced quenching (ACQ) effect through π–π stacking, which substantially impacts device efficiency and color purity. To address these issues, in this work, large steric hindrance groups 4,4′‐bicarbazole derivatives are incorporated into the MR core (BCz‐BN) to obtain two target molecules, namely BCz‐BNDCzMe and BCz‐BNDCzPh. Compared to BCz‐BN, which exhibits an electroluminescence (EL) emission peak at 488 nm with a full width at half maximum (FWHM) of 30 nm, BCz‐BNDCzMe and BCz‐BNDCzPh demonstrate blue‐shifted emissions peaking at 481–482 nm and featuring narrower FWHM values of 25–26 nm. Remarkably, the maximum external quantum efficiency (EQE max ) of 26.75–28.31% is achieved for BCz‐BNDCzMe and BCz‐BNDCzPh based devices at high doping concentrations (10–20 wt.%). These values are significantly higher than the EQE max of 20.50% for BCz‐BN‐based devices at a 3 wt.% doping level. These findings unequivocally indicate that incorporating a bulky steric hindrance group into the MR‐TADF core efficiently mitigates interchromophore interactions, thereby further suppressing the ACQ effect and enhancing device efficiency at high doping concentrations.