The Combination of a Donor–Acceptor TADF and a MR‐TADF Emitting Core Results in Outstanding Electroluminescence Performance

Abstract Here the utility and potential of an emitter design are demonstrated, consisting of a narrowband‐emitting multiresonant thermally activated delayed fluorescent (MR‐TADF) core that is decorated with a suitably higher energy donor‐acceptor TADF moiety. Not only does this D–A TADF group offer additional channels for triplet exciton harvesting and confers faster reverse intersystem crossing (RISC) kinetics but it also acts as a steric shield, insulating the emissive MR‐TADF core from aggregation‐caused quenching. Two emitters, DtCzBN‐CNBT1 and DtCzBN‐CNBT2 , demonstrate enhanced photophysical properties leading to outstanding performance of the organic light‐emitting diodes (OLEDs). DtCzBN‐CNBT2 , containing a D–A TADF moiety, has a faster k RISC (1.1 × 10 5 s −1 ) and higher photoluminescence quantum yield ( Φ PL : 97%) compared to DtCzBN‐CNBT1 (0.2 × 10 5 s −1 , Φ PL : 90%), which contains a D–A moiety that itself is not TADF. The sensitizer‐free OLEDs with DtCzBN‐CNBT2 achieve a record‐high maximum external quantum efficiency (EQE max ) of 40.2% and showed milder efficiency roll‐off (EQE 1000 of 20.7%) compared to the DtCzBN‐CNBT1 ‐based devices (EQE max of 37.1% and EQE 1000 of 11.9%).