The Combination of a Donor-Acceptor TADF and a MR-TADF Emitting Core Results in Outstanding Electroluminescence...

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 × 10⁵ s^-1) and higher photoluminescence quantum yield (Φ_PL: 97%) compared to DtCzBN-CNBT1 (0.2 × 10⁵ 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 (EQE₁₀₀₀ of 20.7%) compared to the DtCzBN-CNBT1-based devices (EQE_max of 37.1% and EQE₁₀₀₀ of 11.9%).