Efficient Cyan Delayed Fluorescence Luminogen as Sensitizing Host for OLEDs with High Efficiencies and Extremely Low Roll‐Offs

Organic light‐emitting diodes (OLEDs) using thermally activated delayed fluorescence (TADF) materials to sensitizing conventional fluorescence dopants (CFDs) have attracted intensive research interest due to the extraordinary device performances. Herein, a cyan luminogen (TCz‐BP‐SFAC) with TADF and aggregation‐enhanced emission (AEE) character is developed as a sensitizing host for CFDs. TCz‐BP‐SFAC owns excellent thermal and electrochemical stabilities, prefers horizontal dipole orientation and demonstrates violent delayed fluorescence with high photoluminescence quantum yields in neat and doped films. It exhibits preeminent electroluminescence (EL) performances with maximum external quantum efficiencies (ηexts) of 25.1% and 30.0% and very low efficiency roll‐offs. TCz‐BP‐SFAC also performs well as a sensitizing host for CFDs of different colors. When TCz‐BP‐SFAC sensitizing green emitter TTPA and orange emitter TBRB, the devices achieve high ηexts of 16.9% and 17.1% as well as very low efficiency roll‐offs of 2.4% and 1.2%, respectively. Moreover, TCz‐BP‐SFAC can serve as a sensitizing host for two‐color all‐fluorescence white OLEDs, resulting in high ηexts of ∼18% and very low efficiency roll‐offs of ∼5%. The outstanding EL performances predict the great potential of TCz‐BP‐SFAC as emitter and host in practical display and lighting devices. Cyan thermally activated delayed fluorescence molecules with aggregation‐enhanced emission and horizontal dipole orientation can function as efficient emitter and sensitizing host to improve electroluminescence efficiencies and reduce efficiency roll‐offs for OLEDs.