Achieving Efficient Blue Delayed Electrofluorescence by Shielding Acceptors with Carbazole Units

The design and synthesis of blue thermally activated delayed fluorescence (TADF) emitters that have high electroluminescence efficiency and low efficiency roll-off features remain a great challenge. Herein, we developed a facile and efficient strategy by shielding acceptors with carbazole units for constructing high-performance blue TADF emitters. Benzonitrile (BN), 9,9-diphenylacridan (DPAc), and carbazole (Cz) were adopted as the acceptor, donor, and protector, respectively, to build two TADF emitters named DPAc-DCzBN and DPAc-DtCzBN. The nondoped organic light-emitting diodes (OLEDs) of DPAc-DCzBN as the emitter exhibited a standard sky-blue emission with Commission Internationale de L'Eclairage (CIE) coordinates of (0.16, 0.26), high external quantum efficiency (EQE) of 20.0%, and low efficiency roll-off (EQEs of 19.5, 16.1, and 12.6% at 100, 500, and 1000 cd m^-2, respectively), which is an outstanding nondoped blue TADF OLED. The doped device of DPAc-DtCzBN displayed a pure blue emission and the corresponding CIE coordinates are (0.16, 0.15). Meanwhile, it also demonstrated high and stabilized EQE values of 23.1, 18.3, and 11.5% at maxima, 100 and 500 cd m^-2, respectively, which is a quite high level among the pure blue TADF OLEDs. This study testifies the feasibility of our strategy in constructing high-performance TADF electroluminescent materials.