High-efficiency orange thermally activated delayed fluorescence by secondary acceptor modification

The secondary donor/acceptor has significant effects on the excited-state properties and efficiencies of thermally activated delayed fluorescent (TADF) emitters. Here, we use two electron-withdrawing units (cyanobenzene and trifluoromethylbenzene) as a secondary acceptor to control the electron-withdrawing ability of the dibenzo[a,c]phenazine (BP) acceptor core based on the BP-triphenylamine (TPA) backbone. The synthesized orange-red TADF emitters (CN-BP-TPA and CF3-BP-TPA) exhibit the singlet-triplet energy splitting (ΔE_ST) values of 0.19 and 0.23 eV, respectively. Both emitters show a high photoluminescence quantum yield up to 92%–98% in the doped films. The organic light-emitting diodes (OLEDs) based on CN-BP-TPA achieve the external quantum efficiency of 26.0%, power efficiency of 76.8 lm/W, and current efficiency of 61.1 cd/A as compared with 16.6%, 59.2 lm/W, and 50.9 cd/A of the CF3-BP-TPA counterpart. In addition, the non-doped OLED involving the non-doped CN-BP-TPA emitter shows an ultralow turn-on voltage of 2.2 V and an external quantum efficiency of 5.0%.