Starburst Type Benzofuroindolocarbazole Donor for High Efficiency and Long Lifetime in Thermally Activated Delayed Fluorescence Emitters

A series of thermally activated delayed fluorescence (TADF) emitters of 5-(4,6-diphenyl-1,3,5-triazin-2-yl)-2-(15-phenylbenzofuro[3,2-a]indolo[3,2-c]carbazol-10(15H)-yl)benzonitrile (TrzCNTrx1) and 5-(4,6-diphenyl-1,3,5-triazin-2-yl)-2-(10-phenylbenzofuro[3,2-a]indolo[3,2-c]carbazol-15(10H)-yl)benzonitrile (TrzCNTrx2) are developed through a molecular design strategy that incorporates a starburst type benzofuro[3,2-a]indolo[3,2-c]carbazole as a rigid donor. The benzofuro[3,2-a]indolo[3,2-c]carbazole is developed as a strong and rigid donor for strong charge transfer character in the TADF emitter in combination with a diphenyltriazine acceptor and a benzonitrile linker. Two starburst type donors with different active sites for the aromatic linker substitution are prepared. Among the TrzCNTrx1 and TrzCNTrx2 emitters, the TrzCNTrx2 emitter shows a small singlet–triplet energy gap below 0.10 eV, high photoluminescence quantum yields over 90%, and short exciton lifetimes less than 5.0 µs. In the device application, the TrzCNTrx2 emitter shows maximum external quantum efficiency of 20.6% with little efficiency roll-off less than 2% at 3000 cd m⁻² and color coordinate of (0.37, 0.57). Moreover, the TrzCNTrx2 device is much better than the TrzCNTrx1 in terms of device lifetime because of short delayed fluorescence lifetime.