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. Two starburst type donors with different active sites for the aromatic linker substitution are developed. Among the TrzCNTrx1 and TrzCNTrx2 emitters, the TrzCNTrx2 emitter shows maximum external quantum efficiency of 20.6% with little efficiency roll‐off less than 2% at 3000 cd m−2. Also, the TrzCNTrx2 device achieves long device operational lifetime.