Design of thermally activated delayed fluorescent sensitizers for high efficiency over 20% and long lifetime in yellow fluorescent organic light-emitting diodes

Yellow thermally activated delayed fluorescence (TADF) materials were synthesized using 2,4,6-triphenylpyrimidine-5-carbonitrile and 4-(3-cyanophenyl)-2,6-diphenylpyrimidine-5-carbonitrile acceptors combined with a very strong donor moiety of 5,10-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole. The strong electron acceptors and the strong donor twisted from the acceptor plane with a large dihedral angle decreased the singlet and triplet energy gaps and managed the Dexter energy transfer. The 4-(4-(10,15-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazol-5-yl)phenyl)-2,6-diphenylpyrimidine-5-carbonitrile yellow TADF emitter showed short delayed fluorescence lifetime, high rate constant of reverse intersystem crossing and a high external quantum efficiency (EQE) of 21.6%. The TADF emitter was used as the sensitizer of the yellow emitting 2,8-di-tert-butyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenyltetracene and the TADF assisted fluorescent organic light-emitting diodes achieved a high EQE of 20.2% and a device lifetime of 1400 h up to 90% of the initial luminance of 1000 cd m−2. This study provides a strategy for developing yellow TADF emitters to assist the fluorescence emission to achieve high EQE and long lifetime simultaneously.