A reverse intersystem crossing managing assistant dopant for high external quantum efficiency red organic light-emitting diodes

Thermally activated delayed fluorescence (TADF) materials that can serve as assistant dopants with reduced Dexter energy transfer rates were designed by replacing the donor moiety of 2,3,5,6-tetra(9H-carbazol-9-yl)terephthalonitrile (4CzTPN) with 5H-benzo[4,5]thieno[3,2-c]carbazole (BTCz). The employment of the BTCz donor strengthened the charge transfer character, inducing high reverse intersystem crossing (RISC) rate. Careful management of the number of BTCz donor moieties optimized the RISC rates and photoluminescence quantum yield value for high efficiency in TADF-assisted fluorescent devices. A TADF-assisted fluorescent device with the BTCz-modified TADF assistant dopant exhibited a high external quantum efficiency of 13.0% compared with 11.3% for that with the 4CzTPN assistant dopant due to suppressed Dexter energy transfer.