Rational Molecular Design Overcoming the Long Delayed Fluorescence Lifetime and Serious Efficiency Roll‐Off in Blue Thermally Activated Delayed Fluorescent Devices

Abstract

Blue thermally activated delayed fluorescent (TADF) devices with short excited-state lifetime, high reverse intersystem crossing rate, and low-efficiency roll-off were developed by managing the molecular structure of donor-acceptor-type blue emitters. Three isomers of blue TADF emitters with a diphenyltriazine acceptor and three carbazole donors were synthesized. The position of the donor moieties in the phenyl linker connecting the donor and acceptor moieties was controlled to devise compounds with a short delayed fluorescence lifetime. A blue TADF emitter with three carbazole donors at 2-, 3-, and 4- positions of a phenyl linker shortened the excited state lifetime to 4.1 μs, showed a high external quantum efficiency of 20.4 %, and low efficiency roll-off of less than 10 % at 1000 cd m^-2 . Therefore, a molecular design distorting the donors by aligning them in a consecutive way is useful to resolve the issues of long delayed fluorescence lifetime and efficiency roll-off of blue TADF devices.