Improved device efficiency and lifetime of green thermally activated delayed fluorescence materials with multiple donors and cyano substitution

Abstract

The challenges associated with thermally activated delayed fluorescence (TADF) materials are the reduced efficiency and lifetime because of the triplet-exciton lifetimes and thermal degradation of the devices owing to low molecular stabilities. To reduce the triplet-exciton lifetime, it is necessary to increase the rate of reverse intersystem crossing (RISC) and the intramolecular C–N bond dissociation energy (BDE) must be increased. In this study, a donor was introduced at the ortho position to reduce the energy gap between the singlet and triplet states and facilitate the expression of TADF properties. Three materials with increasing numbers of carbazole and CN− units and, consequently, increasing molecular stabilities: 4-(9H-carbazol-9-yl)-3-(4,6-diphenyl-1,3,5-triazin-2-yl)benzonitrile (Cz-mCNTrz), 3,3’-(6-phenyl-1,3,5-triazine-2,4-diyl)bis(4-(9H-carbazol-9-yl)benzonitrile) (DCz-mCNTrz), and 3,3′,3”-(1,3,5-triazine-2,4,6-triyl)tris(4-(9H-carbazol-9-yl)benzonitrile) (TCz-mCNTrz), were designed and synthesized. The best device performance of three materials, TCz-mCNTrz, exhibited 25.3% of maximum external quantum efficiency with highest photoluminescence quantum yield(PLQY) and smallest singlet-triplet energy gap. Moreover, lowest efficiency roll-off of 5.92%, as well as the longest lifetime at a 95-h at 5000 nits indicating increased anion BDE and rate of RISC after each addition of the carbazole and cyano moieties.