Lifetime extension in green thermally activated delayed fluorescent organic light-emitting diodes by increasing excited state bond dissociation energy

A lifetime extending molecular design approach by increasing the bond dissociation energy of the thermally activated delayed fluorescent (TADF) emitters was investigated. A TADF emitter with a molecular design having the donors in a planar manner, 4-(9H-carbazol-9-yl)-6-(9,9″-diphenyl-9H,9′H,9″H-[3,3′:6′,3″-tercarbazol]-9′-yl)isophthalonitrile (TCzIPN), was compared with state of the art green TADF emitter. The two emitters were green TADF emitters with similar peak wavelengths, but the TCzIPN exhibited three times longer lifetime than the current TADF emitter. Analysis of bond dissociation energy of the TADF emitters revealed that the weak C–N bond of the TCzIPN was stabilized by small dihedral angle between donor and an aromatic linker. This was confirmed by exciton stability test of the emitters, and the C–N bond stabilization effect in triplet excited state was well correlated with the lifetime improvement of the TCzIPN devices.