Lifetime evaluation of thermally activated delayed fluorescence materials according to the positions of CN substituents

Recently, thermally activated delayed fluorescence (TADF) emitters have been successfully developed for highly efficient organic light-emitting diodes (OLEDs). In this study, we designed and synthesized three green TADF emitters with various positions of the CN group-3,3 ',3"-(1,3,5-triazine-2,4,6-triyl)tris(2-(9H-carbazol-9-yl) benzonitrile) (TCz-2mCNTrz), 3,3 ' ,3"-(1,3,5-triazine-2,4,6-triyl)tris(4-(9H-carbazol-9-yl)benzonitrile) (TCz-4mCNTrz), and 4,4 ',4"-(1,3,5-triazine-2,4,6-triyl)tris(3-(9H-carbazol-9-yl)benzonitrile) (TCz-pCNTrz)-and evaluated their lifetime and efficiency by employing them in green TADF OLEDs. The synthesized materials exhibit different efficiencies and lifetimes owing to different bond dissociation energies (BDEs), which vary according to the position of the CN group. In particular, TCz-pCNTrz exhibits the maximum external quantum efficiency of 19.5% and the lowest efficiency roll-off, among the three materials. Further, TCz-pCNTrz exhibited a lifetime of 352 h in tests conducted under LT50@5000 nits. This resulted in a device with a higher lifespan than those of TCz-2mCNTrz (20.4 h) and TCz-4mCNTrz (50.0 h), in which CN groups were introduced at different positions. Thus, changes in the position of the CN group of TADF emitters can be expected to affect the device lifetime.