15H-diindolo[2,3-b:1',2',3'-lm]carbazole: A Novel Rigid Donor for Highly Efficient Thermally Activated Delayed Fluorescence Emitters

In this contribution, we report a novel rigid 15H-diindolo[2,3-b:1′,2′,3′-lm]carbazole (DICz) donor for designing efficient thermally activated delayed fluorescence emitters named 2-(15H-diindolo[2,3-b:1′,2′,3′-lm]carbazol-15-yl)-5-(4,6-diphenyl-1,3,5-triazin-2)benzonitrile (DICzCNTrz) and unravelled its effects on the physicochemical and electroluminescence properties by comparing it with the control molecule, 2-(9H-carbazol-9-yl)-5-(4,6-diphenyl-1,3,5-triazin-2-yl)benzonitrile (CzCNTrz) featuring a carbazole donor. The photophysical properties revealed that the DICzCNTrz showed small singlet–triplet energy splitting, short delayed fluorescence exciton lifetime and high photoluminescence quantum yield when compared to CzCNTrz. The organic light emitting diode fabricated using the DICzCNTrz-based emitter exhibited superior performance with EQE of 21.4% and reduced efficiency roll-off at high brightness when compared to the device with the CzCNTrz-based emitter, which showed maximum EQE of 13.9%.