Enhanced electroluminescence based on thermally activated delayed fluorescence from a carbazole―triazine derivative

Thermally activated delayed fluorescence (TADF) properties of a dicarbazole-triazine compound, 9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9'-phenyl-3,3'-bicarbazole (CzT), and its OLED characteristics were investigated. An estimated small energy gap of about 90 meV between the singlet and triplet energy states of CzT made the up-conversion of triplet excitons back to a singlet state possible. The origin of the observed delayed fluorescence has been shown to be thermally activated delayed fluorescence. An organic light emitting diode (OLED) with CzT as an emitter showed the maximum external quantum efficiency (EQE) of 6%. For comparison, another carbazole-triazine derivative of 3-(2'-(4,6-diphenyl-1,3,5-triazin-2-yl)-[1,1'-biphenyl]-2-yl)-9-phenyl-9H-carbazole (PhCzTAZ) with a similar structure was also studied. PhCzTAZ showed a low fluorescence quantum yield with no TADF.