High-Efficiency Blue Organic Light-Emitting Diodes Using Emissive Carbazole-Triazine-Based Donor-Acceptor Molecules with High Reverse Intersystem Crossing Rates
We report blue thermally activated delayed fluorescent (TADF) organic light emitting diodes (OLEDs) with high efficiencies which are attributed to employing donor-acceptor type TADF compounds having high reverse intersystem crossing (RISC) rates. Carbazole derivatives and a triphenyltriazine (or a dibenzothiophene) are employed as the donor (D) and the acceptor (A) moieties, respectively. We can achieve four D-A type blue TADF compounds through the combination of the moieties and adjust the energy gap between the lowest excited singlet and triplet states and accordingly the RISC rate (kRISC) for the compounds. A blue OLED using one of the TADF molecules, 9-(9H-carbazol-9-yl)-12-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-5-phenyl-5,12-dihydroindolo[3,2-a]carbazole exhibits high maximum external quantum efficiency (22.3％) due to its short exciton lifetime (6.1 ？s) induced by its high kRISC (1.825 ？ 106 s-1).