Controlling exciton lifetime of blue thermally activated delayed fluorescence emitters using heteroatom contained pyridoindole donor moiety
In this paper, we report a promising molecular design concept to attain short exciton lifetime, appropriate T1 energy and high photoluminescence quantum yield (PLQY) in blue thermally activated delayed fluorescence (TADF) emitters for high quantum efficiency and better efficiency roll-off characteristics in OLEDs. Herein, new TADF molecules with nitrogen at α- and δ-position of carboline donor moiety were synthesized and their effects on the photo-physical and electroluminescent properties were theoretically and experimentally investigated. The TADF molecule with δ-carboline donor moiety was much favorable for short exciton lifetime, high delayed PLQY, good blue color, and small singlet and triplet splitting compared to generally used carbazole. The maximum external quantum efficiency (EQE) of 22.5%, reduced efficiency roll-off and good color purity with Commission Internationale de l’Eclairage (CIE) 1931 color coordinates of (0.19, 0.34) were exhibited in the blue OLED using TADF emitter, 4,5-bis(5H-pyrido[3,2-b]indol-5-yl)phthalonitrile (δ-2CbPN). Moreover, the effectiveness of δ-carboline donor moiety was verified in another deep blue TADF emitter, 5,5'-(2-(9H-carbazol-9-yl)-5-(4,6-diphenyl-1,3,5-triazin-2-yl)-1,3-phenylene)bis(5H-pyrido[3,2-b]indole) (CzDCbTrz). The maximum EQE of 23.4%, deep blue color, and reduced efficiency roll-off were attained in the CzDCbTrz based OLED.