[1,2,4]Triazolo[1,5- a]pyridine-Based Host Materials for Green Phosphorescent and Delayed-Fluorescence OLEDs with Low Efficiency Roll-Off.

Herein, a series of universal bipolar host materials, 9,9'-([1,2,4]triazolo[1,5- a]pyridine-2,6-diylbis(4,1-phenylene))bis(9 H-carbazole) (TP26Cz1), 3-(2-(4-(9 H-carbazol-9-yl)phenyl)-[1,2,4]triazolo[1,5- a] pyridine-6-yl)-9-phenyl-9 H-carbazole (TP26Cz2), 9,9'-([1,2,4]triazolo[1,5- a]pyridine-2,7-diylbis(4,1-phenylene))bis(9 H-carbazole) (TP27Cz1), and 3-(2-(4-(9 H-carbazol-9-yl)phenyl)-[1,2,4]triazolo[1,5- a]pyridin-7-yl)-9-phenyl-9 H-carbazole (TP27Cz2), using [1,2,4]triazolo[1,5- a]pyridine (TP) as electron-transporting moiety and carbazole as hole-transporting moiety, were designed and synthesized. All four compounds possess remarkable carrier-transporting properties and excellent thermal stability with high glass-transition temperature ( Tg ) in the range of 136-144 °C. The hole- and electron-transporting abilities could be regulated by adjusting the linkage mode between the carbazole and TP units, and balanced charge-transporting properties were realized in TP26Cz2 and TP27Cz2. The phosphorescent and thermally activated delayed-fluorescence (TADF) organic light-emitting diodes (OLEDs) based on these host materials exhibit superior performance with high efficiency and low roll-off. For example, TP26Cz2-hosted phosphorescent OLED (PhOLED) exhibits the maximum external quantum efficiency (ηext ) of 25.6%, and at the high luminance of 5000 cd m-2 , the ηext still remained at 25.2%. TP27Cz1-hosted TADF device exhibits the maximum ηext of 15.5% and only dropped to 15.4% at the luminance of 1000 cd m-2 . Moreover, the influence of linking mode of carbazole unit and TP units in these hosts on their photophysical and carrier-transporting properties as well as the electroluminescence (EL) performance of devices was discussed.