1,2,4-Triazole-containing bipolar hosts for blue and green phosphorescent organic light-emitting diodes

Abstract

Three bipolar host materials, 9-(3′,5′-di(1H-1,2,4-triazol-1-yl)-[1,1′-biphenyl]-3-yl)-9H-carbazole (DTzSCz), 9,9′-(3′,5′-di(1H-1,2,4-triazol-1-yl)-[1,1′-biphenyl]-3,5-diyl)bis(9H-carbazole) (DTzDCz), and 9,9′-(5′-(1H-1,2,4-triazol-1-yl)-[1,1′:3′,1″-terphenyl]-3,3″-diyl)bis(9H-carbazole) (STzDCz), with regular 1,2,4-triazole/carbazole ratios of 2 : 1, 2 : 2, and 1 : 2, are developed for blue and green phosphorescent organic light-emitting diodes (PhOLEDs). Different from the 1,3,5-trissubstituted or 3,4,5-trissubstituted configuration of most 1,2,4-triazole derivatives, here 1,2,4-triazole is placed at the molecular periphery by grafting from its N − 1 atom to the biphenyl bridge without substitution on other atoms. Their thermal, physical and electrochemical properties, and bipolar features are characterized using experimental and theoretical means. The designated structures efficiently suppress the intramolecular charge transfer between donor and acceptor and result in high triplet energies. These materials exhibit excellent performances in sky-blue and green PhOLEDs. For example, when using STzDCz as a host that had a triazole/carbazole molar ratio of 1 : 2, the blue and green PhOLEDs exhibited maximum external quantum efficiencies η_ext of 25.0% and 20.3%, respectively. Moreover, the STzDCz hosted green PhOLED exhibited slow efficiency roll-off with a decrease of only 12.8% relative to the maximum value even at an extremely high brightness of 10 000 cd m⁻². A triazole/carbazole ratio of 1 : 2 in the host is proved to be the optimal n-type to p-type ratio favorable for the most balanced charge transportation and highest EL efficiencies.