Star-shaped hexakis(9,9-dihexyl-9H-fluoren-2-yl)benzene end-capped with carbazole and diphenylamine units: Solution-processable, high T_g hole-transporting materials for organic light-emitting devices

Two new hole-transporting materials, namely HFB-Cz and HFB-Dpa, were designed and synthesized by attaching carbazole and diphenylamine units to the hexakis(9,9-dihexyl-9H-fluoren-2-yl)benzene (HFB) core via Buchwald-Hartwig coupling reaction. The long alkyl chain and core rigidity endow these compounds with good solution processability and high thermal stability. HFB-Cz and HFB-Dpa exhibit significantly high glass transition temperatures (225 and 154 °C) relative to widely used hole-transporting materials, such as N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4, 4′-diamine (TPD, 65 °C) and 1,4-bis((1-naphthylphenyl)amino)biphenyl (NPB, 96 °C). Solution-processed green OLED devices using HFB-Cz and HFB-Dpa as hole-transporting materials exhibit very high efficiencies with a maximum current efficiency up to 6.2 cd A^-1. These efficiencies are substantially higher than the NPB-based control device, and are among the highest for the hole-transporting materials in similar device configuration.