Rational design of blocking groups for high triplet energy n-type host materials

Abstract

The effect of blocking groups on the device lifetime and efficiency of blue phosphorescent organic light-emitting diodes (PhOLEDs) was investigated by synthesizing electron transport-type hosts with different blocking groups. Three hosts, 9,9′-(6-(3-(triphenylsilyl)phenyl)-1,3,5-triazine-2,4-diyl)bis(9H-carbazole) (mSi-2CzTrz), 9,9′-(6-([1,1′:3′,1′′-terphenyl]-2′-yl)-1,3,5-triazine-2,4-diyl)bis(9H-carbazole) (tPh-2CzTrz) and 9,9′-(6-(9,9′-spirobi[fluoren]-5-yl)-1,3,5-triazine-2,4-diyl)bis(9H-carbazole) (SBF-2CzTrz) were designed to have both a blocking group and a hole transport-type carbazole group around a triazine core. They had different blocking groups of tetraphenylsilane, terphenyl and spirobifluorene to study their effect. In device applications, the synthesized hosts were used as an electron transport-type host mixed with a hole transport-type 3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl (mCBP) host in PhOLEDs doped with a fac-tris(3-(1-(2,6-diisopropylphenyl)-1H-imidazol-2-yl)benzonitrile)iridium blue phosphorescent emitter. Among the three devices, the mCBP:mSi-2CzTrz mixed host device showed an external quantum efficiency of 22.8% and a device operational lifetime with up to 80% of the initial luminance of over 2000 h at 100 cd m⁻² in blue PhOLEDs.