Bicarbazole based donor–acceptor compound as a host for thermally activated delayed fluorescent emitter

• A bicarbazole derived host material with a benzonitrile substituent showed bipolar charge transport properties.
• A high external quantum efficiency above 20% was obtained in green thermally activated delayed fluorescent device.
• Para substitution of CN was better than metal substitution for high efficiency.

A bicarbazole based donor–acceptor compound, 4,4′-([3,4′-bicarbazole]-9,9′-diyl)dibenzonitrile (4CN34BCz), was synthesized as a host material for thermally activated delayed fluorescent emitter and the device performances of the green devices with the synthesized host and (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) delayed fluorescent emitter were investigated. 4CN34BCz with CN unit at para-position could effectively assist delayed fluorescent emission of 4CzIPN and achieved a high external quantum efficiency of 20.8% compared to 13.8% of the similar host with CN unit at meta-position because the orientation of the CN functional group at para position of the benzonitrile was favourable for charge balance in the emitting layer.

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