Phthalonitrile-based bipolar host for efficient green to red phosphorescent and TADF OLEDs

Abstract

A bipolar host material named 4-(9H-carbazol-9-yl)phthalonitrile (CzPN) was developed by combining carbazole and phthalonitrile as donor and acceptor units, respectively. The introduction of the phthalonitrile unit realized a relatively small energy gap of 3.00 eV as well as a deep lowest unoccupied molecular orbital level of −2.72 eV. The characterization of carrier-only devices demonstrated the bipolar charge transporting property of CzPN. With a triplet energy of 2.70 eV, CzPN was investigated as host in green, yellow, and red phosphorescent organic light-emitting diodes (PhOLEDs) and green thermally activated delayed fluorescence (TADF) devices. A low turn-on voltage of 2.8 V was achieved by all the four diodes. Both PhOLEDs and TADF devices exhibited high external quantum efficiency values over 20% with relatively small efficiency roll-offs. These findings demonstrate that CzPN is an excellent host material for both phosphorescent and TADF emitters, and phthalonitrile is a promising unit to develop universal host materials.