Universal Bipolar Host Materials for Blue, Green, and Red Phosphorescent OLEDs with Excellent Efficiencies and Small-Efficiency Roll-Off

Host materials are
indispensable for the fabrication of organic
light-emitting diodes (OLEDs) with phosphorescent emitters, but high-quality
host materials that can efficiently and simultaneously function in
blue, green, and red phosphorescent OLEDs (PHOLEDs) are much rare.
In this work, four bipolar materials are developed using carbazole
and 9,9-dimethyl-9,10-dihydroacridine as hole-transporting groups,
pyridine as electron-transporting groups, and biphenyl and m-methylbiphenyl as π-spacers. The crystal and electronic
structures indicate that these materials have highly twisted conformations,
which endow them with aggregation-induced emission features, intramolecular
charge transfer processes, wide energy band gaps, and high triplet
energies. The carrier transport ability and energy transfer property
analyses show that these materials are able to achieve balanced hole
and electron transports and can serve as bipolar host materials for
PHOLEDs. A series of monochromatic PHOLEDs with different phosphorescent
dopants, including blue-emissive FIrpic, green-emissive Ir­(ppy)2(acac), and red-emissive Ir­(piq)2(acac), are fabricated
by employing these four host materials. The green PHOLEDs can provide
an impressive luminance of up to 230 200 cd m–2. Based on an identical host material, excellent external quantum
efficiencies as high as 25.12, 24.73, and 19.71%, as well as minor
efficiency roll-off, are attained for blue, green, and red PHOLEDs,
respectively, clearly demonstrating the promising applications as
universal bipolar host materials in PHOLEDs with monochromatic light
and white light.