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 <i>m</i>-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.