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

Abstract

Host materials are indispensable for the fabrication of 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 group, 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 bandgaps 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 materi-als 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 230200 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.