CN-substituted ortho-terphenyl core based high triplet energy bipolar host materials for stable and efficient blue TADF devices

Abstract

Two novel, CN-substituted ortho-terphenyl (OTP) core based bipolar type host materials were designed and synthesized for application in blue thermally activated delayed fluorescence (TADF) devices. Both hosts, 3,3′′-di(9H-carbazol-9-yl)-[1,1′:2′,1′′-terphenyl]-3′-carbonitrile (oCN-OTP) and 3,3′′-di(9H-carbazol-9-yl)-[1,1′:2′,1′′-terphenyl]-4′-carbonitrile (mCN-OTP) with a CN moiety substituted on the ortho- and meta-position of the terphenyl core brought an impressive result in photophysical properties and charge carrier transporting ability. Moreover, the introduction of the CN moiety over the ortho-terphenyl core which connected with carbazole donor units not only enables achieving high triplet energy level but also enhances the bipolar characteristics. As expected, our new design strategy leads to enhancing the device efficiencies when employed with a stable blue TADF dopant of 5-(5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-10,15-diphenyl-10,15-dihydro-5H diindolo [3,2-a:3′,2′-c]carbazole (DBA-DI). The device with oCN-OTP as a host material (30% DBA-DI) exhibited excellent current and external quantum efficiencies of 58.2 cd A⁻¹ and 29.5%, respectively. Interestingly, the oCN-OTP based blue TADF device recorded a longer device lifetime (LT₉₀) of 28.8 hours at an initial luminance of 1000 cd m⁻².