Highly Twisted Carbazole-Oxadiazole Hybrids as Universal Bipolar Hosts for High Efficiency PhOLEDs

Four bipolar molecules oDOXA, mDOXA, oOXA, and mOXA composed of a hole-transport carbazole (donor, D) and an electron-transport 1,3,4-oxadiazole (acceptor, A) bridged with different pi-spacers (biphenyl or o-terphenyl) are synthesized, characterized, and used as host materials for various colored phosphorescent OLEDs (PhOLEDs). The highly twisted geometry established via multiple ortho/meta-connections effectively inhibits direct electronic D-A coupling and gives these bipolar molecules similar high triplet energies (approximate to 2.70 eV). In addition, distinctive bipolar transport capabilities are observed by time-of-flight technique (mu(h) approximate to mu(e) approximate to 10(-5) - 10(-6) cm(2) V-1 s(-1)). The D/A connection topology is found to subtly govern the physical properties, rendering these new molecules suitable for serving as bipolar host materials. Among the four host materials, oOXA using the tandem ortho-linkage terphenyl as a linker outperforms the other three hosts in terms of the device efficiency, in which the maximum external quantum efficiencies (eta(ext)) of the corresponding PhOLEDs are as high as 19.4%, 21.3%, 20.9%, 20.1%, and 19.0% for blue, green, yellow, orange, and red PhOLEDs, respectively. Moreover, a single-host multilayered warm-white OLED based on oOXA also shows remarkable efficiency (19.8%, 42.4 cd A(-1), and 38.6 lm W-1) with high color-rendering index of 86.8 and stable chromaticity.