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

Abstract

Four bipolar molecules o DOXA , m DOXA , o OXA , and m OXA composed of a hole‐transport carbazole (donor, D) and an electron‐transport 1,3,4‐oxadiazole (acceptor, A) bridged with different π‐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 (≈2.70 eV). In addition, distinctive bipolar transport capabilities are observed by time‐of‐flight technique ( μ h ≈ μ e ≈ 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, o OXA 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 ( η 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 o OXA 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.