Influence of the linkage mode and D/A ratio of carbazole/oxadiazole based host materials on phosphorescent organic light-emitting diodes

In the present study, by combining mCP with oxadiazole through tuning the linkage mode and donor/acceptor (D/A) ratio, six different bipolar host materials, o-mCPOXD, m-mCPOXD, p-mCPOXD, o−2mCPOXD, m−2mCPOXD, and p−2mCPOXD were designed and synthesized. Their thermal, photophysical, and electrochemical properties were investigated to reveal the relationship between molecular structure and properties. To evaluate the electroluminescent properties of the six host materials, blue, green, and red phosphorescent organic light-emitting diodes (PhOLEDs) were fabricated and characterized. The compounds with ortho-linkage, o-mCPOXD and o−2mCPOXD, exhibit the highest triplet energy (ET) value of 2.8 eV, which is sufficiently high for blue phosphorescent emitters. The PhOLEDs hosted by hybrids with para-linkage, p-mCPOXD and p−2mCPOXD, show highest current densities but lowest efficiencies due to the relatively low ET value and unbalanced charge flux. The increase of mCP unit leads to lower current densities and device efficiencies. These results demonstrate that bipolar host materials for high-performance PhOLEDs can be constructed by considering the influence of both the linkage mode and D/A ratio.