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Triphenylene-based electron transport materials (ETMs), designated Bpy-TP1-4, with a coplanar molecular structure and a large electron affinity were designed and synthesized for use in organic light-emitting diodes (OLEDs). Spectroscopic ellipsometry measurements clarified that the deposited thin films of these ETMs have optical anisotropy, indicating that the molecules in the deposited thin films tend to be mostly oriented parallel to the substrate. Green OLEDs containing these ETMs allowed a lower driving voltage than that for OLEDs containing tris(8-hydroxyquinolinato)aluminum (Alq3) with a random orientation. In particular, the OLED containing Bpy-TP2 showed a significantly lower driving voltage, achieving lower power consumption when compared with conventional ETMs such as Alq3 and 1,3,5-tris(2-phenyl-1H-benzodimidazol-1-yl)benzene (TPBi). Also, the operational lifetime of a blue OLED containing Bpy-TP2 is equivalent to that of an OLED with TPBi. The increased driving voltage of the device containing Bpy-TP2 is significantly suppressed compared to that of the OLED containing TPBi.
Togashi et al. (Sun,) studied this question.