Enhancement of electron transport by horizontal molecular orientation of oxadiazole planar molecules in organic amorphous films

To demonstrate the effect of molecular orientation on charge transport characteristics in vacuum-deposited organic amorphous films, the electrical and optical properties of films of two oxadiazole derivatives were investigated. One of them has bulky tert-butyl terminals and forms an isotropic film, whereas the other has planar bipyridyl terminals and forms a highly anisotropic film. The very large optical anisotropy of the latter means that the planar molecules stack horizontally, leading to large overlaps of pi-orbitals and more than 30 times higher electron mobility though the geometric and electronic structures of the main conformers of the two derivatives are quite similar.