Study of the ac conductivity of α, α′-dimethyl sexithiophene in pristine and doped states

The study of the electrical and charge transport properties in α, α′-dimethyl sexithiophene in neutral and doped states is addressed by means of dielectric spectroscopy. Doping treatment with iodine gives rise to the generation of radical cations, and also to its π-dimers and π-stacks associated forms. Quite different electric features have been found for the neutral and doped samples. The pristine material present a low conductivity and can be considered into the Mott model of optical phonon assisted hopping of charge carrier between localized states plus the energy difference between two adjacent sites; the charge carrier relaxation time and the polarization relaxation time are different and, moreover, they present different activation energies by which we can assume that both processes are different. The charge transport mechanism in the iodine-doped material has been found to be independent of the applied electric field frequency, thus the charge transport is consistent with the existence of a band-like motion within the planes into the micro-crystals, and a phonon-assisted thermally activated hopping process between the planes, with an activation energy of 0.17 eV.