Mobile iodine dopants in organic semiconductors

The electrical properties of α-quaterthiophene, α-hexathiophene, and pentacene single crystals have been studied for low iodine dopant concentrations. The electronic and ionic $\left({\mathrm{I}}_3^{\mathrm{-}}\right)$ currents have been separated experimentally, providing quantitative information on the ion mobility. The mobility of the ${\mathrm{I}}_3^{\mathrm{-}}$ ions in these layered structures is strongly anisotropic. Parallel to the molecular layers the mobility is of order of ${10}^{-6}$ to ${10}^{-7}{\mathrm{cm}}^2/\mathrm{V}\mathrm{}\mathrm{s},$ whereas perpendicular to the layers it is smaller by about 4 orders of magnitude. In addition, the activation energy for diffusion parallel to the layers is also much smaller (0.2–0.3 eV) than for diffusion perpendicular to the layers (0.85 eV).