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 (I^-₃) currents have been separated experimentally, providing quantitative information on the ion mobility. The mobility of the I^-₃ ions in these layered structures is strongly anisotropic. Parallel to the molecular layers the mobility is of order of 10^-6 to 10^-7 cm²/V 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).