Low frequency ac conduction and dielectric relaxation in pristine poly(3-octylthiophene) films

The ac conductivity sigma(omega)(m), dielectric constant epsilon'(omega) and loss epsilon(omega) of pristine poly(3-octylthiophene) (P3OT) films (thickness similar to 20 mu m) have been measured in wide temperature (77-350 K) and frequency (100 Hz-10 MHz) ranges. At low temperatures, sigma(omega)(m) can be described by the relation sigma(omega)(m) = A omega(s), where s is similar to 0.61 at 77 K and decreases with increasing temperature. A clear Debye-type loss peak is observed by subtracting the contribution of sigma(dc) from sigma(omega)(m). The frequency dependence of conductivity indicates that there is a distribution of relaxation times. This is confirmed by measurement of the dielectric constant as a function of frequency and temperature. Reasonable estimates of various electrical parameters such as effective dielectric constant (epsilon(p)), phonon frequency (nu(ph)), Debye temperature (theta(D)), polaron radius (r(p)), small-polaron coupling constant (Upsilon), effective polaron mass (m(p)), the density of states at the Fermi level N(E-F), average hopping distance (R) and average hopping energy (W) from dc conductivity measurements suggest the applicability of Mott's variable range hopping model in this system.