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

Abstract

The ac conductivity σ(ω)m, dielectric constant ε′(ω) and loss ε″(ω) of pristine poly(3-octylthiophene) (P3OT) films (thickness ∼20 μm) have been measured in wide temperature (77-350K) and frequency (100Hz-10MHz) ranges. At low temperatures, σ(ω)m can be described by the relation σ(ω)m = Aωs, where s is ∼0.61 at 77 K and decreases with increasing temperature. A clear Debye-type loss peak is observed by subtracting the contribution of σdc from σ(ω)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 (εp), phonon frequency (νph), Debye temperature (θD), polaron radius (rp), small-polaron coupling constant (γ), effective polaron mass (mp), the density of states at the Fermi level N(EF), 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. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.