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

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 (77–350 K) and frequency (100 Hz–10 MHz) ranges. At low temperatures, σ(ω)_m can be described by the relation σ(ω)_m = Aω^s, 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 σ_dc 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 (ε_p), phonon frequency (ν_ph), Debye temperature (θ_D), polaron radius (r_p), small-polaron coupling constant , 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.