Hopping transport in doped conducting polymers in the insulating regime near the metal-insulator boundary: polypyrrole, polyaniline and polyalkylthiophenes

Transport data for polypyrrole-hexafluorophosphate (PPy-PF6), protonated polyaniline and iodine-doped regio-regular polyalkylthiophenes (PATs), all in the insulating regime near the disorder-induced metal-insulator (M-I) boundary, are presented and analyzed. These doped conducting polymers have the transport properties of a Fermi glass insulator; the disorder being characterized by the resistivity ratio, ρr  ρ(1.4 K)/ρ(300 K). In the regime close to the M-I transition (ρr < 102), the low-temperature resistivity follows Mottʹs variable range hopping (VRH) conduction law. Samples in the intermediate regime (102 < ρr < 103) show crossover from the Mott to Efros-Shklovskii VRH conduction with a small Coulomb gap (ΔC ∼ 0.3–0.6 meV). For samples farther into the insulating regime (ρr > 103), ρ(T) shows two distinctly different behaviors. In homogeneous material, Mottʹs VRH conduction is recovered. In inhomogeneous samples, where ‘metallic island’ are formed after partial dedoping or by strong morphological disorder, ln ρ ∝ (T0′/T)12, characteristic of a granular system.