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

Abstract

Transport data for polypyrrole-hexafluorophosphate (PPy-PF 6), 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, ρ r  ρ(1.4 K)/ρ(300 K). In the regime close to the M-I transition (ρ r < 10 2), the low-temperature resistivity follows Mott's variable range hopping (VRH) conduction law. Samples in the intermediate regime (10 2 < ρ r < 10 3) show crossover from the Mott to Efros-Shklovskii VRH conduction with a small Coulomb gap (Δ C ∼ 0.3–0.6 meV). For samples farther into the insulating regime (ρ r > 10 3), ρ( T) shows two distinctly different behaviors. In homogeneous material, Mott's VRH conduction is recovered. In inhomogeneous samples, where ‘metallic island’ are formed after partial dedoping or by strong morphological disorder, ln ρ ∝ (T 0′/T) 1 2 , characteristic of a granular system.