Charge transport properties in a one-dimensional description of polymer polyaniline

Abstract

The metal-insulator transition taking place in Polyaniline (PAN) is investigated in a one-dimensional configuration when impurities are introduced by doping. The electronic transport is numerically analyzed, representing the system by a Hückel tight-binding Hamiltonian and using the non-equilibrium Green’s function formalism to obtain the conductance of the system under the presence of polaron and bipolaron doping. In a detailed analysis, we highlight the importance of studying extensive N sites chains, evaluating the conductance as a function of the de-coherence parameter η, decreasing it to the limit of 0+ ; this procedure allows us to study the existence of delocalized states in this system. It was possible to verify that although the bipolaron/polaron doping produces a displacement of the Fermi energy into a region of states outside the gap of the pure polymer, the conductance in the limit of zero inelastic scattering is zero, showing the existence of localized states at the Fermi level, due to disorder. This result indicates that the description of PAN as a linear one-dimensional object analyzed in a extensive N sites disordered chain does not manifest correlated disorder as proposed by the Random Dimer Model (RDM), where transport could occur because of the electronic states are extended. The lack of charge diffusion at the Fermi level in this one-dimensional description of the system shows that it is not an adequate model to study the metal-insulator transition when the polymer is doped. The incorporation of the nature of the polymerization process, introducing higher dimensional effects in its early stages, is possibly an essential ingredient to derive an appropriate model to describe the conducting behavior of the PAN system.