Charge transport of lithium-salt-doped polyaniline

Abstract

Charge transport properties, including temperature-dependent dc conductivity and thermoelectric power are reported for Li-salt $({\mathrm{LiPF}}_{6}{,\mathrm{}\mathrm{LiBF}}_{4}{,\mathrm{}\mathrm{LiAsF}}_{6}{,\mathrm{}\mathrm{LiCF}}_{3}{\mathrm{SO}}_{3},$ or ${\mathrm{LiClO}}_{4})$ -doped polyaniline (PAN) samples. The experiments of electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) are performed for the systems. The electrical and magnetic properties and the doping mechanism of various Li-salt-doped PAN samples are compared with those of hydrochloric-acid (HCl) -doped PAN samples. The PAN materials doped with ${\mathrm{LiPF}}_{6}$ have the highest dc conductivity $({\ensuremath{\sigma}}_{\mathrm{dc}}\mathrm{\ensuremath{\sim}}1\mathrm{}\mathrm{S}/\mathrm{c}\mathrm{m},$ at room temperature) in the Li-salt-doped PAN systems studied here. The temperature dependence of ${\ensuremath{\sigma}}_{\mathrm{dc}}$ of the systems follows a quasi-one-dimensional variable range hopping model, which is similar to that of HCl-doped PAN samples. As the molar concentration increases from $\ensuremath{\sim}{10}^{\ensuremath{-}4}M$ to $\ensuremath{\sim}1M,$ the system is transformed from an insulating to conducting (non-metallic) state. From EPR experiments, we measure the temperature dependence of magnetic susceptibility, and obtain the density of states for various Li-salt-doped PANs with different doping levels. We observe the increase of the density of states as the molar concentration increases. From the analysis of nitrogen $1s$ peak obtained from XPS experiments, we estimate the doping level of the systems. We compare the effective doping thickness between HCl-doped PAN samples and Li-salt-doped PAN ones, based upon the results of XPS argon (Ar) ion sputtering experiments. The diffusion rate of ${\mathrm{Li}}^{+}$ or counterions and the dissociation constants of Li salt in doping solution play an important role for the effective doping and transport properties of the Li-salt-doped PAN samples.