Metallic conductivity beyond the Mott minimum in PEDOT: Sulphate at low temperatures

Abstract

Elastic scattering mechanisms dominate the charge transport in crystalline metals, resulting in a characteristic increase in conductivity at low temperatures. However, disorder – arising, for example, from alloying – can hamper transport and lead to decreased coherence among scattered electrons (i.e. inelastic scattering). This is typically the situation in non-crystalline metals. Likewise, conductive polymers are particularly prone to defect states with decreased carrier mobility (i.e. electrical conductivity). We present the first report of conduction in the elastic scattering regime in conductive polymers without the detrimental effect on conductivity. As in a crystalline metal, conductivity increases upon cooling. More specifically, we observed a minimum conductivity in free-standing metallic PEDOT:sulphate at around 4 K. The polymer chains, which form crystallites of around 800 Å in size, exhibit an extraordinary degree of spatial and energetic order. We show that increasing pressure enabled us to shift the minimum upwards, thus achieving metallic conductivity at up to 10 K with a calculated mean-free path of around 250 Å. These results underline the existence of true metallic states in conductive polymers at low temperatures.