Systematic Conductivity Behavior in Conducting Polymers: Effects of Heterogeneous Disorder

The conduction process in conducting polymers has some unusual features. Even for highly conducting samples, the electronic transport properties show a mixture of metallic and non-metallic character, which is most easily explained in terms of the heterogeneous morphology of the polymers. The Figure shows the characteristic temperature dependence of conductivity as doping level is increased. A key feature of the conductivity of the organic conducting polymers is its surprisingly large magnitude for materials with low carrier density and considerable disorder. For polyacetylene the inferred conductivity of the highly conducting crystalline regions can be greater than that of copper. However, the temperature dependence of the conductivity shows non-metallic sign over a wide range of temperatures in virtually all conducting polymers—a change to metallic sign occurs only at higher temperatures and only in some polymers. This behavior is compared and contrasted with that of carbon nanotubes (which can also be regarded as conducting polymers) and of amorphous conventional metals. The measured conductivities of organic polymers and single-wall carbon nanotube networks are analyzed in terms of a heterogeneous model that gives a good account of the data. The granularity of the superconductivity recently discovered in polythiophene films is also consistent with this heterogeneous model.