High field conductivity in polypyrrole

Abstract

We have measured high electric field conduction properties of polypyrrole films prepared by anodic deposition as a function of temperature. The field dependence is in reasonably good agreement with the model of intergranular tunnelling with Coulomb-like repulsion barriers. The high field results however demonstrate that pure quantum tunnelling probably does not exist at low enough temperatures. The previously observed levelling-off of conductivities as ‘temperature T tends to 0’ is most likely an artefact produced by the finite magnitude of current flowing in the system. Current filaments remove space charge barriers which otherwise would localize carriers at zero temperature ( T=0). Despite this new observation, we argue that the grain tunnelling model is still a better description than variable range hopping. We show that it is reasonable and consistent within this model to expect grains and interfaces to charge up at low temperatures and to produce energy mismatch which then requires inelastic tunnelling. At very high fields, the current has a Fowler—Nordheim tunnelling characteristic which suggests that the grain barriers are slowly varying near the maximum as one might expect.