Electronic Transport in the Conducting Polymer Polypyrrole

Abstract

The mechanism of charge transport in conducting polymers is disputed. Fundamental questions still remain unanswered. These questions include the nature of the charge carrier, the transport mechanism of the charge carrier in conduction processes and the roles of disorder and morphology that ultimately limit the conductivity. This thesis examines the transport properties of the conducting polymer, polypyrrole (PPy), from measurements of the temperature dependent thermoelectric power and conductivity from room temperature down to liquid He temperature (4.2 K). The data in this thesis also complements the optical measurements that were performed by a fellow PhD student on the same samples. We measured the transport properties of a variety of polypyrrole films. The films were grown using chemical and electrochemical polymerisation methods, and were also grown at different temperatures. The films all had varying amounts of disorder, which was shown by a range of conductivities, from insulating to metallic. The observance of metallic behaviour in the conductivity of some polypyrrole films confirms that conductivity cannot be described by phonon mediated hopping alone. Instead, we find that for the low conductivity polypyrrole its conductivity follows a general trend of variable range hopping. This hopping crosses over to fluctuation induced tunnelling for polypyrrole films with higher conductivity. To explain this we employ a heterogeneous model based on a mixture of highly conducting regions that are separated by thin insulating barriers. Thermoelectric power and frequency-dependent conductivity measurements on the same films support this model. This thesis also examines the effect of various gases on the transport properties of polypyrrole. When polypyrrole is exposed to gases, a significant change occurs in the resistance of the polymer. The mechanism that causes the change in the resistance is not understood. We have performed a series of experiments that have examined the effect of nitrogen, oxygen, ammonia and water vapour on the transport properties of polypyrrole. The experiments consisted of measurements of the temperature dependent conductivity, temperature dependent thermoelectric power and Raman spectroscopy.