Investigation of the Electronic Structure of Conducting Polymers by Electron Energy-Loss Spectroscopy

Abstract

For the understanding of the macroscopic properties of conducting polymers (e.g., the high conductivity) the knowledge of the evolution of the π-electron bandstructure as a function of doping concentration is of fundamental importance. We have tried to get information on the electronic structure of conducting polymers by electron energy-loss spectroscopy (ELS). In particular, we have investigated polyparaphenylene (PPP), polypyrrole (PPY) based polymers, and polythiophene (PT) with various counterions. These polymers have the common feature of a non-degenerate ground state; namely, when the positions of the double and single bonds are reversed, an inequivalent structure with different energy results. Theoretical calculations [1–4] have predicted that polarons,and at higher concentrations bipolarons,are the primary defect states created during doping of conjugated polymers with non-degenerate ground states. This leads to two defect states in the gap, the lower one being singly occupied in the polaron case for p-doping. Further oxidation leads to bipolarons with two empty (occupied) states in the gap for p(n) doping. At higher doping level, a broadening of the defect level is predicted and at the highest doping level, an overlap of the bipolaron bands with the π bands should occur,leading to a partially filled valence (conduction) band for p(n) doping. The defect levels,as well as the overlap of bipolaron bands with the π bands,should be observable by optical spectroscopy or by ELS.