Charge Storage in Conducting Polymers: the Effects of Ground State Degeneracy on Doping Induced Electronic States

The development of high power, high current density organic batteries employing conducting polymers as working (and counter) electrodes is not only of technological importance but offers unique advantages to fundamental studies. Through the use of electrochemical techniques it is possible to reversibly dope and "undope" these polymers with precise control over dopant levels. I have used these electrochemical techniques in conjunction with optical spectroscopy to study doping induced electronic states in conducting polymers. In 1979 Su, Schrieffer, and Heeger, proposed the existence of nonlinear topological excitations (bond-alternating domain walls)--solitons--in the quasi one dimensional polymer polyacetylene ({CH}(,x)). Since then, a wealth of experimental evidence has verified their predictions and demonstrated that these solitons are responsible for many of the remarkable properties of (CH)(,x). Among these experiments are the Electrochemical Voltage Spectroscopy (EVS) and opto-EVS studies reported in this thesis. In these studies, the soliton defects are shown to dominate the optical and electrochemical properties of trans-(CH)(,x). The soliton electronic level is expected to exist at the center of the polyacetylene band gap, and that is where an optical absorption peak is observed in the doped polymer. Furthermore, the energy at which dopants are electrochemically removed from polyacetylene is in good agreement with the predicted soliton energy 2/(pi) (DELTA). The existence of solitons in trans polyacetylene is a consequence of the two fold degenerate ground state characteristic of the polymer chain. However, the more general structure of a quasi-one-dimensional polymer has a nondegenerate ground state. In this case, theoretical studies have demonstrated that a more "generic" nonlinear topological charged excitation (a bipolaron) could exist on such a nondegenerate ground state lattice. The next logical step in the study of conducting polymers, therefore, was the study of materials with nondegenerate ground states. Polythiophene and polypyrrole were found to be good candidates for study. The data obtained from these two polymers using EVS and OPTO-EVS have been interpreted in the context of charge storage in bipolarons. The optical studies of polypyrrole also revealed evidence of the transient evolution of thermodynamically unstable polaron states into the more stable bipolaron configuration. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI.