Polarons, bipolarons, and solitons in conducting polymers

On the mechanism of conductivity enhancement in poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) film through solvent treatment

Polyaniline: A historical survey

The effect of charge-transfer doping on the geometric and electronic structures of conjugated polymers has been investigated at the ab initio level with explicit consideration of the doping agents. Three systems were chosen for study as prototypical examples of conjugated polymers with nondegenerate ground states: polyparaphenylene, polypyrrole, and polythiophene. As a result of charge transfer with electron-donating dopants, extra charges appear on the polymer chains and induce strong geometry modifications. The lattice evolves from an aromatic structure towards a quinoid-like structure. Charged defects associated with lattice deformations such as spinless bipolarons are formed. The influence on the electronic structure of the polymer chains is such that with respect to the undoped case, new states appear within the gap. For the maximum doping levels experimentally achieved, band-structure calculations demonstrate that the states in the gap overlap to form bipolaron bands, a few tenths of an electron volt wide. The presence of these bipolaron bands is consistent with optical data as well as with magnetic data which suggest that the charge carriers in the highly conducting regime are spinless.

Highly conducting polyparaphenylene, polypyrrole, and polythiophene chains: An ab initio study of the geometry and electronic-structure modifications upon doping

We present a new method of electrochemical polymerization of poly(thiophene) using dithiophene as the starting material, from which we obtain a high-quality film with a sharp interband absorption edge. An in situ study of the absorption spectrum during the electrochemical doping process has been carried out. In the dilute regime, the results are in detailed agreement with charge storage via bipolarons; weakly confined soliton pairs with confinement parameter $\ensuremath{\gamma}\ensuremath{\cong}0.1\ensuremath{-}0.2$. At the highest doping levels, the data are characteristic of the free-carrier absorption expected for a metal. From a parallel electrochemical voltage spectroscopy study, we find evidence of charge injection near the band edge and charge removal from the bipolaron gap states. In the dilute regime, the position of the chemical potential is consistent with charge storage in weakly confined bipolarons. The high Coulombic recovery over a charge-discharge cycle indicates that poly (thiophene) may be an excellent cathode-active material in battery applications.

Charge storage in doped poly(thiophene): Optical and electrochemical studies

Oxidized and neutral films of polypyrrole have been prepared electrochemically in the absence of oxygen and water. The neutral films are insulating and can be readily oxidized by chemical oxidizing agents to give films of greater conductivity than can be achieved by electrochemical oxidation. Optical spectroscopy provides evidence for the similarity of the polymeric carbonium ion produced by both types of oxidation. NMR studies are consistent with the α,α’ bonding in these polymers; they also show the expected downfield shifts relative to the neutral polymer on both chemical and electrochemical oxidation. ESR studies of both the electrochemically oxidized and the neutral polymer suggest the presence of highly mobile spins.

Preparation and characterization of neutral and oxidized polypyrrole films

ESR measurements have been performed on samples of neutral polypyrrole, polypyrrole-perchlorate, oxygen-doped polypyrrole, and poly($\ensuremath{\beta}$,${\ensuremath{\beta}}^{\ensuremath{'}}$-dimethylpyrrole perchlorate). A narrow Lorentzian line is observed in oxidized, highly conducting films, but no correlation between conductivity and either linewidth or intensity is found. Electrochemically cycled films, which remain highly conducting, show little or no detectable ESR absorption. It is deduced that the ESR signal seen in the as-prepared material does not arise from the current-carrying species, but rather from accidental neutral $\ensuremath{\pi}$-radical defects. The absence of paramagnetism in the metallic state is discussed within the framework of bipolaron formation.

Electron-spin-resonance studies of pyrrole polymers: Evidence for bipolarons

The evidence for bipolarons in pyrrole polymers

: Using Beta/Beta prime dimethypyrrole polymers to reduce the structural disorder it has been possible to interpret the results of electron diffraction data and to propose and idealized structure for neutral polypyrrole. XPS data have been used to define some of the deviations from this ideal structure that are seen in actual films and the origin of some of these defects is related to the mechanism of polymerization. The planarity of the chain structure is shown to be important for high conductivity in pyrrole polymers which appears to involve spinless carriers perhaps associated with bipolarons. (Author)

Characterization of polypyrrole

Polypyrrole is an example of a conjugated polymer with a non-degenerate ground state. It has therefore been suggested that the stable defect states formed upon doping are bipolarons: doubly charged, spinless bound states of two holes with associated deformation of the molecule. ESR, transport, photoelectron spectroscopy and optical absorption data from pyrrole polymers are presented as evidence that indeed polarons and bipolarons are formed upon doping. Electron spin resonance reveals that the conduction mechanism in heavily doped samples involves charge carriers which possess no spin. At doping levels of order tenths of one percent a narrow ESR signal appears, indicating the presence of isolated polarons, but at higher concentrations of oxidant the spin concentration drops again as the polarons bind in pairs to form diamagnetic bipolarons. The transport properties of polypyrrole indicate hopping among localized states, but the data cannot be described in any of the conventional models of disorde...

P. Pfluger

Papers

Preparation and characterization of neutral and oxidized polypyrrole films

Electron-spin-resonance studies of pyrrole polymers: Evidence for bipolarons

The evidence for bipolarons in pyrrole polymers

Characterization of polypyrrole

Evidence for Bipolarons in Pyrrole Polymers