Solitons, Polarons, and Bipolarons in Conjugated Polymers

Refs. [i] Chance RR, Boundreaux DS, Bredas J-L, Silbey R (1986) Solitons, polarons and bipolarons in conjugated polymers. In Skotheim TA (Ed) Handbook of conducting polymers, vol. 2, Marcel Dekker, p 825 [ii] Inzelt G (1994) Mechanism of charge transport in polymer-modified electrodes. In Bard AJ (Ed) Electroanalytical chemistry, vol. 18, Marcel Dekker [iii] Lyons MEG (1994) Charge percolation in electroactive polymers. In Lyons MEG (ed) Electroactive polymer electrochemistry, Parti, Plenum, New York, p 1... 

Bredas, J.L., et al. 1984. The role of mobile organic radicals and ions (solitons, polarons and bipolarons) in the transport properties of doped conjugated polymers. Synth Met 9 265. 

Contrary to inorganic crystalline semiconductors, where charge is transported in general by electrons in the conduction band and holes in the valence band, in doped conjugated polymers charged solitons, polarons, and bipolarons act as charge carriers. These quasi-particles are the direct consequence of the strong electron-phonon interaction present in these quasi-one-dimensional polymers. 

The purpose of this chapter is to illustrate the usefulness of PES for studying the electronic and chemical structure of conjugated polymer surfaces and interfaces. As this handbook deals with all aspects of conjugated polymers, the background information on conjugated polymers is kept to a minimum here. It is further assumed that the reader is familiar with some of the unique electronic structural issues in the physics of conjugated polymers, i.e., the concepts of solitons, polarons, and bipolarons. 

Spectroscopy studies of conjugated polymers have been an important source of information on their electronic structure in particular, photoinduced absorption (excitation spectroscopy) has successfully characterized the gap states associated with the nonlinear excitations (solitons, polarons, and bipolarons) and determined the energies of those states relative to the conduction and valence bands in several conjugated polymers. 

The elementary excitations of a conjugated polymer chain can be described within the mono-electronic approach as electron and hole quasiparticles [74] in a one-dimensional band structure, possibly weakly bound into extended Wannier-type excitons [71,75]. Within this framework, electron-phonon interactions lead to a peculiar family of exotic excitations including solitons, polarons, polaron pairs and bipolarons. In many cases, however, disorder is so significant that the polymer films are better described as an ensemble of relatively short conjugated segments [76], essentially behaving... 

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