Charge storage and charge transport in conducting polymers: solitons, polarons and bipolarons

The results of a series of experiments demonstrate that solitons are the important excitations in trans-(CH)x and that the properties of these nonlinear excitations can be directly studied during photoexcitation or after doping. The importance of these concepts in the more general context of conducting polymers is addressed. Although the twofold degenerate ground state of trans-(CH)x is quite special, the relevant concepts have been generalized to confined soliton pairs (bipolarons). Experimental results that demonstrate electron-hole symmetry and weak confinement in polythiophene make this polyheterocycle a nearly ideal example of a model system in which the ground-state degeneracy has been lifted. In the dilute doping regime, in situ absorption spectroscopy data (during electrochemical doping) strongly suggest charge storage via bipolarons with confinement parameter γ ≈ 0.1–0.2. These results on polythiophene demonstrate that a quantitative fundamental understanding is possible even for relatively complex systems.