Conducting polymers polaron-bipolaron model

These ideas developed by chemists resemble the bipolaron model, which presents the solid-state physicist s view of the electronic properties of doped conducting polymers [96]. The model was originally constructed to characterize defects in inorganic solids. In chemical terminology, bipolarons are equivalent to diionic states of a system (S = 0) after oxidation or reduction from the neutral state. The transition from the neutral state to the bipolaron takes place via the polaron state (= monoion, S = 1/2,... 

Bipolaron — Bipolarons are double-charged, spinless quasiparticles introduced in solid state physics [i]. A bipolaron is formed from two -> polarons (charged defects in the solid). For chemists the double-charged states mean dications or dianions, however, bipolarons are not localized sites, they alter and move together with their environment. By the help of the polaron-bipolaron model the high conductivity of -> conducting polymers can be explained. 

The electronic band structure of a neutral polyacetylene is characterized by an empty band gap, like in other intrinsic semiconductors. Defect sites (solitons, polarons, bipolarons) can be regarded as electronic states within the band gap. The conduction in low-doped poly acetylene is attributed mainly to the transport of solitons within and between chains, as described by the intersoliton-hopping model (IHM) . Polarons and bipolarons are important charge carriers at higher doping levels and with polymers other than polyacetylene. 

Polaron — Polarons are charged quasiparticles with spin lf. This term has been introduced by physicists as one of the possible solutions to the equations of the relevant defect model of solids in order to describe an electron in a dielectric polarizing its environment (electron-phonon coupling), electrically situated below the conduction band, and transported together with its polarized environment. Polarons and -> bipolarons are the charge carriers in oxidized or reduced (doped) -> conducting polymers. A polaron is defined as a neutral and a charged -> soliton in the same... 

An important part of Andre and Bredas s work during the 1980s was to analyze and extend the soliton, polaron and bipolaron models to a number of cases [54] polypyrrole, polyparaphenylene, poly anilines, polythiophenes, etc. More details on the theory of conducting polymers can be found in specialized monographs [55]. 

As discussed in previous sections, the mechanisms for the formation of metallic state and charge conduction in conducting polymers have been the subject of intensive study since the occurrence of an insulator to metal transition was reported upon doping. It is proposed that non linear defects such as polarons, solitons and bipolarons have a major role in these systems [36, 95-97]. In earlier synthesised conducting polymers, inhomogeneities often dominated the transport properties, and metallic island models were proposed to... 

The concept of electric conduction in 7r-conjugated polymers used to be explained in terms of polaron , bipolaron , soliton , and band model [14e]. The iodine-doping of crystalline PTh prepared by the... 

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