Mechanism polaronic conduction

Although the mechanism of conduction in lithium-doped NiO and other low-mobility semiconductors is a controversial matter, the simple polaron hopping model outlined above serves well as a basis for understanding conduction processes in many of the systems discussed later (eg Section 4.4.1). 

There are several distinct mechanisms of conductivity that may occur in these materials. Many conjugated polymers (without metal groups) become conductive when oxidized. Oxidation introduces charge carriers, of which polarons (radical cations) and bipolarons (dications) are the most important [14]. These charge... 

The most widely accepted mechanism of conduction in nondegenerate EAPs, illustrated in Figure 10 for polythiophene, was proposed simultaneously by Bredas and co-workers (240) and Bishop and co-workers (241) in 1981. This mechanism involves a one-electron oxidation to form a radical cation, which is called a po-laron. This radical cation is resonance-stabilized over several rings. Removal of a second electron gives a dicationic species with no impaired electrons, called a bipolaron. The two ions in a bipolaron need to be isolated from each other to minimize unfavorable interactions recent work suggests that at least five rings are needed to stabilize a dicationic species (242,243). The conversion between neutral, polaronic, and bipolaronic species is reversible, using either chemical or electrochemical means to oxidize or reduce the polymer. 

Many other time parameters actually enter - if the molecule is conducting through a polaron type mechanism (that is, if the gap has become small enough that polarization changes in geometry actually occur as the electron is transmitted), then one worries about the time associated with polaron formation and polaron transport. Other times that could enter would include frequencies of excitation, if photo processes are being thought of, and various times associated with polaron theory. This is a poorly developed part of the area of molecular transport, but one that is conceptually important. 

Conjugated conducting polymers consist of a backbone of resonance-stabilized aromatic molecules. Most frequently, the charged and typically planar oxidized form possesses a delocalized -electron band structure and is doped with counteranions (p-doping). The band gap (defined as the onset of the tt-tt transition) between the valence band and the conduction band is considered responsible for the intrinsic optical properties. Investigations of the mechanism have revealed that the charge transport is based on the formation of radical cations delocalized over several monomer units, called polarons [27]. 

Similar investigations have been carried out for the system LaCr xMnx03 [106]. A significant improvement in sinterability appears when Mn is substituted for Cr. For example, densities above 95% of theoretical were achieved at 1475 °C in air for La0.9Sr0 ]Cr03Mn07O3. Electrical conductivity and Seebeck coefficient results are interpreted by a small polaron mechanism for all compositions. This is illustrated for conductivity in Fig. 33. It was also demonstrated that the carrier (electron hole) mobility rather than carrier concentration governs the electronic transport. 

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... 

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