Delocalized transport, polaronic band

At very low temperatures, Holstein predicted that the small polaron would move in delocalized levels, the so-called small polaron band. In that case, mobility is expected to increase when temperature decreases. The transition between the hopping and band regimes would occur at a critical temperature T, 0.40. We note, however, that the polaron bandwidth is predicted to be very narrow ( IO Viojo, or lO 4 eV for a typical phonon frequency of 1000 cm-1). It is therefore expected that this band transport mechanism would be easily disturbed by crystal defects. 

From the combined consideration of [83] and [172] it emerges that the inclusion of the nonlocal electron phonon coupling considerably modifies the Holstein picture. Contrary to what is expected in a pure Holstein model, there is no abrupt change in transport mechanism between delocalized transport and activated transport. The polaronic band mechanism, appropriate at least until 100 K, gives way to the DLTD mechanism until 300 K without approaching the hopping limit. Since both mechanisms result in a similar temperature dependence of the mobility (fj. aT ) they can both be extended beyond their correct limit of applicability, making it relatively easy to interpolate between them (or extrapolate each of them). 

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]. 

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