Band transport mobility

Energy Band Dispersion and Band Transport Mobility... 

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. 

The cubic (ao = 8.441 A) end member Zn[Fe2]04 was seen to be n-type with mobile electrons moving at the bottom of a narrow 3d band. Transport properties... 

Figure 1 Schematic presentation of the two different mechanisms governing charge transport in organic semiconductors. The so-called hopping transport assumes a thermally activated hopping between charge-traps and is always present, the resulting mobilities are very small. For certain materials a mechanism yielding much higher mobilities has been observed, which is commonly referred to as band-like , thus implying a similarity to band-transport observed in conventional semiconductors.

For species that migrate by other mechanisms than diffusion (notably itinerant electrons and holes), the diffusion coefficient is undefined, and instead one uses mobilities with temperature dependences typical of band transport with phonon-, impurity-, or large polaron-Hmited transport [8]. 

The electronic properties of solids were described in Chapter 2 using the band model. A characteristic feature of semiconductors is the separation of the electron energy levels into two bands, the valence band with occupied energy levels and the conduction band with unoccupied energy levels. Both bands are separated by an energy gap. The band gap energy determines the intrinsic conductivity because electricity can only be transported through the semiconductor if some electrons are excited from the valence band to the conduction band. Then either holes in the valence band or electrons in the conduction band become mobile. The mobility of valence band holes and conduction band electrons... 

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