Charge mobility edge

Potential fluctuations due to charged defects and dopants provide the most plausible explanation of the energy difference between the thermopower and the conductivity in doped material. In undoped a-Si H either potential fluctuations or a broadened mobility edge which gives a continuous energy dependence of o( ) could be the explanation. 

We have seen in the previous section that disorder results in the localisation of charge carriers and that the conductivity will fall as a consequence of this. There is a minimum metallic conductivity, which corresponds to the electron mean free path being equal to the lattice repeat distance. The occurrence of the mobility edge means that in an amorphous metal the conductivity can switch... 

It is plausible, and will indeed be demonstrated for our data below, that the charge carrier traps are related to structural defects and/or disorder near the AlOyPc interface. It is further conceivable that there exists some energetic distribution of the traps, and hence has to be seen as a mean energy depth of the traps with respect to the valence band edge, or more generally with respect to some kind of a mobility edge. An often used model which is compatible... 

Such an observation of strong VG and T dependent EA can be explained using the multi trap and release model which assumes a semiconductor with Fermi level closer to the band edge and upon applying VG the Fermi level moves through the distribution of band tail states. As a result EA is reduced as the density of injected charge carriers are increased above the mobility edge [4, 6-11],... 

What is the origin of the charge transport phenomena and what do these experimental observations tell us about the material We show that the Mott-CFO model can answer these questions at least to first order, with the additional assumption that the density of localized band-tail states falls off exponentially away from the mobility edges. In this picture, the time-dependent charge transport is dominated by the statistical process associated with the progressive thermalization of electrons (or holes) into the band-tail states. We confine the discussion to electrons and assume that it can be generalized to holes trivially. 

We note that the transient absorption studies also identified a faster (<400 ps) recombination, which is largely temperature independent but lightly intensity dependent. Nelson explained that the early time recombination events are due to charges before trapping (above mobility edge), and described the recombination dynamics covering the nanosecond-millisecond timescale using a Monte Carlo technique [114]. 

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