Carriers thermal

A key issue is the improvement of solar light harvesting. New active materials with high optical absorption in the visible and good photostability are needed. Implementation of carrier multiplication through impact ionization in quantum dots arrays could mitigate the losses related to carrier thermalization. The alternative approach is the development of vertically stacked tandem systems of increasing band gap active materials, which effect H+ reduction and water oxidation on opposite sides. 

A carrier thermally released from the trap into the transport band may be either retrapped by the same species of traps or a different one and, under the influence of an electric field, may contribute to an externally measurable current. It may either be swept out of the region being probed or recombined with a recombination center. Some of the electrons may even overcome the work function barrier and leave the solid. The traffic of these carriers from traps to the recombination centers or out of the material can be monitored at various stages, and thus, information on the thermal emission rates can be obtained indirectly. 

If the excitation occurred at a low temperature such that the thermal emission rate of carriers from traps is very small, the perturbed equilibrium will exist for a long time and only upon an appropriate increase of the sample temperature can the relaxation process proceed at a rate that permits one to monitor it by measuring the conductivity a(T) = exp(ncfin + Pl p) of the sample (TSC) or the luminescence (TSL) emitted by radiative recombination of carriers thermally released from the traps. 

The study of the dispersion of photoinjected charge-carrier packets in conventional TOP measurements can provide important information about the electronic and ionic charge transport mechanism in disordered semiconductors [5]. In several materials—among which polysilicon, a-Si H, and amorphous Se films are typical examples—it has been observed that following photoexcitation, the TOP photocurrent reaches the plateau region, within which the photocurrent is constant, and then exhibits considerable spread around the transit time. Because the photocurrent remains constant at times shorter than the transit time and, further, because the drift mobility determined from tt does not depend on the applied electric field, the sample thickness carrier thermalization effects cannot be responsible for the transit time dispersion observed in these experiments. 

Low temperature tunneling thermalization in the band tail is observed by time resolved luminescence, which is a transition between band tail electrons and holes, as described in Section 8.3.2. Fig. 8.11 (a) shows the change in energy of the luminescence peak with increasing time after the excitation pulse, measured at 12 K. The decrease of the energy at times up to 10" s is caused by carrier thermalization - the changes at... 

In summary, the experimental data confirm the models of carrier thermalization. Thermalization in extended states is very rapid and is completed in less than 10 s. Thermalization by tunneling between localized states becomes increasingly slow as the carriers move into the band tail and at high temperatures is overtaken by the multiple trapping mechanism of sequential thermal excitation and trapping. 

Free-carrier thermalization with lattice, emission of longitudinal optical phonons... 

Actual calculations for the two limit cases above include also Drude-like intraband contributions, with a plasma frequency calculated self-consistently from the band structure [3]. As already discussed (cf. Eq. (2)), in the U = 0 limit this is the dominating contribution to the optical properties. However, intraband terms are also important in the large-U limit, where they account for the relevant contribution of the carriers thermally excited across the narrow gaps at high temperatures. 

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