Excess charge carriers

Some precautions will be needed for successful measurements. The shorter the time scale the higher the photon densities that will be required. This leads to very high generation of excess charge carriers and to nonlinear phenomena of a complicated nature. 

How can such problems be counterbalanced Since a large capacitance of a semiconductor/electrolyte junction will not negatively affect the PMC transient measurement, a large area electrode (nanostructured materials) should be selected to decrease the effective excess charge carrier concentration (excess carriers per surface area) in the interface. PMC transient measurements have been performed at a sensitized nanostructured Ti02 liquidjunction solar cell.40 With a 10-ns laser pulse excitation, only the slow decay processes can be studied. The very fast rise time cannot be resolved, but this should be the aim of picosecond studies. Such experiments are being prepared in our laboratory, but using nanostructured... 

It has been shown theoretically that an extra electron or hole added to a one-dimensional (ID) system will always self-trap to become a large polaron [31]. In a simple ID system the spatial extent of the polaron depends only on the intersite transfer integral and the electron-lattice coupling. In a 3D system an excess charge carrier either self-traps to form a severely locahzed small polaron or is not localized at all [31]. In the literature, as in the previous sections, it is frequently assumed for convenience that the wavefunction of an excess carrier in DNA is confined to one side of the duplex. This is, of course, not the case, although it is likely, for example, that the wavefunction of a hole is much larger on G than on the complementary C. In any case, an isolated DNA molecule is truly ID and theory predicts that an excess electron or hole should be in a polaron state. 

The question of n- and p-type (excess charge carriers in conductivity and valence band, respectively) mechanisms of semiconductors is shown in Figs. 7.28 and 7.29. For this reason, p-type electrodes will be suitable as anodes,23 i.e., a deelectronation reaction, in which electrons are accepted from ions in the solution layer next to the electrode into the waiting holes in the valence band. Semiconductor doped n will be cathodes. 

Stimulated Emission—This occurs when photons in a semiconductor stimulate available excess charge carriers to the emission of photons. The emitted light is identical in wavelength and phase with the incident coherent light. 

If photons of sufficient energy are incident on a semiconductor, excess electrons and holes are created in the semiconductor conduction and valence bands respectively. Further, if the semiconductor is fabricated to contain one or more p-n junctions, the chemical potential of the excess carriers can be converted into a flow of charges resulting in an electric current. This current can then be used to power the direct electrolysis of water. Alternatively, the excess charge carriers can migrate to the semiconductor surface where they initiate chemical reactions and produce H2 and/or 02 in the surrounding medium either in a PEC or in a suspension of semiconductor particles. 

FIGURE 2.2. (a) Schematic view of SCLC. With an external electric field (V/d), an additional internal field (E ) induced by injected excess charge carriers plays important role for the achievement of high current density. The total electric field, Etotai = V/d + E, significantly enhances current flow. Open symbols (o) corresponds to excess holes, closed symbols ( ) correspond to excess electrons, (b) Comparison of J—V characteristics based on ohmic current (dashed line) and SCLC (solid line). The meshed area shows the J—V requisite for practical devices. 

Space charge polarization is due to the presence of excess charge carriers (electronic or ionic). A macroscopic charge transfer that may be intrinsic (heterocharges) or extrinsic (homocharges) is observed between the electrodes. This polarization is more complex than the dipolar one because it depends on a great number of parameters. 

In the treatment of photoconductivity in Sect. 8.4.1, we simply mentioned in passing the primary process the generation of the excess charge carriers using the internal photoeffect. In this section, we will treat the details of this process and choose again as an example the anthracene crystal. 

The material is assumed to be an insulator, i.e. the thermally-generated charge-carrier density is so small that it makes no noticeable contribution to the transport and therefore can be neglected in the model. All the charge carriers which participate in the transport or are captured in traps are excess charge carriers injected from the contacts. 

For an understanding of charge-carrier mobilities, it is necessary to have some knowledge of the band structures and Eh(k) of the excess charge carriers. Here, Eg and Eh are the energies of the electrons and the holes, and k is their... 

In the tight-binding approximation, the energies of the two excess charge carrier bands of each type, +(k) and (k) for the holes and for the electrons, are determined by the transfer integrals, tf and tf (see below) between the molecular HOMOs or between the molecular LUMOs ... 

With Eqns. (8.69) to (8.72), the band structures for excess charge carriers in several different polyacene crystals were calculated numerically [42]. As an example. 

One result of the Bassler model is the relaxation of the excess charge carriers towards thermal equiUbrium after their production by photoexcitation (Fig. 8.46). The equilibrium energy (Eoo) of the charge carriers which were generated with... 

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