Excess charge carrier bands

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 ... 

Liu CY, Bard AJ (1989) Effect of excess charge on band energetics (optical absorption length and carrier redox potential) in small semiconductor particles. Phys Chem 93 3232-3237. 

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. 

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. 

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... 

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. 

Two types of fluorescence spectra are observed in aqueous solution and in the presence of an excess of cadmium ions. In the absence of any stabilizer the fluorescence is characterized by two very weak bands (41), one centered at 450 nm and attributed to the direct recombination of charge carriers (61) from shallow traps, and the other very broad at about 650 nm, which is not clearly attributed. The presence of a stabilizing agent, such as HMP (59,60), makes it possible to increase the sulfide vacancies at the surface resulting in a more intense fluorescence band, centered in the region of 550 nm. This band is attributed to the recombination of... 

In the presence of an excess of sulfide ions two fluorescence emissions are observed. The first is centered at 450 nm and is attributed to the direct recombination of charge carriers. The second emission band, observed at around 650 nm, depends on the particle size. This second emission band is very weak and is very often quenched by the presence of species absorbed at the interface. By analogy it could be attributed to cadmium ion vacancies. 

The production of free charge carriers during fracture of single crystal silicon has recently been inferred from transient increases in conductivity during this period ( ). phE in the visible portion of the spectrum is not observed from silicon, presumably due to the small band gap of this material. However, EE has been observed (22). This is remarkable considering that the energy required to promote a silicon valence electron to the vacuum level is on the order of 4 eV, well in excess of the band gap energy. ... 

When electrons from the metallic phase are transferred into the n-type semicondnctor, these electrons are not confined to dopant atom sites. Instead, the majority carriers exist as mobile charges in the conduction band. Becanse these excess majority carriers need not reside on dopant atoms, bnt can... 

The measured potential Vm, and thus jEf and K. can be varied through external polarization. Vm is the applied potential when the electrode is externally polarized and is the open-circuit potential without external polarization. When the semiconductor has no excess charge, there is no space charge region and the bands are not bent. The electrode potential under this condition is called the flatband potential Vn,. The flatband potential is an important quantity for a semiconductor electrode because it connects the energy levels of the carriers in the semiconductor to those of the redox couple in the electrolyte and it connects the paramete s that can be experimentally determined to those derived from solid-state physics and electrochemistry. It can generally be expressed as... 

Fig. 4. Band bending in a thin film on a metal substrate where the calculated value of the space-charge layer width, IF, exceeds the film thickness, Lt, so that excess charge appears on the metal. In this situation, the field assists the separation of photoexcited carriers throughout the entire film.

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