Charge carrier generation concentration

Transient Photoconductivity. A solution of neutral molecules in a polar solvent shows only ohmic conductivity, but if ions are formed by the action of the photolytic flash these charge carriers generate an additional current which is proportional to the ion concentration. The observation of such transient photocurrents is the most direct experimental evidence for the formation of free, solvated ions in electron transfer reactions. The quantum yield of ion formation can be obtained through proper calibration procedures and the kinetics of ion recombination can be determined. Figure 7.37 gives an example of such transient photocurrent rise and decay. 

Illumination of the semiconductor leads to charge carrier generation p, which is enhanced by a carrier concentration Ap over the dark carrier concentration Po, for a given generation rate G and recombination rate U ... 

The conduction of liquids depends (i) on the concentration, c, of charge carriers due to autodissociation (e.g., in water c= 10 molxdm ), the concentration of impurities being left after purification, (ii) on the concentration of charge carriers generated when potential is applied to the electrode, and (iii) on the mobility of the existing and newly formed charge carriers. It can be described by the equation... 

In some polymers, the concentration of intrinsic, thermally generated, or photogenerated mobile carriers and their mobility are relatively high and these materials exhibit semiconducting properties. Because properties of polymeric semiconductors cannot be described by classical band theory elaborated for covalent crystalline semiconductors, some specific models used to describe charge carrier generation and transport in polymeric semiconductors will be presented in this section. 

It is important to note that there may be at least two reasons for obtaining deviations from a purely exponential behavior for a PMC transient. These are a too high excess carrier generation, which may cause interfacial rate constants that are dependent on carrier concentration, and an interfacial band bending AU, which changes during and after the flash. For fast charge transfer, a more complicated differential equation has to be solved. 

For p-type electrodes in the dark and in the photoexdted state, the concentration of majority charge carriers (holes) is sufficiently great that the Fermi level eptso of the electrode interior nearly equals the quasi-Fermi level of interfacial holes hence, the overvoltage Up sc for the generation and transport of holes in the space charge layer is zero even as the transfer of anodic holes progresses as expressed in Eqn. 10-30 ... 

Electrochemical properties of silicon single crystals, usually cuts of semiconductor wafers, have to be considered under two distinct respects (1) As an electrode, silicon is a source of charge carriers, electrons or positive holes, involved in electrochemical reactions, and whose surface concentration is a determining parameter for the rate of charge transfer. (2) As a chemical element, silicon material is also involved in redox transformations such as electroless deposition, oxide generation, and anodic etching, or corrosion processes. 

This effect is characterized by light-induced formation of a photo-emf in boundary layer-free systems 8>10>. It is necessary for the generation of the Dember emf to have (a) the formation of a concentration gradient of charge carriers resulting from non-uniform illumination, and (b), electrons and holes of different mobilities ... 

Recall that the concept of Fermi quasilevels, suggested by Shockley (1950), can be introduced as follows. Under steady state photogeneration of charge carriers, a dynamic equilibrium arises in a semiconductor between generation and recombination of electron-hole pairs. As a result, certain steady state (but not equilibrium ) concentration values nj and p are established. The quasiequilibrium concentrations ng and pg are defined by the relations ng = n0 + A and Po = Po + Ap> and since photogeneration of carriers occurs in pairs, we have An = Ap = A. Let the following inequalities be satisfied ... 

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