Majority Carrier Reactions

Dark current-potential curves representing a majority carrier transfer to a redox system have been measured by many research groups. Mostly cathodic currents at n-type electrodes have been studied rather than anodic currents at p-type semiconductors. This is because anodic hole consumption from p-type electrodes usually results in corrosion of the material. At least it is difficult to find a redox system where the oxidation of the redox couple competes sufficiently quickly with the corrosion. 

According to these results, the log j vs. A(psc plots are linear and have a slope of nearly 60 mV/decade of current, as expected from the theory (see Eq. 7.61a). Morrison also evaluated the log 7 vs. Aij)sc plots in terms of rate constants. Taking one example, the reduction of [Fe(CN)(i] at pH 3.8, kl = 1.3 x 10 cm s was obtained. The maximum rate constant /Cc.max defined by Eq. (7.55), can only be calculated if the reorganization energy A is known besides the energy difference El - Ep edox The standard redox potential of the [Fe(CN)6] couple is p,redox = +0-2 eV and El = -0.2 eV as calculated from Mott-Schottky measurements. Two different A values, namely 0.5 and 0.75 eV, were reported for [Fe(CN)6] [14, 28J. The corresponding maximum rate constants are then A e max = 1-6 x 10 and 6.8 x 10 cm s , respectively. The rate constants kl and Aj max do not differ very much because the conduction band is rather close to the maximum of the density of empty states of the redox system. 

The question arises concerning which value of the anodic reverse can then be expected for the same redox system at ZnO. Assuming the same maximum rate con- 

This is a very low value because the occupied states of the redox system are mainly distributed below the conduction band. The anodic current which is expected to be independent of the band bending, is given by Eq. (7.52). Assuming lO cm and 

7 Charge Transfer Processes at the Semiconductor-Liquid Interface 

According to these results, the log j versus A(p plots are linear and have a slope of nearly 60mV/decade of current, as expected from the theory (see Eq. (7.61a)). Morrison also evaluated the logy versus plots in terms of rate constants. Taking one example, the reduction of [Fe(CN)5] at pH 3.8, k = 1.3 X 10 cm s was obtained. The maximum rate constant as 

The question arises concerning which value of the anodic reverse y can then be expected for the same redox system at ZnO. Assuming the same maximum rate constant and the same A values as for the reduction of the [Fe(CN)g] / system, one obtains, according to Eq. (7.53), k = 1.5 X cm s , of course, for both A values. This is a very low value because the occupied states of the redox system are mainly distributed below the conduction band. The anodic current which is expected to be independent of the band bending is given by Eq. (7.52). Assuming 10 cm and = 6x 10 cm (correspondingto 10 M), one obtains 10 A cm , that is, a current density which should be measurable. Morrison tried to measure it unfortunately, however, the currents were not reproducible. 

Redox systems such as ferrocene (Fc / ) and cobaltocene (CoCp ) and their derivatives are usually assumed to be suitable nonadsorbing outer-sphere redox couples for use in nonaqueous solutions such as methanol or acetonitrile. One example is the cathodic reduction of methyl ferrocenium (Me2Fc ) at n-InP in H20-free methanol. The log curve and the corresponding Mott-Schottky 

In addition, Lewis and co-workers have investigated the electron transfer from the conduction band ofn-Si electrodes to viologen in HjO-free CH3OH [30]. Here also the currents vary linearly with the concentration of the corresponding redox 

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Fig. 31. Concept of overpotential for minority carrier reactions at n-type electrode (left) and for majority carrier reactions at p-type electrode (right)...

Concerning valence band processes, one observes the opposite effect. Here the anodic current at a p-type electrode rises with increasing anodic overvoltage because sufficient holes are available. At an n-type electrode, only a small current occurs which can be enhanced again by excitation. In the case of a cathodic current at a p-type electrode, the holes injected into the valence band, are easily transported to the rear contact. Accordingly, valence band processes at p-type electrodes are majority carrier reactions. The kinetics of this process are determined by Eq. (7.61b) and the corresponding theoretical /v- curves are given in Fig. 7.14 for various / values. 

It is well known that photoelectrochemical measurements do not indicate photocurrents in the accumulation region of an illuminated semiconductor. The reason is that majority carriers control interfacial reactions, which... 

Lipid peroxidation is a radical-mediated chain reaction resulting in the degradation of polyunsaturated fatty acids (PUFAs) that contain more than two covalent carbon-carbon double bonds (reviewed by Esterbauer et al., 1992). One of the major carriers of plasma lipids is LDL, a spherical molecule with a molecular weight of 2.5x10 . A single LDL particle contains 1300 PUFA molecules (2700 total fatty-acid molecules) and is... 

The photopontential also approaches to zero when the semiconductor photoelectrode is short-circuited to a metal counterelectrode at which a fast reaction (injection of the majority carriers into the electrolyte) takes place. The corresponding photocurrent density is defined as a difference between the current densities under illumination, /light and in the dark, jDARK ... 

It is possible that electrons are not the major carrier of negative charge in dense interstellar clouds. It has been suggested that if a large fractional abundance of polycyclic aromatic hydrocarbons (PAHs) exists throughout dense interstellar clouds (see Section I), then electron sticking reactions of the type,... 

This is the regime of cathodic currents. The silicon atoms of the electrode do not participate in the chemical reaction in this regime. An n-type electrode is under forward bias and the current is caused by majority carriers (electrons). The fact that photogenerated minority carriers (holes) are detectable at the collector indicates that the front is under flat band or accumulation. A decrease of IBC with cathodization time is observed. As Fig. 3.2 shows, the minority carrier current at the collector after switching to a cathodic potential is identical to that at VQcp in the first moment, but then it decreases within seconds to lower values, as indicated by arrows in Fig. 3.2. This can be interpreted as an increase of the surface recombination velocity with time under cathodic potential. It can be speculated that protons, which rapidly diffuse into the bulk of the electrode, are responsible for the change of the electronic properties of the surface layer [A17]. However, any other effect sufficient to produce a surface recombination velocity in excess of 100 cm s 1 would produce similar results. 

Also of interest in connection with Fig. 16 is a process that has been labeled hot transfer (see, for example, Toyozawa, 1978 Kayanuma and Nasu, 1978 Jortner, 1979). Here it is suggested that in the case AE > A the transition to the ground state can take place during the lattice relaxation, i.e., before the excited state has reached its equilibrium position. This effect was first suggested by Dexter et al. (1955) and further analyzed by Bartram and Stoneham (1975) for F centers. A recent slight modification, in terms of fast capture of a majority carrier subsequent to that of a minority carrier, has been suggested by Sumi (1981) he points out that this process may be active in recombination-enhanced defect reactions. 

Consider now the processes caused by the formation of quasilevels. As was noted above, the shift of Fn relative to F is very small for majority carriers (electrons) and can usually be neglected precisely, this was done in constructing Fig. 16b. But for minority carriers (holes) the shift of Fp can be very large. The shifts of both Fnx F and Fp increase with the growing intensity of semiconductor illumination, so that for a certain illumination intensity Fp may reach the level of the electrochemical potential of anodic decomposition Fdec, p, and Fn—the level of a certain cathodic reaction (for example, reduction of water with hydrogen evolution FHljH20). These reactions start to proceed simultaneously, and their joint action constitutes the process of photocorrosion. 

The occurrence and deactivation of excited states of the first type are schematically shown in Fig. 35. Let the minority carriers (holes) be injected into the semiconductor in the course of an electrode reaction (reduction of substance A). The holes recombine with the majority carriers (electrons). The energy, which is released in the direct band-to-band recombination, is equal to the energy gap, so that we have the relation ha> = Eg for the emitted light quantum (case I). More probable, however, is recombination through surface or bulk levels, lying in the forbidden band, which successively trap the electrons and holes. In this case the excess energy of recombined carriers is released in smaller amounts, so that hco < Eg (case II in Fig. 35). Both these types of recombination are revealed in luminescence spectra recorded with n-type semiconductor electrodes under electrochemical generation of holes (Fig. 

As indicated in Figure 1, if a semiconductor is biased to depletion in contact with an electrolyte, a photocurrent can be generated upon illumination. This occurs because the photo-excited majority carriers are driven by the electric field in the depletion layer to the counter electrode and minority carriers migrate to the interface where they are trapped at the band edge. Nozik has recently speculated that hot minority carrier injection may play a role in supra-band edge reactions.(19)... 

The major functions of pantothenic acid are in CoA (Section 12.2.1) and as the prosthetic group for AGP in fatty acid synthesis (Section 12.2.3). In addition to its role in fatty acid oxidation, CoA is the major carrier of acyl groups for a wide variety of acyl transfer reactions. It is noteworthy that a wide variety of metabolic diseases in which there is defective metabolism of an acyl CoA derivative (e.g., the biotin-dependent carboxylase deficiencies Sections 11.2.2.1 and 11.2.3.1), CoA is spared by formation and excretion of acyl carnitine derivatives, possibly to such an extent that the capacity to synthesize carnitine is exceeded, resulting in functional carnitine deficiency (Section 14.1.2). 

If the density of holes Ps at the surface - or equivalently the quasi-Fermi level Ep p — are equal at the surface of an n- and p-semiconductor electrode, then the same reaction with identical rates, i.e. equal currents, takes place at both types of electrodes (Fig. 15). Since holes are majority carriers in a p-type semiconductor, the position of the quasi-Fermi level Ep,p is identical to the electrode potential (see right side of Fig. 15), and therefore-with respect to the reference electrode - directly measurable. The density of p can easily be calculated, provided that the positions of the energy bands at the surface are known. The measurements of a current-potential curve also yields automatically the relationship between current and quasi-Fermi level of holes. The basic concept implies that the position of the quasi-Fermi level Ep,p at the surface of an n-type semiconductor and the corresponding hole density Ps can be derived for a given photocurrent, since the same relationship between current and the quasi-Fermi level of holes holds. 

In several cases it has been found that the oxidation of the redox system occurs entirely via hole transfer directly from the valence band to the reduced form of the couple. Then both processes, oxidation of the redox system and corrosion, proceed independently. This is usually not visible from measurements with an n-type electrode, because the photocurrent is entirely determined by the light intensity. As already mentioned above, p-type electrodes are more suitable, because the current is determined by majority carrier transfer (reaction rate Vf, in Fig. 21). From the thermodynamic point of view, the oxidation of Cu at GaAs is an interesting case. The corresponding current-potential curves are given in Fig. 22 [93]. The corrosion current is not changed upon addition of Cu, i.e. corrosion and redox process are completely independent. In this case, the kinetics of the direct hole transfer is obviously very fast, i.e. the redox current is considerably larger than the corrosion current. Both processes occur indepen-... 

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