The Flat Band Potential of Semiconductor Electrodes

At the flat band potential of semiconductor electrodes at which the space charge osc is zero, the charge balance is expressed in Eqn. 5-86 ... 

The flat band potential of semiconductor electrodes is determined by the potential across the compact las r at the electrode interface and is characteristic of individual semiconductor electrodes. For semiconductor electrodes in the state of band edge level pinning, the potential across the compact layer remains constant and independent of the electrode potential. For some semiconductor electrodes, however, photon irradiation changes the potential across the compact layer and, hence, shifts the flat band potential of the electrode. 

Consider the interface between a semiconductor and an aqueous electrolyte containing a redox system. Let the flat-band potential of the electrode be fb = 0.2 V and the equilibrium potential of the redox system o = 0.5 V, both versus SHE. Sketch the band bending when the interface is at equilibrium. Estimate the Fermi level of the semiconductor on the vacuum scale, ignoring the effect of dipole potentials at the interface. 

Figure 5-47 shows the Mott-Schottky plot of n-type and p-type semiconductor electrodes of gallium phosphide in an acidic solution. The Mott-Schottl plot can be used to estimate the flat band potential and the effective Debye length I D. . The flat band potential of p-type electrode is more anodic (positive) than that of n-type electrode this difference in the flat band potential between the two types of the same semiconductor electrode is nearly equivalent to the band gap (2.3 eV) of the semiconductor (gallium phosphide). 

Figure 10-17 shows the polarization ciirves for the cathodic hydrogen reaction (cathodic electron transfer) on a p-type semiconductor electrode of galliiun phosphide. The onset potential of cathodic photoexcited hydrogen reaction shifts significantly from the equilibrium electrode potential of the same hydrogen reaction toward the flat band potential of the p-type electrode (See Fig. 10-15.). 

There is, however, a definite difference in the electrode potential between the photoexcited rc-type electrode and the p-type electrode of the same semiconductor. The electrode potential of the former is less positive than that of the latter by a magnitude nearly equal to Ae = n P p P, which is the difference between the flat band potential of the n-type and that of the p-type. The anodic dissolution of the photoexcited w-type semiconductor electrode, as a result, will occur in the potential range less positive by about Ae than that for the p-type electrode of the same semiconductor. Figure 22.10 shows schematic polarization curves of the anodic hole emitting dissolution with an n-type electrode and with a p-type electrode of the same semiconductor under the dark and photoexcitation conditions. Such photoexcited dissolution was observed with w-type GaAs electrodes [14,15],... 

Furthermore, as we saw in a foregoing section, photoexcitation produces in a semiconductor electrode electron-hole pairs and introduces a photo-potential, which reduces the space charge potential in the semiconductor. With an n-type semiconductor in contact with a corroding metal, photoexcitation raises the Fermi level up to the flat band level of the semiconductor, thus shifting the corrosion potential in the less positive direction toward the flat band potential of the n-type oxide as shown in Figure 22.35c. Photoexcitation therefore will shift the corrosion potential in the less positive (more cathodic) direction and the corrosion will then be suppressed. With some n-type oxides such as titanium oxide, photoexcitation brings the interfacial quasi-Fermi level, peF, down to a level lower than the Fermi level, F(redox> of the oxygen electrode reaction ... 

When photoelectrochemical solar cells became popular in the 1970s, many reports appeared concerning the stability, dissolution, and flat-band potential of semiconductors in solutions. These papers investigated parameters such as the energy level of the band edges, which is critical for the thermodynamic stability of the semiconductor and how to determine the potential for the onset of the (photo) electrochemical etching [38-40]. The criterion for thermodynamic stability of a semiconductor electrode in an electrolyte solution is determined by the position of the Fermi level with respect to the decomposition potential of the electrode with either the conduction band electrons or valence band holes E. Under illumination, the quasi-Fermi level replaces the Fermi level. The Fermi level is usually found within the band gap of the semiconductor and its position is not easily evaluated (especially the quasi-Fermi level of minority carriers). Therefore it was found more practical to use the conduction band minimum (Eq) and valence band maximum (Ey) as criteria for electrode corrosion. Thus, a semiconductor will be corroded in a certain electrolyte by the conduction band electrons if its... 

Erne et al. [240] have recently shown that the potential dependence of the DA under the depletion conditions is similar to the Mott-Schottky line, which allows determination of the flat-band potential of the electrode. Other examples of employing DA in spectroelectrochemical studies are analysis of the relaxation process of free carriers in semiconductor electrodes [241] and elucidating the mechanisms of interfacial processes (Section 7.5). 

Here e and so are the permitivity of diamond and free space, respectively, and e is the electron charge the -I- and signs relate to donors and acceptors, respectively. We recall that equation (4.4) reflects the potential dependence of the thickness of the space-charge layer in the semiconductor. By extrapolating the line to C 2 0, the flat band potential of the diamond electrode can... 

Efficient photoelectrochemical decomposition of ZnSe electrodes has been observed in aqueous (indifferent) electrolytes of various pHs, despite the wide band gap of the semiconductor [119, 120]. On the other hand, ZnSe has been found to exhibit better dark electrochemical stability compared to the GdX compounds. Large dark potential ranges of stability (at least 3 V) were determined for I-doped ZnSe electrodes in aqueous media of pH 0, 6.3, and 14, by Gautron et al. [121], who presented also a detailed discussion of the flat band potential behavior on the basis of the Gartner model. Interestingly, a Nernstian pH dependence was found for... 

Here, Ws is the work function of electrons in the semiconductor, q is the elementary charge (1.6 X 1CT19 C), Qt and Qss are charges located in the oxide and the surface and interface states, respectively, Ere is the potential of the reference electrode, and Xso is the surface-dipole potential of the solution. Because in expression (2) for the flat-band voltage of the EIS system all terms can be considered as constant except for tp (which is analyte concentration dependent), the response of the EIS structure with respect to the electrolyte composition depends on its flat-band voltage shift, which can be accurately determined from the C-V curves. 

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