Anodic transfer reactions of photoexcited holes

We consider a redox reaction involving the transfer of anodic holes at semiconductor electrodes as shoMm in Eqn. 10-26  

the reaction rate is proportional to the concentration of interfacial holes in the electrode as described in Sec. 8.3. Since the concentration of interfacial holes depends on the Fermi level er., of the electrode interface, the reaction current due to anodic holes depends on the Fermi level e ,. of the interface in the dark and on the quasi-Fermi level of interfacial holes pCp, in the photoexcited state. 

This conclusion is valid regardless whether the electrode is n-fype or p-fype. Consequently, if the quasi-Fermi level of interfacial holes in a photoexcited n-type semiconductor electrode equals the quasi-Fermi level of interfacial holes pEp, (eq ial to the Fermi level pEp., of the interface) in a p-type electrode of the same semiconductor in the dark, the current due to anodic holes will be the same on the two electrodes and, hence, the curves of the anodic reaction current as a function of the quasi-Fermi level of interfacial holes will be the same for the two electrodes as suggested in Fig. 10-21. The curves of the anodic reaction current represented as a function of the electrode potential (the Fermi level of the electrode), instead of the quasi-Fermi level of interfacial holes, are not the same for the two electrodes, however. 

The quasi-Fermi level of interfacial holes nearly equals the Fermi level pe , ( pEp,.) in photoexcited p-type electrodes, but the quasi-Fermi level pej. of interfacial holes is lower than the Fermi level aSp,. ( p p,) in photoexcited n-type electrodes as shown in Fig. 10-21. It then follows that the range of electrode potential, where the anodic reaction occurs on the photoexcited n-type electrode, shifts itself, from the range of potential where the same anodic reaction occurs on the dark p-type electrode, toward the caliiodic (more negative) direction by an energy equivalent to (nEp - p p,). 

The transfer of anodic holes is associated with the following three processes the generation and transport of holes in the electrode the hole transfer across the compact layer and the diffusion of redox particles in aqueous solution. The total overvoltage, T], is the sum of the three overvoltages np sc for the generation and transport of holes in the electrode, ria for the transfer of holes across the electrode interface, and ii4ur for the diffusion of redox particles in the solution as defined in Eqn. 10-27  

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