Photoexcitation semiconductor electrode

The boundary between effects thus defined is, however, not sharp. If, for instance, light is absorbed by a layer of a photoactive adsorbate attached to the semiconductor electrode, it is apparently difficult to identify the light-absorbing medium as a solution or electrode material . Photoexcited solution molecules may sometimes also react at the photoexcited semiconductor electrode this process is labelled photogalvanovoltaic effect. 

Wilson RH (1977) A model for the current-voltage curve of photoexcited semiconductor electrodes. J Appl Phys 48 4292-4297... 

Figure 6.25 Outline of a cell for the photoelectrolysis of water. The photoexcited semiconductor electrode is made of SrTi02...

The Role of Interface States in Electron-Transfer Processes at Photoexcited Semiconductor Electrodes... 

The process by which the semiconductor carriers reach the surface to react with surface states must be considered. The case of greatest importance under photoexcitation is with the semiconductor biased to depletion as shown in Figure 1. While it is possible for semiconductor carriers to reach the surface of the semiconductor through tunneling, or impurity conduction processes, these processes have not been shown to be important in most examples of photoexcited semiconductor electrodes. Consequently, these processes will be ignored here in favor of the normal transport of carriers in the semiconductor bands. Furthermore, only carriers within a few kT of the band edges will be considered, i.e., "hot" carriers will be ignored. 

Wilson, RH, A model for the Current-Voltage Curve of Photoexcited Semiconductor Electrodes, 3. Appl. Phys., 48, 4292, 1977. 

FIGURE 29.3 Charge separation during photoexcitation of a semiconductor electrode. 

Under light illumination, semiconductor electrodes absorb the energy of photons to produce excited electrons and holes in the conduction and valence bands. Compared with photoelectrons in metals, photoexcited electrons and holes in semiconductors are relatively stable so that the photo-effect on electrode reactions manifests itself more distinctly with semiconductor electrodes than with metal electrodes. 

Fig. 10-S. Photopotential and band bending in a space chaige layer of semiconductor electrodes (a) in the dark and (b) in a photoexcited state - Ae e - pbotopotential.

Fig. 10-9. Photoexcited reaction current (photocurrent) at semicon ductor electrodes (a) photoexcited reaction of cathodic electron transfer (OX + e - RED) at p-type semiconductor electrode, (b) photoexcited reaction of anodic hole transfer (RED - OX + e) at n-type semiconductor electrode, iph = photocurrent.

Fig. 10-10. Polarization curves for electrode reactions at n-type and p type semiconductor electrodes in the dark and in a photoezdted state dashed curve = dark solid curve = photoexcited V (i )= anodic (cathodic) current in the dark tpi, (t ) = anodic (cathodic) current in a photoexcited state.

Fig. 10-11. Anodic photoexcited dissolution current of an n-type semiconductor electrode of gallium arsenide as a function of electrode potential in a 0.6 M sulfuric add solution lo - photon intensity = diotocurrent. [From Memming-Kelly, 1981.]...

With n-type semiconductor electrodes, the anodic oiQ en reaction (Euiodic hole transfer) will not occur in the dark because the concentration of interfacial holes in the valence band is extremely small whereas, the same reaction will occur in the photon irradiation simply because the concentration of interfadal holes in the valence band is increased by photoexcitation and the quasi-Fermi level pEp of interfadal holes becomes lower than the Fermi level the o en redox... 

Similarly, the cathodic photoexcited electron transfer of the h3 rogen reaction shown in Eqn. 10-19 can occur at p-type semiconductor electrodes at which the cathodic hydrogen reaction is thermodynamically impossible in the dark. 

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.). 

A shift of the flat band potential due to photoexcitation of the type shown in Fig. 10-18 results from the capture of holes in the surface state level, e , on the electrode as shown in Fig. 10-19. We now consider a dissolution reaction involving the anodic transfer of ions of a simple elemental semiconductor electrode according to Eqns. 10-24 and 10-25 ... 

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. 

In photoexcited n-type semiconductor electrodes, photoexcited electron-hole pairs recombine in the electrodes in addition to the transfer of holes or electrons across the electrode interface. The recombination of photoexcited holes with electrons in the space charge layer requires a cathodic electron flow from the electrode interior towards the electrode interface. The current associated with the recombination of cathodic holes, im, in n-type electrodes, at which the interfadal reaction is in equilibrium, has already been given by Eqn. 8-70. Assuming that Eqn. 8-70 applies not only to equilibrium but also to non-equilibrium transfer reactions involving interfadal holes, we obtain Eqn. 10-43 ... 

The current i flowing in photoexcited n-type semiconductor electrodes equals the sum of the photoexcited hole current i >h, the limiting current of hole diffusion ip. itB, and the current of hole recombination inc as shown in Eqn. 10—44 ... 

Chapter 10 deals with photoelectrode reactions at semiconductor electrodes in which the concentration of minority carriers is increased by photoexcitation, thereby enabling the transfer of electrons to occur that can not proceed in the dark. The concept of quasi-Fermi level is introduced to account for photoenergy gain in semiconductor electrodes. Chapter 11 discusses the coupled electrode. mixed electrode) at which anodic and cathodic reactions occur at the same rate on a single electrode this concept is illustrated by corroding metal electrodes in aqueous solutions. 

In the case of a semiconductor electrode, the existence of the energy gap makes a qualitatively different location of energy levels quite probable (Figs. 23b, 23c). One of them, either the ground or excited, is just in front of the energy gap, so that the direct electron transition with this level involved appears to be impossible. This gives rise to an irreversible photoelectro-chemical reaction and, as a consequence, to photocurrent iph. The photoexcited particle injects an electron into the semiconductor conduction band... 

Fig. 37. Occurrence of electrogenerated luminescence with the participation of photoexcited reactants in the solution (a) semiconductor electrode, and (b) metal electrode.

Photoelectrochemistry (PEC) is emerging from the research laboratories with the promise of significant practical applications. One application of PEC systems is the conversion and storage of solar energy. Chapter 4 reviews the main principles of the theory of PEC processes at semiconductor electrodes and discusses the most important experimental results of interactions at an illuminated semiconductor-electrolyte interface. In addition to the fundamentals of electrochemistry and photoexcitation of semiconductors, the phenomena of photocorrosion and photoetching are discussed. Other PEC phenomena treated are photoelectron emission, electrogenerated luminescence, and electroreflection. Relationships among the various PEC effects are established. 

Fig. 1. Electron-Hole Generation upon Photoexcitation of a Semiconductor Electrode...

Figure 2.23 Schematic illustrating the dye sensitization of a semiconductor electrode via electron transfer straight lines indicate radiative transitions, curved lines electron transfer, and wavy lines non-radiative (nr) transitions. Photoexcitation into the Si state of the dye may result in charge injection into the conduction band of the semiconductor or fluorescence and inter-system crossing, from where charge injection may occur from the triplet state or phosphorescence...

Fig. 10.19. Representation of a good surface state on a semiconductor electrode, (a) Thermal activation and (b) photoexcitation of an electron from the valence band to surface states. (Reprinted from A. Gonzalez-Martin, thesis, Texas A M University, 1993.)...

Semiconductor electrodes whose band gap is relatively narrow receive photon energy and produce photoexcited electron-hole pairs in the space charge layer. The photoexcited electron-hole pair formation significantly increases the concentration of minority charge carriers (holes in the n-type), but influences little the concentration of majority carriers (electrons in the n-type). The photoexcited electrons and holes set their energy levels not at the electrode Fermi level, ef, but at what we call the quasi-Fermi levels, n p and p p, respectively. The quasi-Fermi level for majority carriers is close to the electrode Fermi level, F, but the quasi-Fermi level for minority carriers is far away from the electrode Fermi level. 

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