Semiconductor electrodes photo excitation

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. 

Pig. 10-18. (a) PolarizatioD curves of anodic dissolution and (b) Mott-Schottky plots of an n-type semiconductor electrode of molybdenum selenide in the dark and in a photo-excited state in an acidic solution C = electrode capacity (iph) = anodic dissolution current immediately after photoexdtation (dashed curve) ipb = anodic dissolution current in a photostationary state (solid curve) luph) = flat band potential in a photostationary state. [From McEv( -Etman-Memming, 1985.]... 

This cell involves the absorption of light by dye molecules spread on the surface of the semiconductor, which upon light absorption will inject electrons into the conduction band of the n-type semiconductor from their excited state. The photo-oxidized dye can be used to oxidize water and the complementary redox process can take place at the counter electrode [46,47]. Tandem cells such as these are discussed in Chapter 8. 

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)... 

Similar to the molecular photosensitizers described above, solid semiconductor materials can absorb photons and convert light into electrical energy capable of reducing C02. In solution, a semiconductor will absorb light, and the electric field created at the solid-liquid interface effects the separation of photo-excited electron-hole pairs. The electrons can then carry out an interfacial reduction reaction at one site, while the holes can perform an interfacial oxidation at a separate site. In the following sections, details will be provided of the reduction of C02 at both bulk semiconductor electrodes that resemble their metal electrode counterparts, and semiconductor powders and colloids that approach the molecular length scale. Further information on semiconductor systems for C02 reduction is available in several excellent reviews [8, 44, 104, 105],... 

Much attention has been devoted to the development of optimal photo sensitizers of semiconductor electrodes [36, 43]. Ruthenium(II) polypyridine complexes are especially well suited for this purpose. They are strong light absorbers in the visible spectral region and bpy or tpy ligands can be easily derivatized with anchoring groups. Moreover, localization of the excited electron on the ligand which is attached to the semiconductor surface facilitates the electron injection. 

A water-splitting device has been invented [4], where photo-semiconductor and platinum are used as the cathode and the anode, respectively, instead of setting both the solar cell and the electrolyzer, separately. This method is called photoelectrochemical (PEC) water-splitting or photo semiconductor electrode method . The key phenomenon of PEC watersplitting is the steep rise (fall) of the potential at the interface between the n-(p-) semiconductor and the liquid electrolyte (e.g., KOH). If photons irradiate onto the interface, both the electrons (e ) and positive holes (IT) are excited to their conductive energy bands where they can move freely, so that e and h+ are separated by the interface potential difference. The h+ react with water by the equation ... 

A second possible approach, based on semiconductor-liquid junctions, is to adsorb dye molecules onto the semiconductor surface which, upon light absorption, will inject electrons (into n-type semiconductors) or holes (into p-type semiconductors) from the excited state of the dye molecule into the semiconductor. In principle, the photo-oxidized (or photoreduced) dye can then oxidize (reduce) water, and the complementary redox process can occur at the counter electrode in the cell. However, this approach has never been demonstrated experimentally. 

Excitation modulation is useful for a semiconductor electrode or for the observation of the photo-induced change in the reflectance [16]. 

Transient grating spectroscopy is relatively easily handled compared with the transient absorption spectroscopy, and is often used to study carrier dynamics at semiconductor electrodes [32]. Figure 14 schematically shows the principle of transient grating spectroscopy. A femtosecond laser pulse for sample excitation is split into two beams, which are crossed again at the semiconductor surface to produce an optical striped interference pattern. The interference pattern produces a striped pattern of the densities of photo-generated electrons and holes near the semiconductor surface. The latter striped pattern gives rise to a striped pattern of optical refractive index near the semiconductor surface, which is monitored by measuring a diffraction pattern of a second probe laser... 

The illumination of semiconductor electrodes can give rise to a photocurrent due to the interband excitation of electrons. Although semiconductor photo-electrochemistry lies outside the scope of this chapter (an excellent review has been published by Morrison [57]), photocurrent spectroscopy has found more general application as an in-situ technique for the characterisation of surface films formed on metal electrodes such as Fe [58] and Pb [59] during corrosion. Quantitative analysis of photocurrent spectra can be used to identify semiconductor surface phases and to characterise their thickness and electronic properties. 

At present there is a sufficiently complete picture of photoelectrochemical behavior of the most important semiconductor materials. This is not, however, the only merit of photoelectrochemistry of semiconductors. First, photoelectrochemistry of semiconductors has stimulated the study of photoprocesses on materials, which are not conventional for electrochemistry, namely on insulators (Mehl and Hale, 1967 Gerischer and Willig, 1976). The basic concepts and mathematical formalism of electrochemistry and photoelectrochemistry of semiconductors have successfully been used in this study. Second, photoelectrochemistry of semiconductors has provided possibilities, unique in certain cases, of studying thermodynamic and kinetic characteristics of photoexcited particles in the solution and electrode, and also processes of electron transfer with these particles involved. (Note that the processes of quenching of photoexcited reactants often prevent from the performing of such investigations on metal electrodes.) The study of photo-electrochemical processes under the excitation of the electron-hole ensemble of a semiconductor permits the direct experimental verification of the applicability of the Fermi quasilevel concept to the description of electron transitions at an interface. 

An alternative inexpensive organic polymer-based photovoltaic solar cell has been invented. In this device, p-type and n-type semiconductors are sequentially stacked on top of each other. In such devices, absorption of a photon by a ji-conjugated polymer results in the formation of an excited state, where coulom-bicaUy bound electron-hole pair (exciton) is created. This exciton diffuses to a region of interface of n-type semiconductor where exciton dissociation takes place and transport of charge to the respective electrodes occurs. For example, the photo-induced electron transfer from a donor layer (p-type) to acceptor layer (n-type) takes place in a polymer/fullerene-based organic bilayer solar cell, MDMO-PPV PCBM, with power conversion efiiciency of 2.5 % (Fig. 11.8) [13]. 

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