Semiconductor Electrodes for Solar Energy Conversion

Generation of photocurrent at the semiconductor/electrolyte interface upon its illumination makes it possible to carry out photoelectrochemical reactions which can be used either for chemical fuel production, or purification of waters. Principles of operation of electrochemical cells with semiconductor electrodes for solar energy conversion to electrical and chemical energy are formulated. Most efficient cells for electricity and hydrogen production are surveyed. Certain processes for photo-destruction of pollutants, recovery of metals, etc. with making use of semiconductor dispersions are briefly discussed. 

Systems that present spectral sensitization, very important when the light absorption properties of a potentially photoactive (generally, luminescent) species does not permit efficient excitation in the desired wavelength range. This kind of phenomenon is crucial for many applications in different fields, such as, for example, the spectral sensitization of semiconductor electrodes in solar energy conversion. 

The stability of semiconductor electrodes, their resistance to photocorrosion, become an especially urgent problem in connection with ever-extending photoelectrochemical applications of semiconductors. This refers, first of all, to electrodes of photoelectrochemical cells for solar energy conversion. 

The heterojunctions of the polyacetylene were realized not only with inorganic photoconductors but also with organic polymers [139]. The results obtained show good similarity with barrier and heterojunction characteristics for inorganic semiconductors. Photoelectrochemical cell for solar energy conversion with polyacetylene electrodes and Na2S, electrolyte had an efficiency of 1 % at 2.4 eV [140], The complicated phenomena take place at the electrodeelectrolyte interface. 

The sensitization of electrodes to visible light by dye molecules is an old area of science with a rich history [2]. A dye-sensitized photoefifect was measured at a semiconductor surface as early as 1887 in Vienna [3]. The accepted mechanisms for the dye sensitization of electrodes emerged from photoelectrochemical studies in the 1960s and 1970s [4-6]. These studies were motivated by a desire to quantify interfacial electron transfer processes and develop cells useful for solar energy conversion. The two most common approaches are shown schematically in Figure 1. 

Such protecting systems are oxidized or reduced, thereby stabilizing semiconductor photoelectrodes, and this effect is used in photoelectrochemical cells for solar energy conversion. The solvent (e.g., water) can also act as a protector if it is oxidized (reduced) easier than the semiconductor electrode material. Situations like this arise with many oxide electrodes (Ti02, SrTi03,... 

Other Semiconductors.—Kennedy et alf have continued to study FcjOj photoelectrodes, and their most recent work shows that high efficiencies are obtained with Si-doped sintered electrodes. Dare-Edwards et alf have characterized lithium-doped NiO in some detail but, as expected, the very low carrier mobility in this material makes it quite unsuitable for solar energy conversion. Gissler has investigated trigonal Se films, and Davidson and Willsher have given further details of the properties of HgS powder photoanodes. Derivatized tin-oxide electrodes have been prepared by Fox et al.f and Janzen et al. have successfully attached the photosynthetic reaction centre molecule isolated from Rhodopseudomones sphaeroides to tin oxide (see also Section 2). 

Jayadevaiah 44 45 has also discussed the use of semiconductor-electrolyte interfaces for solar energy conversion, and Figure 7 gives the cell characteristic for his cell, which contains a silicon electrode. It is clear from the form of this that the internal resistance of the cell is rather high. The power conversion efficiency at the maximum power point is 2.7%. The stability of the Si is not discussed, but is almost certainly poor. 

As Fig. 27 illustrates, there are basically three types of photoelectrochemical devices for solar energy conversion. The first type is regenerative in nature and the species that are photooxidized at the -type semiconductor electrode are simply re-reduced at the counterelectrode (Fig. 27a). Instead of an elec-trocatalytic electrode [291, 292] where the counterelectrode reaction occurs in the dark (this is the situation schematized... 

The band-gap excitation of semiconductor electrodes brings two practical problems for photoelectrochemical solar energy conversion (1) Most of the useful semiconductors have relatively wide band gaps, hence they can be excited only by ultraviolet radiation, whose proportion in the solar spectrum is rather low. (2) the photogenerated minority charge carriers in these semiconductors possess a high oxidative or reductive power to cause a rapid photocorrosion. 

The acceleration of electrode processes at irradiated semiconductors opens the way, at least in principle, for directly converting the energy of ionizing radiation into chemical energy of electrolysis products (quite similar to the case of solar energy conversion) this acceleration can also be used as a means for detecting the radiation. 

---