Photocorrosion solar energy conversion

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 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. 

This catalytic effect of RUO2 has been exploited recently to stabilize small band gap semiconductor particles which from their absorption properties are more suitable for solar energy conversion than Ti02. An undesirable property of these materials is that they undergo photocorrosion under illumination. Holes produced in the valence band migrate to the surface where photocorrosion occurs, l.e.,... 

Another essential requirement for the photocatalyst is its resistance to reactions at the solid/liquid interface that may result in a degradation of its properties. These reactions include electrochemical corrosion, photocorrosion, and dissolution (Morrison, 1980). A large group of photocatalysts with suitable semiconducting properties for solar energy conversion (CdS, GaP, etc.) are not stable in the water-oxidation reaction because the anions of these materials are more susceptible to oxidation than water, causing their degradation by oxidation of the material (Ellis et al., 1977 Williams, 1960). 

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. 

It can be concluded that doped polycrystalline hematite indeed can be interesting for photoelectrolysis of water. The studied area has been intensively researched and some of the findings represent promising photoelectrochemical properties. In combination with the ease of preparation and good stability towards photocorrosion this make doped polycrystalline electrodes of hematite an interesting material for direct conversion of solar energy into dihydrogen. 

Q(quantum)-size semiconductors are of interest, e.g. for novel photonic or electronic applications (refs. 11,12) or as photocatalysts, i.e. for the conversion of solar energy into chemical energy. However, the latter application is complicated namely in the case of sulfides due to their easy susceptibility to photocorrosion. 

The theoretical solar conversion efficiency of a regenerative photovoltaic cell with a semiconductor photoelectrode therefore depends on the model used to describe the thermodynamic and kinetic energy losses. The CE values, which consider all the mentioned losses can generally only be estimated the full line in Fig. 5.65 represents such an approximation. Unfortunately, the materials possessing nearly the optimum absorption properties (Si, InP, and GaAs) are handicapped by their photocorrosion sensitivity and high price. 

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