Solar photocorrosion

Fig. 5.5 SEM surface view and cross section of an electrodeposited, ca. 1 p.m thick, CdSe/li film subjected to accelerated photocorrosion by the apphcation of -0.1 V vs. Pt bias in polysulfide solution under a focused, high-power (1 W cm ) solar illumination for 30 min. The coherence of the as-deposited film morphology is evident. The authors emphasize that, even in this situation, the liquid junction nature prevents the flow of high leakage currents during the process (as it might be the case with a solid junction). (Reprinted from [99], Copyright 2009, with permission from Elsevier)...

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 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. 

Another important photocatalytic application is the synthesis of hydrogen from water. However, low solar efficiency and photocorrosion have proven to be hindrances limiting the process economics of photocatalysis [325], The most efficient systems to date consist of compound semiconductor heterostructures that operate with efficiencies of approximately 16%, however, cost and stability are still problematic [325],... 

Various approaches are being pursued to circumvent the poor solar light response of photocorrosion-stable wide band gap semiconductors. For example, in an experiment carried out by Karn... 

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. 

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 is known that the photoelectrochemical cell (PEC), which is composed of a photoelectrode, a redox electrolyte, and a counter electrode, shows a solar light-to-current conversion efficiency of more than 10%. However, photoelectrodes such as n- and p-Si, n-and p-GaAs, n- and p-InP, and n-CdS frequently cause photocorrosion in the electrolyte solution under irradiation. This results in a poor cell stability therefore, many efforts have been made worldwide to develop a more stable PEC. 

Figure 7. Current-voltage curves in the dark and under illumination (680-nm light, 1 mV [cm2) of a MoS2 electrode of -c orientation at different states of surface perfection (15) (1) freshly cleaved (2) after 25-min photocorrosion (solar light of AM 1) at 1 VSCE in 1M KCl solution (3) after 115-min photocorrosion, same conditions as (2) electrolyte 1M KCl + 0.05M Kl...

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. 

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). 

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