Solar semiconductor-electrolyte interface

Solar energy conversion in photoelectrochemical cells with semiconductor electrodes is considered in detail in the reviews by Gerischer (1975, 1979), Nozik (1978), Heller and Miller (1980), Wrighton (1979), Bard (1980), and Pleskov (1981) and will not be discussed. The present chapter deals with the main principles of the theory of photoelectrochemical processes at semiconductor electrodes and discusses the most important experimental results concerning various aspects of photoelectrochemistry of a semiconductor-electrolyte interface a more comprehensive consideration of these problems can be found in the book by the authors (Pleskov and Gurevich,... 

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. 

Study of the Potential Distribution at the Semiconductor-Electrolyte Interface in Regenerative Photoelectrochemical Solar Cells... 

This volume, based on the symposium Photoeffects at Semiconductor-Electrolyte Interfaces, consists of 25 invited and contributed papers. Although the emphasis of the symposium was on the more basic aspects of research in photoelectrochemistry, the covered topics included applied research on photoelectrochemical cells. This is natural since it is clear that the driving force for the intense current interest and activity in photoelectrochemistry is the potential development of photoelectrochemical cells for solar energy conversion. These versatile cells can be designed either to produce electricity (electrochemical photovoltaic cells) or to produce fuels and chemicals (photoelectrosynthetic cells). 

In this paper our results to simulate the photoactive semiconductor/ electrolyte interface in UHV by adsorbing halogens and H20 on semiconductor surfaces are described. For these experiments layer type compounds and ternary chalcogenides have been considered because clean faces can easily be prepared by cleaving the crystals in UHV and because the reactions with halogens are intensively studied for photoelectrochemical solar cells. 

Next, we define an ideal semiconductor photoanode and photocathode for the solar electrolysis of water. We also briefly examine real world issues related to charge-transfer kinetics at semiconductor/electrolyte interfaces and the need for an external bias to drive the photolysis of water. 

Historically, this is the material which really sparked interest in the solar photoelectrolysis of water. Early papers on TiCh mainly stemmed from the applicability of TiCh in the paint/pigment industry255 although fundamental aspects such as current rectification in the dark (in the reverse bias regime) shown by anodically formed valve metal oxide film/ electrolyte interfaces was also of interest (e.g., Ref. 52). Another driver was possible applications of UV-irradiated semiconductor/electrolyte interfaces for environmental remediation (e.g., Refs. 256, 257). 

However, the last few years have also seen a growing awareness of the problems inherent in using the semiconductor-electrolyte interface as a means of solar-energy conversion. Very long-term stability may not be possible in aqueous electrolytes and no oxide material has been identified that has properties suitable for use as a photoanode in a photoelectrolysis cell. Highly efficient photovoltaic cells are known, both in aqueous and non-aqueous solutions, but it is far from clear that the additional engineering complexity, over and above that required for the dry p-n junction photovoltaic device, will ever allow the "wet photovoltaic cells to be competitive. These, and other problems, have led to something of a pause in the flood of papers on semiconductor electrochemistry in the last two years and the current review is therefore timely. I have tried to indicate what is, and is not, known at present and where future lines of development may lie. Individual semiconductors are not treated in detail, but it is hoped that most of the theoretical strands apparent in the last few years are discussed. 

Other Semiconductors. Gerischer,41 in a very helpful paper on electrochemical photo and solar cells, has explained the mode of action of the semiconductor-electrolyte interface (when the semiconductor is in its depletion mode) as a Schottky barrier, and how this can lead to separation of hole-electron pairs... 

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. 

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. 

In a PEC cell one of the semiconductor-to-metal interfaces in replaced by the semiconductor/electrolyte interface. As a consequence, the price of the device is likely to be reduced since the electrolytic system is much less sensitive to the crystalline perfection and purity of a semiconductor than the solid-state system. Hence, inexpensive and readily available materials can be made use of (It is high price, not poor performance characteristics that narrows down the applicability of e g., silicon solar cells at present.) Moreover, passing of... 

The basis of the processes is the absorption of incident solar photons leading to the generation of electron-hole pairs in a semiconductor in contact with an aqueous electrolyte. It is energetically favorable for the minority carriers (holes) to difEuse to the semiconductor/electrolyte interface, where recombination with electrons from the valence band of water can take place. The hydrogen ions migrate to the metal cathode and are reduced to hydrogen molecules. 

The application of liquid-junction technology to photovoltaic power conversion is limited by problems associated with the semiconductor-electrolyte interface. Primary among these problems is corrosion. Efficient conversion of solar energy requires a band gap between 1.0 and 1.5 eV, and most semiconductors near this band gap corrode readily under illumination. Semiconductors with large band gaps (4-5 eV) tend to be more stable but cannot convert most of the solar spectrum. 

The reactive semiconductor-electrolyte interface makes stability a major issue in photoelectrochemical solar energy conversion devices, and aspects of thermodynamic and kinetic stability are briefly reviewed here. Thermodynamic stability considerations are based on so-called decomposition levels [56, 57] that are determined by combining the decomposition reaction with the redox reaction of the reversible hydrogen reference electrode. The anodic and cathodic decomposition reactions of a compound semiconductor MX can be written for aqueous solutions as... 

FF = 0.68, and rj = 11.7%. The improvement in the photoelectrochemical solar cell properties has been ascribed to the formation of n-CdSe/n-WOs heterojunctions, which enhances the charge transfer at the semiconductor/electrolyte interface. These results indicated for the first time the interesting effects of STA and PTA on chemically deposited CdSe films. This opens up a new method for fabricating mixed electrodes with improved physical properties and photoelectrochemical solar cell performances. 

To facilitate a self-contained description, we will start with well-estahlished aspects related to the semiconductor energy hand model and the electrostatics at semiconductor-electrolyte interfaces in the dark . We shall then examine the processes of light absorption, electron-hole generation, and charge separation at these interfaces. The steady state and dynamic aspects of charge transfer are then briefly considered. Nanocrystalline semiconductor films and size quantization are then discussed as are issues related to electron transfer across chemically modified semiconductor-electrolyte interfaces. Finally, we shall introduce the various types of photoelectrochemical devices ranging from regenerative and photoelectrolysis cells to dye-sensitized solar cells. 

To improve the solar response of a photoelectrode, a proper match between the solar spectrum and the band gap of the semiconductor should be maintained. When a single band gap semiconductor is used, a band gap in the vicinity of 1.4 eV is most desirable from the standpoint of optimum solar-conversion efficiency. An important criterion is that the minority carrier that is driven toward the semiconductor-electrolyte interface should not participate in a photocorrosion reaction that is detrimental to the long-term stability of the photoelectrode. Photocorrosion can be viewed in terms of either kinetic or thermodynamic considerations and the real cause may be a mixture of both. From thermodynamic perspective, a photoanode is susceptible to corrosion if the fermi level for holes is at a positive potential with respective to the semiconductor corrosion potential [21]. The corrosion can be prevented or at least inhibited by choosing a redox couple that has its /ijedox more negative than that for the corrosion process [22,... 

In PEC photoelectrode systems, the semiconductor/electrolyte interface can form a rectifying jtmction, similar to the solid-state pn junctions or Schottky diode jtmctions used in solar cells. Such rectifying junctions exhibit built-in electric fields... 

The efforts to enhance efficiency in solar cells through use of metal nanoparticles should provide a general guide for the application of plasmonic concepts to water photoelectrolysis, although differences in device design, such as the presence of an electrolyte, suggest that there may be additional criteria. When metal nanoparticles are located at the semiconductor/electrolyte interface, considerations of stability, Fermi level, and band bending are essential. Secondary considerations may include the influence of catalytic effects and the possible formation of trap states at the metal/semiconductor interface. 

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