Solar cells 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,... 

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. 

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... 

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. 

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. 

How can such problems be counterbalanced Since a large capacitance of a semiconductor/electrolyte junction will not negatively affect the PMC transient measurement, a large area electrode (nanostructured materials) should be selected to decrease the effective excess charge carrier concentration (excess carriers per surface area) in the interface. PMC transient measurements have been performed at a sensitized nanostructured Ti02 liquidjunction solar cell.40 With a 10-ns laser pulse excitation, only the slow decay processes can be studied. The very fast rise time cannot be resolved, but this should be the aim of picosecond studies. Such experiments are being prepared in our laboratory, but using nanostructured... 

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