In recent years, interest in the use of photoelectrochemical devices for solar energy conversion has grown dramatically. The major advantage over more conventional solar cells (e.g., the silicon p–n junction) is the ease with which the device can be constructed, since the interface between the semiconductor and electrolyte is not subject to such problems as lattice mismatch. Prior to 1969 it was thought that semiconductors invariably degraded when illuminated in an aqueous environment. Such corrosion problems have been overcome by using more rugged semiconductors and adding stabilizing redox ions (e.g.,S2−/S2−2) to the electrolyte. The understanding of semiconductor–electrolyte interfaces has been greatly enhanced by the ability to study stable systems. It is now clear that the conversion efficiency of such devices is a function of the quality of the Schottky barrier at the interface, the band gap and electron affinity of the semiconductor, and the redox level in the electrolyte that it acts upon. Another advantage over the silicon solar-cell is the ability to store solar energy by the direct production of chemicals (e.g., H2). At present the best conversion efficiencies that have been observed have been of the order of 7%–9% (AM 2 sunlight). While this is poor compared to that which can be obtained with silicon solar-cells, cheap thin-film semiconductor techniques could make such devices extremely competitive in terms of economics.
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