Highly Efficient Solar Energy Conversion by Means of Photoelectrochemical Cells Equipped with Metal-coated Silicon Electrodes

Abstract

We entered into the study of the photoelectrochernical processes of various semiconductor electrodes primarily because of its charming aspect in the pure scientific view point. But we were also determined to pursue the way to the industrial utilization of solar energy which should become the main future energy resources of mankind.
For an efficient solar energy conversion, it is substantial to use semiconductors having appropriately small bandgaps, and the main problem in this field is the fact that most of the semiconductor materials having such bandgaps are corrosive in aqueous solutions. Among various methods proposed for the stabilization, we have demonstrated that the application of ultrathin metal layers is very effective. Duration tests of over 5000 h were cleared for n-Si electrodes coated, with several nm layers of Pt and other metals.
Coverage of semiconductor electrodes with continuous metal layers, however, brought about another problem in that the photovoltage arising from the semiconductor-liquid junction is spoiled by the metal coating. We have been working on the effect of discontinuous metal coating on semiconductor photoelectrodes since the discovery of the “misterious” behavior of the metal-coated GaP electrode. The theory for the effect was proposed and supporting experimental results were accumulated. Various trials were made to really construct Ptisland (several nm in size) coated n- or p-Si electrodes and they successfully showed the predicted behavior. It was also shown that these electrodes generate photovoltages of the order of 0.65 V, which are much higher than those of the commercial single crystalline silicon solar cells. The solar to chemical conversion efficiencies of 10.8% for the decomposition of hydrogen iodide and the solar to electrical conversion efficiency of 14.0% in hydrogen iodide have been achieved with these electrodes.
These data, together with the fact that the photoelctrochemical cells do not need energyconsuming p, or n doping processes, indicate the possibility that the photoelectrochemcal cells can be a strong candidate for the industrial application of photovoltaic solar energy converters. The application of our technique to amorphous silicon materials and to new type solid photoelectric converters are also suggested.