New Method of Preparation of Polycrystalline Silicon Thin Films for High-Efficiency and Low-Cost Solar Cells

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Research has been made for the purpose of (1) preparing polycrystalline silicon (Si) thin films by a new electrochemical method and (2) fabrication a high-efficiency and lowcost solar cell of a new type by development of a new junction suitable for use of such polycrystalline Si thin films. A LiCl-KCl eutectic melt at ca. 450ﾟC was used as an electrolyte, and silicon tetrachloride was electrochemically reduced and deposited on a single-crystal Si wafer or a polycrystalline Ni plate as the cathode (and substrate). Scanning electron micrographs and X-ray photoelectron spectroscopy showed that polycrystalline Si thin films were really deposited, but the films were inhomogeneous and deposited only near the upper edge of an electrolyte meniscus on the cathode, probably due to very low solubility of SiCl_4 in the electrolyte. This problem has been solved later by using an organic salt such as n-butylpyridinium chloride as the electrolyte. Basic studies on the mechanism of Si deposition has been made. First, the redox potential for SiCl_4 reduction was determined to be -0.5 V vs.Ag/Ag^+. Next, the mechanism of a current oscillation, found for reduction current corresponding to Si deposition was studied. It has been concluded that progress of the Si-depositing reaction changes the chemical structure of the Si surface, which alters the interfacial tension and hence the height of an electrolyte meniscus, and this causes the current oscillation. The Si surface modified with ultrafine metal particles has been studied as a new junction suitable for use of polycrystalline Si thin films. The deposition of ultrafine platinum particles not only on singlecrystal Si but also multicrystalline Si wafers showed that this method of junction formation is effectively applicable to polycrystalline Si thin films.