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

为了（1）通过新的电化学方法制备多晶硅（Si）薄膜和（2）通过开发适合于使用这种多晶硅薄膜的新的结来制造新型的高效率和低成本太阳能电池的目的，已经进行了研究。LiCl-KCl共晶熔体温度约为100 ℃。使用450 ℃作为电解质，电化学还原四氯化硅并将其沉积在单晶Si晶片或多晶Ni板上作为阴极（和基底）。扫描电子显微镜和X射线光电子能谱分析表明，虽然沉积了多晶Si薄膜，但由于SiCl_4在电解液中的溶解度很低，薄膜不均匀，仅沉积在阴极上的电解液弯月面的上边缘附近。后来通过使用有机盐如氯化正丁基吡啶鎓作为电解质解决了这个问题。对硅沉积机理进行了初步研究。首先，测定了SiCl_4还原的氧化还原电位为-0.5 V vs. Ag/Ag^+。接着，研究了在与Si沉积对应的还原电流中发现的电流振荡的机理。它已得出结论，Si沉积反应的进展改变了Si表面的化学结构，这改变了界面张力，从而改变了电解质弯月面的高度，这导致电流振荡。研究了用金属微粒修饰的硅表面作为一种新的适用于多晶硅薄膜的结。不仅在单晶Si上而且在多晶Si上都沉积了超细铂粒子，表明这种结形成方法对多晶Si薄膜是有效的。

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.