Development of a New Method to Prepare Polycrystalline Silicon Thin Films for High-Efficiency and Low-Cost Solar Cells

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The research has been made with the aim at developing a new electrochemical method to prepare polycrystalline silicon (Si) thin films for high-efficiency and low-cost solar cells of a new type. In the new method, cheap silicon tetrachloride (SiClィイD24ィエD2), as the silicon source, was electrochemically reduced at the cathode (conductive substrate), with a molten salt used as the electrolyte. Much effort has been made to solve the problems of a very low solubility of SiClィイD24ィエD2 in the molten salt and of resulting inhomogeneous deposition of polycrystalline Si thin films. A mixed salt of aluminum trichloride and n-butylpyridinium chloride dissolved SiClィイD24ィエD2 in high concentrations, but its use was limited only below 100℃, resulting in the deposition of amorphous Si films. It was finally found that the use of a rotating cathode, only a half of it being immersed in the electrolyte, together with an eutectic melt of lithium chloride and potassium chloride as the electrolyte, gave a fairly homogeneous polycrystalline Si films. In parallel to these studies, basic studies have been made on the fabrication of a new junction appropriate for the use of polycrystalline Si films. It was found that Si-H termination bond could be converted to Si-X (X=halogen) bonds by immersin in concentrated hydrogen halide solutions containing a small amount of an oxidant, which led to shifts in the Si band edges. Interestingly, immersion of atomically flat n-Si (111) surfaces in a hydrogen iodide solution led to formation of ordered iodine nano-rods. Similar nano-rods of nickel were also formed when it was electrochemically deposited. Solid-state solar cells of a new type, "n-Si/p-CuI/ITO", gave a high open-circuit photovoltage (Voc) of 0.617 V, close to a theoretical value.