High purity silicon materials prepared through wet-chemical and electrochemical approaches

A new approach to produce high purity solar grade silicon (SOG-Si) using wet-chemical and electrochemical processes was proposed. First, diatomaceous earth was used as a source of silica and wet-chemical purification by acid leaching and solvent extraction processes were applied for eliminating heavy metal and light element such as boron, respectively. In particular, the solvent extraction using channel flow reactor demonstrated extremely high efficiency for eliminating boron to 7N (99.99999%) level purity. Secondly, the high-purity silica was reduced to silicon by the direct electrolysis method using molten CaCl₂ as an electrolyte and Si plate as cathode. We proposed the continuous electrolysis system by feeding silica granules to the Si cathode set at the bottom of the cell. It was confirmed that the reduction proceeded steadily to form crystalline Si with a sufficient reduction rate. We also attempted electrodeposition of Si films and have developed a process using KF-KCl + K₂SiF₆ molten salt, from which uniform layer of Si with a thickness larger than 100 μm could be formed.