As the demands for renewable energy have keep increasing, further massive production of solar-grade silicon (SOG-Si), which is key material for large scale solar cells, has been highly required.  Conventional process for its production is based upon carbo-thermic reduction of white silica ore followed by purification process by distilling chlorosilane species, which needs substantial energy consumption and having problems for lack of the resources.  For an alternative approach for this, we have developed a series of process to form the SOG-Si material based on the chemical purification of silica using aqueous solution [1] and electrochemical reduction using molten salt [2].  Here we will introduce our approaches for developing these processes.  For the source of SOG-Si, we focused on diatomaceous earth, which is earth-abundant material consisting of porous silica.  After wet chemical treatment with acid and alkaline solutions, most of the impurities except light elements such as B could be removed.  Then solvent extraction using channel-flow type reactor is applied for eliminating the light element with quite high efficiency, and the 7N level purity has been achieved [3, 4].  Then the silica was reduced by direct electrochemical reduction process using CaCl₂ molten salt as an electrolyte, and crystalline Si was successfully obtained.  To develop a continuous production process, we designed the feeding system for silica powder and confirmed steady reaction for reducing silica to Si, and systematically investigated the reaction process for its optimization [5,6].

This work was supported by CREST, JST.

[1] M. Bessho, Y. Fukunaka, H. Kusuda, T. Nishiyama, Energ. Fuel., 23, 4160 (2009).

[2] T. Nohira, K. Yasuda, Y. Ito, Nature Material, 2, 397 (2003).

[3] N. Matsuo, Y. Matsui, Y. Fukunaka, T. Homma, J. Electrochem. Soc., 161, E93 (2014).

[4] T. Homma, N. Matsuo, X. Yang, K. Yasuda, Y. Fukunaka, T. Nohira, Electrochim. Acta, 179, 512 (2015).

[5]          X. Yang, K. Yasuda, T. Nohira, R. Hagiwara, T Homma, J. Eelectrochem. Soc., 161, D1 (2014).

[6] X. Yang, K. Yasuda, T. Nohira, R. Hagiwara, T. Homma, Metall. Mater. Trans. B, in press.