With the aim to commercialize a new process for making solar-cell grade (SOG) silicon, Chisso Corp. (Tokyo, Japan; edlinks.chemengonline.com/6891-534) is negotiating the possibility of establishing a joint-venture company with Nippon Mining Holdings, Inc. (Tokyo) and Toho Titanium Co., Ltd. (Chigasaki, both Japan). Chisso plants to begin conformation tests of the process this year.
The Chisso SOG-Si (CSS) technology, is based on the reduction reaction of silicon tetrachloride (SiCl4) with zinc (Zn) to produce polysilicon. Although this technology is not new, the CSS project group has refined the reaction to achieve six-nines purity (99.9999%) SOG Si, with a low cost using a totally-closed system. In the CSS process, crude Si is first chlorinated into SiCl4 by reacting with a Cl2-N2 mixture in a fluidized-bed reactor (conditions not disclosed). The reaction has a yield of nearly 100%, says the firm. The SiCl4 vapor is purified by distillation, then reduced with zinc vapor to form ZnCl2 and needle-like crystals of Si, which can easily be separated from the vapors. The byproduct ZnCl2 is separated from the unreacted gas by condensation then solidification, and the unreacted SiCl4 reused in the process. Meanwhile, metallic Zn and Cl2 gas are recovered by electrolysis of molten ZnCl2 — a process that has an electrical efficiency of 80–90%, says the firm.
Product SOG Si — which contains less than 1 ppm of Zn and undetectable levels (below 0.1–0.5 ppm) of all other impurities — has all the desired properties and performances for solar-cell applications. Production costs for the CSS process are estimated to be very competitive for a production scale of approximately 2,000 m.t./yr, says the firm.