Thread Content
Discussion on the Production Technology of Solar-grade Polysilicon, February 27, 2009, Sobey Solar. Solar-grade polysilicon technology can be divided into two main categories: chemical methods and physical methods. The chemical polysilicon production process refers to the need to convert silicon into intermediate compounds during its purification process, and then use pyrolysis or redox reactions to transform these silicon compounds into high-purity polysilicon raw material. The physical processing method, also known as the metallurgical method, involves removing impurities from metallurgical-grade silicon using physical or chemical means, without altering the inherent properties of the silicon. Although chemical reactions are involved, they target impurities such as boron and phosphorus; the chemical properties of silicon remain unchanged. The purity that can be achieved through physical purification has its limits. Due to its low cost, this method is considered a promising alternative to high-purity polycrystalline silicon. The chemical method can produce polysilicon with high purity, but it is technically challenging due to the need to control and handle numerous by-products. Costs vary depending on the technical approach used, and generally they are higher than those of the physical method. Most chemical methods require the use of metallurgical-grade silicon ； MG) is used as a raw material for silicon compounds, with a few cases where silica is directly utilized to prepare silicon compounds. In chemical methods, the Siemens process was widely used by early manufacturers of semiconductor-grade polysilicon; it is a mature technology but comes with high costs. In addition to the Siemens method, there are also the ASiMi method, fluid bed reaction method, and tubular deposition method. The ASiMi method was developed by Advanced Silicon Material (ASiMi) and is a high-purity polycrystalline silicon production technology that uses silane (SiH4) as a raw material. In 2005, ASiMi announced its withdrawal from the polysilicon market, and the majority of its shares were acquired by the Norwegian company REC (Renewable Energy Corp); as a result, this technology is still produced by REC today. The fluidized bed reaction method, like the ASiMi method, uses silomethane as a raw material. It operates at a lower reaction temperature, which allows for a reduction of electricity consumption by nearly 30%. Additionally, due to the larger reaction area provided by the silicon seeds, the gas flow velocity inside the reactor is high, thereby overcoming the issue of slow deposition speed associated with the ASiMi method. Fluidized bed reactions can be carried out on a continuous basis, and this is why fluidized bed reactors are superior to bell-jar Siemens reactors. The tubular deposition method is a polycrystalline silicon production technique used by JointSolarSiliconGmbH (JSSI). The raw materials and principles involved are the same as those in the fluid bed reaction method: polycrystalline silicon is produced through the thermal decomposition of silane and hydrogen. JSSI claims that this method consumes one-tenth of the electricity required by the traditional Siemens method, with a conversion rate of 95–98%; there are already records of mass production using this method. Solar polysilicon produced by physical metallurgical processes began to be manufactured in small quantities in 2008. The technology for purifying solar-grade silicon using metallurgical methods also requires metallurgical-grade silicon with low impurities as raw material, and several processes are needed to complete the purification. In addition, some companies are investing in mass-production technologies for other processes, which are still in the research and development stage. For example, the purification techniques for the sodium reduction method and the thermal carbon reduction method do not require the use of metallurgical-grade silicon; their raw materials, such as sodium fluosilicate, are by-products of fertilizers, while sodium silicate can be obtained directly through chemical reactions involving silica. Overall, the technologies that have been successfully scaled up for production to date are mainly the Siemens method, fluidized bed reaction method, tubular deposition method, and ASiMi method. A small amount of solar-grade polysilicon produced using metallurgical methods was available in 2008, but due to unstable quality, there is still room for improvement in this technology; nonetheless, it has been officially recognized as one of the options for solar material sources. In recent years, various manufacturers have claimed to be investing in the mass production of related technologies; however, before there is a record of stable quality in mass production, any investment in such technologies or large-scale use of such products requires careful evaluation.