When many companies have invested in the construction of polysilicon production lines, information came from Dow Corning, an American silicon material company: a substitute for polysilicon has begun to supply the market. Dow Corning, a well-known American silicon producer, announced that it has successfully used metallurgical-grade silicon to manufacture solar-grade silicon materials, which is expected to solve the bottleneck of polysilicon raw material shortage faced by the development of the solar energy industry. Dow Corning says its solar-grade silicon material called PV1l01 could reduce the solar industry’s reliance on polysilicon. For example, 10t of polysilicon raw material can be mixed with about 2t of PV1101 silicon material to form about 12t of silicon raw material for solar energy. According to Dow Corning, the production of PV1101 silicon is a milestone in the renewal of solar technology. PV1101 not only reduces the amount of polysilicon, but also reduces the production cost of solar cells. The company’s Solar Solutions group in Santos, Brazil, used a new technological approach to extract PV1101 solar-grade silicon from silicon materials. At present, PV1101 hybrid material has been tested by a number of independent institutions and Dow Corning Solar Solutions customer factories around the world. The results show that this hybrid material exhibits performance characteristics similar to polysilicon in terms of solar cell production and efficiency. Dow Corning has started mass production of the PV1101 and has been shipping large quantities to customers since August 2006. Although Dow Corning admits that PV1101 cannot completely replace polysilicon, it is enough to have a deterrent effect on the existing polysilicon market. Analysts believe that the emergence of new alternatives to polysilicon will stifle expectations of rising polysilicon prices.
BP Solar has also developed a new “silicon growth” process, which it says can greatly improve cell efficiency compared to conventional polysilicon solar cells. These solar cells are fabricated from silicon wafers called “Mono2” and combined with other advances in cell process technology from BP Solar. The electricity produced is 5% to 8% higher than that of batteries made by conventional processes. BP Solar has implemented the Mono2 technology on existing equipment at the Frederick plant for mass production, and this type of modular combination production was achieved in 2007.
Chisso Corporation of Japan has commercialized a new process for producing solar cell-grade (SOG) silicon. The company formed a joint venture with Japan Mining Holdings and Toho Titanium to pilot the process in 2007. Chisso’s SOG-Si technology is based on silicon tetrachloride (SiCl), which uses zinc (Zn) reduction to produce polysilicon. Although this technology is not a new technology, it can make the reaction reach the purity of 6 9s (99.9999%) SOG-Si, and the use of a fully closed system has low cost. In this process, crude Si is first chlorinated to SiCl4 with a Cl2-N2 mixture in a fluidized bed reactor. This reaction yields nearly 100%. SiCl4 vapor is purified by distillation, and then reduced with zinc vapor to form needle-like crystals of ZnCl2 and Si. The by-product ZnCl2 is separated from the unreacted gas by condensation, and then solidified. Unreacted siCl4 was reused in the process. In addition, metallic Zn and Cl2 gases were used for the electrolytic recovery of molten ZnCl2, and the electrical efficiency of the process was 80%-90%. The product SOG-Si contains less than 1ppm of Zn, and no other impurities are detected, which fully meets the performance of solar cells. The production cost of this process can fully compete with factories with a production scale of about 2000t/a.
American 1366 technology company announced investment in solar cell technology in early April 2008, aiming to make silicon solar cells compete with coal. The initial investment was $12.4 million. The company will combine innovations in the structure of silicon cells with improvements in manufacturing processes to make polysilicon solar cells cost-comparable to coal-based power generation. 1366 Technologies improved the cell surface structure and metal conductorization to increase the efficiency of silicon solar cells by 25% (to increase the photoelectric conversion efficiency from 15% to 19%), thereby reducing costs. The company plans to transfer its technology in partnership with some solar companies and government departments. In addition, the company plans to build 100MW of industrial-scale facilities in the world.
Blue Squar Energy (BSE) announced on November 6, 2008 that its patented Bright Point technology has produced solar cells with 14% efficiency, the highest conversion efficiency in the world using modified metallurgical grade (UMG) silicon One of the most efficient, certified by the American Renewable Energy Laboratory. The company uses cheap silicon and its Bright Point technology for solar cell production, which the company sees as an important milestone in achieving its goal of low-cost solar power. The uniqueness of BSE’s Bright Point technology lies in its two-part structure, in which a thin layer of high-grade silicon is placed on top of 4NUMG silicon, which has a very different specification than silicon used in the solar industry, and is low-cost and applicable. By using Bright Point technology, BSE can use 100% 4NUMG silicon, which can significantly save costs compared to other UMG silicon solar cell products. Creating low-cost solar cells is the first step in BSE’s eventual production of the world’s highest-efficiency, lowest-cost solar cells, and its Bright Point III technology is already in development.
Researchers at the Institut Fur Solarenergieforschung Hameln (IFSH) in Germany have developed a process for the manufacture of solar cells, the Rear Intersect Single Evaporation (RISE) process. Supplemented by laser processing technology, the photoelectric conversion efficiency of back-contact silicon solar cells fabricated by this process reaches 22%. At present, many manufacturers use laser processing technology to produce silicon solar cells. BP Solar uses laser-grooved buried-gate technology, which means that laser technology is used to groove the silicon surface and then fill it with metal to make the front surface electrically contact the gate. This technique has the advantage of reducing shielding losses compared to standard front surface metallization. Advent Solar uses another technique called emitter wrap through. Through-holes are drilled on the silicon wafer with a laser, and the high-connection wall conducts the current on the front surface of the emission area to the metal contact layer on the back surface, thereby further reducing the shielding loss and improving the photoelectric conversion efficiency. In the process of using the RISE process to produce solar cells In the method, the cross-patterned emission region and base region are fabricated on the backside of the solar cell by laser processing, and laser ablation can also reliably separate the self-aligned contact layer after metal evaporation. Non-contact processing (very important for reducing wafer damage) first uses a frequency tripled 355nm Nd:YVO4 laser to ablate finger-like emitter and base patterns in the silicon nitride or silicon oxide thin layer on the backside of the crystalline silicon wafer . Any damage to the wafer will shorten the lifetime of the carriers and also reduce the photoelectric conversion efficiency. It is then etched with potassium hydroxide (KOH) to remove the damaged portion caused by the laser processing. After etching, the luminescent substance diffuses to form the emission region, leaving the raised silicon nitride and oxide as the base region. Before wet chemical etching, the next processing step is to separate the emitter and base regions using a metal evaporation self-aligned contact layer separation process. In order to optimize the conversion efficiency of solar cells, researchers from the ISFH Institute and the German company Laser Zentrum Hannove found that the lifetime of the carriers is related to the wafer damage and KOH corrosion depth caused by different wavelengths of laser light sources. Moreover, the depth of wafer damage caused by triple frequency Nd:YAG355nm laser ablation is 3um; the depth of wafer damage caused by doubled Nd:YAG 532nm laser ablation is 4μm; and the damage depth caused by Nd:YAG1064nm laser will exceed 20μm. As long as the removed damage layer reaches such a depth, it will not affect the conversion efficiency of the solar cell by more than 20%, but the depth is closely related to the cost of the laser and the thickness of the silicon wafer, which needs to be fully considered in the production process. The researchers of the solar cell research group believe that laser processing technology is the most critical technology in the RISE processing program. Since the non-contact laser processing technology they used is already used in most other industries, the RISE process is suitable for the industrial production of RISE solar cells using large and thin wafers of crystalline silicon, which will make its production cost at least as low as possible. Comparable to the production cost of today’s standard solar cells.
Japan’s Sanyo Electric Corporation announced on May 30, 2009 that it has broken the world’s highest photoelectric conversion efficiency record in the production of crystalline silicon solar cells of practical size (above 100cm²), and its proprietary HIT solar photovoltaic cell photoelectric conversion efficiency has reached 23.0%. The improvement of photoelectric conversion efficiency of solar photovoltaic cells can reduce the production cost of photovoltaic systems and the consumption of raw materials such as silicon.