As the bulk material, the silicon materials used in crystalline silicon solar cells mainly come from the inferior products of semiconductor silicon materials and the head and tail materials of single crystal silicon. At present, the silicon wafers required by the technical production process used in the industrial production of monocrystalline silicon cells are cut from Czochralski monocrystalline silicon rods. The cost of ingot manufacturing and slicing is very large, and the processing cost of silicon wafers accounts for 20%. The cost of silicon materials accounts for 50% to 70% of the cost of solar cells. Because the price of silicon materials is relatively high and the manufacturing process of solar cells is relatively complicated, it is relatively difficult to use this technology to greatly reduce costs. Of course, it is not ruled out that the cost of silicon materials may be greatly reduced due to the revolutionary improvement of the process.
In addition, from the perspective of improving the efficiency of solar cells and reducing costs, solar-grade monocrystalline silicon technology is currently relatively mature, and there is little room for further improvement in the technical level. The current conversion efficiency of monocrystalline silicon cells is 24.7%, which is almost the same as the limit efficiency of monocrystalline silicon cells of 29%. It is very difficult to reduce the price of monocrystalline silicon solar cells to a price that competes with conventional energy by improving the efficiency.
Compared with monocrystalline silicon solar cells, the cost of polycrystalline silicon solar cells has been reduced. The use of cast polycrystalline silicon to make solar cells eliminates the need to pull monocrystalline silicon, and the growth of cast polycrystalline silicon is simple and easy to grow into large-size square ingots, with low growth energy consumption and low cost of silicon wafers, thereby reducing the production cost of solar cells . However, the photoelectric conversion efficiency of polycrystalline silicon solar cells has not been able to exceed 20% for a long time, while monocrystalline silicon solar cells have reached it more than 20 years ago. This is because polycrystalline silicon materials have obvious defects compared with monocrystalline silicon materials (such as grain interface And lattice dislocation), resulting in the photoelectric conversion efficiency of polycrystalline silicon solar cells has been lower than that of monocrystalline silicon solar cells. In recent years, the technical level of polycrystalline silicon solar cells has improved rapidly. The conversion efficiency of its laboratory cells has reached more than 20%, and the conversion efficiency of industrially produced polycrystalline silicon solar cell modules can reach more than 15%, which is only lower than that of monocrystalline silicon solar cells. -2 percentage points. In terms of cost performance, compared with monocrystalline silicon solar cells, polycrystalline silicon solar cells have greater market potential. Therefore, the share of polycrystalline silicon solar cells has reached more than 50% in recent years. However, whether monocrystalline silicon solar cells or polycrystalline silicon solar cells silicon materials account for a large cost, and solar cells are in a period of rapid development, there is a huge gap in the production capacity of silicon materials, so it is necessary to truly reach the ground. The goal of large-scale utilization of solar cells is to make solar cells a civilian battery, and reducing the use of silicon materials has become a necessary development direction.
The target price for solar cells to become civilian batteries is $1/w, which is a price that can compete with conventional energy sources. The cost of photovoltaic power generation is 6~12 cents/(kw·h), which is equivalent to the cost of solar cell modules of approximately US$0.61/w. However, the current international solar cell module cost is about US$2.5/w, which is equivalent to US$0.25/(kw.h) for photovoltaic power generation, which is more than 4 times the required value. Over the years, in order to avoid the disadvantages of expensive processes and waste materials such as drawing single crystal silicon or casting polycrystalline silicon, slicing, etc., scientists from various countries have developed a variety of silicon ribbon preparation technologies, directly drawing from the silicon solution suitable for solar cell preparation. The shape, width and thickness of the silicon tape, the thickness of this silicon tape is more than 2004m, but has not been mass-produced industrially, that is to say, it has not yet been recognized by the photovoltaic industry.
The emerging technology of thinning refers to reducing the thickness of the crystalline silicon cell substrate as much as possible while maintaining the efficiency of the solar cell. It is particularly noteworthy that this thin slice is not processed by a conventional wire saw. According to reports, thin silicon wafers with a thickness of less than 2004m have been commercialized. For example, the Swiss HCT company has realized the cutting of 200um, 150um, and 100pm thin silicon wafers in 2003: in 2004, it has trial-produced silicon wafers with a thickness of 200pm. With the improvement of the level of cutting technology, the thickness of the cut silicon wafer is less than 200um. However, this technology still cannot avoid the expensive process of pulling the single crystal and cannot substantially reduce the use cost of silicon materials. Another development trend is the so-called layer transfer technology, which first deposits a high-quality silicon film on an expensive single crystal silicon substrate, and then separates the silicon film from the single crystal silicon wafer and transfers it to a glass or plastic film On other cheap substrates, single crystal silicon substrates are reused. Its advantages are very obvious. The high-quality monocrystalline silicon film with a thickness of several tens of micrometers ensures the high efficiency of the battery. The cheap substrate is beneficial to reduce the cost, and the repeated use of the monocrystalline silicon substrate will not increase the additional cost. But this kind of craft is too complicated, it is difficult to realize large-scale industrial production.
At present, solar cell-grade polycrystalline silicon is mostly made of monocrystalline silicon rods with slightly lower purity head and tail materials, or leftover materials at the bottom of a monocrystalline furnace to further smelt, dope, blend, and re-melt and cast ingots. Affected by the limitation of monocrystalline silicon output and the substantial price increase, the cost of solar cell-grade polycrystalline silicon is relatively high. There is only one American company, Solar Grade Silicon LLC, a professional manufacturer of solar cell-grade silicon materials. In addition, there are several manufacturers of semiconductor-grade polysilicon that also produce solar cell-grade silicon materials: Tokuyama in Japan, Hemlock in the United States and Wacker in Germany. And the newly entered JSSI, ELKEM, etc. At present, the most advanced countries in the world for polysilicon production technology are a few developed countries such as the United States, Germany, Japan, and Italy. The output of the above four countries accounts for more than 90% of the total output of polysilicon in the world. Chemical purification method.
In recent years, the use of physical purification technology to produce solar cell-grade polysilicon is entering the stage of industrialization. The basic idea of physical purification is to increase the purity from the bottom up (Bottom Up), which is completely different from the current world’s main production method-the improved German Siemens chemical method purity top down (Top Down) model. The advantage of the physical method is low price. Although the product purity is lower than that of the chemical method, after hard work, it can be used as a solar cell-grade polysilicon product. This is another important direction for reducing the cost of solar materials.