The physical method is that the silicon element in the metal silicon does not participate in the chemical reaction, but uses different physical methods to remove different impurities step by step to achieve the purpose of purification. Because this method is similar to the method of refining outside the metallurgical furnace in many aspects, it is also called the metallurgical method. As a silicon material for solar cells, a silicon purity of 6 to 7 9s can meet the requirements. Therefore, from the perspective of reducing the cost of solar cells, focusing on the development of low-cost extraction methods within the allowable impurity range is the future development direction, and physical solar cell polysilicon is one of the most promising methods.
The physical method was tested in the laboratory in the 1980s, but the silicon material of this method was completely unable to meet the application of semiconductors. After the Siemens method purification technology was commercialized, the research was stopped. At the beginning of the 21st century, the amount of silicon used for solar energy has risen, surpassing that of semiconductors, and the research of physical polysilicon has restarted.
Compared with the Siemens method, the physical method consumes less energy and has a lower cost, and may be an ideal method for producing polysilicon for solar cells in the future. At present, the countries that conduct research on the new process of physical purification of industrial silicon to prepare solar-grade silicon are: Japan, China, Norway, and the United States.
For the physical method, metallurgical grade industrial silicon is also used as a raw material to gradually remove impurities to produce polysilicon. As for solar cells, P, B, Co, Fe, Cr, Ni, Cu, Zn, Ca, Mg, Al, etc. are elements to be strictly controlled, so starting from industrial silicon smelting, the process must be adjusted appropriately: The selection of raw materials and the use of tools strictly limit the mixing of the above elements.
In addition to the selection and control of raw materials, the raw materials must be processed. For example, it is easier to remove boron from silicon dioxide than to remove boron from silicon, because silicon and boron easily form compounds. Another example is the addition of some oxidants in the smelting of metallic silicon to increase the oxidation and volatilization of non-metallic elements such as phosphorus and boron, and reduce the content of phosphorus and boron in metallic silicon.
During the smelting process, all measures should be taken to prevent the silicon liquid from absorbing impurities, reduce pollution, and remove impurities in the metal through various refining and purification methods. The impurities in silicon materials are not only from the furnace charge, but also from the equipment itself. The main sources of impurities are as follows:
① Absorb impurities from the furnace lining;
② Absorb impurities from the furnace gas;
③ Absorb impurities from flux and smelting additives;
④ Absorb impurities from the charge and slag;
⑤The impurities accumulated by multiple remelting of old materials, once the content of a certain component or impurity exceeds the relevant standards, waste products will appear;
⑥When the graphite electrode is consumed, the impurities contained in the electrode will also enter the metal silicon product.
Among them, after the furnace lining has been used for several furnaces, a layer of silicon carbide and silicon dioxide will be formed on the furnace wall, which separates the furnace lining material from the furnace charge, and the pollution of the furnace lining to the silicon material will be greatly reduced.
A better metal silicon plant can easily smelt 3N metal silicon. The metal impurities can be controlled within 100×10-6, the phosphorus can be controlled at about 10×10-6, and the boron can be controlled below 1×10-6.
What needs to be mentioned is that some people imagine using high-purity quartz (5 N silica) and high-purity carbon to react directly to produce polysilicon. However, this route is very difficult, because the extensive process of the conventional submerged arc furnace, the purity of the material is more than 5N, and it is very difficult to completely avoid it.
Through the smelting methods and processes of metallurgical silicon, the impurities in metallurgical grade silicon mainly come from the raw materials and equipment in the smelting process. These impurities mainly include the following: One type is represented by C, N, H, etc. The light element impurities; the other is metal impurities, such as Fe, Al, Ca, Cu, Ni, etc.; and the non-metallic compounds in metallurgical silicon, such as oxides, chlorides, sulfides, and silicates, are mostly independent Exist, collectively referred to as non-metallic inclusions, generally referred to as inclusions or slag inclusions for short. The slag inclusions exist in the form of agglomerates or granular slag inclusions of different sizes. If the slag inclusions are dispersed in the metal melt in the form of particles, they are not easy to remove.
The presence of these impurities has a great negative impact on both the semiconductor process and the photovoltaic process. Among them, the excessive content of light elements will cause the silicon wafer to warp, and can introduce secondary defects, etc., while the C in the light elements will reduce the breakdown voltage and increase the leakage current. Transition metal impurities can form deep-level centers or precipitates in Si and affect the electrical properties of materials and devices. In addition, they can significantly reduce the minority carrier lifetime.