Preparation of silicon materials by physical method and Siemens method
1. Physical methods to produce solar-grade (SOG) silicon
In the manufacturing cost of crystalline silicon solar cells, polysilicon raw materials account for a large proportion. In order to reduce the manufacturing cost of polysilicon materials, the polysilicon materials can be purified by physical methods (sometimes called metallurgical methods) or a combination of physical methods and chemical methods. The specific process routes of these methods are different, and their characteristics are to simplify the process, reduce equipment investment, reduce environmental pollution, and ultimately reduce production costs. At present, some physical purification methods have been able to prepare polysilicon materials with a purity close to 6N. For example, metallurgical grade silicon with better purity is selected for horizontal zone melting and unidirectional solidification into a silicon ingot. After removing some metal impurities in the silicon ingot, it is pulverized and cleaned, and boron impurities are removed in a plasma melting furnace, and then zone melting and solidification is performed again. Metal impurities are removed, after crushing and cleaning, phosphorus and carbon impurities are removed in an electron beam melting furnace to directly generate SOG silicon. Metallurgical production of SOG silicon has certain advantages in cost, but the quality needs to be further improved.
2. Siemens method
The Siemens method was first invented and industrialized by the German company Siemens. The Siemens process is to react industrial silicon powder with HCl to form SiHCl3, and then allow SiHCI3 to be reduced and deposited in a H2 atmosphere reduction furnace to obtain polysilicon. Later, Siemens made improvements on this basis and adopted a closed-loop production process to form an improved Siemens method. Improved Siemens method to add reduction tail gas dry recovery system and SiCl4 hydrogenation process. The tail gas H2, SiHCl3, SiCl4, SiH2CL1 and HCl discharged from the reduction furnace are separated and reused, realizing a closed-circuit cycle. Because this method adopts a large reduction furnace, the energy consumption per unit product is reduced; the SiCl4 hydrogenation process and the tail gas dry recovery process are used, which significantly reduces the consumption of raw/auxiliary materials.
First, use coke to reduce quartz sand (SiO2) in an electric arc furnace to obtain elemental silicon at a temperature of 1600 ~ 1800 °C, reduce silicon and CO2 gas, and form industrial silicon with a purity of 98%. Its chemical reaction is
SiO2+C→Si+CO2↑ (1-1)
Then the industrial silicon is pulverized and reacted with anhydrous hydrogen chloride (HCI) in a fluidized bed (ebullated bed) reactor to generate trichlorosilane (SiHCl3). Synthesis of SiHCl3 by fluidized bed has the advantages of large capacity, high content of SiHCl3 in the product, and low cost.
In the fluidized bed, silica fume and HCI react as follows to generate SiHCl:
Si+ 3HCI→(280 ~ 320℃) SiHCl3+H2+50kcal/mol (1-2)
Note that the above reaction is an exothermic reaction, and the reaction temperature must be strictly controlled.
The silicon powder is further decomposed, SiHCl3 is condensed, and SiCl4 returns gaseous H2 and HCI to the reactor, or is discharged into the atmosphere after environmental protection treatment. Then decompose the condensed SiHCl3 and SiCl4 to obtain high-purity SiHCl3.
The methods for purifying SiHCl3 and SiCl4 include distillation, complex method, solid adsorption method and extraction method. Among them, the rectification method of removing impurities by multiple rectification in a distillation column has the characteristics of large processing capacity, convenient operation and high efficiency. Most impurities can be completely separated, but it is more difficult to completely separate boron, phosphorus and chlorides, which are highly polar impurities.
Finally, in the closed polysilicon reaction vessel, high-purity SiHCl is heated to 1050~1100℃ in the H2 atmosphere to reduce polysilicon and deposit on the surface of the silicon core. Through the reaction time of one week or more, the silicon core in the reduction furnace will grow from 8mm to 150~200mm, and obtain high-purity polycrystalline silicon rods.
Its chemical reaction is
SiHCl3+H2→Si→Si+3HCl↑ (1-3)
About 35% of SiHCl3 reacts to form polysilicon, and the remainder is separated from the reaction vessel together with H2, HCI, SiHCl3 and SiCl4 and reused, returning to the entire reaction. Among them, after the main products SiCl4 and SiHCl3 are separated and purified, high-purity SiHCl3 enters the reduction furnace to grow polysilicon. HCI can be recovered by activated carbon adsorption method or cold SiCl4 dissolving HCI method, and then enters the fluidized bed reactor to react with metallurgical grade silicon powder.
The high-purity silicon produced by the improved Siemens method has high purity, and the intermediate product SiHCl3 can be safely transported and stored for a long time (several months), which is suitable for the production of solar-grade polysilicon with an annual output of more than dry tons.