Properties of polysilicon: gray metallic luster (the enlarged polysilicon is shown in Figure 1-1), density is 2.32~2.34g/cm3, melting point is 1410℃, boiling point is as high as 2355℃, insoluble in water, insoluble in nitric acid and hydrochloric acid, But it can be dissolved in the acid that mixes both hydrofluoric acid and nitric acid. The hardness is between germanium and quartz, and the thin silicon is very easy to be brittle at room temperature, so be careful when cutting.
The high temperature plasticity is very good, and it is easy to produce obvious deformation at 1300℃. Silicon is very stable and inactive at room temperature. It has greater chemical activity in the high-temperature molten state. Incorporating certain impurities can become important and excellent semiconductor materials.
Polysilicon is a form of elemental silicon. When molten elemental silicon solidifies under supercooling conditions, silicon atoms are arranged into many crystal nuclei in the form of a diamond lattice. If these crystal nuclei grow into many crystal grains with different crystal lattice orientations, they become polysilicon. Polycrystalline silicon can be used as a raw material for drawing single crystal silicon. The difference between polycrystalline silicon and single crystal silicon is mainly manifested in physical properties. For example, it is far inferior to single crystal silicon in terms of mechanical properties, optical properties, thermal properties, and anisotropy. In terms of properties, the conductivity of polycrystalline silicon crystals is far inferior to that of single crystal silicon, or even almost no conductivity. In terms of chemical activity, the difference between the two is extremely small. Polycrystalline silicon and monocrystalline silicon can be distinguished from the appearance, but the true identification must be identified by analyzing the crystal plane orientation, conductivity type and resistivity of the crystal.
The emergence of polycrystalline silicon production technology is mainly to reduce the cost of crystalline silicon solar cells. Its main advantages are: it can directly draw square silicon ingots, the equipment is relatively simple, and large-scale silicon ingots can be produced to form an industrial production scale, the material power consumption is relatively low, and lower purity silicon can be used as charging material; Take measures to reduce the influence of grain boundaries and other impurities in battery technology. The main disadvantage is that the conversion efficiency of polycrystalline silicon cells produced is slightly lower than that of monocrystalline silicon cells. The polysilicon ingot casting process mainly includes two kinds of directional solidification method and casting method.
There are several ways to purify silicon.
(1) Improved Siemens method
The modified Siemens method is to crush metallurgical silicon powder into fine particles with a particle size of less than 0.5mm, liquefy it into metallurgical grade silicon in a 300-400℃ reactor, and react with HCl to generate SHCl3 and H2 under the catalysis of Cu. The gas is condensed into a liquid through a condenser, and the resulting liquid undergoes multiple fractionation to produce pure trichlorosilane (SiHCI3). In order to extract high-purity silicon, SiHCI3 is reduced by hydrogen in the reactor and pressurized at 1050~1100°C to precipitate fine-grained polysilicon on the pestle. This step not only requires a lot of energy, but also has a low output. Most of the existing polysilicon factories at home and abroad use this method to produce solar-grade polysilicon.
(2) Silane method-silane thermal decomposition method
Silane gas is a material prepared by the silicon tetrachloride hydrogenation method, the silicon alloy decomposition method, the hydride reduction method, and the direct hydrogenation method of silicon. Then, the obtained silane gas is purified to produce rod-shaped polysilicon with higher purity in a thermal decomposition furnace. In the past, only Japan had mastered this technology. Due to serious explosion accidents, it did not continue to expand production. However, two companies in the United States still use silane gas thermal decomposition to produce electronic grade polysilicon products with higher purity.
(3) Fluidized bed method
Put silicon tetrachloride, hydrogen, hydrogen chloride and industrial silicon as raw materials in a fluidized bed (fluidized bed) to generate trichlorosilane under high temperature and high pressure. Then the trichlorosilane and hydrogen are reacted to generate dichlorodihydrosilane, thereby generating silane gas. The prepared silane gas is passed into a reactor with small particle size silicon powder for continuous heating to cause a decomposition reaction to produce a granular polycrystalline silicon product. Because the silicon surface area involved in the reaction in the fluidized bed reactor is large, the production efficiency is high, the power consumption is low, and the cost is low, it is suitable for large-scale production of solar-grade polysilicon. The disadvantage is poor safety, high risk, and product purity is not high enough, but it can basically meet the needs of solar cell production and use performance. At present, only the United States adopts this method to produce granular polysilicon in the world. This method is more suitable for producing low-cost solar-grade polysilicon.
At present, the scale of China’s solar energy industry ranks first in the world. Solar cells are generally produced using polysilicon, but certain problems have also been exposed, mainly due to the soaring international price of high-purity polysilicon, from US$12 per kilogram in 2006 to US$300 in 2009. Despite this, high-purity polysilicon is still in short supply