The Production of Polycrystalline Silicon
About how raw silicon are usually produced.
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Solar photovoltaics is used to generate electricity from the sunlight and is based on the application of the photo-effect. To use it, one needs solar panels made of semiconducting materials to be exposed to the sunlight.
Currently, two kinds of solar panels are being produced: crystalline silicon and thin-film panels. The former are better suited to be integrated into the building structures, while the former are widely used in the solar power plants. Solar DAO’s PV solar plants are also based on crystalline silicon solar panels. But how is it produced? This article explains the process of polycrystalline production.
Polycrystalline Silicon Production Process
Today polycrystalline silicon counts as the main raw material for photovoltaics. If we are to effect the transition from the fossil fuel-based economy to the solar energy-based one, governments should take the polycrystalline silicon industry into serious consideration.
Polycrystalline silicon (polysilicon) production is based on the closed water cycle and recirculation of the starting material. This organization of the production process allows to prevent dangerous pollution into the atmosphere and produce polycrystalline with minimal environmental damage.
In more detailed terms, the production process of polycrystalline silicon, conceived as the raw material for the energy of the future, it is possible to distinguish the following aspects:
- Initially, quartzit (quartz sand) is used to manufacture metallurgic silicon. To do so, determinate quantities of coal, quartz grain, coke, and shiver are getting mixed in an arc furnace under high temperature. Technical silicon enters into a chemical reaction with hydrogen chloride, which result is trichlorosilane. Trichlorosilane is a transparent liquid which turns back into hydrogen chloride even after a minimal interaction with air. In order to prevent the transformation of trichlorosilane into hydrogen chloride, specialized equipment is used in the process of polycrystalline silicon production. Moreover, trichlorosilane is an easy-boiling liquid that needs to be treated with the help of a specialized refrigerator.
- Trichlorosilane is formed at 260–400 °C. Even though the formation of trichlorosilane may involve a series of side reactions, this substance is the most important one for the process of polycrystalline silicon production. Thanks to trichlorosilane, given a series of specialized procedures, silicon rods can be formed.
- In their turn, silicon rods are put into large furnaces (reactors), in which, under the influence of high temperatures (1000 °C), the process of polysilicon sedimentation takes place. The rod is literally growing up and reaches 120 santimeter in diameter and approximately 2 meters in length.
- Later, polycrystalline silicon is getting cut into smaller pieces, packed, and exported.
In sum, polycrystalline silicon is produced by trichlorosilane sedimentation on the silicon rods heated to maximum temperatures.
The installation of the silicon rods into the furnaces and, later, the packaging of the smaller pieces are done manually by workers equipped with specialized costumes to meet the condition of absolute sterility and to protect them from the high temperature.
Polycrystalline silicon belongs to the family of the “purest” products, almost without admixtures. That is, the less admixtures there are in the product, the higher its specific electrical resistivity (SER). High SER allows for a more efficient transformation of the sunlight into electrical energy by the photoelements produced from polysilicon. The purification of polycrystalline silicon, that is, the removal of various admixtures, is done by way of rectification and transition into nonvolatile compounds.
It is also worth noting that the formed metallurgic silicon has the 99.9999% (four decimals — 4N) purity level of the formed raw material. But polycrystalline silicon that is to be applied in the production of solar panels has to be even purer: to have a “solar quality” it has to be 99.999999% (six decimals — 6N) pure. In general, silicon is also used in the production of microelectronic components, but of even higher purity and higher conductivity.
There are a few major producers of polycrystalline silicon in the world, including:
- Mitsubishi, Japan
- Hemlock, USA
- Wacker, Germany
- MEMC, Italy
- Tokuyama Semitomo, Japan
Currently there is a trend on the world market towards an increase in the polysilicon production, but the raw materials deficit on the market is about 8000 tons per year, which drives the raw materials prices up.
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