Polycrystalline silicon (polysilicon) is the most chemically pure type of industrial silicon. Depending on the quality it has different applications:
- Electronic-grade silicon is silicon with a purity of more than 99.999999%. It is used for the production of electric energy converters, microcircuits, transistors, diodes, and so on.
- Solar-grade silicon is silicon with a purity of more than 99.9999%. It is used for the production of photovoltaic converters for solar batteries.
For the most part, polysilicon is obtained from technical silicon by converting it to trichlorosilane gas through catalytic hydrogenation. Then the rectification gas purification and its reduction to crystalline silicon using the Siemens process is conducted: free silicon is precipitated on the silicon wafer surface heated to 650–1,300° C in the flow of a reaction gas-vapor mixture of trichlorosilane and hydrogen.
Technical silicon is obtained by reduction in an ore-thermal furnace from gangue quartz by a carbon reducer. Quartz (SiO2) is a polymorphic modification of silicon dioxide – the most common mineral on Earth. It is also a part of many other minerals, so there is no shortage of raw materials for silicon production: sand can be found almost anywhere in the world. Charcoal and coke obtained from oil are the carbon reducers.
Biochar is the most cost-effective and eco-friendly reducer for the process of silicon crystallization is due to the technology of its production from biomass and waste.