Polycrystalline silicon

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Polycrystalline silicon, or multicrystalline silicon, also called polysilicon, poly-Si, or mc-Si, is a high purity, polycrystalline form of silicon, used as a raw material by the solar photovoltaic and electronics industry.

Polysilicon is produced from metallurgical grade silicon by a chemical purification process, called the Siemens process. This process involves distillation of volatile silicon compounds, and their decomposition into silicon at high temperatures. An emerging, alternative process of refinement uses a fluidized bed reactor which is lower cost.^[1] The photovoltaic industry also produces upgraded metallurgical-grade silicon (UMG-Si), using metallurgical instead of chemical purification processes for lower cost at the expense of purity.^[2][3][4][5] When produced for the electronics industry, polysilicon contains impurity levels of less than one part per billion (ppb), while polycrystalline solar grade silicon (SoG-Si) is generally less pure. In the 2010's, production shifted toward China, with China-based companies accounting for seven of the top ten producers and around 90% of total worldwide production capacity of approximately 1,400,000 MT. German, US and South Korea companies account for the remainder. ^[6]

The polysilicon feedstock – large rods from the Siemens process, usually broken into chunks of specific sizes and packaged in clean rooms before shipment^[7] – is directly cast into multicrystalline ingots which are large square blocks weighing around 800 kg^[8] for making solar wafers^[9] or submitted as-is to a recrystallization process to grow single crystal boules usually with the Czochralski method.^[10][9] The boules are then sliced into thin silicon wafers and used for the production of solar cells, integrated circuits and other semiconductor devices.

Polysilicon consists of small crystals, also known as crystallites, giving the material its typical metal flake effect. While polysilicon and multisilicon are often used as synonyms, multicrystalline usually refers to crystals larger than one millimetre. Multicrystalline solar cells are the most common type of solar cells in the fast-growing PV market and consume most of the worldwide produced polysilicon. About 5 tons of polysilicon is required to manufacture one 1 megawatt (MW) of conventional solar modules.^{[11][citation needed]} Polysilicon is distinct from monocrystalline silicon and amorphous silicon.