Polycrystalline silicon

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 in cost. 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. 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 2010s, 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. Polycrystalline solar cells, often called multi-crystalline panels, are highly cost-effective, budget-friendly, and durable photovoltaic devices made by melting multiple silicon fragments together. Recognizable by their square shape and blue, speckled, mosaic-like appearance, they typically have an efficiency of 14% to 18%. These panels are ideal for large-scale, cost-conscious solar projects. The polysilicon feedstock – large rods from the Siemens process, usually broken into chunks of specific sizes and packaged in clean rooms before shipment – is directly cast into multicrystalline ingots which are large square blocks weighing around 800 kg for making solar wafers or submitted as-is to a recrystallization process to grow single crystal boules usually with the Czochralski method. The boules are then sliced into thin silicon wafers and used for the production of solar cells, integrated circuits...