polysilicon

Polysilicon is silicon refined to high purity, typically 99.9999% (six nines) or better, then solidified into a polycrystalline ingot. It differs from single-crystal silicon (monocrystalline) by containing many small crystal grains with random orientations rather than one continuous lattice. The material is the foundational feedstock for almost all photovoltaic cells and most semiconductor manufacturing, where chemical purity and controlled grain structure matter more than perfect atomic alignment.

Production begins with metallurgical-grade silicon, which is only 98% pure and unsuitable for electronics. Producers use either the Siemens process or fluidized bed reactor (FBR) method. The Siemens route reduces silicon tetrachloride with hydrogen at high temperature, depositing ultra-pure silicon onto heated filaments; FBR suspends fine silicon particles in a reactant gas stream, building the charge in a fluid bed. Both methods yield a silicon core that is then melted, cast into ingots, and cooled. The resulting ingot shows visible grain boundaries under light and breaks along those planes more easily than single-crystal wafers.

For solar applications, polysilicon is melted and cast into square or near-square ingots, then sliced into wafers. The grain boundaries scatter light and trap charge carriers, reducing cell efficiency compared to monocrystalline competitors by 2 to 4 percentage points in standard laboratory tests. However, lower cost per watt has kept polysilicon solar cells dominant in global deployment, especially in utility-scale installations. For semiconductor device manufacturing, polysilicon feedstock is further processed into single-crystal boules via the Czochralski or float-zone method, which is a separate and much more expensive step.

Defects and Performance

Oxygen, carbon, and iron impurities degrade minority carrier lifetime and increase recombination losses in solar cells. Dopant concentration and segregation during solidification create resistivity variations within an ingot; the last material to freeze traps dissolved elements at higher concentrations. Casting residual stress and thermal gradients during cooling introduce dislocation networks that propagate through grain boundaries. Quality control relies on electrical resistivity measurement, minority carrier lifetime mapping (photoluminescence or photoconductance decay), and metallurgical optical inspection.

The name reflects the crystalline structure: many small crystals (poly) of silicon rather than single-crystal (mono). Polysilicon entered mass production in the 1980s as solar photovoltaics scaled; demand exploded after 2000 with government subsidy programs and grid-connected installations. China now supplies roughly 75% of global polysilicon capacity, with major producers running plants in Xinjiang and Sichuan provinces. Typical ingot sizes range from 480 to 530 millimeters on edge and weigh 150 to 200 kilograms; casting cycles take 48 to 72 hours.