Silicon is the second most abundant element on Earth after oxygen. Silicon is usually found in large deposits as quartzite, as a silicate in silicon dioxide (SiO2) form. Although these sources are generally mixed with other elements (such as iron) and therefore impure, silicon as a natural resource is highly abundant.
The production and purification of polysilicon is the first step in the manufacturing process to produce conventional silicon solar cells. The fabrication of polysilicon begins with a carbothermic reduction of SiO2. Quartzite, the source of SiO2, is mixed with carbon—sourced from materials such as coal, coke (fuel), graphite or wood—in a submerged-arc electric furnace. Here, electrodes in the furnace supplying a three-phase current heat the mixture up to approximately 2000 °C, causing the SiO2 to reduce to molten silicon. At the surface of the melt, temperatures are usually lower (approximately 1600 °C) and reactants are reduced to form silicon carbide (SiC),
SiO2 + 3C → SiC + 2CO(g) (1)
In this distillation‑like process, the SiC and SiO2 descend to the lower, hotter parts of the furnace. In the lower regions, where the temperatures are higher (> 1780 °C), SiC and SiO2 react, resulting in elemental silicon and carbon monoxide gas,
SiO2 + 3SiC → 3Si + 2CO(g) (2)
This heavier, molten silicon melt is discharged through the bottom of the furnace. This silicon is approximately 98 % pure and termed metallurgical grade silicon (MGS). Other reactions occurring simultaneously in the furnace make the process self-sufficient. At bottom of the furnace, residual SiO2 and SiC also react to form carbon monoxide and silicon monoxide gas,
SiO2 + SiC → SiO(g) + 2CO (3)
The resulting silicon monoxide and carbon monoxide gasses rise back to the surface where they mix to produce silicon dioxide and carbon, which are recycled as reactants in Eq. 1.
To produce higher purity polysilicon, the MGS needs to be further purified. In this process, MGS is first ground into a powdered form. This powder is then injected into a fluidized bed reactor at high pressure and velocity. Anhydrous hydrochloric acid (HCl) is also injected into the reactor along with a catalyst, forming a series of chlorosilanes and other chlorides. The most important compound formed in this process is trichlorosilane (SiHCl3),
Simgs + 3HCL → 3SiHCl3 + H2 (4)
Through fractional distillation, the trichlorosilane gas is separated from hydrogen and gaseous HCl through a filter at the top of the reactor. The gas is then prepared for the Siemens process. In a Siemens reactor, graphite electrodes pass current through a U-shaped silicon core (seed). Trichlorosilane is injected into the reactor and undergoes hydrogen reduction in a similar process to chemical vapour deposition (CVD) to form solid silicon and gaseous hydrochloric acid. Solid polysilicon deposits onto and grows around the silicon seed. Once the process is complete, the U-shaped core and polysilicon are extracted. The resulting polysilicon is also known as electronic grade silicon with a purity of 9N (99.999999999 % Si) and broken down into smaller pieces ready for ingot production.