Peak Sun's uniform particle size and lack of oxidation make for ideal granular silicon feedstock

Other granular silicon has an irregular size and its oxidized surface makes it difficult to melt

Chunk silicon made using the Siemens Process

Silicon

Silicon is the second most abundant element is the earth’s crust after oxygen. However, silicon does not occur as the pure element in nature, instead it is widely distributed across the globe in various forms of silicon dioxide (sand).

After separation from the oxides in an electric arc furnace, silicon is 99% pure and is commonly known as metallurgical grade silicon. While 99% purity is good enough for industrial applications like producing alloys with aluminum, copper or iron, it is not pure enough for use in solar cells 99.9999% (6N) minimum, or electronic microchips 99.9999999% (9N) minimum purity. 

Silicon is the main component in crystalline solar cells, the dominant form of solar cells on the market today. In order to generate electricity from the sun for a cost equal to or less than carbon emitting sources of electricity like coal fired plants, the cost to manufacture a solar cell must decrease and the efficiency of those cells must increase. This is often called achieving “grid-parity.” A better process to manufacture silicon is the key to solving both of those problems and to making grid-parity a reality.

Purification

Today, most solar and electronic grade silicon is manufactured using the decades old Siemens Process - named after the company that invented the process. In the Siemens Process, high purity silicon rods are exposed to a chemical gas compound trichlorosilane (TCS) at high temperatures inside a bell jar reactor. The TCS gas decomposes due to high temperature and deposits highly pure silicon on the rods. The remaining toxic chlorine byproducts are either disposed of or recycled in an expensive secondary reactor. Once enough high purity silicon has deposited on the rods the reactor is shut down and the silicon is allowed to cool.

Despite the ubiquity of the Siemens Process, there are several problems with the process as it exists today. These include, but are not limited to: 

  • Massive amounts of energy are expended to raise the TCS to the temperature needed for the reaction to occur
  • The batch process nature of the Siemens Process means that there is considerable downtime in the bell jar reactor as the silicon cools and the next batch of rods are raised to high temperature
  • The toxic byproducts tend to form explosive polymers in exhaust ducting which can be catastrophic if not properly monitored by expert technicians
  • A major byproduct of the Siemens Process silicon tetrachloride has been illegally dumped into neighboring watershed areas (see Washington Post Article) by unscrupulous silicon manufacturers giving the solar energy industry a black eye
  • Byproduct recycling efforts are expensive, and thus raise the cost of the silicon brought to market 

While the Siemens Process was effective in providing a high purity silicon suitable for use in the electronics industry, the high energy consumption, the high cost to manufacture, and the toxic byproducts are no longer acceptable for the production of silicon for the solar industry.