Silicon (Si) for solar cells: how is it produced?

Silicon – second only in abundance to oxygen

Silicon (Si) is the second most abundant element in the earth’s crust, oxygen is number one. Si is found abundantly in certain rocks, sand and soil. Traces of Si or silicon compounds are also found in water, atmosphere, plants and even in certain animals. Naturally occurring silicon is found combined with oxygen (oxides of silicon) and with oxygen together with other elements (silicates).

Silicon applications worldwide

Si is used as an important ingredient in many applications worldwide. It is widely used in steel refining industry, aluminum casting industry, fine chemicals industry and of course the electronics industry. The silicon used in the electronics industry is highly purified, and forms the basis of integrated circuits. Si can be considered the building blocks of modern day computers and today’s technology highly dependents on it.

Silicon manufacturing process

Industry grade Si is produce by a chemical process known as reduction processes: In a typical reduction process, one or more elements are displaced when compound is chemically mixed with another. For the manufacturing of Si , silica and coke are chemically treated to very high temperatures inside an electric furnace. Oxygen from silica is displaced, forming Si and an oxide of carbon. See the chemical equation below:

Industrial grade silicon manufacturing process

At very high temperatures (often in excess of 2000 °C or 3600 °F), the carbon displaces oxygen from the silicon compound, forms carbon monoxide and leaves Si behind in a molten state. This liquid Si is collected at the bottom of the furnace, drained and cooled. As this silicon is only about 98% pure, it is called industrial grade silicon(or metallurgical grade Si) and is used in steel and aluminum industry.

Silicon (Si) Ingots

Electronic grade silicon manufacturing process

The use of Si in manufacturing of electronic components requires Si to be free from impurities. The electronic grade Si is generally 99.99% pure. The Si used in the manufacturing of solar cells and solar components has to be even more pure. A purity of 99.9999999% is required by the most advanced solar cells. This is often referred to as "9N" for "9 nines", a process which requires repeated refining.

Refining or purification process

The silicon obtained from the process described earlier, is used in the process of manufacturing of electronic grade silicon. Solid Si undergoes rigorous stages of refinement and purification. Many refining processes are deployed all over the world for this purpose that include zone refining, distill refining, electrolytic refining and Siemens refining process. In a modern day refining process, the silicon is first converted to a silicon compound and then undergoes a distillation process. Most of the impurities are removed by this process and pure Si is obtained as an end product. Without going into many details, silicon is purified by converting it to a Si compound that can be more easily purified by distillation than in its original state, and then exposing that Si to Tri-Chloro-Silane or Si tetrachloride at high temperatures.