How to Process Solar Grade Silicon

Mine and produce either quartzite gravel or crushed quarts. Quartzite gravel or crushed quarts are the foundation for creating solar-grade silicon.

Place the silicon dioxide of either quartzite gravel or crushed quarts into an electric arc furnace. Apply the carbon arc within the furnace to release the oxygen from the quartzite gravel or crushed quarts. The resulting products of this process are carbon dioxide and molten silicon. These special furnaces reach temperatures over 2,500-degrees Fahrenheit. This process purifies the silicon to about 98 percent. However, solar-grade (or semiconductor-grade) silicon requires greater purity. The 98-percent purity level is known as metallurgical-grade silicon, which can be used in steel, aluminum and other industries.

Remove the liquid silicon and place it in a separate, heated chamber. Within the heated chamber, move a rod made of impure silicon across the liquid silicon. Repeat this several times in the same direction. This process further purifies the silicon by dragging away the impurities towards one end of the rod. The impure section of the rod can then be cut away.

Place a silicon seed crystal into the melted silicon. Remove and rotate the silicon seed crystal. As the silicon seed is removed and rotated, it forms a cylindrical silicon ingot or boule. Because the impurities stay behind within the liquid silicon, this ingot is highly purified. This process is called the Czochralski Process. Boron can also be introduced to the silicon at this stage (see Step 6). These polycrystalline silicon ingots are the backbone of solar cells.

Cut the silicon ingot with a specialized circular or multi-wire diamond saw. The resulting highly-purified silicon piece is referred to as a silicon wafer. These wafers are less than a centimeter thick and can be cut out of the ingot in a circular, rectangular or hexagonal shape. Rectangular or hexagonal shapes are better because, unlike circle cuts, they can be pieced together perfectly.

Seal the silicon wafers in another furnace and heat to just below the melting point of silicon, which is about 2,500-degrees Fahrenheit. This step should be done with phosphorous gas surrounding the silicon wafers. As the silicon approaches liquefaction it becomes more porous, and this allows the phosphorous to enter the silicon. Another method for achieving this is to shoot phosphorous ions into the silicon using a small particle accelerator; controlling the speed of the ions controls the depth to which they penetrate. Boron and phosphorous is doped into the silicon to better regulate the electron levels within the silicon. This allows the silicon to conduct electricity at a more efficient level.

Add an anti-reflective coating to the silicon disks. One popular anti-reflective chemical used is titanium dioxide. Introducing an anti-reflective coating to the silicon disks enhances their ability to absorb sunlight.