The basic component of a solar cell is pure silicon, which is not pure in its natural state.
- To make solar cells, the raw materials—silicon dioxide is first placed into an electric arc furnace, where a carbon arc is applied to release the oxygen. The products are carbon dioxide and molten silicon. But still silicon requires further purification.
- This silicon is purified even further using the floating zone technique. A rod of impure silicon is passed through a heated zone several times in the same direction. This procedure “drags” the impurities toward one end with each pass. At a specific point, the silicon is pure, and the impure end is removed.
- Solar cells are made from silicon ingots, polycrystalline structures that have the atomic structure of a single crystal. The most commonly used process for creating the ingot is called the Czochralski method. In this process, a seed crystal of silicon is dipped into melted polycrystalline silicon. As the seed crystal is withdrawn and rotated, a cylindrical ingot of silicon is formed. The ingot withdrawn is usually pure, because impurities tend to remain in the liquid.
- From the ingot, silicon wafers are sliced one at a time using a circular diamond saw whose inner diameter (0.5 mm) cuts into the rod, or many at once with a multiwire saw. Only about one-half of the silicon is lost from the ingot to the finished circular wafer. Rectangular or hexagonal wafers are sometimes used in solar cells because they can be fitted together perfectly, thereby utilizing all available space on the front surface of the solar cell.
- The wafers are then polished to remove saw marks. (It has recently been found that rougher cells absorb light more effectively, therefore some manufacturers have chosen not to polish the wafer).
The traditional way of doping (adding impurities to) silicon wafers with boron and phosphorous is to introduce a small amount of boron during the Czochralski The wafers are then sealed back to back and placed in a furnace to be heated to slightly below the melting point of silicon (2,570 degrees Fahrenheit or 1,410 degrees Celsius) in the presence of phosphorous gas. The phosphorous atoms embed itself into the silicon, which is more porous because it is close to becoming a liquid. The temperature and time given to the process is carefully controlled to ensure a uniform junction of proper depth.
A more recent way of doping silicon with phosphorous is to use a small particle accelerator to shoot phosphorous ions into the ingot. By controlling the speed of the ions, it is possible to control their penetrating depth. This new process, however, has generally not been accepted by commercial manufacturers because it is costly.
- Electrical contacts connect each solar cell to another and to the receiver of produced current. The contacts must be very thin (at least in the front) so as not to block sunlight to the cell. Metals such as palladium/silver, nickel, or copper are used.
- After the contacts are in place, thin strips are placed between cells. The most commonly used strips are tin-coated copper.
- Because pure silicon is shiny, it can reflect up to 35 percent of the sunlight. To reduce the amount of sunlight lost, an anti-reflective coating is put on the silicon wafer. The most commonly used coatings are titanium dioxide and silicon oxide.
- The finished solar cells are then sealed into silicon rubber or ethylene vinyl acetate. The sealed solar cells are then placed into an aluminum frame that has a mylar or tedlar backsheet and a glass or plastic cover.