Silicon: a material with a future

120 crystal-growing machines, also known as pullers, for the manufacture of silicon wafers will be installed in Alzenau. These will supply a total of four production lines. The path from a wafer to a solar cell is technically exacting. The high-purity wafer material, positively doped with boron, is coated on one side with phosphorous atoms. After the coating diffuses into the material, a negative doped layer and the pn-junction (the area between conducting sides of a semiconductor, in which the p-side contains acceptor atoms and the n-side donor atoms) are produced. Metallic circuits for current derivation are then mounted. Using a method called PECVD (Plasma Enhanced Chemical Vapor Deposition), an anti-reflection coating is applied in a final step. This gives the cells their typical blue color and increases their efficiency.

Some 90 percent of all the modules that were sold in 2000 are based on crystalline silicon, which is known as c-Si. Forty percent of the wafers are made of monocrystalline material, which is obtained by the Czochralski process. This method of growing crystal from a molten bath is currently used to produce some 80 percent of semiconductor silicon and is named after the Polish-born Professor Jan Czochralski, who developed the process in 1918 for the “Metallgesellschaft” in Frankfurt am Main. The other 60 percent of wafers are made of multicrystalline silicon, which is produced by casting or ribbon/sheet growth methods. The so-called EFG process, which is only used by RWE SCHOTT Solar, belongs in this category.

The semiconductor material silicon still plays a leading role in thin-layer technologies, which both SCHOTT and RWE already used before the joint venture. This is particularly true for amorphous silicon (ASI) passivated with hydrogen. Other technologies have been developed around the world, such as copper-indium-gallium-diselenide (CIGS) and cadmium-telluride (CdTe).