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Polycrystalline silicon is a type of elemental silicon that has the characteristics of a monocrystalline material. It is used in the manufacture of electronic devices and semiconductors. Unlike single crystals, polycrystalline silicon has a characteristic grain size, which is typically less than 1000 microns. This grain size is important for photovoltaics and MEMS, and it is also used in a wide range of other applications.
In the production of crystalline solar cells, silicon is a common material. The open-circuit voltage is slightly reduced by grain-boundary recombination. Also, defects in the silicon can degrade the efficiency of the cell. These defects can be eliminated through careful processing.
In polycrystalline silicon solar cells, the top layer acts as an emitter. The bottom layer serves as the collector. A thin dielectric coating is applied to the back of the wafers to passivate the surface. To reduce the device dimensions, the surface must be as smooth as possible.
The crystalline grain size of polycrystalline silicon is also important. This size must be optimal for maximum efficiency in a polycrystalline solar cell. Typically, grains are several hundred micrometers, though researchers have grown larger grains.
The microstructure of a polycrystalline Si rod depends on the deposition conditions. Thin rods with square cross section are preferably used. They are heated to deposition temperature by passing current through them. Once the reaction ends, the electrodes for power supply are separated from the rods.
As the thickness of the film increases, the diameter of the rods increases. At least 10% of the processed rods could be refined into a monocrystalline state by the floating zone process.