The most common type of solar cell is the crystalline silicon cell. The first stage in their production is the manufacture of extremely thin wafers of silicon (Si) with a thickness of between 0.2 and 0.3 mm from high purity silicon. These wafers then undergo further processes to be made into solar cells with two semiconductor layers. When light falls on the cells, the electrical charges in the semiconductor layers separate and produce a DC voltage. The voltage produced is specific to the cell material; for silicon this is approximately 0.5-0.6 VDC. This is normally too lowto be of use. In a PV module the cells are connected in series to produce higher voltages. The upper limit on the number of cells and thus the voltage of a module is determined mainly by the practicalities of handling modules.They must be light enough and small enough to enable them to be installed on roofs,
sometimes underdifficultconditions.Toproducethe even higher voltages required in grid-tied PV systems, modules are in turn connected in series. A group of modules in series is called a string. Larger installations will consist of several strings of modules.