When a photon with sufficient energy impinges upon a semiconductor it can transfer enough energy to a electron to free it from the bonds of the semiconductor’s valence band so that it is free to move and thus carry an electric current. The junction in a semiconductor diode provides the necessary electric field to cause the current to flow in an external circuit.
The typical output voltage of a PV cell is between 0.5 and 0.6 Volts and the energy conversion efficiency ranges from less than 10% to over 20%. An array of cells can therefore generate about 200 Watts of electrical power per square meter when illuminated by solar radiation of 1000 Watts per square meter. The corresponding current density will be about 400 Amps/m2. Because of climatic conditions the intensity of the insolation rarely reaches 1000 W/m2.
Practical cells are also much smaller than one square meter with actual sizes of commercially available cells ranging from about one centimeter square to 15 centimeters square. The corresponding output Wattage for these cells range from 20 milliWatts to about 4 Watts.
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