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Another device that harnesses solar energy is the solar cell. Solar cells are expensive and are used only when supplying electricity becomes difficult.
Solar cells are the devices where solar energy is directly converted into electricity.
Sunlight can be used to run machinery and generate electricity.
About hundred years ago it was found that when sunlight falls on a thin layer of selenium electricity is produced. But the efficiency of the conversion was low (as low as 0.6%). Solar cells were not popular for a long time because of this low efficiency.
Solar cells are made of naturally available semiconductors such as silicon and germanium. The resistivity of these materials is between those of conductors and insulators. Therefore they are called semiconductors. They behave like insulators at low temperature and like conductors above room temperature. To increase its conductivity semiconductors are doped with impurities. Semiconductors which are doped with impurities like boron or aluminium are termed as p-type semiconductors and those doped with impurities like phosphorous, are called n-type semiconductors. In p-type semiconductors holes are majority carriers and electrons are minority carriers and in n-type semiconductors electrons are majority carriers and holes are minority carriers.
The first practical solar cell was constructed in 1954 with an efficiency of 1%. Today we have selenium based solar cells with an efficiency of 25%.
Construction and Working of a Solar Cell
Solar cell is a very thin sandwich of n-type and p-type wafer thin semiconductors of silicon. A U-shaped metal grid is embedded into the thin semiconductor chip as shown in the fig. The lower end of the sandwich is fixed to a metal base which has a metal lead. The grid has a lead contact on the top surface which can be connected to another solar cell in series. The upper surface of semi conducting sandwich is coated with anti-reflection coating.
When visible light falls on the solar cell the n-type wafer produces a large number of electrons. These electrons drift towards p-type wafer thereby generating an electric potential.
The potential difference produced by a single solar cell of 4 cm2 is between 0.4 to 0.5 volts and the magnitude of electric current is 60 milli-amperes.
However, when a large number of solar cells are connected in a mixed circuit, i.e., some cells in series and some in parallel, a large potential difference and large current can be obtained.
An assembly of solar cells can be used to generate electricity and such arrangement of cells is known as solar cell panel.
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