About a hundred years ago, it was discovered that electricity is produced when sunlight falls on a wafer (thin layer) of selenium. It was found that only 0.6% of the solar energy falling on selenium is converted into electricity. Thus, the selenium solar cells had very low efficiency and hence no serious efforts were made to produce electricity using them. The first practical solar cell was produced in 1954. It could convert 1% of solar energy into electrical energy. Modern solar cells, which are made of selenium, have efficiencies as high as 25% while those made of semiconductor materials have about 10-15% efficiency. These days’ solar cells are made from semi-conductor materials such as gallium and silicon.
Semiconductors are materials, which have conductivity in between those of conductors and insulators. The conductivity of semi-conductors increases considerably if certain impurities are added to them. The conductivity of semi-conductor also increases when light falls on them.
In solar cells, wafers of semi-conducting materials containing impurities, are so arranged that a potential difference develops between two regions of the semi-conductor when light falls on it. A single solar cell of area 4 cm square can develop a potential difference of about 0.4 to 0.5 volt and supply a current of about 60 mill ampere (mA).
Solar cells are usually arranged on large flat sheet to constitute solar panels. Solar panels can generate much higher power for various used such as operating TV and radio receivers, operating water pumps.