Write properties of solar cells.

Hint: A solar cell is a device which converts solar energy directly into electricity. A solar cell converts sunlight directly to D.C. power without discharging waste products. Solar cells need little maintenance and they have a long life.

Complete answer:
A solar cell is a junction diode which converts light energy into electrical energy. A p-n junction solar cell consists of a large junction with no external biasing.

Working: When photons of energy greater than band gap energy () are made incident on the junction, electron-hole pairs are created which move in opposite directions due to the junction field. These are collected at two sides of the junction, thus producing photo-voltage; this gives rise to photocurrent.

The properties of solar cells are:
-The solar cells are P-N junction diodes connected in reverse bias.
-When light is focused on the junction of the diode there is a generation of current in it.
-This current is reverse current.
-The existence of this reverse current depends on the certain frequency of the light.
-The reverse current is proportional to the intensity of the light focused on it.
-The characteristics Volt-ampere curve of a solar cell or photo diode is given below.

The output voltage of the photodiode is always positive while the current is negative.

Solar cell materials:
-Solar cells made of single crystal Silicon (Si) are commonly used. Actual efficiency for commercially available cells is now 10% to 12% . Raw material for solar cells is high purity silicon. The solar cell is in the form of thin wafers/chips. Silicon cell life ranges from 5 to 8 years. Silicon has an advantage that it is quite cheap as it is available in abundance.
-Another solar cell material is Gallium Arsenide (GaAs). Cells of this material can achieve efficiency of 20% to 25%.
-Selenium (Se) is another material for Solar Cells having efficiency of about 25%.

Uses of solar cell:
-Solar cells are used in artificial satellites
-Solar cells are used as a source of electricity in light houses situated in the sea.
-Solar cells are used for operating electronic watches, calculators and toys.
-Solar cells are used as a source of electricity for radio or wireless transmissions, TV relay stations, traffic lights and research centres located in remote areas.


Note: The solar cell is a special type of p-n junction diode which generates electricity when light is illuminated on it. This special type of diode is called photodiode and is used in solar cells. It is common to confuse the term photodiode to light emitting diode. Light emitting diodes emit light but photo diodes generate electricity when exposed to light.