Describe construction, working and use of the solar cell.

Explanation

A solar cell is basically a p-n junction which generates emf when solar radiation falls on the p-n junction. Sunlight is not always required for solar cells but the energy of the photon of the light which is greater than the band gap energy of semiconductor can also produce the photo voltage.
The principle of the working of photodiode and solar cell are same and it works on the principle of photovoltaic effect but doesn.t require external battery in solar cell.
If you need more power than Sunlight, the area of the junction has to be very large.
The The diagram depicts the appearance of the solar cell and its cross section.

A p - Si wafer of about 300 `mu`m is taken over which a thin layer `(~0.3mum)` of n - Si is grown on one side by diffusion process.
The other side of `p-Si` is coated with a metal (back contact). On the top of `n-Si` layer, metal finger electrode, (or metallic grid) is deposited. This acts as a front contact.
The metallic grid occupies only a very small fraction of the cell area `(lt 15%)` so that light can be incident on the cell from the top.
The generation of emf by a solar cell, when light falls on it is due to the following three basic processes. (In order to produce an emf, a pair of electron hole must be generated and must gather).
(1) generation of e-h pairs due to light (with `hv gt E_(g)`) close to the junction.
(2) Separation of electrons and holes due to electric field of the depletion region. Electrons are swept to n-side and holes to -sides.
(3) Electrons reaching the n-side are collected by the front contact and holes reaching p-side are collected by the back contact. Thus, p-side becomes positive and n-side becomes negative giving rise to photovoltage.
When an external load is connected as shown in the figure (a) a photocurrent `I_(L)` flows through the load due to incident light on solar cell.

`I to V` characteristics of solar cell is drawn in the fourth quadrant of the coordinate axes. This is because a solar cell does not draw current but supplies the same to the load.

Uses of solar cells:
(1) The power electronic devices in satellites and space vehicles.
(2) As a power suupply in calculator, electronic watch and camera and
(3) Many solar cells have a series or parallel connections that can provide the required voltage and current such a solar cell connection is called a solar panel. Solar panels are used to obtain electrical energy in a satellite with the use of such panels, the secondary electric cell can be obtained during the night.