Question

Explain the construction and working of the solar cells.

Answer 1

Construction: 

1. It consists of a p-n junction. The n-side of the junction faces the solar radiation. The p-side is relatively thick and is at the back of the solar cell.
2. Both the p-side and the n-side are coated with a conducting material. The n-side is coated with an anti-reflection coating which allows visible light to pass through it. The main function of this coating is to reflect the IR (heat) radiations and protect the solar cell from heat.
3. This coating works as the electrical contact of the solar cell. The contact on the n-side is called the front contact and that at the p-side is called the back contact or the rear contact.
4. The n-side of a solar cell is thin so that the light incident on it reaches the depletion region where the electron-hole pairs are generated. 
   
   Figure (a): Schematic structure of a solar cell

Working:

1. When light with photon energy greater than the bandgap energy is incident on a solar cell, electron-hole pairs are formed in the depletion region of the diode.
2. The electrons and holes thus formed get recombined and are not available for conduction.
3. However, the photo-generated electrons in the p-type material and the photo-generated holes in the n-type material are spatially separated and prevented from recombination in a solar cell.
4. This separation of carriers is possible due to the intrinsic electric field of the depletion region. Figure (b) shows this schematically.
   
   Figure (b)
5. When the light-generated electron in the p-type region reaches the junction, it crosses the junction due to the electric field at the junction. It reaches the n-type region where it is now a majority carrier.
6. Similarly, the light generated hole reaches the p-type region and becomes a majority carrier in it.
7. The positive and negative charges are thus accumulated on the p-region and the n-region of the solar cell which can be used as a voltage source.
8. When the solar cell is connected to an external circuit, the light-generated carriers flow through the external circuit.