Question

What are solar cells? Explain the structure of a solar panel. List two principal advantages associated with solar cells.

Answer 1

Hint Solar cells are also known as photovoltaic cells. The word photovoltaic means voltage from light. This is enough to understand that when sunlight is incident on these cells, it leads to the production of electricity.

Complete step by step answer
A solar cell is a p-n junction diode that converts solar energy directly into electrical energy and is based on photovoltaic effect.
It consists of a shallow p-n junction formed at a small distance below the surface of a silicon crystal. There is an ohmic contact at the top in the form of a comb-like structure which covers about $7\% $of Si area, and an ohmic contact at the bottom which covers the entire bottom area. The top carries an anti-reflection coating.
When sunlight falls on the top surface, it is absorbed mostly within a small distance below the surface where the p-n junction is located. Each absorbed light photon creates an electron and a hole. An electron in the valence band moves up to the conduction band, leaving a hole behind in the valence band. The built-in electric field forces the electron towards the n-region and the hole towards the p-region. The two sides of the junction thus become oppositely charged, and so a voltage is generated across the junction. This voltage drives a current in the external circuit.
Two principal advantages associated with solar cells are:
It gives rise to a renewable form of energy. Which means that it doesn’t cause any depletion in natural resources.
It produces a clean form of energy. It does not emit harmful gases or any pollutants into the atmosphere.

Note The energy gap between the valence band and the conduction band is the minimum amount of energy which is required for the electrons to be able to jump and become available for conduction. However some of the energy coming from the sun is not enough to overcome this band gap. So that lowers the number of electrons available for conduction by a considerable amount. So, the theoretical efficiency of a silicon photovoltaic cell is about $33\% $.