Solar cell : Structure and working of a Solar Cell .

 Structure and working of a Solar Cell 

Once the layers have been joined, there is a negative charge in the p-layer and a positive charge in the n-layer section of the junction. This imbalance in the charge of the two layers at the p-n junction produces an electric field between the p-layer and the n-layer

If the PV cell is placed in the sun, radiant energy strikes the electrons in the p-n junction and energizes them, knocking them free of their atoms. These electrons are attracted to the positive charge in the n-layer and are repelled by the negative charge in the p-layer

A wire can be attached from the p-layer to the n-layer to form a circuit. As the free electrons are pushed into the n-layer by the radiant energy, they repel each other

The wire provides a path for the electrons to flow away from each other. This flow of electrons is an electric current that we can observe

The electron flow provides the current, and the cell’s electric field causes a voltage

With both current and voltage, we have power, which is the product of the two

 : How a Solar Cell Works

Now it can be seen what happens inside a solar cell. Most solar cells are essentially large area p-n junctions. When light shines on them they can generate current and voltage

The reason this can happen is because of the "built-in" electric field at the junction of the p-type and n-type material

First consider what happens if a silicon solar cell (which is a p-n junction) has a low resistance wire connected externally between the p and n contact. In the dark a solar cell will produce no current. If, however, light shines on the solar cell then current will flow through the wire, from the p-type side to the n-type side (conventional current)

The light has enough energy to break some of the chemical bonds of the silicon crystal

What that means is that electrons, which are normally involved in a silicon bond, are excited by the light into a higher energy state and the bond is broken. Not all of the bonds are broken otherwise the silicon would melt! The sun's light intensity on earth is strong enough to break about 1 bond for every 100 million silicon atoms in the solar cell. So the solar cell doesn't melt under normal conditions

The excited electrons are now like the electrons from phosphorus dopant atoms – they are free to move through the material. Similarly, the broken bonds created by the light act as holes - just like the missing electrons in bonds between silicon and boron atoms -and these holes are also free to move throughout the material. What goes up, however, must eventually come down! Electrons and holes created in this way are physically near each other: for every electron excited by the light there is a corresponding hole generated. These electrons and holes can remain excited only for a short period of time

In a process called recombination, excited electrons stray too close to holes and the two falls back into bonded positions. When this happens the pair's electrical energy is lost as heat. If there is too much recombination, the solar cell won't work very well