Why semiconductors are used in solar cells not conductors

“Why conductors are not used in solar cells?”

2 days back, one of my readers asked me this question on the facebook fan page of Speaking Technology.
why semiconductor only used in solar cell why not conductors?
Frankly speaking this question triggered my mind that though I have studied a lot about semiconductors and solar cells in our engineering course, but never thought of using conductors in the solar cells. Why is that so?

Then I thought there must be some reason behind this and I started hunting for few books on semiconductors in my library and did a few internet search and alas I found the answer!:)

 Why semiconductors used in solar cells instead of conductors

Here is the description of what I understood:

Solar cells or the so called photo-voltaic (PV) cells work on the principle of photo electric effect. In photo electric effect the photons strike the element surface and the energy of the photons is transferred to the electrons present in the valence band of the element. This energy is sufficient to excite the electrons from valence band to conduction band, thus giving rise to free electrons. These free electrons form the base for the electric current in the circuit. The element on which the photons strike can be a conductor, semiconductor or insulator.

Taking the case of insulator, the energy band gap between the valence band and conduction band is very large which means a large amount of energy is needed to eject out the electrons. Generally the photons from the sunlight do not possess this amount of energy. Photons of high frequency like X-rays may be able to perform this task.

Now coming to the case of semiconductors, the energy band gap is quite less. The photons can easily eject the electrons out of it provided the energy of the electrons is more than the threshold energy required.

In case of conductors, the band gap is very little or negligible. So the electrons can be ejected very easily.

Now the question is when photo electric effect can be applied on the conductor, then why it can’t be used in solar cells?

The question to this lies in the ELECTRIC FIELD present within the semiconductors.

The free electrons alone cannot give rise to current, they will just move randomly. To make the electrons move in the circuit, there must be some driving force. This driving force is “Electric Field”.

Due to the presence of PN junction, there is a small electric field present within the semiconductors without which the solar cells won’t work. In the PN junctions, the electrons and holes try to mix with each other but they are not able to completely mix, Otherwise the junction will become neutral and will be of no use. When few electrons and holes combines with each other and rush towards the opposite charge, repulsive force comes into play due to the heavy rush of similar charges.

A potential barrier is formed and when equilibrium is attained, we find the presence of small amount of electric field within it. This acts as a driving force for the free electrons.

But in the case of conductors, since there is no equivalent PN junction, so the free electrons move randomly.

So the conductors cannot be used in the solar cells or the photovoltaic cells.

Currently research is going on to use “conducting polymers” and “transparent conducting films” in solar cells.

If you find any more info related to this interesting topic, then do share with us by commenting below. Do share this post with your friends if you found it interesting.

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