Solar Cells

One of the courses I took in my senior year was a semiconductors course. There was one topic I was really interested in and that was solar cells. I will just go over a little bit of what I learned/remember from class.

A solar cell is a semiconductor device that uses the suns radiation to produce electrical energy. Typically a solar cell is a p-n junction diode. This means that the solar cell is a combination of an n-type semiconductor and p-type semiconductor material. The reason why is because the p-n junctions potential energy barriers are formed and this will facilitate the separation of electron-hole pairs(carriers) generated from photons.

One of the important parameters of a solar cell is the band gap energy. The band gap energy is the minimum energy required to excite an electron from the valence to the conduction band. When the electron is excited into the conduction band it is free to participate in conduction. At the same time though, a 'empty' space is left in the valance band (hole) and as electrons move around in the valence band, it looks like the hole is moving. This means the electron and hole participate in conduction.

Now we can learn about carrier concentration. We learned about free carriers above (electrons and holes participating in conduction). We can call the concentration of these carriers the 'intrinisic carrier concentration'. This tells us the number of electrons in the conduction band and the number of holes in the valence band. The number of carriers will depend on the band gap energy and temperature of the material (High band gap energy leads to less carriers). The band gap energy and carrier concentration will be important in determining the solar cells efficiencies.

When we illuminate light onto a solar cell three things occur. It is either absorbed, reflected or nothing happens. Sunlight contains photons and these photons have different energy levels because of their different wavelengths. E=hc/lambda where h represents Planck constant, c represents the speed of light, and lambda represents the wavelength.

Here is a simple way of looking at it.

If the energy of the photon is greater than the band gap energy of the solar cell that means the electron will be excited out of the valence band and into the conduction band. However, the excess energy of the photon is wasted into heat.

If the energy of the photon is less than the band gap energy of the solar cell nothing will happen to the electron.

If the energy of the photon is equal to the band gap energy the electron will be excited out of the valence band and into the conduction band. This is the most ideal case as no energy is wasted into heat.

I hope this info is useful for anyone who wants to start learning solar cells. Note this is just an intro to solar cells and not in depth information. Solar cells can become much more complex when dealing with other factors like recombination and quantum effieciency.

What do you think of solar cells? Do you believe that they will be important in the future as a clean and renewable energy source? Problems that arise now are the cost of production and the amount of land required to use. Furthermore, the effieciency's of solar cells are still low.

Want to learn more? You can learn more about solar cells by going here

Solar Cells and Photovoltaic Education

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