Band Gap Energy
The electricity generation of a solar cell is not only limited by the quantity, but the type of sunlight it receives. This is because photons emitted by the sun have varying levels of energy. Some photons are higher frequency particles, meaning that they have more energy, while other photons are lower frequency and have less energy.
In order for a photon to release a free carrier from its atom, it must have sufficient energy to do so. This specific energy requirement is called the band gap energy of a material, and varies depending on the type of material hit by a photon.
Since photons must have a specific level of energy for them to be utilized by PV technology, countless photons are unusable for solar cells. If a photon contains less energy then a solar cell's band gap energy, it has no effect on the silicon material and is wasted. This is because multiple photons cannot combine their energy in order to free an electron, making all photons with energies below a solar cell's band gap energy useless for electricity generation.
If a photon's energy is higher than the band gap energy, the excess energy also does not contribute to freeing an electron and is wasted. The exception to this is if a photon has an energy level equal to or higher than a multiple of the band gap energy. For example, if a photon has 2 times the required energy to free an electron, it can free 2 electrons from their atoms; if a photon has 3 times the band gap energy, it can free 3 electrons, etc.
Vocab
Photons
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Particles of light.
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A photon will have a certain level of energy that corresponds with its wavelength (how long the particle is).
Band Gap Energy
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The energy required for a free carrier electron to be freed from its atom in a certain material.
Electronvolts - eV
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A unit of measurement that quantifies the energy gained by an electron.
The electromagnetic spectrum. Photons with shorter wavelengths have more energy.
Image: Cyberphysics
A list of the band gap energies of semiconductor materials.
Chart: MTI Corporation
Now you may be thinking: Why not make solar cells out of a material with a low band gap energy, consequently enabling all photons to generate electricity? At first glance, it seems like a perfect solution. Unfortunately, things aren't so easy like that. The lower a material's band gap energy, the more low energy photons will free electrons, leading to an abundance of electrons with minimal energy.
For example, if we were to make a solar cell out of Indium arsenide (InAs), which has a low band gap energy of about 0.36 eV (electronvolts) at 300 K, we would free tons of electrons, but would barely produce any electricity from them. On the other hand, if we were to make a solar cell out of Gallium nitride (GaN), which has a relatively high band gap energy of about 3.36 eV at 300 K, we would free far fewer electrons, but each one would yield significantly more energy and therefore produce more electricity.
So now that we've identified the problem posed by band gap energy to solar cell efficiency, how can we overcome it? The trick is to look for some kind of middle ground. We need to use a material with a band gap energy low enough to release a large number of free carriers, but high enough to produce a healthy voltage. Keep in mind that the abundance and cost of an element, as well as its compatibility with other substances, affects its practicality. To find the ideal material, scientists will need to continue testing the properties of elements, both alone and as doped substances.
Image: Quora
A Silicon atom.
Silicon is the most widely used semiconductor because of its low cost and high effectiveness.
Image: Socratic Q&A
A Germanium atom.
Germanium is often used an alternative semiconductor.
References
“Constants, Energy Gaps, and Physical Properties of Semiconductor Related Crystals.” MTI Corp - Leading Provider of Lab Equipments and Advanced Crystal Substrates, www.mtixtl.com/bandgap-semiconductor.aspx.
Gregersen, Erik. “Electron Volt.” Encyclopedia Britannica, www.britannica.com/science/electron-volt.
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Stearns, Robert. "Photoelectric Effect." The Gale Encyclopedia of Science, edited by Katherine H. Nemeh and Jacqueline L. Longe, 6th ed., vol. 6, Gale, 2021, pp. 3408-3409. Gale In Context: Science, link.gale.com/apps/doc/CX8124401897/SCIC?u=nysl_me_scarshs&sid=bookmark-SCIC&xid=cd8d20c5. Accessed 25 June 2021.
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Toothman, Jessika, and Scott Aldous. “How Solar Cells Work.” HowStuffWorks Science, 1 Apr. 2000, science.howstuffworks.com/environmental/energy/solar-cell.htm#pt1.