The Anatomy of a Silicon Solar Cell
An important thing to know about PV technology is that solar panels, the world's primary solar energy generators, are not the smallest form of PV technology. A solar panel is actually an array of a smaller type of technology called a solar cell. An average panel contains anywhere between 32 to 96 solar cells; the more cells that make up a panel, the more electricity it can generate.
A commercial solar cell.
Image: Indiamart
Vocab
Valence Electrons
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The outermost electrons of an atom.
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Because of their location, they are the most likely to interact with another atom.
Valence Shell
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The outermost electron shell of an atom.
Photon
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A particle of light.
Semiconductors
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Materials that carry an electrical charge moderately well.
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They are more conductive than insulators, but less so than true conductors like copper.
A traditional solar cell's physical composition is relatively straightforward. Two plates of doped crystalline silicon (Si) stacked on top of each other make up the bulk of the cell. When I say the Si is doped, however, I don't mean drugged. Doping is a process in which "impurities", or trace amounts of atoms of an element, are added to a metal in order to increase its conductivity. In this case, the two plates of Si are doped differently.
The Top Plate
The top plate is doped with Phosphorous (P), an element containing 5 valence electrons. When added to Si, which has 4 valence electrons, the result is a semiconducting material whose atoms have 9 valence electrons.
Here comes some light chemistry. In general, the maximum number of electrons an atom can have in its valence shell is 8. As a result, atoms in the doped material end up having an extra electron. This electron is called a free carrier, and easily separates from its atom when hit by a photon.
Image: Physics Forums
When free carriers are released from their atoms, they roam around the plate, giving it a net negative charge. For this reason, the top plate is known as the n-type side of a solar cell.
The Bottom Plate
The bottom plate is doped with Boron (B), an element containing 3 valence electrons. When added to Si, the result is a semiconducting material whose atoms have 7 valence electrons.
Atoms in the doped material have one less electron than they need to complete their valence shell, creating what's called a free opening. The presence of free openings in the doped material give the plate a net positive charge; consequently, the bottom plate is known as the p-type side of a solar cell.
A simplified cross section of a solar cell.
Image: Pinterest
The Rest of a Solar Cell
Between the two plates of doped Si is an area called the depletion zone, which plays a critical role in the electrochemical processes of a solar cell. Next to the plates, a strong conductor (usually copper, aluminum, or silver) forms an external path for electrons to travel out of the system. Using the flow of electrons on this conductor is how we harness electricity from a solar cell.
Finally, a glass plate is often placed over a cell to keep photons from bouncing off of the silicon, which is decently reflective. The more photons that can be concentrated onto a solar cell's surface, the more electricity it can generate.
The depletion zone of a solar cell.
Image: Images Scientific Instruments
References
Imagesco.com. The PN Junction, www.imagesco.com/articles/photovoltaic/photovoltaic-pg3.html.
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Kerrigoth. “Are the Positive Charges from a Hole and a Proton ?” Physics Forums | Science Articles, Homework Help, Discussion, Physics Forums, 8 Nov. 2016, www.physicsforums.com/threads/are-the-positive-charges-from-a-hole-and-a-proton.851957/.
Toothman, Jessika, and Scott Aldous. “How Solar Cells Work.” HowStuffWorks Science, 1 Apr. 2000, science.howstuffworks.com/environmental/energy/solar-cell.htm#pt3.