How Silicon Solar Cells Function
Vocab
You'll need to know the different parts of an Si solar cell in order to understand its eletrochemical processes, so I encourage you to check out the section on The Anatomy of a Silicon Solar Cell before reading any further.
Done? Awesome, let's get into it.
When the N-type and P-type silicon plates of a solar cell are joined, an electric field instantly develops in the depletion zone between them. This happens even without the introduction of photons to the material.
Photons
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Particles of light.
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Named by Albert Einstein in his explanation of the photoelectric effect.
Diode
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A device that permits electrons to flow in only one direction.
Free Carrier/Photoelectron
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An "extra" electron in the N-type plate of a solar cell that is freed from its atom by a photon.
Free Opening
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The absence of a single electron in an atom's valence, or outermost, electron shell.
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Free carrier electrons tend to fill free openings in order to complete a shell.
The electric field functions as a diode by allowing electrons to flow towards the N-type side but preventing them from reaching the P-type side. To put it simply, the electric field forms an invisible wall, and repels electrons from the negatively charged plate that try to move to the positively charged one. This electric field is important, so keep it in the back of your mind.
The cross section of a traditional solar cell.
Image: Electrical A2Z
When a solar cell is exposed to sunlight, photons hit the N-type plate. If a photon with sufficient energy collides with an atom in the plate, it allows a free carrier to break free from its atom's shell and roam around freely, becoming a photoelectron. This phenomenon, known as the photoelectric effect, happens because photons transfer energy to free carrier electrons, turbocharging them so that they can escape the inward pull of their atom.
Let's use an analogy to simplify this phenomenon. Imagine a parent leaves a shot of espresso on the kitchen countertop, and their curious, mischievous toddler drinks it when no one is looking. A few minutes later, the child is jumping on furniture and bumping into walls. In a similar fashion, when a free carrier electron is energized by a photon, it starts to bounce around rapidly. If it has enough energy, it moves fast enough to break free of its atom, and retains any extra energy it gained from the photon. The more energy a photoelectron retains, the higher voltage of electricity it will produce.
Photon
A free carrier breaking free from its atom.
Image: Physics and Radio-Electronics
Once separated from their atoms, free carriers attempt to move to the P-type plate, as there are free openings there for them to inhabit. Every time a free carrier tries to join the P-type side and rule the galaxy as father and son, however, it can't. Why? Well, its not because of the Sith rule of two, that's for sure. It's because of the electric field that functions as a diode.
Every time an electron from the N-type plate nears the P-type plate, it is instantly repulsed by the electric field. The constant movement of free carriers as they approach the P-type plate and are repelled forms a flow of electrons, which we can harness by putting a strong conductor such as copper, aluminum, or silver in the cell to form an external current path. The path leads electrons out of the solar cell to wherever we want them to go. With control over a flow of electrons, we now have electricity at our disposal.
Image: Basic Electronics
References
“Bypass Diodes in Solar Panels and Arrays.” Electronics Tutorials, 1 Feb. 2021, www.electronics-tutorials.ws/diode/bypass-diodes.html.
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Carts-powell, Yvonne. "Photovoltaic Cell." The Gale Encyclopedia of Science, edited by Katherine H. Nemeh and Jacqueline L. Longe, 6th ed., vol. 6, Gale, 2021, pp. 3423-3425. Gale In Context: Science, link.gale.com/apps/doc/CX8124401905/SCIC?u=nysl_me_scarshs&sid=bookmark-SCIC&xid=c6bcfb07. Accessed 25 June 2021.
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“Photovoltaic (PV) Cell: Structure & Working Principle.” Electrical A2Z, 24 July 2018, electricala2z.com/renewable-energy/photovoltaic-pv-cell-structure-working-principle/.
“Photovoltaics.” SEIA, www.seia.org/initiatives/photovoltaics.
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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.
Toothman, Jessika, and Scott Aldous. “How Solar Cells Work.” HowStuffWorks Science, 1 Apr. 2000, science.howstuffworks.com/environmental/energy/solar-cell.htm#pt1.