Solar 101: How PV Cells Work
Energy generation is a topic that can be very confusing, and is indeed complicated due to the fact that there are so many possible ways to generate it. With renewable energy being a big topic in today’s world, this idea has only grown in relevance and complexity, with solar energy being a very popular and promising subject in this industry. However, despite its popularity, many people never consider the process that occurs to generate electricity from the sun’s seemingly endless source of power. My goal is to give a basic synopsis on how solar PV cells create electricity from the sun’s powerful rays of light.
1. The Outer Layers of a PV Cell
In order for a solar panel to work, it requires a few important subsections that work in conjunction with one another to allow electricity to be generated. The top layer is a protective sheet of glass that is simply meant to allow solar rays in, and keep everything else out (i.e. dirt). Beneath the glass sheet is an antireflection coating whose dark (typically blue), yet transparent design allows light to pass through without being reflected away from the PV cells. Part of this design also involves the precise thickness of the coating that causes any reflected light passing through the coating to be cancelled out by manipulating the phase angles of the reflected light and its incoming counterpart to a point where the waves are perfectly misaligned, with the peak point of one wave corresponding to the trough point of the other. The resulting reflected waves destructively interfere with one another, resulting in zero net reflected energy. This does not cancel out all light reflected however, with the phase distortion effects being dependent on wavelength.
2 . The Inner Layers of a PV Cell
As we continue our way into the PV cell, we reach the most important, and perhaps most complex part of the cell, comprised of semiconductors that are manipulated to allow electrons to flow when light waves reach this point. The most common and most effective semiconductor used is silicon, which is also conveniently the second most abundant element in the Earth’s crust, accounting for roughly 28% of its mass. There are three different layers of silicon that allow for the electricity generation process that occurs within a PV cell. The thin top layer contains silicon that has been chemically doped with a very small amount of an element (usually phosphorus) that contains more electrons than silicon. This layer is also referred to as “Negative Type,” or “N-Type,” as it favors the collection and transportation of electrons. The bottom layer is a bit thicker than the top layer, and instead of being chemically doped with an element that contains more electrons than silicon, it is doped with an element (typically boron) that contains less electrons than silicon. With fewer electrons present in this sector, it is given an overall effective positive charge, and as a result, it is referred to as “Positive Type,” or “P-Type,” where in this case it prefers the collection and transportation of positive charges. Finally, the middle layer contains silicon that has been modified so it has slightly fewer electrons, making it marginally P-Type in nature. Across the N-Type layer, a metallic grid (often made of silver) is laid out to allow light to pass through its openings, as well as electrons to flow through it as a medium for generated electricity. Situated below the P-Type region, an aluminum sheet is connected to the P-Type region to act as the receiving end for electrons flowing from the opposing N-Type region (via a wire connected to the two ends). The properties of these subsections are essential to the process of generating electricity.
3. The Sun and its Different Types of Light Waves
It is common knowledge that the sun produces light that can be seen from all around the world, but in fact this “visible light” is not the only light waveform released through the sun’s rays. The sun releases waves of various different wavelengths with regards to the electromagnetic spectrum, with the ideal wavelength taken in by solar PV cells being between 350 and 1140 nanometers. The reason for this has to do with the properties of the middle silicon layer in between the N-Type and P-Type sections, as this is where we want the light to travel to. If the wavelength of the travelling light is too small, such as with ultraviolet waves, it is unable to reach the middle layer. On the other hand, if the wavelength is too long, as with infrared waves, the wave is too large to be absorbed by the middle layer.
4. Electricity Generation
Now that the inner workings of a solar PV cell are known, we can jump right into the concept of electricity generation. The process is quite simple with everything already set up throughout the different layers of the PV cell. Essentially, when a light wave is absorbed by the middle layer of silicon (between the N-Type and P-Type layers), it breaks an electron off of a silicon atom and causes it to roam freely. It leaves an area of positive charge (also referred to as a “hole” in this case) where it was removed from. As a result, the free electron travels to the N-Type layer, where it is readily accepted, and contrarily, the positively charged hole travels to the P-Type layer. This process continues as long as sunlight is taken in by the PV cell. As a result of this process, when a wire (or conductive medium) is connected to each end, the free electrons will flow from the negatively charged N-Type region to the positively charged P-Type region as a direct electric current. In terms of its overall voltage, one solar PV cell only offers about half a Volt of electric potential; however, when a collection of cells are connected in both series and parallel (via copper wires) to make a solar panel, the system’s overall electric potential is greatly amplified. Lastly, due to the fact that most electronics require an alternating current source, the direct current produced by solar PV cells must be converted to an alternating current using a solar inverter.
Now you know how PV cells work, and hopefully with this knowledge it shows how ideal solar energy can be, especially because it is still at the very beginning of its journey to maximum efficiency.
References:
[1] https://www.pveducation.org/pvcdrom/design-of-silicon-cells/anti-reflection-coatings
[2] https://sciencing.com/eight-abundant-elements-earths-crust-8120554.html
[3] http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/dope.html
[4] https://youtu.be/8RjGHmlOu58
[5] https://news.energysage.com/solar-inverters-comparing-inverter-technologies/
[6] https://youtu.be/UJ8XW9AgUrw
[7] https://sites.energycenter.org/solar/homeowners/frequently-asked-questions
