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[[File:pvcell.jpg|300px|thumb|Figure 1. A solar panel, consisting of many photovoltaic cells.<ref>"20110504-RD-LSC-0621 - Flickr - USDAgov" by U.S. Department of Agriculture. Licensed under CC BY 2.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:20110504-RD-LSC-0621_-_Flickr_-_USDAgov.jpg#/media/File:20110504-RD-LSC-0621_-_Flickr_-_USDAgov.jpg</ref>]]
[[File:pvcell.jpg|300px|thumb|Figure 1. A solar panel, consisting of many photovoltaic cells.<ref>"20110504-RD-LSC-0621 - Flickr - USDAgov" by U.S. Department of Agriculture. Licensed under CC BY 2.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:20110504-RD-LSC-0621_-_Flickr_-_USDAgov.jpg#/media/File:20110504-RD-LSC-0621_-_Flickr_-_USDAgov.jpg</ref>]]


<onlyinclude>A '''photovoltaic cell''' ('''PV cell''') is an [[energy]] harvesting [[technology]], that converts [[solar energy]] into useful [[electricity]]. The vast majority are silicon [[semiconductor]]s, and interact with incoming [[photon]]s in order to generate an electric [[current]].</onlyinclude>
<onlyinclude>A '''photovoltaic''' ('''PV''') '''cell''' is an [[energy]] harvesting [[technology]], that converts [[solar energy]] into useful [[electricity]] through a process called the [[photovoltaic effect]]. There are several different [[types of PV cells]] which all use [[semiconductor]]s to interact with incoming [[photon]]s from the Sun in order to generate an electric [[current]].</onlyinclude>


==Operation==
==Layers of a PV Cell==
The physics of semiconductors requires a minimum [[photon]] energy to remove an [[electron]] from a crystal structure, known as the '''band-gap energy'''. If photon has less energy than the band-gap, the photon gets absorbed as [[thermal energy]]. For [[silicon]], the band-gap energy is 1.12 [[electron volt]]s.<ref name=Wolfson>R. Wolfson, "Photovoltaic Solar Energy" in ''Energy, Environment, and Climate'', 2nd ed., New York, NY: W.W. Norton & Company, 2012, ch. 9, sec. 5, pp. 244-252</ref> Since the energy in the photons from the [[sun]] cover a wide range of energies, some of the incoming energy from the Sun does not have enough energy to knock off an electron in a silicon PV cell. The longest [[wavelength]] (which corresponds to the lowest energy) that is capable removing the is 1.1 [[prefixes|µ]][[meter|m]],<ref><m>\lambda=\frac{hc}{E}=\frac{(6.626kg*m^2/s) \times (3\times10^8 m/s)}{1.12eV}\times \frac{1eV}{1.6\times10^{-19}J}=1.1\mu m</m></ref> this means that ~1/4 of the light from the Sun does not sufficient enough to create electricity.<ref name=Wolfson/>
A photovoltaic cell is comprised of many layers of materials, each with a specific purpose.  The most important layer of a photovoltaic cell is the specially treated [[semiconductor]] layer. It is comprised of two distinct layers ([[p-n junction|p-type]] and [[p-n junction|n-type]]—see Figure 3), and is what actually converts the [[Solar energy to the Earth|Sun's energy]] into useful [[electricity]] through a process called the [[photovoltaic effect]] (see below). On either side of the semiconductor is a layer of [[Conductor|conducting material]] which "collects" the electricity produced.  Note that the backside or shaded side of the cell can afford to be completely covered in the conductor, whereas the front or illuminated side must use the conductors sparingly to avoid blocking too much of the [[Solar spectrum|Sun's radiation]] from reaching the semiconductor. The final layer which is applied only to the illuminated side of the cell is the anti-reflection coating. Since all semiconductors are naturally reflective, reflection loss can be significant. The solution is to use one or several layers of an anti-reflection coating (similar to those used for eyeglasses and cameras) to reduce the amount of solar radiation that is reflected off the surface of the cell.<ref>C. Julian Chen. ''Physics of Solar Energy'', 1st ed. Hoboken, NJ, USA: John Wiley & Sons Inc., 2011.</ref>
[[File:pvcellop.gif|thumbnail|center|800px|Figure 2. The basic operation of a PV cell.<ref>Wikimedia Commons [Online], Available: http://upload.wikimedia.org/wikipedia/commons/7/7d/Operation_of_a_basic_photovoltaic_cell.gif</ref>]]


[[File:pvcellop.gif|500px|center|Figure 2. The basic operation of a PV cell.<ref>Wikimedia Commons [Online], Available: http://upload.wikimedia.org/wikipedia/commons/7/7d/Operation_of_a_basic_photovoltaic_cell.gif</ref>]]
==Photovoltaic Effect==
:: [[photovoltaic effect|''main article'']]


The specific semiconductors used are called [[p-n junction]]s, which essentially work by creating an internal [[electric field]] due to an uneven distribution of [[charge]]s. The [[voltage]] each cell creates is about 0.6 V, which is relatively small. When photons knock an electron free from its atomic shell, it ''can'' flow through this voltage (but won't always, see '''Efficiency''' below) and flow to a [[metal]] contact which can be connected to an external [[electric circuit]]. The PV cell is essentially a solar-powered [[battery]], causing a current to flow just like a chemical battery would.<ref name=Wolfson/>
[[File:photovoltaiceffect.png|400px|thumb|Figure 3. A diagram showing the photovoltaic effect.<ref>''Created internally by a member of the Energy Education team. Adapted from: Ecogreen Electrical. (August 14, 2015). ''Solar PV Systems'' [Online]. Available: http://www.ecogreenelectrical.com/solar.htm''</ref>]]The '''[[photovoltaic effect]]''' is a process that generates [[voltage]] or electric [[current]] in a [[photovoltaic cell]] when it is exposed to [[sunlight]]. These solar cells are composed of two different types of [[semiconductor]]s—a p-type and an n-type—that are joined together to create a '''p-n junction'''. By joining these two types of semiconductors, an [[electric field]] is formed in the region of the junction as [[electron]]s move to the positive p-side and [[electron hole|hole]]s move to the negative n-side. This field causes negatively charged particles to move in one direction and positively charged particles in the other direction.<ref name=boyle>G. Boyle. ''Renewable Energy: Power for a Sustainable Future'', 2nd ed. Oxford, UK: Oxford University Press, 2004.</ref> [[Light]] is composed of [[photon]]s, which are simply small bundles of [[electromagnetic radiation]] or [[energy]]. When light of a suitable [[wavelength]] is incident on these cells, energy from the photon is transferred to an electron of the semiconducting material, causing it to jump to a higher energy state known as the [[conduction band]]. In their excited state in the conduction band, these electrons are free to move through the material, and it is this motion of the electron that creates an electric current in the cell.


==Efficiency==
==Solar Cell Efficiency==
[[Efficiency]] is a design concern for photovoltaic cells, as there are many factors that limit their efficiency. The main factor, as mentioned briefly above, is that 1/4 of the [[solar energy to the Earth]] cannot be converted into electricity by a silicon semiconductor. From Figure 3 it can be seen that the visible[[ light]] and [[ultraviolet]] light have sufficient energy, but a good portion of the most dominant incoming light cannot be used.  
:: [[solar cell efficiency|''main article'']]
[[Efficiency]] is a design concern for photovoltaic cells, as there are many factors that limit their efficiency. The main factor is that 1/4 of the [[solar energy to the Earth]] cannot be converted into electricity by a silicon semiconductor. The physics of semiconductors requires a minimum [[photon]] energy to remove an [[electron]] from a crystal structure, known as the '''band-gap energy'''. If a photon has less energy than the band-gap, the photon gets absorbed as [[thermal energy]]. For [[silicon]], the band-gap energy is 1.12 [[electron volt]]s.<ref name=Wolfson>R. Wolfson, "Photovoltaic Solar Energy" in ''Energy, Environment, and Climate'', 2nd ed., New York, NY: W.W. Norton & Company, 2012, ch. 9, sec. 5, pp. 244-252</ref> Since the energy in the photons from the [[sun]] cover a wide range of energies, some of the incoming energy from the Sun does not have enough energy to knock off an electron in a silicon PV cell. Even from the light that ''can'' be absorbed, there is still a problem. Any energy ''above'' the band-gap energy will be transformed into [[heat]]. This also cuts the efficiency because that heat energy is not being used for any useful task.<ref name=Wolfson/> Of the electrons that are made available, not all of them will actually make it to the metal contact and generate electricity. This is because some of them will not be accelerated sufficiently by the voltage inside the semiconductor. Because of the reasons listed, the theoretical efficiency of silicon PV cells is about '''33%'''.<ref name=Wolfson/>


[[File:PVCELL.png|thumbnail|center|816px|Figure 3. The solar energy received by the Earth, and the corresponding energies of its photons. Efficiency of a PV cell is largely effected by the amount of incoming light that can cause current to flow.<ref>Made internally by member of the Energy Education team, adapted from ''Energy, Environment and Climate'' by R. Wolfson.</ref>]]
There are ways to improve the efficiency of PV cells, all of which come with an increased cost. Some of these methods include increasing the purity of the semiconductor, using a more efficient semiconducting material such as Gallium Arsenide, by adding additional layers or p-n junctions to the cell, or by concentrating the Sun's energy using [[concentrated photovoltaics]]. On the other hand, PV cells will also degrade, outputting less energy over time, due to a variety of factors including UV exposure and weather cycles. A comprehensive report from the National Renewable Energy Laboratory (NREL) states that the median degradation rate is 0.5% per year.<ref>Dirk C. Jordan and Sarah R. Kurtz. ''Photovoltaic Degradation Rates — An Analytical Review'', National Renewable Energy Laboratory, USA, 2012. Accessed April 24, 2018. [Online] Available at https://www.nrel.gov/docs/fy12osti/51664.pdf</ref>


Even from the light that ''can'' be absorbed, there is still a problem. Any energy ''above'' the band-gap energy will be transformed into [[heat]]. This also cuts the efficiency because that heat energy is not being used for any useful task.<ref name=Wolfson/>
==Types of PV Cells==
:: [[types of PV cells|''main article'']]
[[File:Comparison_solar_cell_poly-Si_vs_mono-Si.png|400px|thumb|right|Figure 4. An image comparing a polycrystalline silicon cell (left) and a monocrystalline silicon cell (right).<ref>Wikimedia Commons. (August 18, 2015). ''Comparison of Solar Cells'' [Online]. Available: https://upload.wikimedia.org/wikipedia/commons/7/71/Comparison_solar_cell_poly-Si_vs_mono-Si.png</ref>]]
Photovoltaic cell can be manufactured in a variety of ways and from many different materials. The most common material for commercial solar cell construction is [[Silicon]] (Si), but others include Gallium Arsenide (GaAs), Cadmium Telluride (CdTe) and Copper Indium Gallium Selenide (CIGS).  Solar cells can be constructed from brittle crystalline structures (Si, GaAs) or as flexible thin-film cells (Si, CdTe, CIGS). Crystalline solar cells can be further classified into two categories—''monocrystalline'' and ''polycrystalline'', as shown in figure 4. As the names suggest, monocrystalline PV cells are comprised of a uniform or single crystal lattice, whereas polycrystalline cells contain different or varied crystal structures. Solar cells can also be classified by their number of layers or "p-n junctions". Most commercial PV cells are only single-junction, but multi-junction PV cells have also been developed which provide higher efficiencies at a greater cost.


Of the electrons that are made available, not all of them will actually make it to the metal contact and generate electricity. This is because some of them will not be accelerated sufficiently by the voltage inside the semiconductor.
==For Further Reading==
 
*[[Photovoltaic effect]]
Because of the reasons listed, the theoretical efficiency of these PV cells is about '''33%'''.<ref name=Wolfson/> Realistically this limit cannot be achieved because lots of the radiant energy will reflect off of the surface of the cell. This can be somewhat solved with the use of anti-reflection coatings, but even so the maximum efficiency achieved is about '''half''' of the theoretical limit, about 15%.<ref>UCSD Physics, ''Dont Be a PV Efficiency Snob'' [Online], Available: http://physics.ucsd.edu/do-the-math/2011/09/dont-be-a-pv-efficiency-snob/</ref>
*[[Semiconductor]]
 
*[[P-n junction]]
====Increasing efficiency====
*[[Photon]]
There are ways to increase this efficiency. First of all, using pure silicon can increase the efficiency from 15% to 18%, but this comes at an increased cost (read about the benefits and drawbacks of using pure silicon [http://www.solar-facts-and-advice.com/monocrystalline.html here]). Another way to increase efficiency is by changing the construction of the semiconductor. A standard PV cell has one junction, called a "pn junction". By using different materials and multiple junctions, light that would otherwise generate excess heat as mentioned above can be made more useful. Some cells using this technology can achieve efficiencies up to 44%!<ref>GreenTech, ''Sharp Hits Record 44.4% Efficiency for Triple-Junction Solar Cell'' [Online], Available: http://www.greentechmedia.com/articles/read/Sharp-Hits-Record-44.4-Efficiency-For-Triple-Junction-Solar-Cell</ref> This technology is basically using a stack of pn junctions each one attempting to grab the energy from a passing photon, which is how the limits are increased. This technology is very expensive however, and may have to be cheapened to be of practical use.
*[[Types of PV cells]]
*Or explore a [[Special:Random|random page]]


==References==
==References==
{{reflist}}
{{reflist}}
[[Category:Uploaded]]
[[Category:Uploaded]]

Revision as of 00:28, 9 June 2018

Figure 1. A solar panel, consisting of many photovoltaic cells.[1]

A photovoltaic (PV) cell is an energy harvesting technology, that converts solar energy into useful electricity through a process called the photovoltaic effect. There are several different types of PV cells which all use semiconductors to interact with incoming photons from the Sun in order to generate an electric current.

Layers of a PV Cell

A photovoltaic cell is comprised of many layers of materials, each with a specific purpose. The most important layer of a photovoltaic cell is the specially treated semiconductor layer. It is comprised of two distinct layers (p-type and n-type—see Figure 3), and is what actually converts the Sun's energy into useful electricity through a process called the photovoltaic effect (see below). On either side of the semiconductor is a layer of conducting material which "collects" the electricity produced. Note that the backside or shaded side of the cell can afford to be completely covered in the conductor, whereas the front or illuminated side must use the conductors sparingly to avoid blocking too much of the Sun's radiation from reaching the semiconductor. The final layer which is applied only to the illuminated side of the cell is the anti-reflection coating. Since all semiconductors are naturally reflective, reflection loss can be significant. The solution is to use one or several layers of an anti-reflection coating (similar to those used for eyeglasses and cameras) to reduce the amount of solar radiation that is reflected off the surface of the cell.[2]

Figure 2. The basic operation of a PV cell.[3]

Photovoltaic Effect

main article
Figure 3. A diagram showing the photovoltaic effect.[4]

The photovoltaic effect is a process that generates voltage or electric current in a photovoltaic cell when it is exposed to sunlight. These solar cells are composed of two different types of semiconductors—a p-type and an n-type—that are joined together to create a p-n junction. By joining these two types of semiconductors, an electric field is formed in the region of the junction as electrons move to the positive p-side and holes move to the negative n-side. This field causes negatively charged particles to move in one direction and positively charged particles in the other direction.[5] Light is composed of photons, which are simply small bundles of electromagnetic radiation or energy. When light of a suitable wavelength is incident on these cells, energy from the photon is transferred to an electron of the semiconducting material, causing it to jump to a higher energy state known as the conduction band. In their excited state in the conduction band, these electrons are free to move through the material, and it is this motion of the electron that creates an electric current in the cell.

Solar Cell Efficiency

main article

Efficiency is a design concern for photovoltaic cells, as there are many factors that limit their efficiency. The main factor is that 1/4 of the solar energy to the Earth cannot be converted into electricity by a silicon semiconductor. The physics of semiconductors requires a minimum photon energy to remove an electron from a crystal structure, known as the band-gap energy. If a photon has less energy than the band-gap, the photon gets absorbed as thermal energy. For silicon, the band-gap energy is 1.12 electron volts.[6] Since the energy in the photons from the sun cover a wide range of energies, some of the incoming energy from the Sun does not have enough energy to knock off an electron in a silicon PV cell. Even from the light that can be absorbed, there is still a problem. Any energy above the band-gap energy will be transformed into heat. This also cuts the efficiency because that heat energy is not being used for any useful task.[6] Of the electrons that are made available, not all of them will actually make it to the metal contact and generate electricity. This is because some of them will not be accelerated sufficiently by the voltage inside the semiconductor. Because of the reasons listed, the theoretical efficiency of silicon PV cells is about 33%.[6]

There are ways to improve the efficiency of PV cells, all of which come with an increased cost. Some of these methods include increasing the purity of the semiconductor, using a more efficient semiconducting material such as Gallium Arsenide, by adding additional layers or p-n junctions to the cell, or by concentrating the Sun's energy using concentrated photovoltaics. On the other hand, PV cells will also degrade, outputting less energy over time, due to a variety of factors including UV exposure and weather cycles. A comprehensive report from the National Renewable Energy Laboratory (NREL) states that the median degradation rate is 0.5% per year.[7]

Types of PV Cells

main article
Figure 4. An image comparing a polycrystalline silicon cell (left) and a monocrystalline silicon cell (right).[8]

Photovoltaic cell can be manufactured in a variety of ways and from many different materials. The most common material for commercial solar cell construction is Silicon (Si), but others include Gallium Arsenide (GaAs), Cadmium Telluride (CdTe) and Copper Indium Gallium Selenide (CIGS). Solar cells can be constructed from brittle crystalline structures (Si, GaAs) or as flexible thin-film cells (Si, CdTe, CIGS). Crystalline solar cells can be further classified into two categories—monocrystalline and polycrystalline, as shown in figure 4. As the names suggest, monocrystalline PV cells are comprised of a uniform or single crystal lattice, whereas polycrystalline cells contain different or varied crystal structures. Solar cells can also be classified by their number of layers or "p-n junctions". Most commercial PV cells are only single-junction, but multi-junction PV cells have also been developed which provide higher efficiencies at a greater cost.

For Further Reading

References

  1. ↑ "20110504-RD-LSC-0621 - Flickr - USDAgov" by U.S. Department of Agriculture. Licensed under CC BY 2.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:20110504-RD-LSC-0621_-_Flickr_-_USDAgov.jpg#/media/File:20110504-RD-LSC-0621_-_Flickr_-_USDAgov.jpg
  2. ↑ C. Julian Chen. Physics of Solar Energy, 1st ed. Hoboken, NJ, USA: John Wiley & Sons Inc., 2011.
  3. ↑ Wikimedia Commons [Online], Available: http://upload.wikimedia.org/wikipedia/commons/7/7d/Operation_of_a_basic_photovoltaic_cell.gif
  4. ↑ Created internally by a member of the Energy Education team. Adapted from: Ecogreen Electrical. (August 14, 2015). Solar PV Systems [Online]. Available: http://www.ecogreenelectrical.com/solar.htm
  5. ↑ G. Boyle. Renewable Energy: Power for a Sustainable Future, 2nd ed. Oxford, UK: Oxford University Press, 2004.
  6. ↑ 6.0 6.1 6.2 R. Wolfson, "Photovoltaic Solar Energy" in Energy, Environment, and Climate, 2nd ed., New York, NY: W.W. Norton & Company, 2012, ch. 9, sec. 5, pp. 244-252
  7. ↑ Dirk C. Jordan and Sarah R. Kurtz. Photovoltaic Degradation Rates — An Analytical Review, National Renewable Energy Laboratory, USA, 2012. Accessed April 24, 2018. [Online] Available at https://www.nrel.gov/docs/fy12osti/51664.pdf
  8. ↑ Wikimedia Commons. (August 18, 2015). Comparison of Solar Cells [Online]. Available: https://upload.wikimedia.org/wikipedia/commons/7/71/Comparison_solar_cell_poly-Si_vs_mono-Si.png
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