
A solar cell, or photovoltaic cell, is a semiconductor device consisting of a large-area p-n junction diode, which, in the presence of sunlight is capable of generating usable electrical energy. This conversion is called the photovoltaic effect. The field of research related to solar cells is known as photovoltaics. Solar cells have many applications. They are particularly well suited to, and historically used in situations where electrical power from the grid is unavailable, such as in remote area power systems, Earth orbiting satellites, handheld calculators, remote radiotelephones, water pumping applications, etc. Solar cells (in the form of modules or solar panels) are appearing on building roofs where they are connected through an inverter to the electricity grid in a net metering arrangement. ==Introduction== ===Etymology=== The etymology of the term "photovoltaic" comes from the Greek language ''photos'' meaning light and the name of the Italian physicist Alessandro Volta, after whom the volt (and consequently voltage) are named. It means literally ''of light and electricity''. ===History=== ''Main article'': Timeline of solar cells Russell Ohl is generally recognized for patenting the modern solar cell in 1946 ([http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=2402662.WKU.&OS=PN/2402662&RS=PN/2402662 US2402662], "''Light sensitive device''"). Sven Ason Berglund had a prior patent concerning methods of increasing the capacity of photosensitive cells. ===Materials and efficiency=== Various materials have been investigated for solar cells. There are two main criteria - efficiency and cost. Efficiency is a ratio of the electric power output to the light power input. Ideally, near the equator at noon on a clear day, the solar radiation is approximately 1000 W/m². So a 10% efficient module of 1 square meter can power a 100 W light bulb. Costs and efficiencies of the materials vary greatly. By far the most common material for solar cells (and all other semiconductor devices) is crystalline silicon. Crystalline silicon solar cells come in three primary categories: *Single crystal or monocrystalline wafers made using the Czochralski process. See [http://www.sunpowercorp.com/html/ Sunpower] and [http://www.shell.com/ Shell Solar]. Most commercial monocrystalline cells have efficiencies on the order of 14%; the SunPower cells have high efficiencies around 20%. Single crystal cells tend to be expensive, and because they are cut from cylindrical ingots, they cannot completely cover a module without a substantial waste of refined silicon. Most monocrystalline panels have uncovered gaps at the corners of four cells. *Poly or multi crystalline made from cast ingots - large crucibles of molten silicon carefully cooled and solidified. See [http://www.gtsolar.com/products/hem.php GT Solar HEM Furnace], [http://www.bp.com/modularhome.do?categoryId=4320&contentId=7004540 BP Solar], [http://solar.sharpusa.com/solar/home/0,2462,,00.html Sharp Solar] and [http://www.kyocerasolar.com Kyocera Solar]. These cells are cheaper than single crystal cells, but also somewhat less efficient. However, they can easily be formed into square shapes that cover a greater fraction of a panel than monocrystalline cells, and this compensates for their lower efficiencies. *Ribbon silicon formed by drawing flat thin films from molten silicon and has a multicrystalline structure. See [http://www.evergreensolar.com/ Evergreen Solar], and [http://www.rweschottsolar.com/ RWE Schott Solar]. These cells are typically the least efficient, but there is a cost savings since there is very little silicon waste since this approach does not require sawing from ingots. These technologies are wafer based manufacturing. In other words, in each of the above approaches, self supporting wafers of ~300 micrometres thick are fabricated and then soldered together to form a module. Thin film approaches are module based. The entire module substrate is coated with the desired layers and a laser scribe is then used to delineate individual cells. Two main thin film approaches are amorphous silicon and CIS: *Amorphous silicon films are fabricated using chemical vapor deposition techniques, typically plasma enhanced (PE-CVD). These cells have low efficiencies around 8%. *CIS stands for general chalcogenide films of Cu(InxGa1-x)(SexS1-x)2. While these films can achieve 11% efficiency, their costs are still too high. There are additional materials and approaches. For example, Sanyo has pioneered the HIT cell. In this technology, amorphous silicon films are deposited onto crystalline silicon wafers. The chart below illustrates the various commercial large area module efficiencies and the best laboratory efficiencies obtained for various materials and technologies. ===Interconnection and modules=== Usually, solar cells are electrically connected, and combined into "modules", or solar panels. Solar panels, have a sheet of glass on the front, and a resin encapsulation behind to keep the semiconductor Wafer (electronics) safe from the elements (rain, hail, etc). Solar cells are usually connected in series and parallel circuits in modules, so that their voltages add. ==Theory== ===Background=== In order to understand how a solar cell works, a little background theory in semiconductor physics is required. For simplicity, the description here will be limited to describing the workings of single crystalline silicon solar cells. Silicon is a group 14 element (formerly, group IV) atom. This means that each Si atom has 4 valence electrons in its outer atomic orbital. Silicon atoms can covalently bond to other silicon atoms to form a solid. There are two basic types of solid silicon, amorphous solid (having no long range order) and crystalline (where the atoms are arranged in an ordered three dimensional array). There are various other terms for the crystalline structure of silicon; poly-crystalline, micro-crystalline, nano-crystalline etc, and these refer to the size of the crystal "grains" which make up the solid. Solar cells can be, and are made from each of these types of silicon, the most common being poly-crystalline. Silicon is a semiconductor. This means that in solid silicon, there are certain bands of energies which the electrons are allowed to have, and other energies between these bands which are forbidden. These forbidden energies are called the "band gap". The allowed and forbidden bands of energy are explained by the theory of quantum mechanics. At room temperature, pure silicon is a poor electrical conductor (material). In quantum mechanics, this is explained by the fact that the Fermi level lies in the forbidden band-gap. To make silicon a better conductor, it is "doping (semiconductors)" with very small amounts of atoms from either group 13 element (III) or group 15 element (V) of the periodic table. These "dopant" atoms take the place of the silicon atoms in the crystal lattice, and bond with their neighbouring Si atoms in almost the same way as other Si atoms do. However, because group 13 atoms have only 3 valence electrons, and group 15 atoms have 5 valence electrons, there is either one too few, or one too many electrons to satisfy the four covalent bonds around each atom. Since these extra electrons, or lack of electrons (known as "electron hole") are not involved in the covalent bonds of the crystal lattice, they are free to move around within the solid. Silicon which is doped with group 13 atoms (aluminium, gallium) is known as p-type semiconductor silicon because the majority charge carriers (holes) carry a positive charge, whilst silicon doped with group 15 atoms (phosphorus, arsenic) is known as n-type semiconductor silicon because the majority charge carriers (electrons) are negative. It should be noted that both n-type and p-type silcion are electrically neutral, i.e. they have the same numbers of positive and negative charges, it is just that in n-type silicon, some of the negative charges are free to move around, while the converse is true for p-type silicon. ===Light generation of carriers=== When a photon of light hits a piece of silicon, one of two things can happen. The first is that the photon can pass straight through the silicon. This (generally) happens when the energy of the photon is lower than the bandgap energy of the silicon semiconductor. The second thing that can happen is that the photon is absorbed by the silicon. This (generally) happens if the photon energy is greater than the bandgap energy of silicon. When a photon is absorbed, its energy is given to an electron in the crystal lattice. Usually this electron is in the valence band, and is tightly bound in covalent bonds between neighbouring atoms, and hence unable to move far. The energy given to it by the photon "excites" it into the conduction band, where it is free to move around within the semiconductor. The covalent bond that the electron was previously a part of now has one less electron - this is known as a hole. The presence of a missing covalent bond allows the bonded electrons of neighboring atoms to move into the "hole," leaving another hole behind, and in this way a hole can move through the lattice. Thus, it can be said that photons absorbed in the semiconductor create mobile electron-hole pairs. A photon only needs to have energy greater than the band gap energy to excite an electron from the valence band into the conduction band. However, the solar frequency spectrum approximates a black body spectrum at ~6000 K, and as such, much of the solar radiation reaching the Earth is composed of photons with energies greater than the band gap of silicon. These higher energy photons will be absorbed by the solar cell, but the difference in energy between these photons and the silicon band gap is converted into heat (via lattice vibrations - called phonons) rather than into usable electrical energy. ===The p-n junction=== A solar cell is a large-area semiconductor p-n junction. To understand the workings of a p-n junction it is convenient to imagine what happens when a piece of n-type silicon is brought into contact with a piece of p-type silicon. In practice, however, the p-n junctions of solar cells are not made in this way, but rather, usually, by diffusing an n-type dopant into one side of a p-type wafer. If we imagine what happens when a piece of p-type silicon is placed in intimate contact with a piece of n-type silicon, then what occurs is a diffusion of electrons from the region of high electron concentration - the n-type side of the junction, into the region of low electron concentration - p-type side of the junction. When the electrons diffuse across the p-n junction, they recombine with holes on the p-type side. This diffusion of carriers does not happen indefinitely however, because of the electric field which is created by the imbalance of charge immediately either side of the junction which this diffusion creates. Electrons from donor atoms on the n-type side of the junction are crossing into the p-type side, leaving behind the (extra) positively charged nuclei of the group 15 donor atoms, leaving an excess of positive charge on the n-type side of the junction. At the same time, these electrons are filling in holes on the p-type side of the junction, becoming involved in covalent bonds around the group 13 acceptor atoms, making an excess of negative charge on the p-type side of the junction. This imbalance of charge across the p-n junction sets up an electric field which opposes further diffusion of charge carriers across the junction. This region where electrons have diffused across the junction is called the depletion zone because it no longer contains any mobile charge carriers. It is also known as the "space charge region". The electric field which is set up across the p-n junction creates a diode, allowing current to flow in only one direction across the junction. Electrons may pass from the p-type side into the n-type side, and holes may pass from the n-type side to the p-type side. But since the sign of the charge on electrons and holes is opposite, conventional current may only flow in one direction. ===Separation of carriers by the p-n junction=== Once the electron-hole pair has been created by the absorption of a photon, the electron and hole are both free to move off independently within the silicon latttice. If they are created within a minority carrier diffusion length of the junction, then, depending on which side of the junction the electron-hole pair is created, the electric field at the junction will either sweep the electron to the n-type side, or the hole to the p-type side. ===Connection to an external load=== If Ohmic metal-semiconductor contacts are made to both the n-type and p-type sides of the solar cell, and the electrodes connected to an external load, then electrons which are created on the n-type side, or have been "collected" by the junction and swept onto the n-type side may travel through the wire, power the load, and continue through the wire until they reach the p-type semiconductor-metal contact where they recombine with a hole which was either created as an electron-hole pair on the p-type side of the solar cell, or swept across the junction from the n-type side after being created there. ===Equivalent circuit of a solar cell=== To understand the electronic behaviour of a solar cell, it is useful to create a model which is electrically equivalent, and is based on discrete electrical components whose behaviour is well known. An ideal solar cell may be modelled by a current source in parallel with a diode. In practice no solar cell is ideal, so a shunt resistance and a series resistance component are added to the model. The result is the "equivalent circuit of a solar cell" shown on the left. Also shown on the right, is the schematic representation of a solar cell for use in circuit diagrams. ==Manufacture and devices== Because solar cells are semiconductor devices, they share many of the same processing and manufacturing techniques as other semiconductor devices such as computer and computer storage integrated circuit. However, the stringent requirements for cleanliness and quality control of semiconductor fabrication are a little more relaxed for solar cells. Most large-scale commercial solar cell factories today make screen printed poly-crystalline silicon solar cells. Single crystalline wafers which are used in the semiconductor industry can be made in to excellent high efficiency solar cells, but they are generally considered to be too expensive for large-scale mass production. Poly-crystalline silicon wafers are made by wire-sawing block-cast silicon ingots into very thin (250 to 350 micrometre) slices or wafers. The wafers are usually lightly p-type doped. To make a solar cell from the wafer, an n-type diffusion is performed on the front side of the wafer, forming a p-n junction a few hundred nanometres below the surface. Antireflection coatings, which increase the amount of light coupled into the solar cell, are typically applied next. Over the past decade, silicon nitride has gradually replaced titanium dioxide as the antireflection coating of choice because of its excellent surface passivation qualities (i.e., it prevents carrier recombination at the surface of the solar cell). It is typically applied in a layer several hundred nanometers thick using plasma-enhanced chemical vapor deposition (PECVD). The wafer is then metallised, whereby a full area metal contact is made on the back surface, and a grid-like metal contact made up of fine "fingers" and larger "busbars" is screen-printed onto the front surface using a silver paste. The rear contact is also formed by screen-printing a metal paste, typically aluminum. Usually this contact covers the entire rear side of the cell, though in some cell designs it is printed in a grid pattern. The metal electrodes will then require some kind of heat treatment or "sintering" to make Ohmic contact with the silicon. After the metal contacts are made, the solar cells are interconnected in series (and/or parallel) by flat wires or metal ribbons, and assembled into modules or "solar panels". Solar panels have a sheet of tempered glass on the front, and a polymer encapsulation on the back. Some solar cells have textured front surfaces that, like antireflection coatings, serve to increase the amount of light coupled into the cell. Such surfaces can usually only be formed on single-crystal silicon, though in recent years methods of forming them on multicrystalline silicon have been developed. ===Energy conversion efficiency=== Typical module efficiencies for commercially available screen printed multicrystalline solar cells are around 12%. A solar module's energy conversion efficiency, (or just efficiency) is the ratio of the maximum output electrical power divided by the input light power under "standard" test conditions. The "standard" solar radiation (known as the "air mass 1.5 spectrum") has a power density of 1000 watts per square metre. Thus, a typical 1 m² solar panel in direct sunlight will produce approximately 120 watts of peak power. ==Applications and implementations== See the article solar panel for information about applications and implementations of solar cells and panels. ==Cost analysis== The US retail module costs are in the United States dollar3.50 to $5.00/Wp range ([http://www.solarbuzz.com/ see SolarBuzz]). Additional installation costs for a residential rooftop retrofit in California (CA) is around $3.50/Wp or more. So on the low side, installed system costs are about $7.00/Wp in CA, and probably higher in places with less experience. Federal, state, utility, and other subsidies combined pay about half the cost. So CA rule of thumb is that the installed system PV will cost you at the low end, $3.50/Wp. Under net metering, one offsets regular retail utility rate which for CA is about 11 cents/kWh. Knowing installed system costs, amount of sunshine, and the utility rates, one can figure out the years till payback with or without financing costs. Assuming no financing costs and a $6/Wp installed system cost (lower than current $7), one can take sunshine and utility rate information from around the globe and come up with a payback graph such as shown below. The addition of subsidies brings down the years to payback proportionately. For example, if the years to payback were 24 years at $6/Wp, and subsidies brought that down to $3/Wp, the years to payback would be 12. ==Current research== There are currently many research groups active in the field of photovoltaics at universities and research institutions around the world. Much of the research is focussed on making solar cells cheaper and/or more efficient, so that they can more effectively compete with other energy sources, including fossil energy. One way of doing this is to develop cheaper methods of obtaining silicon that is sufficiently pure. Silicon is a very common element, but is normally bound in sand. Another approach is to significantly reduce the amount of raw material used in the manufacture of solar cells. The various thin-film technologies currently being developed make use of this approach to reducing the cost of electricity from solar cells. The invention of conductive polymers, (for which Alan Heeger was awarded a Nobel prize) may lead to the development of much cheaper cells that are based on inexpensive plastics, rather than semiconductor grade silicon. However, all organic solar cells made to date suffer from degradation upon exposure to UV light, and hence have lifetimes which are far too short to be viable. ===Thin-film solar cells=== The next step in reducing the cost of solar cells and panels seems certain to come from thin-film technology. Thin-film solar cells use less than 1% of the raw material (silicon) compared to wafer based solar cells, leading to a significant price drop per kWh. There are many research groups around the world actively researching different thin-film approaches and/or materials. Thin Film solar cells are mainly deposited by Chemical_vapor_deposition from silane gas and hydrogen. This process produces a material without crystalline orientation : amorphous silicon. Depending on the deposition's parameters nanocrystalline silicon can also be obtained. These types of silicon present dandling and twisted bonds, which results in the aparition of deep defects (energy levels in the bandgap) as well as in the deformation of the valence and conduction bands (band tails). This contributes to reduce the Quantum efficiency of Thin-Film solar cells by reducing the number of collected electron-hole pair by incident photon. Amorphous silicon (a-Si) has a higher bandgap (1.7 eV) than crystalline Silicon (c-Si) (1.1 eV), which means it is more efficient to absorb the visible part of the solar spectrum, but it fails to collect an important part of the spectrum : the infrared. As nano crystalline Si has about the same bandgap as c-Si, the two material can be combined by depositing to diodes on top of each other : the tandem cell. The top cell in a-Si absorbs the visible light and leaves the infrared part of the spectrum for the bottom cell in nanocrystalline Si. One particularly promising technology is crystalline silicon thin-films on glass substrates. This technology makes use of the advantages of crystalline silicon as a solar cell material, with the cost savings of using a thin-film approach. From the [http://www.pacificsolar.com.au/ Pacific Solar] website: :"Crystalline Silicon on Glass (CSG) [is] the photovoltaic technology developed by Pacific Solar that is now being commercialised by CSG Solar. A very thin layer of silicon, less than two micrometres thick, is deposited directly onto a glass sheet whose surface has been roughened by applying a layer of tiny glass beads. The silicon is not crystalline when first deposited, but becomes so after heat treatment in an oven. The resulting layer is processed using lasers and ink-jet printing techniques to form the electrical contacts needed to get the solar-produced electricity out of the thin silicon film." In 2005, a full-scale production factory is being built in Germany to commercialise this technology. CSG Solar expects to release its first product for sale in 2006. Each solar module will have a rated power exceeding 100 watts and will be cheaper than competing solar panels. Another interesting aspect of thin-film solar cells is the possibility to deposit the cells on all kind of materials, including flexible substrates (PET for example), which opens a new dimension for new applications. ===Exotic materials=== For special applications, such as Deep Space 1, high-efficiency cells can be made from gallium arsenide by molecular beam epitaxy. Such cells have many diodes in series, each with a different band gap energy so that it absorbs its share of the electromagnetic spectrum with very high efficiency. Triple junction solar cell have (as the name suggest) 3 diodes layered on top of each other, each absorbing a different spectrum of light, efficiency as high as 28% have been achieved. The multiple junction solar cells may be very efficient, but are prohibitively expensive to make. Cost-effective use of these cells could be achieved with concentrating optics so that less of the array consists of actual semiconductor devices. Experimental non-silicon solar panels can be made of carbon nanotubes or quantum dots embedded in a special conductive polymers. These have only one-tenth the efficiency of silicon panels but could be manufactured in ordinary factories, not clean rooms which should lower the cost. Some of the most efficient solar cell materials are cadmium tellurium (CdTe) and copper indium gallium selenium (CIGS). Unlike the basic silicon solar cell, which can be modelled as a simple p-n junction (see under semiconductor), these cells are best described by a more complex heterojunction model. The best efficiency of a bare solar cell as of April 2003 was 16.5% [Dr IM Dharmadasa, Sheffield Hallam University, UK]. Higher efficiencies (around 30%) can be obtained by using optics to concentrate the incident light. Polymer or organic solar cells are built from ultra thin layers (typically 100 nm) of organic semiconductors such as polyphenylene vinylene and fullerene. The p/n junction model is only a crude description of the functioning of such cells, as electron hopping and other processes also play a crucial role. They are potentially cheaper to manufacture than silicon or inorganic cells, but efficiencies achieved to date are low and cells are highly sensitive to air and moisture, making commercial applications difficult. In the reverse mode, the technology has however already successfully been commercialised in organic LEDs and organic displays, also called polymer displays. Graetzel cells (sometimes called photoelectrochemical cells) have been around for two decades or so. A p/n junction is used here too in the form of a doped solid (normally titanium dioxide) in contact with a solid or liquid electrolyte (for example CuI). In contrast to the classical solar cell not the semiconductor but a dye placed at the p/n interface is used for absorption of radiation, mimicking the process of photosynthesis. As a result, this type of cell allows a more flexible use of materials. Like organic solar cells, Graetzel cells can be manufactured under "dirty" conditions. Commercial applications have failed to appear due to the fast degradation occurring in Graetzel cells. ==Solar cells and energy payback== There is a common but mistaken notion that solar cells never produce more energy than it takes to make them. While the expected working lifetime is on the order of 40 years, the energy payback time of a solar panel is anywhere on the order of 2 to 30 years depending on the type and where it is used, see Net energy gain. ==See also== *Photodiode *Timeline of solar energy *solar power *solar panel *autonomous building *renewable energy *Future energy development *photovore ==External links== * Pennicott, Katie, "''[http://physicsweb.org/article/news/5/12/2 Solar cell edges towards endless energy]''". 7 December 2001. PhysicsWeb. * [http://dcwww.epfl.ch/lpi/solarcellE.html Dye Sensitized Solar Cells] (DYSC) based on Nanocrystalline Oxide Semiconductor Films * News searching: [http://news.google.com/news?hl=da&q=%22Solar+Cell%22 Solar Cell], [http://news.google.com/news?hl=da&q=Photovoltaic Photovoltaic] *[http://www.atse.org.au/index.php?sectionid=391 Historical: Photovoltaic Solar Energy Conversion: An Update] *[http://www.lbl.gov/msd/PIs/Walukiewicz/02/02_8_Full_Solar_Spectrum.html Wladek Walukiewicz, Materials Sciences Division, Berkeley Lab.: Full Solar Spectrum Photovoltaic Materials Identified.] Quote: "... Maximum, theoretically predicted efficiencies increase to 50%, 56%, and 72% for stacks of 2, 3, and 36 junctions with appropriately optimized energy gaps, respectively...." *[http://news.cnet.com/investor/news/newsitem/0-9900-1028-21199489-0.html CNET: 5/12/03 SunPower Announces World's Most Efficient, Low-Cost Silicon Solar Cell] Quote: "...[http://www.nrel.gov/ The National Renewable Energy Laboratory (NREL)] has verified 20.4 percent conversion efficiency for the A-300...." *[http://www.sunpowercorp.com/html/Products/Datasheets/A-300/A-300.pdf SunPower A-300 (pdf)], [http://www.sunpowercorp.com/ SunPower] *[http://www.sciam.com/article.cfm?chanID=sa003&articleID=0004C094-02CC-1CD0-B4A8809EC588EEDF 29 March 2002, Scientists Create New Solar Cell] Quote: "...semiconducting plastic material known as P3HT... 1.7 percent for sunlight..." *[http://www.newscientist.com/news/news.jsp?id=ns99993380 15 February 03, 'Denim' solar panels to clothe future buildings] Quote: "... Unlike conventional solar cells, the new, cheap material has no rigid silicon base..." *[http://www.californiasolarco.com/power-systems-photo-gallery.html Residential Solar Power Systems - Photo Gallery] *[http://www.sma-america.com/installations.html Examples of Photovoltaic Systems ] *[http://science.howstuffworks.com/solar-cell.htm How Solar Cells Work ] *[http://www.azonano.com/news.asp?newsID=548 azonano.com: Carbon Nanotube Structures Could Provide More Efficient Solar Power for Soldiers] 28 February 2005 *[http://www.newton.mec.edu/Brown/TE/HOT/TIMELINES/SOLAR/solar_timeline.html Solar energy timeline] ===Yield data=== * http://www.tectosol.staticip.de/index_en.htm electricity yield of a solar power system * http://www.sunny-portal.de Yield Portal for solar power system users ===Theory=== *[http://www.nrel.gov/buildings/pv/factsheets.html National Renewable Energy Laboratory (NREL): Photovoltaics for Buildings: PV Technology for the Home Factsheets] *[http://www.nrel.gov/research/pv/docs/pvpaper.html 1993, National Renewable Energy Laboratory (NREL): Photovoltaics: Unlimited Electrical Energy From the Sun] BrokenLink *[http://www.cefetba.br/fisica/NFL/PBCN/solar/solardeu.html#ideal Electrical models of solar cells] ===Cost Benefit=== *[http://rredc.nrel.gov/solar/codes_algs/PVWATTS/pvwatts_index.html PVWATTS - A Performance Calculator for Grid-Connected PV Systems] ===Do-it-yourself=== ====PEC (Photo Electro Chromic)==== *[http://www.chemistry.ucsc.edu/teaching/Winter98/Chem1B/photo/Solar_Kit_Word_6.html How to Build Your Own Solar Cell] *[http://www.solideas.com/solrcell/cellkit.html DIY (Do It Yourself): Nanocrystalline Dye-Sensitized Solar Cell Kit] Quote: "... sunlight-to-electrical energy conversion efficiency is between 1 and 0.5 %..." ====Cuprous oxide solar cells==== *[http://www.scitoys.com/scitoys/scitoys/echem/echem2.html#solarcell Make a solar cell in your kitchen], [http://www.scitoys.com/scitoys/scitoys/echem/echem3.html#sflatpanel A flat panel solar battery] *[http://www.zetatalk.com/energy/tengy17f.htm From: How to Build a Solar Cell That Really Works by Walt Noon] ===Indexes=== *Open Directory Project: [http://www.dmoz.org/Business/Energy_and_Environment/Renewable/Solar/ Solar] ===Newsgroups=== *[http://groups.google.com/groups?q=alt.solar.photovoltaic alt.solar.photovoltaic] ===Patents=== * [http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=2402662.WKU.&OS=PN/2402662&RS=PN/2402662 US2402662] -- ''Light sensitive device'' -- R. S. Ohl * [http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=1289369.WKU.&OS=PN/1289369&RS=PN/1289369 US1289369] -- ''Method of increasing the capacity of photosenitive electrical cells'' Electrical components Energy conversion Renewable energy
Solar cells do not use the photoelectric effect. Solar cells are made of semiconductor material, the incoming light (photons) move electrons from the valence band across the band gap to the conduction band. The resulting electron-hole pairs are separated by the internal electrical field of the p-n-junction. In this way different charges on the two electrodes of the solar cell are created, which can be used to drive an current through a wire. ---- Should this be merged with solar panel? User:Rmhermen 17:10, Feb 28, 2004 (UTC) * Definitely. I think the scientific/functional details should be under solar cell and the application and anecdotal information should be under solar panel. Another deficiency is that solar panel completely ignores those (also active solar panels whose purpose is to collect thermal energy only. -User:RatOmeter == Article needs latitude application info... == The Wikipedia is, of course, updatable. A valuable section to add to this article on photovoltaic-energy generation would be something that - even in a very general way - addresses the issue of p.v. applications in various latitudes (or latitude zones). The general info about cents/kwh is fine, but so unspecific as to give the reader little idea of where the technology sits at present. We get the idea that it has application in "desert" areas, but in the English-speaking world, few people live in this climatic zone. Progress in the p.v. field must be pretty continual, so a more general idea of power generation and power storage issues for different latitudes could be further detailed as relevant advances emerge. == expand == "When photons hit the silicon plates, electron-hole pairs are created (with a probability depending on the quantum efficiency) and separated. The electric field across the p-n junction draws the electrons and holes in opposite directions, and they then diffuse to the front and back contacts." :how are the pairs created? can we have more details about this process? - User:Omegatron 22:52, Jan 8, 2005 (UTC) == production energy == I have heard that more energy goes into producing solar cells than they will ever produce. I am doubtful of this but have been unable to find and information on the production of solar cells. Does anyone know anything about the amount of energy required to produce a solar cell or array? :I'm pretty sure this isn't true, though I can't find a reference. I've heard that it takes n years or so for an installation to pay for itself, and they last >n or more, so if money is energy (it isn't, but close enough), then you are making net profit in energy. - User:Omegatron 18:07, Jan 21, 2005 (UTC) :I have some numbers for poly-Si but they are quite old (1994). If someone has better ones, please add them. 16.0 kWh are needed for each Watt-peak installed (of the finished module). This is split in the following way : Production of Silicon ingot 7.9 kWh / production of the wafers 2.9 kWh / Cells production 3.7 kWh / Grouping cells in modules 1.5 kWh. Assuming an efficiency of 14%, 7 Years are needed in central Switzerland to make an installation profitable. Of course it takes a shorter amount of time in sunnier areas at lower latitudes. Furthermore, due to the fact that those figures are pretty old, today's real values are probably lower. Furthermore, thin film technology allows the productions of cells for less energy. Therefore, it is wrong to say that it takes more energy to produce a solar pannel than it will ever give. User:Glaurung 20:59, 21 Jan 2005 (UTC) == LEDs and solar cells == Solar cells seem at first to be kind of a dual of LEDs. They take in light and convert it back to electrical energy. But they aren't really opposites: LED: Has a fixed voltage drop across it. Voltage must increase above this drop for LED to emit light. After switched on, the ''current'' sent through the LED (determined by the resistance of other components in series) determines the intensity of the LED, while the voltage stays constant. Solar cell: Voltage output varies with intensity of light? I think so, anyway... :TMW: No!! The output CURENT of a solar cell depends on the incident light intensity. The voltage remains (alomst) constant. You are right though, they ARE almost like large-area light-emitting diodes operating in reverse. ::So is this just an effect of low load impedance? What kind of source impedance do they have? - User:Omegatron LED: Emits only a single frequency. Voltage drop across LED depends on frequency of light emitted? (Which depends on construction. You can't change the voltage on the fly and change the color emitted (although that would be pretty cool...)) Solar cell: Converts different frequencies into electrical energy? I imagine it has a bandpass response, but I'm not sure. Can we get a physics explanation of the similarities and differences between these processes? - User:Omegatron 15:20, Mar 8, 2005 (UTC) :Hmm... :"Photovoltaic modules (solar panel) convert all wavelengths within the visible light spectrum to DC electricity but are optimized for the wavelengths that occur most commonly. For peak performance a solar module should face the brightest part of the sky. Most modules are installed at a fixed azimuth and tile angle in order to maximize their energy output. :Solar modules are made up of "cells" manufactured from various forms of silicon. The greater the light intensity falling on the cells the greater the current produced (light intensity and output current are proportional). However, the voltage produced is not proportional to light intensity but rises considerably in low light ensuring that charging can take place." - User:Omegatron 15:29, Mar 8, 2005 (UTC) :We should cover the information on this page. http://www.solarserver.de/wissen/photovoltaik-e.html#char - User:Omegatron 15:34, Mar 8, 2005 (UTC) ---- == Full Cost Comparison? == The comparison of the cost per kw/hr between p.v. and nuclear is only a good figure, really, if it includes some accounting of the cost of building the installation and maintaining it (including replacement-component cost) amortized over the expected lifetime of the installation. As an analogy, if you look at the cost of running two different cars for a year with gasoline of equal cost per litre or gallon, over the same roads and streets, with the same number of passengers - but leave out the comparative cost of acquiring and maintaining the two vehicles - it's not a complete comparison. In other words, a comparison could be made more informatively. Hasn't Rocky Mountain Institute (Amory Lovins, et al.) developed the kind of figures I'm referring to? If not, someone else may have. - J.R. == Major Revamp == 10 April 2005 (darkside2010). I am in the process of rewriting and revamping this page "solar cell". The page needs a lot of work. Some of the information on it is just plain wrong, and other things are either poorly described or simply overlooked. (Sorry, no offense intended to the people who already contributed to this page). I am currently doing my PhD in solar cell research, so I think I am fairly well qualified to write about solar cells. I like the page "solar panel". It should definitely not be merged with "solar cell". However, I am a bit concerned about the page "photovoltaic cell". I feel it would be best to make it into a stub which directs readers to "solar cell". I don't see any distinction between photovoltaic cells and solar cells, and solar cell is by far the more commonly used term. I don't see any advantage to duplicating information on these pages. User:Darkside2010 :I agree : one should merge the info on the photovoltaic cell to this page and create a redirect. I also agree that this page needs some clean up and organization. Perhaps we should discuss about a better paragraph layout, starting by explaining the absorption of photons by a semi-c and then move to the main realisation of solar cells (mono, poly, thin film, III-V, Greatcell) User:Glaurung 11:30, 11 Apr 2005 (UTC) ::Ok, I am happy to merge the information on photovoltaic cell (which is not much) to this page, and then create a stub, redirecting here. Do you really think that photovoltaic cell should "automatically" redirect to solar cell? I feel it is better to say a few words about the origin of the term "photovoltaic" on that page, and then redirect readers here. I know that a lot of wikipedians don't like stubs, but I think that an informative stub is better than an automatic redirect. ::Certainly, lets discuss a better paragraph layout for solar cell. As you can see, I went ahead and made some rearrangement of the structure of the article, but I'm sure there is still plenty of room for improvement. ::On another note, I intend on replacing the lonely picture on this page. I think that for the top of the article, a photograph of a solar cell would be more appropriate, and that drawings such as the one currently there would be better in the "workings" section. I intend making a few copyright-free drawings of electron-hole pair creation, the equivalent circuit of a solar cell, etc for the page as I find time in the coming weeks. User:Darkside2010 13:59, 11 Apr 2005 (UTC) ::Another thing is, there are, to my mind, far too many external links on this page. I haven't taken the time to follow any of them yet, But I'm sure they are not all neccessary. :::Photovoltaic cell should redirect here, and any explanatory text should be in the first paragraph of this. - User:Omegatron 15:45, Apr 11, 2005 (UTC) ::::Ok, Omegatron, if that is the way things are done, then let it be so. I have rewritten the first paragraph to include a discussion of the etymology of the term ''photovoltaic''. Perhaps there should be a page called Photovoltaics, though, to describe all the institutions and research departments who work in that field. I noticed that Photovoltaic(s) redirects to solar cell, and i would argue that these are not actually the same thing. User:Darkside2010 13:31, 12 Apr 2005 (UTC) :::::That's a great idea. I'm no expert in the field, so I don't know which are discrete ideas. - User:Omegatron 18:23, Apr 12, 2005 (UTC) ::::Glaurung, Perhaps rather than discuss a new paragraph layout for this page, let's just edit the page itself, and see where the wikipedia process takes us. I mean, we all have the same goal here, right? To produce the best encylopaedic article of a solar cell in the world! So lets not spend our time and energy discussing, but rather making the page the best it can be. If you have an idea for a better structure, just change it. After all, that's what wikipedia is all about, yeah? User:Darkside2010 13:31, 12 Apr 2005 (UTC) I pulled that image which was at the top of the article, and replaced it with one which, in my opinion, ''increases'' understanding for the general reader as to what a solar cell is. The old picture was a baffling barrage of different coloured arrows which, without a detailed accompanying explanation (which was absent), added no understanding as to what a solar cell is, or how it works. At least with the photograph of the solar cell, people can say "Oh, yeah, I have seen one of those somewhere". User:Darkside2010 16:18, 12 Apr 2005 (UTC) :Rather than replace it, just move it down the page and ''add'' the new image. then if you find a better image that covers the same idea as the confusing arrows, use ''that'' to replace them. - User:Omegatron 18:23, Apr 12, 2005 (UTC) :: I reinserted the image where the photon absorption is explained. But I will try to find a better one (or maybe even make one). This one looks nice, but is not really clear. I think a simplier 2D schema would be better. User:Glaurung 20:01, 12 Apr 2005 (UTC) == Tesla == ''Nikola Tesla patented a primitive solar cell in 1901 when he received the patent [http://patimg2.uspto.gov/.piw?Docid=00685957&homeurl=http%3A%2F%2Fpatft.uspto.gov%2Fnetacgi%2Fnph-Parser%3FSect1%3DPTO1%2526Sect2%3DHITOFF%2526d%3DPALL%2526p%3D1%2526u%3D%2Fnetahtml%2Fsrchnum.htm%2526r%3D1%2526f%3DG%2526l%3D50%2526s1%3D685957.WKU.%2526OS%3DPN%2F685957%2526RS%3DPN%2F685957&PageNum=&Rtype=&SectionNum=&idkey=3D035366337E US685957] (Apparatus for the Utilization of Radiant Energy).'' :I believe this is a pretty controversial statement. - User:Omegatron 23:09, May 19, 2005 (UTC) :Yeah, I just read the patent and it's certainly not a solar cell. It appears to collect energy from collisions of ionizing radiation via the photoelectric effect, not light. - User:Omegatron 00:12, May 20, 2005 (UTC) ::but light causes the photoelectric effect to happen so would have powered the apparatus I believe. User:Greenpowered 21:46, 20 May 2005 (UTC) :::I'm reading about this a lot right now and learning a lot of new things. The work function for metal is too high (4.5 eV) for light to cause a photoelectric effect. The radiation hitting it has to be above near UV in frequency. This is really neat. The photoelectric effect actually causes spacecraft to acquire a positive charge and levitates dust particles on the moon! - User:Omegatron 21:50, May 20, 2005 (UTC) :::I'm going to add some details to Talk:Photoelectric effect#Tesla / radiant energy - User:Omegatron 00:12, May 21, 2005 (UTC) == A thought toward the major revamp == The current article says, in one place: "Typical module efficiencies for commercially available screen printed poly-crystalline solar cells are around 17%." What might be a useful and informative addition would be discussion - or preferably a ''graph'' - explaining what the increase in efficiency has been (starting from some point in, say, the 1960s or early '70s) leading up to the currently typical efficiency achievement. It probably goes without saying that this would suggest the positive future of solar-cell applications. I know a lot of people who have languished and rather given up on the potentials of pv, but it is not justified. A discussion or graph could bring home the trajectory toward wide practicality. - J.R. :A graph would indeed be a good idea. If you have the data at hand showing the evolution of efficiency versus time, feel free to make one. User:Glaurung 06:02, 20 May 2005 (UTC) ::I ''don't'' have the data - wish I did. I put the suggestion out for Darkside, since he is working on a PhD on the subject and wants to re-write the Wiki article. I thought he might be in touch with the stats. I do know that at times I've read about current typical solar cells being so much more (i.e., several times more) efficient than in the early 1970s, which was when I, as a kid, first became aware of the exciting idea of "solar energy." I believe a lot of people became aware of the general potentials of pv in the 1970s, which is one reason why I feel the comparative stats would be valuable. :::I have seen books that state it's up to 35% now so a graph could show a lot. Also I have heard solar power was thousands of dollars a watt over 20 years ago, and is now under $4.00. A graph showing the cost prices dropping over the years would really show the doubters solar is the power of the future. User:Greenpowered 21:50, 20 May 2005 (UTC) ==Cost Analysis== I removed this sentence from Cost Analysis, ''The present cost benefit to consumers is pretty bad.'' I do not think that is fair or correct. If you are in an area with subsidies it is not pretty bad, it's pretty good! Even if your not it can still easily be the best option. I know someone the utility companies wanted to charge over $100,000.00 to run grid electricity to his home and solar power was extremely cost effective. And there are even ways to make it cost effective if you are hooked up to the grid and in an area without subsidies. Also the subsidies for solar are compared to the cost of the grid. It does not mention that there are more subsidies on that end then in solar. User:Greenpowered 21:43, 20 May 2005 (UTC)
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Solar cell
A solar cell, or photovoltaic cell, is a semiconductor device consisting of a large-area p-n junction diode, which, in the presence of sunlight is capable of generating usable electrical energy. This conversion is called the photovoltaic effect. The field of research related to solar cells is known as photovoltaics. Solar cells have many applications. They are particularly well suited to, and historically used in situations where electrical power from the grid is unavailable, such as in remote area power systems, Earth orbiting satellites, handheld calculators, remote radiotelephones, water pumping applications, etc. Solar cells (in the form of modules or solar panels) are appearing on building roofs where they are connected through an inverter to the electricity grid in a net metering arrangement. ==Introduction== ===Etymology=== The etymology of the term "photovoltaic" comes from the Greek language ''photos'' meaning light and the name of the Italian physicist Alessandro Volta, after whom the volt (and consequently voltage) are named. It means literally ''of light and electricity''. ===History=== ''Main article'': Timeline of solar cells Russell Ohl is generally recognized for patenting the modern solar cell in 1946 ([http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=2402662.WKU.&OS=PN/2402662&RS=PN/2402662 US2402662], "''Light sensitive device''"). Sven Ason Berglund had a prior patent concerning methods of increasing the capacity of photosensitive cells. ===Materials and efficiency=== Various materials have been investigated for solar cells. There are two main criteria - efficiency and cost. Efficiency is a ratio of the electric power output to the light power input. Ideally, near the equator at noon on a clear day, the solar radiation is approximately 1000 W/m². So a 10% efficient module of 1 square meter can power a 100 W light bulb. Costs and efficiencies of the materials vary greatly. By far the most common material for solar cells (and all other semiconductor devices) is crystalline silicon. Crystalline silicon solar cells come in three primary categories: *Single crystal or monocrystalline wafers made using the Czochralski process. See [http://www.sunpowercorp.com/html/ Sunpower] and [http://www.shell.com/ Shell Solar]. Most commercial monocrystalline cells have efficiencies on the order of 14%; the SunPower cells have high efficiencies around 20%. Single crystal cells tend to be expensive, and because they are cut from cylindrical ingots, they cannot completely cover a module without a substantial waste of refined silicon. Most monocrystalline panels have uncovered gaps at the corners of four cells. *Poly or multi crystalline made from cast ingots - large crucibles of molten silicon carefully cooled and solidified. See [http://www.gtsolar.com/products/hem.php GT Solar HEM Furnace], [http://www.bp.com/modularhome.do?categoryId=4320&contentId=7004540 BP Solar], [http://solar.sharpusa.com/solar/home/0,2462,,00.html Sharp Solar] and [http://www.kyocerasolar.com Kyocera Solar]. These cells are cheaper than single crystal cells, but also somewhat less efficient. However, they can easily be formed into square shapes that cover a greater fraction of a panel than monocrystalline cells, and this compensates for their lower efficiencies. *Ribbon silicon formed by drawing flat thin films from molten silicon and has a multicrystalline structure. See [http://www.evergreensolar.com/ Evergreen Solar], and [http://www.rweschottsolar.com/ RWE Schott Solar]. These cells are typically the least efficient, but there is a cost savings since there is very little silicon waste since this approach does not require sawing from ingots. These technologies are wafer based manufacturing. In other words, in each of the above approaches, self supporting wafers of ~300 micrometres thick are fabricated and then soldered together to form a module. Thin film approaches are module based. The entire module substrate is coated with the desired layers and a laser scribe is then used to delineate individual cells. Two main thin film approaches are amorphous silicon and CIS: *Amorphous silicon films are fabricated using chemical vapor deposition techniques, typically plasma enhanced (PE-CVD). These cells have low efficiencies around 8%. *CIS stands for general chalcogenide films of Cu(InxGa1-x)(SexS1-x)2. While these films can achieve 11% efficiency, their costs are still too high. There are additional materials and approaches. For example, Sanyo has pioneered the HIT cell. In this technology, amorphous silicon films are deposited onto crystalline silicon wafers. The chart below illustrates the various commercial large area module efficiencies and the best laboratory efficiencies obtained for various materials and technologies. ===Interconnection and modules=== Usually, solar cells are electrically connected, and combined into "modules", or solar panels. Solar panels, have a sheet of glass on the front, and a resin encapsulation behind to keep the semiconductor Wafer (electronics) safe from the elements (rain, hail, etc). Solar cells are usually connected in series and parallel circuits in modules, so that their voltages add. ==Theory== ===Background=== In order to understand how a solar cell works, a little background theory in semiconductor physics is required. For simplicity, the description here will be limited to describing the workings of single crystalline silicon solar cells. Silicon is a group 14 element (formerly, group IV) atom. This means that each Si atom has 4 valence electrons in its outer atomic orbital. Silicon atoms can covalently bond to other silicon atoms to form a solid. There are two basic types of solid silicon, amorphous solid (having no long range order) and crystalline (where the atoms are arranged in an ordered three dimensional array). There are various other terms for the crystalline structure of silicon; poly-crystalline, micro-crystalline, nano-crystalline etc, and these refer to the size of the crystal "grains" which make up the solid. Solar cells can be, and are made from each of these types of silicon, the most common being poly-crystalline. Silicon is a semiconductor. This means that in solid silicon, there are certain bands of energies which the electrons are allowed to have, and other energies between these bands which are forbidden. These forbidden energies are called the "band gap". The allowed and forbidden bands of energy are explained by the theory of quantum mechanics. At room temperature, pure silicon is a poor electrical conductor (material). In quantum mechanics, this is explained by the fact that the Fermi level lies in the forbidden band-gap. To make silicon a better conductor, it is "doping (semiconductors)" with very small amounts of atoms from either group 13 element (III) or group 15 element (V) of the periodic table. These "dopant" atoms take the place of the silicon atoms in the crystal lattice, and bond with their neighbouring Si atoms in almost the same way as other Si atoms do. However, because group 13 atoms have only 3 valence electrons, and group 15 atoms have 5 valence electrons, there is either one too few, or one too many electrons to satisfy the four covalent bonds around each atom. Since these extra electrons, or lack of electrons (known as "electron hole") are not involved in the covalent bonds of the crystal lattice, they are free to move around within the solid. Silicon which is doped with group 13 atoms (aluminium, gallium) is known as p-type semiconductor silicon because the majority charge carriers (holes) carry a positive charge, whilst silicon doped with group 15 atoms (phosphorus, arsenic) is known as n-type semiconductor silicon because the majority charge carriers (electrons) are negative. It should be noted that both n-type and p-type silcion are electrically neutral, i.e. they have the same numbers of positive and negative charges, it is just that in n-type silicon, some of the negative charges are free to move around, while the converse is true for p-type silicon. ===Light generation of carriers=== When a photon of light hits a piece of silicon, one of two things can happen. The first is that the photon can pass straight through the silicon. This (generally) happens when the energy of the photon is lower than the bandgap energy of the silicon semiconductor. The second thing that can happen is that the photon is absorbed by the silicon. This (generally) happens if the photon energy is greater than the bandgap energy of silicon. When a photon is absorbed, its energy is given to an electron in the crystal lattice. Usually this electron is in the valence band, and is tightly bound in covalent bonds between neighbouring atoms, and hence unable to move far. The energy given to it by the photon "excites" it into the conduction band, where it is free to move around within the semiconductor. The covalent bond that the electron was previously a part of now has one less electron - this is known as a hole. The presence of a missing covalent bond allows the bonded electrons of neighboring atoms to move into the "hole," leaving another hole behind, and in this way a hole can move through the lattice. Thus, it can be said that photons absorbed in the semiconductor create mobile electron-hole pairs. A photon only needs to have energy greater than the band gap energy to excite an electron from the valence band into the conduction band. However, the solar frequency spectrum approximates a black body spectrum at ~6000 K, and as such, much of the solar radiation reaching the Earth is composed of photons with energies greater than the band gap of silicon. These higher energy photons will be absorbed by the solar cell, but the difference in energy between these photons and the silicon band gap is converted into heat (via lattice vibrations - called phonons) rather than into usable electrical energy. ===The p-n junction=== A solar cell is a large-area semiconductor p-n junction. To understand the workings of a p-n junction it is convenient to imagine what happens when a piece of n-type silicon is brought into contact with a piece of p-type silicon. In practice, however, the p-n junctions of solar cells are not made in this way, but rather, usually, by diffusing an n-type dopant into one side of a p-type wafer. If we imagine what happens when a piece of p-type silicon is placed in intimate contact with a piece of n-type silicon, then what occurs is a diffusion of electrons from the region of high electron concentration - the n-type side of the junction, into the region of low electron concentration - p-type side of the junction. When the electrons diffuse across the p-n junction, they recombine with holes on the p-type side. This diffusion of carriers does not happen indefinitely however, because of the electric field which is created by the imbalance of charge immediately either side of the junction which this diffusion creates. Electrons from donor atoms on the n-type side of the junction are crossing into the p-type side, leaving behind the (extra) positively charged nuclei of the group 15 donor atoms, leaving an excess of positive charge on the n-type side of the junction. At the same time, these electrons are filling in holes on the p-type side of the junction, becoming involved in covalent bonds around the group 13 acceptor atoms, making an excess of negative charge on the p-type side of the junction. This imbalance of charge across the p-n junction sets up an electric field which opposes further diffusion of charge carriers across the junction. This region where electrons have diffused across the junction is called the depletion zone because it no longer contains any mobile charge carriers. It is also known as the "space charge region". The electric field which is set up across the p-n junction creates a diode, allowing current to flow in only one direction across the junction. Electrons may pass from the p-type side into the n-type side, and holes may pass from the n-type side to the p-type side. But since the sign of the charge on electrons and holes is opposite, conventional current may only flow in one direction. ===Separation of carriers by the p-n junction=== Once the electron-hole pair has been created by the absorption of a photon, the electron and hole are both free to move off independently within the silicon latttice. If they are created within a minority carrier diffusion length of the junction, then, depending on which side of the junction the electron-hole pair is created, the electric field at the junction will either sweep the electron to the n-type side, or the hole to the p-type side. ===Connection to an external load=== If Ohmic metal-semiconductor contacts are made to both the n-type and p-type sides of the solar cell, and the electrodes connected to an external load, then electrons which are created on the n-type side, or have been "collected" by the junction and swept onto the n-type side may travel through the wire, power the load, and continue through the wire until they reach the p-type semiconductor-metal contact where they recombine with a hole which was either created as an electron-hole pair on the p-type side of the solar cell, or swept across the junction from the n-type side after being created there. ===Equivalent circuit of a solar cell=== To understand the electronic behaviour of a solar cell, it is useful to create a model which is electrically equivalent, and is based on discrete electrical components whose behaviour is well known. An ideal solar cell may be modelled by a current source in parallel with a diode. In practice no solar cell is ideal, so a shunt resistance and a series resistance component are added to the model. The result is the "equivalent circuit of a solar cell" shown on the left. Also shown on the right, is the schematic representation of a solar cell for use in circuit diagrams. ==Manufacture and devices== Because solar cells are semiconductor devices, they share many of the same processing and manufacturing techniques as other semiconductor devices such as computer and computer storage integrated circuit. However, the stringent requirements for cleanliness and quality control of semiconductor fabrication are a little more relaxed for solar cells. Most large-scale commercial solar cell factories today make screen printed poly-crystalline silicon solar cells. Single crystalline wafers which are used in the semiconductor industry can be made in to excellent high efficiency solar cells, but they are generally considered to be too expensive for large-scale mass production. Poly-crystalline silicon wafers are made by wire-sawing block-cast silicon ingots into very thin (250 to 350 micrometre) slices or wafers. The wafers are usually lightly p-type doped. To make a solar cell from the wafer, an n-type diffusion is performed on the front side of the wafer, forming a p-n junction a few hundred nanometres below the surface. Antireflection coatings, which increase the amount of light coupled into the solar cell, are typically applied next. Over the past decade, silicon nitride has gradually replaced titanium dioxide as the antireflection coating of choice because of its excellent surface passivation qualities (i.e., it prevents carrier recombination at the surface of the solar cell). It is typically applied in a layer several hundred nanometers thick using plasma-enhanced chemical vapor deposition (PECVD). The wafer is then metallised, whereby a full area metal contact is made on the back surface, and a grid-like metal contact made up of fine "fingers" and larger "busbars" is screen-printed onto the front surface using a silver paste. The rear contact is also formed by screen-printing a metal paste, typically aluminum. Usually this contact covers the entire rear side of the cell, though in some cell designs it is printed in a grid pattern. The metal electrodes will then require some kind of heat treatment or "sintering" to make Ohmic contact with the silicon. After the metal contacts are made, the solar cells are interconnected in series (and/or parallel) by flat wires or metal ribbons, and assembled into modules or "solar panels". Solar panels have a sheet of tempered glass on the front, and a polymer encapsulation on the back. Some solar cells have textured front surfaces that, like antireflection coatings, serve to increase the amount of light coupled into the cell. Such surfaces can usually only be formed on single-crystal silicon, though in recent years methods of forming them on multicrystalline silicon have been developed. ===Energy conversion efficiency=== Typical module efficiencies for commercially available screen printed multicrystalline solar cells are around 12%. A solar module's energy conversion efficiency, (or just efficiency) is the ratio of the maximum output electrical power divided by the input light power under "standard" test conditions. The "standard" solar radiation (known as the "air mass 1.5 spectrum") has a power density of 1000 watts per square metre. Thus, a typical 1 m² solar panel in direct sunlight will produce approximately 120 watts of peak power. ==Applications and implementations== See the article solar panel for information about applications and implementations of solar cells and panels. ==Cost analysis== The US retail module costs are in the United States dollar3.50 to $5.00/Wp range ([http://www.solarbuzz.com/ see SolarBuzz]). Additional installation costs for a residential rooftop retrofit in California (CA) is around $3.50/Wp or more. So on the low side, installed system costs are about $7.00/Wp in CA, and probably higher in places with less experience. Federal, state, utility, and other subsidies combined pay about half the cost. So CA rule of thumb is that the installed system PV will cost you at the low end, $3.50/Wp. Under net metering, one offsets regular retail utility rate which for CA is about 11 cents/kWh. Knowing installed system costs, amount of sunshine, and the utility rates, one can figure out the years till payback with or without financing costs. Assuming no financing costs and a $6/Wp installed system cost (lower than current $7), one can take sunshine and utility rate information from around the globe and come up with a payback graph such as shown below. The addition of subsidies brings down the years to payback proportionately. For example, if the years to payback were 24 years at $6/Wp, and subsidies brought that down to $3/Wp, the years to payback would be 12. ==Current research== There are currently many research groups active in the field of photovoltaics at universities and research institutions around the world. Much of the research is focussed on making solar cells cheaper and/or more efficient, so that they can more effectively compete with other energy sources, including fossil energy. One way of doing this is to develop cheaper methods of obtaining silicon that is sufficiently pure. Silicon is a very common element, but is normally bound in sand. Another approach is to significantly reduce the amount of raw material used in the manufacture of solar cells. The various thin-film technologies currently being developed make use of this approach to reducing the cost of electricity from solar cells. The invention of conductive polymers, (for which Alan Heeger was awarded a Nobel prize) may lead to the development of much cheaper cells that are based on inexpensive plastics, rather than semiconductor grade silicon. However, all organic solar cells made to date suffer from degradation upon exposure to UV light, and hence have lifetimes which are far too short to be viable. ===Thin-film solar cells=== The next step in reducing the cost of solar cells and panels seems certain to come from thin-film technology. Thin-film solar cells use less than 1% of the raw material (silicon) compared to wafer based solar cells, leading to a significant price drop per kWh. There are many research groups around the world actively researching different thin-film approaches and/or materials. Thin Film solar cells are mainly deposited by Chemical_vapor_deposition from silane gas and hydrogen. This process produces a material without crystalline orientation : amorphous silicon. Depending on the deposition's parameters nanocrystalline silicon can also be obtained. These types of silicon present dandling and twisted bonds, which results in the aparition of deep defects (energy levels in the bandgap) as well as in the deformation of the valence and conduction bands (band tails). This contributes to reduce the Quantum efficiency of Thin-Film solar cells by reducing the number of collected electron-hole pair by incident photon. Amorphous silicon (a-Si) has a higher bandgap (1.7 eV) than crystalline Silicon (c-Si) (1.1 eV), which means it is more efficient to absorb the visible part of the solar spectrum, but it fails to collect an important part of the spectrum : the infrared. As nano crystalline Si has about the same bandgap as c-Si, the two material can be combined by depositing to diodes on top of each other : the tandem cell. The top cell in a-Si absorbs the visible light and leaves the infrared part of the spectrum for the bottom cell in nanocrystalline Si. One particularly promising technology is crystalline silicon thin-films on glass substrates. This technology makes use of the advantages of crystalline silicon as a solar cell material, with the cost savings of using a thin-film approach. From the [http://www.pacificsolar.com.au/ Pacific Solar] website: :"Crystalline Silicon on Glass (CSG) [is] the photovoltaic technology developed by Pacific Solar that is now being commercialised by CSG Solar. A very thin layer of silicon, less than two micrometres thick, is deposited directly onto a glass sheet whose surface has been roughened by applying a layer of tiny glass beads. The silicon is not crystalline when first deposited, but becomes so after heat treatment in an oven. The resulting layer is processed using lasers and ink-jet printing techniques to form the electrical contacts needed to get the solar-produced electricity out of the thin silicon film." In 2005, a full-scale production factory is being built in Germany to commercialise this technology. CSG Solar expects to release its first product for sale in 2006. Each solar module will have a rated power exceeding 100 watts and will be cheaper than competing solar panels. Another interesting aspect of thin-film solar cells is the possibility to deposit the cells on all kind of materials, including flexible substrates (PET for example), which opens a new dimension for new applications. ===Exotic materials=== For special applications, such as Deep Space 1, high-efficiency cells can be made from gallium arsenide by molecular beam epitaxy. Such cells have many diodes in series, each with a different band gap energy so that it absorbs its share of the electromagnetic spectrum with very high efficiency. Triple junction solar cell have (as the name suggest) 3 diodes layered on top of each other, each absorbing a different spectrum of light, efficiency as high as 28% have been achieved. The multiple junction solar cells may be very efficient, but are prohibitively expensive to make. Cost-effective use of these cells could be achieved with concentrating optics so that less of the array consists of actual semiconductor devices. Experimental non-silicon solar panels can be made of carbon nanotubes or quantum dots embedded in a special conductive polymers. These have only one-tenth the efficiency of silicon panels but could be manufactured in ordinary factories, not clean rooms which should lower the cost. Some of the most efficient solar cell materials are cadmium tellurium (CdTe) and copper indium gallium selenium (CIGS). Unlike the basic silicon solar cell, which can be modelled as a simple p-n junction (see under semiconductor), these cells are best described by a more complex heterojunction model. The best efficiency of a bare solar cell as of April 2003 was 16.5% [Dr IM Dharmadasa, Sheffield Hallam University, UK]. Higher efficiencies (around 30%) can be obtained by using optics to concentrate the incident light. Polymer or organic solar cells are built from ultra thin layers (typically 100 nm) of organic semiconductors such as polyphenylene vinylene and fullerene. The p/n junction model is only a crude description of the functioning of such cells, as electron hopping and other processes also play a crucial role. They are potentially cheaper to manufacture than silicon or inorganic cells, but efficiencies achieved to date are low and cells are highly sensitive to air and moisture, making commercial applications difficult. In the reverse mode, the technology has however already successfully been commercialised in organic LEDs and organic displays, also called polymer displays. Graetzel cells (sometimes called photoelectrochemical cells) have been around for two decades or so. A p/n junction is used here too in the form of a doped solid (normally titanium dioxide) in contact with a solid or liquid electrolyte (for example CuI). In contrast to the classical solar cell not the semiconductor but a dye placed at the p/n interface is used for absorption of radiation, mimicking the process of photosynthesis. As a result, this type of cell allows a more flexible use of materials. Like organic solar cells, Graetzel cells can be manufactured under "dirty" conditions. Commercial applications have failed to appear due to the fast degradation occurring in Graetzel cells. ==Solar cells and energy payback== There is a common but mistaken notion that solar cells never produce more energy than it takes to make them. While the expected working lifetime is on the order of 40 years, the energy payback time of a solar panel is anywhere on the order of 2 to 30 years depending on the type and where it is used, see Net energy gain. ==See also== *Photodiode *Timeline of solar energy *solar power *solar panel *autonomous building *renewable energy *Future energy development *photovore ==External links== * Pennicott, Katie, "''[http://physicsweb.org/article/news/5/12/2 Solar cell edges towards endless energy]''". 7 December 2001. PhysicsWeb. * [http://dcwww.epfl.ch/lpi/solarcellE.html Dye Sensitized Solar Cells] (DYSC) based on Nanocrystalline Oxide Semiconductor Films * News searching: [http://news.google.com/news?hl=da&q=%22Solar+Cell%22 Solar Cell], [http://news.google.com/news?hl=da&q=Photovoltaic Photovoltaic] *[http://www.atse.org.au/index.php?sectionid=391 Historical: Photovoltaic Solar Energy Conversion: An Update] *[http://www.lbl.gov/msd/PIs/Walukiewicz/02/02_8_Full_Solar_Spectrum.html Wladek Walukiewicz, Materials Sciences Division, Berkeley Lab.: Full Solar Spectrum Photovoltaic Materials Identified.] Quote: "... Maximum, theoretically predicted efficiencies increase to 50%, 56%, and 72% for stacks of 2, 3, and 36 junctions with appropriately optimized energy gaps, respectively...." *[http://news.cnet.com/investor/news/newsitem/0-9900-1028-21199489-0.html CNET: 5/12/03 SunPower Announces World's Most Efficient, Low-Cost Silicon Solar Cell] Quote: "...[http://www.nrel.gov/ The National Renewable Energy Laboratory (NREL)] has verified 20.4 percent conversion efficiency for the A-300...." *[http://www.sunpowercorp.com/html/Products/Datasheets/A-300/A-300.pdf SunPower A-300 (pdf)], [http://www.sunpowercorp.com/ SunPower] *[http://www.sciam.com/article.cfm?chanID=sa003&articleID=0004C094-02CC-1CD0-B4A8809EC588EEDF 29 March 2002, Scientists Create New Solar Cell] Quote: "...semiconducting plastic material known as P3HT... 1.7 percent for sunlight..." *[http://www.newscientist.com/news/news.jsp?id=ns99993380 15 February 03, 'Denim' solar panels to clothe future buildings] Quote: "... Unlike conventional solar cells, the new, cheap material has no rigid silicon base..." *[http://www.californiasolarco.com/power-systems-photo-gallery.html Residential Solar Power Systems - Photo Gallery] *[http://www.sma-america.com/installations.html Examples of Photovoltaic Systems ] *[http://science.howstuffworks.com/solar-cell.htm How Solar Cells Work ] *[http://www.azonano.com/news.asp?newsID=548 azonano.com: Carbon Nanotube Structures Could Provide More Efficient Solar Power for Soldiers] 28 February 2005 *[http://www.newton.mec.edu/Brown/TE/HOT/TIMELINES/SOLAR/solar_timeline.html Solar energy timeline] ===Yield data=== * http://www.tectosol.staticip.de/index_en.htm electricity yield of a solar power system * http://www.sunny-portal.de Yield Portal for solar power system users ===Theory=== *[http://www.nrel.gov/buildings/pv/factsheets.html National Renewable Energy Laboratory (NREL): Photovoltaics for Buildings: PV Technology for the Home Factsheets] *[http://www.nrel.gov/research/pv/docs/pvpaper.html 1993, National Renewable Energy Laboratory (NREL): Photovoltaics: Unlimited Electrical Energy From the Sun] BrokenLink *[http://www.cefetba.br/fisica/NFL/PBCN/solar/solardeu.html#ideal Electrical models of solar cells] ===Cost Benefit=== *[http://rredc.nrel.gov/solar/codes_algs/PVWATTS/pvwatts_index.html PVWATTS - A Performance Calculator for Grid-Connected PV Systems] ===Do-it-yourself=== ====PEC (Photo Electro Chromic)==== *[http://www.chemistry.ucsc.edu/teaching/Winter98/Chem1B/photo/Solar_Kit_Word_6.html How to Build Your Own Solar Cell] *[http://www.solideas.com/solrcell/cellkit.html DIY (Do It Yourself): Nanocrystalline Dye-Sensitized Solar Cell Kit] Quote: "... sunlight-to-electrical energy conversion efficiency is between 1 and 0.5 %..." ====Cuprous oxide solar cells==== *[http://www.scitoys.com/scitoys/scitoys/echem/echem2.html#solarcell Make a solar cell in your kitchen], [http://www.scitoys.com/scitoys/scitoys/echem/echem3.html#sflatpanel A flat panel solar battery] *[http://www.zetatalk.com/energy/tengy17f.htm From: How to Build a Solar Cell That Really Works by Walt Noon] ===Indexes=== *Open Directory Project: [http://www.dmoz.org/Business/Energy_and_Environment/Renewable/Solar/ Solar] ===Newsgroups=== *[http://groups.google.com/groups?q=alt.solar.photovoltaic alt.solar.photovoltaic] ===Patents=== * [http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=2402662.WKU.&OS=PN/2402662&RS=PN/2402662 US2402662] -- ''Light sensitive device'' -- R. S. Ohl * [http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=1289369.WKU.&OS=PN/1289369&RS=PN/1289369 US1289369] -- ''Method of increasing the capacity of photosenitive electrical cells'' Electrical components Energy conversion Renewable energy
Solar cell
Solar cells do not use the photoelectric effect. Solar cells are made of semiconductor material, the incoming light (photons) move electrons from the valence band across the band gap to the conduction band. The resulting electron-hole pairs are separated by the internal electrical field of the p-n-junction. In this way different charges on the two electrodes of the solar cell are created, which can be used to drive an current through a wire. ---- Should this be merged with solar panel? User:Rmhermen 17:10, Feb 28, 2004 (UTC) * Definitely. I think the scientific/functional details should be under solar cell and the application and anecdotal information should be under solar panel. Another deficiency is that solar panel completely ignores those (also active solar panels whose purpose is to collect thermal energy only. -User:RatOmeter == Article needs latitude application info... == The Wikipedia is, of course, updatable. A valuable section to add to this article on photovoltaic-energy generation would be something that - even in a very general way - addresses the issue of p.v. applications in various latitudes (or latitude zones). The general info about cents/kwh is fine, but so unspecific as to give the reader little idea of where the technology sits at present. We get the idea that it has application in "desert" areas, but in the English-speaking world, few people live in this climatic zone. Progress in the p.v. field must be pretty continual, so a more general idea of power generation and power storage issues for different latitudes could be further detailed as relevant advances emerge. == expand == "When photons hit the silicon plates, electron-hole pairs are created (with a probability depending on the quantum efficiency) and separated. The electric field across the p-n junction draws the electrons and holes in opposite directions, and they then diffuse to the front and back contacts." :how are the pairs created? can we have more details about this process? - User:Omegatron 22:52, Jan 8, 2005 (UTC) == production energy == I have heard that more energy goes into producing solar cells than they will ever produce. I am doubtful of this but have been unable to find and information on the production of solar cells. Does anyone know anything about the amount of energy required to produce a solar cell or array? :I'm pretty sure this isn't true, though I can't find a reference. I've heard that it takes n years or so for an installation to pay for itself, and they last >n or more, so if money is energy (it isn't, but close enough), then you are making net profit in energy. - User:Omegatron 18:07, Jan 21, 2005 (UTC) :I have some numbers for poly-Si but they are quite old (1994). If someone has better ones, please add them. 16.0 kWh are needed for each Watt-peak installed (of the finished module). This is split in the following way : Production of Silicon ingot 7.9 kWh / production of the wafers 2.9 kWh / Cells production 3.7 kWh / Grouping cells in modules 1.5 kWh. Assuming an efficiency of 14%, 7 Years are needed in central Switzerland to make an installation profitable. Of course it takes a shorter amount of time in sunnier areas at lower latitudes. Furthermore, due to the fact that those figures are pretty old, today's real values are probably lower. Furthermore, thin film technology allows the productions of cells for less energy. Therefore, it is wrong to say that it takes more energy to produce a solar pannel than it will ever give. User:Glaurung 20:59, 21 Jan 2005 (UTC) == LEDs and solar cells == Solar cells seem at first to be kind of a dual of LEDs. They take in light and convert it back to electrical energy. But they aren't really opposites: LED: Has a fixed voltage drop across it. Voltage must increase above this drop for LED to emit light. After switched on, the ''current'' sent through the LED (determined by the resistance of other components in series) determines the intensity of the LED, while the voltage stays constant. Solar cell: Voltage output varies with intensity of light? I think so, anyway... :TMW: No!! The output CURENT of a solar cell depends on the incident light intensity. The voltage remains (alomst) constant. You are right though, they ARE almost like large-area light-emitting diodes operating in reverse. ::So is this just an effect of low load impedance? What kind of source impedance do they have? - User:Omegatron LED: Emits only a single frequency. Voltage drop across LED depends on frequency of light emitted? (Which depends on construction. You can't change the voltage on the fly and change the color emitted (although that would be pretty cool...)) Solar cell: Converts different frequencies into electrical energy? I imagine it has a bandpass response, but I'm not sure. Can we get a physics explanation of the similarities and differences between these processes? - User:Omegatron 15:20, Mar 8, 2005 (UTC) :Hmm... :"Photovoltaic modules (solar panel) convert all wavelengths within the visible light spectrum to DC electricity but are optimized for the wavelengths that occur most commonly. For peak performance a solar module should face the brightest part of the sky. Most modules are installed at a fixed azimuth and tile angle in order to maximize their energy output. :Solar modules are made up of "cells" manufactured from various forms of silicon. The greater the light intensity falling on the cells the greater the current produced (light intensity and output current are proportional). However, the voltage produced is not proportional to light intensity but rises considerably in low light ensuring that charging can take place." - User:Omegatron 15:29, Mar 8, 2005 (UTC) :We should cover the information on this page. http://www.solarserver.de/wissen/photovoltaik-e.html#char - User:Omegatron 15:34, Mar 8, 2005 (UTC) ---- == Full Cost Comparison? == The comparison of the cost per kw/hr between p.v. and nuclear is only a good figure, really, if it includes some accounting of the cost of building the installation and maintaining it (including replacement-component cost) amortized over the expected lifetime of the installation. As an analogy, if you look at the cost of running two different cars for a year with gasoline of equal cost per litre or gallon, over the same roads and streets, with the same number of passengers - but leave out the comparative cost of acquiring and maintaining the two vehicles - it's not a complete comparison. In other words, a comparison could be made more informatively. Hasn't Rocky Mountain Institute (Amory Lovins, et al.) developed the kind of figures I'm referring to? If not, someone else may have. - J.R. == Major Revamp == 10 April 2005 (darkside2010). I am in the process of rewriting and revamping this page "solar cell". The page needs a lot of work. Some of the information on it is just plain wrong, and other things are either poorly described or simply overlooked. (Sorry, no offense intended to the people who already contributed to this page). I am currently doing my PhD in solar cell research, so I think I am fairly well qualified to write about solar cells. I like the page "solar panel". It should definitely not be merged with "solar cell". However, I am a bit concerned about the page "photovoltaic cell". I feel it would be best to make it into a stub which directs readers to "solar cell". I don't see any distinction between photovoltaic cells and solar cells, and solar cell is by far the more commonly used term. I don't see any advantage to duplicating information on these pages. User:Darkside2010 :I agree : one should merge the info on the photovoltaic cell to this page and create a redirect. I also agree that this page needs some clean up and organization. Perhaps we should discuss about a better paragraph layout, starting by explaining the absorption of photons by a semi-c and then move to the main realisation of solar cells (mono, poly, thin film, III-V, Greatcell) User:Glaurung 11:30, 11 Apr 2005 (UTC) ::Ok, I am happy to merge the information on photovoltaic cell (which is not much) to this page, and then create a stub, redirecting here. Do you really think that photovoltaic cell should "automatically" redirect to solar cell? I feel it is better to say a few words about the origin of the term "photovoltaic" on that page, and then redirect readers here. I know that a lot of wikipedians don't like stubs, but I think that an informative stub is better than an automatic redirect. ::Certainly, lets discuss a better paragraph layout for solar cell. As you can see, I went ahead and made some rearrangement of the structure of the article, but I'm sure there is still plenty of room for improvement. ::On another note, I intend on replacing the lonely picture on this page. I think that for the top of the article, a photograph of a solar cell would be more appropriate, and that drawings such as the one currently there would be better in the "workings" section. I intend making a few copyright-free drawings of electron-hole pair creation, the equivalent circuit of a solar cell, etc for the page as I find time in the coming weeks. User:Darkside2010 13:59, 11 Apr 2005 (UTC) ::Another thing is, there are, to my mind, far too many external links on this page. I haven't taken the time to follow any of them yet, But I'm sure they are not all neccessary. :::Photovoltaic cell should redirect here, and any explanatory text should be in the first paragraph of this. - User:Omegatron 15:45, Apr 11, 2005 (UTC) ::::Ok, Omegatron, if that is the way things are done, then let it be so. I have rewritten the first paragraph to include a discussion of the etymology of the term ''photovoltaic''. Perhaps there should be a page called Photovoltaics, though, to describe all the institutions and research departments who work in that field. I noticed that Photovoltaic(s) redirects to solar cell, and i would argue that these are not actually the same thing. User:Darkside2010 13:31, 12 Apr 2005 (UTC) :::::That's a great idea. I'm no expert in the field, so I don't know which are discrete ideas. - User:Omegatron 18:23, Apr 12, 2005 (UTC) ::::Glaurung, Perhaps rather than discuss a new paragraph layout for this page, let's just edit the page itself, and see where the wikipedia process takes us. I mean, we all have the same goal here, right? To produce the best encylopaedic article of a solar cell in the world! So lets not spend our time and energy discussing, but rather making the page the best it can be. If you have an idea for a better structure, just change it. After all, that's what wikipedia is all about, yeah? User:Darkside2010 13:31, 12 Apr 2005 (UTC) I pulled that image which was at the top of the article, and replaced it with one which, in my opinion, ''increases'' understanding for the general reader as to what a solar cell is. The old picture was a baffling barrage of different coloured arrows which, without a detailed accompanying explanation (which was absent), added no understanding as to what a solar cell is, or how it works. At least with the photograph of the solar cell, people can say "Oh, yeah, I have seen one of those somewhere". User:Darkside2010 16:18, 12 Apr 2005 (UTC) :Rather than replace it, just move it down the page and ''add'' the new image. then if you find a better image that covers the same idea as the confusing arrows, use ''that'' to replace them. - User:Omegatron 18:23, Apr 12, 2005 (UTC) :: I reinserted the image where the photon absorption is explained. But I will try to find a better one (or maybe even make one). This one looks nice, but is not really clear. I think a simplier 2D schema would be better. User:Glaurung 20:01, 12 Apr 2005 (UTC) == Tesla == ''Nikola Tesla patented a primitive solar cell in 1901 when he received the patent [http://patimg2.uspto.gov/.piw?Docid=00685957&homeurl=http%3A%2F%2Fpatft.uspto.gov%2Fnetacgi%2Fnph-Parser%3FSect1%3DPTO1%2526Sect2%3DHITOFF%2526d%3DPALL%2526p%3D1%2526u%3D%2Fnetahtml%2Fsrchnum.htm%2526r%3D1%2526f%3DG%2526l%3D50%2526s1%3D685957.WKU.%2526OS%3DPN%2F685957%2526RS%3DPN%2F685957&PageNum=&Rtype=&SectionNum=&idkey=3D035366337E US685957] (Apparatus for the Utilization of Radiant Energy).'' :I believe this is a pretty controversial statement. - User:Omegatron 23:09, May 19, 2005 (UTC) :Yeah, I just read the patent and it's certainly not a solar cell. It appears to collect energy from collisions of ionizing radiation via the photoelectric effect, not light. - User:Omegatron 00:12, May 20, 2005 (UTC) ::but light causes the photoelectric effect to happen so would have powered the apparatus I believe. User:Greenpowered 21:46, 20 May 2005 (UTC) :::I'm reading about this a lot right now and learning a lot of new things. The work function for metal is too high (4.5 eV) for light to cause a photoelectric effect. The radiation hitting it has to be above near UV in frequency. This is really neat. The photoelectric effect actually causes spacecraft to acquire a positive charge and levitates dust particles on the moon! - User:Omegatron 21:50, May 20, 2005 (UTC) :::I'm going to add some details to Talk:Photoelectric effect#Tesla / radiant energy - User:Omegatron 00:12, May 21, 2005 (UTC) == A thought toward the major revamp == The current article says, in one place: "Typical module efficiencies for commercially available screen printed poly-crystalline solar cells are around 17%." What might be a useful and informative addition would be discussion - or preferably a ''graph'' - explaining what the increase in efficiency has been (starting from some point in, say, the 1960s or early '70s) leading up to the currently typical efficiency achievement. It probably goes without saying that this would suggest the positive future of solar-cell applications. I know a lot of people who have languished and rather given up on the potentials of pv, but it is not justified. A discussion or graph could bring home the trajectory toward wide practicality. - J.R. :A graph would indeed be a good idea. If you have the data at hand showing the evolution of efficiency versus time, feel free to make one. User:Glaurung 06:02, 20 May 2005 (UTC) ::I ''don't'' have the data - wish I did. I put the suggestion out for Darkside, since he is working on a PhD on the subject and wants to re-write the Wiki article. I thought he might be in touch with the stats. I do know that at times I've read about current typical solar cells being so much more (i.e., several times more) efficient than in the early 1970s, which was when I, as a kid, first became aware of the exciting idea of "solar energy." I believe a lot of people became aware of the general potentials of pv in the 1970s, which is one reason why I feel the comparative stats would be valuable. :::I have seen books that state it's up to 35% now so a graph could show a lot. Also I have heard solar power was thousands of dollars a watt over 20 years ago, and is now under $4.00. A graph showing the cost prices dropping over the years would really show the doubters solar is the power of the future. User:Greenpowered 21:50, 20 May 2005 (UTC) ==Cost Analysis== I removed this sentence from Cost Analysis, ''The present cost benefit to consumers is pretty bad.'' I do not think that is fair or correct. If you are in an area with subsidies it is not pretty bad, it's pretty good! Even if your not it can still easily be the best option. I know someone the utility companies wanted to charge over $100,000.00 to run grid electricity to his home and solar power was extremely cost effective. And there are even ways to make it cost effective if you are hooked up to the grid and in an area without subsidies. Also the subsidies for solar are compared to the cost of the grid. It does not mention that there are more subsidies on that end then in solar. User:Greenpowered 21:43, 20 May 2005 (UTC)
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