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The most common configuration of this device, the first generation photovoltaic, consists of a large-area, single layer P-n Junction Diode , which is capable of generating usable Electrical Energy from light sources with the Wavelengths of solar light. These cells are typically made using Silicon . However, successive generations of photovoltaic cells are currently being developed that may improve the photoconversion efficiency for future photovoltaics. The '''second generation''' of photovoltaic materials is based on multiple layers of p-n junction diodes. Each layer is designed to absorb a successively longer Wavelength of light (lower energy), thus absorbing more of the solar spectrum and increasing the amount of electrical energy produced. The '''third generation''' of photovoltaics is very different from the other two, and is broadly defined as a semiconductor device which does not rely on a traditional p-n junction to separate photogenerated charge carriers. These new devices include dye sensitized cells, organic polymer cells, and quantum dot solar cells. 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 and Water pumping applications. Assemblies of solar cells (in the form of modules or Solar Panels ) on building roofs can be connected through an Inverter to the electricity grid in a Net Metering arrangement. HISTORY See Also: Timeline of solar cells The term "photovoltaic" comes from the Greek ''photos'' meaning "light", and the name of the Italian physicist Volta , after whom the Volt (and consequently Voltage ) are named. It means literally ''of light and electricity''. The photovoltaic effect was first recognised in 1839 by French physicist Alexandre-Edmond Becquerel . However it was not until 1883 that the first solar cell was built, by Charles Fritts , who coated the Semiconductor Selenium with an extremely thin layer of Gold to form the junctions. The device was only around 1% efficient. Russell Ohl patented the modern solar cell in 1946 ( US2402662 , "''Light sensitive device''"). Sven Ason Berglund had a prior patent concerning methods of increasing the capacity of photosensitive cells. APPLICATIONS AND IMPLEMENTATIONS See Also: solar panel Solar cells are often electrically connected and encapsulated as a module, termed a Solar Panel . Solar panels often have a sheet of glass on the front (sun up) side with a resin barrier behind, allowing light to pass while protecting the semiconductor Wafers from the elements ( Rain , Hail , etc). Solar cells are also usually connected in Series in modules, creating an additive Voltage . THEORY Simple explanation # Photon s in Sunlight hit the solar panel and are absorbed by semiconducting materials, such as Silicon . # Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce Electricity . The complementary positive charges that are also created (like bubbles) are called Holes and flow in the direction opposite of the electrons in a silicon solar panel. # An array of solar panels converts solar energy into a usable amount of Direct Current (DC) electricity. Optionally: # The DC current enters an Inverter . # The inverter turns DC electricity into 120 or 240-volt AC (alternating current) electricity needed for home appliances. # The AC power enters the utility panel in the house. # The electricity is then distributed to appliances or lights in the house. Photogeneration of Charge Carriers When a Photon hits a piece of silicon, one of three things can happen: # 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 photon can reflect off the surface - this (generally) happens if the photon energy is greater than the bandgap energy of silicon. # the photon can be absorbed by the silicon - this happens if the photon energy is within the bandgap energy of silicon. Note that if a photon has ''an integer multiple'' of bandgap energy, it can create more than one electron-hole pair. However, this effect is usually not significant in solar cells. The "integer multiple" part is a result of Quantum Mechanics and the quantization of energy. 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 neighboring 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 need only have greater energy than that of the band gap in order 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. Charge Carrier Separation There are two main modes for charge carrier separation in a solar cell: #drift of carriers, driven by an electrostatic field established across the device #diffusion of carriers from zones of high carrier concentration to zones of low carrier concentration (following a gradient of electrochemical potential). In the widely used ''p-n junction'' designed solar cells, the dominant mode of charge carrier separation is by ''drift''. However, in ''non-p-n junction'' designed solar cells (typical of the third generation of solar cell research such as dye and polymer thin-film solar cells), a general electrostatic field has been confirmed to be absent, and the dominant mode of separation is via charge carrier ''diffusion''. The p-n junction See Also: semiconductor The most commonly known solar cell is configured as a large-area P-n Junction made from silicon. As a simplification, one can imagine bringing a layer of n-type silicon into direct contact with a layer of p-type silicon. In practice, p-n junctions of silicon solar cells are not made in this way, but rather, by diffusing an n-type dopant into one side of a p-type wafer (or vice versus). If a piece of p-type silicon is placed in intimate contact with a piece of n-type silicon, then a Diffusion of electrons occurs 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. The diffusion of carriers does not happen indefinitely however, because of an Electric Field which is created by the imbalance of charge immediately either side of the junction which this diffusion creates. The electric field established across the p-n junction creates a Diode that promotes Current to flow in only one direction across the junction. Electrons may pass from the n-type side into the p-type side, and holes may pass from the p-type side to the n-type side. This region where electrons have diffused across the junction is called the Depletion Region because it no longer contains any mobile charge carriers. It is also known as the "space charge region". Connection to an external load 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. Electrons that 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. Here, they recombine with a hole that 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. SOLAR CELL EFFICIENCY FACTORS Maximum Power Point A solar cell may operate over a wide range of Voltages (V) and Currents (I). By increasing the resistive load (voltage) in the cell from zero (indicating a ''short circuit'') to infinitely high values (indicating an ''open circuit'') one can determine the maximum power point (the maximum output electrical power, Vmax x Imax; or ''Pm'', in W). Energy Conversion Efficiency A solar cell's ''energy conversion efficiency'' (, "eta"), is the percentage of power converted (from absorbed light to electrical energy) and collected, when a solar cell connected to an electrical circuit. This term is calculated using the ratio of ''Pm'', divided by the input light '' Irradiance '' under "standard" test conditions (''E'', in W/m2) and the ''surface area'' of the solar cell (''Ac'' in m2). At solar noon on a clear March or September Equinox day, the solar radiation at the equator is about 1000 W/m2. Hence, the "standard" solar radiation (known as the "air mass 1.5 spectrum") has a power density of 1000 Watt s per square Meter . Thus, as 12% efficiency solar cell having 1 m2 of surface area in full sunlight at solar noon at the equator during either the March or September Equinox will produce approximately 120 watts of peak power. Fill Factor Another defining term in the overall behavior of a solar cell is the the '' Fill Factor '' (''FF''). This is the ratio of the ''maximum power point'' divided by the ''open circuit voltage'' (''Voc'') and the ''short circuit current'' (''Isc''): Quantum Efficiency '' Quantum Efficiency '' refers to the percentage of ''absorbed photons'' that produce ''electron-hole pairs'' (or ''charge carriers''). This is a term intrinsic to the ''light absorbing material'', and not the cell as a whole (which becomes more relevant for ''thin-film'' solar cells). This term should not be confused with energy conversion efficiency, as it does not convey information about the power collected from the solar cell. Comparison of Energy Conversion Efficiencies See Also: Solar panel Silicon solar cell efficiencies vary from 6% for amorphous silicon-based solar cells to 30% or higher with multiple-junction research lab cells. Solar cell energy conversion efficiencies for commercially available ''mc-Si'' solar cells are around 12%. The highest efficiency cells have not always been the most economical -- for example a 30% efficient multijunction cell based on exotic materials such as gallium arsenide or indium selenide and produced in low volume might well cost ''one hundred times'' as much as an 8% efficient amorphous silicon cell in mass production, while only delivering a little under ''four times'' the electrical power. To make practical use of the solar-generated energy, the electricity is most often fed into the electricity grid using inverters (grid-connected PV systems); in stand alone systems, batteries are used to store the electricity that is not needed immediately. A common method used to express economic costs of electricity generating systems is to calculate a price per delivered kilowatt-hour (kWh). The solar cell efficiency in combination with the available irradiation has a major influence on the costs, but generally speaking the overall system efficiency is important. Using the commercially available solar cells (as of 2005) and system technology leads to system efficiencies between 5 and 15%. As of 2005, electricity generation costs ranged from ~ 50 eurocents/kWh (0.60 US$/kWh) (central Europe) down to ~ 25 eurocents/kWh (0.30 US$/kWh) in regions of high solar irradiation. This electricity is generally fed into the electrical grid on the customer's side of the meter. The cost can be compared to prevailing retail electric pricing (as of 2005), which varied from between 0.04 and 0.50 US$/kWh worldwide. The chart at the right illustrates the various commercial large-area module energy conversion efficiencies and the best laboratory efficiencies obtained for various materials and technologies. 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 around 40 years, the energy payback time of a solar panel is anywhere from 1 to 30 years (usually under five) depending on the type and where it is used (see net energy gain). This means solar cells are net energy producers and can "reproduce" themselves (from 6 to more than 30 times) over their lifetime. For details see [http://jupiter.clarion.edu/~jpearce/Papers/netenergy.pdf Net Energy Analysis For Sustainable Energy Production From Silicon Based Solar Cells.] LIGHT ABSORBING MATERIALS All solar cells require a ''light absorbing material'' contained within the cell structure to absorb photons and generate electrons via the ''photovoltaic effect''. The materials used in solar cells tend to have the property of preferentially absorbing the wavelengths of solar light that reach the earth surface; however, some solar cells are optimized for light absorption beyond Earth's atmosphere as well. Light absorbing materials can often be used in ''multiple physical configurations'' to take advantage of different light absorption and charge separation mechanisms (listed in alphabetical order). Many currently available solar cells are configured as bulk materials that are subsequently cut into wafers and treated in a "top-down" method of synthesis (silicon being the most prevalent bulk material). Other materials are configured as '''thin-films''' (inorganic layers, organic dyes, and organic polymers) that are deposited on '''supporting substrates''', while a third group are used as '''quantum dots''' (electron-confined nanoparticles) embedded in a supporting matrix in a "bottom-up" approach. Silicon remains the only material that is well-researched in both ''bulk'' and ''thin-film'' configurations. Bulk These ''bulk'' technologies are often referred to as wafer-based manufacturing. In other words, in each of these approaches, self-supporting wafers between 180 to 240 micrometers thick are processed and then soldered together to form a solar cell module. A general description of silicon wafer processing is provided in ''Manufacture and Devices''. Germanium Germanium is also being investigated as a light absorbing material. Germanium has a smaller band gap than silicon, making it a better material for absorption of longer wavelengths of light (infrared). Wafers can be useful as a multi-layered thin film substrate in this case. Silicon By far, the most prevalent ''bulk'' material for solar cells is crystalline Silicon (abreviated as a group as ''c-Si''). Bulk silicon is separated into multiple categories according to crystallinity and crystal size in the resulting ingot, ribbon, or wafer. See Also: silicon #''monocrystalline silicon'' (mc-Si): often made using the Czochralski Process . Single-crystal wafer cells tend to be expensive, and because they are cut from cylindrical ingots, do not completely cover a square solar cell module without a substantial waste of refined silicon. Hence most ''mc-Si'' panels have uncovered gaps at the corners of four cells. #''Poly- or multicrystalline silicon'' (poly-Si): made from cast square ingots - large blocks of molten silicon carefully cooled and solidified. These cells are less expensive to produce than single crystal cells but are less efficient. #''Ribbon silicon'': formed by drawing flat thin films from molten silicon and having a multicrystalline structure. These cells have lower efficiencies than poly-Si, but save on production costs due to a great reduction in silicon waste, as this approach does not require sawing from ingots. Thin-films The various ''thin-film'' technologies currently being developed reduce the amount (or mass) of light absorbing material required in creating a ''solar cell''. This can lead to reduced processing costs from that of bulk materials (in the case of silicon thin films) but also tends to reduce ''energy conversion efficiency''. but many multi-layer thin films have efficiencies above those of bulk silicon wafers. CIGS CiGS are multi-layered thin-film composites. The abbreviation stands for Copper Indium Gallium Selenide . 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 thin-film solar cell as of December 2005 was 19.5% with CIGS. Higher efficiencies (around 30%) can be obtained by using optics to concentrate the incident light. CIS CIS is an abbreviation for general chalcogenide films of Cu(InxGa1-x)(SexS1-x)2. While these films can achieve 11% efficiency, their manufacturing costs are quite high. CdTe Cadmium Telluride is an efficient light absorbing material for thin-film solar cells. However, Cd is also regarded as a toxic Heavy Metal in the USA, reducing the incentive for development in that country. Conductive polymers Polymer Solar Cells , also called Organic solar cells, are built from thin films (typically 100 nm) of Organic Semiconductors such as Polyphenylene Vinylene and Carbon Fullerenes . Energy conversion efficiencies achieved to date using conductive polymers are very low. GaAs For special applications, such as Deep Space 1 , high-efficiency cells can be made from multiple thin films of Gallium Arsenide , made 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 cells have three diodes layered on top of each other, each absorbing a different spectrum of light, and efficiencies as high as 35.2% 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, as in the recent experimental Sunflower {Link without Title} array. Light Absorbing Dyes See Also: Dye-sensitized solar cells Typically a Ruthenium metalorganic dye (Ru-centered) used as a monolayer of light-absorbing material. The dye-sensitized solar cell depends on a mesoporous layer of Nanoparticulate Titanium Dioxide to greatly amplify the surface area (200-300 m2/gram TiO2, as compared to approximately 10 m2/gram of flat single crystal). The photogenerated electrons from the ''light absorbing dye'' are passed on to the ''n-type'' TiO2, and the holes are passed to an electrolyte on the other side of the dye. The circuit is completed by a redox couple in the electrolyte, which can be liquid or solid. This type of cell allows a more flexible use of materials, and typically are manufactured by screen printing, with the potential for lower processing costs than those used for ''bulk'' solar cells. However, the dyes in these cells also suffer from degradation under heat and UV light, and the cell casing is difficult to seal due to the solvents used in assembly. In spite of the above, this is a popular emerging technology with some commercial impact forecasted within this decade. Silicon Silicon thin-films are mainly deposited by Chemical Vapor Deposition (typically plasma enhanced (PE-CVD)) from Silane gas and Hydrogen gas. Depending on the deposition's parameters, this can yield: # Amorphous Silicon (a-Si or a-Si:H) # Protocrystalline Silicon or # Nanocrystalline Silicon (nc-Si or nc-Si:H). These types of silicon present dangling and twisted bonds, which results in deep defects (energy levels in the bandgap) as well as deformation of the valence and conduction bands (band tails). The solar cells made from these materials tend to have lower ''energy conversion efficiency'' than ''bulk'' silicon, but are also less expensive to produce. The Quantum Efficiency of thin film solar cells is also lower due to reduced number of collected charge carriers per incident photon. Amorphous silicon 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 the Infrared portion of the spectrum. As nc-Si has about the same bandgap as c-Si, the two material can be combined in thin layers, creating a layered cell called a '''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, as pioneered by the Sanyo HIT cell. A patented silicon thin film technology being developed by XsunX, Inc, for building integrated photovoltaics (BIPV) in the form of semi-transparent solar cells which can be applied as window glazing. These cells function as window tinting while generating electricity. Quantum Dots These dimensionally-confined structures make use of some of the same light absorbing materials from the thin-film configuration, but are suspended in a supporting matrix of conductive polymer or mesoporous metal oxide. SILICON SOLAR CELL DEVICE MANUFACTURE Because solar cells are semiconductor devices, they share many of the same processing and manufacturing techniques as other semiconductor devices such as Computer and Memory Chips . 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 micrometer) slices or wafers. The wafers are usually lightly p-type doped. To make a solar cell from the wafer, a surface diffusion of n-type dopants is performed on the front side of the wafer. This forms a p-n junction a few hundred nanometers 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). 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. The wafer is then metallized, 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. Tempered glass cannot be used with amorphous silicon cells because of the high temperatures during the deposition process. CURRENT RESEARCH ON MATERIALS AND DEVICES See Also: Timeline of solar cells There are currently many research groups active in the field of Photovoltaics in Universities and research institutions around the world. This research can be divided into three areas: making current technology solar cells cheaper and/or more efficient to effectively compete with other energy sources; developing new technologies based on new solar cell architectural designs; and developing new materials to serve as light absorbers and charge carriers. Silicon Processing 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 silica, or Silica Sand . Processing silica (SiO2) to produce silicon is a very high energy process, and more energy efficient methods of synthesis are not only beneficial to the solar industry, but also to industries surrounding silicon technology as a whole. The current industrial production of silicon is via the reaction between carbon (charcoal) and silica at a temperature around 1700 Degrees Celsius . In this process, known as carbothermic reduction, each tonne of silicon (metallurgical grade, about 98% pure) is produced with the emission of about 1.5 tonnes of carbon dioxide. Solid silica can be directly converted (reduced) to pure silicon by electrolysis in a molten salt bath at a fairly mild temperature (800 to 900 degrees Celsius). T. Nohira et al, ‘Pinpoint and bulk electrochemical reduction of insulating silicon dioxide to silicon’, Nat. Mater., 2 (2003) 397.X. B. Jin et al, Electrochemical preparation of silicon and its alloys from solid oxides in molten calcium chloride’, Angew. Chem. Int. Ed., 43 (2004) 733. While this new process is in principle the same as the FFC Cambridge Process which was first discovered in late 1996, the interesting laboratory finding is that such electrolytic silicon is in the form of porous silicon which turns readily into a fine powder, with a particle size of a few micrometres), and may therefore offer new opportunities for development of solar cell technologies. Another approach is also to reduce the amount of silicon used and thus cost, as done by Australian National University in production of their "Sliver" cells, by micromachining wafers into very thin, virtually transparent layers that could be used as transparent architectural coverings. Using this technique, two silicon wafers are enough to build a 140 watt panel, compared to about 60 wafers needed for conventional modules of same power output. Thin-film Processing Thin-film solar cells use less than 1% of the raw material (silicon or other light absorbers) 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. 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. 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. Polymer Processing 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. 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. The conjugated double bond systems in the polymers, which carry the charge, are always susceptible to breaking up when radiated with shorter wavelengths. This is due to the highly bipolar nature of the polymers. Additionally, conductive polymers are highly sensitive to Air and Moisture , making commercial applications difficult. Nanoparticle Processing Experimental non-silicon solar panels can be made of Quantum Heterostructure s, eg. Carbon Nanotubes or Quantum Dots , embedded in Conductive Polymers or mesoporous metal oxides. By varying the size of the quantum dots, the cells can be tuned to absorb different wavelengths. If panels that absorb both visible and infrared spectrums are able to be manufactured, the panels may be able to achieve up to 30 percent efficiencyMcDonald, ''et al.'', 2005. SEE ALSO
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