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Silicon Wafer ]] A solar cell or '''photovoltaic cell''' is a device that converts Light Energy into Electrical energy. Sometimes the term ''solar cell'' is reserved for devices intended specifically to capture energy from sunlight, while the term ''photovoltaic cell'' is used when the light source is unspecified. Fundamentally, the device needs to fulfill only two functions: photogeneration of charge carriers ( Electron s and Holes ) in a Light-absorbing Material , and separation of the charge carriers to a conductive contact that will transmit the electricity (simply put, carrying electrons off through a metal contact into a wire or other circuit). This conversion is called the ''photovoltaic effect'', and the field of research related to solar cells is known as Photovoltaics . Solar cells have many applications. They have long been used in situations where electrical power from the Grid is unavailable, such as in remote area power systems, Earth -orbiting Satellites and space probes, consumer systems, e.g. handheld Calculators or wrist watches, remote Radiotelephone s and Water pumping applications. More recently, they are starting to be used in assemblies of solar modules ( Photovoltaic Array s) connected to the Electricity Grid through an Inverter , often in combination with a Net Metering arrangement. FOUR GENERATIONS OF DEVELOPMENT First 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 Wavelength s of sunlight. These cells are typically made using a Silicon wafer. First generation photovoltaic cells (also known as silicon wafer-based solar cells) are the dominant technology in the commercial production of solar cells, accounting for more than 86% of the solar cell market. Second The second generation of photovoltaic materials is based on the use of thin-film deposits of semiconductors. These devices were initially designed to be high-efficiency, multiple junction photovoltaic cells. Later, the advantage of using a thin-film of material was noted, reducing the mass of material required for cell design. This contributed to a prediction of greatly reduced costs for thin film solar cells. There are currently (2007) a number of technologies/semiconductor materials under investigation or in mass production. Examples include , copper indium selenide/sulfide. Typically, the efficiencies of thin-film solar cells are lower compared with silicon (wafer-based) solar cells, but manufacturing costs are also lower, so that a lower cost per watt can be achieved. Another advantage of the reduced mass is that less support is needed when placing panels on rooftops and it allows fitting panels on light or flexible materials, even textiles. Third Third generation photovoltaics are very different from the previous semiconductor devices as they do not rely on a traditional p-n junction to separate photogenerated charge carriers. These new devices include Photoelectrochemical Cell s, Polymer Solar Cell s, and Nanocrystal Solar Cell s. Fourth Fourth generation Composite photovoltaic technology with the use of polymers with nano particles can be mixed together to make a single multispectrum layer. Then the thin multi spectrum layers can be stacked to make multispectrum solar cells more efficient and cheaper based on polymer solar cell and multi junction technology by NASA used on Mars missions. The layer that converts different types of light is first, then another layer for the light that passes and last is an infra-red spectrum layer for the cell - thus converting some of the heat for an overall solar cell composite. Companies working on fourth generation photovoltaics include Xsunx, Konarka Technologies, Inc. , Nanosolar , Dyesol and Nanosys . Research is also being done in this area by the USA 's National Renewable Energy Laboratory (http://www.nrel.gov/). HISTORY See Also: Timeline of solar cells The term "photovoltaic" comes from the physicist Volta , after whom the measurement unit Volts are named. The term "photo-voltaic" has been in use in English since 1849.1 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 (, "''Light sensitive device''"). Sven Ason Berglund had a prior patent concerning methods of increasing the capacity of photosensitive cells. The modern age of solar power technology arrived in 1954 when Bell Laboratories, experimenting with semiconductors, accidentally found that silicon doped with certain impurities was very sensitive to light. This resulted in the production of the first practical solar cells with a sunlight energy conversion efficiency of around 6 percent. This milestone created interest in producing and launching a Geostationary communications Satellite by providing a viable power supply. Russia launched the first artificial satellite in 1957, and the United States' first artificial satellite was launched in 1958. Russian Sputnik 3 ("Satellite-3"), launched on 15 May , 1958 , was the first satellite to use solar arrays. This was a crucial development which diverted funding from several governments into research for improved solar cells. In , pdf, p.6 APPLICATIONS AND IMPLEMENTATIONS See Also: photovoltaic array Solar cells are often electrically connected and encapsulated as a module. PV modules often have a sheet of glass on the front (sun up) side , 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 . Connecting cells in parallel will yield a higher current. Modules are then interconnected, in series or parallel, or both, to create an '''array''' with the desired peak DC voltage and current. The power output of a solar array is measured in Watt s or Kilowatts . In order to calculate the typical energy needs of the application, a measurement in watt-hours, Kilowatt-hours or kilowatt-hours per day is often used. A Rule Of Thumb commonly used is that peak power times 20% gives average power, equating to one kW peak producing 4.8 kWh per day. 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. # The electricity that is not used will be recycled and reused in other facilities. 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 for lower energy photons, # the photon can reflect off the surface, # the photon can be absorbed by the silicon which either:
Note that if a photon has ''an integer multiple'' of band gap 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 fewer 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 solar cells, the dominant mode of charge carrier separation is by drift. However, in non-p-n-junction 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 versa). 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 behavior 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.2 The resulting equivalent circuit of a solar cell is 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 Voltage s (V) and Currents (I). By increasing the resistive load on an irradiated cell continuously from zero (a '' Short Circuit '') to a very high value (an '' Open Circuit '') one can determine the Maximum-power point, the point that maximizes V×I, that is, the load for which the cell can deliver maximum electrical power at that level of irradiation. The maximum power point of a Photovoltaic varies with incident illumination. For systems large enough to justify the extra expense, a Maximum Power Point Tracker tracks the instantaneous power by continually measuring the Voltage and Current (and hence, power transfer), and uses this information to dynamically adjust the load so the maximum power is ''always'' transferred, regardless of the variation in lighting. 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 is 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/m&2) and the ''surface area'' of the solar cell (''Ac'' in m&2). : At solar noon on a clear March or September Equinox day, the solar radiation at the equator is about 1000 W/m&2. 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, a 12% efficiency solar cell having 1 m&2 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 '' 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: Photovoltaics 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 energy 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 2006) and system technology leads to system efficiencies between 5 and 19%. As of 2005, photovoltaic electricity generation costs ranged from ~0.60 US$/kWh (0.50 €/kWh) (central Europe) down to ~0.30 US$/kWh (0.25 €/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. (Note: in addition to solar irradiance profiles, these costs/kwh calculations will vary depending on assumptions for years of useful life of a system. Most c-Si panels are warrantied for 25 years and should see 35+ years of useful life.) 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. Watts peak Since solar cell output power depends on multiple factors, such as the Sun 's Incidence Angle , for comparison purposes between different cells and panels, the measure of watts peak (Wp) is used. It is the output power under these conditions known as STC: mysolar.com FAQ 4 # Insolation (solar Irradiance ) 1000 W/m&2 # solar reference spectrum AM ( Airmass ) 1.5 # cell temperature 25 °C Solar cells and energy payback In the 1990s, when silicon cells were twice as thick, efficiencies 30% lower than today and lifetimes shorter, it may well have cost more energy to make a cell than it could generate in a lifetime. The energy payback time of a modern photovoltaic module is anywhere from 1 to 20 years (usually under five)5 depending on the type and where it is used (see Net Energy Gain ). This means solar cells can be net energy producers, meaning they generate more energy over their lifetime than the energy expended in producing them.6.7 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. 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 configured as ''' Nanocrystals ''' and 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. The following is a current list of light absorbing materials, listed by configuration and substance-name: 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''. Silicon See Also: silicon list of silicon producers By far, the most prevalent ''bulk'' material for solar cells is Crystalline Silicon (abbreviated as a group as ''c-Si''), also known as "solar grade silicon". Bulk silicon is separated into multiple categories according to crystallinity and crystal size in the resulting Ingot , Ribbon , or Wafer . #''monocrystalline silicon'' (c-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 ''c-Si'' panels have uncovered gaps at the corners of four cells. #''Poly- or multicrystalline silicon'' (poly-Si or mc-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. #''New Structures'': These new compounds are special arrangements of silicon that can dramatically improve efficiency such as Ormosil . 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'', although many multi-layer thin films have efficiencies above those of bulk silicon wafers. CdTe Cadmium Telluride is an efficient light-absorbing material for thin-film solar cells. Compared to other thin-film materials, CdTe is easier to deposit and more suitable for large-scale production. Despite much discussion of the toxicity of CdTe-based solar cells, this is the only technology (apart from amorphous silicon) that can be delivered on a large scale, as shown by First Solar and Antec Solar. There is a 40 megawatt plant in Ohio (USA) and a 10 megawatt plant in Germany. First Solar is scaling up to a 100 MW plant in Germany and started building another 100 MW plant in Malaysia (2007). The perception of the toxicity of CdTe is based on the toxicity of elemental Cadmium , a heavy metal that is a Cumulative Poison . Scientific work, particularly by researchers of the National Renewable Energy Laboratories (NREL) in the USA, has shown that the release of cadmium to the atmosphere is lower with CdTe-based solar cells than with silicon photovoltaics and other thin-film solar cell technologies.
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