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PowerPedia:Solar cell

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Solar cell is a device with a pn-junction that converts the luminous Radiant energy directly and efficiently into Electrical energy. A solar cell (or photovoltaic cell) is a device that converts photons from the sun (solar light) into electricity. In general, a solar cell that includes the capacity to capture both solar and nonsolar sources of light (such as photons from incandescent bulbs) is termed a photovoltaic cell. Fundamentally, the device needs to fulfill only two functions: photogeneration of charge carriers (electrons and holes) in a light-absorbing material, and separation of the charge carriers to a conductive contact that will transmit the electricity. 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. Historically solar cells have been used in situations where electrical power from the grid is unavailable, such as in remote area power systems, Earth orbiting satellites or space probes , consumer systems, e.g. handheld calculators or wrist watches, remote radiotelephones and water pumping applications. Recently solar cells are particularly used in assemblies of solar modules (photovoltaic arrays) connected to the electricity grid through an inverter, often in combination with a net metering arrangement. Solar cells are regarded as one of the key technologies towards a sustainable energy supply.

Three generations of development

The first generation photovoltaic (also called wafer solar cells), consists of a large-area, single layer There was an error working with the wiki: Code[48] There was an error working with the wiki: Code[49], which is capable of generating usable There was an error working with the wiki: Code[50] Energy from light sources with the There was an error working with the wiki: Code[51]s of solar light. These cells are typically made using There was an error working with the wiki: Code[52] 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.

The second generation 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. Currently (2007) there are different technologies/semiconductor materials under investigation or in mass production, such as amorphous silicon, poly-crystalline silicon, micro-crystalline silicon, cadmium telluride, copper indium selenide/sulfide. Typically, the efficiencies of thin-film solar cells are lower compared to bulk silicon (=wafer-based) solar cells, but manufacturing costs are also lower, so that a lower price in terms of $/watt of electrical output 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 materials or flexible materials, even textiles. Third generation photovoltaic (also called advanced thin-film photovoltaic) materials are very different from the other two, broadly defined as semiconductor devices which do not rely on a traditional p-n junction to separate photogenerated charge carriers. These new devices include There was an error working with the wiki: Code[53]s, There was an error working with the wiki: Code[54]s, and There was an error working with the wiki: Code[55]s. History The term "photovoltaic" comes from the There was an error working with the wiki: Code[12] ???:phos meaning "light", and the name of the There was an error working with the wiki: Code[13] physicist There was an error working with the wiki: Code[14], after whom the Volt (and consequently Voltage) are named. It means literally of light and electricity. The photovoltaic effect was first recognised in There was an error working with the wiki: Code[15]. However, it was not until There was an error working with the wiki: Code[56] that the first solar cell was built, by There was an error working with the wiki: Code[57], who coated the There was an error working with the wiki: Code[58] There was an error working with the wiki: Code[59] with an extremely thin layer of There was an error working with the wiki: Code[60] to form the junctions. The device was only around 1% efficient. There was an error working with the wiki: Code[61] 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. 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 There was an error working with the wiki: Code[62] communications There was an error working with the wiki: Code[63] 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 There was an error working with the wiki: Code[64] ("Satellite-3"), launched on There was an error working with the wiki: Code[65], There was an error working with the wiki: Code[66], 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. The There was an error working with the wiki: Code[16] of solar cells begins in the 1800s when it is observed that the presence of sunlight is capable of generating usable electrical energy. Solar cells have gone on to be used in many applications. They have historically been used in situations where electrical power from the grid is unavailable. Timeline 1800s There was an error working with the wiki: Code[17] observes the There was an error working with the wiki: Code[67] via an electrode in a conductive solution exposed to light. There was an error working with the wiki: Code[68] - There was an error working with the wiki: Code[69] finds that There was an error working with the wiki: Code[70] is photoconductive. There was an error working with the wiki: Code[71] - W.G. Adams and R.E. Day observed the There was an error working with the wiki: Code[72] effect in solid There was an error working with the wiki: Code[73], and published a paper on the selenium cell. 'The action of light on selenium,' in "Proceedings of the Royal Society, A25, 113. There was an error working with the wiki: Code[74] - There was an error working with the wiki: Code[75] develops a solar cell using selenium on a thin layer of gold to form a device giving less than 1% efficiency. There was an error working with the wiki: Code[76] - There was an error working with the wiki: Code[77] investigates ultraviolet light photoconductivity. There was an error working with the wiki: Code[18] receives patent US389124, "Solar cell", and US389125, "Solar cell". There was an error working with the wiki: Code[78] - There was an error working with the wiki: Code[79] receives patent US527377, "Solar cell", and US527379, "Solar cell". There was an error working with the wiki: Code[80] - There was an error working with the wiki: Code[81] receives patent US588177, "Solar cell".. 1900-1929 There was an error working with the wiki: Code[82] - Nikola Tesla receives the patent US685957, "Apparatus for the Utilization of Radiant Energy", and US685958, "Method of Utilizing of Radiant Energy". There was an error working with the wiki: Code[83] - There was an error working with the wiki: Code[84] observes the variation in electron energy with light frequency. There was an error working with the wiki: Code[85] - There was an error working with the wiki: Code[86] publishes a paper on the photoelectric effect. There was an error working with the wiki: Code[87] makes a semiconductor-junction solar cell (There was an error working with the wiki: Code[88] and There was an error working with the wiki: Code[89]). There was an error working with the wiki: Code[90] - There was an error working with the wiki: Code[91] receives US1077219, "Solar cell". There was an error working with the wiki: Code[92] - There was an error working with the wiki: Code[93] patents "methods of increasing the capacity of photosensitive cells". There was an error working with the wiki: Code[94] - There was an error working with the wiki: Code[95] conducts experiments and proves the photoelectric effect. There was an error working with the wiki: Code[96] - There was an error working with the wiki: Code[97], a Polish scientist, produces a method to grow single-crystal silicon. 1930-1959 There was an error working with the wiki: Code[98] - Audobert and Stora discover the photovoltaic effect in Cadmium-Selenide (CdS), a photovoltaic material still used today. There was an error working with the wiki: Code[99] - There was an error working with the wiki: Code[100] receives patent US2402662, "Light sensitive device". There was an error working with the wiki: Code[101] - There was an error working with the wiki: Code[102] produce solar cells for space activities. There was an error working with the wiki: Code[103] - There was an error working with the wiki: Code[104] begins research into There was an error working with the wiki: Code[105]-There was an error working with the wiki: Code[106] photovoltaic cells. There was an error working with the wiki: Code[107] - There was an error working with the wiki: Code[108] exhibits solar cells at There was an error working with the wiki: Code[109]. Shortly afterwards, AT&T shows them at the <pesn type= forecasts that solar cells will eventually lead to a source of "limitless energy of the sun". There was an error working with the wiki: Code[111] - There was an error working with the wiki: Code[112] licences commercial solar cell technologies. Hoffman Electronics-Semiconductor Division creates a 2% efficient commercial solar cell for$25/cell or $1,785/Watt. There was an error working with the wiki: Code[19]". Hoffman Electronics creates an 8% efficient solar cell. There was an error working with the wiki: Code[113] - T. Mandelkorn, U.S. Signal Corps Laboratories, creates n-on-p silicon solar cells, which are more resistant to radiation damage and are better suited for space. Hoffman Electronics creates 9% efficient solar cells. There was an error working with the wiki: Code[114], the first solar powered satellite, was launched with a 0.1W, 100 cm² solar panel. There was an error working with the wiki: Code[115] - Hoffman Electronics creates a 10% efficient commercial solar cell, and introduces the use of a grid contact, reducing the cell's resistance. 1960-1979 There was an error working with the wiki: Code[116] - Hoffman Electronics creates a 14% efficient solar cell. There was an error working with the wiki: Code[117] - "Solar Energy in the Developing World" conference is held by the There was an error working with the wiki: Code[118]. There was an error working with the wiki: Code[119] - The There was an error working with the wiki: Code[120] communications satellite is powered by solar cells. There was an error working with the wiki: Code[121] - There was an error working with the wiki: Code[122] produces a viable photovoltaic module of silicon solar cells. There was an error working with the wiki: Code[123] - There was an error working with the wiki: Code[124] is the first manned spacecraft to be powered by solar cells There was an error working with the wiki: Code[125] - There was an error working with the wiki: Code[126] is powered by solar cells. There was an error working with the wiki: Code[127] - There was an error working with the wiki: Code[128] is powered by solar cells. There was an error working with the wiki: Code[129] - There was an error working with the wiki: Code[130] begins http://www.fsec.ucf.edu/]. There was an error working with the wiki: Code[131] - David Carlson and Christopher Wronski of RCA Laboratories create first amorphous silicon PV cells, which have an efficiency of 1.1%. There was an error working with the wiki: Code[20] is established at There was an error working with the wiki: Code[132]. World production of solar cells exceeds 500 kW. 1980-1999 There was an error working with the wiki: Code[133] - The Institute of Energy Conversion at University of Delaware develops the first thin-film solar cell exceeding 10% efficiency using Cu2S/CdS technology. There was an error working with the wiki: Code[134] - Worldwide photovoltaic production exceeds 21.3 megawatts, and sales exceed$250 million.

There was an error working with the wiki: Code[135] - 20% efficient silicon cell are created by the There was an error working with the wiki: Code[136] at the There was an error working with the wiki: Code[137].

There was an error working with the wiki: Code[138] - Reflective solar concentrators are first used with solar cells.

There was an error working with the wiki: Code[139] - The There was an error working with the wiki: Code[140] installs solar cells on the roof, marking the first installation on a church in East Germany.

There was an error working with the wiki: Code[141] - Efficient There was an error working with the wiki: Code[142] are developed the There was an error working with the wiki: Code[143] is invented.

There was an error working with the wiki: Code[144] - There was an error working with the wiki: Code[145] There was an error working with the wiki: Code[146] directs the There was an error working with the wiki: Code[147] to establish the There was an error working with the wiki: Code[148] (transferring the existing Solar Energy Research Institute).

There was an error working with the wiki: Code[149] - The National Renewable Energy Laboratory's There was an error working with the wiki: Code[150] is established.

There was an error working with the wiki: Code[151] - NREL develops a GaInP/GaAs two-terminal concentrator cell (180 suns) which becomes the first solar cell to exceed 30% conversion efficiency.

There was an error working with the wiki: Code[152] - The There was an error working with the wiki: Code[153] is established. Graetzel, EPFL, Laussane, Switzerland achieves 11% efficient energy conversion with dye-sensitized cells that use a photoelectrochemical effect.

There was an error working with the wiki: Code[154] - Total worldwide installed photovoltaic power reached 1000 megawatts.

2000-Today

There was an error working with the wiki: Code[155] - Solar cells in modules can convert around 17% of visible incidental Radiant energy to electrical energy.

There was an error working with the wiki: Code[156] - Estimated yearly solar cell production reached 1868 megawatts. Worldwide polysilicon production is projected to grow from 31,000 tons in 2005 to 36,000 tons in 2006.

Future developments

There are currently many research groups active in the field of There was an error working with the wiki: Code[157] in There was an error working with the wiki: Code[158] 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.

There was an error working with the wiki: Code[159]s are proposed satellites to be built in high Earth orbit that would use There was an error working with the wiki: Code[160] to beam Solar power to a very large antenna on Earth where it would be used in place of conventional power sources. There was an error working with the wiki: Code[161] has potential applications in cadmium-telluride solar cells. Some of the highest efficiencies for solar cell electric power generation have been obtained by using this material, but previous applications have not yet caused demand to increase significantly.

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 There was an error working with the wiki: Code[21]. 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 There was an error working with the wiki: Code[162]. 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 There was an error working with the wiki: Code[163] 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 There was an error working with the wiki: Code[164] in production of their "Sliver" cells, by micromachining wafers into very thin, virtually transparent layers that could be used as transparent architectural coverings.http://solar.anu.edu.au/level_1/research/sliver.php 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.

Yet another way to achieve cost improvements is to reduce wastes during the crystal formation by improved modelisation of the process, as done by There was an error working with the wiki: Code[165], spin-off of the There was an error working with the wiki: Code[166].

Another novel approach employed by There was an error working with the wiki: Code[167] is to grow silicon ribbons from specialized 'string puller' furnaces. They claim to be able to produce thinner cells without machining waste plus the resulting cells are naturally rectangular in shape.

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, however it remains to be seen if these solutions can generate the same space-efficiency as traditional silicon processing.

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 (There was an error working with the wiki: Code[22] for example), which opens a new dimension for new applications.

Polymer processing

The invention of There was an error working with the wiki: Code[168] (for which There was an error working with the wiki: Code[169], There was an error working with the wiki: Code[170] and There was an error working with the wiki: Code[171] were awarded a There was an error working with the wiki: Code[172]) may lead to the development of much cheaper cells that are based on inexpensive plastics. However, all There was an error working with the wiki: Code[173] made to date suffer from degradation upon exposure to There was an error working with the wiki: Code[174] 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. Additionally, most conductive polymers, being highly unsaturated and reactive, are highly sensitive to atmospheric moisture and oxidation, making commercial applications difficult.

Nanoparticle processing

Experimental non-silicon solar panels can be made of There was an error working with the wiki: Code[175]s, eg. There was an error working with the wiki: Code[176]s or There was an error working with the wiki: Code[177]s, embedded in There was an error working with the wiki: Code[178] or mesoporous metal oxides. In addition, thin films of many of these materials on conventional silicon solar cells can increase the optical coupling efficiency into the silicon cell, thus boosting the overal efficiency. By varying the size of the quantum dots, the cells can be tuned to absorb different wavelengths. Although the research is still in its infancy, There was an error working with the wiki: Code[177]-modified photovoltaics may be able to achieve up to 42 percent energy conversion efficiency due to multiple exciton generation.There was an error working with the wiki: Code[1]

Transparent conductors

Many new solar cells use transparent thin films that are also conductors of electrical charge. The dominant conductive thin films used in research now are transparent conductive oxides (abbreviated "TCO"), and include fluorine-doped tin oxide (SnO2:F, or "FTO"), doped zinc oxide (e.g.: ZnO:Al), and There was an error working with the wiki: Code[180] (abbreviated "ITO"). These conductive films are also used in the LCD industry for flat panel displays. The dual function of a TCO allows light to pass through a substrate window to the active light absorbing material beneath, and also serves as an ohmic contact to transport photogenerated charge carriers away from that light absorbing material. The present TCO materials are effective for research, but perhaps are not yet optimized for large-scale photovoltaic production. They require very special deposition conditions at high vacuum, they can sometimes suffer from poor mechanical strength, and most have poor transmittance in the infrared portion of the spectrum (e.g.: ITO thin films can also be used as infrared filters in airplane windows). These factors make large-scale manufacturing more costly.

A relatively new area has emerged using There was an error working with the wiki: Code[181] networks as a transparent conductor for organic solar cells. Nanotube networks are flexible and can be deposited on surfaces a variety of ways. With some treatment, nanotube films can be highly transparent in the infrared, possibly enabling efficient low bandgap solar cells. Nanotube networks are p-type conductors, whereas traditional transparent conductors are exclusively n-type. The availability of a p-type transparent conductor could lead to new cell designs that simplify manufacturing and improve efficiency.

Light absorbing dyes

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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 There was an error working with the wiki: Code[220] There was an error working with the wiki: Code[221] 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.

Organic/polymer solar cells

Organic solar cells and There was an error working with the wiki: Code[34]. Energy conversion efficiencies achieved to date using conductive polymers are low at 4-5% efficiency for the best cells to date. However, these cells could be beneficial for some applications where mechanical flexibility and disposability are important.

Silicon

There was an error working with the wiki: Code[222] thin-films are mainly deposited by There was an error working with the wiki: Code[223] (typically plasma enhanced (PE-CVD)) from There was an error working with the wiki: Code[224] gas and Hydrogen gas. Depending on the deposition's parameters, this can yield:

#There was an error working with the wiki: Code[225] (a-Si or a-Si:H)

#There was an error working with the wiki: Code[35] or

#There was an error working with the wiki: Code[226] (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 There was an error working with the wiki: Code[227] 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 absorbs the visible part of the solar spectrum more strongly than the There was an error working with the wiki: Code[228] 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.

Recently, solutions to overcome the limitations of thin film crystalline silicon have been developed. Light trapping schemes where the incoming light is obliquely coupled into the silicon and the light traverses the film several times enhance the absorption of sunlight in the films. Thermal processing techniques enhance the crystallinity of the silicon and passify electronic defects. The result is a new technology - thin film Crystaline Silicon on Glass (CSG)http://www.csgsolar.com/downloads/CSG_Press_PVSC31Jan2005.pdf. CSG solar devices represent a balance between the low cost of thin films and the high efficiency of bulk silicon.

A silicon thin film technology is being developed 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.

Nanocrystalline solar cells

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These structures make use of some of the same thin-film light absorbing materials but are overlain as an extremely thin absorber on a supporting matrix of conductive polymer or mesoporous metal oxide having a very high surface area to increase internal reflections (and hence increase the probability of light absorption).

Concentrating photovoltaics (CPV)

Concentrating photovoltaic systems use a large area of lenses or mirrors to focus sunlight on a small area of photovoltaic cells. If these systems use single or dual-axis tracking to improve performance, they may be referred to as Heliostat Concentrator Photovoltaics (HCPV). The primary attraction of CPV systems is their reduced usage of semiconducting material which is expensive and currently in short supply. Additionally, increasing the concentration ratio improves the performance of general photovoltaic materials and also allows for the use of high-performance materials such as There was an error working with the wiki: Code[36]. Despite the advantages of CPV technologies their application has been limited by the costs of focusing, tracking and cooling equipment. 'On There was an error working with the wiki: Code[229], There was an error working with the wiki: Code[230], Australia announced it would construct a solar plant using this technology to come online in 2008 and be completed by 2013. This plant, at 154 MW', would be ten times larger than the largest current photovoltaic plant in the world.

Solar-powering

A photovoltaic array is a linked collection of photovoltaic modules, one of which is shown in the picture to the right. Each There was an error working with the wiki: Code[37] (PV) module is made of multiple interconnected Solar cell. The cells convert Solar power into There was an error working with the wiki: Code[38]. PV modules distinguish themselves from solar cells in that they are conveniently sized and packaged in weather-resistant housings for easy installation and deployment in residential, commercial, and industrial applications. The application and study of photovoltaic devices is known as There was an error working with the wiki: Code[231].

Solar cells work because of the There was an error working with the wiki: Code[232]. Certain materials are able to convert sunlight into electricity. They absorb some of the energy of the There was an error working with the wiki: Code[233] and cause current to flow between two oppositely charged layers. Individual solar cells provide a relatively small amount of power, but electrical output can be significant when cells are connected together in a PV module. The cells, modules, and arrays can be connected in series or parallel, or typically a combination, to create a desired peak voltage output.

History

In 1839, during the There was an error working with the wiki: Code[39]. As such, photovoltaic cells were used mainly for the purposes of measuring There was an error working with the wiki: Code[234]. Just under one hundred years later, Albert Einstein received the Nobel prize in physics in 1921 for explaining the photoelectric effect, which allowed practical use of photo cells to be put into use. In 1941, There was an error working with the wiki: Code[235] invented the solar cell, following the invention of the There was an error working with the wiki: Code[236].

Applications

Solar photovoltaic panels are frequently applied in There was an error working with the wiki: Code[237] power. However, costs of production have been reduced in recent years for more widespread use through production and technological advances. For example, single crystal silicon solar cells have largely been replaced by less expensive multicrystalline silicon solar cells, and thin film silicon solar cells have also been developed recently at lower costs of production yet (see Solar cell). Although they are reduced in energy conversion efficiency from single crystalline Si wafers, they are also much easier to produce at comparably lower costs.

Together with a storage Battery (electricity), There was an error working with the wiki: Code[238] have become commonplace for certain low-power applications, such as signal There was an error working with the wiki: Code[239]s or devices in remote areas or simply where connection to the electricity mains would be impractical. In Experimental form they have even been used to power automobiles in races such as the There was an error working with the wiki: Code[240] across There was an error working with the wiki: Code[241]. Many There was an error working with the wiki: Code[242]s and land vehicles use them to charge on-board batteries.

PV performance

At high noon on a cloudless day at the equator, the There was an error working with the wiki: Code[40]/m², on the Earth's surface, to a plane that is perpendicular to the sun's rays. As such, PV arrays could track the sun through each day to greatly enhance energy collection. However, tracking devices add cost, and require maintenance, so it is more common for PV arrays to have fixed mounts that tilt the array and face due South in the Northern Hemisphere. In the Southern Hemisphere, they should point due North. The tilt angle, from horizontal, can be varied for season, but if fixed, should be set to give optimal array output during the peak electrical demand portion of a typical year.

Other factors affect PV performance. Accounting for clouds, and the fact that most of the world is not on the equator, and that the sun sets in the evening, the correct measure of solar power is There was an error working with the wiki: Code[243]: the average number of kilowatt-hours per square meter per day. For the weather and latitudes of the United States and Europe, typical insolation ranges from 4 KWh/m²/day in northern climes to 6.5 KWh/m²/day in the sunniest regions. Typical solar panels have an average efficiency of 12%, with the best commercially available panels at 20%. Thus, a photovoltaic installation in the southern latitudes of Europe or the United States may expect to produce 1 KWh/m²/day. A typical "150 Watt" solar panel is about a square meter in size: such a panel may be expected to produce 1 KWh every day, on average, after taking account the weather and the latitude.

In the There was an error working with the wiki: Code[41], and There was an error working with the wiki: Code[244]).http://www.colorado.gov/oemc/presentations/060125-manure.pdf

Photovoltaic cells' electrical output is extremely sensitive to shading. When even a small portion of a cell, module, or array is shaded, while the remainder is in sunlight, the output falls dramatically due to internal 'short-circuiting' (the electrons reversing course through the shaded portion of the P-N junction). Therefore it is extremely important that a PV installation is not shaded at all by trees, architectural features, flag poles, or other obstructions.

Module output and life are also degraded by increased temperature. Allowing ambient air to flow over, and if possible behind, PV modules reduces this problem. However, effective module lives are typically 25 years or more http://www.agr.gc.ca/pfra/water/solardugout_e.htm, so replacement costs should be considered as well.

Solar photovoltaic panels on spacecraft

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Solar panels can be used on There was an error working with the wiki: Code[42] There was an error working with the wiki: Code[245]. Because of these efforts to maximize electric production, and the fact that the Sun is mostly the only source of energy, the There was an error working with the wiki: Code[246] of solar cells on spacecraft could be one of the highest costs. When journeying to outer parts of the solar system (or beyond), nuclear reactors or There was an error working with the wiki: Code[247]s are preferred, as the Sun's rays are too weak at such massive distances to power a spacecraft.

The There was an error working with the wiki: Code[43]. In addition, solar power is being considered to be used as a There was an error working with the wiki: Code[248] mechanism in lieu of There was an error working with the wiki: Code[249] propulsion.

Theory and construction

There was an error working with the wiki: Code[250] and There was an error working with the wiki: Code[251] are typical choices of There was an error working with the wiki: Code[252]s for solar cells. Gallium arsenide crystals are grown especially for photovoltaic use, while silicon crystals are available in less-expensive standard There was an error working with the wiki: Code[253]s. These ingots are produced mainly for consumption in the There was an error working with the wiki: Code[254] industry. There was an error working with the wiki: Code[255] has lower conversion efficiency but also lower cost.

During the manufacturing process, crystalline silicon ingots are sliced into There was an error working with the wiki: Code[44] to remove slicing damage, There was an error working with the wiki: Code[45]s are deposited onto each surface: a thin There was an error working with the wiki: Code[256] on the sun-facing side and usually a flat sheet on the other.There was an error working with the wiki: Code[8] Solar panels are constructed of these cells cut into appropriate shapes, protected from radiation and handling There was an error working with the wiki: Code[257] on the front surface by bonding on a cover There was an error working with the wiki: Code[258], and There was an error working with the wiki: Code[259]ed onto a substrate (either a rigid panel or a flexible blanket). Electrical connections are made in series to achieve a desired output voltage and/or in parallel to provide a desired amount of current source capability. The cement and the substrate must be thermally conductive, because the cells heat up from absorbing There was an error working with the wiki: Code[260] energy that is not converted to electricity. Since cell heating reduces the operating efficiency it is desirable to minimize the heating. The resulting assemblies are called solar panels or solar arrays.

Maximum-power point

A solar cell may operate over a wide range of There was an error working with the wiki: Code[46] (I). By increasing the resistive load on an irradiated cell from zero (a short circuit) to a very high value (an open circuit) one can determine the maximum-power point, that is the maximum output electrical power that the cell can deliver at that level of irradiation. Vm x Im = Pm in There was an error working with the wiki: Code[261].

Energy conversion efficiency

A solar cell's energy conversion efficiency (\eta , "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 There was an error working with the wiki: Code[262] under "standard" test conditions (E, in W/m2) and the surface area of the solar cell (Ac in m²).

:\eta = \frac{P_{m}}{E \times A_c}

At solar noon on a clear March or September There was an error working with the wiki: Code[263] 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 Watts per square Meter. Thus, a 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 There was an error working with the wiki: Code[263] will produce approximately 120 watts of peak power.

Fill factor

Another defining term in the overall behavior of a solar cell is the There was an error working with the wiki: Code[265] (FF). This is the ratio of the maximum power point divided by the open circuit voltage (Voc) and the short circuit current (Isc):

:FF = \frac{P_{m}}{V_{oc} \times I_{sc}} = \frac{\eta \times A_c \times E}{V_{oc} \times I_{sc}}

Quantum efficiency

There was an error working with the wiki: Code[266] 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 There was an error working with the wiki: Code[267] efficiency, as it does not convey information about the power collected from the solar cell.

Comparison of energy conversion efficiencies

Silicon solar cell efficiencies vary from 6% for amorphous silicon-based solar cells to 40,7% with multiple-junction research lab cells. Solar cell energy conversion efficiencies for commercially available mc-Si solar cells are around 14-16%. 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 about 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 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 ~ 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. (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.

Peak watt (or Watt peak)

Since solar cell output power depends on multiple factors, such as the There was an error working with the wiki: Code[268]'s There was an error working with the wiki: Code[269], for comparison purposes between different cells and panels, the peak watt (Wp) is used. It is the output power under these conditions:

# There was an error working with the wiki: Code[270] 1000 W/m²

# solar There was an error working with the wiki: Code[271] AM (There was an error working with the wiki: Code[272]) 1.5

# cell temperature 25There was an error working with the wiki: Code[273]

Solar cells and energy payback

There is a common conception 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 20 years (usually under five) depending on the type and where it is used (see net energy gain). This means solar cells can be net energy producers and can "reproduce" themselves (from just over once to more than 30 times) over their lifetime.

This is disputed, however, by some researchers who object that such analysis doesn't take into account waste, inefficiency, and related energy costs that would come with a real-world solar cell.

Pros and Cons

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List of energy topics

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Renewable energy

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Solar power

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Hybrid vehicle

Solar cell

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General

"Solar Resources". SunPower Corporation, 2004.

"History: Photovoltaics Timeline". About, Inc., 2005.

"Bell Labs Celebrates 50th Anniversary of the Solar Cell - Timeline". Lucent Technologies, 2004.

Lenardic, Denis, "History of photovoltaics". PVResources.com, 2005.