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Solar cell(Redirected from Photovoltaic)
A solar cell, or photovoltaic cell, is a semiconductor device consisting of a large-area p-n junction diode, which, in the presence of sunlight is capable of generating usable electrical energy. The field of research related to solar cells is known as photovoltaics. Solar cells have many applications. They are particularly well suited to, and historically used in situations where electrical power from the grid is unavailable, such as in remote area power systems, Earth orbiting satellites, handheld calculators, remote radiotelephones, water pumping applications, etc. More increasingly, solar cells (in the form of modules or solar panels) are finding their way onto the roofs of houses in cities where they are connected, through a inverter to the electricity grid.
IntroductionEtymologyThe etymology of the term "photovoltaic" comes from the Greek photos meaning light and the name of the Italian physicist Volta, after whom voltage, as well as the SI unit of voltage, the Volt are named. It means literally light electricity. History Russell Ohl is generally recognized for patenting the modern solar cell (US2402662, "Light sensitive device"). Sven Ason Berglund had a prior patent concern methods of increasing the capacity of photosensitive cells. Nikola Tesla patents a primative solar cell in 1901 when he received the patent US685957 (Apparatus for the Utilization of Radiant Energy). He later attains US685958 (Method of Utilizing of Radiant Energy). The patents describe radiation charging and discharging conductors (e.g., a piece of mica) by "radiant energy". MaterialsBy far the most common material that solar cells (and all other semiconductor devices) are made of is silicon. However, there are solar cells which are made from other, more exotic materials such as cadmium telluride (CdTe), or copper indium gallium di-selenide (CIGS). There are (broadly speaking) three types of silicon solar cells:
Interconnection and modulesUsually, solar cells are electrically connected, and combined into "modules", or solar panels. Solar panels, have a sheet of glass on the front, and a resin encapsulation behind to keep the semiconductor wafers safe from the elements (rain, hail, etc). Solar cells are usually connected in series in modules, so that their voltages add. TheoryBackgroundIn order to understand how a solar cell works, a little background theory in semiconductor physics is required. For simplicity, the description here will be limited to describing the workings of single crystalline silicon solar cells. Silicon is a group 14 (formerly, group IV) atom. This means that each Si atom has 4 valence electrons in its outer shell. Silicon atoms can covalently bond to other silicon atoms to form a solid. There are two basic types of solid silicon, amorphous (having no long range order) and crystalline (where the atoms are arranged in an ordered three dimensional array). There are various other terms for the crystalline structure of silicon; poly-crystalline, micro-crystalline, nano-crystalline etc, and these refer to the size of the crystal "grains" which make up the solid. Solar cells can be, and are made from each of these types of silicon, the most common being poly-crystalline. Silicon is a semiconductor. This means that in solid silicon, there are certain bands of energies which the electrons are allowed to have, and other energies between these bands which are forbidden. These forbidden energies are called the "band gap". The allowed and forbidden bands of energy are explained by the theory of quantum mechanics. At room temperature, pure silicon is a poor electrical conductor. In quantum mechanics, this is explained by the fact that the Fermi level lies in the forbidden band-gap. To make silicon a better conductor, it is "doped" with very small amounts of atoms from either group 13 (III) or group 15 (V) of the periodic table. These "dopant" atoms take the place of the silicon atoms in the crystal lattice, and bond with their neighbouring Si atoms in almost the same way as other Si atoms do. However, because group 13 atoms have only 3 valence electrons, and group 15 atoms have 5 valence electrons, there is either one too few, or one too many electrons to satisfy the four covalent bonds around each atom. Since these extra electrons, or lack of electrons (known as "holes") are not involved in the covalent bonds of the crystal lattice, they are free to move around within the solid. Silicon which is doped with group 13 atoms (aluminium, gallium) is known as p-type silicon because the majority charge carriers (holes) carry a positive charge, whilst silicon doped with group 15 atoms (phosphorus, arsenic) is known as n-type silicon because the majority charge carriers (electrons) are negative. It should be noted that both n-type and p-type silcion are electrically neutral, i.e. they have the same numbers of positive and negative charges, it is just that in n-type silicon, some of the negative charges are free to move around, while the converse is true for p-type silicon. Light generation of carriers
A photon only needs to have energy greater than the band gap energy to excite an electron from the valence band into the conduction band. However, the solar spectrum approximates a black body spectrum at ~6000 K, and as such, much of the solar radiation reaching the Earth is composed of photons with energies greater than the band gap of silicon. These higher energy photons will be absorbed by the solar cell, but the difference in energy between these photons and the silicon band gap is converted into heat (via lattice vibrations - called phonons) rather than into usable electrical energy. The p-n junctionA solar cell is a large-area semiconductor p-n junction. To understand the workings of a p-n junction it is convenient to imagine what happens when a piece of n-type silicon is brought into contact with a piece of p-type silicon. In practice, however, the p-n junctions of solar cells are not made in this way, but rather, usually, by diffusing an n-type dopant into one side of a p-type wafer. If we imagine what happens when a piece of p-type silicon is placed in intimate contact with a piece of n-type silicon, then what occurs is a diffusion of electrons from the region of high electron concentration - the n-type side of the junction, into the region of low electron concentration - p-type side of the junction. When the electrons diffuse across the p-n junction, they recombine with holes on the p-type side. This diffusion of carriers does not happen indefintely however, because of the electric field which is created by the imbalance of charge immediately either side of the junction which this diffusion creates. Electrons from donor atoms on the n-type side of the junction are crossing into the p-type side, leaving behind the (extra) positively charged nuclei of the group 15 donor atoms, leaving an excess of positive charge on the n-type side of the junction. At the same time, these electrons are filling in holes on the p-type side of the junction, becoming involved in covalent bonds around the group 13 acceptor atoms, making an excess of negative charge on the p-type side of the junction. This imbalance of charge across the p-n junction sets up an electric field which opposes further diffusion of charge carriers across the junction. This region where electrons have diffused across the junction is called the depletion region because it no longer contains any mobile charge carriers. It is also known as the "space charge region". The electric field which is set up across the p-n junction creates a diode, allowing current to flow in only one direction across the junction. Electrons may pass from the p-type side into the n-type side, and holes may pass from the n-type side to th p-type side. But since the sign of the charge on electrons and holes is opposite, conventional current may only flow in one direction. Separation of carriers by the p-n junctionOnce the electron-hole pair has been created by the absorption of a photon, the electron and hole are both free to move off independantly within the silicon latttice. If they are created within a minority carrier diffusion length of the junction, then, depending on which side of the junction the electron-hole pair is created, the electric field at the junction will either sweep the electron to the n-type side, or the hole to the p-type side. Connection to an external loadIf Ohmic metal-semiconductor contacts are made to both the n-type and p-type sides of the solar cell, and the electrodes connected to an external load, then electrons which are created on the n-type side, or have been "collected" by the junction and swept onto the n-type side may travel through the wire, power the load, and continue through the wire until they reach the p-type semiconductor-metal contact where they recombine with a hole which was either created as an electron-hole pair on the p-type side of the solar cell, or swept across the junction from the n-type side after being created there. Equivalent circuit of a solar cell
Manufacture and devicesBecause 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 considerd 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 (300 -- 500 nm) slices or wafers. The wafers are usually lightly p-type doped. To make a solar cell from the wafer, an n-type diffusion is performed on the front side of the wafer, forming a p-n junction a few tens of nanometres below the surface. The wafer is then metallised, whereby a full area metal contact is made on the back surface, and a grid-like metal contact made up of fine "fingers" and larger "busbars" is screen-printed onto the front surface. The rear contact is usually made by evaporating aluminium, and the front contact is usually made from some kind of silver paste. 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. Optional extra steps in the manufacturing process include texturing the front surface of the wafer, and/or adding an anti-reflection coating to increase the amount of light which is coupled into the device. Energy conversion efficiencyTypical module efficiencies for commercially available screen printed poly-crystalline solar cells are around 17%. A solar module's energy conversion efficiency, (or just efficiency) is the ratio of the maximum output electrical power divided by the input light power under "standard" test conditions. The "standard" solar radiation (known as the "air mass 1.5 spectrum) has a power density of 1000 Watts per square metre. Thus, a typical 1 mē solar panel in direct sunlight will produce approximately 170 Watts of peak power. Applications and implementationsSee the article Solar panel for information about applications and implementations of solar cells and panels. CostIn late 2001, with batteries to provide power at night, desert climates can get power for about 8 cents per kilowatt hour using solar cells, batteries and electronic inverters. By contrast, nuclear and hydroelectric power plants can provide power at 1.5 to 3 cents per kilowatt hour. Solar power is already cheaper than internal combustion generators that use natural gas, diesel or gasoline, and is becoming competitive with the costs of coal power in some areas. Homes in the U.S. use between 5 and 20 kilowatt hours per day, depending on whether they use electricity for lighting, or heating, cooling and cooking as well. If a roof is required for other reasons, and the solar cells are chosen and fabricated to form a weather-resistant roof, the value of the roof reduces solar costs considerably. A well-designed solar cell roof will simply be constructed at the optimal angle to collect power. This saves the cost of mounting brackets to raise the cells to an optimal angle, saving quite a bit of money. Solar panel shingles which replace ordinary shingles have recently become available. The least expensive way to buy a solar power system for a home or small business is as part of a cooperative. Periodically, a group will form on the internet to purchase solar equipment cooperatively. Current researchThere are currently many research groups active in the field of photovoltaics at universities and research institutions around the world. Much of the research is focussed on making solar cells cheaper and/or more efficient, so that they can more effectively compete with other energy sources, including fossil energy. One way of doing this is to develop cheaper methods of obtaining silicon that is sufficiently pure. Silicon is a very common element, but is normally bound in silica sand. Another approach is to significantly reduce the amount of raw material used in the manufacture of solar cells. The various thin-film technologies currently being developed make use of this approach to reducing the cost of electricity from solar cells. The invention of conductive polymers, (for which Alan Heeger was awarded a Nobel prize) may lead to the development of much cheaper cells that are based on inexpensive plastics, rather than semiconductor grade silicon. However, all organic solar cells made to date suffer from degradation upon exposure to UV light, and hence have lifetimes which are far too short to be viable. Thin-film solar cellsThe next step in reducing the cost of solar cells and panels seems certain to come from thin-film technology. Thin-film solar cells use less than 1% of the raw material (silicon) compared to wafer based solar cells, leading to a significent price drop per kWh. There are many research groups around the world actively researching different thin-film approaches and/or materials. Thin Film solar cells are mainly deposited by PECVD from silane gas and hydrogen. This process produces a material without crystalline orientation : amorphous silicon. Depending on the deposition's parameters nanocrystalline silicon can also be obtained. These types of silicon present dandling and twisted bonds, which results in the aparition of deep defects (energy levels in the bandgap) as well as in the deformation of the valence and conduction bands (band tails). This contributes to reduce the efficiency of Thin-Film solar cells by reducing the number of collected electron-hole pair by incident photon. Amorphous silicon (a-Si) has a higer bandgap (1.7 eV) than crytalline Silicon (c-Si) (1.1 eV), which means it is more efficient to absorb the visible part of the solar spectrum, but it fails to collect an important part of the spectrum : the infrared. As nano crystalline Si has about the same bandgap than c-Si, the two material can be combined by depositing to diodes on top of each other : the tandem cell. The top cell in a-Si absorbs the visible light and leaves the infrared part of the spectrum for the bottom cell in nanocrystalline Si. One particularly promising technology is crystalline silicon thin-films on glass subatrates. 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. In 2005, a full scale production factory is being built in Germany to commercialise this technology. Another interesting aspect of Thin-Film solar cells is the possibility to deposit the cells on all kind of materials, including flexibles substrates (PET for example), which opens a new dimension for new applications. Exotic materialsFor special applications, such as Deep Space 1, high-efficiency cells can be made from gallium arsenide by molecular beam epitaxy. Such cells have many diodes in series, each with a different band gap energy so that it absorbs its share of the electromagnetic spectrum with very high efficiency. Triple junction solar cell have (as the name suggest) 3 diodes layered on top of each other, each absorbing a different spectra of light, efficiency as high as 28% have been achieved. The multiple junction solar cells may be very efficient, but are prohibitively expensive to make. Cost-effective use of these cells could be achieved with concentrating optics so that less of the array consists of actual semiconductor devices. Experimental non-silicon solar panels can be made of carbon nanotubes or quantum dots embedded in a special plastic. These have only one-tenth the efficiency of silicon panels but could be manufactured in ordinary factories, not clean rooms which should lower the cost. Some of the most efficient solar cell materials are cadmium telluride (CdTe) and copper indium gallium selenide (CIGS). Unlike the basic silicon solar cell, which can be modelled as a simple p-n junction (see under semiconductor), these cells are best described by a more complex heterojunction model. The best efficiency of a bare solar cell as of April 2003 was 16.5% [Dr IM Dharmadasa, Sheffield Hallam University, UK]. Higher efficiencies (around 30%) can be obtained by using optics to concentrate the incident light. Polymer or organic solar cells are built from thin layers of organic semiconductors such as polyphenylene vinylene and fullerene. The p/n junction model is only a crude description of the functioning of such cells, as electron hopping and other processes also play a crucial role. They are potentially cheaper to manufacture than silicon or inorganic cells, but efficiencies achieved to date are low and cells are highly sensitive to air and moisture, making commercial applications difficult. The technology has however already successfully been commercialised in organic LED's and organic displays , also called polymer displays Graetzel cells (sometimes called photoelectrochemical cells) have been around for two decades or so. A p/n junction is used here too in the form of a doped solid (normally titanium dioxide) in contact with a solid or liquid electrolyte (for example CuI ). In contrast to the classical solar cell not the semiconductor but a dye placed at the p/n interface is used for absorption of radiation, mimicking the process of photosynthesis. As a result, this type of cell allows a more flexible use of materials. Like organic solar cells, Graetzel cells can be manufactured under "dirty" conditions. Commercial applications have failed to appear due to the fast degradation occurring in Graetzel cells. Common misconceptions and myths about solar cellsThere is common misconception and myth regarding solar cells. There is a myth that Solar cells have such a high energy payback time that they never produce more energy than it takes to make them. This is a myth which seems to be put forward by detractors of solar cells. The energy payback time of a solar panel is on the order of six years, while the expected working lifetime is on the order of 40 years. See Net energy gain See alsoExternal links
Yield data
Theory
Cost-BenefitDo-it-yourselfPEC (Photo Electro Chromic)
Cuprous oxide solar cells
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