A photovoltaic cell, often called a solar cell, is a device that converts the energy in light, both photons from the sun (solar light) and non-solar sources, directly into electrical potential energy using a physical process called the photovoltaic effect. Photovoltaic cells
are used to produce electricity in situations where they are more economical than other power generation methods. Occasionally, they are used as photodetectors.
The photovoltaic effect has been known since French physicist Alexandre-Edmond Becquerel (1820–1891) observed light-induced currents in a dilute acid in 1839. Explanation of the effect depends on quantum theories of light and solids that were proposed by German physicist Maxwell Planck (1858–1947) in 1900 and Scottish physicist Charles Thomson Rees Wilson (1869–1959) in 1930.
The first solid-state photovoltaic cells were designed in 1954, after the development of solid-state diodes and transistors. Since then, the number of applications of photovoltaic cells has been increasing, the cost per watt of power generated has been declining, and efficiency has been increasing. Enough photovoltaic modules to provide 50 MW of power were made in 1991. The production rate increases by about 20% each year. As of 2006, efficiencies between 5 to 20% occur within commercially available photovoltaic cells. Costs, at about this same time, run from about 0.60 to 0.25 cents per kilowatt-hour (kWh).
Photovoltaic cells have been used since 1958 to power many satellites orbiting the Earth. On Earth, they are used in remote areas where the cost of transporting electricity to the site is costly. Their use is one of a variety of alternative energy methods being developed that do not depend on fossil fuels. They are also used for low-power mobile applications such as hand-held calculators and wrist watches.
Photovoltaic cells are made of semiconducting materials (usually silicon) with impurities added to certain regions to create either a surplus of electrons (n -type doping) or a scarcity of electrons (p -type doping, also called a surplus of holes). The extra electrons and holes carry electrical charges, allowing current to flow in the semiconducting material.
When a photon hits the top surface of a photo-voltaic cell, it penetrates some distance into the semiconductor until it is absorbed. If the photon’s energy is at least as large as the material’s energy band gap, the energy from the photon creates an electron-hole pair. Usually, the electron and the hole stay together and recombine. In the presence of an electric field, however, the negatively charged electron and the positively charged hole are pulled in opposite directions. This occurs for the same reason that one end of a magnet is attracted to another magnet while the other end is repelled.
Junctions in semiconductors create electrical fields. A junction can be formed at the border between p -and n -doped regions, or between different semiconducting materials (a heterojunction), or between a semiconductor and certain metals (forming a Schottky barrier).
The movement of the charges in the photovoltaic cell creates a voltage (electrical potential energy) between the top and bottom of the cell. Electrical contacts attached to the cell at the p and n sides (the top and bottom) complete the cell. Wires attached to these contacts make the voltage available to other devices.
The distance into the material that a photon goes before being absorbed depends on both how efficient the material is at absorbing light and the energy of the photon—high-energy photons penetrate further than low-energy photons. This is why x rays are used to image bones, but most visible light stops at the skin.
Efficiency of a cell depends on the losses that occur at each stage of the photovoltaic process. Many of the sun’s photons are absorbed or deflected in the atmosphere before reaching Earth’s surface (this is described by a term called air mass). Some photons will reflect off or pass through the cell. Some electron-hole pairs recombine before carrying charges to the contacts on the ends of the cell. Some of the charges at the ends of the cells do not enter the contacts, and some energy is lost to resistance in the metal contacts and wires.
The efficiency of the cell can be increased by shining more light onto it using a concentrator (such as a focusing lens), by adding coatings (such as a mirror to the bottom of the cell to reflect unabsorbed light back into the cell), or by creating heterojunction cells with materials that have different band gaps, and thus are efficient at absorbing a variety of wavelengths. One of the most efficient photovoltaic cells reported was two-junction cell made of gallium arsenide and gallium antimony, coupled with a concentrator that increased the intensity of the light 100 times: it worked with 33% efficiency in a laboratory. In practice, ground-based solar cells tend to have average efficiencies of 13 to 19% or less.
For low-power portable electronics, like calculators or small fans, a photovoltaic array may be a reasonable energy source rather than a battery. Although using photovoltaics lowers the cost (over time) of the device to the user—who will generally never need to buy batteries—the cost of manufacturing devices with photovoltaic arrays is generally higher than the cost of manufacturing devices to which batteries must be added. Therefore, the initial cost of photovoltaic devices is often higher than battery-operated devices.
In other situations, such as solar battery chargers, watches, and flashlights, the photovoltaic array is used to generate electricity that is then stored in batteries for use later.
Electricity for homes or other buildings farther than a couple football fields from the nearest electrical lines, may be cheaper if obtained from photovoltaic cells than by buying electricity from the local power utility, because of the cost of running an electrical line to the house. In most urban areas, however, buying electricity from a utility is much cheaper than using photovoltaics.
The cost of using photovoltaic technology depends not only on the photovoltaic cells themselves but also on the batteries and equipment needed to condition the electricity for household use. Modules made of groups of photovoltaic cells set side-by-side and connected in series generate direct current (DC) electricity at a relatively low voltage, but most household appliances use 120-V alternating current (AC). Inverters and power conditioners can transform DC to AC current at the correct voltage.
The types of appliances in the house are also a consideration for whether to use photovoltaic. Some devices—like televisions, air conditioners, blow-dryers, or laser printers—require plenty of power, sometimes all at once. Because photovoltaic cells do not change the amount of voltage they can supply, this sort of load can drain batteries rapidly. Many people with houses powered by photovoltaic cells buy energy-efficient lights and appliances and limit the number of unnecessary electrical devices in their homes.
In remote parts of the world, entire villages are oftentimes powered by photovoltaic systems. A few utility companies in the United States and Europe run solar farms to produce electricity. Other industrial uses exist for photovoltaic cells, too. These are usually low-power applications in locations inconvenient for traditional electrical sources. Some emergency roadside telephones and other communications devices have batteries that are kept charged by photovoltaic cells. Arrays of cells power cathodic protection: the practice of running current through metal structures to slow corrosion.
Many semiconductor materials can be used to make photovoltaic cells, but silicon is most popular—not because it is most efficient, but because it is inexpensive because much silicon is produced for making microelectronics chips. Semiconductors such as gallium arsenide, cadmium sulphide, cadmium telluride, and copper indium diselenide are used in special-purpose high-efficiency cells, but are more expensive than silicon cells. The highest-efficiency photovoltaic cells are made of such materials.
The least expensive type of solar cell is made of a disordered type of silicon mixed with hydrogen. This hydrogenated amorphous silicon is used in photovoltaic cells for calculators and wristwatches. Amorphous silicon is deposited on a substrate as a coating.
In 1974, David Carlson at RCA’s David Sarnoff Laboratory first made an amorphous silicon photo-voltaic cell. By 1988, amorphous cells with about 13% efficiency were made using a stacked-junction PIN device.
Because large areas can be coated, the cost-per-device is relatively low. Its band gap is 1.7 eV, which means that it absorbs light at shorter wavelengths than the crystalline silicon and that it works well under fluorescent lights. Because it absorbs light efficiently, the cells can be made very thin, which uses less material and also helps make the cells less expensive. These devices, however, degrade in direct sunlight and have a shorter lifetime than crystalline cells.
Cells made of single-crystal silicon, the same material used for microelectronics chips, supply more current than the other types of silicon. Unlike amorphous silicon, the voltage stays constant when different loads are applied. Single-crystal silicon photovoltaic cells that are protected from oxidizing last about 20 years.
Polycrystalline silicon is not uniform enough to make electronic chips, but works well for photovoltaic cells. It can be grown with less stringent control than single-crystal silicon but works nearly as efficiently.
The latest developments of photovoltaic cells include organic polmer cells, dye sensitized cells, and quantum dot solar cells. These photovoltaic cells, and others, do not necessarily rely on a p-n junction but use other devices to function. For example, organic polymer cells use thin-films that are deposited on a supporting substrate, while quantum dot solar cells use quantum dots (a type of nanoparticle) suspended within a support matrix.
In the 2000s, research in photovoltaic cell technology concentrates in refining current technologies to be less expensive and more efficient, while developing new technologies based on innovative designs and materials.
See also Alternative energy sources.
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