A solar cell is a device that uses the photoelectric effect to generate electricity from light. Solar cells are used to power many kinds of equipment, including satellites, calculators, remote radiotelephones, and advertising signs.
The simplest type of solar cell is a silicon diode, but research is continuing into more exotic materials (see below) with greater efficiencies. Modern solar cells are encapsulated in glass-fronted plastic sheets. They have design lifetimes that exceed forty years. Sunlight provides about 1.36 kilowatts per square meter, and most solar cells are between 8 and 12 percent efficient. In desert areas, they can operate for an average of 6 hours per day when mounted in nonrotating brackets.
Solar panels come in four varieties. Most common are rigid monocrystalline and polycrystalline silicon sheets. Monocrystalline silicon provides the highest efficiency of all four types but is also the most expensive. Polycrystalline silicon is lower cost but lower in efficiency. Also available are amorphous silicon solar cells which can be applied to a variety of substrates including flexible ones, like metal foil or plastic foil. The main advantage of amorphous silicon solar cells is that they should eventually be able to be processed at a much lower cost than crystalline solar cells. Nanocrystalline silicon has also been used, in the same processing systems as for amorhpous silicon, and it shows slightly higher efficiency due to the increased absorption in the longer wavelengths. Experimental non-silicon solar panels are made of carbon nanotubes embedded in 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.
In 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.
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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.