Solar cells use a semiconductor to convert the solar energy of sunlight into electricity via the photovoltaic effect. Individual cells are assembled in order to form solar panels and arrays. First observed by French physicist Alexandre-Edmond Becquerel, son of electricity pioneer Antoine Cesar Becquerel, as a teenager in 1839, the photovoltaic effect is the process by which voltage or electric current is created in a material that has been exposed to electromagnetic radiation (such as light). The first solar cell was built in 1883 by American inventor Charles Fritts, who coated selenium (a semiconductor) with a thin layer of gold. The solar cell worked but had an efficiency of only about 1 percent, making it impossible to generate enough electricity to recover the cost of the materials except over a very long period of time. Improvements to solar cells in the mid-20th century brought efficiency up to 5 percent and gradually climbed, with many cells on the 21st-century market exceeding 20 percent efficiency (the most efficient are over 40 percent but are used for satellites). Modern thin-film solar cells can be expected to last at least 20 years and are half the thickness of solar cells used in the 1990s, making them significantly cheaper. The energy payback time of solar cells—the time it takes for an energy system to pay back the energy used to manufacture it—is hard to normalize because location and other factors have such a large impact, but in some cases it has fallen to less than one year, and rarely exceeds four years.
Solar cells are connected in modules, which are connected in an array. There are a number of types of solar cells, often referred to by the material used to make their thin film. Thin film solar cells include thin-film silicon (the same material, just thinner, used in the 1990s), cadmium telluride (CdTe), and copper-indium selenide (CIGS), which vary in efficiency and cost. The most efficient cells are multi-junction devices that use multiple thin films, each of which is most efficient at a certain portion of the electromagnetic spectrum. The cost of gallium and germanium has risen as demand has increased for multi-junction solar cells using gallium arsenide (GaAS), gallium indium phosphide (GaInP), and germanium thin films.
Solar cells can power a specific device—there are, for instance, solar-powered cell phone chargers, lights and lanterns (which charge during the day to be used at night), security cameras, and mp3 players. The U.S. National Aeronautics and Space Administration (NASA) has even developed a solar-powered aircraft. But a more flexible use of solar cells is to feed the collected energy into batteries, or to use an inverter to feed the electricity back into the grid—either way, the electricity is then free to be used by anything that can be plugged in.
Solar cells work because as the photons in sunlight strike the cell, some of them are absorbed by the semiconducting material. As each photon is absorbed, its energy is passed on to an electron. The extra energy frees the electron and allows it to move freely through the semiconductor, while also producing heat from any energy that is not successfully absorbed. This is one of the factors affecting the efficiency of a solar cell, but efficiency is not as critical to practicality in solar energy as it is in the use of nonrenewable fuels, because solar energy itself is free and inexhaustible. Efficiency is primarily of interest in determining how much electricity can be generated by a given number of solar cells, especially in areas with cloud cover where solar collection is not optimal. Furthermore, a more energy-efficient solar cell is not necessarily more cost-efficient if it costs significantly more money to buy or make; in the end, what matters is the cost to the consumer for the kilowatt-hour of energy produced by the cell.
The most significant advance in photovoltaics (the science of solar cells) in recent years has been the thin-film solar cell, which is manufactured by laying thin layers (“thin film,” with a thickness ranging from under a nanometer to several micrometers) of photovoltaic material on a substrate. In 2009, thin-film solar cell sales overtook the older bulk crystalline designs. One of the advantages of thin-film cells is that they can be used to create semitransparent solar cells that can be applied to windows, generating electricity while still allowing light into the house and appearing no different than ordinary window tinting. This greatly increases the available installation area, especially for homeowners, but also opens up the possibility of skyscrapers coated in semitransparent solar cells. Most thin-film cells are not semitransparent, and those that are cost more.
Organic solar cells use conductive organic polymers to absorb light and transport the charge; they have a low production cost when dealing with high volumes but are also less efficient. One class of organic solar cell is the dye-sensitized solar cell, for which French physicist Michael Grätzel won the 2010 Millennium Technology Prize. Though dye-sensitized cells, which use a porous layer of titanium dioxide nanoparticles (the semiconductor) covered in a layer of photosensitive dye (the light absorber), have a lower conversion efficiency than many other thin-film cells, the cost to manufacture is very low, and it is expected that when manufactured in volume (to benefit from economies of scale), the cost per kilowatt-hour will be low, a rate competitive with fossil fuels. Significant development remains to be done in dye-sensitized solar cells, which have only entered the commercial market in the past few years.
Whatever sort of cell is used, interconnected solar cells make up solar panels; multiple panels are joined in a photovoltaic array, and the installation is completed by adding an inverter and batteries. The inverter is necessary because the photovoltaic produces, and batteries store, direct current electricity, while household electrical systems use alternating current. The solar array may be fixed in a specific position, whether on a rooftop, on windows as discussed above, or on a fixed rack mount. When mounted, the array is typically fixed at a tilt angle equivalent to the latitude of the location for maximum solar collection. Moving mounts are also available and may either be manually adjusted (typically along only one axis, adjusted seasonally to maximize sun exposure) or adjusted automatically (often along both axes, to follow the sun's movements over the course of the day). Because solar power is an intermittent source, unavailable at night and low-yield during cloudy conditions, batteries are necessary to store power for times when demand exceeds the solar panels' supply. Alternately, surplus electricity can be fed into the municipal grid and will be credited to the consumer in areas where utility companies offer net metering.
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