Photovoltaic cells convert light energy into electricity at the atomic level. Although first discovered in 1839, the process of producing electric current in a solid material with the aid of sunlight wasn't truly understood for more than a hundred years. Throughout the second half of the 20th century, the science has been refined and the process has been more fully explained. As a result, the cost of these devices has put them into the mainstream of modern energy producers. This was caused in part by advances in the technology, where PV conversion efficiencies have improved considerably.

French physicist Edmond Becquerel first described the photovoltaic (PV) effect in 1839, but it remained a curiosity of science for the next three quarters of a century. At only 19, Becquerel found that certain materials would produce small amounts of electric current when exposed to light. The effect was first studied in solids, such as selenium, by Heinrich Hertz in the 1870s. Soon afterward, selenium PV cells were converting light to electricity at 1% to 2% efficiency. As a result, selenium was quickly adopted in the emerging field of photography for use in light-measuring devices.

Major steps toward commercializing PV were taken in the 1940s and early 1950s, when the Czochralski process was developed for producing highly pure crystalline silicon. In 1954, scientists at Bell Laboratories depended on the Czochralski process to develop the first crystalline silicon photovoltaic cell, which had an efficiency of 4%.

This PV panel, developed by TRW for a communications satellite in 1966, was typical for its day.

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The "photovoltaic effect" is the basic physical process through which a PV cell converts sunlight into electricity. Sunlight is composed of photons, or particles of solar energy. These photons contain various amounts of energy corresponding to the different wavelengths of the solar spectrum (see "Light and the Sun" for more about that). When photons strike a PV cell, they may be reflected or absorbed, or they may pass right through. Only the absorbed photons generate electricity. When this happens, the energy of the photon is transferred to an electron in an atom of the cell (which is actually a semiconductor). With its newfound energy, the electron is able to escape from its normal position associated with that atom to become part of the current in an electrical circuit. By leaving this position, the electron causes a "hole" to form. Special electrical properties of the PV cell—a built-in electric field—provide the voltage needed to drive the current through an external load (such as a light bulb).

The PV cell is the basic unit in a PV system. An individual PV cell typically produces between 1 and 2 watts, hardly enough power for the great majority of applications. But we can increase the power by connecting cells together to form larger units called modules. Modules, in turn, can be connected to form even larger units known as arrays, which can be interconnected for more power, and so on. In this way, we can build a PV system to meet almost any power need, no matter how small or great.

Modules or arrays, by themselves, do not constitute a PV system. We must also have structures on which to put them and point them toward the sun, and components that take the direct-current (dc) electricity produced by the modules or arrays and condition the electricity so it can be used in the specific application. These structures and components are referred to as the balance of system (BOS).

The basic photovoltaic cell typically produces only a small amount of power. To produce more power, cells can be interconnected to form modules, which can in turn be connected into arrays to produce yet more power. Because of this modularity, PV systems may be designed to meet any electrical requirement, no matter how large or how small.

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Flat-plate collectors typically use large numbers or areas of cells that are mounted on a rigid, flat surface. These cells are encapsulated with a transparent cover that lets in the sunlight and protects them from the environment.

One typical flat-plate module design uses a substrate of metal, glass, or plastic to provide back structural support; encapsulant material to protect the cells; and a transparent cover of plastic or glass.

Flat-plate collectors have several advantages in comparison to concentrator collectors. They are simpler to design and fabricate. They do not require special optics, specially designed cells, or mounting structures that must track the sun precisely. Plus, flat-plate collectors can use all the sunlight that strikes them - both the direct sunlight and the diffuse sunlight that is reflected from clouds, the ground, and nearby objects.

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The performance of a PV array can be improved in a number of ways. One option is to employ concentrating optics, which gather sunlight with lenses, thereby increasing the intensity of sunlight striking the PV cell. (This is similar to using a magnifying glass.)

A typical basic concentrator unit consists of a lens to focus the light, a cell assembly, a housing element, a secondary concentrator to reflect off-center light rays onto the cell, a mechanism to dissipate excess heat produced by concentrated sunlight, and various contacts and adhesives. Notice that the module depicted uses 12 cell units in a 2x6 matrix. These basic units may be combined in any configuration to produce the desired module.

The primary reason for using concentration is to decrease the area of solar cell material being used in a system; solar cells are the most expensive components of a PV system, on a per-area basis. A concentrator uses relatively inexpensive materials (plastic lenses, metal housings, etc.) to capture a large area of solar energy and focus it onto a small area, where the solar cell resides. One measure of the effectiveness of this approach is the concentration ratio (how much concentration the cell is receiving).

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We may think of a complete PV system as comprising three subsystems. On one side, we have the PV devices (cells, modules, arrays, etc.) that convert sunlight into direct-current (dc) electricity. On the other side, we have the load, or the application for which the PV electricity is intended. Between these, we need a third subsystem to enable the PV electricity to be properly applied to the load. This third subsystem is generally referred to as the "balance of system" or BOS.

This simple illustration shows the elements required to get the power created by the PV system to the end load (in this example a house). The stand-alone system (a) uses battery storage to provide dependable dc electricity day and night. Even for a home connected to the utility grid (b), PV can produce electricity during the day (converted to ac through the power conditioner). This configuration is desirable because extra electricity can be sold to the utility during the day, and the utility can in turn provide electricity at night or during poor weather. See this in action.

The BOS typically consists of structures for mounting the PV arrays or modules and the power-conditioning equipment that adjusts and converts the dc electricity to the proper form and magnitude required by an alternating-current (ac) load. If required, the BOS also includes storage devices, such as batteries, for storing PV-generated electricity to be used during cloudy days or at night.

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The sun's energy is vital to life on Earth. It determines the Earth's surface temperature and supplies virtually all the energy that drives natural global systems and cycles. Although some other stars are enormous sources of energy in the form of X-rays and radio signals, our sun releases the majority of its energy as visible light. Yet, visible light represents only a fraction of the total radiation spectrum; infrared and ultraviolet rays are also significant parts of the solar spectrum.

The sun emits virtually all of its radiation energy in a spectrum of wavelengths that range from about 2x10-7 to 4x10-6 m. The majority of this energy is in the visible region. Each wavelength corresponds to a frequency and an energy; the shorter the wavelength, the higher the frequency and the greater the energy (expressed in eV, or electron volts).

Each portion of the solar spectrum is associated with a different level of energy. Within the visible portion of the spectrum, for example, red light is at the low-energy end and violet light is at the high-energy end (having half again as much energy as red light). In the invisible portions of the spectrum, photons in the ultraviolet region, which cause the skin to tan, have more energy than those in the visible region. Likewise, photons in the infrared region, which we feel as heat, have less energy than the photons in the visible region.

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