BASIC PHOTOVOLTAIC CELL TECHNOLOGY 101: In 1839, Edmond Becquerel noticed that, in addition to heat, the sunlight that is absorbed by certain materials can produce small quantities of electricity. This curious phenomenon was limited to measuring light levels in photography until the 1950s. Then, the combination of improved purification techniques for semiconductors, the advances in solid state devices beginning with the development of the transistor in 1947, and the needs of the emerging space program, led to the development of photovoltaic cells. In 1954, a 4% efficient silicon crystal photovoltaic cell was demonstrated. By 1958, a small silicon array was used to supply electrical power aboard a U.S. satellite.
Photovoltaic cells convert sunlight directly into electricity by the interaction of photons and electrons within the semiconductor material. To create a photovoltaic cell, a material such as silicon is doped with atoms from an element with one more or less electrons than occurs in its matching substrate (e.g. silicon). A thin layer of each material is joined to form a junction. Photons, striking the cell, cause this mismatched electron to be dislodged, creating a current as it moves across the junction. Through a grid of physical connections, the current is gathered. Various currents and voltages can be supplied through series and parallel arrays of cells.
The DC current produced depends on the material involved and the intensity of the solar radiation incident on the cell. Most widely used today is the single crystal silicon cell. The source silicon is highly purified and sliced into wafers from single-crystal ingots or is grown as thin crystalline sheets or ribbons. Polycrystalline cells are another alternative, which are inherently less efficient than single crystal solar cells, but also cheaper to produce. Gallium arsenide cells are among the most efficient solar cells today, with many other advantages, but are also very expensive.
Another approach to producing solar cells that shows great promise is thin films. Commercial thin films today are principally made from amorphous silicon; however, copper indium diselenide and cadmium telluride also show promise as low-cost solar cells. Thin-film solar cells require very little material and can be easily manufactured on a large scale. Manufacturing lends itself to automation and the fabricated cells can be flexibly sized and incorporated into building components.
Today’s prevalent cell technologies are based on a single junction, which can use only a portion of the sun’s energy spectrum. However, emerging multi-junction cells will allow many layers to use progressive parts of this spectrum, resulting in higher efficiencies. Various means to produce these layers at acceptable costs are being actively pursued.
Solar cells generate current all over their surface. Electrical connections for the photovoltaic cell are necessary in order to utilize the energy in an electric circuit. There is a trade-off between electrical resistance losses and the loss of active surface area on the solar cell from shading by the collector grid. The highest quality grids are produced using photolithography for image transfer. Crystalline cells typically use a layer of aluminum or molybdenum. The typical thin film does not use a metal grid for the electrical contact, but a transparent conducting oxide, such as tin oxide, indium oxide, or zinc oxide.
All of these areas are under active research. Cell improvements have been impressive in recent years, as measured by steadily declining cell costs and increasing efficiencies.
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See You Next Time! Dr. Stripling