trunkmonk not to be over critical but you are splashing around in my field. I have found a good simple explanation of the solar cell that should get the job done.
A Primer on Solar Photovoltaics and PV Systems
A constant and universal sun, a simple device with no fuel and no moving parts, and energy as near as your backyard: this image of photovoltaics has sparked the imagination of millions of people. What this technology is and how it is packaged to produce useful electricity is the subject of this fact sheet.
The Solar Resource
The sun bathes the earth with more energy each minute than the world consumes in one year. But, except in the tropics, the sun is never directly overhead and its intensity varies by season. For example, at a latitude of 45°, solar radiation may vary from 92% (early summer) to 38% (early winter) of theoretical maximum insolation. The average intensity at this latitude is 71% (early spring and fall) of maximum.
At higher latitudes, solar radiation also follows a longer path through the earth's atmosphere. Scattering and absorption of incident and reflected radiation by gases such as CO2, methane, chlorofluorocarbons, and particulates further influence the solar resource available.
These global conditions-- plus local variations such as cloud cover, topography, and altitude-- cause solar to be a variable resource. The figure to the right presents a contour map of the U.S. which shows some of this variability. This type of map is very familiar to anyone who has investigated solar energy. One of the unfortunate aspects of such maps, however, is that they tend to suggest that the solar resource is most important in the Southwestern U.S., much less so in the far Northeast or Northwest. This perception has caused many utilities in those parts of the country to discount the technology.
But straight solar insolation values may be deceptive, especially from a utility standpoint. These maps ignore the capacity value from PV. Recent work by the National Renewable Energy Laboratory (NREL) is showing that high levels of solar insolation are not a necessary condition for finding good sites for PV. A study involving about 35 utilities that correlated system load curves with PV production found many areas of the country with lower insolation levels that received excellent capacity matches. The same study found that the utility's summer/winter peak load ratio provided a good proxy of capacity contribution, with the higher ratios showing higher contributions.
In fact, the Solar Electric Association's market evaluation work has shown that PV can make a contribution to every utility in every part of the country.
Basic Photovoltaic Cell Technology
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 to 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 expensive.
Another approach to producing solar cells that shows great promise are 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 multijunction 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.
