|MadSci Network: Engineering|
The Department of Energy has an excellent web site describing photovoltaic panels. Additional information can be found in the Encyclopedia or a good Physics text on semiconductor devices.
Photovoltaic panels or solar cells are flat panels that convert light directly into electricity. They are made from semiconductor materials like Silicon or Gallium-Arsenide. There are two broad classifications of photovoltaic panels: crystalline and amorphous.
Crystalline panels are made from extremely pure materials grown from seed crystals. The process is very similar to that used for transistors and other semiconductor devices. The high purity and single crystal nature of these panels makes the process costly, but yields panels with generally higher output than non-crystalline types. Amorphous panels are made using simpler processes which reduce cost, but yield lower output for a given amount of light.
There are a number of factors which determine the output of a panel. The dominant factors are the material band gap and recombination. Materials can be classified as conductors, insulators, or semiconductors. A material falls into one of these categories depending on its band gap. The band gap is the difference in energy between the valence band (electron bound to an atom) and the conduction band (electron free to move between atoms). Conductors have very low band gaps (much less than 1 electron-Volt or eV), allowing electrons to flow freely. Copper, Aluminum, and Iron all have low band gaps and conduct electricity very well. Insulators like Calcium and Potassium have high band gaps (much greater than 1 eV) and do not conduct electricity very well. Semiconductors are materials in group IV of the periodic chart and include Silicon and Germanium. These material have a band gap close to 1 eV. Combinations of III-IV materials like Gallium and Arsenic can also yield similar band gaps making them semiconductor materials also.
When a photon of light strikes a semiconductor material, it may kick an electron sitting happily in the valence band into the conduction band where it is free to move from atom to atom. Eventually, the electron may end up at one end of the material where it can be collected by electrical contacts. This collection of electrons results in electrical energy produced from light. The probability of energy transfer depends on the wavelength of incoming light. A photon with the same energy as the band gap is more likely to kick electrons into the conduction band than others. It turns out that the Silicon band gap is 1.1 eV which peaks in the infrared region (980 nm). 980nm is also the peak wavelength of Silicon light emitting diodes (LEDs) and laser diodes. An LED is basically the opposite of a photovoltaic panel: it converts electrical energy to light. Gallium-Arsenide has a slightly higher band gap which is in the visible region. Red, green, and yellow LEDs are made from Gallium-Arsenide and Phosphorous. Sunlight ranges between between 0.5 eV and 2.9 eV, because of this, about 55% of the Suns energy is either too high or too low to kick electrons into the conduction band.
After an electron is kicked into the conduction band, there is a probability that it will fall back into the valence band. If this happens, its energy will be lost as heat. This is known as recombination. The trick to constructing high efficiency solar cells lies in minimizing recombination.
Due to these factors, photovoltaic cells can convert only about 7% to 17% of incoming light into electricity. A typical solar cell about one inch square will deliver about a tenth of a Watt (100mW) in the midday Sun. Variation in sunlight due to the Suns position in the sky and cloud cover reduce the available power. High efficiency cells tend to be too expensive for use as a replacement for convention power genertion plants. Even with these restrictions, solar cells find uses in areas where moderate power (on the order of hundreds of Watts) is needed with little or no maintainence. Last week I climbed to the top of a 500 foot volcanic mound on the island of Tenerife only to find several solar panels using sunlight to charge batteries. The stored energy in the batteries was used to power a night time beacon warning aircraft on the approach path to Tenerife South airport. The beacon works year round, with no power lines and little, if any, maintainence.
Solar cells are sold by a number of companies. You can purchase them at your neighborhood Radio Shack or order them from Edmunds Scientific. They are fun to play with, but tend to be rather delicate and crack easily.
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