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Question

How does a solar cell work? Is it possible to create one using simple lab apparatus?
Asked by: Megha

Answer

Solar cells (photovoltaics), use the energy from light photons to create electrical potential between two layers of silicon crystal. The atomic nature of silicon, with some added impurities, is what makes it all possible. The outer orbital electron shell of a silicon atom contains four electrons. Since it takes eight electrons to fill the electron shell, a silicon atom is continually looking for four electrons to bond with. This it finds by bonding covalently with other atoms of silicon forming a characteristic crystalline structure. Silicon atoms thusly joined are very stable and are not electrically conductive, but this is where the impurities come in. By �doping� the silicon with substances such as phosphorus and boron, entirely different electrical properties are introduced into the silicon creating semi-conductive material.

For instance, when phosphorus joins with silicon, it creates an N-type semi-conductive material because phosphorus has five electrons in its outer shell. The silicon wants four of them but that leaves one electron hanging out by its lonesome and giving the molecule a negative charge. If boron joins with silicon, it creates P-type semi-conductive material (positive charge), as boron has three electrons in its outer shell. Even though silicon bonds with it, it leaves an electron �hole,� where the molecule is positively charged and is still seeking an electron.

If layers of phosphorus impregnated silicon and boron-impregnated silicon are joined together with metal leads or conduits, an electrical potential can be created with some help from light. When light photons strike the phosphorus layer containing the extra electrons, those electrons can be sheered off and freed. When they are, they immediately recognize the potential in the boron layer and head that way. If a load (some work that you want to have done with electricity), happens to be connected in between these two layers where the potential has been created, then the migrating electrons are useful electrical current.

Solar cells are a wonderful alternative energy sources but have definite limitations. Since not all visible light is useful for this process, most of the sunlight energy can not be used to free electrons in the solar cells. Much of it is reflected or passes through not hitting the desired electron target. In addition, the electrical potential is very small and even with the most efficient solar cells; they must be chained together in large arrays to produce enough electricity to be useful. Because of the nature in which they produce their electricity, solar cells do experience a slight drop in effectiveness but they essentially never wear out. Then of course the most obvious problem: what do you do if the sun is not shining?

The nature work required to fabricate semi-conductive materials is probably beyond the realm of simple lab equipment. But many solar cell companies will give away broken cell fragments for the asking if you are looking for something to play with.

Also, if you are an educator, contact the Florida Solar Energy Center.

References:
http://www.nooutage.com/howsolar.htm http://www2.gasou.edu/chemdept/general/molecule/polar.htm
Answered by: Stephen Portz, Technology Teacher, Space Coast Middle School, FL



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