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How Solar Cells Work

Converting Photons to Electrons

Photovoltaics (PV) comes from the roots “photo” (meaning “light”) and “voltaic” (electricity) and its main function is to convert sunlight into electricity that can power almost everything from calculators to homes in remote areas to even satellites.

Current models of PV cells are made from semiconductors like silicon. When light in the form of photons comes into contact with the cell, the cell absorbs some of that energy and this sets the electrons in motion i.e. creating an electrical current. These electrons can also be restricted into flowing in a certain direction depending on whether electric fields are present or not. The addition of metal contacts on the top and bottom of a PV cell makes the electrical energy collected from light available in the form of a current.

Silicon in Solar Cells

Silicon is a semiconductor, meaning that it possesses both metal and non-metal properties. A silicon atom has 14 electrons so its valence shell contains four electrons that are free to form bonds with other silicon atoms. Pure silicon forms a solid, crystalline structure, but the material used in PV cells contains impurities such as one phosphorus atom (with five valence electrons) and one boron atom (three valence electrons) for every million silicon atoms. These impurities provide free carriers (extra electrons) that can carry electrical current, and the result is an electric field that only permits electrons to move in one direction.
Photons striking the semiconductor material will provide enough energy to release electrons and start an electrical current.

An antireflective coating is added to the top of PV cells to fight against the reflection of photons since silicon is a very shiny material, and this reduces reflection losses to less than five percent. A glass cover plate also serves as protection for the PV cell from the elements.

Basic structure of a generic silicon PV cell

Other materials besides single crystal silicon that can be used to manufacture photovoltaic cells include polycrystalline silicon, amorphous silicon, gallium arsenide, copper indium diselenide, and cadmium telluride. Sometimes these materials are used in combination with other semiconductor materials to produce an even more efficient PV cell.

A typical photovoltaic system also uses deep-cycle batteries (batteries that can discharge a fair amount of stored energy while maintaining long life) to store excess electricity that is produced. This is another way to make the cells more efficient especially on cloudy days when there is less exposure to direct sunlight.

1. Aldous, Scott. How Solar Cells Work. How Stuff Works, Inc.      <http://science.howstuffworks.com/solar-cell.htm/printable>


Glenforest Secondary School, Ontario, Canada
ISTF Project #02-541

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