Converting light waves to electricity is the basic function of the photovoltaic (PV) cells within solar panels. Understanding what light wave PV cells use involves both general knowledge of the solar spectrum and background knowledge of the PV effect, which makes electrical conversion possible. The spectrum of useful light varies based on the conducting material of the PV.
The PV effect happens when light waves strike a solar cell. This effect was discovered in 1954 by Bell Telephone scientists, as stated by the National Renewable Energies Laboratory (NREL). They found that silicon, when exposed to sunlight, creates an electric charge. Basically, this happens because a semiconductor (such as silicon) junction breaks. This frees electrons, and facilitates an electric current. Many different forms of solar panels and thin film PV have evolved, though the basic electricity-forming mechanism is the same.
Solar Spectrum Basics
Solar light waves range from tiny ultraviolet waves to relatively large infrared ones. Wavelength is measured from crest to crest or trough to trough on a single wave. Ultraviolet waves are on the small end of the solar spectrum while Infrared are on the larger end. Everything in between is in the visible portion. Ultraviolet waves can range from about 3 nanometers (nm) to 300 nanometers in length. For scale, one nanometer is equal to one billionth of a meter. Infrared waves are about 750 nm to 1,000 nm. The peak efficiency wavelength(s) differ for solar cells depending on the type and arrangement of semiconducting materials.
Traditional Conducting Materials
Amorphous silicon is the traditional semiconducting material used in solar panels. It utilizes the visible portion of the solar spectrum. This portion is only about 7 percent of the entire spectrum. Because of the unstructured nature of amorphous material, silicon has been stacked and crystallized into monocrystalline and polycrystalline forms to increase efficiencies.
Nontraditional Conducting Materials
Thin film technologies and solar panels now use semiconducting materials such as Copper indium diselenide which can use higher energy portions of the solar spectrum. Higher energy wavelengths are found in the ultraviolet spectrum. Cadmium Telluride (CdTe), another conductor, is "matched nearly perfectly to the solar spectrum" according to the U.S. Department of Energy (DOE). This means it utilizes they use the portion of the spectrum with the highest energies. The site also points out that research is focused on "...exploring innovative transparent conducting oxides that allow more light into the cell to be absorbed..."
Gallium arsenide (GaAs) is yet another conductor being used in solar panels. The knock on GaAs is often that it misses out on a good portion of the high energy photons on the lower end of the solar spectrum compared to CdTe.
Multijunction Thin Film and Panels
The future of PV may indeed be in combining semiconducting materials in a way that allows them to efficiently use all portions of the solar spectrum. The DOE points out that a gallium indium phosphide/gallium arsenide tandem solar cell produced a concentrator cell with "efficiency greater than 30 percent," and calls this an "important milestone" of the National Renewable Energy Laboratories (NREL) PV program.
- Photo Credit green-wave image by Lia Marhold from Fotolia.com
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