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Most people do not realize it, but the light that comes from our medium-sized yellow sun is actually made up of all colors ' including those not even visible to the human eye, such as infrared and ultraviolet. (In addition to light waves, or photons, the sun also gives off x-rays and radio waves.) You can see these various wavelengths in the very common and natural phenomenon known as a rainbow, which is nothing more than sunlight hitting water droplets in the atmosphere. You can even create your own miniature rainbow with a garden hose and spray nozzle, or with the use of certain crystals known as prisms.

All well and good ' but what has this to do with photovoltaic solar cells? Quite a bit, actually. One of the historic problems that has limited the efficiency of the typical photovoltaic system is that the solar cells are able to use only a small part of the light spectrum. The standard silicon-based solar cell has an efficiency rate of between twenty-five and forty-five percent. The reason is that such single-junction photovoltaic solar cells utilize only the main, broadest part of the light spectrum. However, because multijunction photovoltaic solar cells can use the entire spectrum, their efficiency ratings are considerably higher, approaching 90%.

Multijunction photovoltaic cells are made up of several layers of semiconductors. Between these layers are what are known as p-n junctions. The "p-n" stands for "positive" and "negative," referring to the types of electrical charges. The science here is rather complicated; suffice it to say that as far as photovoltaic solar cells are concerned, this is where the rubber hits the road (figuratively; rubber is actually an insulator that blocks electrical energy). By literally "tuning" these p-n junctions to different light wavelengths, creating "multiple junctions," it is possible to manufacture a photovoltaic system that utilizes the entire spectrum.

If solar energy is ever able to reach "grid party" with fossil-fuel fired power plants (the virtual "Holy Grail" of solar energy technology), it will be with the use of multijunction photovoltaic technology.

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