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Multijunction

The importance of a multijunction panel is not so much the materials used to make it, but how the materials are employed. The term junction, in a solar panel, refers to the location where p-type and n-type materials come close to one another. These p-n junctions are the entire reason that photovoltaic panels work, but they have a limitation.

The amount of energy needed to excite an electron to leave its atom at a p-n junction is often called the bandgap energy. The overall concept is quite complicated, but the basic premise is that each bandgap and thus each p-n junction, has a specific energy of activation. That is to say, specific wavelengths of light are needed to excite the electrons at specific p-n junctions. If the energy is too low, nothing happens. If the energy from the light is too high, some of it is wasted. Different p-n junctions are tuned to different wavelengths of light, with some operating better in the infrared and some better near the blue/ultraviolet range.

Multijunction panels are just what they sound like. Rather than having one type of p-n junction, they have several. This allows these panels to take advantage of a broader range of light and thus be more efficient. For a single p-n junction silicon cell, theoretical maximum efficiency is 34%. However, for an infinite junction cell, the theoretical limit is nearly 90%. It is through the application of multijunction technology that panels are able to reach higher and higher efficiencies.

Materials Used in Multijunction Cells

There are several different types of materials that can be used to produce multijunction cells. Amorphous silicon is one of the most popular because it is the least difficult and least expensive to produce. The use of multiple thin film a-Si cells that have p-n junctions tuned to different wavelengths of light is cost effective and relatively simple to accomplish.

Other multijunction cells are produced as “monolithically integrated,” which means the various materials and junctions are all in one layer, rather than in several thin layers as with a-Si. This is a more efficient system in terms of energy collection, but also substantially more difficult to construct.

In some panels a combination of indium gallium phosphide, gallium arsenide, and germanium are used to create triple junction panels. Double junction panels are created through the use of gallium arsenide and indium gallium phosphide or indium gallium arsenide are all used. The combination results in highly efficient panels that are also incredibly expensive to manufacture. Most of this type of technology is still in the research and development stage, but triple junction cells have already reached efficiencies of 40% with a theoretical limit of over 60%.

Multijunction Uses

Fabrication of multijunction cells is not all that dissimilar from semiconductor chips in computers. The only major difference is the larger size of photovoltaic cells. This manufacturing process is expensive, which has limited the use of multijunction cells to satellites and other applications where power-to-weight ratios overwhelm almost all cost considerations.
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