Solar Cells Shedding a little light on photovoltaics -- 28 February, 2002 History and background Solar cell physics Available types Performance and use Available types of solar cells -- materials and packaging Materials The most common material used in solar cells is single crystal silicon. Solar cells made from single crystal silicon are currently limited to about 25% efficiency because they are most sensitive to infrared light, and radiation in this region of the electromagnetic spectrum is relatively low in energy. Figure 3 -- Single crystal solar cells (image courtesy ACRE ) But single crystal silicon isn't the only material used to build solar cells. Figure 4 -- Polycrystalline solar cells (image courtesy ACRE ) Polycrystalline ("many crystals") solar cells are made by a casting process in which molten silicon is poured into a mould and allowed to cool, then sliced into wafers. This process results in cells that are significantly cheaper to produce than single crystal cells, but whose efficiency is limited to less than 20% due to internal resistance at the boundaries of the silicon crystals. Figure 5 -- Amorphous solar cells (image courtesy ACRE ) Amorphous cells are made by depositing silicon onto a glass substrate from a reactive gas such as silane (SiH4). This type of solar cell can be applied as a thin film to low cost substrates such as glass or plastic. Thin film cells have a number of advantages, including easier deposition and assembly, the ability to be deposited on inexpensive substrates, the ease of mass production, and the high suitability to large applications. Since amorphous silicon cells have no crystal structure at all, their efficiencies are presently only about 10% due to significant internal energy losses. Aside from the various forms of silicon, a number of other materials can also be used to make solar cells -- gallium arsenide, copper indium diselenide and cadmium telluride to name a few. Note that solar cells are sensitive to different wavelengths of light (i.e., photons of different energies) as a function of the materials they are built from. Accordingly, some cells are better performers outdoors (i.e., optimized for sunlight), while others are better performers indoors (optimized for fluorescent light). Newer, high-tech solar cells have yielded improved energy conversion efficiency by incorporating two or more layers of different materials with different wavelength sensitivities. Top layers are designed to absorb higher energy photons while allowing lower energy photons through to be absorbed by the layers beneath. Double-junction cells are commercially available to BEAMers on occasion (surplus shops often call these "spacecraft cells"). Spacecraft and other mass-sensitive applications have now started to make use of triple-junction cells (but don't expect to find these in surplus shops any time soon). Cell Packaging Solar cells also are available in a variety of packages. Most common are "raw cells," often with some cover sheet attached. One popular line of cells, the Panasonic Suncerams, consist of amorphous silicon cells, deposited on the back of a glass substrate (in this case, the glass functions as both substrate and cover sheet). These are durable and cost-effective cells, if a bit heavy due to the thickness of the glass. Encapsulated solar cells are also sold -- as the name implies, an enclosure (often plastic, often with some sort of concentrator lenses built into the cover sheet) contains a regular (generally multicellular) solar cell or cells. These are extremely durable, if heavy and none too efficient. Recently, flexible solar cells have become available. These are amorphous cells on a thin plastic substrate -- low efficiency, fairly high cost, but light and a very useful package for some applications. Now let's move on to the practical issues involved in using solar cells. History and background Solar cell physics Available types Performance and use

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