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What
are Photovoltaic cells?
Photovoltaic cells were developed at Bell Laboratories in the early 1950s as a spin-of transistor technology. Very thin layers of pure silicon are impregnated with thin amounts of other elements. When exposed to sunlight, small amount of electricity are produced.
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Brief Technical
Explanation
A single PV cell is a thin semiconductor sandwich, with two layers of highly purified silicon. The layers have been slightly doped with boron on one side, and with phosphorous on the other side. Doping produces either a surplus or a deficit of electrons depending on which side we¡¯re looking at. Electronics-savvy folks will recognize these as p-and n-layers. When our sandwich is bombarded by sunlight, photons knock off some of the excess electrons. This creates a voltage difference between the two sides of the wafer, as the excess electrons try to migrate to the deficit side. In silicon this voltage difference is just under half a volt. Metallic contacts are made to both sides of the wafer. If an external circuit is attached to the contacts, the electrons have a complete circuit and a current flows. When enough of these cells are coupled to form a panel, they produce a useable voltage that can be routed to the battery banks. ¡¡ **
Construction
Types
Silicon
is the most common photovoltaic material and it is used to make
the cells in three different ways: the silicon can be cut and
single layers applied to make the cells, known as single/mono
crystalline cells; many smaller slices of crystals are used to
from the cell to make multi or poly crystalline cells; or,
minute particles of silicon are vaporized onto a surface to from
what are known as amorphous cells. It is possible to
differentiate between crystalline and amorphous cells by their
appearance. Crystalline panels show their crystals, while an
amorphous panel looks as if it has been sprayed with metallic
paint.
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Single
or Mono Crystalline This is the oldest and most expensive production technique, but it¡¯s also the most efficient sunlight conversion technology commercially available. Complete modules have sunlight to wire output efficiency averages of about 10% to 12%. Examples are Siemens and Astropower single crystalline produces.
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Multi
or Polycrystalline It
is just slightly lower in conversion efficiency compared to
single crystal, but the process is less exacting. Examples are
Solarex and kyocera multi-crystalline products.
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Amorphous
or Thin-Film In this technique, silicon material is vaporized and deposit on glass. This production technology costs less than any other method, but method, but the cells are less efficient, so more surface area is required. Breakage is more of problem because tempered glass can¡¯t be used with the high-temperature deposition process. Sunlight to wire efficiency averages about 5% to 7%. |