photovoltaic cells :: essays research papers

  Ã‚  Ã‚  Ã‚  Ã‚  Photovoltaic cells are solid state devices that convert light directly into electricity. Photovoltaic literally means â€Å"light electricity.† These devices can be commonly found providing power for small scale devices such as calculators, watches, and small radios. However, they are not limited to small scale systems. They are also used to power satellites, communications equipment, houses and many other things, especially in remote locations where a power grid is not readily available. In isolated locations the only power attainable comes from the sun. The sun shines approximately 1000 watts of energy per square meter of the planets surface, which if harnessed could power any city.   Ã‚  Ã‚  Ã‚  Ã‚  The commercial development of the photovoltaic cell took more then a hundred years to begin. A french physicist Edmond Becquerel first described the photovoltaic effect in 1839. At the age of 19 Becquerel found that certain materials when exposed to light produced small measurable currents. Henrich Hertz also studied the effect in solids in the 1870's and he managed to produce photoelectric cells with an efficiency of about 1%. In the 1940's the new Czochralski process made generating highly pure crystalline silicon possible and furthermore made commercializing photovoltaic cells an option. Development really started however, in 1954 when Bell Laboratories used the Czochralski process to produce a 4% efficient crystalline silicon cell, which was essentially for application in space. This was the beginning of photovoltaic cells.   Ã‚  Ã‚  Ã‚  Ã‚  Photovoltaic cells generally consist of five layers. Two of these layers are semiconductors and the remaining three are merely for protection and to aid absorption of light. The top layer is generally a glass cover for protection against the elements since photovoltaic cells are used outside. Underneath this is an anti-reflective coating. Since most photovoltaic cells are made of silicon and silicon is reflective this prevents the photons from merely being bounced off the cell and allows the highest amount of absorption possible. These two layers are secured with a transparent adhesive. Transparent, obviously, so the photons can pass through it. Surrounding the entire cell is a metallic grid on top and a metal contact on bottom.   Ã‚  Ã‚  Ã‚  Ã‚  The two layers of semiconductor are the most important. Semiconductors are materials that have electrical conductivity intermediate between the high conductivity of metals and the low conductivity of insulators. Conductivity is decided by how many free electrons are available to carry charge, and in turn how tightly electrons are bound to the parent atom.