Saturday, November 29, 2008

Solar Cell

Solar cells use sunlight to produce electricity by the photovoltaic effect, converting photons into electrons. They do this without the use of either chemical reactions or moving parts. Solar cells have many applications. Cells are used for powering small devices such as electronic calculators. Photovoltaic arrays generate a form of renewable electricity, particularly useful in situations where electrical power from the grid is unavailable such as in remote area power systems, Earth-orbiting satellites and space probes.

Solar cells, or Photovoltaic (PV) cells, are made of special materials called semiconductors such as silicon, which is currently the most commonly used. Basically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely. PV cells also all have one or more electric fields that act to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a current, and by placing metal contacts on the top and bottom of the solar cell, we can draw that current off to use externally.

The photovoltaic effect was first recognized in 1839 by French physicist Antoine Cesar Becquerel, while experimenting with a solid electrode in an electrolyte solution. He observed that voltage developed when light fell upon the electrode. However, it was not until 1883 that the first solar cell was built, by Charles Fritts, who coated the semiconductor selenium with a very thin layer of gold to form the junctions. Early solar cells, however, still had energy-conversion efficiencies of less than 1 percent. This difficulty was finally overcome with the development of the silicon solar cell by Russell Ohl in 1941. In 1954, three other American researchers, G.L. Pearson, Daryl Chapin, and Calvin Fuller, demonstrated a silicon solar cell capable of a 6-percent energy-conversion efficiency when used in direct sunlight.

Modern solar cells are based on semiconductor physics. They are basically just P-N junction photodiodes with a very large light-sensitive area. The photovoltaic effect, which causes the cell to convert light directly into electrical energy, occurs in the three energy-conversion layers. The first of these three layers necessary for energy conversion in a solar cell is the top junction layer, which is made of N-type semiconductor. The next layer in the structure is the core of the device; this is the absorber layer, the P-N junction. The last of the energy-conversion layers is the back junction layer, which is made of P-type semiconductor.

The first spacecraft to use solar panels was the US satellite Vanguard 1, launched in March 1958 with solar cells made by Hoffman Electronics. This milestone created interest in producing and launching a geostationary communications satellite, in which solar energy would provide a viable power supply. This was a crucial development which stimulated funding from several governments into research for improved solar cells. In 2007 Scientists at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) have set a world record in solar cell efficiency with a photovoltaic device that converts 40.8 percent of the light that hits it into electricity. This is the highest confirmed efficiency of any photovoltaic device to date.