Solar Cells Shedding a little light on photovoltaics -- 28 February, 2002 History and background Solar cell physics Available types Performance and use History and background Solar cells are near and dear to the hearts of BEAMers since they convert sunlight (or more accurately, any available ambient light) into electricity. For small BEAMbots, this saves you the issues normally associated with battery power (weight, size, the inconvenience of replacing or recharging batteries). Solar cells do their work via the photovoltaic (photo = light, voltaic = electricity) effect; accordingly, you'll sometimes see them referred to as photovoltaic cells, or PV cells. While solar cells can be used to power homes, businesses, and satellites, we'll be more concerned with their BEAMable uses -- but more on that point later. History The development of the solar cell stems from the work of the French experimental physicist Antoine-César Becquerel back in the 19th century. In 1839, Becquerel discovered (although he could not fully explain) the photovoltaic effect while experimenting with an electrolytic cell containing two metal electrodes. The then nineteen year old found that certain metals and solutions would produce small amounts of electric current when exposed to light. In 1877, Charles Fritts constructed the first true solar cells (at least, the first resembling modern cells in that it was made from only solid materials) by using junctions formed by coating the semiconductor selenium with an ultrathin, nearly transparent layer of gold. Fritts's devices were very inefficient, transforming less than 1 percent of the absorbed light into electrical energy, but they were a start. Substantial improvements in solar cell efficiency had to wait for a better understanding of the physical principles involved in their design, provided by Einstein in 1905 and Schottky in 1930. By 1927 another metalÐsemiconductor-junction solar cell, in this case made of copper and the semiconductor copper oxide, had been demonstrated. By the 1930s both the selenium cell and the copper oxide cell were being employed in light-sensitive devices, such as photometers, for use in photography. These early solar cells, however, still had energy conversion efficiencies of less than 1 percent (so they made fine light sensors, but lousy energy converters). Solar cell efficiency finally saw substantial progress with the development of the first silicon cell by Russell Ohl in 1941. In 1954, three other American researchers, G.L. Pearson, Daryl Chapin, and Calvin Fuller, demonstrated a further-refined silicon solar cell capable of a 6% energy conversion efficiency (in direct sunlight). By the late 1980s silicon cells, as well as those made of gallium arsenide, with efficiencies of more than 20% had been fabricated. In 1989 a concentrator solar cell, a type of device in which sunlight is concentrated onto the cell surface by means of lenses, achieved an efficiency of 37% thanks to the increased intensity of the collected energy. Modern solar cells are basically just P-N junction photodiodes with a very large light-sensitive area. So let's move along to get a bit of background on the semiconductor physics involved here. History and background Solar cell physics Available types Performance and use

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