Photovoltaic Materials
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Photovoltaic Materials During the worldwide energy crisis in the 1970s, researchers focused enormous effort toward developing new energy sources, including "clean and inexhaustible" solar power. Unlike electric generators, solar cells have no moving parts, thus eliminating one common wear mechanism. And unlike batteries or fuel cells, solar cells produce no pollution from using chemical reactions to produce power. Small solar cells placed into arrays can be used to generate power in remote locations, such as for water pumps in the deep desert, navigational beacon buoys at sea, and satellites in space. Thousands of cells can work together as central electric power stations, or smaller arrays can power hand calculators and portable telephones. They can operate almost indefinitely if protected from damage. The concepts and crude prototypes for solar power have been around for about a century and a half. The theory behind solar cells was first touched on in 1839 by the physicist Becquerel in France. Becquerel experimented with a solid electrode immersed in an electrolyte solution, observing that a voltage developed when light fell upon the electrode. This is called the photovoltaic effect. In 1887 Heinrich Rudolf Hertz, a physicist in Germany, discovered the photoelectric effect. Hertz observed that ultraviolet light could facilitate a spark across an air gap between metallic electrodes. It wasn't until 1899 that Joseph J. Thomson (discoverer of the electron) and Philipp Edward Anton Lenard determined that light was causing the emission of electrons from the surface of a material. By 1902, Lenard had also shown that the number of electrons emitted per second from a photoelectric material was proportional to the intensity of the incident light, but that the energy of those electrons depended on the wavelength of the incident light. This result was puzzling and incomprehensible to the classical physics community at the time, and was not resolved until 1905, when Albert Einstein proposed a quantum theory on the dual nature of light. It was primarily for this work that Einstein won the Nobel Prize in 1921. Working from the 50-year-old theories of Becquerel, Charles Fritts constructed the first actual solar cell in 1889 by coating the semiconductor selenium with an extremely thin layer of gold. Though Fritts' prototype cells succeeded in converting only 1% of the incident sunlight into electricity, they fostered visions of clean and
inexhaustible power (which must have seemed very desirable in the dirty, early days of the industrial revolution). Two years after Fritts created his first cell, R. Appleyard wrote excitedly of "the blessed vision of the Sun, no longer pouring his energies unrequited into space, but by means of photoelectric cells... gathered into electrical storehouses to the total extinction of steam engines and the utter repression of smoke." Also in 1889, Julius Elster and Hans Gertel published the first of a long series of papers on photoelectricity. They showed that when certain metals (including potassiu
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