Definition   Photovoltaic (PV) Cells convert sunlight directly into electricity. When sunlight strikes a PV cell, electrons are dislodged, creating an electrical current.       The French physicist Edmond Becquerel first described the photovoltaic effect in 1839, when he discovered that certain materials produced small amounts of electric current when exposed to light. Despite these observations, however, photovoltaic cells remained a curiosity of science for the next three quarters of a century.         PV cells first started to be studied seriously in the 1870s by Heinrich Hertz, who examined the photovoltaic effect in solids such as selenium. Soon afterward, selenium PV cells were converting light to electricity at 1 to 2 percent efficiency. Selenium was therefore quickly adopted in the emerging field of photography for use in light-measuring devices.           A major step toward commercial application of photovoltaic cells was taken in the 1940s and early 1950s, when the Czochralski process was developed for producing highly pure crystalline silicon. In 1954, scientists at Bell Laboratories used the Czochralski process to develop the first crystalline silicon photovoltaic cell, which had an efficiency of 4 percent. This launched the development of photovoltaic cells for commercial and residential use. To see how far this industry has come, one may compare these early cells with modern ones: the earliest PV devices converted about 1 to 2 percent of sunlight energy into electric energy, whereas the PV cells of today attain efficiencies of 7 to 17 percent.           The first PV cells were originally developed for use in space, where repair is extremely expensive if not impossible. PV still powers nearly every satellite circling the earth because it operates reliably for long periods of time and requires virtually no maintenance.           The efficiency of a photovoltaic cell is measured by its conversion efficiency, or the proportion of sunlight energy the cell converts to electrical energy. This is very important to the viability of the PV industry, as improving this efficiency makes PV energy competitive with that generated by fossil fuels.