Photovoltaic cells, commonly known as PV cells, are thin layers of pure silicon that are impregnated with tiny amounts of other elements such as boron and phosphorous. When exposed to sunlight, they produce small amounts of electricity. They have been around since the 1950s, and were initially used as a power source for satellites in space. Since then, their prices have steadily come down from $40,000 per watt in the early 1960s to just $1 per watt or less today. PV cells are an efficient and long-lived power source, and are a great alternative to traditional energy sources such as coal and gas.