Cadmium telluride (CdTe) photovoltaics is a technology that uses the compound cadmium telluride (CdTe) as the semiconductor material in photovoltaic cells that convert sunlight into DC (direct current) electricity. Photovoltaic (PV) cells are an integral part of solar-electric energy systems, which are becoming increasingly important as alternative sources of utility power. Cadmium telluride offers some advantages over crystalline silicon (Si), the substance originally used in the manufacture of PV cells. A functional layer of CdTe can be made extremely thin, on the order of a few micrometers (units of 10^-6 meter), far thinner than a typical Si layer. Cadmium telluride offers better efficiency than Si at high temperatures and low levels of illumination. In addition, CdTe PV cells require less energy to produce than Si PV cells do. Cadmium (Cd) and tellurium (Te) are both products of zinc, copper, and gold mining. On the downside, some environmental groups have expressed concern over the existence of Cd in CdTe PV cells. Cadmium is a toxic heavy metal (similar to lead and mercury in this respect). However, the proponents of CdTe technology point out that the compound does not dissolve in water, so the elemental Cd should not pose any hazard to aquifers. Furthermore, CdTe will not vaporize in a fire. Some concern exists over the worldwide Te supply, much of which exists in China. However, the sea floor contains abundant Te, so if a means can be found to mine it, shortages will not pose a problem in the long term. Cadmium telluride technology offers promise for the future of alternative energy, if only because conventional energy sources continue to suffer increasing scrutiny, regulation, and price volatility. At present, PV-derived electricity remains considerably more expensive than electricity produced by coal-fired, natural-gas-fired, or nuclear power plants. Over time, this situation may change as PV technology becomes less costly and conventional energy sources grow ever-more expensive and precarious