A photodiode whose spectral response has been optimized for daytime radiation from the sun. Solar cells are semiconductor devices that provide electrical energy from solar radiation by means of photogeneration of charge carriers at a p-n junction. Early solar cells used compound semiconductors such as cadmium sulphide, which has an energy band gap corresponding to visible light. More modern devices use silicon p-n junctions, where the band gap corresponds to infrared wavelengths, generating a greater electrical output. Compound-semiconductor solar cells based on indium phosphide are used in critical radiation-hard conditions; such devices produce the highest conversion efficiency, about 20%, but are much more costly than silicon devices.
For practical use, large numbers of solar cells can be assembled in flat arrays, known as solar panels. Solar panels are used mainly as a source of power in spacecraft.