Solar cells are optoelectronic devices that convert the energy of solar radiation directly into electricity by the photovoltaic (PV) effect. Assemblies of cells electrically connected together are known as PV modules, or solar panels. The photovoltaic effect was first recognized in the 19th century but the modern PV cells were developed in the mid-1950s. The practical application of photovoltaics started to provide energy for orbiting satellites.