photovoltaic

A photovoltaic material converts light straight into electrical energy. The US Department of Energy puts it simply: PV materials and devices convert sunlight into electrical energy, and a single such device is called a cell.

The word is worth separating from solar power in general, because not all solar energy is photovoltaic. A solar thermal plant uses mirrors to concentrate sunlight, boil a fluid and drive a turbine, which is a heat engine with moving parts. A photovoltaic cell has neither. Light falling on a semiconductor knocks electrons loose and the structure of the material pushes them in one direction, which is a current. That is the whole mechanism.

It also explains why photovoltaic devices scale so oddly compared with other generators. An individual cell is tiny, typically producing about one or two watts, so useful power comes from wiring many of them together into modules and then into arrays. The same principle works far from a rooftop: a photovoltaic device small enough to sit inside the eye can turn light entering it into an electrical signal, which is the idea behind a photovoltaic retinal implant. In each case the device is doing the same job, converting photons into current, at wildly different scales.