Photovoltaic solar cell

A photovoltaic cell converts light directly into electricity, skipping the heat, steam, and spinning machinery that every other kind of power station relies on. It is a slab of silicon with two layers doped to have opposite electrical character — one side made to carry a slight surplus of electrons, the other a slight shortage — meeting at a junction that holds a permanent internal electric field. When a particle of light is absorbed in the silicon it knocks an electron loose; the junction's field sweeps that electron to one side, and if you connect a wire between the two faces, the electrons flow through it as current for as long as the light shines.

Every other generator turns something — a turbine, a dynamo — and needs a source of motion, usually heat from burning fuel or falling water. The solar cell has no moving parts and needs nothing but light; it can sit silent on a rooftop or a satellite for decades. In 1954 it was far too expensive and too weak to matter for ordinary power, but it proved that sunlight could be turned into serious electricity at a useful efficiency. Its first real job was aboard spacecraft, where no other power source could run for years unattended, and over the following decades falling silicon costs carried it from satellites down to household rooftops and utility fields.

Photovoltaics powered the space age first: solar panels have run satellites and probes since the 1950s, from Vanguard 1 onward, because nothing else lasts so long in orbit. On Earth, as manufacturing scaled and the price of a watt of solar fell by orders of magnitude, the same silicon cell became the cheapest new source of electricity in much of the world, spreading across rooftops and deserts alike. It remains the only way we have to make electricity from sunlight with no fire, no turbine, and no moving part — a physics experiment from 1954 now installed by the gigawatt.