Solar cells

A small pocket calculator with no battery compartment still springs to life as soon as light falls on the dark strip above the keys. That strip is a solar cell, and it can do something astonishing: it turns light directly into electric current, with no detour through fire, steam or motion. Nothing spins, nothing burns, nothing wears out. The same principle powers satellites in space, parking meters at the roadside and entire rooftops. How does a silent piece of material manage that?

Light becomes current

A solar cell is almost always made of silicon, the same semiconductor that computer chips are made from. Two wafer-thin layers with different added atoms are built into the cell, and at their boundary a built-in electric field appears. When light hits the cell, its energy packets, the photons, knock individual electrons out of their places in the silicon. The built-in field pushes these free electrons all in the same direction, and as soon as the cell sits in a circuit, a current flows. Researchers at Bell Labs demonstrated the first practical silicon cell in 1954; it turned about 6 percent of the light into electricity and was hailed as a sensation.

How much current the cell delivers depends directly on the light. The brighter the sun, the more photons hit the cell every second, the more electrons get knocked loose, and the larger the current. A weak desk lamp is therefore enough for the calculator and its tiny display, while a rooftop panel only reaches full power in bright sunshine. For the same reason, engineers align solar panels so the sun strikes them as squarely as possible: when light arrives at a slant, the same amount of light spreads over a larger area.

From calculator to power plant

A single cell delivers only about 0.5 volts, less than a flashlight battery. So manufacturers connect many cells in series, letting their voltages add up, and protect them together under glass: the result is a solar panel. A modern rooftop panel is around 2 square metres in size, delivers over 400 watts in full sun and turns a good 20 percent of the incoming light into electricity. That share is called the efficiency. The cell produces direct current, and an inverter turns it into the alternating current your sockets use. As early as 1958 the first solar cells flew into space on the satellite Vanguard 1; today they cover rooftops and vast solar farms.

Powerful, but moody

Everyone knows the solar cell's biggest weakness: at night it delivers nothing, and on a grey winter day often less than a tenth of its full power. That is why storage batteries and other power plants belong at its side. In return, the technology has become unbeatably cheap: panel prices have fallen by more than 90 percent since 2010, and in many countries solar power is now the cheapest way to generate new electricity. The laboratory curiosity of 1954 has become one of the most important energy sources in the world, and it is growing faster than any other.