Solar panels extend from spacecraft like the wings of birds. These have become a familiar feature of the Space Age, although spinning satellites may wear their solar cells like a coat. Certain other spacecraft rely on batteries, fuel cells or radioactive generators.

But converting the free and ever-renewable energy of sunlight into electricity, in photovoltaic cells, is by far the most popular source of power in space.

A junction in a semiconductor, containing different impurities on either side, responds to light by creating an electric voltage.

Until recently, silicon was the favoured semiconductor for solar cells. Now gallium arsenide has begun to replace it. The first gallium-arsenide cells were only slightly more efficient, converting 19 per cent of the solar energy compared with around 16 per cent in good silicon cells. In the latest gallium-arsenide based cells, the efficiency jumps to 27 per cent.

How's the trick done? Each semiconductor junction needs a minimum energy in the particles of sunlight, the photons, if it is to generate a voltage. Engineers can exploit the fact that gallium arsenide is transparent, and stack three junctions on top of each other. The bottom layer absorbs red light and the middle layer green light. The most energetic blue light activates the outermost junction. The result is that more of the available photons are captured, than in a single-junction device.

"Although these triple-junction cells aren't cheap to make, there's a big payoff," says Lothar Gerlach, who is in charge of solar-generator engineering in the power division at ESTEC, ESA's science and technology centre in the Netherlands. "To change and re-qualify an existing satellite design with a new and larger solar array is far more expensive than improving what you have already, just by using more efficient solar cells. Or if you don't need more power, you can have a smaller solar array. Then less fuel is needed to control the satellite's attitude in space, and more payload can go on board."

An ESA contractor, RWE Solar GmbH (formerly called ASE), is Europe's largest producer of solar cells for space. It is now preparing for large-scale production of triple-junction gallium-arsenide based solar cells for space applications, at its plant in Heilbronn, Germany. ESA will use them in spacecraft such as PROBA-2 for technology development, GOCE for charting the Earth's gravity, and Herschel and Planck for astronomy.

A Distinguished History
An artificial sun, intense light flashes, and high-temperature ovens are among the test facilities available for solar-cell engineering at ESTEC. Above the Earth's atmosphere the cells face a solar intensity 36% stronger than on the ground. They are subject to drastic changes of temperature whenever a satellite passes in or out of the Earth's shadow, causing thermal fatigue.

ESTEC's test facilities are used for research, development, evaluation and trouble-shooting, from single solar cells to complete arrays.

Before any solar cells are accepted by ESA for use in space, they must pass qualification tests. The official centre for this purpose is Spasolab, at Spain's Instituto Nacional de T�cnica Aeroespacial (INTA) in Torrej�n de Ardoz. Spasolab also helps with testing during the development of new solar cell components.

ESA and its industrial contractors in Europe have long experience in solar-cell technology. When the Hubble Space Telescope was first launched in 1990, it was powered by solar arrays provided by ESA. In 1993, the first set of solar arrays was replaced by a second ESA set. The astronauts servicing Hubble retrieved one of the older wings and returned it to Earth.

Here was a special chance to investigate the experience of the solar cells in space. Experts examined them for impacts by micrometeorites, manmade debris and high-energy atomic particles.

They also evaluated effects of atomic oxygen and thermal fatigue.

Now Hubble's second set of arrays, both retrieved in March 2002, will undergo a similar post-flight investigation.

Another special effort has met ESA's requirement for solar cells operating far from the Sun, at low light intensities and low temperatures. These will power the Rosetta spacecraft to be launched in January 2003, which will rendezvous with Comet Wirtanen far beyond the orbit of the planet Mars. Hitherto, spacecraft venturing so far afield have had to use radioactive power sources. Europe's engineering and industrial teams have achieved an amazing 25 per cent efficiency in specially designed silicon solar cells operating at low temperatures.

At the other extreme, ESA needs gallium-arsenide based solar cells suitable for very bright light and high temperatures.

The BepiColombo mission to the innermost planet, Mercury, will encounter such severe conditions. The Solar Orbiter spacecraft will swoop even closer to the Sun, where the intensity of sunlight will be 25 times higher than in the Earth's vicinity.

But the solar cells for these missions must also be efficient at the distance of the Earth, because they will propel the spacecraft on their journeys, by electric rockets. In ESA's Cosmic Visions 2020 science programme, these spacecraft are due for launches in 2011 or 2012.

The Next Step: Thin-Film Solar Cells
Weight is seldom a problem in solar-power systems on the Earth's surface, but space engineers have to worry about the total mass of a satellite at launch. They are therefore interested to know, not just how many watts of power they'll get from a square metre of solar cells, but how many watts per kilogram. Typical figures at present are 50 for silicon and 110 for gallium arsenide. That can be pushed to 400 watts per kilogram for thin-film solar cells now under development.

In thin-film devices, the semiconductor is laid as a coating on a thin, flexible but strong support made of metal or plastic. The cell thickness, including the support structure, can be less than three hundredths of a millimetre. The conversion is less efficient than in rigid cells, but that does not matter if the spacecraft can unfurl and control large, lightweight wings of thin-film solar panels.

"For planetary missions, you can even imagine gigantic solar sails coated with thin film solar cells," says Lothar Gerlach. "Then you could use the Sun's energy in two ways at once. The pressure of sunlight on the sail gives direct propulsion, while sunlight converted into electric energy drives an electric rocket and powers the spacecraft with its scientific payload.

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