A DRAMATIC cut in the cost of a super-efficient new breed of solar cell could put domestic solar power on a more economic footing. The cells, which helped take NASA’s electric-powered aircraft Helios to record altitudes, have till now been too expensive. But their manufacturer, SunPower of Sunnyvale in California, has found a way to make them as much as 20 times cheaper.
The cells convert light energy into electricity with an efficiency of 20 per cent – which means they generate one-third more electrical power than conventional silicon solar cells. Last month, delegates at a solar energy conference in Osaka, Japan, heard that the price of the high-quality silicon from which SunPower’s cells are built has fallen, and that has helped to reduce the cost. A new silicon extraction process has driven the price cut. More importantly, the engineering team at SunPower has discovered a simpler manufacturing process that does not compromise the cells’ efficiency.
The key to SunPower’s original success was to build solar cells in which all of the electrical contacts are on the underside. In a conventional cell, a mesh of wires embedded on the top face collects the free electrons liberated when a photon is absorbed by the silicon, while the compensating positive charges are drawn out through an electrode on the back. But this mesh prevents some of the sunlight from reaching the silicon, cutting down the amount of electricity the cell generates. Doing away with this mesh makes SunPower cells about one-third more efficient than conventional cells (see Graphic), but until now their cost has been prohibitive.
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The potential of these cells was demonstrated in August 2001, when NASA’s electrically powered plane Helios soared to altitudes above 96,000 feet (29 kilometres) – a world record for a winged plane not powered by a rocket engine. Its electricity was generated by 62,000 SunPower cells on its 75-metre wing.
But the “back-contact” cells used on Helios were enormously expensive. This is because the electrodes were formed by laying down stripes of silicon just a few micrometres wide using a photolithographic process similar to the one used to make microchips. Photolithography is notoriously slow and expensive. “For a long time we thought it would be impossible to make these solar cells cheaply enough to compete in commercial applications,” says Richard Swanson, SunPower’s chief executive officer.
Now the company has developed and is planning to patent a three-stage screen-printing process that cuts the cost of back-contact cells by 95 per cent. Until patents are safely filed, however, Swanson is not revealing how the printing technique works, beyond saying that it involves “being clever with the features on the back”.
If SunPower’s claims are borne out, its high-efficiency cells will bring solar power closer to becoming a practical option for home owners. The average power demand of a household is 2000 to 3000 watts. With solar cells around 20 per cent efficient, this demand can be met with about 15 square metres of solar panels, which is compact enough to fit on a rooftop. Swanson estimates that ready-to-install arrays will sell for around $10 per watt. That cost may eventually fall further. “The trend is towards higher efficiency” says Swanson. Others agree, but say silicon may not be the material that ultimately delivers it. Thin films of cadmium telluride, for instance, are showing promise in the lab.
But Clive Weatherby of Solar Century, a London-based solar energy systems supplier, says SunPower’s technology is “quite an advance”. In addition to its higher efficiency, he says the cells are very practical. “They are very easy to connect together – you can just put tabs on the sides of the back contacts and connect one cell to another without wires.”