General Electric hopes to rock the solar world with cadmium-based solar cells
Like the mask that conceals a superhero’s dark secret, the glass and plastic sheets encapsulating the world’s fastest-growing solar panel design conceal an unlikely environmental hero: cadmium. Now this toxic heavy metal, together with tellurium, aims to make solar power cheap enough to compete with fossil fuels, perhaps within a decade. The latest sign of the technology’s rising fortunes is the decision by General Electric to enter the business next year.
The design is part of a broader technology known as thin-film photovoltaics. Its main advantage is that the layers in the device—which actually convert sunlight to electricity—can be made far more cheaply than by using the monocrystalline silicon of conventional cells. However, thin-film semiconductors are harder to make in quantity, and they convert electricity less efficiently.
Then came First Solar, based in Tempe, Ariz., which began mass-producing cadmium telluride modules in 2004 and last year produced over 1 gigawatt’s worth, making it the solar market leader [see ”First Solar: Quest for the $1 Watt,” IEEE Spectrum, August 2008]. First Solar drove the price of CdTe down and its efficiency up, to a stable 11 percent. Although that still fell short of the 18 to 21 percent efficiency of conventional silicon, the cost per watt was about 50 cents to $1 cheaper—good enough to score big when growing subsidies from European governments drove module demand beyond what silicon manufacturers could supply in 2008.
CdTe now accounts for three-quarters of the 4.8-GW capacity that utility-scale solar installations are planning and for which they have specified a technology, according to Emerging Energy Research, a consultancy in Cambridge, Mass. And this summer First Solar plans to start building the world’s largest solar power plant: a 2-GW array in Ordos, a center for energy development in China’s Inner Mongolia territory.
Clearly, this is the time for the big boys to enter the game, if they hope to play in it at all. GE says it will challenge First Solar in the utility market next year with a design that boasts 16.5 percent efficiency in small cells—a record for CdTe technology. Part of GE’s trick is to rely more heavily on cadmium than previous designs have been able to manage.
In a conventional CdTe cell, the layers are deposited on a glass sheet, which is flipped over to face the sun. First comes a transparent metal-oxide layer that will form the cell’s top electrode. Next up are the layers—cadmium sulfide, followed by cadmium telluride—that form the p- and n-type semiconductors, whose junction forms the cell’s active, energy-converting region. A coating of silver forms the bottom electrode.
In the GE design, the top electrode consists not of a layer of tin oxide, as in conventional cells, but of a relatively thick layer made of a cadmium-tin oxide called cadmium stannate, plus a thinner layer of zinc-tin oxide. The electrode is therefore more conductive and more transparent.
Observers say GE and other would-be CdTe entrants will need to improve their cells’ performance. First Solar has driven its module cost per watt down 30 percent over the past two years, and it’s planning to cut costs a further 26 to 39 percent by 2014.
Suppliers of conventional silicon cells, meanwhile, are closing the gap. Ken Zweibel, who directs George Washington University’s Solar Institute, estimates that the cost advantage of finished systems using First Solar’s CdTe modules has slipped to about $0.25 per watt.
In the world of green energy, even superheroes must compete.
This article originally appeared in print as "Heavy Metal Power".