Published online 10 July 2008 | Nature | doi:10.1038/news.2008.949


Organic dyes help harvest sunlight

Solar-power costs could be slashed by cheap collectors, claim researchers.

A simple sheet of glass coated with dye could be enough to cut the costs of solar power.

solar cellsConcentrating light onto photovoltaic cells could push down solar power costs.Donna Coveney, MIT

That's the claim from researchers who have created a 'solar concentrator' that harvests photons and funnels them into photovoltaic devices. The device allows relatively small solar cells to harness rays from a much larger area.

Mirrors that track the Sun are already used to deliver extra light onto solar panels and maximize their electricity output. But these mirrors can be costly to deploy and maintain, and the solar cell is prone to overheating.

In the 1970s, scientists tried to develop alternatives that used light-absorbing dyes. Plastic sheets impregnated with these dyes could capture photons and re-emit them at a lower energy. These photons would then bounce along inside the plastic towards a collector at the end, allowing light gathered over a large area to be concentrated at the edges.

But the research stalled because many of the dyes were unstable in sunlight, or because the photons didn’t get very far through the plastic before being reabsorbed.

Researchers led by Marc Baldo at the Massachusetts Institute of Technology in Cambridge have now come up with an alternative that uses a mixture of dye molecules in a thin film coated onto glass. Each dye absorbs light of a different wavelength to make the most of sunlight’s spectrum. By fine-tuning the dye mixture and adding an extra compound to control the re-emission process, ensuring that most of the photons get trapped inside the glass, the team thinks they can boost the power efficiency of a cadmium telluride cell from 9.6% to 11.9%, and a CIGS cell(copper-indium-gallium-selenide materials) from 13.1% to 14.5%. The research is published this week in Science1.

Baldo thinks that the efficiency can still be vastly improved. "We could ultimately double the efficiency of 90% of solar cells used today," he predicts. He also thinks that the system will be easy to commercialize. "It looks very practical to make," he says. Solar cells are very sensitive to defects in the material, but that doesn’t apply for these thin films, says Baldo. That means it could help solar cells to produce electricity at a mere US$1 per watt, which is essential if the solar industry is to be economically sustainable.

Stiff competition

Lawrence Gasman, principal analyst at the market-research company NanoMarkets of Glen Allen in Virginia, is impressed that Baldo’s system seems to be so easy to manufacture, and says that commercial interest in innovations such as these is blooming.

But dye-based systems still face stiff competition from more conventional concentrators. In May, IBM claimed to have used mirrors to concentrate 230 watts of the Sun’s power onto 1 square centimetre of solar cell. Supratik Guha, who leads IBM’s photovoltaics team, points out that this is a much higher concentration than would be possible with Baldo’s dyes.

And Eli Yablonovitch, from the University of California, Berkeley, who looked at the theory of light concentration by dyes back in the 1980s2, is unconvinced that the system will be commercially viable. Baldo’s system absorbs just 80% of photons, he points out, which isn’t good enough.

"The problem with the concept is that 20% of the photons escape back into the air. This is fatal for commercial applications, in which efficiency is king," says Yablonovitch. "Considering the competitive environment in which solar energy finds itself, giving up that much light is intolerable."

Jonathan Mapel, part of Baldo’s research team, disagrees: "That 20% off the top isn’t necessarily fatal," he says, predicting that a solar concentrator based on this technology could be available in three years. 

  • References

    1. Currie, M. J., Mapel, J. K., Heidel, T. D., Goffri, S. & Baldo, M. A. Science 321, 226–228 (2008).
    2. Yablonovitch, E., J. Op. Soc. Am. 70, 1362–1363 (1980).
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