Published online 8 December 2008 | Nature | doi:10.1038/news.2008.1281

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Efficiency records claimed for solar devices

Light concentrator promises higher power output.

A transparent polymer plate has set a new record for concentrating the Sun's light on to a solar cell, boosting the promise of a technology that could lead to vastly improved solar-power capabilities at much cheaper cost.

solar concentratorsSolar concentrator sheets, such as these developed by Marc Baldo at MIT, can boost the output of solar cells.Donna Coveney, MIT

Luminescent solar concentrators (LSCs) are plates of plastic or glass that have solar cells mounted along their edges. The concentrators are coated or impregnated with light-absorbing dye molecules; when the molecules re-emit the light, it bounces through the plate by total internal reflection until it hits the solar cell.

The devices can capture light from almost any angle, and because the area of the solar cells is much smaller than that of the entire plate, fewer cells can catch the same amount of light as a conventional solar panel, improving efficiency and potentially reducing costs.

Amanda Chatten from Imperial College London has now unveiled an LSC with a power conversion efficiency of 7.1%, beating the previous official record-holder by about 0.4%. That efficiency — a measure of how much power the LSC's solar cells generate, compared with the power of the light that hits its face — is still not very high, admits Chatten. "But it could find application where space is not an issue."

"I think the result is very encouraging for the field, and the efficiency is competitive with other technologies," says Marc Baldo of the Massachusetts Institute of Technology, Cambridge, who unveiled glass-based solar concentrators of his own earlier this year (see Organic dyes help harvest sunlight).

Window work

One drawback of Chatten's system is that to operate at its highest efficiency, the LSC must be teamed with expensive gallium arsenide cells, rather than conventional silicon.

Chatten also hopes to move away from using dyes, which degrade over time, and replace them with quantum dots — tiny nanoparticles of semiconductor materials that are longer-lived. Although more expensive than dyes, quantum dots tend to be more effective at capturing the diffuse, highly scattered light typically seen on cloudy days. She hopes that within ten years, windows that can act as solar concentrators will be integrated into buildings.

Chatten presented the results, from her former graduate student Lenneke Slooff, now at the Energy Research Centre of the Netherlands in Petten, at the Materials Research Society meeting in Boston, Massachusetts, on 3 December.

Performance questions

The efficiency of their LSC has been verified by the European Telecommunications Standards Institute in Sophia Antipolis, France1. Verification is important, as there has been growing unhappiness in the solar-power community about efficiency claims. Michael Grätzel of the Swiss Federal Institute of Technology in Lausanne, who pioneered the development of the dye-sensitized solar cell, says that quoted solar-power efficiencies are often imprecise. "Claims of efficiency records are only credible if validated independently by an accredited photovoltaic calibration laboratory," he says.

This concern means that another record claim presented at the same meeting — the most efficient commercial solar cell so far — has prompted a cautious response from some researchers.

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Richard Swanson from SunPower, a solar-panel company based in San Jose, California, announced they had made a protoype silicon solar cell covering an area of 149 square centimetres with a power conversion efficiency of 23.4%.

"As far as commercially produced cells go, it is clearly a record," says David Carlson, from BP Solar in Frederick, Maryland. But Martin Green from the University of New South Wales in Sydney, Australia, who has a lab-based silicon system that works at 25% efficiency, says that some members of the photovoltaic community might be suspicious of Sun Power's claims. "Efficiency is contentious since it is dangerous to let the group who made the cells measure them," he says. 

  • References

    1. Slooff, L. H. et al. Phys. Stat. Sol. 2, 257-259 (2008).
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