Infrared Vision: New material may enhance plastic solar cells
Alexandra Goho
Researchers have moved one step closer to the goal of flexible, low-cost, lightweight solar cells made of plastic. They've created the first polymer-based photovoltaic material that can harness a part of the sun's spectrum that had previously evaded capture.
Because the polymers in plastic solar cells currently under development absorb only visible light, they convert about 6 percent of the sun's energy into electrical power. If the materials could harvest both the visible and infrared parts of the spectrum, plastic solar cells might achieve up to 30 percent efficiency, says Stanford University's Peter Peumans.
Reporting in the February Nature Materials, Ted Sargent of the University of Toronto and his colleagues describe a new polymer material that absorbs infrared thanks to semiconducting nanoparticles called quantum dots (SN: 2/15/03, p. 107: http://www.sciencenews.org/articles/20030215/bob10.asp). The researchers mixed the dots with a conducting polymer, made a thin film, and sandwiched it between two electrodes, one of which was transparent.
When exposed to infrared rays, the quantum dots absorbed the light and gave up electrons, generating a current.
By varying the size of the quantum dots, the researchers tuned the particles to absorb different parts of the infrared spectrum. For instance, films containing particles measuring 6 nanometers in diameter absorbed longer infrared wavelengths than did films with particles only 2 nm across.
To make a solar cell that absorbs both visible and infrared light, Sargent envisions layering polymers that absorb the visible part of the spectrum and polymers containing infrared-absorbing quantum dots.
Neil Greenham of the University of Cambridge in England says that getting polymers to absorb infrared light is an important step but cautions that the material doesn't move electrons efficiently. "You have to make sure you get the charges out of the device," he says.
One approach would be to engineer the particles to point electrons toward only one of the two electrodes, says Greenham. He and others have found that the shape of semiconductor particles used for absorbing visible light influences the direction in which they aim the excited electrons.
Sargent says that in addition to photovoltaic applications, his material might find its way into night-vision cameras for the military. Such devices detect infrared light generated, for instance, by the warmth of people and of moving trucks. Because night-vision cameras currently rely on an expensive semiconductor crystal, "they cost between $10,000 and $100,000," says Sargent. A plastic detector could dramatically bring down the cameras' cost.
An inexpensive detector could also benefit developers of devices that use infrared light to detect early signs of cancer deep within tissues, says Sargent. His group is currently testing its material for this application, and he says that he expects to see the material in commercial medical-imaging devices within 3 to 5 years.
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Letters:
Plastic solar cells may indeed be gaining in efficiency, but here in the Southwest, anything plastic left out in the sun quickly clouds, desiccates, and cracks. Can the new polymer-based material protect against this destruction? It would certainly be cost prohibitive to replace the cells every year.
Stephen Wust
Santa Fe, NM
Researchers are well aware that, left alone, plastic solar cells won't last long in the environment because the combination of light, oxygen, and water vapor degrades polymers. But the solution seems pretty simple. Some kind of transparent sealant or other encapsulating material would block out oxygen and water while letting through light. Companies and academics are working on it.—A. Goho
References:
McDonald, S.A. … and E.H. Sargent. 2005. Solution-processed PbS quantum dot infrared photodetectors and photovoltaics. Nature Materials 4(February):138–142. Abstract available at http://dx.doi.org/10.1038/nmat1299.
Further Readings:
Gorman, J. 2003. Nanolights! Camera! Action! Science News 163(Feb. 15):107–109. Available at http://www.sciencenews.org/articles/20030215/bob10.asp.
McDonald, S.A. … and E.H. Sargent. 2004. Photoconductivity from PbS-nanocrystal/semiconducting polymer composites for solution-processible, quantum-size tunable infrared photodetectors. Applied Physics Letters 85(Sept. 13):2089–2091. Abstract available at http://dx.doi.org/10.1063/1.1792380.
Sources:
Neil Greenham
Department of Physics
Cavendish Laboratory
University of Cambridge
Madingley Road
Cambridge CB3 0HE
United Kingdom
Peter Peumans
Stanford Organic Electronics Lab
Stanford University
330 Serra Mall
Stanford, CA 94305-4075
Ted Sargent
Department of Electrical and Computer Engineering
University of Toronto
10 King's College Road
Toronto, ON M5S 3G4
Canada
From Science News, Volume 167, No. 4, January 22, 2005, p. 53.
