Infrared vision: new material may enhance plastic solar cells.

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). 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 photo-voltaic 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.