ORGANIC CELL: By incorporating nanoscale trees to carry charge researchers at Wake Forest University have created a plastic solar cell more efficient at capturing the energy in sunlight. Image: COURTESY OF WAKE FOREST UNIVERSITY
Solar cells made from cheap, plastic polymer barely capture the energy in sunlight. Photons reflect off the plastic and it is too thin to absorb much, giving the polymers color. "The very fact that it has color is telling you this thing is not working as well as it should," says David Carroll, a physicist at Wake Forest University in Winston-Salem, N.C. But plastic solar cells offer flexibility, are lightweight and, theoretically, low cost, which means they could be incorporated into a range of products. "You can't think of doing anything cheaper than making Saran wrap and that's basically what these are," says Lawrence Kazmerski, director of the Department of Energy's (DOE) National Center for Photovoltaics in Golden, Colo.
Now Carroll and his colleagues at Wake Forest's Center for Nanotechnology and Molecular Materials have shown how to incorporate nanoscale polymer trees to improve the potentially revolutionary solar cell's ability to produce power. "They look kind of like trees going from one contact to the next, grasping this polymer, grabbing the charge and shunting it out of this device," Carroll says. "The material up at the top at the limbs moves charge differently than the material down at the trunk."
Carroll and his colleagues report in Applied Physics Letters that by balancing the charge this way, they boosted the efficiency of such cells to more than 6 percent. That level of efficiency—which would be a world record—has not been independently verified. "I would prefer to see this stuff independently confirmed," Kazmerski says. "The best organic cell we know is 4.8 percent."
The Wake Forest organic cell is much smaller than the standard measure for comparing efficiencies, 5 to 10 millimeters squared rather than 1 centimeter squared. As a result, the DOE's National Renewable Energy Lab (NREL), which independently confirms efficiencies using standard measures, suggests this cell would only reach efficiencies of 3.3 percent. "You do not want to overpromise technologies," Kazmerski says. But "they have a huge potential and we have to give them a chance."
The organic cell was tested using an NREL-calibrated light emitter, however, along with a standard 10 percent discount applied for potential variations in the light spectrum used for measurement. "We feel that the number we have given is lower than the real number," Carroll says. "By increasing the performance to the levels we've raised them, you're really raising the hopes that this is commercial and we've done it with a polymer that is a commodity."
That polymer, known as P3HT, could permit a wide variety of applications, and new nanoengineering technology permits this thin film of plastic to be thicker than ever before—roughly 120 nanometers thick. As a result, the plastic inherently captures more of the energy in light as well as incorporating the current enhancing nanoscale "trees." Furthermore, precursors have shown some durability. "We do have devices that have easily continued to function solidly over a year," Carroll says.
But it is future developments in the way such polymer cells are made that may permit them to become a ubiquitous reality. Carroll and his colleagues have also shown that such polymers can be wrapped around fiber-optics. These fibers guide light into the film and trap it there. "You can hold the photons there until they are absorbed," Carroll says. "Now you don't miss any." Because the fibers guide the photons in, they can be arrayed to catch a much broader range of angled light as well—on a rooftop, for instance. "We're trying to show that these optimal optical geometries can lead to very high-performance devices," he adds.
And if such superefficient solar cells can be made cheaply and easily that would transform the world of energy, especially if they can break the 10 percent efficiency barrier. "If they can have a breakthrough it could kill everything else," Kazmerski says. "It could be the killer technology."