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Scientists at the University of Chicago and Argonne National Laboratory have identified a new kind of polymer that provides an inexpensive way to improve the efficiency of solar cells.
The new compound, called PID2, is a type of polymer that forms familiar materials such as plastics. It allows electrical charges to move more easily through the cell, increasing the production of electrical power -- something that has not been observed previously.
Wei Chen of the Materials Science Division at Argonne National Laboratory said, "This knowledge will serve as a foundation from which to develop high-efficiency organic photovoltaic devices to meet the nation's future energy needs."
The findings were published online in the journal Nature Photonics.
Polymers have recently become a focus of research into solar energy, but so far no one has been able to use them for efficient power generation. Until now, solar cells using polymers emit and receive electrons that generate an electrical current when exposed to light.
Luyao Lu, a graduate student in chemistry who worked on the project, said, "Polymer solar cells have great potential to provide low-cost, lightweight and flexible electronic devices to harvest solar energy," he told UChicago News.
PID2 improves the cells' power-generation efficiency by 15 percent when a mixture of a standard polymer is included along with fullerene, a sphere-shaped carbon molecule.
Lu says fullerene is a standard ingredient in polymer solar cells. "Basically, in polymer solar cells we have a polymer as electron donor and fullerene as electron acceptor to allow charge separation," he says. The researchers from Chicago and Argonne added a second polymer to the device.
With this two-polymer, one-fullerene device, the scientists realized an efficiency of 8.2 percent when they added just the right amount of the new PID2 as a third polymer. This was the greatest efficiency yet for solar cells made of these components.
Luping Yu, a chemistry professor who led the Chicago effort, says his team is encouraged because PID2 used a novel mechanism to enhance the cells' efficiency. Ordinarily efficiency is improved by increasing the absorption of light. But by adding PID2 to the mix, efficiency was improved because electrical charges moved more smoothly throughout the solar cell.
"It's like a step," Yu explains. "When it's too high, it's hard to climb up. But if you put in the middle another step, then you can easily walk up."
Their work demonstrates that even higher efficiencies are possible with further research, and the scientists already are aiming for a goal nearing 10 percent efficiency, which would be essential to make the solar cells commercially viable.
