One way to improve these cells’ performance is by making sure that every single photon that does get absorbed by the polymers is converted into an electron that can be collected. Heeger’s group boosted its cells’ overall efficiency by improving this internal efficiency. How well electrons can move within these films depends on the quality of the interface between the two components that make up the film: in the University of California cell, these are a conductive polymer and a version of a soccer-ball-shaped carbon compound called a fullerene. Heeger’s group tested films made of different ratios of these two components, as well as different solvents for processing them.
The result is a cell with nearly perfect internal efficiency. “All the light that’s been absorbed has been converted into charges,” says Zhenan Bao, an associate professor of chemical engineering at Stanford University, who was not involved in the research. “This group has done very good engineering on the cells.”
“I’m excited about the progress,” says Yang Yang, a professor of materials science and engineering at the University of California, Los Angeles. “You push the record incrementally.” Heeger agrees: “The size of the market depends on the cost in dollars per watt, so every increase in efficiency is important.”
While Yang, Bao, and Heeger say that the figures achieved by Heeger’s group are an important demonstration of the potential of polymer solar cells, all acknowledge that existing materials will not be the ones that will push the industry forward. “Ten percent [efficiency] would be a breakthrough,” says Heeger. “We get there by synthesizing new materials that respond to more of the energy spectrum.”
“Organic materials are still limited to visible light,” says Yang, but much of the sun’s energy is in the neighboring part of the spectrum–the infrared–so polymer scientists are working on solar-cell materials that can also absorb this band. The University of Chicago’s Yu, who is collaborating with Solarmer Energy, says that the company has used his polymers, which absorb shorter-wavelength light, to make cells that should achieve more than 7 percent efficiency, but he cannot disclose the details because the results have not yet been published.