Inside the polymer, electrons go from a low to a high energy level when bombarded by photons. The smaller the difference (or bandgap) between these levels, the more light a cell absorbs, and the higher its efficiency. One way to decrease the bandgap is to bring down the higher energy level. University of Chicago chemistry professor Yu is using this technique to design new types of narrow-bandgap polymers. “The beauty of organic solar cells is that we’re able to engineer new materials that can tailor those energy levels,” Yang says.
The researchers are also trying to improve the interface between the polymer and the carbon nanostructure so that electrons can move faster to the external circuit without getting trapped in the material. And they are developing better electrode materials and improved ways of fabricating the electrodes. Yang says these advances will eventually make it possible to boost the efficiency of individual cells and of stacked cells.
Even if Solarmer reaches its target of 10 percent efficiency, Wu says, it may take as long as three years before the company can print commercial-grade rooftop panels with those ratings. Right now, the company plans to have devices on laptop bags and cell phone back panels in early 2011, followed by awnings and sunshades.
Yang says organic solar cells need not only higher efficiencies but also more stability. “What’s commercialized is not the highest efficiency but most reproducible technology,” he says. Indeed, plastic solar startup Konarka, based in Lowell, MA, is producing flexible panels on a large scale despite just a 3 to 5 percent efficiency.
Adam Moulé, a chemical engineering and materials science professor at the University of California, Davis, says that increasing the lifetime of organic solar cells is now the biggest challenge. Solarmer’s panels have a lifetime of up to three years.
“The 7.9 percent reported efficiency record is really amazing,” Moulé says. “If organic photovoltaic units of more than 5 percent power efficiency could be made that have a guaranteed lifetime of over five years, then I think that they will be competitive with CIGS and silicon because of reduced panel cost.”