Giving Plastic Solar Cells an Energy Boost

Polymer solar cells are finding use in solar charging backpacks and umbrellas, but they still only convert around 6 percent of the energy in sunlight into electricity–or around a third of what conventional silicon panels are capable of. If the efficiency of polymer solar cells–which are cheaper and lighter than silicon cells–can be boosted significantly, they could be ideal for plastering on rooftops or laminating on windows.

Solarmer Energy, based in El Monte, CA, is on target to reach 10 percent efficiency by the end of this year, says Yue Wu, the company’s managing director and director of research and development. Organic cells will likely need at least that efficiency to compete on the photovoltaic market.

In collaboration with Luping Yu, a professor at the University of Chicago, the startup has previously engineered polymers that absorb a broad range of wavelengths and has made cells that convert sunlight to electricity with a record efficiency of nearly 8 percent.

Polymer solar cells with even higher efficiencies are in the works. Solarmer is collaborating with Yang Yang, a materials science and engineering professor at the University of California, Los Angeles. Yang is working on a stack of multiple cells that absorb different bands of light. He expects to achieve 12 to 15 percent efficiencies using this approach along with new polymers and better device design. So far, he has made laboratory prototypes that are better than 6 percent efficient. He presents this work on Tuesday at the American Physical Society meeting.

Polymer solar cells should be cheaper to make than thin-film cadmium-telluride or copper indium gallium selenide (CIGS) ones because they use low-cost materials that are easy to print, says Michael McGehee, a materials science and engineering professor at Stanford University. But McGehee believes polymer cells will need to be more than 15 percent efficient to have a major impact on the solar power market. “We still don’t understand the physics well enough to know what the theoretical limit is,” McGehee says. “I think cells with 15 to 20 percent could be possible.”

Solarmer’s research team has multiple tactics for increasing cell efficiency. Its cells are made of a semiconducting polymer that absorbs sunlight and releases electrons, and a carbon nanostructure that shuttles the electrons to the external circuit.

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.”