Record Efficiency for Plastic Solar Cells

A new process for printing plastic solar cells boosts the power generated by the flexible and cheap form of photovoltaics. Initial solar cells made with the technique can, according to a report in today’s issue of Science, capture solar energy with an efficiency of 6.5 percent–a new power record for photovoltaics that employ conductive plastics to generate electricity from sunlight. Most photovoltaics are made from conventional inorganic semiconductors.

The new process stacks multiple polymer layers within a single photovoltaic device to produce a “tandem” cell. Alan Heeger, who won the 2000 Nobel Prize for his codiscovery of electrically conducting polymers, and his colleagues at the University of California, Santa Barbara (UCSB), created the process with a group from South Korea’s Gwangju Institute of Science and Technology. Heeger says that the tandem architecture offers plenty of room for further improvement–enough to eventually make plastic solar cells practical in rooftop solar panels. “We see a pathway here toward even higher efficiencies,” he says. “We can do significantly better than 6.5 percent in the near future.”

Tandem cells, commonly employed in conventional solar panels, increase power output in two ways. The semiconductors in the different layers can be optimized to capture different bands of light, thus enabling the tandem device to absorb a broader spectrum of sunlight. And the multiple layers boost the voltage of the tandem device, yielding more power from every photon absorbed. “You do a better job of light harvesting and a better job of utilizing the photon energy,” explains Heeger.

Until now, however, the tandem architecture spoiled plastic photovoltaics such as Heeger’s, which are “printed” by spraying solutions of conductive plastics and other materials onto a plastic film. Layers of different plastics sprayed on top tended to mix, degrading rather than enhancing power output. Heeger and his colleagues beat the mixing problem by finding an effective spray-on separator to keep the layers in place.

The bottom cell is filled with a proprietary polymer first disclosed last year by plastic PV developer Konarka Technologies, based in Lowell, MA, which Heeger cofounded and for which he serves as chief scientist. The polymer (a derivative of polythiophene) absorbs both infrared and ultraviolet light. Next comes a titanium-suboxide layer, which seals in the bottom cell, provides a foundation for building the top layer, and, as it’s a metal, efficiently carries away the charged electrons generated in both layers. Finally, the top layer sports a different type of conducting polymer that absorbs mostly blue and green light.

Heeger expects further efficiency strides as device developers gain experience with the cell’s new materials. For example, in May, the UCSB researchers reported a processing tweak that doubles the power output of single cells made with Konarka’s new polythiophene polymer. Heeger says that the processing trick was not used in the tandem cell.

Yang Yang, a physicist at the University of California, Los Angeles, agrees that rapid improvement is likely. He says that such optimization could yield a tandem cell that’s more than 10 percent efficient. “I would call this important progress,” he says.

Not all experts are as optimistic. Sean Shaheen, who recently left a research post at the Department of Energy’s National Renewable Energy Laboratory for the University of Denver, cautions not to overreact to the report. For one thing, says Shaheen, efficiency estimates are notoriously unreliable because each research group tests efficiency under its own approximation of the solar spectrum.

Another hurdle for the tandem cell is manufacturing. Konarka vice president of research Russell Gaudiana expects that the company would be able to produce Heeger’s tandem cells on the same printing lines it now uses to make prototype modules containing single cells of plastic photovoltaics, but he says it will be “trickier” to keep the tandem cell’s layers from intermixing in commercial-scale production. “We anticipate seeing the typical problems that one always sees when putting down multiple layers,” says Gaudiana. “Alan does it in the laboratory and does a very good job at it, but doing it on a coating machine at high speed is a little different.”

For the time being, says Gaudiana, Konarka will stay focused on producing single-cell plastic photovoltaics with 5 percent efficiency. That power output is sufficient for Konarka’s first application, portable battery chargers, which the company hopes to begin selling next year. But tandem cells could help Konarka reach the more demanding rooftop market, which Gaudiana says will require at least 7 percent efficiency.