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.