July 2004

Solar-Cell Rollout

Continued from page 1

By Peter Fairley

Printing Power

In 2003, more conventional solar panels were manufactured than ever before, yet all of them, together, yielded just 750 megawatts of electricity-the equivalent of one average-size coal-fired power plant. What's holding up the solar industry is cost. Most top-of-the-line solar panels are made with 15-centimeter wafers of crystalline silicon, and those materials are very expensive. As a result, solar power is four to ten times more costly to produce than electricity from conventional power plants.

For decades, solar-cell researchers have tried to develop cheaper alternatives to silicon. The problem has been efficiency: other materials just don't generate enough electricity. But Siemens's achievement earlier this year of the highest efficiency to date in plastic solar cells could change that. The Siemens design combined two of the most important advances in materials science in the past 30 years: electrically conducting polymers and buckyballs.

The idea of combining these materials to capture solar power first gained credence in the early 1990s, when physicists Sariciftci and Alan Heeger at the University of California, Santa Barbara, created primitive photovoltaic devices by pouring a solution of conducting plastic and buckyballs onto a glass plate, spinning the plate to spread the solution into a film, and sandwiching the film between electrodes. The conducting polymer absorbed photons, kicking off electrons that were then attracted by the buckyballs and routed to an electrode.

In short, the film acted like a solar cell. Originally, the power output was meager (less than 1 percent of the energy of incoming sunlight). But the principle of the printable solar cell was proved: you could layer a photovoltaic material on a surface and make it work without complex preparations.

For Sariciftci, printable solar cells became an obsession. In 1996, after moving to Kepler University, Sariciftci began assembling a research team to boost the power output of his devices. One of his first recruits was Christoph Brabec, a young polymer scientist. By 2000, Sariciftci and Brabec had found a mix of solvents, temperatures, and drying conditions that delivered a better blend of plastic and buckyballs. The result: more electrons made the jump from plastic to buckyball, more than doubling the power output (see "Solar on the Cheap," TR January/February 2002).

In 2001, Brabec left Sariciftci's lab to head a new research effort in polymer photovoltaics at Siemens. It was his team at Siemens that earlier this year significantly increased the power output of the buckyball-plastic cell by tweaking the nanomaterials and shifting to a more industrial-style coating method. Exactly why the power jumped is not yet clear, says Brabec, though he suspects that the explanation has to do with a more regular structuring of the cell's polymers and buckyballs. What is clear to Brabec is that he and his colleagues can squeeze even more power out of these cells, at least doubling their efficiency once more to capture 10 percent of incoming solar energy-a percentage that experts consider to be a threshold for rooftop applications. "We are absolutely sure that efficiency will continue to climb," says Brabec.

Now, he says, it is time to demonstrate that large-scale production is feasible. "What we did was in a clean room, and the maximum module size is [15 centimeters]," he explains. "The logical next step is to get out of the lab and try reel-to-reel production under industrial conditions." He hopes to get there next year.