Sustainable Energy

Solar-Cell Rollout

Breakthroughs in nanotech are making it possible to churn out cheap, flexible solar cells by the meter. Soon your cell phone may be powered by the sun.

On the test benches of Konarka Technologies in Lowell, MA, a new kind of solar cell is being put through its paces. Strips of flexible plastic all but indistinguishable from photographic film bask under high-intensity lights. These strips, about 10 centimeters long and five centimeters wide, are converting the light into electricity. Wire a few of them together, and they generate enough power to run a small fan.

Solar cells, of course, are nothing new. But until now, solar power has required expensive silicon-based panels that have relegated it, largely, to niche applications like satellites and high-end homes. What’s remarkable about Konarka’s power-producing films is that they are cheap and easy to make, using a production line of coating machines and rollers. The process is more akin to the quick-and-dirty workings of a modern printing press than to the arcane rituals performed in the clean rooms of silicon solar-panel manufacturing. The company literally has rolls of the stuff; its engineers plan to cut off usable sheets as if it were saran wrap.

Konarka’s technology is just one example of a new type of printable solar cell, or photovoltaic, that promises to go almost anywhere, paving the way for affordable and ubiquitous solar power. Not only are the cells inexpensive to produce-less than half the cost of conventional panels, for the same amount of power-but they’re also lightweight and flexible, so they can be built into all sorts of surfaces. Flexible films laminated onto laptops and cell phones could provide a steady trickle of electricity, reducing the need to plug in for power. Solar cells mixed into automotive paint could allow the sun to charge the batteries of hybrid cars, reducing their need for fuel. Eventually, such solar cells could even cover buildings, providing power for the electricity grid.

This story is part of our July/August 2004 Issue
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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.

Shining Startups

At least one startup may beat Siemens to that goal. Konarka is now gearing up to manufacture its novel photovoltaic film, which it expects to start selling next year. Unlike Siemens’s, Konarka’s films don’t use buckyballs, instead relying on tiny semiconducting particles of titanium dioxide coated with light-absorbing dyes, bathed in an electrolyte, and embedded in plastic film. But like Siemens’s solar cells, Konarka’s can be easily and cheaply made.

Konarka sees a short-term payoff in consumer products. Power-hungry electronics such as cell phones and laptops-and anything else with a battery and access to light-could make good use of Konarka’s flexible film, according to executive vice president Daniel McGahn. And the solar films could eliminate the need to run power cords to many other electronic devices installed in homes or businesses, such as the temperature, gas, and process sensors scattered throughout manufacturing plants.

Down the road, researchers hope to boost nano solar cells’ power output and make them even easier to deploy, eventually spraying them directly onto almost any surface. Palo Alto, CA-based startup Nanosolar, which has raised $5 million in venture capital, is working on making this idea practical. The company is exploiting the latest techniques for automatically assembling nanomaterials into precisely ordered architectures-all with a higher degree of control than ever before possible.

Nanosolar’s approach is disarmingly simple. Researchers spray a cocktail of alcohol, surfactants (substances like those used in detergents), and titanium compounds on a metal foil. As the alcohol evaporates, the surfactant molecules bunch together into elongated tubes, erecting a molecular scaffold around which the titanium compounds gather and fuse. In just 30 seconds a block of titanium oxide bored through with holes just a few nanometers wide rises from the foil. Fill the holes with a conductive polymer, add electrodes, cover the whole block with a transparent plastic, and you have a highly efficient solar cell.

In theory, at least, energized electrons in Nanosolar’s columns of plastic need only jump a few nanometers to reach the titanium compounds. From there, the electrons shoot straight through the vertically oriented titanium compounds to an electrode. “It’s a fast path out,” says Nanosolar’s CEO Martin Roscheisen, an Internet entrepreneur who founded the company two years ago.

This technology could enable Nanosolar to spray-paint photovoltaics onto building tiles, vehicles, and billboards, and wire them up to electrodes. At first, the cells would be applied in manufacturing, but eventually they might be sprayed onto existing surfaces. When will this approach become prevalent enough to feed electricity to power grids? Roscheisen won’t say, but he vows that by the end of next year, Nanosolar will have prototypes that capture 10 percent of incoming solar energy.

Catching Some Sun

In their initial applications-such as powering cell phones and laptops, as Konarka envisions-printed solar cells won’t need to produce that much power or run for decades at a time. But scaling them up from personal electronics to rooftops is a whole other story.

Unlike the crystalline silicon in conventional solar panels, the polymers and dyes employed in printable solar cells are exquisitely sensitive to oxygen. Protecting these materials from blowing sand, intense sunlight, extreme temperature shifts, and the myriad other forms of abuse that nature heaps on solar panels will require hermetic seals. But Brian Gregg, a solar expert at the U.S. Department of Energy’s National Renewable Energy Laboratory, predicts that materials scientists will soon develop workable seals that will protect the delicate devices over the long term. “There’s no reason to believe that we can’t make [printed] solar cells that will last for 30 years,” says Gregg.

Indeed, the recent advances in printable solar cells-and the growing possibilities presented by nanotechnology-leave many experts more optimistic than ever that the technology is nearly ready to tackle one of the world’s most troubling problems: how to create a ready and renewable supply of energy. Nanotech pioneer Richard Smalley, for one, is convinced that a solar-powered grid is not just possible but also inevitable-and indispensable. Nanotech could help solve the energy problem, Smalley contends, by providing new tools and materials that make widespread use of solar cells economically viable. But he believes it will take billions of dollars in funding and the focused efforts of the world’s top chemists and physicists to make that happen. So for the past two years, he has been crisscrossing the United States, evangelizing for nothing short of a modern-day Manhattan Project to use nanotech to deliver a sustainable energy system.

That’s the long-term vision. In the meantime, the Konarkas and Siemenses of the world are taking some critical first steps toward changing how we think about harvesting energy from the sun, and how we use electricity in our lives. It may not yet be the Manhattan Project urged by Smalley, but it’s a fast-growing effort that could quickly reach critical mass.

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