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A Sunlit Puzzle: Studying the mystery behind solar panel efficiency

"This is a hot field in the research community around the world"

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Solar energy is considered clean. And it is — if you start counting only after the solar panels are up and running.

That's because, like any industrial process, manufacturing solar cells is dirty business. The silicon for the semiconductors must be mined. The solvents used in processing are toxic. And the whole process uses a lot of water and so much energy that it can take up to four years for one solar cell to generate the energy to manufacture a new one.

Alternatives exist. Solar cells can be made with plastic rather than silicon. True, supplying the raw material for the plastic — usually some kind of fossil fuel — is environmentally damaging, too. But plastic cells are much less energy-intensive, and thus much cheaper, to make. They require so much less material that their "energy debt" is repaid in months, not years. Thinner than plastic food wrap, they're flexible enough to be built into clothing or other portable chargers, and they're easily recyclable.

So why don't we switch? One problem is that plastic solar cells are quite inefficient. At their world-record best, plastic cells converted 10 percent of the sunlight striking them into electricity, compared to about 25 percent for the most common kind of silicon-based cell. And that was under laboratory conditions: Mass-produced plastic cells have proved just 1 percent efficient, compared to 15 percent for factory-made silicon — not good enough to warrant widespread commercial manufacture.

The good news is, that's changing rapidly: The efficiency record for plastic solar is double what it was just six years ago.

The bad news: No one knows exactly how that improvement happened. 

The reason is that some two decades after their first photovoltaic use, the polymers in plastic cells remain mysterious. 

Silicon, a mineral, behaves in regular and well-documented ways. That's not the case with the polymers used in the plastic cells. While scientists know that certain combinations of polymers and other materials generate and conduct electricity, they understand little of how changes in manufacturing techniques affect a plastic solar cell's efficiency. Simply put, the processing causes physical changes at a scale too small to see, even with advanced microscopes.

Thus, efficiency advances have come through what researchers call "blind optimization." Or as Nikolai Zhitenev, an energy researcher at the federal Center for Nanoscale Science and Technology, puts it, "People have to pretty much guess how materials are organized in the mixture."

Among the researchers addressing that problem is a team at the University of Pittsburgh led by Guanyong Li. They're trying to develop new microscopic techniques that see better at the nano scale — where measurements are made down to the one-billionth of a meter.

The technique involves something called Kelvin Probe Force Microscopy, which uses a super-sharp probe and lasers to get images at the nano scale. Li, an assistant professor of electrical and computer engineering, plans to augment its sensitivity by attaching to the tip a "carbon nanotube" — a lab-made object just one atom thick.

If the technique succeeds, researchers will be able to more accurately predict what combination of processing temperature, annealing time and other factors will create the highest efficiency. "If you can see [the interfaces], then you can easily tell, ‘OK, this size gives the best performance,'" says Li.

Scientists elsewhere are exploring solutions, too. "This is a hot field in the research community around the world," says Jim Dietz, of Plextronics, a local firm that specializes in electronic polymers and inks.

But Li's three-year microscopy project has just begun. And plastic solar cells face other hurdles, such as the fact that they last just a year.

All that suggests just how hard it is to build a better solar cell: Research notwithstanding, Dietz says, organic solar cells are still five to 10 years from getting to market.

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