Tuesday, March 20, 2012

Soft cell: what's the future of solar technology? Just one word:plastics

Fossil DSCF2757 by Bill & Mark Bell
Fossil DSCF2757, a photo by Bill & Mark Bell on Flickr.

Earth is awash in energy. Enough of it hits the planet as sunlight in less than 10 seconds to supply humanity's energy needs for a full day. In an age of global concern about the pollution released by burning fossil fuels (coal, oil, and natural gas), harnessing sunlight is more important than ever.

Solar cells such as the kind that now cover many rooftops don't capture much sunlight and are expensive. They convert a little more than 10 percent of the light they receive into electricity and cost thousands of dollars. At those rates, the electricity generated by the burning of coal is 30 times cheaper.

If solar cells are made to convert more light into electricity, the cost goes even higher. "Efficient solar cells are very expensive," says Xiaoyang Zhu, the director of the Center for Materials Chemistry at the University of Texas at Austin. "They might be up to 40 percent efficient, but they'll cost you $10,000 for a square inch."

So Zhu and other scientists have begun chasing another way to make harvesting the sun's energy a reality. "If we can't make solar energy very efficient, maybe we can make it cheaply," he says.


Today, most solar power is captured via photovoltaic technology--photo for "light" and voltaic for "electricity." Light is composed of particles called photons that carry its energy. Photovoltaic cells (otherwise known as solar cells) turn the energy of speeding photons into electricity.

Conventional solar cells do that with the help of the element silicon. Incoming photons strike specially treated silicon and knock some of its electrons (negatively charged particles) free. Charges in another part of the silicon push those electrons away. Wires in the cell allow the electrons to flow in a circuit, generating an electric current.

Because silicon is abundant in Earth's crust, most solar products are made from it. But silicon needs to be refined and treated to be useful. That work is one reason why conventional solar electricity is so expensive.


To overcome the problem, researchers are turning to new materials for solar cells. "We're actually using plastics," Zhu says. "The advantage of plastic is that we can produce it easily and cheaply."

Plastics are made of polymers--long chains of carbon molecules. Depending on how the chains are put together, a plastic will have different properties. Think of the difference between a polyester shirt and a polycarbonate bulletproof window. Both are plastic.

Different polymers have different electrical properties, says Raluca Gearba, a researcher who works with Zhu. "To work well for solar cells," she says, "polymers need to have a particular shape, they need to absorb light, and they need to have a high mobility of charges. Sometimes you get one of those properties, sometimes you get two. We want to know why some polymers are good for solar cells and why some are bad "

Solar cells made of plastic work a bit differently from those based on silicon. When incoming photons strike plastic, electrons aren't knocked free. Instead, they become energized by the photons. Another kind of plastic with different electrical properties must then be used to pull the energized electrons away. Gearba is exploring different kinds of plastic to find which combinations do the best job and then figure out how to make them better. "That's what we're trying to understand," she says. "How do you get free charges in these materials?" Once the electron is free, it can flow through a circuit and become electricity.


As Gearba and other chemists study more plastic polymers, they'll improve how well solar cells made from those polymers work. Right now, commercially available plastic solar cells convert a little more than 3 percent of the energy in light into electricity--about a third as much as silicon-based cells do.

However, the promise of plastic solar cells lies not so much in how efficient they become, but in how easily they can be manufactured and where they can be used. "You can make polymers in large quantities," Gearba says. "You can put them onto flexible surfaces." In fact, layers of polymers can be sprayed or printed like ink.

Zhu is optimistic about the future of plastic solar cells. "If we increase the kind of basic research we're doing right now," he says, "I think in five to 10 years we'll reach a breakthrough and something great is going to happen." He envisions plastic solar cells that will be painted on houses or applied to the windows of skyscrapers as sunshades or even unspooled from huge rolls to cover fields and act as solar farms.

"Scientifically there is no reason we should be using fossil fuels in 50 years," Zhu says. "Politically there may be a reason, or socially there may be a reason, but scientifically there is no reason."

Jozefowicz, Chris

Source Citation
Jozefowicz, Chris. "Soft cell: what's the future of solar technology? Just one word: plastics!" Current Science, a Weekly Reader publication 24 Feb. 2012: 10+. General Science Collection. Web. 20 Mar. 2012.
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