Spray Paint to Soak Up the Sun

January 18, 2016

Susanna ThonImagine a clear solar paint that you could spray onto windows to capture the elusive energy of the sun. Or bright neon paints that could coat sports cars to convert unharnessed solar rays into electricity as the vehicles cruise along the California coast.

These 21st-century products of the future are the visions of Susanna Thon, an assistant professor of electrical and computer engineering. A physicist, Thon has converted her own interest in quantum computing into a passion for the basic science behind nanomaterials—specifically quantum dots, used in sensors and LEDs, and metals, such as silver, gold, and aluminum—to capture solar energy in flexible liquids, such as paints or films, and convert it to electricity.

Today, conventional silicon solar cells—used in brittle, flat panels mounted upon rooftops—are capable of absorbing and converting just about 25 percent of the energy in the sun’s light rays, mostly in the visible wavelengths. Almost half the sun’s untapped energy store resides in the invisible infrared band—those harmless, longer wavelengths used in fiber-optics telecommunications, night vision, and thermal imaging.

Materials such as colloidal quantum dots—nanomaterials composed of tiny chunks of semiconductors—can be engineered to absorb energy from many wavelengths—even the elusive infrared—but they aren’t good at transporting energy as electricity.

“This is our challenge,” says Thon.

In the basement of Barton Hall, in her NanoEnergy Laboratory, Thon is setting out to solve this scientific conundrum. On a recent day, an undergraduate and graduate student in the eight-member lab are making a solar cell by using a syringe to flow a brownish film on a glass slide. The liquid contains trillions of lead sulfide quantum dots. Thon will go on to layer different materials—like a seven-layer torte cake—to optimize the solar cell’s absorption and conversion properties.

“The different layers are needed for the solar cells to work electrically,” explains Thon. “But people haven’t thought about using these same layers as optical layers, too. It turns out, by changing thickness, you can change the optical properties of the whole device as well.”

Thon postulates that metals in nanoparticles could essentially turn themselves into tiny “optical antennas” that could localize light to increase the energy absorption of surrounding materials. She sees great promise in these hybrid materials, and she has submitted an academic paper, currently under review, detailing the potential. “It’s easy to engineer your material to do one thing or the other,” Thon says. “We want to do both.”

This article originally appeared in the Winter 2016 issue of Johns Hopkins Engineering magazine.

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