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Breakthroughs

Spray-On Solar Cells

By Amanda Kolson Hurley
Susanna Thon
Susanna Thon is a physicist at the Whiting School.

We know that our reliance on fossil fuels for energy is a dangerous habit; the related emissions are causing glaciers to shrink and sea levels to rise, making oceans more acidic (which puts marine species in peril) and intensifying droughts and storms (putting us and our homes at risk). Fossil-fuel combustion also causes air pollution, which exacerbates asthma and other health problems. So advances in renewable energy are potential advances for public health, too.

One promising source for that kind of energy comes from nanomaterials. Susanna Thon is a physicist in the Electrical and Computer Engineering Department at the Johns Hopkins Whiting School of Engineering. Thon leads the NanoEnergy Laboratory, where she is working on a liquid that could be coated on almost any surface to capture solar energy. It could be applied to conventional solar cells to boost their efficiency or, in theory, coated directly on a car or a building. This would be a major step forward from today’s solar cells, which are inflexible and brittle, and the bulky panels on which they’re usually mounted.

The breakthrough is down to changes at the nanolevel (i.e., really, really small: one nanometer is one-billionth of a meter). Specifically, Thon and her team are manipulating quantum dots. Quantum dots are tiny chunks of semiconductors—the large class of materials used to make integrated circuits in laptops, cellphones, and other electronic devices.

“Something interesting happens when you structure these types of materials on the nanoscale,” Thon says. “The energy structure of the material changes.” Semiconductors used for solar cells can absorb light in the visible portion of the spectrum, but not the infrared waves the sun emits, which are invisible. At least, they couldn’t until now.

“It turns out that, unlike silicon—which has a set absorption profile—with quantum dots, by just changing their diameter, you can change the portion of the spectrum they absorb,” Thon says. That means the quantum dots can be “tuned” to absorb one part of the spectrum, producing a particular color. Or they can be adjusted to absorb the whole spectrum, including the infrared range, and glean more energy.

Making normal semiconductors is costly and energy-intensive, but Thon and her colleagues can make quantum dots “in a fume hood, in a regular old lab,” and at low cost. They end up with nanoparticles stabilized in a liquid, which turns brown owing to the cells’ color absorption, and this solution can be sprayed onto a surface or painted on as you would paint a piece of furniture with a brush.

The first major impact of the technology might be a better solar cell rather than a replacement for it. Today’s photovoltaic cells tap into only the visible portion of the spectrum, representing about half the potential energy that could be harvested. But coating quantum dots onto the cells would allow them access to infrared waves too, greatly improving their efficiency.

Most commercial silicon cells today have 15 to 20 percent efficiency, Thon estimates. She thinks quantum dots technology could match that soon, and that a tandem silicon-quantum dots cell could get to 30 percent efficiency or higher. “Even improving efficiency by 1 percent is a huge deal,” she notes.

Meanwhile, the lab’s researchers are trying to find alternatives to the current dark brown color of the liquid. If they can optically engineer it to make it appear clear or register as different colors, the possible applications will multiply. Thon envisages a clear quantum dots liquid that could be painted onto windows, capturing infrared sunlight while preventing heat gain inside the building. A liquid that’s green or red or blue could be applied to cars so they generate energy.

Related technologies, like quantum dot displays, are starting to appear on the market, and Thon hopes to see the first products stemming from this research within three to five years (for niche uses). “We’re still in the research and development phase,” she says, “but we hope to get some good results on that time scale and hopefully push things beyond.”

illustration of a man spraying solar cells on a house
Image above by Maggie Chiang; Portrait by Caroline Andrieu

The Johns Hopkins NanoEnergy Laboratory, led by Susanna Thon, is developing a liquid that could harvest solar energy.

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