A Solar Solution, on the Dot

Scientists have spent decades developing technology that harvests sunlight and converts it to electricity. Yet existing solar energy solutions remain cost prohibitive for many applications. Brandi Cossairt hopes to change that.

Cossairt, assistant professor of chemistry, is developing more efficient solar materials that should be significantly less expensive to produce, resulting in low-cost solar power. In May 2014, she received an Innovation Award from the UW to support her research. Innovation Awards provide $500,000 in private support to enable young faculty to pursue higher risk projects that could lead to important breakthroughs.

Why has solar energy been so expensive? It has to do with the manufacturing process. Conventional solar cells are made of silicon, a semiconductor that is abundant but not particularly efficient at absorbing light. “With silicon, the efficiency of a solar cell is about 25 percent,” explains Cossairt, “and to get it to that level of efficiency you need the starting silicon to be very, very pure and then you need to purposefully add impurities in a controlled way. It’s also best if the whole solar cell is a single crystal.” Purifying silicon, and growing crystals large enough for a solar panel, are both costly processes.

Cossairt is exploring a lower cost alternative using quantum dots—nano-sized crystalline semiconductor particles that could be spray-painted on a surface in layers. “With quantum dots, we don’t need a single large crystal and the chemistry used to make them is inherently self-purifying,” she says. “The costs would be limited to the actual materials, where with silicon the major cost is the manufacturing process.”

To appreciate quantum dots, it helps to understand the process by which sunlight converts to energy in semiconductors. When light—sunlight for example—shines on a semiconductor, the energy of that light excites an electron in the material, allowing it to become free to conduct electricity. The excited electron is negatively charged; the hole it leaves behind is positively charged. “In a big piece of silicon, the electron will quickly move away from the hole,” explains Cossairt. “But we make quantum dots so small that the electron and hole can’t get away from each other. They are always bound to one another.”

The confinement of the electron and hole gives rise to one of the most exciting aspects of the quantum dot—its ability to absorb different colors of light. Changing the size of the quantum dot changes the energy, or color, of light absorbed by the material. Given that sunlight is a combination of all the colors in the spectrum, the ability to make a single material that can absorb different colors would translate to far greater efficiency in harvesting solar energy.  (The silicon used today is fixed in terms of the color of light it can absorb.) Cossairt envisions one day being able to paint layers of quantum dots on top of each other, each layer absorbing a different color of light.

Of course harvesting the sun’s power is just one part of the solar energy equation. There’s also the challenge of storing solar energy for future use. This is a separate but related area of research for Cossairt, who is hoping that quantum dots can provide a solution. The goal is to store energy in a chemical bond that could later be broken to release the energy when needed. The same idea powers gasoline, which releases energy when its carbon-carbon bonds are broken through burning.  But while gasoline releases polluting CO₂ along with energy, the solutions Cossairt is exploring would release harmless water as the only byproduct.

“That’s really on the frontiers of research,” says Cossairt. “We’re not the only people thinking about using quantum dots for energy storage, but the research into storage is not nearly as far along as it is for collecting light.” Cossairt believes that we may see quantum dots used to make solar cells within ten years, but the use of quantum dots for storage is “more like a twenty- to thirty-year problem. There’s a lot of science that has to happen before we’ll see sunlight used to make fuel on a commercial scale.”

The Innovation Award is certainly a step in the right direction. Cossairt is using the award to help fund her laboratory team—currently seven graduate students, one undergraduate, and a postdoc—as they tackle these difficult questions, which could dramatically alter how we think about energy.

“Focusing on using completely renewable resources that will never be exhausted, that will never go away, just makes good logical sense,” says Cossairt. “If we can find a way to harvest and store energy that is cheap and efficient, it will be transformational.”