The catalyst for Xiaolin Zheng's groundbreaking work in solar energy began with an offhand comment her father made years ago at her parents' apartment, a 13-story complex in the northeast China city of Anshan.
“In China, the rooftops of many buildings are packed with solar energy devices,” says Zheng. “One day my father mentioned how great it would be if a building’s entire surface could be used for solar power, not just the roof, but also walls and windows.”
An invention from Zheng's research team at Stanford University might someday make that possible. They have created a type of solar cell that is thin, flexible, and adhesive—a solar sticker, in effect, that could help power everything from buildings to airplanes.
“By making solar cells extremely thin and flexible, they can be used in all kinds of new ways," says Zheng, an associate professor at Stanford and recipient of the U.S. Presidential Early Career Award for Scientists and Engineers. "I hope our discovery will dramatically expand the affordable, practical, widespread application of solar power.”
In 2010, a decade after her father’s initial comment, Zheng read a research paper that triggered the idea again. It described an experiment in which the nanomaterial graphene was grown on a layer of nickel atop a silicon wafer. When submerged in water, the nickel separated from the surface, along with the graphene.
“It sounded unbelievable, like a magic trick,” she recalls, “But they had achieved very reliable results.” What if, she wondered, the same principle could be used to yield a thinner, more flexible solar cell that could peel off, attach to adhesive, and stick to virtually any surface?
Because conventional thin-film solar cells are manufactured on glass or silicon wafers, they are rigid, heavy, and quite limited in how and where they can be used. Plastic or paper would be far more flexible, but it cannot withstand the high temperatures and chemicals required for fabrication.
“Our new technique lets us treat the solar cells like a pizza,” explains Zheng. “When you bake pizza, you use a metal pan that can tolerate high temperatures. But when it’s time to distribute the pizza economically, it’s placed in a paper box."
Working with her students, Zheng set out to fabricate solar cells on a silicone or glass surface as usual, but she inserted a metallic layer between the cell and the surface. After some trial and error, the team was finally able to peel away the metallic layer from the surface after soaking the whole structure in water for just a few seconds.
The result was an active solar cell that is only a couple of microns thick—about one-tenth the thickness of plastic wrap, Zheng says. "It’s extremely flexible, so it can be attached to any surface—the back of a mobile phone, a skylight, a wall, a curved column.”
The skinny, bendable cells can produce the same amount of electricity as rigid ones, and they offer cost benefits as well, according to Zheng. “The silicon wafers come through the process clean and shiny,” she says. “So just like a pizza pan, they can be used again and again, which translates to savings.” And because the solar stickers are lighter than conventional panels, they will be easier and less expensive to install.
The stickers might be able to reduce manufacturing costs too, Zheng says. In traditional solar-cell production, the foundation materials account for 25 percent of the cost. The new method will enable that base layer to be removed or replaced with a cheaper material. For example, the windows of a building provide a ready-made base layer, so all that’s needed is the solar cell itself. A cell that could simply be peeled and applied enables that economical shortcut.
Zheng predicts peel-and-stick solar cells could one day paper the sides of buildings, cover sidewalks to light walkways, energize home security systems, and help power solar cars or planes. Along with industrial uses, she envisions being able to stop at your corner store to pick up a pack of solar cells the way you buy batteries today.
To help realize those large-scale applications, Zheng's team wants to test the technology with more efficient cells than the ones used for the initial breakthrough.
"Our cells will also need to be larger, expanding from their current one-square centimeter size to a square foot or even square meter," Zheng says. New equipment will be needed, too, for the peel-off process that the researchers conducted by hand in the lab.
Her research group is also looking into how solar energy could be used to split water atoms, producing hydrogen and providing a potentially cheaper, more efficient way to fuel everything from transportation to home heating.
“As scientists,” she says, “I feel it is profoundly important to use our work to improve the world. For the future of our environment, we need to advance renewable energy rather than heavily relying on fossil fuels to meet growing demand. Solar power has always been my favorite because sunshine is so clean, abundant, and has fewer limitations on where it can be used.”
Exactly how her solar-sticker idea will evolve from a nanoscience lab to the real world is not yet known. Now that the technology has been tested and proven, however, it's a testament to value in pursuing a seemingly outlandish notion.
“Everyone has crazy ideas about some discovery that could change the world,” Zheng says. "Those ‘what if’ questions are always in the back of your mind. You also always keep your eye on the latest emerging technologies. When those two worlds happen to connect, it’s great.”