Transparent solar cells can transform buildings into sky-high power plants, generating electricity from glimmering glass façades. But the see-through technology faces a big barrier to reaching mainstream adoption: organic materials degrade relatively quickly as they bake in the sunlight.
A new approach by the University of Michigan in Ann Arbor might dramatically extend the working life of solar windows. In a new study, researchers said their solar cell design can operate at high efficiencies for up to 30 years—roughly on par with the expected lifetime of conventional silicon solar panels. Now, the team is working to scale their speck-sized device to match the dimensions of a small window.
If they succeed, "It will change the future of buildings," predicted Yongxi Li, an assistant research scientist at University of Michigan and first author of the paper in Nature Communications. "If all the buildings have this transparent device, they'll be able to power themselves."
There's certainly no shortage of real estate. By 2023, global demand for flat glass is forecast to reach 11.9 billion square meters as building construction and manufacturing activity expands worldwide, according to the Freedonia Group.
Xiaoxi He, a technology analyst at IDTechEx in Cambridge, England, described the research as "really innovative work." Still, she added, many steps remain between improving technology in the lab to fabricating devices and, eventually, mass producing solar windows.
Organic solar cells use carbon-based compounds for the semiconducting materials, which are ink-printed or applied in extremely thin layers to a plastic backing. The materials can absorb photons in the near infrared, the invisible part of the light spectrum that accounts for much of the energy in sunlight. Conventional solar cells with silicon semiconductors can harvest more photons, and thus are more efficient at converting sunlight into electricity. But they're also thicker and opaque, which is hardly ideal for windows or skylights.
To block out harmful UV light and keep those bonds from breaking down, engineers added a simple layer of zinc oxide—a common ingredient in sunscreen and diaper rash ointment—to the side of the solar cell that faces the sun.
Li and his colleagues initially designed an organic solar cell with "non-fullerene acceptors." Those materials lie in the molecules that transfer the light-generated electrons to the electrodes. Using an electrode made of indium tin oxide, the researchers created a color-neutral solar cell with an 8.1 percent efficiency rate, a record for this kind of organic design. Swapping in a silver electrode boosted the cell's efficiency to 10.1 percent, though this version had a less desirable greenish tint.
However, researchers found these record efficiencies didn't last for very long. Non-fullerene acceptors contain weak bonds that easily dissociate under ultraviolet light or other high-energy photos, Li said. In lab experiments, the organic cell's efficiency fell to less than 40 percent of its initial value within 12 weeks, when exposed to one sun's worth of illumination.
So researchers devised a set of solutions. The team included engineers at North Carolina State University in Raleigh, as well as Tianjin University and Zhejiang University in China.
A transmission electron microscope (TEM) image of a cross-sectional slice of the organic photovoltaic solar cell. The image reveals an intact organic active region with no breakdown at the edges.Kan Ding/University of Michigan Engineering
To block out harmful UV light and keep those bonds from breaking down, engineers added a simple layer of zinc oxide—a common ingredient in sunscreen and diaper rash ointment—to the side of the solar cell that faces the sun. Next, they determined that most of the solar cell's degradation occurred at the interface between the organic and inorganic materials. By adding ultra-thin "buffer layers" of carbon-based materials, researchers managed to block the chemical changes and improve stability.
They then exposed the small-area solar cell devices—about 0.1-square-centimeters in size—to varying levels of illumination. Typically, nearly 1,000 watts per square meter of solar energy falls on Earth's surface. In the standard spectrum, one sun illumination is defined as 100 milliwatt/cm2 of irradiance.
When the engineering team exposed their device to one sun for 1900 hours, the cell maintained 94 percent of its initial efficiency. They increased sunlight to that of 27 suns, at temperatures of up to 65°C. Based on their experiments, researchers extrapolated that the solar cell would keep operating at 80 percent efficiency after three decades. (Conventional silicon cells are expected to last between 25 to 30 years.)
Li said the Michigan-based team has already developed a 15-square-centimeter "mini" solar window, to test whether the protective layers and buffers can keep materials from degrading at a larger scale. The challenge is that, the larger the active solar cell layer becomes, the greater opportunity for defects to emerge. "We need to demonstrate how to make a uniform field," he said.
Beyond the actual solar cell, many other factors can determine how well a solar window works—and whether it can produce enough electricity to justify the extra expense for building developers or operators, according to He, the IDTechEx analyst. The geography of the building and the amount of local solar resources, along with the angle of windows, the height of the building and its overall design can all significantly influence the solar device's performance.
"When we see a technology improvement, we're always really excited," she said. "But from the lab to the actual adoption for large commercial applications, usually there are lots of considerations and luck involved with that."
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