Research led by the University of Michigan in Ann Arbor has shown that a new transparency-friendly solar cell design could marry high-tech efficiencies with 30-year estimated lifetimes, while also potentially paving the way for windows that provide solar power.
Silicon remains the most efficient material for solar panels, but it isn’t transparent. Stephen Forrest, professor of electrical engineering and research lead at U-M, and his team, were tasked with figuring out how to use transparent organic materials without them degrading quickly during use.
“Solar energy is about the cheapest form of energy that mankind has ever produced since the industrial revolution,” says Forrest. “With these devices used on windows, your building becomes a power plant.”
The strength and weakness of these materials lie in the molecules that transfer the photogenerated electrons to the electrodes, the entrance point to the circuit that wither uses or stores the solar power.
The materials are known generally as non-fullerene acceptors to set them apart from the more robust but less efficient fullerene acceptors made of nanoscale carbon mesh. Solar cells made with non-fullerene acceptors that incorporate sulfur can rival the efficiency of silicon at 18 percent, but do not last long.
“Non-fullerene acceptors cause very high efficiency, but contain weak bonds that easily dissociate under high energy photons, especially the UV [ultraviolet] photons common in sunlight,” says Yongxi Li, U-M assistant research scientist in electrical engineering and computer science and first author of the paper in Nature Communications.
In their experiments, the team that included researchers at North Carolina State University and Tianjin University and Zhejiang University in China, showed that without protecting the sunlight-converting material, the efficiency falls to less than 40 percent of its initial value within 12 weeks under the equivalent of one sun’s illumination.
The team discovered while studying the degradation that only a few places needed reinforcement of repair. First, they needed to block out the UV light causing the damage. To do that, they added an ingredient commonly found in sunscreen — zinc oxide — on the sun-facing side of the glass.
A thinner zinc oxide layer next to the light absorbing regions help conduct the solar-generated electrons to the electrode. Zinc oxide, however, also breaks down the fragile light absorber. To mend this problem, the team added a layer of a carbon-based material called IC-SAM as a buffer.
In addition, the electrode that draws positively-charged holes — essentially spaces vacated by electrons — into the circuit can also react with the light absorber. To protect that flank, they added another buffer layer, this one a fullerene shaped like a soccer ball.
The team tested this solution under different intensities of simulated sunlight, from one all the way up to 27, along with temperatures reaching 150 degrees Fahrenheit. By studying how the performance degraded under these conditions, it was extrapolated that the solar cells would still be running at 80 percent efficiency after 30 years.
Forrest believes these devices will be a reality in people’s homes in the future. His team has increased the transparency from 40 percent and believe they can approach 60 percent.
They’re also working on bumping up the efficiency from the 10 percent achieved in the reported semitransparent modules, closer to the 15 percent believed to be possible at high transparency. Because the materials can be prepared as liquids, the manufacturing costs are expected to be relatively low.
Part of the research was conducted in the U-M Lurie Nanofabrication Facility. The research was funded by the Office of Naval Research and the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy. Universal Display Corp. holds a license to the work. U-M has a financial interest and Forrest has an ownership interest in Universal Display Corp.
