Solar cells have much potential for future uses, but have one crucial engineering constraint. That is that most solar panels are rigid and must be deployed in stiff, frequently heavy, fixed panels, thus limiting their applications. Researchers have been trying to get photovoltaics to more flexible, ideally in the form of decal-like solar panels which can be peeled like band-aids and stuck to practically any surface, from paper to window panes.
Stanford researchers have now developed the world’s first peel-and-stick thin-film solar cells, as reported in the Scientific Reports December 20th issue.
According to Chi Hwan Lee, lead author of the paper, thin-film photovoltaic cells are usually installed on rigid silicon and glass substrates, greatly limiting their uses. While the development of thin-film solar cells promised to introduce some flexibility into the technology, explains Xiaolin Zheng, a Stanford assistant professor and senior author of the paper, scientists discovered that use of alternative substrates was very problematic.
“Nonconventional or ‘universal’ substrates are difficult to use for photovoltaics because they typically have irregular surfaces and they don’t do well with the thermal and chemical processing necessary to produce today’s solar cells,” Zheng explains. “We got around these problems by developing this peel-and-stick process, which gives thin-film solar cells flexibility and attachment potential we’ve never seen before, and also reduces their general cost and weight.”
Using the process, Zheng says, researchers attached their solar cells to paper, plastic and window glass among other materials. “It’s significant that we didn’t lose any of the original cell efficiency,” he adds.
Unlike other thin-film solar cells, Stanford’s peel-and-stick version requires no direct fabrication on the final carrier substrate. This development is far more striking than it may seem. All the challenges associated with putting solar cells on unconventional materials are avoided with the new process, greatly expanding the potential applications of solar technology.
Silicon Metal Sandwich Process
The new process entails a novel silicon, silicon dioxide and metal “sandwich.” First, a 300-nanometer film of nickel (Ni) is deposited on a silicon/silicon dioxide (Si/SiO2) wafer. Then thin-film solar cells are deposited on the nickel layer utilizing standard fabrication techniques, and covered with a layer of protective polymer. A thermal release tape is then attached to the top of the thin-film solar cells to augment their transfer off of the production wafer and onto a new substrate.
The solar cell is now prepared to peel from the wafer. To remove it, the wafer is submerged in water at room temperature and the edge of the thermal release tape is peeled back slightly, allowing water to seep into and penetrate between the nickel and silicon dioxide interface. The solar cell is thus freed from the hard substrate but still attached to the thermal release tape.
Zheng and team then heat the tape and solar cell to 90°C for several seconds, then the cell can be applied to virtually any surface using double-sided tape or other adhesive. Finally, the thermal release tape is removed, leaving just the solar cell attached to the chosen substrate.
Tests have shown that the peel-and-stick process consistently leaves the thin-film solar cells completely intact and functional, Zheng said. “There’s also no waste. The silicon wafer is typically undamaged and clean after removal of the solar cells, and can be reused.”
Commercially Viable Fabrication
While others have been successful in fabricating thin-film solar cells on flexible substrates before, those efforts have required modifications of existing processes or materials, noted Lee. “The main contribution of our work is we have done so without modifying any existing processes, facilities or materials, making them viable commercially. And we have demonstrated our process on a more diverse array of substrates than ever before,” Lee said.
“Now you can put them on helmets, cell phones, convex windows, portable electronic devices, curved roofs, clothing – virtually anything,” said Zheng.
The peel-and-stick technique isn’t limited necessarily to thin-film solar cells, Zheng said. The researchers envison the process can also be applied to thin-film electronics, including printed circuits and ultra thin transistors and LCDs.
“Obviously, a lot of new products – from ‘smart’ clothing to new aerospace systems – might be possible by combining both thin-film electronics and thin-film solar cells,” observed Zheng. “And for that matter, we may be just at the beginning of this technology. The peel-and-stick qualities we’re researching probably aren’t restricted to Ni/SiO2. It’s likely many other material interfaces demonstrate similar qualities, and they may have certain advantages for specific applications. We have a lot left to investigate.”
Image Credit: Chi Hwan Lee, Stanford School of Engineering
Chi Hwan Lee, Dong Rip Kim, In Sun Cho, Nemeth William, Qi Wang & Xiaolin Zheng
Scientific Reports 2, Article number:1000 doi:10.1038/srep01000