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Energy gains from solar cells have been a promising technology for years. But that has been all it has been – promising.

Now it is time to get more innovative and see if solar cells can lose the promising tag and become a reality.

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For years, solar cells have been around, but they have been incredible rigid. They end up deployed in stiff, often heavy, fixed panels, which limits their applications. So researchers have been trying to get photovoltaics to loosen up. The ideal: Flexible, decal-like solar panels that can peel off like band-aids and stuck to virtually any surface.

With that in mind, researcher at Stanford University developed peel-and-stick thin-film solar cells.

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Unlike standard thin-film solar cells, these peel-and-stick cells do not require any direct fabrication on the final carrier substrate. This is a far more dramatic development than it may initially seem. All the challenges associated with putting solar cells on unconventional materials go away with the new process, which expands the potential applications of solar technology.

Thin-film photovoltaic cells traditionally end up fixed on rigid silicon and glass substrates, greatly limiting their uses, said Chi Hwan Lee, lead author of the paper on the subject and a PhD candidate in mechanical engineering. While thin-film solar cells promise to inject some flexibility into the technology, scientists found that use of alternative substrates was problematic in the extreme, said Xiaolin Zheng, a Stanford assistant professor of mechanical engineering and senior author of the paper.

“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 said. “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.”

Utilizing the process, Zheng continues, 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,” Zheng said.

The new process involves a unique silicon, silicon dioxide and metal “sandwich.” First, a 300-nanometer film of nickel (Ni) deposits on a silicon/silicon dioxide (Si/SiO2) wafer. Thin-film solar cells then go on the nickel layer utilizing standard fabrication techniques, and covered with a layer of protective polymer. A thermal release tape then attaches to the top of thin-film solar cells to augment their transfer off of the production wafer and onto a new substrate.

The solar cell is now ready to peel from the wafer. To remove it, the wafer submerges in water at room temperature and the edge of the thermal release tape peels back slightly, allowing water to seep into and penetrate between the nickel and silicon dioxide interface. The solar cell then frees itself from the hard substrate but remains attached to the thermal release tape. Zheng and team then heat the tape and solar cell to 90°C for several seconds, they can then apply the cell to virtually any surface using double-sided tape or other adhesive. Finally, they remove the thermal release tape, leaving just the solar cell attached to the chosen substrate.

Tests show the peel-and-stick process reliably leaves the thin-film solar cells wholly 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.”

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, Lee said. “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,” Zheng said.

Moreover, peel-and-stick technology isn’t necessarily restricted to thin-film solar cells, Zheng said. The researchers believe the process can also apply 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,” Zheng said. “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.”

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