Unlike their stiff, inflexible cousins, new peel-and-stick thin-film solar cells can be peeled off like Band-Aids and stuck to virtually any surface, from paper to windowpanes. Unlike standard thin-film photovoltaic (PV) cells, the peel-and-stick variety developed at Stanford University do not require any direct fabrication on the final carrier substrate, avoiding all the challenges associated with putting solar cells on unconventional materials and vastly expanding the potential applications of solar technology. Thin-film PV cells traditionally are fixed on rigid silicon and glass substrates, greatly limiting their uses, said Chi Hwan Lee, a PhD candidate in mechanical engineering. The development of thin-film photovoltaics promised to inject some flexibility into the technology; however, scientists found that the use of alternative substrates was problematic in the extreme, said Xiaolin Zheng, a Stanford assistant professor of mechanical engineering. Demonstrations of the Stanford peel-and-stick thin-film solar process and various applications. Courtesy of Chi Hwan Lee, Stanford School of Engineering. "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." They have 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-nm film of nickel is deposited on a silicon/silicon dioxide wafer. Thin-film solar cells are then deposited on the nickel layer via 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. To remove the solar cell from the wafer, the wafer is submerged in water at room temperature and the edge of the thermal release tape is peeled back enough to allow water to seep in and penetrate between the nickle and silicon dioxide, freeing the cell from the hard substrate but still leaving it attached to the tape. The tape and solar cell are heated 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 demonstrated that 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," he said. "Now you can put them on helmets, cell phones, convex windows, portable electronic devices, curved roofs, clothing — virtually anything," Zheng said. The researchers also believe the peel-and-stick process can be applied to thin-film electronics, including printed circuits and ultrathin 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. "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." The work is described in a paper in the Dec. 20 issue of Scientific Reports. Co-authors of the paper are Lee, In Sun Cho and Zheng of Stanford, Dong Rip Kim from Hanyang University in Seoul, South Korea, and Nemeth William and Qi Wang from the National Renewable Energy Laboratory in Denver. For more information, visit: www.stanford.edu