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. Standard thin-film photovoltaic (PV) cells require direct fabrication on the final carrier substrate, but the peel-and-stick ones developed at Stanford University in California do not, 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, the use of alternative substrates was problematic in the extreme, said Xiaolin Zheng, a Stanford assistant professor of mechanical 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.” New peel-and-stick thin-film solar cells avoid the challenges associated with putting solar cells on unconventional materials, expanding the potential applications of solar technology. 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 penetrate between the nickel and silicon dioxide, freeing the cell from the substrate but 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. 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.” “Now you can put them on helmets, cellphones, 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. The work is described in Scientific Reports (doi:10.1038/srep01000).