US researchers have found a way to make efficient silicon-based solar cells that are flexible enough to be rolled around a pencil and transparent enough to be used to tint windows on buildings or cars.

The finding, published in the journal Nature Materials, offers a new way to process conventional silicon by slicing the brittle wafers into ultrathin bits and carefully transferring them onto a flexible surface.

"We can make it thin enough that we can put it on plastic to make a rollable system. You can make it grey in the form of a film that could be added to architectural glass," says Professor John Rogers of the University of Illinois at Urbana-Champaign, who led the research.

"It opens up spaces on the fronts of buildings as opportunities for solar energy."

Solar cells, which convert solar energy into electricity, are in high demand because of higher oil prices and concerns over climate change.

Rogers says his technology uses conventional single crystal silicon.