Most solar cells are two-dimensional, and best suited for hard, flat surfaces like solar panels on a roof. Previous attempts to make solar-powered clothing, like this project from Colorado State University, have (somewhat awkwardly) sewed small flat panels into fabric. But there may eventually be a way to make the threads themselves power sources.
Researchers at Penn State University have created the world's first fiber-optic solar cell. The fibers, pictured below, are each thinner than a human hair. They are injected with silicon, a material that can convert sunlight into electricity.
Because the solar cells are three-dimensional, they can absorb light from all directions, unlike normal solar panels. The thin fibers can be woven together into strands, and eventually could possibly be used in clothing so people can generate their own power as they're walking outside, charging phones or other small devices.