They may cause cancer but they're easy (and tasty) to clean*. From PhysOrg:
A new alternative energy technology relies on the element most associated with climate change: carbon.
University of Wisconsin-Madison researchers are studying how to create inexpensive, efficient solar cells from carbon nanotubes, which are sheets of carbon rolled into seamless cylinders one nanometer in diameter. Many researchers are studying how to use nanotubes for mechanical and electronics applications, but materials science and engineering assistant professor Michael Arnold is one of the first to apply them to solar energy.
"We are developing new materials and methods to create scalable, inexpensive, stable and efficient photovoltaic solar cell technologies," Arnold says. "Semiconducting carbon nanotubes have remarkable electronic and optical properties that are ideally suited for photovoltaics, so they are an interesting starting point.">>>MORE
*From the University of Pittsburgh:
...A Pitt research team has found that carbon nanotubes deteriorate when exposed to the natural enzyme horseradish peroxidase (HRP), according to a report published recently in “Nano Letters” coauthored by Alexander Star, an assistant professor of chemistry in Pitt's School of Arts and Sciences, and Valerian Kagan, a professor and vice chair of the Department of Environmental and Occupational Health in Pitt's Graduate School of Public Health. These results open the door to further development of safe and natural methods-with HRP or other enzymes-of cleaning up carbon nanotube spills in the environment and the industrial or laboratory setting....