All-carbon solar cell harnesses infrared light

Researchers at MIT have developed an all-carbon solar cell that harnesses the infrared light that conventional silicon solar cells ignore.

About 40% of the solar energy reaching Earth's surface lies in the near infrared region of the spectrum. The researchers say that tapping into this unused energy could open up the possibility of combination solar cells that make use of almost the entire range of the sun's energy. "It's a fundamentally new kind of photovoltaic cell," commented Michael Strano, the Charles and Hilda Roddey professor of chemical engineering at MIT. The new cell is made of carbon nanotubes and C60, a feat made possible by new developments in the large scale production of purified carbon nanotubes. In order for the new solar cells to work, the nanotubes have to be very pure and of a uniform type: single walled and just one of the two possible symmetrical configurations. Other groups have made photovoltaic cells using carbon nanotubes before, but only by using a layer of polymer to hold the nanotubes in position. The researchers at MIT say their method does not require these extra steps in the production process or any additional coatings to prevent degradation with exposure to air. As the material is transparent to visible light, the cells could be overlaid on conventional solar cells to create a tandem device that could harness most of the energy of sunlight. So far, concept devices have an efficiency of about 0.1%, but the team believes further research and fine tuning could yield cells with much higher efficiency. The system uses layers of nanoscale materials so the cells would be very lightweight and use only small amounts of highly purified carbon. "One of the really nice things about carbon nanotubes is that their light absorption is very high, so you don't need a lot of material to absorb a lot of light," noted graduate student Rishabh Jain. The team says that as the near infrared part of the solar spectrum is completely unused by typical cells, even a low efficiency cell would be worthwhile if it was low cost. The researchers are currently exploring more precise control over the exact shape and thickness of the layers of material.