A new all-carbon solar cell taps into near-infrared light, which is not harnessed by current photovoltaic systems but makes up about 40 percent of the solar energy reaching Earth’s surface. The new cell, developed at MIT, is most effective at capturing sunlight in the near-infrared region. Because the material is transparent to visible light, such cells could be overlaid on conventional solar cells, creating a tandem device that could harness most of the energy available in sunlight. “It’s a fundamentally new kind of photovoltaic cell,” said Michael Strano, a chemical engineering professor at MIT. The MIT cell comprises two types of carbon: carbon nanotubes and C60, also known as buckyballs. Recent advances in carbon-nanotube production enable fabrication of very pure, very uniform single-walled nanotubes needed for this type of solar cell to work. An atomic force microscope image of a layer of single-walled carbon nanotubes deposited on a silicon surface; individual nanotubes are visible. This the first step in manufacturing a new type of solar cell developed at MIT. (Image: Rishabh Jain et al.) Because the new system is made of nanoscale materials, the cells would require only small amounts of the highly purified carbon, and they would be very lightweight. “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,” said Rishabh Jain, a graduate student who was lead author of the paper. But the cells will need refining, Strano said. So far, the early proof-of-concept devices have an energy-conversion efficiency of only about 0.1 percent. Although the system will require further research and fine-tuning, “we are very much on the path to making very high efficiency near-infrared solar cells,” Jain said. The makeup of the nanotubes also means that the researchers know exactly where to make improvements to increase efficiency, including finding solutions to better control the exact shape and thickness of the layers of the materials they produce. “It’s pretty clear to us the kinds of things that need to happen to increase the efficiency,” Jain said. Since the near-infrared region of the solar spectrum is totally unused by current silicon solar cells, Strano points out that even a low-efficiency cell working in that region could be worthwhile as long as its cost is also low. “If you could harness even a portion of the near-infrared spectrum, it adds value,” he said. The research was published this week in Advanced Materials. There are other research projects tapping into the broader solar spectrum. (See: Indium Upped in Nanowire Solar Cell) For more information, visit: www.mit.edu