Approximately 40 percent of the solar energy reaching Earth's surface can be found in the near-infrared region of the spectrum - energy that conventional silicon-based solar cells are unable to harness.
However, a new class of all-carbon solar cell developed by MIT researchers could help tap into that unused energy, opening up the possibility of combination solar cells. Such a component could potentially incorporate both traditional silicon-based cells and the new all-carbon cells - which could make use of almost the entire range of sunlight's energy.
"It's a fundamentally new kind of photovoltaic cell," explains Professor Michael Strano. Indeed, the new cell is made of two exotic forms of carbon: carbon nanotubes and C60, otherwise known as buckyballs.
"This is the first all-carbon photovoltaic cell," Strano says - a feat made possible by new developments in the large-scale production of purified carbon nanotubes. "It has only been within the last few years or so that it has been possible to hand someone a vial of just one type of carbon nanotube."
In order for the new solar cells to work, the nanotubes have to be very pure and of a uniform type: single-walled, and all of just one of nanotubes' two possible symmetrical configurations.
Other researchers have designed photovoltaic (PV) cells using carbon nanotubes, but only by using a layer of polymer to hold the nanotubes in position and collect the electrons knocked loose when they absorb sunlight. But that combination adds extra steps to the production process, and requires extra coatings to prevent degradation with exposure to air.
The carbon-based cell 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 potentially capable of harnessing most of the energy of sunlight.
Unsurprisingly, the MIT researchers acknowledge their carbon cells will need refining, as the early proof-of-concept devices have an energy-conversion efficiency of about 0.1 percent thus far. But while the system requires further research and fine-tuning, "we are very much on the path to making very high efficiency near-infrared solar cells," notes MIT graduate student Rishabh Jain.
Because the new system uses layers of nanoscale materials, producing the cells would require relatively small amounts of highly purified carbon, and the resulting cells 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," Jain adds.
Typically, when a new solar-cell material is studied, there are large inefficiencies, which researchers gradually find ways to reduce. In this case, some of these sources of inefficiency have already been identified and addressed.
For example, scientists already know that heterogeneous mixtures of carbon nanotubes are much less efficient than homogeneous formulations, and material containing a mix of single-walled and multiwalled nanotubes are so inefficient they sometimes don't work at all.
"It's pretty clear to us the kinds of things that need to happen to increase the efficiency," Jain says. To be sure, one area the MIT researchers are currently exploring is more precise control over the exact shape and thickness of the layers of material they produce.
The team hopes that other researchers will join the search for ways to improve their system. "It's very much a model system... and other groups will help to increase the efficiency."
Nevertheless, since the near-infrared part of the solar spectrum is currently entirely unused by typical solar cells, even a low-efficiency cell functioning in that region could be worthwhile - just as long as its cost remains low.