Even the best solar cells can convert only so much of the energy in sunlight into electricity. But researchers in the United States and Canada report today in Science about a new advance that may one day break through that barrier. For now, the devices, which consist of individual carbon nanotubes wired to electrodes, are too small to be considered actual solar cells. But if the technology can be scaled up, it could lead to a new class of high-efficiency photovoltaics.
Most of today's silicon solar cells, the industry standard, convert about 20% of the energy in sunlight to electricity. Although that sounds modest, it's not far from their maximum possible efficiency of 31%. In part, that limit is due to the fact that when a photon of sunlight strikes silicon, it excites just one electron, freeing it to flow in a current. Any excess energy the photon had is lost as heat. For years, researchers have had evidence that tiny nanoparticles could do better, enabling photons to generate two or more electrical charges apiece and thereby increasing the current. But no one has managed to make working solar cells that wrest these excess charges out of the nanoparticles and collect them.
A big part of the challenge is figuring out how to wire nanoparticles to collect the charges. When light strikes these tiny semiconductor specks, it creates one or more "excitons"--negatively charged electrons bound to positive charges in the material called "holes." Working solar cells need to separate the opposite charges and steer them to oppositely charged electrodes.
That job would be easier if the nanoparticles could be stretched into long, thin nanowires or tubes that span a pair of electrodes. Electrons and holes could then travel in opposite directions in the wires or tubes to their preferred electrodes. As a first step, Paul McEuen, a physicist at Cornell University, and colleagues decided to test whether carbon nanotubes were capable of generating more than one electron-hole pair per photon. So they wired individual carbon nanotubes between a pair of oppositely charged electrodes and hit the device with blasts of laser light. They found that the tubes could generate at least two pairs of charges for each photon. And if the researchers added a little electrical juice of their own to the electrodes, the device could collect those excess charges from the tubes.
"That they see this effect so efficiently is really striking," says Philippe Guyot-Sionnest, a chemist at the University of Chicago in Illinois. "If it is confirmed, it is pretty exciting." That still won't make these nanodevices ready for use in actual solar cells. For now, McEuen cautions, the effect works only below 60 kelvin, far too cold to be used in real-world solar cells. But McEuen adds that if he and his colleagues can better understand the effect, they may be able to duplicate it in other materials that work at higher temperatures.