McEuen cautions that his work on carbon nanotube photovoltaics is fundamental. “We’ve made the world’s smallest solar cell, and that’s not necessarily a good thing,” he says. To take advantage of the nanotubes’ superefficiency, researchers will first have to develop methods for making large arrays of the diodes. “We’re not at a point where we can scale up carbon nanotubes, but that should be the ultimate goal,” says Lee, who developed the first nanotube diodes while a researcher at General Electric.
It’s not clear why the nanotube photovoltaic cell offers this two-for-one energy conversion. “It’s mysterious to us,” says McEuen. However, the most likely reason is that while conventional solar materials have only one energy level for electrons to move through, carbon nanotubes have several. And two of them just happen to be very well matched: one of the energy levels, or bandgaps, is twice as high as the other. “We may have gotten lucky, and it has very little to do with the fact that it’s a carbon nanotube,” says McEuen. This means, McEuen hopes, that even if it proves too challenging to make arrays of nanotube solar cells, materials scientists can look for pairs of materials that have these kinds of matched bandgaps, and layer them to make solar cells that do with two materials what the single nanotube cells can do. “Maybe the answer won’t be in nanotubes, but in another pair of materials,” McEuen says.