Sometimes I get tired of writing about carbon nanotubes. I could probably find enough carbon nanotube research papers every week to practically fill Nobel Intent with content by myself. Unfortunately, my constant search for more interesting titanium research often seems unrewarding, because carbon just does so much cool stuff. We’ve shown you magnetic carbon, carbon nanotube gecko adhesives, and improved lasers for telecommunications (to name a few). Now, we bring you carbon nanotubes taking the job of everything but the silicon itself in a silicon-based solar cell—with respectable energy conversion efficiencies to boot.

We've covered solar energy generation recently here at Nobel Intent, so we’ll spare you the down-and-dirty details of how it works, and get right to the good stuff. Here’s how researchers have made what may be a revolutionary new solar cell: they took n-doped silicon (more negative charge carriers), and put webs of double-walled carbon nanotubes on it. That’s pretty much it. Anti-climactic, but surprisingly effective.

The real news isn’t in revolutionary processing techniques—first graders around the country pulling apart cotton balls to make cobwebs for Halloween decorations are well on their way to mimicking the results. The fact that the carbon nanotubes are replacing several key components of the solar cell in one fell swoop is a big deal. Usually, you need to have several material junctions sandwiched together to convert photon energy into electrical energy, move charges around, etc. Here, the double walled carbon nanotubes act as a transparent electrode that allows light to penetrate the cell, as a heterojunction to separate positive and negative charges, and as a conductive network for moving the charges (allowing us to put those electrons to work).

Efficiencies under laboratory conditions that simulated standard solar radiation reached 5%-7%. This is a far cry from the peak efficiencies in the most exotic solar cells, and researchers characterized their cell efficiency as "moderate." But this is still significantly higher than the 1-2 percent efficiencies that have been reported while integrating carbon nanotubes into polymer based cells.

The key here, as with polymer cells, is trading lower efficiency for much cheaper and more sustainable production. Carbon nanotubes themselves are still very expensive, although there are still significant improvements that can be made to current production methods. The carbon nanotubes here did not have to be sorted (carbon nanotubes come in several different flavors, depending on their electrical properties), and they could simply be pulled out of the furnace in their naturally web-like state and put to use after a few chemical washings. The nanotube webs were floated on water and, after a few drops of ethanol coaxed them to spread out, they were transferred directly to the silicon and allowed to air dry. This process is almost guaranteed to scale effectively in an industrial setting.

As with most preliminary work, there are still several paths left to explore that may lead to further performance gains—different nanostructures could be used, and polycrystalline silicon and thin film technologies to further lower costs and increase flexibility. Carbon’s pursuit of functions traditionally held by other materials seems to be relentless. Nanotubes seem to be just like zombies. You’ve been warned.

Advanced Materials, 2008. DOI: 10.1002/adma.200801810