by Rachel Nuwer
Materials Research Society | Published: 07 November 2012
Stanford scientists have developed a new all-carbon solar cell consisting of a photoactive layer, which absorbs sunlight, sandwiched between two electrodes. Image credit: Mark Shwartz / Stanford University. Click image to enlarge.
“For silicon-based solar cells, the performance is very good but the panel is rigid and the material is fragile,” says Zhenan Bao, a professor of chemical engineering at Stanford University. “We were able to apply a number of new developments to demonstrate an all-carbon solar cell that could, in theory, be applied onto curved surfaces or even onto plastic or stretchable substrates,” she says.
Building upon several years of research, Bao and her colleagues set out to fashion a solar cell with all-carbon components: an anode, an active layer and a cathode. Previously, other researchers reported using carbon as a semi-conducting middle layer in solar cells, but carbon-based electrodes for collecting electrons were not available.
To get around this problem, Bao and her colleagues needed to develop a conductive and transparent anode. Her group used solution processed graphene oxide they previously developed in 2008. They also needed high purity, semi-conducting carbon in the form of carbon nanotubes—one-atom thick structures that are about 10,000 times narrower than a human hair. The researchers purchased commercially available nanotubes, which came as a mixture of two-thirds semi-conducting and one-third metallic. In order to remove the metallic tubes, they poured the mix into a solution that included a polymer, which dissolved the semi-conducting nanotubes but not the metallic ones. From there, they could extract only the useful semi-conductors through the solution. For the cathode layer, the researchers applied a special molecule to make the carbon more readily accept electrons.
With each component now available, the researchers had to develop a suitable process to assemble their all-carbon solar cell, which they describe in ACS Nano. First, they deposited a transparent conductive graphene electrode for collecting positive charge. Then, added a layer of semi-conducting nanotubes on top of that. Next, they laid down still another coating of carbon, this time in the form of buckyballs that acted as the active layer. Finally, the cathode—a film of the specially treated carbon nanotubes—joined the mix. “For that electrode, we simply laminate it onto the buckyballs to complete the solar cell,” Bao says.
Bao considers this breakthrough a proof-of-concept rather than a near finished product. Compared to the carbon solar cells with conventional electrodes, the all-carbon solar cell does not perform as well, with an efficiency of less than 1 percent compared to some commercially available products’ 20 percent. The researchers suspect, however, that material roughness may be partly to blame, so smoothing out the layers by more meticulously stacking the nanomaterials could be a start to enhancing performance. “I think we still have quite a lot of work to do for improving the device in terms of both its fabrication as well as structure and the materials themselves,” she says. “But still, this first demonstration shows that it is possible to build an all-carbon solar cell, even if we certainly have a long way to go.”
Though the researchers cannot predict when their all-carbon product may be available on the market, they do speculate that it could eventually be used for harnessing energy over large areas, or for creating solar windows and solar cells on cars. All-carbon solar cells also have the potential to perform well under extreme environments since carbon can remain stable in air temperatures reaching nearly 1,100 degrees Fahrenheit.
I think the more people working on this, the faster we’ll be able to reach the market stage,” Bao says. “I hope our work will inspire others to get interested in this area of research.”
