A major goal of the solar photovoltaic research community is to make solar cells entirely from nanocarbon materials, and therefore free from the polymer binders that currently limit long-term stability. Michael S. Strano and colleagues from the Massachusetts Institute of Technology have now achieved just this, fabricating a solar cell containing only single-walled carbon nanotubes (SWNTs) and C60 as the active photovoltaic material, as reported in the August 22 issue of Advanced Materials (DOI: 10.1002/adma.201202088; p. 4436).
“It wasn’t clear that you could get a photocurrent from a photovoltaic cell that’s built like this—there’s no polymer, there’s no silicon—it’s all carbon,” Strano said. “The major advantages are that you do not have to rely on semiconducting polymers that are unstable, and that the materials are renewable and cheaper than silicon.”
The SWNT active material absorbs near-infrared radiation—a portion of the electromagnetic spectrum not normally absorbed by solar cells. By combining this carbon-based cell with the best available silicon-based cells, Strano envisions a hybrid photovoltaic device that harvests the 40% of the solar spectrum that lies in the near-infrared region. This new class of solar cells could potentially be coupled with silicon technology, which currently dominates the market, to achieve a significant increase in the amount of radiation absorbed and converted to electricity.
The researchers fabricated the layered cell starting with a glass-supporting substrate and a patterned indium-tin-oxide electrode. A 100-nm-thick film of SWNTs, all of the same (6,5) chirality, was deposited on the electrode, followed by a 70-nm-thick layer of C60. The researchers topped the device with a silver film as the second electrode. The SWNT absorbs photons and creates a quasiparticle called an exciton—an electron–hole pair. The exciton moves to the C60 layer, which grabs the electron and sends the hole up to the cathode, thus producing a photocurrent.
A key to the group’s success has been the ability to isolate large amounts of very pure (6,5) SWNTs, based on a process developed by Huaping Liu and colleagues at the National Institute of Advanced Industrial Science and Technology and the Japan Science and Technology Agency. “We’ve shown that if you put in an impurity of (6,4) SWNTs, there’s something about the junction between two dissimilar nanotubes that causes excitons to be irradiatively recombined, instead of separated and harvested for their current,” said Strano. This happens even though (6,5) and (6,4) SWNTs have very similar bandgaps.
Strano acknowledges that this is a “humble advance” in photovoltaic research, because the efficiencies his group has measured to date for the nanocarbon devices are only about 0.1%. However, significant improvements can likely be made by combining the two nanocarbon materials in ways that maximize the surface area and produce continuous phases.
“We see this as a starting point—it expands the tools and the available technologies for the energy engineer to build new kinds of photovoltaic cells,” Strano said. “It carves out a new space in photovoltaic technology.”
