Issue Date: March 11, 2011
A Better Way To Produce Organic Photovoltaics
Though solar energy surrounds us, scientists are still searching for the elusive combination of inexpensive materials and clean processing methods to harness that radiant energy cheaply. Now researchers have demonstrated that they can assemble an all-carbon, proof-of-concept photovoltaic cell by simply suspending the materials in water (J. Am. Chem. Soc., DOI: 10.1021/ja1103734).
Engineers want to produce carbon-based photovoltaic cells because they would be cheaper, more flexible devices than those created with traditional inorganic materials such as silicon. But researchers struggle to manufacture carbon-based devices with the same solar efficiency as typical cells.
Some of these inefficiencies stem from how researchers assemble the all-important layer that converts sunlight into electricity. In any photovoltaic cell, this layer must consist of materials that donate and accept electrons. For carbon-based systems, the electron-donating materials can be thin-walled carbon nanotubes and the electron accepting materials can be C60 fullerenes. But these materials tend not to disperse evenly, especially in water. To get the components to distribute so that they can then assemble in a thin layer, researchers either use reagents to covalently link electron donor to acceptor or mix in surfactants to help solubilize the two types of materials, which then can assemble into composite structures. Unfortunately, the covalent linkage method interferes with the flow of electrons in these surfaces. Meanwhile surfactants, which are insulators, find their way into the photovoltaic layer and disrupt charge transfer.
Jiaxing Huang and his colleagues at Northwestern University thought graphene oxide could solve these issues. Last year, they showed that graphene oxide is a surfactant: though its surface is decorated with polar, oxygen-containing groups, it still contains greasy, hydrophobic patches (J. Am. Chem. Soc., DOI: 10.1021/ja102777p). Unlike other surfactants, when reduced, graphene oxide produces an efficient conductor, graphene.
Building on those findings, Huang and his colleagues mixed graphene oxide with C60 fullerenes and single-walled carbon nanotubes in water. They then coated a glass slide with a thin film of the solution and gently heated it to reduce the graphene oxide. The resulting surface was a smooth, all-carbon composite that integrates grapheme, the nanotubes, and fullerenes, Huang says.
The team then made a proof-of-concept photovoltaic chip with this layer. These chips were up to 1,000 times more efficient at converting light into electricity than are similar organic devices developed with covalent chemistry, Huang says. The researchers now are improving those values by using fullerenes that absorb light more efficiently, such as C70, optimizing the graphene oxide reduction, and removing metallic impurities from the nanotubes.
Edward Samulski of the University of North Carolina, Chapel Hill, says that before this method can become commercially viable, the researchers will need to show that they can control the nanoscale structure of the composite by adjusting the mixture of components. But by moving away from covalent linkages, he adds, the work is "a nice step" toward building more-efficient carbon-based photovoltaic devices.
This story was updated on 3/14/2011. The original version contained an incorrect description of how the researchers produced the photovoltaic layer.
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