Researchers are investigating how carbon can harness the sun's light, potentially replacing more expensive and toxic materials used in conventional photovoltaic technologies.
Now a team at Stanford University has developed a solar cell whose components are made solely from carbon. The scientists published their findings last month in the journal ACS Nano.
"We were interested in forming basically a new type of solar cell in which the materials being used are all carbon materials," said Michael Vosgueritchian, a doctoral student in chemical engineering at Stanford and a co-author.
He explained that carbon materials have several traits that make them appealing to energy developers. "There's no fear of running out of carbon," Vosgueritchian said. "These materials, since they are nanomaterials, they are solution processable. They can be deposited by spraying and coating without high temperatures or vacuums."
Contrast this with typical silicon-based solar panels: Manufacturers need very pure silicon and have to heat it to high temperatures. The devices' electrodes often consist of expensive, rare or dangerous elements like cadmium, tellurium and indium. When a photovoltaic panel wears out, these chemicals also create a disposal hazard.
Working under Zhenan Bao at Stanford, Vosgueritchian said, the research team used several flavors of carbon to construct its device. Graphene, a carbon structure in which the atoms lie in thin sheets of hexagons, formed the anode.
If graphene is rolled into a cylinder, it becomes a carbon nanotube. Nanotubes made up part of the device's active layer, which converts light to electricity. On top was a layer of 60-carbon fullerenes, soccer-ball-shaped arrangements of atoms. The final layer was a cathode composed of nanotubes.
'A long way to go' before practical use
Michael Strano, a professor of chemical engineering at the Massachusetts Institute of Technology, explained that this junction between nanotubes and fullerenes "represents a fundamentally new kind of solar cell." His team developed a device using this system and published its work in Advanced Materials in June.
According to Strano, these exotic carbon structures can come from renewable sources, even pyrolyzed trash. "That's a big difference from solar cell components that would have to be mined from the earth," he said. "It also means that the photovoltaic is more easily recycled." Mining the necessary chemicals for a conventional solar cell is often a destructive process and substantially increases the device's environmental impact throughout its life cycle.
Carbon-based solar cells could also be more stable than silicon or polymer-based solar cells, which can wear out when exposed to the elements. "Graphene and carbon nanotubes are notorious for being fairly inert," Strano said.
However, carbon nanomaterials still face issues with manufacturing expenses. Making structures at the nanometer scale can be labor-intensive, and laboratories have made these carbon solar cells only in small, bench-scale batches. "In order to determine the cost of the material, you have to produce them at scale," Strano said.
Another issue is the device's efficiency. The Stanford researchers reported a maximum power conversion efficiency of 0.46 percent. By comparison, organic solar cells peak just below 10 percent efficiency, and some of the most advanced crystal silicon cells can top 25 percent, according to the National Renewable Energy Laboratory.
"It's low, but it's respectable," said Strano, noting that the technology is still proving concepts and researchers have not yet begun to optimize the cell's components, like blending the fullerenes and nanotubes in the active layer in such a way that it increases the surface area for producing electrons. "Nobody's making a structured heterojunction. [Scientists are] making a kind of peanut butter and jelly sandwich out of it. It's the simplest way," he said.