Lee Rannals for redOrbit.com – Your Universe Online
Creating solar cells from carbon could be a promising alternative to the expensive materials used in photovoltaic devices today.
“Carbon has the potential to deliver high performance at a low cost,” said study senior author Zhenan Bao, a professor of chemical engineering at Stanford. “To the best of our knowledge, this is the first demonstration of a working solar cell that has all of the components made of carbon. This study builds on previous work done in our lab.”
The coating technique also has potential to reduce manufacturing costs, according to Stanford graduate student Michael Vosgueritchian.
“Processing silicon-based solar cells requires a lot of steps,” Vosgueritchian said. “But our entire device can be built using simple coating methods that don’t require expensive tools and machines.”
The group’s experimental solar cell consists of a photoactive layer sandwiched between two electrodes. These electrodes are made of conductive metals and indium tin oxide (ITO).
“Materials like indium are scarce and becoming more expensive as the demand for solar cells, touchscreen panels and other electronic devices grows,” Bao said. “Carbon, on the other hand, is low cost and Earth-abundant.”
The team replaced the silver and ITO used in conventional electrodes with graphene and single-walled carbon nanotubes that are 10,000 times narrower than a human hair.
“Carbon nanotubes have extraordinary electrical conductivity and light-absorption properties,” Bao said.
The scientists used material made of carbon nanotubes and “buckyballs” for the active layer.
“Every component in our solar cell, from top to bottom, is made of carbon materials,” Vosgueritchian said. “Other groups have reported making all-carbon solar cells, but they were referring to just the active layer in the middle, not the electrodes.”
Bao said that they still have a long way to go for efficiency. The all-carbon prototype primarily absorbs near-infrared wavelengths of light, contributing to a laboratory efficiency of less than one percent.
He said with better materials and processing techniques, they will be able to improve the efficiency “dramatically.”
“Roughness can short-circuit the device and make it hard to collect the current,” Bao said. “We have to figure out how to make each layer very smooth by stacking the nanomaterials really well.”
The team is experimenting with carbon nanomaterials that absorb more light in a broader range of wavelengths, including the visible spectrum.
“Materials made of carbon are very robust,” Bao said. “They remain stable in air temperatures of nearly 1,100 degrees Fahrenheit.”
Vosgueritchian said the ability of carbon solar cells to out-perform conventional devices under extreme conditions could overcome the need for greater efficiency.
“We believe that all-carbon solar cells could be used in extreme environments, such as at high temperatures or at high physical stress,” he said. “But obviously we want the highest efficiency possible and are working on ways to improve our device.”
Bao said photovoltaics will be a vital source of power that they plan to tap into in the future.
“We have a lot of available sunlight. We’ve got to figure out some way to use this natural resource that is given to us,” Bao said.
Image 2 (below): The Bao group’s all-carbon solar cell consists of a photoactive layer, which absorbs sunlight, sandwiched between two electrodes. Credit: Mark Shwartz
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