A solar cell made entirely from carbon, one of the most abundant elements on earth, has been developed for the first time by a research team from Stanford University. The discovery opens the door to much cheaper energy generation capabilities in the future. The results are published in the Oct. 31 online edition of the journal ACS Nano.
Stanford Professor Zhenan Bao says:
“Photovoltaics will definitely be a very important source of power that we will tap into in the future,”
said study senior author Zhenan Bao, a professor of chemical engineering at Stanford University .
“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.”
Stanford University scientists have developed the first solar cell made entirely of carbon. This new solar cell promises a cheaper alternative to the expensive, silicon-based photovoltaic solar panels available today.
Conventional solar panels are today made from silicon wafers, one of the more expensive engineering materials available, due to the amount required for solar panels and the intensive processing required. Although silicon is twice as abundant on the earth, and in fact the universe, as carbon, it rarely occurs in its pure form. The demand for silicon, from both the solar energy industry and the semiconductor industry is enormous and the competition between the two industries in 2005 when there was a worldwide shortage of silicon, resulted in the solar industry being unable to afford the price that the supply of silicon demanded.
“Carbon has the potential to deliver high performance at a low cost,” said “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.” said Professor Bao.
One of the most revolutionary benefits that this new technology confers is its flexibility. Unlike the rigid and fragile silicon-based solar panels that are commonly used today, Stanford’s thin film prototype is made of carbon materials in a solution that can be applied like paint to coat things like cars, windows or rooftops.
“Perhaps in the future we can look at alternative markets where flexible carbon solar cells are coated on the surface of buildings, on windows or on cars to generate electricity,” Bao said.
The coating technique also has the potential to reduce manufacturing costs, said Stanford graduate student Michael Vosgueritchian, co-lead author of the study with postdoctoral researcher Marc Ramuz.
“Processing silicon-based solar cells requires a lot of steps,” Vosgueritchian explained. “But our entire device can be built using simple coating methods that don’t require expensive tools and machines.”
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The Bao group’s experimental solar cell consists of a photoactive layer, which absorbs sunlight, sandwiched between two electrodes. In a typical thin film solar cell, the 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.”
For the study, Bao and her colleagues replaced the silver and ITO used in conventional electrodes with graphene – sheets of carbon that are one atom thick –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.
For the active layer, the scientists used material made of carbon nanotubes and “buckyballs” – soccer ball-shaped carbon molecules just one nanometer in diameter. The research team recently filed a patent for the entire device.
“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.”
One drawback of the all-carbon prototype is that it primarily absorbs near-infrared wavelengths of light, contributing to a laboratory efficiency of less than 1 percent – much lower than commercially available solar cells.
“We clearly have a long way to go on efficiency,” Bao said. “But with better materials and better processing techniques, we expect that the efficiency will go up quite dramatically.”
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The Stanford team is looking at a variety of ways to improve efficiency.
“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 researchers are also experimenting with carbon nanomaterials that can 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.”
The properties of carbon are many and varied depending upon its structure. Diamonds, for instance, which are neatly stacked columns of carbon atoms, have the highest thermal conductivity of any known material, at room temperature, with values up to five times that of copper. Graphite, an allotrope or a different physical form in which the element carbon can exist, is used in some forms as a thermal insulator. Diamond is one of the hardest substances known to man, whereas graphite is so soft that the Greeks named it after its use for writing. Diamonds are highly transparent, graphite completely opaque and the list goes on.
The ability of carbon solar cells to out-perform conventional devices under extreme conditions could overcome the need for greater efficiency, according to Vosgueritchian.
“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.”
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Other authors of the study are Peng Wei of Stanford and Chenggong Wang and Yongli Gao of the University of Rochester Department of Physics and Astronomy. The research was funded by the Global Climate and Energy Project at Stanford and the Air Force Office for Scientific Research.
Ramuz, Marc P. (2012-10-31) Evaluation of Solution-Processable Carbon-Based Electrodes for All-Carbon Solar Cells. ACS Nano, 123509. DOI: 10.1021/nn304410w
May, P.W. (2005-3) Thermal conductivity of CVD diamond fibres and diamond fibre-reinforced epoxy composites. Diamond and Related Materials, 13(3-7), 551-603. DOI: 10.1016/j.diamond.2004.10.039