Stanford University scientists have built the first solar cell made entirely of carbon. This discovery could be a promising alternative to the expensive materials used in photovoltaic devices. Reducing material cost of photovoltaic options could make solar and photovoltaic options a very alluring choice for facility managers and building owners worldwide.
The results are published in the Oct. 31 online edition of the journal ACS Nano.
"Carbon has the potential to deliver high performance at a low cost," says 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."
Unlike rigid silicon solar panels that adorn many rooftops, Stanford's thin film prototype is made of carbon materials that can be coated from solution. "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," says Bao.
The coating technique also has the potential to reduce manufacturing costs, says 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 says. "But our entire device can be built using simple coating methods that don't require expensive tools and machines."
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 says. "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," says Bao.
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 says. "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 says. "But with better materials and better processing techniques, we expect that the efficiency will go up quite dramatically."