Scientists at California's Stanford University have managed to construct the first solar cell made entirely of carbon.
If ultimately brought to market, a carbon-based solar cell could offer a potential alternative to the expensive materials currently used in photovoltaic devices.
As Stanford's Professor Zhenan Bao notes, Carbon has the potential to deliver high performance at a low cost.
"Unlike rigid silicon solar panels that adorn many rooftops, [our] thin film prototype is made of carbon materials that can be coated from solution," she explained. "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."
As expected, the coating technique also has the potential to reduce manufacturing costs.
"Processing silicon-based solar cells requires a lot of steps," Stanford graduate student Michael Vosgueritchian confirmed. "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 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. Indeed, carbon nanotubes have extraordinary electrical conductivity and light-absorption properties.
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.
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 acknowledged. "But with better materials and better processing techniques, we expect that the efficiency will go up quite dramatically."
Currently, the Stanford team is attempting to improve the efficiency of its design. "Roughness can short-circuit the device and make it hard to collect the current... We have to figure out how to make each layer very smooth by stacking the nanomaterials really well," said Bao.
The researchers are also experimenting with carbon nanomaterials capable of absorbing more light in a broader range of wavelengths, including the visible spectrum.
"Materials made of carbon are very robust. They remain stable in air temperatures of nearly 1,100 degrees Fahrenheit. We believe that all-carbon solar cells could be used in extreme environments, such as at high temperatures or at high physical stress.
"But obviously we want the highest efficiency possible and are working on ways to improve our device. 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," she added.