For the first time, a solar photovoltaic cell has been produced completely out of carbon. The prototype device, described this week in the journal ACS Nano, offers what could be a promising alternative to the more expensive silicon and other materials used in existing solar cells.

"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," Zhenan Bao, a professor of chemical engineering at Stanford University and the senior author of the paper describing the innovation, said in a prepared release. She also said that carbon has the potential to deliver high-performance photovoltaics at a lower cost than materials currently available.

In addition to the use of carbon, the new cells have another potential benefit: flexibility. Unlike silicon cells, which are rigid, the Sanford prototype was generated from carbon materials that start as a solution that can be coated onto other components. They used carbon nanotubes and one-nanometer diameter carbon molecules called "buckyballs" to create the active photoactive layer for their cell. "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 team also made innovations to the electrodes that attach to photovoltaic layer and transmit the electrical current it generates. Today's electrodes are made from conductive metals and indium tin oxide, a scarce and increasingly expensive substance that is also used in touchscreen devices. Bao and her team replaced those metals with two forms of carbon. First, they used sheets of carbon called graphene that are only one atom thick. Second, they used single-walled carbon nanotubes that are 10,000 times narrower than a human hair. Carbon nanotubes have previously been shown to have tremendous electrical conductivity properties.

"Every component in our solar cell, from top to bottom, is made of carbon materials," said Stanford graduate student Michael Vosgueritchian, another author of the study. "Other groups have reported making all-carbon solar cells, but they were referring to just the active layer in the middle, not the electrodes."

Although the team innovated with the use of carbon, the resulting solar cell isn't quite ready for prime time, as it generates electricity at a much lower level of efficiency than current commercial solar cells. The team says they expect efficiency to increase as they improve their materials and processing techniques. They are already experimenting with several new variations of carbon materials to increase the cells' efficiency.

Another potential advantage of carbon: it is a robust, stable element. Vosgueritchian says all-carbon solar cells could theoretically be of great value in areas where they would be able to withstand extremely high temperatures or physical stress. This might make them more attractive in what he called "extreme environments," regardless of the levels of efficiency achieved.

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