To make large sheets of carbon available for light collection, Indiana University Bloomington chemists have attached what amounts to a 3D bramble patch to each side of the carbon sheet. The scientists say they were able to dissolve sheets containing as many as 168 carbon atoms, which is a first.
"At the moment the most common materials for absorbing light in solar cells are silicon and compounds containing ruthenium. Each has disadvantages," said chemist Liang-shi Li, who led the research.
Their main disadvantage is cost and long-term availability. Ruthenium-based solar cells can potentially be cheaper than silicon-based ones, but ruthenium is a rare metal and will run out quickly when the demand increases.
Carbon is cheap and abundant, and in the form of graphene, capable of absorbing a wide range of light frequencies. Graphene is a single sheet of carbon, one atom thick, and shows promise as an effective, cheap-to-produce, and less toxic alternative to other materials currently used in solar cells.
Though for a sheet of graphene to be of any use in collecting photons of light, the sheet must be large. To use the absorbed solar energy for electricity, however, the sheet can't be too large. Unfortunately, scientists find large sheets of graphene difficult to work with, and their sizes even harder to control. The bigger the graphene sheet, the stickier it is, making it more likely to attract and glom onto other graphene sheets and therefore prevent electricity.
Chemists and engineers have come up with a many strategies for keeping single graphene sheets separate. The most effective solution had been breaking up graphite (top-down) into sheets and wrapping polymers around them to make them isolated from one another. But this makes graphene sheets with random sizes that are too large for light absorption for solar cells.
Li's team tried a different idea. By attaching a semi-rigid, semi-flexible, three-dimensional sidegroup to the sides of the graphene, they were able to keep graphene sheets as big as 168 carbon atoms from adhering to one another. With this method, they could make the graphene sheets from smaller molecules (bottom-up) so that they are uniform in size. To the scientists' knowledge, it is the biggest stable graphene sheet ever made with the bottom-up approach.
The sidegroup consists of a hexagonal carbon ring and three long, barbed tails made of carbon and hydrogen. Because the graphene sheet is rigid, the sidegroup ring is forced to rotate about 90 degrees relative to the plane of the graphene. The three brambly tails are free to whip about, but two of them will tend to enclose the graphene sheet to which they are attached.
The tails also serve as a handle for the organic solvent so that the entire structure can be dissolved. Li and his colleagues were able to dissolve 30 mg of the species per 30 mL of solvent.
"We found a new way to make graphene soluble," Li said. "This is just as important as the relatively large size of the graphene itself."
To test the effectiveness of their graphene light acceptor, the Indiana University Bloomington chemists constructed rudimentary solar cells using titanium dioxide as an electron acceptor. The scientists achieved a 200-microampere-per-square-cm current density and an open-circuit voltage of 0.48 volts. The graphene sheets absorbed a significant amount of light in the visible to near-infrared range (about 200 to 900 nm) with peak absorption occurring at 591 nm.
The scientists are currently redesigning the graphene sheets with sticky ends that bind to titanium dioxide, which will improve the efficiency of the solar cells.