Carbon is cheap, abundant, and in the form of the nanoparticles graphene, capable of absorbing a wide range of light frequencies. Graphene is essentially the same stuff as a pencil’s graphite, except graphene is formed to a single sheet of carbon, just one atom thick. Graphene shows promise as an effective, cheap-to-produce, and less toxic alternative to other materials currently used in solar cells. But it has also vexed scientists.

For a sheet of graphene to be useful as a solar collector of light photons, the sheet must be large enough. To use the absorbed solar energy for electricity the sheet can’t be made too large. Scientists find large sheets of graphene difficult to work with, and the size specification even harder to control.

The bigger the graphene sheet, the stickier it is, making it more likely to attract and attach onto other graphene sheets. Multiple layers of graphene prevent electricity production.

Indiana University at Bloomington chemists have devised an unusual solution – attach a constructed “3-D bramble patch” to each side of the carbon sheet. With a method newly devised, the IU team say they are able to dissolve sheets containing as many as 168 carbon atoms, a first, that may make large sheets of carbon available for light collection.

Graphene Sheets Built Up With Hydrocarbon Cages. Click image for more info.

The IU team’s report, online April 9, will appear in a future issue of Nano Letters.

Chemist Liang-shi Li, who led the research said, “Our interest stems from wanting to find an alternative, readily available material that can efficiently absorb sunlight. At the moment the most common materials for absorbing light in solar cells are silicon and compounds containing ruthenium. Each has disadvantages.”  Ruthenium-based solar cells can potentially be cheaper than silicon-based ones, but ruthenium is a rare metal on Earth, as rare as platinum, and will run out quickly when the demand increases.  Cost and availability loom to discourage investment.

The graphene idea has been around a while.  Chemists and engineers experimenting with graphene have come up with a whole host of strategies for keeping single graphene sheets separated. The most effective solution prior to the IU team’s paper was breaking up graphite from the top-down into sheets and then wrap polymers around them to keep them isolated from one another. But this approach makes graphene sheets with random sizes that are too large for the light absorption needed for solar cells.

Graphene in a Constructed Bramble Patch. Click image for more info.

Li and the team tried an innovative 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 the new dynamic method, they could make the graphene sheets from smaller molecules built 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.  Check the image below for more detail.

The sidegroup consists of a hexagon shaped carbon ring and three long, barbed tails made of carbon and hydrogen. Because the graphene sheet itself 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, with two of them tending to enclose the graphene sheet of which they are attached.  This makes up a dynamic box or cage for the graphene sheet.

But he tails don’t merely act as a cage; they also serve as a handle for an organic solvent so that the entire structure can be dissolved. Li and his colleagues were able to dissolve 30 mg of the specimens per 30 mL of solvent.  The ability to breakdown the graphene is significant as well.

Li said, “In this paper, we found a new way to make graphene soluble. This is just as important as the relatively large size of the graphene itself.”  This new know how may make all the difference for building things using graphene across a range of products.

How well does the new graphene assembly work?  To test the effectiveness of their graphene light acceptor, the scientists constructed rudimentary solar cells using titanium dioxide as an electron acceptor. The team has been able to achieve 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 (200 to 900 nm or so) with peak absorption occurring at 591 nm.  As black as this kind of thing must be – those are good numbers.

The team is in the process of redesigning the graphene sheets with sticky ends so that they bind to titanium dioxide – a construction that will improve the efficiency of the solar cells.

Liang-shi Li’s team includes PhD students Xin Yan and Xiao Cui and postdoctoral fellow Binsong Li.  Along with grants form the National Science Foundation, the American Chemical Society Petroleum Research Fund put money into the research.

This is quite a new start for graphene as a photovoltaic solar collector material.  Kicking up the micro amp from a cm² to a meter² is 100 or 200 micro amps then is .02 amps, not a bad start at nearly a half-volt.


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