Traditionally solar cells are made from either silicon or a ruthenium compound. Unfortunately, ruthenium is relatively rare (a rare Earth element) and will not ‘scale’ to produce the enormous quantities needed for solar energy production. Silicon is what is used now, but silicon is relatively expensive to manufacture correctly. So scientists have been looking for alternatives. An obvious choice is carbon – abundant and cheap. Various approaches to using carbon in some of its many forms have been going on for years. Recently (within the last decade) a new form of carbon, a product of nanotechnology, called graphene appeared. Graphene is essentially the same as graphite (as in pencil lead), but is manufactured in sheets only 1 atom thick. It can absorb light in a number of frequencies. The sheet form would seem to be ideal for solar energy collection; but there are complications.
The problems are inherent in the sheet form. To collect enough photons from sunlight, the sheets need to be (relatively) big. This is a manufacturing problem in its own right, but one that has recently found solutions. To convert the photons into electricity, the graphene sheets can’t be too big because they lose conductivity. To get the most out of graphene sheets it would be appropriate to stack them, but again, the sheet form is so thin that as a sheet gets bigger it becomes ‘sticky’ and insists on grabbing onto nearby sheets. (This is not unlike working with multiple sheets of plastic kitchen wrap.) Scientists have tried to get around this by coating the graphene sheets with a polymer (plastic) layer but the process makes sheets that are too big and come in random sizes.
Chemist Liang-shi Li at Indiana University (USA), who led the research, wanted to create graphene sheets that won’t stick to each other. The solution was to attach to each sheet, almost like a frame, a semi-rigid side group, which is a group of molecules (in this case a hexagonal carbon ring with three carbon-hydrogen ‘tails’) attached to the main molecule – or in this case the carbon of the graphene sheet. The attached side group acts like a kind of cage for the graphene, enforcing a uniform size. The researchers were able to reliably build sheets of precisely 168 carbon atoms that did not stick together.
As a side (group) benefit, the whole assembly could be dissolved with organic solvent. Dissolving carbon structures is not always easy, but often necessary in the manufacturing of specific shapes and configurations.
Making nicely shaped, uniform sheets of graphene is well and good, but the payoff is solar cells.
To test the effectiveness of their graphene light acceptor, the scientists constructed rudimentary solar cells using titanium dioxide as an electron acceptor. The scientists were 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.
The scientists are in the process of redesigning the graphene sheets with sticky ends that bind to titanium dioxide, which will improve the efficiency of the solar cells.
“Harvesting energy from the sun is a prerequisite step,” Li said. “How to turn the energy into electricity is the next. We think we have a good start.”
Graphene is already the principal material in a wide variety of applications, although few may be as important as solar energy collection. This avenue of research is roughly at a ‘mid-point’ between working out the problems in the laboratory and the final road to mass manufacturing. In the end, the graphene solar cell must compete with a wide array of silicon and other materials. It must compete on price, efficiency, and durability – all of which are speculation at this point. Nevertheless, given the advantages of graphene it is, as they say, ‘very promising.’