Hamburg, Germany - Transparent electrodes created from atom- thick carbon sheets could make solar cells and LCDs without depleting precious mineral resources, say researchers in Germany. Solar cells, LCDs and some other devices, must have transparent electrodes in parts of their designs to let light in or out.
These electrodes are usually made from indium tin oxide (ITO) but experts calculate that there is only 10 years' worth of indium left on the planet, with LCD panels consuming the majority of existing stocks.
"There is not enough indium on earth for the future development of devices using it," Linjie Zhi of the Max Planck Institute for Polymer Research in Mainz, Germany, told New Scientist. "It is also not very stable, so you have to be careful during the fabrication process."
Although experimental alternatives to ITO exist, these are also unstable and of unproven efficiency, said the report in New Scientist. Zhi and colleagues Xuan Wang and Klaus Muellen believe they have a cheaper, more stable alternative.
The report said the team is testing solar cells with transparent electrodes made from graphene - flat sheets of carbon atoms arranged in a hexagonal structure. When rolled up, this material makes carbon nanotubes.
The solar panels they created were dye-sensitized solar cells, first invented in 1991 and predicted by some to be the most likely successor to silicon-based solar cells.
Dye-sensitized solar cells use sunlight, a mixture of different dye pigments, and titanium dioxide - the main ingredient in white paint - to excite electrons. This is a process that, in some ways, mimics photosynthesis. It could make solar cells cheaper to manufacture and more efficient, in terms of power collection, than silicon-based ones.
The Max Planck team first coated their cells with a solution of flat graphite oxide flakes, each 10 to 100 nanometres across, leaving a coating on the surface.
Heat treatment was then used to remove the oxygen from the layer. This causes the flakes to merge, leaving behind sheets of graphene.
"It is very stable in the face of heat and acidic conditions," Zhi told New Scientist, "which makes fabrication much easier."
The team is now working on reducing the number of layers to increase transparency, and on "ironing out" the creases that can appear in the sheets. In theory, a single perfect layer of graphene would work well enough to replace ITO.
"Replacing the ITO with graphene is a real great step forward as the transparent current collector is a critical and the most expensive part of our cell," New Scientist quoted Michael Graetzel of the Swiss Federal Institute of Technology in Lausanne, Switzerland, as saying. He invented dye sensitised solar cells in 1991.
He was quoted as saying the search for improved transparent electrodes will be crucial for the success of these solar cells.
"Everything which you do with ITO should be possible (with graphene)," added Muellen, who led the graphene research. That means LCD panels could be made with the graphene-based electrodes, he says, although his team has yet to test this idea.
