All That Matters

Graphene is one of the hottest research areas in nanotechnology, and it may seem slightly surprising it took me a month to write my first blog post on the topic. That moment has now come, with the advance publication of a Nature paper that presents highly attractive graphene transistor, even though in my humble opinion the approach taken seems not the most promising for future highly integrated devices.

There are many reasons why graphene gets researchers so excited. The stability of this single layer of carbon atoms is one of the reasons, promising tough composite materials with increased mechanical strength. The unusual electronic properties that in some respect resemble that of relativistic particles is another. And last but not least, the fact that electrons can travel ballistic, without hitting carbon atoms, for long distances in the micrometer range is another. All these contribute to graphene’s success.

Schematic of the device where a nanowire acts as gate for a graphene transistor. Reprinted by permission from Macmillan Publishers Ltd. Nature (2010). doi:10.1038/nature09405

Two years ago I wrote a feature in New Scientist where I focussed on the potential of graphene to replace silicon logic. The piece is now behind a pay wall, but when talking to Andre Geim at the time, a pioneer in the field, he told me that graphene is uniquely suited to scale down to device dimensions impossible to achieve with silicon. Any transistor needs to support an electric current, that is how you read out its status. However, if you shrink the size of a transistor to only a few nanometers, this electric current will flow across only a small number of atomic bonds. Silicon bonds might not be able to sustain such high current densities. Not so graphene. “The bonds between the carbon atoms in graphene are very strong and can carry exceptionally high currents,” said Geim back then.

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Photo by Scott Robinson (Clearly Ambiguous) via flickr.

Solar energy is a huge market and any improvement to the efficiency of solar cells has a significant impact. In 2008, worldwide photovoltaic solar energy production was about 5 gigawatts, and this is expected to rise to 15 gigawatts in 2015. To put this figure in context, a nuclear reactor produces around 1 to 1.5 gigawatts of electricity.

The overall conversion efficiency of the best solar cell devices, with complex designs that include multiple materials, reaches up to about 50%. For more common silicon-based solar cells efficiencies are around 20%, although commercial photovoltaic panels achieve even less than that. Any enhancement in solar cell efficiency, whether it is in terms of costs or in terms of efficiency, is therefore highly desirable and significant. If all photovoltaic cells that produce those 5 gigawatts of energy were only 5% more efficient, for example with 21% conversion efficiency instead of 20%, it would result in an increase in energy production equivalent to about 250 megawatts.

Another issue is cost. Bulk solar cells are better at catching more sunlight, but use more material and therefore cost a lot more than thin-film films. Albert Polman, Harry Atwater and colleagues have now developed a solar cell design that enhances the power efficiency of thin solar cells. In their study published in Optics Express they are able to enhance the efficiency of a 340 nanometer thick silicon solar cell by 27% when compared to a regular thin-film cell.

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