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 September 17, 2009
More Efficient Solar Cells with Carbon Nanotubes

 Silicon is one of the basic but expensive materials used in designing solar panels. If we can use something else as effective as silicon but more economical, solar power will be within the reach of the common consumers. Cornell researchers are thinking on somewhat similar lines. They are using a carbon nanotube instead of traditional silicon that hopefully will lead to much more efficient ways of converting light to electricity.

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The researchers extracted a simple solar cell known as photodiode and fabricated, tested and measured this photodiode. The specialty of this photodiode is it is made up of an individual carbon nanotube. Paul McEuen, the Goldwin Smith Professor of Physics, and Jiwoong Park, assistant professor of chemistry and chemical biology take pride in their work and elaborate about the device their team has prepared under their leadership. They explained that their device transforms light into electricity in an exceptionally efficient process that multiplies the amount of electrical current that flows. This process could leave its mark for next-generation high efficiency solar cells.

Nathan Gabor who is a graduate student in McEuen's lab, says, "We are not only looking at a new material, but we actually put it into an application --- a true solar cell device."

The researchers utilized a rolled-up sheet of grapheme to create their solar cell. They designed a single-walled carbon nanotube from grapheme. The nanotube was about the size of a DNA molecule. The nanotube was wired between two electrical contacts and close to two electrical gates, one negatively and one positively charged. Other scientists too had created a diode, which is a simple transistor that allows current to flow in only one direction. They too had utilized a single-walled nanotube. The Cornell team went ahead with the basic idea but with a twist. They built something similar, but this time threw light on it. They threw lasers of different colors onto different areas of the nanotube. They were in for a pleasant surprise. They discovered that higher levels of photon energy had a multiplying effect on how much electrical current was produced.

How did this desirable event happen to the research team? When they gave their consideration to the whole event they discovered that the narrow, cylindrical structure of the carbon nanotube forced the electrons to be neatly squeezed through one by one. The electrons running through the nanotube became excited and created new electrons that continued to form a stream. The research team thinks that the nanotube might be a nearly ideal photovoltaic cell because it permitted electrons to produce more electrons by making use of the spare energy from the light.

If we compare carbon nanotubes with the current cells we will find that the latter in fact loses the extra energy as heat. So the conventional cells need cooling factors to function properly. Work is going on for the carbon nanotube but to produce it on commercial scale will still be challenging. The team has just been successful on the theoretical and conceptual fronts only.