15 September 2009

Solar cells take different route

Using a carbon nanotube instead of traditional silicon, there is now a way to create the basic elements of a solar cell that could lead to much more efficient ways of converting light to electricity.

A simple solar cell called a photodiode is formed from an individual carbon nanotube. The device converts light to electricity in an extremely efficient process that multiplies the amount of electrical current that flows, said Paul McEuen, the Cornell University Goldwin Smith Professor of Physics, and Jiwoong Park, the Cornell assistant professor of chemistry and chemical biology.

This process could prove important for next-generation high efficiency solar cells, the researchers said.

“We are not only looking at a new material, but we actually put it into an application, a true solar cell device,” said Nathan Gabor, a graduate student in McEuen’s lab, and first author of a paper on the subject.

Using a single-walled carbon nanotube, which is essentially a rolled-up sheet of graphene, the researchers created their solar cell. About the size of a DNA molecule, they wired the nanotube between two electrical contacts and close to two electrical gates, one negatively and one positively charged. Their work stemmed in part from previous research in which scientists created a diode, which is a simple transistor that allows current to flow in only one direction, using a single-walled nanotube. The Cornell team wanted to see what would happen if they built something similar, but this time shined light on it.

Shining lasers of different colors onto different areas of the nanotube, they found higher levels of photon energy had a multiplying effect on how much electrical current produced.

Further study found the narrow, cylindrical structure of the carbon nanotube caused the electrons to neatly squeeze through one by one. The electrons moving through the nanotube became excited and created new electrons that continued to flow. The nanotube, they discovered, may be a nearly ideal photovoltaic cell because it allowed electrons to create more electrons by utilizing the spare energy from the light.

This is unlike today’s solar cells, in which extra energy is lost in the form of heat, and the cells require constant external cooling.

Though they have made a device, scaling it up to be inexpensive and reliable would be a serious challenge for engineers, Gabor said.

“What we’ve observed is that the physics is there,” he said.

For related information, go to www.isa.org/manufacturing_automation.