Thursday, September 12, 2013

A new method for harvesting energy from light

September 12, 2013
Hybrid optoelectronic nanostructures with controlled variation in photoconduction properties.  Arrays of gold
nanoparticles of various sizes and densities are deposited on insulating substrates and linked with porphyrin molecules. (Credit: American Chemical Society)
Researchers from the University of Pennsylvania have demonstrated a new mechanism for extracting energy from light, a finding that could improve technologies for generating electricity from solar energy and lead to more efficient optoelectronic devices used in communications.
The research centers on plasmonic nanostructures, specifically, materials fabricated from gold particles and light-sensitive molecules of porphyin, of precise sizes and arranged in specific patterns.
Plasmons, or a collective oscillation of electrons, can be excited in these systems by optical radiation and induce an electrical current that can move in a pattern determined by the size and layout of the gold particles, as well as the electrical properties of the surrounding environment.
Because these materials can enhance the scattering of light, they have the potential to be used to advantage in a range of technological applications, such as increasing absorption in solar cells.
Devices incorporating this process of harvesting plasmon-induced hot electrons could be customized for different applications by changing the size and spacing of nanoparticles, which would alter the wavelength of light to which the plasmon responds.
“You could imagine having a paint on your laptop that acted like a solar cell to power it using only sunlight,” Bonnell said. “These materials could also improve communications devices, becoming part of efficient molecular circuits.”
“These results  demonstrate fundamental principles that underlie new behavior that could enable various applications such as  high efficiency energy harvesting (as in solar cells),  optical circuits, and communications devices,” Bonnell explained to KurzweilAI. “Successful application will require further development and engineering.”
Technical details
In 2010, Bonnell and colleagues published a paper in ACS Nano reporting the fabrication of a plasmonic nanostructure, which induced and projected an electrical current across molecules. In some cases they designed the material, an array of gold nanoparticles, using a technique Bonnell’s group invented, known as ferroelectric nanolithography.
The discovery was potentially powerful, but the scientists couldn’t prove that the improved transduction of optical radiation to an electrical current was due to the “hot electrons” produced by the excited plasmons. Other possibilities included that the porphyin molecule itself was excited or that the electric field could focus the incoming light.
“We hypothesized that, when plasmons are excited to a high energy state, we should be able to harvest the electrons out of the material,” Bonnell said. “If we could do that, we could use them for molecular electronics device applications, such as circuit components or solar energy extraction.”
To examine the mechanism of the plasmon-induced current, the researchers systematically varied the different components of the plasmonic nanostructure, changing the size of the gold nanoparticles, the size of the porphyin molecules and the spacing of those components. They designed specific structures that ruled out the other possibilities so that the only contribution to enhanced photocurrent could be from the hot electrons harvested from the plasmons.
“In our measurements, compared to conventional photoexcitation, we saw increases of three to 10 times in the efficiency of our process,” Bonnell said. “And we didn’t even optimize the system. In principle you can envision huge increases in efficiency.”
Duke University.scientists were also involved. The research was supported by the U.S. Department of Energy and the National Science Foundation.

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