Perfecting a solar cell by adding imperfections

Jessika Trancik Postdoctoral Fellow, Santa Fe Institute. Adjunct Associate Research Scholar, Earth Institute, Columbia University. Research Areas: Physics of Complex Systems, Emergence & Innovation in Evolutionary Systems, Emergence, Organization & Dynamics of Living Systems, Focus Areas: Materials science • Technological innovation • Energy systems Contact: Home Page: santafe.edu/~trancik. Email: trancik@santafe.edu. SFI Info: phone & office

Nanotechnology is paving the way toward improved solar cells. New research shows that a film of carbon nanotubes may be able to replace two of the layers normally used in a solar cell, with improved performance at a lower cost. Researchers have found a surprising way to give the nanotubes the properties they need: add defects. Currently, these solar cells, called dye-sensitized solar cells, have a transparent film made of an oxide that is applied to glass and conducts electricity. In addition, a separate film made of platinum acts as a catalyst to speed the chemical reactions involved. Both of these materials have disadvantages, though. The oxide films can't easily be applied to flexible materials: they perform much better on a rigid and heat resistant substrate like glass. This increases costs and limits the kinds of products that can be made. And expensive equipment is necessary to create the platinum films.

Jessika Trancik of the Santa Fe Institute, Scott Calabrese Barton of Michigan State University and James Hone of Columbia University decided to use carbon nanotubes to create a single layer that could perform the functions of both the oxide and platinum layers. They needed it to have three properties: transparency, conductivity, and catalytic activity.

Ordinary carbon nanotubes films are so-so in each of these properties. The obvious ways of improving one, though, sacrifice one of the others. For example, making the film thicker makes it a better catalyst, but then it's less transparent.

Previous theory had suggested that materials may function better as catalysts when they have tiny defects, providing sites for chemicals to attach. So the researchers tried exposing the carbon nanotubes to ozone, which roughs them up a bit. Very thin films, they found, became dramatically better catalysts, with more than ten-fold improvement.

In fact, the performance gets close to that of platinum. "That's remarkable," Trancik says, "because platinum is considered pretty much the best catalyst there is."

In order to address the trade-off between transparency and conductivity, the researchers tried another trick on a bottom layer of tubes: they created carbon nanotubes that were longer. This improved both conductivity and transparency.

The carbon nanotube films might be used in fuel cells and batteries as well.

"This study is an example of using nanostructuring of materials – changing things like defect density and tube length at very small scales – to shift trade-offs between materials properties and get more performance out of a given material," Trancik says. "Making inexpensive materials behave in advanced ways is critical for achieving low-carbon emissions and low cost energy technologies." ###

The researchers published their results recently in Nano Letters. They are currently in the process of filing a patent application for their techniques.

The Santa Fe Institute (SFI) is an acknowledged leader in multidisciplinary scientific research. Its objectives are to discover and understand the common fundamental principles in physical, computational, biological, and social complex systems that underlie many of the most profound issues facing science and society today. By transcending disciplines, breaking academic molds, and drawing together an international network of unorthodox creative thinkers, SFI is an independent non-profit research and education center supported by grants, charitable giving, and corporate relationships.

Contact: Jessika Trancik trancik@santafe.edu 505-946-2794 Santa Fe Institute

Tags: Nano or Nanotechnology and Nanotech

Michigan Tech scientist models molecular switch

Dr. Ranjit Pati Associate Professor PhD in Theoretical Condensed Matter Physics, University at Albany, State University of New York Modeling Molecular Electronics Devices. Current Research: Our current research is mainly focused on addressing some of the fundamental challenges associated with nanoscience. Specifically, we try to answer some of the open questions pertaining to the electron (both charge and spin) transfer process involving nanoscale junction in a non equilibrium condition using quantum many body theory.

HOUGHTON, Mich.--Michigan Technological University physicist Ranjit Pati and his team have developed a model to explain the mechanism behind computing's elusive Holy Grail, the single molecular switch. If born out experimentally, his work could help explode Moore's Law and could revolutionize computing technology. Moore's Law predicts that the number of transistors that can be economically placed on an integrated circuit will double about every two years. But by 2020, Moore's Law is expected to hit a brick wall, as manufacturing costs rise and transistors shrink beyond the reach of the laws of classical physics. A solution lies in the fabled molecular switch. If molecules could replace the current generation of transistors, you could fit more than a trillion switches onto a centimeter-square chip. In 1999, a team of researchers at Yale University published a description of the first such switch,

but scientists have been unable to replicate their discovery or explain how it worked. Now, Pati believes he and his team may have found the mechanism behind the switch.

Applying quantum physics, he and his group developed a computer model of an organometallic molecule firmly bound between two gold electrodes. Then he turned on the juice.

As the laws of physics would suggest, the current increased along with the voltage, until it rose to a miniscule 142 microamps. Then suddenly, and counterintuitively, it dropped, a mysterious phenomenon known as negative differential resistance, or NDR. Pati was astonished at what his analysis of the NDR revealed.

Up until the 142-microamp tipping point, the molecule's cloud of electrons had been whizzing about the nucleus in equilibrium, like planets orbiting the sun. But under the bombardment of the higher voltage, that steady state fell apart, and the electrons were forced into a different equilibrium, a process known as "quantum phase transition."

"I never thought this would happen," Pati said. "I was really excited to see this beautiful result."

Why is this important? A molecule that can exhibit two different phases when subjected to electric fields has promise as a switch: one phase is the "zero" and the other the "one," which form the foundation of digital electronics. ###

Pati is working with other scientists to test the model experimentally. His results appear in the article "Origin of Negative Differential Resistance in a Strongly Coupled Single Molecule-metal Junction Device," published June 16 in Physical Review Letters. The other coauthors are Mike McClain, an undergraduate from Michigan Tech; and Anirban Bandyopadhyay, of the National Institute for Materials Science, Japan. The work of Pati's team was financed by a five-year, $400,000 Faculty Early Career Development Program award he received from the National Science Foundation.

An abstract and a PDF file of the article are available at Origin of Negative Differential Resistance in a Strongly Coupled Single Molecule-Metal Junction Device

Contact: Marcia Goodrich mtunews@mtu.edu 906-487-2343 Michigan Technological University

Tags: Nano or Nanotechnology and Nanotech