Taking Charge of Molecular Wires

In plastic solar cells constructed to date, electrons must jump from one polymer wire to another in order to reach the electrodes. But as the electrons leave one wire in order to jump to the next, they encounter barriers, which require larger amounts of energy to traverse than the barriers that hinder electron movement within typical nanowires. This slows down the electrons.

“Long molecules that can act as molecular wires are one type of nanoscale object with the potential to lead to new technologies.”
— John Miller

Miller and his collaborators want to learn how to eliminate the barriers. But first, they must understand how the electrons move within single polymer wires — the amount of energy the electrons need, for example. Later, this information can be used to choose the best polymer conductors and design structures for plastic solar cells.

The group observed electrons move down a polymer wire by immersing the wire in an organic fluid and shooting high-energy electrons through the fluid. The electrons were supplied by Brookhaven’s Laser-Electron Accelerator Facility (LEAF), which accelerates electrons to high energies for research applications. The energetic LEAF electrons either kick away some of the fluid molecules’ electrons or allow the molecules to give up “holes” — mobile, empty spaces that carry positive charge. As a result, the submerged nanowire receives one of these electrons or holes.

“This new method injects extra negative or positive charges into the wire and allows us to observe the charges quickly diffuse across it. This observation is a key step toward developing polymer nanowires that are good conductors,” Miller said.
In the future, Miller and his group also plan to look for ways to increase the conduction efficiency of the wires.

This research was funded by the Office of Basic Energy Sciences within the U.S. Department of Energy’s Office of Science. Researchers included Alison Funston and Norihiko Takeda from Brookhaven Lab and Kirk Schanze and Eric Silverman from the University of Florida.

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