Molecular Stand-in for Solar’s Polymer Wires

Silicon has long been the catalyst for producing energy from solar photovoltaic (PV) cells, but new molecular technology may change all that. Scientists from the U.S. Department of Energy’s Brookhaven National Laboratory and the University of Florida are studying how electric charge is distributed in polymer molecule chains, and they have uncovered information that may make it easier for molecular wires to replace the silicon conventionally used in components of solar PV cells.

Upton, New York – August 24, 2004 [] In conventional solar cells, incoming solar energy is transferred to the electrons in a semi-conducting material, such as silicon. These electrons are guided to an electrode, creating a current that can be drawn off and used. Plastic solar technology aims to replace materials like silicon with polymer nanowires, which are cheaper and lighter. 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, and so require larger amounts of energy to make the jump. Traversing these barriers uses more of the electron’s energy than is needed for movement within a silicon base. “Long molecules that can act as molecular wires, of which there are many variations, are one type of nanoscale object with the potential to lead to new technologies, due to their ability to conduct electricity,” said Brookhaven chemist John Miller. “But unlike conventional metal wires, polymer nanowires need assistance in order to conduct.” Miller and his collaborators want to learn how to eliminate the barriers. But first, they must understand elements such as how the electrons move within single polymer wires, and the amount of energy the electrons need. This information can help developers in the solar industry choose the best polymer conductors and design structures for plastic solar cells. “Using a cluster of high-energy electrons from an accelerator, we can quickly add an extra negative or positive charge to a polymer molecular wire,” Miller said. “When the end of the wire contains a chemically attached ‘trap’ molecule, one where the electrons will be at a lower, more stable energy, the charge moves to it. This allows us to ‘see’ that the wires conduct electrons quickly, and over long distances.” Advances in the solar PV industry rely on creating a product that is easier to use, has a greater electrical output per area, and is, most importantly, less expensive to manufacture. Fine-tuning the efficiency of electron movement in plastic solar technology could help solar gain greater competitive advantage in the market. Miller and his group plan to look for ways to increase the conduction efficiency of the wires. The Office of Basic Energy Sciences within the U.S. Department of Energy’s Office of Science funded the molecular research. It was performed in collaboration with Alison Funston and Norihiko Takeda (Brookhaven Lab), and Kirk Schanze and Eric Silverman (University of Florida).
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