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CLOSING THE GAP: USING CONDUCTIVE POLYMERS TO MAKE MORE EFFICIENT SOLAR CELLS HARRIS DAVIS
After well over a century of dependence on coal and oil, energy infrastructure in the modern world is devoted to combustion systems. Now, as we look for ways to supply our energy demand from more sustainable sources, we face economic barriers. Solar power is one of the more popular green systems among scientists, but current processes for refining the semiconductors found in most commercial solar cells are expensive. Consider silicon, which covers the planet’s coasts as sand (silicon dioxide). While the material is abundant, using it to build efficient photovoltaics requires an extremely expensive purification process.2 The question becomes: what if there was a material that offered silicon’s conductive properties, but did not require such A A costly refinement? The short answer is that it already exists—in the form of a group of materials called organic polymers (think plastics). However, engineering polymers that will conduct electricity the same way as semiconductors do requires meticulous chemical design, which researchers at UNC-Chapel Hill are currently developing. The common denominator for all solar cells is that they take advantage of a principle known as the photovoltaic effect. Electrons in a valence band become excited and enter the “conduction band” in which they can flow freely; the energy which allows the electron to become excited comes from the absorption of sunlight in the form of a photon.9 This excitation lets electrons move freely and produce a cur-
rent. The energy required to excite an electron from the valence shell to the conduction band is known as the “band gap,” and this requirement determines the colors—or wavelengths of light—a photovoltaic material can absorb. To even consider conduction and band gaps in organic polymers, the compounds must be conjugated—a property which can be synthetically designed by a chemist. A conjugated material has delocalized electrons, similar to those which exist naturally in metals. Scientists engineering organic polymer-based photovoltaics (OPVs) spend much of their time experimenting with energy levels and designing ideal band gaps, as this principle is critical in generating voltage. Generally speaking, deeper band gaps, which involve low-energy valence shells, are associated with higher voltages in photovoltaic materials. More electronegative materials—such as fluorine—offer a way to achieve those deeper band gaps, and thus greater solar cell efficiency. Since 2006, Dr. Wei You and his group of researchers in UNC’s chemistry department have been working to enhance the photovoltaic effect in OPVs. A highlight of their work has been the development of a methodology to add fluorine— the most electronegative element—to polymers to deepen their band gaps. In 2009, the You group had the idea to replace specific hydrogen atoms in conjugated organic polymers with fluorine, as this element is not much larger than hydrogen and can
PRIOR TO THE 1980S, EVERYBODY THOUGHT THAT PLASTICS WERE INSULATORS. BUT WITH HEEGER’S, SHIRAKAWA’S, MACDIARMID’S RESULTS, WE ARE NOW SEEING THAT PLASTICS ACTUALLY CAN CONDUCT ELECTRICITY.
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