“What we’d like to do is absorb every photon from the sun and convert it into electricity,” he said. He said they might also incorporate other materials, such as organic or inorganic semiconductors, to complement CNTs. Each layer would be optimized for a particular portion of the solar spectrum and, thus, absorb more light. Hersam said the next step is to create polychiral CNT solar cells that have multiple layers. We still have to advance this technology by a factor of three to five.” “If you look at our performance, there’s certainly a big jump,” Hersam said. Silicon, for example, can be 15-20 percent efficient, but it is more expensive to manufacture. While this is major progress for CNT solar cells, they still lag behind other materials in efficiency. The cells significantly absorbed near-infrared wavelengths, a range that has been inaccessible to many leading thin-film technologies. This maximized the amount of photocurrent produced by absorbing a broader range of solar-spectrum wavelengths. Hersam’s team made a mixture of polychiral, or multiple chirality, semiconducting nanotubes. “If you make a solar cell out of a single chirality carbon nanotube, you basically throw away most of the solar light.” “The problem is that each nanotube chirality only absorbs a narrow range of optical wavelengths,” Hersam said. In the past, researchers tended to choose one particular chirality with good semiconducting properties and build an entire solar cell out of that one. When a thin sheet of carbon is rolled into a nanotube, several hundred different chiralities are possible. The secret lies in the CNTs’ chirality, which is a combination of the tubes’ diameter and twist. The research is described in the article “Polychiral Semiconducting Carbon Nanotube-Fullerene Solar Cells” in the August 7 issue of Nano Letters. “But we’ve been able to increase it to over 3 percent. “The field had been hovering around 1 percent efficiency for about a decade it had really plateaued,” Hersam said. It is also the first CNT solar cell to have its performance certified by the National Renewable Energy Laboratory. Now a research team led by Mark Hersam, professor of materials science and engineering and the Bette and Neison Harris Chair of Teaching Excellence at the McCormick School of Engineering, has created a new type of CNT solar cell that is twice as efficient as its predecessors. But research stalled when CNTs proved to be inefficient, converting far less sunlight into power than other methods. Lighter, more flexible, and cheaper than conventional solar-cell materials, carbon nanotubes (CNTs) have long shown promise for photovoltaics. Professor Mark Hersam is a faculty member in two portfolio research centers of the Institute for Sustainability and Energy at Northwestern (ISEN) : the Center for Catalysis and Surface Science (CCSS) and the Argonne-Northwestern Solar Energy Research (ANSER) Center. Center for Molecular Quantum Transduction (CMQT).Center for Catalysis and Surface Science (CCSS).Center for Advanced Materials for Energy and the Environment (CAMEE).Great Lakes Circular Economy Partnership. ![]() Master of Science in Energy and Sustainability (MSES).
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