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PHOTOVOLTAICS
A new design for photovoltaic cells may reduce the cost of the expensive devices that convert sunlight into electrical energy. Developed by scientists at the University of California, Santa Barbara, the novel architecture may sidestep the need for costly materials and manufacturing methods. In conventional solar cells, a layer of silicon hosts several key processes simultaneously to derive useful electrical energy. For example, the semiconductor absorbs sunlight and converts the light energy into electrons and holes (positively charged electron vacancies). At the same time, the material separates electrons and holes and delivers the charge carriers to current collectors. The job calls for high-purity materials and methods. But now, chemical engineering professor Eric W. McFarland and graduate student Jing Tang have devised a multilayer photovoltaic-cell structure in which the photon-absorption and charge-separation functions occur in distinct layers that can be prepared by relatively simple means [Nature, 421, 616 (2003)]. In the Santa Barbara design, photons are collected by molecules of merbromin, a fluorescein dye, that are adsorbed on a thin gold film. The film is supported by a layer of TiO2 that, in turn, rests on titanium. As light is absorbed in the dye layer, the molecules liberate energetic electrons that are transported through the gold film, injected into the TiO2 layer, and ultimately conducted to the Ti electrode. Working with Galen D. Stucky, a professor of chemistry and materials, the team is also studying other types of light collectors. McFarland acknowledges that, in their present state, the photo cells' light-conversion efficiencies are too low for commercial applications, but he notes that the work demonstrates a new design concept. Now, the group is investigating several methods for improving performance, such as boosting surface area and concentration of dye molecules and reducing surface reflections.
LOTS O' DOTS UC Santa Barbara researchers use cost-saving techniques to produce arrays of solar cells that use dye molecules or nanometer-sized CdSe-CdS quantum dots to collect light. Micrographs show that the quantum-dot coverage (white specks) can be increased, which, in turn, improves performance. COURTESY OF UC SANTA BARBARA |
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