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See-Through Solar Cells Become Safer

solar cells on house



© CHINCH GRYNIEWICZ; ECOSCENE/CORBIS
Plants are the most efficient converters of sunlight into usable forms of energy, namely, sugar. And our industrialized society benefits from their efforts over the past few billion years: organic products, trapped in the passage of time deep below the earth, were transformed into the oil, coal and natural gas that largely powers our modern world. Cutting out the middle men--plants and tectonic processes--seems like a good solution to providing our energy needs. But man-made cells used to harvest light--so-called solar photovoltaics and other technologies--lose more of the sun's energy than they absorb, and they require tremendous amounts of energy to manufacture. One alternative, known as dye-sensitized solar cells, offers similar energy capture and lower cost but had relied on potentially toxic liquid components, preventing its widespread use. Research presented at the American Chemical Society meeting in San Francisco on September 12, however, has shown how such cells can be made from safer liquids and pointed to possibilities for a more bountiful light harvest.

Chemist Michael Graetzel of the Swiss Federal Institute of Technology in Lausanne developed these dye-sensitized cells more than a decade ago. Made from cheap, abundant components, these cells use a titanium dioxide dye to turn 11 percent of incoming light into electricity--close to the efficiency of standard solar cells used by homeowners today as shown in the photo. Even so, these cells did not find widespread use, primarily because they contained toxic liquids that easily evaporated if not contained. But Graetzel and his colleagues recently discovered that by substituting ionic liquids--liquids made up of charged particles, imidazolium iodide in this case--they could maintain the efficiency and pose no risk of evaporation.

With such solvent issues out of the way, the dye-sensitized cell offers several advantages, according to its creator. "You can look through them," Graetzel notes. "Our cell is not dependent on the angle of light either. It captures light from all angles." These properties make the cells ideal for things like windows or roofs, although they are probably less suitable for large-scale commercial applications, such as solar farms, because they need more area to produce the same amount of electricity as more typical silicon-based solar cells.

But additional research presented at the meeting may boost their efficiency: teams from Tokyo University of Science and Notre Dame both revealed new types of dyes that could extend the spectrum of light captured by such cells into the infrared range. The Tokyo team relied on a dye composed largely of ruthenium to absorb infrared light, though it overlaps too closely with the titanium dye already involved. The Notre Dame researchers, on the other hand, used organic compounds--acetylene and ethylene--to attempt to extend the range of the cells. With or without such improvements, such cells may start appearing in various applications in the next few years, according to Graetzel. But the see-through, reddish cells still have a long way to go to match the light-harvesting power of the original solar cell: green plants.

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