Monday, February 7, 2011

The Key to Better Solar Cells: Bumpy Mirrors

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The Key to Better Solar Cells: Bumpy Mirrors

Stanford researchers develop a trick that could help dye-sensitized solar cells trap more light. By Prachi Patel Dye-sensitized thin-film solar cells are cheaper to make than conventional silicon cells, but they're still relatively inefficient. Now researchers at Stanford University have used a specially designed metal reflector to boost the efficiency of solid electrolyte dye-sensitized solar cells by as much as 20 percent. The reflector is a thin silver film with an array of nanoscale bumps. The researchers use the film to coat the cells' back surface; the film helps trap more light inside the cells. "We get about 5 to 20 percent more absorption depending on the dye," says Michael McGehee, director of the Center for Advanced Molecular Photovoltaics at Stanford. McGehee led the research, which was published online this week in the journal Advanced Energy Materials. Dye-sensitized thin-film cells with a light-to-electricity conversion efficiency of around 11 percent recently made their commercial debut. However, they use liquid electrolytes that are volatile and could leak. Cells with solid electrolytes have only shown efficiencies of about 5 percent. "They took the best solid-state dye cell they could, and made it better," says David Ginger, a chemistry professor at the University of Washington, of the Stanford researchers. "Even better, they did it using technology and methods that could potentially be used in a production environment." Dye-based solar cells are composed of semiconductor nanocrystals (typically titanium dioxide, or titania) that are coated with dye molecules and sandwiched—along with an electrolyte—between glass or plastic sheets. The dye absorbs light and creates electrons and positively charged holes. The crystals transfer the electrons to one electrode to produce an electrical current, while the electrolyte carries the holes to the other electrode. Solid electrolytes are not as efficient as liquid ones, though, and the electrons and holes recombine more easily. To prevent that, the titania layer is very thin—typically two micrometers. But the thinner the cells, the more quickly light passes through them without getting absorbed. Research efforts to improve the efficiency of these cells have typically focused on developing stronger dyes and new types of nanocrystals. But McGehee and his colleagues used plasmonic reflectors to improve their cell's efficiency. Plasmons are the oscillations of electrons at a metal surface when they are excited by light. By controlling the shape of the surface, you can control the type of plasmons created, which in turn influences how light interacts with the material....

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