Monday, February 06, 2006

"OPTICAL: Titania nanotubes could cut solar-cell costs"

Dye-sensitized solar cells present a low-cost option for renewable power generation, but their efficiency has maxed out at a dismal 11 percent. Now a project at Pennsylvania State University suggests that incorporating titania (titanium oxide) nanotube arrays could provide the needed efficiency boost to move the cells toward commercialization, according to Craig Grimes, a professor of electrical engineering and materials science and engineering at Penn State. Solar cells directly convert sunlight into electrical energy by generating free electrons from incident photons. Photovoltaic conversion was discovered as long ago as 1839 by Alexander Bequerel. Dye-sensitized solar cells--invented in the 1990s--depend on a thin-film version of photovoltaic conversion. But, unlike silicon-based thin-film cells, in which light is absorbed by an expensive semiconductor, in dye-sensitized solar cells absorption occurs in an inexpensive thin film comprised of dye molecules attached to titanium oxide nanoparticles in an electrolyte. When the dye cells absorb a photon, the resultant excitation injects electrons into the titanium, which transports them to the negative electrode, with the positive electrode attached to the electrolyte. Today, dye-sensitized solar cells remain a laboratory curiosity, chiefly because no one has come close to achieving their theoretical maximum efficiencies. Many researchers have worked on the problem, because dye-sensitized solar cells could potentially be manufactured very cheaply, but inefficiencies inherent in the transport of electrons to the negative electrode have severely limited their performance. As a result, even the best dye sensitized solar cell prototypes only have about 11 percent photovoltaic conversion efficiency, Grimes noted. To solve the problem, Grimes' research group proposes substituting titania nanotubes for nanoparticles. Consequently, when electrons are liberated by photovoltaic conversion, they can move to the negative electrode via ballistic transport along the titania nanotubes.