Portland, Ore. -- 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.
"We have made highly ordered nanotube arrays, 6.4 microns in length. Integrating these films into dye solar cells should allow us to obtain remarkable photoconversion efficiencies from this low-cost solar cell technology, possibly in the 20 percent range," said Craig Grimes, a professor of electrical engineering and materials science and engineering at Penn State. The "ideal limit" for the material, he said, is 33 percent.
Grimes' transparent nanotubes form the negative electrode for the experimental dye solar cells and provide an efficient technique for electron percolation. He said the group obtained photoconversion efficiencies of 2.9 percent with an electrode length of only 360 nanometers, and he predicts efficiencies of as much as 20 percent when the nanotube length is increased. Grimes' research group also plans to experiment using the technique to produce hydrogen fuel from solar cells.
"This is an amazing material architecture for water photolysis too--that is, the solar generation of hydrogen by water-splitting," said Grimes. "Basically, we are talking about putting water on top of the material so that sunlight can turn the water into hydrogen and oxygen.
"With this technique, our nanotube arrays, under ultraviolet illumination, already have a photoconversion efficiency of 13.1 percent. That means, in a nutshell, that you get a lot of hydrogen out of the system per photon. If we could successfully shift the bandgap into the visible spectrum. we would also have a commercially practical, and scalable, means of generating hydrogen by solar energy."
Grimes' research associates on the Penn State project are electrical engineer Karthik Shankar, physicists Gopal Mor and Oomman Varghese, and materials scientist Maggie Paulose.
Cheap renewable energy
According to the U.S. Department of Energy, almost one-third of America's power generation today is from renewable resources. Of that total, hydroelectric power accounts for 61 percent, geothermal more than 20 percent, wind and wood about 6.5 percent each, municipal waste/ landfill gas about 3 percent, biomass less than 2 percent and solar a mere 1 percent. To reach the strategic national goal of independence from foreign fossil-fuels by 2011, the DOE predicts that the U.S. needs to invent renewable power sources that are cheaper than hydroelectric. The DOE estimates that solar power conversion efficiencies of more than 30 percent would be necessary to enable it to better hydroelectric's cost per kilowatt (see www.eetimes.com/news/latest/showArticle. jhtml?articleID=53700939).
Today, the most expensive single-crystal silicon solar cells can achieve 30 percent efficiencies, but only with direct current. By the time the dc is converted to alternating current and conditioned for the power grid, efficiencies drop to below 17 percent, making solar power generation the most expensive of the renewables. Polysilicon and amorphous silicon solar cells are much less expensive than single-crystal silicon, but their efficiencies range from 15 percent down to 5 percent.
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.