"In dye-sensitized solar cells, the dye is mixed with titania nanoparticles. When a photon hits the dye, it is absorbed, creating an electron-hole pair--the electron goes into the titania and the hole goes into the electrolyte," said Grimes.
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.
"Researchers have been trying to make dye-sensitized solar cells commercially viable for more than a decade. Typically the cells require about a 10-micron layer of titania nanoparticles--which is great at optical absorption," said Grimes. "But when the electrons are generated, they have to hop from nanoparticle to nanoparticle a thousand times or more before reaching the negative electrode. And every time they hop, they have a chance to recombine with a hole in the electrolyte, limiting their efficiencies."
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.
"Everybody has been scratching their heads, saying, 'How can you improve dye solar cells?' So we decided to apply the highly ordered nanotube array we invented in 2001 to the cells. By using titania nanotubes instead of carbon nanotubes, we think we can create the miracle material that everybody thought carbon nanotubes would be," said Grimes.
In 2001, Grimes' research group demonstrated the feasibility of integrating carbon nanotube arrays onto silicon mi- croelectronics by aligning open-tipped carbon nanotube arrays on silicon substrates using a nanoporous template of anodized aluminum oxide (see www.ee.psu.edu/grimes/publications/physlet.pdf). At that time, the researchers demonstrated that they could control the diame- ter, length and density of nanotube ar- rays with a prepatterned aluminum oxide template.
Grimes predicts that the same method will work for growing highly ordered titania nanotube arrays for dye-sensitized solar cells.
"The technological significance of our work is that we can make this really thin negative electrode--360 nanometers thick--which gives you a very big boost in current density compared with what conventional dye solar cells have," said Grimes.
Next, the Penn State group wants to lengthen the titania nanotubes to increase their efficiency.
"We have already grown highly or- dered carbon nanotube arrays up to 6 microns long, and when you extrapolate from our current titania nanotube pro- totype to one with nanotubes that long, you get remarkable photoconversion efficiencies from this geometry," observed Grimes.
Nanotubes also open up the possibility of directly converting water into hydrogen and oxygen. The hollow inner core of the nanotubes would be used to store free hydrogen atoms.
Experiments by the group have shown that highly ordered titania nanotube arrays can produce hydrogen when illuminated with ultraviolet light. The task now is to shift the process into the visible spectrum, potentially creating a commercially practical means of generating hydrogen from solar energy.
Grimes' research team performs its work with funding from the National Science Foundation, the Penn State Hydrogen Center and the Materials Research Institute (www.mri.psu.edu).