Craig Grimes, a professor of electrical engineering and materials science and engineering at Penn State is pioneering a new material for solar-cells, here's a heads-up for those who want to keep up with some of what's happening in this field.
From a report in eetimes.com,
Titania nanotubes could cut solar-cell costs-
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.
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 [conductive solution]. [These are nanoparticles being talked about here, not nanotubes.]
[...]
"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.
[...]
Typically the cells require about a 10-micron [25 microns is about 0.001"] 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.
[...]
Now a project at Pennsylvania State University suggests that incorporating titania (titanium oxide) nanotube arrays [think: very tightly packed bed-of-nails, with hollow nails] could provide the needed efficiency boost to move the cells toward commercialization.
[...]
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[0.36 micron], and he predicts efficiencies of as much as 20 percent when the nanotube length is increased.
[...]
"We have already grown highly ordered carbon nanotube arrays up to 6 microns [6000 nanometers] long, and when you extrapolate from our current titania nanotube prototype to one with nanotubes that long, you get remarkable photoconversion efficiencies from this geometry," observed Grimes.
Grimes' research team performs its work with funding from the National Science Foundation, the Penn State Hydrogen Center and the Materials Research Institute.
Here's a brief description of nanotube cell fabrication-
Conventional solar cells are made from blocks of slowly made silicon boules that are sliced into wafers. Grimes and his team use an easier approach. They coat a piece of glass with a fluorine-doped tin oxide and then sputter on [think: spray paint] a layer of titanium. The researchers can currently lay down a 500-nanometer [1/2 micron] thick titanium layer. They then anodize the layer by placing it in an acidic bath with a mild electric current and titanium dioxide nanotube arrays grow to about 360 nanometers. The tubes are then heated in oxygen so that they crystallize. The process turns the opaque coating of titanium into a transparent coating of nanotubes.
This nanotube array is then coated in a commercially available dye. The dye-coated nanotubes make up the negative electrode and a positive electrode seals the cell which contains an iodized electrolyte. When sun shines through the glass, the energy falls on the dye molecules and an electron is freed. If this electron and others make their way out of the tube to the negative electrode, a current flows. Many electrons do not and are recombined, but the tube structure of the titanium dioxide allows an order of magnitude more electrons to make it to the electrode than with particulate coatings.
[One problem that the researchers mention is establishing and maintaining the cell's thickness-]
Other aspects of the titania nanotube dye sensitive solar cells that need to be optimized include the thickness of the cells. Currently, spacers separate the two layers and provide internal support. These spacers are 25 microns thick. If the spacers could be made as sturdy, but shorter, there would be less of a distance for the electrons to travel and more electrons will make it across the electrodes.
I know the LCD industry uses round, micron-sized spacers to keep the two plates of their cells separated. They can be sprayed-on to any desired density while suspended in a solvent, I wonder how that would work in this case.
Solar-cells seem to have a lot of potential, especially for distributed energy applications. Most of this potential is probably not yet explored and developed. Basic research like this could be very valuable to our survival as far as transitioning away from both petroleum and central generation of energy by helping to make solar-cells more affordable.