Doubling the Efficiency of Dye-Sensitized Solar Cells
by Tim McGee, Helena, MT, USA on 04.10.08
Dye sensitized solar cells (also known as Grätzel cells) continue innovation at a scorching pace. We recently covered how Shaik Zakeeruddin and Michael Grätzel used ionic liquids to make these solar cells flexible and significantly less toxic. Today we learn from the University of Washington (UW) that it's possible to double the efficiency of dye sensitized solar cells by using a novel popcorn-ball design.
"We think this can lead to a significant breakthrough in dye-sensitized solar cells," said lead author Guozhong Cao, "We did not expect the doubling...it was a happy surprise."
Cao, a UW professor of materials science and engineering did not set out to increase the maximum efficiency of dye sensitized solar cells. Instead, his team was investigating the difference in how light is absorbed using different processing and design techniques.
The problem with absorption in dye sensitized solar cells is that you not only want to provide a lot of surface area where light can interact, but you also want the light to have multiple 'attempts' at interacting with one of the dyes. The new solar cell strategy takes advantages of a hierarchical (or fractal if you prefer) design to achieve both goals.
At the smallest scale, Cao and his team developed 15 nanometer diameter 'grains' of the solar cell material. The small grains are then clumped together to form aggregate clusters that are about 300 nanometers across (175 of these clusters lined up are about equal to the width of a human hair). The complex internal structure created within the clusters creates tremendous surface area for light to interact with the dye, about 1000 square feet for each gram of material. Yet, at the same time the larger (300 nanometer) structures scatters more light within the entire solar cell creating more opportunities for interaction with other clusters.
The new design vastly improved the efficiency of the dye sensitized zinc oxide cell Cao was working with for this experiment, from 2.4% to 6.2%. Dye sensitized solar cells today traditionally use Titanium dioxide, and can achieve efficiency conversions around 11%.
"We first wanted to prove the concept in an easier material. Now we are working on transferring this concept to titanium oxide," Cao said.
Cao and team think their new design will also increase the efficiency of titanium dioxide dye sensitized solar cells. If the results are similar to what was achieved in zinc oxide, we may see a 20% efficient dye sensitized solar cell in the near future.
Dye sensitized solar cells are cheaper to produce and easier to manufacture than traditional silicon PV cells. Dye sensitized solar cells also offer low toxicity, creative, flexible, and increasingly efficient designs. The future looks bright for the dye sensitized solar cell to play a significant role in our energy matrix. Photo Credit to University of Washington.
via :: UW News
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While this is certainly interesting, I don't buy the line that "dye sensitized solar cells are cheaper to produce" than conventional solar cells. They might be, but thus far nobody has showed that it actually is. And even if it is, there are a couple of other challenges that have to be overcome before DSC can be commercially significant.
One is scaling up the solar cells from tiny laboratory cells to larger cells more suitable for mass production. Last I looked into it, the efficiency took a nosedive when the cells were scaled up to the size of a standard silicon solar cell (and larger cells are significantly cheaper to produce than small ones). The reasons for this are well understood, so there might have been some progress since I last checked.
Another is the liquid electrolyte. It isn't easy to design a package that will survive outdoors, exposed to the elements 24/7, for decades at a time. When that package has to contain a liquid, the task becomes exponentially more difficult.
Even if such a package can be designed, the liquid seems to have a finite life. That means either developing an electrolyte that can last for decades, or replacing either the electrolyte or the entire module whenever the liquid degrades. A replaceable electrolyte has been developed, but designing a package that withstands the elements and can be opened and re-closed is even more difficult than designing one that withstand the elements in the first place.
Finally, if in the end the module has to be replaced every 5-10 years then its cost per watt will have to be several times lower than that of a silicon module lasting 30 years to equal silicon's total life cycle cost. The electrolyte issues are all related, of course, and solving one will probably go a long way toward solving the others.
I don't think these problems are necessarily insurmountable, but I do think work like that described in this article is a very small part of what has to happen to make DSC commercially significant.
Solar technology is moving forward so quickly!
So that is why they have not installed any on their new compounds right!!??