It was announced recently that engineers at Oregon State University (OSU) have discovered a way to fabricate solar cells that may be up to three times as efficient as conventional solar cells. Amazingly, these new solar cells are made using completely inert organic materials and may be simpler to build than existing cells of their type. Yet even more astonishing though, is that the key to the breakthrough came in the form of tiny algae known as diatoms.
Fossil records indicate diatoms have existed since at least the early Jurassic period. They are producers in the food chain, manufacturing complex organic compounds from simple molecules, and are commonly used by environmental scientists to monitor water quality and environmental conditions. However, it is their characteristic cell wall, a rigid silica shell, and their tiny size that has allowed engineers to use diatoms to form structures at the nanometer scale.
It has been known for some time that the efficiency of solar cells produced using conventional semiconductor production processes would peak. "Most existing solar cell technology is based on silicon and is nearing the limits of what we may be able to accomplish with that," said Greg Rorrer, a professor of chemical engineering at OSU. So the search is on to find new and improved manufacturing methods for solar cells. "There's an enormous opportunity to develop different types of solar energy technology, and it's likely that several forms will ultimately all find uses, depending on the situation," Rorrer says.
The new diatom based process focuses on producing dye-sensitized solar cells, a type of thin film cell in which photons bounce around inside the cell striking photosensitive dyes, which are mere nanometers in size, to produce electrical charge. The difference with the cells produced using the OSU process is made possible because of the unique natural structure of the diatom shell. The diatom shell, or frustule, consists of two asymmetrical sides with a split between them and contains many naturally occurring nanometer sized pores throughout.
In the initial phase of the process the diatoms are placed on a conductive glass surface. The organic components of the diatoms are then removed leaving only the silica shells which form a template for the semiconductor material (titanium dioxide.) In the next step a biological agent is used to precipitate soluble titanium into nanoparticles of titanium dioxide. Once applied to the template the titanium dioxide (TiO2) creates a thin film that acts as the semiconductor for the dye-sensitized solar cell. "Conventional thin-film, photo-synthesizing dyes also take photons from sunlight and transfer it to titanium dioxide, creating electricity," Rorrer says. "But in this system the photons bounce around more inside the pores of the diatom shell, making it more efficient."
Dye-sensitized solar cells have existed for some time and have been demonstrated to work well in lower light conditions. Invented by Michael Grätzel and Brian O'Regan at the École Polytechnique Fédérale de Lausanne in 1991 dye-sensitised solar cells are based on a semiconductor formed between a photo-sensitized anode and an electrolyte.
However, the OSU process is comparatively simple compared to conventional fabrication techniques and uses simple inexpensive and readily available materials. "What's different in our approach are the steps we take to make these devices, and the potential improvements they offer," Rorrer says. Indeed it's thought that the new process may produce solar cells that are three times more efficient than existing dye-sensitised solar cells.
Rorrer says that it's not understood exactly why the new solar cells are so efficient. However, it appears that the nanometre pores that pervade the diatom shell are the key. The pores seem to enhance the interaction between the photons and the dye and increase the conversion rate of light to electricity within the cell. Whatever is actually happening there is no doubt that the process actually works and unlike traditional fabrication techniques this ground-breaking biofabrication process is simple, environmentally benign and takes place at room temperature.
So it seems that Mother Nature has a trick or two to teach mankind yet, but then she has been around for much longer than we have. Perhaps in the future it's not going to be the energy hungry, pollutant rich, industrial fabrication processes we've seen thus far that will yield the technologies to solve our energy requirements. Instead, our salvation may come in the form of a humble algae that has managed to survive for more than 100 million years.
Other cool stuff on Celsias:
"Green Algae Strategy: End Oil Imports and Engineer Sustainable Food and Fuel"
Talking Algae Biofuel with Solazyme and Aquaflow
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