Better Plastic Solar Cells
Improved dyes and electrolytes could make the Grätzel solar-cell design more practical.
Dye-sensitized solar cells, sometimes called Grätzel cells after their inventor, Michael Grätzel, a chemistry professor at the École Polytechnique Fédérale de Lausanne, in Switzerland, have long been considered a promising technology for reducing the cost of solar power. They’re potentially cheaper to make than conventional solar cells and can be quickly printed. But this potential hasn’t been realized because to achieve efficiency levels high enough to compete with conventional solar cells–about 10 percent–it’s been necessary to use volatile electrolytes that need to be carefully sealed inside the cells, an expensive and unreliable step in the manufacturing.
Now Grätzel, along with Peng Wang, a professor at the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, have made efficient solar cells that use nonvolatile electrolytes, with the best achieving efficiencies of 10 percent. They also showed that the solar cells remained stable when exposed to light and high temperatures for 1,000 hours. The advance “pushes the technology close to over the ‘10 percent hump,’ which is where a thin-film technology needs to be to be economically competitive,” says Tonio Buonassisi, a professor of mechanical engineering at MIT.
One of the electrolytes is something called an ionic fluid–a fluid largely made up of ions and often composed of salts that have low melting temperatures. An ionic fluid can be used with plastic electrodes, which would allow for solar cells that are both efficient and flexible, and therefore could be incorporated into clothing, awnings, and covers for cars. “We never dreamt that we could have efficiencies of 9 or 10 percent with ionic liquids,” Grätzel says. “Ten years ago, we had 1 percent efficiency, and we never thought it would get any better.”
The new solar cells were made possible by advances first published this summer. In that work, the researchers increased the conductivity of electrolytes based on ionic fluids and produced solar cells that were 8.2 percent efficient. In the current work, published last month in the Journal of Physical Chemistry, the researchers further increased the efficiency by pairing the ionic liquid electrolyte with a new dye, the part of the dye-sensitized solar cell that absorbs sunlight. The new dye absorbs light far better than the conventional dye. Because the dye absorbs light so well, it’s possible to cut the thickness of the active material in the solar cell in half, which makes it easier for electrons to escape the solar cell and reach an external circuit. That, in turn, increases efficiency, in this case to 9.1 percent.
The researchers also paired the new dye with a nonvolatile solvent-based electrolyte. It’s not quite as stable as an ionic liquid, and it can’t be used with plastic. But it allowed slightly higher efficiencies–up to 10 percent.
Grätzel is working with two companies to commercialize this technology. One, G24 Innovations, based in Cardiff, U.K., is planning to sell dye-sensitized solar cells for applications such as recharging cell phones, especially in countries with unreliable electricity. Another company, Dyesol, based in Queanbeyan, Australia, is planning to sell solar cells that can double as the facades on buildings. Both companies have already developed dye-sensitized solar cells based on earlier technology, but the recent advances could make the cells cheaper and significantly improve performance.
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