Dye-sensitized solar cells could make solar power more affordable: they are cheaper to make than conventional silicon solar cells and can easily be printed on flexible surfaces. But there’s a catch: creating efficient cells of this type has required dyes made of the precious metal ruthenium and volatile electrolytes. Now researchers at the Chinese Academy of Sciences have replaced both of these materials in a new kind of dye-sensitized solar cell that is not only highly efficient: it also promises to be even cheaper and more durable.
The key to the advance is a new organic dye molecule designed by chemistry professor Peng Wang and his colleagues. Organic dyes reduce the cost of making the cells because they are more abundant and cheaper to obtain than ruthenium compounds. The researchers also use a different type of electrolyte called an ionic liquid. This produces a more robust solar cell: the electrolytes that are currently used contain organic solvents that can evaporate and leak out at high temperatures. The ionic liquid can also be used with plastic, opening up the possibility of flexible solar cells. “We demonstrated for the first time that an all-organic dye can be employed to make stable, solvent-free cells exhibiting a high efficiency comparable to ruthenium dyes,” Wang says.
The researchers set a new efficiency record for cells incorporating organic dye molecules. To compete with conventional solar cells, dye-sensitized ones need to be at least 10 percent efficient at converting light into electricity. Wang and his colleagues achieved 9.8 percent efficiency with the new organic dyes. “When you get up to high efficiencies, small increases matter,” says Michael Grätzel, a chemistry professor at the École Polytechnique Fédérale de Lausanne, in Switzerland, who invented dye-sensitized solar cells. He says that it’s exciting to see researchers “getting so close to 10 percent with organic dyes, which is a magic number.”
When the researchers pair the organic dye with an ionic fluid, the efficiency drops to 8.1 percent. But this is still a significant advance, Grätzel says. He published work last year showing similar cells that were 7.2 percent efficient. But while pairing nonvolatile electrolytes with ruthenium dyes, he has achieved 10 percent. “We thought we would never get more than 1 percent 10 years ago,” he says.
In a dye-sensitized solar cell, dye-coated semiconductor nanoparticles are sandwiched, along with the electrolyte, between two glass plates. The dye molecules absorb light and generate electrons, which are transferred to the semiconductor and on to the external circuit. Meanwhile, positively charged holes go to the electrolyte. The big problem with organic dyes in the past has been the difficulty of keeping the charges separated: they tend to recombine and lower currents.
The dye molecule that Wang and his colleagues designed increases the cell’s efficiency in three ways: it quickly shuttles electrons to the semiconductor particles; it keeps the electrons and holes from recombining; and it covers a broader spectrum of light, absorbing more red light than was possible before.
At least two companies are commercializing dye-sensitized solar cells. These devices are up to 11 percent efficient and use ruthenium dyes and a volatile electrolyte. In October 2008, Dyesol opened a factory in Queanbeyan, Australia, to make tiles that can be integrated into building facades. G24 Innovations, in Cardiff, U.K., is making solar chargers for mobile phones.
Wang says that his new work, published online in the journal Chemical Communications, could make the technology cheaper and open up broader applications. “At the moment, the use of toxic and volatile solvents in high-efficiency cells is a big hurdle for the large-scale application of dye-sensitized solar cells,” he says.
The researchers are working to boost efficiency even more. Wang says that will involve “mainly extending the spectral response of sensitizers to the infrared and the design of better solvent-free electrolytes.”
Meanwhile, Grätzel and his colleagues at Lausanne have set themselves a lofty goal. They plan to reach efficiencies of 14 percent by the end of next year using nonvolatile electrolytes. As for the dye, either ruthenium-based dyes or organic dyes could win the race, he says. And he is optimistic about cheaper organic dyes. “If you see how efficiencies of both have gone up, the slope is steeper for organic molecules,” Grätzel says. “If you extrapolate, it could be a year or so before they overtake.”