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.