Solid Advance for Cheap Solar Cells

The price of solar cells has been gliding downward for decades. Now this trend could get a shove from an improvement to a more than 20-year-old solar technology that captures light with dye molecules, an approach that's never managed to catch on.

The advance is "one of the most important breakthroughs in dye cells in the last several years," says Thomas Mallouk, a chemist at Pennsylvania State University, University Park, who was not involved in the study.

Eighty percent of the market for solar cells is taken up by cells made from crystalline silicon wafers, which convert about 20% of the energy in incoming sunlight into electricity. Most of the rest of the market consists of "thin film" cells made from different semiconducting alloys that can be cheaper to produce but require toxic or rare elements.

A third class of solar cells, first developed in 1991 by researchers in Switzerland, are the cheapest to make and are more than 12% efficient. These cells, known as dye-sensitized solar cells (DSSCs), consist of millions of tightly packed titanium dioxide nanoparticles, each coated by a single layer of dye molecules. The titanium dioxide-dye combo is then bathed in an electrically conductive liquid containing mobile ions called an electrolyte.

When photons of light strike these cells, they energize electrons in the dye. These energized electrons immediately hop to the titanium dioxide particles and then to an electrode, where they enter an electrical circuit to provide power. That leaves electron vacancies in the dye molecules, which are filled by less-energized electrons from the electrolyte, which in turn are replenished by electrons from a counter electrode.

Conventional liquid electrolytes are good at giving up their electrons to the dye, but the liquids have other problems. They often leak out of the devices and typically contain compounds that corrode the metal electrodes. To get around these difficulties, researchers have developed solid electrolytes that don't leak or damage other components. Unfortunately, these materials tend to be poor electrical conductors, a limitation that has kept their efficiency to an unremarkable 6%.

Researchers from Northwestern University in Evanston, Illinois, thought they could do better. Chemist Mercouri Kanatzidis knew that scientists at IBM and elsewhere had been developing good solid electrical semiconductors for years. So he teamed up with Northwestern colleague Robert P. H. Chang, a materials scientist, to try one of these compounds, a fluorine-spiked mixture of cesium, tin, and iodine. They dissolved the compound in an organic solvent and poured it over the dye-coated titanium dioxide particles in a DSSC. They then let the solvent evaporate, allowing the cesium-tin-iodine combination to crystallize. As the researchers report online today in Nature, the efficiency of their device was above 10%, often seen as a minimum benchmark for commercial success.

Mallouk cautions that the new work isn't likely to transform the market for solar cells overnight. But the new solid dye cells are a sort of hybrid between conventional solar cells and liquid dye cells. And that, he says, could open new avenues for making solar devices that are cheap, efficient, and durable.