Plants have been using a green pigment for billions of years to capture sunlight, turning it into a flow of electrons and storing its energy in the chemical bonds of big organic molecules (also known as food). Given that successful history, chemist Michael Graetzel of the Swiss Federal Institute of Technology in Lausanne and his colleagues turned to a compound similar in shape and color to chlorophyll when they set out to build a better solar cell.
Graetzel's work could be the precursor to tinted windows that also produce electricity—an advance that could lead to entire buildings generating power, rather than just the rooftops. In a paper in the November 4 issue of Science, Graetzel and his colleagues outline how they took two big steps to making such dye-sensitized solar cells more common in the marketplace: They improved efficiency and lowered the cost of the cells.
Dye-sensitized solar cells, which are named for the colored molecules that enable them to absorb sunlight, are potentially cheaper to make than other types of cells. But researchers and manufacturers had previously employed the rare, expensive metal ruthenium in the dyes and only achieved low voltages in the cells Graetzel invented back in 1991. Graetzel's team found that a zinc-bearing compound, part of a class of molecules known as porphyrins, such as chlorophyll or the iron-bearing heme in hemoglobin that makes blood red, did the job of absorbing sunlight even better, however. Combined with another dye and used in conjunction with a cobalt-based fluid for conducting electrons, the new cell is both cheaper and more efficient.
In fact, the new cell can convert slightly more than 12 percent of the sunlight in the visible spectrum it absorbs into electrical current at a higher voltage than previous such cells. With tweaks to the dyes to help them absorb infrared light, the researchers suggest they may achieve efficiencies of 15 percent. That draws closer to solar cells made from highly purified silicon that can convert roughly 20 percent of incoming sunlight.
"[The] key benefits are light weight and flexibility as well as transparency and multicolor options for building-integrated photovoltaic glass panels," argued the team of researchers in an e-mail to Scientific American (think: color-tinted glass in windows that also produces electricity). "The new cells will be produced ultimately at significantly lower cost than conventional devices."
There are other contenders, though, for the role of photovoltaic devices built into buildings themselves. Plastic solar cells, or organic photovoltaics, do not require any liquids (the cobalt-based fluid for conducting electrons in this cell needs a highly volatile solvent) and they can be manufactured readily using existing machines. Such organic photovoltaics "are more stable and probably easier to fabricate," wrote environmental engineers Vasilis Fthenakis and Annick Anctil of Brookhaven National Laboratory, who were not involved in the research, in an e-mail to Scientific American.
Dye-sensitive cells have efficiency in their favor, however. Both plastic and dye-sensitive cells often rely on the same molecules to absorb light but, once a photon is taken in, a dye-sensitized cell converts almost all of them into electricity, unlike its competitors. Plus, such solar cells work better in weak light—like plants that thrive in the diffuse light of a cloudy day or in the shade of a forest—where they can efficiently absorb much more of the incoming sunlight than other photovoltaics. That means the dye-sensitive design may find a use where and when sunlight is not as intense. "They may not do as well at noon," says materials scientist Michael McGehee of Stanford University, but "they can perform better earlier and later in the day. For that reason, the gap in performance isn't as large as it may seem."
Interactive by Krista Fuentes. Photo of photovoltaic array at Oberlin College courtesy of Robb Williamson