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

6 Comments

1. 1. sault 10:19 PM 11/3/11

   12 percent efficiency is impressive and I hope Graetzel continues the progress he's been doing for a long time now. However, dye-sensitized solar cells also have a problem with stability in direct sunlight and I haven't heard of ANY of these cells coming close to the 20 - 30 year lifetime of semiconductor solar cells.
   
   As First Solar proved with their CdTe solar cells, it's not the efficiency of your solar cell per se that wins over buyers, it's your $/watt and $/kWhr over the life of the solar array. A solar cell that lasts 5 years would have to be 1/6 the cost or make 6x the power of a 30-year cell to be competitive. Even then, you would STILL have to replace the panels 6x as much, which would be a real hassle.
   
   The life expectancy of these cells wasn't mentioned, but I hope these new materials are also more robust than previous dye-sensitized solar cells. These materials would be easier to double as roofing material or building facade, thereby improving their cost metrics even further.

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2. 2. dbiello in reply to sault 09:14 AM 11/4/11

   That is exactly right, sault. Stability, sadly, was only slightly improved (particularly when you take the solvent into account, which essentially nixes these practically speaking, though one would assume a less volatile version could be found). Still, it's ironic that most solar cells don't fare too well in either high heat or direct sunlight (or both). Physics giveth, physics taketh away.

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3. 3. jack.123 06:47 PM 11/7/11

   Here we go again,with another solar cell that doesn't approach even half the full spectrum.Why waste more money something that will be obsolete in few short years,while only gaining a few percent in efficiency?

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4. 4. 13inches 12:17 AM 11/8/11

   jack.123 - think 'baby steps' ... as efficiency percentages increase and costs decrease, solar panels will eventually achieve 100% efficiency and only cost $1...each tiny step on this long path is important.

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5. 5. sault in reply to 13inches 01:15 AM 11/8/11

   Well, the theoretical maximum efficiency of semiconductor cells is around 66% but you'd need an INFINITE amount of junctions to grab each tiny wavelength of light. I do disagree with Jack's disregard for technical progress however. The first solar cells in the late 50s were nowhere near practical for industrial power generation. 60 years of development (and around 10 of actual widespread deployment) and you have solar cells that are less than 1% of the cost of the first space solar cells.
   
   Conversely, after 60 years of trying to develop nuclear power, you have reactors that are more than DOUBLE the cost now (inflation adjusted) than those built in the 1960s! Just throwing out some trends there.

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6. 6. bucketofsquid 03:48 PM 11/9/11

   Can a semiconductor solar cell be dyed? It doesn't sound like it can but if it is possible will it improve output?

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