Easy-to-make solar cells that capture light with dyes have garnered an impressive string of scientific awards, including the Millennium Technology Prize in 2010. Yet they’ve had little commercial impact since their invention in 1988.
A novel design reported by Northwestern University researchers last week could change that, delivering a device that eliminates the dye-sensitized solar cell’s inherent liability: its leak-prone and corrosive liquid electrolyte.
Unlike thin-film and silicon panels, dye-based panels can be produced in cheap roll-to-roll processes akin to printing. So even if they are less efficient than silicon solar cells, they could prove cost-effective.
The Northwestern development is just the latest in a string of advances in what Michael McGehee, director of Stanford University’s Center for Advanced Molecular Photovoltaics, recently dubbed a “renaissance” in dye-sensitized cells. Recent advances in the field could finally transform these elegant scientific curiosities into practical energy-generation devices.
In a dye-sensitized solar cell, incoming light excites a porous layer of titania coated with a dye, generating negative and positive charges. The negative charges—excited electrons—flow out of the cell through the titania, while positive charges flow into a liquid electrolyte. As with electrolyte-filled alkaline batteries, leakage is an ever-present danger, especially in solar panels subject to extreme weathering. Electrolytes heated to 80° C (on a rooftop, for instance) can expand and rupture the panel’s seal. The dye cells’ iodine-based electrolyte is also corrosive enough to eat through even rust-resistant metals such as aluminum and stainless steel.
Northwestern University chemist Mercouri Kanatzidis, materials scientist Robert Chang, and two graduate students replaced the dye cells’ liquid electrolyte with a solid iodine-based semiconductor. While prior solid-state designs have reduced the power output of dye cells, the Northwestern design actually boosts performance, the researchers say, because the cesium-tin-iodine semiconductor that replaces the liquid electrolyte also absorbs light. “Our material actually absorbs more light than the dye itself,” says Kanatzidis.
In a report in Nature last week, the Northwestern team claims its cell converts 10.2 percent of incoming light to electricity—far higher than the 7 percent efficiency of the best existing solid-state dye cells. Sean Shaheen, an expert in organic photovoltaics at Denver University, says that the Northwestern cell’s efficiency would be closer to 8 percent under standard measurement conditions. But Shaheen says it is still an important development for dye cells.
Kanatzidis says it could be possible to commercialize the design if the efficiency of the cells can be pushed above 11 percent. That is lower than the 12 to 16 percent efficiency of commercial thin-film solar panels and far below the efficiency of silicon panels. But the cost of manufacturing dye-based cells should also be lower.
Australian solar developer Dyesol is seeking to exploit low-cost processing to commercialize conventional dye-solar technology—liquid electrolyte included. Its strategy is to integrate dye-based solar into building materials such as glass high-rise panels and steel roofing sheets. This March, Dyesol’s South Korean joint venture partner, Timo Technology, installed glass panels on a building in Seoul. And Dyesol is partnering with India’s Tata Steel to develop dye-solar-coated steel roofing.
Damion Milliken, Dyesol’s research and development manager, insists that liquid electrolytes can be contained. “Dyesol and others have produced devices with excellent long-term stability which have been subjected to accelerated testing equivalent to 25 years’ life and beyond,” Milliken says. “The technology is commercially viable.”