The Key to Better Solar Cells: Bumpy Mirrors
Dye-sensitized thin-film solar cells are cheaper to make than conventional silicon cells, but they’re still relatively inefficient.
Now researchers at Stanford University have used a specially designed metal reflector to boost the efficiency of solid electrolyte dye-sensitized solar cells by as much as 20 percent. The reflector is a thin silver film with an array of nanoscale bumps. The researchers use the film to coat the cells’ back surface; the film helps trap more light inside the cells. “We get about 5 to 20 percent more absorption depending on the dye,” says Michael McGehee, director of the Center for Advanced Molecular Photovoltaics at Stanford. McGehee led the research, which was published online this week in the journal Advanced Energy Materials.
Dye-sensitized thin-film cells with a light-to-electricity conversion efficiency of around 11 percent recently made their commercial debut. However, they use liquid electrolytes that are volatile and could leak. Cells with solid electrolytes have only shown efficiencies of about 5 percent.
“They took the best solid-state dye cell they could, and made it better,” says David Ginger, a chemistry professor at the University of Washington, of the Stanford researchers. “Even better, they did it using technology and methods that could potentially be used in a production environment.”
Dye-based solar cells are composed of semiconductor nanocrystals (typically titanium dioxide, or titania) that are coated with dye molecules and sandwiched—along with an electrolyte—between glass or plastic sheets. The dye absorbs light and creates electrons and positively charged holes. The crystals transfer the electrons to one electrode to produce an electrical current, while the electrolyte carries the holes to the other electrode.
Solid electrolytes are not as efficient as liquid ones, though, and the electrons and holes recombine more easily. To prevent that, the titania layer is very thin—typically two micrometers. But the thinner the cells, the more quickly light passes through them without getting absorbed. Research efforts to improve the efficiency of these cells have typically focused on developing stronger dyes and new types of nanocrystals. But McGehee and his colleagues used plasmonic reflectors to improve their cell’s efficiency.
Plasmons are the oscillations of electrons at a metal surface when they are excited by light. By controlling the shape of the surface, you can control the type of plasmons created, which in turn influences how light interacts with the material.
The reflector made at Stanford has bumps that create plasmons, which turn some of the incoming light rays by 90 degrees. So instead of bouncing off the silver and going back out of the cell, more light scatters back and forth inside the cell, giving the dye a longer time to absorb it.
The researchers made their devices by coating glass with a transparent conductive electrode on which they deposited a layer of titania nanoparticles. Then they took a quartz piece covered with 600-nanometer-wide domes and pressed it into the titania, effectively embossing it with tiny holes. Finally, they added layers of dye and silver.
“This is the first time that plasmonic structures have been applied to solid-state dye-sensitized solar cells, with a substantial increase in cell efficiency being reported,” says Kylie Catchpole, a research fellow at the Australian National University. Catchpole is using light-trapping plasmonics to increase the efficiencies of other types of thin-film solar cells.
A lot of work still needs to be done before the technology makes it to market, says Martin Green, who works on light-trapping photovoltaics at the University of New South Wales. Green says that dye-sensitized cells have “attracted enormous interest from the academic community, but they have made [little] commercial impact due to low efficiencies and doubtful durability,” compared to commercial cells. Liquid electrolyte cells have forayed into the market, but Green is skeptical about their prospects as well.
McGehee, though, is confident that high-enough efficiencies will be possible. The researchers are now looking at creating reflectors with bumps of different sizes, heights, spacing, and patterns. By tweaking these factors, they should be able to increase the amount of light that the cells absorb. They could also explore different dyes. “There definitely seems to be a clear pathway to taking efficiencies up over 20 percent,” he says.
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