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.