Rice University scientists have invented a novel cathode that may make cheap, flexible dye-sensitized solar cells practical.
The Rice lab of materials scientist Jun Lou created the new cathode, one of the two electrodes in batteries, from nanotubes that are seamlessly bonded to graphene and replaces the expensive and brittle platinum-based materials often used in earlier versions.
A sample of the graphene/nanotube hybrid grown on a flexible nickel substrate. Photo by Jeff Fitlow
Dye-sensitized solar cells have been in development since 1988 and have been the subject of countless high school chemistry class experiments. They employ cheap organic dyes, drawn from the likes of raspberries, which cover conductive titanium dioxide particles. The dyes absorb photons and produce electrons that flow out of the cell for use; a return line completes the circuit to the cathode that combines with an iodine-based electrolyte to refresh the dye.
While they are not nearly as efficient as silicon-based solar cells in collecting sunlight and transforming it into electricity, dye-sensitized solar cells have advantages for many applications, according to co-lead author Pei Dong, a postdoctoral researcher in Lou’s lab.
“The first is that they’re low-cost, because they can be fabricated in a normal area,” Dong said. “There’s no need for a clean room. They’re semi-transparent, so they can be applied to glass, and they can be used in dim light; they will even work on a cloudy day.
“Or indoors,” Lou said. “One company commercializing dye-sensitized cells is embedding them in computer keyboards and mice so you never have to install batteries. Normal room light is sufficient to keep them alive.”
Dye-sensitized solar cells (DSSCs) are an emerging photovoltaic technology with both low costs and good efficiency. However, the cathode used in most DSSCs is fluorine-doped tin oxide glass coated with a Pt film, which is both expensive and brittle and therefore limits the flexibility and large-scale implementation of this promising technology. We report here work which showed that flexible, seamlessly covalently bonded, three-dimensional vertically aligned few-walled carbon nanotubes (VAFWCNTs)/graphene on metal foil can act as a novel cathode free from transparent conducting oxide and Pt for application in DSSCs. This cathode has a lower charge transfer resistance and lower contact resistance between the catalyst and the substrate than the conventional combination in a brittle Pt/fluorine-doped tin oxide cathode. The covalently bonded graphene and VAFWCNTs ensure excellent electron transport through the electrode and the large surface area of the hybrid carbon materials rivals the catalytic capability of the Pt analogue. DSSCs utilizing this flexible VAFWCNTs/graphene hybrid cathode outperformed the Pt-based cells in both rigid (8.2% vs. 6.4%) and flexible (3.9% vs. 3.4%) assemblies. The VAFWCNTs/graphene on metal foil combination is a novel, inexpensive, high-performance, flexible cathode for application in solar cells.