What if there was a way to create a material covered with tiny 3D solar collectors instead of the typical 2D flat photovoltaic systems (and in this context flexible PV sheets still count as two-dimensional)? And, what if you could “feed” these collectors with sunlight via optical fibers? Then you might be able to tuck these systems (architecturally speaking) into out-of-the-way locations or sites less obvious than rooftops.
That was some of the thinking motivating a group of researchers at Georgia Tech whose work is reported on in a new paper in Angewandte Chemie International.
The GT group figured out a way to improve upon existing dye-sensitized solar cell technology by growing nanostructures (on the optical fibers) that effectively increase the surface area of a collector. Compared to other approaches, DSSCs, generally speaking, are at a disadvantage because they relatively inefficient. On the other hand, the manufacturing costs of dye-sensitized cells are low. They also tend to be able to take more mechanical abuse.
The group grows the nanostructures by replacing in one section the outer layer of quartz optical fiber with a conductive coating. They then seed the surface with zinc oxide followed by solution-based techniques that grow aligned zinc oxide nanowires that radiate outward around the fiber. Finally, the nanowire–optical fiber is given a dye-sensitized materials coating. Groups of these nanowire-coated fibers are immersed in an electrolyte to harvest electrons. Length improves efficiency and the group has been able to make nanowire sections as long as 20 cm.
According the the GT group, this internal axial illumination in this hybrid system multiplies six-fold the energy conversion efficiency of the DSSC nanowire array. “In each reflection within the fiber, the light has the opportunity to interact with the nanostructures that are coated with the dye molecules,” explains Z.L. Wang, who led the group. “You have multiple light reflections within the fiber, and multiple reflections within the nanostructures. These interactions increase the likelihood that the light will interact with the dye molecules, and that increases the efficiency.”
The team says it has reached an efficiency of 3.3 percent and think efficiencies of 7 to 8 percent are in reach if they make further modifications, such as using a better method for collecting the charges and a titanium oxide surface coating.
These efficiencies are still a long way off of current 2D PV units. But Wang says there would be several advantages to the group’s hybrid DSSC system. The already low production cost could be driven lower by using polymer fibers. The optical fibers used to feed the nanowire fibers could be placed fairly freely, providing a larger area for gathering light, and lenses could also be employed to focus the incoming light.
Another advantage is that it gives building designers new options. “This will really provide some new options for photovoltaic systems,” Wang said. “We could eliminate the aesthetic issues of PV arrays on building. We can also envision PV systems for providing energy to parked vehicles, and for charging mobile military equipment where traditional arrays aren’t practical or you wouldn’t want to use them.”