Gila Stein, assistant professor of chemical and biomolecular engineering, has won a prestigious National Science Foundation CAREER Award, valued at up to $500,000 over five years, to characterize and improve polymer-based solar cells.
Polymer cells have several attributes that could make them a viable alternative to standard silicon-based cells. They are lighter and more durable, easier to produce and have a lower raw materials cost.
Their downside, said Stein, is their ability to convert sunlight into electricity. Commercially available silicon cells hover around 20 percent efficiency, while ten percent efficiency in the field is considered the threshold for a viable product. With polymer-based solar cells, however, the highest reported efficiency is 8 percent in the lab.
The device performance is partly associated with structure of the active layer. It is important to control the interface between the polymer in a cell, which generates electrons when exposed to sunlight, and the material that receives these electrons, in Stein’s research a spherical carbon molecule known as fullerene.
Existing polymer solar cell fabrication techniques don’t control the active layer structure. One popular method, for example, consists of mixing polymer and fullerene in solution, spreading the solution out and evaporating the liquid.
“It’s very disordered and poorly controlled,” said Stein. “We’re focusing on ways to control the distribution of polymer and fullerene instead of just relying on a spontaneous process that is incredibly sensitive to processing conditions and varies substantially from one polymer to another.”
Stein’s efforts start with thin films of the polymer material. Working in the University of Houston’s Nanofabrication Facility, she will then create interconnected nanoscale features, such as dots or lines, on the surface of the polymer film. Next, she will coat the film and fill in the features with fullerene.
She will then determine how efficiently these new solar cells generate electricity, and work to optimize the most promising polymer/fullerene interfaces. Stein will use X-ray scattering techniques to determine with high precision properties such as domain size, domain distribution and interfacial area.
“This is potentially a very simple and very well controlled process. So it offers a nice system to start to figure out what’s most important,” said Stein.
This marks the third CAREER Award won by a Cullen College faculty member this month. Debora Rodrigues from civil and environmental engineering received a grant to examine the impact of graphene toxicity on the environment, while Jiming Bao from electrical and computer engineering won an award to study graphene’s ability to act as a waveguide for surface plasmon.