New Polymer Solar Cells Boost Efficiency

January 19th, 2015 by Glenn Meyers

Research targeting the development of new polymer solar cells — called PID2 — will hopefully contribute to significant increase in solar cell efficiency.

This research, taking place between the University of Chicago’s chemistry department, the Institute for Molecular Engineering, and Argonne National Laboratory, was initially reported last September.

Even with the promise of increasing overall efficiency, “The field is rather immature — it’s in the infancy stage,” said Luping Yu, professor in chemistry, fellow in the Institute for Molecular Engineering, who led the UChicago group carrying out the research.

The new polymer is a type of large molecule that forms plastics and other familiar materials. The polymer allows electrical charges to move more easily throughout the cell, boosting the production of electricity — a mechanism never before demonstrated in such devices.

News was also reported in Photonics.

“The addition of PID2 ultimately caused the polymer blend to form fibers, which improved the mobility of electrons throughout the material. These fibers served as a pathway to allow electrons to travel to the electrodes on the sides of the solar cell, according to the researchers.

“The team is continuing its study, pushing for even higher efficiencies. Reaching 10 percent would allow polymer solar cells to be viable for commercial application, they said.”

Solar cells made from polymers are a popular topic of research due to their appealing properties. But researchers are still struggling to efficiently generate electrical power with these materials.

The active regions of such solar cells are composed of a mixture of polymers that give and receive electrons to generate electrical current when exposed to light. The new polymer developed by Yu’s group, called PID2, improves the efficiency of electrical power generation by 15 percent when added to a standard polymer-fullerene mixture.

“Fullerene, a small carbon molecule, is one of the standard materials used in polymer solar cells,” Lu said. “Basically, in polymer solar cells we have a polymer as electron donor and fullerene as electron acceptor to allow charge separation.” In their work, the UChicago-Argonne researchers added another polymer into the device, resulting in solar cells with two polymers and one fullerene.

While the group reported an efficiency of 8.2% when PID2 was added, the result implies higher efficiencies could be possible with further work. Boosting efficiencies toward 10% represents a benchmark needed for polymer solar cells to be viable for commercial application.

Notably, the research team found that when PID2 was added, charges were transported more easily between polymers and throughout the cell.

In order for a current to be generated by the solar cell, electrons must be transferred from polymer to fullerene within the device. But the difference between electron energy levels for the standard polymer-fullerene is large enough that electron transfer between them is difficult. PID2 has energy levels in between the other two, and acts as an intermediary in the process..

To reveal this structure, Wei Chen of the Materials Science Division at Argonne National Laboratory and the Institute for Molecular Engineering performed X-ray scattering studies using the Advanced Photon Source at Argonne and the Advanced Light Source at Lawrence Berkeley National Laboratory.

“Without that it’s hard to get insight about the structure,” Yu said, calling the collaboration with Argonne “crucial” to the work. “That benefits us tremendously,” he said.

Chen noted that “Working together, these groups represent a confluence of the best materials and the best expertise and tools to study them to achieve progress beyond what could be achieved with independent efforts.”

“This knowledge will serve as a foundation from which to develop high-efficiency organic photovoltaic devices to meet the nation’s future energy needs,” Chen said.

Photo Credit: Solar cell image from Andrew Nelles via the University of Chicago









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