The Advanced Photon Source
a U.S. Department of Energy Office of Science User Facility

Improving Solar Cell Efficiency


New light has been shed on solar power generation using devices made with polymers, thanks to collaboration between scientists in the University of Chicago’s chemistry department, the Institute for Molecular Engineering, and Argonne National Laboratory carrying out research at the U.S. Department of Energy’s Advanced Photon Source (APS).

The researchers identified a new polymer—a type of large molecule that forms plastics and other familiar materials—that improved the efficiency of solar cells. The group also determined the method by which the polymer improved the cells’ efficiency. 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.

“Polymer solar cells have great potential to provide low-cost, lightweight and flexible electronic devices to harvest solar energy,” said Luyao Lu, University of Chicago graduate student in chemistry and lead author of a paper describing the result, published online in the journal Nature Photonics.

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. A new polymer was developed by the group under Luping Yu, professor in chemistry and fellow in the Institute for Molecular Engineering, who led the University of Chicago group carrying out the research. It is called “PID2,” and improves the efficiency of electrical power generation by 15% 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 University of Chicago-Argonne researchers added another polymer into the device, resulting in solar cells with two polymers and one fullerene.

The group achieved an efficiency of 8.2% when an optimal amount of PID2 was added—the highest ever for solar cells made up of two types of polymers with fullerene—and the result implies that even higher efficiencies could be possible with further work. The group is now working to push efficiencies toward 10%, a benchmark necessary for polymer solar cells to be viable for commercial application.

The result was remarkable not only because of the advance in technical capabilities, but also because PID2 enhanced the efficiency via a new method. The standard mechanism for improving efficiency with a third polymer is by increasing the absorption of light in the device. But in addition to that effect, the 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.

“It’s like a step,” Yu said. “When it’s too high, it’s hard to climb up, but if you put in the middle another step then you can easily walk up.”

Thanks to the collaboration with Argonne, Yu and his group were also able to study the changes in structure of the polymer blend when PID2 was added, and show that these changes likewise improved the ability of charges to move throughout the cell, further improving the efficiency. The addition of PID2 caused the polymer blend to form fibers, which improve the mobility of electrons throughout the material. The fibers serve as a pathway to allow electrons to travel to the electrodes on the sides of the solar cell.

“It’s like you’re generating a street and somebody that’s traveling along the street can find a way to go from this end to another,” Yu said.

To reveal this structure, Wei Chen of the Materials Science Division at Argonne and the Institute for Molecular Engineering performed two-dimensional grazing incidence wide-angle x-ray scattering studies using the X-ray Science Division 8-ID-E x-ray beamline at the Argonne APS, and resonant soft x-ray scattering studies at the Lawrence Berkeley National Laboratory Advanced Light Source (ALS) beamline at (both the APS and the ALS are Office of Science user facilities).

“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.

The original University of Chicago version of this article, by Emily Conover, can be read here.

See: Luyao Lu1, Tao Xu1, Wei Chen1,2, Erik S. Landry1,2, and Luping Yu1*, “Ternary blend polymer solar cells with enhanced power conversion efficiency,” Nat. Photonics 8, 716 (2014). DOI: 10.1038/nphoton.2014.172

Author affiliations: 1The University of Chicago, 2Argonne National Laboratory

Correspondence: *

This work is supported by the US National Science Foundation (NSF, grant no. NSF CHE-1229089, DMR-1263006), the Air Force Office of Scientific Research and NSF MRSEC programme at the University of Chicago, the U.S. Department of Energy (DOE) via the ANSER Center, an Energy Frontier Research Center funded by the U.S. DOE Office of Science-Basic Energy Sciences (award no. DE-SC0001059). W.C. acknowledges financial support from the U.S. DOE Office of Science-Basic Energy Sciences (award no. KC020301). The ALS at Lawrence Berkeley National Laboratory is supported by the Director, Office of Science-Basic Energy Sciences, of the U.S. Department of Energy (contract no. DE-AC02-05CH11231). This research used resources of the Advanced Photon Source, a U.S. Department of Energy Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

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