New polymer improves solar cell efficiency

A US research collaboration has developed a new polymer which, when used added to a solar cell, improves the efficiency of that cell. The study has been published in the journal Nature Photonics.

The group comprised researchers from the University of Chicago, the university’s Institute for Molecular Engineering and Argonne National Laboratory, led by Professor Luping Yu. According to lead author Luyao Lu, “Polymer solar cells have great potential to provide low-cost, lightweight and flexible electronic devices to harvest solar energy” - but researchers are still struggling to efficiently generate electrical power with these materials.

The active regions of polymer solar cells are composed of a mixture of polymers that give and receive electrons to generate electrical current when exposed to light. Lu explained that the polymer acts as the electron donor, while fullerene, a small carbon molecule, is the electron acceptor, which allows charge separation.

The group developed a new polymer, called PID2, which was found to improve the efficiency of electrical power generation by 15% when added to a standard polymer-fullerene mixture. This resulted in an overall efficiency of 8.22% - the highest ever for solar cells made up of two types of polymers with fullerene.

This solar cell consists of a new polymer, called PID2, which was developed in the laboratory of Luping Yu. Photo by Andrew Nelles.

Not only did the third polymer increase the absorption of light in the device, but the PID2 additionally enabled electrical charges to be transported more easily between polymers and throughout the cell. The difference between electron energy levels for a standard polymer-fullerene cell is large enough that electron transfer between them is difficult, but PID2 has energy levels in between them and acts as an intermediary in the process.

“It’s like a step,” said Yu. “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.”

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. The structure was revealed through X-ray scattering studies performed by Wei Chen of Argonne National Laboratory and the Institute for Molecular Engineering.

The addition of PID2 caused the polymer blend to form fibres, which improve the mobility of electrons throughout the material. The fibres serve as a pathway to allow electrons to travel to the electrodes on the sides of the solar cell. As explained by Yu, “It’s like you’re generating a street and somebody that’s travelling along the street can find a way to go from this end to another.”

The group is now working to push efficiencies towards 10% - a benchmark necessary for polymer solar cells to be viable for commercial application.

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