Improved Solar Cell Efficiency with New Polymer

PolymerSolarNew 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.

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, graduate student in chemistry and lead author of a paper describing the result, published online last month 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 field is rather immature—it’s in the infancy stage,” said Luping Yu, professor in chemistry and fellow in the Institute for Molecular Engineering, who led the UChicago group carrying out the research.

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% 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.


Luyao Lu, a graduate student in chemistry, works in the solar cell characterization facility of the University of Chicago’s Gordon Center for Integrative Science. Lu is the lead author of a Nature Photonics article describing the development of a new type of polymer solar cell that displays enhanced power conversion efficiency. Image: Andrew NellesLuyao Lu, a graduate student in chemistry, works in the solar cell characterization facility of the University of Chicago’s Gordon Center for Integrative Science. Lu is the lead author of a Nature Photonics article describing the development of a new type of polymer solar cell that displays enhanced power conversion efficiency. Image: Andrew Nelles

8.2% efficiency

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, Yu noted, 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 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 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.

Ternary blend polymer solar cells with enhanced power conversion efficiency

Source: Univ. of Chicago


A Better Low-Cost Organic Solar Cell – Breaking “electrode barrier”

ElectrodeBarrierFor decades, polymer scientists and synthetic chemists working to improve the power conversion efficiency of organic solar cells were hampered by the inherent drawbacks of commonly used metal electrodes, including their instability and susceptibility to oxidation. Now for the first time, researchers at the University of Massachusetts Amherst have developed a more efficient, easily processable and lightweight solar cell that can use virtually any metal for the electrode, effectively breaking the “electrode barrier.”

This barrier has been a big problem for a long time, says UMass Amherst’s Thomas Russell, professor of polymer science and engineering. “The sun produces 7,000 times more energy per day than we can use, but we can’t harness it well. One reason is the trade-off between oxidative stability and the work function of the metal cathode.” Work function relates to the level of difficulty electrons face as they transfer from the solar cell’s photoactive layer to the electrode delivering power to a device.


Russell likes to use a lock-and-dam analogy to talk about electron transfer. “People have thought you’d need to use tricks to help electrons, the water in the lock, over an obstacle, the electrode, like a dam. Tricks like sawing the dam apart to allow the flow. But tricks are always messy, introducing a lot of stuff you don’t need,” he says. “The beauty of the solution reached by these synthetic chemists is to just move the dam out of the way, electronically move it so there is no longer a difference in energy level.”

Synthetic chemist and polymer science professor Todd Emrick agrees, “That challenge was unmet and that’s what this research is all about.” He and polymer chemistry doctoral student Zak Page in his lab had been synthesizing new polymers with zwitterions on them, applying them to several different polymer scaffolds in conjugated systems, also known as semiconductors, in the inter-layer of solar cells. Zwitterions are neutral molecules with both a positive and negative charge that also have strong dipoles that interact strongly with metal electrodes, the scientists found.

Emrick asked Page to see if he could synthesize conjugated polymers, semiconductors, with zwitterionic functionality. With time, and by enlisting a system of multiple solvents including water, Page was able to prepare these new “conjugated polymer zwitterions,” or CPZs.

Emrick explains, “Once we could make CPZs, we were able to incorporate any conjugated backbone we wanted with zwitterionic functionality. That allowed us to make a library of CPZs and look at their structure-property relationship to understand which would be most important in electronics. In particular, we were interested in electron transport efficiency and how well the CPZ could modify the work function of different metals to help move electrons across interfaces towards more powerful devices.

In choosing a metal for use as an electrode, scientists must always negotiate a trade-off, Page says. More stable metals that don’t degrade in the presence of water and oxygen have high work function, not allowing good electron transport. But metals with lower work function (easier electron transport) are not stable and over time will degrade, becoming less conductive.

Guided by UMass Amherst’s photovoltaic facility director Volodimyr Duzhko in using ultraviolet photoelectron spectroscopy (UPS), Page began to categorize several metals including copper, silver and gold, to identify exactly what aided electron transport from the photoactive layer to the electrode. He and Emrick found that “if you want to improve the interlayer properties, you have to make the interface layer extremely thin, less than 5 nanometers, which from a manufacturing standpoint is a problem,” he says.

To get around this, Page and Emrick began to consider a classic system known for its good electron transport: buckyballs, or fullerenes, often used in the photoactive layer of solar cells. “We modified buckyballs with zwitterions (C60-SB) to change the work function of the electrodes, and we knew how to do that because we had already done it with polymers,” Page points out. “We learned how to incorporate zwitterion functionality into a buckyball as efficiently as possible, in three simple steps.”

Here the synthetic chemists turned to Russell’s postdoctoral researcher Yao Liu, giving him two different fullerene layers to test for electron transfer efficiency: C60-SB and another with amine components, C60-N. From UPS analysis of the zwitterion fullerene precursor, Page suspected that the amine type would enhance power even better than the C60-SB variety. Indeed, Liu found that a thin layer of C60-N between the solar cell’s photoactive layer and the electrode worked best, and the layer did not have to be ultra-thin to function effectively, giving this discovery practical advantages.

“That’s when we knew we had something special,” says Page. Emrick adds, “This is really a sweeping change in our ability to move electrons across dissimilar materials. What Zak did is to make polymers and fullerenes that change the qualities of the metals they contact, that change their electronic properties, which in turn transforms them from inefficient to more efficient devices than had been made before.”

Russell adds, “Their solution is elegant, their thinking is elegant and it’s really easy and clean. You put this little layer on there, it doesn’t matter what you put on top, you can use robust metals that don’t oxidize. I think it’s going to be very important to a lot of different scientific communities.”

Source: Univ. of Massachusetts Amherst