Kim group develops practical way to predict and modulate conjugated polymer's energy levels

Kim group develops practical way to predict and modulate conjugated polymer's energy levels

The liquid polymers in the vials toward the left absorb the higher-energy violet to green light while those toward the right capture the lower-energy yellow to red to near-infrared light.

Efficient plastic solar cells could make solar energy cost-effective, but their development has been hindered by the near-impossibility of predicting how much of the sun’s energy a polymer can turn into electricity. Now, Michigan engineers have found a shortcut.


At present, the best light-to-electricity converters are inorganic semiconductors. While the top designs can turn more than 40% of the light that hits them into electricity, their materials must be very pure, and the processing makes them too expensive for most purposes. Plastic, or organic, carbon-based solar cells stand to be much less expensive, but right now, they have relatively low efficiencies.


"Polymer solar cells are going to be cheap," said Jinsang Kim, an associate professor of materials science and engineering. "As we build a better understanding of the type of polymer used in solar cells, we can design custom-made energy conversion materials. That is why I believe the power conversion will continue to increase rapidly over the next several years."


One of the main challenges is aligning the spectrum of light that the polymer can absorb with the most intense part of the solar spectrum, visible light. Researchers are confident that this is possible because polymers are a bit like arrangements of modular furniture – there are many configurations, and it’s easy to swap one unit for another and achieve different functions. But that flexibility comes at the price of complexity. Polymers are so big that researchers can’t directly calculate what part of the spectrum they will absorb.


Fortunately, polymers are the kind of problem that can be broken into manageable chunks. They are chains of less imposing molecules, generically known as monomers. "The monomers are small molecules, so we can calculate the energy levels that determine the absorption range," said Bong-Gi Kim, a doctoral student in Jinsang Kim’s group. "But until now, we didn’t have a way to use simple calculations to predict the energy levels of the polymers they form."


These energy levels can be imagined as a ladder. The bottom rungs are filled with electrons, while the upper rungs are typically empty. Solar cell materials absorb light by promoting an electron from the highest occupied rung to the lowest empty rung, and the distance between those two rungs determines the spectrum of light that a material can absorb.


The relationship between these key energy levels in the monomers and in the resulting polymer wasn’t obvious, but Bong-Gi Kim and his colleagues found the connection by analyzing several prototype polymers. Polymers for solar cells typically combine two different monomers to tailor their absorption ranges, so the team calculated the energy levels in the constituent monomers and measured them in the polymers. Then, they compared the average lower and upper levels of the paired monomers with the measured levels in the polymer.


It turns out that when the two monomers link up in the same way, creating identical "backbone" structures across the polymer, the difference between the polymer’s energy level and average monomer energy level is always the same. This means that with a little data on polymers with the same backbone structure, researchers can now accurately predict the energy levels of a new polymer by calculating the monomers' energy levels.


The team used this strategy to predict the absorption spectrum of a polymer that had not yet been tested. When they made the polymer and measured its energy levels, those levels matched the prediction.


"The bottom line is we need chemicals – either small molecules or giant polymers – with controllable energy levels," said Jinsang Kim. "This is a really powerful and practically useful tool to design polymers for energy harvesting."


The paper describing this work is titled, "Energy Level Modulation of HOMO, LUMO, and Band-Gap in Conjugated Polymers for Organic Photovoltaic Applications" and is published online ahead of print in Advanced Functional Materials.


Jinsang Kim is also an associate professor of chemical engineering, associate professor of macromolecular science & engineering, and associate professor of biomedical engineering.