New polymer property could boost accessible solar energy

07 June 2022

Nanowork News) Lightweight as a window sticker and replicable as a newspaper, organic solar cells are emerging as a viable solution to the country’s growing energy demand.

Researchers at the University of Illinois Urbana-Champaign are the first to observe a biological property called chirality in achiral conjugated polymers used to design flexible solar cells. Their discovery could help increase the charging capacity of the cells and increase access to affordable renewable energy.

The coiled architecture of DNA is recognizable to many as a helix. Structurally, DNA and other spiral molecules are classified as chiral: asymmetric so that superposition on a mirror image is impossible. The term comes from the Greek word for hand, which is also an example. Imagine a left handprint on a sheet of paper, followed by a right handprint directly on top. The two prints are not aligned neatly; your hand, like its DNA, is chiral. Organic solar cells like the one in the photo emerge as a viable solution to meet the country’s growing energy demand. (Image: Azzaya Khasbaatar, a graduate student in the Diao Group)

From hands and feet to carbohydrates and proteins, chirality is woven into the genetic makeup of humans. It is also abundant in nature and even enhances the chemical reaction that stimulates photosynthesis.

“Chirality is a fascinating biological property,” said Ying Diao, an associate professor of chemical and biomolecular engineering and the study’s principal investigator. “The function of many biomolecules is directly linked to their chirality. Take the protein complexes involved in photosynthesis. When electrons move through the spiral structures of the proteins, an effective magnetic field is generated that helps to separate bound charges carried by light. This allows light to be converted more efficiently into biochemicals.”

For the most part, scientists have observed that molecules with similar structures tend to hold their own: chiral molecules assemble into chiral structures (like nucleic acids that make up DNA), and achiral molecules assemble into achiral structures. Diao and her colleagues saw something else. Under the right conditions, achiral conjugated polymers can deviate from the norm and assemble into chiral structures.

Their paper appears in nature communication“Chiral emergence in multistep hierarchical assembly of achiral conjugated polymers”) and introduces new avenues for research on the convergence of biology and electronics. For the first time, scientists can apply a chiral structure to the myriad of materials that achiral conjugated polymers need to function.

In particular solar cells: wafer-thin solar panels reduced to the size of a computer screen. The flexible cells are composed entirely of organic materials and are transparent and light enough to hang from a bedroom window. They are also quick to produce with solution printing, the process used to print newspapers.

“Organic solar cells can be printed at high speed and at low cost, using very little energy. Imagine if solar cells were one day as cheap as newspapers, and you could fold one up and carry it in your backpack,” Diao said.

Conjugated polymers are crucial for the development and design of the cells.

“Now that we have unlocked the potential for chiral conjugated polymers, we can apply that biological property to solar cells and other electronics, learning from how chirality enhances photosynthesis in nature. With more efficient organic solar cells that can be produced so quickly, we can potentially generating gigawatts of energy daily to catch up with rapidly increasing global energy demand,” Diao said.

But renewable energy is just one of many areas that can benefit from the combination of chirality and conjugated polymers. Diverse applications include consumer products such as batteries and smart watches, quantum computers and bio-based sensors that can detect signs of disease in the body.

“This remarkable emergence of chirality in conjugated polymers could open new avenues for applications beyond solar cells. Polarization-sensitive imaging, smart machine vision, chirality-selective catalysis, and even the engineering of new, lightweight topological mechanical metamaterials that can shield shock and minimize impact. work provides direct insight into how to enable these applications,” said Qian Chen, associate professor of materials science and engineering and co-author of this study.

To arrive at their discovery, the researchers first combined achiral conjugated polymers with a solvent. They then added the solution drop by drop to a microscope slide. As the solvent molecules evaporated and the polymers remained, the solution became more and more concentrated. Soon, the compressed achiral polymers began to self-assemble to form structures.

Molecular self-assembly is not an uncommon phenomenon. However, as the concentration of the solution increased, the researchers noted that the achiral polymers did not assemble into achiral structures as expected. Instead, they formed helixes.

“Through the lens of a microscope, we observed the twisted shape and spiral structure of the polymers. The facilities in Beckman’s Microscopy Suite made this discovery possible,” said lead author and postdoctoral researcher Kyung Sun Park.

Furthermore, the researchers found that the chiral to achiral structural evolution does not occur in a single step, but in a multi-step sequence where smaller helixes come together to form increasingly complex chiral structures.

Advanced molecular dynamics simulations helped the researchers confirm the molecular-scale steps in this sequence that cannot be seen with the naked eye.

“Molecular dynamic simulation was essential for this research. Equally important was the collaborative environment of the Beckman Institute that encouraged the merging of molecular dynamics with microscopy and chemistry,” said Diwakar Shukla, associate professor of chemical and biomolecular engineering and co-author of this study.

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