New polymer property could give accessible solar power a boost

June 07, 2022

(News from Nanowerk) Light as glass and reproducible 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 at Urbana-Champaign are the first to observe a biological property called chirality emerging in achiral conjugated polymers, which are used to design flexible solar cells. Their discovery could help improve cell load capacity and increase access to affordable renewable energy.

The coiled architecture of DNA is recognizable to many as a helix. Structurally speaking, DNA and other helical molecules are classified as chiral: asymmetric so that superimposition 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 do not line up perfectly; your hand, like its DNA, is chiral. Organic solar cells like the one pictured are emerging as a viable solution to meet the country’s growing energy demand. (Image: Diao Group graduate student Azzaya Khasbaatar)

From hands and feet to carbohydrates and proteins, chirality is twisted in the genetic make-up of humans. It is also abundant in nature and even enhances the chemical reaction that results in photosynthesis.

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

For the most part, scientists have observed that molecules with similar structures tend to stick together: chiral molecules assemble into chiral structures (like nucleic acids forming DNA) and achiral molecules assemble into achiral structures . Diao and his colleagues observed something different. Under the right conditions, achiral conjugated polymers can deviate from the norm and assemble into chiral structures.

Their article appears in Nature Communication (“Chiral Emergence in Hierarchical Multi-Step Assembly of Achiral Conjugated Polymers”) and introduces new research opportunities to the convergence of biology and electronics. For the first time, scientists can apply a chiral structure to the myriad of materials that require achiral conjugated polymers to function.

In particular, solar cells: very thin solar panels reduced to the size of a computer screen. Composed entirely of organic materials, the flexible cells are transparent and light enough to hang on a bedroom window. They can also be made quickly with solution printing, the process used to print newspapers.

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

Conjugated polymers are crucial for cell development and design.

“Now that we’ve unlocked the potential of chiral conjugated polymers, we can apply this biological property to solar cells and other electronic devices, learning how chirality enhances photosynthesis in nature. With more efficient organic solar cells that can be fabricated so quickly, we can potentially generate gigawatts of energy daily to catch up with rapidly increasing global energy demand,” Diao said.

But renewable energy is just one of many areas to benefit from the union of chirality and conjugated polymers. Various applications could include consumer products such as batteries and smart watches, quantum computing, and bio-based sensors capable of detecting signs of disease in the body.

“This remarkable emergence of chirality in conjugated polymers could open up new avenues of applications beyond solar cells. Polarization-sensitive imaging, intelligent machine vision, selective chirality catalysis, and even the engineering of new lightweight topological mechanical metamaterials that can protect against shock and minimize impact. Our work provides direct insight into how to realize 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, leaving behind the polymers, 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 unusual phenomenon. However, as the concentration of the solution increased, the researchers observed that the achiral polymers did not assemble into achiral structures as expected. Instead, they formed helices.

“Through the lens of a microscope, we observed the twisted shape and helical structure of the polymers. Beckman’s microscopy suite facilities helped make this discovery possible,” said lead author and postdoctoral researcher Kyung Sun Park.

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

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

“The simulation of molecular dynamics has been instrumental in this research. Equally important was the collaborative environment at the Beckman Institute which encouraged the fusion 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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