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New Method Compares the Performances of Organic Materials for Bioelectronics and Energy Storage

Northwestern Engineering’s Jonathan Rivnay has observed a swell in the development of new organic mixed conductors, referring to polymer materials capable of transporting both ions and electrons, over the past five years.

More flexible, lighter and easier to process than their inorganic counterparts, the carbon-based materials prove to be promising in a wide range of applications, starting from energy storage to medical devices. However, with innovation and increased productivity comes a perhaps unexpected problem.

Jonathan Rivnay Credit: Northwestern Engineering

It can be challenging and time consuming to take new materials, put them on a device, and record their performance. But even more challenging is to properly compare the performance of these new materials to each other because there hasn’t been an established benchmarking method.

Jonathan Rivnay, Assistant Professor, Biomedical Engineering, McCormick School of Engineering, Northwestern

This void has now been filled by Rivnay and his team. Rivnay and his team, in order to help researchers pinpoint the best organic mixed conductors for particular applications, have developed a unique framework to target and then compare their performances. This new method allows for the comparison of the presently available materials, and it can also be used for informing the design of new organic materials.

The research has been published online in the Friday, November 24th, issue of Nature Communications. The paper’s corresponding author is Rivnay. Sahika Inal, assistant professor of bioscience at King Abdullah University of Science and Technology, worked as the paper’s first author.

Organic conductors are soft materials capable of conducting electricity. They prove to be promising in lightweight, inexpensive, flexible technologies, including printable electronic circuits, solar cells and organic light-emitting diodes. Very recently, their potential to interact closely with biomolecules and ions has resulted in a major interest in bio-integrated electronics, such as implantable medical devices capable of regulating or monitoring signals inside the human body.

However, it is not possible for one single material to bring all of these applications to reality. Each application needs a material with a specific set characteristics. A sensor, for instance, might need a material with immense sensitivity, while a new class of batteries could need a material that has higher capacity for holding an electronic charge or that which is more stable.

Materials design efforts have accelerated the development of new materials with specific functionalities and performance. But we’re lacking a materials-based figure of merit to benchmark and guide materials design and development.

Jonathan Rivnay

Rivnay and his team solved this problem by looking at the organic electrochemical transistor, a transistor variety in which ions flow between an electrolyte and an organic conductor in order to switch the electrical current passing via the device on or off. Researchers generally have employed a limited set of conducting polymers in these devices over the past 20 years. Rivnay swapped out those polymers for 10 recently developed organic mixed conductors.

After developing electrochemical transistors from 10 varied organic mixed conductors, Rivnay and his team then measured how well each transistor worked, by comparing parameters such as how effortlessly each device transported ions and then stored an electronic charge. By examining every single material’s performance as a transistor, Rivnay then effortlessly rated their weaknesses and strengths.

“We used organic electrochemical transistors as a tool to understand new organic mixed conductors,” Rivnay said. “This tool doesn’t just allow us to see if one material is better than another, it also tells us why.”

Even though Rivnay carried out his experiments with a set of 10 new materials, the method could be employed for any number of freshly developed organic conductors. Rivnay next plans to further discover the properties of the top-performing materials among those which he tested.

We’re looking at the more promising materials and trying to answer more questions, such as how to make them more stable or sensitive. Our work allows us to think about these materials more rationally as we target them for applications such as biosensing.

Jonathan Rivnay

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