New method benchmarks organic mixed conductors

November 27, 2017 by Amanda Morris, Northwestern University
OECT description and operation. a OECT cross section, wiring, and dimensions: channel length (L), and thickness (d). The source, drain, and gate (S, D, G, respectively), and relevant voltages (gate, drain voltage: V G, V D) and currents (drain current, I D) terms are also labeled. b Representative transfer (I D  −  V G) curve for a p-type accumulation mode device (V D < 0 V), and the corresponding transconductance curve (g m). The schematics on the right indicate the doping state of the film, where the “ON” state allows for anion drift/penetration and subsequent stabilization of holes on the semiconducting backbone. In the schematics, cations are orange, anions are blue, and holes are red. Credit: Nature Communications (2017). DOI: 10.1038/s41467-017-01812-w

Within the past five years, Northwestern University's Jonathan Rivnay has noticed a surge in the development of new organic mixed conductors—polymer materials that can transport both electrons and ions. Lighter, more flexible, and easier to process than their inorganic counterparts, the carbon-based materials show promise in a broad assortment of applications, ranging from medical devices to energy storage. But with increased productivity and innovation comes a perhaps unforeseen problem.

"It can be challenging and time consuming to take new , put them on a device, and record their performance," said Rivnay, assistant professor of biomedical engineering in Northwestern's McCormick School of Engineering. "But even more challenging is to properly compare the performance of these to each other because there hasn't been an established benchmarking method."

Now Rivnay and his team have filled this void. To help researchers pinpoint the best organic mixed conductors for specific applications, Rivnay and his team have developed a novel framework to benchmark and compare their performances. Not only does this method allow for the comparison of existing materials, it could also be used to inform the design of new organic materials.

The research was published online Friday, November 24 in Nature Communications. Rivnay is the paper's corresponding author. Sahika Inal, assistant professor of bioscience at King Abdullah University of Science and Technology, served as the paper's first author.

Organic conductors are soft materials that conduct electricity. They show promise in inexpensive, lightweight, flexible technologies, including solar cells, printable electronic circuits, and . More recently, their ability to interact intimately with ions and biomolecules has led to significant interest in bio-integrated electronics, such as implantable that can monitor or regulate signals inside the human body.

One single material, however, cannot bring all of these applications to reality. Each application requires a material with a certain set characteristics. A sensor, for example, might require a material with extreme sensitivity, while a new class of batteries might need a material that is more stable or has higher capacity for holding an .

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

To solve this problem, Rivnay and his team looked to the organic electrochemical transistor, a type of transistor in which ions flow between an organic and an electrolyte in order to switch the electrical current flowing through the device on or off. For the past 20 years, researchers typically have used a limited set of conducting polymers in these devices. Rivnay swapped out those polymers for 10 newly developed organic mixed conductors.

After building electrochemical from 10 different organic mixed conductors, Rivnay and his team measured how well each transistor performed, comparing parameters such as how easily each transported ions and stored an electronic charge. By evaluating each material's performance as a transistor, Rivnay then easily rated their strengths and weaknesses.

"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."

Although Rivnay performed his experiments with a set of 10 new materials, the method could be used for any number of newly developed organic conductors. Next, he plans to further explore the properties of the top-performing materials among those 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," Rivnay said. "Our work allows us to think about these materials more rationally as we target them for applications such as biosensing."

Explore further: Research helps overcome barrier for organic electronics

More information: Sahika Inal et al. Benchmarking organic mixed conductors for transistors, Nature Communications (2017). DOI: 10.1038/s41467-017-01812-w

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