Understanding electron and ion transport in organic semiconductors

Soft organic semiconductors that conduct both electricity and ions could pave the way for new kinds of electrochemical devices such as bioelectronics, biological implants, or energy conversion/storage devices.

Designing materials for such applications requires understanding not only how ions and electrons move within the materials, but also how to optimize the molecular arrangement of the materials for optimum ion/electron transport.

Researchers at the Pritzker School of Molecular Engineering (PME) at the University of Chicago have recently taken an important step toward understanding mixed ion/electron transport in soft materials. 

With collaborators at Cornell University and Argonne National Laboratory, they designed, synthesized, and studied an organic liquid crystal that exhibits both ion and electron transport. By examining the molecular structure and characteristics of the material, they found that adding an ionic dopant unexpectedly affected electronic transport, transforming the material’s capabilities.

“Understanding this relationship between structure and mixed ion/electron transport will help pave the way for more complex engineered material systems,” said Shrayesh Patel, assistant professor of molecular engineering and co-author of the research.

The results were published recently in the journal ACS Nano. The work was supported by the National Science Foundation’s Designing Materials to Revolutionize and Engineer our Future (DMREF) program.

Leveraging liquid crystals to study mixed ion/electron transport

Inorganic materials that conduct electricity have been widely studied, but organic materials are not as well understood, despite the fact that organic electronics have the potential to be cheaper, more flexible, and easier to tailor than silicon-based electronics.

While other researchers have studied mixed ion/electron transport in organic materials such as polymers, the PME team instead looked to liquid crystals, another class of soft materials that have a self-assembled structure and can be easily modeled in computer simulations. While liquid crystals are ubiquitous in electronic displays, their use for other applications is still limited.

The Cornell researchers, led by Fernando Escobedo, conducted a computer simulation to find the optimum chemistry for synthesis and characterization of a mixed-transport material and identified a new liquid crystal: 4T/PEO4, a compound consisting of electron-conducting quarter-thiophene (4T), terminated at both ends by ion-conducting oligoethylenoxide (PEO4).

Tuning structure of 4T/PEO4 for best performance

Following the discovery of 4T/PEO4, Christopher Ober and his group at Cornell successfully synthesized the materials, and the UChicago team, led by Patel and Paul Nealey, the Brady W. Dougan Professor in Molecular Engineering and research co-author, studied the structure of the material to see how it changed when they added a salt called bis(trifluoromethane)sulfonimide lithium (LiTFSI). Surprisingly, the samples underwent a drastic structural transition while still maintaining their ion transport functionality.

To study how these changes in structure influenced both ion and electron transport, the researchers also introduced an electronic dopant called 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and measured ion/electron transport using a technique called impedance spectroscopy.

The team used their newly developed vapor-depositing method to control the amount of induced electronic carrier deposited into the sample and found that structural changes caused by the addition of LiTFSI gave rise to more than an order of increase in electronic conductivity.

“The use of this vapor doping method for electronic dopants is a critical step,” said Ban Dong, a postdoctoral fellow and co-author of the research. “It allows us to investigate mixed ion/electron transport in highly complex liquid crystal systems.”

Developing materials for future electrochemical devices

Understanding materials like these could have implications in lithium-ion battery electrodes and electrochemical transistors for bioelectronic devices. Next the team hopes to characterize more complex materials.

“This shows that liquid crystal are ideal material testbeds to understand mixed conduction in soft materials,” Patel said. “We’re on our way to a better understanding of the behavior of these soft materials.”

Other authors on the paper include Joseph Strzalka, Jens Niklas and Oleg Poluektov of Argonne National Laboratory; and Christopher Ober, Fernando Escobedo, Ziwei Liu and Mayank Misra of Cornell.

Citation: “Structure Control of a π‐Conjugated Oligothiophene-Based Liquid Crystal for Enhanced Mixed Ion/Electron Transport Characteristics.” Dong et al. ACS Nano, doi: 10.1021/acsnano.9b01055

Funding: National Science Foundation’s Designing Materials to Revolutionize and Engineer our Future (DMREF) program.