Understanding Structure and Charge Transport in organic mixed ionic electronic conductors

Organic mixed ionic electronic conductors (OMIECs) have emerged as a promising class of materials for applications in bioelectronics, neuromorphic computing, and polymeric batteries. These materials couple ionic and electronic currents in electrolytes and exhibit dynamic structures under electrolyte swelling. Because of these dynamic structures, mechanistic understanding of how material structure relates to electronic transport properties and device operational stability has been difficult to formalize, as the materials must be probed in their electrolyte-swollen state. To this end, our group works extensively on developing suites of operando and in situ techniques to probe material micro- and nanostructure in conditions relevant for device operation. Through combined structural, photophysical, and electronic characterization, we can paint a cohesive picture of charge transport across a wide range of charge densities and electrolyte environments.

Check out these recent works to get a sense of our group’s research:

(1)Advanced Materials 2022, 34 (21), 2110406. DOI: 10.1002/adma.202110406.

(2)Materials Horizons 2023, 10, 2568-2578. DOI: 10.1039/d3mh00017f.

(3)Nature Materials 2023, 22 (3), 362-368. DOI: 10.1038/s41563-023-01476-6

(4)Advanced Materials 2024, 2310157. DOI: 10.1002/adma.202310157.

Microstructural dynamics probed with Operando X-ray scattering

Operando X-ray scattering tracks material microstructural changes while materials undergo electrochemical charging and discharging. Through this work, OMIEC microstructures are shown to be dynamic with material charge density. Many open questions remain on the mechanisms of these dynamics.

Remarkably, certain systems show evidence of structural ordering increasing with charge density.

Lamellar scattering intensity increases with charge density, coupled with a contraction in the p-p stacking distance, all evidence of increasing structural order.

Spatially resolved microstructural study using cryogenic 4D-STEM

4D-STEM provides local structure information that is used to visualize the materials’ local order revealing heterogeneities, connectivity nature and defects. Using cryo-4D-STEM we study the microstructural transitions that takes place as OMIECs swell and charged and correlate these changes with their properties.

Swelling-induced disorder is predominantly localized within discrete regions, primarily attributed to water uptake rather than added charges.

Check out these 4D-STEM and HRTEM studies of organic semiconducting materials:

(1) PNAS 2023, 119 (46), e2204346119. DOI: 10.1073/pnas.2204346119.

(2) ACS Macro Lett 2021, 10, 2568-2578. DOI: 10.1039/d3mh00017f.

Aggregate interactions probed with spectroelectrochemistry

Spectroelectrochemistry and modeling of material exciton absorption features probe intramolecular interactions and properties of polymer aggregates during electrochemical cycling. Coupled with operando X-ray scattering,  we can form a nearly complete picture of both short-range aggregate order and longer-range microstructural order.

Evidence of increased intramolecular ordering is seen with spectroelectrochemistry, providing evidence alongside operando X-ray scattering that increased structural ordering coincides with material mobility increases

Electrochemical transistor testing studies electronic transport

Electrochemical transistors provide a useful testbed for measuring material electronic transport properties, particularly at high charge densities. The lab has several techniques for accurately measuring charge carrier mobility, allowing us to unravel fundamental links between material structure and electronic transport, as well as probing transport degradation mechanisms. 

Measurements on the same electrochemical transistor reveal two regimes of operation: a “stable” regime, and a “degradation” regime. Stability studies showed that material degradation depended sensitively on the maximum charge density reached.

Active members in this area: Garrett LeCroy, Yael Tsarfati, Adam Marks, Nicholas Siemons, Arianna Magni, Christina Cheng, Kalee Rozylowicz, Ruth Awani