Probing Ion Injection in Organic Electrochemical Transistors

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Plastics that can conduct both electrons and ions are important for numerous applications such as bioelectronic sensors that convert biological nerve impulses into signals readable by digital electronics, energy storage devices that can deliver high currents for short times, and next-generation computers that mimic the function of the brain. One of the factors limiting technological developments in these fields is limited understanding of how ions inject into and transport within these semiconducting plastics. This project addresses this question by analyzing polymers (plastics) at the nanometer scale using advanced microscopy techniques. These methods can determine where polymers are swelling from ion injection and can correlate that response with the chemical signature of the ion using infrared light, also at the nanoscale. The microscopic information is then compared to transistor measurements to gain a fundamental understanding of how the polymer processing and structure influences ion motion. The scientific knowledge from this project enables better design and processing/manufacturing of polymers for the applications noted above. The project additionally builds upon the track record of the principal investigator in education by enabling development of new outreach materials such as polymer electrochemistry kits that are suitable for integration into existing outreach programs and networks. The project also provides direct support for undergraduate research through continuation of the successful partnership with the Rainier Scholars organization to provide pathways to assist under-represented groups and first-generation college students succeed in the sciences. The scientific goal of this project is to gain a fundamental understanding of the structure/function relationships controlling ion injection in pi-conjugated polymers operating as mixed ionic/electronic conductors while using blended organic electrochemical transistors as an experimental testbed. These polymers and blends typically exhibit features on the scale of tens of nanometers, and therefore this project uses advanced scanning probe microscopy tools to investigate the ion transport process at the nanoscale. Conjugated polymers have emerged as promising electronic and photonic materials for transducing signals at the interface between the biological and digital environments, and the proposed project will explore fundamental structure/function properties of these materials relevant to these applications in a way that is distinct from other efforts through a combination of unique local and bulk methods. Specifically, the project will: 1) use a new method, photoinduced force microscopy (PiFM), to make nanoscale maps probing how local chemical structure and morphology combine to affect local ion injection; 2) apply electrochemical strain microscopy (ESM) to measure local swelling due to ion uptake in homopolymers, block copolymers, and blends; and, 3) do so while exploring new blend and composite architectures as a means to overcome the bottlenecks of existing materials performance. Notably, this project uses nanoscale infrared microscopy to probe blends of conducting polymers and ionic conductors to test the hypothesis that decoupling the high electronic mobility component and high ionic mobility component can enable improved electrochemical transistors. The principal investigator has shown in previous work that ion injection and electronic mobility are often anti-correlated in mixed conductors, which serves as a device bottleneck. These experiments yield a distinct set of measurements that enable multimodal analysis of the structure-function relationships underpinning ion injection. The project provides important insight into how mobility and volumetric capacitance in mixed ionic-electronic conductors are related, and whether it is possible to rationally improve conducting polymers and polymer blends design by focusing on how ions move into the polymer from the ground up.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.