| NSF Org: |
ECCS Division of Electrical, Communications and Cyber Systems |
| Recipient: |
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| Initial Amendment Date: | January 5, 2018 |
| Latest Amendment Date: | January 5, 2018 |
| Award Number: | 1750011 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Richard Nash
rnash@nsf.gov (703)292-5394 ECCS Division of Electrical, Communications and Cyber Systems ENG Directorate for Engineering |
| Start Date: | February 15, 2018 |
| End Date: | January 31, 2023 (Estimated) |
| Total Intended Award Amount: | $500,000.00 |
| Total Awarded Amount to Date: | $500,000.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
1500 HORNING RD KENT OH US 44242-0001 (330)672-2070 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
Kent OH US 44242-0001 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | EPMD-ElectrnPhoton&MagnDevices |
| Primary Program Source: |
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| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.041 |
ABSTRACT
Abstract
Nontechnical:
The organic electrochemical transistor (OECT) is one of the most successful organic bio-electronic devices. The list of applications of OECTs ranges from sensors for biomolecules, to electrocardiographic recordings, and in-vivo recording of brain activity. Overall, OECTs will open new ways to study and monitor common diseases, which can lead to an improvement of the health and quality of life of people in the US and around the world.
However, despite the intense interest in OECTs, the understanding of their working mechanism is incomplete. Models used to describe OECTs split the device in two separate parts - one part describing ionic conduction inside the gate electrolyte, and a second part describing the electronic conduction inside the organic semiconductor. This artificial separation between ionic and electronic currents leads to inconsistencies in the description of the device operation, which have to be resolved in order to increase the performance of OECTs.
In its research component, the project aims at enhancing the understanding of the working mechanisms of OECTs. A 2D numerical simulation will be implemented and validated. The model will be used to quantify carrier densities and electric fields inside the devices, to understand the origin of current instabilities of OECTs, and to quantitatively describe the sensing mechanism of OECTs.
The educational goal of this project is to increase the awareness and knowledge of the nature and ethics of science for students of introductory science courses and high school students. Short graphic novels will be developed that explain the nature of science using anecdotes from the lives of famous researchers as examples. These teaching materials will be used in an inverted physics classroom.
Technical:
Organic Electrochemical Transistors (OECTs) hold the promise of enabling new bioelectronic applications and of providing new means to study the working mechanisms of biological systems. OECTs rely on a delicate interplay between ionic and electric currents, which, however, is not sufficiently understood. The research goal of this proposal is to close this research gap and to enhance our knowledge in the working mechanisms of OECTs.
To reach this aim, the following objectives are pursued: i) To formulate and validate a two-dimensional drift-diffusion model that quantitatively describes OECT behavior; ii) to study the origin of hysteresis and gate bias stress effects in OECTs; and iii) to study and model the sensing mechanism of OECTs.
To describe OECT operation, the continuity equations of all involved charge carriers must be solved along with the Poisson equation. A drift-diffusion simulation will be implemented and validated by moving front experiments, impedance spectroscopy of metal-electrolyte-semiconductor junctions, and electric characterization of systematically varied OECTs. A transient model of OECTs will be implemented to explain the origin of hysteresis and gate-bias stress effects in OECTs. Enzymatic reactions are added to the simulation in order to describe the sensing mechanism of OECTs quantitatively.
These experiments have the potential to advance the knowledge in the field: i) the steady state distribution of all charge carriers inside the organic semiconductor will be clarified, which is essential to understand the details of OECT operation. ii) Approaches to avoid instabilities found in current OECTs are proposed, which is essential for a later commercialization. iii) A detailed understanding of the sensing mechanism of OECTs is developed.
The educational goal of the project is to strengthen introductory physics teaching by developing modules that discuss the nature and ethics of science. In collaboration with the Access and Support for Successful Undergraduate Research Experiences (ASSURE) program, summer projects will be offered to students from minority serving universities to support them in their applications for graduate schools.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
Organic Electrochemical Transistors (OECTs) have some key advantages that make them ideally suited for bioelectronic applications. Not only are OECTs mechanically flexible, stretchable, and bio-compatible, but are highly efficient when transducing ionic into electrical currents. Many applications are envisaged, such as wearable and almost imperceptible sensor patches that can be used to monitor the health status and guide treatment of out-of-hospital patients.
However, despite the high interest in OECTs, their working mechanism was surprisingly poorly understood, which made a targeted optimization of OECTs challenging. With the help of funding from the NSF we addressed this gap and developed a two-dimensional drift-diffusion simulation that is capable of describing the detailed interaction of ionic and hole currents inside the OECT.
The model was implemented based on a discretization of Laplace equation and the continuity equations of all species involved. It was thoroughly validated by a comparison of systematically varied OECTs with modeling results (Figure 1). With the help of this comparison, scaling laws of OECTs were deduced, in particular the dependency of the maximum transconductance on the applied gate and drain potential. Furthermore, the origin of the peak in transconductance was explained, and a model to explain contact resistance effect in OECTs was proposed.
With the help of the model and a comparison of the model results to experimentally measured potential profiles inside the transistor channel, we were able to argue that already existing OECT models fail to fully describe the steady-state of OECTs. It was shown that the steady state is given by a balance of lateral ion currents that lead to a more complex potential distribution inside the transistor channel than previously thought.
The maximum switching speed of OECTs is essential for the applicability of OECTs for different applications. To understand the limiting factors of OECT performance, our model was extended to treat transient effects. In particular, we were able to show that the lateral drain potential has an important influence on the switching speed, and were able to propose guidelines to increase OECT speed. Again, the trends observed in the model were verified experimentally (Figure 2).
OECTs not only transduce ionic into electronic currents, but they are often used to sense biomolecules. We implemented protocols to functionalize the gate electrode for a selective detection of various bio-markers. First steps toward modeling this sensing mechanism were taken, in particular towards adding redox-reactions to the gate electrode (Figure 3).
Finally, a new class of gate electrolytes - liquid crystalline elastomers - was proposed by us with the help of support from NSF. We were able to show that not only does this class of electrolyte lead to high performance, but that the characteristic of the OECT can be tuned by the alignment of the liquid-crystalline monomers in the elastomer.
Aside of the scientific output of the project, funding from the NSF helped to train graduate and undergraduate students. Teaching modules were developed for undergraduate students to more efficiently teach introductory physics courses and to teach them about the nature of science as well. A series of graphical stories were developed that discuss the success and struggles of famous scientists.
In summary, funding from the NSF led to a detail understanding of Organic Electrochemical Transistors. In the future, this knowledge might become instrumental in the commercialization of this technology, e.g., in the field of organic bioelectronics.
Last Modified: 04/17/2023
Modified by: Bjorn Lussem
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