| NSF Org: |
ECCS Division of Electrical, Communications and Cyber Systems |
| Recipient: |
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| Initial Amendment Date: | June 12, 2017 |
| Latest Amendment Date: | June 12, 2017 |
| Award Number: | 1709479 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Lawrence Goldberg
ECCS Division of Electrical, Communications and Cyber Systems ENG Directorate for Engineering |
| Start Date: | August 1, 2017 |
| End Date: | July 31, 2021 (Estimated) |
| Total Intended Award Amount: | $360,265.00 |
| Total Awarded Amount to Date: | $360,265.00 |
| 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: |
PO Box 5190 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:
Organic Field-Effect Transistors (OFETs) are a key technology for flexible and low-cost electronics used e.g. for wearable electronics or flexible displays. However, OFETs are facing severe obstacles that delay their commer-cialization. Doping of organic semiconductors opens a new perspective on the OFET technology. This additional degree of freedom allows to realize new device concepts and to overcome current limitations of the OFET tech-nology. To take full advantage of the benefits of doping for OFETs, current doping ratios in the range of 0.1-1% have to be reduced significantly. The project addresses this challenge. Doping processes will be optimized to re-duce the doping level into the sub-100 ppm range. At these ultra-low doping concentrations, the influence of dop-ing on transistor behavior will be studied thoroughly: a) A quantitative model will be developed to describe charge carrier accumulation and depletion in doped organic transistors; b) Generation of minority charge carriers at ul-tralow doping concentrations will be studied; c) A new device concept - the organic tunnel field-effect transistor - will be realized, and its potential will be evaluated.
To broaden the impact of the project, additional measures will be taken to increase the participation of minority students. Summer projects will be offered, which will provide undergraduate students from underrepresented groups with the opportunity to learn about experimental research and to inform them about potential choices for graduate school. Furthermore, research projects will be offered to local high-school students through the college credit plus program, and graduate students will be trained in a highly interdisciplinary field.
Technical:
Doping organic semiconductors provides a new dimension in the design of OFETs and bears the potential of ena-bling new device concepts with increased performance. In light of these prospects, the research goal of this project is to study the influence of doping on OFETs and to provide an improved understanding of majority and minority charge carrier generation and recombination in doped OFETs.
To reach this aim, the following objectives are pursued: a) to develop a consistent and experimentally validated model describing majority charge carrier accumulation and depletion in doped organic transistors; b) to study mi-nority charge carrier dynamics in doped organic transistors and clarify the mechanism of minority charge carrier generation and recombination; and c) to leverage on the potential of doping and realize vertical organic tunnel field-effect transistors (VOTFETs).
Doping organic transistors necessitates the use of much lower doping concentrations as commonly used in organic devices. In this project, a rotating shutter system capable of controlling doping concentrations in the sub 100 ppm regime is introduced, which opens a new regime of doping. The influence of doping on the flatband, threshold, and pinch-off voltage at these ultra-low doping concentrations is studied by capacitance vs. voltage measurements, photoelectron spectroscopy, and transistor characterization. Minority charge carrier generation is studied in organ-ic metal-oxide-semiconductor structures and organic transistors, whereas the lifetime and diffusion length of mi-nority charge carriers are characterized in p-n-i-p structures. The operation mechanism of VOTFETs will be stud-ied by systematic device variations. In particular, the tunnel injection mechanism will be validated by an increase in the thickness of the intrinsic semiconductor layer.
These experiments have the potential to advance the knowledge in the field: a) The mechanism of minority charge carrier generation will be clarified; b) An analytical model describing the influence of the flatband voltage on the threshold and pinch-off voltage will be tested; c) A detailed understanding of minority charge carrier diffusion will be developed and it will be studied how the lifetime of minority charge carriers depends on the doping con-centration and temperature; d) A new analytical solution describing current saturation in doped OFETs will be verified.
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.
Doping of organic semiconductors has become a key technology for highly efficient optoelectronic devices, but was only rarely used in organic field-effect transistors (OFETs). During the course of this project, we were able to show that doping not only improves the performance of OFETs, but opens a new perspective on organic transistor technology: for the first time, doping allows to define the minority and majority charge carrier density and to control the Fermi-Level in different parts of the device.
We were able to develop an optimized deposition technique to reliably dope organic semiconductors in the 100ppm range, which allowed us to take full advantage of doping OFETs. The mechanisms of doping at ultra-low concentrations were studied by a range of different models. An AC drift-diffusion model was implemented to reliably extract doping efficiencies in metal-oxide-doped organic semiconductor (MOS) junctions (Figure 1), a statistical model was proposed to study current bottlenecks of doping and by that formulate design targets for new dopant/semiconductor combinations, and finally an analytic model to discuss doped OFETs was published (Figure 2).
All of these models were thoroughly verified by a range of experiments. Predictions of the analytic model were tested for hole transport (Pentacene, DNTT) and electron transport materials (fullerene and fullerene derivates). All material combinations show a linear shift in threshold voltage with the doping concentration (Figure 1a). Furthermore, it was shown that the threshold voltage shifts linearly with the flatband voltage of the underlying MOS capacitance of the OFET.
These models and test systems allowed us to study minority and majority charge carrier generation in organic semiconductors (Figure 3). The charge generation rates were studied using impedance spectroscopy, which revealed a strong dependence of minority charge generation on the contact material and doping concentrations. In further studies, our drift-diffusion models will be complemented to include different generation mechanisms as well.
Funding from the NSF allowed us to implement a new processing and analysis protocol for OFETs that helped us to identify experimental trends with high statistical confidence. The app developed during the grant period was shared with the community.
Aside of the scientific output of the project, funding from the NSF helped to train graduate and undergraduate students that continue to work in this area after graduation, either in industry or academia.
In summary, funding from the NSF led to a detail understanding of doping at ultra-low concentrations and its influence on organic transistors. In the future, this knowledge might become instrumental in the commercialization of this technology, e.g. in the field of flexible electronics.
Last Modified: 09/30/2021
Modified by: Bjorn Lussem
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