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On the anomalous optical conductivity dispersion of electrically conducting polymers: ultra-wide spectral range ellipsometry combined with a Drude-Lorentz model
Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.ORCID iD: 0000-0002-7626-1181
Univ Nebraska, NE 68588 USA.
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2019 (English)In: Journal of Materials Chemistry C, ISSN 2050-7526, E-ISSN 2050-7534, Vol. 7, no 15, p. 4350-4362Article in journal (Refereed) Published
Abstract [en]

Electrically conducting polymers (ECPs) are becoming increasingly important in areas such as optoelectronics, biomedical devices, and energy systems. Still, their detailed charge transport properties produce an anomalous optical conductivity dispersion that is not yet fully understood in terms of physical model equations for the broad range optical response. Several modifications to the classical Drude model have been proposed to account for a strong non-Drude behavior from terahertz (THz) to infrared (IR) ranges, typically by implementing negative amplitude oscillator functions to the model dielectric function that effectively reduce the conductivity in those ranges. Here we present an alternative description that modifies the Drude model via addition of positive-amplitude Lorentz oscillator functions. We evaluate this so-called Drude-Lorentz (DL) model based on the first ultra-wide spectral range ellipsometry study of ECPs, spanning over four orders of magnitude: from 0.41 meV in the THz range to 5.90 eV in the ultraviolet range, using thin films of poly(3,4-ethylenedioxythiophene): tosylate (PEDOT: Tos) as a model system. The model could accurately fit the experimental data in the whole ultrawide spectral range and provide the complex anisotropic optical conductivity of the material. Examining the resonance frequencies and widths of the Lorentz oscillators reveals that both spectrally narrow vibrational resonances and broader resonances due to localization processes contribute significantly to the deviation from the Drude optical conductivity dispersion. As verified by independent electrical measurements, the DL model accurately determines the electrical properties of the thin film, including DC conductivity, charge density, and (anisotropic) mobility. The ellipsometric method combined with the DL model may thereby become an effective and reliable tool in determining both optical and electrical properties of ECPs, indicating its future potential as a contact-free alternative to traditional electrical characterization.

Place, publisher, year, edition, pages
ROYAL SOC CHEMISTRY , 2019. Vol. 7, no 15, p. 4350-4362
National Category
Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:liu:diva-157203DOI: 10.1039/c8tc06302hISI: 000465303700033OAI: oai:DiVA.org:liu-157203DiVA, id: diva2:1324737
Note

Funding Agencies|Swedish Research Council; Swedish Foundation for Strategic Research; Wenner-Gren Foundations; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]

Available from: 2019-06-14 Created: 2019-06-14 Last updated: 2023-12-28
In thesis
1. Optics of Conducting Polymer Thin Films and Nanostructures
Open this publication in new window or tab >>Optics of Conducting Polymer Thin Films and Nanostructures
2021 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Intrinsically conducting polymers forms a category of doped conjugated polymers that can conduct electricity. Since their discovery in the late 1970s, they have been widely applied in many fields, ranging from optoelectronic devices to biosensors. The most common type of conducting polymers is poly(3,4-ethylenedioxythiophene), or PEDOT. PEDOT has been popularly used as electrodes for solar cells or light-emitting diodes, as channels for organic electrochemical transistors, and as p-type legs for organic thermoelectric generators. Although many studies have been dedicated to PEDOT-based materials, there has been a lack of a unified model to describe their optical properties across different spectral ranges. In addition, the interesting optical properties of PEDOT-based materials, benefiting from its semi-metallic character, have only been rarely studied and utilized, and could potentially enable new applications.

Plasmonics is a research field focusing on interactions between light and metals, such as the noble metals (gold and silver). It has enabled various opportunities in fundamental photonics as well as practical applications, varying from biosensors to colour displays. This thesis explores highly conducting polymers as alternatives to noble metals and as a new type of active plasmonic materials. Despite high degrees of microstructural disorder, conducting polymers can possess electrical conductivity approaching that of poor metals, with particularly high conductivity for PEDOT deposited via vapour phase polymerization (VPP). In this thesis, we systematically studied the optical and structural properties of VPP PEDOT thin films and their nanostructures for plasmonics and other optical applications. 

We employed ultra-wide spectral range ellipsometry to characterize thin VPP PEDOT films and proposed an anisotropic Drude-Lorentz model to describe their optical conductivity, covering the ultraviolet, visible, infrared, and terahertz ranges. Based on this model, PEDOT doped with tosylate (PEDOT:Tos) presented negative real permittivity in the near infrared range. While this indicated optical metallic character, the material also showed comparably large imaginary permittivity and associated losses. To better understand the VPP process, we carefully examined films with a collection of microstructural and spectroscopic characterization methods and found a vertical layer stratification in these polymer films. We unveiled the cause as related to unbalanced transport of polymerization precursors. By selection of suitable counterions, e.g., trifluoromethane sulfonate (OTf), and optimization of reaction conditions, we were able to obtain PEDOT films with electrical conductivity exceeding 5000 S/cm. In the near infrared range from 1 to 5 µm, these PEDOT:OTf films provided a well-defined plasmonic regime, characterized by negative real permittivity and lower magnitude imaginary component. Using a colloidal lithography-based approach, we managed to fabricate nanodisks of PEDOT:OTf and showed that they exhibited clear plasmonic absorption features. The experimental results matched theoretical calculations and numerical simulations. Benefiting from their mixed ionic-electronic conducting characters, such organic plasmonic materials possess redox-tunable properties that make them promising as tuneable optical nanoantennas for spatiotemporally dynamic systems. Finally, we presented a low-cost and efficient method to create structural colour surfaces and images based on UV-treated PEDOT films on metallic mirrors. The concept generates beautiful and vivid colours through-out the visible range utilizing a synergistic effect of simultaneously modulating polymer absorption and film thickness. The simplicity of the device structure, facile fabrication process, and tunability make this proof-of-concept device a potential candidate for future low-cost backlight-free displays and labels.

Place, publisher, year, edition, pages
Linköping University Electronic Press, 2021. p. 114
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 2107
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:liu:diva-173352 (URN)10.3384/diss.diva-173352 (DOI)9789179297459 (ISBN)
Public defence
2021-03-22, K3, Kåkenhus, Campus Norrköping, Norrköping, 10:00 (English)
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Supervisors
Available from: 2021-02-19 Created: 2021-02-17 Last updated: 2023-12-28Bibliographically approved

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