On the anomalous optical conductivity dispersion of electrically conducting polymers: ultra-wide spectral range ellipsometry combined with a Drude–Lorentz model

Abstract

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.Fig. 4 (a) In-plane real optical conductivity dispersion of PEDOT:Tos represented by multiple Lorentz oscillators (the Drude contributions are removed) derived from the DL model. (b) The contributions of the real optical conductivity from THz (blue curve), broad oscillators (brown curve), interband transitions (green curve), and molecular vibrations (grey curves) are indicated. (c and d) Display the amplitude and broadening parameters for each Lorentz oscillator as well as their resonance energy (frequency).Their out-of-plane counterparts are shown in Fig. S10 (ESI †). The parameters for these Lorentz oscillators are listed in Table S2 (ESI †).

Conductive polymer nanoantennas for dynamic organic plasmonics

“…This optically metallic and plasmonic character is related to the high conductivity within the thin film due to high concentration (2.6 ×10 21 cm -3 , determined by ellipsometry, see Supplementary Table.1and Supplementary Information for details) of mobile positive polaronic charge carriers. We also note that the mobility is highly anisotropic 5,6 and the out-of-plane real permittivity (Supplementary Fig. 2a) is primarily positive throughout the measured range, making the conductive polymer thin film a natural hyperbolic material 7 (Supplementary Fig.…”

Poly(3,4 ethylenedioxythiophene)‐tosylate—Its synthesis, properties and various applications

“…In another approach, the plasma treated PVDF films were coated with the oxidant solution of iron(III) p‐toluenesulfonate on both sides, through dip coating method, and the oxidant coated PVDF film was placed into the VPP chamber and exposed to EDOT monomer vapor to achieve the PEDOT‐Tos coated PVDF flexible film for further practical application 63 . Addition of triblock polymer PEG‐PPG‐PEG to the mixture of oxidant solution with alcohols 38,62,64‐69 along with different additives such as deionized water, DMF 70 DMSO or EDTA 71 was also attempted for the synthesis of PEDOT‐Tos films through VPP. Such procedure was repeated for multiple layer synthesis of PEDOT‐Tos by sequential polymerization, until the free standing PEDOT‐Tos film was obtained 72 .…”

Unraveling vertical inhomogeneity in vapour phase polymerized PEDOT:Tos films

“…13 Notably, PEDOT lms prepared by VPP typically possess strong anisotropy, manifested by signicantly lower conductivity in the vertical direction across the lm (out-of-plane). 14,15 The out-of-plane behavior is crucial for many applications and therefore important to understand. Early studies on conducting polymers attributed anisotropy to a preferential orientation of the polymer crystallites.…”