Research ArticleMATERIALS SCIENCE

Thermoelectric properties of a semicrystalline polymer doped beyond the insulator-to-metal transition by electrolyte gating

See allHide authors and affiliations

Science Advances  14 Feb 2020:
Vol. 6, no. 7, eaay8065
DOI: 10.1126/sciadv.aay8065
  • Fig. 1 Simultaneous measurements of S and σ by using the ionic-liquid–gated TFT structure.

    (A) Schematic illustration of the measurement system for the thermoelectric properties under electrolyte gating together with the chemical structures of PBTTT and the ionic liquid [DEME][TFSI]. The ionic-liquid–gated TFT is mounted between two Peltier devices, and the induced temperature difference (ΔT) between the S/D electrodes is monitored by two thermocouples. (B) A photograph of the ionic-liquid–gated TFT. (C) An example of the drain current (Id)–drain voltage (Vd) characteristics. (D) An example of the thermoelectromotive force (ΔV) measurement with respect to ΔT under the application of the gate voltage (Vg). (E) Electrical conductivity (σ) dependence of the Seebeck coefficient (S) and the thermoelectric power factor (P) obtained with the electrolyte gating technique. The dashed lines represent the S ∝ σ−1/4 relation and P ∝ σ1/2 relation expected from the empirical relation, as well as the S ∝ σ−1 relation expected from the Mott equation. The inset shows the S-log σ (Jonker) plot with a dashed line corresponding to the case of the conventional thermally activated process. Photo credit: K. Kanahashi, Waseda University.

  • Fig. 2 GIXD measurements of the PBTTT thin film under electrolyte gating.

    (A) GIXD patterns obtained from the pristine PBTTT film on a glass substrate. (B) Vg dependence of the (100) peak and (C) (010) peak obtained by ex situ measurements, where the ionic-liquid film is removed before the measurements after the application of Vg. (D) The Vg dependence of the interlamellar distance obtained from the (100) peak position and the full width at half maximum (FWHM) of the corresponding peaks. (E) π-π stacking distance obtained from the (010) peak position. (F) Schematic illustration of the molecular arrangements in the doped film, where the TFSI anions are located in the alkyl chain regions.

  • Fig. 3 ESR measurements of the PBTTT thin film under electrolyte gating.

    (A) Schematic illustration of the top-gate bottom contact TFT structure adopted for the ESR measurements under electrolyte gating. Two substrates with a polymer film and an ionic-liquid film on each of them are laminated to form a staggered TFT structure. (B) ESR signal obtained under the application of Vg with a magnetic field (H) perpendicular to the substrate. The background signal at Vg = +1 V was subtracted from all the data. (C) Schematic illustration of the edge-on oriented molecules. (D) σ dependence of the spin susceptibility χ obtained from the integrated ESR intensity. (E) Peak-to-peak ESR linewidth (ΔHpp). The dashed line in (D) shows a guide to the χ ∝ σ relation.

  • Fig. 4 Macroscopic charge transport measurements of doped PBTTT thin films.

    (A) Temperature dependence of σ obtained under the application of Vg. The arrows and filled data points indicate the position of the conductivity maximum. (B) Magnetic field dependence of the magnetoresistance ratio (Δρ/ρ) obtained under Vg = −2.2 V at 150 K. The magnetic field is applied perpendicular to the film plane, as illustrated in the figure. The solid curve shows the fitting of the experimental data by the quadratic relation of H.

  • Fig. 5 Relation of thermoelectric properties and charge transport processes.

    (A) S-σ relations obtained in the present study together with data reported by other groups using different doping methods and dopants (5, 8, 9, 19, 20). The information about the microscopic electronic state obtained from the ESR measurements, as well as the macroscopic transport properties obtained by the temperature dependence of σ, is also shown. The dashed lines represent the same information as in Fig. 1E. (B) Schematic illustration of the ordered (or crystalline) domains and the domain boundaries in the PBTTT thin film. Adjacent domains are connected by tie molecules, shown in red, which enable macroscopic charge transport. (C) Structural optimization of the dimer unit in a single PBTTT chain in the neutral (top) and cationic (bottom) states. The numbers are dihedral angles between the adjacent subunits of thiophene (T) and thienothiophene (TT).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/7/eaay8065/DC1

    Section S1. S-σ relations expected for conventional semiconductors and metals

    Section S2. Effects of high Vg on the transport and thermoelectric properties

    Section S3. In situ GIXD measurements during the application of Vg

    Section S4. AFM imaging of the PBTTT thin film

    Section S5. Anisotropy of the ESR parameters

    Section S6. DFT calculation of backbone rigidity in the neutral and cationic states

    Fig. S1. Thermoelectric properties of the electrolyte-gated PBTTT thin films.

    Fig. S2. In situ GIXD measurements of electrolyte-gated PBTTT thin films.

    Fig. S3. AFM imaging of the PBTTT thin film.

    Fig. S4. Angular dependence of the ESR parameters at various doping levels.

    Fig. S5. DFT calculation of the torsion potential.

    References (4959)

  • Supplementary Materials

    This PDF file includes:

    • Section S1. S-σ relations expected for conventional semiconductors and metals
    • Section S2. Effects of high Vg on the transport and thermoelectric properties
    • Section S3. In situ GIXD measurements during the application of Vg
    • Section S4. AFM imaging of the PBTTT thin film
    • Section S5. Anisotropy of the ESR parameters
    • Section S6. DFT calculation of backbone rigidity in the neutral and cationic states
    • Fig. S1. Thermoelectric properties of the electrolyte-gated PBTTT thin films.
    • Fig. S2. In situ GIXD measurements of electrolyte-gated PBTTT thin films.
    • Fig. S3. AFM imaging of the PBTTT thin film.
    • Fig. S4. Angular dependence of the ESR parameters at various doping levels.
    • Fig. S5. DFT calculation of the torsion potential.
    • References (4959)

    Download PDF

    Files in this Data Supplement:

Stay Connected to Science Advances

Navigate This Article