Research ArticleMATERIALS SCIENCE

Morphology controls the thermoelectric power factor of a doped semiconducting polymer

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Science Advances  16 Jun 2017:
Vol. 3, no. 6, e1700434
DOI: 10.1126/sciadv.1700434
  • Fig. 1 Chemical structure and doping process.

    (A) Chemical structure of PBTTT and FnTCNQ (n = 2 or 4) and the corresponding IE or EA. (B) Solution and vapor doping routes used to achieve doped films.

  • Fig. 2 UV-vis spectra of neat and doped PBTTT thin films.

    UV-vis spectra of (A) neat PBTTT and PBTTT:F4TCNQ thin films and (B) PBTTT:F2TCNQ thin films. Solution-doped films are at a dopant concentration of 10 wt %, and vapor-doped films are for dopant exposure of 10 min (all spectra normalized by thickness). Neat PBTTT films were annealed at 180°C, whereas solution-doped films were annealed at 150°C. Comparison of absorption spectra of annealed neat PBTTT and as-cast neat PBTTT can be found in the Supplementary Materials.

  • Fig. 3 GIWAXS for PBTTT films as a function of processing.

    2D GIWAXS images for (A) annealed neat PBTTT, (B) F4TCNQ vapor-doped annealed film, (C) annealed F4TCNQ solution-doped film, (D) F2TCNQ vapor-doped annealed film, and (E) as-cast F2TCNQ solution-doped film. The (100) reflection in GIWAXS image in (E) was blocked off with lead tape to allow longer exposure time without saturating the detector. Images are obtained at Stanford Synchrotron Radiation Lightsource (SSRL) beamline 11-3.

  • Fig. 4 Out-of-plane and in-plane scattering profiles of annealed neat and doped PBTTT thin films.

    GIWAXS line cuts of (A) out-of-plane scattering and (B) in-plane scattering. Black, neat; orange, F4TCNQ solution doping; purple, F4TCNQ vapor doping; green, F2TCNQ solution doping; blue, F2TCNQ vapor doping. The F2TCNQ-doped film corresponds to as-cast conditions. Dashed red lines are guides to the eye relative to the peak positions for the neat film. All scattering profiles correspond to thermally annealed films, except for the F2TCNQ solution doping, which is for the as-cast case. a.u., arbitrary units.

  • Fig. 5 RSoXS of doped films.

    Lorentz-corrected scattering profiles (log-log scale) from azimuthally averaged RSoXS images at a photon energy of 285.4 eV for (A) annealed and (B) as-cast films. The curves were offset for clarity. Black, neat; purple, F4TCNQ vapor doping; blue, F2TCNQ vapor doping; orange, F4TCNQ solution doping; green, F2TCNQ solution doping. RSoXS experiments were performed at Advanced Light Source (ALS) beamline 11.0.1.2.

  • Fig. 6 The relationship between OCL and thermoelectric material properties.

    (A) Measured electronic conductivity (σ), (B) measured Seebeck coefficient (α), and (C) calculated PF versus the corresponding OCL values, as determined from the RSoXS experiments (table S3).

  • Fig. 7 Trends in Seebeck coefficient and power factor.

    Log-log scale plot showing the trends in (A) Seebeck coefficient (α) and (B) PF (α2σ) versus electronic conductivity (σ) for solution- and vapor-doped films. Orange markers are for vapor-doped films, and blue markers are for solution-doped films. Circle markers are for F4TCNQ-doped films, square markers are for F2TCNQ-doped films, and triangle marker is for the F4TCNQ-doped film on OTS-treated substrate. Open markers correspond to thermally annealed films, and filled markers correspond to as-cast films. The open green diamond is our previously reported FTS-doped PBTTT thin film (33). Dashed lines are empirical trends [α ∝ σ−1/4 and PF ∝ σ1/2] we previously reported on various doped semiconducting polymers (34).

  • Table 1 Summary of electronic conductivity (σ), Seebeck coefficient (α), and PF of doped PBTTT films.

    For the sample on an OTS-treated substrate, σ = 670 ± 4 S/cm, α = 42 ± 6 μV/K, and PF = 120 ± 30 μW m−1 K−2. All other samples reported in this table are on untreated quartz substrates.

    DopantConditionσ (S/cm)α (μV/K)PF (μW m−1 K−2)
    F4TCNQSolution—
    as-cast
    2.08 ± 0.0145 ± 40.42 ± 0.09
    Solution—
    annealed
    3.51 ± 0.0560 ± 91.3 ± 0.4
    Vapor—
    as-cast
    114.1 ± 0.532 ± 412 ± 3
    Vapor—
    annealed
    220.00 ± 0.0239 ± 532 ± 9
    F2TCNQSolution—
    as-cast
    0.41 ± 0.02111.7 ± 0.10.52 ± 0.03
    Solution—
    annealed
    2 × 10−3 ± 2 × 10−4755 ± 1000.11 ± 0.03
    Vapor—
    as-cast
    13.7 ± 0.2130 ± 2023 ± 6
    Vapor—
    annealed
    36 ± 3140 ± 2070 ± 20

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/6/e1700434/DC1

    fig. S1. AFM height and phase images of neat annealed PBTTT and F4TCNQ vapor-doped films at 5 and 10 min.

    fig. S2. Absorption spectra showing the NIR regime for doped PBTTT films and the thermal stability of F2TCNQ-doped films.

    fig. S3. Additional UV-vis-NIR spectra of F4TCNQ vapor-doped films relative to a neat film.

    fig. S4. Williamson-Hall plot for neat (black circle) and F4TCNQ vapor-doped film.

    fig. S5. In-plane scattering profiles of as-cast neat and doped films.

    fig. S6. Thin-film thickness profile of neat and vapor-doped PBTTT:F4TCNQ film.

    fig. S7. Representative 2D RSoXS images for neat PBTTT, F4TCNQ vapor-doped, and F4TCNQ solution-doped thin films (all thermally annealed).

    fig. S8. Lorentz-corrected scattering profiles of neat PBTTT for different annealing temperatures.

    fig. S9. Schematic of the geometry of the contacts for electronic conductivity and Seebeck measurements on thin films of doped polymers.

    table S1. X-ray reflection peaks of annealed PBTTT thin films from GIWAXS.

    table S2. X-ray reflection peaks of as-cast PBTTT thin films from GIWAXS.

    table S3. Summary of OCLs for doped films.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. AFM height and phase images of neat annealed PBTTT and F4TCNQ vapor-doped films at 5 and 10 min.
    • fig. S2. Absorption spectra showing the NIR regime for doped PBTTT films and the thermal stability of F2TCNQ-doped films.
    • fig. S3. Additional UV-vis-NIR spectra of F4TCNQ vapor-doped films relative to a neat film.
    • fig. S4. Williamson-Hall plot for neat (black circle) and F4TCNQ vapor-doped film.
    • fig. S5. In-plane scattering profiles of as-cast neat and doped films.
    • fig. S6. Thin-film thickness profile of neat and vapor-doped PBTTT:F4TCNQ film.
    • fig. S7. Representative 2D RSoXS images for neat PBTTT, F4TCNQ vapor-doped, and F4TCNQ solution-doped thin films (all thermally annealed).
    • fig. S8. Lorentz-corrected scattering profiles of neat PBTTT for different annealing temperatures.
    • fig. S9. Schematic of the geometry of the contacts for electronic conductivity and Seebeck measurements on thin films of doped polymers.
    • table S1. X-ray reflection peaks of annealed PBTTT thin films from GIWAXS.
    • table S2. X-ray reflection peaks of as-cast PBTTT thin films from GIWAXS.
    • table S3. Summary of OCLs for doped films.

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