Research ArticleAPPLIED SCIENCES AND ENGINEERING

Tuning conformation, assembly, and charge transport properties of conjugated polymers by printing flow

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Science Advances  09 Aug 2019:
Vol. 5, no. 8, eaaw7757
DOI: 10.1126/sciadv.aaw7757
  • Fig. 1 Printing methods, model systems, and printing regimes.

    (A) Schematic of blade coating (left) and capillary pen writing (right) processes to produce conjugated polymer films. (B) Chemical structure of the four conjugated polymers used in this study. (C) Schematic illustration of the evaporation profile, flow field, and meniscus shape comparing the three printing regimes. (D) Comparison between experimentally measured (black squares) and simulated (red triangles) film thicknesses as a function of printing speed. Regions of the plot corresponding to evaporation, transition, and LL regimes are colored orange, pink, and blue, respectively. (E) Streamline representation of the flow field comparing three regimes; corresponding printing speeds (Vsub) are labeled. Evaporative flux profile is overlaid (magenta line), showing that the evaporation rate peaks at the contact line in the evaporation and transition regimes. (F) Strain rate (γ̇) in the meniscus comparing three regimes at corresponding printing speeds; peak strain rates (γ̇max) are labeled. (G) Peak strain rate and inverse residence time (τres1) as a function of printing speed. Strain rate (black squares) begins to increase drastically in the transition regime because of increasing viscous forces. Residence time (red triangles) from the position of the maximum strain rate to the contact line is initially short in the evaporation regime and increases with increasing speed. Residence time remains relatively short and finite in the transition regime and then approaches infinity in the LL regime, allowing the relaxation of polymer orientation and conformation. (H) Schematic of PII-2T polymer conformation change across three regimes (see results below).

  • Fig. 2 Flow-induced morphological transition in PII-2T films from molecular, meso- to macroscales.

    (A) Absorption coefficient (α) of the films printed at various printing speeds (0.1 to 5 mm/s). The α values of the PII-2T films are sensitively modulated by the printing speed. The highest αmax value of 1.9 × 105 cm−1 is obtained in the transition regime (1 mm/s) at 719 nm. (B) αmax normalized by the αmax at 0.1 mm/s as a function of printing speed. All αmax values were obtained at ~720 nm corresponding to the 0-0 transition. (C) Absorption spectra calculated for oligomers of PII-2T of increasing size at the time-dependent density functional theory (DFT) level [B3LYP functional/6-31 g(d,p) basis set]. Solid and dashed lines denote spectrum of anti- and syn-conformation, respectively. The transition at 733 nm involves highly delocalized hole and electron wave functions along the conjugated backbone. The second absorption band at 403 nm is attributed to the transition localized on the isoindigo units. (D) SERS spectra of PII-2T films printed (on a thin gold layer) in the evaporation (0.1 mm/s), transition (1 mm/s), and LL (10 mm/s) regimes (left), with magnified spectra for the 1534 and 1704 cm−1 peaks to show the Raman shift (middle). Illustration of representative Raman-active vibrational modes calculated for PII-2T oligomers (right). The peak intensity is normalized by the strongest peak around 1435 cm−1, which is assigned as delocalized C═C stretching over thiophene rings (2T). The peaks around 1534 and 1610 cm−1 are assigned as strong localized C═C stretching, and the peak around 1704 cm−1 is assigned as C═O stretching in isoindigo units. Solid and dashed arrows indicate the relatively stronger and weaker stretching modes, respectively. (E) AFM phase images (left), corresponding orientation mapping analysis (middle), and pole figures of the fibril orientation distribution (right) for the films printed in the evaporation (0.5 mm/s), transition (1 mm/s), and LL (5 mm/s) regimes. The color mapping of fiber orientation ranges from 0° (red), 90° (cyan), to 180° (magenta). The white arrow indicates the printing direction. (F) CPOM images of PII-2T films oriented 0° and 45° relative to the polarizer/analyzer for each regime. The arrows indicate the printing direction, and black cross arrows denote cross-polarizers. The exposure time is 80.3 ms for the film printed at 0.1 mm/s and 161 ms for the films printed at 1 and 5 mm/s. (G) Optical intensity as a function of relative rotational angle between the printing direction and the polarizer. Note that the exposure time is kept constant (147 ms) for direct comparison of the three cases. The film printed at 0.1 mm/s exhibits higher intensity when the printing direction is oriented parallel (0° and 180°) or perpendicular (90°) to the polarizer, indicating that the polymer backbone is aligned diagonal to the printing direction. In contrast, the film printed at 1 mm/s exhibits higher intensity when the printing direction is oriented at 45° or 135° with respect to the polarizer, indicating the polymer backbone alignment parallel or perpendicular to the printing direction. AFM and GIXD analysis confirmed the former case (shown below). At 5 mm/s, the film still exhibits higher intensity at 45° or 135°, albeit with very low birefringence indicating weak alignment. a.u., arbitrary units.

  • Fig. 3 Flow-induced in-plane alignment and out-of-plane orientation distribution in PII-2T films.

    (A) Normalized absorption spectra of polarized UV-vis spectroscopy with the light polarization direction oriented either parallel (||, red line) or perpendicular (⊥, black line) to the printing direction. The dichroic ratio RUV-vis (A||/A) is 0.40 in the evaporation regime (0.1 mm/s), whereas the RUV-vis values are 2.82 and 2.12 for the transition (1 mm/s) and LL (5 mm/s) regimes, respectively. (B) RUV-vis plot as a function of printing speed. The RUV-vis values crossed one from the evaporation to the transition regime, indicating that the polymer backbone is altered from perpendicular to parallel with respect to the printing direction. (C) Comparison of 2D x-ray diffraction patterns with the incident beam oriented || and ⊥ to the printing direction. The red dashed boxes mark edge-on π-π stacking (010) peaks. (D) RGIXD plot representing an in-plane alignment of edge-on π-stacks as a function of printing speed. High strain rate and low residence time in the transition regime lead to highly aligned polymer aggregates with the backbone parallel to the flow direction. (E) Comparison of geometrically corrected intensity of π-π stacking (010) peak as a function of polar angle χ and in-plane rotation angle φ denoting the incident beam directions of φ = 0° (parallel), 30°, 60°, and 90° (perpendicular) with respect to the printing direction. Note that χ = 0° and 90° indicate face-on and edge-on orientation, respectively. Each inset shows the inferred molecular packing structures in PII-2T films printed in three representative regimes. The highest degrees of in-plane and out-of-plane alignment are observed in the transition regime: The backbone is aligned with the printing direction in-plane and adopts a bimodal orientation distribution out-of-plane featuring both edge-on and face-on π-stacks. The rDoC is obtained by integrating the area below each curve. The rDoC is about 480, 835, and 970 for the film produced in the evaporation, transition, and LL regimes, respectively.

  • Fig. 4 PII-2T FET device performance and charge transport anisotropy.

    (A) CPOM image of a set of circularly configured FETs. An array of silver electrodes (black) are deposited on the PII-2T film (orange) to form bottom-gate top-contact FET channels separated by 15°. The channel length (L) and width (W) are 26 and 360 μm, respectively. The charge transport anisotropy of the films can be precisely determined by the relative angle (φ) of S-D locations with respect to the printing direction. Schematic of the device geometry showing typical parallel (φ = 0° or 180°) or perpendicular (φ = 90° or 270°) device measurement. (B) Double-sweep transfer characteristics of PII-2T FET devices in log scale of drain current (black line with dots) and linear scale of the square root of the drain current (red line) at VDS = −100 V. The arrows indicate the direction of the VGS sweep. The dotted gray lines show the gate leakage current IGS, which is consistently low. The electrical parameters calculated for the PII-2T FET printed at 0.1 mm/s are a field-effect mobility (μFET) of 0.39 cm2 V−1 s−1, a threshold voltage (VTH) of −6.6 V, and an on/off current ratio (ION/IOFF) of 4.9 × 103. The FET printed at 1 mm/s resulted in a μFET of 1.1 cm2 V−1 s−1, a VTH of −9.3 V, and an ION/IOFF of 1.3 × 104. The FET printed at 5 mm/s yielded a μFET of 0.41 cm2 V−1 s−1, a VTH of −18 V, and an ION/IOFF of 1.3 × 105. Notably, these results correspond to annealed devices at 200°C to enhance crystallinity and to improve charge transport (fig. S30). (C) Corresponding output characteristics of FET devices. Notably, the contact resistance (inferred from the shape of the output curve near origin) is prominently reduced on the films printed at 1 and 5 mm/s when compared to that of film printed at 0.1 mm/s because of the decreased surface roughness and film thickness. (D) Hole mobility of FET devices as a function of φ. High charge transport anisotropy is observed for films produced in the transition regime because of a high degree of backbone alignment. The extent of charge transport anisotropy is enhanced through thermal annealing.

  • Fig. 5 Molecular structure–dependent flow-induced morphological transition of polymers.

    (A and B) Molecular structures of PII-2T (A) and PTII-2T (B), and corresponding optimized dimers with average torsional angles based on DFT calculations at the B3LYP/6-31 g(d,p) level. Side view of energy-minimized conformers depicts the varying degrees of backbone coplanarity. The systematic torsion within each repeat unit of PII-2T causes its backbone to adopt a slight twist conformation, whereas the backbone of PTII-2T yields relatively high coplanar chains. (C to F) Absorption spectra of PII-2T (C), PTII-2T (D), DPP-BTz (E), and DPP2T-TT (F) polymer films as a function of printing speed. The αmax values obtained for the lowest speed are 3.4 × 104 (PII-2T), 1.2 × 105 (PTII-2T), 4.1 × 104 (DPP-BTz), and 1.8 × 105 cm−1 (DPP2T-TT). (G to J) αmax normalized by the lowest αmax for PII-2T (G), PTII-2T (H), DPP-BTz (I), and DPP2T-TT (J) polymer system. The αmax varies largely with printing speeds for PII-2T and DPP-BTz, showing the highest values for the films produced in the transition regime. In contrast, PTII-2T and DPP2T-TT are less influenced by flow. (K) AFM height images of the polymer films for the three regimes. The white arrow indicates the printing direction. Scale bars, 1 μm. Flow-induced morphological transition is observed for PII-2T and DPP-BTz, in which the twinned domains are diminished upon entering the transition regime.

  • Fig. 6 Flow-controlled polymer assembly mechanism.

    (A to D) CPOM images of PII-2T (A), PTII-2T (B), DPP-BTz (C), and DPP2T-TT (D) solution- to solid-state phase transition in a receding meniscus driven by solvent evaporation. Note that the meniscus receding speeds are approximately 0.01 to 0.05 mm/s, corresponding to evaporation regime. The white arrows indicate the meniscus receding direction. Notably, PII-2T and DPP-BTz exhibit strong birefringence near the contact line, indicating crystalline mesophase formation. In contrast, no such mesophase is observed for PTII-2T and DPP2T-TT. (E to H) CD spectra of PII-2T (E), PTII-2T (F), DPP-BTz (G), and DPP2T-TT (H) polymer films as a function of printing speed. Each low, intermediate, and fast speed corresponds to the evaporation, transition, and LL regime, respectively. Only PII-2T and DPP-BTz films printed in the evaporation regime exhibit CD signals, corroborating the formation of chiral, twinned domains mediated by a twist-bend nematic phase (A and C). In contrast, PTII-2T and DPP2T-TT do not show substantial CD signals, corresponding to the absence of an intermediate twist-bend nematic phase (B and D). (I) Schematic illustration of the inferred flow-controlled polymer assembly mechanism. In the evaporation regime, torsional polymer molecules assemble in a helical fashion to form an NTB mesophase. The helical polymer fibers subsequently assemble into the twinned domains. The chirality in twinned morphology is caused by the helicity of the NTB phase. Given the higher strain in the transition regime, the twisted molecule is stretched out/planarized to eliminate the NTB mesophase, resulting in uniaxially aligned morphology.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/8/eaaw7757/DC1

    Section S1. COMSOL simulation

    Section S2. Spectroscopic measurement and analysis

    Section S3. Characterization of morphology

    Section S4. Concentration, substrate surface, and MW effects

    Section S5. Simulation and experimental results of capillary pen writing

    Section S6. Characterization of molecular orientation and packing structures

    Section S7. Fabrication and measurement of devices

    Section S8. Understanding the molecular origin of flow-induced morphological transition by comparing polymer structures

    Section S9. Evidence of NTB mesophase in solution and in deposited thin films

    Fig. S1. Sensitivity of regimes to geometric parameters in simulation.

    Fig. S2. Complete speed series of velocity fields and strain rates for PII-2T/chloroform blade coating simulations.

    Fig. S3. Meniscus height, polymer volume fraction, and evaporative flux profile for PII-2T/chloroform simulations.

    Fig. S4. Excited-state electronic structure.

    Fig. S5. Normalized solution UV-vis absorption spectra of PII-2T prepared in chloroform with various concentrations from 0.002 to 10 g/liter.

    Fig. S6. UV-vis absorption spectra of PII-2T solutions using different MWs.

    Fig. S7. UV-vis absorption spectra of PII-2T films printed at various printing speeds.

    Fig. S8. PII-2T film preparation for SERS.

    Fig. S9. Experimental and calculated Raman intensity.

    Fig. S10. Illustration of the main bond stretching assignments for Raman-active vibrational modes.

    Fig. S11. Polymer fiber alignment analysis with AFM images as a function of printing speed.

    Fig. S12. Top and bottom surface morphology comparison of PII-2T films.

    Fig. S13. CPOM images of the films as a function of printing speed.

    Fig. S14. CPOM images and thickness of the films as a function of printing speed using various concentrations of PII-2T solutions.

    Fig. S15. CPOM images and thickness of PII-2T films as a function of printing speed using various substrate surfaces.

    Fig. S16. UV-vis absorption spectra of low-MW PII-2T films printed at various printing speeds.

    Fig. S17. Polymer fiber and backbone alignment analysis of low-MW PII-2T films printed at various printing speeds.

    Fig. S18. Flow-induced morphological transition for pen-written PII-2T films and velocity and strain rate from computational fluid dynamics simulations.

    Fig. S19. Complete speed series of velocity fields and strain rates for PII-2T/o-DCB blade coating simulations.

    Fig. S20. Maximum strain rate and inverse residence time as a function of printing speed for PII-2T/o-DCB simulations.

    Fig. S21. CPOM images of the capillary pen–written films as a function of printing speed.

    Fig. S22. Flow-induced morphological transition in PII-2T films printed using o-DCB solvent.

    Fig. S23. Polarized UV-vis absorption spectra and dichroic ratio of PII-2T films printed at various printing speeds with different concentration solutions.

    Fig. S24. Comparison and analysis of 2D x-ray diffraction patterns.

    Fig. S25. GIXD analysis of PII-2T films obtained at multiple in-plane rotation angles.

    Fig. S26. GIXD characterization of PII-2T morphology in the bulk and top surfaces of printed films.

    Fig. S27. GIXD characterization of PII-2T films printed at various printing speeds with different solution concentrations.

    Fig. S28. Electrical characteristics for as-prepared PII-2T devices without post-thermal treatment.

    Fig. S29. Gate voltage–dependent mobility as a function of gate bias, extracted from the transfer curves shown in Fig. 4B.

    Fig. S30. Thermal annealing effect of printed PII-2T films.

    Fig. S31. Calculated torsion potentials of PII-2T and PTII-2T.

    Fig. S32. Normalized solution UV-vis absorption spectra of PTII-2T, DPP-BTz, and DPP2T-TT prepared in chloroform with various concentrations from 0.001 to 10 g/liter.

    Fig. S33. Solution and film UV-vis absorption spectra of PII-2T, PTII-2T, DPP-BTz, and DPP2T-TT.

    Fig. S34. Morphology characterization of PII-2T, PTII-2T, DPP-BTz, and DPP2T-TT polymer films prepared in each regime.

    Fig. S35. Characterization of the polymer film alignment.

    Fig. S36. Formation of chiral, liquid crystalline mesophase in PII-2T solutions with increasing concentrations.

    Fig. S37. GIXD analysis of twinned morphology in PII-2T films.

    Fig. S38. CPOM images of PII-2T twinned morphological films.

    Fig. S39. CPOM images of DPP-BTz twinned morphological films.

    Fig. S40. Multiple CD measurements of PII-2T and DPP-BTz films printed in evaporation regime.

    Table S1. Parameters used in the simulations for PII-2T/chloroform blade coating and PII-2T/o-DCB pen writing.

    Movie S1. Solution- to solid-state phase transition of PII-2T.

    Movie S2. Solution- to solid-state phase transition of PTII-2T.

    Movie S3. Solution- to solid-state phase transition of DPP-BTz.

    Movie S4. Solution- to solid-state phase transition of DPP2T-TT.

    References (4251)

  • Supplementary Materials

    The PDF file includes:

    • Section S1. COMSOL simulation
    • Section S2. Spectroscopic measurement and analysis
    • Section S3. Characterization of morphology
    • Section S4. Concentration, substrate surface, and MW effects
    • Section S5. Simulation and experimental results of capillary pen writing
    • Section S6. Characterization of molecular orientation and packing structures
    • Section S7. Fabrication and measurement of devices
    • Section S8. Understanding the molecular origin of flow-induced morphological transition by comparing polymer structures
    • Section S9. Evidence of NTB mesophase in solution and in deposited thin films
    • Fig. S1. Sensitivity of regimes to geometric parameters in simulation.
    • Fig. S2. Complete speed series of velocity fields and strain rates for PII-2T/chloroform blade coating simulations.
    • Fig. S3. Meniscus height, polymer volume fraction, and evaporative flux profile for PII-2T/chloroform simulations.
    • Fig. S4. Excited-state electronic structure.
    • Fig. S5. Normalized solution UV-vis absorption spectra of PII-2T prepared in chloroform with various concentrations from 0.002 to 10 g/liter.
    • Fig. S6. UV-vis absorption spectra of PII-2T solutions using different MWs.
    • Fig. S7. UV-vis absorption spectra of PII-2T films printed at various printing speeds.
    • Fig. S8. PII-2T film preparation for SERS.
    • Fig. S9. Experimental and calculated Raman intensity.
    • Fig. S10. Illustration of the main bond stretching assignments for Raman-active vibrational modes.
    • Fig. S11. Polymer fiber alignment analysis with AFM images as a function of printing speed.
    • Fig. S12. Top and bottom surface morphology comparison of PII-2T films.
    • Fig. S13. CPOM images of the films as a function of printing speed.
    • Fig. S14. CPOM images and thickness of the films as a function of printing speed using various concentrations of PII-2T solutions.
    • Fig. S15. CPOM images and thickness of PII-2T films as a function of printing speed using various substrate surfaces.
    • Fig. S16. UV-vis absorption spectra of low-MW PII-2T films printed at various printing speeds.
    • Fig. S17. Polymer fiber and backbone alignment analysis of low-MW PII-2T films printed at various printing speeds.
    • Fig. S18. Flow-induced morphological transition for pen-written PII-2T films and velocity and strain rate from computational fluid dynamics simulations.
    • Fig. S19. Complete speed series of velocity fields and strain rates for PII-2T/o-DCB blade coating simulations.
    • Fig. S20. Maximum strain rate and inverse residence time as a function of printing speed for PII-2T/o-DCB simulations.
    • Fig. S21. CPOM images of the capillary pen–written films as a function of printing speed.
    • Fig. S22. Flow-induced morphological transition in PII-2T films printed using o-DCB solvent.
    • Fig. S23. Polarized UV-vis absorption spectra and dichroic ratio of PII-2T films printed at various printing speeds with different concentration solutions.
    • Fig. S24. Comparison and analysis of 2D x-ray diffraction patterns.
    • Fig. S25. GIXD analysis of PII-2T films obtained at multiple in-plane rotation angles.
    • Fig. S26. GIXD characterization of PII-2T morphology in the bulk and top surfaces of printed films.
    • Fig. S27. GIXD characterization of PII-2T films printed at various printing speeds with different solution concentrations.
    • Fig. S28. Electrical characteristics for as-prepared PII-2T devices without post-thermal treatment.
    • Fig. S29. Gate voltage–dependent mobility as a function of gate bias, extracted from the transfer curves shown in Fig. 4B.
    • Fig. S30. Thermal annealing effect of printed PII-2T films.
    • Fig. S31. Calculated torsion potentials of PII-2T and PTII-2T.
    • Fig. S32. Normalized solution UV-vis absorption spectra of PTII-2T, DPP-BTz, and DPP2T-TT prepared in chloroform with various concentrations from 0.001 to 10 g/liter.
    • Fig. S33. Solution and film UV-vis absorption spectra of PII-2T, PTII-2T, DPP-BTz, and DPP2T-TT.
    • Fig. S34. Morphology characterization of PII-2T, PTII-2T, DPP-BTz, and DPP2T-TT polymer films prepared in each regime.
    • Fig. S35. Characterization of the polymer film alignment.
    • Fig. S36. Formation of chiral, liquid crystalline mesophase in PII-2T solutions with increasing concentrations.
    • Fig. S37. GIXD analysis of twinned morphology in PII-2T films.
    • Fig. S38. CPOM images of PII-2T twinned morphological films.
    • Fig. S39. CPOM images of DPP-BTz twinned morphological films.
    • Fig. S40. Multiple CD measurements of PII-2T and DPP-BTz films printed in evaporation regime.
    • Table S1. Parameters used in the simulations for PII-2T/chloroform blade coating and PII-2T/o-DCB pen writing.
    • Legends for movies S1 to S4
    • References (4251)

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.avi format). Solution- to solid-state phase transition of PII-2T.
    • Movie S2 (.avi format). Solution- to solid-state phase transition of PTII-2T.
    • Movie S3 (.avi format). Solution- to solid-state phase transition of DPP-BTz.
    • Movie S4 (.avi format). Solution- to solid-state phase transition of DPP2T-TT.

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