Solution coating of large-area organic semiconductor thin films with aligned single-crystalline domains

Journal name:
Nature Materials
Volume:
12,
Pages:
665–671
Year published:
DOI:
doi:10.1038/nmat3650
Received
Accepted
Published online

Abstract

Solution coating of organic semiconductors offers great potential for achieving low-cost manufacturing of large-area and flexible electronics. However, the rapid coating speed needed for industrial-scale production poses challenges to the control of thin-film morphology. Here, we report an approach—termed fluid-enhanced crystal engineering (FLUENCE)—that allows for a high degree of morphological control of solution-printed thin films. We designed a micropillar-patterned printing blade to induce recirculation in the ink for enhancing crystal growth, and engineered the curvature of the ink meniscus to control crystal nucleation. Using FLUENCE, we demonstrate the fast coating and patterning of millimetre-wide, centimetre-long, highly aligned single-crystalline organic semiconductor thin films. In particular, we fabricated thin films of 6,13-bis(triisopropylsilylethynyl) pentacene having non-equilibrium single-crystalline domains and an unprecedented average and maximum mobilities of 8.1±1.2 cm2 V−1 s−1 and 11 cm2 V−1 s−1. FLUENCE of organic semiconductors with non-equilibrium single-crystalline domains may find use in the fabrication of high-performance, large-area printed electronics.

At a glance

Figures

  1. Fluid-flow-enhanced crystal growth.
    Figure 1: Fluid-flow-enhanced crystal growth.

    a, Schematic of solution shearing using a micropillar-patterned blade. For clarity, the micropillars are not drawn to scale. The arrow indicates the shearing direction. b, A scanning electron micrograph of a micropillar-patterned blade. Inset, top view of the micropillars under an optical microscope. The pillars are 35 μm wide and 42 μm high. c, Streamline representation of simulated fluid flow around the micropillars. The arrow indicates the flow direction. The streamlines are colour coded to indicate the scale of velocity (mm s−1), ranging from 0 (deep blue) to 1.3 mm s−1 (dark red; see Supplementary Fig. S1). df, Cross-polarized optical micrograph of a TIPS-pentacene film coated from its mesitylene solution with (d, right; f) and without micropillars (d, left; e), at a shearing speed of 0.6 mm s−1.

  2. Solution-sheared single-crystalline TIPS-pentacene thin film.
    Figure 2: Solution-sheared single-crystalline TIPS-pentacene thin film.

    ac, Cross-polarized optical micrograph of a TIPS-pentacene film coated from its mesitylene solution with (a,b right and c) and without (a,b left) using FLUENCE, at a shearing speed of 0.6 mm s−1. The region of the film prepared without FLUENCE is labelled as Reference. The crossed arrows indicate the orientation of crossed polarizers. The white arrow in a denotes the shearing direction. The AFM image corresponding to c is shown in Supplementary Fig. S4. d, A photographic image of one 1-mm-wide, 2-cm-long, single-crystal OFET on a Si wafer. The blue regions are TIPS-pentacene single-crystalline domains, corresponding to those shown in the right half of a. The purple regions are the uncoated SiO2 substrate functionalized with OTS, which dewets the solution during coating, designed for controlling nucleation. The Stanford logo is the same size as a dime. e, Schematic of solution shearing with FLUENCE. The arrow indicates the shearing direction.

  3. Domain alignment and crystal quality of the single-crystalline TIPS-pentacene films versus that of reference samples.
    Figure 3: Domain alignment and crystal quality of the single-crystalline TIPS-pentacene films versus that of reference samples.

    a, A schematic illustration of an X-ray -scan performed in grazing-incidence scattering geometry. denotes the in-plane rotation angle. The film orientation shown is at   =  0°. b, In-plane molecular structure of a TIPS-pentacene crystal with the b axis aligned with the shearing direction, at   =  0°. c, Comparison of (010) peak distributions as a function of : single-crystalline (with FLUENCE) versus reference films (without FLUENCE). d, Coherence length comparison between single-crystalline and reference films, both perpendicular and parallel to the shearing direction, obtained from high-resolution X-ray scans (see Methods). e, Tapping-mode AFM image taken on a single-crystalline domain, showing terraces and facets (traced with white lines at a distance). The depth profile beneath is obtained along the white dotted line. The step heights are approximately 1.7 nm, comparable to the height of a single molecular layer of TIPS-pentacene. Crystal terraces are also evident from Supplementary Fig. S4. All characterized samples were solution sheared at 0.6 mm s−1.

  4. Grazing-incidence X-ray diffraction images of TIPS-pentacene films for varying solution concentration.
    Figure 4: Grazing-incidence X-ray diffraction images of TIPS-pentacene films for varying solution concentration.

    The (hk) indexes of the (01), (10) and ( ) Bragg rods are shown on top. Indexes of the degenerate peaks are not shown. TIPS-pentacene films were coated from mesitylene solution of various concentrations at 0.8 mm s−1. The film coated from a lower concentration (1.6 mg ml−1) showed the highest degree of unit-cell distortion, judged from the fact that qxy (01) and qxy (10) exhibit the largest extent of deviation from their respective equilibrium values.

  5. Comparison of transistor characteristics between single-crystalline domain (blue) and reference samples (red) of non-equilibrium TIPS-pentacene.
    Figure 5: Comparison of transistor characteristics between single-crystalline domain (blue) and reference samples (red) of non-equilibrium TIPS-pentacene.

    Reference samples denote those solution-coated without FLUENCE. a, Histograms of hole mobility measured along the solution shearing direction obtained from 50 to 60 transistors. The mobility average, standard deviation and coefficient of variation (CV) are listed in each case. b, Representative transfer characteristics plot on the same scale. The corresponding output curve of the single-crystalline device is shown in Supplementary Fig. S15. c, Typical hysteresis characteristics. Three forward and backward scans are shown. Inset, schematic of the device configuration. G, S and D denote gate, source and drain, respectively. The optical image of the thin-film transistors is shown in Supplementary Fig. S16. Data shown were measured from samples prepared at a shearing speed of 0.6 mm s−1.

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Author information

Affiliations

  1. Department of Chemical Engineering, Stanford University, Stanford, California 94305, USA

    • Ying Diao,
    • Gaurav Giri,
    • Jie Xu,
    • Do Hwan Kim,
    • Hector A. Becerril &
    • Zhenan Bao
  2. Department of Electrical Engineering, Stanford University, Stanford, California 94305, USA

    • Benjamin C-K. Tee &
    • Tae Hoon Lee
  3. Department of Polymer Science and Engineering, Institute of Chemistry and Chemical Engineering, The State Key Laboratory of Coordination Chemistry, The National Laboratory of Nanjing Microstructure Study, Nanjing University, Nanjing 210093, China

    • Jie Xu &
    • Gi Xue
  4. Department of Chemistry, Stanford University, Stanford, California 94305, USA

    • Randall M. Stoltenberg
  5. Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA

    • Stefan C. B. Mannsfeld
  6. Present addresses: Department of Organic Materials and Fiber Engineering, Soongsil University, Seoul, 156-743, Republic Korea (D.H.K); Department of Chemistry at Brigham Young University, Idaho, Rexburg, Idaho 83440, USA (H.A.B.); Lockheed Martin Space Systems Company, 3251 Hanover Street, Palo Alto, California 94304, USA (R.M.S.)

    • Do Hwan Kim,
    • Hector A. Becerril &
    • Randall M. Stoltenberg

Contributions

Z.B., S.C.B.M. and Y.D. conceived and designed the experiments. B.C-K.T. fabricated micropillar-patterned shearing blades, and performed photolithography and scanning electron microscopy. Y.D. and J.X. performed solution coating. Y.D. carried out numerical simulation, performed morphology characterizations and conducted device testing. Y.D., G.G. and S.C.B.M. performed X-ray diffraction measurements and data analysis. D.H.K. and T.H.L. advised on surface functionalization and device testing. H.A.B. and R.M.S. contributed to the initial design of the structured shearing blade. Y.D. wrote the first draft of the manuscript. All authors discussed the results and revised the manuscript. Z.B. and S.C.B.M. directed the project.

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The authors declare no competing financial interests.

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