Research ArticleNANOTECHNOLOGY

Ultrahigh mobility and efficient charge injection in monolayer organic thin-film transistors on boron nitride

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Science Advances  06 Sep 2017:
Vol. 3, no. 9, e1701186
DOI: 10.1126/sciadv.1701186
  • Fig. 1 Device geometry of monolayer C8-BTBT OTFT and its electrical characteristics.

    (A) Schematic illustration of the contact between Au and C8-BTBT. The right part is overlaid by the charge density in the energy range of [EF-0.5 eV, EF]. EF is the Fermi energy of the system. The distribution of charge density along the z direction is plotted. (B) Cross-sectional TEM image of Au/C8-BTBT/BN stack, showing atomically smooth interface. (C) Room temperature double-sweep Ids-Vg characteristics (black line) and the extracted four-terminal field-effect mobility as a function of Vg (red circle). (D) Ids-Vds characteristics of the same device in (C). From top to bottom, Vg = −20, −50, −60, and −70 V, respectively. Inset shows the microscopic image of the device. The source (S), drain (D), and voltage probes (V−, V+) are marked. Scale bar, 6 μm.

  • Fig. 2 Temperature-dependent electrical transport of the monolayer C8-BTBT OTFT in Fig. 1.

    (A) Channel conductance σ4P as a function of Vg under different temperatures. (B) Extrinsic and intrinsic mobility as a function of temperature. (C) Histogram of the intrinsic mobility of monolayer devices at different temperature. During the cooling process, some devices were broken, so the number of devices decreases at low temperature. (D) Low-bias Ids-Vds characteristics at Vg = −70 V under T = 300 K (red), 200 K (blue), 100 K (green), and 80 K (black). Inset shows the contact resistance at Vg = −70 V as a function of temperature, derived from the gFPP measurements.

  • Fig. 3 Thickness-dependent OTFT properties.

    (A) Room temperature Ids-Vds characteristics for typical monolayer, bilayer, and trilayer devices. (B) Arrhenius plot of Ids (Vg = −70 V, Vds = −1 V) of the monolayer (red circle) and bilayer (blue circle) devices in (A). The inset shows energy band diagrams near the contact, with different mechanisms. (C) Arrhenius plot of ln(Ids/T3/2) of the bilayer device in (A). From top to bottom: Vg = −60.4, −56.4, −52.4, −48.4, −44.4, and −40.4 V. (D) Derived SB height as a function of Vg. The true SB height is 140 meV, as pointed by the arrow.

  • Fig. 4 Calculated electronic and contact properties.

    (A) Band alignment of Au/1L and Au/2L metal-semiconductor junction. Four valance states responsible for the hole transport are marked (the band structure is shown in fig. S8). The color of these states indicates different DOSs as shown by the color bar. (B) Total DOS for 1L (blue solid line) and 2L C8-BTBT (red solid line). Blue and red arrows mark the highest-energy valence bands for 1L and 2L, respectively. (C) Spatial distribution of the molecular orbitals at G point for 1LVB1 and 1LVB2. (D) Spatial distribution of the molecular orbitals at G and X point (0.5, 0, 0) for 2LVB1 using an isosurface of 0.0005 e bohr−3.

  • Fig. 5 Electrical characteristics of graphene-contacted bilayer C8-BTBT OTFT.

    (A) Schematic illustration of the device geometry. (B) Cross-sectional TEM image of Au/graphene/C8-BTBT/BN stack. The arrows point at the position of graphene. (C) Raman spectra and AFM (inset) of CVD graphene undergone 20-min UVO treatment. Several characteristic peaks are marked. Scale bar, 3 μm. (D) Room temperature double-sweep Ids-Vg characteristics of a typical graphene-contacted bilayer device. (E) Room temperature Ids-Vds characteristics of the same device in (D). From top to bottom, Vg = −20, −50, −60, and −70 V.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/9/e1701186/DC1

    fig. S1. AFM images of several representative C8-BTBT thin films on BN with different thickness.

    fig. S2. A two-terminal monolayer C8-BTBT OTFT.

    fig. S3. Electrical data of another four-probe monolayer C8-BTBT OTFT.

    fig. S4. Histogram of contact resistance of 12 monolayer C8-BTBT OTFTs.

    fig. S5. Electrical data of another bilayer C8-BTBT OTFT.

    fig. S6. We investigated 14 configurations through rotating β and θ angle of C8-BTBT molecules from 90° to 30° along the x and y axis, respectively.

    fig. S7. Geometric structure of single C8-BTBT molecule and visualized wave functions.

    fig. S8. Electronic band structures of 1L and 2L C8-BTBT.

    fig. S9. Visualized wave functions for six states in the momentum space of the highest valence band of 2L C8-BTBT.

    fig. S10. Visualized G-point wave functions for VB1 and VB2 of 1L and 2L in yz and xy (top view of thiophene part) planes, respectively.

    fig. S11. The contact models of Au electrodes with 1L or 2L C8-BTBT.

    fig. S12. Characterizations of CVD graphene.

    fig. S13. Electrical data of another graphene-contacted bilayer C8-BTBT OTFT.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. AFM images of several representative C8-BTBT thin films on BN with different thickness.
    • fig. S2. A two-terminal monolayer C8-BTBT OTFT.
    • fig. S3. Electrical data of another four-probe monolayer C8-BTBT OTFT.
    • fig. S4. Histogram of contact resistance of 12 monolayer C8-BTBT OTFTs.
    • fig. S5. Electrical data of another bilayer C8-BTBT OTFT.
    • fig. S6. We investigated 14 configurations through rotating β and θ angle of C8-BTBT molecules from 90° to 30° along the x and y axis, respectively.
    • fig. S7. Geometric structure of single C8-BTBT molecule and visualized wave functions.
    • fig. S8. Electronic band structures of 1L and 2L C8-BTBT.
    • fig. S9. Visualized wave functions for six states in the momentum space of the highest valence band of 2L C8-BTBT.
    • fig. S10. Visualized G-point wave functions for VB1 and VB2 of 1L and 2L in yz and xy (top view of thiophene part) planes, respectively.
    • fig. S11. The contact models of Au electrodes with 1L or 2L C8-BTBT.
    • fig. S12. Characterizations of CVD graphene.
    • fig. S13. Electrical data of another graphene-contacted bilayer C8-BTBT OTFT.

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