Liquid crystals for organic thin-film transistors

Abstract

Crystalline thin films of organic semiconductors are a good candidate for field effect transistor (FET) materials in printed electronics. However, there are currently two main problems, which are associated with inhomogeneity and poor thermal durability of these films. Here we report that liquid crystalline materials exhibiting a highly ordered liquid crystal phase of smectic E (SmE) can solve both these problems. We design a SmE liquid crystalline material, 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10), for FETs and synthesize it. This material provides uniform and molecularly flat polycrystalline thin films reproducibly when SmE precursor thin films are crystallized, and also exhibits high durability of films up to 200 °C. In addition, the mobility of FETs is dramatically enhanced by about one order of magnitude (over 10 cm(2) V(-1) s(-1)) after thermal annealing at 120 °C in bottom-gate-bottom-contact FETs. We anticipate the use of SmE liquid crystals in solution-processed FETs may help overcome upcoming difficulties with novel technologies for printed electronics.

Figure 1. Basic characteristic of Ph-BTBT-10.

(a) Chemical structure, (b) texture as determined by polarized optical microscopy at 150 °C, white bar indicates a scale of 50 μm in length, (c) XRD pattern of a liquid crystalline film exhibiting a polydomain texture acquired at 160 °C in an out-of-plain configuration, (d) DSC curves obtained at 2 °C min⁻¹ and single-crystal structures of the (e) ac and (f) ab planes. Cryst., crystalline; Liq., liquid.

Figure 2. Characteristics of polycrystalline thin films of Ph-BTBT-10.

The polycrystalline thin film were fabricated from a 0.5 wt% diethylbenzene solution at ∼110 °C, (a,d) texture as determined by optical microscopy and (b,e) AFM images. Insets show cross-sectional profiles observed by (a,d) confocal laser scanning microcopy and (b,e) AFM. (a,b) As-coated polycrystalline film and (d,e) after thermal stress at 150 °C for 5 min. (a,d) White bars indicate scale of 20 μm in length. (c) Four d-spacings of Ph-BTBT-10 in crystal and SmE phase in wide- and small-angle regions as a function of temperature.

Figure 3. Characteristics of top-contact FET and its change after thermal annealing.

Characteristics of top-contact type FETs made using polycrystalline thin films of Ph-BTBT-10 fabricated from 0.5 wt% p-xylene solution in the SmE phase at ca., 80 °C. Output characteristics of FETs fabricated using the polycrystalline thin films (a) as-coated and (c) after thermal annealing at 120 °C for 5 min, (b) transfer characteristics of FETs fabricated with polycrystalline thin films both as-coated and after thermal annealing, (d) FET mobility as a function of annealing temperature, the error bars calculated from the standard deviations over 10 samples in each annealing temperature, (e) a topographic image of the polycrystalline thin films after annealing at 120 °C, as obtained by AFM, (f) out-of-plane small-angle XRD patterns for crystalline films as-coated and after annealing, with a schematic illustration of the crystalline structure and (g) DSC data for Ph-BTBT-10 during fast cooling and heating at 20 °C min⁻¹.

Figure 4. Characteristics of the polycrystalline thin films and contact resistance.

Depth profile of sulfur atom (a) before, (e) after thermal annealing at 120 °C observed by TOF-SIMS. Depth axis was determined by sputtering rate. Polarized optical microscope images of polycrystalline thin films of Ph-BTBT-10 (b) before and (f) after thermal annealing at 120 °C for 5 min. White bars indicate scale of 20 μm in length. Contact resistance of FETs fabricated with polycrystalline thin film of Ph-BTBT-10 (c,d) before and (g,h) after thermal annealing at 120 °C for 5 min evaluated by transfer line method.

Figure 5. Characteristics of bottom-contact FETs after thermal annealing.

Characteristics of bottom-gate, bottom-contact type FETs made with polycrystalline thin films of Ph-BTBT-10 after thermal annealing at 120 °C. (a) output characteristics and (b–d) transfer characteristics. Transfer characteristics focused on (c) hysteresis and (d) repeated measurement properties. dec, decade.

Figure 6. Reliability of FETs and fabrication of uniform thin films at 40 °C.

FET characteristics of bottom-gate, bottom-contact type FETs made with polycrystalline thin films of Ph-BTBT-10 after thermal annealing at 120 °C. (a) Variations of FET mobility in five films. Thermal durability properties of FET fabricated with polycrystalline thin films of Ph-BTBT-10. (b) FET mobility as a function of stress temperature. Polycrystalline thin films of Ph-BTBT-10 fabricated from a p-xylene solvent mixture. (c) Optical microcopy texture and (d) AFM image. (c) White bar indicates a scale of 20 μm in length. Cryst., crystalline; Liq, liquid.