Liquid crystals for organic thin-film transistors

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 cm2 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.


Supplementary Note 1 | Phase transtion behavior of Ph-BTBT-10
The phase transition temperatures were determined by differential scanning calorimetry (DSC: Shimadzu 60). Their liquid crystal phases were identified by the textures observation under a polarized optical microscope (Nikon OPTIPHOT2-POL and Mettler Toledo FP82HT Hot stage).
Ph-BTBT-10, exhibited two mesophases from 143 to 223 o C as shown in the DSC chart of Fig. 1(d).
The polarized optical microcopy textures at 150, 220 and 26 o C showed disclination line, fan like patterns, and strip texture with cracks as shown in Fig. 1

Supplementary Note 2 | Structure transition of Ph-BTBT-10 from SmE to crystal phases
The liquid crystal and crystal structures of Ph-BTBT-10 are evaluated by X-ray diffraction (XRD) analysis using Rigaku RAD-2X diffractometer with CuKα radiation. The rectangular molecular alignment in SmE phase was very similar to that in crystal phase, and the d-spacing of the molecules within a layer in the wide angles continuously was changed without its abrupt shrinkage in the phase transition from the SmE to crystal phases, while the d-spacing between layers was shortened as shown in Fig. 2(c) and Supplementary Fig. 2. This result explained why no crake was formed in the polycrystalline thin films of Ph-BTBT-10.

Supplementary Note 3 | Molecular alignment of polycrystalline thin films
The molecular alignments of the polycrystalline thin films on a SiO 2 /Si-substrate were evaluated by X-ray diffraction (XRD) analysis, using Rigaku RAD-2X diffractometer with CuKα radiation for out-of-plane measurement and Bruker AXS Co. Ltd. D8 Discover μHR for in-plane measurements.
XRD spectra of out-of-plane and in-plane of the polycrystalline thin films fabricated at 82 o C indicated that molecules are aligned perpendicular on the substrate as shown in Supplementary Fig.   3.

Supplementary Note 4 | Crystal structure of polycrystalline thin film after heating to SmE phase
In order to clarify the mono-layer or bi-layer structures in a bi-layered film of Ph-BTBT-10 after heating at the temperature of over 143 o C, which is the phase transition temperature from crystal to SmE phases of Ph-BTBT-10 in heating process, we evaluated the thin films of Ph-BTBT-10 after thermal heating at 160 o C and fast cooling to room temperature by XRD measurement in small angle region, TOF-SIMS, and AFM images. XRD result showed a peak corresponding to mono-layer structure as shown in Supplementary Fig. 4(a). Furthermore, TOF-SIMS result showed no structure in the depth profile of sulfur atoms as shown in Supplementary Fig. 4(b). This indicates the film had the mono-layer structure as in the case of spin-coated films at a temperature of SmE phase. In fact, the films showed 2.5 nm steps for a mono-layer structure as shown in Supplementary Fig. 4(c) of the surface profile observed by AFM.

Supplementary Note 5 | Contact resistance bottom-gate bottom-contact FET
The HOMO level of Ph-BTBT-10 was estimated to be 5.6eV by cyclic voltammetry which is the same as that of dialkyl-BTBT derivatives. In even bottom-contact FET, the contact resistance with gold electrodes was small, i.e., 3.8kΩcm as shown in Supplementary Fig. 5(a, b), which is much smaller than the contact resistance of dialkyl-BTBT derivatives reported to be 10-100 kΩcm. 1 However, Ph-BTBT-10 FET has high mobility of over 10 cm 2 /Vs and high drain current, 10 -3 A (V DS =-100V, W=500μm) as shown in Fig. 3(b), and the total resistance of 5kΩcm, which was comparable to the contact resistance of several kΩcm. Because of these, FET showed non-ideal characteristics. In order to improve the FET performance, we adopted the bottom-gate bottom-contact FET with Au electrode treated with pentafluorobenzenethiol (PFBT) in order to improve the contact resistance. PFBT-treated Au electrodes gave a considerably small contact resistance of ca., 150 Ωcm as shown in Supplementary Fig. 5(c, d), while bare Au electrodes gave a contact resistance of 3.8kΩcm, leading to improved FET performance as shown in Fig. 5.