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.

1,4,8,11,15,18,22,25-Octaoctylphthalocyanine shows the highest time-of-flight (“long-range”) hole mobility so far reported for the columnar phase of a discotic liquid crystal. Unlike most other high long-range mobility columnar discotics, there is no clear evidence from x-ray diffraction of high order.

The research of materials for organic fi eldeffect transistors (OFETs) has been extended to the exploration for new materials suitable for the fabrication of polycrystalline thin fi lms by solution processes such as spincoating and ink-jet techniques instead of costly vacuum evaporation techniques. In fact, solution-processed polycrystalline thin fi lms from precursor materials, [ 1 , 2 ] such as 6,13-bis(triisopropylsilylethynyl)-pentacene, [ 3 ] and alkylated oligothiophene, [4][5][6] hexabenzocoronene, [ 7 ] and benzothienobenzothiophene (BTBT) [ 8 ] derivatives, attained high fi eld-effect transistor (FET) mobility over 1 cm 2 V − 1 s − 1 comparable to that of vacuum evaporated fi lms of pentacene. [ 9 ] In industrial applications of FETs, uniformity of the fi lms is another important requirement because FETs are often used in a transistor array, for example, for active matrices in display devices. Therefore, the time-consuming optimization of fi lmcoating parameters such as solvents, concentrations and evaporation rates of the solvent, and the surface wetting condition of the substrates is essential. Towards this, polymer materials have some advantages in general thanks to their fi lm-forming ability, as is demonstrated in polythiophenes (P3HT), [ 10,11 ] polyfl uorenes (F8), [ 12 ] and poly(2,5-bis(3-alkylthiophen-2-yl) thieno[3,2-b]thiophene) (PBTTT). [ 13 ] In this paper, we demonstrate the availability of liquid crystalline materials for fabricating polycrystalline thin fi lms for OFET applications in terms of both surface morphology and area uniformity, and discuss its origin.We selected a liquid crystalline terthiophene ( 1 , ω , ω '-dioctylterthiophene (8-TTP-8)) as a model material for liquid crystals and a BTBT ( 2 , 2,7-didecylbenzothienobenzothiophene (C 10 -BTBT)) derivative as another example, the chemical structures of which are shown in Figure 1 . In order to investigate the effect of spin-coating temperature on the resulting fi lms in detail, we paid much attention to the actual temperature at spin-coating, and kept the solution temperature on the substrate before spin-coating and the fi lm temperature after spincoating the same. The procedural details are described in the Supporting Information. Figure 2 shows the optical microscope textures of the fi lms spin-coated with 1 wt% p -xylene solution of 8-TTP-8 at various temperatures. Judging from the textures, the morphology of the resulting polycrystalline fi lms of 8-TTP-8, which exhibited smectic C, F, and G (SmC, SmF, and SmG) phases in a temperature range from 89 to 58 ° C, [ 14 ] strongly depended on the spin-coating temperature: the polycrystalline fi lm spin-coated at 38 ° C had a crystal phase that consisted of many microcrystalline fl akes as shown in Figure 2 a; the fi lm spin-coated at 102 ° C had an isotropic phase, was quite inhomogeneous, and looked like crystal droplets as shown in Figure 2 c. On the other hand, the polycrystalline fi lm spin-coated at 88 ° C had a SmC phase and was quite uniform on the millimeter scale as sho...

The recent development of materials for organic field effect transistors (OFETs), including small-molecule and polymer materials, are briefly reviewed, and the problems that remain to be solved prior to practical application and use are discussed. Liquid crystalline materials are good candidates for OFETs because of their advantageous properties over soluble small-molecule materials. Liquid crystalline materials show good solution processability for the fabrication of uniform crystalline thin films using the precursor of uniform liquid crystalline thin films. In addition, liquid crystalline materials having their highly ordered liquid crystal phases show a high thermal durability despite their high solubility. The novel liquid crystalline material 2-phenyl-7-decyl [1]benzothieno [3,2-b][1]benzothiophene exhibits a highly ordered liquid crystal phase, the Smectic E (SmE) phase, which has good solution processability for uniform crystalline thin films by the precursor of uniform SmE thin films, shows high thermal durability and solubility, and can be used for transistor devices having a high mobility, over 10 cm 2 Vs − 1 , by thermal annealing at 120°C for 5 min. Thus, liquid crystalline materials are good semiconductor materials for organic transistor applications instead of polymer materials. INTRODUCTIONMaterials for organic field effect transistors (OFETs) have attracted considerable attention since their high mobility of over 1 cm 2 Vs − 1 , comparable to that of amorphous silicon FETs, was reported in polycrystalline thin films of pentacene fabricated by vacuum evaporation. 1 OFETs are very attractive for device applications because they can be fabricated even at room temperature to create flexible electronics on plastic substrates. Recent materials research for OFETs has been focused on soluble OFET materials because of their device applications in printed electronics. In this review, the recent development of OFET materials is briefly discussed, including the use of small molecules and polymers, and the remaining problems to be solved for their practical applications are discussed. Then, a new candidate for OFET materials, that is, liquid crystals, as a self-organizing molecular semiconductor, is discussed, focusing on how liquid crystallinity can solve the existing problems, and its high potential for OFET applications is demonstrated with a highly ordered smectic liquid crystal of 2-phenyl-7-decyl[1]benzothieno [3,2-b][1] benzothiophene (Ph-BTBT-10). 2