2,7-Dialkyl[1]benzothieno[3,2-b]benzothiophenes were tested as solution-processible molecular semiconductors. Thin films of the organic semiconductors deposited on Si/SiO2 substrates by spin coating have well-ordered structures as confirmed by XRD analysis. Evaluations of the devices under ambient conditions showed typical p-channel FET responses with the field-effect mobility higher than 1.0 cm2 V-1 s-1 and Ion/Ioff of approximately 10(7).

Although it is widely known that conventional organic semiconductors for solution-processed organic field-effect transistors (OFETs) are conjugated polymers represented by poly(3-alkylthiophenes), [1][2][3][4] highly soluble molecular semiconductors are currently also attracting interest. [5,6] Among these soluble molecular semiconductors 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene, Fig. 1 . [7][8][9][10][11] We recently reported a series of highly soluble molecular semiconductors, 2,7 Fig. 1), of which spin-coated thin films act as superior semiconducting channels, showing m FET values of up to 2.7 cm 2 V À1 s À1 .[12] One of the issues facing C n -BTBT-based OFETs is the effect of alkyl chain length on molecular ordering and device characteristics: out-of-plane X-ray diffraction (XRD) measurements of spin-coated thin films of a series of C n -BTBTs suggested that molecular ordering patterns in the substrate normal direction are identical for all the compounds, regardless of alkyl chain length. Nevertheless, FET devices based on C n -BTBTs with longer alkyl chains tend to show higher m FET values. In particular, devices having alkyl chains longer than C 10 H 21 reproducibly show m FET values higher than 1.0 cm 2 V À1 s À1. [12] It is important to clarify how molecular and structural factors affect the electrical properties of devices. For this reason, we carried out single-crystal X-ray structure analyses of C 8 -, C 10 -, and C 12 -BTBTs and in-plane XRD measurements of C 8 -and C 12 -BTBTs to elucidate thin-film structures and molecular ordering in the solid state.Single-crystal X-ray structure analyses clearly indicate that all the three compounds are isostructural with the same space group (monoclinic, P2 1 /a), have similar cell dimensions (Table 1), and have the same molecular arrangement (Fig. 2). The crystal structures are characterized by a lamella-like alternating structure of alkyl layers and BTBT layers along the crystallographic c-axis direction (Fig. 2a) and a herringbone arrangement of BTBT core parts in the ab plane (Fig. 2b), facilitating two-dimensional carrier transport in this direction. It should be noted that the c-axis lengths of crystallographic unit cells correspond to interlayer distances (d-spacings) elucidated by the out-of-plane XRD of the thin films (29.0, 33.3, and 37.5 Å for C 8 -, C 10 -, and C 12 -BTBTs, respectively). Figure 3 shows the in-plane XRD patterns of spin-coated thin films of C 8 -and C 12 -BTBTs on the Si/SiO 2 substrate. Three characteristic peaks are observed at similar regimes for the films of both C 8 -and C 12 -BTBTs, indicating that the derivatives have crystalline order in the in-plane direction. Because these peaks are assigned to (110), (020), and (120) reflections, based on the structures of bulk single crystals, it is highly likely that the crystal structures in the thin-film phase are identical with those in the bulk single crystals. Taking into consideration the results of out-of-plane and in-plane XRD measurements of the thin films and ...

Vapor-deposited thin films of a newly developed sulfur-containing heteroarene, 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene (DPh-BTBT), were used as an active layer of OFETs, which showed excellent FET characteristics in ambient conditions with mobilities of approximately 2.0 cm2 V-1 s-1 and Ion/Ioff of 107.