Molecular Ordering of High‐Performance Soluble Molecular Semiconductors and Re‐evaluation of Their Field‐Effect Transistor Characteristics

Abstract

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 ...

X-ray Structural Investigation of Nonsymmetrically and Symmetrically Alkylated [1]Benzothieno[3,2-b]benzothiophene Derivatives in Bulk and Thin Films

“…Further more, all observed diffraction peaks in sXRD as well as in GIXD can be indexed using the single crystal structure of C 8 −BTBT− C 8 . 31,32 The specular diffraction peaks correspond to the 00l reflections, while the diffraction rods revealed by GIXD measurements correspond to 11l, 02l, and 12l families of reflections (see Figure 3b−d). It is worth pointing out that no noticeable shift of peak position is observed for the sXRD patterns in the broad range of film thickness investigated.…”

“…The crystalline phase is monoclinic with space group P2 1 /a and consists of a lamellar arrangement of C 8 − BTBT−C 8 molecules with herringbone packing within the lamellae. 31,32 In the case of C 8 −BTBT, the phase sequence as a function of temperature is more complex and involves only crystalline phases: form I, stable at room temperature (present in the crude powder or in samples recrystallized from solution at room temperature), form II, stable at high temperature (obtained either from form I or III upon heating before the melting of C 8 −BTBT), and form III, metastable at room temperature (kinetically favored upon cooling from the melt or from form II). A detailed description of the bulk phase behavior of C 8 −BTBT is not the intent of this Article and is partly dealt with in another publication.…”

“…The bulk crystal structure of C 8 −BTBT−C 8 has previously been solved by Takimiya et al 31,32 Moreover, the same authors also investigated the molecular packing of spin coated thin films of C 8 −BTBT−C 8 by X ray diffraction and conclude on the same molecular packing in the bulk material as in thin films. However, film thickness was not specified nor varied in this study.…”

“…45 Moreover, the dihedral angle around the C−C bond connecting the octyl chain to the BTBT core was allowed to vary in the simulated annealing procedure (together with the three translations and three rotations of the rigid geometry optimized C 8 −BTBT molecule) as it is observed in the crystal structure of dialkylated BTBTs that the planes containing the zigzag of the alkyl chains and the plane of the BTBT moiety are not parallel. 31,32 The final crystal structure was then obtained after Rietveld refinement of the best crystal structure attained from the simulated annealing procedure (low R wp factor and realistic intermolecular interactions), encompassing refinement of the seven degrees of liberty of the rigid body (translations, rotations, and torsion angle), an isotropic mean temperature factor, and parameters describing preferential orientation of crystallites.…”

Small Molecule/Polymer Blend Organic Transistors with Hole Mobility Exceeding 13 cm2 V−1 s−1

“…The small-molecule 2,7- Figure 1(a)] was chosen due to its high solubility in various organic solvents as well as its ability to form large intermolecular overlaps arising from well-ordered molecular arrays, which under certain circumstances can lead to exceptionally high hole mobilities. [5] [7] [22] As the complementary polymer binder we selected the indacenodithiophene-benzothiadiazole (C16IDT-BT) [ Figure 1(a)] primarily due to its good solubility, high hole mobility (3.6 cm 2 /Vs) [23] [24] [25] and comparable highest occupied molecular orbital (HOMO) energy level to that of the C8-BTBT [ Figure 1(b)]. [26] [27] Using this unique combination of materials we have demonstrated solution-processed OTFTs with hole mobility values in excess of 13 cm 2 /Vs.…”