Molecular Orientation Analysis of a C8-BTBT Thin Film Grown under an External Temperature Gradient∗

Organic semiconductors with a high carrier mobility can be realized by controlling a growth direction of the thin film. A new method to control the growth direction is using a temperature gradient. Molecular orientation strongly influences on the carrier mobility. Therefore, the molecular orientation of 2,7-dioctyl benzothieno benzothiophene (C8-BTBT) thin films grown under a temperature gradient, has been evaluated by X-ray absorption fine structure measurements. It is considered that the C8-BTBT has a molecular orientation with standing up on the substrate and is aligned to the temperature gradient direction. [DOI: 10.1380/ejssnt.2018.79]


I. INTRODUCTION
An organic molecule 2,7-dioctyl [1]benzothieno [3,2b]benzothiophene (C 8 -BTBT) has attracted much attention as an organic semiconductor material with high carrier mobility. C 8 -BTBT is superior to conventional organic molecules in atmospheric stability, and research on various applications is proceeding. The previous study has revealed that a herringbone structure of C 8 -BTBT, i.e., the π stacking results in high charge carrier mobility [1]. The herringbone structure of C 8 -BTBT is unique among the molecular packing motifs and has been reported to exhibit a layered-herringbone (LHB) structure which is known to be very suitable for obtaining highperformance organic thin-film transistors (TFTs) [2][3][4]. The crystallinity of the LHB structure enables efficient two-dimensional carrier transport to realize a high performance organic TFT. Moreover, the calculated carrier transport properties indicate that the difference in the type of contact in the LHB structure, i.e., T-shaped (coreto-edge) and slipped parallel (core-to-core) contacts between adjacent planar π-electron skeletons that are taken placement of glide and translational symmetric arrangement, respectively, results in anisotropy in carrier transport [2].
In addition, it is reported that a difference in mobility is observed due to a difference in preparation method [5][6][7]. Differences in preparation methods may lead to molecular orientation differences. Since the molecular orientation can strongly influence on the carrier mobility [6], it is important to quantitatively determine the molecular orientation of C 8 -BTBT. Growth direction of C 8 -BTBT thin films can be controlled by an external temperature gradient in a substrate during drop-casting [8]. This temperature-gradient method is the following procedure; pouring a droplet on a Si substrate carried on a temperature gradient Al plate, then evaporating the droplet and making a thin film. It is expected to align the molecular orientation to some extent by controlling the growth direction of the thin film. In this study, the molecular orientation of the C 8 -BTBT thin film has been evaluated by polarization-dependent X-ray absorption fine structure (XAFS) measurements.

A. Sample preparation by the temperature-gradient method
An external temperature gradient was generated as follows [8]. Two heat stages were kept at different temperatures and a 0.5 mm-thick aluminum plate (10 cm × 2 cm) bridged them. A 1.2 cm-square heavily-doped Si substrate with a 200 nm-thick thermal oxide layer was placed on this Al plate. A solution of 0.2 wt% C 8 -BTBT (Sigma-Aldrich) in chlorobenzene was drop-casted on the substrate with the temperature gradient of ∼ 2.5 • C (65.75-68.25 • C). A thin film of 7 mm in the direction along the temperature gradient and 4 mm in the direction perpendicular to the temperature gradient direction was prepared. The film thickness was approximately 20-50 nm estimated by an atomic force microscopy (AFM) image.
As a contrast experiment, C 8 -BTBT thin films by spin coating without an external temperature gradient were prepared. A solution of 0.4 wt% C 8 -BTBT in chlorobenzene was used. And the Si substrate was placed on the center of the spin-coater. The spinning speed was set to 1000 rpm for the first 5 s, then 3000 rpm for 25 s.

B. XAFS analysis
The XAFS measurements were carried out at the BL-8 of SR Center, Ritsumeikan University. C K-edge XAFS spectra were obtained by the partial electron yield method with retarding voltage of −150 V. The measurements were performed at room temperature under ultrahigh vacuum of ∼ 1 × 10 −7 Pa. In order to see the molecular orientation of the C 8 -BTBT thin films, the incidence angle with respect to the substrate normal was varied. Furthermore, in order to see the in-plane anisotropy of the C 8 -BTBT thin films, the samples were azimuthally rotated around the substrate normal. Although the film thickness was FIG. 1. C K-edge XAFS spectra of C8-BTBT prepared by temperature-gradient (a) and spin-coated (b) methods. The peaks around 285 eV is attributed to the C 1s → π * (C=C) transitions for aromatic rings. The peaks around 292 eV is attributed to the C 1s → σ * transition for aromatic rings and the alkyl chain.
not uniform for the temperature-gradient method, XAFS spectra did not change so much near the center region of the samples (not shown), enabling us to estimate the molecular orientation.

III. RESULTS AND DISCUSSION
C K-edge XAFS spectra are shown in Fig. 1. The structure around 293 eV is attributed to the C 1s → σ * transition for both the aromatic rings and the alkyl chain [9,10]. The structure around 285 eV is attributed to the C 1s → π * (C=C) transitions for aromatic rings [9,10]. The structure around 287 eV is attributed to the C 1s → σ * (C-H) and C 1s → π * (C=C) transitions [9,10]. Since the peak around 287 eV could not be distinguishable, we discuss about the structure around 285 eV. The peak intensity of the C 8 -BTBT thin films fabricated by both the temperature-gradient and the spin-coated methods decrease with increasing the incident angles. From these results combined with the observed step height of ∼ 2.6 nm by AFM [8], which corresponds to the length of a C 8 -BTBT molecule, it can be seen that the C 8 -BTBT molecule in the thin films has a standing up structure with respect to the substrate.
For C 8 -BTBT prepared by the temperature-gradient method, the incidence-angle dependences of the peak intensities around 285 eV were different when the sample was azimuthally rotated (Fig. 2). That is, the polariza-  tion dependence of the intensities with the in-plain polarization parallel to the temperature-gradient direction [ Fig. 2(a)] was stronger than that with the in-plain polarization perpendicular to the temperature-gradient direction [ Fig. 2(c)]. This result indicates that the π * orbital of the C 8 -BTBT molecule preferentially aligns to the temperature-gradient direction. Note that the tilt direction of the π * orbital is irrelevant to the hotter/cooler sides of the temperature-gradient direction since the similar incidence-angle dependences were observed for the opposite grazing incidence (not shown). For C 8 -BTBT prepared by the spin-coated method, on the other hand, the incidence-angle dependences of the peak intensities around 285 eV were almost the same even when the sample was azimuthally rotated (Fig. 3), indicating the isotropic C 8 -BTBT molecular orientation.
In order to evaluate the molecular orientation, fitting was performed on the results obtained from the angular dependence measurements, following the literature [9]. Since in-plane anisotropy was observed for C 8 -BTBT prepared by the temperature-gradient method  ( Fig. 2), the directions of the π * orbitals with respect to the temperature-gradient direction are assumed to be ±Φ. Considering the four molecular orbitals shown in Fig. 4(b), fitting can be done by for each geometry. Here, P is the degree of polarization, θ is the angle between the substrate normal and the incident X-ray, α and Φ are a polar and an azimuthal angles of the molecular orbital, β is the angle between the temperature gradient direction and the projected electric field vector (Fig. 4). We have assumed the degree of polarization P = 0.85, which is typical for a bending magnet source [9]. On the other hand, C 8 -BTBT fabricated by spin-coated methods has isotropic molecular orientation, and fitting can be done by Note that the vector equations in Ref. [9] are applicable because the BTBT core is considered to be rigid, that is, because all the π * orbitals on the rigid BTBT core align to the same direction. The fitting results for the C 8 -BTBT produced by the temperature-gradient and spin-coated methods are shown in Fig. 5 and Fig. 6, respectively. From the obtained parameters, it is found that the tilt angle of a C 8 -BTBT Off-center spin-coated [7] a ∼ 13 • ∼ 55 • a Note that the angles for the off-center spin-coating method from Ref. [7] are re-estimated by using Eq. (1). b Angle between the molecular plane and the growth direction.   molecule is 20±2 • and that the angle between the molecular plane and the temperature-gradient direction is 52±2 • for the temperature-gradient method (Fig. 5). Note that the molecular orientation angles are different from the molecular orbital direction (α and Φ) by π/2. It was found that the C 8 -BTBT produced by the spin-coated method also had the tilt angle of 21 ± 2 • (Fig. 6). As shown in Fig. 5(a, b) and Fig. 6, other values of α, e.g., 80 • and 60 • , do not reproduce well the experimental data. Note that Eq. (1) for β = 45 • does not depend on Φ [ Fig. 5(b)]. For β = 0 • , it does depend on Φ so that the simulation curves of α = 80 • and 60 • with the optimal Φ of 38 • are show in Fig. 5(a). With the optimal α of 70 • , other vales of Φ, e.g., 50 • and 30 • , do not reproduce well the experimental data [ Fig. 5(c)]. The molecular orientation angles of C 8 -BTBT inside the thin films were summarized in Table I. Previous study controlling the growth direction by off-center spin-coating method has also shown the anisotropic molecular packing [7]. Re-estimating the molecular orientation by Eq. (1), the tilt angle is ∼ 13 • and the angle between the molecular plane and the growth direction is ∼ 55 • . As summarized in Table I, these molecular angles are different from the above mentioned angles by the temperature-gradient method though both methods lead to the anisotropic molecular packing. Considering the herringbone angles, which correspond to the angle between the two molecules, are ∼ 55 • and ∼ 125 • in the direction to the a-axis and b-axis of the C 8 -BTBT crystal, respectively [2], the temperature-gradient direction is thought to lead to the preferential b-axis growth. Similarly, the preferred growth direction of the off-center spincoated film was reported to be along the (010) direction of the C 8 -BTBT crystal [7].

IV. CONCLUSION
We have performed polarization-dependent X-ray absorption spectroscopy measurements on C 8 -BTBT films. It was found that the π * intensity of the thin films decreases with grazing incidence and has an anisotropy with respect to the temperature gradient. From these results, it is found that the C 8 -BTBT thin film has a standing-up structure and that the molecules are aligned in the direction of temperature gradient. Furthermore, C 8 -BTBT thin films fabricated by the temperature-gradient method have similar standing up structure, as compared with the C 8 -BTBT produced by the spin-coated method.