9,10-Bis(phenylethynyl)anthracene-based organic semiconducting molecules for annealing-free thin film transistors

Synthetic Metals 160 (2010) 1022–1029

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9,10-Bis(phenylethynyl)anthracene-based organic semiconducting molecules for annealing-free thin film transistors Suk Young Bae a , Ki Hwa Jung a , Mai Ha Hoang a , Kyung Hwan Kim a , Tae wan Lee a , Min Ju Cho a , Jung-Il Jin a , Dong Hoon Lee b , Dae Sung Chung b , Chan Eon Park b , Dong Hoon Choi a,∗ a

Department of Chemistry, Advanced Materials Chemistry Research Center, Korea University, Seoul 136-701, South Korea Organic Electronics Laboratory, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784, South Korea b

a r t i c l e

i n f o

Article history: Received 29 November 2009 Received in revised form 12 February 2010 Accepted 15 February 2010 Available online 15 March 2010 Keywords: X-shaped conjugated molecules Anthracene Absorption Semiconductor Mobility Organic thin film transistor

a b s t r a c t New anthracene-containing conjugated molecules have been synthesized through Stille coupling reaction. 2,6-Dibromoanthracene-9,10-dione was reacted with ethynylbenzene or 1-ethynyl-4hexylbenzene to yield 2,6-dibromo-9,10-bis(phenylethynyl)anthracene 4 and 2,6-dibromo-9,10-bis((4hexylphenyl) ethynyl)anthracene 5. Tributyl(5-hexylthiophen-2-yl)stannane was coupled through Stille reaction to generate two anthracene-based X-shaped molecules. They exhibit good solubility in common organic solvents and good self-film-forming properties. The semiconducting properties of the two molecules were evaluated in organic thin film transistors (OTFTs). Two conjugated molecules 7 and 8 exhibit fairly high charge carrier mobilities—as high as 0.010–0.014 cm2 V−1 s−1 (Ion /Ioff = 1.27 × 107 to 4.38 × 106 ) without thermal annealing process. The X-shaped molecules result in easy crystallization and densely cover the surface of a dielectric layer. This helps in attaining good network interconnection for the carrier transport channel, which is responsible for the relatively high carrier mobility in solution-processed OTFT. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The rapid progress in the development of novel p-type organic semiconductors has resulted in high-performance organic thin film transistors (OTFTs) showing high hole mobilities—of the order of 1 cm2 V−1 s−1 [1–6]. p-Type organic semiconductors, namely, derivatives of pentacene, rubrene, anthracene, or thiophene, have attracted a great deal of attention for applications in highperformance OTFTs [7–18]. However, the deposition methods for the thin films of these molecules—under vacuum or inert atmospheric conditions—suffer severe limitations during large-scale device fabrication, due to the complexity of the process. Organic semiconductors that are soluble in organic solvents facilitate the use of conventional low-cost spin-coating or dropcasting techniques for OTFT device fabrication [19–23]. Compared to vacuum-processed OTFT devices, solution-processed OTFTs usually display inferior properties in terms of device performance. This inferiority stems from the difficulty in forming high-quality

∗ Corresponding author. Tel.: +82 2 3290 3140; fax: +82 2 924 3141. E-mail addresses: [email protected] (C.E. Park), [email protected] (D.H. Choi). 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.02.020

films with desirable anisotropic molecular arrangements from the solution, and from insufficient crystallization. Therefore, the development of solution-processable p-type semiconductor materials is an important issue in the further development of OTFTs. Highly soluble, high-mobility low-molar-mass molecules such as [1]benzotheno[3,2-b]benzothiophene derivatives, selenophenecontaining heteroacene, and silylethynylated polyacenes are not capable of achieving large-area devices since their molecular weights are insufficient for the fabrication of a uniform film with favorable interfacial contact and coverage on the substrate [24–32]. Although some soluble anthracene-based semiconducting molecules have been reported, they exhibit poor performance in OTFT devices [33]. In this paper, we present two different X-shaped conjugated molecules and demonstrate a promising new class of mediumsized molecules for OTFTs. We investigated the optical, thermal, and electrochemical properties of the new conjugated molecules. These solution-processable molecules were employed to fabricate OTFTs and the performance of the devices was investigated. The newly synthesized crystalline molecules containing an anthracene moiety offer not only a high carrier mobility, of the order of 10−2 cm2 V−1 s−1 , but also a high on/off current ratio.

S.Y. Bae et al. / Synthetic Metals 160 (2010) 1022–1029

2. Experimental 2.1. Synthesis All commercially available starting materials and solvents were purchased from Aldrich (Korea), TCI (Korea), and Acros (Korea) Cos. and used without further purification. Compounds 2 and 3 were purchased from Aldrich and TCI Cos., respectively. Compounds 1 and 6 were synthesized by following the literature methods [34–37]. The synthetic procedures for 2,6-dibromo-9,10-bis(phenylethynyl)anthracene (4), 2,6-dibromo-9,10-bis((4-hexylphenyl) ethynyl)anthracene (5) were reported in our previous report [38]. 2.1.1. 5,5 -(9,10-Bis(phenylethynyl)anthracene-2,6-diyl)bis(2hexylthiophene) (7) An oven dried, mag.-stirred, 100 mL RBF was charged with a solution of 4 (1.00 g, 1.86 mmol) and tributyl(5hexylthiophen-2-yl)stannane (3.41 g, 7.46 mmol) in 50 mL freshly distilled N,N-dimethylformamide (DMF). The reaction was allowed to stir for 0.5 h followed by addition of tetrakis(triphenylphosphine)palladium(0) (0.430 g, 0.372 mmol). The mixture was stirred at 90 ◦ C for 12 hours. After completing the reaction, the solution was poured into ethanol to collect the precipitates. Yield 1.10 g, 83% 1 H NMR (400 MHz, CDCl ): ı(ppm) 8.76 (s, 2H), 8.58 (d, J = 8.6 Hz, 3 2H), 7.84 (d, J = 9.0 Hz, 2H), 7.76–7.82 (m, 4H), 7.44–7.51 (m, 6H), 7.37 (d, J = 3.5 Hz, 4H), 6.84(d, J = 3.5, 2H), 2.88 (t, 4H), 1.72–1.79 (m, 4H), 1.32–1.46 (m, 12H), 0.91 (t, 6H). 13 C NMR (100 MHz, CDCl3 ): ı(ppm) 147.02, 141.74, 132.87, 132.35, 131.93, 131.76, 128.96, 128.90, 128.82, 127.93, 125.62, 125.59, 123.99, 123.69, 122.43, 118.19, 102.86, 86.69, 31.86, 30.64, 29.06, 22.85, 14.37. HR-MS (EI) m/z (M+ ): Calcd for C50 H46 S2 , 710.30; found, 710.30. Anal. Calcd. for C50 H46 S2 : C, 84.46; H, 6.52; S, 9.02, found: C, 84.46; H, 6.59; S, 9.22. 2.1.2. 5,5 -(9,10-Bis((4-hexylphenyl)ethynyl)anthracene-2,6diyl)bis(2-hexylthiophene) (8) An oven dried, mag.-stirred, 100 mL RBF was charged with a solution of 5 (0.650 g, 0.923 mmol) and tributyl(5-hexylthiophen2-yl)stannane (1.69 g, 3.70 mmol) in 30 mL freshly distilled DMF. The reaction was allowed to stir for 0.5 h followed by addition of tetrakis(triphenylphosphine)palladium(0) (0.213 g, 0.185 mmol). The mixture was stirred at 90 ◦ C for 12 hrs. After completing the reaction, the solution was poured into ethanol to collect the precipitates. Yield 0.71 g, 87%. 1 H NMR (400 MHz, CDCl ): ı(ppm) 8.80 (s, 2H), 8.61 (d, J = 9.0 Hz, 3 2H), 7.85 (d, J = 9.0 Hz, 2H), 7.73 (d, J = 8.2 Hz, 4H), 7.38 (d, J = 3.5 Hz, 2H), 7.30 (d, J = 8.2 Hz, 4H), 6.84 (d, J = 3.5 Hz, 2H), 2.89 (t, 4H), 2.69 (t, 4H), 1.72–1.80 (m, 4H), 1.64–1.72 (m, 4H), 1.33–1.46 (m, 24H), 0.89–0.93 (m, 12H). 13 C NMR (100 MHz, CDCl3 ): ı(ppm) 146.96, 144.21, 141.82, 132.80, 132.34, 131.83, 131.75, 128.96, 128.02, 125.06, 125.52, 123.94, 122.54, 120.82, 118.28, 103.16, 86.13, 36.29, 31.97, 31.86, 31.55, 31.21, 30.64, 29.22, 29.06, 22.87, 22.85, 14.37. HR-MS (EI) m/z (M+ ): Calcd for C62 H70 S2 , 878.49; found, 878.49. Anal. Calcd. for C62 H70 S2 : C, 84.68; H, 8.02; S, 7.29, found: C, 84.71; H, 7.98; S, 7.14. 2.2. Instrumental analysis 1 H NMR spectra were recorded on a Varian Mercury NMR 400 Hz spectrometer using deuterated chloroform purchased from Cambridge Isotope Laboratories, Inc. Elemental analyses were performed using an EA1112 (Thermo Electron Corp.) elemental

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analyzer. High resolution mass analysis was performed on a JMS700 MStation mass spectrometer (JEOL, resolution 60,000, m/z range at full sensitivity 2400). Thermal properties were studied under a nitrogen atmosphere on a Mettler DSC 821e instrument. Thermal gravimetric analysis (TGA) was conducted on a Mettler TGA50 (temperature rate 10 ◦ C/min under N2 ). The redox properties of X-shaped molecules were examined by using cyclic voltammetry (Model: EA161 eDAQ). Thin films were coated on a platinum plate using chloroform as a solvent. The electrolyte solution employed was 0.10 M tetrabutylammonium hexafluorophosphate (Bu4 NPF6 ) in a freshly dried acetonitrile. The Ag/AgCl and Pt wire (0.5 mm in diameter) electrodes were utilized as reference and counter electrodes, respectively. The scan rate was at 50 mV/s. X-ray diffraction (XRD) experiment was performed using the synchrotron radiation (1.542 Å) of the 3C2 beam line at the Pohang Synchrotron Laboratory, Pohang, Korea. The film samples were fabricated by drop-casting on silicon wafer, followed by drying at 70 ◦ C under vacuum (solvent: chloroform, concentration of the solution: 10 mg/mL). The measurements were obtained in a scanning interval of 2 between 1◦ and 40◦ . Atomic force microscopic (AFM) images were obtained using a XE-100 Advanced Scanning Probe Microscope operating in tapping mode with a silicon cantilever was used to characterize the surface morphologies of the samples. The film samples were fabricated by spin-coating (1500 rpm) on octyltrichlorosilane (OTS) silicon wafer followed by drying at 50 ◦ C under vacuum (solvent: chloroform, conc. of the solution: 10 mg/mL) In order to study absorption behavior, the films of two molecules were fabricated on quartz substrates as follows. The solution (1 wt%) of each molecule in chloroform was filtered through an acrodisc syringe filter (Millipore 0.2 ␮m) and subsequently spincast on the quartz glass. The films were dried overnight at 70 ◦ C for 24 hours under vacuum. Absorption spectra of samples in a film and solution state (chloroform, conc. 1 × 10−5 mol/L) were obtained using a UV–vis spectrometer (HP 8453, photodiode array type) in the wavelength range of 190–1100 nm. 2.3. OTFT fabrication For the characterization of TFT performance, bottom gate top contact device geometry was employed. On the heavily n-doped Si/SiO2 substrate the spin-coated films (thickness ∼40–50 nm) were prepared with chloroform as a solvent. Surface modification was carried out with OTS to make hydrophobic dielectric surface. Source and drain electrodes were then thermally evaporated (100 nm) through shadow mask with the channel width and length of 1500 ␮m and 150 ␮m, respectively. All the field effect mobilities were extracted in the saturation regime using the relationship sat = (2IDS L)/(WC(Vg − Vth )2 ), where IDS means saturation drain current, C is capacitance of SiO2 dielectric, Vg is gate bias, and Vth is threshold voltage. The device performance was evaluated in air using Keithley 237 high voltage sourcemeter at ambient conditions. 3. Results and discussion 3.1. Synthesis We report here the facile and high-yield synthesis of new p-type anthracene-based semiconducting molecules. Scheme 1 illustrates the synthetic routes for the two molecules. 2,6-Dibromoanthracene-9,10-dione was synthesized using an established method [34–35]. The addition of a dione compound to two molar equivalents of ethynyl compounds in the presence

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Scheme 1. Synthesis of the anthracene-based X-shaped molecules. (i) n-BuLi, THF,–78 ◦ C, SnCl2 , HCl, H2 O, 3 h. (ii) Pd(PPh3 )4 , DMF, 90 ◦ C, 12 h.

of BuLi afforded the corresponding enediols. The enediols were obtained as a mixture of two syn- and anti-isomers and were readily reduced by tin chloride in an acidic medium to yield the desired anthracene-based compounds 4 and 5[39–40]. Using tributyl(5hexylthiophen-2-yl)stannane, we performed Stille coupling by the addition of tetrakis(triphenylphosphine) palladium(0) in freshly distilled DMF to yield 7 and 8 in a fairly high yield. Acting as solubilizing groups and crystallization-promoting moieties, two or four hexyl chains were substituted into the phenyl and thiophene groups. In compounds 7, two hexyl groups were aligned in a unidirectional manner. In compounds 8, four hexyl groups were introduced in a biaxial manner. The identity and purity of the synthetic materials were confirmed by 1 H NMR, HRMS, and elemental analysis. They were found to have good self-film-forming properties and dissolved well in various organic solvents such as chloroform, xylene, MC, chlorobenzene, and THF. The compounds had a solubility of >5 wt%, which is comparable to the most soluble organic molecule

6,13-bis(triisopropyl silylethynyl)pentacene, typically employed in OTFTs [41]. 3.2. Thermal analysis When polymers are considered for OTFT applications, their thermal stabilities and dynamic behaviors should be investigated for device fabrication. The thermal properties of the polymers are characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). In this study, DSC measurement was performed at a heating (cooling) scan rate of 10 (−10) ◦ C/min under nitrogen, with the highest temperature limited to a value below the decomposition temperature. Compounds 7 and 8 exhibited distinct crystalline-isotropic transitions in the range of 25–250 ◦ C, including possible liquid crystalline transitions but not showing glass transition temperatures. They showed amphotropic behavior and showed low crystallization temperatures of 156 ◦ C and 166 ◦ C, respectively. Although the liquid crystalline transition seemingly

Fig. 1. DSC traces of 7 (A-1) and 8 (B-1) during the heating (solid line) and cooling (dotted line) cycles. TGA thermograms of 7 (A-2) and 8 (B-2).

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Table 1 Measured and calculated parameters for the two X-shaped conjugated molecules. Td (◦ C)

Tm (◦ C)

Tc (◦ C)

Solution max (nm)

Film max (nm)

cut off (nm)

Before annealing

After annealing

opt

Eg

(eV)

Energy level HOMO (eV)

LUMO (eV)c

7

408

173.5

156.4

412, 437 472, 501

423, 449 463, 493, 533

423, 450, 463, 493, 533

566a 560b

2.19a 2.21b

−5.51

−3.32a −3.30b

8

405

183.1

166.3

415, 439 473, 504

426, 498 539

426, 427 503, 547

563a 572b

2.20a 2.16b

−5.46

−3.26a −3.30b

a b c

Before annealing. After annealing. opt HOMO (eV) − Eg (eV).

took place, no clear optical microscopic images were observed. (see Fig. 1 and Table 1) From the comparison, it is clear that the four hexyl peripheral groups induce higher crystalline melting temperatures, and this is attributed to the biaxial hexyl-hexyl interaction by dense molecular arrays. TGA measurements at a heating rate of 10 ◦ C/min under nitrogen revealed that the molecules had good thermal stabilities and the onset decomposition temperatures (∼405 ◦ C) were enhanced (see Table 1). Although the temperature was increased to 900 ◦ C, some residual weights (50–60% of initial mass) were measured in 7 and 8; these were attributed to char formation resulting from the fused ring structure of the repeating group. 3.3. UV–vis absorption spectroscopy In order to study the interaction between the molecules, the absorption spectra of the samples in chloroform (conc. 1 × 10−5 mol/L) and in thin films were obtained (see Fig. 2) Fig. 2 shows the UV–vis absorption spectra for the solutions, as-coated thin films, and annealed films prepared using the two molecules. The solution spectra for the two molecules are almost identical and the number of hexyl group does not influence the spectra. In two samples, we observed a significant bathochromic shift in the absorption spectra upon film formation, which indicated the generation of strong intermolecular interactions. When the wavelength shift from the absorption maximum in the solution to that in the annealed film is compared, we can conjecture which sample showed the stronger intermolecular interaction in a solid state. Compared to the dihexyl compound, 7, 8 exhibited a larger bathochromic shift (A:  = 2 − 1 =: 32 nm, B:  = 43 nm), which implies the stronger effect of the alkyl-alkyl interaction in a two-dimensional way. From the absorption edge, we were able to calculate the optical bandgap energies for the thin films of the two molecules. Molecules 7 and 8 showed similar bandgap energies together with similar intermolecular interactions. In addition, it was proven that the as-spun films of 7 and 8 prepared at room temperature comprised fully organized molec-

ular structures. Obviously, the presence of four peripheral hexyl moieties further strengthened the bilateral, interlayer interactions thereby driving the formation of interconnected microscopic grains. 3.4. Electrochemical analysis Cyclic voltammograms (CV) were recorded on a solution and film sample and the potentials were obtained relative to an internal ferrocene reference (Fc/Fc+ ). The CV scans in the range of −0.2 V to +1.5 V (versus Ag/AgCl) showed quasi-reversible oxidation peaks. Unfortunately, reduction behaviors were not observed; therefore, we were unable to accurately estimate their lowest unoccupied molecular orbital (LUMO) energies. In order to determine the LUMO levels, we combined the oxidation potential from the CV with the opt optical energy bandgap (Eg ) resulting from the absorption edge in the absorption spectrum. The voltammograms of 7 and 8 in the film state showed their lowest oxidative waves at 1.06–1.11 V. As shown in Table 1, they also have highest occupied molecular orbital (HOMO) levels of −5.46 to −5.51 eV. In addition, they have LUMO energy levels of around −3.26 to −3.32 eV. According to the CV diagram of 6,13-bis(triisopropyl silylethynyl) pentacene (TIPS-pentacene, Eox = 0.80 V, EHOMO = 5.2 eV), the anthracene-based molecules were more persistent from an environmental perspective. The environmental stability of these molecules could be noted, and it was found to be comparable to or better than that of TIPS-pentacene. 3.5. X-ray diffraction analysis In order to study the crystallinity and preferred orientations of the two molecules, X-ray diffraction (XRD) was performed in an out-of-plane mode at room temperature. The molecular layers are thought to be associated with the layered stacking properties resulting from the inclusion of the terminal hexyl groups, which are already known to induce long-range ordering. X-ray diffraction scans of as-cast films of 7 and 8 revealed very distinctive crystalline

Fig. 2. UV–vis absorption spectra of 7 (A) and 8 (B). Solution spectra (solid line), as-coated film (dashed line), and annealed film (dash-dot-dashed line).

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Fig. 3. Cyclic voltammograms of two molecules. Solution sample: 7 (A-1) and 8 (A-2). Film sample: 7 (A-1) and 8 (A-2).

peaks, indicating intermolecular stacking. The edge-on orientation relative to the substrate is evidenced by the absence of diffraction at around 20◦ –25◦ (see Fig. 3). The preferred orientation is inferred through the high reflection (1 0 0) intensity of the peaks at 2 = 3.4◦ –5.7◦ . This result implies that most of the crystallites are preferentially oriented along the (1 0 0)-axis in the plane. When comparing the d-spacing of the two molecules, it is clear that molecules of 8 containing four hexyl peripheral moieties exhibit larger d-spacing than those of 7. The larger lamella ordering distance of 8 is attributed to the two-dimensional ordering induced by the interaction between the biaxial hexyl peripheral moieties. Although the d-spacing was not consistent with the molecular dimension due to the tilted arrangement relative to the substrate,

well-aligned intermolecular stacking was confirmed in the XRD study (Fig. 4). In most of the semiconducting molecules used in device fabrication, thermal annealing is usually required to achieve high TFT-device performance. As we will show, the X-shaped molecules display high device performance without the need for an additional annealing process. This facilitates device fabrication by showing the reproducibility of the carrier transport phenomenon. 3.6. Atomic force microscopy The XRD measurements reveal the type of crystalline structure that prevails throughout the molecular films, while atomic force microscopy (AFM) observations provide images of the top surface. We prepared a solution of each molecule, and then performed spin coating to cover the dielectric surface (SiO2 ) with each solution. After completely drying the semiconducting films, we took the two micrographs shown in Fig. 5. Well-organized crystalline structures and a network of interconnected grains can be observed in the ascast films of two molecules. Of the two molecules, the molecule, 7 shows the compact surface structure, with the smaller crystallites than 8. The highly packed crystalline molecules on the dielectric layer can enhance carrier transport by offering good connection. Moreover, the improved connectivity and densely packed geometry between neighboring domains implies that the resulting charge carrier transport in TFT devices can be expected to be good with few charge traps. If the regions between the crystalline domains are more ordered, we expect that transport should be easier as a result of the improvement in network connectivity and that charge trapping within these regions will be lower [42,43]. 3.7. Properties of OTFTs made of X-shaped molecules

Fig. 4. XRD patterns for as-cast thin films of the two X-shaped conjugated molecules: (A) 7 and (B) 8.

Bottom-gate, top-contact OTFT devices were fabricated under ambient conditions without thermal annealing. A gold source and

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Fig. 5. AFM topographic images (5 ␮m × 5 ␮m) of as-cast films of two X-shaped conjugated molecules: (a) 7 and (b) 8.

1/2

Fig. 6. Electrical characterization of two OTFTs. Left: Plot of drain current (IDS ) versus drain-source voltage (VDS ). Right: Plot of IDS and IDS versus gate voltage (Vg ); VDS = –60 V. (A) device A, (B) device B. Insulator: OTS treated SiO2. Performance was measured under atmosphere.

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Table 2 Performance of TFT devices. TFT characteristics of two devices: FET (cm2 V−1 s−1 ) is the carrier mobility, Ion /Ioff is the on/off current ratio, and Vth (V) is the threshold voltage.

Device A Device B

W/L ratio

Ion /Ioff

10 10

1.27 × 10 4.38 × 106 7

Vth

FET (cm2 V−1 s−1 )

−8.4 −6.2

0.014 0.010

drain electrodes were thermally evaporated using the conventional method. p-Doped polycrystalline silicon was used as the gate electrode, with an octyltrichlorosilane (OTS)-treated SiO2 surface layer used as the dielectric gate insulator. A 300 nm thin film of the semiconductor was deposited by spin coating a 0.5 wt% solution of the molecules in chloroform. After spin coating the solution on the insulator surface, the gold was deposited via thermal evaporation. The channel length was L = 100 ␮m and the channel width was W = 1500 ␮m. It should be noted that we did not anneal the molecular films before measuring the TFT performances, and all measurements were performed under atmosphere. Devices A and B contained semiconducting layers of 7 and 8, respectively. The OTFTs of the molecules exhibited typical p-channel fieldeffect transistor (FET) characteristics. The output characteristics showed very good saturation behaviors and clear saturation currents that were quadratically related to the gate bias (see Fig. 6). The mobilities were obtained from the source (S)–drain (D) current–voltage curves (IDS versus VDS ) in well-resolved saturation regions. The saturated field-effect mobility FET can be calculated from the amplification characteristics using established equations describing field-effect transistors [44]. The mobility values, on/off current ratios, and threshold voltages of the OTFTs obtained by measurements performed on two different devices are listed in Table 2. In Fig. 6, the left side displays the plots of the source-drain current (IDS ) versus the source-drain voltage (VDS ) at various gate voltages (Vg ) for top-contact field-effect transistors using X-shaped conjugated molecules on OTS-treated SiO2 . Transfer characteristics in the saturation regime at a constant source-drain voltage are also shown on the right side. Although the mobility of device A ( = 0.014 cm2 V−1 s−1 ) was slightly higher than that of device B ( = 0.010 cm2 V−1 s−1 ), the TFT performances are comparable. (see Fig. 6 and Table 2). The output curves also showed very good saturation behaviors and very small contact resistance. This might be due to the presence of the ethynyl bond between thiophene and anthracene at the 2 and 6 positions, serving to support the planar configuration of the molecules. The presence of a triple bond can help suppress the rotation of the hexylthiophene peripheral group and can help uniform molecular ordering, thereby resulting in very high crystallinity. Our optical spectroscopy and XRD results could provide indirect evidence of the strong intermolecular interaction and preferred molecular orientation in as-cast crystalline film. Therefore, the high crystallinity and predominant edge-on orientation can explain the high value of the field-effect mobilities. Using UV spectroscopy, CV analysis, XRD and AFM, we could confirm the effect of the molecular ordering and biaxial arrangement of the molecules in the film state.[38] In terms of device configuration, the relationship between the molecular structure and FET characteristics should be studied in more detail. 4. Conclusions We have successfully synthesized and characterized new anthracene-based conjugated molecules that are solutionprocessable. The molecules with hexyl-substituted thiophene peripheral units not only formed smooth films on large surfaces but also showed better homogeneous layer formation with rel-

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