Synthesis and Characterization of Novel Semiconductors Based on

ARTICLE pubs.acs.org/JPCC

Synthesis and Characterization of Novel Semiconductors Based on Thieno[3,2-b][1]benzothiophene Cores and Their Applications in the Organic Thin-Film Transistors Huajie Chen,†,‡ Qingyu Cui,†,§ Gui Yu,*,† Yunlong Guo,*,† Jianyao Huang,†,‡ Minliang Zhu,†,‡ Xiaojun Guo,§ and Yunqi Liu*,† †

Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People's Republic of China ‡ Graduate School of Chinese Academy of Sciences, Beijing 100049, People's Republic of China § Center for Opto-electronic Materials and Devices, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People's Republic of China

bS Supporting Information ABSTRACT:

The synthesis and characterization of four new organic semiconductors with thieno[3,2-b][1]benzothiophene cores and different π-bridge spacers are reported. Cyclic voltammetry measurement indicates that the materials have low energy levels of the highest occupied molecular orbitals and large band gaps. The single-crystal X-ray diffraction experiment reveals that 1,2-di(thieno [3,2-b][1]benzothiophenic-2-)ethylene (DTBTE) molecules have a nearly coplanar structure and crystallize into a herringbone arrangement with strongly intermolecular multiple S 3 3 3 S, S 3 3 3 C, and CH 3 3 3 π interactions. These interactions facilitate charge carrier transport. The DTBTE-based organic thin-film transistors (OTFTs) on an octyltrichlorosilane-modified SiO2/Si substrate exhibit good field-effect performance with the highest mobility of 0.50 cm2 V
1 s
1. Furthermore, the DTBTE-based OTFTs have been fabricated on a flexible polyethylene terephthalate substrate and showed a maximum mobility of up to 0.45 cm2 V
1 s
1, indicating its potential application as an organic semiconductor in the flexible OTFTs.

’ INTRODUCTION Organic thin-film transistors (OTFTs) are regarded as lowcost alternatives to traditional silicon-based transistors for electronic applications. Great progress has been made in their performance and stability since the report in 1998 at the Philips Research Laboratories.1 As the core part of OTFTs, organic semiconductors play a key role in obtaining high performance and have been intensively studied by both industrial and academic researchers worldwide. In the past decades, various novel organic semiconductors with higher charge carrier mobility than that of amorphous silicon (0.1 cm2 V
1 s
1) have been reported.2
4 Among these materials, pentacene has attracted the greatest interest owing to its high performance with a charge r 2011 American Chemical Society

carrier mobility above 1.0 cm2 V
1 s
1.5,6 However, pentacene usually suffers from a fatal disadvantage of being easily oxidized in an ambient condition due to the high energy level of the highest occupied molecular orbital (HOMO) and the formation of dimeric Diels
Alder adducts. Therefore, great efforts have been made to synthesize new heteroacenes with high charge carrier mobility and good environmental stability. Recently, some new heteroacenes, such as thieno[n]acenes7
9 and thiophene
benzene fused compounds,10
13 have been intensely studied Received: August 23, 2011 Revised: October 22, 2011 Published: October 31, 2011 23984

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Scheme 1. Synthetic Routes of Semiconductors Based on Thieno[3,2-b][1]benzothiophene Cores

because the sulfur heteroatom can obviously affect the electronic structures and the molecular interactions in the solid state, therefore, making thiophene-fused aromatics attractive candidates for electronic applications. Nevertheless, further developing new fused heteroacenes with high mobility and good environmental stability and thoroughly characterizing structure
property relationships are still an important task for researchers. Thieno[3,2-b][1]benzothiophene (TBT) is an important building block to synthesize fused-ring thienoacenes. Its perfect planar conjugation structure, large band gap, good redox stability, and facile synthesis have attracted the attention of some researchers.14 However, TBT has not been used as a building block for organic semiconductors in OTFTs yet. The extended π-conjugation system in the fused TBT molecules can provide more efficient orbital overlap and contribute to high charge carrier mobility. In addition, the molecular packing in sulfur-rich TBT-based oligomers may be characterized by the cooperation of intermolecular multiple S 3 3 3 S, S 3 3 3 C, and CH 3 3 3 π interactions, which should lead to high charge carrier mobility for OTFT applications. Herein, we design and synthesize a series of symmetrical organic semiconductors using TBT as an important core with different π-bridge spacers, outlined in Scheme 1. We study the effect of different π-bridge spacers on their electronic and optical properties using several techniques, such as UV
vis absorption spectroscopy, atomic force microscopy (AFM), and X-ray diffraction (XRD). In particular, a two-stage deposition technology was employed to obtain a balance between high film crystallinity and good film grain interconnectivity by properly adjusting the substrate temperature and deposition flux rate. Among the four organic semiconductors synthesized, 1,2-di(thieno[3,2-b][1]benzothiophenic-2-)ethylene (DTBTE) exhibits an excellent field-effect performance with a high mobility of 0.50 cm2 V
1 s
1 and an on/off current ratio of up to 4.8  106. Most importantly, the DTBTE-based flexible OTFTs on a polyethylene terephthalate (PET) substrate were also fabricated and showed a maximum mobility of up to 0.45 cm2 V
1 s
1, which is comparable to that of OTFT fabricated on an octyltrichlorosilane (OTS)modified SiO2/Si substrate.

’ EXPERIMENTAL SECTION Characterization of Materials. Elemental analyses were carried out using a Carlo Erba model 1160 elemental analyzer. Nuclear magnetic resonance (NMR) spectra were obtained in deuterated chloroform (CDCl3) or 1,1,2,2-tetrachloroethane (C2D2Cl4) with a Bruker 300 spectrometer. Electron-impact (EI) mass spectra (EI-MS) were collected on a micromass GCIMS spectrometer. UV
vis absorption spectra were recorded on a Hitachi U-3010 spectrophotometer. Cyclic voltammetry (CV) experiments were conducted on an electrochemistry workstation (CHI660A, Chenhua Shanghai) using a three-electrode cell. The indium tin oxide glass electrode deposited with a thin-film layer of organic semiconductors was used as the working electrode. A Ag/Ag+ (Ag in 0.01 mol/L AgCl) electrode was used as the reference electrode. Platinum wire was used as the counter electrode. An anhydrous and N2-saturated solution of 0.1 M tetrabutylammonium hexylfluorophosphate in acetonitrile was employed as the electrolyte. Diffraction intensities of a single crystal were collected in the reflection mode at room temperature using a Rigaku MM-007 X-ray diffraction (XRD) system (Mo
Kα radiation, λ = 0.71073 Å). XRD of thin films was carried out in the reflection mode at room temperature using a 2-kW Rigaku XRD system. The films were imaged in air using a Digital Instruments Nanoscope III atomic force microscope operated in tapping mode.

’ SYNTHESIS OF MATERIALS Thieno[3,2-b][1]benzothiophene-2-carbaldehyde (2).15

To a solution of TBT14,16 (1.9 g, 10 mmol) in 80 mL of tetrahydrofuran (THF) at
78 °C was added 4.8 mL of a 2.5 M solution of BuLi in n-hexane. The mixture was stirred for 1 h at
78 °C and then was transferred into a vigorously stirred solution of N,N-dimethylformamide (DMF) (0.88 g, 12 mmol) and allowed to stir at room temperature overnight. The resultant mixture was quenched with 10 mL of water and extracted with dichloromethane (3  50 mL) and dried over magnesium sulfate and concentrated in vacuum. The crude product was purified by chromatography using petroleum ether/dichloromethane (3:1) as an eluent to give a light yellow solid (1.74 g, 80%). 1H NMR (300 MHz, CDCl3): 10.0 (s, 1H), 8.0 (s, 1H), 7.96
7.98 (t, 1H), 23985

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7.90 (t, 1H), 7.47
7.49 (t, 2H). MS (EI) m/z: 218 (M+). Calcd for C11H6OS2: C, 60.52 (%); H, 2.77 (%). Found: C, 60.72 (%); H, 2.88 (%). 2-Bromo-thieno[3,2-b][1]benzothiophene (3).15 To a solution of TBT (1.9 g, 10 mmol) in 50 mL of dichloromethane at 0 °C was added (1.6 g, 10 mmol) bromine in 10 mL of dichloromethane. The mixture was stirred for another 2 h at room temperature and then quenched with 10% sodium hydroxide solution, and the aqueous layer was extracted with dichloromethane several times. The combined organic phase was washed with saturated brine, dried over magnesium sulfate, and purified by chromatography using petroleum ether as the eluent to give a light yellow solid (2.6 g, 97%). 1H NMR (300 MHz, CDCl3): 7.83
7.85 (d, 1H), 7.76
7.78 (d, 1H), 7.32
7.43 (m, 3H). MS (EI) m/z: 270 (M+). Calcd for C10H5OS2: C, 44.62 (%); H, 1.87 (%). Found: C, 44.51 (%); H, 2.03 (%). 2,20 -Bis(thieno[3,2-b][1]benzothiophene) (DTBT). A THF (50 mL) solution of compound TBT (0.95 g, 5 mmol) was cooled to
78 °C under a nitrogen atmosphere, and then 2.5 M n-BuLi in n-hexane (2.2 mL, 0.55 mmol) was added over 15 min to the above mixture with the temperature maintained at
78 °C. The mixture was stirred at
78 °C for 1 h and then at
40 °C for 0.5 h. After cooling back to
78 °C, tributyltin chloride (1.63 g, 0.5 mmol) was added, and the solution was stirred at room temperature for 24 h. The reaction was quenched by the addition of 10 mL of water, and the organic product was extracted with dichloromethane. The organic phase was dried over anhydrous MgSO4 and concentrated in vacuum. The resulting green oil was purified by distillation under reduced pressure to give 2.0 g of tributyl-thieno[3,2-b][1]benzothiophenestannane (1) that was used without further purification. MS (EI) m/z: 480 (M+). A THF (40 mL) solution of compound 3 (0.27 g, 1 mmol), compound 1 (0.48 g, 1 mmol), and Pd(PPh3)4 (0.05 g, 0.04 mmol) was then stirred at 70 °C for 10 h under a nitrogen atmosphere. After cooling to room temperature, the resulting solution was poured into 250 mL of petroleum ether. The resulting yellow solid was filtered and then washed with petroleum ether, water, and acetone, respectively. The crude product was further sublimated twice to give a pale yellow solid (0.28 g, 74%). 1H NMR (300 MHz, 380 K, C2D2Cl4): 7.90
7.93 (m, 5H), 7.47
7.62 (m, 5H). MS (EI) m/z: 378 (M+). Calcd for C20H10S4: C, 63.46 (%); H, 2.66 (%). Found: C, 63.03 (%); H, 2.52 (%). trans-1,2-Di[thieno[3,2-b][1]benzothiophenic-2-]ethylene (DTBTE). To a suspension of zinc powder (2.6 g, 41 mmol) in THF (60 mL) was slowly added titanium tetrachloride (2.2 mL) at 0 °C, and then the mixture was refluxed for 3 h under a nitrogen atmosphere. A solution of compound 2 (0.9 g, 4.1 mmol) and pyridine (3.5 g) in THF (30 mL) was slowly added into the mixture, and then the mixture was refluxed for another 6 h. After cooling to room temperature, the mixture was diluted with saturated sodium bicarbonate (50 mL) and stirred for 30 min. The solid was filtered and washed with diluted hydrochloric acid, water, and acetone and then dried. The crude product was sublimated twice to give bright yellow crystals (0.32 g, 39%). 1 H NMR (300 MHz, 380 K, C2D2Cl4): 7.83
7.89 (m, 4H), 7.43
7.46 (m, 2H), 7.38
7.40 (m, 2H), 7.34 (s, 2H), 7.21 (s, 2H). MS (EI) m/z: 404 (M+). Calcd for C22H12S4: C, 65.31 (%); H, 2.99 (%). Found: C, 65.23 (%); H, 3.02 (%). 1,4-Di[thieno[3,2-b][1]benzothiophenic-2-]benzene (DTBTB). A THF (50 mL) solution of compound 3 (0.54 g, 2 mmol),

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1,4-phenylenediboronic acid (0.166 g, 1 mmol), and Pd(PPh3)4 (0.05 g, 0.04 mmol) was stirred at 70 °C for 8 h under a nitrogen atmosphere. After cooling to room temperature, the resulting solution was poured into 200 mL of petroleum ether. The resulting yellow solid was filtered and then washed with petroleum ether, water, and acetone, respectively. The crude product was further sublimated twice to give a green solid (0.36 g, 79%). 1H NMR (300 MHz, 380 K, C2D2Cl4): 7.90
7.93 (m, 4H), 7.79 (s, 4H), 7.63 (s, 2H), 7.39
7.52 (m, 4H). MS (EI) m/z: 454 (M+). Calcd for C26H14S4: C, 68.69 (%); H, 3.10 (%). Found: C, 68.93 (%); H, 3.20 (%). 2,5-Di[thieno[3,2-b][1]benzothiophenic-2-]thiophene (DTBTT). A THF (50 mL) solution of compound 3 (0.54 g, 2 mmol), 2,5-bis(tributylstannyl)thiophene (0.66 g, 1 mmol), and Pd(PPh3)4 (0.05 g, 0.04 mmol) was stirred at 70 °C for 6 h under a nitrogen atmosphere. After cooling to room temperature, the resulting solution was poured into 200 mL of petroleum ether. The resulting yellow solid was filtered and washed with petroleum ether, water, and acetone. The crude product was further sublimated twice to give an orange solid (0.38 g, 82%). 1H NMR (300 MHz, 380 K, C2D2Cl4): 7.86
7.92 (m, 4H), 7.41
7.51 (m, 6H), 7.29 (s, 2H). MS (EI) m/z: 460 (M+). Calcd for C24H12S5: C, 62.57 (%); H, 2.63 (%). Found: C, 62.20 (%); H, 2.59 (%). Device Fabrication and Measurement. OTFTs based on the OTS-modified SiO2/Si substrates were fabricated in a topcontact configuration. Before the deposition of organic semiconductors, OTS treatment was performed on the SiO2 gate dielectrics in a vacuum to form an OTS self-assembled monolayer. The semiconductor thin films (30 nm) were then deposited under vacuum onto OTS-modified SiO2/Si substrates, followed by Au deposition through a shadow mask to define the source-drain electrodes. The channel length (L) and width (W) were 80 and 8800 μm, respectively. Flexible OTFTs were fabricated on the PET substrates. The copper was evaporated to form gate electrodes on the surface of the PET substrate through a metal shadow mask. The substrate was then coated with the polyacrylonitrile (PAN) (bottom layer) and polymethylsilsesquioxane (PMSQ ) (top layer) dielectric layer through spincoating techniques. The dielectric layers were annealed, and then the substrate was treated by ultraviolet/ozone, followed by dipping in the OTS solution. The DTBTE thin film was then deposited onto its surface under vacuum by the two-stage deposition process. Source-drain electrodes were obtained by gold deposition with a shadow mask. The channel length (L) and width (W) of the flexible OTFTs were also 80 and 8800 μm, respectively. The characteristics of the OTFT devices were determined at room temperature in air by using a Keithley 4200 SCS semiconductor parameter analyzer. The mobility of the devices was calculated in the saturation regime. The equation is listed as follows IDS ¼ ðW=2LÞCi μðVGS
Vth Þ2 where W/L is the channel width/length, Ci is the insulator capacitance per unit area, and VGS and Vth are the gate voltage and threshold voltage, respectively.

’ RESULTS AND DISCUSSION Synthesis and Characterization. The synthetic routes of semiconductors based on thieno[3,2-b][1]benzothiophene cores are summarized in Scheme 1. The key building blocks TBT, 14,16 2,15 and 315 were synthesized according to the 23986

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improved methods described in the corresponding literature. Compound 1 was prepared by the reaction of TBT with n-BuLi and tributylchlorostannane. The Stille coupling reaction of compound 1 with compound 3 then afforded the desired product DTBT. Similarly, the Stille coupling reaction of compound 3 with 2,5-bis(tributylstannyl)thiophene was performed to obtain the corresponding product DTBTT. Besides, compound 2 was easily converted into a CdC double bond via the McMurry coupling reaction to afford DTBTE in a moderate yield. The Suzuki coupling reaction of compound 3 and 1,4-phenylenediboronic acid afforded the desired product DTBTB in a high yield. All the synthesized compounds were further purified by twice vacuum sublimations before the devices were fabricated, and their chemical structures were determined by 1H NMR, EI-MS, elemental analyses, and single-crystal XRD. Optical and Electrochemical Properties. The optical data of the four compounds DTBT, DTBTE, DTBTB, and DTBTT in CHCl3 solution and as thin films are provided in Table 1 and Figure 1, respectively. According to the data, it is clear that the UV
vis absorption spectra of the four compounds can be strongly affected by the difference of π-bridge spacers. The absorption spectra of DTBTE and DTBTT in CHCl3 solution show a maximum at 408 and 418 nm, respectively, which are red shifted about 24
34 nm compared with that of DTBT (λmax = 384 nm) and are consistent with a more extended π-conjugation system. Even though a benzene bridge was introduced in the middle of the conjugation system, the compound DTBTB still shows a similar UV
vis absorption behavior in CHCl3 solution with that of DTBT, indicating that the benzene bridge does not extend the effective π-conjugation length compared with that of DTBT. This can be ascribed to the distortion effects of the Table 1. Optical and Electrochemical Properties of the Four Compounds compound

a

λmax

λmax

HOMO

LUMO

Eg

(nm)a

(nm)b

(eV)c

(eV)d

(eV)e

DTBT

370, 384

366

5.71

2.82

2.89

DTBTE

408, 432

366, 444, 476

5.53

2.81

2.72

DTBTB

382

362

5.56

2.68

2.88

DTBTT

404, 418

394, 410, 478

5.49

2.85

2.64

Measurement performed in CHCl3 solution. b Measurement performed in 30 nm thin films. c Determined by cyclic voltammetry. d Estimated by Eg and HOMO energy level. e Determined from the onset of UV
vis absorption spectra in the thin film.

central benzene-bridged single bonds in the molecule, which reduce the molecule's overall planarity and the π-electron conjugation.17 However, the obvious differences were observed between absorption spectra in the thin films and CHCl3 solutions (Figure 1). For the compounds DTBT and DTBTB, the spectra in the thin films show a blue shifted absorption maximum relative to those of the solution due to H-aggregation of molecules. The absorption spectra of compounds DTBTE and DTBTT become broadened with blue shifted absorption peaks and red shifted absorption peaks relative to those of their solutions, respectively, indicating that there are H-aggregation and J-aggregation in the thin films.18 The CV method was employed in the acetonitrile solution, and the data are summarized in Table 1. The HOMO energy levels of the four compounds DTBT, DTBTE, DTBTB, and DTBTT, estimated from the oxidation onset, vary from
5.49 to
5.71 eV, which are much lower than that of pentacene (
5.14 eV). This result implies good stability of the compounds against oxygen under ambient conditions. Besides, the HOMO energy levels match well with the work function of the gold electrode (
5.2 eV) and the hole could be easily injected from the gold source electrode into the organic semiconductor layer. Single Crystal Structure. A single crystal of DTBTE was obtained by slow sublimation in a temperature-gradient furnace. As shown in Figure 2, the crystal structure of DTBTE shows a nearly coplanar conformation with a condensed herringbone arrangement, similar to that of pentacene.19 As expected, the two fused TBT units adopt an anti conformation, which may be favorable to the charge carrier transport.20 The short S 3 3 3 C contact distances between the neighboring molecules are 3.323, 3.364, and 3.497 Å, which are obviously shorter than the nonbonded S 3 3 3 C contact distance (3.61 Å) observed in most of the organic crystal structures.21 Besides, the strong CH 3 3 3 π (2.816 Å) and S 3 3 3 S (3.507 Å) interactions between adjacent molecules were also observed. It should be noted that this short S 3 3 3 S distance is much smaller than the double distances of the van der Waals radius of the S atom (3.7 Å). 22 All these short interactions form an effective three-dimensional (3D) structure, which is beneficial to obtaining a high charge transport performance. OTFT Performance and Film Morphology. The OTFT devices were fabricated in the top contact geometry, as reported previously.22 All the devices were measured in ambient conditions and display p-type properties in air. Typical output and transfer characteristics of the DTBTE-based OTFT devices are

Figure 1. Absorption spectra of the four compounds DTBT, DTBTE, DTBTB, and DTBTT in CHCl3 solutions (a) and as thin films (b). 23987

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Figure 2. (a) Packing structure and (b
d) neighboring interactions of the DTBTE molecules.

Figure 3. (a) Output and (b) transfer characteristics of the DTBTE-based OTFT devices on OTS-treated SiO2/Si substrates fabricated by using a two-stage film deposition process.

presented in Figure 3. Corresponding parameters obtained at different substrate temperatures (Tsub) are summarized in Table 2. At Tsub = 30 °C, the OTFTs based on the compounds DTBT and DTBTT exhibit a relatively low mobility of 0.007 cm2 V
1 s
1 with an on/off current ratio of up to 9.3  107 and a mobility of 0.03 cm2 V
1 s
1 with an on/off current ratio of up to 2.4  106, respectively. However, the compound DTBTE shows a better field-effect performance with a mobility of 0.17 cm2 V
1 s
1

and an on/off current ratio of up to 1.7  107 at Tsub = 30 °C. The mobility of DTBTE is higher by 2
3 orders of magnitude compared with that of the corresponding OTFTs based on the compound DTBT or DTBTT, probably due to the more planar π-conjugation system and the effective 3D intermolecular interactions for the compound DTBTE. After changing the Tsub, the mobility and on/off current ratio for DTBTE are further enhanced to 0.33 cm2 V
1 s
1 and 5.5  106 at Tsub = 100 °C, 23988

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Figure 4. AFM images of the DTBTB and DTBTE thin films grown on SiO2 with OTS treatment at different substrate temperatures.

Table 2. Characteristics of OTFTs Based on the Semiconductors Prepared by Vacuum Deposition on the OTS-Treated SiO2/Si Substrates at Different Substrate Temperatures on/off current

mobility

Tsub (°C)

Vth (V)

ratio

(cm2 V
1 s
1)

DTBT

30


20.5

9.3  107

0.003
0.007

DTBTT

30


15.6

2.4  106

0.02
0.03

DTBTB

30


16

1.5  106

0.13
0.16

80


30

3.5  107

0.16
0.24

100 120


30
33

3.6  107 1.0  107

0.15
0.19 0.19
0.30

compound

DTBTE

30


6.6

1.7  107

0.12
0.17

80


3.3

8.8  106

0.14
0.17

5.5  106

0.21
0.33

3.2  106

0.14
0.24

4.8  106

0.41
0.50

8.2  105

0.20
0.30

100 120 120 + 30 120 + 60


11
7.1
26
6.2

respectively. The effect of Tsub on field-effect performance was also observed in the DTBTB-based OTFTs. The mobility and on/off current ratio for DTBTB are increased from 0.16 cm2 V
1 s
1 and 1.5  106 at Tsub = 30 °C to 0.30 cm2 V
1 s
1 and 1.0  107 at Tsub = 120 °C, respectively. This phenomenon can be readily understood from AFM images (see Figure 4). For the compound DTBTB, with increasing Tsub, the DTBTB film forms larger grains and thus the number of crystalline domains and grain boundaries decreases, which is favorable for the charge transport and leads to an increase of the mobility.23,24 However, for the compound DTBTE, with the Tsub increasing, single crystalline grain sizes of the compound DTBTE remarkably grow up, while the grain boundaries become rather deep at Tsub = 120 °C. Therefore, the mobility has a limited increase just through changing the Tsub. To further improve the device performance of the compound DTBTE, a two-stage deposition technology was employed here. The exactly process was as follows: first, a 10 nm DTBTE film was evaporated on the OTS-modified SiO2/Si substrate under a high Tsub (120 °C) to form large-sized grains on the substrate at a slow flux rate (10 Å/min), and then the grain boundary and charge traps were further filled with a second 30 nm DTBTE film deposited under a lower Tsub (30 °C) at a fast flux rate (40 Å/ min). Because of the enhanced DTBTE film interconnectivity (see Figure 4), the OTFTs based on DTBTE showed a higher mobility of 0.50 cm2 V
1 s
1 and a on/off current ratio of up to 4.8  106 by the two-stage deposition process. Furthermore, in

Figure 5. Mobility distribution for 48 DTBTE-based flexible OTFTs fabricated on the PET substrates by using a two-stage film deposition process.

agreement with their low HOMO energy levels and large band gaps, the OTFTs based on the compound DTBTB or DTBTE films exhibit good environmental stability. After half a year of shelf life in an ambient laboratory, the OTFTs based on the compounds DTBTB and DTBTE still showed a high mobility of 0.12 cm2 V
1 s
1 with an on/off current ratio of up to 3.7  107 and a mobility of 0.21 cm2 V
1 s
1 with an on/off current ratio of up to 1.3  107, respectively. These mobility data are still higher than that of amorphous silicon (0.1 cm2 V
1 s
1).2
4 To further estimate their potential application as the organic semiconductors in the flexible OTFTs, we fabricated DTBTEbased flexible OTFTs on the PET substrates. The gate electrodes were prepared by evaporated copper with a shadow mask. Double dielectric layers composed of PAN and PMSQ were used as insulators. After OTS modification on the PAN/PMSQ dielectric layer, the DTBTE thin film was deposited onto its surface in vacuum by the two-stage deposition process as the same above. Source-drain electrodes were obtained by gold deposition with a shadow mask. Figure 5 shows the statistics chart of mobility distribution for 48 DTBTE-based flexible OTFTs fabricated on the PET substrates. It is clear that all flexible OTFTs exhibit a mobility of more than 0.1 cm2 V
1 s
1 with a maximum mobility of up to 0.45 cm2 V
1 s
1. Meanwhile, 70% of OTFT devices show the mobility between 0.16 and 0.34 cm2 V
1 s
1, which are comparable to those of devices fabricated on the OTS-modified SiO2/Si substrates. All these results indicate great potential application for DTBTE to fabricate the flexible and low-cost OTFT devices. 23989

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Figure 6. XRD diagrams of the DTBTB and DTBTE thin films deposited on SiO2 with OTS treatment at different substrate temperatures.

Film Crystalline Properties. To investigate the crystalline properties of the thin films for the semiconductors based on thieno[3,2-b][1]benzothiophene cores, XRD pattern analysis was performed on the 30 nm films deposited at different temperatures on the OTS/SiO2/Si substrates. As shown in Figure 6, the XRD results of the DTBTB and DTBTE thin films display a series of strong and sharp diffraction peaks, indicating highly ordered films with high crystallinity on the substrates. For the compound DTBTB, the first strong reflection peak in the XRD pattern is at 2θ = 4.8°, corresponding to the d-spacing of 21.74 Å, which is identical to the length of the molecule (21.34 Å) calculated by ChemDraw 3D and optimized by Gaussian 03.25,26 This result indicates that the DTBTB molecules adopt a nearly perpendicular orientation on the substrate. The molecular arrangements in the DTBTE films are in the same manner as those of the DTBTB films; the d-spacing calculated from the first XRD diffraction peak is 18.94 Å. This result is almost identical to the length of the b axis observed in the DTBTE single crystal structure (18.74 Å), indicating that the molecule is also nearly perpendicular to the substrate. This molecular arrangement is well known to achieve high carrier mobility because of the intermolecular interactions in parallel to the direction of the channel current flow.8 In addition, it is clear that the XRD pattern results of the DTBTE film (120 + 30 °C) deposited by the twostage deposition technology show much stronger diffraction peaks compared with those of the films deposited at Tsub = 80 or 100 °C. This result indicates that the two-stage deposited DTBTE films have much higher crystallinity, therefore, obtaining high carrier mobility for the compound DTBTE.

’ CONCLUSIONS In summary, several organic semiconductors with thieno[3,2b][1]benzothiophene cores and different π-bridge spacers were designed and synthesized. These compounds have the relative large band gaps and low HOMO energy levels that should lead to a good environmental stability. Single-crystal XRD data of DTBTE reveal that the strong intermolecular CH 3 3 3 π, S 3 3 3 S, and S 3 3 3 C interactions exist between neighboring molecules. These interactions are helpful to obtain high charge carrier mobility. The two-stage deposition technology of the DTBTE thin film was employed to obtain a balance between high film crystallinity and good film grain interconnectivity. The DTBTEbased OTFTs on the silicon and flexible PET substrates exhibit

excellent field-effect performance with a maximum mobility of 0.50 and 0.45 cm2 V
1 s
1, respectively.

’ ASSOCIATED CONTENT

bS

Supporting Information. Crystallographic data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (G.Y.), [email protected] (Y.L.).

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (20825208, 60736004, and 21021091), the Major State Basic Research Development Program (2011CB808403, 2011CB932303), and the Chinese Academy of Sciences. ’ REFERENCES (1) Drury, C. J.; Mutsaers, C. M. J.; Hart, C. M.; Matters, M.; de Leeuw, D. M. Appl. Phys. Lett. 1998, 73, 108–110. (2) Dong, H. L.; Wang, C. L.; Hu, W. P. Chem. Commun. 2010, 46, 5211–5222. (3) Wu, W. P.; Liu, Y. Q.; Zhu, D. B. Chem. Soc. Rev. 2010, 39, 1489–1502. (4) Murphy, A. R.; Frechet, J. M. J. Chem. Rev. 2007, 107, 1066– 1096. (5) Kelley, T. W.; Boardman, L. D.; Dunbar, T. D.; Muyres, D. V.; Pellerite, M. J.; Smith, T. Y. P. J. Phys. Chem. B 2003, 107, 5877–5881. (6) Klauk, H.; Halik, M.; Zschieschang, U.; Eder, F.; Schmid, G.; Dehm, C. Appl. Phys. Lett. 2003, 82, 4175–4177. (7) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J. J. Am. Chem. Soc. 1998, 120, 664–672. (8) Du, C. Y.; Guo, Y. L.; Liu, Y. Q.; Qiu, W. F.; Zhang, H. J.; Gao, X. K.; Liu, Y.; Qi, T.; Lu, K.; Yu, G. Chem. Mater. 2008, 20, 4188–4190. (9) Du, C. Y.; Guo, Y. L.; Chen, J. M.; Liu, H. T.; Liu, Y.; Ye, S. H.; Lu, K.; Zheng, J.; Wu, T.; Liu, Y. Q.; et al. J. Phys. Chem. C 2010, 114, 10565–10571. (10) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J.; Dodabalapur, A. Adv. Mater. 1997, 9, 36–39. (11) Kim, C.; Marks, T. J.; Facchetti, A.; Schiavo, M.; Bossi, A.; Maiorana, S.; Licandro, E.; Todescato, F.; Toffanin, S.; Muccini, M.; et al. Org. Electron. 2009, 10, 1511–1520. 23990

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