High-performance n-channel field effect transistors based on solution-processed dicyanomethylene-substituted tetrathienoquinoid

Abstract

A solution-processable tetrathienoquinoidal semiconductor CMHT was synthesized and characterized. Single crystal diffraction results showed that CMHT adopted slipped π–π stacking in the crystal structure. Multiple intermolecular interactions, such as S⋯N and S⋯C (where C is the carbon on the cyano group), existed among neighboring molecules, which formed a 2-dimensional charge transport network. Solution-processed thin film transistors of CMHT displayed a high electron mobility of up to 0.22 cm² V⁻¹ s⁻¹ under ambient conditions, one of the highest electron mobilities for quinoidal semiconductors. The correlation between the molecular packing of the CMHT and transistor performance was studied by AFM and XRD.

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Introduction

N-channel organic semiconductors have attracted great attention because of their wide-ranging applications in organic logic circuits, ambipolar transistors and p–n junctions.^1–6 Currently, the development of high performance n-channel organic semiconductors is largely lagging behind that of their p-channel counterparts,^7–12 because of the difficulty of designing and synthesizing strong electron withdrawing units,^13–15 and the low stability of the charge carriers of the n-channel organic semiconductors (electrons) to water and oxygen trapping.¹⁶ Until now, the lack of solution-processable, ambient-stable and high performance n-channel organic semiconductors has become one of the bottlenecks of organic thin film transistors (OTFTs).¹⁷ Hence it is important and necessary to design and synthesize solution-processable, high performance n-channel organic semiconductors with ambient stability.^18,19

Dicyanomethylene-substituted quinoidal compounds have low LUMO energy levels (<−4.0 eV) and are good candidates for ambient-stable n-channel organic semiconductors.^20–23 Recently, we reported a dicyanomethylene-substituted tetrathienoquinoid semiconductor CMUT (see Scheme 1 for the chemical structure).²⁴ This compound displayed one of the highest mobilities reported for solution-processable and ambient-stable n-channel organic semiconductors, suggesting the potential applications of tetrathienoquinoid as a high performance n-channel organic semiconductor. Exploring the molecular packing–property relationships of tetrathienoquinoids will help us to understand their high performance and further enlighten us in the design of new high performance materials. However, due to the long and branched alkyl chains, attempts to obtain single crystals and details of the packing behaviour of CMUT in thin films failed, which hindered the understanding of the correlation between the molecular packing of tetrathienoquinoid and its charge transport properties. With the aim of solving these puzzles and further investigating the applications of tetrathienoquinoid organic semiconductors in organic transistors, herein dicyanomethylene-substituted tetrathienoquinoid with hexyl substituents (CMHT, see Scheme 1 for the chemical structure) was designed and synthesized. Solution-processed TFT based on CMHT displayed a high electron mobility of up to 0.22 cm² V⁻¹ s⁻¹ under ambient conditions. The molecular packing pattern of CMHT in crystals and thin films, and its correlation to device performance, were thoroughly studied.

Scheme 1 The chemical structures of CMUT and CMHT.

Experimental section

Synthesis and characterization

General: all chemicals and solvents were commercially available and used as obtained. 2,3,5,6-tetrabromothieno[3,2-b]thiophene was synthesized according to the reported procedure.^{25 1}H NMR (300 MHz) and ¹³C NMR (100 MHz) spectra were obtained in CDCl₃ on a Varian Mercury (300 MHz or 400 MHz) instrument with TMS as an internal reference. Mass spectra (EI-MS) were performed on a Shimadzu Qp-5050A spectrometer using an electron impact ionization procedure (70 eV), and MALDI-TOF spectra were carried out on a Voyager-DE STR mass spectrometer. Elemental analyses were conducted on an Elementary Vario EL III element analyzer. Out-of-plane X-ray diffraction (XRD) patterns were recorded on a 2 kW Rigaku X-ray diffraction system with Cu Kα radiation (λ = 1.54 Å). Atomic force microscopy (AFM) measurements were carried out on a Nanoscope IIIa atomic force microscopy in tapping mode.

2,4-Di(1,1′-heptanone)-3,6-dibromothieno[3,2-b]thiophene (1)

To a mixture of 2,3,5,6-tetrabromothieno[3,2-b]thiophene (2.28 g, 5 mmol) in 50 mL dry THF, n-butyl lithium (4.2 mL, 2.5 M in hexane) was added dropwise at −78 °C and the resulting mixture was stirred for 2 hours. CuI (1.89 g, 10 mmol) was added and then the temperature was allowed to increase to −23 °C. The mixture was stirred for another 2 hours before being recooled to −78 °C, and heptanoyl chloride (1.49 g, 10 mmol) was then added quickly. The final mixture was allowed to warm to room temperature and was stirred for 2 hours. This reaction was then quenched with 20 mL water and extracted with ethyl acetate. The combined extracts were dried over anhydrous MgSO₄, filtered and the solvent was evaporated under reduced pressure to give the crude products, which were crystallized from ethanol and filtered to afford compound 1 (1.49 g, 57%). Mp, 160–161 °C; ¹H NMR (300 MHz, CDCl₃) δ 0.91 (t, 6H) 1.29–1.46 (m, 12H) 1.72–1.82 (m, 4H), 3.08 (t, 4H); ¹³C NMR (100 MHz, CDCl₃) δ 14.18, 22.64, 24.10, 28.99, 31.75, 41.65, 106.55, 143.55, 143.43, 193.05; MS (EI) m/z = 382 (M⁺ − 2C₅H₁₀); Anal. calcd for C₂₀H₂₆O₂S₂: C, 45.99; H, 5.02%; Found: C, 46.20; H, 4.82%.

2,6,-Dicarboethoxy-3,7-dihexylthieno[3,2-b]thieno[2′,3′:4,5]thieno[2,3-d]thiophe-ne (2)

Compound 1 (1.99 g, 3.82 mmol) was mixed with K₂CO₃ (3.16 g, 138 mmol) and DMF (20 mL). To this mixture, ethyl mercaptoacetate (1.10 g, 9.17 mmol) was added dropwise. The reaction mixture was stirred for 20 hours at 60 °C, and then poured into water (50 mL). A solid was collected by filtration and flash chromatographed on silica. Light petroleum–dichloromethane (ratio 5 : 1) afforded compound 2 (1.36 g, 63%). Mp, 173–174 °C; ¹H-NMR (300 MHz, CDCl₃) δ 0.89 (t, 6H), 1.31–1.44 (m, 18H), 1.71–1.76 (m, 4H), 3.14 (t, 4H); 4.38 (q, 4H); ¹³C NMR (100 MHz, CDCl₃) δ 14.08, 14.33, 22.59, 29.18, 29.31, 29.40, 31.60, 61.10, 127.33, 133.61, 134.52, 143.43, 144.24, 162.49; MS (EI) m/z = 564 (M+); Anal. calcd for C₂₈H₃₆O₄S₄: C, 59.54; H, 6.42%; Found: C, 59.85; H, 6.43%.

3,7-Dihexylthieno[3,2-b]thieno[2′,3′:4,5]thieno[2,3-d]thiophene 2,6,-dicarboxylateacid (3)

To a mixture of THF (18 mL), methanol (3 mL), water (2 mL) and LiOH–H₂O (2.10 g, 50 mmol), compound 2 (1.13 g, 2 mmol) and a catalytic amount of tetrabutylammoniumiodide were added. The mixture was refluxed overnight. The solvent was evaporated and the residue was acidified with concentrated HCl to achieve a pH of 2. The solid was filtered and washed with water three times. The solid was dried to give compound 3 (1.0 g, 98%). Mp, 320–321 °C; ¹H NMR (300 MHz, DMSO-D6) δ 0.85 (t, 6H), 1.26–1.36 (m, 12H), 1.63–1.70 (m, 4H), 3.11 (t, 4H), 13.38 (b, 2H); MS (MALDI-TOF) m/z = 509.4 (M⁺ + H).

2,6,-Dibromo-3,7-dihexylthieno[3,2-b]thieno[2′,3′:4,5]thieno[2,3-d]thiophene (4)

Compound 3 (0.51 g, 1.0 mmol) and NBS (0.53 g, 3 mmol) were added to the mixture of NMP (15 mL) and water (0.75 mL). This mixture was stirred overnight under a nitrogen atmosphere, at room temperature. The mixture was then added to water (40 mL) and extracted with light petroleum. The combined extracts were dried over anhydrous MgSO₄ and filtered. Evaporation of solvent under reduced pressure gave the crude product, which was flash chromatographed on silica. Light petroleum afforded compound 4 (0.46 g, 79%). Mp, 193–194 °C; ¹H NMR (300 MHz, CDCl₃) δ 0.89 (t, 6H), 1.30–1.38 (m, 12H), 1.69–1.74 (m, 4H), 2.74 (t, 4H); MS (EI) m/z = 578 (M⁺); Anal. calcd for C₂₂H₂₆Br₂S₄: C, 45.68; H, 4.53%; Found: C, 45.91; H, 4.75%.

2,6,-Bis(dicyanomethylene)-3,7-dihexylthieno[3,2-b]thieno[2′,3′:4,5]thieno[2,3-d]-thiophene (CMHT)

CMHT was synthesized according to the reported procedure^23,24 with a yield of 73%. Mp = 267–268 °C. The ¹H-NMR and ¹³C-NMR of CMUT were not obtained because of its poor solubility. MS (MALDI-TOF) m/z = 547.44 (M⁺ + H); Anal. calcd for C₂₈H₂₆N₄S₄: C, 61.50; H, 4.79; N, 10.25%; Found: C, 61.74; H, 4.62; N, 10.17%.

Device fabrication and characterization

The Si/SiO₂ substrates were cleaned and modified with octadecyltrichlorosilane (OTS) according to previously reported procedures.^26,27 The devices were fabricated with a top-contact bottom-gate configuration. The gate electrode and gate dielectric layer were n-type heavily doped Si, and a thermally grown SiO₂ layer 300 nm thick (the specific capacitance was measured to be 10 nF.cm⁻²), respectively. Thin films of CMHT were prepared by drop casting 1 mg mL⁻¹ o-dichlorobenzene solution on octadecyltrichlorosilane (OTS)-treated SiO₂/Si wafers at 150 °C. The Au source and drain electrodes were deposited by vacuum evaporation through a shadow mask, which resulted in a channel length of 31 μm and a width of 273 μm. A Keithley 4200 semiconductor parameter analyzer was used to measure the electrical properties of the devices at room temperature under ambient conditions. The field-effect mobility of the electrons (μ_e) and the threshold voltage (V_TH) were calculated from the saturation region, according to the expression I_DS = (W/2L)μ_eC_i(V_G − V_TH)² (I_DS, drain–source current; W, channel width; L, channel length; C_i, capacitance per unit area of the gate dielectric layer; V_TH, threshold voltage).

Results and discussion

The synthetic route of CMHT is illustrated in Scheme 2. Starting from 2,3,5,6-tetrabromothieno[3,2-b]thiophene, compound 1 was obtained through a lithium reaction in a moderate yield (57%). Compound 1 reacted with ethyl mercaptoacetate in the presence of a base to give thieno[3,2-b]thieno[2′,3′:4,5]thieno[2,3-b]thiophene diethyl ester 2 with a yield of 63%. Basic hydrolysis of 2, followed by acidification with HCl, afforded compound 3 in a high yield (98%). Compound 3 was decarboxylized and brominated with NBS/NMP at a high temperature to give 4. The target compound CMHT was synthesized by reacting 4 with malononitrile through a Pd-catalyzed Takahashi coupling reaction, followed by oxidation with bromine. Compared with CMUT, the solubility of CMHT decreased dramatically in common organic solvents such as dichloromethane, chloroform, chlorobenzene and dichlorobenzene (less than 0.5 mg mL⁻¹), which was ascribed to the replacement of the long and branched 3-hexylundecyl substituents with hexyl chains.

Scheme 2 The synthetic route of CMHT.

The combined solution cyclic voltammograms (CVs) of CMHT and CMUT (0.001 mol L⁻¹) in CH₂Cl₂ are shown in Fig. 1a. The much weaker wave peaks of CMHT than CMUT were due to its poor solubility. Similarly to that of CMUT, CMHT displayed two reversible reduction peaks. The first half-wave potential (E₁^1/2) was at −0.10 V and the corresponding LUMO energy level calculated from the CV was −4.3 eV (the same as that of CMUT), suggesting that the difference in the alkyl chains did not affect their LUMO energy levels. Fig. 1b illustrates the absorption spectra of CMHT and CMUT in solution and on the films. They showed the same absorption spectra in CH₂Cl₂ solution. The optical energy gap (E_g^opt) estimated from the onset of the absorption in solution was 1.8 eV. The film of CMHT, prepared by dropping a hot o-dichlorobenzene solution (1 mg mL⁻¹) on to a quartz substrate at 150 °C, exhibited a strong absorption band at λ_max = 498 nm, representing a blue shift of 127 nm from that of the solution. This blue shift is the largest observed in the thienoquinoid compounds and is more than twice as large as that of CMUT (60 nm), indicating that minimizing steric bulkiness is an effective way to enhance the intermolecular interaction. Similarly to CMUT, CMHT displayed a shoulder absorption at 685 nm, which was a typical Davydow splitting and suggested the CMHT adopted H-type aggregation in the thin films.

Fig. 1 (a) Cyclic voltammograms of CMHT and CMUT (10⁻³ mol L⁻¹) with 0.1 mol L⁻¹ Bu₄NPF₆ as an electrolyte in CH₂Cl₂ solutions under a scan rate of 50 mV s⁻¹ (ferrocene was used as an internal standard); (b) UV-vis absorption spectra of CMHT and CMUT in dichloromethane solution (1 × 10⁻³ mol L⁻¹) and on quartz substrate (as deposited thin-film).

To investigate the charge transport properties of CMHT, the foremost task was to prepare large-scale thin films of it on the substrates. As the solubility of CMHT is very low in organic solvents at room temperature, the film was solution-processed at a high temperature, which can dramatically increase the solubility. The SiO₂/Si wafers were treated with octadecyltrichlorosilane (OTS) to avoid electron trapping by hydroxyl groups of the SiO₂ surface. By drop-casting the solution of CMHT in o-dichlorobenzene on an OTS-treated SiO₂/Si substrate at 150 °C, large-scale thin films were successfully prepared. Varying the concentration in the range of 0.1 to 1 mg mL⁻¹ did not change the molecular packing in the films, as they displayed the same family of Bragg reflections.

Transistors with a bottom-gate, top-contact configuration were fabricated and tested under ambient conditions. Fabrication and testing details are described in the experimental section. The devices showed typical n-channel characteristics and the corresponding output and transfer curves are illustrated in Fig. 2. The electron mobility was in the range of 0.08–0.22 cm² V⁻¹ s⁻¹ and the current on/off ratio was about 1 × 10⁴.

Fig. 2 The thin film transistor characteristics of CMHT. (a) Transfer and (b) output curves.

The thin-film microstructures and morphologies, which strongly affect the transistors' performance, were investigated by atomic force microscopy (AFM) and X-ray diffraction (XRD) (Fig. 3). XRD results showed that, similarly to CMUT, the CMHT thin films were highly crystalline, as indicated by the multiple single family Bragg reflections up to the fifth progression. The lattice d-spacing of the CMHT thin films estimated from the diffraction peaks was 18.46 Å. AFM images revealed clearly that grain boundaries existed in CMHT films, and the size of the grain was several nanometers. The smaller grain size, as well as more and larger grain boundaries between the source and drain electrodes, exert negative effects on the transistor performance of CMHT. Even so, the mobility of 0.22 cm² V⁻¹ s⁻¹ is still one of the highest performances for TFT devices based on thiophenequinoidal compounds.

Fig. 3 (a) AFM images (1 × 1 μm) of drop-casted thin films of CMHT; (b) the XRD pattern of drop-casted thin films (the red line), which fit well with the simulated powder pattern from a bulk single crystal of CMHT (the blue line).

With the aim to fully understand the high performance of tetrathienoquinoid compounds, the molecular packing pattern of CMHT was studied with a single crystal. Single crystals of CMHT were grown by recrystallization in a hot o-dichlorobenzene solution. The crystal structure belonged to a triclinic space group with a = 5.2700(8) Å, b = 7.2800(11) Å, c = 18.461(3) Å, α = 92.723(2)°, β = 96.980(3)° and γ = 110.788(3)°. The conjugation core of the CMHT exhibited a planar structure and the hexyl substituents were turned out of the plane, affording a chair-like configuration in the crystals. CMHT adopted face-to-face slipped π–π stacking with a stacking distance of 3.46 Å (a typical π–π stacking distance). Interestingly, multiple strong intermolecular interactions such as S⋯N and S⋯C (where C is the carbon on the cyano group) were found between the neighbouring molecules (Fig. 4). These interactions, along with π–π interactions, formed a 2-dimensional network in the crystals which facilitated charge transport.²⁸ The thin film diffraction peaks of CMHT fit very well with the simulated powder pattern of the single crystals (Fig. 3b), suggesting CMHT adopted the same molecular packing in the bulk single crystals and the thin films. The lattice d-spacing of the CMHT thin films (18.46 Å) was equal to the length of the c-axis (18.46 Å) in the single crystals. Thus, we inferred that the CMHT crystallites on the films had their c axis perpendicular to the substrate surface. This edge-on packing structure, with face-to-face π–π stacking and multiple intermolecular interactions, promoted electron transport and thus should be responsible for the high performance of tetrathienoquinoid semiconductors.

Fig. 4 (a) Intermolecular non-bonded contacts (S⋯N) between the conjugated cores (the sulfur, nitrogen and carbon atoms are marked in yellow, blue and grey, respectively); (b) the π–π stacking of the conjugated backbone and the intermolecular non-bonded contacts (S⋯N and C⋯S) among the conjugated cores; (c) the proposed packing pattern of CMHT on an OTS–SiO₂ substrate in the thin films.

Conclusions

In conclusion, a dicyanomethylene-substituted tetrathienoquinoid semiconductor CMHT was successfully synthesized. A high electron mobility of up to 0.22 cm² V⁻¹ s⁻¹ under ambient conditions was observed, based on solution-processed thin films of CMHT. Single crystal diffraction results showed that CMHT adopted slipped π–π stacking in the crystals, and strong intermolecular interactions such as S⋯N and S⋯C (where C is the carbon on the cyano group) interactions were found among neighbouring molecules. These interactions formed a 2-dimensional electron transporting network in the crystals. Thin film XRD results demonstrated that CMHT adopted the same molecular packing in single crystals and thin films, suggesting that the slipped π–π stacking, as well as the multiple intermolecular interactions among neighbouring molecules, are responsible for the high performance of tetrathienoquinoid organic semiconductors.

Acknowledgements

This work was supported by the National Natural Sciences Foundation of China (21190031, 51273212).

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Footnote

† CCDC 945698. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ra47095d

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