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phenylene co-oligomers: Synthesis, thin film preparation, and transistor device characteristics
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Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel 6 7
Alkyl-monosubstituted thiophene/phenylene co-oligomers: Synthesis, thin film preparation, and transistor device characteristics
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Ryuji Hirase a, Mari Ishihara a, Toshifumi Katagiri b, Yosuke Tanaka c, Hisao Yanagi c, Shu Hotta d,⇑ a
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Hyogo Prefectural Institute of Technology, 3-1-12 Yukihira-cho, Suma, Kobe, Hyogo 654-0037, Japan Sumitomo Seika Chemicals Co., Ltd., 346-1 Miyanishi, Harima-cho, Kako-gun, Hyogo 675-0145, Japan Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan d Kyoto Institute of Technology, Department of Macromolecular Science and Engineering, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan b c
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a r t i c l e
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i n f o
Article history: Received 9 August 2013 Received in revised form 7 April 2014 Accepted 9 April 2014 Available online xxxx
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Keywords: Organic semiconductor Thiophene/phenylene co-oligomer Organic field-effect transistor Molecular bilayer Skim method Free-standing thin film
a b s t r a c t A novel series of alkyl-monosubstituted thiophene/phenylene co-oligomers (TPCOs) has been synthesized and characterized. The introduction of alkyl chains to TPCOs improved the solubility in organic solvents. Thin films of these compounds were prepared by the vacuum-deposition, solution-cast, and skim methods. Of these, the solution-cast films and skim films consist of highly-ordered structures characterized by molecular bilayers. In particular, the skim method produced a large-sized free-standing thin film. We investigated the carrier transport of the thin films on the field-effect transistor (FET) configurations. These FET devices exhibited typical p-channel characteristics with clear saturation region. The introduction of alkyl monosubstituents is responsible for enhancement in the carrier mobility. The device characteristics are contrasted with those of alkyl-disubstituted compounds. Ó 2014 Published by Elsevier B.V.
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1. Introduction
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Organic semiconductors have attracted much attention because of their easier fabrication processes compared with silicon technologies. Among their applications, fieldeffect transistors (FETs) are of particular interest because of the potential for low-cost, large area, flexible, and light-weight electronic devices such as active-matrix flat displays, electronic papers, radio frequency identification tags, and sensors [1–3]. One of the most challenging tasks on organic FETs is the design of excellent organic semiconductors that combine high carrier mobility, good environmental stability, and processability.
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⇑ Corresponding author. Tel.: +81 75 724 7793; fax: +81 75 724 7780. E-mail address: [email protected] (S. Hotta).
Unsubstituted thiophene/phenylene co-oligomers (TPCOs) with various thiophene/phenylene ratios are promising class of organic semiconductors since they offer high carrier mobility and good environmental stability. These materials have been employed in light-emitting diodes [4–6] as well as in p-channel FETs [7–10]. However, the solubility of unsubstituted TPCOs is not good enough for solution processing [11]. Solution-processed organic electronic devices have been attracting much attention because of their low-temperature and easy fabrication process without any vacuum steps, enabling high-throughput fabrication over large areas on flexible substrates [12]. It was reported that the introduction of alkyl chains to oligothiophene backbones improved the solubility of the resulting materials in organic solvents and that the alkyl substitution positions influenced the solubility [13,14].
http://dx.doi.org/10.1016/j.orgel.2014.04.010 1566-1199/Ó 2014 Published by Elsevier B.V.
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Furthermore, researchers studied substituent effects on electronic properties such as carrier mobility of organic semiconductors [15–22]. The intrinsic carrier mobility depends largely on the molecular ordering and p–p stacking extent of organic semiconductors [23]. In this context it is well-known that the introduction of appropriate alkyl chains (such as methyl and hexyl groups) to the terminal positions enhances molecular organization and ordering and improves p–p stacking of aromatic backbones [21,24]. Therefore, their introduction is thought to enhance the carrier mobility. This is indeed the case with a-sexithiophene having relatively shorter alkyl chains (e.g. ethyl and hexyl) [18,21,24]. On the contrary, Halik et al. reported that the FET performance was reduced by introducing longer alkyl chains (i.e. decyl groups) at the molecular terminals [15]. Using a different conjugated backbone [1]benzothieno[3,2-b][1]benzothiophene (BTBT), Ebata et al. reported the FET performance and solubility of its derivatives with various terminal alkyl chains [25]. All the materials mentioned above are characterized by the presence of alkyl chains symmetrically disubstituted at both the molecular terminals. Also Iino et al. [26] studied thermal characteristics and FET performance of a thin film made of a 2-octylthienyl derivative of BTBT. Note in that case that the alkyl substitution was done at a single molecular terminal. Having the aforementioned situations as a background, we have developed TPCO derivatives having an n-alkyl group at a sole molecular terminal (alkyl-monosubstitution) and applied these materials to FET devices. We expect that this modification not only produces good solubility, but alters local chemical environments around both the molecular terminals. This feature is represented in Scheme 1 where an aliphatic part and aromatic part (heterocyclic thienyl or phenyl) are located at either side of the molecule. The interplay of both the parts produces an interesting effect. In fact, we have observed a peculiar morphology characterized by molecular bilayers that results from difference in the local chemical environments. The molecular bilayer structure leads to improved device performance. This is expected to constitute one of possible advantages of the alkyl-monosubstituted compounds over alkyl-disubstituted ones. In this article we present initial results of the synthesis and structural characterization of this new series of alkyl-monosubstituted TPCO materials (Scheme 2). We chose model compounds that contain the aromatic backbone of 2-phenyl-5-[4-(2-thienyl)phenyl] thiophene (4a) or 1,4-bis(5-phenylthiophen-2-yl)benzene (6a) with emphasis upon 4a and its derivatives. Their alkyl-monosubstituted derivatives exhibit a relatively good solubility (up to 10 g L1 in THF). Solution-cast films
Scheme 1. Schematic representation of alkyl-monosubstituted thiophene/phenylene co-oligomers. An aliphatic part and aromatic part (heterocyclic thienyl or phenyl) are located at either side of the molecule.
Scheme 2. Structural formula of (a) 2-phenyl-5-[4-(2-thienyl)phenyl]thiophene (4a) and (b) 1,4-bis(5-phenylthiophen-2-yl)benzene (6a) and their derivatives (4b–4d, 6b–6d). The compound 6d is disclosed in Ref. [35].
of these materials with longer alkyl chains have the abovementioned molecular bilayers and indicate good FET characteristics.
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2. Experimental
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2.1. Synthesis of the alkyl-monosubstituted TPCOs
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General synthetic procedures are summarized in Scheme 3. Alkyl-monosubstituted TPCOs, 4b–4d, were prepared readily through only 3 steps by using 2-(4bromophenyl)thiophene (1) as the starting material. The syntheses of these TPCOs were based on Kumada cross coupling reaction which produced the target compounds in high yields (except for 4d) [27]. In Scheme 3 the compound 1 is the key material. Various target compounds (4b, 4c, and 4d) can be synthesized from 1. The compounds 6b and 6c were synthesized from the Ni(II)-catalyzed coupling of phenylmagnesium bromide with 5b and 5c, which were prepared from bromination of 4b and 4c by 1,3-dibromo-5,5-dimethylhydantoin (DBH), respectively. Unless stated otherwise, starting materials were obtained from Tokyo Chemical Industry Co., Ltd. or Wako Pure Chemical Industries, Ltd. and were used without further purification. The compound 1, 4a, and 6a were prepared according to literature procedures [28]. Cyclopentyl methyl ether (CPME, Zeon Corp.) was distilled on calcium hydride prior to use. All reactions were carried out under nitrogen. All the TPCOs were purified by the recrystallization from CPME and subsequent vacuum sublimation. Synthetic details of individual compounds are described in Appendix A. Supplementary material. 1 H NMR spectra were recorded on a Varian Mercury400BB spectrometer (400 MHz), and deuteriodichloromethane or deuteriotetrahydrofuran were used as a solvent. Chemical shifts were recorded in parts per million (ppm)
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Scheme 3. Synthetic route of alkyl-monosubstituted TPCOs. (i) Mg, Ni(dppp)Cl2, CPME, (ii) DBH, DMF.
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relative to tetramethylsilane as an internal standard. Elemental analyses (for carbon and hydrogen) were carried out on an Elementar Vario EL III. Sulfur was analyzed with a standard oxygen-flask combustion method.
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2.2. Instruments and thin film preparations
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The absorption and photoluminescence (PL) spectra were measured in a THF solution by using a Hitachi U4100 UV–vis spectrometer and a Hitachi F-2500 fluorescence spectrometer, respectively. Thermal gravimetric analysis (TGA) was carried out on a Rigaku TG8120 and differential scanning calorimetric analysis (DSC) was performed on a PerkinElmer DSC8500 instrument under N2 atmosphere at a rate of 10 °C min1. Regarding the thin films, we carried out the X-ray diffractometry (XRD) measurements on a Rigaku SmartLab X-ray diffractometer, using standard h–2h scanning techniques at 45 kV and 200 mA with Cu Ka. We observed morphologies of the thin films by optical microscope, using a Nikon Elipse LV100POL microscope equipped with a mercury lamp as a white light source. We also examined surface morphologies of the thin films in air using a SII NanoTechnology E-sweep scanning probe microscope operated in dynamic force mode. All the XRD, optical, and microscope measurements were carried out at room temperature. With the above XRD measurements and morphology observations as well as the FET device fabrication (see below) we prepared thin films of TPCOs on a Si/SiO2 (500 lm/300 nm) wafer substrate by the vacuum deposition, solution casting, and a skim method (vide infra). The substrates were washed with hot piranha solution (H2SO4/H2O2) followed by exposing the wafers to
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hexamethyldisilazane (HMDS, Aldrich) vapor under nitrogen to give trimethylsilyl-functionalized SiO2 surface of the wafers. The vacuum-deposition of the TPCO materials was carried out at the pressures under 105 Torr at the deposition rate of 2–5 Å s1, yielding an average thickness of ca. 100 nm. The solution-cast films were prepared in a screw vial at 23 °C by drop-casting chlorobenzene solutions of the materials with a concentration varying from 250 to 1000 ppm and successively evaporating the solvent slowly (Fig. 1(a)). Skim films were prepared by skimming off the films grown at the liquid–air interface of a supersaturated chlorobenzene solution. Fig. 1(b) schematically illustrates the fabrication method of the skim films. We prepared chlorobenzene solutions of 4b, 4c, 4d, and 6c with a concentration varying from 250 to 3000 ppm in a screw vial. The solution was heated up to about 70 °C under ultrasonic irradiation to make them supersaturated. The screw vial was allowed to stand overnight in a 23 °C thermostat. Tiny floating crystals began to form at the surface of the solution. These floating crystals acted as nuclei for the further crystallization and underwent subsequent growth to form a floating largesized thin film. Finally the thin film was skimmed off using the substrate from the surface of the solution and the resulting substrate was air-dried at room temperature. Skim films were not available for 4a, 6a, or 6b.
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2.3. FET device fabrications
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FET devices were fabricated with bottom-gate bottomcontact and bottom-gate top-contact configurations. Henceforth we abbreviate these devices as simply the bottom-contact and top-contact devices. For both the types
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Fig. 1. Schematic illustration of fabrication method of (a) a solution-cast film and (b) a skim film. Both methods were used to fabricate films for XRD and FET measurements.
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of devices we used the aforementioned HMDS-treated Si/ SiO2 (500 lm/300 nm) wafer substrates (10 10 mm). We fabricated both the types of devices using vacuumdeposition films, whereas the solution-cast film and skim films were used only for a bottom-contact device. Vacuum deposition of the TPCOs was performed under several substrate temperatures (i.e. room temperature, 130 °C, and 150 °C). With the top-contact devices, source and drain electrodes (100 nm) were deposited on the organic layer by evaporating gold at a pressure under 105 Torr. The channel length and width were 50 lm and 2.5 mm, respectively. For the purpose of making bottom-contact devices, Cr/Au (a few nm/50 nm) interdigitated electrodes (NTT Advanced Technology Corp.) were deposited on the Si/ SiO2 substrate. The channel length and width were 10 lm and 80 mm, respectively. Electrical measurements for FET devices were performed using a Keithley 4200 SCS semiconductor parameter analyzer at room temperature under vacuum. In the saturation regime, VD > (VGVth), we obtained
field-effect carrier mobilities (l) on the basis of the following Eq. (1) [29]:
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ID ¼ ðW=2LÞC i lðV G V th Þ2 ;
ð1Þ
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where ID is the drain-source current in the saturated region; W and L are the channel width and length, respectively; Ci is the capacitance per unit area of the insulating layer; l is the field-effect carrier mobility; VG and Vth are the gate and threshold voltages, respectively.
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2.4. X-ray crystallographic analysis
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For the purpose of determining the crystal structure, we made single crystals of 4a. To this end, 4a (2 mg) was added to chlorobenzene (1.0 g) in a screw vial and the lid of the vial was closed. The solution was heated up to 70 °C under ultrasonic irradiation to make it completely clear. The screw vial was allowed to stand about one month at 23 °C in a thermostated room. During this period
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chlorobenzene was allowed to be evaporated very slowly, leaving yellow crystals at the bottom of vial. The X-ray measurements were carried out at 150 °C with a single crystal of a size 0.180 0.150 0.050 mm. Intensity data were collected using the x-scan technique on a Rigaku R-AXIS RAPID diffractometer with filtered Mo Ka radiation and were corrected for usual Lorentzpolarization effects. Empirical absorption corrections were applied, resulting in transmission factors ranging from 0.806 to 0.983. The structure was solved by the direct method (SIR2008) [30] and refined by the full-matrix least-squares method on F2 (SHELXL-97) [31]. All other calculations were performed using a crystallographic software package (CrystalStructure 4.0) [32]. Some nonhydrogen atoms were refined anisotropically, while the rest were refined isotropically. Hydrogen atoms were refined using the riding model.
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3. Results and discussion
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3.1. Physicochemical properties of the materials
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All the compounds are environmentally stable. In fact, they have not shown any apparent decomposition and discoloration for at least a year in air. Table 1 summarizes physicochemical properties of 4a–4d and 6a–6c. The solubilities of 4b (7.1 g L1) and 4c (8.9 g L1) in THF at room temperature have been improved compared to that of 4.4 g L1 for 4a because of the introduction of n-alkyl substituents. Similarly, the solubilities of 6b (0.41 g L1) and 6c (0.21 g L1) in THF at room temperature were also improved compared to that of 0.04 g L1 for 6a. Contrary to our expectation, however, the solubility of hexadecylmonosubstituted TPCO, 4d (0.18 g L1), was lower than that of the unsubstituted 4a. These results seem to be in agreement with those obtained by Ebata et al. [25] with BTBT derivatives having various alkyl groups of different lengths (pentyl to tetradecyl) at both the molecular terminals. They reported that those compounds indicated increasing solubility with the increased carbon number of the alkyl groups up to nine. With more than nine, however, the solubility rapidly dropped with increasing carbon number. In Table 1 TTGA was increased with increasing chain length of n-alkyl substituent and aromatic core size. DSC
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curves of 4b–4d, 6b, and 6c exhibited plural thermal transitions (Fig. 2(a)), even though those of 4a and 6a showed a single transition. This suggests the presence of mesophases for 4b–4d, 6b, and 6c. The lowest major peaks of DSC reflect the melting points of the compounds. All the compounds showed reversible thermal transitions in the DSC curves. As an example Fig. 2(b) shows the DSC trace of 4d. The reversibility of DSC scans clearly indicates excellent thermal stability of these TPCOs. When comparing 4a–4d, we notice that although melting points of 4b and 4c were only slightly lower (10 °C) than that of 4a, 4d showed a melting point lower than 4a by 100 °C. In relation to these observations, Garnier et al. [21] mentioned that a,x-hexyl-disubstituted sexithiophene showed a melting point of 290 °C, which was higher than that of unsubstituted sexithiophene by 10 °C. They assumed that the a,x-dihexyl substitution enhanced the cohesive force between the conjugated backbones. Meanwhile, Ponomarenko and Kirchmeyer [33] observed that a,x-decyl-disubstituted sexithiophene showed a melting point at 108 °C (i.e. lower than unsubstituted sexithiophene by more than 100 °C). Such relationship between the substituted alkyl chain length and the melting point was noticed with various disubstituted molecules having BTBT, anthradithiophene, and TPCO backbones as well [26,34–36]. Comparing the results of the present studies and previously obtained data for the disubstituted molecules, compounds with appropriately long alkyl groups chosen for monosubstitution are expected to allow us to achieve a good balance between the high solubility and high melting point of the materials. Here the latter requirement is relevant to their application to electronic devices (such as FETs) with good thermal durability [26]. More specifically, if we select a 4a-based compound with high processability and durability in mind, the alkyl groups can be chosen from among e.g. hexyl and octyl (as in 4b and 4c). Thus the alkyl-monosubstitution strategy may well be applied to other molecules and will be useful for future molecular design more widely. The unsubstituted 4a exhibited the absorption peak at 355 nm and the emission peaks at 402 and 425 nm. The absorption and emission peaks of alkyl-monosubstituted 4b, 4c, and 4d, were red-shifted (3–5 nm) compared to those of 4a. We assume that these shifts were associated with the electron-donating alkyl groups in 4b, 4c, and 4d. Similarly, the addition of the phenyl ring to the aromatic
Table 1 Solubility in THF at r.t., absorption maxima (kabs) and emission maxima (kem) in THF solution, and thermal properties of TPCOs, 4a4d and 6a6c.
a b c
Compound
Solubility (g L1)
kabs (nm)
kema(nm)
TTGAb(°C)
TDSCc(°C)
4a 4b 4c 4d 6a 6b 6c
4.4 7.1 8.9 0.18 0.04 0.41 0.21
355 358 358 359 377 379 379
402⁄, 425 406⁄, 429 406⁄, 429 407, 428⁄ 426⁄, 451 429⁄, 454 429⁄, 454
269 289 319 366 366 383 391
233 221, 220, 108, 309 263, 262,
223, 231 224 124, 220 299, 318 299, 306
Absolute maximum indicated by asterisk. TTGA has been defined as a temperature at which 5% weight loss has taken place. TDSC has been determined from a DSC trace and defined as a transition temperature.
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Fig. 2. (a) DSC curves (second heating run) of TPCOs and (b) DSC trace (second cycle) of 4d showing reversible thermal transitions (under nitrogen at a ramp rate of 10 °C min1). 348 349 350
core, which produced 6b and 6c, respectively, resulted in bathochromic shifts of 21–25 nm relative to the absorption and emission peaks observed for 4b and 4c.
3.2. Structural characterizations of thin films
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Fig. 3 represents the optical micrographs of the solution-cast films. Large-sized films were formed for 4b and 4c, in contrast to small crystals for 4a and 4d. In particular, large domains over 1 mm are clearly visible for the film 4c. These observations mean that the length of the n-alkyl chains in TPCOs influences the morphology of solution-cast films. Higher solubility of 4b and 4c is responsible for the formation of large-sized cast films (Fig. 3, Table 1). In Fig. 4 we compare the XRD diagrams of solution-cast films and vacuum-deposition films of 4a–4c. The XRD results of both the vacuum-deposition and solution-cast films are characterized by the first-order reflection together with higher-order reflections. Table 2 collects the XRD results of the thin films formed by the vacuum deposition, solution cast, and skim methods. The dspacings estimated from the first-order reflection of the vacuum-deposition films correspond to the molecular length [35,37]. The corresponding d-spacings for the solution-cast films are about two times those of the vacuum-deposition films and much larger than the molecular lengths. This suggests the presence of a bilayer structure that results from association of two molecules in the solution-cast films [38]. These observations are contrasted with the case of a symmetric compound 6d [35]. The compound 6d shows virtually the same d-spacing for the thin films both solution cast and vacuum deposited [35], thus lacking the bilayer structure. Moreover, the solution-cast films indicated much stronger diffraction peaks and narrower full widths at half maximum (FWHM) than
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Fig. 3. Optical micrographs of solution-cast films of 4a–4d on a Si/SiO2 substrate.
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Fig. 5. XRD patterns of skim and solution-cast films of 4d on a Si/SiO2 substrate. A magnified profile (50) is depicted for the sake of clear visualization.
Fig. 4. XRD patterns of thin films deposited on a Si/SiO2 substrate for (a) 4a, (b) 4b, and (c) 4c. Each diagram includes the results of a vacuumdeposition film (obtained on a substrate held at room temperature) and a solution-cast film. In (a), an XRD diagram of a single crystal of 4a is depicted in an inset. For (b) and (c), magnified profiles (50 or 10) are depicted for the sake of clear visualization.
Table 2 Interplanar distances (d-spacing) for vacuum-deposition (at room temperature of substrate), solution-cast, and skim films of TPCOs. Compound
d-Spacinga (Å) Vacuum-deposition film
4a 4b 4c 4d 6a 6b 6c a b c
17.4 23.8 25.8 34.8 21.6 29.3 31.7
Solution-cast film 34.5 46.5 51.1 70.1 21.6b 29.5b 60.5b
Calculated from the first-order reflections. Many small crystallites were formed. Skim films were not formed.
Skim film c
– 47.0 51.1 66.9 –c –c 60.5
the corresponding vacuum-deposition films. This reflects better molecular ordering and higher crystallinity in the solution-cast films. This unique structure is caused by the difference in local chemical environments caused by aromatic and aliphatic groups and is specific to the alkylmonosubstituted materials. In Fig. 5 we compare the XRD patterns of the skim film and solution-cast film of 4d. Diffraction peaks of the skim film were much stronger than those of the solution-cast film and the relevant peaks were attributed to the 20th order reflections or higher. These observations indicate the highly ordered structure in the skim film. To further characterize the aforementioned unique morphology of the solution-cast and skim films, we show in Fig. 6 the crystallographic structure of 4a [39]. The asymmetric unit contains two crystallographically independent molecules, one of which includes a disordered thiophene ring, with relative occupancies of 72.3 (5)% (S-anti) and 27.7 (5)% (S-syn) for the two components [40]. The two independent molecules can be taken as either M1 and M2 or M1 and M2’. In the former case those molecules can be viewed as if they were a ‘‘joint single molecule,’’ where adjacent thiophene rings come into contact. Half a (0 0 1) -plane spacing is 34.50 Å and this spacing reflects a length of the joint molecule. Consequently, the presence of the two crystallographically independent molecules is the origin of the above-mentioned bilayer structure in the solution-cast and skim films. Moreover, the formation of the bilayer structure is a result of the different chemical environment at both the molecular terminals, as in the case of 4a crystals (where thienyl and phenyl groups are located at the molecular terminals). The difference in the d-spacings of thin films reflects the difference in alkyl chain lengths of TPCOs. We calculated the alkyl chain lengths on the basis of Ref. [41] and estimated those of hexyl, octyl, and hexadecyl groups to be 9.7, 12.2, and 22.2 Å, respectively. We then estimated molecular length of 4b, 4c, and 4d at 25.2, 27.8, and 37.8 Å, respectively. These numbers imply that tilting angles of the molecules against the substrate normal range from 19° to 23°. In the above estimation we took the aromatic backbone geometry from that determined crystallographically for a 4a molecule without disorder and
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Fig. 6. Crystal structure of 4a. M1 and M2 (or M20 ) represent two crystallographically independent molecules; for structural details see text and Ref. [39].
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ignored small difference in the molecular geometry between the two crystallographically independent molecules. When we look closely at the XRD data, we notice the following: (i) Regarding the solution-cast films, 6a and 6b lack the bilayer structure. The crystals of 6a possess
one crystallographically independent molecule as in the cases of other symmetric TPCOs [8,42–44], in agreement with absence of the bilayer structure. This provides a sharp contrast to the case of the 4a crystals outlined above. (ii) The skim films of 4b, 4c, and 4d exhibited FWHMs of 0.16°, 0.12°, and 0.10° for their primary diffraction peaks due to the bilayer structure (see Table 2). This implies that the skim films have increasingly ordered structure with increasing alkyl chain lengths. The FWHM of the skim film of 6c, on the other hand, was 0.24°. We infer from these findings that alkyl-monosubstituted compounds having the asymmetric aromatic backbone and longer alkyl chain tend to show the well-ordered bilayer structure. At the same time, this reasoning probably explains why the solution-cast film of 6b did not have the bilayer structure. We also add that there is room for further development of materials combining high structural order and durability (vide supra). If the crystal structures of alkyl-monosubstituted compounds are available, this will enable us to obtain detailed information on intermolecular interactions between alkyl groups (and aromatic moieties as well) along with those between molecules and the substrate more widely.
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3.3. Device characterizations of thin films
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The output curves of all the devices exhibited the clear p-channel FET properties. As an example, Fig. 7(a) and (b) shows the electrical output and transfer characteristics,
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Fig. 7. (a) Electrical output and (b) transfer characteristics for a bottom-gate bottom-contact device having a vacuum-deposition film of 4c obtained on a substrate held at room temperature. (c) Electrical output and (d) transfer characteristics for a bottom-gate top-contact device having a vacuum-deposition film of 6c obtained at a substrate temperature of 150 °C.
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R. Hirase et al. / Organic Electronics xxx (2014) xxx–xxx Table 3 Carrier mobilities (l) and threshold voltages (Vth) of FET devices with vacuum-deposition films of TPCOs at various substrate temperatures. Compound
4a 4b 4c 4d
6a 6b 6c
457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480
Substrate temperature (°C)
r.t. r.t. r.t. 130 r.t. 130 150 r.t. r.t. 150 r.t. 150
Bottom-gate bottom-contact
Bottom-gate top-contact
l (cm2 V1 s1)
Vth (V)
l (cm2 V1 s1)
Vth (V)
(2.1 ± 1.2) 105 (2.6 ± 0.1) 104 (1.3 ± 0.2) 103 (1.5 ± 0.2) 103 (2.0 ± 0.1) 104 (1.3 ± 0.2) 103 (5.4 ± 0.5) 104 (8.0 ± 0.8) 104 (5.9 ± 0.2) 103 (8.5 ± 0.5) 103 (2.0 ± 0.3) 103 (9.8 ± 0.4) 103
29.7 ± 7.6 20.1 ± 4.4 22.5 ± 0.0 18.7 ± 5.4 19.1 ± 0.8 20.2 ± 1.2 28.0 ± 3.2 21.1 ± 2.1 15.1 ± 0.6 11.9 ± 0.2 14.7 ± 4.5 18.5 ± 1.7
(2.1 ± 0.0) 103 (6.1 ± 3.2) 103 (3.0 ± 2.5) 103 (1.0 ± 0.4) 102 (1.3 ± 1.0) 103 (0.9 ± 0.4) 103 (6.3 ± 1.7) 103 (2.0 ± 0.2) 102 (2.4 ± 0.3) 102 (3.4 ± 0.3) 102 (2.5 ± 0.1) 102 (3.7 ± 0.2) 102
45.5 ± 2.6 31.9 ± 3.3 47.3 ± 8.7 27.1 ± 3.2 35.9 ± 11.7 32.4 ± 14.6 29.8 ± 6.4 16.8 ± 4.3 13.4 ± 2.0 13.0 ± 0.0 19.9 ± 3.8 20.2 ± 1.9
respectively, of the FET measured in the bottom-contact configuration for the 4c film vacuum deposited on a substrate held at room temperature. Fig. 7(c) and (d) represents another device feature in the top-contact configuration with the 6c film vacuum deposited at an elevated substrate temperature of 150 °C. The latter device yielded the highest carrier mobility 3.7 102 cm2 V1 s1. This number is comparable to that for a device of a hexyl-disubstituted TPCO (6d) of 5.4 102 cm2 V1 s1 [35]. Table 3 summarizes the carrier mobilities and the threshold voltages of the FETs for the vacuum-deposition films. We have carried out electrical measurements using 2–6 devices for each entry except for skim film 4c, with which a single device was examined. We indicate an average number and deviations (±). From Table 3 we recognize the following trends: (i) The carrier mobilities of the FETs of 4b and 4c (6b and 6c) were higher than that of the FET of an unsubstituted parent molecule 4a (6a). This can be explained as a result of the improved molecular ordering and p–p stacking caused by the alkyl-monosubstitution [21]. (ii) The 6a–6c devices showed higher mobilities than corresponding 4a–4c for both bottom-contact and top-contact devices. This trend results from the extension in the conjugated system in 6a–6c [45]. (iii) The devices
having the top-contact configuration exhibited higher mobilities than those with the bottom-contact configuration except for the device 4d fabricated at a substrate temperature 130 °C. Related results were described and discussed in the literature in terms of the contact resistance [15]. (iv) The carrier mobilities of the FETs with 4c, 6b, and 6c deposited at higher substrate temperatures were maximally 5 times greater than those deposited on a substrate held at room temperature (see Table 3). This is due to the increase in grain sizes of alkyl-monosubstituted TPCO films deposited at higher substrate temperatures [46,47]. The effect of the substrate temperatures on the grain sizes is evidenced by Fig. 8 that compares surface morphologies of the films 4c deposited at a room temperature and at 130 °C of substrate. Regarding the 4d device, however, the relationship between the mobility and substrate temperatures somewhat deviated from this trend with both the bottom-contact devices and top-contact devices. Such deviation resulted possibly from the fact that the vacuum deposition of 4d was carried out near its melting temperature (see Table 1). As an example of the solution-cast devices, Fig. 9(a) and (b) shows the electrical output and transfer characteristics
Fig. 8. Surface morphologies of a vacuum-deposition films 4c prepared at different substrate temperatures: (a) room temperature, (b) 130 °C. The horizontally long color bars under each micrograph indicate height scale.
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Fig. 9. (a) Electrical output and (b) transfer characteristics for a bottom-contact device having a solution-cast film of 4c.
Table 4 Carrier mobilities (l) and threshold voltages (Vth) of bottom-gate bottom-contact FET devices with vacuum deposition (at room temperature of substrate), solution cast, and skim films of TPCOs. Compound
4a 4b 4c 4d a
Vacuum-deposition film
Solution-cast film
Skim film
l (cm2 V1 s1)
Vth (V)
l (cm2 V1 s1)
Vth (V)
l (cm2 V1 s1)
Vth (V)
(2.1 ± 1.2) 105 (2.6 ± 0.1) 104 (1.3 ± 0.2) 103 (2.0 ± 0.1) 104
29.7 ± 7.6 20.1 ± 4.4 22.5 ± 0.0 19.1 ± 0.8
(9.4 ± 6.6) 104 (1.1 ± 0.3) 102 (1.1 ± 0.0) 102 (1.4 ± 0.4) 103
12.2 ± 7.4 16.6 ± 0.8 19.9 ± 0.4 25.3 ± 0.5
–a –a 1.6 105 (3.2 ± 0.1) 103
2.1 4.6 ± 1.2
Films suitable for the fabrication of devices by the skim method were not obtained.
Fig. 10. Micrographs of (a) a free-standing skim film of 4d obtained by peeling a film off a substrate and (b) a bottom-contact FET device with a skim film 4d. In (a) tweezers are pinching a free-standing film of 6 mm in size. (c) Electrical output and (d) transfer characteristics for a bottom-contact device with a skim film 4d.
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for the solution-cast film of 4c, respectively. Table 4 compares the characteristics of solution-cast devices and vacuum-deposition devices of 4a–4d on the bottom-contact FETs configurations. (i) Their solution-cast devices exhibited one to two orders of magnitude higher carrier mobilities than the vacuum-deposition devices. This is due very likely to the regular molecular alignment attained in the bilayer structure. This forms a striking contrast to the related hexyl-disubstituted symmetric TPCO molecules [35]. Notice here that in contrast to our results, solutioncast films of those hexyl-disubstituted compounds showed lower mobilities than the vacuum-deposition films of the same compound [35]. (ii) The solution-cast 4b and 4c devices produced larger carrier mobility than the solution-cast 4a device as in the case of the vacuum-deposition devices. The value of 102 cm2 V1 s1 was ten times as large as that of 4a. Of these, the solution-cast 4c devices have shown the best reproducibility with the mobility measurements. This is associated with the large domain size of the solution-cast films 4c (Fig. 3). We did not estimate mobilities of solution-cast films of 6a–6c. This is because a solution-cast film of high quality was hard to prepare because of their poor solubility. The skim films of 4d were obtained as a free-standing film by peeling it off a substrate. They were typically several millimeters in size (Fig. 10(a)). Fig. 10(b) shows an optical micrograph of the bottom-contact FET of the skim film 4d. Aside from several wrinkles the uniform thin film covered the entire surface of the substrate. Fig. 10(c) and (d) shows the electrical output and transfer characteristics for the skim film 4d, respectively. This device had a larger carrier mobility of 3.2 103 cm2 V1 s1 than the solution-cast 4d device. Even though the skim film device of 4c exhibited somewhat poor performance, 4c showed the best results in terms of the solution processability and steady device performance among 4a and its alkylmonosubstituted compounds.
due to the regular molecular alignment in the bilayer structure.
563
Acknowledgements
565
We thank Dr. Takeshi Yamao for his helpful discussion and valuable support in electrical measurements. We are grateful to Dr. Shohei Katao for carrying out the X-ray crystallographic analysis. Thanks are also due to Sumika Chemical Analysis Service, Ltd. for elemental analyses (for carbon, hydrogen, and sulfur).
566
Appendix A. Supplementary material
572
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2014.04.010.
573
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542
4. Conclusions
543
We have designed and synthesized a new series of alkyl-monosubstituted TPCOs of 4b–4d, 6b, and 6c as the solution processable semiconducting materials. The introduction of alkyl chains to a sole p-position of the terminal phenyl group improved the solubility in organic solvents, while maintaining good thermal stability and durability. The thin films of these alkyl-monosubstituted TPCOs were obtained by the vacuum-deposition, solution-cast, and skim methods. Of these, the solution-cast films and skim films consist of highly-ordered structures characterized by the molecular bilayer. As a result the skim method produced a large-sized free-standing thin film. The FET devices made of the thin films exhibited typical p-channel characteristics with clear saturation region. A device of the vacuum-deposition film of 6c recorded the highest carrier mobility of 3.7 102 cm2 V1 s1. The solution-cast devices exhibited one to two orders of magnitude higher carrier mobilities than the vacuumdeposition devices, especially the solution-cast 4b and 4c devices producing the mobility 102 cm2 V1 s1. This is
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Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel 6 7
Alkyl-monosubstituted thiophene/phenylene co-oligomers: Synthesis, thin film preparation, and transistor device characteristics
3 4 5 8
Q1
9
Ryuji Hirase a, Mari Ishihara a, Toshifumi Katagiri b, Yosuke Tanaka c, Hisao Yanagi c, Shu Hotta d,⇑ a
10 11 12 13
Hyogo Prefectural Institute of Technology, 3-1-12 Yukihira-cho, Suma, Kobe, Hyogo 654-0037, Japan Sumitomo Seika Chemicals Co., Ltd., 346-1 Miyanishi, Harima-cho, Kako-gun, Hyogo 675-0145, Japan Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan d Kyoto Institute of Technology, Department of Macromolecular Science and Engineering, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan b c
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a r t i c l e
1 2 8 3 19 20 21 22 23
i n f o
Article history: Received 9 August 2013 Received in revised form 7 April 2014 Accepted 9 April 2014 Available online xxxx
24 25 26 27 28 29 30 31
Keywords: Organic semiconductor Thiophene/phenylene co-oligomer Organic field-effect transistor Molecular bilayer Skim method Free-standing thin film
a b s t r a c t A novel series of alkyl-monosubstituted thiophene/phenylene co-oligomers (TPCOs) has been synthesized and characterized. The introduction of alkyl chains to TPCOs improved the solubility in organic solvents. Thin films of these compounds were prepared by the vacuum-deposition, solution-cast, and skim methods. Of these, the solution-cast films and skim films consist of highly-ordered structures characterized by molecular bilayers. In particular, the skim method produced a large-sized free-standing thin film. We investigated the carrier transport of the thin films on the field-effect transistor (FET) configurations. These FET devices exhibited typical p-channel characteristics with clear saturation region. The introduction of alkyl monosubstituents is responsible for enhancement in the carrier mobility. The device characteristics are contrasted with those of alkyl-disubstituted compounds. Ó 2014 Published by Elsevier B.V.
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1. Introduction
49
Organic semiconductors have attracted much attention because of their easier fabrication processes compared with silicon technologies. Among their applications, fieldeffect transistors (FETs) are of particular interest because of the potential for low-cost, large area, flexible, and light-weight electronic devices such as active-matrix flat displays, electronic papers, radio frequency identification tags, and sensors [1–3]. One of the most challenging tasks on organic FETs is the design of excellent organic semiconductors that combine high carrier mobility, good environmental stability, and processability.
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Q2
⇑ Corresponding author. Tel.: +81 75 724 7793; fax: +81 75 724 7780. E-mail address: [email protected] (S. Hotta).
Unsubstituted thiophene/phenylene co-oligomers (TPCOs) with various thiophene/phenylene ratios are promising class of organic semiconductors since they offer high carrier mobility and good environmental stability. These materials have been employed in light-emitting diodes [4–6] as well as in p-channel FETs [7–10]. However, the solubility of unsubstituted TPCOs is not good enough for solution processing [11]. Solution-processed organic electronic devices have been attracting much attention because of their low-temperature and easy fabrication process without any vacuum steps, enabling high-throughput fabrication over large areas on flexible substrates [12]. It was reported that the introduction of alkyl chains to oligothiophene backbones improved the solubility of the resulting materials in organic solvents and that the alkyl substitution positions influenced the solubility [13,14].
http://dx.doi.org/10.1016/j.orgel.2014.04.010 1566-1199/Ó 2014 Published by Elsevier B.V.
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Furthermore, researchers studied substituent effects on electronic properties such as carrier mobility of organic semiconductors [15–22]. The intrinsic carrier mobility depends largely on the molecular ordering and p–p stacking extent of organic semiconductors [23]. In this context it is well-known that the introduction of appropriate alkyl chains (such as methyl and hexyl groups) to the terminal positions enhances molecular organization and ordering and improves p–p stacking of aromatic backbones [21,24]. Therefore, their introduction is thought to enhance the carrier mobility. This is indeed the case with a-sexithiophene having relatively shorter alkyl chains (e.g. ethyl and hexyl) [18,21,24]. On the contrary, Halik et al. reported that the FET performance was reduced by introducing longer alkyl chains (i.e. decyl groups) at the molecular terminals [15]. Using a different conjugated backbone [1]benzothieno[3,2-b][1]benzothiophene (BTBT), Ebata et al. reported the FET performance and solubility of its derivatives with various terminal alkyl chains [25]. All the materials mentioned above are characterized by the presence of alkyl chains symmetrically disubstituted at both the molecular terminals. Also Iino et al. [26] studied thermal characteristics and FET performance of a thin film made of a 2-octylthienyl derivative of BTBT. Note in that case that the alkyl substitution was done at a single molecular terminal. Having the aforementioned situations as a background, we have developed TPCO derivatives having an n-alkyl group at a sole molecular terminal (alkyl-monosubstitution) and applied these materials to FET devices. We expect that this modification not only produces good solubility, but alters local chemical environments around both the molecular terminals. This feature is represented in Scheme 1 where an aliphatic part and aromatic part (heterocyclic thienyl or phenyl) are located at either side of the molecule. The interplay of both the parts produces an interesting effect. In fact, we have observed a peculiar morphology characterized by molecular bilayers that results from difference in the local chemical environments. The molecular bilayer structure leads to improved device performance. This is expected to constitute one of possible advantages of the alkyl-monosubstituted compounds over alkyl-disubstituted ones. In this article we present initial results of the synthesis and structural characterization of this new series of alkyl-monosubstituted TPCO materials (Scheme 2). We chose model compounds that contain the aromatic backbone of 2-phenyl-5-[4-(2-thienyl)phenyl] thiophene (4a) or 1,4-bis(5-phenylthiophen-2-yl)benzene (6a) with emphasis upon 4a and its derivatives. Their alkyl-monosubstituted derivatives exhibit a relatively good solubility (up to 10 g L1 in THF). Solution-cast films
Scheme 1. Schematic representation of alkyl-monosubstituted thiophene/phenylene co-oligomers. An aliphatic part and aromatic part (heterocyclic thienyl or phenyl) are located at either side of the molecule.
Scheme 2. Structural formula of (a) 2-phenyl-5-[4-(2-thienyl)phenyl]thiophene (4a) and (b) 1,4-bis(5-phenylthiophen-2-yl)benzene (6a) and their derivatives (4b–4d, 6b–6d). The compound 6d is disclosed in Ref. [35].
of these materials with longer alkyl chains have the abovementioned molecular bilayers and indicate good FET characteristics.
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2. Experimental
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2.1. Synthesis of the alkyl-monosubstituted TPCOs
131
General synthetic procedures are summarized in Scheme 3. Alkyl-monosubstituted TPCOs, 4b–4d, were prepared readily through only 3 steps by using 2-(4bromophenyl)thiophene (1) as the starting material. The syntheses of these TPCOs were based on Kumada cross coupling reaction which produced the target compounds in high yields (except for 4d) [27]. In Scheme 3 the compound 1 is the key material. Various target compounds (4b, 4c, and 4d) can be synthesized from 1. The compounds 6b and 6c were synthesized from the Ni(II)-catalyzed coupling of phenylmagnesium bromide with 5b and 5c, which were prepared from bromination of 4b and 4c by 1,3-dibromo-5,5-dimethylhydantoin (DBH), respectively. Unless stated otherwise, starting materials were obtained from Tokyo Chemical Industry Co., Ltd. or Wako Pure Chemical Industries, Ltd. and were used without further purification. The compound 1, 4a, and 6a were prepared according to literature procedures [28]. Cyclopentyl methyl ether (CPME, Zeon Corp.) was distilled on calcium hydride prior to use. All reactions were carried out under nitrogen. All the TPCOs were purified by the recrystallization from CPME and subsequent vacuum sublimation. Synthetic details of individual compounds are described in Appendix A. Supplementary material. 1 H NMR spectra were recorded on a Varian Mercury400BB spectrometer (400 MHz), and deuteriodichloromethane or deuteriotetrahydrofuran were used as a solvent. Chemical shifts were recorded in parts per million (ppm)
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Scheme 3. Synthetic route of alkyl-monosubstituted TPCOs. (i) Mg, Ni(dppp)Cl2, CPME, (ii) DBH, DMF.
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relative to tetramethylsilane as an internal standard. Elemental analyses (for carbon and hydrogen) were carried out on an Elementar Vario EL III. Sulfur was analyzed with a standard oxygen-flask combustion method.
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2.2. Instruments and thin film preparations
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The absorption and photoluminescence (PL) spectra were measured in a THF solution by using a Hitachi U4100 UV–vis spectrometer and a Hitachi F-2500 fluorescence spectrometer, respectively. Thermal gravimetric analysis (TGA) was carried out on a Rigaku TG8120 and differential scanning calorimetric analysis (DSC) was performed on a PerkinElmer DSC8500 instrument under N2 atmosphere at a rate of 10 °C min1. Regarding the thin films, we carried out the X-ray diffractometry (XRD) measurements on a Rigaku SmartLab X-ray diffractometer, using standard h–2h scanning techniques at 45 kV and 200 mA with Cu Ka. We observed morphologies of the thin films by optical microscope, using a Nikon Elipse LV100POL microscope equipped with a mercury lamp as a white light source. We also examined surface morphologies of the thin films in air using a SII NanoTechnology E-sweep scanning probe microscope operated in dynamic force mode. All the XRD, optical, and microscope measurements were carried out at room temperature. With the above XRD measurements and morphology observations as well as the FET device fabrication (see below) we prepared thin films of TPCOs on a Si/SiO2 (500 lm/300 nm) wafer substrate by the vacuum deposition, solution casting, and a skim method (vide infra). The substrates were washed with hot piranha solution (H2SO4/H2O2) followed by exposing the wafers to
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hexamethyldisilazane (HMDS, Aldrich) vapor under nitrogen to give trimethylsilyl-functionalized SiO2 surface of the wafers. The vacuum-deposition of the TPCO materials was carried out at the pressures under 105 Torr at the deposition rate of 2–5 Å s1, yielding an average thickness of ca. 100 nm. The solution-cast films were prepared in a screw vial at 23 °C by drop-casting chlorobenzene solutions of the materials with a concentration varying from 250 to 1000 ppm and successively evaporating the solvent slowly (Fig. 1(a)). Skim films were prepared by skimming off the films grown at the liquid–air interface of a supersaturated chlorobenzene solution. Fig. 1(b) schematically illustrates the fabrication method of the skim films. We prepared chlorobenzene solutions of 4b, 4c, 4d, and 6c with a concentration varying from 250 to 3000 ppm in a screw vial. The solution was heated up to about 70 °C under ultrasonic irradiation to make them supersaturated. The screw vial was allowed to stand overnight in a 23 °C thermostat. Tiny floating crystals began to form at the surface of the solution. These floating crystals acted as nuclei for the further crystallization and underwent subsequent growth to form a floating largesized thin film. Finally the thin film was skimmed off using the substrate from the surface of the solution and the resulting substrate was air-dried at room temperature. Skim films were not available for 4a, 6a, or 6b.
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2.3. FET device fabrications
217
FET devices were fabricated with bottom-gate bottomcontact and bottom-gate top-contact configurations. Henceforth we abbreviate these devices as simply the bottom-contact and top-contact devices. For both the types
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Fig. 1. Schematic illustration of fabrication method of (a) a solution-cast film and (b) a skim film. Both methods were used to fabricate films for XRD and FET measurements.
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of devices we used the aforementioned HMDS-treated Si/ SiO2 (500 lm/300 nm) wafer substrates (10 10 mm). We fabricated both the types of devices using vacuumdeposition films, whereas the solution-cast film and skim films were used only for a bottom-contact device. Vacuum deposition of the TPCOs was performed under several substrate temperatures (i.e. room temperature, 130 °C, and 150 °C). With the top-contact devices, source and drain electrodes (100 nm) were deposited on the organic layer by evaporating gold at a pressure under 105 Torr. The channel length and width were 50 lm and 2.5 mm, respectively. For the purpose of making bottom-contact devices, Cr/Au (a few nm/50 nm) interdigitated electrodes (NTT Advanced Technology Corp.) were deposited on the Si/ SiO2 substrate. The channel length and width were 10 lm and 80 mm, respectively. Electrical measurements for FET devices were performed using a Keithley 4200 SCS semiconductor parameter analyzer at room temperature under vacuum. In the saturation regime, VD > (VGVth), we obtained
field-effect carrier mobilities (l) on the basis of the following Eq. (1) [29]:
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244
ID ¼ ðW=2LÞC i lðV G V th Þ2 ;
ð1Þ
246
where ID is the drain-source current in the saturated region; W and L are the channel width and length, respectively; Ci is the capacitance per unit area of the insulating layer; l is the field-effect carrier mobility; VG and Vth are the gate and threshold voltages, respectively.
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2.4. X-ray crystallographic analysis
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For the purpose of determining the crystal structure, we made single crystals of 4a. To this end, 4a (2 mg) was added to chlorobenzene (1.0 g) in a screw vial and the lid of the vial was closed. The solution was heated up to 70 °C under ultrasonic irradiation to make it completely clear. The screw vial was allowed to stand about one month at 23 °C in a thermostated room. During this period
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chlorobenzene was allowed to be evaporated very slowly, leaving yellow crystals at the bottom of vial. The X-ray measurements were carried out at 150 °C with a single crystal of a size 0.180 0.150 0.050 mm. Intensity data were collected using the x-scan technique on a Rigaku R-AXIS RAPID diffractometer with filtered Mo Ka radiation and were corrected for usual Lorentzpolarization effects. Empirical absorption corrections were applied, resulting in transmission factors ranging from 0.806 to 0.983. The structure was solved by the direct method (SIR2008) [30] and refined by the full-matrix least-squares method on F2 (SHELXL-97) [31]. All other calculations were performed using a crystallographic software package (CrystalStructure 4.0) [32]. Some nonhydrogen atoms were refined anisotropically, while the rest were refined isotropically. Hydrogen atoms were refined using the riding model.
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3. Results and discussion
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3.1. Physicochemical properties of the materials
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All the compounds are environmentally stable. In fact, they have not shown any apparent decomposition and discoloration for at least a year in air. Table 1 summarizes physicochemical properties of 4a–4d and 6a–6c. The solubilities of 4b (7.1 g L1) and 4c (8.9 g L1) in THF at room temperature have been improved compared to that of 4.4 g L1 for 4a because of the introduction of n-alkyl substituents. Similarly, the solubilities of 6b (0.41 g L1) and 6c (0.21 g L1) in THF at room temperature were also improved compared to that of 0.04 g L1 for 6a. Contrary to our expectation, however, the solubility of hexadecylmonosubstituted TPCO, 4d (0.18 g L1), was lower than that of the unsubstituted 4a. These results seem to be in agreement with those obtained by Ebata et al. [25] with BTBT derivatives having various alkyl groups of different lengths (pentyl to tetradecyl) at both the molecular terminals. They reported that those compounds indicated increasing solubility with the increased carbon number of the alkyl groups up to nine. With more than nine, however, the solubility rapidly dropped with increasing carbon number. In Table 1 TTGA was increased with increasing chain length of n-alkyl substituent and aromatic core size. DSC
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Q3
curves of 4b–4d, 6b, and 6c exhibited plural thermal transitions (Fig. 2(a)), even though those of 4a and 6a showed a single transition. This suggests the presence of mesophases for 4b–4d, 6b, and 6c. The lowest major peaks of DSC reflect the melting points of the compounds. All the compounds showed reversible thermal transitions in the DSC curves. As an example Fig. 2(b) shows the DSC trace of 4d. The reversibility of DSC scans clearly indicates excellent thermal stability of these TPCOs. When comparing 4a–4d, we notice that although melting points of 4b and 4c were only slightly lower (10 °C) than that of 4a, 4d showed a melting point lower than 4a by 100 °C. In relation to these observations, Garnier et al. [21] mentioned that a,x-hexyl-disubstituted sexithiophene showed a melting point of 290 °C, which was higher than that of unsubstituted sexithiophene by 10 °C. They assumed that the a,x-dihexyl substitution enhanced the cohesive force between the conjugated backbones. Meanwhile, Ponomarenko and Kirchmeyer [33] observed that a,x-decyl-disubstituted sexithiophene showed a melting point at 108 °C (i.e. lower than unsubstituted sexithiophene by more than 100 °C). Such relationship between the substituted alkyl chain length and the melting point was noticed with various disubstituted molecules having BTBT, anthradithiophene, and TPCO backbones as well [26,34–36]. Comparing the results of the present studies and previously obtained data for the disubstituted molecules, compounds with appropriately long alkyl groups chosen for monosubstitution are expected to allow us to achieve a good balance between the high solubility and high melting point of the materials. Here the latter requirement is relevant to their application to electronic devices (such as FETs) with good thermal durability [26]. More specifically, if we select a 4a-based compound with high processability and durability in mind, the alkyl groups can be chosen from among e.g. hexyl and octyl (as in 4b and 4c). Thus the alkyl-monosubstitution strategy may well be applied to other molecules and will be useful for future molecular design more widely. The unsubstituted 4a exhibited the absorption peak at 355 nm and the emission peaks at 402 and 425 nm. The absorption and emission peaks of alkyl-monosubstituted 4b, 4c, and 4d, were red-shifted (3–5 nm) compared to those of 4a. We assume that these shifts were associated with the electron-donating alkyl groups in 4b, 4c, and 4d. Similarly, the addition of the phenyl ring to the aromatic
Table 1 Solubility in THF at r.t., absorption maxima (kabs) and emission maxima (kem) in THF solution, and thermal properties of TPCOs, 4a4d and 6a6c.
a b c
Compound
Solubility (g L1)
kabs (nm)
kema(nm)
TTGAb(°C)
TDSCc(°C)
4a 4b 4c 4d 6a 6b 6c
4.4 7.1 8.9 0.18 0.04 0.41 0.21
355 358 358 359 377 379 379
402⁄, 425 406⁄, 429 406⁄, 429 407, 428⁄ 426⁄, 451 429⁄, 454 429⁄, 454
269 289 319 366 366 383 391
233 221, 220, 108, 309 263, 262,
223, 231 224 124, 220 299, 318 299, 306
Absolute maximum indicated by asterisk. TTGA has been defined as a temperature at which 5% weight loss has taken place. TDSC has been determined from a DSC trace and defined as a transition temperature.
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Fig. 2. (a) DSC curves (second heating run) of TPCOs and (b) DSC trace (second cycle) of 4d showing reversible thermal transitions (under nitrogen at a ramp rate of 10 °C min1). 348 349 350
core, which produced 6b and 6c, respectively, resulted in bathochromic shifts of 21–25 nm relative to the absorption and emission peaks observed for 4b and 4c.
3.2. Structural characterizations of thin films
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Fig. 3 represents the optical micrographs of the solution-cast films. Large-sized films were formed for 4b and 4c, in contrast to small crystals for 4a and 4d. In particular, large domains over 1 mm are clearly visible for the film 4c. These observations mean that the length of the n-alkyl chains in TPCOs influences the morphology of solution-cast films. Higher solubility of 4b and 4c is responsible for the formation of large-sized cast films (Fig. 3, Table 1). In Fig. 4 we compare the XRD diagrams of solution-cast films and vacuum-deposition films of 4a–4c. The XRD results of both the vacuum-deposition and solution-cast films are characterized by the first-order reflection together with higher-order reflections. Table 2 collects the XRD results of the thin films formed by the vacuum deposition, solution cast, and skim methods. The dspacings estimated from the first-order reflection of the vacuum-deposition films correspond to the molecular length [35,37]. The corresponding d-spacings for the solution-cast films are about two times those of the vacuum-deposition films and much larger than the molecular lengths. This suggests the presence of a bilayer structure that results from association of two molecules in the solution-cast films [38]. These observations are contrasted with the case of a symmetric compound 6d [35]. The compound 6d shows virtually the same d-spacing for the thin films both solution cast and vacuum deposited [35], thus lacking the bilayer structure. Moreover, the solution-cast films indicated much stronger diffraction peaks and narrower full widths at half maximum (FWHM) than
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Fig. 3. Optical micrographs of solution-cast films of 4a–4d on a Si/SiO2 substrate.
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Fig. 5. XRD patterns of skim and solution-cast films of 4d on a Si/SiO2 substrate. A magnified profile (50) is depicted for the sake of clear visualization.
Fig. 4. XRD patterns of thin films deposited on a Si/SiO2 substrate for (a) 4a, (b) 4b, and (c) 4c. Each diagram includes the results of a vacuumdeposition film (obtained on a substrate held at room temperature) and a solution-cast film. In (a), an XRD diagram of a single crystal of 4a is depicted in an inset. For (b) and (c), magnified profiles (50 or 10) are depicted for the sake of clear visualization.
Table 2 Interplanar distances (d-spacing) for vacuum-deposition (at room temperature of substrate), solution-cast, and skim films of TPCOs. Compound
d-Spacinga (Å) Vacuum-deposition film
4a 4b 4c 4d 6a 6b 6c a b c
17.4 23.8 25.8 34.8 21.6 29.3 31.7
Solution-cast film 34.5 46.5 51.1 70.1 21.6b 29.5b 60.5b
Calculated from the first-order reflections. Many small crystallites were formed. Skim films were not formed.
Skim film c
– 47.0 51.1 66.9 –c –c 60.5
the corresponding vacuum-deposition films. This reflects better molecular ordering and higher crystallinity in the solution-cast films. This unique structure is caused by the difference in local chemical environments caused by aromatic and aliphatic groups and is specific to the alkylmonosubstituted materials. In Fig. 5 we compare the XRD patterns of the skim film and solution-cast film of 4d. Diffraction peaks of the skim film were much stronger than those of the solution-cast film and the relevant peaks were attributed to the 20th order reflections or higher. These observations indicate the highly ordered structure in the skim film. To further characterize the aforementioned unique morphology of the solution-cast and skim films, we show in Fig. 6 the crystallographic structure of 4a [39]. The asymmetric unit contains two crystallographically independent molecules, one of which includes a disordered thiophene ring, with relative occupancies of 72.3 (5)% (S-anti) and 27.7 (5)% (S-syn) for the two components [40]. The two independent molecules can be taken as either M1 and M2 or M1 and M2’. In the former case those molecules can be viewed as if they were a ‘‘joint single molecule,’’ where adjacent thiophene rings come into contact. Half a (0 0 1) -plane spacing is 34.50 Å and this spacing reflects a length of the joint molecule. Consequently, the presence of the two crystallographically independent molecules is the origin of the above-mentioned bilayer structure in the solution-cast and skim films. Moreover, the formation of the bilayer structure is a result of the different chemical environment at both the molecular terminals, as in the case of 4a crystals (where thienyl and phenyl groups are located at the molecular terminals). The difference in the d-spacings of thin films reflects the difference in alkyl chain lengths of TPCOs. We calculated the alkyl chain lengths on the basis of Ref. [41] and estimated those of hexyl, octyl, and hexadecyl groups to be 9.7, 12.2, and 22.2 Å, respectively. We then estimated molecular length of 4b, 4c, and 4d at 25.2, 27.8, and 37.8 Å, respectively. These numbers imply that tilting angles of the molecules against the substrate normal range from 19° to 23°. In the above estimation we took the aromatic backbone geometry from that determined crystallographically for a 4a molecule without disorder and
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Fig. 6. Crystal structure of 4a. M1 and M2 (or M20 ) represent two crystallographically independent molecules; for structural details see text and Ref. [39].
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ignored small difference in the molecular geometry between the two crystallographically independent molecules. When we look closely at the XRD data, we notice the following: (i) Regarding the solution-cast films, 6a and 6b lack the bilayer structure. The crystals of 6a possess
one crystallographically independent molecule as in the cases of other symmetric TPCOs [8,42–44], in agreement with absence of the bilayer structure. This provides a sharp contrast to the case of the 4a crystals outlined above. (ii) The skim films of 4b, 4c, and 4d exhibited FWHMs of 0.16°, 0.12°, and 0.10° for their primary diffraction peaks due to the bilayer structure (see Table 2). This implies that the skim films have increasingly ordered structure with increasing alkyl chain lengths. The FWHM of the skim film of 6c, on the other hand, was 0.24°. We infer from these findings that alkyl-monosubstituted compounds having the asymmetric aromatic backbone and longer alkyl chain tend to show the well-ordered bilayer structure. At the same time, this reasoning probably explains why the solution-cast film of 6b did not have the bilayer structure. We also add that there is room for further development of materials combining high structural order and durability (vide supra). If the crystal structures of alkyl-monosubstituted compounds are available, this will enable us to obtain detailed information on intermolecular interactions between alkyl groups (and aromatic moieties as well) along with those between molecules and the substrate more widely.
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3.3. Device characterizations of thin films
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The output curves of all the devices exhibited the clear p-channel FET properties. As an example, Fig. 7(a) and (b) shows the electrical output and transfer characteristics,
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Fig. 7. (a) Electrical output and (b) transfer characteristics for a bottom-gate bottom-contact device having a vacuum-deposition film of 4c obtained on a substrate held at room temperature. (c) Electrical output and (d) transfer characteristics for a bottom-gate top-contact device having a vacuum-deposition film of 6c obtained at a substrate temperature of 150 °C.
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R. Hirase et al. / Organic Electronics xxx (2014) xxx–xxx Table 3 Carrier mobilities (l) and threshold voltages (Vth) of FET devices with vacuum-deposition films of TPCOs at various substrate temperatures. Compound
4a 4b 4c 4d
6a 6b 6c
457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480
Substrate temperature (°C)
r.t. r.t. r.t. 130 r.t. 130 150 r.t. r.t. 150 r.t. 150
Bottom-gate bottom-contact
Bottom-gate top-contact
l (cm2 V1 s1)
Vth (V)
l (cm2 V1 s1)
Vth (V)
(2.1 ± 1.2) 105 (2.6 ± 0.1) 104 (1.3 ± 0.2) 103 (1.5 ± 0.2) 103 (2.0 ± 0.1) 104 (1.3 ± 0.2) 103 (5.4 ± 0.5) 104 (8.0 ± 0.8) 104 (5.9 ± 0.2) 103 (8.5 ± 0.5) 103 (2.0 ± 0.3) 103 (9.8 ± 0.4) 103
29.7 ± 7.6 20.1 ± 4.4 22.5 ± 0.0 18.7 ± 5.4 19.1 ± 0.8 20.2 ± 1.2 28.0 ± 3.2 21.1 ± 2.1 15.1 ± 0.6 11.9 ± 0.2 14.7 ± 4.5 18.5 ± 1.7
(2.1 ± 0.0) 103 (6.1 ± 3.2) 103 (3.0 ± 2.5) 103 (1.0 ± 0.4) 102 (1.3 ± 1.0) 103 (0.9 ± 0.4) 103 (6.3 ± 1.7) 103 (2.0 ± 0.2) 102 (2.4 ± 0.3) 102 (3.4 ± 0.3) 102 (2.5 ± 0.1) 102 (3.7 ± 0.2) 102
45.5 ± 2.6 31.9 ± 3.3 47.3 ± 8.7 27.1 ± 3.2 35.9 ± 11.7 32.4 ± 14.6 29.8 ± 6.4 16.8 ± 4.3 13.4 ± 2.0 13.0 ± 0.0 19.9 ± 3.8 20.2 ± 1.9
respectively, of the FET measured in the bottom-contact configuration for the 4c film vacuum deposited on a substrate held at room temperature. Fig. 7(c) and (d) represents another device feature in the top-contact configuration with the 6c film vacuum deposited at an elevated substrate temperature of 150 °C. The latter device yielded the highest carrier mobility 3.7 102 cm2 V1 s1. This number is comparable to that for a device of a hexyl-disubstituted TPCO (6d) of 5.4 102 cm2 V1 s1 [35]. Table 3 summarizes the carrier mobilities and the threshold voltages of the FETs for the vacuum-deposition films. We have carried out electrical measurements using 2–6 devices for each entry except for skim film 4c, with which a single device was examined. We indicate an average number and deviations (±). From Table 3 we recognize the following trends: (i) The carrier mobilities of the FETs of 4b and 4c (6b and 6c) were higher than that of the FET of an unsubstituted parent molecule 4a (6a). This can be explained as a result of the improved molecular ordering and p–p stacking caused by the alkyl-monosubstitution [21]. (ii) The 6a–6c devices showed higher mobilities than corresponding 4a–4c for both bottom-contact and top-contact devices. This trend results from the extension in the conjugated system in 6a–6c [45]. (iii) The devices
having the top-contact configuration exhibited higher mobilities than those with the bottom-contact configuration except for the device 4d fabricated at a substrate temperature 130 °C. Related results were described and discussed in the literature in terms of the contact resistance [15]. (iv) The carrier mobilities of the FETs with 4c, 6b, and 6c deposited at higher substrate temperatures were maximally 5 times greater than those deposited on a substrate held at room temperature (see Table 3). This is due to the increase in grain sizes of alkyl-monosubstituted TPCO films deposited at higher substrate temperatures [46,47]. The effect of the substrate temperatures on the grain sizes is evidenced by Fig. 8 that compares surface morphologies of the films 4c deposited at a room temperature and at 130 °C of substrate. Regarding the 4d device, however, the relationship between the mobility and substrate temperatures somewhat deviated from this trend with both the bottom-contact devices and top-contact devices. Such deviation resulted possibly from the fact that the vacuum deposition of 4d was carried out near its melting temperature (see Table 1). As an example of the solution-cast devices, Fig. 9(a) and (b) shows the electrical output and transfer characteristics
Fig. 8. Surface morphologies of a vacuum-deposition films 4c prepared at different substrate temperatures: (a) room temperature, (b) 130 °C. The horizontally long color bars under each micrograph indicate height scale.
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Fig. 9. (a) Electrical output and (b) transfer characteristics for a bottom-contact device having a solution-cast film of 4c.
Table 4 Carrier mobilities (l) and threshold voltages (Vth) of bottom-gate bottom-contact FET devices with vacuum deposition (at room temperature of substrate), solution cast, and skim films of TPCOs. Compound
4a 4b 4c 4d a
Vacuum-deposition film
Solution-cast film
Skim film
l (cm2 V1 s1)
Vth (V)
l (cm2 V1 s1)
Vth (V)
l (cm2 V1 s1)
Vth (V)
(2.1 ± 1.2) 105 (2.6 ± 0.1) 104 (1.3 ± 0.2) 103 (2.0 ± 0.1) 104
29.7 ± 7.6 20.1 ± 4.4 22.5 ± 0.0 19.1 ± 0.8
(9.4 ± 6.6) 104 (1.1 ± 0.3) 102 (1.1 ± 0.0) 102 (1.4 ± 0.4) 103
12.2 ± 7.4 16.6 ± 0.8 19.9 ± 0.4 25.3 ± 0.5
–a –a 1.6 105 (3.2 ± 0.1) 103
2.1 4.6 ± 1.2
Films suitable for the fabrication of devices by the skim method were not obtained.
Fig. 10. Micrographs of (a) a free-standing skim film of 4d obtained by peeling a film off a substrate and (b) a bottom-contact FET device with a skim film 4d. In (a) tweezers are pinching a free-standing film of 6 mm in size. (c) Electrical output and (d) transfer characteristics for a bottom-contact device with a skim film 4d.
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for the solution-cast film of 4c, respectively. Table 4 compares the characteristics of solution-cast devices and vacuum-deposition devices of 4a–4d on the bottom-contact FETs configurations. (i) Their solution-cast devices exhibited one to two orders of magnitude higher carrier mobilities than the vacuum-deposition devices. This is due very likely to the regular molecular alignment attained in the bilayer structure. This forms a striking contrast to the related hexyl-disubstituted symmetric TPCO molecules [35]. Notice here that in contrast to our results, solutioncast films of those hexyl-disubstituted compounds showed lower mobilities than the vacuum-deposition films of the same compound [35]. (ii) The solution-cast 4b and 4c devices produced larger carrier mobility than the solution-cast 4a device as in the case of the vacuum-deposition devices. The value of 102 cm2 V1 s1 was ten times as large as that of 4a. Of these, the solution-cast 4c devices have shown the best reproducibility with the mobility measurements. This is associated with the large domain size of the solution-cast films 4c (Fig. 3). We did not estimate mobilities of solution-cast films of 6a–6c. This is because a solution-cast film of high quality was hard to prepare because of their poor solubility. The skim films of 4d were obtained as a free-standing film by peeling it off a substrate. They were typically several millimeters in size (Fig. 10(a)). Fig. 10(b) shows an optical micrograph of the bottom-contact FET of the skim film 4d. Aside from several wrinkles the uniform thin film covered the entire surface of the substrate. Fig. 10(c) and (d) shows the electrical output and transfer characteristics for the skim film 4d, respectively. This device had a larger carrier mobility of 3.2 103 cm2 V1 s1 than the solution-cast 4d device. Even though the skim film device of 4c exhibited somewhat poor performance, 4c showed the best results in terms of the solution processability and steady device performance among 4a and its alkylmonosubstituted compounds.
due to the regular molecular alignment in the bilayer structure.
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Acknowledgements
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We thank Dr. Takeshi Yamao for his helpful discussion and valuable support in electrical measurements. We are grateful to Dr. Shohei Katao for carrying out the X-ray crystallographic analysis. Thanks are also due to Sumika Chemical Analysis Service, Ltd. for elemental analyses (for carbon, hydrogen, and sulfur).
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Appendix A. Supplementary material
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Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2014.04.010.
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4. Conclusions
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We have designed and synthesized a new series of alkyl-monosubstituted TPCOs of 4b–4d, 6b, and 6c as the solution processable semiconducting materials. The introduction of alkyl chains to a sole p-position of the terminal phenyl group improved the solubility in organic solvents, while maintaining good thermal stability and durability. The thin films of these alkyl-monosubstituted TPCOs were obtained by the vacuum-deposition, solution-cast, and skim methods. Of these, the solution-cast films and skim films consist of highly-ordered structures characterized by the molecular bilayer. As a result the skim method produced a large-sized free-standing thin film. The FET devices made of the thin films exhibited typical p-channel characteristics with clear saturation region. A device of the vacuum-deposition film of 6c recorded the highest carrier mobility of 3.7 102 cm2 V1 s1. The solution-cast devices exhibited one to two orders of magnitude higher carrier mobilities than the vacuumdeposition devices, especially the solution-cast 4b and 4c devices producing the mobility 102 cm2 V1 s1. This is
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Please cite this article in press as: R. Hirase et al., Alkyl-monosubstituted thiophene/phenylene co-oligomers: Synthesis, thin film preparation, and transistor device characteristics, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.04.010