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Cite This: Chem. Mater. 2018, 30, 5050−5060
Extended and Modulated Thienothiophenes for Thermally Durable and Solution-Processable Organic Semiconductors Satoru Inoue,*,† Shoji Shinamura,† Yuichi Sadamitsu,† Shunto Arai,‡ Sachio Horiuchi,§ Makoto Yoneya,§ Kazuo Takimiya,∥,⊥ and Tatsuo Hasegawa*,‡,§ †
Nippon Kayaku Company, Ltd., 3-31-12 Shimo, Tokyo 115-8588, Japan Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-8656, Japan § Flexible Electronics Research Center (FLEC), National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8565, Japan ∥ Emergent Molecular Function Research Team, RIKEN Advanced Science Institute, Wako, Saitama 351-0198, Japan ⊥ Department of Chemistry, Tohoku University, 6-3, Aramaki, Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan
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‡
S Supporting Information *
ABSTRACT: Herein, we report the rational design of practical small-molecule organic semiconductors based on a π-electron skeleton of benzothieno[3,2-b]naphtho[2,3-b]thiophene (BTNT) whose layered herringbone (LHB) packing is intentionally modulated by the designated asymmetric substitutions with the phenyl group and normal alkyl chains. The thermal stability of the hybrid BTNT core is high enough, as it lies between those of dinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]thiophene (DNTT) and benzothieno[3,2-b]benzothiophene (BTBT), although the solvent solubility for the substituted BTNT at ordinary 2,8substituting positions by the alkyl chain and phenyl group remains extremely low. We show in the BTBT and BTNT derivatives that the tuning of the substituting position works to slightly bend the rodlike organic semiconductor molecules and thus to decrease the cohesive energy of the crystals with retention of the bilayer-type herringbone (b-LHB) packing for the asymmetric rodlike molecules. This modification eventually leads to an increase in solvent solubility, a decrease in phase transition temperature, and the suppression of liquid-crystalline phases at high temperatures. By using the substituting effect, we successfully achieve the organic semiconductors with modulated alkylated Ph-BTNT that exhibits both a sufficiently high solvent solubility and a sufficiently high thermal stability. The variation in the crystal packing also enhances the intermolecular transfer integrals along the T-shaped contacts within the intralayer herringbone packing. Spin coating of the material under ambient conditions affords high-performance bottom-gate, bottom-contact organic thin-film transistors, exhibiting high thermal durability in the device characteristics below 150 °C. The obtained devices also exhibit a higher mobility, a lower threshold voltage, and a smaller subthreshold swing, by initial thermal treatment at 140 °C, composed to those of the as-prepared films, because the thermal treatment stabilizes the b-LHB packing and thus suppresses the residual minority holes and shallow traps. These findings should be crucial in the design and development of organic semiconductor materials for practical printed electronics applications.
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INTRODUCTION In the past decade, considerable progress has been made in the design and development of organic semiconductor (OSC) materials for the realization of high-performance organic thinfilm transistors (OTFTs). Recent attention has increasingly focused on the class of solution-processable OSCs because of the expectations for the applications into the printing-based device production (i.e., printed electronics) technologies.1−7 © 2018 American Chemical Society
Actually, it is demonstrated that some soluble small-molecule OSCs afford high-performance OTFTs whose device mobility reaches 10 cm2 V−1 s−1, which is higher than those obtained generally by vacuum-based thin-film processing.8−13 The Received: March 31, 2018 Revised: July 19, 2018 Published: July 20, 2018 5050
DOI: 10.1021/acs.chemmater.8b01339 Chem. Mater. 2018, 30, 5050−5060
Article
Chemistry of Materials finding that the solution-based thin-film processing is more suitable for achieving high-performance OTFTs than the vacuum-based one has been discussed, as solution-based thinfilm processing can fully take advantage of the oriented and self-organized growth of layered-crystalline thin films via the intermediate (lyotropic) liquid-crystal phases at air−liquid interfaces.14,15 For the actual applications of these solution-processable OSCs, a major challenge is to achieve high thermal stability or durability in the OSC characteristics. This is because the solvent solubility required for the printing processes is closely correlated to the degradation of thermal stability in nature, as it is associated with the decrease in the cohesive energy of the molecular materials. Notable examples include the comparison between benzothieno[3,2-b]benzothiophene (BTBT)8−10,16−21 and dinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]thiophene (DNTT)12,22−25 frameworks, both of which are renowned small-molecule organic semiconductors that form layered herringbone (LHB)-type molecular packing and thus afford high-performance OTFTs. The BTBT with a relatively small π-electron skeleton exhibits a high solvent solubility but a relatively low thermal stability, while the DNTT with a larger π-electron skeleton shows a high thermal stability but an extremely low solvent solubility. It is necessary to address this dilemma to achieve industrial application of printed electronics technology. The solvent solubility of OSCs could be controlled, or should be determined, by several factors, such as the size and shape of the π-electron skeletons, as well as the side-group substitutions. Several studies have been devoted, so far, to increasing the solvent solubility of OSCs composed of relatively large π-electron skeletons, such as by adding bulky substituents26−28 or equipping the π-electron skeletons with electrical polarities.29−31 Another intriguing approach is to use soluble precursor molecules that can be transformed into active OSCs by thermal or photochemical reactions after the precursor solution is deposited on the substrates.32−38 However, this approach is unfavorable in terms of the uniform layered-crystalline film growth by solution processes, because the molecular packing motifs of the active OSCs are generally distinct from those of the precursors. In this respect, it should be necessary to realize a delicate balance between the high solvent solubility and the high thermal durability by a rational design of π-electron skeletons and side-group substitutions for obtaining practical OSCs, especially with preserving the packing motif showing a high layered crystallinity. A recent systematic study of the BTBT derivatives has shown that the bilayer-type LHB (b-LHB) packing, which affords an extremely high layered crystallinity, appears via the asymmetric substitution of the BTBT cores with long normal alkyl chains.9,16,17,39 It is also shown that the cohesive energy of the OSCs is basically determined by the intermolecular interactions between π-electron cores but is enhanced or controlled by the alkyl-chain substitutions. Nevertheless, it is difficult to achieve sufficiently high thermal stability and durability, higher than 150 °C, for practical uses, when using the BTBT-based OSCs, primarily because of the limited size of the π-electron skeleton. In this study, we systematically investigate the effect of substituting positions18 on benzothieno[3,2-b]naphtho[2,3b]thiophene (BTNT40), a hybrid π-electron framework between BTBT and DNTT, on obtaining organic semiconductors for OTFTs showing a unique delicate balance
between the high solvent solubility and the thermal durability in the device characteristics. Scheme 1 presents the chemical Scheme 1. Chemical Structures of BTBT Derivatives 1 and BTNT Derivatives 2
structures of the family of compounds used to investigate the effect of the asymmetric core substitutions by the phenyl group and normal alkyl chains: 2-phenyl-7-decyl-BTBT (Ph-BTBTC10) (1a), 3-phenyl-8-decyl-BTBT (1b), 2-phenyl-8-hexylBTNT (2a), and 3-phenyl-9-decyl-BTNT (2b). It is found that this series of compounds presents systematic variations in thermal and soluble characteristics at the substituting positions, while they keep the b-LHB packing motif. Among them, 2b presents a unique balance between a sufficiently high solvent solubility and thermal durability in the device characteristics. On the basis of the results, we discuss the rational design of practical small-molecule organic semiconductors based on the extended and modulated thienothiophenes.
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EXPERIMENTAL SECTION
Synthesis of Materials. 1a was obtained according to the literature procedure,17 and 1b, 2a, and 2b are original and were obtained according to the reported procedure,40 as summarized in the Supporting Information. The chloroform solution of the final product was concentrated under reduced pressure to obtain pale yellow solids as crude products. Finally, they were purified by repeated recrystallization and sublimation. Solubility. The solubility of all the compounds was evaluated by the same procedure reported previously.17 Organic solvents were added to a powdered sample (3−7 mg) in increments of 40 μL, and the mixture was stirred to dissolve the sample completely at 20 °C. The solubility was calculated from the total amount of solvent and the sample weight. The concentration was checked by fitting a calibration curve for the UV absorbance versus concentration. All the results are summarized in Table S1. Thermal Properties. The samples were thermally analyzed by differential scanning calorimetry (DSC; DSC7000X, Hitachi HighTech Science Co.) at a scanning rate of 5−10 K/min. The measured temperature was calibrated using the melting point of indium (429.8 K), and a synthetic sapphire was used as the standard for determining the heat capacity. For the DSC measurements, the powdered sample was heated and subsequently cooled at rates of 5 K/min (first and second scan) and 10 K/min (third scan). Melting points of all the compounds were determined by visual inspection of the changes in powder samples on a hot plate. Transition enthalpies (ΔH) were estimated at respective phase transitions (at Ttrans) upon heating, whereas the transition entropies (ΔS) were calculated by using the relation ΔS = ΔH/Ttrans.41 All the results are summarized in Table S2. 5051
DOI: 10.1021/acs.chemmater.8b01339 Chem. Mater. 2018, 30, 5050−5060
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Chemistry of Materials Table 1. Crystallographic Data of 1a, 1b, 2a, and 2b chemical formula formula weight crystal system ́ a (Å) ́ b (Å) ́ c (Å) β (deg) ́ V (Å3) space group Z value temperature (K) radiation no. of reflections no. of variables residuals, R residuals, wR2 (all reflections) goodness of fit indicator
1a17
1b
2a
2b
C30H32S2 456.71 monoclinic 6.0471 (3) 7.7568 (4) 53.124 (5) 93.135 (3) 2488.1 (3) P21/a (No. 14) 4 − − − − − − −
C30H32S2 456.71 monoclinic 7.029 (3) 7.391 (2) 48.221 (17) 92.414 (12) 2502.9 (15) P21/a (No. 14) 4 300 MoKα (λ = 0.71075 Å) 5727 290 0.0838 (I > 2σ) 0.1330 0.908
C30H26S2 450.66 monoclinic 6.065 (2) 7.801 (2) 48.772 (19) 92.864 (9) 2304.7 (13) P21/a (No. 14) 4 300 MoKα (λ = 0.71075 Å) 5208 289 0.0497 (I > 2σ) 0.1333 0.681
C34H34S2 506.76 monoclinic 6.931 (10) 7.406 (11) 53.13 (8) 91.153 (8) 2727 (7) P21/a (No. 14) 4 300 MoKα (λ = 0.71075 Å) 6054 325 0.1009 (I > 2σ) 0.2417 0.927
We also conducted thermogravimetric differential thermal analysis (TG-DTA) for all the compounds by using the STARe System instrument (Mettler Toledo Co. Ltd.). Structural Characterization. Single crystals for structural analyses (1b, 2a, and 2b) were grown by recrystallization in a saturated solution of toluene at room temperature. The obtained crystals were thin and flakelike, similar to those reported for 1a.39 The crystal for the structure analyses was carefully collected from the solution using a mounting apparatus (LithoLoops, Molecular Dimensions Ltd.). All the X-ray diffraction experiments were performed with a four-circle diffractometer (AFC10, Rigaku) using monochromated Mo Kα radiation and a hybrid pixel detector (PILATUS200 K, Rigaku Corp.). All the calculations were performed using crystallographic software packages (CrystalClear and CrystalStructure, Molecular Structure Co. and Rigaku Co.)42 The initial structures were modeled using the direct method.43,44 Then sulfur and carbon atoms were refined anisotropically, and hydrogen atoms were refined by the riding model using SHELXL97.45 All the crystallographic parameters are summarized in Table 1. Theoretical Calculation. We conducted empirical force fieldbased calculations to estimate the cohesive energies, based on the obtained crystal structures.46,47 The atomic charges (derived from restrained electrostatic potential48) were obtained by ab initio molecular orbital calculations using the B3LYP/6-31G(d) level in the Gaussian03 program.49 All the simulations were done using molecular dynamics (MD) simulation program GROMACS (version 4.6.7).50 The detailed explanation of the calculation can be found in the Supporting Information of our previous study.17 We also performed Amsterdam density functional (ADF) calculations51,52 based on the crystal structures, to calculate the intermolecular transfer integrals. Photoelectron Yield Spectroscopy. Photoelectron yield spectroscopy (PYS; AC-2, Riken-Keiki) was used to evaluate the highest occupied molecular orbital (HOMO) of the compounds. We prepared the OSC films on Au substrates by a blade-coating technique. The 1/n power of photoelectron yield Y is usually expressed as Y1/n = A(hν − hνth), where hν and hνth are incident and photoemission threshold photon energies, respectively. The PYS spectra were fitted with square-root law Y1/2 and cube-root law Y1/3 for Au and OSCs, respectively.53,54 The results are summarized in Table 2, compared with the values obtained by density functional theory (DFT) calculations. Characterizations of Thin-Film Transistors. We manufactured bottom-gate, bottom-contact OTFTs composed of spin-coated OSC films as channels. We used n+-Si wafers with 300 nm thick silicon dioxide layers as substrates. Source/drain electrodes were fabricated by vacuum deposition of Au through a shadow mask to define the
Table 2. HOMO Levels of 1b, 2a, and 2b energy level (eV) (calcd)
1b
2a
2b
5.29 (5.55)
4.97 (5.20)
5.01 (5.26)
length (L) and width (W) of all the OTFT channels at 20 and 100 μm, respectively. The Au surface was treated with pentafluorobenzenethiol (PFBT) by being soaked in a 10 mM isopropanol solution for 10 min, followed by cleaning with pure isopropanol. Then the OSCs were spin-coated using o-xylene solutions on the substrates. The obtained films were annealed for 10 min on a hot plate. The polycrystalline film morphologies were investigated with an optical microscope (Eclipse L150, Nikon), by atomic force microscopy (AFM; Dimension 3000 Nanoscope IIIa, Bruker Co. Ltd.). We also investigated the effects of initial thermal treatment temperature on Xray diffraction for relatively thick spin-coated films using an X-ray diffractometer (SmartLab, Rigaku Co.). We manufactured single-crystal OTFTs for 2b to investigate the material characteristics without grain boundaries. We used p+-Si wafers with 100 nm thick silicon dioxide layers as substrates. The OSC solution in chlorobenzene (0.1 wt %) was blade-coated at a constant speed and at regulated temperatures on the substrates to form single-crystal films. The top-contact source/drain electrodes (with a thickness of 25 nm) were fabricated by vacuum deposition of Au through a shadow mask to define the L and W of the OTFT channels at 200 and 500 μm, respectively. A micromanipulator (AxisPro, Systems Engineering Inc.) was used to trim away the films outside the channels for the proper evaluation of device mobility. The TFT characteristics were measured using a semiconductor parametric analyzer (E5270A or B1500A; Agilent Technologies Co. Ltd.) under ambient conditions. The field-effect mobility (μ) was defined as the derivative of the transfer curve according to the following equations:
linear regime
saturation regime
μ linear =
L ijj ∂ID yzz jj zz WCiVD jk ∂IG z{
μsaturation =
2L ijj ∂ ID jj WCi jjk ∂VG
yz zz zzz {
2
where ID, Ci, VD, and VG are the drain current, gate capacitance per unit area, drain voltage, and gate voltage, respectively. The measurements were conducted at a VD of −1 V in the linear regime and at a VD of −30 V in the saturation regime. We investigated the effects of the initial thermal treatment temperature on the device characteristics for as-prepared spin-coated films. We also investigated 5052
DOI: 10.1021/acs.chemmater.8b01339 Chem. Mater. 2018, 30, 5050−5060
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Chemistry of Materials the further thermal stress effects on the devices prepared by an initial thermal treatment process at 140 °C for 10 min, where additional thermal stress was applied to the devices for 5 min at various temperatures.
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RESULTS AND DISCUSSION Thermal and Solubility Characteristics. Figure 1 summarizes the solvent solubility and thermal properties of
Figure 2. DSC curves of 1a,17 1b, 2a, and 2b. The scan rate was 5 K/ min (the first cycle of 2a and 2b was 10 K/min).
phases, while 1b and 2b do not, where a single endothermic peak due to solid−liquid melting appears in the DSC curves. 2a presents two LC-like phase transitions at 237 and 285 °C. In contrast, 2b undergoes a melting transition at 180 °C, which is lower than that for 2a, though both of these transition temperatures are high enough for practical uses. Importantly, the solubility of 2b (0.19 wt %) is high enough for its use in printing processes, although that of 2a (10 cm2 V−1 s−1, when the films are spin-coated at high temperatures and are followed by successive quenching and thermal treatment; producing a liquid-crystalline precursor thin film at an elevated temperature (∼110 °C) was found to be necessary. In contrast, the spin-coated films of 2b can be fabricated around room temperature, which is much more useful than those of 1a. Figure 7 presents the out-of-plane X-ray diffraction data for relatively thick spin-coated films of 2b before and after the initial thermal treatment at 140 °C for 10 min. As one can see,
Figure 8. (a) Optical microscope image for a film of 2b, obtained by spin coating and successive thermal treatment. (b) AFM image and a cross-sectional profile for a film of 2b.
the thermal treatment. The films are composed of a step-andterrace structure with large-area terraces and a step height of ∼5 nm that corresponds to the bilayer thickness. We also observe a single-layer step with a step height of ∼2.5 nm in the AFM image. Figure 9a presents the effect of initial thermal treatment temperature on the device characteristics for the OTFTs composed of spin-coated films of 2b. The initial thermal treatment at 140 °C causes a clear improvement in the device characteristics in the subthreshold region (but not sufficiently at 100 and 120 °C). The turn-on voltage (Von) shifts from ∼20 to
