Extended and Modulated Thienothiophenes for Thermally Durable

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Extended and modulated thienothiophenes for thermallydurable and solution-processable organic semiconductors Satoru Inoue, Shoji Shinamura, Yuichi Sadamitsu, Shunto Arai, Sachio Horiuchi, Makoto Yoneya, Kazuo Takimiya, and Tatsuo Hasegawa Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01339 • Publication Date (Web): 20 Jul 2018 Downloaded from http://pubs.acs.org on July 23, 2018

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Chemistry of Materials

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,# Tatsuo Hasegawa*,‡, § †Nippon

Kayaku Co., Ltd., 3-31-12 Shimo, Tokyo 115-8588, Japan of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-8656, Japan §National Institute of Advanced Industrial Science and Technology (AIST), Flexible Electronics Research Center (FLEC), 1-1-1 Higashi, Tsukuba 305-8565, Japan ‡Department

#Emergent

Molecular Function Research Team, RIKEN Advanced Science Institute, Wako, Saitama 351-0198, Japan

ABSTRACT: Herein we report rational design of practical small-molecule organic semiconductors based on a -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 phenyl group and normal alkyl chains. The thermal stability of the hybrid BTNT core is high enough, as it lies between the 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 2,8-substituting positions by the alkyl chain and phenyl group remains extremely low. We show both in the BTBT and BTNT derivatives that the tuning of the substituting position works to slightly bend the rod-like organic semiconductor molecules and thus to decrease the cohesive energy of the crystals with retaining the bi-layer-type herringbone (b-LHB) packing for the asymmetric rod-like molecules. This modification eventually leads to the increase in solvent solubility, the decrease in phase transition temperature, and also 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 sufficiently high solvent solubility and enough high thermal stability. The variation of 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 higher mobility, lower threshold voltage, and smaller subthreshold swing, by initial thermal treatment at 140°C, than those composed of the asprepared 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 into the practical printed electronics applications.

□

INTRODUCTION

In the last decade, considerable progress has been made for the design and development of organic semiconductor (OSC) materials to realize high performance organic thin-film transistors (OTFTs). Recent attentions have increasingly focused on the class of solution-processable OSCs due to the expectations for the applications into the printing-based device production (i.e. printed electronics) technologies.1-7 Actually, it is demonstrated that some soluble small-molecule OSCs afford high performance OTFTs whose device mobility reaches as high as 10 cm2/Vs which is higher than those obtained generally by vacuum-based thin-film processing.8-13 It is discussed that the solution-based thin-film processing is more suitable to achieve high performance OTFTs than the vacuum-based one, as it 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 smallmolecule organic semiconductors that form layered herringbone (LHB)-type molecular packing and thus afford high-performance OTFTs. The BTBT with a relatively small -electron skeleton exhibits high solvent solubility but relatively low thermal stability, while the DNTT with a larger -electron skeleton shows high thermal stability but extremely low solvent solubility. It is necessary to address this dilemma for achieving the 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 -electron skeletons, as well as the side-group substitutions. Several studies have been devoted, so far, to increase the solvent solubility of OSCs composed of relatively large -electron skeletons, such as by adding bulky substituents,26-28 or equipping the -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

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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 -electron skeletons and sidegroup substitutions for obtaining practical OSCs, especially with preserving the packing motif showing high layered crystallinity. A recent systematic study on the BTBT derivatives has shown that the bi-layer-type LHB (b-LHB) packing, that affords extremely high layered crystallinity, appears by 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 are basically determined by the intermolecular interactions between -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 the practical uses, when using the BTBTbased OSCs, primarily because of the limited size of the -electron skeleton. In this study we systematically investigate the effect of substituting positions18 on benzothieno[3,2-b]naphtho[2,3-b]thiophene (BTNT40), a hybrid -electron framework between BTBT and DNTT, to 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 structures of the family of compounds used to investigate the effect of the asymmetric core substitutions by phenyl group and normal alkyl chains; 2phenyl-7-decyl-BTBT (Ph-BTBT-C10) (1a) 3-phenyl-8-decylBTBT (1b), 2-phenyl-8-hexyl-BTNT (2a), and 3-phenyl-9decyl-BTNT (2b). It is found that the series of compounds present systematic variations in thermal and soluble characteristics by the substituting positions, while they keep the b-LHB packing motif. Among them, 2b presents a unique balance between the enough high solvent solubility and the thermal durability in the device characteristics. Based on the results, we discuss rational design of practical small-molecule organic semiconductors based on the extended and modulated thienothiophenes.

□

EXPERIMENTAL SECTION

Materials synthesis. The materials of 1a has been obtained according to the literature procedure17, and 1b, 2a, and 2b are original and have been obtained according to the reported procedure,40 as summarized in the Supporting Information. The chloroform solution of the final product is concentrated under reduced pressure to obtain pale yellow solids as crude products, respectively. Finally, they are purified by repeated recrystallization and sublimation. Solubility. The solubility of all the compounds is evaluated by the same procedure reported previously.17 Organic solvents are added to a powdered sample (3−7 mg) in increments of 40 L, and the mixture is stirred to dissolve the sample completely at 20oC. The solubility is calculated from the total amount of solvent and the sample weight. The concentration is checked by fitting a calibration curve for the UV absorbance versus concentration. All the results are summarized in Table S1. Thermal Properties. The samples are thermally analyzed by differential scanning calorimetry (DSC; DSC7000X, Hitachi High-Tech Science Co.) at a scanning rate of 5−10 K/min. The measured temperature is calibrated using melting point of indium (429.8 K), and a synthetic sapphire is used as the stand-

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ard for determining the heat capacity. For the DSC measurements, the powdered sample is heated and subsequently cooled at rates of 5 K/min (1st and 2nd scan) and 10 K/min (3rd scan). Melting points of all the compounds are determined by the visual inspection of the changes in powder samples on a hotplate. Transition enthalpies (H) are estimated at respective phase transitions (at Ttrans) upon heating, whereas the transition entropies (S) are calculated by using the relation; S = H/Ttrans.41 All the results are summarized in Table S2. We also conducted thermogravimetric differential thermal analysis (TG-DTA) for all the compounds by using the instrument (STARe System, Mettler Toledo Co. Ltd.). Structural Characterizations. Single crystals for the structural analyses (1b, 2a, and 2b) are grown by recrystallization in saturated solution of toluene at room temperature. The obtained crystals are thin and flake-like, similar to those reported for 1a.39 The crystal for the structure analyses is carefully collected from the solution using a mounting apparatus (LithoLoops, Molecular Dimensions Ltd.). All the X-ray diffraction experiments are carried out with a four-circle diffractometer (AFC10, Rigaku) using monochromated MoK radiation and a hybrid pixel detector (PILATUS200K, Rigaku Co.). All the calculations are performed using crystallographic software packages (CrystalClear and CrystalStructure, Molecular Structure Co. and Rigaku Co.)42 The initial structures are modeled using the direct method.43,44 Then sulfur and carbon atoms are refined anisotropically, and hydrogen atoms are refined by the riding model using SHELXL97.45 All the crystallographic parameters are summarized in Table 1. Theoretical calculation. We conduct 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) are obtained by ab initio molecular orbital calculations using the B3LYP/631G(d) level in the Gaussian03 program.49 All the simulations are 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 study17. We also perform Amsterdam density functional (ADF) calculations51,52 based on the crystal structures, to calculate the intermolecular transfer integrals. Photoelectron yield spectroscopy. The photoelectron yield spectroscopy (PYS; AC-2, Riken-Keiki) is used to evaluate the highest occupied molecular orbital (HOMO) of the compounds. We prepare 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(h – hth), where h and hth are incident and photoemission threshold photon energies, respectively. The PYS spectra are fitted with square-root law Y1/2 and cuberoot law Y1/3 for Au and OSCs, respectively.53,54 The results are summarized in Table 2, compared with the values obtained by DFT calculations. Characterizations of thin-film transistors. We manufacture bottom-gate/bottom-contact OTFTs composed of spincoated OSC films as channels. We use n+-Si wafers with 300 nm thick silicon dioxide layers as substrates. Source/drain electrodes are fabricated by vacuum deposition of Au through a shadow mask to define the length (L) and the width (W) of all the OTFT channels at 20 m and 100 m, respectively. The Au surface is treated with pentafluorobenzenethiol (PFBT) by soaking them into 10 mM isopropanol solution for 10 min, followed by cleaning with pure isopropanol. Then the OSCs are spin-coated using o-xylene solutions on the substrates. The obtained films are annealed for 10 min on a hot plate. The polycrystalline film morphologies are investigated by optical microscope (Eclipse L150, Nikon), by atomic force microscope (AFM;


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Dimension 3000 Nanoscope IIIa; Bruker Co. Ltd.). We also investigate the effects of initial thermal treatment temperature on the x-ray diffractions for relatively thick spin-coated films using x-ray diffractometer (SmartLab, Rigaku Co.). We manufacture single-crystal OTFTs for 2b to investigate the materials characteristics without grain boundaries. We use p+-Si wafers with 100 nm thick silicon dioxide layers as substrates. The OSC solution in chlorobenzene (0.1 wt%) is bladecoated at constant speed and at regulated temperatures on the substrates to form single-crystal films. The top-contact source/drain electrodes (with thickness of 25 nm) are fabricated by vacuum deposition of Au through a shadow mask to define the L and W of the OTFT channels at 200 m and 500 m, respectively. A micromanipulator (Axis-Pro; Systems Engineering Inc.) is used to trim away the films outside the channels for the proper evaluation of device mobility. The TFT characteristics are measured using a semiconductor parametric analyzer (E5270A or B1500A; Agilent Technologies Co. Ltd.) under ambient conditions. Field-effect mobility (µ) is defined as the derivative of the transfer curve according to the following equations; (Linear regime) 𝜇linear =

𝐿

(

𝜕𝐼𝐷

𝑊𝐶𝑖 𝑉𝐷 𝜕𝑉𝐺

(Saturation regime) 𝜇saturation =

),

2𝐿 𝑊𝐶𝑖

(

𝜕√𝐼𝐷 𝜕𝑉𝐺

2

) ,

where ID, Ci, VD, and VG are drain current, gate capacitance per unit area, drain voltage, and gate voltage, respectively. The measurements are conducted at VD = −1 V in the linear regime and at VD = −30 V in the saturation regime, respectively. We investigate the effects of initial thermal treatment temperature on the device characteristics for as-prepared spin-coated films. We also investigate the further thermal stress effects on the devices prepared by an initial thermal treatment process at 140°C for 10min, where additional thermal stress is applied on the devices for 5 min at various temperatures.

□

RESULTS AND DISCUSSION

Thermal and solubility characteristics. Figure 1 summarizes the solvent solubility and thermal properties of the compounds; 1a, 1b, 2a, and 2b. The DSC curves of all the compounds used for this plot are presented in Figure 2. Among them, 1a presents moderate solubility of about 0.18 wt% in toluene (and similar solubility in other aromatic solvents), and undergoes two liquid-crystal (LC) phase transitions (at 147°C and 213°C) below the highest transition temperature to the isotropic liquid phase at 225°C.9 In contrast, its isomeric compound of 1b presents much higher solubility, but undergoes direct solid-liquid melting transition at 110°C whose transition temperature is lower than that for 1a. It is clearly seen from the comparison between BTBT and BTNT derivatives that the size of the -electron core considerably affects the overall thermal and solubility characteristics: the BTNT derivatives have higher thermal stability but lower solvent solubility than the BTBT derivatives. Note that the similar trends are also seen in the solubility in other solvents, as presented in Table S1. Furthermore, it is interesting to observe the overall similarity in the phase change behavior at high temperatures as to the substituting positions; both 1a and 2a present two LC 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°C and 285°C. In contrast, 2b undergoes 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 the use in

printing processes, although that of 2a (< 0.01 wt%) is too low. We here note that among the compounds only 2a does not have a decyl group but have a hexyl group. However, it is expected that the hexyl substitution provides higher solubility than the decyl substitution, according to the previous report on the alkyl-chain-length dependence of solubility in Ph-BTBT-Cn’s.17 We notice that slight endothermic and exothermic peaks are observed at around 120°C in the DSC curves of 2b. Similar but only exothermic peak also appear in those of 1b. However, we could not find any change in the appearance of powder sample by the visual inspections at around 120°C in 2b. The origin of the peak will be discussed later, as associated with the thermal treatment effect on the structural and device characteristics of spin-coated films. Nonetheless, thermal durability in the OTFT characteristics is retained in this temperature range. Note that the TG-DTA data, shown in Figure S1, indicate the thermal degradation of molecules for all the compounds undergo at higher temperatures 300°C. All the results presented above imply that the proper choice of substituting positions is effective for providing both suitable solubility and thermal stability for the use in printed electronics technology. Variation of crystal packing motifs. Figure 3(a) presents the molecular packing motifs for 1a, 1b, 2a, and 2b. All the crystals belong to the same crystal symmetry (P21/a), and exhibit a roughly isomorphous b-LHB packing motif: The respective layers are composed of unipolar orientations of the component asymmetric molecules, and the obtained unipolar layers form an alternating antiparallel alignment such that the alkyl chains (and phenyl rings) are in tail-to-tail (head-to-head) contact. The effect of substituting positions is apparently seen in the variation of the packing motif shown in Figure 3(b); the long axes of BTBT and BTNT cores are directed almost perpendicular to the layer (ab plane) in 1a and 2a, while they are tilted by about 20o to the normal of the layer in 1b and 2b (also see the inset in Figure 1(c)). All the crystals form typical herringbone packing motifs within the layers, as presented in Figure 4(a). We also estimated dihedral (herringbone) angles between -electron cores of neighboring molecules with T-shaped contacts; 49° in 1a, 38° in 1b, 51° in 2a, 42° in 2b (see Table S3). The change of the substituting positions from 1a and 2a to 1b and 2b causes lattice constants to be slightly expanded along the slipped-parallel intermolecular contact along a-axis, and to be slightly shrunk along the side-by-side (or T-shaped) intermolecular contact (along b-axis for all), as is originated from the tilt in long axes of -electron skeletons. The calculated intermolecular transfer integrals are summarized in Figure 4(b). The intermolecular transfer integrals are comparable with each other along the slipped-parallel and the T-shaped contacts for all the compounds. This trend is characteristic of the herringbone packings and is advantageous for obtaining two-dimensional carrier transport as channels of OTFTs. We also notice in Figure 4(b) that 2b presents slightly different anisotropy from 1a and 2a; the slipped-parallel contact (t1 and t4) is smaller than T-shaped contact (t2, t3, t5 and t6) in 2b, and the similar trend is seen in 1b. In contrast, the slipped-parallel contact is larger than the T-shaped contact in 1a and 2a. The decreased slipped-parallel contact in 2b (and 1b) can be attributed to the slightly expanded unit cell length (Table 1). Furthermore, we notice that the increase of T-shaped contacts depends on the direction in 2b; t5 (t6) is about two times larger than t2 (t3). Such a notable feature should be ascribed to the asymmetric nature of the BTNT skeletons: Figure 5(a) presents short intermolecular contacts in the case of t5 (t6) and t2 (t3). We labeled the sulfur atoms within the naphthothiophene unit as S1 and within the benzothiophene unit as S2, respectively



(see Figure 5(b)). The S1 has closer contact with -conjugated plane of the neighboring BTNT skeleton, leading to the increase of t5 (t6). We conclude that the large difference between t5 (t6) and t2 (t3) is attributable to the change of molecular orbital in BTNT skeleton, where the S1 has larger amplitude of molecular orbital than S2, as presented in Figure 5(c). We conduct empirical-force-field-based calculations to estimate the cohesive energies, based on the obtained crystal structures, to gain the insight into the substitution effects.17 The calculated cohesive energies of the -electron cores (PhBTBT or Ph-BTNT) are presented in Figure 1(c). The obtained trends for the cohesive energies by the MD calculations are clearly consistent with the experimental results of the solvent solubility and thermal stability. It is most probable that the tilt of the long axes of the -electron cores, by the change of the substituting positions in 2b (1b) as compared to 2a (1a), should lead to the higher solubility and lower thermal stability, through the relaxation of the cohesive nature between the OSC molecules. The discussions as presented above are also basically consistent with the appearance of liquid-crystal phases in 2a (1a), but not in 2b (1b). According to the Onsager’s hard-rod model;55 the appearance of liquid crystallinity depends on the ratio between the overall length and diameter of the component molecules. The substituting positions in 2b (1b) should work as the source for bending positions for the rod-like molecules, which eventually lead to the considerable suppression of the liquid crystallinity. Thin-film transistor characteristics. Figure 6 presents the device characteristics of OTFTs56 composed of the spin-coated films of all the compounds except for 2a; it was not possible to fabricate spin-coated films in 2a due to the insufficient solubility. The average field-effect mobility of 2b over ten devices in the linear regime (VD = −1V) was estimated as 2.8 cm2/Vs with small deviation ( = 15%) and stable turn-on voltage between 5 and 10 V and subthreshold swing of 1.6 V/dec. The estimated values for all the compounds are summarized in Table 3. It is found that 2b affords higher mobility, lower threshold voltage, and smaller subthreshold swing. The results indicate that shallow and deep trap densities are suppressed in 2b. It was reported that 1a affords mobility higher than 10 cm2/Vs, when the films are spin-coated at high temperature and are followed by successive quenching and thermal treatment; it was discussed to be necessary to produce liquid crystalline precursor thin film at elevated temperature (~ 110oC). In contrast, the spin-coated films of 2b can be fabricated at around room temperature, the feature of 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 seen, the observed diffraction pattern feature exhibit considerable variation by the thermal treatment. We find that the d-spacing of the films after thermal treatment is roughly estimated as 5.28 nm, as is consistent with the lattice constant of c-axis for the bilayer-type molecular packing shown in Table 1. In sharp contrast, the d-spacing of the as-prepared spin-coated films is clearly half of the above and is estimated as 2.68 nm. It indicates that the as-prepared polycrystalline films of 2b are mainly composed of microcrystals with single-layer periodicity. It is interesting to point out that the similar variation of the d spacing by thermal treatment is also observed for 1a9. It implies that both 1a and 2b should present rapid-crystallization (or quenching) phases that have single-layer periodicity by the spin coating, although the detailed packing motif is unknown.

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Figure 8(a) and 8(b) present optical-microscope and AFM images of the spin-coated films of 2b after the thermal treatment. The films are composed of step-and-terrace structure with large-area terraces and a step height of about 5 nm that corresponds to the bilayer thickness. We also observe a single-layer step with a step height of about 2.5 nm in the AFM image. Figure 9(a) 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 140oC causes a clear improvement of the device characteristics in the subthreshold region (but not sufficiently at 100oC and 120oC): The turn-on voltage (Von) shifts from about +20 V to less than +10 V, and subthreshold swing becomes also lowered. The estimated values are summarized in Table 4. The effect of the thermal treatment on the device characteristics should be clearly associated with the observed drastic change in the thin-film x-ray diffractions as well as with the slight endothermic and exothermic peaks as observed at around 120°C in the DSC curves. Considering the above results, we conclude that the thermal treatment of 2b at 140°C should allow the structural conversion from the quenched phase with singlelayer periodicity to the b-LHB packing phase, where the residual minority holes and shallow traps are effectively suppressed as observed in the device characteristics. Figure 9(b) and 9(c) summarizes thermal durability of the OTFT characteristics for the spin-coated films of 2b. The mobility was kept above 2 cm2/Vs even after the thermal stress was applied at 160°C. All the results demonstrate that 2b is highly promising to afford solution-processable and thermally durable bottom-contact OTFTs operated at low voltages. We also manufactured single-crystal thin films of 2b by the blade-coating technique. We found from the x-ray diffraction measurements that the film is composed of the b-LHB packing as shown in Table 1 without any thermal treatment. Figure 10(a) and 10(b) present the typical transfer and output characteristics of the single-crystal OTFT. The mobility reaches as high as 6.3 cm2/Vs in the saturation regime, whereas the threshold voltage is as high as −15.6V. The estimated device performance is summarized in Table 5. The large negative threshold voltage in the single-crystal OTFT should be ascribed to the high trap density at the semiconductor/gate dielectric interface. It is interesting to point out that the threshold voltage of thin-film devices is positive as presented in Table 4. The results imply that there must be a built-in mechanism to generate a large number of minority holes in the spin-coated films of 2b, as probably associated with the existence of quenched phase.

□CONCLUSIONS We have successfully developed excellent small-molecule printable organic semiconductor of 2b that exhibits both high solvent solubility and thermal durability in the device characteristics. These properties are achieved by the modified substituting positions on the BTNT skeleton by the phenyl group and the alkyl chain, which slightly modulate the intralayer herringbone packing motif and thus the cohesive energy, and also effectively suppress the appearance of liquid crystal phases at high temperature. We demonstrate that the effect of substituting positions is commonly observed both in alkylated Ph-BTBT and BTNT derivatives, and that the effect can be ascribed to the bending of rod-like OSC molecules, which effectively leads to the modulation of structural characteristics of b-LHB packing. The spin coating of 2b under ambient conditions affords highperformance bottom-contact OTFTs that show enough high thermal durability below 150°C. The initial thermal treatment of the spin-coated films allows the structural conversion from


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the quenched phase with single-layer periodicity to the b-LHB packing phase. Thus, the treatment effectively improves the subthreshold characteristics, probably due to the suppression of residual minority holes and shallow traps in the OSC films. These findings should be crucial in the design and development of organic semiconductor materials into the printed electronics applications.

ASSOCIATED CONTENT Supporting Information Experimental details, evaluation of solubility, measurement of thermal property, crystallographic data (cif), and method of the force-field based calculation. The Supporting Information is available free of charge on the ACS Publications website at DOI:××

Corresponding Authors E-mail: [email protected]

*(T.H.)

(5) Holliday, S.; Donaghey, J. E.; McCulloch, I. Advances in Charge Carrier Mobilities of Semiconducting Polymers Used in Organic Transistors. Chem. Mater. 2014, 26, 647-663. (6) Mei, J.; Diao, Y.; Appleton, A. L.; Fang, L.; Bao, Z. Integrated Materials Design of Organic Semiconductors for Field-Effect Transistors. J. Am. Chem. Soc. 2013, 135, 6724-6746. (7) Kang, B.; Lee, W. H.; Cho, K. Recent Advances in Organic Transistor Printing Processes. ACS Appl. Mater. Interfaces 2013, 5, 2302-2315. (8) Hamai, T.; Arai, S.; Minemawari, H.; Inoue, S.; Kumai, R.; Hasegawa, T. Tunneling and Origin of Large Access Resistance in Layered-Crystal Organic Transistors, Phys. Rev. Appl. 2017, 8, 054011:1-12.

AUTHOR INFORMATION *(S.I.)

(4) Sirringhaus, H. Organic field-effect transistors: The path beyond amorphous silicon. Adv. Mater. 2014, 26, 1319– 1335.

E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This study was partly supported by Grants-in-Aid for Scientific Research (A) (Grant No. 18H03875), for Young Scientist (B) (Grant No. 17K14370), and on Innovative Areas (Grant No. 17H05144) from the Japan Society for the Promotion of Science (JSPS). S. A. also thanks to the supporting from Nanotech CUPAL from JST. The synchrotron X-ray experiment was performed with the approval of the Photon Factory Program Advisory Committee (2014S2-001, and 2017S2-001). The thin-film x-ray diffraction experiments were conducted at Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by "Nanotechnology Platform" of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

(9) Iino, H.; Usui, T.; Hanna, J. Liquid crystals for organic thin-film transistors. Nat. Commun. 2015, 6, 6828. (10) Yuan, Y.; Giri, G.; Ayzner, A. L.; Zoombelt, A. P.; Mannsfeld, S. C. B.; Chen, J.; Nordlund, D.; Toney, M. F.; Huang, J.; Bao, Z. Ultra-high mobility transparent organic thin film transistors grown by an off-centre spincoating method. Nat. Commun. 2014, 5, 3005. (11) Okamoto, T.; Mitsui, C.; Yamagishi, M.; Nakahara, K. Soeda, J.; Hirose, Y.; Miwa, K.; Sato, H.; Yamano, A.; Matsushita, T.; Uemura, T.; Takeya, J. V-Shaped Organic Semiconductors With Solution Processability, High Mobility, and High Thermal Durability. Adv. Mater. 2013, 25, 6392-6397. (12) Nakayama, K.; Hirose, Y.; Soeda, J.; Yoshizumi, M.; Uemura, T.; Uno, M.; Li, W.; Kang, M. J.; Yamagishi, M.; Okada, Y.; Miyazaki, E.; Nakazawa, Y.; Nakao, A.; Takimiya, K.; Takeya, J. Patternable Solution-Crystallized Organic Transistors with High Charge Carrier Mobility. Adv. Mater. 2011, 23, 1626-1629. (13) Minemawari, H.; Yamada, T.; Matsui, H.; Tsutsumi, J.; Haas, S.; Chiba, R.; Kumai, R.; Hasegawa, T. Inkjet printing of single-crystal films. Nature 2011, 475, 364367.

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9

8

6

7

S

4

5

12

11

3

1

S

8

2

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2 6

S

4

5

BTBT

3

BTNT

S

S

C10H21 S

C6H13 S

1a

2a

S S

S C10H21

1b

S

C10H21

2b

Scheme 1. Chemical structure of BTBT derivatives 1 and BTNT derivatives 2.

Table 1. Crystallographic data of 1a, 1b, 2a, and 2b. Compounds 1aref.17 1b Chemical formula C30H32S2 C30H32S2 Formula weight 456.71 456.71 Crystal system Monoclinic Monoclinic a (Ǻ) 6.0471 (3) 7.029 (3) b (Ǻ) 7.7568 (4) 7.391 (2) c (Ǻ) 53.124 (5) 48.221 (17) 93.135 (3) 92.414 (12)  (deg) V (Ǻ3) 2488.1 (3) 2502.9 (15) Space group P21/a (No.14) P21/a (No.14) Z value 4 4 Temperature (K) 300 − Radiation MoK ( = 0.71075 Ǻ) − No. of reflections 5727 − No. of variables 290 − Residuals: R 0.0838 (I>2) − Residuals: wR2 0.1330 − (all reflections) Goodness of 0.908 − fit indicator

Table 2. HOMO levels of 1b, 2a, and 2b. Compounds 1b 2a Energy level / eV 5.29 4.97 (calculated) (5.55) (5.20)

2a 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 MoK ( = 0.71075 Ǻ) 5208 289 0.0497 (I>2)

2b C34H34S2 506.76 Monoclinic 6.931 (10) 7.406 (11) 53.13 (8) 91.153 (8) 2727 (7) P21/a (No.14) 4 300 MoK ( = 0.71075 Ǻ) 6054 325 0.1009 (I>2)

0.1333

0.2417

0.681

0.927

2b 5.01 (5.26)


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Figure 1. (a) Phase transition temperatures (dashed lines) and melting point (full lines) of 1a, 1b, 2a, and 2b determined by DSC measurements. (b) Their solubilities in oxylene at room temperature. (c) Changes of cohesive energies per molecules of Ph-BTBT or Ph-BTNT units by forcefield-based calculations. Figure 2. DSC curves of 1a17, 1b, 2a, and 2b. Scan rate is 5K/min (1st cycle of 2a and 2b is 10K/min).

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Figure 3. Crystal packing structure of 1a17, 1b, 2a, and 2b, projected to (a) the b-c plane (a-b plane), and (b) the a-c plane.

Figure 4 (a) Intralayer molecular arrangements of 2a and 2b. (b) Calculated intermolecular transfer integrals between the neighboring molecules along the slipped-parallel contacts (t1 and t4) and T-shaped contacts (t2, t3, t4, and t5) for 1a17, 1b, 2a, and 2b.

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Figure 5. (a) Appearance of shortest sulfur-carbon contacts of 2b along the two different types of T-shaped intermolecular contacts. (b) Chemical structure of BTNT skeleton. (c) HOMO of BTNT obtained by DFT calculation.

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Figure 6. (a)(b) Transfer characteristics of OTFTs with spin-coated films of 1a, 1b, and 2b in the linear regime, and (d)(e) in saturation regime. (c)(f) A plot of mobility as a function of VG. (g)(h)(i) Output characteristics of OTFTs with films of 1a, 1b, and 2b. Table 3. OTFT parameters with films of 1a, 1b, 2a, and 2b, obtained by spin coating and successive thermal treatment. 1aa 1bb 2b Parameter 2ac (VD = −1V/−30V) (VD = −1V/−30V) (VD = −1V/−30V) Mobility (cm2/Vs) 0.04/0.07 0.24/0.26 NDd 2.8/1.85 Vth (V) 7.1/8.1 ND 8.9/9.8 −1.4/−10.6 Ion/Ioff 104/105 105/106 ND 105/107 Subthreshold 2.11/1.72 1.87/1.62 ND 1.63/1.49 Sweep(V/dec.) aFilm of 1a was fabricated the same procedure as 2b. bThermal treatment is not possible for 1b due to the low thermal durability above 80 oC. cFilm of 2a was not obtained due to its low solubility. dND = not determined.

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Figure 7. Out-of-plane X-ray diffraction profile of a spincoated films of 2b, (a) before and (b) after thermal treatment at 140 oC for 10min.

Figure 8. (a) An optical microscope image for a film of 2b, obtained by spin coating and successive thermal treatment. (b) An AFM image and a cross-sectional profile for a film of 2b.

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Figure 9. (a) Effect of initial thermal treatment on the transfer characteristics, measured at VD = −1V, of an OTFT with a film of 2b. (b) Changes of the transfer characteristics, measured at VD = −1V, of 2b OTFTs after the application of thermal stress at various elevated temperatures for 5 min. (c) The changes of OTFT mobility, plotted as a function of thermal stress temperature.

Table 4. Effect of initial thermal treatment on the OTFT parameters of spin-coated films of 2b, measured at VD = −1V. Thermal treat- Mobility Standard deviaVth Ion/Ioff Subthreshold ment temp. (oC) (cm2/Vs) tion (%) (V) Sweep (V/dec.) 104 100 2.5 18 (8TFTs) 17.8±1.0 2.65 105 120 2.5 26 (7TFTs) 14.9±1.7 2.06 105 8.9±1.2 140 2.8 15 (12TFTs) 1.63

Table 5. OTFT parameters with single-crystal thin film of 2b, measured at VD = −1V. SubMobility Vth threshod Ion/Ioff (cm2/Vs) (V) Sweep (V/dec.) 6.3 15.6 106 1.37

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Figure 10. (a) Transfer characteristics and (b) output characteristics of a single-crystal OTFT of 2b. (c) Crossed-Nicols polarized micrographs of a single-crystal thin film of 2b.

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TOC Figure

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Extended and Modulated Thienothiophenes for Thermally Durable

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