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Soluble Organic Semiconductor Precursor with Specific Phase Separation for High-Performance Printed Organic Transistors Yu Kimura, Takashi Nagase,* Takashi Kobayashi, Azusa Hamaguchi, Yoshinori Ikeda,* Takashi Shiro, Kazuo Takimiya, and Hiroyoshi Naito* There is considerable interest in the development of organic thin-film transistors (OTFTs)[1–3] with high electrical performance because of their potential use in various organic devices, including flexible displays, radio-frequency identification tags, and large-area sensors. OTFTs based on vacuum-deposited pentacene thin films have achieved field-effect mobilities comparable to or higher than those of hydrogenated amorphous silicon (a-Si:H) TFTs at around 1 cm2 V−1 s−1,[4–6] and progress has also been made in the development of high-mobility soluble organic semiconductors, including small molecules[7–11] and polymers,[12,13] over the last decade. As OTFTs move closer to practical use, the instability of organic semiconductors in ambient air becomes a critical issue. For example, pentacene is easily oxidized to form 6,13-pentacenequinone, which leads to reduced field-effect mobility,[14] and a typical polymer semiconductor such as poly(3-hexylthiophene) often suffers carrier doping by oxygen and moisture,[3] which makes it difficult to fabricate OTFTs with low off-state currents under ambient conditions. One promising strategy for enhanced air stability is to increase the ionization potentials of organic semiconductors, and dinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]thiophene (DNTT) has attracted increasing interest because it has a higher ionization potential (5.4 eV) than pentacene (5.0 eV) and forms excellent
Y. Kimura, Dr. T. Nagase, Dr. T. Kobayashi, Prof. H. Naito Department of Physics and Electronics Osaka Prefecture University 1-1 Gakuen-cho, Naka-ku, Sakai 599-8531, Japan E-mail: [email protected]; [email protected] Dr. T. Nagase, Dr. T. Kobayashi, Prof. H. Naito The Research Institute for Molecular Electronic Devices Osaka Prefecture University 1-1 Gakuen-cho, Naka-ku, Sakai 599-8531, Japan A. Hamaguchi, Dr. Y. Ikeda, T. Shiro Electronics Materials Development Project Teijin Limited 4-3-2 Asahigaoka, Hino, Tokyo 191-8512, Japan E-mail: [email protected] Prof. K. Takimiya Department of Applied Chemistry Hiroshima University 1-3-2 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan Prof. K. Takimiya Center for Emergent Matter Science, RIKEN 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
DOI: 10.1002/adma.201404052
Adv. Mater. 2014, DOI: 10.1002/adma.201404052
thin-film microstructures with high field-effect mobility (>1 cm2 V−1 s−1).[15–18] The long-term stability of DNTT TFTs in ambient air has been demonstrated,[16] and recent reports have shown the potential of DNTT for heat-resistant OTFTs.[17] Despite these advantages, DNTT molecules have very low solubility in organic solvents because of their high crystallinity, even when soluble alkyl chains are attached to the molecules,[19] and do not allow DNTT TFT production using conventional solution techniques. In this communication, we report a novel soluble DNTT precursor (5,14-N-phenylmaleimidedinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]thiophene, Figure 1a) that allows the fabrication of high-performance DNTT TFTs using simple solution processes in ambient air. The DNTT precursor enables thin-film formation with excellent microstructures and inherent air stability and can induce specific phase separations when blended with an inert polymer, allowing enhancement of the film crystallinity at interfaces with hydrophobic substrates. We demonstrate that DNTT TFTs processed from a solution of the DNTT precursor blended with a polymer exhibit high field-effect mobilities of up to 1.1 cm2 V−1 s−1 and excellent durability against air exposure and thermal stress. The synthesis of the DNTT precursor was carried out based on a Diels–Alder reaction of DNTT and N-phenylmaleimide, from which a mixture of endo- and exo-isomers (typical ratio of 35.3:64.7) was obtained (for details, please see the Experimental Section). These isomers of the DNTT precursor can be dissolved in organic solvents (e.g., chloroform, dichlorobenzene, or dimethylacetamide) at room temperature because of their low crystallinity, and can be converted into DNTT by thermal annealing above 195 °C through the elimination of N-phenylmaleimide; this was confirmed by thermogravimetric analysis (Figure S8, Supporting Information). A polycrystalline DNTT thin film can be obtained by forming a thin film of the soluble DNTT precursor on a substrate with subsequent annealing at around 200 °C in ambient air. UV– vis absorption measurements show the thermal conversion of the precursor film into the DNTT film in air; the annealed film shows an absorption spectrum that is identical to that of DNTT films processed by vacuum deposition (Figure S9, Supporting Information). To improve the crystallinity of the organic semiconductor films, coating of the organic solutions on hydrophobic substrates is indispensable because the organic semiconductor crystallization process is generally enhanced on substrates with lower surface energies. However, solution-processed DNTT films on hydrophobic substrates often suffer from dewetting, which does not allow continuum film
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Figure 1. a) Chemical structure of the soluble DNTT precursor and its thermal conversion process. b) Polarized optical micrographs, c) out-of-plane XRD patterns, and d) in-plane XRD patterns of DNTT films processed from the precursor on UV/O3-treated Si/SiO2 substrates and DNTT/PS films processed from blends of the precursor and PS with weight ratios of 2:1, 1:1, and 1:2 on OTS-treated Si/SiO2 substrates.
formation. Dewetting problems have frequently been reported in solution processes using soluble organic semiconductors,[20,21] and several film-forming techniques have been developed.[20–23] The most attractive technique uses the vertical phase separation behavior[22,23] that appeared in solution-processed blend films of a soluble small-molecule semiconductor and an insulating polymer, because both excellent film-forming properties and good electrical performance can be achieved using conventional coating and printing processes. Investigation of the capabilities of vertical phase separation in organic films processed from a blend of the soluble DNTT precursor with an insulating polymer is thus of great interest for practical use in printed devices. The blend films were typically processed from the DNTT precursor blended with a representative insulating polymer, polystyrene (PS). The DNTT precursor and the PS were mixed in different weight ratios and dissolved in chloroform at approximately 0.15 wt%. The mixed solutions were drop cast on Si/SiO2 substrates modified with a hydrophobic 2
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self-assembled monolayer (SAM) of octadecyltrichlorosilane (OTS), and the resulting films were subsequently annealed at 200 °C for 10 min in air (the annealing conditions were optimized in TFT experiments). Polarized optical micrographs of films made from the DNTT precursor blended with PS with weight ratios of 2:1, 1:1, and 1:2 on the OTS-treated substrates are shown in Figure 1b, together with an image of the film made from DNTT precursor on UV/ ozone (UV/O3)-treated substrates. The dewetting of the DNTT precursor films on the OTS-modified surfaces is improved remarkably by the addition of PS. The blend films show strong birefringence, indicating the formation of crystalline DNTT grains in the films. The out-of-plane and in-plane X-ray diffraction (XRD) patterns of these films are shown in Figure 1c,d, respectively. The out-ofplane XRD patterns show distinct Bragg reflection peaks at 5.5° and 16.4° and blending of the DNTT precursor with PS does not affect the angles of the diffraction peaks (Figure 1c). The diffraction angles obtained agree well with those of vacuum-deposited
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DNTT films.[15,17] These results indicate that DNTT molecules form well-ordered crystalline grains, in which the long axis of the DNTT molecules preferentially orients perpendicularly on the substrate surface (edge-on molecular orientation). It is also shown that the in-plane XRD pattern of the blend film made from DNTT precursor and PS with a 2:1 ratio on the OTStreated substrate exhibits more intense and narrower diffraction peaks when compared with those of a film made from DNTT precursor on the UV/O3-treated substrate (Figure 1d). This indicates that the DNTT molecules in the blend film crystallize highly to adopt herringbone packing structures parallel to the substrate surface. The improved crystallinity in the blend film is attributable to segregation of the DNTT molecules to the surface of the substrate, because it permits effective crystallization of DNTT molecules using low-energy hydrophobic surfaces. We stress that the formation of both well-ordered edge-on molecular orientation with the surface normal and 2D herringbone packing in the substrate in-plane direction are essential for achieving high field-effect mobility for OTFT applications because π-stacked molecules along the substrate surface provide efficient lateral carrier transport. To confirm formation of the phase-separated structures in the solution-processed DNTT/PS blend films, vertical profiles were investigated via scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) combined with energy dispersive X-ray spectroscopy (EDS). The DNTT and SiO2 dielectric layers can be detected as EDS signals for sulfur (S) and silicon (Si), respectively. Figure 2a,b show crosssectional images from SEM and the element mapping of S of the blend film made from the DNTT precursor and PS before thermal annealing. The S signals are detected from the whole
area of the nonannealed blend film, suggesting that the DNTT precursor molecules are uniformly dispersed in the organic film before thermal annealing. We found that the thermal annealing of the blend film leads to the formation of a distinct layered structure in the vertical direction. Cross-sectional SEM image of the annealed blend film is shown in Figure 2c, in which the formation of a double-layered structure is observed. Figure 2d,e show high-resolution cross-sectional STEM and S mapping images of the annealed blend film, and the vertical profiles of concentrations of S, carbon (C), and Si, along the red line indicated in Figure 2d, are shown in Figure 2f. It can be seen that the S signal from DNTT immediately rises as the profiling reaches the bottom organic layer and drops sharply at the interface with the SiO2 layers. These results clearly show that the DNNT molecules segregate to the interface with the substrate in the annealed blend film and that substrate-mediated molecular ordering is responsible for the improved crystallinity of DNTT/PS blend films on the OTS-treated substrates. It is noted that phase separation behavior observed in the DNTT precursor/polymer blend films slightly differs from that reported in the blend films of a soluble small-molecular semiconductor and an insulating polymer,[22,23] where much of phase separation occurs during coating of the films. In the case of the DNTT precursor/ polymer blend films, the formation of phase-separated structures is caused by thermal annealing, during which the conversion of the DNTT precursor into DNTT and the crystallization of DNTT simultaneously occur. We believe that such moderate crystallization process can allow structural rearrangement prior to structural fixation, which results in
Figure 2. a) Cross-sectional images obtained from SEM and b) element mapping of S in the blend film of the DNTT precursor and PS with a weight ratio of 2:1 on OTS-treated Si/SiO2 substrates before thermal annealing. c) Cross-sectional images obtained from SEM, d) STEM, and e) element mapping of S in the blend film after thermal annealing at 200 °C for 10 min. f) Vertical profiles of concentrations of S, C, and Si along the red line indicated in Figure 2d from the top to the bottom.
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Figure 3. a) Schematic of the device structure of bottom-gate/top-contact OTFTs. b) Typical output characteristics and c) transfer characteristics of OTFTs processed from the DNTT precursor/PS blend (2:1) on OTS-treated Si/SiO2 substrates. A drain voltage (VD) of −100 V was applied when the transfer characteristics were measured. d) Variation in the transfer characteristics of the OTFT processed from the DNTT precursor/PS blend (2:1) on the OTS-treated Si/SiO2 substrate after air exposure for 1100 h. For measurement of the transfer characteristics, a VD of −50 V was applied. e) Transfer characteristics of OTFTs processed from the DNTT precursor/PS blend (2:1) on HMDS-treated and UV/O3-treated Si/SiO2 substrates. f) Field-effect mobility of the OTFT on the OTS-treated substrate as a function of annealing temperature. The mobilities were measured at room temperature in air.
the formation of highly-crystallized DNTT film in the blend films. OTFT devices were fabricated with annealed blend films in a bottom-gate/top-contact configuration with gold source and drain electrodes on the Si/SiO2 substrates, as shown in Figure 3a. The OTFT fabrication and electrical measurement processes are described in the Experimental Section. The typical output and transfer characteristics of OTFTs processed from the 2:1 DNTT precursor/PS blend on OTS-treated Si/ SiO2 substrates are shown in Figure 3b,c, respectively. The OTFTs based on DNTT/PS blend films show good transistor performance with a high on/off current ratio of over 106, a low threshold voltage of less than −10 V and negligible hysteresis in the transfer characteristics between the forward and backward scans of the gate voltage. Also, the electrical characteristics do not show any notable degradation, even after the device is exposed to air for 1100 h (Figure 3d) and heated at 90 °C for 500 h in air (Figure S10, Supporting Information). The electrical performance of OTFTs based on DNTT/ polymer blend films depends on the surface treatment of the substrates, the film annealing temperature and time (Figure S11, Supporting Information), and the concentration and type of insulating polymers to be added (Figures S12, S13, and S14, and Table S1, Supporting Information), but field-effect mobilities of more than 0.2 cm2 V−1 s−1 are routinely obtained in devices fabricated on OTS-treated substrates with annealing temperatures of less than 200 °C. Table 1 summarizes the transistor performances of devices with different surface treatments that were processed under optimized annealing
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conditions (200 °C and 10 min). The field-effect mobility is enhanced remarkably by substrate treatment with SAMs, and devices fabricated on OTS-treated substrates exhibit higher mobility than devices fabricated on hexamethyldisilazane (HMDS)-treated substrates. The observation of such a mobility trend clearly shows that the crystallization process of the DNTT molecules in the blend film is dominated by the substrate surface energy. In contrast to the devices fabricated on SAMtreated substrates, higher hysteresis is observed in devices fabricated on UV/O3-treated Si/SiO2 substrates (Figure 3e). This hysteresis is believed to be caused by long-lifetime trapping of electrons by hydroxyl (OH) groups at the SiO2 surfaces,[3] which can be suppressed by terminating the OH groups with SAMs. These results demonstrate that device fabrication on hydrophobic substrates using blends of DNTT precursor and PS is essential to improve not only their field-effect mobilities but also their electrical stability. The average field-effect Table 1. Electrical performance of solution-processed OTFTs using blend films made from DNTT precursor and PS with weight ratio of 2:1 on Si/SiO2 substrates treated with OTS, HMDS, and UV/O3. Data were obtained from more than nine devices. Surface treatment
Field-effect mobility Threshold voltage [cm2 V−1 s−1] [V]
On/off ratio
OTS
0.76–1.1
−5.7 ± 0.56
106–107
HMDS
0.30–0.63
2.0 ± 1.1
105–106
UV/O3
0.085–0.13
11 ± 1.6
104–105
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Adv. Mater. 2014, DOI: 10.1002/adma.201404052
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Experimental Section Synthesis of DNTT Precursor: A mixture of DNTT (500 mg, 1.47 mmol), N-phenylmaleimide (2.54 g, 14.7 mmol), and hydroquinone (16.2 mg, 0.147 mmol) in mesitylene (100 mL) were stirred for 4 h at 160 °C under a nitrogen atmosphere. The DNTT was removed by filtration, and the residue was purified by column chromatography on a silica gel eluted with chloroform (Rf = 0.37 stereoisomer A and Rf = 0.47 stereoisomer B). 5,14-N-Phenylmaleimidedinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]thiophene (DNTT precursor) with mass of 113.2 mg (Mw = 513.63, yield 15.0 mol%) was obtained. The ratio of the stereoisomers was determined to be A:B = 51.7:48.3 mol% by nuclear magnetic resonance (NMR). A minor
Adv. Mater. 2014, DOI: 10.1002/adma.201404052
compound (Rf = 0.48 stereoisomer C, approximately 1 mol%) was also observed in this synthesis. The minor compound was determined to be 1,4-N-phenylmaleimidedinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]thiophene by NMR spectroscopy. The stereoisomers A and B were separated into the endo-form (stereoisomer A) and the exo-form (stereoisomer B) by high-performance liquid chromatography (Agilent 1100 Series HPLC, Shiseido Capcell Pak C18 Type UG120, solvent:acetonitrile/ water) and the structures of stereoisomers A and B were assigned by X-ray crystallographic analysis. The conformation of stereoisomer C was determined by 1D 1H nuclear overhauser effect difference spectroscopy. The mixture of stereoisomers A and B was repurified only by simple recrystallization from chloroform for OTFT fabrication, and the final ratio of these stereoisomers was A:B = 35.3:64.7. More detailed information including NMR and X-ray crystallographic data is described in the Supporting Information. Materials: PS (average molecular weight (Mw): ~280,000) and poly(methyl methacrylate) (average Mw: ~100,000) were purchased from Sigma-Aldrich and Polymer Science, respectively, and were used as-received. For polycarbonate, Teijin Panlite L-1250 (average Mw: ~60,800) was used. For silane coupling agents for the SAM treatment, OTS (Shin-Etsu Chemical LS6495) and HMDS (Shin-Etsu Chemical LS7150) were used. OTFT Fabrication and Characterization: Discrete OTFTs were fabricated on a heavily doped n-type Si wafer with a thermally-grown silicon dioxide (SiO2) insulator layer (1000 nm thickness). The substrates were washed sequentially with acetone and 2-propanol in ultrasonic baths and were then cleaned with an UV/O3 cleaner to remove any organic contaminants. The substrate surfaces were chemically modified with SAMs of OTS or HMDS. To form the SAMs, the substrates were immersed in ~10 mM SAM solutions in toluene at room temperature overnight in a capped bottle. After the substrates were removed from the SAM solution, they were rinsed with fresh toluene to remove any excess layers. For the semiconductor solution, the DNTT precursor and PS were mixed in chloroform at a weight concentration of ca. 0.15% at room temperature in ambient air. The organic solution was then drop cast on the substrates and the resulting films were subsequently annealed at 200 °C for 10 min in air on a hotplate. Then, gold source and drain electrodes were fabricated on the films by vacuum evaporation through a metal shadow mask to complete the OTFT fabrication process. The channel length (L) and channel width (W) were 200 µm and 1 mm, respectively. The OTFT electrical characteristics were measured at room temperature in the dark and in ambient air using a semiconductor parameter analyzer (Keithley Instruments 4200-SCS). The field-effect mobility and threshold voltage were calculated from the transfer characteristics measured in the saturation regime using the following standard TFT equation: ID =
μC iW (VG − Vth )2 2L
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mobility of optimized devices fabricated on OTS-treated substrates is 0.96 cm2 V−1 s−1 and the maximum mobility reaches 1.1 cm2 V−1 s−1. These mobilities are close to those of DNTT TFTs fabricated by vacuum deposition on OTS-treated Si/SiO2 substrates (1.6–2.9 cm2 V−1 s−1).[15] It is also noted that the maximum mobility of our OTFTs is the highest reported to date for OTFTs based on soluble precursors, including pentacene precursors,[8] for which maximum mobility of 0.89 cm2 V−1 s−1 was achieved after device fabrication under a nitrogen atmosphere. In addition to the excellent carrier transport properties, our approach enables solution processing of DNTT TFTs with high thermal stress durability. Figure 3f shows the field-effect mobility as a function of annealing temperature. The mobilities were measured after the device was annealed at each temperature for 10 min in air and then cooled to room temperature. The field-effect mobility is almost unchanged up to 200 °C. This is in sharp contrast to the thermal stability of vacuum-deposited DNTT TFTs,[17] where the field-effect mobility is significantly degraded by heat treatments at more than 150 °C, because of changes in the packing structure for DNTT molecules in the thin films.[17] The enhanced thermal stability in our devices can be attributed to the distinct layered structures formed in the DNTT/PS blend films, as shown in Figure 2. The results indicate that the spontaneous lamination of the DNTT channel layer by the PS layer greatly reduces the changes in the packing structure, which is qualitatively consistent with the thermal stability reported for OTFTs encapsulated using parylene passivation layers.[17,24] It should be noted that spontaneous formation of the laminated OTFT structures by solution processes is technologically useful, because the solution processing of laminated layers sometimes suffers from dissolution of the underlying semiconductor layers. In summary, we have developed a novel soluble DNTT precursor for solution-processable high-performance OTFTs with good environmental stability. We found that the blend film of the DNTT precursor and electrically inert polymers forms a distinct phase-separated DNTT/polymer structure by thermal annealing, which improves both the film-forming characteristics and the electrical performance of OTFTs. Solutionprocessed DNTT TFTs based on these semiconductor blends exhibit high field-effect mobilities of up to 1.1 cm2 V−1 s−1 and highly stable operation after ambient air exposure for over 1 month and after thermal stress at temperature of up to 200 °C. We believe that the materials and techniques developed here offer new strategies to produce highly reliable flexible electronic devices by low-cost printing processes.
(1)
where ID is the drain current, µ is the field-effect mobility, Ci is the capacitance per unit area for the gate insulator, VG is the gate voltage, and Vth is the threshold voltage. In the air exposure experiment, the OTFT devices were stored in the dark and in ambient air for each period. Film Structure Analysis: DNTT films and DNTT/PS blend films were prepared for structural analyses on UV/O3-treated and OTS-treated Si/SiO2 substrates, respectively, in the same manner as for the OTFT fabrication processes, but without deposition of the Au electrodes on top. Polarized optical micrographs of the DNTT films and DNTT/PS blend films were obtained using a Nikon Eclipse LV100POL polarizing microscope. Out-of-plane and in-plane XRD measurements were conducted under ambient conditions using a Rigaku Raxis Rapid imaging plate area detector with graphite monochromated Cu–Kα radiation (2θmax = 136.4°). SEM and STEM images and element profiles of the cross-sectional structures of the blend films were measured using
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www.MaterialsViews.com a Hitachi FE-SEM S-5200 and a JEOL JEM-2100F equipped with EDS detectors. The samples for SEM and STEM observations were prepared by freeze fracture and ion milling techniques, respectively.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors would like to thank the Nippon Kayaku Corporation for providing the DNTT material. They also thank Midori Kamimura, Matsumoto Yoshiyuki, and Takahiro Takeuchi, of the Teijin Pharma Limited, for performing X-ray single crystal structural analysis and HPLC analysis. H.N., T.N., and T.K. would like to acknowledge partial funding from a Grant-in-Aid for Scientific Research (B) (grant no. 23360140) from the Japan Society for the Promotion of Science and from a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-Blocks (No. 2401)” (grant no. 24102011) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. Received: September 3, 2014 Revised: October 17, 2014 Published online: [1] [2] [3] [4]
C. D. Dimitrakopoulos, P. R. L. Malenfant, Adv. Mater. 2002, 14, 99. G. Horowitz, J. Mater. Res. 2004, 19, 1946. H. Sirringhaus, Adv. Mater. 2005, 17, 2411. Y. Y. Lin, D. J. Gundlach, S. F. Nelson, T. N. Jackson, IEEE Electron Device Lett. 1997, 18, 606. [5] H. Klauk, M. Halik, U. Zschieschang, G. Schmid, W. Radlik, W. Weber, J. Appl. Phys. 2002, 92, 5259. [6] T. W. Kelly, L. D. Boardman, T. D. Dunbar, D. V. Muyres, M. J. Pellerite, T. P. Smith, J. Phys. Chem. B 2003, 107, 5877. [7] P. T. Herwig, K. Müllen, Adv. Mater. 1999, 11, 480.
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[8] A. Afzali, C. D. Dimitrakopoulos, T. L. Breen, J. Am. Chem. Soc. 2002, 124, 8812. [9] M. M. Payne, S. R. Parkin, J. E. Anthony, C.-C. Kuo, T. N. Jackson, J. Am. Chem. Soc. 2005, 127, 4986. [10] S. K. Park, T. N. Jackson, J. E. Anthony, D. A. Mourey, Appl. Phys. Lett. 2007, 91, 063514. [11] H. Ebata, T. Izawa, E. Miyazaki, K. Takimiya, M. Ikeda, H. Kuwabara, T. Yui, J. Am. Chem. Soc. 2007, 129, 15732. [12] I. McCulloch, M. Heeney, C. Bailey, K. Genevicius, I. MacDonald, M. Shkunov, D. Sparrowe, S. Tierney, R. Wagner, W. Zhang, M. L. Chabinyc, R. J. Kline, M. D. McGehee, M. F. Toney, Nat. Mater. 2006, 5, 328. [13] B. H. Hamadani, D. J. Gundlach, I. McCulloch, M. Heeney, Appl. Phys. Lett. 2007, 91, 243512. [14] H. Sirringhaus, Adv. Mater. 2009, 21, 3859. [15] T. Yamamoto, K. Takimiya, J. Am. Chem. Soc. 2007, 129, 2224. [16] U. Zschieschang, F. Ante, D. Kälblein, T. Yamamoto, K. Takimiya, H. Kuwabara, M. Ikeda, T. Sekitani, T. Someya, J. Blochwitz-Nimoth, H. Klauk, Org. Electron. 2011, 12, 1370. [17] K. Kuribara, H. Wang, N. Uchiyama, K. Fukuda, T. Yokota, U. Zschieschang, C. Jaye, D. Fischer, H. Klauk, T. Yamamoto, K. Takimiya, M. Ikeda, H. Kuwabara, T. Sekitani, Y.-L. Loo, T. Someya, Nat. Commun. 2012, 3, 723. [18] K. Takimiya, S. Shinamura, I. Osaka, E. Miyazaki, Adv. Mater. 2011, 23, 4347. [19] M. J. Kang, I. Doi, H. Mori, E. Miyazaki, K. Takimiya, M. Ikeda, H. Kuwabara, Adv. Mater. 2011, 23, 1222. [20] S. Yamazaki, T. Hamada, T. Nagase, S. Tokai, M. Yoshikawa, T. Kobayashi, Y. Michiwaki, S. Watase, M. Watanabe, K. Matsukawa, H. Naito, Appl. Phys. Express 2010, 3, 091602. [21] M. Ikawa, T. Yamada, H. Matsui, H. Minemawari, J. Tsutsumi, Y. Horii, M. Chikamatsu, R. Azumi, R. Kumai, T. Hasegawa, Nat. Commun. 2012, 3, 1176. [22] T. Ohe, M. Kuribayashi, R. Yasuda, A. Tsuboi, K. Nomoto, K. Satori, M. Itabashi, J. Kasahara, Appl. Phys. Lett. 2008, 93, 053303. [23] J. Kang, N. Shin, D. Y. Jang, V. M. Prabhu, D. Y. Yoon, J. Am. Chem. Soc. 2008, 130, 12273. [24] K. Fukuda, T. Yokota, K. Kuribara, T. Sekitani, U. Zschieschang, H. Klauk, T. Someya, Appl. Phys. Lett. 2010, 96, 053302.
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Adv. Mater. 2014, DOI: 10.1002/adma.201404052
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Soluble Organic Semiconductor Precursor with Specific Phase Separation for High-Performance Printed Organic Transistors Yu Kimura, Takashi Nagase,* Takashi Kobayashi, Azusa Hamaguchi, Yoshinori Ikeda,* Takashi Shiro, Kazuo Takimiya, and Hiroyoshi Naito* There is considerable interest in the development of organic thin-film transistors (OTFTs)[1–3] with high electrical performance because of their potential use in various organic devices, including flexible displays, radio-frequency identification tags, and large-area sensors. OTFTs based on vacuum-deposited pentacene thin films have achieved field-effect mobilities comparable to or higher than those of hydrogenated amorphous silicon (a-Si:H) TFTs at around 1 cm2 V−1 s−1,[4–6] and progress has also been made in the development of high-mobility soluble organic semiconductors, including small molecules[7–11] and polymers,[12,13] over the last decade. As OTFTs move closer to practical use, the instability of organic semiconductors in ambient air becomes a critical issue. For example, pentacene is easily oxidized to form 6,13-pentacenequinone, which leads to reduced field-effect mobility,[14] and a typical polymer semiconductor such as poly(3-hexylthiophene) often suffers carrier doping by oxygen and moisture,[3] which makes it difficult to fabricate OTFTs with low off-state currents under ambient conditions. One promising strategy for enhanced air stability is to increase the ionization potentials of organic semiconductors, and dinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]thiophene (DNTT) has attracted increasing interest because it has a higher ionization potential (5.4 eV) than pentacene (5.0 eV) and forms excellent
Y. Kimura, Dr. T. Nagase, Dr. T. Kobayashi, Prof. H. Naito Department of Physics and Electronics Osaka Prefecture University 1-1 Gakuen-cho, Naka-ku, Sakai 599-8531, Japan E-mail: [email protected]; [email protected] Dr. T. Nagase, Dr. T. Kobayashi, Prof. H. Naito The Research Institute for Molecular Electronic Devices Osaka Prefecture University 1-1 Gakuen-cho, Naka-ku, Sakai 599-8531, Japan A. Hamaguchi, Dr. Y. Ikeda, T. Shiro Electronics Materials Development Project Teijin Limited 4-3-2 Asahigaoka, Hino, Tokyo 191-8512, Japan E-mail: [email protected] Prof. K. Takimiya Department of Applied Chemistry Hiroshima University 1-3-2 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan Prof. K. Takimiya Center for Emergent Matter Science, RIKEN 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
DOI: 10.1002/adma.201404052
Adv. Mater. 2014, DOI: 10.1002/adma.201404052
thin-film microstructures with high field-effect mobility (>1 cm2 V−1 s−1).[15–18] The long-term stability of DNTT TFTs in ambient air has been demonstrated,[16] and recent reports have shown the potential of DNTT for heat-resistant OTFTs.[17] Despite these advantages, DNTT molecules have very low solubility in organic solvents because of their high crystallinity, even when soluble alkyl chains are attached to the molecules,[19] and do not allow DNTT TFT production using conventional solution techniques. In this communication, we report a novel soluble DNTT precursor (5,14-N-phenylmaleimidedinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]thiophene, Figure 1a) that allows the fabrication of high-performance DNTT TFTs using simple solution processes in ambient air. The DNTT precursor enables thin-film formation with excellent microstructures and inherent air stability and can induce specific phase separations when blended with an inert polymer, allowing enhancement of the film crystallinity at interfaces with hydrophobic substrates. We demonstrate that DNTT TFTs processed from a solution of the DNTT precursor blended with a polymer exhibit high field-effect mobilities of up to 1.1 cm2 V−1 s−1 and excellent durability against air exposure and thermal stress. The synthesis of the DNTT precursor was carried out based on a Diels–Alder reaction of DNTT and N-phenylmaleimide, from which a mixture of endo- and exo-isomers (typical ratio of 35.3:64.7) was obtained (for details, please see the Experimental Section). These isomers of the DNTT precursor can be dissolved in organic solvents (e.g., chloroform, dichlorobenzene, or dimethylacetamide) at room temperature because of their low crystallinity, and can be converted into DNTT by thermal annealing above 195 °C through the elimination of N-phenylmaleimide; this was confirmed by thermogravimetric analysis (Figure S8, Supporting Information). A polycrystalline DNTT thin film can be obtained by forming a thin film of the soluble DNTT precursor on a substrate with subsequent annealing at around 200 °C in ambient air. UV– vis absorption measurements show the thermal conversion of the precursor film into the DNTT film in air; the annealed film shows an absorption spectrum that is identical to that of DNTT films processed by vacuum deposition (Figure S9, Supporting Information). To improve the crystallinity of the organic semiconductor films, coating of the organic solutions on hydrophobic substrates is indispensable because the organic semiconductor crystallization process is generally enhanced on substrates with lower surface energies. However, solution-processed DNTT films on hydrophobic substrates often suffer from dewetting, which does not allow continuum film
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Figure 1. a) Chemical structure of the soluble DNTT precursor and its thermal conversion process. b) Polarized optical micrographs, c) out-of-plane XRD patterns, and d) in-plane XRD patterns of DNTT films processed from the precursor on UV/O3-treated Si/SiO2 substrates and DNTT/PS films processed from blends of the precursor and PS with weight ratios of 2:1, 1:1, and 1:2 on OTS-treated Si/SiO2 substrates.
formation. Dewetting problems have frequently been reported in solution processes using soluble organic semiconductors,[20,21] and several film-forming techniques have been developed.[20–23] The most attractive technique uses the vertical phase separation behavior[22,23] that appeared in solution-processed blend films of a soluble small-molecule semiconductor and an insulating polymer, because both excellent film-forming properties and good electrical performance can be achieved using conventional coating and printing processes. Investigation of the capabilities of vertical phase separation in organic films processed from a blend of the soluble DNTT precursor with an insulating polymer is thus of great interest for practical use in printed devices. The blend films were typically processed from the DNTT precursor blended with a representative insulating polymer, polystyrene (PS). The DNTT precursor and the PS were mixed in different weight ratios and dissolved in chloroform at approximately 0.15 wt%. The mixed solutions were drop cast on Si/SiO2 substrates modified with a hydrophobic 2
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self-assembled monolayer (SAM) of octadecyltrichlorosilane (OTS), and the resulting films were subsequently annealed at 200 °C for 10 min in air (the annealing conditions were optimized in TFT experiments). Polarized optical micrographs of films made from the DNTT precursor blended with PS with weight ratios of 2:1, 1:1, and 1:2 on the OTS-treated substrates are shown in Figure 1b, together with an image of the film made from DNTT precursor on UV/ ozone (UV/O3)-treated substrates. The dewetting of the DNTT precursor films on the OTS-modified surfaces is improved remarkably by the addition of PS. The blend films show strong birefringence, indicating the formation of crystalline DNTT grains in the films. The out-of-plane and in-plane X-ray diffraction (XRD) patterns of these films are shown in Figure 1c,d, respectively. The out-ofplane XRD patterns show distinct Bragg reflection peaks at 5.5° and 16.4° and blending of the DNTT precursor with PS does not affect the angles of the diffraction peaks (Figure 1c). The diffraction angles obtained agree well with those of vacuum-deposited
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Adv. Mater. 2014, DOI: 10.1002/adma.201404052
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DNTT films.[15,17] These results indicate that DNTT molecules form well-ordered crystalline grains, in which the long axis of the DNTT molecules preferentially orients perpendicularly on the substrate surface (edge-on molecular orientation). It is also shown that the in-plane XRD pattern of the blend film made from DNTT precursor and PS with a 2:1 ratio on the OTStreated substrate exhibits more intense and narrower diffraction peaks when compared with those of a film made from DNTT precursor on the UV/O3-treated substrate (Figure 1d). This indicates that the DNTT molecules in the blend film crystallize highly to adopt herringbone packing structures parallel to the substrate surface. The improved crystallinity in the blend film is attributable to segregation of the DNTT molecules to the surface of the substrate, because it permits effective crystallization of DNTT molecules using low-energy hydrophobic surfaces. We stress that the formation of both well-ordered edge-on molecular orientation with the surface normal and 2D herringbone packing in the substrate in-plane direction are essential for achieving high field-effect mobility for OTFT applications because π-stacked molecules along the substrate surface provide efficient lateral carrier transport. To confirm formation of the phase-separated structures in the solution-processed DNTT/PS blend films, vertical profiles were investigated via scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) combined with energy dispersive X-ray spectroscopy (EDS). The DNTT and SiO2 dielectric layers can be detected as EDS signals for sulfur (S) and silicon (Si), respectively. Figure 2a,b show crosssectional images from SEM and the element mapping of S of the blend film made from the DNTT precursor and PS before thermal annealing. The S signals are detected from the whole
area of the nonannealed blend film, suggesting that the DNTT precursor molecules are uniformly dispersed in the organic film before thermal annealing. We found that the thermal annealing of the blend film leads to the formation of a distinct layered structure in the vertical direction. Cross-sectional SEM image of the annealed blend film is shown in Figure 2c, in which the formation of a double-layered structure is observed. Figure 2d,e show high-resolution cross-sectional STEM and S mapping images of the annealed blend film, and the vertical profiles of concentrations of S, carbon (C), and Si, along the red line indicated in Figure 2d, are shown in Figure 2f. It can be seen that the S signal from DNTT immediately rises as the profiling reaches the bottom organic layer and drops sharply at the interface with the SiO2 layers. These results clearly show that the DNNT molecules segregate to the interface with the substrate in the annealed blend film and that substrate-mediated molecular ordering is responsible for the improved crystallinity of DNTT/PS blend films on the OTS-treated substrates. It is noted that phase separation behavior observed in the DNTT precursor/polymer blend films slightly differs from that reported in the blend films of a soluble small-molecular semiconductor and an insulating polymer,[22,23] where much of phase separation occurs during coating of the films. In the case of the DNTT precursor/ polymer blend films, the formation of phase-separated structures is caused by thermal annealing, during which the conversion of the DNTT precursor into DNTT and the crystallization of DNTT simultaneously occur. We believe that such moderate crystallization process can allow structural rearrangement prior to structural fixation, which results in
Figure 2. a) Cross-sectional images obtained from SEM and b) element mapping of S in the blend film of the DNTT precursor and PS with a weight ratio of 2:1 on OTS-treated Si/SiO2 substrates before thermal annealing. c) Cross-sectional images obtained from SEM, d) STEM, and e) element mapping of S in the blend film after thermal annealing at 200 °C for 10 min. f) Vertical profiles of concentrations of S, C, and Si along the red line indicated in Figure 2d from the top to the bottom.
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Figure 3. a) Schematic of the device structure of bottom-gate/top-contact OTFTs. b) Typical output characteristics and c) transfer characteristics of OTFTs processed from the DNTT precursor/PS blend (2:1) on OTS-treated Si/SiO2 substrates. A drain voltage (VD) of −100 V was applied when the transfer characteristics were measured. d) Variation in the transfer characteristics of the OTFT processed from the DNTT precursor/PS blend (2:1) on the OTS-treated Si/SiO2 substrate after air exposure for 1100 h. For measurement of the transfer characteristics, a VD of −50 V was applied. e) Transfer characteristics of OTFTs processed from the DNTT precursor/PS blend (2:1) on HMDS-treated and UV/O3-treated Si/SiO2 substrates. f) Field-effect mobility of the OTFT on the OTS-treated substrate as a function of annealing temperature. The mobilities were measured at room temperature in air.
the formation of highly-crystallized DNTT film in the blend films. OTFT devices were fabricated with annealed blend films in a bottom-gate/top-contact configuration with gold source and drain electrodes on the Si/SiO2 substrates, as shown in Figure 3a. The OTFT fabrication and electrical measurement processes are described in the Experimental Section. The typical output and transfer characteristics of OTFTs processed from the 2:1 DNTT precursor/PS blend on OTS-treated Si/ SiO2 substrates are shown in Figure 3b,c, respectively. The OTFTs based on DNTT/PS blend films show good transistor performance with a high on/off current ratio of over 106, a low threshold voltage of less than −10 V and negligible hysteresis in the transfer characteristics between the forward and backward scans of the gate voltage. Also, the electrical characteristics do not show any notable degradation, even after the device is exposed to air for 1100 h (Figure 3d) and heated at 90 °C for 500 h in air (Figure S10, Supporting Information). The electrical performance of OTFTs based on DNTT/ polymer blend films depends on the surface treatment of the substrates, the film annealing temperature and time (Figure S11, Supporting Information), and the concentration and type of insulating polymers to be added (Figures S12, S13, and S14, and Table S1, Supporting Information), but field-effect mobilities of more than 0.2 cm2 V−1 s−1 are routinely obtained in devices fabricated on OTS-treated substrates with annealing temperatures of less than 200 °C. Table 1 summarizes the transistor performances of devices with different surface treatments that were processed under optimized annealing
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conditions (200 °C and 10 min). The field-effect mobility is enhanced remarkably by substrate treatment with SAMs, and devices fabricated on OTS-treated substrates exhibit higher mobility than devices fabricated on hexamethyldisilazane (HMDS)-treated substrates. The observation of such a mobility trend clearly shows that the crystallization process of the DNTT molecules in the blend film is dominated by the substrate surface energy. In contrast to the devices fabricated on SAMtreated substrates, higher hysteresis is observed in devices fabricated on UV/O3-treated Si/SiO2 substrates (Figure 3e). This hysteresis is believed to be caused by long-lifetime trapping of electrons by hydroxyl (OH) groups at the SiO2 surfaces,[3] which can be suppressed by terminating the OH groups with SAMs. These results demonstrate that device fabrication on hydrophobic substrates using blends of DNTT precursor and PS is essential to improve not only their field-effect mobilities but also their electrical stability. The average field-effect Table 1. Electrical performance of solution-processed OTFTs using blend films made from DNTT precursor and PS with weight ratio of 2:1 on Si/SiO2 substrates treated with OTS, HMDS, and UV/O3. Data were obtained from more than nine devices. Surface treatment
Field-effect mobility Threshold voltage [cm2 V−1 s−1] [V]
On/off ratio
OTS
0.76–1.1
−5.7 ± 0.56
106–107
HMDS
0.30–0.63
2.0 ± 1.1
105–106
UV/O3
0.085–0.13
11 ± 1.6
104–105
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Adv. Mater. 2014, DOI: 10.1002/adma.201404052
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Experimental Section Synthesis of DNTT Precursor: A mixture of DNTT (500 mg, 1.47 mmol), N-phenylmaleimide (2.54 g, 14.7 mmol), and hydroquinone (16.2 mg, 0.147 mmol) in mesitylene (100 mL) were stirred for 4 h at 160 °C under a nitrogen atmosphere. The DNTT was removed by filtration, and the residue was purified by column chromatography on a silica gel eluted with chloroform (Rf = 0.37 stereoisomer A and Rf = 0.47 stereoisomer B). 5,14-N-Phenylmaleimidedinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]thiophene (DNTT precursor) with mass of 113.2 mg (Mw = 513.63, yield 15.0 mol%) was obtained. The ratio of the stereoisomers was determined to be A:B = 51.7:48.3 mol% by nuclear magnetic resonance (NMR). A minor
Adv. Mater. 2014, DOI: 10.1002/adma.201404052
compound (Rf = 0.48 stereoisomer C, approximately 1 mol%) was also observed in this synthesis. The minor compound was determined to be 1,4-N-phenylmaleimidedinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]thiophene by NMR spectroscopy. The stereoisomers A and B were separated into the endo-form (stereoisomer A) and the exo-form (stereoisomer B) by high-performance liquid chromatography (Agilent 1100 Series HPLC, Shiseido Capcell Pak C18 Type UG120, solvent:acetonitrile/ water) and the structures of stereoisomers A and B were assigned by X-ray crystallographic analysis. The conformation of stereoisomer C was determined by 1D 1H nuclear overhauser effect difference spectroscopy. The mixture of stereoisomers A and B was repurified only by simple recrystallization from chloroform for OTFT fabrication, and the final ratio of these stereoisomers was A:B = 35.3:64.7. More detailed information including NMR and X-ray crystallographic data is described in the Supporting Information. Materials: PS (average molecular weight (Mw): ~280,000) and poly(methyl methacrylate) (average Mw: ~100,000) were purchased from Sigma-Aldrich and Polymer Science, respectively, and were used as-received. For polycarbonate, Teijin Panlite L-1250 (average Mw: ~60,800) was used. For silane coupling agents for the SAM treatment, OTS (Shin-Etsu Chemical LS6495) and HMDS (Shin-Etsu Chemical LS7150) were used. OTFT Fabrication and Characterization: Discrete OTFTs were fabricated on a heavily doped n-type Si wafer with a thermally-grown silicon dioxide (SiO2) insulator layer (1000 nm thickness). The substrates were washed sequentially with acetone and 2-propanol in ultrasonic baths and were then cleaned with an UV/O3 cleaner to remove any organic contaminants. The substrate surfaces were chemically modified with SAMs of OTS or HMDS. To form the SAMs, the substrates were immersed in ~10 mM SAM solutions in toluene at room temperature overnight in a capped bottle. After the substrates were removed from the SAM solution, they were rinsed with fresh toluene to remove any excess layers. For the semiconductor solution, the DNTT precursor and PS were mixed in chloroform at a weight concentration of ca. 0.15% at room temperature in ambient air. The organic solution was then drop cast on the substrates and the resulting films were subsequently annealed at 200 °C for 10 min in air on a hotplate. Then, gold source and drain electrodes were fabricated on the films by vacuum evaporation through a metal shadow mask to complete the OTFT fabrication process. The channel length (L) and channel width (W) were 200 µm and 1 mm, respectively. The OTFT electrical characteristics were measured at room temperature in the dark and in ambient air using a semiconductor parameter analyzer (Keithley Instruments 4200-SCS). The field-effect mobility and threshold voltage were calculated from the transfer characteristics measured in the saturation regime using the following standard TFT equation: ID =
μC iW (VG − Vth )2 2L
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mobility of optimized devices fabricated on OTS-treated substrates is 0.96 cm2 V−1 s−1 and the maximum mobility reaches 1.1 cm2 V−1 s−1. These mobilities are close to those of DNTT TFTs fabricated by vacuum deposition on OTS-treated Si/SiO2 substrates (1.6–2.9 cm2 V−1 s−1).[15] It is also noted that the maximum mobility of our OTFTs is the highest reported to date for OTFTs based on soluble precursors, including pentacene precursors,[8] for which maximum mobility of 0.89 cm2 V−1 s−1 was achieved after device fabrication under a nitrogen atmosphere. In addition to the excellent carrier transport properties, our approach enables solution processing of DNTT TFTs with high thermal stress durability. Figure 3f shows the field-effect mobility as a function of annealing temperature. The mobilities were measured after the device was annealed at each temperature for 10 min in air and then cooled to room temperature. The field-effect mobility is almost unchanged up to 200 °C. This is in sharp contrast to the thermal stability of vacuum-deposited DNTT TFTs,[17] where the field-effect mobility is significantly degraded by heat treatments at more than 150 °C, because of changes in the packing structure for DNTT molecules in the thin films.[17] The enhanced thermal stability in our devices can be attributed to the distinct layered structures formed in the DNTT/PS blend films, as shown in Figure 2. The results indicate that the spontaneous lamination of the DNTT channel layer by the PS layer greatly reduces the changes in the packing structure, which is qualitatively consistent with the thermal stability reported for OTFTs encapsulated using parylene passivation layers.[17,24] It should be noted that spontaneous formation of the laminated OTFT structures by solution processes is technologically useful, because the solution processing of laminated layers sometimes suffers from dissolution of the underlying semiconductor layers. In summary, we have developed a novel soluble DNTT precursor for solution-processable high-performance OTFTs with good environmental stability. We found that the blend film of the DNTT precursor and electrically inert polymers forms a distinct phase-separated DNTT/polymer structure by thermal annealing, which improves both the film-forming characteristics and the electrical performance of OTFTs. Solutionprocessed DNTT TFTs based on these semiconductor blends exhibit high field-effect mobilities of up to 1.1 cm2 V−1 s−1 and highly stable operation after ambient air exposure for over 1 month and after thermal stress at temperature of up to 200 °C. We believe that the materials and techniques developed here offer new strategies to produce highly reliable flexible electronic devices by low-cost printing processes.
(1)
where ID is the drain current, µ is the field-effect mobility, Ci is the capacitance per unit area for the gate insulator, VG is the gate voltage, and Vth is the threshold voltage. In the air exposure experiment, the OTFT devices were stored in the dark and in ambient air for each period. Film Structure Analysis: DNTT films and DNTT/PS blend films were prepared for structural analyses on UV/O3-treated and OTS-treated Si/SiO2 substrates, respectively, in the same manner as for the OTFT fabrication processes, but without deposition of the Au electrodes on top. Polarized optical micrographs of the DNTT films and DNTT/PS blend films were obtained using a Nikon Eclipse LV100POL polarizing microscope. Out-of-plane and in-plane XRD measurements were conducted under ambient conditions using a Rigaku Raxis Rapid imaging plate area detector with graphite monochromated Cu–Kα radiation (2θmax = 136.4°). SEM and STEM images and element profiles of the cross-sectional structures of the blend films were measured using
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www.MaterialsViews.com a Hitachi FE-SEM S-5200 and a JEOL JEM-2100F equipped with EDS detectors. The samples for SEM and STEM observations were prepared by freeze fracture and ion milling techniques, respectively.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors would like to thank the Nippon Kayaku Corporation for providing the DNTT material. They also thank Midori Kamimura, Matsumoto Yoshiyuki, and Takahiro Takeuchi, of the Teijin Pharma Limited, for performing X-ray single crystal structural analysis and HPLC analysis. H.N., T.N., and T.K. would like to acknowledge partial funding from a Grant-in-Aid for Scientific Research (B) (grant no. 23360140) from the Japan Society for the Promotion of Science and from a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-Blocks (No. 2401)” (grant no. 24102011) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. Received: September 3, 2014 Revised: October 17, 2014 Published online: [1] [2] [3] [4]
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Adv. Mater. 2014, DOI: 10.1002/adma.201404052
