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Highly Oriented Polymer Semiconductor Films Compressed at the Surface of Ionic Liquids for HighPerformance Polymeric Organic Field-Effect Transistors Junshi Soeda, Hiroyuki Matsui, Toshihiro Okamoto, Itaru Osaka, Kazuo Takimiya, and Jun Takeya* Solution-processed organic field-effect transistors (OFETs) have been of great interest as basic switching components in the circuitry of future flexible and printed electronics. In this type of next-generation semiconductor technology, since the overall challenge is to attain both simple processability and high device performance at the same time, there has been much interest in maximizing the charge-carrier mobility for solution-processed films of organic semiconductors for applications in fastoperating devices. Such organic semiconductor materials that constitute the active layers of OFETs are generally classified into two groups: small molecules and polymers. High carrier mobility exceeding 10 cm2 V−1 s-1 was first achieved in smallmolecule transistors based on single crystals grown in the vapor phase, such as rubrene and pentacene,[1–4] and became more industrially compatible owing to the recent development of solution-crystallization processes.[5–8] In the small-moleculebased high-mobility OFETs, it is already well established that the charge-transport efficiency is maximized when band transport is realized because of numerous intermolecular charge transfers in the periodic arrangement of π-conjugated molecules.[9,10] On the other hand, it is difficult to obtain such single-crystalline thin films among polymer semiconductors because of the much larger entropy in a polymer system, resulting in a relatively lower mobility of the order of ∼0.1 cm2 V−1 s−1. It should be noted that we find many reports in the literature on significant improvements in the carrier mobility by various efforts such as thermal annealing to increase the local ordering,[11–15] even though the generality of such claims is somewhat questionable for recently developed high-mobility materials such as the copolymer of indacenodithiophene and benzothiadiazole (IDT-BT).[16] Therefore, the mobility can still be improved in

J. Soeda, H. Matsui, Prof. T. Okamoto, Prof. J. Takeya Department of Frontier Sciences The University of Tokyo Kashiwanoha 5–1–5, Kashiwa, Chiba 277–8561, Japan E-mail: [email protected] J. Soeda, Prof. J. Takeya Department of Engineering Osaka University Mihogaoka 8–1, Ibaraki, Osaka 567–0047, Japan I. Osaka, Prof. K. Takimiya Emergent Molecular Function Research Group RIKEN Center for Emergent Matter Science Hirosawa 2–1, Wako, Saitama 351–0198, Japan

DOI: 10.1002/adma.201401495

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conventional π-conjugated polymers such as poly(3-hexylthiophene) (P3HT)[15] or poly[2,5-bis(3-hexadecylthiophen-2-yl) thieno(3,2-b)thiophene] (PB16TTT)[12] by further increasing the orientation of the polymer chain, even if the complete single crystals of the polymers are hard to obtain. Indeed, band calculations assuming single crystals of P3HT and PBTTT predict that the effective mass of holes in these polymers is lighter than that of holes in pentacene by one order of magnitude.[17,18] So far, several methods to align the main chains of semiconducting polymers have been reported.[19–24] Among these techniques, the “zone-casting” method and the Langmuir–Blodgett (LB) technique have attained relatively high degrees of in-plane orientation of polymer main chains for conventional polymeric semiconductors of PBTTT and P3HT, in which the dichroic ratio reached ∼7.[20] Moreover, by employing a specifically designed polymer with a lyotropic liquid-crystal phase, which is intended to align the chain in the phase, a dichroic ratio of over 16 has been obtained using a method that resembles zone-casting.[25] In these films, the hole mobility is higher in the direction of the polymer chain compared to the transverse direction by several orders of magnitude, which is consistent with the prediction of density functional theory (DFT) calculations. Therefore, it has been experimentally suggested that higher orientation of the polymer chains is effective in enhancing intrinsic chargetransport in polymer semiconductors. One drawback of the above methods is the necessity to utilize the liquid-crystalline phase to align the polymer main chains so that the polymers can reorganize on an acceptable time scale. Therefore, it was not easy to extract the maximum performance of high-mobility polymers without a liquid-crystalline phase, such as the recently developed donor–acceptor copolymer semiconductors.[26,27] In such materials, a higher aggregation energy is favorable to realizing effective intermolecular orbital overlap for better charge transport, but it leads to difficulties in rearranging the polymer chains. Furthermore, the relatively low solubility of the highly crystalline polymers makes it difficult to form continuous films by conventional methods, which typically require solubility above 0.1 wt%. In this letter, we report a novel and versatile method to form a well-oriented polymer thin film on the surface of an ionic liquid (IL). On the IL surface, which is stable even at elevated temperatures because of the very low vapor pressure, one can take advantage of the enhanced freedom of motion to arrange the polymer chains. The method was successfully applied to a liquid-crystalline semiconducting polymer as well as a polymer that did not have a liquid-crystalline phase. The solution of the

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COMMUNICATION Figure 1. (a) Molecular structure of poly[2,5-bis(3-hexadecylthiophen-2-yl)thieno(3,2-b)thiophene]. (b) Schematics of the spread-and-compress procedure used to prepare thin films in this study. (c) Evolution of the appearance of the trough as the thin film spread over time.

polymers was released dropwise onto the surface of the IL to spread the thin film. We note that the concentration of the solution could be decreased to ∼0.01 wt% so that this method can be applied to the characterization of polymers with poor solubility. The thin film was subsequently compressed and annealed to align the polymer main chains uniaxially on the surface of the IL. The structure and degree of orientation were evaluated by polarized-light-absorption spectroscopy and X-ray diffraction measurements. Finally, the charge-transport properties in the thin film were also evaluated by thin-film transistors. We first employed PB16TTT as a liquid-crystalline polymer semiconductor, whose molecular structure is depicted in Figure 1a. A schematic of the fabrication procedure for the PB16TTT thin film is also shown in Figure 1b. The powder of the polymer was first dissolved in dehydrated o-dichlorobenzene at a concentration of 0.050 wt% at 100 °C. A 1.5-µL droplet of the solution was then released onto the surface of an IL, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide (EMIM-TFSI, see Supporting Information for the molecular structure), in a square-shaped trough. The volume of the droplet was adjusted so that several molecular layers were formed on the entire surface of the IL. The trough was kept at 120 °C, which is slightly above the liquid-crystal transition temperature of PB16TTT, so that the molecules could move moderately on the surface of the IL. When the solution was released dropwise on the IL, the thin film emerged from the droplet and spread to cover the entire IL surface; the time evolution of the appearance of the thin film is shown in Figure 1c. After the film formation was completed in a few seconds, the film was mechanically compressed in one direction by a glass blade at a speed of 1 mm s−1 on the surface of the IL to align the polymer uniaxially. The resultant thin film was transferred onto the surface of substrates by attaching the substrate horizontally to the

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film floating on the IL as shown in Figure S1. The color of the film darkened in the entire area, indicating its homogeneous compression. The final thickness of the each of the prepared films was 150–200 nm, measured by atomic force microscopy (AFM). Finally, the prepared thin films were transferred onto Si/SiO2 or quartz substrates and rinsed by acetonitrile. All the above processes were conducted in a glove box filled with argon gas. An optical microscope image of a PB16TTT thin film is shown in Figure 2a and b. The shape of the thin film is well defined in a rectangle by the mechanical compression, and the size is limited by the size of the trough. Therefore, this method can be used for large-area fabrication, which is a significant criterion for industrial applications. Figure 2c and d show the cross-polarized microscope image of the polymer thin film. The difference in the color of the thin film seen in Figure 2c corresponds to the interference color because of the difference in film thickness. The reflected light from the film was abruptly extinguished as the sample stage was rotated from the position depicted in Figure 2c to that in Figure 2d. The cross-polarized images indicate the uniaxial alignment of the compressed thin film in a wide area. In the present compression process of the films, the in-plane orientation of the polymer chain is progressed. Similar phenomena of the in-plane orientation of the polymer backbone can be seen in many other literature, in which the lateral pressure is induced on the Langmuir film of the hairy-rod polymers such as poly(3-hexylthiophene), polyimide and polyacetylene derivatives.[21,28,29] We believe that similar mechanism should be applicable to the present progression of the in-plane orientation of the PB16TTT. In order to examine the effect of the temperature during the compression on the degree of orientation, we measured the dichroic ratio of the thin films fabricated at different

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Figure 2. (a, b) Optical microscope images of the compressed PB16TTT thin film on Si wafer with a 500-nm layer of SiO2. (b) A demonstration of fabrication large thin film. The size of the film in the picture reached 0.5 cm by 2 cm. (c, d) Cross-polarized microscope images of the compressed thin film. The appearance changed from (c) to (d) with the rotation of the polarizer. (e) UV–vis absorption spectra of the PB16TTT film which was compressed at 120 °C. The incident light was polarized parallel (blue line) or perpendicular (red line) to the longer side of the film. The insets in (e) are the images of the compressed PB16TTT thin film on a liquid-crystal display. (f) Dependence of the dichroic ratio on the compression temperature. The dichroic ratio was calculated from the absorbance at the photon energy of 2.26 eV.

temperatures. Figure 2e shows the linearly-polarized UV–visible absorption spectrum in an area of 100 µm × 60 µm for the PB16TTT film compressed at 120 °C. The degree of the uniaxial orientation was evaluated by calculating the dichroic ratio A储/A⊥, where A储 and A⊥ denote the absorbance with the polarization parallel and perpendicular to the longer side of the polymer thin film, respectively. The intense absorption at ∼2.3 eV was observed only in the parallel polarization, which indicates that each polymer chain was oriented along the longer side of the rectangular film. The dependence of the dichroic ratio on the compression temperature is plotted in Figure 2f. At around room temperature, the dichroic ratio was ∼1 which indicated no significant progression of the in-plane orientation, while the dichroic ratio increased as the temperature increased. The maximum dichroic ratio as high as 15.6 was obtained by the compression at the 120 °C (Figure 2e). The corresponding in-plane order parameter S, defined as S = (A储 – A⊥)/ (A储 + A⊥), was 0.88. To our knowledge, this is the highest value among uniaxially aligned PBTTT thin films reported in the literature.[20] At the temperature above 120 °C, the PB16TTT is in fluid liquid-crystal phase and it cannot be ordered by the mechanical compression because of excessive fluidity. X-ray diffraction (XRD) measurements were performed on the compressed thin film to evaluate the structure and detailed crystallinity. Figure 3a shows the XRD patterns for out-of-plane measurements. A series of peaks corresponding to d = 24.04 Å, suggesting the lamella distance of PB16TTT, were observed. The rocking curve of the second peak of the lamella stacking in Figure 3b shows a very narrow full-width at half-maximum (FWHM) of 0.023°. The value of the FWHM, which was actually limited by the equipment resolution, is one of the best values among PBTTT thin films in the literature and is even

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comparable to that of the high-quality single crystals of smallmolecular organic semiconductors.[30–34] These results indicate that the so-called edge-on structure was realized in the thin film, which is preferable for the lateral charge conduction in the active layer of thin-film transistors (TFTs). Figure 3c shows the in-plane XRD patterns of the thin film. The peak at 2θ = 24.2° corresponds to a face spacing of 3.67 Å, which corresponds to the π−π stacking distance of PB16TTT. The crystallite size estimated from the sharpness of the peak by Scherrer’s equation was 32 nm, which is as good as well-oriented thin films of PBTTT reported so far.[30] A series of peaks corresponding to the lamella stacking was also observed in the in-plane configuration, which indicates that a small amount of face-on stacked polymers might have formed through the compression process. Figure 3d shows the ϕ-scan on the π−π stacking peak of the uniaxially aligned thin film. This result indicates that the dispersion of the direction of the π−π stacking was within ±20°, which is comparable to the results previously reported using the zone-casting method.[35] These in-plane XRD results clearly indicate that the polymer main chains were aligned along the longer side of the rectangular film. It should be noted that although such experiments are usually conducted at synchrotron light sources, all the XRD measurements in our study could be performed at a laboratory-equipped light source owing to the high crystallinity of the polymer thin films. The TFT characteristics of the compressed PB16TTT thin films were also evaluated. The compressed PB16TTT thin films prepared for our study were transferred onto Si wafers with SiO2 layers as gate insulators, each of which was treated with a fluorinated self-assembled monolayer (triethoxy-1H,1H,2H,2Htridecafluoro-n-octylsilane) to diminish the interfacial trapping state. After the transfer, gold was vacuum-evaporated through

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COMMUNICATION Figure 3. (a–d) XRD patterns obtained in the experiments. The a*, b*, and c* axes are defined as the directions of the lamella, π−π stacking, and polymer backbone, respectively. (a) Out-of-plane XRD pattern of the compressed PB16TTT thin film. (b) Rocking curve for the (2 0 0) peak. (c,d) Grazing-incidence-angle XRD (GIXD) patterns of the PB16TTT thin film. The incident angle was fixed at 0.180°, which is between the total reflection critical angle of PB16TTT and the SiO2 surface, to minimize scattering from the SiO2 surface. (c) 2θχ/ϕ scan patterns for the compressed PB16TTT thin film when the scattering vector was parallel (red line) and perpendicular (blue line) to the direction of the compression. (d) An in-plane rocking curve for the peak corresponding to the π–π stacking.

shadow masks to form source and drain electrodes so that the device had a bottom-gate and top-contact geometry. The current was measured mainly in the direction of the polymer main chains. (The charge-transport properties in the π−π stacking direction measured in TFT configurations are shown in Supporting Information.) The transfer characteristics of the TFT in the saturation and linear regions are shown in Figure 4a and b, respectively. Field-effect mobilities as high as 0.62 and 0.36 cm2 V−1 s−1 were realized in the saturation and linear regions, respectively. These values are comparable to one of the highest mobilities reported for PB16TTT TFTs.[13,14] The output characteristics in Figure 4c show good saturation behavior and negligible non-linear characteristics in the low drain voltage (VD) region. In the spin-coated film, the hole mobility measured by the same TFT configuration was around 0.1 cm2 V−1 s−1,

which is significantly lower than that in compressed PB16TTT thin films (characteristics of the spin-coated samples are shown in Supporting Information). Thus, it is clear that the higher mobility in the study’s thin films should have originated from the high orientation of the polymer chains. The film fabrication method, which spread the polymer film on an ionic liquid was also applied to other polymer semiconductors such as poly(2,7-bis(3-alkylthiophene-2-yl) naphtho[1,2-b:5,6-b′]dithiophene) (PNDTBTC20, inset of Figure 5a) and P3HT to examine the generality of the method. All compounds were successfully spread and compressed on the surface of the EMIM-TFSI to form thin films, following the same procedure for PB16TTT thin films. Both PNDTBTC20 and P3HT were dissolved in o-dichlorobenzene at the concentration of 0.05 wt% and kept at 100 °C to dissolve

Figure 4. (a–b) Transfer characteristics of a PB16TTT transistor in the (a) linear and (b) saturation regions. (c) Output characteristics of the PB16TTT transistor.

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Figure 5. (a) Molecular structure of poly(2,7-bis(3-alkylthiophene-2-yl)naphtho[1,2-b:5,6-b′]dithiophene) (PNDTBTC20). (b, c) Cross-polarized optical microscope images of the compressed PNDTBTC20 thin film. (d) Absorption spectra of the PNDTBTC20 thin film with the incident light polarized parallel (blue) and perpendicular (red) to the longer side of the film. (e) In-plane XRD profiles of the PNDTBTC20 thin film. (f) Transfer characteristics of a PNDT3BTC20 transistor in the saturation region. (g) Output characteristics of the PNDT3BTC20 transistor.

completely. The ionic liquid, EMIM-TFSI, was kept at 240 °C for PNDTBTC20 and 120 °C for P3HT during the compression of the films, respectively. The cross-polarized optical microscope images of the PNDTBTC20 film, which was formed by the spread-andcompress technique, are shown in Figure 5b and c. Anisotropic reflection from the film shown in the images demonstrate the well-ordered structure of the thin film. The highly oriented structure of the polymer chains was confirmed by both the dichroic ratio and in-plane XRD profiles shown in Figure 5d and e, respectively. The dichroic ratio was as high as 5.2 for PNDTBTC20. Although this value is lower than that for PB16TTT, the fabrication method can still align the nonliquid-crystalline polymer in one direction to a high degree. In the in-plane XRD profiles, the peak corresponding to π−π stacking was only observed when the scattering vector was perpendicular to the compression direction (red line), while it was not observed when the scattering vector was parallel to the compression direction (blue line). The azimuthal dispersion (full width at half-maximum of the peak in ϕ-scan) of the π−π stacking peak was determined to be 34.3° by the ϕ-scan measurements, which is almost the same as that for PB16TTT (the plots are shown in Supporting Information.) The transfer characteristics in the saturation region and output characteristics of the TFT fabricated with compressed PNDTBTC20 are shown in Figure 5f and g. mobilities of up to 0.80 cm2 V−1 s−1 for PNDTBTC20 were calculated from the slope in the saturation region. These results clearly demonstrate that the PNDTBTC20 thin film fabricated using our new method exhibited high orientation of the main chain and good mobility despite the absence of the liquid-crystalline phase of the polymer material. The TFT characteristics of the compressed P3HT thin film are also shown in Supporting Information. In conclusion, we have demonstrated a novel fabrication method for polymer thin films in which semiconductor polymers are aligned uniaxially. Thin films of the polymers were first spread on the surface of ILs, and the thin films were subsequently compressed mechanically to align the direction of

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the polymer chains. The polymer orientation and crystallinity were investigated using a cross-polarized microscope, polarized absorption spectroscopy, and XRD measurements. The dichroic ratio of the compressed PB16TTT thin film reached 15.6, which is the highest value among oriented PB16TTT films. The crystallinity was comparable to the best crystallinity among the reported PBTTT thin films. The method was confirmed to be applicable to other typical polymer semiconductors such as P3HT and PNDTBTC20, the latter of which does not exhibit a liquid-crystalline phase. The presented method is a generalized technique to obtain well-oriented polymer thin films that can reveal the charge-transport properties of polymer semiconductors.

Experimental Section The polymer materials PB16TTT and P3HT were purchased from Merck Corporation and Sigma-Aldrich Corporation, respectively. PNDTBTC20 was synthesized according to the method reported previously.[26] The all polymers used in this study were dissolved in o-dichlorobenzene at a concentration of 0.05 wt% and kept at 100 °C to dissolve the polymers completely. EMIM-TFSI was used as the IL to spread and align all the polymer films. The IL was placed in a trough of 1 cm × 1 cm. Then, a solution (1.5 µL) of the polymers was released dropwise on the surface of the ionic liquid to spread the polymer thin film. The polymer thin film on the IL surface was subsequently compressed by a glass blade to align the polymer chains uniaxially so that the final area of the polymer film after compression was 1/10 of the initial area. For the PB16TTT, the thin film was spread and compressed on the IL of 120 °C and the speed of the compression was 1 mm s−1 and 5 mm s−1 for dichroic ratio measurement and for TFT active layer, respectively. For PNDTBTC20, the solution of the PNDTBT was spread at the temperature of 200 °C to avoid the boiling of the solvent and subsequently increased to 240 °C to compress to align the polymers. The compression speed for the PNDTBTC20 thin films was 1 mm s−1. The compressed thin films were transferred onto the surface of substrates for further characterization of the thin films by attaching the substrate to the film floating on the IL as shown in Figure S1. The films were transferred onto a quartz substrate for the optical characterization. For characterization of the field-effect transistors, the films are transferred onto the Si wafer with SiO2 layer as gate dielectrics. The surface of the

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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements We thank Merck & Co., Inc. for providing PB16TTT material. J. S. acknowledges the JSPS research fellowship for young scientists. Received: April 3, 2014 Revised: June 13, 2014 Published online: August 19, 2014

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SiO2 was treated with fluorinated self-assembled monolayer (F-SAM) of triethoxy-1H,1H,2H,2H-tridecafluoro-n-octylsilane). The IL remaining on the substrates after the transfer was removed by rinsing with acetonitrile. The fabrication and measurement processes were all performed under an inert Ar atmosphere. After washing out the IL, the thin film was annealed at 120 °C to remove residual acetonitrile on the substrates and the films. A UV–vis spectrometer (MSV-370, JASCO) was used for the optical spectroscopic measurements. Smart Lab of Rigaku Corporation was used for the XRD measurements (using a Cu Kα light source), and a Keithley SCS4200 semiconductor parameter analyzer was used for the electrical measurements of the TFTs.

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Highly oriented polymer semiconductor films compressed at the surface of ionic liquids for high-performance polymeric organic field-effect transistors.

A novel and versatile method to align polymer semiconductors is demonstrated. Spreading and subsequent mechanical compression of a polymer thin film o...
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