Facile and Microcontrolled Blade Coating of Organic Semiconductor

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Organic Electronic Devices

Facile and Micro-Controlled Blade Coating of Organic Semiconductor Blends for Uniaxial Crystal Alignment and Reliable Flexible Organic Field-Effect Transistors Kyunghun Kim, Jisu Hong, Suk Gyu Hahm, Yecheol Rho, Tae Kyu An, Se Hyun Kim, and Chan Eon Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21130 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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ACS Applied Materials & Interfaces

Facile and Micro-Controlled Blade Coating of Organic Semiconductor Blends for Uniaxial Crystal Alignment and Reliable Flexible Organic Field-Effect Transistors Kyunghun Kim,1 Jisu Hong,1 Suk Gyu Hahm,2 Yecheol Rho,3 Tae Kyu An,4,* Se Hyun Kim,5,* Chan Eon Park1,*

1Department

of Chemical Engineering, Pohang University of Science and Technology, Pohang,

790-784, Korea 2Materials

Research Center, Samsung Advanced Institute of Technology, Suwon, 443-803,

Korea 3Chemical

Analysis Center, Korea Research Institute of Chemical Technology, Daejeon 34114,

Korea 4Department

of Polymer Science & Engineering, Korea National University of Transportation

50 Daehak-Ro, Chungju, 27469, Korea 5School

of Engineering, Yeungnam University, 280 Daehak-Ro, Gyeongsan, Gyeongbuk,

38541, Korea *Corresponding

Authors

E-mail: [email protected] (C. E. Park). E-mail: [email protected] (S. H. Kim) E-mail: [email protected] (T. K. An)

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Abstract The ability to fabricate uniform and high-quality patterns of organic semiconductors using a simple method is necessary to realize high-performance and reliable organic field-effect transistors (OFETs) for practical applications. Here, we report facile fabrication of chemically patterned substrates in order to provide solvent wetting/dewetting regions and grow patterned crystals during blade coating of a small-molecule semiconductor/insulating polymer blend solution. Polyurethane acrylate is selected as the solvent dewetting material, not only because of its hydrophobicity, but also because its patterns are easily produced by selective UV irradiation onto precursor films. 6,13-Bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) crystal patterns are grown on the line-shaped wetting regions of the patterned film, and the crystallinity of TIPS-PEN and alignment of molecules are found using various crystal analysis tools to depend on the pattern widths. The smallest width of 5 μm yielded an OFET showing the highest field-effect mobility value of 1.63 cm2/(V∙s), much higher than the value of the OFET based on the unpatterned TIPS-PEN crystal. Notably, we demonstrate flexible and low voltage-operating OFETs for practical use of the patterned crystals, and the OFETs show highly stable operation under sustained gate bias stress thanks to the patterned crystals. Keywords:

polyurethane

acrylate,

blade

coating;

crystal

alignment,

bis(triisopropylsilylethynyl) pentacene, flexible organic field-effect transistors


6,13-

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1. Introduction Solution-processable organic field-effect transistors (OFETs) have been developed for decades due to their flexible, lightweight properties and low-cost fabrication for potential applications in various electronic devices such as wearable medical devices, radio frequency identification tags and e-papers.1-5 Electrical performance of OFETs have been enhanced by the development of solution processing techniques and dielectric materials, synthesis of highperformance semiconductors, and structural optimization of semiconducting layers.6-12 While there are many important factors related to the realization of high-performance OFETs, the growth of crystalline semiconducting layers is the most basic and essential.10, 13-16 Blends of small-molecule semiconductors and insulating polymers have been expected to serve as ideal platforms for the growth of high-quality semiconducting crystals.17-26 Vertical phase separation during solution coating has been found to induce the small molecules to segregate and self-assemble at the air-film and/or film-substrate interfaces. From intensive studies on phase separation behaviors by controlling solvent evaporation rate, molecular weight of the insulating polymer and surface tension of each component, crystalline morphologies of semiconductors and its interface with the insulating polymer were optimized for fast charge transport, thus yielding high field-effect mobilities (μFETs) in OFETs.23, 27, 28 In addition, these optimizations increased the reproducibility with low performance variation over large areas, which is a critical issue for commercialization of solution-processed OFETs.25 Various solution processing techniques including spin-coating, drop casting, blade coating, ink jet printing and dispenser printing have been introduced for coating the blend solution.18-20, 29, 30

Among them, blade coating has recently attracted particularly considerable attention

because it provides several advantages in solution processing compared to the other methods: i) its directional coating system induces large and uniaxial growth of crystals along the coating 3 ACS Paragon Plus Environment


direction; ii) it is a cost-effective method, which could minimize the wasting of materials; and iii) it is compatible with large-area manufacturing systems such as roll-to-roll processes. Bladecoated blend films have shown crystalline structures favorable for charge transport, yielding high μFET values as well as low threshold voltage (Vth) and subthreshold swing (SS) values for various device structures and even showed μFETs close to the value of its single crystal via an optimization of processing conditions (e.g., blade speed, substrate temperature, and a mixture of solvents).20-22 Considering the application of blade-coated high-performance OFETs in multicomponent integrated circuits and their commercialization, patterning of blade-coated semiconducting films is necessary to reduce crosstalk among the neighboring devices and to stack the subsequent layers.14,

31-37

At this time, systematic investigations on the morphologies and

electrical properties of patterned semiconducting crystals would be helpful in designing their electronic circuits. Here, we developed a facile fabrication of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) semiconductor/insulating polystyrene (PS) blend patterns via blade coating to investigate the dependence of the crystallinity of TIPS-PEN on the pattern widths and to realize high-performance and reliable flexible OFET arrays. Unlike the blade-coated semiconducting crystals that have usually been produced in the form of films, we enabled their patterning by fabricating alternating solvent wetting/dewetting lines on the substrates, where the smallmolecule crystals grew only in the wetting regions (Figure 1). The dewetting lines were easily prepared using hydrophobic UV-curable polyurethane acrylate (PUA) with selective UV exposure. The widths of the wetting lines were varied from 5 μm to 100 μm. We used various crystal analysis tools including polarized optical microscopy (pOM), polarized UV-Vis absorption spectroscopy (pUV) and synchrotron-based grazing incidence wide-angle X-ray scattering (GIWAXS) to investigate the dependence of the crystallinity and 4 ACS Paragon Plus Environment

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alignment of the TIPS-PEN crystals on the pattern widths. It was revealed that as the width was decreased, the crystallinity of TIPS-PEN increased and the π-π conjugations of its molecules tended to be oriented along the coating direction, which was favorable for uniaxial charge transport. Therefore, their OFETs exhibited increasing μFETs with decreasing pattern widths, and the OFETs with a pattern width of 5 μm showed μFETs up to 1.63 cm2/(V∙s). To demonstrate a practical application of the blade-coated TIPS-PEN patterns, we blade-coated 5 μm TIPS-PEN patterns on polyethyl sulfone (PES) substrates, and fabricated flexible OFET arrays. The low voltage-operating flexible OFETs displayed high-performance and reliable operation with a quite low leakage current of about 10-12 A. This low leakage current was attributed to the patterned semiconducting crystals, and suggested the importance of patterning semiconducting layers in fabricating OFETs. From these studies, it was expected that the facile blade-coating method for fabricating organic crystal patterns and in-depth analysis of the dependence of the crystals on the pattern widths would provide insight into integrating them for realizing reliable electronic circuits.

2. Results and Discussion 2.1. Fabrication of hydrophobic patterns Figure 1 schematically illustrates the procedures used to pattern the blend films during blade coating, and to fabricate flexible OFET arrays. The PUA we used to make the dewetting lines on the substrates has been commonly used as a material for replicating flexible stamps using nanoimprint lithography.38-43 The PUA precursors consisted of a prepolymer for PUA, UVcurable releasing agent and a photoinitiator.40 The inherently robust nature of PUA without significant compromises in flexibility has enabled it to be used in the production of stable submicroscale dense structures with high aspect ratios, while maintaining its conformal contact 5 ACS Paragon Plus Environment


with the substrate. In addition, PUA has been considered as a suitable material for replicating flexible stamps because it can be easily manufactured by exposing its precursors to UV light and its surface is hydrophobic enough to be separated from the master mold. We focused on the UV-curable and hydrophobic properties of PUA, and utilized these properties to fabricate hydrophobic patterns onto hydrophilic surfaces (Figure 1). First, PUA precursors diluted by toluene with a precursor:toluene volume ratio of 1 : 40 were spin-coated onto rigid SiO2 substrates to form a 135-nm-thick film, and the film was exposed to UV light with a photomask for crosslinking. The unexposed parts were not cured and, therefore, could be washed out by using toluene, resulting in an alternating hydrophilic SiO2 and hydrophobic PUA lines on the surface. This one-step fabrication of hydrophobic patterns was much simpler and more reliable than previous methods; i) O2 plasma exposure onto hydrophobic selfassembled monolayer (SAM)-treated surfaces, and ii) transfer printing of hydrophobic SAMs because they required complicated steps (e.g., SAM treatment before O2 plasma and fabrication of elastomeric stamps for transfer printing).44-47 The surface energy of the PUA patterns were 26.31 mJ/m2, which was hence hydrophobic as reported previously (Table 1).40 The TIPS-PEN/PS blend solution in toluene was then printed onto the PUA/SiO2 substrates via blade coating with the previously reported experimental setup.48 To determine whether the patterned substrate is effective for the selective coating of the blend solution, the blend solution was also printed onto a control substrate containing alternating lines of SiO2 and SU-8 (Figure S1). SU-8 has been widely used as a negative photoresist.49 In the case of the PUA/SiO2 substrate, the blend solution wetted only the SiO2 region, and TIPS-PEN crystals were selectively grown. By contrast, the blend solution printed on the SU-8/SiO2 substrate wetted both of the regions due to the hydrophilic surface of SU-8, and thus the crystals were not selectively grown (Table 1 and Figure S1). These results indicated that hydrophobic PUA was effective in patterning TIPS-PEN crystals. 6 ACS Paragon Plus Environment

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2.2. Analysis of the dependence of the crystals on pattern widths TIPS-PEN crystals were grown on wetting lines of PUA/SiO2 substrates with various widths from 100 μm to 5 μm, and their crystallinity and orientations were analyzed using pOM, pUV and GIWAXS. pOM images of TIPS-PEN crystals printed on these various PUA/SiO2 substrates showed that the microwire-shaped TIPS-PEN crystals generally grew in the direction of blade coating regardless of the line width (Figures 2a and 2b). The directional growth was also observed in an unpatterned SiO2 substrate (Figure S2). However, the unpatterned crystals showed significant color variations due to the non-uniform crystal growth directions.50-52 Impingements between growing crystallites resulted in many grain boundaries and small crystals with different orientations. On the other hand, the crystals printed on the PUA/SiO2 substrates showed less color variation because their growth directions were uniform by virtue of the lateral confinement on the substrate. As the pattern widths were decreased, the lateral confinement increased and it seemed that larger crystals grew with fewer grain boundaries, and more uniform colors were observed. Especially in the case of the narrowest width of 5 μm, just about only one color was observed throughout the crystal line, indicating a high level of order throughout each line. We also carried out atomic force microscopy (AFM) measurement on the 5-μm-wide patterned TIPS-PEN crystals to estimate their thicknesses. The average step heights of each PUA lines on the PUA/SiO2 substrates were 133 nm, which almost coincided with the thickness measured by an ellipsometer (Figure 2c). Each line showed a steep slope at its edges, indicating well-defined wetting (SiO2) and dewetting (PUA) regions. After blade-coating of the TIPS-PEN/PS blend solution, directionally solidified TIPS-PEN/PS filled only the SiO2 regions, and the heights of that portion increased (Figures 2d and 2e). Therefore, thicknesses of TIPS-PEN/PS patterns were estimated to be 110 - 125 nm. We carried out pUV analyses of the TIPS-PEN crystals to investigate the dependence of their molecular orientations on the pattern widths. Their absorption spectrums were collected 7 ACS Paragon Plus Environment


while the samples were aligned so that the polarization of UV-Vis monochromatic light was incident either parallel or perpendicular to the blade coating direction. We compared the intensities of the π-π* transition absorption peaks of the two alignments, and their molecular orientations were estimated. As shown in Figure 3, all TIPS-PEN crystals showed absorption peaks at wavelengths of about 540, 590, 650 and 697.5 nm, as was found previously for TIPSPEN crystals.53-55 For each sample, the absorption spectrums differed for the two alignments: for example, the intensity of the peak at 697.5 nm corresponding to the π-π* transition was greater for the parallel alignment than for the perpendicular alignment. Considering that the peak at 697.5 nm originated from the π-π conjugations of TIPS-PEN molecules,53 the greater intensity for the parallel alignment indicated that the π-π conjugation in the TIPS-PEN crystals was mainly oriented along the coating directions, which was favorable for rapid charge transport. In addition, it should be noted that the degree of anisotropic π-π conjugation in TIPS-PEN crystals depended on their widths. The dichroic ratio, defined as the ratio of the absorbance at 697.5 nm for the parallel sample alignment to that for the perpendicular alignment, increased with decreasing pattern widths. For 100-μm-wide crystals, the dichroic ratio was 1.32, which was equal to that of unpatterned crystals (Figures 3a and 3b). The 100-μm-wide PUA/SiO2 pattern lines were apparently too wide to effectively align the crystals. As the pattern line widths were decreased, however, the dichroic ratio increased, and reached a value of 1.46 for the 5-μm-wide crystals (Figure 3e). Even though the highest dichroic ratio value in this study was less than the 1.74 value reported previously for TIPS-PEN nanowires,32 the clear tendency of increasing dichroic ratio with decreasing pattern widths showed that micropatterned PUA/SiO2 substrates were effective in aligning TIPS-PEN crystals in one direction (Figure 3f). Therefore, it could be expected that charge transport would be more favorable for the crystals with smaller widths. 8 ACS Paragon Plus Environment

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More details about the crystallinity levels and alignments for the crystals with various widths were obtained by carrying out synchrotron-based GIWAXS experiments (Figure 4 and Figure S3). Diffraction patterns were obtained while the incident X-ray beam was aligned either parallel or perpendicular to the blade coating direction with an incidence angle of 0.12°. First of all, (00l) peaks along the out-of-plane direction at qz = 0.378 were observed for all of the diffraction patterns, yielding a d-spacing value of 16.62 Å. This d-spacing value coincided with the c-axis length of the TIPS-PEN unit cell, suggesting edge-on orientations of TIPS-PEN molecules with respect to the substrates.56 The edge-on orientation was favorable for lateral charge transport of TIPS-PEN crystals. By comparing the diffraction patterns resulting from different incident beam directions, the anisotropic orientations of various TIPS-PEN crystals were corroborated. We focused here on comparing the diffraction pattern of the unpatterned crystal with that of the patterned one with 5 μm pattern widths. In case of the patterned crystal, it was notable that anisotropic crystal orientations were observed: when the crystal was exposed to the parallel incident X-rays, (h0l) diffractions related with only a axis in the crystal lattice were mainly detected; for the perpendicularly incident X-rays, the only intense peaks detected were the b axis-related (0kl) diffraction peaks (Figures 4a and 4b). The apparent discrepancy indicated a uniform and anisotropic arrangement of the crystal where the b axis was oriented along the coating direction. Because the (010) plane has been known as a direction of rapid charge transport, the uniaxial alignment of the crystal was expected to provide a favorable environment for charge transport.48, 55 On the other hand, in the case of the unpatterned crystal, various crystal planes related to both the a and b axes, including (-11l), (11l) and (21l), were detected regardless of the incident beam directions (Figures 4c and 4d). Even though (h0l) and (0kl) diffractions were mainly detected in parallel and perpendicular incidence with strong intensities, respectively, (011) 9 ACS Paragon Plus Environment


diffractions which were only detected in perpendicular incidence for the crystal with 5 μm width were detected in both incidences. (Please see the shaded region in Figure 4d.) This observation indicated a non-uniform and imperfect anisotropic arrangement of the unpatterned crystal, which was not favorable for charge transport. In addition to the poor in-plane order, the out-of-plane ordering of the unpatterned crystal was also lower than that of the crystal with 5 μm width (Figure S4a). Including the GIWAXS results for the other patterned crystals revealed that the uniformity and anisotropy of the TIPS-PEN crystals increased with decreasing pattern widths (Figure S3). The crystal with a 100 μm width showed a diffraction pattern similar to that of the unpatterned one. Various crystal planes were observed along the in-plane direction regardless of incident beam directions. As the pattern line width was decreased, however, the diffraction patterns became similar to that for the crystal with a 5 μm width, meaning that the anisotropic crystal alignment increased. The out-of-plane order of the crystals also increased as the pattern line width was decreased (Figure S4b). These trends suggested that the increased lateral confinement resulting from the decreased pattern widths induced a uniaxial and continuous growth of the TIPS-PEN crystals during the blade coating of the solution. Therefore, from GIWAXS analyses, it could be concluded that the crystal with the narrowest width, i.e., 5 μm, would exhibit the most efficient charge transport along the blade coating direction.

2.3. Electrical characterizations of TIPS-PEN crystals of various widths Electrical properties of various blade-coated TIPS-PEN crystals were evaluated by fabricating typical bottom-gate top-contact OFETs using a 100 nm-thick SiO2 dielectric layer (capacitance, Ci, 27.8 nF/cm2) and Au source/drain (S/D) electrodes (Figure 5). The S/D electrodes were deposited so that the charge carriers could flow along the coating direction (Figure 5a). The channel length (L) and width (W) defined by the S/D electrodes were 100 μm 10 ACS Paragon Plus Environment

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and 1000 μm, respectively. When extracting μFET values for the patterned TIPS-PEN crystals, an effective channel width, Weff, of W/2 was used to exclude the PUA regions. We found that the electrical properties agreed well with the results of pOM, pUV and GIWAXS analyses. The electrical performances of the OFETs were enhanced by employing the crystals with narrower widths. As summarized in Table 2, OFETs for the crystals with 100 μm widths did not show significantly better electrical performances than did the OFETs with the unpatterned crystals. Their average and maximum μFET values collected from 15 (100 μmwide patterned) and 17 (unpatterned) OFET samples were similar, but higher than those of the previously described OFETs with spin-coated and drop-casted crystals.17, 54 This improvement was attributed to directional growth of TIPS-PEN crystals along the coating direction. Their transfer curves were hysteresis free and output curves did not display noticeable contact resistance (Figures S5 and S6), features showing the decent quality of the crystals. As the pattern width was decreased, the electrical performance of their OFETs collected from 15 – 20 samples for each pattern width gradually improved (Figure 5d). OFETs for the crystals with 5 μm widths showed the highest average and maximum μFET values of 1.28 and 1.63 cm2/(V∙s), respectively, which are comparable to the values with previous high quality TIPS-PEN crystals (Figure 5d).14,

21, 22, 55

OFETs with the 5 μm crystals also showed the lowest Vth and SS

magnitudes among various OFETs in this study (Table 2), indicating these devices to be advantageous for low-power-consumption. These results were attributed to the uniaxially grown and highly ordered 5 μm-wide crystals with fewer grain boundaries as revealed by the above-described pOM, pUV and GIWAXS analyses.

2.4. Application to flexible and low-voltage-operating OFET arrays To demonstrate a practical application of the TIPS-PEN crystals with PUA patterns, we fabricated flexible, low-voltage-operating OFET arrays comprised of the crystals with 5 μm 11 ACS Paragon Plus Environment


widths on PES substrates. Bottom-gate top-contact OFET structures were prepared by depositing a 100 nm-thick Al gate electrode and spin-coating a poly(4-vinylphenol) (PVP) solution to form a 72 nm-thick crosslinked PVP (CPVP) dielectric layer (Figure 6a). Thin CPVP layer has been commonly used as an organic dielectric layer for low voltage operation of the OFETs due to its high dielectric strength.57-61 PUA patterns with 5 μm widths were fabricated on the CPVP layer under the same conditions with the rigid substrates. Then, a TIPSPEN/PS blend solution was blade-coated to grow the 5 μm-wide TIPS-PEN crystals on the CPVP surface with the same coating conditions for the PUA/SiO2 surface. The surface energy of the CPVP dielectric layer was high enough for wetting of toluene solvent (Table 1). OFET array fabrication was completed by depositing dense Au S/D dots with Weff and L channel dimensions of 26 μm and 37 μm, respectively (Figure 6b). Flexible OFET arrays based on unpatterned TIPS-PEN crystals were also fabricated (Figure S7a), and their electrical properties were evaluated and compared to those based on the patterned crystals. The Ci value for 72 nm-thick CPVP was 47.2 nF/cm2, which was higher than the value for the 100 nm-thick SiO2, enabling low-voltage operation of the OFETs. The OFETs with the crystals of 5 μm widths exhibited typical transfer and output characteristics (Figures 6c and 6d). It should be noted that the drain current (ID) at low gate voltage (VG) was in the range of 10-12 A in the transfer curve, while it was in the range of 10-10 A for all OFETs with unpatterned crystals (Figure 6c and Figure S7b). This result was attributed to the patterned TIPS-PEN crystals with minimized leakage current for the devices despite the thinness of the CPVP organic dielectric layer, implying that patterning an organic semiconducting layer is efficient at reducing leakage current and thereby static power consumption of integrated circuits for reliable and portable devices. Average and maximum μFETs of 17 OFETs over the area in Figure 6b operated at -5 V were 0.72 and 1.08 cm2/(V∙s), respectively, lower than the values of those on the rigid substrates. 17 devices in the entire OFET arrays (Figure 6b) were selected so that 12 ACS Paragon Plus Environment

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discernible defects were not observed. These lower μFETs could be attributed to the unreacted hydroxyl groups in the CPVP film acting as charge-trapping sites, degrading the electrical performance of the OFETs.60,

61

However, average μFETs higher than unpatterned OFETs,

hysteresis-free transfer curves, low Vth and SS values of -1.24 V and -0.15 V/decade suggested that high-quality TIPS-PEN crystal patterns were fabricated on the flexible substrates (Table 3). To investigate the reliability of the OFETs, we measured the OFETs under a series of drain voltages (VD) values and extracted μFETs from each of the transfer curves (Figure 6c). The average μFETs for VD values of -3, -4 and -5 were 0.69, 0.67 and 0.72 cm2/(V∙s); i.e., they hardly depended on |VD|. In general, for OFETs containing charge-trapping sites localized at the semiconductors or semiconductor/dielectric interface, the generation of mobile charge carriers in the channel has been shown to be delayed and charge transport limited for low values of |VD|, resulting in bias-dependent μFETs even for |VD| ≫ |Vth|.13 In the current study, the biasindependent μFET and low Vth and SS values for the flexible TIPS-PEN OFETs implied that few intrinsic charge-trapping sites existed by virtue of the high-quality 5 μm-wide TIPS-PEN crystals. In addition, the OFETs showed high operational stability under a sustained gate bias stress. The shift of Vth after the application of a VG of -5 V and VD of 0 V for 60 min was 0.11 V, which was sufficiently small considering the operation voltage. It was also notable that the off current was maintained at a level of 10-12 A. Therefore, we expect our demonstration of the flexible and low-voltage-operating TIPS-PEN OFET arrays to provide insight into future fabrications of electronic devices for reliable operation and low power consumption.

3. Conclusions



In conclusion, we introduced a facile way to pattern organic semiconducting crystals during blade coating with the aim of promoting practical uses of OFETs in reliable electronic circuits. We first showed the use of PUA for very easily fabricating hydrophobic linear patterns on a hydrophilic surface via one-step photo-curing. PUA precursors diluted in toluene were spincoated to form thin films on SiO2 substrates, and the films were selectively exposed to UV irradiation to yield alternating PUA and SiO2 lines on the surface. Due to the hydrophobic nature of PUA, the TIPS-PEN/PS blend solution only wetted the hydrophilic SiO2 regions during blade coating, and we were thus able to produce patterns of linear TIPS-PEN crystals of various widths. Crystal analysis tools revealed that the crystallinity and alignment of the crystals depended on the pattern widths. pOM images showed that as the pattern width was decreased, the morphology of the crystal became uniform, with few grain boundaries. pUV and GIWAXS experiments revealed that the order and uniaxial alignment of the crystals increased with decreasing line width. And indeed the electrical performances of the OFETs were observed to improve as the widths of the crystals were decreased. The highest μFET of 1.63 cm2/(V∙s) was obtained from the OFETs with 5 μm-wide crystals. As a practical application of this facile method for patterning organic semiconducting crystals, we fabricated flexible OFET arrays comprised of the 5 μm-wide crystals. They exhibited highly reliable operations with a quite low off current and small Vth shift under sustained gate bias stress. Therefore, we expect our work to not only provide an easy way to produce high-quality organic semiconducting crystal patterns, but also to provide important insight into the fabrication of reliable OFET arrays for their integration into electronic circuits.

4. Experimental


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Materials and OFET fabrication: TIPS-PEN (Aldrich, ≥ 99%), PS (molecular weight = 8,000, Aldrich), toluene (Aldrich), UV-curable PUA precursors (MINS-311RM, Minuta Tech.), PVP (Aldrich), poly(melamine-co-formaldehyde) (PMFA, Aldrich), N,N`-dimethylformamide (DMF, Aldrich) and SU-8 (MicroChem, 2000.5) were purchased and used without further purification. Highly doped n-type Si wafers each with a thermally grown 100 nm-thick SiO2 gate dielectric layer were cleaned with acetone and isopropyl alcohol for 60 min, respectively, and were exposed to UV-ozone for 20 min. PUA/SiO2 substrates were prepared by i) spincoating PUA precursors diluted in toluene at 2000 rpm for 30 s, ii) mild annealing at 70 оC for 10 min to evaporate the residual solvent in the films, iii) UV exposure (wavelength = 365 nm, intensity = 15-18 mW/cm2) for 20 s with a photomask and iv) rinsing off uncured regions with a toluene. As a control sample, SU-8/PUA substrates were prepared by performing conventional photolithography including spin-coating the solution at 3000 rpm for 30 s, soft baking for 1 min at 95 оC, UV exposure with 60 mJ/cm2, post baking for 1 min at 95 оC and development. TIPS-PEN/PS crystals were grown by blade-coating with a coating speed of 0.5 mm/s while maintaining the substrate temperature at 50 оC and the distance between the blade and the substrate of 80 μm. The blade coating was performed in the direction of the lines. Au S/D electrodes were thermally evaporated onto the semiconductors to fabricate bottom-gate top-contact OFETs. PES substrates for flexible OFETs were cleaned with isopropyl alcohol and deionized water each for 3 min, and then exposed to UV-ozone for 3 min. A 100 nm-thick Al gate was thermally evaporated onto the PES substrates. Subsequently, a CPVP layer was spin-coated by applying a 5 wt% PVP solution in DMF, and thermally annealed at 180 °C for 1 h to form a 72 nm-thick robust dielectric film. The PVP solution contained PVP and a PMFA crosslinker with a weight ratio of 5:1. The fabrication of 5 μm-wide TIPS-PEN patterns and Au S/D electrodes for



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bottom-gate top-contact OFET arrays followed the sample procedures on the rigid substrates. We used a 3 mm nickel grid (300 mesh, Ted Pella) as a metal mask for Au S/D arrays.

Characterizations: The thickness of PUA was measured by using an ellipsometer (J.A.Woollam Co. Inc.). Surface energies of the PUA, SU-8 and SiO2 films were obtained from contact angle measurements. Specifically, to calculate the surface energies, we used the contact angles from two test liquids (water and diiodomethane) and the equation 1

1 + cosθ =

1

2(𝛾𝑑𝑠)2(𝛾𝑑𝑙𝑣)2 𝛾𝑙𝑣

1

+

1

2(𝛾𝑝𝑠)2(𝛾𝑝𝑙𝑣)2 𝛾𝑙𝑣

,

where 𝛾𝑑𝑠 and 𝛾𝑝𝑠 indicate the dispersion and polar components of the surface energy, respectively, 𝛾𝑙𝑣 the surface energy of the test liquids, and 𝛾𝑑𝑙𝑣 and 𝛾𝑝𝑙𝑣 the dispersive and polar components, respectively. pOM and OM images of the TIPS-PEN crystals and PUA patterns were obtained by performing optical microscopy (Perkin-Elmer LAMBDA-900). pUV absorption spectra were obtained using a Cary 5000 UV-Vis-near-IR double-beam spectrophotometer. GIWAXS experiments were performed with an X-ray source (wavelength = 0.138 nm) at the 9A beamline in Pohang Accelerator Laboratory, Korea. Electrical properties of various OFETs were measured by using a Keithley 4200 SCS. The equation ID =μFETCi(W/2L)(VG-Vth)2 was used to extract μFET and Vth values in the saturation regime of the transfer curves. All μFET values in this study was extracted based on a previous method to provide the reliability of our result.62 Ci was measured by using an Agilent 4284 precision LCR meter with a metal-gate insulator-metal structure. Supporting Information Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. OM images of SU-8/TIPS-PEN patterns, a pOM image of unpatterned TIPS-PEN crystals, GIWAXS images of TIPS-PEN crystals, extracted 1D profiles, electrical characteristics of TIPS-PEN OFETs on Si and flexible substrates. Notes 16 ACS Paragon Plus Environment

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The authors declare no competing financial interest. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education(2018R1A6A1A03023788). This work was also supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. 2017R1C1B2002888, NRF-2017R1A1A1A05001233). References [1]

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Figure 1. Schematic diagram of micro-controlled blade coating for preparation of flexible TIPS-PEN OFET arrays. Hydrophobic PUA line patterns were fabricated by carrying out a one-step UV curing onto the precursors with the aid of a photomask. Blade coating of the TIPSPEN/PS blend solution was performed along the PUA lines. The solution was only wetted on the CPVP surface and thereby yielded the intended TIPS-PEN crystal patterns. Deposition of Au dot electrodes completed fabrication of the flexible OFET arrays.


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Figure 2. (a) OM images of various PUA/SiO2 substrates. The widths of the SiO2 regions where the TIPS-PEN crystals grew are noted in yellow. (b) pOM images of TIPS-PEN crystals of various widths. The axes of the polarizer (P) and analyzer (A) in the microscope are indicated in the image. (c, d) AFM topographies of PUA/SiO2 and PUA/TIPS-PEN substrates and (e) their cross-sectional height profiles.



Figure 3. (a-e) pUV spectra of blade-coated (a) unpatterned (b) 100 μm-, (c) 20 μm-, (d) 10 μm- and (e) 5 μm-wide TIPS-PEN crystals. (f) Dichroic ratio of absorption intensities at a wavelength of 697.5 nm as a function of crystal width. ‘Parallel’ indicates that polarization of incident monochromatic light was parallel to the coating direction.


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Figure 4. (a,b) GIWAXS images and (c,d) extracted profiles at qxy = 0.75 of blade-coated (a,c) 5-μm-wide and (b,d) unpatterned TIPS-PEN crystals, with the incident X-ray beams either parallel or perpendicular to the blade-coating direction.



Figure 5. (a) pOM image of an OFET with 5 μm-wide TIPS-PEN crystals and Au S/D electrodes. (Inset: Magnified OM image of the channel region.) (b) Typical transfer characteristics of OFETs for the various crystal widths. (c) Output characteristic of OFETs using 5 μm-wide crystals. (d) Corresponding average μFETs as a function of pattern line width.


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Figure 6. Flexible OFET arrays made with 5-μm-wide TIPS-PEN crystals. (a) Schematic diagram and (b) pOM images of the flexible OFET arrays. (c) Transfer characteristics for various VDs. (d) Output characteristic and (e) histograms of extracted μFETs obtained from 17 OFETs. (f) Transfer characteristics of the OFETs with a sustained gate bias stress, VG, of -5 V for 60 min.



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Table 1. Surface energies of various films used in this study.

PUA SiO2 SU-8 CPVP

Contact Angle Water Diiodomethane 96 64 6 4 72 10 74 33

𝜸𝒑𝒔[mJ m-2] 1.62 38.92 4.81 5.79

𝜸𝒅𝒔 [mJ m-2] 24.69 35.13 45.71 38.22

𝜸𝒔 [mJ m-2] 26.31 74.05 50.52 44.01

Table 2. Electrical characteristics of various blade-coated TIPS-PEN OFETs. They were obtained from 15 - 20 OFETs for each crystal. Widths of Crystals 20 μm 10 μm

Unpatterned Crystals

100 μm

0.57 ± 0.12

0.56 ± 0.13

0.80 ± 0.17

0.92 ± 0.19

1.28 ± 0.23

0.70

0.67

1.06

1.07

1.63

5.71105

1.87105

3.12105

3.90105

5.26105

Vth (V)

-1.49 ± 0.10

-1.31 ± 0.17

-1.00 ± 0.12

-0.95 ± 0.10

-0.95 ± 0.09

SS (V/dec)

0.49 ± 0.12

0.32 ± 0.08

0.32 ± 0.06

0.32 ± 0.05

0.25 ± 0.05

Average μFET (cm2/(V∙s)) Maximum μFET On/off ratio

5 μm

Table 3. Electrical characteristics of blade-coated flexible OFET arrays of unpatterned and patterned TIPS-PEN crystals. Unpatterned Crystals Average μFET (cm2/(V∙s))

0.38 ± 0.19

0.72 ± 0.27

0.56

1.08

3.07102

5.72104

-1.26 ± 0.24 2.06 ± 0.57

-1.24 ± 0.16 0.15 ± 0.05

Maximum μFET On/off ratio Vth (V) SS (V/dec)

Patterned Crystals

Table of Contents Graphic


Facile and Microcontrolled Blade Coating of Organic Semiconductor

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