Design Strategies in the Pen-Printing Technique toward Elaborated

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Design Strategies in Pen-Printing Technique toward Elaborated Organic Electronics Hayeong Jang, Seungtaek Oh, Seolhee Baek, Giheon Choi, Hyewon Cho, Heemang Yoo, Yoonseuk Choi, and Hwa Sung Lee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12091 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 14, 2019

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The Journal of Physical Chemistry

Design Strategies in Pen-Printing Technique toward Elaborated Organic Electronics Hayeong Jang1†, Seungtaek Oh1†, Seolhee Baek1, Giheon Choi1, Hyewon Cho1, Heemang Yoo1, Yoonseuk Choi2*, Hwa Sung Lee1* 1Department

of Chemical & Biological Engineering, Hanbat National University, Daejeon 34158, Republic of Korea 2Department

of Electronics and Control Engineering, Hanbat National University, Daejeon, 34158, Republic of Korea

†S.

Baek and H. Jang contributed equally to this work.

*Corresponding author. E-mail: [email protected] (Y. Choi), [email protected] (H. S. Lee)

Abstract Printing techniques suitable for the commercial production of organic electronics, especially organic field-effect transistors (OFETs), has been a topic of intense research interest. In the current work, we carried out systematic studies aimed at improving the printing performance of a facile pen-printing technique. To achieve this goal, two strategies were deployed. In the first strategy, the capillary pen of the pen-printing system was specifically engineered so that the nib angle could be controlled, and the angles 20˚, 25˚, 30˚ and 40˚ were tested; the aim here was to influence the properties of the discharging of the ink solution from the pen to the substrate, such as the amount of ink transferred and the area of contact between the nib and substrate surface. The other strategy involved precisely controlling the surface energy of the target substrate by treating the substrate with hexamethyldisilazane (HMDS)-, octadecyltrichlorosilane (ODTS)-, or HMDS:ODTS-mixed self-assembled monolayers, with the aim of affecting the wetting of the transferred ink. By combining these two strategies, we substantially improved the printing resolution of the pen-printing method; specifically, we were able to reduce the diameters of the patterned deposits from 205 to 31 μm for the water-soluble poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) ink and from 369 to 84 μm for the organic solvent-soluble poly(dimethyl-triarylamine) (PTAA) ink. We also tested the utility of the new pen-printing system by assessing the performances of PTAA transistors made using this system. The highest field-effect mobility (μFET) value we obtained was 0.009 cm2V-1s-1, among the highest reported for any PTAA transistor; this value was obtained using the pen-printing apparatus with a nib angle of 20˚ on a substrate treated with HMDS(1):ODTS(2). We expect these results to promote the use of the pen-printing technique in the commercial fabrication of highly integrated electronic devices in low-cost large-area printed fields.

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Introduction Printed electronics constitute an emerging technology with potential applications in photovoltaics, transistors, displays, batteries, antennas, and sensors.1-9 Recent attention has focused on the realization of low-cost large-area platforms for flexible and disposable devices.10-12 High-resolution direct printing techniques are of particular interest as alternatives to conventional vacuum deposition and photolithographic patterning for the production of functional films such as electrodes, gate dielectrics, and semiconductor layers. Each of the various direct printing techniques, including micro-contact printing (μ-CP), nano-imprinting, screen printing, drop-on-demand (DOD) inkjet printing, and roll-toroll printing, can be categorized as either a non-contact or contact printing method based on the printing procedure used.12-16 In noncontact printing, the patterning process involves discharging the ink solution without making direct contact with the target surface.5,7,9 Inkjet printing, aerosol-jet printing, spraying, and vapor-jet printing are included in this category.1,17-18 However, these techniques are only suitable for certain printing applications because the jetting of the organic ink is highly sensitive to the patterning conditions, including temperature, viscosity, concentration, and humidity.19-22 Furthermore, in noncontact printing, the patterning precision is often limited by the formation of satellites and tails, which degrades the electrical performance of the printed material and the reproducibility of the patterned devices.23-24 Contact printing methods, in which materials are directly patterned on a target substrate using a transfer medium (e.g., a mold, stamp, or roll), have been developed as another technique for fabricating printed electronics.25-26 These methods can afford patterns with greater patterning precision by the contact between the transfer medium and the target substrate, but they have low process rates and may damage to the substrate surface. These drawbacks have prevented the commercial use of contact printing methods. In previous studies, our research group successfully developed a new facile type of contact printing, namely the pen-printing method, in which capillary action is exploited to achieve the direct writing of soluble materials (semiconductors, conductors, or dielectric polymers) onto a variety of substrates without damaging the substrate.27-29 The pen-printing system consists of a capillary nib, an ink reservoir, and a multi-axis position controller that allows the efficient patterning of various soluble materials on the micron scale with a high printing speed. Unlike conventional contact printing systems, the penprinting method is remarkably simple, versatile, and, in particular, compatible with the fabrication of high-performance flexible organic electronic devices. This method has two main advantages. First, the process of printing using a capillary pen is not affected by external processing conditions such as ambient temperature, humidity, or air flow, because the ink solution is transferred to the target substrate through the capillary tube in the pen. And second, the capillary pen approach can suit to print a variety of patterns, from simple dots and lines to more complex patterns such as faces, onto both organic (paper or polymer) and inorganic (glass, ceramic, or metal) substrates. Despite these advantages, the printing 2


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performance of the pen-printing method, especially the pattern resolution, is still insufficient for applications in highly integrated electronic devices. For example, when the capillary pen method was used to print a dot-patterned poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonate) (PEDOT:PSS) electrode, the smallest diameter that could be achieved was around 170±15 μm; and when the method was used to produce a fully drawn organic field-effect transistor (OFET), the diameter was 600±20 μm.27-28 These diameters are much greater than the diameters under 100 μm for the organic transistors, respectively, required for highly integrated organic electronic devices. Therefore, systematic studies to improve the printing performances of the pen-printing technique need to be carried out in order for this technique to find widespread applications in the field of organic electronics. In the current study, through a systematic examination of the pen-printing technique, we improved the printing performance to a level at which it could produce the fine patterns of organic materials (semiconductors or conductors) required for highly integrated organic electronic devices. To achieve this goal, the capillary nib of the pen-printing apparatus was specifically engineered to control the capillary action and the amount of ink solution discharged. The capillary nib plays a key role in transferring the ink solution to the substrate by forming a contact with the substrate. To improve the pen-printing performance, we first investigated the relationship between the nib geometry and the resolution of the deposited pattern, and found it to greatly affect the resolution depending on the ink used. Secondly, by precisely controlling the surface energy of the target substrate, the characteristics related to the spreading (or wetting) of the transferred ink on the substrate were controlled. The combination of these two strategies allowed us to overcome the main drawback of the pen-printing method, namely the low printing resolution, and to specifically reduce the patterned dot size diameter from 205 to 31 μm for water-soluble PEDOT:PSS (polymer conductor) and from 369 to 84 μm for organic-soluble poly(dimethyl-triarylamine) (PTAA) (polymer semiconductor). To show the practical utility of this improved method, the relationships between these characteristics and the electrical performances of the resulting OFET devices were also investigated.

Experimental Pen-printing system. A capillary nib was obtained from a commercially available water-based pen (Monami, plus pen 3000), and any ink in the nib was completely removed by applying more than five washing cycles, each involving an ethanol rinse and sonication in a distilled water bath. Finally, the nib was dried using compressed N2 gas. To adjust the geometric angle of the nib end at which ink was discharged (referred to here as the ‘nib angle’), the end parts of the capillary nib were evenly polished on fine abrasive paper to create nibs with nib angles of 20 to 40˚ (see Fig. 1(c)). The nibs were combined with a home-made pen holder consisting of an ink reservoir and a pen extender fabricated from Teflon 3



and Al, respectively. The combined capillary pen was then mounted on a pen-printing system with a multi-axis motorized position controller and a CCD camera to visualize the drawing process in real time. It was confirmed that the nib touches the surface before starting the pen-printing process using the CCD camera and the travel distance recorded on the monitor, to check the moving distance of the nib when touching the surface. The position of the z-axis where contacts the nib on the substrate was set as a reference position of the pen-printing process, and then the moving distance of the nib was set to 300 μm through the programmed stepper motor, to ensure that pen-printing process works the same way over and over again. The pen-printing of both PEDOT:PSS and PTAA inks has proceeded to the same process as above. The control resolutions of the three-axis position controller were 10 μm in the x- and y-axes and 30 μm in the z-axis. The pen-printing was carried out by moving the stage with the stage speed programmed to be about 15 mm/s. When the nib touched the substrate surface, we immediately stopped the approach of the nib toward the substrate to avoid it from damaging the surface as much as possible. Eventually, the ink solution discharged from the end of the nib was transferred and patterned on the target substrate surface. Materials and OFET fabrications. Poly(3,4-ethylenedioxythiophene) doped with poly(4-styrene sulfonate) (PEDOT:PSS, Clevios PH 1000, Heraeus Deutschland Gmbh & Co.) was used as received without further purification as a conductive ink to form the organic electrodes. The ink containing PEDOT:PSS was printed onto the substrates. PTAA (Sigma-Aldrich Co., Mn = 7–10 kDa, no purification) was used as a p-type polymer semiconducting material. To optimize the drying conditions of the PTAA ink solution, 3 wt% PTAA solution in 1:1 mixtures of chlorobenzene (CB, Aldrich, anhydrous + 99%) and 1,2-dichlorobenzene (DCB, Aldrich, anhydrous +99%) were used in the penprinting system. An appropriate surface wettability of the ink solution was obtained by controlling the surface energy of the substrates using two kinds of silane coupling agents (or self-assembled monolayers, SAMs): hexamethyldisilazane (HMDS) and octadecyltrichlorosilane (ODTS). These SAM materials were purchased from Sigma-Aldrich as received. OFET devices were prepared on the variously treated substrates, each of which was prepared from a highly doped n-Si wafer with a 300 nm-thick thermally grown oxide (SiO2) layer. The wafer served as a gate electrode, whereas the oxide layer played the role of the gate dielectric (capacitance = 10.8 nFcm2).

The HMDS- and ODTS-SAMs were coated onto the SiO2/Si substrates using spin-casting and

solution-dipping to modify the SiO2 surface, respectively. Alternatively, the PEDOT:PSS source/drain electrodes and the PTAA active layers with the dot-shaped deposits were fabricated using a pen-printing system. Characterizations. The pen-printing nib was characterized using field-emission scanning electron microscopy (FE-SEM, Hitachi S-4200). The morphologies of the printed deposits were characterized 4


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using optical microscopy (OM, Eclipse 80i, Nikon) and atomic force microscopy (AFM, Digital Instruments Multimode) operated in tapping mode with an SiNx cantilever and Si tips (42 N/m and 320 kHz, tip radius: 10 nm). Data analysis was performed using Nanoscope 5.30 software. The profiles of the PEDOT:PSS electrodes on the substrate were measured using an alpha-step profilometer (Dektak 150, Veeco). The surface energies of the variously treated substrates were determined by measuring the contact angle, θ, values of two probe liquids, specifically water and diiodomethane, using a contact angle analyzer (Phoenix 300A, SEO Co., Inc.). The surface energy of each electrode (γs) was calculated by fitting the Owens–Wendt equation

1  cos  lv  2

 sd lvd  2  sp lvp

,

where γlv is the surface energy of the probe liquid, and the superscripts d and p refer to the dispersive and polar (nondispersive) terms, respectively. The current-voltage characteristics of the PTAA OFETs were examined by operating the devices under a negative gate voltage. The source electrode was grounded and the drain electrode was negatively biased. The electrical parameters characterizing the OFETs were obtained at room temperature using an HP4156A instrument in a dark environment under ambient conditions.

Results and Discussion

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Figure 1. (a) Schematic diagram of the pen-printing system. The inset shows a schematic of the discharging of an ink solution at the capillary pen nib. (b) Captured CCD snapshot of the sequential pen-printing process used to fabricate the PTAA OFET device; here, PEDOT:PSS electrode deposits were printed on the semiconducting PTAA deposits. The inset shows a schematic diagram of the OFET structure with a bottom-gate top-contact configuration used in our study. (c) Photographs and (d) SEM images of the capillary nibs with nib angles of 20˚ to 40˚ mounted in the pen-printing system. (e) Chemical structures of HMDS- and ODTS-SAMs, and schematic diagrams of representative substrates whose surface energies were modified by using HMDS-, HMDS:ODTS-, and ODTS-SAMs.

The patterning procedure using the pen-printing system is depicted schematically in Figure 1(a). The pen-printing system can provide an efficient printing method for fabricating low-cost large-area organic electronics. In the printing system, ink migrates along the capillary tube from an ink reservoir to the nib, and then flows out from the end of the nib to be deposited on the target substrate.27-28 As the nib approaches and slightly contacts the substrate surface, an ink meniscus forms between the nib end and the substrate through the surface tension of the ink solution, enabling the transfer of ink from the pen 6


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to the substrate with a uniform discharge volume. Our home-built pen-printer combined a pen with a xyz-axis position controller that enabled the drawing of various patterns, with a high degree of control over the interval and size, without damaging the substrate. Figure 1(b) shows a snapshot of the sequential pen-printing process used to fabricate the OFET devices. To improve the pattern resolution of the pen-printing method, we used two strategies: engineering the nib angle and controlling the surface energy of the substrate. In the first strategy, we expected that the size of the printed deposits made by transferring the ink solution onto the substrate could be adjusted by changing the nib angle of the nib, thereby changing the area of the substrate struck by the ink solution during the pen-printing process. Figures 1(c) and (d) show CCD and SEM images of the engineered capillary nibs with nib angles of 20˚ (CP20), 25˚ (CP25), 30˚ (CP30), and 40˚ (CP40). In the second strategy, to control the spreading or wetting properties of the droplets of the ink solution, the surface energies of the substrates were set at values from 30.9 to 45.0 mJ·m-2 by treating them with mixtures of HMDS- or ODTS-SAMs and were finely adjusted by varying the ratio of these SAMs in the mixture. Figure 1(e) shows schematic diagrams of the results of three representative treatments of the substrates. The ODTS-treated substrate had a relatively low surface energy of 30.9 mJ·m-2, whereas the HMDStreated case showed a higher surface energy of 45.0 mJ·m-2. As the mixing ratio of the HMDS-SAMs increased, the surface energies increased stepwise, and this method was found to be efficient in controlling the surface energy. The data for calculating the surface energies were summarized in Table S1.

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Figure 2. (a,b) OM images of pen-printed (a) PEDOT:PSS and (b) PTAA deposits made using the penprinting system using nibs with nib angles of 20 to 40˚. (c) Diameters of the pen-printed PEDOT:PSS and PTAA deposits as a function of the nib angle of the capillary pen. The inset shows the chemical structures of PTAA and PEDOT:PSS.

The effects of nib angle on the sizes of the pen-printed PEDOT:PSS and PTAA deposits were examined by testing nib angles of 20˚, 25˚, 30˚, and 40˚. Figures 2(a) and (b) show optical microscopy (OM) images of patterns of, respectively, 25 PEDOT:PSS and PTAA deposits (with a dot-to-dot distance of 300 μm) prepared using the pen-printing system. Prior to the processing with the pen-printer, the surface energy of the substrate was tuned to be relatively hydrophobic (a surface energy of 45.0 mJ·m-2) by treating the SiO2/Si substrate with HMDS. Previously, HMDS-treated substrates have been shown to display appropriate interaction characteristics with both polar PEDOT:PSS and non-polar PTAA ink solutions.27 Hence, in the present experiments, the ink transferred from the nib end was well pinned at the contact line, and then each printed deposit formed a clear round shape regardless of the nib angle. The morphological properties of the PEDOT:PSS deposits were found to strongly depend on the nib angle, as shown in Figure 2(a): the diameters of the deposits increased from 61 ± 7 μm to 205 ± 18 μm as the nib angle was increased from 20˚ to 40˚. On the other hand, the size of the PTAA deposit (Figure 2(b)) hardly changed as the nib angle was changed: specifically, as the nib angle was increased from 20˚ to 40˚, the diameter of the deposit increased from 355 ± 10 to 362 ± 12 μm, a change of only 1.7%. The deposit diameters as a function of the nib angle of the capillary pen are summarized in Figure 8


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2(c). The factors affecting the spreading properties of solution droplets on a surface can be classified as external variables such as the surface energy of the substrate as well as the ambient temperature and humidity, and internal variables such as the surface tension of the ink solution and the chemical structures of the molecules contained in the ink solution.30-34 In the system described here, internal variables were expected to be the main factors affecting the dependence of the deposit size on nib angle, because the external variable conditions were kept constant. Of the internal variables, the difference between the surface tensions of the PEDOT:PSS and PTAA ink solutions was considered to be the most reasonable cause of the different deposit sizes observed for the PEDOT:PSS and PTAA cases. The PEDOT:PSS ink solution was produced using water as the main solvent. The high surface tension of water (72 mJ⋅m-2) sufficiently affected the spreading (or wetting) behavior to such an extent that the spreading (or wetting) of the ink droplet on the substrate surface was mainly influenced by the amount of ink solution transferred. As the nib angle was increased, the amount of solution transferred to the substrate increased, which subsequently increased the spreadability of the solution droplet on the substrate surface and resulted in the marked increase in diameter of the deposit described above and shown in Figure 2(a). The PTAA ink solution, in contrast, was composed of CB/DCB-mixed solvent with a low surface tension of 35 mJ⋅m-2, so that the spreading (or wetting) properties of the ink droplet on the substrate were apparently hardly affected by the external and internal variables, explaining the very small dependence of pattern deposit size on capillary nib angle, as shown in Figure 2(b).

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Figure 3. CCD-derived snapshots of the ejection of PEDOT:PSS ink from the capillary nib of the developed pen-printing system for the (a) CP30 and (b) CP20 systems. (c) OM images and their height profiles of one PEDOT:PSS deposit pen-printed using the CP20, CP25, CP30, and CP40 nibs.

To confirm the ejection properties of PEDOT:PSS ink through the capillary tube in the nib of the capillary pen, we captured and analyzed CCD images of the end of the nib from which the ink solution was being discharged. In the case of CP30 of Figure 3(a), the PEDOT:PSS ink solution was seen moving through the capillary tubes to the end of the nib at 3 seconds after the ejection process was initiated, and the ink was observed throughout the nib at 13 seconds. At about 20 seconds, the dark blue ink solution was clearly observed to have filled the volume inside the capillary nib, indicating that the PEDOT:PSS ink was fully lodged throughout the nib. In Figure 3(b), the CP20 case as well as the CP30 case showed that the ink migrated throughout the capillary tubes to the end of the nib, although the transfer of the ink solution was relatively slowly for the CP20 case. The ink was finally lodged in the nib at about 240 seconds. We found that the slower movement of ink through the capillary for the CP20 case, relative to the CP30 case, only postponed the process at the initial stages, and did not result in any worse penprinting performance. Figure 3(c) shows OM images and height profiles of the PEDOT:PSS deposits printed with the CP20, CP25, CP30, and CP40 nibs on HMDS-treated substrates. The HMDS-treated substrate was used for evaluating the dependence of deposit shape on nib angle, because this surface has the appropriate interaction characteristics with the polar PEDOT:PSS ink solution as mentioned 10


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above. All PEDOT:PSS deposits were observed to display dot-like shapes when using a nib angle of 20˚ or 25˚, and table-like shapes for a nib angle of 30˚ or 40˚. In general, the process by which a printed ink droplet dries plays a vital role in controlling the deposit morphology and distribution of solute in the deposit, and hence affects its final shape.25,30,35-36 Solution droplets pinned on the surface of a substrate may be affected by two main flows: evaporation-driven and diffusion-driven flows. Evaporation-driven flow is radially directed, from the droplet center to the contact line, to compensate for the difference in evaporation rate and volume change across the drop.32,37-38 On the other hand, diffusion-driven flow is directed toward the droplet center to equilibrate the solute concentration gradient that is set up between the periphery and interior of the droplet.19,39-41 Usually, when evaporation-driven flow is much faster than the diffusion-driven flow, most of the dissolved material is transported to the contact edge and then deposited at the periphery of the drop, resulting in a ring-like deposit. This phenomenon is known as the coffee-stain effect.39-40 In contrast, if the diffusion of the dissolved molecules is much faster than the evaporation-driven flow, the solute concentration is uniform throughout the solution at all times within a solution droplet. Therefore, in this case, the final deposited molecules are gathered mainly in the droplet center (to form a dot-like deposit). A balance between evaporation- and diffusion-driven flows could induce a table-shaped deposit. The formation in our experiments of table-shaped deposits when using large nib angles suggested that both types of flows were operating. In addition, the above-described increase in deposit size with nib angle in our PEDOT:PSS experiments can reasonably be attributed to the contact area between the nib and the surface also having increased with increasing nib angle.

Figure 4. (a) Surface energy as a function of the HMDS-SAM-to-ODTS-SAM blending ratio. (b,c) Sizes (or diameters) of (b) PEDOT:PSS and (c) PTAA deposits pen-printed with various nib angles as a function of surface energy. 11



To determine the effects of surface energy on the sizes of the pen-printed deposits, we first produced substrates having a variety of surface energies. Figure 4(a) shows the surface energies of SiO2 substrates treated with SAMs with various HMDS-to-ODTS blending ratios. The substrate treated with only ODTS SAM, consisting of 18 hydrocarbon chains, showed a relatively low surface energy of 30.9 mJ⋅m2,

while that treated with HMDS SAM alone showed a higher surface energy of 45.0 mJ⋅m-2. And

increasing the relative amount of HMDS SAM in a mixture of these SAMs resulted in a proportional increase in the surface energy. These results revealed a way to precisely control the surface energy. We then plotted the sizes of the deposits made using the pen-printing method as a function of the surface energy for each of the CP20, CP25, CP30, and CP40 nib angles (Figures 4(b) and 4(c)). Also, OM images of these deposits are shown in Figures S1 and S2. When using PEDOT:PSS conducting ink as the deposited polymer electrode, the average deposit diameters were observed to increase more or less continuously throughout the tested range of surface energies, specifically from 31 ± 2 μm to 61 ± 12 μm for the CP20 case and 98 ± 12 to 205 ± 13 μm for the CP40 case as the surface energy was increased from 30.9 to 45.0 mJ⋅m-2 (Figures 4(b) and S1). In contrast, when using PTAA as the deposited polymer semiconductor, the average size of the deposits increased markedly as the substrate surface energy was increased from 30.9 to about 36.1 mJ⋅m-2, but then showed no further increase as the surface energy was increased further to 45.0 mJ⋅m-2 (Figures 4(c) and S2). Note that, as described above and shown in Figure 2, the average size of the PTAA deposits was relatively insensitive to the nib angle, whereas the average size of the PEDOT:PSS deposits showed a stronger dependence on the nib angle, which was understood to be due to the difference between the high surface tension (72 mJ⋅m-2) of the water-based ink and low surface tension (35 mJ⋅m-2) of the organic-based ink. In addition, another interesting result can be seen. The printed sizes of the PEDOT:PSS deposits were continuously increased with the surface energies of the substrates, although the increasing rate was slower from the range of the surface energy of 36.1 or 38.0 mJ⋅m-2. On the other hand, the increases of the printed sizes in the PTAA deposits were saturated at a certain surface energy (36.1 mJ⋅m-2) or more. The results could also be explained by the spreading of the ink solution having been governed by a competitive relationship between the surface tension of the ink solution and the surface energy of the substrate. That is, since the effect of the surface tension of the PEDOT:PSS ink was already saturated due to its high surface tension, it could not be a main cause of differences in the spreading behavior of the ink droplet. Therefore, for this ink, the average size of the printed deposits could continuously increase as the surface energy was increased. Whereas, the PTAA case showed the dramatic size increases of the printed deposits in the lower region than the surface tension (35 mJ⋅m-2) of the ink with 1:1 mixing of CB and DCB, and then the size increase of the PTAA deposits were fully 12


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saturated in the range of high surface energy over 36 mJ⋅m-2 to reduce a total energy having the ink and the substrate surface. To determine the relationship of the nib angle or surface energy with the electrical performances in PTAA OFETs, we obtained their output characteristics, i.e., drain current (ISD) – drain voltage (VD) curves, and transfer characteristics, i.e. ISD – gate voltage (VG) curves. Figure S3 shows the output characteristics of the PTAA OFETs with variable conditions, the nib angle and surface energy. All devices were found to be well-behaved p-type transistors with a clear observation of a linear regime at small VD and saturation regime at VD exceeding VG. High linearity was observed at low VD values for all devices, indicating close-to-Ohmic contact and low contact resistance.42 Figure S4 shows the transfer characteristics of the PTAA OFETs at the saturation regime with a VD of -40 V, which allowed us to extract device parameters including on/off current ratio, threshold voltage (Vth), and most importantly field-effect mobility (μFET). The on/off current ratio was extracted from the curves on a logarithmic scale. The ISD0.5–VG plots (open circles) that were used in extracting Vth and μFET values for the PTAA OFETs exhibited high linearity, indicating an almost VG-independent μFET with high reliability for all devices. Furthermore, negligible hysteresis was observed for all devices and the curves were independent of VG sweep rate. The Vth and μFET values were determined from linear fits of the ISD0.5–VG curves at the on-state, in which the VG-axis intercept (ISD = 0) corresponded to the Vth and the square of the slope was proportional to the μFET = 2L/(CiW)∙(∂ISD0.5/∂VG)2. For the case of PTAA OFETs printed on the ODTS-treated surfaces, somewhat high off-current levels in the transfer curves were observed irrespective of the nib angle, compared to the other surface-treated cases. Given that high off-currents are often attributed mainly to contact problems at the semiconductor/dielectric interface, the low surface energy of the ODTS-treated surfaces presumably provided the poor adhesion behaviors of the penprinted PTAA deposits, leading to charge trapping at the interface.43-44 The OFET parameters obtained from the transfer curves are summarized in Table S2. Note that channel length and width of the penprinted PTAA OFETs depended on the conditions used to print the devices, explaining the inconsistency between the trends of the current levels for the output and transfer characteristics in Figures S3 and S4 and the trends of the μFET values shown in Table S2.

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Figure 5. Graph of the values of μFET for PTAA OFET devices made using various substrate surface treatments (and hence surface energies) and various nib angles.

Figure 5 shows a graphical representation of the results obtained in Table S2 to clearly compare the performances of the various PTAA OFETs. It was difficult to pinpoint a specific relationship between the μFET and PTAA OFET substrate surface energy. However, when comparing the results for one penprinting process nib angle at a time, a specific relationship between the μFET and PTAA OFET substrate surface energy was clearly seen. For the CP20, CP25, and CP30 nib angles, the highest μFET values were found for the OFETS made with substrates treated with HMDS(1):ODTS(2) and thus having a surface energy of 38.0 mJ⋅m-2. And for CP40, while the highest μFET was measured for HMDS(1):ODTS(1), that for HMDS(1):ODTS(2) was nearly as high. Furthermore, as the nib angle of the capillary pen was increased, the effect of the surface energy on the OFET performance was found to have decreased, i.e., the differences between the μFET values decreased. This result may have been due to an increase in the amount of the ink solution ejected from the nib end as the nib angle was increased, with the amount of ink ejected rather than the surface energy having become the predominant factor affecting performance. That is, for the CP20 case, having a small geometry at the nib end, the amount of ink transferred to the substrate surface was small due to a small area of contact between the capillary pen and substrate, and thus a change in the surface energy may have been the more important parameter affecting the electrical performances of the pen-printed PTAA deposits. On the other hand, for the CP40 case, a large amount ink was transferred to the surface, and the surface energy was hence a less significant factor affecting the PTAA OFET performance.

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Figure 6. AFM images of the bottom (dielectric interface) surfaces at the PEDOT:PSS/PTAA interface regions, detached from the (a) HMDS-, (b) HMDS(1):ODTS(2)-, and (c) ODTS-treated surfaces.

As shown in Figure 5, we confirmed the relationships of μFET with nib angle and surface energy. As mentioned above, the highest μFET values of PTAA OFETs were generally found when the OFET substrate surface was treated with HMDS(1):ODTS(2) and hence had a moderate surface energy of 38.0 mJ⋅m-2. The PTAA OFETs fabricated using ODTS, which yielded the lowest surface energy of 30.9 mJ⋅m-2, did not show superior electrical performance, and their μFET values showed some deviations depending on the nib angle. We had expected that the treatment of the substrate surface with HMDS, and the subsequent formation of a surface energy of 45.0 mJ⋅m-2, would yield the highest μFET values, but low electrical performances were observed for this system. Our expectation was due to modifications of surfaces using HMDS SAMs having been generally recognized as a suitable approach to obtain strong adhesion between the organic semiconductor and the substrate surface, and to hence enhance OFET performance. To investigate the cause of these partly unexpected μFET results, AFM analyses were conducted on the bottom surfaces of the interface region between the conducting PEDOT:PSS and semiconducting PTAA deposits, which detached from the various dielectric surfaces in the HMDS, HMDS(1):ODTS(2), and ODTS cases. Figure 6 shows their AFM topography images. On the HMDS-treated surface, shown in Figure 6(a), percolated PEDOT:PSS morphologies were observed in the lower part of the PTAA film. And as shown in Figure 6(c), holes were observed in the ODTS-treated surface; these holes were attributed to insufficient contact between PTAA and PEDOT:PSS at the interface. On the other hand, PTAA and PEDOT:PSS exhibited good contact in the HMDS(1):ODTS(2)-treated system, as shown in Figure 6(b). These results can be explained based on the wetting behavior of the PEDOT:PSS ink solution with high surface tension. Water-based PEDOT:PSS ink discharged onto the high-surfaceenergy HMDS-treated surface showed greater wetting than did the organic solvent-based PTAA ink. Therefore, the PEDOT:PSS ink percolated under the PTAA deposit, and their interface structure 15



became distorted as shown in Figure 6(a). Conversely, the PEDOT:PSS ink did not adhere well to the low-surface-energy ODTS-treated surface. This poor adhesion resulted in the incomplete wetting of the ink at complex parts of the composite, such as at the boundaries between the PTAA and the PEDOT:PSS deposits, and hence also resulted in the formation of holes at the PTAA/PEDOT:PSS interface. Apparently, according to our results, an appropriate interaction occurred between the PTAA ink or the PEDOT:PSS ink and the medium-surface-energy HMDS(1):ODTS(2)-treated surface. This appropriate interaction apparently helped to avoid the problems induced by additional ink percolation or insufficient wetting at their interfaces, and hence formed the best interface structures in the devices. The reasons described above are the rational interpretation that explains why the μFET for the HMDS(1):ODTS(2)treated case yielded the OFETs with the best as shown in Figure 5.

Conclusions In summary, we conducted a systematic study aimed at improving the printing performance of the facile pen-printing technique to permit the fine patterning of (semiconductor or conductor) organic materials on substrates as part of the fabrication of highly integrated electronic devices. To achieve these goals, two strategies were deployed. In one, the capillary pen of the pen-printing system was specifically engineered to vary the nib angle, and tested angles of 20˚ to 40˚. Depending on the ink used, the nib geometry played a key role in the transfer of the ink to the substrate, resulting from the effects of the nib angle on the amount of ink solution discharged and on the area of contact between the nib end and the substrate surface. The other strategy involved precisely controlling the surface energy of the target substrate, accomplished by treating it with HMDS-, ODTS-, or HMDS:ODTS-mixed SAMs, which affected the spreading (or wetting) of the transferred ink. By combining these two strategies, we substantially improved the printing resolution of the pen-printing method, achieving reductions in the diameters of the patterned deposits from 205 to 31 μm when using the water-soluble PEDOT:PSS (polymer conductor) ink and from 369 to 84 μm for organic-soluble PTAA (polymer semiconductor). We also tested the utility of the new pen-printing system by assessing the performances of PTAA OFETs made using this system. The highest μFET value we obtained was 0.009 cm2V-1s-1, among the highest reported for any PTAA OFET; this value was obtained when the CP20 pen-printing apparatus was used on a substrate treated with HMDS(1):ODTS(2). The pen-printing technique could be extended to a variety of site-selective surface pattern structures, including those used in organic electronics, selective surface masking for lithography, and surface-mediated chemical reactions. This pen-printing approach holds great promise for use in the commercialization of low-cost large-area printed fields.

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ASSOCIATED CONTENT Supporting Information: The Supporting Information is available free of charge on the ACS Publications website at DOI: 000000. Contact angles and surface energies of various SAM-treated substrate; additional OM images of PEDOT:PSS and PTAA deposits patterned using pen-printing process; Output and transfer characteristics of PTAA OFETs; summary of PTAA OFET electrical performances.

Acknowledgements: This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF2016R1D1A1B03936094 and 2018R1A6A1A03026005).

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Table of Contents (TOC) Graphic

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Design Strategies in the Pen-Printing Technique toward Elaborated

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