Directionally Aligned Amorphous Polymer Chains via

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Directionally aligned amorphous polymer chains via electrohydrodynamic-jet printing: Analysis of morphology and polymer field-effect transistor characteristics Yebyeol Kim, Jaehyun Bae, Hyun Woo Song, Tae Kyu An, Se Hyun Kim, Yun-Hi Kim, and Chan Eon Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 23, 2017

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ACS Applied Materials & Interfaces

Submitted to ACS Applied Materials & Interfaces, 2017

Directionally aligned amorphous polymer chains via electrohydrodynamic-jet printing: Analysis of morphology and polymer field-effect transistor characteristics Yebyeol Kima,1, Jaehyun Baeb,c,1, Hyun Woo Songb, Tae Kyu And, Se Hyun Kimb,e,*, and YunHi Kimf,*, and Chan Eon Parka,*

a

POSTECH Organic Electronics Laboratory, Polymer Research Institute, Department of

Chemical Engineering, Pohang University of Science and Technology, Pohang, 37673, Republic of Korea b

Department of Advanced Organic materials Yeungnam University, Gyeongsan, 38541,

Republic of Korea c

Korea Dyeing Technology Institution (DYETEC), Deagu 41706, Republic of Korea

d

Department of Polymer Science & Engineering, Korea National University of

Transportation, 50 Daehak-Ro, Chungju, 27469 Republic of Korea 1

ACS Paragon Plus Environment


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e

School of Chemical Engineering, Yeungnam University, Gyeongsan, 38541, Republic of

Korea f

Department of Chemistry and RIGET, Gyeongsang National University, Jin-ju, 52828,

Republic of Korea

2


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ABSTRACT

Electrohydrodynamic-jet (EHD-jet) printing provides an opportunity to directly assembled amorphous polymer chains in the printed pattern. Herein, an EHD-jet printed amorphous polymer was employed as the active layer for fabrication of organic field-effect transistors (OFETs). Under optimized conditions, the field-effect mobility (µFET) of the EHD-jet printed OFETs was five times higher than the highest µFET observed in spin coated OFETs and this improvement achieved without the use of complex surface templating or additional pre- or post-deposition processing. As the chain alignment can be affected by the surface energy of the dielectric layer in EHD-jet printed OFETs, dielectric layers with varying wettability were examined. Near-edge X-ray absorption fine structure measurements were performed to compare the amorphous chain alignment in OFET active layers prepared by EHD-jet printing and spin coating.

KEYWORDS: Organic field-effect transistor (OFET); Electrohydrodynamic-jet printing (EHD-jet printing); Surface treatment; Near-edge X-ray absorption fine structure (NEXAFS); Directional chain alignment

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1. INTRODUCTION The development of organic field-effect transistors (OFETs) has advanced significantly over the last two decades due to new materials synthesis and device engineering technologies. Such progress in the OFET field is expected to fulfill the increasing demand for low-cost switching and amplifying circuits as well as for numerous other applications. Among the OFET fabrication technologies, printing technologies, such as ink-jet, aerosol-jet, and electrohydrodynamic-jet printing (EHD-jet printing), have advantages over vacuum- and photolithography-based technologies, including high throughput and efficient deposition via non-contact, maskless, continuous, and direct patterning.1–4 Such printing processes are also required to fabricate integrated OFET arrays to minimize channel leakage and device-todevice crosstalk. The patterning of the semiconductor is the most indispensable element in the abovementioned fabrication process.5,6 Among the printing technologies, EHD-jet printing has been intensively investigated due to its potential for high-resolution patterning for integrated circuits.4,7,8 In a typical EHD-jet printing process, a jet stream is ejected by balancing various forces acting within an ink drop hanging on a nozzle tip (such as gravity, pressure, surface tension, viscosity, and electrostatic forces) and between the nozzle and the substrate on the stage (the electric field), and printing is implemented by moving the stage.8–10 Fine control of the forces applied allows the meniscus of the ink drop to be deformed, thereby resulting in several jetting modes, namely, dripping, micro-dripping, cone-jet, and multi-jet.11 A detailed explanation of the jetting modes in EHD-jet printing was given by Rogers et al.8 Among the various jetting modes, the cone-jet mode enables a continuous ultrathin jet stream to be ejected from a conical meniscus, which results in the formation of narrow line patterns with line widths down to the 4


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sub-micrometer.8 The continuous jet stream by finely controlled EHD-jet printing process could lead to continuously printed semiconductor patterns, which provides an opportunity to directionally align crystals and polymer chains in the printed pattern. Amorphous polymers are attractive ink materials for EHD-jet printing because of their high solubility in processing solvents, which minimizes nozzle clogging problems. In addition, the amorphous polymer films that are used as the active layers of OFETs exhibit morphological uniformity and reproducibility without energetic complexity. Although amorphous polymers possess the inherent structural feature of short-range ordering between polymer chains, which reduces charge carrier transport in the π-π conjugation system, the charge carrier transport in an amorphous polymer is significantly dependent on the chain features. The directionally extended amorphous polymer chains prepared by EHD-jet printing can facilitate charge carrier transport through the polymer backbones as well as improve the probability of πorbital overlap between neighboring amorphous polymer chains. Unfortunately, however, the relationship between the directional alignment of amorphous polymer chains and the characteristics of the corresponding OFETs has not yet been discussed. In the present study, the aforementioned issues were considered when constructing EHD-jet printed OFETs using an amorphous polymer semiconductor. Previously, we synthesized an amorphous

polymer

semiconductor,

poly[(1,2-bis-(2′-thienyl)vinyl-5′,5′′-diyl)-alt-(9,9-

dioctyldecylfluorene-2,7-diyl] (PFTVT), which was used to fabricate OFETs by spin coating. The OFETs showed field-effect mobilities (µFETs) of up to 10−2 cm2 V−1 s−1.12 In the fabricated patterns using EHD-jet printing, although the amorphous polymer does not exhibit long-range ordering, it is expected that the directional alignment between polymer chains within a very confined area will improve charge carrier transport. In order to obtain a micro5



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patterned OFET array from amorphous and chain-aligned PFTVT, the EHD-jet printing method was applied on the surface-energy-controlled dielectric layers. The directional chain alignment in the amorphous PFTVT micro-patterned OFET array was enabled via the finely controlled jet stream that used the EHD cone-jet mode and the surface-energy-controlled dielectric layer. The electrical performance of the PFTVT patterns was investigated using OFET analysis.

2. EXPERIMENTAL SECTION 2.1. Material Preparation Materials. The solvents and reagents were purchased from Aldrich, Alfa Aesar, or TCI. The catalysts used in the coupling reactions were purchased from Umicore. Other materials were of common commercial grade and used as received. PFTVT was polymerized using two monomers: 1,2-(E)-bis(5′-bromo-2′C-thienyl)ethene and 2,7-bis(4,4,5,5-tetramethyl-1,3,2dioxaborolan2-yl)-9,9-dioctylfluorene via a Suzuki coupling reaction.13 1,2-(E)-Bis(5′bromo-2′C-thienyl)ethene was prepared using 2-bromothiophene, paraformaldehyde, HOAc, 5-bromo-2-thiophenecarboxaldehyde, and sodium hydride. 2,7-Bis(4,4,5,5-tetramethyl-1,3,2dioxaborolan2-yl)-9,9-dioctylfluorene was prepared using 2,7-dibromo-9,9-dioctylfluorene, n-butyllithium, and 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. After the reaction, PFTVT was end-capped with 2-bromonaphthalene. A Soxhlet extraction was performed to remove oligomers. Hexamethyldisilazane (HMDS) and octadecyltrichlorosilane (ODTS) were used to modify the surface of SiO2 dielectrics. HMDS was used as the master solution without any dilution, and ODTS was diluted with toluene [ODTS (70 µL)/toluene (70 mL)]. 6


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To fabricate spin coated films and EHD-jet printed patterns, chloroform was used as the solvent. The number-average molecular weight (Mn) and polydispersity index (PDI) of the synthesized PFTVT were determined by gel permeation chromatography analysis. The Mn was 11 325 g/mol with the PDI of 2.10 with respect to a polystyrene standard.

2.2. Fabrication of spin coated and EHD-jet printed OFETs Both spin coated and EHD-jet printed OFETs were fabricated with the same bottom-gate, top-contact architecture. Highly n-doped silicon and thermally grown 300 nm-thick SiO2 were used as the gate electrode and the dielectric layer, respectively. Self-assembled monolayers (SAM) of HMDS and ODTS were chemically introduced onto bare SiO2 substrates pre-cleaned with piranha solution [H2O2 (40 mL)/concentrated H2SO4 (60 mL)] to control the surface energy of the SiO2 dielectric layer. The PFTVT semiconductor layer was deposited by either spin coating or EHD-jet printing. For the fabrication of spin coated OFETs, a 2 mg/mL PFTVT solution in chloroform was spin coated at 4000 rpm onto bare (non-treated), HMDS-treated, and ODTS-treated SiO2 surfaces, yielding films with an average thickness of 20 nm, as confirmed by ellipsometry (FQTH-100, J. A. Woollam Co., Inc.). For the fabrication of printed OFETs, EHD-jet printing of PFTVT was conducted using an EHD printer (Enjet, Suwon, Korea). A metallic nozzle holder attached to a glass syringe in the EHD printer was filled with a 2 mg/mL PFTVT solution. The PFTVT ink was ejected at a flow rate of 1 µL/min using a motorized pump through a nozzle with a diameter of 220 µm. The substrate temperature was maintained at 50 °C during the printing process. The electrostatic field applied between the nozzle and the Au substrate ground was generated by an installed power supply. An x, y-axis stage was used to control the printing speed and 7



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working distance, which were fixed at 200 mm/s and 100 µm, respectively. The entire process was interfaced with a computer and monitored using a CCD camera. Finally, Au source and drain electrodes (100 nm thickness) were deposited by thermal evaporation through a shadow mask.

2.3. Characterization of OFETs Unencapsulated OFETs were measured under a N2-rich environment using a Keithley 4200 SCS. The transfer characteristics were observed in the saturation region (drain voltage = −80 V). The threshold voltage (Vth) and the µFET were extracted from a plot of IDS1/2–VG (IDS: drain current, VG: gate voltage) using Equation (1): IDS = µFET Cdiel (W/2L)(VG − Vth)2

(1)

where Cdiel is the dielectric capacitance per unit area, W is the channel width, and L is the channel length.

2.4. Morphological Characterization The surface energies of bare, HMDS-treated, and ODTS-treated SiO2 dielectrics were calculated by measuring the contact angles (θ) of two test liquids: deionized water and diiodomethane (DII). The contact angles of water and DII droplets on various surfaces were as follows: bare (water: 44°, DII: 39°), HMDS (water: 75°, DII: 30°), ODTS (water: 110°, DII: 68°), and PFTVT (water: 100°, DII: 35°). The dispersion, polar force, and total surface energy were obtained using Equation (2) 1 + cos =

( ) ( )

+

( ) ( )

(2)

8


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where γsd and γsp are the dispersion and polar force components of the dielectric, γld and γlp are the dispersion (γld_water = 22.0 mJ/m2 and γld_DII = 48.5 mJ/m2) and polar force (γlp_water = 50.2 mJ/m2 and γlp_DII = 2.3 mJ/m2) components of the test liquids, and γl is the surface energy of the test liquids (γl_water = 72.2 mJ/m2 and γl_DII = 50.8 mJ/m2).14 The surface morphologies were observed by optical microscopy (OM: ZEN lite 2011, ZEN) and atomic force microscopy (AFM: VEECO Dimension 3100 + Nanoscope V 7.0, VEECO). A two-dimensional grazing-incidence wide-angle X-ray scattering (2D-GIWAXS) experiment was also performed using synchrotron radiation at an energy of 11.6 keV and a sample-to-detector distance of 235.6 mm at the 6D beamline of the Pohang Accelerator Laboratory (PAL), Pohang, Korea. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was performed to observe the tilt angle and chain alignment of the spin coated PFTVT film and EHD-jet printed PFTVT pattern. Nearly perfectly linearly polarized photons (P = 1) were utilized from an 4D beamline of the PAL. The carbon K-edge (C1s) was collected in the total-electron-yield mode at beam energy of 350 eV. The spectra were obtained at two azimuthal angles (φ: 0° and 90° with respect to the printing direction) with a four normal angle (θ: 30°, 45°, 55°, and 70°). The C1s NEXAFS raw data were normalized with respect to the carbon concentration using their intensity at 324 eV. The EHD-jet printed and spin coated NEXAFS samples were fabricated under the same conditions as OFET samples and were annealed at 100 °C.

3. RESULTS AND DISCUSSION 3.1. Printing Process

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The amorphous polymer PFTVT (Figure 1a) was employed as the active layer of OFETs with a bottom-gate and top-contact structure by EHD-jet printing (Figure 1b) or spin coating. In the EHD-jet printing process, the cone-jet mode was used for PFTVT printing. As mentioned in the introduction, the EHD jetting modes are distinguished by the intensity of the applied electrostatic field. Figure 1c shows the meniscuses at nozzle tips (upper panels) and the shapes of the printed PFTVT patterns (bottom panels) for the dripping, micro-dripping, and cone-jet modes. The working distance (between the nozzle tip and the substrate), printing speed (substrate moving speed), and flow rate were fixed at 100 µm, 200 mm/s, and 5 µL/min, respectively, for all jetting modes. The application of the lowest voltage (ca. 0.3 kV) resulted in a huge bulging drop of ink hanging from the nozzle tip (left panels of Figure 1c). Thus, the applied electrostatic force was not strong enough to overcome the surface tension of the ink to form small droplets. Instead, the size of the large ink drop grew because the gravitational force was dominant under the given flow rate. As a result, the printed PFTVT exhibited a circle-shaped morphology with a diameter of about 170 µm and a drop-to-drop distance of about 150 µm. When the applied voltage was increased to 0.8 kV, the microdripping mode (middle panels of Figure 1c) was activated, and a series of much smaller droplets (about 70 µm) compared with the dripping mode were produced. This may be because the higher electrostatic field can induce net charges at the edges of an ink drop to overcome the surface tension and deform the meniscus from a bulging drop to an ellipsoidal shape. In the stable cone-jet mode (right panels of Figure 1c), achieved at an applied voltage of 1.2 kV, uniform jet streams were ejected out of the conical meniscus, which were elongated by the strong electrostatic field, enabling printing of the desired patterns, i.e., continuous and narrow lines with a width below 15 µm, as shown in the bottom right panel of 10


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Figure 1c. High-dimensional stability of the printed patterns is essential to precisely obtain controlled patterns for the realization of highly functional and integrated OFETs. OFETs based on the EHD-jet printed PFTVT were fabricated under the optimized printing conditions (stable cone-jet mode) by employing a device structure (Figure 1b) in which the charge/current flow occurs parallel to the printing direction. The PFTVT ink was EHD-jet printed onto the SiO2 dielectric layers treated with various SAMs. Treatment of the SiO2 dielectric with a SAM can modify the surface energy, which is an important factor controlling the morphological and electrical characteristics of the printed OFETs. The surface energy not only affects the wettability and trap density of the dielectric layer but also controls the molecular arrangement of the semiconductor layer. Thus, it is necessary to investigate the effect of surface energy to optimize the EHD process for OFETs. The surface energy of the dielectric layer was varied with the kind of surface treated SAM (bare: 56, HMDS: 45, and ODTS: 25 mJ/m2). PFTVT showed a similar surface energy (45 mJ/m2) as that of the HMDS-treated dielectric (Figure 2). The surface treatment of the dielectric layer is directly related to the interface trap distribution of PFTVT.15 The bare SiO2 has numerous randomly oriented hydroxyl groups. The hydroxyl groups near the interface broadened the density of states of semiconductor, thus increasing the significantly higher density of deep trap states of the PFTVT channel region.16,17 Moreover, the surface energy strongly affects the features of the semiconductor pattern because the printing process involves a relatively long solvent-evaporation time compared with spin coating.18 The extremely hydrophobic surface of the ODTS-treated dielectric can cause spontaneous dewetting during EHD-jet printing, resulting in poor interfacial contact between the printed

11



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PFTVT and the ODTS-treated dielectric, which can hinder the formation of a uniform PFTVT pattern. The average line widths and standard deviation values were measured at 20 points of the PFTVT patterns. The average line widths were about 14.0, 14.1, and 11.7 µm, and the standard deviation values were about 0.1, 0.09, and 0.25 µm for the bare, HMDS-, and ODTS-treated dielectrics, respectively (Figure 3a–c). The narrower line width for the ODTS-treated dielectric compared with those for the bare and HMDS-treated ones is a reasonable result when considering the low surface energy of the ODTS-treated dielectric and high surface energy of PFTVT. In the surface profiles observed by OM (Figure 3d–f), the thicker patterns at the edge (marked with a yellow circle) relative to the center (marked with a blue circle) provide information about the fluid flow in the printed patterns during EHD-jet printing and solvent evaporation.19 First, the continuous jet stream ejected from the nozzle was printed onto the dielectric layers. Subsequently, it is expected that fluid flow occurred parallel to the printing direction because the volatile solvent evaporation drives the fluid flow toward the solidification front (Figure 3g, white arrow).20,21 During the evaporation of residual solvent after the printing, fluid flow perpendicular to the printing direction occurred owing to the coffee-ring effect (Figure 3g, yellow arrow).22,23 Consequently, the overall fluid flow was in a direction diagonal to the printing direction (Figure 3h, red arrow), resulting in the edges being thicker than the center in the PFTVT patterns. It is interesting to note that a multiple coffee-ring pattern (Figure 3f, red circle) was produced on the ODTS-treated dielectric, which is a commonly observed phenomenon on hydrophobic surfaces.24-26 In some cases, the initial contact line on a hydrophobic surface does not remain pinned during the entire drying process but it can move to the interior.24,27 12


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Unfortunately, a contact line movement can result in complicated fluid fluxes that can disrupt the directional polymer chain alignment in the printed patterns.28,29

3.2. OFET Characteristics The properties of OFETs based on EHD-jet printed and spin coated amorphous PFTVT were measured (Figures 4 and S2). To fabricate OFETs, the PFTVT active layer was deposited onto various dielectric layers (bare, HMDS-treated, or ODTS-treated), followed by thermal annealing at 80 °C, 100 °C, or 120 °C in vacuum for 10 min. The top contact of the OFETs was formed by depositing Au source/drain electrodes via thermal evaporation. For the OFETs based on HMDS- and ODTS-treated dielectric layers, although the channel length (L) values of both PFTVT layers were about 50 µm, the channel width (W) values for each device are 240 µm (HMDS) and 180.5 µm (ODTS) because of the different pattern shapes of each semiconductor layer. Hence, the IDS was divided by W/unit length (1 m) (W*) to remove the channel width effect. IDS_norm = IDS/W*

(3)

Figures 4a–c show the transfer characteristics of OFETs based on EHD-jet printed PFTVT with bare, HMDS-, and ODTS-treated dielectrics. The device parameters (µFET, Vth, turn-on voltage (Vturn-on), and on/off ratio) extracted from these transfer characteristics are listed in Table 1. The OFET based on EHD-jet printed PFTVT with bare SiO2 dielectric does not appear to behave as a transistor (Figure 4a). The OFET employing a spin coated PFTVT layer on the bare SiO2 dielectric also does not work as a transistor, as shown in Figure S2c. The reason for the absence of OFET characteristics in the device employing bare SiO2 may be because the hydroxyl groups on the SiO2 surface act as a charge carrier trap. In contrast, 13



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both HMDS- and ODTS-treated devices exhibited typical p-type operation at all thermal annealing temperatures (Tanns) and had clear output characteristics with a distinct transition from linear to saturation behavior (Figures 4 and S2). In case of crystalline polymer semiconductor films, thermal annealing generally improve molecular ordering between polymer chains, thereby resulting in significantly improved crystalline structures and superior charge carrier transport.30,31 However, thermal annealing did not cause a significant change in µFET for EHD-jet printed OFETs with HMDS and ODTS dielectrics (ca. 0.06 and ca. 0.01 cm2 V−1 s−1, respectively, Table 1). Meanwhile, as Tann was increased from 80 °C to 120 °C, both Vturn-on and Vth of the HMDS and ODTS devices shifted toward the negative VG direction (Table 1), and in particular, the HMDS devices showed a higher on/off ratio after thermal annealing (from 6 × 104 at 80 °C to 2 × 105 at 120 °C). Thermal annealing at a temperature above the boiling point of the solvent can remove the solvent residues captured in the polymer film. Several polar solvent molecules have been reported to act as electron traps in semiconductor layers, which cause positive shifts of Vturn-on and Vth and an increase in offstate IDS (resulting in a decrease in the on/off ratio) with negligible change on the µFET value.32 Therefore, the effect of thermal annealing seems to be confined to the removal of solvent molecules in PFTVT, not the improvement of their crystalline morphologies. This result is due to the amorphous structure of PFTVT independent of thermal annealing, which will be discussed in the next section. In addition, all HMDS and ODTS devices showed low standard deviation values regardless of Tann and processing methods, indicating good electrical reproducibility of the OFETs employing amorphous PFTVT. It is worth noting that the HMDS devices exhibited average µFET about five times higher than that of the ODTS devices when employing EHD-jet printed PFTVT semiconductor 14


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layers. In contrast, when employing spin coated PFTVT semiconductor layers, µFET of the ODTS devices (ca. 0.01 cm2 V−1 s−1) was found to be much higher than that of the HMDS devices (ca. 0.00035 cm2 V−1 s−1) (Figure S2 and Table 1). In addition, the EHD-jet printed ODTS devices showed a similar µFET value to the spin coated ODTS devices, whereas the EHD-jet printed HMDS devices exhibited a 170 times higher µFET value (annealed at 100 °C) than that of the spin coated HMDS devices. Assuming that the morphologies of the semiconductor layers grown on different dielectric surfaces are identical, the more hydrophobic surface treatments for SiO2 dielectrics promote the more fast charge carrier transport in OFETs by reducing the hydroxyl groups on the SiO2 surface.33 Whereas, with the same surface treatment for SiO2 dielectrics, the improvement in the crystalline morphology of the semiconductor layer (through appropriate methodologies) can also facilitate the charge carrier transport.34 Therefore the careful analysis of the PFTVT morphology may reveal its relationship with charge carrier transport. We considered that EHD-jet printed PFTVT allows a more improved morphology for charge transport than spin coated PFTVT on HMDS-treated dielectrics. In general, spin coating of semiconductor materials (including small molecules, oligomers, and polymers) tends to form the isotropically oriented molecules and crystallites because of the direction of the radial solvent evaporation and/or rotational force. In contrast, quickly dragging a nozzle during EHD cone-jet mode printing involves shear forces that can induce average chain alignment along the printing direction, which is in accordance with the charge carrier transport direction, thereby leading to more pronounced µFET. In the case of ODTS devices, the contact line movement of the printed PFTVT solution droplets due to the low surface energy of ODTS SAM (as discussed above) is likely to interrupt the chain alignment during EHD-jet printing. This may be the reason why EHD-jet printed ODTS 15



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devices did not exhibit higher µFET than spin coated OFETs when employing ODTS-treated SiO2 dielectrics.

3.3. Morphological Characteristics A morphological study was performed to gain an understanding of the directional chain alignment in the PFTVT patterns. The AFM topography images of EHD-jet printed and spin coated PFTVT on various dielectric layers confirmed a smooth, amorphous morphology (Figure S3). Thermal annealing at various temperatures did not result in significant morphological changes. The relatively rough surface of EHD-jet printed PFTVT patterns on ODTS-treated SiO2 dielectrics may be due to the turbulent fluid flow originating from the difference between the surface energies of PFTVT and the surface during EHD-jet printing onto the hydrophobic ODTS surface. For a detailed morphological study, 2D-GIWAXS analysis was conducted into the spin coated and EHD-jet printed PFTVT samples. In particular, EHD-jet printed PFTVT samples were investigated in two incident beam directions: parallel and perpendicular to the printing direction (Figure S4). However, none of the PFTVT samples exhibited any diffraction pattern, confirming its amorphous morphology (Figures S5 and S6). Unlike 2D-GIWAXS, NEXAFS spectroscopy provides information about the average chain arrangement of the polymer in both crystalline and amorphous regions by detecting the absorption of polarized X-rays.35 In this research, to investigate the tilt angle (α) and directional chain alignment of EHD-jet printed and spin coated PFTVT chains on the HMDStreated dielectric, the normal angle (θ = 30°, 45°, 55°, and 70°) and azimuthal angle (φ = 0° and φ = 90°) of the incident X-ray beam was varied during the NEXAFS measurement. The 16


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NEXAFS spectra at φ = 0° and φ = 90° were obtained parallel and perpendicular to the EHDjet printing direction, respectively. For the spin coated PFTVT sample, φ = 0° was randomly defined and φ = 90° was determined relative to φ = 0°. Figures S7 shows the NEXAFS spectra of EHD-jet printed and spin coated PFTVT. An average tilt angle of the transition dipole moment (TDM) of conjugated planes with respect to the dielectric substrate normal was revealed by NEXAFS scans according to normal angle (Figure 5 and Figure S7).36 The C1s-π* resonance at 285.4 eV of NEXAFS spectra over a range of θ was collected and fitted by the following equation.37

I ∝ 1 + (3 cos − 1)(3 cos − 1) +

( )

sin #

(4)

The extracted tilt angles of both EHD-jet printed and spin coated PFTVT indicated the dominant edge-on orientation of PFTVT chains (Figure S7). Furthermore, in the case of EHD-jet printed PFTVT, it should be noted that the C1s-π* resonance was influenced by the directional chain alignment as well as the tilt angle (Figure 5). The observation of the φ angle dependence of the C1s-π* resonance intensity of the NEXAFS spectra can verify the occurrence of directional chain alignment. When the edge-on oriented PFTVT chain directionally aligned with respect to the printing direction, the NEXAFS spectra exhibited maximized C1s-π* resonance intensity when φ = 90°, which was the parallel angle between E-field vector of incidence beam (θ = 70°) and the anisotropically oriented TDM (Figure 5).38 Interestingly, the C1s-π* resonance intensity of EHD-jet printed PFTVT with bare, HMDS, and ODTS and spin coated PFTVT with HMDS dielectric exhibited a different trend of φ angle dependence originated from the directional chain alignment. In the case of EHD-jet printed PFTVT with the bare and HMDS dielectrics, the C1s-π* resonance intensity for 17



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angles of φ = 90° was clearly strong than that of φ = 0° (Figure 6). This observation confirmed that the amorphous PFTVT chains were somewhat directionally aligned in the direction parallel to the EHD-jet printing direction although the existence of the weakest absorption peak at φ = 0° indicates that the chain alignment was not uniform owing to the coffee-ring effect. The directionally aligned PFTVT chains could increase the chance of short-range π-orbital overlap occurring and provide a more effective pathway for charge carrier transport compared with randomly oriented chains. These results account for the 170 times higher µFET observed in the EHD-jet printed HMDS devices compared with the spin coated HMDS devices. Meanwhile, for EHD-jet printed PFTVT with ODTS dielectric and spin coated PFTVT with HMDS dielectric, there were no significant differences between the C1s-π* resonance intensity for angles of φ = 0° and φ = 90°, which indicated that the PFTVT chains were isotropically oriented. This interesting result about EHD-jet printed PFTVT with ODTS dielectric corresponded to the OM observation (Figure 3f). The µFET of EHD-jet printed PFTVT with ODTS was only 1.5 times higher than that of spin coated PFTVT with ODTS because of the wettability problem.

4. CONCLUSION In this study, PFTVT, an amorphous polymer, was applied to the fabrication of an OFET array using EHD-jet printing and spin coating. The dependence of OFET performance on the morphology of the PFTVT pattern was discussed. Careful control of the printing conditions enabled the deposition of directionally aligned amorphous PFTVT chains for the OFET active layers, which led to a five times higher µFET compared with the highest µFET found in layers produced via spin coating. The effect of dielectric surface energy on the directional 18


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chain alignment during printing process was also discussed. For the spin coated OFETs, the extremely hydrophobic surface of ODTS dielectric led to the low density of interface trap distribution of PFTVT, thus improving the µFET than µFET of HMDS dielectric. In contrast, in EHD-jet printed OFETs, it was found that excessively hydrophobic ODTS surfaces led to wettability problem and disrupted the directional chain alignment during the printing process, causing to reduced µFET; however, this can be overcome by treating the HMDS, instead of ODTS, which have similar surface energy with PFTVT.

ASSOCIATED CONTENT Supporting information Cross sectional height profiles obtained by AFM analysis. OFET figures for spin coted samples. AFM and 2D-GIWAXS figures. NEXAFS spectra (TEY).

AUTHOR IMFORMATION Corresponding Authors *Tel.: +82-53-810-2788; Fax: +82-53-810-4686; E-mail addresses: [email protected] (S. H. Kim). *Tel.: +82-55-772-1491; Fax: +82-55-772-1489; E-mail addresses: [email protected] (Y. Kim). * Tel.: +82-54-279-2269; Fax: +82-54-279-8298; E-mail addresses: [email protected] (C. E. Park). Author Contributions 1

Equally contributed as first authors. 19



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ACKNOWLEDGMENT The 2D-GIWAXS and NEXAFS experiments were performed at the 6D and 4D beamline, respectively, of the Pohang Accelerator Laboratory in Korea. This research was supported by a grant (2013M3A6A5073175) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science, ICT and Future Planning, Korea. This research was also supported by the National Research Foundation of Korea (NRF) (2012M3A7B4049647 and 2016R1C1B1009745).

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TABLE LIST Table 1. Transfer characteristics of OFETs employing EHD-jet printed and spin coated PFTVT semiconductor layers on two types of dielectric layers. HMDS *µFET

on/off

2 −1 −1 [cm V s ] ratio

Ann 0.042

6 × 10

0.059

7 × 10

0.048

2 × 10

80 Ann EHD 100 Ann 120

Ann Spin 100

3.50 × 10

−4

9 × 10

4

4

5

3

ODTS *µFET

Vth

Vturn-on

[V]

[V]

1.6

5

0.0091

3 × 10

−3.1

3

0.016

2 × 10

−3.0

−1

0.012

2 × 10

−9.0

−1

0.0106

9 × 10

on/off

2 −1 −1 [cm V s ] ratio

* µFET was obtained by averaging results from 12 individual devices.

24


2

2

2

4

Vth

Vturn-on

[V]

[V]

1.2

3

−2.3

−2

−3.6

−5

0.11

0

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FIGURE LIST

Figure 1. (a) Chemical structure of PFTVT. (b) Architecture of a bottom-gate, top-contact OFET. (c) Images of PFTVT droplets demonstrating the dripping (applied voltage: 0.3 kV), micro-dripping (applied voltage: 0.8 kV), and cone-jet (applied voltage: 1.2 kV) modes of EHD-jet printing (upper panels) and the corresponding printed patterns (bottom panels).

25



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Figure 2. Surface energies of dielectrics with various dielectrics (bare, HMDS, and ODTS) and PFTVT. The inset shows the OM images of water and DII droplets.

26


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Figure 3. OM images of EHD-jet printed PFTVT patterns for various dielectric layers (a–c) and the corresponding surface profiles (d–f). Schematic of EHD-jet printing of PFTVT patterns onto bare (g) and HMDS-treated (h) dielectrics.

27



ann 80 °C ann 100 °C ann 120 °C

ann 80 °C ann 100 °C ann 120 °C

(c)

10 10

4.0x10 2.0x10

-10

-4

0.0 -40

-20 0 VG [ V ]

-40

0.0 (d)

IDS_norm [A]

-8

-4

10

-5.0x10 -1.0x10 -1.5x10 -2.0x10

-20 0 VG [ V ]

-40

-20 0 VG [ V ]

(f) 0.1

-6

10

-12

10

-14

0.01

1E-3

1E-4

EHD-printing ann 80 °C

0.1

0V -10 V -20 V -30 V -40 V

-7

-7

-7

-20

VDS [ V ]

0 -40

-20

E-jet printing ann 100 °C

Spin coating

(e)

-8

-40

2 -1 -1

-4

(b)

µ FET [cm V s ]

ann 80 °C ann 100 °C ann 120 °C

(a)

2 -1 -1

6.0x10

-4

µ FET [cm V s ]

1/2

IDS_norm [A1/2]

8.0x10

IDS_norm [A]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.01

1E-3

1E-4

EHD-printing ann 120 °C

0

VDS [ V ]

Figure 4. Transfer (a–c) and output (d, e) characteristics of EHD-jet printed OFETs for various dielectric layers: bare (a), HMDS (b, d), and ODTS (c, e). (f) Average µFET of OFETs prepared using different methods and with different dielectric layers. The error bars are based on the standard deviation of 12 individual devices measured at each OFET.

28


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Figure 5. Scheme of experimental geometry and TDM of conjugated plans orientation: the incidence beam had two azimuthal angle (φ). (a) φ = 0°; parallel to the printing direction. (b) φ = 90°; perpendicular to the printing direction.

29



0.50

Resonance Intensity 285.4 eV [a.u.]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

φ = 0° φ = 90° 0.45

0.40

0.35

0.30 Bare/ HMDS/ ODTS/ HMDS/ Printing Printing Printing Spin coating

Figure 6. C1s-π* resonance intensity of EHD-jet printed PFTVT for deposit methods (EHDjet printing/ spin coating) and various dielectrics.

30


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GRAPHICAL ABSTRACT

31


Directionally Aligned Amorphous Polymer Chains via

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