Solvent-Free Processable and Photo-Patternable Hybrid Gate

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A Solvent-Free Processable and Photo-Patternable Hybrid Gate Dielectric for Flexible Top-Gate Organic Field-Effect Transistors Jun Seon Kwon, Han-Wool Park, Do Hwan Kim, and Young-Je Kwark ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14500 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 25, 2017

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ACS Applied Materials & Interfaces

A Solvent-Free Processable and Photo-Patternable Hybrid Gate Dielectric for Flexible Top-Gate Organic Field-Effect Transistors Jun Seon Kwon, Han Wool Park, Do Hwan Kim* and Young-Je Kwark* Department of Organic Materials and Fiber Engineering, Soongsil University, 369 SangdoRo, Dongjak-Gu, Seoul 06978, Korea

*Corresponding author. Phone: 82-2-820-0995. Fax: 82-2-817-8346. E-mail: [email protected] (Y.J.K), [email protected] (D.H.K)

Keywords: solvent-free process, photo-patternable, hybrid dielectrics, polysiloxane, flexible top-gate OFETs

Abstract We report a novel solvent-free and direct photo-patternable poly[(mercaptopropyl)methylsiloxane] (PMMS) hybrid dielectric for flexible top-gate organic field-effect transistors (OFETs), utilizing a photo-activated thiol-ene reaction under UV irradiation of 254 nm to induce cross-linking, even in air and at low temperature. In particular, a solvent-free PMMS-f dielectric film, for which an optimal cross-linking density is shown by a well-organized molar ratio between thiol and vinyl in the thiol-ene reaction, exhibited a high dielectric constant (5.4 @ 100 Hz) and a low leakage current (< 1 nA mm-2 @ 2 MV cm-1). The excellent dielectric characteristics of the solvent-free PMMS-hybrid dielectrics, along with their other unique characteristics of a direct photo-patternability for which UV-nanoimprint lithography is used and a high surface energy of 45.6 mJ m-2, allowed the successful application of the dielectrics to flexible solvent-free top-gate OFETs with a high reliability against the radius of curvature (9.5, 7.0, and 5.5 mm) and repetitive bending cycles at the radius of curvature of 5.5 mm. This will eventually enable the proposed dielectric design to be used in a variety of applications such as flexible displays and soft organic sensors including chemical and tactile capability.

1. Introduction ACS Paragon Plus Environment


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Solution-processability, flexibility, and light-weight of organic materials provide these materials with distinctive opportunities to implement low-cost and imperceptible organic fieldeffect transistors (OFETs).1-7

This capability, therefore, enables the applicability of OFETs

in a variety of fields such as flexible display,8-10 radio-frequency identification (RFID) tags11,12, and functional sensors including gas,13,14 chemical,15 temperature,16 and tactile-capability sensors.17 A great care for selecting device types of the OFETs, e.g., bottom-gate and top-gate geometry, however, would be required to be optimal for targeting performances. In general, top-gate OFETs show a favorable aspect compared to bottom-gate ones, in that organic semiconductor layer can be efficiently protected against environmental conditions by gate electrode and gate dielectric layer without additional passivation processes.18

Notably, this

self-passivation would consequently allow for the improved environmental reliability of the organic semiconductors.18,19 Despite this environmental-reliability improvement, the top-gate OFETs suffer from keeping as-deposited organic semiconductor layers chemically robust against subsequent solution processing to form conventional polymer gate dielectrics, which is not at all trivial. This implies that the top-gate geometry can be hardly achieved because the organic semiconductor layer would likely be dissolved or swollen upon the processing of the gate-dielectric layer through a solution.18-20 Progress has been made regarding this issue through the utilization of the orthogonal fluoropolymer dielectric, CYTOP, that does not chemically affect the underlying organic semiconductor layer at all.21-27 This is because the organic semiconductor layers are chemically robust against the solvents that are used for the formation of the CYTOP gate dielectrics. Despite the plausible benefits for the top-gate OFETs, however, a critical drawback is caused upon the implementation of highly flexible and reliable OFETs. Namely, due to a very low surface tension (16.7 mJ m-2) of the CYTOP layer, it suffers from forming the next layer wellorganized on CYTOP surface and allowing good adhesion strength at the interface of both the

ACS Paragon Plus Environment

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layers.28 Thus, an approach focused on orthogonal fluoropolymer-gate dielectrics may not provide the ultimate solution that leads to highly flexible and reliable top-gate OFETs.

In addition, a methodology in the top-gate OFETs that is more impactful should be oriented to the design of direct patternable organic-gate dielectrics for the more-efficient fabrication of OFET backplanes and integrated circuits. The use of direct patternable organicgate dielectrics here is because thin films become not only selectively photo-crosslinked, but also robust to the subsequent post processes. By introducing a thiol-ene chemistry as a photocrosslinking approach, it was possible to successfully fabricate solvent-free direct patternablegate dielectrics.29 Also, these should be introduced generally on the surface of a variety of organic semiconductors while their optoelectrical properties are preserved, which means that all of the patterning processes for the organic dielectrics should be orthogonal to the asdeposited organic-semiconductor layers.

Herein, we describe a hybrid gate dielectric for top-gate OFETs by synthesizing a solventfree processable and directly photo-patternable poly[(mercaptopropyl)methylsiloxane] (PMMS). The synthesized organic/inorganic hybrid PMMS can be easily formed onto an organic semiconductor layer by spin-coating without the use of any solvents, as this material shows a liquid-like viscosity. A photo-activated thiol-ene reaction offers fast curing for the introduction of highly cross-linked networks, and even in air due to its low sensitivity to oxygen.30-33 Also, patterns were successfully formed through UV-nanoimprint lithography (UV-NIL) methods. In particular, the photo activated thiol-ene reaction has been used to implement electrically, chemically, and environmentally stable dielectric layers due to the lowtemperature cross-linking process.30,34 Moreover, the highly dense network afforded by the multiple cross-linking sites in the system can improve the dielectric strength under an electric field.29,34,35



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2. Experimental Section Materials:

(3-Mercaptopropyl)methyldimethoxysilane

(MPMDS)

and

trimethylmethoxysilane (TMMS) were purchased from Gelest. HCl (36.5 % to 38.0 % aqueous solution), barium hydroxide, pentacene, pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), 1,4-cyclohexanedimethanol-divinyl ether (CHDM-DVE), and DAROCUR® 1173 were purchased from Sigma Aldrich. Regioregular poly(3-hexylthiophene) (rr-P3HT) (Mw = 65,200) and diketopyrrolo-pyrrole-dithiophene-thienothiophene (DPP-DTT) (Mw = 86,700) were purchased from Ossila. All the other chemicals were purchased from commercial sources. Measurements: 1H-NMR spectra were recorded on an Avance 400 NMR spectrometer (400 MHz) using CDCl3 as a deuterated solvent. Gel permeation chromatographic (GPC) analyses were performed using Waters GPC equipped with a series of two PL-GEL MIXEDC columns and a Waters 2414 differential refractometer. THF was used as an eluent at a flow rate of 1.0 mL min-1, and the GPC columns were calibrated with polystyrene standards (1,090 Da to 19,640 Da). FT-IR spectra were recorded on a VERTEX70 FT-IR spectrometer. The spectra were obtained in a range of 4000 cm-1 to 400 cm-1 with a resolution of 2 cm-1 for 32 scans. AFM images were taken with a Digital Instruments Multimode-N3-AM under ambient conditions. The AFM was operated in tapping mode by using tapping mode silicon probes with a force constant of 40 N m-1. Viscosity measurements were performed at 25° C using a LVDVI Prime rotational viscometer with the small sample spindle No. 31. The measurement was acquired with 10 mL to 15 mL samples at a rotation speed of 100 rpm, maintaining over 80 % of torque. The surface energy was calculated using deionized water and diiodomethane. The contact angle was observed using a drop-shape analysis system (DSA 100, KRUSS). The capacitance of the PMMS layers with a MIM structure was measured with the Agilent E4980A Precision LCR Meter for frequencies ranging from 20 Hz to 1 MHz under a vacuum condition. The electrical performances of the top-gate OFETs and the current density-electric field (J vs.


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E) of the capacitor were measured with the semiconductor-characterization system Keithley 4200-SCS Agilent at room temperature under a vacuum (~ 10-3 torr) in a dark environment. Synthesis of End-capped PMMS: THF (25.2 mL) and aqueous HCl solution (22.5 mL, pH 2) were mixed in ice bath for 1 h. MPMDS (65 mL, 0.36 mol) was added to induce the hydrolysis of the alkoxy silane, and the mixture was reacted for 6 h. After adding barium hydroxide (3.5 g, 0.02 mol), the mixture was reacted for an additional 28 h at 45° C. TMMS (4.13 mL, 0.03 mol) was then added, and the mixture was stirred for 6 h. The resulting viscous solution was dissolved in dichloromethane and then washed several times with deionized water. An excess amount of MgSO4 was used to remove the water. After filtration, volatiles were removed from the solution by an evaporation process under a reduced pressure. Vacuum drying at 200° C for 12 h resulted in a viscous liquid of PMMS (Mn ~ 5000 g mol-1). UV-nanoimprint lithography: Figure 2a illustrates the UV-NIL process. PMMS-f, filtered through a 0.5 μm syringe filter, was spin-coated onto a Si wafer at a spin rate of 5000 rpm for 1 min to obtain a thin film with a thickness of 1 μm. The PMMS-f film was covered with a PDMS mold, and then exposed to UV light for 10 s (dose ~ 280 mJ cm-2). The mold was removed from the PMMS-f, and the residual resist was removed by O2 plasma. Dielectric film preparation and capacitor fabrication via solvent-free process: PMMS, PETMP, CHDM-DVE, and DAROCUR 1173 were stirred vigorously for 1 h. The highly doped silicon wafers (2.5 cm × 2.5 cm) were cleaned through an ultra-sonication in acetone and isopropanol, respectively, for 15 min, and they were then treated with UV ozone for 10 min. The PMMS mixture was filtered through a 0.5 μm syringe filter and dropped onto a piece of Si wafer. The spin-coating of the mixture was conducted at a spin rate of 5000 rpm for 1 min. The as-prepared PMMS film was treated with UV irradiation at 254 nm. To obtain MIM test structures, gold electrodes with a 1 mm diameter and a 50 nm thickness were deposited



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directly onto the PMMS-dielectric layers through a shadow mask. The thickness of the dielectric film for PMMS-f, measured with a profilometer, was determined to be approximately 1 μm (5000 rpm) or 650 nm (8000 rpm) depending on spin rate during spin-coating. Top-gate OFETs and flexible-device fabrication: Si-wafer with a SiO2 of 300 nm was used as a substrate for the OFETs. They were cleaned through ultra-sonication in acetone and isopropanol for 15 min, respectively. Then, the source and drain electrodes (Cr/Au) were deposited by thermal evaporation through a shadow mask. The Cr electrode was deposited at a rate of 0.1 ~ 0.2 Å s-1 to reach a thickness of 3.5 nm, followed by the deposition of the Au electrode at a rate of 0.7 ~ 0.8 Å s-1 for a thickness of 47 nm under a pressure of 8.0 × 10-6 Torr. The channel length and width are 45 µm and 4000 µm, respectively. They were treated with UV/O3 before the coating of the semiconducting layer. DPP-DTT (5 mg mL-1 in chlorobenzene) and rr-P3HT (10 mg mL-1 in chlorobenzene) were spin-coated as organic semiconductors at 1000 rpm for 60 s, followed by a thermal annealing under N2. Each organic semiconductor was annealed at 180° C and 150° C for 30 min, respectively. The pentacene was deposited by thermal evaporation at a rate of 0.2 Å s-1 to obtain a thickness of 70 nm under a pressure of 8.0 × 10-6 torr. The PMMS-dielectric mixtures were then spin-coated at a rate of 5000 rpm for 30 s. The as-prepared dielectric film was irradiated with UV at 254 nm. Finally, Au (50 nm)-gate electrodes were deposited by thermal evaporation through a shadow mask. The flexible OFETs were fabricated using polyimide (Kapton®) film as a substrate according to the same procedure that is stated above.

3. Results and Discussion Figure 1a shows a design concept of the solvent-free processable and photo-patternable dielectrics, which are composed of the following four components: PMMS, 1, 4-cyclohexanedimethanol-divinyl ether (CHDM-DVE), pentaerythritoltetrakis-(3-mercaptopropionate)


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(PETMP), and photo-initiator (DAROCUR1173). All of the components in the liquid phase could be effectively blended without the use of any solvents for the formation of homogeneous mixtures. Trimethylsilyl (TMS) end-capped PMMS (PMMS) was synthesized by sequential hydrolysis-condensation reactions of methoxysilanes, followed by a reaction with trimethylmethoxysilane (Figure S1). The proton nuclear magnetic resonance (1H-NMR) and fourier transform infrared spectroscopy (FT-IR) data show that all the hydroxyl groups at the end of the PMMS were converted to TMS groups (Figures S2-4). We used PETMP, a commercially available non-volatile tetra-thiol, as an additional cross-linker. PETMP provides four reactive sites, and its provision of more-densely crosslinked networks, which is desirable for electrical and chemical stability, was expected. The photo-initiator, DAROCUR 1173, which can generate radicals under UV irradiation of 254 nm, was chosen as a fast photo-curable and liquid-type photo-initiator. Our novel solvent-free process is shown in Figure 1b. The mixture of the four components was spin-coated on a Si wafer and irradiated with UV light to form a homogeneous thin film at the macroscopic scale (Table 1, Figure 1b and 1c). In particular, an atomic force microscopy (AFM) image reveals that the film represents a very smooth surface (RMS roughness ~ 2.7 Å) without a phase separation of components (Figure 1d). The water-contact angle for the PMMS-f film was approximately 76° (inset image of Figure 1d), which implies that the PMMS-f (45.6 mJ m-2) shows a greater hydrophilic characteristic than CYTOP (16.7 mJ m-2).28 As a consequence, this surface property enables a sequential solution coating onto the PMMS-f surface without de-wetting. To elucidate the kinetics of the thiol-ene reaction, an FT-IR analysis was conducted for the PMMS-f film, as shown in Figure 1e. Upon exposure for 10 s (exposure dose = 280 mJ cm-2) with PMMS-f, the C = C (1610 cm-1) and S-H (2570 cm-1) peaks completely disappeared, indicating the completion of the thiol-ene reaction. The kinetic studies for all of the other compositions with solvent-free PMMS are summarized in Figure S5.



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To observe the patternability of these materials via UV-NIR, silicon maters with a variety of pattern shapes and sizes were fabricated by conventional photolithography, and then PDMS stamps were prepared by drop-casting onto patterned silicon masters, followed by thermal curing (Figure 2a and Figure S6a). Figure 2a also shows the UV-NIL procedure for the implementation of the well-defined micropatterns of the solvent-free processed PMMS-f film for which as-prepared PDMS stamps were used. Figure 2b and Figure S6b show the fieldemission scanning electron microscope (FE-SEM) images of the micropatterned PMMS-f film (strip, circle, and square) without showing any noticeable bugs or distortions. Note that the high quality film-forming of the solvent-free PMMS dielectrics might allow an effective imprinting process on the micron scale.

In general, dielectric characteristics such as the dielectric constant and the break-down voltage should be evaluated for the application of a material as a dielectric layer in OFETs. The capacitance and the leakage current of the photo cross-linked PMMS films were estimated in a metal-insulator-metal (MIM)-device architecture (inset of Figure 3a). A gold electrode with an area of 0.0078 cm2 was thermally evaporated on the photo-crosslinked PMMS thin films on a heavily doped Si substrate, and then the dielectric characteristics were measured using a cryogenic probe station. Figure 3a shows the current density versus the electric-field plots that were extracted from the capacitors of the PMMS films with a versatile composition, and shows that the photo-crosslinked PMMS films exhibit superior dielectric properties compared to the conventional polymer dielectrics. In particular, a leakage-current density for the PMMS-f film was measured as approximately 1.52 × 10-9 A cm-2 at an electric field of 2 MV cm-1, which is much lower than that of conventional dielectrics such as CYTOP,24 poly (4-vinylphenol), and polystyrene.36 In this manner, this improvement of the dielectric strength of the PMMS films is attributed to the smaller free volume that is derived from the highly cross-linked network


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structure. Moreover, note that the PMMS-f films show the lowest leakage current, which might be due to an optimal cross-linking density that was implemented by the well-organized molar ratio (vinyl:thiol = 1:1.1, mol/mol) in the thiol-ene reaction. To demonstrate this feature more clearly, the effect of a tetra-functional PETMP on the crosslinking phenomenon was also explored. As shown in Figure 3a and Table 1, unlike PMMS-c, the PETMP component in the PMMS-f film introduces the networks that are more-densely crosslinked, as evidenced by the leakage-current density that is lower than that of the PMMS-c (1.89 ×10-9 A cm-2), because four of the thiol groups in PETMP can take more chances to be cross-linked even though the molar ratio of the thiol and vinyl groups of the two PMMS films are similar. Figure 3b shows the frequency-dependent dielectric constant as a function of the PMMS-film types. Note that this negative relationship between the dielectric constant and the frequency reflects a representative characteristic that is observed in the conventional polymeric dielectrics, and results from a slow polarization effect.37 All of the dielectric properties for the PMMS films such as thickness, specific capacitance, and dielectric constant are summarized in Table 2. The dielectric constants (measured at 100 Hz) for the PMMS films are observed in the range of 4.9 to 5.4, which is relatively higher than those of most of the polymeric dielectrics as well as SiO2. This is because a sulfur (S) atom in the PMMS film can serve as a high dielectric-constant moiety, and multiple thiol groups can increase a dielectric constant.33 This high k dielectric can afford low operational voltage with high performance complementary electronics and optoelectronic devices.38 Therefore, the approach suggested by us might be highly impactful for the preparation of well-defined gate dielectrics with a high dielectric constant, as the process of this study exhibits a very simple and fast photo-cured system between the thiol and ene linkage even under room temperature.

To investigate the electrical characteristics in the top-gate OFETs using solvent-free



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PMMS dielectrics, diketopyrrolo-pyrrole-dithiophene-thienothiophene (DPP-DTT) and poly (3-hexylthiophene) (P3HT) were used as polymer semiconductors, and pentacene was used as a small molecule semiconductor. The detailed device structure including the molecular schemes of organic semiconductors is shown in Figure 3c. Figure 3d and 3e show the typical p-channel characteristics that are modulated by the gate electrode for OFETs with the PMMSf dielectric layer. The output and transfer characteristics of all of the top-gate OFETs were measured as VDS (0 ~ - 120 V) and VGS (40 ~ - 120 V), respectively. As shown in Figure 3d, the transfer characteristic of the DPP-DTT top-gate OFETs exhibits a steep current increase in the sub-threshold region with low gate-leakage currents, and the output characteristics (Figure S8a) show good saturation behavior with a clear pinch-off. This top-gate OFET exhibits an average hole mobility of 5.5 × 10-2 cm2 V-1 s-1 and a current on-off ratio of 2 × 104. The pentacene-based top-gate OFET also exhibits an average hole mobility of 1.5 × 10-3 cm2 V-1 s1

, a current on-off ratio of 2 × 104, and a well-defined gate modulation, as shown in Figure 3e

and Figure S8b. Both the DPP-DTT- and the pentacene-based top-gate OFETs with the PMMSf dielectric showed a stable device operation. Furthermore, to reduce the operating voltage, the DPP-DTT-based top-gate OFETs were implemented based on thinner PMMS-f gate dielectrics (~650 nm). The transfer characteristic was measured at VDS (-40 V) and VGS (30 ~ -50 V). As shown in Figure S9a, This top-gate OFET exhibits an average hole mobility of 0.11 cm2 V-1 s1

and a current on-off ratio of 1.9 × 104, while maintaining low gate-leakage current. In

particular, as seen in Figure S9b, the hysteresis is hardly observed, owing to almost perfect cross-linking of PMMS-f films. Note that PMMS-f gate dielectric produces a highly crosslinked layer with minimal unreacted thiol groups derived from fast thiol-ene “click” reaction, which prohibits the charge trapping between DTT-DTT polymer semiconductor and PMMS-f gate dielectric. Furthermore, this characteristic enabled the DPP-DTT top-gate OFETs to


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display a very good reliability at ambient condition (@ RH of 35~40% and RT for 10 days) (Figure S10). In addition, in order to demonstrate the charge trapping behavior under continuous bias stress, we evaluated the operating stability of the DPP-DTT top-gate OFETs with PMMS-f gate dielectric (~650 nm) under typical bias stress [On-bias stress (VGS = - 20V) and off-bias stress (VGS = 20V)] for 3h (Figure S11). The threshold voltage shift of the OFETs is relatively small (3V), irrespective of bias stress conditions, which is due to relatively perfect cross-linking of PMMS-f film as well as very stable polymer semiconductor used in this work. As a consequence, this good device characteristic for the top-gate OFETs reveals that the solvent-free PMMS did not deteriorate the underlying organic semiconductor during the solution process. The transfer characteristics of the DPP-DTT and the pentacene top-gate OFETs are shown in Figure S12a and 12b, respectively, in terms of the kinds of solvent-free PMMS (a-e). All of the OFET electrical performances such as mobility, threshold voltage, and on/off ratio are summarized in Table 2, directly reflecting that the OFETs with the PMMS-f dielectric show the best performance in terms of the mobility and the on-off ratio. The output and transfer characteristics of the OFETs with regioregular poly(3-hexylthiophene) (rr-P3HT) are also shown in Figure S13. To investigate the influence of the UV irradiation upon the organic semiconductor during the photo-cross-linking of the PMMS dielectric when the topgate OFETs were fabricated, the comparative bottom-gate OFETs were fabricated using solvent-free PMMS-f and rr-P3HT. As a result, it was possible to observe the absence of any difference in the OFET performance, which implies that a short exposure to UV irradiation has no effect on the organic semiconductors (Figure S14). To demonstrate the compatibility of the solvent-free PMMS dielectric with the flexible OFETs, the DPP-DTT top-gate OFETs were fabricated onto polyimide substrates, as described in Figure 4a. Figure 4b shows the typical p-type characteristic with a mobility of 1 × 10-2 cm2 V−1 s−1, and Figure 4c shows a clear saturated feature in the output curves. However, to be



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applicable to flexible electronics, OFETs should display electrical reliability under mechanical stimuli, especially bending stress, which is regarded as one of the important criteria for the implementation of highly reliable OFETs. Accordingly, the flexible top-gate OFET performance with the PMMS dielectric was evaluated as a function of the radius of curvature (∞, 9.5, 7.0, and 5.5 mm) and the continuous bending reliability under a specific bending radius of 5.5 mm. Figure 4d shows the transfer characteristic of the DPP-DTT top-gate OFET as a function of the radius of curvature. It was possible to observe a stable feature in the transfer curve irrespective of the radius of curvature (9.5, 7.0, and 5.5 mm), which indicates that the devices can be well-operated even under a bending stress that is responding to a bending radius of 5.5 mm. Next, the reliability of the DPP-DTT top-gate OFETs against bending cycles at a radius of curvature of 5.5 mm was evaluated. As depicted in Figure 4e, the devices exhibited a stable operation by 400 bending cycles, showing a consistent mobility (1.0 × 10-2 cm2 V-1 s-1) and threshold voltage (- 40 V). In contrast, over 400 bending cycles, it was found that the mobility decreased up to 0.4 x 10-3 cm2 V-1 s-1. Note that a similar behavior in terms of the radius of curvature (Figure S15a) and the bending cycles (Figure S15b) was also observed for rr-P3HT top-gate OFETs, in which an optimal structure should be required for the implementation of more-reliable top-gate OFET devices by more than 1000 bending cycles. This is beyond the scope of this work, but the behavior of extremely stable top-gate OFETs under mechanical stimuli will be reported in due course. All the data for the flexible top-gate OFETs according to the radius of curvature are summarized in Table S2, wherein a stable device operation is shown.

4. Conclusions In summary, a novel solvent-free and direct photo-patternable hybrid dielectric for highperformance and flexible top-gate OFETs have been demonstrated and implemented via thiol-


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ene “Click chemistry”. The organic/inorganic hybrid-PMMS dielectrics that are suggested here can easily form highly cross-linked networks through a photo-activated thiol-ene reaction, even in air and at low temperature, and this could lead to a chemically, environmentally, and electrically stable gate-dielectric layer with well-defined micron patterns. Moreover, the PMMS-f dielectrics show the lowest leakage current (< 1 nAmm-2 @ 2 MV cm-1), a high dielectric constant (5.4 @ 100 Hz), and a high surface energy of 45.6 mJ m-2 that are derived from the optimal cross-linking density according to the well-organized molar ratio between the thiol and vinyl in the thiol-ene reaction. In this manner, the PMMS-f-based top-gate OFETs with DPP-DTT, rr-P3HT, and pentacene as organic semiconductors revealed electrical characteristics that performed more effectively when compared with those of the other PMMSs with different compositions. Further, the flexible solvent-free top-gate OFETs showed a high reliability against the radius of curvature (9.5, 7.0, and 5.5 mm) and the bending cycles at the radius of curvature of 5.5 mm, and this will eventually enable the proposed dielectric design to be applicable in a variety of fields such as those that produce flexible displays and soft organic sensors including chemical and tactile capability.

■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: GPC, FT-IR, viscosity data of PMMS, FE-SEM images of PMMS-f film, and electrical properties (output and transfer characteristics, storage and bias-stress stabilities) of top-gate OFETs and flexible top-gate OFETs (PDF)

■ AUTHOR INFORMATION Corresponding Authors



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*E-mail: [email protected] *E-mail: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgements: This work was financially supported by the World-Class 300 Project (Development of organic materials with high transmittance, high insulating properties, and high flexibility for next generation display) funded by the Small and Medium Business Administration of Korea (SMBA). D.H.K. acknowledges the support from the Center for Advanced Soft-Electronics under the Global Frontier Project (CASE-2014M3A6A5060932) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning.

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Table 1. Physical and chemical properties of the solvent-free PMMS in terms of the compositions of each component. The weight of the DAROCUR® 1173 as a photo initiator is 5 wt % of the total components.



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Table 2. Gate dielectric characteristics and electrical properties of top-gate OFETs based on solvent-free PMMS films.

a)

The composition of the solvent-free PMMS is a typical formula in our experiment for electrical device

and UV NIR, which is named as PMMS-f.


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Figure 1. Design concept of the solvent-free processable and photo-patternable dielectrics. (a) Components of solvent-free gate dielectric system. (b) Schematic diagram illustrating the solvent-free process. (c) Macroscale morphology of photo-crosslinked PMMS-f film. (d) Tapping-mode AFM image of the PMMS-f film. The inset shows the water contact-angle image. (e) Thiol-ene mechanism and evolution of the FT-IR spectra of the solvent-free PMMSf upon UV exposure (intensity, 28 mW cm-2) in terms of exposure times. Inset: Enlarged FTIR spectra at around 1638 (vinyl) cm-1 and 2556 (thiol) cm-1.



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Figure 2. (a) Schematic picture of the fabrication of a micropattern via UV-nanoimprint lithography (UV-NIL) using solvent-free PMMS-f resist. (b) Field-emission scanning electron microscope (FE-SEM) images of UV-NIL patterns of PMMS-f showing various shapes (line/space, circle and square) and feature sizes: line/space, circle (3 μm), and square (20 μm).


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Figure 3. (a) Current-density versus electric-field (J vs. E) characteristics of the solvent-free PMMS dielectrics. The inset shows the MIM capacitor structure. PMMS films were cured by thiol-ene reaction through UV irradiation at 254 nm. (b) Frequency dependent dielectric constant (f vs. k) characteristics in the range from 20 Hz to 1 MHz. (c) Schematic representation of top-gate bottom-contact OFETs geometry. The left side shows the chemical structures of the corresponding material as an organic semiconductor. Transfer characteristics of (d) DPP-DTT and (e) pentacene top-gate OFETs with solvent-free PMMS-f dielectrics (1.7 μm). The gateleakage currents (dotted line) were plotted.



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Figure 4. Flexible DPP-DTT top-gate OFETs fabricated on flexible polyimide substrate. (a) Schematic representation and photographic image of flexible top-gate OFETs using a polyimide substrate. (b) Transfer and (c) output characteristics of flexible DPP-DTT top-gate OFETs with the solvent-free PMMS-f dielectric. (d) Transfer characteristics of the DPP-DTT top-gate OFETs as a function of the radius of curvature. (e) Mobility and threshold-voltage variations with increasing number of bending cycles (at radius of curvature = 5.5 mm). Error bars denote standard deviation.


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Table of Contents


Solvent-Free Processable and Photo-Patternable Hybrid Gate

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