Solution Processable-, Thin-, and High-Îº Dielectric Polyurea Gate

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Organic Electronic Devices

Solution Processable-, Thin-, and High-# Dielectric Polyurea Gate Insulator with Strong Hydrogen Bonding for Low-voltage Organic Thin-Film Transistors Sungmi Yoo, Dong-Gyun Kim, Taewook Ha, Jong Chan Won, Kwang-Suk Jang, and Yun Ho Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11083 • Publication Date (Web): 03 Sep 2018 Downloaded from http://pubs.acs.org on September 3, 2018

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ACS Applied Materials & Interfaces

Solution Processable-, Thin-, and High-κ Dielectric Polyurea Gate Insulator with Strong Hydrogen Bonding for Low-voltage Organic Thin-Film Transistors Sungmi Yoo,a Dong-Gyun Kim,a,b Taewook Ha,a,c Jong Chan Won,a,b Kwang-Suk Jang,*d and Yun Ho Kim*a,b

a

Advanced Materials Division , Korea Research Institute of Chemical Technology, Daejeon

34114, Republic of Korea. b

Chemical Convergence Materials and Processes, KRICT School, University of Science and

Technology, Daejeon 34113, Republic of Korea c

Department of Chemistry, Korea University, Seoul 02841, Republic of Korea

d

Department of Chemical Engineering, Hanyang University, Ansan 15588, Republic of Korea

* Corresponding author. E-mail address: [email protected] (Y. H. Kim), [email protected] (K.-S. Jang).

Keywords: polyurea, organic gate insulator, solution process, organic thin-film transistor, high capacitance, low-voltage operation

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Abstract We developed a solution-processable, thin, and high-dielectric polyurea-based organic gate insulator (OGI) for low-voltage operation and high performance of organic thin-film transistors (OTFTs). A 60-nm-thick polyurea thin film exhibited a high dielectric constant of 5.82 and excellent electrical insulating properties owing to strong hydrogen bonding. The hydrogen bonding of synthesized polyurea was confirmed using infrared spectroscopy and was quantitatively evaluated by measuring the interactive force using atomic force microscopy. Moreover, the effect of hydrogen bonding of polyurea on the insulating properties was systematically investigated through the combination of various monomers and control of the thickness of the polyurea film. The dinaphtho[2,3-b:2',3'f]thieno[3,2-b]thiophene (DNTT)-based OTFTs with the polyurea gate insulator showed excellent thin-film transistor (TFT) performance with a field-effect mobility of 1.390 cm2/V⋅s and an on/off ratio of ~105 at a low operation voltage below 2 V. In addition, it is possible to fabricate flexible polymer organic semiconductor-based TFT devices using a solution process owing to excellent solvent stability in various organic solvents. We believe that the solution-processable polyurea gate insulator with high dielectric constant and good insulation properties is a promising candidate for lowvoltage operated OTFTs using various organic semiconductors.


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Introduction Over the last two decades, organic thin-film transistors (OTFTs) have been studied extensively owing to their low cost, large-area processing, and compatibility with plastic substrates for flexible electronic applications.1,2 While the development of various organic semiconductors with high charge carrier mobility has been extensively researched, only modest attention has been paid to the gate dielectric material for OTFTs.3 However, for commercial flexible electronic devices driven by OTFTs, it is essential to develop electrically stable organic gate dielectric materials and solution-processing capabilities that can replace inorganic insulators such as SiO2 and SiNx.4,5 Particularly, with the rise of the IoT era, low-voltage transistor operation has attracted interest for low-power applications and portable/wearable devices including various sensors, RFID, and logic circuits. To improve the performance of low-voltage electronics, high-capacitance dielectrics that can serve as gate insulators in OTFTs powered by thin-film and low-capacity batteries are required. High capacitance of dielectrics can be obtained by increasing the dielectric constant or decreasing the thickness of the gate dielectric films. As it is difficult to change the inherent properties of the material with the dielectric constant, various studies are being conducted with the aim of lowering the thickness of the dielectric layer. Several groups have demonstrated that high-capacitance organic gate insulator (OGIs) can be achieved using thin cross-linked polymer films,6 nanometer-thick selfassembled monolayers or multilayers,7,8 and ultrathin polymer insulating layer via initiated chemical vapor deposition (iCVD).9,10 These results show excellent insulating properties and high device characteristics, but fabricating large-area organic devices based on solution process is difficult because they require special surface treatment and deposition processes. In addition, it is difficult to improve the dielectric constant using these techniques because of the limited materials used such as specific self-assembled materials and poly(1,3,5-trimethyl-1,3,5-trivinyl cyclotrisiloxane) for iCVD.9,1 Recently, polyurea-based polymers with strong dipole moments have been reported to achieve higher dielectric constants of above 5.0 than most of the traditional polymer dielectrics including polystyrene (Er~2.6), poly(4-vinylphenol) (Er~4.1), and poly(methyl methacrylate) (Er~3.5).11 As the urea bond has a strong polarity, polyurea materials have a high dielectric constant. Thus, there are many studies



on high-dielectric meta-aromatic or aromatic polyurea materials for high-energy-density capacitors.12,13 In addition, strong intermolecular hydrogen bonding between the urea bonds in the polymer backbone is possible, thus exhibiting high insulating properties by producing dense films with the suppression of free volume.14,15 However, when a DA monomer with a short alkylene spacer is used to increase the insulating property, the solution process is very difficult at ambient conditions owing to excessively strong hydrogen bonding. On the contrary, when the long alkylene spacer is introduced in DA monomer to increase the solubility, the insulation characteristic is remarkably deteriorated. In order to overcome the drawback of solution process for the preparation of polyurea thin film, some groups have reported molecular layer deposition to fabricate a conformal and ultra-thin polyurea film with the thickness of a few hundreds of nanometers.16–18 However, these molecular deposition techniques have limitations in application to the fabrication of OTFTs, which require low-cost and large-area processes, as in the case of monolayer surface treatment or iCVD described above. To the best of our knowledge, there is no reported case of application of high-dielectric polyurea with high insulation characteristics prepared via solution process as a gate dielectric for OTFTs. In this study, we developed a solution-processable, thin, and high-dielectric polyurea-based OGI by systematically controlling molecular design. The synthetic polyurea shows a high dielectric constant of > 5.8 and excellent insulating characteristics obtained at a thickness of under 60 nm without additional cross-linking process owing to strong intermolecular hydrogen bonding of urea groups. It was possible to develop a solution-processable polyurea-based dielectric material by controlling the molecular weight of synthesized polyurea using rational molecular design. The hydrogen bonding of synthesized polyurea was confirmed using Fourier-transform infrared spectroscopy (FTIR) and was quantitatively evaluated by measuring the interactive force using atomic force microscopy (AFM). In addition, the effect of hydrogen bonding of polyurea on the insulating properties was systematically investigated through the combination of various monomers and the control of the thickness of the polyurea film. The 60-nm-thick polyurea was successfully fabricated through a solution process and showed a capacitance of 88 nF/cm2 and a leakage current of < 10-9


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A/cm2 at 2 MV/cm. Consequently, we demonstrated dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT)-based TFTs with high-dielectric polyurea gate insulator, capable of stable operation at a low voltage of < 2 V, exhibiting a charge mobility of 1.36 cm/V⋅s and an on/off ratio of > 105. In addition, as our polyurea thin film showed excellent solvent stability in various organic solvents, it is possible to fabricate flexible polymer organic semiconductor-based TFT devices using a solution process.

Experimental Section Materials 2,4-toluene diisocyanate (DI-1), 1,4-phenylene diisocyanate (DI-2), Ethylene diamine (DA-1), 1,2propylene diamine (DA-2), N,N’-Di-tert-butylethylene diamine (DA-3),

Dinaphtho[2,3-b:2’,3’-

f]thieno[3,2-b]thiophene (DNTT), and Diketopyrrolopyrrole-thienothiophene copolymer (DPPT-TT) were purchased from Sigma-Aldrich. N-methyl-2-pyrrolidinone (NMP) was purchased from Junsei. All chemicals were used as received. Synthesis and characterization of the polyureas (PU-11, PU-12, and PU-13) Polyureas were synthesized based on the step-growth polymerization between diisocyanate monomer (DI-1 and DI-2) and diamine monomer (DA-1, DA-2, and DA-3) in NMP solution. For the solution process, oligomeric polyureas having molecular weight of less than 10,000 were synthesized by controlling the polymerization conditions at a temperature of 10 °C or lower and a solid content of 4 wt% or less in order to prevent gelation by the strong hydrogen bonding of urea bond. For example, for the synthesis of the 3.3 wt% polyurea (PU-11) solutions, 0.5 mmol of diamine monomer (DA-1) was dissolved in 3.43 g of NMP solvent under nitrogen with stirring at 200 rpm. Once the DA-1 monomer was completely dissolved, 0.5 mmol diisocyanate (DI-1) was slowly added at a rate of 0.1 mL/min using a syringe pump and the reaction mixture was cooled to below 10 °C for 2 h with an ice bath; further, this temperature was maintained for the polymerization to occur without sudden gelation. After completion of the reaction, the PU-11 solution was directly used to prepare a thin film on the substrate by spin-coating at room temperature. Other polyurea solutions were prepared in a



similar manner. The concentration of the solutions of PU-12 and PU-13 were adjusted to 3.5 wt% and 4.0 wt%, respectively. To investigate the molecular weight of the synthesized polyureas, gel permeation chromatography (GPC, Waters GPC 1515) analysis was conducted using dimethylacetamide (DMAc) as the mobile phase and polystyrene standards. For measuring the thermal properties, thermogravimetric analysis (TGA) was performed under nitrogen at a heating rate of 10 °C/min using a TA instrument TGA Q500 analyzer. The surface energy was calculated from the contact angles of deionized water and diiodomethane on polyurea films, which were determined using a PHOENIX 450 contact angle analyzer. AFM (Bruker, Multimode 8) images were obtained with a tapping mode microscope. Furthermore, the interactive force owing to hydrogen bonding of polyurea was measured using contact mode AFM. The urea group could be tethered on the AFM tip via the following procedure (see Figure S4 in the Supporting Information). As the urea-modified AFM tip approached each polyurea thin film, an interaction was induced between the surface of polyurea film and the ureamodified AFM tip, inducing a cantilever deflection. The interaction force could be calculated by multiplying the spring constant of the cantilever with the deflection distance. The force could be detected in the same manner as the urea-modified tip was retracted. Thus, the force−extension curve could be constructed from these measurements. We used contact mode AFM tip with a spring constant of 0.9 N/m, supplied by the manufacturer (Bruker, RESP-20). The deflection sensitivity was 103.57 nm/V during the approach and retraction of the surface of polyurea film from the urea-modified tip. All the experiments were carried out in the air at room temperature. Approximately 30 approach/retract cycles were carried out for each polyurea sample. Device fabrication and measurement of OTFTs For the fabrication of the gate insulator film, Si wafer used as a substrate was cleaned using a typical cleaning process: sonication in detergent, deionized water, acetone, and 2-propanol sequentially for every 20 min. A 30-nm-thick aluminum electrode deposited on Si substrate was prepared via the thermal evaporation. Each polyurea solution in NMP was spin-coated on aluminum-electrode-


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deposited Si substrate. The spin-coated films were annealed at 65 °C for 1 min, 90 °C for 10 min, and 160 °C for 40 min on a hot plate in ambient air. The thickness of the polyurea films was controlled to be 60 nm. The thicknesses of the gate insulator films were determined using an alpha-step surface profiler (α-step DC50, KLA-Tencor). To determine the capacitance and leakage current density properties of the polyurea gate insulators, metal-insulator-metal (MIM) capacitor structures were prepared on the aluminum-electrode-deposited Si substrates. For the top electrode, gold was deposited on gate-insulators-coated substrate. The active area of the MIM devices was 50.24 mm2. For electrical characterization, bottom-gate top-contact DNTT TFTs were fabricated. A 60-nm-thick DNTT semiconductor layer was deposited on the gate insulator layer through a shadow mask using thermal evaporation at a pressure of 3 × 10-6 torr. The fabrication of the OTFTs was completed by the deposition of 50-nm-thick source and drain gold electrodes onto the DNTT layer through shadow masks using thermal evaporation, creating transistors with channel lengths and widths of 100 and 1500 µm, respectively. To improve the performance of the OTFTs, the polyurea gate insulator was surface-treated with a self-assembled monolayer of octadecylphosphonic acid (ODPA) assisted by metal-oxide interlayer according to our previous report.19 For the fabrication of flexible (DPPT-TT)TFT, PU-11 gate insulator was spin-coated on aluminum-bottom-electrode-deposited polyethylene terephthalate (PET) substrate. DPPT-TT chloroform solution (30 mg/mL) was spin-coated at 2000 rpm for 60 s on PU-11 gate insulator followed by thermal annealing at 150 °C for 30 min on a hot plate in ambient air. A 50-nm-thick gold top electrode was deposited on the DPPT-TT film via thermal evaporation. The fabricated flexible OTFT device with PU-11 gate insulator was bendingtested 1000 times with a bending radius of 3 mm (Bending Tester, (JUNIL TECH Co., LTD, JIBT610). The capacitance was measured using an Agilent 4294A impedance analyzer. The output and transfer characteristic curve of the OTFTs were measured using an Agilent E5272 semiconductor parameter analyzer. All these electrical measurements were performed in air without any encapsulation.

Results and Discussion



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Polyurea is typically synthesized based on the step-growth polymerization between diisocyanate (DI) with diamine (DA) in polar aprotic solvents such as dimethylformamide (DMF) and NMP. The chemical structures of the monomers and synthesized polyureas (PU-11, PU-12, and PU-13) are presented in Figure 1. The advantage of these step-growth polymerization reactions is that polyureas are formed without the evolution of condensation by-products and the process can be carried out at low temperature without any catalysts.20 Particularly in terms of gate insulator applications, it is a significant advantage to be able to reduce impurities from by-products and traces of residual catalysts, which have a significant effect on coating and insulation properties and device stability.5,21,22 In order to control the hydrogen bonding force and solubility, we have synthesized six kinds of fully aromatic polyureas via polycondensation reaction of the two DIs (DI-1 and DI-2) with three DAs (DA-1, DA2, DA-3) (Figure 1). DI and DA monomers with linear structure were polymerized to create dense films with low degree of free volume for achieving high dielectric constant and better insulation properties. The hydrogen bonding and solubility of the synthesized polyureas were controlled by varying the position of alkyl groups in the DI and DA monomers. As expected, it was frequently observed that gelation progressed very rapidly during the polymerization reaction when the solid content and reaction temperature were not properly controlled owing to strong intermolecular hydrogen bonding. For gelation-free solution processes, it was very important to control the molecular weight of polyurea, which is strongly bound to hydrogen bonds.

Thus, oligomeric

polyureas having molecular weight of less than 10,000 were synthesized by controlling the polymerization conditions at a temperature of under 10 °C or lower and a solid content of < 4 wt% as described in the Experimental section. Three kinds of DI-1 based polyureas (PU-11, PU-12, and PU13) were successfully synthesized without uncontrolled gelation. The number-average molecular weight (Mn) of PU-11, PU-12, and PU-13 were determined to be 5590, 7420, and 2600, respectively, with a polydispersity index of 1.22, 1.16, and 1.01, respectively, measured using GPC with a DMAc eluent. The presence of a methyl group at the ortho-position of the DI-1 monomer improved solubility and prevented gelation. However, all polyureas based on DI-2 (PU-21, PU-22, and PU-23, molecular structure not shown in the main text or Figure 1) were severely gelated during polymerization in the synthetic manner. As DI-2 has a linear molecular structure without a methyl group at the ortho8 Environment ACS Paragon Plus

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position, strong hydrogen bonding between the polymer chains occurred during polymerization, resulting in uncontrolled gelation at room temperature. Further, the gelated DI-2-based polyureas did not dissolve in common polar aprotic solvents such as DMF, NMP, DMAc, dimethyl sulfoxide, and acetone, which have high solubility for conventional polyureas. Thus, the solution process of DI-2based polyureas is impossible. FTIR spectroscopy was used to investigate the chemical bonding and the presence of hydrogen bonding of three DA-1-based polyureas. Figure 2 shows the FTIR spectra of PU-11, PU-12, and PU13. The peak assignments are based on some previous reports for the model compound of polyurea.16,23,24 The characteristic peaks caused by urea coupling are the (C=O) and (N-H), at 1654 and 1540 cm-1, respectively. Moreover, the band at ~2930 cm-1 v(C-H) represents the absorption of CH2 on diamine units. In the FTIR spectrum of PU-11, the very strong NCO peak of DI-1 at 2270 cm1

and the NH stretching peak of DA-1 at 3356 cm-1 and 3280 cm-1 used as monomer were not

observed as shown in Supporting Information Figure S1. The lack of a very strong isocyanate and amine stretch indicates that there are no unreacted isocyanate and amine groups present in the prepared PU-11 film and both monomers (DA and DI) completely reacted to form urea bonds in PU11. The hydrogen bonding can occur between the amide N-H groups and the carbonyl oxygen of the adjacent chain. The sharp v(C=O) peak at 1654 cm-1 in the FTIR spectrum results from well-ordered hydrogen-bonded polyurea chains.24 The effects of the hydrogen bonding are also observed in the v(N-H) modes, which appear in the spectra as a broad peak in the range 3250–3450 cm-1. Similar FTIR results were obtained with PU-12. However, PU-13 was not completely synthesized in this synthetic route. The characteristic peaks of NCO group at 2270 cm-1 from unreacted DI-1 or NCOterminated prepolymer were slightly observed as shown in Fig. 2. The presence of unreacted monomers of PU-13 was also confirmed via TGA analysis (Supporting Information Figure S2). PU11 and PU-12 have heat resistances of 230 °C and 211 °C based on Td of 3%, respectively. In the case of PU-13, the unreacted monomers were evaporated at ~140 °C, and thereafter, the partially synthesized PU-13 thermally decomposed again at ~210 °C. MIM devices were prepared to examine the dielectric and insulating properties of the 60-nm-thick polyurea thin films. Figure 3a shows the frequency-dependent capacitances of the 60-nm-thick PU9 Environment ACS Paragon Plus


11, PU-12, and PU-13 thin films. The dielectric and insulation properties are summarized in Table 1. The capacitance of the PU-11, PU-12, and PU-13 films at 100 Hz was measured to be 88.8, 81.5, and 71.6 nF/cm2, respectively. Moreover, the dielectric constants were calculated to be 5.82, 5.16, and 4.85, respectively. As the additional alkyl substituent chains in the polymer backbone is introduced as PU-12 and PU-13, the dielectric constant decreases. This can be attributed to the increase in free volume, which reduces the number of polarizable groups per unit volume.25–27 While the dielectric constant slightly decreased, the leakage current density increased significantly as the number of alkyl groups in the polymer backbone. At 2 MV/cm2, the leakage current density of PU-11, PU-12, and PU-13 films was 6.2 ×10-10, 5.5 ×10-6, and 6.7 ×10-6 A/cm2, respectively (Figure 3b). The leakage current densities of PU-12 and PU-13 were measured to be 10,000 times greater than the leakage current of PU-11, despite the similar molecular structure. This remarkable difference in the insulating properties is understood to be due to the difference in the hydrogen bonding strength of each polyurea. Further, in order to confirm the effect of the alkylene spacer length in the polymer chain on the dielectric constant and insulation properties more precisely, the polyurea having long alkyl groups with very similar molecular structure to PU-11 was synthesized and its dielectric properties were evaluated. (Supporting Information Figure S3) As expected, when the length of alkylene spacers in the polymer backbone increased, the dielectric constant and insulation properties decreased. It is understood that, as the alkyl chain length increases, the hydrogen bonding force weakens and the chain packing density decreases as the free volume increases. The hydrogen bonding should be an important parameter that determines the insulating property of polyurea thin film as a gate insulator for OTFTs. It is known that the aromatic polyurea forms a readily equivalent ordered structure at room temperature and the strong hydrogen bonds between adjacent urea units can align to form a geometrically dense two-dimensional sheet.23 For the detailed and systematical investigation of the relation between the hydrogen bonding and insulation characteristics of each polyurea thin film, AFM interaction force analysis was carried out. AFM tip was modified to have the urea group by subsequent treatment of 3-(triethoxysilyl)propyl isocyanate and propylamine to create a hydrogen bond with urea bond of the surface of the polyurea thin film (Supporting Information Figure S4). The interactive forces recorded via AFM analysis represent 10 Environment ACS Paragon Plus

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hydrogen bonding between the urea group of modified AFM tip and each polyurea films. Similar AFM analysis has been widely used to measure the interactive forces such as dipole-dipole interaction and hydrogen bonding in the polymer thin film or composites.28–30 Typical force−extension curves are also shown in Supporting Information Figure S4. Small mean interaction forces were recorded for PU-13 film (42.36 ± 7.79 nN), whereas a larger pull-off force was observed for PU-11 (182.13 ± 1.61 nN) and PU-12 (181.59 ± 1.46 nN) films; these value are approximately 4.3 times larger than that for PU-13. PU-11 and PU-12, which differ only in one methyl group on the molecular structure, showed a similar interaction force, whereas for PU-13 containing bulky six-methyl groups, a remarkably low interaction force was obtained. It indicates that the interaction force owing to hydrogen bonding decreased as the number of substituted alkyl groups increased in a polymer main chain. The low interactive force of PU-13 owing to the weak hydrogen bonding is consistent with the poor insulation behavior shown in Fig. 3. However, although AFM measured the interactive forces for PU-11 and PU-12 to be close, the leakage current density, which represents the insulation characteristic, of PU12 is significantly larger than that of PU-11. As the methyl group is asymmetrically placed in the DA2 of PU-12, the packing between 2D-sheets in PU-12 thin film would be imperfect compared with that in PU-11 owing to the methyl group of DA-2. The hydrogen bonds between adjacent units can align to form a sheet, with the methyl group that is part of each segment essentially laying in a plane perpendicular to this structure.23 These results show that the insulation properties of polyurea thin films were affected by not only the hydrogen bonding between the polyurea insulator chains but also the interchain packing structure. Additional systematic studies are required to clarify the relationship between the insulation property of the polyurea film and the interchain packing structure or degree of free volume. However, this is beyond the scope of this study and we plan to carry out further research in the near future. Based on the dielectric and insulating results of Fig. 3, PU-11 is expected to be the most suitable material for gate dielectric material. As high capacitance of dielectrics can be obtained by decreasing the thickness of the gate dielectric films, we aimed to produce the thinnest PU-11 film while maintaining the insulation properties. The thickness of spin-coated PU-11 was easily controlled from 30 nm to 300 nm by controlling the concentration of the coating solution and spin speed. Figure 4 11 Environment ACS Paragon Plus


shows the dielectric and insulating properties of PU-11 films with thicknesses of 30, 60, and 100 nm. As expected, the capacitance increases significantly as the thickness decreases. The dielectric constant of PU-11 was calculated to be 5.82 for the 60-nm-thick film. It has a very high dielectric constant value compared with typical polymeric dielectric materials such as PS and PMMA.31,32 Figure 4b shows the leakage current values of PU-11 films fabricated at different thicknesses. Excellent insulation properties of 10-9 A/cm2 at 2 MV/cm were also observed in thin PU-11 films of thickness 60 nm. However, the PU-11 film with a thick ness of 30 nm showed a significant decrease in the insulating properties, which is understood to be due to the defect structure of the film surface. The dielectric and insulation properties for each thickness are summarized in Table 2. It was unexpected that high insulation properties were maintained even at low thicknesses of PU-11 thin films of less than 60 nm. The excellent insulating properties of PU-11 remain at a thickness of 40 nm (not shown here). In general, it is known that polymer insulating materials without crosslinking are not expected to have high insulation characteristics of less than 10-9 A/cm2 at a thickness of < 100 nm owing to defects existing at interfaces such as pinholes and grain boundaries. The superior insulating properties of PU-11 result from the strong hydrogen bonding between the polyurea main chains. The surface morphology of the PU-11 thin film was investigated using AFM and no pinhole structure was observed as shown in Supporting Information, Figure S5. The surface roughness was 0.34 nm. Interestingly, the surface of the coated films of PU-12 and PU-13, which had poorer insulation properties than PU-11, was uniform and smooth (Supporting Information, Figure S5). It indicates that the high dielectric and insulating properties of PU-11 are due to the strong hydrogen bonding rather than differences in film morphology or coating properties. To investigate the potential of the PU-11 gate insulator, we fabricated DNTT-TFTs. A 60 nm-thick PU-11 film was used as a gate dielectric for OTFTs to improve the reproducibility and stability of fabrication of OTFTs. Fig. 5a shows the transfer characteristic curve (Ids vs. Vgs and Ids1/2 vs. Vgs) of the DNTT-TFT with the PU-11 gate insulator, where Vgs was swept from 0 to 3 V and from 3 to 0 V. The TFT exhibited excellent device performance. Fig. 5b shows the output characteristic curve (drain current vs. drain voltage, Ids vs. Vds) of the DNTT TFT with the 60-nm-thick PU-11 gate insulator. The TFT shows a typical p-type characteristic. To obtain the average and standard deviations of the 12 Environment ACS Paragon Plus

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electrical performance parameters, we prepared twelve DNTT-TFTs with the PU-11 gate insulators. The average mobility, threshold voltage (Vth), sub-threshold slope (S-slope), and on/off current ratio (Ion/Ioff) were measured to be 0.069 ± 0.006 cm2/V⋅s, 1.81 ± 0.06 V, and 0.75 ± 0.05 V/decade, and 4.04 × 105, respectively, as summarized in Table 3. And the device performance of DNTT-TFT with PU-11 gate insulator is comparable to that of the DNTT-TFT using SiO2 or other polymeric gate insulator reported previously.33 When the DNTT TFTs were fabricated using 60-nm-thick PU-12 and PU-13 films as OGIs, the device characteristics and yield were very poor, as expected. In particular, all devices using PU-13 gate insulator were inactive (Supporting Information, Figure S6). Unfortunately, the device performance of the DNTT-TFT with PU-11 gate insulator is much lower than that of the vacuum-deposited DNTT-TFTs with bare or SAM-treated SiO2 gate insulators, which exhibit a field-effect mobility up to 0.74 cm2/V⋅s and an on/off current ratio of ~106.34 The polar urea group included in PU-11 is necessary to realize a high dielectric constant, but the urea group present on the surface of the gate insulator acts as a charge carrier trap to degrade device characteristics.35–37 The negative effects of these surface polar groups could be suppressed through surface treatment and the device characteristics of OTFTs could be further improved.38,39 Recently, we have reported a solution-processable metal-oxide-assisted surface treatment (MAST) method applicable to various organic insulators.19 In the MAST method, α-Al2O3 and ODPA were successively coated on the surface of PU-11. After the MAST surface treatment, the thickness of PU-11/α-Al2O3/ODPA increased from 60 nm to 80 nm, and the insulating and dielectric properties were not affected (Supporting Information Figure S7). However, the water contact angle significantly increased from 61.4° to 101.6°, and the surface energy decreased from 52.5 N/m to 32.7 N/m. It indicates that the surface of PU-11 gate insulator was successfully treated with ODPA with long alkyl chain and the polar urea groups on the surface that could act as charge trap sites were sufficiently suppressed. Figure 5c and 5d show the transfer and output characteristics of the DNTT-TFTs with ODPA/αAl2O3-treated PU-11 gate insulators. Both TFTs exhibit typical p-type characteristics and negligible



hysteresis in the transfer characteristics. As the negative gate voltage (Vgs) increases, the DNTT-TFT with the ODPA/α-Al2O3-treated PU-11 gate insulator exhibits a much higher Ids than that with the non-treated PU-11 gate insulator. This is caused by the improved interfacial properties between DNTT and PU-11 through the surface treatment and the better crystal growth of DNTT as shown in Supporting Information Figure S8. The average field-effect mobility of the DNTT-TFTs with ODPA/α-Al2O3-treated PU-11 gate insulators, i.e., 1.390 ± 0.003 cm2/V⋅s, is much higher than that of the TFTs with non-treated PU-11 gate insulators, i.e., 0.069 ± 0.006 cm2/V⋅s. Other electrical parameters are comparable to those before surface treatment. The PU-11 thin film showed stability to various organic solvents owing to strong hydrogen bonding. Except for highly polar protic solvents such as DMF, NMP, and dimethyl sulfoxide, it showed sufficient solvent stability for general organic solvents such as cyclohexanone, 2-butoxyethanol, tetrahydrofuran, γ-butyrolactone, chloroform, and propylene glycol monomethyl ether acetate, used for polymer organic semiconductor (OSC) coatings in numerous studies. After spin-washing with various organic solvents on spin-cast 60-nm-thick PU-11 film, there was no change in film thickness, surface morphology, and surface roughness (Supporting Information S9). Thus, it is possible to fabricate various polymer OSC-based TFT devices using a solution process and also fabricate the vacuum-deposited DNTT-TFTs with the deposition method described above. To verify the feasibility of the solution-processed flexible TFT, we fabricated poly[[2,5-bis(2-octyldodecyl)-2,3,5,6tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl]-alt-[[2,2′-(2,5-thiophene)bis-thieno(3,2b)thiophene]-5,5′-diyl]] (DPPT-TT)-TFT with PU-11 gate insulator on a 120-µm-thick flexible PET substrate (Figure 6). DPPT-TT is well known as a high-mobility polymer semiconductor.40–42 DPPTTT chloroform solution was successfully spin-coated onto a PU-11 gate insulator to a thickness of 60 nm. The 16 (DPPT-TT)-TFTs fabricated on PET were wound on a glass pipette of diameter 3.5 mm without deformation or destruction of the device (Fig. 6a). Fig. 6c shows the transfer characteristics (Ids vs. Vgs) of the (DPPT-TT)-TFT with the PU-11 gate insulator before bending and after 1,000 bending experiments with a bending radius of 3 mm and a bending speed of 5 mm/s as shown in Fig. 6b. No significant reduction in the performance of the TFT was observed after bending. The flexible


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(DPPT-TT)-TFT showed comparable TFT performance to vacuum-deposited DNTT-TFTs and their electric characteristics are summarized in Table 4.

Conclusions We developed a solution-processable, thin, and high-dielectric polyurea-based gate insulator (PU-11) for achieving low-voltage operation and high performance of OTFTs. The PU-11 thin film shows a high dielectric constant and excellent insulating characteristics obtained at a thickness of under 60 nm without additional cross-linking process owing to strong intermolecular hydrogen bonding of urea groups. The hydrogen bonding of synthesized polyureas was confirmed using infrared spectroscopy and was quantitatively evaluated by measuring the interactive force using AFM technique, which was significantly affected by the number of alkyl group in the polyurea chain. The effect of hydrogen bonding of polyurea on the insulating properties was systematically investigated through the combination of various monomers and control of the thickness of the polyurea film. We observed that the insulation properties of polyurea thin films were affected by not only the hydrogen bonding between the polyurea insulator chains but also the interchain packing structure. In addition, it was very important to control the molecular weight of polyurea, which is strongly bound to hydrogen bonds, for gelation-free solution processes. Thus, oligomeric polyurea with a molecular weight of less than 10,000 was synthesized by controlling the molecular design and polymerization conditions such as temperature below 10 °C and low solid content under 4 wt%. Among the synthesized polyureas, PU-11 polymerized with 2,4-toluene diisocyanate (DI-1) and ethylene diamine (DA-1) showed excellent dielectric and insulation properties. The 60-nm-thick PU-11 thin film was successfully fabricated through a solution process and showed a capacitance of 88 nF/cm2 and a leakage current of < 10-9 A/cm2 at 2 MV/cm. The DNTT-TFTs with the 60-nm-thick PU-11 gate insulator showed excellent TFT performance with field-effect mobility. In addition, it is possible to fabricate flexible polymer organic semiconductor-based TFT devices using a solution process owing to excellent solvent stability to various organic solvents, which were successfully operated after performing bending test for 1000 times at a bending radius of 3 mm. We believe that the solution-processable PU-



11 gate insulator with high dielectric constant and good insulation properties is a promising candidate for low-voltage operated OTFTs using various organic semiconductors.

Acknowledgements This work was supported by the Center for Advanced Soft-Electronics funded by the Ministry of Science, ICT and Future Planning as a Global Frontier Project (2015M3A6A5065315) and Korea Research Institute of Chemical Technology (KRICT) core project (SI-1803-02) Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. FTIR spectra, TGA thermograms, Capacitances and leakage current densities, AFM images, Experimental details for surface modification of AFM tip, Force-extension curve by AFM, Surface energy measurement, and Solvent stability results for synthesized polyureas thin films. Electrical properties of DNTT-TFTs with synthesized polyurea gate insulators.

Author Contributions S. Yoo carried out the overall experiments. T. W. Ha helped with surface treatment of polyurea gate insulator. D.-G. Kim and S. Yoo designed AFM study for measurement of interactive force and characterized molecular weight of polyureas. Y. H. Kim, K.-S. Jang and J. C. Won designed the project and S. Yoo and Y. H. Kim wrote the manuscript.


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Table of Contents



Figures and Tables

Figure 1. Molecular structure of variety of (a) diisocyanates (DI) and diamines (DA) and (b) synthesized polyureas (PU-11, PU-12, PU-13)

Figure 2. FTIR spectra of three synthesized polyureas


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Figure 3. (a) Capacitances (as a function of frequency) and (b) leakage current densities (as a function of electric field) of PU-11, PU-12 and PU-13

Figure 4. (a) Capacitances (as a function of frequency) and (b) leakage current densities (as a function of electric field) of PU-11 thin film with thickness of 30, 60, and 100 nm.



Figure 5. (a, c) Transfer characteristics of the DNTT-TFTs with (a) non-treated PU-11 and (c) surface treated PU-11 gate insulators. Gray lines indicate the gate current. (b, d) Output characteristics of the DNTT TFTs with (b) non-treated PU-11 and (d) surface treated PU-11 gate insulators.

Figure 6. (a, b) Digital images of flexible (DPPT-TT)-TFT on PET substrate and its bending test at 3 mm bending radius equipped with bending tester (c) Transfer characteristics of the (DPPT-TT)-TFTs before and after bending experiments


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Gate insulator

Capacitance at 10 Dielectric constant Leakage current density Surface RMS

(60 nm)

kHz [nF cm-2]

at 10 kHz

at 2 MV cm-1 [A cm-2]

roughness [nm]

PU-11

88.76

5.82

6.2 × 10-10

0.34

PU-12

81.51

5.16

5.5 × 10-6

0.25

PU-13

71.59

4.85

6.7 × 10-6

0.41

Table 1. Dielectric and insulating properties of 60 nm-thick PU-11, PU-12, and PU-13 thin films

Table 2. Dielectric and insulating properties of PU-11 thin films according to thickness variation Thickness

Capacitance at 10 Dielectric constant Leakage current density Surface RMS

[nm]

kHz [nF cm-2]

at 10 kHz

at 2 MV cm-1 [A cm-2]

roughness [nm]

30

130.40

4.76

4.24 × 10-2

0.32

60

88.76

5.82

6.16 × 10-10

0.34

100

50.11

5.60

1.95 × 10-10

0.30



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Table 3. Electrical characteristics of DNTT-TFTs with PU-11, PU-12, PU-13, and surface treatedPU-11 gate insulators Gate Insulator

Mobility a [cm2/Vs]

Vth [V]

S-slope [V/decade]

Ion/Ioff

PU-11

0.066 ± 0.05

-0.10

0.27

1.19 × 105

PU-12

0.005 ± 0.06

-0.61

0.56

1.37 × 103

0.37

5.48 × 105

PU-13 PU-11/surface treatment a

inactive 1.390 ± 0.03

-1.22

Average field-effect mobility of 12 TFT devices.

Table 4. Electrical characteristics of flexible (DPPT-TT)-TFTs with 60 nm-thick PU-11 gate insulators on PET substrate before and after bending experiments

a

(DPPT-TT) TFT

Mobility a [cm2/Vs]

Vth [V]

S-slope [V/decade]

Ion/Ioff

Initial

0.037 ± 0.05

-1.00

0.37

2.01 × 103

After bending (x 1000)

0.036 ± 0.06

-0.88

0.36

2.39 × 103

Average field-effect mobility of 12 TFT devices.


Solution Processable-, Thin-, and High-Îº Dielectric Polyurea Gate

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