Article pubs.acs.org/Macromolecules
Amphiphilic Poly(3-hexylthiophene)-Based Semiconducting Copolymers for Printing of Polyelectrolyte-Gated Organic FieldEffect Transistors Ari Laiho,† Ha Tran Nguyen,‡ Hiam Sinno,† Isak Engquist,† Magnus Berggren,† Philippe Dubois,‡ Olivier Coulembier,*,‡ and Xavier Crispin*,† †
Department of Science and Technology, Organic Electronics, Linköping University, SE-601 74 Norrköping, Sweden Laboratory of Polymeric and Composite Materials, Center of Innovation and Research in Materials and Polymers (CIRMAP), University of MonsUMONS, Place du Parc 23, 7000 Mons, Belgium
‡
S Supporting Information *
ABSTRACT: Polyelectrolytes are promising electronically insulating layers for lowvoltage organic field effect transistors. However, the polyelectrolyte−semiconductor interface is difficult to manufacture due to challenges in wettability. We introduce an amphiphilic semiconducting copolymer which, when spread as a thin film, can change its surface from hydrophobic to hydrophilic upon exposure to water. This peculiar wettability is exploited in the fabrication of polyelectrolyte-gated field-effect transistors operating below 0.5 V. The prepared amphiphilic semiconducting copolymer is based on a hydrophobic regioregular poly(3-hexylthiophene) (P3HT) covalently linked to a hydrophilic poly(sulfonated)-based random block. Such a copolymer is obtained in a three-step strategy combining Grignard metathesis (GRIM), atom transfer radical polymerization (ATRP) processes, and a postmodification method. The structure of the diblock copolymer was characterized using FT-IR, 1H NMR spectroscopy, and gel permeation chromatography (GPC).
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INTRODUCTION Organic field-effect transistors (OFETs) hold great promise for future organic electronic devices such as radio-frequency identification (RFID) tags,1 electronic paper,2 and sensing devices.3 In traditional OFETs, the organic semiconductor film is separated from the gate electrode by a thin insulating dielectric film. The gate−insulator−semiconductor sandwich structure can be seen as a capacitor of capacitance per area Ci = Q/VG, where the charge density Q in the semiconductor, and thus also the conductance of the semiconductor channel, is tuned by the applied gate voltage VG. Tremendous effort has been devoted to reach high capacitance Ci to allow transistors to operate at low voltage.4,5 Since the dielectric constant k of organic materials usually is quite low, very thin gate insulator layers (thickness d) are required in order to obtain a high capacitance Ci ∝ k/d. Molecular assembly and self-organization techniques have been utilized to manufacture gate dielectric layers only a few nanometers thick, resulting in large capacitance (Ci up to 1 μF cm−2).5−7 In the past 10 years, it has been shown that electrolytes can be used as the gate dielectric material in OFETs to obtain low-voltage operation8,9 and decent switching speeds.10 The low operation voltage of the transistor originates from the large capacitance of electric double layer (Helmholtz layer). Those electric double layers are formed at the metal/electrolyte interface and the semiconductor/electrolyte interface upon inducing a gate bias. In the double layer, the distance d between the electronic charges © 2013 American Chemical Society
at the semiconductor surface and the ions in the electrolyte is about 0.5−1 nm, which provides as high capacitance as 10−100 μF cm−2. Beside the low operating voltage, other advantages of electrolytes are that the performance of the electrolyte-gated OFETs is not too sensitive to gate misalignment11,12 or to the variations in thickness12 and roughness13 of the gate dielectric layer, thereby enabling robust manufacturing. Electrolyte-gated organic thin film transistors (EGOTFTs) are all operating at low voltage, but they can be classified according to the permeability of the ions into the semiconductor channel.14 When the anions penetrate in the p-channel, the EGOFET does not switch rapidly completely due to the large charge stored in the semiconducting thin film (Figure 1a). Fully printed electrolyte-gated transistors and circuits have been demonstrated with (hydrophobic) ionic gels printed on (hydrophobic) organic semiconductors.11,15 However, the typical hydrophobic character of the organic semiconductors limits the choice of the electrolyte to ionic liquids and gels, which have poor mechanical strength and can lead to ion penetration in the semiconducting channel. In contrast, polyelectrolytes have been demonstrated to be a key family of truly solid electrolytes that lead to fast electrolyte-gated OTFTs, since the immobile polyions cannot penetrate into the Received: March 13, 2013 Revised: April 23, 2013 Published: May 28, 2013 4548
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semiconductor (Figure 1b).12,16,17 Despite the success in fabrication of polyelectrolyte-gated transistors9,12 as well as PMOS and C-MOS circuits18,19 through spin-coating, the transfer of the manufacturing process to large area processing has remained a great challenge. This challenge stems from the inherent incompatibility of the used materials: a hydrophobic semiconducting layer and a hydrophilic polyelectrolyte layer. One approach that has been tried is to use conjugated polyelectrolytes (polyanionic), where each conjugated monomer is functionalized with an anionic group. Those conjugated polyelectrolytes are hydrophilic and semiconducting. The polyelectrolyte solution wets well the conjugated polyelectrolyte, and printing techniques could be used for manufacturing transistors. However, since the mobile cations of the conjugated polyanions are slowly depleted to open the channel, the transistor displays bulk conduction rather than interfacial transport, which results in an undesirable slow response.16 Another issue is the presence of many polar groups in conjugated polyelectrolyte which deteriorates the charge carrier mobility by the creation of traps.20 Finding a semiconducting polymer with low intrinsic dipole to ensure high charge carrier
Figure 1. Sketch of various operation mechanisms in electrolyte-gated organic transistors. The electrolyte is the layer between the gate and the semiconducting layer connecting the source and the drain. (a) Case of an electrolyte with both mobile cations and anions. The anions penetrate into the positively charged semiconducting polymer channel, resulting in a slow transistor. (b) Case of a polyelectrolyte, where the anions are connected covalently by a polymer backbone. The anions are immobile and cannot penetrate into the semiconductor, resulting in a fast transistor. (c) Case of an insulating layer between the electrolyte and the semiconductor, which will decrease the electric double layer capacitance at the conducting channel.
Figure 2. Summary of the various synthesis steps to fabricate P3HT-b-P(MMA-r-HEMA:SBA) amphiphilic copolymer (H(+)NEt3 cation in structure 9 is omitted for simplicity). 4549
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(vinylphosphonic acid-co-acrylic acid) (P(VPA−AA)) polyelectrolyte was obtained from Rhodia and used without further purification. Synthesis of 2-Bromo-3-hexylthiophene. In a 200 mL flask, a solution of N-bromosuccinimide (5.29 g, 29.7 mmol) was slowly added to a solution of 3-hexylthiophene (5 g, 29.7 mmol) in 50 mL of anhydrous THF (50 mL) at 0 °C under nitrogen. The mixture was stirred at 0 °C for 1 h. After that, 50 mL of distilled water was added to the reaction mixture, and the medium was extracted with 150 mL of diethyl ether. The organic layer was washed with a solution of Na2S2O3 (10%) and a solution of KOH (10%) and dried over anhydrous MgSO4. The organic layer was distilled to give colorless oil (6.7 g, 92%). 1H NMR (300 MHz, CDCl3), δ (ppm): 7.19 (d, 1H), 6.82 (d, 1H), 2.59 (t, 2H), 1.59 (quint, 2H), 1.33 (m, 6H), 0.91 (t, 3H). 13C NMR (300 MHz, CDCl3), δ (ppm): 141.0, 128.2, 125.1, 108.8, 31.6, 29.7, 29.4, 28.0, 22.6, 14.1. Synthesis of 2-Bromo-3-hexyl-5-iodothiophene. Iodine (1.42 g, 11.18 mmol) and iodobenzene diacetate (1.965 g, 6.1 mmol) were added to a solution of 2-bromo-3-hexylthiophene (2.5 g, 11.1 mmol) in dichloromethane (25 mL) at 0 °C. The mixture was stirred at room temperature for 4 h. Then aqueous Na2S2O3 (10%) was added, and the mixture was extracted with diethyl ether and dried over anhydrous MgSO4. Then the solvent was evaporated to obtain crude products, which were purified by silica column chromatography (eluent: heptane) to give pure 2-bromo-3-hexyl-5-iodothiophene as a pale yellow oil (3 g, 86%). 1H NMR (300 MHz, CDCl3), δ (ppm): 6.97 (s, 1H), 2.52 (t, 2H), 1.56 (quint, 2H), 1.32 (m, 6H), 0.89 (t, 3H). 13C NMR (300 MHz, CDCl3), δ (ppm): 144.3, 137.0, 111.7, 71.0, 31.5, 29.6, 29.2, 28.8, 22.5, 14.1. Synthesis of Regioregular Head-to-Tail Poly(3-hexylthiophene) with H/Br End Group (Br-P3HT-H). A dry, 500 mL threeneck flask was flushed with nitrogen and was charged with 2-bromo-3hexyl-5-iodothiophene (15 g, 40 mmol). After three azeotropic distillations by addition of toluene (15 mL), 220 mL of anhydrous THF was added via a previously flamed and nitrogen flushed syringe; the mixture was stirred at 0 °C for 1 h. i-PrMgCl (2 M solution in THF, 19.14 mL, 38.28 mmol) was added via a syringe, and the mixture was continuously stirred at 0 °C. After 1 h, the mixture was transferred to a flask containing a suspension of Ni(dppp)Cl2 (365 mg, 5.5 × 10−1 mmol) in THF (25 mL). The polymerization was carried out for 24 h at 0 °C followed by addition of 20 mL of 5 M HCl solution. After termination, the reaction was stirred for 15 min and extracted with 200 mL of chloroform. The solvent was evaporated, and the polymer was precipitated in cold methanol and washed several times with n-hexane (200 mL). The polymer was characterized by 1H NMR and GPC. Yield: 70%. FT-IR (cm−1): 721, 819, 1376, 1454, 1510, 2853, 2922, 2953. 1H NMR (300 Hz, CDCl3), δ (ppm): 6.96 (s, 1H), 2.90 (t, 2H), 1.79 (sex, 2H), 1.52 (q, 6H), 0.94 (t, 3H). Mn,GPC = 6000 g/mol, ĐM = 1.18. Synthesis of Regioregular Head-to-Tail Poly(3-hexylthiophene) with CHO/Br End Group (Br-P3HT-CHO). Br-P3HT-H (1 g, 16.6 × 10−5 mol) was dissolved in 260 mL of anhydrous toluene under nitrogen. N,N-Dimethylformamide (DMF) (5.12 mL, 66.3 mmol) and phosphorus(V) oxychloride (POCl3) (5.30 mL, 58 mmol) were then added to the solution. The oxidation reaction was performed at 75 °C for 24 h. Then the solution was cooled down to room temperature, followed by the addition of a saturated aqueous solution of sodium acetate (200 mL). The solution was stirred for 4 h. Then, the polymer was extracted with 200 mL of chloroform. The solvent was evaporated under vacuum, and the polymer was precipitated in cold methanol and washed with cold n-hexane (200 mL). After drying under vacuum, 96 mg of polymer was obtained. Yield: 96%. FT-IR (cm−1): 721, 819, 1376, 1453, 1509, 1649, 2854, 2923, 2953. 1H NMR (300 MHz, CDCl3); δ (ppm): 9.99 (s, 1H), 6.96 (s, 1H), 2.78 (t, 2H), 1.69 (sex, 2H), 1.49 (q, 6H), 0.89 (t, 3H). Mn GPC = 6000 g/mol, ĐM = 1.19. Synthesis of Regioregular Head-to-Tail Poly(3-hexylthiophene) with CH2OH/Br End Group (Br-P3HT-CH2OH). Br-P3HTCHO (500 mg, 8.33 × 10−5 mol) was dissolved in 30 mL of anhydrous THF under nitrogen. After addition of NaBH4 (41.8 mg, 1.1 × 10−3 mmol) the mixture was kept stirring at room temperature for 2 h. After
mobility but that can be wetted by a polar solution of polyelectrolyte requires the design for semiconducting polymers. We were inspired by the approach of using amphiphilic copolymers for adaptive polymer surfaces21 where the wetting properties of the surface changes with time due to surface reorganization of soft polymer segments, while hard polymer segments prevent the dissolution of the polymer.22 The analogy led us to imagine a semiconducting copolymer for which the nonpolar conjugated polymer block could provide the hard and nonsoluble structure, while a soft polar block could reorganize at the surface upon solvent wetting. Importantly, if the polar block is an electrical insulator that cannot dissociate in ions, the overall capacitance value of the semiconductor/electrolyte/ metal will be much smaller than in a compact electric double layer (Figure 1b) because the distance between the plane of ions and the electronic charges (holes) in the semiconductor increases due to this insulating spacer overlayer (Figure 1c). The transistor would operate at a higher voltage than in the situation described in Figure 1b. Hence, the polar block needs to be a polyanion block in order to maintain close and intimate contact between the immobile anions and the semiconductor. The literature on the synthesis of block copolymers composed of an insulating polymer segment and a polyelectrolyte segment is vast,23−38 but very little work has been done for the synthesis of block copolymers composed of a semiconducting polymer block combined with a polyelectrolyte block.39 Here, we introduce a novel amphiphilic semiconducting block copolymer composed of a semiconducting polymer block and a polyelectrolyte block. With the right ratio between the two blocks, the copolymer combines a hydrophilic character and a decent semiconducting property (charge carrier mobility) and still prevents the anion penetration into the p-channel of an OFET. The copolymer consists of a hydrophobic semiconducting regioregular poly(3-hexylthiophene) (P3HT) block and a hydrophilic block of randomly alternating repeat units of methyl methacrylate and sulfonated hydroxyethyl methacrylate, i.e., P3HT-b-P(MMA-r-HEMA:SBA), herein simply called the copolymer (Figure 2). Water, as well as the aqueous solution of the polyelectrolyte, wets the surface of the copolymer surface. As a proof of concept, we use this unique combination of properties for a semiconducting polymer to fabricate a polyelectrolyte-gated OFET by inkjet printing.
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EXPERIMENTAL DETAILS
Materials. 3-Hexylthiophene, N-bromosuccinimide, iodine, and iodobenzene diacetate were purchased from Acros and used as received. Ni(dppp)Cl2 and i-PrMgCl in tetrahydrofuran (THF) (2 mol/L) were also purchased from Acros and stored in glovebox at room temperature. 2-Bromoisobutyryl bromide (Br-iBuBr), triethylamine (NEt3, 99%), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, 99%), anhydrous N,N-dimethylformamide (DMF, 99.8%), sodium borohydride (NaBH4, 99%), phosphorus(V) oxychloride (POCl3, 99%), and 2-sulfobenzoic acid cyclic anhydride (SBA) were purchased from Aldrich. Copper(I) bromide (CuBr, 98%) and copper(II) bromide (CuBr2, 98%) were purchased from Fluka and used without further purification. Methyl methacrylate (Aldrich, 98%) was distilled under vacuum and stored under nitrogen at −20 °C. Chloroform (Labscan, 99%), toluene (Labscan, 99%), and THF (Labscan 99%) were dried using an MBraun solvent purification system under N2. Dichloromethane (Chem-Laboratory, 99.8%), nheptane (Labscan, 99%), n-hexane (Labscan, 99%), and methanol (Chem-Laboratory, 99.8%) were used as received. All reactions were performed in oven-dried glassware under purified nitrogen. Poly4550
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through a 0.2 μm PTFE filter. The copolymer solutions were spincoated on the patterned substrates, and the resulting thin films were dried in a nitrogen oven at 110 °C for 10 min. The thickness of the copolymer film was 30 ± 2 nm as measured by an ellipsometer (Sentech SE400). Polyelectrolyte P(VPA−AA) was dissolved in a mixture of 1-propanol and deionized water (5:1 vol:vol, 30 mg/mL), filtered through a 0.2 μm Nylon filter, and finally inkjet printed (Dimatix DMP-2800) on the copolymer surface. The drop spacing setting was set to 40 μm, and the printer platen was kept at 30 °C. Two layers of the polyelectrolyte were printed on top of each other to ensure a full coverage of the polyelectrolyte film over the underlying semiconducting copolymer layer. The thickness of the polyelectrolyte film was 0.5 ± 0.1 μm as measured by a surface profilometer (Veeco, Dektak 3 ST). Finally, the devices were dried in a vacuum oven at 110 °C for 90 s, and to complete the fabrication of the OTFTs, a 60 nm thick Ti gate electrode was thermally evaporated through a shadow mask. All the transistors had the same device geometry with channel length and width of L = 3 μm and W = 15 mm, respectively. Characterization. 1H NMR and 13C NMR spectra were recorded in deuterated chloroform (CDCl3) with TMS as an internal reference, on a Bruker AMX-300 spectrometer at a frequency of 300 MHz. Fourier transform infrared (FT-IR) spectra were recorded using a BIO-RAD Excalibur spectrometer equipped with an ATR Harrick Split PeaTM. Size exclusion chromatography (SEC) of P3HT was performed in THF/NEt3 (98:2; v/v) (sample concentration: 1 wt %) at 35 °C using a Polymer Laboratories (PL) liquid chromatograph equipped with a PL-DG802 degasser, an isocratic HPLC pump LC1120 (flow rate: 1 mL/min), a Basic-Marathon autosampler, a PLRI refractive index detector, and three columns: a guard column PL gel 10 μm and two columns PL gel mixed-B 10 μm. Molecular weight and molecular weight distribution were calculated with reference to polystyrene standards. Contact angle measurements were recorded using a CAM 200 contact angle meter (KSV Instruments), and the photographs of the water droplets (7 μL) were analyzed using Attension Theta software (Biolin Scientific). Optical microscopy was performed using an Olympus MX-50 microscope. AFM micrographs were acquired with Dimension 3100 scanning probe microscope (Digital Instruments) operated in tapping mode with Si cantilevers. Output and transfer characteristics were obtained by using a Keithley semiconductor characterization system (4200-SCS) at a scan rate of 0.3 V s−1. To record switching transients, a square-wave voltage (Agilent 33120A waveform generator) and a dc voltage (Agilent E3631A power supply) were applied to the gate and the drain electrode, respectively, while the voltage drop across a resistor (100 kΩ) connecting the source to the ground was measured with an oscilloscope (Agilent 54832D Infiniium). A charging current of the gate capacitor was extracted from the transients by subtracting transient response at VD = 0 V from the response at VD = −0.5 V.
evaporation of the solvent the polymer was precipitated in cold methanol. After drying under vacuum, 480 mg of the polymer was obtained. Yield: 96%. FT-IR (cm−1): 724, 817, 1376, 1453, 1509, 1561, 2853, 2922, 2953. 1H NMR (300 MHz, CDCl3); δ (ppm): 6.96 (s, 1H), 2.78 (t, 2H), 3.7 (t, 2H), 1.69 (sex, 2H), 1.49 (q, 6H), 0.89 (t, 3H). Mn,GPC = 6000 g/mol, ĐM = 1.19. Synthesis of Bromoester-Terminated Poly(3-hexylthiophene) (P3HT-Macroinitiator). Br-P3HT-CH2OH (480 mg, 8.0 × 10−5 mol) was dissolved in 20 mL of anhydrous THF under nitrogen. To this solution NEt3 (1 mmol) and 2-bromoisobutyryl bromide (0.83 mmol) were added. Then the reaction was carried out at 50 °C overnight; the polymer was extracted by chloroform. The solution was washed two times with distilled water. The polymer was precipitated in cold methanol. After drying under vacuum, 475 mg of the polymer was obtained. Yield: 95%. FT-IR (cm−1): 724, 818, 1376, 1451, 1509, 1561, 1735, 2853, 2922, 2953. 1H NMR (300 MHz, CDCl3); δ (ppm): 6.96 (s, 1H), 5.29 (t, 2H), 2.78 (t, 2H), 1.93 (t, 6H), 1.69 (sex, 2H), 1.49 (q, 6H), 0.89 (t, 3H). Mn,GPC = 7000 g/mol, ĐM = 1.28, Mn,1H NMR = 7200 g/mol. Synthesis of Poly(3-hexylthiophene)-b-poly(methyl methacrylate-random-2-hydroxyethyl methacrylate) (P3HT-b-P(MMA-r-HEMA)). P3HT-b-P(MMA-r-HEMA) was synthesized by ATRP using the P3HT-macroinitiator. 0.1 g of P3HT-macroinitiator (Mn,1H NMR = 7200, ĐM = 1.28) was placed in a 25 mL flask, to which 2 mL of degassed THF was added by syringe. The macroinitiator solution was stirred until it became homogeneous. Monomer solution was prepared separately: methyl methacrylate (0.897 mmol, 90 mg), HEMA (0.08 mmol, 11 mg), PMDETA (0.028 mmol), CuBr (2 mg, 0.014 mmol), and a small amount of Cu(II)Br2 (0.1 mg) were added in a 50 mL round-bottom flask, and the flask was degassed by three freeze−pump−thaw cycles. The monomer solution was stirred until it became homogeneous and then placed in a 70 °C oil bath. When the macroinitiator solution was added by cannula into the monomer solution, the mixture solution became homogeneous with a dark orange color. After the solution was allowed to react for 16 h at 70 °C, the resultant polymer solution was diluted with 20 mL of THF. The solution was then passed through a column of Al2O3 to remove copper. The polymer solution was concentrated and then precipitated into n-hexane (50 mL). The precipitated polymer was collected by vacuum filtration and subsequently washed with n-hexane and methanol, followed by drying under vacuum to give 165 mg of the desired product corresponding to 64% of conversion. FT-IR (cm−1): 724, 818, 1376, 1451, 1509, 1561, 1735, 2853, 2922, 2953, 3550. 1H NMR (300 MHz, CDCl3) δ (ppm): 6.96 (s, 1H), 5.30 (t, 2H), 4.18 (s, 2H), 3.85 (s, 2H), 2.78 (s, 2H), 1.98 (m, 6H), 1.69 (sex, 2H), 1.49 (q, 6H), 0.89 (t, 3H). Mn,GPC = 8550 g/mol, ĐM = 1.39, Mn,1H NMR = 10 500. Composition of the copolymer by 1H NMR: P3HT (69%), PMMA (22%), and PHEMA (9%). Esterification of P3HT-b-P(MMA-r-HEMA) Block Copolymers. All esterification reactions were carried out in THF at room temperature. For a typical reaction, 60 mg of copolymers was dissolved in 20 mL of THF and purged by N2. Once the copolymers were dissolved in the solvent, NEt3 (2 equiv to HEMA) was added. 2Sulfobenzoic acid cyclic anhydride (SBA) (2 equiv to HEMA) in about 10 mL of THF was then slowly added into the reactor with NEt3 as the catalyst. The solution turned turbid immediately, and the reaction was stopped after 24 h. The copolymer was directly precipitated in nhexane to obtain the sulfonated copolymers. Yield: 95%. FT-IR (cm−1): 724, 818, 1090 1376, 1451, 1509, 1561, 1735, 2853, 2922, 2953. 1H NMR (300 MHz, CDCl3); δ (ppm): 9.8 (s, 1H), 8.1 (s, 2H), 7.5 (s, 2H), 6.96 (s, 1H), 4.25 (m, 2H), 4.18 (s, 2H), 3.85 (s, 2H), 3.1 (s, 6H), 2.78 (s, 2H), 1.98 (m, 6H), 1.69 (sex, 2H), 1.49 (q, 6H), 0.89 (t, 3H). Device Fabrication. To fabricate the top-gate/bottom-contact OTFTs, a chromium layer (5 nm thick) and a gold layer (50 nm thick) were first evaporated on borosilicate glass substrates. Interdigitated source and drain electrodes were next patterned on the glass substrates by using standard photolithography and wet-etching. The copolymer was dissolved in a mixture of analytical grade tetrahydrofuran (THF) and methanol (9:1 vol:vol, 5 mg/mL), sonicated, and finally filtered
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RESULTS AND DISCUSSION
Synthesis of the Amphiphilic Copolymer. Poly(3hexylthiophene)-block-poly(methyl methacrylate-random-hydroxyethyl methacrylate) (P3HT-b-P(MMA-r-HEMA)) copolymer and its sulfonated homologue (P3HT-b-P(MMA-rHEMA:SBA)) were synthesized via a multistep procedure consisting mainly of controlled “quasi-living” Grignard metathesis (GRIM) polymerization of 2-bromo-5-iodo-3-hexylthiophene initiated by Ni(dppp)Cl2 to form α-bromopoly(3hexylthiophene) (Br-P3HT-H), followed by a three-step procedure assuring the conversion of Br-P3HT-H into αbromo-ω-bromoisobutyrate P3HT macroinitiator and atom transfer copolymerization of methyl methacrylate (MMA) and hydroxyethyl methacrylate (HEMA) (Figure 2). As far as the GRIM process is concerned, chloromagnesium thiophene, which was prepared in situ by the treatment of 3 with ca. 1 equiv of isopropylmagnesium chloride at 0 °C for 1 h, was polymerized with Ni(dppp)Cl 2 (dppp = 1,3-bis4551
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methylene protons of the poly(thiophene) repeating units at 2.8 ppm (−thiophene−CH2−(CH2)4−CH3). While a Mn,NMR of 10 500 g/mol is calculated (composition P3HT, 69%, PMMA, 22%, and PHEMA, 9%), the SEC analysis reveals an apparent molar mass of 8550 g/mol and confirms the control of the ATRP process by a unimodal chromatogram characterized by a ĐM of 1.39. It is worth noting that the feed ratio of MMA/ HEMA comonomers has to be taken into consideration in order to achieve a well-controlled polymerization; if the MMA/ HEMA feed ratio is lower than 10, discrepancies between theoretical and experimental P(MMA-r-HEMA) molar masses were observed (data not shown here). Such a phenomenon, presumably induced by the difference in terms of chain solubility, is however overcome when a sufficiently low HEMA content relative to MMA is used to allow maintaining a good solubility of the as-obtained diblock copolymer, especially at high monomer conversions. The sulfonated groups were then linked covalently to the HEMA repeating units to form the P3HT-b-P(MMA-rHEMA:SBA) diblock copolymer. As reported by Zhang et al.,41 triethylamine (NEt3) and 2-sulfobenzoic acid cyclic anhydride (SBA) were used as catalyst and reagent, respectively. As soon as a solution of SBA (used in a [SBA]0/[OH]0 ratio of 2) was added, the reaction turned turbid immediately, indicating a clear modification of solubility leading to the progressive precipitation of the diblock copolymer. The reaction was however carried out at room temperature for 24 h. After completion of the reaction, diblock copolymers products were precipitated in n-hexane, filtered, and dried until constant weight. As evidenced by FT-IR, the decrease of OH stretching signal and the appearance of the S O stretching band at around 1090 cm−1 highlights the SN2 reaction between the hydroxyl groups of the HEMA repeating units to the SBA reagent (Figure 3). Despite its apparent good solubility in CDCl3, the presence of dandling −SO3(−)(+)NEt3 salt all along the polymethacrylate block induces a clear modification of the spin−spin relaxation and modifies the line width of all associated signals during a 1H NMR analysis. However, the aromatic proton signals present on the phenyl group of the sulfobenzoic unit (Hk, Figure 4) show up between 7.4 and 8.1 ppm and provide another evidence of the sulfobenzoic acid presence. As tentatively calculated by integration of the appropriate 1H NMR signals, an Mn,NMR of 10 500 g/mol is obtained. Finally, SEC analysis reveals that no change in molar mass dispersity has to be bemoaned, confirming the control of the final step process (ĐM = 1.39). Fabrication and Characterization of the Copolymer Films. The long semiconducting P3HT block is expected to facilitate formation of well-ordered π-conjugated structures, necessary for good electronic charge transport. The hydrophilic HEMA:SBA block, on the other hand, is expected to improve the surface wettability of the semiconducting layer so that the polyelectrolytic gate dielectric can be inkjet printed from an aqueous solution on top of the semiconducting copolymer film. Figure 5 shows an AFM phase image of the copolymer surface taken at 40% relative humidity. The film appears to be partially amorphous with fibrous-like structures where the lateral width of a fiber is 15 ± 5 nm, which is in well accordance with the typical dimensions of homopolymeric P3HT nanofibrillar domains.42,43 The packing of the P3HT segments in fibrills is required to ensure good charge carrier mobility. The pristine copolymer films also show dendritic patterns of a softer phase (regions in yellow) which resemble dewetting patterns of block
Figure 3. FT-IR spectra of P3HT-macroinitiator (a), P3HT-bP(MMA-r-HEMA) (b), and P3HT-b-P(MMA-r-HEMA:SBA) (c).
(diphenylphosphino)propane) at 0 °C for an initial [2]0-to[Ni]0 ratio of 73. To promote protonolysis of the P3HT-Ni complex growing chain instead of competing coupling reactions, the polymerization was quickly quenched with 5 M hydrochloric acid solution after 24 h. The as-obtained polymer was characterized by an experimental molar mass of 6000 g/ mol (as determined by SEC analysis) and showed a unimodal and narrow dispersity value (Mw/Mn = ĐM) of 1.18. Additionally, the 1H NMR analysis attests on the regiocontrol of the process by showing more than 98% of head-to-tail coupling. As already demonstrated by some of us,40 the as-obtained regioregular Br-P3HT-H 4 is easily transformed in three steps on a bromoester-terminated P3HT macroinitiator 7 propitious to an atom transfer radical polymerization (ATRP). The desired copolymer was then synthesized by atom transfer radical copolymerization of methyl methacrylate (MMA) and hydroxyethyl methacrylate (HEMA) from the prepared P3HTmacroinitiator (Mn,NMR = 7200 g/mol, ĐM = 1.28) for an initial [MMA]0/[HEMA]0/[P3HT-Br]0/[CuBr]0/[CuBr2]0/[PMDETA]0 of 64/6/1/1/0.5/2. The polymerization was performed in THF at 70 °C. After 16 h, the copolymer solution was cooled down to room temperature and was diluted with an extra volume of THF. The mixture solution was purified over Al2O3 to remove CuBr and CuBr2. The copolymer was then recovered by precipitation from cold hexane and dried under vacuum until constant weight. As compared to the P3HT-macroinitiator, the FT-IR analysis of the dried poly(3-hexylthiophene)-block-poly(methyl methacrylate-random-hydroxyethyl methacrylate) (P3HT-b-P(MMA-r-HEMA)) copolymer reveals characteristic peaks assigned to the hydroxyl (νOH) and carbonyl (νCO) stretching vibration bands at 3300−3600 and 1735 cm−1, respectively, confirming then the qualitative presence of both MMA and HEMA repeating units (Figure 3). Assuming that each chain of P3HT-b-P(MMA-r-HEMA) is end-capped by the P3HT ATRP initiator, the experimental molar mass of the copolymer can be calculated by 1H NMR spectroscopy (Mn,NMR) from its relative molar composition (Figure 4). This last term was determined from the relative intensity of α-methylene protons of HEMA units at 4.15 ppm (−CO2−CH2−CH2OH), the one of the methyl oxy carbonyl protons of the MMA units at 3.6 ppm (−CO2−CH3) and the 4552
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Figure 4. 1H NMR spectra recorded in CDCl3 (23 °C) of both P3HT-b-P(MMA-r-HEMA) (top) and corresponding P3HT-b-P(MMA-rHEMA:SBA) obtained after sulfonation (bottom).
copolymer thin films after annealing under selective solvent vapor.44,45 These patterns are quite expected since water from the ambient humidity (40% RH) is absorbed by the sulfonated segments and leads to a partial dewetting and swelling of the copolymer. Even more dramatic changes were observed when the copolymer films were exposed to saturated water vapor. As shown in Figure 6, larger bumps started to appear on the copolymer films after 3 h exposure to saturated water vapor. We propose that absorption and infiltration of water into the sulfonated segments of the copolymer and subsequent swelling of the sulfonated regions are responsible for the observed bumps. Figure 7a shows water contact angle as a function of time after deposition of a water droplet on spin-coated surfaces of homopolymeric P3HT and the pristine copolymer. The reference homopolymeric P3HT surface was expectedly hydrophobic, and the water contact angle remained constant
at 104° throughout the duration of the experiment. The water contact angle on the copolymer surface was high, at first, but after a couple of seconds the water droplet collapsed and the surface suddenly changed from hydrophobic to hydrophilic as evidenced by the abrupt drop of contact angle from 95° to 18° (Figure 7a). In order to confirm that these wetting properties are due to the intrinsic properties of the copolymer surfaces and not due to impurities or a dissolution of the copolymer, several experiments were carried out, and the results are reported in the Supporting Information. First, chromatography reveals that the copolymer is pure and does not contain small charged impurities, which could eventually have explained the wetting properties. Second, the copolymer was put in 2 mL of distilled water, and the mixture was stirred for 10 min at room temperature. The copolymer in water was filtered over microfilter. After filtration, the obtained water was checked by 4553
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Figure 7. (a) Water contact angle on P3HT and copolymer surfaces as a function of time. The photographs show profiles of water droplets on the copolymer surface taken at t = 0 s and t = 3 s after deposition of the water droplet. Microscopy images of a polyelectrolyte film inkjet printed on (b) P3HT and (c) copolymer surface. The length of the scale bar is 100 μm.
Figure 5. AFM phase image of the copolymer surface. The phase angle varies from 0 to 15°, and the scan size is 1 μm × 1 μm. 1
H NMR (D2O) and FTIR. The NMR and FTIR spectra of distilled water and the water obtained after filtration of the copolymer are identical and superimposable, thus indicating that the copolymer is not water-soluble (see Figures S1 and S2). Our hypothesis to explain the change in wetting is that the pristine copolymer film is initially hydrophobic because the solvent of the copolymer (tetrahydrofuran and methanol) favors assembly of the hydrophobic P3HT block to the free surface, on top of the film, whereas the sulfonated segments remain mostly buried deeper in the film. This is supported by the AFM images. When stored in a humid environement, the sulfonated regions start to swell and rearrange due to the ambient humidity and dendritic patterns start to appear on the film, which is still mostly hydrophobic at 40% RH. When a water droplet is placed on top of the copolymer film, the sulfonated regions swell further and give rise to infiltration of water through the sulfonated regions deeper into the copolymer film which initiates the wettability change.
Fabrication and Characterization of the Transistor. Finally, we prove for the first time that printing techniques can be extented to hydrophilic electrolytes, which expands the range of materials for the fabrication of EGOFETs from hydrophobic ionic liquids and gels, for the fabrication of EGOFETs. The desirable wettability of the semiconducting copolymer was exploited in fabrication of top-gate/bottomcontact electrolyte-gated OFETs. First, the copolymer thin film was spin-coated on a glass substrate which had prepatterned source and drain electrodes. For the sake of comparison, reference samples were spin-coated from homopolymeric P3HT. Aqueous polyelectrolyte solutions were inkjet printed on the copolymer surface to address whether this change in the wettability of the copolymer film can be exploited in depositing a polyelectrolyte layer on top of the copolymer surface. The polyelectrolyte chosen is a random copolymer poly(vinylphosphonic acid-co-acrylic acid) (P(VPA−AA)) with 70% phosphonic acid groups and 30% acrylic acid groups.
Figure 6. Optical micrographs from copolymer thin films: (a) pristine film and (b) the same film after 3 h of exposure to saturated water vapor. (c) AFM micrograph from a bump in (b). Optical micrographs taken from water-vapor treated thin films show large bumps on the copolymer surface while the pristine films are featureless. 4554
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and 40% RH. Since the P3HT/Au interface shows an Ohmic contact,46 holes are easily injected, and for short channel transistors, the frequency evolution of the impedance is mostly governed by polarization of the polyelectrolyte.10 Figure 8 displays the evolution of the serial capacitance C(f) and the phase angle θ(f). At high frequencies (f > 106 Hz), the capacitance is about 0.1 μF cm−2 and attributed to dipolar relaxation of the material. The polyelectrolyte acts as a dipolar dielectric. The ac frequency is too high for the protons to move. When decreasing the frequency to 104 Hz, the capacitance increases and the phase angle deviates from an ideal capacitor (i.e., θ = −90°). This is attributed to the motion of protons in the oscillating electric field.47 The impedance in that region has a resistive contribution. At frequencies lower than 104 Hz, the capacitance becomes constant and reaches typical value of electric double layer (Ci = 2 μF cm−2). These observations indicate that an electric double layer is formed at the semiconducting copolymer−polyelectrolyte interface. Note that similar capacitances are reached for the homopolymer P3HT/P(VPA−AA) interface.10 Importantly, the phase angle reaches −80° and is also constant, which implies that no slow ion migration occurs in the typical operating frequency of the transistor in circuits. To avoid undesired electrolysis of the absorbed water in the hygroscopic polyelectrolyte, the transistors were operated at less than 0.5 V. The output characteristics of the OFETs showed clear current modulation for drive voltages of less than 0.5 V (Figure 9c). The ratio between the on and off current (ID,on/ID,off) was roughly 67, which was extracted from the transfer characteristics (Figure 7b). The field-effect mobility was determined from a (−ID,sat)1/2 vs VG plot by fitting a straight line for VG < −0.3 V and using the equation
Figure 8. Effective capacitance and phase angle as a function of frequency for a sandwiched capacitor structure of Au/copolymer/ P(VPA−AA)/Ti (size 400 μm × 400 μm). The effective capacitance was calculated from complex impedance by using an equivalent circuit comprising a resistor and a capacitor in parallel. The data were collected with an alpha high-resolution dielectric analyzer (Novocontrol GmbH) by scanning from 1 MHz to 10 Hz and using an rms amplitude of 10 mV.
Figure 7b visualizes the challenge in depositing a uniform polyelectrolyte film on the hydrophobic P3HT homopolymer. Despite using a surfactant to reduce the water contact angle,9 the polyelectrolyte does not form a uniform film on the P3HT surface, but rather it confines to small drops drying in polyelectrolyte islands (Figure 7b), which would inevitably preclude the fabrication of a transistor by inkjet printing. However, the situation is strikingly different for the copolymer surface which allows inkjet printing of a uniform polyelectrolyte film on the copolymer surface as is evidenced in Figure 7c. The fabrication of the transistor was completed by evaporating the titanium top gate electrode. To check if the “semiconducting copolymer/polyelectrolyte/gate” capacitor can lead to high capacitance required for the low operating voltage, the source and gate were connected to an impedance spectrometer. An ac voltage of 10 mV was applied from 106 to 10 Hz, and the impedance was measured at room temperature
μ=
⎞2 2L ⎛ ∂ −ID,sat ⎟ ⎜ WC ⎝ ∂VG ⎠
Figure 9. Electrical characteristics of the OFETs. (a) A schematic and an optical micrograph of the transistor structure where the polyelectrolyte layer has been inkjet printed. (b) Transfer, (c) output, and (d) switching characteristics of the OFETs. 4555
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where C = 2 μF/cm2 is the capacitance per unit area of the gate dielectric (Figure 8), W = 15 mm and L = 3 μm are the channel width and length, and the last term in the parentheses is the slope of the fitted line. The resulting mobility μ = 10−4 cm2 V−1 s−1 is rather low, which is attributed to the structural disorder of the copolymer films. Switching characteristics of the transistors (Figure 9d) were obtained by applying a squarewave potential to the gate electrode while maintaining the drain electrode at a constant potential (VD = −0.5 V) and subsequently measuring the source current. Switch on and off times of the transistors were 1.1 and 0.2 ms, which correspond to 90% and 10% of the final rise and fall response, respectively. The fast transistor response is associated with the absence of anion penetration into the positively charged semiconducting channel,48 due to the fact that the copolymer is composed of polyanions and the polyelectrolyte dielectric is a polaynionic membrane. Conclusion. To conclude, we have introduced a new P3HT-b-P(MMA-r-HEMA:SBA) amphiphilic semiconducting copolymer which, when spread as a thin film, can change from hydrophobic to hydrophilic upon exposure to water. This desirable wettability allows inkjet printing a polyelectrolyte film on the semiconducting copolymer surface and to finally fabricate low voltage (
