Soft Poly(butyl acrylate) Side Chains toward Intrinsically Stretchable

Jun 23, 2017 - Poly(butyl acrylate) (PBA) side chain equipped isoindigo-bithiophene (II2T) conjugated polymers have been designed and synthesized for ...
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Soft Poly(butyl acrylate) Side Chains toward Intrinsically Stretchable Polymeric Semiconductors for Field-Effect Transistor Applications Han-Fang Wen,† Hung-Chin Wu,*,‡ Junko Aimi,∥ Chih-Chien Hung,§ Yun-Chi Chiang,‡ Chi-Ching Kuo,*,† and Wen-Chang Chen*,‡ †

Institute of Organic and Polymeric Materials, National Taipei University of Technology, Taipei 10608, Taiwan Department of Chemical Engineering and §Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan ∥ Molecular Design & Function Group, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ‡

S Supporting Information *

ABSTRACT: Poly(butyl acrylate) (PBA) side chain equipped isoindigobithiophene (II2T) conjugated polymers have been designed and synthesized for stretchable electronic applications. The PBA segment possesses low glass transition temperature and high softness, offering a great opportunity to improve the mechanical property of semiconducting polymer thin films that typically contain lots of rigid conjugated rings. Polymers with 0, 5, 10, 20 and 100% of PBA side chains, named PII2T, PII2T-PBA5, PII2T-PBA10, PII2T-PBA20, and PII2T-PBA100, were explored, and their polymer properties, surface morphology, electrical characteristics, and strain-dependent performance were investigated systematically. The series polymers showed a charge carrier mobility of 0.06−0.8 cm2 V−1 s−1 with an on/off current ratio over 105 dependent on different amounts of PBA chains as probed using a top-contact transistor device. Moreover, we can still achieve a mobility higher than 0.2 cm2 V−1 s−1 even if 10% of PBA side chains were added (i.e., PII2T-PBA10). Such PII2T-PBA polymers, more attractive, exhibited superior thin film ductility with a low tensile modulus down to 0.12 GPa (PII2T-PBA20) due to the soft PBA side chain. The more PBA segment was incroporated, the lower modulus was reached. The mobility performance, at the same time, remained approximately 0.08 cm2 V−1 s−1 based on PII2T-PBA10 films even under a 60% strain and could be simultaneously operated over 400 stretching/releasing cycles without significant electrical degradations. The above results suggest that the rational design of soft PBA side chains provides a great potential for next-generation soft and wearable electronic applications.

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INTRODUCTION Semiconducting polymers have been developed rationally in recent years with their merits of low-cost processing, tunable chemical structure, superior charge transport, and good mechanical tolerance.1−4 Thin films based on such polymeric semiconductors, additionally, are innovated to be deformable and stretchable, leading to next-generation wearable electronics that potentially can be biocompatible and possess distinct functionalities with our body movement.5,6 Active layers in stretchable field-effect transistors (FETs),7−12 light-emitting diodes,13−15 photovoltaics,16−18 or digital data storage memories19−22 have been realized using semiconducting polymers by several chemical or physical approaches, such as buckled and wrinkled structures,23 polymer blends,24 and dynamic bonding functionalization.8 Semiconducting layers that can be intrinsically stretched, in addition, are important to be created and enable large functional area, uniform characteristics, and simple processing integration under strain. However, it is still challenging to maintain the electrical stability and durability when applying forces on such intrinsically strained active layers. © XXXX American Chemical Society

Designing a semiconducting polymer system that possesses not only superior electrical performance but also high thin film ductility and stretchability is crucial for practical applications. Side chain engineering has been focused on the field of polymeric electronics very recently since it can greatly enhance the device performances.25 Rational side chain design, moreover, can also manipulate the solution processability, solid state molecular stacking, and thin film morphology. From the view of soft electronics, we should be able to achieve a flexible or stretchable polymeric semiconducting active layer by controlling the interchain packing interaction and surface morphology based on the side chain substituents. Several polymer side chain modifications have been explored for the intrinsically stretchable device applications, especially for the FETs. For example, our group established an isoindigo-based donor− acceptor polymer bearing long, branched carbosilane side Received: April 26, 2017 Revised: June 13, 2017

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DOI: 10.1021/acs.macromol.7b00860 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Route on Synthesizing the Studied Polymeric Semiconductors

chains, and a charge carrier mobility up to 1 cm2 V−1 s−1 was reported even at strains as large as 60%;26 Bao and co-workers, on the other hand, developed a diketopyrrolopyrrole-based polymer with cross-linkable side chains, leading to a stretchable thin film that could be stabilized under a 150% strain, and a steady mobility ∼0.40 cm2 V−1 s−1 was obtained after 500 strain-and-release cycles of 20% strain.9 Although good charge transport ability was obtained, we still need to involve new synthetic pathway to customize specific side chains. It would be efficient if a common soft and deformable component can be directly incorporate into the conjugated polymer system, facilitating superior mechanical properties in film state with appropriate charge mobility. Poly(butyl acrylate) (PBA) is an elastic compound with a glass transition temperature of approximately −54 °C, which is far below compared to the room temperature. We herein introduce such soft PBA segment with varying amount onto isoindigo-bithiophene backbone as side chains through random copolymerization (Scheme 1). The polymer properties, surface morphologies, molecular packing structures, electrical characteristics, and mechanical endurances are investigated systematically. Linear siloxane-terminated side chain is used to improve the electrical performance and act as a reference polymer. A high modulus of 0.8 GPa is probed for the polymer thin film based on only siloxane side chain. Once the PBA side chain incorporated (i.e., 20%), the thin film tensile modulus can be lowered to 0.12 GPa, demonstrating that the thin film ductility and deformability can be efficiently improved by directly adding soft and elastic component as polymer side chains. This is the first example using such low glass transition temperature element on polymer side chains for intrinsically stretchable transistors, to the best of our knowledge. Stable and reliable charge transport characteristics, more importantly, are

obtained under stretching. A mobility of 0.08−0.1 cm2 V−1 s−1 was achieved under 0−60% strain levels, and such value can be maintained under 400 simultaneous stretching/releasing cycles, indicating our newly designed polymers via side chain engineering possess great potentials for future electronic applications.

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EXPERIMENTAL SECTION

Materials. Magnesium sulfate, potassium carbonate, 6-bromosatin, 6-bromooxindole, 8-bromo-1-octene, trimethyltin chloride, 1,1,3,3,5,5,5-heptamethyltrisiloxane, 5,5′-dibromo-2,2′-bithiophene, Karstedt’s catalyst, tri(o-tolyl)phosphine, tris(dibenzylideneacetone)dipalladium(0), common organic solvents (tetrahydrofuran, toluene, acetic acid, and dimethylformamide) for synthesis, and anhydrous solvents (i.e., chloroform) for device fabrication were purchased from Aldrich (St. Louis, MO) and used as received. 5,5′-Bis(trimethylstannyl)-2,2′-bithiophene (2T)27 and (E)-6,6′-dibromo1,1′- bis(8-(1,1,1,3,5,5,5-heptamethyltrisiloxan-3-yl)octyl)-[3,3′-biindolinylidene]-2,2′-dione (IID-Si)28−30 were synthesized according to the reported procedures. The poly(butyl acrylate) (PBA) homopolymer, in addition, was prepared by atom transfer radical polymerization (ATRP).10 The synthetic routes of studied materials with different amount of PBA side chain are shown in Scheme 1, and the details of monomer syntheses are summarized in the Supporting Information. General Procedures for Polymerization. The general synthetic procedure to polymerize the studied polymers (PII2T, PII2T-PBA5, PII2T-PBA10, PII2T-PBA20, and PII2T-PBA100) is described below: dibromo-isoindigo with siloxane (IID-Si) and PBA (IIDPBA) side chain, bithoiphene-ditin (2T), tri(o-tolyl)phosphine (16 mol % with respect to 2T), and tris(dibenzylideneacetone)dipalladium(0) (2 mol % with respect to 2T) were dissolved in a microwave vessel using chlorobenzene (5 mL) in a nitrogen-filled golvebox. A snap cap was then sealed the vessel, and the target materials were polymerized by Stille coupling reaction at 180 °C for 180 min under the microwave heating. The end-capping process was B

DOI: 10.1021/acs.macromol.7b00860 Macromolecules XXXX, XXX, XXX−XXX

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PII2T-PBA polymers were dissolved in chloroform (3−5 mg mL−1) at 60 °C for 2 h first, and then the polymer thin films were deposited onto the dielectric layer through spin-coating method at 1000 rpm for 60 s. An annealing process (200 °C under vacuum for 1 h), additionally, was introduced to enhance the FET performance. Finally, the top-contact source and drain electrodes (100 nm thick Au) were defined by a regular shadow mask with a 50 μm channel length (L) and a 1000 μm channel width (W). Preparation of Field-Effect Transistors with Stretched Polymer Films. To investigate the charge transport characteristics of these PII2T-PBA polymer-based thin films under a distinct strain ratio (0−100%). The polymer films were also deposited on an OTSmodified SiO2/Si substrate via spin-coating and were annealed at 200 °C for 1 h. Next, such films were transferred onto an elastomeric poly(dimethylsiloxane) (PDMS) (base: cross-linker = 20:1 w/w) slab. Tensile strains (0−100%) were applied to the polymer/PDMS matrix, so that the PII2T-PBA thin films were stretched along with the PDMS slab for any distinct stretching levels. The stretched thin films, afterward, were transferred back to a 300 nm SiO2-equipped Si substrate to act as the active layer in FET devices.26,32 Similarly, the top-contact source/drain Au contacts were defined by the regular shadow mask with 50 μm channel length (L) and 1000 μm channel width (W). It is worth to noting that the direction of Au electrodes could be designed in parallel or perpendicular with respect to the strain direction.

then taken with 2-(tributylstannyl)thiophene and 2-bromothiophene (both 1.1 equiv with respect to 2T and reacted at 185 °C for 10 min), and then the mixture was cooled and precipitated into methanol to form a crude polymer. Such crude polymer was finally purified by Soxhlet apparatus with methanol, acetone, and hexane to remove the oligomers and catalyst residues, and the final product was obtained after drying in a vacuum oven at 40 °C for 16 h. Synthesis of PII2T. Amounts of 197.30 mg (0.4 mmol) of 2T, 435.00 mg (0.4 mmol) of IID-Si, and 5 mL of chlorobenzene were used to give a dark blue solid (398 mg, yield: 90%). 1H NMR (400 MHz, CDCl3), δ (ppm): 9.05−9.03 (br, Ar−H), 7.14−7.11 (br, Ar− H), 6.89 (br, Ar−H), 3.71−3.67 (br, NCH2CH2), 1.39−1.20 (br, CH2CH2CH2), 0.49−0.39 (br, SiCH3), 0.21−0.10 (br, SiCH3). Anal. Calcd for [C54H90N2O6S2Si6]: C, 59.40; H, 7.94; N, 2.57; S, 5.87. Found: C, 59.47; H, 7.68; N, 2.82; S, 5.82. Mn and PDI estimated from gel permeation chromatography are 216 kDa and 1.7, respectively. Synthesis of PII2T-PBA5. Amounts of 172.15 mg (0.35 mmol) of 2T, 360.91 mg (0.33 mmol) of IID-Si, 56.32 mg (0.0175 mmol) of IID-PBA, and 5 mL of chlorobenzene were used to give a dark blue solid (340 mg, yield 98%). 1H NMR (400 MHz, CDCl3), δ (ppm): 9.05−9.03 (br, Ar−H), 7.14−7.11 (br, Ar−H), 6.89 (br, Ar−H), 4.10−3.90 (br, OCH2CH2), 3.63−3.55 (br, NCH2CH2), 2.49−2.20 (br, CH2CH2CH), 2.10−1.95 (br, CH2CH2CH2), 1.39−1.20 (br, CH2CH2CH2), 1.13−1.10 (br, CH3CH), 0.90−0.70 (br, CH3CH2), 0.49−0.39 (br, SiCH3), 0.21−0,10 (br, SiCH3). Anal. Calcd for [C60H96N2O8S2Si6]: C, 59.73; H, 7.97; N, 2.48; S, 5.66. Found: C, 60.12; H, 7.57; N, 2.50; S, 5.66. Mn and PDI estimated from gel permeation chromatography are 198 kDa and 1.8, respectively. Synthesis of PII2T-PBA10. Amounts of 172.15 mg (0.35 mmol) of 2T, 341.91 mg (0.315 mmol) of IID-Si, 112.64 mg (0.035 mmol) of IID-PBA, and 5 mL of chlorobenzene were used to give a dark blue solid (335 mg, yield 98%). 1H NMR (400 MHz, CDCl3), δ (ppm): 9.05−9.03 (br, Ar−H), 7.14−7.11 (br, Ar−H), 6.89 (br, Ar−H), 4.10−3.90 (br, OCH2CH2), 3.63−3.55 (br, NCH2CH2), 2.49−2.20 (br, CH2CH2CH), 2.10−1.95 (br, CH2CH2CH2), 1.39−1.20 (br, CH2CH2CH2), 1.13−1.10 (br, CH3CH), 0.90−0.70 (br, CH3CH2), 0.49−0.39 (br, SiCH3), 0.21−0,10 (br, SiCH3). Anal. Calcd for [C66H105N2O10S2Si5]: C, 60.05; H, 8.01; N, 2.40; S, 5.48. Found: C, 60.38; H, 7.74; N, 2.44; S, 5.37. Mn and PDI estimated from gel permeation chromatography are 133 kDa and 2.5, respectively. Synthesis of PII2T-PBA20. Amounts of 172.15 mg (0.35 mmol) of 2T, 303.92 mg (0.28 mmol) of IID-Si, 225.27 mg (0.07 mmol) of IID-PBA, and 5 mL of chlorobenzene were used to give a dark blue solid (213 mg, yield 74%). 1H NMR (400 MHz, CDCl3), δ (ppm): 9.05−9.03 (br, Ar−H), 7.14−7.11 (br, Ar−H), 6.89 (br, Ar−H), 4.10−3.90 (br, OCH2CH2), 3.63−3.55 (br, NCH2CH2), 2.49−2.20 (br, CH2CH2CH), 2.10−1.95 (br, CH2CH2CH2), 1.39−1.20 (br, CH2CH2CH2), 1.13−1.10 (br, CH3CH), 0.90−0.70 (br, CH3CH2), 0.49−0.39 (br, SiCH3), 0.21−0,10 (br, SiCH3). Anal. Calcd for [C79H124N2O14S2Si5]: C, 60.69; H, 8.08; N, 2.23; S, 5.09. Found: C, 61.40; H, 7.78; N, 2.11; S, 4.79. Mn and PDI estimated from gel permeation chromatography are 49.6 kDa and 2.6, respectively. Synthesis of PII2T-PBA100. Amounts of 191.82 mg (0.39 mmol) of 2T, 820.00 mg (0.39 mmol) of IID-PBA, and 5 mL of chlorobenzene were used to give a dark blue solid (161 mg, yield: 16%). 1H NMR (400 MHz, CDCl3), δ (ppm): 9.05−9.03 (br, Ar−H), 7.14−7.11 (br, Ar−H), 6.89 (br, Ar−H), 4.10−3.90 (br, OCH2CH2), 2.49−2.20 (br, CH2CH2CH), 2.10−1.95 (br, CH2CH2CH2), 1.39− 1.20 (br, CH2CH2CH2), 1.13−1.10 (br, CH3CH), 0.90−0.70 (br, CH3CH2). Anal. Calcd for [C136H206N2O34S2]: C, 65.94; H, 8.38; N, 1.13. Found: C, 66.53; H, 7.10; N, 1.75. Mn and PDI estimated from gel permeation chromatography are 6.9 kDa and 1.3, respectively. Fabrication of Field-Effect Transistors with Pristine Polymer Thin Films. A bottom-gate/top-contact device configuration was used to explore field-effect transistors (FETs) using the studied polymers. The gate dielectric layer was a 300 nm SiO2 layer (capacitance per unit area = 10 nF cm−2) on highly n-doped Si (100) substrates. An octadecyltrimethoxysilane (OTS) self-assembled monolayer was modified onto the SiO2 layer according to the reported method to improve the charge carrier performance.31 For the active layer, our

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RESULTS AND DISCUSSION

Polymer Characterization. The studied isoindigo-bithiophene polymers were designed based on a linear siloxaneterminated side chain (i.e., PII2T), which can facilitate condense interchain packing structure and superior charge transporting.29 Polymers with 5−20% of PBA side chain (respect to siloxane side chain), in addition, were explored to improve the mechanical endurance, named PII2T-PBA5, PII2T-PBA10, and PII2T-PBA20. Note that the polymer equipped only PBA side chain (i.e., PII2T-PBA100) was also synthesized as a comparison. All the studied polymers were successfully polymerized through the Stille coupling reaction under a microwave heating (Scheme 1). 1H NMR was used to confirm the chemical structures of the PII2T-PBA polymers, as depictied in Figure 1 and Figure S2. The signal at approximately 9.2 ppm (peak a) is attributed to the aromatic proton of the benzene ring on isoindigo, and the signals in the range of 6.8−7.4 ppm (peaks b−e) are assigned to the protons on conjugated polymer backbone. For the protrons on side chains, peaks g (ca. 3.8 ppm) and j (ca. 0−0.21 ppm) belong to siloxane-terminated chains, while peaks f (ca. 4.0 ppm), h (ca. 2.20−2.49 ppm), and i (ca. 0.70−1.13 ppm) are represented for PBA chains. The peak intensity changes of both siloxane and PBA-related proton signals are clearly observed with the manipulation of siloxane-to-PBA side chain ratio. It is worth noting that the estimated integration values of the 1H NMR signals for the aliphatic and aromatic protons are consistent with the proposed polymer structures, and the elemental analyses show a good agreement with the theoretical carbon, hydrogen, nitrogen, and sulfur content, demonstrating that our target semiconducting polymers were successfully synthesized. The studied polymers (PII2T, PII2T-PBA5, PII2T-PBA10, and PII2T-PBA20) possess the number-averaged molecular weight in a range of 49.6−216 kDa with polydispersities between 1.7 and 2.6, which are estimated from the size exclusion chromatographic (SEC) measurement using tetrahydrofuran as eluent. Note that PII2T-PBA100 shows a relatively low molecular weight of 6.9 kDa, owing to the bulky PBA side chains diminishing the reactivity in the polymerization. The C

DOI: 10.1021/acs.macromol.7b00860 Macromolecules XXXX, XXX, XXX−XXX

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1b), of PII2T and PII2T-PBA100 was probed at approximately 295 and 396 °C, respectively. The bulky PBA side group is more easily decomposed and vaporized under heating. Although the Td decreased with increased PBA ratio, polymers containing 5−20% PBA side chains (PII2T-PBA5, PII2TPBA10, and PII2T-PBA20) still possess stable thermal stability with a Td higher than 350 °C. The thermal transitions, in addition, are investigated using differential scanning calorimetry (DSC; Figure S3a,b). There is no thermal transition that could be observed under a scanning temperature from 0 to 260 °C (Figure S3a) due to the conjugated polymer backbone being rigid. More interestingly, the glass transition temperature (Tg) can be detected in low temperature range (i.e., below −20 °C) (Figure S3b). The PBA homopolymer presented a low Tg of −57 °C. As inserting PBA onto the II2T polymer backbone as side chains, we could observe a Tg at approximately −30 °C when the amount of PBA side chain was larger than 10%. This thermal transition is mainly contributed by the PBA side chain, which provides the opportunity to reduce the overall polymer thin film modulus and improve the mechanical deformability. Optical and Electrochemical Properties. The solid state UV−vis absorption spectra were recorded in order to realize the effects of PBA side chains on the optoelectronic properties of studied PII2T-PBA polymers, as depicted in Figure 1c. Two distingushable absorption bands could be obtained for our studied polymers, which is typical for the isoindigo-based polymers:29,30,34 (1) the absorption located at approximately 410 nm is associated with the π−π* transitions, while (2) the broad band in a range from 500 to 800 nm is facilitated by the intramolecular charge transfer (ICT) between the electrondonating thiophene rings and electron-accepting isoindigo moieties on the polymer main chain. The relative intensity of the vibrational peak at ∼720 nm, more interestingly, is slightly reduced as the increasing the content of PBA side chain from 0 to 20%. This is mainly due to the soft and bulky PBA groups leading to weaker interchain interactions, so that the polymers form more disorder aggregation domains in the solid state.33 The absorbance of PII2T-PBA100 is also included to demonstrate the effects of PBA side chain. The 720 nm signal, as we expect, cannot be observed once we replace all the polymer side chains into the PBA segment. The results again imply that the PBA side chain can effectively manipulate the solid state polymer characteristics. On the other hand, all studied PII2T-PBA polymers possess a similar optical band gap of 1.6 eV. This value is consistent with the isoindigobithiophene-based polymers in the literature,26,34 indicating that adding PBA groups on polymer side chain can still maintain the polymer conjugations as compared to the classical isoindigo polymers with alkyl side chains. The energy levels of the studied semiconducting materials were further analyzed through cyclic voltammetry (CV; Figure S3b). The highest occupied molecular orbital (HOMO) level, estimated from the onset oxidation potentials in the CV curves, is ranging from −5.16 to −5.27 eV, while the lowest unoccupied molecular orbital (LUMO) level is approximately −3.6 eV, calculated by the difference between the HOMO level and optical band gap. The energy band gaps and energy levels are mainly determined by the backbone structure of conjugated polymers. The studied polymers, thus, possess similar electrochemical properties with the same II2T polymer skeleton. Field-Effect Transistor Characteristics. The charge transport properties of the pristine PII2T, PII2T-PBA5, PII2T-PBA10, and PII2T-PBA20 thin films were characterized

Figure 1. (a) 1H NMR spectrum of PII2T-PBA10 in CDCl3. (b) TGA curves and (c) solid state UV−vis absorption spectra of the studied polymers.

studied PII2T-PBA polymers can be generally dissolved in common organic solvents (e.g., chloroform, chlorobenzene, and tetrahydrofuran), and the polymer solubility is improved with the increase of PBA chain content. The enhanced solubility indicates that the incorporation of PBA chains enlarges the polymer side chain bulkiness and weakens the interchain interaction between the polymer backbones in organic solvents.33 The strength of interaction between polymer chains, moreover, is also highly related to the ductility and deformability of polymer thin film, suggesting the mechanical property of the semiconducting polymers in film state can be also manipulated by tuning the side chain composition of the copolymers. The thermal decomposition temperature (Td, 5% weight loss), based on the thermogravimetric analysis (TGA; Figure D

DOI: 10.1021/acs.macromol.7b00860 Macromolecules XXXX, XXX, XXX−XXX

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Figure 2. (a) Illustration of the studied top-contact/bottom-gate FET device configuration. (b) FET transfer characteristics of the studied polymers and (c) output curves of PII2T-PBA10-based FET device.

Table 1. Charge Transport, Crystallographic, and Mechanical Properties of the Pristine (i.e., Nonstretched) Polymer Thin Films conventional FET polymers PII2T PII2T-PBA5 PII2T-PBA10 PII2T-PBA20 PII2T-PBA100 a

μ (cm V a

2

−1

0.78 0.33 0.22 0.063