Article pubs.acs.org/Macromolecules
Isoindigo-Based Semiconducting Polymers Using Carbosilane Side Chains for High Performance Stretchable Field-Effect Transistors Hung-Chin Wu,*,† Chih-Chien Hung,‡ Chian-Wen Hong,† Han-Sheng Sun,† Jau-Tzeng Wang,† Go Yamashita,§ Tomoya Higashihara,*,§ and Wen-Chang Chen*,† †
Department of Chemical Engineering and ‡Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan § Department of Organic Device Engineering, Graduate School of Science and Engineering, Yamagata University, 4-3-16, Jonan, Yonezawa, Yamagata 992-8510, Japan S Supporting Information *
ABSTRACT: Isoindigo-based conjugated polymers, PII2T-C6 and PII2T-C8, with carbosilane side chains have been designed and synthesized for stretchable electronic applications. The carbosilane side chains offerred a simple synthetic pathway to evaluate long and branched side chains in high yields and were prepared with a six or eight linear spacer plus two hexyl or octyl chains after branching. The studied polymers showed a high charge carrier mobility of 8.06 cm2 V−1 s−1 with an on/off current ratio of 106 as probed using a top-contact transistor device with organized solid state molecular packing structures, as investigated through grazing-incidance X-ray diffreaction (GIXD) and atomic force microscopy (AFM) technique systematically. The studied polymers, more attractive, exhibited superior thin film ductility with a low tensile modulus in a range of 0.27−0.43 GPa owing to the branched carbosilane side chain, and their mobility was remained higher than 1 cm2 V−1 s−1 even under a 60% strain along parallel or perpendicular direction to the tensile strain. Such polymer films, in addition, can be simultaneously operated over 400 stretching/releasing cycles and maintained stable electrical properties, suggesting the newly designed materials possessed great potential for next-generation skin-inspired wearable electronic application with high charge carrier mobility, low tensile modulus, and stable device characteristics during stretching.
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INTRODUCTION Stretchable semiconducting materials have recently attracted extensive research interests due to the development of wearable electronics, which can potentially be biocompatible and integrated to function with human movement.1−4 Conjugated polymers, among several material system in the research community, possess the advantages of low-cost, light weight, solution processability, and thin film ductility, thus providing the opportunity to build the next-generation electronic devices.5,6 Stretchable devices, such as field-effect transistors (FETs),7−12 light-emitting diodes,13−15 solar cells,16−18 and memory devices,19,20 have been explored using buckled structures, wavy structures, or elastic interconnect materials to provide the stretchability. These approaches, however, are not based on semiconducting materials that can be intrinsically stretched. Using an intrinsically strained active layer, it will become challenging to maintain the device performance and electrical durability under tensile strain. Therefore, a semiconducting polymer system that possesses both a superior electrical performance and high stretchability is needed. Isoindigo-based polymers provided considerable electronwithdrawing ability in donor−acceptor conjugated polymer semiconductor systems for a high charge carrier mobility greater than 4 cm2 V−1 s−1.21−25 Moreover, stretchable © XXXX American Chemical Society
materials consisting of an isoindigo−bithiophene backbone and branched alkyl side chain have been shown to have stable charge transport characteristics under a 100% strain.26 On the other hand, it is known that conjugated polymers with branched alkyl groups containing a 2−9 linear carbon spacer could achieve the high field-effect mobility of 5−10 cm2 V−1 s−1.24,27,28 Such long, branched alkyl chains, however, are not easily be synthesized, and the yield is generally low. In order to design a polymer with high mobility and thin film ductility, the carbosilane side chain is introduced.29 We have developed a simple synthetic approach by preparing the isoindigo-based polymers with a branched side chain having a tunable linear carbon spacer group by just replacing the atom of the branching point from carbon to silicon. Combining the merits of the carbosilane side chain and isoindigo-based polymer backbone structure, we have developed novel conjugated polymers consisting of an isoindigo− bithiophene main chain and carbosilane side chain with a six or eight linear spacer plus two hexyl or octyl chains after branching, as shown in Scheme 1. The new semiconducting Received: October 5, 2016 Revised: October 23, 2016
A
DOI: 10.1021/acs.macromol.6b02145 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 1. Synthetic Route on Preparing the Studied Polymeric Semiconductors, PII2T-C6 and PII2T-C8
IID-C8) are shown in Scheme 1, and their 1H NMR spectra are summarized in the Supporting Information (Figures S1−S3). General Procedures for Polymerization. The general procedure of synthesizing the studied polymers is described as follows: Dibromoisoindigos (i.e., IID-C6 or IID-C8), ditin monomer (i.e., 2T), tri(o-tolyl)phosphine (16 mol % with respect to ditin monomer), and tris(dibenzylideneacetone)dipalladium(0) (2 mol % with respect to ditin monomer) were dissolved in a microwave vessel using chlorobenzene (5 mL) under a nitrogen atmosphere. The vessel was then sealed with a snap cap, and the material was polymerized by Stille coupling reaction under microwave heating at 180 °C for 30 min. After end-capping with 2-(tributylstannyl)thiophene and 2-bromothiophene (both 1.1 equiv with respect to ditin monomer and under microwave heating at 185 °C for 10 min on the end-capping), the mixture was cooled and poured into methanol to form a crude polymer. The crude polymer was purified using a Soxhlet apparatus with methanol, acetone, and hexane to remove oligomers and catalyst residues. The final product was obtained after drying in a vacuum at 40 °C. Synthesis of PII2T-C6. Amounts of 260 mg (0.257 mmol) of IIDC6, 126 mg (0.257 mmol) of 2T, and 5 mL of chlorobenzene were used to afford a dark blue solid (yield: 260 mg, 96%). 1H NMR (400 MHz, CD2Cl2), δ (ppm): 9.22−9.01 (br, Ar−H), 7.46−6.85 (br, Ar− H), 3.85−3.60 (br, Ar−CH2), 1.95−0.98 (br, −CH2), 0.98−0.70 (br, −CH3), 0.55−0.40 (br, −CH2), −0.12 (br, −CH3). Anal. Calcd for [C62H94N2O2S2Si2]: C, 73.03; H, 9.29; N, 2.75; S, 6.29. Found: C, 73.56; H, 9.86; N, 2.54; S, 5.41. Number-average molecular weight and polydispersity index estimated from gel permeation chromatographic are 305 kDa and 3.3, respectively. Synthesis of PII2T-C8. Amounts of 446 mg (0.377 mmol) of IIDC8, 185 mg (0.377 mmol) of 2T, and 5 mL of chlorobenzene were used to afford a dark blue solid (yield: 441 mg, 96%). 1H NMR (400 MHz, CDCl3), δ (ppm): 9.12−8.95 (br, Ar−H), 7.82−7.39 (br, Ar− H), 4.05−3.80 (br, Ar−CH2), 1.90−1.05 (br, −CH2), 1.05−0.75 (br,
polymers exhibit a charge carrier mobility as high as 8.06 cm2 V−1 s−1 with an on/off current ratio of 106 as determined using a top-contact FET device, which is the record high value for the isoindigo-based polymers, to the best of our knowledge. Moreover, the tensile modulus of the studied polymer thin films can be lowered to 0.27−0.43 GPa due to the branched carbosilane side chain, and the mobility is remained higher than 1 cm2 V−1 s−1 even under a 60% strain along parallel or perpendicular direction to the tensile strain. The polymer films, in addition, can be simultaneously operated over 400 stretching/releasing cycles while maintainig stable electrical properties. These results demonstrated a promsing semiconducting polymer system possessing a high charge carrier mobility, low tensile modulus, and stable device characteristics during stretching for next-generation skin-inspired wearable electronics.
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EXPERIMENTAL SECTION
Materials. Dichloromethylsilane, hexylmagnesium bromide (2.0 M in diethyl ether), octylmagnesium bromide (2.0 M in diethyl ether), 6bromo-1-hexene, 8-bromo-1-octene, 2-(tributylstannyl)thiophene, trimethyltin chloride, 6-bromoisatin, 6-bromooxindole, Karstedt’s catalyst (platinum divinyltetramethylsiloxane complex in xylene, 3 wt %), tri(o-tolyl)phosphine, tris(dibenzylideneacetone)dipalladium(0), common organic solvents (tetrahydrofuran, toluene, acetic acid, and dimethylformamide), and anhydrous solvents (i.e., chloroform and chlorobenzene) were purchased from Aldrich (St. Louis, MO) and used as received. 6,6′-Dibromoisoindigo (IID)30 and 5,5′-bis(trimethylstannyl)-2,2′-bithiophene (2T)31 were synthesized according to the reported procedures. The synthetic steps of isoindigo monomers with different length of carbosilane side chains (IID-C6 and B
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Figure 1. (a) Illustration of the studied polymer structure and FET device configuration. FET transfer and output characteristics of (b, d) PII2T-C6 and (c, e) PII2T-C8-based FET device, respectively.
Table 1. Electrical Characteristics of PII2T-C6- and PII2T-C8-Based FET Devicesa PII2T-C6 conventional FET device strain levelc (%) strain direction 0 − 10 parallel 10 perpendicular 20 parallel 20 perpendicular 40 parallel 40 perpendicular 60 parallel 60 perpendicular 80 parallel 80 perpendicular 100 parallel 100 perpendicular
PII2T-C8
μ (cm2 V−1 s−1)
Ion/Ioff
Vth (V)
μ (cm2 V−1 s−1)
Ion/Ioff
Vth (V)
2.48 ± 0.28 (3.22)b
∼106
−17 ± 3
5.56 ± 0.95 (8.06)b
∼106
−21 ± 4
∼106 ∼106 ∼106 ∼106 ∼106 ∼105 ∼105 ∼105 ∼105 ∼105 ∼105 ∼105 ∼105
−16 −18 −18 −19 −19 −16 −18 −15 −11 −13 −16 −9 −18
3.24 2.70 2.93 2.30 2.29 1.75 1.74 0.93 1.05 0.78 0.66 0.58 0.54
∼106 ∼107 ∼106 ∼107 ∼106 ∼107 ∼106 ∼107 ∼106 ∼107 ∼106 ∼107 ∼106
−20 −26 −26 −23 −23 −22 −20 −21 −22 −21 −17 −24 −19
1.14 0.78 0.63 0.48 0.51 0.35 0.25 0.25 0.16 0.15 0.10 0.11 0.09
± ± ± ± ± ± ± ± ± ± ± ± ±
0.09 0.06 0.04 0.06 0.07 0.02 0.03 0.07 0.04 0.03 0.02 0.02 0.02
± ± ± ± ± ± ± ± ± ± ± ± ±
3 7 8 6 1 6 2 2 4 6 5 5 5
± ± ± ± ± ± ± ± ± ± ± ± ±
0.11 0.04 0.11 0.10 0.05 0.08 0.05 0.05 0.12 0.05 0.06 0.04 0.04
± ± ± ± ± ± ± ± ± ± ± ± ±
4 4 8 8 7 5 5 3 5 2 6 7 4
a All the FET characteristics was averaged from at least 20 devices from three different batches. bMaximum charge carrier mobility. cThe FET devices were fabricated using a transferred semiconducting thin film with various strain levels.
−CH3), 0.75−0.35 (br, −CH2), 0.02 (br, −CH3). Anal. Calcd for [C74H118N2O2S2Si2]: C, 74.81; H, 10.01; N, 2.36; S, 5.40. Found: C, 75.01; H, 9.70; N, 2.55; S, 5.23. Number-average molecular weight and polydispersity index estimated from gel permeation chromatographic are 434 kDa and 1.6, respectively. Characterization. Microwave polymerization was carried out using a Biotage microwave reactor in sealed vessels. 1H NMR spectra were recorded with a Bruker Advance DRX-400 MHz spectrometer. Elemental analysis was performed by elementary Vario EL cube (for NCSH, Germany) with sulfanilic acid as standard. Size exclusion chromatographic (SEC) analysis was performed on a Lab Alliance RI2000 instrument (two column, MIXED-C and D from Polymer Laboratories) connected with one refractive index detector from Schambeck SFD Gmbh. All SEC analyses were performed on polymer/THF solution at a flow rate of 1 mL min−1 under 40 °C and then calibrated with polystyrene standards. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measure-
ments were probed under a nitrogen atmosphere at a heating rate of 10 °C min−1 using the TA Instruments Q-50 and Q-100, respectively. The UV−vis absorption spectrum was explored using a Hitachi U4100 spectrophotometer. For the thin film spectra, polymers were first dissolved in chlorobenzene (5 mg mL−1) under 90 °C for 2 h and then spin-coated at a speed rate of 1000 rpm for 60 s onto quartz substrate. A rotational polarizer was used to measure the absorption intensity with the incident light polarized parallel (A∥) or perpendicular (A⊥) to the stretching direction and then define the dichroic ratio (R) of the stretched polymer film as R = A∥/A⊥. Cyclic voltammetry (CV) was performed on a CHI 611D electrochemical analyzer using a threeelectrode cell in which ITO was used as a working electrode (polymer films area were about 5 × 7 mm2). A platinum wire was used as an auxiliary electrode. All cell potentials were taken with the usage of a homemade Ag/AgCl, KCl (sat.) reference electrode. The electrochemical properties of the polymer films were detected under 0.1 M dry acetonitrile solution containing tetrabutylammonium perchlorate as the electrolyte. C
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Figure 2. (a) 2D GIXD patterns and (b) AFM topographies of PII2T-C6 and PII2T-C8 thin films, respectively. All the studied polymer thin films are annealed at 200 °C for 1 h under a nitrogen atmosphere. Au contacts were defined through a regular shadow mask with the channel length (L) and width (W) of 50 and 1000 μm, respectively.
Grazing incidence X-ray diffraction (GIXD) measurements were carried out on beamline 17A1 in National Synchrotron Radiation Research Center (NSRRC) of Taiwan. An X-ray wavelength of 1.321 Å was used, and the incident angle was set as 0.12°. The morphology of polymer film surface was obtained with a Nanoscope 3D Controller atomic force microscope (AFM, Digital Instruments, Santa Barbara, CA) operated in the tapping mode at room temperature. Commercial silicon cantilevers with typical spring constants of 21−78 N m−1 were used. Note that the preparation of polymer thin film samples was the same as that of the device fabrications for the measurements of GIXD, UV, and AFM. The electrical characterizations were carried out by a Keithley 4200SCS semiconductor parameter analyzer (Keithley Instruments Inc., Cleveland, OH) in a N2-filled glovebox. Fabrication of Field-Effect Transistors. Field-effect transistors (FETs) were fabricated from the studied polymers with a bottomgate/top-contact configuration. A 300 nm SiO2 layer (capacitance per unit area = 10 nF cm−2) as a gate dielectric was thermally grown onto the highly n-type doped Si (100) substrates. The Si substrates were modified with an octadecyltrimethoxysilane (OTS) self-assembled monolayer according to a reported method.32 The semiconducting polymers (PII2T-C6 and PII2T-C8) were first dissolved in chlorobenzene (5−8 mg mL−1) at 100 °C for at least 2 h, and the polymer thin films were spin-coated onto OTS-modified SiO2/Si substrates using a spin rate of 1000 rpm for 60 s. In addition, the annealing process (200 °C under vacuum for 1 h) was introduced to enhance the FET performance. The top-contact source/drain contacts were defined by 100 nm thick Au through a regular shadow mask, and the channel length (L) and width (W) were 50 and 1000 μm, respectively. For measuring the electrical characteristics of the stretched semiconducting polymer thin film, the polymer films were first spincoated on a OTS-modified SiO2/Si substrate and annealed at 200 °C for 1 h; then those films were transferred to an elastomeric PDMS (20:1 base to cross-linker ratio by mass) slab. Various tensile strain levels were applied to the polymer/PDMS matrix, so that the studied polymer thin films were stretched along with PDMS substrate. Such stretched films, afterward, were transferred back to a silicon substrate with 300 nm SiO2 dielectrci layer. Finally, the top-contact source/drain
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RESULTS AND DISCUSSION Polymer Characterization. The studied isoindigo− bithiophene semiconducting polymers with carbosilane side chains, PII2T-C6 and PII2T-C8, were prepared by the Stille coupling reaction with microwave heating (Scheme 1) and were soluble in the common organic solvents, such as tetrahydrofuran, chloroform, and chlorobenzene. The chemical structures of the studied polymers are confirmed by 1H NMR (Figure S4). The peak at approximately 9.0 ppm is attributed to the aromatic proton of the benzene ring on isoindigo, while the signals in the range of 6.5−7.8 ppm (peaks b−e) are assigned to the protons on the benzene or thiophene rings on the polymer backbone. Peak f at 3.7 ppm and other signals (0−2 ppm) are the −CH2 and −CH3 protons on the carbosilane side chain. Note that the numbers of the aliphatic and aromatic protons estimated from the integration value of the 1H NMR signals are consistent with the proposed polymer structures, and the elemental analyses of the carbon, hydrogen, nitrogen, and sulfur contents are in a good agreement with the theoretical content, demonstrating that our target semiconducting polymers were successfully synthesized. Number-averaged molecular weights of 305 and 434 kDa, moreover, were observed for PII2T-C6 and PII2T-C8 with polydispersities of 3.3 and 1.6, respectively, from the size exclusion chromatographic (SEC) measurements. The studied polymers exhibit a reliable thermal stability with a thermal decomposition temperature (5% weight loss) at approximately 400 °C based on the thermogravimetric analysis (TGA; Figure S5a), which can guarantee a high-performance semiconducting thin film during the device processing. Thermal transitions, however, cannot be obviously exhibited in the differential scanning calorimetry (DSC; Figure S5b) traces of the polymer bulk samples while heating to 230 °C. D
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Figure 3. (a) Schematic illustration of preparing a semiconducting polymer thin film under strain (i.e., 0−100%) through transfer method. (b) Dichroic ratio and (c) lamellar packing spacing of PII2T-C6 and PII2T-C8 thin film under different amounts of applied strain, respectively. Note that the X-ray incident light is controlled to be perpendicular or parallel to the strain direction for the lamellar spacing analysis.
is around −5.2 eV, while the lowest unoccupied molecular orbital (LUMO) level, calculated by the difference between the HOMO level and optical band gap, is −3.6 eV. Note that the optical and electrochemical properties of the studied polymers are mainly determined by the backbone structure; the carbosilane side chain does not disturb the energy levels and electronic vibration state as its chain length is changed. Field-Effect Transistor Characteristics. The charge transport characteristics of the pristine PII2T-C6 and PII2TC8 thin films were determined using a bottom-gate/top-contact FET device, as illustrated in Figure 1a. A 300 nm thick SiO2 dielectric layer with a highly n-doped silicon substrate was modified with a dense crystalline octadecyltrimethoxysilane (OTS) self-assembly monolayer to enhance the electrical performance. The p-channel transfer and output curves of PII2T-C6 and PII2T-C8 thin films are shown in Figure 1b−e, and their device performance parameters (i.e., mobility (μ), on/ off current ratio (Ion/Ioff), and threshold voltage (Vth)) are summarized in Table 1. The averaged charge carrier mobilities of 2.48 and 5.56 cm2 V−1 s−1 were obtained for PII2T-C6 and PII2T-C8-based FET annealed at 200 °C, respectively, with the on/off ratio up to 106. PII2T-C8 thin film, moreover, can
Optical and Electrochemical Properties. The solid state UV−vis absorption spectra were recorded in order to understand the optoelectronic properties of PII2T-C6 and PII2T-C8, as depicted in Figure S6a. The two studied polymers showed similar optical characteristics with two distingushable absorption bands. The absorption located at ∼410 nm is associated with the π−π* transitions, while the broad band in the range of 550−800 nm is from the intramolecular charge transfer (ICT) between the donor and acceptor moiety on the polymer backbone. Typical vibration peaks can be clearly observed due to the ICT effect, which was previously described for the isoindigo-based polymers.21−24 On the other hand, both PII2T-C6 and PII2T-C8 possess a similar optical band gap of 1.6 eV. Such a value is consistent with the isoindigo-based polymers in the literature,21−24 indicating that the incorporation of the carbosilane side chain can still maintain the polymer chain organization as compared to the traditional branched alkyl side chains. The energy levels of PII2T-C6 and PII2T-C8 were further measured by cyclic voltammetry (CV; Figure S6b). The highest occupied molecular orbital (HOMO) level, estimated from the onset oxidation potentials in the CV curves with reference to ferrocence (4.8 eV) of both polymers, E
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Figure 4. AFM topographies of PII2T-C6 and PII2T-C8 thin films under tensile strain.
achieve a maximum mobility of 8.06 cm2 V−1 s−1, which is one of the highest FET performances of the isoindigo-based polymer system, to the best of our knowledge. Such a superior charge transport characteristic is probably attributed to the high molecular weight as well as well-organized solid-state molecular packing with the branched carbosilane side chain containing the linear octyl spacer group, similar to the reported high mobility polymers.24,27,28 Polymer Thin Film Morphologies. To further understand the crystalline structures and surface morphologies of the studied polymer thin films, grazing incidence X-ray diffraction (GIXD) and atomic force microscopy (AFM) analyses were performed, as shown in Figure 2. Based on the 2D GIXD patterns (Figure 2a), both PII2T-C6 and PII2T-C8 thin films possess distinguishable diffraction signals in the Qz direction (i.e., (n00)) with the lamellar spacing of 24.3 and 28.9 Å, respectively. As expected, PII2T-C8 exhibits a greater lamellar packing distance than PII2T-C6 since the length of the carbosilane side chain is longer. The π−π stacking diffraction peak, on the other hand, can be observed in the in-plane direction, indicating that a traditional edge-on packing structure is facilitated in the studied polymer thin film and can generate an effective 2D charge transport between the source and drain contacts in the FET. The π−π stacking distances of the PII2TC6 and PII2T-C8 thin films are 3.61 and 3.55 Å, respectively. The closer stacking of the PII2T-C8 thin film results in a more efficient charge transport ability as well as a higher mobility, as evidenced by the FET device. The AFM topographies (Figure 2b) also indicate that our studied polymer thin films have wellorganized packing structures with a nanofibrillar surface morphology and interconnected networks. With a shorter π−π stacking distance as well as denser molecular packing, PII2T-C8 shows more significant nanofibrillar networks on the thin film surface. Strain-Dependent Properties of Polymer Thin Films. The above results reveal that we can achieve high-performance FETs based on PII2T-C6 and PII2T-C8 with a long, branched carbosilane side chain on the isoindigo−bithiophene backbone.
Nevertheless, another main reason to design such a huge carbosilane side group is to improve the mechanical property of the semiconducting polymer thin film for wearable/stretchable electronic applications. The mechanical, morphological, and electrical properties of the studied polymer thin films under strain were systematically explored, and the stretched thin films were prepared using the second transfer method,33 as illustrated in Figure 3a. The thin film ductility was defined using two specific values, i.e., the dichroic ratio and tensile modulus. The dichroic ratio (R) is calculated from the polarized UV−vis spectroscopy and can correlate with the degree of polymer chain alignment during stretching (Figure 3b). The R value of PII2T-C6 progressively increases from 1 to ∼1.4 as the strain is applied from 0 to 80% and then remains unchanged with any increase in the tensile strain. The R value of PII2T-C8, however, linearly increased over 2.0 with the strain applied from 0 to 100%, indicating that the polymer chain alignment by strain can be continuously achieved. This suggests that the PII2T-C8 thin film possesses a higher mechanical tolerance than PII2T-C6 under strains up to 100%, which can also be demonstrated using optical microscopy (Figure S6). No obvious microscale cracks are formed on the PII2T-C8 thin film under strain; on the contrary, the cracks are observed at ∼60% strain on the PII2T-C6 thin film. To further quantify the thin film ductility, the buckling method was introduced to measure the tensile modulus, as depicted in Figure S8. As a result, the modulus of the PII2T-C6 and PII2T-C8 thin films was 0.43 and 0.27 GPa, respectively. Note that both polymer films can be defined as a ductile thin film when comparing their tensile modulus to other conjugated polymers with the modulus of 0.25−0.8 GPa.34 Again, the PII2T-C8 thin film shows a superior mechanical property to that of PII2T-C6, and the improved ductility of the PII2T-C8 thin film may be mainly attributed to the decrease in the glass transition temperature and reduced volume fraction of the brittle polymer backbone with the longer and branched side chains. The GIXD technique was next used to examine the morphology of the studied polymer films under strain at a F
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Figure 5. (a) Schematic illustration of fabrication a FET device based on the studied semiconducting polymer thin films under strain (i.e., 0−100%). FET transfer characteristics of (b, c) PII2T-C6 and (d, e) PII2T-C8-based stretched films with a charge transport direction (b, d) parallel or (c, e) perpendicular to the strain direction, respectively. Note that the source-to-drain voltage is set as −100 V. (f) Field-effect mobility of the studied polymer thin films under various strain levels. (g) Mechanical endurance of the studied polymer thin films with 60% strain applied for 400 stretching/releasing cycles. The charge transport direction is controlled to be parallel to the strain direction, and both PII2T-C6 and PII2T-C8 possess stable electrical properties, including mobility, on/off ratio, and threshold voltage.
tensile strain on the polymer thin film. Thus, the polymer chain packing distance in the noncracked region is shifted back to its original (i.e., nonstretched) stacking structures. This hypothesis can be proved by the AFM topographies of the stretched polymer thin film surfaces (Figure 4). Under an extremely high strain level (i.e., 100%), cracks perpendicular to the stretching direction are observed on the polymer thin films. Especially, for the PII2T-C6 surface, microscale cracks are observed, affecting the molecular packing structures of the polymer chain in the noncrack region. In addition to comparing the PII2T-C8 thin film with PII2T-C6, it shows a higher degree of lamellar spacing changes below a 40−60% strain, indicating the better tolerance and thin film ductility of PII2T-C8, which is consistent with the dichroic ratio and tensile modulus results. Note that all the π−π stacking distances of the PII2T-C6 and PII2T-C8 thin films
molecular level (Figure 3c, Figures S9 and S10). The incident X-ray beam was controlled to be perpendicular or parallel to the stretching direction. The lamellar spacing distances of the PII2T-C6 and PII2T-C8 thin films remained at approximately 24 and 29 Å in both directions when the applied strain was lower than 20%. As the tensile strain increased to 40−60%, the spacing in the direction of the X-ray light perpendicular to the strain direction decreased, while the spacing did not change in the other direction (Figure 3c). This observation is due to the polymer chains and their crystalline regions being elongated and aligned in the stretching direction, and the layer by layer lamellar stacking is significantly compressed in the perpendicular direction. Interestingly, the lamellar packing distance reverts to the original value when the applied strain greater than 80%, which may be due to crack formation and release of the G
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PII2T-C8 exhibited a field-effect mobility greater than 1 cm2 V−1 s−1 even under a 60% strain and can be simultaneously operated over 400 stretching/releasing cycles, demonstrating a promsing polymeric semiconducting material that possesses high charge carrier mobility, low tensile modulus, and stable device characteristics during stretching. Such a polymer, indeed, can be used for next-generation skin-inspired wearable electronics.
under strain are maintained at 3.61 and 3.55 Å, respectively, meaning that the applied strain did not break the face-to-face stacking between the polymer chains. Charge Transport Characteristics under Stretching. To evaluate the charge transport behaviors of the PII2T-C6 and PII2T-C8 thin films under strain, FETs were fabricated by the transfer method (Figure 5a). Charge transport both perpendicular and parallel to the stretching directions was characterized. The transfer curves of the stretched PII2T-C6 and PII2T-C8 thin film-based devices are shown in Figure 5b− e, and the detailed electrical parameters are listed in Table 1. The field-effect mobility of the PII2T-C6 device along the direction of the applied strain (i.e., parallel) modestly decreases from 1.14 to 0.11 cm2 V−1 s−1 as the strain level increases to 100%. The charge transport ability in the opposite (i.e., perpendicular) direction, in addition, decreased at a similar rate compared to the parallel stretching direction. With a better thin film ductility and denser molecular packing structure (i.e., shorter π−π stacking distance), the PII2T-C8 thin film exhibits an initial mobility of 3.24 cm2 V−1 s−1 and can still maintain the mobility above 1 cm2 V−1 s−1 at a 60% strain in both the parallel and perpendicular directions (Figure 5f). There is no significant difference in the mobility changes between the parallel and perpendicular directions to the strain in the PII2T-C6 and PII2T-C8 thin films, which is similar to the study of a pentacene-based device under strain.35 We not only examined the charge transport characteristics under a single stretching event but also evaluated the PII2T-C6 and PII2T-C8 device during 400 stretching/releasing cycles at 60% (Figure 5g). Both polymer thin films showed stable performances without significant changes in mobility, on/off ratio, and threshold voltage during the testing, and the electrical properties are comparable to the values before the cycling operation commenced, demonstrating that the studied polymer thin films possess an excellent stability and reproducibility under a high tensile strain of 60%. The electrical results suggest that our newly designed isoindigo-based polymers with carbosilane side chains can provide a stable and high-performance charge transport (>1 cm2 V−1 s−1) under a strain up to 60%, which is tolerable to most of the strain on the skin from human movement (generally
