Article Cite This: Macromolecules 2019, 52, 4396−4404
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Tailoring Carbosilane Side Chains toward Intrinsically Stretchable Semiconducting Polymers Yun-Chi Chiang,†,# Hung-Chin Wu,†,# Han-Fang Wen,‡,# Chih-Chien Hung,§,# Chian-Wen Hong,† Chi-Ching Kuo,‡ Tomoya Higashihara,*,∥ and Wen-Chang Chen*,†,⊥
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Department of Chemical Engineering, §Institute of Polymer Science and Engineering, and ⊥Advanced Research Center for Green Materials Science and Technology, National Taiwan University, Taipei 10617, Taiwan ‡ Institute of Molecular Science and Engineering, National Taipei University of Technology, Taipei 10608, 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: Carbosilane side chain-equipped isoindigo−bithiophene semiconducting polymers (PII2T) have been designed and synthesized for stretchable electronics applications. Systematically tailoring the length and branch position of carbosilane side chains (C6 to C10) offers an effective route to optimize charge-transport behavior and improve the mechanical properties of semiconducting polymer thin films. The basic polymer properties, surface morphology, electrical characteristics, and strain-dependent performance of polymers with various lengths of carbosilane side chains were explored. The series of polymers exhibited a field-effect mobility over 2 cm2 V−1 s−1, and an odd−even effect was observed relating to the length of side chains. On the other hand, when the longer side chain was incorporated, a lower thin-film modulus was reached because the extended side chain can dilute the volume of the rigid polymer backbone and open up the space between polymer chains (i.e., larger lamellar spacing). Surprisingly, PII2T-C10 thin films possess desirable electrical and mechanical properties, achieving a mobility of 1 cm2 V−1 s−1 even when stretched under 100% strain, which is the best electrical performance among intrinsically stretchable conjugated polymers in the research community.
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ethylene−butylene−styrene (SEBS) block copolymer.21 Such an elastomeric SEBS is still essential to improve overall thinfilm deformability. Therefore, it is still necessary to develop a semiconducting polymer with excellent thin-film ductility and electrical performance, which can be used in stretchable devices independently. Side chain engineering, a direct pathway to manipulate solution processability, solid-state molecular stacking, and thinfilm morphology of polymers, has been employed numerous times as a method for producing intrinsically stretchable polymer semiconductors.25−28 Morphological, mechanical, and electrical properties can be manipulated simultaneously based on the side chain substituents. The length of alkyl side chains and their branch position have been commonly used to optimize the charge-transport ability of semiconducting polymers,5,29−31 facilely achieving high charge-carrier mobility. Although simple side chain design can tailor and reach outstanding polymer performance, systematic studies of side chain design for stretchable polymer semiconductors with fast charge transport under strain are rare.
INTRODUCTION Nowadays, more and more polymer semiconductors have been explored for next-generation electronics, which possess low cost, large scale processability, good electrical performance, and even mechanical compliance.1−3 The versatility of conjugated polymers has been exploited to design polymer semiconductors with remarkable performance in various electronic devices, such as field-effect transistors (FETs),4−10 solar cells,11,12 light-emitting diodes,13−15 or memories.16−19 Although polymeric semiconductors can achieve comparable electrical performance to their traditional inorganic counterparts, currently, it is still a challenge to simultaneously introduce stretchability at the molecular level, in order to create mechanically robust semiconducting polymers. Chemical or physical approaches, such as buckled and wrinkled structures,20 polymer/elastomer blends,21−23 and dynamic bonding functionalization,24 have been developed to give polymer semiconducting layers desirable electrical properties when stretched by a mechanical force. However, it is still challenging to obtain high device performance using a single semiconducting polymer. For example, the best charge-carrier mobility in a stretchable transistor device, over 1 cm2 V−1 s−1 at 100% strain, was produced by using a conjugated diketopyrrolopyrrole-based polymer blended with a styrene− © 2019 American Chemical Society
Received: March 22, 2019 Revised: May 6, 2019 Published: May 22, 2019 4396
DOI: 10.1021/acs.macromol.9b00589 Macromolecules 2019, 52, 4396−4404
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Figure 1. (a) Chemical structures of PII2T-based polymers with manipulated length of carbosilane side chains. (b) UV−vis spectra and (c) AFM topographies of PII2T-based polymer thin films. (d) 2D GIXD pattern of a PII2T-C8 polymer film. Edge-on packing structure, which is a general packing feature for this set of polymers with long and branched carbosilane side chains, was observed. (e) 1D GIXD profile of PII2T-based polymers extracted in the qz direction. A clear q value change of the lamellar packing signal (n00) was exhibited, especially for PII2T-C6 compared to PII2T-C10 (in-set patterns). (f) Lamellar packing d-spacing and π−π stacking distance of PII2T polymers. The d-spacing became larger as the length of the side chain was increased, whereas a significant odd−even effect was observed on the π−π stacking distance. Side chains possessing even numbers of carbon showed a smaller π−π stacking distance.
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Our group previously established an isoindigo-based donor− acceptor polymer (PII2T) bearing long, branched carbosilane side chains, and a charge-carrier mobility up to 1 cm2 V−1 s−1 was reported even at strains as large as 60%.25 Here, we further extend such a carbosilane side chain system with controlled length and branch position from 6 to 10 carbons (Figure 1a). The polymer properties, surface morphology, thin-film deformability and charge-transport behavior are explored. As expected, increasing side chain length reduced thin-film modulus from 565 to 250 MPa. However, an odd−even effect was discovered to effect charge-carrier mobility of polymer films. Polymers with side chains containing even carbon number exhibited better electrical performance because of a denser packing structure. By balancing electrical and mechanical characteristics, PII2T-C10 showed a mobility of approximately 1 cm2 V−1 s−1 at 100% strain in the directions both parallel and perpendicular to stretching. Such a mobility value is almost the same as the initial (i.e., non-stretched) mobility. More importantly, to the best of our knowledge, the achieved performance is one of the highest device performances among intrinsically stretchable semiconductors, showing great potential for next-generation electronics.
EXPERIMENTAL SECTION
Fabrication of FETs. Bottom-gate/top-contact FETs based on the studied polymers were fabricated. A 300 nm SiO2 layer (capacitance of 10 nF cm−2) was thermally grown onto the highly n-type-doped Si substrates as a gate dielectric layer. An octadecyltrimethoxysilane (OTS) self-assembled monolayer was then modified on the SiO2/Si surfaces to improve molecular packing of the semiconducting polymer layer.34 The semiconducting polymers (PII2T-based polymers) were first dissolved in chlorobenzene (5−8 mg mL−1) at 100 °C for at least 2 h, and the polymer thin films were deposited by spin-coating such polymer solutions onto OTS-modified SiO2/Si substrates. An annealing process (200 °C under vacuum for 1 h), moreover, was introduced to enhance the FET performance. The source and drain contacts were defined by 100 nm thick Au through a regular shadow mask, which defines channel length (L) and width (W) of 50 and 1000 μm, respectively. On the other hand, the transfer method was applied to fabricate FETs using stretched semiconducting polymer thin films. The polymer films were first spin-coated on an OTS-modified SiO2/Si substrate and annealed at 200 °C for 1 h, then such films were transferred to an elastomeric polydimethylsiloxane (PDMS) (20:1 base/cross-linker) slab. Controllable tensile strains (i.e., 0, 20, 60, 100%) were applied to the polymer/PDMS matrix, so that the PII2T thin films could be stretched under a certain strain level along with the PDMS substrate. Such stretched polymer films, afterward, were transferred back to a SiO2/Si substrate, and a functional FET device 4397
DOI: 10.1021/acs.macromol.9b00589 Macromolecules 2019, 52, 4396−4404
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polymers, as shown in Figure 1b, were explored to understand their optoelectronic properties. All studied PII2T polymers possess a similar optical band gap of 1.6 eV, which is consistent with the isoindigo-based polymers in the literature with a similar backbone structure.5,6,32 Using different lengths of carbosilane side chains, indeed, can maintain the polymer electronic properties as compared to the traditional alkyl side chains. On the other hand, two distinguishable absorption bands located at 400−450 nm and 550−800 nm originate from the π−π* transitions and intramolecular charge transfer between the donor (i.e., bithiophene) and acceptor (i.e., isoindigo) moiety on the polymer backbone, respectively. More specifically, a sharp peak at 710 nm, which implies the degree of polymer interchain aggregation or organization, could be observed for all PII2T polymers. As the length of carbosilane side chain was increased (from C6 to C8), the polymer chains showed better self-organization. The extended side chain length facilitates the stacking of the polymer backbone. However, if we further increased the number of carbons on the side chains (from C8 to C10), the degree of aggregation went down. The floppy side chain with a longer chain length may induce additional free volume and push the polymer backbone away from each other. This result suggests that the electronic properties between polymer chains can be manipulated through tailoring the side chain length, leading to desirable electrical and mechanical performances for stretchable device application, as discussed in later sections. Next, the energy levels of PII2T polymers were measured by cyclic voltammetry (CV), as depicted in Figure S14 (Supporting Information). The highest occupied molecular orbital (HOMO) level of polymers is extracted as ∼−5.3 eV, by estimation from the onset oxidation potentials with reference to ferrocene (4.8 eV). Subsequently, the lowest unoccupied molecular orbital level can be calculated by taking the difference between the HOMO level and optical band gap, which is −3.7 eV. The energy levels of the studied polymers are mainly dominated by the backbone structure; the length of the carbosilane side chain thus does not actually influence the energy levels. In addition to the optical and electrochemical properties, surface morphology and molecular packing features in the film state can be directly affected by the polymer structure, such as different side chain lengths. Crystalline structures in the studied PII2T films were elucidated using grazing incidence Xray diffraction (GIXD). A typical edge-on packing [(n00) lamellar packing peaks in the qz direction and (010) π−π stacking signal in the qxy direction on a 2D GIXD pattern] was observed for all PII2T polymers with carbosilane side chains (Figure 1d). Moreover, 1D profiles (Figure 1e) were integrated in the qz direction to extract the lamellar spacing, which changed dramatically from 26.3 to 33.4 Å by extending the carbosilane side chains from C6 to C10 (Figure 1f). Longer side chains on the backbone created a larger lamellar packing distance. The π−π stacking distances, which were determined from the in-plane (010) signal, more interestingly, exhibited a significant odd−even effect (Figure 1f). Polymers with even numbered carbosilane side chains (C6, C8, and C10) possess smaller π−π stacking than those side chains with odd carbon number (C7 and C9). Unlike most polymer designs in the literature for which only the space between the backbone and branch point was changed to be odd or even number of carbons, we tailored both branch position and chain length after branching at the same time. The stacking difference thus
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RESULTS AND DISCUSSION Polymer Syntheses and Characterizations. Isoindigo− bithiophene (PII2T) polymers with various lengths of carbosilane side chains (named PII2T-C6 to PII2T-C10) (Figure 1a) were synthesized following the procedure from our previous work.25 The brominated isoindigo core, first, was functionalized with different carbosilane side chains and then copolymerized with bis(stannyl)-bithiophene through Stille polymerization (Scheme S1 in the Supporting Information). Afterwards, the crude polymers were precipitated in methanol and purified using Soxhlet extraction. With long and branched side chains, our target polymers can be easily dissolved in the common organic solvents (e.g., chloroform or chlorobenzene), securing good solution processability. 1 H NMR was utilized to confirm the chemical structures of the PII2T-based monomers and polymers [Figures S1 and S12 (Supporting Information)]. The numbers of aliphatic and aromatic protons estimated from the integration value of 1H NMR signals are consistent with the proposed chemical structures. Elemental analysis, moreover, was used to investigate the carbon, hydrogen, nitrogen, and sulfur contents of the studied polymers, and the results exhibited good agreement with the theoretical contents, demonstrating that the new PII2T polymers with carbosilane side chains were synthesized successfully. The number-averaged molecular weights, ranging from 85 to 412 kDa, were determined by the size exclusion chromatography (SEC) measurement. Detailed polymer properties are summarized in Table 1. Table 1. Polymer Properties of PII2T Polymer with Carbosilane Side Chains polymer
Mn (kDa)
PII2T-C625 PII2T-C7 PII2T-C825 PII2T-C9 PII2T-C10
305 284 412 85 233
Dispersity 3.3 2.9 3.6 2.1 2.5
Td (°C) 396 395 403 393 412
Note that the molecular weight variations between the studied polymers may be affected by the eluent (i.e., tetrahydrofuran) and temperature (i.e., 40 °C) of SEC measurement, which cannot fully disentangle polymer chains in the solution state. Appropriate thermal stability is required for a semiconducting polymer to maintain its polymer properties during device fabrications. Thermogravimetric analysis (TGA) was conducted (see Figure S13a) and all studied polymers possessed a high thermal decomposition temperature (Td, 5% weight loss) over approximately 400 °C (Table 1), indicating stable polymer properties under heating. On the other hand, differential scanning calorimetry (DSC; Figure S13b) was used to detect thermal transitions of the polymer chain. However, no distinct signals appeared because of the rigid structure of the polymer backbone, while heating up to 250 °C. Optical, Electrochemical, and Morphological Properties. Desirable electronic vibrations, energy levels, and thinfilm morphologies are essential for a semiconducting polymer to achieve good electrical performance and mechanical resilience. Solid-state UV−vis absorption spectra of the studied 4398
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Figure 2. Electrical properties of PII2T-based polymers in a conventional FET device. (a) Illustration of a top-contact/bottom-gate FET device. (b−f) Transfer characteristics of the studied polymers. The source-to-drain voltage was set as −100 V. (g) Summary of averaged charge-carrier mobility, which was averaged from at least 20 devices in three different batches. Polymers with even numbers of carbon on side chains showed higher mobility. (h) FET output curves of the PII2T-C8 thin film.
may be directly generated by the overall length of the side chain.33 The closer stacking between polymer backbones usually corresponds to a more efficient charge-carrier transport. Furthermore, denser molecular stacking (i.e., smaller π−π stacking distance) produced a well-organized nanofibrillar surface morphology and interconnected networks with lower surface roughness, as demonstrated using atomic force microscopy (AFM) (see Figure 1c), which is important for the device applications. Charge Transport of Polymer Thin Films. The chargetransport characteristic of PII2T-based thin films were probed by a bottom-gate/top-contact FET device, as illustrated in Figure 2a. A SiO2 dielectric layer (thickness of 300 nm) acted as the dielectric layer and was modified with an OTS selfassembled monolayer to evoke polymer chain organization and enhance electrical performance.34 Typical p-channel transfer curves of PII2T-C6 to -C10 thin films are shown in Figure 2b− f, respectively, with high on/off current ratios over 105. Output characteristics, moreover, are depicted in Figures 2h and S15 (Supporting Information). Good current modulation as well as well-defined linear and saturation regions were shown. Average mobilities of 2.48, 2.06, 4.67, 0.92, and 2.23 cm2 V−1 s−1 were obtained from PII2T-C6 to -C10-based devices, respectively, as summarized in Figure 2g. It is obvious that polymers with an even number of carbons possessed higher charge-carrier mobility, owing to smaller π−π stacking distance as well as denser molecular stacking as demonstrated by GIXD experi-
ments. Through controlling the length of the carbosilane side chains, the electrical performance of polymers can be adjusted based on morphological modification, resulting in a conventional odd−even effect. PII2T-C8, in addition, exhibited the highest mobility among the studied polymers, which probably can be attributed to its relatively higher molecular weight compared to the other polymers in the series and the fact that its carbosilane side chain contains a linear octyl spacer group between the isoindigo backbone and Si branch point. Such a branch position, around 6−8 carbons’ distance, seems an optimal design for high-performance polymer semiconductors, which has been reported in the literature.5,29,31 If the branch point is close to the polymer backbone, the branched alkyl chains can easily disturb the backbone stacking. On the contrary, a highly extended branch position will largely increase lamellar packing d-spacing and free rotational volume of the side chains, which is insufficient for facilitating chargetransport pathways. Strain-Dependent Properties. In the above discussion we systematically presented the structure−property relationships of PII2Ts containing various lengths of carbosilane side chains, and demonstrated that the physical, morphological, and electrical properties of such polymers can be tailored rationally. Our target by introducing length-controlled carbosilane side chains is not only to boost the polymer performance but also to enhance the resilience and deformability of the semiconducting polymer thin films, which is crucial for future 4399
DOI: 10.1021/acs.macromol.9b00589 Macromolecules 2019, 52, 4396−4404
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Figure 3. Mechanical properties of the studied polymers in the film state. (a) OM images of the PII2T-based polymer thin films under 0 and 100% strain. (b) DMT modulus of PII2T-based thin films. The modulus decreased with increased side chain length, indicating that the polymer films became more ductile when the side chain is lengthened.
Figure 4. Summarized lamellar d-spacing of (a) PII2T-C6, (b) PII2T-C7, (c) PII2T-C8, (d) PII2T-C9, and (e) PII2T-C10 thin films, respectively, under various strain levels. (f) Normalized d-spacing of the studied polymer films under strain. The d-spacing was increased by strain only if the side chain is long (i.e., C8, C9, and C10), indicating that the crystalline domain can be extended by stretching force. Such an observation supports the hypothesis that with longer side chains on the polymer backbone, their thin films have better deformability.
wearable electronics. The mechanical, morphological, and electrical properties of PII2T-based polymer thin films under different strain levels (i.e., from 0 to 100%) were systematically investigated. Stretched polymer films were fabricated using the second transfer method.25,35 The optical microscopy (OM) images, as depicted in Figure 3a, clearly show the impact of carbosilane side chain length on the tolerance of the studied polymer thin films under mechanical strain. The number of micro-cracks formed in the polymer film is an indicator of its thin-film deformability. Cracks can be observed clearly from PII2T-C6 and PII2T-C7 thin films under 100% strain, and as we increase the length of the side chain, the number and size of cracks were both significantly reduced, especially for PII2T-C10 with the longest carbosilane side chain, indicating that the thin film is less damaged by stretching force. Although the OM images can provide a general guideline that the length of the carbosilane side chain modifies the mechanical properties of the PII2T semiconducting thin films, a quantitative analysis is still needed to precisely define the thin-film ductility. Derjaguin, Muller, and Toropov (DMT) modulus, a reduced Young’s modulus calculated according to the DMT model, extracted through AFM measurement is introduced to quantify the polymer thin-film ductility. Unlike
the traditional buckling method for measuring thin-film modulus that involves the transfer printing process28 that leads to a relatively large deviation of modulus values, DMT modulus can be directly probed by AFM tips on freshly coated thin films without additional processes that may induce errors. As summarized in Figure 3b, PII2T-C6 showed a modulus of 565 MPa, which is the highest among the studied polymers. Once the length of the side chain was increased, a significant drop in modulus value was observed (e.g., down to 250 MPa for PII2T-C10), implying that the thin-film ductility is improved, which is consistent with the OM results. Note that although the values of the DMT modulus of PII2T-C6and PII2T-C8-based thin films we measured in this study are slightly higher than that of the tensile modulus we collected using the buckling method in a previous study,25 the trend of modulus value (PII2T-C6 larger than PII2T-C8) is consistent between both methods. Also, thin films based on the studied polymers can be recognized as ductile films as their modulus is comparable to the other conjugated polymer films in the research community (i.e., 250−800 MPa). From OM and DMT modulus results we observe that thin films become softer when the PII2T polymer possesses long side chains. This improved thin-film deformability/ductility may be mainly attributed to increased side chain rotational freedom, reduced 4400
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Figure 5. Electrical properties of stretched polymer films. (a) Schematic illustration of a top-contact/bottom-gate FET device using a stretched PII2T polymer active layer. (b) Transfer characteristics of PII2T-C10-based FETs. The PII2T-C10 thin films were stretched from 0 to 100% and the charge-carrier mobilities in both parallel and perpendicular directions were collected. (c) Mobility distribution in both parallel and perpendicular directions and (d) normalized mobility in the parallel direction of the studied polymer thin films under strain. Note that the FET characteristics were averaged from at least 10 devices in two different batches.
elongated along the direction of stretching, avoiding breakage of packing structures, which can potentially maintain effective charge-transport pathways for stretchable device applications based on polymers equipped with longer side chains. Charge-Transport Characteristics under Strain. Side chain tailoring, indeed, can optimize polymer thin-film properties and improve mechanical stability. Next, the electrical behaviors of the studied polymer-based thin films under strain are evaluated using FET devices, which consist of a stretched active layer fabricated by the transfer method, as illustrated in Figure 5a. The charge-carrier transports in the directions parallel and perpendicular to the stretching direction were both characterized. The transfer characteristics of devices based on stretched PII2T-C10 films are depicted in Figure 5b, and the FET details of other polymer-based films are summarized in the Supporting Information. Figure 5c presents the relationship between average mobility and applied strain of PII2Ts with different side chains. PII2T-C6 and PII2T-C7, which possess short side chains, exhibited a mobility of approximately 1 cm2 V−1 s−1 at 0% strain, and it progressively decreased to 0.1 cm2 V−1 s−1 as the strain level increased to 100%. Note that the mobility trends in both parallel and perpendicular directions are similar, which is comparable to the study of a pentacene-based device under strain.36 By increasing the length of the carbosilane side chain, PII2T-C8 possesses better thin-film ductility and achieved a higher mobility of 0.7 cm2 V−1 s−1 for films under 100% strain. However, the mobility is still at least three times lower than the initial (nonstretched) mobility. Although PII2T-C9 has an even lower thin-film modulus than PII2T-C8, the overall
volume fraction of the brittle polymer backbone, and enlarged lamellar packing distance triggered by the long and extended carbosilane side chains. To further understand the mechanical tolerance of the studied polymer thin films on the molecular scale, GIXD measurements were performed to detect the crystalline structures of polymer films under strain. The incident X-ray beam was controlled to be parallel or perpendicular to the stretching direction, and the lamellar spacing distances measured in both directions are summarized in Figure 4. Figure 4a−e presents the d-spacing of PII2T-based polymers under 0−100% strain. For each PII2T polymer, in general, the spacing showed similar trends of changes in both parallel and perpendicular directions under strain. However, differences can be observed between polymers that are affected by the length of side chains. Normalized d-spacing in the parallel direction of the studied polymer films under strain is shown in Figure 4f. Only if the side chain is longer than 8 carbons (i.e., PII2T-C8, PII2T-C9, and PII2T-C10) does the packing distance increase by stretching forces. With shorter side chains (PII2T-C6 and PII2T-C7), originally the lamellar packing is smaller than other analog polymers with longer side chains, suggesting that the packing between polymer chains may be more compacted. Thus, such polymer films have lower mechanical resilience and higher Young’s modulus, making them easily damaged by strain. Extended long side chains, on the contrary, open the space between polymer chains and reduce the volume fraction of the rigid polymer backbone, significantly improving the overall thin-film deformability. As a result, the polymer chains and their crystalline regions can be slightly stretched and 4401
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systematic design demonstrates that a high-performance polymer semiconductor with high mobility, good thin-film deformability, and stable electrical behavior under stretching can be accomplished through side chain engineering, which is promising for future wearable/stretchable electronics.
electrical performance was poor because of weaker molecular packing (i.e., larger π−π stacking distance), as discussed previously. Surprisingly, stable charge-carrier mobility under strain was discovered in the PII2T-C10-based thin film. The initial mobility of PII2T-C10 is not as high as PII2T-C8 as the longer side chains may disturb backbone organization, but such long and floppy side chains improve the deformability of polymer films. As a consequence, a field-effect mobility up to 1 cm2 V−1 s−1 can be probed in both the parallel and perpendicular directions under 100% strain, which is one of the highest thin-film mobilities of a single conjugated polymer under strain, to the best of our knowledge. Until now, it is rare to see an intrinsically stretchable polymer semiconductor possesses a mobility over 0.5 cm2 V−1 s−1 when the active layer is stretched [listed in Table S1 (Supporting Information)].25−28,37,38 In addition, there is so far no conjugated polymer that can achieve a mobility higher than 0.5 cm2 V−1 s−1 in both parallel and perpendicular directions at the same time under 100% strain. Through rational side chain design, the electrical performance can be maintained upon stretching, as shown in Figure 5d. In addition, stable charge-transport behavior of the PII2T-C10 film was demonstrated under 500 stretching/releasing cycles at 60% strain. Mobility in both parallel and perpendicular directions was maintained at approximately 1 cm2 V−1 s−1, as depicted in Figure S17 (Supporting Information). The results indeed demonstrate the importance of side chains in an intrinsically stretchable polymer semiconductor. Polymers with even longer side chains, moreover, will be explored in the future to further investigate the structure− property relationships. Compared to those reported intrinsically stretchable conjugated polymers, which usually have side chains of branch point at C1 to C3 with two C10 chains after branching,25−28,37,38 our polymer has a much longer side chain (i.e., branch point at C10 plus two C10 chains after branching) because the design of the carbosilane side chain can easily access any length of side chains with high yield. The extremely extended side chains lower the relative volume of a rigid backbone and allow polymer chains to interact with each other, maintaining an effective charge-transport pathway under strain. Hence, a longer side chain can lead to a better device performance at 100% strain and a side chain with even number of carbons is needed to secure a desired surface morphology as well as charge-carrier mobility.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00589.
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Experimental characteristics; 1H NMR spectrum of the studied monomers and polymers; TGA and DSC traces; CV curves; and additional FET characteristics of the studied polymers (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (T.H.). *E-mail:
[email protected] (W.-C.C.). ORCID
Hung-Chin Wu: 0000-0001-6492-0525 Chi-Ching Kuo: 0000-0002-1994-4664 Tomoya Higashihara: 0000-0003-2115-1281 Wen-Chang Chen: 0000-0003-3170-7220 Author Contributions #
Y.-C.C., H.-C.W., H.-F.W., and C.-C.H. contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors appreciate the financial supports by the “Advanced Research Center for Green Materials Science and Technology” from the Featured Area Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (108L9006) and the Ministry of Science and Technology in Taiwan (MOST107-3017-F-002-002). The authors also acknowledge the National Synchrotron Radiation Research Center of Taiwan for facilitating the GIXD experiments.
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CONCLUSIONS We rationally developed carbosilane side chains with various lengths (i.e., from C6 to C10) on the conjugated polymer backbone for achieving semiconducting polymers with high mobility, stretchability, and mechanical stability. By manipulating the side chain branching point and chain length, molecular packing structure, charge-transport properties, and thin-film ductility can be optimized at the same time. Surprisingly, PII2T-C10 presented a charge-carrier mobility of 1 cm2 V−1 s−1 even under a 100% strain, one of the best FET performances of a stretched polymer film, in both the parallel and perpendicular directions. The extended long side chains open the space between polymer chains and reduce the volume fraction of the rigid polymer backbone, significantly improving the overall thin-film deformability. Additionally, the longer side chains also lower the relative volume of the rigid backbone and allow polymer chains to interact with each other, maintaining an effective charge-transport pathway under strain. Indeed, our
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DOI: 10.1021/acs.macromol.9b00589 Macromolecules 2019, 52, 4396−4404
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Macromolecules Chen, W.-C.; Bao, Z. Effects of Molecular Structure and Packing Order on the Stretchability of Semicrystalline Conjugated Poly(Tetrathienoacene-diketopyrrolopyrrole) Polymers. Adv. Electron. Mater. 2017, 3, 1600311.
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DOI: 10.1021/acs.macromol.9b00589 Macromolecules 2019, 52, 4396−4404
