www.afm-journal.de Photo-Patternable Stretchable Semi-Interpenetrating Polymer Semiconductor Network Using Thiol–Ene chemistry 1 Chemistry for Field-Effect Transistors thio - ene Hsin-Chiao Tien, Xin Li, Chang-Jing Liu, Yang Li, Mingqian He, and Wen-Ya Lee* are essential for novel applications, including wearable electronics, implanted devices, electronic skins, electronic eyes, and prosthetics. However, for traditional silicon-based devices, it is challenging to prepare stretchable integrated circuits due to the brittleness of the silicon semiconductor. Therefore, many approaches have been developed for stretchable electronics, including rigid-island architectures,[5–7] buckled surfaces,[8,9] and intrinsically stretchable materials.[4,10–13] Compared to the other approaches, intrinsically stretchable materials have been considered promising for stretchable electronics. They do not need to be like the rigid-island architecture with the large space of serpent-shaped electrodes with the disadvantages of reduced device density and less resolution. Furthermore, the intrinsically stretchable materials would not require a rough, wrinkled surface to release stress. Therefore, the intrinsically stretchable semiconductor is ideal for fabricating stretchable electronics.[4] Intrinsically stretchable semiconductors can be accomplished by either the backbone/side-chain engineering of conjugated polymers[10,14–27] or by blending with elastic polymers.[2,11–13,17,28] Compared with the modification of the chemical structure of conjugated polymer backbone or side chains, which requires complicated synthesis skills, the blending of a conjugate polymer and an insulating elastomer provides a facile way to prepare a stretchable semiconducting layer. Recently, Wang et al. reported a stretchable field-effect transistor (FET) array based on the blend of the donor–acceptor copolymer and polystyrene-b-poly(ethylene-co-butylene)-bpolystyrene (SEBS). The average mobilities of the stretchable transistors are ≈0.82 cm2 V−1 s−1.[11,12] The same group applied solution shearing to align the polymer semiconductor films to further enhance device performance in stretchable polymer transistors.[29] The solution-sheared stretchable transistor showed ideal characteristics with an average mobility of 1.50 cm2 V−1 s−1. Nowadays, the mobility of stretchable transistors has been further improved. However, the fabrication of a large-scale stretchable circuit is still challenging due to the limitation of scalable patterning of stretchable polymer semiconductors. The polymer commonly suffers severe solvent etching during photolithography, dramatically degrading device performance. Stretchable polymer semiconductors are an essential component for skininspired electronics. However, the lack of scalable patterning capability of stretchable polymer semiconductors limits the development of stretchable electronics. To address this issue, photo-curable stretchable polymer blends consisting of a high-mobility donor–acceptor conjugated polymer and an elastic rubber through thiol–ene chemistry are developed. The thiol–ene reaction can selectively cross-link the rubber with alkene or vinyl groups without damaging the electronic properties of the conjugated polymer. The conjugated polymer chains embedded in the elastic polymer matrix induce a semi-interpenetrating polymer network (SIPN). The thiol–ene-cross-linked network provides great solvent resistance and enhances stretchability for the embedded conjugated polymer. The well-defined patterned film with a feature size of ≈10 µm can be obtained using UV light at 365 nm through conventional photolithography processes. Furthermore, the SIPN-based transistors show increased mobilities from 0.61 to 1.18 cm2 V−1 s−1 when applying the strain from 0% to 100%. Moreover, the hole mobility can still maintain at 0.87 cm2 V−1 s−1 after 1000 strain-and-release cycles at the strain of 25%. This study sheds light on the molecular design of photo-curable polymer semiconductors for the mass production of stretchable circuits. 1. Introduction Stretchable electronics have attracted great attention recently due to their great mechanical compliance to tolerate a large degree of deformation without sacrificing electronic properties.[1–4] Deformable devices with high electronic performance H.-C. Tien, C.-J. Liu, W.-Y. Lee Research and Development Center for Smart Textile Technology and Department of Chemical Engineering and Biotechnology National Taipei University of Technology Taipei 106, Taiwan E-mail: wenyalee@mail.ntut.edu.tw X. Li, Y. Li Corning Research Center China Shanghai 201206, P. R. China M. He Corning Incorporated Corning, NY 14831, USA The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202211108. DOI: 10.1002/adfm.202211108 Adv. Funct. Mater. 2023, 2211108 2211108 (1 of 12) © 2023 Wiley-VCH GmbH 16163028, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202211108 by National Taipei University Of Technology, Wiley Online Library on [10/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License RESEARCH ARTICLE 可拉伸介電層 paper phenol Several approaches have been demonstrated to cross-link polymer chains to develop scalable patterning approaches and reduce solvent etching damage. Previously, much effort has been put into the development of dielectric layers in field-effect transistors.[30–37] Lim et al. have reported a cross-linked polymer dielectric consisting of hydrophilic poly(4-vinyl phenol) (PVP) and poly(melamine-co-formaldehyde) with negligible current hysteresis.[38] Yoon et al. have demonstrated a low-voltage-driven transistor using silane-cross-linked gate dielectrics. Using thermal treatments, they employed three trichlorosilanes with various chain lengths to cross-link hydrophilic PVP as gate dielectrics. Low-voltage-driven devices with the leakage ultrathin dielectric material can be achieved by trichlorosilane cross-linking.[34] Cheng et al. cross-linked an amorphous fluoropolymer dielectric, poly(perfluorobutenylvinylether) (Cytop), using 1,6-bis(trichlorosilyl)hexane.[39] The cross-linked Cytop showed great bias and air stability due to its 荣 inert fluorinated chemical structure. Chua et al. used a siloxane-based material, divinyltetramethyldisiloxane-bisbenzocyclobutene (BCB), as a gate dielectric. BCB can form a defect-free surface to facilitate charge transport.[40] However, the BCB film requires a thermal treatment at 290 °C. Hallani et al. synthesized an alkoxy-substituted benzocyclobutene (BCB)-based polymer for gate dielectric materials to reduce process temperatures.[33] The BCB moiety with alkoxy groups can be cross-linked through the electrocyclic opening at 120 °C. Although cross-linked polymer dielectrics are commonly obtained using thermal curing processes, photo-curable polymer dielectrics are preferred for the industry. This photocurable feature provides an efficient direction for patterning a large-scale integrated circuit. Kwon et al. have reported a solventfree photo-patternable dielectric using poly[(mercaptopropyl) methyl-siloxane] based on thiol–ene cross-linking.[31] The poly[(mercaptopropyl)methyl-siloxane] pattern with a feature size of 3 µm can be achieved using UV-nanoimprint lithography. Wang et al. have developed cinnamate-modified carbohydrates as gate dielectrics. The cinnamate-modified carbohydrates can proceed [2+2] cycloaddition after UV exposure and then form a robust cross-linked dielectric.[32] Kwon et al. have demonstrated a cross-linker series with fluorophenyl azide (FPA) groups. The FPA groups can react with alkyl groups of polymer chains.[41] Low/high-k or amorphous/crystalline polymers can be crosslinked as gate dielectrics. Although much progress has been made in cross-linked polymer dielectrics, the development of photo-cross-linked polymer semiconductors significantly lags. The concept of cross-linking semiconductors has been reported in organic solar cells to solve the issues of longterm stability.[42] Cross-linking chemistry is also applicable for stretchable polymer semiconductors. The Bao group first demonstrated an azide-cross-linking dielectric for stretchable transistor array with inkjet printing and traditional photolithography processes. They used azide to allow the photo patterning of the insulating polymer, SEBS, as a stretchable dielectric layer.[12] Sequentially, the stretchable semiconductor is deposited on the dielectrics using inkjet printing. Combining with a fluorinated polymer as a sacrificial layer, photo-patterned stretchable dielectrics, and inkjet-printed polymer semiconductors, a large-scale array of stretchable transistors with a Adv. Funct. Mater. 2023, 2211108 www.afm-journal.de 向 device density of 347 transistor per cm−2 has been made. The same group recently applied a light-triggered carbine insertion chemistry to cross-link stretchable polymer semiconductors by UV light.[13] They synthesized a diazirine cross-linker, which can induce carbene groups by UV light. The carbene group is a highly reactive agent and can react with the CH bonds in the polymer for the cross-linking reaction. The carbene-based photo-cross-linking enables the layer-by-layer deposition of stretchable polymer semiconductors during photolithography. Here, we demonstrate a mild approach to pattern stretchable polymer semiconductors without reducing their charge transport properties using thiol–ene chemistry. The thiol–ene reaction is a highly efficient way of bonding thiols and enes. A donor–acceptor conjugated polymer is embedded in a thiol– ene-cross-linked elastic polymer matrix based on poly(styrenebutadiene-styrene) (SBS) to induce a semi-interpenetrating polymer network (SIPN). The SIPN using thiol–ene chemistry was selected for the following reasons: i) improved stretchability of the polymer semiconductor layer due to the elastic polymer; ii) relative selectivity of cross-linking reaction of thiol– ene chemistry; iii) enhanced solvent resistance due to the semiinterpenetrating polymer network. Compared to the highly reactive agent, azide or carbene group, which can directly react with alkyl groups (CH3), the thiol–ene reaction is a selective reaction, which only allows cross-linking with double bonds to form a thioether. Instead of using SEBS commonly reported in the literature, we selected the elastic polymer, SBS, as our elastic polymer. SBS contains the vinyl and alkene groups, which can act as a cross-linking site for thiol–ene click chemistry. Two thiol reactions have been developed: thiol–ene freeradical and catalyzed thiol-Michael reaction.[43–45] Herein, this work used free-radical thiol–ene addition for the cross-linking reaction. In the free-radical thiol–ene reaction, the reaction rate is dependent on the electron density of the alkene. Vinyl double bonds can react faster than conjugated ones.[43] Therefore, the thiol group has a higher reactivity with the electron-rich ene, e.g., a vinyl group, than electron-poor conjugated alkenes.[45] During the thiol–ene cross-linking reaction, it is expected that the thiol group tends to cross-link with the vinyl group in SBS instead of conjugated polymer chains. Hence, the thiol–ene reaction has less chance of altering the molecular packing structures of the conjugated polymer. Moreover, the selectively cross-linking SBS may form a semi-interpenetrating polymer matrix to encapsulate the conjugated polymer, protecting the conjugated polymer from solvent etching. The main difference between carbene insertion reaction and thiol–ene chemistry is cross-linking target materials. The carbene groups induced from the diazirine cross-linker can insert into the CH bonds of the non-conjugated alkyl chains of a conjugated polymer for cross-linking.[13] The advantage of this carbene reaction is that the carbene groups can react with a large variety of polymers with alkyl chains. However, the carbene insertion reaction may alter the molecular packing of polymer chains since the alkyl chains act an important role in molecular packing. On the other hand, the thiol–ene reaction prefers cross-linking with non-conjugated double bonds of the SBS elastomer instead of the conjugated polymer. Therefore, the thol-ene cross-linking approach should have a lower chance 2211108 (2 of 12) © 2023 Wiley-VCH GmbH 16163028, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202211108 by National Taipei University Of Technology, Wiley Online Library on [10/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de of altering the molecular packing of the conjugated polymer. Furthermore, all the photoinitiator and thiol cross-linkers are commercially available, compared to the above-mentioned carbene cross-linkers require additional chemical syntheses. This commercial availability is another advantage of the thiol–ene cross-linked system for industrial production. In our system, the high mobility donor–acceptor polymer, poly(tetrathienacene-diketopyrrolopyrrole) (P4TDPP), has been chosen as the polymer semiconductor for charge transport.[17] The fused-ring tetrathienacene moiety has high charge mobilities of >1 cm2 V−1 s−1 and provides low resonance energy and great chemical stability.[46,47] It is expected that the combination of the SIPN and highly stable P4TDPP will facilitate the fabrication of scalable photo-patterned stretchable transistors for large-scale production in the industry. Note that our devices are only partially stretchable. The drain, source, gate electrodes, and dielectric layer are not stretchable. However, through the PDMS transfer processes, we can focus on evaluating the polymer semiconductors under various strains without concerning the degradation of drain/source/gate electrodes or dielectrics under deformation. Therefore, this work emphasizes investigating the electronic properties of polymer semiconductors under various strains. Despite lacking the integration of fully stretchable devices, this work can still provide new insight for designing patternable stretchable polymer semiconductors. 2. Results and Discussion 2.1. Photo-Cross-Linking Reaction Based on Thiol–Ene Click Chemistry To incorporate conjugated polymer films with thiol–ene chemistry, we first blended the diketopyrrolopyrrole (DPP)-based conjugated polymers (P4TDPP) with the elastomer (SBS) to enhance the stretchability of the polymer film, as shown in Figure 1a. Then, trimethylolpropane tris(3-mercapto propionate) (TRIS) and trimethyl benzoyl-diphenylphosphine oxide (TPO) were mixed with the polymer blends to form a photocurable polymer network. SBS has both alkene (CHCH) and vinyl group (CHCH2) in the polymer structures, which provide reactive sites for thiol–ene reaction. TPO can produce free radicals initiated by UV light at 365 nm, and the free radicals can be transferred to the thiol group of TRIS. The transferred free radicals form thiyl radical species (Figure 1b). The free-radical thiol–ene reaction is efficiently employed to cross-link SBS as a polymer network. Compared to the air-sensitive azide or carbene reaction, the thiol–ene reaction is insensitive to oxygen or moisture. The whole thiol–ene cross-linking process can be done in the ambient atmosphere condition. Furthermore, the thiol groups prefer reacting with the electronrich group, such as the alkene or vinyl group. The reaction rate of conjugated dienes with the thiol groups is much slower.[48] The different reactivity is a great benefit for photo pattern technology because the cross-linking between the alkene groups of SBS does not change the chemical structure of the conjugated polymer. Furthermore, the different reactivity will cause the formation of the P4TDPP polymer chains embedded in the Adv. Funct. Mater. 2023, 2211108 cross-linked SBS network, the so-called semi-interpenetrating polymer network (SIPN) (Figure 1c). The SIPN structure may offer the encapsulation of the conjugated polymer protected from the etching of organic solvents. To verify the photo-cross-linking reaction of blend film, we employed Fourier transform infrared (FTIR) to analyze the photo-cross-linking reaction of the conjugated polymer blend films. Figure 2a-1 shows the IR absorption peaks of TRIS, the pristine polymer, and the polymer blend films. The signals near 2800–3000 cm−1 are assigned to CH functional groups. The strong absorption peak at 1730 and 1662 cm−1 are assigned to the CO functional group of TRIS and P4TDPP, respectively. The absorption peak at 1600 cm−1 belongs to the CC group of SBS. After exposure to UV light at 365 nm, the peak significantly decreases. The decreased signal indicates that a large amount of the alkene groups forms the thioether group through the thiol–ene reaction. We have also used H-NMR spectra to investigate the cross-linking reaction. However, unfortunately, we could not find any meaningful signal to support the thiol—ene reaction. This may be attributed to the low percentage of the TRIS/TPO (3 wt.%). Despite the lack of the NMR spectra to support our hypothesis, the photocured P4TDPP/SBS films showed great solvent resistance for common solvents, e.g., toluene, p-xylene, butyl acetate, and acetone. The great solvent resistance indicates the successful cross-linking of the P4TDPP/SBS film. Owing to the photocross-linking characteristics, we can successfully prepare a well-defined pattern using UV exposure (Figure 2; Figure S2, Supporting Information). To investigate the influence of thiol– ene cross-linking, we tested the P4TDPP/SBS film without TRIS. Without the thiol cross-linker, the polymer film did not proceed with the thiol–ene cross-linking. As a result, the crosslinking of the film is required a much higher percentage of TPO (≈10 wt.%) and increased exposure time (6 min) to obtain a pattern. However, the device performance of P4TDPP/SBS exhibited a significant decrease. According to these results, we conclude that the addition of the thiol cross-linkers (TRIS) is necessary to realize high-mobility photo-patternable stretchable polymer semiconductors. 2.2. Photo-Patterning of the Conjugated Polymer Blend Films Various pattern sizes of the conjugated polymer blend films were fabricated by conventional photolithography. The desired patterns of the spin-coated P4TDPP/SBS films were obtained via a quartz mask with UV light exposure at 365 nm for a minute. The untreated area of polymer films can be fully removed by immersing it in toluene for 10 min. Figure 2b shows the welldefined patterns of the P4TDPP/SBS blends with various sizes. The rectangular area of the conjugated polymer films with the same length of 1000 µm and various widths of 1000, 200, and 100 µm can be obtained, respectively. The smallest line width based on our current accessible UV curing equipment is ≈10 µm. As shown in Figure 2c, the AFM height image reveals the feature size of the patterned P4TDPP/SBS film with a width of ≈10 µm. Although the etching profile angle is not ideal at 90o, which requires further optimization, a well-defined pattern can still be observed. Figure 2d exhibits The AFM cross-section 2211108 (3 of 12) © 2023 Wiley-VCH GmbH 16163028, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202211108 by National Taipei University Of Technology, Wiley Online Library on [10/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Figure 1. a) Chemical structure of conjugated polymer P4TDPP, poly(styrene-butadiene-styrene) (SBS), and trimethylolpropane tris(3-mercapto propionate) (TRIS). b) Thiol–ene reaction of the light-induced cross-linked polymer network. c) Illustration of SIPN (semi-interpenetrating polymer network) structure. profile of the P4TDPP/SBS spin-coated film. The film thickness estimated from the cross-section profile is ≈25–30 nm (Figure 2d). These results show that the P4TDPP/SBS film can be successfully patterned via the conventional photolithography processes. Furthermore, the success of the small, well-defined pattern indicates that this patterning approach can be applicable to OLED applications, which require a micrometer-scale device array for display panels. Adv. Funct. Mater. 2023, 2211108 2.3. Electrical Characterizations of FETs Based on Patterned Stretchable Polymer Semiconductor Films The electrical performance of the bottom-gate-top-contact FETs is based on the P4TDPP/SBS films, as shown in Figure 3a. All the charge mobilities of FETs are estimated from the saturation regime in the transfer curves. The pristine P4TDPP films and P4TDPP/SBS blend films were prepared by spin coating 2211108 (4 of 12) © 2023 Wiley-VCH GmbH 16163028, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202211108 by National Taipei University Of Technology, Wiley Online Library on [10/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Figure 2. a) Thin film FTIR results of pure P4TDPP, pure SBS, pure TRIS, SBS/P4TDPP/TPO, SBS/TRIS/TPO, SBS/P4TDPP/TRIS/TPO with UV exposure, and SBS/P4TDPP/TRIS/TPO without UV exposure. b) Optical microscope images of patterns (scale bar was 200 µm). c) AFM height image of the SBS/P4DPP/TRIS/TPO pattern with the size of 10 µm × 10 µm. d) The cross-section profile of Figure 2c. from a p-xylene solution with a 5 mg mL−1 concentration. The blend films comprised 60 wt.% SBS and 40 wt.% P4TDPP. All the polymer films were treated with UV light for photo-curing without thermal annealing to investigate the charge transport properties of the photo-cross-linked polymer films. The average mobility (µavg) of the neat P4TDPP film and P4TDPP/ SBS blend film was 0.46 and 0.32 cm2 V−1 s−1, respectively, as shown in Table 1. The blending with SBS showed slightly improved charge mobility (Figure S1, Supporting Information). The performance evaluation indicates that the mixing with the insulating SBS will not obstruct charge transport in the channel of FETs. Furthermore, the 40% to 80% P4TDPP exhibited similar mobilities, while the 20% P4TDPP device showed the lowest performance. However, the charge mobilities of the 60% P4TDPP show a clear degradation under the strain of 60% (Figure S1b, Supporting Information). On the other hand, the 40% P4TDPP exhibited better stretchability and increased mobilities even at the strain of 100%. Considering the mechanical compliance and photo pattern effect, we chose the 40% P4TDPP/60 wt.% SBS as the main blending ratio for stretchable polymer semiconductors. Adv. Funct. Mater. 2023, 2211108 The dual sweeping transfer curves of the blend films and neat films at different strains are given in Figure 3a,b, respectively. All the transfer curves in the dual sweeping showed negligible hysteresis, indicating that the semiconductor/ dielectric interface has a low density of defects even in the SBS blends. Figure 3c,d exhibits the well-defined output curves of blend films at 0 and 100% strains, respectively. The output curves in both conditions showed linear behavior in the low drain voltage, indicating no significant contact resistance even in the blends. Figure 3e shows the mobility trend in blended and neat films with various strains. Note that the average mobilities under various strains were estimated from the charge transport direction along with the stretching direction. The pristine P4TDPP devices showed significant degradation of ≈73% under high-level strains. Compared with the neat film, the blend film with a high ratio of SBS exhibited gradually increased charge mobilities from 0.61 to 1.18 cm2 V−1 s−1 with increased strains from 0% to 100%. This enhanced mobility may be attributed to the improved orientation of the molecular packing of the conjugated polymer during the alignment of polymer chains under stretching. In contrast, the charge 2211108 (5 of 12) © 2023 Wiley-VCH GmbH 16163028, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202211108 by National Taipei University Of Technology, Wiley Online Library on [10/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Figure 3. a,b) Hysteresis curves (VD = −100 V) of the neat film and the blend films under strains. c,d) The output curves of the blend film at strains of 0% and 100%. e) The mobility of the neat film and the blend film at different strain ratios. f) Charge mobilities of the P4TDPP/SBS films before and after solvent etching. mobilities did not show significant enhancement in the direction vertical to the stretching direction (Figure S1c, Supporting Information). This may be attributed to the compression in the vertical direction. When the polymer film is stretched, the film tends to compress in a vertical direction to the stretching force. Therefore, the polymer film did not show significant mobility enhancement in the vertical direction. Note that the average mobilities of the neat and semiinterpenetration polymer semiconductor showed slightly decreased mobility after UV exposure. However, the change of Adv. Funct. Mater. 2023, 2211108 the mobilities is not significant for the neat polymer, indicating that the UV exposure process did not significantly affect the device performance. On the other hand, the semi-interpenetration P4DPP/SBS blends showed a significant decrease after UV exposure. Intriguingly, we observed that the stretching process could recover the reduced performance. This might be affected by the cross-linked SBS matrix and the rearrangement of the conjugated P4TDPP chains. During UV exposure, although the conjugated polymer was not significantly damaged, the formation of the insulating SBS matrix may suppress the charge 2211108 (6 of 12) © 2023 Wiley-VCH GmbH 16163028, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202211108 by National Taipei University Of Technology, Wiley Online Library on [10/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Table 1. A summary of FETs (L = 50 µm, W = 1000 µm) is based on the neat and blended film at different strains, respectively. µavg [cm2 V−1 s−1] Conditions P4TDPP P4TDPP/SBS µmax [cm2 V−1 s−1] Ion/Ioff 4 Vth [V] Without UV 0.58 ± 0.25 0.87 10 0±1 As cast (UV) 0.46 ± 0.21 0.69 104 −6 ± 5 Strain 0% 0.38 ± 0.05 0.46 105 0 ± 10 Strain 50% 0.14 ± 0.01 0.16 104 −4 ± 6 Strain 100% 0.10 ± 0.02 0.12 105 −7 ± 1 4 −19 ± 7 Without UV 0.73 ± 0.08 0.78 10 As cast 0.32 ± 0.03 0.34 104 −12 ± 2 Strain 0% 0.61 ± 0.06 0.72 103 −6 ± 5 3 −11 ± 6 Strain 25% 0.64 ± 0.01 0.69 10 Strain 50% 0.96 ± 0.05 1.06 104 −20 ± 5 Strain 75% 1.11 ± 0.06 1.29 104 −17 ± 7 1.45 4 −19 ± 5 Strain 100% 1.18 ± 0.13 transport in the conjugated polymer. However, the stretching process facilitates the rearrangement of the conjugated polymer chains and rebuilds the channel for charge transport, thus leading to the enhancement of charge mobilities. Figure 3f compares the performance of the photo-cured P4TDPP/SBS FETs before and after solvent etching in toluene for 10 min. The photo-cured polymer blend FETs exhibited 10 great solvent resistance. After immersing in toluene, the FETs can still maintain charge mobilities ≈0.2–0.3 cm2 V−1 s−1 even under the strain of 100%. On the contrary, the non-crosslinked polymer-blend film was fully removed in toluene. This feature indicates that the photo-cross-linked SIPN structure provides a facile way to obtain a selected pattern of the polymer semiconductor film. Additionally, we have applied this SIPN Figure 4. a) Hysteresis curves (VD = −100 V) of the blend film strain to 25% for 100, 500, and 1000 cycles. b) The mobility of the blend film strain to 25% for 100, 500, and 1000 cycles. c,d) The output curves of the blend film strain to 25% for 100 and 1000 cycles. Adv. Funct. Mater. 2023, 2211108 2211108 (7 of 12) © 2023 Wiley-VCH GmbH 16163028, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202211108 by National Taipei University Of Technology, Wiley Online Library on [10/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Figure 5. AFM tapping phase and height images of a,b) P4TDPP neat film, c,d) P4TDPP/SBS blend film under UV exposure for 1 min, and AFM tapping phase images of P4TDPP/SBS blend film e) at 0% strain and f) 100% strain. approach to other conjugated polymers, including p-type poly(diketopyrrolopyrrole-thienothiophene) (PTTDPP) and n-type poly(naphthalene diimide-alt-bithiophene) (PNDI2T), respectively, and evaluate their device performance (Figure S2, Supporting Information). The mobility of these polymers only showed a slight decrease after the photo-curing processes. The mobilities of these photo-cured polymer semiconductors can still maintain the same order of magnitude after immersing in toluene for a few minutes. These results show that our approach can be applied to different conjugated polymers. We also tested the patterning capability of the PNDI2T/SBS film. The polymer blend films showed similar behaviors of great solvent resistance and formed a well-defined polymer semiconductor pattern. These results suggest that the semi-interpenetrating structure has the potential for scalable photo-patterning of polymer semiconductors for stretchable field-effect transistor applications. Adv. Funct. Mater. 2023, 2211108 We also investigated the electrical properties of a cyclic strain of blend films at 25% strain. After the blend film stretching at 25% for 100, 500, and 1000 cycles, the electrical properties show great performance in the transfer curves, as shown in Figure 4a. The mobility was measured parallel to the strain direction. The values of µavg increase from 0.48 cm2 V−1 s−1 at 100 cycles to 0.87 cm2 V−1 s−1 at 1000 cycles (Figure 4b). It is consistent with the enhanced drain current of the transfer and output curves of P4TDPP/SBS FETs with the increased stretching cycles (Figure 4c). This indicates that the P4TDPP/SBS blends have remarkably long-term robustness. 2.4. Morphological Characterizations To further understand the influence of the stretching processes on the morphology, we have investigated the surface 2211108 (8 of 12) © 2023 Wiley-VCH GmbH 16163028, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202211108 by National Taipei University Of Technology, Wiley Online Library on [10/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Figure 6. a) UV–vis spectra of the P4TDPP/SBS film at the strain of 100%. b) The dichroic ratio of blend films at different strain ratios. c) SIMS results of the blended film. morphology of the polymer films by using atomic force microscopy (AFM). The height and phase images of the neat P4TDPP and P4TDPP/SBS films are shown in Figure 5a,b. The P4TDPPonly film showed a smoother surface with small aggregates. On the other hand, the polymer blend film exhibited well-defined nanofiber morphology with large grains. To compare the morphology of the polymer blend films under different strains, we employed Scotch tape to peel off the polymer blend films and measured their backside morphology. The polymer blend films did not show any significant cracking on the surface, even under the strain of 100%. Furthermore, after stretching at the strain of 100%, the nanofibers exhibited slightly aligned wrinkles on the surface, as shown in Figure 5c. Next, we studied their optoelectronic properties using polarized UV–vis spectra. Figure 6a,b shows the polarized UV–vis spectra of the P4TDPP/ SBS films under different strain ratios in the parallel (0°) and perpendicular (90°) directions. The polymer films showed strong anisotropic absorption. We calculate their dichroic ratio from the polarized UV–vis spectra. We can also see that the dichroic ratio increases from 1 to 2.1 when the strain ratio increases from 0 to 100%. The enhanced dichroic ratio indicates that the orientation of polymer chains has been aligned after stretching. The alignment of the polymer chains facilitates better charge transport, which may explain the increased charge carrier mobility with stretching. The surface elemental analysis of blend film was analyzed by SIMS (Secondary Ion Mass Spectroscopy), as shown in Figure 6c. The purple line and blue line represent the content of sulfur and cyanide groups, respectively, which should be Adv. Funct. Mater. 2023, 2211108 contributed from the conjugated polymer, P4TDPP. The red line represents the content of carbon, which can be attributed to both P4TDPP and SBS. The black and orange lines represent the silicon and oxygen content from the Si/SiO2 substrate signals. We found a vertical phase separation in the polymer blend film from the SIMS results. The thickness of the blend film is ≈20–30 nm. The conjugated polymer prefers to distribute in the top and bottom positions, and SBS is rich in the middle of the blend film, which is consistent with the other polymer blends observed in the literature.[11] Because of the vertical phase separation, the conjugated polymer can still form a channel for charge transport even though blending with a high ratio of SBS. The molecular packing of the thin films was analyzed by grazing incidence X-ray diffraction (GIXD). Figure 7a,b showed the 2D-GIXD patterns of the cross-linked P4TDPP/SBS films under 0% and 100% strain. The P4TDPP/SBS films showed well-defined diffraction peaks in the out-of-plane direction, which were attributed to a lamellar structure of the P4TDPP. The estimated lamellar spacing of the P4TDPP/SBS is 31.66 Å, slightly higher than the value of the pristine P4TDPP (30.28 Å). The increased lamellar spacing should originate from the SBS blending, which probably induces a less dense structure. When stretching from 0% to 100%, the lamellar spacing increases to 32.8 Å. Similar behavior has also been observed in the literature. The increased spacing implies that the external mechanical stress may reduce the interdigitated structure of the P4TDPP polymer chains. Additionally, the lamellar spacing of the P4TDPP/SBS blend is slightly higher than the long-branched 2211108 (9 of 12) © 2023 Wiley-VCH GmbH 16163028, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202211108 by National Taipei University Of Technology, Wiley Online Library on [10/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Figure 7. 2D GIXD images of a) P4TDPP/SBS blend film with the strain of 0% and b) 100%, respectively. c) Crystal sizes of the P4TDPP/SBS with various strains. chains of the P4TDPP nanowires reported in the literature.[49,50] This difference may be attributed to the morphology. Compared to our thin-film device, the nanowire of P4TDPP reported in the literature may have a denser interdigitated packing or a tilted backbone. Although the pristine P4DTPP polymer showed a pi–pi stacking distance of 3.72 Å (Figure S4, Supporting Information), we did not observe any (010) signals (π–π stacking) from the P4TDPP/SBS blends. This result should be attributed to the reduced concentration of the conjugated polymer. The blend film consisted of 40 wt.% P4TDPP and 60 wt.% SBS. The concentration of P4TDPP was significantly diluted due to the blending with a large ratio of SBS. Therefore, there are no detectable (010) signals from GIXD patterns of the P4TDPP/ SBS blends. To further investigate the crystallinity, we evaluate the crystal sizes of the polymer films according to the Scherrer equation, Adv. Funct. Mater. 2023, 2211108 which calculates the crystal sizes from the full width at half maximum (FWHM) of the out-of-plane (200) peak. As shown in Figure 7, the crystal sizes increased from 5.13 to 7.37 nm when the applied strain was from 0 to 100%. The larger crystal size facilitates charge transport. This trend agrees with the improved mobility and dichroic ratios with the increased strains. These results indicate that the conjugated polymer embedded in the photo-cross-link SBS network did not cause any degradation of device performance. Instead, the improved stretchability and charge transport properties were observed in the photo-cured P4TDPP/SBS polymer blends. 3. Conclusion This work demonstrates stretchable field-effect transistors using elastic photo-cross-linkable conjugated polymer blends with a 2211108 (10 of 12) © 2023 Wiley-VCH GmbH 16163028, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202211108 by National Taipei University Of Technology, Wiley Online Library on [10/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de semi-interpenetrating polymer network. The photo-cross-linked semi-interpenetrating polymer network provides high charge transport properties, stretchability, and solvent resistance. The feature size of the polymer semiconductor pattern ≈10 µm can be achieved. Moreover, the P4TDPP/SBS blend films exhibit a vertical phase separation and still offer great charge transport. The cross-linked P4TDPP/SBS thin films showed the nanofiber structure and could withstand the stretch of 100% without crack formation. The mobility of P4TDPP/SBS can achieve a value of up to 1.18 cm2 V−1 s−1. Moreover, the P4TDPP/SBS film still has great stability for charge transport under 500 strain-and-release cycles. Most importantly, this approach can obtain the desired pattern without significantly sacrificing their charge mobilities. Therefore, this work shows the great potential of thiol–ene chemistry for the mass production of stretchable transistor arrays. 4. Experimental Section FET Device Fabrication and Characterization: The conjugated polymer P4TDPP (Corning, Mw = 68k) and elastomer Poly(styrene-butadienestyrene) (SBS, Aldrich, Mw = 140k) solutions were prepared by dissolving in p-xylene (5 mg mL−1) and heating overnight at 90 °C, respectively. Photo-cross-linker Trimethylolpropane tris(3-mercapto propionate) (TRIS) and photoinitiator solutions were also prepared by dissolving in p-xylene (4 mg mL−1), respectively. The organic semiconductor thin films were spun-cast on the highly doped n-type Si(100) wafers with 300 nm thick thermal SiO2. The SiO2 surface was modified with OTS self-assembled monolayer according to the reported method.[51] The neat and blend films were spun-cast on the SiO2/Si substrates in the glove box at 1000 rpm for 60 s from the solution, which was pre-heated at 90 °C. The UV curing experiments were accomplished using UVACUBE 100 (Hönle, German). The wavelength of UV light was in the range of 315 to 400 nm (UVA). The calibrated power of the UV light was 5.4 mW cm−2. The P4TDPP/SBS film required an energy of 324 mJ cm−2 for photo patterning. Therefore, the blend films were under UV exposure for 1 min, then used PDMS to peel off the blend films and transfer the films to the FET test bed after a strain at each different strain ratio. Neither the neat films nor the blend films were without annealing procedure. The FETs were bottom-gate top-contact devices. The source and drain electrodes were thermally evaporated 80 nm thick gold electrodes onto the polymer films through a shadow mask with the channel length (L) and width (W) defined as 50 and 1000 µm, respectively. The electrical properties of FETs were using a Keithley 2634B semiconductor parameter analyzer in a nitrogen-filled glovebox. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements W.-Y. L. is thankful for the financial support from the Ministry of Science and Technology in Taiwan (MOST 110-2221-E-027 -008 -MY3; 110-3116-F-011 -003), National Science and Technology Council (111-3116F-011 -006), the Research and Development Center for Smart Textile Technology, and National Taipei University of Technology International Joint Research Project (NTUT-IJRP-109-04; L7101101). Conflict of Interest The authors declare no conflict of interest. Adv. Funct. Mater. 2023, 2211108 Data Availability Statement The data that support the findings of this study are available from the corresponding author upon reasonable request. Keywords field-effect transistors, photolithography, polymer blends, polymer semiconductors, stretchable electronics Received: September 24, 2022 Revised: December 24, 2022 Published online: [1] A. Chortos, J. Liu, Z. A. Bao, Nat. Mater. 2016, 15, 937. [2] C. Lu, W.-Y. Lee, C.-C. Shih, M.-Y. Wen, W.-C. Chen, ACS Appl. Mater. Interfaces 2017, 9, 25522. [3] M. 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