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Organic Electronics

journal homepage: www.elsevier.com/locate/orgel

Highly stretchable fiber transistors with all-stretchable electroniccomponents and graphene hybrid electrodes

Wonoh Leea, Youn Kimb, Moo Yeol Leec, Joon Hak Ohd, Jea Uk Leeb,∗

a School of Mechanical Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju, 61186, South Koreab Carbon Industry Frontier Research Center, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-ro, Yuseong-gu, Daejeon, 34114, South Koreac Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-ro, Pohang, Gyeongbuk, 37673, South Koread School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Gwanak-ro, Gwanak-gu, Seoul, 08826, South Korea

A R T I C L E I N F O

Keywords:GrapheneFiber transistorStretchableTextile electronics

A B S T R A C T

Textile-based electronic devices should be not only bendable but also highly stretchable for human-friendlywearable electronic applications. Herein, a highly stretchable and mechanically durable fiber transistor wasdesigned and prepared by combining all-stretchable electronic components. First, electrically conductive andstretchable electrodes were fabricated by simple prestraining-then-buckling of graphene/silver hybrid fibers on ahighly elastic thermoplastic polyurethane (TPU) monofilament, which maintained a low resistivity (∼30Ω cm)at a high stretching strain (∼70%) and under many stretch/release cycles (∼1,000). A highly stretchable activechannel was prepared by directly blending semiconducting poly(3-hexylthiophene) (P3HT) and viscoelastic TPUwithout using pre-grown P3HT nanofibrils. The P3HT/TPU blend (5/5 w/w blend ratio) active layer possessedreasonably high mobility (1.46× 10−3 cm2 V−1 s−1) and adequate mechanical strength (21MPa) with a highelongation strain (∼120%). In addition, a stretchable dielectric layer and gate electrode were prepared using anelastic ion-gel film and a liquid metal composite, respectively. The assembled fiber transistor exhibited excellentstretchability (∼50%) while maintaining good electrical properties (average charge carrier mobility of1.74 cm2 V−1 s−1, on/off current ratio of 104) and showed outstanding electrical stability up to 1,000 cycles ofstretch/release testing. To the best of our knowledge, these superior stretchability and stability have not beenreported elsewhere in the area of fiber-type transistors. We believe that our work can serve as an important steptoward the development of core components in wearable devices, such as wearable displays, computers, andbiomedical sensors.

1. Introduction

Textile electronics have received considerable attention owing totheir promising applications in multifunctional smart clothes, whichmay revolutionize human life [1–3]. Although various wearable deviceshave been suggested for smart clothes, most are still accessory types inwhich rigid electronic devices are attached to ordinary clothes. Ideally,the ultimate forms of smart clothes are expected to be electronic tex-tiles, in which all the components of electronic devices are intrinsicallyintegrated into three-dimensional textile structures. Like conventionalclothes, textile-based electronic devices should be not only bendableand foldable but also highly stretchable, because the movements ofhuman joints generate strains as high as ∼55% upon stretching andcontracting [4].

In recent years, there have been increasing attempts to integrateelectronic functions into fibers, yarns, and fabrics to develop highly

stretchable textile electronics using various materials such as metalnanostructures [5–7], carbon-based materials [8,9], and their hybrids[10–13]. Among these materials, carbon nanomaterials and their hy-brids have been considered promising building blocks for innovativestretchable electronic components of next-generation textile electronicsbecause of their high mechanical flexibility, superior electrical con-ductivity, large specific surface area, and suitability for textile weaving[14–16]. To date, there have been many attempts to generate textile-based stretchable energy devices such as supercapacitors [17,18], solarcells [19], and tactile sensors [20–22], primarily using carbon nano-materials and their hybrids as stretchable electrodes. However, sur-prisingly, there are only a few reports on developing a textile-basedstretchable transistor, which is one of the most crucial components inwearable devices such as wearable displays, biomedical sensors, andcloth-based computers.

To improve the electrical performance and device features of

https://doi.org/10.1016/j.orgel.2019.03.056Received 19 November 2018; Received in revised form 17 February 2019; Accepted 30 March 2019

∗ Corresponding author.E-mail address: leeju@krict.re.kr (J.U. Lee).

Organic Electronics 69 (2019) 320–328

Available online 01 April 20191566-1199/ © 2019 Elsevier B.V. All rights reserved.

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consumer-level stretchable systems, innovative structural and materialdesigns, and novel processing technologies should be introduced for thedevelopment of wearable transistor devices. Jeong and coworkers de-monstrated stretchable polymer-type transistors constructed using astack of gold nanosheets as electrodes, electrospun poly(3-hex-ylthiophene) (P3HT) fibers as the active material, an ion-gel polyelec-trolyte as the dielectric layer, and an electrospun elastomer nanofibermat as the substrate [23]. Owing to the interpenetrating networkstructure between the ultraviolet (UV)-crosslinked ion-gel polyelec-trolyte and the porous nanofiber mat substrate, the transistors main-tained high electrical performance over 1,500 cycles of stretching at astrain level of ∼70%. Despite this innovative design of the transistorarray on two-dimensional textile substrates, the development of highlystretchable one-dimensional fiber transistors remains technically chal-lenging because of the limited contact area and the high curvature ofthe micrometer-scale fiber substrate. To the best of our knowledge,there have been no reports on the development of one-dimensionalfiber transistors with a stretchability above ∼50%. The deposition ofuniform thin films around fiber substrates, the device structural design,and the selection of appropriate materials have been the main chal-lenges to realization of stretchable fiber transistors [24].

In our previous work, flexible fiber transistors were fabricated usinggraphene/silver hybrid fibers as highly conductive and flexible elec-trodes [25]. Although these transistors showed excellent flexibility(bendability and rollability) and outstanding stability in terms of thedevice performance (the electrical performance was stable after 1,000cycles of bending at a bending radius of 2mm and after 30 days outsideof the glove box), their stretchability was very limited (strain level of∼5%). In this work, we develop fiber transistors that are 10 times morestretchable (strain level of ∼50%) by combining all-stretchable elec-tronic components, including buckled hybrid fibers as electrodes, asemiconducting/elastic polymer blend as the active channel material,an ion-gel polyelectrolyte as the dielectric layer, and an elastic polymer-based monofilament as the substrate. Highly conductive and stretchableelectrodes (strain level of ∼70%) were fabricated by simple pre-straining-then-buckling of graphene/Ag hybrid fibers on the elasticpolymer monofilament. The highly stretchable (strain level of ∼120%)active channel was prepared by combining a semiconducting polymerwith a viscoelastic polymer. The assembled fiber transistor exhibitedexcellent stretchability while maintaining good electrical properties(average charge carrier mobility of 1.74 cm2 V−1 s−1, on/off currentratio of 104) and showed outstanding electrical stability up to 1,000cycles of stretch/release testing.

2. Experimental section

2.1. Materials

All materials were purchased from Sigma-Aldrich, except for thenatural graphite flakes (∼500 μm flakes, Samjung CNG, Inc.), ther-moplastic polyurethane (TPU) monofilament (diameter ∼1mm, HaeKwang Inc.), silicon dioxide (SiO2) wafer (Waferbiz Inc.). RegioregularP3HT was purchased from Rieke Metals Inc. Its molecular weight (Mn)is ∼50,000 gmol−1, and the polydispersity (Mw/Mn) is 1.4–1.6. TPUwas obtained from Lubrizol Inc. (ESTAN S385A-43 N KR). Graphene/Au hybrid fibers were fabricated via the method previously reported byour group [25].

2.2. Fabrication of buckled hybrid fiber electrodes

As illustrated in Fig. 1(a), the TPU monofilament was prestrained upto ∼70% and slightly dissolved by tetrahydrofuran (THF) for partialimpregnation of the graphene/Au hybrid fibers. And then, two straighthybrid fibers were placed parallel to each other with a distance of300 μm on the top surface of the TPU monofilament. After fiber em-bedding and solidification of the dissolved portion, the prestrain of the

TPU monofilament was slowly released, and the hybrid fiber buckled,forming kinked patterns.

2.3. Fabrication of organic field-effect transistor (OFET) based on P3HT/TPU blend electrodes

The OFETs were fabricated in a top-contact bottom-gate config-uration on a highly n-doped Si wafer with thermally grown 300-nm-thick SiO2, which were used as the gate electrode and dielectric layer,respectively. Before deposition of the polymer blend, the surface of thesubstrate was carefully cleaned with solvents, activated with UV–ozoneplasma (10min at 100W), and treated with n-octadecyltrimethox-ysilane (OTS). P3HT/TPU blend films with different P3HT contentswere spin-coated at 1500 rpm for 60 s. The resulting films were dried at150 °C for 10min on a hot plate under nitrogen atmosphere inside aglove box. An Au layer (thickness 40 nm) thermally evaporated througha shadow mask under vacuum (less than 3×10−6 Torr) on top of theactive layer served as the source and drain.

2.4. Fabrication of stretchable fiber transistor devices

Two graphene/Ag hybrid fibers (separated by 300 μm) were alignedon the TPU monofilament, which was prestrained up to ∼70% andslightly dissolved by THF for partial impregnation of the hybrid fiber.After fiber embedding and solidification of the dissolved portion, theprestrain of the TPU monofilament was slowly released, and the hybridfibers buckled, forming kinked patterns. The double layer of poly(vi-nylidene fluoride-co-hexafluoropropylene), P(VDF-HFP), and the ionicliquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide([EMI][TFSA]) ion-gel and P3HT/TPU blend was inversely transferredonto the TPU filament for direct contact between the P3HT/TPU blendlayer and the buckled hybrid fiber electrodes. To complete the fabri-cation of the fiber transistor devices, a stretchable and conductive poly(dimethylsiloxane)/eutectic gallium–indium (75% Ga, 25% In) (PDMS/EGaIn) composite (6.6/1.0 w/w ratio) was applied to the ion-gel layeras a top gate electrode. Each electrode was connected to copper wiresusing silver paste to measure the electrical characteristics of the flexibletransistors during the stretching test.

2.5. Characterization

Optical microscopy (OM) observations were performed using aNikon Eclipse LV150 N. Scanning electron microscopy (SEM) imageswere taken on a JEOL JSM5800 instrument. Atomic force microscopy(AFM) analysis (Nanoscope, Bruker) of the P3HT/TPU blend films onthe Si substrates was performed using a tapping-mode probe tip (scansize 2 μm). X-ray diffraction (XRD) patterns were recorded with aRigaku D/Max-2200 diffractometer using CuKα (λ=0.154 nm) radia-tion. The mechanical properties of the P3HT/TPU blend films weremeasured using a dynamic mechanical analyzer (DMA) with a down-ward displacement ramp rate of 10 μmmin−1. The top and bottomareas of the films were clamped using film tension grips having a clampcompliance of approximately 0.1mN−1, and the load cell was 18 N. Anaverage value from six different samples was determined for eachblending ratio. The electrical properties of the buckled graphene/Aghybrid fibers were characterized using a two-probe method in anelectrical transport property measurement system (Keithley 2100multifunction source meter). The stretching test was conducted using ahome-made two-point stretching device and a high-precision mechan-ical system, where the speed was 2 cm s−1 and the frequency was60 Hz. Capacitance measurements were performed using a BiologicVSP-300 potentiostat at room temperature. Metal–ion gel–metal capa-citors were created, and the experiments were conducted over a fre-quency range of 1–104 Hz at an AC amplitude of 10mV. The curren-t–voltage characteristics of the OFETs and fiber transistor devices weremeasured at room temperature in a nitrogen atmosphere glove box

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using an MST-5500B probe station and Keithley 4200-SCS instrument.

3. Results and discussion

3.1. Preparation and characterization of buckled hybrid fiber electrode

As an electrode material for stretchable fiber-type electronic de-vices, graphene fibers hybridized with Ag nanoparticles (graphene/Aghybrid fibers) were prepared via the method previously reported by ourgroup [25]. First, the graphene fibers were fabricated by wet-spinningof giant graphene oxide (average lateral size ∼40 μm) (Fig. S1(a), (b)).The electrical conductivity of the graphene fibers was then enhanced bychemical reduction using an aqueous solution of hydrogen iodide andsubsequent hybridization with Ag nanoparticles. The resultant gra-phene/Ag hybrid fibers exhibited an extremely high electrical con-ductivity of up to 10,000 S cm−1 and a moderate tensile strength of200MPa. Although the electrical conductivity of the graphene/Ag hy-brid fibers was very stable after repeated bending tests (bending radiusof 2mm) over 1,000 cycles, the hybrid fiber could not withstand thestretching test owing to the very limited failure strain of the graphenelayer (ca. 6%) [26]. When the graphene/Ag hybrid fiber was stretchedto a strain level of even ∼10%, the stacked structure of graphene sheetsin the fiber was damaged and did not recover; the resulting breakage ofthe electrical paths caused a significant increase in resistivity.

To overcome this limitation in the stretchability of the electrodefiber, we introduced a geometrical design with a zigzag buckledstructure into the originally rigid graphene fiber [27,28]. Fig. 1(a)schematically illustrates the fabrication process of the buckled gra-phene/Ag hybrid fiber on a flexible TPU monofilament (diameter∼1mm). Before the hybrid fiber was embedded, the TPU filament wasprestrained up to ∼70% and slightly dissolved by THF so that it im-pregnated the graphene/Ag hybrid fiber. After the fiber was embeddedin the prestrained TPU monofilament, the assembly was left for 10minfor full solidification. The hybrid fiber was then subjected to com-pression by slow release (0.5 cm s−1) and formed a kinked pattern inthe TPU filament.

Fig. 1(b)–(e) show the buckled structures of the graphene/Ag hybridfiber on the TPU monofilament. SEM clearly revealed that the buckledfiber was well aligned along the fiber direction and tightly combinedwith the substrate (TPU filament) after the prestrain was slowly re-leased (Fig. 1(c), (d)). From numerous prestrain/release experiments,we found that the maximum stretchability of our electrode fibers

without any apparent damage was ∼70%. As shown in Fig. 1(e), thebuckled hybrid fiber fabricated at 70% prestrain showed no significantfracture. When the prestrain level exceeded 70% or the release speedwas too fast (5 cm s−1), the resulting buckled fibers exhibited perma-nent breakage in the kinked region owing to severe bending (Fig. S2).Note that no binding polymer resin, such as PDMS [28], was used toattach the hybrid fiber to the TPU filament. Therefore, the upper por-tion of the buckled hybrid fiber could be exposed on the TPU filament,allowing direct contact with the upper layer (active channel material)and stable retention of electrical conductivity in the hybrid fiber.

To investigate the stretchability of the buckled hybrid fiber bothqualitatively and quantitatively, the morphological changes in thebuckled structure were carefully observed, and its electrical resistivitywas measured during stretching tests. When the buckled fiber/TPU fi-lament assembly was stretched back to the prestrain level (∼70%), thehybrid fiber was straightened again without any mechanical failure(Fig. S3). Beyond this strain level (∼80%), the electrode fiber brokeinto several pieces. Furthermore, the electrical resistivity of the hybridfiber changed very little, by only 4Ω cm, at the maximum strain level of∼70% (Fig. 2(a)). Note that the stretchability of the electrode fiber wasdetermined by the prestrain level; when it was restretched beyond theprestrain level, severe rupture occurred at the kinked site, dramaticallyincreasing the resistivity. Although the buckled electrode fiber is elec-trically connected, it has intrinsic failure areas in the kinked regions.Therefore, it is easily separated in the local buckled area when it isrestretched above the prestrain level. For example, the buckled hybridfiber fabricated at a 70% prestrain level was broken after ∼70% re-stretching, whereas the fiber fabricated at 50% prestrain was cut after∼50% restretching. Therefore, we could design and fabricate astretchable conductor with a specific stretchability by controlling theprestrain level of the buckled hybrid fiber.

The stability of the buckled hybrid fiber/TPU filament assembly(70% prestrain) was further examined by repetitive stretch/releasecycles up to a strain level of ∼55%, which is the maximum strain thathuman joints can generate upon stretching and contracting. Fig. 2(b)shows that the electrical resistivity remained stable up to 1,000 stretch/release cycles. Because there was no slippage between the buckled fiberand the TPU filament, the change of the graphene interlayer distancebetween stretch and release of the fiber was negligible, where the d-spacing distance values were 8.21 Å and 8.24 Å, respectively (Fig. S4).Therefore it can be concluded that highly stable electrical properties ofthe hybrid fiber could be obtained. The excellent morphological and

Fig. 1. Fabrication of buckled graphene/Ag hybrid fiber. (a) Schematic illustration of fabrication procedure for buckled graphene/Ag hybrid fiber on TPU filament.Photographs of (b) stretch/release fixture with (c) buckled graphene/Ag hybrid fiber on a TPU monofilament. (d) Low-magnification and (e) high-magnification SEMimages of the buckled graphene/Ag hybrid fiber.

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electrical stability of the buckled graphene/Ag hybrid fibers are criti-cally important for maintaining consistent performance when the fibersare used as stretchable electrodes in wearable electronic devices.

3.2. Preparation and characterization of stretchable semiconductingcomposites

To realize stretchable smart electronics, every component in theelectronic devices should be stretchable and maintain high-quality in-terfaces with the other components. There have been many attempts todevelop highly flexible and even stretchable semiconducting materialsby combining conventional semiconducting polymers with viscoelasticpolymers. For example, Cho group fabricated a flexible organic thin-film transistor based on P3HT/poly(methylmethacrylate) and P3HT/polystyrene blend films, which recorded field-effect mobilities com-parable to that obtained from the pristine P3HT film [29,30]. Lee et al.developed a stretchable polymer transistor based on P3HT/poly(e-ca-prolactone) composite fibers [31]. Very recently, many research groupshave also considered blending conjugated polymers with conventionalelastomers or other conjugated polymers to impart elasticity or enhancecharge transport [32–35]. They proved that, despite the presence of theinsulating polymer phase, the semiconducting polymer/insulatingpolymer blend can possess good semiconducting characteristics whenprepared under carefully controlled conditions. In this work, we se-lected the regioregular P3HT/TPU blend system as a stretchable semi-conducting active material because the addition of TPU was expected toprovide both stretchability and good interfacial bonding with the TPUfilament substrate.

To evaluate the electrical properties, P3HT/TPU blend films withdifferent P3HT contents were coated on a highly n-doped silicon waferwith an OTS-treated SiO2 layer (300 nm in thickness). As source anddrain (S/D) electrodes, 40-nm-thick Au was deposited on the SiO2/Siwafer by thermal evaporation using a patterned shadow mask(Fig. 3(a)). Fig. S5 shows the typical output (ID–VD, where ID is the draincurrent and VD is the drain voltage) and transfer (ID–VG, where VG is thegate voltage) characteristics of the resulting OFET based on the P3HT/TPU blend; the device parameters are summarized in Table 1. Here, theaverage value of the mobility (μ) was calculated from the slope of VG vs.|ID|1/2 transfer curves in the saturation regime obtained from more thanfive transistors using the following equation [36]:

= −I μ WL

C V V2

( )D i G T2

(1)

where W and L are the width and length of the channel, respectively,and Ci and VT correspond to the capacitance of the dielectric layer(11.5 nF cm−2 for SiO2) and the threshold voltage, respectively. As theP3HT/TPU ratio increased from 2/8 to 5/5 w/w, both the charge car-rier mobility and on/off current ratio (Ion/Ioff) increased, and thehighest average mobility of 1.46× 10−3 cm2 V−1 s−1 (maximum

mobility of 1.56×10−3 cm2 V−1 s−1) and Ion/Ioff=5.83× 103 wererecorded in the device with a 5/5 w/w blend ratio (Fig. 3(c) andTable 1). Furthermore, the thin-film transistor with the highest P3HTcontent (5/5 w/w blend ratio) in the blend film showed the most stableperformance with the smallest variation (Fig. S5), presumably becausethe P3HT domain formed an interconnected network structure withoutany isolated phases in the entire area when the P3HT/TPU ratioreached 5/5 w/w (Fig. S6). The nanoscale morphologies of the blendfilms were investigated using AFM, as shown in Fig. S7. The phaseimage of the blend film with a 5/5 w/w blend ratio showed continuousnanofibril features despite the presence of 50% TPU. In contrast, theblend film with a 1/9 w/w blend ratio exhibited an isolated P3HT phaseowing to the much larger proportion of TPU. In many works on theP3HT blend system, the P3HT:elastomer composites reported were allbased on P3HT nanofibers that were pre-grown, prior to blend and filmdeposition [32,33]. Therefore, such blend systems could exhibit thecharge transport despite the very low portion of P3HT. However, ourapproach is totally different from such methods to use pre-grown P3HTnanocrystals because P3HT and TPU are directly blended and casted.Further investigation is underway to elucidate the mechanism of fi-brillar structure formation and charge transport in P3HT in the blendfilms.

After obtaining the desired P3HT/TPU blend film with satisfactorysemiconducting properties, we focused on the mechanical properties, inparticular the stretchability of the blend film, by performing tensiletests using a DMA. The mechanical properties of the blend films werealso influenced by the blending ratio, as shown in Fig. 3(d). As theP3HT/TPU ratio increased, both the Young's modulus and tensilestrength increased; however, excessive loading of P3HT in the blendfilm led to a significant decrease in elasticity owing to the brittleness ofP3HT (Table 2). Although the film with a 5/5 w/w blend ratio showedthe lowest limit strain among the blend films, it could endure a sub-stantial tensile strain of up to ∼120% before fracture, which is suffi-cient for our stretchable devices, because the buckled hybrid fiberelectrodes can be stretched only up to ∼70% strain. Owing to the su-perior electrical performance and adequate stretchability of the P3HT/TPU blend at a 5/5 w/w blending ratio, this blend film was selected as astretchable semiconducting layer for preparation of the stretchable fibertransistors.

3.3. Preparation and characterization of stretchable fiber transistors

We prepared the stretchable fiber transistors using the buckledhybrid fibers and the P3HT/TPU blend film as S/D electrodes and asolution-processable p-type semiconductor, respectively. The deviceswere fabricated in a bottom-contact top-gate architecture, as schema-tically depicted in Fig. 4(a). First, two buckled hybrid fibers werepartially embedded parallel to each other on the stretchable TPUmonofilament to expose their upper portions on the TPU filament and

Fig. 2. Electrical resistivity of the buckled graphene/Ag hybrid fiber on TPU filament as a function of (a)applied strain and (b) number of repetitive stretch/release cycles up to a stretching level of ∼55%. Theinsets are OM images of the stretched buckled fibers.The scale bars are 500 μm. The symbols and errorbars in (a) and (b) represent the average values andstandard deviations, respectively, of 10 samples.

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allow direct contact with an upper layer. Second, the P3HT/TPU blendfilm was transferred, along with an elastic ion-gel layer (forming asemiconductor/ion-gel double layer). Using our previously reportedmethod [25], we selected an elastic ion-gel layer P(VDF-HFP) and theionic liquid [EMI][TFSA] as both the high-capacitance gate dielectriclayer and mechanically robust transporter for the channel material[37]. The specific capacitance of the ion-gel dielectric layer is as largeas 9 μF cm−2 at low frequency (corresponding to a thickness of 11 μm),as shown in Fig. S8. Finally, the fabrication of fiber transistors wascompleted by simply laying a stretchable and conductive PDMS–EGaIn(75% Ga, 25% In) composite on the ion-gel layer as the top gate elec-trode [38]. The PDMS/EGaIn composite ensures high electrical con-ductivity with minimal resistivity change at high tensile strain(∼100%). Fig. 4(b) shows OM images of the OFET developed in thiswork, where the buckled-hybrid-fiber-based S/D electrodes were wellaligned along the TPU filament direction. The OM images confirm thatthe P3HT/TPU blend channel and ion-gel layers covered the TPU fila-ment. The channel length, defined as the average distance between twoaligned buckled S/D fiber electrodes, was approximately 300 μm, andthe channel width, defined as the width of the transferred P3HT/TPUblend layer, was 2mm. To measure the electrical characteristics of theprepared fiber transistors, Ag paste was applied to the ends of the S/D

electrodes to obtain stable contact with the probe tips (Fig. 5(a), (b)).Fig. 5(c) shows the typical output characteristics of the stretchable

fiber transistors at different VG. Saturation of the drain current wasobserved at low gate and drain bias (e.g., |ID| > 0.20mA atVG=−4 V), indicating huge modulation of charge carriers by the ion-gel dielectric because of the large capacitance and highly efficientcoupling of the ion-gel dielectric [39–41]. Typical transfer curves on alogarithmic scale (left axis) and a linear scale (right axis) are providedin Fig. 5(d). From the slope of the VG vs. |ID| curves obtained for morethan five devices, the average charge carrier mobility was estimated inthe linear regime (VD=−1 V) using the following equation [36]:

Fig. 3. (a) Device configuration and OM image of thin-film transistor based on P3HT/TPU blend film. (b) Photographs showing uniaxial tensile tests of the P3HT/TPU blend film using a DMA with a downward displacement ramp rate of 10 μmmin−1. (c) Charge carrier mobility (VG=40 to−100 V, VD=−100 V) as a functionof P3HT content. The symbols and error bars in (c) represent the average values and standard deviations, respectively, of five devices. (d) Typical strain–stress curvesof P3HT/TPU blend films with blending ratios of 5/5, 4/6, and 3/7, obtained using a DMA.

Table 1Charge carrier mobility, on/off current ratio, and threshold voltage of thin-film transistors based on P3HT/TPU blends with different blending ratios. The deviceswere fabricated in a top-contact bottom-gate configuration on a highly n-doped Si wafer with an OTS-treated SiO2 dielectric (thickness 300 nm); the channel length is50 μm, and the channel width is 1,000 μm aAverage mobility value obtained from more than five transistor devices.

P3HT/TPU ratio Average mobilitya [cm2 V−1 s−1] Max. mobility [cm2 V−1 s−1] Ion [A] Ioff [A] Ion/Ioff VT [V]

1/9 Cannot be measured2/8 9.70× 10−6 3.42× 10−5 −6.21×10−9 −3.66×10−11 1.70× 102 −25.463/7 4.53× 10−4 6.20× 10−4 −3.81×10−7 −9.96×10−11 3.83× 103 −4.414/6 7.69× 10−4 8.87× 10−4 −4.70×10−7 −8.49×10−11 5.53× 103 −9.835/5 1.46× 10−3 1.56× 10−3 −7.49×10−7 −1.64×10−10 5.83× 103 −4.37

Table 2Mechanical properties of P3HT/TPU blend films with different P3HT/TPU ra-tios obtained using a DMA.

P3HT/TPUratio

Tensile strength[MPa]

Young's modulus[MPa]

Elongation at break[%]

3/7 4.7 ± 0.2 14.4 ± 0.5 220 ± 154/6 12.8 ± 0.6 67.7 ± 2.5 164 ± 115/5 20.6 ± 0.9 127.2 ± 3.8 121 ± 7

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= −I μWL

C V V V( )D i D G T (2)

The average charge carrier mobility was calculated to be1.74 cm2 V−1 s−1 (maximum mobility of 1.92 cm2 V−1 s−1), which ismuch higher than those reported in other P3HT-based transistors gatedwith conventional dielectrics (0.1–0.01 cm2 V−1 s−1) [42,43], butcomparable to other recent results for ion-gel electrolyte-gated polymertransistors [31], owing to penetration of ions from the ion-gel dielectricinto the active channel, which fill the carrier traps and act as a dopant

in the P3HT film [44,45]. Furthermore, all the devices showed a rea-sonably high Ion/Ioff of ∼104 and low VT values of approximately −1.5to 2.0 V. As shown in Fig. S9, our fiber transistor showed negligiblehysteresis during operation in a nitrogen atmosphere. Furthermore, theleakage current was 1.5 μA at VG= 4 V and was about three orders ofmagnitude smaller than the channel current (Fig. S9), which guaran-teed that the device performance was not affected by the leakage. Theseresults confirm that the buckled hybrid fibers and P3HT/TPU blend filmworked well as highly conductive S/D electrodes and a semiconductingactive material, respectively, and all the components were well

Fig. 4. (a) Schematic illustration and (b) OM images of fiber transistor fabrication. The scale bars are 1,000 μm.

Fig. 5. (a) Schematic illustration and (b) OM image of the fiber transistor based on buckled hybrid fiber electrodes. The scale bar is 1,000 μm. (c) ID–VD and (d) ID–VG

characteristics of the fiber transistors.

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assembled with high-quality interfaces to form high-performance fibertransistors.

To focus on the effect of the ion-gel dielectric on the mobility of theOFETs, we fabricated ion-gel-gated OFETs based on a pure P3HT orP3HT/TPU (5/5 w/w) channel layer with a Si gate electrode and Au S/D electrodes. The device configuration and photographs of the ion-gel-gated OFETs are shown in Fig. S10. The output and transfer curves (Fig.S11) and electrical characteristics (Table S1) of the OFET devices basedon pure P3HT and P3HT/TPU (5/5 w/w) channel layers with the ion-gel (gate dielectric), Si (gate electrode), and Au (S/D electrodes) ex-hibited electrical characteristics similar to those of the fiber transistorswith the ion-gel dielectric, confirming that the mobility of the OFETsbased on the ion-gel dielectric can be much higher than that of the SiO2

gate dielectric device with the same channel layer, in agreement withprevious reports [46,47].

To investigate the stretchability of the fiber transistors devices,uniaxial tensile tests were performed up to ∼50% strain, as shown inthe OM images in Fig. S12. Because all the components of the device,including the buckled hybrid fiber electrodes, P3HT/TPU blendchannel, ion-gel dielectric layer, and PDMS/EGaIn composite gate,were stretchable and mechanically stable, on average more than 90% ofthe fiber transistors showed stable device performance and no visiblepeeling or mechanical failure were observed up to ∼50% strain.Fig. 6(a) shows the transfer curves as a function of strain. The transfercurves of the fiber transistor showed negligible change at all appliedstrains, and the device performance indicators (μ, Ion/Ioff, and VT) cal-culated from the transfer curves were similar to those of the initialdevices during the stretching test and after release (Table 3). Althoughthe average charge carrier mobility decreased slightly from 1.74 to1.60 cm2 V−1 s−1 at ∼50% strain (the mobility values were calculatedby ignoring the change in the channel dimensions), it recovered to theinitial value after the strain was released (Fig. 6(b)). On the other hand,the fiber transistor fabricated with a pure P3HT channel showed very

unstable device performance after both stretching and release (Fig.S13), which is in agreement with the findings of Chortos et al. [48],who reported that microscale cracking of P3HT transferred to a PUsubstrate started at less than ∼15% strain. From these results, weconfirmed that the stretchability of each component plays a critical rolein the stable performance of the assembled transistors at high strain.

The durability under repeated use represents the robustness offlexible devices to long-term stretch/release cycles in terms of stabledevice performance and mechanical integrity. Because robust deviceperformance is critical for practical realization of wearable electronicdevices, we also measured the changes in the transfer curves andelectrical characteristics after the cyclic stretching test. Fig. 6(c) showsthe transfer curves after 1, 10, 100, and 1,000 cycles of stretching at∼50% strain, from which the electrical characteristics were calculated(Fig. 6(d) and Table 3). The transfer curve changed slightly and theaverage device mobility steadily decreased to 1.39 cm2 V−1 s−1 during

Fig. 6. (a) Transfer curves and (b) changes inthe charge carrier mobility (μ) and thresholdvoltage (VT) of the device as a function of strain.(c) Transfer curves and (d) changes in μ and VT

of the device obtained after cyclic stretchingtests at ∼50% strain. The symbols and errorbars in (b) and (d) represent the average valuesand standard deviations, respectively, of fivedevices.

Table 3Charge carrier mobility, on/off current ratio, and threshold voltage of fibertransistors as a function of strain and stretching cycle. The devices were fab-ricated in a bottom-contact top-gate architecture with an 11-μm-thick ion-gel-based dielectric layer; the channel length is 300 μm, and the channel width is2mm.

Strain Cycles Average mobilitya

[cm2 V−1 s−1]Max. mobility[cm2 V−1 s−1]

Ion/Ioff VT [V]

0% – 1.74 1.92 9.8× 103 −1.8530% – 1.62 1.83 8.5× 103 −1.8050% – 1.60 1.81 6.0× 103 −1.75

Released 1 1.73 1.91 8.8× 103 −1.84– 10 1.52 1.71 6.0× 103 −1.89– 100 1.39 1.60 5.7× 103 −1.88– 1,000 1.36 1.59 5.2× 103 1.88

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100 cycles of stretching. After 100 cycles, however, the device perfor-mance stabilized, and the hole mobility remained unchanged until thenumber of stretching cycles reached 1,000. Such an electrical perfor-mance degradation before 100 cycles may be caused by the slight po-sition change of the buckled graphene hybrid fibers. Also, initiallyunstable surface morphologies of other components in the device maybecome stabilized after a certain number of cycles, which were com-monly observed in several stretchable devices [19,22–24]. Once theirpositions and surface roughness are stabilized without any furtherchange after large number of cycles, the device performance becomesstabilized. Therefore, we may consider the first 100 cycles as a deviceaging stage. The small increase in VT may be related to contact effects.As the fiber transistor device was stretched, the contact resistance in-creased with increasing strain, which could induce a negative shift of VT

[49]. The stretchability and durability of the fiber transistor could beexplained as follows: all the components of the device were flexible andstretchable, and they maintained high-quality interfaces during me-chanical deformation. We already confirmed that the buckled hybridfiber electrodes and P3HT/TPU blend channel could be stretched up to∼70% and ∼120% strain, respectively, without electrical loss or me-chanical failure. Furthermore, both the active channel layer and fila-ment substrate are composed of elastic TPU. The similarity in elasticityand flexibility enables synchronized deformation of the substrate andchannel layer. This avoids exfoliation of each layer from the TPU sub-strate and contributes to the excellent stability of the device perfor-mance during many stretch/release cycles.

Another interesting discovery is that the device performance re-mained nearly unchanged when the device was left to stand in air for 10days without a passivation (Fig. S14). Many research groups reportedthat the off-state current of polythiophene-based transistors typicallyincreases with exposure to the ambient because of contamination byoxygen [50,51]. The outstanding stability of our fiber devices underenvironmental conditions is attributed to the capping effect of the ion-gel dielectric layer on the channel layer and the encapsulating effect ofthe TPU matrix [52,53]. To the best of our knowledge, the stretchabilityand stability of our fiber transistor with all-stretchable electroniccomponents are superior to those of other published fiber transistors.Table S2 compares our device with other reported devices.

4. Conclusions

In summary, we demonstrated a route to fabricate highly stretch-able fiber transistors by combining all-stretchable electronic compo-nents. We used a TPU monofilament as the substrate, buckled gra-phene/Ag hybrid fibers as the S/D electrodes, a P3HT/TPU polymerblend as the active material, an ion-gel polyelectrolyte as the dielectriclayer, and a liquid metal/polymer composite as the gate electrode. Inparticular, the buckled graphene/Ag hybrid fiber electrode maintaineda low resistivity (∼30Ω cm) at a high stretching strain (∼70%) andunder many stretch/release cycles (∼1,000). The P3HT/TPU activelayer possessed reasonably high mobility (1.46× 10−3 cm2 V−1 s−1)and adequate mechanical strength (21MPa) with a high elongationstrain (∼120%), which is a new stretchable blending system by justblending and casting the two components without using the pre-grownP3HT nanoscrystals. Finally, the assembled fiber transistor exhibitedexcellent stretchability (strain level of ∼50%) while maintaining goodelectrical properties (average hole mobility of 1.74 cm2 V−1 s−1, on/offcurrent ratio of 104) and showed outstanding electrical stability up to1,000 cycles of stretch/release tests, which is superior to those of otherpublished fiber transistors. These results are attributed to the stretch-ability of all the components in the device and the high-quality inter-faces between them. We believe that our work can serve as an im-portant step toward the development of core components in wearabledevices, such as wearable displays, computers, and biomedical sensors.

Author contribution

Wonoh Lee: Investigation, Writing-Original Draft.Youn Kim: Investigation, Visualization.Moo Yeol Lee: Investigation, Visualization.Joon Hak Oh: Investigation, Writing-Review&Editing.Jea Uk Lee: Conceptualization, Investigation, Writing-Original

Draft.

Acknowledgements

This work was supported by the Principal Research Program(KK1801-G01) in the Korea Research Institute of Chemical Technology.This work was also supported by the National Research Foundation ofKorea grant funded by the Ministry of Science, ICT and Future Planning(2018R1A2A2A15020973 and 2016M3A7B4021149).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.orgel.2019.03.056.

References

[1] C. Wu, T.W. Kim, T. Guo, F. Li, Wearable ultra-lightweight solar textiles based ontransparent electronic fabrics, Nanomater. Energy 32 (2017) 367–373.

[2] A.I.S. Neves, T.H. Bointon, L.V. Melo, S. Russo, I. de Schrijver, M.F. Craciun,H. Alves, Transparent conductive graphene textile fibers, Sci. Rep. 5 (2015) 09866.

[3] Y.J. Yun, W.G. Hong, W.J. Kim, Y. Jun, B.H. Kim, A novel method for applyingreduced graphene oxide directly to electronic textiles from yarns to fabrics, Adv.Mater. 25 (2013) 5701–5705.

[4] T. Yamada, Y. Hayamizu, Y. Yamamoto, Y. Yomogida, A. Izadi-Najafabadi,D.N. Futaba, K. Hata, Stretchable carbon nanotube strain sensor for human-motiondetection, Nat. Nanotechnol. 6 (2011) 296–301.

[5] M. Park, J. Im, J.J. Park, U. Jeong, Micropatterned stretchable circuit and strainsensor fabricated by lithography on an electrospun nanofiber mat, ACS Appl. Mater.Interfaces 5 (2013) 8766–8771.

[6] N. Matsuhisa, M. Kaltenbrunner, T. Yokota, H. Jinno, K. Kuribara, T. Sekitani,T. Someya, Printable elastic conductors with a high conductivity for electronictextile applications, Nat. Commun. 6 (2015) 7461.

[7] M. Park, J. Im, M. Shin, Y. Min, J. Park, H. Cho, S. Park, M.–B. Shim, S. Jeon,D.–Y. Chung, J. Bae, J. Park, U. Jeong, K. Kim, Highly stretchable electric circuitsfrom a composite material of silver nanoparticles and elastomeric fibres, Nat.Nanotechnol. 7 (2012) 803–809.

[8] M. Alshrah, H.E. Naguib, C.B. Park, Reinforced resorcinol formaldehyde aerogelwith Co-assembled polyacrylonitrile nanofibers and graphene oxide nanosheets,Mater. Des. 151 (2018) 154–163.

[9] Y. Ma, D. Bai, X. Hu, N. Ren, W. Gao, S. Chen, H. Chen, Y. Lu, J. Li, Y. Bai, Robustand antibacterial polymer/mechanically exfoliated graphene nanocomposite fibersfor biomedical applications, ACS Appl. Mater. Interfaces 10 (2018) 3002–3010.

[10] K.-Y. Chun, Y. Oh, J. Rho, J.–H. Ahn, Y.–J. Kim, H.R. Choi, S. Baik, Highly con-ductive, printable and stretchable composite films of carbon nanotubes and silver,Nat. Nanotechnol. 5 (2010) 853–857.

[11] Z. Xu, Z. Liu, H. Sun, C. Gao, Highly electrically conductive Ag-doped graphenefibers as stretchable conductors, Adv. Mater. 25 (2013) 3249–3253.

[12] R. Ma, J. Lee, D. Choi, H. Moon, S. Baik, Knitted fabrics made from highly con-ductive stretchable fibers, Nano Lett. 14 (2014) 1944–1951.

[13] B. Pant, M. Park, R.S. Jang, W.C. Choi, H.Y. Kim, S.J. Park, Synthesis, character-ization, and antibacterial performance of Ag-modified graphene oxide reinforcedelectrospun polyurethane nanofibers, Carbon Lett 23 (2017) 17–21.

[14] X. Du, I. Skachko, A. Barker, E.Y. Andrei, Approaching ballistic transport in sus-pended graphene, Nat. Nanotechnol. 3 (2008) 491–495.

[15] J.W. Lee, J.U. Lee, J.W. Jo, S. Bae, K.T. Kim, W.H. Jo, In-situ preparation of gra-phene/poly(styrenesulfonic acid-graft-polyaniline) nanocomposite via direct ex-foliation of graphite for supercapacitor application, Carbon 105 (2016) 191–198.

[16] Z. Xu, H. Sun, X. Zhao, C. Gao, Ultrastrong fibers assembled from giant grapheneoxide sheets, Adv. Mater. 25 (2013) 188–193.

[17] Y. Meng, Y. Zhao, C. Hu, H. Cheng, Y. Hu, Z. Zhang, G. Shi, L. Qu, All-graphenecore-sheath microfibers for all-solid-state, stretchable fibriform supercapacitors andwearable electronic textiles, Adv. Mater. 25 (2013) 2326–2331.

[18] J. Sun, Y. Li, Q. Peng, S. Hou, D. Zou, Y. Shang, Y. Li, P. Li, Q. Du, Z. Wang, Y. Xia,L. Xia, X. Li, A. Cao, Macroscopic, flexible, high-performance graphene ribbons,ACS Nano 7 (2013) 10225–10232.

[19] Z. Yang, J. Deng, X. Sun, H. Li, H. Peng, Stretchable, wearable dye-sensitized solarcells, Adv. Mater. 26 (2014) 2643–2647.

[20] J.J. Park, W.J. Hyun, S.C. Mun, Y.T. Park, O.O. Park, Highly stretchable andwearable graphene strain sensors with controllable sensitivity for human motionmonitoring, ACS Appl. Mater. Interfaces 7 (2015) 6317–6324.

W. Lee, et al. Organic Electronics 69 (2019) 320–328

327

[21] X. Wang, Y. Qiu, W. Cao, P.A. Hu, Highly stretchable and conductive core−sheathchemical vapor deposition graphene fibers and their applications in safe strainsensors, Chem. Mater. 27 (2015) 6969–6975.

[22] Y. Ding, J. Yang, C.R. Tolle, Z. Zhu, A highly stretchable strain sensor based onelectrospun carbon nanofibers for human motion monitoring, RSC Adv. 6 (2016)79114–79120.

[23] M. Shin, J.H. Song, G.–H. Lim, B. Lim, J.-J. Park, U. Jeong, Highly stretchablepolymer transistors consisting entirely of stretchable device components, Adv.Mater. 26 (2014) 3706–3711.

[24] J.S. Heo, J. Eom, Y.-H. Kim, S.K. Park, Recent progress of textile-based wearableelectronics: a comprehensive review of materials, devices, and applications, Small14 (2018) 1703034.

[25] S.S. Yoon, K.E. Lee, H.J. Cha, D.G. Seong, M.K. Um, J.H. Byun, Y. Oh, J.H. Oh,W. Lee, J.U. Lee, Highly conductive graphene/Ag hybrid fibers for flexible fiber-type transistors, Sci. Rep. 5 (2015) 16366.

[26] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, J.H. Ahn, P. Kim, J.Y. Choi,B.H. Hong, Large-scale pattern growth of graphene films for stretchable transparentelectrodes, Nature 457 (2009) 706–710.

[27] Y. Wang, R. Yang, Z.W. Shi, L.C. Zhang, D.X. Shi, E. Wang, G.Y. Zhang, Super-elasticgraphene ripples for flexible strain sensors, ACS Nano 5 (2011) 3645–3650.

[28] M. Zu, Q. Li, G. Wang, J.–H. Byun, T.–W. Chou, Carbon nanotube fiber basedstretchable conductor, Adv. Funct. Mater. 23 (2013) 789–793.

[29] L. Qiu, X. Wang, W.H. Lee, J.A. Lim, J.S. Kim, D. Kwak, K. Cho, Organic thin-filmtransistors based on blends of poly(3-hexylthiophene) and polystyrene with a so-lubility-induced low percolation threshold, Chem. Mater. 21 (2009) 4380–4386.

[30] X. Wang, W.H. Lee, G. Zhang, X. Wang, B. Kang, H. Lu, L. Qiu, K. Cho, Self-stratifiedsemiconductor/dielectric polymer blends: vertical phase separation for facile fab-rication of organic transistors, J. Mater. Chem. C 1 (2013) 3989–3998.

[31] S.W. Lee, H.J. Lee, J.H. Choi, W.G. Koh, J.M. Myoung, J.H. Hur, J.J. Park, J.H. Cho,U. Jeong, Periodic array of polyelectrolyte-gated organic transistors from electro-spun poly(3-hexylthiophene) nanofibers, Nano Lett. 10 (2010) 347–351.

[32] G.-J.N. Wang, A. Gasperini, Z. Bao, Z., Stretchable polymer semiconductors forplastic electronics, Adv. Electron. Mater. 4 (2018) 1700429.

[33] D. Choi, H. Kim, N. Persson, P.-H. Chu, M. Chang, J.-H. Kang, S. Graham,E. Reichmanis, Elastomer−polymer semiconductor blends for high-performancestretchable charge transport networks, Chem. Mater. 28 (2016) 1196–1204.

[34] J. Xu, S. Wang, G.-J.N. Wang, C. Zhu, S. Luo, L. Jin, X. Gu, S. Chen, V.R. Feig,J.W.F. To, S. Rondeau-Gagné, J. Park, B.C. Schroeder, C. Lu, J.Y. Oh, Y. Wang,Y. Kim, H. Yan, R. Sinclair, D. Zhou, G. Xue, B. Murmann, C. Linder, W. Cai, J.B.-H. Tok, J.W. Chung, Z. Bao, Highly stretchable polymer semiconductor filmsthrough the nanoconfinement effect, Science 355 (2017) 59–64.

[35] S. Savagatrup, A.D. Printz, D. Rodriquez, D.J. Lipomi, Best of both worlds: con-jugated polymers exhibiting good photovoltaic behavior and high tensile elasticity,Macromolecules 47 (2014) 1981–1992.

[36] N. Arora, MOSFET Models for VLSI Circuit Simulation Theory and Practice,Springer, New York, 1993.

[37] K.H. Lee, M.S. Kang, S. Zhang, Y. Gu, T.P. Lodge, C.D. Frisbie, “Cut and Stick”rubbery ion gels as high capacitance gate dielectrics, Adv. Mater. 24 (2012)4457–4462.

[38] A. Fassler, C. Majidi, Liquid-phase metal inclusions for a conductive polymer

composite, Adv. Mater. 27 (2015) 1928–1932.[39] J.H. Cho, J. Lee, Y. Xia, B.S. Kim, Y. He, M.J. Renn, T.P. Lodge, C.D. Frisbie,

Printable ion-gel gate dielectrics for low-voltage polymer thin-film transistors onplastic, Nat. Mater. 7 (2008) 900–906.

[40] L.–a. Kong, J. Sun, C. Qian, G. Gou, Y. He, J. Yang, Y. Gao, Ion-gel gated field-effecttransistors with solution-processed oxide semiconductors for bioinspired artificialsynapses, Org. Electron. 39 (2016) 64–70.

[41] L.-a. Kong, J. Sun, C. Qian, C. Wang, J. Yang, Y. Gao, Spatially-correlated neurontransistors with ion-gel gating for brain-inspired applications, Org. Electron. 44(2017) 25–31.

[42] B.S. Ong, Y. Wu, P. Liu, S. Gardner, High-performance semiconducting poly-thiophenes for organic thin-film transistors, J. Am. Chem. Soc. 126 (2004)3378–3379.

[43] Y.J. Song, J.U. Lee, W.H. Jo, Multi-walled carbon nanotubes covalently attachedwith poly(3-hexylthiophene) for enhancement of field-effect mobility of poly(3-hexylthiophene)/multi-walled carbon nanotube composites, Carbon 48 (2010)389–395.

[44] Y. Xia, J.H. Cho, J. Lee, P.P. Ruden, C.D. Frisbie, Comparison of the mobility–carrierdensity relation in polymer and single-crystal organic transistors employing vacuumand liquid gate dielectrics, Adv. Mater. 21 (2009) 2174–2179.

[45] K.H. Lee, S. Zhang, Y. Gu, T.P. Lodge, C.D. Frisbie, Transfer printing of thermo-reversible ion gels for flexible electronics, ACS Appl. Mater. Interfaces 5 (2013)9522–9527.

[46] S.-Y. Min, T.-S. Kim, B.J. Kim, H. Cho, Y.-Y. Noh, H. Yang, J.H. Cho, T.-W. Lee,Large-scale organic nanowire lithography and electronics, Nat. Commun. 4 (2013)1773.

[47] J. Lee, L.G. Kaake, J.H. Cho, X.-Y. Zhu, T.P. Lodge, C.D. Frisbie, Ion gel-gatedpolymer thin-film transistors: operating mechanism and characterization of gatedielectric capacitance, switching speed, and stability, J. Phys. Chem. C 113 (2009)8972–8981.

[48] A. Chortos, J. Lim, J.W.F. To, M. Vosgueritchian, T.J. Dusseault, T.-H. Kim,S. Hwang, Z. Bao, Highly stretchable transistors using a microcracked organicsemiconductor, Adv. Mater. 26 (2014) 4253–4259.

[49] A. Chortos, G.I. Koleilat, R. Pfattner, D. Kong, P. Lin, R. Nur, T. Lei, H. Wang, N. Liu,Y.–C. Lai, M.-G. Kim, J.W. Chung, S. Lee, Z. Bao, Mechanically durable and highlystretchable transistors employing carbon nanotube semiconductor and electrodes,Adv. Mater. 28 (2016) 4441–4448.

[50] M.S.A. Abdou, F.P. Orfino, Y. Son, S. Holdcroft, Interaction of oxygen with con-jugated polymers: charge transfer complex formation with Poly(3-alkylthiophenes),J. Am. Chem. Soc. 119 (1997) 4518–4524.

[51] M.L. Chabinyc, R.A. Street, J.E. Northrup, Effects of molecular oxygen and ozone onpolythiophene-based thin-film transistors, Appl. Phys. Lett. 90 (2007) 123508.

[52] H.C. Avila, P. Serrano, A.R.J. Barreto, Z. Ahmed, C. de P. Gouvêa, C. Vilanic,R.B. Capaz, C.F.N. Marchiori, M. Cremona, High hole-mobility of rrP3HT in organicfield-effect transistors using low polarity polyurethane gate dielectric, Org.Electron. 58 (2018) 33–37.

[53] S. Lee, H. Jeon, M. Jang, K.-Y. Baek, H. Yang, Tunable solubility parameter of poly(3-hexyl thiophene) with hydrophobic side-chains to achieve rubbery conjugatedfilms, ACS Appl. Mater. Interfaces 7 (2015) 1290–1297.

W. Lee, et al. Organic Electronics 69 (2019) 320–328

328