Flexible Textile-Based Organic Transistors Using Graphene/Ag ... - MDPI

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Flexible Textile-Based Organic Transistors Using Graphene/Ag Nanoparticle Electrode Youn Kim 1,† , Yeon Ju Kwon 1,† , Kang Eun Lee 2 , Youngseok Oh 2 , Moon-Kwang Um 2 , Dong Gi Seong 2, * and Jea Uk Lee 1, * 1 2

* †

C-Industry Incubation Research Center, Korea Research Institute of Chemical Technology (KRICT), Daejeon 34114, Korea; [email protected] (Y.K.); [email protected] (Y.J.K.) Composites Research Division, Korea Institute of Materials Science (KIMS), Changwon 51508, Korea; [email protected] (K.E.L.); [email protected] (Y.O.); [email protected] (M.-K.U.) Correspondence: [email protected] (D.G.S); [email protected] (J.U.L.); Tel.: +82-55-280-3266 (D.G.S); +82-42-860-7392 (J.U.L.) These authors are contributed equally.

Academic Editor: Thomas Nann Received: 1 June 2016; Accepted: 29 July 2016; Published: 16 August 2016

Abstract: Highly flexible and electrically-conductive multifunctional textiles are desirable for use in wearable electronic applications. In this study, we fabricated multifunctional textile composites by vacuum filtration and wet-transfer of graphene oxide films on a flexible polyethylene terephthalate (PET) textile in association with embedding Ag nanoparticles (AgNPs) to improve the electrical conductivity. A flexible organic transistor can be developed by direct transfer of a dielectric/semiconducting double layer on the graphene/AgNP textile composite, where the textile composite was used as both flexible substrate and conductive gate electrode. The thermal treatment of a textile-based transistor enhanced the electrical performance (mobility = 7.2 cm2 ·V−1 ·s−1 , on/off current ratio = 4 × 105 , and threshold voltage = −1.1 V) due to the improvement of interfacial properties between the conductive textile electrode and the ion-gel dielectric layer. Furthermore, the textile transistors exhibited highly stable device performance under extended bending conditions (with a bending radius down to 3 mm and repeated tests over 1000 cycles). We believe that our simple methods for the fabrication of graphene/AgNP textile composite for use in textile-type transistors can potentially be applied to the development of flexible large-area electronic clothes. Keywords: e-textile; graphene oxide; textile composite; textile transistor

1. Introduction Electrical textile (e-textile) has been considered as an ideal platform for flexible and wearable electronic devices on account of its light weight, flexibility, and comfort [1]. To date, many research groups have shown the feasibility of e-textile structures in various applications in organic field-effect transistors (OFETs) [2], organic photovoltaics (OPVs) [3], rechargeable batteries [4], and organic light-emitting diodes (OLEDs) [5]. From these studies, it has been determined that developing electronic materials with textile form and a multilayered assembly are key requirements for the realization of e-textile-based clothes. In previous reports [6], electrolyte-gated textile transistors were demonstrated by using Au microwire as gate electrode. However, these devices showed poor flexibility and limitation for mass production, owing to the use of expensive Au wire electrode and complex micro-fabrication processes. Very recently, Oh et al. reported the fabrication of highly flexible photosensors on a commercially available textile substrate using photoresponsive semiconducting polymer nanofibers as the photoactive layers [7]. They also fabricated large-area photosensor arrays on textile substrate,

Nanomaterials 2016, 6, 147; doi:10.3390/nano6080147

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Nanomaterials 2016, 6, 147

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as the photoactive layers [7]. They also fabricated large-area photosensor arrays on textile substrate, which could be very practical for use as wearable photo imaging or scanning devices. However, for which could be very practicalon forthe use as wearable photo imagingprocedures or scanning devices. However, forthe the the construction of devices textile substrate, additional are required, including construction of devices on the textile substrate, additional procedures are required, including the flattening of polyethylene terephthalate (PET) textile with a poly(dimethylsiloxane) (PDMS) layer flattening polyethylene terephthalate (PET) textile withona poly(dimethylsiloxane) layer and and gateof electrode deposition by thermal evaporation buffered PET textile. To(PDMS) make the textile gate electrode thermal evaporation onfor buffered PET textile.textile To make the textile devices devices moredeposition practical, aby facile fabrication process a multifunctional composite should be more practical, a facile fabrication process for a multifunctional textile composite should be developed. developed. InInthis conductive and and flexible flexible thiswork, work,we wereport reporton onaaquite quitesimple simplemethod method of of fabricating fabricating highly conductive textile forfor the the realization of wearable textile-based OFETs.OFETs. The textile prepared textilecomposites composites realization of wearable textile-based Thecomposites textile composites prepared vacuumand filtration and wet-transfer ofoxide graphene (GO) on textile, in by vacuumby filtration wet-transfer of graphene (GO)oxide on flexible PETflexible textile,PET in association association with Ag embedding Ag nanoparticles (AgNPs),very exhibited very lowresistance electrical resistance (4 with embedding nanoparticles (AgNPs), exhibited low electrical (4 Ω/γ) and Ω/γ) and outstanding 7.5%of increase of resistance 1000 of bending A outstanding flexibility flexibility (only 7.5%(only increase resistance after 1000after cycles ofcycles bending test). A test). flexible flexible organic transistor can be developed by direct transfer of a dielectric/semiconducting double organic transistor can be developed by direct transfer of a dielectric/semiconducting double layer on layer on the graphene/AgNP textile composite, textile composite usedflexible as bothsubstrate flexible the graphene/AgNP textile composite, where thewhere textilethe composite was usedwas as both substrate and gate conductive gate The electrode. The textile-based transistor high electrical and conductive electrode. textile-based transistor showed highshowed electrical performance 2·V−1·s−1, on/off current ratio =54 × 105, and threshold voltage = −1.1 V) −1 ,cm performance (mobility = 7.2(mobility cm2 ·V−1=·s7.2 on/off current ratio = 4 × 10 , and threshold voltage = −1.1 V) and and stable performance cycles of bending. We believe simple methods stable devicedevice performance after after 1000 1000 cycles of bending. We believe thatthat our our simple methods for for the the fabrication of graphene/AgNP textile composite for use in textile-type transistors can potentially fabrication of graphene/AgNP textile composite for use in textile-type transistors can potentially be be applied to development the development of flexible large-area electronic clothes. applied to the of flexible large-area electronic clothes. Results 2.2.Results

2.1. 2.1.Prepratation PrepratationofofGraphene/Ag Graphene/AgNanoparticle NanoparticleElectrode Electrode on on PET PET Textile Textile Figure films on on aa flexible flexible textile textile Figure11shows showsthe theoverall overall process process for for the the fabrication fabrication of graphene films substrate methods. filtration method is is one of substrateusing usingvacuum vacuumfiltration filtrationand anddirect directtransfer transfer methods.The Thevacuum vacuum filtration method one of most the most efficient convenient techniques fabricatethin thingraphene grapheneoxide oxide films. films. GO films the efficient andand convenient techniques toto fabricate films were were transferredonto ontothe themono-filament mono-filament PET textiles, according method reported Jung’s group transferred PET textiles, according to to thethe method reported byby Jung’s group [8]. [8]. Commercially available PET textile (filament diameter ~40 µ m,size pore µ m) was utilized Commercially available PET textile (filament diameter ~40 µm, pore ~60size µm)~60 was utilized because its high flexibility (see Supplementary Information; Figure S1). ofbecause its highofflexibility (see Supplementary Information; Figure S1).

Figure illustrationofofthe thevacuum vacuum filtration subsequent transfer of graphene Figure1.1.Schematic Schematic illustration filtration andand subsequent transfer of graphene oxide oxide (GO) (GO) film on a flexible textile substrate. film on a flexible textile substrate.

Figure films on onanodic anodicaluminum aluminum oxide (AAO) membranes textiles. Figure22shows shows the the GO films oxide (AAO) membranes andand PETPET textiles. To Toprepare prepare the transparent and stable film, various solutions were vacuum-filtered through the transparent and stable GOGO film, various GOGO solutions were vacuum-filtered through the the AAO membrane varying the concentration GOsolutions. solutions.We Weused usedan anaqueous aqueoussolution solutionof of AAO membrane byby varying the concentration ofofGO NaOH NaOH solution solutionnot notonly only NaOHininorder ordertotosafely safelytransfer transferthe theGO GOfilms films to to the the PET PET textiles, textiles, since the NaOH removes films onto onto the the PET PET textiles textilesby by removesthe theAAO AAOmembrane membrane filter filter but but also also assists assists the fusion of GO films slightly bottom layer of of GO films (see Supplementary Information; Figure S2).S2). When the slightlydissolving dissolvingthe the bottom layer GO films (see Supplementary Information; Figure When thefilm GO was film too wasthin too (GO thin 0.2 (GOmg, 0.2thickness mg, thickness ~50 nm), we observed the film GO film easily GO ~50 nm), we observed that that the GO was was easily torn torn down the transfer andnot could notthe cover theofpores of the PETcompletely, textile completely, down duringduring the transfer processprocess and could cover pores the PET textile whereas whereas the film thick(GO GO2.0 film (GO 2.0 mg,~560 thickness ~560 nm) could not with the PET textile the thick GO mg, thickness nm) could not merge withmerge the PET textile because of because the lack ofThe flexibility. The thin0.6 film 0.6 (thickness mg of GO (thickness ~170 nm)good showed good the lack ofofflexibility. thin film with mgwith of GO ~170 nm) showed coverage, coverage,and flexibility, and transparency the PET textile. flexibility, transparency on the PET on textile.

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Figure 2. Photographs of GO films on anodic aluminum oxide (AAO) membranes with different Figure films on aluminum oxide membranes with Figure 2.2.ofPhotographs Photographs of GO microscope on anodic anodic aluminum oxide (AAO) membranes with different different amounts GO (top). Optical images of GO films on(AAO) polyethylene terephthalate (PET) amounts of GO (top). of GO films on polyethylene terephthalate (PET) amounts of GO (top). Optical microscope images on polyethylene terephthalate (PET) textiles (bottom). textiles (bottom). textiles (bottom).

To improve the electrical conductivities of the GO films on PET textiles for flexible electronic To improve improve the electrical conductivities of the the GO GO films films on on PET PET textiles for flexible flexiblewith electronic To conductivities of for electronic applications, thethe GOelectrical films (0.6 mg) were chemically reduced, and textiles then embedded silver applications, GO films (0.6 mg) were chemically reduced, and then embedded with silver applications, the GO films (0.6 mg) were chemically reduced, and then embedded with silver nanoparticles (AgNPs). By introducing a silver precursor (AgCF3COO in ethanol) to the reduced nanoparticles (AgNPs). By and silverinto precursor (AgCF ethanol) to the the methods reduced nanoparticles (AgNPs). introducing a silver precursor in ethanol) to reduced graphene oxide (rGO) films then converting AgNPs(AgCF following previously reported 33COO graphene oxide (rGO)films filmsand andthen then converting into AgNPs following previously reported methods graphene oxide (rGO) converting into AgNPs following previously reported methods [9], [9], we could develop graphene/AgNP composite films in which the AgNPs embedded onto both the [9], we could develop graphene/AgNP composite films in which the AgNPs embedded onto both we could develop graphene/AgNP composite films in which the AgNPs embedded onto both the outer outer surface and the inside of the graphene films. Figure 3 exhibits the composite film composedthe of outerand surface thePET inside of the graphene films.textile 3 exhibits the film composed of surface and theand inside of the graphene films. Figure 3Figure exhibits the composite film composed of rGO film and rGO AgNPs on textile (graphene/AgNP composite). Thecomposite rGO/AgNP composite rGO and AgNPs on PET textile (graphene/AgNP textile composite). The rGO/AgNP composite film AgNPs on PET textile (graphene/AgNP textile composite). The rGO/AgNP composite film coating on coating on the PET textile maintained well without any distinguishable damages after chemical coating onand themaintained PET textilewell maintained without any distinguishable after chemical the PET textile without well any distinguishable damages after damages chemical reduction and reduction embedding of AgNPs. Furthermore, when we examined the microstructure of reduction and embedding of AgNPs. Furthermore, when we examined the microstructure of embedding of AgNPs. Furthermore, when we examined the microstructure of graphene/AgNP graphene/AgNP textile composite by scanning electron microscopy (SEM, Figure 3c), we textile could graphene/AgNP textile composite by scanning electron microscopy (SEM, Figure 3c), we could composite by scanning electron microscopy (SEM, Figure 3c), we could observe that the pores of the observe that the pores of the PET textile were completely covered by composite film. Although an observe that the pores of was thecovered PET textile were completely covered byexcess film. an PET textile were by composite film. Although an amount of Although AgNPs was excess amount ofcompletely AgNPs not observed on the graphene film due tocomposite the mild reduction reaction excess amount ofthe AgNPs was not observed the of graphene film due to AgNPs, the mild reduction reaction not observed graphene due toresistance theonmild reduction reaction of the mean values of of AgNPs, theon mean values of film electrical the composite film were dramatically reduced of AgNPs, the mean values of electrical resistance of the composite film were dramatically reduced electrical resistance of the composite film were dramatically reduced from 5 kΩ/sq to 4 Ω/sq after from 5 kΩ/sq to 4 Ω/sq after embedding the AgNPs. Through the simple transfer of GO film and the from 5 kΩ/sq toAgNPs. 4 Ω/sq after embedding theprepare AgNPs. Through the simple of GO film and the embedding the the simple transferthe ofhighly GO film and the transfer subsequent embedding of subsequent embedding ofThrough AgNPs, we could conductive graphene/AgNP electrode subsequent embedding of the AgNPs, weconductive could prepare the highly conductive AgNPs, we could prepare highly graphene/AgNP electrodegraphene/AgNP on PET textile. electrode on PET textile. on PET textile.

Figure 3. (a,b) Optical microscopy (OM) images and (c) scanning electron microscopy (SEM) image Figure 3. (a,b) Optical microscopy (OM) images and (c) scanning electron microscopy (SEM) image of Figure 3. (a,b) Optical microscopy (OM) images and (c) scanning electron microscopy (SEM) image of graphene/AgNP textile composite. graphene/AgNP textile composite. of graphene/AgNP textile composite.

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Since high flexibility is one of the important requirements for wearable devices, we investigated graphene/AgNP textile composites. Figure S3 (see the effect of of bending bendingon onthe theelectrical electricalproperties propertiesofof graphene/AgNP textile composites. Figure S3 Supplementary Information) displays the change of the electrical resistanceresistance of the textile (see Supplementary Information) displays the change of the electrical of composites the textile as the number of bending (bending radius = 3 mm) increased. The electrical of the composites as the numbercycles of bending cycles (bending radius = 3 mm) increased.resistance The electrical textile composites showed a small increase 20increase cycles ofafter bending test and then maintained the resistance of the textile composites showed aafter small 20 cycles of bending test and then stable valuethe of stable 4.3 Ω/sq. Theofhigh electrical durabilityand to the repeated was maintained value 4.3 Ω/sq. The conductivity high electricaland conductivity durability tobending the repeated attributed to attributed the highlytoporous structures rGO films, which provide to incorporate enough bending was the highly porousof structures of rGO films, whichroom provide room to incorporate AgNPs inside the graphene layers. layers. enough AgNPsofinside of the graphene 2.2. 2.2. Prepratation Prepratation of of Transistor Transistor Devices Devices Using Using Graphene/AgNP Graphene/AgNPTextile TextileComposites Composites In In order order to to utilize utilize the the outstanding outstanding electrical electrical conductivity conductivityand andflexibility flexibilityof ofthe thegraphene/AgNP graphene/AgNP textile composite, composite, we wefabricated fabricatedananorganic organictransistor transistor using textile composite as both by by using the the textile composite as both gate gate electrode and flexible substrate. Transistor devices were fabricated with a bottom-gate, electrode and flexible substrate. Transistor devices were fabricated with a bottom-gate, top-contact top-contact geometry with the composite film gate electrode, as schematically 4. geometry with the composite film gate electrode, as schematically depicted depicted in Figurein4.Figure Poly(3Poly(3-hexylthiophene) was chosen the solution-processable p-channel semiconductor hexylthiophene) (P3HT)(P3HT) was chosen as theas solution-processable p-channel semiconductor [10].[10]. We We also used an elastic ion gel layer based on poly(vinylidene fluoride-co-hexafluoropropylene) also used an elastic ion gel layer based on poly(vinylidene fluoride-co-hexafluoropropylene) P(VDFP(VDF-HFP) and the ionicliquid liquid 1-ethyl-3-methylimidazolium 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide HFP) and the ionic bis(trifluoromethylsulfonyl)amide ([EMI][TFSA]) ([EMI][TFSA]) [11] [11] as as both both high capacitance gate dielectric layer and mechanically robust transporter for the the channel channelmaterial. material.After After successive spin-coating of ion thegel ionand gelthe and the P3HT layersa thethe successive spin-coating of the P3HT layers onto onto a washed the double layer cut waswith cut with a razor transferred washed silicon silicon wafer, wafer, the double layer was a razor bladeblade and and transferred ontoonto the the graphene/AgNP electrode on textile PET textile for direct contact between the dielectric layer and the graphene/AgNP electrode on PET for direct contact between the dielectric layer and the textile textile electrode [12]. This procedure provided a convenient and solvent-freeroute routeto to simultaneously electrode [12]. This procedure provided a convenient and solvent-free incorporate the active channel and the ion gel layers in the transistors without any contamination of each component. component. The fabrication of textile-based transistors was completed by simply patterning the Au Au source source and and drain drain electrodes electrodes (thickness (thickness ~40 ~40 nm) through a shadow shadow mask mask by by thermal thermal evaporation evaporation onto the P3HT P3HT layer. layer.

Figure 4. Schematic illustration of the fabrication process of the transistor device based on the Figure 4. Schematic illustration of the fabrication process of the transistor device based graphene/silver nanoparticle (AgNP) textile composites. P3HT: Poly(3-hexylthiophene); S: Source on the graphene/silver nanoparticle (AgNP) textile composites. P3HT: Poly(3-hexylthiophene); electrode; D: Drain electrode. S: Source electrode; D: Drain electrode.

Figure 5 displays a photograph and optical microscopy images of the transistor device Figure 5 displays a photograph and optical microscopy images of the transistor device developed developed in this study. For an accurate measurement of the electrical characteristics of the textilein this study. For an accurate measurement of the electrical characteristics of the textile-based based transistors, the device was fixed on the glass slide using 3M tape. The thickness of the P3HT transistors, the device was fixed on the glass slide using 3M tape. The thickness of the P3HT and ion and ion gel layer was 60 nm and 11 µ m, respectively. The channel length (L) and width (W) was 50– gel layer was 60 nm and 11 µm, respectively. The channel length (L) and width (W) was 50–100 µm 100 µ m and 800–1000 µ m, respectively. The projection of the weaving morphology of the PET textile and 800–1000 µm, respectively. The projection of the weaving morphology of the PET textile after the after the gold electrode deposition confirms that every layer of the device was compactly assembled by the simple transfer and deposition procedures.


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gold electrode deposition confirms that every layer of the device was compactly assembled by the Nanomaterials 2016, 6, 147deposition procedures. simple transfer and Nanomaterials 2016, 6, 147 5 of5 9of 9

Figure 5. Photograph optical microscopy image of textile-based transistor device. Figure 5. Photograph (a) (a) andand optical microscopy image (b)(b) of textile-based transistor device.

Figure 66 shows the typical output where the drain current the drain voltage) Figure shows the typical output -Vwhere D, where IDDthe isisthe drain current and isisthe drain voltage) Figure 6 shows the typical output (ID(I(I -V D, ID is drain current and VDVisDDthe drain voltage) DD-V D, and transfer (I -V where V is the drain voltage) characteristics of the textile-based transistors. transfer (I D -V G, where V G is the drain voltage) characteristics of the textile-based transistors. The and transfer (ID-VDG, where VG is the G, G drain voltage) characteristics of the textile-based transistors. The The well-defined gate modulation in output the output curves (Figure 6a) reveals the ohmic contact between well-defined gate modulation curves (Figure reveals ohmic contact between well-defined gate modulation in in thethe output curves (Figure 6a)6a) reveals thethe ohmic contact between thethe the composite film electrode and the transferred semiconductor/dielectric layers. The transistor composite film electrode and the transferred semiconductor/dielectric layers. The transistor device composite film electrode and the transferred semiconductor/dielectric layers. The transistor device exhibited a saturation current mA and − V. To To electrical saturation current of 1.0 mA =− −3 V=D D −1 == −1 evaluate exhibited a saturation current ofof 1.01.0 mA atatat VV GV =G =−3 V3 V and VDV V.1V. To evaluate thethe electrical G characteristics the transistors, such the charge carrier mobility (µh(μ current ratio ), characteristics of the transistors, such as the charge carrier mobility ),on/off on/off current ratio /I),off off), characteristics ofof the transistors, such asas the charge carrier mobility (μ current ratio (I(Ion(I /Ion/I off on h),),hon/off and threshold-voltage (V ), the drain current was measured while sweeping V from 0 V to − 4 a threshold-voltage (V T current was measured while sweeping V G from 0 V to −4 V at and threshold-voltage (VT),Tthe drain current was measured while sweeping VG G from 0 V to −4 V at a − −11and 33 mV· aaconstant DDvalue VV(Figure 6b). Despite manual fabrication process rate mV and constant V value of−1 −1(Figure (Figure 6b). Despite the manual fabrication rate of of 3333 mV· s−1·ssand a constant VDV value of of −1 V 6b). Despite thethe manual fabrication process ambient conditions, transistors made the composite film electrode showed a reasonably high ambient conditions, the transistors made the composite film electrode showed a reasonably in in ambient conditions, thethe transistors made of of theof composite film electrode showed a reasonably high 4 and 4 and 4 I on /I off of 4.5 × 10 low V T values around −1.5 V. From the slope of the V G vs. |I D | curves obtained high I /I of 4.5 × 10 low V values around − 1.5 V. From the slope of the V vs. |I Ion/Ioff of 4.5 −1.5 V. From the slope of the VG vs. |ID| curves obtained on × T G D| off 10 and low VT values around 2 −1 −1 2calculated for more than five devices, the average field-effect mobility was calculated 2.7 ·V·s−1·,swhich , which obtained for more five field-effect devices, themobility average field-effect mobility to be forcurves more than five devices, thethan average was calculated to to bebe 2.7was cmcm ·V−1 2 ·Vhigher −1 ·s−1 , than is much those reported in other P3HT-based transistors gated with conventional 2.7 cm which is much higher than those reported in other P3HT-based transistors gated is much higher than those reported in other P3HT-based transistors gated with conventional −comparable 1 ·s−1 ) [13], but −1) [13], but 2·V2−1 dielectrics (0.1–0.01 cm ·V·s−1−1·)s(0.1–0.01 other recent results on electrolyte-gated with conventional dielectrics are recent comparable toon other recent results on ·V dielectrics (0.1–0.01 cm [13], butcm are2are comparable to to other results electrolyte-gated polymer transistors [14]. Ittransistors has been speculated that high mobility value this result is due to electrolyte-gated polymer [14]. It has been speculated thatvalue the high mobility value into this polymer transistors [14]. It has been speculated that thethe high mobility in in this result is due the penetration of ions from the ion gel dielectric into the active channel that fills the carrier traps and result is due to the penetration of ions from the ion gel dielectric into the active channel that fills the the penetration of ions from the ion gel dielectric into the active channel that fills the carrier traps and acts a dopant the film [15,16]. carrier traps and acts asP3HT a dopant in the P3HT film [15,16]. acts as as a dopant in in the P3HT film [15,16].

Figure 6.(a) (a) IDD-Vand D and IDG-Vcharacteristics G characteristics of the textile-based transistor device. Figure 6.6.(a) IDI-V (b)(b) ID-V of the textile-based transistor device. Figure D -V D and (b) ID -V G characteristics of the textile-based transistor device.

One approach toward improving performances electronic devices (e.g, organic One approach toward improving thethe performances of of thethe organic devices (e.g, organic One approach toward improving the performances oforganic the electronic organic electronic devices (e.g, thin film transistors, organic light emitting diode [17], and organic photovoltaics [18]) is thin film transistors, organic light emitting diode [17], and organic photovoltaics [18]) is is organic thin film transistors, organic light emitting diode [17], and organic photovoltaics [18]) postproduction heat treatment. order enhance device performance textile-based postproduction heat treatment. In In order to to enhance thethe device performance of of thethe textile-based transistors, thermally annealed device min, which a higher temperature than transistors, wewe thermally annealed thethe device at at 80 80 °C°C forfor 15 15 min, which is aishigher temperature than glass transition temperature g) of P3HT = 12 and PVDF = −115 polymer layers thethe glass transition temperature (Tg(T ) of P3HT (Tg(T=g 12 °C)°C) and PVDF (Tg(T=g −115 °C)°C) polymer layers [19,20]. From optical microscopy image textile-based transistor device after thermal [19,20]. From thethe optical microscopy image of of thethe textile-based transistor device after thermal annealing (see Supplementary Information; Figure S4), clearer weaving morphology PET annealing (see Supplementary Information; Figure S4), clearer weaving morphology of of thethe PET


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postproduction heat treatment. In order to enhance the device performance of the textile-based transistors, we thermally annealed the device at 80 ◦ C for 15 min, which is a higher temperature than the 2016, glass6,transition temperature (Tg ) of P3HT (Tg = 12 ◦ C) and PVDF (Tg = −115 ◦ C) polymer Nanomaterials 147 6 of 9 layers [19,20]. From the optical microscopy image of the textile-based transistor device after thermal annealing (see Supplementary Information; Figure weaving morphology ofbetween the PET textile textile was observed over the gold electrodes, sinceS4), theclearer stronger interfacial adhesion the was observed over gold electrodes, since theannealing. stronger interfacial adhesion between the component component layers wasthe developed by the thermal Furthermore, VT was decreased to −1.1 V layers was developed the the thermal annealing. Furthermore, V T was decreased to −1.1 V afterin thermal after thermal annealing,byand decrease of off-current and increase of on-current resulted the 5 2 −1 −1 annealing, and the decrease off-current increase on-current resulted enhancement enhancement of on/off current of ratio and holeand mobility of of 4 × 10 and 7.2 cm ·Vin·sthe , respectively 2 ·V−1 ·s−1 , respectively (Figure 7). of on/off current ratio and mobility of 4 ×was 105 ascribed and 7.2 to cmthe (Figure 7). The improvement ofhole device performance crystallization and molecular The improvement of device performance was ascribed to the crystallization molecular orientation orientation of the semiconducting polymer and the decrease of the contact and resistance between the of the semiconducting polymer the decrease of the contact resistance component component layers induced by theand thermal annealing. These results verify between that the the textile-based layersmade induced by the thermal resultstoverify that the textile-based devices made devices by simple transfer annealing. method areThese applicable the field-effect transistor (FET) device. by simple transfer method are applicable to the field-effect transistor (FET) device. Table 1 lists Table 1 lists the detailed electrical properties of the textile-based transistors according to the thermalthe detailed electrical properties of the textile-based transistors according to the thermal annealing. annealing.

Figure 7. I7.D-V characteristics of the textile-based transistor device after thermal annealing at 80 °C◦ Figure IDG-V G characteristics of the textile-based transistor device after thermal annealing at 80 C forfor 15 15 min. min. Table 1. 1.Average hole-mobilities, on/off current ratios, andand threshold voltages forfor textile-based Table Average hole-mobilities, on/off current ratios, threshold voltages textile-based transistors according to thermal annealing. transistors according to thermal annealing. Treatment Treatment Without annealing

Without Withannealing annealing With annealing

μh

µh (cm2 ·V−1 ·s−1 )

(cm ·V−1·s−1) 2

2.7

2.7 7.2 7.2

IIon on/I /Ioff off

(V) VVTT(V)

104

4.5 × 4.5 10 54 4 ××10

−1.5 −1.5 − 1.1

4 × 105

−1.1

In addition, we measured the changes of device performance of the textile-based transistor after In addition, we measured the changes of device performance of the textile-based transistor after a cyclic bending test, because flexibility is critical for practical applications (Figure 8). In order to a cyclic bending test, because flexibility is critical for practical applications (Figure 8). In order to develop organic electronic devices with high flexibility, all of the components should be flexible and develop organic electronic devices with high flexibility, all of the components should be flexible and mechanically stable with intimate interfacial adhesion. The transistors without thermal treatment mechanically stable with intimate interfacial adhesion. The transistors without thermal treatment showed very unstable device performance; a considerable decrease of hole mobility was observed showed very unstable device performance; a considerable decrease of hole mobility was observed after bending the device to a radius of 3 mm for 10 cycles, which was because the upper layer (ion gel, after bending the device to a radius of 3 mm for 10 cycles, which was because the upper layer (ion P3HT, and Au electrodes) was peeled off from the graphene/AgNP textile composites. In contrast, gel, P3HT, and Au electrodes) was peeled off from the graphene/AgNP textile composites. In contrast, the device performance of the annealed textile-based transistors maintained up to 70% of the original the device performance of the annealed textile-based transistors maintained up to 70% of the original values through 1000 cycles of bending. From the bending radius of 3 mm and device thickness values through 1000 cycles of bending. From the bending radius of 3 mm and device thickness (90 (90 µm), it is calculated that the annealed device underwent a strain of 0.014 repeatedly during the µ m), it is calculated that the annealed device underwent a strain of 0.014 repeatedly during the bending test. The outstanding durability of the annealed textile-based transistors originates from the good mechanical flexibility of every component and the closely-packed device structure of the textilebased transistor.


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bending test. The outstanding durability of the annealed textile-based transistors originates from the good mechanical flexibility of every component and the closely-packed device structure of the Nanomaterials 2016, 6, 147 7 of 9 textile-based transistor.

Figure 8.8. Changes Changes ininthe thehole hole mobilities of the thermally-annealed transistor square) Figure mobilities of the thermally-annealed transistor (blue(blue square) and and not-annealed transistor (red square) depending onbending the bending not-annealed transistor (red square) depending on the cycle.cycle.

3. Materials 3. Materialsand andMethods Methods Materials. All All materials materials were were purchased purchased from from Sigma-Aldrich Sigma-Aldrich (St. (St. Louis, Louis, MO, MO, USA), USA), except except for for Materials. the GO aqueous solution, which was purchased from Angstron Materials Inc. (Dayton, OH, USA). the GO aqueous solution, which was purchased from Angstron Materials Inc. (Dayton, OH, USA). AAO membrane membrane (200 (200 nm nm pore pore size, size, 47 47 mm mm diameter; diameter; Whatman Whatman (Buckinghamshire, (Buckinghamshire, UK), UK), PET PET textile textile AAO (Textoma, Inc., Daegu, Republic of Korea), and Poly(3-hexylthiophene) (M W = 50 K, Rieke Metals Inc., (Textoma, Inc., Daegu, Korea), and Poly(3-hexylthiophene) (MW = 50 K, Rieke Metals Inc., Lincoln, Please write which city, NE, USA). NE, USA). GO film film transfer transfer on on PET PET textile. filtration of GO textile. GO GO films filmson onPET PETtextiles textileswere wereprepared preparedvia viavacuum vacuum filtration aqueous GO solution and direct transfer onto PET textile, using the previously reported method [8]. of aqueous GO solution and direct transfer onto PET textile, using the previously reported method [8]. Briefly, GO GO (0.2, (0.2, 0.6, 0.6, 1.0, 1.0, 2.0 2.0 mg) mg) in in aqueous aqueous solution solution was was first first vacuum-filtered vacuum-filtered through through an an AAO AAO Briefly, membrane. The dried GO film was placed into a bath of 3 M NaOH. The AAO membrane was then membrane. The dried GO film was placed into a bath of 3 M NaOH. The AAO membrane was then dissolved, and the thin GO film was floated on the surface of the NaOH solution. The NaOH solution dissolved, and the thin GO film was floated on the surface of the NaOH solution. The NaOH solution was exchanged exchanged with with deionized deionized water water by by recirculation recirculation until until the the pH pH was was near near 7.0. 7.0. A A piece piece of of the the PET PET was textile (4 × × 44cm cm22))was wasimmersed immersed into into the the water water and and allowed allowed to to sink sink to to the the bottom bottom of of bath. bath. As As the the textile water water was was drained, drained, the the floating floating GO GO film film slowly slowly descended descendedand and attached attachedonto ontothe the PET PET textile. textile. The The GO GO film-PET film-PET textile textile was was dried dried in in ambient ambient conditions. conditions. Graphene/AgNP Graphene/AgNP composite composite film filmpreparation preparation on on PET PET textile. textile. The dried GO films films on on PET PET textile textile were C for werechemically chemicallyreduced reducedby byexposing exposingthe thefilms filmsto tohydrazine hydrazinevapor vapor(100%) (100%)at at90 90◦°C for 22 h. To fabricate fabricate graphene/AgNP rGO films on PET were dipped into a AgCF solution graphene/AgNP composite compositefilms, films,thethe rGO films on textile PET textile were dipped into 3aCOO AgCF 3COO in ethanolin(15 wt %) for min of drying in air, filmsin were to were hydrazine vapor solution ethanol (1530 wtmin. %) After for 305 min. After 5 min of the drying air,exposed the films exposed to ◦ C for 2 h to reduce the adsorbed Ag ions to AgNPs. (100%) at 90 hydrazine vapor (100%) at 90 °C for 2 h to reduce the adsorbed Ag ions to AgNPs. Fabrication Fabrication of textile-based transistor. transistor. Silicon wafer was cleaned by piranha solution solution to remove remove any oxygen plasma to introduce a hydrophilic surface. The The ion any organic organiccontamination contaminationand andtreated treatedwith with oxygen plasma to introduce a hydrophilic surface. gel was prepared by first P(VDF-HFP) and theand ionic liquid, in acetone ionlayer gel layer was prepared bycodissolving first codissolving P(VDF-HFP) the ionic [EMI][TFSA] liquid, [EMI][TFSA] in (the weight between the polymer–ionic liquid–solvent was kept at kept 1:4:7),atand then spin-coated acetone (theratio weight ratio between the polymer–ionic liquid–solvent was 1:4:7), and then spinon the washed Si wafer Si at wafer 1000 rpm for 1rpm min.for Spin-coated ion gel layers placed in a placed vacuum coated on the washed at 1000 1 min. Spin-coated ion were gel layers were inata ◦ 70 C for at 2470 h to residualthe solvent. Regioregular P3HT was spin-coated 2000 rpm for 60 s vacuum °Cremove for 24 hthe to remove residual solvent. Regioregular P3HT was at spin-coated at 2000 −1) on ion from chloroform (10 mg ·mL−1 )(10 on mg· ion gel Thegel double layer ion gellayer and P3HT cut rpm for 60 s fromsolution chloroform solution mLlayer. layer. Theof double of ion was gel and with a razor blade, and then transferred onto the graphene/AgNP textile composites for direct contact P3HT was cut with a razor blade, and then transferred onto the graphene/AgNP textile composites between dielectric and the composite filmand electrode. To complete fabrication textile-based for direct contactlayer between dielectric layer the composite filmthe electrode. Toofcomplete the transistor devices, Au sourcetransistor and draindevices, electrodes a thickness ofelectrodes 40 nm were thermally deposited fabrication of textile-based Auwith source and drain with a thickness of 40 nm were thermally deposited onto the P3HT through a shadow mask. The devices were then annealed at 80 °C for 15 min on a digital hot plate under nitrogen atmosphere inside a glove box. Characterization. Optical microscope observations were performed with a Nikon ECLIPSE LV150N (Tokyo, Japan). SEM images were taken on a JEOL JSM5800 (Tokyo, Japan). The electrical properties of graphene/AgNP composite films were characterized using a four-point probe measurement system (Napson, CRESBOX, Tokyo, Japan). The bending test was carried out with a


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onto the P3HT through a shadow mask. The devices were then annealed at 80 ◦ C for 15 min on a digital hot plate under nitrogen atmosphere inside a glove box. Characterization. Optical microscope observations were performed with a Nikon ECLIPSE LV150N (Tokyo, Japan). SEM images were taken on a JEOL JSM5800 (Tokyo, Japan). The electrical properties of graphene/AgNP composite films were characterized using a four-point probe measurement system (Napson, CRESBOX, Tokyo, Japan). The bending test was carried out with a home-made two-point bending device and a high-precision mechanical system. The measurement of the current–voltage characteristics of the textile-based transistor devices were carried out at room temperature in a N2 -atmosphere glove box using a MST-5500B probe station (Gyeonggi-Do, Korea) and Keithley 4200-SCS (Solon, OH, USA). From the slope of the V G vs. |ID | curves obtained for more than five devices, the average field-effect mobility was estimated in the linear regime (V D = −1 V) from the following equation [21]: ID = µ

W Ci VD (VG − VT ) L

(1)

where ID is the drain current, µ is the field-effect mobility, Ci (9 µF·cm−2 ) is the specific capacitance of the ion gel dielectric film, V D is the drain voltage, V G is the gate voltage, V T is the threshold voltage, and W and L are the channel width and length, respectively. 4. Conclusions In summary, we fabricated multifunctional textile composites by vacuum filtration and wet-transfer of graphene oxide on flexible polyethylene terephthalate textile in association with embedding of Ag nanoparticles, which showed high electrical conductivity and outstanding flexibility. A flexible organic transistor can be developed by direct transfer of a dielectric/semiconducting double layer on the graphene/AgNP textile composite, where the textile composite was used as both flexible substrate and conductive gate electrode. The thermal treatment of the textile-based transistor enhanced the electrical performance and bending durability due to the improvement of interfacial properties between conductive textile electrode and ion-gel dielectric layer. We believe that our simple methods for the fabrication of graphene/AgNP textile composite for use in textile-type transistors can potentially be applied to the development of flexible large-area electronic clothes. Supplementary Materials: The following are available online at http://www.mdpi.com/2079-4991/6/8/147/s1. Acknowledgments: This work was supported by the Principal Research Program in the Korea Research Institute of Chemical Technology (KRICT) and Korea Institute of Materials Science (KIMS); the Global Research Laboratory Program (K20704000090). Author Contributions: M.-K.U and J.U.L. designed the research; Y.K., Y.J.K., and K.E.L. performed the experiments; Y.O. and D.G.S. analyzed the data; Y.K., Y.J.K., and J.U.L. wrote the manuscript; All authors discussed and commented on the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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