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Highly Stretchable Polymer Transistors Consisting Entirely of Stretchable Device Components Minkwan Shin, Jun Hyuk Song, Guh-Hwan Lim, Byungkwon Lim, Jong-Jin Park, and Unyong Jeong* Stretchable electronics have received a great deal of attention due to their use in deformable devices such as tactile sensors for electronic skin,[1–9] stretchable displays,[10,11] and wearable devices based on stretchable textiles.[12–15] The fabrication of stretchable devices has been explored via two approaches: wavy design of non-stretchable materials or elastomeric electronic materials. Rogers group fabricated an array of field-effect transistors (FETs) using wrinkled Si nanoribbons.[16,17] They also demonstrated stretchable displays utilizing buckled metal interconnects bridging flat non-stretchable device units.[18] Such wavy structures minimize the accumulation of stress on the device units at large strains. Someya group developed a stretchable electrode composed of a carbon nanotube (CNT)/polymer gel composite.[19] They exhibited an array of light emitting diodes operated by FETs mounted on stretchable textiles.[20] In an approach based on stretchable materials, each component of the device must maintain its performance up to a critical strain. There have been many attempts to generate stretchable electrodes, primarily using composites of rubber and conductive fillers such as CNTs,[21–24] Ag nanowires,[25,26] Au nanoplates,[27] and metal nanoparticles.[28,29] There have been a limited number of reports investigating stretchable gate dielectric layers. Ahn and coworkers demonstrated stretchable FETs using an ion-gel dielectric layer on graphene electrodes.[30] In their study, the obtained reliable level of strain (ε) without severe degradation of performance was ε = ∼0.05, which is not large enough for stretchable devices. To the best of our knowledge, there have been no reports on a stretchable organic channel layer to date. Due to the limited materials which are useful as gate dielectric and active channel materials, a stretchable device constructed entirely of stretchable components has not been reported yet. In this study, building upon our previous works,[31] we demonstrated the fabrication of an array of highly stretchable polymer transistors made entirely of stretchable components.
M. Shin, J. H. Song, Prof. U. Jeong Department of Materials Science and Engineering Yonsei University 134 Shinchon-dong, Seoul, Korea E-mail: [email protected] G.-H. Lim, Prof. B. Lim School of Advanced Materials Science and Engineering Sungkyunkwan University Suwon 440–746, Korea Dr. J.-J. Park Samsung Advanced Institute of Technology Mt.14–1, Nongseo-Dong, Giheung-Gu, Yongin-Si, Gyeonggi-Do, 446–712, Korea
DOI: 10.1002/adma.201400009
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The transistors were constructed of stretchable Au nanosheet electrodes,[27] polyelectrolyte gel for the gate dielectric,[31,32] and electrospun poly(3-hexylthiophene) (P3HT) nanofibers for the active channel material.[33] We used a poly(styrene-b-butadieneb-styrene) (SBS) electrospun elastomer fiber mat as a substrate to enhance the mechanical stability of the transistors fabricated on the substrate. The combination of the four stretchable components provided a high hole mobility (μ = 18 cm2/V·s) at ε = 0.7 and showed excellent electrical stability over 1500 stretching cycles. Figure 1A illustrates the device structure developed in this study. A thick electrospun SBS nanofiber mat (∼500 µm thick) was used as the stretchable substrate. The average diameter of the nanofibers was ∼300 nm. The nanofiber mat was completely elastic up to ε = 0.4.[28] Thermal evaporation of Au on the nanofiber mat was not suitable because the electrodes were torn apart at ε = 0.07. Highly stretchable Au nanosheet electrodes were adopted in this study by following a previously reported procedure.[27] Au was selected because of the low energy barrier for charge injection to organic semiconductors, which was P3HT in this study.[34] On average, the Au nanosheets used in this study were 20 nm thick and 10 µm wide. The Au nanosheets were floated on a water surface to form a monolayer.[27] The monolayer was transferred repeatedly to a PDMS stamp with an array of rectangular pillars. The stacks of Au nanosheets on the PDMS pillars were transferred to the SBS fiber mat substrate. The electrical conductivity of the electrode was saturated when the monolayer was transferred six times on the fiber mat (Figure S1). To enhance the adhesion between the SBS substrate and the stack of Au nanosheets, the substrate was preheated at 90 °C and the stacked Au nanosheets on the PDMS pillars were contacted with the substrate by applying gentle pressure. This heating process reduced the roughness of the fiber mat substrate. Due to the van der Waals forces between the nanosheets, the stack of nanosheets was transferred cleanly to form rectangular electrodes. The thickness of the resulting electrodes was ∼80 nm and its root-mean-square roughness was ∼214 nm (Figure S2). The channel length and width of the electrodes were 100 and 800 µm, respectively. P3HT fibers were directly electrospun on the substrate with the Au electrode pattern. The average diameter of the fibers was 2.5 µm. In order to guarantee continuous production of the fibers without nozzle-tip blockage caused by fast crystallization of P3HT, a polymer solution containing poly(ε-carprolactone) (PCL) (P3HT:PCL = 7:3, w/w) was electrospun. Despite the presence of the insulating PCL phase, the fibers possessed good semiconducting characteristics.[33] Through this approach, large-area printing is achievable because the P3HT fibers can be electrospun continuously, concentrated solution of the Au
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COMMUNICATION Figure 1. (A) Schematic illustration of the transistor on a paper substrate. The Au nanosheet S/D electrodes were transferred to the polystyrene-bpolybutadiene-b-polystyrene (SBS) nanofiber mat and the P3HT fibers were collected directly between them. A solution mixture containing the ionic molecules and crosslinker was absorbed on the surface of the SBS substrate. UV irradiation formed a network between the dielectric layer and substrate. Au nanosheets were transferred to the dielectric surface for the gate electrode. (B) The optical microscopy (OM) image shows an array of S/D electrodes on the SBS nanofiber mat and the P3HT fibers in between (with a length of 100 µm and width of 800 µm). (C) SEM image of the SBS fiber mat, Au plate electrode, and P3HT fibers. (D) Representative SEM image of the device. The inset shows a part of the array of 180 devices (15 × 12).
nanosheets can be directly printed by stamping method on the SBS fiber mat substrate, and the ioninc molecules can be continuously patterned by UV exposure through metallic masks. Figure 1B displays an optical microscopy (OM) image of an array of Au nanosheet electrodes and electrospun P3HT fibers on the SBS substrate. Figure 1C shows a magnified SEM image of the electrode. Typically, 30 P3HT fibers were connected between the Au nanosheet electrodes. To fabricate the ion-gel dielectric layer, a few drops of a solution mixture consisting of poly(ethylene glycol)-diacrylate (PEG-DA), 2-hydroxy-2-methyl propiophenone (HOMPP), and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([EMIM][TFSI]) (88:8:4, w/w) were placed on the substrate. The solution embedded the P3HT fibers and penetrated the top surface of the SBS nanofiber mat substrate, as indicated by the shaded region in Figure 1A. UV crosslinking through a patterned mask followed by washing the unexposed mixture solution beneath the mask produced ion-gel dielectric layers which bridged the source and drain electrodes.[31] The ion-gel and substrate formed an interpenetrated network structure, which is the key factor for good mechanical properties without being peeled off from the substrate. The Au nanosheets were transferred on the gel surface as a gate electrode by touching the Au nanosheet layer (∼80 nm thickness) to the ion-gel patterns. Figure 1D shows an SEM image of the transistor. We fabricated an array of 180 devices (15 × 12) on one SBS substrate. The inset SEM image of Figure 1D shows a part of the device array.
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The SBS nanofiber mat substrate was completely elastic up to ε = 0.4 and it had a small amount of remnant strain when it was stretched further. The remnant strain was removed by placing the nanofiber mat on a thin sheet of PDMS, which allowed complete elasticity up to ε = 1.0. The ion-gel dielectric layer was also completely elastic up to ε = 1.0, but it tore often at ε > 1.0. The stretchability of the Au nanosheet electrode and P3HT fibers on the fiber mat substrate was evaluated. Figure 2A displays the change of the resistivity of the Au nanosheet electrodes as the uniaxial strain was increased. We measured the resistivity change in the transverse direction of the electrodes because the electrical behavior of the active layer was more sensitive to elongation in that direction compared to the longitudinal direction. The stretchability of the electrode on the fiber mat was significantly enhanced compared to that on the flat PDMS substrate. The resistivity of the electrode on the PDMS substrate showed an abrupt increase at ε = 0.6 with a propagation of cracks, whereas the electrode on the fiber mat showed no difference (∼0.3 × 10−6 Ω·m) in the stretched state at ε = 0.9 and the released states. The enhanced electrical stretchability of the electrode on the SBS substrate was attributed to the observation that the first nanosheet imitated the surface topology of the fiber mat (Figure S3). The topological structure on the fiber mat had better tolerance to external strain than the fully extended nanosheets on a flat PDMS.[28] Figure 2B exhibits the resistivity of the electrode on the fiber mat over 3000 cycles of stretching at ε = 0.7. The
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Figure 2. Changes of the resistivity and conductivity of the Au electrode and P3HT active layer upon stretching. (A) Resistivity change of the Au nanosheet S/D electrodes on the SBS nanofiber mat as a function of strain. (B) Durability test of the Au nanosheet electrode on the mat over 2000 cycles of stretching at ε = 0.7. The resistivity was measured after relaxing the applied strain. (C) Conductivity change of the P3HT fibers connecting the S/D electrodes as a function of strain. The open circles show the measured values and the solid circles are corrected values considering the dimensional change of the substrate due to strain.
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electrode exhibited negligible changes of the resistivity during the cycling test. The excellent stretchability of the Au nanosheet electrode is attributed to the sliding between the nanosheets during stretching and to the stress reduction by following the topology of the mesoporous nanofiber mat.[27] To date, there have been no reports of flat semiconducting thin films which are stretchable up to a notable strain (for example, ε = 0.1). Recent studies employing a honeycomb-structured thin film of a polymer semiconductor exhibited enhanced stability of the transistor to uniaxial strain, but the maximum strain at which the device could operate was ε < 0.1.[35] To obtain sufficient stretchability of the active layer, we used electrospun P3HT fibers directly collected on the source and drain electrodes. Figure 2C shows the I–V characteristics of the P3HT fibers depending on the tensile strain. The conductivity gradually decreased as the strain increased, without the sudden drop which is typically observed in film-type active layers.[36] The slight decrease of conductivity is reasonable because the distance between the source and drain electrodes was increased by stretching. Considering the increase of the length and decrease of the width of the active layer, the corrected conductivity of the P3HT fiber layer was negligible at ε = 0.7, as shown in Figure 2C (right axis). Whenever gel electrolytes are used in electronic devices, the switching speeds are constrained because the movement of ionic molecules limits the frequency at which the devices operate reliably. The transfer curves of the transistors in this study were checked with varying frequency up to 100 Hz (Figure S4A,B). The on-current and off-current of the devices were not dependent on the frequency (Figure S4C). It is notable that we found the devices became unstable at 200 Hz and higher frequencies. The reason is not clear yet. The frequency dependence may be further reduced by decreasing the thickness of the ion-gel layer. We checked the change in capacitance of the ion-gel when the thickness was 100 nm (Figure S4D). The capacitance at 50 um thickness showed severe decrease from 500 Hz, while the capacitance at 100 nm thickness showed a good stability up to 1000 Hz and the decreasing slope at higher frequencies was much smaller. Figure 3 shows the performance of the transistors as a function of strain. An array of transistors on the fiber mat substrate was stretched uniaxially, as shown in the optical image in Figure 3A. Figure S5 in the supporting information displays the dimensional change of a transistor by stretching. Because all of the components of the transistor were stretchable, any symptoms of wrinkling were not detected up to ε = 1.0. The interpenetrated network structure between the gel dielectric and the SBS nanofibers provided high mechanical stability without causing peeling or mechanical damage on the transistors. Device failure was caused in the mechanically weak polyelectrolyte gel at ε > 1.0. The stable operation of the transistors was guaranteed up to ε = 0.7 in all of the transistors (15 × 12 device array). Figure 3B shows the transfer curves as a function of strain. The transfer curve and the output curve at ε = 0 are provided in the supporting information (Figure S6). Every device in the array showed the similar transistor performance. The average on-current value at 0% strain from the 180 devices was 2 ± 0.46 mA. More than 80% of the devices were within ±0.2 mA. The average mobility of the device at 0% strain was 22 ± 0.71 cm2V−1s−1, which indicates the mobility of the devices was highly reproducible. The transfer curves maintained the
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COMMUNICATION Figure 3. (A) Digital image exhibiting the dimensional change of the device array observed by stretching at ε = 0.7. (B) Transfer curves of the device obtained at different strains. (C) Mobility change as a function of strain. The measured mobilities (hollow) were converted into corrected values (solid) by considering the dimensional change of the channel layer. (D) Changes of the threshold voltages (Vth) of the device as a function of strain.
typical transistor behavior at all strains tested. The on-current of the transistor decreased as the strain increased, which is attributed to the increased channel length and decreased channel width. The transfer curve obtained after release from ε = 0.7 was the same as the curve obtained before stretching. The uniaxial stretching of ε = 0.7 generates ∼30% compression in the perpendicular direction. To investigate the effect of compression on the device performance, the device array on the SBS substrate was mounted on a thick PDMS, and compression was exerted in the direction of source and drain electrodes. An optical image of the compression at ε = –0.3 and the corresponding transfer curves are presented in supporting image (Figure S7). The device performance showed negligible change during repeated compression at ε = –0.3. The hole mobility (µ) and threshold voltage (Vth) were calculated from the transfer curve obtained during stretching. These results are displayed in Figure 3C and D, respectively. Without external strain, the mobility was 23 cm2/Vs in the saturation regime (VD = −1 V) based on the equation: IDS = (WCµ/2L)·(VG – Vth).[37,38] The fiber construction of the semiconductor calls a simple estimation of mobility based on parallel plate models of the capacitance should be compensated. Because the device mobility is more valuable than the mobility of each fiber, we calculated the mobility simply based on the channel dimension defined by the area covered by the ion gel dielectric (200 µm × 150 µm). So, the mobility calculated in this study is an effective mobility of the device. By applying the same channel dimension during uniaxial stretching, the apparent measured mobility (hollow circle) of the device continuously decreased as
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the strain increased, with a value of 7 cm2/Vs at ε = 0.7. This decrease was caused by the decrease of the on-current, as shown in Figure 3B. The absolute value of the threshold voltage (Vth) increased as the strain increased for the same reason. After releasing strain, the mobility and threshold voltage recovered the initial values obtained before stretching. By considering the change of the channel dimensions during stretching, the corrected mobility (solid square) maintained the same value up to ε = 0.4 and slightly decreased to 16 cm2/Vs at ε = 0.7. To estimate the correction, we measured the width and length of the channel. The measured channel dimensions were in good agreement with the dimensions calculated by assuming a Poisson’s ratio of an elastomer (0.5) for the substrate. Bao and coworkers reported that a large strain applied to a film of an organic semiconductor caused a large decrease of charge mobility even at ε < 0.1 because the increase of the inter-grain gap resulting from the strain can increase the energy barrier for hole-hopping.[39] Once the performance was largely degraded in the transistors using a thin film organic semiconductor, the transistors did not recover the initial performance.[36] The small mobility change of the P3HT fiber active layer even at a large strain (ε < 0.7) results from the curved contour path of the fibers. Durability of the device performance is critical for practical realization. Figure 4 summarizes the changes of the transfer curve (A), and the hole mobility and threshold voltage (B) during 1500 cycles of stretching at ε = 0.7. The transfer curves obtained during cycling at ε = 0.3 and 0.5 are shown in the supporting information (Figure S8). The electrical characteristics were measured after releasing the strain. The device mobility
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Experimental Section
Figure 4. Changes of the electrical performance of the device after repeated cycling at ε = 0.7. Each measurement was carried out at ε = 0 after relieving the strain. (A) Changes of the transfer curve, and (B) changes of the charge mobility and threshold voltage.
decreased slightly until the number of stretching cycles reached 500 (15.5 cm2/Vs). Thereafter, the mobility changed negligibly (18 cm2/Vs at 1500 cycles). The performance degradation exhibited during the small number of stretching events may be caused by the position change of the P3HT fibers while they were straightened repeatedly at large strains. Once their positions were stabilized without any further change at high strains, the device performance stabilized. The on-off current ratio and Vth remained unchanged during the cycling tests. In summary, we fabricated an array of high-performance stretchable polymer transistors constructed entirely of stretchable components of the transistors. We used a SBS electrospun elastomer nanofiber mat as the substrate, a stack of Au nanosheets as the electrodes, electrospun P3HT fibers as the active material, and ion-gel polyelectrolyte as the dielectric layer. The interpenetrating network structure between the ion-gel and the porous substrate provided the transistors with high mechanical stability under severe stretching events. The high electrical performance (∼18 cm2/Vs mobility, 105 on-off ratio) of the device was stable over 1500 cycles of stretching at ε = 0.7. 3710
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Materials: A silicon wafer was treated by oxygen plasma (O2 flow rate: 22 sccm, power: 50 W, time: 50 s) and dipped in a toluene solution containing octadecyltrichlorosilane (OTS, 0.3 wt%) for 5 min. The residual OTS was washed by fresh toluene. Polystyrene-b-polybutadieneb-polystyrene (SBS) block copolymer (Asahi Kasei Chemicals, volume fraction of polybutadiene = 70%, Mn = 130 000) was dissolved in a mixture solvent (THF:DMF = 3:1). For electrospinning, a 13 wt% SBS solution was fed through a metallic nozzle (diameter: 200 µm) at a rate of 15 µl/min under 18 kV DC. Electrospun SBS fibers were collected on the OTS-treated silicon wafer because the SBS fiber mat was easily peeled off on hydrophobic surfaces. The thickness of the SBS fiber mat substrate was 0.3 mm. Poly(3-hexylthiophene) (P3HT, Mw = 87 000, 95% regioregularity, Rieke Metal Co.) and poly(ε-caprolactone) (PCL, Mw = 80 000, Aldrich) were dissolved in chloroform at a mixing ratio of 7:3 (w/w). The total polymer weight fraction in the solution was 12 wt%. The solution was electrospun to the nanofiber mat substrate at 18 kV and Au nanosheet electrodes were patterned on it. The diameter of the metal nozzle was 21 G and the nozzle-to-collector distance was 20 cm.[33] For the ion-gel dielectric pattern, the ionic molecule ([EMIM][TFSI]), crosslinker (PEG-DA), and initiator (HOMPP) were mixed at the ratio applied in previous studies ([EMIM][TFSI]:PEG-DA:HOMPP = 88:8:4, w/w).[31,32] Device fabrication: Patterned electrodes were prepared with Au nanosheets (∼10 µm diameter, 20 nm thick) by following the procedure reported previously by our group.[27] The Au nanosheets were dispersed in butanol. Several drops of the suspension were dropped on water in a beaker. The Au nanosheet monolayer floated on water was transferred to the PDMS stamp with embossed rectangular patterns. The transfer process was repeated 6 times to obtain metallic conductivity of the electrode. The patterned stack of the Au nanosheets on the PDMS stamp was transferred onto the SBS substrate preheated at 90 °C. Electrospun P3HT fibers were collected on the substrate after the electrode was transferred followed by annealing at 100 °C for 1 h under vacuum. The solution mixture for the dielectric layer was deposited after electrospinning the P3HT fibers. To obtain patterns of the ion-gel layer, UV (365 nm, 300 mW/cm2) was irradiated for 2 s through a mask with square holes. The ionic solution in the unexposed area was removed by washing with ethanol. The same stack of Au nanosheets were collected on a PET substrate and then transferred onto the dielectric pattern. The device performance was characterized at ambient conditions using a semiconductor parameter analyzer (Agilent 4156A). Stretching tests were carried out using a ZBT-200 (Z-TEC.Co).
Supporting Information The electrical resistivity of the Au nanosheet electrode, surface roughness of the electrode on the SBS substrate, dimensional change of the device depending on strains, frequency dependence of the devices, effect of compression on the devices, and device characteristics. Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgments This work was supported by Samsung Research Funding Center of Samsung Electronics under Project Number SRFC-MA1301–07. B.L. thanks the support from the Center for Advanced Soft Electronics under the Global Frontier Research Program (No. 2011–0031628).
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Received: January 1, 2014 Revised: February 7, 2014 Published online: March 24, 2014
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Highly Stretchable Polymer Transistors Consisting Entirely of Stretchable Device Components Minkwan Shin, Jun Hyuk Song, Guh-Hwan Lim, Byungkwon Lim, Jong-Jin Park, and Unyong Jeong* Stretchable electronics have received a great deal of attention due to their use in deformable devices such as tactile sensors for electronic skin,[1–9] stretchable displays,[10,11] and wearable devices based on stretchable textiles.[12–15] The fabrication of stretchable devices has been explored via two approaches: wavy design of non-stretchable materials or elastomeric electronic materials. Rogers group fabricated an array of field-effect transistors (FETs) using wrinkled Si nanoribbons.[16,17] They also demonstrated stretchable displays utilizing buckled metal interconnects bridging flat non-stretchable device units.[18] Such wavy structures minimize the accumulation of stress on the device units at large strains. Someya group developed a stretchable electrode composed of a carbon nanotube (CNT)/polymer gel composite.[19] They exhibited an array of light emitting diodes operated by FETs mounted on stretchable textiles.[20] In an approach based on stretchable materials, each component of the device must maintain its performance up to a critical strain. There have been many attempts to generate stretchable electrodes, primarily using composites of rubber and conductive fillers such as CNTs,[21–24] Ag nanowires,[25,26] Au nanoplates,[27] and metal nanoparticles.[28,29] There have been a limited number of reports investigating stretchable gate dielectric layers. Ahn and coworkers demonstrated stretchable FETs using an ion-gel dielectric layer on graphene electrodes.[30] In their study, the obtained reliable level of strain (ε) without severe degradation of performance was ε = ∼0.05, which is not large enough for stretchable devices. To the best of our knowledge, there have been no reports on a stretchable organic channel layer to date. Due to the limited materials which are useful as gate dielectric and active channel materials, a stretchable device constructed entirely of stretchable components has not been reported yet. In this study, building upon our previous works,[31] we demonstrated the fabrication of an array of highly stretchable polymer transistors made entirely of stretchable components.
M. Shin, J. H. Song, Prof. U. Jeong Department of Materials Science and Engineering Yonsei University 134 Shinchon-dong, Seoul, Korea E-mail: [email protected] G.-H. Lim, Prof. B. Lim School of Advanced Materials Science and Engineering Sungkyunkwan University Suwon 440–746, Korea Dr. J.-J. Park Samsung Advanced Institute of Technology Mt.14–1, Nongseo-Dong, Giheung-Gu, Yongin-Si, Gyeonggi-Do, 446–712, Korea
DOI: 10.1002/adma.201400009
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The transistors were constructed of stretchable Au nanosheet electrodes,[27] polyelectrolyte gel for the gate dielectric,[31,32] and electrospun poly(3-hexylthiophene) (P3HT) nanofibers for the active channel material.[33] We used a poly(styrene-b-butadieneb-styrene) (SBS) electrospun elastomer fiber mat as a substrate to enhance the mechanical stability of the transistors fabricated on the substrate. The combination of the four stretchable components provided a high hole mobility (μ = 18 cm2/V·s) at ε = 0.7 and showed excellent electrical stability over 1500 stretching cycles. Figure 1A illustrates the device structure developed in this study. A thick electrospun SBS nanofiber mat (∼500 µm thick) was used as the stretchable substrate. The average diameter of the nanofibers was ∼300 nm. The nanofiber mat was completely elastic up to ε = 0.4.[28] Thermal evaporation of Au on the nanofiber mat was not suitable because the electrodes were torn apart at ε = 0.07. Highly stretchable Au nanosheet electrodes were adopted in this study by following a previously reported procedure.[27] Au was selected because of the low energy barrier for charge injection to organic semiconductors, which was P3HT in this study.[34] On average, the Au nanosheets used in this study were 20 nm thick and 10 µm wide. The Au nanosheets were floated on a water surface to form a monolayer.[27] The monolayer was transferred repeatedly to a PDMS stamp with an array of rectangular pillars. The stacks of Au nanosheets on the PDMS pillars were transferred to the SBS fiber mat substrate. The electrical conductivity of the electrode was saturated when the monolayer was transferred six times on the fiber mat (Figure S1). To enhance the adhesion between the SBS substrate and the stack of Au nanosheets, the substrate was preheated at 90 °C and the stacked Au nanosheets on the PDMS pillars were contacted with the substrate by applying gentle pressure. This heating process reduced the roughness of the fiber mat substrate. Due to the van der Waals forces between the nanosheets, the stack of nanosheets was transferred cleanly to form rectangular electrodes. The thickness of the resulting electrodes was ∼80 nm and its root-mean-square roughness was ∼214 nm (Figure S2). The channel length and width of the electrodes were 100 and 800 µm, respectively. P3HT fibers were directly electrospun on the substrate with the Au electrode pattern. The average diameter of the fibers was 2.5 µm. In order to guarantee continuous production of the fibers without nozzle-tip blockage caused by fast crystallization of P3HT, a polymer solution containing poly(ε-carprolactone) (PCL) (P3HT:PCL = 7:3, w/w) was electrospun. Despite the presence of the insulating PCL phase, the fibers possessed good semiconducting characteristics.[33] Through this approach, large-area printing is achievable because the P3HT fibers can be electrospun continuously, concentrated solution of the Au
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COMMUNICATION Figure 1. (A) Schematic illustration of the transistor on a paper substrate. The Au nanosheet S/D electrodes were transferred to the polystyrene-bpolybutadiene-b-polystyrene (SBS) nanofiber mat and the P3HT fibers were collected directly between them. A solution mixture containing the ionic molecules and crosslinker was absorbed on the surface of the SBS substrate. UV irradiation formed a network between the dielectric layer and substrate. Au nanosheets were transferred to the dielectric surface for the gate electrode. (B) The optical microscopy (OM) image shows an array of S/D electrodes on the SBS nanofiber mat and the P3HT fibers in between (with a length of 100 µm and width of 800 µm). (C) SEM image of the SBS fiber mat, Au plate electrode, and P3HT fibers. (D) Representative SEM image of the device. The inset shows a part of the array of 180 devices (15 × 12).
nanosheets can be directly printed by stamping method on the SBS fiber mat substrate, and the ioninc molecules can be continuously patterned by UV exposure through metallic masks. Figure 1B displays an optical microscopy (OM) image of an array of Au nanosheet electrodes and electrospun P3HT fibers on the SBS substrate. Figure 1C shows a magnified SEM image of the electrode. Typically, 30 P3HT fibers were connected between the Au nanosheet electrodes. To fabricate the ion-gel dielectric layer, a few drops of a solution mixture consisting of poly(ethylene glycol)-diacrylate (PEG-DA), 2-hydroxy-2-methyl propiophenone (HOMPP), and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([EMIM][TFSI]) (88:8:4, w/w) were placed on the substrate. The solution embedded the P3HT fibers and penetrated the top surface of the SBS nanofiber mat substrate, as indicated by the shaded region in Figure 1A. UV crosslinking through a patterned mask followed by washing the unexposed mixture solution beneath the mask produced ion-gel dielectric layers which bridged the source and drain electrodes.[31] The ion-gel and substrate formed an interpenetrated network structure, which is the key factor for good mechanical properties without being peeled off from the substrate. The Au nanosheets were transferred on the gel surface as a gate electrode by touching the Au nanosheet layer (∼80 nm thickness) to the ion-gel patterns. Figure 1D shows an SEM image of the transistor. We fabricated an array of 180 devices (15 × 12) on one SBS substrate. The inset SEM image of Figure 1D shows a part of the device array.
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The SBS nanofiber mat substrate was completely elastic up to ε = 0.4 and it had a small amount of remnant strain when it was stretched further. The remnant strain was removed by placing the nanofiber mat on a thin sheet of PDMS, which allowed complete elasticity up to ε = 1.0. The ion-gel dielectric layer was also completely elastic up to ε = 1.0, but it tore often at ε > 1.0. The stretchability of the Au nanosheet electrode and P3HT fibers on the fiber mat substrate was evaluated. Figure 2A displays the change of the resistivity of the Au nanosheet electrodes as the uniaxial strain was increased. We measured the resistivity change in the transverse direction of the electrodes because the electrical behavior of the active layer was more sensitive to elongation in that direction compared to the longitudinal direction. The stretchability of the electrode on the fiber mat was significantly enhanced compared to that on the flat PDMS substrate. The resistivity of the electrode on the PDMS substrate showed an abrupt increase at ε = 0.6 with a propagation of cracks, whereas the electrode on the fiber mat showed no difference (∼0.3 × 10−6 Ω·m) in the stretched state at ε = 0.9 and the released states. The enhanced electrical stretchability of the electrode on the SBS substrate was attributed to the observation that the first nanosheet imitated the surface topology of the fiber mat (Figure S3). The topological structure on the fiber mat had better tolerance to external strain than the fully extended nanosheets on a flat PDMS.[28] Figure 2B exhibits the resistivity of the electrode on the fiber mat over 3000 cycles of stretching at ε = 0.7. The
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Figure 2. Changes of the resistivity and conductivity of the Au electrode and P3HT active layer upon stretching. (A) Resistivity change of the Au nanosheet S/D electrodes on the SBS nanofiber mat as a function of strain. (B) Durability test of the Au nanosheet electrode on the mat over 2000 cycles of stretching at ε = 0.7. The resistivity was measured after relaxing the applied strain. (C) Conductivity change of the P3HT fibers connecting the S/D electrodes as a function of strain. The open circles show the measured values and the solid circles are corrected values considering the dimensional change of the substrate due to strain.
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electrode exhibited negligible changes of the resistivity during the cycling test. The excellent stretchability of the Au nanosheet electrode is attributed to the sliding between the nanosheets during stretching and to the stress reduction by following the topology of the mesoporous nanofiber mat.[27] To date, there have been no reports of flat semiconducting thin films which are stretchable up to a notable strain (for example, ε = 0.1). Recent studies employing a honeycomb-structured thin film of a polymer semiconductor exhibited enhanced stability of the transistor to uniaxial strain, but the maximum strain at which the device could operate was ε < 0.1.[35] To obtain sufficient stretchability of the active layer, we used electrospun P3HT fibers directly collected on the source and drain electrodes. Figure 2C shows the I–V characteristics of the P3HT fibers depending on the tensile strain. The conductivity gradually decreased as the strain increased, without the sudden drop which is typically observed in film-type active layers.[36] The slight decrease of conductivity is reasonable because the distance between the source and drain electrodes was increased by stretching. Considering the increase of the length and decrease of the width of the active layer, the corrected conductivity of the P3HT fiber layer was negligible at ε = 0.7, as shown in Figure 2C (right axis). Whenever gel electrolytes are used in electronic devices, the switching speeds are constrained because the movement of ionic molecules limits the frequency at which the devices operate reliably. The transfer curves of the transistors in this study were checked with varying frequency up to 100 Hz (Figure S4A,B). The on-current and off-current of the devices were not dependent on the frequency (Figure S4C). It is notable that we found the devices became unstable at 200 Hz and higher frequencies. The reason is not clear yet. The frequency dependence may be further reduced by decreasing the thickness of the ion-gel layer. We checked the change in capacitance of the ion-gel when the thickness was 100 nm (Figure S4D). The capacitance at 50 um thickness showed severe decrease from 500 Hz, while the capacitance at 100 nm thickness showed a good stability up to 1000 Hz and the decreasing slope at higher frequencies was much smaller. Figure 3 shows the performance of the transistors as a function of strain. An array of transistors on the fiber mat substrate was stretched uniaxially, as shown in the optical image in Figure 3A. Figure S5 in the supporting information displays the dimensional change of a transistor by stretching. Because all of the components of the transistor were stretchable, any symptoms of wrinkling were not detected up to ε = 1.0. The interpenetrated network structure between the gel dielectric and the SBS nanofibers provided high mechanical stability without causing peeling or mechanical damage on the transistors. Device failure was caused in the mechanically weak polyelectrolyte gel at ε > 1.0. The stable operation of the transistors was guaranteed up to ε = 0.7 in all of the transistors (15 × 12 device array). Figure 3B shows the transfer curves as a function of strain. The transfer curve and the output curve at ε = 0 are provided in the supporting information (Figure S6). Every device in the array showed the similar transistor performance. The average on-current value at 0% strain from the 180 devices was 2 ± 0.46 mA. More than 80% of the devices were within ±0.2 mA. The average mobility of the device at 0% strain was 22 ± 0.71 cm2V−1s−1, which indicates the mobility of the devices was highly reproducible. The transfer curves maintained the
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COMMUNICATION Figure 3. (A) Digital image exhibiting the dimensional change of the device array observed by stretching at ε = 0.7. (B) Transfer curves of the device obtained at different strains. (C) Mobility change as a function of strain. The measured mobilities (hollow) were converted into corrected values (solid) by considering the dimensional change of the channel layer. (D) Changes of the threshold voltages (Vth) of the device as a function of strain.
typical transistor behavior at all strains tested. The on-current of the transistor decreased as the strain increased, which is attributed to the increased channel length and decreased channel width. The transfer curve obtained after release from ε = 0.7 was the same as the curve obtained before stretching. The uniaxial stretching of ε = 0.7 generates ∼30% compression in the perpendicular direction. To investigate the effect of compression on the device performance, the device array on the SBS substrate was mounted on a thick PDMS, and compression was exerted in the direction of source and drain electrodes. An optical image of the compression at ε = –0.3 and the corresponding transfer curves are presented in supporting image (Figure S7). The device performance showed negligible change during repeated compression at ε = –0.3. The hole mobility (µ) and threshold voltage (Vth) were calculated from the transfer curve obtained during stretching. These results are displayed in Figure 3C and D, respectively. Without external strain, the mobility was 23 cm2/Vs in the saturation regime (VD = −1 V) based on the equation: IDS = (WCµ/2L)·(VG – Vth).[37,38] The fiber construction of the semiconductor calls a simple estimation of mobility based on parallel plate models of the capacitance should be compensated. Because the device mobility is more valuable than the mobility of each fiber, we calculated the mobility simply based on the channel dimension defined by the area covered by the ion gel dielectric (200 µm × 150 µm). So, the mobility calculated in this study is an effective mobility of the device. By applying the same channel dimension during uniaxial stretching, the apparent measured mobility (hollow circle) of the device continuously decreased as
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the strain increased, with a value of 7 cm2/Vs at ε = 0.7. This decrease was caused by the decrease of the on-current, as shown in Figure 3B. The absolute value of the threshold voltage (Vth) increased as the strain increased for the same reason. After releasing strain, the mobility and threshold voltage recovered the initial values obtained before stretching. By considering the change of the channel dimensions during stretching, the corrected mobility (solid square) maintained the same value up to ε = 0.4 and slightly decreased to 16 cm2/Vs at ε = 0.7. To estimate the correction, we measured the width and length of the channel. The measured channel dimensions were in good agreement with the dimensions calculated by assuming a Poisson’s ratio of an elastomer (0.5) for the substrate. Bao and coworkers reported that a large strain applied to a film of an organic semiconductor caused a large decrease of charge mobility even at ε < 0.1 because the increase of the inter-grain gap resulting from the strain can increase the energy barrier for hole-hopping.[39] Once the performance was largely degraded in the transistors using a thin film organic semiconductor, the transistors did not recover the initial performance.[36] The small mobility change of the P3HT fiber active layer even at a large strain (ε < 0.7) results from the curved contour path of the fibers. Durability of the device performance is critical for practical realization. Figure 4 summarizes the changes of the transfer curve (A), and the hole mobility and threshold voltage (B) during 1500 cycles of stretching at ε = 0.7. The transfer curves obtained during cycling at ε = 0.3 and 0.5 are shown in the supporting information (Figure S8). The electrical characteristics were measured after releasing the strain. The device mobility
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Experimental Section
Figure 4. Changes of the electrical performance of the device after repeated cycling at ε = 0.7. Each measurement was carried out at ε = 0 after relieving the strain. (A) Changes of the transfer curve, and (B) changes of the charge mobility and threshold voltage.
decreased slightly until the number of stretching cycles reached 500 (15.5 cm2/Vs). Thereafter, the mobility changed negligibly (18 cm2/Vs at 1500 cycles). The performance degradation exhibited during the small number of stretching events may be caused by the position change of the P3HT fibers while they were straightened repeatedly at large strains. Once their positions were stabilized without any further change at high strains, the device performance stabilized. The on-off current ratio and Vth remained unchanged during the cycling tests. In summary, we fabricated an array of high-performance stretchable polymer transistors constructed entirely of stretchable components of the transistors. We used a SBS electrospun elastomer nanofiber mat as the substrate, a stack of Au nanosheets as the electrodes, electrospun P3HT fibers as the active material, and ion-gel polyelectrolyte as the dielectric layer. The interpenetrating network structure between the ion-gel and the porous substrate provided the transistors with high mechanical stability under severe stretching events. The high electrical performance (∼18 cm2/Vs mobility, 105 on-off ratio) of the device was stable over 1500 cycles of stretching at ε = 0.7. 3710
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Materials: A silicon wafer was treated by oxygen plasma (O2 flow rate: 22 sccm, power: 50 W, time: 50 s) and dipped in a toluene solution containing octadecyltrichlorosilane (OTS, 0.3 wt%) for 5 min. The residual OTS was washed by fresh toluene. Polystyrene-b-polybutadieneb-polystyrene (SBS) block copolymer (Asahi Kasei Chemicals, volume fraction of polybutadiene = 70%, Mn = 130 000) was dissolved in a mixture solvent (THF:DMF = 3:1). For electrospinning, a 13 wt% SBS solution was fed through a metallic nozzle (diameter: 200 µm) at a rate of 15 µl/min under 18 kV DC. Electrospun SBS fibers were collected on the OTS-treated silicon wafer because the SBS fiber mat was easily peeled off on hydrophobic surfaces. The thickness of the SBS fiber mat substrate was 0.3 mm. Poly(3-hexylthiophene) (P3HT, Mw = 87 000, 95% regioregularity, Rieke Metal Co.) and poly(ε-caprolactone) (PCL, Mw = 80 000, Aldrich) were dissolved in chloroform at a mixing ratio of 7:3 (w/w). The total polymer weight fraction in the solution was 12 wt%. The solution was electrospun to the nanofiber mat substrate at 18 kV and Au nanosheet electrodes were patterned on it. The diameter of the metal nozzle was 21 G and the nozzle-to-collector distance was 20 cm.[33] For the ion-gel dielectric pattern, the ionic molecule ([EMIM][TFSI]), crosslinker (PEG-DA), and initiator (HOMPP) were mixed at the ratio applied in previous studies ([EMIM][TFSI]:PEG-DA:HOMPP = 88:8:4, w/w).[31,32] Device fabrication: Patterned electrodes were prepared with Au nanosheets (∼10 µm diameter, 20 nm thick) by following the procedure reported previously by our group.[27] The Au nanosheets were dispersed in butanol. Several drops of the suspension were dropped on water in a beaker. The Au nanosheet monolayer floated on water was transferred to the PDMS stamp with embossed rectangular patterns. The transfer process was repeated 6 times to obtain metallic conductivity of the electrode. The patterned stack of the Au nanosheets on the PDMS stamp was transferred onto the SBS substrate preheated at 90 °C. Electrospun P3HT fibers were collected on the substrate after the electrode was transferred followed by annealing at 100 °C for 1 h under vacuum. The solution mixture for the dielectric layer was deposited after electrospinning the P3HT fibers. To obtain patterns of the ion-gel layer, UV (365 nm, 300 mW/cm2) was irradiated for 2 s through a mask with square holes. The ionic solution in the unexposed area was removed by washing with ethanol. The same stack of Au nanosheets were collected on a PET substrate and then transferred onto the dielectric pattern. The device performance was characterized at ambient conditions using a semiconductor parameter analyzer (Agilent 4156A). Stretching tests were carried out using a ZBT-200 (Z-TEC.Co).
Supporting Information The electrical resistivity of the Au nanosheet electrode, surface roughness of the electrode on the SBS substrate, dimensional change of the device depending on strains, frequency dependence of the devices, effect of compression on the devices, and device characteristics. Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgments This work was supported by Samsung Research Funding Center of Samsung Electronics under Project Number SRFC-MA1301–07. B.L. thanks the support from the Center for Advanced Soft Electronics under the Global Frontier Research Program (No. 2011–0031628).
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Received: January 1, 2014 Revised: February 7, 2014 Published online: March 24, 2014
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