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Highly Stretchable Transistors Using a Microcracked Organic Semiconductor Alex Chortos, Josh Lim, John W. F. To, Michael Vosgueritchian, Thomas J. Dusseault, Tae-Ho Kim, Sungwoo Hwang,* and Zhenan Bao* Liberating electronic devices from the confines of traditional rigid substrates can improve mechanical robustness and enable new applications and manufacturing methods. Stretchability facilitates electronics that can be mounted on unconventional substrates,[1] such as lenses and human bodies,[2] and allows dynamic tuning of devices such as electronic eye cameras[3] and lasers.[4] Accommodating complex movements of supporting structures facilitates integration with moving entities and is critical for biointerfacing applications[5] and electronic skins[6–10] for prosthetics and robotics. Arrays of electronic devices often include transistors as active addressing elements in order to improve the signal collection process.[5,8] Furthermore, many applications, including sensor arrays[6,11] and displays,[12] require large area coverage and therefore benefit from low-cost, high-throughput fabrication methods. In this communication, we report a stretchable organic transistor that maintains transistor behavior to >250% strain, which is several times larger than previous reports.[13–16] Strain-independent characteristics are achieved by “programming” the device with an initial strain that causes the formation of microcracks in the semiconductor layer. Crack formation accommodates strain, while maintaining a percolating pathway. Similar microcracking[17,18] or void formation[19] strategies have been employed successfully in stretchable conductors used in applications such as strain sensors[19] and neuroprosthetic devices.[20] The fabrication process involves cost efficient solution methods including spraycoating and spincoating. Stretchable electronics can be fabricated using two main methods. The first involves geometrical patterning of conventional electronic materials such as metals and inorganic semiconductors into meandering patterns or buckles in order to reduce deformation in the active material.[21–24] Stiff islands connected with stretchable conductors have produced arrays of high-performance devices including transistors,[25]

A. Chortos, J. Lim, J. W. F. To, Dr. M. Vosgueritchian, T. J. Dusseault, Prof. Z. Bao Department of Chemical Engineering Stanford University 381 North-South Mall, Stanford, CA, USA E-mail: [email protected] T.-H. Kim, S. Hwang Nano Electronics Laboratory Samsung Advanced Institute of Technology Suwon 443–803, Korea E-mail: [email protected]

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photodetectors,[3] and LEDs.[12] However, because stretchability is imparted by the regions between islands, there is a trade-off between device density and stretchability. Buckling involves the formation of wavy structures that flatten out to accommodate applied strain. The optical properties of these wavy structures could benefit the performance of stretchable solar cells,[2,24] but may be undesirable for other optoelectronic applications,[26] or for devices that require planar interfaces. The second method of imparting stretchability is to fabricate devices composed of elastic materials. Because elastomers are typically insulators, electronic functionality is often imparted by blending with electronic materials[12,26] or applying thin films of compliant electronic materials.[15] Several intrinsically stretchable electronic devices have been reported, including stretchable light emitters based on electrochemical active layers[26,27] and graphene-[13] and MoS2-based[14] transistors that stretch to 5%. A hybrid method was reported by Chae et al. in which an intrinsically stretchable graphene-based gate electrode was combined with a buckled inorganic dielectric layer to make high-performance transistors that could sustain repeated strain cycles to 20%.[15] Inorganic semiconductors with proper fabrication schemes and device design can provide exceptional performance in stretchable electronic devices,[3,28] but their high processing costs limit implementation in applications where devices need to be disposable, cheap, or cover large areas. Organic semiconductors (OS) are an alternative that have been touted for their materials availability and compatibility with highthroughput, room-temperature deposition methods such as slot die coating[29] and inkjet printing.[30] While the performance of OS has traditionally lagged behind that of their inorganic counterparts, their electrical characteristics have been steadily improving,[31] and their mechanical properties may be more suitable for compliant electronics. The strain tolerance of OS can vary; research by O’Connor et al. suggests that a rigid, 3D packing structure results in a very low strain at fracture, while the 2D packing structure of a common polymeric semiconductor poly(3-hexylthiophene) (P3HT) allows deformation to greater than 150%.[32] Consequently, P3HT was chosen as the semiconductor in our stretchable transistors due to favorable mechanical characteristics, well-characterized properties,[33] and ready availability. Devices were fabricated in a bottom contact, top gate architecture, as depicted in the schematic in Figure 1a. The source and drain (S/D) electrodes were composed of carbon nanotubes embedded in a polyurethane elastomer (PU), similar to electrodes first published by Pei and coworkers.[27] The sheet resistance of the electrodes before and after embedding in PU

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to 20% are provided in Figure 1d. Additionally, exposure to air is known to result in a lower on/off ratio for P3HT transistors due to oxygen and water doping.[36] Furthermore, the loose packing and flexibility of PU results in a tendency to absorb polar solvents, which can have several potential effects on the device performance: (1) solvents can disrupt charge transport, reducing charge mobility[13] (2) absorbed water increases the polarization in the dielectric and consequently the measured on current (ION), distorting mobility values,[37] and (3) polar groups can increase leakage, off currents (IOFF), and hysteresis.[38] While there is much room for improvement in dielectric selection, the main purpose of this report is to analyze the relative performance of the device with applied strain. The changes in P3HT morphology with strain were investigated using three structures: (1) P3HT transferred onto a PU substrate (PU/P3HT-t) (Figure 2a-c), (2) PU Spincoated onto P3HT and then transferred onto a PU substrate (PU/P3HT-s) (Figure 2d-f) and (3) (PU/P3HT-t) with a dielectric spincoated on top (PU/P3HT/PU) (Figure 2g-i). In the PU/P3HT-t structure, optical microscopy revealed that cracking started in the semiconductor at strains less than 15% strain. At 65% Figure 1. Summary of device fabrication and performance a) Schematic of the fabrication pro- strain, large cracks with widths in the range cess. Microscope image of a device with b) zero and c) 150% applied strain. d) Transfer curves of 10 μm were observed (Figure 2b). In addifor a device stretched to 160% and released to 20%. The source drain bias was −20 V and the tion to the microscale cracks observable with W/L is 40. optical microscopy, smaller nanoscale cracklike defects could be resolved using AFM (Figure S3). Continued stretching increased the crack width was ∼155 Ω/sq and 945 Ω/sq, respectively. P3HT films were (Figure 2c). With PU/P3HT-s, the onset of observable microtransferred from a wafer coated with a hydrophobic silane scale cracking occurred between 40% and 65% (Figure 2e). monolayer[34] onto the embedded CNT electrodes, and a 4 μm Similar to the PU/P3HT-t structure, the crack length and width thick PU dielectric layer (Figure S1) was spincoated on top. increased with strain (Figure 2f). However, the widths of the The device was completed by applying eutectic gallium indium cracks in the PU/P3HT-s structure were in the range of hun(EGaIn), a liquid metal, as the gate electrode. Figure S2 provides dreds of nanometers to several microns. The improved stretchmicroscope images of a device at different stages throughout ability may be due to a better adhesion of P3HT on the spin the fabrication process, and a more thorough description is coated PU inhibiting crack formation in the semiconductor. given in the experimental section. Figure 1b and 1c depict the Observations for PU/P3HT/PU were similar to those from PU/ device at 0% and 150% strain. P3HT-s. The crack size in this structure is much smaller than The electrical characteristics of the assembled transistors the device dimensions, indicating that crack formation does not were collected using a probe station in a nitrogen environinduce substantial variability between devices. ment, and a typical transfer curve is provided in Figure 1d. The average and standard deviation of the transistor characteristics The orientation of polymer backbones can be investigated include mobility (μ) values of μ = 3.4•10−2 ± 1.63•10−2 cm2/Vs using UV-Vis measurements polarized perpendicular and parallel to the stretching directions. The dichroic ratio (R) and an on/off ratio of 591 ± 461. The large variation in device is defined as the ratio of the peak intensity of the absorption characteristics was a consequence of the manual fabrication polarized parallel to the stretching direction divided by the peak processes, which included transferring the semiconductor, intensity of the perpendicular absorption (R = A储/A⊥). When spincoating the dielectric, and applying the top gate. The device performance is expected to be closely related to the nature of strain is accommodated by continuous plastic deformation the dielectric. A large dielectric thickness and concomitant low in P3HT, alignment of the polymer chains in the direction of capacitance (∼1 nF/cm2) can lead to impaired mobility values, stretching causes anisotropy in the optical absorption, resulting in a dichroic ratio larger than 1.[39] The trend in R with strain due to a lower applied electrical field. Correspondingly, the on/ off ratio was also lower than some other reports.[35] Transfer displayed key differences between the different structures with P3HT (Figure 2j). The PU/P3HT-t sample exhibited a slight characteristics of a device stretched to 160% and released back

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COMMUNICATION Figure 2. Effect of strain on the structure and morphology of P3HT. a-c) PU/P3HT-t, d-f) PU/P3HT-s, and g-i) PU/P3HT/PU under different strain values. j) The dichroic ratio (R) extracted from UV-Vis measurements.

increase in R at small strains followed by a gradual return to 1. This suggests that a small amount of plastic deformation is accommodated before cracking occurs. Subsequent stretching resulted in misorientation of the partially-aligned domains, causing R to approach 1. In contrast, the PU/P3HT-s and PU/ P3HT/PU structures exhibited a linear increase in R up to ∼1.3 at 50% strain, followed by a region with relatively little change in R. The UV-Vis observations are consistent with those from optical microscopy; both characterization methods indicate that large-scale cracking begins below 15% strain for P3HT transferred onto PU (PU/P3HT-t) and in the range of 50% strain for the systems in which spincoating was used (PU/P3HT-s and PU/P3HT/PU). Adhesion of a plastic material to a deformable substrate improves ductility by limiting the localization of strain that is responsible for crack formation.[40] The spincoating process may facilitate excellent adhesion by conformally coating the P3HT, limiting delamination. UV-Vis spectra collected

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before and after spincoating the dielectric (Figure S4) suggest that the semiconductor film is largely unchanged. Several reports have described the continuous deformation of P3HT to large strain values without the formation of cracks. However, these reports used high temperatures during stretching[41] or higher molecular weight P3HT with improved adhesion promoted by UV/Ozone treatments.[39] UV/Ozone treatments were detrimental to the device performance in this work because of doping effects that increased IOFF. Electrical measurements were collected while stretching the devices both perpendicular and parallel to the direction of current flow in the channel (Figure 3a). When stretching was undertaken parallel to the charge transport direction (perpendicular to the orientation of the S-D electrodes), the ION decreased rapidly. The experiment was concluded at 140% strain when the On/Off ratio reached below 10. Compared to the parallel direction, the ION decreased more slowly while stretching in

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Figure 3. Electrical characteristics of devices with stretching. a) On and off currents for devices stretched parallel and perpendicular to the charge transport direction. The currents are reported as measured, and are not adjusted for changes in device dimensions. b) Calculated gauge factors. c-d) Mobility and leakage currents with strain. e) Strain series indicating reversible characteristics at low strains and increasingly irreversible characteristics at higher strains. f) Normalized ION for three cycles to 100% strain demonstrating reproducible, strain-independent characteristics.

the perpendicular direction, and the devices exhibited transistor characteristics to ∼265% strain (Figure 3a). These uniaxial strain values are significantly larger than reported in previous work.[13–16] The output characteristics displayed in Figure S6 showed that there is little contact resistance in the measured strain range. Contact resistance is discussed more thoroughly in the supplementary. The gauge factor (GF) is a parameter that is commonly used to quantify the sensitivity of strain sensors. GF is defined as (ΔR/R0)/ε, where ΔR is the change in resistance, R0 is the initial resistance, and ε is the strain. A small GF is sought for application in stretchable active matrices. In the parallel stretching direction, the GF began at ∼7 at low strains, and slightly decreased before increasing at high strains (Figure 3b). In contrast, devices stretched in the perpendicular direction exhibited a GF close to 2 throughout the measured strain range. These values compare favorably to a GF of >10 estimated from published data on stretchable graphene transistors.[13] The mobility of the devices decrease at a similar rate for both the perpendicular and parallel stretching directions (Figure 3c). The method of calculating mobility is included in

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the supplementary information. The increase in leakage current (Figure 3d) is consistent with a reduction in the dielectric thickness. The threshold voltage does not show a stable trend with strain (Figure S7). A strain series was conducted in the parallel direction (Figure 3e), which shows reversible characteristics at small strains, with the degree of irreversible changes increasing with strain. Strain-independent ION was achieved during multiple perpendicular stretching cycles (Figure 3f); ION decreased by ∼50% during the first cycle and remained relatively constant throughout the subsequent cycles. Similar strain-independent properties after an initial prestretch have been observed in CNTbased stretchable electrodes[6] and in pentacene transistors.[42] Reversible changes in OS transistors have been observed only at low strains,[43,44] which is consistent with our observations. In both stretching directions, IOFF was limited by leakage through the dielectric, which changed modestly with strain (Figure 3c). Consequently, the trend in the on/off ratio followed ION. The collected data facilitate the consideration of possible mechanisms for strain-dependence in the electrical properties. Microscale cracking was not observed until ∼50% strain,

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indicating that the rapid reduction in ION at low strain values must be attributed to other processes. One possible process could be the formation of nanoscale cracks observed by AFM in Figure S3, which could not be resolved using optical microscopy. Alternatively, work on flexible devices has suggested that strain sensitivity can arise from both changes in the separation between molecules[44,45] and changes in the separation between crystallites.[43,46] The relatively constant shape of the UV-Vis spectra with strain (Figure S9) suggests that increased separation between crystallites is more likely than changes in intermolecular distance. At larger strain values, irreversible characteristics begin to be observed (Figure 3b), which we attribute to the onset of cracking. The irreversible behavior associated with crack formation allows the devices to be “programmed” to have strain-independent properties within a chosen strain range (Figure 3d), providing a method to create reliable stretchable transistors consisting of conventional organic semiconductors. Despite the formation of cracks, the change in mobility shows the same trend in the perpendicular and parallel stretching directions. This is contrary to the findings of O’Connor et al.,[39] who found that the mobility increases in the perpendicular direction and decreases in the parallel stretching direction. However, it is consistent with some studies on strained pentacene devices.[45] Additional considerations include the role of the electrodes and dielectric. While the conductivity of the S/D electrodes decreased with strain (Figure S10), this is not expected to significantly affect the device performance due to the small currents extracted from the device. The strain-induced changes in dielectric capacitance were consistent with what would be expected based on a Poisson ratio of 0.5 (Figure S11). While increasing capacitance should improve device performance, the effect was evidently overshadowed by changes in the semiconductor layer. The cycling stability of the electrical characteristics was investigated by repeatedly applying 40% strain perpendicular to the charge transport direction. The device characteristics were measured in the unstretched state ∼5 min after completing 1, 10, and 100 cycles, and the transfer curves are depicted in Figure 4a. IOFF remained constant throughout the cycling measurements. After the first cycle (initial programming), ION decreased by 17% after cycle 10 and 28% after cycle 100. This cycling performance could be related to the changes in P3HT morphology and the viscoelastic properties of the substrate. R decreased from ∼1.3 after one cycle to ∼1.2 after 300 cycles, indicating some reorganization of the P3HT (Figure 4b). Observations from AFM (Figure S12) provide support for the morphology change with increased cycle number. In addition to the changes in semiconductor morphology, the viscoelastic properties of the substrate can affect cycling results. Physical crosslinking elastomers, such as the thermoplastic PU used in this work, are known to exhibit large mechanical hysteresis,[47] and the stress-strain behavior of the substrate (Figure S13) indicated that viscous deformation increases with increasing cycles. The extension ratio (λ) at zero stress was taken as a measure of the extent of viscous deformation. A plot of 1/λ vs strain displays a similar trend as the normalized ION (Figure 4c), providing support for a relationship between the substrate viscosity and the cycling performance. To further

Figure 4. Effect of cycling and viscoelastic properties of the substrate on the electrical characteristics. a) Transfer curves collected after releasing the device following the designated number of cycles. b) Effect of strain cycles on the dichroic ratio (R), indicating rearrangement of P3HT chain conformation. c) A comparison showing excellent correlation between ION and 1/λ. d) Effect of time on the transfer characteristics measured after the completion of 100 cycles.

characterize the effect of substrate viscosity, transfer characteristics were collected at different times after the completion of 100 cycles to 40% strain (Figure 4d). Over a 40 minute period, ION increased from 0.65 μA to 0.80 μA, which can be attributed to the continued contraction of the substrate toward its initial dimensions. In contrast, pentacene devices operated within the elastic strain range of the substrate (2.6%) have shown no cycling dependence.[42] The observation of a significant timedependence in the electrical properties emphasizes the importance of elastomer viscoelastic properties in stretchable devices. Chemical crosslinking elastomers, which typically display less viscous response,[48] may be more appropriate for compliant electronics. In summary, stretchable transistors have been fabricated that exhibit transistor characteristics to large strains. Embedding the semiconductor between two elastomer layers was found to modify the deformation processes and suppress the formation of cracks. Strain-independent characteristics were achieved by programming microcracks with an initial strain. Subsequently, measurements within the range up to the initial strain were relatively constant (Figure 1d). The cycling performance of the device is related to both the change in P3HT morphology and the viscous deformation in the substrate, highlighting the importance of implementing elastomers with minimal viscous response. Further work is required to gain a complete understanding of the origin of the strain-dependent electrical properties of the devices. While the described devices provide

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an initial platform to study several phenomena in stretchable electronics, there are many potential avenues for improvement. Replacing the dielectric with a more resistive elastomer that absorbs fewer solvents could improve the device characteristics. Furthermore, the stretching performance could be improved by implementing a more elastic semiconducting film. Lastly, for practical applications, the liquid metal electrodes must be replaced with a solid state conductor.

Experimental Section All solvents were purchased from commercial sources and used as received. P3HT (Sepiolid P200, MW = 20 to 30 K, ave PDI = 2.0) and soluble polyurethane (Tecoflex SG80A) were kindly donated by BASF and Lubrizol, respectively. Device Fabrication: Polyurethane (PU) substrates were created by dissolving PU in dimethylformamide (DMF) at a concentration of 50 mg/mL. The solution was cast into a glass petri dish and the solvent was evaporated at 120 °C. The final substrate thickness was 500 μm. Arcdischarge CNTs (Hanwaha Nanotech Corp.) in N-methylpyrrolidinone (NMP) at a concentration of 0.165 mg/mL were ultrasonicated (Cole Parmer ultrasonic processor 750 W) for 30 min at 30% power. The resulting dispersion was centrifuged at 8000 RPM for 30 min to remove bundles and contaminants, and the top 75% of the supernatant was retained for spraycoating. Si wafers were cleaned in UV/Ozone for 20 min prior to deposition. The substrates were heated at 200 °C on a hotplate and a commercial airbrush (Master Airbrush, Model SB844-SET) was used to spraycoat the CNT/NMP dispersion through a magnetic shadow mask to define the source and drain electrodes. Due to limitations in the spraycoating process, the channel length was 400 μm. PU in tetrahydrofuran (THF) (10 mg/mL) was cast on the electrodes and the solvent was allowed to evaporate while covered with a glass dish. The electrodes were transferred onto a 500 μm thick PU substrate for easy handling. Silicon wafers treated with a self-assembled layer of octadecyltrimethoxysilane (OTS) were prepared by spincoating a solution of OTS in trichloroethylene followed by a vapour treatment in ammonium hydroxide. P3HT in chloroform (10 mg/mL) was spincoated onto the OTS-coated wafers using a two-step program: (1) 0 RPM for 30 s, and (2) 1000 RPM with 500 RPM s−1 acceleration for 1 min. The resulting P3HT films were manually transferred onto the CNT/ PU electrodes at 60 °C by applying gentle pressure for 2 min. The PU dielectric was deposited by spincoating from THF (40 mg/mL) at 1000 RPM. During the spincoating process, the device was placed off-center from the spincoater chuck to ensure that a portion of the electrodes remained exposed. To complete the device, eutectic gallium indium (EGaIn) liquid metal was applied as a top gate using a syringe and needle. The manual process of applying the EGaIn resulted in variation in the W/L of the devices, and the channel width was later measured using optical microscopy. The W/L ratio was ∼12. Capacitors used to measure the dielectric properties of the elastomer with strain were fabricated by spraycoating CNTs onto Si without a shadow mask and then embedding them in PU. PU was spincoated on top from THF (40 mg/mL), and EGaIn was used as the top electrode. Characterization: The sheet resistance of the CNT electrodes was measured using a four point probe setup connected to a Keithley 2400 sourcemeter. The resistance of the CNT electrodes was investigated by securing the film into a linear actuator (Newmark Systems) that had been modified in lab to include clamps. EGaIn was used to make electrical contact to the clamped sections of the conductor. For the PU capacitors, EGaIn was again used to make contact to the clamped, bottom CNT electrode, and a micromanipulator was used to make contact to the top EGaIn electrode. The electrical characteristics were collected using an LCR (inductance, capacitance, resistance) meter (Agilent E498E precision LCR meter) controlled with a LabView Script.

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To collect UV-Vis and optical microscope images, thin PU substrates (∼80 μm) were used to facilitate easy deformation. Strain was applied by stretching the substrate and applying Kapton tape to secure it to a glass slide. Microscope images were collected using a Leica DM4000 M microscope equipped with a digital camera and operated in Differential Interference Contrast (DIC) mode. UV-Vis spectra were collected using a Cary 6000i UV/Vis/NIR Spectrometer equipped with a polarizer that could be rotated relative to the sample. Electrical characteristics of the transistors were collected using a probe station in a nitrogen environment connected to a Keithley 4200. The source and drain electrodes were probed using 50 μm gold wire attached to the tungsten probes of the micromanipulators, while contact to the EGaIn gate was made using a tungsten probe. Stretching measurements were facilitated by a homemade, hand operated stretching apparatus. The engineering strain in the device was measured directly using digital calipers. Cycling measurements were conducted in a nitrogen environment by hand using a second stretching apparatus. Electrical measurements were collected in the released state after completing the designated number of cycles. Mechanical properties of the PU substrate were collected using an Instron 5565 with a 5 kN load cell. 100 cycles were completed at a strain rate of 1 mm s−1.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work is partially supported by the Samsung Advanced Instituted of Technology and Air Force Office of Scientific Research (FA9550–12– 1–0190). This research was partially supported by the MSIP (Ministry of Science, ICT and Future Planning), Korea, under the “IT Consilience Creative Program” (NIPA-2014-H0201–14–1001) supervised by the NIPA (National IT Industry Promotion Agency). Received: November 3, 2013 Revised: March 18, 2014 Published online:

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Highly stretchable transistors using a microcracked organic semiconductor.

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