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Highly stretchable and waterproof electroluminescence device based on superstable stretchable transparent electrode Banseok You, Youngmin Kim, Byeong-Kwon Ju, and Jong-Woong Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14535 • Publication Date (Web): 19 Jan 2017 Downloaded from http://pubs.acs.org on January 21, 2017

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ACS Applied Materials & Interfaces

Highly stretchable and waterproof electroluminescence device based on superstable stretchable transparent electrode

Banseok You,1,2 Youngmin Kim,1 Byeong-Kwon Ju,2,* and Jong-Woong Kim1,*

1

Display Materials & Components Research Center, Korea Electronics Technology Institute,

68 Yatap-dong, Bundang-gu, Seongnam 463-816, South Korea

2

Display and Nanosystem Laboratory, College of Engineering, Korea University, Seoul 136-

713, Korea

KEYWORDS: Stretchable device; waterproof; electroluminescence; silver nanowire; polyurethane urea

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ABSTRACT Realization of devices with enhanced stretchability and waterproof properties will significantly expand the reach of electronics. To this end, we herein fabricate an elastic transparent conductor that comprises silver nanowires (AgNWs) on a hydroxylated polydimethylsiloxane (PDMS) substrate covered by polyurethane urea (PUU), which is fully compatible with both materials. Carboxylic acid groups of PUU was designed to form hydrogen bonds with the carbonyl groups of poly(vinylpyrrolidone) on the AgNW surface, resulting in an enhanced affinity of AgNWs for PUU. Exceptionally strong hydrogen bonds between PUU and the hydroxylated PDMS thus facilitate the achievement of water sealable, mechanically stable, and stretchable transparent electrodes. To fabricate stretchable electroluminescence (EL) devices, ZnS particles were mixed with PUU, and the mixture was coated onto the AgNWs/hydroxylated PDMS, followed by a face-to-face lamination with another identical electrode. The devices could be stretched up to 150% without a severe reduction in the emission intensity, and they survived 5000 cycles of 100% stretch–release testing. The high adhesion between PUU and PDMS even in water is responsible for the good waterproof characteristics of the EL devices. These results pave the way for realization of fully stretchable and waterproof electronic devices.


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INTRODUCTION

Technologies for realizing optoelectronic devices on stretchable or nonplanar plastic substrates have been widely studied1-3 because of the tremendous potential of such devices for novel applications, such as stretchable displays, solar cells on curved three-dimensional (3D) objects, light emitting skins providing biomedical functionalities, and conformal in vivo devices to interface with various internal organs,4 that cannot be achieved using rigid electronics. The largest obstacle, however, has been the development of stretchable and transparent conductors that can be used for stretchable optoelectronic devices.5 Transparent electrodes are ubiquitously used in devices in which light must pass through a layer to which either a current or voltage is applied.6 Various types of stretchable transparent electrodes,7 such as graphene,8-14 carbon nanotubes,15-19 conductive polymer,20,21 ionic conductor,22 biomimicking composite materials,23,24 and metallic nanomaterials formed on stretchable polymers,25-29 have been developed for optoelectronic devices . Most of these electrodes had been demonstrated to function in light emitting diodes, solar cells, or sensors; however, they all have limited stretchability and mechanical stability.

The percolated network structure of silver nanowires (AgNWs) has emerged as a paradigmatic one-dimensional metallic nanomaterial and is attracting considerable attention because of its outstanding electrical and optical merits.30-38 The ductile nature of AgNWs has led them being considered as a suitable material for achieving stretchable transparent electrodes. Embedding AgNWs into the surface of a stretchable polymer has reportedly resulted in mechanically robust electrodes that can withstand large strains of up to 30–80%, without a significant loss of conductivity.26,39-41 Liang et al. reported that an intrinsically stretchable transparent electrode was attained by embedding AgNWs in their own polyurethane acrylate (PUA) material, which was used in an elastomeric stretchable polymer 3 Environment ACS Paragon Plus


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light emitting diode.40 Their device can emit light when exposed to strains as large as 100% and survive repeated continuous stretching cycles. However, the electrical conductivity of the electrodes, and thus the emission intensity of the polymer light emitting diodes, decreased considerably with the stretching cycles because of a variety of reasons, such as the viscoelastic nature of the PUA and damage to the light emitting polymer and the AgNWs. We should therefore note that the light emitting materials as well as the transparent electrodes should be sufficiently stretchable in order to achieve stretchable light emitting devices. In an attempt to resolve this, Wang et al. dispersed ZnS:Cu in polydimethylsiloxane (PDMS) and sandwiched the material between two AgNWs-based electrodes formed directly on the PDMS.42 Using their approach, the large deterioration of the emission intensity upon stretching could be resolved, but damage to the AgNW electrodes remained an issue. Some other studies have shown that AgNWs-based elastomeric electrodes are one of the best choices for stretchable transparent electrodes,27-35 but thorough studies on the damage to the AgNWs caused by high strain or repeated stretching have not been reported.

Keeping in mind that the reported fracture strain range of AgNWs does not exceed 4%,43 the reported stretchability of AgNWs-based elastomeric electrodes of up to 100% seems unreasonable. Revisiting the study by Liang et al., the resistance of the electrodes in fact increased continuously with an increasing number of stretching cycles, possibly owing to an accumulation of damaged AgNWs, e.g., disconnected wires.40 An interesting point is that the resistance of an electrode with a lower sheet resistance (Rs) increased at a more moderate pace with increasing strain compared with an electrode with a higher Rs. This may due to the undamaged nanowire parts cancelling the stretching effect on damaged wires at higher densities. But even for a high density, low Rs electrode, resistance gradually increased for an increasing number of repeated stretching cycles. Unless this issue is resolved, mechanically reliable transparent electrodes for stretchable optoelectronic devices cannot be realized. 4 Environment ACS Paragon Plus

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Herein, we introduce a novel approach to achieve AgNWs-based elastomeric transparent electrode that has an ultrahigh mechanical stability even under severe stretching. For this, we designed and synthesized a stretchable transparent polymer, named polyurethane urea (PUU), to enhance the mechanical stability of the AgNWs that were directly deposited onto a PDMS film. A thin PUU overcoat layer on the AgNWs/PDMS structure effectively preserved AgNW conductivity under various harsh testing conditions, such as tape-testing, repeated stretch testing, and dipping in water. PUU was also used to fabricate stretchable light emitting materials by dispersing ZnS particles in it, which resulted in a ZnS-PUU composite. By combining the electrode with this composite material, a highly stretchable and waterproof light emitting device was achieved: the device emitted light even when it was subjected to 150% strain or immersed in water for 30 min.



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RESULTS AND DISCUSSION

Before fabrication of the stretchable transparent electrodes employing PUU, we needed to investigate the effects of placing significant strain on the microstructure of the AgNWs-based elastomeric electrodes. We had to know what sort of failures might occur in the nanowire network or at its interface with the underlying substrate. For this, we first prepared a stretchable transparent electrode with AgNWs formed on a stretchable polymer according to a method illustrated in Figure S1. A PDMS film was formed on glass via spin coating from solution followed by thermal curing; this resulted in a film thickness of 240 µm. Instead of synthesizing a new elastomeric polymer, we employed PDMS because of its high transparency, neutral coloration, high elasticity, large elongation up to 160–180%, and biocompatibility.44 Among these properties, we were particularly interested in its high elasticity. In our pretesting to compare the elasticity of various elastomers including PDMS, polyurethane (PU), PUA, and some other silicone-based polymers, we found that the PDMS has the lowest viscosity in its elasticity and the smallest plastic deformation when it was stretched up to 150% strain. This should contribute to the reversible deformation of PDMSbased electrodes during stretch–release testing, which should guarantee it a high mechanical stability. One demerit that can be found in using PDMS with AgNW dispersions is its hydrophobicity, which originates from its low surface energy. To facilitate the wetting of AgNW dispersions and promote nanowire adhesion to PDMS, the PDMS films were oxygen plasma treated to form hydroxyl functional groups on the PDMS film surface. Then, AgNWs dispersed in isopropanol (in 0.9 wt%) were deposited on the PDMS and dried via infrared radiation; the resulting film was peeled from the glass substrate, resulting in a stretchable AgNWs/PDMS transparent electrode with a simple structure.


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We investigated the surfaces of the AgNW electrodes under varying levels of strain; using scanning electron microscopy (SEM), and the results are summarized in Figure 1. As shown in Figures 1a and 1f, a dense nanowire forest is safely deposited on the PDMS surface, resulting in a porous, networked structure. Using atomic force microscopy (AFM), the film was determined to be rather rough, with peak-to-valley (Rpv) and root-mean-square (RRMS) roughness values of around 331 nm and 48 nm, respectively. With such a rough structure, many parts of the nanowire networks would not be able to form a stable contact with the functionalized PDMS surface and might therefore be easily delaminated by external stresses. Both at low and high magnification, no defects such as disconnections of the nanowires or polymer cracking were observed. However, the surface of the electrode that was stretched under a strain of 25% differed from that of the pristine electrode, as shown in Figures 1b and 1g. When stretched under a strain of 25%, the AgNW forest forms as an ordered network of nanowire ligaments separated with vertically aligned microcracks in the stretching direction. These microcracks formed even under a smaller strain of 5%, as shown in Figure S2, implying that a strain larger than the fracture strain of the AgNWs (4%) causes a specific type of defect. The width and density of the cracks increased along with the applied strain, creating a fine ligament network of AgNWs at high strain levels (Figures 1 and S2). The PDMS substrate elongates in a specific direction and contacts with AgNWs during stretching up to 4% strain, but higher strain levels are believed to detach some sections of the nanowire network from the underlying PDMS, causing the crack-like structures in the stretched state. We suggest that specific sections of the nanowire network that do not contact the PDMS may cause the microcracks.

An interesting point from this result is that the microcracks formed a regular pattern in a spontaneous manner to compensate the large strains induced. The regularly separated nanowire ligaments should increase the electrode's resistance, but also allow room for it to 7 Environment ACS Paragon Plus


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recover its original shape and resistance. The microstructural images in Figures 2a and 2b support this hypothesis. Figure 2b shows a cracked nanowire electrode after being released; the distinct black-colored cracks have faded to gray, implying that the separated ligaments were rejoined. We measured the resistance of the electrode during a stretch-release test employing a maximum strain of 50%, as shown in Figure 2c. The resistance increased and decreased with each stretch and release, respectively, with negligible change over the first few cycles. In this investigation, we assume that the AgNW ligaments form an electrically percolated network. When the elastomeric substrate is stretched by more than 4%, the previously mentioned regularly shaped ligaments are formed. But then, as the stretching increases, the ligament network twists and deflects out of plane so that a large elongation of the elastomeric substrate induces only weak elastic strains in the AgNWs network.45 From these results, we derived general principles for designing elastic transparent conductors on elastomeric substrates. Considering that there are very large resistance increases, the structure we fabricated could be used as a stretchable piezoresistive strain sensor that measures the deformation of stretchable objects only if the resistance trends can be stably reproduced.

The resistance measurement in Figure 2c, however, shows that the resistance variation was not reversible: the curve's peaks gradually shift to higher values. After about 50 cycles, the peak resistance was significantly higher and the recovery time to the lowest value was significantly prolonged. This means that further variations in the network structure occurred, revealing a poor mechanical stability of the electrodes. Therefore, they would be unsuitable for the fabrication of both stretchable optoelectronic devices and sensors. To improve their mechanical stability, we buried the surface nanowires in stretchable polymers by employing an inverted layer processing technique.46 The main point of this approach is that this process enlarges the contact areas between the AgNWs and the polymer, resulting in enhanced adhesion. We investigated the embedded electrodes during the stretching process, but 8 Environment ACS Paragon Plus

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interestingly, the formation of the ligament network was not observed, possibly because in this case the nanowires do not behave as a body but individually. The individual nanowires were considered to be disconnected underneath the surface of the polymer during stretching greater than 4%; therefore, they should affect the conductivity of the electrodes.

To evaluate the mechanical stability of the embedded structures, we used a tape adhesion and a cyclic stretch–release test employing 50% strain, as shown in Figure 3. Unfortunately, the embedding approach did not effectively enhance the mechanical stability of the AgNWs/PDMS electrodes. After only a few cycles of tape attach–release testing the resistance of the electrodes increased significantly, while the resistance variation continuously increased as more stretch–release cycles were completed. This poor stability has two origins: the first one is the weak adhesion between the nanowires and the PDMS and the second being the low strength of the PDMS. We should note that the PDMS layer covering the embedded AgNWs was very thin and therefore not strong enough to sufficiently protect the underlying nanowires from external stresses. Nanowires that were detached from the PDMS by tape testing are shown in Figure S3.

Sometimes, a cover layer can be useful limiting stress singularities, which can form at the interfaces between the nanowires and the polymer when the electrode is mechanically strained.37 Therefore, in order to further improve the mechanical stability of the AgNWs/PDMS electrodes, we also prepared a PDMS/AgNWs/PDMS sandwich structure using the method schematically outlined in Figure S4. A PDMS layer was partially cured at 70°C for 20 min before an overcoat of liquid PDMS was applied on top of it. To integrate the two layers, the sample was fully cured at 70°C for 24 h. The mechanical stability of the sample was then tested by repeated stretching with a strain of 50% for 5000 cycles, as shown in Figure S5. However, the resistance measurement showed that the electrodes were not fully 9 Environment ACS Paragon Plus


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stabilized even with this approach owing to the lack of anchoring sites remaining in the partially cured PDMS. After 5000 cycles of stretching, the two PDMS film can be easily separated even with mild peeling conditions using tweezers.

To address all these issues, we employed a transparent and stretchable polymer, named PUU, which is fully compatible with both AgNWs and PDMS.44 The synthesis scheme is shown in Figure S6, and its characteristics are described in our previous studies.44,47 PUU displayed stronger hydrogen bonding than PU owing to the urea moieties, which donate two hydrogen atoms.44,48 PUU was designed to enhance the compatibility of the AgNWs for PUU since we can expect that poly(vinylpyrrolidone)'s carbonyl groups, located on the surface of the AgNWs, form hydrogen bonds with the carboxylic acid groups of PUU.44,47 Here, a PUU overcoat was applied to the AgNWs, which were deposited on hydroxylated PDMS (Figure 1a), where hydrogen bonding between the hydroxyl groups and urea (or urethane) was expected to enhance the inter-layer interactions.47 By employing PUU as a cover protection layer, the AgNWs could be very stably adhered to the PUU and in turn contacted with the PDMS, resulting in a mechanically stabilized stretchable electrode.

Figure 3 shows the beneficial effects of the PUU covering on the mechanical stability of the electrodes. The resistance of the electrodes did not vary at all even after conducting 15 tape tests, and the resistance variation with a repeated stretching of 5000 cycles employing 100% strain was very uniform. Furthermore, the resistance dynamic range was much smaller than for the embedded case. To analyze this, we investigated the electrodes after stretching with varying strain loads as shown in Figures 4a–4j. Because of the formation of the thin PUU layer on the AgNWs, the image contrast of the nanowires to the PDMS was reduced. As in the preceding case without PUU, the nanowire formed a percolated network of nanowire ligaments with vertically aligned microcracks in the stretching direction. However, 10 Environment ACS Paragon Plus

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interestingly, the microcracks were less ordered so that the percolating paths were more abundant even for the highest applied strain values. The magnified images in Figures 4g–4j indicate that the disconnected nanowires were still in contact with each other and the gaps between them were less severe. This was unaffected by repeated stretch–release testing, hinting at why the resistance dynamic range was smaller and the electrodes more stable than bare AgNWs/PDMS. In order to utilize this structure in optoelectronic devices, its conductivity and transparency must also be evaluated: Figure S7 shows the transmittance and haziness of the PUU/AgNWs/PDMS structure along with its corresponding Rs. Considering that the resistance of the electrodes should increase dramatically when a high strain is applied, we designed the electrode to exhibit a Rs of around 3 ohm/sq. The transmittance and haziness of the electrode were 70.1% and 8.9%, respectively. These results are much better than those of commercial indium tin oxide films, which are not even flexible.38

To evaluate the performance of the fabricated electrode, it was used in the fabrication of intrinsically stretchable electroluminescent (EL) devices. The devices utilized alternating current electroluminescent (ACEL) materials, which were deposited along with the other layers via an all-solution processable approach (Figure S8). Since we were aiming for a device in which all components are stretchable, ZnS microparticles (Figure S9) were dispersed in PUU, resulting in an intrinsically stretchable EL material. An overcoating of this material onto the AgNWs deposited on hydroxylated PDMS caused the AgNWs to be embedded in the PUU resulting in a highly stretchable EL layer after curing. To confirm the stretchability of the EL layer, we investigated a specific location on the layer by inducing strains of up to 100%, as shown in Figures 4k–4o. We observed that the distance between two specific particles could be precisely determined by the applied strain. Because of the broader rubber plateau of PUU and its strong adhesion to both the AgNWs and PDMS, the EL layer was intact even after 5000 cycles of 100% applied strain. Face-to-face lamination with 11 Environment ACS Paragon Plus


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another identical AgNWs/PDMS film completed the highly stretchable ACEL device. A cross-sectional view of the final design of the stretchable EL device is schematically shown in Figure S10.

After a thorough study to optimize the device architecture, we fabricated a device in which the thickness of each layer was optimized with excellent stretchability characteristics and efficient EL performances. A cross-sectional view (SEM) of the optimized device is shown in Figure 5a; the figures show that the ZnS particles uniformly dispersed in the cured PUU layer (thickness of 45 µm) was sandwiched between two identical AgNWs/PDMS films. Thanks to the high wettability and adhesion of PUU to PDMS, voids or other severe defects were not observed. The PUU layer penetrates through the AgNW network, resulting in a stable AgNW electrode that is fully embedded in the PUU and adheres well to the bottom PDMS substrate. Strong and stable bonding between each layer is crucial to maintain optimum device performances under different mechanical deformations as the interfaces are more vulnerable compared with other parts in the device during compressing, twisting, and stretching.42 Here, it was impossible to delaminate the EL layer from the PDMS surface because of the strong interfacial bonding between the PUU and the PDMS: a decapsulation of the upper section of the PDMS layer revealed that the fracture surface was solely composed of PUU. We measured the emission intensity of the fabricated devices against applied voltage and electric field under varying frequencies up to 500 Hz, as shown in Figure 5b. The emission intensity over the whole device increased as the applied frequency and voltage was increased. Beyond a certain bias voltage, the probability of electrons being accelerated to a given energy and subsequently exciting the luminescent centers will increase steeply; correspondingly, the luminescent intensity will also increase.42 This is in good agreement with the graph shown in Figure 5b, revealing that an electrically stable ACEL device was successfully fabricated via our approach. Thanks to the transparency of the all-organic materials employed, the 12 Environment ACS Paragon Plus

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percolated AgNW networks, and the low concentration of ZnS particles in PUU, the ACEL device was also semitransparent (Figure S11) with a transmittance of around 50% and a 17% in haziness. This demonstrates the potentially wide applicability of the device.

Given that the main goal of developing the EL device was to obtain a highly reliable mechanical performance under continuous large-strain deformation, repeated stretch–release tests were carried out for the devices fabricated in this study. First, we investigated the stress– strain behavior by employing continuous stretch–release testing without an interval between cycles. The strain used for this test was 50%. Figure 6a shows the hysteresis curves for a fabricated ACEL device, in which the loading and unloading behavior of the device was nonlinear and showed a small residual strain of approximately 6% after an applied strain of 50%, mainly due to the viscoelastic nature of PUU. The residual strain was defined as the extension when the load (stress) was equal to zero during the release of the stretched devices. From a practical perspective, the residual strain issue after the first stretch and release can simply be settled by pre-straining the device before use. Through further repetitions using the same amount of stretching, the residual strain did not, however, noticeably increase. In our pretests with some other polymers such as PU, PUA, and silicone-based stretchable polymers, inelastic deformation was found to be one of the most critical issues that needed to be resolved. Residual strain continuously formed after each stretch–release cycle with the other tested materials, revealing that they are not perfectly suited for fabricating mechanically stable stretchable devices.

We measured the luminance of the EL devices under stretching of up to 150%, as shown in Figures 6b and 6d. The devices could be perfectly stretched without exhibiting any critical damages and stably emit light, even at strain values of 150%. The emission intensity continuously decreased with increasing strain mostly due to an enlargement of the emission 13 Environment ACS Paragon Plus


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area, which corresponds with a decrease in ZnS particle density. It should be noted that the reduction ratio of the emission intensity is changed for a strain of 120%: before the strain reaches 120% the ratio is 0.125 and between 120 and 150% it is 0.667. The emission intensity slightly decreased (by around 15% compared with the initial intensity) at a strain of 120%. A further increase in the stretching strain up to 150% leads to a 35% decrease of the emission intensity. The smaller reduction ratio in the lower strain range may be linked to the decreased emission layer thickness, which contributes to the larger electrical field that is formed. The emission layer thickness dependency appeared to diminish at higher strain values, resulting in a larger decrease in the emission intensity. Even if we consider all these reduction mechanisms alongside the experimental results, the stretchability of our devices is excellent when compared with previous reports employing similar ACEL materials as well as polymer light emitting layers.40,42,49 To reveal the mechanical stability of the EL devices, repeated stretch–release testing for 5000 cycles was also employed using various strains (25%, 50%, 75%, and 100%), as shown in Figure 6c. The emission intensity was measured for each strain value after release. The tests with 25% and 50% strains did not severely deteriorate the EL performance of the devices (less than 10% decrease in emission intensity), and even 75% strain led to a decrease in the emission intensity of only around 15%. Only after repeated stretch–release testing with 100% strain for 5000 cycles did the emission intensity decreased by about 50%. To the best of our knowledge, this is the most severe stretching condition that has been reported for such a material, demonstrating the devices we fabricated had ultrahigh mechanical stabilities.

Waterproof EL devices can be mounted on the curved fingertips of vinyl glove for possible use in robotics or advanced surgical devices.50 Many other practical applications can be envisioned such as devices adhered onto cloths, tents, human skin or internal organs.51-54 For instance, the waterproof EL devices can be utilized in optogenetic systems to afford 14 Environment ACS Paragon Plus

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minimally invasive operation in the soft tissues of the mammalian brain.54 Achieving waterproof EL devices is considered to be one of the most difficult challenges because of the well-known vulnerability of organic light emitting materials to water and oxygen. The chemical stability of inorganic ZnS particles and the hydrophobicity of PDMS, however, were expected to enable the devices to be waterproof, but only if the interfaces between the PUU and PDMS were mechanically stable in water. Fortunately, the adhesion between the two layers was strong enough to resist severe stretching in water (insets in Figure 7 and Movie S1); this may be due to the reliable hydrogen bonds between the hydroxyl groups on PDMS and urea (or urethane). The emission intensity was not varied in water for up to 30 min, as plotted in Figure 7. Movie S1 demonstrates our superstable, stretchable EL device, which can be severely elongated and twisted in water without the emission performance suffering. Such a device could also be cuttable. Considering that we did not employ any additional materials or processes to seal the devices (only one process we performed to seal the AgNWs from exposure to water is described in Figure S12), the approach proposed here shows significant potential for the fabrication of waterproof and stretchable EL devices.



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CONCLUSION

We have demonstrated a stretchable AgNW-based transparent electrode that shows excellent mechanical properties with a sustained conductivity under repeated applied mechanical strain up to 100%. For this, we employed a stretchable transparent adhesive (PUU) as an encapsulant, which was synthesized to enhance the mechanical stability of the AgNWs/PDMS electrode. An overcoating of PUU on the AgNWs deposited on the hydroxylated PDMS resulted in a superstable, stretchable transparent electrode, in which hydrogen bonding between the hydroxyl groups and urea (or urethane) potentially enhances the inter-layer interactions. Based on these experimental achievements, we could successfully fabricate a waterproof, semitransparent, and stretchable EL device. ZnS microparticles dispersed in PUU were coated onto the AgNWs/PDMS film, followed by a face-to-face lamination with another identical AgNWs/PDMS film, thereby completing the EL devices. Enabled by the strong adhesion between the PUU and PDMS, the devices could be stretched by up to 150% and survived 5000 cycles of 100% applied strain. The adhesion was not affected even by immersion in water for 30 min, which enables the EL devices to be waterproof. To the best of our knowledge, this is the first demonstration of a mechanically stable, stretchable light emitting device that is waterproof. The developed method solves major challenges in the field of metal nanowire-based stretchable electrodes and enables their use in waterproof, stretchable EL devices. The approaches demonstrated in this study are projected to be guidelines for the future fabrication of various stretchable electronic devices.


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EXPERIMENTAL SECTION Materials and Synthesis: Materials and synthesis for PUU were described in our previous studies.44,47 A Sylgard 184 elastomer kit was obtained from Dow Corning, USA. Pure PDMS was prepared by mixing the base and curing agent with a weight ratio of 10:1.

Fabrication of stretchable electrodes and EL device: The fabrication of the stretchable light emitting devices is schematically illustrated in Figure 5. A glass substrate was first cleaned using detergent, de-ionized water, isopropanol, and acetone. Pure PDMS was spin-coated onto the glass, degassed, and thermally cured at 70°C for 12 h. The thickness of the PDMS layer after curing was around 240 µm. Oxygen plasma was treated to form hydroxyl functional groups on the PDMS film (see Figure S13). The O2 gas flow rate, gas pressure, power and treating time were controlled to be 30 ml/min, 15 Pa, 50 W, and 120 s, respectively. A solution of AgNWs dispersed in isopropanol (Nanopyxis Ltd., Korea) was spin-coated onto the PDMS film and heated on a hot plate at 60°C for 10 min to remove any remaining organic solvent from the coated layer. ZnS microparticles (National EL Technology, Korea) mixed with the synthesized PUU liquid (in a weight ratio of 1:1) were spin-coated (spin velocity: 400 rpm, duration time: 60 sec, temperature: 25°C) onto the AgNWs/PDMS film, followed by drying at 25°C for 30 min; this resulted in a thickness of 45 µm. Another AgNWs/PDMS film prepared in a separate procedure was laminated with this film at 80°C using a roll-laminator. After cooling, the fabricated device was peeled from the glass substrate from both sides.

Evaluation of electrodes and EL devices: A field-emission scanning electron microscope (FESEM; JSM6700F, JEOL Ltd., Japan) was used to investigate the microstructure of the AgNW networks. The optical transmission was also measured using a UV–visible spectrophotometer (V-560, Jasco, Japan), while the sheet resistance (Rs) was measured using



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a non-contact measurement system (EC-80P, Napson Corporation, Japan). The surface morphology was measured by an atomic force microscope (AFM; XE-100TM, Park Systems, USA). An automatic stretch-testing machine (Stretching Tester, Jaeil Optical System, Korea) was used to measure the long-term reliability under repeated cycles of stretching. The electrodes and devices were stretched and released at a rate of 0.5 mm/s under a varying strain to measure the stress–strain behavior, and the resistance and luminance during testing were measured. A long-term cyclic test of up to 5000 cycles was also conducted. This test used tensile strains of 25%, 50%, 75% and 100% and stretched the samples at a cycle rate of 1.2 cycles/min. An adhesive tape (Scotch Magic Tape, 3M, USA) was used to evaluate the adhesion between the AgNWs and PDMS. An AC power source (6600 series, Extech Electronics, Taiwan) was used to power the emitting devices, and the luminance was measured by a luminance meter (LS-100, Konica Minolta, Japan). More than 10 samples were fabricated and measured to determine most of the parameters. More than 10 samples were fabricated and their performance was evaluated.

Figure 1. SEM micrographs for AgNWs deposited on hydroxylated PDMS: panels (a)–(e) are low magnification and panels (f)–(j) high magnification images. Panels (a) and (f) represent the pristine state, (b) and (g) the 25% strain stretched state, (c) and (h) the 50% strain stretched state, (d) and (i) the 75% strain stretched state, and (e) and (j) show images during stretching at 100% strain.


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Figure 2. AgNWs deposited on hydroxylated PDMS (a) under stretching with 100% strain and (b) after release. Panel (c) shows the resistance of the AgNWs/hydroxylated PDMS measured during 50% strain stretch–release testing.



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Figure 3. Resistance change of AgNWs/PDMS (embedded) and PUU/AgNWs/PDMS measured during (a) a repeated tape test and (b) a cyclic stretch–release test employing 100% strain for up to 5000 cycles.


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Figure 4. Micrographs for AgNWs on hydroxylated PDMS with a PUU cover layer: (a)–(e) show low magnification and (f)–(j) high magnification. (a) and (f) represent the pristine state, (b) and (g) the 25% strain stretched state, (c) and (h) the 50% strain stretched state, (d) and (i) the 75% strain stretched state, and (e) and (j) the 100% strain stretched state. Micrographs for ZnS particles dispersed in cured PUU are shown in (k)–(o). Panel (k) shows the pristine state, and panel (l) the 25% strain stretched state, (m) the 50% strain stretched state, panel (n) the 75% strain stretched state, and panel (o) showing 100% strain stretched state.



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Figure 5. (a) Cross-sectional view (SEM image) of the fabricated ACEL device; magnified images are presented in the insets. (b) Measured luminance with increasing bias voltage under varying frequency. Inset photographs in (b) show optical images of the emitting samples with specific biasing conditions marked in the graph.


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Figure 6. (a) Stress–strain hysteresis curves for a fabricated EL device. Luminance change with (b) applied strain up to 150% strain and (c) number of stretch–release tests under various applied strain values. (d) Optical images of a light emitting device under various applied strain values.



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Figure 7. Luminance of a fabricated EL device measured during immersion in water for up to 30 min. Inset photographs show an unstretched and stretched emitting device in water.


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ASSOCIATED CONTENT Supporting Information including synthetic scheme of polymer, fabrication procedures of electrodes, additional experimental results and movies for waterproof/stretchable lighting device is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * J.-W. K.: Tel: +82-31-789-7438, E-mail address: [email protected] * B.-K. J.: Tel: +82-2-3290-3237, E-mail address: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Global Excellent Technology Innovation of the Korea Institute of Energy Technology Evaluation and Planning, granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20165020301170).



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(52) Yi, F.; Wang, J.; Wang, X.; Niu, S.; Li, S.; Liao, Q.; Xu, Y.; You, Z.; Zhang, Y.; Wang, Z. L. Stretchable and Waterproof Self-Charging Power System for Harvesting Energy from Diverse Deformation and Powering Wearable Electronics. ACS Nano 2016, 10, 6519-6525. (53)

Lee, H.; Choi, T. K.; Lee, Y. B.; Cho, H. R.; Ghaffari, R.; Wang, L.; Choi, H. J.; Chung, T. D.; Lu, N.; Hyeon, T.; Choi, S. H.; Kim, D. A Graphene-Based Electrochemical Device with Thermoresponsive Microneedles for Diabetes Monitoring and Therapy. Nat. Nanotechnol. 2016, 11, 566-574.

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Kim, T.; McCall, J. G.; Jung, Y. H.; Huang, X.; Siuda, E. R.; Li, Y.; Song, J.; Song, Y. M.; Pao, H. A.; Kim, R.; Lu, C.; Lee, S. D.; Song, I.; Shin, G.; Al-Hasani, R.; Kim, S.; Tan, M. P.; Huang, Y.; Omenetto, F. G.; Rogers, J. A.; Bruchas, M. R. Injectable, Cellular-Scale Optoelectronics with Applications for Wireless Optogenetics. Science 2013, 340, 211-216.


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