Stretchable Electroluminescent Display Enabled by Graphene-Based

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Functional Nanostructured Materials (including low-D carbon)

Stretchable Electroluminescent Display Enabled by Graphene-based Hybrid Electrode Heechang Shin, Bhupendra Kumar Sharma, Seung Won Lee, Jae-Bok Lee, Minwoo Choi, Luhing Hu, Cheolmin Park, Jin Hwan Choi, Tae Woong Kim, and Jong-Hyun Ahn ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019

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ACS Applied Materials & Interfaces

Stretchable Electroluminescent Display Enabled by Graphene-based Hybrid Electrode Heechang Shin,† Bhupendra K. Sharma,† Seung Won Lee,‡ Jae-Bok Lee,† Minwoo Choi,† Luhing Hu,† Cheolmin Park,‡ Jin Hwan Choi,§ Tae Woong Kim,§ and Jong-Hyun Ahn†,*

†School of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-

gu, Seoul, 03722 (Republic of Korea) ‡Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro,

Seodaemun-gu, Seoul, 03722 (Republic of Korea) §Product Research Team, Display Research Center, Samsung Display, 1 Samsung-ro, Kiheung-

Gu, Yongin, 17113 (Republic of Korea)

*Corresponding Author: [email protected]

KEYWORDS: Stretchable electrode, Graphene, PEDOT: PSS, Silver nanowires, ZnS, Stretchable display, Alternating current electroluminescence device.

ACS Paragon Plus Environment

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ABSTRACT Stretchable alternating-current electroluminescent (ACEL) devices are required due to their potential in wearable, biomedical, e-skin, robotic, lighting, and display applications; however, one of the main hurdles is achieving uniform electroluminesence, demanding an optimal combination of transparency, conductivity, and stretchability in electrodes. We therefore propose a fabrication scheme involving strategically combining two-dimensional graphene layers with a silver nanowires (Ag NWs) embedded PEDOT: PSS film. The developed hybrid electrode overcomes the limitations of commonly known metallic NWs and ionic conductor-based electrodes for ACEL applications. Furthermore, the potential of the hybrid electrode is realized in demonstrating large-area stretchable ACEL devices composed of an 8 × 8 passive array. The prototype ACEL passive array demonstrates efficient and uniform electroluminescence under high levels of mechanical deformation such as bending, rolling, twisting, and stretching.


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INTRODUCTION Stretchable electroluminescent (EL) devices are attracting much interest due to their emerging applications in decorative lighting, liquid crystal backlighting, volumetric displays, electronic skins, robotics, and biomedical devices that integrate with internal tissue imaging and photosensitive drug activated apparatus.1-6 For such unprecedented applications, the mechanical deformation of these devices must go beyond the simple stretchable behavior and confront with flexing, rolling, folding, twisting, crumpling, and adhere to curved surfaces.7 The strains that the EL devices must withstand in such an extraordinary mechanically deformed shapes exceed 10%.8 Alternating-current EL devices (ACEL) are more attractive than other EL devices in certain cases due to their simple structure (an emissive layer composed of phosphor material sandwiched between two electrodes) and uniform emission.9 More importantly, these do not have a harsh requirement on conductivity and work functions of electrodes due to their high voltage operating range, while other EL devices such as light-emitting diodes and light-emitting electrochemical cells are operated at low voltage; thus, they are sensitive to the conductivity and work functions of the electrodes.10-12 Therefore, the first step to realize ACEL-based stretchable electronic applications is developing highly stretchable, transparent, conformally adherent, ultrathin electrodes.13 Transparent, stretchable, highly conductive electrodes based on metallic nanowires (NWs),14,15 carbon nanotube,16-18 two-dimensional (2D) graphene films,19 and hybrids of metallic NWs and graphene films,20-22 have been demonstrated and exhibited excellent stretchability. In ACEL-based applications, reasonable progress has been made with respect to rigid and flexible substrates; however, few attempts have been made to achieve stretchability. Overall, two types of transparent, stretchable electrodes based on Ag NWs and ionic conductors have been exploited in stretching ACEL circuitry.23-25 Both types of electrodes demonstrated


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promising stretchability (≥100%); particularly, the stretching limit reached an exceptionally high value of 700% with ionic conductor-based electrodes.26,27 However, ACEL devices exhibited a lack of stable light emission with strain due to the intrinsic limitations of the electrodes. For instance, Ag NWs-based electrodes exhibit an inhomogeneous network structure in which open areas among NWs are quite big and uneven, allowing light transmission. The applied voltage created a maximum electric field around the NWs, which decreases towards the open regions. Thus, the non-uniform electric field is generated in such electrodes, and the non-uniformity increases more and more with stretching, resulting in unstable light emission. Additionally, the dispersed Ag NWs exhibited local aggregation, which also contributed to the non-uniform electric field. The ionic conductors adhere to the whole surface of the emissive layer, creating a uniform electric field; however, its impedance under high strains becomes comparable to that of the emissive layer thus significantly affect the emitted luminescence. Furthermore, an electronic double layer (EDL) formed at the interface of electronic and ionic conductors may become unstable if bias voltage exceeds the electrochemical window. The unstable EDL allows the flow of electrons and ions through it and as a result, induces the electrolysis of ionic conductor.24 Thus, transparent, highly stretchable electrodes for ACEL applications are still in their infancy and must be developed with optimal combinations of transparency, conductivity, and stretchability, resulting in high and uniform performances under stretching. 2D graphene has been realized as the potential candidate for transparent, conductive electrode in flexible/stretchable format,28,29 with stretching limit of ≤ 4%.30-33 Recently, polypropylene textile fiber coated with graphene sheets was used to fabricate the ZnS: Cu based ACEL light emitting devices for a textile-based wearable device application.34 As an alternative, the stretchability of metallic NWs-based thin film (100%) fulfills the requirement for ACEL devices without significantly compromising its


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conductivity.23 Here we propose a scheme that efficiently overcomes the issues arising from the inhomogeneous network structure of Ag NWs-based electrodes, such as the non-uniform electric field, oxidation, and aggregation. The proposed scheme comprises a network of Ag NWs embedded in a matrix of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS), a transparent conductive polymer, which is then strategically sandwiched between bilayer graphene (Bi-Gr) films to form transparent, conductive, highly stretchable, ultra-thin hybrid electrodes dedicated to ACEL-based applications. The fabricated hybrid electrodes showed good transparency (78% transmittance), high conductivity (58 ohm/sq), and excellent stretchability ( 85%), which was employed to demonstrate large-area, stretchable ZnS:Mn emissive layer-based ACEL devices composed of 8 × 8 passive matrix arrays.

EXPERIMENTAL SECTION Preparation of Ag NWs–PEDOT: PSS composite compound. Ag NWs with an average length and diameter of 20 μm (±5 μm) and 23 nm (±5 nm) dispersed in distilled water (0.3 wt%) were purchased from Nanopyxis Co., Ltd. (Product name: Ag NWs solution (LOT No.: S 23I)). The nanofibril structure of PEDOT: PSS was formed by treating an aqueous dispersion of PEDOT: PSS (Clevios AI 4083) with a 1 wt% solution of Triton X-100 (polyethylene glycol p-(1,1,3,3tetramethylbutyl)-phenyl ether) by stirring at room temperature for 30 minutes and filtering with a 0.42 μm PVDF syringe filter. The dispersed Ag NWs were then mixed into the treated nanofibril structure of PEDOT: PSS in the desired weight ratio, forming a tangled structure of PEDOT: PSS nanofibrils and Ag NWs. The Ag NWs and PEDOT: PSS nanofibril compound solution was stored at 5°C and gently stirred at room temperature for 10 minutes before spincoating.


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Fabrication of Hybrid Bi-Gr/Ag NWs-PEDOT: PSS/Bi-Gr Stretchable electrode. Monolayer graphene was grown on a Cu foil (25 μm thick) by chemical vapor deposition. Briefly, Cu foil was thermally annealed at 1,000°C for 30 minutes in the presence of H2 gas (8 sccm) at 80 mtorr. CH4 was then injected as a precursor gas (20 sccm) at 1.6 torr for 1 h deposition. Finally, the Cu foil was allowed to cool rapidly. After reaching to the room temperature, monolayer graphene was transferred to another monolayer graphene on Cu foil to make Bi-Gr. The other side of the Bi-Gr on the Cu foil was etched away with O2 plasma and the Ag NWs–PEDOT: PSS composite solution was spin-coated on top of the Bi-Gr. Spin-coating was performed at 2,000 rpm for 30 sec, and the resultant thickness of the Ag NWs–PEDOT: PSS composite layer was measured as 101 nm (±5 nm) using a surface profiler (Dektak XT). Afterwards, the Ag NWs–PEDOT: PSS composite layer on Bi-Gr/Cu foil was partially dried and the Cu was etched with 0.1 M ammonium persulfate [(NH4)2S2O8] solution. The floating Ag NWs–PEDOT: PSS composite layer on Bi-Gr was reversibly transferred to another Bi-Gr/Cu foil, followed by Cu etching, washing, and transfer of the floating Bi-Gr/Ag NWs–PEDOT: PSS/BiGr hybrid electrode to PDMS. PDMS was prepared by mixing a base with a curing agent (Sylgard 184, Dow Corning) at a weight ratio of 17:1. The hybrid electrode on PDMS was kept in a vacuum at 70°C overnight to remove residual trapped water at the electrode/PDMS interface. The stretchable hybrid electrode on PDMS was attached to a glass carrier substrate, and electrode lines were patterned using O2 plasma with a shadow mask. Introduction of Stretchable Interconnect. Cu wires as interconnects to the external electrical circuit were attached to both ends of the electrode lines and bonded with the GaIn eutectic paint (EGaIn), purchased from Sigma-Aldrich. The connection between stretchable electrode lines and


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Cu wires in the presence of EGaIn was encapsulated by dropping PDMS liquid and drying it at 70°C for 3 h. Fabrication of the Stretchable EL Devices. ZnS:Mn microparticle (particle size: 29 μm) powder was purchased from Shanghai KPT company. The ZnS:Mn powder was mixed with PDMS liquid in a 2:1 weight ratio as a precursor for the luminescent active layer. The liquid mixture of active/emissive layer was degassed for 30 minutes and then spin-coated (3,000 rpm for 30 sec) onto the bottom strip patterned stretchable electrodes attached to the glass carrier substrate. The emissive layer coated electrodes were peeled off from the glass substrate and thermally cured at 80°C for 1 h. The thickness of the resulted emissive layer was 100 µm. During curing, the identical patterned stretchable electrode strips were gently attached to the top of the active layer perpendicularly to the bottom electrode strips. The complete stretchable EL device was kept in a vacuum at 80°C overnight. Characterization Methods: Topographic images of the stretchable electrode were taken using field-emission scanning electron microscopy (FESEM) (TESCAN MIRA3). Transmittance spectra were recorded as a function of the Ag NWs concentration using a UV/Vis spectrophotometer (V-650, JASCO Corporation), and the Rs was evaluated using a standard four-point probe measurement system (CMT-SR1000N, AiT). The surface profile of the hybrid electrode, Gr/Ag NWs - PEDOT: PSS/Gr was characterized by AFM (Park Systems, NX-10). Stretching and cycling tests were conducted on the stretchable electrode and EL device using a home-made stretching machine at room temperature. A high voltage power amplifier (Trek 623B) connected to a function generator (Agilent 33220A Digital Waveform Generator) was used to apply an output alternating voltage to the stretchable electroluminescent device. The voltage-luminance characteristics of the stretchable EL devices were determined with a


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multichannel precision AC power analyzer (ZIMMER Electronics Systems LMG 500), and the EL spectra and luminance characteristics were determined with a spectroradiometer (Konica CS 2000).

RESULTS AND DISCUSSION Initially, aqueous dispersions of PEDOT: PSS was treated with a nonionic surfactant (Triton X100),35,36 in order to make nano-fibril structure.37 Afterwards, Ag NWs dispersed in deionozed water was mixed in PEDOT: PSS solution and their composite was stirred and spin-coated on bilayer graphene (Bi-Gr) on Cu foil followed by moderate heat-treatment. The Cu foil was then etched away and the floating Bi-Gr film supported by Ag NWs embedded in PEDOT: PSS matrix was washed thoroughly by transferring it to DI water. The washed floating film was reversely transferred onto another Bi-Gr on Cu to make a sandwich of Ag NWs embedded in PEDOT: PSS matrix film between Bi-Gr films.38 Cu etching and washing steps were then performed and the floating hybrid structure comprising Bi-Gr/Ag NWs-PEDOT: PSS/Bi-Gr (HGrAgPGr) was transferred to a thin layer of polydimethylsiloxane (PDMS) (Figure 1a and b). Structural, optical, and electrical characterization was conducted for different Ag NWs densities to achieve an optimal H-GrAgPGr. The thickness of H-GrAgPGr was estimated to be 100 nm and this remained similar as the Ag NWs density changed. Scanning electron microscopy (SEM) imaging indicated that the surface morphology of H-GrAgPGr was moderately smooth, with no cracks or voids (Figure 1c). The optical transmittance of H-GrAgPGr in the visible region decreased slightly as the Ag NWs density increased (Figure 1d); however, it remained adequate (≥75%) even for high density (30×10-2 µm-2). The optical transmittance for the Ag NWs dispersed in PEDOT: PSS (AgP), i.e. without the top and bottom graphene is shown in Figure


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S1. Although the usage of Ag NWs does not significantly affect the thickness, surface morphology, and optical transmittance, a large variation was observed in sheet resistance (Rs) and surface roughness (Figure 1e). The Rs of the Bi-Gr/PEDOT: PSS/Bi-Gr film was estimated to be 157 ohm/sq, which decreased with the increase of NWs density. Above the NWs density of 15×10-2 μm-2, a gradual decrease occurred, reaching a minimum value of 29 ohm/sq for BiGr/Ag NWs/Bi-Gr film. The measured Rs values are much lower than those reported in graphene based electrodes,22,37 and comparable to the electrodes used in for the ACEL devices.23,39 The values of surface roughness at different NWs densities were estimated by atomic force microscopy (AFM) (Figure 1f). The Bi-Gr/PEDOT: PSS/Bi-Gr film was observed to be moderately smooth with low roughness (10 nm), which increased with the insertion of Ag NWs as expected. A drastic increase in surface roughness was noted above the NWs density of 20×102

μm-2. Here, the conductivity and surface roughness exhibit an inverse behavior, i.e., improving

and degrading, respectively, with the density of Ag NWs. The Rs did not change significantly above 15×10-2 μm-2; however, the surface roughness degraded considerably. Therefore, the optical transmittance, Rs, and surface roughness analyses indicate that the optimum density of Ag NWs is 15×10-2 μm-2 for H-GrAgPGr; hereafter, this was used for further mechanical investigations and ACEL device fabrication. The developed hybrid electrode (H-GrAgPGr) was compared with earlier reported Ag NWs/PEDOT: PSS based electrodes (AgP) (Table S1). The sandwich structure, H-GrAgPGr, containing a Ag NWs in PEDOT: PSS film between graphene layers on PDMS substrate (3 × 5 cm) was photolithographically patterned into 2 × 40 mm long strips and subjected to mechanical testing. Prior to detailed investigations of H-GrAgPGr strips under stretching and cyclic testing, the robustness of soldering paste (for attaching the electrode to external interconnect wires) on H-GrAgPGr electrode strips under stretching was tested. In


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common practice, methods such as Ag-paste, copper tape and anisotropic conductive film (ACF) bonding are used to take out the external electrical connections. However, the bonded region resulted by these methods is too rigid to withstand high strain over 50%, and limit the mechanical stretchability range of the resulting devices. On the other hand, liquid metallic paint such as GaIn eutectic (EGaIn) paint may show robust behavior under mechanical deformation, while retaining its high electrical conductivity. Here, we compared the electrical performances of H-GrAgPGr strips under stretching for two types of external electrical interconnects formed with different types of metallic paint: Ag-paste, which becomes brittle after drying, and EGaIn paint, which maintains in liquid form at room temperature. The formation of electrical interconnects on H-GrAgPGr strips involves sequentially placing a drop of the desired metallic paint at two end points, gently attaching a Cu wire, and then laminating with the PDMS thin film followed by curing to fasten the contact regions. Schematics and a photograph of the top view of the formed interconnects are shown in Figure 2a and b, respectively. The schematics indicating the X-axis uniaxial stretching (indicated by arrow) for both Ag-paste and EGaIn interconnects are shown in Figure 2c. During the stretching, the cracks were generated in Ag-paste at much lower strain (≥ 40%), compared to EGaIn (≥ 75%). The Ag-paste based interconnects exhibited sudden increase in resistance above 54% while this failure occured in EGaIn paint above 85% (Figure 2c and d). The electrical outputs of the H-GrAgPGr strips measured with EGaIn based interconnects showed 32.8% higher mechanical endurance against stretching strains compared to Ag-pastebased electrical interconnects (Figure 2d). A ten-fold increase in the normalized resistance was observed at 85% strain for EGaIn-based interconnects; however, similar degradation occurs at a much lower strain of 54% for Ag-paste-based interconnects. This indicates that the EGaIn-based interconnects are more mechanically robust compared to the Ag-paste. Thus, EGaIn-based


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external electrical connections were formed on lithographically patterned long strip H-GrAgPGr electrodes. Since the transparency and Rs decrease and the surface roughness increases as a function of the NWs density, an identical structure of Bi-Gr/PEDOT: PSS/Bi-Gr (H-GrPGr) strips was also fabricated for comparison. Both types of electrodes, H-GrPGr and H-GrAgPGr were tested under stretching and repeated cycling (Figure 3a, b, and c). The H-GrAgPGr electrode was found to be considerably electrically superior under stretching strain (≤85%) compared to H-GrPGr (≤ 29%) (Figure 3a). It is clear that adding Ag NWs decreases the Rs, but it should be noted that such brittle metallic NWs increase the electrical elastic limit of the hybrid electrode due to its mesh structure. A similar trend was observed in electrical conductivity with respect to stretching for both electrodes (Figure 3b). Thus, the electrical endurance of HGrAgPGr electrodes against stretching is much higher (57%) than that of H-GrPGr electrodes (Figure 3a). This indicates that the proposed hybrid electrode H-GrAgPGr containing the Ag NWs density of 15×10-2 μm-2 in PEDOT: PSS sandwiched between Bi-Gr is electrically considerably superior under high mechanical deformation. Moreover, the H-GrAgPGr electrode showed excellent recovery of relative resistance under repeated (50) cyclic tests up to 65% strain; however, a slight increase was noticed above this strain (Figure 3c). The maximum deviation in retention for higher strains of 80% was observed to be four times higher, resulting in a 10% change in resistance, which is acceptable for ACEL devices. The developed hybrid electrode, H-GrAgPGr, is dedicatedly optimized in terms of conductivity, transparency and more importantly, in achieving electroluminescence under a high level of stretching for ACEL applications. The fabricated H-GrAgPGr patterned electrodes were then integrated to form large-area, stretchable 8 × 8 passive arrays of ACEL devices. Thus, uniformly distributed ZnS:Mn-based phosphor microparticles in PDMS solution were spin-coated as an


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emissive layer on top of H-GrAgPGr patterned electrodes followed by moderate curing. Subsequently, an identical set of H-GrAgPGr patterned electrodes was gently attached to the emissive layer coated patterned electrodes such that the top electrodes were perpendicularly aligned to the bottom ones. The details of these steps are schematically presented in Figure 4a. The assembled passive matrix ACEL array was efficiently lit by applying an AC voltage (405 V at 1 kHz) as revealed by their OFF and ON states (Figure 4b). The applied electric field accelerated the carriers which excite the luminescence centers and generate the electron-hole pairs. These electron-hole pairs once relaxed under radiative recombination, produce the luminescence. The electroluminescence can further be enhanced by embedding the emissive layer (ZnS:Mn) in high-K dielectric medium.39,40 Furthermore, the passive array exhibited typical characteristics of ACEL devices. The variation in emission intensity under increasing bias voltage conformed to the commonly used expression, L = Lo exp(−β/V1/2), where L is the luminescence, V is the applied voltage, and β and Lo are the fitting parameters. The electroluminescence exponentially increased with the applied voltage, but increased moderately with modulated frequencies (Figure 4c). Furthermore, the luminescence is low at low and high frequencies, but presents a maximum value at a specific value (Figure S2). The experimental results shown in Figure 4c were fitted using the expression, L = Lo exp(−β/V1/2) with varied values of Lo and β. The fitting parameters, Lo and β are empirical constants and depend on the device.31,41,42 It is evident that a specific applied voltage increases the acceleration of electrons which activates more luminescence centers. It results in a steep increment in electroluminescence as indicated by all three curves at 1, 5 and 10 kHz. Next, the increase in the frequency of applied bias increases the number of accelerations of electrons in a unit time, which increases the number of emitted photons and subsequently intensity of electroluminescence. However, due to the


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sinusoidal behavior of the AC signal, the increment in frequency reduces the width of the signal. Therefore, the time it takes for electrons to accelerate reduces, which resultantly decreases the electroluminescence. So, initially electroluminescence increases with frequency and reaches to maximum value, afterwards it decreases with further increment of frequency (Figure S2). The voltages are applied upto 350V with the step size of 50 V across the emissive layer with a thickness of 100 µm. The electroluinescence of H-GrPGr based ACEl device with applied voltages is also shown in Figure S3a. Under the present measurement conditions, the obtained maximum electroluminescence under a particular set of electric field and frequency values (i.e., E = 3.5 V/µm and f = 10 kHz) was noted as 80 cd/m2. This value is much higher than that for the earlier reported Ag NWs and ionic conductor electrode-based ACEL devices,23,24 for identical set of electric field and frequency values, revealing the superiority of our proposed ultra-thin hybrid electrode structure. The measured electroluminescence intensity was monitored as a function of wavelength (Figure 4d), resulting in a peak at 495 nm, a typical feature of ZnS:Mn phosphor material supported by the International Commission on Illumination (Figure S4). Moreover, the ACEL passive array showed a uniform and clearly distinguished electroluminescence with various applied electric field strengths (Figure 4e). Furthermore, the electroluminescence performances of ACEL passive array devices were evaluated under exceptionally high mechanical deformations including bending, rolling, twisting, and stretching. The strains generated by these deformations can be understood by considering two scenarios: bending and stretching (Figure 5a). During uni- or biaxial- stretching, the device undergoes tensile strain; however, during bending, the inward layer bears compressive and outward layer tensile strains. The representative prototype ACEL passive arrays exhibited excellent electroluminescence with uniform intensities for each pixel during bending, rolling,


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twisting, and stretching deformation (Figure 5b and c). Furthermore, the pixel can be controlled and displayed in desired shapes (Figure 5d and Figure S5). Usually, Ag NWs and ionic conductor electrode-based ACEL devices show uncontrolled luminescence under stretching;41,42 however, our developed hybrid electrode efficiently overcomes this issue. In the present case, the integrated electroluminescence of ACEl device was recorded under continous uniaxial tensile stretching with the step size of 5% strain which showed moderately stable luminescence with respect to stretching, even up to 80% (Figure 5e). The electroluinescence of H-GrPGr based ACEl device under stretching strain is also shown in Figure S3b for comparision purpose. Ag NWs network exhibited a mesh-like structure. Therefore, the surface belonging to open regions among the NWs network has insulating properties, compared to nearby surface filled with Ag NWs. Such electrodes in ACEL device, lead to non-uniform distribution of potential upon applying the voltage. Therefore, in the Ag NWs mesh-based electrodes, the applied voltage created a maximum electric field centered on the NWs region, which diminishes towards open regions.24 Thus, the created electric field strength becomes non-uniform throughout the electrode surface. Hence, the embedded phosphor materials in the active layer exhibit an uneven field strength, resulting in non-uniform electroluminescence. In case of ionic conductor based electrode, its resistance increases with the applied strain and become comparable to the active layer resistance, which may result in non-uniform electroluminescence. However, in the present case, the developed hybrid electrode exploits the Ag NWs network embedded in PEDOT: PSS, acting as a good conductor, while the graphene layers provide a smooth, continuous surface for uniformly distributing the applied potential, creating a uniform electric field strength throughout the active layer (Figure S6). Additionally, graphene layers prevent oxidation of the Ag NWs, maintaining the conductivity of the hybrid electrode against exposure to humidity and moderate


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heating (Figure S7).22,43 Next, the hygroscopic and acidic nature of PEDOT: PSS cause water absorption and corrosion of Ag NWs which can result in electrical instability of H-GrAgPGr electrode. However, the sheet resistance of H-GrAgPGr electrode does not change much with acidic and neutral nature of PEDOT: PSS below 150 C.44 In the present case, the temperature was kept below 100 C during the whole device fabrication and operation, which rules out any possibility of such degradation in electrode. Furthermore, they showed robustness under repeated cyclic stretching (Figure 5f). Finally, the functionality of the proposed hybrid electrode, HGrAgPGr, was realized by peeling off and reattaching the top electrode pattern during operation (Figure 5g, supplementary movie). This revealed the easy handling and excellent robustness of our large-area, stretchable ACEL passive array.

CONCLUSIONS A transparent, conductive, and stretchable hybrid electrode comprising Ag NWs dispersed in PEDOT: PSS film sandwiched between Bi-Gr is developed for ACEL-based applications. This hybrid electrode maintains good conductivity and transparency under high levels of stretching and efficiently overcomes the limitations of commonly known ACEL electrodes based on metallic NWs and ionic conductors. Integrating graphene layers in hybrid electrodes helps to achieve a uniform electric field, preventing oxidation of Ag NWs, which plays a key role in achieving uniform emission under stretching. The developed hybrid electrode allows high and uniform electroluminescence of a prototype large-area ACEL passive array and makes it suitable for operation even under high levels of mechanical deformation including bending, rolling, twisting, and stretching.


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ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charges on the ACS Publications websites at DOI: Transmittance spectra, color space chromaticity diagram, ACEL passive matrix, schematics showing uniform electric field, movie showing the stretching of unit pixel and 8×8 passive array, and peeling off and reattaching the top electrode pattern during operation showing the functionality of the proposed hybrid electrode.

AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected] Office phone: +82-2-2123-8286, Fax:+82-2-313-2879 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF2015R1A3A2066337) and Samsung Display.


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Figure Captions

Figure 1. Fabrication and characterization of hybrid stretchable electrode; Bi-Gr/Ag NWsPEDOT: PSS/Bi-Gr (H-GrAgPGr). Schematic illustration of (a) sequential fabrication steps and (b) vertically stacked sandwich structure of H-GrAgPGr . (c) SEM image of top surface (scale bar 10 µm). (d) Transmittance spectra for different Ag NWs densities. The inset shows an optical photograph of an electrode on PDMS with a size of 3 × 3 cm (scale bar 1 cm). (e) Rs (black) and surface roughness (blue) as a function of the Ag NWs densities. (f) AFM topography images with different Ag NWs densities (scale bar 1 µm).


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Figure 2. Evaluation of mechanical robustness of soldering paste based interconnects against stretching. (a) Schematic, and (b) optical images (scale bar 1 cm). (c) Schematics representing the cracks (≥ 54%) for Ag-paste (upper), and no cracks (≤ 85%) for EGaIn (lower) based interconnects. (d) Variation in normalized resistance against stretching for Ag-paste and EGaIn paint based interconnects.

Figure 3. Mechanical and electrical characterization of H-GrAgPGr stretchable electrode. (a) Normalized resistance change and (b) Conductivity change with respect to stretching strain with and without Ag NWs in PEDOT: PSS. (c) Repeated cycling test for 50 cycles at different stretching strains.

Figure 4. Characterization of H-GrAgPGr electrode based stretchable ZnS:Mn ACEL device. (a) Schematic illustration of stretchable ZnS:Mn electroluminescent device. (b) Optical photograph of EL device in OFF and ON states (scale bar 1 cm). (c) Luminance of the stretchable ACEL device with applied voltages at different frequencies. Solid lines present the fitted curves. (d) Electroluminescence spectrum of stretchable ACEL device. Inset shows luminescence from a single pixel (scale bar 5 mm). (e) Photographs of passive matrix ACEL device at various applied voltages (scale bar 1 cm).

Figure 5. Characterization of H-GrAgPGr electrode-based stretchable ZnS:Mn ACEL device in deformation states. (a) Schematic illustration of resting, stretching, and rolling states. Photographs of the luminescence of an ACEL device in (b) bending, rolling, and twisting of array (scale bar 1 cm); (c) stretching of unit pixel under different strain levels (scale bar 1 cm);


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and (d) initial state (upper) and biaxially stretched (below) for array. (e) Variation in electroluminescence intensity of the ACEL device under different stretching strains. (f) Mechanical stability test of the ACEL device for 200 cycles. (g) Photographs of ACEL device in peeling off and reattaching states (scale bar 1 cm).


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Figure 1. Fabrication and characterization of hybrid stretchable electrode; Bi-Gr/Ag NWsPEDOT: PSS/Bi-Gr (H-GrAgPGr). Schematic illustration of (a) sequential fabrication steps and (b) vertically stacked sandwich structure of H-GrAgPGr . (c) SEM image of top surface (scale bar 10 µm). (d) Transmittance spectra for different Ag NWs densities. The inset shows an optical photograph of an electrode on PDMS with a size of 3 × 3 cm (scale bar 1 cm). (e) Rs (black) and surface roughness (blue) as a function of the Ag NWs densities. (f) AFM topography images with different Ag NWs densities (scale bar 1 µm).


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Figure 2. Evaluation of mechanical robustness of soldering paste based interconnects against stretching. (a) Schematic, and (b) optical images (scale bar 1 cm). (c) Schematics representing the cracks (≥ 54%) for Ag-paste (upper), and no cracks (≤ 85%) for EGaIn (lower) based interconnects. (d) Variation in normalized resistance against stretching for Ag-paste and EGaIn paint based interconnects.


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Figure 3. Mechanical and electrical characterization of H-GrAgPGr stretchable electrode. (a) Normalized resistance change and (b) Conductivity change with respect to stretching strain with and without Ag NWs in PEDOT: PSS. (c) Repeated cycling test for 50 cycles at different stretching strains.


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Figure 4. Characterization of H-GrAgPGr electrode based stretchable ZnS:Mn ACEL device. (a) Schematic illustration of stretchable ZnS:Mn electroluminescent device. (b) Optical photograph of EL device in OFF and ON states (scale bar 1 cm). (c) Luminance of the stretchable ACEL device with applied voltages at different frequencies. Solid lines present the fitted curves. (d) Electroluminescence spectrum of stretchable ACEL device. Inset shows luminescence from a single pixel (scale bar 5 mm). (e) Photographs of passive matrix ACEL device at various applied voltages (scale bar 1 cm).


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Figure 5. Characterization of H-GrAgPGr electrode-based stretchable ZnS:Mn ACEL device in deformation states. (a) Schematic illustration of resting, stretching, and rolling states. Photographs of the luminescence of an ACEL device in (b) bending, rolling, and twisting of array (scale bar 1 cm); (c) stretching of unit pixel under different strain levels (scale bar 1 cm); and (d) initial state (upper) and biaxially stretched (below) for array. (e) Variation in electroluminescence intensity of the ACEL device under different stretching strains. (f) Mechanical stability test of the ACEL device for 200 cycles. (g) Photographs of ACEL device in peeling off and reattaching states (scale bar 1 cm).


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Table of Contents Stretchable alternating-current electroluminescent (ACEL) device exhibiting relatively stable electroluminescence against stretching strain was demonstrated. The stable luminescence was realized by developing a hybrid electrode consisting of Ag NWs dispersed in PEDOT: PSS sandwiched between bilayer graphene layers. The top and bottom graphene layers assist to create uniform electric field which results in stable electroluminescence.


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Stretchable Electroluminescent Display Enabled by Graphene-Based

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