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Applications of Polymer, Composite, and Coating Materials
Bright Stretchable Electroluminescent Devices based on Silver Nanowire Electrodes and High-k Thermoplastic Elastomers Yunlei Zhou, Shitai Cao, Jing Wang, Hangyu Zhu, Jiachen Wang, Sennan Yang, Xiao Wang, and Desheng Kong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17423 • Publication Date (Web): 28 Nov 2018 Downloaded from http://pubs.acs.org on December 4, 2018
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ACS Applied Materials & Interfaces
Bright Stretchable Electroluminescent Devices based on Silver Nanowire Electrodes and High-k Thermoplastic Elastomers Yunlei Zhou,1,2 Shitai Cao,3 Jing Wang,1,2 Hangyu Zhu,3 Jiachen Wang,1,2 Sennan Yang,4 Xiao Wang,3 and Desheng Kong1,2,* 1College
of Engineering and Applied Sciences, National Laboratory of Solid State
Microstructure, and Collaborative Innovation Centre of Advanced Microstructures, Nanjing University, Nanjing 210093, China. 2Jiangsu
Key Laboratory of Artificial Functional Materials, Nanjing University, Nanjing 210093,
China 3Kuang
4School
Yaming Honors School, Nanjing University, Nanjing 210023, China. of Marine Science and Engineering, Hebei University of Technology, Tianjin 300130,
China KEYWORDS. Stretchable electronics, silver nanowire, stretchable electrode, light-emitting device, electroluminescent
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ABSTRACT. Stretchable electroluminescent device is a compliant form of light emitting device to expand the application areas of conventional optoelectronics on rigid wafers. Currently, practical implementations are impeded by the high operating voltage required to achieve sufficient brightness. In this study, we report the fabrication of an intrinsically stretchable electroluminescent device based on silver nanowire electrodes and high-k thermoplastic elastomer. The device exhibit bright emission with low driving voltage by using polar elastomer as the dielectric matrix of the electroluminescent layer. Highly stretchable silver nanowire electrodes contribute to the exceptional elasticity and durability of the device in spite of bending, stretching, twisting, puncturing and cutting. Stretchable electroluminescent devices developed here may find potential uses in wearable displays, deformable lightings, and soft robotics.
INTRODUCTION Stretchable electronic devices are able to be bent, twisted, stretched and interfacing with irregular and moving objects, which are promising for emerging areas such as biomedical instruments, artificial skin for prosthetics and robotics, and wearable systems.1-9 As a fundamental building component, stretchable light-emitting device is a key enabler of visual information display and solid-state lighting with a deformable form factor. Light emitting diodes have been interconnected with elastic conductors on elastomer substrates to create stretchable displays with discrete emissive elements.10-12 Another approach is to transfer ultrathin organic light emitting diodes (LEDs) onto pre-stretched substrates, thereby forming stretchable devices through buckling.13-14 Intrinsically stretchable light-emitting devices, on the other hand, require all active components to exhibit compliant mechanical properties, which have been recently demonstrated as light-emitting electrochemical cells, organic LEDs and alternating current electroluminescent (ACEL) devices.15-22 Stretchable ACEL devices are particularly attractive
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with simplified structure and robust construction, in which light emission come from phosphor microparticles excited by alternating electric field. Stretchable and flexible ACEL devices have already been implemented as stretchable light source, addressable display and soft electronic skin.18, 20-21, 23-25 In spite of significant progress, the widespread applications of stretchable ACEL devices are hindered by the high operating voltage to achieve sufficient brightness. The issue is largely associated with the extensive use of chemically crosslinked silicone with low polarizability in these devices,18, 20-21, 23 which can be improved by high loading of ceramic nanoparticles to achieve high dielectric constants (high-k)21 or introducing functional groups with high dipole moment thought chemical modification.26-27 In addition, the use of large phosphor particles with the dimension in tens of microns also limits the minimal thickness of electroluminescent (EL) layer, thereby raising the required driving voltages for sufficient excitation electrical fields.18 In this work, we report the successful development of an intrinsically stretchable ACEL device based on silver nanowire electrodes and high-k thermoplastic elastomer. The device employs a polar thermoplastic elastomer and fine phosphor microparticles to prepare the EL layer, which effectively enhances the light emission under low drive voltages. Highly compliant electrodes consisting of AgNW network embedded in thermoplastic polyurethane (TPU) substrate contribute to the excellent stretchability and durability of the device. Our stretchable ACEL device achieves sufficient brightness for indoor lighting conditions with a driving voltage of 100V. The device is durable to withstand repetitive stretching and various forms of external damages. A solid-state display with patterned illumination is prepared and integrated into a pneumatic actuator. Intrinsically stretchable ACEL device developed in this study may find potential applications in smart wearables, biomimetic devices, and soft robotic systems.
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EXPERIMENTAL SECTION Materials. The thermoplastic elastomers used in this study are commercially available and acquired from corresponding vendors, including TPU (Tecoflex SG80A) from Lubrizol Inc., ePVDF-HFP (Daiel G801) from Daikin Industries, H-SEBS (Tuftec H1052) from Asahi Kasei elastomers. ZnS:Cu phosphor microparticles were synthesized by Shanghai Keyan Phosphor Technology Company. AgNWs were synthesized in our lab using polyol process (120–150 nm ×40–70 μm).28 Emissive composite preparation. The emissive composites were prepared by dispersing phosphor powders into thermoplastic elastomers. The elastomers were initially dissolved in selected solvents and then mixed with ZnS:Cu powders in a weight ratio of 1:2. The choice of solvents was adjusted to control the evaporation rate, with 2-butanone /dimethylformamide (10/1 by weight) for e-PVDF-HFP, tetrahydrofuran /dimethylformamide (10/1 by weight) for TPU, and toluene for H-SEBS. The mixture was initially homogenized at 2000 rpm for 5 min in a planetary mixer (JF-RVITV-150, Shenzhen Junfeng Technology Company). Subsequently, the slurry was further homogenized in a three roll mill for five passes (ST65, Shanghai Chi Le Machinery Technology Co., Ltd.), in which the two gaps between the rollers were set as 30 μm and 80 μm, respectively. The mixture was dried in a vacuum oven at 100 °C for 2 h to obtain the emissive composite in the form of rubbery solid. Device Fabrication. TPU elastomer was dissolved in tetrahydrofuran at a concentration of 100 mg/mL and then casted onto octadecyltrichlorosilane (OTS)-modified glass. The solvent was evaporated at room temperature to produce a ~300 μm thick substrate. As synthesized AgNWs were dispersed in isopropyl alcohol and spray coated onto TPU substrate, thereby producing stretchable electrodes with area density of ~40 μg/cm2. Patterned AgNW electrodes were defined
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by shadow masks during spray coating. AgNW electrodes were annealed in an oven at 140160°C for 20 min and naturally cooled down. The emissive composite was dissolved in suitable solvents and spin casted into a ~25μm thick EL layer over OTS-modified glass. The solvents were toluene for H-SEBS, tetrahydrofuran/dimethylformamide (10/1 by weight) for TPU, and 2butanone/dimethylformamide (10/1 by weight) for e-PVDF-HFP. For device assembly, AgNW/TPU transparent electrodes were thermally bonded to EL layer to form the sandwiched devices. The lamination was carried out with 70 to 120 kPa pressure at 100 to 120 oC for 5 min in an automatic vacuum laminator (Shenzhen Jingkeda Machinery and Electronic Equipment), which was fine-tuned for each emissive composite. The vacuum environment is essential to ensure intimate contacts without trapped bubbles. Material characterizations. SEM images were taken using Zeiss Ultra55 field-emission scanning electron microscope. Digital images and videos were acquired by a Fujifilm X-T10 camera. The sheet resistance of AgNW electrodes was measured using Keithley 2110 digital multimeter with four-point probe configuration to eliminate contact resistance. The optical transmittance at 550 nm was measured by an UV−vis spectrophotometer (Jena Specord 200 plus) A pure TPU substrate with transmittance of 92.3% at 550 nm was used as the reference. The dielectric constants of the thermoplastic elastomers were measured using parallel plate capacitors. A heavily doped silicon substrate (
