MoirÃ©-Free Imperceptible and Flexible Random Metal Grid Electrodes

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Moiré-Free Imperceptible and Flexible Random Metal Grid Electrodes with Large Figure-of-Merit by Photonic Sintering Control of Copper Nanoparticles

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Jinwook Jung,† Hyunmin Cho,† Seok Hwan Choi,† Dongkwan Kim,† Jinhyeong Kwon,‡ Jaeho Shin,† Sukjoon Hong,§ Hyeonseok Kim,† Yeosang Yoon,† Jinwoo Lee,† Daeho Lee,∥ Young D. Suh,*,† and Seung Hwan Ko*,†,⊥,# †

Department of Mechanical Engineering, ⊥Institute of Advanced Machinery and Design (SNU-IAMD), and #Institute of Engineering Research, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea ‡ Manufacturing System R&BD Group, Korea Institute of Industrial Technology, 89 Yangdaegiro-gil, Ipjang-myon, Seobuk-gu, Cheonan, Chungcheongnam-do 31056, Republic of Korea § Department of Mechanical Engineering, Hanyang University, Sangnok-gu, Ansan, Gyeonggi-do 15588, Korea ∥ Department of Mechanical Engineering, Gachon University, 1342 Seongnamdaero, Sujeong-gu, Seongnam 461-701, Gyeonggi-do, Korea S Supporting Information *

ABSTRACT: Flexible micro/nano metal grid transparent conductors emerged as an alternative to the fragile/rigid indium tin oxide electrode. They are usually fabricated by the combination of the conventional photolithography and the vacuum deposition of regular metal grid patterns, however, seriously suffer from moiré and starburst problems induced by periodic regular pattern structures. In this paper, we demonstrated flexible and imperceptible random copper microconductors with an extremely high figure-of-merit (∼2000) by the thermal conduction layer-assisted photonic sintering of copper nanoparticles without damages in the plastic substrate. This process can be easily applied to complicated structures and surfaces including a random pattern which is imperceptible and free of interferences. As a proof-of-concept, a transparent windshield defogger in a car was demonstrated with a Cu transparent random conductor at an extreme and reversible fogging state. KEYWORDS: copper nanoparticle, photonic sintering, flexible, imperceptible electrode, moiré-free, transparent heater

1. INTRODUCTION In recent years, transparent conducting electrodes (TCEs) have become an indispensable material in various technologies including solar cells,1,2 organic light-emitting diodes,3,4 smart windows,5 touch panels,6,7 and high-end home appliances. The most commonly used TCE material is indium tin oxide (ITO);8 however, scarcity of the indium raw material9,10 and high sheet resistance, which increases the response time especially for large area applications, makes it difficult to be used in displays larger than 15 in.11 For this reason, many alternative materials such as the silver nanowire12−14 or copper nanowire (CuNW)15−17 percolation network as well as graphene layers,18,19 carbon nanotubes,19,20 ultrathin metal films,21,22 and metal grids23,24 have been studied extensively by various research groups. The graphene-based TCEs have relatively high sheet resistances with low transmittance in the visible regime and the metal nanowires have high raw material prices or yield rate problems25,26 as compared to the metal grid, which has the highest figure of merit (FOM) value24,27,28 and mass production compatibility.29,30 Despite such advan© XXXX American Chemical Society

tages, its application has been limited by perceptibility of the electrodes especially when the linewidth is larger than 7 μm.27 Several approaches have achieved imperceptibility by using conventional photolithography followed by vacuum deposition, inkjet printing, and electrohydrodynamic (EHD) jet printing. However, the vacuum deposition process increases the price of the overall process, while inkjet printing and EHD jet printing have low writing speed31 and difficulties in forming a high aspect ratio and complex metal grid patterns.32 In addition to line visibility, imperceptibility of the metal gridbased TCE has been hindered by the problems from moiré and starburst effects, which are induced by periodic pattern structures. Although few research studies reported that these problems can be improved by nonperiodic structures,33,34 most of the studies involve costly deposition processes.35,36 The process cost and selection of low-cost materials, such as copper Received: January 29, 2019 Accepted: April 5, 2019

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DOI: 10.1021/acsami.9b01893 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic fabrication process: a random patterned master mold is used for pattern transfer onto the PET film using UV-curable epoxy. CuNPs are filled in the trench. The nanoparticles are treated by lactic acid and sintered by the TCL-assisted laser sintering process. The microscope image of the (b) conventional laser-sintered electrode and (c) TCL-assisted laser-sintered electrode. The inset images depict relative heat transfer rates between materials. (d) Highly transparent fabricated electrode. (e) Narrow linewidth (∼2.4 μm) of fabricated electrode. (f) TCITE overlaid on high-resolution (518 ppi) display.

nanoparticles (CuNPs), are important for the mass production compatibility. Unlike silver nanoparticles, sintering of CuNPs requires more careful treatments because of their native oxide shells and higher sintering temperatures. For this reason, most of the CuNP applications are focused in which relatively thick electrode patterns such as radio frequency identification tags (RFID) with a high temperature compatible but expensive and nontransparent polyimide film substrates. Recently, several studies have shown a metal grid-based transparent electrode fabrication technique using low-temperature sintering of CuNPs on the transparent film.37,38 However, the fabricated metal grids have a linewidth of 7−20 μm, which is not only easily visible to human eyes but also constrained to large aperture loss when placed onto high-resolution display. Generally, narrowing the linewidth of nanoparticle patterns is challengeable because the number of percolation paths between sintered CuNPs decreases at the same time. For this reason, a dense nanostructure which is formed by successive necking is important for enhancing the electrical conductivity. To create the dense nanostructure, a high sintering temperature, which usually damages the polymerbased substrates, is required. Here, we presented, for the first time, using low-cost CuNPs, fabrication of a random-patterned Cu microgrid-based flexible transparent electrode with an extremely high FOM (∼2000) and narrow linewidth, thus imperceptible to the human eyes and inducing very low aperture loss when combined with displays. In addition, using nonperiodic patterns, the fabricated transparent electrode has no moiré effect and starburst effect which inevitably appears by superposition of reflected or diffracted lights at the periodically aligned metal lines, thus it is also imperceptible at diffraction-induced conditions. In this process, by adapting a thermal conduction layer (TCL) during the photonic sintering of nanoparticles, the heat generated during the photonic sintering process rapidly dissipates through the TCL and a dense nanostructure can be formed without damaging the transparent polymer-based substrate film. As an application of this transparent electrode, a

transparent defogger for a car windshield was demonstrated by simulating extreme conditions.

2. EXPERIMENTAL SECTION 2.1. Master Mold Fabrication and Replication. A master mold was fabricated using a silicon wafer by the conventional photolithography process, followed by directional etching (Plasma-Therm, SLR-770-10R-B). For the nonperiodic and imperceptible pattern, a photomask was designed by mimicking a random crack network on a highly stressed silicon nitride thin film on silicon38 with narrow linewidth. A detailed process for fabricating the master mold is illustrated in Figure S1a. An UV-curable epoxy resin (CCTECH, SEA-1) was poured on the master mold and a polyethylene terephthalate (PET) substrate (125 μm) was placed on top. Then, UV-curable epoxy with the PET film was cured under UV light (400 W, 350 nm) for 120 s. Subsequently, the cured film was detached from the master mold. Because the UV-curable epoxy bonded well to the plastic substrate, patterns in the mater mold were transferred as detached. Consequently, a random structure consisting of narrow trenches was fabricated on the plastic substrate. 2.2. CuNP Synthesis and Patterning Process. CuNPs were synthesized by modified polyol synthesis. All chemicals were purchased from Aldrich without further treatment. A 0.75 M of sodium hypophosphite monohydrate (≥99%), 0.005 M of polyvinylpyrrolidone (K-30, average MW 40 000), and 0.8 M of Cu(II) sulfate pentahydrate (ACS reagent, ≥98%) were dissolved in ethylene glycol (≥99%) at 90 °C to fabricate CuNPs having approximately 40 nm in diameter. The particles were then cleaned with acetone, ethanol, and deionized (DI) water and redispersed in DI water to attain 30 wt % solution. Then, 30 μL of CuNP paste is poured on the O2 plasma-treated (100 mW, 5 min) random-patterned substrate and bladed on a 50 °C hot plate to fill nanoparticles into the trench of the plastic substrate. 2.3. Thermal Conducting Layer-assisted Laser Sintering Process. In order to eliminate surface oxide, the CuNP-filled trench was dipped into diluted lactic acid (1:1 ratio in DI water) for 3 min, followed by gentle washing with DI water. Then, transparent glass (1 mm in thickness) was placed on the Cu-filled substrate as a thermal conducting layer. A continuous wave ND:YAG laser (532 nm) was selected for sintering nanoparticles. Through the half-wave plate, polarizing beam splitter, beam expander, and galvano scanner with telecentric lens, the beam size of 20 μm is obtained. Laser scan speed and hatch size is set to 100 mm/s and 5 μm, respectively, while the B


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Figure 2. Effect of a thermal conducting layer. Thermal simulation of sintering (a) with TCL and (b) without TCL. (c) Simulated temperature of CuNPs and surrounding substrates of two sintering cases is plotted. (d) Porous structure of without TCL sintering and (e) dense structure of with TCL sintering is presented. (f) XRD θ−2θ scan data of fabricated electrode and the inset shows around Cu(111) peaks. (g) fwhm of Cu(111) peaks and the calculated crystal size of three samples: as-filled, without TCL, and with TCL. (h) Sheet resistance with transparency of the fabricated electrode by the two sintering conditions is plotted with other copper nanomaterial-based transparent electrodes and ITO. FOM values indicating 2000, typical industry standard (∼350) and minimum industry standard (∼35) are plotted in the graph. laser power of 60−100 mW without TCL and 200−240 mW with TCL is used. 2.4. Simulation of Thermal Conduction. The heat conduction effect with or without TCL during the laser sintering process was simulated by using commercial software COMSOL multiphysics. For simplification, the simulation was conducted for the single CuNP line. The CuNP line (2.4 × 3.6 × 40 μm3) was imbedded in the polyurethaneacrylate (PUA) substrate (40 × 40 × 10 μm3) covered with the upper layer (air or glass of 40 × 40 × 10 μm3). To simulate laser irradiation, a Gaussian-profiled heat flux (24.5 and 9.5 mW for glass and air, respectively) for 20 μs was applied on the surface of copper lines. Thermal conductivity of CuNP layers was determined to be 4.7 W/m K by considering previous reports.39 2.5. Characterization. Transparency was measured by UV−vis spectroscopy (Jasco V-770) with using the PET film as the reference, sheet resistance was measured by the van der Pauw method (more than five samples for each cases), X-ray diffraction (XRD) θ−2θ scan data were obtained by D8-Advance (Bruker) with a Ge monochromator, standard deviation of moiré was captured by a DinoCapture 2.0 digital microscope at a distance of 40 cm and intensities were analyzed. For quantified defogging measurements, a miniature car was used as a humid chamber. The warm and humid environment was created by continuously injecting hot steam into the chamber through the PVC tube. A PUA-encapsulated TCITE defogger was attached to the windshield of the miniature car. Then, the laser (output power < 1 mW) illuminated the photodetector (PD) through the defogger to evaluate the defogging state.

S4]. Before the sintering process, lactic acid treatment is conducted to convert the copper oxide to copper followed by the sintering process as shown in eqs 1 and 2.40,41 2CH3CHOHCOOH + CuO → Cu(CH3CHOHCOO)2 + H 2O

(1)

Cu(CH3CHOHCOO)2 → Cu + 2CO2 + H 2 + 2CH3CHO

(2)

Without lactic acid, the CuNPs are easily ablated during the sintering process because of low thermal conductivity of oxide shells which lowers ablation threshold,42 as shown in Figure S2. For successful sintering of narrow line patterns of nanoparticles which has lower percolation paths than thicker lines,43 establishing a dense nanostructure is important for enhancing electrical conductivity. To form such a nanostructure, a TCL is introduced to increase effective sintering temperature. As a TCL material, SiO2 glass is chosen because it is highly transparent at the visible wavelength regime, thus photothermal heat is selectively generated at CuNPs. The generated heat transfers to surroundings (upper layer and substrates) and the rate of heat transfer between two materials can be explained by eq 3 dQ kAΔT AΔT AΔT = = = dt L L /k L1/k1 + L 2 /k 2

3. RESULTS AND DISCUSSION In order to fabricate a TCL-assisted copper imperceptible transparent electrode (TCITE), the mold-based pattern transfer method is used to fabricate fine linewidth random patterns. As shown in Figure 1a, using a master mold which has embossed random patterns, an inverse of the mold which has trenches having 2.4 μm linewidth and 3.5 μm height can be facilely transferred using UV-curable epoxy (Figure S1b,c). CuNPs are then selectively filled into the trenches by the blading process and sintered [wide view and cross-sectional scanning electron microscopy (SEM) images, Figures S3 and

(3)

where dQ/dt, A, T, L, and k represent heat transfer rate, contact area, temperature, thickness, and thermal conductivity (TCL: 0.8−1.4 W/m K, air: 0.024 W/m K, polymer substrate: 0.1−0.5 W/m K44), respectively. Assuming that other conditions are the same, the rate of heat transfer can be compared easily by input thermal conductivities of materials. In case of no TCL process, the heat transfer rate dQ/dtCu−air is lower than dQ/dtCu‑substrate, thus heat transfer to the polymer substrate is preferred and therefore damages can occur easily C


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Figure 3. Moiré effect comparison of periodic and random metal patterns. (a) Visual image of the periodic pattern overlapped on the LCD monitor. The curved line at the upper left image (crossing angle: 0°) represents the intensity of the image which means moiré effect. The right image represents (crossing angle: 90°) same moiré direction and the period of ∼2 mm. The lower left image (crossing angle: 6°) represents narrower moiré, which is 18° rotated having a period of ∼0.7 mm. (b) Random pattern overlapped on the LCD monitor. Moiré is not observed at any crossing angles. (c) Moiré effect analyzed by standard deviation of intensity according to the crossing angle. The periodic pattern shows high standard deviation at the crossing angle which enhances moiré effect while random pattern does not. (d) Geometric location of frequency vectors of LCD and periodic grid (80 μm pitch) structure with crossing angle 0° and 6°. (e) Geometric location of convoluted frequency which represents generated moiré period and the angle between the LCD and periodic grid. (f) Fabricated TCITE touch panel with no moiré effect on the LCD screen (inset).

XRD analysis is conducted. Though the copper peaks are weak compared to substrate peaks (Figure 2f) due to small amount of CuNPs filled in the PUA substrate, it is possible to compare the crystal structures by analyzing Cu(111) peaks around 43.4° (inset of Figure 2f). The peak intensity of with TCL, which is proportional to the number of scatterer, is highest among other conditions, meaning that the amount of scattering point especially for the copper (111) plane is increased with TCLassisted sintering (Figure 2f). Moreover, the crystal size calculated by the full width at half-maximum (fwhm) and Scherrer formula shows that the sintered with TCL sample has the largest crystal size (Figure 2g) which supports SEM images. As a consequence, the fabricated TCITE has a high transparency of 96.2% and an average sheet resistance of 4.7 Ω/sq, which is about 10 times lower than no TCL samples (49 Ω/sq) (standard deviation of 0.66 and 10.81 Ω/sq, respectively). In order to compare this result with other solution-processed copper transparent electrodes46 and conventional ITO films, total transmittance against sheet resistances of the transparent electrodes is plotted as shown in Figure 2h with FOM curves which are usually used for comparison of various TCEs. The FOM curves in the graph are defined to be 2000, 350 (typical industry standard), and 35 (minimum industry standard)24 using following eq 4

(Figure 1b and inset). However, with TCL under the same laser condition, the heat transfer rate dQ/dtCu−TCL is greater than the dQ/dtCu‑substrate, thus heat is rather easily transferred to TCL than the polymer substrate and no substrate damages (Figure 1c and inset) occur. Moreover, the substrate heat also easily transfers to the TCL because dQ/dtsubstrate−air < dQ/ dtsubstrate−TCL. The fabricated transparent electrode has a high transparency of 96% (Figures 1d and S5) at 550 nm because the CuNPs are only filled into the narrow line patterns as shown in Figure 1e. This narrow metal lines barely block subpixels of high-resolution display (518 ppi, magnified image for Figure S7a), thus the visibility of display is not hindered (Figure 1f). As described above, because the laser power threshold values without damaging the polymer substrate are different under the two sintering conditions (without and withTCL), the sintering characteristics need to be compared based on equivalent substrate damage. As a theoretical approach to confirm the effect of TCL on the sintering process, the heat transfer simulation result of two cases during laser sintering is shown in Figure 2a,b. In order to simulate the same substrate damage condition, the laser power is modulated to equalize the maximum temperature of the substrate which is set to 130 °C near the laser irradiation region. At this condition, the sintering temperature of CuNP (234 °C) is obtained for with TCL while the without TCL condition shows the sintering temperature of 207 °C (Figure 2c). It is known that the higher sintering temperature forms the denser nanostructure,43 thus this simulation result well matches our experimental sintering characteristics which show the substrate damaging laser power limit of 100 mW for without TCL and 240 mW for with TCL, and thus with TCL leads to the denser nanostructure than without TCL cases (Figure 2d,e). This nanostructure might be the reason of smaller resistance changes of with TCL according to bending radius because of the resistance change (Figure S6).45 In addition, to quantify the effect of TCL on the nanostructure of the copper matrix,

ij 188.5 σOP yzz T = jjj1 + z j R S σDC zz{ k

−2

(4)

where σDC/σOP, T, and RS represent FOM, transmittance resistance, and sheet resistance, respectively. The resultant FOM values of TCITE is above 2000, which are above the industrial standards and the highest among other types of TCEs including ITO and other solution-based electrodes using CuNPs and CuNWs. In addition to the advantage of fine linewidth and high optoelectrical performance, a random pattern TCITE is D


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Figure 4. TCITE application as a car windshield defogger: (a) Rectangular grid has starburst effect induced by periodic patterns while (b) TCITE has no starburst effect. The inset images show diffraction properties of laser, respectively. The image of the humid car chamber (c) before and (d) after operating the TCITE windshield defogger and the corresponding infrared camera images, respectively (bottom) in a miniature car model. Details of the car are blurred. (e) Continuous reversible defogging ability measurement. The fogged state (e-off) shows low PD intensity and the defogged state (e-on) shows high PD intensity. (f) Time-dependent heating profiles of TCITE and the corresponding PD intensity. (g) In situ measurement of defogging ability of TCITE at extreme conditions for various target temperatures represented with defogging and internal temperatures. (h) Response times of the defogging windshield for various target temperatures. All figures are captured by the authors.

nonperiodic, which can prevent many problems arising from repeated pattern structures such as moiré and starburst effects. For comparison, a periodic square Cu grid having several micrometers linewidth is fabricated. As shown in Figure 3a, the periodic moiré patterns having white and gray lines appeared when the periodic grid is placed over the LCD monitor as indicated by the red intensity profile shown at the middle of the image (top left). This gray-scale moiré is appeared because the metal lines periodically blind a small portion of pixels (Colorful moiré patterns are observed when thick metal lines are overlapped onto the LCD, thus blinding a large portion of pixels, Figure S8d). The moiré patterns are changed by rotating 6° (bottom left) and 90° (right). However, as shown in Figure 3b, the TCITE, which has a random pattern structure, shows no moiré pattern at different rotation angles. In order to quantify the reduction of moiré effect, two transparent electrodes are placed on the LCD monitor and a standard deviation of intensities is analyzed with various crossing angles as represented in Figure 3c. The result shows that the periodic pattern has high standard deviation at a low crossing angle and similar value with the random pattern at about 45° while the random pattern has similar values at overall crossing angles which means no observation of moiré patterns is possible. An interesting factor in this moiré pattern is that the period (Tm) of moiré patterns is different than the calculated value by the well-known eq 5 as follows

Tm =

T1 × T2 2

T1 + T2 2 − 2TT 1 2 cos(θc)

(5)

where T1, T2, and θc represents the period of repetitive structure 1, period of repetitive structure 2, and crossing angle between the structures. Using T1 (period of LCD, supporting Figure S8b,c) ≈250 μm, T2 = 80 μm, and θc = 0, Tm of 6 mm is obtained by calculation while the measured moiré period is about 2 mm. This discrepancy is described by high-order moiré, which the moiré period must be calculated by the spectral approach based on the Fourier theory.47 The two repetitive layers can be assumed as transmittance functions r1(x,y) and r2(x,y), which are the binary square wave and its overlapped transmittance is represented as following eq 6. roverlappped(x , y) = r1(x , y) × r2(x , y)

(6)

Using the Fourier transform, the functions can be represented in the frequency domain as R1(u,v) and R2(u,v) which is infinite series of weighted sine and cosine functions at fundamental frequency of 1/T and all its harmonics n/T.47 To visualize the frequencies of the layers, the geometric location of frequency vectors for three individual layers (0°, 6° rotated grid, and LCD layer) are plotted in Figure 3d (only vertical periodic lines are considered for simplicity). By taking the Fourier transform of both sides of eq 6, the frequency domain E


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ACS Applied Materials & Interfaces representation of overlapped transmittance function Roverlapped can be obtained as eq 7 R overlappped(u , v) = R1(u , v)*R 2(u , v)

heating condition [stage (iii)] but also under the cooling down condition [stage (iv)]. After the temperature reaches to internal temperature [stage (v)], the fog starts to form. According to this result, the temperature is the most important parameter in this experiment, thus the defogger is operated at various target temperatures (Figure 4g). In every cases, the defogging state appears and maintains at slightly higher temperature than internal temperature though the response time varies with target temperatures (Figure 4h). The confirmation of this phenomenon suggests that the TCITE defogger can be effectively applied to electric cars, which has no waste heat from engines.

(7)

where * means convolution notation. By convolution of two individual layers, new frequency vectors which are allocated in the u−v space can be generated. Figure 3e represents the geometrical location of generated frequency vectors attributed by third harmonic frequency of the LCD monitor and the first harmonic frequency of the 80 μm pitch grid, which is closest from the origin and easily observed by eye because the visibility circle forms near the origin.47 The calculated moiré period and angles by size and orientation of the generated frequency vectors is equivalent to ∼2 mm with 90° and ∼0.7 mm with 18° for 0° and 6° rotated grids, respectively. The calculated value is analogous to the measured value from moiré images. In addition, the random pattern TCITE shows no moiré frequencies when overlaid on the periodic LCD pixels, thus it is a good candidate for the transparent electrode of a touch panel display. Figure 3f shows a fabricated resistive-type touch panel and its operation without moiré effect when overlaid on the LCD using TCITE. Another advantage of using the random pattern is reduced starburst effect. As shown in Figure 4a, the incident light is diffracted at the periodic metal lines and the so-called starburst effect is observed because the periodic metal lines work as a diffraction grating (Figure 4a, inset image). This effect is another critical obstacle for the metallic grid-type TCEs to be imperceptible. A random pattern TCITE, meanwhile, achieves visual imperceptibility because the randomness of the pattern annuls the starburst effect (Figure 4b). Thanks to this advantage, TCITE is appropriate for windshield defogger which requires clear vision for safe driving. Though various defogging experiments using the transparent electrode have been previously conducted by several research groups, most of the experiments are conducted in irreversible fogging conditions, thus it is difficult to distinguish the contributions of heater from natural defogging. In this study, a reversible fogging condition where the fogging continuously occurs is constructed and tested. By injecting humid and warm steam into the miniature car chamber, an extreme fogging condition (65 °C with RH > 90%) is made. For this process, the PUA overcoating layer is applied though TCITE has fair oxidation stability (Figure S7) at ambient conditions (25 °C and RH 30−45%). With the continuous mist supply experiment setup, it is possible to analyze the defogging characteristics of TCITE separately from the natural defogging by ambient air. The car chamber is defogged by an operating heater (Figure 4d) and fogged again (Figure 4c) when the heater is turned off. To evaluate the response time and temperature condition for defogging and fogging, a laser and PD are utilized, as depicted in Figure 4e. In this setup, when fog precipitates on the windshield (defogger off), the laser signal is lost at the PD, and thus the laser signal is detected only when defogged. Figure 4f shows five conditions in this experiment. At stage (i), PD intensity is low because the windshield is fogged. When the voltage is applied, stage (ii), the heater generates Joule heat, thus temperature increases with increasing PD signal. It is noteworthy that the decrease in the temperature slope at maximum PD intensity implies that generated heat is used for latent heat which is required for evaporation of fogs on the windshield, thus this point is defined as defogging temperature. This defogging effect persists not only under the continuous

4. CONCLUSIONS In conclusion, by introducing a TCL to the photonic sintering process of very narrow CuNP pattern which has linewidth as low as 2.4 μm, imperceptible transparent electrodes can be successively fabricated on the plastic substrates by increasing effective sintering temperature. This process can be easily applied to complicated random pattern structures, thus interference-free conditions such as moiré and starburst can be achieved. These features are advantageous to be used especially for windshield defogger. This imperceptible, costeffective copper electrode has a great potential to be applied to the large area mass production process such as roll-to-roll printing because it is fabricated by the all-solution-based process on a plastic film. TCITE can be a promising substitute for ITO in wide ranges of TCE applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b01893.

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Detailed schematic image for the master mold fabrication process, effect of lactic acid treatment, wide-view and cross-sectional SEM images, UV−vis spectrum, stabilities, and magnified image of the display pixel array (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.D.S.). *E-mail: [email protected] (S.H.K.). ORCID

Seung Hwan Ko: 0000-0002-7477-0820 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Research Foundation of Korea (NRF) Grant funded through Basic Science Research Program (2017R1A2B3005706).

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REFERENCES

(1) Angmo, D.; Larsen-Olsen, T. T.; Jørgensen, M.; Søndergaard, R. R.; Krebs, F. C. Roll-to-Roll Inkjet Printing and Photonic Sintering of Electrodes for ITO Free Polymer Solar Cell Modules and Facile Product Integration. Adv. Energy Mater. 2013, 3, 172−175. (2) Krebs, F. C.; Espinosa, N.; Hösel, M.; Søndergaard, R. R.; Jørgensen, M. 25th Anniversary Article: Rise to Power - OPV-Based Solar Parks. Adv. Mater. 2014, 26, 29−39.

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MoirÃ©-Free Imperceptible and Flexible Random Metal Grid Electrodes

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