Selective Wavelength Plasmonic Flash Light Welding of Silver

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Functional Nanostructured Materials (including low-D carbon)

Selective wavelength plasmonic flash light welding of silver nanowires for transparent electrodes with high conductivity Yong-Rae Jang, Wan-Ho Chung, Yeon-Taek Hwang, Hyun-Jun Hwang, Sang-Ho Kim, and Hak-Sung Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03917 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 26, 2018

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ACS Applied Materials & Interfaces

Selective Wavelength Plasmonic Flash Light Welding of Silver Nanowires for Transparent Electrodes with High Conductivity Yong-Rae Jang1, Wan-Ho Chung1, Yeon-Taek Hwang1, Hyun-Jun Hwang1, Sang-Ho Kim2, 3 and Hak-Sung Kim1, 4,*

1Department

of Mechanical Engineering, Hanyang University, 17 Haengdang-Dong, Seongdong-Gu,

Seoul 133-791, South Korea 2

Department of Chemistry, Kongju National University, Gongju-si, Chungcheongnam-do, 32588, South

Korea 3

N&B Co. Ltd., 125-10, Techno 2-ro, Yuseong-gu, Daejeon, 34024, South Korea

4Institute

of Nano Science and Technology, Hanyang University, Seoul, 133-791 South Korea

* Corresponding author. Tel: +82-2-2220-2898 E-mail address: [email protected] (Hak-Sung Kim) ABSTRACT In this work, silver nanowires (AgNWs) printed on a polyethylene terephthalate (PET) substrate using a bar coater were welded via selective wavelength plasmonic flash light irradiation. To achieve high electrical conductivity and transparent characteristics, the wavelength of the flash white light was selectively chosen and irradiated by using high-pass filters, low-pass filters, and band-pass filters. The flash white light irradiation conditions such as on-time, off-time, and number of pulses were also optimized. The wavelength range (400~500 nm) corresponding to the plasmonic wavelength of the AgNW could efficiently weld the AgNW films and enhance its conductivity. To carry out in-depth study of the welding phenomena with respect to wavelength, a multi-physics COMSOL simulation was conducted. The welded AgNW films under selective plasmonic flash light welding conditions showed the lowest sheet resistance (51.275 Ω/sq), and noteworthy transmittance (95.3 %). Finally, the AgNW film, which was welded by selective wavelength plasmonic flash light with optical filters, was successfully used to make a large area transparent heat film and dye-sensitized solar cells (DSSCs) showing superior performances.

KEYWORDS: silver nanowire, flash light welding, intensive plasmonic, optical filter, band-pass filter, wavelength range, printed electronics

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INTRODUCTION Recently, flexible transparent electrodes have attracted increasing interest because of the expanding printed electronics market, which includes heat films1-2, solar cells3-4, organic light emitting diodes (OLED)5-6, touch screens7-8, transparent displays9-10, etc. To fabricate transparent electrodes, indium tin oxide (ITO)11-12, carbon nanotubes (CNTs)7, 10, 13

, graphene14-15, and metal nanowires3, 16-18 are widely used. However, ITO cannot be used for flexible

transparent electrodes because of its brittleness and high cost19-21. On the other hand, CNTs and graphene with ductile characteristics can be used for flexible transparent electrodes. However, these materials are not suitable for transparent electrodes due to their low conductivity (CNTs: over 500 Ω/sq at the transmittance of 80%, graphene: over 3000 Ω/sq at the transmittance of 80%)22. Therefore, to solve these problems (i.e., brittleness, low conductivity, and high cost), the silver nanowire has been used owing to its advantageous characteristics, including its ductility, low sheet resistance, and low cost.23-24 To improve the electrical and mechanical characteristics of the transparent electrode using silver nanowires (AgNWs), various welding processes (such as heat, microwave, laser25, plasma26, and flash white light welding27-29) have been developed. However, these welding processes, except for the flash white light process, are not suitable for the welding of AgNWs on a flexible substrate because of the damage caused to the polymer substrates by the high temperature (230 °C)30, long process time, or very small welding area (line width, 5–17 μm)31. On the other hand, the flash white light process can weld AgNW films with a large surface area at room temperature and under ambient conditions32-34.

When the AgNWs are irradiated by the flash white light, the AgNWs absorb a specific wavelength range because of the surface plasmon effect17, 32, 35. The generated surface plasmon effect assisted the local welding of the AgNWs at their contact junctions. Therefore, to enhance the welding of the AgNWs at a specific wavelength range by the surface plasmon effect, the wavelength of the flash white light should be controlled using various optical filters (such as high-pass, low-pass, and bandpass filters)36. In this work, the wavelength of the flash light was selectively chosen and irradiated to enhance the welding of the AgNWs using surface plasmonic resonance. Various optical filters were used, and their effect on the welding of the AgNWs, and the damage on the PET substrate film were investigated. A multi-physics simulation was also conducted using COMSOL Multiphysics 5.0 to study the surface plasmon welding phenomena among the AgNWs with respect to the wavelength of the irradiated flash light. Finally, a large area flexible transparent heat film and a flexible dye-sensitized solar cell (DSSC) were successfully produced with the fabricated welded AgNW films.


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RESULTS AND DISCUSSION To fabricate the conductive flexible transparent film, an AgNW solution was diluted with deionized (D.I.) water. It was then coated onto the PET substrates by using the bar-coating process and dried using near infrared (NIR) lamps. Then, the flash white light with a wavelength range of 350–850 nm without an optical filter was used to irradiate the AgNW films. Note that the flash white light without an optical filter covers the entire wavelength range of visible light. The AgNWs were irradiated with the flash white light with various irradiation energies, ranging from 4 J/cm2 to 12 J/cm2 (pulse number: 1, on-time: 10 ms). As shown in Figure 1(a), the initial sheet resistance of the AgNW film before the flash white light welding was 100 Ω/sq, with an error of±0.1%. Initially, the AgNW solution was coated on the PET substrate with the thickness of 20μm. In this solution state, AgNWs are gathered and connected to each other to form a multi-layer web shape, whereas the polymer binder was sunken and accumulated on the PET substrate. After that, the solvent is evaporated through the NIR drying process, the polymer binder layer was accumulated to a thickness of 200 nm (Figure S1(a)) while the AgNWs still maintain their contacts due to Van der Waals forces. Therefore, these mechanical contacts among the AgNWs is the reason why the not-welded AgNW film shows the conductivity even though the polymer binder covers the AgNWs thick. On increasing the flash white light energy, the sheet resistance of the AgNW films decreased because the AgNWs were welded to each other and the polymer binder was removed. Figure 1(b) shows the scanning electron microscopy (SEM) image of the non-welded AgNWs covered with the polymer binder. In Figure 1(c), polymer binders which covered the AgNWs were reduced by 8 J/cm2 of flash light irradiation. The polymer binder around the AgNWs was removed because of the decomposition of the polymeric chain of the polymer binders with heat generated by photo-thermal effect (Figure S8)28. Once PVP is decomposed as the gaseous alcohol, evaporation of polymer binder and welding of AgNWs can be occurred simultaneously. These discussions were added in the supporting information of the manuscript. Moreover, the polymer binder on the AgNW film was fully evaporated and AgNWs were welded together when the film was irradiated with the flash white light with energy of 10 J/cm2 (Figure 1(d))37-39. This means that the energy from the 10 J/cm2 flash white light was sufficient to remove the polymer binder and weld the AgNWs. These phenomenon were observed with TEM analysis (Figure S2) and it could be confirmed that two AgNWs having different lattice directions made neck-junction with each other at the irradiation energy of 10 J/cm2 (Figure S2(c)). Thus, the sheet resistance of the welded AgNW film became lower than that of the non-welded AgNW films. However, when irradiating with a flash white light with 12 J/cm2 energy, the sheet resistance of the welded AgNW film increased again as the AgNWs were melted in a bubble shape by the excessive flash white light energy (Figure 1(e)). Therefore, the optimum flash white light energy for the welding of the AgNWs was chosen to be 10 J/cm2. To use selectively the wavelength of the flash white light, various optical filters were used. Various high-pass filters (400 nm, 500 nm, and 600 nm high-pass filters), which can block the



wavelength range from 350 nm to 400 nm, 500 nm, and 600 nm and pass the higher wavelength range, were used with the xenon lamp. To confirm the irradiated wavelength range, the wavelength spectrum of the irradiated flash white light was measured by a wave spectrometer (UV-NIR spectroradiometer USB2000+, Ocean Optics) under the same irradiation energy (10 J/cm2) (Figure 2(a-c)). The total irradiation energy was measured in all cases using an energy meter (Nova II, Ophir Optronics Solutions Ltd). In Figure 2(a-c), the black line shows the entire wavelength range of the flash white light, irradiated without a filter; the red lines show the low wavelength light blocked by the high-pass filters. It was found that the intensity of the flash light with high-pass filters was higher than that of the nofilter case. It is to make the equal light energy (10 J/cm2) in all cases by compensating the cut energy in the range of the blocked low wavelength range light. As shown in Figure 2(h), the sheet resistance of the welded AgNWs in the 400 nm high-pass filter case was a little lower than that of the welded AgNWs without a filter. It means that the neck-junction between the welded AgNWs in the 400 nm high-pass filter case grew larger than that of those without a filter, and the 500 nm and 600 nm high-pass filter cases (see and compare between Figure 1(d) and Figure 2(d)). It is because in the 400 nm high-pass filter case, the plasmonic absorption wavelength of the AgNWs exists in the range of the wavelength passing through the 400 nm high-pass filter. The absorption wavelength of the AgNWs is represented as 415 nm, as shown in the UV-vis spectra of Figure 3. Therefore, the irradiated flash light passing through the 400 nm high-pass filter assisted the welding of the AgNWs with a higher intensity of the specific plasmonic wavelength (415 nm) of the AgNWs. Meanwhile, the sheet resistance of the welded AgNWs using the 500 nm and 600 nm high-pass filters increased because the AgNWs and PET substrate were damaged by the high intensity of the high wavelength range (Figure 2(d)-(f)). For an in-depth study of the relationship between the wavelength of the flash light and the welding of the AgNWs, a multi-physics computer simulation (COMSOL Multiphysics) was performed. As shown in Figure S4, two AgNWs with a diameter of 50 nm and length of 1000 nm were modeled, and the heat generation between the two AgNWs was analyzed with respect to the wavelength of the irradiated light (electromagnetic wave). As shown in Figure S5, in the wavelength range of less than 500 nm, concentrated heat could be generated at the junctions between the AgNWs. It was also found that at a wavelength of 420 nm, the heat generation at the junction between the AgNWs was at a maximum as their plasmon wavelength (415 nm) was coincident with the irradiated light wavelength (Figure S5(ac)). However, when the wavelength range was higher than 500 nm, a significant amount of heat was generated on the entire surface of the AgNWs, which would damage the AgNWs and the PET substrates simultaneously (Figure S5(d-f)). All of the simulation results agreed well with those from the experiments discussed above. Again, it is noteworthy that the AgNWs could be effectively welded at its surface plasmon wavelength range (around 400 nm), owing to the effective heat generation in the junction between the AgNWs. Further, to study the wavelength effect on the welding of the AgNWs, low-pass filters (i.e. 400


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nm, 500 nm, 600 nm, and 700 nm low-pass filters) were used. The flash light with the low-pass filters was irradiated on the AgNW film (energy: 10 J/cm2, on-time: 10 ms, Pulse number: #1). As shown in the wave spectrum of Figure 4(a-d), the intensity of the irradiated flash light with a low-pass filter was higher than that of the no-filter case to compensate for the energy of the blocked wavelength range of light, similarly to the high-pass filter cases above. Figure 4(i) shows the sheet resistance of the welded AgNW films with and without the low-pass filters. When the 600 nm and 700 nm low-pass filters were used, the sheet resistance of the welded AgNWs slightly decreased as compared to that of the welded AgNWs without a filter (Figure 4(g, h)). In addition, the damage to the PET substrate was decreased by blocking the high wavelength range. However, when the 400 nm low-pass filter was used, the sheet resistance of the welded AgNWs increased and was higher than that of the no-filter. In the case of the non-welded AgNWs, almost all of the AgNWs were completely melted with bubble shapes (Figure 4(e)). It is because the plasmonic wavelength of the AgNWs (around 415 nm) was not included in the irradiated flash light passing through the 400 nm low-pass filter (see Figure 4(a)). However, when the 500 nm low-pass filter was used, the sheet resistance of the AgNWs dramatically decreased as shown in Figure 4(i). In addition, the junctions among the AgNWs were firmly welded, as seen in Figure 4(f). Similarly, the 600 nm and 700 nm low-pass filters showed lower sheet resistances than the no-filter case shown in Figure 4(i). It is noteworthy that the sheet resistance decreased as the cut-off wavelength of the low-pass filter was decreased from 700 nm to 500 nm. It is because the plasmon wavelength intensity of the AgNWs around 415 nm increased as the cut-off wavelength of the low-pass filter was decreased to 500 nm to make the total energy of the flash light(10 J/cm2). Note that the heat at the junction between the AgNWs could be generated when the plasmonic wavelength of around 415 nm is irradiated. When using 400 nm high-pass filter, the sheet resistance of the AgNW film was little less than that of ‘no filter’ case (Figure 2(h)). However, there were some damages on the AgNW film because of near IR wavelength range of flash white light. On the other hand, when using 500 nm low-pass filter, the sheet resistance was remarkably decreased from 100 Ω/sq to 56.38 Ω/sq while the sheet resistance of ‘400 nm LPF’ case was increased to 106.81 Ω/sq (Figure 4(i)). It means that the wavelength under 500 nm was very effective to welding of AgNWs, but the wavelength under 400 nm was very harmful to welding of AgNWs and made damages on them. From these reasons, to verify the combined results of two experiments and to maximize the plasmonic wavelength around 415 nm, the band-pass filter with a wavelength range of 400–500 nm was used. In addition, to determine the optimum flash light energy conditions, the AgNWs were irradiated with the flash light containing the band pass filter under energy ranging from 6 J/cm2 to 10 J/cm2 (Figure 5(a)). As shown in Figure 5(b), when the flash light of 8 J/cm2 was used, the sheet resistance of the welded AgNW films was the lowest. In addition, the junction between the AgNWs was fully welded without damage to the PET substrate (Figure 5(d)). This phenomenon was attributed to the significant effect of the selective plasmonic wavelength light for the



welding of the AgNWs. Meanwhile, at the irradiation energy of 6 J/cm2, the sheet resistance of the AgNW film slightly decreased because the welding of the AgNWs was weakly generated (Figure 5(c)). In addition, when the flash light of 10 J/cm2 was used, the sheet resistance of the welded AgNW films increased. It was observed that the AgNWs were twisted because of the excessive flash light energy (Figure 5(e)). Figure 6(a)-(f) show the cross-sectional views of the heat generation of the AgNWs by a COMSOL multiphysics 5.0 simulation. Because the electromagnetic wavelength was oscillated on x-axis and yaxis, the visualizations of heat generation on x-axis, and y-axis were represented at the upwards, and downwards each. As shown in Figure 6(b), the heat generated by the absorbed light was concentrated around the junction area. This indicates that a local hot spot known as the plasmon-enhanced area occurred at the spherical nanowire junction point40-41. The average heat-generation value of each junction point of the AgNWs with respect to the wavelength of the light was calculated as shown in Figure 6(g). It should be noted that the localized surface plasmon (LSP) induced around the junctions of the AgNWs (especially the hot spot area) was greatly enhanced at the plasmon resonant wavelength (400–500 nm), as expected. Therefore, when the flash light with the band-pass filter from 400 nm to 500 nm was used, the AgNWs could be effectively welded owing to the hot spot effect at the junctions by the surface plasmon effect. The transmittance of three AgNW films (a non-welded AgNW film, a welded AgNW film without an optical filter, and with the band-pass filter from 400 nm to 500 nm) was evaluated with a spectrophotometer (UV-vis, Lambda 650S, PerkinElmer). As shown in Figure 7(a), after flash light irradiation, the transmittance of the AgNW films increased from 92.904 % to 94.181 %. Compared with the thickness of the AgNW which was 4~50 nm thick, the polymer binder layer whose thickness was 4 times thicker than the thickness of the AgNW (Figure S1(a)). This thick thickness of polymer binder might be sufficient to reduce the transmittance of the AgNW film. From the UV-vis results, it can be confirmed that the 1.4 % reduction in transmittance in the case of a film coated only with the polymer binder was occurred compared to that of the pure PET substrate (Figure S7). From this reason, it can be concluded that evaporating and removing polymer binder was important factor to increase the transmittance. On the other hand, when AgNW film was welded without optical filters, the transmittance of the AgNW film (average transmittance: 94.181%) was lower than that of the band-pass filter case (average transmittance: 95.343%) when it was irradiated at a wavelength of 400~500 nm. It is because the PET substrate was slightly damaged by the flash light. It is noteworthy that the PET substrate can be damaged at both the low wavelength range (near UV range) and the high wavelength range near the IR range 42. Therefore, the selective wavelength (400–500 nm) plasmonic welding method proposed in this work could not only weld the AgNW junctions efficiently, but also prevent the damage of the PET substrate by blocking unnecessary and harmful light to the PET substrate. In addition, after flash light welding without optical filter and with 400~500nm band-pass filter, the surface roughness Rq decreased


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to 7.136 nm and 6.523 nm, respectively (Figure S3(b,c)). It was because the selective wavelength plasmonic flash light irradiation could make the AgNWs welded together and embedded into the PET substrates. Also, bending fatigue characteristics were remarkably improved by flash light welding process (Figure 7(b)). The sheet resistance of the AgNW film which was not welded was highly increased almost 58.72 % while no filter case had a little of 9.46 % of sheet resistance increase ratio. Furthermore, when the selective plasmonic flash light was irradiated through the 400~500 nm bandpass filter, the sheet resistance of the AgNW increased only 6.33 % after 10000 cycles of the bending fatigue test. It shows that the selective wavelength of 400~500 nm could efficiently weld the junction of the AgNWs, the AgNW networks were embedded into PET and endure the external stress. The applicability of the welded AgNW flexible transparent film was demonstrated. Firstly, a large area flexible transparent heat film was fabricated with the AgNW transparent electrodes. To measure the efficiency of the heat films, three samples (i.e. a non-welded AgNW film, a welded AgNW film by flash light without a filter, and with a band-pass filter from 400 nm to 500 nm) were prepared. An electrical current was applied to each electrode attached on both sides of the film and the potential was increased gradually from 0 V to 18 V by a power supply (Figure 8(a)). To measure the temperature of the heat films, a thermographic camera (FLIR-E63900, FLIR) was used. As shown in Figure 8(c), the initial temperature of the AgNW transparent heat film was almost 30±0.5 °C. When the applied voltage was 18 V, the temperature of each of the heat films (i.e. the non-welded AgNW film, the welded AgNW film by a flash light without a filter, and with a band-pass filter from 400 nm to 500 nm) were 77.1 °C, 80.8 °C, and 91.9 °C ,respectively (Figure 8(d-f)). These results show that the AgNW heat film welded with the 400–500 nm band-pass filter had the highest efficiency for increasing the temperature (91.9 °C), which is 21.8% higher than the case without an optical filter. This is because the AgNW heat film welded with the selective wavelength (400–500 nm with band-pass filter) plasmonic flash light has the lowest sheet resistance among them. The power applied to the AgNW films could be easily calculated with the following equation: P = VI =

𝑉𝑉 2 𝑅𝑅

(1)

As the second application of the welded AgNW transparent conductive film (TCE), a DSSC was fabricated (Figure 9(a)) and its performance was demonstrated. The short-circuit photocurrent density-Voltage (J-V) curves of the DSSCs with the AgNW transparent conductive electrodes were measured as shown in Figure 9(b). It was found that the short-circuit photocurrent density (Jsc) of the photo-anodes increased considerably after flash white light irradiation, resulting in better efficiency (52% increase). This is because the AgNWs welded by flash white light irradiation had reduced the sheet resistance of the transparent electrodes. The lower electrical resistance of the electrodes in the DSSCs enables a longer lifetime of the photo-excited electrons and higher efficiency of the DSSCs. It was also



found that the DSSC with the selective wavelength (400–500 nm with a band-pass filter) and plasmonic flash light-welded AgNWs had the highest efficiency (1.6%), approximately 13% higher than that obtained without an optical filter. Therefore, it was demonstrated that the selective wavelength (400– 500 nm) plasmonic flash light-welding technique of AgNW TCE is expected to be a promising approach to realize many applications, including flexible transparent heat films and flexible dye-sensitized solar cells.

CONCLUSIONS In this work, a selective wavelength (400–500 nm) plasmonic flash white light welding of the AgNW film was developed and demonstrated. The optimized flash light-welding conditions (energy: 8 J/cm2, on-time: 10 ms, pulse number: #1) and wavelength range (from 400 nm to 500 nm) was achieved and an in-depth study of the relationship between the welding phenomena of AgNWs and the wavelength of the flash light was conducted via a multi-physics COMSOL simulation. The optimized AgNW films have a high conductivity (51.275 Ω/sq) with high transparency (95.343 %) and without damage to the PET substrate.

METHODS Fabrication of the silver nanowire solution. For fabrication of the AgNWs solution, a commercialized AgNWs concentrate was used (0.6 wt%, 40-50 nm in diameter, 30-40 μm in length; Co. Nanotech & Beyond). For the solution, 80 g of AgNWs

concentrate was diluted with 30 g of deionized (DI) water (99%; Samchun Chemical). The AgNWs solution was stirred for 30 min to disperse the AgNWs in the solvent and polymer binder.

Preparation of AgNW films. To fabricate the AgNW films, a polyethylene terephthalate (PET) substrate (thickness: 185 μm) was

used. To remove the impurities and dust, the surface of the PET substrate was cleaned with ethanol and DI water using an ultra-sonicator for 15 min. The fabricated AgNWs solution was coated on a PET substrate using a bar coater (Wireless No. 9, 20.57 μm in Wet Thickness; iNexus, Inc.). In this coating process, a surface roughness of the AgNWs film was Rq = 10.474 nm (Figure S3(a)). The AgNWs films were dried with an infrared lamp (NIR, wavelength range: 800~1500 nm, 500 W; Adphos L40) at a power of 350 W for 10 s.

Wavelength-controlled photonic welding process using various optical filters. The fabricated AgNW films were welded using white light from an Intense Pulsed Light (IPL) equipment at room temperature under ambient conditions (Figure S6(a)). This equipment consisted of a reflector, a xenon lamp (First Light Co.), a power supply, capacitors, and a pulse controller (Pstek


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Co.). The xenon lamp emitted a wavelength range from 350 nm to 900 nm. In this study, the wavelength of the flash white light was controlled using optical filters (i.e. low-pass filter, high-pass filter, and bandpass filter) (Figure S6(b-d)). Then, the AgNW films were welded with the wavelength-controlled flash light. To analyze the wavelength of the flash white light passing through the optical filter, a wave spectrometer (UV-NIR spectroradiometer USB2000+, Ocean Optics) was employed. The flash white light and wavelength-controlled flash light by the optical filter were irradiated under various conditions such as the energy (from 4 to 12 J/cm2), the irradiation time (10 ms) and pulse number (1 pulse).

Characterization. The sheet resistances of the welded AgNW films were measured with a four-point probe (diameter of probe tip: 1 μm, interval of probe tips: 1 mm; Modusystems, Inc.) with a source meter (2015 THD, Keithley). The welded AgNW films were observed using a scanning electron microscope with an operating voltage of 15 kV (SEM, S4800 HITACHI). In addition, the absorption wavelength of the AgNWs was analyzed using a UV-visible spectrometer (UV-vis, Lambda 650S, PerkinElmer). To evaluate the transmittance of the AgNW films, a spectrophotometer (Libra S70, Bruker Co.) was used. Finally, to analyze the welding mechanism of the AgNWs welded by flash light at the various wavelengths, commercial COMSOL Multiphysics 5.0 based on the finite element method (FEM) was used (Figure S4).

Fabrication of heat films and solar cell applications. To demonstrate the applications of these highly conductive flexible transparent electrodes, a large area heat film was fabricated (100 mm × 100 mm) using three AgNW films (a non-welded

AgNW film, a welded AgNW film without an optical filter, and with a band-pass filter from 400 nm to

500 nm). The flexible transparent large area heat film was fabricated with a silver paste and copper tape. To connect the heat film and copper tapes, the silver paste was painted between the edges of the heat film and copper tape. Copper tape was used for the anode and cathode of the heat film. Then, the power supply (SDP 30-3DT, SM Techno) applied a voltage through the copper tape to the AgNW film. Finally, the temperature change was measured by a thermographic camera (FLIR-E63900, FLIR). For fabrication of the flexible DSSCs, a TiO2 nanoparticle paste (Solaronix; 15-20 nm in diameter) was printed on the AgNW-coated PET substrates by the doctor blade method, followed by thermal sintering (120 °C) and N-719 dyeing. The AgNW TCE was also applied to the counter electrode. Sandwich type DSSCs were assembled by using the N-719 dye-coated TiO2 as a photo anode and Pt as the counter electrode.

AUTHOR INFORMATION

Corresponding Author



*Tel:+82-2-2220-4898 E-mail: [email protected]

ORCID Hak-sung Kim: 0000-0002-6076-6636

Notes. The authors declare no competing financial interest.

ACKNOWLEDGMENTS In this study, COMSOL Multiphysics 5.0 simulation program was offered from Hanyang University, Department of mechanical engineering, Multiphysics System Design Laboratory, Prof. Gil-Ho Yoon (http://mpsd.hanyang.ac.kr).

SUPPORTING INFORMATION Supporting Information Available: FIB cross-section SEM images of the AgNW films to explain the mechanism of removing polymer binder layer during flash light welding process, Figure S1; TEM images of the AgNW films to show welded junction between AgNWs, Figure S2; Surface roughness measurement of the AgNW films using AFM before, after flash light welding and selective wavelength plasmonic welding process, Figure S3; Detail information about COMSOL multiphysics simulation to demonstrate the wavelength effect on the AgNW, Figure S4; The wavelength effect on heat generation at the overall AgNWs with respect to the wavelength using COMSOL multiphysics, Figure S5; Real images of researcher’s flash light welding equipment and optical filters, Figure S6; Transmittance spectra of the polymer binder on the PET substrate to investigate the influence of the polymer binder on the transmittance of the AgNW films, Figure S7; The reference figure which explain a photo degradation of PVP by flash light irradiation, Figure S8.


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FIGURE LIST

Figure 1. The sheet resistances of the flash light welded AgNW without optical filter (Pulse number, 1; ontime, 10 ms). (a) The sheet resistances of the AgNWs film and SEM images of the AgNWs. (b) The SEM image of the AgNWs film which was not welded, and the SEM images of the AgNWs welded with respect to the flash light welding energy of (c) 8 J/cm2, (d) 10 J/cm2, (e) 12 J/cm2 Figure 2. The effect of high-pass filters for welding of AgNWs. The wavelength spectrum of flash light passed by (a) 400 nm, (b) 500 nm, (c) 600 nm high-pass filter compared with the spectrum of flash white light without optical filter. SEM images of the AgNWs films welded by flash light using (d) 400 nm, (e) 500 nm, (f) 600 nm high-pass filters. (g) The damaged AgNWs film which was welded by flash light through 600 nm high-pass filter. (h) Sheet resistance of the AgNWs without filter and with high-pass filters (irradiation energy, 10 J/cm2; pulse number, 1; on-time, 10 ms), Figure 3. The UV-vis result, which represented the absorbance intensity of AgNWs film with respect to wavelength range. Figure 4. The effect of low-pass filters for welding of AgNWs. The wavelength spectrum of flash light passed by (a) 400 nm, (b) 500 nm, (c) 600 nm, (d) 700 nm low-pass filter compared with the spectrum of flash white light without optical filter. SEM images of the AgNWs films welded by flash light using (e) 400 nm, (f) 500 nm, (g) 600 nm, and (h) 700 nm low-pass filters. (i) Sheet resistance of the AgNWs without filter and with low-pass filters (irradiation energy, 10 J/cm2; pulse number, 1; on-time, 10 ms) Figure 5. The effect of band-pass filter for welding of AgNWs. The wavelength spectrum of flash light passed by (a) 400 nm ~ 500 nm band-pass filter compared with the spectrum of flash white light without optical filter. (b) sheet resistance of the AgNWs without filter and with low-pass filter (irradiation energy, 6~10 J/cm2; pulse number, 1; on-time, 10 ms), SEM images of the AgNWs films welded by flash light using 400 nm ~ 500 nm band-pass filter with the irradiation energy of (c) 6 J/cm2, (d) 8 J/cm2, (e) 10 J/cm2. Figure 6. Heat generation at the junction point between two crossed AgNWs with respect to the wavelength. Heat generation at the junction point when the wavelength was (a) 400 nm, (b) 420 nm, (c) 450 nm, (d) 500 nm, (e) 650 nm, and (f) 950 nm. (g) The averaged heat generation value at the junctions of the Ag NWs Figure 7. (a) Transmittance and (b) bending fatigue test of AgNW films which were not welded, welded without optical filters, and welded using 400~500 nm band-pass filter. Figure 8. AgNWs film application for large area heat film. (a) Temperature changes of 3 case of AgNWs heat film (i.e. not welding, welding without optical filter, welding with 400~500 nm band-pass filter), (b) the temperature measurement with thermo-graphic camera in cases of (c) 0 V, (d) 18 V to not welded film, (e) 18 V to the AgNWs film welded without optical filter, (f) 18 V to the AgNWs film welded with 400~500 nm band-pass filter. Figure 9. Ag NW TCE application for dye-sensitized solar cell (DSSC). (a) An image of flexible DSSC fabricated with Ag NW films. (b) Photocurrent density–voltage curves for DSSCs with Ag nanowire transparent electrodes before and after flash white light irradiation (energy: 8 J/cm2, on-time: 10 ms, pulse number: #1) with band-pass filter (wavelength range from 400 nm to 500 nm).


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Figure 1. The sheet resistances of the flash light welded AgNW without optical filter (Pulse number, 1; ontime, 10 ms). (a) The sheet resistances of the AgNWs film and SEM images of the AgNWs. (b) The SEM image of the AgNWs film which was not welded, and the SEM images of the AgNWs welded with respect to the flash light welding energy of (c) 8 J/cm2, (d) 10 J/cm2, (e) 12 J/cm2 358x284mm (150 x 150 DPI)


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Figure 2. The effect of high-pass filters for welding of AgNWs. The wavelength spectrum of flash light passed by (a) 400 nm, (b) 500 nm, (c) 600 nm high-pass filter compared with the spectrum of flash white light without optical filter. SEM images of the AgNWs films welded by flash light using (d) 400 nm, (e) 500 nm, (f) 600 nm high-pass filters. (g) The damaged AgNWs film which was welded by flash light through 600 nm high-pass filter. (h) Sheet resistance of the AgNWs without filter and with high-pass filters (irradiation energy, 10 J/cm2; pulse number, 1; on-time, 10 ms) 352x361mm (150 x 150 DPI)



Figure 3. The UV-vis result, which represented the absorbance intensity of AgNWs film with respect to wavelength range. 230x169mm (150 x 150 DPI)


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Figure 4. The effect of low-pass filters for welding of AgNWs. The wavelength spectrum of flash light passed by (a) 400 nm, (b) 500 nm, (c) 600 nm, (d) 700 nm low-pass filter compared with the spectrum of flash white light without optical filter. SEM images of the AgNWs films welded by flash light using (e) 400 nm, (f) 500 nm, (g) 600 nm, and (h) 700 nm low-pass filters. (i) Sheet resistance of the AgNWs without filter and with low-pass filters (irradiation energy, 10 J/cm2; pulse number, 1; on-time, 10 ms) 541x475mm (150 x 150 DPI)



Figure 5. The effect of band-pass filter for welding of AgNWs. The wavelength spectrum of flash light passed by (a) 400 nm ~ 500 nm band-pass filter compared with the spectrum of flash white light without optical filter. (b) sheet resistance of the AgNWs without filter and with low-pass filter (irradiation energy, 6~10 J/cm2; pulse number, 1; on-time, 10 ms), SEM images of the AgNWs films welded by flash light using 400 nm ~ 500 nm band-pass filter with the irradiation energy of (c) 6 J/cm2, (d) 8 J/cm2, (e) 10 J/cm2. 360x243mm (150 x 150 DPI)


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Figure 6. Heat generation at the junction point between two crossed AgNWs with respect to the wavelength. Heat generation at the junction point when the wavelength was (a) 400 nm, (b) 420 nm, (c) 450 nm, (d) 500 nm, (e) 650 nm, and (f) 950 nm. (g) The averaged heat generation value at the junctions of the Ag NWs 618x256mm (150 x 150 DPI)



Figure 7. (a) Transmittance and (b) bending fatigue test of AgNW films which were not welded, welded without optical filters, and welded using 400~500 nm band-pass filter. 352x153mm (150 x 150 DPI)


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Figure 8. AgNWs film application for large area heat film. (a) Temperature changes of 3 case of AgNWs heat film (i.e. not welding, welding without optical filter, welding with 400~500 nm band-pass filter), (b) the temperature measurement with thermo-graphic camera in cases of (c) 0 V, (d) 18 V to not welded film, (e) 18 V to the AgNWs film welded without optical filter, (f) 18 V to the AgNWs film welded with 400~500 nm band-pass filter. 374x170mm (150 x 150 DPI)



Figure 9. Ag NW TCE application for dye-sensitized solar cell (DSSC). (a) An image of flexible DSSC fabricated with Ag NW films. (b) Photocurrent density–voltage curves for DSSCs with Ag nanowire transparent electrodes before and after flash white light irradiation (energy: 8 J/cm2, on-time: 10 ms, pulse number: #1) with band-pass filter (wavelength range from 400 nm to 500 nm). 334x165mm (150 x 150 DPI)


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Selective Wavelength Plasmonic Flash Light Welding of Silver

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