Silver

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

Highly Transparent Conductive Reduced Graphene Oxide/Silver Nanowires/Silver Grid Electrodes for Low-Voltage Electrochromic Smart Windows Koduru Mallikarjuna and Haekyoung Kim* School of Materials Science and Engineering, Yeungnum University, Gyeongsan 712 749, Republic of Korea

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S Supporting Information *

ABSTRACT: Transparent conductive electrodes (TCEs) based on hybrid structures (silver nanowires) have been compressively reconnoitered in next-generation electronics such as flexible displays, artificial skins, smart windows, and sensors because of their admirable conductivity as well as flexibility, which make them favorable substitutes to replace ITO (indium tin oxide) as a transparent conductor. Nevertheless, silver-based TCEs grieve from poor stability because of the corrosion and oxidation of silver in electrolytes. To overcome these issues, a RGO (reduced graphene oxide) layer on silver was promoted to resolve the difficulties of corrosion and oxidation in the electrolyte. Moreover, we successfully designed and demonstrated low-voltage WO3-based electrochromic devices (ECDs) with fabricated hybrid TCEs. The hybrid electrodes with RGO/silver nanowires/metal grid/PET (RAM) electrode exhibited improvements in the switching stability and optoelectronic properties, such as the sheet resistance (0.714 ohm/sq) as well as optical transparency of 90.9%. The coloration and bleaching behavior of the ECD was observed in an applied low-voltage range of −1.0 to 0.0 V with a maximum optical difference of 72% at 700 nm, which yielded a coloration efficiency (η) of ∼33.4 cm2/ C. The highly conductive hybrid TCEs exhibit favorable features for numerous embryonic flexible electronics and optoelectronic devices. KEYWORDS: silver nanowires, corrosion, oxidation, reduced graphene oxide, electrochromic devices very hard to apply in flexible applications because of its fragility.13 Moreover, from the functioning point of view in ECDs, the usage of ITO is problematic because at high transmittance (>90%) the low concentration of charge transports results in high sheet resistance (ohms on order of tens/sq) for these ITO-TCEs.14,15 It has been recognized in ECDs the sheet resistance has a noteworthy consequence on the reaction time and color contrast. The electrodes with large sheet resistance lead to nonuniform potential and current throughout the TCE due to high ohmic losses.16 When electrodes with high conductivity are used, low potentials are desirable to achieve a fast response time and color modulation, as the ionic mobility depends on the electric field between electrodes. Thus, several materials such as metal nanowires, metal grid patterns, conducting polymers, and carbon-based materials like graphene and CNTs have been progressively studied as candidates to replace ITO in next-generation flexible devices.17−24 Among these, flexible transparent electrodes with silver nanowires (Ag NWs) and metal grid transparent electrodes (AM electrodes) are of specific desire because of the high transparency and conductivity recognized over

1. INTRODUCTION Eco-friendly energy conservation, management, production, and storage are harvesting snowballing apprehensions because of the diminution of fossil-fuel resources.1,2 Energy conversion and administration in houses are very essential; in developed countries, >40% of the total energy is required for electrical appliances used in heating and cooling and for ventilation.3,4 To overcome these energy crisis issues, many researchers put forth significant efforts to improve innumerable unconventional tools, such as reflective displays, smart windows/mirrors, batteries, and supercapacitors for energy convertible strategies. It makes a huge difference in energy adeptness and management in buildings.5 Electrochromic technology, which is capable of alternatively moving color under an external electric field, has initiated a extensive range of forthcoming appliances like smart windows, antiglare mirrors, display devices, adaptive IR camouflaging, and sensors.6−9 To use the electrochromic devices (ECDs) efficiently, TCEs must be highly transparent and highly conductive to fulfill the necessities of high coloration switching speeds. ITO has been extensively used as conventional TCE material for optoelectronic devices because of its high transmittance and low sheet resistance.10−12 The necessity of portable, stretchable, and flexible devices is of great desire for application in numerous displays and smart electronics. Nonetheless, ITO is © XXXX American Chemical Society

Received: August 16, 2018 Accepted: December 20, 2018 Published: December 20, 2018 A

DOI: 10.1021/acsami.8b14086 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

imprinting method on poly(ethylene terephthalate) (PET). The grid/ PET was obtained from Jusung Engineering (Korea), and width and length of the grid pattern were ∼10 and 500 μm, respectively. The Ag NW active grid ink was obtained from C3nano (USA), which has length and diameter of the nanowires in the ranges of 20−30 μm and 45−60 nm, respectively. Graphene oxide (GO) was obtained from Angston Materials (Korea). To prepare RGO, the GO was distributed in ethanol at concentration of 1 mg/mL with an ultrasonic bath for 30 min. Then, 10 μL/mL hydrazine was added to the solution; the mixture was followed by again sonication for 60 min at room temperature. Preparation of Ag NW/Metal Grid/PET (AM) and RGO/Ag NW/Metal Grid/PET (RAM) Hybrid Electrodes. A dispersed Ag NW ink solution was drop-casted on the one end of the metal grid substrate and the Mayer rod coating drawdown machine to distribute the solution to form a uniform coating. After the substrate was coated, TCE films were dried for 10 min at room temperature followed by a heat treatment in an oven at 120 °C for another 10 min. Subsequently, the RGO was coated on the Ag NW/metal grid substrate, monitored by drying again at 120 °C for 10 min. The Ag NW/metal grid/PET and RGO/Ag NW/metal grid/PET electrodes are denoted as AM and RAM electrodes, respectively. Preparation of Tungsten Oxide (WO3) and Electrochromic Devices. The electrochromic material (WO3) was synthesized according to our previously described method.49 First, 6.8 g of metallic tungsten powder was added to the 93.2 g (31% in water) of hydrogen peroxide, which reacted at room temperature until a pure transparent colorless solution formed. This mixture was heated at 100 °C in a closed vessel with continuous stirring, forming a yellow solution after 2 h. Then, the vessel cap was opened under the same conditions, the resulting solvent was evaporated, and dry yellow precipitate was obtained. A suspension of WO3 (30%) in a blend of isopropanol and deionized water (1:1 ratio) was arranged and dispersed by sonication for 4 h. The obtained sonicated solution was coated onto the TCE by spin coating at 5000 rpm for 20 s. Afterward, the electrochromic material-coated films were dried in a vacuum oven at 60 °C for overnight. The electrolyte suspension containing 0.5 M lithium perchlorate and 0.05 M ferrocene (Fc) was prepared in dry propylene carbonate solution. To fabricate the ECD, a 30 μm thick (2 × 2 cm2) gasket was placed on the WO3-coated TCE and filled with the electrolyte solution; afterward, another TCE was placed on the gasket, which was wrapped with silicon resin to prevent the outflow of electrolyte during the analysis. For comparison, we prepared AMECD (AM as the reference and working electrodes), ITO-ECD (ITO (15 ohm/sq) as the reference and working electrodes), ITO-RAMECD (RAM as reference and ITO as working electrodes), and RAMECD (RAM as working and reference electrodes) devices. Characterization. The transmittance of the prepared TCEs was studied using a GENESIS 10S UV−vis spectrophotometer (Thermo Scientific). The sheet resistances of TCEs were further analyzed with the four-probe technique (Loresta EP MCP-T360). The size and shape of the TCE materials were determined using scanning electron and atomic force microscopes (SEM, S-4800-Hitachi; AFM Nanoscope V BRUKER). The optical properties like transmittance and absorption of the prepared ECDs were also recorded with a UV−vis spectrophotometer coupled with a potentiostat (Weis 500WonA Tech).

percolating nanowires on a grid pattern. However, silver-based flexible TCEs suffer from a long switching time for coloration and bleaching states in electrochromic applications because of corrosion and oxidation of Ag under positive (+ve) potentials in an electrolyte.25 To overcome these issues, researchers have directed efforts toward the use of conducting polymers, metal oxides, and carbon-based materials to protect silver-based TCEs.26 In this regard, RGO used as a protective layer for the silver-based TCE, as it can suppress the oxidation and corrosion problems in ECDs. A worldwide effort is ongoing on new and improved procedures of generating thin films of graphene and RGO; the solution process procedure conveyed hitherto has enormous benefits like low cost, ease of preparation, and large-scale applications.27,28 However, the solution process of RGO-based conducting electrodes is not compatible due to the high sheet resistance at high transmittance. Nevertheless, a high-temperature annealing of such RGO helped to considerably decrease its sheet resistance. Unfortunately, flexible substrates cannot sustain at high temperatures like 250 °C and beyond.29 In this connection, to improve the conductance of the flexible electrode at lower temperature, silver nanowires/RGO is used where its optical transmittance gets negligibly affected.30−33 However, in spite of fascinating properties, RGO/silver nanowire electrodes have various challenges in implementation of TCE in smart devices for commercial purposes because of their high sheet resistance.34 To overcome aforementioned issues, the addition of multilayer RGO/silver nanowire/silver grid pattern resulted in a hybrid transparent electrode films. These prepared hybrid electrodes can fulfill requirement of conductivity−transparency constraint of pristine graphene and metallic nanowire-based electrodes.35 The reduction of GO was prepared by different methods like chemical, microwave, photocatalytic, thermal, and electrochemical procedures.36 Among these, chemical reduction of GO was satisfied by consuming broad variety of reducing agents like dimethylhydrazine, hydroxylamine, hydrazine, sodium borohydride, aluminum/iron powder, vitamin C, and hydrohalic acid.37−45 Hydrazine has been used as reductant because of their capability to attain a high degree of reduction in the process of RGO solution in ultrasonication. The efforts were made to fabricate multilayers to enhance the overall properties like physical parameters, such as folding, bending, contact scratching, and moreover chemical reactions of oxidation and corrosion.46−48 Herein, the efforts were made to fabricate novel multilayer hybrid electrodes with an RGO/Ag NW/metal grid/PET by using the bar coating method. The presence of both Ag NW and metal grid with RGO reduces the sheet resistance and oxidation problem of this hybrid electrode. In addition, it also helps to enhance the overall performance, including optoelectronic constraint (sheet resistance−transmittance) and switching time. Through the protection of an encapsulating RGO layer on silver mesh and nanowires, the switching time of the coloration and bleaching state and the coloration efficiency were improved. Owing to their stability, the metal grid and metal nanowires provide highly conductive pathways connected to RGO domains. Moreover, the RGO protects the metal patterns and nanowires from oxidation and corrosion in an electrolyte.

3. RESULTS AND DISCUSSION To estimate the consequence of the multilayer of the hybrid electrodes on the optical and electrical performance of the prepared TCE, we measured transmittance and sheet resistance. The optoelectrical properties of different hybrid TCEs are shown in Figure 1. The sheet resistance and transmittance of the metal grid electrode were 18 ohm/sq and 95.5%, respectively. Generally, the optoelectrical properties of transparent conductor are most crucial requirements for ECDs.The transmittance and sheet resistance of single and multilayer prepared electrodes are

2. EXPERIMENTAL DETAILS Materials and Methods. All the chemicals were used as analytical grade, and the metal grid (silver mesh) was prepared via the B


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in Figure S1. The electrons can freely transfer from the metal grid/Ag NW to RGO, which can reduce the contact resistance between the NWs and the RGO, which reduces the overall resistance of the electrode. Moreover, the coating of RGO and Ag NW multilayers on metal grid patterns gives the shortest pathways for moving electrons to improve the electrical conductivity. Furthermore, by comparing the measurement of transmittance of RAM electrode with the AM electrode, we find that there is a slight decrease in transmittance (Figure 1b). After heating the sample, we assume that the reason for the transmittance of hybrid electrodes are enhanced because of solvent was evaporated under heating.57,58 The flattering and diffusion at intersections significantly enhance the optoelectrical properties of nanowire-based TCEs; the bridge effect on RGO/Ag NW domains reduces the sheet resistance of hybrid film under heating.59 The structural characteristics and morphology, connections, and degree of dispersion for AM and RAM electrodes are depicted in Figure 2. The SEM images of the Ag NWs show long, thin, and well-defined nanowire structures. The SEM images in Figure 2a indicate that the Ag NWs were coated on the metal grid, that the metal grid was wellconnected with wire-to-wire confluence, and that the percolation of Ag NWs in the cavity of the metal grid improved the sheet resistance (Figure 2b). Usually, silverbased electrical networks illustrate sluggish and irregular electrochromic toggling (Blooming effect) because of corrosion and oxidation under +ve potential voltage in an electrolyte solution.42 The microscopic structures of the RGO/ Ag NW/metal grid network were analyzed via SEM (Figure 2c,d). The white shade on the network denotes the RGO layer on the Ag NW/metal grid pattern. The Ag NW junctions were wrapped and covered with RGO sheetssimilar to a blanketon the top side of the Ag NW/metal grid networks. This reduced the sheet resistance because of the RGO conduction channel and reduced the internanowire contact resistance because of the amalgamation of wire-to-wire interfaces.60,61 The cross-sectional SEM images of the RAM electrode are shown in Figure S2. The RGO sheets shown on the surface of nanowires that were in contact with the PET substrate as well as thin layer beneath led to a significant decrease of the overlapped NW’s height and surface roughness. The surface topography and roughness of the prepared hybrid electrodes with Ag NWs and RGO coatings were studied using AFM. Figure 3a,c presents the AM electrode, and Figure 3b,d depicts the RAM electrode 2D and 3D topographical AFM images. The AM electrode consists of an isolated island of nanoparticles which are present in Ag NW ink. The overlapping of Ag NWs led to relative surface roughness; the root-mean-square roughness of AM electrode was observed to be 15.95 nm. The surface is very important when creating TCEs because electrical shorts can be caused by protuberant Ag NWs. The well-organized and well-dispersed Ag NWs and RGO coatings were extensively connected without aggregation. We can find the roughness of the surface drives to a flat plane when the Ag NWs network was covered with the RGO layer. Therefore, the results reveal that the roughness of the prepared electrode changed from 15.95 to 8.92 nm with the RGO coating on the Ag NW/metal grid electrode, and the thickness of the RGO layer is around 3−10 nm. Therefore, the low surface roughness value associated with the RGO coating can be critical for the fabrication of highquality devices.62 The RGO layer on the Ag NW/metal grid

Figure 1. Optoelectrical properties of prepared hybrid electrodes: (a) sheet resistance; (b) transmittance of the prepared electrodes at 550 nm.

presented in Table S1 (Supporting Information). The change in sheet resistance from 18 to 2.53 ohm/sq of TCE was reduced by inserting a layer of Ag NW on the metal grid pattern by the bar coating method. In the process of bar coating, we find that the coating of a uniform layer of Ag NWs on metal grid pattern enables the shorter pathways of electrons by bridging the gap (cavity) of grid by Ag NWs and thereby enhances the electrical conductivity with less loss in transmittance of 95.5% to 91.2%. Moreover, various methods have been investigated to decrease the sheet resistance of the film, e.g., pressing, heating, doping, laser activation, plasma treatment, and photonic welding.50,51 Among these, the heating method is easy to apply for reducing the sheet resistance and increases the optical transmittance.52−54 However, silver has problems such as corrosion and oxidation; therefore, the electrode was protected with an RGO layer, which can prevent corrosion and oxidation problems.55,56 When coated with the RGO layer, the Ag NW/metal grid electrode exhibited a worthy performance with a very low sheet resistance of 0.715 ohm/sq with transmittance of 90.9%. The transmittance spectra of RAM and AM electrodes are depicted C


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Figure 2. Tilted SEM images of hybrid electrodes: (a, b) Ag NW/metal grid electrode; (c, d) RGO/Ag NW/metal electrode (arrows indicate that Ag NW junctions wrapped and covered by RGO sheets).

were analyzed with contrast of ∼60% and a slow decrease in further cycles. The Ag NW/metal grid ECD exhibited electrochemical instability, poor coloration stability, corrosion, and proneness to oxidation, suffering the damage of electrical conductivity, which is difficult for counter and working electrodes through the regular face-to-face device configuration, although the sheet resistance of AM electrode was very low (2.53 ohm/sq).62 To overcome this issue, the Ag NW/ metal grid electrode was designated by the coating of an electrochemically stable RGO layer as a protecting layer.13,64 Compared with the AM electrode, the RAM electrode exhibited switching stability and good optical contrast with coloring and bleaching states (Figure 5b). The optical contrast of RAM as counter and ITO as working electrodes was also studied, as depicted in Figure 5c. Moreover, Fc was introduced as an anodic species in electrolyte to enhance the optical contrast at low potentials. The optical coloration ∼72% of the RAM electrode device was witnessed by applying a low potential of −1.0 V, which is greater than those of the ITO device (Figure 5d) and other recently reported devices (see Table 1).25,49,60,65−73 The WO3-coated films at 0.0 and −1.0 V are bleaching and coloration states and are presented in Figure 6, indicating the reversibly modulated optical characteristic photographs of the prepared devices. It is revealed that the color contrast was vice versa under an applied potential. During the bleaching state

not only prevented oxidation but also enhanced the electrical conductivity and reduced the blooming effect of the ECD, which was studied via electrochromic analysis. To assemble ECDs, electrochromic (WO3) films were deposited on the prepared Ag NW/metal grid and RGO/Ag NW/metal grid electrodes using the sol−gel procedure. For comparison, we required to fabricate the ITO electrode (15 ohm/sq) with the same conditions. Figure 4 shows a schematic diagram of the hybrid TCE-based flexible ECD. To simplify the device configuration, the Fc (anodic species) was incorporated into the electrolyte layer, yielding a simple structure without a secondary electrode, which is different from traditional WO3-based ECDs. To elucidate the device dynamics, we recorded the transmittance spectra and contrast with respect to the square potential (−1.0 to 0.0 V) and measured the profiles at 700 nm. The electrochemical properties such as the contrast coloring and bleaching states of different prepared AM, ITO, ITO-RAM, and RAM based with respect to the potential were measured. The coloration process of the WO3-based electrochromic properties was explained by the transformation of electrons to their respective oxidation states under applied potentials.27 As shown in Figure 5a, the optical transmittance spectra of the electrochromic layer coating on AM electrode and its dynamics of colored and bleached states were recorded under different applied potentials (−0.4, −0.6, −0.8, and −1.0 V). The first cycles D


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Figure 3. (a, b) Two-dimensional and (c, d) 3-dimensional topographical AFM images of AM and RAM electrodes, respectively.

On the other hand, the coloration efficiency (η) is an imperative metric for determining the accomplishment of ECDs, which can be expressed as follows:49 η = ΔOD/ΔQ ΔOD = log(Tb/Tc)

Here, ΔOD is the change in optical density, ΔQ is the amount of applied charge for the respective ΔOD, and Tb and Tc are optical transmittance values at bleaching and colored states, respectively. ΔOD is plotted with respect to the inserted optical density at applied potential of −1.0 V, and coloration efficiency is derived (Figure 7). The derived coloration efficiency is 53.1, 21.7, 27.3, and 33.4 cm2/C for the ITO, AM, ITO-RAM, and RAM ECDs, respectively. The coloration efficiency of the RAM-ECD is lower than that of the ITO-ECD but higher than recently reported values (see Table 1). The transmittance spectra for the bleaching and coloration states of prepared devices depicted in Figure 8. The high coloration efficiency, high transmittance intonation, and cycle stability of the ECDs matched with ITO ECDs are attributed to numerous factors. It is estimated that the RGO layer is effective for controlling the Li+ ions at the interface between the TCE and electrolyte.

Figure 4. Schematic representation of RGO/Ag NWs/metal grid/ PET electrode-based WO3 electrochromic device.

(0.0 V) the device exhibited a pale yellow color due to the characteristic absorption of Fc. After an applied voltage of −1.0 V, the pale yellow color changed to dark blue color (colored state). Furthermore, the colored state disappeared when the potential was increased from −1.0 to 0.0 V. Among the all prepared devices, the RAM-ECD showed the most intense blue color at −1.0 V. E


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Figure 5. Coloring and bleaching of WO3 electrochromic transmittance spectra at different potentials: (a) AM-ECD, (b) RAM-ECD, (c) ITORAM-ECD, and (d) ITO-ECD.

Table 1. Optical Modulation, Coloration Time, Bleaching Time, and Coloration Efficiency of Various Reported ECDs in Comparison with This Work no.

material

optical modulation (%)

coloration time (tc, s)

bleaching time (tb, s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

WO3/Ag grid PEDOT/graphene/Ag NW WO3/Ag NW WO3/Ag NW heptyl viologen/Ag NW WO3/Ag NW WO3/ITO WO3/ITO WO3/ITO WO3/ITO WO3/FTO WO3 WO3/ITO WO3/RGO/Ag NW/Ag grid

92 25 68 55.9 74 50 65.2 25 61 52 53.3 51.2 62 72

9 4.1 4.1 3.9 32 1 6 7 15 2.5 5.9 11.8 8 12

15 3.4 8.5 3.2 43 4 3 8.5 15 7.6 4.3 14.6 6 10

coloration efficiency (η, cm2/C) 51 35.7 54.8 31.82 12.6 60.2 29.03 16.3 34.3 48 34.5 53.1 33.4

ref 25 60 65 66 67 68 69 70 71 49 72 73 this work this work

coloration transient time from one state to another state with changing potential has significant limitation for the ECD, which represents the direct optical signature for the dynamics in the electrochemical process. In particular, the transient time requires 90% of transmittance modulation between colored and bleached states, also denoted as the switching time. The switching time for the WO3 film on the ITO film was measured

Additionally, it is known that the RGO is chemically inert and has a large specific surface area and high electrical conductivity, which reduces the oxidation and corrosion inhibiting coating for silver. Therefore, it can increase the electron transfer rate and switching stability. Furthermore, drying the sample at 60 °C possibly will lead to a crystalline structure that significantly progresses the stability. The F


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Figure 6. Digital photographs of colored and bleaching states of different prepared ECDs.

Figure 8. Transmittance spectra of different prepared ECDs at colored and bleaching states. Figure 7. Optical density versus charge density of different ECD with applied voltage −1.0 V.

optical contrast of the RAM-ECDs is more than that of the ITO-ECD, which are comparable to the reported values of high-performance ECDs.13,49 However, the AM electrodebased ECD showed lesser optical contrast than that of ITO ECD, which may have lesser sheet resistance than the ITO electrode. Such poor optical contrast of AM-ECD might be due to the instability of Ag NWs in the electrolyte because of their oxidation and corrosion. In this connection, the rapid accumulation of heat at Ag NW junctions facilitates the interruption at the junctions and raises the electrode resistance under applied electrical bias, which influences the optical contrast of ECD under the applied potential.74 In the case of RGO-ECD, the presence of RGO (2D materials) at the junction of NWs reduces the resistance, which increases the electrical conductivity of TCE. Moreover, the generated heat would be rapidly spread over to the surroundings because of the high thermal conductivity of carbon 2D materials, essential to improve RGO-based TCEs stability and protected from

as 8 and 6 s for coloration and bleaching states; in the case of RAM-ECD the switching time was 12 s for coloration and 10 s for the full bleaching state. Moreover, the electrochromic behavior of the different ECDs depended on the sheet resistance of the prepared electrodes. Figure 9 depicts the EC toggling behavior of the hybrid electrodes with different sheet resistances. The RAM electrode (0.715 ohm/sq) exhibited a high optical contrast (∼72%) compared with the AM (2.53 ohm/sq) and ITO (15 ohm/sq) electrodes (57% and 62%, respectively). The results clearly show that the optical contrast depends on sheet resistance of the electrodes. A low sheet resistance electrode yields a more uniform potential and current distribution throughout the electrode. It further enables the electric field and hence the ionic mobility in the electrolyte between two electrodes, resulting in high optical contrast. The G


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4. CONCLUSIONS An RGO/Ag NW/metal grid hybrid electrode was successfully prepared via a low-cost and simple approach. The prepared electrode was highly conductive, with satisfactory optical transmittance and high stability against oxidation. Optical modulation, excellent switching stability, fast switching, and high coloration efficiency were attained on a flexible substrate when the electrode was coated with electrochromic WO3 for ECDs. A considerably good coloration efficiency of 33.4 cm2/ C obtained under low voltage of −1.0 V, fast switching response (12 s for coloration and 10 s for bleaching), and a device responsivity of 90% were achieved. The current procedure establishes a novel, low-cost synthesis route for fabricating ECDs utilizing the electrochemical redox of hybrid structures and which can be further adopted to several sizeand shape-dependent transition-metal oxides.

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Figure 9. Switching performance of the prepared hybrid electrodes with different sheet resistances of RAM electrode (0.715 ohm/sq) compared with AM (2.53 ohm/sq) and ITO (15 ohm/sq) electrodes.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b14086. Optoelectronic constraints of sheet resistance and transmittance features of single-layered Ag NWs, RGO, and bilayered RGO/Ag NWs; transmittance spectra of RGO/AgNW/metal grid/PET and AgNW/metal grid/ PET transparent conducting electrode before and after drying; cross-sectional morphological features of RGO/ AgNW/metal grid/PET electrode (PDF)

oxidation of Ag NWs in electrolyte.70 In this connection, the rapid accumulation of heat at Ag NW junctions facilitates the interruption at the junctions and raises the electrode resistance under applied electrical bias, which influences the optical contrast of ECD under applied potential.69 In the case of RGO-ECD, the presence of RGO (2D materials) at the junction of NWs reduces the resistance, which increases the electrical conductivity of TCE. Moreover, the generated heat would be rapidly spread over to the surroundings because of the high thermal conductivity of carbon 2D materials, essential to improve RGO-based TCEs stability and protected from oxidation of Ag NWs in electrolyte.75 The optical contrast of the prepared ECDs as a function of applied potential is depicted in Figure 10. The transmittance degrades continuously with decrease of applied voltage. Nonetheless, the results imply that the EC properties not only are a function of applied voltage but also depend strongly on substrate materials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Haekyoung Kim: 0000-0002-9870-3905 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been financially supported by the Ministry of Education and National Research Foundation of Korea (NRF2018R1A1A3A04076752).

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REFERENCES

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Figure 10. Variation of transmittance at 700 nm of different ECDs under different applied potentials. H


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