A partial etching mechanism is proposed to meet the requirement for low-visibility patterning of silver nanowire (AgNW)-based transparent conductive electrodes (TCEs) by reducing the difference in optical properties between conductive and nonconductive regions of the pattern. Using the finite difference time domain (FDTD) method, etched geometries that provide the smallest difference in transmittance after etching are theoretically determined. A sodium hypochlorite-based etchant capable that allows the etched geometry to be varied by controlling the pH is used to create a low-visibility pattern with a transmittance and haze difference of 0.07 and 0.04%, respectively. To the best of our knowledge, this is the first time that a partial etching mechanism such as this has been studied in relation to AgNW-based TCEs.
© 2015 Optical Society of America
Transparent conductive materials are now being extensively used as an essential component in optoelectronic devices such as liquid-crystal displays (LCDs), touch panels, organic light emitting diodes (OLEDs) and solar cells. In such applications, the high conductivity and high transparency of indium tin oxide (ITO) has seen it become the standard choice of transparent conductive electrode material [1,2]; however, it is still greatly limited by its high raw material cost, poor mechanical flexibility due to its inherent brittleness, and the requirement for high-temperature deposition conditions . These problems have created a need for alternative materials for use in newly emerging flexible optoelectronic devices, as well as current applications that require high-quality transparent electrodes. Of the various materials that have been explored, films consisting of random networks of solution-synthesized silver nanowires (AgNWs) have emerged as one of the most promising candidates [3,4]. Unlike other alternatives such as carbon nanotubes, graphene and conductive polymers, AgNWs offer an optical transparency and conductivity comparable to ITO. Moreover, AgNW electrodes offer the advantage of being inexpensive, flexible and compatible with the easily scalable roll-to-roll deposition process.
Transparent conducting electrodes (TCEs) based on AgNWs can be patterned to form the conductive and nonconductive regions required for most display devices. Screen printing can be used for low-resolution patterns, but high-resolution patterning is typically achieved through photolithography, in which an etch mask is applied and unwanted areas are removed by chemical etching. The complete removal of AgNWs by etching increases the optical transmittance and reduces haziness, resulting in a difference in optical properties between etched and non-etched areas that makes the pattern created visible to the human eye. As this fails to satisfy a key requirement for patterned layers used in projected-capacitive touch sensors, an alternative approach has been suggested in which nanowires in the etched area are partially cut into smaller segments, thereby reducing their average length without a significant loss of electrical conductivity . This partial etching process creates minimal difference in the AgNW coverage, thus ensuring that optical properties such as transmission, color, haziness and reflectivity remain nearly identical across all regions. Moreover, it allows patterns to be produced that are near-invisible to the naked eye without the need for additional index-matching layers that would normally be required for an ITO film. However, this process can be difficult to employ in practice, as insufficient patterning risks losing electrical conductivity, while excessive patterning degrades the optical characteristics. Spatial variables such as lot-to-lot variations in the film’s electrical conductivity and run-to-run variation in the patterning solution concentration, temperature and/or exposure time can also make it difficult to achieve consistent partially-patterned film products.
Transparent conductive films can be fabricated using standard wet-coating methods, whereby a coating of AgNWs is first applied by drying an aqueous dispersion, and then covered with a thin (100–150 nm) UV-curable polymer layer. This overcoat layer helps impart mechanical strength to the AgNW coating, including some degree of scratch resistance, while also protecting it from direct environmental exposure. In the polyol process normally used to produce AgNWs from decahedral seeds , this overcoating is provided by a monolayer of polyvinylpyrrolidone (PVP) that absorbs through Ag-O coordination, with the <1 nm thick skeleton chain of the PVP molecule lying on the surface of the Ag nanowire . During conventional wet etching, this PVP layer can be damaged; however, if sodium hypochlorite (NaOCl) is used it instead results in an opening of the pyrrolidone ring and chain scission of the PVP that alters the pH of the solution . The overall reaction is therefore considered to be the oxidation of PVP in alkaline solution.
As definitive studies into the low-visibility patterning of transparent conductive AgNW films are sadly lacking, this study investigates the theoretical optical properties of various AgNW geometries that can be produced by partial etching with a view to creating low-visibility patterns. For this, a sodium hypochlorite-based wet etchant was developed to more easily control the etched geometry, allowing for the first experimental demonstration of areliable partial etching mechanism for AgNWs. The optical properties of the patterns produced are herein discussed in relation to their potential to replace existing TCEs in touch panel devices.
2. Relation between the geometry of AgNWs and their optical properties
In an AgNW film, electrical current travels through the nanowires, whereas light passes through the open spaces between the individual nanowires. Consequently, although an increase in the density of nanowires leads to a more uniform electrical conductivity, and can reduce the sheet resistance to ~10 Ω/sq, the subsequent decrease in open space reduces the transmittance of light . Since the theory of percolation states that the density of nanowires needed for a continuous conductive path is inversely proportional to the square of their length (N ~1/L2), achieving the highest electrical conductivity for the lowest loss of light transmission requires nanowires with a high-aspect-ratio . Another critical property to consider is the haziness of the film, which is simply a measure of the amount of incident light scattering. For a given nanowire length, this is proportional to the cube of the nanowire diameter, with the Rayleigh light scattering induced by a wire with a cross section of ~d4 able to be treated as an electric linear dipole . Thus, applications such as displays and touch screens that demand a minimal amount of light scattering require very thin nanowires be used; however, the diameter, length and density of the wires must also be carefully chosen to ensure optimum electrical properties.
The optical properties of AgNWs with various geometries was calculated and analyzed using a custom-made simulation tool based on the FDTD method, wherein a cylindrical AgNW with a large aspect ratio (20 nm in diameter and 4 μm in length) and resting on a semi-infinite dielectric plane was used as a reference. A plane wave with either TE or TM polarization illuminates this nanowire from above, as illustrated in Fig. 1(a), with the AgNW gap and diameter then being varied to investigate the difference this has on the optical properties in Fig. 1(b). It is assumed here that the diameter and length of the AgNWs decrease as Δd and Δl during etching. Moreover, in this calculation it is assumed that the cross section of nanowire is always circular (regardless of the etching process), and only the diameter and length of the nanowires are changed due to the etching. Therefore, the angular distribution of the scattered light from the silver nanowires before and after the etching process show typical angular distribution of the scattered light from cylindrical silver nanowire (L >> a). The haze is calculated as the ratio of forward scattered transmission and total transmission. The forward scattered light is integrated over 5 ~90° in the polar coordinate system, and the value is integrated out in the wavelength between 300 ~800 nm. The optical properties of the various AgNW geometries modeled are shown in Fig. 2, which reveals an increase in transmittanceand decrease in haziness as the length and diameter of the AgNWs is reduced by etching. If the reduction in the length or diameter is fixed, then we see that the optical properties converge regardless of the aspect ratio. Moreover, the diameter has a much greater effect on the optical properties, as can be seen from the fact that the length can be reduced to 90% of its original value without significantly altering the transmittance or haziness in Fig. 2(c). This means that if an etching process can reduce the length of AgNWs without isolating them or affecting their diameter, then the patterns before and after etching should be indistinguishable to the naked eye.
3. Partial etching of AgNWs for low-visibility patterning
In the patterning of transparent conductive films by conventional photolithography, after applying an etch mask in Fig. 3(a) the film is exposed to a liquid etchant that penetrates a thin protective overcoat layer and dissolves the underlying NWs. This makes the maximum etching time before electrical isolation of the nanowires occurs dependent on such parameters as the etchant chemistry and temperature. The scanning electron microscopy (SEM) images in Figs. 3(b) and 3(c) show AgNW films after being fully etched with HNO3 and H3PO4-based solutions (PMA-17A, Soulbrain Co., Ltd.) for 7 minutes, from which it is clear that AgNWs are perfectly removed from the etched regions while remaining in the masked regions. For measurements of transmission and haze, we run standard transmission and haze measurement using the commercial haze meter with an integrating sphere (NDH-7000SP, Nippon denshoku). The transmittance and haze in the masked region are 90.4 and 1.04%, respectively, whereas in the fully etched region these values are 92.4 and 0.6%. This difference of ΔT = 1.8% and ΔH = 0.44% makes it possible to recognize with the naked eye the patterned and unpatterned regions.
The transmission electron microscopy (TEM) image in Fig. 4(a) shows a 20-nm-thick Ag NW uniformly covered by PVP, a covering that was demonstrated in previous research to be effectively removed after 20 min in 5 M HNO3 solution in Fig. 4(b) . Here, the nitric acid plays a role not only in removing the PVP, but also produces irregular oxidation of the AgNW. As a result, the etched geometry (e.g., the diameter and length of the NW) cannot be accurately controlled using nitric acid alone, and so a NaOCl-based etchant (EO-P100, EO Tech) diluted to 3.3 wt% with deionized water was used in this study to remove the PVP layer and rapidly etch the exposed AgNWs. Figure 4(c) shows partial removal of the PVP layer after ~10 minutes, which is clearly more uniform than that seen with nitric acid in Fig. 4 (b). Moreover, after etching with NaOCl, Cl- ions were confirmed to exist in the vicinity of theetched AgNWs, as seen in the energy-dispersive X-ray spectroscopy (EDS) maps in Fig. 4(d). It should be noted here that an insulator capping was applied to the etched AgNWs to prevent electromigration caused by the localized heating of high-resistance contact points between nanowires, which can remove surface ligands, weld nanowires and reshape the contact pathway to create a desirable geometry for low-resistance interwire conduction .
The SEM images in Figs. 5(a–c) show an AgNW film after etching in HNO3 and H3PO4-, with the contrast in the secondary electron (SE) image in Fig. 5(a) being primarily associated with changes in the topography of the film surface. However, the backscattered electron (BSE) image in Fig. 5(b) more clearly shows a 100-μm-wide etched region, with Fig. 5(c) showing that this region has been only partially etched by being irregularly cut into smaller segments. Note that this irregular etched geometry results in irregular removal of the PVP layer, as was previously mentioned in relation to Fig. 3(b). Partial etching using NaOCl for 15 minutes, on the other hand, resulted in no damage to the overcoat layer and no clear distinction between etched and unetched regions, as demonstrated by the SE image in Fig. 5(d). It is also difficult to differentiate between the macro-etched regions in Fig. 5(e), though in the magnified SEM BSE image in Fig. 5(f) the etched AgNWs can be seen to have regular segments with a fairly consist diameter. The invisible patterning of Fig. 5(e) is therefore attributed to the unique etched geometry of the AgNWs when using NaOCl, with the etching mechanism of the PVP layer clearly having an important influence. Thus, as predicted by the FDTD simulation, low-visibility patterning can be achieved by minimizing the change in the diameter of the AgNWs.
4. Optical and electrical characterization of partially etched AgNWs
The PVP coating on the AgNWs can be controllably removed by adjusting the pH of the NaOCl-based etching solution (EO-P100), with the initial of 6 being reduced to 4 by adding 20% by volume of 1 M CH3COOH to make hypochlorous acid (HOCl) the predominant species. Solutions with a pH greater than 6 were synthesized by adding 0.1M NaOH, giving a pH of 11 at 10% addition by volume and making hypochlorite ions (OCl−) the dominant oxidant and stabilizing the hypochlorite ion . An additional etchant was prepared by mixing ammonium hydroxide, hydrogen peroxide and methanol to a ratio of 4:1:1, with the results for each of these etchants presented in Table 1. This shows that the etchant chemistry cannot only alter the etched shape, but can also reduce the etching time by more aggressively attacking the overcoat layer. The etching times for partial etching are therefore determined based on the minimum time for electrical insulation. Thus, although AgNWs completely disappear after 3 min of full etching with PMA-17A, resulting in optical differences sufficiently large to see with the naked eye, disorderly lengths similar to Fig. 5(c) remain after partial etching for 30 s that considerably minimize the optical difference. The use of a NH4OH-based etchant can provide longer Ag NW segments, which results in a further decrease in the optical difference compared to PMA-17A. Furthermore, reducing the etching time in the case of PMA-17A to increase NW length produces non-isolated regions, making it impossible to precisely control the etched geometry for low-visibility patterning. A NaOCl-based etchant, on the other hand, allows for a variety of different etched shapes simply by varying the pH of the solution. For example, at a pH of 6 the etched Ag NWs are cut into dense segments like those shown in Fig. 5(f), whereas an alkaline solution (a pH of 11) creates insulation between patterns without significantly altering the geometry. This greatly reduces the optical differences and allows the etching time to be reduced, but the pH has a tendency to drop due to the consumption of active chlorine. This can see a shift from OCl− as the predominant species to HOCl, however this can be countered by adding an alkali like sodium hydroxide (or any other strong base) to keep the pH high and increase the osmotic pressure for a given concentration of available chlorine . Using an acidic solution (a pH of 4) also produces etched geometries that are similar and allows the etching time to be reduced, as the predominant oxidizing agent (hypochlorous acid) can be generated by a reaction between NaOCl and CH3COOH. Thus, electrically insulated etched structures can be created with minimum change in geometry using either a strongly alkaline or acidic sodium hypochlorite-based etchant. Indeed, with a solution pH of 4, the change in transmittance and haze afteretching are only ΔT = 0.07% and ΔH = 0.041%; these values agree well with the FDTD simulation results.
In partial etching, any non-uniformity to the coating of the AgNW network can create variation in the insulation depending on location, which means that the process has a very narrow window with regards to the etching time. The results in Fig. 4(d) suggest that an insulating AgCl by-product is produced on the etched AgNWs when using a NaOCl-based etchant, which should prevent any additional etching reactions that may have a bad influence on low-visibility patterning. To confirm this, a series of experiments were performed in which the etching time was progressively increased, the results of which are shown in Fig. 6. Without photoresist patterning the AgNW was fully insulated within 1 min of being immersed in a NaOCl-based etchant with a pH of 4, yet no appreciable change was seen in the etched geometry and optical properties until 5 minutes. The newly developed NaOCl-based wet etchant therefore clearly provides a much greater process window for low-visibility patterning.
To test the long-term stability of the overcoating layer, various patterns with sizes ranging from 10 to 100 μm were fabricated with a resistance of over 1 GΩ between them. The change in normalized line resistance for each pattern was then tested in an environment of 85 °C and 85% RH over 10 days and found to change by less than 4.6% regardless of the pattern size in Fig. 7(a). Subsequent evaluation of the reliability of the patterned AgNW electrodes under the same conditions in Fig. 7(b) found that transparent electrodes laminated on a glass substrate with an optically clear adhesive exhibit remarkable reliability, with a change in sheet resistance of less than 1% after 30 days. This change in resistance increased dramatically when they were exposed to the same conditions without a cover glass, indicating the glass lamination after patterning is needed to ensure stability against thermal oxidation. To demonstrate the potential for these AgNW films to be used as an electrode in touch screen devices, a prototype 7” touch screen device was fabricated using an AgNW electrode on a polyethylene terephthalate (PET) instead of a conventional ITO electrode in Fig. 7(c). The effective operability of this device was confirmed by connecting it with a desktop PC, thus demonstrating its potential to replace ITO electrodes.
A low-visibility patterning method based on partial etching has been developed for creating transparent AgNW electrodes using FDTD simulation for the first time to determine the optimal etched geometry to minimize the optical differences between patterns. To create these geometries in practice, a NaOCl-based wet etchant was created that allows for the controlled removal of the protective PVP layer and creation of specific geometries by adjusting the pH of the solution. In this way, a low-visibility pattern with a transmittance and haze difference of 0.07 and 0.04%, respectively, was successfully achieved. The insulating AgCl by-product produced by this process has also been found to greatly reduce the sensitivity of the process to etching time when compared to conventional etchants. When laminated on a glass substrate these AgNW-based TCEs have proven to be remarkably reliable, with a change in sheet resistance of less than 1% after 30 days. More importantly, their successful use in a 7” touch screen prototype demonstrates that this approach to low-visibility patterning could be used to create viable alternatives to existing TCEs and facilitate the development of new optoelectronic devices.
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