Wet-Processable Transparent Conductive Materials
A novel wet-processable transparent electrode material exhibits significant performance advantages over ITO. This material has recently achieved mass production in smart phones.
by Michael Spaid
NEARLY ALL DISPLAY DEVICES, including TFT-LCDs, OLEDs, and e-paper, contain one or more layers of a transparent conductive material, most commonly indium tin oxide (ITO). Currently, virtually all applications of ITO for transparent conductors require deposition by a sputtering method in a vacuum chamber. Due to the significant growth of large-area displays, smart phones, and tablet devices that often incorporate projected-capacitive touch sensors, the market for the ITO sputtering targets has become several billion dollars as of 2011, with steady growth expected in the next 10 years.1 If one considers not just the material cost of ITO, but also the deposition and patterning costs that are necessary to make display devices, it is reasonable to assume that the worldwide cost for creating a useful ITO layer is a significant multiple of the cost of the ITO sputtering targets. While the need for high-quality transparent conductive materials has grown dramatically, desirable alternatives to ITO have not emerged in the marketplace.
Drawbacks of ITO and Potential Alternatives
There are many factors driving the need for an ITO replacement material from the standpoint of raw-material supply, cost of ownership, and overall performance. Within the last 10 years, the price of ITO has been highly volatile, driven both by the increased consumption of the material as well as the geopolitically sensitive nature of the indium supply chain.1 In addition to the high cost of the raw materials, capital investments for the vacuum-sputtering equipment necessary to deposit ITO are multiples of the cost of wet-coating tools with equivalent coating capacity. In addition, there are certain performance attributes of ITO that are undesirable, such as its brittleness and tendency to crack – a key factor with regard to making displays on flexible substrates. ITO also has a high refractive index that results in poor light transmission and the need for additional index-matching layers to achieve acceptable optical performance. Finally, high-temperature deposition conditions coupled with post-deposition annealing processes cause significant issues with achieving acceptable optical and electrical properties on plastic films.
In addition to current applications that require high-quality transparent electrodes, emerging applications such as OLED lighting, thin-film photovoltaics, and new types of flexible displays require a higher-performing transparent electrode, driven by both cost and performance. Wet-processed alternatives such as carbon nanotubes and conductive polymers have not been competitive with the optical and electrical properties established by ITO. Graphene has been widely discussed as an alternative; however, its development is in its infancy.2 In this article, the transparent conductive material produced by the author's company, Cambrios Technologies Corporation, will be described as the first commercially available wet-processed alternative to ITO. Smart phones incorporating ClearOhm* materials have been mass produced since early 2011.3,4
Silver Nanowire Technology
ClearOhm conductive material consists of a wet-processable dispersion of high-aspect-ratio silver nanowires. Starting from silver salts, twinned-crystal silver nanowires are grown via the polyol process, a patented method5 described by Xia.6 By carefully controlling the process parameters, high-aspect-ratio silver nanowires can be synthesized at high yield, with an average diameter in the low tens of nanometers and an average length in excess of 10 μm. Independent control of nanowire length and diameter is possible, allowing the tailoring of morphology-dependent optical and electrical properties for specific applications. These nanostructures are then purified and formulated into a coatable suspension that is compatible with industry-standard coating methods such as roll-to-roll slot die coating or spin coating.
The transparent conductive layer is created by coating the formulated suspension of nanowires on the surface of a substrate such as glass or plastic. Upon drying of the solvent, the nanowires form an interconnected, two-dimensional mesh on the surface. Controlling the sheet resistivity of the layer of interconnected nanowires is accomplished by changing the number density of nanowires on the surface, as is shown in Fig. 1. The electrical properties of the interconnected mesh are well described by the theory of percolation, in which the number density of nanowires required to achieve a continuously conductive path on the substrate scales inversely with the square of their length (N ~ 1/L2). Thus, high-aspect-ratio nanowires are uniquely suited to achieve high electrical conductivity with a minimal amount of metal.
Fig. 1: Scanning electron microscope (SEM) images of transparent conductive layers show how they are created by an interconnected network of silver nanowires. By controlling the nanowire surface coverage, different sheet resistances can be produced.
In addition to the efficiency of the mesh structure, a large benefit in material usage relative to sputtered metal oxides arises due to the large difference in electrical conductivity, as silver is 50–100x more conductive than ITO. This difference in conductivity allows the material to cover only a few percent of the surface to achieve equivalent electrical properties. The remaining 95–99% of the surface consists of void space, which to the the first order determines the light transmission of the layer. Comparing material costs, indium tin oxide consists primarily of indium metal (>90% typically), and the price of indium and silver over the past 5 years has been roughly equivalent. Thus, the 50–100x reduction in material utilization due to the higher electrical conductivity of silver directly translates into a similar-fold reduction in material costs relative to ITO sputtering targets.
The materials used to create the transparent conductive layer can be scaled efficiently to volumes required to serve entire industries. For example, the estimated market size for ITO-coated plastic film in 2010 was 11 million square meters. The first manufacturing facility at Cambrios was sized to produce enough coating material for 30 million square meters of coated substrates at the relevant sheet resistance for touch panels.
Roll-to-Roll Wet-Coating Process
Wet coating as a deposition technology for key layers in consumer-electronic display devices has proven cost effective and robust for the most demanding applications. Examples of wet coatings in TFT-LCDs include the color resists and the color-filter planarization layer, both of which must be deposited defect-free at high yield. On plastic film, scratch-resistant hard coats and optical coatings such as anti-glare and anti-reflection are routinely applied at high yield using high-speed roll-to-roll coating methods. Cambrios' transparent conductive materials can be coated using a variety of industry-standard techniques, including roll-to-roll methods such as slot-die coating on plastic film. For sheet processing, standard wet-coating methods used in the LCD industry for coating color resists and photoresist are applicable, such as slit-coating or spin-coating. Capital equipment costs for equivalent coating capacity are typically 3–5x lower for wet-coating methods relative to vacuum-coating methods.
The Cambrios transparent conductive film is created using roll-to-roll slot-die coating. As shown schematically in Fig. 2, two coating steps are used to produce the transparent conductive layer, with the aqueous dispersion of silver nanowires coated and dried in the first step, followed by the coating of a thin (100–150 nm), transparent, UV-curable polymer layer on top of the nanowire layer. The purpose of the second layer is two-fold: (1) to impart mechanical strength to the coating, including some degree of scratch resistance, and (2) to protect the nanowire layer from direct environmental exposure. All processing steps can be performed at low temperature (<100°C) and are thus compatible with most types of optical-grade plastic film such as polyethylene terephthalate (PET), polycarbonate (PC), triacetyl cellulose (TAC), or cyclic olefin polymer (COP).
Once coated, the film does not need the subsequent 60–90-minute annealing step that is commonly required with coated ITO film to achieve the optimized optical and electrical properties. Contrary to sputter coating of ITO, the throughput of the roll-to-roll wet-coating process is not linked to the amount of conductive material to be deposited. At an equivalent line speed, it is possible to adjust the roll-to-roll coating parameters to deliver a thicker wet film on the substrate that, in turn, results in a higher density of nanowires (and higher conductivity) in the dry film. Conductivity levels of the coatings can also be adjusted without sacrificing throughput by changing the nanowire concentration in the coating material to create a layer with a higher or lower density of nanowires using the identical wet-film thickness. Whether the conductive coating is at resistivity of 10 or 300 Ω/sq., the throughput remains the same. For sputtered metal oxides, higher conductivity necessitates longer sputtering times, resulting in a reduction in coating throughput.
Optical, Electrical, and Mechanical Properties
Three key optical properties that govern the performance of a transparent conductive layer at a given sheet resistivity are light transmission, color, and haze. A relatively low refractive index (~1.5) is desirable for many applications, as this minimizes reflection losses and optical appearance issues arising from patterned layers on glass or plastic due to differences in refractive index. Since the index of refraction of the transparent conductive materials layer is dominated by the index of the void space, it is possible to tune the refractive index by choosing an overcoat material with the desired index. As with most optical layers that appear in a display stack, the uniformity of the optical properties is critical, and the defect level of coated substrates needs to conform to industry standards.
Figure 3 shows the transmission of ClearOhm PET film as a function of sheet resistivity spanning a range from 10 to 250 Ω/sq. Including reflection losses associated with the PET substrate, the transmission of the film is in excess of 90% at 40 Ω/sq. or >98% for the conductive coating itself. Comparing this to ITO film that includes a multilayer anti-reflection coating, the Cambrios film has equivalent transmission at 50 Ω/sq. as compared to ITO film at a 3x higher sheet resistance. For standard-grade ITO film that does not include anti-reflection coatings, the transmission advantage of this film would be significantly larger. ClearOhm film also provides improvements in reflection, primarily due to its lower effective refractive index. In Fig. 4, the total reflection, or combination of diffuse and specular reflection, is plotted as a function of wavelength for both ClearOhm film and high-quality multilayered ITO film. ClearOhm film has a lower total reflectivity across the entire spectrum, with notable improvements in the blue and red regions of the visible spectrum.
Fig. 3: Transmission vs. sheet resistance of the ClearOhm conductive material layer is shown on 125-μm single-side hard-coated PET film (blue curve) and for the ClearOhm layer by itself (red curve). For reference, commercially available ITO film is shown.
Fig. 4: Shown is the total reflection of multilayered ITO film at 150 and 285 Ω/sq., versus ClearOhm film at 130 Ω/sq. Oscillations in the spectra for ITO are due to the presence of a hard coating layer.
In addition to its high transmission, ClearOhm transparent conductive film exhibits a more neutral color relative to ITO film, as measured by the color indices b* and a*, metrics that are derived from the film transmission spectrum. ITO film typically has a yellowish appearance corresponding to b* values of 2.0–2.5 in the resistance range of 150–270 Ω/sq., whereas ClearOhm transparent conductive film is less yellow, with values ranging from 1.0 to 1.5 over a wider resistance range from 80 to 250 Ω/sq.
In most display applications, a high degree of optical clarity is necessary, which, in turn, requires coated substrates to have minimal haze. Transparent conductive materials that are particulate in nature will typically scatter more light than a smooth continuous film, with the amount of light-scattering proportional to the particle surface coverage. However, the level of light scattering can be controlled through careful engineering of the nanowire morphology.7 For a fixed nanowire length, the amount of light scattering is proportional to the cube of the nanowire diameter, or the scattering cross section of ~d3. Thus, for applications that demand a minimal amount of light scattering such as displays and touch screens, very thin nanowires should be used. For applications such as thin-film photovoltaics or OLED displays and lighting where a high degree of haze is desirable to allow more efficient light input or output coupling, larger-diameter nanowires are appropriate. Commercially available ClearOhm film for display applications has achieved very low haze with typical values ranging from 0.6 to 0.9%, corresponding to a resistance range of 80–250 Ω/sq. These values are only a few tenths higher than the haze of the optical-grade PET substrate.
Flexible, curved, or 3-D shaped displays and touch screens have been discussed for a number of years; however, the advent of these types of devices has been hindered by a lack of availability of the appropriate materials. Since ITO is a ceramic material, it is inherently brittle and cracks when exposed to a minimal amount of bending or strain. In addition to enabling new types of flexible devices, ClearOhm's flexibility and compatibility with mild strain should enhance manufacturing yields for existing devices manufactured with flexible conductive films. The silver nano-wires in the material are inherently flexible, and conform to surfaces in a manner more similar to well-cooked pasta than rigid beams. Figure 5 compares the flexibility of ClearOhm transparent conductive film and ITO film. In this test, a piece of conductive PET film is rolled around an 8-mm mandrel, subsequently unrolled, and the resistance of the film is compared to its initial value. Within the first few cycles of this process, the ITO-coated PET film cracks, resulting in orders of magnitude increases in sheet resistance. Conversely, the ClearOhm transparent conductive film sheet resistance remains unchanged after 50 wrapping cycles.
Fig. 5: The flexibility testing results shown above are for conductive films wrapped around an 8-mm mandrel. On the y axis, the percent change in sheet resistance is plotted on a logarithmic scale versus the number of wrap cycles on the x-axis. The starting resistance is normalized to 100%.
Transparent conductors based on metal nanowires can be patterned8,9 into conductive and non-conductive regions as is required for most display devices. Examples of common transparent conductor patterns are the drive and sense lines in a projected-capacitive touch screen or the pixel electrode in an LCD. The most common method to create patterned ITO is lithography with removal of material by wet chemical etching. Etch masks are typically applied by a photo process for high-resolution patterns or by a screen-printing process for low-resolution patterns. Alternatives to wet etching such as laser patterning or other dry-etching methods are much less common, and thus new types of transparent conductive materials should be patternable by wet etching to achieve immediate adoption.
ClearOhm transparent conductive film can be patterned by wet chemical etching using established volume-production processes. After applying the etch mask, the film is exposed to a liquid etchant that penetrates the thin overcoat layer and dissolves the underlying nanowires. The etching time for electrical isolation is dependent on the typical wet-etching parameters, including the etchant chemistry and temperature. The pattern resolution that can be achieved on ClearOhm transparent conductive film by wet etching is the same as with ITO film and is governed by the resolution of the applied etch mask. Figure 6 shows a dark-field microscope image of an etched 30-μm line gap that electrically isolates adjacent conductive regions. At this resolution, the pattern created in the transparent conductive film is not visible by the human eye, a key requirement for the patterned layers in a projected-capacitive touch sensor.
Fig. 6: This dark-field microscope image of a 30-μm gap separating two adjacent conductive regions. The pattern was created by photolithography and wet chemical etching of the transparent conductive film.
In addition to wet-etching processes where the nanowires are completely removed during the etching process, novel methods are possible that exploit the percolative nature of conductive nanowire networks.9 In the "partial etching" process for the transparent conductive film, nanowires in the etched area are cut into smaller segments, effectively reducing their average length. Due to the strong dependence of electrical percolation on nanowire length, the network can be rendered non-conductive with a minimal amount of material removed.
Figure 7 shows a dark-field microscope image of the resulting pattern created using the partial etching technique. In both the conductive and non-conductive regions, the outlines of the nanowires are clearly visible. However, in the etched area, numerous small breaks in the nanowires that electrically disconnect the network are observed all along their length. Optical properties such as trans-mission, color, haze, and reflectivity are nearly identical for the conductive and non-conductive regions resulting from the minimal differences in silver nanowire coverage. Patterns that are nearly invisible to the human eye can be produced with this method without the need for additional index-matching layers to minimize the optical differences of the patterned conductor, as is required for ITO film.
Fig. 7: A dark-field microscope image of ClearOhm film that has been etched using the "partial etching" method. The solid yellow line indicates the border between the conductive and non-conductive region. Red arrows indicate locations of nanowire discontinuities created during the partial etching process.
Although not described in this article, alternative methods for patterning the ClearOhm layer are possible, including laser-patterning, photosensitive transfer films10,11 and even direct printing. Direct patterning of the layer via printing represents the most compelling value proposition for cost reduction, by eliminating the numerous process steps and materials necessary for traditional wet-etching methods. Printable transparent conductive inks have been demonstrated using a variety of printing methods, including ink-jet, screen, gravure, and reverse offset. Future implementations of ClearOhm materials in mass-produced consumer-electronic devices should be able to take advantage of this unique value proposition.
The author would like to thank the entire Product Development team at Cambrios for developing the silver-nanowire transparent conductor technology discussed in this article. Special thanks to Dr. Michael Knapp for his insightful suggestions in the preparation of the manuscript.
*ClearOhm is a trademark of Cambrios Technologies Corp.
1NanoMarkets, Transparent Conductor Markets, 2011 (2011).
2B. Mackey, SID Symposium Digest 42, 617 (2011).
3Nissha Printing Co., Ltd. March 30, 2011 Press Release: http://www.nissha.co.jp/english/news/2011/03/news-559.html.
4Synaptics Inc., April 6, 2011 Press Release: http://www.synaptics.com/about/press/press-releases/cambrios-clearohm™-film-used-smart-phone.
5U.S. Patent 7,585,349.
6Y. Sun, B. Gates, B. Mayers, and Y. Xia, Nano. Lett. 2, 165 (2002).
7U.S. Patent 7,849,424.
8U.S. Patent 8,049,333.
9U.S. Patent 8,018,568.
10Hitachi Chemical Corp., July 21, 2011 Press Release: http://www.hitachi-chem.co.jp/japanese/information/2011/n_110721.html.
11Cambrios Technologies Corp., July 25, 2011 Press Release: http://www.cambrios.com/212/HCC_Release.htm. •