This article discusses the potential of flexible transparent conductive film technology from the characteristics of transparent conductive films, describes the current state of development of various technologies, and analyzes the development trends of various technologies from the characteristics of materials, mass production technology and the advancement of commodity industrialization. At the occasion of the rise of sex electronics, the industry can lay out materials, processes, and equipment, and master the great opportunities of soft electronics.
Transparent conductive film is the basis of optoelectronic products
Optoelectronic products require light penetration and electrical conduction, so the transparent conductive film is the basis of optoelectronic products, flat display, touch panel, solar cells, electronic paper, OLED lighting and other optoelectronic products need to use transparent conductive film. According to a market survey released by the research agency Research and Markets in 2017, the average annual growth rate of the market for transparent conductive films is estimated to exceed 9% from 2017 to 2026, regardless of the industrial chain or market size of optoelectronic products. Transparent conductive film is an important material that can not be ignored in the photoelectric industry.
“Transparency” and “conductivity” are physically two mutually exclusive features. “Transparency” represents the amount of visible light that can penetrate the medium, while “conductivity” represents the carrier (including electrons and holes). The number of carriers is related to carrier concentration.
In terms of optical properties, carriers can be considered to be in a plasma state and interact strongly with light. When the frequency of incident light is less than the plasma frequency of the material carrier, incident light is reflected. Therefore, the carrier-plasmon frequency of the material in the spectral position is the decisive factor for whether the visible light band (380nm-760nm) can penetrate.
Generally, the plasma frequency of the metal film is in the ultraviolet region, so the visible light cannot penetrate the metal. This is the reason why the metal exhibits opaque optical properties in the visible region, and the plasma frequency of the metal oxide falls in the infrared region, so the visible region is Light can pass through the metal oxide, showing a transparent state.
However, the metal oxide energy gap (Gap) is too large and the carrier concentration is limited, resulting in poor electrical conductivity of the metal oxide. From the physical characteristics of the material, "transparency" and "conductivity" are difficult It is relatively difficult to develop a material with high electrical conductivity and high light transmittance at the same time.
Reducing the thickness of metallic materials is a method of increasing the light transmittance. However, the thickness of metallic films is too thin to be processed easily. For example, film formation by evaporation forms island-like discontinuous growth; on the other hand, because of thin film thickness, Oxidation is easy to occur in the air, resulting in drastic changes in resistance, poor film stability, is not conducive to subsequent processing applications.
Increasing the carrier concentration of the metal oxide to increase its conductivity is the other direction of the transparent conductive film. The oxide material is stable and the film has good film forming properties. Doping or manufacturing defects can be used to increase the carrier concentration. To improve the conductivity, it is the ideal material for transparent conductive film.
Such as doped tin oxide, zinc oxide, etc. have high transparency, high conductivity characteristics, which in turn uses indium tin oxide (Indium Tin Oxide, ITO) is the most widely used. ITO conductivity is good, visible light transmittance is high, at the same time Membrane technology and subsequent etching patterning process are mature and reliable, and are the main materials of transparent conductive film.
Although the ITO transparent conductive film is widely used, the ITO is a brittle ceramic material and it is susceptible to embrittlement. From the viewpoint of the functional requirements of the flexible electrons, the bending and cracking characteristics of the ITO make the ITO flexible. The component application encounters a bottleneck and has a flexible characteristic. The product that replaces the ITO transparent conductive film must be the basic material of the future flexible optoelectronic products, and is the strategic material of the soft optoelectronic products.
Increased demand for flexible transparent conductive films Diversification of manufacturing materials
In recent years, soft electronic products have gradually become commercialized, soft displays, soft lighting to soft sensors, soft solar cells and other technologies are changing with each passing day, and these soft products are eagerly demanding flexible transparent conductive films.
According to the report of Touch Display Research 2015, the market demand for non-ITO transparent conductive films will gradually increase (Figure 1). It is expected that by 2018, the market value of transparent conductive film replacing ITO will reach 4 billion US dollars; by 2022, it will More than ten billion US dollars. Such a large market scale mainly comes from soft touch, soft displays, soft solar cells and other flexible electronic components that will flourish in the coming years, resulting in the market demand for flexible transparent conductive films.
Figure 1 Touch Display Research predicts the market size of non-ITO transparent conductive film.
Although theoretically a material has high light transmittance at the same time, high conductivity and flexible properties are difficult, but through the material design such as metal film, oxide/thin metal/dielectric (DMD) Composite structure, doped organic conductive polymer with conjugated bonds; conductive conductive carbon material such as graphene, carbon nanotube (CNT); or design Mesh-invisible structures such as Metal Mesh and Metal Web can be made into soft transparent conductive films (Figure 2). The following is a review of current research and development achievements of these technologies.
Figure 2 Various potential transparent flexible conductive film technologies
Metal film
Reducing the thickness of metallic materials can increase the penetration of light. However, when the thickness of a metallic film is too thin, the stability of the material is poor, and it is easily oxidized, resulting in drastic changes in the resistance value. Japanese TDK replaces silver metal with a thin silver alloy and overcomes the upper and lower protective layers to overcome Metal film stability problems. As shown in Figure 3, the unique Ag-Stacked Film still has a penetration of up to 90% with a resistance of 9 Ω/sq.
Fig. 3 TDK flexible silver alloy soft transparent conductive film structure
Reducing the oxide thickness to the nanometer level can improve the brittleness of the oxide. However, the decrease in the thickness will inevitably reduce the electrical conductivity. When the metal film with excellent electrical conductivity is sandwiched in the oxide, there is a chance that the oxide film will have a certain degree of flexibility. Maintain applicable light transmittance and conductivity.
DMD structural materials also include ZnS/Ag/WO3; MoOx/Au/MoOx. These DMD structures are particularly suitable for components that require energy-level matching, such as stacked-layer OLEDs and solar cells, which can be selected by the choice of oxides Matching, in order to increase the photoelectric conversion efficiency of the module. Both the metal film and the DMD structure require a complicated vacuum process. The manufacturing cost is higher than that of ITO, which is more suitable for optoelectronic products with high added value.
Conductive polymer
With conjugated polymer materials, the electrons are less bound by the π-bond junction and can increase the carrier concentration under proper doping to become a conductive polymer. The conductive polymer film with flexible properties is coated Film-forming method, low processing cost, is an ideal material for soft transparent conductive film.
Polyaniline (PANI) doped with camphorsulfonic acid (CSA) was prepared by microemulsion polymerization method. Polypyrrole (PPY) and AuCl3 doped poly-3-hexylthiophene (Poly (3) Poly(3,4-ethylenedioxythiophene), PEDOT, which is doped with polystyrene sulfonate (PSS), can form soft transparent conductive Membrane, which has been commercialized PEDOT: PSS material in the application of transparent conductive film is the most widely studied.
After adding PEDOT:PSS modified with Dimethyl Sulfoxide (DMSO) and fluorine-containing surfactant, Vosgueritchian developed a 46%/sq resistor and a 82% transparent soft transparent conductive film.
In addition, there are also Methanesulfonic Acid (MSA) treatments, for example, some scholars published under the 50Ω/sq resistance, 92% light transmittance of the film manufacturing technology; or control PEDOT: PSS molecules Arranged to create a record of 17Ω/sq, penetration rate of up to 97.2% of the film.
The conductive polymer transparent conductive film is formed by coating, which has the advantages of production cost, but the stability of the conductive polymer material is poor. Under UV irradiation, the conjugated bond is easily broken to generate free radicals and the material is irreversibly destroyed. , Reduce the conductivity.
In addition, doping materials are generally charged ions, which easily absorb moisture and cause variation in the resistance of the conductive film. Although there are many methods for increasing the stability of conductive polymers in development, it is still not practical to replace ITO.
Conductive carbon material
Carbon is a variety of materials, carbon allotropes can have excellent insulation properties such as diamond film, but also can have excellent conductive properties such as graphene, depending on the end of carbon bonds. Conductive Carbon materials include graphite, Carbon Nanotube (CNT) and Graphene, etc. Among them, carbon nanotubes and graphene have a certain degree of electrical conductivity, which is smaller than the nanometer-scale structure of visible light wavelength, and can have high light. Penetration and flexibility, with the potential to be applied to flexible transparent conductive films.
Carbon nanotubes
Carbon nanotubes are tubular structures composed of carbon atoms. They have single wall CNTs (SWCNTs) and multi-wall CNTs (MWCNTs). The carbon nanotubes are chemically treated or are Doping can make carbon nanotubes have high conductivity characteristics. Apply these fibrous, conductive carbon nanotubes can form a conductive network by staggered overlapping.
Some scholars use the dry transfer method to directly transfer high-temperature-grown SWCNTs to soft substrates formed at 110Ω/sq, with a light transmittance of 90%. If a transparent coating is formed at a lower cost Membrane, it is more difficult to achieve the photoelectric properties of the direct transfer method, this is because the Van Dewa force between the CNT is strong, easy to form in the liquid CNT bundles (Bundle), to be made into a coatable suspension is necessary Add some additives to the liquid to make the CNTs evenly dispersed. These additives will affect the photoelectric properties of the film.
Using a non-ionic surfactant as a dispersant, scholar Woong used a spin-coating method to produce a film with a light transmittance of 71% at 59 Ω/sq. Another scholar, Kim, mixed SWCNT with hydroxypropylcellulose. Modulated into a blade coating slurry, after coating and after pulsed light after treatment to obtain a soft transparent conductive film, light transmittance of 89% at 68Ω/sq.
FIG. 4 is a schematic diagram of a process for producing a flexible CNT transparent conductive film for industrial use. Among them, ink dispersion, coating film formation and post-treatment are three key technologies for the industrialization of CNT transparent conductive film.
Figure 4 Process diagram of soft CNT transparent conductive film
. Graphene
Graphene is one of the most notable materials of this century. Since 2004, Andre Geim and Konstantin Novoselov succeeded in separating single-layer graphene materials from highly oriented pyrolytic graphite. The high conductivity characteristics of its two-dimensional special structures are attracting attention. The application of transparent conductive films has naturally become a research and development project. Similar to CNTs, direct dry transfer of graphene films and modulation into ink coating are two transparent conductive films. Film formation method.
Using a high-temperature CVD process and proper doping, a transparent conductive film of graphene with a light transmittance of 87% can be produced at 150Ω/sq. However, a high-molecular-flexible substrate cannot withstand the CVD high-temperature process.
Sony developed a transfer method to overcome this problem by using high-quality graphene grown on a copper foil substrate and transferring it to a PET film, and then dissolving copper to obtain a soft graphene transparent conductive film (Fig. 5). The cost of a continuous transfer process is high, and industrial production is more complex and difficult.
Figure 5 SONY uses a development transfer method to make a soft graphene transparent conductive film.
The graphene coating process is similar to CNTs in that they are all ink-based, coated and film-formed to remove additives and post-treatments. Due to the graphene sheet structure, the aggregation caused by van der Waals force is more severe than that of CNTs, making graphene in Dispersion in liquid is more difficult than CNT.
Therefore, the development of dispersion technology for graphene is the key to the fabrication of transparent conductive films for soft graphene. Researchers used graphite suspensions to transfer and disperse them directly into water/alcoholic solutions, strip graphene, and obtain graphene inks (Figure 6). , is to avoid the difficulty of dispersing graphene.
Figure 6 Graphite liquid stripping method to make coated graphene ink.
In addition, Graphene Oxide (GO) has more polar oxygen bonds and is easier to make into a stable ink, which contributes to the coating process. However, graphene oxide (GO) needs to be applied after coating. It is reduced to a conductive graphene film, and the milder reduction process is still under development.
Metal Network
The eye's identification of the lines is about 6um, so a metal wire with a diameter of less than 6um can be formed into a transparent conductive film with no visible metal lines. Since metal has excellent conductivity, only a small amount of metal material can be formed. Highly conductive thin film, is a potential technology.
The metal mesh film can be etched, screen printed to form a pattern-controlled metal mesh (Metal Mesh), or metal mesh or nanowires can be interwoven to form a patterned metal network (Metal Web).
Metal Mesh
The etched copper metal grid is a mature product. In the past, Plasma Display used copper metal meshes for electromagnetic shielding (EMI). The metal grids with traditional exposure, development, etching, and other yellow light processes are transparent and conductive. The film has been commercialized and applied to the touch panel industry. Using the Cu2O/Cu/Cu2O structure, scholar Kim showed a metal mesh transparent conductive film with a line width of 7 μm and a lattice spacing of 450 μm, which can be transmitted at a resistance of 15.1 Ω/sq. Up to 89%.
Different from the yellow etching process, the process of printing the mesh directly on the substrate is more varied. Fujifilm has developed silver salt exposure technology. First, it performs silver bromide coating on the substrate, and then it undergoes exposure and washing. Silver and other programs make grid patterns and then chemically thicken silver metal grids.
Or use precision printing (Direct Printing Technology, DPT) printing 20um line width of the silver mesh, sheet resistance 0.5 ~ 1.6Ω/sq, light transmittance of 78% to 88%. Komura-Tech Japan gravure transfer (Gravure Offset) Printed transparent conductive film up to 5um line width.
Some scholars also print directly out of the grid by inkjet printing, with a surface resistance of 0.3 Ω/sq. The biggest challenge in the printing process is the large area, and it is challenging to print a line width of 5 um or less. In addition, no matter which type In the printing method, the nano metal paste must be sintered to form a grid with good conductivity. The heat resistance of the polymer flexible substrate is poor, and the nano metal can easily be oxidized during sintering.
Laser sintering can simultaneously achieve the purpose of grid patterning and high-temperature sintering. It can be laser sintered with copper nanoparticles, or laser sintered with nano-silver particles, to make copper metal grids respectively, and silver metal grids are Figure 7). The sheet metal resistance of the silver metal grid is below 30Ω/sq, and the light transmittance is greater than 85%.
Figure 7 Laser sintered copper metal grid with silver metal grid
. Metal Web
Compared to the metal mesh that is designed and passed through the process, the naturally formed metal network can be omitted in the patterning process, but it can achieve the purpose of forming a conductive network. When the suspension is dried, the solids will aggregate to form a coffee ring (Coffee Ring). The effect is that a suitable suspension can be formed into a self-sequestered, naturally-occurring metal network after the film has been formed into a dry film; a conductive metal network can also be formed using nanowire interlacing, as described below.
When the suspension dries, the solids will aggregate to form a ring called the coffee ring effect. The nanosilver is designed with a special ink to allow the silver to automatically form a network after the liquid is volatilized and dried, eliminating the need for a printed patterning process.
Scholar Tokuno's use of bubble bursts to automatically form a network of nanosilver filaments. After sintering, a transparent conductive film with a sheet resistance of 6.2Ω/sq and a penetration of 84% can be formed (Fig. 8). US Cima Nano Tech also uses similar methods. The principle of making transparent conductive film. Figure 9 is the use of the company developed special ink formed metal network.
Figure 8 Nano-silver film is automatically gathered into the network to form a transparent conductive film.
Fig. 9 Cima Nano Tech in the United States automatically integrates metal networks with nanosilver
Another type of metal network is composed of nanowires. The nanowires are very thin, and the presence of wires is invisible to the naked eye. The metal network interwoven with nanowires forms a transparent conductive film with excellent conductivity. The metal network formed by the overlapping of metal wires (Fig. 10) has a simpler manufacturing process and lower cost.
Chemically synthesize nano copper wire, scholar Guo published in 51.5Ω/sq, light transmittance can reach 93.1% of transparent conductive film; silver conductivity better than copper, a small amount of nano silver wire can be interwoven into high Conductivity, high transmittance transparent conductive film. Another scholar Jia published a resistance of 21Ω/sq, light transmittance of 93% of the soft transparent conductive film, its superior flexibility and touch panel display as shown 11 shows.
Figure 10 Interlaced metal network of nanosilver wires
Figure 11 shows the flexible flexible silver nanowire transparent conductive film and touch panel with excellent flexibility
The continuous production technology of large-area nano-silver transparent conductive film has become more and more mature. The researchers used a continuous roll-to-roll slit coating (Slot-die Coating) to produce a 400-nm-wide soft silver nanowire transparent conductive layer. When the film has a surface resistance of 30Ω/sq, the light transmittance can reach 90%. However, the high aspect ratio material properties of the nanosilver wire make it difficult to control the coating uniformity. Therefore, it is necessary to develop a process and apparatus capable of controlling the uniformity. One of the key points in the industrialization of the transparent silver conductive film products.
Three major trends in the development of flexible transparent conductive film technology
Looking at the development of the above several kinds of soft transparent conductive film technology, there are certain development achievements in the three characteristics of flexibility, light penetration, and conduction. The following will discuss its future development from material characteristics, mass production process, and technology maturity.
Material properties
Conductivity and light transmittance are the most important photoelectric characteristics of soft transparent conductive film, and high light transmittance can still maintain high light transmittance is the trend of product development. In order to compare the above-mentioned several kinds of soft transparent conductive film technology, the author The results of surface resistance and light penetration published by various research institutions over the past few years have been used to evaluate various flexible transparent conductive film technologies, as shown in Figure 12.
Figure 12 evaluates various soft transparent conductive films with surface resistance and light transmittance.
From this figure, it can be found that if the light transmittance is greater than 80%, the above technologies can meet the requirements when the resistance is greater than 100Ω/sq; but when the resistance is less than 100Ω/sq, graphene and carbon nanotubes must be Growth by vacuum method, and then by the transfer technology film can meet the demand.
Conductive macromolecules and metal meshes, metal networks can reach this specification, and below 10Ω/sq, only metal meshes can meet the metal network. The nanosilver network below 100Ω/sq, even lower Outstanding characteristics, this is due to the excellent conductivity of silver, a small amount of nanosilver can achieve low resistance and high penetration of the photoelectric properties.
Production process
The complexity of the mass production process is closely related to the cost of the flexible transparent conductive film. The mass production process analysis of the above several flexible transparent conductive film technologies is shown in Table 1. Both the thin metal film and the oxide/metal film/oxide are Vacuum coating process, equipment and manufacturing costs are the highest.
Carbon nanotubes, the dry transfer process of graphene are special, and new equipment needs to be developed. Although the metal mesh of the etching process is complicated, the exposure, development, etching, and the yellow light-splitting equipment are expensive, but the manufacturing technology is mature. Mesh transparent conductive film has been mass production applied to the touch panel industry.
The metal mesh of the printing method replaces the yellow light patterning process with printing, and it is expected that the investment in patterning equipment can be further simplified, but the low-temperature sintering process and equipment must be increased. The metal network in the pre-assembled sequence also omits the patterning process and its manufacture The cost is simpler than printing metal grids.
Coated carbon nanotubes must be doped after they are formed into films. Graphene must be reduced after the graphene oxide coating is formed. The equipment and manufacturing costs should be similar to those of the metal network assembled randomly. Nanowire bonding The metal network and conductive polymer can be manufactured by using coating film-forming equipment. It is the most competitive technology for equipment and manufacturing cost.
Progress of commodity industrialization
The industrialization of new technologies is a process that requires material development, process development, and mass production. In this process, “volume production development” is an important key. Mass production development involves the integration of materials, processes, and equipment. It is also a new technology. The key to commercialization.
The copper metal grid touch panel is on the market. It is the fastest growing technology in all flexible transparent conductive film technologies; Nano silver touch panels are displayed at many professional display exhibitions by many professional touch panel manufacturers. Industrialization of commodities.
Although conductive polymer transparent conductive films are exhibited by many film manufacturers, practical applications are still being developed. The metal network of printing and self-assembly processes has made some progress in materials and processes, and related mass production processes and equipment Still under development. Graphene is still in the development stage for ink materials and process technology. Qualitative progress is shown in Figure 13.
Fig.13 Progress of commercialization of various soft transparent conductive films at present
From the point of view of material properties, mass production process and technology maturity, nanosilver transparent conductive film is the most competitive. In terms of photoelectric characteristics, it has excellent light penetration across several Ω/sq to hundreds Ω/sq range. The low-cost coating film process, combined with a complete industrial chain from nanosilver, ink, and flexible transparent conductive film to touch panel applications, the only need to be strengthened is equipment and process integration.
Nanosilver ink is a special ink with low viscosity and high aspect ratio. Uniformity is not easily controlled during film coating. Special coating equipment developed for nano silver wire conductive network is to open nano silver wire soft transparent conductive A key to the bottleneck of membrane production.
Optoelectronic products from hard to soft grasp key materials for development opportunities
Since the 1990s, transparent conductive films were made by sputtering, ITO was synonymous with transparent conductive films. However, the trend of optoelectronic products from small to large, from hard to soft, made the characteristics of ITO transparent conductive films unable to meet future optoelectronic products. demand.
In the development of new materials, soft transparent conductive films, carbon nanotubes, graphene, and conductive polymers have made some progress. However, various technologies still have process development before application to the market, and equipment integration and other technical problems remain. get over.
In addition, the manufacturing cost is still an important factor for each technology to win in the end. From the comparison of the characteristics of materials, the ease of process comparison, and the progress of commodity industrialization, this article summarizes and reviews the progress of commodity industrialization. We look forward to the application of commercialization in the optoelectronics industry. At the critical moment of hard-to-soft, related industries can master the strategic key materials of soft optoelectronic products and become the opportunity for the development of soft transparent conductive films.