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 trend of various technologies from material characteristics, mass production technology and industrialization progress of products. 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.
The transparent conductive film is the basic optoelectronic product. The optoelectronic products require light penetration and electrical conduction. Therefore, the transparent conductive film is the basis of optoelectronic products. Photoelectric products such as flat panel displays, touch panels, solar cells, electronic paper, and OLED lighting are required. A transparent conductive film is used. Market research released by market research agency Research and Markets in 2017 pointed out that the market for global transparent conductive film is expected to grow at an average annual rate of more than 9% from 2017 to 2026, whether from the optoelectronic product industry chain or It is a market scale to evaluate, transparent conductive film is an important material in the photoelectric industry can not be ignored.
“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 position of the carrier's plasma frequency in the spectrum is the determinant of 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, it is difficult to see the "transparency" and "conductivity" of the metal oxide. 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, and 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 is susceptible to embrittlement. From the perspective of the functional requirements of the flexible electrons on flexure, the stress bending and cracking characteristics make ITO in soft electrons. The bottleneck of the component application, with flexible features, replace the ITO transparent conductive film products must be the basic material of the future soft optoelectronic products, is the strategic material of the soft optoelectronic products.
Demand for Flexible Transparent Conductive Films Diversifies Manufacturing Materials In recent years, soft electronic products have gradually become commercialized, soft displays, soft lighting to soft sensors, and soft solar batteries are rapidly changing. These soft products are The demand for soft transparent conductive films is growing.
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.
Although it is theoretically difficult for a material to have high light transmittance at the same time, high conductivity and flexible characteristics, 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 designed for the naked eye The structure of the grid, such as Metal Mesh and Metal Web, can be made into a soft transparent conductive film (Figure 2). The following is a review of the current research and development results of these technologies.
Metal films can reduce the thickness of metal materials and increase the penetration of light. However, when the thickness of metal film is too thin, the material is poor in stability and easily oxidized, resulting in a drastic change in the resistance value. TDK Japan replaces silver metal with a thin silver alloy, and the upper and lower protective layers To overcome the stability of the metal film. As shown in Figure 3, the unique Ag-Stacked Film still has a 90% penetration rate with a resistance of 9 Ω/sq.
Reducing the thickness of the oxide 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 to maintain it under a certain degree of flexibility. 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 through the choice of oxides. Matching, in order to increase the photoelectric conversion efficiency of the component. The metal film and DMD structure need complicated vacuum process, the manufacturing cost is higher than that of ITO, and it is more suitable for optoelectronic products with high added value.
Conductive polymers Polymers with conjugated bonds, where the electrons are less constrained by the π-bond junction, and the concentration of carriers can be increased with proper doping to become conductive polymers. Conductive polymer films with flexible properties Is the use of coating film, low processing costs, is an ideal material for soft transparent conductive film.
Polyaniline (PANI) doped with camphorsulfonic acid (CSA) was prepared by microemulsion polymerization method. Polypyrrole (PPY) nanospheres were prepared, and Poly(3-hexylthiophene) doped with AuCl3 (Poly(3-). Both hexylthiophene, P3HT) and Polystyrene Sulfonate (PSS)-poly(3,4-ethylenedioxythiophene, PEDOT) can form soft transparent conductive films. Among them, the commercially available PEDOT:PSS material has the most extensive application in the transparent conductive film.
After the addition of dimethylsulfoxide (DMSO) and fluorine-containing surfactant-modified PEDOT:PSS, Vosgueritchian developed a 46Ω/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 easily breaks to generate free radicals and causes irreversible damage to the material. , 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 Carbon is a versatile material, carbon allotropes can have excellent insulating properties such as diamond film, but also can have excellent conductive properties such as graphene, depending on the carbon bond The conductive carbon materials include graphite, Carbon Nanotube (CNT) and Graphene. Among them, carbon nanotubes and graphene have a certain degree of electrical conductivity. They are smaller than the wavelength of visible light and have a nano-scale structure. High light transmittance and flexible properties, with potential for flexible transparent conductive film.
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 doping can make carbon nanotubes have high conductivity characteristics. Apply these fibrous, conductive carbon nanotubes to form a conductive network.
Some scholars use the dry transfer method to directly transfer high-temperature-grown, high-quality SWCNTs to a 110-Ω/sq.-per-millimeter-thick transparent conductive film with a light transmittance of 90%. Membrane, it is more difficult to achieve the photoelectric properties of the direct transfer method, this is because the Vandev force between the CNT is strong, easy to form in the liquid CNT bundles (Bundle), to be made into a coating suspension need to 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 process 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 in industry. Among them, ink dispersion, coating film formation and post-treatment are the three key technologies for the industrialization of CNT transparent conductive film.
Graphene graphene is one of the most notable materials of this century. After Andre Geim and Konstantin Novoselov succeeded in separating monolayer graphene material from highly oriented pyrolytic graphite in 2004, Graphene has attracted attention due to its high conductivity characteristics of its two-dimensional special structures. 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 film forming 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 CVD high-temperature processes.
Sony developed a transfer method to overcome this problem by growing high-quality graphene on a copper foil substrate, 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.
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 of graphene is the key to the fabrication of transparent conductive film for soft graphene. The researchers used graphite suspension to transfer and disperse directly into water/alcohol solution and strip graphene to make graphene ink (Figure 6). , is to avoid the difficulty of dispersing graphene.
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) still needs to be coated after coating. It is reduced to a conductive graphene film, and a milder reduction process is still under development.
The metal network (Metal Network) human eye about the line of identification of about 6um, so the diameter of less than 6um metal mesh can be made into the naked eye can not see the metal wire transparent conductive film. Because of the excellent conductivity of metal, as long as a small amount Metal materials can be formed into highly conductive films, which is a potential technology.
Metal mesh thin films can be etched, screen printed to form a pattern-controlled metal mesh (Metal Mesh), or metal meshes or nano metal wires can be interwoven into a patterned metal network (Metal Web).
Metal mesh etched copper mesh is a mature product. In the past, plasma displays used copper meshes for electromagnetic shielding (EMI). Traditional exposure, development, etching, etc. Optical process metal mesh transparent conductive films have been commercialized and applied to the touch panel industry. Using the Cu2O/Cu/Cu2O structure, Kim published a transparent conductive film of a metal mesh with a line width of 7 μm and a grid pitch of 450 μm at the resistance of 15.1. The penetration rate of Ω/sq can reach 89%.
Different from the yellow etching process, the process of printing the grid 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 ink jet printing, with a surface resistance of 0.3Ω/sq. The biggest challenge in the printing process is the large area, and it is challenging to print a line width of 5um or less. In addition, regardless of which 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 achieve mesh patterning and high-temperature sintering at the same time. It can be laser sintered with copper nanoparticles, or laser sintering with nano-silver particles, to make copper metal meshes respectively, as with silver metal meshes (Figure 7). ). Among them, the sheet metal resistance of the silver metal grid is below 30Ω/sq, and the light transmittance is more than 85%.
The metal web is designed relative to the metal mesh that has been designed and passed through the process. The naturally formed metal network can eliminate the patterning process, but it can achieve the purpose of forming a conductive network. The solids will aggregate when dried by the suspension. The effect of forming a coffee ring (Coffee Ring), the appropriate suspension can be formed into a self-sequester (self-alignment) naturally formed metal network after the appropriate suspension is dried, and a conductive metal network can also be formed by interdigitation of nano-metal wires, as described below.
When the suspension dries, the solids will aggregate to form a ring called the coffee ring effect. The nano-silver is specially designed for inks. It can automatically form nano-silver 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 nanosilver aggregate network, which can be sintered to form a transparent conductive film with a surface resistance of 6.2Ω/sq and a penetration of 84% (Figure 8). US Cima Nano Tech also uses similar principles Making a transparent conductive film. Figure 9 is the use of the company developed special ink formed metal network.
Another type of metal network is composed of nanowires. The nanowires are very thin, and the presence of lines cannot be detected by the naked eye. The metal network interwoven by nanowires can form a transparent conductive film with excellent conductivity. The use of nanowires The lap-joined metal network (Figure 10) results in 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 is 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 shown in Figure 11 Show.
The continuous production technology of large-area nano-silver wire 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 nano-silver wire transparent conductive film. When the surface resistance is 30Ω/sq, the light transmittance can reach 90%. However, the material characteristics of the nano-silver wire with a high aspect ratio make it difficult to control the coating uniformity. Therefore, the development of a process and device capable of controlling the uniformity is the nanosilver wire. One of the key to the industrialization of transparent conductive film products.
The three major trends in the development of soft transparent conductive film technology The development of several types of flexible transparent conductive film technology has been reviewed. The three major characteristics of flexibility, light penetration, and conduction have certain development results. The following are the material characteristics, mass production process , The maturity of technology explores its future development.
Material properties Conductivity and light transmittance are the most important photoelectric properties of soft transparent conductive film. High light transmittance can still maintain high light transmittance is the trend of product development. To compare the above-mentioned several kinds of soft transparent conductive film technology, The author has evaluated the various kinds of flexible transparent conductive film technologies with the results of surface resistance and light transmittance published by various research institutions in recent years, as shown in Figure 12.
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 The vacuum method grows, and then the film can only be achieved by the transfer technology.
Conductive macromolecules and metal meshes, metal networks can reach this specification, and below 10Ω/sq, only metal meshes can meet the metal network. Among them, silver nanowire networks can display below 100Ω/sq, even lower. Excellent 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.
The complexity of the mass production process mass production process is closely related to the cost of the soft transparent conductive film. The mass production process analysis of the above several soft transparent conductive film technologies is shown in Table 1. Thin metal film and oxide/metal film/ Oxide is a vacuum coating process, with the highest equipment and manufacturing costs.
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-removing equipment are expensive, but the manufacturing technology is mature. Grid transparent conductive film has been mass production applied to the touch panel industry.
The metal mesh of the printing method replaces the patterning process of yellow light 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 self-assembled sequence also omits the patterning process and its manufacture The cost is simpler than printing metal grids.
Coating 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 cost should be similar to those of the metal network assembled randomly. Nanowire bonding 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.
Industrialization of commodity development 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. , is also an important key to the commercialization of new technologies.
The copper metal grid touch panel is on the market and is the fastest growing technology in all soft transparent conductive film technologies. Nano silver touch panels are displayed at many professional display exhibitions by many professional touch panel manufacturers, and are also close to commodities. Industrialization.
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.
From the point of view of material characteristics, mass production process and technology maturity, nano-silver wire transparent conductive film is the most competitive. In terms of photoelectric characteristics, it has excellent light transmittance across several Ω/sq to hundreds of Ω/sq range. Low-cost coating film-making process, together with the industrial chain from nanosilver, ink, soft transparent conductive film to touch panel application, the only thing to be strengthened is the integration of equipment and process.
Nano-silver ink is a special ink with low viscosity and high aspect ratio. It is difficult to control the evenness of the film during coating. The development of special coating equipment for nano-silver wire conductive network is a bottleneck for the opening of nano-silver soft transparent conductive film. One of the keys.
Photovoltaic products from hard to soft to grasp key materials for the development of opportunities from the 1990s on the production of transparent conductive film by sputtering, ITO is a synonym for transparent conductive film, however, optoelectronic products from small to large, from hard to soft trend The characteristics of ITO transparent conductive film can't meet the needs of future optoelectronic products.
With the development of new materials, flexible transparent conductive films, carbon nanotubes, graphene, and conductive polymers have made some progress. However, various technologies still have process development before they are put on the market, and equipment integration and other technical problems need to be overcome. .
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.