Transparent electrode is the electronic component normally has high-transparency of 80% or more and conductivity of 500 Ω/sqm sheet resistances or less, and the technology widely used in display such as LCD front electrodes, and OLED electrodes, and electronics such as touch screen, solar cells, and optoelectronic devices.
|Required Characteristics of Transparent Electrode Materials|
- Transmittance (@ 550nm): ≥ 80 %, (based on base film transmittance 100%)
- Sheet Resistance: ≤ 103 Ω/sq
- Uniformity: ≥ 99 %
- Flexibility: diameter 10.0mm, same sheet resistance value after 10,000 bending tests
- Pattern Accuracy: ≤ 2 μm
In general, mainly adopted technology is ITO (Indium Tin Oxide) film, manufactured by vapor deposition or sputtering. However, ITO, inorganic, has a limit in flexibility since it is weak in deflection and indium itself, a rare metal, has concerns of resource depletion. Organic conductive substance-using films are also being developed, but if the conductivity is improved, there are problems in coloring and durability.
Japanese companies such as Nitto Denko and Toyo Boseki account for 70% of overall ITO film market. Nitto Denko accounts for 30%, of touch panel products suppliers, followed by Oike Kogyo. Samsung Corning, U.S. CP Film, and Neo Pack are also considered as ITO film representatives.
Next-generation materials of transparent electrode, which demands are increasing, are being actively developed to reduce cost, and improve conductivity contrast to transparency. Of these, development trends and the status of related companies/agencies of Graphene, CNT, and Ag Nano Wire are discussed here.
The origin of the word, Graphene, was coined as a combination of graphite and the suffix -ene, which means molecules with a double bond of carbon. Its structure is one-atom-thick (0.35nm) planar sheets of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. It is Fullerene if carbon atom is in the form of sphere, Carbon Nanotube if it is in cylindrical nanostructure, and Graphene if it is unfolded.
Graphene is twice stronger than diamond, and 200 times stronger than steel. Also, diamond is a nonconductor that does not conduct electricity, but graphene conducts electricity very well, and has electrical conductivity that is more than 100 times higher than copper. For example, if light travels at the constant speed of 300,000km in a vacuum, electron travels at the constant speed of 1000km/sec in graphene. The reason for the difference is the difference in the bonding that connects carbon and carbon. Compare to diamond, a single bond, graphene has a double bond. As the name implies, double bond has two bridges connecting carbon and carbon. Because double bond is a stronger bond than a single bond, which has one bridge to connect carbon and carbon, graphene is stronger than diamond when compared and through one of two bonds, electrons can go back and forth between graphene. It also means diamond, which has a single bond, does not conduct electricity, but graphene does.
It is also resistant to shock due to the hexagonal honeycomb shape because just as a net, which changes shape when bended or stretched but does not change the connection, empty space of hexagonal structure works as a buffer. Thus, it has the elasticity good enough to be stretched to 20% of the area. The electrical conductivity does not go away when bended or stretched and it is transparent enough to pass 98% of light. That is why graphene is in the limelight as the next-generation device that can replace transparent ITO.
(Source: Bae, S., Nature nanotechnology, 5, 574, (2010))
In Korea, Sungkyunkwan University Department of Chemistry Professor Byung-Hee Hong developed transparent conductive film, which bends about 2cm in length and width, for the first time in the world in 2009 and published in Nature, the scientific journal, and then developed 30” touch screen with graphene for the first time in June 2010. Korea is also ranked at No.2, after the U.S., in the number of key patents; mainly 3 institutions, Samsung, Sungkyunkwan University, and KIST. For the number of academic papers related to graphene, Korea is ranked at No. 4 after China, U.S., and EU. Samsung Electronics developed graphene-applied flexible nano-power plant with Sungkyunkwan University and Samsung Techwin built a pilot line (sample production line) that can synthesizes graphene and mass produces using chemical vapor deposition (CVD). Besides Samsung Techwin, Dongjin Semichem is looking for ways to apply to coating and printed electronics graphene ink, POSCO is looking for ways to apply to low-cost production technology, and LG Chem is looking for ways to apply to display and the secondary cells.
EU Economic Committee is pushing EU Graphene Flagship project, worth total € 1B (KRW 1.6T) in the next 10 years, and 60 institutions from 29 countries are participating. Representatively, the UK already has poured 5M pounds (KRW 90B), Denmark already has poured KRW 12B, and Sweden already has poured KRW 7B in the R&D to commercialize graphene.
Led by NSF (National Science Foundation), U.S. also has been investing USD 28M (KRW 3.2T) for the last 5 years to develop graphene-used RF device, and 8” graphene wafer and high-speed RF device developments are now underway with the USD 30M investment in 2011.
Japan supports the basic of graphene and application research around National Institute of Advanced Industrial Science and Technology (AIST) under the Ministry of Economy, and is currently promoting the synthesis of graphene and application technology development with the KRW 140B government funded R&D investment in 2011.
Singapore is spurring to lead the commercialization of graphene by investing KRW 100B for next 5 years around the Graphene Center in National University of Singapore, and plans to intensively promote the basic of graphene and application research by inviting Nobel Prize winning-worthy researchers.
2. CNT (Carbon Nano Tube)
Carbon nanotube, which discovered before graphene, was discovered 20years ago, in 1991, by Dr. lijima in Japan, accidently from the material attached to graphite electrode. As the name, nano (1/billion), implies, carbon nanotube has the cylindrical nanostructure with carbons connected with hexagonal rings.
The reason carbon nanotube has received much attention previously is because of the excellent material properties. It has more than 1000 times higher thermal conductivity than copper, which is being used as a wire, shows superconducting property at ultralow temperature, and while heat generation problem becomes severe as copper wire gets thinner, carbon nanotube can stably transmit electricity because it emits heat well. Also, because it is stronger than steel, and has good elastic modulus, it bends well. Of course it is a lot lighter than steel because it is hollow.
In particular, because it can control the sheet resistance value of the thin film surface depending on the thickness of thin film formation, it received more attention as flexible transparent electrode.
CNT-applied transparent electrode technology is being led by U.S. and Japan. Eikos of U.S. developed transparent conductive film using single-walled CNT, applied related patents, and released product named Invisicon. Unidym of U.S merged with CNI, CNT manufacturer, developed flexible single-walled CNT transparent conductive electrode, and the research to replace ITO transparent electrode used in touch screen, LCD, and OLED is underway.
In the case of Japan, Mitsubishi Rayon announced single-walled transparent conductive electrode, but failed to commercialize since the physical property of thin film fell behind U.S products. However, these attempts are evaluated as the opportunities to develop CNT transparent electrode and related researches to the next level.
In Europe, the study to develop flexible display is being carried out through ‘FlexDis’ project, created by 20 companies, research institutions, and universities. Small number of research teams such as Germany Bayer, which recently started on CNT mass production, is close to the commercialization of CNT transparent conductive thin film, a core material.
Development fever is also heating up in Korea. The basic research is in progress around universities such as Korea University, Sungkyunkwan University, and KAIST and enterprises and venture companies are jumping into the R&D on CNT transparent electrode. Research is being actively promoted around some venture companies such as DPI Solution, and Top Nanosys. Top Nanosys released touch screen CNT transparent electrode prototype, which durability, conductivity, and transmittance are improved, and conducting CNT large-area coating research to lead this to mass production system. Already, commercialization movement is active around decompression TSP market. Wise Power agreed to supply CNT as the material for Nissha-developed 3D TSP and Top Nanosys also plans to mass produce for navigation-purpose. It is not easy to apply to TSP since film pattern is difficult but recently, Sangbo signed a contract on CNT transparent electrode film production technology transfer with Korea Electrotechnology Research Institute (KERI), and preparing capacitive CNT film mass production from 2013.
3. Ag Nanowire
(Source: Professor Sangho Kim Research Team, Kongju National University in South Korea)
Silver has the best conductivity of all metals. Nanoparticle surface of silver is composed of crystal plane and elongated wire shape can be formed, using the differences in reactivity, by inducing anisotropic growth. Ag nanowire is favorable to larger-area since its resistance value, 80~120Ω, is lower than ITO film, 200~400Ω, can be applied with printing methods, not deposition, and also can be applied to flexible display since it can be made into a curved surface.
Cambrios Technologies Corp., U.S electronic material manufacturing venture, has silver nanowire ink source technology (ClearOhmTM coating film) and Samsung Venture Investment in Korea has invested USD 5M. Shin-Etsu Polymer of Japan also introduced Cambrios ink, and has been exclusively supplying to Hansung Elcomtec. Hansung Elcomtec has announced that it will mass produce silver nano TSP for the first time in the world. Pure metal is favorable in low-resistance, but because of the disadvantage that it oxidized in the air, it has been commercialized. However, as Hansung Elcomtec developed antioxidant technology, the possibility of commercialization got bigger. When production line equipments are completed, 100K/month production will be possible on a 10.1” TSP basis. Inktec, a company specialized in ink material, has developed silver nano ITO film of over 90% transmittance, and built to the mass production stage in Q3 2011, and has introduced at the exhibition. Recently, as antioxidant treatment technology was developed, Hansung Elcomtec developed silver nano film, which implemented 80Ω per 1m2 and transmittance of 92%. Inktec hybrid transparent electrode film is an ITO alternative material that added a small amount of metal material to silver nanowire.
(Source: Toray Industry)
Toray Film of Japan also produced new transparent electrode film in April 2011 by introducing Cambrios ink and combining film processing technology. Toray film has product design-suggesting technology matches to (1) coating thickness control technology of nano-level, (2) multilayer laminating technology comply with wet process, and (3) machining process of each user. Compare to ITO film that needs vacuum process such as sputtering, it is favorable of mass production, and can be applied to touch panel mounted on mobile phones, smartphones, and Tablet PCs since it has high transparent conductivity, and excellent invisible patterning. In addition, low-resistance, which is required in large-area touch panel, is possible, and can be applied to 3D shape touch panel, solar cells, or AMOLED electrode in the future.
Next-generation transparent electrode material development, to replace ITO, is being continued. Graphene, CNT, and Ag nanowire are necessary fields for ITO alternative as well as the development of next-generation such as flexible display. The importance of transparent electrode material will increase even more as current LCD, AMOLED-oriented display market switches to next-generation display such as flexible display.