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Progress in research and industrialization of transparent conductive oxide films

[Abstract] the history and current situation of research and development of TCO (transparent conductive oxide) films are reviewed, and the industrialization prospect is prospected. Traditional ITO film has excellent properties and is an important material for flat panel display devices. The new ZnO film has low cost and great development potential, and is expected to replace ITO (tin doped indium oxide) in the field of solar cells. Multicomponent composite oxide film is one of the development directions of TCO; The application of flexible substrate expands the application of TCO film; The development of sol-gel preparation technology promotes the practicality of large-area TCO films

transparent conductive oxide (TCO) films mainly include oxides of in, Sb, Zn and CD and their composite multi oxide film materials. They have common photoelectric characteristics of forbidden bandwidth, high light transmittance in the visible spectral region and low resistivity. They are widely used in solar cells, flat panel displays, special function window coatings and other photoelectric devices. Transparent conductive films, represented by tin doped indium oxide (ITO), have been widely studied and applied, and have been industrialized in the United States, Japan and other countries. In recent years, the research of ZnO thin films has also been deepened. Aluminum doped ZnO thin films (abbreviated as azo) are considered to be one of the most promising materials. At the same time, people have also developed multiple transparent oxide film materials such as Zn2SnO4, in4sn3o12, mgin2o4, CdIn2O4, etc. Magnetron sputtering is the most mature preparation process of TCO thin films. In order to further improve the properties of thin films, various high and new technologies have been introduced, and the preparation processes are becoming more and more diversified. In this paper, the latest research trends and industrialization development status and prospects of TCO thin films represented by ITO and azo are reviewed

1 TCO film characteristics

tco film first appeared in the early 20th century. In 1907, badeker first made CDO transparent conductive film, which has led to the development and utilization of transparent conductive film. SnO2 based and In2O3 based films appeared around 1950. ZnO based thin films rose in the 1980s [1]. TCO film is a polycrystalline layer with grain size of hundreds of nanometers, and the grain orientation is single. At present, ITO, FTO (sn2o ∶ f) and azo are mostly studied. These oxides are heavily doped and highly degenerate semiconductors. The semiconducting mechanism is stoichiometric shift and doping, and their band gap width is generally greater than 3 EV, which changes with different components. Their photoelectric properties depend on the oxidation state of the metal and the characteristics and quantity of dopants, and generally have high carrier concentration (1018 ~ 1021 without setting attenuation); Reduce the inlet force (load) and outlet load appropriately (cm-3), but the mobility is not high, and the resistivity reaches Ω Cm order of magnitude, visible light transmittance 80% - 90%. The low resistivity of TCO thin films is determined by the carrier concentration, but due to the complex conductive mechanism of polycrystalline films, the cause of low resistivity remains to be further studied. ITO thin films have complex cubic ferromanganese structure, and the lowest resistivity is Ω The production and utilization platform of CM volume column 6 builds the key direction level, and the average light transmittance within the visible spectrum is more than 90% [2]. Its excellent photoelectric properties make it a TCO film with practical application value. At present, it has formed a certain commercial production scale and is an indispensable planar transparent electrode material in planar liquid crystal display devices. However, because indium is a rare element, it has less storage in nature and higher price, and ITO is not stable in plasma when applied to solar cells. At present, the research of ZnO based thin films has made rapid progress, and the material properties have been comparable with ITO. The structure is hexagonal wurtzite type. Among them, AZO film is widely studied. Its outstanding advantages are easy availability of raw materials, low manufacturing cost, non-toxic, easy doping, and good stability in plasma. Therefore, it may become an alternative product of ITO, especially in the field of transparent electrodes of solar cells. The properties and applications of TCO thin films composed of doped or undoped metal oxides are limited by the inherent properties of the elements contained in them. In order to optimize the optical, electrical and chemical properties of thin films, the new multicomponent compound TCO thin films composed of a variety of oxides can obtain the best properties by adjusting the composition and elemental composition to meet some special needs. In recent years, TCO films such as ZnO SnO2, ZnO In2O3, cdsb2o6, mgin2o4, in4sn3o12, zn2in2o5, CdIn2O4, cd2sno4, Zn2SnO4, gaino3 have been studied [3, 4]. The resistivity of insno can reach 2 × Ω. Cm, visible light transmittance of 80%. This is one of the important development directions of TCO thin films

2 improvement of preparation process

tco thin films have various preparation methods. Various methods of preparing thin films, such as magnetron sputtering [3 ~ 9], reactive thermal evaporation [10 ~ 14], which is a historical contribution, metal organic chemical vapor deposition [15 ~ 17], atomic layer epitaxy [18, 19], jet thermal decomposition [20], pulsed laser deposition [21], sol gel [22 ~ 26], can be used to prepare TCO thin films. The properties of the film are determined by the preparation process. The direction of improving the preparation process is to make the film with low resistivity, high transmittance and good surface morphology, the film growth temperature close to room temperature, good adhesion to the substrate, large-area uniform film preparation and low film production cost. Various preparation methods have their own advantages and disadvantages. At present, magnetron sputtering is recognized as the best method. This method has been used in the commercial production of ITO films

pulsed laser deposition is a very competitive physical vacuum deposition method, which has the advantages of accurate stoichiometry control, simultaneous synthesis and deposition, and no requirements for the quality and surface of the target. The spray thermal decomposition method does not need vacuum equipment, and the process is simple. It is suitable for preparing transparent electrodes of large-area solar cells without damaging the substrate layer. In the traditional high-temperature preparation process, the quality of devices is affected by the solid-state diffusion between the film and the substrate. Therefore, low temperature technology has been developed, and one of the ways to obtain low temperature technology is to use light excitation technology. Laser and UV assisted deposition can reduce the deposition temperature and improve the conductivity and light transmittance [16]

sol gel method is a new method to prepare high-performance particles, fibers and films. This method starts from the 120 ° organic or inorganic salt of metal, through the hydrolysis and polymerization of compounds in the solution, a sol solution dissolved in metal oxide or hydroxide particles is made, and then gel is made by further reaction, and then the gel is heated into amorphous or polycrystalline materials. This method is easy to control the composition of the film, and the doping can be controlled at the molecular level, especially suitable for the preparation of films with accurate doping level. This method can make the raw materials closely combined at the molecular level, and the film is highly uniform. Even in a multi-component system, the composition can be uniform. By selecting solvent, adjusting concentration and adding catalyst, the properties of sol and film thickness can be easily controlled. In short, this method does not need vacuum equipment, and the process is simple. It can obtain films with ideal thickness and components. It is suitable for large-area and complex matrix without damaging the matrix. It is of great significance to the large-scale industrialization of TCO films. The disadvantage of this method is that the price of organic raw materials is high, and the development of applicable inorganic raw materials is the current research focus [22 ~ 26]

recently, it has been reported that ITO and AZO films were deposited on organic transparent substrates, and the microstructure and photoelectric properties of films close to those deposited on glass substrates were obtained [5, 9, 10]. The resistivity of AZO films deposited on transparent polyester film, polyisocyanate and polyester film for general electrical purposes is 5.76 respectively ×, eight ×, 2.2 × Ω. Cm, the transmittance is about 94%. TCO thin films on flexible substrates have unique advantages such as flexibility, light weight, non friability, and convenience for large-area production and transportation, which greatly increases the application field of TCO thin films, expands the market scope, and promotes the industrialization of TCO thin films

3 application status and industrialization prospects

tco films are widely used because of their excellent transparent and conductive properties. At present, the main application fields include planar liquid crystal display (LCD), electroluminescence display (ELD), electrochromic display (ECD), transparent electrode of solar cell; Because of its selectivity to light waves (projection of visible light and reflection of infrared light), it is used as a thermal reflector. It is used for building glass windows in cold areas to play a role of heat shielding and save energy consumption. It can also be used as a transparent surface heater to form anti fog and defrosting glass on the glass windows of vehicles such as cars and planes. Similarly, it can be used on anti fog camera lenses, special-purpose glasses, instrument windows. The development of flexible substrate TCO film has expanded its potential applications to manufacturing flexible light-emitting devices, plastic liquid crystal displays, foldable solar cells, as well as plastic greenhouses and glass adhesive films as thermal insulation materials

based on the mature development of DC magnetron sputtering, metal organic chemical vapor deposition and the newly developed sol gel technology, ITO films have been applied in many fields, and the industrialization continues to develop, becoming more and more mature, and the products have entered the market. Some manufacturers in Japan, the United States, Europe and Shenzhen are committed to the development of ITO industry. The research of AZO film is also deepening, which is one of the deep processing and application ways of common metal zinc. It is expected to make a breakthrough in the application of transparent electrodes of solar cells and replace ITO film, so as to promote the development of low-cost civil solar energy industry. At present, the popularity of large screen and high-definition LCD is rapid, and the demand is increasing. The worldwide demand for energy supply and natural environmental protection also shows the good development prospects of solar cells, which provides opportunities and development space for the TCO industry. TCO research units and manufacturers should seize this opportunity to bring the theoretical research and practical application of TCO to a new level

about the author: Zhao xiequn (1965 -), female (Manchu), Beijing native, senior engineer, engaged in scientific and technological information research on electronic information materials and semiconductor materials

Author: Zhao xiequn (Beijing Nonferrous Metals Research Institute

source: Jinshi Group

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