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Company News >> Research and Application of TFT-LCD LCD Optical Film 12th,June,2018
                                             First, optical film profile
1, the definition of optical film
Optical films are ubiquitous in our lives, from precision and optical devices and display devices to optical film applications in everyday life; for example, glasses, digital cameras, all kinds of home appliances, or banknotes that are usually worn on a banknote. Technology can be called an extension of the application of optical thin film technology. Without optical thin film technology as the basis for development, modern optoelectronics, communications, or laser technology will not be able to make progress. This also shows the importance of research and development of optical thin film technology.
Optical film refers to the coating or coating of one or more dielectric films or metal films or a combination of these two types of films on an optical element or a separate substrate to change the light wave transmission characteristics, including the transmission of light, Reflection, absorption, scattering, polarization and phase change. Therefore, through proper design, the transmittance and reflectance of the surface of different waveband components can be adjusted, and the light of different polarization planes can also have different characteristics.
In general, the production methods of optical films are mainly divided into dry and wet production processes. The so-called dry type is that no liquid appears in the entire processing process. For example, vacuum evaporation is a vacuum environment in which the solid raw material is heated by electrical energy, and after sublimation into a gas, it is attached to the surface of a solid substrate to complete coating. Processing. The gold, silver, or metal-like packaging film that is seen in everyday life is manufactured by dry coating. However, in the consideration of actual mass production, the scope of application of dry coating is less than wet coating. The general practice of wet coating is to mix components with various functions into a liquid coating, apply it on a substrate in different processing methods, and then dry and solidify the liquid coating to make a product. Only the optical film industry of wet coating technology is discussed in this article.
2, optical film types
Optical films can be classified according to their use classification, characteristics and applications: reflective film, anti-reflection/anti-reflective film, filter, polarizer/polarizer film, compensation film/phase plate, alignment film, diffusion film/sheet, Brightening film / prism sheet / condenser film, shading film / black and white glue. Related derivatives include optical grade protective films, window films, and the like.

2.1, reflective film
Reflective films can be generally classified into two types, one is a metal reflective film, and the other is an all-dielectric reflective film. In addition, there is a metal dielectric reflective film that combines the two functions, which increase the reflectivity of the optical surface. General metals have a large extinction coefficient. When the light beam is incident on the metal surface by air, the amplitude of the light entering the metal is rapidly attenuated, so that the energy of light entering the inside of the metal is reduced accordingly, and the energy of the reflected light is increased. The greater the extinction coefficient is, the faster the light amplitude decays, the less light energy enters the metal, and the higher the reflectivity. People always choose metals with large extinction coefficients and relatively stable optical properties as metal film materials. The commonly used metal thin material in the ultraviolet region is aluminum, commonly used in the visible region of aluminum and silver, in the infrared region commonly used gold, silver and copper, in addition, chromium and platinum are often used as a film of some special films. Since materials such as aluminum, silver, and copper are easily oxidized in the air to degrade performance, they must be protected by a dielectric film. Commonly used protective film materials include silicon oxide, magnesium fluoride, silicon dioxide, aluminum oxide, and the like.

The advantage of the metal reflective film is that the preparation process is simple and the wavelength range of operation is wide; the disadvantage is that the optical loss is large and the reflectance cannot be high. In order to further increase the reflectance of the metal reflective film, a certain number of dielectric layers may be additionally coated on the outside of the film to form a metal dielectric reflective film. It should be pointed out that the metal dielectric film increases the reflectivity of a certain wavelength (or a certain wave region), but it destroys the characteristics of neutral reflection of the metal film. All-dielectric reflective films are based on multiple beam interference. In contrast to antireflection coatings, the optical surface is coated with a film with a higher refractive index than the base material, which increases the reflectivity of the optical surface. The simplest multi-layer reflection is formed by alternate evaporation of high and low refractive index materials. The optical thickness of each film is one quarter of a certain wavelength. Under this condition, the reflected light vectors on the overlapping surfaces of the various parties face the same vibration direction. The synthetic amplitude increases with the number of thin film layers.
Aluminum foil reflective film Dike aluminum foil insulation material, also known as barrier film, insulation film, heat insulation foil, heat extraction film, reflective film and so on. By aluminum foil veneer + polyethylene film + fiber knit fabric + metal film laminated by hot melt adhesive, aluminum foil coil has thermal insulation, waterproof, moisture and other functions. The solar radiation absorption coefficient (solar radiation absorption coefficient) of aluminum foil insulation coil is extremely low (0.07), it has excellent thermal insulation performance, can reflect more than 93% of radiant heat, and is widely used in building roof and exterior wall insulation Insulation.
Corresponding to it is an anti-reflection film, the main effect is to improve the diffraction of light, so that people can watch text and graphics for a long time. This requires an anti-reflection film with less surface smooth reflection.
2.2, AR/ATR
Anti-reflection coating, also known as anti-reflection coating, its main function is to reduce or eliminate the reflected light of the lens, prism, plane mirror and other learning surface, thereby increasing the light transmission of these components, reduce or eliminate the system's stray light. Anti-reflection films are based on the fluctuation of light and interference phenomena. Two waves with the same amplitude and the same wavelength overlap, then the amplitude of the light wave increases; if the two waves are originally the same, the wave paths are different, and if the two light waves overlap, they cancel each other out. Anti-reflection coating is the use of this principle, the surface of the lens plated with anti-reflection coating (AR-coating), so that the reflected light generated by the front and back surface of the film interfere with each other, thus canceling the reflected light to achieve the effect of anti-reflection. The simplest antireflection film is a single film. In general, single-layer anti-reflection film is difficult to achieve the desired anti-reflection effect. In order to achieve zero reflection at a single wavelength, or to achieve a good anti-reflection effect in a wide spectral region, double, three, or even more often used. Multiple layers of antireflection film.
The practical application of anti-reflection coatings is very extensive. The most common are anti-reflection coatings for lenses and solar cells. By building anti-reflection coatings, the power wattage of photovoltaic modules is increased. At present, the anti-reflection film material used in crystalline silicon photovoltaic cells is silicon nitride. Plasma-enhanced chemical vapor deposition technology is used to ionize ammonia and silane, deposit on the surface of silicon wafers, have a high refractive index, and can play a role. Better anti-reflection effect. Early photovoltaic cells used silica and titania films as antireflection layers.
2.3, filter
Filters are made of plastic or glass and then add special dyes, red filters only allow red light to pass, and so on. The refractive index of the glass sheet is almost the same as air, and all shades can pass through, so it is transparent, but after dyeing, the molecular structure changes, the refractive index changes, and the passage of certain color light changes. For example, when a white light passes through a blue filter, it emits a beam of blue light, while the green and red light are rare and most of them are absorbed by the filter. Filter products are mainly classified according to spectral bands, spectral characteristics, film materials, and application characteristics. Spectral bands: UV filters, visible filters, infrared filters; Spectral characteristics: bandpass filters, cutoff filters, spectroscopic filters, neutral density filters, reflective filters; membranes Layer material: soft film filter, hard film filter. The hard-coated filter not only refers to the hardness of the film, but more importantly to its laser damage threshold, so it is widely used in laser systems. Soft membrane filters are mainly used in biochemical analyzers.
Bandpass type: Light passes through the selected band, and light outside the passband is cut off. Short-pass type (also called low-pass): Light shorter than the selected wavelength passes, and light longer than this wavelength is cut off. For example, infrared cut filter, IBG-650. Long-pass type (also known as high-pass): light longer than the selected wavelength passes, shorter than the wavelength of light cutoff, such as infrared transparent filter, IPG-800. Color filters are an important part of TFT-LCD backlight modules. See Chapter 2 for details.

2.4. Polarizer
The full name of the Polarizing Film should be a polarizer. Imaging of liquid crystal displays must rely on polarized light. The main role of the polarizer is to make the non-polarized natural light become polarized, converted to polarized light, plus the torsional characteristics of liquid crystal molecules, to control the passage of light, thereby increasing the transmittance and viewing angle range, forming Anti-glare and other functions. Polarizers can be widely used in modern LCD products: LCD TVs, notebook computers, mobile phones, PDAs, electronic dictionaries, MP3s, instrumentation, projectors, etc., and can also be used for fashion polarized glasses. Among them, the application of LCD is the main force that drives the development of polarizer industry. See Chapter 2 for details.

2.5, compensation film / phase difference board
The compensation principle of the compensation film is to correct the phase difference produced by each viewing angle in various display modes (TN/STN/TFT (VA/IPS/OCB)) liquid crystals. In short, it is the birefringence of liquid crystal molecules. The nature is compensated for by symmetry. To distinguish between its functional purposes, the phase difference film, the chromatic aberration compensation film, and the viewing angle expansion film can be divided into simple phases. The compensation film can reduce the amount of light leakage when the liquid crystal display is in a dark state, and can significantly improve the contrast, chroma, and overcome some gray-scale inversion problems within a certain viewing angle.
2.6, Alignment film
The alignment film is a thin film with straight stripe scratches, which acts to guide the orientation of the liquid crystal molecules (Figure 1.1). On a glass substrate on which a transparent conductive film (ITO) has been evaporated, a parallel groove is printed on the ITO film using a PI coating liquid and a roller, and the liquid crystal can be oriented in the direction of the groove. Lying horizontally in the grooves, the liquid crystals are arranged in the same direction. This has one direction of the film, which is the alignment film. The reason why the liquid crystal can be applied to the screen is that it can be driven by the electric field because it has different energizing rates in the parallel molecular direction and the vertical molecular direction. On the other hand, since the liquid crystal also has a refractive index that varies depending on the direction of the visual molecule (also With birefringence, the polarization direction of the polarized light can be changed. Finally, due to the strong interaction between the liquid crystal and the alignment film, the liquid crystal is restored by the elasticity (recovery force) after the electric field is turned off. From the original arrangement, it can be seen that there is no alignment film and the liquid crystal cannot work. However, in the application of liquid crystal screens, the liquid crystal molecules and the surface of the alignment film are inclined at an angle (ie, PretiltAngle), so that uniform alignment effects can be achieved. The alignment films involve coated non-wrap wet coating methods, including conventional directional brushing and current UV light alignment methods, electron paste alignment, and ion beam alignment.

2.7, diffusion film
The diffusion film is a key component of the TFT-LCD backlight module, and can provide a uniform surface light source for the liquid crystal display. Generally, the conventional diffusion film is mainly in the diffusion film substrate, and one particle of chemical particles is added as scattering particles. However, in the existing diffusion plate, the fine particles are dispersed between the resin layers. Therefore, when the light passes through the diffusion layer, it will continue to pass through two different refractive index media, so that the light will undergo many refraction, reflection and scattering. Phenomenon, this will cause the effect of optical diffusion. See Chapter 2 for details.
2.8, brightness enhancement film / prism sheet / condenser sheet
The brightness enhancement film is also called Prism Sheet, often abbreviated as BEF (Brightness Enhancement Film), and is a key component in the TFT-LCD backlight module. It mainly uses the principle of light refraction and reflection to correct light using a prism sheet. Direction, making the front of the light concentrated, and the light that has not been used outside the viewing angle can be recycled and used, while improving the overall brightness and uniformity to achieve the effect of brightening, also known as the condenser film. The composite optical film mainly integrates the functions of the original condenser sheet and the diffusion function. This will reduce the use of one diffusion sheet, which is beneficial to the downstream manufacturers to simplify the design of the backlight, save the processes, and reduce the cost. At the same time, the brightness efficiency can also be improved. . For optical film manufacturers, although the composite brightness enhancement film will replace the traditional condenser film (brightness enhancement film), the unit price and profit are better.
2.9, shading film / black and white glue
Black and white shading glue | shading film is mainly used in the backlight source, from the fixed, shading effect (cover off the side light and light position of the light), also known as shading film, black and white film, referred to as black and white glue (can be said to be a kind of double-sided tape) . Relative to the backlight used by the TFT-LCD, the blackout requirement is high, so most black and white adhesives are applied to the TFT-LCD backlight. In addition to black and white glue, there is black plastic (black on both sides), the main role is still fixed, shading; black and silver plastic (single black, single silver), in addition to shading, the silver surface has a reflective effect. Relative black and white glue is the mainstream product of the LCD market. The stickiness of the black and white surfaces is greater, because the white surface needs to be connected to the rubber frame, and the black surface is connected to the glass. The adhesion of the glass to the glue is worse, so the white surface is more viscous. Large to ensure the stability of the entire module.

Second, TFT-LCD industry chain optical film
1, TFT-LCD industry overview
The TFT (Thin Film Transistor)-LCD means that each liquid crystal pixel on the liquid crystal display is driven by a thin film transistor integrated behind the liquid crystal display, which can display screen information at high speed, high brightness, and high contrast. The most mature mainstream technology in display technology (FPD) is the most widely used in the market. The TN type, STN type LCD is relatively backward. The TN type (twisted nematic type) uses liquid crystal molecules that are twisted at 90 degrees to achieve display, and the STN type (super-twisted nematic type) uses liquid crystal molecules that are twisted 180-270 degrees to display.

According to DisplaySearch, LCD monitors account for about 88% of the flat panel display market. Although PDP has formed a certain industrial scale, it is far behind the industry scale of TFT-LCD, OLED is in the early stage of industrialization, and FED, EPD are in the stage of technology development and pilot test. It can be considered that in the future will still occupy a large share of the flat panel display market for a considerable period of time. The material cost structure of the 42” TV panel is generally 25% for the backlight module, 16% for the color filter, 8% for the polarizer, 7% for the glass substrate, 3% for the liquid crystal, and 3% for the IC driver. In the TFT-LCD industry chain (Figure 2.1), upstream costs account for 75-80%, TFT industry profits are mainly concentrated in the upstream materials area, gross margin is relatively stable, generally around 15%.Our target customers It is the manufacturer of the upstream materials used in roll-type wet-coated polarizers and diffusers and brightness enhancement films in backlight modules.
1.1 TFT-LCD Structure and Process
At present, the LCD mainly consists of three parts: a color filter (CF) substrate, a TFT array substrate, and a backlight module (Backlight).
There are countless pixel arrays on the TFT Array glass, and color filters are the source of picture color. The LCD is generally filled with a liquid crystal layer between the upper and lower transparent electrodes and sandwiched between the two glass substrates of the TFT Array glass and the color filter. When the voltage is applied to the TFT (transistor), the liquid crystal turns and the light passes through the liquid crystal to generate a pixel on the panel. 

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