Structure and application of polarizer film for thin ... - Semantic Scholar
Displays 32 (2011) 49–57
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Review
Structure and application of polarizer film for thin-film-transistor liquid crystal displays Ji Ma a,b,⇑, Xin Ye b, Bo Jin b a b
Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA Technology Center, Shanghai SVA-Optronics Co., Ltd, Shanghai 200233, China
a r t i c l e
i n f o
Article history: Received 23 June 2010 Received in revised form 24 November 2010 Accepted 22 December 2010 Available online 18 January 2011 Keywords: Liquid crystal display Polarizer film Compensation film
a b s t r a c t Thin-film-transistor liquid crystal displays (TFT-LCDs) are the most popular flat panel displays now. Polarizer film is one of the most important components in the TFT-LCDs, which is a multi-layered complex film developed by the technology of stretching film with dichroic materials. In this paper, a systematic review about polarizer film used for TFT-LCDs is given. Structure, property, function and material of each layer and detailed explanations of compensation films as well as its types, especially for the twisted nematic (TN) mode LCD, are summarized. Manufacturing processes of the tri-acetyl cellulose (TAC) film and the polarizer film, attachment process of a polarizer film to a LCD panel and the key technologies in these processes are illustrated. Examples in practical applications and technology development trends in the future are also presented. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Thin-film-transistor liquid crystal displays (TFT-LCDs) are the most popular flat panel displays till now because of their excellent features of light weight, low power consumption, compact size and so on. TFT-LCDs have been exploited for several commercial display modes including twisted nematic (TN), in-plane switching (IPS), multi-vertical alignment (MVA), etc. [1–8]. Polarizer film (POL) is an essential and important component in the TFT-LCDs, which is developed by the technology of stretching film with iodine or other dichroic dye molecules [9,10]. Liquid crystal (LC) is sandwiched between two polarizer films. Taking a normally-white transmissive TN mode TFT-LCD as an example, the LC director configurations in the voltage-off state and voltage-on state are shown in Fig. 1. In the voltage-off state, the LC is aligned parallel to the transmitted optical axis of the polarizer films and is rotated 90° in the bulk by the alignment boundary on the substrates. The incident linearly polarized light is produced by the rear polarizer film and follows the twisted LC molecular structure to transmit through the front polarizer film, resulting in a white state. In the voltage-on state, the LC is reoriented vertical to the substrates upon the electric filed. The front polarizer film blocks the incident linearly polarized light produced by the rear polarizer film, resulting in a black state [11,12]. The driving voltage applied to the LC is used to modulate the output-light intensity through the LCD cell to generate gray-levels. ⇑ Corresponding author at: Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA. E-mail address: [email protected] (J. Ma). 0141-9382/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.displa.2010.12.006
Observed from a normal direction of a LCD cell, the angle between the absorption axes of the crossed polarizer films is 90°, which can yield a good black state. However, observed from an oblique direction, the effective angle is not 90°, as shown in Fig. 2. Therefore light leakage occurs in the black state [13–16]. The contrast ratio (CR) is decreased. CR = LWhite/LBlack, wherein LWhite and LBlack are the luminance in the white state and black state. The field of viewing angle at which the CR is greater than 10:1 is also decreased. In order to reduce light leakage in the black state, retardation films (compensation films) are introduced into polarizer film [17]. The compensation film has a certain retardation value, which optically compensates the retardation of the LC in the black state to increase the CR at oblique direction and the field of viewing angle. In this paper, based on the discussion of layered structure of polarizer film, property, function and manufacturing process are summarized. Principle of compensation film, examples in practical applications and technology development trends are also presented.
2. Polarizer film The working principles of polarizer film are similar for various transmissive TFT-LCDs except that different types of compensation films will be used for different display modes. The layered structure of a TN-type polarizer film is shown in Fig. 3. From the surface of the polarizer film to the LCD cell, the structure includes surface protection film (PF), tri-acetyl cellulose (TAC) film with surface treatment, uni-axially polyvinyl alcohol (PVA) layer, TAC film with wide viewing (WV) compensation layer, pressure sensitive
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Fig. 1. LC director and polarizer film configurations of a normally-white TN mode TFT-LCD in voltage-off state (left) and voltage-on state (right).
Fig. 2. Effective angle between the absorption axes of crossed polarizer films observed from a normal direction (left) and an oblique direction (right).
When the LCD panel is used, the PF is removed. In this case, the quality requirement of the PSA in the PF is high, where it should avoid residual sealant on the polarizer film front surface. TAC film is widely used as a protective and supporting film for the PVA layer by virtue of high light transmittance (>93%), low birefringence, high moisture resistance, high thermal-stability, high uniformity and good curling ability [23–26]. The chemical structure of the TAC is shown in Fig. 5, which has bulky, rigid main chain and bulky acetyl groups that give large intermolecular free volume [27]. The PVA layer is sandwiched between two TAC films, where the TAC prevents swelling and shrinking of the PVA and prevents the evaporation of iodine or dyes in the PVA when temperature and humidity of the polarizer film are changed. The TAC film is manufactured by solvent casting technology [28]. Plasticizer, dissolving agent, wetting agent, lubrication prescription and UV-absorber are added to help casting process. PVA layer is the functional layer to produce polarized light. The chemical structure of the PVA is shown in Fig. 6. The film of the semicrystalline PVA polymers containing iodine or other dichroic materials is stretched to obtain uni-axial orientation molecular film. During this stretching process, the dichroic materials are oriented with the linear PVA chains (Fig. 6). Thereby anisotropic absorption of light is generated due to the dichroic absorption, resulting in polarized light [29–33]. Iodine-PVA polarizer film is the most common commercial polarizer film for TFT-LCDs because of high transmittance, high alignment ability, good mechanical property and ease of manufacturing. Dye-doped PVA polarizer film is more suitable for out-door applications because it has high temperature and humidity resistance. Release film is located in the bottom of the polarizer film. It would be peeled off when the polarizer film is attached to a LCD panel. The chemical structure of the PET and the structure of the release film are shown in Fig. 7. Similar with the PET base film in the PF, the release film is a film with a silicone coating on the PET surface. The function of the release film is to protect the PSA layer before the polarizer film is attached. The silicone coating can help the base PET film easily to be removed from the PSA without residual sealant [34,35].
Fig. 3. Layered structure of a typical polarizer film for a TN mode TFT-LCD.
adhesive (PSA) layer and release film [18]. These functional layers compose a complex multi-layered polarizer film. Surface protection film (PF) is a flexible, heat-durability film of polyesters such as polyethylene (PE) or polyethylene terephthalate (PET). For high-quality TFT-LCD products, PET film is used. The structure of the PF is composed of anti-polluting layer, anti-static layer, PET, PSA and PET base film with silicone coating, as shown in Fig. 4. The function of the PF is to prevent damage, soiling or electrostatic discharge of the polarizer film during the processes of cutting, transporting and attaching to a LCD cell [19,20]. Different marks may be printed on the PFs on the front and rear polarizer films to avoid confusion. The functional treatments of anti-polluting and anti-static are introduced to enhance the protection for the polarizer film [21,22]. When the PF is laminated with the polarizer film in the manufacturing process, the PET base film is peeled off.
Fig. 4. Structure of the surface protection film.
Fig. 5. Chemical structure of the TAC molecule.
Fig. 6. Chemical structure and stretched uni-axial orientation of the PVA molecule.
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Fig. 7. Chemical structure of the PET (upper) and structure of the release film (lower).
PSA is a kind of bonding material, which adheres upon a contact force or a slight pressure [36]. PSA is acrylic layer formed by alkyl acrylate monomer and cross-linking agent co-polymer such as epoxy-based and isocyanate-based compounds. It is used to attach the polarizer film to a LCD panel. It is also used as bonding material among the other layers in the polarizer film [37–39]. The sufficient bond strength and clean removability are two essential factors to optimize PSA property. It needs maintenance of sufficient bonding strength to avoid detaching the polarizer film from the LCD panel when the polarizer film is on the panel. It also needs clean removability from the LCD panel when the polarizer film is removed or replaced due to wrong-position or other failure issues. These two factors can be influenced by chemical composition, amount of cross-linking agent, use of plasticizer, surface treatment, thickness and construction of the PSA [40,41]. As mentioned above, the types of compensation films are different for various transmissive TFT-LCDs such as TN, IPS or MVA mode. Compensation film can be manufactured by LC coating, stretching, or addition of a retardation film into the polarizer film. When a compensation film is used, the total phase retardation d of the LCD is expressed as 0
d ¼ 2pðdDn d Dn0 Þ=k
ð1Þ
where d and Dn are the thickness and birefringence of the LC in the LCD cell, d0 and Dn0 are the thickness and birefringence of the Table 1 Classification of compensation films.
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compensation film in the polarizer film and k is the incident wavelength. To match Eq. (1), several types of compensation films have been proposed, for example, uni-axial (A-plate and C-plate), oblique (O-plate) and biaxial plate [42–46]. Table 1 shows the classifications of compensation films according to refractive indices [47]. For uni-axial anisotropic birefringent material, if ne (extraordinary refractive index) > no (ordinary refractive index), it is positive while if ne < no, it is negative. A-plate means the optical axis of the film is parallel to the film surface and C-plate means the optical axis of the film is perpendicular to the film surface. We can find, for C-plate, nx = ny. The examples of LC-type compensation film as A-plate and C-plate are given in Table 1. When the optical axis of positive rod-shaped LC is parallel or perpendicular to the film surface, the type is (+) A-plate or (+) C-plate. When the optical axis of negative disc-shaped LC is parallel or perpendicular to the film surface, the type is () A-plate or () C-plate. According to Eq. (1) and Table 1, the conceivable conjugations of compensation films and LC configurations in different LCD modes can be obtained. For IPS mode TFT-LCDs, positive LCs in the LCD cell are aligned and rotated parallel to the substrate. (+) C-plate, () A plate, biaxial compensation film or combination of multicompensation film may be used [48–53]. For MVA mode TFT-LCDs, negative LCs are aligned vertical to the substrate, thereby (+) Aplate, () C-plate, biaxial compensation film or combination of multi-compensation film may be adopted [54–60]. Different compensation films are used depending on sheet-to-sheet or rollto-roll manufacturing process and display performances. The stretching-type biaxial compensation film becomes a mainstream for high-performance IPS and MVA mode TFT-LCDs. TN mode TFT-LCDs are widely manufactured in the industry, of which the manufacturing process is more mature and simpler than IPS and MVA mode. They are suitable for many low-cost panels from small size displays (in several inches) for cell phones and cameras to large size displays (up to 26 inch) for monitors and televisions [61–63]. Nowadays the CR and viewing angle of TN mode TFT-LCD with compensation film have already exceeded 1000:1
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and 160° [64]. The key factor on continuous promotion of CR and field of viewing angle lies in LC coating compensation film technology. Here we discuss TN-type compensation film in detail. The TNtype compensation film (WV film) is a hybrid-aligned discotic LC (DLC) coating on an alignment layer on the TAC film, which three-dimensionally compensates the light leakage in all viewing directions of a TN mode TFT-LCD in the black state [65–71]. The chemical structure of the DLC and the structure of the WV film are shown in Fig. 8. The DLC is one of triphenylene derivatives, which has a disc-like molecular structure, high birefringence and is crossing-linking. The DLC right next to the alignment layer has a high degree of randomness. However, the DLC in the vicinity of the alignment layer tends to align with the molecular plane parallel to the alignment layer surface with few degrees of pretilt angle in the alignment direction, while the DLC in the vicinity of the air surface tends to align with the molecular plane almost perpendicular to the air surface [72–74]. Therefore, the DLC exhibits a hybrid alignment structure. This structure would be fixed by photo-polymerization of the cross-linking group in the DLC using UV light irradiation and this layer is so-called polymerized discotic material (PDM) [75]. When a WV-polarizer film is attached to a TN mode LCD panel, O-mode display is used, in which the absorption axis of the PVA is parallel to the adjacent rubbing direction of the polyimide (PI) on the substrate [76–78]. The alignment direction of DLC is parallel to the absorption axis of the PVA layer so that the WV film can be laminated in the longitudinal direction in the roll-to-roll process. The configuration of alignment direction of the DLC in the WV film, absorption axis of the PVA layer and rubbing direction of the PI is shown in Fig. 9. The idealized and simplified model of optical compensation for the TN mode TFT-LCD using a WV film is shown in Fig. 10. In the black state, the orientation of the LC continuously changes from the homogeneous alignment near the PI on the substrate to the homeotropic alignment in the LC bulk along the thickness direction, while the direction of each DLC layer in the hybrid alignment continuously changes in the WV layer thickness direction consisting of splay and bend deformation to match and compensate the LCs in the TN mode LCD cell. The LC in the LCD cell has a positive birefringence while the DLC in the WV film has a negative birefringence. Since they have complicated opposite alignment structure, they can compensate each other, resulting in higher CR and larger field of viewing angle. Based on different designs and specifications, WV film have been developed three generations till now, i.e., wide viewing angle (WV-A), super wide viewing angle (WV-SA) and excellent wide viewing angle (WV-EA) film [79–82]. The WV-EA film is more suitable for large size, wide aspect ratio LCD panels. Typical viewing angles at the contrast ratio of 10:1 for horizontal (H) and vertical (V) direction are 130°/100°, 145°/135° and >160°/>160° for WVA, WV-SA and WV-EA film, respectively. The iso-contrasts of the
Fig. 9. Relationship of the alignment direction of the DLC in the WV film, the absorption axis of the PVA layer and the rubbing direction of the PI in the TN mode TFT-LCD with compensation film.
Fig. 10. Idealized and simplified model of optical compensation for a TN mode TFTLCD using a WV film.
TN mode TFT-LCD compensated with these three types of WV films are shown in Fig. 11. The differences among these WV films are the thickness of the PDM layer, the orientation angle of the DLC and thickness retardation Re = [(nx + ny)/2–nz]d0 . Here, nx is main refractive index within the WV film plane, ny is refractive index perpendicular to the direction of nx within the plane and nz is refractive index in the thickness direction of the WV film. The thickness of the PDM, Re and average angle of the DLC of WV-A, WV-SA and WV-EA film are measured and summarized in Table 2. To design new type WV film, the mura issue (light leakage induced by temperature gradient and non-uniform thermal stress distribution), CR and color shift at oblique viewing angle should be considered more. Some surface treatments have been introduced to the polarizer film, such as anti-reflection (AR) [83,84], anti-glare (AG) [85–87] to
Fig. 8. Chemical structure of the DLC molecule (left) and structure of the WV film (right).
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Fig. 11. Iso-contrast of a TN mode TFT-LCD: (a) without WV film, (b) with 1st generation WV-A, (c) with 2nd generation WV-SA and (d) with 3rd generation WV-EA. (This Figure was reproduced from Review of viewing angle compensation of TN mode LCDs using WV film, S. Yasuda, T. Ito, T. Oikawa, Y. Ito, Y. Takahashi, published in Journal of the Society for Information Display, vol. 17, Issue 4, 2009, with permissions by The Society for Information Display.)
Table 2 Summary of WV films for TN mode TFT-LCDs.
Thickness of PDM (lm) Re (lm) Average angle of DLC (degree) Field of viewing angle (H/V, degree)
WV-A
WV-SA
WV-EA
1.5 136 16.0 130/100
2 156 18.9 145/135
1.7 155 16.0 >160/>160
prevent or decrease the glare of LCDs and hard coating [88] to prevent scrapes for the front polarizer film. Dual brightness enhancement film (DBEF) have been developed to increase the utilized efficiency of backlight for the rear polarizer film [89]. As examples, the working principles of the AG and DBEF are shown in Fig. 12. The AG-type polarizer film has a dispersion layer with tiny particles on the surface. The formed fine undulations in the rough surface scatter light in multiple directions to avoid the reflective light directly entering eyes. The directly reflective light is fatiguing to eyes whereas the AG treatment makes the displays pleasant. The DBEF-type polarizer film can recycle the reflected backlight to increase the polarized light transmittance through the rear polarizer film. It also reduces the backlight unit thickness and cuts assembly cost. These treatment technologies enhance the functions of polarizer films and make the display performances better.
3. Manufacturing process of polarizer film 3.1. TAC film manufacturing process Solvent casting technology is versatile for thin film production. The TAC film is produced by this technology [23–28]. Fig. 13 shows the TAC film manufacturing process. The solution containing tips of the TAC, dichloromethane solvent and additive is filtrated and cast on a metal band. The formed polymer film is peeled off from the metal band after some drying. In the following drying zone, residual solvent (>99%) is dried and the film is made for a roll. Uniform, transparent TAC film with very small fluctuation of thickness is obtained.
Fig. 13. Illustration of solvent casting process of a TAC film.
3.2. Polarizer film manufacturing process The iodine-doped production method is used to fabricate polarizer film for TFT-LCDs. The main manufacturing process is pretreatment, dyeing and stretching, lamination, cutting and inspection of defects [90,91], as shown in Fig. 14. Because of the flexibility of polymer film, a roll-to-roll process can be used. During the roll-to-roll production, recipe, consistency and temperature of the I2 solvent, rate of stretching process and drying temperature are critical processes. Polyvinyl alcohol polymer, water-soluble cross-linking agent, plasticizer and surfactant are dissolved in water and this aqueous polyvinyl alcohol polymer solution is cast to form the PVA film by the process of casting, drying and heattreating. Then the PVA film is dyed by iodine and stretched to form uni-axial orientation molecular film. The TAC, the PF and the release film are laminated to the PVA layer sequentially. At last the wide polarizer film is cut as favorite size and is inspected under a light to remove defective products. Polarization degree of a polarizer film can be decided by
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðT p T c Þ 100% Polarization degree ð%Þ ¼ ðT p þ T c Þ
ð2Þ
where Tp is parallel transmittance of horizontally overlapped polarizer films and Tc is crossed transmittance of vertically overlapped polarizer films [92]. Usually the specification of the polarization degree is >99.7%. Typical Tp and Tc of a WV-EA type polarizer film are measured and shown in Fig. 15. If the transmittance of Tc at blue or red region is high, the hue of the LCD is bluish or reddish in the black state. This issue can be adjusted by dyeing and stretching process to control the polarization degree. 3.3. Attachment method and process
Fig. 12. Principles of the AR (left) and DBEF (right) in the polarizer film.
Attachment method and process of a polarizer film to a LCD panel are shown in Fig. 16. The release film of the polarizer film is peeled off and the PSA layer is attached to the panel through a set of rollers. The equipment can auto-position the polarizer film
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Fig. 14. Schematic manufacturing process of a polarizer film.
certain temperature and pressure. The autoclave equipment can also increase the adhesive strength in an instant. If there is a demand to remove or replace a polarizer film from a panel due to defects, wrong-position or evaluations of different types of polarizer films using a same panel, special tools or peeling machines are needed [94,95]. These tools can peel the polarizer film from the panel and prevent damage to the panel. 4. Examples and discussions 4.1. Light leakage
Fig. 15. Transmittance (Tp and Tc) of a WV-EA type polarizer film.
Fig. 16. Attachment method and process of a polarizer film to a LCD panel.
and the panel, separate the release film from the polarizer and attach the PSA layer to the panel [93]. After the rear polarizer film is attached to the panel, the panel is turned over and the front polarizer film is attached to the opposite side of the panel. The last procedure of the polarizer film attachment is to remove air bubbles using autoclave equipment (bubble-removing machine) under
Light leakage of polarizer film induced by temperature gradient and non-uniform thermal stress distribution from backlight is the most normal and serious problem for a LCD panel [96–98]. The light leakage of a TN mode LCD panel in the black state is shown in Fig. 17 (left). The light leakage phenomenon is around the edge of the panel, especially after long-term durability test of high temperature and high humidity. The stretched PVA layer tends to relax back to original random alignment state upon heat and humidity that causes shrinkage of the polarizer film. Non-uniform thermal stress distribution makes the polarizer film distortion. The degree of the shrinkage is different between the TAC film and the PVA layer, where the shrinkage force of the PVA layer is larger than the TAC film (Fig. 18). The uniform of the PVA alignment is broken and the retardation of the TAC is changed. Therefore light leakage occurs. The behavior of light leakage under a hot air blower can prove it (Fig. 17 right). To enhance the durability of polarizer film, PSA recipe and process are adjusted to alleviate light leakage [99,100]. The distorted retardation R can be described as
R ¼ E F dt
Fig. 17. Light leakage of a TN mode LCD panel in the black state (left) and behavior under a hot air blower (right).
ð3Þ
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4.3. Polarizer film bubbles When polarizer film is stacked up or stored in the package, particles or residual sealants on the polarizer film surface will produce dents in the flexible films due to the gravity. The dents on the release film side could induce random polarizer film bubbles after attachment process, as shown in Fig. 20. The dent is vital if the bubble induced by the dent cannot be removed through auto-clave process. To decrease dents, the number of the particle should be under control in the clean-room. In addition, the vacuum degree of the polarizer film package can be decreased and fewer packages can be put in one tray to alleviate dents (Fig. 21).
Fig. 18. Distortion of a polarizer film upon high temperature.
where E is photoelastic coefficient of the TAC film, F is the shrinkage stress on the TAC film and dt is the thickness of the TAC film. The stress F can be defined as
F ¼ rðA A0 Þ
ð4Þ
where r is coefficient of the TAC film, A is the shrinkage of the polarizer film without the PSA and A0 is the shrinkage of the polarizer film with the PSA [99]. We can find that the property of the PSA is very important to reduce the shrinkage stress between the glass and the TAC film according to Eqs. (3) and (4). Large relaxation capacity of the PSA is considered as a key parameter to alleviate the deformation of each film. Low glass transition temperature copolymers, doping of plasticizers or low molecular weight acrylic polymers and decreasing cross-linking agents have been introduced to the PVA recipe and process [101,102]. Through these methods, the light leakage can be improved.
4.4. PSA bubbles After durability test of high temperature and high humidity, a cluster of bubbles in the PSA layer were observed. The shape of the bubble near the polarizer film edge was long and inclined from the corner to the center of the panel. Whereas the shape of the bubble near the center was more circular, as shown in Fig. 22. The inclined directions are same with the stretching direction of the polarizer film (45° and 135° for the front and rear polarizer film, TN mode LCD panel). The focus distance of an optical microscopy can deduce the bubble positions exactly in the PSA layer. The formation mechanism of this PSA bubble occurred after durability test is explained using moisture and gasification
4.2. Foreign materials During the processes of manufacture and attachment of polarizer film, foreign materials involved would degrade display performance if the sizes of the foreign materials exceed the limitation samples. Several foreign materials such as fiber, residual sealant, particle and dust are shown in Fig. 19(a)–(d), respectively. Most of foreign materials locate in the PSA layer. These foreign materials may block the light or change the retardation of the TFT-LCDs, resulting in light spots. A Fourier Transform Infrared (FTIR) spectrometer can be used to detect the element of the foreign material. The spectrum of the foreign material is compared with the spectrums in the database to distinguish the elemental composition. Thus the foreign material sources can be found. It needs rework (replace) the polarizer film if the foreign materials degrade the displays.
Fig. 20. Bubbles in the polarizer film due to dent.
Fig. 21. Tray and vacuum packaging of the polarizer film.
Fig. 19. Foreign materials in the polarizer film.
Fig. 22. Bubbles in the PSA layer in the panel (left) and under a microscope (right). Red (Blue) arrow means the direction of the front (rear) polarizer film shrinkage. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 23. Formation hypothesis of the PSA bubble.
hypothesis [100]. Fig. 23 shows the formative process. Moisture, residual solvent or monomer within the PSA layer is gathered around foreign object or dent. When the temperature is increased, gasification occurs. The bubble forms and further expands. The shrinkage of the polarizer film upon high temperature makes the bubble elongate along the stretching direction. The stress near the edge of the polarizer film is larger. Consequently the bubble near the edge is longer than those near the center. Reducing the substances of moisture, residual solvent or monomer, and adopting polymer with lower level of water absorbability in the PVA can prohibit gasification and expansion, reducing PSA bubbles. 5. Conclusions Polarizer film is an essential component for TFT-LCDs. The layered structure, property, function, process and applications of polarizer film are summarized and discussed in this paper. These factors should be considered when new type polarizer film is designed and developed. With the expansion of TFT-LCDs as monitors and TVs, development of polarizer film continues to focus on highperformance and low-cost products. Technology development trends of polarizer film include durability improvement to reduce light leakage, contrast ratio improvement to obtain high contrast, viewing angle improvement by design of new type compensation film, hue improvement to obtain neutral color by suppression of high transmittance in the short-wavelength range and surface treatment technology developments such as high-resolution AG treatment, AR treatment and hardness enhancement. The size of polarizer film needs to be increased to fit larger TFT-LCD panel. New processes, new materials and new structures of polarizer film also need to be developed to expand production capacity. Thinner thickness, higher durability and more function properties will be introduced to polarizer film to meet the requirements of future high-performance TFT-LCDs. References [1] S.-T. Wu, D.-K. Yang, Reflective Liquid Crystal Displays, John Wiley & Sons Inc., New York, 2001. [2] K.-H. Kim, J.-K. Song, Technical evolution of liquid crystal displays, NPG Asia Mater. 1 (2009) 29–36. [3] C.T. Liu, Revolution of the TFT LCD technology, J. Display Technol. 3 (2007) 342–350. [4] M. Oh-e, M. Ohta, S. Aratani, K. Kondo, Principles and characteristics of electro-optical behavior with in-plane switching mode, Asia Display’95 Digest (1995) 577–580. [5] A. Takeda, S. Kataoka, T. Sasaki, H. Chida, H. Tsuda, K. Ohmuro, Y. Koike, T. Sasabayashi, K. Okamoto, A super-high-image-quality multi-domain vertical alignment LCD by new rubbing-less technology, SID’98 Digest (1998) 1077– 1079. [6] R.W. Sabnis, Color filter technology for liquid crystal displays, Displays 20 (1999) 119–129. [7] S.S. Kim, The word’s largest (82-in.) TFT-LCD, SID’05 Digest (2005) 1842– 1847. [8] J. Ma, Y.-C. Yang, Z. Zheng, J. Shi, W. Cao, A multi-domain vertical alignment liquid crystal display to improve the V-T property, Displays 30 (2009) 185– 189. [9] E.H. Land, Light polarizer and process of manufacturing the same, US Patent, 2237567, 1941. [10] E.H. Land, Light polarizer and process of manufacture, US Patent, 2328219, 1943. [11] P. Yeh, C. Gu, Optics of Liquid Crystal Displays, John Wiley & Sons Inc., Hoboken, 2009.
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Displays journal homepage: www.elsevier.com/locate/displa
Review
Structure and application of polarizer film for thin-film-transistor liquid crystal displays Ji Ma a,b,⇑, Xin Ye b, Bo Jin b a b
Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA Technology Center, Shanghai SVA-Optronics Co., Ltd, Shanghai 200233, China
a r t i c l e
i n f o
Article history: Received 23 June 2010 Received in revised form 24 November 2010 Accepted 22 December 2010 Available online 18 January 2011 Keywords: Liquid crystal display Polarizer film Compensation film
a b s t r a c t Thin-film-transistor liquid crystal displays (TFT-LCDs) are the most popular flat panel displays now. Polarizer film is one of the most important components in the TFT-LCDs, which is a multi-layered complex film developed by the technology of stretching film with dichroic materials. In this paper, a systematic review about polarizer film used for TFT-LCDs is given. Structure, property, function and material of each layer and detailed explanations of compensation films as well as its types, especially for the twisted nematic (TN) mode LCD, are summarized. Manufacturing processes of the tri-acetyl cellulose (TAC) film and the polarizer film, attachment process of a polarizer film to a LCD panel and the key technologies in these processes are illustrated. Examples in practical applications and technology development trends in the future are also presented. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Thin-film-transistor liquid crystal displays (TFT-LCDs) are the most popular flat panel displays till now because of their excellent features of light weight, low power consumption, compact size and so on. TFT-LCDs have been exploited for several commercial display modes including twisted nematic (TN), in-plane switching (IPS), multi-vertical alignment (MVA), etc. [1–8]. Polarizer film (POL) is an essential and important component in the TFT-LCDs, which is developed by the technology of stretching film with iodine or other dichroic dye molecules [9,10]. Liquid crystal (LC) is sandwiched between two polarizer films. Taking a normally-white transmissive TN mode TFT-LCD as an example, the LC director configurations in the voltage-off state and voltage-on state are shown in Fig. 1. In the voltage-off state, the LC is aligned parallel to the transmitted optical axis of the polarizer films and is rotated 90° in the bulk by the alignment boundary on the substrates. The incident linearly polarized light is produced by the rear polarizer film and follows the twisted LC molecular structure to transmit through the front polarizer film, resulting in a white state. In the voltage-on state, the LC is reoriented vertical to the substrates upon the electric filed. The front polarizer film blocks the incident linearly polarized light produced by the rear polarizer film, resulting in a black state [11,12]. The driving voltage applied to the LC is used to modulate the output-light intensity through the LCD cell to generate gray-levels. ⇑ Corresponding author at: Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA. E-mail address: [email protected] (J. Ma). 0141-9382/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.displa.2010.12.006
Observed from a normal direction of a LCD cell, the angle between the absorption axes of the crossed polarizer films is 90°, which can yield a good black state. However, observed from an oblique direction, the effective angle is not 90°, as shown in Fig. 2. Therefore light leakage occurs in the black state [13–16]. The contrast ratio (CR) is decreased. CR = LWhite/LBlack, wherein LWhite and LBlack are the luminance in the white state and black state. The field of viewing angle at which the CR is greater than 10:1 is also decreased. In order to reduce light leakage in the black state, retardation films (compensation films) are introduced into polarizer film [17]. The compensation film has a certain retardation value, which optically compensates the retardation of the LC in the black state to increase the CR at oblique direction and the field of viewing angle. In this paper, based on the discussion of layered structure of polarizer film, property, function and manufacturing process are summarized. Principle of compensation film, examples in practical applications and technology development trends are also presented.
2. Polarizer film The working principles of polarizer film are similar for various transmissive TFT-LCDs except that different types of compensation films will be used for different display modes. The layered structure of a TN-type polarizer film is shown in Fig. 3. From the surface of the polarizer film to the LCD cell, the structure includes surface protection film (PF), tri-acetyl cellulose (TAC) film with surface treatment, uni-axially polyvinyl alcohol (PVA) layer, TAC film with wide viewing (WV) compensation layer, pressure sensitive
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Fig. 1. LC director and polarizer film configurations of a normally-white TN mode TFT-LCD in voltage-off state (left) and voltage-on state (right).
Fig. 2. Effective angle between the absorption axes of crossed polarizer films observed from a normal direction (left) and an oblique direction (right).
When the LCD panel is used, the PF is removed. In this case, the quality requirement of the PSA in the PF is high, where it should avoid residual sealant on the polarizer film front surface. TAC film is widely used as a protective and supporting film for the PVA layer by virtue of high light transmittance (>93%), low birefringence, high moisture resistance, high thermal-stability, high uniformity and good curling ability [23–26]. The chemical structure of the TAC is shown in Fig. 5, which has bulky, rigid main chain and bulky acetyl groups that give large intermolecular free volume [27]. The PVA layer is sandwiched between two TAC films, where the TAC prevents swelling and shrinking of the PVA and prevents the evaporation of iodine or dyes in the PVA when temperature and humidity of the polarizer film are changed. The TAC film is manufactured by solvent casting technology [28]. Plasticizer, dissolving agent, wetting agent, lubrication prescription and UV-absorber are added to help casting process. PVA layer is the functional layer to produce polarized light. The chemical structure of the PVA is shown in Fig. 6. The film of the semicrystalline PVA polymers containing iodine or other dichroic materials is stretched to obtain uni-axial orientation molecular film. During this stretching process, the dichroic materials are oriented with the linear PVA chains (Fig. 6). Thereby anisotropic absorption of light is generated due to the dichroic absorption, resulting in polarized light [29–33]. Iodine-PVA polarizer film is the most common commercial polarizer film for TFT-LCDs because of high transmittance, high alignment ability, good mechanical property and ease of manufacturing. Dye-doped PVA polarizer film is more suitable for out-door applications because it has high temperature and humidity resistance. Release film is located in the bottom of the polarizer film. It would be peeled off when the polarizer film is attached to a LCD panel. The chemical structure of the PET and the structure of the release film are shown in Fig. 7. Similar with the PET base film in the PF, the release film is a film with a silicone coating on the PET surface. The function of the release film is to protect the PSA layer before the polarizer film is attached. The silicone coating can help the base PET film easily to be removed from the PSA without residual sealant [34,35].
Fig. 3. Layered structure of a typical polarizer film for a TN mode TFT-LCD.
adhesive (PSA) layer and release film [18]. These functional layers compose a complex multi-layered polarizer film. Surface protection film (PF) is a flexible, heat-durability film of polyesters such as polyethylene (PE) or polyethylene terephthalate (PET). For high-quality TFT-LCD products, PET film is used. The structure of the PF is composed of anti-polluting layer, anti-static layer, PET, PSA and PET base film with silicone coating, as shown in Fig. 4. The function of the PF is to prevent damage, soiling or electrostatic discharge of the polarizer film during the processes of cutting, transporting and attaching to a LCD cell [19,20]. Different marks may be printed on the PFs on the front and rear polarizer films to avoid confusion. The functional treatments of anti-polluting and anti-static are introduced to enhance the protection for the polarizer film [21,22]. When the PF is laminated with the polarizer film in the manufacturing process, the PET base film is peeled off.
Fig. 4. Structure of the surface protection film.
Fig. 5. Chemical structure of the TAC molecule.
Fig. 6. Chemical structure and stretched uni-axial orientation of the PVA molecule.
J. Ma et al. / Displays 32 (2011) 49–57
Fig. 7. Chemical structure of the PET (upper) and structure of the release film (lower).
PSA is a kind of bonding material, which adheres upon a contact force or a slight pressure [36]. PSA is acrylic layer formed by alkyl acrylate monomer and cross-linking agent co-polymer such as epoxy-based and isocyanate-based compounds. It is used to attach the polarizer film to a LCD panel. It is also used as bonding material among the other layers in the polarizer film [37–39]. The sufficient bond strength and clean removability are two essential factors to optimize PSA property. It needs maintenance of sufficient bonding strength to avoid detaching the polarizer film from the LCD panel when the polarizer film is on the panel. It also needs clean removability from the LCD panel when the polarizer film is removed or replaced due to wrong-position or other failure issues. These two factors can be influenced by chemical composition, amount of cross-linking agent, use of plasticizer, surface treatment, thickness and construction of the PSA [40,41]. As mentioned above, the types of compensation films are different for various transmissive TFT-LCDs such as TN, IPS or MVA mode. Compensation film can be manufactured by LC coating, stretching, or addition of a retardation film into the polarizer film. When a compensation film is used, the total phase retardation d of the LCD is expressed as 0
d ¼ 2pðdDn d Dn0 Þ=k
ð1Þ
where d and Dn are the thickness and birefringence of the LC in the LCD cell, d0 and Dn0 are the thickness and birefringence of the Table 1 Classification of compensation films.
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compensation film in the polarizer film and k is the incident wavelength. To match Eq. (1), several types of compensation films have been proposed, for example, uni-axial (A-plate and C-plate), oblique (O-plate) and biaxial plate [42–46]. Table 1 shows the classifications of compensation films according to refractive indices [47]. For uni-axial anisotropic birefringent material, if ne (extraordinary refractive index) > no (ordinary refractive index), it is positive while if ne < no, it is negative. A-plate means the optical axis of the film is parallel to the film surface and C-plate means the optical axis of the film is perpendicular to the film surface. We can find, for C-plate, nx = ny. The examples of LC-type compensation film as A-plate and C-plate are given in Table 1. When the optical axis of positive rod-shaped LC is parallel or perpendicular to the film surface, the type is (+) A-plate or (+) C-plate. When the optical axis of negative disc-shaped LC is parallel or perpendicular to the film surface, the type is () A-plate or () C-plate. According to Eq. (1) and Table 1, the conceivable conjugations of compensation films and LC configurations in different LCD modes can be obtained. For IPS mode TFT-LCDs, positive LCs in the LCD cell are aligned and rotated parallel to the substrate. (+) C-plate, () A plate, biaxial compensation film or combination of multicompensation film may be used [48–53]. For MVA mode TFT-LCDs, negative LCs are aligned vertical to the substrate, thereby (+) Aplate, () C-plate, biaxial compensation film or combination of multi-compensation film may be adopted [54–60]. Different compensation films are used depending on sheet-to-sheet or rollto-roll manufacturing process and display performances. The stretching-type biaxial compensation film becomes a mainstream for high-performance IPS and MVA mode TFT-LCDs. TN mode TFT-LCDs are widely manufactured in the industry, of which the manufacturing process is more mature and simpler than IPS and MVA mode. They are suitable for many low-cost panels from small size displays (in several inches) for cell phones and cameras to large size displays (up to 26 inch) for monitors and televisions [61–63]. Nowadays the CR and viewing angle of TN mode TFT-LCD with compensation film have already exceeded 1000:1
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and 160° [64]. The key factor on continuous promotion of CR and field of viewing angle lies in LC coating compensation film technology. Here we discuss TN-type compensation film in detail. The TNtype compensation film (WV film) is a hybrid-aligned discotic LC (DLC) coating on an alignment layer on the TAC film, which three-dimensionally compensates the light leakage in all viewing directions of a TN mode TFT-LCD in the black state [65–71]. The chemical structure of the DLC and the structure of the WV film are shown in Fig. 8. The DLC is one of triphenylene derivatives, which has a disc-like molecular structure, high birefringence and is crossing-linking. The DLC right next to the alignment layer has a high degree of randomness. However, the DLC in the vicinity of the alignment layer tends to align with the molecular plane parallel to the alignment layer surface with few degrees of pretilt angle in the alignment direction, while the DLC in the vicinity of the air surface tends to align with the molecular plane almost perpendicular to the air surface [72–74]. Therefore, the DLC exhibits a hybrid alignment structure. This structure would be fixed by photo-polymerization of the cross-linking group in the DLC using UV light irradiation and this layer is so-called polymerized discotic material (PDM) [75]. When a WV-polarizer film is attached to a TN mode LCD panel, O-mode display is used, in which the absorption axis of the PVA is parallel to the adjacent rubbing direction of the polyimide (PI) on the substrate [76–78]. The alignment direction of DLC is parallel to the absorption axis of the PVA layer so that the WV film can be laminated in the longitudinal direction in the roll-to-roll process. The configuration of alignment direction of the DLC in the WV film, absorption axis of the PVA layer and rubbing direction of the PI is shown in Fig. 9. The idealized and simplified model of optical compensation for the TN mode TFT-LCD using a WV film is shown in Fig. 10. In the black state, the orientation of the LC continuously changes from the homogeneous alignment near the PI on the substrate to the homeotropic alignment in the LC bulk along the thickness direction, while the direction of each DLC layer in the hybrid alignment continuously changes in the WV layer thickness direction consisting of splay and bend deformation to match and compensate the LCs in the TN mode LCD cell. The LC in the LCD cell has a positive birefringence while the DLC in the WV film has a negative birefringence. Since they have complicated opposite alignment structure, they can compensate each other, resulting in higher CR and larger field of viewing angle. Based on different designs and specifications, WV film have been developed three generations till now, i.e., wide viewing angle (WV-A), super wide viewing angle (WV-SA) and excellent wide viewing angle (WV-EA) film [79–82]. The WV-EA film is more suitable for large size, wide aspect ratio LCD panels. Typical viewing angles at the contrast ratio of 10:1 for horizontal (H) and vertical (V) direction are 130°/100°, 145°/135° and >160°/>160° for WVA, WV-SA and WV-EA film, respectively. The iso-contrasts of the
Fig. 9. Relationship of the alignment direction of the DLC in the WV film, the absorption axis of the PVA layer and the rubbing direction of the PI in the TN mode TFT-LCD with compensation film.
Fig. 10. Idealized and simplified model of optical compensation for a TN mode TFTLCD using a WV film.
TN mode TFT-LCD compensated with these three types of WV films are shown in Fig. 11. The differences among these WV films are the thickness of the PDM layer, the orientation angle of the DLC and thickness retardation Re = [(nx + ny)/2–nz]d0 . Here, nx is main refractive index within the WV film plane, ny is refractive index perpendicular to the direction of nx within the plane and nz is refractive index in the thickness direction of the WV film. The thickness of the PDM, Re and average angle of the DLC of WV-A, WV-SA and WV-EA film are measured and summarized in Table 2. To design new type WV film, the mura issue (light leakage induced by temperature gradient and non-uniform thermal stress distribution), CR and color shift at oblique viewing angle should be considered more. Some surface treatments have been introduced to the polarizer film, such as anti-reflection (AR) [83,84], anti-glare (AG) [85–87] to
Fig. 8. Chemical structure of the DLC molecule (left) and structure of the WV film (right).
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Fig. 11. Iso-contrast of a TN mode TFT-LCD: (a) without WV film, (b) with 1st generation WV-A, (c) with 2nd generation WV-SA and (d) with 3rd generation WV-EA. (This Figure was reproduced from Review of viewing angle compensation of TN mode LCDs using WV film, S. Yasuda, T. Ito, T. Oikawa, Y. Ito, Y. Takahashi, published in Journal of the Society for Information Display, vol. 17, Issue 4, 2009, with permissions by The Society for Information Display.)
Table 2 Summary of WV films for TN mode TFT-LCDs.
Thickness of PDM (lm) Re (lm) Average angle of DLC (degree) Field of viewing angle (H/V, degree)
WV-A
WV-SA
WV-EA
1.5 136 16.0 130/100
2 156 18.9 145/135
1.7 155 16.0 >160/>160
prevent or decrease the glare of LCDs and hard coating [88] to prevent scrapes for the front polarizer film. Dual brightness enhancement film (DBEF) have been developed to increase the utilized efficiency of backlight for the rear polarizer film [89]. As examples, the working principles of the AG and DBEF are shown in Fig. 12. The AG-type polarizer film has a dispersion layer with tiny particles on the surface. The formed fine undulations in the rough surface scatter light in multiple directions to avoid the reflective light directly entering eyes. The directly reflective light is fatiguing to eyes whereas the AG treatment makes the displays pleasant. The DBEF-type polarizer film can recycle the reflected backlight to increase the polarized light transmittance through the rear polarizer film. It also reduces the backlight unit thickness and cuts assembly cost. These treatment technologies enhance the functions of polarizer films and make the display performances better.
3. Manufacturing process of polarizer film 3.1. TAC film manufacturing process Solvent casting technology is versatile for thin film production. The TAC film is produced by this technology [23–28]. Fig. 13 shows the TAC film manufacturing process. The solution containing tips of the TAC, dichloromethane solvent and additive is filtrated and cast on a metal band. The formed polymer film is peeled off from the metal band after some drying. In the following drying zone, residual solvent (>99%) is dried and the film is made for a roll. Uniform, transparent TAC film with very small fluctuation of thickness is obtained.
Fig. 13. Illustration of solvent casting process of a TAC film.
3.2. Polarizer film manufacturing process The iodine-doped production method is used to fabricate polarizer film for TFT-LCDs. The main manufacturing process is pretreatment, dyeing and stretching, lamination, cutting and inspection of defects [90,91], as shown in Fig. 14. Because of the flexibility of polymer film, a roll-to-roll process can be used. During the roll-to-roll production, recipe, consistency and temperature of the I2 solvent, rate of stretching process and drying temperature are critical processes. Polyvinyl alcohol polymer, water-soluble cross-linking agent, plasticizer and surfactant are dissolved in water and this aqueous polyvinyl alcohol polymer solution is cast to form the PVA film by the process of casting, drying and heattreating. Then the PVA film is dyed by iodine and stretched to form uni-axial orientation molecular film. The TAC, the PF and the release film are laminated to the PVA layer sequentially. At last the wide polarizer film is cut as favorite size and is inspected under a light to remove defective products. Polarization degree of a polarizer film can be decided by
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðT p T c Þ 100% Polarization degree ð%Þ ¼ ðT p þ T c Þ
ð2Þ
where Tp is parallel transmittance of horizontally overlapped polarizer films and Tc is crossed transmittance of vertically overlapped polarizer films [92]. Usually the specification of the polarization degree is >99.7%. Typical Tp and Tc of a WV-EA type polarizer film are measured and shown in Fig. 15. If the transmittance of Tc at blue or red region is high, the hue of the LCD is bluish or reddish in the black state. This issue can be adjusted by dyeing and stretching process to control the polarization degree. 3.3. Attachment method and process
Fig. 12. Principles of the AR (left) and DBEF (right) in the polarizer film.
Attachment method and process of a polarizer film to a LCD panel are shown in Fig. 16. The release film of the polarizer film is peeled off and the PSA layer is attached to the panel through a set of rollers. The equipment can auto-position the polarizer film
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Fig. 14. Schematic manufacturing process of a polarizer film.
certain temperature and pressure. The autoclave equipment can also increase the adhesive strength in an instant. If there is a demand to remove or replace a polarizer film from a panel due to defects, wrong-position or evaluations of different types of polarizer films using a same panel, special tools or peeling machines are needed [94,95]. These tools can peel the polarizer film from the panel and prevent damage to the panel. 4. Examples and discussions 4.1. Light leakage
Fig. 15. Transmittance (Tp and Tc) of a WV-EA type polarizer film.
Fig. 16. Attachment method and process of a polarizer film to a LCD panel.
and the panel, separate the release film from the polarizer and attach the PSA layer to the panel [93]. After the rear polarizer film is attached to the panel, the panel is turned over and the front polarizer film is attached to the opposite side of the panel. The last procedure of the polarizer film attachment is to remove air bubbles using autoclave equipment (bubble-removing machine) under
Light leakage of polarizer film induced by temperature gradient and non-uniform thermal stress distribution from backlight is the most normal and serious problem for a LCD panel [96–98]. The light leakage of a TN mode LCD panel in the black state is shown in Fig. 17 (left). The light leakage phenomenon is around the edge of the panel, especially after long-term durability test of high temperature and high humidity. The stretched PVA layer tends to relax back to original random alignment state upon heat and humidity that causes shrinkage of the polarizer film. Non-uniform thermal stress distribution makes the polarizer film distortion. The degree of the shrinkage is different between the TAC film and the PVA layer, where the shrinkage force of the PVA layer is larger than the TAC film (Fig. 18). The uniform of the PVA alignment is broken and the retardation of the TAC is changed. Therefore light leakage occurs. The behavior of light leakage under a hot air blower can prove it (Fig. 17 right). To enhance the durability of polarizer film, PSA recipe and process are adjusted to alleviate light leakage [99,100]. The distorted retardation R can be described as
R ¼ E F dt
Fig. 17. Light leakage of a TN mode LCD panel in the black state (left) and behavior under a hot air blower (right).
ð3Þ
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4.3. Polarizer film bubbles When polarizer film is stacked up or stored in the package, particles or residual sealants on the polarizer film surface will produce dents in the flexible films due to the gravity. The dents on the release film side could induce random polarizer film bubbles after attachment process, as shown in Fig. 20. The dent is vital if the bubble induced by the dent cannot be removed through auto-clave process. To decrease dents, the number of the particle should be under control in the clean-room. In addition, the vacuum degree of the polarizer film package can be decreased and fewer packages can be put in one tray to alleviate dents (Fig. 21).
Fig. 18. Distortion of a polarizer film upon high temperature.
where E is photoelastic coefficient of the TAC film, F is the shrinkage stress on the TAC film and dt is the thickness of the TAC film. The stress F can be defined as
F ¼ rðA A0 Þ
ð4Þ
where r is coefficient of the TAC film, A is the shrinkage of the polarizer film without the PSA and A0 is the shrinkage of the polarizer film with the PSA [99]. We can find that the property of the PSA is very important to reduce the shrinkage stress between the glass and the TAC film according to Eqs. (3) and (4). Large relaxation capacity of the PSA is considered as a key parameter to alleviate the deformation of each film. Low glass transition temperature copolymers, doping of plasticizers or low molecular weight acrylic polymers and decreasing cross-linking agents have been introduced to the PVA recipe and process [101,102]. Through these methods, the light leakage can be improved.
4.4. PSA bubbles After durability test of high temperature and high humidity, a cluster of bubbles in the PSA layer were observed. The shape of the bubble near the polarizer film edge was long and inclined from the corner to the center of the panel. Whereas the shape of the bubble near the center was more circular, as shown in Fig. 22. The inclined directions are same with the stretching direction of the polarizer film (45° and 135° for the front and rear polarizer film, TN mode LCD panel). The focus distance of an optical microscopy can deduce the bubble positions exactly in the PSA layer. The formation mechanism of this PSA bubble occurred after durability test is explained using moisture and gasification
4.2. Foreign materials During the processes of manufacture and attachment of polarizer film, foreign materials involved would degrade display performance if the sizes of the foreign materials exceed the limitation samples. Several foreign materials such as fiber, residual sealant, particle and dust are shown in Fig. 19(a)–(d), respectively. Most of foreign materials locate in the PSA layer. These foreign materials may block the light or change the retardation of the TFT-LCDs, resulting in light spots. A Fourier Transform Infrared (FTIR) spectrometer can be used to detect the element of the foreign material. The spectrum of the foreign material is compared with the spectrums in the database to distinguish the elemental composition. Thus the foreign material sources can be found. It needs rework (replace) the polarizer film if the foreign materials degrade the displays.
Fig. 20. Bubbles in the polarizer film due to dent.
Fig. 21. Tray and vacuum packaging of the polarizer film.
Fig. 19. Foreign materials in the polarizer film.
Fig. 22. Bubbles in the PSA layer in the panel (left) and under a microscope (right). Red (Blue) arrow means the direction of the front (rear) polarizer film shrinkage. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 23. Formation hypothesis of the PSA bubble.
hypothesis [100]. Fig. 23 shows the formative process. Moisture, residual solvent or monomer within the PSA layer is gathered around foreign object or dent. When the temperature is increased, gasification occurs. The bubble forms and further expands. The shrinkage of the polarizer film upon high temperature makes the bubble elongate along the stretching direction. The stress near the edge of the polarizer film is larger. Consequently the bubble near the edge is longer than those near the center. Reducing the substances of moisture, residual solvent or monomer, and adopting polymer with lower level of water absorbability in the PVA can prohibit gasification and expansion, reducing PSA bubbles. 5. Conclusions Polarizer film is an essential component for TFT-LCDs. The layered structure, property, function, process and applications of polarizer film are summarized and discussed in this paper. These factors should be considered when new type polarizer film is designed and developed. With the expansion of TFT-LCDs as monitors and TVs, development of polarizer film continues to focus on highperformance and low-cost products. Technology development trends of polarizer film include durability improvement to reduce light leakage, contrast ratio improvement to obtain high contrast, viewing angle improvement by design of new type compensation film, hue improvement to obtain neutral color by suppression of high transmittance in the short-wavelength range and surface treatment technology developments such as high-resolution AG treatment, AR treatment and hardness enhancement. The size of polarizer film needs to be increased to fit larger TFT-LCD panel. New processes, new materials and new structures of polarizer film also need to be developed to expand production capacity. Thinner thickness, higher durability and more function properties will be introduced to polarizer film to meet the requirements of future high-performance TFT-LCDs. References [1] S.-T. Wu, D.-K. Yang, Reflective Liquid Crystal Displays, John Wiley & Sons Inc., New York, 2001. [2] K.-H. Kim, J.-K. Song, Technical evolution of liquid crystal displays, NPG Asia Mater. 1 (2009) 29–36. [3] C.T. Liu, Revolution of the TFT LCD technology, J. Display Technol. 3 (2007) 342–350. [4] M. Oh-e, M. Ohta, S. Aratani, K. Kondo, Principles and characteristics of electro-optical behavior with in-plane switching mode, Asia Display’95 Digest (1995) 577–580. [5] A. Takeda, S. Kataoka, T. Sasaki, H. Chida, H. Tsuda, K. Ohmuro, Y. Koike, T. Sasabayashi, K. Okamoto, A super-high-image-quality multi-domain vertical alignment LCD by new rubbing-less technology, SID’98 Digest (1998) 1077– 1079. [6] R.W. Sabnis, Color filter technology for liquid crystal displays, Displays 20 (1999) 119–129. [7] S.S. Kim, The word’s largest (82-in.) TFT-LCD, SID’05 Digest (2005) 1842– 1847. [8] J. Ma, Y.-C. Yang, Z. Zheng, J. Shi, W. Cao, A multi-domain vertical alignment liquid crystal display to improve the V-T property, Displays 30 (2009) 185– 189. [9] E.H. Land, Light polarizer and process of manufacturing the same, US Patent, 2237567, 1941. [10] E.H. Land, Light polarizer and process of manufacture, US Patent, 2328219, 1943. [11] P. Yeh, C. Gu, Optics of Liquid Crystal Displays, John Wiley & Sons Inc., Hoboken, 2009.
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