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Effects of thermomechanical properties of polarizer components on light leakage in thin-film transistor liquid-crystal displays

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Japanese Journal of Applied Physics 54, 076701 (2015) http://dx.doi.org/10.7567/JJAP.54.076701

Effects of thermomechanical properties of polarizer components on light leakage in thin-film transistor liquid-crystal displays Taiy-In Lin1, Alexander Chen1, Shou-I Chen2, and Jihperng Leu1* 1

Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan 30049, R.O.C. Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu, Taiwan 30261, R.O.C. E-mail: [email protected]; [email protected] 2

Received January 21, 2015; accepted April 14, 2015; published online June 3, 2015 In this paper, we present static thermal analysis of stress and strain on a thin-film transistor liquid-crystal display (TFT-LCD) panel and their correlation with light leakage phenomena under high-temperature durability test. Three-dimensional (3D) finite element analysis (FEA) is coupled with experimental parameters of key components of the TFT-LCD panel for the analysis. A strong correlation exists between light leakage and retardation difference induced by stress on triacetyl cellulose (TAC) films. Moreover, shrinkage in stretched poly(vinyl alcohol) (PVA) film and modulus of the adhesive layer are key factors affecting stress distribution and displacement of polarizer stack. An increase in Young’s modulus (E) of the adhesive layer effectively reduces polarizer shrinkage and light leakage at the center of the panel. A TAC film with lower Young’s modulus and/or coefficient of thermal expansion (CTE) is also an effective solution. © 2015 The Japan Society of Applied Physics

1.

Introduction

Contrast ratio is one of the important indicators of quality of a thin-film transistor liquid-crystal display (TFT-LCD). For instance, light leakage occurs when light from the LCD unit is not completely blocked and leads to loss of detail and contrast of display.1–3) Such problems are generally identified and rectified during product durability test. Typically, TFTLCDs are aged at a standard high temperature (60 °C) for durability evaluation.4,5) Depending on the Young’s modulus of pressure-sensitive adhesives (PSA) layer, two light leakage phenomena are observed in twisted nematic (TN) mode displays after durability test: (1) light leakage that is concentrated on the edges of LCD panel having hard PSA (PSA-h) and (2) funnel-shaped light leakage in LCD with soft PSA (PSA-s), as shown in Fig. 1.6) Product reliability concerns have stimulated significant interest in stress characteristics and their impact on durability and reliability of LCDs. The effect of stress on light leakage phenomenon has been explained with numerous mechanisms including thermally induced light leakage due to phase retardation of liquid crystal (LC),1) polarizer shrinkage induced bending of poly(vinyl alcohol) (PVA) absorption axis,7) warpage of LCD panel,8,9) disturbed orientation of LC due to stress,10) and thermal stress on glass due to nonuniform temperature distribution.11,12) Recently, three-dimensional (3D) finite element analysis (FEA) has been employed to analyze the probable causes for light leakage. Chen et al. used computational fluid dynamics tools to analyze light leakage in TN mode LCDs and found that uneven temperature distribution in LCD modules with backlight generates a nonuniform thermal stress distribution on the glass substrate, thus resulting in light leakage.13) Chu et al. used a thermal fluid analysis model (ANSYS CFX) to analyze a 23-in. TFT-LCD panel. They attributed the degrading image quality of LCD TVs to thermal deformation due to heat sources including cold cathode fluorescent lamps (CCFLs).14) Moreover, Leu et al. performed transient thermal analysis on a 13-in. TFT-LCD using a 3D FEA model and reported strong correlation between chip-on-glass (COG) package-induced light leakage phenomena and localized warpage or principal stress.9) These studies have reported that stress=strain distribution is a major contributor to light

Fig. 1. (Color online) Light-leakage phenomena of TFT-LCD with different PSA layers after durability test: (a) panel=PSA-h and (b) panel= PSA-s.

leakage. Nevertheless, the root cause of polarizer leakage phenomenon in TFT-LCD panels under high-temperature reliability tests and the effects of thermomechanical properties of polarizer components on light leakage have not yet been fully clarified. In this study, a 3D simulation model coupled with static thermal analysis using finite element method is employed to examine stress distribution in multilayered structures consisting of components of a TFT-LCD under high-temperature reliability test. We first focus on identifying the root cause of the polarizer leakage phenomenon. Thereafter, we investigate the effect of temperature-dependent material properties of polarizer components on light leakage and determine the key material properties affecting light leakage. The selection and design of key components such as PSA and TAC films is also recommended. 2.

Experimental methods

The modulus of the triacetyl cellulose (TAC) film was meas-

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Table I. Material properties of the glass, LC, PSA, and polarizer components (PVA and TAC) at room temperature. Material

t (µm)

E (MPa)

CTE (ppm=°C)

ν

Tg (°C)

Glass15)

700

70000

3.25

0.23

N=A

4

2000

0

0.3

N=A N=A 190

15)

LC

PVAa) TAC

20

MD: 16000 TD: 3140

MD: 1 TD: 60

PRXY: 0.3 PRYZ: 0.49 PRZX: 0.3

40

0.3

20–90

0.49

40

3500

b)

PSA-h

25

0.25

PSA-sb)

25

0.04

−26 −37

a) Ref. 8 (MD is the direction of stretching and TD is the vertical direction in PVA). b) Supplied by Innolux Corporation.

ured with a dynamic mechanical analyzer (TA Instruments DMA 2980) under N2 atmosphere from room temperature to 250 °C at a heating rate of 3 °C=min and a frequency of 1 Hz. The coefficient of thermal expansion (CTE) of the TAC film was obtained with a thermomechanical analyzer (TMA; Mettler-Toledo TMA=STDA 841e) from room temperature to 100 °C at a heating rate of 3 °C=min. The elastic modulus of the PSA film was measured with a microforce testing system (MTS Systems Tytron™ 250) at a force of 0.001 N. The material properties of glass, LC, PSA, and the polarizer components (PVA and TAC) are summarized in Table I.8,15) Thermomechanical stress analysis of the LCD panel was performed via 3D FEA coupled with a static thermaldependent analysis model using the ANSYS™ computer simulation tool. A 3D full structural model is developed for the TFT-LCD panel consisting of a basic polarizer (one stretched PVA film sandwiched by two TAC films) with PSA on both sides of the LC cell, as shown in Fig. 2. For the panel of size 417.15 × 237.8 mm2, the elemental mesh density is 123200 elements with 137052 nodes, as shown in Fig. 3. The selected solid element of stress analysis model is SOLSH190, which is used for simulating shell structures with a wide range of thicknesses (thin to moderately thick). The element

Fig. 2. Cross-sectional diagram of Polarizer=PSA multi-layered structure attached on both sides of LC cell.

possesses continuum solid element topology and features eight-node connectivity with three degrees of freedom at each node, namely, translations in the nodal x-, y-, and zdirections. Therefore, SOLSH190 can be easily connected with other continuum elements. In order to achieve good balance between accuracy and efficiency, the following assumptions are made in this numerical model. (1) All the materials except the PVA film are elastic and isotropic owing to their limited displacement in the temperature range. In the PVA film, the direction of stretching and its perpendicular direction are referred to as MD and TD directions, respectively. The MD directions of the upper and lower PVA films are orthogonal. (2) All the components in the global model have perfect adhesion between one another. (3) For static thermal analysis, the temperature is reduced uniformly from 60 °C to room temperature (25 °C) to simulate durability test under varying thermal loads.

Fig. 3. (Color online) Schematic diagram illustrating the full structure model used in 3D finite element computations.

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Simple symmetric boundary conditions are applied on the four bottom edges of the glass substrate to avoid rigid motion. The mid-nodes on these four bottom edges are also fixed in the x- or y-directions to avoid rotation induced by the local coordination system difference in PVA films. The twist angle in the xy plane of the upper PVA film is 45° whereas that of the lower PVA film is −45°. 3.

(a)

Results and discussion

3.1 Static temperature-dependent stress analysis of full multilayered structure

In a transparent material exhibiting temporary birefringence, variations in refractive indices are linearly proportional to applied stresses. This is referred to as stress-optic law16) and can be expressed in terms of relative retardation given by n1 n0 ¼ C1 1 þ C2 ð2 þ 3 Þ;

ð1Þ

n2 n0 ¼ C1 2 þ C2 ð1 þ 3 Þ; ð2Þ n3 n0 ¼ C1 3 þ C2 ð1 þ 2 Þ; ð3Þ where σ 1 , σ 2 , σ 3 are the principal stresses, and n0 is the index of refraction of the material in unstressed state. Further, n1, n2 , n3 are the refractive indices in the direction of principal stresses, and C1, C2 are stress optical coefficients. When plane-polarized light passes through, the phase retardation (R) due to the difference in refraction indices can be expressed by R¼

2h 2hc n ¼ ;

(b)

Fig. 4. (Color online) Retardation graphs predicted by the static thermal stress analysis for various TFT-LCDs with different PSA layer: (a) PSA-h and (b) PSA-s.

ð4Þ

where h is the length of the path, i.e., film thickness in this case, and λ is the wavelength of light. Further, Δσ is the principal stress difference, i.e., the difference between the maximum and minimum principal stress. Correspondingly, the intensity of light leakage (I) through the LC medium under crossed linear polarizers can be given by17,18) 1 R I ¼ sin2 ð2’Þ sin2 ; ð5Þ 2 2 where φ is the twist angle relative to the transmission axis of the polarizer caused by the applied field. In a basic TN mode panel, linearly polarized light is generated when incident light passes through the rear polarizer and follows the twisted LC molecular structure to transmit (normally white mode) or block (normally black mode) light through the front polarizer film. Driving voltage is applied to the LC to modulate the output-light intensity through the LCD cell to generate gray levels. Therefore, stress-induced retardation only appears on the stack of layers between two separated polarizers, i.e., the two TAC films, two vertically separated PVA films, two PSA layers, glass layers, and LC cell. Specifically, stress on the PSA layers is approximately 30 times lower than that on the TAC films from static thermal simulation analysis. In addition, in the case of the LC cell, stress is generated only on the glass surface. For all analyzed conditions, maximum warpage of glass is 0.042 µm, i.e., only 0.003% of substrate thickness. It should be noted that these results have not been included. Therefore, the majority of stress-induced retardation occurs on the two TAC films. From Eqs. (4) and (5), light leakage intensity primarily depends on φ and Δσ. In order to save calculation time, henceforth, retardation is expressed as the

difference in principal stresses (Δσ) between the two TAC films in the polarizer stack. Figures 4(a) and 4(b) show the retardation simulation results in terms of the principal stress difference (Δσ) between the two TAC films in the TFT-LCD panel with PSA-h and PSA-s, respectively. The principal stress difference (Δσ) has a wide range of 0 to 9 MPa. Nevertheless, the higher Δσ values (Δσ ≧ 1.125 MPa is arbitrarily chosen for illustration purpose) are primarily limited to a few millimeters from the edges of the panel. From Eq. (5), light leakage intensity will be greater in areas having higher Δσ values. The simulation contour plots of principal stress difference shown in Figs. 4(a) and 4(b) are in good agreement with the light leakage phenomena shown in Figs. 1(a) and 1(b). For the PSA-h in Fig. 4(a), the higher Δσ values (Δσ ≧ 1.125 MPa), i.e., larger retardation, are concentrated on the edges of the panel rather than that at the center, thus showing substantial inhomogeneity. Contrarily, for the LCD panel with PSA-s, the area of higher Δσ values (Δσ ≧ 1.125 MPa) increases from the edges to the center. The extent of principal stress difference expands several centimeters from the edges into the center, thus affecting a large area of the panel. Therefore, our 3D simulation model based on static thermal analysis for light leakage phenomena is validated by the retardation prediction in terms of principal stress difference. In addition, stressinduced retardation on TAC films is the most critical contributor to the light leakage phenomenon. We further extend this 3D simulation model to examine the effects of thermomechanical properties (Young’s modulus and CTE) of polarizer components such as PSA layer and TAC film on light leakage. Selection of materials and design of polarizer components for reducing light leakage will be also addressed in the following sections.

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Fig. 5. (Color online) The prediction of the retardation graphs (TAC) and mechanical strain (ε) graphs (PSA) with varying the modulus of PSA.

3.2 Effect of Young’s modulus of PSA layer on light leakage

The effect of Young’s modulus of the PSA layer on light leakage was investigated by varying the modulus from 0.001 to 0.5 MPa. The simulation results of retardation graphs (TAC) and the mechanical strain (ε) maps of the upper and lower PSA layers are listed in Fig. 5. The shape of the prediction graph depends strongly on the elastic modulus of the PSA layer. For a very soft PSA system (0.001 MPa), light leakage is nearly uniform over the entire LCD panel. In contrast, light leakage is concentrated on the edges of the LCD panel for the PSA-h (0.5 MPa) whereas it is funnel shaped for the PSA with medium modulus (0.04 MPa). The effect of PSA modulus on light leakage can be understood from strain (ε) mapping shown in Fig. 5. The PSA with higher modulus exhibits higher resistance to deformation. High strain is concentrated only near the edges of the panel. For an elastic modulus ≤0.04 MPa, strain spreads inward and diagonally. In addition, maximum strain is found to occur in the direction parallel to the transmittance axis (TD), which is perpendicular to the direction of PVA film stretching. Moreover, the strain mapping results of the upper and lower PSA layers are in mirror symmetry by position. This can be attributed to the higher resistance to thermal deformation in MD direction of the PVA film, which has a higher molecular orientation parallel to the stretching direction. Therefore, a softer PSA results in higher strain defor-

Fig. 6. Shrinkage values of the polarizer predicted by the static thermal stress analysis with different moduli of PSA.

mation in the region shown (Fig. 5), thus leading to a higher degree of light leakage in its corresponding patterns. After determining the effect of PSA on TAC films, namely light leakage, the role of PVA polarizer film is further examined. The impact of PSA modulus on PVA polarizer film shrinkage is shown in Fig. 6. Polarizer shrinkage induced by a PSA-s of 0.001 MPa is approximately 480 µm, which is approximately 6 times greater than that in the case of the hardest PSA of 0.5 MPa. Therefore, the elastic modulus of PSA restricts the PVA film in the polarizer stack. Greater

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shrinkage of the polarizer causes larger deformation of the PSA layer, as summarized in Fig. 5. The softer PSA tends to induce larger deformation, thus leading to more uniform light leakage in the whole panel. Contrarily, the harder PSA induces much less deformation and causes the least shrinkage of the PVA film, thus restricting light leakage near the edges of the panel. In the TFT-LCD panel shown in Fig. 2, the PSA layer serves as a stress buffer layer and dissipates the stress induced by a mismatch between the modulus and the CTE between the polarizer and the LC cell (glass=LC=glass). For PSA-h, the rigidity of its relatively high modulus transfers the induced stress to the TAC film with minimal deformation in the PSA, thus resulting in a relatively high stress on the TAC film. In contrast, the PSA-s with relatively low modulus readily undergoes plastic deformation to minimize the stress induced by the mismatch between the CTE and the elastic modulus between the LC cell and the polarizer, thus leading to weak restriction of the polarizer, namely, more shrinkage in the stretched PVA film. The simulation results show that the hard PSA layer induces limited PVA shrinkage and reduces the retardation at the center of the panel. Therefore, a PSA layer with higher mechanical strength provides better structural integrity and optical performance in terms of light leakage. Recently, Nagamoto et al. reported an LCD having a PSA with storage elastic modulus higher than 0.38 MPa to be practically usable with significantly reduced light leakage.19) Numerous methods for enhancing the mechanical strength of PSA such as adding an oligomer crosslinker to PSA20) and adding spherical poly(methyl methacrylate) microbeads to PSA21) have also been proposed. 3.3 Effects of thermomechanical properties of TAC film on light leakage

The role of TAC film on light leakage is discussed in this section. In addition, we will explore a strategy of materials selection and design of TAC film by analyzing the effect of elastic modulus and=or CTE of TAC film on polarizer light leakage in an LCD with PSA-h. Figure 7(a) shows the iso-retardation simulation graphs for an LCD panel with a PSA layer of 0.25 MPa (PSA-h) with different modulus and CTE values of TAC film. In order to quantitatively evaluate the extent of light leakage, we use specific area percentage (%), which is the ratio of the number of qualified nodes (Δσ ≧ 1.125 MPa) to the total number of nodes. Figure 7(b) shows the extent of light leakage as a function of CTE and modulus of TAC film. A linear relationship can be seen between light leakage and the modulus and the CTE of the TAC film. Therefore, the linear curve-fit equation shown in Fig. 7(b) can be used to predict the corresponding extent of light leakage once the thermomechanical properties of the TAC film are experimentally determined. For example, for a TAC film with a CTE of 40 ppm=°C and E of 3.5 GPa, the light leakage area is approximately 19.5%. When the modulus of the TAC film is increased to 5.25 GPa (CTE maintained at 40 ppm=°C) or its CTE is increased to 60 ppm=°C (E maintained at 3.5 GPa), the area is increased to 30.8 and 25.6%, respectively. Specifically, as the modulus or CTE of the TAC film decreases, light leakage area percentage decreases. A smaller CTE of the TAC film implies smaller CTE mismatch with the glass substrate, thus leading

(a)

(b)

Fig. 7. (Color online) (a) Iso-retardation simulation graphs and (b) predict specific area (Δσ ≧ 1.125 MPa) with varying the Young’s modulus (E) and the CTE of TAC film.

to lower induced stress or retardation. In summary, the TAC film serves as a stress reliever and reduces the thermal stress created by the mismatch between the PVA and the glass and the shrinkage stress in the stretched PVA film. A TAC film with lower modulus has better stress relieving ability and thus, minimizes light leakage. Therefore, a reduction in CTE or modulus of TAC film is effective in reducing light leakage. Light leakage can be more effectively minimized by lowering the modulus of the TAC film. 4.

Conclusions

In this study, we performed static thermal analysis on the stress=strain of a TFT-LCD panel using 3D FEA to identify the root cause of light leakage and recommend solutions for minimizing it. Excellent correlations exist between simulated retardation graphs and light leakage in panels with two different PSA layers. The FEA analysis showed that stressinduced retardation on TAC films is the most critical factor contributing to light leakage phenomenon. Furthermore, the effect of Young’s modulus of the PSA layer on light leakage phenomena was investigated. For PSA-h, the area of higher principal stress difference is relatively small and localized on the edges of the panel, thus indicating low dimensional variations. In contrast, funnel-type light leakage and greater polarizer shrinkage were found for PSA-s. This warrants new material design and chemistry to ensure higher modulus of PSA film to minimize light leakage. Furthermore, a TAC film with lower Young’s modulus (E ) and=or CTE is an effective solution for reducing light leakage. Acknowledgment

The authors thank the Ministry of Science and Technology, Taiwan, R.O.C. (NSC 103-2221-E-009-181-MY3), Innolux Corporation for the financial support in past, and the National Center for High-Performance Computing (NCHC), Taiwan, R.O.C. for the support of ANSYS™ software and computation analysis.

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