We propose a new electrode structure for the fringe-field switching mode, which not only has a smaller color shift but also transmits more light than the chevron-type structure. While the chevron-type electrode structure mainly uses the different directions of the electric field, the proposed structure makes use of position-dependent strengths as well as different directions of the electric field. Position-dependent electric field strengths bring about rotation of LCs different from position to position in a pixel, which further reduce the color shift by using the multi-domain effect.
© 2007 Optical Society of America
Cathode ray tube displays (CRTs) have been replaced by liquid crystal displays (LCDs) during the past 10 years. LCDs have many advantages over CRTs; they are lighter, consume less power, and are slimmer. However, since LCDs utilize the anisotropy of liquid crystals, LCDs have always had problems with viewing-angle tone and color shift when viewed from oblique directions. Limited viewing angles have been overcome by introducing several LC modes. Among them, horizontal switching LC modes, which are represented by in-plane switching (IPS) and fringe-field switching (FFS) modes, show the best performance for wide viewing angles [1–2].
The wide viewing angle of the IPS mode is based on the in-plane rotation of the LCs between the electrodes. The LCs above the electrodes, however, are rarely rotated by the applied electric field so that the amount of light transmitted in the IPS mode is less than that in the twisted nematic (TN) LC mode [2, 3]. However, in the FFS mode, a horizontal electric field as well as a vertical electric field exist over the entire liquid crystal medium so that even the LCs above the electrodes are rotated, resulting in a greater transmission of light .
Nevertheless, in the conventional single-domain electrode structure where the LCs are rotated in only one direction, color shift occurs at off-normal directions, especially when viewed perpendicular or parallel to the LC director in the bright state. The chevron-type electrode structure was proposed to overcome this problem, where two different directions of electric fields are formed in each pixel so that the LCs can be rotated in two opposite directions, i.e. clockwise or counterclockwise [4–6]. The chevron-type electrode structure results in much smaller color shift than the single-domain electrode structure. However, the color shift does not fully compensate for all gray levels.
In order to reduce the color shift further, we propose a new electrode structure for the FFS mode which not only has a smaller color shift but also a greater light transmission than the chevron-type structure does. The chevron-type electrode structure reduces color shift in off-normal directions by utilizing two different directions of the electric field. In contrast, the proposed structure is based on modulating the distance between electrodes, which brings about position-dependent strengths as well as different directions of the electric field. Position-dependent electric fields contribute to generate rotation of LCs different from position to position in a pixel, which further reduces the color shift by using the multi-domain effect. Moreover, the amount of transmitted light is greater because of the strong in-plane electric field, while maintaining as wide a viewing angle as the chevron-type electrode structure does.
2. Electrode structure for color shift reduction
The normalized light transmission of a nematic LC layer under crossed polarizers is given by
where Ψ is the angle between the input polarizer and the LC director, d is the cell gap, Δn(θ,φ) is the birefringence of the LC layer dependent on polar (θ) and azimuth (φ) angles of the incident light, and λ is the wavelength of the incident light. There are two independent factors that influence the total light transmission, which are the angle, Ψ, and the phase retardation, πdΔn(θ,φ)/λ.
Given that the phase retardation is a constant, light transmission can be increased only by the angle, Ψ. In that case, light transmission is not dependent on wavelength. On the other hand, the change of the phase retardation, d Δn(θ,φ), shifts the wavelength of peak transmission so that the color is changed. The phase retardation, πdΔn(θ,φ)/λ, influences color characteristics as well as the transmittance, whereas the angle, Ψ, changes only the amount of light transmitted. Thus, white shifts to bluish or yellowish colors as the value of dΔn decreases and increases, respectively. In other words, white can look bluish or yellowish depending on viewing angle in a single-domain structure, as shown in Fig. 1. This color shift problem can be corrected by the chevron-type electrode structure .
In the chevron-type structure, pixel electrodes are patterned in chevron shapes as shown in Fig. 2(a) so that two different directions of electric field are generated in each pixel, which can rotate the LCs in two opposite directions, i.e. clockwise or counterclockwise. That is, each pixel is divided into two domains where LCs can be rotated in two opposite directions. LCs aligned in a single direction in the off-state are rotated to two opposing directions with the applied electric field so that it compensates the retardation dependent on viewing directions. Even though the LC directors of the two domains are perpendicular to each other in the full white state, they can not form right angles at all gray levels, as the schematic in Fig. 2(b) shows. Here, V10, V50 and V100 correspond to voltage levels of 10, 50 and 100% of the maximum transmission, respectively. As can be seen, a color shift problem still occurs in the gray levels, although it has been reduced much more than in the conventional single-domain electrode structure.
The proposed diaper-type electrode structure is shown in Fig. 3, which is based on modulating the distance between electrodes to generate position-dependent strengths of the electric field. There are two effects contributing to the suppression of the color shift in the proposed structure. One is the different electric field directions, in which the LCs rotate in opposite directions as in the chevron-type electrode structure. The other is position-dependent electric field strengths in the area of a unit diaper, which is related to the position-dependent rotation angle of LCs.
LC directors are rotated by the dielectric torque, M⃗, whose magnitude can be expressed by
where D⃗ is the electric displacement vector, E⃗ the electric field, ε 0 dielectric constant in vacuum, Δε the dielectric anisotropy, n̂ the LC director, and φ the angle between n̂ and the electric field . In the diaper-type electrode structure, the voltages generate in-plane electric fields in four directions as shown in Fig. 3. The LCs in regions 1 and 3 rotate counterclockwise because of the dielectric torque, while those in regions 2 and 4 rotate clockwise. Together, four domains are produced so that the color shift can be effectively reduced. This is the same effect as in the chevron-type electrode structure.
3. Numerical results and discussion
To analyze the electro-optic and color characteristics of the newly proposed electrode structure in detail, the commercial software ‘Techwiz LCD’ (Sanayi System Co., Ltd.) was used. This software bases its calculations on the Eriksen-Leslie theory for the motion of LC directors and the extended 2×2 Jones matrix for optical analysis. LC material with positive dielectric anisotropy (Δε = 13.1 and Δn = 0.0778 at λ = 550 nm) is assumed; the cell gap is 3.89 um; and the surface pretilt generated by rubbing is 1° for numerical calculations.
Parameters for the chevron-type electrode are: the width of electrodes is 3 um and the distance between them is 5 um, which are typical values being used for fabrication of FFS panels. Retardation of the chevron-type electrode, dΔn, is chosen to be 300 nm for maximum transmission. The electrode is inclined at 7° (or -7°) with respect to the rubbing direction, as shown in Fig. 2(a). For the diaper-type electrode, the width and the inclined angle of electrodes are chosen to be 4 um and 10° (or -10°) with respect to the rubbing direction, as shown in Fig. 3. The distances between electrodes are covering from 4 um to 10 um.
The twist angle distributions of the LCs at three different positions at the same distance from an electrode are shown in Fig. 4. While twist angle of the LCs at three different positions in the chevron-type structure is the same, the twists differ from position to position in the diaper-type electrode structure. As the distance between the electrodes is dependent on position, the strength of the in-plane electric field is varied with positions even at the same distance from an electrode. Equation (2) shows us that the rotation of LCs is mainly affected by the strength of electric field between electrodes. The strength of the position-dependent electric field brings about rotation of LCs different from position to position, which takes advantage of the multi-domain effect to further reduce color shift.
The principle of color shift compensation in the diaper-type electrode is schematically explained in Fig. 5. Electric fields in regions A and B have different directions, while those in region C and D have different intensities. It is structurally analogous to a chevron-type electrode with non-uniform electrode distance shown in Fig. 5(b). It is believed that these two effects contribute to suppress color shift more effectively in the diaper-type electrode.
Figure 6 shows the color differences in the chevron-type electrode structure and the diaper-type electrode structure in CIE 1976 USC diagram. Color difference is defined as the distance between sampled positions and the reference in CIE 1976 USC diagram. It can be expressed by 
where Δu=u′sample -u′reference and Δv = v′sample -v′reference
Figures 6(a) and 6(b) show azimuth and polar dependence of color differences, respectively. The diaper-type electrode structure has smaller color differences than the chevron-type structure does at off-normal directions as expected.
LCs with positive dielectric anisotropy align parallel to the applied electric field. In the horizontal switching modes, the LCs rotates in-plane if the vertical field component can be ignored. However, the LCs near the surfaces are not rotated by the electric field due to the surface anchoring force, which results in a twisted alignment of the LCs. That is, rotation angles are dependent on z/d, where z is the distance from the bottom substrate surface and d is the cellgap. Therefore, it makes sense to consider the average director angle and the effective retardation of the LC layer in transmitted light. If all other conditions are held constant, the nearer the average director angle is to 45° with respect to the input polarizer and the nearer the effective retardation of the LC layer is to the half-wave condition, the more light can be transmitted.
In FFS mode the vertical and horizontal components of the electric field depend on the ratio, l/d, where l is the distance between electrodes and d is the cellgap . When this ratio is greater than 1, the horizontal field is mainly generated between electrodes so that the LCs has mainly twist deformation. On the other hand, when the ratio l/d is less than 1, the fringe field exists so that the LCs not only has twist deformation but also is tilted upwards.
In the chevron-type electrode structure, the l/d ratio is usually less than 1. The generated fringe field makes LCs twisted and tilted upwards at the same time. In contrast, in the diaper-type electrode structure, the l/d ratio differs from position to position. It can be less than or greater than 1. In some regions where the l/d ratio is less than 1, there the vertical and horizontal fields exist simultaneously, as in the conventional FFS mode. In other regions where it is greater than 1, the horizontal field is mainly generated so that the LCs are rotated more effectively, which is similar to the conventional IPS mode.
In the chevron-type electrode, intensity of the lateral electric field decreases sharply as we move along the vertical direction from the bottom substrate, as shown in Fig. 7. It means that the portion of LC layer which contributes to the effective retardation mainly comes from the lower part of LC layer. On the other hand, in diaper-type electrode, the lateral field is still strong enough to give twist deformation on LCs near the top substrate so that the portion of LC layer contributing to the effective retardation is bigger than that of the chevron-type electrode. Even though LCs just above the electrodes rarely rotate, the effective retardation and the average director angle of the LC layer are closer to a half-wave condition and 45° than those of the chevron-type structure.
Figure 9 shows the calculated sub-pixel images of the two structures. As shown in Fig. 9(a), both structures have dead zones due to LCs rotating in opposite directions. This reduces the aperture ratio, the area that can transmit light in a pixel. Even though the diaper-type electrode structure has a larger area of dead zone, the image of a pixel is brighter than that of the chevron-type. In the diaper-type electrode structure, 47% of the pixel area transmits light over 0.15 of the input, while only 30% of the pixel area transmits light over 0.15 of the input in the chevron-type, as shown in Fig. 9(b). Therefore, light transmission in the diaper-type electrode, which is defined as the average value of transmitted light over the whole area, is higher than that in the chevron-type electrode, as shown in Fig. 10. Because the LCs are aligned homogeneously along the transmission axis of the polarizer in the dark state, the viewing angle is as wide as that of the chevron-type structure, as shown in Fig. 11.
Color shift and transmission characteristics can be affected by the electrode distance and the inclined angle of electrodes. To achieve the best performance, dimensions of a unit diaper can be varied considering the pixel size. From numerical calculation, the optimum distance between electrodes of a unit diaper is between 12 um and 15 um in a pixel of 100 um × 300 um. Inclination angles of electrodes with respect to the rubbing direction are between 5° and 15°. Figure 12 shows structural dependence of color shift and transmission characteristics. Color shift along the azimuth directions and transmission characteristics rarely show structural dependence within the range of dimensions mentioned above. Along the polar directions, however, color shift characteristics are influenced a little by the structure of a unit diaper. In short, non-uniform distances between electrodes resulted in color shift reduction and higher transmission in the diaper-type electrode.
We proposed a new type of electrode structure for the FFS mode, the diaper-type electrode structure. The proposed diaper-type electrode structure makes use of not only different directions but also position-dependent strengths of the lateral electric field. Position-dependent electric fields generate rotation of LC different from position to position in a pixel, which can reduce the color shift further by the multi-domain effect. Due to the multi-domain effect, which can effectively reduce the dependence of the retardation on viewing directions, the diaper-type electrode structure has a smaller color shift than the chevron-type structure. Furthermore, a strong lateral field can easily allow more light to be transmitted because the average LC director angle is near 45° with respect to the input polarizer and the effective retardation of the LC layer is near half-wave retardation. The proposed structure shows greater light transmission than the chevron-type electrode structure. With the proposed electrode structure, it is expected that we can realize LCDs which shows wide-viewing angle, higher transmission, and small color shift.
This work was supported in part by the Information Display R&D Center, one of the 21st Century Frontier R&D Program funded by the MOCIE of Korea.
References and links
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