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When an electric field is applied to in-plane switching (IPS) and fringe-field switching (FFS) cells with zero rubbing angle, virtual walls are built such that the switching speed can be increased several-fold. In this study, we investigate the dependence on the interdigitated electrode structure of the electro-optical characteristics of IPS and FFS cells with zero rubbing angle. We found that when the rubbing angle is zero, the single-layered IPS electrode structure provides a higher transmittance than the double-layered FFS electrode structure because of the reduced width of dead zones at domain boundaries between interdigitated electrodes. Single-layered IPS electrodes not only minimize the transmittance decrease but also provide a shorter response time than double-layered FFS electrodes, although the operating voltage is higher and fabrication requires a more precise rubbing process. The transmittance decrease due to the zero rubbing angle in an IPS cell can be minimized using optimization of the electrode structure while retaining a short response time.
© 2016 Optical Society of America
1. Introduction
In-plane switching (IPS) and fringe-field switching (FFS) liquid crystal displays (LCDs) have been widely used in various display devices because of their advantageous features, such as wide viewing angle, small color shift, and pressure resistance (in touch panels) [1–6]. The FFS mode [3, 4] is superior to other display modes because of its high transmittance and low operating voltage, and because of the embedded storage capacitor (under the interdigitated electrodes). The FFS mode has been widely used in mobile displays, where power consumption is a critical issue. The increase of LCD panel size accompanies an increase in the pixel size, which leads to a large storage capacitor in an FFS LCD. Therefore, for large-sized display applications, the charging time could become a problem in an FFS LCD [4, 7]. The IPS mode [1, 2] exhibits features similar to the FFS mode, except that it has a slightly lower transmittance and a higher operating voltage. The IPS mode, which does not use a built-in storage capacitor, is widely applied in large LCD panels because the storage capacitance can easily be optimized for a given LCD panel size.
The slow liquid crystal (LC) response time, which causes motion blur and deteriorated image quality, remains a critical issue in the wider application of LCDs. The response times of IPS and FFS cells are relatively slow because the restoring elastic torque is governed primarily by the twist elastic constant K22, which is roughly one half of the value of the other two elastic constants, namely, the splay elastic constant K11 and the bend elastic constant K33. The response time of the LC can be reduced by decreasing the cell gap, because it is proportional to the square of the cell gap [8–10]. However, this approach imposes several challenges, such as a low manufacturing yield and the requirement of a high birefringence LC, which typically has a large rotational viscosity. Another approach is to create polymer networks or walls in an LC cell to impose a strong restoring force [11–17].
Recently, an FFS cell with zero rubbing angle was reported to achieve a short response time [17]. In an FFS cell with zero rubbing angle, virtual walls are built when an electric field is applied between the interdigitated electrodes and the common electrode such that the switching speed could be increased by several times. The device exhibits several favorable characteristics, such as a low operating voltage (similar to a conventional FFS cell), reduced color shift because of the multi-domain effect, and a simple fabrication process that does not require additional steps. However, a challenge in an FFS cell with zero rubbing angle is transmittance decrease when compared to a conventional FFS cell with non-zero rubbing angle. There is an urgent need to minimize the transmittance decrease while maintaining a fast response.
In this paper, we investigate the electro-optical characteristics of IPS and FFS cells with zero rubbing angle. We found that, when the rubbing angle is zero, the IPS electrode structure provides higher transmittance than the FFS electrode structure because of the reduced width of the dead zones at domain boundaries between interdigitated electrodes. The IPS electrodes suppress the transmittance decrease while maintaining a short response time. However, there is a tradeoff: an increased operating voltage exists. Through optimization of the interdigitated electrode structure, we can minimize the transmittance decrease in an IPS cell with zero rubbing angle while retaining a short response time.
2. Operational principle
To apply an in-plane electric field to a homogeneously aligned LC cell, two types of electrode structures are widely used, as shown in Fig. 1. The double-layered electrode structure shown in Fig. 1(a), which is known as the FFS mode, is widely used for mobile applications. The other type is the single-layered electrode structure shown in Fig. 1(b), which is known as the IPS mode. This mode is widely used in large-sized display applications. In the single-layered IPS electrode structure, the pixel and common electrodes are placed in an interdigitated pattern on the bottom substrate. The in-plane component of the electric field is dominant between adjacent interdigitated electrodes, whereas the vertical component is dominant above the interdigitated electrodes. In the double-layered FFS electrode structure, in which a common electrode is separated from the interdigitated pixel electrodes by an insulating layer, the vertical and in-plane components are generated between and above the interdigitated electrodes.
Fig. 1 Cross-sectional views of electrode structures with equipotential lines. (a) Double-layered and (b) single-layered electrodes.
The rubbing direction of the LC and the transmission axis of one of the crossed polarizers are chosen to be parallel to each other. In conventional FFS and IPS cells employing LCs with positive dielectric anisotropy, the rubbing angle α, which is defined as the angle between initial LC director and interdigitated electrodes, is chosen to be between 5° and 15° with respect to the interdigitated electrodes, such that all of the LC molecules are rotated in the same direction when an in-plane electric field is applied to an LC cell. When an electric field is applied to a conventional IPS cell with α ≠ 0°, LC molecules between the interdigitated electrodes are rotated by the applied field whereas LC molecules above the interdigitated electrodes are not fully rotated. On the other hand, in a conventional FFS cell with α ≠ 0°, the LC molecules near the electrode edges (where the in-plane component of the applied electric field is high) are rotated by the dielectric torque, whereas LC molecules at boundaries A and B in Fig. 1(a) (where the applied electric field has no in-plane component) are rotated by the elastic torque caused by neighboring rotated molecules. Thus, when α ≠ 0°, an FFS cell exhibits a higher transmittance than an IPS cell.
In contrast, in an LC cell with α = 0°, the LC molecules in region II of Fig. 1(b) are rotated in the direction opposite to those in region I when an electric field is applied to the LC cell. At boundaries A and B between regions I and II, there is no change in the azimuth angle of the LC director because of the applied electric field; therefore, boundaries A and B can be treated as virtual walls. When α = 0°, the LC molecules are confined not only by the two substrates but also by these virtual walls [17]. There is no change of the azimuth angle by the applied field at boundaries A and B in an FFS cell with α = 0°. The LC molecules are tilted at these boundaries and tilted LC molecules suppress the rotation of LC molecules such that the transmittance is very low near the boundaries. Although the switching mechanism of LC molecules in an IPS cell with α = 0° is similar to that of an FFS cell with α = 0°, there is a difference at the boundary B between interdigitated electrodes. In contrast to an FFS cell in which there is no in-plane component of the applied electric field at boundary B, a strong in-plane component exists at boundary B between interdigitated electrodes in an IPS cell. LC molecules are tilted at boundary B in an FFS cell, whereas LC molecules at boundary B are not tilted in an IPS cell by the in-plane component of the applied electric field; therefore, LC molecules around boundary B in an IPS cell are rotated more than those of an FFS cell, resulting in a higher transmittance.
3. Results and discussion
To confirm the dependence on the electrode structure of electro-optical characteristics in homogeneously aligned LC cells, LC cells were fabricated. Transparent interdigitated and common electrodes with a 0.15-µm-thick insulating layer between them were formed on the bottom substrate of the FFS cell, whereas single-layered interdigitated electrodes were formed on the bottom substrate of the IPS cell. The width W of the interdigitated electrodes and spacing L between them were 2.8 µm and 6 µm, respectively. The top and bottom glass substrates were spin-coated with homogeneous polyimide alignment layers and then subjected to a baking process. The anti-parallel rubbing process was conducted along the directions of either 0° or 10° with respect to the interdigitated electrodes. The pretilt angle, which is defined as the angle between the LC director and the substrate surface, generated by the rubbing process was 2°. The two substrates were then assembled using silica spacers to maintain a cell gap of 3.5 µm. Nematic LCs with positive dielectric anisotropy were injected into each cell using capillary action. The physical properties of the LC used are as follows: optical anisotropy ∆n = 0.119; dielectric anisotropy ∆ε = 5.1; elastic constants K11 = 11.3 pN, K22 = 5.9 pN, K33 = 14 pN; and rotational viscosity γ1 = 47.6 mPa·s.
To evaluate the electro-optical characteristics of the fabricated LC cells, we measured the voltage-transmittance curves of the fabricated LC cells, as shown in Fig. 2. When α = 10°, the FFS cell showed 9.8% higher transmittance and 17% lower operating voltage than the IPS cell. The FFS cell had a transmittance of 23.0% at an applied voltage of 4.8 V, whereas the IPS cell had a transmittance of 21.9% at an applied voltage of 5.8 V. When α = 0°, the IPS cell had 9.8% higher transmittance and a 60% higher operating voltage than the FFS cell. The FFS cell had a transmittance of 17.4% at an applied voltage of 4.5 V, whereas the IPS cell had a transmittance of 19.1% at an applied voltage of 7.2 V.
Fig. 2 Measured transmittances as functions of the applied voltage of the fabricated IPS and FFS cells with either α = 10° or 0°.
To measure the response time of the fabricated cells, we applied a voltage corresponding to the maximum transmittance to each cell and then removed it after several seconds. The measured temporal switching behaviors of the LC cells are shown in Fig. 3. We defined the turn-on time as the transient time from 10% to 90% of the maximum transmittance (and vice versa for the turn-off time). The response times of FFS and IPS cells were almost the same when the rubbing angle α was 10°. The measured turn-on (turn-off) times of FFS and IPS cells were 11.26 (12.33) ms and 11.64 (12.41) ms, respectively. When α was 0°, the measured turn-on (turn-off) times of FFS and IPS cells were 7.56 (5.24) ms and 5.57 (4.98) ms, respectively. The turn-off times of FFS and IPS cells are almost the same, whereas the turn-on time of an IPS cell is 36% shorter that of an FFS cell. An IPS (FFS) cell with α = 0° has a 2.28 (1.84) times faster response time than that of cells with α = 10°.
To better understand the effects of the electrode structure on the electro-optical characteristics of the LC cells, the Ericksen-Leslie equation coupled with the Laplace equation was solved numerically using the finite-element method. The Ericksen-Leslie equation is traditionally used to describe the motion of an LC director. We performed numerical calculations using commercial software, i.e., TechWiz LCD 2D. The parameters used for numerical calculations were the same as those of the LC used for experiments. Figure 4(a) shows the calculated transmittance distributions of FFS and IPS cells with α = 0° at applied voltages corresponding to the maximum transmittance. No light is transmitted at boundaries A and B of either FFS or IPS cells because there is no change in the azimuth angle of the LC director. In an FFS cell with α = 0°, there is no in-plane component of the applied electric field, but a weak vertical component exists at boundaries A and B such that the LC molecules at the boundaries are tilted when an electric field is applied. The tilted LC molecules at boundaries A and B suppress the rotation of LC molecules near the boundaries, as shown in Fig. 4(b); therefore, the transmittance is relatively low around the boundaries. On the other hand, the tilt and azimuth angles of the LC molecules at boundary B between interdigitated electrodes are not changed in an IPS cell; therefore, the LC molecules are rotated more near boundary B, although switching of the LC molecules at boundary A above the interdigitated electrodes is similar to that in an FFS cell, as shown in Fig. 4(b). Thus, when the rubbing angle α is 0°, the transmittance of an IPS cell is higher than that of an FFS cell, although the operating voltage is higher.
Fig. 4 (a) Calculated transmittance distributions of IPS and FFS cells with α = 0° and (b) twist angle distributions at three different electrode positions.
When an electric field is applied to conventional FFS and IPS cells with α ≠ 0°, all of the LC molecules are rotated in the same direction. Therefore, the total elastic energies are very low, because LC molecules are rotated by the elastic torque, as shown in Fig. 5. In contrast, in FFS and IPS cells with α = 0°, LC molecules in region II are rotated in the direction opposite to those in region I when an electric field is applied to the LC cell. LC molecules at boundaries A and B between regions I and II are not rotated by the elastic torque, resulting in an elastic energy much higher than that of LC cells with α ≠ 0°. The LC molecules are confined not only by the two substrates but also by virtual walls so that both splay and twist deformation occur when an electric field is applied to LC cells with α = 0°. The response time can be reduced as a result of the enhanced anchoring provided by the virtual walls. Although the response time of LC cells with α = 0° is shorter than that of LC cells with α ≠ 0° because of virtual walls, the transmittance of LC cells with α = 0° is smaller than that of LC cells with α ≠ 0° because of dead zones around these virtual walls, as shown in Figs. 5 and 6.
Fig. 5 Transmittance distributions, elastic energy profiles and LC director orientations of (a) FFS and (b) IPS cells.
In FFS and IPS cells with α = 0°, in which LCs are two-dimensionally confined, the turn-off time can be expressed as [16, 17]
The first term in the denominator of Eq. (1) is governed by anchoring between the two substrates. The second term is associated with the restoring force due to anchoring between the virtual walls. As expected from Eq. (1), the response time can be significantly reduced as the distance D between virtual walls, which is a half of the pitch of the interdigitated electrodes, is decreased. Although the reduced pitch of interdigitated electrodes is highly favorable for the short response time, its reduced pitch could lead to a transmittance decrease. Therefore, a delicate balance between these two parameters should be taken into consideration.Next, we discuss how we can minimize the transmittance decrease in an LC cell while retaining the short response time. An IPS cell with α = 0° has a higher transmittance near the boundary B between interdigitated electrodes than near the boundary A above interdigitated electrodes; therefore, the transmittance decrease can be minimized via optimal design of the electrode structure. Figure 7 shows the calculated electro-optical characteristics of IPS cells with a spacing L between interdigitated electrodes as the parameter. As the spacing between interdigitated electrodes is increased, the maximum transmittance of an IPS cell with α = 0° is increased, as shown in Fig. 7(a), because the width of dead zones around the virtual walls is reduced for a fixed pixel area, whereas the operating voltage is increased. When the spacing between interdigitated electrodes was 8 µm, the maximum transmittance of the IPS cell with α = 10° was 39.5% at an applied voltage of 7 V, whereas that of the IPS cell with α = 0° was 33.4% at an applied voltage of 8.2 V. The transmittance of the IPS cell with α = 0° is 15.4% smaller than that of the IPS cell with α = 10°.
Fig. 7 (a) Transmittance, operating voltage, and (b) response times as functions of spacing L between interdigitated electrodes. The width W of interdigitated electrodes was fixed at 2 µm.
Multi-domain electrode structures are often employed to reduce color shift in an IPS cell with α = 10°. The transmittance of a multi-domain IPS cell is dependent on the pixel size and the pixel design because no light is transmitted at domain boundaries. As an example, we calculated the transmittance of a chevron-shaped two-domain IPS cell [18] whose pixel size is 180 μm × 60 μm or 60 μm × 20 μm, as shown by dark cyan lines in Fig. 7(a). The transmittance of an IPS cell with α = 0° is 13.0% smaller than that of a chevron-type two-domain IPS cell at the pixel size of 180 μm × 60 μm whereas the transmittance of an IPS cell with α = 0° is 7.2% smaller than that of a chevron-type two-domain IPS cell at the pixel size of 60 μm × 20 μm. As the pixel size is reduced, the transmittance difference between a chevron-shaped IPS cell and an IPS cell with α = 0° is reduced. On the other hand, as the pixel size is increased, the transmittance of a chevron-shaped IPS cell approaches that of a single-domain IPS cell.
Although the increase of spacing between interdigitated electrodes sacrifices the response time, as shown in Fig. 7(b), the response time of an IPS cell with α = 0° is still much shorter than that of an IPS cell with α = 10°. When the spacing between interdigitated electrodes was 8 µm, the turn-on (turn-off) time of the IPS cell with α = 10° was 9.21 (13.26) ms, whereas that of the IPS cell with α = 0° was 9.01 (7.19) ms. If IPS cells with α = 0° are used for digital signage and automotive display applications, the reduced pitch of interdigitated electrodes is preferable for fast response, because being able to operate well under low temperature environments is very important for these applications. On the other hand, the increased pitch of the interdigitated electrodes is preferable for high transmittance applications such as large-sized TVs. A trade-off exists between transmittance and response time.
As an alternative method, we can consider the change in the ratio L/W between the width W of the interdigitated electrodes and the spacing L while the pitch L + W is maintained at a fixed value. As the ratio L/W is increased, both the transmittance and the operating voltage of the IPS cell with α = 0° were increased, as shown in Fig. 8(a), because the width of the dead zone above interdigitated electrodes is reduced and the width between interdigitated electrodes is widened. On the other hand, the response time was almost the same because of the fixed distance between virtual walls, as shown in Fig. 8(b). Therefore, via optimization of the electrode structure, the transmittance decrease in an IPS cell with α = 0° can be minimized while retaining a short response time.
Fig. 8 (a) Transmittance, operating voltage, and (b) response times as functions of L/W. The pitch L + W of interdigitated electrodes was fixed at 7 µm.
In addition to a short response time, small color shift and wide viewing angle are the major requirements for large-sized LCD applications. Thanks to in-plane reorientation of the LC molecules, the IPS mode exhibits wider viewing angle than other LCD modes. However, a single-domain IPS LCD which uses striped electrodes exhibits color shift at off-axis because the LCs are rotated in one direction. To reduce the color shift, chevron-type electrode structure has been widely used. In a chevron-type two-domain IPS cell, the LCs in one domain are rotated in the direction opposite to those in the other domain when an in-plane electric field is applied to the LC cell [18]. In LC cells with α = 0°, off-axis color shift is reduced when compared to a single-domain IPS cell because LCs in region II is rotated in the direction opposite to those in region I [17]. To confirm viewing angle characteristics of an IPS cell with α = 0°, we calculated the contrast contour of a chevron-type IPS cell and an IPS cell with α = 0°, as shown in Fig. 9. In order to reduce the off-axis light leakage at the dark state, both cells was compensated by adding a half-wave biaxial film [19]. The IPS cell with α = 0° shows wide veiwinsg angle characteristics comparable with that of a chevron-type IPS cell, as shown in Fig. 9.
Fig. 9 Iso-contrast contour plots of biaxial film-compensated (a) chevron-type IPS cell and (b) IPS cell with α = 0°. The chevron-shaped electrode has a bending angle of 10°.
One challenge of an IPS cell with α = 0° is the need for a more precise rubbing process than that used for an FFS cell with α = 0°; this process provides a symmetric LC director configuration. Figure 10 shows the maximum acceptable rubbing angles as functions of the pretilt angle in FFS and IPS cells with α = 0°. Non-zero pretilt angle is required for switching of an LC cell with α = 0° without creating unintended random domains. As the pretilt angle is reduced, which affects the viewing-angle characteristics, the allowable deviation of the rubbing angle of both cells decreases, as shown in Fig. 10. The rubbing angle dependence of an IPS cell is more sensitive than that of an FFS cell; therefore, an IPS cell with α = 0° requires a more precise rubbing process.
Fig. 10 Calculated maximum acceptable rubbing angle vs. the pretilt angle in FFS and IPS cells with α = 0°.
4. Conclusion
In conclusion, we have investigated dependence on the interdigitated electrode structure of the electro-optical characteristics in a homogeneously aligned LC cell with zero rubbing angle. We confirmed that when the rubbing angle is zero, the transmittance of an IPS cell is higher than that of an FFS cell, although the former has a higher operating voltage and requires a more precise rubbing process than the latter. The electrode structure of an IPS cell with zero rubbing angle can be optimized such that the transmittance decrease can be minimized while retaining a short response time. In addition to providing a short response time, an LC cell with zero rubbing angle retains wide-viewing-angle characteristics because of the multi-domain effect. Thus, the proposed method can be a candidate for large-sized, high-resolution, and high-frame-rate TV applications, in addition to outdoor applications, such as digital signage and automotive displays.
Acknowledgement
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2014R1A2A1A01004943).
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