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We investigated the two-dimensional (2-D) confinement effect of liquid crystals (LCs) on the switching of vertically aligned LCs by an in-plane electric field. When an in-plane field is applied to a vertical alignment (VA) cell, virtual walls are built at the center of the interdigitated electrodes and at the middle of the gaps between them. The LC molecules are confined not only by the two substrates but also by the virtual walls so that the turn-off time of a VA cell driven by an in-plane field is dependent on the pitch of the interdigitated electrodes as well as the cell gap. Therefore, the turn-off time of a VA cell driven by an in-plane field can be reduced simply by decreasing the pitch of the interdigitated electrodes as a result of the enhanced anchoring provided by the virtual walls. The experimental results showed good agreement with a simple model based on the 2-D confinement effect of LCs.
© 2016 Optical Society of America
1. Introduction
In the past decades, narrow viewing angles and slow response have been the critical issues faced by high-performance liquid crystal displays (LCDs). Among various types of commercialized LC modes, such as twisted nematic [1], in-plane switching (IPS) [2–4], and vertical alignment (VA) [5], the VA mode has been widely used in large sized LCD devices owing to several attractive features, such as, a high contrast ratio over 100,000:1, rubbing-free fabrication process, and advantages in fabrication of curved panels. The dependence of the VA mode on the viewing angle is significant because the phase retardation of vertically aligned LCs is highly dependent on the viewing angle. To improve the viewing angle characteristics, various multi-domain VA modes have been proposed [6–11]. By employing these multi-domain VA modes, viewing angle characteristics have been considerably improved. However, the slow response time still remains a critical issue. The response time of these modes is not sufficiently fast for complete elimination of motion blur.
In the meantime, in-plane switching of vertically aligned LCs with positive dielectric anisotropy, known as the VA-IPS mode [12, 13], has been proposed. The VA-IPS mode itself provides two-domains in each pixel because of the multi-domain-like switching behavior of LCs. By employing multi-domain electrode structures, the VA-IPS mode can provide wide viewing angle characteristics, small gamma distortion, and small color shift, as the conventional multi-domain VA modes [12–16].
In a conventional LC cell, although the turn-on time can be reduced by overdrive schemes, turn-off switching remains rather slow because it relies on the slow relaxation of LCs. We can expect that a VA-IPS cell that uses LCs with positive dielectric anisotropy can provide a response time shorter than a conventional VA cell that uses LCs with negative dielectric anisotropy because of the smaller rotational viscosity of positive LCs. It has been reported that the turn-off time of a VA-IPS cell is shorter than that of a conventional VA cell [16]. However, the unusually short turn-off time of a VA-IPS cell cannot be explained simply by the conventional theoretical analysis. The underlying mechanism for fast turn-off switching of a VA-IPS cell has not been clearly explained until now. In a VA-IPS cell, turn-on switching is much faster than turn-off switching because of a high operating voltage, and so the response time is primarily determined by the turn-off time. To further improve the switching speed in a VA-IPS cell, better understanding of the switching mechanism is essential.
In this study, we investigate the underlying mechanism for fast turn-off switching of a VA-IPS cell. We found that the turn-off time of a VA-IPS cell is dependent on the pitch of the interdigitated electrodes as well as the cell gap. When an in-plane electric field is applied to a VA cell, virtual walls are built at the center of interdigitated electrodes and at the middle of the gaps between them. The LC molecules are confined not only by two substrates but also by the virtual walls. We confirmed that the experimental results are in good agreement with the results of a simple model based on the two-dimensional (2-D) confinement effect of LCs.
2. Operational principle
Figure 1 shows the structure and operational principle of a VA-IPS cell with interdigitated electrodes placed on the bottom substrate. In the initial state, LC molecules are vertically aligned between crossed polarizers, so that the cell is in the dark state, as shown in Fig. 1(a). When an in-plane electric field is applied between the neighboring interdigitated electrodes, the LC molecules in region I are tilted down in the clockwise direction, whereas those in region II are tilted down in the counter-clockwise direction, as shown in Fig. 1(b). The LC molecules are tilted down along the diagonal direction with respect to the transmission axes of the two polarizers such that the cell switches to the bright state. At boundaries A and B between regions I and II, the LC molecules remain vertically aligned and there is no change in the polar angle of the LC director, and hence no light is transmitted there. Therefore, boundaries A and B can be treated as virtual walls such that the LC molecules are two-dimensionally confined not only by the two substrates but also by these virtual walls. Similar behavior has been observed in a homogeneously aligned LC cell with zero rubbing angle [17, 18].
Fig. 1 Device structure of a VA-IPS cell with LC director configurations in (a) the off state and (b) the on state.
3. Results and discussion
To confirm the 2-D confinement effect of LCs on the response time of a VA-IPS cell, we fabricated VA cells with the interdigitated electrode structure shown in Fig. 1. We coated a homeotropic alignment layer on each substrate, and the substrates were baked for 1 h at 180°C. Then, a cell was assembled using silica spacers such that the cell gap was maintained at 4.1 μm. Finally, LCs with positive dielectric anisotropy were injected by capillary action into the cell. The physical properties of the LC material used in the fabrication were as follows: the splay elastic constant K11 = 13.1 pN, the twist elastic constant K22 = 5.95 pN, the bend elastic constant K33 = 13.6 pN, the optical anisotropy ∆n = 0.1169, the dielectric anisotropy ∆ε = 7, and the rotational viscosity γ = 61 mPa·s. We used interdigitated electrodes with width W and spacing L between them. The pitch P ( = W + L) of the interdigitated electrodes was varied from 8 μm (W: L = 3 μm: 5 μm) to 13 μm (W: L = 6 μm: 7 μm).
To confirm the dynamic switching behavior of the LCs, the Ericksen-Leslie equation coupled with the Laplace equation were solved numerically using the finite-element method. The Ericksen-Leslie equation is generally used to describe the motion of LC molecules. Numerical calculations were performed using the commercial software TechWiz LCD 2D (Sanayi System Company, Ltd., Korea). The parameters used for numerical calculation were the same as those used in the experiments.
Under the one-dimensional confinement of LCs where the LCs are sandwiched between two substrates, the turn-off time of a VA cell can be written under small angle and single elastic constant approximations as.
As shown in Eq. (1), the turn-off time of a VA cell is proportional to the product of the rotational viscosity γ and the square of the cell gap d. It is inversely proportional to the bend elastic constant K33. The turn-off time is largely dependent on the cell gap d, but it is not dependent on the pitch of the interdigitated electrodes, as shown by the black line in Fig. 2. In contrast, experimental and numerical results show that the turn-off time in a VA-IPS cell is largely dependent on the pitch of the interdigitated electrodes, as shown in Fig. 2. As the pitch of the interdigitated electrodes was reduced, the turn-off time decreased. These results suggest that the LC molecules in a VA-IPS cell may be confined not only by the two substrates but also by the virtual walls, as will be discussed later.Fig. 2 Turn-off times of VA-IPS cells obtained by 1-D model, 2-D model, numerical calculation and the experiment as functions of the pitch of interdigitated electrodes.
In a VA-IPS cell, the polar angle of the LC director does not change at boundaries A and B even when an electric field is applied between neighboring interdigitated electrodes, as shown in Fig. 1(b). If we consider boundaries A and B as virtual walls, by which the LC molecules are confined not only by the two substrates but also by these virtual walls, the turn-off time of a VA-IPS cell can be written as [17–19]
The first term in the denominator of Eq. (2) is governed by the anchoring of the two substrates. The second term is associated with the restoring force owing to the anchoring by neighboring virtual walls. The distance between the neighboring virtual walls is a half of the pitch P ( = W + L) of the interdigitated electrodes. According to Eq. (2), the turn-off time of a VA-IPS cell is dependent on the pitch of interdigitated electrodes as well as the cell gap; therefore, as the pitch of interdigitated electrodes is reduced, the turn-off time is significantly reduced as a result of the decreased distance between neighboring virtual walls, as shown by the red line in Fig. 2. Although the turn-off time obtained with Eq. (2) shows deviation from both numerical and experimental results, all of them show the same trend. The turn-off time decreases as the pitch of the interdigitated electrodes is reduced. In other words, we need to consider the 2-D confinement effect to evaluate the dynamic response of a VA-IPS cell because the LC molecules are two-dimensionally confined not only by the two substrates but also by the virtual walls.In a VA-IPS cell, in which LCs are two-dimensionally confined, both bend and splay deformation occur when an electric field is applied between neighboring interdigitated electrodes. After the applied electric field is removed, the deformed LC molecules relax to the initial state because of the elastic restoring torque. In the IPS electrode structure, the electric field distribution in the longitudinal direction between the two substrates is less uniform than that in the lateral direction between interdigitated electrodes. As a result, the effect of anchoring by the virtual walls on the elastic restoring torque of LC molecules may be different from the effect of anchoring by the two substrates. To account for this difference, we modified Eq. (2) as
Here, A is a proportional constant extracted through fitting the measured results with Eq. (3). We fitted the numerical and measured results with Eq. (3), as shown by blue and dark cyan lines in Fig. 3. A for the numerical results was found to be 1.717, whereas A for the measured results was found to be 2.835. Although there is a deviation between the two, they can be represented with a proportionality constant A larger than unity. These results show that the anchoring effect of the virtual walls on the turn-off time is much more profound than the effect of the two substrates in a VA-IPS cell.Fig. 3 Dependence of the turn-off time on the pitch of interdigitated electrodes in VA-IPS cells; the dots represent numerical and measured results whereas the solid lines were obtained through fitting with Eq. (3).
To investigate the deviation between numerical and measured results, we first calculated the turn-off times of VA-IPS cells by using the modified 2-D model with the cell gap as a parameter and compared them with numerical results, as shown in Fig. 4. Here, we defined the turn-off time as the transient time from 90% to 10% of the maximum transmittance. To investigate the dependence of A on LC material, we also numerically calculated the turn-off times of VA-IPS cells by employing three different LC materials, as shown in Table 1. The turn-off times calculated with Eq. (3) are in good agreement with the numerical results. As the cell gap is increased, the turn-off time obtained with the modified 2-D model increased more slowly than that obtained with the original 2-D model. A good agreement between the modified 2-D model and numerical results was obtained, indicating that there is indeed a non-unity proportionality constant A. Moreover, A is independent of LC material properties, as shown in Fig. 4, which is an attractive feature because it implies that Eq. (3) is universally applicable to all VA-IPS cells, regardless of the LC employed.
Fig. 4 Dependence of the turn-off time on the cell gap in VA-IPS cells fabricated by using three different LC materials. (a) LC I, (b) LC II, and (c) LC III. (W: L = 3 μm: 5 μm)
Table 1. Physical Properties of the Three Different LC Materials used in Our Experiment
We also calculated the response time of a VA-IPS cell as we vary the ratio L/W for a fixed pitch P ( = W + L). As shown in Fig. 5, the response time was not significantly affected by the ratio L/W when the pitch P was maintained at a fixed value of 8 µm. This result clearly confirms that Eq. (3) is universally applicable to all VA-IPS cells, regardless of the electrode structure employed.
Fig. 5 Numerically calculated response time of a VA-IPS cell vs. the ratio L/W for a fixed pitch of 8 µm.
Next, we will experimentally evaluate this proportionality constant A. As shown in Fig. 3, the measured turn-off times were shorter than the numerical results. Further study is needed to investigate the cause of the difference between numerical and experimental results. As mentioned above, by fitting the experimental results with Eq. (3), A was found to be 2.835. We measured the temporal switching behavior of VA-IPS cells during the turn-off process from the full-bright to the dark state with the cell gap as a parameter. The measured turn-off times were compared with the modified 2-D model, as shown in Fig. 6. Experimental results showed a good agreement with the modified 2-D model.
Fig. 6 Dependence of the turn-off times on the cell gap in VA-IPS cells obtained through experiment, by the 2-D model, and by the modified 2-D model. (W: L = 3 μm: 5 μm)
We also measured the temporal switching behavior of VA-IPS cells fabricated by employing the three different LC materials listed in Table 1, as shown in Fig. 7. The VA-IPS cells showed a very fast turn-on time of 500 μs because of their high operating voltages, as shown in Fig. 7(a). The turn-on times were almost the same for all three LC materials. The measured turn-off times were compared with the 2-D model and the modified 2-D model, as shown in Table 2. For all three LC materials, the measured turn-off times are in good agreement with those obtained using the modified 2-D model. A is independent of LC material parameters, which implies that the modified 2-D model can be used to account for the turn-off times in a VA-IPS cell. This model can provide a useful guideline for optimization of the performance of a VA-IPS cell. It provides a simple method for reducing the turn-off time of a VA-IPS cell without requiring additional fabrication steps or complicated drive schemes. The turn-off time can be reduced simply by decreasing the pitch of the interdigitated electrodes owing to the enhanced anchoring provided by the virtual walls. Although zero rubbing angle can increase the in-plane switching speed of homogeneously-aligned LCs by several times because of the 2-D confinement effect [17, 18], the devices require a precise rubbing process in order to provide a symmetrical LC director configuration. On the other hand, a VA-IPS cell is robust against such a problem owing to its rubbing-free fabrication process.
Fig. 7 Measured temporal switching behaviors during (a) the turn-on and (b) turn-off processes in VA-IPS cells with three different LC materials. (W: L = 5 μm: 5 μm)
Table 2. Turn-off Times of VA-IPS Cells Employing Three Different LC Materials Obtained by the 2-D Model, Modified 2-D Model, and through Experiment
4. Conclusion
In summary, we have investigated the turn-off switching mechanism of a VA-IPS cell by considering the 2-D confinement effect of LCs. When an in-plane electric field is applied to a VA cell, virtual walls are built at the center of interdigitated electrodes and at the middle of the gaps between them so that the turn-off time of a VA-IPS cell is dependent on the pitch of the interdigitated electrodes as well as the cell gap. Therefore, the turn-off time of a VA-IPS cell can be reduced simply by decreasing the pitch of the interdigitated electrodes. We confirmed that experimental results are in good agreement with the theoretical results obtained by considering the 2-D confinement effect of LCs. We believe that the VA-IPS mode can be a potential candidate for realizing large-sized, high-resolution and high-frame-rate TV applications, in addition to outdoor applications, such as automotive displays and digital signage.
Funding
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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