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We propose an interdigitated pixel electrode structure with alternating tilts for fast fringe-field switching of liquid crystals (LCs). In contrast to an LC cell, where the pixel electrodes are parallel to the LC alignment direction, this device does not require a non-zero pretilt angle, owing to an obliquely applied electric field; thus, it can retain a much wider viewing angle by aligning the LCs without a pretilt. In addition to a short response time and wide viewing angle, the proposed device allows a much larger deviation of the LC alignment direction, which is essential for mass production. Moreover, LCs with negative dielectric anisotropy can be used to minimize the transmittance decrease.
© 2016 Optical Society of America
1. Introduction
Liquid crystal displays (LCDs) are widely used for several reasons, e.g., their high resolution, low power consumption, light weight, and thinness. However, a major drawback is their slow response time, causing severe image degradation of fast-moving pictures. Emerging outdoor applications, e.g., automotive displays and digital signage, require a much faster response at room temperature so they can operate well in low-temperature environments where the rotational viscosity of liquid crystals (LCs) is dramatically increased [1–4]. To realize augmented/virtual reality with low-latency and see-through LCDs with high transparency, the slow response has been a technical challenge and is currently drawing considerable attention [5,6].
Among successfully commercialized LC modes, the in-plane switching (IPS) mode [7] and the fringe-field switching (FFS) mode [8, 9] are widely used because they can provide the widest viewing characteristics as a result of their homogeneous alignment and the in-plane reorientation of the LCs. However, LCDs suffer from a slow response time, which makes it difficult to use them in the above-mentioned applications. To reduce the response time of IPS and FFS modes, various approaches have been proposed, e.g., a thin cell gap [10], low viscosity LCs [11], control of anchoring energy [12, 13], three-terminal electrode structures [14–18], polymer-stabilized cells [19, 20], and inserted vertical walls [21, 22].
It was recently demonstrated that the LC response time can be reduced several fold by introducing pixel electrodes parallel to the LC alignment direction [23, 24]. In addition to a very short response time, the device exhibits several outstanding features, e.g., a wide viewing angle, small color shift, simple drive scheme, etc. Although fast switching can be achieved by employing homogeneously aligned LCs with pixel electrodes parallel to the LC alignment direction, it can only be applied to LCs with positive dielectric anisotropy (p-LCs) with a non-zero pretilt angle. As the pretilt angle is reduced, the allowable deviation of the LC alignment direction also decreases, which may be unfavorable for mass production of the devices. Recently, photo-alignment technology has been actively studied and widely used in various LCD panels because it offers several advantages, such as high alignment uniformity, high contrast ratio, and wide viewing angle. The pretilt angle of LCs induced by the photo-alignment is near-zero, and so it can provide wide-viewing-angle characteristics [25].
In this paper, we propose an interdigitated pixel-electrode structure with alternating tilts for fast fringe-field switching of LCs to overcome technical issues in an LC cell that has the pixel electrodes parallel to the LC alignment direction. This device allows a much larger deviation of the LC alignment direction than that of an LC cell with pixel electrodes parallel to the LC alignment direction. It can provide a much wider viewing angle because it does not require a non-zero pretilt angle. Moreover, in contrast to an LC cell with parallel pixel electrodes for which only p-LCs can be used, LCs with negative dielectric anisotropy (n-LCs) can be employed in this device to minimize the transmittance decrease.
2. Operational principle
Figure 1 shows the device structure and operational principle of a conventional FFS cell with angle α ≠ 0° between the pixel electrodes and the LC alignment direction, an FFS cell with α = 0°, and an FFS cell using pixel electrodes with alternating tilts. Transparent interdigitated and common electrodes with an insulating layer between them are formed on the bottom substrate in all the FFS cells. Whether α = 0° or not, adjacent interdigitated electrodes are parallel to each other, as shown in Fig. 1(a) and Fig. 1(b). When an electric field is applied between the interdigitated electrodes and the common electrode, all the LC molecules in an FFS cell with α ≠ 0° are rotated in the same direction, as shown in Fig. 1(a); whereas, in an FFS cell with α = 0°, the LC molecules in region II of Fig. 1(b) are rotated in the direction opposite those in region I.
Fig. 1 (a) Cross-sectional view with equipotential lines. Top view of LC cells with pixel electrodes (b) not parallel to the LC alignment direction (α ≠ 0°), (c) parallel to the LC alignment direction (α = 0°), and (d) alternating tilt angles of ± β.
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; thus, the LC molecules are confined not only by the two substrates but also by these boundaries [23, 24], where the distance between virtual walls is half of the pitch of the interdigitated electrodes. In an FFS cell with α = 0°, a non-zero pretilt angle is required for switching LCs without creating unintended random domains.
The low pretilt angle required to widen the viewing angle can decrease the allowable deviation of the LC alignment direction in an FFS cell with α = 0° [24], which may be a challenge for mass production. Moreover, photo-alignment techniques, which were introduced to replace the rubbing method for high performance, cannot be used for an FFS cell with α = 0° because of the very low pretilt angle. Using pixel electrodes with alternating tilts, as shown in Fig. 1(c), can be a candidate to solve the problem. In contrast to a conventional FFS structure, where adjacent pixel electrodes are parallel to each other, the interdigitated pixel electrodes in region I of Fig. 1(c) are tilted counterclockwise by a small angle β, whereas those in region II are tilted clockwise by the same angle β. Thanks to pixel electrodes with alternating tilts, an electric field is obliquely applied to the LC molecules. Thus, the device can operate well without requiring the non-zero pretilt.
When an electric field is applied between pixel electrodes and the common electrode, the LC molecules in region II of Fig. 1(c) are rotated in the direction opposite those in region I. Therefore, the LC molecules at boundary A are rotated by the elastic torque caused by rotated neighboring molecules, whereas boundary B can be treated as a virtual wall. Here, the distance between virtual walls is the pitch of the interdigitated electrodes. Although a tradeoff still exists between the transmittance and the response time, pixel electrodes with alternating tilts provide several favorable features, e.g., a large allowable deviation of the LC alignment direction for easier mass production, allowing zero-pretilt alignment for a wider viewing angle, and allowing n-LCs for higher transmittance, as will be discussed later.
3. Results and discussion
To confirm the electro-optic performance of an FFS cell using pixel electrodes with alternating tilts, the Ericksen-Leslie equation, coupled with the Laplace equation, was solved numerically using the finite element method. The Ericksen-Leslie equation is generally used to describe the motion of an LC director. Numerical calculations were performed using the commercial software TechWiz LCD 3D.
The parameters used for numerical calculations were as follows. Both the width W of the electrodes and the gap L between them were 2 μm. The pitch P ( = W + L) of the interdigitated electrodes was 4 μm. The thickness of the insulation layer between the pixel and common electrodes was 150 nm. The tilt angle β of the pixel electrodes was set to ± 1° with respect to the alignment direction of the LC molecules. The LC material parameters used in the calculations are shown in Table 1. The thickness of the LC layer was 3.5 μm and the pretilt angle was set to 0°. For comparison, the electro-optic performance of FFS cells with α = 10° or 0° was also calculated. The pretilt angle in a conventional FFS cell with α = 10° was set to 0°, whereas an FFS cell with α = 0° was set to 2°.
Table 1. Physical properties of the LC materials used in numerical calculations.
We calculated the voltage-transmittance curves of FFS cells using p-LCs, which are shown in Fig. 2. Whereas a conventional FFS cell with α = 10° showed a maximum transmittance of 31.3% at an applied voltage of 6.4 V, an FFS cell with α = 0° showed a much lower maximum transmittance of 16.4% at an applied voltage of 6.6 V, when the same electrode structure (W = L = 2 μm) was employed. As the gap L between the interdigitated electrodes was increased from 2 μm to 6 μm, the transmittance of an FFS cell with α = 0° increased to 22.2% at an applied voltage of 5.8 V, as shown in Fig. 2, because the dead zones around the virtual walls were reduced for a fixed pixel area. However, increasing the gap between the interdigitated electrodes sacrifices the response time; thus, a trade-off exists between the transmittance and the response time. In the case of an FFS cell using pixel electrodes with alternating tilts, virtual walls are built only at boundary B in the middle of the gaps between pixel electrodes, in contrast to an FFS cell with α = 0°, where virtual walls are built at boundary A in the center of the interdigitated electrodes, as well as boundary B in the middle of the gaps between them, as shown in Fig. 1. Therefore, the transmittance of an FFS cell using pixel electrodes with alternating tilts is higher than that of an FFS cell with α = 0°, when the pitch of the interdigitated electrodes is the same. The maximum transmittance of an FFS cell using pixel electrodes with alternating tilts was 21.8% at an applied voltage of 6 V.
To calculate the response time, a voltage corresponding to the maximum transmittance condition was applied to the FFS cells. Their calculated temporal switching behaviors are shown in Fig. 3. We defined the turn-on time as the transient time for the transmittance to rise from 10% to 90% of the maximum value, and vice versa for the turn-off time. The calculated turn-on [turn-off] time of the FFS cell with α = 10° was 11.9 ms [12.9 ms], whereas the turn-on [turn-off] times of the FFS cell with α = 0° with electrode gaps of 2 μm and 6 μm were 4.3 ms [1.5 ms] and 8.3 ms [5.0 ms], respectively. The shorter response time of the FFS cells with α = 0° is caused by the effect of the two-dimensional confinement with virtual walls [23, 24, 26].
Although the distance between the virtual walls in an FFS cell using pixel electrodes with alternating tilts was twice as long as that of an FFS cell with α = 0°, the response time was much shorter than that of a conventional FFS cell with α = 10°. The response time of an FFS cell using pixel electrodes with alternating tilts was slightly shorter than that of an FFS cell with α = 0° for the electrode gap of 6 μm for which the distance between virtual walls was the same. The turn-on [turn-off] time of an FFS cell using pixel electrodes with alternating tilts was 6.6 ms [4.5 ms]. The turn-off switching was 2.8 times faster than that of an FFS cell with α = 10°. The reduced pitch of the interdigitated electrodes is preferable for a faster response, whereas the increased pitch of the interdigitated electrodes is preferable for a higher transmittance.
When compared to an FFS cell with α = 0°, an FFS cell using pixel electrodes with alternating tilts can provide a much wider viewing angle and fabrication process margin. A non-zero pretilt angle is required in an FFS cell with α = 0° to suppress the creation of unintended random domains. On the other hand, a low pretilt angle is preferable for a wider viewing angle. However, it will make the process margin narrower, as shown in Fig. 4. In contrast to an FFS cell with α = 0°, an FFS cell using pixel electrodes with alternating tilts operates well with a zero pretilt angle and allows a much larger deviation of the LC alignment direction, as shown in Fig. 4, which is very useful for mass production. As shown in Fig. 5, the wider-viewing-angle characteristics can be achieved by applying a zero pretilt alignment of the proposed device [27–29].
Fig. 4 Calculated maximum acceptable deviation of the LC alignment direction vs. the pretilt angle in an FFS cell with α = 0° and the proposed FFS cell using pixel electrodes with an alternating tilt angle of β = ± 1°.
Fig. 5 Iso-contrast contour plots of biaxial film-compensated FFS cells using pixel electrodes with alternating tilts when the pretilt angle of the LC layer is (a) 2°, (b) 1°, and (c) 0°.
Although the switching speed can be increased several fold in an FFS cell with α = 0° through two-dimensional confinement of the LCs, it can only be applied to p-LCs. Recently, the FFS mode using n-LCs has drawn considerable attention because of its higher transmittance and less image flickering, which is due to the small flexoelectric polarization, in comparison to the FFS mode using p-LCs [9, 30, 31]. Both n-LCs and p-LCs can be used in a pixel electrode structure with alternating tilts. For numerical calculations, the tilt angle β of the pixel electrodes in the proposed device using n-LCs was set to ± 89° with respect to the LC alignment direction, whereas in a conventional FFS cell, the angle α between the interdigitated electrodes and the LC alignment direction was set to 80°.
We calculated the voltage-transmittance curves of FFS cells using p-LCs and n-LCs, whose material parameters are summarized in Table 1, as shown in Fig. 6. The transmittance of a conventional FFS cell using n-LCs was 33.5% at an applied voltage of 6.8 V, which is 7.0% higher than that of an FFS cell using p-LCs. The transmittance of an n-LC FFS cell using pixel electrodes with alternating tilts was 24.7% at an applied voltage of 8 V, which is 13.3% higher than that of a p-LC FFS cell with alternating-tilt pixel electrodes.
Fig. 6 Calculated voltage-transmittance curves of conventional FFS cells and the proposed cells using p-LCs and n-LCs.
We also calculated the temporal switching behaviors of FFS cells, as shown in Fig. 7. The response time of a conventional FFS cell using n-LCs was very slow because of its high rotational viscosity. The turn-on [turn-off] time of an FFS cell using n-LCs was 15.4 ms [26.1 ms]. On the other hand, the turn-on [turn-off] time of an FFS cell using n-LCs with alternating-tilt pixel electrodes was 11.6 ms [8.7 ms]. The turn-off time of an FFS cell using n-LCs with alternating-tilt electrodes was three times shorter than that of a conventional FFS cell using n-LCs. Interestingly, despite using n-LCs with high rotational viscosity, an FFS cell with alternating-tilt pixel electrodes showed a much shorter response time than that of a conventional FFS cell using p-LCs. These results show that using n-LCs can help minimize the transmittance decrease while maintaining a short response time.
Fig. 7 Calculated switching behavior of conventional FFS cells and the proposed cells using p-LCs and n-LCs.
4. Conclusion
In conclusion, we proposed an interdigitated pixel electrode structure for fast fringe-field switching of LCs. In contrast to an FFS cell with α = 0°, where a non-zero pretilt angle was required for switching LCs without creating unintended random domains, this device did not require a non-zero pretilt angle; therefore, it could retain much wider-viewing-angle characteristics by employing a zero pretilt alignment. Moreover, n-LCs could be used in this device to minimize the transmittance decrease while maintaining the short response time.
Funding
National Research Foundation of Korea (NRF) (2014R1A2A1A01004943).
References and links
1. L. Rao, S. Gauza, and S.-T. Wu, “Low temperature effects on the response time of liquid crystal displays,” Appl. Phys. Lett. 94(7), 071112 (2009). [CrossRef]
2. T.-H. Choi, Y.-J. Park, J.-W. Kim, and T.-H. Yoon, “Superfast low-temperature switching of nematic liquid crystals using quasi-impulsive driving and overdrive,” J. Disp. Technol. 12(1), 17–21 (2016). [CrossRef]
3. Y. Iwata, M. Murata, K. Tanaka, A. Jinda, T. Ohtake, T. Shinomiya, and H. Yoshida, “Novel super-fast-response, ultra-wide temperature range VA-LCD,” SID Int. Symp. Dig. Tech. Pap. 44, 431–434 (2013). [CrossRef]
4. T. Matsushima, K. Okazaki, Y. Yang, and K. Takizawa, “New fast response time in-plane switching liquid crystal mode,” SID Int. Symp. Dig. Tech. Pap. 46, 648–651 (2015). [CrossRef]
5. Y. Iyama, T. Sasaki, I. Aoyama, K. Hanaoka, T. Ishihara, M. Yashiki, K. Takase, H. Miyata, and H. Yoshida, “Transparent liquid crystal display with three states: transparent, white and black,” Proc. 22nd IDW, 1291–1294 (2015).
6. J.-H. Lee, K.-B. Park, S.-I. Jo, S.-S. Yoon, M.-J. Lee, M. Jun, and I.-B. Kang, “The study on the capability of VR with AH-IPS mode,” Proc. 26th ILCC (2016).
7. M. Oh-e and K. Kondo, “Electro-optical characteristics and switching behavior of the in-plane switching mode,” Appl. Phys. Lett. 69, 623–625 (1996). [CrossRef]
8. S. H. Lee, S. L. Lee, and H. Y. Kim, “Electro-optic characteristics and switching principle of a nematic liquid crystal cell controlled by fringe field switching,” Appl. Phys. Lett. 73(20), 2881–2883 (1998). [CrossRef]
9. D. H. Kim, Y. J. Lim, D. E. Kim, H. Ren, S. H. Ahn, and S. H. Lee, “Past, present, future of fringe-field switching liquid crystal display,” J. Inf. Disp. 15(2), 99–106 (2014). [CrossRef]
10. Z. Ge, S.-T. Wu, S. S. Kim, J. W. Park, and S. H. Lee, “Thin cell fringe-field-switching liquid crystal display with a chiral dopant,” Appl. Phys. Lett. 92(18), 181109 (2008). [CrossRef]
11. H. Chen, F. Peng, Z. Luo, D. Xu, S.-T. Wu, M.-C. Li, S.-L. Lee, and W.-C. Tsai, “High performance liquid crystal displays with a low dielectric constant material,” Opt. Mater. Express 4(11), 2262–2273 (2014). [CrossRef]
12. D. Andrienko, F. Barbet, D. Bormann, Y. Kurioz, S.-B. Kwon, Y. Reznikov, and M. Warenghem, “Electrically controlled director slippage over a photosensitive aligning surface; in-plane sliding mode,” Liq. Cryst. 27(3), 365–370 (2000). [CrossRef]
13. Y. Kim, Y.-J. Lee, D.-H. Kim, J.-H. Baek, J.-H. Lee, B.-K. Kim, C.-J. Yu, and J.-H. Kim, “Fast response time of fringe-field switching liquid crystal mode devices with reactive mesogens in a planar alignment layer,” J. Phys. D Appl. Phys. 46(48), 485306 (2013). [CrossRef]
14. J.-I. Baek, K.-H. Kim, J. C. Kim, T.-H. Yoon, H. S. Woo, S. T. Shin, and J. H. Souk, “Fast in-plane switching of a liquid crystal cell triggered by a vertical electric field,” Jpn. J. Appl. Phys. 48(10), 104505 (2009). [CrossRef]
15. Y. Li, Z. Ge, R. Lu, M. Jiao, and S.-T. Wu, “Fast-response liquid-crystal displays using crossed fringe fields,” J. Soc. Inf. Disp. 16(10), 1069–1074 (2008). [CrossRef]
16. T.-H. Choi, J.-W. Kim, and T.-H. Yoon, “Fast in-plane switching of negative liquid crystals using crossed patterned electrodes,” Jpn. J. Appl. Phys. 53(8), 081701 (2014). [CrossRef]
17. J.-W. Kim, T.-H. Choi, and T.-H. Yoon, “Fast switching of nematic liquid crystals over a wide temperature range using a vertical bias electric field,” Appl. Opt. 53(26), 5856–5859 (2014). [CrossRef] [PubMed]
18. T.-H. Choi, Y.-J. Park, J.-W. Kim, and T.-H. Yoon, “Fast grey-to-grey switching of a homogeneously aligned liquid crystal device,” Liq. Cryst. 42(4), 492–496 (2015). [CrossRef]
19. M. J. Escuti, C. C. Bowley, G. P. Crawford, and S. Zumer, “Enhanced dynamic response of the in-plane switching liquid crystal display mode through polymer stabilization,” Appl. Phys. Lett. 75(21), 3264–3266 (1999). [CrossRef]
20. T.-H. Choi, Y. Choi, Y.-J. Park, J.-W. Kim, and T.-H. Yoon, “Cell gap effects on electro-optic performance of a polymer-stabilized liquid crystal cell,” IEEE Photonics Technol. Lett. 28(10), 1138–1141 (2016). [CrossRef]
21. H. Kim and J.-H. Lee, “Fast falling time of fringe-field-switching negative dielectric anisotropy liquid crystal achieved by inserting vertical walls,” Appl. Opt. 54(5), 1046–1050 (2015). [CrossRef] [PubMed]
22. T.-H. Choi, Y.-J. Park, J.-W. Kim, and T.-H. Yoon, “A liquid crystal cell with double-side protrusion electrodes for fast response and low-voltage operation,” Liq. Cryst. 43, 411–416 (2016).
23. T.-H. Choi, S.-W. Oh, Y.-J. Park, Y. Choi, and T.-H. Yoon, “Fast fringe-field switching of a liquid crystal cell by two-dimensional confinement with virtual walls,” Sci. Rep. 6, 27936 (2016). [CrossRef] [PubMed]
24. T.-H. Choi, Y. Choi, J.-H. Woo, S.-W. Oh, and T.-H. Yoon, “Electro-optical characteristics of an in-plane-switching liquid crystal cell with zero rubbing angle: dependence on the electrode structure,” Opt. Express 24(14), 15987–15996 (2016). [CrossRef] [PubMed]
25. S.-W. Oh, A.-K. Kim, B. W. Park, and T.-H. Yoon, “Optical compensation methods for the elimination of off-axis light leakage in an in-plane-switching liquid crystal display,” J. Inf. Disp. 16(1), 1–10 (2015). [CrossRef]
26. T.-H. Choi, J.-H. Woo, Y. Choi, and T.-H. Yoon, “Effect of two-dimensional confinement on switching of vertically aligned liquid crystals by an in-plane electric field,” Opt. Express 24(18), 20993–21000 (2016). [CrossRef] [PubMed]
27. O. Yaroschchuk, V. Kyrychenko, D. Tao, V. Chigrinov, H. S. Kwok, H. Hasebe, and H. Takatsu, “Stabilization of liquid crystal photoaligning layers by reactive mesogens,” Appl. Phys. Lett. 95(2), 021902 (2009). [CrossRef]
28. S.-W. Oh, J.-H. Park, and T.-H. Yoon, “Near-zero pretilt alignment of liquid crystals using polyimide films doped with UV-curable polymer,” Opt. Express 23(2), 1044–1051 (2015). [CrossRef] [PubMed]
29. S.-W. Oh, D.-J. Lee, M.-K. Park, K. H. Park, J.-H. Lee, B. K. Kim, and H.-R. Kim, “Enhancement of viewing angle properties of a single-domain fringe-field switching mode using zero pretilt alignment,” J. Phys. D Appl. Phys. 48(40), 405502 (2015). [CrossRef]
30. L. M. Blinov and V. G. Chigrinov, Electrooptic Effects in Liquid Crystal Materials (Springer, 1994).
31. Y. Chen, Z. Luo, F. Peng, and S.-T. Wu, “Fringe-field switching with a negative dielectric anisotropy liquid crystal,” J. Disp. Technol. 9(2), 74–77 (2013). [CrossRef]
