Transient flickering behavior in fringe-field switching ... - OSA Publishing

Transient flickering behavior in fringe-field switching liquid crystal mode analyzed by positional asymmetric flexoelectric dynamics Dong-Jin Lee,1,3 Gyu-Yeop Shim,2 Jun-Chan Choi,2 Ji-Sub Park,2 Joun-Ho Lee,3 Ji-Ho Baek,3 Hyun Chul Choi,3 Yong Min Ha,3 Amid Ranjkesh,2 and Hak-Rin Kim1,2,* 1

Department of Sensor and Display Engineering, Kyungpook National University, Daegu 702-701, South Korea 2 School of Electronics Engineering, Kyungpook National University, Daegu 702-701, South Korea 3 LG Display Co., Ltd., Gumi, Gyeoungsangbuk-do 730-340, South Korea * [email protected]

Abstract: We analyzed a transient blinking phenomenon in a fringe-field switching liquid crystal (LC) mode that occurred at the moment of frame change even in the optimized DC offset condition for minimum image flicker. Based on the positional dynamic behaviors of LCs by using a highspeed camera, we found that the transient blink is highly related to the asymmetric responses of the splay-bend transitions caused by the flexoelectric (FE) effect. To remove the transient blink, the elastic property adjustment of LCs was an effective solution because the FE switching dynamics between the splay-enhanced and bend-enhanced deformations are highly dependent on the elastic constants of LCs, which is the cause of momentary brightness drop. ©2015 Optical Society of America OCIS codes: (160.3710) Liquid crystals; (230.3720) Liquid-crystal devices.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

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Received 10 Nov 2015; revised 18 Dec 2015; accepted 20 Dec 2015; published 24 Dec 2015 28 Dec 2015 | Vol. 23, No. 26 | DOI:10.1364/OE.23.034055 | OPTICS EXPRESS 34055

18. D.-J. Lee, G.-Y. Shim, S.-H. Yoo, J.-H. Lee, J.-H. Baek, H. C. Choi, Y. M. Ha, and H.-R. Kim, “Analysis on flexoelectric effect in AH-IPS LC mode under low frame rate driving using a high speed camera,” in Proceedings of The 21st International Display Workshops (2014), pp. 155–158. 19. K.-C. Chu, C.-W. Huang, R.-F. Lin, C.-H. Tsai, J.-N. Yeh, S.-Y. Su, C.-J. Ou, S.-C. F. Jiang, and W.-C. Tsai, “A method for analyzing the eye strain in fringe-field-switching LCD under low-frequency driving,” SID Int. Symp. Digest Tech. Pap. 45(1), pp. 308–311 (2014). 20. J.-W. Kim, T.-H. Choi, T.-H. Yoon, E.-J. Choi, and J.-H. Lee, “Elimination of image flicker in fringe-field switching liquid crystal display driven with low frequency electric field,” Opt. Express 22(25), 30586–30591 (2014). 21. D.-J. Lee, M.-K. Park, J.-T. Kim, J.-H. Baek, J.-H. Lee, H. C. Choi, A. Ranjkesh, and H.-R. Kim, “Electro-optic variation in AH-IPS liquid crystal mode by controlling the flexoelectric effect of liquid crystal,” SID Int. Symp. Digest Tech. Pap. 46(1), 1573–1576 (2015). 22. S.-W. Oh, M.-K. Park, H. J. Lee, J. M. Bae, K. H. Park, J.-H. Lee, B. K. Kim, and H.-R. Kim, “Improvement of asymmetric viewing angle properties in single-domain fringe-field-switching liquid crystal mode by using parallel-rubbed alignment surfaces,” Liq. Cryst. 41(4), 572–584 (2014). 23. 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). 24. D.-K. Yang and S.-T. Wu, Fundamentals of Liquid Crystal Devices (Wiley, 2006), Chap. 4. 25. H. J. Yun, M. H. Jo, I. W. Jang, S. H. Lee, S. H. Ahn, and H. J. Hur, “Achieving high light efficiency and fast response time in fringe field switching mode using a liquid crystal with negative dielectric anisotropy,” Liq. Cryst. 39(9), 1141–1148 (2012). 26. 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). 27. Y. Chen, F. Peng, T. Yamaguchi, X. Song, and S.-T. Wu, “High performance negative dielectric anisotropy liquid crystals for display applications,” Crystals 3(3), 483–503 (2013). 28. C. S. Lim, J.-H. Lee, H. C. Choi, C. H. Oh, and S. D. Yeo, “Fast response time in IPS mode using LC mixtures with high elastic constant,” in Proceedings of The 4th International Meeting on Informational Display (2004), pp. 843–846. 29. D. K. Kim, C. S. Lim, D.-J. Lee, J. I. Hwang, H. G. Jung, S. W. Lee, C. H. Oh, and I. B. Kang, “The improvement of GTG response time using new concept LC mixture in S-IPS mode for high frame frequency technologies,” in Proceedings of The 6th International Meeting on Informational Display (2006), pp. 864–867.

1. Introduction Fringe-field switching (FFS) liquid crystal (LC) mode has been widely used for mobile phones, tablets, and high-end notebook displays owing to its high optical efficiency and low power consumption properties required for ultra-high resolution displays [1]. In particular, many efforts in the past decades have attempted to further reduce the power consumption, and this research will continue in the future [2]. Because the power consumption to operate the LC display (LCD) panel is linearly proportional to the driving frequency, recently, a lowfrequency driving method has been applied to reduce the power consumption [2]. However, the flexoelectric (FE) effect is easily observed in the FFS LC mode because the highly distorted and spatially asymmetric LC distribution on application of an electric field causes polarization by asymmetric splay and bend deformations in a cell [3–13]. Since the reorientation of LC directors affected by the FE effect depends on the sign of the applied field (in contrast to the dielectric behavior, which does not show signal-polarity dependency), the brightness level is influenced by the polarity of the driving signal. This electro-optic variation has not been recognized at the frame rate of 60 Hz, which has been broadly applied to most LC displays so far, because of the perceptual integration of the intensity change in the human visual system [14]. However, this temporal brightness variation is more easily observed at lower-frequency driving [15–17] and leads to severe flickering problems [11–21]. Moreover, when a positive dielectric LC (p-LC) is used, this problem becomes more serious owing to the large deformational variation of the LC directors between the splay and the bend elasticity, which causes a polar angle distribution change of LC directors depending on the polarity of the driving field. This is a critical obstacle to reducing power consumption in the FFS LC mode even though the p-LC has many advantages such as low cost, low viscosity, high dielectric constant, and high reliability. Therefore, the FE effects in the FFS LC mode have been studied by some researchers [18– 21]. They demonstrated the brightness variation depending on the signal polarity at lowfrequency driving, and proposed methods to analyze and reduce the brightness variation. Our

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group has also researched an influence of DC offset on the image flicker in the FFS pixel and suggested a controllable method of the FE polarization to reduce the dependency of signal polarity on the positional brightness variation [18,21]. However, these studies were mainly focused on the flickering phenomena generated by the static behavior, i.e., the flicker that is recognized mostly in the range showing saturated LC orientation after a signal polarity change. The static flicker based on the average brightness level over a whole pixel area can be removed by the DC offset adjustment and the control of FE polarization [20,21]. By contrast, the dynamic flicker still occurs in general even after the DC offset adjustment because of the dynamic behavior of LC directors caused by the FE effect between both static LC orientations at the moment of frame change, which has been frequently mentioned as “transient blink”. Even if the static flicker is completely removed, the dynamic flicker would behave as an additional flickering source despite a short moment. Although the dynamic flicker as well as the static flicker should be eventually solved together to eliminate image flicker at lowfrequency driving, an analysis on the cause of the dynamic flicker arising at the moment of frame change and a promising solution to remove the dynamic flicker have not been sufficiently reported yet. In this paper, we investigated the causes of the static and dynamic flickers in the FFS LC mode by using a polarizing microscope equipped with a high-speed camera. We could analyze the positional and time-varying brightness variation by the FE effect in detail. First, we measured the positional brightness in an FFS pixel depending on the polarity and magnitude of the DC offset. This result could be explained by the deformational difference of LC directors according to the direction and magnitude of the electric field at the moment showing the saturated brightness after a signal polarity change. This corresponds to the static flicker. Second, we focused on the dynamic flicker that appears as a transient blinking phenomenon caused by the FE effect at the moment of frame change even in the optimized DC offset condition for the minimum image flicker. We analyzed this transient phenomenon in terms of time-varying change of LC directors and its asymmetric responses of the splaybend transitions caused by the FE effect at the moment of frame change. Finally, we proposed a promising solution to eliminate the dynamic flickering problem based on this analysis. 2. Experimental procedure The pixel structure of FFS LC mode used for this work is described in Fig. 1. The bottom substrate was composed of the stacked layers with electrodes and an insulator without a thin film transistor (TFT) to exclude the charging and holding effects of the TFT in our flickering analysis. In this structure, operating signal can be directly applied the top pixel and bottom common electrodes. The width of the patterned indium-tin-oxide (ITO) for the top pixel electrode was 3 μm and the spacing between the pixel electrodes was 4 μm. A SiNx (εr = 6.7) layer was used as an insulator between the top pixel and the bottom common electrodes, and its thickness was 0.5 μm. To obtain a uniform cell gap, we patterned column spacers (CS) of 3.4 μm in height on the bare glass and this CS-coated glass was used for the top substrate. We applied PT114 (JNC Corporation) for the planar alignment layers of both substrates, and used ML0643 (Merck) for a p-LC. The physical properties of the employed LC material are as follows: dielectric anisotropy of Δε = 7.9; birefringence of Δn = 0.1026; and elastic constants of K11 = 10.2 pN, K22 = 6.9 pN, and K33 = 13.6 pN. The alignment layers were spin-coated on both substrates. These substrates were prebaked at 80 °C for 60 s to evaporate the solvent, and then post-baked at 250 °C for 30 min for polyimidization. Both substrates were rubbed to generate an easy axis of LC directors of 7° by using a commercial rubbing cloth. Both rubbed substrates were assembled, and the LC material was filled into the cells in the isotropic phase.

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Fig. 1. Structure and cross-sectional view of the FFS pixel.

We operated the FFS LC cells by using a square wave signal of 0.5 Hz at a voltage of V20. V20 is the voltage for 20% transmittance of full brightness level, and it corresponds to the halfgray level of gamma 2.2 grayscale. Since the brightness variation is very sensitive to human eyes at this V20, the flicker is accurately observable at V20, while the temporal flickering phenomena around the full white level cannot be observed considering the human eyes’ light sensitivity depending on the average brightness levels. Figure 2 shows our evaluation system. We used a high-speed camera (FASTCAM MC2.1, Photron) at a rate of 250 frames per second (fps) to obtain the moving and still images of the FFS pixel. The positional brightness distributions of the pixel images obtained with the high-speed camera were extracted through image analysis. These positional distributions were analyzed to evaluate the brightness variations in static flicker depending on the DC offset conditions at the saturated moment within each frame and the FE dynamics of the LC directors at the moment of frame change.

Fig. 2. Schematic image of high-speed camera evaluation system and operating signal waveform. (V20: voltage for 20% transmittance of full brightness level, Voffset: induced offset voltage, and Vmin: the optimum DC offset condition showing the minimum static flicker.)

3. Results 3.1 DC offset dependence of positional brightness In general, an electrical bias occurs in the LCD panels owing to the parasitic capacitance in TFT area, the asymmetry of electric field, the properties of stacked layers etc. Thus, during the mass production process, DC offset should be optimized over a pixel area to compensate for the electrical bias and minimize the brightness variation depending on the signal polarity.

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Figure 3 shows the variation of average brightness over a pixel area observed at the different DC offset values in our FFS cell that is operated at V20 with a square wave signal of 0.5 Hz. Because our FFS cell was designed for a normally black mode, the brightness in the signal polarity strengthened by induced DC offset increases and the image flicker becomes severe as shown in Figs. 3(a) and 3(c). To minimize the image flicker, the DC offset should be optimized based on the average brightness level over a pixel area. The brightness result balanced by the optimized DC offset between the positive and negative frames is shown in Fig. 3(b). Since our FFS cell does not have a TFT layer, and the difference between the widths of pixel electrode and interelectrode, related with the asymmetry of the positional fringe fields, is not much, the optimum DC offset value did not deviate too much from 0 V. The value was 0.19 V.

Fig. 3. The variation of average brightness level over a pixel area observed at the different DC offset values in our FFS LC cell that is operated at V20 with a square wave signal of 0.5 Hz. (a) Negative DC offset of 1 V from Vmin (Voffset – Vmin < 0). (b) Optimum DC offset (Voffset – Vmin = 0). (c) Positive DC offset of 1 V from Vmin (Voffset – Vmin > 0). (V20: voltage for 20% transmittance of full brightness level, Voffset: induced offset voltage, and Vmin: optimum DC offset showing the minimum flicker.)

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Fig. 4. (a) Positional brightness distribution depending on DC offset condition and signal polarity. (Contrast of images was adjusted from original images for the vivid comparison.) (b) DC offset sensitivity on brightness and magnitude of flicker in the FFS pixel. (Voffset: induced offset voltage, Vmin: optimum DC offset showing the minimum flicker, Voffset – Vmin: additional DC offset deviated from Vmin.)

In the FFS LC mode, because the orientation of the LC director is not uniform within a pixel because of the asymmetric fringe-field and the FE effect, we need to investigate in depth the effect of DC offset on the positional brightness variation and LC orientation by the FE effect. This investigation will provide a fundamental understanding of the static flickering behavior and will be helpful to look for the solution for much improved flickering performance at low-frequency driving. Figure 4(a) shows the positional brightness

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distributions and still images depending on Voffset – Vmin in both positive and negative frames. As defined in Fig. 2, Voffset is the induced offset voltage in the square wave signal, and Vmin corresponds to the optimum DC offset showing the minimum flicker. Thus, Voffset – Vmin of the horizontal axis in Fig. 4(a) means that the DC offset deviated from Vmin, and the origin corresponds to the condition that exhibits the optimum DC offset showing the minimum static flicker (Voffset = Vmin). As shown in Fig. 4(a), for the condition of Voffset – Vmin < 0, the brightness variation between positive and negative frames on the interelectrode was reduced, but the brightness variation on the pixel electrodes became more severe. Thus, the flicker on the pixel electrodes got worse. By contrast, for the condition of Voffset – Vmin > 0, the brightness variation on the pixel electrodes was reduced, and the flicker on the interelectrode got worse. This means that the actual Vmin, at which the brightness variation is minimized, is locally different, although we cannot help fixing only one Vmin value in the display panel considering the average brightness over the entire pixel area. This detailed positional brightness variation could not be detected with the measurement of the average brightness level within the entire pixel area as shown in Fig. 3. The plot of Fig. 4(b) shows the measured flickering values with changing Voffset – Vmin. For the brightness measurement, we used a He-Ne laser (633 nm) as a light source, and the brightness variation was detected by a photo detector. The degree of flicker can be calculated on the basis of the measured brightness variation by using Eq. (1):

(Tmax − Tmin ) AC component = × 100 DC component  (Tmax + Tmin )    2 (1)   : Maximum and minimum values of measured average brightness

Degree of flicker (%) =

Tmax and Tmin

over an entire pixel in fluctuating brightness In the average view of the entire pixel area, the degree of flicker was 0.035% for the optimum condition of Voffset = Vmin, which is a very small value and is close to 0. However, as shown in the middle image of Fig. 4(a), we can observe the positional brightness fluctuation whenever the field polarity changes, even in the optimum condition of Voffset = Vmin. Although the positional brightness fluctuation itself might not be conceived by human eyes considering human eyes’ resolution if the average brightness levels are balanced by the optimized DC offset condition once, this positional brightness fluctuation showing the locally different Vmin, which was mentioned previously, becomes a cause of the DC charge accumulation at the interface of the LC and PI surface and results in a narrow adjustable range for the optimum common voltage in mass production. Figure 4(a) depicted a severe brightness variation on the middle points of the electrode and interelectrode owing to the reoriented configuration of LC directors, which is extremely varied according to the signal polarity. This variation is caused by the coupling effect between the polarity of the fringe-field and the FE polarization by the asymmetrically deformed LC structure. In Fig. 4(a), considering that a brightness is lower in the position where an electric field heads upward and higher where an electric field heads downward for both positive and negative frames, the FE polarization by bend deformation is more dominant and the sign of es−eb, which is the difference between splay and bend FE coefficients described in Eq. (2) [9], is estimated at “–” for ML0643 used in this study.

Pf = es ⋅ n ( ∇ ⋅ n ) + eb ⋅ n × ( ∇ × n ) Pf : Flexoelectric polarization es and eb : Splay and bend flexoelectric LC coefficients

(2)

n : Vector description of the LC director As illustrated in Fig. 5(a), on the middle points of the electrode and interelectrode of the FFS LC mode, the tangential vector component of the fringe-field is important near the

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surface. This makes the polar angle of the LC directors larger, while the horizontal field on the middle points is weak [22,23]. The LC directors at these points induce splay or bend deformations and the transition between these two deformations also occurs whenever the signal polarity changes. A schematic image of splay-bend transition and the measured brightness profiles in this study are shown in the cross-sectional view of Fig. 5(b). For example, when an electric field heads upward along the “+” direction of the z-axis on the middle point of the pixel electrode, the LC directors at this point easily form a bend structure coupling with the electric field. This behavior corresponds to the blue line of Fig. 5(b). Similarly, when an electric field heads downward along the “–” direction of the z-axis, the LC directors shows easily the splay structure as the red line in Fig. 5(b). When we evaluated the DC offset value for the minimum static flickering condition using the E7 LC, the “–” DC offset voltage was required, which was the opposite sign condition to ML0643 shown in Fig. 5. Considering that the relative FE coefficient of the E7 is es−eb > 0 [11,12], the polarity of the DC offset condition, measured experimentally, and the FE-coupled LC distribution, explained here, matched well each other. Since the effective Δn (Δneff) is closely affected by the polar angle of the LC director [24], this deformational change leads to a brightness variation between the splay and bend deformations. Therefore, whenever the splay-bend transition occurs by a directional change of the electric field, the positional brightness profile will be varied and this change emerges as flickering behavior, consequently.

Fig. 5. (a) Reorientation of LC directors and observed positional brightness image under fringe-fields. (b) Schematic image of splay-bend transition and measured brightness profiles depending on the signal polarity.

The positional brightness fluctuation caused by the signal polarity change arises because the FE polarization is not eliminated even though the DC offset for the minimum static flicker is optimized based on the average brightness over an entire pixel area. This positional fluctuation cannot be improved without a fundamental approach to reduce or remove the FE polarization in the FFS LC mode. To improve the static flicker, the following two approaches have been proposed:

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i) To prevent the Δneff variation of LC directors, the polar angle variation caused by the signal polarity should be suppressed. ii) Despite the asymmetric polar distribution of the LC directors, the FE polarization caused by the asymmetric distribution should be minimized. For the first case, the negative dielectric LC (n-LC) can be tried and can be a very helpful solution. The deformational change of the n-LC in the polar direction is much less sensitive to the electric field, unlike the p-LC [25–27], because its dipole moment that responds to the electric field is located in the lateral position of the LC molecule, unlike the p-LC having the longitudinal dipole moment. For the second case, our group has been researching a controllable method of the FE polarization by doping the bent-core LCs (BLCs). In that work, we could confirm that the BLCs, used in appropriate ratios to dope a nematic host LC, can modify the FE polarization of a host LC, reducing it, cancelling it, or even changing its sign owing to the remarkable ability of the BLC to change the FE coefficients and the spatial distribution of the host LC. In this section, we showed that the positional brightness variation in the FFS pixel, which occurs even in the optimum DC offset condition and could not be detected in the average brightness level within the entire pixel area. This investigation on the time-varying behavior is very helpful to analyze the cause for flickering phenomenon. 3.2 Dynamic behavior of LC directors at the moment of frame change

Fig. 6. Transient blinking phenomenon of a conventional p-LC observed at the moment of frame change in FFS LC mode that is operated with a square wave signal of 0.5 Hz. (Wi→ j and Ii→j: temporal width and intensity of momentary brightness drop peaks, respectively. i→j denotes the direction of signal change between positive (P) and negative (N) frames).

Even if a minimum static flickering condition is achieved by removing difference of the average brightness level between the positive and negative frames, the temporal brightness change would take place in a moment owing to the different transition responses between the splay-enhanced and bend-enhanced deformations at the moment of frame change as shown by the momentary brightness drops between frames in Fig. 3(b). This leads to the additional blinking problem in the low-frequency-driven FFS LC mode using a p-LC. To distinguish this

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from the static flicker, which was explained in the previous section, we defined this transient blinking phenomenon as a dynamic flicker in this paper. The dynamic flicker in the FFS LC mode should be also solved for the application of the low-frequency driving solution. Figure 6 shows the time-varying brightness under the optimum DC offset condition corresponding to the middle image of Fig. 4(a). As we can see in Fig. 6, even though the brightness levels become equivalent between both signal polarities, the transient blink is frequently observed whenever the signal polarity changes. This momentary brightness drop results in the dynamic flickering problem. Although the transient blink cannot be observed in the severe static flickering situation as shown in Figs. 3(a) and 3(c) since the intensity of transient blink is not large, it can be easily observed in the small static flickering condition as shown in Fig. 3(b). The magnitude of the transient blink is determined in proportion to the area of momentary brightness drop peaks. The momentary brightness drop peaks that cause the transient blink show the similar temporal duration between positive and negative frame changes (WN→P = WP→N), however, they show the different peak intensities (IN→P < IP→N). This different peak intensity is related to both the pixel design and the LC dynamics. This will be discussed in this section in detail. To verify the cause of the transient blinking phenomenon, we monitored the moving image with a high-speed camera and analyzed the positional brightness variation at the moment of frame change. The positional brightness distribution in the FFS LC mode depends on Voffset – Vmin, as shown in Fig. 4(a). Figures 7(a) and 7(b) show the temporal variation of positional brightness distributions at the moment of frame change for the condition showing the minimum static flicker corresponding to the middle image of Fig. 4(a). The “Moment of transition” plots in Figs. 7(a) and 7(b) were obtained at the moment of 8 ms after changing a signal polarity that corresponded the moment showing the largest brightness drop as shown in Fig. 6 with the temporal variation of the average brightness level within a pixel. Figure 7(a) is for the moment of frame change from positive to negative, and Fig. 7(b) is for the opposite condition. In Fig. 7(a) we can see that the brightness on the interelectrodes drops rapidly; however, the brightness on the pixel electrodes increases slowly. This behavior originates from that the FEcoupled transition response from the bend-enhanced deformation to the splay-enhanced deformation, which induces the brightness increase, is slower than the opposite situation. In Fig. 7(b) under the opposite field direction condition, because the brightness recovery on the interelectrodes is relatively slow, it also shows a brightness drop similar to that in Fig. 7(a). The configuration of LC directors on the pixel electrodes and the interelectrodes depending on the field direction, and the mechanism inducing the momentary brightness drop, are illustrated in Fig. 7(e). From this point of view, Wi→j in Fig. 6 is related to the response for the splay-bend transition, and the difference according to the changing direction between WN→P and WP→N is indistinguishable. However, the intensity of the momentary brightness drop, Ii→j, is different depending on the state-changing direction. The reason why IP→N is larger than IN→P in Fig. 6 is because of the pixel electrode design of the FFS mode. In the conventional FFS mode, because the interelectrode distance is generally larger than the pixel electrode width as described in Fig. 1 for the maximized transmittance performance, IP→N induced by slow splay-to-bend transition (which leads to brightness reduction as mentioned before) on the interelectrodes shows a larger influence on the brightness drop of an entire pixel than IN→P by splay-to-bend transition on the pixel electrodes.

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Fig. 7. Measured LC dynamics at the moment of frame change under the minimum static flickering condition using the conventional p-LC. (a,c) Positional and time-varying views for the frame change from positive to negative. (b,d) Positional and time-varying views for the frame change from negative to positive. (e) Schematic image of asymmetric dynamic behavior between splay and bend deformations. (τi→j: response for the splay-bend transition. i→j denotes the change between splay (S) and bend (B) deformations).

Figures 7(c) and 7(d) show the dynamic brightness changes at specific positions during the moment of frame change. As shown in Figs. 7(a) and 7(b), positions 1 and 3 are the edges of the pixel electrodes and correspond to the area exhibiting the highest brightness. The brightness in these positions is insensitive to the polarity change of the fringe-field, and is not affected by the FE polarization since the LC directors rotate azimuthally not showing the

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rotation to polar direction [23]. Thus, the brightness variations of positions 1 and 3 were almost flat irrespective of the field polarity, as shown in Figs. 7(c) and 7(d). However, positions 2 and 4 are the middle points on the pixel electrodes and the interelectrodes, respectively. In these positions, the polar distribution of the LC directors near the surface is easily affected by the tangential vector component of the fringe-field because of the weak horizontal field [22,23]. Thus, the brightness in these positions is significantly affected by the polarity of the fringe-field because of the notable transition between the splay-enhanced and bend-enhanced LC deformations. As we can see in Figs. 7(c) and 7(d), noticeable brightness variations were observed in positions 2 and 4, as expected. Significantly, the decreasing slope was steeper than the increasing one regardless of the changing direction of the signal polarity. We calculated the decreasing and increasing slopes after the frame change based on the positional brightness values at 4 ms and 8 ms after a signal polarity change showing the steepest variation. The calculated results were –1.427 /ms and 0.814 /ms, and –1.663 /ms and 0.653 /ms in Figs. 7(c) and 7(d), respectively. The rapid brightness drops of positions 2 and 4 were reflected to the total sum of brightness values shown in the top plots of Figs. 7(c) and 7(d), with the largest brightness drop at the point of 8 ms after the frame change. This transient blink was caused by the different LC dynamic responses between the splay-to-bend and bend-to-splay transitions. This was clearly confirmed through an evaluation by using the high-speed camera. The transient blink should be properly eliminated for the improved flickering performance even under the DC offset condition that is optimized based on the static flickering analysis for the entire pixel area. 3.3 Reduction of transient blinking phenomenon In the middle points on the pixel electrodes and interelectrodes of the FFS LC mode, two saturated LC orientations (i.e. splay-enhanced and bend-enhanced) exist depending on the direction of signal polarity and the FE coefficients of the LC due to the FE effect. More importantly, the transient blinking phenomenon occurs because of the different FE-coupled LC dynamic responses between these two LC deformations. Because the dynamic response of the LC molecules is highly related to their elastic properties [24,28,29], the mismatch of the responses between the splay-bend transitions can be balanced by adjusting K11 and K33 values of LC material. In our experimental case, to compensate for the relatively slow response of the bent-to-splay transition, we adjusted the elastic constants such as the next two types of cases and evaluated the transient blinking phenomenon in the same way as the previous section. (i) For the faster transition from bend to splay deformations, we reduced K11 as compared with the reference p-LC (ref. p-LC), which was named LC-B. (ii) For the slower transition from splay to bend deformation, we increased K33, named LC-C. Besides these two types of LCs, we prepared one more LC sample, which has very similar K11 and K33 values to the ref. p-LC for reproducibility, named LC-A. Other properties such as Δn, Δε, and transition temperature of the prepared LC mixtures remained the similar as those for the ref. p-LC. The properties of the ref. p-LC and prepared LCs are summarized in Table 1. Table 1. Properties of reference p-LC and three LC samples employed for evaluation of the transient blinking phenomenon. LC Ref. p-LC LC-A LC-B LC-C

K11 [pN] 13.1 13.9 10.8 14.0

K33 [pN] 13.6 13.8 13.9 16.0

Δε 7.9 7.9 7.7 8.0

Δn 0.1026 0.1029 0.1029 0.1027

TNI [°C] 80.0 80.4 79.8 82.3

The K11 value for the LC-B was 10.8 pN with a reduction by 17.6% from 13.1 pN for the ref. p-LC, but its K33 value was similar to that of ref. p-LC. The K33 value for the LC-C was 16.0 pN with an increase of 17.6% from 13.6 pN for the ref. p-LC, but its K11 value was similar to that of ref. p-LC. The variation of the LC elastic constants over 17% was reported to affect the response time performance of the LC display [28,29]. This variation assumed to be large enough to change the dynamic response of the LC directors in our study.

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Fig. 8. Transient blinking phenomena and its magnitude depending on elastic constants observed at the moment of frame change in FFS LC mode that is operated with a square wave signal of 0.5 Hz.

Figure 8 shows the time-varying brightness under the minimum static flicker condition for each evaluated LC. As we can see in Fig. 8(b), the LC-A that has a similar K11 and K33 reproduced the transient blink with a similar width and intensity to those of the ref. p-LC in Fig. 8(a). We expect that the LC-A with similar elastic constants to the ref. p-LC shows a similar asymmetric dynamic response between the splay and bend deformations. In Fig. 8(c), we can observe a larger transient blink for the LC-B. Although the K11 of the LC mixture was reduced as planned, the reduced elastic constant affects the second dynamics for the reorientation to the saturated position in the midplane after the transition to splay deformation in the surface, while preserving the fast response to the bend deformation showing the low brightness. Because of the slowed second dynamics, the brightness drop increased twice compared with that of the ref. p-LC, and the recovery to the static state after orientation of the LC director also became very slow, as shown in Fig. 8(c). We assert that this is strongly related to the very slow increasing result that was observed after the transient blink in Fig. 8(c). Therefore, the dynamic flicker cannot be solved with a reduced K11 owing to the side effect showing a slow elastic recovery. By contrast, a significantly reduced transient blink

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could be observed with the LC-C that has a larger K33, as shown in Fig. 8(d). This result can be explained by two probable causes: the fast response to the bend deformation was suppressed, or the response to the splay was accelerated owing to the relatively increased K33. Enlarged transient peaks of the evaluated LCs are summarized in Fig. 8(e) for comparison. To distinguish the two probabilities in detail on the results of the LC-C that shows the reduced transient blink, we again investigated the positional and time-varying brightness variation at the moment of frame change by using a high-speed camera. Figures 9(a) and 9(b) show the positional brightness distributions using the LC-C for the moment of frame change from positive to negative states and for the opposite condition, respectively. In Figs. 9(a) and 9(b), we can see that the deformational transition responses on the pixel electrodes and the interelectrodes were also observed at the moment of frame change; however, detail dynamics were different from the previous results shown in Figs. 7(a) and 7(b) in that the transition response became similar between the splay-to-bend and bend-to-splay transitions. Figures 9(c) and 9(d) show the dynamic behaviors of brightness for positions 1–4 at the moment of frame change. Positions 2 and 4 are also sensitive points to FE polarization such as the ref. p-LC shown in Figs. 7(c) and 7(d). Moreover, these also exhibit noticeable brightness variations in our approach. However, the decreasing and increasing slopes, which were calculated in the same way as those of Figs. 7(c) and 7(d), were almost equalized at the moment of change. The values of decreasing and increasing slopes after the frame change in Figs. 9(c) and 9(d) were –0.821 /ms and 0.784 /ms, and –1.024 /ms and 0.996 /ms, respectively. Significantly, with a larger K33, the brightness-decreasing responses corresponding to the splay-to-bend transition became slower than those of the ref. p-LC for both signal polarity changes. The high bend elastic property means “hard to bend,” and this makes the transition from splay to bend deformations slower. Especially, as shown at the points of 8 ms after the frame change in position 4 of Fig. 9(c) and position 2 of Fig. 9(d), where the LC directors change from splay to bend deformations and the largest brightness drops were observed in Figs. 7(c) and 7(d), we could confirm that each brightness drop became slow enough to compensate the slowly rising brightness unlike the aspect observed in Figs. 7(c) and 7(d). This is a highly meaningful result that realizes a slower response from the splay to bend deformation owing to a larger K33, and validates the mechanism of the transient blink mentioned in subsection 3.2. As a result, in the total sum of brightness values shown in the top plots of Figs. 9(c) and 9(d), the transient blinks disappeared at the moment of frame change by exactly balancing the FE-coupled dynamic responses of splay-to-bend with bendto-splay symmetrically, as illustrated in Fig. 9(e). These dynamic behaviors matched well with the measured result using the photo detector of Fig. 8(d). Note that the results using the LC-C showed the removed transient blinking property due to the balanced splay-to-bend and bend-to-splay FE-coupled dynamics but the brightness level differences depending on the signal polarities still remained in the initial and saturated time conditions as shown in Figs. 9(c) and 9(d). That means that the es−eb value of the LC-C was not zero. After the DC offset adjustment for the minimum static flickering condition, our results revealed that the symmetric FE-coupled dynamic behavior, highly dependent with K11 and K33 values of an LC material, is important to solve the dynamic flickering problem at the moment of frame change.

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Fig. 9. Measured LC dynamics at the moment of frame change under the minimum static flickering condition using the LC-C. (a,c) Positional and time-varying views for the frame change from positive to negative. (b,d) Positional and time-varying views for the frame change from negative to positive. (e) Schematic image of asymmetric dynamic behavior between splay and bend deformations. (τi→j: response for the splay-bend transition. i→j denotes the change between splay (S) and bend (B) deformations).

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4. Conclusion In this paper, we discussed the static and dynamic flickers in the low-frequency-driven FFS LC mode using a p-LC. In the FFS LC mode, both flickering effects are highly related to the FE effect coupled with the fringe fields. The static flicker means a brightness variation between positive and negative frames at the range of the saturated LC orientation within each frame. By contrast, dynamic flicker is caused by the dynamic behavior of LC directors changing between both static LC orientations at the moment of the frame change. This has been frequently mentioned as “transient blink.” Even under the minimum static flickering condition, which is achievable by optimizing the DC offset, dynamic flicker in the FFS LC mode remains and it should also be clearly solved to introduce an extremely low-frequency driving technique to decrease power consumption of the LCD panel. We analyzed the causes and mechanisms of both flickers by using a high-speed camera. We then discussed the effects on the display image, and solutions to eliminate both flickers. First, we observed the positional brightness variation caused by the FE effect and depending on the field direction. Although the difference of the average brightness level over the entire pixel area between positive and negative frames was minimized by the optimum DC offset condition, we could figure out that the positional brightness distribution within a pixel was significantly changed depending on the field direction. This positional brightness fluctuation occurs because the FE polarization could not be fundamentally eliminated in spite of tuning the DC offset for a minimum static flickering condition and the splay-bend transition by the FE effect occurs in every directional change of electric field. To remove the positional brightness fluctuation, the FE polarization in the FFS LC mode should be controlled, but the positional brightness fluctuation itself might not be conceived by human eyes considering human eyes’ resolution if the average brightness levels are balanced by the optimized DC offset condition once. Second, even if the minimum static flickering condition is achieved by removing the brightness difference between positive and negative frames based on the average brightness over the entire pixel area, the transient blinking phenomenon could be observed owing to the different FE-coupled transition responses between the splay-enhanced and bend-enhanced LC deformations at the moment of frame change. The transient blink was highly related to the LC dynamics varying with changes in signal polarity. This dynamic behavior leads to the additional flickering problem in the low-frequency-driven FFS mode. By analyzing the images that were monitored by the polarizing optical microscope equipped with a high-speed camera, we confirmed that the transient blink was caused by a faster splay-to-bend response than the bend-to-splay response at the moment of frame change. Based on this analysis result, we evaluated the LC samples, of which elastic constants were adjusted to reduce the transient blinking phenomenon. A higher K33 slowed down the response of the splay-to-bend transition and balanced the dynamics between splay-to-bend and bend-to-splay transitions. As a result, we could achieve symmetric dynamic behavior between the splay and bend deformations. With the proposed method, the stable image without the blinking problem is able to be obtained in the low-frequency-driven FFS LC mode. We expect that the optimization of the DC offset for the minimum static flicker and the adjustment of the material properties for elimination of the dynamic flicker, presented here, would be very essential process to solve the image flickering problems. Acknowledgment This work was supported by Institute for Information & communications Technology Promotion (IITP) grant funded by the Korea government (MSIP) (No.B0101-15-295, Development of UHD Realistic Broadcasting, Digital Cinema, and Digital Signage Convergence Service Technology).

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