1. Introduction

Liquid crystal displays (LCDs) have several attractive characteristics, such as a wide viewing angle, high brightness, and high contrast ratio. Among the liquid crystal (LC) modes, the in-plane-switching (IPS) and fringe-field switching (FFS) modes exhibit the widest viewing angle characteristics because the LCs are, initially, homogeneously-aligned, and they are rotated within a plane parallel to the substrates when an in-plane electric field is applied [1–4]. The FFS mode has become a mainstream approach for mobile displays owing to its high transmittance. However, some technical issues in this mode remain to be solved, such as the slow response time, especially at low temperatures [5], and non-zero pre-tilt angle of LCs, which causes off-axis dark state light leakage [6–8].

Besides the above-mentioned issues, image flicker under low-frequency driving is an important issue in the FFS mode as it can affect the display quality significantly [9–11]. Recently, low-frequency driving of a display panel to reduce the power consumption, especially in mobile devices, has drawn much attention. When an FFS cell using LCs with positive dielectric anisotropy is driven by a low-frequency electric field, image flicker due to the flexoelectric effect in the LCs can be observed [12–16]. On the other hand, in an FFS cell using LCs with negative dielectric anisotropy, the tilt angle is much less sensitive to the sign of the applied electric field, which, in turn, results in a much weaker flicker [17]. However, FFS cells using LCs with positive dielectric anisotropy are still being widely employed for most high-resolution displays because of their low operating voltages and fast response times.

Several approaches to eliminate the image flicker in an FFS cell using LCs with positive dielectric anisotropy have been studied, but they are mainly focused on the material parameters of LC, such as the flexoelectric anisotropy and the dielectric anisotropy [16,17]. In this work, we have shown that the image flicker in an FFS cell under low-frequency driving can be eliminated by applying a bipolar voltage wave. We found that, by applying a bipolar voltage wave to an FFS cell, the image flicker induced by the transmittance difference can be completely eliminated and the ripple phenomenon can be significantly reduced.

2. Image Flicker in a FFS cell

Figure 1 shows the measured voltage-transmittance curves of an FFS cell during the positive and negative frames when it is driven by a 2-Hz unipolar voltage wave. The transmittance during the negative frame is different from that during the positive frame. This transmittance difference causes image flicker. In addition to the transmittance difference, there is another problem, the so-called ripple phenomenon. The transmittance of a cell drops for a short time between frames. Both phenomena can be interpreted in terms of the flexoelectric effect of the LC. The transmittance difference between the positive and negative frames is caused by the difference in their transmittance profiles [16]. The ripple phenomenon is caused by the difference in the transition time between bend and splay deformations [11].

 

Fig. 1 Measured voltage-transmittance curves of an FFS cell driven by a 2-Hz unipolar voltage wave during the positive and negative frames. LC (ML-0648): Δn = 0.1029 and Δε = 10.3.

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The difference in transmittance between the positive and negative frames is related to the flexoelectric effect. Figure 2 shows a schematic illustration of the transmittance and the LC director distributions in an FFS cell during the positive and negative frames. Splay deformation, resulting in lower transmittance, occurs above the interdigitated electrodes during the positive field frame, whereas it is induced between the electrodes during the negative field frame. A spatial shift of the transmittance minimum was observed when the sign of the applied field was reversed. Furthermore, the transmittance profile during the negative frame is different from that during the positive frame.

 

Fig. 2 Schematic illustration of splay deformation in an FFS cell during the positive and negative frames.

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Although an easy way to reduce the transmittance difference is to apply a bias voltage such that the transmittance during the positive frame is the same as that during the negative frame, this technique is difficult to apply to a real panel because it needs different bias voltages depending on the gray level. Another approach to reducing the transmittance difference is to control the flexoelectric anisotropy Δe [16]. Because the image flicker induced by the flexoelectric effect is related to the flexoelectric anisotropy Δe ≡ e_s - e_b, where e_s and e_b are the splay and bend flexoelectric coefficients, it can be decreased by decreasing Δe. Chen et al. found that a low-dielectric-anisotropy LC (low-Δε LC) can help to suppress image flicker [17]. For low-Δε LCs, the tilt angle is less sensitive to the applied electric field, which, in turn, results in weaker splay deformation.

However, although the transmittance difference may be reduced via the above-mentioned methods, the ripple phenomenon, which is also highly related to the flexoelectric effect, still exist. When the sign of the applied electric field is reversed, the position of deformations is shifted. Because the duration of the bend-to-splay transition is different from the duration of the splay-to-bend transition, the transmittance of a cell drops for a short time. This ripple may cause blinking in low-frequency operation and deteriorate the display quality [11]. Because the ripple phenomenon is related to splay deformation, it can be reduced by increasing the gap between the interdigitated electrodes. However, this method is difficult to apply because the operating voltage may increase as the gap between the interdigitated electrodes increases.

In this paper, we have shown that the image flicker can be eliminated without changing the material parameters of the LC. The transmittance difference in an FFS cell can be eliminated by applying a bipolar voltage wave. The ripple phenomenon can also be reduced significantly by applying a bipolar voltage wave instead of increasing the gap between the interdigitated electrodes. An FFS device driven by a bipolar voltage wave for high transmittance was proposed [18]. The in-plane electric field induced among the three electrodes using three voltage levels rotates the LC molecules more effectively than in an FFS cell driven by a unipolar voltage wave, as shown in Fig. 3.

 

Fig. 3 Electric field lines during positive (or odd) and negative (or even) frames in FFS cells driven by (a) unipolar and (b) bipolar voltage waves.

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3. Results and discussion

To investigate the image flickering phenomenon, an FFS cell was fabricated. Dielectric and optical anisotropies of the LC used in the fabrication (Merck ML-0648) were 10.3 and 0.1029, respectively. The flexoelectric anisotropy Δe of the LC was 9.5 pC/m [16]. The thickness of the LC layer was 3.6 μm. The width of the patterned electrodes and the gap between them were 2.8 and 4 μm, respectively. To align the LCs homogeneously, polyimide (JNC PIA-5310-GS10) was spin-coated on each substrate. We applied unipolar and bipolar voltage waves to an FFS cell to measure the transmittance.

Figure 4 shows the measured dynamic response and polarized optical microscopy images of an FFS cell driven by 3.25-V-unipolar and bipolar ± 2.2-V-bipolar voltage waves of 2 Hz. When it is driven by a unipolar voltage wave, the FFS cell showed a 0.91% change of transmittance between neighboring frames. This change of transmittance results in noticeable image flicker [17]. Whether it is driven by the unipolar or the bipolar wave, the spatial shift of the transmittance minimum can be observed in an FFS cell when the sign of the applied field is reversed, as shown in Fig. 4(b). However, when the cell is driven by a bipolar voltage wave, the transmittance difference under low-frequency driving was effectively eliminated. The ripple phenomenon was also reduced significantly when a bipolar voltage wave was applied. The ripple phenomenon will be discussed later in terms of splay deformation.

 

Fig. 4 (a) Measured dynamic response and (b) polarized optical microscopy images during positive (or odd) and negative (or even) frames of an FFS cell driven by unipolar and bipolar voltage waves at 2 Hz.

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To confirm the decrease of the transmittance difference between frames, we performed a numerical calculation using a commercial simulation program, TechWiz LCD (Sanayi System Co., Korea), with which we can examine the flexoelectric effect in an LC cell [16, 17]. The LC parameters used for numerical calculation are as follows: the birefringence Δn = 0.1029, the dielectric anisotropy Δε = 7.3, and the elastic constants K₁₁ = 12.6 pN, K₂₂ = 5.9 pN, and K₃₃ = 13 pN. The splay and bend flexoelectric coefficients were e_s = 12 pC/m and e_b = −12 pC/m, respectively. The cell gap, the width of the interdigitated electrodes, and the gap between them were 3.8 μm, 5 μm, and 5 μm, respectively.

The spatial shift of the transmittance minimum can be observed in an FFS cell when the sign of the applied field is reversed, whether it is driven by the unipolar wave or the bipolar wave, as shown in Fig. 5. To eliminate the transmittance difference, the shifted LC director distribution during the even frame must be the same as that during the odd frame. Although the LC director distribution and the point of minimum transmittance are shifted in an FFS cell, the LC director and transmittance distribution during the even frame were the same as those of the odd frame when driven by a bipolar voltage wave. Note that this difference between the unipolar and bipolar waves is caused by the difference in the LC director distribution. For an FFS cell driven by a unipolar voltage wave, a fringe field exists between the pixel and the ground electrodes. Therefore, the LC director distribution is changed when the sign of the applied field is reversed. On the other hand, for an FFS cell driven by a bipolar voltage wave, an in-plane transverse electric field exists between the two neighboring pixel electrodes above the ground electrode, whereas a fringe field exists between the ground electrode and the pixel electrodes. Therefore, when the sign of the applied field was reversed, the shifted LC director distribution does not change.

 

Fig. 5 Calculated LC director and transmittance distributions during positive (or odd) and negative (or even) frames in an FFS cell driven by (a) unipolar and (b) bipolar voltage waves.

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To reduce the ripple phenomenon, we must decrease the effective splay and bend deformation. If an FFS cell has a gap between the interdigitated electrodes that is twice as big, the spatial splay deformation will be reduced by half and the ripple phenomenon will be reduced. However, the driving voltage increases as the gap increases, which may cause higher power consumption. Instead of increasing the gap between the interdigitated electrodes, the ripple phenomenon can be reduced in an FFS cell by applying a bipolar voltage wave. To explain the reduced ripple phenomenon in an FFS cell driven by a bipolar voltage wave, we examined the splay deformation of the LC directors through measurement of the transmittance distribution. Figures 6(a) and 6(b) show measured transmittance distributions and schematic illustrations of the splay deformation in an FFS cell driven by unipolar and bipolar voltage waves, respectively. Because the fringe electric field is induced among three electrodes using three voltage levels, the splay deformation in an FFS cell driven by a bipolar voltage wave is reduced by half, relative to an FFS cell driven by a unipolar voltage wave. This decrease in the effective splay deformation causes the decrease in the ripple phenomenon, as shown in Fig. 4.

 

Fig. 6 Measured transmittance distributions and schematic illustrations of the splay deformation during positive (or odd) and negative (or even) frames in an FFS cell driven by (a) unipolar and (b) bipolar voltage waves.

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Figures 7 shows the calculated spatial distributions of the transmittance when the sign of the applied field is reversed with time as a parameter. As clearly shown by these two cases, splay deformation resulting in the transmittance minimum occurred within 20 ms of the sign of the applied electric field being reversed. However, the relaxation of the splay deformation took roughly 60 ms. These results show that the duration of the bend-to-splay transition is different from the duration of the splay-to-bend transition. Therefore, the total transmittance of a cell dropped for a short time, which led to the ripple phenomenon. For an FFS cell driven by a bipolar voltage wave, the ripple phenomenon is reduced by the decrease in effective splay deformation.

 

Fig. 7 Calculated transmittance distributions with time as a parameter when the sign of the applied electric field was reversed in an FFS cell driven by (a) unipolar and (b) bipolar voltage waves.

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4. Conclusion

In this paper, we have shown that image flicker can be eliminated without changing the material parameters of LCs. Our results show that the transmittance difference in an FFS cell can be effectively eliminated by applying a bipolar voltage wave. We observed that the ripple phenomenon can be also reduced significantly by applying a bipolar voltage wave, instead of increasing the gap between the interdigitated electrodes.