Recently, a low frequency driving of a fringe-field switching-liquid crystal display (FFS-LCD) draws much attention to minimize the power consumption. In the low frequency driving of FFS-LCD, an image flickering effect occurs when the sign of the electric field is reversed. We suggested a method to eliminate the image flickering effect by doping small amount of bent-core liquid crystal (BLC) molecules. The BLC molecules have an opposite sign of flexoelectric polarization and reduce the flexoelectric polarization of the host liquid crystal. By adding 2.0 wt% of BLC, the total transmittance during a positive and a negative electric field could be balanced and the image flickering effect was not observed by eyes.
© 2014 Optical Society of America
For the past decades, the liquid crystal display (LCD) devices have been surprisingly developed and are widely used in our daily lives. The contrast ratio of the current commercial LCD is over 1000:1 and the narrow viewing angle problem was resolved using compensation films and multi-domain technologies [1,2]. Recently, the LCD is faced to new technical challenges such as a high resolution and low power consumption along with the rapid growth of the smart phone market. For the high resolution without a loss of transmittance (TR), the fringe-field switching (FFS)-LCD is superior compared to the other display modes due to the embedded storage capacitor under the interdigitated electrodes [3,4]. For the power consumption issue, a low frequency driving of the display panel can be a solution and draws much attention in these days [5,6]. However, when the FFS mode LCD is driven with a low frequency electric field, image flickering problem occurs due to the flexoelectric effect of the liquid crystal (LC) [7–11].
Figure 1 shows the optical simulation results of the TR and the LC director distribution of the FFS-LCD during the positive and negative electric field frame. The width and the separation of the interdigitated electrodes of the bottom substrate were 2.8 and 4.0 μm, respectively. The splay and bend flexoelectric coefficients were es = 8 pC/m and eb = −8 pC/m, respectively. 2 Hz 2.2 V voltage was applied and the data was obtained 200 ms after applying the voltage. Two kinds of changes were observed in Fig. 1. First, the integrated total TR during the positive and negative electric field frame was different . Total TR during the positive field [Fig. 1(a)] was greater than the one during the negative field [Fig. 1(b)]. Second, a spatial shift of the TR minimum to the x-direction was observed when the field direction was reversed . The splay deformation showing lower TR was induced between the electrodes during the positive field frame [Fig. 1(a)], whereas it was found on the interdigitated electrode during the negative field frame [Fig. 1(b)]. In particular, the change of total TR results in the image flicking effect, while the spatial shift is hardly distinguished by eyes due to the narrow periodicity of the electrodes about 7 μm [4–6].
The change of TR during the positive and negative field frame can be interpreted with the flexoelectric effect of LC [Fig. 2]. When the positive electric field is applied [Fig. 2(a)], the electric field is spread from the interdigitated electrodes and the splay deformation of LC is induced between the interdigitated electrodes. If the LC molecule has a quadrupole moment depicted in Fig. 2(a) [11,12], nonzero flexoelectric polarization to the -z-direction can be shown by the spatial gradient of the charge density [Fig. 2(a)]. When the electric field is reversed, the flexoelectric polarization direction in Fig. 2(a) becomes antiparallel to the electric field and the free energy density is increased in this configuration. The increased free energy can be reduced by forming the splay orientation on the interdigitated electrodes [Fig. 2(b)]. The flexoelectric polarization formed on the interdigitated electrodes in Fig. 2(b) is then parallel to the electric field and can decrease the free energy density.
According to the Meyer’s notation , the flexoelectric polarization is theoretically given by . Thus, the image flickering effect induced by the flexoelectric effect is related to the flexoelectric anisotropy Δe≡es-eb and the LC director orientation determined by the electric field. Because the LC director profile is different during the positive and negative frame, the flexoelectric polarization during the positive electric field frame is also different from the one during the negative field frame. Consequently, the change of TR profile is shown when the electric field is reversed. In this study, we mixed a small amount of bent-core liquid crystal (BLC) into the conventional rod-like nematic LC host and experimentally varied Δe. Consequently, we could obtain the equal total TR during the positive and negative electric field frame.
2. Experimental procedure
We tested two kinds of BLC molecules, B57 and B81 [Fig. 3(a)]. The B57 and B81 molecules have a pair of hydrogen and fluorine atoms at the lateral position X, respectively. Thus, the B81 molecule has a stronger dipole and quadrupole moment compared to the B57 molecule. The BLC is wedge-shaped with a kink angle of 130°, hence easily adapt the splay orientation [Fig. 3(b)] . If the flexoelectric polarization of the BLC molecules is opposite to that of host LC, the macroscopic flexoelectric polarization can be compensated, thus reducing the image flickering effect [Fig. 3(b)].
We mixed 2.0 wt% of BLC with a commercial LC mixture ML0648 (Merck). The dielectric anisotropy of the ML0648 was Δε = 10.3 and the birefringence was Δn = 0.10. The LC mixtures were injected into an empty sample with a cell gap of 3.8 μm by a capillary action. The width and separation of the interdigitated electrodes were 2.8 and 4.0 μm, respectively. A commonelectrode was separated from the interdigitated electrodes with an insulating layer [Fig. 2]. No electrode was deposited onto the top substrate. The substrates were coated with a planar alignment polyimide and baked at 200 °C for 1 h. Then, the substrates were unidirectionally rubbed with a cotton cloth. The rubbing direction was at 10° from the longitudinal direction of the interdigitated electrodes. To measure TR of the sample, a white light source consecutively passed through a polarizer, the sample, an analyzer, and a detector. The beam diameter was 10 mm much larger than the pixel size 7 μm. 2 Hz of square voltage was applied across the interdigitated electrodes and the common electrode. We also investigated the spatial distribution of TR in each pixel by analyzing the polarizing optical microscopy (POM) image. The optical simulation was calculated using a commercial LCD simulator Techwiz LCD 2D (Sanayi System).
3. Results and discussion
Figure 4 shows TR of the pure, B57-, and B81-mixed LC samples in the presence of 2 Hz square electric field. The pure LC sample showed 1.33% change of TR when the sign of the electric field was reversed. This change of TR gives noticeable image flickering effect . On the other hand, the change of TR in the B57- and B81-mixed LC sample was 0.42 and 0.021%, respectively. Thus, the image flickering effect during the low frequency driving was remarkably eliminated by doping BLC molecules. In particular, the B81 molecule with a greater flexoelectric effect than B57 showed less image flickering effect.
Figure 5(a)-(c) show the POM image of the pure LC, B57-, and B81-mixed LC samples during the positive and negative frame of the electric field, respectively. To investigate the spatial distribution of TR, we captured the POM images and analyzed the TR profile along a single slice cut normal to the interdigitated electrodes. Similar to the results of Fig. 4, the pure LC showed a noticeable decrease of TR when the negative electric field was applied [Fig. 5(a)]. On the other hand, total TR difference of the B57- and B81-mixed sample during the positive and the negative field frame [Fig. 5(b), 5(c)] was decreased. In particular, the B81-mixed sample showed similar total TR during the positive and the negative field frame, while the shift of TR was still observed [Fig. 5(c)].
Figure 6 shows the simulation results of the spatial TR profile with es and eb varied. The TR difference during the positive and negative field frame was prominent with Δe = 20 pC/m, where es = 10 pC/m, eb = −10 pC/m [Fig. 6(a)]. The TR difference decreased with smaller Δe = 10 pC/m [Fig. 6(b)], Δe = 5 pC/m [Fig. 6(c)], and completely disappeared Δe = 0 pC/m [Fig. 6(d)]. When the sign of Δe was reversed [Fig. 6(e), 6(f), 6(g)], the TR profile showed opposite dependence on the field polarity to the experimental results [Fig. 5]. Although the experimental results showed some deviation to the simulation results, it was seen that the image flickering effect was decreased with smaller Δe. Thus, we think Δe was reduced by doping the BLC molecules with an opposite sign of the flexoelectric polarization to host LC. Consequently, the difference of the net electric field during the positive and negative frame becomes smaller, resulting in a smaller image flickering effect. To confirm this model, we separately measured Δe value according to the experimental method using a hybrid aligned cell method . Δe of the pure LC was 9.5 pC/m, while that of B81-mixed LC was 6.1 pC/m. Thus, the experimental Δe of the B81-mixed LC was also smaller than the pure LC. Thus, the decreased flexoelectric effect after mixing BLC was qualitatively confirmed. From the simulation results, it was found that each magnitude of es and eb had negligible effect on the image flicker whereas Δe significantly affected on it.
Meanwhile, the TR profile shift phenomenon to the x-direction was not completely removed even in the BLC-mixed sample [Fig. 5]. As described above, the splay deformation is inevitably shown in the FFS mode sample and the deformation position is changed depending on the electric field direction. For the perfect elimination of the TR shift, i.e., identical spatial TR distribution, Δe should be converged to zero as shown in Fig. 6(d). One can expect that higher concentration of BLC can induce such an effect. However, the BLC molecules were separated from host LC when the concentration was over 2.0 wt%. With a viewpoint of material science, BLC with a good miscibility and an opposite sign of Δe to host LC can be a solution for the complete elimination of the image flickering effect. We also simulated the dependence of the image flicker on the Δε of the host LC. The TR difference during the positive and negative field frame was increased with greater Δε value under the same voltage. This is due to the larger field-induced deformation of the LC molecules. Meanwhile, the image flickering effect was negligible when LC with a negative dielectric anisotropy Δε<0 was used due to the small splay and bend deformation of the LC molecules .
We suggested a method to eliminate the image flickering effect of the FFS-LCD driven with a low frequency electric field. We mixed a small amount of BLC molecules and experimentally reduced Δe value. When 2.0 wt% of B81 molecule was doped, total TR during the positive and negative electric field frame became similar, thus no flickering effect was seen to eyes. The proposed method can be a useful solution for the low power consumption of the display devices.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the ministry of Science, ICT & Future Planning (NRF-2013R1A1A1058681 and 2014R1A2A1A01004943).
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