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An effective molecular alignment method and appropriate monomer material for governing the liquid crystal (LC) molecular configuration and enhancing the electro-optical performance of in-plane-switching vertically aligned LC devices are proposed. Three kinds of monomer materials are selected and separately mixed with the nematic LC. A functional and stabilized small pretilt angle of 2° is constructed in the cells by using the U-shaped-alignment electrical field and 2-wt.% mixed concentration during the photo-curing process. Compared to the pure cell, the fabricated cell with the surface-anchored cross-linking polymers respectively achieves over 30% and 60% improvement in the optical-switch and gray-level responses.
© 2016 Optical Society of America
1. Introduction
Liquid crystal (LC) displays, due to their low power consumption, light weight, and compact size, have been widely employed in information and electronic devices. Of all the advanced development display modes, the in-plane-switching (IPS) mode has attracted much industrial and academic attention since it possesses two advantages at the application level, namely a wide viewing angle and faster image response [1]. To effectively align the LC molecules, the mechanical rubbing process is applied to the conventional IPS cells [2]. However, the created dust and particles are deposited in the alignment layer. Furthermore, for an IPS cell, light leakage in the dark state, resulting in a poor contrast ratio, has also been revealed [3]. In order to solve these problems, the vertically aligned (VA) display mode without the rubbing process not only has the dust-free property, but also shows a high contrast ratio because the LC molecules are aligned vertically on the substrate surface. The major drawback of this display mode is a long response time [4], especially for the fall time (tf), which is attributed to the fact that a negative dielectric anisotropy LC (negative LC) is usually applied to the VA cells. Although the rise time (tr) can be reduced by the overdrive method [5], an additional driving scheme is required. Regarding the tf, it is independent of the driving voltage and is only related to the cell gap and the material parameters of negative LCs, such as the rotational viscosity and the bend elastic constant [6]. To annihilate the disadvantages of the IPS and VA display mode, the IPS-VA display mode using positive LC material has been demonstrated [7], but some of the electro-optical properties need to be modified, such as the high driving voltage and low output light transmittance.
The mixture of LC and monomer, owing to its low cost, simple and rapid fabrication, mass production, and high repeatability, has recently been considered as a potential candidate for controlling the LC molecular orientation and enhancing the device performance of VA-based cells [8–16]. Through the polymerization process, the surface pretilt angle and specific LC molecular configuration are constructed using in-cell stabilized polymer networks. Although this created pretilt angle in the VA-based cells can remarkably improve the tr response, the slow tf response, obvious light leakage in the dark state, and low contrast ratio are also revealed [17]. To solve the light leakage problem, a method combining lower monomer concentration (~0.1 wt.%) of the reactive mesogens (RM) monomer, which is widely applied to the LC cells, and an in-plane electrical field during the curing process is applied to the VA-based cell. This makes the LC molecules near the substrate surface tilt, building a small pretilt angle with respect to the substrate surface [18]. However, the required curing time for RM monomer is very long in order to develop the polymer networks (as sometimes a second curing process is needed), and the tf of this kind of cell cannot be effectively reduced, so it is even longer than that of the unprocessed cell due to the lack of a sufficient anchoring energy effect on the LC molecular configuration. Although the higher monomer concentrations (≧ 6 wt.%) forming three-dimensional polymer networks can improve the tf response of VA-based LC cells due to the bulk polymer anchoring effect, a very high driving voltage (~80 V) is necessary for performing a faster tr response [19]. Thus, developing a practical and powerful fabrication approach with the appropriate monomer material is necessary for improving and enhancing the electro-optical properties of VA-based LC cells.
In this paper, an effective alignment method for LC molecules is demonstrated by employing the U-shaped vertical electric field and lower monomer concentration (~2 wt.%) in the IPS-VA cell during the photo-curing process. Three kinds of monomer materials are selected to investigate the effects of surface-anchored polymer morphology on the LC molecular configuration and the electro-optical properties. To further reduce the Vth and tr of the IPS-VA LC/polymer cells, the “functional” pretilt angle fabricated on the substrate surface is introduced. A functional small pretilt angle of 2° with respect to the normal direction is constructed, which is located at the interdigital-electrode edge, and provides the LC molecules with a certain pre-direction. During the analysis of the device performance, the prepared cells are driven by an IPS electrical field. The cell with planar cross-linking polymer morphology exhibits the fastest optical switch and gray-level responses at a low driving voltage (~12 V). In addition, with the small pretilt angle effect, the threshold voltage (Vth) of the planar cross-linking polymer cell with the stronger anchoring energy effect is comparable to that of the pure cell. More than 30%, 60%, and 70% improvement in the optical switch, gray-level tr, and gray-level tf responses, respectively, is revealed for the planar cross-linking polymer cell, as compared to the pure cell. This proposed method, creating a small pretilt angle effect and stronger anchoring effect on the IPS-VA LC cell with an appropriate polymer morphology, can effectively govern the LC molecular orientation and greatly improve the electro-optical properties.
2. Experimental
In the experiment, the proposed U-shaped-alignment (USA) IPS-VA LC cells with a functional small pretilt angle are fabricated by employing the U-shaped vertical electrical field and surface-anchored polymer morphology. Indium tin oxide (ITO) material is selected to process the interdigital electrodes on the bottom-glass substrate. The period of interdigital electrodes is 12 μm; 4-μm width (w) and 8-μm separation (s). To produce the USA field, the ITO thin film is deposited on the top-glass substrate. Both substrates are coated with the AL60101L VA polyimide (V-PI) and heated at 200 °C for 1 hr. 4-μm spacers are applied to the USA IPS-VA LC/polymer cells for controlling the cell gap (d). Three kinds of monomer materials, TA-9164 from Tatung University, UCL002 from DIC Corp., and NOA65 from Norland Inc., are selected and separately mixed with the nematic E7 LC from Merck at a specific concentration. The TA-9164 monomer, possessing the crosslinking, bifunctional acrylate, and anisotropy properties, was selected and used in this paper. The chemical structure of the TA-9164 monomer has been studied and published elsewhere [20]. In addition, two kinds of commercial monomer materials, UCL002 (from Dainippon Ink and Chemicals) and NOA65 (from Norland), due to the fact that they are easily accessible and their morphology is quickly formed, were also applied to the LC cells for a more thorough investigation. Differing from the TA-9164 monomer, the UCL002 monomer is a mono-functional acrylate monomer which has the linearly linking property, whereas the NOA65 monomer is a multifunctional acrylate monomer and has a crosslinking network formation. This monomer is essentially made of four constituents [21–23], trimethylolpropane diallyl ether, trimethylolpropane tristhiol, isophorome diisocyanate ester, and benzophenone. The latter is a photoinitiator at a concentration of 5 wt.%. The birefringence (Δn) and dielectric anisotropy (Δε) of E7 LC are around 0.218 and + 14.5, respectively. The mixed concentration is defined as the ratio of the monomer weight to the sum of the monomer and E7 LC weight. To control the produced polymers with a stabilized and surface-anchored morphology, the mixed concentration in this paper is selected to be 2 wt.%. The mixture is then injected into the prepared cell by capillary action. During the photo-curing process, the USA vertical field and 365-nm ultraviolet (UV) light with 16 mW/cm2 are applied to these cells for 30 min. After polymerization, the U-shaped LC molecular configuration with a stable pretilt angle (θ) with respect to the normal direction of the substrate surface is constructed. The layer structure and LC molecular configuration of the USA IPS-VA LC/polymer cell under the curing process is illustrated in Fig. 1(a). Once the stabilized polymer networks have been formed, the stable multi-pretilt LC molecular structure and a small pretilt angle (θ) with respect to the normal direction of the substrate surface are successfully achieved. The pretilt angle value is estimated and determined through the relationship between the transmittance and the phase retardation [24, 25]. The created pretilt angle is approximately 2° at the curing voltage (VC) of 2 V to avoid severe light leakage and poor tf response. After the USA process, the IPS voltage (VIPS) is applied to the fabricated cell, making the LC molecules reorient, as illustrated in Fig. 1(b). Changing the LC molecular reorientation is not only dependent on the VIPS value, but also on the polymer morphology, which will be shown later. The USA IPS-VA E7/NOA65 cell, the USA IPS-VA E7/TA-9164 cell, and the USA IPS-VA E7/UCL002 cell are respectively called the USA NOA65 polymer cell, the USA TA-9164 polymer cell, and the USA UCL002 polymer cell in the following content. In the electro-optical measurement, these USA IPS-VA cells are sandwiched between the crossed polarizers and are oriented to 45° with respect to the polarizer. Normalized output light transmittance-voltage (T-V) curves, optical-switch responses (switching between the maximum brightness state and the dark state), specific gray-level responses (driving between the zeroth state and the specific state), and total gray-level responses (driving between the specific gray-level states) are performed by employing a diode laser with a 650-nm wavelength and ac voltage with 1 kHz square waveform in these cells.
Fig. 1 (a) Layer structure and USA process of a USA IPS-VA cell. During the curing process, the VC and UV light are applied to the cell. The U-shaped vertical electrical field is shown by dashed lines. A small and functional pretilt angle is symbolized by θ with respect to the normal direction of the substrate surface. (b) LC molecular reorientation by employing higher VIPS in a USA IPS-VA cell. As the higher VIPS is applied to the cell, the LC molecules will further reorient, and the LC molecules located in the s region will be parallel to the substrate surface.
3. Results and discussion
Figures 2(a), 2(b), and 2(c) respectively show the polymer morphologies of the TA-9164, UCL002, and NOA65 monomers on the interdigital-electrode substrate, recorded by field emission scanning electron microscopy (FE-SEM). Due to lower monomer concentration (~2 wt.%), the developed polymer morphologies are almost anchored on the substrate surface, which differs from the findings of other research groups [19]. In Fig. 2(a), the TA-9164 polymers have the planar cross-linking morphology and uniform connection with their neighbors. This polymer morphology is expected to introduce a stronger anchoring effect on the LC molecules, to successfully govern the LC molecular reorientation, and to enhance the electro-optical performance, which will be shown later. Differing from the TA-9164 polymer morphology, the UCL002 and NOA65 polymers, shown in Fig. 2(b) and Fig. 2(c), respectively, have the sphere- and stone-like polymer morphology on the substrate surface. Although the morphology of UCL002 polymers is a spherical and uniform distribution, the sizes of the polymer spheres are not the same. In addition, because UCL002 and NOA65 polymers have three-dimensional morphology, the original VA LC configuration will be slightly destroyed during the LC molecular alignment process, as is shown later in the conoscopic images [Fig. 3(A), 3(B), 3(C), and 3(D)]. Both polymer morphologies could disturb the VA LC molecular alignment, resulting in unexpected LC molecular directors in the cell. The sphere- and stone-like polymer morphology applied to the USA IPS-VA cells will reduce the reaction probability with the LC molecules and change the electro-optical properties, as revealed in Fig. 4, Fig. 5, and Fig. 6.
Fig. 2 (a)-(c) Polymer morphologies for the TA-9164, NOA65, and UCL002 monomer materials. These images are recorded by FE-SEM. The monomer concentration is 2 wt.%. TA-9164, NOA65, and UCL002 monomer materials respectively show the cross-linking, stone-like, and sphere-like polymer morphologies.
Fig. 3 POM images of four kinds of IPS-VA-based cells and for the variation of cell transmittance as a function of applied voltage: (A)–(Aiv) pure E7 LC cell, (B)–(Biv) USA NOA65 polymer cell, (C)–(Civ) USA TA-9164 polymer cell, and (D)–(Div) USA UCL002 polymer cell. The mixed concentration is of 2 wt.%, and the cell is sandwiched at 45° between the crossed polarizers. The red arrows shown in (Aiii) and (Aiv) represent the change in molecular orientation. The conoscopic images, showing the effect of polymer morphology on the alignment quality of IPS-VA cells, are inserted in (A), (B), (C), and (D). Of all the cells, the planar cross-linking TA-9164 polymer cell has a good vertical aligned quality because the crosshair is located at the center of the circle.
Fig. 4 (a) Normalized T-V curves for different kinds of USA IPS-VA cells with a pretilt angle of 2° and a mixed concentration of 2 wt.%. (b) Stronger anchoring effect on the LC molecular reorientation of the USA IPS-VA LC/polymer cells with the TA-9164 surface-anchored cross-linking polymer morphology at a higher driving voltage (VIPS > VTmax). (c) Weaker anchoring effect on the LC molecular reorientation of the USA IPS-VA LC/polymer cells with the NOA65/UCL002 surface-anchored stone-like/sphere-like polymer morphology at a higher driving voltage (VIPS > VTmax).
Fig. 5 (a) tr and (b) tf responses for the pure E7 LC cell, and the USA NOA65, USA TA-9164, and USA UCL002 polymer cells. Specific gray-level tr and tf responses for the pure E7 LC cell, USA NOA65, USA TA-9164, and USA UCL002 polymer cells in (c) and (d), respectively. The pretilt angle is around 2° with respect to the normal direction of the substrate surface, and the mixed concentration is around 2 wt.% for these USA IPS-VA LC/polymer cells.
Fig. 6 Total gray-level tr and tf responses for (a)/(e) the pure E7 LC cell, (b)/(f) the USA NOA65 polymer cell, (c)/(g) the USA UCL002 polymer cell, and (d)/(h) the USA TA-9164 polymer cell.
The voltage-dependent textures for the pure E7 LC, USA NOA65 polymer, USA TA-9164 polymer, and USA UCL002 polymer cell are shown in Figs. 3(A)-3(Aiv), 3(B)-3(Biv), 3(C)-3(Civ), and 3(D)-3(Div), respectively, which were recorded by polarized optical microscopy (POM). During the observation period, the driven voltage, labeled as VIPS, is applied to these four kinds of cells. At the initial state (VIPS = 0 V), the POM images of the pure E7 LC cell and the USA TA-9164 polymer cell, which are respectively shown in Fig. 3(A) and 3(C), are dark due to the LC molecular director normal to the substrate surface. However, for the USA NOA65 and UCL002 polymer cell, the POM images shown in Fig. 3(B) and 3(D) are not dark (navy blue), suggesting that the VA LC configuration is slightly destroyed. Of all the polymer cells, the USA TA-9164 polymer cell shows the best dark state, indicating that the planar cross-linking polymers do not severely deteriorate the original VA LC configuration. The alignment quality can be observed by the conoscope, and the conoscopic images for these four kinds of cells are inserted into Fig. 3(A), 3(B), 3(C), and 3(D). In Fig. 3(C), the USA TA-9164 polymer cell with the planar cross-linking polymers, fabricated on the substrate surface, can effectively align the LC molecules and make a good vertical LC configuration. The crosshair in the USA TA-9164 polymer cell is definitely located at the center of the circle, indicating that the alignment quality is very good. Such a highly orderly aligned cell has stronger anchoring energy against the LC molecular rotation [26, 27]. In addition, the USA TA-9164 polymer cell does not perform obvious light scattering in the dark state. The light scattering in the LC/polymer cell is related to the monomer concentration and polymer morphology. However, in this study, the light doping monomer concentration, which is about 2 wt.%, is applied to the cells, and so can avoid the significant light scattering effect on the cell. Thus, the light scattering can be attributed to the polymer morphology. In contrast, the stone- or sphere-like polymers applied to an LC cell (named as the USA NOA65 and USA UCL002 polymer cell, respectively) not only form the light scattering center, but also slightly destroy the original VA configuration, as shown in the inserted conoscopic images of Figs. 3(B) and 3(D), respectively. The original VA LC configuration of the USA NOA65 and UCL002 polymer cell could be changed because the crosshair is slightly tilted, that is, the molecular alignment of this kind of cell is disorderly as compared to that of the pure E7 LC cell. The polymer morphology of either UCL002 or NOA65 applied to the VA cell will slightly perturb the original alignment. This also causes the USA UCL002 and NOA65 polymer cell to have a slightly poor dark state and poor contrast ratio. With the VIPS increased to less than 10V, the IPS-electrode edge becomes bright and the central dark line is observed on these four types of cells. The brightness of the pure E7 LC cell is superior to that of the other cells, which is attributed to the surface polymer effect. When the VIPS is sufficiently increased, the alignment of the LC molecules between the IPS electrodes will become parallel to the substrate surface for the pure E7 LC cell, as illustrated in Fig. 1(b), which will make the central dark line disappear. The change in molecular alignment, labeled by the red arrow in Fig. 3(Aiii), is observed at ~10V, indicating that the LC molecules have started rotating. Such an alignment transformation will appear in more areas when the VIPS is more than 10 V, as revealed in Fig. 3(Aiv). However, this transformation is not observed on the polymer cells. Different polymer morphologies lead to different electro-optical properties, which is mainly attributed to the polymer anchoring effect on the LC molecular configuration and reorientation. Applying the TA-9164 monomer material to the LC cells increases the anchoring energy and improves the image-response performance, but applying the UCL002 and NOA65 monomer material to the LC cells decreases the anchoring energy and degrades the image-response performance, which will be shown in Fig. 5 and Fig. 6.
Figure 4(a) shows the normalized T-V curves for the pure E7 LC cell and different types of USA IPS-VA LC/polymer cells with a 2° pretilt angle. The normalized output light transmittance (Norm. T.) treated by each maximum transmittance (Tmax) is shown on the vertical axis. Of all the USA IPS-VA cells, the USA TA-9164 polymer cell has the highest Vth (3.71 V), which is determined by the maximum tangent line’s slope, whereas the NOA65 polymer cell has the lowest Vth (2.45 V). This results from the polymer morphology and LC molecular configuration. The planar cross-linking morphology of TA-9164 polymer anchoring on the substrate surface can effectively align the LC molecules, indicating that the cell possesses a stronger anchoring energy (W) effect (Vth ∝ W1/2), and additional electric potential energy is necessary for changing the LC molecular configuration [28, 29]. However, for the USA UCL002 and NOA65 polymer cells, the original vertical alignment of LC molecules, due to their clustered (stone- and sphere-like) polymer morphology, could be destroyed. This implies that the LC molecular alignment of these two kinds of cells is slightly disorderly as compared to that of the pure E7 LC cell. Such stone- or sphere-like polymer morphology applied to the USA IPS-VA LC cell will perturb the original VA LC configuration, making the LC molecules easier to reorient. Thus, the T-V curves of the NOA65 and UCL002 polymer cells shift left, and the threshold voltages of the NOA65 and UCL002 polymer cells are smaller than those of the pure E7 LC cell (Vth|NOA65 < Vth|UCL002 < Vth|E7). This phenomenon also suggests that the USA NOA65 and UCL002 polymer cells have a weaker anchoring energy effect than the pure E7 LC cell. In addition, it is noted that the value of Vth of the USA TA-9164 polymer cell is just slightly more than that of the pure E7 LC cell. This is attributed to the small pretilt angle effect, reducing slightly the anchoring energy effect introduced by the cross-linking TA-9164 polymers [29, 30]. The small pretilt angle gives the specific direction to the LC molecules for simply driving at the initial state. Although the large pretilt angle effect can further benefit the LC molecular reorientation, the remarkable light leakage in the dark state and the poor switching response can also be revealed, which are adverse to the device performance [17, 25, 30]. As the applied voltage is more than Vth, the LC molecules located at the electrode edge start reorienting, and the output light transmittance is gradually increased. Of all the proposed cells, one can find that the maximum-light-transmittance driving voltage (VTmax) of the USA TA-9164 polymer cell, due to the stronger polymer anchoring effect, is the largest (VTmax|TA-9164 = 11.86 V), which is advantageous to the switching and gray-level responses, as will be shown later. The lowest value of VTmax ( = 7.30 V) is measured on the USA NOA65 polymer cell because its polymer anchoring effect is weaker and the LC alignment is relatively disorderly, which will lead to poor response performance. Once the applied voltage is over VTmax, the LC molecular configuration of the pure E7 LC cell will have a more parallel alignment, causing the light transmittance to be lower than the maximum light transmittance [31]. This indicates that the value of phase retardation is beyond the optimum value (0.5 π). In addition, since the USA NOA65 and UCL002 polymer cells do not have a sufficient anchoring energy effect to control the LC molecular reorientation, their LC molecular configuration at higher driving voltages will be easily changed, that is, the LC molecules will further reorient. This causes the output light transmittance to rapidly decrease. The decay rates of the output light transmittance of these IPS-VA-based cells can be estimated using a tangent line at a specific interval. The smallest slope can be observed in the USA TA-9164 polymer cell, suggesting that the LC molecular reorientation is effectively governed and the output light transmittance is moderately decreased. Of all the proposed cells, the USA TA-9164 polymer cell performs a more stable configuration of LC molecules, which is suitable for operating at higher driving voltages (> VTmax). The stronger and weaker anchoring effects, resulting from the polymer morphology, on the LC molecular reorientations of the USA IPS-VA LC/polymer cells at high VIPS are illustrated in Figs. 4(b) and 4(c), respectively. Some of the electro-optical properties of these USA IPS-VA cells are summarized in Table 1. Regarding the hysteresis effect on the LC/polymer cell, it is significantly related to the value of the switch voltage. When the cell is operated between 0 V and VTmax, the hysteresis phenomenon can be suppressed because the LC molecules are anchored by the polymer networks, and the LC molecular reorientation is not dramatic. One transmittance value corresponds to one applied voltage value. However, as the cell is operated between 0 V and 30 V (>VTmax), the LC molecules further reorient, and the hysteresis phenomenon will be observed. This is due to the fact that the transmittance values correspond to the two applied voltage values. In this paper, the cell is only switched between 0 V and VTmax, and the hysteresis phenomenon is not existent in the display devices.
Table 1. Electro-Optical Properties of the USA IPS-VA Cells with Different Polymer Morphologies.
Figures 5(a) and 5(b) show the normalized optical-switch responses for different types of USA IPS-VA cells, switched between the dark state and the maximum bright state. The tr response is defined as the optical light transmittance from 10% Tmax to 90% Tmax, whereas the tf response is defined inversely. In Fig. 5(a), as the driving voltage is changed from 0 V to VTmax, the USA TA-9164 polymer cell shows the fastest tr response (2.64 ms) at the applied driving voltage (VTmax = 11.86 V), which is associated with the polymer anchoring effect and pretilt angle effect on the LC molecules [25, 29, 30], making the optical light transmittance of the cell rapidly reach the Tmax level. Different from the bulk polymer cells, this kind of cell is operated at a low driving voltage, which is smaller than that applied in other published works (> 30 V) [18, 19]. For the USA UCL002 polymer cell, although the VTmax|UCL002 ( = 9.07 V) is lower than the VTmax|E7 ( = 10.50 V), the tf response of the USA UCL002 polymer cell is slightly faster than that of the pure E7 LC cell, which is due to the polymer morphology effect. The spherical polymer morphology, prepared on the substrate surface of the USA UCL002 polymer cell, can slightly disturb the LC molecular configuration and then reduce the alignment energy [32, 33]. Therefore, as the driving voltage is applied to the USA UCL002 polymer cell, the LC molecular reorientation of the USA UCL002 polymer cell is easier than that of the pure E7 LC cell. Of all the cells, the USA NOA65 polymer cell exhibits the slowest tr response, which is attributed to the VTmax value. Although the change in LC molecular configuration of the USA NOA65 polymer cell is easier than that of the pure E7 LC cell due to the fact that the value of Vth|NOA65 is smaller than that of Vth|E7, the value of VTmax|NOA65 is too small to rapidly reach the Tmax level. Despite the fact that the higher driving voltage can improve the tr response of the USA NOA65 polymer cell, the output light transmittance will be sacrificed and will be lower than Tmax. In Fig. 5(b), when the driving voltage is released, the USA TA-9164 polymer cell shows the fastest tf response of all the cells. This is because the fabricated planer cross-linking polymer morphology introduces a stronger anchoring energy effect (Wan) on the LC molecules, making the driven LC molecules rapidly reorient to the original configuration (tf ∝ 1/Wan) [33]. It is noted that the small pretilt angle is not adverse to the tf response of the USA TA-9164 polymer cell due to the fact that the polymer anchoring effect is stronger than the pretilt angle effect [25]. In contrast, applying the UCL002 and NOA65 polymers to the USA IPS-VA cells is not beneficial to the tf response. From the T-V-curve behavior, it can be understood that the UCL002 and NOA65 polymers cannot increase the anchoring energy, indicating that the LC molecular reorientation will not be effectively controlled. Compared to the pure E7 LC cell, the USA UCL002 polymer cell shows similar tf-response behavior (tf |UCL002 = 10.66 ms, tf |E7 = 10.72 ms), which can be attributed to the fact that the spherical polymer morphology of the UCL002 monomer can play a role in reducing the cell gap, and can compensate for the insufficient anchoring effect (tf ∝ d) [33]. The slowest tf response is revealed on the USA NOA65 polymer cell. The stone-like polymers located on the substrate surface of the NOA65 polymer cell might not only disturb the LC molecular alignment, but may also create unwanted pretilt angles, which will significantly affect the reorientation process of the LC molecules. Based on the above results, the sphere- or stone-like polymer morphology cannot enhance the cell response due to the lack of a stronger anchoring effect on the LC molecular reorientation. According to the above results, the USA TA-9164 polymer cell can significantly enhance the tr and tf responses. The USA IPS-VA optical-switch responses, including tr, tf, and the total response time (tt = tr + tf), are listed in Table 1. Over 30% enhancement in the tt on the USA TA-9164 polymer cell is achieved.
To further investigate the effects of polymer morphology on the response performance of the USA IPS-VA cells, the specific gray-level tr responses from the zeroth state to each gray-level state are shown in Fig. 5(c). The gray difference between the adjacent levels is equal, and the value of 100% labeled on the horizontal axis means that the output light transmittance has already reached the Norm. Tmax level. Of all the cells, the USA TA-9164 polymer cell shows the shortest tr for all states. The gray-level tr responses of the USA TA-9164 are significantly improved, which is attributed to the high applied driving voltage and small pretilt angle, giving the USA TA-9164 polymer cell a rapid LC molecular response. Compared to the pure E7 LC cell, the USA UCL002 and NOA65 polymer cells show faster gray-level tr responses when the cells are operated in lower gray-level states. This is because of the USA and polymer morphology effect on the LC molecular reorientation, resulting in a smaller Vth value causing a rapid change in the LC molecular configuration. However, when the cell is operated in the higher gray-level states, the gray-level tr responses of the USA UCL002 and NOA65 polymer cells change. The gray-level tr of the USA UCL002 polymer cell is slightly shorter than that of the pure E7 LC cell, but the gray-level tr of the USA NOA65 polymer cell is longer than that of the pure E7 LC cell. Although the gray-level driving voltage in the higher gray-level states of the USA UCL002 polymer cell is largely the same as that of the pure E7 LC cell, the change in the LC molecular configuration of the USA UCL002 polymer cell is still faster than that of the pure E7 LC cell. This is due to the fact that the spherical polymers formed from the UCL002 monomer perturb the LC molecular alignment, causing the LC molecules to be simply driven. The main reason for the USA NOA65 polymer cell showing the longest gray-level tr at the higher gray-level states is that the value of the gray-level driving voltage is the lowest, which cannot make the cell rapidly reach the specific gray level. Figure 5(d) shows the specific gray-level tf responses of the pure E7 LC cell and the USA IPS-VA cells from the specific gray state to the zeroth state as the driving voltage is released. Of all the cells, the USA TA-9164 polymer cell exhibits the shortest gray-level tf, indicating that the stronger anchoring effect from the planer cross-linking polymer morphology makes the LC molecules reorient instantly. Since the tf response is only related to the in-cell anchoring effect, it can be found that the curve behavior is independent of the change in the gray-level state, meaning that the gray-level tf curve is independent of the gray-level driving voltage. The USA UCL002 and NOA65 polymer cells are adverse to the gray-level tf responses, especially for the USA NOA65 polymer cell. This is due to the fact that the sphere- and stone-like morphology cannot provide sufficient anchoring energy to the cell to effectively govern the LC molecular reorientation. According to the above results, the USA TA-9164 polymer cell with cross-linking polymer morphology not only has excellent response performance, but also has an appropriate Vth value, which is beneficial to the enhancement of the electro-optical properties of the IPA-VA-based devices.
Figure 6 shows the total gray-level responses of the pure E7 LC cell and the USA IPS-VA LC/polymer cells, which are equally divided into eight levels by the Norm. Tmax value of each cell. The gray-level tr responses of the pure E7 LC cell, and the USA NOA65, USA UCL002, and USA TA-9164 polymer cells, are respectively presented in Figs. 6(a)-6(d). Of all the cells, the USA TA-9164 polymer cell performs the fastest tr response to adjacent levels (from one of the start levels to one of the target levels) owing to the small pretilt-angle effect and the slightly higher driving voltage, leading to the rapid reorientation of the LC molecules. Since the USA NOA65 and USA UCL002 polymer cells have a lower Vth property as compared to the pure E7 LC cell (Vth|NOA65 < Vth|E7, Vth|UCL002 < Vth|E7), indicating that the LC molecular configuration is relatively easily changed, one can find that some of the gray-level tr responses of the USA NOA65 and the USA UCL002 polymer cells are faster than that of the pure E7 LC cell, and the overall performance of the USA NOA65 and USA UCL002 polymer cells is comparable to that of the pure E7 LC cell. Figures 6(e), 6(f), 6(g), and 6(h) individually present the total gray-level tf responses of the pure E7 LC cell, the USA NOA65, the USA UCL002, and the USA TA-9164 polymer cells, once the gray-level driving voltage is released. The USA TA-9164 polymer cell significantly enhances the gray-level tf responses (from one of the target levels to one of the start levels) and exhibits the fastest tf responses for all gray levels, mainly due to the stronger anchoring effect on the LC molecular reorientation. However, the USA UCL002 and NOA65 polymer cells cannot improve the gray-level tf responses, especially for the USA NOA65 polymer cell. The stone-like polymers in the USA NOA65 polymer cell perturb the reorientation path of the LC molecules due to the insufficient anchoring effect and the multi-pretilt effect, making the driven LC molecules require a longer tf time to carry out the reorientation process. Based on the above results, the USA TA-9164 polymer cell demonstrates over 60% and 70% improvement in the gray-level tr and tf responses respectively, which can be used to boost the dynamic electro-optical performance.
4. Conclusion
The effects of U-shaped alignment and surface-anchored polymer morphology on the electro-optical properties of IPS-VA LC cells are investigated in detail. With the U-shaped vertical electrical field and TA-9164 cross-linking polymers, the small and functional pretilt angle of 2° is successfully built near the interdigital-electrode edge. Of all the developed polymer morphologies, the surface-anchored cross-linking polymers, formed by the TA-9164 monomer, significantly enhance the electro-optical performance of the IPS-VA LC cells. The USA TA-9164 polymer cell not only shows the shortest tr due to the higher driving voltage and small pretilt angle effect, but also shows the shortest tr, which is attributed to the stronger polymer anchoring energy effect. In addition to the excellent response performance, the value of Vth of the USA TA-9164 polymer cell, because of the small pretilt angle effect giving the pre-director to the LC molecules, is comparable to that of the pure E7 LC cell. However, the stone- and sphere-like polymer morphology, which respectively correspond to the USA NOA65 and USA UCL002 polymer cell, are not beneficial to the electro-optical performance. This indicates that an appropriate polymer morphology, applied to the USA IPS-VA LC cell, cannot only effectively govern the LC molecular reorientation and configuration, but can also boost the device performance. As compared to the pure E7 LC cell, the USA TA-9164 polymer cell with the planar cross-linking polymer morphology achieves more than 30%, 60%, and 70% improvement in the optical-switch response, gray-level tr, and gray-level tf responses, respectively. The proposed USA IPS-VA LC/polymer devices are suitable for display applications and are compatible with ordinary wide viewing angle solutions.
Acknowledgments
This research work was supported by the Ministry of Science and Technology of Taiwan under Grant No. MOST 104-2221-E-027-091 and NO. MOST 105-2221-E-027-056.
References and links
1. M. Oh-e and K. Kondo, “Electro-optical characteristics and switching behavior of the in-plane switching mode,” Appl. Phys. Lett. 67(26), 3895–3897 (1995). [CrossRef]
2. R. H. Guan, Y. B. Sun, and W. X. Kang, “Rubbing angle effect on in-plane switching liquid crystal displays,” Liq. Cryst. 33(7), 829–832 (2006). [CrossRef]
3. Y. Momoi, K. Tamai, K. Furuta, T.-R. Lee, K. J. Kim, C. H. Oh, and T. Koda, “Mechanism of image sticking after long-term AC field driving of IPS mode,” J. Soc. Inf. Disp. 18(2), 134–140 (2010). [CrossRef]
4. Z. Ge, X. Zhu, T. X. Wu, and S. T. Wu, “High-transmittance in-plane-switching liquid-crystal displays using a positive-dielectric-anisotropy liquid crystal,” J. Soc. Inf. Disp. 14(11), 1031–1037 (2006). [CrossRef]
5. K. Kawabe, T. Furuhashi, and Y. Tanaka, “New TFT-LCD driving method for improved moving picture quality,” SID Symp. Dig. Tech. Pap. 32(1), 998–1001 (2001). [CrossRef]
6. M. L. Dark, M. H. Moore, D. K. Shenoy, and R. Shashidhar, “Rotational viscosity and molecular structure of nematic liquid crystals,” Liq. Cryst. 33(1), 67–73 (2006). [CrossRef]
7. S. H. Lee, H. Y. Kim, I. C. Park, B. G. Rho, J. S. Park, H. S. Park, and C. H. Lee, “Rubbing-free, vertically aligned nematic liquid crystal display controlled by in-plane field,” Appl. Phys. Lett. 71(19), 2851–2853 (1997). [CrossRef]
8. K. Hanaoka, Y. Nakanishi, Y. Inoue, S. Tanuma, Y. Koike, and K. Okamoto, “A new MVA-LCD by polymer sustained alignment technology,” SID Symp. Dig. Tech. Pap. 35(1), 1200–1203 (2004). [CrossRef]
9. S. G. Kim, S. M. Kim, Y. S. Kim, H. K. Lee, S. H. Lee, G. D. Lee, J. J. Lyu, and K. H. Kim, “Stabilization of the liquid crystal director in the patterned vertical alignment mode through formation of pretilt angle by reactive mesogen,” Appl. Phys. Lett. 90(26), 261910 (2007). [CrossRef]
10. S. H. Lee, S. M. Kim, and S. T. Wu, “Review Paper: Emerging vertical-alignment liquid-crystal technology associated with surface modification using UV-curable monomer,” J. Soc. Inf. Disp. 17(7), 551–559 (2009). [CrossRef]
11. S. M. Kim, I. Y. Cho, W. Kim, K. U. Jeong, S. H. Lee, G. D. Lee, J. Son, J. J. Lyu, and K. H. Kim, “Surface-modification on vertical alignment layer using UV-curable reactive mesogens,” Jpn. J. Appl. Phys. 48(3), 032405 (2009). [CrossRef]
12. J. J. Lyu, H. Kikuchi, D. H. Kim, J. H. Lee, K. H. Kim, H. Higuchi, and S. H. Lee, “Phase separation of monomer in liquid crystal mixtures and surface morphology in polymer stabilized vertical alignment liquid crystal displays,” J. Phys. D Appl. Phys. 44(32), 325104 (2011). [CrossRef]
13. S. H. Lim, D. H. Kim, S. J. Shin, W. C. Woo, H. S. Jin, S. H. Lee, E. Y. Kim, and S. E. Lee, “Polymer Stabilized In-Plane Field Driven Vertical Alignment Liquid Crystal Device,” SID Symp. Dig. Tech. Pap. 42(1), 1645–1647 (2011). [CrossRef]
14. C. Y. Huang, W. Y. Jhuang, and C. T. Hsieh, “Switching of polymer-stabilized vertical alignment liquid crystal cell,” Opt. Express 16(6), 3859–3864 (2008). [CrossRef] [PubMed]
15. J. K. Song, K. E. Lee, H. S. Chang, S. M. Hong, M. B. Jun, B. Y. Park, S. S. Seomun, K. H. Kim, and S. S. Kim, “Novel method for fast response time in PVA mode,” SID Symp. Dig. Tech. Pap. 35(1), 1344–1347 (2004).
16. K. Hanaoka, Y. Nakanishi, Y. Inoue, S. Tanuma, Y. Koike, and K. Okamoto, “A new MVA-LCD by polymer sustained alignment technology,” SID Symp. Dig. Tech. Pap. 35(1), 1200–1203 (2004). [CrossRef]
17. S. H. Kim and L. C. Chien, “Electro-optical characteristics and morphology of a bend nematic liquid crystal device having templated polymer fibrils,” Jpn. J. Appl. Phys. 43(11A), 7643–7647 (2004). [CrossRef]
18. S. W. Kang, Y. E. Choi, B. H. Lee, J. H. Lee, S. Kundu, H. S. Jin, Y. K. Yun, S. H. Lee, and L. Komitov, “Surface polymer-stabilised in-plane field driven vertical alignment liquid crystal device,” Liq. Cryst. 41(4), 552–557 (2014). [CrossRef]
19. Y. J. Lim, Y. E. Choi, J. H. Lee, G. D. Lee, L. Komitov, and S. H. Lee, “Effects of three-dimensional polymer networks in vertical alignment liquid crystal display controlled by in-plane field,” Opt. Express 22(9), 10634–10641 (2014). [CrossRef] [PubMed]
20. T. J. Chen and K. L. Chu, “Pretilt angle control for single-cell-gap transflective liquid crystal cells,” Appl. Phys. Lett. 92(9), 091102 (2008). [CrossRef]
21. G. W. Smith, “Mixing and phase separation in liquid crystal/matrix systems: Determination of the excess specific heat of mixing,” Phys. Rev. Lett. 70(2), 198–201 (1993). [CrossRef] [PubMed]
22. T. Kyu and D. Nwabunma, “Simulations of microlens arrays formed by pattern-photopolymerization-induced phase separation of liquid crystal/monomer mixtures,” Macromolecules 34(26), 9168–9172 (2001). [CrossRef]
23. B. P. Iguanero, A. O. Pérez, and I. F. Tapia, “Holographic material film composed by Norland NOA 65® adhesive,” Opt. Mater. 20(3), 225–232 (2002). [CrossRef]
24. G. Baur, V. Wittwer, and D. W. Berreman, “Determination of the tilt angles at surfaces of substrates in liquid crystal cells,” Phys. Lett. 56(2), 142–144 (1976). [CrossRef]
25. G. J. Lin, T. J. Chen, Y. W. Tsai, Y. T. Lin, J. J. Wu, and Y. J. Yang, “Performance enhancement using a non-uniform vertical electric field and polymer networks for in-plane switching of multi-pretilt, vertically aligned liquid crystal devices,” Opt. Lett. 39(21), 6225–6228 (2014). [CrossRef] [PubMed]
26. A. D. Garbo and M. Nobili, “Order parameter dependence of the nematic liquid crystal anchoring energy: A numerical approach,” Liq. Cryst. 19(2), 269–276 (1995). [CrossRef]
27. F. Akkurt, N. Kaya, and A. Alicilar, “Phase transitions, order parameters and threshold voltages in liquid crystal systems doped with disperse orange dye and carbon nanoparticles,” Fuller. Nanotube. Car. N. 17(6), 616–624 (2009). [CrossRef]
28. X. Nie, Y. H. Lin, T. X. Wu, H. Wang, Z. Ge, and S. T. Wu, “Polar anchoring energy measurement of vertically aligned liquid-crystal cells,” J. Appl. Phys. 98(1), 013516 (2005). [CrossRef]
29. A. Murauski, V. Chigrinov, A. Muravsky, F. S. Y. Yeung, J. Ho, and H. S. Kwok, “Determination of liquid-crystal polar anchoring energy by electrical measurements,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 71(6 Pt 1), 061707 (2005). [CrossRef] [PubMed]
30. X. Nie, H. Xianyu, R. Lu, T. X. Wu, and S. T. Wu, “Pretilt angle effects on liquid crystal response time,” J. Disp. Technol. 3(3), 280–283 (2007). [CrossRef]
31. G. J. Lin, T. J. Chen, M. J. Lee, J. J. Wu, K. Y. Lai, and Y. J. Yang, “Effect of cross-linking polymer networks on the molecular reorientation and electro-optical performance of in-plane switching vertically aligned liquid crystal devices,” J. Polym. Sci. Part B Polym. Phys. 53(16), 1123–1130 (2015).
32. G. J. Lin, T. J. Chen, B. Y. Chen, J. J. Wu, and Y. J. Yang, “Enhanced electro-optical properties of vertically aligned in-plane-switching liquid crystal displays employing polymer networks,” Opt. Mater. Express 4(8), 1657–1667 (2014). [CrossRef]
33. X. Nie, R. Lu, H. Xianyu, T. X. Wu, and S. T. Wu, “Anchoring energy and cell gap effects on liquid crystal response time,” J. Appl. Phys. 101(10), 103110 (2007). [CrossRef]
