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Abstract
Liquid crystal (LC) cells with photopolymers usually exhibit a fast response time but inevitably present decreased optical transmittance and lower contrast ratio due to incomplete dark states. In this study, we show that this issue can be improved when photopolymerization at low temperature is considered. Comparing performance with the 4 wt% RM257-doped fringe-field switching (FFS) LC cell photopolymerized at room temperature, the 1.4 wt% RM257-doped FFS LC cell photopolymerized at low temperature (273 K) shows better contrast ratio and lower operating voltage. In addition, the electrostriction effect can be also reduced in LC cells with lower RM257-doped concentration. As a result, the 1.4 wt% RM257-doped FFS cell shows a response time as fast as that in the 4 wt% RM257-doped FFS cell. Meanwhile, the average response time for gray-to-gray switching of the optimal FFS cell is 4.9 ms.
© 2017 Optical Society of America
1. Introduction
Techniques for the production of liquid crystal displays (LCDs) have been developed, and three valuable modes, namely, multidomain vertical alignment (MVA) [1,2], in-plane switching [3, 4], and fringe-field switching (FFS) [5, 6], have been mostly used in commercial fabrication of LCDs. Upgraded FFS shows excellent characteristics, including wide viewing angles, small color shifts versus various viewing directions, and robust features for touch panel applications.
LCDs generally exhibit slow response time, so they often show blurred images and have limited operation [7, 8]. The response time of LCDs is generally divided into two parts, namely, rising time and falling time (similar to relaxation time). Both parts are sensitive to liquid crystal (LC) physical parameters, cell conditions, and electrical operations. For example, the relaxation time in FFS mode is obtained by restoring the elastic torque, which is mainly related to twist elastic constant (K22), of LCs. Given that its value is smaller than those of splay elastic constant (K11) and bend elastic constant (K33), the response time in FFS mode is about 2–3 times slower than that in MVA mode [12]. Numerous methods, such as using multielectrodes to improve relaxation time [9, 10], using LCs with ultralow viscosities [11, 12], and using thin cell gaps [13, 14], were proposed to improve the response time of LCDs.
Polymer network liquid crystal is another method that is usually investigated and remarkably efficient to achieve fast response time; in this method, LC cells doped with a suitable concentration of anisotropic monomer are exposed to UV light. Consequently, the response time, especially the relaxation time in LC cells, shows significant improvement [15–17]. However, light scattering issues attributed to the refractive index mismatch of ingredients [15], incomplete dark states due to polymer networks affecting uniform LC alignments, and instability of electrically dynamic response due to electrostriction effect [17, 18] should also be addressed. Furthermore, using high operating voltages greatly limits LCD applications [16, 17]. To date, a number of methods with improved response time and low threshold voltage have been proposed. These methods include stabilizing LCs on substrate surfaces [19] and increasing azimuthal anchoring energy with a doped monomer in LC alignment materials [20, 21].
However, the tradeoff issue between response time and optical transmittance inevitably occurs in photopolymerized liquid crystal cells with high concentrations of photocurable monomer. In the present study, we demonstrate that the available fast response time in FFS LC cells is achieved if UV photopolymerization at low temperature and decreased concentration of reactive mesogen are used. A lowly concentrated polymer network can penetrate the LC layer that presents constraints to LCs and thereby effectively improve response time. In addition, polarization optical microscopic images, scanning electron microscopy (SEM) images, and theoretical models are used to illustrate and discuss the possible mechanism involved in this improvement. Except response time, FFS LC cells exposed to low-temperature UV exhibit considerable advantages. In particular, these cells exhibit higher transmittance, improved dark states, and lower operating voltages than FFS LC cells exposed to UV light at room temperature.
2. Experimental
The specifications of FFS LC cells used in this study were as follows: electrode width, 3 μm; gap between neighbor electrodes, 4 μm; rubbing angle with respect to electrode direction, 7°; and cell gap, 3.1 μm. The positive dielectric anisotropy of LCs, named LC377, possessed the following physical parameters: elastic constants (K11 = 11.5 pN, K22 = 6 pN, and K33 = 12.5 pN); optical birefringence (Δn), 0.1094; dielectric anisotropy (Δε), 9.5; and rotational viscosity (γ1), 55 mPas. The LC mixture filled in cells was composed of LC377 LCs, reactive mesogen RM257 (AUO Ltd.), and photoinitiator Igracure-651 (Ciba Ltd.). The prepared LC mixture was filled into FFS cells through capillarity effect. RM257 weight percentage and temperature during photoexposure can affect the electro-optical performance of FFS LC cells. Table 1 shows a list of all cell conditions in the experiments. A fixed 0.5 wt% photoinitiator Igracure-651 was included in all cells. The cell temperature was controlled by a temperature controller (T95-PE, LINKAM). The UV (λ = 365 nm) exposure process was conducted using a power intensity of 250 mW/cm2 for 300 s.
Table 1. RM257 percentages and processing temperatures for all FFS LC cells in the experiments
The performance of FFS LC cells exposed to UV light under various experimental conditions was observed. First, the applied voltage versus optical transmittance (V–T) curves were measured by using white LEDs as backlight and photoreceiver (New Focus, 2001-FS) to measure the optical transmittance of FFS LC cells with applied AC voltages (1 kHz square waveform). The measured FFS cell was located between a pair of crossed polarizers with 0° polarization directions with respect to the LC alignment direction. Given that the formation of polymer network in FFS cells generally affects performance, incomplete dark in OFF-state and transmittance decrease in ON-state for FFS cells were both considered and measured by V-T curves and normalized transmittance spectra. The setup for measuring transmittance spectra was composed of a white light source (StellarNet, SL1), fiber spectrometer (StellarNet, Blue-Wave), and special fixture used to prevent scattering light induced by LC cells. The ON-state voltage indicated Von was operated in FFS cells with maximum transmittance from the measurements of V-T curves. The FFS cells were applied with individual Von voltage when measuring the transmittance spectra. Finally, the experimental cells were decomposed and subsequently scanned through SEM (JEOL-6340F) to identify the differences among the polymer networks generated by various concentrations of RM257 in FFS LC cells under various processing temperature levels. The FFS cells were immersed in n-hexane solution for 24 h to dissolve LCs and obtain only the polymer networks.
3. Results and discussion
Figure 1(a) shows the individually measured V–T curves with respect to five FFS cells, of which four were RM257-doped cells and one was filled with only LC377 LCs. In addition, various percentages of RM257-doped FFS cells were exposed to UV light at room temperature (298 K). All RM257-doped cells significantly possess lower Tmax values than cells filled only with LC377 LCs. The comparisons among the electro-optical performances of the cells are summarized in Table 2. To investigate why transmittance decrease exists in all RM257-doped cells, we measured their normalized transmittance spectra. Figure 1(b) shows the measured results of normalized transmittance spectra and denotes a ratio in comparison with the transmittance of FFS cell filled with LC377 only with respect to various wavelengths. Evidently, transmission decreased because of the increase in serious light scattering caused by polymer networks with increasing RM257 percentages in cells, especially those in the short wavelength range. Microscopic observations of both OFF- and ON-state FFS cells with respect to zero and individual Von voltages were recorded via polarization optical microscopy (POM, Nikon Eclipse 50iPOL). In the OFF-state, the POM images in RM257-doped cells showed significant incomplete dark at a high incident light intensity, as shown in Fig. 2(a). This finding strongly suggests that incomplete dark states are caused by the disturbances caused by polymer networks. These disturbances affect LC molecular orientations without completely uniform alignments in all the RM257-doped cells. Thus, residual transmittance, indicated by Tmin, exists in the OFF-state (Table 2). By contrast, Fig. 2(b) shows the POM images in the ON-state with respect to individual Von in FFS cells. Light scattering issue is not the main cause of transmittance decrease because of the extremely large difference between the transmittances of the POM images of highly concentrated RM257-doped cells and FFS cell filled with LC377 only. We observed that the FFS cell filled with LC377 only was in the ON-state, and the POM image showed periodic areas with low generated transmittance, where LC molecules were only slightly rotated after the application of electric fields [22, 23]. By contrast, high concentrations of RM257-doped FFS cells possessed considerably low transmittance areas, because high-density polymer networks penetrated into the LC layers such that LC molecules were much difficult to reorient after the operation of electric fields. Finally, the RM257-doped FFS cells showed evidently low transmittance.
Fig. 1 Comparisons of V–T curves and normalized transmittance spectra for FFS cells. One cell was filled with LC377 LCs only, whereas the other cells were filled with various percentages of RM257 dopant and photopolymerized at room temperature (298 K). (a) V–T curves (b) normalized transmittance spectra.
Table 2. Electro-optical performance of FFS LC cells (the same as in Fig. 1)
Fig. 2 Comparisons between POM images in OFF- and ON-states with respect to FFS cells with various percentages of RM257 dopants. (a) OFF-state with zero applied voltage. (b) ON-state with individual applied Von voltages, as shown in Table 2. Red dash lines indicate the centers of neighbor electrodes in the FFS cells.
Therefore, the decreased transmittance of RM257-doped FFS cells in V–T curves was attributed to the mismatch between the refractive indices of LCs and polymer networks and locally constrained LCs due to increased density polymer networks. The former induced light scattering and decreased transmittance, while the latter hindered the reorientation of LC molecules following the operation of electric fields.
The response time was available and based on normalized transmittance in cells electrically operated between the OFF-state with zero voltage and ON-state with individual Von voltages, and the results are shown in Fig. 3. Rising time was defined as the spent time during 10%–90% transmittance variation, and falling time as the spent time during 90%–10% transmittance variation. Given that the performance of F-298-1.2 cell closely approximates the performance of LC cell with LC377 only, the value was disregarded in the response time curve of F-298-1.2 cell. Comparisons among the response times of the FFS cells are summarized in Table 2. The labeled LC377 cell showed a rising time of 9.3 ms and falling time of 10.4 ms between 0 and 5 Vrms electrically switching. The labeled F-298-4 cell showed a rising time of 4.5 ms and falling time of 1.8 ms between 0 and 10 Vrms electrically switching. These results show that the generated polymer networks of RM257 not only improved falling time in FFS cells but also seriously degraded maximum transmittance Tmax and incomplete dark states with residual transmittance Tmin. In addition, multiple RM257 percentages in F-298-4 cell exhibited a slower rising time (4.5 ms) than that of F-298-3 cell (4.2 ms; Table 2). This phenomenon was attributed to the electrostriction effect and that can be observed and illustrated in Fig. 3(a), in which the behavior of two steps of dynamical response for rising time in the F-298-4 cell is evident. During the first step of dynamical response, the normalized transmittance rapidly achieved 85% value with an applied Von of 10 Vrms. In the second step, the much denser polymer networks generally provide more constrains on LC molecules to reorient during electric driving, extremely slow transmittance increase was apparent and finally available to 90% normalized transmittance because of the strong constraints on LCs and the electrical deformation of polymer network [17, 18].
Fig. 3 Comparisons of response time in FFS LC cells (the same as in Fig. 1) (a) rising time (b) falling time.
Although the labeled F-298-4 cell showed good performance in falling time, numerous issues were also apparent, including low contrast ratio (CR), high operating voltage, and tradeoff between response time and transmittance in FFS cells doped in high concentrations of RM257. Therefore, we investigated the electro-optical performance of the FFS cells at low RM257 concentrations (1.2, 1.3, and 1.4 wt%). Simultaneously, the process of UV photopolymerization was executed at low temperature (273 K) to reduce the reaction rate of polymerization in order to change the morphology of the polymer networks. Two 2 wt% RM257-doped FFS cells were individually processed at 298 and 273 K to compare the transmittances that are related only to photopolymerization temperature. The resulting V–T curves and response time for FFS cells are shown in Fig. 4 and summarized in Table 3. The labeled F-273-1.2 and F-273-2 cells showed better response time compared with F-298-1.2 and F-298-2 cells. The rising time and falling time of labeled F-273-2 cell were 0.76 and 2.1 ms, respectively. However, high applied voltages (~17 Vrms) was required to achieve maximum transmittance (the detailed curves related to performance of F-273-2 cell is not shown in Fig. 4). Therefore, only the FFS cells with low RM257 concentrations (1.2, 1.3, and 1.4 wt%) and photopolymerization at low temperature (273 K) were further investigated and discussed. In Table 3, the decrease in maximum transmittance (Tmax) can be attributed to light scattering loss and abundant areas containing LCs that are difficult to reorient electrically due to the strong constraints exerted by the polymer networks. In general, in cell doped with low concentrations of RM257-doped cells, polymer networks are usually formed on the surfaces of glass substrates via phase separation when the cells are exposed to UV at room temperature [19, 24]. Although the polymer networks on the glass substrates possibly provide strong anchoring energy that aligns the LC molecules, the response time is sensitive and mainly governed by restoring the elastic torque of the entire LC molecules in the cell. Therefore, polymer networks formed on substrates cannot achieve extremely fast response time, especially falling time [19, 20]. At low photopolymerization temperature, high LC viscosity induces significant difference on polymer networks through phase separation between RM257 and LCs [25–27]. Evidently, polymer networks not only are present on substrate surfaces but also penetrate the LC layer to constrain LC molecular reorientations. This result was confirmed by SEM images.
Fig. 4 Comparisons of V–T curves and response time in lower RM257-doped FFS LC cells and one LC377-only-filled cell with photopolymerization at low temperature (273 K). (a) V–T curves; (b) response time curves.
Table 3. Electro-optical performance of FFS LC cells (the same as that in Fig. 4)
The normalized transmittance spectra for FFS cells with low concentrations of RM257 and under UV photopolymerization at 273 K are shown in Fig. 5(a). Given that the three FFS cells have nearly similar low RM257 concentrations, the issue of transmittance decreased because of the seemingly equal degrees of light scattering. Furthermore, higher transmittance was observed in each cell than that in the F-298-4 cell. Figure 5(b) shows the comparisons among the POM images for the FFS cells, and the optical performances and response times of the cells are summarized in Table 3. Evidently, all the cells showed significant improvements in transmittance of Tmin and Tmax. Furthermore, incomplete dark states are probably correlated to the polymer network that slightly disturbs the LC orientation in the cell. Notably, the F-273-1.4 cell showed excellent characteristics, including a CR value of 169.8, Tmax with lower operating voltage (Von) of 7 Vrms, and response time of 6.7 ms. In addition, the F-273-1.4 cell presented a faster rising time of 2.9 ms compared with the F-298-4 cell. Therefore, the two steps of dynamical response that affected rising time can be improved via low temperature photopolymerization in FFS cells doped with low concentrations of RM257.
Fig. 5 Comparisons of normalized transmittance spectra and POM images in low concentration RM257-doped FFS cells photopolymerized at 273 K. (a) normalized transmittance spectra. (b) POM images of FFS cells with respect to individual OFF- and ON-states.
To further illustrate and discuss what mechanism improves response time for RM257-doped FFS cells, we consider the dynamics of LC molecular reorientation according to the Erickson–Leslie equation [28]. The POM images of the FFS cells in ON-state show numerous areas with low transmittances where are named Region-1 as schematically shown in Fig. 6. A location in the FFS cell shows the fringe electric fields mainly varied along both z-axial and x-axial directions, which are indicated with Ez and Ex, respectively. In Region-1 areas, the Ex is negligible and simultaneously the Ez is weak so that the LC molecules are electrically reoriented with only a small angle. By contrast, larger Ex electric fields exist in the areas nearby the edge of electrodes so that LC molecules are largely reoriented to achieve bright areas [22, 23]. In RM257-doped FFS cells, the polymer networks penetrate the LC layer to provide more constraints for all LC molecules. Therefore, the LC molecules located in Region-1 areas will be more difficult to be electrically reoriented due to weak electric fields. Therefore, the dynamics of LC reorientation can be simulated using a two-dimensional model, and strong anchoring force was assumed to exist on cell substrates and Region-1 areas, where only the LCs located area bounded by the red dot line can be reoriented after the application of electric fields. The Erickson–Leslie equation can be expressed as follows by considering the small-angle approximation:
where ϕ is the rotational angle of LC director, K11 and K22 are the splay and twist elastic constants, respectively, ε0ΔεE2 is the electric field energy density, ε0 is the electric permittivity, Δε is the dielectric anisotropy, E is the applied electric field in cell, and γ1 is the rotational viscosity. Considering Eq. (1) in the static case, the Freedericksz transition threshold voltage can be approximated as Eq. (2) [28], where l is the gap between neighbor electrodes, d is the cell gap, and D is the width of brightness area (Region-2 area; Fig. 6). The measured D values are summarized in Table 4. In the absence of an electric field, the solution of Eq. (1) can be simply expressed as Eq. (3), where ϕm and τ0 present the maximum rotational angle and free relaxation time of LC, respectively. In general, the falling time of LC cell is similar to the free relaxation time (τ0) [29]. Rising time is more complicated and its value related to the τ0 and applied voltages. Therefore, we only focused on and discussed the falling times and threshold voltages in FFS cells. From Eqs. (2) and (3), the reduction of cell gap and width of the reoriented LC area improves falling time but increases threshold voltage (Vc). The Vc values are available from the V-T curves of the FFS cells and summarized in Table 4. In addition, the theoretical values of τ0 and Vc were calculated using Eqs. (2) and (4), and the results are also listed in Table 4. For the numerous RM257-doped cells, high concentrations of RM257 in FFS cells significantly decreased the reoriented LC areas, resulting in improved response time. The deviation between the calculated τ0 value and falling time in RM257-doped FFS cells may be attributed to the unsatisfactory hypothesis with the two-dimensional model, where the anchoring energy to constrain LC molecules is not infinite. For the cells doped in low concentrations of RM257, the POM images also showed dark areas, where LCs appear to be not reoriented. Evidently, falling time is sensitive to the widths of the reoriented LC areas. Although Eqs. (2) and (4) from the theoretical two-dimensional model cannot completely describe the dynamics of LC molecular reorientations and threshold values of Freedericksz transition in the RM257-doped FFS cells, the equations provided information that can contribute to the improvement of response time and increase of threshold voltage. The improved response time of RM257-doped FFS cells can be attributed to the increase of anchoring energy in Region-1 areas, except the part of anchoring energy from polymer networks.Fig. 6 Local scheme of LC reorientations in a FFS cell from the top viewing direction. The Region-2 area means that the incident light can pass through the cell with a pair of crossed polarizers to show a bright area in the POM image. The z-axis direction is perpendicular to the glass substrates of the FFS cell.
Table 4. Parameter list for discussing mechanism of response time based on theoretical two dimensional Erickson-Leslie equation.
Given the optimal performance in the F-273-1.4 cell, the V–T curve was divided into eight gray levels, which were labeled G1–G8, to obtain the response time under gray-to-gray switching. All transmittance values of the gray levels under applied voltages are listed in Table 5. Response time performance under gray-to-gray switching is summarized in Table 6. The average response time for gray-to-gray switching showed a rising time of 4.6 ms and falling time of 5.2 ms. In order to get much better improvements of contrast ratio and transmittance in photopolymerized FFS cells at low temperature, it is a possible way to use lower viscosity LC materials and dope with lower concentration of RM257 [11] to achieve higher transmittance along with ideal dark states in cells.
Table 5. Transmittance versus applied voltages for eight gray levels of the F-273-1.4 cell
Table 6. Response time (ms) under gray-to-gray switching of the F-273-1.4 cell
The experimental cells were decomposed for SEM analysis. As shown in Figs. 7(a)–(d), polymer networks formed in the FFS cells with various RM257 concentrations and under photopolymerization at room temperature (298 K). Evidently, polymer networks were formed on the substrate surface of the F-298-1.2 cell processed at room temperature. The polymer networks (fibrils) became markedly coarse and penetrated the cell gap when RM257 concentration was increased from 2 wt% to 4 wt%. By contrast, Figs. 7(e)–(g) show the SEM images of the FFS cells doped with low concentrations of RM257 and processed at 273 K. In general, the viscosities of LCs increase at low temperature to induce the slow diffusion of photocurable monomers during phase separation [30, 31]. Polymer networks with shapes of small fibril bundles in cells then form. Meanwhile, only countable fibrils are observed because of the use of substantially low RM257 concentrations.
Fig. 7 SEM cross-section images of decomposed FFS cells. The cells are labeled with (a) F-298-1.2, (b) F-298-2, (c) F-298-3, (d) F-298-4, (e) F-273-1.2, (f) F-273-1.3, and (g) F-273-1.4. The area between both parallel green dash lines mean LC layer for each cell.
4. Conclusion
We demonstrate that the well-known tradeoff issue of response time versus transmittance in photopolymerized FFS LC cells can be improved by performing photopolymerization at low temperatures. The transmittance reduction of RM257-doped FFS cells is reasonably illustrated according to normalized transmittance spectra and POM images. A lot of issues in photopolymerized FFS cells surely have been improved in this study including lower light scattering, improved dark states, electrostriction effect reduction, and lower operating voltage. In addition, the SEM images show that photopolymerization at low temperature 273 K leads the lowly concentrated polymer networks to penetrate the LC layer to form the two-dimensional constraints, so that the falling time is effectively improved from 10.4 to 3.8 ms. An optimal 1.4 wt% RM257-doped FFS cell photopolymerized at 273 K showed fast response time of 6.7 ms and simultaneously achieved Tmax value that was twofold larger than that of an FFS cell with high concentrations of RM257 photopolymerized at 298 K. Such result can be possibly used to further improve the Tmin and Tmax values of FFS LC cells, that is, when LCs with low viscosities are used to reduce the RM257 concentration. The proposed method is also suitable for most LC electro-optical devices with fast response time, except FFS LC cells.
Funding
AU Optronics (AUO, Taiwan, Grant No. B104-K035).
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