1. Introduction

Liquid crystal displays (LCDs) are widely used due to their low operating voltage, low power consumption, and thinness [1]. However, their slow response is considered as the major drawback of the three-dimensional dynamic display application [2, 3], which is attributed to the properties of LC molecules [4]. To improve the display performance, several methods have been proposed, while some disadvantages, such as low transmittance [5], complicated fabrication [6–8], and difficult control of the LC molecular conformation [9, 10], have also been revealed.

Among the various advanced display modes, in-plane switching (IPS) mode [11] is one of the most promising technologies in the current LCD industry. This technology provides a wide viewing angle, good image quality, color accuracy and low operating voltage [12]. However, to effectively govern the arrangement of the LC molecules, the conventional IPS mode is usually combined with the rubbing process during cell fabrication [13]. This causes light leakage in the dark state and a poor contrast ratio (CR) [14, 15]. The vertical alignment (VA) LC cell exhibits a high CR because the LC molecules are perpendicular to the substrate surface, but such an alignment of negative dielectric anisotropy LC (negative LC) usually has a long response time [16], especially for the falling time (τ_f). The τ_f is only related to the cell gap thickness and material parameters of the negative LC, such as rotational viscosity and the bend elastic constant [16], which cannot be improved by overdriving. The negative LC, which usually has higher viscosity, is disadvantageous to the τ_f and more expensive than the positive LC. Although the rising time (τ_r) can be reduced by the overdrive method [17, 18], sophisticated driving schemes are needed. To eliminate the drawbacks of the conventional IPS and VA modes, an experiment on the VA-IPS display mode using a positive LC has been demonstrated [19], but the poor device characteristics, such as high operating voltage and low transmittance, need improving. In 2011, Yang and Sun proposed a high-transmittance VA-IPS LCD model, and its electro-optical properties were also simulated [20, 21].

Recently, the LC cell with polymer-stabilized networks has been considered as a potential candidate for controlling the molecular orientation [22–26] in the multi-domain vertical alignment (MVA), patterned vertical alignment (PVA), and VA modes. Although this method can reduce the τ_r, the surface pretilt could be adverse to the τ_f response. Moreover, light leakage in the dark state is also observed, which will sacrifice the high CR. In addition, the in-cell polymer networks inducing the additional anchoring effect with respect to the substrate surface are detrimental to the other electro-optical properties, such as poor CR [27], higher threshold voltage (V_th), and higher addressing voltage in the gray-level responses and switching response (~80 V) [28]. These are associated with the mixed concentration and type of monomer. Thus the process and electro-optical parameters of the LC cell with polymer networks require optimizing. To align the LC molecules well, Lee et al. used the in-plane electric field during the process of polymerization to produce a small pretilt angle in the VA-IPS mode [29, 30]. With this configuration, the τ_r response can be improved, but a higher driving voltage (~30 V) applied to the VA-IPS cell is required. Moreover, due to the pretilt angle effect, the τ_f response is degraded and is slower than the unprocessed cell for all gray levels.

In this paper, there are three kinds of monomer materials separately employed in the pure VA-IPS LC cells, and the superior electro-optical characteristics of the cell resulting from one of these monomer materials are demonstrated. The polymer networks used here can effectively govern the LC molecular reorientation, which is mainly attributed to the polymer anchoring energy, and maintains good optical transmittance under the high voltage applied. In terms of dynamic response, the polymer cells using a relatively small driving voltage (≤ 12 V) as compared to the other works [28–30] exhibit a shorter τ_r, and the stronger polymer anchoring effect (TA-9164) leads to a fast τ_f response. In addition, although the overdrive method applied to the pure VA-IPS LC cell can realize a more rapid τ_r response, low optical transmittance is also revealed. Without the additional driving circuit, the τ_r of the TA-9164 polymer cell under normal drive is much closer to that of the pure VA-IPS LC cells under overdrive.

2. Experimental

In the experiment, VA-IPS LC cells with polymers are driven by the indium-tin-oxide interdigital electrodes, which are fabricated on the bottom-glass substrate. The electrode width (w) is 4 μm and the separation (s) is 8 μm. Pure glass is used as the top substrate. Both substrates are coated with the VA polyimide (AL60101L) and heated at 200 °C for 1 hour. After that, 4-μm spacers are used on the substrate surface in order to control the cell gap. The LC E7 (Δn = 0.2179, Δε = + 14.5, K₁₁ = 12.0 pN, K₂₂ = 9.0 pN, K₃₃ = 19.5 pN, and γ₁ = 232.6 mPa·s at room temperature from Merck) and the monomer are mixed at a specific concentration at the mixing temperature of 80 °C to get uniform mixing before the process of polymerization. The mixed concentration is defined as the ratio of the monomer weight to the sum of the monomer and E7 LC weight. Three kinds of monomer, TA-9164 (with cross-linking from Tatung University), UCL002 (with linear-linking from DIC Corp.), and NOA65 (with cross-linking from Norland Inc.), are selected and separately mixed to the nematic E7 LC, then injected into the prepared cell by capillary action. To control polymer networks with a stabilized and functional morphology, the mixed concentration is selected of 2 wt% for above-mentioned monomer materials, and the top and bottom substrate surfaces of cell are both exposed to 365-nm-ultraviolet (UV) light for 30 minutes. The intensity of UV light is controlled at 16 mW/cm² to avoid degrading the polymer quality and morphology. This design fabricated process and conditions can stabilize the morphologies of TA-9164, UCL002, and NOA65 monomer materials (well-developed) as well as it can enable the cell to show good electro-optical performance, as will be shown later. The cell structures before and after the UV illumination are respectively illustrated in Fig. 1(a) and Fig. 1(b). A molecular alignment and the driving mode of the E7 LC cell with polymer networks are shown in Fig. 1(c). The polymer morphologies for TA-9164, UCL002, and NOA65 monomer materials are recorded by field emission scanning electron microscopy (FE-SEM), and the effect of polymer morphology on the alignment quality of LCs is inspected by the conoscope under the two crossed polarizers. During the measurement, the IPS voltage (V_IPS) is applied to these cells, sandwiched between the two crossed polarizers and orientated to 45° with respect to the polarizer. Transmittance-voltage (T-V) curves, optical switch and gray-level responses, and the overdriving τ_r response are demonstrated using a laser light with a 650-nm wavelength and ac voltage with 1 kHz square waveform.

 

Fig. 1 Schematic illustrations of the process of polymerization and LC molecular orientation for a VA-IPS LC cell with polymers. (a) Well mixed monomer in the LC cell and UV-light exposure. (b) Polymers assembling on the top- and bottom-substrate surface after UV curing. (c) A cell sandwiched between the polarizer and analyzer, and the configuration of LC molecules under the connection with V_IPS.

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In order to compare the effect of different morphologies of polymers on electro-optical properties of VA-IPS cells, it is worth mentioning that the reliable process route must be explored. A well-developed process of polymerization that makes the LC/polymer cell have a good phase separation is very important because it can significantly reduce the effect of LC/monomer on LC basic parameters (changing LC basic parameters). Thus the curing time plays a key role in this treatment. In addition, the employed monomer material and its concentration also have to be taken into account due to the fact that they affect the device properties. Thus selecting an appropriate monomer material and its concentration can effectively improve the display electro-optical performance, such as reducing the driving voltage, increasing light transmission efficiency, and enhancing the τ_f response. For the VA-IPS cells, the design of interdigital electrode pattern has to be considered as well because it affects the driving voltage and light transmittance of cell. Therefore, the above design treatment conditions can satisfy these considerations, and can benefit the reliability of VA-IPS LC/polymer systems.

3. Results and discussions

The morphologies of the TA-9164, UCL002, and NOA65 polymer networks, recorded by FE-SEM, are shown in Fig. 2(a), 2(b), and 2(c), respectively. In Fig. 2(a), the TA-9164 polymers exhibit the planar cross-linking network morphology and uniform connection with their neighbors, expected to benefit the electro-optical performance of VA-IPS LC cell. In contrast, the UCL002 and NOA65 polymers individually shown in Fig. 2(b) and 2(c) present the self-assembling morphology and the uniform distribution on the substrate surface. Different from the TA-9164 polymer morphology, the UCL002 polymer morphology is spherical (spheres), and the NOA65 polymer morphology is clustered (small sticks). Such these two kinds of polymer morphologies relatively lessen the probability of the reaction with LC molecules and result in different electro-optical properties in comparison with the TA-9164 polymers. The polymer morphologies of TA-9164, UCL002, and NOA65 monomer materials are well controlled and well developed by the above demonstrated experimental parameters (monomer concentration, mixing temperature, curing time, and UV light intensity), which stabilize the polymer morphology and make the monomer material present the typical morphology. It is worth mentioning that: from the point of view of the polymer process, the different treatment on the different polymer materials could possibly yield the similar morphology. However, since the TA-9164 polymers and the UCL002 polymers, in essence, are totally different, their morphologies present in their own ways and will not be changed even if they are treated in different conditions. Also, their morphology will not be the same even if the treatment parameters are changed. This indicates that the TA-9164 and UCL002 polymers have the stable morphology, not easily changed as the different treatment parameters are applied. Although the NOA65 polymer morphology is slightly sensitive to the treatment parameters [31–34], it still shows its typical morphology (“clustered” small sticks) on the glass substrate surface. However, from the viewpoint of display application, the polymers under the different treatment parameters might not benefit the device performance. Further detailed discussion about the morphology and device characteristics of TA-9164, UCL002, and NOA65 polymer networks treated in different conditions will be reported elsewhere.

 

Fig. 2 (a)-(c) FE-SEM images recording the TA-9164 with planar cross-linking, UCL002 with spherical and NOA65 with clustered polymer morphologies, respectively. For these three images, the scale bar is the same, as represented in (b). (d) T-V curves for different types of VA-IPS cells: pure E7 LC, NOA65 polymer, TA-9164 polymer, and UCL002 polymer cells. The horizontal and vertical dotted lines represent the T_max and V_Tmax of each cell, respectively. (e) Conoscope images showing the effect of polymer morphology on the alignment quality of VA-IPS cells. Of all the cells, the planar cross-linking TA-9164 polymer cell has the highest brightness, indicating that the alignment quality is the best (high orderly vertical configuration). (f) Estimation on the V_th of the VA-IPS E7 LC cell with different monomer materials. The V_th value for each cell is shown in parentheses, determined by the maximum tangent line’s slope.

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Figure 2(d) shows the T-V curves for the different types of VA-IPS cells. Compared to the maximum transmittance (T_max) of the pure E7 LC cell, the T_max is decreased by 16% in the NOA65 polymer cells and 11% in the UCL002 polymer cells, which is due to the light scattering effect generated from the interface between the LCs and polymers [35]. However, this effect can be suppressed by the TA-9164 polymer cell, and the decrease in T_max is within 4.5%. The light scattering is attributed to the morphology of polymers [5, 35] and refractive index mismatch between polymer and liquid crystals [36]. Both polymer morphology and index mismatch can cause the optical phase retardation in the cell space, and could be adverse to the output light transmittance. The “spherical” or “clustered” morphology, forming the center of light scattering, can easily change the incident light wavefront with different phase retardations at any position between the top and bottom substrates. This could cause the destructive interference occurring at the LC/polymer medium where the interfering waves are without the same optical displacement. In contrast, the light scattering centers can be released by the “planar” cross-linking morphology of TA-9164 polymers, suggesting that the light loss of TA-9164 polymer cell is primarily related to the refractive index mismatch between TA-9164 and E7. Of all polymer cells, it is found that the transmittance of TA-9164 polymer cell is close to that of pure E7 LC cell. In addition, the Fresnel diffraction caused by the interdigital electrode pattern is the other factor, and usually it exists in the IPS-based cells and will reduce their transmittance [37]. However, as compared to the transmittance of the other developed IPS cells (> 70%) [38], the transmittance of the proposed LC/polymer cells has to be improved. Thus, to have a better device transmittance, there are some improvement methods that can be used during the device fabrication as follows: 1) Anneal ITO thin film: After the interdigital electrode pattern is processed, the quality of ITO thin film can be improved through the annealing treatment, which can reduce surface state and roughness. With this treatment, the output light transmittance can be increased. 2) Uniform polymer morphology: By an appropriate thermal treatment, the monomer and E7 LC can be mixed evenly. This can avoid polymer networks forming unexpected morphology, and can reduce the center of light scattering. 3) Reduce the refractive index difference: Minimizing the difference of refractive index between the monomer material and nematic E7 LC can make lower reflectance. 4) Optimize the interdigital electrode pattern: Designing the optimum interdigital electrode pattern, including the electrode width (w), electrode length, and the separation (s) between electrodes, can effectively increase the light transmittance of cell.

Regarding the electrical properties of these VA-IPS cells, the TA-9164 polymer cell has the highest V_th (3.9 V), whereas the NOA65 polymer cell has the lowest V_th (3.3 V), which is attributed to the polymer morphology and LC molecular configuration. The planar cross-linking TA-9164 polymer networks on the substrate surface can trap the LC molecules and make the good vertical configuration, proved by the conoscopic images under the two crossed polarizers, as shown in Fig. 2(e). In this figure, of all VA-IPS cells, the TA-9164 polymer cell shows the highest brightness and clearest edge on the four lobes, indicating that the VA quality is the best. Such a high orderly aligned cell has the stronger anchoring energy againstthe LC molecular rotation [39, 40]. This makes the T-V curve of the TA-9164 polymer cell shift right, indicating that additional electric potential energy is necessary for changing the molecular alignment of the cell. In contrast, the original VA configuration of the cell with spherical or clustered polymers could be changed because the four-lobe brightness of the conoscopic image is reduced, that is, the molecular alignment of this kind of cell is slightly disorderly as compared to that of the pure E7 LC cell. Such the polymer morphology of either UCL002 or NOA65 applied to the VA cell will “slightly” perturb the original alignment. Since the decrease in the order parameter (S) of LCs can result in the V_th decrease (V_th ∝ S^1/2), the threshold voltages (V_ths) of both UCL002 and NOA65 polymer cells are smaller than that of pure E7 LC cell, meaning that the LC molecules are simply driven. The V_ths of VA-IPS cells are determined by the maximum tangent line’s slope, as shown in Fig. 2(f). This figure can also be used for estimating the relationship between the V_th and anchoring energy (W) effect on the different polymer cells (V_th ∝ W^1/2) [41]. According to the above statements, the highest maximum-transmittance driving voltage (V_Tmax) applied to the cell with the maximum bright state will be observed in the TA-9164 polymer cell (V_Tmax = 12 V). In addition, the dependence of basic parameters of LCs (elastic constant and dielectric permittivity) on the morphology of polymers can also be used to explain the V_th behavior of cell. For the VA-IPS cells, the V_th is proportional to the ratio of bend elastic constant (K₃₃) to dielectric anisotropy (Δε), or properly saying (V_th ∝ (K₃₃/Δε)^1/2), and can be influenced either by changing K₃₃ or changing Δε. With the 2 wt% planar cross-linking TA-9164 polymers having the LCs align vertically, the K₃₃ of LCs is equivalently increased leading to the increment of V_th, based on the Maier-Saupe theory (K₃₃ ∝ S²) [42, 43]. On the other hand, for the spherical/clustered morphology of 2wt% UCL002/NOA65 polymer cells, the K₃₃ of LCs is to be equivalently decreased because the original perpendicular orientation of LCs is perturbed. Such an alignment with a slightly non-vertical configuration can also alter Δε and make the cell Δε decrease due to Δε ∝ S [44]. With the relationship between V_th and the ratio of K₃₃ (∝ S²) /Δε (∝ S), doping the NOA65 or UCL002 polymers to E7 LC can reduce the V_th of cell. The electro-optical characteristics (T_max, V_th, and V_Tmax) of VA-IPS LC cells with different polymers are summarized in Table 1.Moreover, when the V_IPS is more than 20 V, the transmittance of the polymer cells will be superior to that of the pure E7 LC cell, especially for the TA-9164 polymer cell. This is due to the fact that the stronger anchoring effect of TA-9164 polymer networks can limit the LC molecules to further reorient.

Table 1. Electro-Optical Properties for Different Kinds of VA-IPS Cells.

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The normalized optical switch responses of VA-IPS cells, addressed by the respective V_Tmax, are shown in Fig. 3(a) and Fig. 3(b), corresponding to τ_r and τ_f, respectively. τ_r is defined as the transmittance from 10% T_max to 90% T_max, whereas τ_f is defined as the transmittance from 90% T_max to 10% T_max. In Fig. 3(a), the TA-9164 polymer cell has the shortest τ_r (3.6 ms) at the applied driving voltage (V_Tmax = 12 V), making the light transmittance of cell rapidly reach the T_max level. This driving voltage is smaller than that used in other published works (> 30 V) [28–30]. It is noted that although the V_Tmax value of the pure E7 LC cell is larger than that of the NOA65 and UCL002 polymer cells, the pure E7 LC cell has the slowest response. The change of τ_r response is associated with not only V_Tmax, but also degree of LC molecular reorientation [45], which have to be taken into account simultaneously. Furthermore, from the point of view of the polymer morphology effect on the LC basic parameters, because the TA-9164 cross-linking polymers align a high orderly configuration of LCs, the rotational viscosity (γ₁), related to the intermolecular interactions and to the molecular reorientation around an axis, can be equivalently increased (γ₁ ∝ S) [46, 47]. However, to a fast response LCD, the high γ₁ effect should be lessened. Due to the fact that the τ_r response is proportional to the γ₁ (∝ S) /Δε (∝ S) and 1/(V_IPS-V_th)², the increased γ₁ effect can be reduced by increasing Δε effect and can be easily smoothed out by applying the higher V_IPS to the cell. According to the T-V curves shown in Fig. 2(d), of all the difference between the applied V_IPS ( = V_Tmax) and V_th on all VA-IPS cells, the maximum difference can be found in the TA-9164 cross-linking polymer cell. As compared to the pure E7 LC cell, this type of cell can significantly improve the τ_r response. On the other hand, the improvement in the τ_r response for the UCL002 or NOA65 clustered polymer cells is not obvious although their LC molecules are relatively simply driven. This is because the differences between V_Tmax and V_th of the UCL002 and NOA65 polymer cells are smaller than that of pure E7 LC cell. Despite the fact that applying the higher V_IPS to these two polymer cells can further enhance the τ_r response, their light transmittance in the steady state will be reduced. Such a consequence will be observed obviously in the pure E7 LC cell and will be shown later. In Fig. 3(b), the planar cross-linking TA-9164 polymer cell has the fastest τ_f response, attributed to the stronger polymer anchoring effect (τ_f ∝ 1/W) [48]. This provides the driven LC molecules with strong recovered energy to rapidly return to their original configuration. In contrast, the UCL002 and NOA65 clustered polymer cells are not beneficial to the τ_f response, which is attributed to the disorderly configuration of LCs and the larger visco-elastic coefficient [49]. The τ_f response of an LCD is proportional to the square of cell gap (d²) and to the visco-elastic coefficient, γ₁/K₃₃ (∝ 1/S). Thus, as the aligned order of LCs of cell is decreased, it will take longer time for LCs to reorient back to the non-driving configuration. One interesting thing is that the uniform distribution of polymer spheres of UCL002 polymer cell might play a role in reducing the cell gap. So the effect of γ₁/K₃₃ could be slightly compensated by d², making the τ_f of UCL002 polymer cell close to that of pure E7 LC cell (τ_{f (UCL002)} = 10.70 ms, τ_{f (E7)} = 10.69 ms). In addition, from the viewpoint of cell anchoring energy, the weaker anchoring effect of UCL002 polymer cell can be compensated by the equivalent shrinkage effect of cell gap (τ_f ∝ d/W) [48]. The τ_r, τ_f, and total response time (τ = τ_r + τ_f) of these VA-IPS cells are listed in Table 1. Based on above results, the spherical or clustered polymer morphology can produce a low V_th, but cannot enhance the cell response. Furthermore, the prepared VA-IPS LC/polymer cells after the continuous switching test at the high driving voltage (~ac 120 V) are still with the reliable electro-optical performance, especially for the TA-9164 cross-linking polymer cell.

 

Fig. 3 (a) τ_r and (b) τ_f responses for the pure E7 LC cell, NOA65 polymer, TA-9164 polymer, and UCL002 polymer cells.

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Figure 4 shows the gray-level responses for the pure E7 LC cell and the TA-9164 polymer cell. According to the T-V curve, the gray level is equally divided into eight levels by each cell T_max. τ_r here represents the response from the zero state to each gray-level state, while τ_f is the inverse process. In Fig. 4(a), the τ_r of the TA-9164 polymer cell is shorter than that of the pure E7 LC cell. This is because the difference in applied voltage between these two cells, or properly saying (V_{g (TA-9164)} - V_{th (TA-9164)}) > (V_{g (E7)} - V_{th (E7)}), where V_g is the gray-level-response voltage that can be obtained from Fig. 2(d), makes the TA-9164 polymer cell have a rapid LC molecular response. The cell with TA-9164 polymers has a faster response to the specific grey level, especially at a high level. As the applied voltage is released, the τ_f is only associated with the ratio of γ₁/K₃₃ and with the anchoring energy of the cell [48]. Based on this point, in Fig. 4(b), the τ_f of the TA-9164 polymer cell is shorter than that of the pure E7 LC cell, indicating that the stronger anchoring effect causes the LC molecules to reorient instantly. In general, the τ_r response of the pure E7 LC cell can be improved by using the overdrive method, whereas the τ_f response is only related to the in-cell anchoring effect.

 

Fig. 4 Gray-level responses of (a) τ_r and (b) τ_f for the pure E7 LC cell and the TA-9164 polymer cell.

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Figure 5 shows that the pure E7 LC cell applied with overdrive (V_IPS = 15 V) can significantly enhance the τ_r response as compared with the other cells applied with normal drive (V_IPS = V_Tmax; V_{Tmax (E7)} = 10.5 V, V_{Tmax (TA-9164)} = 12 V). With the large electric potential energy (15 V), the anchoring-energy effect from the LC molecules and substrate surface can be smoothed away, making the LC molecular reorientation more rapid. However, this method causes phase retardation over the optimum phase retardation and leads to the lowest transmittance in the steady state. Moreover, a complex driving scheme for the E7 LC cell applied with overdrive is also needed. In contrast, for the TA-9164 polymer cell, the fast τ_r response can be achieved at the normal drive (12 V) applied, and the light transmittance can be sustained at good transmittance level (maximum bright state) in the steady state. In this figure, it can be found that the τ_r response of the TA-9164 polymer cell at the normal drive applied is comparable to that of the pure E7 LC cell at the overdrive applied (τ_{r (E7) | 15 V} = 3.4 ms, τ_{r (TA-9164) | 12 V} = 3.6 ms). Since the τ_f response is independent on the driving voltage, τ_f = d²γ₁/π²K₃₃, the τ_f response of the pure E7 LC cell addressed by the overdrive is equal to that of the pure E7 LC cell addressed by the normal drive (τ_{f (E7) | 15 V} = τ_{f (E7) | 10.5 V} = 10.7 ms), which is slower than that of the TA-9164 polymer cell driven by the normal drive (τ_{f (TA-9164) | 12 V} = 9.0 ms). Thus as the driving voltage, transmittance, and response ( = τ_r + τ_f) are considered simultaneously, the TA-9164 polymer cell is more suitable for display devices, as compared to the pure E7 LC cell applied with normal drive and overdrive.

 

Fig. 5 τ_r response behavior for the pure E7 LC cell applied with normal drive (V_Tmax = 10.5 V), the pure E7 LC cell applied with overdrive (15 V), and the TA-9164 polymer cell applied with normal drive (V_Tmax = 12 V).

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4. Conclusion

Enhanced electro-optical properties of VA-IPS cells are shown by incorporating the TA-9164 monomer material with nematic E7 LC. With polymer networks, the V_th can be changed and the configuration of the LC molecules can be effectively governed. Due to the polymer anchoring effect, the polymer cells have phase retardation close to the optimum value, resulting in higher transmittance under high voltage operation, as compared with the pure E7 LC cell. Of all these VA-IPS cells, the TA-9164 polymer cell has the strongest polymer anchoring energy, which is observed from the T-V curve, and has the fastest τ_f response. For the τ_r response, the τ_r of each polymer cell is shorter than that of the pure E7 LC cell. This is related to the fact that both the degree of the LC molecular reorientation and the strength of the IPS electric field need taking into account simultaneously during operation. In addition, the gray-level τ_r and τ_f response of the TA-9164 polymer cell are both superior to that of the pure E7 LC cell. Although the overdrive used to the pure E7 LC cell can significantly improve the τ_r response, the lower transmittance and sophisticated driving circuit are encountered. In contrast, the τ_r response of the TA-9164 polymer cell with the normal drive is comparable to that of the pure E7 LC cell with the overdrive method. This study demonstrates that the appropriate polymer applied to the pure VA-IPS LC cells can benefit the dynamic display to have the superior electro-optical properties and fast response.