November 1, 2014 / Vol. 39, No. 21 / OPTICS LETTERS
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Performance enhancement using a non-uniform vertical electric field and polymer networks for in-plane switching of multi-pretilt, vertically aligned liquid crystal devices G. J. Lin,1 T. J. Chen,2,* Y. W. Tsai,2 Y. T. Lin,2 J. J. Wu,2 and Y. J. Yang1 1
2
Graduate Institute of Electronics Engineering, National Taiwan University, Taipei 10617, Taiwan Department of Electro-Optical Engineering, National Taipei University of Technology, Taipei 10608, Taiwan *Corresponding author: [email protected] Received September 18, 2014; accepted September 30, 2014; posted October 6, 2014 (Doc. ID 223076); published October 23, 2014
A simple and reproducible alignment method for fabricating vertically aligned (VA) liquid crystal (LC) cells with a multi-pretilt structure is developed. A non-uniform vertical electric field is employed in the LC/monomer mixed cells during the photocuring process, and two pretilt domains with a functional small pretilt angle (∼1.6°) in the stabilized VA LC/polymer cells are achieved. The enhanced electro-optical performance of the cell driven by an in-plane switching field is demonstrated. Compared to the pure cell, the 2 wt.% pretilt angle cell shows 36%, 64%, and 76% improvement in the optical switch, the gray-level rise time, and the gray-level fall time responses, respectively, which are obtained at a low driving voltage (≤12 V). When applied to LC devices, the proposed method not only effectively benefits the LC molecular alignment, but it also significantly boosts the electro-optical performance. © 2014 Optical Society of America OCIS codes: (160.3710) Liquid crystals; (230.3720) Liquid-crystal devices. http://dx.doi.org/10.1364/OL.39.006225
Of all the advanced display modes, the in-plane switching (IPS) mode used in display devices has attracted much industrial and academic attention because of its wide viewing angle and good image response [1]. To align the liquid crystal (LC) molecules effectively, a mechanical rubbing process is needed for conventional IPS cells [2]. Although this treatment is suitable for mass production, contamination from particles and electrostatic charges exist in the alignment layer. The photoalignment method is an alternative way to achieve LC molecular alignment while avoiding the above problems [3], but it requires linearly polarized light during the alignment process. Also, it has low anchoring energy, which could degrade the device response. Further, for an IPS cell, light leakage in the dark state and a poor contrast ratio (CR) have also been revealed [4]. In contrast, a vertically aligned (VA) LC display (LCD) with the rubbing-free process exhibits a high CR because the LC molecules are aligned vertically on the substrate surface. However, this kind of cell with a negative dielectric anisotropic LC (negative LC) usually has a long response time [5], especially for the fall time (tf ). This is considered to be the major drawback for dynamic image applications. The tf is only related to the cell gap thickness and the material parameters of negative LCs, such as the rotational viscosity and the bend elastic constant, which cannot be improved by overdriving [6]. Despite the fact that the rise time (tr ) can be reduced by the overdrive method [7], additional driving schemes are needed. To eliminate the disadvantages of the IPS and VA display mode, an IPS-VA display mode using positive LC material has been demonstrated [8], but poor device properties such as the high threshold voltage (Vth ) and low transmittance still have to be improved. In 2011, a high-transmittance IPS-VA LC device structure was proposed, and its electro-optical properties were simulated [9,10]. 0146-9592/14/216225-04$15.00/0
The pretilt angle (PA) of the LC is a critical factor that affects the electro-optical properties of LCDs. To control the LC molecular orientation and improve the display performance, various methods for making a PA with respect to the substrate surface have been proposed [11–16]. However, these methods are not favorable for multi-pretilt structure fabrication because of complex process and stability issues. Recently, polymer networks have been considered as a potential candidate for adapting the LC molecular configuration owing to their low cost, rapid prototyping, and effective distribution. Polymer networks applied to the device to produce the surface PA of LCs can obviously improve the tr in VA display modes [17–22], whereas the PA could degrade the tf response and sacrifice the high CR [23]. In 2013, Lee et al. [24] used an in-plane field and lower polymer concentration (∼0.1 wt:%) in the monomer curing process to have the LC molecules near the substrate surface tilt. This approach can make a small PA and avoid light leakage in the dark state, but the tf of this kind of device cannot be effectively reduced, and it is in fact even longer than that of the unprocessed device. Although higher monomer concentrations applied to the VA LC devices for forming three-dimensional polymer networks can reduce the tf because of the bulk polymer anchoring effect, a very high driving voltage (∼80 V) is necessary for performing a short tr [25]. In this Letter, a simple alignment method for producing a multi-pretilt VA LC cell is demonstrated. This method applies a non-uniform vertical electric field (Env ) and a higher monomer concentration (≥1 wt:%) to a “positive” LC VA cell during the photocuring process. A functional small PA near the electrode edge is formed on the interdigital-electrode substrate surface. The fabricated device driven by an IPS field exhibits excellent optical-switch and gray-level responses at the low driving voltage © 2014 Optical Society of America
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(≤12 V). As compared to the pure (0 wt.%) cell, the 2 wt.% polymer cell with 2° pretilt (called the 2 wt.% PA cell) shows about 36%, 64%, and 76% improvement in the optical switch, gray-level tr , and gray-level tf responses, respectively. In contrast to the conventional device fabrication process with low monomer concentration, the application of the proposed method to devices effectively governs the LC molecular orientation and greatly enhances the electro-optical performance. In the experiment, the proposed VA LC cells with a small PA are demonstrated by employing the Env and polymers. Indium tin oxide (ITO) material is selected to make the interdigital electrodes, which are processed on the bottom-glass substrate. The period of interdigital electrodes is 12 μm: 4 μm width (w) and 8 μm separation (s). Besides, the ITO thin film is also deposited on the topglass substrate for the following aligned treatment of LC molecules. Both substrates are coated with VA polyimide (AL60101L) and heated at 200°C for 1 h. Then, 4 μm spacers are used to control the cell gap (d). Positive nematic E7 LC (with Δn 0.218 and Δε 14.5 from Merck) and TA-9164 monomer (with bifunctional acrylate and anisotropy from Tatung University) are mixed at a specific concentration and injected into the prepared cell by capillary action. During the polymerization process, Env and ultraviolet (UV) light (365 nm, 16 mW∕cm2 ) are applied to these cells for 30 min. Then, the stable multi-pretilt LC molecular structure and a small PA (θ) with respect to the normal direction of the substrate surface are achieved. The PA value is estimated through the relationship between the transmittance and the phase retardation [26]. The curing process and the cell structure are illustrated in Figs. 1(a)–1(c). The inserted images of polymer morphology show that TA-9164 polymers are anchored on the top- and bottom-substrate surfaces. After that, the IPS voltage (VIPS ) is applied to this fabricated cell, which makes the LC molecular orientation parallel to the IPS field direction, as illustrated in Fig. 1(d). To observe this PA effect on the electro-optical properties of the polymer cell (called the wt.% PA cell) clearly, another type of polymer cell without the PA (called the wt.% VA cell) is also prepared. In the device measurements, these types of IPS-VA cells are sandwiched between the crossed polarizers and are orientated to 45° with respect to the polarizer. Normalized transmittance–voltage (T–V) curves, optical-switch responses, and gray-level responses are performed by applying the diode laser with a 650 nm wavelength and ac voltage with 1 kHz square waveform to the cells. To make an appropriate PA in the IPS-VA LC/polymer cells, the Env is first applied to the pure E7 LC cell. The dependence of light transmittance on the curing voltage (VCU ) is shown in Fig. 2(a), and the cell texture and brightness are recorded by polarized optical microscopy (POM). The POM images for the cell at different curing voltages are inserted in Fig. 2(a). As the VCU is less than 1.00 V, the LC molecules are almost perpendicular to the substrate surface, which leads to extremely small light transmittance and the dark-state image. However, when the VCU is gradually increased, the light transmittance of the cell will be obvious because of the fact that LC molecules start tilting to the substrate surface. At the applied VCU of 2.00 V, the image state of the cell is no longer dark
Fig. 1. Schematic illustrations of the fabrication process of PA and LC molecular orientation for an IPS-VA cell. (a) Layer structure of an IPS-VA LC cell. (b) Application of the VCU and UV light to the cell during the curing process; the Env is shown by dashed lines. (c) Multi-pretilt LC molecular configuration with the small PA duplicated and polymers assembling on the top- and bottom-substrate surfaces, as shown in the inset images recorded by scanning electron microscopy. (d) The LC molecules aligned by the IPS electric field (EIPS ) as the VIPS is applied.
and the light transmittance is about 0.05. In this case, the functional small PA is revealed, which is approximately 1.6°. Although the transmittance of the cell is remarkable as the VCU is more than 3.00 V, the relatively large PA will cause light leakage and further reduce the CR. Based on these results, a VCU of 2.00 V is selected and employed in the cell during the process of polymerization. Once the stabilized polymer networks are formed, the stable IPS-VA LC molecular structure with a small PA can be achieved. From the POM image, the bright state along the IPS-electrode edge is revealed, and it can be attributed to this small PA effect. In Figs. 2(b)–2(d), it is found
Fig. 2. (a) Transmittance–voltage curve for the pure E7 LC cell in the presence of Env . The inserted POM images show the variation of light brightness in the cell with the VCU . (b), (c), and (d) the POM images after the curing process for pure E7 LC cells with different polymer concentrations of 0, 1, and 2 wt.%, respectively. The yellow dash lines added in (a), (c), and (d) indicate the center of the IPS electrode (w) and the bright state along the w edge is revealed.
November 1, 2014 / Vol. 39, No. 21 / OPTICS LETTERS
that as the polymer concentration increases, the color of the initial state of an IPS-VA LC/polymer cell becomes a little bluish, which indicates that the small and stable PA has been constructed. This is different from the 0 wt.% (pure E7 LC) cell and will alter the electro-optical performance. Figure 3 shows the normalized T–V curves for IPS-VA cells during different processing conditions. The normalized voltage (Norm. V.) labeled on the horizontal axis is defined as the VIPS ∕Vth0 , where Vth0 is the threshold voltage of the 0 wt.% (E7 LC) cell and is equal to 3.50 V. The normalized light transmittance (Norm. T.) treated by each maximum transmittance (Tmax ) is shown on the vertical axis. The value of Vth , which is associated with the polymer concentration and the LC molecular configuration, is determined by the maximum tangent line’s slope. Of all the IPS-VA cells, the 2 wt.% VA cell has the highest Vth (1.12 Vth0 ), which indicates that the cell has a stronger anchoring energy (W) effect (Vth ∝ W1∕2 ), and additional electric potential energy is required to change the LC molecular configuration of the cell [27,28]. However, with the PA effect, the Vth can be significantly reduced to 1.02 Vth0 for the 1 wt.% PA cell and to 1.06 Vth0 for the 2 wt.% PA cell, which are close to that of the 0 wt.% cell. Such behavior is related to two competing mechanisms: (1) the PA effect, which pushes the T–V curve toward the left [29], and (2) the anchoring effect, which pulls it toward the right [28]. Because the existing PA is not very large and the monomer used has a stronger cross-linking ability [30], the T–V curve of the cell will shift slightly to the right. This suggests that the anchoring effect is stronger than the PA effect, which is expected to improve the cell tf response. Once the applied VIPS is more than the Vth and equal to the Tmax driving voltage (VTmax ), one can find that the VTmax of the 2 wt.% PA cell is almost the same as that of the 1 wt.% PA cell. This is because the more stable multi-pretilt structure in the 2 wt.% PA cell causes the LC molecules to be effectively aligned, and the anchoring effect on the LC molecular reorientation will be weaker as the VIPS is more than Vth . When the VIPS is over VTmax , the LC molecular configuration of the 0 wt.% cell will have a more parallel alignment, which leads to a value of phase retardation beyond the optimum value (0.5π) and lower light transmittance as compared with the polymer cells. On the other hand, the LC molecular reorientation for polymer cells will be relatively restricted by the polymer anchoring effect, and the light transmittance will be decreased moderately, especially for the 2 wt.% cells. Thus, the cell with polymer networks can perform a more stable configuration of LC
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molecules at high driving voltage (>6 Vth0 ). Some of the electro-optical properties of these IPS-VA cells are summarized in Table 1. Figure 4 shows the normalized optical-switch responses for different types of IPS-VA cells, which are switched between the maximum bright state and the dark state. In Fig. 4(a), the tf response of the 1 wt.% PA cell is close to that of the 0 wt.% (E7 LC) cell, which suggests that the cell with low polymer concentration cannot obviously improve the tf response. This is due to the lack of a stronger anchoring effect on the driven LC molecules (tf ∝ 1∕W) [31]. However, as the polymer concentration is increased to 2 wt.%, the faster tf response can be carried out no matter whether the cell has a small PA or not. Such a higher polymer concentration mixed in the LC cell can make the driven LC molecules reorient rapidly to the original configuration. In Fig. 4(b), when the operating voltage is changed from 0 V to the VTmax , the tr response of the 2 wt.% PA cell is faster than that of the 2 wt.% VA cell, which is attributed to the small PA effect that makes the LC molecular reaction faster [28]. Despite the fact that the 2 wt.% VA cell has a higher VTmax (3.65 Vth0 ) than the 1 wt.% PA cell, the tr response is slightly slower because of the stronger anchoring effect and the LC configuration of the cell without a pre-director. According to the above results, the 2 wt.% PA cell can significantly improve the tf and enhance the tr , and thus it is the best candidate for dynamic responses. The IPS-VA opticalswitch responses, including tr , tf , and the total response time (tt tr tf ), are listed in Table 1. A 36% enhancement in the tt on the 2 wt.% PA cell is achieved. The cell responses can be further enhanced if the mixed concentration is more than 2 wt.%; however, this will cause serious light scattering and additional potential energy will be needed to drive the LC molecules [32]. The gray-level tr responses, which are equally divided into eight levels by each cell Tmax , are shown in Figs. 5(a) and 5(b); these figures correspond to the 0 wt.% cell and the 2 wt.% PA cell, respectively. Compared to the 0 wt.% cell, the 2 wt.% PA cell performs a faster tr response to adjacent levels owing to the small PA effect and the slightly higher driving voltage, which leads to the rapid reorientation of the LC molecules. Figures 5(c) and 5(d) individually present the gray-level tf responses for the 0 and 2 wt.% PA cells, respectively, as the gray-level voltage is turned off. The 2 wt.% PA cell exhibits the fastest tf responses for all gray levels because of the stronger anchoring effect on the LC molecular reorientation. Through this fabrication method, around 64% and 76% improvements in the gray-level tr and tf responses on Table 1. Electro-Optical Properties for Different Cell Conditions
Fig. 3. Normalized T–V curves for the pure E7 LC with different polymer concentrations.
Alignment Vth V V Tmax V tr ms tf ms tt ms a
Vth0 3.50 V.
0 wt.%
1 wt.%
VA 1.00 Vth0 a 3.04 Vth0 7.41 10.72 18.13
w/PA 1.02 Vth0 3.38 Vth0 3.53 10.38 13.41
2 wt.% w/PA 1.06 Vth0 3.39 Vth0 2.64 8.96 11.60
VA 1.12 Vth0 3.65 Vth0 3.62 8.91 12.53
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Fig. 4. (a) tr and (b) tf responses for the pure E7 LC with different polymer concentrations.
Fig. 5. Gray-level tr and tf responses for the (a), (c) 0 wt. % cell and (b), (d) 2 wt. % PA cell, respectively.
the 2 wt.% PA cell are demonstrated, respectively, which can be utilized to effectively improve the dynamic image quality. In summary, the proposed simple fabrication method for VA LC molecular alignment with a functional PA structure is implemented, and the enhanced electro-optical properties of the cell are demonstrated. The stable and multi-pretilt-angle configuration of the LC molecules is successfully achieved by employing the TA-9164 monomer and the Env in the pure E7 LC cell (0 wt.% cell). Compared to the 0 wt.% cell, the LC cell with the lower polymer concentration of 1 wt.% performs a shorter tr , but it cannot obviously reduce the tf because of the lack of a sufficient anchoring effect on the driven LC molecules. As the mixed polymer concentration is increased to 2 wt.%, the cell has a higher transmittance at the high operating voltage, and this significantly improves the tf response. This kind of cell with a stronger polymer anchoring effect can effectively govern the LC molecular reorientation and configuration. With a small PA effect, the tr response of the cell can be further enhanced at a low switching voltage (
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Performance enhancement using a non-uniform vertical electric field and polymer networks for in-plane switching of multi-pretilt, vertically aligned liquid crystal devices G. J. Lin,1 T. J. Chen,2,* Y. W. Tsai,2 Y. T. Lin,2 J. J. Wu,2 and Y. J. Yang1 1
2
Graduate Institute of Electronics Engineering, National Taiwan University, Taipei 10617, Taiwan Department of Electro-Optical Engineering, National Taipei University of Technology, Taipei 10608, Taiwan *Corresponding author: [email protected] Received September 18, 2014; accepted September 30, 2014; posted October 6, 2014 (Doc. ID 223076); published October 23, 2014
A simple and reproducible alignment method for fabricating vertically aligned (VA) liquid crystal (LC) cells with a multi-pretilt structure is developed. A non-uniform vertical electric field is employed in the LC/monomer mixed cells during the photocuring process, and two pretilt domains with a functional small pretilt angle (∼1.6°) in the stabilized VA LC/polymer cells are achieved. The enhanced electro-optical performance of the cell driven by an in-plane switching field is demonstrated. Compared to the pure cell, the 2 wt.% pretilt angle cell shows 36%, 64%, and 76% improvement in the optical switch, the gray-level rise time, and the gray-level fall time responses, respectively, which are obtained at a low driving voltage (≤12 V). When applied to LC devices, the proposed method not only effectively benefits the LC molecular alignment, but it also significantly boosts the electro-optical performance. © 2014 Optical Society of America OCIS codes: (160.3710) Liquid crystals; (230.3720) Liquid-crystal devices. http://dx.doi.org/10.1364/OL.39.006225
Of all the advanced display modes, the in-plane switching (IPS) mode used in display devices has attracted much industrial and academic attention because of its wide viewing angle and good image response [1]. To align the liquid crystal (LC) molecules effectively, a mechanical rubbing process is needed for conventional IPS cells [2]. Although this treatment is suitable for mass production, contamination from particles and electrostatic charges exist in the alignment layer. The photoalignment method is an alternative way to achieve LC molecular alignment while avoiding the above problems [3], but it requires linearly polarized light during the alignment process. Also, it has low anchoring energy, which could degrade the device response. Further, for an IPS cell, light leakage in the dark state and a poor contrast ratio (CR) have also been revealed [4]. In contrast, a vertically aligned (VA) LC display (LCD) with the rubbing-free process exhibits a high CR because the LC molecules are aligned vertically on the substrate surface. However, this kind of cell with a negative dielectric anisotropic LC (negative LC) usually has a long response time [5], especially for the fall time (tf ). This is considered to be the major drawback for dynamic image applications. The tf is only related to the cell gap thickness and the material parameters of negative LCs, such as the rotational viscosity and the bend elastic constant, which cannot be improved by overdriving [6]. Despite the fact that the rise time (tr ) can be reduced by the overdrive method [7], additional driving schemes are needed. To eliminate the disadvantages of the IPS and VA display mode, an IPS-VA display mode using positive LC material has been demonstrated [8], but poor device properties such as the high threshold voltage (Vth ) and low transmittance still have to be improved. In 2011, a high-transmittance IPS-VA LC device structure was proposed, and its electro-optical properties were simulated [9,10]. 0146-9592/14/216225-04$15.00/0
The pretilt angle (PA) of the LC is a critical factor that affects the electro-optical properties of LCDs. To control the LC molecular orientation and improve the display performance, various methods for making a PA with respect to the substrate surface have been proposed [11–16]. However, these methods are not favorable for multi-pretilt structure fabrication because of complex process and stability issues. Recently, polymer networks have been considered as a potential candidate for adapting the LC molecular configuration owing to their low cost, rapid prototyping, and effective distribution. Polymer networks applied to the device to produce the surface PA of LCs can obviously improve the tr in VA display modes [17–22], whereas the PA could degrade the tf response and sacrifice the high CR [23]. In 2013, Lee et al. [24] used an in-plane field and lower polymer concentration (∼0.1 wt:%) in the monomer curing process to have the LC molecules near the substrate surface tilt. This approach can make a small PA and avoid light leakage in the dark state, but the tf of this kind of device cannot be effectively reduced, and it is in fact even longer than that of the unprocessed device. Although higher monomer concentrations applied to the VA LC devices for forming three-dimensional polymer networks can reduce the tf because of the bulk polymer anchoring effect, a very high driving voltage (∼80 V) is necessary for performing a short tr [25]. In this Letter, a simple alignment method for producing a multi-pretilt VA LC cell is demonstrated. This method applies a non-uniform vertical electric field (Env ) and a higher monomer concentration (≥1 wt:%) to a “positive” LC VA cell during the photocuring process. A functional small PA near the electrode edge is formed on the interdigital-electrode substrate surface. The fabricated device driven by an IPS field exhibits excellent optical-switch and gray-level responses at the low driving voltage © 2014 Optical Society of America
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(≤12 V). As compared to the pure (0 wt.%) cell, the 2 wt.% polymer cell with 2° pretilt (called the 2 wt.% PA cell) shows about 36%, 64%, and 76% improvement in the optical switch, gray-level tr , and gray-level tf responses, respectively. In contrast to the conventional device fabrication process with low monomer concentration, the application of the proposed method to devices effectively governs the LC molecular orientation and greatly enhances the electro-optical performance. In the experiment, the proposed VA LC cells with a small PA are demonstrated by employing the Env and polymers. Indium tin oxide (ITO) material is selected to make the interdigital electrodes, which are processed on the bottom-glass substrate. The period of interdigital electrodes is 12 μm: 4 μm width (w) and 8 μm separation (s). Besides, the ITO thin film is also deposited on the topglass substrate for the following aligned treatment of LC molecules. Both substrates are coated with VA polyimide (AL60101L) and heated at 200°C for 1 h. Then, 4 μm spacers are used to control the cell gap (d). Positive nematic E7 LC (with Δn 0.218 and Δε 14.5 from Merck) and TA-9164 monomer (with bifunctional acrylate and anisotropy from Tatung University) are mixed at a specific concentration and injected into the prepared cell by capillary action. During the polymerization process, Env and ultraviolet (UV) light (365 nm, 16 mW∕cm2 ) are applied to these cells for 30 min. Then, the stable multi-pretilt LC molecular structure and a small PA (θ) with respect to the normal direction of the substrate surface are achieved. The PA value is estimated through the relationship between the transmittance and the phase retardation [26]. The curing process and the cell structure are illustrated in Figs. 1(a)–1(c). The inserted images of polymer morphology show that TA-9164 polymers are anchored on the top- and bottom-substrate surfaces. After that, the IPS voltage (VIPS ) is applied to this fabricated cell, which makes the LC molecular orientation parallel to the IPS field direction, as illustrated in Fig. 1(d). To observe this PA effect on the electro-optical properties of the polymer cell (called the wt.% PA cell) clearly, another type of polymer cell without the PA (called the wt.% VA cell) is also prepared. In the device measurements, these types of IPS-VA cells are sandwiched between the crossed polarizers and are orientated to 45° with respect to the polarizer. Normalized transmittance–voltage (T–V) curves, optical-switch responses, and gray-level responses are performed by applying the diode laser with a 650 nm wavelength and ac voltage with 1 kHz square waveform to the cells. To make an appropriate PA in the IPS-VA LC/polymer cells, the Env is first applied to the pure E7 LC cell. The dependence of light transmittance on the curing voltage (VCU ) is shown in Fig. 2(a), and the cell texture and brightness are recorded by polarized optical microscopy (POM). The POM images for the cell at different curing voltages are inserted in Fig. 2(a). As the VCU is less than 1.00 V, the LC molecules are almost perpendicular to the substrate surface, which leads to extremely small light transmittance and the dark-state image. However, when the VCU is gradually increased, the light transmittance of the cell will be obvious because of the fact that LC molecules start tilting to the substrate surface. At the applied VCU of 2.00 V, the image state of the cell is no longer dark
Fig. 1. Schematic illustrations of the fabrication process of PA and LC molecular orientation for an IPS-VA cell. (a) Layer structure of an IPS-VA LC cell. (b) Application of the VCU and UV light to the cell during the curing process; the Env is shown by dashed lines. (c) Multi-pretilt LC molecular configuration with the small PA duplicated and polymers assembling on the top- and bottom-substrate surfaces, as shown in the inset images recorded by scanning electron microscopy. (d) The LC molecules aligned by the IPS electric field (EIPS ) as the VIPS is applied.
and the light transmittance is about 0.05. In this case, the functional small PA is revealed, which is approximately 1.6°. Although the transmittance of the cell is remarkable as the VCU is more than 3.00 V, the relatively large PA will cause light leakage and further reduce the CR. Based on these results, a VCU of 2.00 V is selected and employed in the cell during the process of polymerization. Once the stabilized polymer networks are formed, the stable IPS-VA LC molecular structure with a small PA can be achieved. From the POM image, the bright state along the IPS-electrode edge is revealed, and it can be attributed to this small PA effect. In Figs. 2(b)–2(d), it is found
Fig. 2. (a) Transmittance–voltage curve for the pure E7 LC cell in the presence of Env . The inserted POM images show the variation of light brightness in the cell with the VCU . (b), (c), and (d) the POM images after the curing process for pure E7 LC cells with different polymer concentrations of 0, 1, and 2 wt.%, respectively. The yellow dash lines added in (a), (c), and (d) indicate the center of the IPS electrode (w) and the bright state along the w edge is revealed.
November 1, 2014 / Vol. 39, No. 21 / OPTICS LETTERS
that as the polymer concentration increases, the color of the initial state of an IPS-VA LC/polymer cell becomes a little bluish, which indicates that the small and stable PA has been constructed. This is different from the 0 wt.% (pure E7 LC) cell and will alter the electro-optical performance. Figure 3 shows the normalized T–V curves for IPS-VA cells during different processing conditions. The normalized voltage (Norm. V.) labeled on the horizontal axis is defined as the VIPS ∕Vth0 , where Vth0 is the threshold voltage of the 0 wt.% (E7 LC) cell and is equal to 3.50 V. The normalized light transmittance (Norm. T.) treated by each maximum transmittance (Tmax ) is shown on the vertical axis. The value of Vth , which is associated with the polymer concentration and the LC molecular configuration, is determined by the maximum tangent line’s slope. Of all the IPS-VA cells, the 2 wt.% VA cell has the highest Vth (1.12 Vth0 ), which indicates that the cell has a stronger anchoring energy (W) effect (Vth ∝ W1∕2 ), and additional electric potential energy is required to change the LC molecular configuration of the cell [27,28]. However, with the PA effect, the Vth can be significantly reduced to 1.02 Vth0 for the 1 wt.% PA cell and to 1.06 Vth0 for the 2 wt.% PA cell, which are close to that of the 0 wt.% cell. Such behavior is related to two competing mechanisms: (1) the PA effect, which pushes the T–V curve toward the left [29], and (2) the anchoring effect, which pulls it toward the right [28]. Because the existing PA is not very large and the monomer used has a stronger cross-linking ability [30], the T–V curve of the cell will shift slightly to the right. This suggests that the anchoring effect is stronger than the PA effect, which is expected to improve the cell tf response. Once the applied VIPS is more than the Vth and equal to the Tmax driving voltage (VTmax ), one can find that the VTmax of the 2 wt.% PA cell is almost the same as that of the 1 wt.% PA cell. This is because the more stable multi-pretilt structure in the 2 wt.% PA cell causes the LC molecules to be effectively aligned, and the anchoring effect on the LC molecular reorientation will be weaker as the VIPS is more than Vth . When the VIPS is over VTmax , the LC molecular configuration of the 0 wt.% cell will have a more parallel alignment, which leads to a value of phase retardation beyond the optimum value (0.5π) and lower light transmittance as compared with the polymer cells. On the other hand, the LC molecular reorientation for polymer cells will be relatively restricted by the polymer anchoring effect, and the light transmittance will be decreased moderately, especially for the 2 wt.% cells. Thus, the cell with polymer networks can perform a more stable configuration of LC
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molecules at high driving voltage (>6 Vth0 ). Some of the electro-optical properties of these IPS-VA cells are summarized in Table 1. Figure 4 shows the normalized optical-switch responses for different types of IPS-VA cells, which are switched between the maximum bright state and the dark state. In Fig. 4(a), the tf response of the 1 wt.% PA cell is close to that of the 0 wt.% (E7 LC) cell, which suggests that the cell with low polymer concentration cannot obviously improve the tf response. This is due to the lack of a stronger anchoring effect on the driven LC molecules (tf ∝ 1∕W) [31]. However, as the polymer concentration is increased to 2 wt.%, the faster tf response can be carried out no matter whether the cell has a small PA or not. Such a higher polymer concentration mixed in the LC cell can make the driven LC molecules reorient rapidly to the original configuration. In Fig. 4(b), when the operating voltage is changed from 0 V to the VTmax , the tr response of the 2 wt.% PA cell is faster than that of the 2 wt.% VA cell, which is attributed to the small PA effect that makes the LC molecular reaction faster [28]. Despite the fact that the 2 wt.% VA cell has a higher VTmax (3.65 Vth0 ) than the 1 wt.% PA cell, the tr response is slightly slower because of the stronger anchoring effect and the LC configuration of the cell without a pre-director. According to the above results, the 2 wt.% PA cell can significantly improve the tf and enhance the tr , and thus it is the best candidate for dynamic responses. The IPS-VA opticalswitch responses, including tr , tf , and the total response time (tt tr tf ), are listed in Table 1. A 36% enhancement in the tt on the 2 wt.% PA cell is achieved. The cell responses can be further enhanced if the mixed concentration is more than 2 wt.%; however, this will cause serious light scattering and additional potential energy will be needed to drive the LC molecules [32]. The gray-level tr responses, which are equally divided into eight levels by each cell Tmax , are shown in Figs. 5(a) and 5(b); these figures correspond to the 0 wt.% cell and the 2 wt.% PA cell, respectively. Compared to the 0 wt.% cell, the 2 wt.% PA cell performs a faster tr response to adjacent levels owing to the small PA effect and the slightly higher driving voltage, which leads to the rapid reorientation of the LC molecules. Figures 5(c) and 5(d) individually present the gray-level tf responses for the 0 and 2 wt.% PA cells, respectively, as the gray-level voltage is turned off. The 2 wt.% PA cell exhibits the fastest tf responses for all gray levels because of the stronger anchoring effect on the LC molecular reorientation. Through this fabrication method, around 64% and 76% improvements in the gray-level tr and tf responses on Table 1. Electro-Optical Properties for Different Cell Conditions
Fig. 3. Normalized T–V curves for the pure E7 LC with different polymer concentrations.
Alignment Vth V V Tmax V tr ms tf ms tt ms a
Vth0 3.50 V.
0 wt.%
1 wt.%
VA 1.00 Vth0 a 3.04 Vth0 7.41 10.72 18.13
w/PA 1.02 Vth0 3.38 Vth0 3.53 10.38 13.41
2 wt.% w/PA 1.06 Vth0 3.39 Vth0 2.64 8.96 11.60
VA 1.12 Vth0 3.65 Vth0 3.62 8.91 12.53
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OPTICS LETTERS / Vol. 39, No. 21 / November 1, 2014
Fig. 4. (a) tr and (b) tf responses for the pure E7 LC with different polymer concentrations.
Fig. 5. Gray-level tr and tf responses for the (a), (c) 0 wt. % cell and (b), (d) 2 wt. % PA cell, respectively.
the 2 wt.% PA cell are demonstrated, respectively, which can be utilized to effectively improve the dynamic image quality. In summary, the proposed simple fabrication method for VA LC molecular alignment with a functional PA structure is implemented, and the enhanced electro-optical properties of the cell are demonstrated. The stable and multi-pretilt-angle configuration of the LC molecules is successfully achieved by employing the TA-9164 monomer and the Env in the pure E7 LC cell (0 wt.% cell). Compared to the 0 wt.% cell, the LC cell with the lower polymer concentration of 1 wt.% performs a shorter tr , but it cannot obviously reduce the tf because of the lack of a sufficient anchoring effect on the driven LC molecules. As the mixed polymer concentration is increased to 2 wt.%, the cell has a higher transmittance at the high operating voltage, and this significantly improves the tf response. This kind of cell with a stronger polymer anchoring effect can effectively govern the LC molecular reorientation and configuration. With a small PA effect, the tr response of the cell can be further enhanced at a low switching voltage (
