1. Introduction

In-plane switching (IPS) display mode has attracted much industrial and academic attention because of its wide viewing angle and good image response [1]. The mechanical rubbing process is usually employed in conventional IPS-based liquid crystal (LC) devices to effectively align the LC molecular configuration [2]. Although this treatment is suitable for mass production, the produced particles and the electrostatic charges are revealed in the alignment layer. In addition, the drawbacks of the IPS-based LC devices are the light leakage in the dark state and the poor contrast ratio (CR) [3]. In contrast, the vertically aligned (VA) LC devices do not need the rubbing process, and exhibit a high CR, which is because the LC molecules are vertically aligned on the substrate surface. However, the VA-based devices that use a negative dielectric anisotropy LC (negative LC) usually have a long response time [4], especially for the fall time (t_f). The t_f response, obtained when the driving voltage is released, is only related to the device thickness and the material parameters of negative LCs, such as the rotational viscosity and the bend elastic constant, which cannot be improved by overdriving [5]. In contrast, the rise time (t_r) response can be improved by the overdrive method [6], but additional driving schemes are needed. To eliminate the disadvantages of the IPS- and VA-based devices, VA-IPS devices using “positive” LC material have been demonstrated [7], but their higher threshold voltage (V_th) and lower output light transmittance need improvement.

To effectively govern the LC molecular configuration and improve the electro-optical performance of LC-based devices, several methods of producing the pretilt structure in the device have been proposed [8–13]. However, these methods have very complex process and stability issues, which are not favorable for multi-pretilt structure fabrication. Recently, polymer networks, due to their low cost and simple development, have been wieldy employed in the LC molecular alignment process. Applying the polymer networks in the VA display modes can obviously improve the t_r response [14–19], whereas the produced pretilt structure leads to a slow t_f response and low CR [20]. In 2013, Lee et al. used an in-plane field and lower monomer concentration (~0.1 wt.%) to give the LC molecules near the substrate surface a small pretilt angle [21], which avoids light leakage in the dark state, but degrades the t_f response due to the lack of a sufficient anchoring effect on the LC molecular reorientation. Although applying higher monomer concentration (three-dimensional polymer networks) to the VA-based LC devices can improve the t_f response owing to the stronger polymer anchoring effect on the LC molecular configuration, a very high driving voltage (~80 V) is required to ensure a fast t_r response [22].

To adjust and improve the electro-optical properties of VA-IPS LC devices, an effective method, doping the TA-9164 monomer into the pure E7 LC device at the mixed concentration of 2 wt.%, is demonstrated by our research group. The developed crosslinking polymer networks from TA-9164 monomer material are anchored on the substrate surface and observed by the scanning electron microscope (SEM). This kind of VA-IPS LC/polymer device not only produces faster t_r and t_f responses, but also significantly reduces the driving voltage (< 15 V) [23], as compared to the other research results (≥ 30 V) [21,22]. Although the increment of the mixed monomer concentration (≥ 4 wt.%) can further effectively govern the LC molecular dynamic alignment, obvious light scattering at the initial state and higher driving voltage (≥ 20 V) are revealed [24]. Thus, the appropriately mixed monomer concentration applied to the VA-IPS LC device is 2 wt.%. Moreover, during the polymerization process, applying the 2-wt.% TA-9164 polymer networks and a small curing voltage (V_CU = 2 V) creating the non-uniform vertical electric field to the VA-IPS E7 LC device cannot only achieve a stable and functional multi-pretilt structure, but can also further refine the electro-optical properties [25]. The produced pretilt angle (θ) is around 2° with respect to the normal direction of the substrate surface, which solves the problems of light leakage and the slow t_f response. In addition to the TA-9164 monomer material, two types of monomer materials, UCL002 and NOA65, are also used in the VA-IPS E7 LC devices. However, the electro-optical properties of these two kinds of devices (VA-IPS E7 LC/UCL002 and /NOA65 polymer devices) are not superior to those of the VA-IPS E7 LC/TA-9164 polymer device [26]. Thus, understanding the effects of LC molecular configuration and monomer concentration on the device properties is very important, and is significantly related to optical material science. Based on our previous research results, employing the TA-9164 monomer material in the VA-IPS E7 LC device can be significantly beneficial to its electro-optical performance.

In this paper, to develop the tunable electro-optical properties of U-shaped-alignment (USA) IPS LC devices, four kinds of curing voltages (V_CU = 2 V, 4 V, 6 V, and 8 V) and two kinds of mixed concentrations (Conc. = 1 wt.% and 2 wt.%) are selected and applied to the VA-IPS-based LC devices to provide the reader with the complete information and to benefit readers in the application and design fields. During the polymerization process, the V_CU and TA-164 monomer are simultaneously applied to the VA-IPS LC devices to the control of the LC molecular dynamic alignment of the VA-IPS-based LC devices with the lower monomer concentration (≤ 2 wt.%). The original VA LC molecular configuration through this process is changed to a stable U-shaped configuration. Different curing voltages make different USA LC molecular configurations, which are observed from the polarized optical microscope (POM) and conoscope. The electro-optical properties of the proposed USA-IPS E7 LC/TA-9164 polymer devices, V_th, V_Tmax, output light transmittance, optical switch, and gray-level responses, not only depend on the V_CU, but also on the mixed monomer concentration. This indicates that the tunable electro-optical properties of USA-IPS LC devices can be successfully achieved by appropriately adjusting the V_CU and mixed monomer concentration. Of all kinds of devices, the fastest and slowest responses are revealed in the 2-wt.% USA-IPS device with the V_CU of 2 V and in the 1-wt.% USA-IPS device with the V_CU of 8 V, respectively, which results from the small pretilt and stronger polymer anchoring effects on the LC molecular reorientation. The proposed fabrication method will be beneficial to the optimized performance design of optical components and systems, due to the fact that the effects of LC/polymer mixture are employed in many kinds of optical components to effectively change their electro-optical properties.

2. Device fabrication and measurement

In the experiment, the USA-IPS LC devices with the tunable electro-optical properties are fabricated by applying the curing voltage and surface-anchored crosslinking monomer to the VA-IPS LC devices during the polymerization process. This process changes the LC molecular alignment from VA to USA, and makes a pretilt configuration at one of the substrates. The detailed device structure and fabrication are shown in Fig. 1 and are described below. Indium tin oxide (ITO) thin film is deposited on the top- and bottom-glass substrates. The interdigital electrodes, which have 4-μm width (w) and 8-μm space (s), are created on the bottom-glass substrate surface. Both prepared substrates are coated with the AL60101L VA polyimide (V-PI), and then heated at 200 °C for 1 hr. 4-μm spacers are employed in the devices to control the device thickness. The TA-9164 monomer material from Tatung University is selected and mixed with the nematic E7 LC (Δn = 0.218 and Δε = + 14.5) from Merck at a specific concentration. The mixed concentration is defined as the ratio of the monomer weight to the sum of the monomer and E7 LC weight. Based on the previous study [23–26], two types of mixed concentrations, 1 wt.% and 2 wt.%, were chosen in this research. The mixture is then injected into the prepared devices by capillary action. During the polymerization process, 365-nm ultraviolet (UV) light with 16 mW/cm² and the specific value of V_CU inducing the USA electric field are applied to the prepared VA-IPS E7 LC/TA-9164 monomer devices for 30 min. After the polymerization process, the V_CU is removed and the surface-anchoring crosslinking TA-9164 polymers recorded by SEM and inserted in step 4 of Fig. 1 are formed. The produced pretilt angle and the USA LC molecular configuration are stable without a holding voltage. Different curing voltages create different USA LC molecular configurations, which leads to the USA-IPS LC/polymer device with the tunable electro-optical properties. Once the USA LC molecular structure is achieved, the IPS voltage (V_IPS) is applied to the USA-IPS LC/polymer devices, making the LC molecules reorient and parallel to the direction of the IPS electric fields. The USA-IPS LC device with 1-wt% TA-9164 polymers and the USA-IPS LC device with 2-wt% TA-9164 polymers are respectively named as the 1-wt.% USA-IPS device and the 2-wt.% USA-IPS device in the following content. In the electro-optical measurement, these different kinds of USA-IPS LC/polymer devices are sandwiched between the crossed polarizers, and are oriented to 45° with respect to the polarizer. V_IPS-dependent light transmittance curves, optical-switch response (switching between the maximum brightness state and the dark state), and specific gray-level responses (driving between the zeroth state and the specific state) are performed by employing a diode laser with a 650-nm wavelength and ac voltage with 1 kHz square waveform in these devices.

 

Fig. 1 Schematic illustrations of the fabrication process of USA (pretilt) configuration and LC molecular orientation for a USA-IPS device. Step 1 shows that the V-PI is coated on the ITO and IPS substrate surfaces. The processed substrates are baked at 200 °C for 1 hr as shown in Step 2. Step 3 shows the UV curing process by applying the TA-9164 monomer, V_CU and UV light to the pure VA-IPS LC device. The induced U-shaped curing electric field is shown with dashed lines. Step 4 shows the LC molecular reorientation when the IPS electric field induced by V_IPS is applied to the fabricated USA-IPS LC/polymer device. The inserted SEM image shows the crosslinking morphology of TA-9164 polymers on the substrate surface.

Download Full Size | PDF

3. Results and discussions

To produce the USA-IPS devices with the tunable electro-optical properties, firstly, the pure VA-IPS LC device without the TA-9164 polymers is selected to investigate the effects of V_CU on the LC molecular configuration. The V_CU-dependent transmittance, brightness recorded by POM, and LC molecular configuration recorded by conoscope are shown in Fig. 2(a). The normalized output light transmittance (Norm. T.) is shown on the vertical axis. As the V_CU is less than 1 V, the VA LC molecules, which are almost perpendicular to the substrate surface, are not obviously driven, as shown from the inserted conoscopic images. With the red-line reticle labeled on the conoscopic images, the device at the small V_CU (< 1 V) shows an excellent VA quality for LC molecules and a clear edge on the four lobes, indicating that the strength of the induced electric field is insufficient to make the original VA LC molecules reorient. In this case, the extremely small light transmittance and the dark-state image are revealed and shown from the inserted POM images. However, when the V_CU is gradually increased, the LC molecules located on the IPS-electrode edge start tilting to the substrate surface, leading to the fact that the light transmittance will be increased. As the applied V_CU is more than 2 V, the distortion and deformation for the reticle and lobes in the conoscopic images will become obvious, and the brightness in the POM images will no longer be dark. This suggests that the LC molecular configuration starts changing from VA to USA. The light transmittance of the device at the higher V_CU (> 4 V) is remarkable because the increased strength of the induced electric field can drive more LC molecules, which are not only located at the IPS-electrode edge, but also at the IPS-electrode space (s). A higher V_CU (> 4 V) results in the notable USA LC molecular configuration, but the produced larger pretilt angle with respect to the normal direction of the substrate surface will cause light leakage and further reduce the response (as will be shown later). The USA LC molecular configurations under the low and high V_CU are illustrated in Fig. 2(b) and 2(c), respectively. In this paper, two kinds of mixed concentration, 1 wt.% and 2 wt.%, are selected. The VA-IPS LC/polymer devices are fabricated at four kinds of curing voltage, 2 V, 4 V, 6 V, and 8 V, during the process of polymerization. Once the stabilized crosslinking polymers are well developed, the LC molecular configuration will be changed from the VA to the USA profile, and the stable USA-IPS LC molecular structure will be achieved without any holding voltage [25].

 

Fig. 2 (a) T-V curve for the pure E7 LC device in the presence of different curing voltages. The inserted POM and conoscopic images show the V_CU-dependent brightness and pretilt configuration of the pure E7 LC device. As the V_CU is increased, the reticle, labeled by the red line in the conoscopic image at a V_CU of 0 V, will be distorted and eliminated. (b) and (c) effect of the small and large V_CU on the LC molecular configuration, respectively. The small V_CU makes the small U-shaped (pretilt) structure, and most LC molecules are vertically aligned and perpendicular to the substrate surface. However, the large V_CU drives more LC molecules located on the IPS-electrode edge and space (s), making the obviously U-shaped (pretilt) structure.

Download Full Size | PDF

Figure 3 shows the normalized transmittance-V_IPS (T-V) curves for the USA-IPS devices with different fabricated conditions. Four kinds of curing voltages, 2 V, 4 V, 6 V, and 8 V, are selected and applied to the 1- and 2-wt.% USA-IPS devices. Norm. T., treated by each maximum output light transmittance (T_max), is shown on the vertical axis. In Fig. 3(a) and 3(b), there are some common phenomena and properties that are independent of the mixed monomer concentration. The threshold voltage (V_th) determined by the maximum tangent line’s slope and the T_max driving voltage (V_Tmax) are gradually decreased when the applied V_CU is gradually increased, which results from the alignment of the LC molecular configuration. The higher applied V_CU forces the LC molecules near the IPS-electrode edge to further tilt towards the substrate surface, and the USA profile will become obvious. This reduces the value of V_th which makes the LC molecules start reorienting, indicating that the change in the LC molecular configuration is relatively easy [27] and the output light transmittance is gradually increased (V_IPS > V_th) because of the accumulation of the optical phase retardation. Moreover, the value of V_Tmax driving the device with the maximum brightness is also reduced, suggesting that the driving potential energy can be saved due to the fact that the pre-optical phase retardation in the device is produced by the USA molecular configuration. However, once the applied V_IPS is more than V_Tmax, the output light transmittance will be lower than the T_max. This is because the LC molecules are further driven and aligned with respect to the direction of the IPS electric field, leading to the fact that the value of phase retardation is beyond the optimum value (0.5 π) [25, 28]. The behavior of the T-V curve at higher driving voltage (V_IPS > V_Tmax) is related to the value of V_CU. Higher V_CU (≧ 6 V) causes the rapid decay of the output light transmittance when the USA-IPS device is operated at a higher V_IPS value. The decay rates of the output light transmittance of these USA-IPS devices can be estimated by using a tangent line at a specific interval. In Fig. 3(a), the 1-wt.% USA-IPS device at the V_CU of 2 V has the highest V_th ( = 3.42 V) and the highest V_Tmax ( = 12.92 V). However, as the V_CU is increased to 8 V, the 1-wt.% USA-IPS device not only has the lowest V_th ( = 1.17 V) and V_Tmax ( = 7.98 V), but also has obvious light transmittance (leakage) in the initial (dark) state (V_IPS = 0 V) [25]. Producing the larger pretilt USA LC molecular configuration can benefit the LC molecular reorientation and reduce the driving voltage, but the remarkable light leakage in the dark state and the poor switching response (as will be shown later) can also be revealed, which are adverse to the device performance [25–28]. Compared to the 1-wt.% USA-IPS devices, it can be found that the 2-wt.% USA-IPS devices possess higher V_Tmax, as shown in Fig. 3(b), which is attributed to the polymer anchoring effect. The higher mixed monomer concentration leads to the stronger polymer anchoring energy effect on the device, and additional electric potential energy is required to change the LC molecular configuration of the device [29, 30]. However, the slightly higher driving voltage and stronger polymer anchoring effect will benefit the response performance of the USA-IPS device, as will be shown later. In addition, for the 2-wt.% USA-IPS devices, the light leakage is revealed and the V_th is eliminated as the V_CU is over 6 V. The value of the light leakage of the 2-wt.% USA-IPS device is higher than that of the 1-wt.% USA-IPS device, which is also attributed to the polymer anchoring effect on the USA LC molecular configuration. The higher mixed monomer concentration gives the device a more stable USA LC molecular configuration after the polymerization process. Thus, the USA effect on the 2-wt.% USA-IPS devices will be more remarkable than that on the 1-wt.% USA-IPS devices. The tunable electro-optical properties of the USA-IPS devices are achieved by changing the monomer concentration and V_CU. Some of the electrical properties of these USA-IPS devices are summarized in Table 1.

 

Fig. 3 Normalized T-V curves for the 1- and 2-wt.% USA-IPS devices with different curing voltages during the USA configuration process, are shown in (a) and (b), respectively.

Download Full Size | PDF

Table 1. Electrical properties of the USA-IPS devices with different kinds of fabrication conditions.

View Table | View all tables in this article

Figure 4 shows the normalized optical-switch responses for different types of USA-IPS devices, switched between the initial state at V_IPS = 0 V and the maximum bright state at V_IPS = V_Tmax. In the response measurement, the t_r response of the device with free-light-leakage in the dark state is estimated as the optical light transmittance from 10% T_max to 90% T_max, whereas the t_f response is estimated inversely. In Figs. 4(a) and 4(b), the t_r responses of the proposed USA-IPS devices are addressed from V_IPS = 0 V to V_IPS = V_Tmax. It can be found that when the value of V_CU is increased, the time for LC molecules reorienting from the dark state to the maximum bright state is increased, meaning that the t_r response is slow. This phenomenon, independent of the mixed monomer concentration, is revealed in both Figs. 4(a) and 4(b). The higher V_CU makes an obvious USA structure in the proposed device, but the average value of this kind of V_CU-dependent pretilt structure is not very large, and its region ranges from 2° at a V_CU of 2 V to 16° at a V_CU of 8 V [31]. However, the value of V_Tmax is decreased significantly when the applied V_CU is increased. Thus, the performance of t_r response is dominated by the applied value of V_Tmax. Regarding the same USA structure, meaning that the same value of V_CU is applied to the device during the polymerization process, the t_r responses of 2-wt.% USA-IPS devices shown in Fig. 4 (b) are faster than those of 1-wt.% USA-IPS devices shown in Fig. 4 (b), which is related to the TA-9164 crosslinking polymer anchoring effect on the LC molecular reorientation. The increased TA-9164 crosslinking monomer concentration results in a stronger anchoring energy effect on the proposed USA-IPS devices, indicating that additional potential energy to make the LC molecules reorient is required (V_th and V_Tmax) as shown in Fig. 3. Since the t_r response is inversely proportional to the applied value of V_Tmax, the fastest and slowest t_r responses are revealed in the 2-wt.% USA-IPS device at V_Tmax of 13 V and V_CU of 2 V and in the 1-wt.% USA-IPS device at V_Tmax of 5 V and V_CU of 8 V, respectively. In addition, the initial state at V_IPS of 0 V is not dark as the USA-IPS device uses the polymerization process with the value of V_CU over 6 V, which can be applied to long-term and energy-saving information applications. As the V_Tmax is released, the performance of t_f response of the USA-IPS device is only associated with the USA structure and TA-9164 monomer concentration. The obvious USA structure with the pretilt angle over 5° produced by the larger V_CU (> 4 V) will reduce the t_f response, meaning that the time for the driven LC molecules that reorient from the maximum-brightness state to the as-fabricated state is longer. This is attributed to the fact that no stronger recovery force can make the driven LC molecules rapidly reorient to the original configuration [25, 32]. Regarding the TA-9164 polymer anchoring effect on the t_f response of the USA-IPS device, the previous research results show that the IPS-based device with the mixed monomer concentration of 1 wt.% cannot obviously improve the t_f response as compared to the pure (0 wt.%) IPS-based LC devices, which is owing to the lack of a stronger polymer anchoring effect (W_an) on the driven LC molecules (t_f ∝ 1/W_an) [32]. However, once the mixed concentration of TA-9164 monomer is increased to 2 wt%, the faster t_f response can be carried out, which is shown in the inserted figure of Fig. 4(d). From Figs. 4(c) and 4(d), one can find that the t_f responses of the 2-wt.% USA-IPS devices are superior to those of the 1-wt.% USA-IPS devices, which is due to the fact that a stronger polymer anchoring effect makes the driven LC molecules rapidly reorient to the original configuration. It is noted that the USA structure produced at V_CU of 2 V does not degrade the t_f response of the USA-IPS device since the polymer anchoring effect in this case is stronger than the USA effect [25]. However, when the value of V_CU is increased, the effect of the USA structure on the t_f response of the USA-IPS device gradually becomes more obvious, especially for the 1-wt.% USA-IPS devices with a weaker polymer anchoring effect. Of all the proposed USA-IPS devices, the fastest and lowest t_f responses are revealed in the 2-wt.% USA-IPS device with the V_CU of 2 V and in the 1-wt.% USA-IPS device with the V_CU of 8 V, respectively. Based on the above discussion, the tunable optical-switch response of the USA-IPS device can be achieved by controlling the TA-9164 monomer concentration and the value of V_CU during the polymerization process. The optical-switch responses of the proposed USA-IPS devices, including t_r, t_f, and the total response time (t_t = t_r + t_f), are summarized in Table 2.

 

Fig. 4 Optically switching t_r/t_f responses for (a)/(c) 1-wt.% and (b)/(d) 2-wt.% USA-IPS devices with different kinds of curing voltages, respectively.

Download Full Size | PDF

Table 2. Optical-switch responses of the USA-IPS devices with different kinds of fabrication conditions.

View Table | View all tables in this article

To further investigate the effects of TA-9164 monomer concentration on the response performance of the proposed USA-IPS devices, the value of V_CU is fixed at 2 V, and the specific gray-level t_r responses from the zeroth state to each gray-level state are shown in Fig. 5(a). The gray difference between the adjacent levels is equal, and the value of the gray state labeled on the horizontal axis is from 0 to 7, meaning that the output light transmittance is from 0 to the T_max level. Compared to the 0 wt.% USA-IPS device (also called the pure E7 LC device), the 1 wt.% and 2 wt.% USA-IPS devices show the faster gray-level t_r responses for all states, which is because of the USA structure and polymer-increased driving voltage effect on the LC molecular reorientation, resulting in a smaller value of the V_th value and a rapid change in the LC molecular configuration. Of all the devices, the 2 wt.% USA-IPS devices exhibit the shortest gray-level t_r, which is attributed to the stable USA structure and the highest gray-level driving voltage divided from V_Tmax (V_{Tmax|2 wt.%} > V_{Tmax|1 wt.%} > V_{Tmax|0 wt.%}, as shown in the inserted figure), making the LC molecules in the 2-wt.% USA-IPS device reorient rapidly. Figure 5(b) shows the specific gray-level t_f responses for the different kinds of USA-IPS devices from the specific gray state to the zeroth state once the driving voltage is released. Due to the fact that the pretilt angle in the device is small, the t_f response is only related to the anchoring energy effect, and the curve behavior of the t_f response is almost independent of the change in the gray-level state, meaning that the gray-level t_f curve is independent of the gray-level driving voltage. Of all the devices, the 2-wt.% USA-IPS device shows the shortest gray-level t_f, indicating that the introduced stronger polymer anchoring effect makes the driven LC molecules reorient faster. According to the above results, the proposed USA-IPS devices not only have excellent response performance, but also have tunable electro-optical properties, which is beneficial to the design and application of the IPS-based devices.

 

Fig. 5 Gray-level t_r and t_f responses for the different kinds of IPS-based devices, shown in (a) and (b), respectively. The V_CU of the selected 1- and 2-wt.% USA-IPS devices is 2 V. The inserted T-V curve in (a) shows that the 2-wt.% USA-IPS devices at the V_CU of 2 V has the largest V_Tmax value.

Download Full Size | PDF

4. Conclusion

The tunable electro-optical properties of USA-IPS devices are demonstrated and well developed by adjusting the value of V_CU and the mixed concentration of TA-9164 monomer during the polymerization process. The V_CU effects on the USA structure and LC molecular pretilt level are observed from the output light transmittance, POM, and conoscopic images. Through the curing process, TA-9164 crosslinking polymers change the original VA configuration and provide a stable USA configuration to the IPS-based LC devices, forming the USA-IPS devices. The V_th, V_Tmax, light transmittance, and optical response not only depend on the V_CU, but also on the mixed monomer concentration. As the value of V_CU is increased, the V_th and V_Tmax are decreased, which is attributed to the obvious USA structure providing a pre-direction to the LC molecules. The change in V_Tmax can adjust the optical response of the proposed USA-IPS device from fast to slow. Moreover, the higher mixed concentration of TA-9164 monomer, due to the introduced stronger anchoring energy effect on the LC molecular reorientation, is beneficial to the improvement in the optical response of the USA-IPS device. Of all the different kinds of devices, the fastest response is achieved in the 2-wt.% USA-IPS device under the process condition of 2-V V_CU, whereas the lowest response is revealed in the 1-wt.% USA-IPS device under the process condition of 8-V V_CU. This proposed fabrication method not only effectively changes the electro-optical properties of IPS-based devices, but also benefits the performance design of the information and optical system.