Abstract: We propose an alignment method for the near-zero pretilt angle of liquid crystals (LCs) using polyimide films doped with a UV-curable polymer. The near-zero pretilt angle can be obtained by UV curing of reactive mesogen monomers mixed with planar alignment material while a vertical electric field is applied to an LC cell assembled after the rubbing process. We demonstrated that the pretilt angle can be decreased from 2.390° to 0.082° by employing the proposed method.
© 2015 Optical Society of America
Liquid crystal displays (LCDs) have attractive characteristics such as a wide viewing angle, high brightness, and high contrast ratio. Of the different liquid crystal (LC) modes, the in-plane switching (IPS) mode exhibits the widest viewing angle characteristics because the LCs are initially homogeneously aligned, and they rotate within a plane parallel to the substrates when an in-plane electric field is applied [1–4]. Nowadays, IPS or fringe-field switching (FFS) cells, in which LC molecules with positive dielectric anisotropy are homogeneously aligned, are mass-produced because of their low operation voltage and high reliability. In the meantime, the demand for high-resolution displays has continued to rapidly increase. However, a higher resolution is accompanied by a lower aperture ratio or lower light efficiency. To maintain the brightness in a high-resolution display panel, we need to enhance the luminance of the backlight, which leads to an increase in power consumption. In order to reduce the power consumption, we need to improve the light efficiency of LC devices. Because a FFS cell using LCs with negative dielectric anisotropy has a higher transmittance than a FFS cell using LCs with positive dielectric anisotropy , the use of LCs with negative dielectric anisotropy may be suitable for this purpose.
Many LC alignment methods have been developed, including the directional rubbing of polymer films, the evaporation of silicon monoxide (SiOx), irradiation by plasma beam, the UV exposure of photopolymers, and ion-beam treatment on polymer substrates [6–12]. Among these methods, the rubbing method is the most widely used because it exhibits excellent electro-optic performance with good thermal stability. However, the pretilt angle of LCs and the non-uniformity of their alignment in large-size substrates may limit the performance of display panels.
Recently, photo-alignment technology has been the focus of research efforts to achieve high levels of display performance in homogeneously aligned LC devices. Photo-aligned LC cells can provide excellent viewing angle characteristics and a high contrast ratio owing to the advantages of photo-alignment, namely the low pretilt angle of LCs and high alignment uniformity [13–15]. Even though photo-alignment has various advantages that enable us to overcome the drawbacks of the rubbing process, it has not yet been widely applied in mass production because of its weak surface anchoring, the complexity of the process, high curing energy, and strong image sticking. Many approaches have been employed to develop various materials and their irradiation methods for photo-alignment, but most of these technologies have not been successfully applied for commercial application.
In this paper, we propose an alignment method for the near-zero pretilt angle of LCs using UV-curable reactive mesogen (RM) mixed with the planar alignment material. Because the proposed alignment method relies on the rubbing process, it has a strong azimuthal anchoring energy. The near-zero pretilt angle can be obtained by the UV curing of RM monomers while a vertical electric field is applied to an LC cell assembled after the rubbing process.
2. The device fabrication
To demonstrate the performance of the proposed LC alignment method, we fabricated an LC cell with the structure shown in Fig. 1. In order to apply a vertical electric field during the UV exposure, the device contains the top electrode. The structure of the bottom substrate is the same as that of a conventional FFS cell. The patterned electrodes, which are separated by a thin dielectric layer from the bottom common electrode, are placed on the bottom substrate [16, 17]. The width of the patterned electrodes and the gap between them were 2.8 μm and 6 μm, respectively. A mixture of the planar alignment material PIA-5310-GS10 (JNC, Japan), RM257 (Merck, Germany), and the photo-initiator IRGACURE 651 (Chiba chemical, Japan) was spin-coated on each substrate. Then, the alignment layer was baked at 180°C for 1 h. The mixture of the planar alignment material that was spin-coated on each substrate was rubbed with cotton after the baking process, as shown in Fig. 2(a). The rubbing direction was set to 80° with respect to the patterned electrode. Then, the cell was assembled using 4 μm diameter silica spacers, and the negative LC MLC-6608 (Δn = 0.083, Δε = −4.2, Merck) was injected.
To achieve near-zero pretilt of the LCs, a UV irradiation step that lasted for 10 min was added to the conventional rubbing process. In the initial state, the LCs were aligned homogeneously with a non-zero pretilt θ, as shown in Fig. 2(b). The RM monomers in planar alignment layer are easily dissolved and diffused in the LCs, and are movable because of the liquid crystalline property of RMs . These dissolved RM monomers are polymerized when exposed to UV light. When a vertical electric field larger than a certain threshold voltage is applied, the LC molecules are realigned along the direction perpendicular to the applied electric field, or the LCs are aligned with near-zero pretilt angle, as shown in Fig. 2(c). Under the applied electric field, the LC cell was exposed to the 365-nm UV light with intensity 10 mW/cm2 using a mercury arc lamp (Osram HBO 103 W/2) to polymerize the RM monomers. Near-zero pretilt was maintained after curing of the RM monomers was completed and the applied voltage was removed, as shown in Fig. 2(d). Finally, near-zero pretilt was obtained in the homogeneously aligned negative LC cell.
3. Experimental results and discussion
To confirm the electro-optic characteristics of the homogeneously aligned LC cell that was fabricated using the proposed alignment method, we operated the cell in the FFS mode. For comparison, an FFS cell with parameters that were the same as those used for the proposed method was also fabricated using the conventional rubbing method.
To determine the pretilt angles as a function of the UV exposure time, we measured them using the crystal rotation method with an accuracy of ± 0.1° . As the UV curing time was increased, the pretilt angle of a LC cell using polyimide (PI) mixed with RM was decreased, as shown in Fig. 3. The pretilt angle was decreased to 0.132° (RM 0.5 wt%, 30 min) and 0.082° (RM 1.0 wt%, 30 min), which are much lower than the 2.390° obtained without RM. The RM monomers were polymerized when a vertical electric field was applied to the assembled LC cell. The pretilt angle of a LC cell using pure PI does not change when the UV curing time was increased. Without UV curing, the pretilt angle of a LC cell using PI mixed with RM also does not change. Without applying a vertical field during the UV curing, the pretilt angle was decreased slightly to 1.810° (RM 1.0 wt%, 30 min). Reduction of the pretilt angle in a LC cell fabricated without applying a vertical field may be caused by increased azimuthal anchoring energy. To confirm the increase of the azimuthal anchoring energy, we measured it using the torque balance method with an accuracy of ± 0.5° . The azimuthal anchoring energy was increased from 1.178 × 10−4 Jm−2 without RM to 1.604 × 10−4 Jm−2 (RM 0.5 wt%, 30 min) and 1.651 × 10−4 Jm−2 (RM 1.0 wt%, 30 min).
To identify a physical reason for the change in the azimuthal anchoring energy of the PI-RM mixture samples, we investigated the surface morphology of the pure PI and PI-RM mixture-coated surfaces using an atomic force microscope (AFM). The vertical scales of all images in Fig. 4 were set to be identical for fair comparison of the morphology. As shown clearly in Fig. 4, the surface morphology becomes rougher as the curing time is increased. In addition, the striped morphology becomes more prominent as the curing time is increased. Although the surface of the PI-RM mixture is less flat than that of the pure PI, the unidirectional striped morphology may enhance the azimuthal anchoring energy because directionally polymerized RMs on the surface can induce strong interactions with the LC molecules, and consequently, the uniform chain ordering of the alignment layer along the rubbing direction. The phase separation between RM and PI will be investigated in future work.
Measured voltage-transmittance curves of the fabricated LC cells are shown in Fig. 5. For AC driving, a 1-kHz square voltage wave was applied between the patterned electrodes and the bottom common electrode, with the top electrode being floated. The FFS cell that was fabricated using the conventional rubbing method showed a transmittance of 29.98% at an applied voltage of 6.5 V, while the FFS cell that was fabricated using the proposed method with the floated top electrode showed nearly the same maximum transmittance of 29.51% at an applied voltage of 7.0 V. We measured the response time of the FFS cell that was fabricated using the conventional rubbing method and the proposed method. The turn-on and turn-off times of the FFS cell that was fabricated using the proposed method were 24.42 ms and 14.70 ms, respectively, whereas those of the FFS cell that was fabricated using the conventional rubbing method were 23.96 ms and 18.32 ms, respectively. The enhanced azimuthal anchoring energy is believed to have led to a decrease in the falling time in the FFS cell that was fabricated using the proposed method.
We measured the viewing-angle characteristics of the dark state light leakage in a homogeneously aligned LC cell, and this is shown in Fig. 6. There is little light leakage at normal incidence, but the light leakage can be observed along the diagonal directions with respect to the transmission axes of the crossed polarizers. The viewing-angle dependence of the light leakage in an LC cell fabricated by the proposed method is more symmetric than that of an LC cell fabricated by the conventional rubbing method. The symmetric luminance distribution of the dark state was obtained from the lower pretilt angle, and this was confirmed by the calculated dark-state luminance distribution shown in Fig. 7. The symmetric luminance distribution is key to ensuring easy elimination of the off-axis light leakage by optical compensation [21, 22], as can be seen in the next paragraph.
To study the effect of the pretilt angle on the viewing angle characteristics in a homogeneously aligned LC cell, we calculated the luminance distributions of the dark state using a simulation program named “TechWiz LCD 2D” in the double biaxial compensation structure. Using this approach, we can effectively eliminate the wavelength dependence through the symmetrical rotation of the polarization state by each biaxial film . As shown in Fig. 8, a higher pretilt angle results in an increased off-axis light leakage. The light leakage is smaller than 0.0005% over the entire viewing cone when the pretilt angle is 0°. However, we observed a light leakage higher than 0.0005% at a polar angle of ± 55° and ± 40° when the pretilt angle was 1° and 2°, respectively. These results indicate that a lowering of the pretilt angle is essential for the complete elimination of the off-axis light leakage.
For practical display applications, electrical characteristics of the alignment materials are very important. We measured the voltage holding ratio (VHR) using a commercial equipment (VHR-200, Sesim corp., Korea). To measure the VHR, 60 Hz ac square voltage was applied with 60μs duration. The measured VHR value was 98.83%, which may be acceptable for display applications . We also measured the residual direct current (RDC). To measure the RDC, we applied a dc voltage of 4 V to a LC cell for 1 h before we measure it for 10 min. The measured RDC value was 617 mV, which may be rather high for display applications. To decrease the RDC value, we need to optimize the concentration of the monomer and the UV irradiation condition.
We proposed an LC alignment method for near-zero pretilt angle using UV-curable RM mixed with a planar alignment material. We showed that we can obtain near-zero pretilt angle by the UV curing of RM monomers while a vertical electric field is applied. We confirmed the elimination of the asymmetric luminance distribution induced by the surface pretilt. Because the proposed method includes the rubbing process, it has strong azimuthal anchoring energy, which may suffice for commercial applications. The proposed alignment method includes a 10-min UV irradiation step, with which we can easily fabricate an LC device with near-zero pretilt at a relatively low additional manufacturing cost.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2014R1A2A1A01004943).
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