We present a single cell-gap transflective liquid crystal (LC) device using a homogeneous alignment polyimide (H-PI) mixed with a liquid crystalline reactive monomer that is able to vertically align the LC. We obtain two different pretilt angles in each pixel through the region by region control of the UV exposure time. The smaller pretilt angle is used to obtain a half-wave phase retardation for the transmissive part, whereas the larger pretilt angle is used to obtain a quarter-wave phase retardation for the reflective part.
© 2011 OSA
With the recent high demand for various mobile devices, such as smart phones, tablet PCs, and digital multimedia broadcasting applications, transflective liquid crystal displays (LCDs) have received increased attention due to their high performance in both indoor and outdoor environments. Transflective LCD structures can be classified into two types: the double cell-gap structure and the single cell-gap structure. The double cell-gap structure requires a complicated manufacturing process because the maximum brightness is only obtained when the cell-gap of the reflective part (R-part) is half of that of the transmissive part (T-part). In addition, the cell-gap difference between these two parts results in a difference in the response time and color saturation. Therefore, the single cell-gap structure has been considered to be more desirable.
Various methods have been proposed to create the single cell-gap transflective structure [1–5]. We need to apply electric fields with different amplitudes or directions to the two parts of each pixel, which may result in a complicated manufacturing process in regards to the patterned electrodes or the requirement of two thin film transistors for each pixel. A single cell-gap transflective liquid crystal (LC) device using the LC mixed with a liquid crystalline reactive monomer (LC-RM) was recently presented [6, 7]. However, it can suffer from a critical problem in that the residual LC-RM in the LC layer may degrade the electro-optical characteristics of the device. Moreover, it is difficult to control the pretilt angle of the LC over the desired wide 10° to 90° range.
We presented a method that uses vertically aligned PI (V-PI) mixed with the homogeneously aligned (positive A symmetry, +A) LC-RM when it is coated on the substrate . Using this method, we can control the pretilt angle of the LC by changing the mixing ratio of the LC-RM or the UV exposure time used to cure the LC-RM, ensuring that no residual LC-RM is left in the LC layer. However, high concentrations of +A LC-RM may cause light leakage.
In this paper, we present a method that creates a single cell-gap transflective LC device through the control of the pretilt angle of the LC by employing homogeneous alignment PI (H-PI) mixed with the vertically aligned (positive C symmetry, +C) LC-RM. By controlling the UV exposure time, we can obtain the pretilt angle required for each pixel part. The smaller pretilt angle is used to obtain a λ/2 phase retardation for the T-part, whereas the larger pretilt angle is used to obtain a λ/4 phase retardation for the R-part.
2. The device fabrication
In order to control the pretilt angle of the LC, we employed a mixture of two materials with opposite aligning characteristics, H-PI and +C LC-RM, by which an intermediate pretilt angle can be obtained through the anchoring competition between the two materials . The pretilt angle can be controlled by changing the mixing ratio of the H-PI/+C LC-RM or through the UV curing conditions. Figure 1 shows the fabrication process of the proposed method. First, a mixture of H-PI and the +C LC-RM is prepared, which is stirred for several hours. The mixture is then spin-coated onto the indium tin oxide (ITO) glass, as shown in Fig. 1(a). The mixture-coated substrates are exposed to a UV source in order to cure the LC-RM, as shown in Fig. 1(b). In this process, the pretilt angle of the LC can be controlled by changing the UV curing conditions. A baking process used for the polyimidization of the H-PI is employed, followed by a rubbing process that determines the alignment direction of the LC, as shown in Fig. 1(c). The rubbed substrates are then assembled so that the rubbing directions are anti-parallel. Pure LC is then injected into the empty cell. Finally, an intermediate pretilt angle is obtained, as shown in Fig. 1(d).
By using a photo mask during the UV exposure, we can generate regions with two different pretilt angles, which can be used to fabricate the single cell-gap transflective device. The device with two domains is manufactured through the following steps: in the step shown in Fig. 1(b), we expose the UV light region by region using the mask to obtain two different pretilt angles. The region exposed to UV light has a large pretilt angle determined by the H-PI and cured LC-RM, whereas the region not exposed to UV light has a small pretilt angle because the homogeneous alignment is dominated by the H-PI. The region with the large pretilt angle is used as the R-part, whereas the region with the small pretilt angle is used as the T-part.
3. The principle of operation
The optical configuration of the proposed single cell-gap transflective LC device is shown in Fig. 2 . An LC layer is placed between the crossed polarizers. In the R-part, a reflector and a compensation film with a λ/4 phase retardation are added. In the T-part, the incident light from the backlight system becomes vertically polarized light (VPL) when passing through the bottom polarizer whose transmission axis (TA) is 90°, which is changed to horizontally polarized light (HPL) during its propagation through the LC layer whose phase retardation is λ/2 and optic axis (OA) is 45°. The HPL passes through the top polarizer (TA: 0°), through which the bright state is obtained in the T-part. In the R-part, the ambient light becomes HPL in passing through the top polarizer (TA: 0°), and is changed to circularly polarized light (CPL) by the λ/4 film whose OA is 45°. While passing through the LC layer (λ/4, OA: 45°), the light is rotated to VPL, and is reflected by the reflector at the bottom. The reflected VPL passes back through the LC layer and the film. It changes back to HPL and passes through the top polarizer, by which the bright state is obtained. In the presence of an applied electric field, the LCs are aligned vertically in both the T- and R-parts so that the LC layer has no retardation. In the T-part, the VPL transmitted through the bottom polarizer passes through the LC layer with no change in its polarization state, and is blocked by the top polarizer because the top polarizer TA and the VPL are perpendicular to each other. In this manner the dark state is obtained. In the R-part, the light passes through the top polarizer and it becomes HPL. It is then changed to CPL by the film. The polarization state is maintained until the light is changed to VPL by the film again, after it is reflected by the reflector. Finally, the VPL is absorbed by the top polarizer, and so the dark state is obtained.Fig. 3 . The light from the source (543.5 nm) sequentially passes through a polarizer, an LC cell, a Soleil-Babinet compensator (Sigma Koki), an analyzer, and finally reaches a detector. We can measure the retardation of the cell through controlling the retardation of the compensator. Once the phase retardation is obtained, the pretilt angle θ can be estimated using Eq. (1).
4. Experimental results
In order to verify the pretilt angle control, a mixture of H-PI (SE-150, Nissan Chemical) and + C LC-RM (RMS03-015, Merck) was prepared. The LC-RM was exposed to a UV source with an irradiance of 20 mW/cm2 for 1 min, followed by baking for 1 h at 200 °C for the polyimidization of the H-PI. The cell-gap was maintained at 4 µm and positive LC (MLC-0223, Δn: 0.0809, Δε: 7.2, Merck) was injected into the empty cell.
Figure 4 shows the measured pretilt angles as a function of the mixing ratio of the LC-RM. The LC-RM that we used aligns the LC vertically when it is coated on the substrate. The greater the LC-RM concentration, the less effect the H-PI has on the homogenous alignment region. As the LC-RM concentration increases, the pretilt angle increases from 5.4° to 78.2°, as shown in Fig. 4(a). The images shown in Fig. 4(b) are photographs of each sample between the crossed polarizers. When the pretilt angle of the sample cell is 5.4°, the cell has a retardation of about λ/2, through which the bright state is obtained, as shown in the bottom image. When the pretilt angle of the cell is increased, its retardation is decreased, so that the cell shows the dark state under the crossed polarizers. We can achieve the alignment uniformity by controlling the parameters, such as the coating condition, the rubbing condition, the size of the substrates, and the uniformity of UV light source.
Although we confirm that the intermediate pretilt angles are obtained by controlling the mixing ratio of the LC-RM, it is difficult to control the LC-RM concentration used to form the multi-domain in each pixel for a single cell-gap transflective device. Therefore, we controlled the UV exposure times region by region in order to form the multi-domain structure. We fabricated the sample using a H-PI mixed with 20 wt% LC-RM solution. The other parameters are the same as those of experiments shown above. The measured voltage-birefringence curves of the UV-exposed and non-exposed regions in a single cell are shown in Fig. 5(a) . The measured pretilt angles of the T-part and R-part are 6° and 45°, respectively. At zero voltage, the birefringence of the T-part is 307 nm and that of the R-part is 153 nm. Figure 5(b) shows the images of the fabricated cell between crossed polarizers at various applied voltages. Initially, as shown in the leftmost image, the T-part (left) is much brighter than the R-part (right) because of the difference in their retardation due to the pretilt angle. However, at the applied voltage of 7 V, the cell shows a very similar dark state, as shown in the third image.
Figure 6 shows a fabricated single cell-gap transflective LC cell (20 mm × 15 mm) under various environments. In the presence of a backlight, the T-part passes light (see Figs. 6(a) and 6(b)), whereas the R-part (right) is dark because there is no ambient light. In the presence of the amount of ambient light found in an outdoor environment, the R-part shows bright and dark states (see Figs. 6(c) and 6(d)); the T-part remains dark because of the absence of a reflector. The dark state of the R-part can be improved through the optical design found in . In the presence of both a backlight and the ambient light, both the T- and R-parts show bright and dark states, as shown in Figs. 6(e) and 6(f). Therefore, we confirm that the multi-domain structure can be easily achieved using the proposed method, through which a single cell-gap transflective LC cell can be created.
In conclusion, we presented a method used to create a single cell-gap transflective LC device by controlling the pretilt angle using a mixture of H-PI and +C LC-RM. The pretilt angles were controlled by varying the UV exposure time region by region in order to form a multi-domain structure. We fabricated cells possessing intermediate pretilt angles and a single cell-gap transflective LC device. The proposed method can be applied to various display devices used in outdoor applications.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No.2011-0029198).
References and links
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