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In this paper an optical configuration of a transflective liquid crystal (LC) cell driven by a horizontal electric field is proposed, which shows high contrast, high cellgap tolerance, and single gamma, simultaneously. The dark state of the reflective part is realized by a polarizer (0°), a half-wave plate (15°), LC layer (120°), and a quarter-wave layer (-15°), while a wide-band quarter-wave plate (45°) and a polarizer (90°) are added for the dark state of the transmissive part. Since the optic axis of the homogeneously aligned LC layer is set to be parallel to the polarization direction of the light passed through the half-wave plate, the dark state is rarely affected by the cellgap of the LC layer. Due to the different directions of the electric fields, LCs are rotated to 97.5° for the bright state of the reflective part, but to 75° for that of the transmissive part. With the proposed configuration, a high contrast single-gamma transflective display with high cellgap tolerance can be realized in a single-cellgap structure.
©2009 Optical Society of America
1. Introduction
Nematic liquid crystals are uniaxial media that exhibit the birefringence effect when light propagates through them. The birefringence effect results in limited viewing-angle characteristics and wavelength-dependent light transmission of liquid crystal displays (LCDs). Various studies on LCD modes have been carried out to improve the viewing angle characteristics and wavelength dependence of LCDs. Among these studies horizontal switching modes, such as in-plane switching (IPS) and fringe field switching (FFS) modes, were proposed for transmissive displays [1-3]. Compared to vertical switching LC modes, the effective optical retardation of the LC layer in horizontal switching LC modes is almost identical regardless of the viewing directions. This intrinsic property of horizontal switching modes can be beneficial not only to display wide-viewing angles, but also to realize high contrast. Especially, in reflective LCDs, since ambient light comes from every direction, the incident lights feel the optical retardation dependent on their optical paths. As a result, a reflective LCD in the horizontal switching mode, whose optical retardation is relatively insensitive to viewing directions, shows reduced light leakage in the dark state, as shown in Fig. 1. Reduction of light leakage at the dark state is essential for a high contrast display.
Fig. 1. Light leakage at the dark state in reflective LCDs. With the increase of the incident angle, light leakage at the dark state in vertical switching modes gets higher than that in horizontal switching modes.
There have been several attempts to realize transflective LCDs with horizontal switching modes [4-11]. However, these have required a two-layered in-cell retarder or have shown low cellgap tolerance. In the former, with a two-layered in-cell retarder, optical retardation of the LC layer rarely affects the dark state, since the LCs are aligned parallel to the transmission axis of the polarizer, resulting in high contrast ratios and high cellgap tolerances, as shown in Fig. 2(a) [4]. However, because of its thermal and chemical weaknesses, the in-cell retarder is generally coated on top of the electrodes so that the two-layered in-cell retarder serves as a dielectric layer to greatly increase the operating voltage, as shown in Fig. 2(b). Also, the two-layered in-cell may complicate the fabrication process, resulting in increased manufacturing costs. Besides, multiple stacks of in-cell retarder may degrade the dark state since the thickness control of in-cell retarder in a high accuracy is very difficult during fabrication process as shown in Fig. 3, resulting in poor contrast. In the latter, one of the in-cell layers of the former is replaced by the LC layer to reduce the number of the in-cell layers, as shown in Fig. 4(a) [6]. Compared with the two-layered in-cell retarder, it shows relatively low operating voltage, as shown in Fig. 4(b), and makes fabrication process simpler. However, since the LC layer should have retardation of λ/2 at the dark state, the change in optical retardation of the LC layer is fatal to the contrast ratio. With low cellgap tolerances, any small change in the cellgap of the LC layer can significantly affect the contrast ratio. Moreover, considering LC molecules near the substrates, which are rarely rotated by a horizontal electric field, the constraint on the LC layer (λ/2 retardation at the dark state) makes retardation of LC at the bright state less than λ/2. It may reduce the brightness, as shown in Fig. 4(b). Therefore, fewer layers of the in-cell retarder and a high cellgap tolerance are very desirable for reducing manufacturing cost and to obtain low operating voltage devices with high contrast ratio and high brightness.
In this paper an optical configuration of a single-cellgap transflective LC cell is proposed, which needs just one layer of an in-cell retarder and also shows high cellgap tolerance. Since the LC layer plays the role of a null layer at the dark state, cellgap change of the LC layer rarely affects the dark state, which guarantees high contrast ratio. The wavelength dispersion can be suppressed effectively through optimization of the optic axis of each optical component by using the Muller matrix method. Besides, single gamma can be achieved by applying different directions of electric fields to the reflective part and the transmissive part. The proposed optical configuration can be used to realize a high contrast transflective display in single-cellgap structure with high cellgap tolerance.
Fig. 2. Optical configuration and electro-optic characteristics of a transflective LCD with two-layered in-cell retarder. Since LC layer is aligned parallel to the transmission axis of the top polarizer, optical retardation of LC layer rarely affects optical properties at the dark state. However, in-cell retarder serves as a dielectric layer that makes the operating voltage higher.
Fig. 3. Light leakage averaged over the entire range of visible wavelengths at the dark state of the reflective part of two-layered configuration (Fig. 2) as a function of tλ/4 with tλ/2 as a parameter. tλ/4 and tλ/2 are the thickness of in-cell retarder for λ/4 and λ/2 retardations, respectively.
Fig. 4. Optical configuration and electro-optic characteristics of a transflective LCD configured with one-layered in-cell retarder. While one-layer in-cell retarder makes the increase in the operating voltage relatively low, change of the optical retardation of LC layer is fatal to contrast ratio.
2. Design the reflective LC cell with high cellgap tolerance and wideband operation
In order to make the LC layer serve as a null layer in the dark state, the angle between the optic axis (OA) of an homogeneously aligned LC layer, and the transmission axis (TA) of the top polarizer (TP), was set to be twice that between the OA of a half-wave plate (HWP) and the TA of the TP, as shown in Fig. 5. Since the LC layer does not affect the polarization state of the incident light, the dark state of the reflective LC cell is realized by a HWP and a quarter-wave (QW) layer. The relation between OA (θH) of HWP, and OA (θQ) of QW layer for the dark state of the reflective LC cell has been known to be 2θH = θQ ± 45° [12].
Fig. 5. Optical configuration of a reflective LC cell at the dark state. So as to make LC layer serve as a null layer, the angle between OA of the LC layer and TA of TP is set to be twice of that between OA of HWP and TA of TP.
The optimum relation between θH, θLC, and θQ for wideband characteristics can be found by using the Muller matrix, where θLC is the optic axis angle of the LC layer. The Muller matrix for the light propagating through HWP, LC layer, and QW layer can be expressed by the following equation
where Si, MQ, MLC, and MH are the input Stokes vector and Muller matrices for QW layer, LC layer, and HWP, respectively. To investigate the wavelength dependence over the entire range of visible wavelengths, the polarization deviation (ΔS) was defined, which describes how much the output Stokes vector So = (S 0, S 1, S 2, S 3) deviates from the desired Stokes vector on the Poincare sphere [13]. As the circularly polarized light can be expressed as (1, 0, 0, ±1) in the Stokes vector, the polarization deviation on the Poincare sphere can be defined as
through a simple geometric calculation. Figure 6 shows the polarization deviation as a function of the wavelength calculated by the Muller matrix method. The spectral characteristics of the polarization deviation are improved by increasing the angle between the TA of the TP, and the OA of HWP, up to 15°. However, at an angle over 15°, the spectral characteristics deteriorate. Considering the results shown in Fig. 6, the optimum angle between the TA of the FP, and the OA of the HWP, has been selected as 15°.
Finally, θH, θLC and θQ are chosen to be 15°, 120°, and -15° for wideband characteristics respectively, as shown in Fig. 7(a). An excellent dark state has been achieved over the entire range of visible wavelengths through suppression of wavelength dispersion in the dark state, as shown in Fig. 8(a). To obtain the bright state, a horizontal electric field can be applied to rotate the LCs to 97.5°. Since the polarization direction of the light, passed through the LC layer, is orthogonal to the OA of the QW layer, the polarization state does not change by the round trip through the QW layer, as shown in Fig. 7(b). Therefore, the reflected light can pass through the top polarizer freely, resulting in the bright state.
Fig. 8. Reflection and transmission spectra of the proposed transflective LC cell with broadband operation.
3. Design the transflective LC cell with the designed reflective LC cell
By introducing the optical configuration of the reflective LC cell designed above, a transflective LC cell with high cellgap tolerance and wideband characteristics can be configured easily. In Fig. 7(a), the combination of TP, HWP, LC layer, and the QW layer in the reflective part plays the role of a wideband circular polarizer. The addition of an orthogonal circular polarizer, behind the patterned reflector, can make the crossed polarizer configuration in the transmissive part, resulting in the wideband dark state of the two parts, as shown in Fig. 8(b). Here, since the LC layer works as a null layer at the dark state, any cellgap change of the LC layer rarely affects wideband operation at the dark state. Therefore, not only can a high cellgap tolerance be realized, but also higher transmittance can be obtained by increasing the cellgap, as shown in Fig. 9. To obtain the bright state, a horizontal electric field can be applied to rotate the LCs to 75° for the transmissive part, as shown in Fig. 7(b). Combination of the HWP, the rotated LC layer, and the QW layer plays the role of a QWP with an optic axis of 45°. As a result, retardation between the crossed polarizers becomes nearly a half wave with an optic axis of 45°. The polarization state of the incident (90° linearly polarized) light passed through the bottom polarizer is rotated by 90°, to 0° linear polarization, so that it can pass through the top polarizer freely to make the bright state.
Fig. 9. Maximum and minimum brightness, and contrast ratio as functions of cellgap deviation. Since the LC layer in the proposed structure plays a role of a null component at the dark state, changes in cellgap of the LC layer rarely affects the contrast ratio.
Contrast ratio is defined by the ratio of the light output efficiency at the brightest state (Lmax) and the light output efficiency at the darkest state (Lmin) that the system is capable of producing. While Lmax and Lmin can affect the contrast ratio, higher contrast can be achieved more effectively by lower Lmin rather than by higher Lmax. As shown in Fig. 9(a), the maximum light transmission shows an increased output as the cellgap increases, while the change in the maximum light reflection of the proposed structure is negligible. However, the change in cellgap of LC layer can induce polarization changes dependent on wavelengths at the dark state, since the polarization states just before passing through the LC layer do not show the same polarization state over the entire range of visible wavelengths. In other word, the minimum light reflection and transmission slightly increase as cellgap deviation increases, as shown in Fig. 9(b). As a result, the reflective part and the transmissive part of the proposed structure show a decreased contrast ratio as cellgap deviation increases.
Along with high contrast ratio, wide viewing-angle characteristics are also very important for mobile displays. While multiple retardation films can provide good wideband characteristics over the entire range of visible wavelength, multiple retardation films for wideband operation can deteriorate the inherent wide viewing-angle characteristics of horizontal switching modes. However, a simple optical compensation can guarantee the wide viewing-angle characteristics in the proposed configuration, especially in the transmissive part, as shown in Fig. 10.
4. Design of the electrode structure for single gamma
As expected, the proposed configuration with a one-layered in-cell retarder shows relatively low threshold voltage Vth and voltage at peak brightness Vpeak, as shown in Table 1. However, the electro-optic characteristics of the reflective part are different from those of the transmissive part. To make the electro-optic performances of the two parts the same, two driving circuits, one for the reflective part and the other for the transmissive part, are needed. When two driving circuits are used, there will be degradation in the performance of the system. In addition to economic problems and difficulties in fabrication, two thin-film transistors for each pixel may reduce the aperture ratio, or it will decrease the brightness. From this point of view, single gamma driving is necessary to realize good performance in the transflective LCD.
Table 1. Threshold voltages and voltages for peak brightness.
Fig. 11. Operating voltage and the peak reflectance (transmittance) as functions of electrode directions.
The relation between the motions of LC molecules and the electric field can be explained with the dielectric torque D on the polarization of nematic liquid crystal medium defined as
where E 0, Δε, and β are the amplitude of the electric field, the dielectric anisotropy of the LC, and the angle between the LC molecules and the electric field, respectively. In contrast to the vertical switching modes, horizontal switching modes have several factors by which E 0 and β can be controlled [2]. E 0 can be controlled by varying the distance between electrodes, and β can be controlled by changing the direction of electrodes. Therefore, by introducing a different electrode structure, which represents the distance and the direction, for the reflective part and the transmissive part, single gamma in horizontal switching modes can easily be realized [14]. Here, the electrode structure will be designed in the IPS mode for single gamma of the proposed optical configuration.
To investigate the effects of the direction of electrodes on the electro-optic characteristics of the proposed configuration in IPS mode, the direction of the electrodes is varied between 125° and 160° with respect to TA of TP. It is assumed that the electrode width is 4 μm, and the distance between electrodes was varied from 6 to 18 μm. Here a positive LC was used, which has a birefringence Δn of 0.0778 at 589 nm, and a dielectric anisotropy Δε of +13.1. The cellgap was chosen to be 4.1 μm to guarantee high brightness, which corresponds to a LC retardation of 320 nm. All the calculations were performed using a commercial simulator, LCD MASTER (Shintech).
The effects of the electrode structures on the electro-optic characteristics are shown in Figs. 11 and 12, where the thickness of in-cell retarder for QW layer is set to be 1 μm in order to take into account the voltage shielding effect of in-cell retarder. With the increase in electrode angle, Vth decreases but Vpeak increases, which corresponds to a decrease in the steepness of the V-R(T) curve, as shown in Fig. 10. As the distance between electrodes is increased, Vth and Vpeak are increased, which means a higher operating voltage, as shown in Fig. 11. That is, the threshold voltage, and the steepness of the V–R(T) curve for an IPS cell, can be controlled separately by varying the distance between electrodes and the electrode direction, respectively.
It was found that single gamma can be obtained when electrode direction and electrode distance are 140° and 16 μm for the reflective part, and 125° and 4 μm for the transmissive part, as shown in Fig. 13. Under these conditions, reflectance of 27% and transmittance of 24% can be obtained, which give light efficiencies of 84% and 76% of the maximum transmittance of two parallel linear polarizers themselves, respectively. Since there is a tradeoff between high brightness and single gamma, higher brightness can only be obtained if single gamma is sacrificed.
Fig. 12. Operating voltage and the peak reflectance (transmittance) as functions of distance between electrodes.
3. Conclusion
An optical configuration of a single-gamma transflective LC cell, driven by a horizontal electric field, in single cellgap structure has been proposed. Since the LC layer plays the role of a null layer at the dark state, any cellgap change of the LC layer rarely affects the light leakage at the dark state so that a high cellgap tolerance can be realized and high brightness can be obtained by increasing the cellgap. With a broadband structure designed by the Muller matrix method, an excellent dark state was achieved over the entire range of visible wavelengths, to obtain high contrast ratios. Finally, through optimizing the electrode structure, the V-R curve was well matched with the V-T curve to realize single gamma. With the proposed transflective LC cell, a single-gamma transflective display in single cellgap structure, with high contrast ratio, can be realized. High cellgap tolerance of the LC cell guarantees high yields in the fabrication process to realize lower manufacturing costs.
Acknowledgment
This work was supported by Samsung Electronics.
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