Polymer-stabilized blue-phase liquid crystals (PS-BPLCs)^[1–5] have unique advantages, such as sub-millisecond response time which offers the possibility to realize color-sequential displays using LEDs without noticeable color breakup,^[6,7] an isotropic dark state which helps to achieve an inherently wide and a symmetric viewing angle,^[8] and no need of alignment layer,^[9] so they have aroused much interest of researchers. However, some drawbacks including high operating voltage, low transmittance, and hysteresis need to be improved. In order to overcome these technical barriers, the protrusion electrode,^[10,11] wall-shaped electrode,^[12] and corrugated electrode^[13] have been proposed.

Transflective liquid crystal displays (TR-LCDs) have been widely applied in mobile electronic products such as smart phones and tablet PCs owing to their low power consumption and good readability of the sunlight environment.^[14] The TR-LCD consists of transmissive (T) and reflective (R) regions, and the backlight passes through the LC layer only once in the T region, while in the R region the ambient light passes through the LC layer twice. In order to balance the optical phase retardation between the T and R regions, various double-cell-gap^[15–18] and single-cell-gap^[19–22] TR-LCDs have been proposed. Compared with double-cell-gap TR-LCDs, single-cell-gap TR-LCDs have a huge advantage in fabrication. However, several single-cell-gap TR-LCDs using polymer-stabilized blue-phase liquid crystals (PS-BPLCs)^[23,24] are still have a high operating voltage. For example, a single-cell-gap transflective blue-phase liquid crystal display (BP-LCD) with the etched in-plane switching structure^[23] exhibits well-matched voltage-dependent transmittance (VT) and reflectance (VR) curves and a wide viewing angle, but the voltage is high, that is 16 V. For another single-cell-gap TR-LCD using the polymer-stabilized BPLC,^[24] the structure makes use of the space above the pixel electrodes as reflective regions and the space between pixel electrodes as transmissive regions. The operating voltage is 17 V, but it is still high.

In this paper, we propose a single-cell-gap transflective BP-LCD with a non-uniform etching substrate. In the T and R regions, in-plane switching (IPS) electrodes are put on different gaps, and the bottom substrate between the electrodes is etched into different depths. We design different electrode gaps and etching depths to make the T and R regions to obtain the same optical phase retardation. Moreover, the etched IPS structure can generate a deep penetrating fringe field which can lower the operating voltage. As a result, the well-matched VT and VR curves and low operating voltage are obtained.

Figure 1 shows the proposed single-cell-gap transflective BP-LCD. In order to obtain a normally black state and a relatively better viewing angle, we use two broadbands and wide-view circular polarizers consisting of positive and negative A-plates.^[25] The BPLC layer is placed between two crossed circular polarizers. Each pixel is divided into the T and R regions. IPS electrodes are put on different gaps, and the bottom substrate between the electrodes is etched into different depths in two regions. Here, W₁ is the electrode width, which is the same in the T and R regions, W₂ is the width between the electrodes in the R region, and W₃ is the width between the electrodes in the T region. The bottom substrate between the electrodes is etched with depth h₁ in the R region and depth h₂ in the T region, and d is the cell gap. The phase retardation is different in the T and R regions because the times of the backlight and ambient light pass through the LC layer are different. To achieve the same phase retardation in both T and R regions, we make the T and R regions to have different electrode gaps and etching depths. The gap W₃ is intentionally made smaller than W₂, and the depth h₂ is larger than h₁. This design can generate a stronger fringing field in the T region to balance the phase retardation between the T and R modes and match the VT and VR curves.

Fig. 1.

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	Fig. 1. Schematic structure of the transflective BP-LCD with a non-uniform etching substrate.

Unlike the conventional LCD which is based on molecular reorientation from anisotropy to anisotropy, for a BP-LCD, the induced birefringence is governed by the Kerr effect. At the voltage-off state, the BP-LC is optically isotropic, so there will be a good dark state. When a voltage is applied, the BP-LC is optically anisotropic and the induced birefringence emerges. The induced birefringence (Δn)_ind can be expressed by the following equation which is known as the extended Kerr effect model:^[26]

To validate the device design, we used the commercial simulation software TechWiz LCD 3D (SANAYI System Co., Ltd., Incheon, Korea) to simulate the electro–optic characteristics of the transflective BP-LCD. In our simulations, we assume K = 16.3 nm/V², E_s = 4.15 V/μm, and (Δn)_s = 0.15, at λ = 550 nm. The transmittance is normalized to that of two parallel polarizers (G-1220DU) (28.4%). Here, we choose the cell gap d in two regions to be 8 μm.

Figure 2 shows the simulated VT and VR curves for the proposed transflective BP-LCD at λ = 550 nm. The parameters of the device: W₁ = 1 μm, W₂ = 5 μm, W₃ = 3 μm, h₁ = 0.5 μm, and h₂ = 3 μm. The device is an optimized structure, which can achieve low operating voltage and high optical efficiency. The red and blue solid lines represent the simulated VT and VR curves, respectively. We can see that the operating voltage at the peak of the transmittance (∼79%) and reflectance (∼90%) is equal at 6 V, and both two regions have high optical efficiency at this voltage. The red line with closed circles and the blue line with triangles represent the normalized transmittance and reflectance, respectively. A good match between the VT and VR curves is obtained to make the device single gamma driving.

Fig. 2.

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	Fig. 2. Simulated VT (red line) and VR (blue line) curves for the proposed transflective BP-LCD, and the red circles and blue triangles represent the normalized transmittance and reflectance, respectively.

Tables 1 and 2 summarize the operating voltage, transmittance, and reflectance with different electrode gaps and etching depths. In these structures, the electrode width W₁ is set as 1 μm. When the etching depths h₁ and h₂ remain unchanged, as shown in Table 1, the operating voltages of the T and R regions become larger as the electrode gaps W₂ and W₃ increase. This is because the electric fields between electrodes become weaker when the electrode gaps increase. Therefore, we should choose the structure with the smaller electrode gap. As shown in Table 2, we keep the electrode gaps (W₂ and W₃) unchanged, and we can find that the voltage of the T region decreases, and the voltage of the R region increases, when the etching depth h₂ increases and h₁ decreases. As a result, the operating voltages at the peak of the transmittance and reflectance will be equal at 6 V, which contributes to make the VT and VR curves to match with each other tolerably well. From the two tables, it is found that the reflectance is larger than the transmittance. According to our analysis, the disparity is caused by the electrode width W₁ in the T region which leads to low transmittance. To reduce the disparity, we can decrease the electrode width W₁ in the T region.

Table 1.

Table 1.

Table 1. Operating voltage (VT and VR) and the corresponding optical efficiency (T and R) of the proposed transflective BP-LCD with different electrode gaps (W2 and W3). . h1/μm h2/μm W1/μm W2/W3/(μm/μm) VT/Vrms Vr/Vrms T/% R/% 0.5 3 1 4/2 5 5.4 55 92.2 0.5 3 1 5/3 6 6 79 90 0.5 3 1 6/4 8.6 6.4 77 90.3

Table 1. Operating voltage (VT and VR) and the corresponding optical efficiency (T and R) of the proposed transflective BP-LCD with different electrode gaps (W2 and W3). .

Table 2.

Table 2.

Table 2. Operating voltage (VT and VR) and corresponding optical efficiency (T and R) of the proposed transflective BP-LCD with different etching depths (h1 and h2). . h1/μm h2/μm W1/μm W2/W3/(μm/μm) VT/Vrms Vr/Vrms T/% R/% 0.5 2.5 1 5/3 7.4 6.2 66.5 92.4 0.5 3 1 5/3 6 6 79 90 1 3 1 5/3 5.2 4.4 88 92

Table 2. Operating voltage (VT and VR) and corresponding optical efficiency (T and R) of the proposed transflective BP-LCD with different etching depths (h1 and h2). .

Figures 3(a) and 3(b) show the simulated VT and VR curves with different electrode widths W₁ at W₂ = 5 μm, W₃ = 3 μm, h₁ = 0.5 μm, and h₂ = 3 μm. When the electrode width W₁ decreases from 1.5 μm to 0.5 μm, the transmittance becomes higher, while the reflectance is insensitive to the electrode width. Moreover, we can find that the operating voltage at the maximum of the transmittance is reduced as the electrode width W₁ becomes smaller, but the operating voltage at the maximum of the reflectance remains the same. At W₁ = 0.5 μm, the transmittance is approximately 88%, the disparity of the transmittance and reflectance can be eliminated, and the VT and VR curves match with each other very well. However, the fabrication of the small electrode width using the current technology is very complicated.

Fig. 3.

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	Fig. 3. Simulated (a) VT curves and (b) VR curves with different electrode widths for the proposed transflective BP-LCD at W2 = 5 μm, W3 = 3 μm, h1 = 0.5 μm, and h2 = 3 μm.

Figures 4(a) and 4(b) show the simulated iso-contrast plots of the T and R regions at λ = 550 nm, respectively. In order to solve some light leakage at the off-axis direction of the two crossed polarizers, λ/4 biaxial compensation film with N_z = (n_x − n_z)/(n_x − ny) = 0.5 is used to obtain a good wide viewing angle. From Fig. 4, we can find that the viewing angle is reasonably wide. The 1000:1 contrast ratio (CR) is over 40° viewing cone and CR=100:1 is nearly over the entire viewing cone in the T region. In the R region, CR=10:1 is over 55°.

Fig. 4.

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	Fig. 4. Simulated iso-contrast plots for (a) T mode and (b) R mode of the proposedtransflective BP-LCD.

We proposed a wide-view single-cell-gap transflective display with a non-uniform etching substrate based on polymer-stabilized BPLCs. The etched IPS structure was formed in the T and R regions. By designing a set of optimal electrode gaps and etching depths, we can not only lower the operating voltage by generating a deep penetrating fringe field, but also make the VT and VR curves to match with each other very well which makes the device a single gamma curve driving. Both the T and R regions of this BP-LCD exhibit reasonably high optical efficiency. Moreover, the fabrication of the etched substrate and IPS electrodes is simple which makes this transflective device to have a great application potential in portable electronics.