Fast-switching initially-transparent liquid crystal light shutter with crossed patterned electrodes

Recently, see-through displays have received increasing attention as next-generation displays.^1,2 Most of current researches are focused on see-through displays using organic light emitting diodes (OLEDs).^3,4 A pixel in a see-through OLED display is divided into transparent and light-emitting parts. One can see the background image through the transparent part. However, it is not possible to obtain black color in a see-through OLED display because the transparent part is continuously open to the background. As a result, see-through OLED display panels exhibit poor visibility. This inevitable problem can be solved by placing a light shutter at the back of a see-through display.

Several types of light shutters using liquid crystals (LCs), such as polymer-dispersed LC, LC gel, cholesteric LC (ChLC), and dye-doped LC, have been proposed.^5–12 These light shutters are switchable between their transparent and translucent (or opaque) states by scattering (or absorbing) the incident light. The translucent or opaque state can be used to increase the visibility of a see-through display, whereas the transparent state can be used to view the background together with the displayed images. These light shutters, however, suffer from a slow response time because of their reliance on the slow relaxation of the LCs. Moreover, the light shutters require a high operating voltage because of their helical structure or polymer material. Light shutters based on light scattering cannot provide black color, whereas light shutters based on light absorption cannot completely block the background. In addition, power consumption is one of the key issues related to a display device. Because most light shutters are initially in the translucent or opaque state, power must be continuously supplied to a display panel in order to maintain the transparent state while the display panel is not being used.

In this paper, we propose an initially transparent light shutter using a polymer-networked LC (PNLC) cell containing patterned electrodes. We studied the operating characteristics of the light shutter when various parameters, such as the UV curing condition, the structure of the patterned electrodes, and the concentration of the dye molecules, were changed. As a result of both the turn-on and the turn-off switching of the proposed light shutter being created by applying an electric field, the light shutter can provide a fast response time. In addition, the light shutter has a low operating voltage and a high transmittance in the transparent state. When operating in the opaque state, the light shutter blocks the background completely and provides black color.

To improve the electro-optic characteristics of a light shutter in the opaque state, we propose a method that uses scattering and absorption simultaneously in a LC light shutter. The structures of the proposed light shutter are shown in Fig. 1. We use LC with positive dielectric anisotropy. Patterned electrodes are used to rotate the vertically-aligned LCs. We can use an electrode structure in which the top and bottom patterned electrodes are parallel (as shown in Fig. 1(a)) or crossed (as shown in Fig. 1(b)) with each other.¹³ Polymer networks are employed to scatter the incident light. Dichroic dyes are used to absorb the incident light. When the absorption axes of the dye molecules and the polarization of the incident light are aligned, dye molecules strongly absorb the incident light. On the other hand, dye molecules weakly absorb the incident light when the axes of the dye molecules and the polarization of the incident light are crossed with each other. Dichroic dyes are convenient for use in the control of light transmission through an LC cell because dye molecules are easily oriented along the LC molecules.^14,15

Figure 2 illustrates the operating principle of the proposed light shutter schematically. In the initial state, the LC and the dye molecules are vertically aligned on each substrate; this allows most of the arbitrarily polarized light to pass through the LC cell. In order to switch to the opaque state, we apply an in-plane electric field between the neighboring electrodes of the top and bottom patterned electrodes; in this way, the LC and dye molecules are randomly oriented by the polymer networks.⁸ The incident light is simultaneously scattered by LCs and absorbed by dye molecules while in this state. Fast turn-off switching back to the initial transparent state is achieved by applying a vertical trigger pulse for a short period of time between the top and bottom patterned electrodes.

To confirm the electro-optic characteristics of the proposed light shutter, polymer-networked LC cells were fabricated. The parameters for the fabrication of the cell are as follows: Positive nematic LCs (E7, Δ n: 0.223, Δε: 13.5, Merck) were mixed with 5 wt% of UV curable monomers (Bisphenol A dimethacrylate, Sigma-Aldrich) and different concentrations (0 wt%, 1 wt%, and 2 wt%) of black dichroic dyes (S-428, Mitsui). The width and gap of the patterned electrodes were 2.8 μm and 4 μm, respectively. The top and bottom substrates were spin-coated with a homeotropic alignment material and baked at 200 °C for 1 h. The LC mixtures were injected into an empty cell. The thickness of the LC layer was chosen to be 10 μm. Finally, we exposed UV light of different intensities and durations (0.7 mW/cm² for 2 h, 1.5 mW/cm² for 1 h, and 5 mW/cm² for 20 min) to the cell.

The electro-optic properties of the fabricated PNLC cells were studied by measuring the transmittance using an unpolarized white light source. The transmitted light was detected by using a photodiode placed 20 cm away from a PNLC cell. We measured the transmission spectra of the fabricated PNLC cells for visible wavelengths ranging from 400 nm to 650 nm in the transparent and opaque (or translucent) states; these spectra are shown in Fig. 3.

To compare the switching behavior of PNLC cells with parallel versus crossed electrode structures, we measured the transmittance of each structure. These cells were UV-cured using an intensity of 0.7 mW/cm² for 2 h and the dichroic dyes were not doped. The transmission spectra of these cells are shown in Fig. 3(a). The measured transmittances in the transparent state were 67.0% in both electrode structures. To switch to the translucent state, we applied an in-plane voltage of 10 V between the neighboring electrodes of both the top and bottom patterned electrodes. This applied voltage is much lower than the operating voltage (>50 V) of LC light shutters using LC gel or ChLCs.^6,7 In the translucent state, the measured transmittance of 3.2% transmittance in the PNLC cell with crossed electrodes was much lower than that of 30.6% in the cell with parallel electrodes. As can be seen in the polarized optically microscopy (POM) images shown in Fig. 4, a PNLC cell with crossed electrodes scatters more light than a cell with parallel electrodes. In a cell with crossed electrodes, the higher scattering results in a low value of the measured transmittance.

To confirm the dependence of the electro-optical characteristics of the fabricated PNLC cells on the UV curing conditions, we cured the cells using three different conditions that utilize the same total energy (0.7 mW/cm² for 2 h, 1.5 mW/cm² for 1 h, and 5 mW/cm² for 20 min). The voltages applied to these cells for switching to the transparent state were 10 V, 20 V, and 50 V, respectively. The measured transmittances in the transparent state were 67.0%, 65.6%, and 53.5%, respectively. The measured transmittances in the translucent state were 3.2%, 3.2%, and 7.8%, respectively. In summary, a PNLC cell cured using a low UV intensity has a higher transmittance in the transparent state, a lower transmittance in the translucent state, and a lower operating voltage than a cell cured using a high UV intensity. These results in the translucent state show the same tendency as that reported in a previous study.¹⁶ In the transparent state, polymerization at a low UV intensity does not disturb the initial alignment of LCs so that we can obtain a high transmittance.

To confirm the dependence of the transmittances on the concentration of the dye molecules, we measured the transmittances of dye-doped PNLC cells containing different concentrations of the dye molecules. These cells with crossed patterned electrodes were UV-cured at an intensity of 0.7 mW/cm² for 2 h. When the concentrations of the dye molecules were 1 wt% and 2 wt%, the measured transmittances in the transparent state were 65.5% and 57.4%, respectively. The measured transmittances in the opaque state were 2.3% and 1.8%, respectively. These cells were all switched by applying the same voltage: 10 V. When the concentration of the dye molecules was increased, the transmittance in the opaque state decreased. However, because the dichroic dyes weakly absorb the incident light when they are aligned vertically on each substrate, the transmittance in the transparent state also decreased. In addition, as the incident angle is increased, absorption by dye molecules increases and the transmittance decreases in the initial transparent state. The measured transmittance of the light shutter at a polar angle of ±60° was about a half of the on-axis transmittance, as shown in Fig. 5.

We took photographs of the PNLC cells by placing them on a printed image. The photographs of PNLC cells with parallel and crossed patterned electrodes are displayed in Fig. 6. For both cells, we can view the clear background image in the transparent state because the cells have high transmittances in this state. In the translucent state, a PNLC cell with crossed patterned electrodes can block the background image, whereas a PNLC cell with parallel patterned electrodes cannot block the background image.

The photographs of dye-doped (2 wt%) PNLC cells are shown in Fig. 7. In the transparent state, the clear background image can be seen because of the high transmittance of this cell. A dye-doped PNLC cell can block the background image completely and provide black color in the opaque state because the incident light is simultaneously scattered and absorbed.

We measured the response time of the fabricated PNLC cells. The various light shutters using nematic LCs, such as LC gel and ChLC, suffer from a slow response time greater than a few tens of milliseconds because they rely on the slow relaxation of the LCs.^6,7 The proposed light shutter showed a slow turn-off time of 10.0 ms because polymerization at a low UV intensity increased the turn-off time.¹⁶ For faster switching of the proposed light shutter, both the turn-on and turn-off switching were tightly controlled by applying an electric field.^17–20 To measure the response time of a PNLC cell for the case of turn-on switching, we applied 10 V between the neighboring patterned electrodes of both the top and bottom substrates. To provide for fast turn-off switching, a vertical trigger pulse of 30 V was applied between the top and bottom patterned electrodes for 1 ms. In addition to the existing elastic restoring torque, the applied vertical electric field can exert a strong torque to pull the LC directors back to the vertical direction. The measured turn-on and turn-off time were 0.47 ms and 0.46 ms, respectively, as shown in Fig. 8. This is much faster than other LC technologies such as LC gel and ChLCs.^6,7

In summary, we demonstrated an initially transparent light shutter using polymer-networked liquid crystals with crossed patterned electrodes. The electro-optic characteristics of the proposed light shutter were optimized by changing the electrodes structure, the UV curing conditions, and the concentration of the dye molecules. Both the turn-on and turn-off switching of the proposed light shutter can be controlled by applying an electric field that exhibits a fast response time of less than 1 ms. The turn-on and turn-off switching times of the proposed light shutter are much faster than other LC technologies such as LC gel and ChLCs. The light shutter has a low operating voltage and a high transmittance in the transparent state, and it can block the background image completely and provides black color when in the opaque state. We expect that the proposed light shutter can be applied to see-through displays and smart windows in automobiles, buildings, airplanes, and in other applications as well.

This work was supported by the IT R&D program of MOTIE/KEIT [10042412, More than 60″ Transparent Flexible Display with UD Resolution, Transparency 40% for Transparent Flexible Display in Large Area] and the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. 2014R1A2A1A01004943).