We propose a light shutter device using dichroic-dye-doped liquid crystals (LCs) whose Bragg reflection wavelength is set to be infrared by controlling the pitch of cholesteric liquid crystals (ChLCs). A dye-doped long-pitch ChLC cell is switchable between the dark planar state and the transparent homeotropic state. It has the advantages of high transmittance, low operation voltage, and an easy fabrication process relative to previous LC light shutter devices. The proposed light shutter device is expected to achieve high visibility for transparent organic light-emitting diode displays and emerging smart windows, which can be used in airplanes, cars, and other similar applications.
© 2013 Optical Society of America
Recently, transparent displays have drawn much attention as next-generation displays. In particular, transparent displays using organic light-emitting diodes (OLEDs) are being studied actively [1, 2]. However, since transparent OLEDs cannot provide the black color, they exhibit poor visibility characteristics. This inevitable problem can be solved by placing a light shutter at the back of a transparent OLED display. In order to realize a light shutter, several methods have been proposed, including polymer dispersed liquid crystals (PDLC), LC gel, and cholesteric liquid crystal (ChLC) devices [3–5]. However, these devices cannot provide the black color, as they only switch between transparent and opaque states by the scattering effect. Therefore, such methods may not be suitable for achieving high visibility in a transparent OLED display.
In this work, we demonstrate a light shutter device using dye-doped LCs to achieve high visibility for a transparent OLED display. Dye-doped LC devices were developed and attract much attention because of their high transmittance and polarizer-free structure. Because of their dichroism, dye molecules strongly absorb the incident light polarized parallel to their absorption axes and weakly absorb the incident light polarized perpendicular to their absorption axes. Moreover, a dye-doped LC device is convenient for switching because dye molecules are easily aligned by LC molecules [6, 7].
Achieving a good dark state in dye-doped LC devices is not easy while operating in such modes as the vertical alignment (VA) mode and the electrically controlled birefringence (ECB) mode because dye molecules absorb only the light linearly polarized along the rubbing direction in the dark state. A dye-doped twisted nematic (TN) cell also shows a poor dark state in spite of its twisted structure, because it absorbs the light polarized along a specific direction owing to the wave-guiding effect. In order to obtain a dark state independent of the incident light polarization, several methods have been proposed, including the dye-doped PDLC, the dye-doped blue phase LC, and the dye-doped polymer-networked LC [8–12]. However, these methods have some disadvantages such as hysteresis, high operation voltage, low transmittance, and a complicated fabrication process owing to the in-cell monomer structure. Moreover, the addition of the UV curing process for photo-polymerization of a pre-polymer material can be a serious disadvantage in mass production.
The periodic structure of ChLCs in the planar state causes Bragg reflection, where the wavelength of the reflected light is determined by its pitch. Therefore, long-pitch ChLCs reflecting infrared are transparent in both planar and homeotropic states [13, 14]. In this paper, we propose a light shutter device in which dichroic dyes are doped to long-pitch ChLCs whose Bragg reflection wavelength is set to be infrared by controlling the pitch of ChLC. By doping dye molecules, we can achieve the dark planar state and the transparent homeotropic state. In contrast to the TN mode, the wave-guiding effect can be ignored owing to the periodically twisted structure and the small pitch of the ChLCs. The proposed light shutter device has the advantages of high transmittance and an easy fabrication process. A dye-doped long-pitch ChLC cell shows a good dark state regardless of the polarization direction of the incident light owing to the helical structure of the LC mixture in the planar state. Moreover, it is switchable at a much lower operating voltage than seen in previous studies. We expect that the proposed light shutter device can be applied to obtain high visibility for a transparent OLED and to applications such as emerging “smart windows” for airplanes, cars, et cetera.
2. Cell fabrication
Figure 1 shows the configuration of a dye-doped long-pitch ChLC cell. In the planar state, LCs and dye molecules have a periodic structure, as shown in Fig. 1(a). Dye molecules absorb the arbitrarily polarized light because they are twisted along the helical axis perpendicular to the substrate. Bragg reflection by ChLCs in the planar state can be ignored owing to the infrared reflection characteristics. In the homeotropic state, LCs and dye molecules are aligned perpendicularly to the substrate, allowing most of the arbitrarily polarized light to pass through, as shown in Fig. 1(b). The focal-conic state is not used in the proposed light shutter because light scattering by randomly distributed molecular domains results in poor visibility.
To confirm the electro-optic characteristics of the proposed configuration, a dye-doped long-pitch ChLC cell was fabricated. The top and bottom indium-tin-oxide glass substrates are spin-coated with a homogeneous polyimide alignment material (AL16301K, JSR Micro Korea), and the cell-gap is maintained at 10 or 20 μm by using ball-type spacers. Positive nematic LCs (Δn: 0.159, Δε: 13.5) are mixed with a chiral material (S811, Merck). The mixing ratio is chosen so as to reflect infrared light of wavelength 2,000 nm. Two types of dyes, 2 wt% of S-428 (Mitsui) and 1 wt% of AC1 (Nematel), are doped to the LC mixture to obtain the black color. Then the LC mixture is stirred for 24 hours and an ultrasonic wave is applied to it for 3 hours.
3. Experimental results and discussion
The measured transmission spectra of the fabricated dye-doped long-pitch ChLC cells in the planar and homeotropic states are shown in Fig. 2.When the cell-gaps of the fabricated cells are 20 μm, the measured transmittance in the planar state is 3.7%. By applying 50 V, the cells can be switched to the homeotropic state, where measured transmittance is 42.3%. By lowering the cell-gap to 10 μm, the measured transmittance in the homeotropic state is increased to 58.2%. The voltage applied for switching to the homeotropic state is decreased to 35 V. The transmittance in the planar state also increases to 15.6%.
We fabricated dye-doped LC cells operating in VA, ECB, and TN modes to confirm the effect of the helical structure of the LC mixture on transmittance in the dark state. Table 1 shows the measured electro-optic characteristics of dye-doped LC cells. Cell-gap (10 μm) and the amount of dyes are identical in all fabricated cells. The measured transmittances in the transparent state are nearly identical in all the fabricated cells due to the same amount of dyes and the equivalent arrangement of LC and dye molecules. However, the transmittance in the dark state of a dye-doped long-pitch ChLC cell is about 50% smaller than that of other dye-doped LC cells. Other dye-doped cells show a poor dark state because they only absorb a specific linearly polarized light. Owing to its helical structure, a dye-doped long-pitch ChLC cell absorbs incident light regardless of polarization direction; thus, it can provide a much better dark state than other LC cells.
The proposed light shutter can absorb arbitrary polarized light because of the weak wave-guiding effect resulting from the small pitch. We fabricated test cells with a fixed amount of dyes and cell-gap to check the dependence of transmittance on pitch in the planar state. The measured transmittances versus the number of pitches in ChLC cells are shown in Fig. 3.The measured transmittances of the fabricated cells are nearly identical in the homeotropic state due to the same amount of dyes being used. However, as the number of pitches is increased, the dark state transmittance decreases. Additionally, the operating voltage increases owing to an increased amount of chiral material.
We performed numerical calculations using the commercial software Techwiz LCD (Sanayi System Co., Ltd., Korea) to confirm the experimental results. The parameters for the numerical calculations are obtained from the experimental results and the LC parameters. The imaginary parts of ordinary and extraordinary refractive indices of the LC mixture used in the numerical calculation are 0.0012 and 0.014, respectively. Figure 4 shows the dependence of the measured and calculated transmittances in the planar state on the number of pitches. As number of pitches is increased, both measured and calculated dark state transmittances decrease with almost the same slope.
To investigate the reason for the decrease of transmittance in the planar state, we measured the transmittance of the fabricated 20 μm cells using a polarized light source as shown in Fig. 5. As the number of pitches is increased, the transmittance in the planar state decreases when the polarization of the incident light is perpendicular to the rubbing direction. When the polarization of the incident light is parallel with the rubbing direction, transmittance in the planar state increases slightly. These results show that the decrease of transmittance in the planar state is caused by the decrease of the wave-guiding effect. As the number of pitches is increased, a dye-doped long-pitch ChLC cell shows a much better dark state regardless of the polarization direction of the incident light.
Figure 6 shows photographs of a 20 μm dye-doped long-pitch ChLC cell in the planar and homeotropic states. The cell whose Bragg reflection wavelength is chosen to be 2,000 nm was placed on a backlight unit. In the planar state, the fabricated cell can provide a superior dark state because it absorbs the arbitrarily polarized light from a backlight unit. However, the homeotropic state is transparent because the incident light is allowed to pass through.
In summary, we demonstrated a light shutter device using dye-doped long-pitch ChLCs. It is switchable between the dark planar and transparent homeotropic states. Our experimental results show a high transmittance, a good dark state, and a low operating voltage. The proposed light shutter is suitable to achieve high visibility for a transparent OLED display. We expect that the proposed light shutter is applicable to emerging smart windows, which can be used in various applications.
This work was supported by the Global Leading Technology Program of the Office of Strategic R&D Planning (OSP) funded by the Ministry of Trade, Industry & Energy, Republic of Korea (10042412) and the National Research Foundation of Korea(NRF) grant funded by the Korea government (MSIP) (No. 2011-0029198).
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