- Email: [email protected]
Dye-doped cholesteric liquid crystal light shutter with a polymer-dispersed liquid crystal film
Dyes and Pigments 134 (2016) 36e40
Contents lists available at ScienceDirect
Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Dye-doped cholesteric liquid crystal light shutter with a polymerdispersed liquid crystal film Seung-Won Oh, Jong-Min Baek, Joon Heo, Tae-Hoon Yoon* Department of Electronics Engineering, Pusan National University, Busan 46241, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 February 2016 Received in revised form 19 June 2016 Accepted 27 June 2016 Available online 28 June 2016
A light shutter, which consists of a dye-doped cholesteric liquid crystal (ChLC) layer and a polymerdispersed liquid crystal (PDLC) film, for simultaneous control of haze and transmittance is demonstrated. In the opaque state, it can not only provide a black color by using the dye-doped ChLCs but also hide the objects behind the display panel by using the PDLC film. The proposed light shutter shows a high haze value of 90.7% with a low specular transmittance of 1.20%. By switching the proposed light shutter placed at the back of a see-through display, we can choose between the see-through mode and the high-visibility mode in a see-through display. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Light shutter Polymer-dispersed liquid crystal Cholesteric liquid crystal See-through display
1. Introduction See-through displays are considered as one of the highly promising next-generation displays [1e4]. Currently, a see-through display using organic light-emitting diodes (OLEDs) are being widely studied [2e4]. However, a see-through display using OLEDs cannot represent black color because each pixel includes a transparent window area. Therefore, a see-through display using OLEDs has poor visibility characteristics. To overcome these drawbacks, a light shutter is placed at the back of a see-through display. To realize a high-visibility see-through display, it is necessary to use light scattering and absorption effects simultaneously to hide the objects behind the display panel and provide the black color. Light shutters that uses both effects simultaneously, such as dye-doped polymer-networked liquid crystal (PNLC) [5e7] and dye-doped cholesteric liquid crystal (ChLC), have been reported recently [8e12]. However, light shutters that use dye-doped ChLC or dyedoped PNLC exhibit a rather high transmittance in the opaque state because dichroic dye molecules are not aligned parallel to the substrate. Recently, our group proposed a light shutter device using two ChLC cells [12]: a light-scattering cell and a light-absorption cell. Although this double-cell light shutter has low transmittance in the
* Corresponding author. E-mail address: [email protected] (T.-H. Yoon). http://dx.doi.org/10.1016/j.dyepig.2016.06.045 0143-7208/© 2016 Elsevier Ltd. All rights reserved.
opaque state because the dye molecules are aligned parallel to the substrate, it suffers from disadvantages that include low transmittance in the transparent state, high thickness, and high fabrication cost because of the double-cell structure. Thus, for practical application of a light shutter in a see-through display, a single-cell light shutter in which dye molecules are aligned parallel to the substrates is desirable. In this paper, we demonstrate a dye-doped ChLC light shutter containing a PDLC film within a single cell. The proposed light shutter can provide light absorption and scattering simultaneously, which in turn results in low transmittance with high haze in the opaque state. We expect that the proposed light shutter can be used widely for see-through displays that can hide the objects behind the display panel while providing the black color. 2. Principle of operation The structure of a dye-doped ChLC light shutter with a PDLC film is shown in Fig. 1. For absorption of the incident light, we use dichroic dyes. Dye molecules are convenient for switching because they are easily aligned by LC molecules [5e12]. When the polarization direction of the incident light is parallel with the absorption axis of dye molecules, the incident light is strongly absorbed. Conversely, the incident light is weakly absorbed when the direction of polarization is perpendicular to the absorption axis. To absorb the incident light regardless of polarization direction, we can use dye-doped ChLCs, which have a helical structure [10]. For
S.-W. Oh et al. / Dyes and Pigments 134 (2016) 36e40
37
Fig. 1. Structure and operation of the proposed light shutter in the (a) opaque and (b) transparent states.
scattering of the incident light, we use a PDLC film, which can scatter the incident light owing to the refractive index difference between the LC and the polymer [13,14]. The operation of a dye-doped ChLC light shutter with a PDLC film is shown schematically in Fig. 1. It is switchable between the opaque and transparent states. The opaque state can be realized using absorption by the dye-doped ChLCs and scattering by the PDLC film. In the opaque state, the incident light is absorbed by dye molecules and backward- and forward-scattered by the PDLC film. To switch to the transparent state, we apply a vertical electric field between the top and bottom electrodes. In the transparent state, light absorption is minimized in the dye-doped ChLC because it is switched to the homeotropic state so that the incident light is weakly absorbed by dye molecules [10]. Moreover, light scattering by the PDLC film is minimized because the LC molecules inside the film are aligned parallel to a vertical electric field so that the refractive indices of the LC and polymer are the same.
total transmittance Tt is the sum of the specular transmittance Ts and the diffuse transmittance Td. The haze H can be calculated as H ¼ Td/Tt. Fig. 2(a) depicts the total transmittance and haze of PDLC films as a function of the monomer concentration. The maximum haze value was obtained with 40 wt% of UV-curable monomer. Fig. 2(b) depicts the total transmittance and haze of PDLC films as a function of the UV intensity with 40 wt% of UV curable monomer. As the intensity of UV light was increased, the measured total transmittance decreased because of the increase in backward scattering. When the UV intensity was greater than 100 mW/cm2, the haze value rapidly decreased. The fabrication condition was chosen as
3. Cell fabrication To verify the electro-optical characteristics of the proposed light shutter, a dye-doped ChLC cell with a PDLC film was fabricated. To prepare a PDLC film, we mixed positive LC (E7, Dn: 0.223; Dε: 13.5) with UV curable monomer (NOA65, Norland Products). We studied the characteristics of the fabricated PDLC films as a function of the monomer concentration and the intensity of UV light. We introduced total, specular, and diffuse transmittance and haze for evaluation of the optical performance. The specular [diffuse] transmittance Ts [Td] refers to the ratio of the power of the beam that emerges from a sample cell, which is parallel (within a small range of angles of 2.5 ) [not parallel] to a beam entering the cell, to the power carried by the beam entering the sample. The
Fig. 3. Total transmittance and haze versus number of pitches.
Fig. 2. Total transmittance and haze versus (a) the ratio of NOA65 and (b) the intensity of UV light.
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Fig. 4. Fabrication process of the proposed light shutter.
Fig. 5. SEM image of the cross section of the fabricated light shutter.
40 wt% of UV-curable monomer and 100 mW/cm2 of the UV light intensity. To prepare a dye-doped ChLC cell, we mixed positive LC (E7) with a chiral dopant (S811, Merck). Because the absorption increases with increasing number of pitches but saturates when the number of pitches reaches 10 as shown in Fig. 3, the mixing ratio was chosen to reflect infrared light of 1640 nm (number of pitches: 10; pitch: 1 mm). In addition, 0.7 wt% of black dichroic dye molecules (S-428, Mitsui) were doped to the LC mixture. The mixture was stirred for 24 h, followed by application of an ultrasonic wave for 3 h. The fabrication process flow of a dye-doped ChLC light shutter with a PDLC film is shown schematically in Fig. 4. First, a PDLC cell is assembled using silica spacers to maintain a cell gap of 10 mm, as shown in Fig. 4(a). The cell is then exposed to UV light of 100 mW/ cm2 for 1 min. After UV curing, we remove the indium-tin-oxide glass substrate, as shown in Fig. 4(c). The PDLC film remained on the top indium-tin-oxide glass substrate with little damage thanks to the monomers diffused to the top side [15]. In the actual process, the Meyer rod coating method may be used for easy fabrication of a PDLC film [16]. The cell with a PDLC film is assembled using silica spacers to maintain a cell gap of 20 mm. Dye-doped ChLCs are then
Fig. 6. Total transmittance, specular transmittance, and haze of the fabricated light shutter as functions of the applied voltage. Fig. 7. Specular transmittance of the proposed light shutter vs the applied voltage with the cell gap as a parameter.
S.-W. Oh et al. / Dyes and Pigments 134 (2016) 36e40
Fig. 8. Transmission spectra of the proposed light shutter (filled circle) and the doublecell light shutter (empty circle).
injected into the cell. Finally, a dye-doped ChLC light shutter with a PDLC film can be obtained, as shown in Fig. 4(e). 4. Experimental results and discussion We observed the fabricated structure through the scanning electron microscopy (SEM). The fabricated cell was immersed in isopropyl alcohol for one day to remove the LCs while maintaining the polymer structure. Fig. 5 shows the cross-sectional SEM image of the fabricated structure. The black-colored layer represents the dye-doped ChLC layer, and the other layer represents the PDLC film. The total cell thickness is approximately 20 mm. It is evident that thickness of both the PDLC film and the dye-doped ChLC layer is 10 mm. The optical performance of the proposed light shutter was measured using a haze meter (HM-65W, Murakami Color Research Laboratory). Fig. 6 shows the total transmittance, specular transmittance, and haze of the fabricated light shutter as a function of
39
the applied voltage. In the initial opaque state, the incident light is absorbed by dye molecules in the ChLC layer and backward- and forward-scattered by the PDLC film. The measured total transmittance, specular transmittance, and haze of the fabricated light shutter were 12.9%, 1.20%, and 90.7% in the opaque state, respectively. Haze was considerably high, with a low specular transmittance. We should note that not only the specular transmittance but also the total transmittance was low because of the absorption by dye molecules in the ChLC layer and the backward scattering by the PDLC film. As the applied voltage increased, the total transmittance and specular transmittance increased but the haze decreased gradually. In the transparent state, light absorption in the ChLC layer and light scattering in the PDLC film were minimized. The measured total transmittance, specular transmittance, and haze of the fabricated light shutter were 72.5%, 71.8%, and 0.965% in the transparent state, respectively. To study the dependence of the transmittance on cell gap, we fabricated the proposed light shutters with different cell gaps of 10, 20, and 40 mm and measured their specular transmittance as a function of the applied voltage. As shown in Fig. 7, the transmittance of a light shutter in the opaque state can be decreased simply by increasing the cell gap. However, the transmittance in the transparent state decreases at the same time as the cell gap is increased. Moreover, the operating voltage increases as the cell gap is increased. Therefore, we need to choose a proper cell gap considering the application. To compare the transmission characteristics of the fabricated dye-doped light shutter containing a PDLC film with a double-cell light shutter [12], we fabricated a double-cell ChLC light shutter. To prepare a scattering cell, we mixed positive LC (E7) with chiral dopant (pitch: 0.62 mm; reflection wavelength: 1000 nm). An absorption cell was prepared using the same positive LC and chiral dopant (pitch:1.2 mm; reflection wavelength: 2000 nm). In addition, 3.0 wt% of black dichroic dye molecules (S-428, Mitsui) were doped to the LC mixture. The cell gaps of the cells were 10 and 20 mm. The transmission spectra of the proposed light shutter and the double-cell light shutter measured with a spectrometer (MCPD 3000, Photal) are shown in Fig. 8. The specular transmittances of the proposed light shutter and the double-cell light shutter were 0.795% and 1.94% in the opaque state, respectively. Although the dye concentration of 0.7 wt% used for the proposed light shutter
Fig. 9. Photographs of the fabricated light shutter in the (a) opaque and (b) transparent states.
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S.-W. Oh et al. / Dyes and Pigments 134 (2016) 36e40
was much lower than 3 wt% for a double-cell light shutter, the specular transmittance of the proposed light shutter, which contains a PDLC film, was lower than that of the double-cell light shutter, which contains a ChLC cell, because the transmittance [haze] of a PDLC cell can be lower [higher] than that of a ChLC cell in the opaque state [17]. Moreover, the proposed light shows very low specular transmittance over the entire visible spectrum, as shown in Fig. 8. The specular transmittances of the proposed light shutter and the double-cell light shutter were 72.7% and 34.7% in the transparent state, respectively. Although light absorption and scattering in both light shutters were minimized in the transparent state, the specular transmittance of the double-cell light shutter was much lower than that of the proposed light shutter because 3 wt% of dye was doped in the double-cell light shutter for low specular transmittance in the opaque state. Moreover, optical losses when passing through the additional two glass substrates and the reflection by the air gap between the two cells lowered the transmittance in the transparent state. Photographs of the proposed dye-doped ChLC light shutter with a PDLC film in the opaque and transparent states are shown in Fig. 9. In the opaque state, the proposed light shutter can hide the objects behind a display panel because the dye molecules are aligned parallel to the substrate in the ChLC layer and can scatter the incident light by the refractive index difference between LC and polymer in the PDLC film. In the transparent state, we can identify characters behind a display panel clearly.
5. Conclusion A light shutter consisting of a dye-doped ChLC layer and a PDLC film, by which we can simultaneously control haze and transmittance, was demonstrated. It can not only provide the black color with low transmittance using dye-dope ChLCs but also hide the objects behind the display panel with a high haze value using a PDLC film while still featuring high transmittance in the transparent state. By switching the proposed light shutter placed at the back of a see-through display, we can choose between the seethrough mode and the high-visibility mode in a see-through display. We expect that the proposed device can be a new candidate for smart window technology applicable to various fields, such as automotive, architectural, aerospace, and marine.
Acknowledgements This work was supported by the IT R&D program of MOTIE/KEIT [10042412, More than 6000 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).
References [1] Tang CW, VanSlyke SA. Organic electroluminescent diodes. Appl Phys Lett 1987;51(12):913e5. [2] Gu G, Bulovic V, Burrows PE, Forrest SR, Thompson ME. Transparent organic light emitting devices. Appl Phys Lett 1996;68(19):2606e8. [3] Lin C-H, Lo W-B, Liu K-H, Liu C-Y, Lu J-K, Sugiura N. Novel transparent LCD with tunable transparency. SID Dig Tech Pap 2012;43(1):1159e62. [4] Yeon J, Koh T-W, Cho H, Chung J, Yoo S, Yoon J-B. Actively transparent display with enhanced legibility based on an organic light-emitting diode and a cholesteric liquid crystal blind panel. Opt Express 2013;21(8):10358e66. [5] Lin Y-H, Ren H, Gauza S, Wu Y-H, Liang X, Wu S-T. Reflective direct-view displays using a dye-doped dual-frequency liquid crystal gel. J Disp Technol 2005;1(2):230e3. [6] Lin Y-H, Yang J-M, Lin Y-R, Jeng S-C, Liao C-C. A polarizer-free flexible and reflective electro-optical switch using dye-doped liquid crystal gels. Opt Express 2008;16(3):1777e85. [7] Heo J, Huh J-W, Yoon T-H. Fast-switching initially-transparent liquid crystal light shutter with crossed patterned electrodes. AIP Adv 2015;5:047118. [8] Uchida T, Katagishi T, Onodera M, Shibata Y. Reflective multicolor liquidcrystal display. IEEE Trans Electron Dev 1986;33(8):1207e11. [9] Wang C-T, Lin T-H. Bistable reflective polarizer-free optical switch based on dye-doped cholesteric liquid crystal. Opt Mater Express 2011;1(8):1457e62. [10] Yu B-H, Huh J-W, Kim K-H, Yoon T-H. Light shutter using dichroic-dye-doped long-pitch cholesteric liquid crystals. Opt Express 2013;21(24):29332e7. [11] Yu B-H, Huh J-W, Heo J, Yoon T-H. Simultaneous control of haze and transmittance using a dye-doped cholesteric liquid crystal cell. Liq Cryst 2015;42(10):1460e4. [12] Huh J-W, Yu B-H, Heo J, Yoon T-H. Double-cell light shutter using long-pitch cholesteric liquid crystal cells. Appl Opt 2015;54(12):3792e5. [13] Doane JW, Vaz NA, Wu BG, Zumer S. Field controlled light scattering from nematic microdroplets. Appl Phys Lett 1986;48(4):269e71. [14] Smith GW. Cure parameters and phase behavior of an ultraviolet-cured polymer-dispersed liquid crystal. Mol Cryst Liq Cryst 1991;196(1):89e102. [15] Wang Q, Guo R, Kumar S. Method for fabrication of liquid-crystal cells with narrow gap, fast switching, and flexible plastic substrates. J Soc Inf Disp 2006;14(6):545e50. [16] Yoo S-H, Park M-K, Park J-S, Kim H-R. Enhanced adhesion and transmittance uniformity in laminated polymer-dispersed liquid crystal films. J Opt Soc Korea 2014;18(6):753e61. [17] Yoon T-H, Heo J, Yu B-H, Huh J-W. Liquid crystal light shutter for simultaneous control of haze and transmittance. In: Proc SPIE, 9769; 2016. 97690W.
Contents lists available at ScienceDirect
Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Dye-doped cholesteric liquid crystal light shutter with a polymerdispersed liquid crystal film Seung-Won Oh, Jong-Min Baek, Joon Heo, Tae-Hoon Yoon* Department of Electronics Engineering, Pusan National University, Busan 46241, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 February 2016 Received in revised form 19 June 2016 Accepted 27 June 2016 Available online 28 June 2016
A light shutter, which consists of a dye-doped cholesteric liquid crystal (ChLC) layer and a polymerdispersed liquid crystal (PDLC) film, for simultaneous control of haze and transmittance is demonstrated. In the opaque state, it can not only provide a black color by using the dye-doped ChLCs but also hide the objects behind the display panel by using the PDLC film. The proposed light shutter shows a high haze value of 90.7% with a low specular transmittance of 1.20%. By switching the proposed light shutter placed at the back of a see-through display, we can choose between the see-through mode and the high-visibility mode in a see-through display. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Light shutter Polymer-dispersed liquid crystal Cholesteric liquid crystal See-through display
1. Introduction See-through displays are considered as one of the highly promising next-generation displays [1e4]. Currently, a see-through display using organic light-emitting diodes (OLEDs) are being widely studied [2e4]. However, a see-through display using OLEDs cannot represent black color because each pixel includes a transparent window area. Therefore, a see-through display using OLEDs has poor visibility characteristics. To overcome these drawbacks, a light shutter is placed at the back of a see-through display. To realize a high-visibility see-through display, it is necessary to use light scattering and absorption effects simultaneously to hide the objects behind the display panel and provide the black color. Light shutters that uses both effects simultaneously, such as dye-doped polymer-networked liquid crystal (PNLC) [5e7] and dye-doped cholesteric liquid crystal (ChLC), have been reported recently [8e12]. However, light shutters that use dye-doped ChLC or dyedoped PNLC exhibit a rather high transmittance in the opaque state because dichroic dye molecules are not aligned parallel to the substrate. Recently, our group proposed a light shutter device using two ChLC cells [12]: a light-scattering cell and a light-absorption cell. Although this double-cell light shutter has low transmittance in the
* Corresponding author. E-mail address: [email protected] (T.-H. Yoon). http://dx.doi.org/10.1016/j.dyepig.2016.06.045 0143-7208/© 2016 Elsevier Ltd. All rights reserved.
opaque state because the dye molecules are aligned parallel to the substrate, it suffers from disadvantages that include low transmittance in the transparent state, high thickness, and high fabrication cost because of the double-cell structure. Thus, for practical application of a light shutter in a see-through display, a single-cell light shutter in which dye molecules are aligned parallel to the substrates is desirable. In this paper, we demonstrate a dye-doped ChLC light shutter containing a PDLC film within a single cell. The proposed light shutter can provide light absorption and scattering simultaneously, which in turn results in low transmittance with high haze in the opaque state. We expect that the proposed light shutter can be used widely for see-through displays that can hide the objects behind the display panel while providing the black color. 2. Principle of operation The structure of a dye-doped ChLC light shutter with a PDLC film is shown in Fig. 1. For absorption of the incident light, we use dichroic dyes. Dye molecules are convenient for switching because they are easily aligned by LC molecules [5e12]. When the polarization direction of the incident light is parallel with the absorption axis of dye molecules, the incident light is strongly absorbed. Conversely, the incident light is weakly absorbed when the direction of polarization is perpendicular to the absorption axis. To absorb the incident light regardless of polarization direction, we can use dye-doped ChLCs, which have a helical structure [10]. For
S.-W. Oh et al. / Dyes and Pigments 134 (2016) 36e40
37
Fig. 1. Structure and operation of the proposed light shutter in the (a) opaque and (b) transparent states.
scattering of the incident light, we use a PDLC film, which can scatter the incident light owing to the refractive index difference between the LC and the polymer [13,14]. The operation of a dye-doped ChLC light shutter with a PDLC film is shown schematically in Fig. 1. It is switchable between the opaque and transparent states. The opaque state can be realized using absorption by the dye-doped ChLCs and scattering by the PDLC film. In the opaque state, the incident light is absorbed by dye molecules and backward- and forward-scattered by the PDLC film. To switch to the transparent state, we apply a vertical electric field between the top and bottom electrodes. In the transparent state, light absorption is minimized in the dye-doped ChLC because it is switched to the homeotropic state so that the incident light is weakly absorbed by dye molecules [10]. Moreover, light scattering by the PDLC film is minimized because the LC molecules inside the film are aligned parallel to a vertical electric field so that the refractive indices of the LC and polymer are the same.
total transmittance Tt is the sum of the specular transmittance Ts and the diffuse transmittance Td. The haze H can be calculated as H ¼ Td/Tt. Fig. 2(a) depicts the total transmittance and haze of PDLC films as a function of the monomer concentration. The maximum haze value was obtained with 40 wt% of UV-curable monomer. Fig. 2(b) depicts the total transmittance and haze of PDLC films as a function of the UV intensity with 40 wt% of UV curable monomer. As the intensity of UV light was increased, the measured total transmittance decreased because of the increase in backward scattering. When the UV intensity was greater than 100 mW/cm2, the haze value rapidly decreased. The fabrication condition was chosen as
3. Cell fabrication To verify the electro-optical characteristics of the proposed light shutter, a dye-doped ChLC cell with a PDLC film was fabricated. To prepare a PDLC film, we mixed positive LC (E7, Dn: 0.223; Dε: 13.5) with UV curable monomer (NOA65, Norland Products). We studied the characteristics of the fabricated PDLC films as a function of the monomer concentration and the intensity of UV light. We introduced total, specular, and diffuse transmittance and haze for evaluation of the optical performance. The specular [diffuse] transmittance Ts [Td] refers to the ratio of the power of the beam that emerges from a sample cell, which is parallel (within a small range of angles of 2.5 ) [not parallel] to a beam entering the cell, to the power carried by the beam entering the sample. The
Fig. 3. Total transmittance and haze versus number of pitches.
Fig. 2. Total transmittance and haze versus (a) the ratio of NOA65 and (b) the intensity of UV light.
38
S.-W. Oh et al. / Dyes and Pigments 134 (2016) 36e40
Fig. 4. Fabrication process of the proposed light shutter.
Fig. 5. SEM image of the cross section of the fabricated light shutter.
40 wt% of UV-curable monomer and 100 mW/cm2 of the UV light intensity. To prepare a dye-doped ChLC cell, we mixed positive LC (E7) with a chiral dopant (S811, Merck). Because the absorption increases with increasing number of pitches but saturates when the number of pitches reaches 10 as shown in Fig. 3, the mixing ratio was chosen to reflect infrared light of 1640 nm (number of pitches: 10; pitch: 1 mm). In addition, 0.7 wt% of black dichroic dye molecules (S-428, Mitsui) were doped to the LC mixture. The mixture was stirred for 24 h, followed by application of an ultrasonic wave for 3 h. The fabrication process flow of a dye-doped ChLC light shutter with a PDLC film is shown schematically in Fig. 4. First, a PDLC cell is assembled using silica spacers to maintain a cell gap of 10 mm, as shown in Fig. 4(a). The cell is then exposed to UV light of 100 mW/ cm2 for 1 min. After UV curing, we remove the indium-tin-oxide glass substrate, as shown in Fig. 4(c). The PDLC film remained on the top indium-tin-oxide glass substrate with little damage thanks to the monomers diffused to the top side [15]. In the actual process, the Meyer rod coating method may be used for easy fabrication of a PDLC film [16]. The cell with a PDLC film is assembled using silica spacers to maintain a cell gap of 20 mm. Dye-doped ChLCs are then
Fig. 6. Total transmittance, specular transmittance, and haze of the fabricated light shutter as functions of the applied voltage. Fig. 7. Specular transmittance of the proposed light shutter vs the applied voltage with the cell gap as a parameter.
S.-W. Oh et al. / Dyes and Pigments 134 (2016) 36e40
Fig. 8. Transmission spectra of the proposed light shutter (filled circle) and the doublecell light shutter (empty circle).
injected into the cell. Finally, a dye-doped ChLC light shutter with a PDLC film can be obtained, as shown in Fig. 4(e). 4. Experimental results and discussion We observed the fabricated structure through the scanning electron microscopy (SEM). The fabricated cell was immersed in isopropyl alcohol for one day to remove the LCs while maintaining the polymer structure. Fig. 5 shows the cross-sectional SEM image of the fabricated structure. The black-colored layer represents the dye-doped ChLC layer, and the other layer represents the PDLC film. The total cell thickness is approximately 20 mm. It is evident that thickness of both the PDLC film and the dye-doped ChLC layer is 10 mm. The optical performance of the proposed light shutter was measured using a haze meter (HM-65W, Murakami Color Research Laboratory). Fig. 6 shows the total transmittance, specular transmittance, and haze of the fabricated light shutter as a function of
39
the applied voltage. In the initial opaque state, the incident light is absorbed by dye molecules in the ChLC layer and backward- and forward-scattered by the PDLC film. The measured total transmittance, specular transmittance, and haze of the fabricated light shutter were 12.9%, 1.20%, and 90.7% in the opaque state, respectively. Haze was considerably high, with a low specular transmittance. We should note that not only the specular transmittance but also the total transmittance was low because of the absorption by dye molecules in the ChLC layer and the backward scattering by the PDLC film. As the applied voltage increased, the total transmittance and specular transmittance increased but the haze decreased gradually. In the transparent state, light absorption in the ChLC layer and light scattering in the PDLC film were minimized. The measured total transmittance, specular transmittance, and haze of the fabricated light shutter were 72.5%, 71.8%, and 0.965% in the transparent state, respectively. To study the dependence of the transmittance on cell gap, we fabricated the proposed light shutters with different cell gaps of 10, 20, and 40 mm and measured their specular transmittance as a function of the applied voltage. As shown in Fig. 7, the transmittance of a light shutter in the opaque state can be decreased simply by increasing the cell gap. However, the transmittance in the transparent state decreases at the same time as the cell gap is increased. Moreover, the operating voltage increases as the cell gap is increased. Therefore, we need to choose a proper cell gap considering the application. To compare the transmission characteristics of the fabricated dye-doped light shutter containing a PDLC film with a double-cell light shutter [12], we fabricated a double-cell ChLC light shutter. To prepare a scattering cell, we mixed positive LC (E7) with chiral dopant (pitch: 0.62 mm; reflection wavelength: 1000 nm). An absorption cell was prepared using the same positive LC and chiral dopant (pitch:1.2 mm; reflection wavelength: 2000 nm). In addition, 3.0 wt% of black dichroic dye molecules (S-428, Mitsui) were doped to the LC mixture. The cell gaps of the cells were 10 and 20 mm. The transmission spectra of the proposed light shutter and the double-cell light shutter measured with a spectrometer (MCPD 3000, Photal) are shown in Fig. 8. The specular transmittances of the proposed light shutter and the double-cell light shutter were 0.795% and 1.94% in the opaque state, respectively. Although the dye concentration of 0.7 wt% used for the proposed light shutter
Fig. 9. Photographs of the fabricated light shutter in the (a) opaque and (b) transparent states.
40
S.-W. Oh et al. / Dyes and Pigments 134 (2016) 36e40
was much lower than 3 wt% for a double-cell light shutter, the specular transmittance of the proposed light shutter, which contains a PDLC film, was lower than that of the double-cell light shutter, which contains a ChLC cell, because the transmittance [haze] of a PDLC cell can be lower [higher] than that of a ChLC cell in the opaque state [17]. Moreover, the proposed light shows very low specular transmittance over the entire visible spectrum, as shown in Fig. 8. The specular transmittances of the proposed light shutter and the double-cell light shutter were 72.7% and 34.7% in the transparent state, respectively. Although light absorption and scattering in both light shutters were minimized in the transparent state, the specular transmittance of the double-cell light shutter was much lower than that of the proposed light shutter because 3 wt% of dye was doped in the double-cell light shutter for low specular transmittance in the opaque state. Moreover, optical losses when passing through the additional two glass substrates and the reflection by the air gap between the two cells lowered the transmittance in the transparent state. Photographs of the proposed dye-doped ChLC light shutter with a PDLC film in the opaque and transparent states are shown in Fig. 9. In the opaque state, the proposed light shutter can hide the objects behind a display panel because the dye molecules are aligned parallel to the substrate in the ChLC layer and can scatter the incident light by the refractive index difference between LC and polymer in the PDLC film. In the transparent state, we can identify characters behind a display panel clearly.
5. Conclusion A light shutter consisting of a dye-doped ChLC layer and a PDLC film, by which we can simultaneously control haze and transmittance, was demonstrated. It can not only provide the black color with low transmittance using dye-dope ChLCs but also hide the objects behind the display panel with a high haze value using a PDLC film while still featuring high transmittance in the transparent state. By switching the proposed light shutter placed at the back of a see-through display, we can choose between the seethrough mode and the high-visibility mode in a see-through display. We expect that the proposed device can be a new candidate for smart window technology applicable to various fields, such as automotive, architectural, aerospace, and marine.
Acknowledgements This work was supported by the IT R&D program of MOTIE/KEIT [10042412, More than 6000 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).
References [1] Tang CW, VanSlyke SA. Organic electroluminescent diodes. Appl Phys Lett 1987;51(12):913e5. [2] Gu G, Bulovic V, Burrows PE, Forrest SR, Thompson ME. Transparent organic light emitting devices. Appl Phys Lett 1996;68(19):2606e8. [3] Lin C-H, Lo W-B, Liu K-H, Liu C-Y, Lu J-K, Sugiura N. Novel transparent LCD with tunable transparency. SID Dig Tech Pap 2012;43(1):1159e62. [4] Yeon J, Koh T-W, Cho H, Chung J, Yoo S, Yoon J-B. Actively transparent display with enhanced legibility based on an organic light-emitting diode and a cholesteric liquid crystal blind panel. Opt Express 2013;21(8):10358e66. [5] Lin Y-H, Ren H, Gauza S, Wu Y-H, Liang X, Wu S-T. Reflective direct-view displays using a dye-doped dual-frequency liquid crystal gel. J Disp Technol 2005;1(2):230e3. [6] Lin Y-H, Yang J-M, Lin Y-R, Jeng S-C, Liao C-C. A polarizer-free flexible and reflective electro-optical switch using dye-doped liquid crystal gels. Opt Express 2008;16(3):1777e85. [7] Heo J, Huh J-W, Yoon T-H. Fast-switching initially-transparent liquid crystal light shutter with crossed patterned electrodes. AIP Adv 2015;5:047118. [8] Uchida T, Katagishi T, Onodera M, Shibata Y. Reflective multicolor liquidcrystal display. IEEE Trans Electron Dev 1986;33(8):1207e11. [9] Wang C-T, Lin T-H. Bistable reflective polarizer-free optical switch based on dye-doped cholesteric liquid crystal. Opt Mater Express 2011;1(8):1457e62. [10] Yu B-H, Huh J-W, Kim K-H, Yoon T-H. Light shutter using dichroic-dye-doped long-pitch cholesteric liquid crystals. Opt Express 2013;21(24):29332e7. [11] Yu B-H, Huh J-W, Heo J, Yoon T-H. Simultaneous control of haze and transmittance using a dye-doped cholesteric liquid crystal cell. Liq Cryst 2015;42(10):1460e4. [12] Huh J-W, Yu B-H, Heo J, Yoon T-H. Double-cell light shutter using long-pitch cholesteric liquid crystal cells. Appl Opt 2015;54(12):3792e5. [13] Doane JW, Vaz NA, Wu BG, Zumer S. Field controlled light scattering from nematic microdroplets. Appl Phys Lett 1986;48(4):269e71. [14] Smith GW. Cure parameters and phase behavior of an ultraviolet-cured polymer-dispersed liquid crystal. Mol Cryst Liq Cryst 1991;196(1):89e102. [15] Wang Q, Guo R, Kumar S. Method for fabrication of liquid-crystal cells with narrow gap, fast switching, and flexible plastic substrates. J Soc Inf Disp 2006;14(6):545e50. [16] Yoo S-H, Park M-K, Park J-S, Kim H-R. Enhanced adhesion and transmittance uniformity in laminated polymer-dispersed liquid crystal films. J Opt Soc Korea 2014;18(6):753e61. [17] Yoon T-H, Heo J, Yu B-H, Huh J-W. Liquid crystal light shutter for simultaneous control of haze and transmittance. In: Proc SPIE, 9769; 2016. 97690W.