- Email: [email protected]
Fabrication of a dye-doped liquid crystal light shutter by thermal curing of polymer
Optical Materials 69 (2017) 164e168
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Fabrication of a dye-doped liquid crystal light shutter by thermal curing of polymer Byeong-Hun Yu, Seong-Min Ji, Jin-Hun Kim, Jae-Won Huh, Tae-Hoon Yoon* Department of Electronics Engineering, Pusan National University, Busan 46241, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 March 2017 Received in revised form 11 April 2017 Accepted 12 April 2017
We report a thermal curing method for fabrication of a dye-doped polymer-stabilized liquid crystal (PSLC) light shutter, which can prevent the decrease in absorption and discoloration of the dye caused by the UV curing process. We found that the measured transmittance in the opaque state of a dye-doped PSLC cell fabricated by thermal curing was approximately 35% lower than that of a dye-doped PSLC cell fabricated by UV curing. Thermal curing can be an alternative approach for fabrication of a dyedoped PSLC light shutter which can be used to provide high visibility of a see-through display. © 2017 Published by Elsevier B.V.
Keywords: Liquid crystals Dichroic dye Light shutter See-through display Smart window
1. Introduction Recently, see-through displays using organic light-emitting diodes (OLEDs) have drawn much attention as one of the nextgeneration displays [1,2]. However, see-through displays exhibit poor visibility because objects behind the display panel are also seen along with the displayed images. Moreover, the color black cannot be displayed because the operation of an OLED relies on light emission. The poor visibility of a see-through OLED display can be overcome by placing a light shutter at the back of the display panel. By switching a light shutter, we can operate a see-through display in either high-visibility or see-through modes [3e9]. The high-visibility mode of a see-through display can be realized by using a light shutter that simultaneously uses light scattering [10e14] and absorption effects [15e17]. For this reason, dye-doped liquid crystal (LC) light shutters are being extensively studied [18e21]. In particular, an initially-transparent dye-doped polymerstabilized liquid crystal (PSLC) light shutter has been actively studied [8,9,20,21]. To fabricate a dye-doped PSLC light shutter, polymerization of the monomer material doped in the LC mixture is indispensable. Among the various polymerization methods, such as
* Corresponding author. E-mail address: [email protected] (T.-H. Yoon). http://dx.doi.org/10.1016/j.optmat.2017.04.029 0925-3467/© 2017 Published by Elsevier B.V.
photopolymerization, thermal polymerization, and electron beam polymerization [22,23], UV curing has been most widely used in various devices because of its easy, fast, and efficient process [24]. When UV light is incident to an LC mixture doped with photoinitiators, it is absorbed by photo-initiators to yield excited species that easily decompose to free radicals. A free radical has an unpaired valence electron, which makes free radicals chemically very reactive towards other substances. These free radicals initiate polymerization by reaction with monomer molecules, and this process is followed by successive addition of monomer molecules to growing polymer chains or networks [25e28]. During the UV curing process, the doped dye molecules will inevitably be exposed to the UV light [5,18] and damaged by free radicals, which can lead to the decreased absorption and discoloration of the dye molecules. As a result, a dye-doped PSLC cell fabricated by UV curing can exhibit performance degradation and discoloration of dye molecules. In this work, we demonstrate fabrication of a dye-doped PSLC light shutter by thermal curing, which can prevent the decrease in absorption and discoloration of dye molecules caused by the UV curing process. Because polymerization by thermal curing does not require the UV exposure, a dye-doped PSLC light shutter fabricated by thermal curing can provide a superior black color in the opaque state.
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2. Absorption decrease and discoloration of dye Fig. 1 shows the configuration of a dye-doped PSLC cell. Initially, the light shutter is transparent because the LC and dye molecules are aligned perpendicular to the substrate. Therefore, both light scattering and absorption by LC and dye molecules are minimized in the initial state. When a vertical electric field is applied, the cell is switched to the opaque state. Randomly oriented LC and dye molecules cause light scattering and absorption to occur at the same time. When the applied voltage is removed, the cell returns to its initial transparent state. To fabricate a dye-doped PSLC cell by UV curing, an LC mixture including monomer, photo-initiator, and dye can be used. Such an LC mixture is injected by capillary action between the two substrates, and exposed to the UV light to polymerize the inherent monomer. However, the color of a dye-doped PSLC cell can change from black to blue-green by the UV exposure process, as shown in Fig. 2. We prepared the two kinds of LC mixtures to investigate the discoloration of dye molecules. One is negative LC (RTA93000-100, TNI: 71 C, Dn: 0.2, Dε: 5.5, HCCH) doped with 1% of black dye (S428, Mitsui) and 0.5% of photo-initiator (Irgacure 651, Ciba). The other is the same LC doped with 1% of the same black dye but without photo-initiator. Each LC mixture was injected into an empty cell with a cell gap of 10 mm. Each substrate for an empty cell was prepared by coating a homogeneous polyimide alignment layer and rubbing in the anti-parallel direction to align LC and dye molecules. Finally, each cell was exposed to UV light with an intensity of 5 mW/cm2 using a mercury arc lamp. To investigate the degradation of dye molecules by the UV exposure, we measured the polarization-dependent transmission spectra of the fabricated test cells by using the linearly polarized light. Fig. 3 shows a schematic view of the transmittance measurement of the fabricated cells. The polarizer located in front of an unpolarized white light source (MC-916C, Photal Otsuka Electronics) will allow only the component polarized along its transmission axis to pass through it. To measure the polarizationdependent transmission spectra of the fabricated cells, the transmission axis of the polarizer was arranged perpendicular or parallel to the rubbing direction of the fabricated cells. To confirm the degradation of dye molecules by the UV exposure, we measured the transmittance T⊥ [Tll] of a test cell when the polarization of the incident light is perpendicular [parallel] to the rubbing direction as we varied the UV exposure time. As shown in Fig. 4(a), the measured T⊥ and Tk of a test cell without photo-initiator were rarely changed by the UV exposure. Likewise, the measured T⊥ of a test cell with photo-initiator also rarely changed by the UV exposure. Whereas, the measured Tk of the cell with photo-initiator was considerably increased by the UV
Fig. 2. Photographs of a dye-doped PSLC cell before and after the UV exposure.
exposure, as shown in Fig. 4(b). These results indicate that absorption of the incident light polarized along the absorption axis of dye molecules was decreased seriously by the UV exposure in the LC cell with photo-initiator. Especially, the increased transmittance for wavelengths ranging from 400 to 550 nm was considerably high, as shown in Fig. 4(b). Consequently, we can observe the discoloration from black to blue-green caused by the UV exposure. In summary, the UV exposure to an LC mixture including photoinitiator caused the decreased absorption and discoloration of the dye molecules. We fabricated additional test cells to investigate the dependency of T⊥ and Tk on the UV intensity. An LC mixture including 1% of dye and 0.5% of photo-initiator was characterized with the above-mentioned measurement method. Fig. 5 shows the measured dependence of the transmittance of the fabricated test cells on the UV intensity. The measured T⊥ values of the test cells were not affected by the UV intensity. Whereas, as the UV intensity was increased, the measured Tk of the test cells was increased rapidly. To check the dependence of the transmittance on the amount of photo-initiator, we fabricated test cells with a fixed amount of dye. Fig. 6 shows dependence of the measured transmittances of dyedoped cells on the amount of photo-initiator. We can prevent the degradation of dye during the UV exposure by reducing the amount of photo-initiator. However, as mentioned above, the photoinitiator is used to form polymer chains or networks in the UV polymerization method. Therefore, reducing the amount of photoinitiator to prevent degradation of dye is not acceptable in the fabrication of a dye-doped PSLC cell by UV curing. To avoid the degradation of dye molecules in a dye-doped PSLC cell, a polymerization method which does not rely on UV curing is necessary. For this purpose, we demonstrate the fabrication of a dye-doped PSLC cell by thermal curing. 3. Experimental results and discussion We fabricated test cells to verify the performance of a PSLC cell fabricated by thermal curing. A vertical alignment polyimide layer was spin-coated to the top and bottom indium-tin-oxide glass substrates, and ball-type spaces were used to maintain the cell gap
Fig. 1. Configuration of a dye-doped PSLC cell.
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Fig. 3. Schematic view of the transmittance measurement of fabricated cells.
Fig. 4. Measured transmission spectra of test cells (a) without and (b) with photo-initiator.
Fig. 5. Dependence of the measured transmittance of the fabricated test cells on the UV exposure time with the UV intensity as a parameter.
Fig. 6. Dependence of the measured transmittance of the fabricated test cells on the UV exposure time with the amount of photo-initiator as a parameter.
at 10 mm. To fabricate PSLC cells, a negative LC (RTA93000-100) was mixed with 2, 3, 4, 5, and 10% of polymer (NOA83H, Norland
Products). The LC mixtures were stirred for 24 h and an ultrasonic wave was applied for 3 h. Then, the LC mixture was injected into an
B.-H. Yu et al. / Optical Materials 69 (2017) 164e168
Fig. 7. Measured voltageehaze curves of PSLC cells fabricated using 2, 3, 4, 5, and 10% of polymer (without dye).
Fig. 8. Measured Tt, Td, Ts, and haze of dye-doped PSLC cells fabricated by thermal (solid lines) and UV curing (dotted lines).
Table 1 Measured Tt, Td, Ts, and haze of dye-doped PSLC cells fabricated by thermal and UV curing.
Transparent State Opaque state
Curing Method
Tt
Td
Ts
Haze
Thermal UV Thermal UV
41.1% 41.7% 8.0% 12.4%
0.8% 0.9% 5.5% 8.6%
40.3% 40.8% 2.5% 3.8%
1.9% 2.1% 68.8% 69.4%
167
concentrations of 2, 3, 4, 5, and 10%, the measured maximum haze of the PSLC cells were 59.2, 63.1, 67.0, 73.5, and 52.4%, respectively. We used 5% of monomer to fabricate a dye-doped PSLC cell by thermal curing because the PSLC cell with 5% of polymer showed the highest haze value among the fabricated test cells. To fabricate a dye-doped PSLC cell by thermal curing, negative LC (RTA93000-100) was mixed with 5% of thermo-curable polymer (NOA83H, Norland Products) and 5% of black dye (S-428, Mitsui). The LC mixture was cured in a convection oven at 125 C for 2 h. To identify the degradation of the dye by the UV exposure in the opaque state, we fabricated additional dye-doped PSLC cells by UV curing. For UV curing, 4.5% of UV curable polymer (RM257, Merck), 0.5% of photo-initiator (Irgacure 651, Ciba), and 5% of black dye (S428, Mitsui) were doped to the negative LC (RTA93000-100). We exposed the cell to UV light with an intensity of 20 mW/cm2 for 10 min using a mercury arc lamp. The solid and dotted lines in Fig. 8 show the measured total transmittance (Tt), diffuse transmittance (Td), specular transmittance (Ts), and haze of dye-doped PSLC cells fabricated by thermal and UV curing. The cells showed low haze in the initial transparent state owing to the uniform alignment of LC and dye molecules. As mentioned above, the absorption of the incident light polarized perpendicular to the absorption axis of dye molecules was not changed by the UV exposure. Therefore, the measured Tt, Td, and Ts of dye-doped PSLC cells fabricated by the two methods were similar in the initial transparent state. When an electric field was applied to the fabricated cells, the LC and dye molecules became randomly distributed. As the voltage applied to the fabricated test cells was increased, the measured haze and Td also increased. The measured Tt and Ts decreased owing to the increased haze and light absorption by dye molecules. Table 1 lists the measured Tt, Td, Ts, and haze of the dye-doped PSLC cells fabricated by thermal and UV curing in the transparent and opaque states. The measured maximum haze of the dye-doped PSLC cells fabricated by thermal and UV curing were almost the same. However, in the opaque state, the measured Tt, Td, and Ts of a dye-doped PSLC cell fabricated by thermal curing were approximately 35% lower than those of a dye-doped PSLC cell fabricated by UV curing. These results can be attributed to dependence of absorption of the incident light by dye molecules on the curing method. The doped dye molecules were damaged during the UV curing process. Therefore, a cell fabricated by UV curing shows a poor opaque state. Whereas, a cell fabricated by thermal curing shows a superior opaque state because there is no UV exposure. Fig. 9 shows photographs of dye-doped PSLC cells fabricated by thermal and UV curing. Because the doped dye molecules were not damaged by UV curing, the PSLC cells fabricated by thermal curing showed a superior opaque state without discoloration. In contrast, a blue-green color was observed in dye-doped PSLC cells fabricated by UV curing because the dye molecules were discolored during the UV curing process. 4. Conclusions
empty cell. Each fabricated cell was cured in a convection oven at 125 C for 2 h. Fig. 7 shows the measured voltageehaze curves of the PSLC cells fabricated by thermal curing. The measurements of the fabricated cell were performed using a haze-meter (Haze-gard Dual, BYK Gardner) as we vary the amplitude of a 1 kHz square-wave voltage pulse. As the polymer concentration was increased, the operating voltage of the PSLC cell also increased. For the polymer
In summary, we investigated the fabrication of a dye-doped PSLC light shutter by thermal curing for high-visibility of a seethrough display. Because thermal curing does not require UV curing during the cell fabrication process, a dye-doped PSLC cell fabricated by thermal curing shows a superior black color in the opaque state without discoloration or decreased absorption of dye molecules. We can see-through or hide objects behind a seethrough display by switching the light shutter. Dye-doped PSLC cells fabricated by thermal curing can be used in various
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Fig. 9. Photographs of dye-doped PSLC cells fabricated by (a) thermal and (b) UV curing.
applications that serve as substitutes for curtains, blinds, and motorized light screens. Acknowledgements 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. 2017R1A2A1A05001067]. References [1] C.W. Tang, S.A. VanSlyke, Organic electroluminescent diodes, Appl. Phys. Lett. 51 (1987) 913. [2] G. Gu, V. Bulovi c, P.E. Burrows, S.R. Forrest, M.E. Thompson, Transparent organic light emitting devices, Appl. Phys. Lett. 68 (1996) 2606. [3] B.-H. Yu, J.-W. Huh, K.-H. Kim, T.-H. Yoon, Light shutter using dichroic-dyedoped long-pitch cholesteric liquid crystals, Opt. Express 21 (2013) 29332. [4] J.-W. Huh, B.-H. Yu, J. Heo, T.-H. Yoon, Double-layered light shutter using longpitch cholesteric liquid crystal cells, Appl. Opt. 54 (2015) 3792. [5] J.-W. Huh, S.-M. Ji, J. Heo, B.-H. Yu, T.-H. Yoon, Bistable light shutter using dyedoped cholesteric liquid crystals driven with crossed patterned electrodes, J. Disp. Technol. 12 (2016) 779. [6] B.-H. Yu, J.-W. Huh, J. Heo, T.-H. Yoon, Simultaneous control of haze and transmittance using a dye-doped cholesteric liquid crystal cell, Liq. Cryst. 42 (2015) 1460. [7] B.-H. Yu, S.-M. Ji, J.-H. Kim, J.-W. Huh, T.-H. Yoon, Light shutter using dyedoped cholesteric liquid crystals with polymer network structure, J. Inf. Disp. 18 (2016) 13. [8] J. Heo, J.-W. Huh, T.-H. Yoon, Fast-switching initially-transparent liquid crystal light shutter with crossed patterned electrodes, AIP Adv. 5 (2015) 047118. [9] S.-M. Ji, J.-W. Huh, J.-H. Kim, Y. Choi, B.-H. Yu, T.-H. Yoon, Fabrication of flexible light shutter using liquid crystals with polymer structure, Liq. Cryst. (2017), http://dx.doi.org/10.1080/02678292.2017.1281452 (online publish). [10] J.W. Doane, N.A. Vaz, B.-G. Wu, S. Zumer, Field controlled light scattering from nematic microdroplets, Appl. Phys. Lett. 48 (1986) 269. [11] J.W. Lee, J.K. Kim, F. Ahmad, M. Jamil, Y.J. Jeon, Properties of thiol-vinyl PDLC films without additional photoinitiator, Liq. Cryst. 41 (2014) 1109.
[12] Y. Geng, P. Brogueira, J.L. Figueirinhas, M.H. Godinho, P.L. Almeida, Light shutters from nanocrystalline cellulose rods in a nematic liquid crystal, Liq. Cryst. 40 (2013) 769. [13] R. Hikmet, Electrically induced light scattering from anisotropic gels, J. Appl. Phys. 68 (1990) 4406. [14] D.-K. Yang, J.L. West, L.-C. Chien, J.W. Doane, Control of reflectivity and bistability in displays using cholesteric liquid crystals, J. Appl. Phys. 76 (1994) 1331. nchez-Pena, D. Barrios, C. V [15] R. Vergaz, J.-M. Sa azquez, P. Contreras-Lallana, Modelling and electro-optical testing of suspended particle devices, Sol. Energ. Mat. Sol. C 92 (2008) 1483. [16] D. Barrios, R. Vergaz, J.M. Sanchez-Pena, C.G. Granqvist, G.A. Niklasson, Toward a quantitative model for suspended particle devices: optical scattering and absorption coefficients, Sol. Energ. Mat. Sol. C 111 (2013) 115. [17] A. Azens, C. Granqvist, Electrochromic smart windows: energy efficiency and device aspects, J. Solid State Electr. 7 (2003) 64. [18] A.Y.-G. Fuh, C.-C. Chen, C.-K. Liu, K.-T. Cheng, Polarizer-free, electrically switchable and optically rewritable displays based on dye-doped polymerdispersed liquid crystals, Opt. Express 17 (2009) 7088. [19] Y.-H. Lin, H.-S. Chen, T.-H. Chiang, C.-H. Wu, H.-K. Hsu, A reflective polarizerfree electro-optical switch using dye-doped polymer-stabilized blue phase liquid crystals, Opt. Express 19 (2011) 2556. [20] Y.-H. Lin, J.-M. Yang, Y.-R. Lin, S.-C. Jeng, C.-C. Liao, A polarizer-free flexible and reflective electro-optical switch using dye-doped liquid crystal gels, Opt. Express 16 (2008) 1777. [21] G.H. Lee, K.Y. Hwang, J.E. Jang, Y.W. Jin, S.Y. Lee, J.E. Jung, Characteristics of color optical shutter with dye-doped polymer network liquid crystal, Opt. Lett. 36 (2011) 754. [22] Q. Meng, H. Cao, M. Kashima, H. Liu, H. Yang, Effects of the structures of epoxy monomers on the electro-optical properties of heat-cured polymer-dispersed liquid crystal films, Liq. Cryst. 37 (2010) 189. [23] J.V. Crivello, Advanced curing technologies using photo-and electron beam induced cationic polymerization, Radiat. Phys. Chem. 63 (2002) 21. [24] C. Decker, The use of UV irradiation in polymerization, Polym. Int. 45 (1998) 133. [25] H.F. Gruber, photoinitiators for free radical polymerization, Prog. Polym. Sci. 17 (1992) 953. [26] L. Lecamp, B. Youssef, C. Bunel, Photoinitiated polymerization of a dimethacrylate oligomer: 1. influence of photoinitiator concentration, temperature, and light intensity, Polymer 38 (1997) 6089. ci, I. Reetz, Externally stimulated initiator systems for cationic poly[27] Y. Yag merization, Prog. Polym. Sci. 23 (1998) 1485. [28] J.V. Crivello, T.P. Lockhart, J.L. Lee, Diaryliodonium salts as thermal initiators of cationic polymerization, J. Polym. Sci. Pol. Chem. 21 (1983) 97.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Fabrication of a dye-doped liquid crystal light shutter by thermal curing of polymer Byeong-Hun Yu, Seong-Min Ji, Jin-Hun Kim, Jae-Won Huh, Tae-Hoon Yoon* Department of Electronics Engineering, Pusan National University, Busan 46241, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 March 2017 Received in revised form 11 April 2017 Accepted 12 April 2017
We report a thermal curing method for fabrication of a dye-doped polymer-stabilized liquid crystal (PSLC) light shutter, which can prevent the decrease in absorption and discoloration of the dye caused by the UV curing process. We found that the measured transmittance in the opaque state of a dye-doped PSLC cell fabricated by thermal curing was approximately 35% lower than that of a dye-doped PSLC cell fabricated by UV curing. Thermal curing can be an alternative approach for fabrication of a dyedoped PSLC light shutter which can be used to provide high visibility of a see-through display. © 2017 Published by Elsevier B.V.
Keywords: Liquid crystals Dichroic dye Light shutter See-through display Smart window
1. Introduction Recently, see-through displays using organic light-emitting diodes (OLEDs) have drawn much attention as one of the nextgeneration displays [1,2]. However, see-through displays exhibit poor visibility because objects behind the display panel are also seen along with the displayed images. Moreover, the color black cannot be displayed because the operation of an OLED relies on light emission. The poor visibility of a see-through OLED display can be overcome by placing a light shutter at the back of the display panel. By switching a light shutter, we can operate a see-through display in either high-visibility or see-through modes [3e9]. The high-visibility mode of a see-through display can be realized by using a light shutter that simultaneously uses light scattering [10e14] and absorption effects [15e17]. For this reason, dye-doped liquid crystal (LC) light shutters are being extensively studied [18e21]. In particular, an initially-transparent dye-doped polymerstabilized liquid crystal (PSLC) light shutter has been actively studied [8,9,20,21]. To fabricate a dye-doped PSLC light shutter, polymerization of the monomer material doped in the LC mixture is indispensable. Among the various polymerization methods, such as
* Corresponding author. E-mail address: [email protected] (T.-H. Yoon). http://dx.doi.org/10.1016/j.optmat.2017.04.029 0925-3467/© 2017 Published by Elsevier B.V.
photopolymerization, thermal polymerization, and electron beam polymerization [22,23], UV curing has been most widely used in various devices because of its easy, fast, and efficient process [24]. When UV light is incident to an LC mixture doped with photoinitiators, it is absorbed by photo-initiators to yield excited species that easily decompose to free radicals. A free radical has an unpaired valence electron, which makes free radicals chemically very reactive towards other substances. These free radicals initiate polymerization by reaction with monomer molecules, and this process is followed by successive addition of monomer molecules to growing polymer chains or networks [25e28]. During the UV curing process, the doped dye molecules will inevitably be exposed to the UV light [5,18] and damaged by free radicals, which can lead to the decreased absorption and discoloration of the dye molecules. As a result, a dye-doped PSLC cell fabricated by UV curing can exhibit performance degradation and discoloration of dye molecules. In this work, we demonstrate fabrication of a dye-doped PSLC light shutter by thermal curing, which can prevent the decrease in absorption and discoloration of dye molecules caused by the UV curing process. Because polymerization by thermal curing does not require the UV exposure, a dye-doped PSLC light shutter fabricated by thermal curing can provide a superior black color in the opaque state.
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2. Absorption decrease and discoloration of dye Fig. 1 shows the configuration of a dye-doped PSLC cell. Initially, the light shutter is transparent because the LC and dye molecules are aligned perpendicular to the substrate. Therefore, both light scattering and absorption by LC and dye molecules are minimized in the initial state. When a vertical electric field is applied, the cell is switched to the opaque state. Randomly oriented LC and dye molecules cause light scattering and absorption to occur at the same time. When the applied voltage is removed, the cell returns to its initial transparent state. To fabricate a dye-doped PSLC cell by UV curing, an LC mixture including monomer, photo-initiator, and dye can be used. Such an LC mixture is injected by capillary action between the two substrates, and exposed to the UV light to polymerize the inherent monomer. However, the color of a dye-doped PSLC cell can change from black to blue-green by the UV exposure process, as shown in Fig. 2. We prepared the two kinds of LC mixtures to investigate the discoloration of dye molecules. One is negative LC (RTA93000-100, TNI: 71 C, Dn: 0.2, Dε: 5.5, HCCH) doped with 1% of black dye (S428, Mitsui) and 0.5% of photo-initiator (Irgacure 651, Ciba). The other is the same LC doped with 1% of the same black dye but without photo-initiator. Each LC mixture was injected into an empty cell with a cell gap of 10 mm. Each substrate for an empty cell was prepared by coating a homogeneous polyimide alignment layer and rubbing in the anti-parallel direction to align LC and dye molecules. Finally, each cell was exposed to UV light with an intensity of 5 mW/cm2 using a mercury arc lamp. To investigate the degradation of dye molecules by the UV exposure, we measured the polarization-dependent transmission spectra of the fabricated test cells by using the linearly polarized light. Fig. 3 shows a schematic view of the transmittance measurement of the fabricated cells. The polarizer located in front of an unpolarized white light source (MC-916C, Photal Otsuka Electronics) will allow only the component polarized along its transmission axis to pass through it. To measure the polarizationdependent transmission spectra of the fabricated cells, the transmission axis of the polarizer was arranged perpendicular or parallel to the rubbing direction of the fabricated cells. To confirm the degradation of dye molecules by the UV exposure, we measured the transmittance T⊥ [Tll] of a test cell when the polarization of the incident light is perpendicular [parallel] to the rubbing direction as we varied the UV exposure time. As shown in Fig. 4(a), the measured T⊥ and Tk of a test cell without photo-initiator were rarely changed by the UV exposure. Likewise, the measured T⊥ of a test cell with photo-initiator also rarely changed by the UV exposure. Whereas, the measured Tk of the cell with photo-initiator was considerably increased by the UV
Fig. 2. Photographs of a dye-doped PSLC cell before and after the UV exposure.
exposure, as shown in Fig. 4(b). These results indicate that absorption of the incident light polarized along the absorption axis of dye molecules was decreased seriously by the UV exposure in the LC cell with photo-initiator. Especially, the increased transmittance for wavelengths ranging from 400 to 550 nm was considerably high, as shown in Fig. 4(b). Consequently, we can observe the discoloration from black to blue-green caused by the UV exposure. In summary, the UV exposure to an LC mixture including photoinitiator caused the decreased absorption and discoloration of the dye molecules. We fabricated additional test cells to investigate the dependency of T⊥ and Tk on the UV intensity. An LC mixture including 1% of dye and 0.5% of photo-initiator was characterized with the above-mentioned measurement method. Fig. 5 shows the measured dependence of the transmittance of the fabricated test cells on the UV intensity. The measured T⊥ values of the test cells were not affected by the UV intensity. Whereas, as the UV intensity was increased, the measured Tk of the test cells was increased rapidly. To check the dependence of the transmittance on the amount of photo-initiator, we fabricated test cells with a fixed amount of dye. Fig. 6 shows dependence of the measured transmittances of dyedoped cells on the amount of photo-initiator. We can prevent the degradation of dye during the UV exposure by reducing the amount of photo-initiator. However, as mentioned above, the photoinitiator is used to form polymer chains or networks in the UV polymerization method. Therefore, reducing the amount of photoinitiator to prevent degradation of dye is not acceptable in the fabrication of a dye-doped PSLC cell by UV curing. To avoid the degradation of dye molecules in a dye-doped PSLC cell, a polymerization method which does not rely on UV curing is necessary. For this purpose, we demonstrate the fabrication of a dye-doped PSLC cell by thermal curing. 3. Experimental results and discussion We fabricated test cells to verify the performance of a PSLC cell fabricated by thermal curing. A vertical alignment polyimide layer was spin-coated to the top and bottom indium-tin-oxide glass substrates, and ball-type spaces were used to maintain the cell gap
Fig. 1. Configuration of a dye-doped PSLC cell.
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Fig. 3. Schematic view of the transmittance measurement of fabricated cells.
Fig. 4. Measured transmission spectra of test cells (a) without and (b) with photo-initiator.
Fig. 5. Dependence of the measured transmittance of the fabricated test cells on the UV exposure time with the UV intensity as a parameter.
Fig. 6. Dependence of the measured transmittance of the fabricated test cells on the UV exposure time with the amount of photo-initiator as a parameter.
at 10 mm. To fabricate PSLC cells, a negative LC (RTA93000-100) was mixed with 2, 3, 4, 5, and 10% of polymer (NOA83H, Norland
Products). The LC mixtures were stirred for 24 h and an ultrasonic wave was applied for 3 h. Then, the LC mixture was injected into an
B.-H. Yu et al. / Optical Materials 69 (2017) 164e168
Fig. 7. Measured voltageehaze curves of PSLC cells fabricated using 2, 3, 4, 5, and 10% of polymer (without dye).
Fig. 8. Measured Tt, Td, Ts, and haze of dye-doped PSLC cells fabricated by thermal (solid lines) and UV curing (dotted lines).
Table 1 Measured Tt, Td, Ts, and haze of dye-doped PSLC cells fabricated by thermal and UV curing.
Transparent State Opaque state
Curing Method
Tt
Td
Ts
Haze
Thermal UV Thermal UV
41.1% 41.7% 8.0% 12.4%
0.8% 0.9% 5.5% 8.6%
40.3% 40.8% 2.5% 3.8%
1.9% 2.1% 68.8% 69.4%
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concentrations of 2, 3, 4, 5, and 10%, the measured maximum haze of the PSLC cells were 59.2, 63.1, 67.0, 73.5, and 52.4%, respectively. We used 5% of monomer to fabricate a dye-doped PSLC cell by thermal curing because the PSLC cell with 5% of polymer showed the highest haze value among the fabricated test cells. To fabricate a dye-doped PSLC cell by thermal curing, negative LC (RTA93000-100) was mixed with 5% of thermo-curable polymer (NOA83H, Norland Products) and 5% of black dye (S-428, Mitsui). The LC mixture was cured in a convection oven at 125 C for 2 h. To identify the degradation of the dye by the UV exposure in the opaque state, we fabricated additional dye-doped PSLC cells by UV curing. For UV curing, 4.5% of UV curable polymer (RM257, Merck), 0.5% of photo-initiator (Irgacure 651, Ciba), and 5% of black dye (S428, Mitsui) were doped to the negative LC (RTA93000-100). We exposed the cell to UV light with an intensity of 20 mW/cm2 for 10 min using a mercury arc lamp. The solid and dotted lines in Fig. 8 show the measured total transmittance (Tt), diffuse transmittance (Td), specular transmittance (Ts), and haze of dye-doped PSLC cells fabricated by thermal and UV curing. The cells showed low haze in the initial transparent state owing to the uniform alignment of LC and dye molecules. As mentioned above, the absorption of the incident light polarized perpendicular to the absorption axis of dye molecules was not changed by the UV exposure. Therefore, the measured Tt, Td, and Ts of dye-doped PSLC cells fabricated by the two methods were similar in the initial transparent state. When an electric field was applied to the fabricated cells, the LC and dye molecules became randomly distributed. As the voltage applied to the fabricated test cells was increased, the measured haze and Td also increased. The measured Tt and Ts decreased owing to the increased haze and light absorption by dye molecules. Table 1 lists the measured Tt, Td, Ts, and haze of the dye-doped PSLC cells fabricated by thermal and UV curing in the transparent and opaque states. The measured maximum haze of the dye-doped PSLC cells fabricated by thermal and UV curing were almost the same. However, in the opaque state, the measured Tt, Td, and Ts of a dye-doped PSLC cell fabricated by thermal curing were approximately 35% lower than those of a dye-doped PSLC cell fabricated by UV curing. These results can be attributed to dependence of absorption of the incident light by dye molecules on the curing method. The doped dye molecules were damaged during the UV curing process. Therefore, a cell fabricated by UV curing shows a poor opaque state. Whereas, a cell fabricated by thermal curing shows a superior opaque state because there is no UV exposure. Fig. 9 shows photographs of dye-doped PSLC cells fabricated by thermal and UV curing. Because the doped dye molecules were not damaged by UV curing, the PSLC cells fabricated by thermal curing showed a superior opaque state without discoloration. In contrast, a blue-green color was observed in dye-doped PSLC cells fabricated by UV curing because the dye molecules were discolored during the UV curing process. 4. Conclusions
empty cell. Each fabricated cell was cured in a convection oven at 125 C for 2 h. Fig. 7 shows the measured voltageehaze curves of the PSLC cells fabricated by thermal curing. The measurements of the fabricated cell were performed using a haze-meter (Haze-gard Dual, BYK Gardner) as we vary the amplitude of a 1 kHz square-wave voltage pulse. As the polymer concentration was increased, the operating voltage of the PSLC cell also increased. For the polymer
In summary, we investigated the fabrication of a dye-doped PSLC light shutter by thermal curing for high-visibility of a seethrough display. Because thermal curing does not require UV curing during the cell fabrication process, a dye-doped PSLC cell fabricated by thermal curing shows a superior black color in the opaque state without discoloration or decreased absorption of dye molecules. We can see-through or hide objects behind a seethrough display by switching the light shutter. Dye-doped PSLC cells fabricated by thermal curing can be used in various
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Fig. 9. Photographs of dye-doped PSLC cells fabricated by (a) thermal and (b) UV curing.
applications that serve as substitutes for curtains, blinds, and motorized light screens. Acknowledgements 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. 2017R1A2A1A05001067]. References [1] C.W. Tang, S.A. VanSlyke, Organic electroluminescent diodes, Appl. Phys. Lett. 51 (1987) 913. [2] G. Gu, V. Bulovi c, P.E. Burrows, S.R. Forrest, M.E. Thompson, Transparent organic light emitting devices, Appl. Phys. Lett. 68 (1996) 2606. [3] B.-H. Yu, J.-W. Huh, K.-H. Kim, T.-H. Yoon, Light shutter using dichroic-dyedoped long-pitch cholesteric liquid crystals, Opt. Express 21 (2013) 29332. [4] J.-W. Huh, B.-H. Yu, J. Heo, T.-H. Yoon, Double-layered light shutter using longpitch cholesteric liquid crystal cells, Appl. Opt. 54 (2015) 3792. [5] J.-W. Huh, S.-M. Ji, J. Heo, B.-H. Yu, T.-H. Yoon, Bistable light shutter using dyedoped cholesteric liquid crystals driven with crossed patterned electrodes, J. Disp. Technol. 12 (2016) 779. [6] B.-H. Yu, J.-W. Huh, J. Heo, T.-H. Yoon, Simultaneous control of haze and transmittance using a dye-doped cholesteric liquid crystal cell, Liq. Cryst. 42 (2015) 1460. [7] B.-H. Yu, S.-M. Ji, J.-H. Kim, J.-W. Huh, T.-H. Yoon, Light shutter using dyedoped cholesteric liquid crystals with polymer network structure, J. Inf. Disp. 18 (2016) 13. [8] J. Heo, J.-W. Huh, T.-H. Yoon, Fast-switching initially-transparent liquid crystal light shutter with crossed patterned electrodes, AIP Adv. 5 (2015) 047118. [9] S.-M. Ji, J.-W. Huh, J.-H. Kim, Y. Choi, B.-H. Yu, T.-H. Yoon, Fabrication of flexible light shutter using liquid crystals with polymer structure, Liq. Cryst. (2017), http://dx.doi.org/10.1080/02678292.2017.1281452 (online publish). [10] J.W. Doane, N.A. Vaz, B.-G. Wu, S. Zumer, Field controlled light scattering from nematic microdroplets, Appl. Phys. Lett. 48 (1986) 269. [11] J.W. Lee, J.K. Kim, F. Ahmad, M. Jamil, Y.J. Jeon, Properties of thiol-vinyl PDLC films without additional photoinitiator, Liq. Cryst. 41 (2014) 1109.
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