- Email: [email protected]
Contact-free method to prepare photoalignment layers with spacers for flexible liquid crystal displays
Microelectronic Engineering 96 (2012) 67–70
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Accelerated Publication
Contact-free method to prepare photoalignment layers with spacers for flexible liquid crystal displays Hyundae Hah a,1, Jihye Lee a,1, Shi-Joon Sung b, Kuk Young Cho c, Jung-Ki Park a,d,⇑ a
Department of Chemical & Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea Green Energy Research Division, Daegu Gyeongbuk Institute of Science and Technology (DGIST), 50-1, Sang-Ri, Hyeonpung-Myeon, Dalsung-Gun, Daegu 711-873, Republic of Korea c Division of Advanced Materials Engineering, Kongju National University, 275 Budae-dong, Cheonan, Chungnam 331-717, Republic of Korea d Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 3 February 2012 Accepted 23 February 2012 Available online 30 March 2012 Keywords: Contact-free Flexible liquid crystal displays Photoalignment Linear UV light Microstructure
a b s t r a c t The alignment layers and spacers between two polymeric substrates are indispensable components for flexible liquid crystal displays (LCDs). Photoalignment layers with spacers were prepared by a contactfree method using UV patternable and UV-curable materials on polyethersulfone (PES) films. Wellaligned liquid crystal with the direction of the photoalignment layer was observed by polarized microscope. Spacers played a significant role in preventing disruption of the ordering of the liquid crystal. The photoalignment layer with spacers prepared on a polymeric substrate without any physical contact was free from problems that are found in the rubbing method or the micro stamp replication method. Additionally this method can be used in wide areas for the manufacturing of large sized displays. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction UV curable materials are gaining much interest due to their expansion to wider application fields [1]. UV curable materials are now used in the liquid crystal displays (LCD), which is a major commercialized display device. Sealing materials for the panel and prism of brightness enhancement films are good examples of UV curable materials. Recently, liquid crystal-based flexible displays have been highlighted because they offer the potential of developing display devices that are thinner, lighter, and more conformable than rigid glass-based displays [2–5]. Generally, in flexible displays, plastic film is used as a substrate rather than a rigid glass plate. In spite of the high flexibility of a plastic film, there are some obstacles to overcome: plastic substrates exhibit low thermal stability and are susceptible to generating static electricity, which leads to dust contamination [6]. This implies that it is difficult to fabricate the alignment layer for liquid crystal ordering because the conventional alignment layer is formed through imidization of poly(amic acid) at high temperature followed by rubbing with a cloth [7]. Thus, imprinting [8] and photoalignment [9] have been extensively ⇑ Corresponding author at: Department of Chemical & Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea. Tel.: +82 42 350 3965; fax: +82 42 350 3910. E-mail address: [email protected] (J.-K. Park). 1 These authors contributed equally to this work. 0167-9317/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.02.034
investigated as alternative techniques to the rubbing method to fabricate the liquid crystal alignment layers for flexible LCDs. For the photoalignment layer, we previously investigated photoreactive oligomeric materials using a linearly polarized UV light aligning mechanism [10–13]. Considering the flexible characteristics of the display panel of flexible LCDs, dimensional stability is necessary to maintain the cell gap, which results in stable and uniform liquid crystal orientation in a bending environment [14,15]. To overcome the problems generated by high temperature during the processing of plastic substrates for flexible LCDs, we have reported the fabrication of UV embossed alignment layers with phase-separated polymer walls as well as the production of UV embossed alignment layers with patterned spacers in our previously studies, and showed that the procedure is effective [16,17]. However using specific molds inevitably requires a contact process that is not completely free from contamination, and there is difficulty in increasing the size of the master mold for large area displays. In this report, we propose a completely contact-free method to fabricate photoalignment layers. We demonstrate photoalignment layers with spacers that are made by a photolithography process and a photo-induced method using UV curable materials. The alignment layer, formed by photo-irradiation using linearly polarized UV, exhibited a stable liquid crystal orientation. In addition, the flexible liquid crystal cell with spacers had an excellent alignment of liquid crystal even with the external deformation of the liquid crystal cell.
68
H. Hah et al. / Microelectronic Engineering 96 (2012) 67–70
Fig. 1. Schematic illustration of the procedures for the fabrication of PES films and photoalignment layers with spacers.
Fig. 2. Scanning electron microscope images of PES film with the spacers.
2. Experimental 2.1. Materials SU-8 (Microchem Co.) and polyethersulfone (PES) (i-component) films were used as a negative-thick photoresist and plastic substrates, respectively. Poly(vinyl cinnamate) (PVCi) was selected as a photo-reactive polymer. Cyclohexanone was used as a solvent to dissolve the PVCi. They were purchased from Aldrich Co. and used without further purification. The nematic liquid crystal (E7) was purchased from Merck Co.
ing step, photoreaction of the PVCi was induced by irradiating polarized UV for 10 min onto the PVCi coated PES film. Polarized UV light was obtained by passing light from a 300 W high-pressure mercury arc (Oriel) through a UV linear dichroic polarizer (27320, Oriel) and a UV filter 59800 (Oriel). The intensity of the irradiated UV light was 4 mW cm2. 2.4. Liquid crystal orientation and characterization
Fig. 1 illustrates the procedures for fabricating the PES films and photoalignment layers with spacers. The PES films with spacers were fabricated by a photolithography process. Negative photoresist SU-8 was spin-coated onto the PES film and the coated film was then baked for 10 min. The photoresist-coated PES film was irradiated under UV light (Contact aligner EVG640, EV group) through a photomask containing transparent areas. Unexposed areas were not cured and easily removed by a developing solution. As a result, a PES film with spacers was obtained.
A homogeneously aligned liquid crystal cell was made by placing nematic liquid crystal (E7) between a pair of PES films assembly coated with photoalignment layers with spacers. Liquid crystals were injected into the cell by capillary motion at 65 °C which induces the isotropic phase transition of liquid crystal. The texture of the liquid crystal in the cell was investigated with a Nikon Optophoto2-Pol polarized optical microscopy under the cross Nicoles. The director of the nematic liquid crystals in the liquid crystal cell was determined from the dichroic absorption of a dichroic dye (disperse blue 14, 0.2 wt.%) included in the nematic liquid crystals. The order parameter (S) of the liquid crystal cell was measured using one polarizer and a UV–Vis spectrometer (Shimadzu UV-1601) [18]. The azimuthal anchoring energy was estimated by measuring of the width of Neel wall (x) [19].
2.3. Fabrication of the photoalignment layer
3. Results and discussion
As demonstrated in Fig. 1, 2 wt.% solution of PVCi in cyclohexanone was spin-coated on the PES film with spacers at the speed of 1800 rpm followed by prebaking at 60 °C for 1 h. After the anneal-
Utilization of UV curable materials can provide a method free from physical contact to create the alignment structure on the plastic films. Using photoreactive materials, we fabricated
2.2. Preparation of PES films with a spacer structure
H. Hah et al. / Microelectronic Engineering 96 (2012) 67–70
photoalignment layers with spacers that have good cell gap uniformity. Fig. 2 shows the images of the scanning electron microscopy (SEM) for the photoalignment layers with spacers prepared with our proposed contact-free procedure. The height, the width, and the intervals of the spacer were 6 lm, 20 lm, and 300 lm, respectively. The most important issue of the flexible display based on liquid crystal is preventing distortion of the cell-gap during the deformation of the cell, and the spacer can support the regular liquid crystal cell gap even under an external bending force. In this report, a spacer with an isotropic configuration was used to re-
69
move the effect of the direction of bending on the interaction between the alignment layer and liquid crystal molecules. Fig. 3 shows polarized optical microscope images of a photoinduced liquid crystal cell with spacers. The photodimerization reaction of PVCi by polarized UV irradiation is a [2 + 2] cycloaddition reaction based on the mechanism of a pericyclic reaction. This reaction occurs in the photoalignment layer resulting in the generation of structural anisotropy of the photo-reactive materials [20,21]. Orientation of liquid crystal molecules is usually induced by van der Waals interaction between liquid crystal molecules
Fig. 3. Polarized optical microscope images of a photo-induced liquid crystal cell with spacers. The directions of polarizer (A and P) and the photoalignment direction of LC (L) are noted as arrows in the images.
Fig. 4. Change of the (a) order parameter, (b) azimuthal anchoring energy, and (c) polar plot in a photo-induced liquid crystal cell with spacers with different bending conditions.
70
H. Hah et al. / Microelectronic Engineering 96 (2012) 67–70
and photopolymer substrates. A highly uniform display quality of a black and white state was observed from photoalignment layers with spacers under crossed polarizer. Light was transmitted in the state where the liquid crystals on the photoalignment layer were aligned by 45° between the polarizer and the analyzer. On the other hand, light was blocked when the light transmission axis of the polarizer corresponded to the photoalignment direction. Hence, it is clear that the anisotropic structure of the photoalignment layer was constructed by the unidirectional photodimerization of PVCi without any defects. In order to elucidate the change of liquid crystal orientation during the bending procedure of photo-induced liquid crystal cells with the spacers, we examined the order parameter (S) and azimuthal anchoring energy of the liquid crystal. The degree of bending is represented by the radius of the curvature of the cell (R) [22]. A smaller R value corresponds to a larger degree of bending. The photo-induced liquid crystal cell with spacers demonstrated good alignment properties in the liquid crystal under the bent condition of the cells as presented in Fig. 4. These results clearly reflect the impact of the spacers in photoalignment. Fig. 4a shows that the order parameter (S) of the dichroic dye for a photo-induced liquid crystal cell with the spacers decreased much less with curvature compared to that of a normal photo-induced liquid crystal cell. Order parameter, S, is defined as S = (Ak A\)/(Ak + 2A\), where Ak and A\ represent the absorbance of the doped dye parallel and perpendicular to the polarization direction of the irradiated UV light, respectively [18]. Fig. 4b is a plot of the azimuthal anchoring energy of the photo-induced liquid crystal cell with spacers and the normal photo-induced liquid crystal cell against the radius of curvature. The azimuthal anchoring energy was calculated using the following formula: Eu = 2dK1/x2 [19]. The x is defined as the distance between two black blushes where the direction is rotated from 45° to 135° with respect to the direction in a uniformly aligned region. K1 and d indicate the elastic constant for splay deformation and the thickness of the liquid crystal layer, respectively. In the photo-induced liquid crystal cell with spacers, the azimuthal anchoring energy was reasonably maintained as R decreased, while the azimuthal anchoring energy of the normal photo-induced liquid crystal cell decreased. Considering these results, we propose that sufficient mechanical strength of the spacers contributes to the preservation of the direction and stability of the liquid crystal molecular orientation in spite of application of external bending pressure. Fig. 4c indicates the polar plot of a normal photo-induced liquid crystal cell and a photo-induced liquid crystal cell with spacers. A small amount of dichroic dye shows a strong absorption at 653 nm, and from the angular dependence of the absorption at 653 nm in the polarized UV spectra of the liquid crystal cell, the distribution of the liquid crystal directors was determined. The liquid crystal orientation in the photoinduced liquid crystal cell with spacers was found to be quite stable, whereas the liquid crystal orientation in the normal photoinduced liquid crystal cell was almost completely destroyed as the degree of bending of the PES substrate increased. We attributed the deterioration in orientation to the randomized location of liquid crystal in the normal photo-induced liquid crystal cell due to the absence of spacers that sustain the uniform liquid crystal cell
gap. As a consequence, our results confirm that photo-induced liquid crystal cell with spacers maintains stable liquid crystal alignment against bending deformation.
4. Conclusion We demonstrated a new contact-free method of designing flexible LCDs with photoalignment layers with spacers on a plastic substrate for good liquid crystal orientation properties in the bending conditions. These photoalignment layers with spacers can effectively prevent the problems caused by the rubbing process, such as the need of a high process temperature and contamination. This method also provides mechanical stability for maintaining the stable orientation of liquid crystal against external forces. This approach is promising for practical application to flexible displays especially with a low temperature process, less contamination, and large scale displays using UV light. Acknowledgments This work was supported by a Grant from by the Korea Science and Engineering Foundation (KOSEF) Grant (WCU Program, 312008-000-10055-0) and Basic Research Science Program (20110026669) of National Research Foundation (NRF) funded by the Ministry of Education, Science, and Technology (MEST) of Korea. The authors also appreciate partial support from the Brain Korea 21 Program at KAIST. References [1] C. Dekker, Macromol. Rapid. Commun. 23 (2002) 1067. [2] P. Wang, A.G. MacDiarmid, Displays 28 (2002) 101. [3] Q. Wang, R. Guo, M.R. Daj, S.W. Kang, S. Kumar, Jpn. J. Appl. Phys. 46 (2007) 299. [4] S.C. Jeng, L.P. Hsin, Y.R. Lin, J.M. Ding, C.C. Liao, Jpn. J. Appl. Phys. 45 (2006) 6340. [5] H. Sato, H. Fujikake, Y. Iino, M. Kawakita, H. Kikuchi, Jpn. J. Appl. Phys. 41 (2002) 5302. [6] J.V. Haaren, Nature 411 (2001) 29. [7] K. Ichimura, Chem. Rev. 100 (2000) 1847. [8] H. Hah, S.-J. Sung, M. Han, S. Lee, J.-K. Park, Mater. Sci. Eng. C-Bio S 27 (2007) 798. [9] M. Schadt, H. Seiberle, A. Schuster, Nature 381 (1996) 212. [10] J. Lee, S. Lee, Y.-C. Jeong, K.Y. Cho, J.-K. Park, Opt. Exp. 17 (2009) 23565. [11] S.-J. Sung, M.R. Kim, D.-H. Kim, K.Y. Cho, Macromol. Res. 18 (2010) 614. [12] S.-J. Sung, J.-H. Yun, S. Lee, J.-K. Park, D.-H. Kim, K.Y. Cho, React. Funct. Polym. 70 (2010) 622. [13] S.-J. Sung, E.A. Jung, D.-H. Kim, D.-H. Son, J.-K. Kang, K.Y. Cho, Opt. Exp. 18 (2010) 11737. [14] B.M.I. van der Zande, S.J. Roosendaal, C. Doornkamp, J. Steenbakkers, J. Lub, Adv. Funct. Mater. 16 (2006) 791. [15] R. Penterman, S.I. Klink, H. de Koning, G. Nisato, D.J. Broer, Nature 417 (2002) 55. [16] H. Hah, S.-J. Sung, J.-K. Park, Appl. Phys. Lett. 90 (2006) 063508. [17] H. Hah, S.-J. Sung, M. Han, S.S. Lee, J.-K. Park, Displays 29 (2008) 478. [18] N. Kawatsuki, K. Goto, T. Kawatsuki, T. Yamamoto, Macromolecules 35 (2002) 706. [19] X.T. Li, D.H. Pei, S. Kobayashi, Y. Iimura, Jpn. J. Appl. Phys. 36 (1997) L432. [20] S.-J. Sung, K.Y. Cho, H. Hah, J. Lee, H.K. Shim, J.-K. Park, Polymer 47 (2006) 2314. [21] M. Schadt, K. Schmitt, V. Kozinkov, V. Chigrinov, Jpn. J. Appl. Phys. 31 (1992) 2155. [22] H. Sato, H. Fujikake, H. Kikuchi, T. Kurita, J. Appl. Phys. 42 (2003) 476.
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Accelerated Publication
Contact-free method to prepare photoalignment layers with spacers for flexible liquid crystal displays Hyundae Hah a,1, Jihye Lee a,1, Shi-Joon Sung b, Kuk Young Cho c, Jung-Ki Park a,d,⇑ a
Department of Chemical & Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea Green Energy Research Division, Daegu Gyeongbuk Institute of Science and Technology (DGIST), 50-1, Sang-Ri, Hyeonpung-Myeon, Dalsung-Gun, Daegu 711-873, Republic of Korea c Division of Advanced Materials Engineering, Kongju National University, 275 Budae-dong, Cheonan, Chungnam 331-717, Republic of Korea d Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 3 February 2012 Accepted 23 February 2012 Available online 30 March 2012 Keywords: Contact-free Flexible liquid crystal displays Photoalignment Linear UV light Microstructure
a b s t r a c t The alignment layers and spacers between two polymeric substrates are indispensable components for flexible liquid crystal displays (LCDs). Photoalignment layers with spacers were prepared by a contactfree method using UV patternable and UV-curable materials on polyethersulfone (PES) films. Wellaligned liquid crystal with the direction of the photoalignment layer was observed by polarized microscope. Spacers played a significant role in preventing disruption of the ordering of the liquid crystal. The photoalignment layer with spacers prepared on a polymeric substrate without any physical contact was free from problems that are found in the rubbing method or the micro stamp replication method. Additionally this method can be used in wide areas for the manufacturing of large sized displays. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction UV curable materials are gaining much interest due to their expansion to wider application fields [1]. UV curable materials are now used in the liquid crystal displays (LCD), which is a major commercialized display device. Sealing materials for the panel and prism of brightness enhancement films are good examples of UV curable materials. Recently, liquid crystal-based flexible displays have been highlighted because they offer the potential of developing display devices that are thinner, lighter, and more conformable than rigid glass-based displays [2–5]. Generally, in flexible displays, plastic film is used as a substrate rather than a rigid glass plate. In spite of the high flexibility of a plastic film, there are some obstacles to overcome: plastic substrates exhibit low thermal stability and are susceptible to generating static electricity, which leads to dust contamination [6]. This implies that it is difficult to fabricate the alignment layer for liquid crystal ordering because the conventional alignment layer is formed through imidization of poly(amic acid) at high temperature followed by rubbing with a cloth [7]. Thus, imprinting [8] and photoalignment [9] have been extensively ⇑ Corresponding author at: Department of Chemical & Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea. Tel.: +82 42 350 3965; fax: +82 42 350 3910. E-mail address: [email protected] (J.-K. Park). 1 These authors contributed equally to this work. 0167-9317/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.02.034
investigated as alternative techniques to the rubbing method to fabricate the liquid crystal alignment layers for flexible LCDs. For the photoalignment layer, we previously investigated photoreactive oligomeric materials using a linearly polarized UV light aligning mechanism [10–13]. Considering the flexible characteristics of the display panel of flexible LCDs, dimensional stability is necessary to maintain the cell gap, which results in stable and uniform liquid crystal orientation in a bending environment [14,15]. To overcome the problems generated by high temperature during the processing of plastic substrates for flexible LCDs, we have reported the fabrication of UV embossed alignment layers with phase-separated polymer walls as well as the production of UV embossed alignment layers with patterned spacers in our previously studies, and showed that the procedure is effective [16,17]. However using specific molds inevitably requires a contact process that is not completely free from contamination, and there is difficulty in increasing the size of the master mold for large area displays. In this report, we propose a completely contact-free method to fabricate photoalignment layers. We demonstrate photoalignment layers with spacers that are made by a photolithography process and a photo-induced method using UV curable materials. The alignment layer, formed by photo-irradiation using linearly polarized UV, exhibited a stable liquid crystal orientation. In addition, the flexible liquid crystal cell with spacers had an excellent alignment of liquid crystal even with the external deformation of the liquid crystal cell.
68
H. Hah et al. / Microelectronic Engineering 96 (2012) 67–70
Fig. 1. Schematic illustration of the procedures for the fabrication of PES films and photoalignment layers with spacers.
Fig. 2. Scanning electron microscope images of PES film with the spacers.
2. Experimental 2.1. Materials SU-8 (Microchem Co.) and polyethersulfone (PES) (i-component) films were used as a negative-thick photoresist and plastic substrates, respectively. Poly(vinyl cinnamate) (PVCi) was selected as a photo-reactive polymer. Cyclohexanone was used as a solvent to dissolve the PVCi. They were purchased from Aldrich Co. and used without further purification. The nematic liquid crystal (E7) was purchased from Merck Co.
ing step, photoreaction of the PVCi was induced by irradiating polarized UV for 10 min onto the PVCi coated PES film. Polarized UV light was obtained by passing light from a 300 W high-pressure mercury arc (Oriel) through a UV linear dichroic polarizer (27320, Oriel) and a UV filter 59800 (Oriel). The intensity of the irradiated UV light was 4 mW cm2. 2.4. Liquid crystal orientation and characterization
Fig. 1 illustrates the procedures for fabricating the PES films and photoalignment layers with spacers. The PES films with spacers were fabricated by a photolithography process. Negative photoresist SU-8 was spin-coated onto the PES film and the coated film was then baked for 10 min. The photoresist-coated PES film was irradiated under UV light (Contact aligner EVG640, EV group) through a photomask containing transparent areas. Unexposed areas were not cured and easily removed by a developing solution. As a result, a PES film with spacers was obtained.
A homogeneously aligned liquid crystal cell was made by placing nematic liquid crystal (E7) between a pair of PES films assembly coated with photoalignment layers with spacers. Liquid crystals were injected into the cell by capillary motion at 65 °C which induces the isotropic phase transition of liquid crystal. The texture of the liquid crystal in the cell was investigated with a Nikon Optophoto2-Pol polarized optical microscopy under the cross Nicoles. The director of the nematic liquid crystals in the liquid crystal cell was determined from the dichroic absorption of a dichroic dye (disperse blue 14, 0.2 wt.%) included in the nematic liquid crystals. The order parameter (S) of the liquid crystal cell was measured using one polarizer and a UV–Vis spectrometer (Shimadzu UV-1601) [18]. The azimuthal anchoring energy was estimated by measuring of the width of Neel wall (x) [19].
2.3. Fabrication of the photoalignment layer
3. Results and discussion
As demonstrated in Fig. 1, 2 wt.% solution of PVCi in cyclohexanone was spin-coated on the PES film with spacers at the speed of 1800 rpm followed by prebaking at 60 °C for 1 h. After the anneal-
Utilization of UV curable materials can provide a method free from physical contact to create the alignment structure on the plastic films. Using photoreactive materials, we fabricated
2.2. Preparation of PES films with a spacer structure
H. Hah et al. / Microelectronic Engineering 96 (2012) 67–70
photoalignment layers with spacers that have good cell gap uniformity. Fig. 2 shows the images of the scanning electron microscopy (SEM) for the photoalignment layers with spacers prepared with our proposed contact-free procedure. The height, the width, and the intervals of the spacer were 6 lm, 20 lm, and 300 lm, respectively. The most important issue of the flexible display based on liquid crystal is preventing distortion of the cell-gap during the deformation of the cell, and the spacer can support the regular liquid crystal cell gap even under an external bending force. In this report, a spacer with an isotropic configuration was used to re-
69
move the effect of the direction of bending on the interaction between the alignment layer and liquid crystal molecules. Fig. 3 shows polarized optical microscope images of a photoinduced liquid crystal cell with spacers. The photodimerization reaction of PVCi by polarized UV irradiation is a [2 + 2] cycloaddition reaction based on the mechanism of a pericyclic reaction. This reaction occurs in the photoalignment layer resulting in the generation of structural anisotropy of the photo-reactive materials [20,21]. Orientation of liquid crystal molecules is usually induced by van der Waals interaction between liquid crystal molecules
Fig. 3. Polarized optical microscope images of a photo-induced liquid crystal cell with spacers. The directions of polarizer (A and P) and the photoalignment direction of LC (L) are noted as arrows in the images.
Fig. 4. Change of the (a) order parameter, (b) azimuthal anchoring energy, and (c) polar plot in a photo-induced liquid crystal cell with spacers with different bending conditions.
70
H. Hah et al. / Microelectronic Engineering 96 (2012) 67–70
and photopolymer substrates. A highly uniform display quality of a black and white state was observed from photoalignment layers with spacers under crossed polarizer. Light was transmitted in the state where the liquid crystals on the photoalignment layer were aligned by 45° between the polarizer and the analyzer. On the other hand, light was blocked when the light transmission axis of the polarizer corresponded to the photoalignment direction. Hence, it is clear that the anisotropic structure of the photoalignment layer was constructed by the unidirectional photodimerization of PVCi without any defects. In order to elucidate the change of liquid crystal orientation during the bending procedure of photo-induced liquid crystal cells with the spacers, we examined the order parameter (S) and azimuthal anchoring energy of the liquid crystal. The degree of bending is represented by the radius of the curvature of the cell (R) [22]. A smaller R value corresponds to a larger degree of bending. The photo-induced liquid crystal cell with spacers demonstrated good alignment properties in the liquid crystal under the bent condition of the cells as presented in Fig. 4. These results clearly reflect the impact of the spacers in photoalignment. Fig. 4a shows that the order parameter (S) of the dichroic dye for a photo-induced liquid crystal cell with the spacers decreased much less with curvature compared to that of a normal photo-induced liquid crystal cell. Order parameter, S, is defined as S = (Ak A\)/(Ak + 2A\), where Ak and A\ represent the absorbance of the doped dye parallel and perpendicular to the polarization direction of the irradiated UV light, respectively [18]. Fig. 4b is a plot of the azimuthal anchoring energy of the photo-induced liquid crystal cell with spacers and the normal photo-induced liquid crystal cell against the radius of curvature. The azimuthal anchoring energy was calculated using the following formula: Eu = 2dK1/x2 [19]. The x is defined as the distance between two black blushes where the direction is rotated from 45° to 135° with respect to the direction in a uniformly aligned region. K1 and d indicate the elastic constant for splay deformation and the thickness of the liquid crystal layer, respectively. In the photo-induced liquid crystal cell with spacers, the azimuthal anchoring energy was reasonably maintained as R decreased, while the azimuthal anchoring energy of the normal photo-induced liquid crystal cell decreased. Considering these results, we propose that sufficient mechanical strength of the spacers contributes to the preservation of the direction and stability of the liquid crystal molecular orientation in spite of application of external bending pressure. Fig. 4c indicates the polar plot of a normal photo-induced liquid crystal cell and a photo-induced liquid crystal cell with spacers. A small amount of dichroic dye shows a strong absorption at 653 nm, and from the angular dependence of the absorption at 653 nm in the polarized UV spectra of the liquid crystal cell, the distribution of the liquid crystal directors was determined. The liquid crystal orientation in the photoinduced liquid crystal cell with spacers was found to be quite stable, whereas the liquid crystal orientation in the normal photoinduced liquid crystal cell was almost completely destroyed as the degree of bending of the PES substrate increased. We attributed the deterioration in orientation to the randomized location of liquid crystal in the normal photo-induced liquid crystal cell due to the absence of spacers that sustain the uniform liquid crystal cell
gap. As a consequence, our results confirm that photo-induced liquid crystal cell with spacers maintains stable liquid crystal alignment against bending deformation.
4. Conclusion We demonstrated a new contact-free method of designing flexible LCDs with photoalignment layers with spacers on a plastic substrate for good liquid crystal orientation properties in the bending conditions. These photoalignment layers with spacers can effectively prevent the problems caused by the rubbing process, such as the need of a high process temperature and contamination. This method also provides mechanical stability for maintaining the stable orientation of liquid crystal against external forces. This approach is promising for practical application to flexible displays especially with a low temperature process, less contamination, and large scale displays using UV light. Acknowledgments This work was supported by a Grant from by the Korea Science and Engineering Foundation (KOSEF) Grant (WCU Program, 312008-000-10055-0) and Basic Research Science Program (20110026669) of National Research Foundation (NRF) funded by the Ministry of Education, Science, and Technology (MEST) of Korea. The authors also appreciate partial support from the Brain Korea 21 Program at KAIST. References [1] C. Dekker, Macromol. Rapid. Commun. 23 (2002) 1067. [2] P. Wang, A.G. MacDiarmid, Displays 28 (2002) 101. [3] Q. Wang, R. Guo, M.R. Daj, S.W. Kang, S. Kumar, Jpn. J. Appl. Phys. 46 (2007) 299. [4] S.C. Jeng, L.P. Hsin, Y.R. Lin, J.M. Ding, C.C. Liao, Jpn. J. Appl. Phys. 45 (2006) 6340. [5] H. Sato, H. Fujikake, Y. Iino, M. Kawakita, H. Kikuchi, Jpn. J. Appl. Phys. 41 (2002) 5302. [6] J.V. Haaren, Nature 411 (2001) 29. [7] K. Ichimura, Chem. Rev. 100 (2000) 1847. [8] H. Hah, S.-J. Sung, M. Han, S. Lee, J.-K. Park, Mater. Sci. Eng. C-Bio S 27 (2007) 798. [9] M. Schadt, H. Seiberle, A. Schuster, Nature 381 (1996) 212. [10] J. Lee, S. Lee, Y.-C. Jeong, K.Y. Cho, J.-K. Park, Opt. Exp. 17 (2009) 23565. [11] S.-J. Sung, M.R. Kim, D.-H. Kim, K.Y. Cho, Macromol. Res. 18 (2010) 614. [12] S.-J. Sung, J.-H. Yun, S. Lee, J.-K. Park, D.-H. Kim, K.Y. Cho, React. Funct. Polym. 70 (2010) 622. [13] S.-J. Sung, E.A. Jung, D.-H. Kim, D.-H. Son, J.-K. Kang, K.Y. Cho, Opt. Exp. 18 (2010) 11737. [14] B.M.I. van der Zande, S.J. Roosendaal, C. Doornkamp, J. Steenbakkers, J. Lub, Adv. Funct. Mater. 16 (2006) 791. [15] R. Penterman, S.I. Klink, H. de Koning, G. Nisato, D.J. Broer, Nature 417 (2002) 55. [16] H. Hah, S.-J. Sung, J.-K. Park, Appl. Phys. Lett. 90 (2006) 063508. [17] H. Hah, S.-J. Sung, M. Han, S.S. Lee, J.-K. Park, Displays 29 (2008) 478. [18] N. Kawatsuki, K. Goto, T. Kawatsuki, T. Yamamoto, Macromolecules 35 (2002) 706. [19] X.T. Li, D.H. Pei, S. Kobayashi, Y. Iimura, Jpn. J. Appl. Phys. 36 (1997) L432. [20] S.-J. Sung, K.Y. Cho, H. Hah, J. Lee, H.K. Shim, J.-K. Park, Polymer 47 (2006) 2314. [21] M. Schadt, K. Schmitt, V. Kozinkov, V. Chigrinov, Jpn. J. Appl. Phys. 31 (1992) 2155. [22] H. Sato, H. Fujikake, H. Kikuchi, T. Kurita, J. Appl. Phys. 42 (2003) 476.