An in-plane switching liquid crystal (LC) cell using a glass substrate and a photoinduced polymer layer is demonstrated. The fabrication process is based on the electrodynamics of dielectric fluids. When the fringing field is present, the LC molecules tend to aggregate in the strong electric field regions while the monomers diffuse to the weak field regions. After photopolymerization, the LC molecules are confined by a thin polymer layer and polymer walls which define the cell gap. This approach enables single-substrate large panel display devices to be fabricated

Lin, Yi-Hsin

Ren, Hongwen

Wu, Shin-Tson

English

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University of Central Florida STARS Faculty Bibliography 2000s Faculty Bibliography 1-1-2007 Single glass substrate liquid crystal device using electric field-enforced phase separation and photoinduced polymerization Hongwen Ren University of Central Florida Shin-Tson Wu University of Central Florida Yi-Hsin Lin Find similar works at: https://stars.library.ucf.edu/facultybib2000 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 2000s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Ren, Hongwen; Wu, Shin-Tson; and Lin, Yi-Hsin, "Single glass substrate liquid crystal device using electric field-enforced phase separation and photoinduced polymerization" (2007). Faculty Bibliography 2000s. 7568. https://stars.library.ucf.edu/facultybib2000/7568 Appl. Phys. Lett. 90, 191105 (2007); https://doi.org/10.1063/1.2737366 90, 191105© 2007 American Institute of Physics.Single glass substrate liquid crystaldevice using electric field-enforced phaseseparation and photoinduced polymerizationCite as: Appl. Phys. Lett. 90, 191105 (2007); https://doi.org/10.1063/1.2737366Submitted: 24 March 2007 . Accepted: 13 April 2007 . Published Online: 07 May 2007Hongwen Ren, Shin-Tson Wu, and Yi-Hsin LinARTICLES YOU MAY BE INTERESTED INSingle-substrate cholesteric liquid crystal displays by colloidal self-assemblyApplied Physics Letters 88, 043502 (2006); https://doi.org/10.1063/1.2167398Network morphology of polymer stabilized liquid crystalsApplied Physics Letters 71, 2454 (1997); https://doi.org/10.1063/1.120087Fast-switching initially-transparent liquid crystal light shutter with crossed patternedelectrodesAIP Advances 5, 047118 (2015); https://doi.org/10.1063/1.4918277Single glass substrate liquid crystal device using electric field-enforcedphase separation and photoinduced polymerizationHongwen Ren and Shin-Tson WuaCollege of Optics and Photonics, University of Central Florida, Orlando, Florida 32816Yi-Hsin LinDepartment of Photonics, Institute of Electro-Optical Engineering, National Chiao Tung University,Hsinchu 30010, TaiwanReceived 24 March 2007; accepted 13 April 2007; published online 7 May 2007An in-plane switching liquid crystal LC cell using a glass substrate and a photoinduced polymerlayer is demonstrated. The fabrication process is based on the electrodynamics of dielectric fluids.When the fringing field is present, the LC molecules tend to aggregate in the strong electric fieldregions while the monomers diffuse to the weak field regions. After photopolymerization, the LCmolecules are confined by a thin polymer layer and polymer walls which define the cell gap. Thisapproach enables single-substrate large panel display devices to be fabricated. © 2007 AmericanInstitute of Physics. DOI: 10.1063/1.2737366Liquid crystals LCs have been widely used fordisplays,1 phase modulators,2,3 adaptive-focus lens,4,5 andmany other optical components. Due to the fluidic nature,usually LC molecules are confined between two glass sub-strates. Recently, LC devices using only one glass substratehave been demonstrated in an in-plane switching IPScell.6–9 The reasons for choosing IPS cells are obvious: 1the interdigitated electrodes are in the same substrate whichcan be a glass or plastic substrate overcoated with a LCalignment layer, and 2 the device exhibits a wide viewingangle. Compared to conventional two glass substrate LC de-vices, the single substrate LC devices are attractive becauseof their lighter weight and potentially lower cost.A single-substrate IPS cell was first demonstrated usingphotoenforced anisotropic phase separation between LC andprepolymer.6 After photopolymerization, the prepolymerforms a thin layer which is floated on the LC surface. Thestability of the device is a concern due to the lack of a strongsolid support to bear the weight of the polymer layer. Toovercome this problem, a two-step photopolymerization pro-cess was developed.8 The first step is to selectively exposemonomers through a photomask for forming polymer walls.The second step is to remove the photomask and expose theuncured monomers so that they could undergo anisotropicpolymerization. Although these devices possess a good me-chanical stability, this approach encounters two problems.Firstly, an error contraposition of the photomask would causethe formed polymer walls to occupy the active regions. Sec-ondly, a high resolution photomask can severely diffract lightwhich, in turn, widens the polymer walls during phase sepa-ration and decreases the aperture ratio of the active pixels.Recently, our group developed an IPS LC cell using asingle glass substrate and an anisotropic polymer film.9 Thiscell exhibits almost the same electro-optical properties asthat of a conventional IPS LC cell using two glass substrates.However, this kind of LC cell also suffers from weak me-chanical stability because it has no spacers to support the softpolymer film. Therefore, it is difficult to control the unifor-mity while making a large LC panel.In this letter, we demonstrate a single-substrate IPS cellbased on electric field-enforced anisotropic phase separationbetween the LC and the liquid monomer. Using this ap-proach, the LC and monomer mixture can be phase separatedon a substrate surface. After photopolymerization, both poly-mer walls and top polymer layer are formed simultaneously.Such a LC device possesses a good mechanical stabilitywhich enables the fabrication of large panels with high reso-lution. Moreover, the fabrication process is fairly simple.Electric field-induced anisotropic phase separation wasfirst demonstrated in reflective displays employing two glasssubstrates.10 In this approach, the polymer in the cell wasdistributed in the interpixel regions working as walls. How-ever, in our device a portion of the polymers are used aswalls and the majority are used as top cover layer to preventthe LC from leaking.Figures 1a–1d illustrate the fabrication procedures ofour LC cell. A glass substrate with interdigitated indium tinoxide ITO electrodes was chosen as a bottom substrate.The ITO glass was overcoated with a thin polyimide align-ment layer and buffed along the ITO strip direction. A mix-ture of LC and photocurable monomer was spin coated overaElectronic mail: swu@mail.ucf.eduFIG. 1. Color online Procedures for fabricating a single glass substrate IPScell: a coating film on the substrate surface, b applying a voltage to theITO strips, c UV curing, and d removing the voltage.APPLIED PHYSICS LETTERS 90, 191105 20070003-6951/2007/9019/191105/3/$23.00 © 2007 American Institute of Physics90, 191105-1the substrate surface, as shown in Fig. 1a. When a voltageis applied to the ITO strips, the dielectric fluid experiencesan electric field and bears a force. The force can be expressedas11F = P · E , 1where P is the polarization of dielectric material and E theelectric field strength. Further, P is related to the electric fieldasP = 0 − 1E , 2where 0 is the permittivity of free space and 1 repre-sents the dielectric constant of the LC or the liquid monomer.Since the LC employed has a larger dielectric constant thanthe liquid monomer, the LC molecules will experience agreater force from the electric field. Under such a circum-stance, the LC molecules would drift toward the high electricfield regions while the monomers aggregate in the weak elec-tric field regions, as Fig. 1b shows. In an IPS cell, theweakest electric field is in the centers of ITO electrodes.After phase separation, the liquid monomers are cured by anUV light, as shown in Fig. 1c. After polymerization, bothpolymer layer and polymer walls are formed at the sametime. The polymer layer covers the active LC regions, whilethe polymer walls sit on top of ITO electrodes, as shown inFig. 1d. In a real display panel, these inactive regions arecovered by the absorbing black matrices for improving con-trast ratio. When the voltage is removed, the LC moleculesare reoriented along the rubbing direction due to the anchor-ing effect of the alignment layer. As indicated in Fig. 1d,the rubbing direction of the bottom substrate is pointing intothe paper.Based on the above mechanism, we fabricated an IPSLC cell with a single glass substrate. The ITO electrode onthe bottom substrate surface was etched to form interdigi-tated chevron shapes in order to create multiple domainstructures for widening the viewing angle. The angle of thezigzag ITO strips is 150°. The width of the electrode strip is4 m and the electrode gap is 10 m. The surface of thesubstrate was coated with a thin polyimide layer and buffedin one direction at 15° with respect to the electrode strips. Amixture consisting of 55 wt % nematic LC E7, Merck LC12 and 45 wt % photocurable monomer NOA65 NorlandAdhesive p5 at f =1 kHz was spin coated at4000 rpm for 15 s on a glass substrate at room tempera-ture T21 °C. The volume of the LC is approximately thesame as that of the monomer because E7 has a slightlyhigher density than NOA65. An electric field of 5Vrms/mwas applied to the ITO strips for 1 min. Afterwards, the mix-ture was exposed to an UV light =365 nm at intensity of35 mW/cm2 for 15 min.After polymerization the cell is transparent in thevoltage-off state. To observe the phase separation result, weplaced the sample under a polarizing optical microscope witha white light lamp. The axes of the polarizer and analyzer arecrossed. When the sample was rotated along the azimuthaldirection, the transmitted light intensity changed correspond-ingly. When the rubbing direction of the IPS cell was ori-ented parallel to the optic axis of the entrance polarizer, adark state was obtained, as shown in Fig. 2a. Figure 2bshows the on-state transmittance of the IPS cell atV=20Vrms. The chevron structures are clearly observed. Theblack grids indicate that the polymer walls formed on themiddle of ITO strips are isotropic. The width of the polymerwall is about 2 m which is less than the width 4 m ofthe ITO strips. In in-plane switching, the fringing field isinhomogeneous and the weakest field occurs at the middle ofthe ITO electrodes. These polymer walls function as spacersfor controlling the cell gap and supporting the top polymerlayer. Due to the polymer networks, the IPS cell should havea good mechanical stability and uniform cell gap.The electro-optical properties of the LC cell were stud-ied by measuring the transmittance of the cell using a He–Nelaser =633 nm beam. The cell was placed between twocrossed polarizers. The transmitted light was detected by aphotodiode set at 20 cm away from the sample. The responsetime of the cell was recorded by a digital oscilloscope. Thevoltage dependent transmittance was collected by a LABVIEWdata acquisition system.Figure 3 shows the voltage dependent transmittance ofthe IPS cell. At V=0, a fairly good black state is observed.As the applied voltage exceeds 5Vrms, the transmission be-gins to increase indicating that the LC directors are reori-ented by the electric field. At 20Vrms, the transmissionreaches a maximum. Continue to increase voltage leads to adecreased transmittance. This is because the total phase re-tardation of the LC cell exceeds 1.The contrast ratio was measured to be 200:1. Forcomparison, the contrast ratio of a conventional IPS cell us-ing two glass substrates exceeds 500:1. By a large, the bot-tom alignment layer still aligns the bulk LC directors reason-ably well. At a closer look on Fig. 2a, we find a weak lightleakage in the LC-rich regions. To investigate what causesthis light leakage, we zoomed into the top polymer layer ofthe LC cell. As shown in Fig. 2c, some LC bubbles with2–3 m in size are kept within the top polymer layer. Theseresults imply that the LC and monomer are not completelyphase separated so that a small amount of LC bubbles areretained in the polymer matrices. These micron-sized LCdroplets cause light leakage between crossed polarizers and alittle bit of light scattering. As a result, the device contrastratio is degraded.FIG. 2. Color online Microscope photos of the IPS LC cell: a V=0, bV=20Vrms, and c magnified image of the top polymer layer.FIG. 3. Color online Voltage dependent transmittance of the IPS cell: LCemployed is E7 and =633 nm.191105-2 Ren, Wu, and Lin Appl. Phys. Lett. 90, 191105 2007The response time of the IPS cell was measured using asquare voltage burst at f =1 kHz and 20Vrms. Results areshown in Fig. 4. The measured rise and decay times are 10and 75 ms, respectively. In a vertical alignment or homo-geneous LC cell, the optical decay time decay which is de-fined as 100%–10% transmittance change is related to theLC directors decay time 0 as follows:12decay =02ln o/2sin−10.1 sino/2 . 3In Eq. 3, 0=1d2 /k222 is the LC director relaxation timewhere 1 is the rotational viscosity, d is the LC cell gap, k22is the twist elastic constant, and 0 is the initial phase of theLC cell which is usually 1. If we neglect the logarithmicterm in Eq. 3, we find decay0 /2. In our LC cell, 1 andk22 are 0.25 Pa s and 8.8 pN, respectively. From Eq. 3, d isestimated to be 7.2 m. Because the monomer NOA65and E7 have about the same volume, the thickness of the toppolymer layer is 7 m.The E7 IPS cell we fabricated is for proving conceptonly because its response time is too slow for practical ap-plications. Two factors causing the observed slow responsetime are thick cell gap and high viscosity. In reality, the thinfilm transistor–grade LC mixture has a two to three timeslower viscosity than E7 and the cell gap is about 4 m. Tomake the cell gap thinner while retaining a robust top poly-mer layer and rigid polymer walls, we should decrease theLC concentration in the LC/monomer mixture. A thinner LCcell improves not only response time but also molecularalignment because the influence of bottom alignment layer tothe bulk LC molecules is stronger. Better LC alignmentwould result in a higher contrast ratio.In conclusion, we have demonstrated a single glass sub-strate LC cell using electric field-enforced stratificationmethod. Such a LC cell exhibits advantages in simple fabri-cation, lightweight, and good mechanical stability. This ap-proach opens a possibility for making single-substrate largepanel display devices.The authors would like to thank H. Xianyu and S. Gauzafor their experimental assistance and useful discussions.1S. T. Wu and D. K. Yang, Reflective Liquid Crystal Displays Wiley, NewYork, 2001.2D. P. Resler, D. S. Hobbs, R. C. Sharp, L. J. Fridman, and T. A.Dorschner, Opt. Lett. 21, 689 1996.3R. L. Sutherland, L. V. Natarajan, V. P. Tondiglia, and T. J. Bunning,Chem. Mater. 5, 1533 1993.4S. Sato, Jpn. J. Appl. Phys. 18, 1679 1979.5H. Ren, Y. H. Fan, S. Gauza, and S. T. Wu, Appl. Phys. Lett. 84, 47892004.6V. Vorflusev and S. Kumar, Science 283, 1903 1999.7I. Kim, J. H. Kim, D. Kang, D. M. A. Kooijman, and S. Kumar, J. Appl.Phys. 92, 7699 2002.8R. Penterman, S. T. Klink, H. de Koning, G. Nisato, and D. J. Broer,Nature London 417, 55 2002.9Y. H. Lin, H. Ren, S. Gauza, Y. H. Wu, Y. Zhao, J. Y. Fan, and S. T. Wu,J. Disp. Technol. 2, 21 2006.10Y. Kim, J. Francl, B. Taheri, and J. L. West, Appl. Phys. Lett. 72, 22531998.11P. Penfield and H. A. Haus, Electrodynamics of Moving Media MIT,Cambridge, 1967.12H. Wang, T. X. Wu, X. Zhu, and S. T. Wu, J. Appl. Phys. 95, 5502 2004.FIG. 4. Measured response times of the IPS cell: rise time10 ms anddecay time75 ms.191105-3 Ren, Wu, and Lin Appl. Phys. Lett. 90, 191105 2007

Single glass substrate liquid crystal device using electric field-enforced phase separation and photoinduced polymerization

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Single glass substrate liquid crystal device using electric field-enforced phase separation and photoinduced polymerization

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