A wide-view transflective liquid crystal display (TR-LCD) using a single cell gap multidomain vertical alignment (MVA) cell is proposed. Unlike a conventional MVA which uses physical protrusions to form multidomain structures, our TR-LCD uses slit-induced longitudinal and fringe electric fields to generate multidomains. As a result, wide-view, high contrast ratio, and high transmittance are obtained. By varying the slit density in the transmissive and reflective regions, the optical path-length disparity between these regions can be balanced. Potential application of this TR-LCD for mobile displays is emphasized. (C) 2008 American Institute of Physics

Ge, Zhibing

Lu, Ruibo

Wu, Shin-Tson

English

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University of Central Florida STARS Faculty Bibliography 2000s Faculty Bibliography 1-1-2008 Wide-view and single cell gap transflective liquid crystal display using slit-induced multidomain structures Ruibo Lu University of Central Florida Zhibing Ge University of Central Florida Shin-Tson Wu University of Central Florida 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 Lu, Ruibo; Ge, Zhibing; and Wu, Shin-Tson, "Wide-view and single cell gap transflective liquid crystal display using slit-induced multidomain structures" (2008). Faculty Bibliography 2000s. 652. https://stars.library.ucf.edu/facultybib2000/652 Appl. Phys. Lett. 92, 191102 (2008); https://doi.org/10.1063/1.2926669 92, 191102© 2008 American Institute of Physics.Wide-view and single cell gap transflectiveliquid crystal display using slit-inducedmultidomain structuresCite as: Appl. Phys. Lett. 92, 191102 (2008); https://doi.org/10.1063/1.2926669Submitted: 19 February 2008 . Accepted: 21 April 2008 . Published Online: 13 May 2008Ruibo Lu, Zhibing Ge, and Shin-Tson WuARTICLES YOU MAY BE INTERESTED INWide-view transflective liquid crystal display for mobile applicationsApplied Physics Letters 91, 231108 (2007); https://doi.org/10.1063/1.2822410Transflective liquid crystal display using commonly biased reflectorsApplied Physics Letters 90, 221111 (2007); https://doi.org/10.1063/1.2745206Electro-optic characteristics and switching principle of a nematic liquid crystal cell controlledby fringe-field switchingApplied Physics Letters 73, 2881 (1998); https://doi.org/10.1063/1.122617Wide-view and single cell gap transflective liquid crystal displayusing slit-induced multidomain structuresRuibo Lu, Zhibing Ge, and Shin-Tson WuaCollege of Optics and Photonics, University of Central Florida, Orlando, Florida 32816, USAReceived 19 February 2008; accepted 21 April 2008; published online 13 May 2008A wide-view transflective liquid crystal display TR-LCD using a single cell gap multidomainvertical alignment MVA cell is proposed. Unlike a conventional MVA which uses physicalprotrusions to form multidomain structures, our TR-LCD uses slit-induced longitudinal and fringeelectric fields to generate multidomains. As a result, wide-view, high contrast ratio, and hightransmittance are obtained. By varying the slit density in the transmissive and reflective regions, theoptical path-length disparity between these regions can be balanced. Potential application of thisTR-LCD for mobile displays is emphasized. © 2008 American Institute of Physics.DOI: 10.1063/1.2926669Transflective liquid crystal displays TR-LCDs havebeen widely used for mobile displays because of their sun-light readability. Some general requirements for a TR-LCDare wide viewing angle, high optical efficiency, high contrastratio, low power consumption, and good color performance.To meet these challenges, several LC configurations, e.g.,twisted-nematic,1,2 homogeneous alignment,3 vertical align-ment VA,4 in-plane switching,5,6 and fringe field switching7have been proposed. Among these approaches, TR-LCDs canbe further divided into double cell gaps versus single cellgap, and single thin film transistor TFT versus doubleTFTs.8 Presently, most TR-LCDs adopt the double cell gapapproach for two reasons: 1 both transmissive T and re-flective R modes can achieve maximum light efficiency,and 2 the gamma curve matching between the voltage-dependent transmittance VT and reflectance VR is almostperfect. However, two shortcomings are found: 1 the T re-gion has a slower response time than the R region because itscell gap is about twice thicker than that of the R region, and2 the viewing angle is relatively narrow, especially whenthe homogeneous cells are employed.9 To widen viewingangle, a special rodlike LC polymeric compensation film hasto be used.10Film-compensated multidomain vertical alignmentMVA cells offer an excellent contrast ratio and wide view-ing angle, and have been extensively used in LCD TVs.11,12Both double and single cell gaps, wide-view TR-LCDs usingMVA have been demonstrated.13,14 In the single cell gap ap-proach, two reflective regions are required in order to matchthe VR with VT curves. Besides the fabrication complexityof such a device structure, a MVA TR-LCD has an intrinsicproblem: It uses physical protrusions to form multiple do-mains. The light leakage from the protrusion edges degradesthe contrast ratio and the dead zones above the opaque pro-trusions in the voltage-on state lower the optical efficiency.These problems become more pronounced in small sizedLCD panels because of the smaller aperture ratio and in-creased protrusion density.In this paper, we demonstrate a wide-view, single cellgap TR-LCD using MVA in which multidomain structuresare formed by slit-induced longitudinal and fringe electricfields. Without using physical protrusions, this MVA TR-LCD exhibits a high optical efficiency, high contrast ratio,and wide viewing angle. Moreover, by optimizing the slitwidth and gap, the VT and VR curves reasonably matchwell.Figure 1 shows a typical unit pixel and device configu-ration of the proposed MVA TR-LCD between two crossedcircular polarizers. Each unit pixel is divided into T and Rsubpixels. As shown in Fig. 1a, transparent indium tin ox-ide ITO layer is first deposited on the inner surface of thebottom glass substrate. Then, a portion of the ITO layer isetched off and covered with a metal layer such as aluminumAl to form bumpy reflector for the R mode. The region notcovered by the reflector works as T mode. The Al reflectorand ITO are connected to form pixel electrode which is con-nected to the same TFT. Therefore, the R and T subpixels aredriven by a single TFT while keeping the same cell gap.On the top substrate, some slits on the R and T regionsare formed in the ITO common electrode. Here, the reflectiveopening ratio ORR is defined as the effective slit projectionaElectronic mail: swu@mail.ucf.edu.FIG. 1. Color online a The unit pixel and b device configuration of theproposed VA TR-LCD with crossed circular polarizers. The alignment layersare not shown here.APPLIED PHYSICS LETTERS 92, 191102 20080003-6951/2008/9219/191102/3/$23.00 © 2008 American Institute of Physics92, 191102-1area corresponding to that of the bottom R region, and trans-missive opening ratio ORT as the effective slit projectionarea corresponding to that of the bottom T region. The open-ing ratio is mainly determined by the slit shape, size, and theneighboring slit gap, D1 and D2, where the slit gap is usuallylarger than the slit width, S1 and S2, and the cell gap, d. Theinner sides of both top and bottom substrates have a thinpolyimide film not shown to align the LC molecules.As Fig. 1b shows, the LC directors are verticallyaligned on the substrate surfaces without rubbing processand the cell is in the normally black mode under crossedcircular polarizers. When the applied voltage exceeds athreshold, the fringe electric fields surrounding the slits aregenerated while the longitural electric fields mainly exist in-between the flat common and pixel ITO electrodes, as shownin Fig. 1a, where the red dashed lines with arrows repre-sent the electric field directions. The LC with 0 direc-tors would be reoriented perpendicular to the electric fields.Therefore, the backlight transmits through the crossed circu-lar polarizers in the T mode, and the ambient light is re-flected by the bumpy reflectors and exits through the topcircular polarizer in the R mode. The longitudinal electricfields play the primary role to bend the LC directors. How-ever, this directors bending is also guided by the fringe fieldsnearby the slit edges which force the LC directors to tilt todifferent directions. As a result, multidomains, e.g., four do-mains, are formed in a unit pixel when zigzag electrodes areconfigured.11,15To obtain maximum transmittance and reflectance, theeffective phase retardation in the T region and R regionshould satisfy the following relationship: dneff,T=2dneff,R /2, where n is the LC birefringence and is the wavelength of the incident light. It is possible to tunethe phase retardation by varying the opening areas or slitdensity as Fig. 1 shows. Because the ambient light traversesthe LC layer twice in the R region, we should design the slitsto have ORRORT in order to reduce the effective LC retar-dation in this region. Correspondingly, the longitudinal elec-tric fields in the R region are weaker than those in the Tregion. The less reoriented LC directors in the R region leadto a smaller phase retardation. However, because of doublepass, its resultant phase retardation is similar to that of the Tregion. Thus, a good match between VT and VR curves canbe achieved.Let us take one example to illustrate the design principleand device performance. Here, we select the repeated unitpixel size of the strip-shaped structure at 7525 m2, andchoose the slit width S1=4 m and slit gap D1=18 m forthe T region, and slit gap D2=12 m for the R region. Aguiding slit S2 whose width is 2 m is located at the centerof D2 in the R region to avoid the domain corruption in thevoltage-on state. Correspondingly, the reflective opening ra-tio ORR is 31% and the ORT is 18%. The cell gap betweenthe top and bottom substrates is 4 m. A negative LC mix-ture Merck MLC-6608 n=0.083 at =550 nm, =−4.2and rotational viscosity 1=0.186 Pa s is vertically alignedto the substrates in the initial state. Its azimuthal angle is 0°and the pretilt angle is 90°.Figure 2 shows the simulated LC director distribution ofthe proposed transflective VA cell with the strip-shaped mul-tislits at V=5 Vrms between the common and pixel elec-trodes. The distribution is the side view cut from the centerof the pixel unit along the Y-axis direction. It is observed thatthe LC directors are reoriented perpendicular to the electricfields due to the combined effect from the fringing field andthe longitural electric field in both R and T parts. Therefore,in a voltage-on state a two-domain VA TR-LCD is formedfrom the strip-shaped multislits in each unit pixel.Figure 3 is a plot of VT and VR curves of the transflec-tive VA cell with the strip multislits under crossed circularpolarizers at =550 nm. The T and R parts have the samethreshold voltage at 2.25 Vrms. Both VT and VR curves showa high transmittance/reflectance at 5 Vrms a typical TFTdriving voltage, which is 30.2% for the T region and 30.1%for the R region. The employed circular polarizer pair has amaximum transmittance of 35%. Therefore, the strip-shapedmultislit transflective VA cell has a high normalized opticalefficiency of 86% for both T and R regions at 5 Vrms.From the inset in Fig. 3, the VT and VR curves fairlymatch well when the applied voltage is below 5 Vrms. Toquantitatively characterize the gamma curve matching be-tween the VT and VR curves, the root-mean-square rmsvalue of the difference between the sampled transmittanceand reflectance values is evaluated using rms=i=1N Ti−Ri2 /N, where N is the number of samplepoints, Ti the transmittance, and Ri the reflectance at a givenFIG. 2. Color online The simulated LC director distribution of the two-domain VA TR-LCD with strip-shaped multislits. The LC employed isMerck MLC-6608 and the applied voltage is V=5 Vrms.FIG. 3. Color online The VT and VR curves of the two-domain VA TR-LCD with the strip-shaped multislits under crossed circular polarizers. TheLC material is MLC-6608 and =550 nm. The inset shows the normalizedVT and VR curves.191102-2 Lu, Ge, and Wu Appl. Phys. Lett. 92, 191102 2008voltage Vi. Based on this definition, a smaller rms value im-plies to a better match between the VT and VR curves. Forthe VT and VR curves shown in Fig. 3, by sampling N=256 voltage points between V=0 Vrms corresponding togray level 0 and V=5 Vrms gray level 255, the calculatedrms difference between the VR and VT curves is 4%. Itindicates that the TR-LCD can be driven by a single TFTwhile possessing the matched VT and VR curves.It is commonly known that optical phase compensationfilms are required in order to reduce the light leakage atoblique angles of a VA LC cell for achieving wide viewingangle.16 In our simulations, we use the wide-view circularpolarizers17 which consist of a series of uniaxial compensa-tion films to work as broadband circular polarizers for wid-ening the viewing angle of the two-domain VA TR-LCD, asFig. 1b shows. A negative c-plate whose dn=−332 nm isadded after the first wide-view circular polarizer to cancelthe off-axis light leakage from the LC layer. The contrastratio is calculated between 0 and 5 Vrms under a white lightsource mixed with the red, green, and blue primaries at 3:6:1weighting ratios.Figure 4a is a plot of the viewing angle of the TR-LCDin the T region. The isocontrast contour exceeds 3000:1within the 	30° viewing cone. Within the 	80° view cone,the contrast ratio keeps higher than 75:1. As for the R region,the 250:1 isocontrast contour line almost covers the wholehorizontal directions, as shown in Fig. 4b, but the viewingcone shrinks to 30° at the 45°/225° direction. For the 10:1contrast ratio, the viewing cone is as wide as 	80°. Thisdemonstrates that the proposed TR-LCD has a high contrastratio and wide-view angle in both T and R modes. Theseproperties are especially desirable for high performance mo-bile displays.In conclusion, we have proposed a wide-view single cellgap TR-LCD which utilizes the slit-induced fringe and lon-gitudinal electric fields to form multiple LC domains withoutusing physical protrusions. This TR-LCD exhibits a high op-tical efficiency, high contrast ratio, wide viewing angle inboth T and R regions, and matched gamma curves. Its appli-cation for mobile LCDs is foreseeable.The authors are indebted to the financial support fromChi-Mei Optoelectronics, Taiwan.1M. Schadt and W. Helfrich, Appl. Phys. Lett. 18, 127 1971.2S. T. Wu and C. S. Wu, Appl. Phys. Lett. 68, 1455 1996.3M. Shibazaki, Y. Ukawa, S. Takahashi, Y. Iefuji, and T. Nakagawa, SIDInt. Symp. Digest Tech. Papers 34, 90 2003.4Z. Ge, X. Zhu, R. Lu, T. X. Wu, and S. T. Wu, Appl. Phys. Lett. 90,221111 2007.5O. Itou, S. Hirota, Y. Sekiguchi, S. Komura, M. Morimoto, J. Tanno, K.Fukuda, T. Ochiai, H. Imayama, T. Nagata, and T. Miyazawa, SID Int.Symp. Digest Tech. Papers 37, 832 2006.6R. Lu, Z. Ge, Q. Hong, and S. T. Wu, J. Disp. Technol. 3, 15 2007.7J. H. Song, Y. J. Lim, M.-H. Lee, S. H. Lee, and S. T. Shinb, Appl. Phys.Lett. 87, 011108 2005.8X. Zhu, Z. Ge, T. X. Wu, and S. T. Wu, J. Disp. Technol. 1, 15 2005.9S. T. Wu and C. S. Wu, Liq. Cryst. 24, 811 1998.10T. Uesaka, S. Ikeda, S. Nishimura, and H. Mazaki, SID Int. Symp. DigestTech. Papers 38, 1555 2007.11Y. Koike and K. Okamoto, Fujitsu Sci. Tech. J. 35, 221 1999.12Q. Hong, T. X. Wu, X. Zhu, R. Lu, and S. T. Wu, Appl. Phys. Lett. 86,121107 2005.13S. H. Lee, H. W. Do, G. D. Lee, T. H. Yoon, and J. C. Kim, Jpn. J. Appl.Phys., Part 2 42, L1455 2003.14C. Y. Tsai, M. J. Su, C. H. Lin, S. C. Hsu, C. Y. Chen, Y. R. Chen, Y. L.Tsai, C. M. Chen, C. M. Chang, and A. Lien, Proceedings of the AsiaDisplay, 2007 unpublished, Vol. 1, p. 24.15R. Lu, X. Zhu, S. T. Wu, Q. Hong, and T. X. Wu, J. Disp. Technol. 1, 32005.16D. K. Yang and S. T. Wu, Fundamentals of Liquid Crystal Devices Wiley,New York, 2006.17Q. Hong, T. X. Wu, X. Zhu, R. Lu, and S. T. Wu, Opt. Express 13, 83182005.FIG. 4. Color online The isocontrast contour of the two-domain VA TR-LCD between 0 and 5 Vrms: a T mode and b R mode. A white lightsource, two wide-view circular polarizers, and a negative c-plate withdn=−332 nm are used for calculations.191102-3 Lu, Ge, and Wu Appl. Phys. Lett. 92, 191102 2008

Wide-view and single cell gap transflective liquid crystal display using slit-induced multidomain structures

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Wide-view and single cell gap transflective liquid crystal display using slit-induced multidomain structures

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