Application of antiferroelectric liquid crystals with high tilt

Application of Antiferroelectric Liquid Crystals with High Tilt Koen D’havé Promotor: prof dr ir H Pauwels Proefschrift ingediend tot het behalen van de graad van Doctor in de Toegepaste Wetenschappen: Elektrotechniek Vakgroep Elektronica en Informatiesystemen Voorzitter: prof dr ir J Van Campenhout Faculteit Toegepaste Wetenschappen Academiejaar: 2001-2002 Tai ngay!!! Ban co the xoa dong chu nay!!! i Acknowledgement ii Table of contents Introduction 1.1 The current display market 1.2 Liquid crystal displays 1.2.1 The liquid crystal phase 1.2.2 Liquid crystal displays; layer by layer 1.3 Quantification of the image quality 10 1.4 An overview of this work 12 Ferroelectric and antiferroelectric liquid crystals 2.1 SmC and SmC* phases 2.1.1 Structure 2.1.2 Dielectric tensor 2.1.3 Optical properties 2.1.4 Ferroelectric liquid crystal displays 2.2 The SmCa and SmCa* phases 2.2.1 Structure 2.2.2 Dielectric tensor 2.2.3 Optical properties 2.2.4 Antiferroelectric liquid crystal displays 2.3 The first goal 15 15 15 17 21 22 28 28 30 33 36 38 Alignment of AFLCs 3.1 Prototype cells 3.2 Optimization of the buffing parameters 3.3 Compensating the rubbing directions 3.4 Obliquely evaporated SiOx 3.5 Conclusion 39 39 42 45 49 51 iv Orthoconic antiferroelectric liquid crystals 4.1 Uniaxial anticlinic conditions 4.2 Isotropic anticlinic conditions 4.3 A solution for the dark state problem of an AFLCD 4.4 Orthoconic antiferroelectric liquid crystals 4.5 Phase modulation by means of OAFLCs 4.5.1 Polarisation switches 4.5.2 An alternative construction for an OAFLC display 4.5.3 Ternary phase modulation 4.6 The pretransitional effect in AFLCDs 4.7 Conclusion 4.8 The second goal 53 53 56 56 58 61 61 63 64 70 73 74 Reflective AFLCDs 5.1 Normally bright mode 5.2 Normally dark mode 5.2.1 A λ/4 film between liquid crystal layer and mirror 5.2.2 A λ/4 film between polariser and liquid crystal layer 5.3 Conclusion 5.4 The third goal 75 75 77 Light scattering polymer dispersions of OAFLC 6.1 Polymer Dispersed Liquid Crystals 6.2 OAFLC in a polymer matrix 6.3 The influence of the material parameters 6.3.1 Extinction in a light scattering medium 6.3.2 Anomalous diffraction 6.3.3 Influence of the tilt angle 6.3.4 Influence of the birefringence 6.3.5 Viewing angle dependency of the transparent state 6.3.6 Conclusion 83 83 85 86 86 87 91 93 Conclusions 7.1 Achievements 7.2 Some remarks 97 97 98 77 80 82 82 93 96 v A Data sheets A.1 CS4001 A.2 W107 A.3 W107a A.4 W107b A.5 W123 A.6 W124 A.7 W129 Bibliography 99 99 100 102 104 106 108 110 113 vi List of figures 1.1 1.2 1.3 1.4 2.1 2.2 2.3 2.4 2.5 2.6 2.7 A schematic comparison between the nematic (N), the cholesteric (N*) and the smectic A (SmA) phases A schematic cross section of the pixels of a typical liquid crystal display The purpose and the materials used for the different layers are further explained in the text A schematic comparison between (a) a passive matrix and (b) an active matrix The wavelength dependency of the transmission of a birefringent layer between crossed polarisers The optical thickness of the layer was optimised for a wavelength of 550 nm The best choice is k=0, corresponding to a optical thickness of λ/2 That plot shows least curvature and hence has the lowest wavelength dependency The curve with k=1 corresponds to an optical thickness of 3λ/2 A schematic illustration of the difference between (a) the SmA and (b) the SmC phases A representation of a full pitch length of the helix in the SmC* phase A review of the phenomenological angles The direction of the optic axes for a ferroelectric liquid crystal The bookshelf geometry of an SSFLCD and its electro-optic application between crossed polarisers The chevron profile in an SSFLCD and the stable states in which the material can be switched In order to keep a constant layer spacing at the surfaces, the layers have to tilt when the layer thickness decreases due to the increasing tilt angle of the material If this phenomenon occurs in opposite directions at both surfaces a chevron with a sharp tip is created For the situation represented here, without surface pretilt, both chevron directions are equally probable and optically identical If both directions are present in one cell, they give rise to the characteristic zig-zag or lightning defects The operation of the Twisted Smectic Mode in the SmC* phase This mode is similar to the twisted nematic mode The electrooptic behaviour is ‘normally bright’ 10 13 16 16 18 23 23 25 27 viii 2.8 2.9 2.10 2.11 2.12 2.13 3.1 3.2 3.3 3.4 3.5 3.6 3.7 The operating principle of the V-shaped switching mode in the SmC* phase Note that the choice of the directions of polariser and analyser differ from the TS-mode The electro-optic behaviour is also reversed A schematic illustration of the difference between (a) synclinic and (b) anticlinic behaviour A representation of a full pitch length of the helix in the SmCa* phase The helix represented here is an idealized structure To be more precise the director in adjacent layers of the unit cell does not make a phase angle difference of exactly 180° The position of the glide mirror plane and the symmetric deformation with respect to the ideal anticlinic structure When the symmetric deformation reaches 90°, one obtains a synclinic state At that moment the angle ψ describes the phase angle of the SmC phase This description will allow us to determine the direction of the principle axes and their matching refractive indices in a more simplified manner The direction of the optic axes for an antiferroelectric liquid crystal The working principle of an AFLCD An adapted buffing machine which allows for easy control over the buffing directions on the substrates An image of the alignment for (left) parallel and (right) anti-parallel assembly The rubbing directions as well as the directions of polariser and analyser are indicated in the top right corner of each picture The alignment (left) obtained for a cell of which only one substrate received an optimal buffing treatment The right part of that picture shows the deformation of the structure when applying even small electric fields The other picture (right) shows the structure after prolonged addressing of such a cell The straightening of the smectic layers by an electric field The chevrons, which are created due to shrinkage of the layers during cool down, endure a straightening torque Instead of creating a bookshelf structure, a defect structure in the plane of the cell, which is called the striped texture, is obtained Between crossed polariser one sees a grid of alternating bright and dark lines Rubbing and assembly of the substrates for compensation experiments Structures obtained for (a) a too small, (b) an almost ideal and (c) a too large angle between the buffing directions When the directions on both substrates are switched we obtain a structure as shown in (d) Typical texture for hybrid cells for which the evaporation angle is (a) lower than 70°, (b) between 70° and 83° and (c) higher than 83° with respect to the substrate normal 28 29 30 32 35 37 41 43 43 45 47 48 50 108 A.6 W124 Manufacturer Military University of Technology, Warsaw, Poland (W Drzewinski & R Dabrowski) Phases and transition temperatures X (6/24) SmCa* (94.1) SmC* (102.6) SmA* (117/123) I temperatures in °C, obtained from DSC measurements SmA* SmCa* Ps (nC/cm2) 300 SmC* Spontaneous polarisation 200 100 20 40 60 80 100 Temperature (°C) Figuur A.14: Spontaneous polarisation of W124, measured with capacitance bridge method P = 299 nC ⁄ cm at 25°C Apparent tilt angle 50 30 20 40 60 80 SmA* 10 SmC* 20 SmCa* Tilt (°) 40 100 Temperature (°C) Figuur A.15: Apparent tilt angle of W124, measured with polarizing microscope and rotation stage θ ≈ 42.8° at 25°C 109 Helical pitch The marble like appearance suggests an extremely short pitch At room temperature bright lines along the layers appear, which signifies that surface stabilization is lost even in very thin cells Threshold SmC* SmCa* Threshold (V/µm) SmA* Threshold up Threshold down 20 40 60 80 100 Temperature (°C) Figuur A.16: Threshold field of W124, measured with a square wave of 10 Hz E th = 7.4 V ⁄ µm at 25°C with a square wave of 10 Hz 110 A.7 W129 Manufacturer Military University of Technology, Warsaw, Poland (W Drzewinski & R Dabrowski) Phases and transition temperatures X (13/15) SmCa* (84.5) SmC* (102.3) SmA* (117/125.2) I temperatures in °C, obtained from DSC measurements SmA* SmCa* Ps (nC/cm2) 300 SmC* Spontaneous polarisation 200 100 20 40 60 80 100 Temperature (°C) Figuur A.17: Spontaneous polarisation of W129, measured with capacitance bridge method P = 309.6 nC ⁄ cm at 25°C Apparent tilt angle 50 30 20 40 SmC* 10 60 80 SmA* 20 SmCa* Tilt (°) 40 100 Temperature (°C) Figuur A.18: Apparent tilt angle of W129, measured with polarizing microscope and rotation stage θ ≈ 42.6° at 25°C 111 Helical pitch Undetermined, estimated higher than W107 and W107a due to the absence of coloured reflection SmA* SmC* SmCa* Threshold (V/µm) Threshold Threshold up Threshold down 20 40 60 80 100 Temperature (°C) Figuur A.19: Threshold field of W129, measured with a square wave of 10 Hz E th = 6.1 V ⁄ µm at 25°C with a square wave of 10 Hz 112 Bibliography [1] J Castellano, “Current and Future Market Opportunities for Active Matrix LCDs”, 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