LIQUID CRYSTAL DISPLAYS FOR PIXELATED GLARE SHIELDING EYEWEAR

Transcription

1 LIQUID CRYSTAL DISPLAYS FOR PIXELATED GLARE SHIELDING EYEWEAR A dissertation submitted to Kent State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy by Shawn Patrick Hurley August 2010

2 Dissertation written by Shawn Patrick Hurley B.S., Ohio University, 2005 B.S., Ohio University, 2005 Ph.D., Kent State University, 2010 Approved by, Chair, Doctoral Dissertation Committee Deng-Ke Yang, Members, Doctoral Dissertation Committee Philip Bos, Liang-Chy Chien, Christopher Mullin, Jing Li, Arden Ruttan Accepted by, Chair, Department of Chemical Physics Liang-Chy Chien, Dean, College of Arts and Sciences John R.D. Stalvey ii

3 TABLE OF CONTENTS TABLE OF CONTENTS... iii LIST OF FIGURES... ix LIST OF TABLES... xiii ACKNOWLEDGEMENTS... xiv REFERENCES Chapter Page 1. Introduction Glare Natural Response To Glare Non-Pixelated Glare Shielding Eyewear Pixelated Glare Shielding Eyewear Dissertation Overview Fundamentals of Liquid Crystals and Liquid Crystal Displays Properties of Liquid Crystals Liquid Crystal Director Order Parameter Liquid Crystal Phases Cholesteric Liquid Crystals Elastic Deformations and Elastic Constants Elastic Energy Density iii

4 2.1.7 Dielectric Anisotropy Optical Anisotropy Liquid Crystals in an External Field: The Frederiks Transition Surface Alignment, Rubbing, and Pretilt Angle The Design of a Single LCD Pixel Electrode Structures and Driving Schemes Direct Drive Scheme Active Matrix Drive Scheme Passive Matrix Drive Scheme Block Addressing Drive Scheme Bistable Drive Scheme Experimental Methods Mixture Preparation Experimental Equipment and Setup Transmission Measurement and Calculation Contrast Ratio from Static Response of Transmission vs. Voltage Response Time from the Dynamic Response of Transmission vs. Time Which LCDs Can Work in Pixelated Glare Shielding Eyewear? LCD Designs Liquid Crystal Displays Without a Polymer Network Liquid Crystal Displays With a Polymer Network Conclusion Super Twisted Nematic for Polarizer Based Absorptive Displays Motivation iv

5 3.2 Overview Measuring the Birefringence Dispersion Theory Experiment Simulation Director Relaxation Light Transmission Calculation of Color, Luminance, and Contrast Ratio Compensation Film Experiment Using Liquid Crystal MLC Optimizing the Material Properties and Cell Design Using Simulation Electro-Distortional Curve Birefringence Magnitude and Dispersion Driving Voltages Polarizer and Analyzer Angles The Best Cell Gap Response Time of STN Summary Experiment Using Liquid Crystal 13B Viewing Angle of STN Further Improvements Conclusion Normal Mode Polymer Stabilized Cholesteric Texture for Scattering Displays Overview v

6 4.2 Experiment Results Chiral Concentration Varied Monomer Concentration Varied Liquid Crystal Material Varied Conclusion Reverse Mode Polymer Stabilized Cholesteric Texture for Scattering Displays Overview Experiment Over Drive for a Faster Turn On Response Time Results Chiral Dopant Concentration Varied Monomer Concentration Varied UV Curing Intensity Varied UV Curing Time Varied Alignment Layer Varied Haze in the Transparent State Background Haze Haze From Filling Defects Type A Defect Prevention and Removal Methods Type B Defect Prevention and Removal Methods Thermal Annealing Method to Remove Type B Defects Voltage Annealing Method to Remove Type B Defects Type B Defect Removal Summary vi

7 5.4.8 Haze From Spacers Cell Gap Varied: Wedge Cell Experiment Motivation Experiment Contrast Ratio vs. Thickness Transparency vs. Thickness Response Time vs. Thickness Performance of Reverse Mode PSCT Using Monomer RM A Double Cell Design Conclusion Scattering Profile of Polymer Stabilized Cholesteric Textures Introduction Experiment Normal Mode Results Reverse Mode Results Conclusion Dye-Doped Normal Mode Polymer Stabilized Cholesteric Texture Introduction Formulation and Experiment Glass Substrate Results Flexible Substrate Results Flexible Display Prototype Conclusion Conclusion vii

8 8.1 Dissertation Summary Scattering vs. Absorption Final LCD Recommendation viii

9 LIST OF FIGURES Figure 1 Molecule of pentylcyanobiphenyl (commonly known as 5CB) Figure 2 Various phases of rod-like thermotropic liquid crystals Figure 3 The twisted structure of the cholesteric liquid crystal phase Figure 4 The elastic distortions of the director: splay (K 11 ), twist (K 22 ), and bend (K 33 ) Figure 5 Cross section of the layers in a LCD, which happens to be cholesteric Figure 6 Definition of the polar and azimuthal angles of the director Figure 7 An example of a direct drive connection with a seven segment display Figure 8 A high resolution direct drive pixel layout which uses a common electrode.. 23 Figure 9 A four row high resolution direct drive pixel layout Figure 10 An equivalent circuit of an active matrix pixel: a) TFT b) MIM Figure 11 A passive matrix drive scheme: a) electrode layout b) equivalent circuit Figure 12 Demonstration of the voltages used while addressing a single line Figure 13 Maximized voltage ratio given the number of rows in a passive matrix Figure 14 Block addressing demonstration Figure 15 Drawing of the setup measuring contrast ratio and response time of PSCT. 37 Figure 16 Definition of the collection angle Figure 17 General form of the RMS voltage applied to a sample during a response time measurement. The inset is a reminder that the pulses consist of many cycles Figure 18 Transmission curve (bold) of a typical response time measurement with the applied voltage pulse (dashed) shown underneath Figure 19 Photoinitiator benzoin methyl ether (BME) Figure 20 Output spectrum of UV source used with the most relevant peak at 365nm. 48 Figure 21 Some of the LCDs along the continuum of the cholesteric pitch Figure 22 STN diagrams: a) cross section of two optical states, b) the angular convention used in the equations and simulations Figure 23 An example of the experimental data and fit Figure 24 a) Birefringence dispersion curves for various liquid crystals. b) Same data as in a) normalized at 589nm Figure 25 The director orientation angles vs. thickness Figure 26 The 1931 (x,y) CIE Diagram Figure 27 Color Matching Functions of the 1931 CIE 2 Standard Observer Figure 28 The 1976 (u,v ) CIE Diagram a Figure 29 Dominant (or complimentary) wavelength and purity ( purity ) on the a b a) 1931 CIE diagram and b) 1976 CIE diagram ix

10 Figure 30 Contrast ratio vs. polarizer and analyzer angles & transmission spectrum of simulation compared to experiment: a) 4.4μm b) 3.7μm Figure 31 Contrast ratio vs. compensation film angle & transmission spectrum of simulation compared to experiment: a) 4.4μm b) 3.7μm Figure 32 Simulations of midlayer tilt angle, θ m, show how the STN electro-distortional curve varies with one device parameter: a) twist angle: Φ (d/p is also varied to match twist), b) pretilt angle: θ p, c) thickness/pitch ratio: d/p Figure 33 Simulations of midlayer tilt angle, θ m, show how the STN electro-distortional curve varies with one material parameter: a) bend/splay elastic constant ratio: K 33 /K 11, b) twist/splay elastic constant ratio: K 22 /K 11, c) dielectric parameter: / Figure 34 Simulations of midlayer tilt angle, θm, show that 13B-000 is steeper than MLC-6080 using a common set of cell design parameters: Φ = 240, θ p = 7, d/p = Figure 35 Simulated transmission spectra of: E44, MLC-6080, & 13B-000 with Δnd(589nm) = 0.79, α i = -44, and α o = Figure 36 Simulated transmission spectra of 13B-000 for cell gaps 5-7μm Figure 37 Contrast ratio and luminance of 13B-000 for cell gaps 5-7μm Figure CIE chromaticity coordinates of 13B-000 for cell gaps 5-7μm Figure 39 Purity of 13B-000 for cell gaps 5-7μm Figure 40 Two normalized figures of merit for 13B-000 for cell gaps 5-7μm Figure 41 The spectrum at various voltages: 0V, 1.17V and 1.5V using 13B-000 liquid crystal in a 240 STN LCD Figure 42 Transmission vs. voltage for 13B Figure 43 Response times, contrast ratio and luminance for 13B-000 as V off is varied Figure 44 Iso-contrast viewing angle of the 13B-000 STN as seen by the wearer Figure 45 Polymer network formation in normal mode PSCT by UV light exposure. 115 Figure 46 Optical states of the normal mode PSCT Figure 47 Molecular structure of monomer RM257 used in normal mode PSCT Figure 48 Chiral dopant in use R Figure 49 Typical experimental result used for calculating contrast ratio Figure 50 Contrast ratio, saturation voltage, and hysteresis width data when monomer concentration is fixed at 2.0% for normal mode PSCT Figure 51 Response time data when monomer concentration is fixed at 2.0% for normal mode PSCT Figure 52 Contrast ratio, saturation voltage, and hysteresis width data when chiral concentration is fixed in the range % for normal mode PSCT Figure 53 Response time data when chiral concentration is fixed in the range % for normal mode PSCT Figure 54 Response time data for E44 compared to BL Figure 55 Polymer network formation in reverse mode PSCT by UV light exposure. 130 Figure 56 Optical states of the reverse mode PSCT Figure 57 Monomer RM257 was abandoned in the reverse mode experiments x

11 Figure 58 Monomer BMBB-6 had the best results in the reverse mode experiments. 133 Figure 59 Chiral dopant first studied in reverse mode PSCT, R Figure 60 Final chiral dopant studied in reverse mode PSCT, R Figure 61 Comparison of two different over drive durations to no over drive Figure 62 Response time vs. over drive voltage using Von=21V & Voff=7V Figure 63 Transmission data for various R-811 chiral dopant concentrations Figure 64 Contrast and voltage vs. monomer concentration Figure 65 Response time vs. monomer concentration Figure 66 Transmission curves at two UV intensities Figure 67 Response time vs. UV curing time Figure 68 Type B filling defects and background haze are visible by eye Figure 69 Type A defect (Note: Type B defects are in image also) Figure 70 Type B defect Figure 71 After filling Figure 72 After slow annealing Figure 73 After fast annealing Figure 74 Microscope view of a spacer in RM-PSCT Figure 75 Wedge cell with 15V applied. The number of π turns is labeled in each zone Figure 76 Measured and calculated contrast ratios vs. cell gap Figure 77 Contrast ratio vs. cell gap at constant V on = 30V & V off = 10V Figure 78 Images of the wedge cell: a) sample in focus b) background in focus at selected voltages shows the contrast and switching behavior Figure 79 Transparent state transmission vs. cell gap Figure 80 Response times vs. cell gap. Filled marks use the voltages for best contrast ratio and the open marks use constant V on =30V & V off =10V Figure 81 Driving voltage of the wedge cell vs. thickness Figure 82 T-V curves of reverse mode samples comparing RM257 to BMBB Figure 83 Increasing T-V curves of three RM257 reverse mode samples with the same mixture as sample 4A at different UV intensities Figure 84 The turn off time vs. driving voltage duration for two monomers Figure 85 Microscope photo of BMBB-6 monomer crystals in a PSCT sample under crossed polarizers Figure 86 Diagram of the experimental setup for measuring the scattering profile Figure 87 Normal mode PSCT scattering profile (log) Figure 88 Contrast ratio variation with total collection angle in normal mode PSCT. 188 Figure 89 Contrast ratio as the scattering state voltage varies with a 3 degree total collection angle in normal mode PSCT Figure 90 The 3-D scattering profiles of the normal mode PSCT samples Figure 91 Reverse mode PSCT scattering profile (log) Figure 92 Contrast ratio variation with total collection angle in reverse mode PSCT. 193 Figure 93 Contrast ratio as the scattering state voltage varies with a 3 degree total collection angle in normal mode PSCT Figure 94 The 3-D scattering profiles of the reverse mode PSCT samples xi

12 Figure 95 Dye-doped PSCT display cell geometry and operation principle Figure 96 Molecular structure of the dye 1,5-bis-phenylsulfanyl-anthraquinone Figure 97 Contrast ratio and saturation voltage vs. chiral concentration with monomer fixed at 2% and dye fixed at 3% Figure 98 Contrast ratio and saturation voltage vs. dye concentration with monomer fixed at 2% and chiral dopant fixed at 5.5% Figure 99 Transmission measurement of a plastic display with 2% monomer, 4% chiral dopant and 6% dye Figure 100 Spectra of a) glass PSCT at 30V, b) glass dye-doped PSCT at 30V and c) plastic dye-doped PSCT at 20V Figure 101 Photograph of a dye-doped PSCT display while bent in hand Figure 102 Photograph of a dye-doped PSCT display bent vertically to an 85mm radius of curvature a) transparent (20V) with scenic background b) scattering/absorbing (0V) with black background xii

13 LIST OF TABLES Table 1 Parameters in the response time measurements Table 2 Response time calculations Table 3 List of LCDs considered for pixelated glare shielding eyewear Table 4 Birefringence dispersion fitting parameters for various liquid crystals Table 5 Summary of the results of the STN LCDs made with MLC-6080 liquid crystal Table 6 Parameters of 13B-000 and MLC-6080 from the Merck datasheet [37] Table 7 Purity of E44, MLC-6080, & 13B-000 from the spectra in Figure Table 8 Chiral concentration and intrinsic pitch of normal mode samples Table 9 Parameters of BL036 and E44 from the Merck datasheet [37] Table 10 Percent chiral dopant, intrinsic pitch, turns, and contrast ratios for the samples Table 11 Comparison of results from two different alignment layers at 0-30V driving Table 12 Summary of the two defect annealing methods to remove Type B oily streaks Table 13 Detailed comparison of the molecular structures of RM257 and BMBB Table 14 The double stacked reverse mode PSCT results Table 15 Chiral concentrations and intrinsic pitch of each sample xiii

14 ACKNOWLEDGEMENTS During the course of my study at Kent State University s Liquid Crystal Institute (LCI) there have been many people who have helped me reach my goals. I shall first distinguish my advisor and mentor, Professor Deng-Ke Yang, who taught me everything he knew about liquid crystals and polymers. When I approached him with a question, he often provided the answer by sharing a paper with me from his collection, the majority of which were authored under his leadership. He oftentimes ended a difficult lesson with a simple joke or short story that made comprehension easier. I will miss that very much. I appreciate Professors Phil Bos and Antal Jakli for lab rotation experience as well as all of the professors who taught me so much. While it was difficult at the time, it is those assignments that provided a foundation when I needed it later. Finally, I thank the professors on my dissertation committee for committing their time to reading and evaluating this work. Several of the staff, scientists, and technicians at LCI have been there for me when I needed them. The office staff have always steered me to the right place when I had a non-technical question and their planned social events were always a welcome release. I want to mention Doug Bryant and Dave Hayes for sharing their time and skills with me in the clean room as well as Merrill Groom for his equipment expertise. A thank you is due to Kevin Ballard for his clear instruction in cell fabrication so I could make my own cells; however, he deserves even more accolades for saving me time by actually making xiv

15 most of the cells used in this dissertation. I express my gratitude for the many hallway discussions I have had with Bentley Wall. His balanced perspective and unbiased attitude often made the difference in solving difficult problems. Finally, I thank Chris Mullin for envisioning a pixelated glare shielding device and his confidence in me to help him design it. His attention to detail and technical explanations have been an invaluable resource. I want to thank my classmates at LCI. It was their support during group study and scientific discussions that really enabled me to understand the most important concepts in liquid crystals. Thanks to Shin-Ying Lu for her comments on this dissertation. I also would like to specially recognize the following students and post-doctoral staff members in our lab: Sarah Hicks, Rafael Zola, Ji Ma and Young-Cheol Yang, for their valuable discussions and collaborations. Professionally and financially, I owe recognition to those organizations that have enabled my travel to conferences and published my research, such as Kent State s Graduate Student Senate and the Society for Information Display (SID). So much support and preparation has come from outside the walls of the LCI. I would like to thank Ohio University for the education that gave me an opportunity to earn a Ph.D. at Kent State University. I thank my parents, Patrick and Cheryl, and my sister Kathleen for their guidance. My parents advice has helped me succeed and their encouragement and love throughout my schooling has been superior. Finally, I want to emphasize my best friend and number one supporter, my fiancée Tracey. Her love and devotion means everything to me. xv

16 Chapter 1 1. Introduction 1.1 Glare In human vision, glare is caused by a significantly high ratio of luminance between the source of bright light and a nearby area of interest. Glare impairs visibility by reducing contrast from light scattering in the eye, reducing brightness due to pupil restriction, and reducing sensitivity to dark scenes from the eye s saturated chemical response. Furthermore, if glare isn t impairing vision, it may simply be perceived as discomforting or distracting. 1.2 Natural Response To Glare Human vision can be effective over a very wide range of luminance values that spans nearly twelve orders of magnitude from the dim starlit nighttime (10-6 cd/m 2 ) to the bright sunlit daytime (10 5 cd/m 2 ). However, the visual system can not process information over the entire range all at once, but instead operates within a smaller range of discrimination. Light brighter than the discrimination range is perceived as glare and light dimmer than the discrimination range is perceived as shadow. A real world example is the appearance of car headlights in two different scenarios. Despite having a constant luminance, headlights will appear to increase in brightness as day turns to night which can shift the perception from detectable to glaringly bright. During this time, the visual system has 1

17 2 repositioned itself to detect dimmer light through a continuous and reversible process called adaptation [1]. 1.3 Non-Pixelated Glare Shielding Eyewear Although polarized sunglasses can selectively reduce most reflected glare, traditional sunglasses have the undesirable effect of reducing the brightness of the entire field of view. Newer sunglasses can alter their transmission with a passively or a manually switchable device. Photochromic lenses are one passive design which reversibly decreases transmission after exposure to ultra-violet (UV) light [2]. One manual design uses the reorientation of liquid crystal molecules to change the absorption of an incorporated dye [3]. However, both switchable designs uniformly darken the entire field of view just like traditional sunglasses. 1.4 Pixelated Glare Shielding Eyewear The solution to avoid darkening the entire field of view is to pixelate the display so that different regions can be independently switched on and off to shield the source of glare. A passively switchable device like the photochromic lens can not operate in this fashion, because even if the display is broken up into pixels, all areas of the lens would respond to glare equally. Therefore, the lens must consist of a switchable device such as a liquid crystal display (LCD) made up of many pixels in a grid. However, if the display is still operated manually it would be cumbersome to operate. The key component that makes the device effective is a method to detect the direction of incoming glare light, determine the pixels between the light source and the wearer s pupil, and darken only those pixels. If this process can be repeated fast enough, then it becomes an automatic

18 3 reaction to any changes in the wearer s field of view. This dissertation is an investigation of the best LCD design for a pixelated glare shielding lens with the assumption that the automatic glare detection device is already available. 1.5 Dissertation Overview This dissertation presents the results of using a liquid crystal display (LCD) for pixelated glare shielding eyewear. Chapter 2 is an introduction to liquid crystals and their most important properties for use in displays. Chapter 2 also includes an overview of the many different LCD designs with a consideration for each design s suitability in glare shielding eyewear. The rest of the dissertation discusses the results from experimental measurements and computer simulations of some of the best choices. All the designs are characterized for the best electro-optic performance as defined by driving voltage, brightness and color, contrast ratio, response time, and viewing angle. The results show how the performance varied with changes in mixture formulation, cell construction and fabrication procedures. The dissertation investigated two distinct glare shielding methods: absorption and scattering. The first method uses light absorption performed by attached polarizers which dims the field of view to a constant baseline and pixels between the eye and glare will reduce or eliminate glare by darkening further. The super-twisted nematic (STN) mode uses this method and is discussed in Chapter 3. The second glare shielding method uses light scattering performed by a polymer stabilized cholesteric texture (PSCT) liquid crystal configuration which has a transparent field of view and pixels between the eye and glare will reduce or eliminate glare by scattering light away from the pupil. The two

19 4 modes that use this method are the normal mode, which is discussed in Chapter 4, and the reverse mode, which is discussed in Chapter 5. The two scattering modes are further compared in Chapter 6 using a measurement of the scattering profile. In Chapter 7 there is data for the normal mode PSCT after adding a dye to the chemical mixture and includes the results from a flexible display fabricated on plastic substrates. Finally, the conclusion in Chapter 8 summarizes the performance of all the LCDs investigated in this dissertation by reviewing each design s strengths and weaknesses. The final statements will then formulate a recommendation of the best choice to use in the plans for a pixelated glare shielding device.

20 Chapter 2 2. Fundamentals of Liquid Crystals and Liquid Crystal Displays 2.1 Properties of Liquid Crystals Many are familiar with the three phases of matter: solid, liquid, and gas. For example, upon heating water it transforms from the solid phase (ice) to the liquid phase at 0 C and then to the gaseous phase at 100 C. Solids are characterized by a repeating structure that constitutes an orientational and positional order. Liquids are characterized as having the ability to flow, meaning they take the shape of the container in which they are placed. Also, liquids are isotropic, which means their physical properties are uniform in all directions. Gases are characterized by the ability to be compressed because of the large separation between particles. Liquid crystals are an intermediate phase of matter, a mesophase, between a solid and a liquid, which possesses some properties of each state. Liquid crystals are fluid and flow just like a liquid. However, liquid crystals behave like a crystal because they have orientational order in one or more dimensions. This orientational order is the attribute that enables liquid crystals to be anisotropic, which means they have different physical properties in different directions. The anisotropy of liquid crystals is the distinction that makes them both scientifically intriguing and technologically functional. Liquid crystals were discovered in 1888 by an Austrian botanist Friedrich Reinitzer when he observed that a sample of cholesteryl benzoate appeared to possess two distinct melting points. Upon heating, it would first transform from a solid to a cloudy liquid at 5

21 C and then at a higher temperature, C, it would change into a clear liquid. He sent a sample to a German physicist, Otto Lehman, for further investigations. It was Lehman who described the material as flowing crystals and would eventually be credited for coining the term liquid crystals [4]. Liquid crystals are organic compounds composed of molecules that are elongated and rod-like (calamitic) or flat and disc-like (discotic). This dissertation exclusively studied calamitic liquid crystals, which are composed of molecules that are typically five or more times longer (~2-10nm) than they are wide (~0.5nm). One of the simplest and most common liquid crystals is pentylcyanobiphenyl, more commonly known as 5CB, as shown in Figure 1. 5CB has a permanent dipole from the cyano group CN. Although the molecules themselves are not cylindrical, they can still be regarded as such because of the rapid rotation about the long axis (~GHz) due to thermal motion. In non-ferroelectric liquid crystal phases the dipole has an equal probability to point either up or down, so the cylindrical representation has no head or tail. The basic molecular components considered necessary for liquid crystallinity are a flexible tail(s) and a rigid core(s). If the molecule is purely rigid, then it will directly transform from an isotropic liquid to a crystalline solid. However, if it is completely flexible, then it cannot maintain orientational order over a long enough length scale. The balance of these two components creates liquid crystals phases [5].

22 7 Figure 1 Molecule of pentylcyanobiphenyl (commonly known as 5CB) Liquid Crystal Director The macroscopic orientation of calamitic liquid crystals is defined by the director, ( n ). Although individual molecules can deviate from the director, it points along the average orientational direction of the long axis of the liquid crystal molecules. The director is a double arrowed vector because of symmetry; there is an equal probability of a molecule to point up or down. However, the director is often specified as a single arrowed vector for convenience and it is simply noted that n and - n are equivalent. This dissertation is confined to uniaxial liquid crystals which have only one director since there is no preferred orientation of either of the molecule s short axes. Unless otherwise specified, all references to the orientation of a liquid crystal in this dissertation refer to the director s orientation rather than an individual molecule s orientation Order Parameter The overall deviation of the liquid crystal molecules from the director is quantitatively defined by the order parameter, S. The order parameter of an isotropic fluid devoid of order is 0. Perfect orientational order of all molecules has an order parameter of 1. The value of the order parameter is found using Equation 1, where

23 8 P (cos ) is the second order Legendre polynomial and indicates a temporal average 2 over all orientations [6]. 1 2 S P2 (cos ) 3cos 1 (1) 2 is the polar angle between the instantaneous direction of the long molecular axis and the average direction, n. The order parameter of a liquid crystal strongly depends on temperature and typical values are in the range at low temperatures. Upon heating a liquid crystal, the order parameter abruptly decreases to 0 at the nematicisotropic phase transition temperature, T NI, also called the clearing point [5]. Many anisotropic properties of liquid crystals, which will be discussed later in this dissertation, are dependent on the order parameter, which means they are also temperature dependent. However, unless otherwise discussed, all experiments were undertaken at room temperature in the range C, which was far enough from the clearing point that small day-to-day temperature deviations aren t significant Liquid Crystal Phases Liquid crystal materials can form many different phases which are distinguished by the degree of positional and orientational order. Lyotropic liquid crystals are a mixture of liquid crystal and solvent which changes phase as the concentration of solvent varies. Thermotropic liquid crystals changes phase as the temperature is varied and are the sole focus of this dissertation because of their application in displays. The most common phases of thermotropic liquid crystals are shown in Figure 2.

24 9 n n n Isotropic Nematic Smectic-A Smectic-C Solid Crystal Decreasing Temperature Figure 2 Various phases of rod-like thermotropic liquid crystals. Upon cooling a thermotropic liquid crystal from the isotropic fluid, which has no orientational or positional order, it will undergo a transition to the nematic phase where the molecules become aligned along a preferred direction, the director. The nematic phase is characterized by its orientational order in one dimension and no positional order. Upon further cooling, some liquid crystals form the smectic-a phase, which is a two dimensional fluid because it has one dimensional positional order as well as orientational order. This positional order is characterized by an undulating density distribution which is shown in the simplified structure of Figure 2 as a layered spacing. The director in smectic-a is perpendicular to the layers. Cooling further, the smectic-c phase may form where the director is tilted with respect to the layers. Finally, at the lowest temperature the crystalline solid forms where the molecules exhibit both positional and orientational order and they no longer diffuse. Other more complicated liquid crystal phases exist, which are not relevant to this dissertation and will not be discussed here. However, a special case of the nematic phase exists, the cholesteric nematic, which is important enough that it is treated separately in the next section.

25 Cholesteric Liquid Crystals Even before the first observation of liquid crystals, Louis Pasteur discovered a natural consequence of symmetry that is critical to understanding how most LCDs work. He found that a salt of tartaric acid formed two distinct crystals where one was the mirror image of the other. He observed that a solution of one type rotated linearly polarized light clockwise and a solution of the other type would rotate linearly polarized light counterclockwise. This was a landmark investigation in the early studies of chirality. A chiral object is an object that can not be superimposed on its mirror image by any combination of rotations and/or translations. What this means is that chiral objects have two forms that can be identified by a handedness, which is either left or right. Chirality is a universal principle that applies to objects of all sizes from the macroscopic scale to the atomic scale [7]. For this dissertation, the most interesting chirality is molecular chirality and its effect on the orientation of the liquid crystal director. Molecular chirality is mapped onto the phase of the liquid crystal system. The system can be a liquid crystal composed of chiral molecules or the addition of a guest compound composed of chiral molecules (chiral dopant) into an achiral (non-chiral) nematic liquid crystal host. Both methods will induce the director to twist into a helical structure where the director uniformly rotates azimuthally about the helical axis, which is perpendicular to the long molecular axis as shown in Figure 3. The first observations of chirality in liquid crystals were in cholesteryl derivatives so the chiral nematic phase is more commonly known as the cholesteric phase.

26 11 z Pitch y x Cholesteric Figure 3 The twisted structure of the cholesteric liquid crystal phase. Oftentimes, naturally chiral liquid crystals do not have properties that will create high performance LCDs, so cholesteric liquid crystals are usually formed by adding a chiral dopant to an achiral nematic host. The twisting rate of the cholesteric phase is critical to display applications and is quantified by a length parameter: pitch (P), defined as a 360 (2π) rotation of the director. The intrinsic (unconstrained) pitch depends on the amount of chiral dopant present (fraction by weight) in the mixture and its helical twisting power (HTP) according to the relationship in Equation 2. P 1 HTP c (2)

27 12 The HTP is the degree of chirality; essentially a measure of how much twist can be induced by the dopant and has units of µm -1. The HTP varies from dopant to dopant and can even vary slightly according to the host. Therefore, in designing LCDs it is important to know the HTP of the chiral dopant/liquid crystal system in order to achieve the required pitch Elastic Deformations and Elastic Constants The director of liquid crystals in the nematic phase is uniform throughout space in the lowest energy state. However, an applied field or external confinement geometry will cause a deformation of the liquid crystal director and the elastic forces create a torque on the liquid crystal, which resists the deformation. All static deformations can be decomposed into three basic forms: splay, twist, and bend. Each deformation has its own elastic constant, which is a measure of the energy cost of each type of deformation. These are shown pictorially in Figure 4 alongside the mathematical description of the director and the elastic constant [8]. K 11 K 22 K 33 n 0 n n 0 n n 0 Splay Twist Bend Figure 4 The elastic distortions of the director: splay (K 11 ), twist (K 22 ), and bend (K 33 ).

28 13 It is usually found that the elastic constants follow the relationship K 22 < K 11 < K 33, which makes sense qualitatively if considering the molecules as closely packed rigid cylinders. Although the elastic constants are called constants, they are not really constant. First, they vary from material to material and so they must be experimentally measured. Secondly, they are temperature dependent because they are proportional to S 2. The order of magnitude of the three elastic constants is ~10-11 N. This is a good fit with the estimated order of magnitude based on a dimensional analysis that K ii ~ U/a where U is the interaction energy between molecules (~ J) and a is the molecular length (1nm) [5, 9] Elastic Energy Density The equilibrium director distribution is the one that minimizes the free energy of the system, which consists of contributions from elastic energy, electric energy, and surface anchoring energy components. The elastic energy can be written down in terms of the three basic director distortions (splay, twist, and bend) given the following assumptions: 1) the director distortions are smooth at the molecular scale so the order parameter will remain constant and 2) the bulk to surface ratio is high enough to exclude the surface elastic terms. The Oseen-Frank elastic energy density is then given by Equation 3. f elastic K11 ( n) K 22( n n) K33( n n) (3) However, because the lowest energy state of this equation corresponds to a uniformly aligned nematic it will not hold true for the cholesteric liquid crystal. Recall that the ground state of the cholesteric is a uniformly twisting director field. Therefore, if we

29 14 define a chirality term q 0 = 2π/P, where P is the pitch, then we can rewrite the elastic energy density in Equation 3 to account for the cholesteric liquid crystals as shown in Equation 4. f elastic K11 ( n) K22( n n q0) K33( n n) (4) Dielectric Anisotropy The dielectric constant of liquid crystals exhibits a uniaxial symmetry that follows from the orientational symmetry. The dielectric constant parallel to the long molecular axis differs from the dielectric constant perpendicular to the long axis, which leads to the definition of the dielectric anisotropy as shown in Equation 5. // (5) The sign and magnitude of the dielectric anisotropy is a very critical parameter to the performance of LCDs. For example, when an electric field is applied to a uniformly aligned liquid crystal, the induced dipole moment is not parallel to the field (unless the molecules are parallel or perpendicular to the field). This induced dipole moment will cause a net torque on the liquid crystal molecules which tends to align them along the direction of the field. When the dielectric anisotropy is positive, the liquid crystal molecules will reorient to align with their long axis parallel to the electric field. Conversely, when the dielectric anisotropy is negative, the liquid crystal molecules will reorient to align with their long axis perpendicular to the electric field.

30 Optical Anisotropy Light impinging on a uniaxially aligned nematic liquid crystal will decompose into two principle rays, the extraordinary and ordinary rays, where each experiences its own principle index of refraction. The birefringence is defined as the difference between the extraordinary index of refraction, n e, and the ordinary index of refraction, n o, as shown in Equation 6. n n e n o (6) The birefringence is responsible for most of the optical effects seen in liquid crystals so it is exploited by most LCD designs in order to modulate the light transmission of the display. The birefringence depends on the chemical structure of the liquid crystal molecules. The ordinary index, n o, is usually ~1.5 and the birefringence is commonly in the range from 0.05 to 0.3 although some liquid crystal compounds can have higher or lower values. The birefringence normally decreases with temperature due to the decrease in density with temperature, but the more dominant cause is the direct dependence of birefringence on the order parameter [5] Liquid Crystals in an External Field: The Frederiks Transition When a liquid crystal is confined between conducting layers in a typical liquid crystal display and connected to a voltage source then it will have a dielectric interaction with the electric field. This section describes how the equilibrium orientation of the liquid crystal director is determined, which involves minimizing the free energy in the presence of an electric field. The free energy is an integral of the free energy density, for example

31 16 the elastic free energy is F elastic f elastic dv. The free energy density of the electric field is shown below in Equation 7: f electric 1 E D 2 (7) The voltage source maintains a constant voltage across the liquid crystal, so it must supply an amount of energy equal and opposite to the work necessary to maintain a constant voltage due to the change of the component of the electric displacement normal to the conductive surfaces (along the electric field). The minimum of the total bulk free energy is achieved when the sum of the variation in energy of all three components is zero, as shown below in Equation 8. F F F 0 (8) elastic electric Following the description in [6], it was found that source F 2 (9) source F electric which means that the free energy density supplemented with the dielectric term becomes f 1 f eleastic E D 2 (10) where f eleastic comes from Equation 4. Using the components of the electric displacement along the director and perpendicular to the director can rewrite the free energy density as f f eleastic ( n E) (11) 2 The Frederiks transition occurs when the liquid crystal molecules begin to reorient due to an applied electric field. The voltage at which this occurs is called the threshold voltage and is labeled: V th. The physical meaning of the threshold voltage is based on the

32 17 free energy discussion above. When a weak electric field is applied, the decrease in electric energy is not enough to overcome the increase in elastic energy of a deformation. However, at a stronger electric field above the threshold, the decrease in electric energy is enough to reduce the free energy of the deformed system, so the liquid crystal will reorient. The threshold voltage depends on the initial geometry of the liquid crystal compared to the electric field. In the simple case of a uniform planar aligned nematic which has a positive dielectric anisotropy, when the electric field is perpendicular to the molecules (and above the threshold) then a splay deformation is induced. Using an approximation that the angle of reorientation at the threshold is small, the threshold voltage of the splay geometry is obtained as shown below [5]. K 11 V th (12) 2.2 Surface Alignment, Rubbing, and Pretilt Angle A uniform orientation of the liquid crystal is desired for most LCDs to avoid light scattering and repeatable operation. The role of the surface of the display is to induce a uniform alignment of the liquid crystal, which ensures a single domain free of defects such as disclinations where the director is discontinuous between two domains of uniform 0 orientation. Two basic polar alignment conditions exist: 1) homeotropic (vertical) alignment where the director is perpendicular to the substrates and 2) homogenous (planar) alignment where the director is parallel to the substrates. The azimuthal orientation of planar alignment is controlled by treating the surface. This is commonly performed by physically rubbing it with a linen cloth. The mechanism behind this is still

33 18 not fully understood, but it likely involves the formation of microgrooves and alignment of dangling bonds and polymer side chains of the alignment layer at the surface [8]. 2.3 The Design of a Single LCD Pixel The following description of a single LCD pixel will help illustrate the components of the basic display for future reference later in this dissertation. Figure 5 shows a simplified cross sectional view of a single LCD pixel that utilizes the top-down electrode structure. Many other electrode designs are possible, but they are generally more complex and will not work with the driving schemes that are compatible with glare shielding eyewear (as will be discussed later). Therefore, all the cells used in the experiments and simulations in this dissertation will follow the top-down electrode structure described here. The electrode material employed is a transparent conductive metal such as indium tin oxide (ITO) or indium zinc oxide (IZO) which is deposited onto a glass or plastic substrate. A barrier layer can be optionally applied over the ITO to protect the display from accidental short circuits. Most liquid crystal modes require a uniform director alignment at each surface which is performed by an alignment layer. The most common alignment layer material in use is a polyimide (PI) that is spin coated in solution with a solvent onto the substrate, baked to induce thermal imidization, and then (optionally) rubbed with a rubbing cloth.

34 19 Barrier Layer (Optional) Glass or Plastic Substrate Indium Tin Oxide (ITO) Liquid Crystal (Cholesteric) Glue Spacer Glue Polyimide (PI) Alignment Layer (Optional) Barrier Layer (Optional) Glass or Plastic Substrate Indium Tin Oxide (ITO) Colors have been added to aid the eye. Components are not drawn to scale. Figure 5 Cross section of the layers in a LCD, which happens to be cholesteric. The rubbing step is used to control the initial azimuthal (in-plane) director orientation, φ, by inducing the liquid crystal director to align uniformly along the rubbing direction. The initial polar (out-of-plane) angle of the director orientation, θ, depends on the material used, baking conditions, and rubbing conditions. This initial polar angle will be referred to as the pretilt angle and is designated with a subscript: θ p. The diagram showing the relationship between the rubbing direction and director orientation angles is shown in Figure 6.

35 20 z y director orientation θ rubbing direction φ x Figure 6 Definition of the polar and azimuthal angles of the director. The final steps of LCD assembly involve dispersing spacers to control the thickness of the liquid crystal layer (commonly 3-30μm), denoted as the cell gap, and sealing the two substrates together with glue that is cured either thermally or by ultra-violet (UV) light exposure. If two openings are left in the glue border then liquid crystal can fill into the cell by capillary action and the second opening allows air to escape. Unless otherwise noted, the displays tested in this dissertation have only one opening and must be filled under vacuum. This allows the liquid crystal to be degassed, which means all the dissolved gas is removed because it can interfere with the display s performance. 2.4 Electrode Structures and Driving Schemes Each pixel in a pixelated LCD shares the same basic design as was described previously (Figure 5) for the top-down electrode structure. An important step in the design of the LCD to be used as a pixelated device for glare shielding eyewear is the electrode structure of the pixels because this determines how the pixels will be connected to the circuit which switches them off and on. This design must place an emphasis on

36 21 creating a field of view which is optically uniform and unobstructed. This optical requirement places restrictions on which driving schemes are compatible, which then places a restriction on which LCD modes will be acceptable. This is because each LCD mode s performance is affected by the driving scheme employed. Another factor to consider is that the chosen driving scheme will dictate a limit to the resolution of the display. It is important to note that the pixel sizes in glare shielding eyewear (approximately 10mm 10mm) are going to be much larger than pixels in a display showing information content such as a computer monitor (approximately 80μm 240μm). Therefore, the discussions in this dissertation about resolution and limits to resolution refer to the number of pixels, not the pixel density. The rest of this section will describe the four major driving schemes for LCDs and which one is the best choice for pixelated glare shielding eyewear Direct Drive Scheme The simplest driving scheme of a LCD is the direct driving scheme. Each pixel of the display is able to be driven at any voltage independently from the other pixels. This arrangement allows independent control of each pixel and the voltage applied is only limited by the capability of the circuitry components and power source. A well known example of this type of driving scheme is the 7 segment LCD seen on watch and clock displays as shown in Figure 7. The pixels are the components used to make up the numbers 0 through 9, but they can also include arbitrarily shaped patterns such as icons or fixed letters in a word. The gaps between the elements allow supply lines to reach each pixel of the design on one substrate. The other side is patterned with the same

37 22 placement of pixel elements, but they are all interconnected to form a common electrode. Care is taken not to overlap the interconnections with the supply lines on the other substrate since these will form unwanted pixels [10]. Legend To Individual Connections (Top Substrate) To Common Connections (Bottom Substrate) ITO Electrodes (Both Substrates) Figure 7 An example of a direct drive connection with a seven segment display. The downside of the direct driving scheme, as shown in Figure 7, is that space is needed between pixels along the outside border to reach pixels near the center. The only workaround that would avoid the gaps and achieve full area coverage involves connecting all the pixels to the edge of the display. There are two versions of this type of display and they will be described next with the resolution maximized. The first version is simpler because it has one common electrode on the bottom substrate and each pixel on the top substrate is independent of the others. A diagram of this design is shown in Figure 8. Figure 8 (a) shows the most general structure where just the top substrate pattern is shown and the lines represent the interpixel gaps where the

38 23 ITO has been removed. The number of rows is unlimited in the first and last columns. The number of columns in the first and second rows is also unlimited. However, there cannot be anymore than two rows in columns 2 through (N C -1) where N C is the total number of columns. The total number of connections required is 2 (N C -2) + N R1 + N R2 + 1, where N R1 and N R2 are the number of rows in column 1 and column N C and the additional connection is for the common bottom electrode. The design can be made to accommodate only square pixels of the same size with an aspect ratio fixed at 1:1, but this further limits the overall resolution by reducing the number of rows in the first and last column to 2 as shown in Figure 8 (b). It does reduce the connections required, however, to 2 N C + 1. a) b) N R1 N R2 2 N C N C Both designs employ a common bottom electrode. Figure 8 A high resolution direct drive pixel layout which uses a common electrode. The second version is more complex because it has requires multiple connections on each substrate and the voltage applied to each electrode might depend on the voltage applied to another electrode. However, each pixel is still able to have any voltage applied to it independent of the other pixels and the resolution is improved to a maximum of four rows of pixels. A diagram of the second version design is shown in Figure 9. The top

39 24 electrode pattern is shown above the bottom electrode pattern. To save space, only the diagram with all pixels of the same size and a 1:1 aspect ratio is drawn; however, as in the first design, the first and last columns can have an unlimited resolution if the aspect ratios are stretched or the pixel size is changed. The number of connections required is, in general, 4 (N C -2) + N R1 + N R2 + 5 and if simplified using square pixels of the same size, 4 (N C -2) To drive this display, the voltage is first set on the long horizontal electrodes in the center as well as the long vertical electrode on the edge, each marked with an asterisk (*) in Figure 9. Then the electrodes above them are set according to the pixel voltage required in the center pixels. Then the voltage on the outside electrodes on the bottom substrate depends on the voltages of the electrodes above them and the desired pixel voltages; however, any voltage can be applied to any pixel as long as these dependencies are solved by the driving circuitry before each addressing frame. Top Substrate N R1 N R2 * * * Bottom Substrate N C Figure 9 A four row high resolution direct drive pixel layout.

40 25 In summary, the resolution of the direct drive scheme with a common electrode maximizes at two rows and with patterning on both substrates maximizes at four rows. If the display is rotated then the description is the same with rows exchanged for columns. Both direct drive designs allow an independent voltage to be applied to each pixel, but without a common electrode on one substrate, the voltage applied to each electrode will depend on each other. In both cases, if pixel aspect ratios besides 1:1 or unequal pixel sizes are acceptable, then the first and last column will have an unlimited resolution Active Matrix Drive Scheme The active matrix drive scheme allows the individual pixels of the LCD to be switched on and off independently of each other. The vast majority of LCDs made for high resolution applications, such as televisions and computer screens, employ an active matrix drive scheme because it can achieve a higher performance and a higher pixel density (more pixels per unit area) than any other method. Each pixel of an active matrix display resides at the crossover of the row and column bus lines and is composed of a thin film transistor (TFT) and a capacitor as seen in Figure 10 (a). There are several downsides to the active matrix display that render them unsuitable for an application in glare shielding eyewear. For example, the TFT fabrication procedure requires several photolithography steps which increase manufacturing cost. The cost is increased even further if the assembly uses plastic substrates, since common TFT processing temperatures are high enough to ruin plastic by exceeding the glass transition temperature of the material. Even if manufacturing costs and processing issues could be overcome, certain characteristics of the active matrix

41 26 design are unbefitting of an eyewear application. As an example, it was previously stated that the field of view must be free of obstructions, but the active (switchable) area of each pixel is diminished by the space consumed by the driving circuits, storage capacitor (C S ) and scan and data bus lines. Many of these elements can not be made to be transparent, since the elements of the TFT in an active matrix display are sensitive to stray light that will induce a photocurrent in the semiconductor material. Therefore, a black mask must be included to block this light and although it is quite small and outside the wearer s focus, it might still be perceptible. It is important to note that light striking a scattering mode LCD can be scattered to a large enough angle to be guided along the inside of the liquid crystal material by total internal reflections. This means that if the black mask resides on the substrate opposite the TFT as is common in active matrix designs, it would not be able to shield against large angle scattered light. Another opaque component of the active matrix structure is the storage capacitor, which is used to reduce the TFT off current (leakage current) and increase the RC (resistance capacitance) time constant to reduce flicker. However, for pixel densities lower than 200 lines per inch (as would be the case in glare shielding eyewear) the capacitance of the liquid crystal pixel itself (C LC ) is large enough, so this storage capacitor (C S ) is not required [8].

42 27 a) b) Column Line Column Line Row Line C S C LC C MIM Row Line TFT C LC Figure 10 An equivalent circuit of an active matrix pixel: a) TFT b) MIM. Another possible design of an active matrix display will use metal-insulator-metal (MIM) pixel switches (C MIM ), as shown in Figure 10 (b), instead of the more common TFT. There are several differences between displays using MIMs compared to those using TFTs. Displays using MIMs have the advantage of lower processing temperatures than TFTs, which make them more compatible for fabrication on plastic substrates. MIMs can also require many fewer photolithography steps which can reduce complexity and cost. MIM displays place the column lines on the opposite substrate which can increase the aperture ratio compared to TFTs, but might introduce complexity if crossover connections are desired to place all the row and column pads which link to the driving circuitry (via flex cable) on one substrate. MIM displays are not able to incorporate storage capacitors because the lines are not grounded. MIM displays do not require shielding from stray light using a black mask since they are not photosensitive. A disadvantage of MIMs that use ITO electrodes is a pronounced asymmetry that may be compensated by using a pair of MIMs connected back-to-back [10]. Finally, two other

43 28 possible switches exist, one using a diode ring and the other using a nonlinear resistor called a varistor. The diode ring performs like the MIM, but is nearly as expensive to fabricate as TFTs. The varistor substrate is not transparent, so it was never commercialized [10]. Some physical limitations exist with an active matrix design. Firstly, it requires liquid crystal materials with a high resistivity (low conductance) in order for the voltage to remain on the pixel during addressing. Since the conductance of liquid crystals can vary widely, this requirement will place a large restriction on the list of liquid crystal materials that are compatible with an active matrix. Secondly, although the voltages that can be applied are in principle unlimited, they are in fact limited by the sensitive components of the switching element to be, in general, less than 10V [11]. In summary, the active matrix drive scheme has many obstacles to overcome in order to be compatible with glare shielding eyewear. Examples include reducing, eliminating or replacing the light sensitive components, improving the optical uniformity so the driving circuitry at each pixel is not visible, finding suitable materials with high resistivity and low driving voltage, and limiting the processing steps and temperatures to reduce cost and allow fabrication on plastic substrates. These obstacles are not surmountable within the scope of the research in this dissertation, therefore the active matrix drive scheme is not considered Passive Matrix Drive Scheme The passive matrix drive scheme is a design that utilizes two substrates with stripe electrodes oriented perpendicularly so that a matrix of pixels is formed at the overlapping

44 29 regions. An example of the electrode design is shown in Figure 11, where Figure 11 (a) shows the ITO pattern outlined by stripes and the pixels are shaded in the diagram with the gaps between the electrodes exaggerated (not to scale). Figure 11 (b) shows the electrodes as lines so you can visualize the liquid crystal at each pixel as its circuit equivalent: a capacitor. The liquid crystal is a dielectric medium that creates a capacitor (Figure 11 (b)) at each pixel, but the liquid crystal fluid itself is not pixelated; it fills the display and connects to all the pixels. When there are N R rows and N C columns, there are N R N C pixels and N R + N C connections. Addressing of the display is carried out a row at a time; the row voltage is set to the scanning voltage and the columns are individually addressed with the data voltage. If the time to address a row is t, then addressing N R rows takes T f N t time, called the frame time. R a) b) Top Substrate Top Substrate N R Bottom Substrate N C Bottom Substrate Figure 11 A passive matrix drive scheme: a) electrode layout b) equivalent circuit.

45 30 There are three main concerns with this type of addressing: 1) the frame time should be shorter than the temporal response time of the human eye (~40ms) in order to avoid flicker and shorter than the response time of the liquid crystal, 2) the rows not being addressed may have a different voltage applied to them compared to the row being addressed, and 3) the changes in column voltages will affect the pixels not being addressed (in the other rows). The last two points can cause an undesired voltage on some pixels. For example, if the intent is to apply a voltage (V) to only one pixel, then a voltage (V) is applied to one row and a voltage (0) to one column. However, there is an electrical path that can be drawn through all the pixels that distributes the voltage (V) over a circuit of series capacitors which means that every pixel will experience some fraction of the total voltage (V) applied These effects that cause undesired voltages are called crosstalk [5]. During operation, the pixels that are switched on have a desired voltage V on applied and the other pixels will have (at most) an undesired voltage V off applied. The display is still effective as long as V off causes an acceptably small amount of reorientation of the liquid crystal, which is true if V off is near or below the threshold voltage V th. When addressing a single row, the voltages applied to the pixels are demonstrated in Figure 12 where the selected row is set to V sel and the other rows are set to V nsel and the columns with pixels on are set to V on and the columns with pixels off are set to V off. Then the voltage on pixels in the non-selected rows is at either V 1 or V 2 and the voltage on pixels in the selected row is at either V 3 or V 4. The maximal ratio between the value of V on and V off is achieved when V 1, V 2, and V 3 are minimized, which makes the ratio of V on :V off =

46 31 3:1. This assumes V sel = 0V, but it doesn t change the ratio if V sel 0V, therefore, in this dissertation, V on and V off always refer to the value relative to V sel. Selected Rows V 4 V 3 Non-Selected Rows V 1 V 2 V on (columns with pixels on) V nsel (non-selected rows) V off (columns with pixels off) V sel (the selected row) Figure 12 Demonstration of the voltages used while addressing a single line. However, when addressing multiple lines a row at a time, the maximized ratio between the root mean square (RMS) values of V on and V off is limited by the number of rows, N R, in the display. This is calculated using the method of Alt and Pleshko [12] and the result is shown in Equation 13. V V on off R m N N R R 1 1 (13) The maximized ratio decreases rapidly as the number of rows increases as seen in Figure 13. The contrast ratio normally has the opposite dependence, which means that it increases with increasing applied voltage. Therefore, the highest achievable contrast ratio decreases with increasing display resolution. However, more rows can be addressed with no loss in contrast ratio if the steepness of the electro-optical response curve can be increased, which will be described in more detail in Chapter 3. The row-at-a-time passive matrix driving scheme only turns a pixel on and off, so to achieve an intermediate

47 32 transmission level, called grey scale levels, the display can vary the ratio of on-time to off-time between frames (frame rate control) or vary the ratio of on-time to off-time within the applied voltage waveform (pulse width modulation). The use of grey scale levels might improve the aesthetics and/or user comfort in pixelated glare shielding eyewear, but that is beyond the scope of this dissertation. 2.4 Maximized Voltage Ratio, R m Number of Rows, N R Figure 13 Maximized voltage ratio given the number of rows in a passive matrix Block Addressing Drive Scheme Block addressing is a special type of passive matrix driving that can achieve a higher voltage ratio (V on /V off ) than the standard row-at-a-time passive matrix addressing scheme (Equation 13) because it addresses the entire display at once. Block addressing can then reach the maximum voltage ratio described in Figure 12, an on state voltage (V on ) that is three times higher than the off state voltage (V off ), which is why it is also called the 3:1 addressing scheme. However, this larger ratio comes with a tradeoff. Whereas the

48 33 standard row-at-a-time passive matrix addressing scheme can place an arbitrary pattern on the display, the block addressing scheme addresses the entire display at once, so its patterns are limited to continuous blocks of pixels with any number of entire row or entire column slices through them. This gives the patterns a block appearance (for which the scheme is named) as can be visualized in Figure 14, which shows examples of allowed and disallowed patterns. The lines represent the interpixel gaps and the darkened squares represent pixels in the on (selected) state at voltage, V on. Off pixel On pixel Allowed NOT Allowed Allowed NOT Allowed Allowed Figure 14 Block addressing demonstration. The examples in Figure 14 show that a pixel must be turned on if it shares both a row and a column with any other pixel that is also turned on. However, pixels that share only a row or only a column with a pixel that is turned on are able to be turned off (to the unselected state voltage, V off ). Unless otherwise specified, this dissertation uses passive matrix addressing/drive scheme to refer to the row-at-a-time (resolution dependent) voltage ratio and block addressing/drive scheme to refer to the fixed 3:1 voltage ratio.

49 Bistable Drive Scheme Most liquid crystal display designs must have the voltage applied continuously in order for the orientation of the molecules (and therefore the optical state) to remain fixed. If the voltage is not applied continuously, then it can be applied at a regular interval faster than the response time of the liquid crystal, which is usually often enough to keep the orientation at equilibrium. This will work because the liquid crystal molecules are sensitive to the root mean square (RMS) of the applied voltage over time. Using a liquid crystal with a slow response time can help reduce the frequency of the updates, but will still require power to stay in place. The solution is to use LCDs that are able to remain in place after being initially reoriented without the voltage being constantly reapplied. These types of LCDs are called bistable because they only require a voltage to be applied to change the orientation, not to maintain it. For the purpose of this dissertation, we consider bistability between two optical states (on and off) although in principle grey level states may be possible. The driving scheme for a bistable LCD utilizes the same matrix of row and column electrodes as a passive matrix does and operates by addressing the display a row-at-atime. The exact scheme used depends on the LCD employed, for example the voltages for on and off states might be two different amplitudes of the same polarity, two different polarities of similar amplitude, or two different frequencies [13] of the same polarity and similar amplitude. The bistable driving scheme can also be associated with a hysteresis in the transmission characteristics of the LCD. This means that if the hysteresis is large enough so that two states of the LCD are stable at a bias voltage then the bistable driving

50 35 scheme can be implemented so that the on and off state are above and below the bias voltage. Admittedly, using a bias voltage will not save power because the bias voltage must be maintained at all times, so these types of LCDs are not strictly bistable, but they are compatible with the bistable driving scheme [10]. The resolution of the bistable driving scheme is not limited by the steepness of the electro-distortional curve (transmission vs. voltage) like a passive matrix driving scheme. However, since the truly bistable LCDs can leave darkened pixels in place at 0V by design, they will not become uniformly transmissive if the power source fails so they are not applicable to glare shielding eyewear. Therefore, only those LCDs that operate with the bistable driving scheme at a bias voltage (and are also transmissive at 0V) can be considered. 2.5 Experimental Methods This section will provide a description of the experimental methods, including the equipment used, how the performance parameters are measured, and the relevant terminology Mixture Preparation When a new liquid crystal mixture was prepared for use in an experiment, the same procedure was used to minimize experimental error. Two possible mistakes that could cause trouble are: incorrect or unknown amounts of each component and improper or incomplete mixing. The first mistake was prevented by using a scale with a digital readout in grams that had 5 significant figures after the decimal point. Each mixture was weighed according to

51 36 the actual amount placed in the bottle and the percentage by weight is calculated (wt.%) after all components are added. All references to the amount of a component in a mixture in this dissertation are wt.%, but for simplicity shall just be stated as %. Each mixture reached a minimum total weight of 100mg, so each component in the mixture had two digits of precision after the decimal point when converted to percent. Controlling the weight of a solid powder component is easier than a liquid because the solid can be added in smaller increments. Therefore, a greater accuracy in reaching a target weight was maintained by adding the solid components first and starting with the component having the smallest contribution to the total. If a major deviation from the desired weight occurred, the sample could be thrown out before the mixture was complete, which minimized waste. The second mistake was avoided by alternating steps of vigorous mixing of the mixture with heating the mixture up to ~60-80 C (lower than the clearing point). The mixing was carried out by shaking the bottle from the outside. These steps expedited the formation of a homogenous solution by improving the solubility of the solids in the liquid crystal host. The process was repeated until no more solid was detected in the bottle by eye and then performed one more time for good measure Experimental Equipment and Setup The system used to measure the scattering PSCT displays is shown in Figure 15. Working from right to left, an unpolarized white light source whose beam diameter is controlled by aperture #1 is collimated and focused onto the sample by lens #1. The typical value of the beam diameter incident on the sample is 2.5mm. The light scattered

52 37 by the sample is collected by lens #2 and focused into the photodiode detector by lens #3. A photopic filter calibrated to the sensitivity of the human visional system was used to ensure the measurement mimicked the sensitivity of the human eye while also attenuating light outside the visible range. computer amplifier detector photopic filter lens #3 lens #2 aperture #2 sample lens #1 aperture #1 white light source Figure 15 Drawing of the setup measuring contrast ratio and response time of PSCT. The most critical adjustment in this system is the diameter of aperture #2 as well as the distance between it and the sample because they determine the collection angle. The scattered light intensity varies with collection angle and thus, the contrast ratio will vary with collection angle. The collection angle of our measurements (unless otherwise stated) is a total angle of θ = 2 as defined in the drawing shown in Figure 16.

53 38 aperture r θ 2 tan 1 r ( ) d sample light d Figure 16 Definition of the collection angle. Also shown in the diagram of Figure 15 are the computer and amplifier which supply the voltage to the sample. The computer has a data acquisition (DAQ) card from National Instruments, NI PCI-6221, controlled by LabVIEW software designed by the author that outputs a square wave of variable frequency up to a 833kS/s sampling rate and amplitude up to 10V RMS (both specifications are from the manufacturer). The amplifier can then increase the amplitude further, up to 74V RMS for all frequencies up to 20kHz (both verified from measurement). The software also reads the voltage from the detector at a variable sampling rate up to 250kS/s (from the manufacturer specifications). Unless otherwise specified, the voltage source used to switch the liquid crystal samples was a 1kHz square wave. This is a practical choice because much of the published research (including the material constants) is measured with this waveform and frequency. Another factor affecting the choice of driving voltage is the limitation of the driving circuitry. For example, it is highly desirable to minimize the power consumption of the LCD in a battery powered portable device (such as pixelated glare shielding

54 39 eyewear). Power can be saved if the peak voltage amplitude and driving frequency required are as low as possible without sacrificing performance. A square wave is desired instead of a sine wave or other waveform because it does not require the peak amplitude to be higher than the root mean square (RMS) value. Unfortunately, the square wave is oftentimes incompatible with the LCD driving scheme, but the goal to limit the peak amplitude still applies. Additional power savings is realized by minimizing the applied frequency. However, if the frequency is reduced too far, a larger number of ions dispersed in the liquid crystal material will begin to collect at the top and bottom of the cell effectively cancelling a portion of the electric field that reorients the liquid crystal molecules. This effect will lower the effective voltage on the cell and causes a residual voltage to remain after the applied voltage is removed (called image sticking ). Some LCD modes require high frequencies (>5kHz) to operate and these will draw too much power to be considered for an application in pixelated glare shielding eyewear. Consequently, a frequency of 1kHz is used to reduce power consumption and avoid performance degrading ionic effects Transmission Measurement and Calculation The first experiment to be performed on a sample is the measurement of the transmission vs. voltage. The transmission of the sample is recorded by the software using the following calculation shown in Equation 14. VSAMPLE VBACKGROUND T 100% (14) V V REFERENCE BACKGROUND

55 40 The voltage of the photodiode detector, discussed previously as shown in Figure 15, will output a DC voltage from 0-10V that is linearly proportional to the light intensity. This is V SAMPLE when the sample is being measured. Just before the experiment is undertaken, the reference, V REFERENCE, and background, V BACKGROUD, measurements are performed. The light intensity is adjusted so it is high enough for a good signal to noise ratio, but not so high that it causes a loss of information due to saturation of the detector. The reference measurement, V REFERENCE, is usually made with no sample present, but it can be taken with some other configuration. The detector has a constant nonzero voltage output when no light is present. Furthermore, stray light in the laboratory will add an additional voltage to the detector output even if the experimental light source is switched off. Both of these effects are measured with the direct light from the light source blocked and stored as V BACKGROUD to be subtracted from the sample and reference voltage readings. All experiments referring to transmission, T, are values in percent referenced to no sample present unless otherwise specified Contrast Ratio from Static Response of Transmission vs. Voltage A static response measurement results in a curve of transmission vs. voltage (T-V curve) and is defined by the following variable parameters set in the software (typical values in parenthesis): starting voltage (0V), ending voltage (50V), number of steps (51), delay time (1000ms), and frequency (1kHz). An experiment is carried out automatically where the voltage to the cell is instantly updated number of steps times from the starting voltage to the ending voltage, then back down. For example, 0V to 10V with 11 steps would measure transmission at 0,1,2,3,4,5,6,7,8,9,10V, then repeat those 11 steps in

56 41 reverse order. A delay between each step is incurred; however, the voltage value applied to the sample is maintained throughout the delay. During the final moments of the delay time a transmission measurement is stored as an average encompassing several samples of the detector voltage. All experiments used a delay time of 1000ms and averaged 15,000 samples at a rate of 100kS/s unless otherwise noted. The contrast ratio is the performance metric that summarizes the darkening ability of a display with a single number. The contrast ratio (sometimes shortened to just contrast) is the ratio of the transmission of the bright state to the transmission of the dark state and is shown in Equation 15. TBright Contrast Ratio (15) T Dark The bright state of a normally white display is at 0V or at V off <V on and the dark state is at the voltage where the transmission minimizes (and therefore, contrast ratio maximizes) if possible. It is important to note that the transmission might minimize at a particular voltage or it might continuously decrease with voltage (approaching 0%). The dark state of a normally black display is at 0V or at V off <V on and the bright state is at the voltage where the transmission maximizes (and therefore, contrast ratio also maximizes). Similarly to normally white displays, the transmission of normally black displays might maximize at a particular voltage or it might continuously increase with voltage (approaching 100%).

57 Response Time from the Dynamic Response of Transmission vs. Time The total response time of glare shielding eyewear includes the time needed for the electronics to detect glare, calculate what pixels to darken, address the pixels of the display, and wait for the liquid crystal molecules to completely finish reorientation to an equilibrium state. If the entire field of view is darkened (all pixels bulk switched as a single pixel) first, then when the individual pixels are addressed, the glare shielding pixels stay dark and the remaining pixels are switched back to the original bright state. This method (single pixel response) can be used to eliminate the addressing time, which depends on the driving scheme and resolution of the display. The response time of typical LCD electronics can occur very rapidly: ~100μs [14]. Furthermore, all but the fastest liquid crystal displays will have a director reorientation that is much higher: most often in the range ms, which is the limiting contribution to the total response time. Therefore, all response times in this dissertation refer to the liquid crystal response time and this section describes the details of how it is measured experimentally. The response times of the liquid crystal can be measured by analyzing the dynamic response using the measurement of the light transmission as a function of time. As mentioned previously, the experiment was setup to achieve a temporal resolution (sampling rate) of 100kS/s or one measurement every 0.01ms. It was confirmed that the equipment was capable of detecting sub-millisecond transitions by using it to measure a high speed liquid crystal display: the pi cell (OCB) mode.

58 43 Table 1 Parameters in the response time measurements. Label Parameter Specification Typically V off Pre-Pulse Voltage V off (@3:1 V ratio for max contrast) V off or 0V 1 Pre-Pulse Time Long enough for stable reading 500ms V OD Over Drive Pulse Voltage As high as possible Limited to 30V 2 Over Drive Time As long as T is still decreasing <2ms V on Pulse Voltage V on (@3:1 V ratio for max contrast) V on 3 Pulse Time Long enough to reach saturation 500ms V off Post Pulse Voltage V off (@3:1 V ratio for max contrast) V off or 0V 4 Post Pulse Time Long enough for stable reading 500ms Inset Frequency Same as operation frequency 1kHz A dynamic response measurement results in a curve of transmission vs. time. An experiment will measure both the response to a rising voltage and a falling voltage using one pulse and is defined by the variable parameters set in the software. These parameters are described in Table 1 which lists the description of the specification alongside a typical value and the label used to identify that region in Figure 17. An experiment is carried out automatically with attention paid to make sure it lasted long enough for the transmission to reach saturation and the response times are calculated from the curve. Most experiments set the value of the pre-pulse and post-pulse voltage to V off and the pulse voltage to V on to test the display under operating conditions. These values are taken from the voltage where the maximum and minimum in transmission occur in the static response measurement described earlier. However, the maximum and minimum transmission that can actually be obtained will likely be constrained by the voltage ratio V ratio = V on /V off (for example given by Equation 13), or the voltage capabilities of the power source or even a maximum voltage imposed by strict power

59 RMS Voltage (V) 44 consumption limits. Therefore, the response time is sometimes measured at other voltages for comparison. V OD f Inset: Square Wave at frequency = f V on V off Time (ms) Figure 17 General form of the RMS voltage applied to a sample during a response time measurement. The inset is a reminder that the pulses consist of many cycles. This dissertation shall state the response time with two separate numbers: the turn on (τ on ) and turn off (τ off ) time, which refer to the voltage transitions from V off < V on and from V on > V off respectively and if a voltage other than one of these operation voltages is used, it shall be specified. The calculation of response time is found from the curve of

60 Transmission (%) 45 transmission vs. time as shown by the cartoon of a typical curve in Figure 18. T max T min + 90% of (T max - T min ) T min + 10% of (T max - T min ) T min τ on Time (ms) τ off Figure 18 Transmission curve (bold) of a typical response time measurement with the applied voltage pulse (dashed) shown underneath. First, the values of the maximum transmission (T max ) and minimum transmission (T min ) are found by averaging over several points once saturation has been reached. Then, the standard response time is calculated to be the time to transition from a point on the curve that is 10% of the range T max T min higher than T min to a point that is 90% of the range T max T min higher than T min and visa-versa. In other words, τ on is from 10% to 90% for a normally black display and from 90% to 10% for a normally white display.

61 46 During the course of this study, it was observed that some LCD modes have a hesitation time preceding the transition which is not contained in the standard response time. Therefore, this dissertation includes a modified calculation to report response times which include any hesitation time immediately after the voltage is changed. The calculation is modified so it can be thought of as the transition from 0% to 90% or from 100% to 10% and is calculated as the time needed to reach a point 10% (or 90%) of the range T max T min higher than T min from the point the voltage is switched (V on V off or V off V on ). To avoid confusion related to whether the specific mode being discussed is normally white (bright) or normally black (dark), the designation or 0-90 is added to the response time label according to whether the transition is to a dark state or a bright state respectively. A summary of the response times reported in this dissertation are shown in Table 2. Table 2 Response time calculations Transition Designation Definition Dark to Bright τ on 0-90 Time required to switch up to 90% of the range (T max T min ) from the instant the pulse V on is applied. Dark to Bright τ off 0-90 Time required to switch up to 90% of the range (T max T min ) from the instant the pulse V off is applied. Bright to Dark τ on Bright to Dark τ off Time required to switch down to 10% of the range (T max T min ) from the instant the pulse V on is applied. Time required to switch down to 10% of the range (T max T min ) from the instant the pulse V off is applied. The previous description of the calculation assumed that the duration of V OD was 0ms. However, if V OD > 0ms, then it does not affect the calculation except to clarify that

62 47 the beginning of the pulse for the purpose of the calculation starts when V OD is applied. The over drive portion of the pulse is precisely the voltage used during the single pixel method of driving the entire display before addressing individual pixels. The response time is usually improved with an over drive voltage V OD > V on since the single pixel driving is only limited by the maximum capability and/or design of the electronics (instead of being limited by the addressing scheme). It can be assumed that over drive is not used in an experiment unless it is otherwise stated. Figure 19 shows the photoinitiator used in all the polymer stabilized cholesteric texture display experiments, benzoin methyl ether (BME) (Aldrich). For reference, the spectral output of the UV light source used to cure the polymer network in our samples is shown in Figure 20 and the detector used to measure UV intensity in mw/cm 2 is model XRD-140B (International Light) with a sensitivity range from nm and a peak sensitivity at 365nm attached to the IL1350 Radiometer/Photometer (International Light). Figure 19 Photoinitiator benzoin methyl ether (BME).

63 Intensity (unreferenced counts) Wavelength (nm) Figure 20 Output spectrum of UV source used with the most relevant peak at 365nm. 2.6 Which LCDs Can Work in Pixelated Glare Shielding Eyewear? The intent of this section of the dissertation is to present a review of different transmissive LCD technologies, within the context of their relevancy to pixelated glare shielding eyewear. Each display s operation principle and design is specifically considered for compatibility with glare shielding eyewear and if necessary, the expected performance of the contrast ratio, response time, and viewing angle are also considered. This means the best performing LCDs used in information displays may not work for a glare shielding display application. Specifically, a display is eliminated from consideration if any of the following criteria are NOT met:

64 49 The display must be operable using a unobstructed and nearly continuous pixel layout and have satisfactory performance at a high enough resolution to constitute a significant improvement over a single pixel, i.e. >4 rows. This limits the available driving schemes to the passive matrix row-at-time (maximum V ratio from Equation 13), passive matrix display-at-once block addressing (maximum V ratio 3:1), or a bistable drive scheme. The display must be failsafe, so if the power source fails, it will revert to a transmissive (bright) state (referred to as normally white), which means all truly bistable designs are not considered. The display must have a transmissive state that has a reasonable uniformity across the visible spectrum, i.e. close to a white, neutral grey, or color-less appearance. The display must be able to switch on and off using a portable power source (for example a rechargeable battery). The display s response time should be faster than the eye saccade latency [15] or the blink reaction time [16], both of which are ~200ms to avoid discomfort. The evaluation of each LCD s suitability will classify a design as eliminated if it doesn t meet one of the aforementioned criteria or undesirable if other shortcomings may present impractical complications. Those LCDs that remain are classified as possible. A summary at-a-glance of each display is included at the end of this chapter in Table 3.

65 LCD Designs Since the invention of the first LCD by Heilmeier [17], the dynamic scattering mode (DSM), decades of research and billions of dollars of product sales have helped to spur an industry that has spawned a multitude of LCD designs. Each of these designs is derived from changes in the electrode layout, liquid crystal alignment, pitch, material properties, chemical mixture, and many others. For example, by varying the pitch in a cholesteric liquid crystal from a few tenths of a micron to infinity, as shown in Figure 21, several types of LCD designs have been developed (although some require other changes too). However, one common theme among all the LCDs considered here is that they must be switchable using an applied voltage which reorients the liquid crystal. (PSI) Polymer Stabilized Isotropic <400nm (Ch) Reflective Cholesteric nm (PSCT) Normal Mode Polymer Stabilized Cholesteric Texture ~0.5-2μm (R-PSCT) Reverse Mode Polymer Stabilized Cholesteric Texture (STN) Super- Twisted Nematic (TN) Twisted Nematic ~5μm ~5-15μm ~20μm (VA) Vertically Aligned & (ECB) Electrically Controlled Birefringence Pitch Figure 21 Some of the LCDs along the continuum of the cholesteric pitch Liquid Crystal Displays Without a Polymer Network The dynamic scattering mode (DSM) LCD relies on the conductivity of the liquid crystal or ionic dopants to stimulate turbulence which forms a multi-domain structure composed of regions of different liquid crystal director orientation where the change in

66 51 index of refraction from one domain to another scatters light. All scattering mode LCDs use the same optical effect, but in the DSM LCD the domains are continually changing with time from the motion of the fluid. The DSM LCD requires a much higher current to drive than would be reasonable for a battery powered device, which eliminates it from consideration in this dissertation. The guest-host (GH) LCD uses an incorporated dye in the liquid crystal mixture which absorbs light. The dye molecules align with the liquid crystal and change orientation with the liquid crystal when a voltage is applied. The dyes absorb light more strongly when it is polarized along the long axis of the dye molecule than when polarized along the short axis of the molecule (or visa-versa). Therefore, some GH LCDs require polarizers to absorb the polarized component of light that is not absorbed by the dye in the mixture, but other methods can be used to absorb unpolarized light such as a twisted structure (induced by a chiral dopant), a double stacked design, or a multi-domain structure. The basic GH LCD suffers from low contrast because it is limited by the ratio of the absorption along each molecular axis (dichroic ratio) of the dye [5] and many dyes only absorb in a certain wavelength range, so a black dye would be necessary to absorb all visible wavelengths. This makes the GH LCD undesirable; however, dyes can be doped into other LCDs to possibly improve the performance (as will be described in Chapter 7). When the pitch of a liquid crystal is so long it becomes infinite, then either no chiral dopant is present or there are exactly equal amounts of right and left handed dopants. This is simply the case of the nematic display and many displays can be designed using

67 52 the (untwisting) nematic such as the two simple geometries listed in Figure 21 among many others. The electrically controlled birefringence (ECB) LCD places a homogenously aligned liquid crystal whose total optical retardation is an integer of a half wave at 45 between crossed polarizers. At 0V it transmits light and at high voltage ( 0 ) it switches to black. This design is eliminated from consideration because the electro-distortional curve is not steep enough for the passive matrix drive scheme and it is too colorful in the transmissive state. The vertically aligned (VA) LCD operates has a homeotropic alignment layer on both substrates to induce a uniform alignment perpendicular cell surfaces. When placed between crossed polarizers the optical retardation is nearly zero for all wavelengths at normal incidence and thus the display has an excellent dark state. The VA LCD uses a liquid crystal with a negative dielectric anisotropy ( 0) so that when a field is applied, the liquid crystal is reoriented to become parallel to the display substrate at 45 between crossed polarizers and the display becomes bright [18]. There is, however, a problem if the liquid crystal in the field-off state is exactly perpendicular to the cell substrate because then the liquid crystal is initially parallel to the applied field. This means there is no preferred direction for the reorientation, which causes a slow turn-on time, low bright state transmittance and non-uniformity due to defects. The solution is to create a preferred direction for the liquid crystal to reorient under the field. One method uses patterned electrodes or protrusions to generate tilted electric fields such that the applied field is no longer parallel to the liquid crystal. Unfortunately, that type of

68 53 geometry wouldn t be feasible in a LCD used in eyewear. Another method uses alignment layers with a very small pretilt angle (as defined from the perpendicular) induced by rubbing the homeotropic alignment layer. An investigation by the author showed that increasing the number of rubbings will increase the pretilt angle and this decreases the response time [19]. Although the rubbing technique can produce a defect free VA LCD it is still inherently a normally black LCD, meaning it is dark at 0V, so this eliminated it from any further investigation in this dissertation. Another type of nematic LCD is the optically compensated bend (OCB) LCD other wise known as the pi cell LCD [20]. The OCB LCD consists of a homogenously aligned nematic with a pretilt angle in opposite directions at each surface. This gives the cell a splay director configuration at 0V. When a voltage above the critical voltage is applied, the liquid crystal slowly transforms to the bend director configuration. While in this configuration, if the rubbing direction is at 45 between crossed polarizers, then it will be transmissive. At a high voltage, the bend state reorients further towards a nearly homeotropic orientation, which switches the display to black. The viewing angle is symmetric due to the self compensation from the opposing pretilts and the director profile eliminates the backflow effect resulting in a very fast response time. The downside of the OCB LCD is that even though the splay state is transmissive, it takes too long to transform to the bend state (tens of seconds) because the transition is a nucleation process. However, a bigger downside is that the electro-distortional curve is very gradual, which renders it incompatible with passive matrix addressing.

69 54 Two more nematic LCDs are: the in plane switching (IPS) [21] and the fringe field switching (FFS) [22] LCDs, which operate using electrodes patterned on only one substrate of the display. These electrodes are inter-digitated such that the electric field lines curve from one electrode to the other and are mostly in the plane and parallel to the substrates between the electrodes and mostly perpendicular to the electrodes in the region just above them. The benefit of the in plane design allows for an excellent viewing angle improvement compared to many other LCDs. Unfortunately, the IPS and FFS LCDs can not be used as a display for pixelated liquid crystal eyewear because the inter-digitated electrode structure must be driven using an active matrix driving scheme. The twisted nematic (TN) LCD uses two rubbed alignment layers oriented perpendicularly to make the liquid crystal rotate 90. A small amount of chiral dopant ensures one handedness is favorable and the pitch is commonly ~20μm so that the eigenmodes of light propagating parallel to the helical axis are decomposed into two linearly polarized modes: one parallel to the director and one perpendicular to it. This means that the plane of polarization of linearly polarized light will follow the twist of the liquid crystal. The TN is placed between crossed polarizers, which are commonly aligned with the rubbing directions at each substrate. This configuration is normally white and an applied voltage reorients the liquid crystal to a nearly homeotropic texture for the dark state. (A TN LCD between parallel polarizers is normally black). The TN LCD suffers from poor viewing angles and the electro-distortional curve is not normally steep enough to be driven using a passive matrix, so it is undesirable.

70 55 If the twist angle is increased above 90, placing the pitch in the range 5-15μm, then the super twisted nematic (STN) LCD can be made. The STN has a steeper electrodistortional curve and can be driven by a passive matrix, so it is ideally suited for pixelated glare shielding eyewear and it is the subject of a detailed study discussed in Chapter 3. Although any twisted nematic structure is technically a subset of cholesteric liquid crystals, the cholesteric (Ch) LCD commonly refers to a pitch within the visible range from nm. This region is special because incident light will be reflected from the periodic structure of the twisted planar texture. In general, the optics of light propagation in a Ch LCD may be described as a superposition of two eigenmodes which are elliptically polarized. However, within the narrow reflection band the eigenmodes are circularly polarized. The circular component having the same handedness of the liquid crystal twist will be reflected due to Bragg diffraction within the range of wavelengths shown in Equation 16 where P is the pitch, Δn is the birefringence taken from Equation 6, and θ is the angle of incidence with respect to the helical axis [23]. P n cos (16) Reflective cholesteric displays switch from a planar orientation, which is the reflective state, to the focal conic orientation, which is weakly scattering, through an intermediate state, the clear and transmissive homeotropic texture. In a reflective display application, the focal conic state will appear dark if the display is placed in front of a black absorbing background, but in a transmissive application, the reflective planar texture must be the dark state. The Ch LCD is not useful for glare shielding eyewear for many reasons. The

71 56 reflection bandwidth is too narrow to cover the entire visible spectrum, but improved designs that are able to reflect at all wavelengths [24] would still need to be double stacked to reflect both circular polarizations of light. The worst drawback, however, is the bistability of the focal conic and reflective states and the fact that the colorless, transmissive homeotropic state is not stable at 0V, so the Ch LCD can not be used in this dissertation. Many bistable LCDs do have a transparent state that is stable at 0V, including designs such as the surface stabilized ferroelectric liquid crystal (SSFLC), surface stabilized nematic, bistable twisted nematic (BTN), and others. However, these were also not seriously considered for this dissertation; even though the addressing scheme of truly bistable LCDs saves power, those pixels that are in the dark state would not return to the bright state when the power source or driving circuitry fails Liquid Crystal Displays With a Polymer Network When a polymer network is added to a liquid crystal, the resulting liquid crystal/polymer composite can be used to incrementally improve the performance of an existing LCD design by the nature of bulk interactions between the polymer and the liquid crystal. They also can be used to create entirely new LCDs, many of which are based on the formation of a static multi-domain structure that scatters light. The following discussions consider those polymer/liquid crystal composites that form new LCD designs. The polymer dispersed liquid crystal (PDLC) display consists of isolated droplets of liquid crystal within a continuous polymer binder. The droplet size is usually distributed

72 57 around 1μm order and the display scatters light from a mismatch in refractive index from droplet to polymer and droplet to droplet. The droplets are formed by phase separation from the polymer and the polymer concentration is comparable to that of the liquid crystal. In a simplified concept of the PDLC, droplets of nematic liquid crystal each have a uniform, but randomly oriented director so light is scattered at 0V. An applied voltage above the threshold will reorient the droplets so they are all aligned along the electric field and light is transmitted when the ordinary index of the liquid crystal matches the (isotropic) index of refraction of the polymer. The downside of the PDLC is that the indices are only well matched for the on-axis viewing directions; off-axis viewing angles are hazy because the indices of refraction between liquid crystal and polymer begin to mismatch each other. The PDLC is eliminated because it is not transparent at 0V. The solution to the off-axis haze in PDLC displays is to reduce the polymer content. The polymer stabilized liquid crystal (PSLC) displays are fabricated by adding a small amount (typically <10% by weight) of polymerizable monomer to the liquid crystal mixture. Because PSLCs have a smaller amount of polymer network than PDLCs, haze from the mismatch in refractive index between the liquid crystal and the polymer network is reduced to an insignificant amount. Oftentimes, the best results are achieved when using mesogenic monomers that have a rigid core instead of completely flexible isotropic monomers to increase the interaction between the liquid crystal and the polymer network, which plays an important role during and after polymerization. Polymerization occurs in the liquid crystal phase, so the polymer network formation will mimic the orientation of the liquid crystal, which is controlled by the alignment layers and/or electric field. After

73 58 polymerization, the anisotropic network will continue to influence the liquid crystal orientation in the bulk [25, 26]. When polymer stabilization is added to a homogenously aligned nematic ( 0 ) it is transparent at 0V and at high voltages it reorients unevenly within the plane defined by the rubbing direction [27]. This forms a multi-domain structure that primarily scatters light polarized along the long axis of the liquid crystal (rubbing direction). To scatter both linear polarizations (unpolarized light), two cells must be stacked together with the rubbing directions crossed. The polarization sensitivity can be avoided by adding polymer stabilization to a vertically aligned (VA) nematic liquid crystal ( 0 ). At 0V the display is highly transparent and at high voltages the liquid crystal reorients unevenly in all directions azimuthally (no preferred direction) to form a multi-domain structure that scatters unpolarized light. A dichroic dye can be added to the PSLC VA scattering LCD (in a manner similar to the method in Chapter 7) to improve the contrast ratio [28], but the drawback still remains that the transmission vs. voltage curve of the PSLC (VA) LCD is too gradual to achieve the best performance using passive matrix addressing, which makes it undesirable. When polymer stabilization is added to a cholesteric liquid crystal it is called a polymer stabilized cholesteric texture (PSCT) LCD and two designs are possible. The first design stabilizes the scattering focal conic texture at 0V and is called the normal mode (NM-PSCT). Unfortunately, this means it is not transparent 0V, so it won t be able to be used in a final design for pixelated glare shielding eyewear, but nonetheless a detailed study was conducted and the results are in Chapter 4. However, the reverse

74 59 mode (RM-PSCT) is transparent at 0V because a rubbed anti-parallel homogeneous alignment layer stabilizes the planar texture with a pitch longer than wavelengths of visible light. The polymer network helps stabilize the planar texture and the liquid crystal will form a scattering focal conic texture when a high voltage is applied. The reverse mode PSCT can be driven with a 3:1 block addressing scheme and it is the subject of an intensive study that is discussed in Chapter 5. For completeness, the reflective Ch LCD can also be polymer stabilized [29], but all references in this dissertation to a polymer stabilized cholesteric will refer to the design with a longer pitch that scatters light. Two designs of a liquid crystal/polymer composite display use a dual-frequency liquid crystal: one is a reverse mode PDLC [30] and the other is a PSLC [31]. A dual frequency liquid crystal has a positive dielectric anisotropy ( 0 ) below the crossover frequency (5kHz typical) and a negative dielectric anisotropy ( 0 ) above the crossover frequency. The reverse mode PDLC design has a high polymer concentration and liquid crystal droplet structure like the regular PDLC, but it does meet the requirement to be transparent at 0V. However, it suffers from the need to operate with applied voltages at high frequencies (~50kHz). For the PSLC design, a high voltage at a low frequency is applied during curing to form a homeotropic liquid crystal texture and polymer network. The network is strong enough to support the texture after the field is removed and the display is transparent. When a high voltage at a high frequency is applied it switches the texture to a multidomain scattering texture. If a rubbed anti-parallel homogeneous alignment layer is used,

75 60 the scattering will only occur for light polarized along the rubbing direction and if no alignment layer is used the scattering is polarization independent. The display can switch in less than a millisecond, but it suffers from the need to drive at high frequencies (~50kHz) and high amplitudes (>50V are typical at 8μm). These high frequencies and amplitudes are necessary when the magnitude of the dielectric anisotropy is low (as is common for the dual frequency liquid crystal materials) and tends to increase the power consumption beyond compatibility with a battery powered device. Therefore, all designs based on dual-frequency liquid crystal materials are eliminated from consideration in this dissertation. Some polymer stabilized LCDs are not a scattering mode display (they use polarizers) and are a totally new design (not incremental improvements). For example, the polymer stabilized isotropic (PSI) [32] LCD has a cholesteric liquid crystal with a pitch below 400nm (shorter than the wavelengths of visible light) and it uses a polymer network to assemble very small domains of liquid crystal that can still reorient allowing the display to operate using the electro-optic Kerr effect. These displays are optically isotropic when no field is applied because the length scale of the local anisotropy is smaller than the wavelengths of visible light. An applied field will induce birefringence along the field direction. The polymer stabilized blue phase cholesteric (PS-BP) [33] LCD operates by a similar design. The advantage of these two displays is a super fast response time which can be less than one millisecond. However, they need an in-plane electrode structure, which requires an active matrix to achieve a pixelated display. Therefore, the PSI and PS-BP LCDs will be eliminated in this dissertation.

76 61 A reflective LCD can be made by forming the polymer into a spatially varying structure with a non-uniform polymerizing condition as can be introduced using the interference pattern of coherent laser light [5]. The result is called the holographic PDLC (H-PDLC) LCD which has alternating regions of polymer rich and liquid crystal rich layers. At 0V the liquid crystal is randomly oriented, so the display has a periodic index of refraction and if the periodicity is d, then light satisfying the Bragg condition, d cos, will be reflected. As in the regular PDLC, the H-PDLC uses a liquid crystal with an ordinary index of refractions that matches the index of refraction of the polymer. Therefore, at a high voltage, the liquid crystal reorients to create a uniform index of refraction and the display is transparent. Unfortunately, the reflection band of the H-PDLC is very narrow, which will cause the contrast ratio to be very poor when used in a transmissive application such as pixelated glare shielding eyewear. 2.8 Conclusion Liquid crystals molecules have distinctive anisotropic optical and dielectric properties which make them ideally suited for display applications. LCDs are a good choice for pixelated glare shielding eyewear because of their fast response to a driving voltage, ability to be battery powered by a portable device, high contrast ratio between a bright state and a dark state, and the ability to be segmented into pixels. The pixelated electrode structure for this dissertation is the passive matrix which has more than 4 rows of resolution and saves cost and complexity over an active matrix design. The best driving method is either the row-at-a-time resolution-dependent voltage ratio or the block

77 62 addressing 3:1 voltage ratio. The best possible LCD modes from those considered in this chapter are the STN LCD and the reverse mode PSCT as summarized below in Table 3. Table 3 List of LCDs considered for pixelated glare shielding eyewear. LCD* Operation Classification Most Significant Reasoning DSM Scattering Eliminated High power requirements ECB Absorbing Eliminated Colorful transmission VA Absorbing Eliminated Not transparent at 0V IPS & FFS Absorbing Eliminated In-plane electrodes required OCB Absorbing Eliminated Warm up time required PSI Absorbing Eliminated In-plane electrodes required PSBP Ch Absorbing Eliminated In-plane electrodes required Ch Reflecting Eliminated Not transparent at 0V Bi-Nem Absorbing Eliminated Not transparent at 0V PDLC Scattering Eliminated Not transparent at 0V PSLC (ECB) Scattering Eliminated Polarization dependent H-PDLC Reflecting Eliminated Narrow reflection band All Bistable Varies Eliminated May leave pixels on if power fails Dual Frequency Varies Eliminated High frequencies use too much power NM-PSCT Scattering Eliminated** Not transparent at 0V GH Absorbing Undesirable Low contrast ratio PSLC (VA) Scattering Undesirable Poor performance on a passive matrix TN Absorbing Undesirable Poor performance on a passive matrix STN Absorbing Possible Steep curve for a passive matrix RM-PSCT Scattering Possible Will work using a 3:1 passive matrix *Incremental improvements not listed (i.e. adding a dichroic dye or compensation film) **However, it is still included in this dissertation.

78 Chapter 3 3. Super Twisted Nematic for Polarizer Based Absorptive Displays 3.1 Motivation Within the realm of LCDs for information display, recent trends have seen the rapid growth of thin film transistor (TFT) active matrix displays with an ever continuing emphasis on improving the contrast ratio, viewing angle, and response time [34]. However, the passive matrix displays, many of which utilize the STN liquid crystal technology, are still important because they provide acceptable optical performance and save manufacturing cost and complexity in applications with low information content [35]. The conclusion discussed in Chapter 2 stated that the most effective electrode design for a LCD in pixelated glare shielding eyewear must utilize the passive matrix with includes both voltage ratio driving schemes. This constraint was the biggest motivation behind choosing the STN liquid crystal display to be the best suited polarizer based absorptive LCD to study in this dissertation. The remainder of this chapter reveals the results of computer simulations and experiments to discover the best design of the STN LCD for this unique application. 3.2 Overview The arrangement of the liquid crystal molecules in the STN LCD is the planar twisted structure where the director twists an angle ( / 2 3 / 2) from the bottom 63

79 64 substrate to the top substrate of the cell. The STN studied in this dissertation is the normally white mode which switches between the two optical states shown in Figure 22 (a). At V = Voff, the voltage applied is lower than the threshold voltage, so the STN display is in the bight state where the liquid crystal director is unchanged and light is transmitted. When the applied voltage exceeds the threshold voltage at V = Von > Vth, the STN display is in the dark state where the liquid crystal director is tilted toward the cell normal direction and very little light is transmitted. d Bright Dark Analyzer Compensation film (optional) Liquid Crystal Polarizer Analyzer o y Compensation film (optional) i LC director at exit plane Polarizer LC director at entrance plane V=V off V=V on x a) b) Light propagation direction is out of the page. Figure 22 STN diagrams: a) cross section of two optical states, b) the angular convention used in the equations and simulations The STN LCD structure sandwiches the liquid crystal layer between two polarizer films. However, due to the large twist angle and thin cell gap, the Mauguin condition shown in Equation 17 is usually not satisfied in STN LCDs, meaning the waveguiding effect utilized in TN LCDs is no longer present. (2 nd) 1 (17)

80 65 Thus, in general, the polarizer and analyzer are not perpendicular to each other, nor are they parallel or perpendicular to the director at the entrance or exit plane of the liquid crystal layer. Therefore, determining the best configuration of the components of the STN is the critical factor in determining the transmission in both optical states. In this dissertation, the first film encountered by light traveling along the propagation direction is labeled as the polarizer and the second polarizing film is labeled as the analyzer. Another STN component that is (optionally) included to improve optical performances is a compensation film, which in this study was inserted between the liquid crystal and the analyzer. Figure 22 (b) shows the angular convention used in the equations and simulations throughout this chapter. All angles are referenced with respect to the azimuthal angle of the liquid crystal director at the entrance plane, which is defined to be zero degrees. In the bright state, linearly polarized light incident on the STN liquid crystal layer excites two elliptically polarized eigenmodes which interfere after exiting the cell to produce light that is nearly, but not completely, linearly polarized. Therefore, the analyzer will absorb some of the incident light and the transmittance will not be 100% at every wavelength in the visible light region. This limits the maximum luminance of the STN. Similarly, in the dark state, light exiting the liquid crystal is nearly linearly polarized parallel to the absorption axis of the analyzer; however, the transmittance can not be 0% at every wavelength. This is because the applied voltage will not be able to align the liquid crystal fully homeotropic, which leaves a residual optical retardation that causes light leakage. This limits the maximum contrast ratio [8].

81 66 For a given liquid crystal, the display cell parameters such as cell gap, polarizer and analyzer angles, applied voltage, and compensation film, must be carefully chosen to minimize the wavelength-dependence thereby optimizing the performance of the display. The first part of this study describes the methods used to simulate the results of the STN design. Then the computer s processing power is harnessed to determine the best combination of display cell parameters and material parameters such as birefringence, elastic constants, and dielectric constants that will optimize the performance. We then built the STN LCD according to the theoretical results and experimentally characterized the performance of the display. In addition, it was determined early on that the birefringence dispersion of the liquid crystal must be measured and this process will be described first. 3.3 Measuring the Birefringence Dispersion Theory The birefringence (Δn) of liquid crystals is defined as the difference between the extraordinary and the ordinary index of refraction as shown previously in Equation 6. For most commercially available liquid crystals (and mixtures of liquid crystals) the birefringence is measured at the specific wavelength emitted by sodium vapor lamps of 589nm. Unfortunately, the birefringence is not constant over the visible light region, which means it is a function of wavelength: Δn(λ). Therefore, all simulations of the transmission spectrum of a liquid crystal in polarized light that use a single value for the birefringence will not be accurate. Fortunately, it is straightforward to determine Δn(λ)

82 67 by experiment. A simple model for Δn(λ) proposed by S.T. Wu [36] is used in this dissertation as shown in Equation ( * ) n ( ) G (18) 2 2 * According to Wu, the dimensionless factor, G in Equation 18, depends on molecular packing density, available electrons per molecule, degree of molecular order, and resonance anisotropy of electronic transitions. In this dissertation, the temperature dependence of G is ignored for simplicity. Wu also states that this model is applicable to liquid crystals and liquid crystal mixtures that have an electronic absorption band in the UV or near-uv region comprised of one peak or two closely spaced and overlapping peaks. This is represented by the term λ* in Equation 18, which is the mean resonant wavelength. In general, the dispersion of n e and n o may not be the same; however, the dispersion of the optical retardation, Δn(λ), is all that is necessary to compare different STN designs Experiment It is not simple to measure the parameters G and λ* directly. However, it is simple to determine Δn(λ) with the following experiment. First, the exact cell gap of an ITO coated cell with 10μm spacers and an anti-parallel rubbed low pretilt planar alignment layer is measured. After vacuum filling the cell with liquid crystal, it is placed between crossed polarizers with the rubbing direction aligned along the polarizer transmission axis to achieve a minimum transmission. This means the liquid crystal director (controlled by the rubbing direction) is parallel to the polarizer. A high voltage (>10V) is applied to the

83 Transmission (%) 68 cell and the analyzer is rotated by 90 to obtain parallel polarizers for the spectral reference measurement. Then the cell is rotated by 45 and the analyzer is rotated back 90 (crossed polarizers) for the transmission spectrum measurement. The transmitted light intensity, T%, is modulated by the acquired phase shift which depends on the birefringence, wavelength and thickness as described by Equation 19 [6]: 2 nd T% sin ( ) (19) Equation 18 is inserted into Equation 19 along with the measured value of cell gap, d. Then the parameters G and λ* become fitting parameters using a minimization algorithm such as Nelder-Mead included with the Matlab computational software Measured Fit Wavelength (nm) Figure 23 An example of the experimental data and fit.

84 69 The result is checked for consistency by comparing the value of Δn(589nm) found by the fit with the reported value in the literature. An example of the measured data and fit is shown in Figure 23. The fitting parameters G and λ* found for different liquid crystals are listed in Table 4 which correspond with the curves of Δn(λ) plotted in Figure 24 (a). All these liquid crystals are from Merck; however, 6A-100 and 13B-000 use an internal naming unique to this dissertation. These results show that lower values of λ* result in a flatter curve with less dispersion. This is verified more easily by observing the same curves re-plotted in Figure 24 (b) after normalizing the data using Δn(589nm). In general, it is predicted that a lower overall birefringence results in a flatter dispersion curve and the materials studied agree with this hypothesis. Table 4 Birefringence dispersion fitting parameters for various liquid crystals. Liquid Crystal λ* G Δn(589nm) E44 255nm MLC nm A nm B nm E nm Data from S.T. Wu [36]. Value determined using fitting parameters in Equation 18. Values reported on the Merck datasheet for each liquid crystal material [37].

85 n( ) Normalized at 589nm n( ) a) b) E Wavelength (nm) E7 6A 1.25 MLC B Wavelength (nm) Figure 24 a) Birefringence dispersion curves for various liquid crystals. b) Same data as in a) normalized at 589nm.

86 Simulation Computer simulations of the STN performance were carried out using three steps. The first step determines the equilibrium director configuration of the liquid crystal at some applied voltage, then the second step uses that director configuration to calculate the light transmission of the display vs. wavelength, and then the third step uses the transmission spectrum to calculate the performance such as contrast ratio, luminance, and color. The software used in the modeling was an implementation of the finite difference method written by the author using the Matlab programming language Director Relaxation Following the STN structure described in Figure 22 (a) and (b), the LC director n is described by the polar angle (the angle between n and the x y plane which is parallel to the cell substrates) and azimuthal angle (the angle between the projection of n on the x y plane and the x axis). and are functions of the coordinate z which is perpendicular to the cell substrates, so the liquid crystal is discretized by dividing it into several slabs. The equilibrium director configuration of the liquid crystal at some applied voltage is obtained by minimizing the total free energy of the system, which consists of the elastic energy (Equation 3) plus the electric energy, f 1 2 K K cos ' K sin ' cos ' ( cos ' ) P o ( cos ) o (20)

87 , Polar Angle ( ), Azimuthal Angle ( ) 72 where ' / z, ' / z, P o is the intrinsic pitch of the liquid crystal, and is the surface charge density. The minimization uses the Euler-Lagrange equation and the over relaxation method with constant charge to find the equilibrium director configuration as defined by the two angles, (z) and (z), and the voltage is calculated. The process reiterates by calculating all the slabs during each loop and continues until the change of the mid-layer tilt angle, θ m, is less than 10-8 radians from one loop to the next. The director angles are plotted for a 240 STN in Figure 25 at four different voltages (5V) (3V) (5V) (3V) (2V) (0V) (2V) (0V) z/d Figure 25 The director orientation angles vs. thickness.

88 Light Transmission The 2x2 Jones matrix method is used to calculate the light transmission of the STN LCD. The liquid crystal orientation is already known within each individual layer from the director relaxation simulations. Each layer represents a thickness of slabs. The Jones vector of the incident light is, E i (cos,sin ) i i z z / N for N (21) The Jones vector of the light exiting the liquid crystal depends on the polar angle,, and the azimuthal angle, j, of the director in the j th layer as given by, j Eo 1 R( j ) G( j ) R ( j Ei N ) j 1 (22) where the phase retardation is defined as j 2 ( ) z /, the rotation matrix is n eff cos j R( j ) sin j sin j 1 0 cos, the retardation matrix is G( j ) i j, and the j 0 e effective extraordinary index of refraction, n eff (λ), is given by, n eff ( n ( ) n e o ) n (23) o (( ne ( )) sin j no cos j ) which depends on the polar angle,. The (constant) ordinary index of refraction is j taken from the material s data sheet and the extraordinary index of refraction is calculated as a function of wavelength using Equation 18 to get Δn(λ) and adding it to n o. The transmission is given by: 2 T E ox cos E sin (24) o oy o

89 74 The transmission spectrum output is a value in percent referenced to perfect parallel polarizers, which transmit 50% of unpolarized light at every wavelength. The downside is that this idealized calculation will overestimate the contrast ratio of a real device because of the inefficiencies of real polarizers and it does not take into account the light leakage due to spacers. Light leakage from spacers is a concern in STN LCDs since the polarizers are not crossed. However, even real polarizers will leak some light when crossed due to incomplete absorption and they will not transmit 50% of unpolarized light when parallel due to excess absorption. Therefore, the simulated contrast ratio will be used to compare different designs, but not used to directly verify an experimental result. The transmission spectrum is analyzed to find the color, luminance, and contrast ratio using the human eye response characteristics and calculations to be described in the next section Calculation of Color, Luminance, and Contrast Ratio As previously described, the larger twist angle of the STN places it outside the Mauguin waveguiding regime (Equation 17) which means the transmittance in the bright state is not 100% at every wavelength in the visible light region [8]. The birefringence dispersion of the liquid crystal only compounds this issue by making the birefringence also dependent on wavelength. It is desirable to transmit as much light as possible because a higher overall transmission will allow the device to be worn in lower ambient lighting conditions. The overall transmission is defined as integrated luminance. It is also desirable to have as white a spectral transmission as possible because a whiter spectral transmission allows the color of objects viewed through the display to remain as

90 y 75 natural as possible and gives the lens itself a more neutral colorless appearance. This dissertation uses the 1931 CIE color analysis to calculate and compare the effective color as described in the next section nm 510nm 532nm Diode Laser Pointers 540nm 543nm HeNe Gas Laser 0.6 Green 560nm 500nm Yellow 580nm 0.4 Cyan 490nm White Pink Orange 600nm Red 620nm 633nm HeNe Gas Laser 0.2 Magenta nm Blue Purple 480nm 650nm DVD Laser Diode Violet 460nm nm nm Blu-Ray x Laser Diode Figure 26 The 1931 (x,y) CIE Diagram In 1931 the Commission Internationale de l Éclairage (CIE) developed a standard to represent perceived color by averaging the visual characteristics from measurements using several groups of people. This standard is used in this dissertation and it includes a calculation that converts a spectral measurement of intensity to the perceived color

91 76 defined by its two chromaticity coordinates, (x,y). These coordinates are plotted on the 1931 CIE diagram, as shown in Figure 26, which encompasses all the perceivable colors. The outer border of the diagram contains the pure spectral colors that consist of a single wavelength and the values of these are periodically labeled on the diagram. The output of a laser is almost exactly at a single wavelength (monochromatic) and a few common laser wavelengths are plotted on the diagram in Figure 26 along the outer border. A spectrum that has color coordinates that are inside the diagram will contain a mixture of light of different wavelengths and the basic colors are written on the figure in the general area where they reside. Another important reference, the white point, is the near the center of the diagram located at x = 0.33 and y = The white point represents a spectrum with equal energy at all wavelengths, completely achromatic and colorless [38]. The chromaticity coordinates, (x,y), are found from the tristimulus values (X,Y,Z), which are calculated using the following equations [8], X x X Y Z Y y X Y Z (25) Z z X Y Z X k S( ) T( ) x( ) d Y k S( ) T( ) y( ) d (26) Z k S( ) T( ) z( ) d

92 Color Matching Functions (CIE Observer) 77 where x ( ), y ( ), and z( ) are the color matching functions for a standard observer (plotted in Figure 27), S ( ) is the spectrum of the display s source of light, and T ( ) is the transmission spectrum of the display (i.e. STN LCD). 2.0 z( ) y( ) x( ) Wavelength (nm) Figure 27 Color Matching Functions of the 1931 CIE 2 Standard Observer Whereas STN LCDs for information display have a backlight, a transmissive STN LCD in pixelated glare shielding eyewear doesn t, so the S ( ) depends on the reflection spectrum of the object the user is observing and the spectral output of the light source (i.e. the sun, fluorescent light, incandescent light, etc.). The intent is to see how chromatic (colored) the STN LCD is, i.e. how far the transmission spectrum is from the

93 78 white point; therefore, the variability of the source can be ignored ( S ( ) = 1 for all ). The normalizing factor, k, is defined so that the Y tristimulus value is equal to 100 if the transmission spectrum of the device is completely achromatic ( T ( ) 100% for all ). k 100 S( ) y( ) d (27) The Y tristimulus value is the luminous intensity (luminance) of the STN LCD, because y( ) is the response function (sensitivity) of the human eye. The sensitivity of the human eye is highest for green wavelengths ( y ( ) in Figure 27) which means if a red, a green, and a blue light source have equal absolute intensities, the green one is perceived as being the brightest (has the highest luminance). Understanding the importance of the human eye sensitivity is the major motivation for using the CIE calculations to design STN LCDs when trying to maximize the brightness of the bright state and minimize the darkness of the dark state. The luminance compares the brightness of the STN designs to the brightness of a STN LCD with T ( ) 100% (of ideal parallel polarizers). The luminous intensity, Y, is also used in calculating the contrast ratio of the STN LCD (following Equation 15) defined as the luminance of the bright state divided by the luminance of the dark state as shown below in Equation nm T YBright 400nm Contrast Ratio 700nm YDark T Bright Dark 400nm ( ) y( ) d ( ) y( ) d (28) All that remains is to quantify the color and relative colorfulness of the spectrum. Unlike the connection between Y and luminance, there is no similar connection between

94 79 X and Z and color perception. However, the chromaticity coordinates of the LCD can be used instead along with the chromaticity coordinates of the illuminant. In this dissertation, the illuminant is simply the white point based on the assumption that S ( ) = 1 for all. In order to quantify the color, however, one must assume that the chromaticity system is equally spaced so that equal distances correspond to equal differences in perception. Unfortunately, the 1931 CIE diagram is visually non-uniform; it is distorted in size and shape similarly to the way the Mercator projection of a map of the earth distorts the size and shape of the continents. In 1976, the CIE adopted a new chromaticity diagram called the uniform chromaticity scale (UCS) diagram with improved perceptual uniformity and a linear transformation from the 1931 chromaticity coordinates (x,y) to the 1976 chromaticity coordinates (u,v ) as shown in Equation 29 [39]. u 4x 2x 12y 3 9y v (29) 2x 12y 3 A caveat of the 1976 CIE diagram is that it is still not perceptually uniform as a function of luminance factor; however, this was not considered to be critical to this dissertation since the luminance of the chromaticity coordinates being compared is of the same order of magnitude. The 1976 CIE chromaticity diagram is shown in Figure 28.

95 v nm Diode Laser Pointers nm540nm 560nm 580nm 600nm Green 510nm 500nm 490nm Cyan 543nm HeNe Gas Laser Yellow White Orange Pink Magenta 633nm HeNe Gas Laser 620nm Red 650nm DVD Laser Diode nm nm Blue Purple Violet 460nm 405nm Blu-Ray Laser Diode nm u Figure 28 The 1976 (u,v ) CIE Diagram For this dissertation, we define the color of the STN LCD as the hue and this is provided by the dominant wavelength, λ d, of the spectrum. The dominant wavelength is defined as the monochromatic stimulus that, when mixed with a pure white stimulus in a suitable proportion, will match the chromatic stimulus under analysis. More simply, the dominant wavelength is the wavelength of the point on the border of the CIE diagram that lies on a line drawn through the white point and the chromaticity coordinates of the STN LCD as shown for Sample 1 in Figure 29. There are no pure wavelengths that can represent mixtures of red and blue (such as purple, magenta, or pink), which means that

96 y v 81 dominant wavelengths that lie on the dashed line in Figure 29 are undefined. In these situations, the wavelength that lies on the border from the opposite side of the diagram is used and called the complimentary dominant wavelength, λ c, as shown for Sample 2 in Figure 29. The colorfulness or saturation of the hue of a STN LCD is defined by the excitation purity (purity for short). The purity ranges from 0 if it is purely white to 1 if it is purely monochromatic. Therefore, the purity is calculated as the distance from the chromaticity of the sample to the chromaticity of the white point over the total distance from the white point to the edge of the diagram along the line connecting the coordinates of the white point and the sample [40]. 0.8 a) 1931 CIE diagram d = 543nm 0.8 b) 1976 CIE diagram c = 500nm White Point b a a Sample 1 b Sample d = 543nm Sample 1 b a a b White Point Sample 2 c = 500nm x u a Figure 29 Dominant (or complimentary) wavelength and purity ( purity ) on the a b a) 1931 CIE diagram and b) 1976 CIE diagram For example, in Figure 29, the purity is calculated using the lengths of line segments a and b and the dominant or complimentary wavelength for Sample 1 and Sample 2

97 82 are shown on both the 1931 CIE (Figure 29 (a)) and the 1976 CIE (Figure 29 (b)) diagrams. Both diagrams are shown in order to emphasis the differences previously mentioned; however, all references to the dominant wavelength and purity will be calculated using the 1976 CIE diagram in this dissertation for better perceptual uniformity. To summarize, minimizing the purity will be how the colored appearance of the STN LCD is minimized in this dissertation. 3.5 Compensation Film The STN can be fabricated with a compensation film to improve the contrast ratio, improve the whiteness of the bright state, or improve the viewing angle. Some compensation schemes involve stacking two displays (D-STN) where the second display is passive (not driven) with the exact same cell gap, but with a reverse twist handedness [35]. This type of compensation adds lots of weight, has tight manufacturing tolerances, and only applies to the normally black STN, so it will not be pursued here. A much simpler option would be to add a full wave plate between the STN and the analyzer. The full wave plate does not change the polarization state of light at the designed wavelength which can be designed to be near the center of the visible light spectrum, for example 560nm is a common retardation value. Therefore, green light which is already nearly linearly polarized orthogonally to the analyzer (after exiting the STN) stays that way upon exiting the compensation film. The light at other wavelengths is, in general, elliptically polarized after exiting the STN (which causes light leaking) so the wave plate will help to transform more of this light to a polarization state that is better absorbed by the analyzer in the dark state to improve the contrast ratio. Alternatively, it can be used

98 83 to transform more light in the bright state to a polarization state that is better transmitted by the analyzer, improving whiteness and brightness. In either case, the wave plate accomplishes this transformation because it is oriented in such way that its wavelength dependent optical retardation effect will partially cancel some of the residual retardation of the STN liquid crystal. Analytical techniques exist to optimize the orientation of the wave plate [41]; however a numerical trial and error method was employed in this dissertation. Upcoming results with liquid crystal MLC-6080 show the potential for improvement using a compensation film. 3.6 Experiment Using Liquid Crystal MLC-6080 STN LCDs with a 240 twist were constructed using the liquid crystal material MLC The samples had a thickness of 4.4μm and 3.7μm. The off state voltage was chosen to be V off = 2.2V, which is near the threshold voltage. This STN was designed for a display with 17 rows of resolution, so according to Equation 13, V ratio = 1.28, which makes V on = 2.82V. The contrast ratio was calculated from the simulations of the spectra as the polarizer and analyzer angles were varied on a 1 grid as shown in Figure 30. The simulations of the spectrum used the measured wavelength dependent birefringence as shown in Table 4 and described by Equation 18. Then the spectrum of the sample was measured and compared to the simulation as also shown in Figure 30.

99 Contrast Ratio Contrast Ratio 84 Contrast Ratio vs. Polarizer and Analyzer Transmittance vs. Wavelength (a) 4.4 μm Highest Contrast of 48 at: α i = -39 α o = V 2.82 V 4.4 μm (b) 3.7 μm α i (degree) α o (degree) Highest Contrast of 84 at: 2.2 V 3.7 μm α i = -46 α o = V α i (degree) α o (degree) Simulation Experiment Figure 30 Contrast ratio vs. polarizer and analyzer angles & transmission spectrum of simulation compared to experiment: a) 4.4μm b) 3.7μm The 3.7μm sample had a higher simulated contrast ratio because the reduced thickness lessened the residual optical retardation, which reduced the light leakage. Also, the transmission spectrum of the 3.7μm sample in the bright state is shifted towards shorter wavelengths, which shifted the color from green to blue. The color shift can be seen in the inset of Figure 30 where the sample is shown correctly aligned between the polarizer and analyzer at the STN drive voltages. The simulations of the spectra using a compensation film inserted between the liquid crystal cell and the analyzer were re-calculated as the azimuthal angle of the film was varied on a 1 grid.

100 85 Contrast Ratio vs. Compensation Film Angle Retardation Angle = 2π Transmittance vs. Wavelength 4.4 μm Highest Contrast of 99 at: σ = V 2.82 V 4.4 μm 3.7 μm Highest Contrast of 148 at: σ = V 2.82 V 3.7 μm Simulation Experiment Figure 31 Contrast ratio vs. compensation film angle & transmission spectrum of simulation compared to experiment: a) 4.4μm b) 3.7μm The compensation film used was a full wave plate that was (2π) retardation at 550nm so it would not change the polarization state at that wavelength and the birefringence of the wave plate was characterized to be n [199 /( 199 )] no, where n o was the birefringence at 550nm. Then the spectra of the best result were recalculated and plotted as shown in Figure 31. The spectrum of the sample with the compensation film was measured and compared to the simulation as also shown in Figure 31 and good agreement was obtained. The simulated contrast ratio was improved using a compensation film.

101 86 These results using MLC-6080 are summarized below in Table 5 and were presented at the Society for Information Display Conference [42]. Table 5 Summary of the results of the STN LCDs made with MLC-6080 liquid crystal. Cell Experiment Simulation Adjusted Component Gap Results Results Results* Polarizer, α i μm Analyzer, α o Compensation Film, σ Polarizer, α i μm Analyzer, α o Compensation Film, σ *Better agreement is possible if the input rubbing direction was actually at -1.5 for 4.4μm and -3.5 for 3.7μm. In the experimental measurements, the final alignment angles between the polarizers, rubbing directions and compensation films (if used) did not exactly match the theoretical results. The twist angle of the samples was well controlled, so the discrepancy was likely due to experimental error in the exact alignment of the input rubbing direction relative to the other components. The column labeled adjusted results in Table 5 demonstrates that if the input rubbing direction was really located at -1.5 for 4.4μm and -3.5 for 3.7μm, then the results are in better agreement. The comparison from this experiment as well as elsewhere in the literature [43] demonstrates the need to fine tune the alignment angles around the theoretical orientations for the best performance during actual product manufacturing.

102 Optimizing the Material Properties and Cell Design Using Simulation The results of the STN LCD study using MLC-6080 only took into account one liquid crystal material and two cell thicknesses. The purpose of the remainder of this chapter is to design and implement an optimization that will incorporate all the elements of the STN LCD including the effect of the material parameters of the liquid crystal and the cell design parameters. Some parameters are linked together, such as the cell gap and the birefringence Δn(λ), which both affect the transmission of the display; therefore each parameter is considered individually. The evaluation of performance will find the best compromise to maximize contrast ratio, maximize luminance, and minimize purity with much less emphasis on improving the response time or viewing angle. A compromise between device simplicity and performance will also be made. For example, the results of the compensation film simulation in Figure 31 suggest that the best improvements in contrast ratio require another component to have precise alignment with the cell. In addition, the results of optimizing the liquid crystal material and cell design (as will be discussed in the following sections) will shown an improvement in measured contrast ratio that compares to the film compensated contrast ratio simulation results from using MLC Although a compensation film would continue to improve the performance of the STN LCD, it will be excluded from the remainder of the dissertation to reduce the cost and complexity of device fabrication Electro-Distortional Curve All things being equal, the best contrast ratio is achieved when light leaking in the high voltage (V on ) state is minimized. Light leaking is caused by residual birefringence

103 88 due to incomplete reorientation of the liquid crystal molecules where they are not fully homeotropic throughout the cell s thickness. Therefore, the best contrast ratio can be achieved by maximizing the reorientation of the director. The defect free equilibrium director distribution inside the cell when a voltage is applied changes smoothly and continuously from the strong anchoring points at the cell surfaces. This places the maximum deviation in the center layer (mid-layer) of the cell which makes the extent of the reorientation in the mid-layer of liquid crystal a benchmark for comparison. When the reorientation of this center layer is maximized, the best contrast ratio can be achieved. If the center layer has reached saturation and reoriented to fully homeotropic, then the width of this saturation region (along the cell thickness z) should be maximized. To achieve the maximal director reorientation we must apply as high a voltage as possible. The row-at-a-time passive matrix driving scheme has a physical limitation on the highest voltage ratio between the off state voltage and the on state voltage which depends on the number of rows (Equation 13). Therefore, the higher the resolution of the display (in one direction) the lower the best contrast ratio will be. If a suitable resolution is chosen and then fixed, then the only way to improve the contrast ratio is to steepen the change of the mid-layer director orientation with respect to applied voltage: the electrodistortional curve. To summarize, when the voltage ratio is fixed, choosing a liquid crystal material and cell design with a steeper electro-distortional curve will yield an increased contrast ratio. The following material parameters affect the steepness of the electro-distortional curve: the elastic constant ratio K 33 /K 11, the elastic constant ratio K 22 /K 11, and the ratio of

104 89 the dielectric anisotropy to the perpendicular dielectric constant, the dielectric parameter /. The following cell design geometry parameters also affect the steepness of the electro-distortional curve: the total twist angle Φ, the ratio of thickness to pitch d/p, and the pretilt angle at the surfaces θp. Each of these parameters were simulated independently to discern their effects on the steepness and the results are shown in Figure 32 and Figure 33. The standard condition curve is common to all the figures and refers to the set of parameters: Φ = 210, θp = 5, d/p = 0.58, K 33 /K 11 = 1.5, K 22 /K 11 = 0.6 and γ = 2.5. The only exception is the study of the twist angle which varied d/p so that it was equal to the equilibrium condition, Φ/360. As the total twist angle increases, the steepness of the electro-distortional curve increases as shown in Figure 32 (a). Twist angles higher than 270 exhibit bistability where the curve inverts and both the planar undistorted state and the reoriented state are stable at the same voltage. It is not acceptable for the state of a real device to depend on its history or contain defects, so a large total twist of 270 or less will avoid bistability and improve steepness. The pretilt angle has only a slight affect on the steepness as seen in Figure 32 (b); however, higher pretilts will reduce the threshold voltage and tend to smooth out the transition. The thickness/pitch ratio, d/p, also changes the electro-distortional curve as seen in Figure 32 (c). The d/p value in a real device is controlled by the chiral dopant concentration, which allows the thickness to be chosen independently for other reasons (which will be discussed later). Lower values of d/p have a slightly steeper curve, but will also reduce the threshold and tend to smooth out the transition. Deciding which

105 90 pretilt and d/p to choose will depend on how they influence stripe defect formation, which will be discussed next. It is desirable to avoid stripe domains in STN LCDs, which are regions where the applied voltage doesn t reorient the liquid crystal in the usual manner. Instead, the entire domain is flipped on its side meaning the natural twist is intact and the helical axis is parallel to the cell surfaces. Stripe domains scatter light, so to avoid this problem, the d/p should be as small as possible (by under-doping the mixture) in order to raise the threshold of the stripe defects above the threshold of the Frederiks transition. A lower d/p, however, might introduce a state that is under-twisted [44]. For example, if the pretilt angle favors a 270 twist then a 90 twist of the same handedness would be possible with a slight splay in the cell to match the pretilt angle imposed at the substrate surfaces. Since increasing the pretilt angle also lowers the threshold of the Frederiks transition, it can be used to discourage stripe defects. Increasing the pretilt angle also discourages under-twisted domains by increasing the cost of adding a splay distortion [35]. Given the risk of bistability and stripe defects, the best choices are to have the highest pretilt possible with commercially available polyimide and standard rubbing alignment techniques which is in the vicinity of 7-8 (SE-3510 from Nissan Chemical [37]), a twist of 240 since it eliminates a chance for bistability and reduces the chiral dopant needed (d/p) while still resulting in a fairly steep electro-distortional curve and a d/p that is less than the perfectly matched d/p (for a 240 that is d/p < 240 /360 = 0.67).

106 91 The other parameters that affect the steepness of the electro-distortional curve are all a consequence of the liquid crystal material used and shown in Figure 33. The steepness increases as K 33 /K 11 increases as shown in (Figure 33 (a)) and the steepness decreases as either the K 22 /K 11 increases (Figure 33 (b)) or the γ increases (Figure 33 (c)). Therefore, the choice of liquid crystal material should be one that maximizes the elastic constant ratio K 33 /K 11, minimizes the elastic constant ratio K 22 /K 11, and minimizes the dielectric parameter γ. In practice, the data sheets that report the liquid crystal material s parameters are often missing numbers for K 22 /K 11 and/or K 22. In general, K 22 doesn t vary as much as the other elastic constants from material to material so K 22 will be estimated from K 22 /K 11 = 0.6 (given K 11 ). In order to improve the performance of STN, the following liquid crystal, 13B-000 (Merck) is evaluated as a possible replacement for MLC-6080 for its material properties. For example, the elastic constant ratio K 33 /K 11 is higher. Despite the fact that the dielectric parameter γ is somewhat high, 13B-000 is predicted to have a steeper electrodistortional curve than MLC The materials properties from the Merck datasheets are shown in Table 6. A simulation of the electro-distortional curves of these two materials was carried out and the results confirm that the electro-distortional curve for 13B-000 is steeper than MLC-6080 as seen in Figure 34 using a common set of cell design parameters: Φ = 240, θ p = 7, d/p = Therefore, 13B-000 is chosen as a replacement for MLC-6080 to improve the performance of the STN LCD.

107 Midlayer Tilt Angle, m ( ) Midlayer Tilt Angle, m ( ) Midlayer Tilt Angle, m ( ) a) = b) Reduced Voltage (V/V o ) 70 p = m = p c) Reduced Voltage (V/V o ) Reduced Voltage (V/V o ) 0 d/p = Figure 32 Simulations of midlayer tilt angle, θ m, show how the STN electro-distortional curve varies with one device parameter: a) twist angle: Φ (d/p is also varied to match twist), b) pretilt angle: θ p, c) thickness/pitch ratio: d/p

108 Midlayer Tilt Angle, m ( ) Midlayer Tilt Angle, m ( ) Midlayer Tilt Angle, m ( ) a) K 33 /K 11 = b) Reduced Voltage (V/V o ) K /K 11 = c) Reduced Voltage (V/V o ) = Reduced Voltage (V/V o ) Figure 33 Simulations of midlayer tilt angle, θ m, show how the STN electro-distortional curve varies with one material parameter: a) bend/splay elastic constant ratio: K 33 /K 11, b) twist/splay elastic constant ratio: K 22 /K 11, c) dielectric parameter: /

109 Midlayer Tilt Angle ( ) 94 Table 6 Parameters of 13B-000 and MLC-6080 from the Merck datasheet [37]. Parameter Symbol 13B-000 MLC-6080 Phase Transition Temperature T N-I 95 C 85 C Flow Viscosity [mm 2 s -1 ] 20 C η Dielectric Anisotropy (20 C, 1kHz) Δε ε// ε Dielectric Parameter γ Optical Anisotropy (20 C, 589nm) Δn n e n o Elastic Constants (20 C) K pN 14.4pN K 22 N/A 7.1pN K pN 19.1pN Elastic Constant Ratios K 33 / K K 22 / K 11 N/A MLC B Reduced Voltage (V/V o ) Figure 34 Simulations of midlayer tilt angle, θm, show that 13B-000 is steeper than MLC-6080 using a common set of cell design parameters: Φ = 240, θ p = 7, d/p = 0.53.

110 Birefringence Magnitude and Dispersion Earlier results described the process of measuring the variation of the liquid crystal birefringence with wavelength and why it was important for obtaining an accurate simulation of the transmission spectrum. It is hypothesized that less dispersion from a lower value of λ* will produce a broader spectrum in the bright state of a STN LCD leading to a less colorful appearance. This hypothesis will be tested using three liquid crystal materials with different λ*: E44, MLC-6080, and 13B-000 from Table 4. The experiment will simulate the bright state transmission spectrum of a 240 STN LCD using the dispersion curves from Figure 24 (b). To ensure a fair and equal comparison, each material will have the same optical retardation at 589nm, Δnd(589nm) = 0.79; because Δn(589nm) varies, the cell thickness must change to compensate. Also, the same polarizer (α i = -44 ) and analyzer angles (α o = 14 ) will be used to ensure the transmission at 589nm is the same. These conditions ensure the only difference between the materials will be the shape of the dispersion curve as influenced by λ*. The results of the simulation are shown in Figure 35 where it is seen that lower values of λ* tend to broaden the transmission spectrum. The calculation of the purity of the three spectra using the 1976 CIE chromaticity coordinates is shown in Table 7 and it has the trend that lower λ* have lower purity, which also confirms that in this case the broader spectra are less colorful. It should be noted that the trend of purity with λ* shown here might not prove to be a true in all cases, for example if the spectrum has an unusual shape that causes a large shift in the dominant wavelength when λ* changes. However, to design an STN LCD with high luminance, the dominant wavelength should

111 Transmission (%) 96 be close to the peak of the human eye sensitivity around 550nm; therefore, this trend of purity with λ* should apply to most STN LCD designs. The previous study in this chapter found that, in general, a lower overall birefringence results in a lower λ* (flatter dispersion curve). Therefore, it can be concluded that liquid crystal materials with a lower birefringence are (in general) more desirable for designing less colorful STN displays. This is another criterion where 13B-000 will be an improvement over MLC in the STN LCD E44, * = 255nm MLC-6080, * = 231nm 13B-000, * = 196nm Wavelength (nm) Figure 35 Simulated transmission spectra of: E44, MLC-6080, & 13B-000 with Δnd(589nm) = 0.79, α i = -44, and α o = 14.

112 97 Table 7 Purity of E44, MLC-6080, & 13B-000 from the spectra in Figure 35. Liquid Crystal λ* λ d Purity (1976 CIE) E44 255nm 566nm MLC nm 565nm B nm 563nm Driving Voltages Choosing the best voltage for driving the STN LCD involves determining two data points: V ratio, the voltage ratio, which depends on the resolution of the display and V th, the threshold voltage, which depends on the STN LCD material and cell design. The voltages are set so that V off = V th and V on = V off V ratio. The threshold voltage of the 13B-000 liquid crystal was calculated using the method published by Breddels [45] which states that the threshold voltage is the voltage at which the middle layer tilt angle equals the pretilt angle: θ m = θ p. The result is shown by Equation 30 below. V th K 0 K 33 K22 2 K33 K22 d 2 cos p (30) K11 K11 K11 K11 p 11 4 Using Equation 30 with the following cell design parameters, Φ = 240, θ p = 7, d/p = 0.53 and the material parameters for 13B-000 (Table 9), the V off = V th = 1.17V. Using the V ratio = 1.28 for a 17 row display gives V on = V = 1.50V Polarizer and Analyzer Angles In the previous studies described in this chapter, the polarizer and analyzer angles were determined by a brute force method that checked all possible combinations for the highest contrast ratio. In practice, the efficiency of the simulation can be improved by taking some shortcuts that do not sacrifice accuracy and will find the solution much

113 98 faster. Instead of maximizing the contrast ratio, we find the polarizer and analyzer angles that minimize the dark state transmission at a single wavelength, 550nm. This simplification is sufficient because the contrast ratio is more heavily influenced by the dark state transmission and the human eye sensitivity nears a maximum at 550nm The Best Cell Gap Until now, the cell gap has been ignored in the display design for simplicity because the equilibrium director profile is independent of the cell gap. The director profile does depend on d/p; however, this parameter can first be considered fixed with respect to changes in thickness, d, by changing the chiral dopant concentration. Once the voltages are chosen and the director profile is determined for both states, then the transmission spectrum of the on and off states at the best polarizer and analyzer orientation can be found for several cell thicknesses (with d/p fixed). In general, it is expected that smaller cell gaps will have higher contrast ratios since the residual retardation will be reduced (Equation 31 below), thereby reducing the light leaking in the dark state. 2 n ( ) d (31) It is not obvious how the bright state spectrum (which determines the purity and luminance) will change with cell thickness. This is because the polarizer and analyzer angles are only optimized for the dark state and the bright state transmission spectrum will not only increase or decrease, but it will also shift left and right as the cell thickness (along with polarizer and analyzer angles) changes. The following experiment is

114 Transmission (%) 99 designed to simulate the transmission spectrum at several cell thicknesses for the 13B- 000 material and cell parameters to find the best compromise to maximize contrast ratio, maximize luminance, and minimize purity. The parameters that are fixed are Φ = 240, θ p = 7, V off = 1.17V, V on = 1.50V and d/p = The polarizer and analyzer were optimized for the lowest transmission at 550nm in the dark (V = V on ) state μm μm Wavelength (nm) Figure 36 Simulated transmission spectra of 13B-000 for cell gaps 5-7μm. For the material 13B-000, as the cell gap increases, the transmission spectrum will shift towards longer wavelengths. This can be seen in Figure 36, where the RGB color of each spectrum as printed in the figure is tuned to be close to the actual perceived color to aid the reader. In Figure 37 (a), the contrast ratio vs. cell gap confirms the hypothesis

115 Luminance Contrast Ratio 100 that the larger cell gaps have more light leakage in the dark state, which reduces the contrast ratio. The trend that contrast ratio will increase with decreasing cell thickness should be universal in the STN LCD a) b) Cell Gap ( m) Cell Gap ( m) Figure 37 Contrast ratio and luminance of 13B-000 for cell gaps 5-7μm.

116 v 101 For the material 13B-000, the luminance doesn t continuously increase with increasing cell gap, but it tends to have a peak value as seen in Figure 37 (b). This effect occurs because the peak of the transmission spectra is shifting towards higher wavelengths with increasing cell gap and the luminance is highly dependent on the human eye response (shown as y ( ) in Figure 27) which peaks at around 550nm. For this material, the peak of the transmission also increases with increasing cell gap, however this does not change the luminance as significantly as the overall shift with wavelength does (with respect to the human eye response). 0.6 d = 7.0 m White Point 0.4 d = 5.0 m u Figure CIE chromaticity coordinates of 13B-000 for cell gaps 5-7μm.

117 Purity (1976 CIE) 102 The purity of the material 13B-000 depends on the dominant wavelength and the chromaticity coordinates of the spectrum. The 1976 CIE chromaticity coordinates are plotted in Figure 38. As the cell gap increases, the distance from each coordinate to the white point first decreases then increases and a few coordinates at the intermediate cell gaps are nearly the same distance from the white point. The purity takes into account the dominant wavelength to determine the perceived coloration and the dominant wavelength trends the same as the shift of the transmission spectrum and steadily increases with increasing cell gap. The cell gap with the lowest purity (least perceived coloration) as seen in Figure 39 is determined to be 5.8μm Cell Gap ( m) Figure 39 Purity of 13B-000 for cell gaps 5-7μm. To optimize a combination of factors, they need to be combined into a single figure of merit and the form of this combination is completely subjective. We chose the following

118 Normalized Figure of Merit 103 two combinations to compare: contrast-brightness and contrast-brightness-purity. The best compromise between high contrast and high brightness is obtained using a figure of merit which is defined as the contrast ratio times the luminance. This calculation is shown normalized to the maximum value in Figure 40 and it peaks at 5.4μm (α i = -50.0, and α o = 20.0 ). The best compromise between high contrast, high brightness, and less perceived coloration is obtained using a figure of merit which is defined as the contrast ratio times the luminance divided by the purity. This calculation is also shown normalized to the maximum value in Figure 40 and it peaks at 5.8μm (α i = -48.1, and α o = 18.1 ), which is chosen to be the optimized cell gap for this material Contrast*Luminance/Purity Contrast*Luminance Cell Gap ( m) Figure 40 Two normalized figures of merit for 13B-000 for cell gaps 5-7μm Response Time of STN The response time of STN displays can be calculated by the dynamics of the director reorientation. The rotation of the director is controlled by the balance between the

119 104 electric torque and elastic torques with the rotational viscosity torque. The coupling between the rotational motion and the translational motion is neglected. The rotation is over-damped because the viscosity is high, so the inertial term is neglected [46]. For the case of infinitely strong anchoring and zero twist angle at the surface during the field off transition, the turn off time is described by Equation 32, where γ r is the rotational viscosity, K 22 is the twist elastic constant and d is the cell gap [5] r d off (32) K The turn on time also depends on the strength of the electric field as described by Equation on off (33) ( V / V ) Summary The best recommendation for a STN liquid crystal material for pixelated glare shielding eyewear will have a steep electro-distortional curve and a broad transmission th spectrum. The previous results have shown that this can be done by maximizing K 33 /K 11 and minimizing γ as well as keeping Δn lower to maintain a lower λ* for a whiter transmission spectrum and higher contrast. Also, the simulations that varied the pretilt angle and d/p show it has less effect on the steepness of the electro-distortional curve as shown in Figure 32 (b-c). However, to prevent stripe defects, the d/p is will remain at a common value used for 240 STN LCDs in manufacturing, d/p = 0.53, which is less than the equilibrium value of 0.67 and the pretilt angle will be high around 7-8.

120 105 One consequence of a lower Δn is that the thickness must be larger to maintain enough optical retardation for high brightness in the off state. Fortunately, a thicker cell is a benefit in the construction of a real device since it is easier to maintain a uniform cell gap at larger thicknesses. However, the downside is that a larger thickness will have a longer response time which increases rapidly ~d 2 (Equation 32). 3.8 Experiment Using Liquid Crystal 13B-000 A cell was constructed using SE-3510 with pretilt of 7-8 and filled with liquid crystal 13B-000 doped for a pitch of 10μm. The cell was constructed using 5.3μm spherical spacers to give a d/p of The spectrum was measured using an Ocean Optics USB4000 spectrometer and a Nikon polarizing microscope. The cell was placed between parallel polarizers and switched to a high voltage (30V) for the reference spectrum. (Using the cell during the reference reading allows some of the excess reflections and interference to be taken into account). The cell was re-oriented along with the polarizer and analyzer to the orientation which was predicted by the simulation to produce the lowest transmission at 550nm (α i = -50.4, and α o = 20.4 ) at 1.5V. However, this orientation did not minimize the transmission. Then, the orientation of the polarizers and STN cell were adjusted to minimize the transmission; however, at this orientation the bright state spectra at 1.17V and 0V did not match the simulation results well. After carefully considering the possible sources of the error, it was determined that the cell gap did not maintain the 5.3μm spacing after being pressed down on the spacers after filling during the manufacturing. The spherical spacers can respond to pressure with a small deformation

121 Dark State Transmission % Bright State Transmission % 106 more easily than rod shaped fiber spacers due to the surface contact area being smaller. If the spectrum is simulated using a 5.0μm cell gap as shown in Figure 41, then the simulations match the measurements well for the voltages: 0V, 1.17V and 1.50V Experiment Simulation m, d/p=0.5, V=1.17V, p =9 5.0 m, d/p=0.5, V=0V, p = m, d/p=0.5, V=1.50V, p = Wavelength (nm) Figure 41 The spectrum at various voltages: 0V, 1.17V and 1.5V using 13B-000 liquid crystal in a 240 STN LCD. One important distinction is that the d/p must be updated in the simulation to account for the thinner cell gap. An empty STN cell made in the same batch as the filled STN cell was pressed using the same pressure and sealed while empty to verify the deformed spacer hypothesis. The cell gap was measured in this empty cell and it varied from μm depending on localized spacer density; therefore, it is reasonable to consider that the region of the cell measured and shown in Figure 41 had a reduced cell gap. The dark

122 Transmission % 107 state at 1.5V also shows a good match to the theoretical transmission and the uniformly higher transmission in the experimental data is likely due to light leaking through the spacers. The transmission vs. voltage curve was measured using the experimental setup described in this dissertation that used a white light source and detector with a photopic filter. The result is shown in Figure 42. The reference measurement was taken with the cell placed between parallel polarizers at 30V and the polarizer and analyzer were adjusted to α i = -55, and α o = 18 which was the orientation that minimized the transmission at 1.5V Voltage (V) Figure 42 Transmission vs. voltage for 13B-000. As mentioned previously, the spacers were deformed slightly during pressing the cell, and the cell gap was really 5.0μm, therefore the d/p was actually 0.50 and the pretilt was

123 108 closer to 9. For these parameters, the simulations predicted α i = -52, and α o = 22, which means the input rubbing alignment was not exactly at 0. If the input rubbing was actually near 3 it would reduce the error in each polarizer angle to ~1 or less. The contrast ratio found experimentally was 150 using V off = 0V and V on = 1.5V, but was only 123 using V off = 1.17V and V on = 1.5V due to the partial reorientation at V off which reduced the luminance at that level from 50% to 41% in this experiment. Note that the luminance value at 0V (50%) more closely matches the luminance value predicted by the simulations for 5.0μm (58%) than for 5.3μm (66%) as shown in Figure 37. Unfortunately, the transmission vs. voltage curve is not as steep as the prediction from the simulation shows in Figure 34. This is most likely due to the inaccuracy in the K 22 elastic constant or the pretilt angle, both of which had to be estimated for the simulations. Further accuracy would be obtained if these parameters were measured empirically. For example, the simulated spectrum at 1.17V in Figure 41 was done using a pretilt of 9 instead of 7 in order to better match the experimental result. However, after reviewing the experimental data in Figure 42 and comparing it to the simulation in Figure 34, it appears that the lack of steepness in the experimental data is not due solely to an error in the estimation of the pretilt angle. The same cell described previously was used to measure the response time, contrast ratio and luminance using several different voltages with the experimental setup used to take the transmission vs. voltage curve. Each pair of voltages, V off and V on, were arranged at the same V ratio = 1.28, while V off was varied from 1.09V to 1.20V. The contrast ratio is shown in Figure 43 and it peaked at V off = 1.17V as expected because (as

124 109 previously mentioned) the polarizer and analyzer were oriented to minimize the transmission at V on = 1.5V. The luminance steadily decreased as the V off increased because at each increase in V off, the polar tilt angle of the STN midlayer increased a little further from the undisturbed planar state. The turn on time (τ on 0-90 ) and turn off time (τ off ) are also plotted in Figure 43. The turn on time steadily decreases with increasing V off because V on is increasing. The turn off time steadily increases with increasing V off because the difference between the initial and final liquid crystal director profile becomes smaller, which means the restoring force as the voltage is switched from V on to V off becomes smaller. However, the total response time, τ on τ off , remains nearly constant. The response times are quite long; however, all of them are close to or less the human eye response time discussed in Chapter 2. Unfortunately, the response times at temperatures lower than room temperature (22 C) or at larger cell gaps will likely be longer due to increasing viscosity at low temperatures and the cell gap dependence (Equation 32) respectively.

125 Contrast Ratio Luminance Response Time (ms) off 0-90 on V OFF (V) Contrast Ratio Luminance V OFF (V) 30 Figure 43 Response times, contrast ratio and luminance for 13B-000 as V off is varied.

126 Viewing Angle of STN The viewing angle of pixelated glare shielding eyewear benefits from the curvature of the lens. This curvature will help compensate the viewing angle since it orients the substrates at each pixel closer to being perpendicular to the propagation direction of incoming glare. However, a limitation of the design is that the LCD in pixelated glare shielding eyewear must place the (input) polarizer s transmission axis vertical in order to absorb directly reflected glare. This is because reflected glare is partially polarized parallel to the surface of reflection, a consequence of Brewster s angle. If the polarizer in the front (input) surface is vertical, then the possible orientational configurations are limited, so choosing an LCD with a symmetric viewing angle is desired. It is well known that the viewing angle of the STN LCD is not as good as other LCD modes; fortunately however, the viewing angle in STN is much more symmetric than the uncompensated TN viewing angle. The TN viewing angle is easily compensated to become more symmetric using discotic films made from polymerized discotic materials (PDM) [47]; however, these films will only compensate a dark state that is fully saturated (>5V). As mentioned previously, the fully saturated state can not be reached even with 3:1 block addressing due to the extremely low threshold voltages and lack of steepness in the electro-distortional curve of the TN LCD. The STN viewing angle was measured using the same sample which used a 13B-000 STN and a different experimental setup. This setup had the same white light source and a similar detector with a photopic filter, but it had automated rotation stages to acquire the contrast ratio using the same driving voltages 1.17V and 1.50V for several orientations.

127 112 The polarizer and analyzer were adhered to the cell at the orientation which minimized the on axis transmission at 1.5V. The results are shown in Figure 44 where the isocontrast is plotted from the wearer s point of view and the input polarizer was vertical. It can be seen that a contrast ratio of 5 or greater exists out to a polar angle of 30 for all azimuthal directions with a contrast over 100 on axis and slightly off axis below center (which corresponds to light entering from above as direct sunlight would enter) Figure 44 Iso-contrast viewing angle of the 13B-000 STN as seen by the wearer Further Improvements It is well known that the 90 twisted nematic (TN) can obtain higher contrast ratios than the STN; however, the contrast is not higher at the resolutions necessary for

128 113 pixelated glare shielding eyewear to be effective (using a passive matrix). The 180 STN using the optical mode interference (OMI) effect can achieve a higher resolution compared to TN and higher contrast compared to STN for the same resolution; however, it suffers from low brightness. This brightness is <50% of parallel polarizers, which would be typically <20% of unpolarized light overall due to polarizer inefficiencies and this is too dim for the bright state of glare shielding eyewear. In fact, all the polarizer based absorption displays that switch from high optical retardation to low optical retardation (ECB, TN, STN) will improve in contrast ratio as driving voltage is increased. For example, some compensated LCDs have achieved contrast ratios high enough (>2000) to be commercialized in single pixel glare shielding eyewear devices by switching to V = 8V th [48, 49], but this voltage ratio is not compatible with a passive matrix. Therefore, the 240 STN is better; however, further improvements in contrast of a 240 STN would come from increasing the voltage ratio to the 3:1 block addressing scheme, since the residual light leaking would be further suppressed by additional reorientation of the director at higher voltages. Block addressing also has the added benefit of allowing the display resolution to scale to whatever number of rows will optimize the effectiveness of the glare shielding without disrupting the voltage ratio Conclusion A systematic procedure to optimize STN LCDs and film compensated STN LCDs was developed. The best twist angle is shown to be 240 to have the best compromise that achieves a steep electro-distortional curve for a passive matrix design, high

129 114 brightness for usability, and a relatively symmetric viewing angle. A theoretical and experimental study showed the effects of changing the cell parameters, such as, polarizer and analyzer angles, cell thickness, and compensation film angle, on the transmission spectra of STN LCDs. Methods to reduce light leakage in the dark state and improve the contrast ratio of STN LCDs were demonstrated by choosing material properties that steepen the electro-distortional curve. It was shown that reducing the birefringence dispersion can improve the bright state by increasing luminance and decreasing the perceived coloration. If all the experimental variables are known accurately, then the simulation results agree well with the experimentally measured data. The best STN design using the material 13B-000 was a 240 STN LCD with d/p = 0.53, a pretilt angle 7-9 at driving voltages of V off = 1.17V and V on = 1.5V for a 17 row display with V ratio = The polarizer and analyzer angles should be adjusted for the minimum transmission at 550nm and 1.5V near the simulated orientation due to manufacturing tolerances in alignment of the components and/or variations in the cell gap. The cell gap should be 5.4μm (simulation results are α i = -50.0, and α o = 20.0 ) to emphasize high contrast or 5.8μm (simulation results are α i = -48.1, and α o = 18.1 ), to emphasize high contrast and brightness as well as less perceived coloration.

130 Chapter 4 4. Normal Mode Polymer Stabilized Cholesteric Texture for Scattering Displays 4.1 Overview The polymer stabilized cholesteric texture (PSCT) display was developed by researchers at Kent State University [25, 50, 51]. The normal mode PSCT is formed from a mixture of liquid crystal, chiral dopant, monomer and photoinitiator. The mixture is vacuum filled into ITO coated cells with no alignment layer. Polymerization occurs with a high voltage placed on the liquid crystal cell which unwinds the chiral nematic structure and reorients the liquid crystal molecules to the homeotropic state (perpendicular to the substrate) as shown in Figure 45. V UV Irradiation V Liquid Crystal Polymer Network Monomer Photoinitiator Figure 45 Polymer network formation in normal mode PSCT by UV light exposure. When UV light is incident on the sample, the photoinitiator will become electronically excited, decompose into free radicals, undergo a photocleaving process, and react with the double bonds at the end of the monomer molecules. Only a very small amount of 115

131 116 photoinitiator (<15% of the monomer s concentration) is necessary to start a chain reaction that forms a three dimensional cross-linked anisotropic polymer network. A diagram of the two optical states of the normal mode PSCT is shown in Figure 46. After polymerization, the display is still transparent because the liquid crystal is aligned homeotropically due to the applied voltage and there is not enough dispersed polymer to disrupt the index of refraction and cause light scattering. As mentioned in the overview of the normal mode PSCT in Chapter 2, the PSCT does not absorb light (like the polarizer based LCDs), but instead will scatter it. As shown in Figure 46, when the voltage is removed the display scatters light from the boundaries between cholesteric focal conic domains because of an abrupt change in the index of refraction. Transmitted Scattered V V Liquid Crystal Polymer Network Figure 46 Optical states of the normal mode PSCT. It is desirable to optimize the average domain size for the best scattering efficiency. This is achieved when the focal conic domain size is comparable to the wavelength of visible light, but there are several parameters which can affect the domain structure and

132 117 size, which includes the properties of the materials employed, the concentrations of the materials, and the polymer network curing conditions. 4.2 Experiment This dissertation evaluated the performance of the normal mode PSCT with the following experiments: The chiral dopant concentration was varied (which also varied the pitch). The monomer concentration was varied. The liquid crystal material was changed. Each experiment independently varied one parameter while keeping the other parameters fixed. The standard condition was a cell constructed from 10µm thick ITO coated cells with no alignment layer. The UV curing conditions exposed the cells at room temperature for 20 minutes with a UV light source which had a measured intensity of 3.15mW/cm 2. The liquid crystal used was BL036 (Merck) which is a commercial liquid crystal mixture whose molecular components and ratios are not published. The monomer chosen was a commercially available liquid crystalline monomer RM257 (Merck) with reactive double bonds at both ends of its chemical structure as shown below in Figure 47 [52]. Figure 47 Molecular structure of monomer RM257 used in normal mode PSCT.

133 118 The chiral dopant (shown in Figure 48) used was R-811 (Merck), chosen because of its good solubility in the liquid crystal host at higher concentrations and a reasonable HTP, which was measured at 10µm -1. Figure 48 Chiral dopant in use R-811. The photoinitiator was previously shown in Figure 19 which is benzoin methyl ether (BME) (Aldrich) and it was added at a concentration that was 10% of the monomer concentration used. The collection angle for all measurements was 9.5 total or equivalently a cone of 4.75 from the beam center in all directions. Three cells of each formulation were made and the data was averaged before being plotted. Error bars indicate the deviation and connecting lines are included to show the trends. The hysteresis, ΔV, is defined as the distance between the points where the increasing and decreasing transmission curves pass through the value that is 0.5 (T max - T min ) + T min. The delay between measurement points was 1000ms and the voltage was ramped from 0 to 60V in increments of 1V. 4.3 Results All calculations of the contrast ratio used the maximum transmission and the minimum transmission as found in the T-V curve like the one shown in Figure 49 below. If the contrast ratio in normal mode PSCT is calculated when the voltage is removed

134 119 gradually it is oftentimes lower than the contrast ratio calculated when the voltage is removed suddenly. This is because the transition from the homeotropic texture to the focal conic texture is not a homogenous transition (occurring everywhere simultaneously), but is nucleation process that starts at defects in the sample such as spacers and the strands of the polymer network [53]. As voltage decreases slowly, the focal conic texture grows from just a few places so it forms larger domains. If the voltage is removed suddenly, the domains form in many more places at once and therefore the average domain size tends to be smaller, which scatters light better. This phenomenon is similar to the cause of the bistable textures in the reflective cholesteric where the sudden removal of voltage forms the planar texture and the gradual removal forms the focal conic texture [53]. Before a T-V curve is measured the sample is switched off to 0V from a high voltage >50V suddenly. Then the increasing voltage curve (shown in blue in Figure 49) is used to find the sudden contrast ratio. The gradual contrast ratio is found from the decreasing voltage curve (shown in red in Figure 49). The saturation voltage (V 90 ) is a number that represents the driving voltage necessary to reach 90% of the range T max T min higher than T min (the same point used in response time calculations). The higher the saturation voltage is, the more voltage is required to reach the bright transparent state. The saturation voltage will still have some scattering and appear hazy so it is NOT to be confused with the driving voltage, which should be higher to achieve complete transparency.

135 Transmission % Contrast Ratio = 51.4 Contrast Ratio = o Collection Angle Voltage Decreasing V = 9.7 Voltage Increasing Voltage (V) Figure 49 Typical experimental result used for calculating contrast ratio Chiral Concentration Varied Perhaps the most critical factor in the normal mode PSCT for controlling the domain size is the pitch of the cholesteric liquid crystal. According to Figure 21 from Chapter 2, the pitch of normal mode PSCT should be within the range between 0.5-2µm. In the experimental results shown in Figure 50 the monomer concentration was fixed at 2.0% and the chiral dopant was varied. The result of the pitch calculation using the measured HTP of 10µm -1 is shown in Table 8.

136 121 Table 8 Chiral concentration and intrinsic pitch of normal mode samples. Normal Mode R811 Pitch 4.90% 2.04 µm 7.28% 1.37 µm 8.72% 1.15 µm 11.10% 0.90 µm 13.10% 0.76 µm As shown in Figure 50 the contrast ratio increases with decreasing pitch (chiral dopant increasing) for both the sudden and the gradual contrast ratio. The saturation voltage increased with increasing chiral dopant concentration because the unwinding of a tighter pitch with an applied voltage becomes more difficult as the pitch decreases. This is expected due to the equation for the critical field necessary to unwind a cholesteric completely to the homeotropic texture, which is inversely proportional to the pitch as seen in Equation 34 [5]. 2 K22 E c (34) P The hysteresis, ΔV, did not change significantly with chiral dopant concentration. The response times vs. chiral concentration are shown in Figure 51. Both the turn on and the turn off times decreased with increasing chiral concentration. The turn on time is the transition from scattering focal conic to transparent homeotropic. This transition time decreased because the driving voltage increased proportionally to the 0 saturation voltage, V 90. The turn off time is the transition from the transparent homeotropic to the scattering focal conic texture. This transition is driven by the

137 Saturation Voltage (V 90 ) (V) Hysteresis Width ( V) (V) Contrast Ratio 122 relaxation of the liquid crystal as well as the tendency to form a twisted structure. The smaller the pitch, the greater is the energy of the untwisted state when the voltage is removed, so the turn off response time decreases with increasing chiral dopant (decreasing pitch) Sudden Gradual V 90 R811 V Chiral Dopant Concentration (wt.%) R811 Chiral Dopant Concentration (wt.%) Figure 50 Contrast ratio, saturation voltage, and hysteresis width data when monomer concentration is fixed at 2.0% for normal mode PSCT.

138 Turn On Time (0-90%) (ms) Turn Off Time (100-10%) (ms) R811 Chiral Dopant Concentration (wt.%) R811 Chiral Dopant Concentration (wt.%) Figure 51 Response time data when monomer concentration is fixed at 2.0% for normal mode PSCT Monomer Concentration Varied The monomer concentration (along with photoinitiator at a fixed ratio of monomer:photoinitiator of 10:1) was varied when chiral concentration is fixed in the range %. As shown in Figure 52 both the sudden and the gradual contrast ratio

139 124 first increases with increasing monomer concentration and then decreases with a peak at 2.8%. The domain size decreases with increasing polymer concentration. When the domain size is around the wavelength of incident light, the scattering is the strongest. When the domain size is too small or too large, the scattering decreases. The more polymer network dispersed in the cell the stronger the aligning effect will resist a return to the focal conic texture from homeotropic state, so a larger hysteresis would be expected. This resistance is due to the aligning effect of the polymer network which prefers the orientation of the liquid crystal near polymer bundles to be parallel to the bundles. Indeed as expected, Figure 52 shows that the measured hysteresis, ΔV, increases with increasing monomer concentration. It is also predictable that this aligning force of the polymer would tend to decrease the saturation voltage and this is in fact seen up to 2.5% monomer at which point additional monomer no longer has an effect. The response times vs. monomer concentration are shown in Figure 53. The turn on time is the transition from scattering focal conic to transparent homeotropic and it peaks around 2.5% monomer. Faster response times are found with more or less monomer which is not easily explainable. The turn off time is the transition from the transparent homeotropic to the scattering focal conic texture. This transition is driven by the relaxation of the liquid crystal as well as the tendency to form a twisted structure. The higher the polymer concentration, the slower the relaxation becomes and this trend is confirmed.

140 Saturation Voltage (V 90 ) (V) Hysteresis Width ( V) (V) Contrast Ratio Sudden Gradual RM257 Monomer Concentration (wt.%) 20 V V RM257 Monomer Concentration (wt.%) Figure 52 Contrast ratio, saturation voltage, and hysteresis width data when chiral concentration is fixed in the range % for normal mode PSCT.

141 Turn Off Time (100-10%) (ms) Turn On Time (0-90%) (ms) RM257 Monomer Concentration (wt.%) RM257 Monomer Concentration (wt.%) Figure 53 Response time data when chiral concentration is fixed in the range % for normal mode PSCT.

142 Liquid Crystal Material Varied In principle, almost any liquid crystal can be used in a PSCT display. However, there are certain physical properties that are critical to display operation and others that are desirable for good display performance. The material must have a positive dielectric anisotropy so it can be aligned homeotropically to obtain the transparent state. The higher it is then the lower the voltage necessary to unwind the helix and switch to the homeotropic state (as shown previously in Equation 34), which will reduce the display s power requirements. It has been shown that a high birefringence (Δn > 0.2) liquid crystal should be used to ensure more efficient light scattering, which improves the contrast ratio; however the highest Δn liquid crystals are not commercially available [54]. Table 9 Parameters of BL036 and E44 from the Merck datasheet [37]. Parameter Symbol BL036 E44 Phase Transition Temperature T N-I 95 C 100 C Flow Viscosity [mm 2 s -1 ] 20 C η Dielectric Anisotropy (20 C, 1kHz) Δε ε // ε Optical Anisotropy (20 C, 589nm) Δn n e n o Elastic Constants (20 C) K pN 15.5pN K pN 28.0pN Viscosity is an important property to consider as well because the response time of the liquid crystal is proportional to the viscosity as was described earlier in Equations 32 and 33. Ideally, the rotational viscosity of each liquid crystal should be compared because that is the viscosity parameter in these equations, but this data is not always available. As

143 Turn On Time (0-90%) (ms) Turn Off Time (100-10%) (ms) 128 a substitute, the flow viscosity was compared between the two liquid crystals BL036 and E44. It was hypothesized that since the birefringence, dielectric anisotropy, and elastic constants do not differ by very much, the lower viscosity of E44 should improve the response time in normal mode PSCT. The values of the important parameters are taken from the Merck data sheet and compared in Table E44 BL R811 Chiral Dopant Concentration (wt.%) R811 Chiral Dopant Concentration (wt.%) Figure 54 Response time data for E44 compared to BL036.

144 129 In the data shown in Figure 54, the response time data for E44 is shown directly compared to the data for BL036. It is seen that the new material had a measurable improvement in the response time, which is attributed to the lower viscosity. The contrast ratio of E44 was comparable to BL Conclusion The trends of the normal mode PSCT have been experimentally measured with the variation of chiral dopant and monomer concentration as well as the substitution of the liquid crystal host. The trends agree well with previous works [55]. The best formulation found in this dissertation of the normal mode PSCT for high contrast, fast response time, and lower driving voltage would be around 13% R811 (pitch = 0.75µm), 2.8% RM257 monomer (with photoinitiator at 1/10th as much) cured for 20 minutes at 3.15mW/cm 2, and the liquid crystal E44. As mentioned in the overview of the PSCT in Chapter 2, the PSCT scatters light, but despite the fact it doesn t absorb light the normal mode can be considered normally black. This means if the power source fails it will revert to the scattering state (which is the stable state at 0V). Recalling the LCD summary in Chapter 2, this means the normal mode has a fatal flaw which has gotten the design eliminated from the possible choices for use in glare shielding eyewear. What is needed is a design that reverses the optical states so that it will be transparent in the stable state at 0V, which would be considered normally white. Just such a design is possible and it is the subject of a detailed study described in Chapter 5.

145 Chapter 5 5. Reverse Mode Polymer Stabilized Cholesteric Texture for Scattering Displays 5.1 Overview As mentioned in Chapter 4, the polymer stabilized cholesteric texture (PSCT) display was developed by researchers at Kent State University [25, 50, 51]. The reverse mode PSCT is also formed from a mixture of liquid crystal, chiral dopant, monomer and photoinitiator like the normal mode. However, the cell is coated with a planar (homogenous) alignment layer which has been rubbed, usually in an antiparallel orientation. No voltage is applied during polymerization. Instead, the cell is polymerized in the planar twisted structure that is naturally present due to the influence of the alignment layers as shown in Figure 55. UV Irradiation Liquid Crystal Polymer Network Monomer Photoinitiator Figure 55 Polymer network formation in reverse mode PSCT by UV light exposure. The same polymerization will occur in reverse mode as occurred in normal mode, whereby the formation of the polymer network occurs by the photocleaving process 130

146 131 induced by UV light irradiation. The photoinitiator becomes electronically excited and decomposes into free radicals which react with the double bonds at the ends of the monomer molecules. Similarly to the normal mode PSCT, only a very small amount of photoinitiator (<15% of the monomer s concentration) is necessary to start a chain reaction in the material forming a three dimensional polymer network. The monomer molecules have a similar structure to the liquid crystal molecules which causes them to assemble into a network that mimics the orientation of the liquid crystal. The display is called reverse mode because the two optical states: 1) switched off (0V) or low voltage (V<V th ) and 2) under an applied voltage higher than the threshold voltage (V>V th ) have traded places compared to the normal mode PSCT as shown in the diagram of the two optical states of the reverse mode PSCT is shown in Figure 56. Although the optical states are reversed, the liquid crystal orientation in reverse mode is different from normal mode. After polymerization, the display is (still) transparent because the liquid crystal is aligned in the twisted planar structure with a pitch longer than the wavelength of visible light, which pushes the cholesteric reflection band to infrared light. Also, there is not enough dispersed polymer to significantly disrupt the index of refraction and cause light scattering as is common in the PDLC. As shown in Figure 56, when the voltage is applied, the display scatters light from the boundaries between cholesteric focal conic domains because of an abrupt change in the index of refraction. In the absence of a polymer network, the planar alignment layer will exert enough influence to induce the focal conic texture to relax to the planar texture; however, this transition is very slow due to such a small energy difference between them and an

147 132 energy barrier which favors the metastable focal conic state. The polymer network orientation is the critical phenomenon that enables the electric field induced focal conic texture to switch back to the planar texture (when the voltage is removed) fast enough for display applications (ms). Moreover, the polymer network controls the focal conic domain size. Transmitted Scattered V V Liquid Crystal Polymer Network Figure 56 Optical states of the reverse mode PSCT. 5.2 Experiment The reverse mode PSCT LCD consists of several components and processing steps that are added together to make the display. Because there is more than one material that can fulfill the function of each component, this study focused on varying these materials and processing parameters to determine the trends of each and the combination which had the best electro-optical performance. The rest of this section will discuss the reasoning behind the choice of what specific chemical components to add to the mixture. Then the next section describes the results from varying the concentrations of these componenents, varying the polymer network curing conditions, and varying other cell fabrication components and steps.

148 133 Two monomers were studied for use in the reverse mode PSCT. These monomers are shown below by the molecular formula. The first monomer tried, shown in Figure 57, was the same commercially available monomer used in the normal mode study in Chapter 4, so the earliest experiments conducted with reverse mode for this dissertation used it and found it to produce unsatisfactory results. After a few trials with RM257 fell short of the expected performance, a second monomer was tried. This monomer, shown in Figure 58, is a laboratory synthesized monomer described in early reports by Deng-Ke Yang [25]. Figure 57 Monomer RM257 was abandoned in the reverse mode experiments. Figure 58 Monomer BMBB-6 had the best results in the reverse mode experiments. The differences between these two monomers will be discussed later in this chapter for two reasons. Firstly, monomer RM257 was used early in the study and not revisited, so a comparison between the two monomers requires an understanding of the trends using the monomer BMBB-6 as will be described in this chapter of the dissertation. Secondly, the comparison draws on the structural differences between RM257 and

149 134 BMBB-6, both on the molecular level and the network level, which will benefit from the results gained in the network curing experiments. Two chiral dopants were studied for use in the reverse mode PSCT. These dopants are shown below by the molecular formula. Both dopants gave us satisfactory results; however, the accuracy of the pitch was more easily controlled using R-811 because the HTP of R-811 was measured to be 10μm -1 compared to the HTP of R-1011 which is ~3 times higher at ~30μm -1. Therefore, the amount of R-811 necessary is about three times larger making it a concentration that is easier to control precisely when fine tuning the pitch. Figure 59 Chiral dopant first studied in reverse mode PSCT, R Figure 60 Final chiral dopant studied in reverse mode PSCT, R Over Drive for a Faster Turn On Response Time Following the discussion in Chapter 2, an over drive pulse (at amplitude V OD ) can be added to the waveform to speed up the turn on time. In reverse mode PSCT, this is the

150 Transmission % 135 transition to the scattering state. The effectiveness of this over drive voltage depends on the driving voltage (V on ) being used, which means that for some fixed value of V OD, the improvement in response time is greater for lower driving voltages V on. The width of the over drive pulse must be long enough to ensure the maximum benefit is obtained, but not so long that the transmission reaches a minimum and begins to increase again. An experiment was conducted using cell 15C which consisted of: 0.15% BME, 7.10% BMBB-6, 2.02% R811, and 90.73% E44. The 10μm cell was coated with an SE-3510 alignment layer and was cured for 5 hours at an intensity of 0.015mW/cm 2. The over drive pulse was set to the maximum amplitude possible from the amplifier, 74V, and the duration was varied V OD = 21V (No OD) V OD = 74V 3.0ms V OD = 74V 0.5ms Time (ms) Figure 61 Comparison of two different over drive durations to no over drive.

151 Response Time (ms) 136 Data with the over drive pulse in Figure 61 shows an incredible improvement in the turn on time, which results in sub-millisecond switching! It was determined that the duration of the over drive pulse has an effect on the switching behavior. For example, in Figure 61 if the duration of the over drive pulse increased from 0.5ms to 3ms then the transmission halts abruptly before reaching the minimum and begins to increase because some of the focal conic domains have reoriented so much they are beginning to unwind to the homeotropic texture. 10 No change in Turn Off Time Off 0-90% On % (OD = 0.5ms) On % (OD = 1ms) On % (OD = 3ms) 1 No "Over Drive" at this voltage because V OD = V on Sub-millisecond switching past V OD =37V Over Drive Voltage (V) Figure 62 Response time vs. over drive voltage using Von=21V & Voff=7V. The results of varying the over drive pulse from V on = 21V to 74V at three constant durations of 0.5, 1 and 3ms is shown in Figure 62. The data in Figure 62 shows submillisecond switching times past V OD = 37V using a 3ms duration with no measureable change to the turn off time across the entire range of over drive amplitudes measured.

152 137 The data in Figure 62 also shows that it is important to use an over drive duration that is long enough to get the maximum benefit, for example 0.5ms is not long enough below V OD = 50V and 1ms is not long enough below V OD = 35V. However, the response curve, such as the one shown previously in Figure 61 should also be inspected to ensure the transmission smooth decreases with increasing during the over drive. As a reminder, using an over drive in a passive matrix requires the entire display to be switched on when glare is detected before individual pixels can be addressed because the over drive voltage is beyond the voltage ratio allowed. 5.3 Results Chiral Dopant Concentration Varied The chiral dopant concentration was varied to study its effect. All samples were made using nearly the same amount of the monomer BMBB-6 ~ % and cured at a similarly low curing intensity and duration; however, they were not made at the same time. All cells did have the same SE-3510 alignment layer and 16μm cell gap. A summary of the concentrations and cells are in Table 10 below. The pitch and number of turns are calculated from the HTP of R-811 in E44, 10μm -1, as was measured in the lab. Recalling the discussion in Chapter 2, the quantization of the pitch is forced by the requirement that the director must match the boundary conditions imposed by the rubbing direction. When the intrinsic pitch doesn t fit evenly, then two (or more) configurations of the helical pitch may exist in the same sample that will differ by a 180 (π) twist of the director. This is a defect that can be avoided by more closely matching the chiral dopant to the cell gap. The defect is undesirable because the two regions of different pitch will

153 Transmission % 138 not switch at the exact same voltage and this could be noticeable during display operation. See the defect treatment section for a more in depth discussion. Table 10 Percent chiral dopant, intrinsic pitch, turns, and contrast ratios for the samples. Cell ID R811 Pitch 180 (π) Turns Contrast 13B 1.24% 8.06μm A 1.56% 6.41μm A 1.89% 5.29μm A 2.34% 4.27μm 7 or A 2.65% 3.77μm 8 or P 0 = 8.06 m P 0 = 6.41 m P 0 = 5.29 m P 0 = 4.27 m P 0 = 3.77 m Voltage (V) Figure 63 Transmission data for various R-811 chiral dopant concentrations. The possibility of having a defect where two different pitches exist in the same sample is the main difference between an experiment that varies the chiral dopant concentration in the normal mode and in the reverse mode. Therefore, it isn t the best chiral dopant

154 139 percentage we are varying, but the pitch. Since the pitch is quantized it would be equally useful to consider the number of turns. The T-V curves of the samples in Table 10 are shown in Figure 63. When you compare the data it is seen that increasing the number of turns will increase the threshold voltage. The best contrast ratio and therefore the best scattering efficiency came from the sample with 6 turns which corresponds to a ~5μm pitch, which in this specific experiment was 1.89% chiral dopant Monomer Concentration Varied The monomer concentration of BMBB-6 was varied to study its effect. All samples were made using the same pre-mixture of: 0.14% BME, 1.94% R811, and 97.92% E44. All cells were constructed using SE-2170 alignment layers on top of the AT-720 barrier layer with 10μm spacers. The cells were all voltage treated for defects and cured using 0.016mW/cm 2 (using detector B).

155 Contrast Ratio & Drive Voltage V on (V) Drive Voltage V on (V) Contrast Ratio BMBB-6 (%) Figure 64 Contrast and voltage vs. monomer concentration. The results shown in Figure 64 and Figure 65 are an average of two cells using the same mixture (except 6.36% which had only one cell). From the data in Figure 64, it appears that the minimum transmission point in the T-V curve (defined as the drive voltage, V on ) increases with steadily with increasing monomer concentration. This is reasonable because the polymer network induces a restoring force which opposes the voltage induced reorientation of the liquid crystal. The contrast ratio is plotted on the same figure (using the same scale) and appears to not change significantly with monomer concentration, which means the domain size is not heavily influenced by the polymer network. The anomalous data at the concentration higher than 7.5% BMBB-6 likely

156 Turn Off Time (ms) Turn On Time (ms) 141 occurred because the monomer could not be fully dissolved in the material since the concentration is very near the solubility limit Turn On % Turn Off 0-90% All data uses V OD =30V OverDrive at start of pulse BMBB-6 (%) Figure 65 Response time vs. monomer concentration From the data in Figure 65, we see the turn off time (plotted on the left y-axis) increases with decreasing monomer concentration. The turn off time was the time to switch from V on to V off where V on is equal to the driving voltage in Figure 64 and V off = V on /3. The turn off time in reverse mode is a liquid crystal relaxation process, but one where the polymer network introduces a restoring force. This force is greater at higher monomer concentrations, thereby reducing the turn off time. The turn on time will depend on the voltage used to switch the display on, V on. Higher voltages will naturally reduce the turn on time (the principle on which the over drive method is based). Therefore, in order to

157 142 remove this variable, each sample was measured using a V OD =30V over drive pulse long enough for the transition to be complete before switching to V on. The turn on time is shown in Figure 65 as plotted on the right y-axes. The turn on time is more or less independent of the monomer concentration. This is somewhat surprising because it would seem that the lower monomer concentrations that have a lower driving voltage V on would switch on faster. Instead it appears that turn on time is nearly independent of monomer concentration. From this data, it appears that 6.25% BMBB-6 is the best concentration to use for high contrast, fast response time, and low switching voltage, although it is not significantly better than other nearby concentrations UV Curing Intensity Varied The same reverse mode mixture of 0.10% BME, 6.77% BMBB-6, 2.23% R811, and 90.9% E44 was split into two different cells. One cell was cured at a very low intensity of 0.020mW/cm 2 and the other at a very high intensity of 7.67mW/cm 2 and both cells cured for 4 hours. Also, both cells had SE-3510 alignment layers and a 10μm cell gap. It is expected that curing at higher UV intensity will cause the polymer network to form much more quickly because all the available photoinitiator is activated in a short time period. The lower UV intensity should activate the photoinitiator more slowly, which should regulate the network formation speed as well. The network that forms more quickly is expected to have many more bundles and each bundle is smaller in diameter (larger surface area to volume ratio). A slow forming network will have fewer bundles, but each will be larger in diameter (smaller surface area to volume ratio). Also,

158 143 a larger surface area to volume ratio should influence the liquid crystal more heavily and cause more resistance to reorientation [56]. Therefore, the higher UV intensities should exhibit higher driving voltages and higher hysteresis. Figure 66 shows the T-V curves of the two samples. As expected from the network formation model, the sample cured at high intensity shows a much higher driving voltage and hysteresis about three times higher that the sample cured at low intensity. The contrast ratio (9.8) of the sample at cured at high intensity was nearly half as much as the contrast ratio (19.5) of the sample cured at low intensity. This indicates that the polymer network that formed more quickly created domains that were not as well optimized for scattering light efficiently and due to the higher surface area to volume ratio these domains were likely too small. It might be possible to reduce the polymer concentration at the higher UV intensity to improve the results, however at very low polymer concentrations the stabilization can not be maintained over time and the network can be damaged. A study of the polymer concentration was carried out and described later in the chapter, but it was not tested at high UV intensities. In conclusion, lower UV intensities will reduce hysteresis, increase contrast, and reduce the driving voltages.

159 Transmission % Low UV Contrast = 19.5 V = High UV Contrast = 9.8 V = Voltage (V) Figure 66 Transmission curves at two UV intensities UV Curing Time Varied The UV curing time was studied using the same mixture (Mix 8) of 0.098% BME, 6.91% BMBB-6, 0.66% R1011, and % E44 in five different cells. All cells had SE-3510 alignment layers and a 10μm cell gap. All cells started curing at a UV intensity of 0.018mW/cm 2 at the beginning of the experiment and one cell was removed every hour until no cells remained when the intensity was re-measured to be 0.015mW/cm 2. The measurements were taken at the same time relative to the beginning of the curing time so that the total time since the cell started curing was the same for all samples.

160 Contrast Ratio Response Times (ms) Contrast Ratio Turn Off 0-90% Turn On % Curing Time (hours) Figure 67 Response time vs. UV curing time. As seen in the results of Figure 67, the contrast ratio did not change significantly positive or negative with curing time. Similarly, the turn on response time did not trend significantly positive or negative with curing time. However, the turn off time improved significantly from the first to the second hour and continued to improve incrementally thereafter. Therefore, the best UV curing time in this experiment is 5 hours. These results are consistent with the hypothesis that the polymer network is mostly cured after the first hour of exposure, but is not completely cured unless the curing time is at least 5 hours. Regrettably, the curing time was not increased further to determine the point of saturation in exposure time when no further improvements in turn off time are detected.

161 Alignment Layer Varied There are two independent angles of molecular orientation at the interior surface of the display that is controlled by the alignment layer. These are the azimuthal (in-plane) angle and the polar (out-of-plane) angle. The azimuthal angle is controlled by the rubbing direction and we use an anti-parallel rubbing configuration in all samples. The polar angle (called the pretilt angle when it is located at the surface) is mainly determined by the type of polyimide (PI) used for the alignment layer. Two samples with different alignment layers were made: one using SE-2170 (Nissan Chemical) which had a low pretilt angle (~2 ) and the other one using SE-3510 (Nissan Chemical) which had a high pretilt angle (~7-8 ) [37]. Then an experiment investigated any differences in performance using the same exact mixture in a both cells, Mix 22: 0.13% BME, 6.72% BMBB-6, 1.99% R811, and 90.97% E44. The experiment measured both samples to have the exact same cell gap at 10.4μm. In order to eliminate differences due to driving voltage, they both were tested using the same voltages of V on = 30V and V off = 0V, which was over driven for this cell gap on purpose to eliminate any bias toward one cell or the other. The results are shown in Table 11. Table 11 Comparison of results from two different alignment layers at 0-30V driving. Cell ID Barrier Polyimide Pretilt Angle* CR Turn On % Turn Off 0-90% 0V 22A None SE-3510 ~ ms 7.46ms 86.44% 22B AT-702 SE-2170 ~ ms 7.59ms 84.33% *Pretilt angles from Nissan Chemical [37].

162 147 The data indicates that there is not a measureable difference in contrast ratio or response time between the two alignment layers. The only exception was that SE-3510 had a higher transmission in the transparent state; however, this might be due to the barrier layer in the SE-2170 cell decreasing transmission from extra reflections and/or absorption. 5.4 Haze in the Transparent State Background Haze The area of the cell where liquid crystal and cured polymer is present is hazier than the perimeter where the glue outline is very clear and free of haze. The uniform haze in the sample doesn t vary measurably across the sample so it can be considered as the background haze and it is due to a slight mismatching of the index of refraction from the liquid crystal to the polymer network. The reverse mode is hazier than the normal mode PSCT because normal mode PSCT has a mismatch of only one index, the n o of the liquid crystal to n o of the polymer. Both of these indices are close to 1.5 so there is very little haze. The reverse mode PSCT might have some mismatching of both indices, the n o of both liquid crystal and polymer and n eff (angularly dependent extraordinary index) of both liquid crystal and polymer. However, the mismatch in reverse mode is still only slight because both have n o close to 1.5 and the deviation between n eff of the two components will not worsen with increasing angle because the liquid crystal and monomers are aligned with each other. Reverse mode has less haze than the PDLC display, because the PDLC has an isotropic polymer filled with nematic droplets. The mismatch in PDLC is between the n eff of the liquid crystal and the isotropic index n of the

163 148 polymer (which is usually chosen to match the n o of the liquid crystal). Therefore, the mismatch in PDLC can be very large at increasingly off-axis angle because the polymer s index of refraction doesn t change and the liquid crystal s index gets higher. Additionally the monomer concentration in PDLC is considerably higher than PSCT which creates significantly more instances of mismatching index within the structure of the display. The background haze is identified in the photograph shown in Figure 68. There is a major distinction between the background haze and haze caused by defects, which is that the background haze is dependent on choice of materials, their concentrations, the network curing conditions, and is highly dependent on cell thickness (see Figure 79). However, this dissertation did not directly focus on minimizing background haze, but instead choose to prioritize minimizing defect haze as will be discussed in the following sections Haze From Filling Defects When filling a cholesteric liquid crystal cell, defects will form in the otherwise uniform planar cholesteric texture due to the fluid flow. It should be noted that all the cells filled in this dissertation were done under vacuum in the liquid crystalline phase at room temperature. Filling defects are unwanted because they strongly scatter light in the transparent state of the display. Filling defects can be on the order of mm which gives them a tendency to sparkle making them easily visible by eye. A photograph of these defects is shown in Figure 68 with a scale bar added for comparison. The light source was positioned near the sample, but outside the direct view of the camera to scatter light

164 149 from the sample into the image. One of the filling defects is labeled in the photograph of Figure 68 and these defects are called oily streaks [57]. The oily streak is a defect in the layered structure where the cholesteric layers bend around and reconnect causing two parallel disclinations with a wall between them. Two broad categories of oily streak defects can occur. The first type, called Type A in this dissertation, inserts at least one extra layer into the cholesteric structure. In other words, it is a dislocation because it separates two regions that are not topologically equivalent; they differ by at least one turn 180 (π) turn. Therefore, Type A defect lines must form closed loops and can not have a free end. Type A defects will form when the variation of thicknesses in the sample and the chiral dopant concentration does not match well. It is this Type A defect that is exactly the lines that occur between two zones with a different number of turns in a Cano wedge cell [9]. The second type, called Type B in this dissertation, is simpler because the same disclination line separates two areas with the same pitch (number of turns). This type of oily streak can have a free end and it usually has a positive line tension meaning it will coarsen and shrink over time [58]. One way that Type B oily streaks form is during filling as the liquid crystal flows into the cell. The cholesteric liquid crystal forms a focal conic domain along the leading edge of the fluid flow and when it encounters a spacer it can draw out an oily streak that connects the spacer to the filling front edge. These defects are not energetically favored, but spacers and other debris in the cell act to stabilize them because a small focal conic domain will always form around the spacer and the oily streak will becomes stabilized because it replaces a portion of the

165 150 disclination ring around the spacer. If another spacer is encountered by the filling front it may stabilize the oily streak by stringing it between two spacers. Therefore spacers (and other debris) will form the nodes of the oily streak network. When more spacers are in the cell then a larger oily streak network is formed and this network of defects was shown to alter the fluid properties of a cholesteric to make it behave more gel-like [59]. Background Haze Type B Oily Streak Defects Figure 68 Type B filling defects and background haze are visible by eye. Type A defects very easy to distinguished from Type B defects under a polarizing microscope. Recall that the Type A defects have no beginning or end and because they separate two areas of different pitch (number of turns) the two regions will have a different color (under crossed polarizers). This color is directly caused by the difference in optical retardation which causes a difference in the transmission spectrum. The Type

166 151 B defects are stretched between spacers and often form straight lines parallel to the original filling flow. Type B Defect Type A Defect A Spacer P Figure 69 Type A defect (Note: Type B defects are in image also). Both types of defects can be observed in Figure 69 where they are individually labeled. In either case, if the defects are not removed before curing they will be permanently frozen in by the polymer network! The next section is the description of the filling defect removal process Type A Defect Prevention and Removal Methods Recall that the Type A defect occurs when another turn (half pitch layer) is inserted or removed from the cholesteric in one region. They are due to the quantization of the

167 152 number of turns of the cholesteric pitch and regions with mismatched numbers of turns are topologically inequivalent. This means a defect must form at the boundary, but this defect does not span vertically from one surface to another, meaning it scatters much less light compared to the Type B defect. However, Type A defects are very detrimental to the display because the two regions will switch at different voltages, which can cause parts of the display to exhibit non-uniformity in contrast ratio and/or switching speed. For example, an extreme case where the thickness is too high will cause holes that remain transparent because they are unswitchable and where the thickness is too low will cause blotches of haze from areas that are prematurely switched on by the off state voltage, V off. Unfortunately, the Type A defect can never be removed; there is no treatment for this type of defect once it occurs because the chiral concentration is fixed. There are however, ways to prevent the formation of the defects in first place. Just like in the Cano wedge cell, the probability of the defect forming is highest when the local thickness of the sample is midway between the two thicknesses where N and N±1 180 (π) turns are most stable. For example, if the intrinsic pitch of the sample is P 0 =10μm then the most stable thicknesses (with lowest free energy) are exact multiples of P 0 /2 such as 5, 10, 15, 20 μm. In this example, the thickness most likely to form a Type A defect is near the in between thicknesses of 2.5, 7.5, 12.5, 17.5 μm, which is exactly where the lines would occur in a wedge cell filled with a cholesteric of P 0 =10μm. The shorter the pitch, the closer the defect lines occur in the Cano wedge which means there is less tolerance in the cell gap variation when avoiding defects. The optimal reverse mode PSCT pitch found to

168 153 be P 0 =5μm is long enough to allow for a deviation of ±1.25μm from the optimal cell gap. This is a reasonable tolerance in the construction of cells that can be achieved by distributing the spacers evenly to avoid low cell gap regions and pressing the substrates together to make sure there are no high cell gap regions before sealing it. Once the cell gap is known, the final step in Type A defect prevention is to create a good liquid crystalline mixture by controlling the chiral dopant concentration accurately using materials with a previously measured HTP of the chiral dopant in the liquid crystal host Type B Defect Prevention and Removal Methods Type B defects are very easily distinguished under a polarizing microscope because they terminate on a physical intrusion in the cell such as a spacer or dirt as is shown in Figure 70. Type B defects will separate two areas of the same color, since the liquid crystal has the same helical pitch on either side. Type B Defect 20 μm Spacer A P Figure 70 Type B defect.

169 154 Type B defects can be prevented by eliminating spacers altogether, however this is not practical because it will encourage a non-uniform cell gap, which would promote type A defects and an optically non-uniform performance of the display. However, minimizing the spacer densities is the preferred method to prevent Type B defects. Another method of prevention is to fill the cell rapidly because the excess turbulence allows the defects to annihilate more easily. However, these two preventative methods only reduce the defect density; they can not guarantee a defect free result after filling Thermal Annealing Method to Remove Type B Defects It was hypothesized that a thermal annealing method could be used to remove the Type B defects. The process first involves bringing the sample above the clearing temperature where the liquid crystal transforms into a disordered isotropic fluid since the defects are not able to exist in the isotropic state because they require order to exist. Then it was hypothesized that upon cooling the liquid crystal, it would produce a single domain of cholesteric liquid crystal in the planar texture. In practice, the transition returning the liquid crystal to its ordered cholesteric state upon cooling occurs in many places within the cell at once, due to the nature of a first order phase transition [9]. It was observed that droplets of cholesteric liquid form in the isotropic background and grow as the temperature continues to drop. When two droplets grow large enough to touch each other an oily streak (of either Type A or B) forms between them creating two domains of planar texture. This oily streak can sometimes be drawn towards the edge of the new larger droplet so that the two domains become a larger single uniform domain. However, spacers and surface alignment irregularities can

170 155 cause them to be trapped and stabilized. When the phase transition occurs very rapidly, the defects have more difficulty becoming stabilized by spacers due to the turbulence from the rapid phase transition. However, in some cases an unidentified phenomenon occurs whereby the uniform anchoring of the alignment layer is disrupted, which causes an extreme stabilization of new defects that form during the cooling process. In other words, when two droplets of cholesteric liquid crystal suspended in the isotropic background merge into one cholesteric droplet, the surface stabilizes the defect between them and a single domain will not form. This can also cause mismatched twist domain defects (type A) to arise which may not have been present before the thermal annealing process was attempted. Several combinations of alignment layer, barrier layer, liquid crystal material, cholesteric pitch, and cooling rate were studied and although some combinations might be more likely to form a defect free planar state, none of those studied were consistently able to do so. Filling the cell in the isotropic state initially and then cooling (both rapidly and slowly) would also cause similar problems. One example from many experiments during this systematic study used liquid crystal E7 with 2.54% R811, where the T NI = 60ºC and with 2.54% R811 the T NI = 56ºC. The experiment consisted of a ramp up to 5ºC/min, then a ramp down to room -0.05ºC/min, then this process was repeated very rapidly. In this experiment four cells were studied with the following layers: SE-3510 alignment layer only, SE-2170 alignment layer only, AT720 barrier & SE-3510 alignment layer, both SE & SE-2170 alignment layers (one on each side). Photographic evidence of the

171 156 results is shown over the next 3 figures. Figure 71 shows the microscope texture between crossed polarizers after filling, Figure 72 shows the microscope texture between crossed polarizers and the macroscopic view after the slow annealing, and Figure 73 shows the microscope texture between crossed polarizers and the macroscopic view after the fast annealing. Although the sample with SE-2170 alignment layer was almost perfectly free of defects in this experiment, repeating the experiment with the same material and alignment layer combination was not able to repeat these results consistently. A untested hypothesis is that ion contamination in the polyimide causes the surface defects to form, which could be tested in the future by using a fluorinated liquid crystal with a low conductance. A P Figure 71 After filling.

172 157 Edge of Cell A P Center of Cell Figure 72 After slow annealing.

173 158 A P Figure 73 After fast annealing.

174 159 Two notable studies have had success using a thermal annealing process to remove oily streak defects, but they both used a cholesteric layer with a pitch short enough for a reflective cholesteric display (<1μm), whereas in the thermal annealing study in this dissertation the pitch was tuned to be near 5μm for the reverse mode PSCT. Ozaki et al. had success achieving the defect free planar state with liquid crystal E44 by using a rapid thermal annealing and cooling process performed several degrees below the transition temperature and repeating it over and over ( cycles) [60]. Tien and Huang achieved the defect free state using liquid crystal E7 mixed with monomer RM257 (before polymerization) when the annealing temperature was at the transition temperature (but not above it) and the annealing time was long (>10min) [61]. In both cases, the reduced viscosity at higher temperatures was postulated as one of the mechanisms responsible for the defect annihilation. Both studies admitted that exceeding the transition temperature caused more defects to appear Voltage Annealing Method to Remove Type B Defects The voltage annealing method removes Type B defects by applying a series of pulses to the cell. These pulses are precisely controlled so that the voltage is high enough to disturb the liquid crystal, but does not last long enough to reorient it substantially. This disturbance causes the defect to buckle [62] which can cause it to detach from a spacer at one end or split somewhere in the middle. The voltage just acts as a mechanical stimulation to disrupt the defect causing it to annihilate. This works because a free end is

175 160 unstable causing the defect to shrink and disappear. It should be noted that the same principle can be applied by mechanical pressure to the outside of the cell, but this is not practical for a real device. The voltage annealing method follows the general description from the dissertation of Zhang [63]. The best combination of voltage amplitude, duration, and delay between pulses must be optimized to achieve the best result. However, the best pulse structure depends on the thickness to pitch ratio, d/p, as well as the threshold field of the liquid crystal used (Equation 34). Rapid fire pulses tend to have had better results than isolated pulses Type B Defect Removal Summary The summary of the pros and cons of both methods are shown in Table 12. The thermal annealing method could be the best choice due to the ability to clear several displays at once without any visual feedback required; however, it is not the best choice because none of the combinations of materials studied could produce reliable or reproducible results. The root cause of the formation of irreversible surface defect structures by the thermal cycling of the cholesteric liquid crystal is still unsolved.

176 161 Table 12 Summary of the two defect annealing methods to remove Type B oily streaks. Thermal Annealing Pros Works on the entire display, including areas outside the active ITO area. Process can be standardized so that many displays are annealed at once in large batches without visual feedback required. Cons Can fail catastrophically causing more defects than were present before. The root cause of this failure is so far inconclusive. Voltage Annealing Pros Works on all material combinations studied. Doesn t cause additional defects (when properly implemented) Cons Won t work outside the active ITO area. Process is display specific, labor intensive, and requires visual inspection for feedback. The voltage method is the best method because for removing Type B defects because it works every time and when implemented cautiously it won t cause additional defects. However, if new defects are introduced inadvertently, they are not irreversible and can be removed with patience. The inability to clear regions without ITO is not a substantial downside because the ITO area can be easily extended to the edge of most displays and the interpixel gaps can be smaller than the cell gap which prevents trapped oily streaks between pixels Haze From Spacers For completeness, the scattering from the spacers shall be mentioned here. Normally, the spacers in a liquid crystal cell cause negligible scattering of light, but as previously stated, a focal conic region surrounds the spacers in the cholesteric cell which scatters

177 162 light as seen in Figure 74. No method is known to reduce the scattering from the liquid crystal surrounding spacers, other than to reduce their lateral size by choosing spherical spacers over glass fiber rods. 20 μm Clumping makes haze worse due to larger defects Focal Conic Domain A Spacer P Figure 74 Microscope view of a spacer in RM-PSCT. Therefore, limiting the density of the spacer distribution is the best known method to reduce the overall scattered light. A lower density of spacers will also help by discouraging clumping into groups. A group of spacers that touch each other can scatter enough light in the transparent state to be noticeable, but will also not scatter enough when switched on. This spacer clump region will then be a large distraction that sparkles in bright light in the transparent state and appears as a pinhole that leaks light in the scattering state. Limiting the spacer density also helps discourage filling defects, but the tradeoff is a loss of cell gap control.

178 Cell Gap Varied: Wedge Cell Experiment Motivation As previously discussed, the scattering efficiency of the display is highest when the focal conic texture domain size is optimized. The results from the experiments in this dissertation have studied how the reverse mode PSCT scattering depends on the polymer concentration, curing conditions, the pitch of the liquid crystal (chiral concentration), and the applied voltage. One interesting point is that the voltage required to obtain the optimal domain size distribution at a fixed mixture recipe also depends on the ratio of the cell thickness (d) to the cholesteric pitch (p), d/p. Therefore, a fixed mixture will have a fixed pitch and it is expected that thicker cells will require a higher absolute voltage to obtain the best light scattering. Additionally, the total light scattered (contrast ratio) will increase with cell thickness because light has a longer path length through the material giving it a higher probability to scatter away. This means for a fixed recipe and a fixed voltage, only one specific cell gap will obtain the optimized scattering efficiency. However, the question is: will a cell gap thicker than that optimal cell gap still scatter more light and have a higher contrast ratio, despite not being driven at the optimal voltage for the best scattering efficiency? Experiment Construction of a wedge cell would allow the same mixture and curing conditions to span several cell thicknesses in one sample. As the cell thickness increases, the boundary conditions at the top and bottom surfaces will influence the total twist of the director to be quantized to integers of π (180 ) and with each incremental increase in the total twist

179 164 an incrementally higher voltage must be applied to achieve the best domain size distribution. In this experiment the effect of a theoretical maximum voltage of 30V is investigated because beyond some particular thickness this limit will prevents the domain size distribution from being optimized. Measurements beyond this particular thickness will determine if the contrast continues to improve because a longer path length contributes more additional scattering to make up for the loss in scattering efficiency. A wedge cell, WD2, was prepared with the following formulation, Mix 22: 0.13% BME, 6.72% BMBB-6, 1.99% R811, and 90.97% E44. The wedge cell was coated with an SE-2170 alignment layer rubbed anti-parallel on top and bottom in a direction perpendicular to the cell thickness gradient. After vacuum filling the cell it was cured for 6 hours at an intensity of 0.016mW/cm 2. The wedge cell thickness varied from nearly zero up to 28μm. The pitch (2 turns) was ~5μm, so there were 10 measurable zones with 2,3,4,5,6,7,8,9,10 & 11 total turns each as shown in Figure 75. To avoid a short circuit in the sample the first two zones with 0 and 1 turn respectively were not in the active ITO area at the center of the sample, so they were not able to be switched on.

180 165 1π 2π 3π 4π 5π 6π 7π 8π 9π 10π 11π 15V Applied Figure 75 Wedge cell with 15V applied. The number of π turns is labeled in each zone Contrast Ratio vs. Thickness When multiple scattering is neglected, the transmitted light intensity, I out, scales exponentially as [5]: I out I in e d (35) Where d is the cell thickness and σ is the scattering cross section. Then by the definition of the contrast ratio we have: CR I in d ( d) e (36) I If the thickness, d, changes by a factor, x, then the contrast ratio will change by the exponential of that factor: out CR ( x )] x d) [ CR( d (37)

181 Contrast Ratio 166 The T-V curves were measured versus thickness, d, in the wedge cell and using the curves the best contrast ratio versus thickness was found. The contrast ratio at the thinnest measurement was used to generate a calculated contrast ratio versus thickness using the model in Equation 37. As shown in Figure 76, this fit is a fairly good prediction of the measured data until thicknesses greater than 20μm, where it is hypothesized that multiple scattering interferes with the model s assumptions. Multiple scattering will tend to reduce the contrast from the predicted value because a photon that was scattered out of the detection range may get scattered back into the detection range Calculated Contrast Ratio Measured Contrast Ratio 22.2V 28.8V 32.1V 40.2V 55.5V 43.8V 64.2V 56.1V 11.4V 17.1V The labeled voltages are V on Cell Gap ( m) Figure 76 Measured and calculated contrast ratios vs. cell gap.

182 Contrast Ratio 167 The experiment in Figure 76 only considers the optimized contrast ratio, so according to this data the best contrast ratio is 25 at a 12.8μm cell gap if limited to a maximum of 30V. To study the hypothesis that a longer path length can compensate for an underdriven cell whose scattering efficiency is not maximized (for its cell thickness) we applied a constant test signal of V off = 10V and V on = 30V to each cell thickness in the wedge cell. The measured contrast ratio in this experiment is plotted in Figure 77. This data shows that our hypothesis is confirmed because an 18μm cell gap improved the best achievable contrast ratio to Cell Gap ( m) Figure 77 Contrast ratio vs. cell gap at constant V on = 30V & V off = 10V. The contrast ratio as well as the required driving voltage vs. thickness can be visualized in Figure 78 which shows two view of the wedge cell at selected voltages.

183 168 a) b) 0V 10V 20V 35V Figure 78 Images of the wedge cell: a) sample in focus b) background in focus at selected voltages shows the contrast and switching behavior.

184 Transmission % Transparency vs. Thickness The transmission of the reverse mode PSCT decreases with increasing thickness due to the accumulation of background haze. The results of the measurement are shown in Figure 79. We see that this curve appears to change slope for data at 16μm and higher, which might be due to non-uniformity in the UV intensity along the cell thickness that becomes more pronounced as thickness increases. Although points are placed on this graph, the experimental beam was really an average over a region that varied by ±1μm (in thickness) centered at the thickness where the data point is placed Cell Gap ( m) Figure 79 Transparent state transmission vs. cell gap Response Time vs. Thickness It is expected that the as the cell gets thicker, the turn on time when using the voltage that gives the best contrast is relatively unchanged. This is because each increase in cell

185 170 thickness is almost completely offset by a requisite increase in driving voltage needed to obtain the best contrast. The turn off response time from the same best driving voltage is also expected to be relatively unchanged relaxation of the liquid crystal is more heavily influenced by the bulk polymer network than the surface alignment. The turn on and turn off response times were measured at each thickness using two different combinations of V on and V off. Both had a voltage ratio of V on :V off = 3:1 voltage ratio, but one was fixed at V on = 30V & V off = 10V and the other one varied V on to equal the voltage of maximum contrast at that thickness. The results are shown in Figure 80. The data in Figure 80 shows that at the fixed voltages of V on = 30V & V off = 10V the turn on time (open circles) increases dramatically with thickness because the driving voltage is insufficient; it is under driving the cell. However, the response time at the optimal driving voltage is not over driving or under driving the cell so the turn on time (closed circles) has no trend with cell thickness. As the cell gets thicker, the turn off response time at the fixed voltages of V on = 30V & V off = 10V (open squares) decreases because the driving voltage is insufficient; therefore, the liquid crystal doesn t have as far to reorient back to the planar state. However, the turn off response time at the optimal driving voltage (solid squares) is not over driving or under driving the cell and it increases with cell thickness. It is not expected to increase with cell thickness as ~d 2 like the STN system does (Equation 32) because the polymer network should have a stronger aligning effect in the bulk. The explanation for the observed increase in turn off time with cell gap might come from non-uniformity in the UV intensity along the cell thickness that becomes more pronounced as thickness increases.

186 Response Time (ms) V:30V Turn On % 10V:30V: Turn Off 0-90% Best V: Turn On % Best V: Turn Off 0-90% "Under Driven" Region Cell Gap ( m) Figure 80 Response times vs. cell gap. Filled marks use the voltages for best contrast ratio and the open marks use constant V on =30V & V off =10V. Another observation that should be mentioned from the data in Figure 80 is the oscillations in both the turn on and turn off response times at the optimal driving voltage. There is an alternating pattern where each subsequent data point has a tendency to trend oppositely from the trend preceding it. It is also apparent that when one response time trends up the other trends down. The reason for this effect is the same reason that under driven (10V:30V) turn off times are lower and turn on times are higher. In other words the response times mimic the trend in the applied voltage necessary to reach the optimal scattering, which oscillated about a trend that increased linearly with thickness as shown in Figure 81.

187 Driving Voltage Cell Gap ( m) Figure 81 Driving voltage of the wedge cell vs. thickness. Other than a coincidence, the following (untested) explanation is offered as a possible cause for this effect. The cell was rubbed anti-parallel which means only even multiples of the number of turns will correctly match the boundary conditions imposed by the pretilt angle. In other words, zero twist, two turns, four turns, etc. will fit without a splay component, but one, three, five, etc. turns must have a splay component to match the boundaries. This effect would be less pronounced when the pretilt is small (such as in this cell); however, if it is measureable then it means that those regions with splay would need slightly more voltage to overcome the defect. In this experiment, thicknesses at even multiples of half the pitch (2.5μm) have an even number of turns, so if these regions had splay to fit the boundaries and they needed slightly more voltage to reach optimal

188 173 scattering, it fits with the explanation. However, constructing the same exact wedge cell rubbed using parallel rubbing is necessary to confirm (or deny) this hypothesis. 5.6 Performance of Reverse Mode PSCT Using Monomer RM257 The differences in performance between the two monomers RM257 and BMBB-6 in reverse mode PSCT may be attributed to the difference in polymer network morphology. Morphology is a term that is used to describe the structure of the polymer network as defined by features such as bundle size, shape, and orientation. Other studies polymer stabilization in liquid crystals have shown that optical measurements such as the threshold voltage and hysteresis are affected by polymer concentration and that the morphology of the network may also depend on concentration [64]. Previous studies have found that changes in polymer network formation conditions can affect its morphology. For example, changing the curing temperature, UV intensity, UV curing time, or the ratio of photoinitiator to monomer will influence the curing rate, which then can alter the network morphology. However, given the same curing conditions, changing the percentage of monomer or the molecular makeup of the monomer itself can also control the network structure. An example of the under-performance of RM257 can be seen in Figure 82, which compares the T-V curves of three samples. The discussion begins with a description of the samples and their preparation. The first sample (labeled with the open triangles) was sample 2A which consisted of: 0.29% BME, 2.98% RM257, 2.96% R811, and 93.77% E44. The 10μm cell was coated with an SE-3510 alignment layer and was cured for 20 minutes at an intensity of 4.37mW/cm 2. The second sample (labeled with the open

189 Transmission % 174 circles) was sample 4A which consisted of: 0.44% BME, 4.82% RM257, 2.79% R811, and 91.95% E44. The 10μm cell was coated with an SE-3510 alignment layer and was cured for 20 minutes at an intensity of 1.02mW/cm 2. The third sample (from much later experiments) was sample 15C and consisted of: 0.15% BME, 7.10% BMBB-6, 2.02% R811, and 90.73% E44. The 10μm cell was coated with an SE-3510 alignment layer and was cured for 5 hours at an intensity of 0.015mW/cm % RM % RM % BMBB V=0.1V V=4.8V V=2.6V 30 Contrast Ratio = Contrast Ratio = Contrast Ratio = Voltage (V) Figure 82 T-V curves of reverse mode samples comparing RM257 to BMBB-6. The analysis of the curves in Figure 82 shows that the samples prepared using RM257 had a drastically larger hysteresis. The contrast ratio was also much worse for the same sample thickness and base liquid crystal material, therefore the focal conic

190 175 domain size was too large or too small when the RM257 monomer was used. The hypothesis is that increasing the monomer concentration would break up the focal conic domains, making them smaller. Since both RM257 samples have less monomer concentration than the BMBB-6 sample and increasing the RM257 from 2.98% to 4.82% decreased the contrast ratio, then both RM257 samples must have focal conic domains that are too small. The response times of the RM257 monomer were longer than the BMBB-6 and the turn on time did not improve by using an over drive pulse. This hints that the polymer network morphology of RM257 has a stronger aligning effect on the liquid crystal in both the transparent planar state and the scattering focal conic state despite the higher concentration of BMBB-6! This is likely due to a combination of the two factors affecting the morphology: the curing conditions and the molecular structure. The curing conditions for RM257 consisted of shorter times and higher intensities than the optimal results with BMBB-6 suggested. However, when the curing intensity was varied using the same mixture as sample 4A with the RM257 monomer at three levels, 1.02, 2.33, and 4.37mW/cm 2, the T-V curve didn t change significantly as shown below in Figure 83.

191 Transmission % mW/cm mW/cm mW/cm Voltage (V) Figure 83 Increasing T-V curves of three RM257 reverse mode samples with the same mixture as sample 4A at different UV intensities. It may be possible that all three UV intensities were still too high, which caused the rate of polymerization to be too high. In addition the concentration of photoinitiator used with RM257 was about 2-3 times as high as the concentrations used with BMBB-6 which would lead to a faster rate of polymerization similar to effect of a higher UV intensity. In addition to RM257 having a longer response time, it also suffered from a memory effect whereby the scattering state would noticeably persist after the voltage was applied for a long time. This meant that the turn off time depended on how long the driving voltage was applied. Switching to the BMBB-6 monomer reduced this problem to become barely measureable. The analysis of this effect is shown in Figure 84 which

192 Time (ms) to reach an absolute T=60% 177 compares samples 5H (0.31% BME, 3.23% RM257, 3.19% R811, 93.27% E44, 10μm cell gap, SE-3510 alignment layer, cured for 20 minutes at 4.32mW/cm 2 ) to Mix 8E (0.10% BME, 6.91% BMBB-6, 0.66% R1011, 92.33% E44, 10μm cell gap, SE-3510 alignment layer, cured for 5 hours at 0.018mW/cm 2 ). The measurement of response time was tweaked so it measured the time required to reach a transmission of 60% of the reference. This was chosen because it was an absolute benchmark instead of the usual response times which are relative to the transparent and scattering transmission levels ms RM257 (Sample 5H) BMBB-6 (Sample 8E) 32.9ms ms ms 7.84ms 8.08ms Driving Voltage Duration (ms) Figure 84 The turn off time vs. driving voltage duration for two monomers. According to previous studies, BMBB-6 has better performance because its network morphology is strikingly different from other monomer molecules like RM257 [65-74]. These other studies used comparisons from SEM imaging of the network after the liquid

193 178 crystal was removed. One study by Dierking et al. [74], shows SEM pictures of the BMBB-6 monomer compared to RM206 (Merck), which is structurally similar to RM257. He found that monomer BMBB-6 forms rice grain-like bundles and monomer RM206 forms smooth polymer strands. The rice grain-like bundles have larger voids between them because they have a larger volume to surface area ratio. This causes less polymer/liquid crystal interaction which lowers the threshold voltage and removes much of the hysteresis in the system. It also tends to reduce the turn on response time (the field on transition). A study by Huang, et al. [75] shows SEM images of PSCT samples with RM257 at 2.5% concentration and the network formed by RM257 has the same smooth strands as seen in the images of the RM206 network did in the study previously mentioned by Dierking [74]. Work by Held et al. of the transition in reverse mode PSCT were carried out using confocal microscopy [72]. It showed that the monomer RM206 (Merck), which is structurally similar to RM257, has a fine polymer network that permeates the cell while BMBB-6 has a polymer network that varies enough to produce polymer rich and polymer poor regions. The hypothesis is that the morphological differences are caused by the lower solubility of BMBB-6 in the liquid crystal mixture. If all other factors are equal, it is expected that for the monomers in this dissertation, the solubility of BMBB-6 is lower than RM257 because of the differences shown in Table 13. Each difference results in an increase in molecular weight of BMBB-6 over RM257 and the general trend of solubility decreases with increasing molecular weight.

194 179 Table 13 Detailed comparison of the molecular structures of RM257 and BMBB-6. RM257 BMBB-6 Structure Description Structure Description Rigid Core one phenyl ring two phenyl rings with a methyl attachment Flexible Tail 3 hydrocarbons 6 hydrocarbons Terminal Group acrylate methacrylate Qualitatively, this low solubility has been observed when crystals of the BMBB-6 monomer form within the cell after filling. These crystals remain after polymerization and are undesired. Many fewer crystals will form if the mixture is heated to C (below the curing temperature) for 4 or more hours and then mixed vigorously for several minutes before filling the cell. If the cell still has monomer crystals, then they can be redissolved using the same heat treatment on the cell, C, for 5 minutes. An extreme example of the monomer crystals can be seen in the cell shown below in Figure 85 where the cell was never cured with UV light, which caused all of the monomer to crystallize out.

195 μm A P Figure 85 Microscope photo of BMBB-6 monomer crystals in a PSCT sample under crossed polarizers. 5.7 A Double Cell Design A double cell stack was assembled to test the contrast and response time. Recall that increasing the cell gap caused an increase in the response time (turn on time increases if under driven and turn off time increases if fully driven) and an increase in driving voltage as shown in the wedge cell experiment. The motivation for this double cell stack experiment is to see if the contrast can increase as though the cell thickness doubled without worsening the response time and driving voltage. This stack was merely two test cells placed in contact using an index-matching immersion oil to reduce the reflections between them. This experiment tested two 10μm

196 181 and two 16μm cells together which were all filled and cured together with the same mixture, Mix 15: 0.15% BME, 7.10% BMBB-6, 2.02% R811, and 90.73% E44. The cells were coated with an SE-3510 alignment layer rubbed anti-parallel and after vacuum filling were cured for 5 hours at an intensity of 0.015mW/cm 2. The results of this experiment are shown in the summary of Table 14. The data shows that the contrast can be improved without sacrificing response time or driving voltage for the same cell thickness. Additionally, it shows that two 10μm cells can have a lower driving voltage, a faster response time, and a higher contrast ratio than a single 16μm cell! However, the results show that the contrast ratio of a double cell will not simply be the contrast ratio of a single cell squared due to the effect of multiple scattering. Table 14 The double stacked reverse mode PSCT results. Layers Gap V on Contrast Turn Off Turn On (No Over Drive) Turn On (30V Over Drive) Single 10μm 22V ms 2.96ms 1.39ms Single 16μm 30V ms* 4.81ms N/A** Double 10μm 22V ms 2.53ms 1.22ms Double 16μm 30V ms* 3.88ms N/A** *16μm is faster to relax because it is under driven. **Not applicable, since we already driving V on at 30V, no over drive is possible. 5.8 Conclusion The trends of the reverse mode PSCT have been experimentally measured with the variation of chiral dopant, monomer concentration, and cell thickness as well as the substitution of the monomer incorporated. The best formulation found in this dissertation of the reverse mode PSCT for high contrast, fast response time, and lower driving voltage

197 182 would be around 2.00% R811 (pitch = 5.00µm), 6.25% BMBB-6 monomer cured for 5 hours at 0.015mW/cm 2, photoinitiator BME at 0.15% and the liquid crystal E44. If a maximum output of 30V from a display s drive circuitry [76] is considered, then the best cell gap of a single cell is 18μm for maximum contrast. If manufacturability is deemed possible, then a double stacked cell will improve the contrast ratio using the same driving voltage without significantly altering the response time. As mentioned in the overview of the PSCT in Chapter 2, the PSCT scatters light and the reverse mode will revert to a transparent state if the power source failed, making it ideally suited to pixelated glare shielding liquid crystal eyewear.

198 Chapter 6 6. Scattering Profile of Polymer Stabilized Cholesteric Textures 6.1 Introduction Changes in fabrication procedures, material properties, and polymer network formation all affect the scattering efficiency of PSCT displays. Normally the scattering efficiency is reported using a single value: contrast ratio. Unfortunately, the contrast ratio depends on the experimental conditions since the scattered light intensity varies with collection angle and the unscattered light intensity is essentially unchanged. Therefore, it is difficult to compare results from different studies if the collection angles are different or are unreported. However, a measurement of the scattered light as a function of angle, the scattering profile, can make direct comparisons possible. Additionally, the contrast ratio can be calculated from the scattering profile for many different collection angles. This chapter presents one of the first investigations of the scattering profile of the PSCT displays in both normal and reverse modes. PSCT samples with different concentrations of chiral dopant as listed in Table 15 were used to study the effect of pitch and focal conic domain size on the scattering efficiency. The results and analysis are discussed along with the calculated contrast ratio as a function of collection angle. 183

199 184 sample mirror detector rotation direction rotation stage laser Figure 86 Diagram of the experimental setup for measuring the scattering profile 6.2 Experiment The experimental setup used to measure the scattering profile is shown in Figure 86 and it used an unpolarized gas He-Ne laser emitting green light (543.5 nm) for the light source. The detector was a 9mm 2 photodiode and it was rotated about a vertical axis located under the sample while set at a fixed distance of 346mm away from the sample. This geometry provided a 1 total collection angle (± 0.5 ) and the total solid angle of the detector was calculated as shown in Equation det 4sin (sin sin ) (38) Not shown in the diagram of Figure 86 are the apertures which were used to control stray light and beam intensity. Data was taken on a 1 grid from 0 to 80 (the setup couldn t reach angles higher than 80 ). The experiments measured the transmission through the cell referenced to the full intensity of the laser with no sample present. The detector didn t have enough dynamic range to avoid saturation so a neutral density filter was placed in the beam path for the reference measurement. Then the data was rescaled to account for the filter s reduction

200 185 factor of 1000 (ND3). The scattering intensity was measured as the voltage applied to the sample varied from 0 to 40V using a 1 khz square waveform. To calculate the contrast ratio at a given total collection angle we must divide the incident light intensity by the sum of the total light intensity scattered into this collection angle. However, the light intensity at each position of the detector is only a sample of the total light scattered into a ring of solid angle defined by Equation 39, where the angles θ 1 and θ 2 are the angles from the center of the unscattered light beam to the first and second edges of the ring respectively. ring (cos 1 cos ) (39) 2 2 By applying a normalizing factor to each intensity measurement then the sum of the total light intensity into a given collection angle can be accurately found. This normalizing factor is a ratio of the solid angle of the ring to the solid angle of the detector, which is simply Equation 39 divided by Equation 38. (Note: This normalizing factor is only applied to the contrast ratio data.) Table 15 Chiral concentrations and intrinsic pitch of each sample. Normal Mode Reverse Mode ID R811 Pitch ID R811 Pitch N1 4.90% 2.04 µm R1 1.24% 8.06 µm N2 7.28% 1.37 µm R2 1.56% 6.41 µm N3 8.72% 1.15 µm R3 1.89% 5.29 µm N % 0.90 µm R4 2.34% 4.27 µm N % 0.76 µm R5 2.65% 3.77 µm

201 Normal Mode Results The entire scattering profile of normal mode PSCT is shown in Figure 87 on a log scale. Data at negative angles is a copy (mirror image) of the data at positive angles and each scattering profile shown was taken at 0V. The inset of Figure 87 shows the same data rescaled on a log-log plot for scrutiny. Samples N1 and N2 have more light scattered into small angles. As the concentration of chiral dopant increases, the pitch decreases and the focal conic domain size becomes smaller. Therefore, more light is scattered into larger angles while light scattered into smaller angles decreases. The other data shown is the contrast ratio which is calculated as a function of angle (Figure 88) and voltage (Figure 89). For all samples in Figure 88, the data indicates that the smaller collection angles have the higher contrast ratios; however for most projection display applications the smaller collection angles will limit the power efficiency by lowering the brightness of the display. For this experiment, the most efficient focal conic domain size distribution was formed when the pitch was 1.15 µm (N3) because it had a monotonically decreasing contrast vs. voltage as shown in Figure 89. The shorter pitch samples, N4 and N5 both reached a maximum contrast at a voltage higher than 0V, which indicates that the domain size distribution was too small at 0V. However, if the application needs high contrast at larger collection angles then a pitch of 0.76 µm (N5) is a better choice. Additionally, the increase of contrast with voltage in samples N4 and N5 would be a benefit to a bias-voltage bistable drive scheme that capitalizes on the normal mode s hysteresis. The entire 3-D scattering profile is shown in Figure 90 for each sample.

202 Normalized Intensity N N1 N N3 N4 N5 N N3 N4 N Angle ( ) Figure 87 Normal mode PSCT scattering profile (log).

203 Contrast Ratio N5 N3 N4 50 N2 N Total Collection Angle ( ) Figure 88 Contrast ratio variation with total collection angle in normal mode PSCT.

204 Contrast Ratio N N N N1 N Voltage (V) Figure 89 Contrast ratio as the scattering state voltage varies with a 3 degree total collection angle in normal mode PSCT.

205 Figure 90 The 3-D scattering profiles of the normal mode PSCT samples. 190

206 Reverse Mode Results The entire scattering profile of reverse mode PSCT is shown in Figure 91 on a log scale. Data at negative angles is a copy (mirror image) of the data at positive angles and each scattering profile is at the voltage which had the best contrast ratio. In the inset of Figure 91 the same data is rescaled on a log-log plot for scrutiny. The data indicates that the focal conic domain size distribution is only slightly influenced by the pitch. It is hypothesized that the monomer concentration and polymer network formation and morphology will have a greater effect on the domain size in reverse mode PSCT. The calculated contrast ratio is shown as a function of angle (Figure 92) and voltage (Figure 93). For all samples in Figure 92, the data indicates that the smaller collection angles have the higher contrast ratios; however, compared to normal mode PSCT the change in reverse mode PSCT is more gradual. For this experiment, the best focal conic domain size distribution was formed when the pitch was 6.41 µm; however, the distinction was not very great. For example, a pitch of 5.29 µm reaches nearly the same contrast, but at a lower voltage. The major difference between the samples was the amount of unscattered light. For instance, if the pitch is too long (R1) or too short (R5), there is considerably more unscattered light in the central peak of the scattering profile as the small angle contrast ratio in Figure 92 suggests. Because the actual confined pitch (determined by the number of director rotations) of reverse mode is quantized, a different cell gap would have a slightly different optimal intrinsic pitch for the most efficient light scattering, but it would still be close to 5 µm. The entire 3-D scattering profile is shown in Figure 94 for each sample.

207 Normalized Intensity R5 R1 R3 R2 R R1R2R3 R4 R Angle ( ) Figure 91 Reverse mode PSCT scattering profile (log).

208 Contrast Ratio R2 R3 R4 R5 R Total Collection Angle ( ) Figure 92 Contrast ratio variation with total collection angle in reverse mode PSCT.

209 Contrast Ratio R3 R2 R R5 R Voltage (V) Figure 93 Contrast ratio as the scattering state voltage varies with a 3 degree total collection angle in normal mode PSCT.

210 Figure 94 The 3-D scattering profiles of the reverse mode PSCT samples. 195

211 Conclusion The technique of measuring the scattering profile is useful to compare the scattering efficiency of different chiral dopant concentrations in normal mode and reverse mode PSCT and it allows the contrast ratio to be calculated for many different collection angles. The best formulation of the normal mode depends on the application and the collection angle, but for small collection angles it has a pitch of 1.15 µm and for large collection angles it has a pitch 0.76 µm. The best formulation of the reverse mode has a pitch of ~5 µm, but the exact value will depend on the cell gap of the application. The contrast of reverse mode varied less with angle than the contrast of normal mode did. Both modes had similar contrast ratios at high collection angles ~10, but the normal mode has a higher contrast ratio at small collection angles. The total light scattering should increase with cell thickness and birefringence. Both modes used liquid crystals with a similar birefringence, but reverse mode had a significantly larger cell gap in this experiment (16µm) than normal mode (10µm). Therefore, the results of this experiment indicated that the normal mode PSCT has a higher scattering efficiency than reverse mode PSCT. The results from the scattering experiment have been presented at the Society for Information Display conference [77]. The experimental results shown here are the source of motivation for simulations carried out by Yang using a theoretical model to describe the light scattering as a function of focal conic domain size distribution, the results of which are awaiting publication.

212 Chapter 7 7. Dye-Doped Normal Mode Polymer Stabilized Cholesteric Texture 7.1 Introduction Recent LCD research has focused on reducing power consumption, making lightweight flexible displays, and reducing manufacturing complexity [78-81]. The normal mode PSCT display is a good design to realize these needs since it does not require polarizers or alignment layers. This not only simplifies display fabrication by reducing the necessary components, but it also allows the use of flexible plastic substrates. The power consumption of the normal mode PSCT is not itself very low, but it can be decreased if the driving voltage necessary to achieve a specified contrast ratio can be reduced. The addition of a dichroic dye to the mixture that absorbs light predominantly in the off state can achieve this effect [28, 82, 83] as shown in the diagram of Figure 95. This chapter describes the experimental measurements of the dye-doped PSCT and the results of the fabrication of the display using flexible plastic substrates. 197

213 198 Transmitted Scattered V V Liquid Crystal Dye Absorbed Polymer Network Figure 95 Dye-doped PSCT display cell geometry and operation principle. 7.2 Formulation and Experiment The PSCT mixture consisted of chiral dopant R811 (Figure 48), monomer RM257 (Figure 47), and photoinitiator BME (Figure 19) added to the host liquid crystal E44 (Merck). The dye-doped mixtures also contained a yellow dye 1,5-bis-phenylsulfanylanthraquinone (laboratory synthesized) and the molecular structure of the dye is shown in Figure 96. concentration. The photoinitiator concentration was fixed at 10% of the monomer The mixture was vacuum filled into cells made from ITO coated substrates (glass or plastic) of thickness 10μm with no alignment layer. The cells were cured in UV light of high intensity (14mW/cm 2 ) for 20 minutes while a high voltage (70V glass and 20V plastic) was applied. The aligning effect of the liquid crystal on the monomer caused the polymer network to form perpendicular to the cell substrates.

214 199 Figure 96 Molecular structure of the dye 1,5-bis-phenylsulfanyl-anthraquinone A few different mixtures (with and without dye) were characterized for contrast ratio and saturation voltage. The experimental setup was the same as previously described in this dissertation and had a collection angle of 2. The contrast ratio was calculated using the maximum transmission divided by the minimum transmission and the saturation voltage is defined as the voltage needed to reach 90% of maximum transmission. The transmission through the cell was referenced to the transmission with no sample present as the voltage applied to the sample varied from 0 to 50V using a 1kHz square waveform. 7.3 Glass Substrate Results The first series of mixtures held the monomer concentration fixed at 2% and the dichroic dye concentration fixed at 3% and studied the variation of the contrast ratio and saturation voltage with chiral dopant concentration as shown below in Figure 97. The same trend as seen in Chapter 4 was repeated here with the dye added; as the chiral dopant concentration increased the pitch and domain size was reduced. This caused the scattering efficiency to increase and therefore the contrast ratio improved. However, there is a trade-off because improvements in contrast come at the price of higher driving voltage and higher power consumption.

215 Contrast Ratio Saturation Voltage (V) Data For 2% Monomer & 3% Dye R811 Chiral Dopant (%) Figure 97 Contrast ratio and saturation voltage vs. chiral concentration with monomer fixed at 2% and dye fixed at 3% The second series of mixtures studied the effect of the dye. It held the monomer concentration fixed at 2% and the chiral dopant concentration fixed at 5.5% while varying the dichroic dye concentration as shown below in Figure 98. The contrast ratio of the sample with 6% dye improved two-fold over the sample with no dye without a measurable change in the saturation voltage. Consequently, to reduce the power consumption, a mixture with a low chiral concentration and high dye concentration should be chosen.

216 Contrast Ratio Saturation Voltage (V) Data For 2% Monomer & 5.5% Chiral Dichroic Dye (%) Figure 98 Contrast ratio and saturation voltage vs. dye concentration with monomer fixed at 2% and chiral dopant fixed at 5.5%. 7.4 Flexible Substrate Results Flexible plastic substrate cells of thickness 10μm with no alignment layer were filled with a mixture of 2% monomer, 4% chiral dopant, and 6% dye. The transmission vs. voltage measurement of a plastic substrate display is shown below in Figure 99 where it can be seen that the driving voltage was low by using a lower chiral dopant concentration in order to keep power consumption lower. However, the higher dye concentration only slightly improved the contrast ratio at this chiral concentration. The transparent state has a lower overall transmittance because the plastic substrates transmit less light than ITO on glass. This was confirmed by the data of transmission vs. wavelength for displays using different substrates as shown in Figure 100 (referenced to no sample present). The

217 Transmission % 202 absorption of the dye occurs mainly in the region from nm which gives it a yellow appearance Contrast Ratio = 8.9 Voltage Decreasing Voltage Increasing Voltage (V) Figure 99 Transmission measurement of a plastic display with 2% monomer, 4% chiral dopant and 6% dye. 7.5 Flexible Display Prototype A flexible plastic display prototype was constructed for evaluation. UV light exposure was used to pattern the photoresist and the ITO was etched away in a region which formed the letters LCI. Since the LCI region had no ITO on one substrate, it was expected that very little electric field was applied there during curing. However, the liquid crystal formed very large domains there and hence remained optically transparent. Figure 101 shows a photograph of a plastic display that is bent while Figure 102 shows the same display in operation while bent in a metal press to an 85mm radius of curvature. The transparent state shown in Figure 102 (a) is in front of a scenic

218 Transmission % 203 background to show the clarity and slight coloration of the display. The scattering/absorbing state shown in Figure 102 (b) is in front of a black background to accentuate the transparent patterned letters. The flexible display research discussed in this dissertation was presented at the 2009 SID Display Week Conference [84] a) b) 60 c) Wavelength (nm) Figure 100 Spectra of a) glass PSCT at 30V, b) glass dye-doped PSCT at 30V and c) plastic dye-doped PSCT at 20V. 7.6 Conclusion The results show that adding a dichroic dye to the normal mode PSCT display improves the contrast ratio without overly degrading the bright state transmission or increasing the driving voltage. Although the normal mode PSCT is not applicable to pixelated glare shielding eyewear because the scattering state is at 0V, the feasibility of creating a flexible display by making cells on ITO coated plastic substrates with

219 204 patterned letters was demonstrated. These displays were bent to a radius of 85mm and were still operable at this curvature. Figure 101 Photograph of a dye-doped PSCT display while bent in hand. a) b) Figure 102 Photograph of a dye-doped PSCT display bent vertically to an 85mm radius of curvature a) transparent (20V) with scenic background b) scattering/absorbing (0V) with black background.