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
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
TABLE OF CONTENTS
TABLE OF CONTENTS ................................................................................................... iii
LIST OF FIGURES ........................................................................................................... ix
LIST OF TABLES ........................................................................................................... xiii
ACKNOWLEDGEMENTS ............................................................................................. xiv
REFERENCES ............................................................................................................... 212
Chapter
1.
2.
Page
Introduction ................................................................................................................. 1
1.1
Glare .................................................................................................................... 1
1.2
Natural Response To Glare ................................................................................. 1
1.3
Non-Pixelated Glare Shielding Eyewear ............................................................ 2
1.4
Pixelated Glare Shielding Eyewear .................................................................... 2
1.5
Dissertation Overview ........................................................................................ 3
Fundamentals of Liquid Crystals and Liquid Crystal Displays .................................. 5
2.1
Properties of Liquid Crystals .............................................................................. 5
2.1.1
Liquid Crystal Director ................................................................................... 7
2.1.2
Order Parameter .............................................................................................. 7
2.1.3
Liquid Crystal Phases ..................................................................................... 8
2.1.4
Cholesteric Liquid Crystals........................................................................... 10
2.1.5
Elastic Deformations and Elastic Constants ................................................. 12
2.1.6
Elastic Energy Density.................................................................................. 13
iii
2.1.7
Dielectric Anisotropy .................................................................................... 14
2.1.8
Optical Anisotropy ........................................................................................ 15
2.1.9
Liquid Crystals in an External Field: The Frederiks Transition ................... 15
2.2
Surface Alignment, Rubbing, and Pretilt Angle ............................................... 17
2.3
The Design of a Single LCD Pixel ................................................................... 18
2.4
Electrode Structures and Driving Schemes....................................................... 20
2.4.1
Direct Drive Scheme ..................................................................................... 21
2.4.2
Active Matrix Drive Scheme ........................................................................ 25
2.4.3
Passive Matrix Drive Scheme ....................................................................... 28
2.4.4
Block Addressing Drive Scheme .................................................................. 32
2.4.5
Bistable Drive Scheme .................................................................................. 34
2.5
2.5.1
Mixture Preparation ...................................................................................... 35
2.5.2
Experimental Equipment and Setup.............................................................. 36
2.5.3
Transmission Measurement and Calculation ................................................ 39
2.5.4
Contrast Ratio from Static Response of Transmission vs. Voltage .............. 40
2.5.5
Response Time from the Dynamic Response of Transmission vs. Time ..... 42
2.6
Which LCDs Can Work in Pixelated Glare Shielding Eyewear? ..................... 48
2.7
LCD Designs ..................................................................................................... 50
2.7.1
Liquid Crystal Displays Without a Polymer Network .................................. 50
2.7.2
Liquid Crystal Displays With a Polymer Network ....................................... 56
2.8
3.
Experimental Methods ...................................................................................... 35
Conclusion ........................................................................................................ 61
Super Twisted Nematic for Polarizer Based Absorptive Displays ........................... 63
3.1
Motivation ......................................................................................................... 63
iv
3.2
Overview ........................................................................................................... 63
3.3
Measuring the Birefringence Dispersion .......................................................... 66
3.3.1
Theory ........................................................................................................... 66
3.3.2
Experiment .................................................................................................... 67
3.4
4.
Simulation ......................................................................................................... 71
3.4.1
Director Relaxation ....................................................................................... 71
3.4.2
Light Transmission ....................................................................................... 73
3.4.3
Calculation of Color, Luminance, and Contrast Ratio .................................. 74
3.5
Compensation Film ........................................................................................... 82
3.6
Experiment Using Liquid Crystal MLC-6080 .................................................. 83
3.7
Optimizing the Material Properties and Cell Design Using Simulation ........... 87
3.7.1
Electro-Distortional Curve ............................................................................ 87
3.7.2
Birefringence Magnitude and Dispersion ..................................................... 95
3.7.3
Driving Voltages ........................................................................................... 97
3.7.4
Polarizer and Analyzer Angles ..................................................................... 97
3.7.5
The Best Cell Gap ......................................................................................... 98
3.7.6
Response Time of STN ............................................................................... 103
3.7.7
Summary ..................................................................................................... 104
3.8
Experiment Using Liquid Crystal 13B-000 .................................................... 105
3.9
Viewing Angle of STN ................................................................................... 111
3.10
Further Improvements ..................................................................................... 112
3.11
Conclusion ...................................................................................................... 113
Normal Mode Polymer Stabilized Cholesteric Texture for Scattering Displays .... 115
4.1
Overview ......................................................................................................... 115
v
4.2
Experiment ...................................................................................................... 117
4.3
Results ............................................................................................................. 118
4.3.1
Chiral Concentration Varied ....................................................................... 120
4.3.2
Monomer Concentration Varied ................................................................. 123
4.3.3
Liquid Crystal Material Varied ................................................................... 127
4.4
5.
Conclusion ...................................................................................................... 129
Reverse Mode Polymer Stabilized Cholesteric Texture for Scattering Displays ... 130
5.1
Overview ......................................................................................................... 130
5.2
Experiment ...................................................................................................... 132
5.2.1
5.3
Over Drive for a Faster Turn On Response Time ....................................... 134
Results ............................................................................................................. 137
5.3.1
Chiral Dopant Concentration Varied .......................................................... 137
5.3.2
Monomer Concentration Varied ................................................................. 139
5.3.3
UV Curing Intensity Varied ........................................................................ 142
5.3.4
UV Curing Time Varied ............................................................................. 144
5.3.5
Alignment Layer Varied ............................................................................. 146
5.4
Haze in the Transparent State ......................................................................... 147
5.4.1
Background Haze ........................................................................................ 147
5.4.2
Haze From Filling Defects .......................................................................... 148
5.4.3
Type A Defect Prevention and Removal Methods ..................................... 151
5.4.4
Type B Defect Prevention and Removal Methods ..................................... 153
5.4.5
Thermal Annealing Method to Remove Type B Defects ........................... 154
5.4.6
Voltage Annealing Method to Remove Type B Defects ............................ 159
5.4.7
Type B Defect Removal Summary ............................................................. 160
vi
5.4.8
5.5
6.
7.
8.
Haze From Spacers ..................................................................................... 161
Cell Gap Varied: Wedge Cell Experiment...................................................... 163
5.5.1
Motivation ................................................................................................... 163
5.5.2
Experiment .................................................................................................. 163
5.5.3
Contrast Ratio vs. Thickness....................................................................... 165
5.5.4
Transparency vs. Thickness ........................................................................ 169
5.5.5
Response Time vs. Thickness ..................................................................... 169
5.6
Performance of Reverse Mode PSCT Using Monomer RM257 .................... 173
5.7
A Double Cell Design ..................................................................................... 180
5.8
Conclusion ...................................................................................................... 181
Scattering Profile of Polymer Stabilized Cholesteric Textures .............................. 183
6.1
Introduction ..................................................................................................... 183
6.2
Experiment ...................................................................................................... 184
6.3
Normal Mode Results ..................................................................................... 186
6.4
Reverse Mode Results..................................................................................... 191
6.5
Conclusion ...................................................................................................... 196
Dye-Doped Normal Mode Polymer Stabilized Cholesteric Texture ...................... 197
7.1
Introduction ..................................................................................................... 197
7.2
Formulation and Experiment .......................................................................... 198
7.3
Glass Substrate Results ................................................................................... 199
7.4
Flexible Substrate Results ............................................................................... 201
7.5
Flexible Display Prototype ............................................................................. 202
7.6
Conclusion ...................................................................................................... 203
Conclusion .............................................................................................................. 205
vii
8.1
Dissertation Summary..................................................................................... 205
8.2
Scattering vs. Absorption ................................................................................ 208
8.3
Final LCD Recommendation .......................................................................... 209
viii
LIST OF FIGURES
Figure 1 – Molecule of pentylcyanobiphenyl (commonly known as 5CB). ....................... 7
Figure 2 – Various phases of rod-like thermotropic liquid crystals.................................... 9
Figure 3 – The twisted structure of the cholesteric liquid crystal phase. .......................... 11
Figure 4 – The elastic distortions of the director: splay (K11), twist (K22), and bend (K33).
....................................................................................................................... 12
Figure 5 – Cross section of the layers in a LCD, which happens to be cholesteric. ......... 19
Figure 6 – Definition of the polar and azimuthal angles of the director. .......................... 20
Figure 7 – An example of a direct drive connection with a seven segment display. ........ 22
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. ...................................... 24
Figure 10 – An equivalent circuit of an active matrix pixel: a) TFT b) MIM. ................. 27
Figure 11 – A passive matrix drive scheme: a) electrode layout b) equivalent circuit. .... 29
Figure 12 – Demonstration of the voltages used while addressing a single line. ............. 31
Figure 13 – Maximized voltage ratio given the number of rows in a passive matrix....... 32
Figure 14 – Block addressing demonstration.................................................................... 33
Figure 15 – Drawing of the setup measuring contrast ratio and response time of PSCT. 37
Figure 16 – Definition of the collection angle. ................................................................. 38
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.
....................................................................................................................... 44
Figure 18 – Transmission curve (bold) of a typical response time measurement with the
applied voltage pulse (dashed) shown underneath. ....................................... 45
Figure 19 – Photoinitiator benzoin methyl ether (BME). ................................................. 47
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. ................. 50
Figure 22 – STN diagrams: a) cross section of two optical states, b) the angular
convention used in the equations and simulations ......................................... 64
Figure 23 – An example of the experimental data and fit................................................. 68
Figure 24 – a) Birefringence dispersion curves for various liquid crystals. b) Same data as
in a) normalized at 589nm. ............................................................................ 70
Figure 25 – The director orientation angles vs. thickness. ............................................... 72
Figure 26 – The 1931 (x,y) CIE Diagram ......................................................................... 75
Figure 27 – Color Matching Functions of the 1931 CIE 2° Standard Observer ............... 77
Figure 28 – The 1976 (u′,v′) CIE Diagram ....................................................................... 80
a
Figure 29 – Dominant (or complimentary) wavelength and purity ( purity 
) on the
ab
a) 1931 CIE diagram and b) 1976 CIE diagram ............................................ 81
ix
Figure 30 – Contrast ratio vs. polarizer and analyzer angles & transmission spectrum of
simulation compared to experiment: a) 4.4μm b) 3.7μm .............................. 84
Figure 31 – Contrast ratio vs. compensation film angle & transmission spectrum of
simulation compared to experiment: a) 4.4μm b) 3.7μm .............................. 85
Figure 32 – Simulations of midlayer tilt angle, θm, 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: θp, c) thickness/pitch ratio: d/p ................... 92
Figure 33 – Simulations of midlayer tilt angle, θm, show how the STN electro-distortional
curve varies with one material parameter: a) bend/splay elastic constant ratio:
K33/K11, b) twist/splay elastic constant ratio: K22/K11, c) dielectric
parameter:    /   .................................................................................... 93
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°, θp = 7°,
d/p = 0.53. ...................................................................................................... 94
Figure 35 – Simulated transmission spectra of: E44, MLC-6080, & 13B-000 with
Δnd(589nm) = 0.79, αi = -44°, and αo = 14°. ................................................. 96
Figure 36 – Simulated transmission spectra of 13B-000 for cell gaps 5-7μm. ................. 99
Figure 37 – Contrast ratio and luminance of 13B-000 for cell gaps 5-7μm. .................. 100
Figure 38 – 1976 CIE chromaticity coordinates of 13B-000 for cell gaps 5-7μm. ........ 101
Figure 39 – Purity of 13B-000 for cell gaps 5-7μm........................................................ 102
Figure 40 – Two normalized figures of merit for 13B-000 for cell gaps 5-7μm. ........... 103
Figure 41 – The spectrum at various voltages: 0V, 1.17V and 1.5V using 13B-000 liquid
crystal in a 240° STN LCD. ......................................................................... 106
Figure 42 – Transmission vs. voltage for 13B-000......................................................... 107
Figure 43 – Response times, contrast ratio and luminance for 13B-000 as Voff is varied.
..................................................................................................................... 110
Figure 44 – Iso-contrast viewing angle of the 13B-000 STN as seen by the wearer. ..... 112
Figure 45 – Polymer network formation in normal mode PSCT by UV light exposure. 115
Figure 46 – Optical states of the normal mode PSCT. ................................................... 116
Figure 47 – Molecular structure of monomer RM257 used in normal mode PSCT....... 117
Figure 48 – Chiral dopant in use R-811. ......................................................................... 118
Figure 49 – Typical experimental result used for calculating contrast ratio................... 120
Figure 50 – Contrast ratio, saturation voltage, and hysteresis width data when monomer
concentration is fixed at 2.0% for normal mode PSCT. .............................. 122
Figure 51 – Response time data when monomer concentration is fixed at 2.0% for normal
mode PSCT. ................................................................................................. 123
Figure 52 – Contrast ratio, saturation voltage, and hysteresis width data when chiral
concentration is fixed in the range 10.4-11.1% for normal mode PSCT. .... 125
Figure 53 – Response time data when chiral concentration is fixed in the range 10.411.1% for normal mode PSCT..................................................................... 126
Figure 54 – Response time data for E44 compared to BL036. ....................................... 128
Figure 55 – Polymer network formation in reverse mode PSCT by UV light exposure. 130
Figure 56 – Optical states of the reverse mode PSCT. ................................................... 132
Figure 57 – Monomer RM257 was abandoned in the reverse mode experiments.......... 133
x
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-1011. ....................... 134
Figure 60 – Final chiral dopant studied in reverse mode PSCT, R-811. ........................ 134
Figure 61 – Comparison of two different over drive durations to no over drive. ........... 135
Figure 62 – Response time vs. over drive voltage using Von=21V & Voff=7V. .......... 136
Figure 63 – Transmission data for various R-811 chiral dopant concentrations. ........... 138
Figure 64 – Contrast and voltage vs. monomer concentration. ...................................... 140
Figure 65 – Response time vs. monomer concentration. ................................................ 141
Figure 66 – Transmission curves at two UV intensities. ................................................ 144
Figure 67 – Response time vs. UV curing time. ............................................................. 145
Figure 68 – Type B filling defects and background haze are visible by eye. ................. 150
Figure 69 – Type A defect (Note: Type B defects are in image also). ........................... 151
Figure 70 – Type B defect. ............................................................................................. 153
Figure 71 – After filling. ................................................................................................. 156
Figure 72 – After slow annealing.................................................................................... 157
Figure 73 – After fast annealing. .................................................................................... 158
Figure 74 – Microscope view of a spacer in RM-PSCT. ................................................ 162
Figure 75 – Wedge cell with 15V applied. The number of π turns is labeled in each zone
..................................................................................................................... 165
Figure 76 – Measured and calculated contrast ratios vs. cell gap. .................................. 166
Figure 77 – Contrast ratio vs. cell gap at constant Von = 30V & Voff = 10V................... 167
Figure 78 – Images of the wedge cell: a) sample in focus b) background in focus at
selected voltages shows the contrast and switching behavior. .................... 168
Figure 79 – Transparent state transmission vs. cell gap. ................................................ 169
Figure 80 – Response times vs. cell gap. Filled marks use the voltages for best contrast
ratio and the open marks use constant Von=30V & Voff=10V. .................... 171
Figure 81 – Driving voltage of the wedge cell vs. thickness. ......................................... 172
Figure 82 – T-V curves of reverse mode samples comparing RM257 to BMBB-6. ...... 174
Figure 83 – Increasing T-V curves of three RM257 reverse mode samples with the same
mixture as sample 4A at different UV intensities. ....................................... 176
Figure 84 – The turn off time vs. driving voltage duration for two monomers. ............. 177
Figure 85 – Microscope photo of BMBB-6 monomer crystals in a PSCT sample under
crossed polarizers......................................................................................... 180
Figure 86 – Diagram of the experimental setup for measuring the scattering profile .... 184
Figure 87 – Normal mode PSCT scattering profile (log). .............................................. 187
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. ...................................................... 189
Figure 90 – The 3-D scattering profiles of the normal mode PSCT samples. ................ 190
Figure 91 – Reverse mode PSCT scattering profile (log). .............................................. 192
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. ...................................................... 194
Figure 94 – The 3-D scattering profiles of the reverse mode PSCT samples. ................ 195
xi
Figure 95 – Dye-doped PSCT display cell geometry and operation principle. .............. 198
Figure 96 – Molecular structure of the dye 1,5-bis-phenylsulfanyl-anthraquinone ....... 199
Figure 97 – Contrast ratio and saturation voltage vs. chiral concentration with monomer
fixed at 2% and dye fixed at 3%. ................................................................. 200
Figure 98 – Contrast ratio and saturation voltage vs. dye concentration with monomer
fixed at 2% and chiral dopant fixed at 5.5%. ............................................... 201
Figure 99 – Transmission measurement of a plastic display with 2% monomer, 4% chiral
dopant and 6% dye....................................................................................... 202
Figure 100 – Spectra of a) glass PSCT at 30V, b) glass dye-doped PSCT at 30V and c)
plastic dye-doped PSCT at 20V. .................................................................. 203
Figure 101 – Photograph of a dye-doped PSCT display while bent in hand. ................. 204
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. ...................................... 204
xii
LIST OF TABLES
Table 1 – Parameters in the response time measurements. ............................................... 43
Table 2 – Response time calculations ............................................................................... 46
Table 3 – List of LCDs considered for pixelated glare shielding eyewear. ...................... 62
Table 4 – Birefringence dispersion fitting parameters for various liquid crystals. ........... 69
Table 5 – Summary of the results of the STN LCDs made with MLC-6080 liquid crystal.
....................................................................................................................... 86
Table 6 – Parameters of 13B-000 and MLC-6080 from the Merck datasheet [37]. ......... 94
Table 7 – Purity of E44, MLC-6080, & 13B-000 from the spectra in Figure 35. ............ 97
Table 8 – Chiral concentration and intrinsic pitch of normal mode samples. ................ 121
Table 9 – Parameters of BL036 and E44 from the Merck datasheet [37]. ..................... 127
Table 10 – Percent chiral dopant, intrinsic pitch, turns, and contrast ratios for the samples.
..................................................................................................................... 138
Table 11 – Comparison of results from two different alignment layers at 0-30V driving.
..................................................................................................................... 146
Table 12 – Summary of the two defect annealing methods to remove Type B oily streaks.
..................................................................................................................... 161
Table 13 – Detailed comparison of the molecular structures of RM257 and BMBB-6. 179
Table 14 – The double stacked reverse mode PSCT results. .......................................... 181
Table 15 – Chiral concentrations and intrinsic pitch of each sample. ............................ 185
xiii
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
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
Chapter 1
1.
1.1
Introduction
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 (10-6cd/m2) to the bright
sunlit daytime (105cd/m2). 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
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
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
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.
Chapter 2
2.
2.1
Fundamentals of Liquid Crystals and Liquid Crystal Displays
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
6
145.5°C and then at a higher temperature, 178.5°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.
components creates liquid crystals phases [5].
The balance of these two
7
Figure 1 – Molecule of pentylcyanobiphenyl (commonly known as 5CB).
2.1.1
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.
2.1.2
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
8
P2 (cos ) is the second order Legendre polynomial and
indicates a temporal average
over all orientations [6].
S  P2 (cos  ) 
1
3 cos 2   1
2
(1)
 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 0.4-0.6 at low temperatures. Upon
heating a liquid crystal, the order parameter abruptly decreases to 0 at the nematicisotropic phase transition temperature, TNI, 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 20-22°C, which was far enough from the clearing point that
small day-to-day temperature deviations aren‟t significant.
2.1.3
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.
9

n
Isotropic

n
Nematic

n
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.
10
2.1.4
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.
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)
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.
2.1.5
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].
K11

n 0
Splay
K22


n   n  0
Twist
K33


n n 0
Bend
Figure 4 – The elastic distortions of the director: splay (K11), twist (K22), and bend (K33).
13
It is usually found that the elastic constants follow the relationship K22 < K11 < K33,
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 S2.
The order of magnitude of the three elastic constants is ~10-11N. This is a good fit with
the estimated order of magnitude based on a dimensional analysis that Kii ~ U/a where U
is the interaction energy between molecules (~0.1×10-19J) and a is the molecular length
(1nm) [5, 9].
2.1.6
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 





1
1
1
K11 (  n ) 2  K 22 (n    n ) 2  K 33 (n    n ) 2
2
2
2
(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
14
define a chirality term q0 = 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 
2.1.7
1

1


1


K11(  n ) 2  K 22 (n    n  q0 ) 2  K33 (n    n ) 2
2
2
2
(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.
15
2.1.8
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, ne, and the ordinary index of refraction, no, as shown in
Equation 6.
n  ne  no
(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, no, 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].
2.1.9
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
16
the elastic free energy is Felastic   f elasticdV . 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.
Felastic  Felectric  Fsource  0
(8)
Following the description in [6], it was found that
Fsource  2Felectric
(9)
which means that the free energy density supplemented with the dielectric term becomes
f  f eleastic 
1  
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
 
1
f  f eleastic   0  (n  E ) 2
2
(11)
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: Vth. The physical meaning of the threshold voltage is based on the
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].
Vth  
2.2
K11
 0 
(12)
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
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
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.
19
Glass or Plastic Substrate
Barrier Layer
(Optional)
Liquid Crystal
(Cholesteric)
Indium Tin Oxide (ITO)
Spacer
Glue
Barrier Layer
(Optional)
Glue
Polyimide (PI)
Alignment Layer
(Optional)
Indium Tin Oxide (ITO)
Glass or Plastic Substrate
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: θp. The diagram
showing the relationship between the rubbing direction and director orientation angles is
shown in Figure 6.
20
z
director
orientation
y
rubbing
direction
θ
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
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.
2.4.1
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
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
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 (NC-1) where NC is the total
number of columns. The total number of connections required is 2 × (NC-2) + NR1 + NR2
+ 1, where NR1 and NR2 are the number of rows in column 1 and column NC 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×NC + 1.
a)
b)
…
…
…
…
NR1
NR2
2
NC
NC
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
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 × (NC-2) + NR1 + NR2 + 5 and if simplified using square pixels of the same
size, 4 × (NC-2) + 13. 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
…
NR1
NR2
*
*
…
*
Bottom
Substrate
…
NC
Figure 9 – A four row high resolution direct drive pixel layout.
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.
2.4.2
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.
fabrication procedure
requires
several
For example, the TFT
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
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 (CS)
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 (CLC)
is large enough, so this storage capacitor (CS) is not required [8].
27
a)
b)
Column Line
Column Line
Row Line
CS
CLC
TFT
CMIM
CLC
Row Line
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 (CMIM), 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
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.
2.4.3
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
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 NR rows and NC columns, there are
NR × NC pixels and NR + NC 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 NR
rows takes T f  N R t time, called the frame time.
a)
b)
Top
Substrate
Top
Substrate
NR
Bottom
Substrate
NC
Bottom
Substrate
Figure 11 – A passive matrix drive scheme: a) electrode layout b) equivalent circuit.
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 Von applied
and the other pixels will have (at most) an undesired voltage Voff applied. The display is
still effective as long as Voff causes an acceptably small amount of reorientation of the
liquid crystal, which is true if Voff is near or below the threshold voltage Vth. When
addressing a single row, the voltages applied to the pixels are demonstrated in Figure 12
where the selected row is set to Vsel and the other rows are set to Vnsel and the columns
with pixels on are set to Von and the columns with pixels off are set to Voff. Then the
voltage on pixels in the non-selected rows is at either V1 or V2 and the voltage on pixels
in the selected row is at either V3 or V4. The maximal ratio between the value of Von and
Voff is achieved when V1, V2, and V3 are minimized, which makes the ratio of Von :Voff =
31
3:1. This assumes Vsel = 0V, but it doesn‟t change the ratio if Vsel ≠ 0V, therefore, in this
dissertation, Von and Voff always refer to the value relative to Vsel.
Selected
Rows
Non-Selected
Rows
Von (columns with pixels on)
V1
V4
Vnsel (non-selected rows)
V2
Voff (columns with pixels off)
V3
Vsel (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 Von and Voff is limited by the number of rows, NR,
in the display. This is calculated using the method of Alt and Pleshko [12] and the result
is shown in Equation 13.
Von
 Rm 
Voff
NR  1
NR  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
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.
Maximized Voltage Ratio, Rm
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0
10
20
30
40
50
60
70
Number of Rows, NR
80
90
100
Figure 13 – Maximized voltage ratio given the number of rows in a passive matrix.
2.4.4
Block Addressing Drive Scheme
Block addressing is a special type of passive matrix driving that can achieve a higher
voltage ratio (Von/Voff) 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 (Von) that is
three times higher than the off state voltage (Voff), which is why it is also called the 3:1
addressing scheme. However, this larger ratio comes with a tradeoff. Whereas the
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, Von.
Off pixel
Allowed
On pixel
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, Voff). 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.
34
2.4.5
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
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.
2.5.1
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
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.
2.5.2
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
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
white
light
source
detector
photopic
filter
lens
#3
lens
#2
aperture
#2
sample
lens
#1
aperture
#1
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.
38
aperture
sample
r
d
  2 tan 1 ( )
r
θ
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 10VRMS (both specifications are from the manufacturer).
The
amplifier can then increase the amplitude further, up to 74VRMS 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
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.
2.5.3
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.
T
VSAMPLE  VBACKGROUND
100%
VREFERENCE  VBACKGROUND
(14)
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
VSAMPLE when the sample is being measured. Just before the experiment is undertaken,
the reference, VREFERENCE, and background, VBACKGROUD, 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, VREFERENCE, 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 VBACKGROUD 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.
2.5.4
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
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.
Contrast Ratio 
TBright
TDark
(15)
The bright state of a normally white display is at 0V or at Voff<Von 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 Voff<Von 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%).
42
2.5.5
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 100-102 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.
43
Table 1 – Parameters in the response time measurements.
Label
Voff
1
VOD
2
Von
3
Voff
4
Inset
Parameter
Pre-Pulse Voltage
Pre-Pulse Time
Over Drive Pulse Voltage
Over Drive Time
Pulse Voltage
Pulse Time
Post Pulse Voltage
Post Pulse Time
Frequency
Specification
Typically
Voff (@3:1 Vratio for max contrast)
Voff or 0V
Long enough for stable reading
500ms
As high as possible
Limited to 30V
As long as T is still decreasing
<2ms
Von (@3:1 Vratio for max contrast)
Von
Long enough to reach saturation
500ms
Voff (@3:1 Vratio for max contrast)
Voff or 0V
Long enough for stable reading
500ms
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
Voff and the pulse voltage to Von 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 Vratio = Von/Voff (for example given by Equation 13), or the voltage
capabilities of the power source or even a maximum voltage imposed by strict power
44
consumption limits.
Therefore, the response time is sometimes measured at other
voltages for comparison.
VOD
f
RMS Voltage (V)
Inset: Square Wave
at frequency = f
Von
Voff
1
2
3
4
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
(τon) and turn off (τoff) time, which refer to the voltage transitions from Voff < Von and
from Von > Voff 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
45
transmission vs. time as shown by the cartoon of a typical curve in Figure 18.
Tmax
Transmission (%)
Tmin + 90% of (Tmax - Tmin)
Tmin + 10% of (Tmax - Tmin)
Tmin
τon
Time (ms)
τoff
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 (Tmax) and minimum transmission (Tmin)
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 Tmax – Tmin higher than Tmin to a point that is 90% of the range
Tmax – Tmin higher than Tmin and visa-versa. In other words, τon is from 10% to 90% for a
normally black display and from 90% to 10% for a normally white display.
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 Tmax – Tmin higher than Tmin from the point the voltage is switched (Von  Voff or
Voff  Von). To avoid confusion related to whether the specific mode being discussed is
normally white (bright) or normally black (dark), the designation 100-10 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 τon 0-90
Time required to switch up to 90% of the range (Tmax –
Tmin) from the instant the pulse Von is applied.
Dark to Bright τoff 0-90
Time required to switch up to 90% of the range (Tmax –
Tmin) from the instant the pulse Voff is applied.
Bright to Dark τon 100-10
Time required to switch down to 10% of the range (Tmax
– Tmin) from the instant the pulse Von is applied.
Bright to Dark τoff 100-10
Time required to switch down to 10% of the range (Tmax
– Tmin) from the instant the pulse Voff is applied.
The previous description of the calculation assumed that the duration of VOD was
0ms. However, if VOD > 0ms, then it does not affect the calculation except to clarify that
47
the beginning of the pulse for the purpose of the calculation starts when VOD 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 VOD > Von 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/cm2 is model
XRD-140B (International Light) with a sensitivity range from 326-401nm and a peak
sensitivity at 365nm attached to the IL1350 Radiometer/Photometer (International Light).
Figure 19 – Photoinitiator benzoin methyl ether (BME).
48
5
Intensity (unreferenced counts)
10
4
10
3
10
2
10
1
10
200 250 300 350 400 450 500 550 600 650 700 750 800 850
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:
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 Vratio from Equation 13), passive matrix display-at-once block
addressing (maximum Vratio 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.
50
2.7
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)
(Ch)
Polymer Reflective
Stabilized Cholesteric
Isotropic
(PSCT)
Normal
Mode
Polymer
Stabilized
Cholesteric
Texture
(R-PSCT)
Reverse
Mode
Polymer
Stabilized
Cholesteric
Texture
(STN)
(TN)
SuperTwisted
Twisted Nematic
Nematic
(VA)
Vertically
Aligned &
(ECB)
Electrically
Controlled
Birefringence
Pitch
<400nm
400-700nm
~0.5-2μm
~5μm
~5-15μm ~20μm
∞
Figure 21 – Some of the LCDs along the continuum of the cholesteric pitch.
2.7.1
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
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
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
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.
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.
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 400-700nm. 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
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.
2.7.2
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
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
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
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,
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.
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
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*
DSM
ECB
VA
IPS & FFS
OCB
PSI
PSBP Ch
Ch
Bi-Nem
PDLC
PSLC (ECB)
H-PDLC
All Bistable
Dual Frequency
Operation
Scattering
Absorbing
Absorbing
Absorbing
Absorbing
Absorbing
Absorbing
Reflecting
Absorbing
Scattering
Scattering
Reflecting
Varies
Varies
Classification
Eliminated
Eliminated
Eliminated
Eliminated
Eliminated
Eliminated
Eliminated
Eliminated
Eliminated
Eliminated
Eliminated
Eliminated
Eliminated
Eliminated
Most Significant Reasoning
High power requirements
Colorful transmission
Not transparent at 0V
In-plane electrodes required
Warm up time required
In-plane electrodes required
In-plane electrodes required
Not transparent at 0V
Not transparent at 0V
Not transparent at 0V
Polarization dependent
Narrow reflection band
May leave pixels on if power fails
High frequencies use too much power
NM-PSCT
Scattering
Eliminated**
Not transparent at 0V
GH
PSLC (VA)
TN
Absorbing Undesirable
Scattering Undesirable
Absorbing Undesirable
Low contrast ratio
Poor performance on a passive matrix
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.
Chapter 3
3.
3.1
Super Twisted Nematic for Polarizer Based Absorptive Displays
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
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.
y
Bright
Analyzer
Dark
Compensation film (optional)
LC director at
exit plane
Analyzer
Compensation
film (optional)
d
o
Liquid Crystal
a)
Polarizer

i
Polarizer
V=Voff

LC director at
entrance plane
x
V=Von
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)
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].
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
3.3.1
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)
67
by experiment. A simple model for Δn(λ) proposed by S.T. Wu [36] is used in this
dissertation as shown in Equation 18.
 2 ( * 2 )
n( )  G 2
   *2
(18)
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 ne and no may not be the same; however, the
dispersion of the optical retardation, Δn(λ), is all that is necessary to compare different
STN designs.
3.3.2
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
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]:
T %  sin 2 (
nd
)

(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.
100
90
Measured
Fit
Transmission (%)
80
70
60
50
40
30
20
10
0
400
450
500
550
600
650
Wavelength (nm)
Figure 23 – An example of the experimental data and fit.
700
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.
1
Liquid Crystal
E44
λ*
255nm
G
3.14 ×10-6
MLC-6080
231nm
3.05 ×10-6
6A-100
233nm
2.65 ×10-6
13B-000
196nm
2.82 ×10-6
E71
250nm
3.06 ×10-6
Δn(589nm)
0.2513†
0.2627††
0.1923†
0.1984††
0.1708†
0.1708††
0.1220†
0.1204††
0.2333†
0.224††
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].
†
70
0.35
a)
0.30
n()
0.25
0.20
0.15
0.10
n() Normalized at 589nm
1.40
400
1.35
b) 450
500
550
600
650
700
E44
E7
6A
MLC6080
13B
600
650
Wavelength (nm)
1.30
1.25
1.20
1.15
1.10
1.05
1.00
0.95
0.90
400
450
500
550
700
Wavelength (nm)
Figure 24 – a) Birefringence dispersion curves for various liquid crystals. b) Same data as
in a) normalized at 589nm.
71
3.4
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.
3.4.1
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
1
2
K11 cos 2  '2  K 22 (
 cos 2  ' ) 2 
2
2
Po
1
2
K33 sin 2   '2  cos 2  '2 
2
2 o (    cos 2  )


(20)
72
where  '   / z ,  '   / z , Po 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, θm, 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.
90
240
 (5V)
 (3V)
200
 (5V)
 (3V)
 (2V)
 (0V)
60
160
45
120
30
80
 (2V)
15
40
 (0V)
0
0
0.1
0.2
0.3
0.4
0.5
z/d
0.6
0.7
0.8
0.9
Figure 25 – The director orientation angles vs. thickness.
0
1.0
, Azimuthal Angle ()
, Polar Angle ()
75
73
3.4.2
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 z  z / N for N
slabs. The Jones vector of the incident light is,

Ei  (cos  i , sin  i )
(21)
The Jones vector of the light exiting the liquid crystal depends on the polar angle,  j , and
the azimuthal angle,  j , of the director in the jth layer as given by,


j 1

N 




Eo 
R( j )  G (  j )  R 1 ( j )  Ei
(22)
where the phase retardation is defined as j  2neff ( )z /  , the rotation matrix is

cos  j
R( j )  
 sin  j
 sin  j 

1 0 
, the retardation matrix is G ( j )  

i  j  , and the
cos  j 
0 e 
effective extraordinary index of refraction, neff(λ), is given by,
neff ( ) 
n e ( ) n o
((ne ( )) sin 2  j  no2 cos 2  j )
2
 no
(23)
which depends on the polar angle,  j . The (constant) ordinary index of refraction is
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 no.
The transmission is given by:
T  Eox cos  o  Eoy sin  o
2
(24)
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.
3.4.3
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
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.
520nm
532nm Diode Laser Pointers
0.8
540nm
543nm HeNe Gas Laser
510nm
560nm
Green
0.6
500nm
580nm
y
Yellow
0.4
600nm
Orange
White
Cyan
633nm HeNe
Gas Laser
620nm
490nm
Pink
Magenta
0.2
Blue
480nm
0
Red
650nm DVD Laser Diode
Violet
460nm
0
Purple
680-780nm
380-410nm
0.2
405nm Blu-Ray
Laser Diode
0.4
0.6
0.8
x
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
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 = 0.3 3 and y = 0.3 3 . 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),
X
X Y  Z
Y
y
X Y  Z
Z
z
X Y  Z
x
(25)
which are calculated using the following equations [8],
X  k  S ( )T ( ) x ( )d
Y  k  S ( )T ( ) y ( )d
Z  k  S ( )T ( ) z ( )d
(26)
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
Color Matching Functions (CIE 1931 2 Observer)
z ( )
1.5
y ( )
x ( )
1.0
0.5
0
400
500
600
Wavelength (nm)
700
800
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
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
(27)
 S ( ) y ( ) d 
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 28.
700nm
Contrast Ratio 
YBright
YDark

T
Bright
400nm
700nm
T
Dark
( ) y ( ) d 
(28)
( ) y ( ) d 
400nm
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
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
 2 x  12 y  3
v 
9y
 2 x  12 y  3
(29)
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.
80
532nm Diode Laser Pointers
543nm HeNe Gas Laser 633nm HeNe
520nm540nm 560nm
Gas Laser
0.6
580nm
600nm
650nm DVD
Green
620nm
Laser Diode
510nm
Yellow
500nm
Orange
Red
White
490nm
Cyan
0.4
Magenta
v
680-780nm
Pink
Blue
480nm
Purple
0.2
Violet
0
460nm
405nm Blu-Ray
Laser Diode
0
380-410nm
0.2
u
0.4
0.6
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, λd, 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
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, λc, 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].
a) 1931 CIE diagram
0.8
Sample 1
c = 500nm b
b
Sample 2
a
0.4
d = 543nm
0.6
v
0.6
y
b) 1976 CIE diagram
0.8
d = 543nm
White
Point
0.2
c = 500nm
0.2
0
0
0.4
x
b
White Point
b
0
a
0.4
a
0.2
a
Sample 1
0.6
0.8
0
0.2
Sample 2
0.4
u
Figure 29 – Dominant (or complimentary) wavelength and purity ( purity 
0.6
0.8
a
) 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”
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
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-
6080. The samples had a thickness of 4.4μm and 3.7μm. The off state voltage was
chosen to be Voff = 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, Vratio = 1.28, which
makes Von = 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.
84
Contrast Ratio vs. Polarizer and Analyzer
(a)
4.4 μm
Transmittance vs. Wavelength
4.4 μm
Highest
Contrast
of 48 at:
2.2 V
Contrast
Ratio
αi = -39°
αo = 9°
2.82 V
αo (degree)
αi (degree)
3.7 μm
(b)
3.7 μm
Highest
Contrast
of 84 at:
2.2 V
Contrast
Ratio
αi = -46°
αo = 16°
αo (degree)
αi (degree)
2.82 V
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.
85
Contrast Ratio vs. Compensation Film Angle
Transmittance vs. Wavelength
Retardation Angle = 2π
4.4 μm
4.4 μm
Highest
Contrast
of 99 at:
2.2 V
σ = -1°
3.7 μm
2.82 V
3.7 μm
Highest
Contrast
of 148 at:
2.2 V
σ = -69°
2.82 V
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  2.1926105 [1992  2 /(2  1992 )]no , where
no
was the birefringence at 550nm.
Then the spectra of the best result were re-
calculated 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.
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
Gap
Experiment Simulation
Adjusted
Results
Results
Results*
Polarizer, αi
-40°
-39°
-38.5°
4.4μm Analyzer, αo
7°
9°
8.5°
Compensation Film, σ
-5°
-1°
-3.5°
Polarizer, αi
-48°
-46°
-44.5°
11°
16°
14.5°
3.7μm Analyzer, αo
Compensation Film, σ
-73°
-69°
-69.5°
*Better agreement is possible if the input rubbing direction was actually at
-1.5° for 4.4μm and -3.5° for 3.7μm.
Component
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.
87
3.7
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-6080. 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.
3.7.1
Electro-Distortional Curve
All things being equal, the best contrast ratio is achieved when light leaking in the
high voltage (Von) state is minimized. Light leaking is caused by residual birefringence
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 K33/K11, the elastic constant ratio K22/K11, and the ratio of
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, K33/K11 = 1.5, K22/K11 = 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
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).
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 K33/K11 increases as shown in (Figure 33 (a)) and the steepness decreases as
either the K22/K11 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 K33/K11, minimizes the elastic constant ratio K22/K11, and minimizes the dielectric
parameter γ.
In practice, the data sheets that report the liquid crystal material‟s
parameters are often missing numbers for K22/K11 and/or K22. In general, K22 doesn‟t
vary as much as the other elastic constants from material to material so K22 will be
estimated from K22/K11 = 0.6 (given K11).
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 K33/K11 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-6080. 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°, θp = 7°, d/p = 0.53. Therefore, 13B-000 is chosen as a
replacement for MLC-6080 to improve the performance of the STN LCD.
92
Midlayer Tilt Angle, m ()
90
70
60
50
Midlayer Tilt Angle, m ()
270
240
40
210
180
30
 = 90
20
10
0
80
0
b)
70
0.5
1.0
3.0
3.5
3.0
3.5
3.0
3.5
50
50
40
m = p
40
30
30
20
20
10
5
1
10
80
1.5
2.0
2.5
Reduced Voltage (V/Vo)
p = 60
60
0
Midlayer Tilt Angle, m ()
a)
80
0
c)
0.5
1.0
70
60
0
1.5
2.0
2.5
Reduced Voltage (V/Vo)
d/p = 0.333
0.583
0.833
50
40
30
20
10
0
0
0.5
1.0
1.5
2.0
2.5
Reduced Voltage (V/Vo)
Figure 32 – Simulations of midlayer tilt angle, θm, 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: θp, c) thickness/pitch ratio: d/p
93
Midlayer Tilt Angle, m ()
Midlayer Tilt Angle, m ()
Midlayer Tilt Angle, m ()
90
a)
80
70
60
50
40
30
K33/K11 = 0.5
20
1.0
1.5
2.0
3.0
2.5
10
0
80 0
b)
0.5
1.0
1.5
2.0
2.5
Reduced Voltage (V/Vo) 0.8
3.0
3.5
1.5
2.0
2.5
Reduced Voltage (V/Vo)
3.0
3.5
3.0
3.5
K22/K11 = 0.4 0.6
70
60
50
40
30
20
10
0
0
80
c)
0.5
1.0
5
70
1
60
2.5
=0
50
40
30
20
10
0
0
0.5
1.0
1.5
2.0
2.5
Reduced Voltage (V/Vo)
Figure 33 – Simulations of midlayer tilt angle, θm, show how the STN electro-distortional
curve varies with one material parameter: a) bend/splay elastic constant ratio: K33/K11, b)
twist/splay elastic constant ratio: K22/K11, c) dielectric parameter:    /  
94
Table 6 – Parameters of 13B-000 and MLC-6080 from the Merck datasheet [37].
Parameter
Phase Transition Temperature
Flow Viscosity [mm2s-1] 20˚C
Dielectric Anisotropy
(20˚C, 1kHz)
Dielectric Parameter
Optical Anisotropy
(20˚C, 589nm)
Elastic Constants (20˚C)
Elastic Constant Ratios
Symbol
TN-I
η
Δε
13B-000
95˚C
41
+27.0
MLC-6080
85˚C
18
+7.5
ε//
ε┴
γ
Δn
33.7
6.7
4.0
0.1204
11.4
3.9
1.9
0.1984
ne
no
K11
K22
K33
K33/ K11
K22/ K11
1.6089
1.4885
9.6pN
N/A
21.5pN
2.24
N/A
1.7025
1.5041
14.4pN
7.1pN
19.1pN
1.33
0.49
90
MLC-6080
13B-000
Midlayer Tilt Angle ()
80
70
60
50
40
30
20
10
0
0
0.5
1.0
1.5
2.0
2.5
Reduced Voltage (V/Vo)
3.0
3.5
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°, θp = 7°, d/p = 0.53.
95
3.7.2
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 (αi = -44°) and analyzer angles (αo = 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
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 MLC6080 in the STN LCD.
100
90
Transmission (%)
80
70
60
50
40
30
E44, * = 255nm
MLC-6080, * = 231nm
13B-000, * = 196nm
20
10
0
400
450
500
550
600
Wavelength (nm)
650
700
Figure 35 – Simulated transmission spectra of: E44, MLC-6080, & 13B-000 with
Δnd(589nm) = 0.79, αi = -44°, and αo = 14°.
97
Table 7 – Purity of E44, MLC-6080, & 13B-000 from the spectra in Figure 35.
Liquid Crystal
E44
MLC-6080
13B-000
3.7.3
λ*
255nm
231nm
196nm
λd
566nm
565nm
563nm
Purity (1976 CIE)
0.6858
0.6470
0.5167
Driving Voltages
Choosing the best voltage for driving the STN LCD involves determining two data
points: Vratio, the voltage ratio, which depends on the resolution of the display and Vth, the
threshold voltage, which depends on the STN LCD material and cell design.
The
voltages are set so that Voff = Vth and Von = Voff × Vratio. 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: θm = θp. The result is shown by Equation 30 below.
Vth  
K11
 0 
   K33 K 22  2
K33  
K 22 d  
 2


cos



4

p
  K11 K11 
K11  
K11 p  

(30)
Using Equation 30 with the following cell design parameters, Φ = 240°, θp = 7°, d/p =
0.53 and the material parameters for 13B-000 (Table 9), the Voff = Vth = 1.17V. Using
the Vratio = 1.28 for a 17 row display gives Von = 1.28×1.17V = 1.50V.
3.7.4
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
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.
3.7.5
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
99
designed to simulate the transmission spectrum at several cell thicknesses for the 13B000 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°,
θp = 7°, Voff = 1.17V, Von = 1.50V and d/p = 0.53. The polarizer and analyzer were
optimized for the lowest transmission at 550nm in the dark (V = Von) state.
100
90
80
7.0μm
Transmission (%)
70
60
50
40
5.0μm
30
20
10
0
400
450
500
550
Wavelength (nm)
600
650
700
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
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.
240
a)
220
Contrast Ratio
200
180
160
140
120
100
5.0b) 5.2
90
5.4
5.6
5.8
6.0
6.2
Cell Gap (m)
6.4
6.6
6.8
7.0
5.4
5.6
5.8
6.0
6.2
Cell Gap (m)
6.4
6.6
6.8
7.0
80
Luminance
70
60
50
40
30
20
10
0
5.0
5.2
Figure 37 – Contrast ratio and luminance of 13B-000 for cell gaps 5-7μm.
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
d= 5.0m
v
0.4
0.2
0
0
0.2
u
0.4
0.6
Figure 38 – 1976 CIE chromaticity coordinates of 13B-000 for cell gaps 5-7μm.
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.
1.1
1.0
Purity (1976 CIE)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
5.0
5.2
5.4
5.6
5.8
6.0
6.2
Cell Gap (m)
6.4
6.6
6.8
7.0
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
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 (αi = -50.0°,
and αo = 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 (αi = -48.1°, and αo
= 18.1°), which is chosen to be the optimized cell gap for this material.
1.1
Normalized Figure of Merit
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
Contrast*Luminance/Purity
Contrast*Luminance
0.2
0.1
5.0
5.2
5.4
5.6
5.8
6.0
6.2
Cell Gap (m)
6.4
6.6
6.8
7.0
Figure 40 – Two normalized figures of merit for 13B-000 for cell gaps 5-7μm.
3.7.6
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
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 γr is the rotational
viscosity, K22 is the twist elastic constant and d is the cell gap [5].
 off
 d 
 r  
K 22   
2
(32)
The turn on time also depends on the strength of the electric field as described by
Equation 33.
 on   off
3.7.7
1
(V / Vth ) 2  1
(33)
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
spectrum. The previous results have shown that this can be done by maximizing K33/K11
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°.
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 ~d2 (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 0.53. 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 (αi =
-50.4°, and αo = 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
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.
100
Experiment
Simulation
Dark State Transmission %
18
16
80
14
12
10
90
5.0m, d/p=0.5,
V=1.17V, p=9
70
5.0m, d/p=0.5,
V=0V, p=9
60
50
8
40
6
30
4
2
0
400
20
5.0m, d/p=0.5,
V=1.50V, p=9
450
500
550
600
Wavelength (nm)
Bright State Transmission %
20
10
650
0
700
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 5.05.3μm 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
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 αi = -55°, and αo = 18° which was the orientation that minimized the
transmission at 1.5V.
50
Transmission %
40
30
20
10
0
0
0.2
0.4
0.6
0.8
1.0
1.2
Voltage (V)
1.4
1.6
1.8
2.0
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
108
closer to 9°. For these parameters, the simulations predicted αi = -52°, and αo = 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 Voff = 0V and Von = 1.5V, but was
only 123 using Voff = 1.17V and Von = 1.5V due to the partial reorientation at Voff 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
K22 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, Voff and Von, were
arranged at the same Vratio = 1.28, while Voff was varied from 1.09V to 1.20V. The
contrast ratio is shown in Figure 43 and it peaked at Voff = 1.17V as expected because (as
109
previously mentioned) the polarizer and analyzer were oriented to minimize the
transmission at Von = 1.5V. The luminance steadily decreased as the Voff increased
because at each increase in Voff, the polar tilt angle of the STN midlayer increased a little
further from the undisturbed planar state. The turn on time (τon 0-90) and turn off time (τoff
100-10)
are also plotted in Figure 43. The turn on time steadily decreases with increasing
Voff because Von is increasing. The turn off time steadily increases with increasing Voff
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 Von to Voff
becomes smaller. However, the total response time, τon 0-90 + τoff 100-10, 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.
110
250
off 0-90
on 100-10
225
Response Time (ms)
200
175
150
125
100
75
50
25
50
1.11
1.13
1.15
VOFF (V)
1.17
1.19
100
46
75
42
50
38
Contrast Ratio
Luminance
25
0
1.09
1.11
1.13
1.15
VOFF (V)
1.17
34
1.19
30
Figure 43 – Response times, contrast ratio and luminance for 13B-000 as Voff is varied.
Luminance
Contrast Ratio
0
1.09
111
3.9
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.
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).
90
60
10
120
60
40
150
30
20
180
0
100
5
2
5
210
100
3010
5
240
2
330
2
300
270
Figure 44 – Iso-contrast viewing angle of the 13B-000 STN as seen by the wearer.
3.10 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
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 = 8Vth [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.
3.11 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
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 Voff = 1.17V and Von = 1.5V for a 17 row
display with Vratio = 1.28. 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 αi = -50.0°, and αo = 20.0°) to
emphasize high contrast or 5.8μm (simulation results are αi = -48.1°, and αo = 18.1°), to
emphasize high contrast and brightness as well as less perceived coloration.
Chapter 4
4.
4.1
Normal Mode Polymer Stabilized Cholesteric Texture for Scattering Displays
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
Liquid Crystal
Monomer
Polymer Network
Photoinitiator
V
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
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
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/cm2. 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.
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 × (Tmax - Tmin) + Tmin.
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
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 (V90) is a number that represents the driving voltage necessary to
reach 90% of the range Tmax – Tmin higher than Tmin (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.
120
100
Contrast Ratio = 51.4
Contrast Ratio = 18.1
90
Transmission %
80
o
9.50 Collection Angle
70
60
50
Voltage
Decreasing
40
V = 9.7
Voltage
Increasing
30
20
10
0
0
5
10
15
20
25
30 35
Voltage (V)
40
45
50
55
60
Figure 49 – Typical experimental result used for calculating contrast ratio
4.3.1
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.
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].
Ec 
2
P
K 22
 0 
(34)
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
saturation voltage, V90.
The turn off time is the transition from the transparent
homeotropic to the scattering focal conic texture.
This transition is driven by the
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).
80
Sudden
Gradual
70
Contrast Ratio
60
50
40
30
20
10
Hysteresis Width (V) (V)
Saturation Voltage (V90) (V)
0
40
4.5
5.5
6.5
7.5
8.5
9.5
10.5 11.5
V90
V Chiral Dopant Concentration (wt.%)
R811
12.5
13.5
5.5
6.5
12.5
13.5
30
20
10
0
4.5
7.5
8.5
9.5
10.5
11.5
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.
123
Turn Off Time (100-10%) (ms)
100
90
80
70
60
50
40
30
20
10
Turn On Time (0-90%) (ms)
350
0
4.5
300
5.5
6.5
7.5
8.5
9.5
10.5
11.5
12.5
13.5
12.5
13.5
R811 Chiral Dopant Concentration (wt.%)
250
200
150
100
50
0
4.5
5.5
6.5
7.5
8.5
9.5
10.5
11.5
R811 Chiral Dopant Concentration (wt.%)
Figure 51 – Response time data when monomer concentration is fixed at 2.0% for normal
mode PSCT.
4.3.2
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 10.4-11.1%. As shown in Figure 52 both the sudden and the gradual contrast ratio
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.
125
80
70
Sudden
Gradual
Contrast Ratio
60
50
40
30
20
10
Hysteresis Width (V) (V)
Saturation Voltage (V90) (V)
0
40
1.0
1.5
2.0
2.5
3.0
3.5
4.0
3.5
4.0
RM257 Monomer Concentration (wt.%)
30
V
20
V90
10
0
1.0
1.5
2.0
2.5
3.0
RM257 Monomer Concentration (wt.%)
Figure 52 – Contrast ratio, saturation voltage, and hysteresis width data when chiral
concentration is fixed in the range 10.4-11.1% for normal mode PSCT.
126
Turn Off Time (100-10%) (ms)
Turn On Time (0-90%) (ms)
100
90
80
70
60
50
40
30
20
50
1.0
1.5
2.0
2.5
3.0
3.5
4.0
3.5
4.0
RM257 Monomer Concentration (wt.%)
40
30
20
10
0
1.0
1.5
2.0
2.5
3.0
RM257 Monomer Concentration (wt.%)
Figure 53 – Response time data when chiral concentration is fixed in the range 10.411.1% for normal mode PSCT.
127
4.3.3
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
Phase Transition Temperature
Flow Viscosity [mm2s-1] 20˚C
Dielectric Anisotropy (20˚C, 1kHz)
Optical Anisotropy (20˚C, 589nm)
Elastic Constants (20˚C)
Symbol
TN-I
η
Δε
ε//
ε┴
Δn
ne
no
K11
K33
BL036
95˚C
67
+17.0
23.1
6.1
0.2670
1.7940
1.5270
15.10pN
27.00pN
E44
100˚C
47
+16.8
22.0
5.2
0.2627
1.7904
1.5277
15.5pN
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
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 9.
Turn Off Time (100-10%) (ms)
30
E44
BL036
20
10
0
Turn On Time (0-90%) (ms)
50
10.5
11.5
12.5
R811 Chiral Dopant Concentration (wt.%)
13.5
11.5
12.5
R811 Chiral Dopant Concentration (wt.%)
13.5
40
30
20
10
0
10.5
Figure 54 – Response time data for E44 compared to BL036.
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 BL036.
4.4
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/cm2, 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.
Chapter 5
5.
5.1
Reverse Mode Polymer Stabilized Cholesteric Texture for Scattering Displays
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
Monomer
Polymer Network
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
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<Vth) and 2) under an applied voltage higher than the threshold
voltage (V>Vth) 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
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.
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
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-1011.
Figure 60 – Final chiral dopant studied in reverse mode PSCT, R-811.
5.2.1
Over Drive for a Faster Turn On Response Time
Following the discussion in Chapter 2, an over drive pulse (at amplitude VOD) can be
added to the waveform to speed up the turn on time. In reverse mode PSCT, this is the
135
transition to the scattering state. The effectiveness of this over drive voltage depends on
the driving voltage (Von) being used, which means that for some fixed value of VOD, the
improvement in response time is greater for lower driving voltages Von. 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/cm2. The over
drive pulse was set to the maximum amplitude possible from the amplifier, 74V, and the
duration was varied.
100
Transmission %
80
60
40
VOD = 21V (No OD)
20
0
VOD = 74V 3.0ms
VOD = 74V 0.5ms
0
5
10
15
20
25
Time (ms)
Figure 61 – Comparison of two different over drive durations to no over drive.
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.
No change in Turn Off Time
Response Time (ms)
10
Off 0-90%
On 100-10% (OD = 0.5ms)
On 100-10% (OD = 1ms)
On 100-10% (OD = 3ms)
1
No "Over Drive"
at this voltage
because
VOD = Von
0.1
20
Sub-millisecond switching past VOD=37V
30
40
50
Over Drive Voltage (V)
60
70
Figure 62 – Response time vs. over drive voltage using Von=21V & Voff=7V.
The results of varying the over drive pulse from Von = 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 VOD = 37V using a 3ms duration with no measureable
change to the turn off time across the entire range of over drive amplitudes measured.
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
VOD = 50V and 1ms is not long enough below VOD = 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
5.3.1
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 ~6.5-7.0% 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
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
13B
9A
10A
14A
11A
R811
1.24%
1.56%
1.89%
2.34%
2.65%
Pitch
8.06μm
6.41μm
5.29μm
4.27μm
3.77μm
180° (π) Turns
4
5
6
7 or 8
8 or 9
Contrast
49
65
70
62.5
47
Transmission %
100
P0 = 8.06m
P0 = 6.41m
P0 = 5.29m
P0 = 4.27m
P0 = 3.77m
10
1
0
5
10
15
20
25
Voltage (V)
30
35
40
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
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.
5.3.2
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/cm2 (using detector B).
140
Contrast Ratio & Drive Voltage Von (V)
28
Drive Voltage Von (V)
Contrast Ratio
24
20
16
12
3.0
3.5
4.0
4.5
5.0
5.5
6.0
BMBB-6 (%)
6.5
7.0
7.5
8.0
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, Von) 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
141
occurred because the monomer could not be fully dissolved in the material since the
concentration is very near the solubility limit.
36
6.0
33
5.5
Turn On 100-10%
Turn Off 0-90%
5.0
27
4.5
24
4.0
21
3.5
18
3.0
15
2.5
12
2.0
9
1.5
6
1.0
All data uses VOD=30V OverDrive at start of pulse.
3
0
3.0
Turn On Time (ms)
Turn Off Time (ms)
30
3.5
4.0
4.5
5.0
5.5
6.0
BMBB-6 (%)
6.5
7.0
0.5
7.5
0
8.0
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
Von to Voff where Von is equal to the driving voltage in Figure 64 and Voff = Von/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, Von. Higher voltages will naturally reduce the turn
on time (the principle on which the over drive method is based). Therefore, in order to
142
remove this variable, each sample was measured using a VOD=30V over drive pulse long
enough for the transition to be complete before switching to Von. 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 Von
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.
5.3.3
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/cm2 and the other at a very high intensity of 7.67mW/cm2 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,
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.
144
100
Transmission %
80
60
Low UV
Contrast = 19.5
V = 0.122
40
High UV
Contrast = 9.8
V = 0.444
20
0
0
5
10
15
Voltage (V)
20
25
30
Figure 66 – Transmission curves at two UV intensities.
5.3.4
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 92.332% 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/cm2 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/cm2.
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.
145
50
25
Contrast Ratio
Turn Off 0-90%
Turn On 100-10%
20
30
15
20
10
10
5
0
0
1
2
3
4
Curing Time (hours)
5
6
Response Times (ms)
Contrast Ratio
40
0
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.
146
5.3.5
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 Von = 30V and Voff = 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 Barrier
Polyimide Pretilt CR
Turn On
Turn Off
ID
Angle*
100-10%
0-90%
22A None
SE-3510
~7-8°
16.04
1.76ms
7.46ms
22B AT-702 SE-2170
~2°
16.17
1.86ms
7.59ms
*Pretilt angles from Nissan Chemical [37].
T% @
0V
86.44%
84.33%
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
5.4.1
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 no 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 no
of both liquid crystal and polymer and neff (angularly dependent extraordinary index) of
both liquid crystal and polymer. However, the mismatch in reverse mode is still only
slight because both have no close to 1.5 and the deviation between neff 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 neff of the liquid crystal and the isotropic index n of the
148
polymer (which is usually chosen to match the no 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.
5.4.2
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
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
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
151
B defects are stretched between spacers and often form straight lines parallel to the
original filling flow.
Type A
Defect
Type B
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.
5.4.3
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
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,
Voff.
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 P0=10μm then the most
stable thicknesses (with lowest free energy) are exact multiples of P0/2 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 P0=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
153
be P0=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.
5.4.4
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.
20 μm
Type B
Defect
Spacer
A
P
Figure 70 – Type B defect.
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.
5.4.5
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
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 TNI = 60ºC and with 2.54% R811 the TNI = 56ºC. The
experiment consisted of a ramp up to 70ºC @ 5ºC/min, then a ramp down to room
temperature @ -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 SE3510 & SE-2170 alignment layers (one on each side). Photographic evidence of the
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.
157
Edge of
Cell
A
Center of
Cell
P
Figure 72 – After slow annealing.
158
A
P
Figure 73 – After fast annealing.
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 (20-160 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.
5.4.6
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
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.
5.4.7
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.
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.
5.4.8
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
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.
163
5.5
Cell Gap Varied: Wedge Cell Experiment
5.5.1
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?
5.5.2
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
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/cm2. 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.
165
1π
2π
3π
4π
6π 7π 8π 9π 10π 11π
5π
15V Applied
Figure 75 – Wedge cell with 15V applied. The number of π turns is labeled in each zone
5.5.3
Contrast Ratio vs. Thickness
When multiple scattering is neglected, the transmitted light intensity, Iout, 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(d ) 
I in
 e  d
I out
(36)
If the thickness, d, changes by a factor, x, then the contrast ratio will change by the
exponential of that factor:
CR( x  d )  [CR(d )]x
(37)
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.
1000
Calculated Contrast Ratio
Measured Contrast Ratio
64.2V
56.1V
55.5V
43.8V
Contrast Ratio
100
40.2V
32.1V
28.8V
10
22.2V
17.1V
11.4V
The labeled voltages are Von
1
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30
Cell Gap (m)
Figure 76 – Measured and calculated contrast ratios vs. cell gap.
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 Voff = 10V and Von = 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 57.
Contrast Ratio
60
50
40
30
20
10
0
0
2
4
6
8 10 12 14 16 18 20 22 24 26 28 30
Cell Gap (m)
Figure 77 – Contrast ratio vs. cell gap at constant Von = 30V & Voff = 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.
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.
169
5.5.4
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.
87
Transmission %
86
85
84
83
82
81
80
79
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
Cell Gap (m)
Figure 79 – Transparent state transmission vs. cell gap.
5.5.5
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
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 Von and Voff. Both had a voltage ratio of Von:Voff = 3:1 voltage
ratio, but one was fixed at Von = 30V & Voff = 10V and the other one varied Von 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 Von = 30V & Voff = 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 Von = 30V & Voff = 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 ~d2 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.
171
50
Response Time (ms)
10V:30V Turn On 100-10%
10V:30V: Turn Off 0-90%
Best V: Turn On 100-10%
Best V: Turn Off 0-90%
20
10
"Under Driven"
Region
5
2
0
2.5
5.0
7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 30.0
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 Von=30V & Voff=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.
172
75
Driving Voltage
60
45
30
15
0
2.5
5.0
7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 30.0
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
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/cm2. The second sample (labeled with the open
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/cm2. 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/cm2.
100
2.98% RM257
4.82% RM257
7.10% BMBB-6
90
80
Transmission %
70
60
50
40
V=0.1V
V=4.8V
V=2.6V
30
Contrast Ratio = 6.55
20
Contrast Ratio = 9.32
10
0
Contrast Ratio = 17.77
0
5
10
15
20
25
30 35 40
Voltage (V)
45
50
55
60
65
70
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
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/cm2, the T-V curve didn‟t change significantly as shown below in Figure 83.
176
100
2
4.37mW/cm2
2.33mW/cm2
1.02mW/cm
90
Transmission %
80
70
60
50
40
30
20
10
0
0
5
10
15
20
25 30 35
Voltage (V)
40
45
50
55
60
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
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/cm2) 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/cm2). 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.
Time (ms) to reach an absolute T=60%
200
148.7ms
100
RM257 (Sample 5H)
BMBB-6 (Sample 8E)
50
32.9ms
20
10.07ms
10
7.33ms
100
7.84ms
1000
Driving Voltage Duration (ms)
8.08ms
10000
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
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.
179
Table 13 – Detailed comparison of the molecular structures of RM257 and BMBB-6.
RM257
Structure
Flexible Tail
Description
one phenyl ring
with a methyl
attachment
3 hydrocarbons
Terminal Group
acrylate
Rigid Core
BMBB-6
Structure
Description
two phenyl rings
6 hydrocarbons
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 60-80°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, 60-80°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.
180
20 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
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/cm2. 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
Von
Contrast
Single
Single
10μm
16μm
22V 17.9
30V 60
Double
Double
10μm
16μm
22V 130
30V 391
Turn Off
21.7ms
15.2ms*
Turn On (No
Over Drive)
2.96ms
4.81ms
Turn On (30V
Over Drive)
1.39ms
N/A**
26.4ms
17.0ms*
2.53ms
3.88ms
1.22ms
N/A**
*16μm is faster to relax because it is “under driven”.
**Not applicable, since we already driving Von 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
182
would be around 2.00% R811 (pitch = 5.00µm), 6.25% BMBB-6 monomer cured for 5
hours at 0.015mW/cm2, 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.
Chapter 6
6.
6.1
Scattering Profile of Polymer Stabilized Cholesteric Textures
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
184
sample
rotation
direction
detector
mirror
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 9mm2 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 38.
 det
1
1
 4 sin (sin sin )  0.305  10  4
2
2
1
(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
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 θ1
and θ2 are the angles from the center of the unscattered light beam to the first and second
edges of the ring respectively.
 ring  2 (cos 1  cos  2 )
(39)
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.
ID
N1
N2
N3
N4
N5
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
Reverse Mode
ID
R811
Pitch
R1 1.24% 8.06 µm
R2 1.56% 6.41 µm
R3 1.89% 5.29 µm
R4 2.34% 4.27 µm
R5 2.65% 3.77 µm
186
6.3
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.
187
10
0
N1
0.001
Normalized Intensity
10
10
-1
N2
N3
N4
N5
N1
-2
0.0001
N2
10
N3
-3
1
2
5
10
20
50
N4
N5
10
10
-4
-5
-20
0
20
40
Angle ()
60
Figure 87 – Normal mode PSCT scattering profile (log).
80
188
350
300
Contrast Ratio
250
200
N5
N3
150
N4
100
50
N2
N1
0
1
2 3 4 5 6 7 8 9 10 11
Total Collection Angle ()
Figure 88 – Contrast ratio variation with total collection angle in normal mode PSCT.
189
250
225
Contrast Ratio
200
N5
175
N4
150
125
N3
100
75
50
N1
N2
25
0
0
5
10 15 20 25 30 35 40
Voltage (V)
Figure 89 – Contrast ratio as the scattering state voltage varies with a 3° degree total
collection angle in normal mode PSCT.
190
Figure 90 – The 3-D scattering profiles of the normal mode PSCT samples.
191
6.4
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.
192
10
0
0.0003
R1
Normalized Intensity
10
0.0002
-1
R3
10
10
-2
10
R4
R5
0.0001
-3
1
R1 R2 R3
10
R2
2
5
10
20
R4 R5
-4
-5
-20
0
20
40
Angle ()
60
Figure 91 – Reverse mode PSCT scattering profile (log).
80
193
200
Contrast Ratio
150
R2
R3
R4
100
R5
R1
50
0
1
2 3 4 5 6 7 8 9 10 11
Total Collection Angle ()
Figure 92 – Contrast ratio variation with total collection angle in reverse mode PSCT.
194
150
R2
R3
125
Contrast Ratio
R4
R5
100
R1
75
50
25
0
0
5
10 15 20 25 30 35 40
Voltage (V)
Figure 93 – Contrast ratio as the scattering state voltage varies with a 3° degree total
collection angle in normal mode PSCT.
195
Figure 94 – The 3-D scattering profiles of the reverse mode PSCT samples.
196
6.5
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.
Chapter 7
7.
7.1
Dye-Doped Normal Mode Polymer Stabilized Cholesteric Texture
Introduction
Recent LCD research has focused on reducing power consumption, making light-
weight 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
198
Scattered
Transmitted
V
Liquid Crystal
V
Absorbed
Dye
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/cm2) 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.
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.
200
100
90
80
70
60
50
40
30
20
10
0
50
40
30
20
10
0
2
4
6
8
10
R811 Chiral Dopant (%)
Saturation Voltage (V)
Contrast Ratio
Data For 2% Monomer & 3% Dye
0
12
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.
201
30
30
25
25
20
20
15
15
10
10
5
0
1
2
3
4
5
Dichroic Dye (%)
6
7
Saturation Voltage (V)
Contrast Ratio
Data For 2% Monomer & 5.5% Chiral
5
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
202
absorption of the dye occurs mainly in the region from 400-500nm which gives it a
yellow appearance.
100
Contrast Ratio = 8.9
90
Transmission %
80
70
60
Voltage
Decreasing
50
Voltage
Increasing
40
30
20
10
0
0
5
10
Voltage (V)
15
20
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
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].
100
Transmission %
90
a)
80
70
b)
60
50
400
c)
450
500
550
600
Wavelength (nm)
650
700
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
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.
Chapter 8
8.
8.1
Conclusion
Dissertation Summary
In Chapter 1 the benefits of eyewear that uses a pixelated device for shielding glare is
discussed. Glare is defined as areas of undesirably high luminance in the field of view.
A display that darkens the entire field of view can reduce glare to comfortable levels, but
the area of focus is reduced as well. A pixelated design enhances the wearer‟s vision
because the accommodation of the eye can remain mostly undisturbed. This happens
because the overall luminance level perceived by the eye won‟t change drastically since
some of the pixels are turned on to reduce the glare.
Many liquid crystal displays (LCDs) have been developed that take advantage of the
distinctive anisotropic optical and dielectric properties of liquid crystals molecules.
LCDs are a good choice for pixelated glare shielding eyewear because they have a fast
response to a driving voltage, the ability to be battery powered in a portable device, a
high contrast ratio between a bright state and a dark state, and the ability to be segmented
into pixels. The best pixelated electrode structure to use for a glare shielding device is
the passive matrix because can address a high resolution display and it saves cost and
complexity over an active matrix design. The best driving method is either the row-at-atime resolution-dependent voltage ratio or the block addressing 3:1 voltage ratio. The
205
206
best possible LCD modes from those considered in this dissertation and shown in Table 3
(in Chapter 2) are the STN LCD and the reverse mode PSCT.
A systematic procedure to optimize STN LCDs and film compensated STN LCDs
was developed in Chapter 3. 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 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. It was shown that reducing the birefringence dispersion can
improve the bright state by increasing luminance and decreasing the perceived coloration.
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 electrodistortional curve. If the 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 Voff = 1.17V and Von = 1.5V for a 17 row
display with Vratio = 1.28. 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 αi = -50.0°, and αo = 20.0°) to
emphasize high contrast or 5.8μm (simulation results are αi = -48.1°, and αo = 18.1°), to
emphasize high contrast and brightness as well as less perceived coloration.
207
The normal mode PSCT was shown in Chapter 4 where the performance was
experimentally measured with the variation of chiral dopant and monomer concentration
as well as the substitution of the liquid crystal host. 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/cm2, and the
liquid crystal E44. As mentioned in the overview of the PSCT in Chapter 2, the normal
mode PSCT is considered “normally black”; therefore, the normal mode has a fatal flaw
which eliminates the design for use in pixelated glare shielding eyewear.
The reverse mode PSCT was experimentally measured with the variation of chiral
dopant, monomer concentration, and cell thickness as well as the substitution of the
monomer incorporated and shown in Chapter 5. The best formulation found in this
dissertation of the reverse mode PSCT for high contrast, fast response time, and lower
driving voltage would be close to 2.00% R811 (pitch = 5.00µm), 6.25% BMBB-6
monomer cured for 5 hours at 0.015mW/cm2, 0.15% photoinitiator BME, and the
remainder liquid crystal E44. If the maximum output from the display‟s drive circuitry is
30V [76] then the best cell gap of a single cell is 18μm for maximum static contrast. A
double stacked cell will improve the contrast ratio using the same driving voltage without
significantly altering the response time.
The technique of measuring the scattering profile described in Chapter 6 proved
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
208
many different collection angles. The best formulation of the normal mode depends on
the collection angle, but for small collection angles it has a pitch of 1.15µm and large
collections angles it was 0.76µm. 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 normal
mode has a higher scattering efficiency than reverse mode because it scatters more light
for the same driving voltage.
Adding a dichroic dye to the normal mode PSCT display as illustrated in Chapter 7
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 display is in the scattering state is at 0V,
the feasibility of creating a flexible display by making cells with a patterned ITO design
and plastic substrates was demonstrated. These displays were bent to a radius of 85mm
and were still operable at this curvature.
8.2
Scattering vs. Absorption
Even if the contrast ratio, response time, and viewing angles of the scattering and
polarizer based absorption based displays were equal, there would still be some
fundamental differences between them. For example, if a pixel in an absorptive LCD is
not able to switch completely black and allows some light from a glare source to reach
the eye, then the outline and shape of the glaring light may still be determinable.
However, if a pixel in a scattering LCD has some light that scatters forward allowing
light from a glare source to reach the eye, then it might obscure the shape or outline of
209
the light source the same way a large cloud can obscure the outline of the sun, but still
allows some light to reach the earth.
Also, the appearance of the display while powered off is going to be different. The
polarizer based display will transmit no more than 50% of the light from the absorption
of the polarizer and it will realistically be <40% due to polarizer inefficiencies and
reflection losses making them mainly suitable for use outdoors in daytime.
The
scattering display‟s transparency is much higher because it is only limited by reflection
losses and background haze.
This means they will transmit ~90% or more of all
wavelengths which makes them suitable for applications in low light such as indoors or at
night. The scattering displays can even be used in new applications besides eyewear
which would require a colorless and transparent field of view, such as car windshields
and windows. For example, imagine a window that tracks the sun and always blocks just
the right size spot to keep the direct sunlight glare off of the television or your favorite
chair.
8.3
Final LCD Recommendation
The best choice for glare shielding eyewear will depend on the application. For an
application that requires fast response times, the best result would come from a reverse
mode PSCT using the over drive method which can achieve sub-millisecond response
times. (Note: The STN might also achieve sub-millisecond response times if sufficiently
over driven, but this was not studied in this dissertation). If nighttime use is required,
then high transparency is needed, which also dictates the use of a reverse mode PSCT. If
long battery life is required, then the STN is the best choice since the driving voltages are
210
lower. If the design requires an arbitrary pixel pattern, then the STN is the best option
because the switching behavior is steep enough for row-at-a-time addressing.
When comparing the light attenuating results directly with no regard to the
application, then the best uncompensated contrast ratio of the STN was measured to be
~125. However, if block addressing is acceptable, then the STN contrast ratio will
increase since the voltage ratio is increased. An additional benefit of block addressing is
that the resolution can be adjusted to suit the application since the entire display is
addressed during each frame. The reverse mode PSCT, which already must use block
addressing, has a contrast ratio that is highly dependent on cell gap. In the range 1618µm the contrast will be around 60 when using the same measurement system as the
STN and limited to 30V.
However, when measuring the scattering profile and
calculating the contrast ratio, it can be as high as 150 if the collection angle is ~2°, but is
reduced to 50 if the collection angle is ~10°. Therefore, the contrast ratio of the reverse
mode PSCT also highly depends on the distance of the lens from the eye (vertex distance)
and the diameter of the pupil. If the vertex distance is 15mm (a common distance for
corrective lenses) and the pupil diameter varies in a reasonable range from 3 to 9mm
[85], then the total collection angle of the eye (as calculated using the configuration in
Figure 16) varies from 11.4° to 33.4°. If it is assumed that all the scattered light collected
by the eye is “detected” then, at best, it reduces the contrast ratio of reverse mode PSCT
to <50. Therefore, it is concluded that the contrast ratio of the STN will be higher than
the reverse mode PSCT in a real device.
211
Finally, if considering manufacturability, both PSCT designs have a drawback
because they are susceptible to damage caused by pressure.
This damage causes
undesirable haze in the transparent state which is permanent.
Although some
investigations have determined ways to help prevent or partially repair this problem [86],
more work is needed to find a complete solution. Both the STN and the PSCT displays
have been implemented in real products; however, the STN LCD is a more mature
technology and it doesn‟t require UV curing or defect clearing procedures during
production.
Consequently, the final recommendation of this dissertation has determined that the
best LCD is the uncompensated STN using 13B-000 liquid crystal redesigned with the
3:1 block addressing (Voff = 1.17V and Von = 3.51V) for an outdoor/daytime application
of a robust pixelated glare shielding eyewear device with long battery life, a reasonably
large viewing angle, tolerable coloration, and a response time adequate for everyday use.
212
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