1 examples of flat-panel liquid crystal displays
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Examples of flat-panel liquid crystal displays
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glass pixelated color filter + black mask Planarization layer (acrylate network) ITO electrode Polyimide orientation layer Liquid crystal Polyimide orientation layer ITO electrode pattern glass
polarizer
polarizer
Acrylic waveguide with scattering elements
glass pixelated color filter + black mask Planarization layer (acrylate network) ITO electrode Polyimide orientation layer Liquid crystal Polyimide orientation layer ITO electrode pattern glass
polarizer
polarizer
Acrylic waveguide with scattering elements
Display design
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Display element Efficiency (%) individual cumulative
lamp reflector + in-coupling 80 80 backlight waveguide 70 56 diffuser + air gap 90 50 back polarizer 40 20display aperture 80 16 color filters 28 5effective area for color 33 2 front polar 95 1
Display element Efficiency (%) individual cumulative
lamp reflector + in-coupling 80 80 backlight waveguide 70 56 diffuser + air gap 90 50 back polarizer 40 20display aperture 80 16 color filters 28 5effective area for color 33 2 front polar 95 1
Light efficiency of flat-panel TN LCDs
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Optical films for LCDs
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Polymers in LCD displays
• Polarizers• Color Filters• Viewing angle compensation• Prism films• Multi-layer optical films• Specular reflectors• Liquid crystal alignment layers
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Ceramics in LCD displays
• Display quality glass• Transparent conductive oxides• Spacers• Viewing angle compensation• Prism films• Multi-layer optical films• Specular reflectors
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Quenched-in misorientation of InO6 structural units leads to amorphous films
300
250
200
150
100
50
0co
un
ts
4035302520two theta (degrees)
{222}
{400}{211}
Bixbyite crystal structure: c-type rare earth sesquioxide
•Ia3 with 80 atom unit cell consisting of InO6 structural units
Indium oxide
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Sputter Target Qualification
•Film purity
•Film properties
•Step coverage
•System throughput
•Uniformity of deposition
•Target integrity (& utilization)
•Process repeatability
Sputter Deposition Challenges
•Compositional analysis
•standard test conditions:•Sputter system•Power density•Voltage•Sputter gas•Film thickness
•Resistivity (film)
•Surface asperities (nodules)
•Sputter rate
Time dependence of...
Transparent Conducting Oxides
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NS
NS
NS
DC Power supply
erosion center
substrate
target
sputtered atoms
incident ion
Target
Sequence of collisions results in ejection of target atom (sputtering)
sputtered atom
reflected ions and neutrals
secondary electrons
implanted ion
Magnetron Sputter Deposition
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Crystalline and wt%SnO2 Indium Oxide
0.2 µm
Pure In2O3 In2O3:9.8 wt%SnO2
0.2 µm
Pure In2O3 In2O3:9.8 wt%SnO2
Crystallinity Wt% SnO2
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• Erosion profile
•Surface profile
•Nodule formation
•Composition
•Toughness
Sputter target removed from service
Sputter target characterization
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TFT Foils
TFT Structure
• Flexible polyimide substrates• On surface minimum radius of curvature
depends on TFT strain to failure• Inside substrate minimum radius of curvature
depends on substrate
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By an external field • electrical field (e.g. 1 V/m) • magnetic field (e.g. 5 kG) • mechanical field (e.g. flow)
At an oriented surface • buffed substrate for planar alignment • surfactants for homeotropic alignment
form anisotropy anisotropic molecular properties
combined action of sterical and dispersive interfacial interactions
E
Liquid crystal alignment
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Rubbing directions and chiral dopants determines rotation direction
Twisted nematic displays
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rubbing
Tilt angle by selection of alignment material and rubbing
Rubbing direction in accordance with twist sense
90o twist by adding chiral dopant
Pretilt and chiral additives to prevent domain formation
Tilt, twist and rubbing directions
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N O
O
O
C14H19
CCH3
CH3
O N
O
O
N
O
O
O CCH3
CH3
O N
O
O C14H19
Alignment mechanism: • at nano grooves by excluded volume effects
of rod-like molecules at interface • at preferentially orientated chain segments
by anisotropic dispersive interactions with LC molecules
• apolar side/end groups provide pre-tilt control
n
Rubbing of polyimide provides liquid crystal orientation
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•Use polarized UV light to modify polymer surface in order to control liquid-crystal alignment
linearly polarized UV-light
*
Recent development: photo-alignment
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Problems with mechanical rubbing: • static electricity • dust formation • uniform rubbing of large surface area • uniform rubbing of irregular surface
Photo-alignment is a non-contact method that avoids these problems !
Why photoalignment ?
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• Orientation perpendicular to polarization direction • No or small pretilt
( )n
O
C O
C
C
( )n
O
C O
C
C
( )n
( )n
O
O
C O
C O
LP-UV
Photo-alignment using polyvinylcinnamates
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• Orientation parallel to polarization direction • Pretilt
O
O O
O
O
O O
O
LP-UV
Coumarin-based photoalignment (Rolic)
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• Multi-component mix for: • low melting temperature • high nematic to isotropic transition temperature • optimized optical anisotropy • small dispersion of refractive indices • low viscosity for fast response • small elastic constants • high dielectric anisotropy / low threshold voltage • low conductivity
LC mixtures for displays contain many components
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FC5H11
FC7H15
FC6H13
C3H7 OCF3
C5H11 OCF3
OCF3CH2CH2C3H7
OCF3CH2CH2C5H11
CH2CH2C3H7 F
F
C3H7
F
C3H7
C3H7
F
C5H11
C5H11
F
C5H11
12 w%
10 w%
10 w%
13 w%
12 w%
11 w%
9 w%
13 w%
3 w%
4 w%
3 w%
FC5H11
FC7H15
FC6H13
C3H7 OCF3
C5H11 OCF3
OCF3CH2CH2C3H7
OCF3CH2CH2C5H11
CH2CH2C3H7 F
F
C3H7
F
C3H7
C3H7
F
C5H11
C5H11
F
C5H11
12 w%
10 w%
10 w%
13 w%
12 w%
11 w%
9 w%
13 w%
3 w%
4 w%
3 w%
Example of LC mixture
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Column electrodes
Row electrodes
Active matrix LCD (TN)
Direct addressingPassive matrix LCD (STN) - row and column electrodes - LC responds to RMS voltage
switch at each pixel
Passive plate with counter electrode
Drive Schemes
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24macroorganisch 6C275 / kernkeuze college 6C270 24-04-01 home
0
25
50
75
100
0 2 4 6
Applied voltage (VRMS)
Tra
nsm
issi
on (
%) TN cell
STN cell
TN STN for polars: (Super Twisted Nematic) 90o twist 180o-270o twist white off state colored on/off state black-on state compensation foil
for B/W for // polars: (poor) black off white on state
twist angle
Electro-optic response of TN and STN LCD’s
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active matrix:high end computer monitors, video ‘emissive’ by back light
passive matrix: simpler displays
??
passive matrix, bistable effect, reflective color: simpler displays, extremely low power consumption
paper-white reflective effects
• Polarizer-based LC effects • Twisted-nematic • In-plane switching of nematics • Vertically aligned nematics • Ferroelectric (SC*) • Supertwisted-nematic
• Polarizer-free LC effects • Polymer dispersed liquid crystals • LC gels • surface or polymer-stabilized cholesterics • guest-host LC’s
Other liquid crystal display effects
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h
E
transparent
scattering
Polymer stabilized liquid crystals
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• Schematic representation of different types of liquid crystal polymers • Network formation by photo-initiated polymerization • Formation of ordered networks by photopolymerization of liquid-crystalline
monomers • Example of photo-initiated polymerization in the liquid-crystalline state • Typical processing sequence • Reactive liquid crystals • Influence of functional moiety on mesomorphism of reactive liquid crystals • Refractive indices before and after polymerization • Order parameter of LC diacrylates before and after polymerization • Liquid crystalline networks for advanced optics • Three-dimensional polymer architectures • Combination of different alignment principles • Photopolymerization of a chiral monomer • Pitch of the helix can be freely chosen by blending chiral with nematic monomers • Reflection band of sample containing 62 % chiral diacrylate • Photo-induced diffusion in z-direction • Gradient in UV light by strong absorbing dye • Modulation of properties in z-direction • Cholesteric network with a pitch gradient
Polymeric liquid crystals and liquid crystal networks
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rod
rigid main chain
flexible main chain
disk
LC MAIN CHAIN POLYMERS
LC SIDE CHAIN POLYMERS
LC NETWORKS
Schematic representation of different types of liquid crystal polymers
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OO O
O
O
O
O
O
O
O
h
aligned LC monomer
Formation of LC networks by photopolymerization of LC monomers
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Iso-contrast lines
Grey scale inversion
Contrast and grey-scale inversion as function of viewing angle
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high contrastlow contrast
low V medium V high V
low low contrasthigh
dn >> 0dn = 0
Grey scale inversion • Contrast degradation
Viewing angle of TN-LCD’s
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towards towards homeotropic orientation homeotropic orientation at air interfaceat air interface
planar orientation planar orientation at rubbed substrateat rubbed substrate
with WVA
without WVA
Compensation foils to improve on viewing angle
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Tilted discotic networks to improve on viewing angle
O
O
C
C
O
H3C
OR
O CH3
OR
OO CC
OO
OR
H3C
RO
H3C
O
O
C
C
O
O
RO
CH3
CH3
RO
R = -C10H21 and/or
(CH2)11O C
O
CH CH2
Fuji film
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Reflecting polarizer, e.g. wide-band cholesteric film
Depolarizing or polarization converting reflector
100% polarized light instead of 50% by recycling principle
Non-absorbing polarizer improves on light-efficiency
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wideband cholesteric polarizer
Display partly provided with wide-band cholesteric polarizer
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monomer with steep temperature dependence
polymer with flat
temperature
dependence
300
400
500
600
700
800
20 40 60 80 100 120
Temperature (oC)
Ref
lect
ion
wav
elen
gth
(nm
) monomer
polymer 1
polymer 2
polymer 3
Cholesteric color filters: color generation without absorption improves LCD’s on light efficiency