Examples of flat-panel liquid crystal displays

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Examples of flat-panel liquid crystal displays
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Display design
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Light efficiency of flat-panel TN LCDs
Display element Efficiency ()
individual cumulative lamp reflector
in-coupling 80 80 backlight waveguide
70 56 diffuser air gap 90 50 back
polarizer 40 20 display aperture 80 16
color filters 28 5 effective area for
color 33 2 front polar 95 1
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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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Indium oxide

• Bixbyite crystal structure c-type rare earth
  sesquioxide
• Ia3 with 80 atom unit cell consisting of InO6
  structural units

Quenched-in misorientation of InO6 structural
units leads to amorphous films
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Transparent Conducting Oxides
Sputter Target Qualification
Sputter Deposition Challenges

• Compositional analysis
• standard test conditions
• Sputter system
• Power density
• Voltage
• Sputter gas
• Film thickness

• Film purity
• Film properties
• Step coverage
• System throughput
• Uniformity of deposition
• Target integrity ( utilization)
• Process repeatability

Time dependence of...

• Resistivity (film)
• Surface asperities (nodules)
• Sputter rate

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Magnetron Sputter Deposition
reflected ions and neutrals
secondary electrons
incident ion
sputtered atom
sputtered atoms
substrate
erosion center
target
Target
implanted ion
Sequence of collisions results in ejection of
target atom (sputtering)
DC Power supply
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Crystalline and wtSnO2 Indium Oxide
Crystallinity
Wt SnO2
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Sputter target characterization

• Erosion profile
• Surface profile
• Nodule formation
• Composition
• Toughness

Sputter target removed from service
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TFT Foils

• Flexible polyimide substrates
• On surface minimum radius of curvature depends on
  TFT strain to failure
• Inside substrate minimum radius of curvature
  depends on substrate

TFT Structure
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Liquid crystal alignment

• By an external field
• electrical field (e.g. 1 V/mm)
• magnetic field (e.g. 5 kG)
• mechanical field (e.g. flow)

E
form anisotropy anisotropic molecular
properties

• At an oriented surface
• buffed substrate for planar alignment
• surfactants for homeotropic alignment

combined action of sterical and dispersive
interfacial interactions
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Twisted nematic displays

Rubbing directions and chiral dopants determines
rotation direction
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Tilt, twist and rubbing directions
Pretilt and chiral additives to prevent domain
formation
Rubbing direction in accordance with twist sense
rubbing
?
Tilt angle ? by selection of alignment material
and rubbing
90o twist by adding chiral dopant
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Rubbing of polyimide provides liquid crystal
orientation
n

• 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

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Recent development photo-alignment

• Use polarized UV light to modify polymer surface
  in order to control liquid-crystal alignment

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Why photoalignment ?

• 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 !
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Photo-alignment using polyvinylcinnamates
n
C
O
O
O
LP-UV
C
O

• Orientation perpendicular to polarization
  direction
• No or small pretilt

n
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Coumarin-based photoalignment (Rolic)
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LC mixtures for displays contain many components

• 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

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Example of LC mixture
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Drive Schemes
Column electrodes
Row electrodes
Passive matrix LCD (STN) - row and column
electrodes - LC responds to RMS voltage
Direct addressing
Active matrix LCD (TN)
switch at each pixel
Passive plate with counter electrode
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Electro-optic response of TN and STN LCDs
twist angle
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
24-04-01
home
macroorganisch 6C275 / kernkeuze college 6C270
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Other liquid crystal display 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 LCs

active matrixhigh end computer monitors, video
emissive by back light
??
passive matrix simpler displays
paper-white reflective effects
passive matrix, bistable effect, reflective
color simpler displays, extremely low power
consumption
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Polymer stabilized liquid crystals
h?
transparent
E
scattering
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Polymeric liquid crystals and liquid crystal
networks

• 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

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Schematic representation of different types of
liquid crystal polymers
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Formation of LC networks by photopolymerization
of LC monomers
h?
aligned LC monomer
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Contrast and grey-scale inversion as function of
viewing angle
Iso-contrast lines
Grey scale inversion
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Viewing angle of TN-LCDs

• Grey scale inversion

• Contrast degradation

high contrast
low contrast
low
low contrast
high
dDn gtgt 0
dDn 0
low V medium V high V
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Compensation foils to improve on viewing angle
towards homeotropic orientation at air interface
planar orientation at rubbed substrate
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Tilted discotic networks to improve on viewing
angle
Fuji film
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Non-absorbing polarizer improves on
light-efficiency
Reflecting polarizer, e.g. wide-band cholesteric
film
100 polarized light instead of 50 by recycling
principle
Depolarizing or polarization converting
reflector
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Display partly provided with wide-band
cholesteric polarizer
wideband cholesteric polarizer
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Cholesteric color filters color generation
without absorption improves LCDs on light
efficiency
monomer

• monomer with steep temperature dependence
• polymer with flat temperature dependence

polymer 1
polymer 2
polymer 3