Liquid Crystal And its Applications on Displays

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Liquid Crystal And its Applications on Displays

• 0411 Presentation
• Lai Kwing To
• Chan Yat Long

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What is Liquid Crystal (LC)?

• Liquid- in which molecules can flow- molecules
  can move relative to each other
• Crystal - molecules have regular, ordered
  arrangement
• Liquid Crystal (LC)- molecules can flow AND-
  have ordered arrangement at the same time

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What is Liquid Crystal (LC)?

• Accidentally found at 1888,
• By biologist Friedrich Reinitzer
• What was found was
• A cholesterol derivative
• Has crystallite observed
• Has 2 melting point
• solid -gt cloudy liquid
• cloudy -gt clear liquid

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What is Liquid Crystal (LC)?

• Two types of LC
• Thermotropic
• Phase change with temp
• Lyotropic
• Phase change with concentration of LC molecules
  in the solvent
• Now on, LC refer to thermotropic LC

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What is Liquid Crystal (LC)?

• LC is a phase
• Only some molecules CAN have LC phase
• These special molecules are - usually organic
  - highly anisotropic eg. Rod shape Plate
  shape

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A typical LC molecule

• Shape Rod shape
• Ringsprovide intermolecular force, contribute
  to 'dielectric anisotropy
• Terminal GroupsCan Contribute to 'dielectric
  anisotropy'

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Further Details of LC phase

• LC phase can be further classified to
• Smectic
• Nematic
• Chiral(Cholesteric)

Recall LC can flow but still have ordered
arrangement
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Further details of LC phase

• As temp ?, orderness ?
• The LC molecules can undergo phase
  transiteg.
• Other transition sequence are possible,depending
  on the LC molecules used

Low Temp
High Temp
Solid Crystal ?Smentic LC ?Nementic LC ?Isotropic
Liquid
Ordered
Disorder
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Optics of anisotropic media

• LC molecules are anisotropic,therefore
• Different direction ? Different Polarizability
• ? Different e (dielectric constant)
• Different e for Light with
• Different Polarization
• Different travel direction

Feels e1
Feels e2
Feels e2
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Optics of anisotropic media

• Recall
• Different e
• Different wave speed
• Different refractive index n
• Different optical path length
• ,for different light.

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Optics of anisotropic media

• Different polarization lightgets different
  optical path length
• ? Can manipulate polarization state
• How?

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Example 1- simple LC layer
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Example 1- simple LC layer

• At 1st x, y-components in phase ?45deg
  polarized
• Afterwards x, y-components go through diff.
  optical path length - If y-component lags p
  (antiphase) ?135deg polarized- If y-component
  lags p/2 ?circular polarized (Recall Sec.
  School CRO knowledge)

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Example 2 TN-LC

• TN Twisted Nematic
• Similar to chiral, but twisted by us
• If initially,the polarization direction //LC
  orientation
• ? The polarization direction will keep // LC
  orientation
• Similar effect for - case
• For other case, not so simple

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Example 2 TN-LC

• Poor explanation
• Consider a thin slide of LC

If polarization // LC orientation gt Feels only
one e gt No change in polarization
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Example 2 TN-LC
Afterwards Polarization(// y-axis)
OriginalPolarization
gt Pass through LC,x-component leadsy-component
a bit
gt Assume the ellipse is very thin
- Remark LC orientation // y-axis

• If theres a little deviation between
  polarizationand LC orientation
• ? Polarization twist towards LC orientation
  (y-axis)

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Example 2 TN-LC

• In every thin LC slide,polarization twist
  towards LC orientation Total effect
  Polarization twist with LC orientation
• The approx. is good when FltltG
• F twist rate of LC orientation
• G(2pd/?)(n1-n2)
• The LC orientation twist slowly
• The refractive index for the 2 polarization
  direction is very different

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Example 3 Nothing Happens

• x,y-component experience same e
• ? same optical path length
• ? no change in polarization

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Using LC for display

• Different LC orientation
• ? different result polarization
• If use with polarizer
• ? different amount of transmitted light
• If we can control LC orientation
• ? Display application
• How to control LC orientation?
• E-field

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Using LC for display

• Recall
• For LC,different direction ? different
  polarizability
• For minimum energy,the most polarizable
  direction should // E-field

No Good
Good
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Using LC for display

• For most rod shape LC,the rod will point // to
  E-field
• By applying E-field, we can control the LC
  orientatione.g. TN operation

OFF
ON
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What is LCD?

• LCD (Liquid Crystal Display)-display panel using
  LC-LC first considered for display in
  1950s-1960s-first produced in 1970s-smallest
  unit for display is pixel-for color display, one
  pixel consists of 3 elements Red, Green, Blue

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How Display becomes possible

• Limitation of Human vision
• Human eyes detect only 3 colors (RGB)-display
  color mixed from RGB is detail enough-achieve
  full color display

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How Display becomes possible

• For human eyes, motioncontinuous pictures-if
  refresh rate is fast enough,?motion can be
  displayed smoothly.
• However, human eyes can detect any defects less
  than 1 pixel in size-display panel have to be
  produced with accurate techniques.-cannot make a
  large panel by merging small panels

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LC Cell

• Major unit of LCD
• Have many types7-segment, active matrix.
• But their structure is similar

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Structure of LC cell
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Manufacture of LC cell

• 1. Glass preparation-alkali-free glass with
  higher melting point is chosen-uniformity of
  flatness should be controlled within 0.2 µm (cell
  thickness6-8 µm)gtsurface polishing-passivation
  layer is added (SiO2 film)

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Manufacture of LC cell

• 2. color filter layer-thickness2-3 µm-good
  smoothness and strength are needed

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Manufacture of LC cell

• 3. electrode-transparent ITO (Indium Tin Oxide)
  film-active matrix TFT (Thin Film Transistor)
  is added on each pixel
• 4. alignment layer-put polyimides on glass and
  make pretilt angle by rubbing

LC
alignment layer
glass
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Manufacture of LC cell

• 5. sealing and spaceing-sealing prevent LC from
  reacting with air-area inside is
  vaccum-spacing control cell thickness-use
  micro glass fibres/plastic balls

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Manufacture of LC cell

• 6. filling LC-inject LC into sealing area
  inside-usually several kinds of LCs are
  used-with some chiral components-common type
  Twisted Nematic (TN),Supertwisted Nematic (STN)
• 7. polarizers and backlight system-two
  polarizers penpedicalur to each other-light
  which peaks at RGB and saves power is preferred

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Input of video signal to LCD

• Different E-field gt Pixel ON/OFF
• But, how to control Vdiff at the 2
  electrodes for EVERY Pixel?
• Current methodActive Matrix Addressing with
  help of Thin Film Transistor (TFT)

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Input of video signal to LCD

• The layout of the control structure

Data Lines
Gate Lines
TFT
LC cell / pixel
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Input of video signal to LCD

• TFT
• Compose of thin films of semiconductor
• gtSmall size
• Same as ordinary transistor
• Gate(G) voltage controlsSource(S) to Drain(D)
  resistance
• Works like water tapGate Voltage On/Offgt
  current flow/no flow

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Input of video signal to LCD

• The control process

Data Lines control this row pixels independently
This Gate Line is ON
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Input of video signal to LCD

• The LC cell has some capacitance,
• even when data lines are controlling another row,
• some charge maintains the E-field.
• By refreshing row by rowthe image was inputted
  to the LCD

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Contribution of LCD to HDTV

• Now, HDTV is a big-hit topic in HK.Yet, do you
  know what is HDTV?And how LCD contributes in
  this way?
• HDTV (High Definition television)-resolution
  1920x1080 pixels-high contrast ratio, large
  size-digital signal (less loss)-people talked
  about it since 1980s, but it was developed well
  in about 10 years ago

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Contribution of LCD to HDTV

• Due to HDTV, a strange phenomenon appears
  CRT-gtLCD, why?
• First, let see CRT (Cathode Ray Tube)-good
  reaction time (lt1ms)-wide viewing angle-good
  gray scales, color saturation-low cost

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Contribution of LCD to HDTV

• Yet,-take up many spaces, heavy-consume energy
  -produce X-ray and low-frequency B-field-image
  burn-in problem-overlapping pixelsgtx high
  resolution-difficult to make a large
  screengtbrightness depends on sizegtdeflection
  angle of electron beam limits the size
• Therefore, CRT is not good for HDTV

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Contribution of LCD to HDTV

• Now, let see LCD-distinct pixels-large panel
  is possible-thin, light panel-low power
  consumption-no radiation produced
• Although LCD is more expensive than CRT, LCD is a
  good choice for HDTV

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Contribution of LCD to HDTV

• In market, there are PDP (Plasma Display Panel)
  and OLED (Organic Light Emitting Diode)
• So, why LCD dominates?

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Contribution of LCD to HDTV
Which one will you choose?
RemarkRecent technology overcomes viewing angle
problem of LCD (178o)
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Contribution of LCD to HDTV

• The relatively low price of high-quality LCD TV
  gives chance to more people to afford the cost of
  watching HDTVgtpopularize HDTV broadcast

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Contribution of LCD to HDTV

• HDTV has come to PC
• LCD-thin, light, low power consumption, low
  pricegtgood for both desktop and notebook

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Summary

• LC can flow and is ordered
• LC has anisotropic optical properties related to
  its alignment
• Anisotropic optical property lead to change of
  light polarization
• LCD achieve display by controlling LC alignment
  by E-field and using polarizer
• Due to human vision, display can be made in much
  easier way
• Structure of LC cells is similar
• LCD contributes a lot to HDTV

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Game over
Continue? 9
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Optics of anisotropic media

• Isotropic Different Direction are same
• Anisotropic Different Direction are different
• Two Types of Anisotropic media
• 1.Uniaxial
• If xyz axis are well chosen,
• View from x-direction
• View from y-direction

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Optics of anisotropic media

• 2) Biaxial
• No matter how you chose axis,
• no x-y symmetry as in uniaxial
• case

In display, usually uniaxial LC are used.