LCD Panel

TTsn Tsn T Tni Tni T
Solid
Liquid Crystal
• Tsn : Solid to nematic transition temperature • Tni : Nematic to isotropic transition temperature
Liquid
Phase transition of nematic LC by temperature
Parrallel Polarizer
Cross Polarizer
3. Liquid Crystal Display
• Alignment of LCs : In the presence of an applied electric field, LC molecules are aligned parallel to the applied electric field to minimize the electrostatic energy. • Normally White Mode : Light is transmitted in the absence of an electric field and is blocked in the presence of a field.
Silicon Oxide : Poor uniformity and difficulty in mass production.
Polyvinyl alcohol : Poor electric and chemical characteristics.
• Alignment Methods
Rubbing : UV Exposure : Inclined SiO2 deposition :
H
H
C
C
Liquid Crystal Phase Change by temperature Water
Solid Solid
Liquid Crystal
at 0 ° C
Liquid Liquid
• LC are organic materials : a rod-like shape or flat(disk like) shape : Nematic liquid Crystal(LC) is used for display application.
Δε=ε//-εㅗ :
<ε> = (ε//-2εㅗ)/3 ne : no : Δn = ne - no : K11, K22, K33 :
Dielectric Anisotropy
Mean dielectric constant Refractive index parallel to LC director Refractive index perpendicular to LC director Birefringence Splay, twist, bend elastic constants
Why needs pretilt angle
: Prevention of reversetilt flaw in voltage impressing. • If TN liquid crystal having no polarity and no chirality, the rodlike molecules can twist in either direction in the absence of a small pretilt angle;both right handed left handed twist of 90° are possible. • When a small pretilt angle exist, only the right handed twist is possible. Aligning flaw • In the case of the pretilt angle 0° Zero Pretilt Angle
• Molecular Orientation in Liquid Crystal Phases
n
n
F
F
n
n Director
Nematic LC • Chemical structure of a Liquid Crystal
Smectic LC
Cholesteric LC
Ring : Benzene rings or cyclohexanes X A Y X : Tetminal group, CnH2n+ 1, CnH2n+1O Y : Terminal group, CnH2n+ 1(Non polar), CN, F(polar) A : Linking group, C2H4, -C=C-, -N=N-, CH=N, COO….. • Liquid Crystal Parameters ε// : εㅗ : Dielectric constant parallel to LC director Dielectric constant perpendicular to LC director
Nematic LC • Anisotropy
Discotic LC
Due to the orientational ordering of the rod-like shaped molecules, the nematic LCs are uniaxially symmetric. As a result of the uniaxial symmetry, the electro-optical characteristics are different in value along the preferred axia and perpendicular to this axis.
5. How to make a single cell to cell array
• Classification of Liquid Crystal Display LCD Passive Matrix LCD or simple Matrix LCD TN(Twist Nematic) STN ( Super twist nematic) ECB( Electrically controlled birefraingence) Active Matrix LCD Two terminal Device : MIM diode, pin diode Three terminal Device: a-Si TFT Poly-Si TFT : High temperature poly Si TFT : Low temperature poly Si TFT Cdse TFT MOSFET: silicon substrate ex. LC on Silicon(LCOS
4-1. Liquid Crystal Display(Normally White)
•How to control light intensity
Transparent Electrodes
Transmittance
100 (%)
50
0 Applied Voltage

Typical T-V Characteristics (Normally white mode).
4-2 Alignment Materials
• Without AL: Liquid Crystal molecules become randomly oriented on the substrate which does not have alignment material.
• AL Coated surface Liquid crystal become mono -domain (or single domain) on the substrate which has surface treated alignment material by rubbing.
a
b
Aligning direction
c
Reverse tilt disclination
a) W/O Alignment layer b) With Alignment layer and surface treated uniformly c) With Alignment layer but surface treated nonuniformly • Alignment materials Polyimide : Good electric and chemical characteristic and good uniformity and suitable for the mass production.
• LC molecules can have a long range order in the case of imposing boundary condition (ex.rubbing). As a consequence of the molecules alignment, Nematic liquid crystals exhibit a strong optical birefringence. • Within the optically active media(ex.LC), the plane of polarization of a beam of light traversing through them rotates.
TFT-LCD
1.What is Liquid Crystal
• Liquid + Crystal Properties • Liquid crystal is a state of matter that is intermediate between the crystalline solid and the amorphous liquid. It may also be viewed as a liquid in which an ordered arrangement of molecules exist.