LCD Or 3 M DEBF Twisted Nematic Liquid

簡介 LCD結構圖 Or 3 M DEBF

Twisted Nematic Liquid Crystal Displays Color Filter Conducting Layer (ITO) Alignment Layer Backlight TFT: Thin Film Transistor

¡ Classification of liquid crystal: Based on their symmetry of molecular arrangements, liquid crystals are divided into three major classes: (1) Nematic Phase (向列相) (2) Cholesteric Phase (膽固醇相) A distorted form of the nematic phase (1) Smectics (近晶相)

Liquid Crystals (LCs) LC mesophases Solid Smectic C Crystalline Tm Smectic A Liquid Nematic Isotropic TNI Temp.

The Chiral Nematic Ordinary Nematic Chiral Nematic CN CN director n pitch P

Chirality H H H C N H-C-C-C H H H non-chiral H H H C N H-C-C-C CH 3 H H mirror images chiral (RH) The methyl group on the 2 nd carbon atom on the alkyl chain of the molecules extends out of the plane of the paper and the hydrogen atom extends into the plane of the paper. Therefore the 2 nd carbon can be thought of as a right or left handed coordinate system

Phase shift in a birefringent material E-wave O-wave Phase retardation Δn= ne-no>0 正單光軸 Δ = e- o =2 πd ( 1/λe-1/λ 0) =2πd(ne-no)/ λv =2πdΔn/λv Important parameter Δn= ne-no<0 負單光軸

Tipping the director by the angle Ө E-wave O-wave

Birefringence of LC ITO Glasses Index Ellipsoid: x 2/no 2 + y 2/no 2 + z 2/ne 2 = 1 ( For an uniaxial crystal: nx=ny=no, nz= nene) z (optical axis) Ex k Ey Inclined plane ne ITO Glasses O n Ez 45 o Ex k x=x’ d =2 nd/ =π/2 z’ no y o y’ neff ( ) none/(no 2 sin 2 + ne 2 cos 2 )1/2

LC mode?

LCD Modes Source: Sharp

SURVEY OF LIQUID CRYSTAL DISPLAY MODES　 1. 2. 3. Birefringence mode Dichroic Dye mode Scattering mode

Birefringence Mode ▬ TN , STN (Polarization Rotation ) ▬ ECB-electrically controlled birefringence (or phase retardation) – VA, IPS, HC, Chiral-VA ▬ FLC-Ferroelectric LC ▬ Mixed mode

Twisted Nematic Liquid Crystal Displays Color Filter Conducting Layer (ITO) Alignment Layer Backlight

A Twisting Nematic (扭曲型LCD ) Normally Black : P//A V=0 , Dark State Normally White : P⊥A V=0 , transparent State

TN-LCD作法 1. 2. 3. 4. 5. ITO- cotaed glass Alignment layer (spin coating) Baking Rubbibg Pretilt~20 避免reverse tilt Cell thickness? Rubbing It depends on the birefringence ∆n of LC For 900 TN-LCD between two parallel ideal polarizers (normally Black mode), the transmission is (unpolarized monochromatic)

Transmission of TN LCDs --- NB mode (Gooch and Tarry) Where: X 2 = 2 + 2/4 = 2 nd/ = /2 (Minimum conditions)

Transmission of TN LCDs --- NW mode (Gooch and Tarry) Where: = 2 nd/ = /2 (Maximum conditions)

∆n. d=0. 48 m, 1. 09 m or 1. 68 m 選LC ∆n已知 d可知 6. Nematic LC+少許chiral component 避免reverse twist 7. 灌充即可

Typical cell gap for TN LCDs First minimum condition : 5 m is a typical cell gap for first minimum TN LCDs

TN LCDs ---- threshold voltage Using the previous equations, one can derive the redistribution of the director in a TN cell requires a threshold voltage give by 90 T For most nematic mixtures formulated for TN displays, Vth is typically around 1 V. 100 80 60 40 TN LCD (NW) T-V curve 20 0 0 Vth 0. 5 1 1. 5 2. 0 V

TN LCDs ---- Contrast Ratio →Commonly used

Performance of TN-LCD standard products Viewing angle needs to improve: will discussed later

(2) STN: twisting angle 180 o-270 o STN & TN : Twisting angle ↑ , E-O curve sharpness ↑ → multiplexing capability ↑, but Gray scale more difficult

Multiplexing Capability : Number of multiplexing lines N vs Selection ratio

Electrically controllable birefringence (ECB) (or phase retardation mode) VA (Vertical Alignment) LC ň 投影 Θ=45 o

Performance of MVA-LCD Fujitsu products MVA: Multi-domain VA (wide view angle)

Optical Switching of Surface Stabilized Ferroelectric Liquid Crystal (SSFLC) (Phase retardation mode) ¡ Surface interactions can be used to unwind the spontaneous helix, which yields a uniform FLC alignment with l Fast response l Bistability l Wide viewing angle (IPS)

SSFLCs ‧Book shelf structure Rubbing direction LC molecule

2. Dichroic Dye Liquid Crystal Displays Dichroic dye molecule A∥: Elight ∥ , absorb light A⊥: Elight ⊥ , do not absorb light • Heilmeier dichroic display • Quarter wave dichroic display • Phase change dichroic display Dichroic Ratio (DR) ,