液晶物理学_Chapter_5_Fast_response_liquid_crystal.ppt

1、液晶材料特性及顯示原理-9,Fast Response Liquid Crystals,LCD TV :video rate (rise+ decay16ms) Color-sequential display (rise+ decay5ms),Optical comparison,Response time: TN MVAIPS,Ideal Candidate,Ferroelectric LCD, e.g. Surface-Stabilized Ferroelectric LC (SSFLC) 100s range Bistable,Optical Switching ofSurface S

2、tabilized Ferroelectric Liquid Crystal,Surface interactions can be used to unwind the spontaneous helix, which yields a uniform FLC alignment with Fast response Bistability Wide viewing angle (IPS),SSFLC problems,1. DC voltage effect: Accumulation of ions, and causes serious image sticking after pro

3、longed operation. 2. Thin cell: Limit manufaturing yield.,Major LCDs: nematic LCDs Two major R&D jobs: Response time, viewing Angle,Response Time,Small tilt angle approximation: d:cell gap r1:rotation viscosity K:elastic const. Vth:threshold voltage Vb:bias voltage V:applied voltage,For T-900TN LCD,

4、 dn = 1st Gooch-Tarry Min = Let Vb=0 (free relaxation process) F0MK n2/1 In nematic LCDs, switching between different gray levels=real response time.,900TN LCD,1 2: slowest, 7 8:fastest,8 gray levels,How to improve response time,Temperature effect Material Effects Viscosity effect Elastic constant e

5、ffect Birefringence effect Voltage Effect Bias voltage effect Overdrive effect Undershoot,Surface Stabilization Effect Cell Gap Effect -cell Blinking backlight/Scanning backlight/Black Frame insertion/Gray frame insertion /Black data insertion/Increase frame rate, Temperature effect,Viscosity effect

6、,Macroscopic properties of a LC,These properties (table shown below) are closely related OCF3 group :Good Candidate,Elastic Constant Effect,rise & decay1/K Homeotropic cell: K33 Twist cell:K22 Homogeneous cell:K11 K33K11K22 for most nematics To take advantage of a large K33,the homeotropic alignment

7、 needs to be implemented.,Birefringent Effect n,d2,but d n=constant 1/( n)2 i.e. larger n for thinner cell,Voltage Effect,Shorten rise: Bias Voltage effect:Vb VbVth: LC molecules are already tilted to a certain degree by Vb. Thus, they quickly follow the incoming activation pulses. The higher the Vb

8、, the faster the response time.,Overdrive voltage e.g. switching from gray scale 12 we apply VV2 but a narrower pulse width, and then back to V2 rise pulse,Overdrive Voltage effectEx : 12 1 3 2,8 gray levels,Overdrive Voltage effectEx : 12 1 3 2,Undershoot driving scheme: Shorten decay,decay1.5ms,No

9、 undershoot decay4ms,Surface Stabilization,In PSNLC & PSCT display Polymer network alignment force due to the surface effect shorten decay,Scanning backlight,Black data insertion,Gray frame insertion,Cell Gap Effect,d2,Ex. d=4m, response time =30ms and how to achieve 16ms by using cell gap reduction

10、?,IPS (In-Plane Switching) LCD,Response time limit of the IPS,response time 需d 但d時driving voltage會,Response timedriving voltage 兩者trade off,FFS (fringe field switching) structure,Response time and driving voltage can be optimized at the same time,Response time and driving voltage for Ultra FFS,OCB c

11、ell (-Cell ),The OCB cell is a nematic LC device which can readily be made to exhibit combined “on+off” responses time less 10ms. Wide viewing angle: 160o cone,OCB cell (or -cell),Why fast response? Why wide viewing angle?, -Cell (OCB cell) OCB (Optically Compensated Bend),Self-compensation: Wide vi

12、ewing angle Fast switching: Eliminates backflow,OCB mode: operated in phase retardation mode,V=0,Pre-tilt 2-5o,V=Vbias2V,Von 5 V,Problem: difficult to obtain uniform splay to bend transition among pixels.,Bend 1 state,The director at the cell center aligns along the electric field, perpendicular to

13、the cell surface (refer to last pape). Thus, during switching, the applied voltage causes director distortion only in the vicinity of the cell surfaces, with the center remaining permanently along the filed. It means that, during switch, the two surface regions of an OCB cell are thus decoupled, it is this feature, toge