当代显示技术 第一章

当代显示技术 第一章

H S KWOK

Copyright 9/1998Hong Kong University of Science & Technology

当代显示技术 第一章

1. Introduction

1.1 Displays

1.2 Various kinds of displays

1.3 Display market

1.4 Cathode ray tubes

2. Basic structure of LCDs

3. Material science One

3.1 Properties of LC materials

3.2 Classification of LC

3.3 LC mixtures

4. Fundamentals of optics

4.1 Colours

4.2 Reflection and refraction

4.3 Light polarization

4.4 Birefringence

4.5 Retardation plates

5. LCD director and alignment

5.1 LC director

5.2 Director alignment on the boundary surface

5.3 Elastic deformation of the LC director

5.4 Some special examples

5.5 Details on T-cells

5.6 Bistability of LC alignment

6. Electro-mechanics of LCD – director deformation

under an electric field

6.1 Introduction

6.2 LC dielectric anisotropy

6.3 Dielectric energy

6.4 Euler-Lagrange equations

6.5 Threshold voltage and Frederick transition

6.6 Response times

当代显示技术 第一章

7. Polarization optics – Jones Matrix

7.1 2x2 matrix

7.2 Coordinate transformation

7.3 LCD Optics modeling

7.4 Jones matrix of uniformly twisted nematic cells

7.5 Eigenvalues and eigenvectors of MLC

7.6 Parameter space

8. LCD optical modes

8.1 ECB modes

8.2 Waveguiding modes

8.3 Mixed modes

8.4 Reflective modes

9. Electro-optic properties of LCD

9.1 Transmission voltage curve

9.2 Contrast ratio

9.3 Viewing angle

10. Several LCD types

10.1 TN

10.2 ECB

10.3 Guest-host display

10.4 Cholesteric (Phase change) display

10.5 PDLC

10.6 Ferroelectric LC

11. Material science Two

11.1 Glass

11.2 Polarizer

11.3 Alignment layer

11.4 Spacers

11.5 ITO

11.6 Retardation film

12. LCD measurements

12.1 Tilt angle

12.2 Cell gap

当代显示技术 第一章

12.3 Electro-optic curve

12.4 Viewing angle

13. LCD electronics

13.1 Driving frequency

13.2 Direct drive

13.3 Multiplexing – segment displays

13.4 Alt and Pleshko law

13.5 Multiline addressing

13.6 Grayscale

14. STN displays

14.1 Twist angles

14.2 Different STN modes

14.3 STN LC mixture

14.4 Film compensated STN

15. Active matrix LCD

15.1 Microelectronics

15.2 Amorphous silicon

15.3 Diode addressing

15.4 Thin film transistors

15.5 Poly-silicon TFT

15.6 Chip on glass

16. LCD manufacturing processes

17. Radiometry and photometry

17.1 Ocular response

17.2 Blackbody radiation

17.3 Photometric units

17.4 Color charts

当代显示技术 第一章

Chapter 1. IntroductionDisplays are the conduits to the information superhighway.Monitors, TV, sign boards, etc

Display requires knowledge in materials science, opticalphysics and electronics.

Electronics

Materials

ScienceOpticsProgress in each area can affect the performance of thedisplay. e.g. new LC materials, new polarizer, new electronicdriving schemes, new optical modes for LCD.

当代显示技术 第一章

Information displays:Fixed displays

Electronic displays

Electronics displays:Emissive

Non-emissive, light control

Emissive displays:Neon light

Cathode ray tube (CRT)

Light emitting diode (LED)

Electroluminescent display (ELD)

Plasma display panels (PDP)

Vacuum fluorescent

display (VFD)

Field emission display (FED)

Flat CRT

Organic ELD (OELD)

Organic LED (OLED)

Non-emissive display:Mechanical displays

Liquid crystal display (LCD)

Digital mirror display (DMD)

Electrochromic display (ECD)

Electronic ink (e-ink)

Liquid crystal light valve (LCLV)

当代显示技术 第一章

Display classification:

Direct view: All of the above

technologies

Projection:

Based on LCLV or CRT

Front projection

Rear projection

当代显示技术 第一章

The deflection coils may be replaced by electrostatic fields.The einzel lens is like optical lens for light.

Phorsphor efficiency: near 90%.

当代显示技术 第一章

Chapter 2. Basic structure of a liquidcrystal display:

SpacerCell gap = 2 - 20 microns

Other materials used but not shown:

Silver dots for connecting top ITO ground to bottom

glass plate

Epoxy perimeter seal

Epoxy end seal

Dopant for liquid crystal

PI adhesion promoter

Light diffuser/reflector

Connectors to electronics

当代显示技术 第一章

LCD operates by polarization manipulation of light using

electrical pulses. Polarization change => brightness changes.The effect of the applied voltage is first felt by the LC

molecules, which rotate and align themselves in the electricfield. This alignment change will then affect the optical

properties and the polarization of the light passing through it.

Common model of a TN display:

We shall see that this picture is slightly inaccurate later!

Need to understand several things for the basic operation ofLCD:

1. What is liquid crystal?

Phase transition

当代显示技术 第一章

2. How can we align liquid crystals?

Boundary conditions

3. How does an applied voltage affect the alignment?

Elasticity, dielectric anisotropy

4. How does the alignment affect the optics?

Optics of anisotropic and electro-active

materials

For further understanding and design of LCD:

1. Flow dynamics of LC – response time

2. Optical modeling of LCD

3. Optical modeling with additional optical elements

such as retardation films1. Transmittive: The LCD is viewed in transmission. This isoften the case for LCD with backlighting.

PolarizerLCAnalyzer

cell

当代显示技术 第一章

There is no reflector.

2. Transflective: This is the most popular. The rear reflector is

often a diffusive type reflector to eliminate mirror effects.

Reflector

PolarizerLC

cellAnalyzer

Ambient light or front lighting is used.

This transflective LCD is simply 2 transmittive LCD in tandem.3. Reflective: In a truly reflective LCD, there is only one frontpolarizer. This is a new development with HKUST being oneof the active participants.

当代显示技术 第一章

Reflector

PolarizerLC

cell

The advantages are obvious. There is one less polarizer andalso the reflector can be placed inside the LC cell.

当代显示技术 第一章

Chapter 3. Material Science One:A liquid that has partial crystalline structure.

Ordinary liquid: molecules are randomly oriented

(amorphous)

Perfect crystal: all atoms are fixed at regular intervals.

Only in solids (crystalline).

当代显示技术 第一章

Usual behavior of liquid:

melting

point

LiquidSolid

(am

(crystalline)

Liquid crystal material:

clearingmelting

pointpoint

LiquidLiquidSolid

(amorphous)crystal

(crystalline)

Melting temperature Tm

Clearing temperature Tcl

Liquid crystal is useful only between Tm and Tocl.

The typical useful range is between –20 C and 80o CCan be as wide as –30o to 120o C

当代显示技术 第一章

Thermotropic: State of the liquid is determined by

temperature. All liquid crystal displays are of this

type.

Lyotropic:State of the liquid is determined by concentration.

Useful in biological systems.

Amphitropic: Both temperature and concentration are

important.

Shapes of the liquid crystal molecule:

Calamitic: Rod shape. All common LCDs are of this type.Discotic:Disk like shape.

Phases of liquid crystals:

Nematic (N): All the LC molecules point in the same

direction. All common LCD are of this type.

Smectic (Sm): The LC molecules point in the same direction

and form layers.

Cholesteric (Ch): Twist angle between molecules. Same as

chiral nematic.

Structures of the liquid crystal “crystal”:

In a perfect crystal, all the molecules are fixed in space and

orientation – no freedom

当代显示技术 第一章

In a liquid, all the molecules are free to move and rotate –

complete freedom

Nematic LC are fixed in orientation, free in translation

Smectic LC are fixed in orientation, fixed in z direction motion,

and somewhat fixed in the x-y plane.

当代显示技术 第一章

Smectic B (SmB) forms a hexagonal structure in the x-y plane.(almost like a perfect crystal)

Chiral Smectic C (SmC*) is also called ferroelectric LC. Moreon that later.

Structurally, cholesteric = chiral nematic (N*) = twisted nematicTN LCD = twisted nematic LCD

STN LCD = supertwisted nematic LCD

(All nematic.)

Chiral = twist. Can be induced by chiral dopants, or intrinsic inthe LC molecule itself.

Whether a cholesteric is called a TN or STN or SSTN (Ch)

depends on the natural pitch.

p

TN pitch – 100 µm

STN pitch – 10 µm

Chpitch – 1 µm

当代显示技术 第一章

Common LCDs are made with nematic, thermotropic and

calamitic (rod shape) liquid crystals.

Examples of common LC materials:

MBBA (p-methyoxybenzylidene-p’-butylaniline)

CH3

O -

- CH = N -- C4H9

However, most nematic LCDs are doped to have a twiststructure. TN, STN.

For any LC, we can describe the orientation of the moleculesby the director n. For the simplest case

n = n(z)

where z is distance perpendicular to the LC cell. The directorn is fully defined by the polar angle φ and the azimuthal angleθ. For a LC cell, the tilt angle is defined as 90-θ. It is the anglebetween the director and the x-y plane.For nematic LC, we can define an order parameter S todescribe the degree of ordering:S = 13cos2θ 12

当代显示技术 第一章

where we have defined the direction of the LC as the z-axis.Note that this is different from a LC cell. The averaging over φis zero. S is a measure of randomness of n. n is distributedabout the z-axis in a cone. Thereforeπ1π2S = ∫(3cosθ 1)f(θ)sinθdθ/∫f(θ)sinθdθ200

where f(θ) is the distribution function of θ.

Check: if f=constant, i.e. the director is randomly distributed,

S = 0

If f(θ) = δ(0), i.e. the director is perfectly pointing in the samedirection,

S = 1.

Maier-Saupe Theory shows that in general yTS = 1 T cl β

当代显示技术 第一章

where y is of the order of 0.98 and β is in the range of 0.13-0.18. Hence the S for all LC are more or less the same in Tdependence if plotted against normalized temperature T/Tcl.

The following is the case for M15 and M21.Most LC molecules are of the structure:

AB

Y - -X- - Z

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