Liquid Crystals for Large Display&

Basic Theory

Dr. Md. Lutfor Rahman

FIST, UMP

Introduction

• LCDs based on the twisted nematic (TN) mode are the most common flat panel displays.

• The conventional TN LCDs utilized in wristwatches or desk calculators can only contain a small number of matrix segments (e.g. 8 X 8), and therefore cannot be applied for displays with high information content.

• Further development of the TN cell resulted in thin film transistor (TFT) LCDs.

• This technology enables active switching of a large number of segments (e.g. 640 X 1024) by integrated TFTs.

• Even though TFT LCDs made a great leap forward in performance compared to the conventional TN LCDs, they kept their advantages like low weight, low space requirement and low power consumption.

• Based on the unique combination of properties of LCs and TFTs, a totally new product, the notebook came up in the beginning of the 1990s.

• The display performance and size of TFT LCDs had been dramatically improved since that time as a consequence of the continuous development of electronics and materials.

• Whereas in the first half of the 1990s the cursor could not follow fast movement of the mouse, multimedia applications were realized for the first time by the introduction of fast switching 15 inch TFT LCDs in 1998.

• As the visible area of a 15 inch TFT LCD corresponds to that of a 17 inch cathode ray tube (CRT) it was obvious to consider use in desktop monitors.

• However, for this application the viewing angle dependency of the optical effect, especially contrast and colour shift, had to be improved.

• This was possible by the introduction of optical compensation films. Even better optical performance is possible after the development of new switching modes, the so-called in plane switching (IPS) and the vertically aligned (VA) mode in the end of the 1990s.

• Computer monitors using these techniques have already replaced CRT monitors to a large extent.

• However, the hurdles of LCDs for TV use were far higher than for notebooks and monitors, because for the first time the display of full moving pictures had to be realized.

• Therefore a switching time under the so-called frame time (vide infra) of 16 ms is required. This one frame switching time should be achieved together with high brightness, high contrast and good colour quality.

• Before reviewing the LCDs material development we give an overview on basic requirements of the TV application and the essential physical properties of liquid crystals.

Basic requirements and physical properties

• The prerequisite for commercial LCs is a broad nematic phase range of -40 to 100°C in order to guarantee the so-called Operating temperature range of LCDs.

• The clearing point of a liquid crystal is the temperature at which the liquid crystal phase vanishes.

• It has to be at least 10°C higher than the operating temperature of the device.

• In order to respond to an applied switching voltage LCs must exhibit a dielectric anisotropy (Δε), defined as the difference of the dielectric constants parallel and perpendicular to the director of the nematic phase.

• Depending on the molecular structure the dielectric anisotropy can be positive (molecular dipole parallel to the long axis of the molecule) or negative (molecular dipole perpendicular to the long axis of the molecule).

• Elastic constants Ki are the proportional constant between the force (the electric field in case of LCDs) and the deformation of director fields.

• There exist three Ki (i = 1.2.3) dependent on the deformation of the director (splay, twist, bend).

• The operating voltage is proportional to the square root of the fraction between elastic constants and dielectric anisotropy.

• The reorientation of the LCs upon switching depends on the configuration of the display and the switching mode.

• The basis of the visible electro-optical effect is the birefringence (∆n), which is defined as the difference between the extraordinary refractive index (light propagation parallel to the director) and the ordinary refractive index (light propagation perpendicular to the director).

• Upon switching the reorientation of the LC molecules leads to an effective change of the optical path which is defined as d∆n (d = cell gap).

• This results in a change of the transmission of the display between 0 and 100%.

• The switching time of an LCD is proportional to the rotational viscosity (γl) of the liquid crystal.

• Low γl is an absolute perquisite for the TV application. Each pixel in a display is driven by a TFT charging the pixels by signal pulses.

• The voltage has to be sustained till the next refresh signal pulse arrives (this time is one frame time).

• The voltage drop during one frame time is characterized by the voltage holding ratio (VHR), which is defined as the ratio of the voltages at a pixel at the end and the beginning of the frame time.

• A high VHR is important for a flicker free picture.

• All structural elements (side chain, rings, linking groups,

terminal groups) of a liquid crystal molecule contribute to the

physical properties (Table 1).

• For example benzene rings with polar substituents contribute

to the dielectric aniosotropy significantly.

• In addition the aromatic ring contributes to higher values of

the optical aniosotropy compared to a cyclohexane ring.

Δn Δε γ1

0.15 – 0.26 8 – 35-2- (-7)

80 – 260

0.23 – 0.28 15 – 20 250 – 500

Z

Table 1: Characteristic Δn, Δε and γ1 ranges for different core structures with polar substitutions. Z can be a COO group, a CF2O group, a CH2CH2 group or a single bond.

• As a consequence we empirically find limitations for high polar liquid crystals with low ∆n values.

• Other contradictory requirements are the coexistence of high clearing point and low viscosity.

• Thus, it is very difficult to achieve LCs with a high clearing point (broad operating temperature range) and low viscosity (fast switching) or LCs with high polarity (low operating voltage) and low viscosity.

• However, these are the decisive LC properties for LCD products (Table 2). It means that we have to achieve breakthroughs to overcome these phenomenological limitations of LC material properties.

• In Table 1 characteristic property combinations for the most important LC parameters ∆n, ∆ε and γl for different core structures are shown.

• Additional properties like solubility etc. play a decisive role for practical use.

• Moreover liquid crystals must be chemically, photochemically and electrochemically stable.

• Empirically VHR values decrease with increasing dielectric anisotropy of the LCs.

• The combination of high polarity with high VHR is therefore

another usually contradictory material requirement.

• Only few super fluorinated materials (SFM) have suitable

property combinations.

• The main material research target is synthesis and

identification of compounds with lower viscosity maintaining

or even improving the other properties.

• Computer simulations for the calculation of ∆ε, ∆n based on

molecular properties such as dipole moments and

polarizabilities are often in good agreement with measured

values of LCs.

• Calculated electrostatic potentials give a very useful hint for the so-called reliability parameters.

• Computer simulation of bulk properties like elastic constants, viscosities and melting points on the other hand are still in an early stage.

• A single liquid crystal compound cannot fulfill the complex requirements of the displays.

• The wide operating temperature range in combination with other

properties requires mixtures of typically 10 to 20 compounds.

• Nevertheless melting points of LC mixtures are not thermodynamically defined phase transitions but metastable states with a very long lifetime.

 

Technology Material requirements

Characteristics

VA – TFT Δε -3 ~ -4Δn ~ 0.08γ1 ~ 100

Low rotational viscosity difficult to achieve for lateral highly flourinated compounds

IPS Δε 7 – 11Δn 0.09 – 0.12

γ1 ~ 70

Inter – digital electrodes / high aperture ratio requires high Δε values

TN – TFT Δε 5 – 6Δn 0.09 – 0.10

γ1 ~ 70

Low Δn values difficult to combine with high positive Δε and low rotational viscosity

Projection Δε ~ 8Δn 0.21γ1 200

High Δn materials with very high light stability required

Table 2: Overview material requirements for the most important active matrix addressed LCDs use in LCD TVs

The Nematic Director n

LongMolecular

Axis

H H

H H

H H

H H

C NOC C

H H

HH

C C

H H

HH

H

n

The local average axis of the long molecular axis

director

n

Temperature Smectic C Smectic A Nematic

nz

n

Other Liquid Crystal Phases

n

The Order Parameter

n

22

1(cos ) (3 cos 1)

2 S P

2

2

2

cos1

cos3

cos ( 0 ) 1

o

d

dno order

perfect order

2

2

(cos ) 1

(cos ) 0

S P

S P

perfect crystal

isotropic fluid

The Order Parameter

It is sometimes difficult to determine whether a material is in a crystal or liquid crystal state.

Crystalline materials demonstrate long range periodic order in three dimensions.

By definition, an isotropic liquid has no orientational order.

Substances that aren't as ordered as a solid, yet have some degree of alignment are properly called liquid crystals.

The Order Parameter

To quantify just how much order is present in a material, an order parameter (S) is defined.

Traditionally, the order parameter is given as follows:

The Order Parameter

where theta is the angle between the director and the long axis of each molecule.

The brackets denote an average over all of the molecules in the sample.

In an isotropic liquid, the average of the cosine terms is zero, and therefore the order parameter is equal to zero.

For a perfect crystal, the order parameter evaluates to one.

The Order Parameter

Typical values for the order parameter of a liquid crystal range between 0.3 and 0.9, with the exact value a function of temperature, as a result of kinetic molecular motion.

Interactions between individual molecules are represented by a potential of average force

2 2cos cosV vP P

Maier-Saupe Theory - Mean Field Approach

• {V: minimum} when phase is ordered (-P2(cos))• {V: V=0} when phase is disordered (<P2(cos)>)• factor for intermolecular strength ( )

n

The Order Parameter: How does it affects display performance ?

The order parameter, S, is proportional to a number of importantparameters which dictate display performance.

Parameter Nomenclature Elastic Constant Kii S2

Birefringence n SDielectric Anisotropy SMagnetic Anisotropy SViscosity Anisotropy S

Example: Does the threshold switching voltage for a TN increase or decrease as the operating temperature increases.

Scales as the square root of S therefore lowers with increasing temperature

2

TH

K SV S

S

proportional to

Anisotropy: Dielectric Constant

Off-axis dipole moment, angle with molecular axis

2

23cos 12o B

NhFS F

k T

N: number densityh,f: reaction field, reaction

cavity parametersS: order parameter: anisotropy in polarizability: molecular dipole momentkB: Boltzman constantT: Temperature

For values of the angle , thedipolar term is positive, and forvalues , the dipolar term isnegative, and may result in a materials with an overall -.

Anisotropy: Dielectric Constant

+++++

- -- --

E

E

++++

----

positive

negative

all angles inthe plane to E arepossible for the- materials

E

Dielectric Constants (@20oC, 1kHz)

*Mixture Application

BL038 PDLCs 16.7 21.7 5.3MLC-6292 TN AMLCDs 7.4 11.1 3.7ZLI-4792 TN AMLCDs 5.2 8.3 3.1TL205 AM PDLCs 5 9.1 4.118523 Fiber-Optics 2.7 7 4.395-465 - material -4.2 3.6 7.8

Materials Dielectric ConstantVacuum 1.0000Air 1.0005Polystyrene 2.56Polyethylene 2.30Nylon 3.5Water 78.54

*EM Materials

Magnetic Anisotropy: Diamagnetism

Diamagnetism: induction of a magnetic moment in opposition to an applied magnetic field. LCs are diamagnetic due to thedispersed electron distribution associated with the electron structure.

Delocalized charge makesthe major contribution to diamagnetism.

Ring currents associated witharomatic units give a largenegative component to for directions to aromatic ringplane. is usually positive since:

0ll ll

Magnetic Anisotropy: Diamagnetism

C 5 H 1 1

C 7 H 1 5

C N

C N

C N

C 5 H 1 1

C N

C 7 H 1 5

C 7 H 1 5

C N

9 3 1/ 1 0 m k g

1 . 5 1

1 . 3 7

0 . 4 6

0 . 4 2

- 0 . 3 8

Compound

Optical Anisotropy: Birefringenceordinary ray (no, ordinary index of refraction)

extraordinary ray (ne, extraordinary index of refraction)

Optical Anisotropy: Birefringenceordinary wave

extraordinary wave

on n2 2

2 2 2

1 cos sin

o en n n

For propagation along the opticaxis, both modes are no

optic axis

Birefringence (20oC @ 589 nm)

EM Industry n ne no Application Mixture BL038 0.2720 1.7990 1.5270 PDLCTL213 0.2390 1.7660 1.5270 PDLCTL205 0.2175 1.7455 1.5270 AM PDLCZLI 5400 0.1063 1.5918 1.4855 STNZLI 3771 0.1045 1.5965 1.4920 TNZLI 4792 0.0969 1.5763 1.4794 AM TN LCDsMLC-6292 0.0903 1.5608 1.4705 AM TN LCDsZLI 6009 0.0859 1.5555 1.4696 AN TN LCDsMLC-6608 0.0830 1.5578 1.4748 ECB95-465 0.0827 1.5584 1.4752 - devicesMLC-6614 0.0770 --------- --------- IPSMLC-6601 0.0763 --------- --------- IPS18523 0.0490 1.5089 1.4599 Fiber OpticsZLI 2806 0.0437 1.5183 1.4746 - device

Birefringence Example: 1/4 Wave Plate

Unpolarized

linear polarized

circular polarized

polarizerLC: n=0.05d

What is minimum d forliquid crystal 1/4 wave plate ?

1

41

41 589

2,950 2.954 4 0.05

e o

e o

N N

n d n d

nmd nm m

n

Takes greater number of e-waves than o-waves to span d, use n=0.05

Surface Anchoring

microgrooved surface -homogeneous alignment (//)rubbed polyimide

ensemble of chains -homeotropic alignment ()surfactant or silane

Alignment at surfaces propagates over macroscopic distances

Surface Anchoring

N

n

polar anchoring W

azimuthalanchoring W

surfa

ce

Strong anchoring 10-4 J/m2

Weak anchoring 10-7 J/m2

W, is energy needed to move director n from its easy axis

Creating Deformations with a Field and Surface - Bend Deformation

E or B

Creating Deformations with a Field and Surface - Splay Deformation

E or B

Creating Deformations with a Field and Surface - Twist Deformation

E or B

Magnitudes of Elastic Constants

EM Industry K11 K22 K33

Mixture (pN) (pN) (pN) Application

BL038 13.7 ------ 27.7 PDLCTL205 17.3 ------ 20.4 AM PDLCZLI 4792 13.2 6.5 18.3 TN AM LCDZLI 5400 10 5.4 19.9 TNZLI-6009 11.5 5.4 16.0 AM LCD

Order of magnitude estimate of elastic constant

U: intermolecular interaction energy: molecule distance

146 11

8

1010 10 10

10ii

U ergsK dynes N pN

cm

Elastic Constant K22: Temperature Dependence

7

6

5

4

3

2

-30 -20 -10 0T-TNI (°C)

K22

(x

10

-12

Ne

wto

n)

P-azoxyphenetole

P-azoxyanisole (PAA)

2( )K S T

Viscosity: Shear Flow Viscosity Coefficient

n

v

v n v nvn v

Typically > >

( )

( )

shear stress

velocity gradient

v

n nn

Viscosity: Flow Viscosity Coefficient

Dynamic Viscosity 1 kg/m·s = 1 Pa·s 0.1 kg/m·s = 1 poise

Kinematic Viscosity 1 m2/s

31000

kg

m

LC specification sheets givekinematic viscosity in mm2/s

Approximate density

Viscosity: Flow Viscosity Coefficient

2

2 2 3 33

120 / 20 / 10 / 0.02 / 0.2

10ii

mmm s mm s kg m kg ms poise

mm

Typical Conversion Density Conversion Flow 0.1 kg/ms = 1 poiseViscosity

EM Industry Kinematic () Dynamic () MIXTURE CONFIGURATION (mm2/s) (Poise)

ZLI-4792 TN AM LCDs 15 0.15ZLI-2293 STN 20 0.20MLC-6610 ECB 21 0.21MLC-6292 TN AM LCDs (Tc=120oC) 28 0.28

18523 Fiber Optics (no=1.4599) 29 0.29

TL205 PDLC AM LCD 45 0.45BL038 PDLCs (n=0.28) 72 0.72

Viscosity: Temperature Dependence

For isotropic liquids

0 expisoB

E

K T

E is the activation energy for diffusion of molecular motion.

H3CON C4H9

1.0

0.7

0.4

0.2

0.120 30 40 50 60

2

3

1

TNI

Vis

cosi

ty (

pois

e)

Temperature (°C)

n

Viscosity: Rotational Viscosity CoefficientT

ime

n

n

Rotation of the director n bv externalfields (rotating fields or static).

Viscous torque's v are exerted on a liquidcrystal during rotation of the director n and by shear flow.

1v

d

dt

rotational viscosity coefficient

n

Viscosity: Rotational Viscosity Coefficient

nn

EM Industry Viscosity Viscosity MIXTURE CONFIGURATION (mPas) (Poise)

ZLI-5400 TN LCDs 109 1.09ZLI-4792 TN AM LCDs 123 1.23ZLI-2293 STN 149 1.4995-465 - Applications 185 1.85MLC-6608 TN AM LCD 186 1.86

1 3

1109 109 0.109 0.109 / 1.09

10

PamPa s mPa s Pa s kg m s poise

mPa

Viscosity: Comparisons

Material Viscosity (poise)

Air 10-7

Water 10-3

Light Oil 10-1

Glycerin 1.5

LC-Rotational (1) 1< 1 < 2LC-Flow (ii) 0.2< ii<1.0

Sur

face

x

Relaxation from Deformation

E

Sur

face

x

field on state

zero field state

Relaxation when field is turned off Relaxation time

Defects

s=+1 s=+1 s=+1

s=1/2 s=-1/2 s=-1

s=3/2 s=+2

The singular line(disclination) is pointing out of the page, and director orientation changes by2s on going around the line (s is the strength)

AX Y

Z Z’

• Aromatic or saturated ring core• X & Y are terminal groups• A is linkage between ring systems• Z and Z’ are lateral substituents

CH3 - (CH2)4C N

4-pentyl-4’-cyanobiphenyl (5CB)

General Structure

Mesogenic Core Linking Groups Ring Groups

N

N

phenyl

pyrimidine

cyclohexane

biphenylterphenyldiphenylethanestilbenetolaneschiffs baseazobenzeneazoxyben-zenephenylbenzoate(ester)phenylthio-benzoate

CH CH2 2

CH CH CH CH CH N

N N

N N

O

C O

C S

O

O

Common Groups

NomenclatureMesogenic Core

phenylbenzylbenzene

biphenyl terphenyl

phenylcyclohexane (PCH)cyclohexane cyclohexyl

Ring Numbering Scheme

3’ 2’

1’

6’5’

4’

32

1

6 5

4

Terminal Groups

(one terminal group is typically an alkyl chain)

CH3

CH2

CH2

CH2

CH3

CH2

C*H

CH2

CH3

straight chain

branched chain (chiral)

Attachment to mesogenic ring structureDirect - alkyl (butyl)Ether -O- alkoxy (butoxy)

CH3-

CH3-CH2-

CH3-(CH2)2-

CH3-(CH2)3-

CH3-(CH2)4-

CH3-(CH2)5-

CH3-(CH2)6-

CH3-(CH2)7-

methyl

ethyl

propyl

butyl

pentyl

hexyl

heptyl

octyl

CH3-O-

CH3-CH2-O-

CH3-(CH2)2-O-

CH3-(CH2)3-O-

CH3-(CH2)4-O-

CH3-(CH2)5-O-

CH3-(CH2)6-O-

CH3-(CH2)7-O-

methoxy

ethoxy

propoxy

butoxy

pentoxy

hexoxy

heptoxy

octoxy

Terminal Groups

Second Terminal Group andLateral Substituents (Y & Z)

H -F flouroCl chloroBr bromoI iodoCH3 methylCH3(CH2)n alkylCN cyanoNH2 aminoN(CH3) dimethylaminoNO2 nitro

phenyl

cyclohexyl

Odd-Even EffectClearing point versus alkyl chain length

0 1 2 3 4 5 6 7 8 9 10 11 carbons in alkyl chain (n)

cle

arin

g po

int

18

16

14

12

10

CH3-(CH2)n-O O-(CH2)n-CH3C-O

O

CH3-(CH2)4C N

CH3-(CH2)4-O C N

4’-pentyl-4-cyanobiphenyl

4’-pentoxy-4-cyanobiphenyl

Nomenclature

Common molecules which exhibit a LC phase

Structure - Property

N

N

CH3-(CH2)4C N

vary mesogenic core

A

A C-N (oC) N-I(oC) n

22.5 35 0.18 11.5

71 52 0.18 19.7

31 55 0.10 9.7

Structure - Property

CH3-(CH2)4COO

vary end group

X

X C-N (oC) N-I (oC)

HFBrCNCH3

C6H5

87.592.0115.5111.0106.0155.0

114.0156.0193.0226.0176.0266.0

Lateral Substituents (Z & Z’)

AX Y

Z Z’

• Z and Z’ are lateral substituents • Broadens the molecules• Lowers nematic stability • May introduce negative dielectric anisotropy

S-N <-40 C solid nematic transition (< means supercools)

Clearing +92 C nematic-isotropic transition temperature

Viscosity (mm2 /s) flow viscosity, some materials may stipulate the+20 C 15 rotational viscosity also. May or may not give 0 C 40 a few temperatures

K33/K11 1.39 ratio of the bend-to-splay elastic constant

5.2 dielectric anisotropy

n 0.0969 optical birefringence (may or may not give ne, no)

dn (m) 0.5 product of dn (essentially the optical path length)

dV/dT (mV/oC) 2.55 how drive voltage changes as temperature varies

V(10,0,20) 2.14V(50,0,20) 2.56 threshold voltage (% transmission, viewing angle,V(90,0,20) 3.21 temperature)

EM Industry Mixtures

Property ZLI 4792 MLC 6292/000 MLC 6292/100S-N <-40 C <-30 C <-40 C

Clearing +92 C +120 C +120 C

Viscosity (mm2 /s)+20 C 15 28 25 0 C 40 95 85 -20 C 160 470 460 -40 C 2500 7000 7000

K33/K11 1.39 ------- ------

5.2 7.4 6.9n 0.0969 0.0903 0.1146

dn (m) 0.5 0.5 0.5dV/dT (mV/C) 2.55 1.88 1.38

V(10,0,20) 2.14 1.80 1.38V(50,0,20) 2.56 2.24 2.25V(90,0,20) 3.21 2.85 2.83

EM Industry Mixtures