Chapter 10. Several LCD types

Chapter 10. Several LCD types 10.1 TN displays 90o TN LCDs are by far the most common LCD. It is based on the waveguiding mode, either in the first or second minimum. It is used in watches, calculators and simple games. AMLCD also uses the TN mode. The optics of the TN mode has been discussed in section 8.2. Nomenclature: TN = 90o LTN = Low TN HTN = High TN STN = Super TN GTN = General TN More nomenclatures: Normally white (NW) mode: No voltage = bright. This corresponds to ⊥ polarizers. Normally black (NB) mode: parallel polarizers. Sometimes called the negative mode. Observations about viewing angles: The viewing angles of NB modes are usually much wider than the NW modes. But most displays are NW for convenient driving. The contrast at normal viewing however, is better with NW than with NB mode. The reason is quite simple. For NW mode, the black state is at high voltage. It can be made very dark at very high voltages (homeotropic state). Hence the contrast can be high. Contrasts of >1000 are common.

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For NB mode, the blackness of the LCD depends on the waveguiding mode and the cross polarizer efficiency. So it is limited to ~100. Methods to improve the negative mode normal state darkness: Use low twist angles, add dichroic dyes (similar to guest-host displays). Another observation that you can verify by optical modeling: the first minimum has wider viewing angle than the second minimum.

10.2 ECB displays The ECB mode has been discussed in section 8.1. ECB modes with no twist can be (1) H-cell, Np mode, (2) H-cell, Nn mode, or the (3) HAN cell. The Nn mode requiring a negative ∆ε is also called the DAP (dynamic aligned phase) mode. Transmission of a ECB cell T = sin 2 δ where

δ =

πd∆n λ

Interesting application of the ECB LC cell: tunable Lyot filter: A Lyot filter consists of N retardation plates with increasing retardations of δ, 2δ, 4δ, 16δ,.... Each retardation plate is placed between parallel polarizers at 45o to the retardation plate c-axis. The overall transmission is

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T = cos2δ cos22δ cos24δ cos216δ ....cos2 2N-1δ Some simple expansion leads to 1  sin 2 N δ T= 2  2 N sin δ

   

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This is the same function as in diffraction gratings and in mode locking analysis of lasers. It is a saw tooth function or comb function or sinc function. The transmission peaks are at

λ=

d∆n M

M=1,2,3...

Therefore, the transmission peak can be tuned as we adjust ∆n. Such tunable optical filters can have applications in WDM communications. The width of the transmission peak is given by ∆δ = 21−Nπ (Exercise) Show that the bandwidth (full width at half maximum) of the Lyot filter is given by ∆λ =

0.886λ2 2 N d∆n

For example, if N=5, d∆n =1.1µm, ∆λ = 7.6nm. Another application of ECB displays – color displays. Color displays without color filters: 3

ECB are intrinsically dispersive. Therefore there are colors. ECB with a high twist is usually preferred because of the better viewing angle. Examine the parameter space diagram of the 45o polarizer case. One can see that the color changes as the voltage is applied, or ∆n changes. Thus the ECB can have voltage controlled colors.

10.3 Guest-host displays Guest-host with polarizer

V=0

V=high

The LC is dye doped to 1-2%. The dye molecules are dichroic, i.e. the absorption depends on the polarization of light. α// > α⊥ positive dichroism (usually the case) How to calculate dye absorption: absorption coefficient = α = Nσ where N = molecular concentration and σ = absorption cross section of each dye molecule. Common dye has σ = 10-15 cm2. For example, a 0.001 Mole solution contains 10-3 x

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A/1000 molecules/cm3. It will have an α of 600 cm-1. For a 10 µm cell, the transmission is e-600x0.001=0.55. Contrast and peak transmission tradeoff: T⊥ = e-α⊥d CR = where

e−α ⊥ d e−α // d

= (T⊥ )1− c

c = α// / α⊥

For example, c=5 Peak transmission(x2) 0.9 0.8 0.7 0.6 0.5 0.4

contrast ratio 1.5 2.4 4.2 7.7 16 39

Therefore one has to do some tradeoff between peak transmission and desired contrast ratio. Another parameter to characterize dichroism: S=

c −1 c +1

Some common dyes: Dye Merocyanine StyrlAzo-

Colour blue-green yellow-orange orange

c 4.5 5.7 5.3

S 0.64 0.7 0.68 5

Azomethine Anthraquinone

yellow blue

5.7 7.3

0.7 0.76

Guest-host display without polarizers: It is possible to eliminate the polarizer by having a twist for the no-voltage state. Common: 180o or 360o twist so that all polarization directions will see α//. In that case, the average absorption is given by . This implies a smaller absorption. But the on-state is brighter due to lack of polarizer. Tradeoff between brightness and contrast is till needed. Two-colour dyes Some dyes can be switched between 2 absorption bands. Therefore it is possible to switch the colour instead of switching the brightness.

α⊥

α//

λ 10.4 Phase change (cholesteric) displays: Cholesteric to nematic phase change can be induced by a voltage, The cholesteric phase can exist in 2 states: focal conic (FC) and planar (P) texture.

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Recall: Cholesteric is just chiral nematic with a short pitch near 0.3 µm. At near normal incidence, the resonant wavelength is given by λ = np This is quite dramatic if λ is near the optical wavelength. The LC cell will appear to have bright color.

R

np

λ

The peak reflectance is 50% which makes the cholesteric display very bright. The cholesteric display tends to form a lot of domains – textures. For the planar texture, all the pitch axes are near the z-direction. In the focal conic texture, the pitch axes are random, making the display scattering instead of reflecting. Bistable cholesteric display (BCD): Both the FC and P state are stable at zero volt. As the display is subjected to high V so that it is in the homeotropic state, it may relax to either the FC state or P state as V drops to 0. The FC state will appear transparent, and very dark if a dark cloth is placed behind the display. The P state will appear to have bright color. The color depends on the pitch.

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fast Homeotropic

Planar

Transient Planar slow

Focal conic

Details of the switching dynamics are not clear yet. Several schemes for driving the BCD have been proposed.

Planar

Focal conic

Focal conic

Also, a small pulse can transform the P state to the FC state directly. It is because that the FC state has the lowest entropy.

10.5 Polymer dispersed liquid crystal display: PDLC or PNLC (polymer network) are made from nematic LC and a polymer matrix. 8

High V, index matched

Low V, index not matched

In PDLC, the LC are confined in small ~1 µm droplets. The director orientations are random. Hence there is strong scattering of the incident light if the index of the polymer is different from ne. The display appears opaque. When a voltage is applied, all the directors are aligned in the zdirection. The incident light sees an index of no. If the polymer index is also near no, then there is no scattering. This is called dynamic scattering (DS).

T 0o 80% 30o

2-5% V

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PDLC is also called NCAP (nematic curvilinear aligned phase). It is because of the curvilinear alignment of the 0droplets:

Production: 1. Emulsion, not popular 2. Phase separation, most popular. Just mix the polymer precursor and the LC together. Then do either UV or thermal curing of the polymer slowly. The LC will phase separate out into droplets. The size and size distribution depends on the curing parameters. Usual voltage required: 1 V/µm to make the LC aligned. In a polymer network LC, the polymer concentration is much reduced. Instead of forming droplets, the LC occupies most of the space. Advantages of PDLC: no polarizer, bright, flexible substrate, large area possible, no rubbing and generally easier to make Disadvantages of PDLC: slow response (100 ms), high operating voltages, low contrast, unstable to UV and heat

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Why slow? For large scattering, need large difference in n(polymer) and ne, i.e. large ∆n. Large ∆n implies large η, which means slow LC response. Problem with off-axis scattering: n(polymer) matches no only for normal incidence. Ideally, the polymer should also be birefringent with ne and no matching those of the LC. Then at the high voltage state, the entire matrix becomes homogeneous. Formulas for PDLC: 2

a K  −1 d 2ε + 1 b Vth = ∆ε a 3 where a, b are diameters of the droplet in the z and x directions.

τ off =

γa 2  a 2  K   − 1  b  

γd 2 τ on = ∆εV where γ is the rotational viscosity. Recently, it was found that adding a chiral dopant can increase the speed of PDLC, (with an increase in the threshold voltage as well.) For example, at 1% dopant, the

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threshold can increase by 2x, but the speed decreases by 15x, from 150ms to 10ms. This is almost video rate. τd Vth

1%

10.6 Ferroelectric LC (FLC) FLC is the same as the SmC* phase (chiral smectic C). The LC molecules have a permanent dipole moment. The direction of the dipole is such that p=pzXn

Therefore it is like a side chain. z

n p

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z

Each layer contributes a dipole moment. If the LC is thick enough, all the dipole moments cancel each other and there is no macroscopic dipole. There is one special case that the permanent dipole can be observed – the Clark-Lagerwell cell. Surface stabilized FLC (SSFLC): Use the cell surface to squeeze the FLC from the side. This prevents the chiral smectic phase from forming chiral structures.

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P2

n2 z

n1

P1 The only possible alignment of the LC is shown below, viewing from the top of the display:

x up

n2, P up z n1, P down y

Possible states: U1 state: bottom director = n1, top director = n1. U2 state: bottom director = n2, top director = n2. T1 state: bottom director = n1, top director = n2. T2 state: bottom director = n2, top director = n1. For FLC, it is desired to have the U1 and U2 state operation. The stability diagram of the various states is shown below.

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pitch (µm) 6 helix T1 2 T2

U1+ U2 2

6 d (µm)

It can be seen that in order to have the U1 and U2 states, the cell gap has to be ~2µm. This makes the LC difficult to manufacture. It is also sensitive to shock. Switching between the U1 and U2 states can be achieved simply by applying a DC voltage to the LC cell. If the voltage is +, then the polarization should be UP, and the U2 state will be obtained. For – voltage, the U1 state will be obtained. Switching is not to overcome the elastic energy as in nematics. It is to overcome the surface energy anchoring. Threshold is very low: Vth ~ 0.1V. Notice that the FLC is bistable. Once the desired U state is achieved, the applied voltage can be switched off and the LC remains in that state until further voltage is applied. Also, the speed of switching of the FLC is very fast. (~10µs). It is because that the electric field interacts with the permanent dipole moment. The electric energy is F=p.E

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This is different from the nematic LCD case where the dielectric energy is proportional to E2. The FLC usually operates in the ECB mode. It is placed between cross polarizer as shown:

Output polarizer n2 2θ n1

Input polarizer

The transmission of the FLC is given by T = sin 4 θ sin 2

πd∆n λ

For maximum efficiency, the apex cone angle of the smectic C phase should be 22.5o. FLC are used in applications where speed is important, such as optical signal processing, spatial light modulators etc, and also for time sequential color displays. Disadvantage: FLC is binary. There is no grayscale. Grayscale has to be achieved by frame modulation. Recent invention: antiferroelectric LC. In this case, grayscale can be achieved because of the “linear VT curve”. (so-called τV mode). 16

11. Materials Science Two Recall the basic structure of a LCD:

Polarizer Glass

Patterned ITO Alignment layer

Spacer

Liquid Reflector Analyzer

We shall discuss the materials used to form a functional LCD in more detail here.

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11.1 Glass Two kinds of glass are in common use: soda lime and borosilicate.

Manufacturing methods: 1. Horizontal float zone 2. Vertical flow

Standard sizes: many, especially AMLCD. e.g. 14x14, 14x16

Standard thicknesses: 1.1mm, 0.7mm, 0.5mm, 0.4mm

Surface smoothness requirements: TN Surface roughness (microscopic) Waviness (cm scale) Warp (entire panel)

STN

200-300A 200-300A

AMLCD 100-150A