Liquid Crystals & LCDsThey’re all around you

What are liquid crystals used for?

• Special types of liquid crystals, called lyotropic liquid crystals, are used in biology for cell membranes

• Liquid crystal’s main commercial use, though, is for creating Liquid Crystal Displays – LCDs. You may know them as TFT monitors, or simply flatscreen monitors.

LCDs

• This presentation will briefly investigate how LCDs operate with liquid crystals

What are liquid crystals?

• Liquid crystals are often described at the 4th state of matter between liquids and solids.

• They are closer to liquids than solids, as less energy is required to change liquid crystals to liquids than solids to liquid crystals.

• Thermotropic liquid crystals can behave like solids at cold temperatures and behave like liquids at warmer temperatures

Liquid crystal molecules

• Liquid crystal molecules are long and thin with a rigid centre. They come in varying arrangements, which are essential to their functions.

The director

• Liquid crystals all have one thing in common: all the molecules in their natural liquid crystal state tend to point in one direction. This direction is called the director. The amount the molecules are oriented in this direction is referred to as the liquid crystal’s orientational order

The 3 main types of liquid crystal

• Nematic – has orientational order

• Smectic – has orientational order & varying degrees of positional order

We will focus on:-

• Twisted Nematic – layers of nematic liquid crystal molecules

Twisted Nematic molecules

• Each 2-dimensional layer has nematic liquid crystals (LCs) aligned roughly with the director.

• However, each layer rotates the director slightly as shown, so you get an altering helical director through the liquid crystal. The distance for 1 entire director rotation is the pitch.

• This is why these LCs are called twisted.

Director alignment

• LC molecules have an intrinsic dipole

• Should we apply an electric field to the molecule, the dipole will align along the field

• In this way, we are able to alter the director to point in a specific direction

What happens in Twisted Nematics

• In Twisted Nematic LCs, this has the logical effect of aligning all the directors in all the layers, if the electric field is strong enough.

• RULE 1: The Twisted Nematic LC untwists when electricity is applied.

• On its own, maybe this isn’t very useful. However, LCs have another useful property to do with optics.

Birefringence

• At a basic level, a refraction index simply relates to the speed a wave will travel through a substance in comparison to travelling without impedance through nothing (a vacuum)

• Liquid crystals are birefringent

• This means they have two separate refraction indices – one perpendicular and one parallel to the director

Twisted Nematics and Light

• All EM waves are composed of perpendicular electric and magnetic waves

• When circularly polarised light enters a TN LC, the two components can be said to be left and right circularly polarised; both components rotate circularly in opposite directions.

Twisted Nematics and Light

• The two components will experience different refraction indices, because both the two indices of refraction are perpendicular to one another and the two components of the wave are perpendicular to one another.

• Therefore, by the very definition of refraction indices, one wave will travel faster than the other through the TN LC.

Twisted Nematics and Light

• When the components emerge, their sum gives the final wave. As one of the components got ahead of the other, the final wave will have rotated through an angle.

• RULE 2: TN LCs rotate light through an angle, directly depending on the depth of the LC the light passes through

LCD Construction

• LCDs use rules 1 and 2 to operate.

• LCDs have two polarization plates – one at the front, one at the rear. They are at 90° to each other. There is a mirror behind the rear polarization plate.

TN LCs in LCDs

• It should be apparent that, with both polarization plates perpendicular, no light will pass.

• TN LCs are then placed in between the two plates. They are attached to both plates by surface anchoring so they can’t move. The TN LCs will be twisted by 90° (1/4 pitch) between the two plates.

TN LCs in LCDs

• Linearly polarized light coming in from the first polarization filter is a case of circularly polarized light.

• The TN LCs will therefore turn this light by an angle of 90°, so it passes through the second polarization filter too. It is then reflected off of the back mirror to make its way back through the filters and into the eyes of the user, making this LCD natively light instead of dark.

Adding the Electronics

• A conductive material is spread all over the back polarization plate between the plate and the TN LC.

• Two electronic contacts are placed on the front polarization plate between the plate and the TN LC.

• When an electric field is applied, the TN LC untwists. Light is no longer twisted through 90° and gets blocked by the second filter.

Putting it into Context

• If we repeat this situation over and over, we can get a grid of TN LCs and electric contacts, making up a screen, like that of a digital watch or calculator.

• By altering the size and shape of the electronic contacts, and using a circuit to control when the LCs receive electricity through the contacts (direct addressing), things like 7-segment displays can be created.

7 Segment Displays

From elzet80.com

Modern LCD Screens

• LCD screens have too many picture elements (pixels) – the small dots that make up an image – to have them all controlled directly by wire contacts.

• Modern LCD screens use a grid of wires which connect to transistors at each and every pixel.

• The transistors act as switches, turning power on and off to the TN LC below it.

Adding Colour

• Creating colour displays is quite easy. Colour filters are put onto the front polarization filter over the top of every filter.

• Three colour filters are used: red, green and blue. It is by combining these colours in various configurations that millions of unique colours can be created for the user to see on their screen.

LCDs versus CRT monitors

• CRT (Cathode Ray Tube) monitors are the large, bulky, old-fashioned monitors that fire photons at the screen. There are advantages and disadvantages of using CRTs.

• Advantages of CRTs: price, colour range, no dead pixels, no native resolution

• Advantages of LCDs: size, weight, power consumption, no flicker, price (for passive matrices).

Alternatives to Liquid Crystals

• Plasma displays – expensive, worse quality, high power consumption, not as thin as LCDs.

• LED (dot matrix) displays – awful quality, hardwired colours, unrealistic proposal for monitors.

• OLED (Organic Light Emitting Diode) displays: high quality, cheap, versatile, good power efficiency. However, it’s an emerging technology.