How LCD (Liquid Crystal Display) Works

How LCD (Liquid Crystal Display) Works

1. Introduction 2. What is Liquid Crystal? 3. Nematic Liquid Crystal 4. How to Make LCD? 5. Homemade LCD

6. Backlit LCD and reflective LCD 7. LCD system 8. Passive matrix LCD 9. Active matrix LCD

10. Color LCD 11. The future of LCD

introduction

You probably use items that contain LCDs (liquid crystal displays) every day. They're everywhere - in laptops, clocks, watches, microwaves, CD players, and many other electronic devices. LCDs do have some real advantages over other display technologies, which is why they're so popular. For example, they're thinner and lighter than cathode ray tubes (CRTs), and they use less power.

Simple LCD display on a calculator

But what exactly are these things called liquid crystals? The name "liquid crystal" sounds like a contradiction in terms. We think of crystals as solid materials like quartz, which are usually as hard as stone, and liquids are obviously different. How can a material contain both "liquid" and "crystalline"?

In this article, you'll learn how liquid crystals have this property, and we'll explore the technology that makes LCDs work. You'll also learn how the properties of liquid crystals were used to create a new type of light valve, and how these opening and closing light valves can form a grid of patterns to represent numbers, words, and pictures!

What is Liquid Crystal?

Matter has three common states: solid, liquid, and gas. Solids are solid because their molecules stay in the same direction and in the same position relative to each other. The molecules in a liquid are the opposite: they can change direction and move around in the liquid. But some substances can be in a strange state, some like liquids and some like solids. When they are in this state, their molecules tend to keep their direction like in a solid, but at the same time they have the properties of liquid molecules and can move to different positions. In other words, liquid crystal is neither solid nor liquid. That's why it is called the somewhat contradictory name "liquid crystal".

So, do liquid crystals behave like solids, liquids, or something else entirely? It turns out that liquid crystals are closer to a liquid than a solid. It takes quite a bit of heat to turn a solid from a solid to a liquid crystal, and only a tiny bit more to turn it from a liquid crystal to a true liquid. This explains why liquid crystals are so sensitive to temperature, and why they're used to make thermometers and mood rings. It also explains why your laptop screen might look weird on a really cold day or on a hot beach!

Nematic Liquid Crystal

Just as there are many different kinds of solids and liquids, there are also many kinds of liquid crystal materials. Liquid crystals can be in one of a number of different phases, depending on the temperature and properties of the material. In this article, we will discuss the liquid crystals used to make LCDs, namely liquid crystals in the nematic phase.

One of the main characteristics of liquid crystals is that their properties are affected by electric current. There is a special type of nematic liquid crystal called twisted nematic (TN) liquid crystal, which is twisted in its natural state. When an electric current is applied to this type of liquid crystal, it will reversely twist to a corresponding angle according to the magnitude of the applied voltage. This type of liquid crystal reacts very precisely to electric current, so it can be used to control the flow of light and thus be used to manufacture LCDs.

Types of LCD Most liquid crystal molecules are rod-shaped, and can be roughly divided into thermotropic liquid crystals and lyotropic liquid crystals. Image courtesy of Dr. Oleg Lavrentovich, Institute of Liquid Crystals Thermotropic liquid crystals react to changes in temperature (or pressure). Lyotropic liquid crystals react depending on the solvent they are mixed with, and are used in soaps and detergents. Thermotropic liquid crystals can be isotropic or nematic. The main difference between the two is that the molecules in isotropic liquid crystals are arranged randomly, while the molecules in nematic liquid crystals have a specific order or pattern. The orientation of the molecules in a nematic phase is determined by a director. This director can be anything, such as a magnetic field or a surface with microscopic grooves. Nematic phases can be further classified based on the relative orientation of the molecules. The most common arrangement is the smectic phase, where the molecules are arranged in layers. There are many variations of smectic phases, such as the C-type smectic phase, where the molecules in each layer are tilted relative to the layer above. Another common phase is the cholesterol phase, or chiral nematic phase. In this phase, the molecules in each layer are slightly twisted relative to the adjacent layers, forming a spiral structure. Ferroelectric liquid crystal (FLC) uses liquid crystal materials with chiral molecules arranged in a C-type smectic phase. Because the helical characteristics of the molecular arrangement make the switching reaction time in the microsecond range, FLC is particularly suitable for advanced display screens. Surface-stabilized ferroelectric liquid crystal (SSFLC) uses a glass plate to apply an adjustable pressure to suppress the helicity of the molecules, further shortening the response time.

How to make LCD?

Manufacturing an LCD is much more complicated than manufacturing a piece of liquid crystal. The conditions required to manufacture an LCD are:

• Light has polarization. (See the article on polarization in Aspects of Sunglasses.)
• Liquid crystals can transmit and change the polarization of light.
• The structure of liquid crystal can be changed by electric current.
• There are transparent substances that can conduct electricity.

LCD devices cleverly exploit these four conditions.

To make an LCD, you need two sheets of polarized glass. A special polymer that can make tiny grooves on the surface of an object is rubbed on the side of the glass that does not have a polarizing film. The grooves must be in the same direction as the polarizing film. Then a layer of nematic liquid crystal is added to a filter. The grooves will cause the orientation of the first layer of molecules in the liquid crystal to be the same as the filter. Next, a second piece of glass is added so that the direction of its polarizing film is at right angles to the direction of the polarizing film on the first piece of glass. Therefore, each subsequent layer of nematic molecules will be twisted at an angle, until the top layer and the bottom layer are 90° apart, thus matching the polarizing glass filter.

When light hits the first filter, it is polarized. Each layer of molecules directs the light they receive to the next layer. As the light passes through each layer of liquid crystal, the corresponding molecules also change the light's polarization plane to match the angle of the molecule's own orientation. When the light reaches the farthest end of the liquid crystal material, its polarization direction is the same angle as the last layer of molecules. If the last layer of liquid crystal matches the orientation of the second polarizing glass filter, the light can pass through.

Homemade LCD

Making a simple LCD is easier than you think. Start by making a glass-liquid crystal-glass sandwich as described above, and then add two transparent electrodes. For example, let's say you want to make the simplest LCD, consisting of just a rectangular electrode. Its layers look like this:

This is the very basic function of an LCD. At the end is a mirror (A), which reflects light. Next, we add a piece of glass with a polarizing film on the bottom (B), and a regular ITO electrode plate on top of the glass (C). The regular electrode plate covers the entire LCD. On top of this is a layer of liquid crystal material (D). Next is another piece of glass (E), which has a rectangular electrode on the bottom and another polarizing film on top (F), which is oriented at right angles to the first polarizing film.

The electrodes are connected to a power source such as a battery. When there is no current flowing, light coming in from the front of the LCD simply hits the mirror and reflects. But when there is current flowing, the distortion of the liquid crystal between the ordinary electrode plate and the rectangular electrode plate will be eliminated, thus preventing light from passing through this area. This makes the rectangular part of the LCD appear as a dark area.

Backlit LCD vs. Reflective LCD

Please note that our simple LCD requires an external light source. The liquid crystal material itself does not emit light. Small and cheap LCDs are usually reflective, which means that they must reflect light from an external source to display an image. Look at this LCD watch: small electrodes charge the liquid crystals to eliminate the distortion of the molecules in the liquid crystal layer, and the light cannot pass through the polarizing film, so the numbers can be displayed.

In the example above, we used a common electrode plate and a single electrode strip to control which liquid crystals are affected by the charge. If we add other electrodes to the layer of single electrode strips, we can create more complex displays.

LCD System

Ordinary flat-panel LCDs are suitable for simple displays that show the same pattern over and over, such as the screens on watches and microwave timers. In these devices, the hexagonal rod shape mentioned above is the most common electrode arrangement, but in fact the electrodes can be arranged in any shape. Just look at those ordinary handheld game consoles, such as card games, alien game consoles, fishing game consoles and slot machines. The various patterns in them are just electrodes of various shapes.

There are two main categories of LCDs used in computers: passive matrix and active matrix. In the following two sections, we will introduce these two categories of LCDs respectively.

Passive Matrix LCD

Passive matrix LCDs use a simple grid to deliver power to specific pixels on the display. Making this grid is a pretty complicated process! First there must be two layers of glass called substrates. A transparent conductive material is arranged in columns on one substrate and in rows on the other. The conductive material is usually indium tin oxide. These rows and columns are connected to an integrated circuit, which controls when charge is sent to a specific column or row. The liquid crystal material is pressed between the two glass substrates, and polarizing films are attached to the outer surface of each substrate. To light up a pixel, the integrated circuit sends charge to a specific column on one substrate and grounds a specific row on the other substrate. The row and column intersect at the designated pixel point, and the resulting voltage untwists the liquid crystal molecules in that pixel area.

Active Matrix LCD

The basis of active matrix LCDs is thin film transistors (TFTs). Simply put, TFTs are tiny switching transistors and capacitors. They are arranged in a matrix on a glass substrate. To find a specific pixel, the corresponding row is turned on and then charge is injected into the corresponding column. Because the other rows intersecting that column are all turned off, only the capacitor at the specified pixel can receive the charge. This capacitor can hold this charge until the next refresh cycle. If we carefully control the voltage value applied to the crystal, we can adjust the degree of distortion elimination, thereby allowing only a portion of the light to pass through.

This creates extremely small and fine increments, allowing LCDs to display grayscale. Most displays today can offer 256 different levels of brightness per pixel.

Color LCD

Each pixel of a color LCD must have three sub-pixels, each of which contains a red, green, and blue filter.

The brightness of each subpixel can be varied in 256 levels by finely adjusting and controlling the applied voltage. The permutations of the subpixel color intensities can produce 16.8 million colors (256 reds x 256 greens x 256 blues), as shown below. Color displays require an extremely large number of transistors. For example, the average laptop computer has a resolution of 1,024x768. If we multiply 1,024 rows by 768 columns by 3 subpixels, we get a number of 2,359,296 transistors that need to be etched into the glass! A problem with any one of these transistors will create a "bright spot" on the display. Most active matrix displays will have several bright spots scattered across the screen.

The Future of LCD

LCD technology is constantly evolving. Today’s LCDs use many different liquid crystal technologies, including super twisted nematic (STN), double scan twisted nematic (DSTN), ferroelectric liquid crystal (FLC), and surface stabilized ferroelectric liquid crystal (SSFLC).

The size of a display is limited by the quality control issues of the manufacturer. In short, to increase the size of a display, you have to add more pixels and transistors. And as the number of pixels and transistors in a display increases, the chance that one of them is bad also increases. For today's large LCDs, 40% of the panels in the production line are discarded by the manufacturer. The level of discarding directly affects the price of LCDs, because the sales revenue of good LCDs must cover the combined production costs of good and bad LCDs. Only advances in manufacturing technology can produce large-size displays at reasonable prices.