TFT-LCD working principle

Optical and electrical properties of liquid crystal

Since the structure of liquid crystal molecules is anisotropic, the photoelectric effect caused will be different due to different directions. Simply put, liquid crystal molecules have different photoelectric properties such as dielectric coefficient and refractive index. Therefore, we can use these properties to change the intensity of incident light to form gray scales and apply them to display components.

Refractive index: Since liquid crystal molecules are mostly formed by rod-shaped or disc-shaped molecules, there will be some differences in physical properties parallel to or perpendicular to the long axis of the molecule, so liquid crystal molecules are also called anisotropy Crystal. Like the dielectric coefficient, the refractive index is divided into two vectors according to the direction perpendicular and parallel to the director. They are n // and n⊥ respectively.

Polarizer 

The traveling direction of light waves is perpendicular to the electric and magnetic fields. At the same time, the electric and magnetic field components of the light wave itself are also perpendicular to each other. That is to say, the direction of travel and the components of the electric field and magnetic field are parallel to each other. The polarizer acts like a fence, blocking the components perpendicular to the fence and only allowing the components parallel to the fence to pass. So if we pick up a polarizer and look at the light source, it will feel like wearing sunglasses and the light will become darker. But if you stack two polarizers together, it will be different. When you rotate the relative angle of the two polarizing plates, you will find that with the relative angle, the brightness of the light will become darker and darker. When the fence angles of the two polarizing plates are perpendicular to each other, the light cannot pass through at all. The liquid crystal display uses this feature to complete. Use the upper and lower fences between the vertical polarizers to fill the liquid crystal, and then use the electric field to control the rotation of the liquid crystal to change the direction of light. In this way, different electric field sizes will result in different grayscale brightness.

Two layers of glass and alignment film 

The upper and lower layers of glass are mainly used to clamp the liquid crystal. There are thin film transistors (TFT) on the bottom layer of glass, and the color filter is attached to the top layer of glass. The two pieces of glass are not smooth on the side that touches the liquid crystal, but have jagged grooves. The main purpose of this groove is to hope that the long rod-shaped liquid crystal molecules will be arranged along the groove. In this way, the arrangement of the liquid crystal molecules will be neat. Because if it is a smooth plane, the arrangement of the liquid crystal molecules will be uneven, causing light scattering and light leakage. In fact, this is just a theoretical explanation. It tells us that we need to deal with the contact surface of the glass and the liquid crystal to make the arrangement of the liquid crystal have a certain order. However, in the actual manufacturing process, the glass cannot be made into such a groove-like distribution. Generally, a layer of PI (polyimide) is coated on the surface of the glass, and then a cloth is used for rubbing. So that the surface molecules of PI are no longer strayly distributed, and will be arranged in a fixed and uniform direction. And this layer of PI is called an alignment film, and its function is like a groove in glass, providing the interface conditions for the uniform arrangement of liquid crystal molecules, so that the liquid crystals are arranged in a predetermined order.

TN(Twisted Nematic) LCD

When no voltage is applied between the upper and lower glass, the arrangement of the liquid crystal will be determined by the alignment film of the upper and lower glass. For TN-type liquid crystal, the angle difference between the upper and lower alignment films is exactly 90 degrees. Therefore, the arrangement of the liquid crystal molecules will automatically rotate 90 degrees from top to bottom. When the incident light passes through the upper polarizer At this time, there will only be light waves polarized in one direction. When passing through the liquid crystal molecules, since the liquid crystal molecules have rotated a total of 90 degrees, when the light wave reaches the lower polarizer, the polarization direction of the light wave turns by exactly 90 degrees. The angle between the lower polarizer and the upper polarizer is exactly 90 degrees.

Back light (BL)

In a general CRT screen, a high-speed electron gun is used to emit electrons to hit the phosphors on the silver screen to generate bright light to display the screen. However, the liquid crystal display itself can only control the brightness of light passing through, and has no light-emitting function. Therefore, a backlight must be added to the LCD to provide a light source with high brightness and uniform brightness distribution. The main parts that make up the backlight panel are lamp (cold cathode tube), reflector, light guide, prism sheet, diffuser, etc. The lamp tube is the main light-emitting part, and the light is distributed everywhere by the light guide plate. The reflective plate restricts the light to the direction of the TFT LCD.