Liquid crystal and its display principle

Liquid crystal is the functional material of liquid crystal display. It is a "liquid crystal" with a certain regularity in the arrangement of molecules, referred to as "liquid crystal". Liquid crystal is an organic compound. During the melting process, it first appears as an opaque turbid liquid, and then becomes a transparent liquid when heated. Liquid crystal has similar properties to crystals, such as optical anisotropy, dielectric and magnetic anisotropy, and electrical properties, while also having the fluidity of liquid. If you want to change a solid crystal, you must rotate the entire crystal; unlike liquid crystals, the polarization direction of liquid crystal molecules can be controlled by electric or magnetic fields.

According to the conditions and composition of liquid crystal formation, liquid crystal can be divided into two categories: thermotropic liquid crystal and lyotropic liquid crystal. The former is caused by the change of liquid crystal phase temperature, while the latter is composed of compounds and solvents that meet certain structural requirements. Thermotropic liquid crystal is further divided into smectic phase, nematic phase and cholesteric phase, as shown in Figure 1.

Smectic is also called lipidic, nematic is also called rod (filament) and cholesteric is also called alcoholic. Nematic (i.e. rod or filament) liquid crystal is used in twisted nematic display (TN-LCD), super twisted nematic display (STN-LCD) and active matrix display (AM-LCD). TN-LCD, STN-LCD and TFT-LCD all use similar manufacturing processes except for the thin film transistor (TFT) array. Figure 2 shows the structure of a TN-LCD box.

Figure 1 Schematic diagram of the arrangement of thermotropic liquid crystal molecules

Figure 2 TN-LCD cell structure

The display principle of TN-LCD is shown in Figure 3.

The friction directions of the upper and lower glass sheets of TN-LCD differ by 90° (orthogonal). When natural light passes through the upper polarizer, it becomes linearly polarized light, and the vibration direction of the polarized light is consistent with the polarization direction of the polarizer, and is also consistent with the arrangement direction of the liquid crystal molecules near the upper glass substrate. When the light passing through the upper polarizer enters the liquid crystal layer, due to the refractive index of the liquid crystal n0≠ne, the linearly polarized light is decomposed into O light and E light, and their propagation speeds are different, but the phases are the same. Therefore, the result of the synthesis of O light and E light at any moment is that the vibration direction of the polarized light has changed, and the light passing through the liquid crystal layer has also been gradually distorted. Due to the limitations of the boundary conditions of TN-LCD, when the light reaches the lower polarizer, the vibration direction of its optical axis is twisted by 90°, which is consistent with the polarization direction of the lower polarizer, and the light can pass through the lower polarizer to become a bright field. When the voltage is applied, the hydraulic molecules are oriented under the action of the electric field, and the twisted structure disappears. The linearly polarized light passing through the upper polarizer no longer rotates after entering the liquid crystal layer, so it cannot pass through the lower polarizer, forming a dark field. The electro-optical response characteristic curve of this display mode is shown in FIG4 .

In Figure 4, V90 is called the threshold voltage. The ratio of V10/V90 is called the threshold voltage sharpness. The smaller the ratio, the more scanning lines. However, when the V10/V90 value is too small and the number of scanning lines increases too much, the viewing angle and contrast of the LCD will be significantly deteriorated.

Figure 3 TN-LCD display principle

Figure 4 shows the electro-optical response characteristic curve of TN-LCD

The display principle of STN-LCD is the same as that of TN-LCD, except that it increases the distortion angle between liquid crystal and incident light from 90° to 180°~270°. TN-LCD usually has only two changes, light and dark, while STN-LCD displays are mainly light green and orange. As long as a color filter is added to the STN-LCD display, and each pixel in the monochrome display matrix is ​​divided into three sub-pixels, and the three primary colors of red, green and blue are displayed through the color filter, the color can be displayed.

TFT-LCD uses a combination of "back-transmission" and "reflection" and an "active matrix" driving method, and its light-emitting principle is shown in Figure 5.

Figure 5 TFT-LCD light-emitting principle

In the TFT-LCD active matrix, the TFT transparent electrode matrix is ​​made on the lower glass plate, the upper glass plate is the filter plate, the liquid crystal material is sandwiched between the two glass plates, the long axis of the nematic liquid crystal molecules is twisted 90° between the upper and lower glass plates, and the glass plate is sandwiched between two polarizers. The function of the entire structure is like a voltage-controlled optical wave.

The main features of LCD are low power consumption, light weight, ultra-thin, ultra-fine, realistic images, rich layers, delicate picture quality, strong three-dimensional sense, no flicker, no radiation and can reduce people's visual fatigue. Therefore, it has become a very important flat panel display and has been widely used in products such as watches, calculators, cameras, measuring instruments, automobile instruments, word processors, PCs (personal computers), mobile phones, personal digital assistants (PDAs), digital cameras, digital video cameras, computer monitors and LCD TVs.