Working principles of several modes of liquid crystal

Working principles of several modes of liquid crystal

1. Liquid crystal material is the main body of liquid crystal display devices. Regardless of the type of liquid crystal display, it works based on the following principle, that is, through the action of external fields such as electric field or heat, the liquid crystal molecules are transformed from a specific initial arrangement state to other arrangement states. As the arrangement of liquid crystal molecules changes, the optical properties (birefringence properties) they exhibit change accordingly, and finally transform into light and dark visual changes.

2. The working mode adopted by common TFT active matrix liquid crystal displays is the normally white mode (Normally White) of TN (Twisted Nematic) mode. The most important feature of TN mode is that the setting of liquid crystal box meets the Morgan condition (specifically stated as: the product of the twist pitch of liquid crystal molecules and their refractive index anisotropy is much larger than half of the wavelength of incident light, that is, Δnd »λ/2), so that when light passes through the liquid crystal layer, the rotation of its polarization plane is independent of the wavelength (or when the Morgan condition is met, the rotation angle of each polarization plane of incident light of different wavelengths after passing through the liquid crystal layer is the same); the liquid crystal box is filled with Np (positive nematic phase) liquid crystal, the liquid crystal molecules are arranged along the surface, and the long axis of the molecules is continuously twisted 90º between the upper and lower glass substrates, and the upper and lower polarizers are set orthogonally.

The working principle of the TN box is shown in Figure 1.1 below: In the disconnected state, since the Morgan condition is satisfied and the polarization direction of the polarizer is parallel to the pointer of the liquid crystal molecule on the surface of the lower substrate, the incident linear polarized light obtained by the polarizer will rotate synchronously with the gradual twisting of the liquid crystal molecules after entering the liquid crystal layer (this is the so-called: optical rotation effect). When it reaches the upper substrate, its polarization plane rotates to 90°, and its polarization direction becomes parallel to the polarization direction of the analyzer, so that the linear polarized light can pass through the analyzer and show a bright state display (because it is a white screen when there is no electric field, it is called "normally white mode"). When we apply a voltage greater than the threshold value Vth to the liquid crystal box, the twisted structure of the Np-type nematic liquid crystal molecules will be destroyed and become tilted in the direction of the electric field; when the external voltage reaches 2Vth, all liquid crystal molecules except the molecules on the upper and lower substrate surfaces become rearranged in the direction of the electric field. At this time, the 90° optical rotation performance of the TN box disappears, and the liquid crystal box between the orthogonal polarizers loses its light transmission effect, resulting in a dark state display.

3. Currently, there is also the STN mode, which uses the birefringence of liquid crystal molecules to work (while the TN mode mentioned above uses the optical rotation characteristics of a specially set liquid crystal molecule layer to work). This mode is only used for low-end displays, such as mobile phones, PDAs, etc., because of its complex process and unsatisfactory color display (there are interference colors, that is, poor color reproduction).

The differences between the STN mode liquid crystal cell and the TN mode are as follows: (1) The polarization direction of the polarizer and the long axis of the liquid crystal molecules at the lower substrate (i.e., the optical axis direction) are not parallel to each other but form a 30° angle. In this way, the linear polarization obtained by the polarizer will cause birefringence when it enters the liquid crystal layer. (2) The long axis direction of the liquid crystal molecules at the upper and lower substrates is continuously twisted by 270°, while in the TN cell it is 90°.

The working principle of STN mode is as follows: when no power is applied, the liquid crystal molecules are twisted and arranged (the long axis direction of the liquid crystal molecules at the upper and lower substrates is twisted continuously by 270°). Since the polarization directions of the liquid crystal molecules at the lower substrate and the polarizer are not parallel to each other but at an angle of 30°, the linear polarized light obtained by the polarizer will undergo birefringence when it enters the liquid crystal layer. The two electric vector components of the refracted light are recombined at the upper electrode plate to become elliptically polarized light, and eventually a part of the light is emitted from the analyzer. When power is applied, the twisted structure of the liquid crystal molecules is disintegrated and becomes a vertical arrangement state. The orthogonal polarizers can block the projection of light, resulting in a dark display.

4. In the above two modes, the larger the applied voltage, the larger the tilt angle of the liquid crystal molecules (the closer to the vertical arrangement state), and the greater the intensity of the corresponding transmitted light; the smaller the applied voltage, the smaller the tilt angle of the liquid crystal molecules (the closer to the planar arrangement state), and the smaller the corresponding transmitted light intensity. In other words, by controlling the size of the applied voltage, the desired grayscale display can be achieved.

5. Color display Mechanism:
The color display of current liquid crystal display devices is achieved by using color filter films. The color filter film production process is shown in Figure 2.7:

The structure of the combined liquid crystal box can be seen in the figure below (this is the structure diagram of the MVA mode liquid crystal box in my graduation project. The TN mode liquid crystal box just does not have the small upper and lower protrusions, and the rest is basically the same). In other words, the three primary colors of R, G, and B of the color filter film are arranged in a certain pattern and correspond one by one to the TFT sub-pixels on the TFT substrate (note: a pixel is composed of three sub-pixels). The white light emitted by the backlight source is converted into corresponding R, G, and B color light after passing through the color filter film. The voltage value applied to each sub-pixel can be adjusted through the TFT array, thereby changing the transmission intensity of each color light. When RGB color lights of different intensities are mixed together, color display is achieved.

6. There are other modes, such as MVA mode, IPS mode, etc. These are newly developed LCD working modes to improve viewing angle characteristics and increase response speed. They are just improvements on the TN mode.

7. The setting of the upper and lower polarizers (i.e., the polarizer and analyzer) determines the brightness and darkness of the liquid crystal box in the powered and unpowered states: when the upper and lower polarizers are set orthogonally (i.e., the polarization directions of the polarizer and analyzer are perpendicular to each other), the TN and STN modes are bright in the unpowered state (so it is called the normally white mode); and dark in the powered state. When the upper and lower polarizers are set in parallel (i.e., the polarization directions of the polarizer and analyzer are parallel to each other), the TN and STN modes are dark in the unpowered state (so it is called the normally black mode), and bright when powered.