Study on Photoelectric Characteristics of Liquid Crystals

The structure of liquid crystal molecules is anisotropic, so the photoelectric effect caused by the difference is different. In other words, liquid crystal molecules have anisotropy in photoelectric properties such as dielectric constant and refractive index. Sex, so we can use these properties to change the intensity of the incident light to form a gray scale for use on display components. The photoelectric properties of liquid crystals are as follows:

1. Refractive index:

Since liquid crystal molecules are mostly formed of rod-like or dish-shaped molecules, there are some differences in physical properties parallel or perpendicular to the long axis of the molecule, so liquid crystal molecules are also referred to as anisotropic crystals. Like the dielectric constant, the refractive index is also divided into two directions of vector according to the direction perpendicular and parallel to the director, respectively n // and n⊥.

In addition, for a uniaxial crystal, there are originally two definitions of different refractive indices. One is no, it refers to the refractive index of ordinary ray, so it is abbreviated as no. Ordinary ray means that the electric field component of its light wave is perpendicular to the optical axis. The other is ne, which refers to the refractive index for extraordinary ray, and the extraordinary ray means that the electric field component of its light wave is parallel to the optical axis. Also defines birefringence (birefrigence) n = ne-no is the refractive index difference between the above two.

According to the above, in the case of the layered liquid crystal , the linear liquid crystal, and the cholesteric liquid crystal , since the liquid crystal molecules have a long rod shape, the direction of the director is parallel to the long axis of the molecule. Referring again to the definition of the refractive index of a single optical axis crystal, it has two refractive indices, which are perpendicular to the long axis direction of the liquid crystal n ⊥ (= ne) and parallel liquid crystal long axis direction n / / (= no), so When light enters the liquid crystal, it is affected by two refractive indices, which causes the speed of light in the direction of the long axis of the vertical liquid crystal and the long axis of the parallel liquid crystal to be different.

If the velocity of the light traveling parallel to the long axis of the molecule is less than the velocity perpendicular to the long axis of the molecule, this means that the refractive index in the long axis direction of the parallel molecule is greater than the refractive index in the vertical direction (because the refractive index is inversely proportional to the speed of light) ), that is, ne-no > 0. Therefore, the birefringence n > 0, we call it an optical positive liquid crystal, and the layered liquid crystal and the linear liquid crystal are almost all optically positive liquid crystal. If the speed at which the traveling direction of the light is parallel to the long axis is faster, the refractive index representing the direction of the parallel long axis is smaller than the refractive index in the vertical direction, so the birefringence n < 0. We call it an optical negative liquid crystal. The cholesteric liquid crystal is mostly an optical negative liquid crystal.

2. Dielectric permittivity:

We can separate the dielectric coefficient into two components, which are // (the component parallel to the director) and ⊥ (the component perpendicular to the director). When //> ⊥ is called a liquid crystal with a positive dielectric anisotropy, it can be used in parallel coordination. And / / < ⊥ is called a liquid crystal with a negative dielectric anisotropy, which can only be used in vertical coordination to achieve the desired photoelectric effect.

When there is an applied electric field, the liquid crystal molecules will determine whether the steering of the liquid crystal molecules is parallel or perpendicular to the electric field due to whether the dielectric anisotropy is positive or negative, to determine whether the light penetrates or not. Most of the TN-type liquid crystals commonly used on TFT LCDs are liquid crystals of positive dielectric constant type. When the dielectric anisotropy (=//- ⊥) is larger, the threshold voltage of the liquid crystal is smaller. In this way, the liquid crystal can be operated at a lower voltage.

3. Other features :

For the photoelectric characteristics of liquid crystals, in addition to the above two important characteristics, there are many different characteristics.

For example, like elastic constant (elastic constant: 11, 22, 33), it contains three main constants, respectively, 11 refers to the elastic constant of splay, and 22 refers to twist. The elastic constant, 33 refers to the elastic constant of the bend. In addition, like the viscosity coefficient, it will affect the rotational speed and response time of the liquid crystal molecules, and the smaller the value, the better. However, this characteristic is most affected by temperature. In addition, there is magnetic susceptibility, and because of the anisotropy of liquid crystal, it is divided into c // and c⊥. The magnetic anisotropy is defined as c = c // -c⊥ .

In addition, there are photoelectric properties such as conductance and the like.

The most important of the liquid crystal characteristics is the dielectric constant and refractive index of the liquid crystal . The dielectric constant is the characteristic that the liquid crystal is controlled by the electric field to determine the steering of the liquid crystal molecules, and the refractive index is an important parameter that affects the path of the light when the light penetrates the liquid crystal. The liquid crystal display utilizes these characteristics of the liquid crystal itself, and appropriately uses the voltage to control the rotation of the liquid crystal molecules, thereby affecting the traveling direction of the light to form different gray scales as a tool for displaying images. Of course, the LCD itself alone cannot be used as a display, and other materials are needed to help.