Liquid crystals are organic compounds that are in a state between solid and liquid, have anisotropic optical properties, and are sensitive to external electric, magnetic, and temperature conditions. The liquid crystal display uses liquid crystal as a display material, and controls the light through the array of liquid crystal shutters to display text, graphics and images. At present, the liquid crystal display has developed a Cholesteric Liquid Crystal Display (ChLCD), a Zenithal Bistable Display (ZBD), a Surface Stabilized Ferroelic Liquid Crystal Display (SSFLCD), and a polymer dispersion. Liquid crystal display (Polymer Dispersed Liquid Crystal Display, PDLCD), Nematic Liquid Crystal Display (NCLD), and the like.
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1.1 Technical principles
The cholesteric liquid crystal display is composed of optically active molecules, and the arrangement of liquid crystal molecules has a periodic helical structure. In the absence of an applied electric field, there are two stable states of the planar state and the focal conic state; in the planar state, the periodic arrangement of the cholesteric liquid crystal is like the regular lattice arrangement of the crystal, and the light in the incident light satisfies the Bragg reflection condition. The wavelength will form constructive interference, and the incident light of this wavelength will be reflected back, which is a bright state, thereby changing the period of the cholesteric liquid crystal to change the reflection color. In the focal conic state, since the cholesteric liquid crystal will exhibit an irregular arrangement, the incident light will be exposed, and the absorbing layer attached below will be in a dark state. The technology can add red, green, blue and other colors by adding optical swirling agents with different rotating pitches to meet the needs of color display. The advantage of this display mode is that it is less affected by the spacing of the upper and lower plates, does not require a polarizing plate and a color filter, and can be driven by a passive matrix. There are two ways in which cholesteric liquid crystals can achieve the bistable effect, one is surface stability type, and the other is polymer stability type. These two technologies are quite popular cholesteric liquid crystal display technologies in recent years. . The apex bistable liquid crystal display has liquid crystal molecules perpendicular or parallel to the inner panel. These liquid crystal molecules are in one of two stable directions of black or white, and the transition between the two states can be achieved by the polarized voltage pulse. This bistable state is unaffected by heat or pressure, and once determined, the state will remain, even if the power is turned off. The arrangement of ferroelectric liquid crystal molecules is arranged in a spiral layer. When any two layers of liquid crystal molecules exhibit the same oblique arrangement, the distance between them is called a pitch.
When the ferroelectric liquid crystal is sandwiched between two transparent conductors having a gap less than one pitch, the interaction between the boundary and the liquid crystal molecules limits the arrangement of the molecules and cannot form a layered arrangement of spirals. The liquid crystal molecules in the layer have the same direction, the long axis of the liquid crystal molecules and the normal direction of the layer structure. Under the symmetry of the energy distribution, the liquid crystal molecules have two stable states. As for which state of the liquid crystal molecules is selected, it is subject to Control of the electric field. Polymer-dispersed liquid crystal works between light scattering mode and transparent mode: when there is no electric field, liquid crystal droplets are randomly distributed in the polymer, and almost all refractive index mismatch occurs between the liquid crystal droplets and the polymer. The light is scattered, and the liquid crystal film is opaque and called opaque. When an electric field is applied to the polymer dispersed liquid crystal film, the orientation of the liquid crystal molecules is distributed along the direction of the electric field. At this time, the refractive index of the liquid crystal molecules and the polymer The refractive index is equal, and the liquid crystal film exhibits a transparent state, that is, an open state. The molecules of the nematic liquid crystal are arranged in parallel in a rod-shaped rigid portion. The liquid crystal molecules have a large degree of freedom of movement and are the liquid crystals having the best fluidity. This type of liquid crystal has a small viscosity and a fast response speed, and is the first liquid crystal to be applied. It is used in LCD TVs, notebook computers, and various types of display elements.
1.2 Advantages of LCD as a flexible display
Currently, cholesteric liquid crystals are often used as flexible displays. The technology is a bistable display technology, characterized in that two optical states can be stably existed for a period of time without an external electric field, and the two optical states can be mutually converted under the action of an external electric field. Since the two optical states used for the display are stable without an external field, it is not necessary to add an external field for a long time to maintain the display state, and there is a relatively power saving feature. This kind of display technology can show the advantage of power saving in the static display. In addition, the research of liquid crystal on the flexible display side is relatively mature. The flexible liquid crystal display can continue many traditional manufacturing techniques and is relatively easy to cut into the product market.
1.3 technical difficulties
The development and basic research of the liquid crystal display in the process and equipment is quite complete, and the glass substrate is already a mature industry. Some products are applied on the rigid plastic substrate. At this stage, the display mode of the liquid crystal is applied to the flexible plastic substrate. The resources required are relatively small. In addition, the liquid crystal display mechanism has the following difficulties: the cholesteric liquid crystal display and the polymer dispersed liquid crystal display have prominent problems such as slow response speed, high driving voltage, and low contrast; the apex bistable liquid crystal flexible display is difficult to exist. Realize the problem of full color display and narrow viewing angle; ferroelectric liquid crystal display has fast response speed and can achieve high quality animation display effect. Since ferroelectric liquid crystal molecules are arranged in smectic form, the texture is easily exposed to external pressure or vibration. damage. Since the nematic liquid crystal image depends on the cell gap between the polymers, the bending of the panel causes the gap to change, thereby affecting the image quality, and the design difficulty of the backlight module is greatly increased, and it is particularly difficult to ensure the brightness uniformity of the screen. Further research is to develop new liquid crystal materials, improve drive technology, and improve the performance of flexible liquid crystal displays.
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