Cholesteric liquid crystals (ChLCD) are materials with bistable properties. Their excellent reflective properties make them a key display technology. They are widely used in electronic displays, including monochrome, multi-color, and even full-color paper-like displays. Utilizing ambient light as a reflective source, they do not emit light themselves. The origins of liquid crystals can be traced back to cholesteric materials in 1888, and the term "cholesteric" is still used today to describe the liquid crystal's structure.
ChLCD's bistable nature
ChLCDs exhibit bistable properties, meaning they can naturally remain stable in two states. One state is the planar state, where the liquid crystal molecules are aligned in an orderly manner, reflecting specific wavelengths of light, often referred to as the bright state. The other state is the focal conic state, where the liquid crystal molecules are arranged in a disordered manner, scattering incident light and allowing most of it to pass through, often referred to as the dark state. In this state, the color of the substance beneath the liquid crystal layer, usually black, can be seen. Additionally, there is a temporary state known as the homeotropic state, where all liquid crystal molecules are aligned perpendicularly, allowing all light to pass through and revealing the color of the substance beneath the liquid crystal layer.
The three states can be altered by applying an electric field to the cholesteric liquid crystal:
- When the cholesteric liquid crystal is in a planar state, a relatively small electric field can be applied to change it to a focal conic state. When a higher electric field is applied, all the liquid crystals can be aligned perpendicularly, transitioning to a homeotropic state.
- When the liquid crystal is in the homeotropic state, if the electric field is removed quickly, the liquid crystal returns to the planar state. If the electric field is removed slowly, the liquid crystal will become focal conic.
Therefore, by applying an electric field and controlling the speed of its removal, the state of the cholesteric liquid crystal can be changed.
Full-color applications of cholesteric liquid crystals
The ability of cholesteric liquid crystals to display color is fundamentally linked to their reflective state (bright state) which follows Bragg's Law. This law indicates that when light interacts with a crystalline lattice, the first light beam reflects at point A and the second at point B. The difference in the paths of these beams is characterized by the distances CB and BD, which together equal 2d × sinθ, where d is the distance between the periodic lattice points and θ is the angle of incidence. If the difference in path lengths (2d × sinθ) is an integer multiple of the wavelength of the incident light (λ), constructive interference is achieved. Consequently, by altering the pitch of the liquid crystal, one can adjust the wavelength of the reflected light and, as a result, its color.
When the pitch of the liquid crystal is adjusted to enable constructive interference for blue light, it can reflect blue light, resulting in the display of blue color. Similarly, by adjusting the pitch of the liquid crystal, it is possible to achieve reflection for green and red wavelengths. This allows cholesteric liquid crystals to exhibit various colors through pitch adjustments. By stacking three layers of cholesteric liquid crystals—red, green, and blue—along with a black absorbing layer at the bottom, a full-color ChLCD is capable of producing over 16 million colors.
Full-color images are achieved based on the additive principle of the three primary colors. For instance, yellow is produced by the combination of green and red. When the red, green, and blue colors are fully illuminated (reflective mode), they display as white. Conversely, when all three colors are turned off (transmissive mode), the black color of the bottom layer is revealed.
Extended applications of ChLCD
Further extending the transmissive properties of ChLCD, one could replace the black absorption layer with a solar panel. This modification would enable the display to not only present images but also store electrical energy simultaneously. The visible light portion of outdoor sunlight would be utilized for image reflection, while the infrared light would penetrate the liquid crystal layers, reaching the underlying solar components where it would be converted into electricity. Thus creating a display that is capable of both showcasing visuals and generating power.
Similarly, when integrating it with light-emitting displays, such as MiniLED panels, one can switch between the ChLCD and the MiniLED display to display static or dynamic content.
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