Cholesteric liquid crystals - special liquid crystal modes arranged in a helix

Cholesterol liquid crystal is a special liquid crystal mode arranged in a helical pattern. It is a kind of organic material display technology. This technology can mix red, green, blue and other colors by adding optically active agents with different rotational pitches to meet the needs of color display. demand. However, the cholesterol referred to here is only molecularly similar, not real cholesterol. Cholesteric liquid crystals are mainly used in e-book displays, and their color panels have faster progress and better effects.

The name of cholesteric liquid crystal is because most of the components of the liquid crystal are produced by cholesteric substances. The structure of cholesteric liquid crystal is shown in Figure 1. Its hierarchical structure is similar to that of linear liquid crystal, but if viewed in the Z-axis direction, Its directors are distributed in a spiral with different layers, and the direction of rotation of the director is located on the X and Y planes, but the direction is different, and the director rotates 360. The required molecular thickness is called the "pitch". Because each layer of it is similar to linear liquid crystal, it is also called "C crystal phase". However, due to the different directors, different optical and electrical differences will occur, and different performance characteristics will appear.

When liquid crystals were first discovered, there were many types of liquid crystals, so people in different research fields had different classification methods for liquid crystals.

(1) Layered liquid crystal (Sematic)
The structure of lamellar liquid crystal is composed of liquid crystal rod-shaped molecules gathered together to form a layer-by-layer structure. The long axis directions of the molecules of each layer are parallel to each other, and the direction of the long axis is perpendicular to the plane of each layer. Tilt angle. Because its structure is very similar to crystal, it is also called smectic phase.

(2) Linear liquid crystal (Nematic)
Linear liquid crystal is a TN type liquid crystal commonly used in TFT liquid crystal displays. This liquid crystal looks like a silk thread, hence the name. The linear liquid crystal molecules have a one-dimensional regular arrangement in space. All rod-shaped liquid crystal molecules will choose a specific direction (that is, the director) as the main axis of the long axis, and are arranged parallel to each other, and do not have the same separation as the layered liquid crystal. Compared with the layered liquid crystal, the layered structure has a relatively disordered arrangement. In addition, its viscosity is small, so it is easier to flow (its mobility mainly comes from the free movement of the long axis of the molecule).

(3) Cholesteric liquid crystal (cholesteric)
The name of cholesteric liquid crystals comes from the fact that most of them are generated from derivatives of cholesterol, but some liquid crystals without a cholesteric structure also have this liquid crystal phase. If you look at the cholesteric liquid crystal layer by layer, it will look like a linear liquid crystal, but its directors will be distributed like a spiral with the different layers.

(4) Disc-shaped liquid crystal (disk)
Discotic liquid crystal is also called columnar liquid crystal. For each liquid crystal, it is long like a disk, but its arrangement is like a discoid.

Cholesteric liquid crystal is one of the main technologies used in e-book displays. Because it is similar to electrophoresis technology, it has the characteristics of bistable display, that is, when the screen is still, it hardly needs electricity, so it has the same ability as electrophoresis technology. Power saving features.
At the same time, cholesteric liquid crystal, like electrophoresis technology, is a reflective display technology, so it needs to be displayed with external light source illumination, and it also has the characteristics of being visible under strong light, and there is no problem of whitening under strong light.
The response speed of cholesteric liquid crystal is similar to or slightly better than that of electrophoresis technology. When used in text reading, the response speed is not a problem. However, in terms of supporting handwriting recognition, it has the disadvantage of not keeping up with the screen display.
The performance of cholesteric liquid crystals on a white screen is still far inferior to electrophoresis. However, in terms of color development, the electrophoresis technology with E-ink as the mainstream must achieve color through color filters, which not only increases the cost, but also reduces the reflectivity performance, and the threshold for entering soft display in the future is higher; In contrast, cholesteric liquid crystals are progressing faster in developing color panels and have relatively better effects.

Flexible LCD Technology

Liquid crystal display technology, as the most mainstream technology in the current flat panel display field, has promoted the great progress of the flat panel display industry, and also has a place in the field of flexible display. The liquid crystal technologies currently used for flexible displays mainly include cholesteric liquid crystal display (Ch-LCD), ferroelectric liquid crystal display and vertex bistable liquid crystal display. At present, the most mature display of flexible trial products launched by many enterprises or research institutions is cholesteric liquid crystal display. This technology is a reflective display technology that does not require polarizers and color filters, and can be driven by passive matrix. , This liquid crystal display technology does not need a backlight and has bistable characteristics. The display screen can be displayed stably for a long time without an external electric field, so the power consumption is very low, and it is widely used in high-end electronic paper and RFID tags.

Due to the application potential of liquid crystal display in the field of electronic paper, Fujitsu, LG, Samsung, Toshiba, Taiwan Industrial Technology Research Institute and other companies or research institutions have carried out a lot of research on this, and have launched trial products of flexible liquid crystal display.