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>> Detailed history of LCD development 23th,Apr,2018
Liquid crystals are organic substances that have a certain degree of order between the solid and liquid states, and have dynamic photoelectric scattering properties; it has a variety of liquid crystal phases such as cholesteric phase, various smectic phases, and nematic equality. According to different material properties, various phase-state liquid crystal materials have been developed for use in flat panel display devices. Various nematic liquid crystals, polymer-dispersed liquid crystals, dual (multi-) steady state liquid crystals, and ferroelectrics have been developed. Liquid crystal and anti-ferroelectric liquid crystal displays, among which the most successful development, the largest market share and the fastest-growing are nematic liquid crystal displays. The liquid crystal material for display is composed of a variety of small-molecule organic compounds whose main structural features are rod-like molecular structures. With the rapid development of LCD, people are becoming more and more interested in the development and research of liquid crystal materials.
1. TN-LCD liquid crystal material
TN liquid crystal materials for LCDs have been developed in many types. The structure of these liquid crystal compounds is very stable, the nematic phase temperature range is wide, and the relative viscosity is low. Not only can meet the high-definition bright point of the mixed liquid crystal, low viscosity requirements of 20 ~ 30mPa8226; S (20 °C) and △ n ≈ 0.15, but also can guarantee the system has good low temperature performance. The △n value of the biphenyl-containing liquid crystal compound is large, and it is an effective component for improving the steepness of the liquid crystal. The K33/K11 values of pyrimidines are small, only about 0.60. In TN-LCD and STN-LCD liquid crystal material formulations, they are often used to adjust the temperature order and Δn values. The dioxane liquid crystal compound is an essential component for adjusting "multiple drive" performance.
The development of TN-type liquid crystal materials originated in 1968, when the United States announced the dynamic scattering liquid crystal display (DSM-LCD) technology. However, due to the structural instability of the liquid crystal materials provided, their use as display materials is greatly limited. After the introduction of twisted nematic liquid crystal displays (TN-LCDs) in 1971, TN-type liquid crystal materials with positive dielectric anisotropy were quickly developed; in particular, the relatively structurally stable biphenyl eye series of liquid crystal materials in 1974 consisted of GW. After Gray and others synthesized, they met the performance requirements of LCD devices such as electronic watches, calculators and instrument displays at the time, thus truly forming the TN-LCD industry era.
2. STN-LCD liquid crystal material
Since the invention of the super twisted nematic liquid crystal display (STN-LCD) in 1984, due to its expanded display capacity, the electro-optic characteristic curve has become steeper and the contrast has been improved, nematic phase liquid crystal material used has been required to have better electro-optic performance. In the late 1980s, the STN-LCD industry was formed. Its products are mainly used in BP machines, mobile phones and notebook computers, and portable computer terminals.
The STN-LCD liquid crystal compounds mainly include diphenylacetylenes, ethyl bridges, and alkenyl liquid crystal compounds. Diphenylacetylene compounds: The response speed of STN-LCD is increased from 300ms to 120~130ms, which greatly improves the performance of STN-LCD, and is therefore more used in today's STN-LCDs. The current STN-LCD liquid crystals are used. About 70% of the materials in the material contain diphenylacetylene compounds. Ethyl-bridged liquid crystals: These liquid crystals have lower viscosity and Δn values than corresponding other liquid crystals; the phase transition temperature and melting point of the corresponding compounds are relatively low, and they are low-temperature TN and STN mixed liquid crystal materials. The important component of low temperature performance. Alkenyl-based liquid crystals: Since the STN-LCD requires steep threshold characteristics, only the increase of the elastic constant ratio of the liquid crystal material K33/K11 can achieve this goal. The olefin-based liquid crystal compound has an unusually large elastic constant ratio K33/K11, which is used in the STN-LCD and gives very satisfactory results.
The mixed crystal materials for STN-LCD generally have the following properties: low viscosity; large K33/K11 value; Δn and Vth (threshold voltage) are adjustable; the clearing point is higher than the upper limit of the working temperature by 30°C or more. Modulation of mixed-crystal materials often uses a "four-bottle system." This modulation method can independently change the threshold voltage and birefringence without significantly changing other characteristics of the liquid crystal.
In recent years, STN displays have shown significant improvements in contrast, viewing angle and response time. Due to the impact of TFT-LCD, STN-LCD has gradually lost its market in notebook computers and LCD TVs. In view of cost factors, TFT-LCDs will not be able to completely replace STN-LCD's existing applications in mobile communications and game consoles.
3, TFT-LCD liquid crystal material
Compared with TN and STN materials, TFTs require higher and stricter material properties. The mixed liquid crystal is required to have good light, heat, chemical stability, high charge retention and high resistivity. Hybrid liquid crystals are also required to have low viscosity, high stability, appropriate optical anisotropy, and threshold voltage.
TFTLCD liquid crystal material features:
With the rapid development of thin-film transistor TFT array-driven liquid crystal display (TFT-LCD) technology, TFTLCD has occupied not only high-end display markets such as portable notebook computers in recent years, but also has become a desktop monitor with the improvement of manufacturing process and cost reduction. issue a challenge. The use of a thin-film transistor array to directly drive liquid crystal molecules eliminates the cross-over distortion effect and thus the display information capacity is large. With the use of a low-viscosity liquid crystal material, the response speed is greatly improved, and the video image display needs can be satisfied. Therefore, TFTLCDs have made a qualitative leap over TN-type and STN-type liquid crystal displays and have become one of the most promising display technologies in the 21st century.
The TFT-LCD also uses the TN-type electro-optical effect principle, but the TFT-LCD liquid crystal material is different from the conventional liquid crystal material. In addition to the requirement for good materialization stability and a wide operating temperature range, TFTLCD liquid crystal materials must also have the following characteristics:
(1) low viscosity, viscosity at 20 °C should be less than 35mPa8226; s, to meet the needs of rapid response;
(2) Lower threshold voltage (Vth) to achieve low voltage drive and reduce power consumption;
(3) High voltage retention (V.H.R.) This means that liquid crystal materials must have a high resistivity, generally requiring at least a large
In 1012Ω8226cm;
(4) Optical anisotropy (△n) matched to the TFTLCD to eliminate the rainbow effect and obtain a large contrast and
Wide-angle view. The Δn range should be between 0.07 and 0.11.
At present, among the liquid crystal display materials, TN-LCD has gradually entered the recession period, the market demand has gradually shrunk, and the production capacity is excessive, the price competition is fierce, and it has no investment value. The STN-LCD will gradually enter the mature stage, the market demand will increase steadily, and the production technology will be fully mature. The TFT-LCD is entering a new period of rapid growth on a global scale, and the market demand is rapidly increasing, which is expected to become one of the most promising display materials in the 21st century.
Liquid crystal materials whose terminal group is cyano are widely used in TN and STN liquid crystal displays, such as biphenyls containing cyano groups, phenylcyclohexane liquid crystals, although they have high Δε and good electro-optical performance, Studies have shown that compounds containing terminal cyano groups tend to introduce ionic impurities and have low voltage retention; their viscosity is still higher than that of fluorinated liquid crystals with the same molecular structure. These adverse factors have limited the use of such compounds in TFT-LCDs. Application. Ester liquid crystals have the advantages of simple synthesis methods and various types of phase transitions, and have a wide range of phase transitions, but their higher viscosity leads to a significant reduction in the amount of TFT LCD formulations. Therefore, the development of novel liquid crystal compounds that meet the above requirements has become the focus of liquid crystal chemistry research.
Conclusion:
The development of liquid crystal materials has progressed rapidly with the development of LCD devices, from the gradual development of biphenylnitriles, esters, oxygen-containing heterocyclic benzenes, pyrimidine ring-like liquid crystal compounds to cyclohexylbenzenes, diphenylacetylenes, Ethyl bridges and various fluorine-containing aromatic liquid crystal compounds have recently been synthesized in Japan as a structurally stable vinylidene fluoride liquid crystal compound, whose molecular structure has become more and more stable, and has continuously satisfied the performance requirements of STN and TFT-LCD. Although the market volume of the world's LCD monitors is increasing, the share of the LCD industry in China is very small, and it is still concentrated in the TN liquid crystal materials. There are certain developments in the TFT liquid crystal materials, but it is currently in the world LCD market. China lacks competitiveness and strongly urges the state to adopt positive measures to strengthen the human and financial investment in the research and development of liquid crystal display devices and materials, and to tilt upstream in the flat panel display industry in order to revitalize the Chinese liquid crystal display industry.
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