Research status and development trend of liquid crystal materials

Liquid crystal is an organic substance with a certain order between solid and liquid, and has photoelectric dynamic scattering characteristics; it has a variety of liquid crystal phases, such as cholesteric phase, various smectic phases, nematic phase, etc. According to the different properties of its materials, most of the liquid crystal materials in various phases have been developed for use in flat panel display devices. Various nematic liquid crystals, polymer dispersed liquid crystals, bi-(multi-)stable liquid crystals, ferroelectric liquid crystals and antiferroelectric liquid crystal displays have been developed. Among them, the most successful, the largest market share and the fastest growing are nematic liquid crystal displays. Liquid crystal materials for display are composed of a variety of small molecule organic compounds, and the main structural feature of these small molecules is the rod-like molecular structure. With the rapid development of LCD, people are more and more interested in the development and research of liquid crystal materials.

1. Liquid crystal materials for TN-LCD

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 provided liquid crystal materials, their use as display materials was greatly limited. After the advent of twisted nematic liquid crystal display (TN-LCD) in 1971, TN-type liquid crystal materials with positive dielectric anisotropy were quickly developed; especially in 1974, after the relatively structurally stable biphenylnitrile series of liquid crystal materials were synthesized by GWGray and others, 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.

Many types of TN liquid crystal materials for LCD have been developed. The structures of these liquid crystal compounds are very stable, the nematic phase temperature range is wide, and the relative viscosity is low. Not only can they meet the requirements of high-definition bright spots, low viscosity of 20~30mPa•S (20℃) and △n≈0.15 of mixed liquid crystals, but they can also ensure that the system has good low-temperature performance. The △n value of biphenyl ring-containing liquid crystal compounds is large, and they are effective components for improving the steepness of liquid crystals. The K33/K11 value of pyrimidine compounds is small, only about 0.60. In the formulation of TN-LCD and STN-LCD liquid crystal materials, they are often used to adjust the temperature ordinal and △n value. Dioxane liquid crystal compounds are essential components for adjusting the "multi-channel drive" performance.

2. Liquid crystal materials for STN-LCD

Since the invention of the super twisted nematic liquid crystal display (STN-LCD) in 1984, its display capacity has expanded, its electro-optical characteristic curve has become steeper, and its contrast has increased, requiring the nematic liquid crystal materials used to have better electro-optical properties. By the end of the 1980s, the STN-LCD industry was formed, and its products are mainly used in BP machines, mobile phones, notebook computers, and portable microcomputer terminals.

Mixed crystal materials for STN-LCD generally have the following properties: low viscosity; large K33/K11 value; adjustable △n and Vth (threshold voltage); clearing point is 30°C above the upper limit of the operating temperature. The modulation of mixed crystal materials often adopts the "four-bottle system". This modulation method can independently change the threshold voltage and birefringence without significantly changing other properties of the liquid crystal.

The liquid crystal compounds used in STN-LCD mainly include diphenylacetylene, ethyl bridge 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. Therefore, they are widely used in today's STN-LCD. About 70% of the current liquid crystal materials for STN-LCD contain diphenylacetylene compounds. Ethyl bridge liquid crystal: Compared with other corresponding liquid crystals, the viscosity and △n value of this type of liquid crystal are relatively low; the phase transition temperature range and melting point of the corresponding compounds are relatively low, and they are important components for adjusting the low-temperature performance of low-temperature TN and STN mixed liquid crystal materials. Alkenyl liquid crystal: Since STN-LCD requires steep threshold characteristics, only by increasing the elastic constant ratio K33/K11 of the liquid crystal material can the purpose be achieved. The ene-terminated liquid crystal compounds have an abnormally large elastic constant ratio K33/K11, which is used in STN-LCD and obtains very satisfactory results.

In recent years, STN displays have made significant progress in contrast, viewing angle and response time. Due to the impact of TFT-LCD, STN-LCD has gradually lost the market in areas such as notebook computers and LCD TVs. Due to cost factors, TFT-LCD will not be able to completely replace STN-LCD's original applications in areas such as mobile communications and game consoles.

3. Liquid crystal materials for TFT-LCD

With the rapid development of thin film transistor TFT array driven liquid crystal display (TFT LCD) technology, TFT LCD has not only occupied the high-end display market such as portable notebook computers in recent years, but also has challenged desktop monitors with the improvement of manufacturing technology and the reduction of costs. Since the thin film transistor array directly drives the liquid crystal molecules, the cross distortion effect is eliminated, so the display information capacity is large; with the use of low viscosity liquid crystal materials, the response speed is greatly improved, which can meet the needs of video image display. Therefore, TFT LCD has made a qualitative leap compared to TN and STN liquid crystal displays, and has become one of the most promising display technologies in the 21st century.

Compared with TN and STN materials, TFT has higher and stricter requirements on material performance. The mixed liquid crystal is required to have good light, heat, and chemical stability, high charge retention rate and high resistivity. The mixed liquid crystal is also required to have low viscosity, high stability, appropriate optical anisotropy and threshold voltage. Characteristics of liquid crystal materials for TFT LCD:

TFT LCD also uses the TN type electro-optical effect principle, but the liquid crystal materials used in TFT LCD are different from traditional liquid crystal materials. In addition to having good physical and chemical stability and a wide operating temperature range, the liquid crystal materials used in TFT LCD must also have the following characteristics:

(1) Low viscosity: the viscosity should be less than 35 mPa•s at 20°C to meet the needs of rapid response;

(2) High voltage holding ratio (VHR), which means that the liquid crystal material must have a high resistivity, generally required to be at least greater than 1012Ω•cm;

(3) Lower threshold voltage (Vth) to achieve low voltage drive and reduce power consumption;

(4) Optical anisotropy (△n) that matches the TFT LCD to eliminate the rainbow effect and achieve greater contrast and a wide-angle field of view. The △n value should be between 0.07 and 0.11.

Liquid crystal materials with cyano end groups are widely used in TN and STN liquid crystal displays, such as biphenyl and phenylcyclohexane liquid crystals containing cyano groups. Although they have high △ε and good electro-optical properties, studies have shown that compounds containing cyano end groups are prone to introduce ionic impurities and have low voltage retention rates; their viscosity is still higher than that of fluorine-containing liquid crystals with the same molecular structure. These unfavorable factors limit the application of such compounds in TFT LCDs. Ester liquid crystals have the characteristics of simple synthesis methods, a wide variety of types, and a wide phase transition range, but their high viscosity leads to a significant reduction in their use in TFT LCD formulations. Therefore, the development of new liquid crystal compounds that meet the above requirements has become the focus of liquid crystal chemistry research.

At present, among liquid crystal display materials, TN-LCD has gradually entered a period of decline, with market demand gradually shrinking, and overcapacity and fierce price competition, which makes it no longer worth investing. STN-LCD will gradually enter a mature period, with market demand steadily increasing and production technology fully mature. TFT-LCD is entering a new round of rapid growth globally, with market demand increasing sharply, and is expected to become one of the most promising display materials in the 21st century.

Conclusion

Liquid crystal materials are developing rapidly along with the development of LCD devices, from diphenyl nitrile, esters, oxygen-containing heterocyclic benzene, pyrimidine ring liquid crystal compounds to cyclohexyl (biphenyl), diphenyl acetylene, ethyl bridge bonds and various fluorine-containing aromatic ring liquid crystal compounds. Recently, Japan has synthesized a stable difluoroethylene liquid crystal compound, whose molecular structure is becoming more and more stable, and constantly meets the performance requirements of STN and TFT-LCD. Although the market volume of liquid crystal displays in the world is getting bigger and bigger, the share of my country's liquid crystal industry is very small, and it is still concentrated in TN liquid crystal materials. There is a certain development in TFT liquid crystal materials, but it lacks competitiveness in the world liquid crystal market. It is strongly called on the country to take 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 to revitalize the Chinese liquid crystal display industry.