What Are The Common Materials Used in LCD Displays?

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What Are The Common Materials Used in LCD Displays?

TFT is an abbreviation for "Thin Film Transistor," also known as "true color," and it belongs to the active matrix LCD. It is a screen composed of thin film transistors, and each of its liquid crystal pixels is driven by a thin film transistor. Each pixel is behind four independent thin film transistors that drive the pixel to emit color light, and it can display 24-bit color depth true color. Four independent thin-film transistors drive each pixel to emit color light, allowing it to display a true 24-bit color depth. TFT LCDs offer resolutions of up to UXGA (1600×1200).TFT arrangement has memory, so when the current disappears, it does not immediately return to its original state, effectively improving the effect of the LCD display dynamic picture, the ability to display static images is also more prominent, TFT LCD has the advantage of short response time than efficiency, and colorful, so it is widely used in notebook computers and DV, DC-DC. The downside of TFT LCD is increased power consumption and a relatively high cost.


Liquid crystal is an organic material that alternates between solid and liquid states and possesses photoelectric dynamic scattering properties. It has several liquid crystal phase states, including the cholesteric phase, different near-crystalline phases, the nematic phase, and so on. According to its material properties, most liquid crystal materials of various phase states have been developed for flat panel display devices, which have been developed with various nematic liquid crystals, polymer dispersion liquid crystals, double (multi) stable liquid crystals, ferroelectric liquid crystals, and anti-ferroelectric liquid crystal displays, etc., of which the most successfully developed, the largest market share, and the fastest development is the one. Liquid crystal display materials are made up of a range of small-molecule organic compounds, with the major structural feature being a rod-like molecular structure. With the rapid advancement of LCDs, there is an increased interest in producing and studying liquid crystal materials.


1. Liquid Crystal Materials for TN-LCD

The development of TN-type liquid crystal materials began in 1968 when the Dynamic Scattering Liquid Crystal Display (DSM-LCD) technology was introduced in the United States. However, because of the structural instability of the available liquid crystal materials, their application as display materials was severely limited. Following the introduction of distorted columnar liquid crystal display (TN-LCD) in 1971, TN-type liquid crystal materials with positive dielectric anisotropy were rapidly developed; in particular, G.W. Gray and others synthesized the relatively stable structure of the biphenyl eye series liquid crystal materials in 1974. The performance criteria of LCD devices such as electronic watches, calculators, and instrument displays were met at the time, ushering in the TN-LCD industry era.


A variety of TN liquid crystal materials for LCDs have been produced. These liquid crystal compounds are all structurally stable, with a diverse range of nematic phase temperatures and low relative viscosity. They not only meet the specifications of high-definition brightness, low viscosity in 20-30mPaoS (20℃), and △n≈0.15 for hybrid liquid crystals but also ensure the system's low-temperature performance. Biphenyl ring-containing liquid crystal compounds exhibit higher △n values and enhance the steepness of liquid crystals. Pyrimidine compounds, with a K33/K11 value of approximately 0.60, are commonly employed to alter temperature order and △n value in TN-LCD and STN-LCD liquid crystal material compositions. The dioxyhexacyclic liquid crystal compounds are required to control the performance of a "multiplex drive".


2. Liquid Crystal Materials for STN-LCD

Since the invention of super twisted nematic liquid crystal display (STN-LCD) in 1984, due to its expanded display capacity, steeper electro-optical characteristic curve, and increased contrast, requiring the use of nematic liquid crystal materials with better electro-optical properties, to the end of the 1980s, the formation of the STN-LCD industry, its products are mainly used in beepers, cell phones, notebook computers, and portable microcomputer terminals.


STN-LCDs with mixed crystal materials often feature low viscosity, a high K33/K11 value, adjustable △n and Vth (threshold voltage), and a bright point above 30 ℃ or higher. Mixed crystal material modulation is commonly referred to as the "four-bottle system". This modulation approach can modify the threshold voltage and birefringence without adversely affecting the liquid crystal's other properties.


The most common liquid crystal compounds utilized in STN-LCD include diphenylacetylene, ethylene bridge bonds, and chain alkene liquid crystal compounds. Diphenylacetylene-type compounds: STN-LCD reaction speed has been enhanced from 300ms to 120~130ms, leading to greater performance and increased utilization in modern STN-LCDs. Approximately 70% of the present liquid crystal materials for STN-LCD contain diphenylacetylene-type chemicals in their formulation. Ethylene-bridge bonded liquid crystals have low viscosity, △n value, phase transition temperature range, and melting point, making them ideal for regulating low-temperature performance in TN and STN hybrid liquid crystals. Liquid crystals of the chain alkenyl group Because STN-LCD requires sharp threshold characteristics, simply doubling the elastic constant ratio K33/K11 of liquid crystal materials is sufficient. The alkene-terminated liquid crystal compounds have an extraordinarily high elastic constant ratio of K33/K11 and are employed in STN-LCDs with excellent results.


In comparison to viewing angle and response time, STN displays have advanced significantly in recent years. Because of the influence of TFT-LCD, STN-LCD gradually lost market share in laptop computers and LCD televisions. Given the expense, TFT-LCD will not be able to fully replace the original STN-LCD in mobile communications, game consoles, and other applications.


3. TFT-LCD using liquid crystal materials

With the rapid development of thin-film transistor TFT array-driven liquid crystal display (TFT LCD) technology, TFT LCD has recently occupied not only the portable notebook computer and other high-end display markets but has also launched a challenge to desktop monitors through improved manufacturing processes and cost reduction. The use of thin-film transistor arrays to directly drive the liquid crystal molecules eliminates the cross-distortion effect, resulting in a large information capacity; the use of low-viscosity liquid crystal materials improves response speed and meets the needs of video image display. As a result, TFT LCD has advanced significantly beyond TN-type and STN-type liquid crystal displays, emerging as one of the most promising display technologies of the twenty-first century. Click here for 10.1 Lvds TFT LCD Modules.


TFT materials must meet greater and more demanding material performance standards than TN and STN materials. The hybrid liquid crystal must be optically, thermally, and chemically stable, with high charge retention and resistance. Hybrid liquid crystals must also have low viscosity, excellent stability, correct optical anisotropy, and a threshold voltage.


TFT LCD also uses the TN-type electro-optic effect principle, although the liquid crystal materials used are not the same as those used in traditional liquid crystal displays. In addition to strong physical and chemical stability and a wide operating temperature range, TFT LCD liquid crystal materials must have the following properties.


(1) Viscosity at 20 ℃ should be less than 35 mPaos for quick response.


(2) A high voltage retention rate (V.H.R) requires a liquid crystal material with a resistivity of at least 1012Ω.cm.


(3) Reduce the threshold voltage (Vth) to obtain low-voltage driving and decrease power usage.


(4) Optical anisotropy (△n) matches the TFT LCD to eliminate the rainbow effect and achieve a high contrast and broad field of view. The △n-value range should be 0.07–0.11.


TN and STN LCDs commonly use liquid crystal materials with cyano end groups, such as biphenyl-like and phenyl cyclohexane liquid crystals containing cyano. Terminal cyano compounds have high △ε and good electro-optical properties, but they attract ionic impurities and have low voltage retention. Additionally, their viscosity is higher than fluorine-containing liquid crystals with the same molecular structure, limiting their application in TFT LCDs. Ester liquid crystals have straightforward synthesis processes, a large range of kinds, and a long phase transition interval, but their higher viscosity causes a significant reduction in the amount needed in TFT LCD formulations. As a result, the creation of new liquid crystal molecules that meet the aforementioned conditions has become the primary focus of liquid crystal chemistry research.


TN-LCD is currently in decline among liquid crystal display materials, with market demand gradually shrinking and surplus manufacturing capacity and pricing rivalry strong, making it uninvestable. While STN-LCD will gradually reach maturity, market demand is continually increasing, and manufacturing technology is fully established. TFT-LCD is entering a new era of rapid expansion around the world, and market demand is increasing rapidly; it is predicted to become one of the most promising display materials in the twenty-first century.

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