Liquid Crystal Materials
Experimental material physics
- Table of contents -
Chapter
1- Introduction and historical overview
2- Different liquid crystal materials, briefly
2.1 - STN and TN display materials
2.2 - FLC display materials
2.3 - PDLC display materials
2.4 - ECB display materials
2.5 - NCPT display materials
3 - Physical LCD properties
3.1 - Active matrix LCDs
3.1.1 - Direct ? Active addressing
3.2 - Passive matrix LCDs
3.3 - Twisted nematic devices
3.3.1 - How polarizers work
3.3.2 - Twisted nematic liquid crystal cells
3.4 - Supertwisted nematic LCDs
3.5 - Ferroelectric liquid crystals
3.5.1 - What are ferroelectric liquid crystals ?
3.5.2 - What are surface-stabilized ferroelectric liquid crystals ?
3.6 - Polymer-dispersed LCDs
3.7 - Ferroelectric liquid crystal textures
4 - Status and prospects for the future for the different LCD materials
4.1 - Twisted nematic LCDs
4.2 - Fast supertwisted nematic LCDs
4.3 - Vertically aligned nematic LCDs
4.4 - Ferroelectric LCDs
5 - Active or passive LCD displays ?
5.1 - Active matrix (TFT) LCDs
5.2 - Passive matrix LCDs
5.3 - Comparison tables of passive matrix LCD technologies
6 - Manufacturing of LCDs and TFTs
6.1 - Manufacturing in general
6.2 - Manufacturing equipment
6.2.1 - PECVD
6.2.2 - Sputtering
6.2.3 - Lithography
6.2.4 - Wet processing and cleaning
6.2.5 - Dry etching
6.2.6 - Drivers and packaging
7 - Suppliers and markets
8 - Some commercial products and prototypes
1 - Introduction and historical overview
Liquid crystal materials were first discovered in 1888 by an Austrian botanist, F. Renitzer. However those liquid crystals were not suitable for any commercial usage, and it is only 25 years ago since the first material suitable for electronically driven displays, was developed. The first room-temperature nematic liquid crystal was observed in the late 1960s. Unfortunately this crystal had quite a short temperature range as it was affected by impurities. Occasionally in homologous series the temperature range could reach from ?40 to +100 degrees Celsius. Unfortunately these mixtures were very unstable and they possessed a negative dielectric anisotropy not useful in the twist cell .
The major breakthrough came when cyanobiphenyl materials were discovered a few years later. The more stable phase had a large positive dielectric anisotropy as well as a strong birefringence nearly ideal for the twist cell .
During the 1970s and 1980s several liquid crystal compounds and phases were discovered, primarily by the industry, but also in several research programs on liquid crystal materials in colleges and universities around the world.
The ferroelectric chiral smecic (FLC) phase was discovered in 1975 and proved to have a unique form of ferroelectricity. The first display based on the FLC phase was actually patented in 1980. Another example of new liquid crystal phases also discovered during this intense research period, are the forms of polymer dispersions. Also a new effect, the electroclinic effect, was discovered during this period and is now being carefully studied for possible future display applications.
Lately several new materials has been discovered such as the retardation film which is extremely important for the supertwisted nematic (STN) and twisted nematic (TN) displays.
Most of this research is situated in Japan due to strong manufacturing capability and high research funds. The most frequently used liquid crystalline phase used today in display devices is the nematic phase. The phase is used both in the TN cell as well as in the active matrix (AM) TN cell. About 60 % of all nematic materials supplied by Merck-Japan, which is the biggest producer of such materials, goes to these applications. The active matrix TN cell is expected to grow substantially during the next 5 years as this technology dominates the manufacturing industry today. Other types of rapidly growing display types are the electrically controlled birefringence (ECB) and polymer-dispersed liquid crystals (PDLCs). Due to the relatively recent technology discovered in producing these cells, they have not yet reached a fully commercial usage.
2. Different liquid crystal display materials
2.1 STN and TN display materials
There has been considerable research over the past 20 years in the development of low-molecular-weight nematic compounds with improved characteristics such as lower viscosity, increased temperature range, larger birefringence and dielectric anisotropy. However the TN and STN cell is about to get as far as research can provide, and only minor changes like improvements of existing cells may be possible. Instead new cells and compounds are being examined such as the FLC phase and the PDLC material. These materials shows substantially improved features in certain areas such as speed and brightness.
Unfortunately the low response speed, lower power consumption and low cost still makes TN and STN materials suitable for slow displays. Some basic properties are compared in figure 2.1 and 2.2 below.
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