LIQUID CRYSTAL MATERIALS

Introduction and Historical Overview

Liquid crystal materials were discovered in 1888 by an Austrian botanist, F. Renitzer (Kelker, 1988), but only in the last 25 years have these materials been developed sufficiently to be used in electronically driven displays (Bahadur, 1983). In the early 1960s, when RCA was first considering using liquid crystals for dynamic scattering displays, a room-temperature nematic liquid crystal did not exist. The first room- temperature nematic phase was observed in the compound MBBA, but the temperature range was short and strongly affected by impurities (Demus, 1988). It was then discovered that eutectic mixtures of MBBA with other compounds in its homologous series could broaden the temperature range to extend from below -40 degrees centigrade to over 100 degrees centigrade. However, these mixtures were very unstable, and they also possessed a negative dielectric anisotropy not useful in the twist cell.

It was therefore a major breakthrough when cyanobiphenyl materials discovered by Professor George W. Gray of Hull University in England were found to exhibit room- temperature nematic phases. These materials were not only more stable, but they also possessed a large positive dielectric anisotropy and strong birefringence nearly ideal for the twist cell, which had been invented only a few years earlier. Patents on these materials gave English and European industries a leading edge in the manufacturing and marketing of nematic materials for displays. E. Merck of Darmstadt and F. Hoffmann-LaRoche, Ltd. of Basel remain leading suppliers of nematic materials today. Both companies have established divisions or joint ventures in Japan: Merck-Japan and RODIC, the latter name an acronym derived from Hoffmann-LaRoche and Dainippon Ink and Chemicals, Inc. The cyanobiphenyl patents are due to expire around 1993.

During the 1970s and 1980s, nematic liquid crystal compounds and mixtures for displays were developed primarily by industry. Almost totally disconnected from this effort were very strong research programs on liquid crystal materials in colleges and universities around the world. These programs explored not only nematic phases but also other kinds of liquid crystal materials, studying both the physics and the chemistry of the materials. In fact, the 1991 Physics Nobel Laureate, Pierre-Gille de Gennes, performed his prize-winning work on liquid crystals during this time. Out of this work came many new kinds of materials and liquid crystal phases, some of which have found applications in displays. One is the ferroelectric chiral smectic (FLC) phase. The pure smectic C was discovered at Kent State University (Saupe, 1969). Chirality was later added by R. Meyer (Meyer et al., 1975), and the resulting material was discovered to have a unique form of ferroelectricity. Noel Clark and S. Lagerwall (Clark & Lagerwall, 1980) patented an FLC display using this technology. Other examples are some forms of polymer dispersions that have links to basic research programs in the university environments. Currently, new effects, such as the electroclinic effect, are being studied for display applications.

Japan has a strong display manufacturing capability and the associated infrastructure; now, with the leadership of such professors as S. Kobayashi in university circles, it is developing strong material research components. New, important materials are being discovered there. An example is the retardation film, which is extremely important for supertwisted nematic (STN), twisted nematic (TN), and other displays. This is truly a Japanese invention and is currently produced only by Japanese industry. From Chemical Abstracts it can be noted that Japanese scientists lead those in the United States and Europe combined by a ratio of 3:1 in applications for patents on liquid crystal materials for displays. U.S. and European researchers show much less awareness of or concern about applying liquid crystal materials in displays. Much of the new chemistry is published immediately in the open literature. The development of polymer liquid crystals (PLCs) is perhaps an example of this. Many new PLC materials are developed everyday that could be of value in the display industry if polymer workers were more aware of the uses of these materials in displays.

The nematic phase is the liquid crystalline phase most used in display devices. Several different types of displays make use of this phase; some of them have been well developed for commercial devices. The most frequently used and best developed is the TN cell, which has been and still remains the workhorse of the industry. Nearly 50% of nematic materials supplied by Merck-Japan go toward TN displays, and another 10% toward TN active matrix (AM) displays. The latter type is expected to grow substantially in the next 10 years as the TN AM technology dominates the display manufacturing industry in Japan. Currently, the STN is a widely used display for laptop computers and consumes 40% of Merck-Japan nematic materials. Other display types, such as electrically controlled birefringence (ECB) or polymer-dispersed liquid crystals (PDLCs), currently consume only a small percentage of the nematic materials market. PDLC-type displays (Doane, et al., 1982) are the most recent liquid crystal technology and have not yet reached use in a commercial product. An interesting facet of the PDLC technology is its use in switchable windows, which use large quantities of nematic materials and thus could, in the long term, drive down the cost of such materials.

Suppliers and Markets

The Merck group, which consists of E. Merck Darmstadt, Merck-Japan, Ltd., and Merck Ltd., Poole, holds early patents and is a major supplier (50% worldwide) of nematic liquid crystal materials. Their sales breakdown is as follows: 70% to Japan, 25-30% to Southeast Asia, and 1-2% each to Europe and the United States.

The joint venture RODIC claims 30% of the liquid crystal material market in Japan. Hoffman-LaRoche in Basel, Switzerland, supplies Southeast Asia, the United States, and Europe. Other suppliers of nematic materials are listed in Table 2.1. Current costs of nematic materials supplied by E. Merck range between $2.85 and $10.00 per gram, depending upon the materials used. This is actually a small percentage of the total cost of an STN or TN AM thin-film transistor (TFT) display. A significant part of these costs results from formulation of mixtures. According to Merck, each sale normally involves mixtures of different liquid crystal compounds prepared to meet the specifications each company wants for its displays. Custom- designed mixtures are code-named to keep a customer's mixture proprietary. It can take as long as a year to get a mixture correct. Merck assumes the responsibility of meeting a manufacturer's specifications. Often the specs are tightened on the next order. Mixtures have involved as many as several hundred compounds. Some customers remix materials or change mixtures supplied by Merck. Purity is always an important issue.

TN and STN Display Materials

Table 2.2 outlines the physical properties of a nematic material that is desired for an STN display, along with values typically provided by manufacturers. These data were graciously supplied by Dr. H. Takatsu of Dainippon Ink and Dr. B. Rieger of Merck-Japan. Table 2.2 clearly illustrates why different STN manufacturers desire different material characteristics: One manufacturer may desire fast response time, whereas another seeks better contrast. Uniformity may be an issue that alters the value of the pretilt used, but not without a compromise in speed. High resistivity, approx. 10(superscript 12) ohms cm, is normally sought for all displays. This parameter is controlled by ionic impurities, hence the demand for highly purified materials. The widest possible temperature range is often desired, and approx. -30 to 80 degrees centigrade is normally achieved and accepted. There can be a sacrifice in temperature range to achieve lower drive voltage in TN displays. Temperature ranges beyond -30 to 100 degrees centigrade are difficult to achieve.

Table 2.3 shows material characteristics desired and achieved for the TN cell on the AM TFT and metal-insulator-metal (MIM) display.

Each company has its own proprietary compounds for mixing to meet desired characteristics. There has been considerable research over the past 20 years in the design and synthesis of low-molecular-weight nematic compounds with improved characteristics, such as lower viscosity, increased temperature range, larger birefringence, and dielectric anisotropy. It is generally believed that further research on the low-molecular-weight compounds will not provide substantial improvements in the nematic physical parameters. Improvements in the STN or TN cells will come primarily from improvements in display design or in other materials used in the display, such as the alignment layers, which control pretilt and molecular anchoring strengths (discussed later), or retardation films.

For displays with substantially improved features in certain areas such as speed or brightness, display technology generally looks toward other promising kinds of liquid crystal phases, such as the FLC phase, or toward different kinds of materials, such as PDLCs.

Table 2.1
Suppliers of Nematic Liquid Crystal Materials for Displays

E. Merck group ---------------------------------claims 40% market share

E. Merck, Darmstadt, Germany ---all synthesis at Darmstadt
Merck Poole, England ---------------focuses on PDLC materials
Merck-Japan
EM Chemicals, U.S.

RODIC joint venture, Tokyo -----------------claims 30% Japan market share

Hoffmann-LaRoche, Switzerland --supplier of Southeast Asia
Dainippon Ink, Japan

Chisso, Tokyo

Other Japanese companies

Bohusui
Hoechst, Japan
Kohusai Electric
Mitsubishi Kasei
Mitsui Toatsu Chemicals
Nagase Sangyo
Samco International
Sumitomo Chemical

Table 2.2
Nematic Materials Properties and Display Parameters for STN Displays

Table 2.3
Nematic Materials Properties and Display Parameters for a TN Cell

FLC Display Materials

The FLC display offers substantially improved switching times and bistability. The latter feature permits the use of the LCD display for a passive matrix with reduced display cost. A commercial product from this technology has been slow in coming for several reasons: It is difficult to fabricate because of small cell spacing; it is easily destroyed by mechanical shock because molecular anchoring at the surface is unstable; and gray scale is not easily achieved. FLC materials have also not met desired specifications. Although the response time is fast, it is marginal in most materials for line-at-a-time addressing at TV rates on a passive matrix. Mr. Mochizuki of Fujitsu, for example, reports a 120-micro sec response for a 20V drive and 80-micro sec response for a 30V drive; but 30 micro sec is required for addressing 1000 lines at video rates. Higher resolution requires shorter response. A preferred FLC material of Dainippon Ink shows a 60-micro sec response time that could be reduced to 29 micro sec with a sacrifice in contrast. The FLC display recently reported by Canon does not exhibit video rates. Another material problem is temperature range because of the extreme sensitivity of viscosity (and resulting response time) to temperature. There is probably more promise in improving the FLCs with new synthesis and molecular design than there is in nematics for TN and STN, because less has been done. For example, work at Fujitsu and Dainippon Ink showed new materials with a wide smectic A range about the smectic C. Scientists at Fujitsu described how this feature can lead to improved surface stabilization. There is substantial research on FLC compounds in universities and industry around the world. New variations of FLC materials, such as antiferroelectric materials or FLCs from side-chain polymers, do not appear to receive as much enthusiasm from Japanese scientists as from European and U.S. scientists. This is perhaps because scientists in Japan are closer to manufacturers and thus are aware of manufacturing problems. A representative of Fujitsu commented that antiferroelectric materials showed improved stability because of the soft layers, but their contrast ratio was not as good. Possible improvements from other display materials such as alignment layers will be discussed later in this report.

PDLC Display Materials

The area of PDLC materials is a recent technology that has been rapidly picked up, improved upon, and developed for display application by Japanese scientists (Doane, 1991). The physical concepts behind this technology have origins in English patent literature, but materials and processes to bring it about largely began in the United States. The team found that nearly all display companies in Japan had an interest in and maintained a research and development program on these materials. In displays, these materials offer improved brightness because they do not require polarizers and they are relatively simple to fabricate. They are principally interesting for use in projection television, but many companies foresee their use in direct-view displays. Since PDLC materials require the active matrix for high resolution, most research programs focus on efforts to lower the drive voltage and increase resistivities required for the AM TFT. Two companies have made significant strides in this direction: Asahi Glass reported a full-color video projection prototype using PDLC materials, and Dainippon Ink has developed a PDLC material and is now working with other display companies to develop display products. Both companies show materials that can be used on AM TFT substrates.

There are variations in the amount and type of polymer used in PDLC materials; the amount generally varies from 20% to 70% polymer by weight. Recent materials using gel polymers contain approx. 2% polymer. Both aqueous and nonaqueous polymers have been used. For high resistivities, both the polymer and liquid crystal material must be of high purity. Hysteresis can be a problem. Nematic materials most desired are those with large delta e and delta n. Dainippon Ink reports the use of fluorinated materials to achieve high-purity nematic materials. An example is the fluorinated tolans, which also exhibit a high delta n:

The characteristics of materials developed by Asahi Glass and Dainippon Ink are shown in Table 2.4.

Although it is still too early to determine all the problem areas in PDLC materials, they include control of hysteresis and polymer chemistry problems. While the use of PDLC technology offers the potential for substantial improvements in the brightness of projection television, there has not been sufficient development time for commercial-grade prototypes to appear.

ECB Display Materials

Materials for ECB LCDs are a very small part of the liquid crystal materials market. Normally, large delta e and delta n materials are desired. Vertically aligned nematics (VANs) require a negative delta e.

NCPT Display Materials

Fujitsu plans production of a 5M-pixel black-and-white overhead projection system using a cholesteric nematic phase change (NCPT) display. Under a suitable bias voltage, the material possesses a bistable memory, needing only a passive matrix. It works on a light-scattering principle, providing for a bright projection display (no polarizers). It has several advantages over the STN projection system: It does not degrade in the center of the picture as the STN has been reported to do; also, according to Fujitsu, its manufacturing cost is lower and high definition is possible. Fujitsu is now developing a 7M-pixel system. Color is possible but not yet fully developed.

A key material to the success of the NCPT is a chiral material that possesses a temperature-independent pitch length over a wide temperature range and a pitch length, p, of approx. 1.0 micron in a cell with an inner electrode spacing, d, of 5-6 microns. The memory depends on ratio p/d, which can limit the thickness of the cell and ultimately the contrast of the display. Research efforts underway in the United States are using polymer gel dispersions in the NCPT cell to eliminate this shortcoming.

Table 2.4
Performance Characteristics of Polymer Dispersions
by Asahi Glass and Dainippon Ink for Cells with a
Spacing of 8 micronsUsing a Light Collection Angle of 5 degrees

University LC Materials Research in Japan

There is considerable research in Japan aimed at developing new types of liquid crystal materials and displays with improved features. This research is conducted in both industry and universities, but the more basic work is being done at universities. The team visited and interviewed two leading professors in Japan involved in display materials, Professor S. Kobayashi of Tokyo University of Agriculture and Technology and Professor T. Uchida of Tohoku University. Professor Kobayashi outlined the fundamental issues important in a display (Table 2.5). In Dr. Kobayashi's view, no technology can cover all of these issues well; but he claims that all liquid crystal technologies can cover these issues sufficiently well and if pursued strongly enough could become marketable technologies. Therefore, he said, all LCD technologies should be explored.

Professor Kobayashi pointed out the need to develop a direct-view display without a backlight. In none of the companies the JTEC team visited in Japan were there any discussions on this topic. However, Professor Uchida showed an interest in reflective color displays; he has achieved a reflectivity of about 20% and a contrast of 5:1 using a dichroic dye guest-host type display.

Table 2.5
Basic Issues Important for a Display of Commercial Value

  1. Information content (resolution)
  2. Viewability
    1. Legibility (contrast ration and luminescence)
    2. Full color capability
    3. Gray scale
    4. View angle
  3. Cost of driving circuits
  4. Production costs (yield, throughput)
  5. Space (flat-panel, weight)

Often trade-offs between 1 and 2; 1,2 and 3,4

Professor Uchida estimated that about 100 physics and 100 chemistry faculty members in Japanese universities are working on materials and chemical physics problems related to displays. Few are actually developing a display. Table 2.6 lists areas of interest in Japan mentioned by Professors Kobayashi and Uchida.

Table 2.6
Areas of Research Interest in Japanese Universities

Ferroelectric Liquid Crystals
Bistability
Gray scale

Surface Alignment Materials and Problems

Langmuir-Blodgett films
Polar anchoring
Torsional anchoring
Conductive orientation films

Retardation Films

Impurity Films

Polymer Dispersions, PDLCs

Electroclinic Effects

SA-SC* phase transition

The funding for university research in Japan comes primarily through the Ministry of Education (MOE) through such agencies as Japan Society for the Promotion of Science (JSPS). Each agency has many committees in areas such as materials science, laser technology, and so forth. These committees are supported in part by the government and in part by industry. Professor Kobayashi maintains an effective grant on "Cooperative Research with Incorporated Organizations" through MOE. In this grant effort Dr. Kobayashi has projects with five companies on FLCs, AM LCDs, High Definition (HD) LCDs, LC alignment layers, and flexible displays.


Published: June 1992; WTEC Hyper-Librarian