Blue Phase Mode LCD
Encyclopedia
A Blue Phase Mode LCD is a liquid crystal display
technology that uses highly twisted cholesteric phases in a blue phase. It was first proposed in 2007 to obtain a better display of moving images with, for example, frame rates of 100–120 Hz to improve the temporal response of liquid crystal displays (LCDs). This operational mode for LCDs also does not require anisotropic alignment layers (e.g., rubbed polyimide) and thus theoretically simplifies the LCD manufacturing process.
In Reinitzer's reports from 1888 on the melting behaviour of cholesteryl benzoate
, there is a note that the substance briefly turned blue as it changed from clear to cloudy upon cooling. This subtle effect however remained unexplored for more than eighty years until experimental results were published during the late 1960s and early 1970s that indicated that the blue color
was due to at least two new and very different liquid crystalline phases
.
For almost one hundred years, scientists assumed that the most stable cholesteric
helical structure could be described by a single helical axis about which the director rotates. It turned out that in the new structure the director rotates in a helical fashion about any axis perpendicular to a line as illustrated in fig. 1. Although an unlimited number of helical axes are actually present, this structure was named double twist structure.
This double twist structure is more stable than the single twist structure (i.e., the normal helical structure of chiral nematics) only up to a certain distance from the line at the center. Since this distance is on the order of the pitch of the chiral nematic liquid crystal
(typically 100 nm) and since the geometries of usual liquid crystal samples are much larger, the double twist structure occurs only rarely.
Blue phases are special cases when double twist structures fill up large volumes. When double twist structures are limited in all directions to the distance from the center line where the twist amounts to 45° a double twist cylinder results. Because of its small radius, such a cylinder is more stable than the same volume filled with a single twist chiral nematic liquid crystal.
A large structure can be composed from these double twist cylinders, but defects
occur at the points where the cylinders are in contact as illustrated in fig. 5. These defects
occur at regular distances and tend to make the structure less stable, but it is still slightly more stable than the single twist structure without defects, at least within a temperature range of about 1 K below the transition from the chiral nematic phase to an isotropic liquid.
The defects
that occur at regular distances in three spatial dimensions form a cubic lattice
just as we know it from solid crystals. Blue phases are thus formed by a regular three-dimensional lattice of defects
within a chiral liquid crystal. Since the spacings between the defects
of a blue phase are in the range of the wavelength of light (several hundred nanometers), for certain wavelength ranges of the light reflected from the lattice constructive interference occurs (Bragg reflection) and the blue phase reflects colored light (note that only some of the blue phases actually reflect blue light).
Furthermore, electro-optical switching with response times of the order of 10−4 s for the stabilized blue phases at room temperature has been shown.
of 240 Hz. Samsung unveiled a 15 inch prototype model of its Blue Phase LCD panel at the SID (Society for Information Display
) 2008 international Symposium, Seminar and Exhibition, which was held in Los Angeles from May 18 to 23, 2008 .
Developed with a look at cost-efficiency, Samsung's Blue Phase mode does not require liquid crystal
alignment layers, unlike today's most widely used TFT LCD modes such as Twisted Nematic (TN), In-Plane Switching (IPS) or Vertical Alignment (VA). The Blue Phase mode can make its own alignment layers, eliminating the need for any mechanical alignment and rubbing processes. This reduces the number of required manufacturing steps, resulting in savings on production costs. Additionally it has been claimed that Blue Phase panels would reduce the sensitivity of the liquid crystal layer to mechanical pressure which could impair the lateral uniformity of display (e.g. luminance
, chromaticity).
In a blue phase based LC-display for TV applications it is not the selective reflection of light according to the lattice pitch (Bragg reflection) that is used for display of visual information, but an external electric field induces a birefringence
in the liquid crystal
via the Kerr effect
. That field induced birefringence becomes apparent as a change of transmission when the Blue Phase Mode LC layer is placed between crossed polarizer
s.
Response time
compensation by voltage overdriving as currently applied in many LCD panels with 120 Hz frame frequency for improvement of the display of moving images in LCD TVs could become obsolete since the Blue Phase mode features a superior response speed (in the range of 10 - 100 microseconds, see ref. 1), allowing images to be reproduced at 240 Hz frame rate or higher without the need for any overdrive circuit.
For a detailed discussion of the blue phase LCs in in-plane switching (IPS) structures and related modeling method based on the Kerr effect in a macroscopic scale see references . With an isotropic dark state, blue phase LCDs show many interesting electro-optic performances. Presently, the driving voltage of blue phase LCs in IPS structures is still a little bit too high. To reduce the voltage, material engineering for developing high Kerr constant mixtures is critically important.
See page 124, Figure 5.4 for details on the disclination formed in the gusset (i.e., triangular area where three double twist cylinders are in contact).
Liquid crystal display
A liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....
technology that uses highly twisted cholesteric phases in a blue phase. It was first proposed in 2007 to obtain a better display of moving images with, for example, frame rates of 100–120 Hz to improve the temporal response of liquid crystal displays (LCDs). This operational mode for LCDs also does not require anisotropic alignment layers (e.g., rubbed polyimide) and thus theoretically simplifies the LCD manufacturing process.
History
Cholesteryl benzoate
Cholesteryl benzoate, also called 5-cholesten-3-yl benzoate, is an organic chemical, an ester of cholesterol and benzoic acid. It is a liquid crystal material forming cholesteric liquid crystals with helical structure....
, there is a note that the substance briefly turned blue as it changed from clear to cloudy upon cooling. This subtle effect however remained unexplored for more than eighty years until experimental results were published during the late 1960s and early 1970s that indicated that the blue color
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
was due to at least two new and very different liquid crystalline phases
Phase (matter)
In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...
.
For almost one hundred years, scientists assumed that the most stable cholesteric
Cholesteric liquid crystal
A cholesteric liquid crystal is a type of liquid crystal with a helical structure and which is therefore chiral. Cholesteric liquid crystals are also known as chiral nematic liquid crystals. They organize in layers with no positional ordering within layers, but a director axis which varies with...
helical structure could be described by a single helical axis about which the director rotates. It turned out that in the new structure the director rotates in a helical fashion about any axis perpendicular to a line as illustrated in fig. 1. Although an unlimited number of helical axes are actually present, this structure was named double twist structure.
Liquid crystal
Liquid crystals are a state of matter that have properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of LC phases, which can be...
(typically 100 nm) and since the geometries of usual liquid crystal samples are much larger, the double twist structure occurs only rarely.
Crystallographic defect
Crystalline solids exhibit a periodic crystal structure. The positions of atoms or molecules occur on repeating fixed distances, determined by the unit cell parameters. However, the arrangement of atom or molecules in most crystalline materials is not perfect...
occur at the points where the cylinders are in contact as illustrated in fig. 5. These defects
Crystallographic defect
Crystalline solids exhibit a periodic crystal structure. The positions of atoms or molecules occur on repeating fixed distances, determined by the unit cell parameters. However, the arrangement of atom or molecules in most crystalline materials is not perfect...
occur at regular distances and tend to make the structure less stable, but it is still slightly more stable than the single twist structure without defects, at least within a temperature range of about 1 K below the transition from the chiral nematic phase to an isotropic liquid.
The defects
Crystallographic defect
Crystalline solids exhibit a periodic crystal structure. The positions of atoms or molecules occur on repeating fixed distances, determined by the unit cell parameters. However, the arrangement of atom or molecules in most crystalline materials is not perfect...
that occur at regular distances in three spatial dimensions form a cubic lattice
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
just as we know it from solid crystals. Blue phases are thus formed by a regular three-dimensional lattice of defects
Crystallographic defect
Crystalline solids exhibit a periodic crystal structure. The positions of atoms or molecules occur on repeating fixed distances, determined by the unit cell parameters. However, the arrangement of atom or molecules in most crystalline materials is not perfect...
within a chiral liquid crystal. Since the spacings between the defects
Crystallographic defect
Crystalline solids exhibit a periodic crystal structure. The positions of atoms or molecules occur on repeating fixed distances, determined by the unit cell parameters. However, the arrangement of atom or molecules in most crystalline materials is not perfect...
of a blue phase are in the range of the wavelength of light (several hundred nanometers), for certain wavelength ranges of the light reflected from the lattice constructive interference occurs (Bragg reflection) and the blue phase reflects colored light (note that only some of the blue phases actually reflect blue light).
Wide temperature range blue phases
In 2005, researchers from the Centre of Molecular Materials for Photonics and Electronics at the University of Cambridge reported their discovery of a class of blue-phase liquid crystals that remain stable over a range of temperatures as wide as 16-60 °C. The researchers showed that their ultrastable blue phases could be used to switch the color of the reflected light by applying an electric field to the material, and that this could eventually be used to produce three-color (red, green, and blue) pixels for full-color displays. The new blue phases are made from molecules in which two stiff, rod-like segments are linked by a flexible chain, and are believed to be stabilized due to flexoelectricity.Furthermore, electro-optical switching with response times of the order of 10−4 s for the stabilized blue phases at room temperature has been shown.
First blue phase LC-display
In May, 2008 Samsung Electronics announced that it has developed the world's first Blue Phase LCD panel which can be operated at an unprecedented refresh rateRefresh rate
The refresh rate is the number of times in a second that a display hardware draws the data...
of 240 Hz. Samsung unveiled a 15 inch prototype model of its Blue Phase LCD panel at the SID (Society for Information Display
Society for Information Display
The Society for Information Display is an industry organization for displays, generally electronic displays such as televisions and computer monitors. SID was founded in 1962. Its main activities are publishing technical journals and running Display Week, its main conference, held in May each year...
) 2008 international Symposium, Seminar and Exhibition, which was held in Los Angeles from May 18 to 23, 2008 .
Developed with a look at cost-efficiency, Samsung's Blue Phase mode does not require liquid crystal
Liquid crystal
Liquid crystals are a state of matter that have properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of LC phases, which can be...
alignment layers, unlike today's most widely used TFT LCD modes such as Twisted Nematic (TN), In-Plane Switching (IPS) or Vertical Alignment (VA). The Blue Phase mode can make its own alignment layers, eliminating the need for any mechanical alignment and rubbing processes. This reduces the number of required manufacturing steps, resulting in savings on production costs. Additionally it has been claimed that Blue Phase panels would reduce the sensitivity of the liquid crystal layer to mechanical pressure which could impair the lateral uniformity of display (e.g. luminance
Luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
, chromaticity).
In a blue phase based LC-display for TV applications it is not the selective reflection of light according to the lattice pitch (Bragg reflection) that is used for display of visual information, but an external electric field induces a birefringence
Birefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
in the liquid crystal
Liquid crystal
Liquid crystals are a state of matter that have properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of LC phases, which can be...
via the Kerr effect
Kerr effect
The Kerr effect, also called the quadratic electro-optic effect , is a change in the refractive index of a material in response to an applied electric field. The Kerr effect is distinct from the Pockels effect in that the induced index change is directly proportional to the square of the electric...
. That field induced birefringence becomes apparent as a change of transmission when the Blue Phase Mode LC layer is placed between crossed polarizer
Polarizer
A polarizer is an optical filter that passes light of a specific polarization and blocks waves of other polarizations. It can convert a beam of light of undefined or mixed polarization into a beam with well-defined polarization. The common types of polarizers are linear polarizers and circular...
s.
Response time
Response time
In technology, response time is the time a system or functional unit takes to react to a given input.- Data processing :In data processing, the response time perceived by the end user is the interval between the instant at which an operator at a terminal enters a request for a response from a...
compensation by voltage overdriving as currently applied in many LCD panels with 120 Hz frame frequency for improvement of the display of moving images in LCD TVs could become obsolete since the Blue Phase mode features a superior response speed (in the range of 10 - 100 microseconds, see ref. 1), allowing images to be reproduced at 240 Hz frame rate or higher without the need for any overdrive circuit.
For a detailed discussion of the blue phase LCs in in-plane switching (IPS) structures and related modeling method based on the Kerr effect in a macroscopic scale see references . With an isotropic dark state, blue phase LCDs show many interesting electro-optic performances. Presently, the driving voltage of blue phase LCs in IPS structures is still a little bit too high. To reduce the voltage, material engineering for developing high Kerr constant mixtures is critically important.
Further reading
- O.D. Lavrentovich, M. Kleman: Defects and Topology of Cholesteric Liquid Crystals" in "Chirality in Liquid Crystals, 5", Springer Verlag: New York (2001), excerpt available here.
See page 124, Figure 5.4 for details on the disclination formed in the gusset (i.e., triangular area where three double twist cylinders are in contact).
External links
- Cambridge University, Department of Engineering http://www.admin.cam.ac.uk/news/dp/2005081802
- Cambridge University, Centre of Molecular Materials for Photonics and Electronics http://www-g.eng.cam.ac.uk/CMMPE/bluephase.html
- World's First 'Blue Phase' Technology LC TV http://www.physorg.com/news129997960.html