Liquid crystal display

Alternative Title: LCD


Electro-optical effects in liquid crystals

Liquid crystals are materials with a structure that is intermediate between that of liquids and crystalline solids. As in liquids, the molecules of a liquid crystal can flow past one another. As in solid crystals, however, they arrange themselves in recognizably ordered patterns. In common with solid crystals, liquid crystals can exhibit polymorphism; i.e., they can take on different structural patterns, each with unique properties. LCDs utilize either nematic or smectic liquid crystals. The molecules of nematic liquid crystals align themselves with their axes in parallel, as shown in the figure. Smectic liquid crystals, on the other hand, arrange themselves in layered sheets; within different smectic phases, as shown in the figure, the molecules may take on different alignments relative to the plane of the sheets. (For further details on the physics of liquid crystalline matter, see the article liquid crystal.)


Twisted nematic displays

The first LCDs became commercially available in the late 1960s and were based on a light-scattering effect known as the dynamic scattering mode. These displays were used in many watches and pocket calculators because of their low power consumption and portability. However, problems connected with their readability and the limited lifetime of their liquid crystal materials led to the development during the 1970s of twisted nematic (TN) displays, variants of which are now available in computer monitors and flat-panel televisions.

A TN cell, as shown in the figure, consists of upper and lower substrate plates separated by a narrow gap (typically 5–10 micrometres; 1 micrometre = 10−6 metre) filled with a layer of liquid crystal. The substrate plates are normally transparent glass and carry patterned electrically conducting transparent coatings of indium tin oxide. The electrode layers are coated with a thin aligning layer of a polymer that causes the liquid crystal molecules in contact with them to align approximately parallel to the surface. In most currently manufactured displays, the alignment layers consist of a layer of polymer a few tens of nanometres thick (1 nanometre = 10−9 metre) that has been rubbed with a cloth in only one direction. In assembling the cell, the top and bottom substrate plates are arranged so that the alignment directions are perpendicular to each other. The whole assembly is then contained between a pair of sheet polarizers, which also have their light-absorption axes perpendicular to each other. In the absence of any voltage, the perpendicular alignment layers cause the liquid crystal to adopt a twisted configuration from one plate to the other. With no liquid crystal present, light passing in either direction through the cell would be absorbed because of the crossed polarizers, and the cell would appear to be dark. In the presence of a liquid crystal layer, however, the cell appears to be transparent because the optics of the twisted liquid crystal match the crossed arrangement of the polarizers. Application of three to five volts across the liquid crystal destroys the twisted state and causes the molecules to orient perpendicular to the substrate plates, giving a dark appearance to the cell, as shown in the diagram. For simple displays, the liquid crystal cell is operated in a reflective mode, with a diffuse reflector placed behind the display, and the activated parts of the electrode pattern appear as black images on a gray background provided by the diffuse reflector. By patterning the electrodes in segments or as an array of small squares, it is possible to display alphanumeric characters and very low-resolution images—for example, in digital watches or calculators.

More-complex images can be displayed using a technique known as passive-matrix addressing (described below). However, even with this technique, 90° TN displays can produce images consisting of only about 20 rows of picture elements, known as pixels.

Supertwisted nematic displays

It was discovered in the early 1980s that increasing the twist angle of a liquid crystal cell to about 180–270° (with 240° being fairly common) allows a much larger number of pixel rows to be used, with a consequent increase in the complexity of images that can be displayed. These supertwisted nematic (STN) displays achieve their high twist by using a substrate plate configuration similar to that of TN displays but with an additional optically active compound, known as a chiral dopant, dissolved in the liquid crystal. The display is activated using passive-matrix addressing, for which the pixels are arranged in rows and columns; selective application of a voltage to a particular row and column will activate the corresponding element at their intersection. The supertwist causes a larger relative change in optical transmission with applied voltage, compared with 90° twisted cells. This reduces the illumination of unwanted pixels, so-called “cross talk,” which controls the number of rows that can be activated in passive-matrix addressing. Colour STN displays have been produced for computer monitors, but they are being replaced in the market by more modern thin-film transistor TN displays (described below), which have better viewing angles, colour, and response speed. Monochrome STN displays are still widely used in mobile telephones and other devices that do not require colour.

Thin-film transistor displays

The display of complex images requires high-resolution dot-matrix displays consisting of many thousands of pixels. For example, the video graphics array (VGA) standard for computer monitors consists of an array of 640 by 480 picture elements, which for a colour LCD translates to 921,600 individual pixels. Excellent images can be built up from arrays of this complexity by using thin-film transistor (TFT) TN displays, in which each pixel has associated with it a silicon transistor that acts as an individual electronic switch. (A cutaway portion of a TFT display is illustrated in the figure.) The use of a transistor for each pixel makes the TFT an active-matrix display, as opposed to the passive-matrix display described in the previous section. The TN effect produces black-and-white images, but, as shown in the diagram, colour images can be generated by forming three-pixel groups using red, blue, and green filters. The displayed image is bright by virtue of a flat backlight placed behind the liquid crystal panel.

Introduced at the end of the 1980s, TFT displays are now widely used in portable computers and as space-saving flat-screen monitors for personal computers. Some aspects of TFTs, such as viewing angle, speed, and the manufacturing cost of large-area displays, have slowed their full commercial exploitation. Nevertheless, these LCDs are increasingly entering the home television market.

Other transmissive nematic displays

In recent years a number of alternatives to the 90° TN have been commercialized for use on active-matrix substrates. For example, in-plane switching (IPS) displays operate by applying a switching voltage to electrodes on a single substrate to untwist the liquid crystal. IPS displays have a viewing angle intrinsically superior to that of TFT TNs; however, the requirement for more electrode circuitry on their substrate can result in a less efficient use of the backlight. Twisted vertically aligned nematic (TVAN) displays utilize molecules that tend to orient with their long axes perpendicular to the direction of an applied electric field. A small quantity of an optically active material is added to the liquid crystal, causing it to adopt a twisted configuration upon the application of voltage. TVAN displays can show very high contrast and good viewing-angle characteristics.

Reflective displays

The backlight of LCDs typically accounts for more than 80 percent of the display’s power consumption. For mobile complex displays, battery lifetime is of great importance, and clearly the development of products that can be viewed in ambient light without recourse to backlighting is highly desirable. Such displays are known as reflective displays, and they can be realized in a number of ways. Some commercial reflective displays operate much like the transmissive STN. The liquid crystal again acts as an electro-optical layer between two polarizers. In place of a backlight, however, an aluminum mirror is used to reflect ambient light back toward the viewer when the liquid crystal is switched to a bright (or transmissive) state. Polarizers absorb about 50 percent of unpolarized light passing through them, and the removal of one or both polarizers can increase the brightness of the reflective displays. Indeed, active-matrix devices with single polarizers have begun to dominate the high-quality reflective display market—for example, in mobile phones and handheld electronic games.

Another type of reflective device, known as a guest-host reflective display, relies on dissolving “guest” dye molecules into a “host” liquid crystal. The dye molecules are selected to have a colour absorption that depends on their orientation. Variations in an applied electric voltage change the orientation of the host liquid crystal, and this in turn induces changes in the orientation of the dye molecules, thus changing the colour of the display. Guest-host devices may use one or no polarizers, but again they require a mirror. They can show high brightness, but generally they exhibit poorer contrast than optimized TN single-polarizer devices.

Truly reflective displays (not requiring a mirror) have been manufactured using optically active liquid crystals known as chiral nematics or cholesteric liquid crystals. (The first chiral nematics were based on derivatives of cholesterol, hence the now-obsolete term cholesteric.) The molecules of such optically active liquid crystals spontaneously order into helical structures that are found to reflect light of a specific wavelength (i.e., a specific colour) that is approximately equal to the pitch of the helices. Changing the orientation of the helices by an electric field can switch the liquid crystal from a coloured reflective state to a scattering or black state. The devices have a high resolution and acceptable contrast, but they are rather slow and are typically used in static displays.

Transflective displays have been developed that combine some of the features of polarizer-based reflective displays and transmissive displays. Transflective devices use a mirror that is partially reflective and partially transmissive, situated between the liquid crystal layer and a backlight. When ambient light levels are high, the backlight may be turned off and the display operated as a reflective device, saving battery power. When light levels are low, the backlight may be turned on to increase the brightness of the display. This clearly has advantages, although transflective displays by their nature represent a compromise and cannot readily match the reflectivity of a dedicated reflective display or the brightness of a transmissive device.

Projection displays

The LCDs used in projection systems are typically small reflective or transmissive panels illuminated by a powerful arc lamp source. A series of lenses magnifies the reflected or transmitted image and casts it onto a screen. In front-projection systems the LCD is situated on the same side of the screen as the viewer, while in rear-projection systems the screen is illuminated from behind. Projectors of higher cost and performance may use three separate LCD panels, forming separate red, green, and blue images that combine to form a coloured image on the screen.

Smectic LCDs

The increasing demand for video displays has placed a growing emphasis on the switching speed of liquid crystals. This has led to the development of devices employing smectic liquid crystals, certain of which have a faster electro-optical response than nematic liquid crystals. The surface-stabilized ferroelectric liquid crystal (SSFLC) display is currently the most developed smectic device. In it the liquid crystal molecules are arranged in layers perpendicular to the substrate planes, which are separated by one or two micrometres, and within the layers the molecules are tilted, as illustrated in the figure. The host liquid crystal contains optically active molecules, and a subtle consequence of the optical activity and the tilt of the molecules is the appearance of a permanent charge separation, or ferroelectric dipole, analogous to the ferromagnetic dipole of a magnet. The direction of this dipole is perpendicular to the tilt direction of the molecules and in the plane of the layers. Thus, there is a permanent charge separation across the liquid crystal layer in the SSFLC, and its sign is directly coupled to the tilt direction of the molecules. An applied voltage of the correct sign can reverse the direction of this dipole in tens of microseconds and hence reverse the tilt direction of the molecules. The corresponding change in optical properties can cause a change from light to dark when one or more polarizers are used.

SSFLC devices have been commercialized for large passive-matrix displays, but their cost and complexity have prevented them from making any significant impact on the market. Small transmissive and reflective active-matrix SSFLC displays, however, show some promise for use as elements in projection systems or as viewfinders in digital cameras. Their fast response allows them to be used in time-sequential colour systems, in which costly colour filters are replaced by a coloured backlight that flashes red, green, and blue in rapid succession (about 100 cycles per second). For example, the liquid crystal can be switched to a transmissive state during the red and green periods and to a nontransmissive state during the blue period, with the result that the eye sees an average of red and green light, or the colour yellow.

David Dunmur Harry G. Walton

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