PASSIVE LIQUID CRYSTAL DISPLAYS
Twisted Nematic LCDs
The name "liquid crystal" was first suggested by the physicist O. Lehman in 1889 to characterize a peculiar state of matter whose properties are intermediate between a crystalline solid and an isotropic liquid. This state of matter combines many of the mechanical properties of ordinary fluids (they often adopt the shape of their container, form droplets, etc.) with optical and electromagnetic anisotropic properties characteristic of crystals. Terms like mesophases or ordered liquids are also used in the literature to describe this state.
The molecules forming liquid crystals are often characterized by cigar-shaped or elongated molecules. The direction of the elongation defines the long axis of the molecules. As mentioned above, the essential properties of a liquid crystal are its optical and electromagnetic anisotropy. The manifestation of this property at the molecular level is that the long axis of the molecules tend to align in a preferred direction, that is, they have orientational order. Depending on the type of this orientational order, there are a number of distinct phases of liquid crystals materials: nematic, smectic, and cholesteric.
For display applications, the most useful are the nematic and twisted nematic phases. The nematic phase is characterized by long-range orientational order. The long molecular axes possess a preferred orientation, so that on the average they are positioned parallel to this preferred direction, called the director. This phase is optically uniaxial, with the optical axis parallel to the director. The director field is easily distorted by electromagnetic fields or by surfaces that have been properly prepared. Preparation of the surfaces is done by special coatings and rubbing techniques. The nematic liquid in contact with this surface will align parallel to the surface in a preferred direction.
By introducing a nematic liquid between two surfaces with the alignment preparation perpendicular to each other, a peculiar situation is achieved where the director is seen to rotate in a regular fashion from one plate to another as one progresses along the twist axis (See part (a) of Figure 3.1). This is the known 90 degree twisted nematic phase that is widely used for liquid crystal displays. It can be shown that in certain conditions, the plane of polarization of linearly- polarized light propagating parallel to the helical axis follows the twist of the phase, and as a result, the polarization of the light also turns by 90 degrees.
Figure 3.1. Principle of TN-type LCDs.
The twisted nematic liquid crystal (TN LCD) operation is depicted in Figure 3.1. Each substrate generally consists of glass overcoated with a transparent conductive layer (for example, indium tin oxide or ITO). The innermost surfaces are prepared with a special alignment layer so that a 90 degree twist of the TN molecules is achieved. The cell is then sandwiched between two polarizers. The polarizing axes of the external polarizer can be parallel or perpendicular to the alignment layer rubbing direction. In the perpendicular mode, Figure 3.1 (used for watches and calculator instruments displays), when no voltage is applied to the two transparent electrodes, light passes through the TN structure because the polarized light will follow the twist, as explained above. When a voltage is applied, because of the electric anisotropic properties of the liquid crystal phase, the TN molecules will unwind and align parallel to the direction of the electrical field (See part (b) of Figure 3.1). In this situation, the polarization of the light remains unchanged and the light transmission through the TN cell is negligible. The transmission curve of the TN cell, as a function of the rms voltage, is shown in Figure 3.2.
Figure 3.2. The transmission of TN cell as function of the applied rms voltage.
Simple liquid crystal displays using seven segments per numeric character have one wire connection for each addressable segment (direct drive mode). As displays become larger, requiring more characters, it becomes difficult and uneconomical to make a connection to each segment. To reduce the number of connections, a matrix addressed scheme has evolved called time multiplexing. For example, in a full graphic display of (N x M) segments, multiplexing will reduce the number of connections from (M x M + 1) to (N + M + 1). These displays suffer from low contrast ratio and reduced viewing angle; these properties degrade further as the multiplex ratio increases. The quality of a 640 x 480 laptop computer display using TN mode is basically unacceptable.
The pixels (segments in a matrix display) respond to the applied rms voltage. Alt and Pleshko (1974) showed that as the multiplex ratio (i.e., the number of lines to be addressed in the matrix) increases, the contrast ratio becomes poorer until no distinction is possible between ON and OFF pixels:
Von / Voff - selection ratio
Von / Voff - the rms "on" and "off" pixel voltages
N - number of multiplexed lines
From the formula above it is obvious that there are a very limited number of lines that one can multiplex without destroying the contrast ratio. For a given selection ratio, the contrast ratio of TN display can be improved by increasing the steepness of the transmission curve (Figure 3.2). Unfortunately, the steepness is a function of the liquid crystal material parameters, and today it appears unlikely that further material improvements will lead to significant changes in the performance of multiplexed TN displays.
Supertwisted Nematic Displays
Through computer modeling it was found (Amstutz et al. 1983) that the steepness of the transmission curve for a twisted nematic structure is dramatically increased by increasing the layer twist angle from 90 degrees to 180 degrees and 270 degrees. Under these conditions one can achieve a large number of multiplexed lines, higher contrast, and, as a result, a large number of gray scale, requirements that are a precondition for a high information content display. While TN displays are limited to about 64:1 multiplexing, STN LCDs can achieve multiplexing ratios of 480:1. To sustain a twisted structure greater than 90 degrees, the nematic materials are doped with an optically active material. The chirality of the dopant molecules imparts an intrinsic helical twist to the whole nematic structure, and the new nematic solution is known as chiral nematic.
The first successful STN LCD used the birefringence mode (Scheffer and Nehring 1984), in which the contrast results from the interference of two optical modes. By adjusting the polarizers axis with respect to the rubbing directions, one can obtain a yellow mode or a blue mode. These colors are not well accepted (dþn ÷ 0.8-1 þm). In 1987 a neutral black-on-white STN was demonstrated by reducing the spacing between the glass plates such that dþn (the optical path difference) has values between 0.4-0.6 þm. Another approach to remove the interference colors consists of a retardation compensation that results from using a double-layer STN LCD (DSTN). The same compensation can be achieved using a polymer film with double refraction (film supertwisted nematic -- FSTN). STN displays are extensively used in all portable applications, although they suffer from slow response time ( approx. 150 msec), small spacing impacting the yield, and a limited number of gray scale levels. Currently, the best solution for achieving high information content, TV speed, and full-color LCDs are active matrix LCDs.
Published: December 1994; WTEC Hyper-Librarian