Twisted nematic liquid crystal display 1(3)
	History of LCD-technique   In 1888, an Austrian named Friedrich Reinitzer discovered that cholesterol benzoate melted to form a milky liquid. Upon heating the liquid further, it became transparent. Later, it was shown that the liquid was birefingent, a property previously known only to certain crystals, but until then not observed in liquids.   Normally, the molecules in these organic materials have rod-like structures that are not completely randomly orientated above the melting point but form various structures with some degree of long range ordering within certain temperature ranges. The possibility of using liquid crystals as optical shutters has been known for over 50 years. However, the development of displays did not start until the beginning of the 1960’s. The principle of the first liquid crystal display was based on light scattering. In 1971, a description of the twisted nematic (TN) effect was published by Schadt and Helfrich. A few years later, this initiated a revolution in LCD-watches and calculators.   The word liquid is often associated with something fluid and the word crystal something solid; the expression liquid crystal therefore appears to be contradictory. The word crystal implies that there is some degree of long range order in the structure. A crystal such as quartz has a regular structure where the molecules are held in an ordered arrangement. However, glass is an amorphous material where the molecules are arranged in random positions and orientations with no order being present.     Phases of matter   Substances normally occur in three different phases; solid, liquid & gas. The solid phase can be either crystalline or amorphous (super-cooled liquids). When crystalline solids are heated, they normally melt to form an amorphous liquid with an irregular structure. Lowering the temperature of the liquid causes it to return to the solid phase with a crystalline structure.   Nematic liquid crystals are organic materials and the molecules often have rod-like forms. One example of a liquid crystal molecule is the low molar mass material p-n pentyl-p’-cyanobiphenyl (5CB).           Phases of liquid crystal (LC) materials   In the solid phase, the liquid crystal (LC) molecules are arranged in a crystalline structure. At the melting point, the material transforms between a solid phase and a liquid crystal (LC) phase. In this phase, there is no short range stacking order but a long range macro-structure does exist. Unlike other materials, a liquid crystal material possesses several different liquid phases depending upon the temperature. Heating the liquid crystal phase further transforms the material into an amorphous phase.       Liquid crystal materials often consist of elongated, rod-shaped molecules that resemble a liquid in that they are free to flow, but differ from it in that within a certain temperature range, the molecular shape gives rise to a long range stacking order. Other molecular shapes that display this phenomenon include discs and rectangular formed molecules.     Nematic phase of liquid crystal materials   The long range stacking sequence displays several different phases depending upon the temperature and geometry of the liquid crystal molecules. The most commonly used phase as far as LCD’s are concerned is the nematic phase. Here, the LC molecules possess a natural tendency for the molecular director axes to align parallel with each other, whilst their centre of masses remain randomly distributed throughout the material.       Smectic phases of liquid crystal materials   A lowering of the temperature from the nematic phase may produce a further ordering of the LC structure by causing the molecular centre of masses to align in layers, although the molecules still remain randomly distributed within each individual sheet of LC material. This produces both the Smectic A and Smectic C phases in which the average molecular axes are aligned respectively perpendicular to and with a small tilt angle away from the planes of these layers. Further temperature reduction beyond this point finally produces the crystalline, solid phase in which the material also possesses stacking order within the individual layers themselves and are hence held rigidly in an ordered lattice.           Elastic constants of liquid crystal materials   We can define an elastic constant for a liquid crystal material that gives the relative ease of structure deformation with an externally applied force. However, the amount of structure deformation depends upon the nature of the deformation process. This leads to there being three elastic constants for a liquid crystal material, one for each of the three deformation mechanisms. In general, it is relatively easy to induce twist deformation and consequently k22 < k11 < k33           Order parameter of liquid crystal materials   To quantify the degree of stacking order present in a liquid crystal material, an order parameter (S) is usually defined where S = ½ <3cos2q – 1>. Here, q is the angle between the director and the long molecular axis and an average is taken for all molecules in the sample. The average of the cosine term is zero for an isotropic liquid, giving the order parameter as zero. For a perfect crystal, the order parameter evaluates to one. Typical values for the order parameters of LC materials lie between 0.3 and 0.9, with the value being a function of temperature due to kinetic molecular motion.     Most liquid crystal materials possess a working temperature range of about -20oC to +70oC. This temperature range covers the majority of both commercial and industrial applications. In applications where the liquid crystal display will be out-door or in direct sunlight, a higher temperature range may be required. The storage temperature is typically -40oC to +90oC.     Structure of a liquid crystal cell   Twisted nematic (TN) liquid crystal displays (LCD) are devices used for displaying images and are found in many different consumer products such as digital clocks, microwave ovens, CD players and many other modern electronic devices. LCD’s are common because they offer many advantages over other display technologies; they are thinner & lighter and require much less power than other display technologies such as Cathode Ray Tubes (CRTs), etc. LCD’s are also highly cost effective.       In a liquid crystal display, the liquid crystal material is enclosed in a cell consisting of two (2) substrates bonded together by the gasket along the edges. An opening in the gasket allows the liquid crystal material to be injected into the cell via the filling-hole. The gap between the substrates is very thin and typical values lie between 2µm and 20µm. To maintain the distance between the substrates, spacers are often used. Spacers are small glass spheres or rods with uniform diameters.                   Surface molecular alignment in liquid crystal cells   The inner-surfaces of the liquid crystal cell are usually treated with an alignment layer. The purpose of the alignment layer is to orientate the surface liquid crystal molecules in a pre-defined direction. One method of alignment is to deposit a thin polyimide layer (100nm) over the inner-surfaces and form a series of microscopic scratches in a uniform direction by gentle rubbing of a velvet cloth. In such case, the liquid crystal molecules at the polyimide surface show a tendency to align parallel to the microscopic groves with a small tilt angle of between approximately 1.5° and 8°.   Other alignment techniques also exist such as oblique vacuum deposition procedures. These orientation methods generate different molecular tilt angles but the cell still functions in essentially the same physical manner.   The liquid crystal alignment at both sides of the cell is hence defined during cell manufacturing. By careful control, any twist-angle can therefore be induced in the helical structure across the liquid crystal layer. With a twist-angle of exactly 90°, the standard 90° twisted nematic (TN) cell is formed. Twist-angles of less than 90° form the low-twist (LT) cell whereas by definition, super-twist cells are cells that possess twist-angles exceeding 180°.     Twisted-nematic liquid crystal displays (TN-LCD)   The twisted nematic (TN) liquid crystal cell is frequently used in liquid crystal displays (LCD’s). Here, the liquid crystal material is in the nematic phase at room temperature. Characteristic for this type of liquid crystal cell is that the orientation of the rod-like liquid crystal molecules strive to form a 90° twisting helical structure.       Twisted Nematic (TN), Super Twisted Nematic (STN) and Active Matrix (AM-LCD) displays all use the nematic liquid crystal phase for their operation. This phase is selected for its physical properties and the widest temperature range. It should be noted that the fluids used for display purposes are often mixtures of several different liquid crystal materials; by mixing several liquid crystals together, a eutectic mixture will result with the melting point depressed well below room temperature.     Optical activity of liquid crystal materials   In the limit of large cell thickness, a twisting helical structure is capable of rotating the plane of linearly polarised light with nearly 100% efficiency. Light rotation occurs due to the inherent anisotropic index of refraction (birefringence) present in the molecules of the liquid crystal material. Here, incident radiation experiences two refractive indices depending upon the oscillation direction; no corresponding to the ordinary ray with the electric field vector oscillating perpendicular to the molecular director and ne for the extra-ordinary ray oscillating parallel to this axis. A phase factor or retardation is therefore introduced between the two light components upon passage through the liquid crystal material, inducing a rotation of the polarisation.   The birefringence or anisotropic index of refraction, Dn, is defined as being Dn = (ne - no). The majority of liquid crystal materials possess positive anisotropic indices of refraction, typically lying between +0.05 and +0.30 (i.e  ne > no).   The optic-axis of the liquid crystal material is defined as being in a direction parallel with the rod-like molecules and along this axis there is no birefringence (retardation). However, the liquid crystal material exhibits birefringence in directions perpendicular to the optic axis.         In general, the refractive index, n, is both wavelength and temperature dependent, although in practice the variations are relatively modest, n becoming somewhat smaller both with increasing wavelength and temperature until the clearing point is reached. Above the clearing point, rapid molecular rotations average out the refractive indices for both light components and the anisotropic index of refraction for the material approaches zero.   Click here for technical article discussing the efficiency of the twisting nematic helical structure to rotate the plane of polarisation of incident light.     Dielectric anisotropy of liquid crystal materials   The molecules of a liquid crystal material also possess a dielectric anisotropy; hence can be predominantly aligned upon application of an external electric field across the cell. This destroys the natural helical stacking structure of the device and forms the homeotropic phase. In the homeotropic texture, the optical activity of the device is predominately removed.           Click here to continue to page two of this section. © Stephen Palmer 2005