FERROELECTRIC LIQUID CRYSTAL DISPLAYS (FLCD) Visit www.seminarlinks.blogspot.com to download
Introduction FLCD, is a next generation display device that uses the interactions between spontaneous polarization of the ferrodielectric liquid crystal and the electric field and provides rapid response characteristics.
Ferrofluid • A ferrofluid is a liquid which becomes strongly magnetized in the presence of a magnetic field. • Ferrofluids are colloidal liquids made of nanoscale ferromagnetic, or ferrimagnetic, particles suspended in a carrier fluid (usually an organic solvent or water).
Advantages over traditional LCDs Traditional LCDs • Traditional LCD displays are usually of the Super Twisted Nematic (STN) variety. • Response time in tens of miliseconds (ms) • Causes video anomalies • Narrow visual angle • Generates cross-talk among pixels within a certain distance • Lead to difficulties in reducing a pixel size below a certain size. FLCD • Spontaneous polarization of the ferrodielectric liquid crystal and the electric field . • provides rapid response characteristics below 1 ms. • Rapid response is essential to eliminating video anomalies. • Very wide viewing angles • Can implement a high resolution with a more reduced pixel size . • Prevent cross-talk from occurring due to strong interactions between molecules
The graphic, provided by Displaytech, nicely illustrates the comparison between images produced by their FLC with VLSI backplane devices and the typical AMLCD device. Notice that the FLC device (left) allows full color on each pixel while each pixel has only one color for the AMLCD device (right).
Working of Ferro Electric Crystals • FLCDs are smectic liquid crystals that have a natural layered order. • Most FLCDs are • of the smectic C phase (SmC*) • i.e., they are tilted away from the layer normal (90°) and • possess a chiral behaviour • i.e., they have a layered structure with the molecules at some angle (the "cone angle") away from the layer normal, and there is some inherent twist in the structure.
Geometry of chiral smectic C phase
• So, an unconstrained system, the azimuthal direction in which the molecules tilt away from the layer normal will differ slightly from one layer to the next. • Typically, the FLCDs are built with cell gaps less than 2 µm for stable molecular alignment. • Alignment layer causes perpendicular stacked alignment. • The cell's polarisation is determined by the magnetic field applied. • That in turn results in opaque or transparent layer when used in combination with polarised layers as in LCD.
FLCD Operating Principle • An FLCD acts as a classical half-wave plate, one whose optic axis can be reoriented by an applied field: • If the optic axis is parallel or perpendicular to incoming polarized light, the light passes through the FLCD unchanged and is blocked by the exit polarizer(oriented at 90o to the entrance polarizer) • If the optic axis makes an angle of 45o to the incoming polarized light, the direction of polarization changes by 90o and is able to pass through the exit polarizer • Orientation of FLF molecules is changed by applying a voltage pulse of suitable polarity.
FLCD Operation Exit Polarizer Glass LC Molecule (Rotates through an angle of 45O) Glass Entrance Polarizer Optical Axis Cell Spacing OFF State ON State DARK BRIGHT
FLCD Operation • In thin FLC cells, a bistability appears with two bistable states as shown in Fig. • Ferroelectric liquid crystals have a spontaneous polarization (Ps) whose direction is perpendicular to the layer. • When the electric field is applied, molecules re-align in a way that the direction of the spontaneous polarizations is the same as that of the electric field. • Combining a pair of polarizers (polarizer and analyzer), FLCDs can realize dark and bright states. Principle of FLCD
Molecular orientation control • It is one of the most important key technologies for the development of practical FLCDs. • The molecular orientations of FLCDs are classified as two layer structures: Bookshelf Layer Chevron Layer
Molecular Orientation Control • The FLC cells with parallel rubbing, in which the rubbing directions of both substrates are the same, have been known to show four orientational states with a chevron layer structure - • C1-uniform (C1U), C1-twisted (C1T), C2-uniform (C2U) and C2- twisted (C2T). • Among them, the C1U and C2U orientations are useful for practical applications because of their extinction positions between cross nicols.
The C2U orientation
• On a color filter substrate and a glass substrate with ITO electrodes there are an insulating film and an aligning films coated. • The material of the aligning film was polyimide. • The rubbing direction of both substrates is in the same direction (parallel rubbing). • An aligning film with a medium pretilt angle (about 3°) was utilized in order to gain 100% of the C2U state without any zigzag defects or any C1 states.
• Each pixel is divided into two areas with 1:2 ratio so as to realize spatial dither gray scale. • Spacer walls were constructed within the panel in order to show high shock stability. • The cell spacing was 1.3µm. • Optimal adhesion between both substrates was obtained, yielding higher shock stability more than 20kg/cm2.
Properties and uses • Very thin layer (less than 2 µm thick) can help produce a 90° polarisation twist. High density displays with small display areas can be produced. DisplayTECH claims that a stamp sized FLCD can drive resolutions needed for 50 inch screens. • Switching time is less than 100 µs High frame rate video displays are possible. • Magnetic polarisation effect is bistable. Can be used for low frame rate displays that can run on very low power This property can help build display with non-volatile memory with the advantage that the memory can be changed easily.
Cont.. • Viewing angle is greater than 120° This makes it suitable for commercial TV applications. • Some commercial products do seem to utilize FLCD. • High switching allows building optical switches and shutters in printer heads.
Challenges • The problems facing ferroelectric researchers are numerous. • Alignment defect control (sensitive to shock and vibration) • Cell spacing control • Temperature range • Response time, and Gray Scale New fluorinated liquid crystal compounds are being developed to help decrease the response time and improve the contrast ratio (contrast is limited by defects).
Conclusion • Prospects for the future are mixed for this technology. • While much research continues, it is unclear what market the ferroelectric LCD will serve. • Certainly, if the problems can be solved, then the high contrast and wide viewing angle achieved with ferroelectric LCDs will put them in competition with active matrix LCDs.