<strong>Antiferroelectric</strong> <strong>liquid</strong> <strong>crystal</strong> <strong>displays</strong>Fig. 1. Technologies and applications of <strong>displays</strong>. A trend for increasing size and resolution is highlighted.At the same time as AM-TFT LCDs were developed, analternative way to prepare high resolution LCDs arose froma different approach. Bistability was demonstrated insmectic LC phases, associated with ferroelectricity. BistableLCs are able to hold pixel information in the absenceof power supply. Therefore, they can be driven, in principle,by passive matrices. Since then, new ferroelectric families– antiferroelectrics, V-shape smectics, orthoconic materials– have widen the number of possible applications forthese materials. In the last few years, ferroelectric-baseddisplay prototypes ranging from micro<strong>displays</strong> to large direct-view<strong>displays</strong> have been created [8], demonstrating thefeasibility of such technology. Development of these <strong>displays</strong>has been precluded by the lack of suitable materialsTable 1. Display capabilities of different LCD technologies.Material/technologyMultiplexabilityGreyscaleOpticalstabilityColourditheringSwitchingtypeViewingangleDynamicresponsePassive twistednematicVerylimitedAnalogue Monostable Spatial Out-of-plane Poor 10’s msPassive supertwistednematicActive matrix twistednematicActive matrixhomogeneousnematicLimited Analogue Monostable Spatial Out-of-plane Very poor 10’s msUnlimited Analogue Monostable Spatial Out-of-plane Good 10’s msUnlimited Analogue Monostable Spatial In-plane Excellent 10’s msPassive ferroelectricUnlimitedSpatial/temporalditheringBistable Temporal In-plane Excellent 10’s µsPassiveantiferroelectricUnlimited Analogue MultistableSpatial/temporalIn-plane Excellent 10’s µsActive matrixV-shape smecticUnlimited Analogue MonostableSpatial/temporalIn-plane Excellent 10’s µs264 Opto-Electron. Rev., 12, no. 3, 2004 © 2004 COSiW SEP, Warsaw
XV Liquid Crystal Conferenceand some difficulties in the manufacturing process.AM-TFTs, on the other hand, have experienced an extraordinarygrowth in many different applications areas [9].In this work, the current state of the art and possibleevolution of <strong>displays</strong> made of several ferroelectric kinds isreviewed. The work is focused on the above mentionedthree families, whose electrooptic characteristics bestowgood performance for high-end display applications.2. Electrooptical behaviour of ferroelectricfamiliesAn increasingly growing number of smectic LC phaseshave been described [10]. A number of them showferroelectricity. Depending on their spontaneous orientationin the absence of voltage, ferroelectric LC materials,similarly to solid ferroelectric materials, are classified inthree different groups:• regular ferroelectrics, in which all the molecular dipolemoments are parallel to each other, and the macroscopicspontaneous polarization, therefore, is maximum,• antiferroelectrics, in which the dipole moments areantiparallel, therefore the spontaneous polarization iscancelled out,• ferroelectrics, intermediate phases with net spontaneouspolarization lower than the corresponding ferroelectricmaterial. No practical applications of these phases havebeen described yet.In the last few years, other ferroelectric subfamilies havebeen included:• V-shape smectics, a configuration showing thresholdlessswitching. Besides the controversy about the originof this effect, and the involved smectic phase [11],their electrooptical (EO) behaviour seems quite appropriatefor display applications.• orthoconic smectics, tilted smectic phases like C phase(SmC) are characterized by the angle formed betweenthe molecular director and the smectic layer normal. Inorthoconic materials, this angle is 45°. Ferroelectricorthoconics switch between states located at 90° to eachother. This confers unique EO properties to these materials[12], as commented below. Ferroelectric andantiferroelectric orthoconic materials have been found.Displays made of ferroelectric LCs are usually preparedin thin (1.5–2 µm) cells with the LC material oriented inhomogeneous configuration (i.e., parallel to the plates).These conditions lead to surface stabilization and bistabilityor multistability. Moreover, all the configurationsachieved by the material upon switching are oriented parallelor nearly parallel to the outer plates. This feature iscalled in-plane switching (IPS) and results advantageousfrom the optical point of view. All kinds of ferroelectric<strong>displays</strong> show excellent optical performance as comparedto twisted LCDs. The viewing angle is high, and colourdegradation for oblique light incidence is less noticeablethan in twisted nematic LCDs – not to mentionsupertwisted nematics. Some recent realizations onhigh-end <strong>displays</strong> include special arrangements of electrodesto achieve IPS with nematic LCs.2.1. Switching ferroelectric <strong>displays</strong>All ferroelectric families in surface-stabilized homogeneousconfigurations can be switched by DC signals. Thesimplest case is the bistable switching of regular FLCs. Theorientation achieved upon the application of an electricfield is kept in the absence of voltage. This intrinsic memorycan be used to generate permanent images in <strong>displays</strong>with no power consumption, and to prepare high rate multiplexed<strong>displays</strong> with passive matrices.The main disadvantage of FLC materials in high-end<strong>displays</strong> arises from its own bistability. Indeed, permanentorientation avoid the generation of intermediate transmissionlevels (i.e., grey levels). A full greyscale is requiredfor the display to render full colour. High-end <strong>displays</strong> requireat least 64 grey levels (256 kcolours); 256 grey levels(16.7 Mcolours) are customarily assumed in most applications.Although several alternatives have been proposed[13,14], regular FLC <strong>displays</strong> can only produce grey levelsby spatial or temporal dithering, i.e., sharing either thepixel area between several subpixels or the frame time betweenON and OFF states. These dithering techniques increasethe number of addressed subpixels (spatial) or reducethe frame time (temporal). In either case the data raterequired for any given application is substantially increased.Data rates as high as 2 Gbps may be required toprovide true colour (24 bits) in a high definition (SXGA)video rate application.2.2. Analogue switchingSeveral ferroelectric families, like antiferroelectrics andV-shape smectics, show intrinsic analogue greyscale. Thisis a decisive advantage for these materials to be employedin high-end <strong>displays</strong>. Figure 2 shows the electrooptical responseof these two materials when driven by low frequencyAC voltage signals of different amplitudes.V-shape smectics show thresholdless switching, the greyscalebeing developed at low voltages. Tristate antiferroelectricsdevelop greyscale above a voltage threshold. Greylevels can be stabilized by applying a constant DC voltage(bias voltage) below a threshold. Bias voltage is the samefor any grey level, this is important because bias voltage isthen compatible with passive multiplexing. The greyscalevoltage range in both cases is just a few volts, therefore itcan be generated with standard electronics.As it can be seen in Fig. 2, V-shape smectics do not showhysteresis. Grey levels are kept as long as the correct voltageis applied to every pixel. As a consequence, multiplexing ofthese materials requires the use of active matrices. TristateAFLCs, on the other hand, can be driven either with active orpassive matrices, for grey levels can be maintained over thewhole display with a constant DC voltage.Opto-Electron. Rev., 12, no. 3, 2004 J.M. Otón 265
Change language