Photonic Materials - Liquid Crystal Display 1
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Liquid Crystal Liquid crystals are organic compounds that flow like a liquid while maintaining a long range orderliness of a solid. The molecules of liquid crystal compound are in the form of long cigar shaped rods. 3
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Based on the orientation of these rods like polar molecules, the liquid crystals are classified into three basic types. Types of Liquid Crystal 1)Smectic: The Smectic phase consists of flat layers of cigar shaped molecules with their long axes oriented perpendicular to the plane of the layer. The molecules within each layer remain oriented within each layer and do not move between layers. This most ordered smectic mesophase structural model. 2)Nematic: The nematic phase also has molecules with their long axes parallel to each other, but they are separated into layers. In the nematic mesophase, while the molecules maintain their orientation, the individual molecules can move freely up and down.the nematic liquid crystal molecule consists of two benzene rings linked with a central group. A typical example is 4-methoxybenzenylidene-4-butylanaline (MBBA). The nematic liquid exhibits crystalline property over the temperature range 20 C to 47 C. 3)Chloesteric: Chloesteric mesophase can be defined as a special type of nematic in which the thin layers of mostly parallel molecules have their longitudinal axes twisted (rotated) in adjacent layers at a definite angle. This is the most ordered phase. Each layer is basically nematic. 5
Liquid Crystal Orientation In LCD s two preferred orientations of LC molecules are used: Homeotrophic [With long axis of the molecules parallel to the glass plates and electrodes] Homogeneous[With long axis of the molecules perpendicular to the glass plates and electrodes] 6
Liquid Crystal Orientation The orientation of the LC molecules parallel to the glass plates is achieved by the deposition of a layer of dielectric over the transparent electrodes. The dielectric layer near the electrodes has preferred orientation which in turn aligns LC molecules. The top and bottom dielectric layers are by 90 with respect to one another. Therefore the direction of the crystal is rotated 90 with respect to the bottom of the liquid crystal. The liquid crystal thus acts like a set of polarizers whose optic axes are parallel to each other in the presence of electric field and in crossed position in the absence of electric field. 7
Effect of electric field The fundamental property of LCs that makes them useful as display device is that they are sensitive to an external electric field. The nematic liquid crystal finds its applications increasingly in electro-optic devices since their molecules can be aligned by electric and magnetic fields to produce sufficient change in their optical properties. Liquid crystal molecules rotate as a result of external electric field. The behaviour of initially ordered liquid crystal material due to increase in electric field. 8
LCD Cell Construction The most important structure of twisted nematic mesophase liquid crystal cell. It consists of a thin layer of LC material between two glass plates that are fused together. The thickness of the LC is 10 to 25 m. The two glass plates have transparent electrodes on their inside faces made of conducting material indium tin oxide. 9
Operation of LCD The cell is assembled so that LC molecules undergo 90 twist from the top plate to the bottom plate. The cell is sandwiched between two polarizers with their polarization direction is parallel to the LC direction of each plate. The incident un-polarized light on the cell is polarized linearly as indicated and undergoes 90 rotation as it passes through the LC before exiting the bottom of the polarizer. In this mode of operation, frequently used in LCDs, the cell is transmissive, in the absence of electric field. 10
Effect of electric field 11
Effect of electric field 12
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When voltage is applied to the electrodes the LC molecules will align with field. Now the incident light do not undergo rotation in polarization direction due to liquid crystal and therefore absorbed by the exit polarizer. Thus the twisted nematic liquid crystal cell is opaque in driven state and transmissive in non driven state. The LC display device can be operated either in transmission or reflection mode. In the reflector mode, a reflector is placed below the bottom of the polarizer. With no field, therefore, the device reflects the incident light and appears bright. When the field is applied, the direction of polarization of light travelling across the cell is not rotated and cannot pass through the second polarizer and the device will appear dark. 16
Limitation of twisted nematic displays oviewing angle is restricted to ±45 ouse of polarizers reduces the maximum amount of light that can be reflected. Super twisted nematic displays othey are basically TN displays only but they have a twist of 90 to 270 from top to bottom plates, osuper twisted nematic displays have greater image contrast and wider range of viewing angles. High resolution displays are developed using STN. 17
Comparison between LED and LCD Sl. No. LED Demerits LCD Merits 1 Cost is high compared to LCD Cost is very low. 2. Not suitable for large area display Suitable for large areas display 3. High consumption Power (milliwatts) Low power consumption (microwatts) Merits Demerits 4. Operating temperature is 0 to 70 C. Operating temperature is 10 C to 47 C. 5. Response time is in nano seconds (10-9 sec) Response time is in microseconds (10-6 sec) 6. Intensity of light can be controlled Intensity of light cannot be controlled. 7. Different colour displays are available at low cost. Colour displays will not be available at low cost. 18
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