Liquid Crystal Displays Cosmin Ioniţă - Spring 2006 -
A brief history 1888 - Friedrich Reinitzer, an Austrian chemist working in the Institute of Plant Physiology at the University of Prague, discovered a strange phenomenon. While trying to find out the melting point of a cholesterol based substance, he noticed that at 145.5 C the solid crystal melted into a cloudy liquid which existed until 178.5 C where the cloudiness suddenly disappeared, giving way to a clear transparent liquid. Puzzled by his discovery, Reinitzer turned for help to the German physicist Otto Lehmann, who was an expert in crystal optics. Lehmann realized that the cloudy liquid was an entire new state of matter and he coined the name liquid crystal, because it shares both properties of the solid state and of the liquid state: Crystaline solids have positional and orientational order. Conventional liquids don t present any type of order. Liquid crystals might have no positional order but they present orientational order, which usually persists for a fairly narrow temperature range. Their work was continued most significantly in the 1960s by the French theoretical physicist Pierre-Gilles de Gennes. His work was rewarded with the Nobel Prize in Physics in 1991, deeply influencing the modern development of liquid crystal science.
Fig. 1 Ordering of liquid crystalline molecules (mesogens)
But how is this helpful? How do we build a display? Hint: The molecules change their orientation if an electric field is applied
In 1969, the nematic field effect in liquid crystals was discovered by James Fergason at Kent State University in the USA 1971 his company ILIXCO produced the first LCDs based on the nematic field effect In order to understand this effect we have to be comfortable with the concept of light polarization Unpolarized light Polarized light Fig. 2 Light polarization
Fig. 3 Twisted nematic
1 Vertical polarizer film 2 Glass substrate with ITO (Indium Tin Oxide) electrodes 3 Twisted nematic liquid crystals 4 Glass substrate with common electrode film (ITO) with horizontal ridges to line up with the horizontal film 5 Horizontal filter film to block/allow light through. 6 Reflective surface to send light back to viewer Fig. 4 Reflective twisted nematic liquid crystal display
LCDs can be transmissive or reflective, depending on the location of the light source. Transmissive LCDs illuminated from the back by a backlight and viewed from the opposite side (front) used in applications requiring high luminance levels such as computer displays, televisions, personal digital assistants, and mobile phones the illumination device used to illuminate the LCD usually consumes much more power than the LCD itself Reflective LCDs often found in digital watches and calculators illuminated by external light reflected by a reflector behind the display can produce darker 'blacks' than the transmissive type since light must pass through the liquid crystal layer twice and thus is attenuated twice image contrast is usually poorer than a transmissive display the absence of a lamp significantly reduces power consumption, allowing for longer battery life in battery-powered devices small reflective LCDs consume so little power that they can rely on a photovoltaic cell
How do we get color displays? In color LCDs, each pixel is divided in 3 subpixels: one for red, one for green and one for blue. The 3 colors are generated by filters. Each subpixel is controlled independently, therefore we can get thousands or millions of possible color combinations for the same pixel. Through the careful control and variation of the voltage applied, the intensity of each subpixel can range over 256 shades. Combining the subpixels produces a possible palette of 16.8 million colors (256 shades of red x 256 shades of green x 256 shades of blue). These color displays take an enormous number of transistors. For example, for a typical laptop computer display that supports resolutions up to 1024x768: 1024 columns x 768 rows x 3 subpixels = 2,359,296 transistors If something goes wrong with one transistor, then the LCD display will present a bad pixel. Most LCDs have some bad pixels scattered across the display. Color components may be arrayed in various pixel geometries, depending on the monitor's usage. If the software knows which type of geometry is being used in a given LCD, this can be used to increase the apparent resolution of the monitor through subpixel rendering.
Adressing the display Direct adressing each segment requires an independent circuit drive element each segment requires continuous application of voltage or current for a M rows x N columns display: M x N electrodes are required Matrix adressing time is multiplexed row at a time scanning for a M rows x N columns display: M + N electrodes are required
Direct addressed LCD
Passive-matrix and Active-matrix displays Passive-matrix LCDs use electric grids made of ITO to define each pixel by row and column a pixel is activated by applying charge at the corresponding column and grounding the corresponding row Advantages: very simple design inexpensive reduced number of electrodes Disadvantages slow response time, which implies a slow image refresh rate and hence lower quality of changing display problems arise as the number of rows and columns increase: with higher pixel density, the electrode size must be reduced and the amount of voltage necessary to drive the display rapidly increases. Also the liquid crystal material near the row and column being charged is affected by the pulse, generating partially active pixels. This effect is known as cross talk.
Active-matrix LCDs (AMLCDs)
How TFT LCDs work The TFT acts as switch. The gate electrode of TFT is connected to the scan line, the source electrode is connected to the data line and the drain electrode is connected to a capacitor and then to ground. When the gate is selected, the channel of the TFT will open and then the capacitor corresponding to the selected pixel will be charged. The capacitor retains the charge until the next duty cycle. When the gate is unselected, the channel of the TFT will be closed.
AMLCDs Advantages: better voltage control which results in better gray scaling of the display device active-matrix displays are much brighter and sharper than passive-matrix displays of the same size, and generally have quicker response times Disadvantages: high quality TFTs remain expensive Why are TFT LCDs expensive? With the increasing competition in the market of LCDs, quality control has become a very important issue. A few years ago the presence of 10-15 bad pixels was considered acceptable, but nowadays, if a SVGA LCD presents 4 bad pixels, it will be considered defective and will not go on sale. Some manufacturers, such as Samsung, now have "zero defective pixel guarantee" and would replace a product even with one defective pixel. Even where such guarantees do not exist, the location of defective pixels is important. A display with only a few defective pixels may be unacceptable if the defective pixels are near each other. Manufacturers of existing large LCDs often reject about 40 percent of the panels that come off the assembly line. The level of rejection directly affects LCD price since the sales of the good LCDs must cover the cost of manufacturing both the good and bad ones. Only advances in manufacturing can lead to affordable displays in bigger sizes.
Thank you for your attention!
References MIT Open Courseware 6.976 Flat Panel Display Devices Toshihisa Tsukada Active-Matrix Liquid-Crystal Displays http://nobelprize.org/physics/educational/liquid_crystals/history/index.html http://en.wikipedia.org/wiki/liquid_crystal_display http://www.barrett-group.mcgill.ca/teaching/liquid_crystal/lc02.htm http://moebius.physik.tu-berlin.de/lc/lcd.html