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LCDTFTLED-OLEDCCDCMOS

Display Systems and photosensors (Part 1)LCD Display

M.A.MOEENI

2Discovery of liquid crystals (LC)

- Between 1850 and 1888, people from different fields like chemistry, biology observed strange behavior in some materials when the temperature is approaching the melting point. W. Heintz, a German biochemist, reported in 1850 that stearin melted from a solid to a cloudy liquid at 52 C, changed at 58 C to an opaque and at 62.5 C to a clear liquidIn 1888, Friedrich Reinitzer, an Austrian botanist observed that there was two melting points for a certain material while he was making esters of cholesterol for studying cholesterol in plant. He also observed birefringence and iridescent colors between these two melting points.Lehmann and Reinitzer may thus with some justification be called the grandfathers of liquid crystal science

Discovery of liquid crystals (LC)

-In 1922, Georges Friedel suggested a classification scheme to name different phases of liquid crystals called nematic, smectic and cholesteric, which are still used today- Carl Oseen in Sweden worked on elastic properties of liquid crystals and his results were used on the continuum theory by England's F.C. Frank. This theory became one of the fundamental theories in liquid crystals today.

Alfred SaupeV. Freedericksz Discovery of liquid crystals (LC)

From 1945 to 1958, everything seemed slow down in the liquid crystal field. People thought they knew everything about liquid crystals and that nothing new could be expected in this area. No one could yet imagine how large a role liquid crystals would play in technical applications todayIn 1958 Glenn Brown, an American chemist, published an article in Chemical Reviews on the liquid crystal phase and subsequently sparked an international resurgence in liquid crystal research.

Pierre-Gilles de Gennes, the laureate of Nobel Prize in Physics 1991, becomes the first and so far the only person receiving this prize in the field of liquid crystals.

Simple types of lc

-Three major characteristics of Liquid Crystal

- The Thermal NatureHigh TempClearing PointMelting PointLiquid StateLiquid Crystaline StateSolid State (Crystal)- The Optical Nature of a LC moleculeLightNO change in polarization statePhase retardation will existLow Temp- The Electrical Nature of the LC moleculesAC potentialNo potential fieldElectrodesproperty of liquid crystals (LC)

optical properties of lcLC is a birefringent medium.The electric and magnetic susceptibilities (the susceptibility of a material or substance describes its response to an applied field) are different along the director and perpendicular to the director.if the incoming light's polarization is at an angle other than 0 or 90 to the director, then a phase retardation will exist at the exit, and the light becomes elliptically polarized. These optical properties are used in the LC displays.Chiral nematic and smectic C phases are optically active, i.e. a linearly polarized light is rotated as it passes through such a medium.The history of liquid crystal display LCDBefore the appearance of LCD, it is the Cathode Ray Tube (CRT) kingdom. The CRT monitors dominate almost all the display applications. The CRT, invented by Karl Ferdinand Braun, is also called Braun tube.Cathode rays exist in the form of streams of high speed electrons emitted from the heating of cathode inside a vacuum tube at its rear end. The released electrons form a beam within the tube due to the voltage difference applied across the two electrodes, and the direction of this beam is then altered either by a magnetic or electric field to trace over the inside surface of the phosphorescent screen (anode), covered by phosphorescent material.Light is emitted by that material at the instant that electrons hit it.

- good image quality- no problem with response time - no problem with viewing angle But it is bulky and high power consumption, So Display engineers tried looking for alternative technologies like flat panel display.In 1990s, technology breakthrough brought the birth of active matrix LCD, along with the plasma display, both of which become the main stream of the flat display markets, replacing the CRT

LCD display

(A) It has a mirror which makes it reflective(B) & (F) piece of glass with a polarizing film on the bottom side(C) & (E) a common electrode plane made of indium-tin oxide on top(D) layer of liquid crystal substance

THREE COMMON TYPES OF LCDLight(Back Light)- TRANSMISSIVE TYPE- REFLECTIVE TYPEPOLARIZER ON THE FRONT SIDEREFLECTOR ON THE BACK SIDEIncident Light- TRANSFLECTIVE TYPEDay LightNight Light(Back Light)TRANSFLECTOR ON THE BACK SIDEPOLARIZER ON BOTH SIDESPOLARIZER ON THE FRONT SIDELCDLCDLCDEyes

An alternative way to achieve high-resolution LCD is to use the Liquid Crystal on Silicon (LCOS) devices. LCOS devices use only one glass substrate, and employ a silicon wafer for the back substrate. The pixels are then generally coated with a reflective aluminum layer, and then a polyimide alignment layer.This technology can also be used in personal viewer such as viewfinder in digital cameras and camcorders. Display Types( commonly used Display )EmitsLightVoltageCurrentPatterningFlexibilityCRT ( Cathode Ray Tube )YesHigh DCLow( scanning )VFD ( Vacuum Florescent Displays )YesHigh DCLowMediumLED ( Light Emitting Diode )YesLow DCMediumLowPlasma DisplaysYesHigh DCLowLowEL ( Electro Luminescent Displays )YesHigh DCLowHighLCD ( Liquid Crystal Display )NOLow ACLowHighOLED (Organic LED)YesLow DCLow-mediumHighCompetition between FLAT PANEL DISPLAYSLCD modesAlong with the development of LCD's driving infrastructure, different LCD modes were introduced to improve the image quality.

-ASV-BINEM-Cholesteric-ECB-SSFLC-Guest-Host-IPS-LCOS-MVA-PDLC-Pi-Cell-PVA-STN-TN

George Heilmeier in 1968TODAYS LCDDuty RatioPanel Size 1/1Static1/21/31/41/81/161/321/641/25610 mm21001,00010,000100,000Time piecesHand Held GamesFilm CameraCalculatorData BankMobile Phone Digital InstrumentsPDADigital CameraOffice EquipmentNotebook PanelsProjector Portable TVActive LCDPassive LCDSTNTN1M+ Pixels100K Pixels10K Pixels1K Pixels100 PixelsLCD TV & Monitors1/128Twisted Nematic (TN) modeThe TN mode is the "workhorse" for the LC display. It was first introduced by Schadt and Helfrich, and also by Fergason in 1971

Twisted Nematic (TN) mode

The gray scale is achieved by applying intermediate voltages between 0 and the value at which light is completely blocked.A TYPICAL TN TYPE LCD CELLWith AC VoltsconnectedNO powersupplyPolarizer(Axis 90 degrees)LightCell Gap The separation between two glassesDepending on how the LCD fluid is formulated.Glass with electrodesPolarizer(Axis 0 degree)The smaller the cell gap, the faster response.Twisted Nematic (TN) mode

Twisted Nematic (TN) mode

For a LCD, each pixel is divided into three subpixels, which have red, green and blue color filters.The exact color coordinates of the white point depend on the relative transmission and color purity of the red green and blue subpixels.HTN (Highly Twisted Nematic) & FSTN (Film STN)TN90 deg TwistedSTN180 deg or higher degTwistedHTN110 degTwistedNarrow View AngleWide View Angle BUT with Darker Color Background &Blue or dark blue patterns.Wider View Angle than TNbut narrower than STNFSTN240 deg or higher degTwisted View angle same as 240 deg STN BUT in Grey Background Color& Black patterns.LCD CellDSTN (Double STN Cells)1st Cell with patterns Same as usual STN 2nd Cell without patternBut in reverse twisting Old way when NO Retardation filmPolarizerElectrically Controlled Birefringence (ECB) modeThe ECB mode uses the applied voltage to change the tilt of the liquid crystal molecules, as a result, the birefringence is changed as a function of the tilt angle.In the TN mode, the transmission maxima will be reached when the condition is met:

If we use applied voltage to change the tilt angle, we are able to shift the wavelength of the maxima.In general, ECB can be used to generate color to replace the color filter, by applying different voltages on each pixel corresponding the desired colorthe color obtained from ECB are not fully saturated because the spectrum curve are not sharp (narrow) enough, with some overlapping with neighbor colors. Therefore, it is not used often in practical applications for color generations.

Positive Mode (Pattern on a Clear Background)EitherTNHTNSTNFSTNDeg Twisted90110180-240240BackgroundColorGreyGreyYellow Greenor GreyGreyPattern ColorBlackBlackDark Blueor BlueBlackTemp Range-40C to +85C-20C to +40C-20C to +70C-20C to +70CView Angle 60 deg80 deg120 deg110 degVoltage2.5v min 5v typical3v min5v typical3v min5v typical5v typical, (higher duty, higher volts)Negative Mode (Clear Pattern on a Color Background)TNHTNSTNFSTNDegree Twisted90 deg110 deg180-240deg240 degBackgroundColorBlackBlack(Seldom used)BlackPattern ColorClearClear(Seldom used)ClearAdvanced Super View (ASV) modeThe ASV mode was developed by Sharp.

Because of the full circle rotation of the director, the viewing cone is very symmetric and viewing angle performance is excellent.Polymer Dispersed Liquid Crystal (PDLC) mode-The PDLC display consists of droplets of liquid crystals inside a polymer network

In the off state, the droplets are randomly aligned hence the light is scattered in a large angle towards the viewer.In the on state, light can be transmitted with a very high transmission.Polymer Dispersed Liquid Crystal (PDLC) modeThe working voltage and response time of the PDLC can be affected by:- the resistive and dielectric properties of LC inside the droplet and the polymer properties- size of the droplets - shape of the droplets- the viscosity of the of the dropletsThere are a few factors influencing the contrast ratio of the PDLC display:the cell gap the density of the dropletsGuest - Host (GH)modeIn a Guest - Host system, the mixture is prepared by mixing LC and dichroic dyes.The dichroic dyes absorb the light whose E-field is along the long axis of the dye.When the LC molecules change their orientation, the dye will also change along with LC molecules, consequently, the absorption axis is changing, a light transmission can be modulated.There are three simple GH displays:- The Heilmeier type GH- The Double Layer type GH - The PDLC type GH.

Proceedings of the Liquid Crystal Seminar HK by E. Merck, Darmstadt, Germany.

Various articles in the SID International Symposium and Information Display by the Society forInformation Display, Inc. USA

LCD Displays, the leading edge in flat panel displays, by Sharp Technical Library, Vol. 1, ofSharp Corporation, Osaka, Japan. 6) polymerizability.com The information in above are partly referring to the following documents