Post on 29-Jun-2020
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Liquid Crystal Displays
Organic LED Displays
Electronic Paper Displays
12/05/2019 1Input/Output Systems and Peripheral Devices (05-3)
Organic LED DisplaysTypes of Organic LEDs
Structure and Operation
Passive-Matrix Displays
Active-Matrix Displays
Color Generation Techniques
Transparent Displays
Sub-Pixel Layouts
Advantages and Disadvantages 12/05/2019 2Input/Output Systems and Peripheral Devices (05-3)
OLED – Organic Light Emitting DiodeComposed of layers of organic materials
Based on electroluminescence
In the 1970s OLEDs based on conductive polymers were developed
The first practical OLED was developed at Eastman Kodak (1987)
In 1990 a material based on polyphenylenevinylene was developed layer of 100 nm
12/05/2019 3Input/Output Systems and Peripheral Devices (05-3)
Depending on the size of molecules, there are two types of OLEDs:
With small molecules: SM-OLED (Small-Molecule OLED)
With polymers: P-OLED (Polymer OLED), LEP (Light Emitting Polymer)
Both types generate light by forming electrons and holes, and then by their recombination
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Small-Molecule OLEDs (SM-OLED)Used for most of OLED displays
An evaporation process under vacuum is usedAdvantages: homogeneous films and complex multi-layer structures can be formed
Disadvantage: expensive process
Materials: fluorescent dyesAbsorb light and re-emit it at different wavelengths
Research to develop soluble SM-OLED materialsEnable to use inexpensive technologies
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Polymer OLEDs (P-OLED)Require lower voltagesCan be processed from solutions
Technologies: inkjet printing; spin-coatingAdvantage: lower cost than evaporation in vacuum
Materials: polyphenylene vinylene (PPV), polyfluorene (PF)
Printable P-OLED materials (Image credit Sumitomo Chemical)
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Based on the type of emission, there are fluorescent and phosphorescent OLEDsFluorescent OLEDs
Fluorescence: emission of visible light by a material due to absorption of energyThe energy is re-emitted when the electrons return back to the original energy levelThe return occurs almost immediately (10-8 s)Stops as soon as the energy source is removed
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Phosphorescent OLEDsPhosphorescence: emission of light by a material exposed to a form of radiationThe emission persists after the radiation has been removed
Concepts related to particle physicsSpin
Angular momentum carried by elementary and composite particlesMeasured in multiples of a unit called Dirac (ħ) usually, the unit ħ is omittedVector quantity: it has direction and magnitude
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Spin direction: direction in which the spin vector is pointingSpin magnitude: specified by the spin quantum number (s) For fermions, particles that make all known matter: s is 1/2, 3/2 Spin-½ particles: one of two orientations in a magnetic field, with the spin pointing in the +zor –z directionWhen two fermions reside on a single orbital, they must have different quantum states (the Pauli exclusion principle) s = 0
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Singlet state Obtained when two spin-½ particles are combined
If the particles have opposite spins, the total spin is s = 0 only one quantum state
Triplet stateSet of three quantum states of an elementary particle or combination of particles
Each state has a total spin of s = 1
Combination of two spin-½ particles: the spin directions are the same
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ExcitonsFormed when electrons and electron holes in a semiconductor absorb energy
Electron-hole pairs excitons in singlet or tripletstate
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Formation of a triplet state is more probable
Triplet state: set of three quantum states 75% of the excitons are in triplet state
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Fluorescent OLEDs:Only singlet states contribute to light emission
Efficiency is limited to 25%
Phosphorescent OLEDs:Introduction of heavy-metal atoms into the emitting layer facilitates transition from the triplet to the singlet state light emission
The singlet state also contributes to light emission
Efficiency approaches 100%
12/05/2019 13Input/Output Systems and Peripheral Devices (05-3)
Organic LED DisplaysTypes of Organic LEDs
Structure and Operation
Passive-Matrix Displays
Active-Matrix Displays
Color Generation Techniques
Transparent Displays
Sub-Pixel Layouts
Advantages and Disadvantages12/05/2019 14Input/Output Systems and Peripheral Devices (05-3)
SM-OLED devicesFirst OLED devices: used a single organic layer inserted between an anode and a cathodeOLED devices developed at Kodak: two layersCurrent OLED devices: multiple layers
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P-OLED devicesUse simpler structuresMay contain only two polymer layersCathode: metallic mirror (e.g., LiF)Anode: transparent (ITO)
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If a voltage is applied between electrodes: A current of electrons flows through the organic layers (cathode anode)
Electrons and holes are attracted towards each other by electrostatic forces
An electron and a hole may recombine exciton in a singlet state or triplet state
Depending on the type of emissive material, decay of the singlet state or triplet state releases the extra energy as a photon
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Top-emitting OLED displaysTransparent non-metallic cathode (top)Reflective anode (bottom)Advantage: easy integration of transistors for active-matrix displays
Bottom-emitting OLED displaysReflective metallic cathode (top)Transparent anode (bottom)Luminosity is limited by the transparency of the anode and driver circuitry (active-matrix)
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Structure of a bottom-emitting OLED display
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Organic LED DisplaysTypes of Organic LEDs
Structure and Operation
Passive-Matrix Displays
Active-Matrix Displays
Color Generation Techniques
Transparent Displays
Sub-Pixel Layouts
Advantages and Disadvantages12/05/2019 20Input/Output Systems and Peripheral Devices (05-3)
PMOLED (Passive-Matrix OLED)
Drivers attached to each electrodeThe pixel rows are selected successively
A certain voltage is applied to the columns of selected row an electric current
Advantage: manufacturing costs are low
Disadvantages: relatively intensive currents are required high power consumption; only suitable for small screens
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Original image © HowStuffWorks, Inc.
12/05/2019 22Input/Output Systems and Peripheral Devices (05-3)
Organic LED DisplaysTypes of Organic LEDs
Structure and Operation
Passive-Matrix Displays
Active-Matrix Displays
Color Generation Techniques
Transparent Displays
Sub-Pixel Layouts
Advantages and Disadvantages12/05/2019 23Input/Output Systems and Peripheral Devices (05-3)
AMOLED (Active-Matrix OLED)
Array of thin film transistors (TFTs)
At least two transistors and a storage capacitor are needed for each sub-pixel
First TFT: charges the storage capacitor
Second TFT: provides a correct voltage
Advantages: higher refresh rates; higher luminosity; reduced power consumption; displays are not limited in size
12/05/2019 24Input/Output Systems and Peripheral Devices (05-3)
Original image © HowStuffWorks, Inc.
12/05/2019 25Input/Output Systems and Peripheral Devices (05-3)
Organic LED DisplaysTypes of Organic LEDs
Structure and Operation
Passive-Matrix Displays
Active-Matrix Displays
Color Generation Techniques
Transparent Displays
Sub-Pixel Layouts
Advantages and Disadvantages12/05/2019 26Input/Output Systems and Peripheral Devices (05-3)
Direct-emission OLED (RGB OLED)
Uses R, G, and B sub-pixels patterning of organic materials
High luminous efficiency
More complex manufacturing process
Color balance may change in time
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Original image © LG Display
White-emitting OLED (WOLED)
The emitter layers are deposited uniformly white light
Two layers: blue and yellow
Color filters patterned into sub-pixels (R, G, B) are appliedA fourth white sub-pixel (W) is added increases the efficiency Original image © LG Display
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Color filter deposition: photolithographic methods, also used for LCDs
Advantages:Simpler and scalable manufacturing process
Lower production costs
No color balance problems occur
Disadvantages:Lower efficiency due to the color filters
Additional cost of the color filters and the more complex addressing (four sub-pixels)
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Stacked OLEDs (SOLED - Stacked OLED)Each pixel contains R, G, and B emittersThe emitters are stacked vertically, separated by transparent intermediate electrodesAdvantage: potential increase of resolutionA white-emitting OLED can be added
12/05/2019 30Input/Output Systems and Peripheral Devices (05-3)
Organic LED DisplaysTypes of Organic LEDs
Structure and Operation
Passive-Matrix Displays
Active-Matrix Displays
Color Generation Techniques
Transparent Displays
Sub-Pixel Layouts
Advantages and Disadvantages12/05/2019 31Input/Output Systems and Peripheral Devices (05-3)
TOLED (Transparent OLED)Both the anode and cathode are transparent
Original image © Universal Display Corporation12/05/2019 32Input/Output Systems and Peripheral Devices (05-3)
OFF pixels: transparency may reach 85%Active-matrix addressing: transparency is slightly reducedExample of material: PEDOT:PSS
Polymer based on polythiophene and sulfonated polystyreneConductive material with high efficiencyTransparent and easy to processCan be used as hole transport layer and replacement for ITO electrodes
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© Samsung Display
12/05/2019 34Input/Output Systems and Peripheral Devices (05-3)
Organic LED DisplaysTypes of Organic LEDs
Structure and Operation
Passive-Matrix Displays
Active-Matrix Displays
Color Generation Techniques
Transparent Displays
Sub-Pixel Layouts
Advantages and Disadvantages12/05/2019 35Input/Output Systems and Peripheral Devices (05-3)
Conventional layout: RGB
RG-B-RG PenTile LayoutInspired by peculiarity of the human retina fewer sensors for perceiving blue colors
Uses proprietary algorithms for sub-pixel rendering
Any input pixel is mapped to a logical pixel either a red-centered or a green-centered logical pixel
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RG-BG PenTile LayoutG sub-pixels, alternating R and B sub-pixels
The input image is mapped to sub-pixels 1:1 mapping only for G sub-pixels
Only two sub-pixels are used for a pixel the sub-pixel density can be reduced
Resolution of the luminance information is not affected significantly
Disadvantage: the pixel structure may be more visible
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RGB layout (left) and RG-BG PenTile layout (right)(Image credit Stuff-Review)
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Diamond Pixel LayoutDeveloped by Samsung Electronics
The number of G sub-pixels is double than that of R and B sub-pixels
Oval shape for G sub-pixels
Diamond shape for R and B sub-pixels
Image credit DisplayMateTechnologies Corporation
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Modified Diamond Pixel layout
First used with the Galaxy S5 seriesAll sub-pixels are diamond-shapedB sub-pixels have the same size as the R sub-pixels improved efficiency of B emitterDensities of over 400 or 500 pixels/inch (PPI)
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Image credit DisplayMateTechnologies Corporation
Organic LED DisplaysTypes of Organic LEDs
Structure and Operation
Passive-Matrix Displays
Active-Matrix Displays
Color Generation Techniques
Transparent Displays
Sub-Pixel Layouts
Advantages and Disadvantages12/05/2019 41Input/Output Systems and Peripheral Devices (05-3)
Advantages High contrast ratio (>1,000,000:1), both static and dynamic Wide viewing angles no color shifting Wide color gamut Fast response time (0.01 ms .. 1 ms) On average, power consumption is lower compared to LCDs (40% .. 80%) The plastic substrate is lightweight Flexible and transparent displays can be built
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Disadvantages Currently, the cost of the manufacturing process is relatively high
The lifetime of some organic materials (blue OLEDs) is limited (e.g., between 20,000 and 50,000 hours)
Color balance may change in time
Biasing the color balance towards blue
Optimizing the size of R, G, and B sub-pixels larger blue sub-pixels
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Image persistence may occur
The display may be damaged by prolonged exposure to ultraviolet rays
The organic materials can be damaged by water
Readability in outdoor conditions may be limited
Circular polarizer; anti-reflective coating
Power consumption is increased when displaying images on white background
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Liquid Crystal Displays
Organic LED Displays
Electronic Paper Displays
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Electronic Paper DisplaysPrinciple of Operation
Electrophoretic Technology
Color Electrophoretic Technologies
Advantages and Disadvantages
Applications
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Also called electronic ink (e-ink) displaysReflective displaysMay use one of several technologies
ElectrophoreticElectrophoresis: motion of particles in a fluid under the influence of an electric field
Electro-fluidic: uses a pigment dispersion placed inside a small reservoirInterferometric modulation: colors created through interference of reflected light
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Electronic Paper DisplaysPrinciple of Operation
Electrophoretic Technology
Color Electrophoretic Technologies
Advantages and Disadvantages
Applications
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First electrophoretic displaysSuspension of charged particles with pigmentsin a dyed insulating fluidTwo conducting electrode platesApplying a voltage across the plates: the particles migrate towards the plate with opposite charge
Either the color of the pigment or of the fluid becomes visible
Limitations: particle migration to the electrode edges; particle settling or floating; particle sticking
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Microencapsulated electrophoretic displaysInitially developed at E Ink Corp.
Top electrode: continuous and transparent
Bottom electrode layer: divided into pixels
Electrophoretic ink: transparent liquid + charged microscopic pigment particles
Black particles negatively charged
White particles (titanium dioxide – TiO2) positively charged
The ink is enclosed within microcapsules12/05/2019 50Input/Output Systems and Peripheral Devices (05-3)
The ink is laminated to a plastic film
Applying a voltage to the bottom electrode: the particles move to the top or bottom
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Electronic Paper DisplaysPrinciple of Operation
Electrophoretic Technology
Color Electrophoretic Technologies
Advantages and Disadvantages
Applications
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Three-Pigment InkUses red or yellow pigments in addition to black and white
Example: E Ink Spectratechnology
The pigments are enclosed in microcups
The red and black pigments have different mobility © E Ink Holdings Inc.
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Using Color FiltersR, G, B color filter array
Disadvantage: reflected light is reduced
Example: E Ink TritonActive matrix backplane
Electronic ink layer
Color filter array layer
16 levels of grayscale, 4096 colors
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Advanced Color ePaper (ACeP)Technology developed by E Ink Corp.Does not use a color filter arrayIt eliminates the disadvantages of filters
Filters absorb light limit reflectivitySide-by-side combinations of the primary colors reduces resolution and color saturation
Three transparent, colored pigments (C, M, Y) and an opaque white pigmentTwo pigments: positively charged; two pigments: negatively charged
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Operation is based on selective electrophoretic motion of pigments
Pigments have different electrophoretic mobility
Another phenomenon: pigment aggregationTwo oppositely charged pigments group togetherMay be controlled by the electric fieldsThere are four possible groupings of the C, M, Y pigments (C + M + Y Black)
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Experimental display32 native colors 32 different voltages applied to each pixelA full-color image is produced from the native colors using spatial dithering techniques
© E Ink Holdings Inc.12/05/2019 57Input/Output Systems and Peripheral Devices (05-3)
Electronic Paper DisplaysPrinciple of Operation
Electrophoretic Technology
Color Electrophoretic Technologies
Advantages and Disadvantages
Applications
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AdvantagesExtremely low power consumption
Bi-stable technology: a static image is retained even when the power source is removed
No need for constantly refreshing the screen
No need for backlight
High contrastReflective displays
Same pigments are used as in the printing industry same readability as printed paper
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Wide viewing angle
Very good visibility in direct sunlight or in dimmed light
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High resolution
Robustness: plastic film substrate; plastic-based TFT backplane
Light and thin displaysPlastic materials can be used as conductors and semiconductors; e.g., PEDOT PSS
Organic TFTs
Flexible displaysFlexible and transparent substrates obtained by printing or vapor deposition processes
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DisadvantagesVery low refresh rate
Not suitable for interactive applications
Shadow images Visible after refreshing the screen
The screen needs to be refreshed several times
Difficulty to build color displaysColor technologies that do not use filter arrays are complex
Color displays are considerably more expensive12/05/2019 62Input/Output Systems and Peripheral Devices (05-3)
Electronic Paper DisplaysPrinciple of Operation
Electrophoretic Technology
Color Electrophoretic Technologies
Advantages and Disadvantages
Applications
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E-Book ReadersAdvantages compared to tablets:
Better screen readability
Longer battery life
Low weight
Examples:Amazon Kindle (Paperwhite, Oasis)
Barnes & Noble NOOK
Kobo Aura H2O Edition 2
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Mobile devicesSecondary display for smartphones and tablets
Information displayed: date/time, maps, boarding cards, tickets
Can be used as a mini e-book reader
Example: Yota3 phone
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Accessories for mobile devicesDisplays included in cases for smartphonesWireless connectivity with the smartphoneVarious functions: e-reader for books and articles, displaying images and informationExamples: OAXIS InkCase i7, InkCase i7 Plus
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KeyboardsEvery key has an e-paper screen
The image displayed can be changed dynamically
Example: E-Inkey keyboard
Smart CardsMay contain embedded e-paper displays
Multi-function cards, e.g. Fuze Card
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Digital signageSuitable for indoor and outdoor displays
Legible, robust, durable, weatherproof
Electronic shelf labels
Public transportation signs: timetables, maps
Public information display: events, wayfinding signs
Whiteboards and e-writers
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Wearable AccessoriesSmart watches
Examples: Sony FES Watch U, Pebble Time Round, Seiko Spirit
Smart jewelryBracelets with flexible displays
Adaptable to a variety of designs
Example: Tago Arc (L!BER8 Technology)
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Types of OLEDs based on the size of molecules: SM-OLED and P-OLED
SM-OLED: manufacturing process based on evaporation under vacuumP-OLED: can be processed from solutions
Based on the type of emission: fluorescent and phosphorescent OLEDsTheir operation is based on forming electrons and holes, and then recombining them
Decay of the singlet and/or triplet state releases photons
12/05/2019 70Input/Output Systems and Peripheral Devices (05-3)
Active-matrix OLED displays require two transistors and a capacitor for each pixel
Advantages: higher luminosity; reduced power consumption
Color generation techniques: direct-emission(RGB OLED); white-emitting OLED (WOLED); stacked OLEDs (SOLED)Advantages: high contrast; wide viewing angles; fast response timeDisadvantages: limited lifetime of blue OLED materials; color balance may change in time
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Electronic paper displays are reflectiveIn practice, microencapsulated electrophoreticdisplays are builtColor technologies that have been developed:
Three-pigment inkUsing color filtersAdvanced Color ePaper (ACeP)
Advantages: low power consumption; good visibility; robustnessDisadvantages: low refresh rate; shadow images; complexity of color technologies
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Small-molecule and polymer OLEDs
Fluorescent and phosphorescent OLEDs
Structure and operation of an OLED cell
Structure of a bottom-emitting OLED display
Passive-matrix OLED displays
Active-matrix OLED displays
Direct-emission OLEDs
White-emitting OLEDs
Stacked OLEDs
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Sub-pixel layouts
Advantages of OLED displays
Disadvantages of OLED displays
Microencapsulated electrophoretic e-paper display technology
Three-pigment e-ink technology
Advanced Color ePaper technology
Advantages of e-paper displays
Disadvantages of e-paper displays
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1. What is the difference between fluorescent and phosphorescent OLEDs considering the efficiency of emission?
2. What are the advantages of OLED displays?
3. What are the phenomena the Advanced Color ePaper technology is based on?
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