Amoled Term Report

7/28/2019 Amoled Term Report

1/24

Report Heading

AMOLED DISPLAY

TECHNOLOGY

(ACTIVE MATRIX LIGHT

EMMITING DIODE)

Submitted By:

Abhishek Kumar

(A20405309002)

B.Tech (I.T), III Semester

Under The Guidance Of:

Mr. Anil Saroliya

Amity School of Engineering


2/24

AMITY UNIVERSITY RAJASTHAN

Contents

0. Certificate

1. AMOLED (Active Matrix Organic Light Emitting

Diode) :

(a). AMOLED In-Cell Touch Panels

(b). Thin Film Transistor (TFT)

(c). Structure

(d). How Does OLED Emit Light?

(e). Working Principle

2. Device Architecture:

(a). Structure

(b). Patterning technologies

(c). Backplane Technologies

3. Advantages

4. Disadvantages

5. Manufactures and Commercial Uses

6. Active Matrix Addressing

7. Passive Matrix Addressing

8. Commercial Devices


3/24

9. References

CERTIFICATE

Certified that the Report entitledAMOLED DISPLAY TECHNOLOGY (ACTIVEMATRIX LIGHT EMMITING DIODE) submitted by Abhishek Kumar with

Enrollment No. A20405309002 on (Nov / 2010) is his own work and hasbeen carried out under my supervision. It is recommended that the

candidate may now be evaluated for his work by the University.

(STUDENT) (GUIDE)

Signature:Signature:

Designation:

Date:


4/24

AMOLED: Active Matrix

Organic Light

Emitting Diode

Active-matrix OLED (Active-matrix organic light-emitting diode or AMOLED) is a display technology for use

in mobile devices and televisions. OLED describes a specific

type of thin film display technology in which organic

compounds form the electroluminescent material, and active

matrix refers to the technology behind the addressing of

pixels. AMOLED technology is currently used in mobile phone

and media players and continues to make progress towards

low power, low cost and large size (for example 40 inch) forapplications such as televisions.

An active matrix OLED display consists of a matrix of OLED

pixels that generate light upon electrical activation that have

been deposited or integrated onto a thin film transistor (TFT)

array, which functions as a series of switches to control the

current flowing to each individual pixel.

Typically, this continuous current flow is controlled by at

least two TFTs at each pixel, one to start and stop the

charging of a storage capacitor and the second to provide a

voltage source at the level needed to create a constant


5/24

current to the pixel and eliminating need for the very high

currents required for passive matrix OLED operation.

TFT backplane technology is crucial in the fabrication of

AMOLED displays. Two primary TFT backplane technologies,

namely polycrystalline silicon (poly-Si) and amorphous

silicon (a-Si), are used today in AMOLEDs. These

technologies offer the potential for fabricating the active

matrix backplanes at low temperatures (below 150C)

directly onto flexible plastic substrates for producing flexible

AMOLED displays.

AMOLED In-Cell Touch Panels

Manufacturers have developed in-cell touch panels, integrating the production

of capacitive sensor arrays in the AMOLED module fabrication process. In-cell

sensor AMOLED fabricators include AU Optronics and Samsung. Samsung

has marketed their version of this technology as Super AMOLED.

Thin-film transistor

A thin-film transistor (TFT) is a special kind of field-effect transistor made by

depositing thin films of a semiconductor active layer as well as the dielectric

layer and metallic contacts over a supporting substrate. A common substrate is

glass, since the primary application of TFTs is in liquid crystal displays. This

differs from the conventional transistor where the semiconductor material

typically is the substrate, such as a silicon wafe


6/24

Structure

1 - Glass plates

2/3 - Horizontal and vertical polarisers

4 - RGB colour mask

5/6 - Horizontal and vertical command lines

7 - Rugged polymer layer

8 - Spacers

9 - Thin film transistors

10 - Front electrode

11 - Rear electrodes


7/24

How does an OLED emit light?

OLEDs basic structure consists of organic materials

positioned between the cathode and the anode, which iscomposed of electric conductive transparent Indium Tin

Oxide (ITO). The organic materials compose a multi-layered

thin film, which includes the Hole Transporting Layer (HTL),

Emission Layer (EML) and the Electron Transporting Layer

(ETL). By applying the appropriate electric voltage, holes and


8/24

electrons are injected into the EML from the anode and the

cathode, respectively. The holes and electrons combine

inside the EML to form excitons, after which

electroluminescence occurs. The transfer material, emission

layer material and choice of electrode are the key factorsthat determine the quality of OLED components.

Working principle

A typical OLED is composed of a layer of organic materialssituated between two electrodes, the anode and cathode, alldeposited on a substrate. The organic molecules areelectrically conductive as a result of delocalization of pi


9/24

electrons caused by conjugation over all or part of themolecule. These materials have conductivity levels rangingfrom insulators to conductors, and therefore are consideredorganic semiconductors. The highest occupied and lowest

unoccupied molecular orbitals (HOMO and LUMO) of organicsemiconductors are analogous to the valence andconduction bands of inorganic semiconductors.

Originally, the most basic polymer OLEDs consisted of asingle organic layer. One example was the first light-emittingdevice synthesised by J. H. Burroughes et al., which involveda single layer of poly(p-phenylene vinylene). However

multilayer OLEDs can be fabricated with two or more layersin order to improve device efficiency. As well as conductiveproperties, different materials may be chosen to aid chargeinjection at electrodes by providing a more gradualelectronic profile,or block a charge from reaching theopposite electrode and being wasted. Many modern OLEDsincorporate a simple bilayer structure, consisting of aconductive layer and an emissive layer.

During operation, a voltage is applied across the OLED suchthat the anode is positive with respect to the cathode. Acurrent of electrons flows through the device from cathodeto anode, as electrons are injected into the LUMO of theorganic layer at the cathode and withdrawn from the HOMOat the anode. This latter process may also be described asthe injection of electron holes into the HOMO. Electrostaticforces bring the electrons and the holes towards each other

and they recombine forming an exciton, a bound state of theelectron and hole. This happens closer to the emissive layer,because in organic semiconductors holes are generally moremobile than electrons. The decay of this excited state resultsin a relaxation of the energy levels of the electron,accompanied by emission of radiation whose frequency is inthe visible region. The frequency of this radiation depends


10/24

on the band gap of the material, in this case the difference inenergy between the HOMO and LUMO.

As electrons and holes are fermions with half integer spin, anexciton may either be in a singlet state or a triplet statedepending on how the spins of the electron and hole havebeen combined. Statistically three triplet excitons will beformed for each singlet exciton. Decay from triplet states(phosphorescence) is spin forbidden, increasing thetimescale of the transition and limiting the internal efficiencyof fluorescent devices. Phosphorescent organic light-emittingdiodes make use of spinorbit interactions to facilitate

intersystem crossing between singlet and triplet states, thusobtaining emission from both singlet and triplet states andimproving the internal efficiency.

Indium tin oxide (ITO) is commonly used as the anodematerial. It is transparent to visible light and has a high workfunction which promotes injection of holes into the HOMOlevel of the organic layer. A typical conductive layer may

consist of PEDOT:PSS as the HOMO level of this materialgenerally lies between the workfunction of ITO and theHOMO of other commonly used polymers, reducing theenergy barriers for hole injection. Metals such as barium andcalcium are often used for the cathode as they have lowwork functions which promote injection of electrons into theLUMO of the organic layer. Such metals are reactive, sorequire a capping layer of aluminium to avoid degradation.

Single carrier devices are typically used to study the kineticsand charge transport mechanisms of an organic material andcan be useful when trying to study energy transferprocesses. As current through the device is composed ofonly one type of charge carrier, either electrons or holes,


11/24

recombination does not occur and no light is emitted. Forexample, electron only devices can be obtained by replacingITO with a lower work function metal which increases theenergy barrier of hole injection. Similarly, hole only devices

can be made by using a cathode comprised solely ofaluminium, resulting in an energy barrier too large forefficient electron injection.

Device Architectures

Structure

* Bottom or top emission: Bottom emission devices usea transparent or semi-transparent bottom electrode to getthe light through a transparent substrate. Top emissiondevices use a transparent or semi-transparent top electrodeemitting light directly. Top-emitting OLEDs are better suitedfor active-matrix applications as they can be more easilyintegrated with a non-transparent transistor backplane.

* Transparent OLEDs use transparent or semi-transparentcontacts on both sides of the device to create displays thatcan be made to be both top and bottom emitting(transparent). TOLEDs can greatly improve contrast, makingit much easier to view displays in bright sunlight. Thistechnology can be used in Head-up displays, smart windowsor augmented reality applications. Novaled's OLED panelpresented in Finetech Japan 2010, boasts a transparency of60-70%.


12/24

* Stacked OLEDs use a pixel architecture that stacks thered, green, and blue subpixels on top of one another insteadof next to one another, leading to substantial increase ingamut and color depth, and greatly reducing pixel gap.

Currently, other display technologies have the RGB (andRGBW) pixels mapped next to each other decreasingpotential resolution.

* Inverted OLED: In contrast to a conventional OLED, inwhich the anode is placed on the substrate, an InvertedOLED uses a bottom cathode that can be connected to thedrain end of an n-channel TFT especially for the low cost

amorphous silicon TFT backplane useful in themanufacturing of AMOLED displays.

Patterning technologies

Patternable organic light-emitting devices use a light or heatactivated electroactive layer. A latent material (PEDOT-TMA)is included in this layer that, upon activation, becomeshighly efficient as a hole injection layer. Using this process,light-emitting devices with arbitrary patterns can beprepared.

Colour patterning can be accomplished by means of laser,such as radiation-induced sublimation transfer (RIST).

Organic vapour jet printing (OVJP) uses an inert carrier gas,such as argon or nitrogen, to transport evaporated organic


13/24

molecules (as in Organic Vapor Phase Deposition). The gas isexpelled through a micron sized nozzle or nozzle array closeto the substrate as it is being translated. This allows printingarbitrary multilayer patterns without the use of solvents.

Conventional OLED displays are formed by vapor thermalevaporation (VTE) and are patterned by shadow-mask. Amechanical mask has openings allowing the vapor to passonly on the desired location.

Backplane technologies

For a high resolution display like a TV, a TFT backplane isnecessary to drive the pixels correctly. Currently, Low

Temperature Polycrystalline silicon LTPS-TFT is used forcommercial AMOLED displays. LTPS-TFT has variation of theperformance in a display, so various compensation circuitshave been reported. Due to the size limitation of the excimerlaser used for LTPS, the AMOLED size was limited. To copewith the hurdle related to the panel size, amorphous-

silicon/microcrystalline-silicon backplanes have beenreported with large display prototype demonstrations.

Advantages

The different manufacturing process of OLEDs lends itself to

several advantages over flat-panel displays made with LCDtechnology.

* Future lower cost: Although the method is not currentlycommercially viable for mass production, OLEDs can be


14/24

printed onto any suitable substrate using an inkjet printer oreven screen printing technologies,they could theoreticallyhave a lower cost than LCDs or plasma displays. However, itis the fabrication of the substrate that is the most complex

and expensive process in the production of a TFT LCD, soany savings offered by printing the pixels only work to offsetthe OLED's costly LTPS substrate - a fact that is borne out bythe significantly higher initial price of AMOLED displays thantheir TFT LCD competitors. A mitigating factor to this pricedifferential going into the future is the cost of retoolingexisting lines to produce AMOLED displays over LCDs to takeadvantage of the economies of scale afforded by massproduction.

* Light weight & flexible plastic substrates: OLED displayscan be fabricated on flexible plastic substrates leading to thepossibility of Organic light-emitting diode roll-up displaybeing fabricated or other new applications such as roll-updisplays embedded in fabrics or clothing. As the substrateused can be flexible such as PET., the displays may beproduced inexpensively.

* Wider viewing angles & improved brightness: OLEDs canenable a greater artificial contrast ratio (both dynamic rangeand static, measured in purely dark conditions) and viewingangle compared to LCDs because OLED pixels directly emitlight. OLED pixel colours appear correct and unshifted, evenas the viewing angle approaches 90 degrees from normal.

* Better power efficiency: LCDs filter the light emittedfrom a backlight, allowing a small fraction of light through sothey cannot show true black, while an inactive OLED elementproduces no light and consumes no power.

* Response time: OLEDs can also have a faster responsetime than standard LCD screens. Whereas LCD displays are


15/24

capable of a 1 ms response time or less offering a frame rateof 1,000 Hz or higher.

Disadvantages

* Lifespan: The biggest technical problem for OLEDs was the

limited lifetime of the organic materials. In particular, blue OLEDs

historically have had a lifetime of around 14,000 hours to half

original brightness (five years at 8 hours a day) when used for

flat-panel displays. This is lower than the typical lifetime of LCD,

LED or PDP technologyeach currently rated for about 60,000

hours to half brightness, depending on manufacturer and model.

However, some manufacturers' displays aim to increase thelifespan of OLED displays, pushing their expected life past that of

LCD displays by improving light outcoupling, thus achieving the

same brightness at a lower drive current.[57][58] In 2007,

experimental OLEDs were created which can sustain 400 cd/m2 of

luminance for over 198,000 hours for green OLEDs and 62,000

hours for blue OLEDs.

* Color balance issues: Additionally, as the OLED material used

to produce blue light degrades significantly more rapidly than the

materials that produce other colors, blue light output will

decrease relative to the other colors of light. This differential color

output change will change the color balance of the display and is

much more noticeable than a decrease in overall luminance. This

can be partially avoided by adjusting colour balance but this may

require advanced control circuits and interaction with the user,which is unacceptable for some users. In order to delay the

problem, manufacturers bias the colour balance towards blue so

that the display initially has an artificially blue tint, leading to

complaints of artificial-looking, over-saturated colors. More

commonly, though, manufacturers optimize the size of the R, G


16/24

and B subpixels to reduce the current density through the

subpixel in order to equalize lifetime at full luminance. For

example, a blue subpixel may be 100% larger than the green

subpixel. The red subpixel may be 10% smaller than the green.

* Efficiency of blue OLEDs: Improvements to the efficiency and

lifetime of blue OLEDs is vital to the success of OLEDs as

replacements for LCD technology. Considerable research has

been invested in developing blue OLEDs with high external

quantum efficiency as well as a deeper blue color. External

quantum efficiency values of 20% and 19% have been reported

for red (625 nm) and green (530 nm) diodes,respectively.However, blue diodes (430 nm) have only been able

to achieve maximum external quantum efficiencies in the range

between 4% to 6%.This is primarily due to two factors. Firstly, the

human eye is less sensitive to the blue wavelength compared to

the green or red, so lower efficiency is expected. Secondly, by

calculating the band gap (Eg = hc/), it is clear that the shorter

wavelength of the blue OLED results in a larger band gap at 2.9

eV. This leads to higher barriers, so less efficiency is alsoexpected.

* Water damage: Water can damage the organic materials of

the displays. Therefore, improved sealing processes are important

for practical manufacturing. Water damage may especially limit

the longevity of more flexible displays.

* Outdoor performance: As an emissive display technology,

OLEDs rely completely upon converting electricity to light, unlike

most LCDs which are to some extent reflective; e-ink leads the

way in efficiency with ~ 33% ambient light reflectivity, enabling


17/24

the display to be used without any internal light source. The

metallic cathode in an OLED acts as a mirror, with reflectance

approaching 80%, leading to poor readability in bright ambient

light such as outdoors. However, with the proper application of a

circular polarizer and anti-reflective coatings, the diffusereflectance can be reduced to less than 0.1%. With 10,000 fc

incident illumination (typical test condition for simulating outdoor

illumination), that yields an approximate photopic contrast of 5:1.

* Power consumption: While an OLED will consume around 40% of

the power of an LCD displaying an image which is primarily black,

for the majority of images it will consume 6080% of the power of

an LCD - however it can use over three times as much power to

display an image with a white background such as a document or

website. This can lead to disappointing real-world battery life in

mobile devices.

* Screen burn-in: Unlike displays with a common light source,

the brightness of each OLED pixel fades depending on the content

displayed. The varied lifespan of the organic dyes can cause a

discrepancy between red, green, and blue intensity. This leads to

image persistence, also known as burn-in

Manufacturers and Commercial Uses

OLED technology is used in commercial applications such asdisplays for mobile phones and portable digital mediaplayers, car radios and digital cameras among others. Suchportable applications favor the high light output of OLEDs forreadability in sunlight and their low power drain. Portabledisplays are also used intermittently, so the lower lifespan oforganic displays is less of an issue. Prototypes have been


18/24

made of flexible and rollable displays which use OLEDs'unique characteristics. Applications in flexible signs andlighting are also being developed. Philips Lighting havemade OLED lighting samples under the brand name

'Lumiblade' available online.

OLEDs have been used in most Motorola and Samsungcolour cell phones, as well as some HTC, LG and SonyEricsson models. Nokia has also recently introduced someOLED products including the N85 and the N86 8MP, both ofwhich feature an AMOLED display. OLED technology can alsobe found in digital media players such as the Creative ZEN V,

the iriver clix, the Zune HD and the Sony Walkman X Series.

The Google and HTC Nexus One smartphone includes anAMOLED screen, as does HTC's own Desire and Legendphones. However due to supply shortages of the Samsung-produced displays, certain HTC models will use Sony's SuperLCD technology in the future.

Other manufacturers of OLED panels include AnwellTechnologies Limited, Chi Mei Corporation, LG, and others.

DuPont stated in a press release in May 2010 that they canproduce a 50-inch OLED TV in two minutes with a newprinting technology. If this can be scaled up in terms ofmanufacturing, then the total cost of OLED TVs would begreatly reduced. Dupont also states that OLED TVs madewith this less expensive technology can last up to 15 years ifleft on for a normal eight hour day.


19/24

Handheld computer manufacturer OQO introduced thesmallest Windows netbook computer, including an OLEDdisplay, in 2009.

The use of OLEDs may be subject to patents held byEastman Kodak, DuPont, General Electric, Royal PhilipsElectronics, numerous universities and others. There are bynow literally thousands of patents associated with OLEDs,both from larger corporations and smaller technologycompanies .

Active matrix addressing

Active matrix addressing is an addressing scheme used invideo displays. Given a m n matrix, the number ofconnectors needed to address the display is m + n.

Each pixel is attached to a switch-device, which activelymaintains the pixel state while other pixels are beingaddressed, which also prevents crosstalk from inadvertentlychanging the state of an unaddressed pixel. The mostcommon switching devices are Thin Film Transistors (TFT),i.e. a FET based on either the cheaper non-crystalline thin-film silicon (a-Si), polycrystalline silicon (poly-Si), or CdSesemiconductor material. The latter is rarely used any longerin favour of the former two.

Passive matrix addressing

Passive matrix addressing is an addressing scheme used inearlier LCD displays, and may be used in future LCD


20/24

displays. This is a matrix addressing scheme meaning thatonly m + n control signals are required to address a m ndisplay. A pixel in a passive matrix must maintain its statewithout active driving circuitry until it can be refreshed

again.

A new display technology uses a bi-stable pixel, whichmaintains its state indefinitely without the need forindividual transistor elements at each pixel.

The signal is divided into a row or select signal and a columnor video signal. The select voltage determines the row that isbeing addressed and all m pixels on a row are addressedsimultaneously. When pixels on a row are being addressed, aVsel potential is applied, and all other rows are unselectedwith a Vunsel potential. The video signal or column potentialis then applied with a potential for each m columnsindividually. An on-lighted pixel corresponds to a Von, an off-switched corresponds to a Voff potential.

for the unselected rows.

Passive matrix addressed displays such as ferroelectricLiquid crystal display do not need the switch-component ofan active matrix display because it has a built-in bistability.

Technology for electronic papers also have a form ofbistability. Displays with bistable pixel elements areaddressed with passive matrix addressing scheme, whereas

TFT-LCD-displays are addressed using active addressing.


21/24

Commercial devices

Phones:

* Dell Venue Pro

* Google Nexus one

* HTC Desire

* HTC Droid Incredible

* HTC Legend

* Nokia C7-00* Nokia C6-01

* Nokia E7-00

* Nokia N8

* Nokia N85

* Nokia N86 8MP

* Samsung i7500 Galaxy

* Samsung i8910

* Samsung Jet


22/24

* Samsung Omnia 2

* Samsung Rogue

* Samsung Galaxy S series (Super AMOLED)

* Samsung Wave S8500 (Super AMOLED)

* Samsung Focus (Super AMOLED)

* Samsung Omnia 7 (Super AMOLED)

Music Players:

* Cowon S9

* Cowon J3

* Iriver Clix

* Zune HD

Super AMOLED

Super Active-Matrix Organic Light-Emitting Diode or SuperAMOLED is a display technology for use in mobile devicessuch as mobile phones. It differs from many other displaytechnologies in that the layer which detects touch isintegrated into the screen rather than being overlaid on top.

The first company known to produce display units of this

type is Samsung, when it introduced its Samsung Wavemobile phone on 14 February 2010. The Samsung i9000Galaxy S series, including the Captivate, Epic 4G, Fascinate,and Vibrant, also use this display.


23/24

Compared with the first-generation AMOLED, the SuperAMOLED advantages are:

* 20% brighter screen

* 80% less sunlight reflection

* 20% reduced power consumption


24/24

References

http://www.pcworld.com/article/192176/first_look_samsung_

galaxy_s_android_phone_with_smart_life.html

http://www.engadget.com/2010/06/28/sprint-lines-up-epic-4g-against-the-competition-likes-its-chanc

http://www.samsung.com/au/smartphone/technology/super-

amoled.html

http://www.oled-info.com/super-amoled

Amoled Term Report

Documents

Transcript of Amoled Term Report