65118773 Oled Seminar Report

8/2/2019 65118773 Oled Seminar Report

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A

SEMINAR REPORT

ON

OLED

( )ORGANIC LIGHT EMITTING DIODE


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ORGANIC LIGHT EMITTING DIODE

CONTENT

S

1 History

2 Working principle 3 Material technologies3.1 Small molecules

3.2 Polymer light-emitting diodes

3.3 Phosphorescent materials

4 Device Architectures

4.1 Structure4.2 Patterning technologies

4.3 Backplane technologies

5 Advantages6 Disadvantages7 Manufacturers and commercial uses

7.1 Samsung applications

7.2 Sony applications

7.3 LG applications

8 References


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Organic light-emittingdiodeIN

TRODUCTIO

NDemonstration of a flexible OLED device

An organic light emitting diode (OLED) is a light-emitting diode (LED) in whichthe emissive electroluminescent layer is a filmof organic compounds which emit light in responseto an electric current. This layer of organicsemiconductor material is situated between twoelectrodes. Generally, at least one of theseelectrodes is transparent.


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OLEDs are used in television screens, computermonitors, small, portable system screens suchas mobile phones and PDAs, watches, advertising,information, and indication. OLEDs are also used inlight sources for space illumination and in large-area light-emitting elements. Due to their early

stage of development, they typically emit less lightper unit area than inorganic solid-state based LEDpoint-light sources.

An OLED display functions without a backlight.Thus, it can display deep black levels and can bethinner and lighter than liquid crystal displays. In

low ambient light conditions such as dark rooms,an OLED screen can achieve a higher contrastratio than an LCD using either cold cathodefluorescent lamps or the more recentlydeveloped LED backlight.

There are two main families of OLEDs: those basedupon small molecules and those

employing polymers. Adding mobile ions to anOLED creates a Light-emitting ElectrochemicalCell or LEC, which has a slightly different mode ofoperation.

HISTOR

YThe first observations of electroluminescence inorganic materials were in the early 1950s by A.

Bernanose and co-workers at the Nancy- Universit,France. They applied high-voltage alternatingcurrent(AC) fields in air to materials such


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as acridine orange, either deposited on ordissolved in cellulose or cellophane thin films. Theproposed mechanism was either direct excitationof the dye molecules or excitation of electrons

In 1960, Martin Pope and co-workers at New YorkUniversity developed ohmic dark-injecting

electrode contacts to organic crystals They further .]

described the necessary energetic requirements(work functions) for hole and electron injectingelectrode contacts. These contacts are the basis ofcharge injection in all modern OLED devices.Pope's group also first observed direct current (DC)

electroluminescence under vacuum on a puresingle crystal of anthracene and on anthracenecrystals doped with tetracene in 1963 using a small.

area silver electrode at 400V. The proposedmechanism was field-accelerated electronexcitation of molecular fluorescence.

Pope's group reported in 1965 that in the absence.

of an external electric field, theelectroluminescence in anthracene crystals iscaused by the recombination of a thermalizedelectron and hole, and that the conducting level ofanthracene is higher in energy than the excitonenergy level. Also in 1965, W. Helfrich and W. G.

Schneider of the National Research Council inCanada produced double injection recombination

electroluminescence for the first time in ananthracene single crystal using hole and electroninjecting electrodes,the forerunner of moderndouble injection devices. In the same year, Dow


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Chemical researchers patented a method ofpreparing electroluminescent cells using highvoltage (5001500 V) AC-driven (1003000 Hz)electrically-insulated one millimetre thin layers of amelted phosphor consisting of ground anthracenepowder, tetracene, and graphite powder.Their

proposed mechanism involved electronic excitationat the contacts between the graphite particles andthe anthracene molecules.

Device performance was limited by the poorelectrical conductivity of contemporary organicmaterials. This was overcome by the discovery and

development of highly conductive polymers For .]

more on the history of such materials,see conductive polymers.

Electroluminescence from polymer films was firstobserved by Roger Partridge at the NationalPhysical Laboratory in the United Kingdom. Thedevice consisted of a film of poly(n-vinylcarbazole)

up to 2.2 micrometres thick located between twocharge injecting electrodes. The results of theproject were patented in 1975 and published in[13]

1983 .

The first diode device was reported at EastmanKodak by Ching W. Tang and Steven VanSlyke in

1987.This device used a novel two-layer structurewith separate hole transporting and electron

transporting layers such that recombination andlight emission occurred in the middle of the organiclayer. This resulted in a reduction in operatingvoltage and improvements in efficiency and led to


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the current era of OLED research and deviceproduction.

WORKING

PRINCIPLE

Schematic of a bilayer OLED: 1. Cathode (-), 2.Emissive Layer, 3. Emission of radiation, 4.Conductive Layer, 5. Anode (+)

A typical OLED is composed of a layer of organicmaterials situated between two electrodes,the anode and cathode, all deposited ona substrate. The organic molecules are electricallyconductive as a result of delocalization of pielectrons caused by conjugation over all or part of

the molecule. These materials have conductivitylevels ranging from insulators to conductors, andtherefore are considered organic semiconductors.The highest occupied and lowest unoccupiedmolecular orbitals (HOMO and LUMO) of organicsemiconductors are analogous to

the valence and conduction bands of inorganicsemiconductors.

Originally, the most basic polymer OLEDs consistedof a single organic layer. One example was the firstlight-emitting device synthesised by J. H.Burroughes et

al.

, which involved a single layer


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of poly(p- phenylene vinylene) . However multilayer OLEDs can be fabricated with two or more layers inorder to improve device efficiency. As well asconductive properties, different materials may bechosen to aid charge injection at electrodes byproviding a more gradual electronic profile,or block

a charge from reaching the opposite electrode andbeing wasted Many modern OLEDs incorporate a.

simple bilayer structure, consisting of a conductivelayer and an emissive layer.

During operation, a voltage is applied across theOLED such that the anode is positive with respect

to the cathode. A current of electrons flowsthrough the device from cathode to anode, aselectrons are injected into the LUMO of the organiclayer at the cathode and withdrawn from theHOMO at the anode. This latter process may alsobe described as the injection of electron holes intothe HOMO. Electrostatic forces bring the electrons

and the holes towards each other and theyrecombine forming an exciton, a bound state of theelectron and hole. This happens closer to theemissive layer, because in organic semiconductorsholes are generally more mobile than electrons.The decay of this excited state results in arelaxation of the energy levels of the electron,accompanied by emission

of radiation whose frequency is in the visibleregion. The frequency of this radiation depends onthe band gap of the material, in this case thedifference in energy between the HOMO and LUMO.


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As electrons and holes are fermions with halfinteger spin, an exciton may either be in a singletstate or a triplet state depending on how the spinsof the electron and hole have been combined.Statistically three triplet excitons will be formed foreach singlet exciton. Decay from triplet states

(phosphorescence) is spin forbidden, increasing thetimescale of the transition and limiting the internalefficiency of fluorescent devices. Phosphorescentorganic light-emitting diodes make use of spinorbit interactions to facilitate intersystemcrossing between singlet and triplet states, thusobtaining emission from both singlet and tripletstates and improving the internal efficiency.

Indium tin oxide (ITO) is commonly used as theanode material. It is transparent to visible light andhas a high work function which promotes injectionof holes into the HOMO level of the organic layer. Atypical conductive layer may consist

ofPEDOT

:PSS as the HOMO level of this material[22]generally lies between the workfunction of ITO andthe HOMO of other commonly used polymers,reducing the energy barriers for hole injection.Metals such as barium and calcium are often usedfor the cathode as they have low workfunctions which promote injection of electrons intothe LUMO of the organic layer. Such metals are[23]

reactive, so require a capping layerof aluminium to avoid degradation.

Single carrier devices are typically used to studythe kinetics and charge transport mechanisms of


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an organic material and can be useful when tryingto study energy transfer processes. As currentthrough the device is composed of only one type ofcharge carrier, either electrons or holes,recombination does not occur and no light isemitted. For example, electron only devices can be

obtained by replacing ITO with a lower workfunction metal which increases the energy barrierof hole injection. Similarly, hole only devices can bemade by using a cathode comprised solely ofaluminium, resulting in an energy barrier too largefor efficient electron injection .


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Material technologies

Polymer light-emitting diodes

poly(-

phenylene vinylene) , used in the first PLED.

Polymer light-emitting diodes (PLED), also light-emitting polymers (LEP), involvean electroluminescent conductive polymer thatemits light when connected to an external voltage.

They are used as a thin film for full-spectrum colour displays. Polymer OLEDs are quiteefficient and require a relatively small amount ofpower for the amount of light produced.

Vacuum deposition is not a suitable method for

forming thin films of polymers. However, polymers

can be processed in solution, and spin coating is acommon method of depositing thin polymer films.This method is more suited to forming large-areafilms than thermal evaporation. No vacuum isrequired, and the emissive materials can also beapplied on the substrate by a technique derived


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from commercial inkjet printing. However, as[31][32]

the application of subsequent layers tends todissolve those already present, formation ofmultilayer structures is difficult with thesemethods. The metal cathode may still need to bedeposited by thermal evaporation in vacuum.

Typical polymers used in PLED displays includederivatives of poly(

-phenylene

inylene)

and polyfluorene. Substitution of sidechains onto the polymer backbone may determinethe colour of emitted light or the stability and[33]

solubility of the polymer for performance and ease

of processing.[34]

While unsubstituted poly(p-phenylene vinylene)(PPV) is typically insoluble, a number of PPVs andrelated poly(naphthalene vinylene)s (PNVs) thatare soluble in organic solvents or water have beenprepared via ring opening metathesispolymerization. [35][36][37]

Phosphorescent materials

Ir(mppy) , a phosphorescent dopant which emits green light.3

Phosphorescent organic light emitting diodes usethe principle of electrophosphorescence to convert


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electrical energy in an OLED into light in a highlyefficient manne with the internal quantumr.

efficiencies of such devices approaching 100%.

Typically, a polymer such as poly(n-vinylcarbazole)is used as a host material to which anorganometallic complex is added as a

dopant. Iridiumcomplexes such as[40]

Ir(mppy) are currently the focus of research,3[38]

although complexes based on other heavy metalssuch as platinum have also been used.[39]

The heavy metal atom at the centre of thesecomplexes exhibits strong spin-orbit coupling,

facilitating intersystemcrossing between singlet and triplet states. Byusing these phosphorescent materials, both singletand triplet excitons will be able to decayradiatively, hence improving the internal quantumefficiency of the device compared to a standardPLED where only the singlet states will contribute

to emission of light.

Applications of OLEDs in solid state lighting requirethe achievement of high brightness with good CIEcoordinates (for white emission). The use ofmacromolecular species like polyhedral oligomericsilsesquioxanes (POSS) in conjunction with the use

of phosphorescent species such as Ir for printedOLEDs have exhibited brightnesses as high as

10,000 cd/m .

Device

ArchitecturesStructur

e


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Bottom emissionBottom or top emission:

devices use a transparent or semi-transparent bottom electrode to getthe light through a transparentsubstrate. Top emission devices [43]

use a transparent or semi-[44]

transparent top electrode emittinglight directly. Top-emitting OLEDsare better suited for active-matrixapplications as they can be moreeasily integrated with a non-transparent transistor backplane.

use transparent orTransparent OLEDssemi-transparent contacts on bothsides of the device to createdisplays that can be made to beboth top and bottom emitting(transparent). TOLEDs can greatlyimprove contrast, making it much

easier to view displays in brightsunlight. This technology can be[45]

used in Head-up displays, smartwindows or augmentedreality applications.Novaled's OLED panel presented[46]

in Finetech Japan 2010, boasts atransparency of 6070%.

use a pixel architectureStacked OLEDsthat stacks the red, green, and bluesubpixels on top of one anotherinstead of next to one another,


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leading to substantial increase ingamut and color depth, and greatlyreducing pixel gap. Currently, otherdisplay technologies have the RGB(and RGBW) pixels mapped next toeach other decreasing potential

resolution.: In contrast to aInverted OLED

conventional OLED, in which theanode is placed on the substrate, anInverted OLED uses a bottomcathode that can be connected to

the drain end of an n-channel TFTespecially for the lowcost amorphous silicon TFTbackplane useful in themanufacturing of AMOLED displays.

[47]

Patterning technologies

Patternable organic light-emitting devices use alight or heat activated electroactive layer. A latentmaterial (PEDOT-TMA) is included in this layer that,upon activation, becomes highly efficient as a holeinjection layer. Using this process, light-emittingdevices with arbitrary patterns can be prepared. [48]

Colour patterning can be accomplished by meansof laser, such as radiation-induced sublimationtransfer(RIST).

[49]

Organic vapour jet printing (OVJP) uses an inertcarrier gas, such as argon or nitrogen, to transport


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evaporated organic molecules (as in Organic VaporPhase Deposition). The gas is expelled through amicron sized nozzle or nozzle array close to thesubstrate as it is being translated. This allowsprinting arbitrary multilayer patterns without theuse of solvents.

Conventional OLED displays are formed by vaporthermal evaporation (VTE) and are patterned byshadow-mask. A mechanical mask has openingsallowing the vapor to pass only on the desiredlocation.

Backplane technologies

For a high resolution display like a TV,a TFT backplane is necessary to drive the pixels

correctly. Currently, LowTemperature Polycrystalline silicon LTPS-TFT isused for commercial AMOLEDdisplays. LTPS-TFThas variation of the performance in a display, sovarious compensation circuits have been reported.

Due to the size limitation of the excimer[43]

laser used for LTPS, the AMOLEDsize was limited.To cope with the hurdle related to the panel size,amorphous-silicon/microcrystalline-siliconbackplanes have been reported with large displayprototype demonstrations. [50]

Advantages


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Demonstration of a 4.1" prototype flexible display from Sony

The different manufacturing process of OLEDslends itself to several advantages over flat-paneldisplays made with LCD technology.

Lower cost in the

future:

OLEDscan be printed onto any

suitable substrate by an inkjetprinter or even by screen printing,

theoretically making them[51]

cheaper to produce than LCDor plasma displays. However,fabrication of the OLED substrate is

more costly than that of a TFT LCD,until mass production methodslower cost through scalability. Roll-roll vapour-deposition methods fororganic devices do allow massproduction of thousands of devicesper minute for minimal cost,

although this technique alsoinduces problems in that multi-layerdevices can be challenging to make.

Light weight & flexible plastic

substrates

:

OLED displays can befabricated on flexible plastic

substrates leading to the possibilityofflexibleorganic light-emittingdiodes being fabricated or othernew applications such as roll-updisplays embedded in fabrics orclothing. As the substrate used can


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be flexible such as PET., the[52]

displays may be producedinexpensively.

Wider viewing angles &

improved brightness: OLEDs canenable a greater artificial contrast

ratio (both dynamic range andstatic, measured in purely darkconditions) and viewing anglecompared to LCDs because OLEDpixels directly emit light. OLED pixelcolours appear correct and

unshifted, even as the viewingangle approaches 90 from normal.

Better power efficiency: LCDsfilter the light emitted froma backlight, allowing a smallfraction of light through so theycannot show true black, while an

inactive OLED element does notproduce light or consume power. [53]

Response time: OLEDs can alsohave a faster response time thanstandard LCD screens. WhereasLCD displays are capable of

between 2 and 8 ms responsetime offering a frame rate of +/-

200 Hz, an OLED can[citation needed ]

theoretically have less than 0.01 msresponse time enabling100,000 Hz refresh rates.


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Disadvantages

Currentcosts: OLED manufacture currentlyrequires process steps that make it extremely

expensive. Specifically, it requires the use of Low-Temperature Polysilicon backplanes; LTPSbackplanes in turn require laser annealing from anamorphous silicon start, so this part of themanufacturing process for AMOLEDs starts with the

process costs of standard LCD, and then adds anexpensive, time-consuming process that cannotcurrently be used on large-area glass substractors.

Color balance issues: Additionally, as the OLEDmaterial used to produce blue light degrades

significantly more rapidly than the materials thatproduce other colors, blue light output willdecrease relative to the other colors of light. Thisdifferential color output change will change thecolor balance of the display and is much morenoticeable than a decrease in overall luminance.


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This can be partially avoided by adjusting colour[60]

balance but this may require advanced controlcircuits and interaction with the user, which isunacceptable for some users. In order to delay theproblem, manufacturers bias the colour balancetowards blue so that the display initially has an

artificially blue tint, leading to complaints ofartificial-looking, over-saturated colors. Morecommonly, though, manufacturers optimize thesize of the R, G and B subpixels to reduce thecurrent density through the subpixel in order toequalize lifetime at full luminance. For example, ablue subpixel may be 100% larger than the greensubpixel. The red subpixel may be 10% smallerthan the green.

Water damage: Water can damage the organicmaterials of the displays. Therefore, improvedsealing processes are important for practicalmanufacturing. Water damage may especially limit

the longevity of more flexible displays. [66]

Outdoor performance: As an emissive displaytechnology, OLEDs rely completely uponconverting electricity to light, unlike most LCDswhich are to some extent reflective; e-ink leads theway in efficiency with ~ 33% ambient light

reflectivity, enabling the display to be used withoutany internal light source. The metallic cathode in

an OLED acts as a mirror, with reflectanceapproaching 80%, leading to poor readability inbright ambient light such as outdoors. However,with the proper application of a circular polarizer


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and anti-reflective coatings, the diffuse reflectancecan be reduced to less than 0.1%. With10,000 fc incident illumination (typical testcondition for simulating outdoor illumination), thatyields an approximate photopic contrast of 5:1.

Power consumption: While an OLED will

consume around 40% of the power of an LCDdisplaying an image which is primarily black, forthe majority of images it will consume 6080% ofthe power of an LCD however it can use overthree times as much power to display an imagewith a white background such as a document or [67]

website. This can lead to reduced real-worldbattery life in mobile devices.

UV sensitivity: OLED displays can be damaged byprolonged exposure to UV light. The mostpronounced example of this can be seen with anear UV laser (such as a Bluray pointer) and candamage the display almost instantly with more

than 20 mW leading to dim or dead spots wherethe beam is focused. This is usually avoided byinstalling a UV blocking filter over the panel andthis can easily be seen as a clear plastic layer onthe glass. Removal of this filter can lead to severedamage and an unusable display after only a few

months of room light exposure.Manufacturers and commercial uses


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OLED technology is used in commercialapplications such as displays for mobile phonesand portable digital media players, car radiosand digitalcameras among others. Such portableapplications favor the high light output of OLEDsfor readability in sunlight and their low power

drain. Portable displays are also usedintermittently, so the lower lifespan of organicdisplays is less of an issue. Prototypes have beenmade of flexible and rollable displays which useOLEDs' unique characteristics. Applications inflexible signs and lighting are also beingdeveloped. Philips Lighting have made OLED[69]

lighting samples under the brand name 'Lumiblade'available online. [70]

OLEDs have been used inmost Motorola and Samsung colour cell phones, aswell as some HTC, LG and Sony Ericsson models.

Nokia has also recently introduced some OLED[71]

products including the N85 and the N86 8MP, bothof which feature an AMOLED display. OLEDtechnology can also be found in digital mediaplayers 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 an AMOLED screen, as does HTC'sown Desire and Legend phones. However due to

supply shortages of the Samsung-produceddisplays, certain HTC models will useSony's SLCD displays in the future, while the[72]


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Google and Samsung Nexus S smartphone will use"Super Clear LCD" instead in some countries. [73]

Other manufacturers of OLED panelsinclude Anwell Technologies Limited, Chi Mei[74]

Corporation, LG, and others.[75] [76] [77]

DuPont stated in a press release in May 2010 thatthey can produce a 50-inch OLED TV in two

minutes with a new printing technology. If this canbe scaled up in terms of manufacturing, then thetotal cost of OLED TVs would be greatly reduced.Dupont also states that OLED TVs made with thisless expensive technology can last up to 15 years if

left on for a normal eight hour day. [78][79]

The use of OLEDs may be subject to patents heldby Eastman Kodak, DuPont, General Electric, RoyalPhilips Electronics, numerous universities andothers. There are by now literally thousands of[80]

patents associated with OLEDs, both from largercorporations and smaller technologycompanies [1].

Samsung applications

By 2004 Samsung, South Korea'slargest conglomerate, was the world's largest OLEDmanufacturer, producing 40% of the OLED displaysmade in the world, and as of 2010 has a 98%[81]

share of the global AMOLED market. The[82]

company is leading the world OLED industry,


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generating $100.2 million out of the total $475million revenues in the global OLED market in2006. As of 2006, it held more than 600[83]

American patents and more than 2800international patents, making it the largest ownerof AMOLED technology patents. [83]

Samsung SDI announced in 2005 the world'slargest OLED TV at the time, at 21 inches (53 cm).

This OLED featured the highest resolution at the[84]

time, of 6.22 million pixels. In addition, thecompany adopted active matrix based technologyfor its low power consumption and high-resolution

qualities. This was exceeded in January 2008, whenSamsung showcased the world's largest andthinnest OLED TV at the time, at 31 inches and4.3 mm. [85]

Samsung's latest AMOLED smartphones usetheir Super AMOLED trademark, with the SamsungWave S8500 and Samsung i9000 Galaxy S being

launched in June 2010. In January 2011 Samsungannounced their Super AMOLED Plus displays -[92]

which offer several advances over the older SuperAMOLED displays - real stripe matrix (50% moresub pixels), thinner form factor, brighter image anda 18% reduction in energy consumption.

Sony applications


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Sony XEL-1 , the world's first OLED TV. (front)[93]

The Sony CLI PEG-VZ90 was released in 2004,being the first PDA to feature an OLED screen.

Other Sony products to feature OLED screens[94]

include the MZ-RH1 portable minidisc recorder,released in 2006 and the Walkman X Series.[95] [96]

At the Las Vegas CES 2007, Sony showcased 11-inch (28 cm, resolution 960540) and 27-inch(68.5 cm, full HD resolution at 19201080) OLEDTV models. Both claimed 1,000,000:1 contrast[97]

ratios and total thicknesses (including bezels) of5 mm. In April 2007, Sony announced it would

manufacture 1000 11-inch OLED TVs per month formarket testing purposes. On October 1, 2007,[98]

Sony announced that the 11-inch model, nowcalled the XEL-1, would be released commercially;

the XEL-1 was first released in Japan in[93]

December 2007. [99]In May 2007, Sony publicly unveiled a video of a2.5-inch flexible OLED screen which is only 0.3millimeters thick. At the Display 2008[100]

exhibition, Sony demonstrated a 0.2 mm thick3.5 inch display with a resolution of 320200


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pixels and a 0.3 mm thick 11 inch display with960540 pixels resolution, one-tenth the thicknessof the XEL-1. [101][102]

In January 2011, Sony announced the NextGeneration Portable handheld game console (thesuccessor to the PSP) will feature a 5-inch OLED

screen. [106]

On February 17, 2011, Sony announced its 25"OLED Professional Reference Monitor aimed at theCinema and high end Drama Post Productionmarket. [107]

Reference

s

.

Kallmann, H.; Pope, M. (1960). "Positive Hole Injection into Organic1.

Crystal.

^Mark, Peter; Helfrich, Wolfgang (1962). "Space-Charge-Limited2.Currents in Organic crystal.

Pope, M.; Kallmann, H. P.; Magnante, P. (1963).3.

"Electroluminescence in Organic Crystals".

^Sano, Mizuka; Pope, Martin; Kallmann, Hartmut (1965).4.

"Electroluminescence and Band Gap in Anthracene".

Helfrich, W.; Schneider, W. (1965). "Recombination Radiation in5.

Anthracene Crystal.


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