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PRESENTED BY-

Manjari Upasana Sahanyregd- 0501214052

TALKFLOW Introduction History Components Principle Working Types Applications Advantages Disadvantages Conclusion References 3

INTRODUCTION An OLED is any LED whose emissive

electroluminescent layer comprises a film of organic compounds.

The layer usually contains a polymer substance that allows suitable organic compounds to be deposited.

They are deposited in rows and columns onto a flat carrier by a simple printing process.

The resulting matrix of pixels can emit light of different colours.

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HISTORY The first EL from a an organic molecule, anthracene,

was reported by Pope and coworkers in 1963 . Its thickness was between 10μm-5mm when a bias of several hundred volts was applied across it.

P. S. Vincent achieved bright blue EL from vacuum-deposited 0.6μm thick anthracene crystal films with an applied bias of less than 100V.

The breakthrough was achieved by Tang and VanSlyke in 1987, who made a bilayer structure by thermally evaporating the small molecular weight organic materials.

In 1989, Tang developed a laser-dye doped Alq3 multilayer structure ,in which the fluorescent efficiency was improved and the emission color varied from the original green to the dopant emission color.

M. A. Baldo fabricated and demonstrated phosphorescent OLEDs , by doping phosphorescent molecules, where the EL is due to triplet emission, into a fluorescent host layer.

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FROM INVENTION TO MARKET… …TAKES A LONG TIME IN DISPLAY TECHNOLOGIES Cathode Ray Tube(CRT) - 1897 Plasma display - 1927 CRT television - 1929 Liquid Crystal Display(LCD) - (1970’s -

1980’s) Plasma Display Panels(PDP) - 1990’s Organic Light Emitting Diodes(OLED) and

Electronic Paper - 1990’s

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OLED COMPONENTS Substrate (clear plastic,

glass, foil) - The substrate supports the OLED.

Anode (transparent) -

Indium tin oxide (ITO) is utilized as the

transparent anode electrode .

Organic layers - These layers are made of organic molecules or polymers.

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Conducting layer - This layer is made of organic plastic molecules that transport "holes" from the anode. One conducting polymer used in OLEDs is polyaniline.

Emissive layer - This layer is made of organic

plastic molecules (different ones from the conducting layer) that transport electrons from the cathode; this is where light is made. One polymer used in the emissive layer is polyfluorene.

Cathode - The cathode injects electrons when a current flows through the device.

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PRINCIPLE 1: CARRIER INJECTION:1.1: IMAGE FORCE LOWERING : Injected carriers from a metal electrode into

the organic layers encounter the injection barrier which is the energy difference between the Fermi level (Ef) of the metal and the LUMO level ELUMO for electron injection.

Holes encounter a barrier, which is the difference between Ef and EHOMO.

After injection many electrons remain on the surface of the organic layer at distance +x from the metal-organic interface. These electrons induce equivalent hole charges in the metal layer at –x. 9

1.2: THERMIONIC EMISSION: Charge injection at low applied bias is

primarily due to thermal emission of charge carriers over the interface potential barrier when the barrier is not too high for thermal injection.

1.3: Field Emission: Field emission is the process whereby

carriers tunnel through a barrier in the presence of a high electric field.

For low fields the thermionic current dominates; for high fields the tunneling current prevails . 10

2:CARRIER TRANSPORT IN OLEDS Two molecules are separated by a

potential barrier, a carrier on one side can move to the other either by tunneling through the barrier or by moving over the barrier via an activated state (hopping).

2.1:Field Dependent Mobility: Field emission is the process whereby

carriers tunnel through a barrier in the presence of a high electric field.

When the barrier is triangular, the tunneling is called Fowler- Nordheim (FN) tunneling.

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2.2:SPACE-CHARGE LIMITED CURRENT:

For an ohmic contact, the current-voltage relation of an organic semiconductor is linear at low fields but becomes non-ohmic at higher fields.

This behavior is due to two effects: i) At the higher current densities corresponding to

higher values of field, a relatively large concentration of charge carriers in transit to the collector electrode is present between the electrodes. These carriers constitute the space charge.

ii) The existence of traps which are due to the disorder within the organic semiconductor gives rise to highly localized energy states within the energy gap. The traps filled by injected charge carriers become electrically charged centers, thus contributing to the formation of the space charge . 12

3:CHARGE RECOMBINATION AND EFFICIENCY:

3.1:Charge recombination: Necessary condition for recombination

is that the separation λh-e between the hole and the electron must be less than the Coulomb capture or Onsager radius rC .

Onsager radius is the distance where the Coulomb attractive energy and the thermal dissociation energy are equal .

That is,

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3.2:EFFCIENCY: Factors determining efficiency: Luminescence

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fluorescence phosphorescence

HOW DO OLEDS EMIT LIGHT?

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[Schematic energy level diagram of an optimized small molecule OLED employing the phosphorescent complex]

[Schematic energy level diagram of a single layer polymer light emitting device]

WORKING continued …

TYPES OF OLEDS 1. Passive-matrix OLED:

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The anode strips are arranged perpendicular to the cathode strips.The intersections of the cathode and anode make up the pixels where light is emitted.PMOLEDs are most efficient for text and icons .These are best suited for small screens like cell phones and mp3 players.

2. Active-matrix OLED AMOLEDs have full layers

of cathode, organic molecules and anode, but the anode layer overlays a thin film transistor (TFT) array that forms a matrix.

The TFT array itself is the circuitry that determines which pixels get turned on to form an image.

These are used in computer monitors, large screen TVs and electronic signs or billboards.

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3. Transparent OLED

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4. Top Emitting OLED

5. Foldable OLED

6. White OLED

POTENTIAL APPLICATIONS

Domestic and commercial lighting with controllable color

Television Computer display which wont require

back light as in conventional LCDs Flat panel display Digital camera Mobile phones Flexible displays

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WHY OLEDS? Robust Design Viewing Angle High Resolution Electronic Paper Production Advantages Video Capability Hardware Content Power Usage

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ADVANTAGES Thinner, lighter and more flexible Plastic substrates rather then glass High resolution (<5um pixel size) and fast

switching (1-10um) Do not require backlight, light generated Low voltage, low power and emissive source Robust Design (Plastic Substrate) Larger sized displays Brighter- good daylight visibility Larger viewing angles -170o

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DISADVANTAGES Lifetime

White, Red, Green 46,000-230,000 hours About 5-25 years

Blue 14,000 hours About 1.6 years

Expensive Susceptible to water Overcome multi-billion dollar LCD market

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CONCLUSION Limited use caused by degradation of

materials. OLED will replace current LED and LCD

technologies Expensive Flexibility and thinness will enable

many applications

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REFERENCES1) Donald A Neamen,”SEMICONDUCTOR PHYSICS AND

DEVICES Basic Principles,”TATA McGraw-HILL ,Third Edition,2003.

2) G. Parthasarathy, J. Liu, and A. R. Duggal,” Organic Light Emitting Devices From Displays to Lighting,” The Electrochemical Society Interface • Summer 2003.

3) Bhaskar Choudhury,” Organic light emitting devices (OLEDs) and Structurally integrated photoluminescence based chemical and biological sensors excited by OLEDs,” Iowa State University Ames, 2005.

4) Craiq Freudenrich, ”How OLEDs Work,” [Online] .Available:

http://www.electronics.howstuffworks.com/oled.html25

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