Organic Light Emitting Diodes (OLEDs)

Presented by M.Satish Kumar(08-437)

What is an oled?
an oled is a light emiting diode (led)which emissive electroluminesent layer is composed of a film of organic compunds

Histroy of oled

First developed in the year 1950 in france In 1960 ac driven electroluminescent cells using doped anthracene was developed In 1987 the first light emitting diode was introduced from thin organic layers In 1990 electroluminesence was discovered

Architecture of OLEDs
    

Cathode Emissive layer Conductive layer Aode Substrate

Why OLEDs


Lighting efficiency
   

Incandescent bulbs are inefficient Fluorescent bulbs give off ugly light LEDs (ordinary light emitting diodes) are bright points; not versatile OLEDs may be better on all counts

Displays: Significant advantages over liquid crystals

  

Faster Brighter Lower power

Cost and design



LEDs are crystals; LCDs are highly structured; OLEDs are not
 

Malleable; can be bent, rolled up, etc. Easier to fabricate

In general, OLED research proceeds on many fronts

The Bands on Stage

E E E E E

Gap

No Gap

Small Gap

Insulator

Conductor

Semiconductor

Doped Semiconductors

Doping
N-type

Add Impurities
P-type

The Bands on Stage

E E E E E

N-type No Gap Small Gap

P-type

Gap

Insulator

Conductor

Semiconductor

Doped Semiconductors

Diode: p-type meets n-type

E E

Diode: p-type meets n-type

E E

Diode: p-type meets n-type

E E

Diode: p-type meets n-type

Try to make current flow to right? Current Flows! Electrons in higher band meet Holes in lower band

Electric Field

Current

Excitons
N-type


Electron in higher band meets a hole in lower band The two form a hydrogen-like bound state! Exciton!


Like positronium
 

Can have any orbital angular momentum Can have spin 0 or spin 1 Rate is slow Electron falls into hole Energy emitted

Annihilation
  

Energy released as electron falls into hole

 

May turn into vibrations of lattice ( phonons ) heat May turn into photons (only in some materials)
 

Infrared light (if gap ~ 1 eV) Visible light (if gap ~ 2-3 eV)

remote control LED

May excite other molecules in the material (if any; see below)

Organic Semiconductors


These are not crystals! Not periodic structures Band structure is somewhat different


Orbitals determined by shape of organic molecule



Quantum chemistry of pi bonds, not simple junior QM

Polymers are common

Conduction is different
 

Electrons or holes may wander along a polymer chain As with inorganic conductors
 

Some materials allow electrons to move Some materials allow holes to move typical for organics!!

Doping is more difficult

 

Doping typically not used Instead electrons/holes are provided by attached metals

The basic OLED

Anode

Cathode

Conductive Layer

Emissive Layer

The basic OLED

The holes move more efficiently in organics

Anode

Cathode

Conductive Layer

Emissive Layer

The basic OLED

The holes move more efficiently in organics Excitons begin to form in emissive layer

Anode

Cathode

Conductive Layer

Emissive Layer

The Exciton Exits in a Flash



As before, excitons eventually annihilate into

 

Molecular vibrations heat (typical) Photons (special materials, rare)

But with organics, can add

 

Fluorescent molecules Phosphorescent molecules

e.g. attach to end of polymer



Light can be generated indirectly:

  

Exciton can transfer its energy to this molecule Molecule is thus excited Returns to ground state via fluorescence or phosphorescence

 

Greatly increases likelihood (per exciton) of light emission Also allows for different colors


determined by the light-emitting molecule(s), not the exciton

Advantages of OLEDs
     

Faster response time than lcd Consume significantly less energy Can be transparent when off Thinner display-no back light Safer for environment Wider viewing angles up to 170 degrees

Disadvantages of OLEDs
Oled seems to be the perfect tewchnology for all types of displays,but it also has some problems *Currently manfacturing is more expensive *water can easily damage OLEDs *limited market availability *when red and green OLEDs films have longer life time(46000-230000hrs

Applications of OLEDs
   

Telivision (sony,LG) Cell phone screens Computer screens Digital camera

Some references


How Stuff Works

http://electronics.howstuffworks.com
 

Craig Freudenrich, How OLEDs work Tom Harris, How LEDs Work The P-N Junctions , by R Nave The Diode , by Don Johnson Scientific American, pp 5-9, Feb 2004

Hyperphysics Website


http ://hyperphysics.phy-astr.gsu.edu/hbase/solids/pnjun.html

Connexions Website
http://cnx.org


Webster Howard, Better Displays with Organic Films



M.A. Baldo et al, Highly efficient phosphorescent emission from organic electroluminescent devices


Nature 395, 151-154 (10 September 1998)

Various Wikipedia articles, classes, etc.