Light Emitting Diodes(LED) and Organic Light Emitting Diodes(OLED)

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Light Emitting Diodes(LED) and Organic Light Emitting Diodes(OLED)

Plan of talk Light-Emitting Diode Organic Light-Emitting Diode Bands and Conduction Semiconductor Standard Diode Light Emission Organic Light-Emitting Diode Organic Semiconductors Organic Diode

The Bands on Stage E E E E E No Gap Small Gap Gap Insulator Conductor Semiconductor Doped Semiconductors

Doping – Add Impurities N-type P-type

The Bands on Stage E E E E E N-type P-type No Gap Small Gap Gap Insulator Conductor Semiconductor Doped Semiconductors

Diode: p-type meets n-type

Diode: p-type meets n-type

Diode: p-type meets n-type

Diode: p-type meets n-type Electric Field Excess Negative Ions Excess Positive Ions

Diode: p-type meets n-type Try to make current flow to left? Depletion Zone Grows Electric Field

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

A light-emitting diode (LED) is a two-lead semiconductor light source A light-emitting diode (LED) is a two-lead semiconductor light source. It is a p–n junction diode, which emits light when activated. When a suitable voltage is applied to the leads, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons.

Organic Semiconductors These are not crystals! Not periodic structures Band structure is somewhat different “Orbitals” determined by shape of organic molecule Polymers are common Conduction is different Electrons or holes may wander along a polymer chain 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

OLEDs Similar physics to LEDs but Manufacturing advantages Non-crystalline No doping; use cathode/anode to provide needed charges Manufacturing advantages Soft materials – very malleable Easily grown Very thin layers sufficient Many materials to choose from Relatively easy to play tricks To increase efficiency To generate desired colors To lower cost Versatile materials for future technology

Some references How Stuff Works Hyperphysics Website http://electronics.howstuffworks.com Craig Freudenrich, “How OLEDs work” Tom Harris, “How LEDs Work” Hyperphysics Website http ://hyperphysics.phy-astr.gsu.edu/hbase/solids/pnjun.html “The P-N Junctions”, by R Nave Connexions Website http://cnx.org “The Diode”, by Don Johnson Webster Howard, “Better Displays with Organic Films” Scientific American, pp 5-9, Feb 2004 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.