Organic light emitting diodes

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Information about Organic light emitting diodes

Published on March 15, 2014

Author: karthikkumar167


Organic Light Emitting Diodes (OLEDs) .

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

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

Electrons in a Lattice  Atom has bound states  Discrete energy levels  Partially filled by electrons  Periodic array of atoms (cf. QM textbook)  Effectively continuous bands of energy levels  Also partially filled V(r) r E V(x) r E

The Bands on Stage E E E EE Gap No Ga p Smal l Gap Insulator Conductor Semiconductor Doped Semiconductors

Doping – Add Impurities N-type P-type

The Bands on Stage E E E EE Gap No Ga p Smal l Gap Insulator Conductor Semiconductor Doped Semiconductors N-type P-type

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 E E Electric Field Excess Positive Ions Excess Negative Ions

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

Diode: p-type meets n-type Electric Field Try to make current flow to right? Current Flows! Electrons in higher band meet Holes in lower band Current

Excitons  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  Annihilation  Rate is slow  Electron falls into hole  Energy emitted  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) – remote control  Visible light (if gap ~ 2-3 eV) – LED  May excite other molecules in the material (if any; see below) E N-type

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 Anode Cathode • The holes move more efficiently in organics Conductive Layer Emissive Layer

The basic OLED Anode Cathode Conductive Layer Emissive Layer • The holes move more efficiently in organics • Excitons begin to form in 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

OLEDs  Similar physics to LEDs but  Non-crystalline  No doping; use cathode/anode to provide needed charges  Fluorescence/phosphorescence enhance excitonlight probability  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  Craig Freudenrich, “How OLEDs work”  Tom Harris, “How LEDs Work”  Hyperphysics Website http ://  “The P-N Junctions”, by R Nave  Connexions Website  “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.

A neat trick  Exciton  Spin 0 (singlet)  Spin 1 (triplet)  Can transfer its energy but not its spin to molecule  Thus spin-1 can’t excite fluorescents  Lose ¾ of excitons  But  Use phosphors  Bind to polymer so that exciton can transfer spin  Then 4 times as many excitons cause light emission P

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