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Organic Light Emitting Diodes(OLEDs)
Physics 496/487Matt Strassler
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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
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Plan of talk Light-Emitting Diode
Bands and Conduction Semiconductor Standard Diode Light Emission
Organic Light-Emitting Diode Organic Semiconductors Organic Diode Light Emission
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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
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The Bands on Stage
E E E EE
Gap
No Gap
Small Gap
Insulator Conductor Semiconductor Doped Semiconductors
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Doping – Add Impurities
N-type
P-type
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The Bands on Stage
E E E EE
Gap
No Gap
Small Gap
Insulator Conductor Semiconductor Doped Semiconductors
N-type
P-type
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Diode: p-type meets n-type
EE
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Diode: p-type meets n-type
EE
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Diode: p-type meets n-type
EE
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Diode: p-type meets n-type
EE
Electric Field
Excess Positive
Ions
Excess Negative Ions
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Diode: p-type meets n-type
Electric Field
Try to make current flow to left?
Depletion Zone Grows
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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
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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
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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
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The basic OLED
Anode Cathode
Conductive Layer Emissive Layer
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The basic OLED
Anode Cathode
• The holes move more efficiently in organics
Conductive Layer Emissive Layer
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The basic OLED
Anode Cathode
Conductive Layer Emissive Layer
• The holes move more efficiently in organics
• Excitons begin to form in emissive layer
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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
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OLEDs Similar physics to LEDs but
Non-crystalline No doping; use cathode/anode to provide needed charges Fluorescence/phosphorescence enhance excitonlight
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
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Some references How Stuff Works
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 Websitehttp://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.
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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