07-02-2013, 09:02 AM
Organic Light Emitting Diodes(OLEDs)
1Organic Light.ppt (Size: 490.5 KB / Downloads: 69)
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
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
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)
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
1Organic Light.ppt (Size: 490.5 KB / Downloads: 69)
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
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
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)
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