Paul E. Burrows PhD Paul E. Burrows PhD Energy Sciences and Technology Directorate Energy Sciences and Technology Directorate Manager, Nanoscience and Technology Initiative Manager, Nanoscience and Technology Initiative Pacific Northwest National Laboratory Pacific Northwest National Laboratory OLEDs: OLEDs: A bright opportunity A bright opportunity for vacuum technology for vacuum technology
OLEDs: A bright opportunity for vacuum technology. Paul E. Burrows PhD Energy Sciences and Technology Directorate Manager, Nanoscience and Technology Initiative Pacific Northwest National Laboratory. Disclaimer: this is not the whole story…. - PowerPoint PPT Presentation
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Paul E. Burrows PhDPaul E. Burrows PhD
Energy Sciences and Technology DirectorateEnergy Sciences and Technology Directorate
Manager, Nanoscience and Technology InitiativeManager, Nanoscience and Technology Initiative
Pacific Northwest National LaboratoryPacific Northwest National Laboratory
Disclaimer:Disclaimer:this is not the whole story…this is not the whole story…
Disclaimer:Disclaimer:this is not the whole story…this is not the whole story…
"Never try to tell everything you know. It may take
too short a time." - Norman Ford
• What are they?• A sense of history• LED : OLED… key differences• What we don’t understand, why it’s interesting• Making OLEDs: Large area and manufacturing• The lure of plastic
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The organic “zoo”: Phylum
Small molecule
Polymer
Dendrimer
This lecture will mostly focus on these
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Class: “Small Molecule” Organics
The History of Manufacturing
1. Stone Age
3. Molecular Age
2. Micro-Stone AgeIntel 4004
Why OLEDs are not LEDs
Inorganic LEDs(e.g. InGaN)
Crystalline, epitaxial
OLEDsAmorphous, flexible, weak
adhesion, structural complexity
p,n-doping Generally can be either p- or n-doped with substitutional
dopant atoms at 1015 – 1020/cm3
Materials are either electron or hole conducting. Negligible background charge carrier density. Electronic
doping requires 1 – 5% loading and chemically changes the host
molecules
mobility up to ~ 1000 cm2/Vs Holes: 10-3 cm2/VsElectrons: 0 – 10-4 cm2/Vs
voltage and field dependent
excited states Electronic: light generated by band-to-band recombination, weakly bound excitons, weak
Trap Charge LimitedBurrows, et al, J. Appl. Phys. (1996) 79, 7991
En
ergy
Distance
LUMO
Trapdistribution
EF
MOTIVATION: Correlate current conduction w/ molecular structure
Alq3 – Do we know what we have?
mer-Alq3
Higher symmetry
More polar ( ~ 7D vs. 5.3D)
Higher energy (4.7kcal/mol)
Trap state for electron ? (Curioni et al. Chem. Phys. Lett. (1998) 294, 263)
Several polymorphic phases, all involve interactions of mer enantiomeric pairs
Brinkman, et al., JACS, 122, 5147 (2000)
C1
fac-Alq3
Interconversion?
C3
Braun, et al, J. Chem. Phys. (2001) 114, 9625.
Amati & Lelj, Chem. Phys. Lett. (2002) 358, 144
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Degrees of Freedom: Dynamical Motions for AlQ3
Single frame Overlaid Trajectory Frames
• Dynamical trajectory shows quinolate ring motion about Al coordination
Organic Electroluminescence
2.Excitons transfer to
luminescent dye
1.Excitons formed
from combinationof electrons and
holes
6.0 eV
a-NPD
2.6 eV
5.7eV
Alq3
2.7 eV electrons
exciton
trap states
low work functioncathode
transparent anode holes
dopant molecule(luminescent dye)
host molecules(charge transport
material)
+
-
Why it’s important to put the right spin on your excitons:
Optical excitation is spin-conserved– a spin zero ground state produces a spin zero
excited state which can vertically relax back to the ground state with unit quantum efficiency
Electrical excitation is spin-random–Simple statistics 25% singlets, 75% high spin triplet state (vertical recombination to ground state “forbidden”)–e-h correlation may change this ratio–some evidence of > 25% singlets in polymers–remains a controversial area
Fluorescence
ground state(singlet)
singletexcited state triplet
excitedstate
FLUORESCENCE
singlet exciton
symmetry conserved
tripletexciton
PHOSPHORESCENCE
Phosphorescence
triplet to ground state transition is not permitted
fast process ~10-9s slow process ~ 1s
From fluorescence towards phosphorescenceFrom fluorescence towards phosphorescenceCollect all the singlets and triplets: 100% efficiency
S0
S1
T1
S0
S1
T1
kDD
kDD : dipole-dipole (Forster) long range 1/R6
kD
kD : Dexter transfer, short range exp(- r)
ISC through spin-orbit coupling Z5
Baldo et al., Nature 395, 151 (1998), Susuki et al. APL 69 224 (1996) El in
benzophenone at 100 K.
N
N
N
N
Et Et
Et
Et
Et Et
Et
Et
Pt
N
Ir
R 3
R = F, OMe, ...
Phosphorescent molecules enable triplet state recombinationPhosphorescent molecules enable triplet state recombinationPhosphorescent molecules enable triplet state recombinationPhosphorescent molecules enable triplet state recombination
Heavy metal ion causes spin-orbit coupling with organic ligandSymmetry broken allowed phosphorescent recombinationColor tuning by ligand choice
2000 hours at L0 = 600 cd/m2 for green phosphorescent OLED display on plastic (passive matrix 128 x 64)
(A. Chwang et al. Materials Research Society Conference, April 2003Collaboration between Universal Display Corporation, Pacific Northwest
National Laboratories and Vitex Systems Inc.)
Opportunities and Challenges(by way of conclusion)
Flat Panel Displays: $70B worldwide market OLEDS: $2B by 2006 (by some estimates) Next Generation Lighting
Practical if we can reach 50 lm/W 22% of US electricity generation goes for lighting Luminescent wallpaper? Dual or multi use windows using transparent OLEDs?
Lifetime, particularly in blue Large area scale-up at very high yield and low cost Commercial scale-up… production lines with minimal
downtime Supply infrastructure?? Materials purity assay etc. Still insufficient understanding of basic material