OLEDs: A bright opportunity for vacuum technology
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Transcript of OLEDs: A bright opportunity for vacuum technology
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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
OLEDs:OLEDs:A bright opportunityA bright opportunity
for vacuum technologyfor vacuum technology
OLEDs:OLEDs:A bright opportunityA bright opportunity
for vacuum technologyfor vacuum technology
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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
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• 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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6/19/03 4
The organic “zoo”: Phylum
Small molecule
Polymer
Dendrimer
This lecture will mostly focus on these
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6/19/03 5
Class: “Small Molecule” Organics
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The History of Manufacturing
1. Stone Age
3. Molecular Age
2. Micro-Stone AgeIntel 4004
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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
exciton-phonon coupling
Excitonic: correlated e--h+ pairsconduction bands meaningless, strongly bound excitons, strong
exciton-phonon coupling
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W. Helfrich & W.G. SneiderPhys. Rev. Lett. 14(7), 229 (1965)
Anthracene (C14H10)
electrode electrode+ -
5 mm
1000V
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J. Dresner, RCA Rev. 30, 322 (1969)
Anthracene (C14H10)
Thingold
electrodeAg Pasteelectrode
+ -
50 m
100-1000V
8% external quantum efficiency
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Hole transporter
Light
Transparent conductor
Electron transporter
Cathode
C.W. Tang, U.S. Patent # 4,356,429 (1980)
• Vacuum deposition enabled thin electron transport layer• Hole transport layer was spin-coated polymer: 10 – 20 V, 15cd/m2 brightness
• All vacuum device: 10 – 20 V, 100 cd/m2 using Alq3 emission layer
• C.W. Tang and S.A. VanSlyke Appl. Phys. Lett. 51, 913(1987)
100 nm
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OLED products available:
Kodak LS633 Camera2.2 inch, 512x218 OLED screen,~ $500 (in partnership with Sanyo)
Not yet available in USA
Optrex Instrument ClusterBMW 7 series
$85,000 (car included)
Not shown: Philips OLED-equipped electric shaver
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OLEDs: The Future…
Kodak/Sanyo active-matrix display features full-color, 1280 x 720
(HDTV) resolution
Sony: 13 inches,800 x 600, low temperature poly-silicon TFT active matrix using organic
phosphorescence
Not shown: Toshiba 17inch AM OLEDwith resolution of 1280 x 768 pixels.
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Complexity of Molecular Systems
There has been an alarming increase in the number of things we don’t understand…
Why we need more research!
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The effects of traps…
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• Assumes bulk effects limit current conduction
• Assumes trap energies are exponentially distributed below LUMO
• Neglects voltage and temperature dependence of mobility (secondary to trap effects)
• Assumes charge separation at the metal-organic interface, which creates dipole layer
• Assumes dipolar disorder in the bulk
Both models only fitted to Alq3 dataAre extracted parameters meaningful?
Metal Organic
Trap Charge LimitedBurrows, et al, J. Appl. Phys. (1996) 79, 7991
En
ergy
Distance
LUMO
Trapdistribution
EF
Interface Limited InjectionBaldo & Forrest Phys Rev. B. (2001), 64, 085201
Metal Organic
Interfacial Dipole layer
En
ergy
Distance
LUMO
EF
Metal Organic
Trap Charge LimitedBurrows, et al, J. Appl. Phys. (1996) 79, 7991
En
ergy
Distance
LUMO
Trapdistribution
EF
MOTIVATION: Correlate current conduction w/ molecular structure
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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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6/19/03 18
Degrees of Freedom: Dynamical Motions for AlQ3
Single frame Overlaid Trajectory Frames
• Dynamical trajectory shows quinolate ring motion about Al coordination
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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)
+
-
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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
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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
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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, ...
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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
Ir
N
N
C OOC
0
0.5
1
450 500 550 600 650 700
Wavelength (nm)
O NIr
S NIr
S NIr
O NIr
PL eff. = 0.35 = 4 sec (77K)max = 525 nm
PL eff. = 0.4 = 2 secmax = 555 nm
PL eff. = 0.05 = 2 secmax = 590 nm
PL eff. = 0.2 = 2 secmax = 605 nm
M.E. ThompsonUniversity of Southern California
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0.16, 0.37
0.30, 0.63
0.65, 0.35
0.57, 0.430.61, 0.38
Phosphorescent OLED Status*
0.70, 0.30+
0.15, 0.22
+
0.14, 0.23
1931 CIE chart
*Subset of PHOLEDs
Courtesy Universal Display Corporation
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Xxxxxx
PhOLED Technology (Phosphorescent OLED)Courtesy Universal Display Corporation
PHOLED
Color
CIE (x, y) 0.65, 0.35 0.61, 0.38 0.30, 0.65 0.14, 0.37 0.14, 0.23Luminous Efficiency
(cd/A) at 1 mA/cm2 12 22 24 16 10
Luminance (cd/m2)
at 1 mA/cm2 120 220 240 160 100
Lifetime (hours)
15,000 @
300 cd/m 2
> 10,000 @
300 cd/m 2
13,000 @
600 cd/m 2
800 @
600 cd/m2* *
White PHOLEDs • CIE = (0.37, 0.40), CRI = 83• 31,000 cd/m2 at 14V• 6.4 lm/W US patents: 6,303,238
6,097,147
* Under development
14 lm/W6 lm/W
Breaking news: lower voltage structures further improve power efficiencies by 20 – 50%
no data
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What is the limit of the possible?20% of the light from a simple OLED escapes a planar device
Existing: 14 lm/W green at 250 cd/m2
Outcoupling x5: 70 lm/WVoltage decrease, 140 lm/W ÷ 2 possible This assumes no further
increase in quantum efficiency!
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Manufacture and Scale-Up
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Assembling OLEDs at PNNLAssembling OLEDs at PNNL
System by Angstrom Engineering Inc. Andrew Bass et al.
4” substrate, organic deposition (thermal), oxides (sputtering), metal (thermal)
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People are serious about OLED!
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Large area? Kodak thermal deposition
Society for Information DisplayAnnual Meeting 2002
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Alternative: OVPD, The R&D Concept
CooledSubstrateCarrier
Source 1(Host)
Source 2(Dopant)
Gas Phase Transport by Inert Carrier Gas, ~ 1 Torr
Multiple Zone Heater
Sublimation Transport Condensation
"Low Pressure Organic Vapor Phase Deposition of Small Molecular WeightOrganic Light Emitting Device Structures.“
Appl. Phys Lett. 71, 3033 (1997) Courtesy Universal Display Corporation
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OVPD scaleup vs thermal evaporation
Substrate
Substrate
Showerhead ShadowMask
•Highly efficient deposition•Gas phase controlled•No bowing of shadow mask
•Inefficient deposition (wall coating)•Temperature controlled•Bowing of shadow mask
Close Coupled
Courtesy Universal Display Corporation
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- Web-based processing- Cost-effectiveness
What about plastic?What about plastic?What about plastic?What about plastic?
SupplyRoll
ProductRoll
OLEDDeposition
Patterning
EncapsulationTensioner
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So…What’s the problem?
Photos: Courtesy of Dupont Displays
Photo: Courtesy of Universal Display CorporationU.S. Patent
No. 5,844,363
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LightLight
Oxide
H2O, O2
Degradation of Organic Devices
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Rigid OLED Architecture:
Stainless steel can
Glass
ITO
OLED layers
desiccant
Epoxy adhesivemembrane
Pioneer Patent EP 0 776 147 A1
Flexible (FOLED) Architecture:
Flexible moisture barrier substrateFlexible thin film encapsulation
Typical lifetimes 5k – 100k hoursBlue is generally the least stable
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10-6
10-4
10-2
100
102
104
PN
B, A
rton
PE
T (
hard
coat
)
Org
anic
Coa
tings
Inor
gani
c C
oatin
gsP
EC
VD
Barix™
Limit ofMOCON
measurement
OLEDRequirement
H2O Permeation Rate (g/m2/day at 25ºC)
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PET
High Speed, Large Area…
MonomerLiquid Cure
CeramicDeposition
Multilayer Barrier Deposition:
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0
0.2
0.4
0.6
0.8
1
0 500 1000 1500 2000 2500 3000 3500 4000
Nor
mal
ized
lum
inan
ce [
arb.
uni
ts]
Time [hours]
(i) (ii)
L0 = 400 cd/m2
ITO/CuPc(10nm)/NPD(30nm)/CBP:Irppy[6%](30nm)/BAlq(10nm)/Alq3(40nm)/LiF(1nm)/Al(100nm)
1200 hr
3000 hr
2 mm pixel
Irppy-based OLED: PET substrate, glass lidConstant current, DC drive
Appl. Phys. Lett. 81, 2929 (2002)
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PNNL Rollcoating
• 7” web• 2 monomer sources• 3 inorganic sources• UV, ebeam or plasma cure
• Polymer evaporation• Composite extrusion• Oxide deposition
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Latest Flexible Display Results:
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.)
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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
structure-property relationships