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…
"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
The organic “zoo”: Phylum
Small molecule
Polymer
6/19/03
Dendrimer
This lecture will mostly
focus on these
4
Class: “Small Molecule” Organics
6/19/03
5
The History of Manufacturing
1. Stone Age
2. Micro-Stone Age
Intel 4004
3. Molecular Age
Why OLEDs are not LEDs
Inorganic LEDs
(e.g. InGaN)
Crystalline, epitaxial
OLEDs
Amorphous, 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/Vs
Electrons: 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+ pairs
conduction bands meaningless,
strongly bound excitons, strong
exciton-phonon coupling
W. Helfrich & W.G. Sneider
Phys. Rev. Lett. 14(7), 229 (1965)
1000V
5 mm
Anthracene (C14H10)
electrode
+
-
electrode
J. Dresner, RCA Rev. 30, 322 (1969)
100-1000V
50 mm
Anthracene (C14H10)
8% external
quantum efficiency
Thin
gold +
electrode
-
Ag Paste
electrode
C.W. Tang, U.S. Patent # 4,356,429 (1980)
Cathode
Electron transporter
100 nm
Hole transporter
Transparent conductor
Light
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Vacuum deposition enabled thin electron transport layer
Hole transport layer was spin-coated polymer: 10 – 20 V, 15cd/m2 brightness
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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)
OLED products available:
Kodak LS633 Camera
2.2 inch, 512x218 OLED screen,
~ $500 (in partnership with Sanyo)
Not yet available in USA
Optrex Instrument Cluster
BMW 7 series
$85,000 (car included)
Not shown: Philips OLEDequipped electric shaver
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 OLED
with resolution of 1280 x 768 pixels.
Complexity of Molecular Systems
 There
has been an alarming
increase in the number of things we
don’t understand…
 Why
we need more research!
The effects of traps…
MOTIVATION: Correlate current conduction w/
molecular structure
Trap Charge Limited
Interface Limited Injection
Burrows, et al, J. Appl. Phys. (1996) 79, 7991
Baldo & Forrest Phys Rev. B. (2001), 64, 085201
LUMO
LUMO
EF
EF
Trap
distribution
Metal
Organic
Metal
Organic
Interfacial Dipole layer
Distance
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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)
Distance
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Assumes charge separation at the metalorganic interface, which creates dipole
layer
Assumes dipolar disorder in the bulk
Both models only fitted to Alq3 data
Are extracted parameters meaningful?
Alq3 – Do we know what we have?
C1
Higher symmetry
More polar (m ~ 7D vs. 5.3D)
Higher energy (4.7kcal/mol)
mer-Alq3
Trap state for electron ?
(Curioni et al. Chem. Phys.
Lett. (1998) 294, 263)
Int
erc
on
v
ers
i
on
?
fac-Alq3
C3
Several polymorphic phases, all
involve p-p interactions of mer
enantiomeric pairs
Brinkman, et al., JACS, 122, 5147 (2000)
Braun, et al, J. Chem. Phys. (2001) 114, 9625.
Amati & Lelj, Chem. Phys. Lett. (2002) 358, 144
Degrees of Freedom: Dynamical Motions for AlQ3
Single frame
Overlaid Trajectory Frames
• Dynamical trajectory shows quinolate ring motion about Al coordination
6/19/03
18
Organic Electroluminescence
1.
2.6 eV
Excitons formed
from combination
of electrons and
holes
transparent anode
electrons
2.7 eV
trap states
a-NPD
holes
low work function
cathode
exciton
Alq3
5.7eV
6.0 eV
-
+
host molecules
(charge transport
material)
dopant molecule
(luminescent dye)
2.
Excitons transfer to
luminescent dye
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

singlet
excited state
triplet
excited
state
Phosphorescence

triplet
exciton
FLUORESCENCE
ground state
(singlet)
PHOSPHORESCENCE
singlet exciton
symmetry conserved
triplet to ground state
transition is not permitted
fast process ~10-9s
slow process ~ 1s
From fluorescence towards phosphorescence
Collect all the singlets and triplets: 100% efficiency
kDD
S1
S1
ISC through spin-orbit
coupling Z5
Et
Et
N
T1
Et
T1
Et
N
Pt
N
Et
Et
N
kD
Et
S0
S0
kDD :
Et
dipole-dipole (Forster) long range 1/R 6
N
Ir
kD : Dexter transfer, short range exp(-  r)
R
Baldo et al., Nature 395, 151 (1998), Susuki et al. APL 69 224 (1996) El in
benzophenone at 100 K.
3
R = F, OMe, ...
Phosphorescent molecules enable triplet state recombination
N
C
C
Ir
O
O
O
N
S
Ir
N
O
Ir
N
S
Ir
N
Ir
N
PL eff. = 0.35
 = 4 msec (77K)
max = 525 nm
Heavy metal ion causes spinorbit coupling with organic
ligand
Symmetry broken  allowed
phosphorescent recombination
Color tuning by ligand choice
M.E. Thompson
University of Southern California
PL eff. = 0.4
 = 2 msec
max = 555 nm
PL eff. = 0.05
 = 2 msec
max = 590 nm
PL eff. = 0.2
 = 2 msec
max = 605 nm
Phosphorescent OLED Status*
1931 CIE chart
0.57, 0.43
0.61, 0.38
0.30, 0.63
0.65, 0.35
0.16, 0.37
++
0.70, 0.30
0.15, 0.22
0.14, 0.23
*Subset of PHOLEDs
Courtesy Universal Display Corporation
PhOLED Technology (Phosphorescent OLED)
Courtesy Universal Display Corporation
PHOLED
Xxxxxx
Color
CIE (x, y)
Luminous Efficiency
(cd/A) at 1 mA/cm 2
Luminance (cd/m 2)
at 1 mA/cm 2
Lifetime (hours)
0.65, 0.35
0.61, 0.38
0.30, 0.65
0.14, 0.37
0.14, 0.23
12
22
24
16
10
6 lm/W
14 lm/W
no data
120
220
240
160
100
15,000 @
300 cd/m 2
> 10,000 @
300 cd/m 2
13,000 @
600 cd/m 2
800 @
600 cd/m2*
*
* Under development
White PHOLEDs
• CIE = (0.37, 0.40), CRI = 83
• 31,000 cd/m2 at 14V
US patents: 6,303,238
• 6.4 lm/W
6,097,147
Breaking news: lower voltage structures further
improve power efficiencies by 20 – 50%
What is the limit of the possible?
20% of the light from a simple OLED escapes a planar device
Existing:
Outcoupling  x5:
Voltage decrease,
 ÷ 2 possible
14 lm/W green at 250 cd/m2
70 lm/W
140 lm/W
This assumes no further
increase in quantum efficiency!
Manufacture and Scale-Up
Assembling OLEDs at PNNL
System by Angstrom Engineering Inc.
Andrew Bass et al.
4” substrate, organic deposition (thermal), oxides (sputtering), metal (thermal)
People are serious about OLED!
Large area? Kodak thermal deposition
Society for Information Display
Annual Meeting 2002
Alternative: OVPD, The R&D Concept
Multiple Zone Heater
Source 1
(Host)
Cooled
Substrate
Carrier
Source 2
(Dopant)
Sublimation
Transport
Condensation
Gas Phase Transport by Inert Carrier Gas, ~ 1 Torr
"Low Pressure Organic Vapor Phase Deposition of Small Molecular Weight
Organic Light Emitting Device Structures.“
Appl. Phys Lett. 71, 3033 (1997)
Courtesy Universal Display Corporation
OVPD scaleup vs thermal evaporation
Substrate
Close Coupled
Showerhead
Shadow
Mask
Substrate
• Highly efficient deposition
• Gas phase controlled
• No bowing of shadow mask
• Inefficient deposition (wall coating)
• Temperature controlled
• Bowing of shadow mask
Courtesy Universal Display Corporation
What about plastic?
Tensioner
OLED
Deposition
Supply
Roll
Encapsulation
Patterning
- Web-based processing
- Cost-effectiveness
Product
Roll
So…
What’s the problem?
Photos: Courtesy of Dupont Displays
U.S. Patent
No. 5,844,363
Photo: Courtesy of Universal
Display Corporation
Degradation of Organic Devices
Oxide
H2O, O2
Light
Rigid OLED Architecture:
Glass
Pioneer Patent EP 0 776 147 A1
OLED layers
ITO
Stainless steel can
Epoxy adhesive
membrane
desiccant
Typical lifetimes 5k – 100k hours
Blue is generally the least stable
Flexible (FOLED) Architecture:
Flexible moisture barrier substrate
Flexible thin film encapsulation
-6
10
-4
-2
0
PNB, Arton
PET (hardcoat)
Barix™
Organic Coatings
OLED
Requirement
PECVD
Inorganic Coatings
Limit of
MOCON
measurement
2
10
10
10
10
H2O Permeation Rate (g/m2/day at 25ºC)
4
10
Multilayer Barrier Deposition:
Monomer
Liquid
Cure
Ceramic
Deposition
PET
High Speed, Large Area…
Normalized luminance [arb. units]
Irppy-based OLED: PET substrate, glass lid
Constant current, DC drive
1
L0 = 400 cd/m2
0.8
0.6
(i)
(ii)
0.4
1200 hr
0.2
3000 hr
0
0
500
Appl. Phys. Lett. 81, 2929 (2002)
2 mm pixel
1000 1500 2000 2500 3000 3500 4000
Time [hours]
ITO/CuPc(10nm)/NPD(30nm)/CBP:Irppy[6%](30nm)/BAlq(10nm)/Alq 3(40nm)/LiF(1nm)/Al(100nm)
PNNL Rollcoating
• 7” web
• 2 monomer sources
• 3 inorganic sources
• UV, ebeam or plasma cure
• Polymer evaporation
• Composite extrusion
• Oxide deposition
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 2003
Collaboration between Universal Display Corporation, Pacific Northwest
National Laboratories and Vitex Systems Inc.)
Opportunities and Challenges
(by way of conclusion)
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Flat Panel Displays: $70B worldwide market
OLEDS: $2B by 2006 (by some estimates)
Next Generation Lighting
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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