oleds: a bright opportunity for vacuum technology

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

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

6/19/03 4

The organic “zoo”: Phylum

Small molecule

Polymer

Dendrimer

This lecture will mostly focus on these

6/19/03 5

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

exciton-phonon coupling

Excitonic: correlated e--h+ pairsconduction bands meaningless, strongly bound excitons, strong

exciton-phonon coupling

W. Helfrich & W.G. SneiderPhys. Rev. Lett. 14(7), 229 (1965)

Anthracene (C14H10)

electrode electrode+ -

5 mm

1000V

J. Dresner, RCA Rev. 30, 322 (1969)

Anthracene (C14H10)

Thingold

electrodeAg Pasteelectrode

+ -

50 m

100-1000V

8% external quantum efficiency

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

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

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.

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…

• 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

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

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

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

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

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

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

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!

Manufacture and Scale-Up

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)

People are serious about OLED!

Large area? Kodak thermal deposition

Society for Information DisplayAnnual Meeting 2002

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

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

- Web-based processing- Cost-effectiveness

What about plastic?What about plastic?What about plastic?What about plastic?

SupplyRoll

ProductRoll

OLEDDeposition

Patterning

EncapsulationTensioner

So…What’s the problem?

Photos: Courtesy of Dupont Displays

Photo: Courtesy of Universal Display CorporationU.S. Patent

No. 5,844,363

LightLight

Oxide

H2O, O2

Degradation of Organic Devices

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

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)

PET

High Speed, Large Area…

MonomerLiquid Cure

CeramicDeposition

Multilayer Barrier Deposition:

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)

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 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

structure-property relationships