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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….

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oleds a bright opportunity for vacuum technology

OLEDs:A bright opportunityfor 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
Disclaimer:this is not the whole story…

"Never try to tell everything you know. It may take

too short a time." - Norman Ford

slide3

OLED: Organic Light Emitting Device

  • 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
slide4

Small molecule

This lecture will mostly focus on these

Polymer

The organic “zoo”: Phylum

Dendrimer

slide6

1. Stone Age

2. Micro-Stone Age

Intel 4004

3. Molecular Age

The History of Manufacturing

j dresner rca rev 30 322 1969

100-1000V

Thin

gold

electrode

-

Ag Paste

electrode

+

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

50 mm

Anthracene (C14H10)

8% external

quantum efficiency

slide10

100 nm

Cathode

Light

Electron transporter

Hole transporter

C.W. Tang, U.S. Patent # 4,356,429 (1980)

Transparent conductor

  • 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)
slide11

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 OLED-equipped electric shaver

oleds the future
OLEDs: The Future…

Sony: 13 inches,800 x 600, low temperature poly-silicon TFT active matrix using organic phosphorescence

Kodak/Sanyo active-matrix display features full-color, 1280 x 720 (HDTV) resolution

Not shown: Toshiba 17inch AM OLED

with resolution of 1280 x 768 pixels.

complexity of molecular systems
Complexity of Molecular Systems
  • There has been an alarming increase in the number of things we don’t understand…
  • Why we need more research!
slide16

Trap Charge Limited

Burrows, et al, J. Appl. Phys. (1996) 79, 7991

Trap Charge Limited

Burrows, et al, J. Appl. Phys. (1996) 79, 7991

Interface Limited Injection

Baldo & Forrest Phys Rev. B. (2001), 64, 085201

LUMO

LUMO

LUMO

EF

EF

EF

Trap

distribution

Trap

distribution

Energy

Energy

Energy

Metal

Metal

Organic

Organic

Metal

Organic

Interfacial Dipole layer

Distance

Distance

Distance

  • 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 data

Are extracted parameters meaningful?

MOTIVATION: Correlate current conduction w/ molecular structure

slide17

mer-Alq3

Higher symmetry

More polar (m ~ 7D vs. 5.3D)

Higher energy (4.7kcal/mol)

Trap state for electron ? (Curioni et al. Chem. Phys. Lett. (1998) 294, 263)

C1

Several polymorphic phases, all involve p-p interactions of mer enantiomeric pairs

Brinkman, et al., JACS, 122, 5147 (2000)

fac-Alq3

C3

Alq3 – Do we know what we have?

Braun, et al, J. Chem. Phys. (2001) 114, 9625.

Amati & Lelj, Chem. Phys. Lett. (2002) 358, 144

slide18

Degrees of Freedom: Dynamical Motions for AlQ3

Single frame Overlaid Trajectory Frames

  • Dynamical trajectory shows quinolate ring motion about Al coordination
slide19

-

+

host molecules

(charge transport

material)

dopant molecule

(luminescent dye)

Organic Electroluminescence

1.

Excitons formed

from combination

of electrons and

holes

2.6 eV

electrons

2.7 eV

trap states

low work function

cathode

exciton

a-NPD

Alq3

holes

transparent anode

5.7eV

6.0 eV

2.

Excitons transfer to

luminescent dye

why it s important to put the right spin on your excitons

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
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
slide21

Fluorescence

Phosphorescence

singlet

excited state

triplet

exciton

triplet

excited

state

FLUORESCENCE

PHOSPHORESCENCE

ground state

(singlet)

singlet exciton

symmetry conserved

triplet to ground state

transition is not permitted

fast process ~10-9s

slow process ~ 1s

slide22

From fluorescence towards phosphorescence

Collect all the singlets and triplets: 100% efficiency

N

Ir

3

R

Baldo et al., Nature 395, 151 (1998), Susuki et al. APL 69 224 (1996) El in benzophenone at 100 K.

R = F, OMe, ...

phosphorescent molecules enable triplet state recombination

PL eff. = 0.35

  • = 4 sec (77K)
  • max = 525 nm
  • PL eff. = 0.4
  • = 2 sec

max = 555 nm

  • PL eff. = 0.05
  • = 2 sec

max = 590 nm

  • PL eff. = 0.2
  • = 2 sec

max = 605 nm

Phosphorescent molecules enable triplet state recombination
  • Heavy metal ion causes spin-orbit coupling with organic ligand
  • Symmetry broken  allowed phosphorescent recombination
  • Color tuning by ligand choice

M.E. Thompson

University of Southern California

slide24

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

slide25

PhOLED Technology (Phosphorescent OLED)

Courtesy Universal Display Corporation

Xxxxxx

no data

6 lm/W

14 lm/W

* Under development

  • 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

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
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/W

Voltage decrease, 140 lm/W

÷ 2 possible

This assumes no further

increase in quantum efficiency!

slide28

Assembling OLEDs at PNNL

System by Angstrom Engineering Inc.

Andrew Bass et al.

4” substrate, organic deposition (thermal), oxides (sputtering), metal (thermal)

large area kodak thermal deposition
Large area? Kodak thermal deposition

Society for Information Display

Annual Meeting 2002

alternative ovpd the r d concept

Multiple Zone Heater

Sublimation

Transport

Condensation

Alternative: OVPD, The R&D Concept

Source 1

(Host)

Cooled

Substrate

Carrier

Source 2

(Dopant)

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
OVPD scaleup vs thermal evaporation

Substrate

Close Coupled

Shadow

Mask

Showerhead

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

OLED

Deposition

Encapsulation

Tensioner

Patterning

Supply

Roll

Product

Roll

What about plastic?

- Web-based processing

- Cost-effectiveness

slide34

U.S. Patent

No. 5,844,363

So…

What’s the problem?

Photos: Courtesy of Dupont Displays

Photo: Courtesy of Universal Display Corporation

slide35

Light

Light

Degradation of Organic Devices

Oxide

H2O, O2

slide36

Glass

OLED layers

ITO

Stainless steel can

Epoxy adhesive

membrane

desiccant

Rigid OLED Architecture:

Pioneer Patent EP 0 776 147 A1

Typical lifetimes 5k – 100k hours

Blue is generally the least stable

Flexible (FOLED) Architecture:

Flexible moisture barrier substrate

Flexible thin film encapsulation

slide37

Limit of

MOCON

measurement

OLED

Requirement

Inorganic Coatings

Organic Coatings

PET (hardcoat)

PNB, Arton

Barix™

PECVD

-6

-4

-2

0

2

4

10

10

10

10

10

10

H2O Permeation Rate (g/m2/day at 25ºC)

slide38

Multilayer Barrier Deposition:

Monomer

Liquid

Ceramic

Deposition

Cure

PET

High Speed, Large Area…

slide39

Irppy-based OLED: PET substrate, glass lid

Constant current, DC drive

L0 = 400 cd/m2

(i)

(ii)

1200 hr

3000 hr

2 mm pixel

Appl. Phys. Lett. 81, 2929 (2002)

ITO/CuPc(10nm)/NPD(30nm)/CBP:Irppy[6%](30nm)/BAlq(10nm)/Alq3(40nm)/LiF(1nm)/Al(100nm)

slide40

PNNL Rollcoating

  • 7” web
  • 2 monomer sources
  • 3 inorganic sources
  • UV, ebeam or plasma cure
  • Polymer evaporation
  • Composite extrusion
  • Oxide deposition
slide41

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
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