Organic Light Emitting Diodes: Display and Lighting Technology of the Future

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Organic Light Emitting Diodes:

Display and Lighting Technology of the Future

Karl Leo

Institut für Angewandte Photophysik, TU Dresden, and

Fraunhofer-Institute für Photonische Mikrosysteme, Dresden

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Motivation:

What is possible with OLED:

Samsung 40” TV display

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Organic Semiconductors

Photovoltaic cells

Organic materials Integrated circuits

• Large area & flexible substrates possible

• Large variety of materials

• Low cost

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Small-Molecules

Organic semiconductors: Two variants

Polymers

Technology: Spin-On Technology: Evaporation

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p_z p_z

sp₂ sp₂

σ σ^∗

π π^∗ p lan e of th e

sp₂-orb ita ls

p_z-orb ita l π -bo n d

π -bo n d σ -bo n d

p_z-orb ita l

The basics of organic semiconductors:

Conjugated π-electron systems

Sp

₂

-hybridised Carbon:

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d e lok a lisie rte π − E lektro nen

p_z

σ^∗

sp₂ 6 x

18 x

LUMO (π*) => E

_C

„conduction band“

HOMO (π) => E

_V

„valence band“

π-electron systems delocalize!

• VdW crystals

• small π−π-overlap, narrow bands

• saturated electron system

delocalized π-electrons

Molecules with conjugated π-electron system

Technology: vapor deposition

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Device energy diagram E

x

Cathode

LUMO

HOMO

Anode

Device structure V

A

Light emission

Organic Light Emitting Diodes OLEDs: Basic Principles

Glass substrate Transparent anode

Emissive layer Cathode

+ + +

- - -

≈300nm

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Organics Collaboration Dresden

Institut für Angewandte Photophysik

IAPP, University of Dresden (www.iapp.de, approx. 40 people)

• Basic research on novel device concepts

• Low-voltage OLED

Fraunhofer-IPMS Dresden (www.ipms.fraunhofer.de, approx. 25 people)

• Process development with highly efficient OLEDs

• OLED-Inline-deposition set-up (30x40cm Substrate)

• Driving Electronic for OLED-Displays

• Small to medium volume fabrication

Novaled GmbH (www.novaled.com, approx. 45 people)

• Highly efficient OLED-stacks for RGB-Displays

• Technology licensing

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• Exponential current-voltage relation

• Flat-band under operation

• Low work-function contacts not needed !

p n

Why doped transport layers:

Inorganic vs. Organic LED

Inorganic LED (e.g., GaAs/AlGaAs) Organic LED

CB

VB E_Fe

E_Fh

Metal

• space charge limited currents

• low work function metals needed ITO-preparation necessary

HTL

ETL

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Double emission layer (D-EML) structure

D-EML structure Al

BPhen:Cs BPhen TAZ:Ir(ppy)₃ TCTA:Ir(ppy)₃

Spiro-TAD MeO-TPD:F₄-TCNQ

ITO substrate

n-doped ETL HBL PET-EML PHT-EML EBL p-doped HTL

p -H T L

EBL HBLEML2

ITO

Al

n-ETL

EML1

• Self-balancing EML system

• no charge accumulation at interfaces

X. Zhou et al., Appl. Phys. Lett. 81, 4070 (2002)

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2.6 2.8 3.0 3.2 10

100 1,000 10,000

Luminance (cd/m2 )

Voltage (V)

1,000 cd/m² @ 2.9V 61.5 cd/A; 67 lm/W

100 cd/m² @ 2.68V 64 cd/A; 75 lm/W

Highly efficient & stable PIN Triplet OLED:

World record in efficiency

G. He et al., Appl. Phys. Lett. 85, 3911 (2004)

• Highly efficient devices are also most stable devices

• Efficiency higher than for best green nitride LED

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Lifetime of PIN-OLEDs

OLEDAsia Seoul 2005

Current status:

phosphorescent red (0.68, 0.32):

100,000 h at 500 cd/m²

phosphorescent green (0.38, 0.59): 25,000

h at 500 cd/m² (Irppy₃ based)

fluorescent blue (0.15, 0.17):

6,000 h at 500 cd/m²

• Novaled pin-OLEDs have reached state of the art lifetimes for emitter systems in all three colors

• samples still contain Cs as n-dopant

100 1.000 10.000

1.000 10.000 100.000 1.000.000

Lifetime / h

Luminance / cd/m² 200,000 h @ 150 cd/m² 30,000 h @ 500 cd/m² 8,000 h @ 1000 cd/m²

n = 1.7

Red triplet pin OLED

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OLEDFAB Project

• OLEDFAB Projekt iniated by Thomson und BMWA/SMWA

• Novaled leads Saxonian consortium with IPMS und IAPP

• Goals:

- Demo of a 5 inch active-matrive display with highly efficient pin technology - Display manufactured using novel inline technology

• Other partners:

- Applied Films

- Labor für Bildschirmtechnik Stuttgart

- Deutsche Thomson Brandt

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Full color AM display based on pin technology

Performance:

operating voltage for 100cd/m

²

:

around 7V

(OLED + backplane)

power consumption:

240mW@100cd/m

²

(in forward direction only, video mode)

Æ about 60% of equivalent LCD display

• Active matrix OLED made in joint project by ITRI (Taiwan), Thomson (France) and Novaled

• Panel emitting in both directions on n-channel LTPS backplane (3.25”, QVGA, 130dpi) based on PIN structure

OLEDFAB: Display with improved backplane

in preparation

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Even higher Light Efficiencies: out-coupling enhancement

OLEDAsia Seoul 2005

Efficiency at 1000 cd/m²: 110 lm/W (with outcoupling enhancement)

Novaled focuses on further improvement of their record breaking efficiencies by out-coupling enhancements (target 2005: 140lm/W)

10 100 1000 10000

0 20 40 60 80 100 120

Power efficiency (lm/W)

Luminance (cd/m²)

Green phosphorescent PIN OLED with outcoupling enhancement

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Institut für Angewandte Photophysik Technische Universität Dresden

LED performance vs. time

1965 1970 1975 1980 1985 1990 1995 2000 2005 0

20 40 60 80 100 120

0 20 40 60 80 100 120

OLED white Red Filtered

Tungsten Bulb (unfiltered)

Fluorescent Lamp

InGaN green OLED

PLED AlInGaP Red/Yellow

Power Efficiency (Lumen/Watt)

Year

Dresden leading

in both RGB and

white

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OLED Lighting Applications

source: Philips Lighting March

2004

GE demonstrator

Signage General lighting

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Technology Roadmap (USDC)

(Diffuse lighting)

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OLLA project

• OLLA EU 6th Framework Integrated Project initiated by Philips

• OLLA: Organic LED for Lighting Applications

• Aims:

- Highly Efficient white OLED - Large Area OLED

- Low-Cost manufacturing

• Other German partners:

- Siemens-Osram, Aixtron, Covion, HCStarck, Syntec, Novaled - FhG-IPMS, TU Dresden, Uni Kassel

• about 15 further partners from other European Universities

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• Achieve Smooth Transition from R&D to production: Close Collaboration of Fraunhofer Society and Industry

• Topics: OLED for Displays and Lighting Organic Solar Cells

Memory and other Applications

• Tools:

- Gen 2 Inline Pilot Production Tool (Displays&Lighting) - Reel-to-Reel R&D tool

• Location: New IPMS cleanroom: Virtually no building investment

A new Initiative: Center for Organic Materials and

Electronic Devices Dresden (COMEDD)

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Outline of COMEDD

• Investment by FhG/state of saxony

• Operations organized in projects

• Projects are defined by advisory committee

• OLED foundry does pilot manufacturing

Projects Tools

Inline Reel-to-Reel

Manufacturing Manufacturing

Center for Organic Materials and Electronic Devices Dresden (COMEDD)

Advisory Board Funding agencies,

FhG, Industry

COMEDD Operations Funding

agencies:

Saxony BMBF, EC

OLED Foundry

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• OLED have still a long way to go, but

• Lifetime problem is close to being solved even for demanding applications

• Efficiency:

OLED displays: will beat all other FPDs

OLED lighting: should be able to surpass fluorescent tube

• COMEDD: New initiative to establish OLED manufacturing in Saxony

Conclusions

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