Polymeric Electroluminescent Devices K W Wong Department of Physics The Chinese University of Hong Kong

PolymericPolymericElectroluminescent DevicesElectroluminescent Devices

K W Wong

Department of Physics

The Chinese University of Hong Kong

ContentsContents

Introduction– Why PLED/OLED?– Business & Academic

Historical background Present status in research– Underlying physics– Materials research– Device technology– Device failure

Our current status

IntroductionIntroduction…why PLED/OLED

Simple & robust device structure Good processability Low production cost Theoretically full colour range Workable at low temperature Low electrical requirement Applicable on flexible substrate

(e.g. plastics)

Metallic cathodePolymeric / Organic layerTransparent ITO anode

Glass

…typical structure

IntroductionIntroduction

ITOHole

transportlayer

CathodeElectrontransport

layer

Holeinjection

Electroninjection

…EL mechanism


…applications


IntroductionIntroduction…business

IntroductionIntroduction…business

IntroductionIntroduction…academic


During 1990s, there were over 5000 titles registered in Chemical Abstracts and over 500 related US Patents.

The publications and patents ranged from the areas of physics, chemistry, materials science, electronic engineering, and etc.

…academic

History – “High driven voltage”History – “High driven voltage”

1950s Bernanose - High-voltage AC field to crystalline thin film of acridine orange & quinacrine.

EL in organic materials 1960 Gurnee & Fernandez - AC-driven EL cells using anthracene.

(DOW) emit blue light 1963 Pope - DC-driven EL cells using single crystals

of anthracene.

REMARKS...

During this period, EL with organic materials were demonstrated. The EL cells were firstly driven by AC-field, while DC-field was employed in later stages. However, a HIGH driven voltage (~102 V) were required.

History – “Use of electrodes”History – “Use of electrodes”

During late 60s and early 70s, the idea of using of metal electrodes to enhance carrier injection. This significantly reduces the driven voltage to the order to ~10 V.

REMARKS...

1969 Digby & Schadt - First use of a reactive cathode in organic EL cell to facilitate e- injection. US Patent 3,621,321

1975 Partridge - First EL cell with polymer: polyvinyl carbozole (PVK) doped with perylene.- First use of alkali metal cathode. US Patent 3,995,299

History – “Turning point”History – “Turning point”

1987 C. W. Tang - A two-layer structure of vacuum-deposited (Eastman Kodak Co.) small-molecules film;

- Use of indium-tin-oxide as hole-injection anode;- Driven voltage < 10 V. US Patent 4,164,431, 4,356,429

1990 R. H. Friend et. al. - fabricated green-yellow EL cell using poly (Cambridge University) (p-phenylene vinylene), PPV in a single

layer structure. Nature 347, 539 (1990)

REMARKS...

The pioneering works from Tang and Friend have aroused worldwide interests from physicists, materials scientists and engineers.

History – “Commericalization”History – “Commericalization”

1980s Eastman Kodak Co. - actively initiated related intensive research programmes.

1991 UNIAX - a new company spun out of UC Santa Barbara, the company was later acquired by DuPoint.

1992 Cambridge Display Technology - a company established by Friend et. al. based on their patents on material synthesis and technologies.

1997 Pioneer Electronics Co. - manufactured car radio consoles based on organic EL materials.

1999 Covion Organic Semiconductors - a spin-off company of Hoechst.

Others: Dow Chemicals, DuPont Chemicals, Philips Electronics, Epson Groups, IBM, and etc.

Present status in research – Present status in research –

Efficiency > 20 Im/W, Lifetime ~ 10,000 hours at 200 cd/m2,

(target: 100,000 hours) Achieved emission of green, blue, red, yellow

and white light, Already have simple commercialized

products.

…achievement

Present status in research – Present status in research – Area of interests:

– Insertion of buffer layers• C. W. Tang et. al. from Eastman Kodak Co.

Most buffer layers used are inorganic materials, e.g. LiF, SiO2, and etc.– Surface and interface modification

• R. H. Friend et. al. from Cambridge University Most treatments are restricted to plasma and chemical treatment on ITO.

– Energy level alignment at the interfaces• A. Kahn et. al. from Princeton University• Y. Gao et. al. from U. of Rochester, C. W. Tang et. al. from Eastman Kodak Co.• S. Forrest et. al. from Princeton University

Experimental findings based on different system leads to very different proposed mechanism.

Although lots of studies have been performed, there is still no unified picture on the energy level alignment between the electrode/polymer and polymer/polymer interfaces. Also, the use of buffer layers provides a large platform for research.

PENDING ISSUES...

…underlying physics

Present status in research –Present status in research –…materials research

Leading polymeric emissive materials:

– Poly (p-phenylene vinylene), PPV – Poly (phenylene), PPP– Polyfluorenes, PFO

Y

Y

R

RR R R R

R RR

PPV PFO PPP


Carrier transport materials:– Poly (3,4-ethylene dioxythiophene), PEDOT (HTL)– Polyaniline, PANI (HTL)– Arylamine, -NPB (HTL)– Copper Phthalocyanine, CuPc (HTL)– Tris-(8-hydroxyquinoline) aluminium, Alq (ETL)– Bathophenanthroline, BPhen (ETL)

O O

S

PEDOT

N

N

PANI


Dopants:– Tetrephenylporphyrin, TPP (Red dye)– Rubrene (Red dye)– Inorganic dopants, e.g. Li, LiF

Composite materials:– Polymer/polymer composite– Polymer/organic composite

Rubrene TPP


– Transparent stacked layer device structure• Forrest and Kahn et. al., from Princeton University

…device technology


– Microcavity EL devices

• Tokito et. al., from TOYOTA, Inc.

– Inkjet printing of polymers

• Yang Yang et. al., from UCLA

– EL devices on flexible substrate

• Universal Display Co.

…device technology

Present status in research – Present status in research –

Area of interests:

– Inter-diffusion between layers– Kato et. al., J. Appl. Phys. 81, 7313 (1997)

– Emergence of “dark” spot– Ch. Jonda et. al., J. Appl. Phys. 85, 6884 (1999)

– Life-time related problems– Parker et. al., J. Appl. Phys. 85, 2441 (1999)

– Thermal breakdown of organic materials– Z. Zhou et. al., Adv. Mater. 12, 265 (2000)

– Quenching effects by ambient species (e.g. O2)– J. Yu et. al., Science 289, 132 (2000)

– Susceptibility of certain materials towards environment– PEDOT absorbs water quickly from ambient and degrade the device

…device failure

Our current statusOur current status

Insertion rod (48" travel)

HV chamber for depositionand surface treatment with

evaporation sourceand RF ion source

Central UHV chamberfor evaporation of metals

& Kelvin probe measurement

UHVAFM/STM

Spincoater

Nitro

gen g

love bo

xfor sp

in-co

ating

and

samp

le prep

aration

Fast entryloadlock

…integrated coating & analysis system

Our current statusOur current status…integrated coating & analysis system

Our current statusOur current status…green emitting PLED fabricated here

Our current statusOur current status

On-going research

Interface characterization of PEDOT:PSS/ITO,Blocking of In diffusion from ITO to

PEDOT:PSS by using “Self Assembly Monolayer (SAM)”,

Solvent effect on ITO,Study on polymer / ITO contact under different

spin-casting conditions.

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Polymeric Electroluminescent Devices K W Wong Department of Physics The Chinese University of Hong Kong