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OLEDs OLEDs Basic principles, technology and applicationsBasic principles, technology and applications

Sébastien FORGET

Laboratoire de Physique des Lasers

Université Paris Nord – P13

www-lpl.univ-paris13.fr:8088/lumen/

Introduction Basics Technology Applications

Paris Nord University (Paris 13)

2Sébastien Forget, Univ. Paris 13

S Chenais S ForgetThis course gathers slides taken from various presentations by those guys :

« copyright » : Some slides were also illustrated with images from the web.

When known, the origin of the pictures is given as a reference


OutlineOutline

Introduction

Basic principles

.

.


.

.

Technology : state of the art and bottlenecks

Applications : Displays, Lighting, Lasers (?)


OutlineOutline

Introduction

Basic principles

.

.


.

.





L.H

irsc

h, IM

S b

ord

ea

ux


What about Organic SCs and OLEDs ?What about Organic SCs and OLEDs ?

Organic Electronics: building basic (opto)electronic components with organicsemiconductors : transistors, photovoltaic cells, light-emitting diodes (OLEDs)…

OLEDs specific properties:

Low electric consumption/ high efficiency

Emission all over the visible spectrum

Compatibility with flexible substrates

Low cost (compared to inorganic)

Large areas with uniform luminance © UDC


Applications : ultra-flat displays / lighting

© Novaled© Sony


19621963

1962 : First inorganic LED (General Electrics)

1963 : Electroluminescence in anthracène (Pope)

1977 : Electronic conduction in polyacetylene films

A. Hegger

A. McDiarmid

H. Shirakawa

2000 Nobel Prize (chemistery)

1987 : First organic light-emitting diode with a several-layer design (C.Tang and S. Van Slyke, Eastman Kodak)

Some history : breakthroughsSome history : breakthroughs


1977

19871990

1990 : Electroluminescence in polymers (Cambridge)

1997

1997 : First commercial product(Pioneer)

2002

2003

2002 : flat screen 15” (Kodak, Sanyo)

2003 : Camera (Kodak) and…

Crystals

Thin Films Heterojonctions

Applications

Polymers


As for a LED, several layers are superimposed :

Organic Materials (small

molecules or polymers)

What does an OLED look like ?What does an OLED look like ?

Metalic Cathode

• Electrons Injection

•Al, Au, Ag…

• Electron transport,

•Multilayers

•Molecules/Polymers


Light

Substrat

Transparent AND conductive Anode = ITO

Total thickness ~ 200 nm : high F with reasonable V

•Molecules/Polymers

• Hole injection

•Recombination

•Ligth emission through ITO


Organic materials :

MaterialsMaterials

« Small » molecules


Polymers

polyethylene


Organic materials :

How can we make it ?How can we make it ?

Can be thermally evaporated

• Small Molecules only

• « complex »

• Very fine thickness control

•Multilayer possible


Can be spin-casted

• Polymers only

• Very simple and cheap

• Multilayer ? Control ?

•Multilayer possible


OutlineOutline

Introduction

Basic principles

.

.


.

.




« Plastic » is a priori an insulator… but organic semi-conductors do exist

Back to basicsBack to basics

1977 : Discovery of the electronic conduction in polyacetylene films

A. Hegger

A. McDiarmid

H. Shirakawa

Chemistry Nobel Prize 2000


Thoses molecules can conduct electricity (badly !)

How ? Some basic chemistry is needed…


Back to basics : some chemistryBack to basics : some chemistry

The Carbon – Carbon bond

Sp² Hybridation

Pz

4 valence electrons and 3 atoms around :

4 valence electrons and 4 atoms around :

Sp3 Hybridation : INSULATOR

π, liante

π*, anti-lianteE


C : 1s² 2s12px2py2pz

4 valence e-

Pz

SP2

SP2

SP2

C CH H

HH


What is π-conjugation ?

C C CC

HH

H

H

HC

HH

6 electrons delocalised over the whole molecule

Benzène C6H6

What happens when a pi-conjugated molecule absorbs an electron ?

« Classical view » (here on polyacetylene)


From ISS, B.Wright (http://www.isstavanger.no)


C C C C

H HH H

H

H

HC

H

…Or a more « quantical » one : the electron is delocalized over the whole molecule like in a quantum well (here with anthracène)


Energy bands

HOMO = Highest Occupied Molecular Orbital

= highest π orbital occupied by a pair of electrons

LUMO = Lowest Unoccupied Molecular Orbital

= lowest unoccupied π* orbitale

pzπ

π* = conduction band

π

π*

= valence bandHOMO

LUMOGAP


•The emitted photon has ~ the gap

17

Sébastien Forget, Univ. Paris 13

LUMO

HOMO

•The emitted photon has ~ the gap energy : mostly in the visible spectrum

•λ is proportionnal to the length of the polymeric chain

17

LUMO

HOMO


Organic luminescence



http://micro.magnet.fsu.edu

Also see the animation at http://micro.magnet.fsu.edu/primer/java/jablonski/jabintro/index.html


Material panel : huge !

Gap Energies for some polymers




ClassicalClassical OLED StructureOLED Structure

Tang et VanSlyke, 1987

LUMO2.3

3.0

Vacuum level (E = 0)

20

E (eV)

ITO

NPB

HOMO

Alq3

3.0

5.5 5.7

4.6

Al

X (nm)40 nm 60 nm

N

O

AlO

N

O

N

N N


GaussianGaussian disorderdisorder

LUMO

Weak electronic coupling between two molecules

→ Random positioning during deposition

→ Energetic and geometric disorder


21

E (eV)

ITO

NPB

HOMO

Alq34.6

4.3Al


ContactContact


22

ITO

NPB Alq3 4.3Al


ContactContact

+Vapplied

V0

-

Work function W

Electronic Affinity


23

NPB

(HTL)

Alq3

(ETL)

ITO

+Vapplied

Al


InjectionInjection

W

+

1/r

F=0F≠0 Thermoelectronic injection : J ≡ T² exp(-E/kT)

Schottky effect : image potential

Total potential

MODEL 1 (Richardson-Schottky)

The total energy barrier is lowered by the attractive potential : J ≡ T².exp(-(E-bF1/2)/kT)


V=-eFr

METAL Distance r

potentialThis model (Richardson-Schottky) is valid

essentially when F and T are weak


InjectionInjection

W

F=0F≠0 Tunneling injection : J ≡ F² exp(-b/F)

The Schottky effect (image potential) is hereneglected

MODEL 2 (Fowler-Nordheim)

Tunneling


V=-eFr

METAL Distance r

This model (Fowler-Nordheim) is validessentially when F and T are high

More complex effects can be considered to get more subtle models : still an active research area…


Transport Transport : «: « hoppinghopping »»

e-

Initiation of a Initiation of a Initiation of a Initiation of a polaronpolaronpolaronpolaron

Al

1) Spatial re-organization

2) Polarisation

-

26

NPB

(HTL)

Alq3

(ETL)

ITO

+

-Al

Molecules are fairly independant of each other and are bonded via weak Van der Waals interactions.

Molecules can hence undergo large amplitude vibrations


e-

Al

-



2) Polarisation


27

NPB

(HTL)

Alq3

(ETL)

ITO

+

-Al


e-

Al

-



2) Polarisation


28

NPB

(HTL)

Alq3

(ETL)

ITO

+

-Al


e-

Polaron Polaron Polaron Polaron Transport by Transport by Transport by Transport by «««« hoppinghoppinghoppinghopping »»»»

Al

Transport is thermally activated

-


29

NPB

(HTL)

Alq3

(ETL)

ITO

+

-Al


Key parameter for transport Key parameter for transport : : mobilitymobility

( ) FpTEepj .,,.. µ=Current density (A/m²) Charge carrier

density (e- or h+)

Electric field (V/m)

Mobility

(m²/V.s)

Mobility model

H. Bässler, Phys. Stat. Sol. B 175, 15 (1993)

( )22 2

02

30,

C EkTkTT E e e

σσ

µ µ −∑ − =

= average velocity of the charge carriers per unit of electrical field

FF

30

Martens et al, Phys. Rev. B 61, 7489 - 7493 (2000)

Temperature dependence of the zero-field mobility of four PPV derivatives

( ) 0,T E e eµ µ=σ = width (RMS) of the density of state

Σ = parameter for geometric disorder

Orders of magnitude : µ ~ 10-7 – 10-3 cm².V-1.s-1

Silicium : µ ~ 103 cm².V-1.s-1

µ with Twith Twith Twith T (hopping evidence) and with Fwith Fwith Fwith F

Generally µelectron << µhole

F

(Poole-Frenkel)


Mobilities :Very low/ inorganic semi-conductors

electrons and holes exhibit very different mobilities



Martens et al, Phys. Rev. B 61, 7489 - 7493 (2000)

Temperature dependence of the zero-field mobility of four PPV derivatives


Mobilities :Very low/ inorganic semi-conductors

electrons and holes exhibit very different mobilities




RecombinaisonRecombinaison : : exciton formationexciton formation

electrons

Eexciton < E polaron because the exciton is « stabilised » by the Coulomb interaction

+ +→

« electron » « hole »

LUMO

HOMO

EXCITON

-

33

NPB

(HTL)

Alq3

(ETL)

ITO

+

holes

-Al


ExcitonsNeutral Quasi-particule : electron-hole pair linked by Coulombic interaction

Spatially limited to a single molecule (in a first approach)

ExcitonsExcitons

Wannier-Mott excitons

10 nm

hole electron exciton

INORGANIC

Fundamental

34

Frenkel excitons

1 nm

hole electron exciton

+- ORGANIC

Fundamental

V. M. Agranovich and G. F. Bassani, ed., Electronic Excitations in Organic Based Nanostructures, in Thin Films and Nanostructures Vol. 31,

(Elsevier Academic Press, Amsterdam, 2003)


Photon Photon emissionemission

electrons

+ +→

« electron » « hole »

LUMO

HOMO

EXCITON

-

Eexciton < E polaron because the exciton is « stabilised » by the Coulomb interaction

35

NPB

(HTL)

Alq3

(ETL)

ITO

+

holes

Photon-

Al


DifferencesDifferences IISC /SC /OOSCSC

Organic Semiconductors / OLEDs Inorganic Semiconductors / LEDs

Electrons (holes) localised on ONE molecule

(=polarons) : charges are hopping from one

molecule to another

Electrons (holes) delocalised in the crystal :

energy bands

36

Very low mobility, increasing with T (hopping)

No doping needed : charges are directly coming

from the electrodes

Wide choice of structures and materials

Emission over the whole visible spectrum,

possible mixing…

High Mobility decreasing with T (phonons)

Doping is needed ! The free charges are inside

the material

Limited Heterostructures design (crystalline

structure must fit !)

Emission only for a given set of λ (gap)


Some definitions :External quantum efficiency ηext

= number of emitted photons / number of injected e-

ηext = ηrad. .ΦPL. ηcoupling

ηrad= probability of exciton formation (from one e- and one h+) (~ 1)

What is the « OLED efficiency » ?What is the « OLED efficiency » ?


א = probability that the exciton is emissive (~ 0.25)

ΦPL= luminescence quantum yield (> 80%)

ηcoupling = fraction of photons escaping from the OLED (~ 0.20)


Electron HolePolaron - Polaron +

transport

RecombinaisonExciton creation

S T75%

Diffusion

ηrad ~ 100%

א ~ 25%

ΦPL ~ 80%

Cathode Anode



Desexcitation (non-radiative)

Out CouplingWaveguiding by Total Internal Reflection

Emitted Photon

ηcouplage ~ 20%

TOTAL ~ 4%

radiative

Desexcitation











SOLUTION : PHOSPHORESCENCE

25% of singlets excitons (antiparalleles spins)

75% of triplets excitons (paralleles spins)

S0

S1

T1

Emission

No emission


S0

S1

T1

No emission

S = +1/2 + 1/2 = 1

1- 0 Forbidden

S0

S1

Emission

S = +1/2 -1/2 = 0

0 -0 Authorised

Rule (Pauli)

Two electrons with same spin CANNOT occupy the same energetic level.

One electron CANNOT change its spin during a transition

T1

PhosphorescencePhosphorescence


Phosphorescence

Idea : inserting a heavy element (high Z) to by-pass the selection rule ! (The spin-orbit coupling becomes non negligible and Triplet-Singulet transitions becomes allowed)

Fluorescence


Glass Substrat ~ 2 mm ; n = 1,5

Modes guided in the

Modes guided in the substrat

Outcoupled Modes

Light extractionLight extraction


Organic layers+ITO ~ 300 nm

Refractive index ~ 1.7

Localisation of the excitons (~ 10 nm)

cathode

Modes guided in the organic layers+ITO


Outcoupled Fraction of the light

7.1%204

12

2=≈×≈ nfor

n

With reflection on a perfect mirror and neglecting the losses.

Light extractionLight extraction


Solutions :

• Optical Microcavity

• Diffraction gratings, corrugation, microlenses…

Rapid proof :

Ω=2π (1- cosθ) ~ π θ² ~ π/n²

Then Ω/(4π)=1/4n²




ηext = ηrad. .ΦPL. ηcoupling = 1 x 0.25 x 0.8 x 0.2 =4%

1 x 1 x 0.8 x 0.35 =28%







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