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Les diodes électroluminescentes organiques ou l’émergence de l’électronique organique

Matériaux, dispositifs et applications

Bernard GEFFROY

Laboratoire Composants Hybrides

CEA/DRT/LITEN/DTNM

CEA/SACLAY 91191 GIF Sur YVETTE

L’électronique organique

Ø    Les transistors organiques (OTFTs)

Ø    Les diodes électroluminescentes       organiques (OLEDs/PLEDs)

Ø    Les cellules solaires organiques (OPVs)

Plan

ØGénéralités

ØLes diodes électroluminescentes organiques

ü   Quelques généralités sur les matériaux organiques

ü   Principe de fonctionnement des OLEDs

ü   Dispositifs et matériaux électroluminescents organiques

ü   Adressage des écrans OLEDs

ü   Réalisation de dispositifs couleurs

ü   Techniques de dépôt des matériaux organiques

ü   Démonstrateurs et réalisations industrielles

ü   Aplication à l’éclairage

ØConclusions

Le CEA, un fort ancrage régional

 biomédical ILE-DE-FRANCE

Les quatre missions du CEA

 

Le LITEN

Laboratoire d’Innovation pour les Technologies des

Énergies nouvelles et les Nanomatériaux

Activités du LITEN

 

Electronique organique: définition


 

Cellule solaire                             FOLED

photovoltaïque organique         Universal Display

(CEA)         Corporation Electronique organique (EO) 

l’élément actif est un matériau constitué d’une grande assemblée de molécules  ordonnées ou non.

 Electronique plastique

i.e. souple, pas nécessairement performante en terme de densité d’intégration, mais facile à produire, bas coût et qui vise des applications grand public.


Electronique Moléculaire (EM) basée sur des composants actifs constitués d’un édifice moléculaire : molécule organique (petite molécule, oligomère ou polymère), fullerène, nanotube de carbone … L’échelle de ces composants se situe dans la gamme de un à quelques dizaines de nanomètres.

Source: OMNT Electronique Organique


NTC connecté entre deux électrodes métalliques

J. P. Bourgoin et coll. Phys. Rev. Lett. 95, 185504 (2005)

 

Plan

Les diodes électroluminescentes organiques

Introduction

Basic device structure

Electroluminescence : Generation of light with electric field

consists of:

3.A transparent electrode (ITO)

4.An emissive layer

5.A reflective electrode (metal)

Oxide)

Thin layer devices from organic dyes or conjugated polymers

Organic layer thickness :  ~ 150 nm

History of organic electroluminescence

 

History of organic electroluminescence

Electroluminescence was observed  from single crystals of anthracene.

W. Helfrich et al.

Phys. Rev. Lett. 14, 229 (1965)

5 mm thick crystal

El quantum efficiency ~ 1-5% High driving voltage

Good understanding of the basic physical processes involded in electroluminescence like double injection, charge carrier migration, electron-hole capture (exciton formation), and light emission (fluorescence)

OLEDs roadmap

 

Forecast display production

Strong increase of OLEDs displays production

OLED unit forecast

 

2008

Plan

Les diodes électroluminescentes organiques

Généralités sur les matériaux organiques Conjugated molecules

Electronic structure of carbon

Isolated carbon atom: 1s2 2s1 2p3    à valence of 4

Hybridized spn orbitals (superposition of s & 2p orbitals)

Sp2 hybridization (double bond)

 

Molecules with delocalized ? orbitals

Semiconducting properties

HOMO-LUMO Bands

 

HOMO : Highest Occupied Molecular Orbital

(The highest energy molecular orbital that contains a pair of electrons)

LUMO : Lowest Unoccupied Molecular Orbital

(The lowest energy molecular orbital that contains no electrons)

Organic semiconductors

Small molecule organic semiconductors

 

Polymer organic semiconductors

 

Source:

Electron affinity & ionization potential

Electron affinity

Ionization potential

                              2.5 – 3 eV                                  EAIP       4.5 - 6 eV

Evaluated by cyclic or photoelectron spectroscopy voltametry in solution

Electronic transitions

Polyatomic molecule

H

C

 O..

                                                                                                                                                         H                    ..

E

Ground state

? à ?*

 

n(p) à ?* à *

 

Excited states

n(p) à *

 

                                                                                  ?*                                                                                                        

LUMO ?*

                       HOMO    n(p)

?

?

                                                                                                                                                                                                                                     ? ?                                                                          ?

Optical properties of molecules

                                                                                                                                                                            PHBN : R=n-hexyl

Organic materials are characterized by a large Stockes shift between absorption and emision spectra à they are almost transparent to their own emitted light

Singlet – triplet states

Excitons

Singlet excited   Triplet excited state           state

                                                                                                                                                                                                                              S=0                                                                                                S=1

                                                                                                                                                                                                                                   25 %                                                                                        100 %

Singlet decay (radiative) is called fluorescence

Triplet decay (forbidden process) is called phosphorescence

Ir(ppy)3

           Strong spin-orbit coupling mixes singlet and triplet states                                                  H3C



Heavy metals (Ir, Pt…) impove triplet emission

Characteristic times

Absorption

Vibrational relaxation

Internal conversion

Fluorescence (decay of excited state S1)

 10-15 s

 10-12 -10-10s

 10-11 -10-9s

 10-10 -10-7s

10-10 -10-8 s

Intersystem crossing (ISC)

10-6 -1s

 

Phosphorescence

 (decay of excited state T1)

Lifetimes and quantum yields

Effect of molecular structure on fluorescence

Molecule

?f

?p

?T (s)

Naphthalene

0.55

0.051

2.3

1-Fluoronaphthalene

0.84

0.056

1.5

1-Chloronaphthalene

0.06

0.30

0.29

1-Bromonaphthalene

0.0016

0.27

0.02

1-Iodonaphthalene

< 0.0005

0.38

0.002

Source Wehry 1990

Charge transport in organic solids

                                                           Periodic lattice       Amorphous lattice

Delocalized    Localized chargescharges

Crystals : periodic structures band model (conduction & valence bands) delocalized charges (electrons in CB, holes in VB)

Amorphous organic materials : band model ?

localized charges (radical ions) transport through intersite hopping charge traps (defects)

Charge transport in conjugated polymers

In conjugated polymers the charges are partially transported via delocalisation along the HOMO and LUMO levels.

Transport properties are usually determined by defects in the 1D-chains (intra molecular) or by hopping from chain to chain (inter molecular)

 

Charge transport in small molecules

Charge transport in small molecules is via hopping, i.e. the charges have to jump from one molecule to the neighbouring one to be transported. 

 

Charge transport

 Charge transport via hopping  Low mobility (disorder)  µh+ # µe-

 Challenge for

High EL efficiency :

Charge Carrier Balance

Plan

Les diodes électroluminescentes organiques

Principe de fonctionnement des OLEDs

Organic Light Emitting Diode : Principle

 

1   à Charge carrier injection

2   à Charge carrier transport

3   à Charge recombination (exciton formation)

4   à Exciton diffusion

5   à Exciton recombination and photon emission

I-V-L characteristics

Diode behavior

Brightness is proportional  to the current flow

OLEDs conduct in forward bias and do not conduct under reverse bias. The impedance drops exponentially with V for V>Vth.

OLEDs : 2 main technologies

 

Charge injection : holes

Anode : ITO

Small barrier for holes injection into HOMO level of HTL organic material

Use of  materials with high work function (ideal ~ 5 eV)

Typically use of transparent ITO as anode

Need ITO surface treatment to enhance holes injection (i.e. Oxygen plasma treatment), ITO fermi level stabilization around 5 eV.

Réf.: Kim et al., Appl. Phys. Lett., 74, N°21 (1999) 3084

Charge injection : electrons

Cathode

Small barrier for electrons injection into LUMO level of ETL organic material (ideal ~ 2.5 to3 eV) Use of metals with low work function (Ca, Mg…)

But such metals are very sensitive to oxidation

Use alloys such as Mg/Ag or Al in combination with alkali metals like Li, Cs,

K, Na…

Barrier, dipole vs injection

EF

Metal-organic interfaces are varied and complex

Interface chemistry and interdiffusion can play key roles

-   change with interface processing (deposition sequence)

-   affect interface barriers (gap states, doping effects, dipoles)

Source: A. Kahn, Summer school, Aussois, 2005

Quantum efficiency

External quantum efficiency

 

?qext  =   Number of emitted photons Number of injected electrons  =  ?r . ?????PL .?ext

(%)


?r : probability that charges recombine to excitons

??: probability of production of emissive species

?PL : quantum efficiency of luminescence

?ext : fraction of generated photons leaving device

Generally, only singlet excitons are radiative

?r ~ 1 ? = 1/4

             ?ext ~ 1/2n2

?q ext :  ~ 5 % max


Fluorescence efficiency in solide state

 

Power efficiency

External power efficiency

 

Power efficiency: light power versus electrical power

?e =                        Output light power Input electrical power = nphq.V . h?= ?qext.e.Vh?

WL/W

Luminous efficiency (lm/W)

?L =

?e . ???. km

luminous flux versus electrical power

With  km = 683 lm/W

Device efficiency

Other useful units

Characterization of device efficiency : cd/A

L?cd/? cd/A=

10?J ?mA/cm² ?

lm/W=cd/A??

V ?V ?

Luminance-efficiency vs Applied voltage

 

Aging mechanisms

 

Device Lifetime

Degradation of OLED devices is one of the main issues. Degradation phenomena occur both under operating condition as well as under storage.

   No really standardized measurement method

(DC vs pulsed constant current, brightness level …)

 Device lifetime usually  defined as : 

Mean time to half-brightness

Advantages of OLEDs for Displays

•       Very thin    • RGB, white

•       Light weight   • Low DC drive voltage

•       Fast response time  • Structural flexibility

•       High brightness  • Large operating

temperature range

•       Large viewing angle

•       Low power consumption

Plan

Les diodes électroluminescentes organiques

Dispositifs et matériaux électroluminescents Diode structures

Efficiency & stability

Multilayers

Cathode

EML/doped

           Monolayer                                                                       HTL

Anode

Doped transport

~~                                                     layers K. Leo, U. Dresden



                  Anode                                                                                        HBL

Hole Blocking layer:

                 1965                  PLED 1985              Exciton confinement        2002

Single layer device : recombination zone

Balanced charge transport

Imbalanced charge transport

Recombination zone

   

e-/h+ recombination occursin the organic material bulk.

Not many organic materials have electron and hole mobilities that are in the same order of magnitude

e-/h+ recombination occurs near an electrode.

Reduction device efficiency due to quenching of luminescence by the electrode (cathode).

 

Bilayered device : recombination zone

                                                         HTL : hole transporting layer  ETL : electron transporting layer

 

*   e-/h+ recombination occurs away from the device electrodes.

*   Broadens the number of useful organic materials (only single carrier type per layer).

*   Allows reduction of the barrier for charge injection.

 More efficient device

Hole and electon mobilities

 

Bilayered device : emissive zone

 

*   The emissive zone is confined to a small section of the device and usually near the HTL/ETL heterojunction.

*   Color tuning and luminance efficiency can be improved by doping the emissive zone with a highly luminescent molecule.

Exciton transfer through doping

                  Dopant                  Dopant                        H3C                                                                                                                                                               3

RUBRENE                                                                       (25%)                                      (100%)

Exiton transfer via Förster transfer (dipole-dipole)   H3C or Dexter transfer (charge transfer)

ISC : Intersystem crossing (via spin orbit coupling)

Doping effects

 
   

4 0 0

5 0 0

                  6 0 0                       7 0 0                       8 0 0

W a v e l e n g t h ( n   m         )

Efficiency improvement

Color tuning

(energy transfer)

 

PL ~ 1 (avoid quenching/low concentration)

       Lifetime improvement

 

Device engineering: RGB stack OLED

 

G. Gu et al., Appl. Phys. Lett., Vol. 74, 305 (1999)

Device engineering: HBL & EBL

 

V.I. Adamovich et al., Organic Electronics 4 (2003) 77–87

Les matériaux : petites molécules

Alq3

                                                                       Dérivé oxadiazoleN

Un point clé : la pureté des matériaux

Material purification

The purity of the material is a main issue Purification by train sublimation

                                                                              Quartz tubes                                  Heating zone

Argon or

Nitrogen flux

Cooling zone (water)

Alq3

?PL

As received

13%

Purified

25%

Les matériaux : polymères

   

All colours are avaliable

 

Material requirements

High luminescence efficiency (PL, EL)

Adequate conductivity (p or n type)

Good temperature stability (high Tg)

Good radical cation/anion stability

Good oxidative stability (water, oxygen)

Good coatability (thin, uniform films with no pinhole defects or impurities)

                                                                            PLED :        Good film formatiom from solutionNo side reactions with solvents

                                                                            OLED :         Does not degrade during evaporationNo catastrophic film crystallization

 Color saturation and purity (narrow spectra and correct CIE coordinates)

Les matériaux

Films minces de matériaux organiques ?-conjugués  2 classes de matériaux :

 1987 :  diodes efficaces à base de

           OLED petites molécules’

C.W. Tang, S.A. Vanslyke, Appl. Phys. Lett. 51

                                (1987) 913                                                        H3CCH3

Films préparés par évaporation sous vide

 1990 : Electroluminescence dans les polymères

PLED

J.H. Burroughes et al.,

Nature 347 (1990) 539

Films préparés par spincoating

Poly(p-Phenylen-Vinylen)

Les matériaux : Génération 2

Utilisation de matériaux phosphorescents pour augmenter l‘efficacité

 Première réalisation:

M.A. Baldo, M.E. Thompson, S.R. Forrest et al., Nature 395 (1998) 152

 

H3C

Doped transport layers

ITO /p-TDATA (100nm, doped F4-TCNQ) / TPD (5nm) / Alq3 (65nm) / LiF (1nm) / Al

 

Improved OLEDs using doped hole transport layers

Ref.: K. Leo et al., Univ. Dresden

Structure PIN : 2nd generation

 

Ref. : M. Pfeiffer et al., Adv. Mater. 14 (2002) 1633

Efficacité et stabilité

Small molecules : fluorescent materials

Colour

Red

Green

Blue

L (cd/m²)

@ 20 mA/cm²

400

1500

600

Cd/A @ 20 mA/cm²

3

7

3

T ½ (h) @ 100 cd/m², 20°C

30 000

100 000

25 000

Ref. Eastman Kodak, 2002

Matériaux phosphorescents

Small molecules : phosphorescent materials

   

UDC



CIE (x, y)

Luminous

Lifetime

at luminance

PHOLED materials

Efficiency (cd/A)

(hrs)

cd/m²

 Red: RD15

(0.67, 0.33)

12

100 000

500

Red: RD07

(0.65, 0.35)

18

40 000

500

Green: GD29

(0.30, 0.63)

24

10 000

600

Green: GD33

(0.31, 0.64)

40

20 000

1000

Green: GD48

(0.32, 0.63)

37

25 000

1000

RD61

(0.62, 0.38)

30

40 000

500

GD107

(0.35, 0.60)

40

25 000

1000

YD85

(0.41, 0.58)

65

under test

1000

New green

(0.32, 0.63)

80

15 000

1000

New green

(0.32, 0.63)

57

40 000

1000

New blue

(0.16, 0.37)

22

15 000

200

New blue

(014, 0.13)

9

under development

200

new blue

(0.16, 0.10)

3

under development

200

Source: M.S. Weaver et al., Proceeding Eurodisplay 2005, 188 (2005)

Efficacité et stabilité

Polymer performances

Color

At 100 cd/m²

Lifetime at RT (hrs)

 

CIE

(x, y)

Luminous

efficiency (cd/A)

measured at L (cd/m²)

extrapolated a

at 100 cd/m²

Red

Green

Blue

Yellow

Orange

White

(0.68, 0.32)

(0.43, 0.55)

(0.16, 0.20)

(0.50, 0.49)

(0.58, 0.42)

(0.30, 0.36)

1.7

7.7

4.8

2.1

0.9

5.1

1790

2000

2867

2000

510

1425

2420 4000

8138 1000

290

1600

~210 000

~255 000

~100 000

~290 000

~320 000

~40 000

a assuming that lifetime is proportional to 1/(luminance)n with 1.3 < n < 2

Source: N. Patel, CDT Workshop Notes, Eurodisplay 2005

Dark spots in OLEDs

 

                                                                       To                 After storing for 24  in ambient conditions

Ref.: Liew et al., Appl. Phys. Lett., Vol. 77, N° 17, 23 October 2000

Dégradation : veillissement Alq3

Hole-only device

 Eviter formation espèces cationiques

Alq3+

Principaux effets de la dégradation

                                                  Diminution de la luminance

 vieillissement des matériaux  vieillissement différentiel (RGB)

                                                      Augmentation de la tension de fonctionnement

 barrière injection (électrodes, interfaces)

 Apparition et croissance de ‘points noirs’  électrodes, environnement

Encapsulation

 

Pioneer Patent EP 0 776 147 A1

Thin film encapsulation

Barrier coating

OLED

Source : Vitex

Flexible displays

 

                     PLED Dupont Plastic Substrate             Universal Display Corporation

L=200 cd/m2, e= 175 µm

Pixels : 400 µm x 500 µm

Plan

Les diodes électroluminescentes organiques

Adressage des écrans OLEDs

Adressage passif

Avantages :

*     Bien adapté aux OLEDs

*     Simple / bas coût

Inconvénients :

 Nécessite forte brillance crête

Pour chaque ligne : Lcrête = Nligne * L moy. soit écran VGA : si  Lmoy. = 300 cd/m2 d’où Lcrête=72000 cd/m2

Limitation : ~100 – 150 lignes max (résolution limitée)

Adressage par  matrice active

   

Avantages :



*     Découplage adressage / excitation

*     Courant plus faible

*     Tension compatible TFTs

 

Inconvénients :

*     Silicium poycristallin

*     Nécessite transistor type p

*     Faible taux ouverture (4 TFTs/pixel)

Fluorescence vs Phosphorescence

 

Réf: ELIATECH Co., Ltd., OLED ASIA 2004

Plan

Les diodes électroluminescentes organiques

Réalisation de dispositifs couleurs

Full-colour display

Side-by-side patternOLED with

Blue, Green and Red emissive sub-pixels

Red, Green  and BlueFilters

Colour filter

White OLED

Red and Green

Colour Conversion Converters

Blue OLED

RGBW display (Eastman Kodak)

 

Source: ASIA Display IMID’04

Colour by blue approach

Colour Conversion Media (CCM)

Blue OLED

Blue OLED

Ø Multilayer structure based on evaporated small molecules

Converting Layers Composition

Ø Host Photopatternable Polymer (transparent)

Patterning of Red and Green sub-pixels

Ø Dye (Green or Red emission)

           Good absorption of the blue light from the blue OLED (OD > 2.5) Efficient emission in green or red (hight PL yield and acceptable CIE coordinates)

Color Conversion Media

Photopatternable resin

Bisphenol A ethoxylate diacrylate +

Photoinitiatorr Irgacure 186 (1% wt / monomer)


-  UV photopaternable resin

-  Transparent resin

O                                                           - Film thickness  ~ 5 µm

 

O

Fluorescent dyes

GREEN: Coumarin 6 (C6)

 

RED: mixture of green (C6) and red (Nile Red or NR) dyes

RGB demonstrator

 
 

Dispositif

Luminance (cd/m2) at 10 mA

x

y

B

1779

0.154

0.128

G

1459

0.244

0.609

R

63

0.663

0.329

 

Plan

Les diodes électroluminescentes organiques

Techniques de dépôt des matériaux organiques RGB patterning

material

Cluster tool for organic deposition

 

Polymer deposition

The most common technique for polymer RGB applications is inkjetting.

Inkjet printing of LEP Colour Displays

 

Ink-jet printing

Some RGB ink-jetted pixels

 

2001

Démonstrateur 2,5’’ diag.

200 x 150 pixels (x 9)

Pixel : 10µm x 86 µm

Pas : 52 x 133 µm

Laser Induced Thermal Imaging (LITI)

Principe

                                                                                               95 µm                                                  


Samsung SDI & 3M Display


3.6” QVGA full color AMPLED

Pixel pitch 80 x 240 µm

Organic Vapour Phase Deposition

 

Linear Deposition

                        Image: Fraunhofer IPMS                                                                

Source : 24 february 2006

Plan

Les diodes électroluminescentes organiques

Démonstrateurs et réalisations industrielles First OLEDs product on the market

   

Kodak commercial product

 

LPTS poly-Si Active Matrix

Objets  commerciaux à afficheur OLED/PLED

 

Sanyo / Kodak  OLED display

 

                                                   5.5 in. diagonal        Poly-Si active matrix

320 × 240 pixels Sub-pixel size : 116µm x 348µm 150 cd/m2

Sony full color display

                                        13 in. Diagonal            Poly-Si active matrix

                                       SVGA 800 x 600 pixels Pixel size : 330µm x 330µm

 

Color

Efficiency

Cd/A

CIE x

y

Blue

4.5

0.145

0.086

Green

45

0.230

0.667

Red

7

0.703

0.297

20’’ a-Si AMOLED

‘Top emission’

Source: K. Micha et al., IDTech


2005

SAMSUNG

1 dalle de 40’’

 

Prototypes écran OLED

 

2004

EPSON

4 dalles de 20’’


OLED main manufacturers

Table 1: Top Four OLED Manufacturers' Q1'05 Revenue and Growth (US$ Millions)

   

Revenue

Rank

Manufacturer

US $M

 

1            Samsung SDI

37

 

 

2             RITdisplay

28

 

 

3               Pioneer

20

 

 

4              Univision

14

 

 

5                LGE

7

 

 

Others

19

 

 

Total

125

 

         

OLED production

SAMSUNG débute la construction d'une usine d'OLEDs  à matrice active

Après la production d’afficheurs OLEDs à matrice passive,

SAMSUNG vient d’annoncer la construction d’une usine (450 millions de dollars) pour produire des écrans OLEDs à matrice active en silicium polycristallin basse température.

Le marché visé concerne les écrans pour téléphones portables. La production devrait démarrer début 2007 et produire 20 millions d’écrans sur l’année.

Source: Electronique International novembre 2005



Plan

Les diodes électroluminescentes organiques

Application à l’éclairage

Nouvelles sources d’éclairage

SSL (Solid State Lighting)

                  LED

AlGaInN

Source ponctuelle

OLED

Film mince

Petites molecules  Ep ~ < 1 mm polymères Surface conformable

Source étendue

Evolution de l’éclairage

 

OLED éclairage

 

WOLED: état de l’art

Performances à 1000 cd/m²

15 lm/W

CCT: 4400 K

Equivalent ampoule 80W

CRI: 88

CIE: x= 0.36; y= 0.36 Source: General Electrics

OLED  Eclairage

NOVALED : record du monde

Développement d’une OLED verte pour l’éclairage avec une efficacité de 110 lm/W at 1000 Cd/m2 : c’est 50% de mieux que les LEDs inorganiques

Objectif de NOVALED : dépasser les tubes fluorescents dans le blanc

PRESS RELEASE

Dresden, February 16th 2005

 

Flexible Organic-Based Displays

 

Single colour passive matrix flexible display

Vitex/Universal Display Corp. collaboration

Conclusions

Matériaux organiques (petites molécules et polymères) sont très prometteurs pour une nouvelle technologie d’affichage.

ü    Forte croissance prévue dans les 4 prochaines années.

ü    Petites molécules permettent de réaliser des structures plus complexes et constituent actuellement la technologie la plus avancée.

ü    Les polymères semblent mieux appropriés pour de grandes surfaces.

La 2nde génération de matériaux (phosphorescents) ou de structure (dopage couche de transport) permettent d’atteindre des rendements lumineux très élevés.

Points importants :

üpuissance lumineuse

üdurée de vie

üCIE (pureté couleur)

Possibilté de fabriquer des dispositifs souples ou conformables.

D’autres secteurs industriels envisageables comme l’éclairage.

Réduction des coûts de production nécessaires pour être compétitif par rapport aux LCDs



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