Organic Light Emitting Diodes

for lightening (WOLEDs)

ORGANIC ELECTRONICS : principles, devices and applications

Chiara Botta, ISMAC, CNR via Bassini 15, 20133 Milanoc.botta@ismac.cnr.it

WOLEDs

Emitting species:Singlet and Triplet exciton, excimer, exciplex

Conclusion and outlooks

Materials: Organic molecules and polymers, Hybrid materials

White light OLED (WOLED)

•White light: fundamental parameters (efficiency, CIE, CRI)

OUTLINE

•White light with molecules, polymers and hybrid systems

ELETTROLUMINESCENCE

+ -1-+

1

1.Injection

+ -3

3.Exciton Formation

4 4.Emission

2

2

2.Transport

ITO

HOMO

LUMO

vacuum

1

+

-1

metal

φm

φITO IP

EA

∆∆∆∆e

∆∆∆∆h

2

3

24

-

+

P-

P+ hννννΦ metals should fit theHOMO and LUMOlevels

ηηηηextEL ≈≈≈≈ Φ Φ Φ Φ γγγγ ηηηηEX ηηηηint

PL

Eff. of Photoluminescence

Eff. Exciton formation per couple of injected charges

µe, µh

∆h ≈≈≈≈ ∆e

e-, h+ injection

max 1

Quantum Efficiencycharges injected n.

photons emitted n.≡ELη

Prob. Radiative recombination

output light

~ 1/2n2 ~ 20%

P+ + P- → Exc↓↑ max 1/4

P+ + P- → Exc↑↑ max 3/4

EFFICIENCY of ELECTROLUMINESCENCE

Th

S0

S1

S2

10-12secIC

S0

S1

S2

10-9

secFluorescence

ISC

10-3

sec

Phosphorescence

S0

T

Tn

10-15 Absorption

Photoluminescence

Jablonski Diagram

S0 →S1, S2 S1 →S0 T1 →S0

fluorescein-5-isothiocyanate

Th

S0

S1

S2

IC

Fluorescence

250 300 350 400 450 500 550 600 650

fluorescina"abs"

Opt

ical

Abs

orpt

ion

Wavelength (nm)

S0→→→→S2

S0→→→→S1

Fluorescence

Em

S1→→→→S0

Fluorescence = radiative transition S1 →→→→ S0

Specular Simmetry between S1 →→→→ S0 and S0 →→→→ S1

Fluorescence

S0

S1

S2

ISC

S0

T1

Phosphorescence

S(H2C)11

O

OO

F3C

N

NEu

S

(CH2)11

Br

O

OCF3

S

(H2C)11

O

O

OCF3

Ligand Eu

300 400 500 600 700

Abs

orpt

ion E

mission

Wavelength (nm)

No specular symmetry

PhosphorescenceHeavy atom favours

Inter System CrossingISC

broad emissionat low energy

EXCIMER

+-

dimer in the excited state complex between one

molecule in the excitedstate and one in the ground state

complexes betweentwo identical molecules

complexes betweentwo different molecules + -

EXCIPLEX

complex between onemolecule in the excitedstate and one in the ground state d~3-4Å

ELECTROPLEX

complex with charge transfer character d~4-7Å

Emission from excited states in organic interacting species

ELECTROMER

as electroplex, for twoidentical molecules

Electrical excitation

Optical Excitation

emission at different energy

Excimer= physical dimer created in the excited state

configurationalcoordinate

Q

physical dimer

dissociative state

broad, red-shiftedemission

no absorption(only monomer)

Eonly for short distances (3.5Å)

extension towards the redof the emission

LED: emission from Exciplex in heterostructure

OLED ITO/ PAT/Alq 3/Al

h+ e-

200 300 400 500 600 700 800 900

Ass

orbi

men

to (

u.a.

)

Intensità PL (u.a.)

Lunghezza d'onda (nm)

PAT

Alq3

Al

PATITOGlass

+

-Alq3

0

0.5

300 400 500 600 700 800 900

Ass

orba

nza

Intensità PL (u.a.)

Lunghezza d'onda (nm)

Alq3 P5OMe

Exciplex

OLED: Singlet/Triplet Emission

FLUORESCENT Materials (organic molecules and polymers)

Singolet S: FluorescenceFL , fast emission (ns), efficientTriplet T : PhosforescencePH, slow emission (µs-ms), not efficient

ηηηηPL= ηηηηFL

Fluorescence

hνννν

S0

S1

T

ISC

FL PH

singlet and tripletgeneration is differentfor PL and EL •PL: all S excitons•EL: 1S/ 3T

FL PH4e-

S0

S1

T4h++< >

Electroluminescence

ηηηηELmax = 0.25-0.50ηηηηPL

polymersmolecules

σσσσT< σσσσS in polymers

Cross section for S/T formation

PA and PADMR measurements

Measurements of the ratio ofsinglet/triplet formationPRL 88, 197401 (2002) M. Wohlgenannt, XM. Jiang, Z V. Vardeny, RAJ Janssen

Exciton formation is spin-independent for oligomers with length lessthan 7A, because for themexciton delocalization is low for both S and T excitons. When conjugation is higher (polymers) Singlet are more easly generated since e and h bound at distances longer than for Triplets

σσσσT/S ÷ Eg÷1/conjug. length

PHOSPHORESCENT Materials (organometallic compounds)

e-

h+ELECTROPHOSPHORESCENCECAN REACH INTERNAL EFFICIENCY OF 100% !

ηηηηEPmax = ηηηηPL

Electrophosphorescence

S1spin-orbit coupling in heavy atoms

}MLCT

ligand

PH

S0

S1

T1

ISC

hνννν

Phosphorescence

OLED: Singlet/Triplet Emission

Complexes ofmetalswith organic ligands

Complexes of Transition Metals (partially occupied d orbitals ): Emission occurs from a mix of metal and ligand states : - Emission color can be tuned by changing the ligand- Emission is a broad band

Complexes of Lanthanides (partially occupied f orbitals ): Emission occurs from the metal : - emissione caratteristica del Metallo- Emission is formed by narrow peaks

ORGANOMETALLIC COMPLEX

Iridium based phosphorescentcomplexes: Emission from metal-ligand states

Complexes of transition metals

Strong spin–orbit coupling induced by heavy atom →

Efficient Inter Sistem Crossing from Singlet to Triplet: Triplet Population. -Mixing of S and T states via spin–orbit coupling relaxes the selection rule of the spin-

forbidden T1→S0 transition→ phosphorescence quantum efficiency increases

-transitions πd: LMCT (Ligand-to-Metal-Charge-Transfer): Transitions of e- from ligand statesto the metal centered orbital

•Rhenium, Ruthenium, Osmium, Rhodium, Iridium, Palladium, Platinum

Ions of transizion metal + organicligand able to coordinate the ion

Organo-Lanthanides Complexes

Partial Level Diagram of Lanthanide ionsGround statesEmitting Levels

Chem. Soc. Rev., 2005, 34, 1048.

f → f transizion are forbidden: Low absorption coefficient of Ln

Excitation occurs through the ligand: sensitization or antenna effectLigand → Ln*

NIR emittersH2O

NIR

T1 ≈ D → Back ET

Sharp emission lines of the metals

How can we obtain Triplet emission in WOLEDs?

Generally phosphorescent materials do not possessgood charge transport properties

Need of dispersion of phosphorescent emitters in matrix possessing good charge transport propertiesand able to harvest the triplets of the phosphorescent material

Phosphorescent molecule

4e-

S0

S1

T+< >

PH

S0

S1

T

4h+

Matrix

Energy Trasfers Further Advantage:red-shifted emission with respect to bulk⇒⇒⇒⇒ Reduced self-absorption

•Foerster transfer: Resonant (dipole-dipole) long range (up to 100 Å) ~1/R6

mostly Singlet excitons

•Dexter trasfer (electron exchange between D and A)short range (up to 10 Å) ~ e-R

Singlet and Triplet excitons

601)(

R

Rk

DDA τ

=Transfer rate

( ) ( )dEEFEn

R DAD

o ∫Γ≈ εη

4

2FoersterRadius

spectral overlap

R0≈≈≈≈10-100 Å

Energy Trasfer

Best FRET efficiencyD

AθA

θD φ

ΓΓΓΓ2 = 4

ΓΓΓΓ2 = 1

ΓΓΓΓ2 = 2/3

ΓΓΓΓ2 = 0

Phosphorescent molecules in polymer matrices

Energy Transferhost→dye

Exciton is generated on the matrix and is transferredto the dye through Foerster or Dexter transfer

Long lifetimes (∼∼∼∼100µµµµs) of T excitonfavours T-T annihilation for high current densities→ need of low concentrations of dyes

S1 and T1 of the matrtix must besuitable for ET to blue dyes, avoiding backtransfer→ need of hosts with high T energy level

PH

ISCS1

S0

T1

S1 legante

} CT legante-metallo

dye

S1

+< >

host

4e-

4h+

T1 host dye

h+

e-

One or both the charges are trapped on the dyewhere exciton is formed and recombine

Exciton is generated on the dye

Charge Trapping on the dye

Polymer = hostEmitting Molecule = dye

Transfer Processes in Semiconducting Polymer Porphyrin Blends Adv. Mater. 13, 44 (2001)By V.Cleave, G. Yahioglu, P. Le Barny, D-H Hwang, A.B. Holmes, R.H. Friend, and N. Tessler

1- Offset livelli HOMO e LUMO Ricombinazione di carica

2- Sovrapposizione spettrale Trasferimento di energia

Dye= Shallow trap for both e- and h+ (e-h+) formed at Host

Spectral matching ET Host� Guest

Dye= Deep trap for h+ (e-h+) ricombination at Guest

1Separazione di caricaquenching emissione

2En. polimero < En. dye

ET Guest � hostquenching emissione

WOLED

Multilayers WOLEDs ( deposition of low molecular weight molecules by thermalevaporation under vacuum)Single layerWOLED (solution processable materials)

Outlook:EL from nanostructured organic and hybrid materials ?

•White light: fundamental parameters (efficiency, CIE, CRI)

•White light with molecules, polymers and hybrid systems: -balance of emissions from Singlet and Triplet states-control of Energy Transfer

CIE (x;y) coordinatesColor is exactly identified

(0.33;0.33) white light

pure colors are on the borders

Color

Quality of white light: CRI and CCT

CRI = Color Rendering IndexHow good the color of an object is obtained by illumination (CRI = 0-100)

CCT = Correlated Color TemperatureBlach body Temperature“warm” Light CCT=2500-3700K“cold” Light CCT=4700-7000K

Wien law: λmax = b/T

CIE coordinates similar to those of a black body with CCT between 2500 and 6500 K

•Color Rendering Index CRI > 80

White Light: Requests for lightening

Fluorescent50-100 lm/W, 20000h

Power consumption

Lightening absorbs about 22% of the produced total electricity→ a small increase of the lightening efficiencycan induce largeenergy saving

Incandescent Lamps12 lm/W , <2000h

Losses due to heating

100% Efficiency means 680 lm/W

Discharge Lamps use Hg gasUV emission converted by

Fluorescent materials

problems for the environment !

Inorganic LEDs

disadvantages:Low color qualityPoint sources, needs diffusorsSensible to heatingCost

Solutions:Phosphors to increase CRIOptical systems as diffusersHeat dissipatorsCost is balanced by long lifetimes

New technology of remote phosphors

Lumen is photometric equivalent to Watt:it is wheighted with the spectral response of uman eye

1 watt at 555 nm = 683.0 lumensNight vision Day vision

ILLUMINANCE:1 lm/m2 (lumens per square meter)= 1 lux (lx)= 10-4 lm/cm2

IRRADIANCE:1 W/cm 2 = 10 4 W/m 2

= 6.83 x 106 lux at 555 nm

Efficienies:ηquantum~nphoton/n charge (%)ηcurrent~ IL/Jc (cd/A)ηpower ~ IL/JcV (lm/W)

1 lumen=1/(4ππππ) candela (isotropa)

Lightening needs :ηpower =100 lm/W at 1000 cd/m 2

CRI > 80, lifetimes > 20000h

Lamps Efficiencyη~ηηηηpow/ηηηη max (%)

UNIT OF MEASURE OF LIGHT INTENSITY AND EFFICIENCIES

lightening with LED or OLED?

WOLED:Universal Display:

100 lm/W (at 1000 cd/m 2)CRI = 70, CCT 3900 K

INORGANIC LED packages:134 lm/W (cool:CRI=70-80;CCT=4746-7040K)96 lm/W (warm: CRI=80-90;CCT=2580-3710K)

Large AreasColor QualityLow Cost

06.04.2008 OLED Lighting - Vision becomes reality

OSRAM and Ingo Maurer mark lighting history

Philips

Novaled

Performances of different lightening systems

Adv. Mater. 2011, 23, 233

Companies investing in WOLEDS

Strategies for WOLED

• Stacked devices (two or more) each of one emits a differentcolor (only molecules)

• Device with multiple layers (only molecules)

• LED with single layer emitting white light (molecules and/or polymers)– Mix of 2 or 3 materials– One material : emission from monomer + excimer– One material: copolymer

White light can be obtained as sum of two complementary colorsor three primary colors

Low cost deposition techniques (deposition from solution)

High cost deposition techniques (evaporation in vacuum)

•SOLED: Stacked OLEDs

In the mid 1990’s, a highly-innovative SOLED™ stackedOLED was conceived by Dr. Stephen R. Forrest and histeam at Princeton University.

the SOLED is based on stacking the red, green, and blu e subpixels on top of one another, instead of side-by-s ideas is the common configuration for CRTs and LCDs, in a vertically-integrated OLED structure where intensity, color and gray scale can be independently tuned toachieve high-resolution full-color. While SOLED architectures may find application as a high-resolutionpixel approach, there are a number of new possibiliti esfor stacked and hybrid structures in lighting and otherdevices that build on these earlier innovations.

S.Tokito et al.,Curr. Appl. Phys. 5 (2005) 331

blu emissionηηηηEXT=10 %, 20cd/Anew host withET=3eV

intermediate layerto avoid voltage

dependence of the color! critical growthconditions

White light from 2 colors

•Multiple layers: two colors

Management of singlet and triplet excitons for efficien twhite organic light-emitting devicesStephen R. Forrest et al. Nature, 440, 908 (2006)

ηηηηp = 37.6 lm/W, ηηηηext = 18.7 % At a practical surface luminance of 500 cd/m 2, ηηηηp = 23.8 lm/W, approximately 50% greaterof common incandescent lighting.

Different harvesting of singlet and triplets gives high CRI = 85 at all current densities studied

Triplet diffusion length L D = (460 ± 30) Å, (75 ± 5)% of the phosphoresce results from Tripletexciton diffusion from the adjacent EML interfaces

CRI = 85, CIE (0.40;0.40)reduced sensitivity of ηηηηextto current densityabsence of triplet–tripletannihilation suggests that the highest density of triplet excitonsis at the interfaces in the fluorescent doped regions

Low doping 5%

Layer undopedX>R Foester

•Multiple layers: three colors

Triplet-Harvesting ConceptK. Leo et al. Adv. Funct. Mater. 19, 1319 (2009);Nature 459,234 (2009); Adv. Mater. 19, 3672 (2007).

•Multiple layer: three colors

Fluorescent blue emitterwith hight T level

Phosphorescent orangeemitter

Phosphorescent green emitter

←e-

h+→

←

T

S T

single layer WOLED

•BLENDS with different MATERIALS•Polymer Blends of 2 or 3 polymers•Blend of a polymer with fluorescent and/or phosphorescent dye/dyes

Segregation and demixingControl of the relative concentration(very low concentration of one or two species)

•SINGLE MATERIAL•Block Copolymers with 2 or 3 blocks•Copolymers- fluorescent and/or phosphorescent dye/dyes

Difficult synthesis

Film from solution Low cost technologies

Strategies for single layer Strategies for single layer WOLEDsWOLEDs

Multi-components system (blends) Single compound

+

+

++

+

white emission

0

1

300 400 500 600 700 800Wavelength (nm)

PL

ELelectroplexPVK:FSB

electroplexPVK:PBD

electromerPVK

ηELEXT = 0.52 %

ITO/PEDOT:PSS/blend/Ca/Al

CIE (0.22;0.23)

PVK : FSB

h-transp fluorescentdye

N

FF

F

FF

PBD :

e-transp

N

O

N

Broadening of the emission zone from blue towards white

Appl. Phys. Lett. 87, 171910 (2005)

Blend one polymer with two molecules

0

500

1000

1500

2000

400 450 500 550 600 650 700 750

PL of Tb(hfa)4P(ph)

4 in CH

2Cl

2

PL

PL

(a.u

.)

wavelength (nm)

5D4

7F6

5D4

5D4

5D4

5D4

7F5

7F4

7F3

7F2

0

5000

1 104

1,5 104

2 104

2,5 104

450 500 550 600 650 700 750

PL of Eu(tta)4N(et)

4 in CH

2Cl

2

PL

PL

(a.u

.)

wavelength (nm)

5D0

5D0

5D0

5D0

7F1

7F2

7F3

7F4

NCH

CH2

n

300 400 500 600 700

PL

inte

nsity

(a.

u.)

Wavelength (nm)

PVK

350 400 450 500 550 600

PL

inte

nsity

(a.

u.)

Wavelength (nm)

PBD

O

NNt-Bu

Blend one polymer with two Org-Ln

Phys. Chem. Chem. Phys., 2009, 11, 10152

9,5 V 18 V

low Vhigh V

x=0,33y=0,45

EL 14V

0

0,2

0,4

0,6

0,8

1

350 400 450 500 550 600 650 700 750

EL

inte

nsity

(a.

u.)

wavelength (nm)

Single copolymer compound

PFTPA

balanced BG light forend-capped copolymer

F8BT polymer

ROD-ROD Block-Copolymer Max EQE

1-2%

Max EQE

1-2%

Macromol. 42, 1107 (2009)

Device optimization

Max EQE >5.5%

33lm/W

50000cd/m2

Max EQE >5.5%

33lm/W

50000cd/m2

Chem Mater. 23, 810 (2011)

NNn

C8H17C8H17

CH3

CH3

CH3

CH3

+

+MultiMulti --componentcomponent systemsystem

Single Single compoundcompound

CIE .31 .24

J. Phys. Chem. C, 113, 2290 (2009)

Max EQE

1%

Max EQE

1%

ITO/PEDOT:PSS/blendblend /Ca/Al

N N

S

O

n

OF3CEu

N

N

S

O

O

F3C

S

O

OCF3

Polymer with Phosphorescent compounds

MultiMulti --componentcomponent systemsystemall polymeric, fully compatible mixture

++

Comp.EQEMAX

[%]L (at 7V)[cd/m2]

CIE (1931)

EL

PFTPA 1.5 295 (0.18;0.11)

G core 0.7 20200 (0.37; 0.58)

R core 0.6 5900 (0.62; 0.37)

blend1 0.2 8300 (0.30;0.42)

blend2 1.3 10000 (0.32; 0.33)

Organic Electronics 11, 2012 (2010)

• High-quality white emission (CRI 90,

CCT 5893K) for (R:G:B) of (0.3:0.45:99.25)

• High chromatic time stability

• Bias dependent color

N N

N

SN

NN

SS

n

n

Single material: Copolymer with RGB components

C-F. Shu and co-workers.Adv. Funct. Mater. 2007

Al/CsF/polymer/PEDOT/ITO

Chlorobenzene spin-coatedCarrier balance by oxadiazoles and triphenylamine

main chain type3-colours

EQE 4%, 8.2 cd/A,

7.2 lm/W, 1000 cd/m2

CIE (0.33,0.36),

CRI 82

ET HT

Chem.Phys.Chem. 11, 683 (2010)

excimerelectromer

systems containing pyrene units (excitonand excimers) as energy donors and a styrylpyridiniumderivative as energyacceptor

Towards single Towards single dendronicdendronic materialmaterial

Dendronicantennae

White lightHost PVK matrix for electroluminescence

(0.35; 0.40)

Stefano CicchiUniv. Firenze

Self-organized micro- nano- structuresfor OLEDs applications

Device reduced sizes improve integration capabiliy and increasesnumber of functions/volume

Organic/Organic or Organic/Inorganic nanostructures can improve OLED’s performances by:• separation of radiative recombination from charge transport, • ordered micro-pattern useful for pixeling

The auto-assembly ability of organics allows to easilyobtain ordered micro and nano-structures for a device

The auto-organization in hybrid nanostructures improvesthe stability of organic materials

4 µµµµm

25 µµµµm

10µµµµm

Fluorescent Nanofibres embedding dye-loaded zeolites

Elettrospun FluorescentNanofibres

1 µm

Zeolites with StopcockThe stopcock has a head that can not enter the channel : � provides a link between the outside and the inside of the zeolite

Conjugated polymer fibres� Polymer in contact with the zeolite

How to obtain ET from a polymer to the dyes into the zeolites?

Thanks to the presence of the stopcock we get a two step energytransfer from polymer to the dyeinside the zeolite

V. Vohra, A. Devaux, L-Q. Dieu, M. Catellani, G. Calzaferri, C. Botta, Adv. Mater. 21, 2009, 1146

Functionalized zeolite crystals

Embedded in PFTPA/PEO

Advantages of this system: - larger choice acceptor molecules (no specific orientation)- more donor/acceptor pairs - molecules covalently bound � more stable system- increased compatibility with the polymer � higher zeoliteconcentrations without aggregate formation

Oxonine loadedzeolite L crystal

Ox+ Ox+ Ox+

Functionalized with tetra-hexylsexithiophene

Efficient Two step energy transfer

Förster radiusPFTPA/T6 : R0 = 57,3 ÅT6/oxonine: R0 = 50,1 Å

4 50 5 00 5 50 6 00w av e le ngth

PLE: Excitation profiles:Spotted: PFTPA/PEO/OxZLT6 (film)Dashed: OxZLT6 (powder)Solid: OxZL (powder)

PLPLE

V. Vohra et al. ACS Nano, 41409 (2010)

White light emissionfrom the nanofibers with a single

excitation @ 380 nm CIE :(0,33; 0,26)

Electroluminescence from a conjugated polymer nanofiber

V. Vohra et al. ACS Nano, 7, 5572 (2011)

EQE 1.1% and 2300 cd/m2 at 6 V

EL from the ribbon-like(110nm-700nm cross section)

electrospun nanofibres

Acknowledgements

• Varun Vohra, Dr. Sami Yunus, Dr. Fabrice Spano, Dr. Marinella Catellani, Dr. Umberto Giovanella, Dr. Alberto Bolognesi, Dr. Williamo Porzio, Dr. Silvia Destri, Dr M Pasini, Dr. Guido Scavia(ISMac-CNR, Milano)

• Prof. Gion Calzaferri, Dr. André Devaux(University of Bern)

• Prof. Luisa De Cola, Fabio Cucinotta (Univ. of Muenster)

• S. Cicchi (UNIFI and INSTM), M. Melucci (ISOF), G. Farinola(UNIBA), D. Roberto (UNIMI), G. Zotti, B. Vercelli (IENI-CNR)

•the European Commission through the Human Potential Program (Marie-Curie RTN “Nanochannel” and 'Nanomatch' Contracts)•CARIPLO Foundation – SOLCO

WOLEDs: MIUR -FIRB RBNE03S7XZ SINERGY -FIRB-RBIP06JWBH NODIS -FIRB-RBIP0642YL LUCI -PRIN 2007PBWN44

CARIPLO Foundation –WOLED

Regione Lombardia - IndoLED