organic light-emitting diodes: basic concepts

1

Organic Light-Emitting Diodes:Organic Light-Emitting Diodes:

Basic ConceptsBasic Concepts

Bernard Kippelen

2

Organic Display TechnologiesOrganic Display Technologies

Uniax/Dupont

CDT/Seiko Epson

Philips

Pioneer

eMaginUDC

3

4

Flat panel displaysFlat panel displays

• Wall-mount TV• Computers• Car Navigators• Replace paper ?

$20 billion market

LCD

86%

Tremendous Market in theinformation-oriented society

5

Flat panel display technologies

• Liquid Crystal Displays- Backlight- High power consumption- Limited viewing angle- Slow response - High manufacturing cost

• Emissive technologies- Plasma- Field Emission

- AC thin film EL (ACTFL)- Organic LEDs

Source: SHARP

6

Design of Organic LEDsDesign of Organic LEDs

7

Light weight Structural flexibility Low power consumption Low dc drive voltage High brightness (100,000 cd/m2)

Fast response time (ns) Thin (< 1 m) RGB, white Large viewing angle Large operating temperature range

AdvantagesAdvantages

8

Introduction to organic electroluminescenceIntroduction to organic electroluminescence

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Charge and Energy TransferCharge and Energy Transfer

10

Anode

Cathode

ETL

ETL

HTL

HTL

++

En

erg

y

TPDX+ + AlQ- TPDX + AlQ*

Introduction to organic electroluminescenceIntroduction to organic electroluminescence

11

EC

H ole tran sp ort

H o le In jec tion

E lec tron tran sp ort

E lec tron In jec tion

O L E D s

L an g evinR ecom b in a tion

S in g le t/Trip le tB ran ch in g

E xte rn a lC ou p lin g

F lu orescen ceE ffic ien cy

EA

h e

R S

F

E

Device Quantum Efficiency:

= R . S . F . E

Physics of OLEDsPhysics of OLEDs

12

Fundamentals of Charge Transport in Organic Fundamentals of Charge Transport in Organic SolidsSolids

Crystals: periodic structures, band model, delocalization, electron in conduction band, hole in valence band

Amorphous organic materials: localized charge in the form of a radical ion, intersite hopping through a hopping site manifold

13

Hole transport and electron transportHole transport and electron transportD+ + D D + D+ A- + A A + A-

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Transport in Organic Transport in Organic SemiconductorsSemiconductors

Benchmark: amorphous silicon 0.5 –Benchmark: amorphous silicon 0.5 –1 cm1 cm22/Vs/Vs

15

TOF experimentsTOF experiments

2

Vv E

L

L L

v V

NN22 laser, 337 nm, 6 ns laser, 337 nm, 6 ns

R = 10R = 1022 –10 –1044 , C = 10 pF, RC , C = 10 pF, RC << <<

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0 50 100 150 200 250 300

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8110801aMB7_b

No

rma

lize

d p

ho

tocu

rre

nt

(a.u

.)

Time (s)

70 V/m 60 " 50 " 40 " 30 " 20 "

0 20 40 60 80 100 120-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4110801bMB7_b

No

rma

lize

d p

ho

tocu

rre

nt

(a.u

)

Time (s)

E = 40 V/m

30 0C 40 " 50 " 60 " 70 " 80 " 90 "

Field dependenceField dependence Temperature Temperature dependencedependence

Field and temperature Field and temperature dependencedependence

17

2 2

2 1/ 20

2( , ) exp exp

3 B B

E T C Ek T k T

The disorder formalism The disorder formalism (Bassler and (Bassler and

Borsenberger)Borsenberger)Transport occurs by hopping through a manifold of Transport occurs by hopping through a manifold of localized states with energetic and positional localized states with energetic and positional

disorderdisorder

Energetic Energetic disorder: width disorder: width

Positional Positional disorder width: disorder width:

Distributions are Distributions are Gaussian Gaussian

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0 200 400 600 800 1000

1E-6

1E-5

1E-4

III

II

I

TPD

(c

m2 /V

s)

E1/2 (V/cm)1/2

Field dependenceField dependence

Mobility Mobility follows field follows field dependence dependence predicted by predicted by the disorder the disorder formalismformalism

2 2

2 1/ 20

2( , ) exp exp

3 B B

E T C Ek T k T

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Dipolar contribution to the Dipolar contribution to the energetic disorder: energetic disorder:

Random distribution of dipoles Random distribution of dipoles generates fluctuations in generates fluctuations in electrostatic potential electrostatic potential

2 2 2 2d vdW D

dd molecular molecular dipoledipole

vdWvdW van der van der Waals Waals contributioncontribution

DD matrix matrix (=0)(=0)

To appear in Chem. of Mater.

20

1E-7 1E-6 1E-5 1E-4 1E-3 0.01 0.1 1

Mobility (cm2/Vs)

TPD*

TPD:PC* (50wt.%)

*Values measured at 20V/

PMPS*PVK

DPQ*

AlQ*

NTDI* PyPySPyPy* Bphen

Hole and Electron Mobility in Non-Crystalline MaterialsHole and Electron Mobility in Non-Crystalline Materials

PBD

21

Fundamentals of radiometryFundamentals of radiometry

OpticsOptics RadiometryRadiometry

Power: [Watt]

Intensity: [Watt/cm2]

Energy: [Watt] x [time] = [Joule]

Energy Q: [Joule]

Flux (power) : dQ/dt [Watt]

Intensity I: d /d [W/sr]

Radiance L: dRadiance L: d22 /dAcos/dAcosdd

[W/sr.m[W/sr.m22]]

: angle between the normal of the surface and the line of sight.

Radiance: power per unit area per unit of projected solid angle

A source is characterized by its radiance

22

Fundamentals of radiometryFundamentals of radiometry

Formula for radiative transfer:Formula for radiative transfer: 1 1 2 22

cos cosdA dAd L

1

2

dA1

dA2

Exitance E = d/dAPower radiated per unit area

Incidance M = d/dAPower received per unit area

23

Fundamentals of radiometryFundamentals of radiometry

For a point source:For a point source:

2

S LE

Z [W/m2]

S: source area with radius r

Z: distance from source to detector

L: radiance of the source

S

Z

For an area source:For an area source:

Z/r >5

M L

(and not 2L)

In both cases, one measures intensity (in the optics definition in W/m2) and deduce the radiance of the source

24

Fundamentals of photometryFundamentals of photometry

In radiometry: radiance given in W.m-2. sr-1

In photometry: radiance given in lmlm.m-2.sr-1lm = lumenlm = lumen

lm.m-2.sr-1 = cd.m-2 = nit

( ) ( )mK V d With Km = 683 lm/W at 555 nm

1 W of optical power per cm2 per steradian of monochromatic light at 555 nm has a radiance of 683 cd/cm2 = 6.83 x 106 cd/m2

25

Photopic response of the human eyePhotopic response of the human eye

400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

1.2041200a

Photopic response of the human eye

Nor

mal

ized

res

pons

e

Wavelength (nm)

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Examples of luminance levelsExamples of luminance levels

3,000,000

30,000

300

3

0.03

0.0003

0.000003Threshold of

vision

Moonless clear starlight

Snow in full moon

Neon lampSky heavily overcast

day

Fluorescent lamp

Upper limit for visual tolerance

cd/mcd/m22

The sun: 900,000,000 cd/m2

27

CIE color chartCIE color chart

( ) ( ) (

( ) ( ) (

)

)

( ) ( ( )

)Y

X k S T x

Z k

k S T y

d

T z

d

S d

CIE: Commission Internationale de l’Eclairage

Xx

X Y Z

Yy

X Y Z

Tristimulus values

x + y + z = 1

Color coordinates

3 kind of sensors in

the eye

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Light Emission in Organic SolidsLight Emission in Organic SolidsSelection rules

Spin selection ruleSpin selection rule Parity selection ruleParity selection rule

Forbids electronic transitions between levels

with different spin

S0

S1

T1

T2

1Ag

1Bu

2Ag

Forbids electronic transitions between

levels with same parity

29

Fundamentals of Energy Transfer Fundamentals of Energy Transfer

D* + A D + A*

D and A molecules separated in space but coupled by the electric field associated with the excited molecule. Interaction hamiltonian has two

contributions:

Coulomb interactionCoulomb interaction Exchange interactionExchange interaction

* (1)D (2)A(1)D

* (2)AInitial

Final

* (1)D (2)A(2)D

* (1)A

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Förster transfer: long range interactionFörster transfer: long range interaction

2 2

6A D

ETAD

kR

i transition dipole moment

2 00

1i i i

i

f k

0i pure radiative lifetime

fi transition oscillator strength

02

6

( )T

DA

D AE

Jk k

k

R

Geometrical factorOverlap integral

Constant

Emission of D

Absorption of A

31

Dexter transfer: short range interaction Dexter transfer: short range interaction

( ) exp( 2 / )ET DAk exchange K R LJ

Dexter transfer is based on two electron transfer reactions and requires proximity of the two molecules short range interaction

Singlet-singlet transfer: allowed both by Forster and Dexter

Triplet-triplet: allowed only by Dexter

J: normalized overlap integral

Not that both Förster and Dexter transfer rates depend on the overlap integral. However, in the case of Dexter the rate is independent of the

amplitude of the extinction coefficient of A.

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Spin considerations Spin considerations EC

H ole tran s p ort

H o le In jec tion

E lec tron tran s p ort

E lec tron In jec tion

O L E D s

L an g evinR ecom b in a tion

S in g le t/Trip le tB ran ch in g

E xtern a lC ou p lin g

F lu orescen ceE ffic ien cy

EA

h e

R S

F

E

P+• + P-• P + P*

+

+

Singlet state

Triplet states

S = 0.25 ? No because singlet and

triplet wavefunctions are different

33

From fluorescence towards phosphorescenceFrom fluorescence towards phosphorescenceCollect all the singlets and triplets: 100% efficiency

S0

S1

T1

S0

S1

T1

kDD

kDD : dipole-dipole (Forster) long range 1/R6

kD

kD : Dexter transfer, short range exp(- r)

ISC through spin-orbit coupling Z5

Baldo et al., Nature 395, 151 (1998), Susuki et al. APL 69 224 (1996) El in

benzophenone at 100 K.

N

N

N

N

Et Et

Et

Et

Et Et

Et

Et

Pt

N

Ir

R

3

R = F, OMe, ...