Organic Electroluminescence by Dieter Neher

8/7/2019 Organic Electroluminescence by Dieter Neher

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2Dieter Neher Organic Electroluminescence

Overview

Introduction into LEDsSome basic considerations and equationsThe real device

cathode-quenching

recombinationsinglet-triplet ratio

Outcoupling of lightexternal efficiencies

microcavity effectsluminance and luminous efficienciesPhosphorescent LEDs


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Organic Light-Emitting Diodes

counter electrodee. g. Ca or Al

-V

glass substrate

transparent electrode

hole transport layer

emission layer

electron transport layer

Typical active layer thickness: 100-250 nm

j

h

T

h+

e

++++

++++


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Organic Light-Emitting Diodes (OLEDs)

N

OAl N

ON

O

Alq3

Poly(p-Phenylen-Vinylen)

First demonstration of efficient LED from organic dye(layers prepared by thermal evaporation)C.W. Tang, S.A. Vanslyke, Appl. Phys. Lett. 51 (1987) 913

Thin layer devices from organic dyes or conjugated polymers

First demonstration of electroluminescence from a conjugated polymer(layers prepared by spincoating)

J.H. Burroughes et al., Nature 347 (1990) 539n


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Basic Mechanisms in Electroluminescence

1 2

3 4

Charge injection Charge diffusion and recombination

Exciton diffusion Exciton recombination and photon emission

+ -

12

3

4

1 2

+ -

h

T

h+e-


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Radiative Recombination

)( fl

T

)()( phfl TSr ++++==== )(25.0)(int PLEL R ====

)(ph

S

?

simple spin statistic: S : T= 1 : 3

pure hydrocarbon materials:(ph)


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Charge Carrier Balance

major

orr

majoror

orR

j

j

jjr

jr min1

min

min

)1(

+= =

HOMO

LUMO

jmajor

ITO 4,7 eV

Ca 2,9 eV

Al 4,3 eV

Au 5,2 eVPolymer

jminorSimplifications:

injection limited currentsdetermined by barrier heights

electrons are minority carriers


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Tuning of Color and Electronic Properties

PPP PPV

O

O

R

R

n

O

O

R

R

n

kn

O

O

Variation of the electronic structure by the defined control of the chemicalcomposition

M. Remmers, D. Neher, R.H. Friend, J. Warman,J.-L. Bredas, Macromolecules 29 (1996) 7432


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Efficiency of Charge Recombination

-5

-4

-3

-2

-1

0

PPP P3A P3V P2VMEH-PPV

h+

e-

Ene

rgy[eV]

Ca

ITO 0.01

0.1

1

10

100

Recombin

ationProbablity[%

]

O

O

R

R

n

O

O

R

R

n

PPP PPV

Compare (PL) with (EL)

)(25.0)(int PLEL R ====


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The Real Device

Questions:

is the spin statistics: S : T= 1:3 correct

is recombination and emission homogenous throughout the layer

under which conditions do all electrons recombine with holes (or vice versa)

is it correct to assume (fl) =(PL)

how many photons leave the device

the following sheets will give some answers!!!!


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Determination of the Emission Zone

ITO

Aluminium

light output

n monolayersof parallelorientation

O

O n

100 - n monolayersof perpendicularorientation

Detection of the polarization of emitted light from a supramolecular stack ofmolecules

Spreading

Transfer


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Recombination and Quenching

0 20 40 60 80 100 120

0%

10%

20%

30%

40%

50%

60%

ITOAl PPP e miss ion Zone

Dis tanc e from Al-e le c trod e [nm]

Quenching

Percentageoflightemittedper

10monolayers(12nm)

J. Grner, M. Remmers, D. Neher, Adv. Mater. 9 (1997) 964

quenchingat cathode exponential

decay


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Effect of Metal Cathodes on Radiative Emission

Quartz substrate

Metal layer

SiO2 spacer

CN-PPV

Laser

light

H. Becker, S.E. Burns, R.H. Friend,PRB 56 (1997) 1893

nrr

r

kk

kPL

++++====)(


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Langevin Recombination

kTr

e

co

====4

2

ceee Aveni ====

(((( )))) )( che rrF ====++++

+

-

-

-

-

E

rc 2

4 cr

(((( ))))

eo

heeeR eneenii ====++++========

(((( ))))

o

he e++++====

Coulomb radius rc:

Electron current towards countercharge:

Recombination current per cation iR:

F(rc)


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Recombination Cross Section

e

he

o

hRhE

enn

++++======== ~

ee

ndx

dn

====

he

he

o

h

E

j

++++====

2

+ -

+ -j

h+ e-

0 2 4 6 80

5

10

15

20

25

30

Efficiency[cd/A

]

Current density [mA/cm2]

R

current density of electrons towards cations

electron absorption coefficient

x


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Recombination Considerations

he

he

o

h

xE

xjx

++++====

)(

)()(

2

injection limited currentsconstant nh, constant E

exponential recombination profile

small jj, < d-1

inefficient recombination

large, > d-1

recombination close to cathode

space charge limited currentsnh(x), E(x)

0 20 40 60 80 100

cathodeanode

x[nm]

nh

[a.u.]

E[a.u.]

largest close to anodeideal recombination conditions


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The Singlet-Triplet Branching Ratio

h+ + e-

(((( )))) (((( ))))

++++

2

1

2

1

spin statistics: S : T= 1 : 3, ifsinglet and triplet excitons have

same formation cross-section: S : T

M. Wohlgenannt, S. Mazumdar, Z.V. Vardeny et al.Nature 409 (2001) 494


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Few Remarks on the Quantum Efficiency of Emission

singlet excitons (and triplet excitons) can be quenching in the presence

of an electric field (exciton dissociation)

Singlet excitons are quenching by charges (via energy transfer)

Is(fl) =(PL)?

In comparison with PL experiments, emission in a LED is in the presence ofan electric field and charge carriers

M. Deussen, M. Schneidler, H. Bssler, Synth. Met. 73 (1995) 123.K.E. Ziemelis,D. D. C. Bradley, R. Friend, J. Rhe, G. Wegner, Phys. Rev. Lett. 66 (1991) 2231.D. Fichou, F. Charra, Synth. Met. 76 (1996) 11.


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External Efficiencies

External quantum efficiency:only fraction of generated photons leave LED

ideal fluorescent emitterideal recombination conditions

n = 1.7:%5)( =EL

ext

SubstrateAnode

Emitter

Cathode

2

int

2

)()(

n

ELEL

ext

=

LED is a Lambert emitter

N.C. Greenham et al., Adv. Mater 6 (1994) 491


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Microcavity Effects

Z

de

Spectral Tuning: I() at fixed angle Angular Tuning: I() at fixed wavelength Efficiency Tuning Avoid emission into leaky modes

de< lc

self-interference of emittedphotons by multiple reflections

because of energy conservation and Fermis Golden Rule, photons can onlybe emitted under conditions of constructive interference

I(,)


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Microcavity Color Tuning

200 300 400 500 600 700 800

0

1

2

3

4

Absorptioncoe

fficient(107m

-1)

W avelength (nm)

EmissionIntens

ity(arb.unit)

H R'

R'

RR'

R'

HR

H Rn

400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

NormalizedIntensity

Wavelength (nm)

Blue (= 460 nm) for d= 60 nmGreen (= 565 nm) for d= 250 nmRed (= 695 nm) for d= 150 nm

LPPP

glass/Au/LPPP/Al

V. Cimrova, U. Scherf, D. Neher,Appl. Phys. Lett. 69 (1996) 608


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Microcavity Field Distribution

25 nm Aluminium

230nm

4 layers SPPP+ 114 layers

Polyglutamate

FWHM

~20nm

Wavelength (nm)

Inten

sityo

fonax

isem

ission

25 nm Aluminium

m=2 mode

~387nm

0

0.05

0.1

0.15

0.2

360 370 380 390 400 410 420 430 440

0 50 100 150 200 250

SimulationExperiment

Intensity

ofon-ax

is

emission

Distance into sample, nm

0.30.7

CHNH CO

CH2

CH2CH2

COOC18H37COOCH3

CH2

CONH CH

O

On

(a)

(b)

S. E. Burns, D. Neher, et al., Adv. Mater. 9 (1997) 395


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Microcavity Optimization of Efficiency

450 500 550 600 6500

2

4

6

8

10

12

Wavelength [ nm ]

Intensity

[a.u.]

solid: exp

dashed: sim

ZnSe

0 nm

30 nm

40 nm50 nm

60 nm

100 nm

top-emitting microcavity diodes

dielectric layer: ZnSecathode: Ca/Mganode Al/Ni H. Riel, S. Karg, T. Beierlein, W. Rie, K. Neyts, JAP in press


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Luminance and Luminous Efficiencies

[[[[ ]]]] ][]/)[( WWlmVlumen EV ====

[[[[ ]]]] PV VWlm ==== )(/

Power efficiency:light power versus electrical power

)(

)()(

EL

ELeUeU

EL

ext

extE

p

========

Luminous efficiency:

luminous flux versus electrical power

Luminous flux V

400 500 600 700

V

(lm

/W

)

max

= 555 nm680

Wavelength (nm)


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Phosphorescent Emitters

400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

Intensity(a.u)

Wavelength (nm)

FirpicIr(ppy)

3

btp2ir(acac)

F

O

F

F

N

N

Ir

NF

O

CH3

CH3

CH3

N

N

N

Ir

CH3

CH3

O

OS

Ir

N

2

Firpic Ir(mppy)3 btp2ir(acac)

Increase phosphorescent rate by addition of heavy metal atoms

Pt(II) Octaethylporphine,

* transition in ligandN

N

NN P t

Ir-complexes,ligand-to metal transition

PtOEP

M. A. Baldo, S. Sibley, M. E. Thompson,S. R. Forrest et al. Nature 395 (1998) 151.


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Guest-Host Systems

Pure phosphorescent dyes can not be used due to triplet-triplet annihilation

HOMO

S1

T1

LUMO

HOMO

S1

T1

LUMO

Host Guest

e

-

h+

3 major processes: charge transfer to guestDexter and Frster transfer for singlet excitonsDexter transfer of triplet excitons


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Selection of Host

Guest T1 energy must be below host T1 level!!

HOMO-LUMO (single particle) gap versus S1 energyS1 gap versus T1 energy

A.P. Monkman, A.D. Burrows et al.,PRL 86 (2001) 1358

T1 = (1.13 S1 1.45 eV)

e.g. green emitting guest

requires large-bandgap host


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Charge and Energy Transfer

Hole-trapping

Exciton-transfer

Phosphor.-quenching

Electron-transfer

Exciton-transfer

V. Cleave, N. Tessler et al.,Adv. Mater 13 (2001) 44

Variation of polymer matrix:


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Charge Carrier Trapping on Guest

P.A. Lane et al., PRB 63 (2001)235206

n

N

N

NN P t

PFO PtOEP

EL spectrum dominated by guestalready at very low concentration

Increase in driving voltage with

increasing guest concentration

ext= 3.5 % V> 0.2 lm/W


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PVK-based Green Phosphorescent Diode

doped into PVK-PBD (40 wt.%)

X. Gong, D. Moses, A.J. Heeger et al., Adv. Mater, 14 (2002) 581

ITO/PEDOT/Emitter/Ca/Al


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Device with Improved Carrier Injection

Use 70 nm PEDOT layer, CsF/Al cathode

improved electron injection: compensation of positive space chargePeak luminance efficiency: 27 cd/A, EQE 7.6 % at 610 cd/m22

Peak PCE: 14 lm/W at 170 cd/m2

0 20 40 60 80 1000

5

10

15

20

25

30

0

5

10

15

PCE[lm/W]

Lumin

anceefficiency[cd/A]

Current density (mA/cm2)

(b)

0 2 4 6 8 10 120

20

40

60

80

0

5000

10000

15000

Currentdensity(mA/cm

2)

Voltage (V)

(a)

B

rightness(cd/m

2)

X. Yang, D. NeherAdv. Mater.


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The Efficiency Record

M. Ikai et al., APL 79 (2001) 15670 lm/W @ 65 cd/A, ext = 19 %

Ir(ppy)3 in TCTA


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Doped Injection Layers

Problem:depending on treatment, ITO has a work function of 4.3-4.9 eVinefficient hole injection into many organic semiconductors

One solution:chemical or electrochemicaldoping of semiconductor

One example:PEDOT:PSSwork function ca. 5.2 eVnot soluble in organic solventsquite transparent

hole-injection layer


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Control of Workfunction

Electrochemical doping ofpolythiophene hole-injection layer

M. Gross, U. Scherf, D. Neher, K. Meerholz,Nature 405 (2000) 661-665.


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Efficiency of Light-Emitting Diodes

Polymer OLED

Small molecule OLED


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eMagin 0.6 Microdisplay

Sony 13

Kodak Camera

Thanks to:

Heike Riel (IBM)Andreas Elschner (HC Starck)

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Organic Electroluminescence by Dieter Neher