the chemistry, physics and engineering of organic light ... · the chemistry, physics and...

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The Chemistry, Physics and Engineering

of Organic Light Emitting Diodes

George G. MalliarasDepartment of Materials Science and Engineering

Cornell University

Electronics go everywhere

Pioneere-Ink & Lucent

Electrolux

2

Outline

• Materials

• Device Principles

• Device Physics

• Degradation

• Applications

Organic semiconductors

Molecularly Dispersed Functional Small MoleculesPolymers (MDP) Polymers

3

Materials for OLEDs

Carbon as a semiconductor

CH2=CH2

En = n2

n=1,2,3,...

ħ2π2

2mL2

LUMO

HOMOEG ≈ ħ2π2

2maN

• Hybridization: sp2 and pZ

• Particle in a box:

EG

4

Tuning of optical propertiesBlue

Red

R.E. Gill et. al., Adv. Mater. 6, 132 (1994).

Covion

Optical properties of polythiophenes

R.E. Gill et. al., Adv. Mater. 6, 132 (1994).

5

PC:TPD hole mobility

10-6

10-5

10-4

10-3

Mobil

ity (

cm2

/Vse

c)

10008006004002000

E0.5

((V/cm)0.5

)

30%TPD

40%TPD

60%TPD

80%TPD

50%TPD

100%TPD

Materials for OLEDs (II)

6

Materials for OLEDs (III)

Advantages

• Ease of processinglarge area filmsflexible substrates

• Optoelectronic propertiestrap free transporttunable energy gaphigh luminescence efficiencylarge absorption coefficient

7

Applications

• Light emitting diodes

• Thin film transistors

• Photovoltaic devices

Summary I: Materials

• Different families of organic semiconductors. Main difference is in processing.

• Major advantages: Processing, taylor-made properties.

8

Outline

• Materials

• Device Principles

• Device Physics

• Degradation

• Applications

OLED structure and operation

ITO

Ca

h+

e-

9

Model for single layer devices

Anode

PolymerLUMO Cathode

PolymerHOMO

1 2

3

12

Mechanism involves:

1: Charge injection

2: Charge transport

3: Charge recombination

OLED characteristics

-10 -5 0 5 1010

-11

10-9

10-7

10-5

10-3

Curr

ent

(A)

Voltage (V)

10-11

10-9

10-7

10-5

10-3

Au/MEH-PPV/Ca

Rad

iance (W

)

10

OLED characteristics (2)

ITOAg

Ca

11

Bilayer devices

Anode

Hole-transport layer

Cathode

Electron-transportlayer

Other architectures

Emissive Layer

Dopants

12

Summary II: Device principles

• Electroluminescence involves injection, transportand recombination of opposite charges.

• High and low work-function metals needed for electrodes.

Outline

• Materials• Device Principles• Device Physics

• Introduction• Built-in potential• Charge transport• Charge injection• Ionic space charge effects

• Degradation• Applications

13

Charge injection vs. transport

Is the flow limited by the valve or the hose?

Pedagogical analogue:Water hose and valve

Is the performance injection limited?

After I.D. Parker, J. Appl. Phys. 75, 1656 (1994).

0.0 0.2 0.4 0.6 0.8 1.0

10-4

10-3

Fowler-Nordheim:

I=!V2exp("/V)

Au/MEH-PPV/Au

I/Va

pp

l2

(A

/V2

)

1/Vappl

(V-1)

14

..or is it trap limited?

After A.J. Campbell et.al., J. Appl. Phys. 82, 6326 (1997).

0.1 1 1010

-11

10-9

10-7

10-5

10-3

10-1

Trap limited conduction:

I=!V"

Au/MEH-PPV/Au

I (A

)

Vappl

(V)

Bipolar current more complicated

0.1 1 1010

-8

10-6

10-4

10-2

100

102

Au

Al

Ca

J (m

A/c

m2

)

Vappl

(V)

15

Electron-hole recombination

Jh/JJe/J

Anode Cathodex

b

External Quantum Efficiency: η = b⋅Φ/2n2

Outline

• Materials• Device Principles• Device Physics

• Introduction• Built-in potential• Charge transport• Charge injection• Ionic space charge effects

• Degradation• Applications

16

The built-in potential

It controls the I-V characteristics of OLEDs !

Au

MEH-PPV

Ca

Contact

Electroabsorption

0 1 2 3 40.0

0.5

1.0ITO/MEH-PPV/Ca

|!T

|/T

(ar

b.

un

its)

Vappl

(V)

After I.H. Campbell et.al., Phys. Rev. Lett. 76, 1900 (1996).

|ΔT|/T ∝ |Vbi-Vappl|

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Energy level diagram

0.0

0.5

1.0

1.5

2.0

-0.5

Vbi(V)

LUMO

HOMO

Au

AgAl

Mg

Ca

Outline

• Materials• Device Principles• Device Physics

• Introduction• Built-in potential• Charge transport• Charge injection• Ionic space charge effects

• Degradation• Applications

18

Charge transport in semiconductors

Figure of merit:mobility, µ (cm2/V·sec)

Conductionband

Valenceband

Shallowtrap

Deeptrap

v=µ·E

ε

x

+-

Charge transport in semiconductors (II)

Log(J)

Log(V)

JOHM

JSCL

Lower voltages: Ohm’s lawJOHM = e⋅Ν0⋅µ⋅V/L

Higher voltages: Space charge limitedcurrentJSCL = (9/8)⋅ε⋅ε0⋅µ⋅V2/L3

V0

Question: What is the maximum current that canflow through a (trap-free) semiconductor?

L

JV

Lampert and Mark, Current Injection in Solids (Academic Press,1970).

19

Time-of-flight (TOF)

Light pulse

OSC

RV

20x10-6

15

10

5

0

-5

Curr

ent

(A)

100x10-6

6040200-20

Time (sec)

80% TPD

ttr

µ=L2/Vttr

µ = _______tTR · V

L2

Great book on transport:

P. M. Borsenberger, D. S. Weiss, Organic photoreceptorsfor Xerography (Marcel Decker, Inc., New York, 1998).

Hopping transport

W.D. Gill, J. Appl. Phys. 43, 5033 (1972).

νij ~ exp-(Rij)

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Dispersive transport and universality

F.C. Bos and D.M. Burland, Phys. Rev. Lett. 58, 152 (1987).

t-(1-a)

t-(1+a)

a=0.66

Disorder formalism (I)

H. Scher and E.W. Monrtoll, Phys. Rev. B 12, 2455 (1975).

Gaussian transport

ψ(t) ~ e-t/τ

<l> ~ t

σ ~ t½

σ/<l> ~ t-½

Disorder formalism

ψ(t) ~ t-(1+a)

<l> ~ ta

σ ~ ta

σ/<l> ~ const.

Universality!

(0<a<1)

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Disorder formalism (II)

H. Scher and E.W. Monrtoll, Phys. Rev. B 12, 2455 (1975).

Transport in MPDs

P.M. Borsenberger et al., Jpn. J. Appl. Phys. 37 (1998)

Increase

mobility

Decrease

distance

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Gaussian disorder model

LUMO

HOMO

ε

x

ε

σ = 0.1 eV

DOS

• Energetic disorder • Positional disorder

Gaussian disorder model (II)

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

Density of states:

DOS(ε) = (2·π·σ2)-0.5·exp[-(ε2/2σ2)]

Hopping rate:

νij = ν0·exp-(2·γ·a·ΔRij/Rij)·

Mobility:

µ=µ0·exp[-(2σ/3kT)2]·exp{C·[(σ/kT)2-Σ2] ·E0.5}

{ exp[-(εj-εi)/kT] ;εj>εi

1 ;εj>εi

εiεj

Rij

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Gaussian disorder model (III)

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

Carriers relax at:

σ2/kT

µ ~ exp[-(σ/kT)2]

Comparison with experiment

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

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Correlated disorder (I)

Molecules carry a largedipole moment.

Charge dipole interactioncauses spatial correlationin the energy of hoppingsites.

Correlated disorder (II)

LUMO

HOMO

ε

x

Deeper valleys are also wider.

D.H. Dunlap, P.E. Parris and V.E. Kenkre, Phys. Rev. Lett. 77, 542 (1996).

µ=µ0·exp[-(σ/kT)2 + 2·(σ/kT)·(e·a·E/kT)0.5]

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Correlated disorder (III)

S.V. Novikov, J. Polym. Sci. Part B: Polym. Phys., to appear.

Black/white:sites with energyabove/below the

mean.

Hole mobility in MEH-PPV

1500 2000 2500 3000 3500 4000 4500 5000

1

µ=µ0exp(!E0.5

)

µ0=(2+/-1) 10

-7 cm

2/Vsec

!=(5+/-1) 10-4 (m/V)

0.5

µ (

10-6

cm2

/Vse

c )

E0.5

((V/m)0.5

)

26

µ=µ0exp(γ√E)

JSCL≈(9/8)εε0µ0V2exp[0.89γ(V/L)0.5]/L3

Charge transport

P.N. Murgatroyd, J. Phys. D: Appl. Phys. 3, 151 (1970).

0.1 eV

Electrical characteristics of OLEDs

0 200 400 600 80010

-16

10-15

10-14

Ca

Al

Au (98 nm)

Au (168 nm)

JL3

/(Va

pp

l-V

bi)2

(m

Acm

/V2

)

((Vappl

-Vbi)/L)

0.5 ((V/cm)

0.5)

G.G. Malliaras et.al., Phys. Rev. B 58, R13411 (1998).

JSCL≈(9/8)εε0µ0V2exp[0.89γ(V/L)0.5]/L3

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Efficiency of MEH-PPV OLEDs

0 1 2 3 410

-5

10-4

10-3

10-2

10-1

100

101

102

Ca

Al

! q (

%)

Vappl

-Vbi (V)

Outline

• Materials• Device Principles• Device Physics

• Introduction• Built-in potential• Charge transport• Charge injection• Ionic space charge effects

• Degradation• Applications

28

Quantifying the injection process

Injection efficiency: η = JINJ / JSCL

(Contact supply / Bulk demand)

JSCL = (9/8)⋅ε⋅ε0⋅µ⋅V2/L3

JINJ = ??? (Thermionic emission, Tunneling)

Injection efficiency measurements

M. Abkowitz et.al., J. Appl. Phys. 83, 2670 (1998).

_ + _+

Time-of-flight Injection

Blockingcontact

Contactunder test

• Measure mobility → Calculate SCLC• Measure injected current

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Hole injection in PC:TPD

10-8

10-6

10-4

10-2

100

J (A

/cm

2

)

12008004000

Voltage (V)

50% TPD JSCL

100% TPD JSCL

50% TPD JINJ

100% TPD JINJ

A simple injection model

J = Cexp(-ϕB/kT) -en0S(E)

• Surface recombination as a hopping process in

the image charge potential.

• No current flow at zero field.

C = 16πεε0N0(kT)2µ/e2

S(0) = 16πεε0 (kT)2µ/e3

J.C. Scott et al., Chem. Phys. Lett. 299, 115 (1999).

_+

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Outline

• Materials• Device Principles• Device Physics

• Introduction• Built-in potential• Charge transport• Charge injection• Ionic space charge effects

• Degradation• Applications

Chelated complexes of Osmium

-

2+ LUMO

HOMO

π* of ligand

t2g ofmetal

S. Bernhard et. al., Adv. Mater. 14, 433 (2002).

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Device model

Anode

Cathode

Anode

Cathode

t = 0 sec t >> 0 sec

–

+

h+

e-

Device characteristics

At 3V:

ηQE = 1%

300 cd/m2

ITO/[Os(bpy)2(L)]2+(PF6-)2/Au

10-7

10-6

10-5

10-4

10-3

10-2

10-1

Curr

ent (A

)

806040200

Time (min)

10-7

10-6

10-5

10-4

10-3

10-2

10-1

Rad

iance (W

)

4V

6V

Slow response, indicative of ionic motion

S. Bernhard et al., Adv. Mater. 14, 433 (2002).

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Summary III: Device physics

• Built-in potential important for understanding OLED characteristics.

• Physics of charge injection and transport is different than in crystalline semiconductors.

• Ionic space charge can lead to ohmic contacts.

Outline

• Materials

• Device Principles

• Device Physics

• Degradation

• Applications

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Degradation of the cathode

(a) 2 min (b) 10 h (c) 20 h

(d) 30 h (e) 40 h

Courtesy of Dr. Homer Antoniadis

CsF/Al cathodes

10-10

10-8

10-6

10-4

10-2

Cur

rent

(A

)

86420

Voltage (V)

ITO/MEH-PPV/CsF(2Å)/Al ITO/MEH-PPV/Al

10-8

10-7

10-6

10-5

Rad

ianc

e (W

)

86420

Voltage (V)

34

Thickness dependence

10-3

10-2

10-1

100

Qua

ntum

Eff

icie

ncy

(%)

6040200

CsF Thickness (Å)

Ca cathode

ITO/MEH-PPV/(xÅ)CsF/Al

Degradation of the organic

H. Aziz et. al., Science 283, 1900 (1999).

Layered device Mixed emission layer

35

Encapsulation

glass

glass

sealant

OLED

Outline

• Materials

• Device Principles

• Device Physics

• Degradation

• Applications

36

RGB schemes

Patterned emitters

Microcavities

Color filters

Fluorescent converters

Stacked

Organic light emitting diodes (OLEDs)

Pioneer(2001 - demo)Pioneer

(1997)

Motorola(2001)

Kodak(2004)

Sony(2004)

37

OLEDs vs. liquid crystals

OLEDs vs. Liquid Crystals

• Low power consumption

• High intensity/low voltage

• 180o viewing angle

• Very flat

• No backlights/polarizers/color filters

38

OLEDs for lighting

GE

Summary IV: Prospects

• After ~15 years of development OLEDs are on the threshold of widespread commercialization.

• Key areas for fundamental research:• New materials• Interfaces