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1
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|
17
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)
20
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)
21
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
22
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
23
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).
24
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]
25
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
27
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
29
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).
_+
30
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).
31
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).
32
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
33
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