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-
14
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
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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
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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
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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
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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
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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
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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
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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, ...