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Organic ElectronicsStephen R. Forrest
Organic Electronics: Foundations to Applications
This course is divided into two semesters with the following objectives:
• Semester 1-Foundations topics: Crystal structure and binding, Optical and electronic properties of organics, and materials growth and patterning. This semester covers material in Chapters 1-5.
• Semester 2-Applications topics: Light emitters, light detectors, transistors (including phototransistors), and selected other topics. This semester covers material in Chapters 1-9.
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Organic ElectronicsStephen R. Forrest
Week 2-1
Review of Semester 1: Foundations
Light Emitters 1OLED Basics
DisplaysCh. 6.1, 6.4
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Organic ElectronicsStephen R. Forrest
3
Organic Materials are Interesting Because…
• They have properties that bridge between their individual molecular
and collective (solid state) properties
• They provide deep insights into how the properties of molecules
transform into band structure (via tight binding), conductivity and
excitonic states
• Almost all physical properties result from electrostatic, van der Waals
bonds (vs. chemical bonds) between molecules in the solid state
• Disorder governs characteristics in the solid state
• Their mechanical fragility leads to film growth and patterning that
differ from more robust, inorganic semiconductors
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Organic ElectronicsStephen R. Forrest
Band Structure is Replaced by Energy Levels
ConventionalSemiconductor
OrganicSemiconductor
LUMO: Lowest unoccupied molecular orbital
HOMO: Highest occupied molecular orbital
It is essential to keep your terminology clear: Band gaps exist in inorganics, energy gaps without extended bands are the rule (but with important exceptions) in organics.4
EG
BWConduction Band
ValenceBand
DOS
Energy
EGHOMO
Energy
HOMO-1
HOMO-n••••
••••
••••
LUMO+n
LUMO+1LUMO
••••
DOS
ECBM
EVBM
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Organic ElectronicsStephen R. Forrest
This Chart Explains Why Organic Semiconductors are Unique
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Property Organics Inorganics
Bonding van der Waals Covalent/Ionic
Charge Transport Polaron Hopping Band Transport
Mobility ~1 cm2/V·s ~1000 cm2/V·s
Absorption 105-106 cm-1 104-105 cm-1
Excitons Frenkel Wannier-Mott
Binding Energy ~500-800 meV ~10-100 meV
Exciton Radius ~10 Å ~100 Å
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Organic ElectronicsStephen R. Forrest
Review of optical and electronic properties• What makes organics interesting and unique and
adapted to device applications?
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Organic ElectronicsStephen R. Forrest
van der Waals bonding• Purely electrostatic instantaneous induced dipole-induced dipole interaction
between π-systems of nearby molecules.
A"
B"
A"
B"
t=t0" t>t0"
Medium around the dipole is polarized
U (r12 ) = −
Adisp
r126
: Dispersion interaction
U(r) = 4ε σr
⎛⎝⎜
⎞⎠⎟12
− σr
⎛⎝⎜
⎞⎠⎟6⎡
⎣⎢
⎤
⎦⎥ : Lennard-Jones 6-12 potential (includes core repulsion)
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Organic ElectronicsStephen R. ForrestGROUND STATE WANNIER EXCITON
MOLECULAR PICTURE
GROUND STATE FRENKEL EXCITON
S1
S0
SEMICONDUCTOR PICTURE
CONDUCTIONBAND
VALENCEBAND
Wannier excitonInorganic semiconductors
Frenkel excitonOrganic materials
Dielectric constant ~15binding energy ~10meV (unstable at RT)
radius ~100Å
Dielectric constant ~2binding energy ~1eV (stable at RT)
radius ~10Å
treat excitons as chargeless
particlescapable of diffusion.
Transport of energy (not
charge)
Charge Transfer (CT) Exciton
(bridge between W and F)
Organic Semiconductors are Excitonic MaterialsInorganics Organics
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Organic ElectronicsStephen R. Forrest
Band Structure is Replaced by Energy Levels
ConventionalSemiconductor
OrganicSemiconductor
LUMO: Lowest unoccupied molecular orbital
HOMO: Highest occupied molecular orbital
It is essential to keep your terminology clear: Band gaps exist in inorganics, energy gaps without extended bands are the rule (but with important exceptions) in organics.9
EG
BWConduction Band
ValenceBand
DOS
Energy
EGHOMO
Energy
HOMO-1
HOMO-n••••
••••
••••
LUMO+n
LUMO+1LUMO
••••
DOS
ECBM
EVBM
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Organic ElectronicsStephen R. Forrest
TripletS=1
ms=±1, 0
Singlet and triplet states
(b)$
S=1$mS=1$
S$
(a)$
S=0$mS=0$
ms=)½$
z$ms=½$
S=1$mS=0$
S=0$mS=)1$
SingletS=0
ms= 0
Spatially symm. Spin antisymm.
Pauli Exclusion Principle: Total wavefunctions must be antisymmetric
In phase180o out of phase
10
S mS
ψ (r1,r2;0,0) =12φa r1( )φb r2( ) +φa r2( )φb r1( )( ) α1β2 −α 2β1( )
√
√
S=1
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Organic ElectronicsStephen R. ForrestConfiguration coordinate (Q)
n = 2n = 1
n = 0
n = 2n = 1
n = 0
DFC
DQ
𝑆! → 𝑆"
𝑆! ← 𝑆"
Understanding molecular spectraStatistics of vibronic state filling:
N nl( ) = N 0( )exp −nl!ω l / kBT( )
Vibronicmanifold
Electronicstates
Vibr
onic
prog
ress
ion
Spectral replicas
Stokes, or Franck-Condon
Shift
Stokes, or Franck-Condon
Shift
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Organic ElectronicsStephen R. Forrest
Jablonski Diagrams: Life Histories of Excitons
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Kasha’s ruleThe radiative transition froma given spin manifold occursfrom the lowest excited state.
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Organic ElectronicsStephen R. Forrest
GROUND STATEspin anti-symmetric
Singletspin anti-symmetric
Tripletspin symmetric
Relaxation allowedfast, efficientʻFluorescenceʼ
25% 75%
Phosphorescence enhanced by mixing S+T eg: spin-orbit
coupling via heavy metal atom
100% Internal Efficiency via Spin-Orbit Coupling Heavy metal induced electrophosphorescence ~100% QE
Relaxation disallowedslow, inefficientʻPhosphorescenceʼ
Relaxation allowednot so slow, efficientʻPhosphorescenceʼ
- + +
Baldo, et al., Nature 395, 151 (1998)
100%
x
MOLECULAR EXCITED STATESAFTER ELECTRICAL EXCITATION
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Organic ElectronicsStephen R. Forrest
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Contact'zone'(Exchange:'Dexter)'
Near'field'zone'(FRET:'Förster)'
Intermediate''zone'
Far'field'zone'(Radia?ve:'1/r)'
Energy Transfer
• If excitons are mobile in the solid, they must move from molecule to molecule² The microscopic “hopping” between neighboring molecules = energy transfer
Different transfer ranges accessed by different processes
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Organic ElectronicsStephen R. Forrest
Energy Gap Law
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• The larger the energy gap, the lower the probability for non-radiative recombination. ⇒ As the energy gap of a molecular species decreases, radiative transitions
have a higher probability for non-radiative decay.
kif = Aexp −γ Eg / !ω p( )
γ = logEg
ΩEp
⎛
⎝⎜⎞
⎠⎟−1
Ω= number of modes contributing to the maximum phonon energy,= ½ the Stokes shift.
1.6 2.0 2.4 2.8
104
105
106
107
1
6
5
43
k nr (s
-1)
E00 (eV)
(b)
2
400 500 600 700 8000.0
0.5
1.065432
No
rmal
ized
em
issi
on
(A
U)
Wavelength (nm)
RT 77 K
1(a)
1.6 2.0 2.4 2.8
104
105
106
107
1
6
5
43
k nr (s
-1)
E00 (eV)
(b)
2
400 500 600 700 8000.0
0.5
1.065432
No
rmal
ized
em
issi
on
(A
U)
Wavelength (nm)
RT 77 K
1(a)
1.6 2.0 2.4 2.8
104
105
106
107
1
6
5
43
k nr (s
-1)
E00 (eV)
(b)
2
400 500 600 700 8000.0
0.5
1.065432
No
rmal
ized
em
issi
on
(A
U)
Wavelength (nm)
RT 77 K
1(a)
Shi, S., et al. 2019. J. Am. Chem. Soc., 141(8), pp.3576-3588.
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Organic ElectronicsStephen R. Forrest
Förster:- resonant dipole-dipole coupling- donor and acceptor transitions must be allowed
Acceptor(dye )
Donor
up to ~ 100Å
Donor* Acceptor Donor Acceptor*
Electron Exchange (Dexter):- diffusion of excitons from donor to acceptor
by simultaneous charge exchange: short range
Acceptor(dye)
Donor
~ 10ÅDonor* Acceptor Donor Acceptor*
spin is conserved: e.g. singlet-singlet or triplet-triplet
Energy Transfer from Host to Dopant: A Review
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Organic ElectronicsStephen R. Forrest
Modes of Conduction
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(a) ECBM
(b) (c)
ELUMO
EEAEIP
EVAC
(a) ECBM
(b) (c)
ELUMO
EEAEIP
EVAC
Band transport
Hopping and tunneling transport
• Coherent• Charge mean free path λ>>a•
BW
BW > kBT , !ω 0
Molecule
• Incoherent (each step independent of previous)• Charge mean free path λ~a• Tunneling between states of equal energy is band-like• BW < kBT , !ω 0
EB
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Organic ElectronicsStephen R. Forrest
Transport Bands in Organics• Tight binding approximation is useful due to importance of only nearest
neighbor interactions
• Recall case of dimers and larger aggregates on exciton spectrum. Close proximity of neighbors results in:
• Coulomb repulsion• Pauli exclusionØ Splitting leads to broadening of discrete energies into bands
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Organic ElectronicsStephen R. Forrest
Light Emitters: Objectives
• Learn about vision: what makes a good display or lighting fixture?• Gain a knowledge of how fundamental properties of
organics leads to two important light emitting device types• OLEDs• Organic lasers
• Learn about challenges yet to be met before OLEDs completely dominate the display market• Learn about the challenges for lighting and lasing
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Organic ElectronicsStephen R. Forrest
OLEDs• Basic concepts• Displays and Lighting• R-G-B pixellation• WOLEDs• TOLEDs
• Getting light out• Intensity roll-off and annihilation• Device reliability• Lasing
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Organic ElectronicsStephen R. Forrest
2010: Galaxy Phones Phosphorescent R,G>2 Billion sold ?!
2014-15: 65” and 77” OLED TVs2016: 4K OLED TV
2012: LG 55” & SamsungPhosphorescent TV, $15002017: iPhone X
$7.2
$10.8$12.7
$16.2$18.6
$20.6
$22.8$24.6
$25.8$27.0
$0
$5
$10
$15
$20
$25
$30
2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
$ billion
s
Mobile TV Tablet / PC Other
Source: DisplaySearch, Q2’14
Mobile Growth Tablet /PC Growth TV Growth
AMOLED Display Market Potential
9
Panasonic, Sony, Toshiba….(2017)
LG
AMOLED Displays: Driving the Technology
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Organic ElectronicsStephen R. Forrest
The Future is Flexible
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Organic ElectronicsStephen R. Forrest
Virtual and Augmented Reality Enabled by OLEDs
RequirementsFastBrightUltrahigh resolution
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Organic ElectronicsStephen R. Forrest
White Lighting is Rapidly Becoming a Reality
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Organic ElectronicsStephen R. Forrest
Organic Light Emitting Diode(OLED)
+
electrons and holesform excitons
(bound e--h+ pairs)
some excitons radiate
HOMO
LUMO
recombination region
ETLHTL
E
_
V
+
-
GlassITO
Alq3
TPD
Mg:AgAg
400 500 600 700 800
PL
EL
Wavelength [nm]
Alq3
AlN
O3
TPDN N
Tang & van Slyke, Appl. Phys. Lett., 51, 913 (1987)
hEQE = 1%
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Organic ElectronicsStephen R. Forrest
PPV
GlassGlass
ITO
Al
EL
Energy (eV)
PL a
nd E
L (a
.u.)
-
+
Recombination
region
LUMO
HOMO
hEQE = 0.1%
Recombination zone not well-defined
First Polymer OLED
Burroughes, et al. 1990. Nature, 347, 539.
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Organic ElectronicsStephen R. Forrest
Benefits of OLEDs
• Can be prepared on any substrate - active materials are amorphous
• Low cost materials and fabrication methods, scalable to large area
• Readily tuned color and electronic properties via chemistry
• Can be transparent when off
• Device characteristics• Efficiency ~ 100% demonstrated, white > 120 lm/W• > 1,000,000 hour (100 years) lifetime• Can be very bright: 106 cd/m2, CRT = 100 cd/m2, fluorescent panel = 800 cd/m2
• Turn-on voltages as low as 3 Volts
+
metal cathode
(-)
(+)
substrate
transparent conductor
Organic layers, total thickness < 200 nm
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Organic ElectronicsStephen R. Forrest
OLED vs. Liquid Crystal Displays (LCDs)
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Color Mixing
LCD OLEDPlasmaCRT
Display Technologies
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Organic ElectronicsStephen R. Forrest
Colorfilters
CF glass
TFT glass
Front Polarizer
Rear Polarizer
Backlight
LCCF
LCD/LED Displays
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Organic ElectronicsStephen R. Forrest
Front Polarizer
Cover glass
TFT glassOLED
WOLEDpanel
WOLEDCF
Front PolarizerCover glass
TFT glass
Two Types of OLED Displays
• RGB pixels• Top emitting• Dominates mobile (Samsung)
• WOLED pixels + Color filters• Top emitting• Dominates TVs (LG)
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