Organic LEDs – part 7
• Solvation Effect - Review
• Solid State Solvation
• Exciton Dynamics in Disordered Organic Thin Films
• Quantum Dot LEDs
Handout on QD-LEDs: Coe et al., Nature 420, 800 (2002).
@ MIT
April 29, 2003 – Organic Optoelectronics - Lecture 20
Electroluminescence in Doped Organic Films
1.
2.6 eV
Excitons formed
from combination
of electrons and
holes
transparent anode
electrons
2.7 eV
trap states
a-NPD
low work function
cathode
Alq3
exciton
holes
5.7eV
6.0 eV
host molecules
(charge transport
material)
2.
Excitons transfer to
luminescent dye
dopant molecule
(luminescent dye)
1
Electroluminescence of x% DCM2 in Alq3 OLEDs
tuning range
35 nm
Intensity [a.u.]
1.0
0.5 %
1.5 %
2.5 %
4.5 %
6%
0.8
0.6
Alq3
0.4
0.2
0.0
400
500
600
700
800
Wavelength [nm]
Alq3
DCM2 in Alq3
low DCM2
high DCM2
2
... change in the spectral position of
aborption/luminescence
aborption/luminescence band
due to change in the polarity of the medium
dipolar
molecule
dipolar
lumophore
dipole - dipole interaction
modifies the energy structure
of the molecules
Ö solvation is a physical perturbation of lumophore’s molecular states
Ö isolated molecule (in a gas phase) and solvated molecule
are in the same chemical state
(no solvent induced proton or electron transfer, ionization, complexation, isomerization)
Solid State Solvation (SSS)
∆E = ∆ ⟨µ • Eloc⟩
Eloc ~ 107 V/cm
⟨µ⟩ > 0
R
Eloc
⟨µ⟩ → 0
as R → large
polar
lumophore
“self polarization”
for strongly dipolar lumophores
dipolar host
with moment µ
(aggregation possible for highly polar molecules)
3
Influence of µ0 and µ1
on Chromatic Shift Direction
∆E = ∆ ⟨µ • E⟩
solute (chromophore)
WITH DIPOLE MOMENT
µ
solvent
µ0 < µ1
excited
state
µ0 > µ1
S1
S1
S0
S0
ground
state
SOLVENT POLARITY
Æ Bathochromic (red) PL shift
SOLVENT POLARITY
Æ Hypsochromic (blue) PL shift
PL of DCM2 Solutions and Thin Film
Bulović et al., Chem. Phys. Lett. 287, 455 (1998).
SOLVENT
DIPOLE MOMENT
0
1.15
1.69
3.9
C6H6 CHCl3 C2H5OH (CH3)2S:O
~ SOLVATOCHROMIC ~
SHIFT
[bathochromic (red) shift]
1.0
Intensity [a.u.]
N
O
NC
thin film
0.5
0.0
CN
600
700
800
Wavelength [nm]
4
Dynamic Relaxation Picture (a.k.a. solvation)
µe, E*g,loc
µe, Ee,loc
2
3
Excited State
Excited State
(non-equilibrium)
(equilibrium)
µg, Eg,loc
µg, E*e,loc
1
4
Ground State
Ground State
(equilibrium)
(non-equilibrium)
Continuum, Dipole in Spherical
Cavity Model:
≡
r
r
2(ε − 1) µ
E loc =
2ε + 1 a 3
Γε
Thin Film Photoluminescence
1% DCM2 in Alq3
Intensity [a.u.]
1.0
0.8
0.6
polar host
µ ~ 5.5 D
635 nm
0.4
0.2
0.0
Intensity [a.u.]
1.0
1% DCM2 in Zrq4
0.8
0.6
non-polar host
605 nm
0.4
0.2
0.0
500
600
700
800
900
Wavelength [nm]
5
A “Cleaner” Experiment
• Employ trace DCM2 so as to effectively eliminate aggregation
• But still appreciably change local medium ⇒ use another dopant!
• should be polar and optically inactive (i.e. wide band gap)
DCM2
Camphoric
Anhydride (CA)
Normalized Intensity
PL Spectra of Different PS:CA:DCM2 Films
1.0
CA %: 25%
0.8
0.6
0%
0.4
0.2
0.005% DCM2 Doping
0.0
1.6
Polystyrene (PS)
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.4
2.5
Energy (eV)
CA Doping and Electronic Susceptibility
Peak PL Energy of PS:CA:DCM2 Films
2.22
Energy (eV)
42 nm red shift from 0 to 25% CA
563 nm
2.20
0.005% DCM2 Doping
2.18
2.16
Results unchanged even for 10x
higher DCM2 concentration:
2.14
2.12
2.10
2.08
DCM2 aggregation not the answer
2.06
605 nm
2.04
0
5
10
15
20
25
Camphoric Anhydride Concentration (%)
Bulk Electronic Susceptibility of PS:CA:DCM2 Films
Epsilon (at 100 KHz)
6
0.005% DCM2 Doping
5
Local fields are responsible for the
spectral shifts…
4
3
ε = 2.44 + 0.13 x (CA %)
2
0
5
10
15
20
25
… and dielectric measurements
suggest a “solvatochromic” effect.
Camphoric Anhydride Concentration (%)
6
Solvation Theory
Dynamic Relaxation Picture (a.k.a. solvation)
µe, E*g,loc
µe, Ee,loc
2
3
Excited State
Excited State
(non-equilibrium)
(equilibrium)
µg, Eg,loc
µg, E*e,loc
Continuum, Dipole in Spherical
Cavity Model:
1
4
Ground State
Ground State
(equilibrium)
(non-equilibrium)
≡
r
r
2(ε − 1) µ
E loc =
2ε + 1 a 3
Γε
Connecting Theory to Experiment
E↓ = E
gas
↓
Evolution of Peak PL Energy for PS:CA:DCM2 Films
r
r
r
− ∆µ ⋅ (Γε µe + Γn 2 µg ) a 3
2.22
Experiment
2.20
2.18
E↓gas constant with CA%
Energy (eV)
n nearly constant with CA%
(ranging from ~1.55 to ~1.65)
∆E↓ ≈ −Γε A
r r
∆µ ⋅ µ e
A=
a3
where
A=
0.62 eV
0.57 eV
0.52 eV
2.16
2.14
2.12
2.10
2.08
2.06
2.04
0.005% DCM2 Doping
2.02
-5
0
5
10
15
20
25
30
Camphoric Anhydride Concentration (%)
Gamma Functional Shape
2.20
1.0
0.7
0.6
2(ε − 1) 

 Γε ≡

2ε + 1 

0.5
0.4
0.3
0.2
0.1
0.0
DCM2 Peak PL in Various Solvents
Slope gives
A ~ 0.55 eV
2.15
0.8
Energy (eV)
Γ (no units)
0.9
2.10
2.05
2.00
1.95
1.90
0
2
4
6
8
ε (no units)
10
12
0.4
Benzene
Toluene
Chloroform
Dichlormethane
Acetone
Acetonitrile
Methanol
0.5
0.6
0.7
0.8
Γ (no units)
0.9
1.0
7
gth
time
len
ve
wa
wavelength
25 nm
Exciton Dynamics in Time Dependant PL
2 ns
time
Dynamic Spectral Shifts of DCM2 in Alq3
• Measurement performed on doped DCM2:Alq3 films
• Excitation at λ=490 nm (only DCM2 absorbs)
10
Alq3
DCM2
8
6
4
2
0
1.7
1.8 1.9 2.0
Energy [ eV ]
2.1
Normalized Intensity
Intensity [ arb. units ]
12
720
Wavelength [ nm ]
680
1.0
2% DCM2:Alq3
0.8
time
0.6
0.4
640
600
0 ns
0.1 ns
0.3 ns
0.6 ns
1.0 ns
1.6 ns
2.2 ns
3.4 ns
6.2 ns
0.2
0.0
1.7
1.8 1.9 2.0 2.1
Energy [ eV ]
~ DCM2 PL red shifts > 20 nm over 6 ns ~
8
Time Evolution of 4% DCM2 in Alq3 PL Spectrum
Wavelength [nm]
Normalized Intensity
750
700
650
600
1.0
550
Spectrum
at 4 ns
0.8
Integrated Spectrum
(0-10 ns)
0.6
Spectrum
at 1 ns
0.4
0.2
0.0
1.6
1.8
2.0
2.2
Energy [eV]
Electronic Processes in Molecules
INTERNAL
CONVERSION
S1
density of available
S1 or T1 states
1-10 ns
PHOSPHORESCENCE
ENERGY TRANSFER
FLUORESCENCE
FÖRSTER, DEXTER
or RADIATIVE
ABSORPTION
Energy
10 ps
T1
>100 ns
S0
9
Time Evolution of DCM2 Solution PL Spectra
2.20
Benzene
2.15
Energy [eV]
2.10
2.05
CHCl3
2.00
CHCl2
1.95
1.90
CH3CN
1.85
600
620
Acetone
640
DMSO
660
Wavelength [nm]
580
680
1.80
0
1
2
3
4
5
Time [ns]
Spectral Shift due to
~ Exciton Diffusion ~
~ Intermolecular Solid State Interactions ~
wavelength shift
35 nm
5
10% DCM2 in Alq3
Time [ns]
4
3
2
1
0
600
650
700
750
Wavelength [nm]
10
Excitonic Energy Variations
Each dye molecule experiences a
different local medium
⇒ variations in excitonic energies
1
2
3
Non-zero width to excitonic
density of states
1
2
Excitonic Density of States
3
Frequency
Intensity
Molecular PL Spectra
Energy
Energy
Exciton Distribution in the Excited State (S1 or T1)
S1 or T1 Energy
~ Time Evolved Exciton Thermalization ~
fwhm
1
hν
2
3
4 log(Time)
EXCITON DIFFUSION LEADS TO REDUCTION IN FWHM
11
2.10
Low E
midpoint
4% DCM2 in Alq3
High E
midpoint
FWHM
600
High E
midpoint
2.05
Energy
620
Energy [eV]
2.00
1.95
640
1.90
Low E
midpoint
Wavelength [nm]
T = 15~125 K
660
1.85
680
1.80
0
1
2
3
4
5
6
7
Time [ns]
x% DCM in Alq3
2.30
540
550
Alq3
560
Energy [eV]
2.20
570
0.2 %
2.15
580
0.4 %
2.10
590
600
2.05
0.6 % 610
2.00
620
2%
630
4.5 %
1.95
0
1
2
3
Wavelength [nm]
2.25
640
4
5
Time [ns]
12
X% DCM in Alq3
2.16
570
580
Energy [eV]
2.12
590
2.08
600
2.04
610
Final Peak PL
2.00
620
630
1.96
0.1
Wavelength [nm]
Initial Peak PL
0.2
0.5
1
2
4
% of DCM in Alq3
Time Evolution of Peak PL in Neat Thin Films
2.45
Alq3
Energy [eV]
2.40
2.35
2.30
Ir(ppy)3
2.25
1
10
Time [ns]
100
13
Parameters for Simulating Exciton Diffusion
Normalized Integrated
Spectral Intensity
observed radiative lifetime (τ)
Förster radius (RF)
1
► Assign value for allowed transfers:
decreasing doping
(5% down to 0.5%)
ΓF
0.1
1  RF 


τ R 
τ = 3.3 ns
0.01
-2 0
2 4
6
8 10 12 14 16
6
0
Time [ ns ]
∆E
Wavelength [ nm ]
Normalized Intensity [ arb. units ]
excitonic density of states (gex(E))
► Assume Gaussian shape of width, wDOS
► Center at peak of initial bulk PL spectrum
► Molecular PL spectrum implied…
750 700
650
600
550
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
FWHMs
Sinit(E) : 0.27 eV
Smol(E) : 0.22 eV
gex(E) : 0.15 eV
1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4
Energy [ eV ]
2.04
2.02
2.00
1.98
620
630
1.96
1.94
610
0.5% DCM2:Alq3
0
1
2
3
4
5
6
Time [ ns ]
7
8
9
Energy [ eV ]
Energy [ eV ]
RF=0.88 wDOS=0.185 eV
RF=0.98 wDOS=0.161 eV
RF=1.08 wDOS=0.146 eV
RF=1.18 wDOS=0.136 eV
RF=1.28 wDOS=0.130 eV
Wavelength [ nm ]
600
2.06
2.06
2.04
2.02
2.00
1.98
1.96
1.94
1.92
1.90
1.88
1.86
1.84
Peak PL of x%
DCM2:Alq3
610
620
0.5%
630
640
1%
650
2%
660
5%
670
0
1
2
3
4
5
6
7
8
Wavelength [ nm ]
Fitting Simulation to Experiment – Doped Films
9
Time [ ns ]
•
Good fits possible for all data sets
•
RF decreases with increasing doping,
falling from 52 Å to 22 Å
•
wDOS also decreases with increasing doping,
ranging from 0.146 eV to 0.120 eV
14
Fitting Simulation – Neat Films
2.40
Ir(ppy)3
wDOS~0.21 eV
2.38
Energy [ eV ]
2.36
2.34
2.32
Spectral shift observed in each material system
•
Molecular dipole and wDOS are correllated: lower
dipoles correspond to less dispersion
•
Even with no dipole, some dispersion exists
•
Experimental technique general, and yields first
measurements of excitonic energy dispersion
in amorphous organic solids
2.30
2.28
2.26
2.24
0
50
100
150
200
250
300
Time [ ns ]
Alq3
wDOS~0.15 eV
2.36
2.80
Energy [ eV ]
2.34
Energy [ eV ]
•
2.32
2.30
2.28
0
10
20
30
40
2.78
2.76
2.74
2.72
2.26
50
NPD
wDOS~0.10 eV
-2
0
2
4
6
8
10
12
14
Time [ ns ]
Time [ ns ]
Temporal Solid State Solvation
upon excitation both magnitude and direction
of lumophore dipole moment can change
FOR EXAMPLE for DCM: µ1 – µ0 > 20 Debye !
~ from 5.6 D to 26.3 D ~
µ0
Eloc
t<0
µ1
Eloc
t=0
µ1
Eloc
t ~ 1 ns
following the excitation the environment surrounding the
excited molecule will reorganize to minimize the overall
energy of the system (maximize µ • Eloc)
15
Exciton Distribution in the Excited State (S1 or T1)
Energy
~ Time Evolved Molecular Reconfiguration ~
∆E
1
2
3
4
log(Time)
DIPOLE-DIPOLE INTERACTION
LEADS TO ENERGY SHIFT IN DENSITY OF EXCITED STATES
Fusion of Two Material Sets
Efficient
Emissive
Photo by F. Frankel
Organic
Semiconductors
Flexible
Absorption [a.u.]
Tunable
Nanoscale
500
600
LEDs, Solar Cells,
Photodetectors,
Modulators, and
Lasers
which utilize the
best properties of
each individual
material.
Colloidal
QDs
400
Hybrid devices
could enable
700
Fabrication of
rational structures
has been the main
obstacle to date.
date
Wavelength [nm]
16
Inorganic Nanocrystals – Quantum Dots
ZnS
CdSe
E
Lowest allowed
energy level
D=
120Å
Absorption [a.u.]
80Å
D
72Å
55Å
45Å
33Å
Quantum Dot SIZE
29Å
20Å
17Å
400
500
600
700
Wavelength [nm]
Synthetic route of Murray
et al, J. Am. Chem. Soc.
115, 8706 (1993).
Photo by F. Frankel
Fusion of Two Material Sets
Quantum Dots
Organic Molecules
NANOSCALE
Alq3
EMISSIVE
N
O
O
Al
N
N
Absorption Spectra
O
CH3
TPD
H3C
N
N
TAZ
N N
TUNABLE
N
NPD
400
500
600
700
Wavelength [nm]
N
PbSe
CdSe(ZnS)
0.9
1.2
1.5
1.8
2.1
N
2.4
Wavelength [µm]
17
Integration of Nanoscale Materials
Quantum Dots and Organic Semiconductors
Oleic Acid or
TOPO caps
ZnS overcoating shell
(0 to 5 monolayers)
Trioctylphosphine oxide
Synthetic routes of Murray et
al, J. Am. Chem. Soc. 115,
8706 (1993) and Chen, et al,
MRS Symp. Proc. 691,G10.2.
HO
O
PbSe or
CdSe Core
Alq3
Oleic Acid
10Å
N
O
O
D = 17-120Å
Al
N
N
O
TPD
(~50Å pictured)
Tris(8-hydroxyquinoline)
Aluminum (III)
CH3
N
TAZ
H3C
N
N,N'-Bis(3-methylphenyl)N,N'-bis-(phenyl)-benzidine
N N
N
3-(4-Biphenylyl)-4-phenyl-5tert-butylphenyl-1,2,4-triazole
NPD
poly-TPD
N
N
N
N
Differences in:
Chemistry
Size
Molecular Organics
Aromatic
“Small”
Quantum Dots
Aliphatic Caps “Big”
N,N'-Bis(naphthalen-1-yl)N,N'-bis(phenyl)benzidine
ÎPhase Segregation
Phase Segregation and Self-Assembly
QD Solution
1
TPD Solution
2
1. A solution of an organic
material, QDs, and solvent…
2. is spin-coated onto a clean
substrate.
3. During the solvent drying
time, the QDs rise to the
surface…
4. and self-assemble into
grains of hexagonally close
packed spheres.
Organic hosts that deposit as flat films
allow for imaging via AFM, despite the
AFM tip being as large as the QDs.
Phase segregation is driven by a
combination of size and chemistry.
chemistry.
QDs
3
TPD/solvent
Substrate
close packed QDs
4
TPD
Substrate
~8nm PbSe QDs
Flat TPD
background
50nm
18
Monolayer Coverage – QD concentration
QD coverage = 20%
50%
80%
500nm
250nm
500nm
90%
100%
As the concentration of
QDs in the spin-casting
solution is increased,
the coverage of QDs on
the monolayer is also
increased.
QD-LED Performance
50nm Ag
50nm Mg:Ag
30nm Alq3
10nm TAZ
Current Density [A cm-2]
100nm
500nm
10
0
10
-2
10
-4
10
-6
10
-8
Control
QD-LED
1
40nm TPD
ITO
Glass
Voltage [V]
10
PbSe/oleic acid
CdSe(ZnS)/TOPO
EA=2.1
TPD
QD
EA=3.0 EA=3.1
TAZ
Alq3 E =3.7
F
Mg
EF=4.7
EA=4.6
ITO
IE=5.4
CdSe
IE=6.8
IE=6.5
IE=5.8
Normalized counts
Electron Energy [eV]
Vacuum Level = 0 eV
ZnS
TOPO
400
500
600
1200
1400
1600
W avelength [nm ]
19
Structure of a
Single Nanocrystal
HRTEM
4 nm
Jonny Steckel and Seth Coe
8 nm
Absorbance (arbitrary units)
Full Size Series of PbSe Nanocrystals
from 3 nm to 10 nm in Diameter
9
peak
absorption
8
2208 nm
7
2036 nm
6
1812 nm
5
1708 nm
4
1600 nm
3
1504 nm
2
1404 nm
1
1268 nm
0
1136 nm
Ordered Monolayer of
PbSe Nanocrystals
AFM
QDs are poor charge
transport materials...
1000
2500
Design of Device Structures
CdSe core
TOPO
ZnS
caps
shell
QD
50nm Ag
50nm Mg:Ag
30nm Alq3
-
e
But efficient emitters…
10nm TAZ
ZnS
40nm TPD
ITO
Glass
QD intersite spacing
Isolate layer functions of maximize
device performance.
1. Generate excitons on organic sites.
2. Transfer excitons to QDs via Förster
or Dexter energy transfer.
3. QD electroluminescence.
Need a new fabrication method in
order to be able to make such double
heterostructures:
Phase Segregation.
Segregation
Use organics for charge transport.
Vacuum Level = 0 eV
Electron Energy [eV]
TOPO caps
2000
QD monolayer
QD
CdSe
1500
Wavelength (nm)
EA=2.1
TPD
QD
EA=3.0 EA=3.1
TAZ
Alq3 E =3.7
F
Mg
EF=4.7
EA=4.6
ITO
IE=5.4
CdSe
IE=6.8
IE=6.5
IE=5.8
ZnS
TOPO
20
A general method?
Phase segregation occurs
for different
• This process is robust, but further exploration
is needed to broadly generalize these findings.
1) organic hosts: TPD,
NPD, and poly-TPD.
• For the explored materials, consistent
description is possible.
2) solvents: chloroform,
chlorobenzene, and
mixtures with toluene.
• We have shown that the process is not
dependent on any one material component.
3) QD core materials:
PbSe, CdSe, and
CdSe(ZnS).
Phase segregation Æ QDQD-LED structures
5) QD size
4) caps
shell
4) QD capping molecules:
oleic acid and TOPO.
QD monolayer
7) S
pin
P
5) QD core size: 4-8nm.
6) substrates: Silicon,
Glass, ITO.
ara
me
ters
2) Solvent
QD
3) core
Cathode
ETL
1) Organic Host
7) Spin parameters:
speed, acceleration
and time.
6) Substrate/Anode
For more details on phase segregation process,
come to the talk tomorrow - P10.7.
depth
EL Recombination
Region Dependence
on Current
Cathode
increasing
current
Alq3 (35 nm)
RF
2% DCM2 in Alq3 (5nm)
exciton density
J [mA/cm ]
DCM2
1300
130
13
1.3
0.13
80
DCM EL
Anode
60
Glass
40
Alq3 EL
20
0
-1 0
Alq3
400
TPD (40 nm)
100
EL Fraction [%]
Normalized Intensity
2
1
2
3
log(J[mA/cm2])
500
600
700
Wavelength [nm]
800
Coe et al., Org. Elect. (2003)
21
Spectral Dependence on Current Density
J [mA/cm2]
TOP DOWN VIEW of the QD MONOLAYER
Alq3
380
Device Area
Grain Boundary
RF
130
QD
13
1.3
0.38
500
600
700
Interstitial Space
Wavelength [nm]
Exciton recombination
width far exceeds the QD
monolayer thickness at
high current density.
density
To achieve true
monochrome emission,
new exciton confinement
techniques are needed.
Monolayer Void
Cathode
ETL
electron
Alq3 EL
depth
400
increasing
current
RF
exciton density
hole
QD EL
HTL
Anode
Glass
CROSS-SECTIONAL VIEW of QD-LED
Benefits of Quantum Dots in Organic LEDs
Demonstrated:
•Spectrally Tunable – single material set can access most of visible range.
•Saturated Color – linewidths of < 35nm Full Width at Half of Maximum.
•Can easily tailor “external” chemistry without affecting emitting core.
•Can generate large area infrared sources.
Potential:
CIE Diagram
0.9
0.8
0.6
0.5
0.4
•Inorganic – potentially more stable,
longer lifetimes.
The ideal dye molecule!
y
•High luminous efficiency LEDs possible
even in red and blue.
Potential QD-LED
0.7
colors
0.3
0.2
standard CRT
color triangle
FWHM 32nm
0.1
0.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
x
QD
Coe et al, Nature 420, 800 (2002).
400 450 500 550 600 650
22