Organic LEDs – part 8
• 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 20b
1
wavelength
gth
len
ve
wa
time
25 nm
Exciton Dynamics in Time Dependant PL
2 ns
time
2
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 ~
3
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]
4
Electronic Processes in Molecules
INTERNAL
CONVERSION
ENERGY TRANSFER
1-10 ns
density of available
S1 or T1 states
FLUORESCENCE
S1
PHOSPHORESCENCE
FÖRSTER, DEXTER
or RADIATIVE
ABSORPTION
Energy
10 ps
T1
>100 ns
S0
5
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]
6
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]
7
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
3
Excitonic Density of States
Frequency
Intensity
Molecular PL Spectra
Energy
Energy
8
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
9
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]
10
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 %
2.00
610
620
2%
630
4.5 %
1.95
0
1
2
3
Wavelength [nm]
2.25
640
4
5
Time [ns]
11
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
12
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
decreasing doping
(5% down to 0.5%)
► Assign value for allowed transfers:
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
excitonic density of states (gex(E))
► Assume Gaussian shape of width, wDOS
► Center at peak of initial bulk PL spectrum
► Molecular PL spectrum implied…
Normalized Intensity [ arb. units ]
Wavelength [ nm ]
750 700
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
650
600
550
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 ]
14
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
15
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 ]
2.36
Alq3
wDOS~0.15 eV
2.80
Energy [ eV ]
2.34
Energy [ eV ]
•
2.32
2.30
2.28
2.26
0
10
20
30
Time [ ns ]
40
50
NPD
wDOS~0.10 eV
2.78
2.76
2.74
2.72
-2
0
2
4
6
8
Time [ ns ]
10
12
14
16
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)
17
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
18
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]
19
Inorganic Nanocrystals – Quantum Dots
ZnS
CdSe
E
Lowest allowed
energy level
80Å
Absorption [a.u.]
D
D=
120Å
72Å
55Å
45Å
33Å
Quantum Dot SIZE
29Å
20Å
17Å
400
500
600
700
Wavelength [nm]
Photo by F. Frankel
Synthetic route of Murray
et al, J. Am. Chem. Soc.
115, 8706 (1993).
20
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
PbSe
CdSe(ZnS)
400
500
600
700
Wavelength [nm]
N
0.9
1.2
1.5
1.8
2.1
N
2.4
Wavelength [µm]
21
Integration of Nanoscale Materials
Quantum Dots and Organic Semiconductors
Oleic Acid or
TOPO caps
ZnS overcoating shell
(0 to 5 monolayers)
Trioctylphosphine oxide
HO
Synthetic routes of Murray et
al, J. Am. Chem. Soc. 115,
8706 (1993) and Chen, et al,
MRS Symp. Proc. 691,G10.2.
O
Oleic Acid
PbSe or
CdSe Core
Alq3
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,N'-Bis(naphthalen-1-yl)N,N'-bis(phenyl)benzidine
N
Differences in:
Chemistry
Size
Aromatic
“Small”
Molecular Organics
Quantum Dots
Aliphatic Caps “Big”
ÎPhase Segregation
22
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.
QDs
3
TPD/solvent
Substrate
close packed QDs
4
TPD
Substrate
~8nm PbSe QDs
Flat TPD
background
50nm
23
Monolayer Coverage – QD concentration
QD coverage = 20%
50%
500nm
250nm
90%
80%
500nm
100%
As the concentration of
QDs in the spin-casting
solution is increased,
the coverage of QDs on
the monolayer is also
increased.
500nm
100nm
24
50nm Ag
50nm Mg:Ag
30nm Alq3
10nm TAZ
Current Density [A cm-2]
QD-LED Performance
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 ]
25
Structure of a
Single Nanocrystal
HRTEM
Jonny Steckel and Seth Coe
8 nm
AFM
Ordered Monolayer of
PbSe Nanocrystals
Absorbance (arbitrary units)
4 nm
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
1000
1500
2000
2500
Wavelength (nm)
26
QDs are poor charge
transport materials...
CdSe core
ZnS
shell
50nm Ag
50nm Mg:Ag
30nm Alq3
QD monolayer
QD
e-
CdSe
Design of Device Structures
But efficient emitters…
QD
TOPO
caps
10nm TAZ
ZnS
40nm TPD
ITO
Glass
TOPO caps
QD intersite spacing
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]
Isolate layer functions of maximize
device performance.
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
27
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 QD-LED structures
5) QD size
QD monolayer
7) S
p
5) QD core size: 4-8nm.
6) substrates: Silicon,
Glass, ITO.
7) Spin parameters:
speed, acceleration
and time.
4) caps
shell
4) QD capping molecules:
oleic acid and TOPO.
2) Solvent
in P
ara
me
ters
Cathode
QD
3) core
ETL
1) Organic Host
6) Substrate/Anode
28
depth
EL Recombination
Region Dependence
on Current
Cathode
increasing
current
Alq3 (35 nm)
RF
2% DCM2 in Alq3 (5nm)
J [mA/cm2]
DCM2
1300
130
13
1.3
0.13
TPD (40 nm)
100
EL Fraction [%]
Normalized Intensity
exciton density
80
DCM EL
60
Glass
40
Alq3 EL
20
0
-1 0
Alq3
400
Anode
1
2
3
log(J[mA/cm2])
500
600
700
Wavelength [nm]
800
Coe et al., Org. Elect. (2003)
29
Spectral Dependence on Current Density
J [mA/cm2]
380
TOP DOWN VIEW of the QD MONOLAYER
Alq3
Device Area
Grain Boundary
RF
130
QD
13
1.3
0.38
500
600
700
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.
Interstitial Space
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
30
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
31