Organic LEDs – part 7 2. 1.
Transcript of Organic LEDs – part 7 2. 1.
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@ MITApril 29, 2003 – Organic Optoelectronics - Lecture 20
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).
Electroluminescence in Doped Organic Films
2.Excitons transfer to
luminescent dye
1.Excitons formed
from combinationof electrons and
holes
6.0 eV
a-NPD
2.6 eV
5.7eV
Alq3
2.7 eV electrons
exciton
trap states
low work functioncathode
transparent anode holes
dopant molecule(luminescent dye)
host molecules(charge transport
material)
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400 500 600 700 8000.0
0.2
0.4
0.6
0.8
1.0
Inte
nsity
[a.u
.]
Wavelength [nm]
0.5 %1.5 %
2.5 %4.5 %
6 %
Electroluminescence of x% DCM2 in Alq3 OLEDs
Alq3
35 nm35 nmtuning range
AlqAlq33 DCM2 in AlqDCM2 in Alq33
low DCM2low DCM2 high DCM2high DCM2
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... change in the spectral position of... change in the spectral position ofaborptionaborption/luminescence band/luminescence band
due to change in the polarity of the mediumdue to change in the polarity of the medium
solvationsolvation is a physical perturbationphysical perturbation of lumophore’s molecular statesisolated molecule (in a gas phase) and solvated molecule
are in the same chemical state(no solvent induced proton or electron transfer, ionization, complexation, isomerization)
dipolarlumophore
dipolarmolecule
dipole - dipole interactionmodifies the energy structuremodifies the energy structure
of the molecules
∆E = ∆ ⟨µ • Eloc⟩∆E = ∆ ⟨µ • Eloc⟩Solid State Solvation (SSS)
Eloc
R
dipolar hostwith moment µ
polarlumophore
⟨µ⟩ > 0⟨µ⟩ → 0as R → large
“self polarization”for strongly dipolar lumophores
(aggregation possible for highly polar molecules)
Eloc ~ 107 V/cm
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Influence of µ0 and µ1on Chromatic Shift Direction
solvent
solute (chromophore)WITH DIPOLE MOMENT µ
µ0 < µ1µ0 < µ1
SOLVENT POLARITY
S0
S1
groundstate
excitedstate
µ0 > µ1µ0 > µ1
SOLVENT POLARITY
S0
S1
∆E = ∆ ⟨µ •E⟩∆E = ∆ ⟨µ •E⟩
Bathochromic (red) PL shift Hypsochromic (blue) PL shift
C6H6 CHCl3 C2H5OH (CH3)2S:O
600 700 8000.0
0.5
1.0
Inte
nsit
y [a
.u.]
Wavelength [nm]
C6H6 CHCl3 C2H5OH (CH3)2S:O
thin filmthin film
0 1.15 1.69 3.9
O
CNNC
N
SOLVENTDIPOLE MOMENT
Bulović et al., Chem. Phys. Lett. 287, 455 (1998).
PL of DCM2 Solutions and Thin Film
~ SOLVATOCHROMIC ~SHIFT
[bathochromic (red) shift]
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Ground State(equilibrium)
µg, Eg,loc1
µe, E*g,loc
Excited State(non-equilibrium)
2
Excited State(equilibrium)
µe, Ee,loc
3
µg, E*e,loc
Ground State(non-equilibrium)
4Continuum, Dipole in Spherical
Cavity Model:
312)1(2
aµ
εεEloc
rr
+−=
≡Γε
Dynamic Relaxation Picture (a.k.a. solvation)
500 600 700 800 900
0.0
0.2
0.4
0.6
0.8
1.0
Inte
nsity
[a.u
.]
Wavelength [nm]
0.0
0.2
0.4
0.6
0.8
1.0
Inte
nsity
[a.u
.]
1% DCM2 in Alq3Alq3
1% DCM2 in Zrq4Zrq4
polar hostµ ~ 5.5 D
non-polar host
635 nm
605 nm
Thin Film Photoluminescence
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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)
Polystyrene (PS) 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5
0.0
0.2
0.4
0.6
0.8
1.0
25%
0%
0.005% DCM2 Doping
PL Spectra of Different PS:CA:DCM2 Films
Nor
mal
ized
Inte
nsity
Energy (eV)
CA %:
CA Doping and Electronic Susceptibility
0 5 10 15 20 252
3
4
5
6
ε = 2.44 + 0.13 x (CA %)
0.005% DCM2 Doping
Bulk Electronic Susceptibility of PS:CA:DCM2 Films
Epsi
lon
(at 1
00 K
Hz)
Camphoric Anhydride Concentration (%)
42 nm red shift from 0 to 25% CA
Results unchanged even for 10x higher DCM2 concentration:
DCM2 aggregation not the answer
Local fields are responsible for the spectral shifts…
… and dielectric measurements suggest a “solvatochromic” effect.
0 5 10 15 20 25
2.042.062.082.102.122.142.162.182.202.22
605 nm
563 nm
0.005% DCM2 Doping
Peak PL Energy of PS:CA:DCM2 Films
Ener
gy (e
V)
Camphoric Anhydride Concentration (%)
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Dynamic Relaxation Picture (a.k.a. solvation)Solvation Theory
Ground State(equilibrium)
µg, Eg,loc1
µe, E*g,loc
Excited State(non-equilibrium)
2
Excited State(equilibrium)
µe, Ee,loc
3
µg, E*e,loc
Ground State(non-equilibrium)
4Continuum, Dipole in Spherical
Cavity Model:
312)1(2
aµ
εεEloc
rr
+−=
≡Γε
constant with CA%
n nearly constant with CA% (ranging from ~1.55 to ~1.65)
gasE↓
AE εΓ∆ −≈↓ where 3∆
aµµA err
⋅=
( ) 32ΓΓ∆ aµµµEE gneε
gas rrr+⋅−= ↓↓
Connecting Theory to Experiment
0.4 0.5 0.6 0.7 0.8 0.9 1.0
1.90
1.95
2.00
2.05
2.10
2.15
2.20
Benzene Toluene Chloroform Dichlormethane Acetone Acetonitrile Methanol
DCM2 Peak PL in Various Solvents
Slope givesA ~ 0.55 eV
Ener
gy (e
V)
Γ (no units)
( )
+−≡
1212Γ
εε
ε
0 2 4 6 8 10 120.00.10.20.30.40.50.60.70.80.91.0
ε (no units)
Γ(n
o un
its)
Gamma Functional Shape
-5 0 5 10 15 20 25 30
2.02
2.04
2.06
2.08
2.10
2.12
2.14
2.16
2.18
2.20
2.22
0.52 eV
0.62 eV0.57 eV
A =
Evolution of Peak PL Energy for PS:CA:DCM2 Films
0.005% DCM2 Doping
Experiment
Ener
gy (e
V)
Camphoric Anhydride Concentration (%)-5 0 5 10 15 20 25 30
2.02
2.04
2.06
2.08
2.10
2.12
2.14
2.16
2.18
2.20
2.22
Evolution of Peak PL Energy for PS:CA:DCM2 Films
0.005% DCM2 Doping
Experiment
Ener
gy (e
V)
Camphoric Anhydride Concentration (%)
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Exciton Dynamics in Time Dependant PL
wavelength time
2 ns
25 n
m
time
wav
elen
gth
Dynamic Spectral Shifts of DCM2 in Alq3
1.7 1.8 1.9 2.0 2.10.0
0.2
0.4
0.6
0.8
1.0
720 680 640 600
2% DCM2:Alq3
6.2 ns3.4 ns2.2 ns
1.6 ns1.0 ns
0.6 ns0.3 ns
0.1 ns0 ns
time
Nor
mal
ized
Inte
nsity
Energy [ eV ]
Wavelength [ nm ]
DCM2Alq3
~ ~ DCM2 PL red shifts > 20 nm over 6 ns ~DCM2 PL red shifts > 20 nm over 6 ns ~
• Measurement performed on doped DCM2:Alq3 films
• Excitation at λ=490 nm (only DCM2 absorbs)
1.7 1.8 1.9 2.0 2.10
2
4
6
8
10
12
Energy [ eV ]
Inte
nsity
[ ar
b. u
nits
]
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1.6 1.8 2.0 2.20.0
0.2
0.4
0.6
0.8
1.0
Integrated Spectrum(0-10 ns)
Spectrumat 1 ns
Nor
mal
ized
Inte
nsity
Energy [eV]
750 700 650 600 550Wavelength [nm]
Spectrumat 4 ns
Time Evolution of 4% DCM2 in Alq3 PL Spectrum
T1S1
S0
FL
UO
RE
SC
EN
CE
PH
OS
PH
OR
ES
CE
NC
E
ENERGY TRANSFER
FÖRSTER, DEXTERor RADIATIVE
INT
ER
NA
LC
ON
VE
RS
ION
AB
SO
RP
TIO
N
10 ps
1-10 ns
>100 ns
Ener
gy
density of availableS1 or T1 states
Electronic Processes in Molecules
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0 1 2 3 4 51.80
1.85
1.90
1.95
2.00
2.05
2.10
2.15
2.20
CH3CN
Acetone
DMSO
CHCl3
CHCl2
BenzeneEn
ergy
[eV
]
Time [ns]
680
660
640
620
600
580
Wav
elen
gth
[nm
]
Time Evolution of DCM2 Solution PL Spectra
Wavelength [nm]
Tim
e [n
s]
0
1
2
3
4
5
600 650 700 750
10% DCM2 in Alq310% DCM2 in Alq3
Spectral Shift due to
~ Exciton Diffusion ~~ Intermolecular Solid State Interactions ~
35 nm35 nmwavelength shift
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Each dye molecule experiences a different local medium
⇒ variations in excitonic energies
Inte
nsit
y
Energy
12
3
Molecular PL Spectra
Freq
uenc
yEnergy
Excitonic Density of States
Non-zero width to excitonicdensity of states
Excitonic Energy Variations
12 3
log(Time)
S 1or
T1
Ener
gy
1 2 3 4
Exciton Distribution in the Excited State (S1 or T1)
fwhm
~ Time Evolved Exciton Thermalization ~
EXCITON DIFFUSION LEADS TO REDUCTION IN FWHM
hν
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Ener
gy [e
V]
Time [ns]
680
660
640
620
600
Wav
elen
gth
[nm
]
0 1 2 3 4 5 6 7
High Emidpoint
Low Emidpoint
1.80
1.85
1.90
1.95
2.00
2.05
2.10
T = 15~125 K
Energy
Low Emidpoint
High Emidpoint 4% DCM2 in Alq3
FWHM
0 1 2 3 4 5
1.95
2.00
2.05
2.10
2.15
2.20
2.25
2.30
Ener
gy [
eV]
Time [ns]
640
630
620
610
600
590
580
570
560
550
540
Wav
elen
gth
[nm
]x%x% DCM in AlqDCM in Alq33
0.2 %
2 %
0.6 %
0.4 %
Alq3
4.5 %
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1.96
2.00
2.04
2.08
2.12
2.16
Ener
gy [
eV]
% of DCM in Alq3
630
620
610
600
590
580
570
Wav
elen
gth
[nm
]
0.1 1 2 40.50.2
Initial Peak PL
Final Peak PL
X% DCM in AlqX% DCM in Alq33
1 10 100
2.25
2.30
2.35
2.40
2.45
Ener
gy [
eV]
Time [ns]
Alq3
Ir(ppy)3
Time Evolution of Peak PL in Neat Thin Films
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Parameters for Simulating Exciton Diffusion
observed radiative lifetime (τ) Förster radius (RF)
► Assign value for allowed transfers:
0.01
0.1
1
-2 0 2 4 6 8 10 12 14 16
decreasing doping(5% down to 0.5%)
τ = 3.3 ns
Nor
mal
ized
Inte
grat
edSp
ectra
l Int
ensi
ty
Time [ ns ]
1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.40.00.10.20.30.40.50.60.70.80.91.0
750 700 650 600 550
FWHMsSinit(E) : 0.27 eVSmol(E) : 0.22 eVgex(E) : 0.15 eV
Nor
mal
ized
Inte
nsity
[ ar
b. u
nits
]Energy [ eV ]
Wavelength [ nm ]excitonic density of states (gex(E))
► Assume Gaussian shape of width, wDOS
► Center at peak of initial bulk PL spectrum
► Molecular PL spectrum implied…
∆E0
ΓF61
RR
τF
Fitting Simulation to Experiment – Doped Films
1.94
1.96
1.98
2.00
2.02
2.04
2.06
630
620
610
600
0 1 2 3 4 5 6 7 8 9
0.5% DCM2:Alq3
RF=0.88 wDOS=0.185 eVRF=0.98 wDOS=0.161 eVRF=1.08 wDOS=0.146 eVRF=1.18 wDOS=0.136 eVRF=1.28 wDOS=0.130 eV
Ener
gy [
eV ]
Wav
elen
gth
[ nm
]
Time [ ns ]1.841.861.881.901.921.941.961.982.002.022.042.06
670660
650
640
630
620
610
0 1 2 3 4 5 6 7 8 9
5%
2%1%
0.5%
Ener
gy [
eV ]
Wav
elen
gth
[ nm
]Peak PL of x%DCM2:Alq3
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
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0 10 20 30 40 502.26
2.28
2.30
2.32
2.34
2.36
Ener
gy [
eV ]
Time [ ns ]
0 50 100 150 200 250 300
2.24
2.26
2.28
2.30
2.32
2.34
2.36
2.38
2.40
Ener
gy [
eV ]
Time [ ns ]
-2 0 2 4 6 8 10 12 142.72
2.74
2.76
2.78
2.80
Ener
gy [
eV ]
Time [ ns ]
Alq3wDOS~0.15 eV NPD
wDOS~0.10 eV
Ir(ppy)3wDOS~0.21 eV
Fitting Simulation – Neat Films
• 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 dispersionin amorphous organic solids
upon excitation both magnitude and directionof lumophore dipole moment can change
FOR EXAMPLE for DCM: µ1 – µ0 > 20 Debye !~ from 5.6 D to 26.3 D ~
following the excitation the environment surrounding the excited molecule will reorganize to minimize the overall
energy of the system (maximize µ • Eloc)
Eloc
µ0
t < 0
Eloc
µ1
t = 0
Eloc
µ1
t ~ 1 ns
Temporal Solid State Solvation
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∆ E
log(Time)
Ener
gy
1 2 3 4
Exciton Distribution in the Excited State (S1 or T1)
~ Time Evolved Molecular Reconfiguration ~
DIPOLE-DIPOLE INTERACTIONLEADS TO ENERGY SHIFT IN DENSITY OF EXCITED STATES
Fusion of Two Material Sets Hybrid devices could enable
LEDs, Solar Cells, Photodetectors, Modulators, and
Lasers
which utilize the best properties of
each individual material.
Fabrication of rational structures has been the main obstacle to dateto date.
Organic Semiconductors
Abso
rptio
n [a
.u.]
400 500 600 700Wavelength [nm]
Emissive
Tunable
Nanoscale
Efficient
Flexible
Colloidal QDs
Photo by F. Frankel
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Inorganic Nanocrystals – Quantum Dots
Quantum Dot SIZE
D
Lowest allowed energy level
E
Abso
rptio
n [a
.u.]
400 500 600 70017Å20Å
29Å
33Å
45Å55Å72Å
80Å
D=120Å
Wavelength [nm]
Synthetic route of Murray et al, J. Am. Chem. Soc. 115, 8706 (1993).
CdSeZnS
Photo by F. Frankel
Fusion of Two Material Sets
Organic Molecules
Abso
rptio
n Sp
ectra
Quantum Dots
TAZ
N
NN
TPD
N
CH3
N
H3C
Alq3
N
OAl
NO
NO
N N
NPD
TUNABLE
PbSe
Wavelength [µm]0.9 1.2 1.5 1.8 2.1 2.4
NANOSCALE
CdSe(ZnS)
Wavelength [nm]400 500 600 700
EMISSIVE
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ZnS overcoating shell (0 to 5 monolayers)
Oleic Acid or TOPO caps
Integration of Nanoscale MaterialsQuantum Dots and Organic Semiconductors
D = 17-120Å(~50Å pictured)
TAZ
3-(4-Biphenylyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole
N
NN
TPD
N,N'-Bis(3-methylphenyl)-N,N'-bis-(phenyl)-benzidine
N
CH3
N
H3C
Alq3
Tris(8-hydroxyquinoline)Aluminum (III)
N
OAl
NO
NO
10Å
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. PbSe or
CdSe Core
O
HO Oleic Acid
N N
NPD
N,N'-Bis(naphthalen-1-yl)-N,N'-bis(phenyl)benzidine
N
N
poly-TPD
Phase Segregation
Chemistry SizeMolecular Organics
Differences in:
Aliphatic CapsAromatic
Quantum Dots“Small”“Big”
Phase Segregation and Self-Assembly
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 chemistrysize and chemistry..
50nm
~8nm PbSe QDs
Flat TPD background
TPD/solvent
QD Solution TPD Solution
11 22 33 44Substrate Substrate
QDs
TPDclose packed QDs
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Monolayer Coverage – QD concentration
250nm
500nm
500nm500nm
100nm
As the concentration of QDs in the spin-casting solution is increased, the coverage of QDs on the monolayer is also increased.
QD coverage = 20% 50% 80%
90% 100%
400 500 600 1200 1400 1600
Nor
mal
ized
cou
nts
W avelength [nm ]
QD-LED Performance
CdSe(ZnS)/TOPO PbSe/oleic acidGlassITO
50nm Ag50nm Mg:Ag30nm Alq3
40nm TPD
10nm TAZ
EF=3.7
EA=4.6
IE=6.8
QD
EF=4.7
Vacuum Level = 0 eV
ITO
Mg
Elec
tron
Ener
gy [e
V] EA=2.1
IE=5.4
TPD
IE=5.8
EA=3.1
Alq3
EA=3.0
IE=6.5
TAZ
ZnS
CdSe
TOPO
1 10
10-8
10-6
10-4
10-2
100
Voltage [V]
Curre
nt De
nsity
[A cm
-2]
Control
QD-LED
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4 nm HRTEMHRTEM
AFMAFM
Full Size Series of PbSe Nanocrystalsfrom 3 nm to 10 nm in Diameter
1000 1500 2000 2500
0
1
2
3
4
5
6
7
8
9
1136 nm
1268 nm
1600 nm
1404 nm
1504 nm
1708 nm
1812 nm
2036 nm
2208 nm
Wavelength (nm)
Abso
rban
ce (
arbi
trar
y un
its)
peak absorption
8 nmOrdered Monolayer ofOrdered Monolayer ofPbSePbSe NanocrystalsNanocrystals
Structure of aSingle Nanocrystal
Jonny Steckel Jonny Steckel and Seth Coeand Seth Coe
Design of Device Structures
QD
ZnS
CdSe
TOPO capsQD intersite spacing
QDs are poor charge transport materials...
e-
But efficient emitters…
1. Generate excitons on organic sites.2. Transfer excitons to QDs via Förster
or Dexter energy transfer.3. QD electroluminescence.
Isolate layer functions of maximize device performance.
Need a new fabrication methodnew fabrication method in order to be able to make such double heterostructures:
Phase SegregationPhase Segregation.
EF=3.7
EA=4.6
IE=6.8
QD
EF=4.7
Vacuum Level = 0 eV
ITO
Mg
Elec
tron
Ener
gy [e
V] EA=2.1
IE=5.4
TPD
IE=5.8
EA=3.1
Alq3
EA=3.0
IE=6.5
TAZ
ZnS
CdSe
TOPO
Use organics for charge transport.GlassITO
50nm Ag50nm Mg:Ag30nm Alq3
40nm TPD
10nm TAZ
QD monolayer ZnS shell
TOPO caps
QD
CdSe core
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A general method?
Phase segregation occurs for different
1) organic hosts: TPD, NPD, and poly-TPD.
2) solvents: chloroform, chlorobenzene, and mixtures with toluene.
3) QD core materials: PbSe, CdSe, and CdSe(ZnS).
4) QD capping molecules: oleic acid and TOPO.
5) QD core size: 4-8nm.
6) substrates: Silicon, Glass, ITO.
7) Spin parameters: speed, acceleration and time.
• This process is robust, but further exploration is needed to broadly generalize these findings.
• For the explored materials, consistent description is possible.
• We have shown that the process is not dependent on any one material component.
3) core
2) Solvent
7) Spin Parameters
Phase segregation QDQD--LED structuresLED structures
6) Substrate/Anode
Cathode
ETL
1) Organic Host
shell
QD monolayer
4) caps
QD
5) QD size
For more details on phase segregation process, come to the talk tomorrow - P10.7.
GlassAnode
Cathode
exciton density
dept
h
Alq3 (35 nm)increasing current
2% DCM2 in Alq3 (5nm)RF
TPD (40 nm)
Coe et al., Org. Elect. (2003)
Wavelength [nm]
Nor
mal
ized
Inte
nsity
400 500 600 700 800
Alq3
DCM2
130013013
1.30.13
EL F
ract
ion
[%]
log(J[mA/cm2])
100
80
60
40
20
0
100
80
60
40
20
0
-1 0 1 2 3-1 0 1 2 3
J [mA/cm2]DCM EL
Alq3 EL
EL Recombination Region Dependence
on Current
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Interstitial Space Monolayer Void
Grain Boundary
RF
Device AreaTOP DOWN VIEW of the QD MONOLAYER
Exciton recombination width far exceeds the QD monolayer thickness at high current densityhigh current density.To achieve true monochrome emission, new exciton confinement techniques are needed.
400 500 600 700
1.3
Alq3
QD
Wavelength [nm]
380
130
13
0.38
J [mA/cm2]
Spectral Dependence on Current Density
CROSS-SECTIONAL VIEW of QD-LED
dept
h
GlassAnode
Cathode
HTL
exciton densityRF
QD EL
Alq3 EL
hole
electronincreasing
currentETL
Demonstrated: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.
Benefits of Quantum Dots in Organic LEDs
Coe et al, Nature 420, 800 (2002).
400 450 500 550 600 650
FWHM 32nm0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
y
x
standard CRTcolor triangle
CIE Diagram
Potential:Potential:•High luminous efficiency LEDs possible even in red and blue.
•Inorganic – potentially more stable, longer lifetimes.
The ideal dye molecule!The ideal dye molecule!
Potential QD-LED colors
QD