Organic LEDs – part 6
• Exciton Recombination Region in Organic LEDs
• White OLED
• Flexible OLEDs
• Solvation Effect
• Solid State Solvation
Handout: Bulovic, et al., Chem. Phys. Lett. 287, 455 (1998); 308, 317 (1999).
@ MIT
April 17, 2003 – Organic Optoelectronics - Lecture 19
1
Exciton can transfer its energy to:
Bulović
´ et al., Phys. Rev. B 58, 3730 (1998).
RADIATIVE modes:
SURFACE
emission
* radiation modes
* waveguide modes
NON-RADIATIVE modes:
* internal conversion,
intersystem crossing
* surface plasmons
* losses due to absorption
in metal electode
RADIATION
Modes
Glass
Θ
k
EDGE
emission
ITO
WAVEGUIDE
Modes
α - NPD
Alq3
Glass Substrate
OLED
- EL Region Metal
Losses
Surface
Plasmons
Mg:Ag Electrode
2
ExternaL EL Quantum and Power Efficiency (ηR and ηP)
for ITO/ 350Å NPD/ xÅ Alq / Mg:Ag / Ag
ηR (SURFACE) [%]
0
500
1000
1500
0.6
0.4
0.2
200
0.6
150
0.4
100
0.2
50
0.0
0
500
1000
0
1500
Power Consumed
(@10mA/cm2) [mW/cm2]
ηP (SURFACE)
(@10mA/cm2) [lm/W]
0.0
Alq3 Thickness [Å]
3
PL Dependence on Alq3 Thickness
Bulović
´ et al., Phys. Rev. B 58, 3730 (1998).
PL
540
PL Peak [nm]
Normalized Intensity
1.0
0.8
NO ELECTRODE
530
ELECTRODE
0.6
520
0.4
400
600
800
Alq3 Thickness [Å]
300 Å Alq3
600 Å Alq3
Θ = 0º
800 Å Alq3
0.2
0.0
450
200
500
550
600
650
700
750
Wavelength [nm]
4
EL Dependence on Alq3 Thickness
Bulović
´ et al., Phys. Rev. B 58, 3730 (1998).
550
EL
EL Peak [nm]
Normalized Intensity
1.0
0.8
THEORY
540
530
0.6
200
0.4
400
500
Alq3 Thickness [Å]
200 Å Alq3
350 Å Alq3
Θ = 0º
500 Å Alq3
0.2
300
Theoretical Fit
(to 200 Å Alq3 spectrum)
0.0
450
500
550
600
650
700
750
Wavelength [nm]
5
Top to Side EL Ratio - Dependence on Alq3 Thickness
Bulović
´ et al., Phys. Rev. B 58, 3730 (1998).
ΦSURFACE / ΦEDGE
2.0
Experiment
Theory
1.5
1.0
SURFACE
Detector
EDGE
Detector
0.5
OLED
Glass Substrate
0.0
100
200
300
400
500
600
700
Alq3 Thickness [Å]
6
EL Recombination Region Dependence on Current
Bulović
´ et al., Phys. Rev. B 58, 3730 (1998).
LREC-1 [nm-1]
0.05
0.03
0.01 0
0.06
1.62
(a)
(b)
0.05
0.04
1.58
0.03
1.56
0.02
1.54
0.01
1.52
1.50
10-4
LREC-1 [nm-1]
ΦSURFACE / ΦEDGE
1.60
0.00
10-3
10-2
Current [A/cm2]
10-4
10-3
10-2
Current [A/cm2]
7
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)
8
Wavelength [nm]
Tuning
Emission of
White OLEDs
500
600
700
1.0
0.6%
1.5%
3%
6%
0.8
Intensity [a.u.]
Ag
Mg:Ag
400
Alq3
BCP
0.6
changing
DCM2 in α-NPD
concentration
(with 40A BCP)
0.4
0.2
NPD
0.0
NPD:DCM2
1.0
BCP – exciton
blocking layer
0.8
Intensity [a.u.]
glass
DCM2
120 Å BCP
80 Å BCP
40 Å BCP
TPD
ITO
Alq3
White OLED
(0.33,0.33)
0.6
changing
BCP layer
thickness
0.4
(with 0.6% DCM2)
0.2
0.0
400
500
600
Wavelength [nm]
700
9
Flexible OLED (FOLED)
-
Ultra lightweight
Thin form factor
Rugged
Impact resistant
Conformable
Manufacturing Paradigm Shift
Web-Based Processing
10
FOLED-based Pixelated, Monochrome Display
Source: UDC, Inc.
11
Transparent FOLED-based Pixel
Source: UDC, Inc.
12
Packaging of OLEDs – Multilayer Coatings
Barix is a coating composed of alternating layers
of polymer and ceramic thin films that can be
deposited on a plastic substrate or directly on an
OLED display. The technology breakthrough that
enables this to be used as the packaging
material for flat panel displays is the creation of
a barrier layer 10,000 times better than anything
currently produced.
From Vitex, makers of barix coating
13
The PRESENT …
… and the nearby FUTURE …
… of ORGANIC DISPLAY TECHNOLOGY
14
Luminescence of Molecules in Solid Matrices
• Solid State Solvation (SSS) •
• Time-Resolved SSS •
@ MIT
15
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)
16
Effect of Dopants on the OLED EL Spectrum
1.0
N
Normalized EL Intensity
O
0.8
0.6
Al
N
O
Alq3
NC
3
CN
DCM2:Alq3
PtOEP:Alq3
0.4
N
N
Pt
N
N
0.2
0.0
400
500
600
700
800
Wavelength [nm]
17
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
0.4
Alq3
0.2
0.0
400
500
600
700
800
Wavelength [nm]
18
Alq3
DCM2 in Alq3
low DCM2
high DCM2
19
Solvatochromism - Historical Perspective
It has been long recognized that
UV/visible absorption spectra
can be influenced by:
- THE PHASE (gas or liquid)
- SOLVENT
1878 - Kundt established relationship between
solvent effect and solvent properties (Kundt’s Rule)
using chlorophyl, fuchsin, aniline green,
cyanine, quinizarine, and egg yolk,
in twelve different solvents he concluded that
increasing index of refraction of the solvent
is related to the red-shift in the absorption spectrum of the solute
20
... change in the spectral position of
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)
21