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Chapter 4: Electroluminescence
ZnS
/Cu/Cl/I/ Mn
Sylvania
100V 500 cd/m2
Fluorescence and Phosphorescence
Excimer Formation
Exciplex Formation
History of Organic Electroluminescence
1963 Pope
1965 Helfrich
1970 Williams
1982 Vincett
1983 Partridge
400V 10-20 um anthracene
100V 5 % efficiency
30V 50 nm low efficiency
Polymeric materials
Basic Principle of Organic EL
ITO
4.9-5.1 eV
Metal (eV)
Ca 2.9
Mg 3.7
In
4.2
Al 4.28
Ag 4.6
Cu 4.7
Au 5.1
Charge recombination leads to emission of fluorescence
Fowler-Nordheim Equation: I = AF2exp(-kf3/2/F)
F: field strength, A: material constant, f: energy difference
across the interface
Efficiency:
= Number of photons emitted/Number of electrons injected
I/V relationship and B/V relationship
Tang etal, Kodak
ETL
Electron Transporting
Layer
HTL
Hole Transporting
Layer
Hole Transporting
Layer
Electron Transporting Materials
Criteria for the Materials of Emitting Layer
Matching of Energy Levels
TPD
ITO Surface Modification Layer for Hole Injection
O
O
S
PEDOT.PSS
N
H
PANI
N
H
Addition of
Hole Injection Layer
TPD
Fluorescence Dye as Dopant:
A Yellowish Light Emitting Device
Rubene
Red light
emitting materials
Dopant amounts and
Performance of the EL device
Rubrene as a medium for energy transfer
Green emitters
Blue Light Emitting Device
460-480 nm, 4000 cd/m2
White Light OLED
White = Blue + Red
Blue
Red
Device 1 Undoped; Device 2 Doped with 5% of red DCM2
Highly-bright white organic light-emitting diodes based on
a single emission layer
C. H. Chuen and Y. T. Tao
Trilayer Device Structure
Recent advances on the Interfacial Problems
X. Zhou, M. Pfeiffer, J. Blochwitz, A. Werner, A. Nollau, T. Fritz, and K. Leo APL
2001 410
They demonstrated the use of a p-doped
amorphous starburst amine, 4, 48, 49-tris(N, Ndiphenylamino triphenylamine )(TDATA),
doped with a very strong acceptor,
tetrafluorotetracyanoquinodimethane by
controlled coevaporation as an excellent hole
injection material for organic light-emitting
diodes (OLEDs). Multilayered OLEDs
consisting of double hole transport layers of pdoped TDATA and triphenyldiamine, and an
emitting layer of pure 8-tris-hydroxyquinoline
aluminum exhibit a very low operating voltage
(3.4 V) for obtaining 100 cd/m2 even for a
comparatively large (110 nm) total hole
transport layer thickness.
Low voltage organic light emitting diodes featuring doped
phthalocyanine as hole transport material
J. Blochwitz, M. Pfeiffer, T. Fritz, and K. Leo
Rough estimates lead to values of about
0.2% luminescence efficiency for the highest
doped case. However, those devices use
sophisticated multi-layer designs and lowwork function contacts. We believe that the
major reason for the lower efficiency of our
diodes is that the simple two-layer design
does not prevent negative carriers injected
from the Al electrode from reaching the
opposite electrode due to the missing energy
barrier for electrons at the Alq3–VOPc
interface. This limits the probability of
exciton formation and their radiative decay.
Graded mixed-layer organic light-emitting devices
Anna B. Chwang,a) Raymond C. Kwong, and Julie J. Brown
Improved efficiency by a graded emissive region in organic lightemitting diodes
Dongge Ma, C. S. Lee, S. T. Lee, and L. S. Hung
Metal Complexes
Al Complexes
Organic light-emitting diodes using a gallium complex
210 cd/m2 with Al
2500 cd/m2 with LiF
Red Light Emitting Device
Based on Eu Complexes
7-137 cd/m2
Thickness Effect
Better ET, 820 cd/m2
Hole Blocking Layer
Phosphorescent Devices
100000cd/m2
Shizuo Tokito APL 2003 569
Controlling Exciton Diffusion in Multilayer White
Phosphorescent Organic Light Emitting Devices
Brian W. D'Andrade, Mark E. Thompson, Stephen R. Forrest*
Adv. Mater. 2002
The color balance (particularly enhancement of blue emission) can be improved by
inserting a thin BCP, hole/excitonblocking layer between the FIrpic and Btp2Ir(acac)
doped layers in Device 2. Thislayer retards the flow of holes from the FIrpicdoped
layer towards the cathode and thereby forces more excitons to form in the FIrpic
layer, and it prevents excitons from diffusing towards the cathode after forming in
the FIrpic doped layer. These two effects increase FIrpic emission relative to Btp2Ir(acac).
Device 2 is useful for flat-panel displays since the human perception of white from
the display will be unaffected by the lack of emission in the yellow region of the
spectrum.
Electroluminescence in conjugated polymers
R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C.
Bradley,
D. A. Dos Santos, J. L. Bre¬ das, M. Lo» gdlund & W. R. Salaneck Nature 1999 397 121
Wessling Approach
Solubilizing Groups
Red
Green
Red
Blue
Figure 6 Energy levels for electroluminescent diodes. a±c, An ITO-PPV-Ca diode before contact between
the three layers, illustrating the energies expected, a, from the metal Fermi energies, assuming no chemical
interactions at the interface, b, after some `doping' of the interfacial layer of PPV by Ca, setting up
bipolaron' bands within the PPV semiconductor gap (note that the Fermi energy for the `doped' PPV lies
between the upper bipolaron level and the conduction band), and c, after interfacial chemistry which sets
up a blocking layer at the interface (as expected in the presence of oxygen). d, Energy levels for the
components of a two-layer heterojunction diode fabricated with PPVand CN-PPV.
Unexpectedly high efficiency