Chapter 4: Electroluminescence

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Chapter 4 Electroluminescence
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ZnS /Cu/Cl/I/ Mn
Sylvania
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100V 500 cd/m2
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Fluorescence and Phosphorescence
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Excimer Formation
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Exciplex Formation
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History of Organic Electroluminescence
1963 Pope 400V 10-20
um anthracene 1965 Helfrich
100V 5 efficiency 1970 Williams 1982 Vincett
30V 50 nm low
efficiency 1983 Partridge
Polymeric materials
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Basic Principle of Organic EL
Metal (eV) Ca 2.9 Mg 3.7 In 4.2 Al
4.28 Ag 4.6 Cu 4.7 Au 5.1
ITO 4.9-5.1 eV
Charge recombination leads to emission of
fluorescence
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Fowler-Nordheim Equation I AF2exp(-kf3/2/F) F
field strength, A material constant, f energy
difference across the interface
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• Efficiency
• Number of photons emitted/Number of electrons
  injected
• I/V relationship and B/V relationship

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Tang etal, Kodak
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ETL Electron Transporting Layer HTL Hole
Transporting Layer
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Hole Transporting Layer
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Electron Transporting Materials
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Criteria for the Materials of Emitting Layer
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Matching of Energy Levels
TPD
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ITO Surface Modification Layer for Hole Injection
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Addition of Hole Injection Layer
TPD
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Fluorescence Dye as Dopant A Yellowish Light
Emitting Device
Rubene
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Red light emitting materials
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Dopant amounts and Performance of the EL device
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Rubrene as a medium for energy transfer
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Green emitters
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Blue Light Emitting Device
460-480 nm, 4000 cd/m2
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White Light OLED White Blue Red
Blue
Red
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Device 1 Undoped Device 2 Doped with 5 of red
DCM2
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Highly-bright white organic light-emitting diodes
based on a single emission layer C. H. Chuen and
Y. T. Tao
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Trilayer Device Structure
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Recent advances on the Interfacial Problems
X. Zhou, M. Pfeiffer, J. Blochwitz, A. Werner, A.
Nollau, T. Fritz, and K. Leo APL 2001 410
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They demonstrated the use of a p-doped amorphous
starburst amine, 4, 48, 49-tris(N,
N-diphenylamino triphenylamine )(TDATA), doped
with a very strong acceptor, tetrafluorotetracyano
quinodimethane by controlled coevaporation as an
excellent hole injection material for organic
light-emitting diodes (OLEDs). Multilayered OLEDs
consisting of double hole transport layers of
p-doped 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.
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Low voltage organic light emitting diodes
featuring doped phthalocyanine as hole transport
material J. Blochwitz, M. Pfeiffer, T. Fritz, and
K. Leo
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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 low-work 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
Alq3VOPc interface. This limits the probability
of exciton formation and their radiative decay.
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Graded mixed-layer organic light-emitting
devices Anna B. Chwang,a) Raymond C. Kwong, and
Julie J. Brown
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Improved efficiency by a graded emissive region
in organic light-emitting diodes Dongge Ma, C. S.
Lee, S. T. Lee, and L. S. Hung
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Metal Complexes
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Al Complexes
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Organic light-emitting diodes using a gallium
complex
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210 cd/m2 with Al
2500 cd/m2 with LiF
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Red Light Emitting Device Based on Eu Complexes
7-137 cd/m2
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Thickness Effect
Better ET, 820 cd/m2
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Hole Blocking Layer
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Phosphorescent Devices
100000cd/m2
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Shizuo Tokito APL 2003 569
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Controlling Exciton Diffusion in Multilayer White
Phosphorescent Organic Light Emitting
Devices Brian W. D'Andrade, Mark E. Thompson,
Stephen R. Forrest Adv. Mater. 2002
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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.
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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
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Solubilizing Groups
Red
Red
Blue
Green
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Figure 6 Energy levels for electroluminescent
diodes. ac, 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.
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Unexpectedly high efficiency
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