THESIS Integrated Thin Film Electroluminescence Displays LETFEL
Chapter 4: Electroluminescence. Sylvania ZnS /Cu/Cl/I/ Mn.
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Transcript of Chapter 4: Electroluminescence. Sylvania ZnS /Cu/Cl/I/ Mn.
Chapter 4: Electroluminescence
Sylvania
ZnS/Cu/Cl/I/ Mn
100V 500 cd/m2
Fluorescence and Phosphorescence
Excimer Formation
Exciplex Formation
History of Organic Electroluminescence
1963 Pope 400V 10-20 um anthracene1965 Helfrich 100V 5 % efficiency1970 Williams1982 Vincett 30V 50 nm low efficiency1983 Partridge Polymeric materials
Basic Principle of Organic EL
Charge recombination leads to emission of fluorescence
ITO4.9-5.1 eV
Metal (eV)Ca 2.9Mg 3.7In 4.2Al 4.28Ag 4.6Cu 4.7Au 5.1
Fowler-Nordheim Equation: I = AF2exp(-k3/2/F)F: field strength, A: material constant, energy difference across the interface
Efficiency: = Number of photons emitted/Number of electrons injected
I/V relationship and B/V relationship
Tang etal, Kodak
ETLElectron TransportingLayer
HTLHole 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
S
OO
PEDOT.PSS
NH
NH
PANI
TPD
Addition of Hole Injection Layer
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, N-diphenylamino 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 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.
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 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 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 light-emitting 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
2500 cd/m2 with LiF
210 cd/m2 with Al
Red Light Emitting DeviceBased 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 DevicesBrian 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 polymersR. 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
Red Red
Green Blue
Solubilizing Groups
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