Transcript Outline 1. 2. 3. 4. 5. 6. 7. 8. 9. Chronology of display technology Advantages of LED’s Definition of OLED Principles of operation Technology Branches SMOLED’s LEP’s Effect of dopant Other applications Corporations in this field Conclusion.

Outline
1.
2.
3.
4.
5.
6.
7.
8.
9.
Chronology of display technology
Advantages of LED’s
Definition of OLED
Principles of operation
Technology Branches
SMOLED’s
LEP’s
Effect of dopant
Other applications
Corporations in this field
Conclusion
Basic Idea Behind Emission
Molecular
Energy
Systems
Light
Beginning of LED
www.kodak.com
Advantages of LED’s over LCD
1. Brighter, thinner, lighter, faster
2. Bright from all viewing angles
3. Need less power to run
4. A lot cheaper to produce
5. Expanding memory capability - coating new layer on top of existing one
6. Wider temperature range
7. Doping or enhancing organic material helps control
Brightness
Color of light.
Semiconductor LED’s
LED’s work on the principle of injection
luminescence.
Conventional LEDs are made of :
(AlGaAs) - red and infrared
(GaAs/P) - red, orange,yellow
(GaN) - green
(GaP) - green
(ZnSe) - blue
(InGaN) - blue
(SiC) - blue
diamond (C) - ultraviolet
OLED is a display
device that
sandwiches carbon
based films between
the two electrodes
and when voltage is
applied creates light.
www.ol-ed.com
Single Layer Device
Organic electroluminescene (EL) is the electrically driven
emission of light from non-crystalline organic materials
Energy level diagram of a two-layer OLED
•
HOMO, LUMO of the HTL is
slightly above that of the ETL
L.S.Hung et al.,Materials Science and Engineering R 39, (2002), 143
Chemistry behind Emission
Electrons and holes recombine :
singlet state, triplet state
- +
+
S+T
S+T
S0 + h
Formation of triplet is 3 times
more feasible than singlet
Photoluminescence vs. Electroluminescence
When a radical anion and a
radical cation combine on a
single conjugated segment,
singlet and triplet excited
states are formed, of which
the singlets can emit light.
A.B.Holmes et al., Angew. Chem. Int. Ed. 37, 1998, 402
R.H.Friend et al., Nature 413, 2001, 828
Thermodynamics of Electroluminescence
A + e-
A-
E reduction
(- )
A +e
A
E oxidation
(+)
A + hv
A*
E emission
A+ + A-
2A or A + A*
+
When E oxidation -
E reduction > or = E emission
Ru(bpy)32++ e-
Ru(bpy)3+
E reduction
-1.4 V
Ru(bpy)33++ e
Ru(bpy)32+ E oxidation
+ 1.2 V
Ru(bpy)3 + hv
Ru(bpy)33* Eemission
2.05 V
Factors influencing efficiency
1.
Efficiency of electrons and holes recombination
2.
Efficiency of excited state formation upon annihilation.
3.
Quantum yield of emission of excited state.
Two Principle Branches
1. Light-Emitting Polymers (LEPs)
Or Polymer Light Emitting Diode
(PLEDs)
Using relatively large molecules
eg :Conjugated molecules
2. Small Molecule Organic Light
Emitting Diodes (SMOLEDs).
Using relatively small
molecules (even monomers)
eg: Metal chelates
SMOLEDs
Criteria Metal chelates must satisfy
•
Thermally stable,
•
Highly luminescent in the solid state,
•
Thin-film forming upon vacuum deposition
•
Capable of transporting electrons
.
C.H.Chen et al., Coordination Chemistry Reviews 171, (1998), 161
Early thin film organic device
Mg:Ag – 10:1
Luminescent film - 600A
Diamine – 750A
•
Relatively High voltage (80-100 V)
- Inject charge into organic
crystals
•
Low work function alloy-cathode
•
Organic layers, cathode were
vacuum deposited.
C.W. Tang & S.A. VanSlyke, Kodak Research Laboratories
Emission Spectrum of the EL Diode.
EL emission spectrum is sensitive to
thickness of organic layer.
Diamine layer transports holes and blocks
electrons injected from Mg:Ag
electrode.
Brightness-Current-Voltage Characteristics
Most of the bias voltage is across AlQ3
EL diode can be driven to produce
high brightness.
Key Factors
•
Morphological properties of organic layers are critical.
•
Thin films must be smooth and continuous .
•
Mg is susceptible to atmospheric oxidation and corrosion
•
Ag improves the sticking coefficient of the metal to the organic layer.
•
A dc voltage of less than 10V drives the diode.
Full-Color Displays
•
Development of red, green, and blue
emitting electroluminophores
•
Photophysical properties of Alq3-type
complexes are dominated by ligandcentered excited states
Pavel Jr.et al., J. Org. Chem. 69, 2004, 1723
Varying degree of
electronic density in the
quinolinolate ligand,
Excitation of dichloromethane
solutions at 365 nm.
Preliminary experiments with fabrication of
OLED devices
•
All complexes are
electroluminescent
•
They can be processed via
vapor deposition
The emission maxima of the OLEDs
are very close to the maxima recorded
in solution
Other Materials
Abhishek et al., Chemistry of Materials, 2004 ASAP
Rules governing the fluorescence of metal
chelates
(1)
Paramagnetic metal ions : Essentially non-fluorescent
(2)
Increasing atomic number : Fluorescence reduced
InQ3 < GaQ3 <AlQ3
(3)
Covalent nature of the metal-ligand bonding increased : Emission shifts
to longer wavelength.
Light Emitting Polymers
1. Dendrimers:
They are highly branched structures
built up from monomer units with
precisely controlled architectures.
2. Long chain conjugated molecules:
Semiconducting property
Electroluminescent behavior
•
Semiconducting properties :delocalised -electron bonding
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 and * orbitals form delocalised valence and conduction
wavefunctions, which support mobile charge carriers.
•
Electrons and holes capture : polymer film
•
Form neutral bound excited state: Exciton
•
Due to confinement, energy difference between singlet and triplet may
be large.
R.H.Friend et al., Nature 397, (1999), 121
J.H. Burroughes et al., Nature 347, (1990), 539
Perfluorinated Phenylene Dendrimers
•
Good Electron-transport materials for OLEDs
(1) Low-lying LUMOs and HOMOs
(2) Relatively low sublimation temperature
(3) Good thermal and chemical stability
(4) Soluble in CHCl3, THF and aromatic solvents
such as toluene.
Suzuki et al.,J. Am. Chem. Soc. 122, 2000, 1832
Luminance-voltage characteristics
Performance of the devices
3 < 2 < 4 < 5.
2 and 3 (biphenyl)< 4 (p-terphenyl)
< 5 (p-quaterphenyl)
When the LUMO energy level of the
electron-transport material
becomes lower, the electron
injection from the metal layer to
the electron-transport layer should
be easier
www.iitk.ac.in
Perfluorinated Oligo(p-Phenylene)s:
PF-5P <1< PF-6P = PF-7P = PF-8P <2
•
A perfluoro-2-naphthyl group turned out to be an excellent building block for
constructing n-type semiconductors
•
This might indicate that the LUMO level is low enough rate of electron
injection is not affected by the LUMO energy
Sophie B. Heidenhain et al.,J. Am. Chem. Soc.122, 2000, 10240
•
Inorganic semiconductors , organic dyes : deposited sublimation or
deposition
vapor
Fluorescent conjugated polymers : deposited from solution by spin-coating or
Langmuir Blodgett technique
A.B.Holmes et al., Angew. Chem. Int. Ed. 37, 1998, 402
Multilayer Devices
Increase efficiency of devices electron injection has to be
significantly boosted.
Electron-conducting/holeblocking
(ECHB) layer
Design of ECHB
Electron-deficient and poor hole
acceptor
Work on electron hopping mechanism
Fu Wang et al., Adv. Mater. 11, 1999, No. 15
Polymers with higher electron affinity
Ideal light-emitting polymer should
be both fluorescent and avoid the
need for an extra electrontransporting material.
Electron-withdrawing groups on the
ring or vinylene moiety of PPV
A.B.Holmes et al., Angew. Chem. Int. Ed. 37, 1998, 402
_
Effect of Dopant (Organic Fluorescent dyes)
Dyes in solid state suffer from
 Quenching
 Broadening of emission bands
 Bathochromic Shifts
Rubrene
Doping fluorescent dye as guest in a
host matrix
Increase in lifetime
Peter Baeuerl et al.,J. Mater. Chem., 10, 2000 , 1471
Other applications
•
FOLED: Flexible OLED
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PHOLED :Phosphorescent OLED
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TOLED: Transparent OLED
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SOLED: Stacked OLED
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PMOLED: Passive Matrix OLED
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AMOLED: Active Matrix OLED
Future Research
Solutions for the following:
• Susceptibility towards oxidative degradation
• Lifetimes remains lower
• Photooxidation produces carbonyl defects that quench fluorescence
Corporations in OLED’s
Small Molecule
Kodak
IBM
UDX
Ritek
Polymer
CDT
Dupont
Philips
Dow Chemicals
Conclusion
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OLED is a display device that sandwiches carbon based films between the
two electrodes and when voltage is applied creates light
•
SMOLED’s & LEP’s are its technology branches.
•
Chemical modifications to the structure can tune the emission over the
entire visible region.
•
Multilayer devices and dopants also play a role in tuning emission.
The dynamic interplay of chemistry with device physics results in these
remarkable displays.
Acknowledgments
Prof. Russell.H.Schmehl
Group Members : Dr.Sujoy Baitalik
Heidi Hester
Kalpana Shankar
Rupesh Narayana Prabhu
David Karam
Chemistry Department
All of You
Different forms of luminescence
Luminescence type
Excitation Source
Application
Catholuminescene
Electrons
TV sets, monitors
Photoluminescene
(UV) Photons
Fluorescent lamps,
plasma displays
Chemiluminescene
Chemical reaction energy
Analytical chemistry
Bioluminescence
Biochemical reaction energy Analytical chemistry
Electroluminescene
Electric field
LEDs, EL displays
Triboluminescence
Mechanical energy

Hole-Injection Materials
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Anode buffer layer- reduces the
energy barrier in-between
ITO/HTL.
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Enhances charge injection at
interface.
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CuPc,p-doped aromatic amines,