7/29/2019 Srivathsa Dept Seminar
1/50
7/29/2019 Srivathsa Dept Seminar
2/50
Outline
1. Chronology of display technology2. Advantages of LEDs
3. Definition of OLED
4. Principles of operation
5. Technology Branches
SMOLEDs
LEPs
6. Effect of dopant
7. Other applications
8. Corporations in this field
9. Conclusion
7/29/2019 Srivathsa Dept Seminar
3/50
Energy Molecular
Systems
Basic Idea Behind Emission
Light
7/29/2019 Srivathsa Dept Seminar
4/50
Beginning of LED
www.kodak.com
7/29/2019 Srivathsa Dept Seminar
5/50
Advantages of LEDs 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.
7/29/2019 Srivathsa Dept Seminar
6/50
7/29/2019 Srivathsa Dept Seminar
7/50
Semiconductor LEDs
LEDs work on the principle of injectionluminescence.
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
7/29/2019 Srivathsa Dept Seminar
8/50
OLED is a display
device that
sandwiches carbon
based films between
the two electrodesand when voltage is
applied creates light.
www.ol-ed.com
7/29/2019 Srivathsa Dept Seminar
9/50
Single Layer Device
Organic electroluminescene (EL) is the electrically drivenemission of light from non-crystalline organic materials
7/29/2019 Srivathsa Dept Seminar
10/50
Energy level diagram of a two-layer OLED
HOMO, LUMO of the HTL isslightly above that of the ETL
L.S.Hung et al.,MaterialsScience and EngineeringR 39, (2002), 143
7/29/2019 Srivathsa Dept Seminar
11/50
Chemistry behind Emission
Electrons and holes recombine :singlet state, triplet state
Formation of triplet is 3 times
more feasible than singlet
- + + S + T
S + T S0 + h
7/29/2019 Srivathsa Dept Seminar
12/50
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
7/29/2019 Srivathsa Dept Seminar
13/50
R.H.Friend et al., Nature 413, 2001, 828
7/29/2019 Srivathsa Dept Seminar
14/50
Thermodynamics of Electroluminescence
A + e- A- Ereduction (- )
A+ + e- A E oxidation
(+)
A + hv
A+ + A-
When E oxidation - Ereduction > or = E emission
A*
2A or A +A*
E emission
Ereduction -1.4 V
e- E oxidation + 1.2 V
Eemission 2.05 V
Ru(bpy)32+
+ e-
Ru(bpy)33++ Ru(bpy)3
2+Ru(bpy)3
+
Ru(bpy)3 + hvRu(bpy)3
3*
7/29/2019 Srivathsa Dept Seminar
15/50
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.
7/29/2019 Srivathsa Dept Seminar
16/50
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 LightEmitting Diodes (SMOLEDs).
Using relatively small
molecules (even monomers)
eg: Metal chelates
7/29/2019 Srivathsa Dept Seminar
17/50
7/29/2019 Srivathsa Dept Seminar
18/50
Criteria Metal chelates must satisfy
Thermally stable,
Highly luminescent in the solid state,
Thin-film forming upon vacuum deposition
Capable of transporting electrons.
SMOLEDs
C.H.Chen et al., Coordination Chemistry Reviews171, (1998), 161
7/29/2019 Srivathsa Dept Seminar
19/50
Early thin film organic device
Relatively High voltage (80-100 V)
- Inject charge into organic
crystals
Low work function alloy-cathode
Organic layers, cathode were
vacuum deposited.
Mg:Ag 10:1
Luminescent film - 600A
Diamine 750A
C.W. Tang & S.A. VanSlyke, Kodak Research Laboratories
7/29/2019 Srivathsa Dept Seminar
20/50
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.
7/29/2019 Srivathsa Dept Seminar
21/50
Brightness-Current-Voltage Characteristics
Most of the bias voltage is across AlQ3
EL diode can be driven to produce
high brightness.
7/29/2019 Srivathsa Dept Seminar
22/50
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.
7/29/2019 Srivathsa Dept Seminar
23/50
Full-Color Displays
Development of red, green, and blueemitting electroluminophores
Photophysical properties of Alq3-typecomplexes are dominated by ligand-centered excited states
Pavel Jr.et al., J. Org. Chem. 69, 2004, 1723
7/29/2019 Srivathsa Dept Seminar
24/50
Varying degree of
electronic density in the
quinolinolate ligand,
Excitation of dichloromethane
solutions at 365 nm.
7/29/2019 Srivathsa Dept Seminar
25/50
Preliminary experiments with fabrication ofOLED 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
7/29/2019 Srivathsa Dept Seminar
26/50
Other Materials
Abhishek et al., Chemistry of Materials, 2004 ASAP
7/29/2019 Srivathsa Dept Seminar
27/50
Rules governing the fluorescence of metalchelates
(1) Paramagnetic metal ions : Essentially non-fluorescent
(2) Increasing atomic number : Fluorescence reduced
InQ3 < GaQ3
7/29/2019 Srivathsa Dept Seminar
28/50
Light Emitting Polymers
1.Dendrimers:They are highly branched structures
built up from monomer units withprecisely controlled architectures.
2. Long chain conjugated molecules:
7/29/2019 Srivathsa Dept Seminar
29/50
Semiconducting property
7/29/2019 Srivathsa Dept Seminar
30/50
Electroluminescent behavior
Semiconducting properties :delocalised -electron bonding
and * orbitals form delocalised valence and conductionwavefunctions, 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 maybe large.
R.H.Friend et al., Nature 397, (1999), 121
J.H. Burroughes et al., Nature347, (1990), 539
7/29/2019 Srivathsa Dept Seminar
31/50
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 solventssuch as toluene.
Suzuki et al.,J. Am. Chem. Soc. 122, 2000, 1832
7/29/2019 Srivathsa Dept Seminar
32/50
7/29/2019 Srivathsa Dept Seminar
33/50
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 theelectron-transport materialbecomes lower, the electroninjection from the metal layer to
the electron-transport layer shouldbe easier
7/29/2019 Srivathsa Dept Seminar
34/50
www.iitk.ac.in
7/29/2019 Srivathsa Dept Seminar
35/50
Perfluorinated Oligo(p-Phenylene)s:
PF-5P
7/29/2019 Srivathsa Dept Seminar
36/50
A perfluoro-2-naphthyl group turned out to be an excellent building block forconstructing n-type semiconductors
This might indicate that the LUMO level is low enough rate of electroninjection is not affected by the LUMO energy
Sophie B. Heidenhain et al.,J. Am. Chem. Soc.122, 2000, 10240
7/29/2019 Srivathsa Dept Seminar
37/50
Inorganic semiconductors , organic dyes : deposited sublimation or vapor
deposition
A.B.Holmes et al., Angew. Chem. Int. Ed. 37, 1998, 402
Fluorescent conjugated polymers : deposited from solution by spin-coating or
Langmuir Blodgett technique
7/29/2019 Srivathsa Dept Seminar
38/50
Multilayer Devices
Increase efficiency of devices -
electron injection has to be
significantly boosted.
Electron-conducting/holeblocking
(ECHB) layer
7/29/2019 Srivathsa Dept Seminar
39/50
Design of ECHB
Electron-deficient and poor holeacceptor
Work on electron hopping mechanism
Fu Wang et al., Adv. Mater. 11, 1999, No. 15
7/29/2019 Srivathsa Dept Seminar
40/50
Polymers with higher electron affinity
Ideal light-emitting polymer should
be both fluorescent and avoid the
need for an extra electron-
transporting material.
Electron-withdrawing groups on the
ring or vinylene moiety of PPV
A.B.Holmes et al., Angew. Chem. Int. Ed. 37, 1998, 402
7/29/2019 Srivathsa Dept Seminar
41/50
_
7/29/2019 Srivathsa Dept Seminar
42/50
Effect of Dopant (Organic Fluorescent dyes)
Dyes in solid state suffer from
Quenching
Broadening of emission bands
Bathochromic Shifts
Doping fluorescent dye as guest in a
host matrix
Increase in lifetime
Peter Baeuerl et al.,J. Mater. Chem., 10, 2000 , 1471
Rubrene
7/29/2019 Srivathsa Dept Seminar
43/50
Other applications
FOLED: Flexible OLED
PHOLED :Phosphorescent OLED
TOLED: Transparent OLED
SOLED: Stacked OLED
PMOLED: Passive Matrix OLED
AMOLED: Active Matrix OLED
7/29/2019 Srivathsa Dept Seminar
44/50
Future Research
Solutions for the following:
Susceptibility towards oxidative degradation
Lifetimes remains lower
Photooxidation produces carbonyl defects that quench fluorescence
7/29/2019 Srivathsa Dept Seminar
45/50
Corporations in OLEDs
Small Molecule
Kodak
IBM
UDX
Ritek
Polymer
CDT
Dupont
Philips
Dow Chemicals
7/29/2019 Srivathsa Dept Seminar
46/50
7/29/2019 Srivathsa Dept Seminar
47/50
Conclusion
OLED is a display device that sandwiches carbon based films between the
two electrodes and when voltage is applied creates light
SMOLEDs & LEPs are its technology branches.
Chemical modifications to the structure can tune the emission over theentire visible region.
Multilayer devices and dopants also play a role in tuning emission.
The dynamic interplay of chemistry with device physics results in theseremarkable displays.
7/29/2019 Srivathsa Dept Seminar
48/50
Acknowledgments
Prof. Russell.H.Schmehl
Group Members : Dr.Sujoy Baitalik
Heidi Hester
Kalpana Shankar
Rupesh Narayana Prabhu
David Karam
Chemistry Department
All of You
7/29/2019 Srivathsa Dept Seminar
49/50
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
7/29/2019 Srivathsa Dept Seminar
50/50
Hole-Injection Materials
Anode buffer layer- reduces theenergy barrier in-betweenITO/HTL.
Enhances charge injection atinterface.
CuPc,p-doped aromatic amines,