Srivathsa Dept Seminar

7/29/2019 Srivathsa Dept Seminar

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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


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Energy Molecular

Systems

Basic Idea Behind Emission

Light


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Beginning of LED

www.kodak.com


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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.


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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


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OLED is a display

device that

sandwiches carbon

based films between

the two electrodesand when voltage is

applied creates light.

www.ol-ed.com


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Single Layer Device

Organic electroluminescene (EL) is the electrically drivenemission of light from non-crystalline organic materials


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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


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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


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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


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R.H.Friend et al., Nature 413, 2001, 828


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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*


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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.


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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


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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


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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


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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.


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Brightness-Current-Voltage Characteristics

Most of the bias voltage is across AlQ3

EL diode can be driven to produce

high brightness.


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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.


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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


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Varying degree of

electronic density in the

quinolinolate ligand,

Excitation of dichloromethane

solutions at 365 nm.


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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


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Other Materials

Abhishek et al., Chemistry of Materials, 2004 ASAP


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Rules governing the fluorescence of metalchelates

(1) Paramagnetic metal ions : Essentially non-fluorescent

(2) Increasing atomic number : Fluorescence reduced

InQ3 < GaQ3


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Light Emitting Polymers

1.Dendrimers:They are highly branched structures

built up from monomer units withprecisely controlled architectures.

2. Long chain conjugated molecules:


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Semiconducting property


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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


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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


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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


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www.iitk.ac.in


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Perfluorinated Oligo(p-Phenylene)s:

PF-5P


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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


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Inorganic semiconductors , organic dyes : deposited sublimation or vapor

deposition


Fluorescent conjugated polymers : deposited from solution by spin-coating or

Langmuir Blodgett technique


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Multilayer Devices

Increase efficiency of devices -

electron injection has to be

significantly boosted.

Electron-conducting/holeblocking

(ECHB) layer


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Design of ECHB

Electron-deficient and poor holeacceptor

Work on electron hopping mechanism

Fu Wang et al., Adv. Mater. 11, 1999, No. 15


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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



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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


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Other applications

FOLED: Flexible OLED

PHOLED :Phosphorescent OLED

TOLED: Transparent OLED

SOLED: Stacked OLED

PMOLED: Passive Matrix OLED

AMOLED: Active Matrix OLED


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Future Research

Solutions for the following:

Susceptibility towards oxidative degradation

Lifetimes remains lower

Photooxidation produces carbonyl defects that quench fluorescence


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Corporations in OLEDs

Small Molecule

Kodak

IBM

UDX

Ritek

Polymer

CDT

Dupont

Philips

Dow Chemicals


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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.


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Acknowledgments

Prof. Russell.H.Schmehl

Group Members : Dr.Sujoy Baitalik

Heidi Hester

Kalpana Shankar

Rupesh Narayana Prabhu

David Karam

Chemistry Department

All of You


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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


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Hole-Injection Materials

Anode buffer layer- reduces theenergy barrier in-betweenITO/HTL.

Enhances charge injection atinterface.

CuPc,p-doped aromatic amines,

Srivathsa Dept Seminar

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