OutlineOutline

1. Chronology of display technology

2. Advantages of LED’s

3. Definition of OLED

4. Principles of operation

5. Technology Branches

SMOLED’s

LEP’s

6. Effect of dopant

7. Other applications

8. Corporations in this field

9. Conclusion

EnergyMolecular

Systems

Basic Idea Behind EmissionBasic Idea Behind Emission

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’sSemiconductor 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 DeviceSingle Layer Device

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

Energy level diagram of a two-layer OLEDEnergy 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 EmissionChemistry 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

Photoluminescence vs. ElectroluminescencePhotoluminescence 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 ElectroluminescenceThermodynamics of Electroluminescence

A + e- A- E reduction(- )

A+ + e- A E oxidation(+)

A + hv

A+ + A-

When E oxidation - E reduction > or = E emission

A*

2A or A + A*

E emission

E reduction -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 + hv Ru(bpy)33*

Factors influencing efficiencyFactors 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 BranchesTwo 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

Criteria Metal chelates must satisfy

• Thermally stable,

• Highly luminescent in the solid state,

• Thin-film forming upon vacuum deposition

• Capable of transporting electrons.

SMOLEDsSMOLEDs

C.H.Chen et al., Coordination Chemistry Reviews 171, (1998), 161

Early thin film organic deviceEarly 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 – 10:1Luminescent film - 600ADiamine – 750A

C.W. Tang & S.A. VanSlyke, Kodak Research Laboratories

Emission Spectrum of the EL Diode.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 CharacteristicsBrightness-Current-Voltage Characteristics

Most of the bias voltage is across AlQ3

EL diode can be driven to produce high brightness.

Key FactorsKey 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 DisplaysFull-Color Displays

• Development of red, green, and blue emitting electroluminophores

• Photophysical properties of Alq3-type complexes are dominated by ligand-centered excited states

Pavel Jr.et al., J. Org. Chem. 69, 2004, 1723

Varying degree of electronic density in the quinolinolate ligand,

Excitation of dichloromethanesolutions at 365 nm.

Preliminary experiments with fabrication of Preliminary experiments with fabrication of OLED devicesOLED 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 MaterialsOther Materials

Abhishek et al., Chemistry of Materials, 2004 ASAP

Rules governing the fluorescence of metal Rules governing the fluorescence of metal chelateschelates

(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 PolymersLight Emitting Polymers

1. Dendrimers:

They are highly branched structures built up from monomer units with precisely controlled architectures.

2. Long chain conjugated molecules:

Semiconducting propertySemiconducting property

Electroluminescent behaviorElectroluminescent behavior

• Semiconducting properties :delocalised -electron bonding

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 DendrimersPerfluorinated 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 characteristicsLuminance-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(Perfluorinated Oligo(pp-Phenylene)s:-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 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

Multilayer DevicesMultilayer Devices

Increase efficiency of devices - electron injection has to be significantly boosted.

Electron-conducting/holeblocking (ECHB) layer

Design of ECHBDesign 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 affinityPolymers 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

_

Effect of Dopant (Organic Fluorescent dyes)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

Other applicationsOther applications

• FOLED: Flexible OLED

• PHOLED :Phosphorescent OLED

• TOLED: Transparent OLED

• SOLED: Stacked OLED

• PMOLED: Passive Matrix OLED

• AMOLED: Active Matrix OLED

Future ResearchFuture Research

Solutions for the following:

• Susceptibility towards oxidative degradation

• Lifetimes remains lower

• Photooxidation produces carbonyl defects that quench fluorescence

Corporations in OLED’sCorporations in OLED’s

Small Molecule

Kodak IBM UDX Ritek Polymer CDT Dupont Philips Dow Chemicals

ConclusionConclusion

• 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.

AcknowledgmentsAcknowledgments

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

• Anode buffer layer- reduces the energy barrier in-between ITO/HTL.

• Enhances charge injection at interface.

• CuPc,p-doped aromatic amines,