introduction - latest seminar topics for engineering cs|it

1

Organic lightOrganic light--emitting diodes emitting diodes

Fang-Chung ChenDepartment of Photonics and Display Institute

National Chiao Tung University

IntroductionIntroduction

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Organic light-emitting diodes--The emerging technology

OLED Displays

Pull out travel guide

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3

OLED Revenue Forecast by Application

http://www.displaysearch.com/press/2003/122303.htm

Future Star !!

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3

5http://www.epson.co.jp/e/newsroom/news_2004_05_18.htm

commercialization in 2007Screen size 40-inch diagonalNumber of pixels 1280 x RGB x 768dots (W-XGA)Driving method Active matrixPixels per inch 38No. of colors 260,000

Main SpecificationsScreen size 40-inch diagonalNumber of pixels 1280 x RGB x 768dots (W-XGA)

Seiko Epson: The largest OLED Display using IJPSeiko Epson: The largest OLED Display using IJP

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Candle

year

Per

form

ance

(lm

/W)

OLEDsPLE

Ds

Efficiencies of Efficiencies of LEDsLEDs

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7

History of organic lightHistory of organic light--emitting devices emitting devices ((OLEDsOLEDs))

1963 --- First organic electroluminescent (EL) based on anthracene single crystal; (Pope et al.)Problem: low quantum efficiency and high

operating voltage (>100V)

1987 --- First organic electroluminescent (EL) based on amorphous organic molecules; (Kodak; C. W. Tang et al.)high quantum efficiency (~1%); low driving voltagebi-layer structure; thin amorphous organic films

1990 --- First organic electroluminescent (EL) based on polymer; (Cambridge University; Burroughes et al.

polymer light-emitting diodes (PLEDs)

8

+

Mg Ag

Alq3

Diamine

ITO

Glass

Devices were fabricated by thermal evaporation

Drive voltage ~5VQE: ~1%; 3 cd/A (green)Fast response time (<1 μsec)

Device structure of Device structure of OLEDsOLEDs

5

9

Mechanism involves:

1: Charge injection

2: Charge transport

3: Charge recombination(Exciton formation)

cathode

anode+

- -

ITO++

diamine(hole-transporting layer)

Alq3electron-transporting layer

Mg/Ag

HTL ETL Electrical field: >105 V/cm

100 nm; @~1V

+

Mg Ag

Alq3

Diamine

ITO

Glass

Operating mechanism of Operating mechanism of OLEDsOLEDs

10

MEH-PPV

H3CO

O

n

O O

S

S

O O

O O

S

S

O O

O O

S

SO3-

SO3H

n+

n m

PEDOT:PPS

Ca/Al

ITO

+ -

glass

MEH-PPV

PEDOT:PPS

Devices were fabricated by spin-coatingSingle emissive layer was used

Operating mechanism of Operating mechanism of PLEDsPLEDs

6

11

LUMO

HOMO

2.9eV

4.7eV

π* (2.8eV)

π (4.9eV)

Ca

ITO

e-

h+

Mechanism involves:

1: Charge injection

2: Charge transport

3: Charge recombination

Anode

1

PolymerLUMO

PolymerHOMO

1 2

3

2Cathode

Device mechanismDevice mechanism

12

-10 -5 0 5 1010-11

10-9

10-7

10-5

10-3

Curre

nt (A

)

Voltage (V)

10-11

10-9

10-7

10-5

10-3

Au/MEH-PPV/Ca

Radiance (W)

ITO/MEH-PPV/Ca

Diodes!!!II--V characteristicsV characteristics

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13

• Easy and low-cost fabrication

• Solution processability

• Light weight and flexible

• Easy color tuning

Spin-coating for mono-color display

Ink Jet printing for multi-colors display

http://www.nobel.se/chemistry/laureates/2000/illpres/7.html http://www.epson.co.jp/e/newsroom/news_2004_05_18.htm

40-inchcommercialized in 2007

Why Why PLEDsPLEDs??

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Family of lightFamily of light--emitting emitting poly(thiophenepoly(thiophene))

8

15

H3CO

O

n

MEH-PPV

n

nn

PPV

PA

Eg = 1.4 eV Eg = 3.0 eV

PPP

Eg = 2.4 eV

Eg = 2.1 eV

Molecular engineering of lightMolecular engineering of light--emitting polymersemitting polymers

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Flat and thinWider viewing angle (> 160o)Saturated emissive colorWide operating temperatureHigh contrastFlexible, Plastic can be used as substratesLight weightFast response time (~μs)Low temperature processing Low cost

The merits of The merits of OLEDsOLEDs over other over other display technologiesdisplay technologies

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17

< 1μsec

Comparison of O/PLED with other Comparison of O/PLED with other display technologydisplay technology

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OLED DisplaysOLED Displays

OLEDLCD

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19

OLEDLCD


20


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21


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1.8 mmModule thickness>250Contrast ratio

670 mWPower Cons.

262K (6 bits)Color Number300 cd/m2Brightness

Bottom EmissionEmission Type

a-Si TFTSubstrate

2-TFT Voltage ProgrammingDriving Method

171um×264umSub-Pixel Pitch160×(RGB)×234Resolution

4-inchDisplay SizeFeaturesParameter

AUO – world first 4” a-Si AMOLED (2003)


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23http://www.universaldisplay.com/

OLED built on flexible substrates

FlexibilityUltra-lightweight and Thin

Flexible Flexible OLEDsOLEDs




Chemical and electronicChemical and electronicStructure of organic materials Structure of organic materials

13

25

C 、H、O、N、S…..

Organic metallics

Alq3、CuPc…..

α-NPD

Organic MaterialsOrganic Materials

26

Comparison of Bohr and waveComparison of Bohr and wave--mechanical mechanical atom modelsatom models

14

27

ϕ2 : electron density

ϕ : one-electron wave function

Atomic Atomic OrbitalsOrbitals

28

Bonding

Anti-bondingEnergy levels

Δ: depends on the degree to which the orbitalsoccupy the same space or “overlap”

Δ

Molecular Molecular OrbitalsOrbitals

15

29

Molecular Molecular OrbitalsOrbitals

30

Electronic energy Electronic energy vsvs interatomicinteratomicseparation of an aggregate of 12 atomsseparation of an aggregate of 12 atoms

16

31

single bond double bond triplet bond

π orbitals

Carbon atom bonding configurationsCarbon atom bonding configurations

32

Orbital structure of benzene (Six Carbons)Orbital structure of benzene (Six Carbons)

17

33

The The ππ--molecular molecular orbitalsorbitals and and energy levels for benzeneenergy levels for benzene

34

The lowest electronic transition (band gap, Eg)

Ethylene (C2H4) : Eg1 = 6.9 eV

Benzene (C6H6) : Eg2 = 4.6 eV

More delocalized π electrons, the lower the band gap energy

Eg1Eg2

Chemical structures of common Chemical structures of common organic semiconductorsorganic semiconductors

18

35

Electron band structures in solids at 0 K

Filled states

Emptyband

Band gap

Empty states

Filled states

Emptyband

Emptyconduction

band

Ef

Ef Band gap

Filledvalence

band

Emptyconduction

band

Band gap

Filledvalence

band

Metal (Cu) Metal (Mg) Insulator Semiconductor

(Eg >2 eV)

107 Ω-1 cm-1 10-10 - 10-20 Ω-1 cm-1 10-6 - 104 Ω-1 cm-1

36

Chemical structures of common Chemical structures of common organic semiconductorsorganic semiconductors

19

37

p-doped polyacetylene

Conductivity domain of metals, Conductivity domain of metals, semiconductors, and insulatorssemiconductors, and insulators

38

Small Molecules Functional Polymers

Weak bonding(van der Waals force)

Low melting point

Low conductivity10-8 - 10-12 Ω-1 cm-1

Organic (Molecular) SemiconductorsOrganic (Molecular) Semiconductors

20

39

A conjugated system is one having alternating single and double bonds

Conjugated PolymerBackbones:alternating single-double bonds

n

Eg = 3.0 eVn

Eg = 1.4 eV

PPPPA

n

Eg = 2.4 eV

PPV

Delocalized π electron clouds

polyacetylene

ConjugationConjugation

40

Polymer, Macromolecules

Historically, molecules larger than 10k (10000 g/mole) belong to this group

Technically, all polymers are mixtures

Polymers show isomers, and polymers having the same Chemical formula can show different properties

different

Regioregular - Polypropylene Random - Polypropylene

Polymer Polymer vsvs Small MolecularSmall Molecular

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41

ExcitonsExcitons in Organic Materialsin Organic Materials

Electronic excitation is considered as a quasi-particle, capable of migrating. This is termed as “Exciton”

Excitons can be regarded as bounded electron-hole pairs.Also can be viewed as the excited states of molecules

Frenkelexciton

Wannier-Mottexciton

Charge-transferexciton

lattice constant

42

+-

Frenkel Excitonq 1 q 2E ∝

ε r

Coulombic interaction

(binding energy 0.2 - 1.0 eVRadius ~ 10Å)

hv

-

+

LUMO

HOMO

organic molecules

The Nature of The Nature of ExcitonsExcitons in Organic Materialsin Organic Materials

22

43

+ hν

σ∗

σ

E

ground state excited state

UltravioletUltraviolet--visible (UVvisible (UV--visvis) Spectroscopy) Spectroscopy

44

λ ~ 150 nm, λ < 200 nm, vacuum ultraviolet, strongly absorbed by the oxygen

λ = 200 - 400 nm, ultraviolet, λ = 400 - 750 nm, visible, π – π* transition

σ – σ* transition


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45The longer the chain of conjugation

The longer the wavelength of the absorption band

ππ –– ππ* transitions* transitions

46

Light source

Sample

hνmonochromator

I0 I

hν

A = - log ( )I0

I

detector


PF

C8H17C8H17

n

UV-vis Spectroscopy of polyfluorene

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47

UVUV--visvis Spectroscopy of Spectroscopy of polyfluorenepolyfluorene---- another exampleanother example

48

Light source

Samplehν

Monochromator&

detector

hν’

Photoluminescence (PL)Photoluminescence (PL)

25

49

Energy

Typical energy levels and energyTypical energy levels and energy--transfertransferprocess of a moleculeprocess of a molecule

50

Only two spin states (α, β) are stable

Vector representation of an electronVector representation of an electron’’s spin magnet moments spin magnet moment

26

51

T1

X

S1

S0

S = 0 S = 1

S1

S0

S = 0

ground state

single excited state

triplet excited state

Fluorescent Phosphorescent

Single and TripletSingle and Triplet

52

“up” stateα

“down” stateβ

Single and triplet statesSingle and triplet states

1




Basic Device PhysicsBasic Device Physics

+

Mg Ag

Alq3

Diamine

ITO

Glass

Devices were fabricated by thermal evaporation

Drive voltage ~5VQE: ~1%; 3 cd/A (green)Fast response time (<1 μsec)

Device structure of Device structure of OLEDsOLEDs

2

Mechanism involves:

1: Charge injection

2: Charge transport

3: Charge recombination(Exciton formation)

cathode

anode+

- -

ITO++

diamine(hole-transporting layer)

Alq3electron-transporting layer

Mg/Ag

HTL ETL Electrical field: >105 V/cm

100 nm; @~1V

+

Mg Ag

Alq3

Diamine

ITO

Glass


+

- -

+

- hν

LUMO(Conduction band)

HOMO(Valence band)

+anode

cathode


3

Metal

ITOHTL

ETL(EML)

Metal

ITO

HTL(EML)

ETL

Metal

ITO

HTL

ETL

EML

ETL, electron-transport layer EML, emissive layerHTL, hole-transport layer

Typical multilayerTypical multilayer--device structuresdevice structures

MEH-PPV

H3CO

O

n

O O

S

S

O O

O O

S

S

O O

O O

S

SO3-

SO3H

n+

n m

PEDOT:PPS

Ca/Al

ITO

+ -

glass

MEH-PPV

PEDOT:PPS

Devices were fabricated by spin-coatingSingle emissive layer was used

Operating mechanism of Operating mechanism of PLEDsPLEDs

4

What is PEDOT:PSS?

O O

S

S

O O

O O

S

S

O O

O O

S

SO3-

SO3H

n+

n m

PEDOT:PPS

PEDOT:PSS is a hole-transporting conductive polymerDeposited from an aqueous suspension

ρ ~ 1000 to 100000 Ω-cm

Work function ~ 5.0±0.2 eV

ITO work function depends on the surface treatmentITO surface is often full of spikes

PEDOT:PSS (~ 100 nm) both planarizes the surfaceand stablizes the work function of the anode of the PLEDsIt is one of the keys to reproducible devices

Very common for PLEDsThe material should be “bi-polar”

Anode

1

PolymerLUMO

PolymerHOMO

1 2

3

2Cathode

Single layer organic EL deviceSingle layer organic EL device

5

Metal

ITO

HTL

ETL

EML

Metal

ITO

Hole-injection layer

Emitting polymer

smOLEDs:Evaporation of a multilayer stack of small organic molecules(Mw ~ several 100)

PLEDs:Spincoating/inkjet printing of polymers (Mw ~ 50,000 – 500,000)

Small molecule and Polymer Small molecule and Polymer OLEDsOLEDs

-10 -5 0 5 1010-11

10-9

10-7

10-5

10-3

Curre

nt (A

)

Voltage (V)

10-11

10-9

10-7

10-5

10-3

Au/MEH-PPV/Ca

Radiance (W)

ITO/MEH-PPV/Ca

Diodes!!!

II--V characteristicsV characteristics

6

• Easy and low-cost fabrication

• Solution processability

• Light weight and flexible

• Easy color tuning

Spin-coating for mono-color display

Ink Jet printing for multi-colors display

http://www.nobel.se/chemistry/laureates/2000/illpres/7.html http://www.toshiba.co.jp/about/press/2001_05/pr_j3001.htm

Why Why PLEDsPLEDs??

ηext = ηint ηp = γ ηr φf ηp

ηext : external quantum efficiencyηint : internal quantum efficiencyηp : light out-coupling efficiencyγ: charge carrier balance factor (e/h)ηr : efficiency of exciton productionφf : internal quantum efficiency of luminescence

Maximum external quantum efficiency is ~5%

~100% ~25% ~100% ~20%

Efficiency of Organic EL DevicesEfficiency of Organic EL Devices

7

ηp : light out-coupling efficiency

Mirror

Front view

Organic layern ~ 1.5

n = 1.0

due to total internal reflection loss ηp = 1 / (2n2)

n : reflection index of the emissive medium

If n ~ 1.5 ηp = 22%

γ: charge carrier balance factor (e/h)

ITO anode Metal cathod

Jh

Je

J’h

J’eJ

J : circuit currentJr : current used for charge recombination

γ= Jr / J

J = Jh + J’e = Je + J’hJr = Jh - J’h = Je - J’e

8

√2

√2

1/ +

-

3 symmetric statesTriplets

1 antisymmetric stateSinglet1/

+ + or

singlettriplet

hole (+) electron (-) exciton (*)

ηr : efficiency of exciton production

φf : internal quantum efficiency of luminescence

T1

X

S1

S0

intersystemcrossing

phosphorescentfluorescenceThermal deactivation

other deactivationprocesses

kF

kO

kT

kI

kP

φF =kF + kI+ kT + kO

kF

9

L. S. Hung and C. H. Chen, Mater. Sci. & Eng. R 39, 143 (2002)

75 nm NPD/75 nm Alq3

Typical ITypical I--LL--V curves of an Alq3V curves of an Alq3--based OLEDbased OLED

substrate

Cathodematerial

Organics

Metal mask

Thermal evaporation

Manufacture of Manufacture of OLEDsOLEDs

10

Spin-coating or ink-jet printing

Manufacture of Manufacture of PLEDsPLEDs

InkInk--jet printing to pattern polymersjet printing to pattern polymers

11

ηext = ηint ηp = γ ηr φf ηp

ηext : external quantum efficiencyηint : internal quantum efficiencyηp : light out-coupling efficiencyγ: charge carrier balance factor (e/h)ηr : efficiency of exciton productionφf : internal quantum efficiency luminescence

Quantum efficiency:

Power efficiency:

ηpow = ηext EpU-1

electrical power inputoptical power output

Ep : the average energy of the emitted photons

U : the known values of the applied voltage

Efficiency of organic EL DevicesEfficiency of organic EL Devices

(lm/W), important for engineer and system design

Luminous efficiency: ηlum = ηpow S

S : the eye sensitivity curves

Efficiency of organic EL DevicesEfficiency of organic EL Devices

Current efficiency (Cd/A), important for material evaluation

12

Efficiency of organic EL Devices Efficiency of organic EL Devices –– an Examplean Example

Device current density : 50 mA/cm2 at 10VBrightness : 3500 cd/m2

Current Efficiency :3500 cd/m2

50 mA/cm2x 1

10= 7 cd/A

Power Efficiency :7 cd/A10 V

x π = 2.2 lm/W

Definitions of Efficiencies of Definitions of Efficiencies of OLEDsOLEDs

S. R. Forrest et al. Adv. Mat. 15, 1043 (2003)

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