Organic Photonic Materials

Nonlinear Optics Materials

Organic Light Emitting Diode (OLED)

1

Nonlinear optics

The interaction of electromagnetic fields with various media to produce new electromagnetic fields altered in

phase, frequency, amplitude

from the incident fields 2

Inversion asymmetry materials

Second harmonic generation (SHG), the conversion of coherent light of frequency into light of frequency 2

The electro-optic effectallows one to change the refractive indexof a material by simply applying a DC electric field to the material; thus, one can utilize the modulation of an electrical signal to activate an optical switch.

3

The electro-optic effect is a change in the optical properties of a material in response to an electric fieldthat varies slowly compared with the frequency of light.

a) change of the absorptionelectroabsorption: general change of the absorption

constantsFranz-Keldysh effect: change in the absorption shown

in some bulk semiconductors Quantum-confined Stark effect: change in the

absorption in some semiconductor quantum wellselectro-chromatic effect: creation of an absorption band

at some wavelengths, which gives rise to a change in colour

4

http://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/w/index.php?title=Electroabsorption&action=edithttp://en.wikipedia.org/wiki/Franz-Keldysh_effecthttp://en.wikipedia.org/wiki/Stark_effecthttp://en.wikipedia.org/wiki/Quantum_wellhttp://en.wikipedia.org/w/index.php?title=Electro-chromatic_effect&action=edit

b) change of the refractive indexPockels effect (or linear electro-optic effect): change in the refractive index linearly proportional to the electric field. Only certain crystalline solids show the Pockels effect, as it requires inversion asymmetry

Kerr effect (or quadratic electro-optic effect, QEO effect): change in the refractive index proportional to the square of the electric field. All materials display the Kerr effect, with varying magnitudes, but it is generally much weaker than the Pockels effect

electro-gyration optical activity: change in the .

5

http://en.wikipedia.org/wiki/Pockels_effecthttp://en.wikipedia.org/wiki/Kerr_effecthttp://en.wikipedia.org/w/index.php?title=Electro-gyration&action=edithttp://en.wikipedia.org/wiki/Optical_activity

6

In harmonic generation, multiple photons interact simultaneously with a molecule with no absorption events. Because n-photon harmonic generation is essentially a scattering process, the emitted wavelength is exactly 1/n times the incoming fundamental wavelength. When the excitation color is changed, the emission color changes also. In contrast the wavelength of fluorescence emission is Stokes-shifted to a longer wavelength; the line shape is determined strictly by the molecular energy levels.

The polarization P induced in a molecule by a local electric field E

P= E + E2 +E3+

linear polarizability (the origin of refractive index)

second order hyperpolarizability (the origin of the second order nonlinear polarization response)

7

Push-Pull in a Donor-Acceptor

8

Values of Some Organic Chromophores(10-30 esu, 1064 nm)

9

Charge Transfer Resonance Structures

First, the greater the charge separation in the charge transfer state (Dm), the larger the Second, the closer the frequency of the incident light is to the resonant frequency of the charge transfer, the larger the

10

Organic Electro-Optic MaterialsA Historical Perspective

Statistical mechanical calculations suggested a new paradigm optimization of electro-optic activity: Control chromophore shape!

11

N

O

NCCN

NC

N SO

CNNCNC

N

O

NCCN

NC

R' R'

R

R

R

R

R

R

CLD-1

CLD-2

CLD-3

R = OTBDMS

R=H

FTC-1 R = OAc, R' = H

FTC-2 R = OAcR' = CH2CH2CH2CH 3

R = H

0

20

40

60

80

100

120

140

0 20 40 60

No. Density (10^19/cc)

CLD-2

CLD-3

Disperse Red (1995)

For Bulk Materials

P = (1) E + (2)E2 + (3) E3+ ...

12

Fabrication of organic second order NLO materials

organic crystal growth, inclusion complexes, mono- and multilayered assemblies

(e.g. Langmuir-Blodgett films), poled polymers

13

Polymer poling

The polymer is heated above the glass transition temperature and placed in a strong external electric field; this process is termed poling.

The field serves to orient the chromophorewith its dipole moment parallel to the applied field.

14

light-emitting diode (LED)

A light-emitting diode (LED) is a semiconductor device that emits incoherent narrow-spectrum light when electrically biased in the forward direction. This effect is a form of electroluminescence. The color of the emitted light depends on the chemical composition of the semiconducting material used.

AlGaAs - red and IRAlGaP - green AlGaInP - high-brightness orange-red, orange, yellowyellow, and greenGaAsP - red, orange-red, orange, and yellowyellowGaN - green, and blueInGaN - near UV, bluish-green and blueAlN, AlGaN - near to far UV

15

16

(A) a band diagram and (B) absorption spectrum of a semiconductor.

17

18

ZnZn

SS

Zinc blende structureDiamond structure

19

Table 7.3 Periodic Properties of a Family of Isoelectronic, Tetrahedral Semiconductors

Material Cubic Unit Cell Parameter,

Eg, eV (, nm)

Ge 5.66 0.0 0.66 (1900)GaAs 5.65 0.4 1.42 (890)ZnSe 5.67 0.8 2.70 (460)CuBr 5.69 0.9 2.91 (430)

20

21

22

23

24

Paulings Electronegativities

25

Length of unit cell = 5.658 0.004

Increasing ionic character

26

Emission Spectra of LED

27

28

Progress of LED, OLED, and PLED

29

Units of LED Efficiency

External Quantum Efficiency (%)= (Photon# / Electron#) 100%

Luminance Efficiency (cd/A)(Photometric Efficiency)

Power Efficiency (lm/W)Luminance (Lm) : cd/m2Current density (J) : mA/cm2

Luminous flux Luminous IntensityLumen

Name:Unit: Candela

LuminanceCandela/m2 (nit)

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Scale of Light Intensity

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30,000,000 -300,000,0000 -

3,000,000 -300,000 -30,000 -3,000 -300 -3 -0.3 -0.03 -0.003 -0.0003 -0.00003 -0.0000003 -

cd/m2

There are two main directions in OLED: Small Molecules and Polymers.

The first technology was developed by Eastman-Kodak and is usually referred to as "small-molecule" OLED. The production of Small-molecule displays requires vacuum deposition which makes the production process expensive and not so flexible.

A second technology, developed by Cambridge Display Technologies or CDT, is called LEP or Light-Emitting Polymer, though these devices are better known as Polymer Light Emitting Devices(PLEDs). No vacuum is required, and the emissive materials can be applied on the substrate by a technique derived from commercial ink-jet printing.

Recently a third hybrid light emitting layer has been developed that uses nonconductive polymers doped with light-emitting, conductive molecules. The polymer is used for its production and mechanical advantages without worrying about optical properties. The small molecules then emit the light and have the same longevity that they have in the Small-Molecule OLEDs. 32

Organic Light-Emitting Diodes (OLEDs)

OLEDs operate at substantially lower efficiency than inorganic (crystaline) LEDs. The best efficiency of an OLED so far is about 10%.

It is much cheaper to fabricate OLED than inorganic LEDs, and large arrays of them can be deposited on a screen using simple printing methods to create a color graphic display.

OLEDs will have most impact on markets for small, high information content display required low to medium brightness (mobile phone, PDA, lap-top computer).

33

Organic Light-Emitting Diodes (OLEDs)

vs. inorganic LEDsFlexibilitySimple and easy thin film fabrication and micronscale patterning (vs. wire-bonded epitaxial AlGaAs or group III nitride discrete semiconductor LEDs)

vs. liquid crystal display, LCDWide viewing angleVery bright and highly contrastNo back-lighting needed (low energy consumption)Fast switching times (video-rate display)Multicolor emission (RGB)Thin and light weightFoldable, very thin screen possible

34

35

(

)

(

)

Configuration of LCD and OLED

LCD

OLED

36

37

Stokes shiftPhotoexcitation and Relaxation

Jablonski Diagram

38

S1

S2

T1

S0

vc

ISC

vcvc

vc

ISC

IC

vc

vc haha hf

hp

IC

vc : vibrational cascade

ha: absorption energyhf : fluorescence energyhp : phosporescence energy

IC: internal conversionISC : intersystem crossing

Illustrating possibleelectronic process following absorption of a photon with energy ha

S0: singlet ground stateS2: second lowest singlet excited stateS1: lowest singlet excited stateT1: lowest triplet excited state

Competition Among Flat Panel Displays (FPDs)

39

Thin-film transistor (TFT)

From Wikipedia, the free encyclopedia.

A thin film transistor (TFT) is special kind of field effect transistor made by depositing thin films for the metallic contacts, semiconductor active layer, and dielectric layer. Most TFTs are not transparent themselves, but their electrodes and interconnects can be. The first transparent TFTs, based on zinc oxide were reported in 2003. The best known application of thin-film transistors is in TFT LCDs. Transistors are embedded within the panel itself, reducing crosstalk between pixels and improving image stability. As of 2004, all but the cheapest color LCD screens use this technology.

40

http://en.wikipedia.org/wiki/Zinc_oxidehttp://en.wikipedia.org/wiki/TFT_LCD

CIE 1931 (x, y) Chromaticity DiagramInternational Commission on Illumination

41

The human eye has receptors for short (S), middle (M), and long (L) wavelengths, also known as blue, green, and red receptors. That means that one, in principle, needs three parameters to describe a color sensation. In the CIE diagram, those parameters are not the M, S, and L stimuli, but rather a more abstract x and y parameter, and an implicit luminosity (brightness) parameter, that is not shown

42Adv. Mater. 2000, 12, 1737

43

Comparison of OLEDs with the Other FPDs

* US Billion; source from Standford Resource, 2000, 8

Item LCD PDP VFD FED Inorg. EL OLED

View Angle Improving Excellent Excellent Excellent Excellent Excellent

Efficiency(lm/W) 2 - 3 1 0.8 - 14 7 2 4 5 - 10

Full color Excellent good Limited Limited Limited Improving

Size (in.) < 21 > 40 Small 5 - 20 2 - 20 2 - 20

VoltageTFT: 2 5BL: 1000

AC90 - 150

DC10 - 40

DC1000

AC200

DC

44

Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913.

indium oxide

45

Indium tin oxide (ITO) is a mixture of (III) (In2O3) and tin(IV) oxide (SnO2), typically 90% In2O3, 10% SnO2 by weight. ITO is mainly used to make transparent conductive coatings for electronic displays, and heat-reflecting coatings for architectural, automotive, and light bulb glasses.

Physical PropertiesState of matter SolidMelting point 1800-2200 K (2800-3500 F)Density 7120-7160 kg/m3 at 293 KColor (in powder form)

Pale yellow to greenish yellow, depending on SnO2concentration

http://en.wikipedia.org/wiki/Indiumhttp://en.wikipedia.org/wiki/Oxidehttp://en.wikipedia.org/wiki/Tinhttp://en.wikipedia.org/wiki/Light_bulbhttp://en.wikipedia.org/wiki/Colorhttp://en.wikipedia.org/wiki/Kilogram_per_cubic_metrehttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Kelvinhttp://en.wikipedia.org/wiki/Melting_pointhttp://en.wikipedia.org/wiki/State_of_matter

Electrochemical and Light-Emitting of OLED

Element Work Function (eV) ElementWork

Function (eV)Cs 2.14 Ag 4.26K 2.30 Al 4.28

Ba 2.70 Nb 4.30Na 2.75 Cr 4.50Ca 2.87 Cu 4.65Li 2.90 Si 4.85

Mg 3.66 Au 5.10In 4.12 46

emitting layer

hole-transporting layer

Adv. Mater. 2000, 12, 173747

Al

NO

N

O

NO

Alq3o

(600 )Diamine

o

N N

(750 )

anthracenecrystal

( 10~20 m)

GlassITO

turn-on voltage < 10 V

1% external quantum efficiency1.5 lm/W luminous efficiency

Mg:Ag

Ag

Ag

turn-on voltage > 400 V

external quantum efficiency ~5%

Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913.Pope, M.; Kallmann, H. P.; Magnante, P. J. Chem. Phys.1963, 38, 2042. 48

Layer Structures of OLEDUnbalanced charge-mobility (10-5 cm2/Vs for electron and 10-3 cm2/Vs for hole) requires electron- or hole-transporting materials to balance the charges

Glass

Glass Glass

Glass

ITO

ITO ITO

ITO

Single-Layer Device

Double-Layer Device

Double-Layer Device

Triple-Layer Device

Metal Metal

Metal Metal

Metal Metal

Metal Metal

Electron-Transporting (Hole-Blocking) MaterialLight-Emitting MaterialHole-Transporting (Electron-Blocking) Material

49

OLED Efficiency

50

The Magic of Alq3

Al

NO

N

O

NO

High Td > 350 oC: thermally stable

1. Ball-Shape Molecule: Hard to crystallizeExciplex formation prohibited: efficient fluorescence

in solid stateVoltile under reduced pressureHigh Tg ~ 175 oC: stable glass phase

defect-free amorphous film

2. Six-Coordinated Metal : Chemically inert

Six-Coordinated Octahedron

3. Availability: Very easy to synthesize

51

O OAiO + N

OH

Aluminium isoproxide 8-Hydroxyquinoline

39 USD/ Kg 79 USD/ 500 gAlq3

45 USD/ 5 g (99%)

toluene

66 USD/ 5 g (99.9995%)

Al

NO

N

O

NO

Alq3Metal stabilizes chelating ligand

Fine Tuning Color of Alq3

Al

NO

N

O

NO

Cl

Cl

Cl

Al

NO

N

O

NO

532 nm 542 nm

Al

NO

N

O

NO

Al

N

N

O

NN

O

NN O

580 nm

Al

NO

N

O

NO

563 nm

Alq3 LUMO

Alq3 HOMO

522 nm

Al

N

NO

NN

O

N

NO

440 nm

52Burrows, P. E.; Shen, Z.; Bulovic, V.; McCarty, D. M.; Forrest, S. R.; Thompson, M. E. J. Appl. Phys. 1996, 79, 7991

Chen, C. H.; Shi, J. Coord. Chem. Rev. 1998, 171, 161.

Tuning of Energy Gap by Donor and Acceptor

LUMO

HOMO

Donor on HOMOAcceptor on LUMO

LUMO

HOMO

Donor on LUMO

Acceptor on HOMO

Red-Shifted Blue-Shifted

53

Enhancing Performance of OLED by Dopants

ON

CNNC

DCM1

Fluorescent Red Dopant:

Glass

ITOMg:Ag

Highly Fluorescent Green Dopant:

Mg:Ag/Alq3:dopant/diamine/ITO

ON

S

ONCoumarin 540

54Tang, C. W.; VanSlyke, S. A.; Chen, C. H. J. Appl. Phys.1989, 65, 3610

Cascade Frster Energy Transfer

Alq3DCM1

excitation bycharge-

recombination

greenemission

redemission

excitation by Forster energytransfer from Alq3

the overlap of donoremission with acceptorabsorption spectra

absorption

emission

a through spaceCoulombic dipole-dipole interaction

400 500 600 70055nm

Color Dopant Materials for OLED

NN

N NPt

PtOEP

N

O

NC CN

DCJT

N

O

NC CN

DCM

NH

HN

O

OQuinacridone

Rubrene

O

N

S

N O

Coumarin 6

NO

NO

AlO

BAlq

N

NEu

O

OF3C

S

3

Eu complex

perylene

in

400 500 600 700nm 56

N N

O

PBDElectron Transporting Layer (ETL)

N

OCH3

OCH3NSD

Hole Transporting / Light Emitting Layer (EML)

SNN

NOC2H5C2H5O

H3CO

Orange Dopant

The Width of Recombination Region in OLED

Virtually all radioative recombination occurs in the HTL, within 100 A of the HTL/ETL interfaces

Adachi, C.; Tsutsui, T.; Saito, S. Optoelectron. Dev. Technol. 1991, 6, 25.

57

Theoretical Efficiency (el) of OLEDs

el = r pl

: Light output coupling factor = 1/(2n2) 20%n: refractive index of the emission medium (n = 1.7 in Alq3-based devices)

: Probability of carrier recombinationmaximum ~ 100% (balanced hole and electron in OLED)

25% for singlet-state (fluorescence) 75% for triplet-state (phosphorescence)

el : Production efficiency of an exciton

pl: Fluorescence or Phosphorescence quantum yields50% ~100% for most organic compounds

Maximumel is

2.5%~5% for fluorescent materials7.5%~15% for phosphorescent materilas

_ _ _ _

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First Polymer-Based OLED (PLED)

60

Current-voltage-luminance determinations for two PLED devices:a) employing a green emitter, and b) using a red one. c) EL spectra for the two emitting materials.

61

Due to the disorder of the polymer matrix, emission peaks will be broad, with a full width at half maximum(FWHM) approaching 60 to 70 nm for monochromaticsources.

62

Narrow Emission Band from PLED with Microcavities

Distributed Bragg Reflector (DBR): a stack of layers having alternating high (PPV doped with nanoparticles of SiO2) and low refractive indexes

Ho, K. H.; Thomas, D. S.; Friend, R. H.; Tessler, N. Science, 1999, 285, 233.63

Issues need to be Solved for OLEDs

Reliability (operation lifetime) 10000 (polymeric film) ~ 35000 (molecular film) hours @ 200 cdm-2

Encapsulation problems: H2O and O2 from air kill OLED devices

Material problems:Crystallization (Low Tg) of molecular materials

Electrode problems:Charge-injection interface barrierDiffusion and degradation of ITO anode and metal cathode)

Efficiency (photon/electron) 10% of commercial light bulbs

64

Decay of OLED

ITO

Mg : AgAlq3 : rubrene

CuPc-NPD

Glass

Initial luminance of 100 cd/m2

65

Methods for Full Color OLEDs

66

(a) Side-by-side patterning of RGB emitters

(b) Color passband filting of white emitters

(c) Wavelength down-conversion of blue emitters

(d) Microcavity-filtered white emitters

(e) Color-tunable of stacked emitters

Bulovic, V.; Burrows, P. E.; Forrest, S. R. Semicond. Semimetal. 2000, 64, 255.

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Disadvantages of OLED:

- Engineering Hurdles OLEDs are still in the development phases of production. Although they have been introduced commercially for alphanumeric devices like cellular phones and car audio equipment, production still faces many obstacles before production.

- Color lifetime The reliability of the OLED is still not up to par. After a month of use, the screen becomes nonuniform. Reds, and blues die first, leaving a very green display. 100,000 hours for red, 30,000for green and 1,000 for blue. Good enough for cell phones, but not laptop or desktop displays.

- Overcoming Commercial development of the technology LCDs have predominately been the preferred form of display for the last few decades. Tapping into the multi-billion dollar industry will require a great product and continually innovative research and development. Furthermore, the basics of OLED technology is heavily patented by Kodak and other firms, requiring outside research teams to acquire a license.

Organic Photonic MaterialsNonlinear opticsPush-Pull in a Donor-AcceptorValues of Some Organic Chromophores (10-30 esu, 1064 nm)Charge Transfer Resonance StructuresFor Bulk MaterialsP = (1) E + (2)E2 + (3) E3 + ...Fabrication of organic second order NLO materialsPolymer polingPaulings Electronegativities