On the influence of Temperature and UV – Irradiation on Polyfluorene Michael Graf Christian Doppler Laboratory Advanced Functional Materials Institut für

On the influence of Temperature On the influence of Temperature and UV – Irradiation on and UV – Irradiation on

PolyfluorenePolyfluorene

Michael Graf

Christian Doppler LaboratoryAdvanced Functional Materials

Institut für FestkörperphysikTechnische Universität Graz

Institut für Nanostrukturierte Materialien und Photonik Joanneum Research Forschungsgesellschaft mbH.

Austria Technologie und Systemtechnik AG.

Seminar, 06.02.2008

Christian-Doppler Laboratory Advanced Functional Materials

Outline

Introduction

“The Keto Story”: Emissive Defects in Polyfluorenes (PF)Identification and Formation of Keto-DefectFluorene/fluorenone Copolymers:Emission from Ketos versus Excimer Emission Strategies to Avoid or Use Defect Emission

Excitation Energy Migration in PF – Fluorenone Copolymers

Influence of UV Irradiation on PF – Fluorenone CopolymersSuppression of Defect EmissionCross – LinkingStructured OLEDs Built from PF – Fluorenone Copolymers

Conclusion and Outlook

Acknowledgements


Motivation: Organic Light Emitting Displays Based on Printed PLEDs

Schematic image of the inkjet printing process of a color PLED display and the 40 inch full color inkjet printed polymer light-emitting display prototype fabricated by Seiko Epson Corporation.

OLED display: Inkjet printed conjugated polymers


Polyfluorenes

Blue electroluminescencee.g. A.W. Grice, D.D.C. Bradley, M.T. Bernius, M. Inbasekaran, W.W. Wu, E.P. Woo, Appl. Phys. Lett. 73, 629 (1998)

Full color emission byChemical tuningI.S. Millard, Synth. Met. 111-112, 397 (2000)Color conversion

Polarized emittersM. Grell et al. Adv. Mater. 11, 671 (1999)

RR

** n

400 450 500 5500,0

0,2

0,4

0,6

0,8

1,0

1,2

Nor

mal

ized

PL

(a.u

.)

(nm)


What is the Major Degradation Mechanism in Polyfluorenes ?

?

400 450 500 550 600 6500,0

0,2

0,4

0,6

0,8

1,0

1,2

No

rma

lize

d P

L (

a.u

.)

(nm)

400 450 500 550 600 6500,00,51,01,52,02,53,03,54,04,55,05,56,06,57,07,58,0

No

rma

lize

d P

L (

a.u

.)

(nm)


Excimer Emission

Excimer emission appears redshifted in addition to the backbone emission

Excimer emission is broad and structureless

An excimer can not be directly excited as the ground state is non binding

Excimer emission in solution depends strongly on the material concentration in the solvent

M.Pope and C.E.Swenberg,”Electronic processes in organic crystals and polymers” Oxford University Press 1999


Photoluminescence and OD of PF/Fluorenone Copolymer Solutions

PF/fluorenone copolymer can be regarded as model polymer for degraded PF

Green to orange emission increases with increasing fluorenone content

Fluorenone absorbance at 2.8 eV increases with increasing content

y x

R RO

U. Scherf et al. E-polymers, 009, (2002)L. Romaner et al. Adv. Func. Mater. 13, 597, (2003)


Identification of Emission Band at ca. 2.3 eV as Keto-Site

Emission in PL @ 2.3 eV in PF is identified as the emission from a fluorenone (keto-defect)[M. Ilharco et al. Langmuir. 13, 3787 (1997). ]

Carbonyl stretching mode (>C=O)of the fluorenone building block @ ca. 1721 cm-1 in IR [R. M. Silverstein, et. al., SpectroscopicIdentification of Organic Compounds4th ed. ; Wiley: New York 1981. ]

O

IR- spectrum

E.J.W. List et al. Advanced Materials 14, 374 (2002)


Single Molecule Spectroscopy

Presence of carbonyl units accepted as a must for defect emission.

BUT: Excimer formation at fluorenone sites still considered responsible for the observed effect (Sims et al., Adv. Func .Mater. 14, 765 (2004).

UNTIL: Spectroscopy on single molecules of PF – fluorenone copolymers displayed green emission.

K. Becker et al. Adv. Func .Mater. 16, 364 (2006)


Singlet Exciton Trapping by Keto-Defects in Films and Solutions

Intra- and interchain exciton migration in PF segments Exciton localisation on keto

h E h

Stronger interchain than and intrachain migration

Only weak on chain Förster type energy transfer to keto

Exciton localisation at keto-defect

E.J.W. List et al. Chem. Phys. Lett. 325, (2000) 132.U. Scherf et al. E-polymers, 009, (2002)

L. Romaner et al. Adv. Func. Mater. 13, 597, (2003)


Functionalization of Conjugated Polymers: The Concept of Dendronic Side Chains

h E h

Excitation energy migration prior to energy transfer to defectReducing the migration possibilities hindered energy transfer to defect sites

n

n

A. Pogantsch et al. J.Chem. Phys. 119, 6904 (2003)E. List et al. Mat. Res. Soc. Symp. Proc. 665, C5.47.1 (2001)

O

O


Fully Aryl-Substituted Ladder-Type Pentaphenylenes

400 450 500 550 600 6500,0

0,2

0,4

0,6

0,8

1,0 2 min. 3 min. 4 min. 6 min. 8 min. 10 min.

Nor

mal

ized

Ele

ctro

lum

ines

cenc

e

Wavelength (nm)

J. Jabob, et al. Macromolecules 38, 9933 (2005)

* *n

C8H17C8H17C8H17

C8H17 C8H17 C8H17

C8H17

C8H17

Arylic substituents provide much higher stability against oxidation at the 9 positions than their alcylic counrterparts.


Outline

Introduction


Excitation Energy Migration in PF – Fluorenone Copolymers



Acknowledgements


Population of the KETO-defect states

Fluorenone units can be treated as guest emitters in the polyfluoren backbone

Defects are populated via an excitation energy migration process and a subsequent energy transfer of Förster type


Transfer efficiency and rate equations

The relative Fluorenone emission component to the total photoluminescence is a direct measure for the transfer efficiency

Rate equations:

.

Flo

Flo

Flo Pf

PLrel

PL PL

( )

( )

Pf Pf Pfnr r trans Pf

Flo FloFlotrans Pf nr r Flo

dNG k k c k N

dtdN

c k N k k Ndt


Transfer efficiency

Data can be modeled as follows:

1.

1Flo Pf

r

trans PLFlo

relk

c k

Radiative lifetime 800 psS.Khan, et.al., Phys Rev B, 2004, 69, 85201

Transfer rate measured by femtosecond pump probe spectroscopy

ktrans=1.9*10-20 cm3ps-1

C.Gadermaier, et.al., Phys Rev Letters, accepted

Photoluminescence quantum yield

Fluorenone concentration

2*1019 cm-3 for 5% fluorenone


Model and measurements

Model fits the measured data with an assumed fluorenone quantum efficiency of 20%

400 450 500 550 600 650 7000.0

0.5

1.0

1.5

2.0

0.1% Fluorenone 0.5% Fluorenone

norm

alis

ed

phot

olum

ines

cenc

e

wavelength / [nm]

400 450 500 550 600 650 7000

10

20

30

40

50

1% Fluorenone 5% Fluorenone

norm

alis

ed p

hoto

lum

ines

cenc

e

wavelength / [nm]


Temperature dependence

Slight bathochromic shift

Decrease of FWHM

Decrease of the relative fluorenone emission

Photoluminescence quantum efficiency

is temperature independent

(A.Monkman, et.al., Jour Chem Phys, 2003, 119,22)

Decrease of the relative Fluorenone emission caused

by decrease of temperature dependent excitation

energy migration

1 %

0,1 %


Temperature dependence

Temperature dependent transfer rate:

Model fits the data with an activation energy of 35 meV

Rel. Φ changes from 0,58 to 0,25 at 0.1%

Rel. Φ changes from 0,95 to 0,88 at 1%

Fraction of migration much greater at low Fluorenone concentration

( )

0

A

B

E

k Ttrans Tk k k e

1 %

0,1 %


Difference between Electroluminescence and Photoluminescence

Fluorenone emission much more pronounced in EL than in PL

This was attributed to charge carrier trapping

Enhanced temperatures in polymer films due to high current densities(J.Lupton, Appl. Phys. Lett., 2002, 80, 2)

Enhanced temperatures lead to an enhanced excitation energy migration

0,1 %

1 %


Electroluminescence and Photoluminescence at low temperatures

Difference of relative fluorenone emission between EL and in PL at low temperatures is about 0.2

The same difference can be observed at room temperature

Difference can be attributed to charge carrier trapping

400 450 500 550 600 650 7000,0

0,5

1,0

1,5

2,0

2,5

electroluminescence photoluminescence

nor

m.

lum

ine

scen

ce /

[a

.u.]

wavelength / [nm]

0,1 %

0,1 %


Fluorenone emission during operation of the device

Operating the device leads to an enhanced Fluorenone emission

Relative Fluorenone emission increases by 0,04 (current density 200 mA/cm2)

Increase is fully reversible

Can be attributed to enhanced excitation energy migration

Extrapolation leads to an increase in temperature of 40K

0,1 %

0,1 %


Conclusion: Change in the spectra

Difference of the Fluorenone Emission between electroluminescence and photoluminescence results from charge carrier trapping

Enhancement of the Fluorenone emission in electroluminescence during the operation of the device can be attributed to an temperature activated excitation energy migration


Outline

Introduction


Excitation Energy Migration



Acknowledgements


Materials and Experimental Setup

Statistical Polyfluorene (PF2/6) – Fluorenone copolymers with fluorenone contents of 0.1, 0.5, and 5 mol%.

Setup:Ar ion Laser Alternatively: 1000 W Xe lamp(De)focusing LensSample holder (inert) with heating plate and thermo coupleFilter and CCD camera

O

n m

stat


Temperature Dependent PL

At elevated temperatures green emission @ 540nm starts to decrease again.

Stays decreased after cooling down again.

400 450 500 550 600 650 7000,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0 22°C 40°C 60°C 20°C cooled

No

rma

lize

d P

L (

a.u

.)

(nm)

0.5% Flo


Structuring of Thin Films

400 450 500 550 600 650 7000

100000

200000

300000

400000

500000

600000

700000

800000

0min 10min

PL

(a

.u.)

(nm)

0,5% FLO

400 450 500 550 600 650 7000

100000

200000

300000

400000

500000

600000

700000

800000

0min 3min

PL

(a

.u.)

(nm)

0,1% FLO

Relative AND absolute intensity of blue peak @ 412nm increase.

Relative AND absolute intensity of green peak @ 540nm decrease.


Cross - Linking

After washing irradiated samples with toluene, parts of the films remain on the substrate.

A UV induced cross – linking effect is held responsible for this behavior.

PL spectra of these insoluble areas display relative green intensities between pristine and cured samples.

Curing and cross – linking seem to be 2 counteracting effects!

400 450 500 550 600 6500,0

0,4

0,8

1,2

pristine cured insoluble

No

rma

lize

d P

L (

a.u

.)

(nm)

0.1% Flo


Optical Absorption

Absorption peak @ 380nm for pristine and cured samples.

No difference in spectral position for 0.1% and 5% films.

After washing, a red shift of approx. 5nm occurs.

Absolute intensities similar for pristine and cured samples.

40 – 50% of the films remain on the substrate after washing for both samples.

Cross linking ratio is independent of fluorenone content!

250 300 350 400 4500,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

360 370 380 390 4000,8

0,9

1,0

1,1 pristine cured insoluble

Opt

ical

Abs

orpt

ion

(a.u

.)

(nm)

a)

250 300 350 400 4500,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

360 370 380 390 4000,8

0,9

1,0

1,1 pristine cured insoluble

Opt

ical

Abs

orpt

ion

(a.u

.)

(nm)

b)

0.1% Flo

5% Flo


Fourier Transform Infra Red

Problem: film thickness! Too thick: no effective curing.Too thin or low fluorenone content: no sufficient signal intensity for carbonyl stretching mode @ 1721cm-1.

Carbonyl stretching intensity remains constant or increases slightly.

NO break up of C=O double bond.

Peak position remains constant. NO photo fragmentation at C=O

double bond.

Impeded energy transfer towards defect sites must be responsible for recovered blue emission!

3000 2500 2000 15000,85

0,90

0,95

1,00

1,05

1,10

1,15

0,75

0,80

0,85

0,90

0,95

1,00

1,05

0,99

1,00

1,011800 1750 1700 1650

cured

Tra

nsm

ittan

ce (

%)

wavenumber (cm-1)

C=

C s

tret

chin

g

C-H

str

etch

ing

pristine

C=

O s

tret

chin

g

?


Irradiation of Solutions

Only on-chain transfer in solutions (less green intensity)

No cross – linking effects.

PL spectra of a solution with 5% FLO shows almost total suppression of ketonic emission after curing.

5nm blue shift in absorption.

Reduced conjugation length (chain scission) causes significant reduction of energy transfer onto keto sites!

350 400 450 500 550 600 6500,0

0,2

0,4

0,6

0,8

1,0

1,2

0,0

0,2

0,4

0,6

0,8

1,0

1,2 pristine cured

No

rma

lize

d A

bso

rptio

n (

a.u

.)

(nm)

No

rma

lize

d P

L (

a.u

.)

5% Flo

350 360 370 380 390 4000,6

0,7

0,8

0,9

1,0

1,1 pristine cured

No

rma

lize

d A

bso

rptio

n (

a.u

.)

(nm)


Proposed Mechanism


Model System

A model trimer (FLO – FL – FLO) was synthesized and blend into pure PF2/6.

Low absorption cross – section.Simulates the situation of inhibited on – chain transfer.

PL of this system (1.6% FLO) similar to cured 0.5% or pristine 0.1% films.

Inhibited relaxation on excited segments leads to significant reduction of defect emission!

OO

400 450 500 550 600 6500

1

2

3

4

5 PF05 pristine PF01 pristine 1.6mol% FLO in PF2/6 PF05 cured for 3h

No

rma

lize

d P

L (

a.u

.)

(nm)


Device Fabrication

OLEDs in standard geometry: curing was performed prior to top electrode evaporation.

Green emission in EL is enhanced compared to PL due to charge carrier trapping.

Significant reduction can be achieved by curing.

400 450 500 550 600 650 7000,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0 pristine cured

No

rma

lize

d E

L (

a.u

.)

(nm)

a)

400 450 500 550 600 650 7000

2

4

6

8

10

12

14 pristine cured

No

rma

lize

d E

L (

a.u

.)

(nm)

b)

Ca/Al cathode

PF – FLO PEDOT:PSS HTL ITO

glass


Structured Devices

Structured OLED devices fabricated by curing with a shadow mask.

Critical, since “over curing” increases the onset of the cured area by almost 20V.

Device performance:4 V onsetLuminance of 2000cd/m² @ 7.4VEfficiency of 2cd/A @ 7.4 V



Recovery of blue emission in statistical fluorene – fluorenone copolymers.

Accompanying cross – linking effect is observed independent of fluorenone concentration.

FTIR reveals that no modification of the C=O double bonds occurs.

A suppressed on – chain relaxation is held responsible for the reduced green emission, which is confirmed with a fluorenone containing model system.

The observed effect allows for the fabrication of structured OLEDs.

A next step is to break – up the double bonds with reducing silane – groups for more effective curing.


Acknowledgements

Emil J. W. List and the whole CDL – AFM crew.

Horst Scheiber – for lots of the UV – curing data

Christoph Gadermaier and Florian Grasse – for help and discussions

Peter Pacher for low temperature PL support.

Ullrich Scherf and coworkers for the copolymers.


Thank You for Your Attention!

Documents

On the influence of Temperature and UV – Irradiation on Polyfluorene Michael Graf Christian Doppler Laboratory Advanced Functional Materials Institut für