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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
Christian-Doppler Laboratory Advanced Functional Materials
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
Christian-Doppler Laboratory Advanced Functional Materials
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)
Christian-Doppler Laboratory Advanced Functional Materials
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)
Christian-Doppler Laboratory Advanced Functional Materials
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
Christian-Doppler Laboratory Advanced Functional Materials
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)
Christian-Doppler Laboratory Advanced Functional Materials
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)
Christian-Doppler Laboratory Advanced Functional Materials
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)
Christian-Doppler Laboratory Advanced Functional Materials
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)
Christian-Doppler Laboratory Advanced Functional Materials
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
Christian-Doppler Laboratory Advanced Functional Materials
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.
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
Christian-Doppler Laboratory Advanced Functional Materials
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
Christian-Doppler Laboratory Advanced Functional Materials
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
Christian-Doppler Laboratory Advanced Functional Materials
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
Christian-Doppler Laboratory Advanced Functional Materials
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]
Christian-Doppler Laboratory Advanced Functional Materials
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 %
Christian-Doppler Laboratory Advanced Functional Materials
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 %
Christian-Doppler Laboratory Advanced Functional Materials
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 %
Christian-Doppler Laboratory Advanced Functional Materials
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 %
Christian-Doppler Laboratory Advanced Functional Materials
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 %
Christian-Doppler Laboratory Advanced Functional Materials
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
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
Influence of UV Irradiation on PF – Fluorenone CopolymersSuppression of Defect EmissionCross – LinkingStructured OLEDs Built from PF – Fluorenone Copolymers
Conclusion and Outlook
Acknowledgements
Christian-Doppler Laboratory Advanced Functional Materials
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
Christian-Doppler Laboratory Advanced Functional Materials
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
Christian-Doppler Laboratory Advanced Functional Materials
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.
Christian-Doppler Laboratory Advanced Functional Materials
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
Christian-Doppler Laboratory Advanced Functional Materials
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
Christian-Doppler Laboratory Advanced Functional Materials
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
?
Christian-Doppler Laboratory Advanced Functional Materials
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)
Christian-Doppler Laboratory Advanced Functional Materials
Proposed Mechanism
Christian-Doppler Laboratory Advanced Functional Materials
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)
Christian-Doppler Laboratory Advanced Functional Materials
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
Christian-Doppler Laboratory Advanced Functional Materials
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
Christian-Doppler Laboratory Advanced Functional Materials
Conclusion and Outlook
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.
Christian-Doppler Laboratory Advanced Functional Materials
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.
Christian-Doppler Laboratory Advanced Functional Materials
Thank You for Your Attention!