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Frédéric Laquai – MPIP Mainz
Ultrafast Optical Spectroscopy of Excited States in Conjugated Polymers
Frédéric LaquaiMax Planck Research Group „Photophysics
of Conjugated
Materials“
Max Planck Institute for
Polymer Research Mainz, Germany
–
JST-DFG Workshop, Kyoto, Japan, January 2009 –
Frédéric Laquai – MPIP Mainz
Outline
•
Introduction –
Photophysics
•
Photophysical
properties of step-ladder polymers
•
Light amplification in thin films of poly(ladder-type phenylene)s
•
First results on polymer / fullerene organic solar cells
•
Summary and Outlook
Frédéric Laquai – MPIP Mainz
Excited states in conjugated materials
τFl
= ps
-
ns τPh
= µs –
stAbs
= fs
Ene
rgy
Frédéric Laquai – MPIP Mainz
Poly(ladder-type
phenylene)s
P1
P2
R =
C8H17Ar =
Ar Ar
Ar Ar
RR
n
n
P3
P4
P5
R1
R1
R1
R1
R2 R2Me Me
Me MeR2 R2n
Ar ArAr Ar
ArAr ArArn
Ar ArAr Ar
ArAr n
1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.60.0
0.2
0.4
0.6
0.8
1.0
700 600 500 400
PL E
mis
sion
Inte
nsity
(nor
mal
ised
)
Energy [eV]
Polymer P1 Polymer P2 Polymer P3 Polymer P4 Polymer P5
Wavelength [nm]
•
chemically
well-defined
•
no keto
defects
•
improved
stability
•
good solubility
and film forming properties
Frédéric Laquai – MPIP Mainz
Poly(ladder-type
phenylene)s
P1
P2
R =
C8H17Ar =
Ar Ar
Ar Ar
RR
n
n
P3
P4
P5
R1
R1
R1
R1
R2 R2Me Me
Me MeR2 R2n
Ar ArAr Ar
ArAr ArArn
Ar ArAr Ar
ArAr n
0.0 0.1 0.2 0.3 0.4 0.5 0.62.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
Polymers Monomers Kuhn fitMeLPPP
PF2/6
Ener
gy [e
V]
1 / N
N = 11
1cos'21
00 +
+=Nk
kEE π
Kuhn model
J. Gierschner, J. Cornil, H.-J. Egelhaaf, Adv. Mater. 2007, 19, 173.
Frédéric Laquai – MPIP Mainz
Ultrafast Fluorescence Spectroscopy (Streak Camera)
0 200 400 600 800 1000 1200
10-2
10-1
100
solution 296 K solution 80 K film 296 K
PL In
tens
ity /
norm
aliz
ed
Time / ps
Ar ArAr Ar
ArAr n
C8H17Ar =
1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.20.0
0.2
0.4
0.6
0.8
1.0
650 600 550 500 450 400Wavelength [nm]
PL
Inte
nsity
(nor
mal
ised
)
Energy [eV]
Frédéric Laquai – MPIP Mainz
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.40.0
0.2
0.4
0.6
0.8
1.0
1.2700 600 500 400
2.61 eV2.12 eV
1.94 eV 2.42 eV
3.06 eV
PL
inte
nsity
/ no
rmal
ized
Energy / eV
2.90 eV2.78 eV
Wavelength / nm
Ar ArAr Ar
ArAr n
1.2 1.4 1.6 1.8 2.0 2.2 2.40.0
0.2
0.4
0.6
0.8
1.0
1.21000 900 800 700 600
Polymer P1 Polymer P2 Polymer P3 Polymer P4
ΔT/
T (n
orm
alis
ed)
Energy [eV]
Wavelength [nm]
Delayed photoluminescence (solution) Steady-state PIA spectra (film)
Delayed photoluminescence and photoinduced
absorption (PIA)
Frédéric Laquai – MPIP Mainz
Polymer S1 S0
[eV]τFl
[ps]T1 S0
[eV]τPh
[s]T1 Tn
[eV]P1 (N=2) 2.97 422 2.18 1.0 1.51
P2 (N=3) 2.86 379 2.13 1.2 1.40
P3 (N=4) 2.81 390 2.12 1.3 1.37
P4 (N=5) 2.78 332 2.06 1.0 1.37
P5 (MeLPPP) 2.69 390 2.08 1.1 1.35
Comparison of photophysical
properties
F. Laquai, A.K. Mishra, M.R. Ribas, A. Petrozza, J. Jacob, L. Akcelrud, K. Müllen, R.H. Friend, G. Wegner,
Adv. Funct. Mater. 2007, 17, 3231-3240.
Frédéric Laquai – MPIP Mainz
Amplified spontaneous emission of conjugated polymers
n
Poly(9,9‘-dioctyl-fluorene) / PFO
Methyl-substituted
ladder-type
PPP
R1
R1
R1
R1
R2 R2Me Me
Me MeR2 R2n
G. Heliotis
et al., Appl. Phys. Lett. 2002, 81, 415.
C. Zenz
et al., Appl. Phys. Lett. 1997, 18, 2566.
Frédéric Laquai – MPIP Mainz
nsubstrate
npolymer
nair
Pump laser beam
nair
< npolymer
> nsubstrate
scattered light
scattered light
Amplified spontaneous emission (ASE) in polymer waveguides
Frédéric Laquai – MPIP Mainz
Raw laser beam profile
Beam profile after homogenizer
Lens arrays
Experimental setup
Frédéric Laquai – MPIP Mainz
Characterisation
of ASE parameters
sample
laser striperazor blade
1. Gain coefficient g(λ):
2. Absorption coefficient α:
sample
laser stripe
)1()(
)( )( −= lgp egAI
I λ
λλ
xout eII α−= 0
Frédéric Laquai – MPIP Mainz
1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.20.0
0.2
0.4
0.6
0.8
1.0
1.2650 600 550 500 450 400
PL
Em
issi
on In
tens
ity /
a.u.
Energy / eV
11.3 μJ/cm2 x pulse 28.3 μJ/cm2 x pulse 283 μJ/cm2 x pulse
Wavelength / nm
Amplified spontaneous emission (ASE) / slab waveguide
F. Laquai, P.E. Keivanidis, S. Baluschev, J. Jacob, K. Müllen, G. Wegner, Appl. Phys. Lett. 87, 261917 (2005).
Ith
= 3 µJ/cm2
~ 375 W/cm2
Ar Ar
Ar Ar n
4-level laser
system
Frédéric Laquai – MPIP Mainz
1 10 100 1000
103
104
105
106
1 10 100
5
10
15
3.0 µJ/cm²
Wid
th o
f Gau
ss F
it (n
m)
Pump Energy Density (µJ/cm2 x pulse)
ASE onset
m = 2
AS
E P
eak
Inte
nsity
(a.u
.)
Pump Energy Density (µJ/(cm2 x pulse))
Amplified spontaneous emission (ASE) / thresholds
Frédéric Laquai – MPIP Mainz
Amplified spontaneous emission (ASE) / gain coefficient
0.0 0.1 0.2 0.3 0.4 0.5
103
104
105
0.0 0.1 0.2
0
5
10
15
20
25
ASE
Pea
k In
tens
ity (a
.u.)
Excitation Stripe Length (cm)
g = 21 cm-1
Stripe Length (cm)
AS
E P
eak
Inte
nsity
(a.u
.)
sample
laser striperazor blade
)1()(
)( )( −= lgp egAI
I λ
λλ
g(λ) = σSE (λ) ×
Nexc
Frédéric Laquai – MPIP Mainz
ASE of Poly(ladder-type phenylene)s
R =
NAr Ar
NAr ArR
R
n
NR
Ar Ar Ar Ar
nC1
C2
Aryl-PF
P2C8H17Ar =
Ar Ar
Ar Ar
ArAr
n
n
P3
P4
P5
R1
R1
R1
R1
R2 R2Me Me
Me MeR2 R2n
Ar ArAr Ar
ArAr ArArn
Ar ArAr Ar
ArAr n
1 2 3 4 5 6
10
100
1000
P2
carbon-bridged carbazole-containing
C2
P4
C1
P3
AS
E th
resh
old
[μJ
cm-2 p
ulse
-1]
N (number of bridged phenyl rings)
Aryl-PF
2.3 2.4 2.5 2.6 2.7 2.8 2.90.0
0.2
0.4
0.6
0.8
1.0
1.2520 500 480 460 440
P5C2 P4
P3
PL
Inte
nsity
(nor
mal
ised
)
Energy [eV]
Wavelength [nm]
Aryl-PFP2
Frédéric Laquai – MPIP Mainz
Comparison of ASE properties
Polymer λASE
[nm]
τFl
[ps]
Ith
[μJ/cm2]g(λASE
) [cm-1]
α(λASE
) [cm-1]
τR
[ps]
ΦF
[%]Aryl-PF 449 318 24 18 <1 558 57
P2 (N=3) 468 240 3 21 7 571 45
P3 (N=4) 475 170 10 15 1 630 23
P4 (N=5) 479 122 100 6 5 349 35
P5 (MeLPPP)
489 130 20 16 2 500 26
C1 (N=4) 480 93 110 - - 620 15
C2 (N=5) 492 110 150 - - 786 14
Frédéric Laquai – MPIP Mainz
Excited state absorption (ESA)
U. Scherf, S. Riechel, U. Lemmer, R.F. Mahrt, Current Opinion in Solid State and Materials Science 5 (2001) 143.
Frédéric Laquai – MPIP Mainz
Ultrafast transient absorption spectroscopy
Change of transmission (ΔT) of samplein the presence of pump beam: ΔT = Tpump
on
- Tpump
off
Pump Pulse (fs)Supercontinuum
Probe Pulse
Frédéric Laquai – MPIP Mainz
Excited states in conjugated materials
SE
PA
PA-
ΔT/
TWavelength
0
+ SE
PA: Photoinduced
absorption negative ΔT/T
SE: Stimulated Emission positive ΔT/T
Frédéric Laquai – MPIP Mainz
ASE of Poly(ladder-type phenylene)sWhat influences the threshold of light amplification in conjugated polymers?
Ith
= 20 µJcm-2pulse-1 Ith
= 150 µJcm-2pulse-1
Ith = Minimum pump pulse intensity for amplification of light to occur
450 500 550 600 6500.0
0.2
0.4
0.6
0.8
1.0
1.22.6 2.4 2.2 2
PL
Inte
nsity
(nor
mal
ised
)
Wavelength [nm]
10 μJcm-2pulse-1
31 μJcm-2pulse-1
100 μJcm-2pulse-1
0-0
0-1
0-2
Energy [eV]
450 500 550 600 6500.0
0.2
0.4
0.6
0.8
1.0
1.22.6 2.4 2.2 2
PL
Inte
nsity
(nor
mal
ised
)
Wavelength [nm]
44 μJcm-2pulse-1
351 μJcm-2pulse-1
3500 μJcm-2pulse-1
Energy [eV]
0-00-1
0-2
R1
R1
R1
R1
R2 R2Me Me
Me MeR2 R2n
NAr Ar
NAr ArR
R
n
Frédéric Laquai – MPIP Mainz
ASE of Poly(ladder-type phenylene)sWhat influences the threshold for light amplification in conjugated polymers?
Ith
= 20 µJcm-2pulse-1 Ith
= 150 µJcm-2pulse-1
Strong overlap of SE region and PI absorption increases threshold !
480 500 520 540 560 580 600 620
-0.02
0.00
0.02
0.04
0.062.6 2.5 2.4 2.3 2.2 2.1 2
0-1
Stimulated Emission
ΔT/
T
Wavelength [nm]
PA
0-2
Energy [eV]
460 480 500 520 540 560 580 600
-0.06
-0.04
-0.02
0.00
0.02
0.04
0.06
0.08
0.102.6 2.5 2.4 2.3 2.2 2.1
0-1
ΔT/T
Wavelength [nm]
0-2
Photoinduced Absorption
SE
Energy [eV]
Frédéric Laquai – MPIP Mainz
Transient absorption experiments
ASE leads to rapid depopulation of excited singlet excitons
0.1 1 10 100 1000
-1.0
-0.5
0.0
0.5
1.0
SE (0-1) SE (0-2) PA
ΔT/T
(nor
mal
ised
)
time [ps]1 10 100 1000
-1.0
-0.5
0.0
0.5
1.0
SE (d~50 nm) PA (d~50 nm) SE (d~120 nm)
ΔT/T
(nor
mal
ised
)
time [ps]
R1
R1
R1
R1
R2 R2Me Me
Me MeR2 R2n
NAr Ar
NAr ArR
R
n
Frédéric Laquai – MPIP Mainz
Photovoltaic blends –
Operating principle
HOMO
LUMO
anode cathode
Electron Donor Electron Acceptor
η
~ 5 % (power conversion)
Frédéric Laquai – MPIP Mainz
Photovoltaic blends –
Limiting processes
HOMO
LUMO
anode cathode
Electron Donor Electron Acceptor
• exciton
decay (non-radiative
/ radiative) ?
• trapping of charge carriers ?
• backtransfer
/ geminate recombination ?
• non-geminate recombination ?
Frédéric Laquai – MPIP Mainz
Layout of Organic
Solar Cells
Glass
ITO ~100 nmPEDOT:PSS ~50 nm
Photoactive
layer
~200 nm
Aluminum
~100nm
Frédéric Laquai – MPIP Mainz
MaterialsName Chemical structure Specifications
P3HTPoly(3-hexylthiophene-
2,5-diyl)
Mw = 60.000PDI = 2.2
RR = 94 %
P3HTPoly(3-hexylthiophene-
2,5-diyl)
Mw = 25.000PDI = 1.6
RR > 98 %
PCBM[6,6]-Phenyl C61 butyric
acid methyl ester
C72H14O2 M =
910.88 g/molPurity > 99 %
Frédéric Laquai – MPIP Mainz
IV-Curve
and Efficiency
Power Conversion
Efficiency:
PCE = Electrical
Power/ Solar Power
= MPP / (1000 W/m²
* Sample Area)
= FF * Voc
* Isc
/ (1000 W/m²
* Sample Area)
Voc
= Open Circuit
Voltage
Isc
= Short Circuit
Current
FF= Fill
Factor
Cur
rent
I [m
A]
& P
ower
P [m
W]
Voltage V [V]
Frédéric Laquai – MPIP Mainz
Experimental Results
Improvement of PCE upon annealing
0 20 40 60 80 100 120 140 160 1800,0
0,4
0,8
1,2
1,6
2,0
2,4 thick thin
Pow
er C
onve
rsio
n E
ffici
ency
[%]
Annealing Temperature T [°C]
Cells prepared in ambient conditions.
Frédéric Laquai – MPIP Mainz
Experimental Results
Increase in absorption not sufficient to explain improvement in PCE
300 400 500 600 700 8000
25
50
75
100
125
150
Abs
orpt
ion
[a.u
.]
Wavelength [nm]
annealed pristine
~30% increase in absorption of solar radiation upon annealing
Frédéric Laquai – MPIP Mainz
2.) Amplified
spontaneous
emission
(ASE) and lasing
in ladder-type
polymers
•
New materials
with
low
thresholds
and high charge
carrier
mobilities
F. Laquai, A.K. Mishra, M.R. Ribas
et al., Adv. Funct. Mater. 2007, 17, 3231-3240.
F. Laquai, A.K. Mishra, K. Müllen, R.H. Friend, Adv. Funct. Mater. 2008, 18, 3265-3275.
4.) Charge transport
in conjugated
materials
•
Time-of-flight
technique
for
charge
carrier
mobility
measurements
3.) Charge generation
and recombination
in organic
solar cells
•
Time-resolved
absorption
spectroscopy
•
Structure-property-efficiency
relationships
1.) Photophysical
processes
in conjugated
materials
(Donor-Acceptor
Systems)
•
Ultrafast
(fs-ns) time-resolved
photoluminescence
and absorption
spectroscopy
•
Delayed
(ns-ms) photoluminescence
and photoinduced
absorption
spectroscopy
Outlook –
Ongoing research activities
F. Laquai, G. Wegner, H. Bässler, Phil. Trans. R. Soc. A 2007, 365, 1473–1487.
Frédéric Laquai – MPIP Mainz
Prof. Sir Richard Friend
Prof. Gerhard Wegner
Prof. Heinz Bässler
Clare Hall College Research Fellowship
Acknowledgements
Dr. Justin Hodgkiss
Dr. Annamaria
Petrozza
Ian Howard
Material provider
Prof. Klaus Müllen (MPIP)
Cambridge Display Technology (CDT)
BASF SE
Postdoctoral Research Scholarship
Independent Max Planck Research Group
Ralf Mauer
Hun Kim