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Ultrafast Optical Spectroscopy of Excited States in Conjugated Polymers Frdric Laquai Max Planck Research Group Photophysics of Conjugated Materials Max Planck Institute for Polymer Research Mainz, Germany JST-DFG Workshop,

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Frédéric Laquai – MPIP Mainz

Ultrafast Optical Spectroscopy of Excited States in Conjugated Polymers

Frédéric Laquai

Max Planck Research Group „Photophysics

  • f Conjugated

Materials“ Max Planck Institute for Polymer Research Mainz, Germany

– JST-DFG Workshop, Kyoto, Japan, January 2009 –

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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
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Frédéric Laquai – MPIP Mainz

Excited states in conjugated materials

τFl = ps

  • ns

τPh = µs – s tAbs = fs

Energy

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Frédéric Laquai – MPIP Mainz

Poly(ladder-type phenylene)s

P1 P2 R = C8H17 Ar =

Ar Ar Ar Ar R R n n

P3 P4 P5

R1 R1 R1 R1 R2 R2 Me Me Me Me R2 R2 n Ar Ar Ar Ar Ar Ar Ar Ar n Ar Ar Ar Ar Ar Ar n

1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 0.0 0.2 0.4 0.6 0.8 1.0 700 600 500 400

PL Emission Intensity (normalised) 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

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Frédéric Laquai – MPIP Mainz

Poly(ladder-type phenylene)s

P1 P2 R = C8H17 Ar =

Ar Ar Ar Ar R R n n

P3 P4 P5

R1 R1 R1 R1 R2 R2 Me Me Me Me R2 R2 n Ar Ar Ar Ar Ar Ar Ar Ar n Ar Ar Ar Ar Ar Ar n

0.0 0.1 0.2 0.3 0.4 0.5 0.6 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8

Polymers Monomers Kuhn fit MeLPPP PF2/6 Energy [eV] 1 / N N = 11

1 cos ' 2 1 + + = N k k E E π

Kuhn model

  • J. Gierschner, J. Cornil, H.-J. Egelhaaf, Adv. Mater. 2007, 19, 173.
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Frédéric Laquai – MPIP Mainz

Ultrafast Fluorescence Spectroscopy (Streak Camera)

200 400 600 800 1000 1200 10

  • 2

10

  • 1

10

solution 296 K solution 80 K film 296 K

PL Intensity / normalized

Time / ps

Ar Ar Ar Ar Ar Ar n

C8H17 Ar =

1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 0.0 0.2 0.4 0.6 0.8 1.0 650 600 550 500 450 400

Wavelength [nm] PL Intensity (normalised) Energy [eV]

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Frédéric Laquai – MPIP Mainz

1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 700 600 500 400

2.61 eV 2.12 eV 1.94 eV 2.42 eV 3.06 eV PL intensity / normalized Energy / eV 2.90 eV 2.78 eV Wavelength / nm

Ar Ar Ar Ar Ar Ar n

1.2 1.4 1.6 1.8 2.0 2.2 2.4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1000 900 800 700 600 Polymer P1 Polymer P2 Polymer P3 Polymer P4 ΔT/T (normalised)

Energy [eV] Wavelength [nm]

Delayed photoluminescence (solution) Steady-state PIA spectra (film)

Delayed photoluminescence and photoinduced absorption (PIA)

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Frédéric Laquai – MPIP Mainz

Polymer S1S0 [eV] τFl [ps] T1S0 [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.
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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 R2 Me Me Me Me R2 R2 n

  • G. Heliotis

et al., Appl. Phys. Lett. 2002, 81, 415.

  • C. Zenz

et al., Appl. Phys. Lett. 1997, 18, 2566.

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Frédéric Laquai – MPIP Mainz

nsubstrate npolymer nair Pump laser beam

nair < npolymer > nsubstrate

scattered light scattered light

Amplified spontaneous emission (ASE) in polymer waveguides

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Frédéric Laquai – MPIP Mainz

Raw laser beam profile Beam profile after homogenizer

Lens arrays

Experimental setup

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Frédéric Laquai – MPIP Mainz

Characterisation

  • f ASE parameters

sample laser stripe razor blade

  • 1. Gain coefficient g(λ):
  • 2. Absorption coefficient α:

sample laser stripe

) 1 ( ) ( ) (

) (

− =

l g p

e g AI I

λ

λ λ

x

  • ut

e I I

α −

=

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Frédéric Laquai – MPIP Mainz

1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 650 600 550 500 450 400

PL Emission Intensity / a.u. Energy / eV

11.3 μJ/cm

2 x pulse

28.3 μJ/cm

2 x pulse

283 μJ/cm

2 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

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Frédéric Laquai – MPIP Mainz

1 10 100 1000 10

3

10

4

10

5

10

6 1 10 100 5 10 15 3.0 µJ/cm² Width of Gauss Fit (nm) Pump Energy Density (µJ/cm

2 x pulse)

ASE onset m = 2 ASE Peak Intensity (a.u.) Pump Energy Density (µJ/(cm

2 x pulse))

Amplified spontaneous emission (ASE) / thresholds

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Frédéric Laquai – MPIP Mainz

Amplified spontaneous emission (ASE) / gain coefficient

0.0 0.1 0.2 0.3 0.4 0.5 10

3

10

4

10

5

0.0 0.1 0.2 5 10 15 20 25

ASE Peak Intensity (a.u.) Excitation Stripe Length (cm)

g = 21 cm

  • 1

Stripe Length (cm) ASE Peak Intensity (a.u.)

sample laser stripe razor blade

) 1 ( ) ( ) (

) (

− =

l g p

e g AI I

λ

λ λ

g(λ) = σSE (λ) × Nexc

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Frédéric Laquai – MPIP Mainz

ASE of Poly(ladder-type phenylene)s

R =

N Ar Ar N Ar Ar R R n N R Ar Ar Ar Ar n

C1 C2 Aryl-PF P2 C8H17 Ar =

Ar Ar Ar Ar Ar Ar n n

P3 P4 P5

R1 R1 R1 R1 R2 R2 Me Me Me Me R2 R2 n Ar Ar Ar Ar Ar Ar Ar Ar n Ar Ar Ar Ar Ar Ar n

1 2 3 4 5 6 10 100 1000 P2

carbon-bridged carbazole-containing

C2 P4 C1 P3

ASE threshold [μJ cm

  • 2 pulse
  • 1]

N (number of bridged phenyl rings)

Aryl-PF 2.3 2.4 2.5 2.6 2.7 2.8 2.9 0.0 0.2 0.4 0.6 0.8 1.0 1.2 520 500 480 460 440 P5 C2 P4 P3

PL Intensity (normalised) Energy [eV] Wavelength [nm]

Aryl-PF P2

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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

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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.
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Frédéric Laquai – MPIP Mainz

Ultrafast transient absorption spectroscopy

Change of transmission (ΔT) of sample in the presence of pump beam:

ΔT = Tpump

  • n
  • Tpump
  • ff

Pump Pulse (fs) Supercontinuum Probe Pulse

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Frédéric Laquai – MPIP Mainz

Excited states in conjugated materials

SE PA

PA

  • ΔT/T

Wavelength

+

SE

PA: Photoinduced absorption negative ΔT/T SE: Stimulated Emission positive ΔT/T

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Frédéric Laquai – MPIP Mainz

ASE of Poly(ladder-type phenylene)s

What 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

450 500 550 600 650 0.0 0.2 0.4 0.6 0.8 1.0 1.2 2.6 2.4 2.2 2

PL Intensity (normalised) Wavelength [nm]

10 μJcm

  • 2pulse
  • 1

31 μJcm

  • 2pulse
  • 1

100 μJcm

  • 2pulse
  • 1

0-0 0-1 0-2 Energy [eV]

450 500 550 600 650 0.0 0.2 0.4 0.6 0.8 1.0 1.2 2.6 2.4 2.2 2

PL Intensity (normalised) Wavelength [nm]

44 μJcm

  • 2pulse
  • 1

351 μJcm

  • 2pulse
  • 1

3500 μJcm

  • 2pulse
  • 1

Energy [eV] 0-0 0-1 0-2

R1 R1 R1 R1 R2 R2 Me Me Me Me R2 R2 n N Ar Ar N Ar Ar R R n

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Frédéric Laquai – MPIP Mainz

ASE of Poly(ladder-type phenylene)s

What 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 0.06 2.6 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 0.10 2.6 2.5 2.4 2.3 2.2 2.1

0-1

ΔT/T

Wavelength [nm] 0-2 Photoinduced Absorption SE Energy [eV]

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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 (normalised)

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 (normalised)

time [ps]

R1 R1 R1 R1 R2 R2 Me Me Me Me R2 R2 n N Ar Ar N Ar Ar R R n

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Frédéric Laquai – MPIP Mainz

Photovoltaic blends – Operating principle HOMO LUMO

anode cathode Electron Donor Electron Acceptor

η ~ 5 % (power conversion)

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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 ?
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Frédéric Laquai – MPIP Mainz

Layout of Organic Solar Cells

Glass ITO ~100 nm PEDOT:PSS ~50 nm Photoactive layer ~200 nm Aluminum ~100nm

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Frédéric Laquai – MPIP Mainz

Materials

Name Chemical structure Specifications

P3HT

Poly(3-hexylthiophene- 2,5-diyl)

Mw = 60.000 PDI = 2.2 RR = 94 % P3HT

Poly(3-hexylthiophene- 2,5-diyl)

Mw = 25.000 PDI = 1.6 RR > 98 % PCBM

[6,6]-Phenyl C61 butyric acid methyl ester

C72H14O2 M = 910.88 g/mol Purity > 99 %

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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 Current I [mA] & Power P [mW] Voltage V [V]

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Frédéric Laquai – MPIP Mainz

Experimental Results

Improvement of PCE upon annealing

20 40 60 80 100 120 140 160 180 0,0 0,4 0,8 1,2 1,6 2,0 2,4

thick thin

Power Conversion Efficiency [%] Annealing Temperature T [°C]

Cells prepared in ambient conditions.

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Frédéric Laquai – MPIP Mainz

Experimental Results

Increase in absorption not sufficient to explain improvement in PCE

300 400 500 600 700 800 25 50 75 100 125 150

Absorption [a.u.] Wavelength [nm] annealed pristine ~30% increase in absorption of solar radiation upon annealing

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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.
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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