Ultrafast Optical Spectroscopy of Excited States in ...Frédéric Laquai – MPIP Mainz Ultrafast Optical Spectroscopy of Excited States in Conjugated Polymers Frédéric Laquai Max

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 –


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


Excited states in conjugated materials

τFl

= ps

-

ns τPh

= µs –

stAbs

= fs

Ene

rgy


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


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.


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]


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)


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.


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.


nsubstrate

npolymer

nair

Pump laser beam

nair

< npolymer

> nsubstrate

scattered light

scattered light

Amplified spontaneous emission (ASE) in polymer waveguides


Raw laser beam profile

Beam profile after homogenizer

Lens arrays

Experimental setup


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


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


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


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


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


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


Excited state absorption (ESA)

U. Scherf, S. Riechel, U. Lemmer, R.F. Mahrt, Current Opinion in Solid State and Materials Science 5 (2001) 143.


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


Excited states in conjugated materials

SE

PA

PA-

ΔT/

TWavelength

0

+ SE

PA: Photoinduced

absorption negative ΔT/T

SE: Stimulated Emission positive ΔT/T


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


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]


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


Photovoltaic blends –

Operating principle

HOMO

LUMO

anode cathode

Electron Donor Electron Acceptor

η

~ 5 % (power conversion)


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 ?


Layout of Organic

Solar Cells

Glass

ITO ~100 nmPEDOT:PSS ~50 nm

Photoactive

layer

~200 nm

Aluminum

~100nm


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 %


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]


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.


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


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.


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

Documents

Ultrafast Optical Spectroscopy of Excited States in ...Frédéric Laquai – MPIP Mainz Ultrafast Optical Spectroscopy of Excited States in Conjugated Polymers Frédéric Laquai Max