TRANSIENT PHOTOCONDUCTIVITY AND CHARGE GENERATION IN A THIN FILMS OF Π -CON JUGATED POLYMERS

LAMINATE meeting Norrköping, September 2004 1

TRANSIENT PHOTOCONDUCTIVITY AND CHARGE GENERATION IN A THIN FILMS OF Π-CONJUGATED POLYMERS

Outline:1. introduction2. transient photoconductivity3. charge photogeneration4. effect of doping5. reexcitation of GP‘s6. conclusion

Martin Weiter, Heinz BäßlerInstitute of Physical, Macromolecular and Nuclear Chemistry, Philipps University, Marburg, Germany


motivation

focus on photoconductivity in Π-conjugated materials: the origin of the optical charge generation and following charge transport

1. What is the origin of observed transient photocurrent?

2. What is the rate limiting field dependent step for photoionization in conjugated polymers: exciton dissociation into geminate e-h pair or full dissociation into free charge carriers?

3. Is the photoionization tractable in terms of Onsager theory?OC10

OC10

OMe

n

0.50.5

phenyl‑substituted poly‑phenylenevinylene

RMe

R'

R'

RMe

Me R

R'

R'

n

R = 1,4-C6H4-n-C10H12

R' = n-C6H13

methyl‑substituted ladder type polyphenylene


charge photogeneration – classic Onsager concept

0

(E ,T )

),(),( 0 TETE esc

0),(lim),(lim TETE TE

),(0 TEf

1. creation of e-h pairs - dependent on photon energy - independent on field and temperature

2. dissociation of e-h pairs - dependent on field and temperature

Two step process:

1. 2.


motivation

charge generation at variance with Onsager concept:

M.A. Stevens, C. Silva, D.M. Russell, R.H. Friend, Phys. Rev. B 63 (2001) 165213.

W. Graupnerat all, Phys. Rev. Lett. 81 (1998) 3259.

M. Gailberger, H. Baessler, Phys. Rev. B 44 (1991) 8643.

S. Barth, H. Baessler, Phys. Rev. Lett. 79 (1997) 4445.

S. Barth, H. Baeassler, U. Scherf, K. Muullen, Chem. Phys.Lett. 288 (1998) 147.

J. Pan, U. Scherf, A. Schreiber, R. Bilke, D. Haarer, Synth. Met. 115 (2000) 79.

D. Hertel, E. V. Soh, H. Bässler and L. J. Rothberg, Chem. Phys. Lett 361, (2002), 99

Measured by different techniques at variety of materials


question

What is the rate limiting field dependent step for photoinization in

conjugated polymers?

0

(E ,T )

?

Exciton dissociation into geminate e-h pair?

Full disssociation into free charge carriers ?OR


h

Ito / (semitr. Al) Al

Scope

photoconductive Time of Flight method

time

photocurrent

• electrodes (ITO, Al, semitransparent Al, SiO, Mg)

• wide range of excitation light wavelengths

• different field strength with both polarity

• temperature influence

• recombination


experiment 1

• electrodes (ITO+semitansparent Al, Al)

• laser wavelengths 453 nm (2.7 eV), 355 nm (3.5 eV),


• temperature 150K – 320 K

RMe

R'

R'

RMe

Me R

R'

R'

n

R = 1,4-C6H4-n-C10H12

R' = n-C6H13

methyl‑substituted ladder type polyphenylene

MeLPPP

300 350 400 450 500 5500.0

0.2

0.4

0.6

2

1

optic

al d

ensi

ty

wavelength (nm)


transient photocurrents

10-8 10-7 10-6 10-5 10-410-3

10-2

10-1

100

5·105Vcm-1

2·105Vcm-1

1·106Vcm-1

2·106Vcm-1

Pho

tocu

rren

t [m

A]

Time [s]

RC constant Transit time

Ito:Al-MeLPPP-Al,100 nm thickT=295K, 355 nm, 5ns 40 uJ/pulse (6e12 photons)

1123 sVcm102 h

MeLPPP:•Hole trasporting material

low degree of disorderweek T and F dep. of mobility.No electr. transport observed

Carrier transport is controlled by charge carrier generation rather by their transport

Duration field independent


escGP kteNti )()(

phescdiss

T

NtiQ 0

d

source of the transient photocurrent must be the final dissociation of metastable geminately bound electron‑hole pairs (GPs) whose existence has been well established by

•delayed field collection experiment • delayed fluorescence • thermally stimulated luminescence • two color photoconduction

NGP -number of GPs which survived until time t kesc -rate constant of their subsequent escape

ηdiss -the initial yield of dissociation of singlet excitations,

Nph -produced by the laser and

ηesc is the fraction of GP,

ηdiss·Nph is the number of GP which escape geminate recombination.


10-8 10-7 10-6 10-5 10-410-3

10-2

10-1

100

5·105Vcm-1

2·105Vcm-1

1·106Vcm-1

2·106Vcm-1

Pho

tocu

rren

t [m

A]

Time [s]


charge generation vs. light intensity

0.1 1 10 100 1000108

109

1010

1011

C*V/e=4,7*1010

C*V/e=1,87*1011

C*V/e=1,03*1011

C*V/e=1,9*1010

2.106Vcm-1

1,1.106Vcm-1

5.105Vcm-1 2.105Vcm-1

gene

rate

d ch

arge

(-)

laser intensity (J/pulse)

Consistent with: D. Hertel, Yu. V. Romanovski, B. Schweitzer, U. Scherf and H. Bässler, Synth. Met. 116, (2001), 1391 A. Haugeneder, M. Neges, C. Kallinger, W. Spirkl, U. Lemmer, …, J. Appl. Phys 85, (1999), 1124

• meaningful information on monomolecular dissociation of singlet excitons requires experiments with laser intensities sufficiently low that bimolecular reaction of neither singlet excitons nor charge carrier can occur

• GP dissociation yielding quickly moving holes and trapped electrons. The latter create a negative space charge that act as recombination centers.


charge generation vs. electric field

104 105 106

108

109

1010

1011

420 J/pulse 44 J/pulse 4,1 J/pulse

Num

ber

of g

ener

ated

cha

rges

Electric field [Vcm-1]

Q=CV/e


charge generation - yield

104 105 106 10710-4

10-3

10-2

10-1

100

Yie

ld


10-3

10-2

10-1

100

101

Pea

k ph

otoc

urre

nt [

mA

]

•yield normalizing Q to the number of singlet pairs generated

•field dependence of photocurrent maximum is very similar to the field dependence of the number of charges collected the field dependence of the escape rate of GP from initial coulombic well must be vanishingly small

squares: 4.1 μJ/pulse, circles: 44 μJ/pulse


charge generation vs. temperature

4.0 4.5 5.0 5.5 6.0 6.5 7.0

2x1010

4x1010

6x1010

8x1010

1x1011

1x1011

Charge generation vs. temperature parametric in field

char

ge g

ener

atio

n /

-

1/T (.10-3 K )

4e5 Vcm -1

1.1e6 Vcm -1

2e6 Vcm -1

4.0 4.5 5.0 5.5 6.0 6.5 7.02.0x1010

4.0x1010

6.0x1010

8.0x1010

1.0x1011

1.2x1011

Charge generation vs. temperature parametric in exc. intensity

char

ge g

ener

atio

n /

-

1/T (.10-3 K )

0.8J 3.8J 26J


ultrafast absorption spectroscopy

Additional information:

V. Gulbinas at all: Phys. Rev. Lett. 89, (2002), 107 401

0 50 100 150 2000.0

0.1

0.2

0.3

0.4

0.5

Cha

rge

sepa

ratio

n yi

eld

Electric field (V/m)

the number of GP generated within time domain of 200 fs to 500 ps which is too short for their subsequent escape from the coulombic potential


charge generation - yield

104 105 106 10710-4

10-3

10-2

10-1

100

Yie

ld


10-3

10-2

10-1

100

101

Pea

k ph

otoc

urre

nt [

mA

]

It confirms that 1. the rate limiting field assisted

step is the dissociation of relaxed singlet excitons into GPs rather than their subsequent escape from coulombic well,

2. dissociation of relaxed singlet excitons proceeds on a 200 fs to 0.5 ns time scale while the dwell time of GPs extends to the microsecond regime

3. approximately 10% of initially generated GP are liable to complete dissociation at a field of 3 MV/cm.


conclusion1

the field dependence of the intrinsic photogeneration in the π‑conjugated polymer MeLPPP is controlled by the initial step of the dissociation of a relaxed singlet exciton into a geminate pair rather than its escape from the coulombic potential

0

(E ,T )

M. Weiter, H. Bässler, V. Gulbinas, U. Scherf, Chem. Phys. Lett.379 (2003) 177.


experiment 2

OC10

OC10

OMe

n

0.50.5

phenyl‑substituted poly‑phenylenevinylene

(Ph‑PVV)

300 350 400 450 500 5500.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

2

1

optic

al d

ensi

ty

wavelength (nm)

• electrodes (ITO+semitansparent Al, Al)

• laser wavelengths 450 nm (2.7 eV), 355 nm (3.5 eV),


• temperature 150K – 320 K

• doping by TNF


transient photocurrents Ph-PVV

-hole mobility 2×10-6 V/(cm2s) Transit time (2-40) μs-j(t) shape invariant with light intensity, field, polarity-transient is a convolution of time dependent charge carrier generation, transport and discharge

10-3

10-2

10-1

100

101

36 J

3.3 J

352 J a.) F=3.5 105 Vcm-1

ph

oto

curr

en

t (m

A)

10-8 10-7 10-6 10-5 10-410-3

10-2

10-1

100

101

352 J

36 J

3.3 J

b.) F=1.3 106 Vcm-1

ph

oto

curr

en

t (m

A)

time (s)

10-8 10-7 10-6 10-5 10-410-3

10-2

10-1

100F=10 6 Vcm-1

F=10 5 Vcm-1

ph

oto

curr

en

t (m

A)

time (s)


temperature dependence

at E=2.2·105 V/cm Q is weakly temperature dependent features activation energy of about 0.07 eV for T>200 Kat lower temperatures Q tends to saturate

3 4 5 6 7 8

10-4

10-3

10-2

Yie

ld

1000/T(K-1)

1.1·106 Vcm-1

6.1·105 Vcm-1

2.2·105 Vcm-1

calculated

model: V. I. Arkhipov, E. V. Emelianova and H. Bässler, Chem. Phys. Lett. 372, (2003), 886


comparison of the yield of photogeneration

•the photoresponse in PhPPV is larger than in MeLPPP and is associated with weaker field dependence •The doping increase the photogeneration by about a factor of 2

105 10610-4

10-3

10-2

10-1

Yie

ld

Field (Vcm-1)

MeLPPP PhPPV PhPPV + 1 % TNF

Taking into account previous experimental results :

in this case the field assisted and rate-limiting step for photogeneration must be the subsequent escape of the pair from its coulombic well


conclusion2

There are two processes occurring in the bulk of conjugated polymer that can give rise to photogeneration:

1. the field assisted dissociation into geminate pairs that subsequently can separate fully

2. sensitized photogeneartion at either non‑intentional or intentional dopants that can act as electron scavengers

0

(E ,T )

M. Weiter, V.I. Arkhipov, H. Bässler, Synth. Met. 141 (2004) 165


Dou

ble

sh

ot:

Tim

e d

ela

y

Scope

ITO

+ s

em

itr.

Al

Al e

lect

rodeh1

h2

experiment 3

OC10

OC10

OMe

n

0.50.5

(Ph‑PVV)

„Double shot“ photocurrent technique

To elucidate whether or not there are longer-lived geminate e-h pairs that can be dissociated optically by a second laser pulse rather than decay by geminate pair recombination.



0 10 20

0

1

2

3

4

0.1 1 10

0.2

2

ph

oto

curr

en

t [m

A]

time [s]

Double shots photocurrents at el field 7.3 105 V/cm. The energy of the first pulse at 3.49 eV was 35 J/pulse, the energy of the second pulse at 2.74 eV was 0.7

J/pulse.



1 10

0.01

0.1

phot

ocur

rent

pea

k (a

.u.)

time (s)

2.7 107 V/m Ito plus 2.7 107 V/m Ito minus 4.7 107 V/m, Ito plus 4.7 107 V/m, Ito minus 7.3 107 V/m, Ito plus 7.3 107 V/m, Ito minus 1.0 108 V/m, Ito plus 1.0 108 V/m, Ito minus 1.3 108 V/m, Ito plus 1.3 108 V/m, Ito minus

At low electric fields and low light intensities the result signal is not the superposition of both signals but the photocurrent is controlled by charge carrier generation (and their transport) originate on dissociation of metastable geminately bound electron‑hole pairs (GPs) caused by the reexciting signal.

At higher intensities of electric field and/or higher laser intensities the influence of bimolecular recombination is major.


final conclusion



3. Is the photoionization tractable in terms of Onsager theory?

1. The observed current transient is a convolution of time

dependent charge generation (dominant at MeLPPP) ,

transport and discharge.


final conclusion




2. There are two processes occurring in the bulk of

conjugated polymer: the field assisted dissociation into

geminate pairs that subsequently can separate fully

(significant for MeLPPP) and sensitized photogeneration at

either non‑intentional or intentional dopants that can act as

electron scavengers (significant for PhPPV).


final conclusion




3. We emphasize that in neither case the Onsager

description is applicable because in the intrinsic case the

field assistance in the primary event of exciton breaking is

disregarded in the Onsager formalism, while in the other

case GP disssociation is aided by the finite energy of the

charge carrier - usually the hole - that resides on a

segment of the conjugated polymer next to the counter

charge.

TRANSIENT PHOTOCONDUCTIVITY AND CHARGE GENERATION IN A THIN FILMS OF Π -CON JUGATED POLYMERS

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