transient photoconductivity and charge generation in a thin films of Π -con jugated polymers
DESCRIPTION
TRANSIENT PHOTOCONDUCTIVITY AND CHARGE GENERATION IN A THIN FILMS OF Π -CON JUGATED POLYMERS. M artin Weiter , Heinz Bä ß ler Institute of Physical, Macromolecular and Nuclear Chemistry, Philipps University, Marburg, Germany. Outline: introduction transient photoconductivity - PowerPoint PPT PresentationTRANSCRIPT
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
LAMINATE meeting Norrköping, September 2004 2
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
LAMINATE meeting Norrköping, September 2004 3
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.
LAMINATE meeting Norrköping, September 2004 4
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
LAMINATE meeting Norrköping, September 2004 5
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
LAMINATE meeting Norrköping, September 2004 6
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
LAMINATE meeting Norrköping, September 2004 7
experiment 1
• electrodes (ITO+semitansparent Al, Al)
• laser wavelengths 453 nm (2.7 eV), 355 nm (3.5 eV),
• different field strength with both polarity
• 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)
LAMINATE meeting Norrköping, September 2004 8
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
LAMINATE meeting Norrköping, September 2004 9
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.
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]
LAMINATE meeting Norrköping, September 2004 10
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.
LAMINATE meeting Norrköping, September 2004 11
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
LAMINATE meeting Norrköping, September 2004 12
charge generation - yield
104 105 106 10710-4
10-3
10-2
10-1
100
Yie
ld
Electric field [Vcm-1]
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
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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
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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
LAMINATE meeting Norrköping, September 2004 15
charge generation - yield
104 105 106 10710-4
10-3
10-2
10-1
100
Yie
ld
Electric field [Vcm-1]
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.
LAMINATE meeting Norrköping, September 2004 16
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.
LAMINATE meeting Norrköping, September 2004 17
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),
• different field strength with both polarity
• temperature 150K – 320 K
• doping by TNF
LAMINATE meeting Norrköping, September 2004 18
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)
LAMINATE meeting Norrköping, September 2004 19
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
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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
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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
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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.
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transient photocurrents
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.
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transient photocurrents
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.
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final conclusion
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?
1. The observed current transient is a convolution of time
dependent charge generation (dominant at MeLPPP) ,
transport and discharge.
LAMINATE meeting Norrköping, September 2004 26
final conclusion
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?
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).
LAMINATE meeting Norrköping, September 2004 27
final conclusion
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?
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.
LAMINATE meeting Norrköping, September 2004 28