Photocurrent response of bipyridine containing poly(p-phenylene-vinylene) derivatives

Article abstract of DOI:10.1021/jp0112931

The photoinduced charge separation and subsequent transport under an external electric field is studied in the family of poly[bipyridine/(p-phenylene-vinylene)_n] derivatives having n = 0, 1, and 3, respectively, p-phenylene-vinylene subunits separating the bipyridylene vinylene skeleton. Steady-state photocurrent of the polymers is studied in sandwich and surface configurations and correlated with transient photocurrent measurements. The results reveal the facile electric-field-induced separation of the electron-hole pair for n = 1 samples relative to n = 3 samples. We also estimate the energy barriers involved in the process of carrier generation and transport in these systems.

Full text of DOI:10.1021/jp0112931

J. Phys. Chem. B 2001, 105, 7671-7677
7671
Photocurrent Response of Bipyridine Containing Poly(p-phenylene-vinylene) Derivatives
K. S. Narayan* and K. V. Geetha
Chemistry and Physics of Materials Unit, Jawaharlal Nehru Center for AdVanced Scientific Research,
Jakkur, Bangalore 560064, India
G. Nakmanovich, E. Ehrenfreund, and Y. Eichen
Solid State Institute, Technion - Israel Institute of Technology, Haifa, 32000 Israel
ReceiVed: April 9, 2001
The photoinduced charge separation and subsequent transport under an external electric field is studied in the
family of poly[bipyridine/(p-phenylene-vinylene) ] derivatives having n ) 0, 1, and 3, respectively, p-phenylene-
n
vinylene subunits separating the bipyridylene vinylene skeleton. Steady-state photocurrent of the polymers is
studied in sandwich and surface configurations and correlated with transient photocurrent measurements.
The results reveal the facile electric-field-induced separation of the electron-hole pair for n ) 1 samples
relative to n ) 3 samples. We also estimate the energy barriers involved in the process of carrier generation
and transport in these systems.
Introduction
of mobile charges through an interband π-π* transition.8,9 On
the other hand, on the basis of a variety of complementary
Conjugated aromatic p-phenylene ring-containing oligomers
and polymers provide model systems in the current effort to
elucidate the interconnection between the chemical structure of
a conjugated material, and its electronic, optical, and nonlinear
optical properties. There has been sustained interest in the
ordering and energies of electronic states, quantum efficiencies
for charge separation and recombination processes, and their
dependence on the chemical structure. Charge generation and
charge transport processes have been extensively studied in the
poly(p-phenylene-vinylene) (PPV) and its derivatives. In this
paper, we study the effect of introducing bipyridylene vinylene,
experiments, it is argued that the photocurrent (Iph) originates
from secondary processes, where the initial intrachain excitons
1
0,11
dissociate to free charges.
The evolution of Iph upon
photoexcitation and the subsequent decay is, therefore, a
valuable tool to explore various processes and develop a deeper
understanding of phenomena such as electroluminescence in
these systems. Extrinsic mechanisms are most likely to affect
the relatively long time (> nanosecond) transient photocon-
ductivity measurements. This relatively slow decay process is
interpreted as being extrinsic due to dissociation of polaron pairs
1
2-5
(
or interchain excitons) through interaction with oxygenated
(
bpv) moieties into the phenylene-vinylene skeleton (poly(5-
vinylene-5′-vinylenephenylene-2,2′-bipyridine), 1, poly(p-tris-
phenylenevinylene)-2,2′-bipyridine), 2, and poly(5,5′-vinylene-
,2′-bipyridylene), 3, on the photophysical properties. An
1
2
defects to create positive polarons. The slow component has
been modeled in terms of a recombination limited dispersive
decay.
(
2
The spectral responses of Iph in these PPV-bipyridine systems
is studied in planar or sandwich configuration, where the
incident light propagation direction is perpendicular or parallel
to the electric field, respectively. We use both steady-state and
transient measurements to study these polymers. The depend-
encies of Iph on the electric field, light intensity, and temperature
are measured in order to probe the mechanisms of charge
generation and transport in such polymers.
incentive to the synthesis of such polymers is their ability to
exhibit reversible and tunable optical and electrooptical proper-
ties, depending on their protonation state at the nitrogen atoms
of the bipyridine moieties.6 The tunability of these novel
polymers was clearly demonstrated to originate from a combi-
nation of changes in the intrachain (“molecular”) as well as
interchain (“supramolecular”) electronic properties arising from
a significant change in aggregation upon protonation-depro-
6,7
tonation processes. Such reversible systems can be effectively
utilized in sensor applications. This unique class of polymers
allows the study of charge separation properties in a homologous
series where the asymmetry of the charge density distribution
in the repeat unit can be controlled. In this paper we focus
largely on the free base form of these polymers and explore
the effect of the variation in charge density distribution in the
repeat unit on bulk photoconductivity properties.
Experimental Section
Materials. Polymers poly(5-vinylene-5′-vinylenephenylene-
2
,2′-bipyridine), 1, poly(p-tris(phenylenevinylene)-2,2′-bipyri-
dine), 2, and poly(5,5′-vinylene-2,2′-bipyridylene), 3, were
prepared as per the scheme mentioned in ref 6.
5
,5-Bis(carboxyethyl)-2,2′-bipyridine, 4. This polymer was
13
prepared according to a modified literature procedure.
On the basis of the coincidence of the onsets of the
photoconductivity and absorption in MEH-PPV, it was con-
cluded that photoexcitation of PPV leads to a direct generation
5
,5′-Bis(chloromethyl)-2,2′-bipyridine dihydrochloride, 5.
Sodium borohydride (6.8 g, 180 mmol) was added to an ice-
cold solution of 5,5′-bis(carboxyethyl)-2,2′-bipyridine (10.4 g,
3
4 mmol) in 150 mL of ethanol. The solution was left to reach
*
Corresponding author. Fax: 91 80 8462766. E-mail: narayan@
jncasr.ac.in.
room temperature, then refluxed overnight under an inert
1
0.1021/jp0112931 CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/25/2001

7
672 J. Phys. Chem. B, Vol. 105, No. 32, 2001
Narayan et al.
atmosphere. The solvent was removed under reduced pressure
and the residue was refluxed in acetone for 1 h, then refluxed
in aqueous K2CO3 for an additional hour, then dried. In view
powder, 90 mg (75%), MWAv ∼5500-20000. Elemental
analysis for free-base and acid-saturated (“protonated”) forms,
respectively: Anal. calcd for C20H14N2: C, 85.08; H, 5.00;
N, 9.92. Found: C, 79.89; H, 5.16; N, 9.05. Anal. calcd for
C20H14N2.2HCl: C, 67.62; H, 4.54; N, 7.89. Found: C, 60.83;
of its low solubility in most organic solvents, crude 5,5′-bis-
1
(
(
hydroxymethyl)-2,2′-bipyridine, 6, (NMR: H (CDCl3): δ 4.58
s, 4H), 7.84 (d, 2H), 8.33 (d, 2H), 8.59 (s, 2H); 13C (DMSO):
1
H, 4.48; N, 6.66. NMR H(CF COOD): δ 9.92 (brs, aldehyde
3
δ 60.54, 119.83, 135.40, 137.83, 147.70, 153.99; mp: 155-
end groups), 9.20 (brs, 2Hpy), 8.89 (brs, 2Hpy), 8.57 (brs,
+
13
1
57 °C; MS: (CI) 217, M+H ) was used “as is” for the next
2Hpy), 7.71-7.40 (m, 8Hph+vinyl). Solid-state NMR C: δ
step. Freshly distilled thionyl chloride (120 mL) was added
slowly to the grounded crude 5,5′-bis(hydroxymethyl)-2,2′-
bipyridine. The solution was stirred for 2 days at 60 °C and
then evaporated to dryness. Dichloromethane (100 mL) and 50
mL of water were added to the residue and the aqueous solution
was neutralized using ammonia solution. The product was
extracted to dichloromethane, then dried over sodium sulfate,
filtered, and precipitated from solution using dry HCl gas, in
the form of hydrochloric salt, yielding 3.8 g, 40%.
153.35 (py), 149.80 (py), 137.70 (py), 135.35 (ph), 131.10 (py),
129-123 (ph+vinyl), 120.0 (py). IR (KBr): 1696, 1589, 1537,
1505, 1469, 1418, 1376, 1275, 1207, 1107, 1053, 1021, 959,
-
1
840, 803, 739, 651 cm . Luminescence quantum yield (Ex:
-7
-5
355 nm): 0.50, 0.20 (solutions of 10 M and 10 M in formic
acid, respectively).
Poly(p-tris-(phenyleneVinylene)-2,2′-bipyridine), 2. A solution
of sodium methoxide (27 mg, 0.5 mmol) in 10 mL of dry ethanol
was added dropwise to a solution of 1,4-bis(4-formylstyryl)-
benzene, 6, (81 mg, 0.23 mmol) and 5,5′-bis(triphenyl-phos-
phonium-methyl)-2,2′-bipyridine dichloride, 4, (178 mg, 0.23
mmol) in a mixture of DMF (25 mL) and ethanol (8 mL) at 50
°C. The reaction mixture was stirred overnight at room
temperature, then the precipitate was collected, successively
refluxed with several portions of dichloromethane and THF to
remove short oligomers, and dried under reduced pressure,
yielding 96 mg (90%) of the polymer as bright yellow powder.
MWAv ∼ 2000-5500. Elemental analysis for free-base form:
Anal. calcd for C36H26N2: C, 88.86; H, 5.39; N, 5.76.
5
,5′-Bis(formyl)-2,2′-bipyridine, 7. A solution of 5,5′-bis-
hydroxymethyl)-2,2′-bipyridine, 6, (0.81 g, 3.7 mmol) in 25
mL of dry pyridine was added dropwise to a solution of Pb-
CH3CO2)4 (5.1 g, 11.5 mmol) in 25 mL of dry pyridine. The
(
(
solution was stirred at 90 °C for 2 h, then the solvents were
reduced under reduced pressure. Chromatography (silica, 98:2
dichloromethane/ethanol) afforded 120 mg (15% yield) of pure
7
as a colorless solid.
NMR: 1H (CDCl3): δ 8.32 (dd, 2H), 8.70 (d, 2H), 9.16 (d,
13
2
1
H), 10.19 (s, 2H); C (CDCl3): δ 122.36, 131.69, 137.12,
+
1
51.55, 159.14, 190.28; mp: 228 °C; MS: (CI) 212.4, M .
Found: C, 69.78; H, 4.79; N, 4.45. NMR H(CF3COOD): δ
9
2
.71 (brs, aldehyde end group), 8.71 (brs, 2Hpy), 8.53 (brs,
5
,5′-Bis(triphenyl-phosphonium-methyl)-2,2′-bipyridine di-
Hpy), 8.37 (brs, 2Hpy), 7.54-7.08 (m, 20Hph+vinyl). Solid-
chloride, 8. An amount of 2.5 g (7.4 mmol) of the hydrochloride
salt of 5,5′-bis(chloromethyl)-2,2′-bipyridine was neutralized
with NH3(aq), extracted with dichloromethane, dried with
sodium sulfate, filtered, and freed from organic solvents under
reduced pressure. An amount of 2 g (7 mmol) of 5,5′-bis-
13
state NMR C: δ 153.35 (py), 149.80 (py), 137.70 (py), 135.35
(
1
1
ph), 131.10 (py), 129-123 (ph+vinyl), 120.0 (py). IR (KBr):
696, 1589, 1537, 1505, 1469, 1418, 1376, 1275, 1207, 1107,
-
1
053, 1021, 959, 840, 803, 739, 651 cm .
(chloromethyl)-2,2′-bipyridine and 4 g (15 mmol) of tri-
Poly(5,5′-Vinylene-2,2′-bipyridylene), 3. A solution of sodium
phenylphosphine were dissolved in 60 mL of DMF and stirred
overnight at 80 °C. The resulting white precipitate was filtered
and washed with DMF and ether yielding 4.1 g (75%) of
pure 5,5′-bis(triphenyl-phosphonium-methyl)-2,2′-bipyridine di-
methoxide (122 mg, 1.8 mmol) in 20 mL of dry ethanol was
added dropwise to a solution of 5,5′-diformyl-2,2′-bipyridine,
7, (106 mg, 0.5 mmol) and 5,5′-bis(triphenyl-phosphonium-
methyl)-2,2′-bipyridine dichloride, 4, (397 mg, 0.5 mmol) in a
mixture of dichloromethane (8 mL) and ethanol (20 mL) at 50
°C. The reaction mixture was stirred overnight at room
temperature, then the precipitate was collected, successively
refluxed with several portions of dichloromethane, water,
DMSO, ethanol, and THF to remove short oligomers, and dried
under reduced pressure, yielding 113 mg (50%) of the polymer
1
chloride. NMR H(CDCl3): δ 5.30 (d, 4H), 7.44 (d, 2H), 7.67-
7
1
.95 (m, 30H), 8.13 (d, 2H), 8.24 (s,2H); 13C(DMSO): δ 116.5,
18.2, 120.4, 125.1, 130.2, 134.0, 135.2, 139.4, 150.7, 154.0;
+
mp: 267-270 °C (dec); MS: (CI) 705.5 M+H .
1
,4-Bis(4-formylstyryl) benzene, 9. A solution of sodium
methoxide (360 mg, 6.7 mmol) in 50 mL of dry ethanol was
added dropwise to a solution of 1,4-bis(triphenylphosphonium)-
p-xylenedichloride (2 g, 3 mmol) and terephthalaldehyde (0.8
g, 6 mmol) in 30 mL of dry ethanol. The solution was stirred
overnight at room temperature, then the solvent was removed
under reduced pressure and the residue chromatograph (silica:
1
4-17
as bright yellow powder.
Measurements. The polymers isolated in the form of powders
were soluble in formic acid yielding dark orange luminescent
solutions. Thin films of the appropriate polymer were deposited
from formic acid solution on glass-coated ITO substrates by
drop casting and this process was followed by drying in an oven
1
:2 hexane/dichloromethane and then dichloromethane). An
-
5
amount of 400 mg of 6 (40%) was collected as a white solid.
under reduced pressure (10 mmHg, 70 °C, 5 days). It has
been shown using IR studies that this process of prolonged
heating under reduced pressure results in a gradual disappear-
1
HNMR (CDCl3): δ 7.20 (d, 4H), 7.55 (s, 4H), 7.64 (d, 4H),
7
3
.86 (d, 4H), 9.98 (s, 2H); mp: 247-249 °C; MS: (High Res.)
+
6
39.1, M+H .
ance of the formate absorption bands. It was shown that
Poly(5-Vinylene-5′-Vinylenephenylene-2,2′-bipyridine), 1. A
reexposing the films to formic acid vapors results in the
reappearance of the formate band. All Iph measurements referred
6
solution of 68 mg of sodium ethoxide (1 mmol) in 15 mL of
dry ethanol was added to a solution of 318 mg (0.4 mmol)
of 5,5′-bis(triphenyl-phosphonium-methyl)-2,2′-bipyridine di-
chloride and 45 mg (0.4 mmol) of terephthalaldehyde in 40 mL
of ethanol. The solution was stirred overnight at room temper-
ature and the yellow precipitate was collected, washed with
several portions of dichloromethane and THF, and dried under
reduced pressure. The polymer was isolated as bright yellow
in the results and discussion sections refer to that of the free-
2
base samples. Aluminum films (area of electrode ) 0.3 cm ,
typical thickness of 500 nm) were deposited on the polymer
films using standard thermal evaporation techniques. Iph mea-
surements were carried out in vacuum conditions using sandwich-
type devices of ITO|Polymer|Al with the light incident on the
ITO side, using sensitive lock-in techniques as well as dc

Bipyridine Containing Poly(p-phenylene-vinylene) Derivatives
J. Phys. Chem. B, Vol. 105, No. 32, 2001 7673
Figure 2. Comparison of Iph(λ) in polymers 1, 2, and 3 sandwich
devices at low forward bias voltage, inset Iph(λ) at higher reverse bias
voltage.
Figure 1. Chemical structure along with the absorption spectra of
polymers a- 1, b- 2, and c- 3 and the emission spectra (λexc ) 355
nm): a- 1, b- 2, c- 3. Solid lines represent free base form, dashed lines
represent acid-saturated forms.
Figure 3. Dark current and Iph as a function of applied voltage for
polymer 1 (A) and polymer 2 (B) devices.
methods.18 In the planar (surface) geometry, the polymer was
respective protonated forms which are red shifted and broader
with no vibronic features. (ii) Polymer 1 in its free base form
has a more well defined absorption onset and a lesser degree
of inhomogeneous broadening compared to polymers 2 and 3.
Detailed studies on the changes in the absorption and PL of the
films, which occur upon protonation-deprotonation processes,
have been shown to originate from both molecular level and
interchain (supermolecular) interactions. The increased inter-
chain interaction in the accompanied protonated form by the
larger charge delocalization effectively causes the red-shift.
Additionally, the PL band in the case of the protonated film is
broader without any vibronic features and can be attributed to
the increased disorder which can also arise from increased
interchain interactions.
deposited on quartz substrates. After processing it to the base
form, metal electrodes, 0.1 mm apart, were deposited on the
surface. Spectral response of the photocurrent was carried out
using a 150 W xenon lamp along with a monochromator using
the lock-in technique. Temperature-dependent measurements Iph-
(T) were carried using a closed cycle refrigerator offering a wide
temperature range capability of 10-500 K, with the sample
mounted alongside a calibrated thermometer, on the cold head
of the cryostat fitted with quartz windows. 5 mW diode lasers
at 532 and 473 nm were used as the light source for the Iph(T)
measurements. Transient decay measurements were performed
on samples in the planar auston-strip configuration using a
nanosecond pulse laser at 355 nm, 10 Hz repetition rate, and a
5
4520A HP digital oscilloscope. The temporal resolution for
The spectral response of the photocurrent, I , in sandwich-
ph
the transient measurements was ∼2 ns. Iph of the protonated
films was accompanied by large noise-to-signal ratio and
systematic measurements were not possible.
type devices having polymers 1, 2, and 3 as active layers is
shown in Figure 2. Clearly, the spectral response indicates an
onset roughly around the optical absorption edge. The response
Iph for polymer 1 is an order of magnitude higher than in similar
Results and Discussion
2
devices of polymer 2. The magnitude of Iph is 20 nA/cm at an
2
Steady-State Photoconductivity. Figure 1 depicts the ab-
sorption and luminescence spectra of thin films of the three
polymers 1, 2, and 3 along with their chemical structure in their
free base and protonated forms. The results of absorption and
incident photon density of 73 µW/cm at 400 nm and applied
bias of -2 V. The Iph-V response in devices of polymer 1,
shown in Figure 3, is asymmetric with an open circuit voltage,
Voc, of 0.2 V and short circuit current, Isc, of 0.8 nA. The
asymmetry represents the photodiode type behavior of these
devices with the reverse bias mode indicating the negatively
biased ITO with respect to the Al electrode. The Iph-V of
6
emission spectra presented in Figure 1 and in previous studies
show the following. (i) The free base forms of these polymers
have sharper absorption and luminescence profiles than their

7674 J. Phys. Chem. B, Vol. 105, No. 32, 2001
Narayan et al.
Figure 4. Iph(λ) in the planar-surface geometry for polymer 1, polymer
, and polymer 3; inset is the plot of the normalized Iph versus relative
2
intensity of polymer 1 in the log-log scale.
polymer 2 is less asymmetric with Voc ∼ 0.1 V and Isc ∼ 0.6
2
2
nA/cm at light intensity of ∼50 µW/cm at 400 nm. An increase
in Voc is expected with increasing photon density until reaching
saturation. At higher voltages, > 4 V, injection currents develop
and the device exhibits the characteristic LED response.
Figure 5. (A) Temperature dependence of the dark current I and the
photocurrent Iph in the planar geometry of polymer 1 at high field, HF
(
400 V) and low field, LF (100 V) voltage bias conditions. (B)
Arrheneius plot of Iph at high and low field, inset is the voltage
dependence on the Iph
The intensity dependence of Iph for input power of 0.01 to
.
2
1
0 µW/cm is linear in the entire voltage range for all the devices
in this configuration. The linear behavior of Iph as a function of
photon density for the three samples indicates monomolecular
kinetics; the photocurrent directly measures the efficiency of
the charge carrier generation. The Iph(λ) spectral response of
devices of polymer 1 depends strongly on the polarity and
magnitude of induced bias as is evident from the inset in Figure
dependence of Iph on moisture conditions is observed in this
planar geometry for all the samples. Iph drops by a factor of
100 as the vapor pressure is reduced, probably due to the
removal of traces of water molecules from the surface of the
polymer. This effect is found to be reversible. Upon exposure
to ambient atmosphere, the polymers regain their initial proper-
ties, revealing the extrinsic nature of such a defect-mediated
transport process in the systems. The intrinsic component of
the photocurrent processes is reflected in the measurements
carried out after extensive pumping at elevated temperatures.
The magnitude of the spectral response Isurph(λ) after this
treatment with measurements in vacuum conditions, which is
scaled down by a factor of 100, was in the same range as the
2
. Clearly, a red shift of ∼0.3 eV in the onset of the Iph(λ) is
observed under high reverse bias. A similar, but considerably
smaller, effect was observed for polymers 2 and 3. Such red-
shifted bias polarity-dependent features can be qualitatively
explained on the basis of exciton quenching at the Al interface
and the higher hole mobility of the polymer. However, a detailed
bias-dependent spectral analysis requires the knowledge of
various parameters, such as exciton and/or free carrier diffusion
2
noise fluctuation amplitude at low light levels < 1 µW/cm .
1
9
lengths, and barrier height at the interface junctions.
Isurph(λ) after careful analysis qualitatively resembled ambient
condition responses. However, measurements using laser sources
at fixed wavelengths of 473 and 532 nm were possible with
large signal-to-noise ratio even in vacuum conditions.
Iph(λ) in these polymers clearly consists of both extrinsic and
intrinsic contributions. The Iph(λ) from intrinsic charge carrier
is expected to be blue shifted relative to the absorption
20
spectrum. Results from the sandwich devices can be explained
on this basis of bulk photoionization and interfacial exciton
quenching processes. The photocarrier generation rate is clearly
higher for polymer 1 based devices compared to polymer 2 and
polymer 3, with Iph(λ) in polymer 1 samples strongly dependent
on the bias conditions. At low bias voltage, interfacial processes
for charge separation appear to be dominant as indicated by
the red shift in the reverse bias Iph(λ) while at high voltage bulk
photoionization takes over with the forward and reverse bias
Iph(λ) appearing to be identical.
Isurph in the planar geometry varies significantly with ambient
conditions, in a reversible manner, with the extrinsic processes
governed by the surface-defect-mediated photocurrent in the
samples. Iph(T), in the first approximation, can be analyzed as
a process composed of a thermally activated barrier component
for photoinduced charge generation and a complex hopping
transport or a activated charge-transport component. The
relatively weak temperature dependence of the dark current
indicates an activation energy smaller than 0.1 eV. The much
stronger Iph(T) dependence on T is indicative of a barrier limited
free carrier generation mechanism in polymer 1 . This inter-
pretation is supported by the strong dependence of Iph(T) on
Measurements in the planar geometry were carried out in a
vacuum as well as in ambient conditions. Figure 4 depicts the
steady-state spectral response of Isurph(λ) in a planar geometry
3
the electric field, above a threshold value (∼3 × 10 V/cm).
for the three polymers in ambient conditions under a field of
The higher barrier energy of ∼0.2 eV, at V > 300 V (∼ 5 ×
3
3
1
0
V/cm. Measurements in this geometry are devoid of
10 V/cm), for Iph(T) along with the fact that the similar
problems related to electrode modifications and interfacial
effects; however, since the electric field magnitudes are quite
low the magnitude of Iph decreases proportionately. Figure 4
reveals the sharp onset of Iph(λ) at the absorption edge. The
results are qualitatively similar to the response of Iph(λ) to
reversed bias in a sandwich-type geometry. However, a strong
temperature dependence for Idark(T) at different voltage bias also
strongly indicates the dominant charge carrier generation limited
current rather than a transport limited process in this field and
temperature regime.
The temperature, T dependence of Isurph of the vacuum heated
3
samples of polymer 1 at F < 10 V/cm shown in Figure 5 can

Bipyridine Containing Poly(p-phenylene-vinylene) Derivatives
J. Phys. Chem. B, Vol. 105, No. 32, 2001 7675
be fitted to an Arrhenius behavior with an activation energy
∼
0.1 eV. The plot of Isurph(F) as a function of applied bias
indicates a cross over to a higher nonlinear regime as F
3
approached 5 × 10 V/cm. This behavior is also observed in
3
the response of Isurph(T) at F ∼ 5 × 10 V/cm, showing more
than an order of magnitude increase in Iph, compared to the low
field regime. A respective barrier of 0.2 eV is measured over
the entire temperature range. The intensity dependence of the
steady-state photocurrent shows a square root dependence over
2
the entire measured range of incident photon density (10 W/cm
2
R
to 1 mW/cm ) at 473 nm. Isurph ∼ (intensity) with R ∼ 0.5
(Figure 5, inset).
The observed square root dependence of Iph as a function of
incident photon flux (Figure 4, inset), can be due to the
bimolecular photocarrier recombination caused by their spatial
restriction, often observed in this type of planar geometry.
On the other hand, a linear relation has been observed in the
sandwich configuration of these devices.21,22 The transport and
2
1
Figure 6. Transient decay current I of polymer 1 after a pulse
excitation, inset is the log(I) versus time and the initial current (at time
t ) 0) as a function of electric field F.
recombination mechanisms of the photogenerated carriers in the
two geometries are clearly different with a surface mediated
process in the planar geometry, and a sizable electric field/
Shotkky processes in the sandwich geometry. The results in
Figure 5 indicating the Iphsur(T) and sub-linear intensity depen-
dence in the coplanar geometry can also be interpreted on the
22
basis of trap-controlled mechanisms. A modified version of
the model proposed by Rose23 can possibly explain the field-
dependent Iphsur(T) and also the intensity dependence. In this
model the square root dependence of Iphsur(intensity) is modified
by the electron traps in energy.23 The density of traps in any
small energy interval through which the steady-state fermi level
sweeps can be typically taken to be an exponential function of
the energy of traps below the band edge. The trap filling and
emptying kintetics are also dependent on the electric field and
may be source for the field-dependent Iphsur(T).
Figure 7. Transient decay current I (on a normalized scale) of polymers
1, 2, and 3 after a pulse excitation at the same electric field. Inset is
the plot on the log scale.
Transient Photoconductivity. The difference in the photo-
induced charge generation process is significantly apparent in
the transient measurements. These measurements offer additional
insight in the photophysics of these materials, with the results
yielding a closer estimate for the efficiency of photocarrier
generation and transport. The estimate involves the measurement
of the total charge collected at the electrode immediately after
an incident short laser pulse and the yield æ can be expressed
as æ ) J/eNp, where Np is the photon flux, J is the current
density with the cross sectional area L/R for the carrier transport,
where R is the absorption coefficient, and L is the electrode
3
dimension.
Figure 6 depicts the transient decay, Iph(t) of the current after
a pulsed photoexcitaion of polymer 1. The time constant for
the initial fast decay, indicated in the figure, puts an upper limit
on the actual decay constant in the Iph(t) response due to
experimental limitations. Nevertheless, the measurements can
explicitly separate the initial fast process, which are in the 0.5-
1
0 ns range with the slower decay kinetics in the 100 ns-1 s
Figure 8. (A) Charge carrier yield as a function of electric field for
polymers 1, 2, and 3 samples from transient decay measurements along
with fits to Onsager’s 3D model (solid lines) for different thermalization
regime, observed in such systems. The results indicate the
predominant contribution of the photocurrent from the fast decay
process (<40 ns) and a relatively small contribution from longer
processes (>40 ns), which constitute <5% of the overall
integrated response. The field dependence of the transient
response shows the following interesting features. (i) The current
at t ) 0 shows a dramatic difference with respect to field for
polymer 1 compared to polymer 2 as in Figure 6 inset (ii). There
is no significant variation in the time constants involving the
initial decay with respect to varying electric field. The initial
0
distance, r . (B) Fits to a power law behavior.
decay in polymer 1 is fast characterized by a shorter time
constant compared to polymer 2 and polymer 3 as depicted in
Figure 7.
The values for the quantum yield æ are plotted as a function
of field in Figure 8 for the different samples. A reasonable
approach to understand the nonlinear dependence of æ(F) is
the Onsager three-dimensional (3D) model for geminate re-

7676 J. Phys. Chem. B, Vol. 105, No. 32, 2001
Narayan et al.
combination limited charge carrier transport.24 An essential
assumption of Onsager’s theory of the dissociation of short-
lived charge pairs is that photon absorption creates with
probability æ0 a Coulombically bound e-h pair with initial pair
distance r0. It can either recombine or dissociate with a
probability in the course of a temperature- and field-assisted
diffusion process. The activation energy Ea of the quantum yield,
supramolecular factors also need to be considered, to comment
on the possible origins of the field dependent primary yield.
Previous results on such family of polymers have shown that
insertion of BPY units in an otherwise well conjugated polymer
chain brings about structural changes that diminish π-conjuga-
2
6
tion in the polymer chain. This was argued on the basis that
the electronic coupling is reduced because the disparity in the
electronic structure of the two units and the rotation about the
C-C bond connecting the pyridyl rings of BPY results in
nonplanar conformation between the two adjacent conjugated
segments of the polymer also causing a reduced π-orbital
overlap. These studies were done on dilute solutions of the
polymer where interchain interactions were absent.26 It is
reasonable to expect an increased interaction in the solid/
concentrated form when additional nonplanar degrees of free-
dom of BPY units are present. This feature of higher charge-
separation yield in the nanosecond time regime and the field
dependence in polymer 1 compared to polymer 2, can be
attributed to an higher interchain interaction in polymer 1, which
has a higher content of BPY units. The interchain processes
effectively result in electrons and holes generated on separate
chains, which have a lower probability of recombination than
that of intrachain charge-separation processes. In other words,
increased interchain charge-transfer processes can enhance
photocurrent efficiency. The nonplanar geometry introduced by
insertion of BPY units in a PPV chain can facilitate the
interchain interaction in the condensed phase. This qualitative
picture can also explain the strong electric field dependence of
the photocurrent in polymer 1, since the electric field influences
the charge carrier separation across the chains. However, a
systematic study of the packing arrangement with varying units
of BPY is needed to conclusively establish this correlation.
In conclusion, we have studied charge generation and
separation in a novel series of PPV-BPY polymers by
measuring the photocurrent response at various temperatures,
electric fields, and light intensity. The efficiency for photocurrent
generation is clearly higher in polymer 1 (PV-BPY) than in
2
measured at low fields, is related to r0 via r0 ) e /(4πꢀꢀ0Ea)),
in the limit of the homogeneous medium approximation. Figure
8
3
also compares the experimentally obtained efficiencies with
D Onsager model with assumptions and calculations as
25
reported by Pai and Enck and the following additional
assumptions for the dielectric constant, with the primary
dissociation yield ) 1 at the high-temperature limit. The model
also does not take into account any disorder effects. The 3D
model yields good qualitative fits to the experimental data for
polymer 2 and polymer 3 with the value of thermalization
distance ∼1.2 nm corresponding to an activation energy from
eq 2 to be ∼0.3 eV. However, close fits to the 3D to the
experimental data for polymer 1 were not possible with a
reasonable wide range of values for the parameters involved in
the model. It has been argued that the early stage of e-h pair
dynamics should be dominated by intrachain rather than
4
interchain processes justifying a need for a 1D model. Attempts
to fit the results of polymer 1 to an ideal 1D Onsager model
were also unsuccessful. However, it is noted that the low field
photocurrent measurements shown in Figure 5b (inset), which
4
is essentially low field data < 5 × 10 V/cm, yields a better fit
to a 1D model with a reasonable estimate of r0 ∼ 1.2 nm. The
strong, and the sizable nonlinear dependence of the yield φ(F)
for polymer 1 in the intermediate and high field regime is
â
indicated by the power law fit in this range with φ ≈ AF with
â ≈ 3.5 for polymer 1 while the values of â for polymer 3 and
polymer 2 are ≈1.5 and 1, respectively, as shown in Figure
8
B. The measure of â reveals the strong field dependence of
the photocurrent for polymer 1 and is indicative of field-induced
dissociation processes in this system. The observation and
interpretation of the transient measurements is consistent with
the observations from the bias-dependent spectral responses Iph-
polymer 2 ((PV) -BPY) as observed by steady-state photo-
3
conductivity measurements and transient photoconductivity
(
λ) and the activated behavior observed in the temperature
measurements. A strong electric-field dependence of the yield
for photoinduced charge generation is also observed in polymer
1 compared to polymers 2 and 3. These observations can arise
from the increased interchain interaction in polymer 1. The
results indicate a strong correlation of the bulk property of
photoconductivity with the basic structure of the tunable repeat
unit.
dependence of Iph at high F.
These experimental observations of the field dependence of
polymer 1 can be argued on the basis that the primary ionization
process is field dependent, which can be expressed accordingly;
æ0(F) ) æ0 + æ1(F). The importance of this field-dependent
term can be evaluated on the basis of the value of the exponent
â
â where æ1(F)/ æ0 ) AE . The effective probability for charge-
esc
esc
carrier separation æ(F) ∼ æ0(F) × j (F) where æ (F) is the
Acknowledgment. The authors (K.S.N. and K.V.G.) thank
the Department of Science and Technology, Govt. of India. The
work at Technion was also supported by Israel Ministry of
Science (2046-1-99).
dissociation probability for a geminate pair at an initial
2
4
separation. The ratio of the measured field-dependent photo-
current and the photocurrent expected from an initial field-
independent process can then be expressed as
References and Notes
esc
æ(F)/æ æ (F) ) (1 + æ (F)/æ )
0
1
0
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