Photovoltaic response and values of state dipole moments in single-layered pyrazoloquinoline/polymer composites

Article abstract of DOI:10.1016/j.saa.2007.07.014

We report the photovoltaic response of composite films formed by polymer transport matrices poly(3-octylthiophene) (P3OT) and poly(3-decylthiophene) (PDT) with incorporated 1H-pyrazolo[3,4-b]quinoline (PAQ) chromophore (see the first figure). The photovol

Full text of DOI:10.1016/j.saa.2007.07.014

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 70 (2008) 117–121
Photovoltaic response and values of state dipole moments
in single-layered pyrazoloquinoline/polymer composites
E. Gondeka, I.V. Kitykc,∗, A. Danelb, J. Sanetraa
a Institute of Physics, Technical University of Cracow, Podchrazych 1, Krakow, Poland
b Department of Chemistry, Agricultural University of Krakow, Al. Mickiewicz 24/28, Krakow 30-059, Poland
c Institute of Physics, J. Dlugosz University Cze˛stochowa, Al. Armii Krajowej 13/15, Cze˛stochowa, Poland
Received 15 June 2007; received in revised form 9 July 2007; accepted 11 July 2007
Abstract
We report the photovoltaic response of composite films formed by polymer transport matrices poly(3-octylthiophene) (P3OT) and poly(3-
decylthiophene) (PDT) with incorporated 1H-pyrazolo[3,4-b]quinoline (PAQ) chromophore (see the first figure). The photovoltage (PV) data
were obtained for different substituted PAQ possessing different state dipole moments. The photovoltaic cells were formed between ITO and
aluminum electrodes. We found that the PV signal of polymer/PAQ substantially depends on the state dipole moments of the pyrazoloquinoline
chromophore. This fact indicates on a possibility of significant enhancement of PV efficiency by appropriate variations of the state dipole moments
of chromophore. This results in photoinduced electron transfer from polymer serving as donors to PAQ being the electron acceptor. Despite an
efficiency of the PV devices is below 1%, however, it may be substantially enhanced in future varying the chromophore state dipole moments
appropriately.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Photovoltaic response; Pyrazoloquinoline
1. Introduction
dependent on the dipole–dipole interactions between the trans-
port polymer chains and the PQ chromophore. This interaction
seem to be crucial for polarons (electron vibration autolocalised
states) forming effective trapping centers, that determine all the
photo-kinetics processes.
In this article we have chosen the PQ chromophore molecules
(see Fig. 1a) with different state dipole moments to study the PV
parameters such as open circuit voltage (VOC) and short current
(ISC) for different length polymer transport matrices—PDT and
P3OT (Fig. 1b).
ItiswellknownthatPAQchromophoredemonstrateexcellent
optoelectronic and carrier transport properties [1], which allow
using them as materials for creation of single-layer light emitting
diodes. In the PAQ-based LEDs devices, the PVK or polysilane
polymers were used as a conductive transport matrices [2].
Both of the matrices exhibit good transport properties
required for efficient LED devices development, however they
are not applicable yet for the photovoltaic. We decided to use
the PDT and P3OT transport matrices with HOMO values of
about −5.2 to 5.3 eV and LUMO levels of about −2.85 eV. The
latter values correlate well with the energies of the LUMO in the
PQ donors, providing an efficient charge transfer. The relatively
large charge mobility in the PDT and P3OT polymer matri-
ces allows expecting a high charge carrier separation. During
exciton transport through the polymer chains, the main source
of the exciton recombination of the transport kinetics will be
The overall efficiency ηeff of a solar cell can be calculated by
the following formula:
VOC SC FF
I
ηeff
=
Ilight
where VOC is the open circuit voltage, ISC the short circuit cur-
rent, FF the fill factor, and I is the incident light solar radiation.
All the parameters are indicated in Fig. 2. The fill factor of solar
cells, which reflects their diode properties is determined by
I
mppVmpp
∗
Corresponding author.
E-mail address: i.kityk@ajd.czest.pl (I.V. Kityk).
FF =
I
SCVOC
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.07.014

118
E. Gondek et al. / Spectrochimica Acta Part A 70 (2008) 117–121
Fig. 1. (a) Investigated chromophore molecule of pyrazoloquinolines. (b) The principal molecular chemical formula for the transport polymers.
conducting polymers can be used in combination with PAQ
as electron acceptors to construct polymer photovoltaic cells.
Poly(3-octylthiophene) P3OT and poly(3-decylthiophene) PDT
with region regular structures were purchased from Aldrich.
Pyrazolo[3,4-b]quinolines can be prepared by various
withImpp andVmpp beingthecurrentandvoltageatthemaximum
power point in the fourth quadrant of the I/V curve.
2. Experimental
¨
method. The most important one is to include Fridlander con-
2.1. Materials
densation of o-aminobenzophenones, o-aminoacetophenones
and o-aminobenzaldehydes with pyrazolones [4–6]. There
may be some difficulties with the availability of ortho-amino
carbonyl component (aldehyde or ketone) (see Scheme 1).
The next one is the reaction of substituted anilines with
5-chloro-4-formyl-pyrazoles [7]. This method is very conve-
nient and is characterized with short time of the reaction
and relatively high yields. Formyl pyrazoles (2, Scheme 1)
are prepared by DMF/POCl3 formylation of pyrazolones. The
third one is a three-component reaction of aldehyde (aromatic
or aliphatic), substituted aniline and pyrazolone. In spite of
low yield (30–33%) it is very versatile one-step procedure
for pyrazolo[3,4-b]quinoline preparation substituted in pyridine
ring (alkyl or aryl) [8]. This procedure avoids the synthesis of
o-amino carbonyl component which is usually prepared in two
to three stages.
Polythiophenes are among the most promising conduct-
ing polymers, as they combine chemical stability, good melt
and solution process ability with the electronic and optical
properties interesting for their various applications [3]. Semi-
For the synthesis of methoxy substituted pyrazolo[3,4-
b]quinolines we have chosen the second procedure developed
by Brack [7] (Scheme 1). Aniline 1 (R3 H) or p-anisidine
(R3 OMe) reacts with aldehyde 2 at 140–190 ◦C for 20–40 min.
After cooling the reaction mixture, it is dissolved in chloroform
and filtered by short pad of alumina and finally subjected to
column chromatography on silica gel (Merck 60, 70–230 mesh).
Fig. 2. The current vs. applied voltage of a solar cell. The extracted current is
nagative. The fourth quadrant represents the voltage and current generated by
the cell. An externally applied voltage is necessary to obtain date points in the
first and third quadrants.

E. Gondek et al. / Spectrochimica Acta Part A 70 (2008) 117–121
119
Scheme 1. 140–180 ◦C.
Thin polymer films were prepared on glass substrate cov-
ered partially with ITO. The films were obtained by spin
coating from solutions of mixture of polymer and PAQ (poly-
thiophene/PAQ = 1/1). The photovoltaic cell configuration was
ITO/polymer + PAQ/Al. Aluminium electrode was evaporated
on the surface of polymer under high vacuum conditions. The
photocurrent as well as current–voltage (IV) characteristic were
measured using the Kithley 2400 source meter (Fig. 3).
In general, enhancement of the efficiency may be achieved by
formation of the many-layered structure with large number of
the heterojunction to increase the charge separation, aggrega-
tion of the molecular complexes, incorporation of inorganic
nanoparticles, etc. One of the restrained factors is relatively low
mobility favoring a charge gathering near the electrodes and the
thermo-destruction of the composites.
It is clear that the system, covered by donor-electron with-
drawing side chains and acceptor-chromophore with electron
donating side chains, enhances the process of carrier transport-
ing. Consequently, the system has optimum performance of
photoelectric conversion when the energy differences between
the electrodes and the chromophore are small. However, pro-
cesses of exciton transport are determined by dipole moments
of the polymer chains and by the efficiency of the hole–electron
interactions between the transporting materials and the chro-
mophore. One can expect that for every type of donor polymers
and of the acceptor chromophore it should be an optimal value of
state dipole moments determining the optimal charge separation
responsible for the open circuit voltage and the Isc currents.
For the investigation in the present work we have chosen the
PQ chromophore with relatively stable positions of the HOMO
and LUMO levels, however due to the substitution of the back-
side groups there exists a possibility to operate by their state
dipole moments. So there appears a rare possibility to operate
by the dipole–dipole interactions without a necessity to change
the positions of the HOMO and LUMO levels. Another factor
was to investigate how to influence the space separation for the
particular transport donor polymers that would influence on the
principal parameters of the charge separation determining effi-
ciency of the PV cells. For these reason two types of polymers
3. Results and discussion
Main factor responsible for quenching of the power conver-
sion according to the current state-of-the-art for single-layer
solar cells is relatively low energy conversion efficiency during
the transformation process of an absorbed photon to an exci-
ton (electron–hole coulomb pair) gathered at the electrodes. The
absorption of light within the spectral range of 450–570 nm usu-
ally leads to an open circuit voltage of up to 760 mV, exhibiting
approximately 80% energy losses. Contribution to the open cir-
cuit voltage of donor–acceptor heterojunction cell give both the
photocurrent and the dark current. The LED efficiency influ-
ences different factors such as illumination intensity, organic
heterojunction structure, electrode properties, and operating
temperature. We also show that the common knowledge of using
the “effective” gap of a donor–acceptor heterojunction used for
determination for the maximum open circuit voltage needs to be
carefully re-examined. Traditionally, dipole–dipole interactions
between the chromophore and the transport polymer matrices
are not considered in the sufficient way [9–15].
The proposed approach includes variation of position of the
acceptor LUMO levels with respect to the donor HOMO ones.
Fig. 3. Typical architecture of the photovoltaic devices.

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E. Gondek et al. / Spectrochimica Acta Part A 70 (2008) 117–121
Fig. 4. Typical current–voltage features of the investigated PV devices without
thephotoexcitation(stars)andafterthephotoexcitationsunderthe1315 ␮W/cm2
light.
Fig. 6. I–V characteristics under illumination of ITO/active layer/Al devices
(Plight = 1315 ␮W/cm2 light).
materials, however one can see that the differences between the
different chromophore are sufficiently large.
Energy conversion efficiencies of various organic solar cells
including small molecular [16] as well as polymeric solar cells
[17] are of the order of 1%. So, appropriately varying the state
dipole moments one can achieve the desired effects in enhance-
ment of the corresponding VOC
.
Following the fact that we deal with almost identical HOMO
and LUMO parameters of the chromophores, one can under-
stand that principal difference will be due to the polarizability
of the chromophores and their interactions both with the same
molecule within the aggregated molecular complexes as well as
between the chromophore and the polymer chains.
For convenience of readers in Figs. 7 and 8 the dependences
of the efficiency versus the dipole moments of the particular
chromophore in different matrices are given.
Fig. 5. I–V characteristics under illumination of ITO/active layer/Al devices
(Plight = 1315 ␮W/cm2 light).
From Figs. 7 and 8 one can see that for the both matrices
with increase of the state dipole moments an enhancement of
the VOC is observed. Principal difference is that in the PDT
matrix this dependence shows saturation whereas for the P3OT
there is a drastic jump of VOC for the molecule with larger dipole
moments. The calculations of dipole moments were performed
by B3LYP DFT method by taking into account of surrounding
polymer matrix modifying the electronic states of the chro-
with different chain sizes—PDT and P3OT were chosen (see
Fig. 1b).
Typical dependence for the PV current voltage features is
given in Figs. 4–6. One can see that there exists photovoltaic
response of 1315 ␮W/cm2 light. From Table 1 one can see that
theefficiencyofthePVdevicesisbelowthethresholdfororganic
Table 1
Photovoltaic parameters for a sequenced of layers
Sequence of layers
VOC (A)
ISC (␮A/cm2)
FF
Rs (ꢀ)
Rsh (ꢀ)
QE (%)
DK11 + PDT
DK16 + PDT
DK7 + PDT
DK8 + PDT
DK6 + PDT
DK12 + PDT
DK10 + P30T
DK16 + P30T
DK7 + P30T
DK6 + P3OT
DK12 + P30T
0.382
0.131
0.337
0.39
0.388
0.207
0.407
0.713
0.401
0.63
9.84
2.95
5.83
9.85
6.69
15.00
5.51
22.00
2.83
0.32
6.70
0.25
0.25
0.32
0.249
0.209
0.255
0.271
0.236
0.714
0.19
1.0 × 105
7.6 × 104
5.5 × 104
1.1 × 106
2.5 × 104
4.5 × 104
1.8 × 104
2.94 × 105
5.5 × 104
2.50 × 104
4.55 × 104
3.69 × 105
2.93 × 105
2.96 × 105
2.87 × 106
7.15 × 10−2
7.35 × 10−3
4.78 × 10−2
7.27 × 10−2
4.12 × 10−2
6.02 × 10−2
4.62 × 10−2
0.28
6.16 × 10−2
2.91 × 10−3
3.61 × 10−2
1.10 × 105
3.67 × 105
4.29 × 105
2.96 × 105
1.35 × 105
1.10 × 105
0.279
0.254

E. Gondek et al. / Spectrochimica Acta Part A 70 (2008) 117–121
121
the chromophore and the transport polymers. Variation of the
HOMO and LUMO positions is not crucial for these kinds of
devices, however it may become important during substantial
substitution for the copolymer groups.
Following the obtained results one can expect that the appro-
priate operation by the values of the state dipole moments may
open additional possibility to enhance the PV efficiency. From
this point of view, promising materials may be considered as
photopolymer matrices [18], allowing to enhance the efficient
state dipole moments of the chromophore.
4. Conclusions
Photovoltaic effects in single-layer pyrazoloquinoline chro-
mophore incorporated into thiophene-containing polymer
matrices with different separation of the backbone molecule
are reported. We have established principal role of the state
dipole moments of the PQ dyes in the observed dependences of
the open current voltage. Enhanced separation of the molecule
leads to drastic enhancement of the open circuit voltage of up to
two times achieving of about 750 mV. The performed quantum
chemical calculations show a large potential of the investigated
chromophore after appropriate modification of the backside
group. The advantage for the use of the PQ molecule consists
of an establishment of appropriate dipole–dipole interactions
between the chromophore and the transport polymers.
Fig. 7. Dependence of the VOC vs. the state dipole moments in PDT matrix.
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mophore. So there is drastic enhancement of the VOC or the
molecule with the lowest sizes. POT may indicate that less
separation of the thiophene rings may favor substantial enhance-
ment of the charge transfer separation due to enhanced dipole
moments. At the same time the excited dipole and transition
dipole moments do not show any correlation with the observed
PV efficiencies.
It is crucial that we did not find any significant correlation
with the excited and transition dipole moments. This fact may
reflect that the free carriers propagating in the polymer chains
may be determined by the electrostatic dipole–dipole interac-
tions.
Another crucial fact is a substantially lower Isc for the P3OT
matrices compared to the PDT ones. This fact may reflect
shorter sides of the P3OT chains compared to the PDT. So the
main potential for the use of the PQ molecule consists of an
establishment of appropriate dipole–dipole interactions between