Structural optimization of thiophene-(N-aryl)pyrrole-thiophene-based metal-free organic sensitizer for the enhanced dye-sensitized solar cell performance

Article abstract of DOI:10.1016/j.tet.2013.11.050

In this study, we prepared thiophene-(N-aryl)pyrrole-thiophene (TPT)-based two new metal-free organic sensitizers (TPTDYE 2 and TPTDYE 3) with the aim of improving the dye-sensitized solar cell (DSSC) performance of recently reported TPT-based organic sen

Full text of DOI:10.1016/j.tet.2013.11.050

Tetrahedron 70 (2014) 371e379
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Structural optimization of thiophene-(N-aryl)pyrrole-thiophene-
based metal-free organic sensitizer for the enhanced dye-sensitized
solar cell performance
Vellaiappillai Tamilavan a, A-Young Kim b, Hye-Bin Kim a, Misook Kang b,
,
Myung Ho Hyun a
*
a Department of Chemistry, Chemistry Institute for Functional Materials, Pusan National University, Busan 690-735, Republic of Korea
b Department of Chemistry, Yeungnam University, Gyeongsan 712-749, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, we prepared thiophene-(N-aryl)pyrrole-thiophene (TPT)-based two new metal-free organic
sensitizers (TPTDYE 2 and TPTDYE 3) with the aim of improving the dye-sensitized solar cell (DSSC)
performance of recently reported TPT-based organic sensitizer (TPTDYE 1). The molecular structure of
TPTDYE 1 was tuned by decreasing the distance between the donor and acceptor groups (TPTDYE 2) or
by introducing a fluoride atom on the phenyl ring near to the electron accepting cyanoacrylic acid group
(TPTDYE 3). The photophysical and electrochemical studies of the newly synthesized sensitizers revealed
that their absorption and energy levels were significantly altered compared to those of TPTDYE 1. The
DSSC performance of each of sensitizers TPTDYE 2 and TPTDYE 3 was investigated with and without
coadsorbent and compared with those of TPTDYE 1 and standard N719. Between the two DSSCs, the one
sensitized by TPTDYE 2 offered greatly improved solar to electrical energy conversion efficiency of 6.85%
without coadsorbent and 7.06% with coadsorbent. The overall conversion efficiency of the DSSC sensi-
tized by TPTDYE 2 without and with coadsorbent was found to be improved by 32% and 20%, re-
spectively, compared with that of the DSSC sensitized by TPTDYE 1 and almost equal (98.7%) to that of
the standard cell prepared from N719 under an identical condition.
Received 9 September 2013
Received in revised form 15 November 2013
Accepted 17 November 2013
Available online 22 November 2013
Keywords:
Dye-sensitized solar cells
Metal-free organic sensitizers
1-(2,6-Diisopropylphenyl)-2,5-di(2-thienyl)
pyrrole
Thiophene-pyrrole-thiophene-based
sensitizers
Triphenylamine-based sensitizers
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
two different sensitizers show their absorption band at different
intervals of the solar spectra (co-sensitization).9,10 Among them
Dye-sensitized solar cells (DSSCs) are considered as one of the
promising green energy production to solve world energy needs due
to their high incident solar light to electrical energy conversion ef-
ficiency and easy device fabrication at low cost.1 In DSSCs, typically
four important components such as mesoporous semiconducting
metal oxide (TiO2), a sensitizer, an electrolyte/hole transporter, and
counter electrode are employed in the solar light to electrical energy
conversionprocess.1 Currently, several research groups are devoting
many efforts to enhance the DSSC performance through the utili-
zation of any one of the optimized four components mentioned
above or enhancing the light absorption of the DSSCs by making
tandem structured or co-sensitized DSSCs. For example, the stan-
dard TiO2 was replaced or modified with ZnO,2,3 structurally new
sensitizers were developed,4e6 standard I3/Iꢀ electrolyte was
replaced with cobalt electrolyte7,8 and the DSSCs constructed with
developing structurally new sensitizers for DSSCs is believed to be
an attractive and essential research area, since the sensitizers play
a crucial role in determining the conversion efficiency as well as the
stability of the DSSCs. In this instance, recently variety of organo-
metallic complex sensitizers based on rutheniumepolypyridyl11
and zinceporphyrin complexes12 and metal-free organic sensi-
tizers based on coumarin,13,14 indoline,15,16 phenothiazine,17,18 car-
bazole,19,20 fluorene,21,22 tetrahydroquinoline,23,24 squaraine,9,25
cyanine,10,26 ullazine,27,28 triphenylamine,29e33 perylene,34,35 spi-
robifluorene,36,37 stilbene,38 anthracene,39,40 dithienothiophene,41
dithiensilole,42,43 cyclopentadithiophene,44,45 dithienopyrrole,29,46
benzothiadiazole,47,48 and diketopyrrolopyrrole49,50 were re-
ported. At present, the organometallic complex-based DSSCs were
found to show a maximum conversion efficiency in the range of
12e13%7,8,51 while the DSSCs made from metal-free organic sensi-
tizers offered
a
maximum efficiency in the range of
9e10%.7,23e33,47,49 The overall energy conversion efficiency of the
DSSCs made from organic sensitizers is quite close to that of the
organometallic complex-based DSSCs. The high energy conversion
* Corresponding author. Tel.: þ82 51 510 2245; fax: þ82 51 516 7421; e-mail
address: mhhyun@pusan.ac.kr (M.H. Hyun).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.11.050

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V. Tamilavan et al. / Tetrahedron 70 (2014) 371e379
efficiency of organic sensitizers induced us to develop new organic
sensitizers for DSSCs because they have many advantages over or-
ganometallic complexes such as easy preparation and purification,
higher molar extinction coefficient, easier structural modification,
and unlimited resources because they do not contain noble metals.
Recently, we reported the synthesis and DSSC applications of
a series of 1-(2,6-diisopropylphenyl)-2-(5-phenylthiophen-2-yl)-
5-(thiophen-2-yl)pyrrole (thiophene-(N-aryl)pyrrole-thiophene,
TPT)-based metal-free organic sensitizers.52,53 Among them,
a DSSC sensitized by TPTDYE 1 (Fig. 1) was found to give high solar
2. Results and discussions
2.1. Synthesis and characterization
In this study, we devoted our efforts to improve the photovoltaic
properties of TPT-based sensitizers through the structural optimi-
zation of TPTDYE 1. The p-conjugation distance between the donor
and acceptor group of TPTDYE 1 is found to be considerably wider
than that of the organic sensitizers showing high conversion
efficiency.4e6,13e50 In order to facilitate the electron flow from do-
to electrical energy conversion efficiency (
h
) of 6.71% using I3/Iꢀ
nor to acceptor group of TPTDYE 1, we intended to decrease the p-
electrolyte.52 The high molar absorptivity, planar structure and
high energy conversion efficiency of TPTDYE 1 induced us to op-
timize its molecular structure. The sensitizers obtained through
the structural modification of TPTDYE 1 are expected to show
different opto-electrical properties, which might enhance their
photovoltaic performances. In this instance, in this study, we
prepared two new TPT-based organic sensitizers such as TPTDYE 2
and TPTDYE 3 (Fig. 1), studied their opto-electrical properties and
compared their photovoltaic properties with those of TPTDYE 1
and the most successful inorganic sensitizer N719 (Fig. 1) as
a standard.
conjugation distance or introduce an electron attracting fluoride
atom on the phenyl group near to the cyanoacrylic acid. In this
instance, TPTDYE 2 was prepared by removing the phenyl ring in
between the TPT and cyanoacrylic acid unit of TPTDYE 1. On the
other hand, TPTDYE 3 was prepared by introducing a fluoride atom
on the phenyl ring near to the cyanoacrylic acid group of TPTDYE 1.
The synthetic routes to TPT-based organic sensitizers, TPTDYE 2
and TPTDYE 3, are outlined in Fig. 2.
The key intermediates, 1 and 5, were synthesized by treating 1-
(2,6-diisopropylphenyl)-2,5-di(thiophen-2-yl)pyrrole with
1 or
2
equiv of NBS, respectively, according to the known
Fig. 1. Molecular structures of sensitizers TPTDYE 1, TPTDYE 2, TPTDYE 3, and N719.
Fig. 2. Scheme for the preparation of sensitizers TPTDYE 2 and TPTDYE 3.

V. Tamilavan et al. / Tetrahedron 70 (2014) 371e379
373
procedures.52e55 The formylation of compound 1 was performed by
treating compound 1 with n-BuLi and then DMF to afford com-
pound 2. The bromination of compound 2 by using NBS offered
compound 3. Then, the Suzuki coupling reaction between com-
pound 3 and 4-(diphenylamino)phenylboronic acid yielded com-
pound 4. On the other hand, compound 6 was obtained from the
Suzuki coupling reaction of compound 5 with 4-(diphenylamino)
phenylboronic acid and 3-fluoro-4-formylphenylboronic acid. The
Knoevenagel condensation reactions of each of compounds 4 and 6
with cyanoacetic acid in the presence of piperidine afforded
TPTDYE 2 and TPTDYE 3, respectively, in good yields.
dependent absorption behaviors as reported previously.52 The
red-shifted absorption of TPTDYE 2 compared with that of TPTDYE
1 is expected to be originated from the efficient ICT between the
donor and acceptor group because the ICT usually improves when
the distance between the donor and acceptor group is decreased.
On the contrary, the introduction of fluoride group on the phenyl
ring of TPTDYE 1 seems to diminish the ICT in THF, resulting in the
narrow and blue shifted absorption band for TPTDYE 3. To make
sure the blue shifted absorption of TPTDYE 3, we measured the
absorption spectra of all three sensitizers TPTDYE 1, TPTDYE 2, and
TPTDYE 3 in chloroform (CHCl3) as shown in Fig. 3bed. In-
terestingly, the absorption range of TPTDYE 3 was much broader in
CHCl3 than in THF while the absorption ranges of TPTDYE 1 and
TPTDYE 2 in CHCl3 were almost identical with those in THF. The
change of solvent from THF to CHCl3 seems to enhance the ICT in
TPTDYE 3 even though the reason is not clear and, consequently,
improve the absorption range of TPTDYE 3 significantly in CHCl3.
The absorption spectra of sensitizers TPTDYE 1, TPTDYE 2, and
TPTDYE 3 adsorbed on TiO2 with and without chenodeoxycholic
acid (DCA) are also displayed in Fig. 3bed, respectively. The ab-
sorption bands of sensitizers TPTDYE 1, TPTDYE 2, and TPTDYE 3
adsorbed on TiO2 were found to be significantly broadened and red
shifted compared with those in solution and the absorption bands
appeared in the range of 350 nme600 nm for TPTDYE 1 and
350e650 nm for TPTDYE 2 and TPTDYE 3. The absorption maxima
of TPTDYE 1 and TPTDYE 3 adsorbed on TiO2 were found to be
475 and 500 nm, respectively, while the absorption maximum of
TPTDYE 2 appeared quite broad from 390 nm to 540 nm. The red-
shifted absorption of dyes adsorbed on TiO2 might be attributed to
the formation of a J-type aggregate.56
2.2. Optical properties
The optical properties such as absorption and photo-
luminescence of sensitizers TPTDYE 1, TPTDYE 2, and TPTDYE 3
were measured in 1.0ꢁ10ꢀ5 M tetrahydrofuran (THF) solution and
their corresponding spectra are presented in Fig. 3a. The relatively
less
p-conjugated sensitizer (TPTDYE 2) showed red-shifted ab-
sorption band with maximum absorption at 512 nm
(
3
w4.8ꢁ104 Mꢀ1 cmꢀ1) compared with that of TPTDYE 1 (maximum
absorption at 455 nm,
quite similarly structured sensitizer TPTDYE 3 containing a fluoride
atom showed narrower absorption band than that of TPTDYE 1
3
¼5.8ꢁ104 Mꢀ1 cmꢀ1).52 On the other hand,
with maximum absorption at 423 nm (
3
¼5.4ꢁ104 Mꢀ1 cmꢀ1). The
absorption bands of the sensitizers are expected to be originated
from the overlap of the
pep transition and the donoreacceptor
*
intramolecular charge transfer (ICT) between the donor and the
cyanoacrylic acid anchoring moiety.52 The latter phenomenon was
evidently documented for sensitizer TPTDYE 1 using solvent-
Fig. 3. UVevisible absorption spectra of sensitizers TPTDYE 1, TPTDYE 2, and TPTDYE 3 measured in THF (1.0ꢁ10ꢀ5 M). Inset: Normalized absorption and photoluminescence
(excited at their absorption maxima) spectra of TPTDYE 1, TPTDYE 2, and TPTDYE 3 in THF solution (a). UVevisible absorption and photoluminescence (PL) spectra of TPTDYE 1 (b),
TPTDYE 2 (c), and TPTDYE 3 (d). UVevisible absorption spectra were measured in chloroform (CHCl3), as film adsorbed on TiO2 or as film adsorbed on TiO2 with DCA. Photo-
luminescence (PL) spectra were measured in chloroform.

374
V. Tamilavan et al. / Tetrahedron 70 (2014) 371e379
All three sensitizers adsorbed on TiO2 along with DCA showed
slightly blue shifted absorption bands compared with those of the
sensitizers adsorbed on TiO2 without DCA. The absorption maxima
of TPTDYE 1 and TPTDYE 3 adsorbed on TiO2 with DCA were found
to be 471 nm and 497 nm, respectively, while the absorption
maximum of TPTDYE 2 was appeared quite broad from 390 nm to
540 nm. DCA added is expected to compete with the sensitizers to
bind on TiO2 surface, and consequently, the aggregation of the
sensitizers on TiO2 is reduced, which leads blue shift in their ab-
sorption bands.57
The normalized photoluminescence (PL) spectra of TPTDYE 1,
TPTDYE 2, and TPTDYE 3 in THF (inset of Fig. 3a) and in CHCl3
(excited at their absorption maximum, Fig. 3bed) are displayed
in Fig. 3. The PL maxima of TPTDYE 1, TPTDYE 2, and TPTDYE 3 are
appeared at 615, 661, and 575 nm, respectively, in CHCl3 and their
band gaps (E0e0) were estimated from the wavelength at the in-
tersection of absorption and emission spectra to be 2.34, 2.08, and
2.42 eV, respectively. The optical properties of sensitizers TPTDYE 1,
TPTDYE 2, and TPTDYE 3 are summarized in Table 1.
Fig. 4. Cyclic voltammograms of sensitizers TPTDYE 1, TPTDYE 2, and TPTDYE 3 ad-
sorbed on TiO2 in 0.1 M Bu4NBF4/acetonitrile at 100 mV/s, potential versus Ag/AgCl.
Table 1
Optical and electrochemical properties of TPTDYE 1, TPTDYE 2, and TPTDYE 3
PL maxe
(nm)
E0e0
(eV)
Eox
ELUMO
(V)
a
b
c
d
f
g
h
Sensitizer
lmax,soln
3
lmax,dye film
lmax,dyeþDCA film
(nm)
(nm)
(nm)
(V)
TPTDYE 1
TPTDYE 2
TPTDYE 3
455
512
423
5.8ꢁ104
4.8ꢁ104
5.4ꢁ104
475
390e540
500
471
390e540
497
615
661
575
2.34
2.08
2.42
0.96
1.02
0.95
ꢀ1.38
ꢀ1.06
ꢀ1.47
a
Measurements from dye dissolved in THF (1ꢁ10ꢀ5 M).
b
c
Molar extension coefficient (Mꢀ1 cmꢀ1) of the dyes measured in THF (1ꢁ10ꢀ5 M).
Measurements from dye adsorbed on TiO2.
d
e
f
Measurements from dye and DCA adsorbed on TiO2.
Measurements from dye dissolved in CHCl3 (1x10ꢀ5 M).
The optical band gap (E0e0) estimated at the intersection of absorption and emission spectra in CHCl3 solution.
g
h
The onset oxidation potentials were estimated from cyclic voltammetry analysis.
The ELUMO level was calculated using the following equation, ELUMO¼EoxꢀE0e0
.
2.3. Electrochemical properties
TPTDYE 3 was more negatively shifted compared with that of
TPTDYE 1. The LUMO energy levels of all three sensitizers are lo-
cated well above the LUMO level of TiO2 and the HOMO energy
levels are positioned below the redox potential of electrolyte
ðIꢀ3 =IꢀÞ. The HOMO and LUMO energy levels of TPTDYE 2 and
TPTDYE 3 revealed that they are also suitable candidates for DSSC
applications. The electrochemical properties of sensitizers TPTDYE
1, TPTDYE 2, and TPTDYE 3 are summarized in Table 1.
The CV analysis is one the most common and simplest method
to determine the energy levels including the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO) of the sensitizers. It is essential to determine the
energy levels of the sensitizers to ensure their suitability for DSSC
applications. To ensure the efficient electron injection from sensi-
tizer to TiO2 and from electrolyte ðIꢀ3 =IꢀÞ to sensitizer, the HOMO
level of the sensitizer should be positioned below the redox po-
tential of electrolyte (0.4 V, Iꢀ3 =Iꢀ) while the LUMO energy of the
sensitizer should be positioned above the LUMO level of TiO2
(ꢀ0.5 V).52,53 Cyclic voltammetry (CV) experiments were con-
ducted for the sensitizers adsorbed on TiO2 as an working electrode,
platinum wire as a counter electrode and Ag/AgCl as an reference
electrode in acetonitrile (ACN) containing 0.1 M tetrabutylammo-
nium tetrafluoroborate (TBATFB) at room temperature and ambient
atmosphere. Based on the CV analysis shown in Fig. 4, the Eox
(oxidation potential) values reflecting the HOMO levels of sensi-
tizers TPTDYE 1, TPTDYE 2, and TPTDYE 3 were estimated to be
0.96, 1.02, and 0.95 V, respectively. On the other hand, the LUMO
levels of the sensitizers were calculated by using the known
equation (ELUMO¼EoxꢀE0e0),52,53 where Eox is the oxidation poten-
tial or HOMO energy level and E0e0 is the band gap of the sensi-
tizers. The LUMO energy levels of sensitizers TPTDYE 1, TPTDYE 2,
and TPTDYE 3 were calculated to be ꢀ1.38, ꢀ1.06, and ꢀ1.47 V,
respectively. The CV analysis indicates the HOMO energy level of
TPTDYE 2 was slightly higher than that of TPTDYE 1, but the LUMO
energy level of TPTDYE 2 was significantly lowered than that of
TPTDYE 1. On the contrary, the HOMO level of TPTDYE 3 was almost
identical with that of TPTDYE 1, but the LUMO energy level of
2.4. Computational studies
The structures of sensitizers TPTDYE 2 and TPTDYE 3 were
optimized by using B3LYP hybrid functional and 6-31G basis sets
*
with the aim of understanding the correlation between their
structures, optical properties, and electronic distribution at valance
(HOMO) and conduction (LUMO) bands. The optimized structures
and their electronic distribution in the HOMO and LUMO levels of
sensitizers TPTDYE 1, TPTDYE 2, and TPTDYE 3 are shown in Fig. 5.
The geometrically optimized structures of sensitizers TPTDYE 1,
TPTDYE 2, and TPTDYE 3 clearly indicate that the sensitizers ex-
hibit planar structures except for the triphenylamine (TPA) unit. In
addition, the dihedral angles between the aromatic groups are not
much different for the three sensitizers. These results suggest that
the planarity of TPTDYE 1 is not much affected by the decreased
distance between the electron donor and acceptor group or by the
incorporation of fluoride atom on the back bone of TPTDYE 1.
The electron density was found to be distributed throughout the
electron donor groups such as TPA and N-aryl TPT in the HOMO
energy levels of the sensitizers while it was found to be distributed
on the electron acceptor group (cyanoacrylic acid) along with
benzene and N-aryl TPT in the LUMO energy levels. The electronic

V. Tamilavan et al. / Tetrahedron 70 (2014) 371e379
375
Fig. 5. The optimized structures and frontier orbital plots of the HOMO and LUMO of sensitizers TPTDYE 1, TPTDYE 2, and TPTDYE 3.
distribution over the N-aryl TPT unit in both HOMO and LUMO
suggests that an effective ICT occurs from the donor to acceptor
through the N-aryl TPT unit. In particular, the electronic overlap
between the HOMO and LUMO level of the sensitizers was found to
that of the monomer) on the TiO2, and consequently, increase the
electron injection efficiency from the sensitizer into the TiO2,
resulting in the improved Jsc values.57e59 In contrast, the rate of
recombination of the injected electrons in the DSSCs prepared with
DCA seems to be quite similar with that in the DSSCs prepared
without DCA and/or the influence of DCA on the conduction band
movement of sensitizer-adsorbed TiO2 might be negligible for the
DSSCs based on sensitizers TPTDYE 1, TPTDYE 2, and TPTDYE 3,
be excellent for relatively less
p-conjugated sensitizer, TPTDYE 2,
and these results suggest that the ICT can occur more efficiently for
sensitizer TPTDYE 2 than for sensitizer TPTDYE 3 or TPTDYE 1.
2.5. Photovoltaic performance of DSSCs
The solar to electrical energy conversion ability of sensitizers
TPTDYE 2 and TPTDYE 3 was tested by preparing DSSCs with each
of the sensitizers. Under the identical condition DSSCs were also
prepared with the reported sensitizer, TPTDYE 1, and the most
successful rutheniumepolypyridyl complex, N719, as a standard to
briefly compare their energy conversion efficiencies. The current-
density/voltage (JeV) curves of the DSSCs sensitized by each of
sensitizers TPTDYE 1, TPTDYE 2, TPTDYE 3, and N719 without DCA
are shown in Fig. 6a and the detailed photovoltaic parameters such
as open-circuit voltage (Voc), short-circuit photocurrent density
(Jsc), fill factor (FF), and energy conversion efficiency (h) are sum-
marized in Table 2. The DSSC performances were measured at
100 mW/cm2 under simulated AM 1.5 G solar light conditions. The
DSSC sensitized by TPTDYE 2 without DCA offered maximum en-
ergy conversion efficiency (
13.80 mA/cm2 and an FF of 0.68, while the DSSC sensitized by
TPTDYE 3 without DCA gave a of 2.63% with a Voc of 0.65 V, a Jsc of
5.95 mA/cm2, and an FF of 0.68. Under the identical condition, the
h) of 6.85% with a Voc of 0.73 V, a Jsc of
h
DSSCs sensitized by TPTDYE 1 and N719 without DCA gave an
h of
5.19% (Voc¼0.68 V, Jsc¼11.93 mA/cm2, and FF¼0.64) and 7.15%
(Voc¼0.67 V, Jsc¼16.16 mA/cm2, and FF¼0.66), respectively.
Our previous studies revealed that the photovoltaic perfor-
mances of TPTDYE-based DSSCs were significantly improved with
the use of coadsorbent such as DCA.53 To investigate the DCA in-
fluence on the performances of the DSSCs sensitized by TPTDYE 1,
TPTDYE 2, and TPTDYE 3, we prepared the DSSCs from each of the
sensitizers along with DCA (10 mM). The JeV curves of the DSSCs
made from each of sensitizers TPTDYE 1, TPTDYE 2, and TPTDYE 3
with DCA are shown in Fig. 6b, and those DSSCs were found to show
maximum
h
value of 5.87% (Voc¼0.71 V, Jsc¼12.72 mA/cm2, and
FF¼0.65), 7.06% (Voc¼0.72 V, Jsc¼14.86 mA/cm2, and FF¼0.66) and
4.42% (Voc¼0.65 V, Jsc¼10.15 mA/cm2, and FF¼0.67), respectively.
The DSSCs prepared with DCA from each of three sensitizers
TPTDYE 1, TPTDYE 2, and TPTDYE 3 showed improved
h with quite
similar Voc and FF values and improved Jsc values compared with
those of the DSSCs prepared without DCA. It is worth to notice that
the bulky DCA molecules prevent the dye aggregation (dimer or
trimer formation, whose electron injection efficiency is lower than
Fig. 6. JeV Characteristics of the DSSCs made from each of sensitizers TPTDYE 1,
TPTDYE 2, TPTDYE 3, and N719 without (a) and with DCA (b) under AM 1.5 irradiation
(100 mW/cm2).

376
V. Tamilavan et al. / Tetrahedron 70 (2014) 371e379
Table 2
Photovoltaic properties of TPTDYE 1, TPTDYE 2, TPTDYE 3, and N719
Name of the sensitizer
Without DCA
With DCA
Jscb (mA/cm2)
FFc (%)
h
d (%)
IPCEe (%)
Voc (V)
Jscb (mA/cm2)
FFc (%)
h
d (%)
IPCEe (%)
a
a
Voc (V)
TPTDYE 1
TPTDYE 2
TPTDYE 3
N719
0.68
0.73
0.65
0.67
11.93
13.80
5.95
0.64
0.68
0.68
0.66
5.19
6.85
2.63
7.15
57
65
40
64
0.71
0.72
0.65
d
12.72
14.86
10.15
d
0.65
0.66
0.67
d
5.87
7.06
4.42
d
58
68
49
d
16.16
a
Open-circuit voltage.
Short-circuit current density.
Fill factor.
Power conversion efficiency.
Incident photon to collected electron efficiency.
b
c
d
e
showing similar Voc values in the DSSCs prepared with and without
DCA.57e59
In comparison with the DSSC sensitized by TPTDYE 1, the pho-
tovoltaic parameters such as Voc, Jsc, and FF were found to be higher
for the DSSC sensitized by TPTDYE 2 with and without DCA. We
expect that the positive effects such as red-shifted absorption
maximum along with broad absorption ability (more effective light
harvesting than TPTDYE 1 and TPTDYE 3) and the enhanced elec-
tron injection from sensitizer TPTDYE 2 to TiO2 as well as the
electron regeneration efficiency from electrolyte to sensitizer
TPTDYE 2 due to its suitable HOMO and LUMO energy levels might
offer significantly improved device performances compared with
that of TPTDYE 1. The DSSC sensitized by TPTDYE 3 was found to
give the FF and Voc values similar with those of the DSSC prepared
from TPTDYE 1 with and without DCA. However, the DSSC sensi-
tized by TPTDYE 3 was found to give significantly lower Jsc value
compared with that of the DSSC prepared from TPTDYE 1 with and
without DCA. The exact reason for the lower Jsc value of the DSSC
sensitized by TPTDYE 3 is not clear, but might be due to the rela-
tively increased recombination process. There are two possible
reasons for the increased recombination process in the DSSC sen-
sitized by TPTDYE 3. The first reason is that the relatively high
energy difference (DEg¼0.97 V) between the LUMO levels of
TPTDYE 3 and TiO2 might reduce the electron injection from the
LUMO level of TPTDYE 3 to the LUMO level of TiO2, and conse-
quently, increases the recombination process. The second reason is
that the presence of fluoride atom having the tendency of strong
electron affinity might restrict the electron transfer process as
previous reports evidently documented that the incorporation of
electron withdrawing moiety near to the cyanoacrylic acid in-
creases the recombination process or jeopardize the electron in-
jection to TiO2.47,60
The solar to electrical energy conversion efficiency of the DSSC
sensitized by TPTDYE 2 without and with DCA was found to be
improved by 32% and 20%, respectively, compared to that of the
DSSC sensitized by TPTDYE 1. In addition, the overall photovoltaic
performance of the DSSC sensitized by TPTDYE 2 was found to be
quite similar (98.7%) to that of the DSSC sensitized by N719. These
results reveal that TPTDYE 2 is a promising candidate for DSSC
applications.
The monochromatic IPCEs (Incident-Photon-to-electron Con-
version Efficiencies) of the DSSCs are presented in Fig. 7. The IPCE
spectra of DSSCs prepared with each of sensitizers TPTDYE 1,
TPTDYE 2, TPTDYE 3, and N719 without DCA were found to show
the maximum IPCE value of 57% at 450 nm, 65% at 480 nm, 40% at
415 nm, and 64% at 550 nm, respectively, while those of the cor-
responding DSSCs made from each of sensitizers TPTDYE 1, TPTDYE
2, and TPTDYE 3 with DCA were found to show the increased IPCE
maximum values of 58% at 465 nm, 68% at 485 nm, and 49% at
465 nm, respectively. The IPCE maximum value was found to be 10%
higher for the DSSC sensitized by TPTDYE 2 and 9% lower for the
Fig. 7. IPCE characteristics for the DSSCs made from each of sensitizers TPTDYE 1,
TPTDYE 2, TPTDYE 3, and N719 without (a) and with DCA (b).
DSSC sensitized by TPTDYE 3 compared to that for the DSSC sen-
sitized by TPTDYE 1. The relatively broader IPCE spectrum and
higher IPCE maximum value of the DSSC sensitized by TPTDYE 2
evidently support its higher energy conversion efficiency compared
with that of the DSSC sensitized by TPTDYE 1 or TPTDYE 3.
3. Conclusions
We prepared 1-(2,6-diisopropylphenyl)-2,5-di(2-thienyl)pyr-
role-based two new metal-free organic sensitizers TPTDYE 2 and
TPTDYE 3 with the aim of improving the DSSC performance of
reported TPTDYE 1 through the structural modifications. New
sensitizers TPTDYE 2 showing less
p-conjugation distance between
the donor and acceptor units and TPTDYE 3 containing fluoride

V. Tamilavan et al. / Tetrahedron 70 (2014) 371e379
377
atom near to the cyanoacrylic acid group showed the red-shifted
absorption bands compared with that of TPTDYE 1 in chloroform
and as film on TiO2. The electrochemical study indicates that the
HOMO and LUMO energy levels of TPTDYE 2 were significantly
compressed than those of TPTDYE 1, while the HOMO energy level
of TPTDYE 3 was found to be almost identical with that of TPTDYE
1, but the LUMO energy level was more negatively shifted. We
found that the incorporation of electron attracting fluoride atom
near to the cyanoacrylic acid group of TPTDYE 1 is not satisfactory
in terms of increasing the energy conversion efficiency. In-
terestingly, the energy conversion efficiency was notably enhanced
15 min, and at 500 ꢂC for 15 min. The resulting film was composed
of a 6.0 m thick transparent layer and a 7.0 m thick scattering
layer. Then, they were immersed into the dye solution [0.3 mM
solution of dye in ethanol or ethanol containing 0.3 mM of dye with
m
m
10 mM of 3a,7b-dihydroxy-5b-cholanic acid (chenodeoxycholic
acid, DCA)] and kept at room temperature for 24 h. FTO plates for
the counter electrodes were cleaned in an ultrasonic bath in H2O
and acetone, subsequently. The counter electrodes were prepared
by Pt vacuum coating using a JEOL-400 vacuum evaporator
(7.5ꢁ10ꢀ6 Torr, 20 V, 30 A, and 250 L/min) for 30 s on an FTO plate.
The dye-adsorbed TiO2 electrodes and the Pt counter electrodes
were assembled into a sealed sandwich-type cell by heating at
80 ꢂC using a hot-melt ionomer film (Solaronix Surlyn, Meltonix
when the
p-conjugation distance between donor and acceptor
units was decreased. The maximum energy conversion efficiency of
7.06% and 4.42% was achieved for the DSSCs made from the sen-
sitizers TPTDYE 2 and TPTDYE 3, respectively, along with DCA. The
maximum conversion efficiency of the DSSC sensitized by TPTDYE
2 with DCA was found to be 20% higher than that of the DSSC
sensitized by TPTDYE 1 and almost equal (98.7%) to that of the DSSC
sensitized by N719 prepared under the identical device preparation
condition.
1170-25, 25 mm) as a spacer between the electrodes. A drop of the
electrolyte solution was placed in the drilled hole of the counter
electrode and was driven into the cell via vacuum backfilling. Fi-
nally, the hole was sealed using additional Surlyn and a cover glass.
The electrolyte was then introduced into the cell, which is com-
posed of 1.0 M 3-propyl-1,2-dimethyl imidazolium iodide, 0.03 M
iodine, 0.05 M LiI, 0.1 M guanidinium thiocyanate, and 0.5 M 4-tert-
butylpyridine in 85:15(v/v) acetonitrile and valeronitrile. The cell
performance was measured using 150-W xenon light source,
whose power of an AM 1.5 SUN 2000 solar simulator (ABE tech-
nology) was calibrated by using KG5 filtered Si reference solar cell.
Incident light intensity and active cell area were 100 mW cmꢀ2 (one
sun illumination) and 0.40 cm2 (0.5 cmꢁ0.8 cm), respectively. The
incident photon-to-current conversion efficiency (IPCE) spectra for
the cells were measured on an IPCE measuring system (PV
Measurements).
4. Experimental section
4.1. Materials and instruments
Reagents were received from Aldrich or TCI chemicals and used
without further purification. The purification of the compounds
was performed on silica gel (Merck Kieselgel 60, 70e230 mesh)
column chromatography. The proton (1H, 300 MHz) and carbon
(
13C, 75 MHz) NMR spectra were recorded using a Varian Mercury
Plus spectrometer (300 MHz) in deuterated solvents. Infrared
spectra were obtained on a Nicolet 380 FTIR spectrophotometer
with samples prepared as KBr pellets. The absorption and photo-
luminescence spectra were recorded using JASCO V-570 and Hita-
chi F-4500 fluorescence spectrophotometer, respectively, at 25 BC.
Sensitizer-adsorbed TiO2 films were prepared as follows: TiO2
coated FTO plates were prepared by using a doctor blade printing
technique and then dried at 450 ꢂC for 30 min. Subsequently, FTO/
TiO2 substrates were immersed in 0.3 mM sensitizer solution
(without or with 10 mM DCA) for 24 h. The dye-adsorbed TiO2
plates were dried at 40 ꢂC for 10 min and at rt for 3 h and then used
for measurements. The cyclic voltammograms were recorded by
using a CH Instruments Electrochemical Analyzer. The CV in-
strument calibration was performed by using the ferrocene/ferro-
cenium (FOC) redox couple as an external standard.
4.3. Synthesis of dyes
New organic sensitizers TPTDYE 2 and TPTDYE 3 were synthe-
sized as outlined in Fig. 2. Compounds 1 and 5 were synthesized
from
1-(2,6-diisopropylphenyl)-2,5-di(thiophen-2-yl)pyrrole
according the procedures reported.52e55 The detailed synthetic
procedure and characterization of the intermediates and final
sensitizers are given below.
4.3.1. Synthesis of 5-(1-(2,6-diisopropylphenyl)-5-(thiophen-2-yl)pyr-
rol-2-yl)thiophene-2-carbaldehyde (2). To a stirred solution of com-
pound 1 (0.90 g, 1.91 mmol) in dry diethyl ether (60 mL) was added
n-BuLi (2.5 M in hexane, 1.0 mL, 2.5 mmol) drop by drop at 0 ꢂC
under argon atmosphere. The solution was stirred for 30 min in the
same bath and then N,N-dimethylformamide (DMF, 2.96 mL,
38.2 mmol) was added in one potion. The solution was slowly
warmed to room temperature (rt) and stirred for overnight. The
solution was poured into 2 N HCl (50 mL), and the aqueous solution
was extracted with ethyl acetate (3ꢁ50 mL). The combined organic
layer was dried over anhydrous Na2SO4. The solvent was removed
and the residue was purified by silica gel column chromatography
(hexane/ethyl acetate, 95:5, v/v) to afford compound 2. Yield is
4.2. Fabrication and characterization of dye-sensitized solar
cells (DSSCs)
Fluorine-doped tin oxide (FTO) glass plates (Hartford FTO,
30
U
cmꢀ2, 80% transmittance in visible region) were cleaned in
a detergent solution using an ultrasonic bath for 30 min and then
rinsed with water and ethanol. Then, the plates were immersed in
40 mM TiCl4 (aqueous) at 70 ꢂC for 30 min and washed with water
and ethanol. A transparent nanocrystalline layer was prepared on
the FTO glass plates by using a doctor blade printing TiO2 paste
(Dyesol, DSL 18NR-T), which was then dried for 2 h at 25 ꢂC. The
TiO2 electrodes were gradually heated under an air condition at
325 ꢂC for 5 min, at 375 ꢂC for 5 min, at 450 ꢂC for 15 min, and at
500 ꢂC for 15 min. The thickness of the transparent layer was
measured by using an Alpha-step 250 surface profilometer (Tencor
Instruments, San Jose, CA). A paste containing 400 nm sized anatase
particles (Dyesol. DSL 18NR-AO) was deposited by means of doctor
blade printing to obtain the scattering layer, and then dried for 2 h
at 25 ꢂC. The TiO2 electrodes were gradually heated under an air
condition at 325 ꢂC for 5 min, at 375 ꢂC for 5 min, at 450 ꢂC for
0.40 g (50%). 1H NMR (300 MHz, CDCl3):
2.41e2.50 (m, 2H), 6.48e6.52 (m, 2H), 6.72 (d, 1H), 6.791e6.795(m,
d (ppm) 0.91 (t, 12H),
1H), 6.90(d, 1H), 7.03(d, 1H), 7.31 (d, 2H), 7.41(d, 1H), 7.60 (t, 1H),
9.69 (s, 1H); 13C NMR (75 MHz, CDCl3)
d (ppm) 23.8, 23.9, 28.6, 110.1,
112.4,123.0,124.0,124.4, 125.3,127.3, 129.0,131.3, 132.9, 133.7, 134.4,
137.2, 139.9, 145.4, 148.0, 182.5; HRFAB-MS m/z [Mþ] calcd for
C25H25NOS2 419.1378, found: 419.1374.
4.3.2. Synthesis of 5-(5-(5-bromothiophen-2-yl)-1-(2,6-diisopropyl
phenyl)pyrrol-2-yl)thiophene-2-carbaldehyde (3). To a stirred solu-
tion of compound 2 (0.20 g, 0.48 mmol) in DMF (10 mL) was added
a solution of N-bromosuccinimide (NBS) (0.10 g, 0.52 mmol) in DMF
(5 mL) drop by drop at 0 ꢂC. The solution was slowly warmed to rt and
stirred for overnight. The solvent was completely removed by using

378
V. Tamilavan et al. / Tetrahedron 70 (2014) 371e379
rotary evaporator and then the residue was dissolved in ethyl acetate
(50 mL). The organic solution was washed with water and brine and
dried over anhydrous Na2SO4. The solvent was removed and the
residue was purified by flash column chromatography (silica gel,
hexane) to afford compound 3. Yield is 0.18 g (75%). 1H NMR
argon atmosphere for 16 h. Under reflux condition, 3-fluoro-4-
formylphenylboronic acid (0.34 g, 2.00 mmol) was added to the
reaction mixture. The reaction mixture was refluxed again for ad-
ditional 16 h. Then, the reaction mixture was allowed to cool to
room temperature and the solvent was completely removed by
using a rotary evaporator. The residue was dissolved in ethyl acetate
(50 mL) and washed with water and brine, and then dried over
anhydrous Na2SO4. The solvent was concentrated and the residue
was purified by column chromatography (silica gel, hexane/ethyl
acetate¼90:10, v/v) to afford compound 6. Yield is 0.25 g (40%). 1H
(300 MHz, CDCl3):
d (ppm) 0.93 (t, 12H), 2.40e2.44 (m, 2H), 6.25 (d,
1H), 6.52 (d, 1H), 6.66 (d, 1H), 6.74(d, 1H), 6.87 (d, 1H), 7.32 (d, 2H),
7.42 (d, 1H), 7.61 (t, 1H), 9.69 (s, 1H); 13C NMR (75 MHz, CDCl3)
d
(ppm) 22.9, 23.9, 28.6, 31.8, 110.1, 111.1, 112.4, 123.3, 123.9, 124.9,
125.5, 129.4, 130.1, 131.6, 131.8, 133.2, 136.0, 137.1, 140.1, 144.9, 147.9,
182.5; HRFAB-MS m/z [Mþ] calcd for C25H24BrNOS2 497.0483, found:
497.0485.
NMR (300 MHz, CDCl3):
2H), 6.30 (d, 1H), 6.72e6.77 (m, 2H), 6.91 (d, 1H), 6.99e7.16 (m,
10H), 7.20e7.35 (m, 10H), 7.60 (t, 1H), 7.79 (t, 1H), 10.27 (s, 1H); 13
NMR (75 MHz, CDCl3): (ppm) 23.9, 28.6, 109.6, 110.6, 112.0, 112.3,
d (ppm) 0.95e0.98 (m, 12H), 2.51e2.56 (m,
C
4.3.3. Synthesis of 5-(5-(5-(4-(diphenylamino)phenyl)thiophen-2-
yl)-1-(2,6-diisopropylphenyl)pyrrol-2-yl)thiophene-2-carbaldehyde
(4). A stirred solution of compound 3 (0.18 g, 0.36 mmol) and 4-
(diphenylamino)phenylboronic acid (0.12 g, 0.43 mmol) in toluene
(30 mL) was purged well with argon for 45 min and then Pd(PPh3)4
(5 mol %) and aqueous 2 M K2CO3 solution (7 mL) were added. The
reaction mixture was refluxed for 24 h under argon atmosphere.
The solvent was completely removed by using a rotary evaporator
and the residue was dissolved in ethyl acetate (50 mL). The organic
solution was washed with water and brine, and then dried over
anhydrous Na2SO4. The solvent was concentrated and the residue
was purified by column chromatography (silica gel, hexane/ethyl
acetate¼95:5, v/v) to afford compound 4. Yield is 0.18 g (77%). 1H
d
121.2, 122.5, 122.9, 123.3, 123.9, 124.1, 124.4, 124.5, 124.7, 125.1,
126.4, 128.4, 128.8, 129.4, 129.5, 131.0, 131.5, 132.4, 132.6, 133.4,
134.2, 138.0, 142.0, 142.6, 147.3, 147.7, 148.1, 186.5; HRFAB-MS m/z
[Mþ] calcd for C49H41FN2OS2 756.2644, found: 756.2646.
4.3.6. Synthesis of (E)-3-(4-(5-(5-(5-(4-(diphenylamino)phenyl)thi-
ophen-2-yl)-1-(2,6-diisopropylphenyl)pyrrol-2-yl)thiophen-2-yl)-2-
fluorophenyl)-2-cyanoacrylic acid (TPTDYE 3). TPTDYE 3 was syn-
thesized by using the synthetic procedure similar to that for
TPTDYE 2. Compound 6 (0.25 g, 0.34 mmol) was subjected to
Knoevenagel condensation with cyanoacetic acid in the presence of
piperidine to afford TPTDYE 3. Yield is 0.22 g (80%). 1H NMR
NMR (300 MHz, CDCl3):
d
(ppm) 0.91e0.97 (m, 12H), 2.44e2.53 (m,
(300 MHz, DMSO-d6) d (ppm) 0.95e0.98 (m, 12H), 2.49e2.58 (m,
2H), 6.33 (d, 1H), 6.54 (d, 1H), 6.74 (d, 1H), 6.90e6.93 (m, 2H),
6.99e7.11 (m, 6H), 7.18e7.34 (m, 10H), 7.42 (d, 1H), 7.60 (t, 1H), 9.69
2H), 6.34 (d, 1H), 6.67e6.75 (m, 2H), 6.91 (d, 2H), 7.01e7.09 (m,
10H), 7.21e7.34 (m, 10H), 7.57e7.60 (m, 1H), 8.02 (s, 1H); IR (KBr,
cmꢀ1): 3440, 3061, 3030, 2961, 2927, 2868, 2217,1610, 1592, 1489;
HRFAB-MS m/z [Mþ] calcd for C52H42FN3O2S2 823.2702, found:
823.2703.
(s,1H); 13C NMR (75 MHz, CDCl3)
d (ppm) 22.9, 23.9, 28.6, 29.9, 31.2,
31.8, 110.0, 112.6, 116.5, 122.1, 122.5, 123.0, 123.1, 123.4, 123.8, 124.8,
125.4, 126.4, 127.4, 128.2, 129.1, 129.3, 129.5, 131.4, 132.8, 132.9,
133.7, 137.2, 139.7, 142.7, 145.4, 147.5, 147.6, 147.9, 148.4, 182.5;
HRFAB-MS m/z [Mþ] calcd for C43H38N2OS2 662.2426, found:
662.2432.
Acknowledgements
This research was supported by the National Research Founda-
tion of Korea (NRF-2013R1A2A2A04014576).
4.3.4. Synthesis of (E)-3-(5-(5-(5-(4-(diphenylamino)phenyl)thio-
phen-2-yl)-1-(2,6-diisopropylphenyl)pyrrol-2-yl)thiophen-2-yl)-2-
cyanoacrylic acid (TPTDYE 2). To a stirred solution of compound 4
(0.18 g, 0.28 mmol) in a mixture of chloroform and acetonitrile
(30 mL, 1:1, v/v) was added piperidine (0.14 mL, 1.40 mmol) and
cyanoacetic acid (0.12 g, 1.40 mmol) at rt under argon atmosphere.
The solution was refluxed for 24 h and then cooled to room tem-
perature. The solvent was concentrated by using a rotary evapo-
rator and the residue was dissolved into 10 mL of methylene
chloride (CH2Cl2) and the resulting mixture was poured into
100 mL water. To the stirred mixture was added 2 N HCl until the
reaction mixture became acidic. The aqueous layer was extracted
with CH2Cl2 and the organic layer was washed with water and
brine, and then dried over anhydrous Na2SO4, filtered, and evapo-
rated on a rotary evaporator to dryness. The residue was purified by
column chromatography (silica gel, CH2Cl2/methanol¼90:10, v/v)
to afford TPTDYE 2. Yield is 0.16 g (80%). 1H NMR (300 MHz, DMSO-
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