Influence of solvent and bridge structure in alkylthio-substituted triphenylamine dyes on the photovoltaic properties of dye-sensitized solar cells

Article abstract of DOI:10.1002/asia.201100814

Three new triphenylamine dyes that contain alkylthio-substituted thiophenes with a low bandgap as a π-conjugated bridge unit were designed and synthesized for organic dye-sensitized solar cells (DSSCs). The effects of the structural differences in terms of the position, number, and shape of the alkylthio substituents in the thiophene bridge on the photophysical properties of the dye and the photovoltaic performance of the DSSC were investigated. The introduction of an alkylthio substituent at the 3-position of thiophene led to a decrease in the degree of redshift and the value of the molar extinction coefficient of the charge-transfer band, and the substituent with a bridged structure led to a larger redshift than that of the open-chain structure. The introduction of bulky and hydrophobic side chains decreased the short-circuit photocurrent (J_sc), which was caused by the reduced amount of dye adsorbed on TiO₂. This resulted in a decrease in the overall conversion efficiency (η), even though it could improve the open-circuit voltage (V_oc) due to the retardation of charge recombination. Furthermore, the change in solvents for TiO₂ sensitization had a critical effect on the performance of the resulting DSSCs due to the different amounts of dye adsorbed. Based on the optimized dye bath and molecular structure, the ethylene dithio-substituted dye (ATT3) showed a prominent solar-to-electricity conversion efficiency of 5.20%.

Full text of DOI:10.1002/asia.201100814

DOI: 10.1002/asia.201100814
Influence of Solvent and Bridge Structure in Alkylthio-Substituted
Triphenylamine Dyes on the Photovoltaic Properties of Dye-Sensitized Solar
Cells
Chun Sakong,[a] Se Hun Kim,[a] Sim Bum Yuk,[a] Jin Woong Namgoong,[a]
Se Woong Park,[b] Min Jae Ko,[b] Dong Hoe Kim,[a] Kug Sun Hong,[a] and Jae Pil Kim*[a]
Abstract: Three new triphenylamine
dyes that contain alkylthio-substituted
thiophenes with a low bandgap as a p-
conjugated bridge unit were designed
and synthesized for organic dye-sensi-
tized solar cells (DSSCs). The effects
of the structural differences in terms of
the position, number, and shape of the
alkylthio substituents in the thiophene
bridge on the photophysical properties
of the dye and the photovoltaic perfor-
mance of the DSSC were investigated.
The introduction of an alkylthio sub-
stituent at the 3-position of thiophene
led to a decrease in the degree of red-
shift and the value of the molar extinc-
tion coefficient of the charge-transfer
band, and the substituent with a bridged
structure led to a larger redshift than
that of the open-chain structure. The
introduction of bulky and hydrophobic
side chains decreased the short-circuit
photocurrent (Jsc), which was caused by
the reduced amount of dye adsorbed
on TiO2. This resulted in a decrease in
the overall conversion efficiency (h),
even though it could improve the
open-circuit voltage (Voc) due to the re-
tardation of charge recombination. Fur-
thermore, the change in solvents for
TiO2 sensitization had a critical effect
on the performance of the resulting
DSSCs due to the different amounts of
dye adsorbed. Based on the optimized
dye bath and molecular structure, the
ethylene dithio-substituted dye (ATT3)
showed a prominent solar-to-electricity
conversion efficiency of 5.20%.
Keywords: adsorption
·
dyes/
pigments · solvent effects · thio-
phene · triphenylamine
Introduction
a relatively lower efficiency than ruthenium-complex dyes.
Recently, many groups have studied the synthesis of metal-
free organic dyes due to their advantages of low cost, ease
of structural modifications, ease of synthesis, and high molar
extinction coefficients. These groups have published results
on triphenylamine, indoline, coumarin, cyanine, perylene,
and phenothiazine.[4] Among them, indoline-[5] and triphe-
nylamine-based[6] organic dyes showed conversion efficien-
cies of approximately 10%, which are close to those of
ruthenium-complex dyes.
Some of the main drawbacks of organic dyes are that the
absorption wavelength is in the blue region and the narrow
absorption band shape. Therefore, the optimal organic dyes
for efficient DSSCs should have a broad and redshifted ab-
sorption spectrum. Most organic dyes have a molecular
structure with donor (D)-to-acceptor (A) moieties connect-
ed by a p-conjugated bridge unit such as a methine chain,
thiophene, and furan to obtain a broad and redshifted ab-
sorption spectrum. Recently, the most successful approaches
have been the introduction of thiophene and furan as a p-
conjugated bridge unit into the organic dye framework,
which not only increased the molar extinction coefficient
but also broadened and redshifted the absorption spectrum
of the organic dye.[7] As a result, the investigated organic
dyes showed noticeably improved solar-to-electric conver-
sion efficiencies.
Dye-sensitized solar cells (DSSCs) have attracted considera-
ble attention in the field of renewable energy because of
their easy processing and low production cost.[1] The most
typical sensitizers used in DSSCs are the ruthenium(II)
polypyridyl complex series, and these sensitizers show a high
solar-to-electric conversion efficiency above 10% under the
conditions of AM 1.5-simulated irradiation (100 mWcmꢀ2).[2]
Although ruthenium-complex dyes exhibit high efficiency
and superior stability,[3] they have the disadvantages of high
production cost and difficulties in synthesis and purification.
On the other hand, metal-free organic dyes have shown
[a] C. Sakong, S. H. Kim, S. B. Yuk, J. W. Namgoong, D. H. Kim,
Prof. K. S. Hong, Prof. J. P. Kim
Department of Materials Science and Engineering
Seoul National University
30-402, Daehak-dong, Kwanak-gu
Seoul 151-744 (Korea)
Fax : (+82)2-885-1748
E-mail: jaepil@snu.ac.kr
[b] S. W. Park, Dr. M. J. Ko
Solar Cell Research Center
Materials Science and Technology Division
Korea Institute of Science and Technology (KIST)
Seoul 136-791 (Korea)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200814.
Chem. Asian J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
&
&
&
These are not the final page numbers! ÞÞ

FULL PAPERS
In recent years, Liu and co-
workers have reported a series
of organic dyes with 3,4-ethyle-
nedioxythiophene (EDOT) as
a
p-conjugated bridge unit[8]
that have been widely used as
monomer for conductive
a
polymers, poly(ethylene-3,4-di-
oxythiophene) (PEDOT). They
have exceptional hole-injection
properties, and high conductivi-
ty and physical/chemical stabili-
ty.[9] EDOT has a lower oxida-
tion potential and bandgap
properties than thiophene due
to the strong electron-donating
effect of two alkoxy groups at
the 3- and 4-positions of the
thiophene ring.[10] Through the
introduction of EDOT with
high electron density and low
bandgap properties as a p-con-
jugated bridge unit, the dye
showed a significant redshift in
the absorption band and a high
Scheme 1. Synthetic routes for the synthesis of the dyes: a) 3-methylbutylthiol or 1,2-dithiolethane, p-toluene-
sulfonic acid, toluene, 80–908C; b) NBS, 408C, CH2Cl2; c) 1.6m nBuLi, DMF, THF, ꢀ788C!RT; d) NBS,
DMF, RT; e) [Pd
ACHTNUGTNERN(NUG PPh3)4], 2m K2CO3 aqueous solution, THF, 758C; and f) cyanoacetic acid, piperidine, aceto-
nitrile, heated to reflux.
solar-to-electric conversion efficiency of up to 7.33%, which
is slightly inferior to ruthenium dyes, N719.
In this study, to broaden and redshift the absorption spec-
tra of the dyes, alkylthio-substituted thiophenes with a low
bandgap were introduced as a p-conjugated bridge unit to
connect the triphenylamine donor and cyanoacrylic acid an-
choring group in the dye molecules. In the thiophene series,
alkylthio substituents are stronger donors than alkoxy ones.
Therefore, 3,4-ethylenedithiathiophene (EDTT), in which
two oxygen atoms of EDOT are replaced by sulfur, shows
an oxidation potential 0.13 V lower than that of EDOT
(EDTT: 1.22 V versus EDOT: 1.35 V).[11] Moreover, to ex-
amine the effect of the structural difference on the photo-
physical properties and conversion efficiency, dyes with
open-chain, bridged, symmetric, and asymmetric bridge-unit
structures were designed and their photophysical, electro-
chemical, and photovoltaic properties were analyzed.
Scheme 2. Molecular structures of the triphenylamine dyes with alkylth-
io-substituted thiophene bridges.
These nucleophilic substitutions take place upon reaction
with approximately 5 equivalents of the appropriate thiol in
the presence of p-toluenesulfonic acid at 808C. On the other
hand, in the case of a cyclic compound, a higher tempera-
ture and larger excess amount of 1,2-dithiolethane
(10 equiv) is essential for obtaining the intermediate 7 in
29% yield. Bromination and formylation reactions of com-
pounds 1, 4, and 7 were performed with N-bromosuccini-
mide (NBS) at ambient temperature and nBuLi in N,N-di-
methylformamide (DMF) at ꢀ788C, respectively. Suzuki
coupling was carried out with 4-(diphenylamino)phenyl bor-
onic acid and intermediates 3, 6, and 9 to directly yield the
aldehydes 10–12, which are necessary in the final reactions.
Finally, intermediates 10–12 with an aldehyde group were
condensed with cyanoacetic acid by using the Knoevenagel
Results and Discussion
Synthesis
The synthetic routes and structures of the triphenylamine
dyes with alkylthio-substituted thiophene bridges are shown
in Schemes 1 and 2, respectively. Alkylthio-substituted thio-
phene derivatives as the important intermediates in this
study have been synthesized by means of several common
reactions and the detailed synthetic procedures are de-
scribed in the Supporting Information.
Intermediates 1, 4, and 7 were obtained by the substitu-
tion of the methoxy group starting from 3-methoxythio-
phene and 3,4-dimethoxythiophene in reasonable yields.[12]
&
&
&2
&
www.chemasianj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 0000, 00, 0 – 0
ÝÝ These are not the final page numbers!

Triphenylamine Dyes
reaction in the presence of piperidine and converted to
ATT1–ATT3.
Photophysical Properties of the Dyes in Solution and TiO2
Film
The absorption spectra of a 5ꢁ10ꢀ5 m solution of the three
dyes in dichloromethane are shown in Figure 1a, and the
corresponding data are listed in Table 1. All the dyes exhib-
ited two major prominent absorption bands, which appear at
below 350 nm and at 450–550 nm, respectively. The former
is due to a localized p–p* transition, and the latter is attrib-
uted to the intramolecular charge-transfer transition (ICT)
from the triphenylamine donor part to the cyanoacrylic acid
acceptor group.[13] This assignment is supported by the solva-
tochromic behavior of the dyes. As shown in Figure 1c, the
p–p* transition bands at 302 nm are nearly solvent-polarity
independent, whereas the ICT bands at 490 nm exhibit neg-
ative solvatochromism (i.e., a blueshift of lmax in more polar
solvents). A comparative examination of the absorption
spectra between ATT2 (or ATT3) and ATT1 indicates that
the absorption bands have significant contributions from the
substituent at the C(3) position of the 2,3,4-substituted thio-
phene. ATT2 and ATT3 showed a blueshift and relatively
weaker redshift in the ICT band, respectively. They also had
smaller extinction coefficients in the ICT band relative to
ATT1 with only a C(4)-donating substituent, even though
they had two donating substituents at the C(3) and C(4) of
the thiophene bridge. This might be due to the decreased
overlap of delocalized p electrons between the triphenyla-
mine donor and alkylthio-substituted thiophene bridge,
which results from the effect of the C(3) substituent on the
coplanarity of the dye. As shown in Figure 2, the coplanarity
of ATT2 and ATT3 between the triphenylamine donor and
thiophene bridge decreased significantly, and the dihedral
angles between the benzene of the donor group and thio-
phene bridge were 39.2 and 40.98, respectively. On the other
hand, ATT1 has a flatter molecular geometry, and the dihe-
dral angle between the benzene of the donor group and the
thiophene bridge was 18.18. Therefore, it can be stated that
ATT2 and ATT3 had a more twisted molecular geometry
than ATT1 because of the steric hindrance caused by the
substituent at the C(3) position, and the overlap of the delo-
calized p electron would be decreased significantly. In the
case of ATT3, although the dihedral angle between the ben-
zene of the donor group and the thiophene bridge is close
to that of ATT2, its absorption band was redshifted consid-
erably relative to that of ATT2. This might be due to the en-
hancement of p-electron delocalization by the bridged struc-
ture of EDTT.[14]
The absorption spectra of the dyes adsorbed on TiO2 are
shown in Figure 1b and the corresponding data are listed in
Table 1. All the dyes showed a blueshift in absorption com-
pared to those in solution, and there was a larger shift in
ATT1 (56 nm) and ATT3 (54 nm) than in ATT2 (38 nm).
Such blueshifts in the absorption spectra by adsorption onto
TiO2 have also been observed in other organic dyes. Either
Figure 1. Absorption spectra of the dyes in a) CH2Cl2, b) adsorbed onto
~
&
*
TiO2
( ATT1, ATT2, ATT3), and c) absorption spectra of ATT1 in
~
!
&
*
different solvents ( toluene, THF, EtOH, DMF).
Chem. Asian J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
3
&
&
&
These are not the final page numbers! ÞÞ

Jae Pil Kim et al.
FULL PAPERS
TiO2 surface.[15] Therefore, opti-
mization of the solvent system
for TiO2 sensitization is very
important for obtaining good
conversion efficiency (h) in
DSSCs. To optimize the solvent
systems for the dyes synthe-
sized, TiO2 films were sensi-
tized with the dyes in various
solvents (EtOH, mixed solvent
Table 1. Photophysical and electrochemical properties of the synthesized dyes.
[a]
lmax [nm] (eꢁ10ꢀ4 mꢀ1 cmꢀ1
)
lmax (TiO2) [nm]
Eox [eV][b]
E0–0 [eV][c]
EoxꢀE0–0 [eV]
ꢀ1.02
ATT1
ATT2
ATT3
302 (2.08)
490 (3.68)
322 (2.70)
478 (2.31)
280 (1.79)
492 (2.30)
434
1.19
2.21
440
438
1.21
1.15
2.22
2.18
ꢀ1.01
ꢀ1.03
[a] Absorption and emission spectra were measured for solutions in CH2Cl2. [b] The oxidation potentials (Eox
)
of the dyes were obtained by cyclic voltammetry in CH2Cl2 with 0.1m tetrabutylammonium tetrafluoroborate
(TBATFB) with a scan rate of 100 mVsꢀ1 and converted to NHE. [c] The zeroth–zeroth transition E0–0 values
were estimated from the intersection of the normalized absorption and emission spectra.
(MeOH/CHCl3
1:1),
and
CHCl3). Figure 4 shows the de-
pendence of the photocurrent
a polar interaction with TiO2 and/or deprotonation of car-
boxylic acid or H-aggregation of the dyes has been suggest-
ed to account for such a blueshift.[15]
density–voltage (J–V) curves of the DSSCs in three different
solvents, and corresponding photovoltaic performance data
are collected in Table 2. The DSSCs fabricated in the three
different solvents showed large variations in their perform-
Electrochemical Properties of the Dyes
When the LUMO level of a dye is more negative than the
conduction band of TiO2, the electrons of the dye excited by
light are injected into TiO2 effectively. In addition, oxidized
dyes can be easily regenerated by the electrolyte when the
HOMO level of a dye is more positive than the redox po-
tential of the electrolyte.
Table 2. Photovoltaic performances of the DSSCs based on the synthe-
sized dyes sensitized in different solvents.[a]
Solvent
Jsc [mAcmꢀ2
]
Voc [mV]
FF
h [%]
ATT1
ATT2
ATT3
N719
EtOH
MeOH+CHCl3
CHCl3
EtOH
MeOH+CHCl3
CHCl3
EtOH
MeOH+CHCl3
CHCl3
10.40
10.06
5.62
8.10
7.53
2.49
10.47
9.85
7.70
14.11
664.9
646.6
655.8
680.9
664.3
613.2
650.9
619.0
617.2
822.6
72.69
75.67
76.27
76.90
76.21
72.89
76.25
75.41
74.52
67.62
5.03
4.92
2.81
4.24
3.81
1.11
5.20
4.60
3.54
7.85
The oxidation potential (Eox) that corresponds to the
HOMO level of the dye was measured in dichloromethane
by using cyclic voltammetry (CV). The CV curves of the
dyes are shown in Figure 3 and the corresponding data are
collected in Table 1. The HOMO levels of the three dyes
were positive enough relative to the redox potential of the
electrolyte, 0.4 V (versus the normal hydrogen electrode
(NHE)), thereby indicating that the oxidized dyes could be
regenerated effectively by the electrolyte. From these
values, the different shapes (open-chain and bridged struc-
ture) of the substituents in the thiophene bridge affected the
HOMO levels, and the ring-closed structure of the substitu-
ents made the HOMO levels of the dyes more negative. For
example, ATT3 showed a more negative HOMO, 1.15 V
(versus NHE), than those of ATT1 and ATT2, 1.19 and
1.21 V (versus NHE), respectively.
The LUMO levels of these dyes were calculated by
EoxꢀE0–0, in which E0–0 is the zeroth–zeroth energy of the
dyes estimated from the intersection between the normal-
ized absorption and emission spectra, and the corresponding
data are listed in Table 1. The LUMO levels of the three
dyes were more negative than the conduction band of TiO2,
ꢀ0.5 V (versus NHE), thus indicating that the excited elec-
trons of the dyes could be injected effectively into the con-
duction band of TiO2.
EtOH
[a] The concentration was maintained at 5ꢁ10ꢀ4 m in different solvents,
with 0.5m 1-methyl-3-propylimidazolium iodide (PMII), 0.2m LiI, 0.05m
I2, and 0.5m 4-tert-butylpyridine (TBP) in acetonitrile/valeronitrile
(85:15) solution. Performance of the DSSC was measured with a 0.45 cm2
working area.
ances. When EtOH was used to sensitize TiO2, the DSSCs
based on ATT1–ATT3 showed the highest h values of 5.03,
4.24, and 5.20%, respectively. The DSSCs fabricated in
a mixed solvent had a slightly lower performance than those
in EtOH. On the other hand, the DSSCs fabricated in
CHCl3 gave the lowest h values. In particular, among the
photovoltaic performance parameters, short-circuit photo-
current density (Jsc) values of the DSSCs fabricated in
CHCl3 decreased dramatically relative to those in the other
solvents.
The absorption spectra of the three dyes on TiO2 films
sensitized in different solvents were measured to determine
the reason for such variations in their performances. The re-
sults are shown in Figure 5. Also, the amounts of anchored
dyes on TiO2 films were estimated by the methods ex-
plained in the Experimental Section, and the corresponding
data are collected in Table 3. The amounts of dyes adsorbed
onto TiO2 films were higher in EtOH and mixed solvent
than in CHCl3. This result shows that the strength of the Jsc
of the DSSCs fabricated in various solvents has a direct rela-
Dye Adsorption in Various Solvents and the Performances
of DSSCs
According to the literature, dyes in various solvents exhibit
diverse interactions with the solvents, which could alter the
physical and chemical interactions between the dyes and the
&
&
&4
&
www.chemasianj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 0000, 00, 0 – 0
ÝÝ These are not the final page numbers!

Triphenylamine Dyes
Figure 3. Cyclic voltammograms (CV) curves of the three dyes in CH2Cl2
~
&
*
(
ATT1, ATT2, ATT3).
Photovoltaic Properties of DSSCs Based on the Synthesized
Dyes
Three DSSCs were fabricated by using the synthesized dyes
according to the methods described in the Experimental
Section. The photovoltaic performances of these DSSCs fab-
ricated in EtOH under standard AM 1.5 G irradiation con-
ditions are listed in Table 2, and the respective J–V curves
are shown in Figure 6. The incident monochromatic photon-
to-current conversion efficiencies of the DSSCs fabricated
in EtOH are shown in Figure 7. The range of the IPCE
spectrum based on ATT1 was up to 660 nm, and a high
IPCE value (>70%) was observed from 410 to 550 nm with
the maximum value of 78.57% at 470 nm. The range of the
IPCE spectrum based on ATT3 was up to 680 nm, and
a high IPCE value (>70%) was observed from 410 to
550 nm, with the maximum value of 77.12% at 470 nm.
These two DSSCs have high and broad IPCE spectra. On
the other hand, the range of the IPCE spectrum based on
ATT2 (up to 640 nm) was narrower than those of the
former two dyes. Moreover, it also exhibited a lower IPCE
value (the maximum IPCE value of 71.94% at 470 nm). The
rather narrow and low IPCE value for ATT2 is responsible
for the lower Jsc and inferior h. It is well known that a larger
amount of dye adsorbed on TiO2 leads to a broader IPCE
spectrum and higher IPCE value of the DSSC, which con-
tribute to the higher h. Therefore, the results might be due
to the smaller amount of ATT2 adsorbed onto TiO2 caused
by its bulky and twisted molecular structure.
Figure 2. a) Optimized molecular geometry and b) Frontier molecular or-
bitals of the HOMO (left) and LUMO (right) of the synthesized dyes cal-
culated with DFT at the B3LYP/6-31+GACTHNUTRGNEU(GN d,p) level.
tionship with the amounts of dyes adsorbed. This is because
the increased amounts of dyes adsorbed onto TiO2 films
leads to a broader incident monochromatic photon-to-cur-
rent conversion efficiency (IPCE) spectrum and higher
IPCE value of the resulting DSSC.[16]
The performance of the DSSCs improved in the order of
ATT2<ATT1<ATT3. It is important to note that the
amounts of dye adsorbed vary according to the structure of
the thiophene bridge (7.20ꢁ10ꢀ8 (ATT1), 7.00ꢁ10ꢀ8
(ATT2), 1.34ꢁ10ꢀ7 molcmꢀ2 (ATT3)), hence the influence
of the amount of dye adsorbed on the cell performance was
considered important in our discussion (Table 3).
Chem. Asian J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
5
&
&
&
These are not the final page numbers! ÞÞ

Jae Pil Kim et al.
FULL PAPERS
Figure 5. Absorption spectra of a) ATT1, b) ATT2, and c) ATT3 ad-
Figure 4. J–V curves of the DSSCs based on a) ATT1, b) ATT2, and
~
&
*
sorbed onto TiO2 in different solvents ( EtOH,
MeOH+CHCl3,
~
&
*
c) ATT3 in different solvents ( EtOH, MeOH+CHCl3, CHCl3).
CHCl3).
As shown in Scheme 3, it seems that the different amount
of dye adsorbed is due to the different coverage area per
molecule on the TiO2 surface according to the molecular
size of the dye. ATT3 showed the highest amount of adsorp-
&
&
&6
&
www.chemasianj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 0000, 00, 0 – 0
ÝÝ These are not the final page numbers!

Triphenylamine Dyes
Table 3. The amount of anchored dyes on TiO2 films in various solvents.
EtOH [molcmꢀ2
]
MeOH+CHCl3 [molcmꢀ2
]
CHCl3 [molcmꢀ2
]
ATT1
ATT2
ATT3
7.20ꢁ10ꢀ8
7.00ꢁ10ꢀ8
1.34ꢁ10ꢀ7
3.36ꢁ10ꢀ8
2.64ꢁ10ꢀ8
1.28ꢁ10ꢀ7
6.50ꢁ10ꢀ10
2.70ꢁ10ꢀ10
1.53ꢁ10ꢀ9
Figure 7. Incident photon-to-current conversion efficiencies spectra for
~
!
&
*
the DSSCs based on the synthesized dyes ( ATT1, ATT2, ATT3,
N719).
Scheme 3. Schematic diagram of the TiO2 surface sensitized with ATT2
and ATT3.
ilar to that of ATT3 due to the higher molar extinction coef-
ficient.
The open-circuit voltage (Voc) increased in the order of
ATT3<ATT1<ATT2. It is considered that the bulky and
hydrophobic 3-methylbutylthio substituents of ATT1 and
ATT2 suppressed the diffusion of the electrolyte to the TiO2
surface, thereby resulting in a retardation of charge recom-
bination.[17] To elucidate the correlation of Voc with the dyes,
electrochemical impedance was measured in the dark. The
Nyquist plots of the DSSCs based on the three dyes under
a forward bias of ꢀ0.65 V with a frequency range of 0.1 Hz
to 100 kHz are shown in Figure 8a. The Bode phase plots
are also shown in Figure 8b. Three semicircles were ob-
served in the Nyquist plots. The smaller semicircle at high
frequency is assigned to the redox charge-transfer response
at the Pt/electrolyte interface.[18] The large one at the inter-
mediate frequency represents the electron-transfer impe-
dance at the TiO2/dye/electrolyte interface, and the last one
at low frequency shows the Nernst diffusion response of the
electrolyte. The charge recombination resistance at the TiO2
surface, Rrec, can be estimated from the radius of the middle
semicircle in the Nyquist plots. The charge recombination
resistance is related to the charge recombination rate, such
that a smaller Rrec means a large charge recombination rate.
The Rrec values increased in the order of ATT3<ATT1<
ATT2. The result appears to be consistent with the increase
of Voc in the DSSCs based on ATT1 and ATT2 that bear
a 3-methylbutylthio side chain. The increased Rrec values
imply the retardation of charge recombination between the
Figure 6. Photocurrent density–voltage (J–V) curves for the DSSCs based
on the synthesized dyes under AM 1.5 G simulated solar light
(100 mWcmꢀ2) ( ATT1, ATT2, ATT3, N719).
~
!
&
*
tion on TiO2 although it had the most twisted molecular
structure. This might be due to the smaller molecular size of
ATT3 caused by the bridged structure of the substituent. A
larger amount of dye adsorbed onto TiO2 leads to a higher
Jsc and superior h of the DSSC. On the other hand, ATT1
and ATT2 had a larger molecular size than ATT3 because
of the bulky open-chain structure of the 3-methylbutylthio
substituent and thus they had a smaller amount of dye ad-
sorbed on TiO2, thereby leading to a lower Jsc. Although
ATT1 had a smaller amount of dye adsorbed, its Jsc was sim-
ꢀ
injected electron and electron acceptor (I3 ) in the electro-
lyte, with a consequent increase in the Voc. Correspondingly,
the frequency of the characteristic peak in the Bode plots
increased in the order of ATT3<ATT1<ATT2, thereby
suggesting that the DSSCs based on ATT1 and ATT2 that
bear a 3-methylbutylthio side chain have longer electron
Chem. Asian J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
7
&
&
&
These are not the final page numbers! ÞÞ

Jae Pil Kim et al.
FULL PAPERS
caused by the blueshift of the absorption spectrum and de-
crease in the amount of dye adsorbed on TiO2. On the other
hand, the introduction of a substituent with a bridged struc-
ture at the same position led to an improvement in Jsc due
to the relatively redshifted absorption spectrum and the
larger amount of dye adsorbed, which resulted in the in-
crease in h. In addition, the DSSCs fabricated in different
solvents showed large variations in their performances due
to the amount of dye adsorbed. Therefore, designing the
molecular structure to have a larger amount of dye adsorbed
and optimizing the solvent system for TiO2 sensitization is
needed to obtain highly efficient DSSCs.
Experimental Section
General Information
3-Methoxythiophene, 3,4-diACTHNUGTRNEUNG(methoxy)thiophene, 3-methylbutylthiol, 1,2-
dithiolethane, and p-toluenesulfonic acid purchased from Sigma–Aldrich;
and 4-(diphenylamino)phenyl boronic acid from TCI were used without
further purification. All chemicals used in this study were of synthesis
1
grade. H and 13C NMR spectra were recorded with a Bruker Avance 500
at 500 MHz using CDCl3 and [D6]DMSO as the solvent and TMS as the
internal standard. Mass spectra were measured with a JEOL JMS-600W
Agilent 6890 Series, and FTIR spectra were measured with a Thermo Sci-
entific Nicolet 6700. UV-visible absorption spectra were measured with
an HP 8452A spectrophotometer and emission spectra were recorded
with a QuantaMaster UV/Vis spectrofluorometer. Cyclic voltammetry
was performed with a three-electrode electrochemical cell on a Potento-
stat/Gavanostat PAR Model 273A. Glassy-carbon, platinum wire, and
Ag/Ag+ were employed as the working, counter, and reference electro-
des, respectively. Tetrabutylammonium tetrafluoroborate (TBATFB) was
used as the supporting electrolyte and ferrocene was added as an internal
reference for calibration.
Synthesis of 2-Cyano-3-{5-[4-(diphenylamino)phenyl]-3-(3-
methylthio)thiophene-2-yl}acrylic acid (ATT1)
Figure 8. Electrochemical impedance spectra of DSSCs based on the
A mixture of the intermediate 10 (0.6 g, 1.3 mmol), cyanoacetic acid
(0.28 g, 3.25 mmol), and piperidine (0.51 mL, 5.2 mmol) was dissolved in
acetonitrile (60 mL) and heated at reflux for 6 h at 928C under nitrogen.
The reaction mixture was cooled to room temperature and the solvent
was removed under reduced pressure. The crude product was purified by
column chromatography on silica gel using dichloromethane/methanol
(8:1, v/v). The product was dissolved in dichloromethane (120 mL) and
then 0.1m HCl aqueous solution was added to this solution. The solution
was vigorously stirred for 2 h and the organic layer was dried over anhy-
drous MgSO4. The solvent was removed by rotary evaporation and subse-
quently dried in a vacuum oven to afford ATT1 (0.46 g, 67%) as a dark
red solid. 1H NMR (CDCl3, 500 MHz): d=8.69 (s, 1H), 7.54 (d, J=
8.7 Hz, 2H), 7.31 (t, J=7.9 Hz, 4H), 7.13 (m, 7H), 7.05 (d, J=8.7 Hz,
2H), 3.02 (m, 2H), 1.74 (m, 1H), 1.57 (m, 2H), 0.93 ppm (m, 6H);
13C NMR ([D6]DMSO, 500 MHz): d=163.38, 151.30, 148.54, 148.70,
145.71, 141.51, 129.24, 127.04, 126.94, 124.62, 123.99, 123.79, 120.81,
116.07, 95.84, 37.23, 31.74, 26.23, 21.51 ppm; HRMS (FAB+): m/z calcd
for C31H28N2O2S2: 524.1637; found: 524.1635; elemental analysis calcd
(%) for C31H28N2O2S2: C 70.96, H 5.38, N 5.34; found: C 70.59, H 5.17, N
5.06.
three dyes measured in the dark under ꢀ0.65 V bias: a) Nyquist plots,
~
&
*
b) Bode phase plots ( ATT1, ATT2, ATT3).
lifetimes due to the slower recapture of the conduction-
ꢀ
band electron by I3 . This result corresponded to the se-
quence of the Voc in the photovoltaic performances of
DSSCs based on the three dyes.
Conclusion
A new series of novel triphenylamine dyes with different al-
kylthio-substituted thiophene moieties as a p-conjugated
bridge were designed and synthesized for applications in
DSSCs. The photophysical properties of these dyes and the
photovoltaic performances of the DSSCs markedly depend-
ed on the molecular structure of the dyes in terms of the po-
sition, number, and shape of the alkylthio substituents in the
thiophene bridge. The introduction of an additional bulky
alkylthio substituent with an open-chain structure at the 3-
position of the thiophene led to a decrease in h, despite the
enhancement of Voc. This was attributed to the lower Jsc
Synthesis of 2-Cyano-3-{5-[4-(diphenylamino)phenyl]-3,4-bis(3-
methylthio)thiophene-2-yl}acrylic acid (ATT2)
This compound was synthesized by using the method established for
ATT1. The crude product was purified by column chromatography on
silica gel using dichloromethane/methanol (8:1, v/v) to afford ATT2
(0.34 g, 55%) as a red solid; 1H NMR (CDCl3, 500 MHz): d=9.02 (s,
1H), 7.68 (d, J=8.5 Hz, 2H), 7.32 (t, J=7.7 Hz, 4H), 7.16 (d, J=8.2 Hz,
&
&
&8
&
www.chemasianj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 0000, 00, 0 – 0
ÝÝ These are not the final page numbers!

Triphenylamine Dyes
4H), 7.12–7.07 (m, 4H), 3.02 (m, 2H), 2.73 (m, 2H), 1.72 (m, 1H), 1.53
(m, 1H), 1.46 (m, 2H), 1.28 (m, 2H), 0.91 (m, 6H), 0.79 ppm (m, 6H);
13C NMR ([D6]DMSO, 500 MHz): d=162.87, 151.32, 148.21, 147.64,
145.86, 143.48, 133.99, 131.84, 129.72, 129.23, 124.47, 123.67, 120.66,
115.57, 99.51, 37.38, 36.95, 34.38, 32.65, 26.03, 25.84, 21.48, 21.39 ppm;
HRMS (FAB+): m/z calcd for C36H38N2O2S3: 626.2112; found: 626.2108;
elemental analysis calcd (%) for C31H28N2O2S2: C 68.97, H 6.11, N 4.47;
found: C 68.66, H 5.93, N 4.23.
estimated by measuring the absorption spectrum of the resulting solution
after desorption of the dye in a basic solution (0.1m NaOH aqueous/etha-
nol (1:1) mixed solvent).
Computational Study
All calculations were carried out with the Gaussian 09 software pro-
gram.[19] The molecular geometry and the electron distributions of the
synthesized dyes for the HOMO and LUMO state were calculated with
DFT at the B3LYP/6-31+G(d) level.
Synthesis of 2-Cyano-3-{5-[4-(diphenylamino)phenyl]-3,4-
(ethylenedithio)thiophene-2-yl}acrylic acid (ATT3)
This compound was synthesized by using the method established for
ATT1. The crude product was purified by column chromatography on
silica gel using dichloromethane/methanol (5:1, v/v) to afford ATT3
(0.19 g, 52%) as a dark red solid. 1H NMR (CDCl3, 500 MHz): d=8.50
(s, 1H), 7.49 (m, 2H), 7.32 (t, J=7.5 Hz, 4H), 7.16 (d, J=8.0 Hz, 4H),
7.12 (m, 2H), 7.07 (t, J=7.5 Hz, 2H), 3.38 (m, 2H), 3.24 ppm (m, 2H);
13C NMR (CDCl3, 500 MHz): d=168.26, 155.03, 151.23, 149.47, 147.20,
146.91, 134.67, 133.20, 130.30, 129.51, 125.37, 124.05, 121.52, 115.55, 97.08,
35.78, 34.02 ppm; HRMS (FAB+): m/z calcd for C28H20N2O2S3: 512.0715;
found: 512.0711; elemental analysis calcd (%) for C31H28N2O2S2: C 65.60,
H 3.93, N 5.46; found: C 65.31, H 3.78, N 5.28.
Acknowledgements
This work was supported by a grant from the Korean Evaluation Institute
of Industrial Technology funded by the Korean government (MKE).
[1] a) B. OꢂRegan, M. Grꢃtzel, Nature 1991, 353, 737–740; b) M. Grꢃt-
zel, Nature 2001, 414, 338–344.
[2] a) Y. Chiba, A. Islam, Y. Watanabe, R. Komiya, N. Koide, L. Han,
Jpn. J. Appl. Phys. 2006, 45, L638–L640; b) M. K. Nazeeruddin,
F. D. Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito, B.
Takeru, M. Grꢃtzel, J. Am. Chem. Soc. 2005, 127, 16835–16847;
c) Z.-S. Wang, T. Yamaguchi, H. Sugihara, H. Arakawa, Langmuir
2005, 21, 4272–4276.
[3] P. Wang, S. M. Zakeeruddin, J. E. Moser, M. K. Nazeeruddin, T. Se-
kiguchi, M. Grꢃtzel, Nat. Mater. 2003, 2, 402–407.
[4] A. Mishra, M. K. Fischer, P. Bꢃuerle, Angew. Chem. 2009, 121,
2510–2536; Angew. Chem. Int. Ed. 2009, 48, 2474–2499.
Fabrication of Dye-Sensitized Solar Cells
Fluorine–tin oxide (FTO) glass plates (Pilkington, TEC-8, 8 W/square,
2.3 mm thick) were cleaned with ethanol by ultrasonication for 10 min,
and then treated in a UV-O3 system for 20 min. The FTO layer was first
covered with 7.5% TiIV bis(ethyl acetoacetato)diisopropoxide solution by
using a spin-coating method. For the transparent nanocrystalline layer,
TiO2 paste (230ACHTUNGTRENNUNG(M2331)-2T) was coated on the FTO glass plates by
doctor blade printing, and then sintering was carried out at 5008C for
30 min. TiO2 paste (CCIC-1T) for the scattering layer was prepared using
the same method. The resulting layer was composed of a 9 mm-thick
transparent layer and 4 mm-thick scattering layer. The active area of the
TiO2 films was about 0.45 cm2. The TiO2 electrodes were immersed into
the different solutions (0.5 mm in EtOH, MeOH/CHCl3, and CHCl3) and
kept at room temperature for 24 h. Counter electrodes were prepared by
[5] S. Ito, H. Miura, S. Uchida, M. Takata, K. Sumioka, P. Liska, P.
Comte, P. Pꢄchy, M. Grꢃtzel, Chem. Commun. 2008, 5194–5196.
[6] G. Zhang, H. Bala, Y. Cheng, D. Shi, X. Lv, Q. Yu, P. Wang, Chem.
Commun. 2009, 2198–2200.
[7] a) H. Choi, C. Baik, S. O. Kang, J. Ko, M. S. Kang, M. K. Nazeerud-
din, M. Grꢃtzel, Angew. Chem. 2008, 120, 333–336; Angew. Chem.
Int. Ed. 2008, 47, 327–330; b) K. Hara, M. Kurashige, Y. Dan-Oh,
C. Kasada, A. Shinpo, S. Suga, K. Sayama, H. Arakawa, New J.
Chem. 2003, 27, 783–785; c) S.-L. Li, K.-J. Jiang, K.-F. Shao, L.-M.
Yang, Chem. Commun. 2006, 2792–2794; d) D. P. Hagberg, T. Ed-
vinsson, T. Marinado, G. Boschloo, A. Hagfeldt, L. Sun, Chem.
Commun. 2006, 2245–2247; e) R. Chen, X. Yang, H. Tian, X. Wang,
A. Hagfeldt, L. Sun, Chem. Mater. 2007, 19, 4007–4015; f) J. T. Lin,
P.-C. Chen, Y.-S. Yen, Y.-C. Hsu, H.-H. Chou, M.-C. Yeh, Org. Lett.
2009, 11, 97–100.
dropping a 0.7 mm H2ACHTUNGTRENNUNG[PtCl6] solution on FTO glass and heating it at
4008C for 20 min. The dye-adsorbed TiO2 electrode and the counter elec-
trode were sealed using 25 mm-thick Surlyn film (Dupont 1702). An elec-
trolyte solution was introduced through a drilled hole on the counter
electrode, in which the electrolyte solution consisted of 0.5m 1-methyl-3-
propylimidazolium iodide (PMII), 0.2m LiI, 0.05m I2, and 0.5m 4-tert-bu-
tylpyridine (TBP) in acetonitrile/valeronitrile (85:15).
Photovoltaic Measurements
[8] a) W.-H. Liu, I.-C. Wu, C.-H. Laia, C.-H. Lai, P.-T. Chou, Y.-T. Li,
C.-L. Chen, Y.-Y. Hsu, Y. Chi, Chem. Commun. 2008, 5152–5154;
b) K.-J. Jiang, K. Manseki, Y.-h. Yu, N. Masaki, J.-B. Xia, L.-M.
Yang, Y.-I. Song, S. Yanagida, New J. Chem. 2009, 33, 1973–1977.
[9] S. Kirchmeyer, K. J. Reuter, J. Mater. Chem. 2005, 15, 2077–2088.
[10] a) N. Sakmeche, J. J. Aaron, M. Fall, S. Aeiyach, M. Jouni, J. C. La-
croix, P. C. Lacaze, Chem. Commun. 1996, 2723–2724; b) B. Sankar-
an, J. R. Reynolds, Macromolecules 1997, 30, 2582–2588; c) F. Jonas,
L. Schrader, Synth. Met. 1991, 41, 831–836; d) H. Huang, P. G.
Pickup, Chem. Mater. 1998, 10, 2212–2216; e) Y. Fu, H. Cheng,
R. L. Elsenbaumer, Chem. Mater. 1997, 9, 1720–1724.
[11] a) J. Roncali, Chem. Rev. 1992, 92, 711–738; b) C. Wang, J. L. Schin-
dler, C. R. Kannewurf, M. G. Kanatzidis, Chem. Mater. 1995, 7, 58–
68; c) P. Blanchard, A. Cappon, E. Levillain, Y. Nicolas, P. Frꢅre, J.
Roncali, Org. Lett. 2002, 4, 607–609.
[12] F. Goldoni, B. W. Langeveld-Voss, E. W. Meijer, Synth. Commun.
1998, 28, 2237–2244.
Photovoltaic measurements were performed with a Keithly model 2400
source measuring unit. A 1000 W Xe lamp (Spectra-physics) served as
a light source and its light intensity was adjusted using a national renewa-
ble energy laboratory (NREL)-calibrated silicon solar cell equipped with
a KG-5 filter to approximate AM 1.5 G of sunlight intensity. IPCE was
measured as a function of wavelength from 300–800 nm using a specially
designed IPCE system for dye-sensitized solar cells (PV Measurements,
Inc.). A 75 W Xe lamp was used as the light source for generation of
a monochromatic beam. Calibrations were performed using a silicon pho-
todiode, which was calibrated using NIST-calibrated photodiode G425 as
a standard, and IPCE values were collected at a low chopping speed of
10 Hz.
Measurements of the Amount of Dye Adsorbed and Absorption Spectra
on TiO2
The preparation method of TiO2 films was the same as the procedure
above. The 9 mm-thick (area: 20ꢁ10 mm) TiO2 film without the scatter-
ing layer was immersed into the different solutions (0.5 mm in EtOH,
MeOH/CHCl3, and CHCl3) and kept at room temperature for 24 h. The
dye-adsorbed TiO2 films were washed with the solvent and subsequently
dried. Absorption spectra of dye-adsorbed TiO2 films were measured
with an HP 8452A spectrophotometer. The amount of dye adsorbed was
[13] a) S. Roquet, A. Cravino, P. Leriche, O. AlꢄvÞque, P. Frꢅre, J. Ron-
cali, J. Am. Chem. Soc. 2006, 128, 3459–3466; b) K. R. Justin Tho-
mas, Y.-C. Hsu, J. T. Lin, K.-M. Lee, K.-C. Ho, C.-H. Lai, Y.-M.
Cheng, P.-T. Chou, Chem. Mater. 2008, 20, 1830–1840.
[14] J.-M. Raimundo, P. Blanchard, N. Gallego-Planas, N. Mercier, I.
Ledoux-Rak, R. Hierle, J. Roncali, J. Org. Chem. 2002, 67, 205–218.
Chem. Asian J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
9
&
&
&
These are not the final page numbers! ÞÞ

Jae Pil Kim et al.
FULL PAPERS
[15] a) R. Chen, G. Zhao, X. Yang, X. Jiang, J. Liu, H. Tian, Y. Gao, X.
Liu, K. Han, M. Sun, L. Sun, J. Mol. Struct. 2008, 876, 1–8; b) H.
Tian, X. Yang, R. Chen, R. Zhang, A. Hagfeldt, L. Sun, J. Phys.
Chem. C 2008, 112, 11023–11033.
[16] E. L. Tae, S. H. Lee, J. K. Lee, S. S. Yoo, E. J. Kang, K. B. Yoon, J.
Phys. Chem. B 2005, 109, 22513–22522.
[17] a) J.-H. Yum, D. P. Hagberg, S.-J. Moon, K. M. Karlsson, T. Marina-
do, L. Sun, A. Hagfeldt, M. K. Nazeeruddin, M. Grꢃtzel, Angew.
Chem. 2009, 121, 1604–1608; Angew. Chem. Int. Ed. 2009, 48, 1576–
1580; b) Z.-S. Wang, N. Koumura, Y. Cui, M. Takahashi, H. Sekigu-
chi, A. Mori, T. Kubo, A. Furube, K. Hara, Chem. Mater. 2008, 20,
3993–4003.
[18] a) J. van de Lagemaat, N.-G. Park, A. J. Frank, J. Phys. Chem. B
2000, 104, 2044–2052; b) R. Kern, R. Sastrawan, L. Ferber, R.
Stangl, J. Luther, Electrochim. Acta 2002, 47, 4213–4225; c) Q.
Wang, J.-E. Moser, M. Grꢃtzel, J. Phys. Chem. B 2005, 109, 14945–
14953; d) D. Kuang, S. Ito, B. Wenger, C. Klein, J. E. Moser, R.
Humphry-Baker, S. M. Zakeeruddin, M. Grꢃtzel, J. Am. Chem. Soc.
2006, 128, 4146–4154.
[19] Gaussian 09, Revision A.01, M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V.
Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X.
Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Son-
nenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven,
J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J.
Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J.
Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A.
Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O.
Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian,
Inc., Wallingford CT, 2009.
Received: September 28, 2011
Revised: January 3, 2012
Published online: && &&, 0000
&
&
&10
&
www.chemasianj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 0000, 00, 0 – 0
ÝÝ These are not the final page numbers!

FULL PAPERS
Structure and solvent effects: Three
novel triphenylamine dyes with differ-
ent alkylthio-substituted thiophene
bridges were synthesized for dye-sensi-
tized solar cells (DSSCs). The photo-
voltaic performances of the DSSCs
showed a marked dependence on the
bridge structure and solvent system
due to the different amounts of dye
adsorbed (see scheme).
Solar Cells
Chun Sakong, Se Hun Kim,
Sim Bum Yuk, Jin Woong Namgoong,
Se Woong Park, Min Jae Ko,
Dong Hoe Kim, Kug Sun Hong,
Jae Pil Kim*
&&&& — &&&&
Influence of Solvent and Bridge Struc-
ture in Alkylthio-Substituted Triphe-
nylamine Dyes on the Photovoltaic
Properties of Dye-Sensitized Solar
Cells
Chem. Asian J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
11
&
&
&
These are not the final page numbers! ÞÞ