Enhancing Dye-Sensitized Solar Cell Performances by Molecular Engineering: Highly Efficient π-Extended Organic Sensitizers

Article abstract of DOI:10.1002/cssc.201402164

This study deals with the synthesis and characterization of two π-extended organic sensitizers (G1 and G2) for applications in dye-sensitized solar cells. The materials are designed with a D–A–π–A structure constituted by i) a triarylamine group as the donor part, ii) a dithienyl-benzothiadiazole chromophore followed by iii) a further ethynylene-thiophene (G1) or ethynylene-benzene (G2) π-spacer and iv) a cyano-acrylic moiety as acceptor and anchoring part. An unusual structural extension of the π-bridge characterizes these structures. The so-configured sensitizers exhibit a broad absorption profile, the origin of which is supported by density functional theory. The absence of hypsochromic shifts as a consequence of deprotonation as well as notable optical and electrochemical stabilities are also observed. Concerning the performances in devices, electrochemical impedance spectroscopy indicates that the structural modification of the π-spacer mainly increases the electron lifetime of G2 with respect to G1. In devices, this feature translates into a superior power conversion efficiency of G2, reaching 8.1 %. These results are comparable to those recorded for N719 and are higher with respect to literature congeners, supporting further structural engineering of the π-bridge extension in the search for better performing π-extended organic sensitizers.

Full text of DOI:10.1002/cssc.201402164

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DOI: 10.1002/cssc.201402164
Enhancing Dye-Sensitized Solar Cell Performances by
Molecular Engineering: Highly Efficient p-Extended
Organic Sensitizers
Roberto Grisorio,[a] Luisa De Marco,*[b] Rita Agosta,[b] Rosabianca Iacobellis,[b, d]
Roberto Giannuzzi,[b] Michele Manca,[b] Piero Mastrorilli,[a] Giuseppe Gigli,[b, c, d] and
Gian Paolo Suranna*[a]
This study deals with the synthesis and characterization of two
p-extended organic sensitizers (G1 and G2) for applications in
dye-sensitized solar cells. The materials are designed with a D–
A–p–A structure constituted by i) a triarylamine group as the
donor part, ii) a dithienyl-benzothiadiazole chromophore fol-
lowed by iii) a further ethynylene-thiophene (G1) or ethyny-
lene-benzene (G2) p-spacer and iv) a cyano-acrylic moiety as
acceptor and anchoring part. An unusual structural extension
of the p-bridge characterizes these structures. The so-config-
ured sensitizers exhibit a broad absorption profile, the origin
of which is supported by density functional theory. The ab-
sence of hypsochromic shifts as a consequence of deprotona-
tion as well as notable optical and electrochemical stabilities
are also observed. Concerning the performances in devices,
electrochemical impedance spectroscopy indicates that the
structural modification of the p-spacer mainly increases the
electron lifetime of G2 with respect to G1. In devices, this fea-
ture translates into a superior power conversion efficiency of
G2, reaching 8.1%. These results are comparable to those re-
corded for N719 and are higher with respect to literature con-
geners, supporting further structural engineering of the p-
bridge extension in the search for better performing p-extend-
ed organic sensitizers.
Introduction
Fully organic sensitizers[1] continue to capture the attention of
the scientific community devoted to the development of dye-
sensitized solar cells (DSSC)[2] as a competitive alternative to
ruthenium(II)-based dyes.[3] Purely organic sensitizers bring ad-
vantages in terms of availability of the starting materials,
design flexibility, and higher molar absorptivity with respect to
the inorganic dyes, providing access to several possible archi-
tectures and clearing the path towards more and more effi-
cient DSSCs. However, compared to ruthenium dyes, most or-
ganic sensitizers generally exhibit narrower absorption bands
at shorter wavelengths, resulting in less effective light-harvest-
ing. This disadvantage is related to their commonly accepted
design rule (relying on the well-known D–p–A concept) in
which an electron-donating group (D) and an electron-with-
drawing group (A) are connected through a suitable p-spacer
(p).[4] In fact, small p-bridges are usually preferred in compari-
son to more p-extended spacers that do increase the portion
of absorbed sunlight but, at the same time, facilitate the for-
mation of undesired aggregates[5] and are relatively unstable
when irradiated with high-energy photons.[6] However, the for-
mation of dye aggregates can be limited by the use of suitable
co-adsorbents,[7] although with unpredictable results in terms
of device efficiency. Therefore, to further improve the per-
formance of an organic dye, new design concepts should prin-
cipally aim at achieving a broad spectral response, while at the
same time preserving the optical and electrochemical stability
of the molecule.
[a] Dr. R. Grisorio,+ Prof. P. Mastrorilli, Prof. G. P. Suranna
DICATECh—Dipartimento di Ingegneria Civile, Ambientale, del Territorio,
Edile e di Chimica
Politecnico di Bari
Via Orabona, 4 I-70125 Bari (Italy)
E-mail: surannag@poliba.it
[b] Dr. L. De Marco,+ Dr. R. Agosta, Dr. R. Iacobellis, Dr. R. Giannuzzi,
Dr. M. Manca, Prof. G. Gigli
Center for Biomolecular Nanotechnology (CBN)
Fondazione Istituto Italiano di Tecnologia (IIT)
Via Barsanti 1, 73010, Arnesano (Italy)
E-mail: luisa.demarco@iit.it
[c] Prof. G. Gigli
NNL—Istituto Nanoscienze, CNR c/o distretto tecnologico Lecce
via Arnesano 16, 73100 Lecce (Italy)
[d] Dr. R. Iacobellis, Prof. G. Gigli
Department of Mathematics and Physics “E. De Giorgi”
University of Salento, Campus Universitario
via Monteroni, 73100 Lecce (Italy)
To this end, the incorporation of an electron-withdrawing
unit into the p-bridge of a new generation of organic dyes (in-
dicated as D–A–p–A structures) can conveniently modulate the
breadth of the light-harvesting range, introducing new elec-
tronic transitions with an intramolecular charge-transfer char-
[+] These authors contributed equally to the work.
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201402164.
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Results and Discussion
acter.[8] Furthermore, placing an acceptor moiety within the p-
bridge might lead to a remarkable improvement of the dye
photo-stability by increasing its oxidation potential.[9] Among
the electron-withdrawing structures (such as benzotriazole,[10]
quinoxaline,[11] phtalimide,[12] or diketopyrrolopyrrole[13]) suc-
cessfully employed for the preparation of low band-gap organ-
ic sensitizers, benzothiadiazole can certainly be considered the
most widespread unit for the obtainment of high photovoltaic
performance.[14] Until now, however, little attention has been
paid to the role of the p-conjugated spacer between the two
acceptor units. Even a small modification of its structure could,
in fact, influence both the photophysical properties of the dye
and the degree of its intermolecular interactions.
Synthesis
The synthetic sequence followed for the obtainment of G1 and
G2 is illustrated in Scheme 1. A selective mono-bromination of
4,7-bis-thiophen-2-yl-benzo[2,1,3]thiadiazole was achieved with
an equimolar amount of N-bromo-succinimide (NBS) to afford
the corresponding bromo-derivative 1 which, in turn, was con-
verted into the corresponding iodo-derivative 2 by reaction
with N-iodo-succinimide (NIS). A Suzuki coupling between 4-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-N,N-bis(4-(2-ethyl-
hexyloxy)phenyl)-aniline and 2 allowed the binding of the triar-
ylamine electron-donating group to the benzothiadiazole chro-
mophore, with the obtainment of 3, endowed with the bro-
mide leaving group. The introduction of the ethynylene
moiety was accomplished by a Pd-catalyzed Sonogashira cross-
coupling between 7 and trimethylsilyl-acetylene, to yield the
alkyne 4. The trimethylsilyl protecting group in 4 was easily re-
moved by reaction with potassium fluoride affording the ter-
minal alkyne 5. At this stage, the synthetic procedure diverged
for the obtainment of G1 and G2. The assembly of the p-
bridges was, in fact, completed by a further Sonogashira cross-
coupling of 5 with either 5-bromo-thiophene-2-carbaldehyde
or with 4-bromobenzaldehyde to obtain aldehydes 6 and 7, re-
spectively. The introduction of the cyano-acrylic functionality
was carried out subjecting 6 and 7 to a Knoevenagel conden-
sation with cyanoacetic acid in the presence of ammonium
acetate eventually yielding the target molecules G1 and G2, re-
spectively. The chemical structures of the dyes as well as those
of their precursors were confirmed by a combination of ele-
mental analysis, multinuclear NMR spectroscopy, IR spectrosco-
py, and mass spectrometry.
Within this context, we embarked in the synthesis, character-
ization, and application in DSSC of the new p-extended benzo-
thiadiazole-based structures (G1 and G2, Figure 1), aiming at
evaluating the impact of the p-bridge extension of the dye on
the optical properties and device performances. In particular,
the synthetic design envisaged the functionalization of a di-
thienyl-benzothiadiazole chromophore with a suitable elec-
tron-donating triarylamine group on one side and with an eth-
ynyl-thiophene (G1) or with an ethynyl-benzene (G2) spacer,
both carrying the cyano-acrylic anchoring group, on the other
side. With respect to the low band-gap D–A–p–A sensitizer
proposed by Grꢁtzel and Kim (HKK-BTZ4, Figure 1),[14g] the in-
troduction of the ethynylene-arylene spacer was motivated by
its tendency to favor the electron-transfer process,[15] promot-
ing the planarity of the structure and the electronic communi-
cation between the donor and the acceptor parts of the mole-
cules. Compared to G1, the apparently innocent structural
modification of the p-bridge (replacement of the thiophene
unit with the benzene one) resulted in a remarkable improve-
ment of the power conversion efficiencies of DSSC devices em-
bodying G2, reaching a value of 8.1% representing one of the
best result thus far reported for p-extended organic sensitizers.
Photophysical and electrochemical properties
The UV/Vis absorption spectra of G1 and G2 measured in THF
are shown in Figure S1A, Supporting Information. The absorp-
Figure 1. Chemical structure of G1, G2, and the congener HKK-BTZ4 (R=2-ethyl-hexyl).
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Scheme 1. Synthetic sequence for the obtainment of G1 and G2. i: NBS, DMF, 08C (35%); ii: NIS, CH2Cl2, CH3COOH, rt (62%); iii: 4-(4,4,5,5-tetramethyl-1,3,2-di-
oxaborolan-2-yl)-N,N-bis(4-(2-ethyl-hexyloxy)phenyl)-aniline (1 equiv), Pd(PPh3)4, THF, 1m NaHCO3, reflux (74%); iv: trimethylsilyl-acetylene, PdCl2(PPh3)2, CuI,
Et2NH, reflux (55%); v: KF, CH2Cl2, MeOH, 558C (88%); vi: 5-bromo-thiophene-2-carbaldehyde, Pd(PPh3)4, CuI, Et2NH, reflux (59%); vii: 4-bromobenzaldehyde,
Pd(PPh3)4, CuI, Et2NH, reflux (53%); viii: cyano-acetic acid, CH3COONH4, CH3COOH, reflux; yield: 40% (G1) and 38% (G2); R=2-ethyl-hexyl.
tion profiles of G1 and G2 are characterized by two main ab-
sorption bands, in analogy with other benzothiadiazole-based
organic dyes.[14]
ticular, that the presence of the thiophene unit in G1 seems to
bring about stronger intermolecular interactions, favoring ac-
cumulated p–p interactions and leading to dye aggregation.
The solvatochromic behavior of an organic sensitizer is par-
ticularly useful in order to anticipate the modifications of the
absorption profile consequent to the dye adsorption on TiO2.
Since in the absorption process the free ꢀCOOH group is very
likely converted into a titanium carboxylate,[16] a detrimental
blue-shift of the dye absorption maximum can commonly be
observed, caused by the lower electron acceptor character of
the carboxylate moiety with respect to the carboxylic acid
group. In order to rule out aggregation and scattering effects,
it was chosen to record the UV/Vis spectra of the dyes in THF
solution and in the presence of an excess of a base (triethyla-
mine, TEA) or of an acid (formic acid), as shown in Figure S2.
The use of a large excess (ꢁ105 mol per mol) of TEA was con-
ceived to directly investigate in solution the optical behavior
of the deprotonated form of the dye. We were pleased to find
that the absorption profiles of G1 and G2 recorded either in
the presence or in the absence of TEA were very similar and,
practically superimposable in the visible region. It can be de-
duced that, in these D–A–p–A structures, the presence of ben-
zothiadiazole as additional electron-withdrawing group as well
as of the further p-bridge extension, reduces the contribution
of the carboxylic functionality to the electronic transitions,
thereby preserving the light-harvesting properties of these
molecules from the blue shift induced by the deprotonation.
These results were substantiated by the optical responses of
G1 and G2 in the presence of formic acid. Again, the absence
Concerning the lower energy band, the structural modifica-
tion of the p-bridge did not lead to substantial differences in
the absorption maxima of the two dyes, observed at 539 and
538 nm for G1 and G2, respectively. Consequently, their ab-
sorption onset in THF solution was very similar (636 nm). Fur-
thermore, the broad absorption profile in the visible region
was also accompanied by high molar extinction coefficients
(e=24000 and 20000mꢀ1 cmꢀ1 for G1 and G2, respectively).
The absorption peak in the UV region was characterized by
a relatively higher intensity and appeared at 373 nm for both
molecules (e=28500mꢀ1 cmꢀ1 and 23200mꢀ1 cmꢀ1 for G1 and
G2, respectively).
Considering that the absorption spectra of organic sensitiz-
ers on mesoporous TiO2 can be heavily affected by scattering
effects and in order to predict the aggregation behavior of the
p-extended dyes once absorbed onto the TiO2 surface, the op-
tical properties of G1 and G2 were investigated in the solid
state as thin films on quartz substrates. Significant differences
can be observed in the optical behavior of the two dyes in the
solid state: in fact, inspection of Figure S1B in the Supporting
Information reveals a red-shift of the absorption maximum and
a remarkable broadening of the absorption profile of the thio-
phene-based dye G1 (lmax =551 nm) with respect to its ben-
zene-based counterpart G2 (lmax =541 nm). This result indi-
cates that the nature of the p-bridge has a remarkable influ-
ence on the solid state properties of the materials and, in par-
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of a significant modification in the absorption profiles rules
out a dependence of the dyes absorption behavior on the pro-
tonated or deprotonated form of the carboxylic functionality.
Another photophysical issue to be investigated for a promis-
ing organic sensitizer is the dye optical stability under the op-
erating conditions of the device. An appropriate photo-stability
accelerated test was therefore carried out. The photochemical
stability of G1 and G2 was, therefore, studied by submitting
a film of the dye to a high intensity UV-irradiation in air for
20 min. The high-energy photons contained in the UV radia-
tion constitute a significant portion (ꢁ5%) of the solar spec-
trum and can be responsible for a degradation of the dye, in-
ducing modifications on its absorption intensity and profile.[17]
Moreover, possible degradation mechanisms of the dye might
involve radical pathways that can be promoted by singlet
oxygen formation under UV-light irradiation.[18] The photo-oxi-
dation treatment (Figure S3) did not promote substantial
changes of either the absorbance or the absorption profile of
G1 and G2. Although some regions of the sensitizers, such as
the electron-rich triarylamine group or the ethynylene moiety,
can be considered reasonably susceptible to an easy oxidative
degradation, the presence of the electron-withdrawing benzo-
thiadiazole unit in the structure of the extended p-bridge
seems to impart a remarkably high intrinsic photo-stability to
the molecules.
dye might also occur in its oxidized state. In solid state CV ex-
periments, the molecules of the first active layer are not re-
placed at the electrode interface and, therefore, repeated
anodic scans mimic the oxidation and regeneration cycles oc-
curring in the device. As evident from Figure S8, the CV anodic
waves of G1 and G2 did not undergo substantial modifications
after repeated scans, revealing their electrochemical stability
under oxidative conditions in the time scale of the experiment.
This result hints that no decomposition occurs while G1 and
G2 are in their oxidized states. It is reasonable to suppose that,
in these sensitizers, the sufficient stabilization of the dye oxi-
dized form, as a result of hole delocalization, inhibits degrada-
tion pathways, which, conversely, are highly favored when
holes are localized on specific carbon atoms.[19] Considering
that, during a single CV oxidation scan, the formed cations per-
sist for approximately 2–3 s and that seven oxidation cycles
were performed, G1 and G2 remained stable in their oxidized
states for ꢁ15–20 s. Applying Katoh’s evaluation[6a] to the elec-
trochemical stability, a dye lifetime of 15–20 year can be con-
jectured for G1 and G2.
Theoretical calculations
In order to provide a theoretical basis to the structure–proper-
ty relationships emerging from the previously described inves-
tigations, as well as to properly discuss the behavior of the
synthesized dyes in DSSC devices, we investigated appropriate
models of G1 and G2 by density functional theory (DFT) and
time-dependent DFT (TD-DFT) calculations. The calculations
were carried out at the ground state on isolated molecules at
the B3LYP/6-31G(d,p) level of the theory. To reduce the compu-
tational load, the 2-ethyl-hexyl chains were replaced by methyl
groups. The study of the optimized geometry of the molecules
was addressed first. It was found that the torsion angles be-
tween the aryl planes present on the p-bridges ranged be-
tween 0.428 and 1.918, indicating their near-planarity, favored
by the presence of the ethynylene spacers between the aryl
moieties.[20]
In order to verify the feasibility of the electron injection
from the excited states of an organic dye to the TiO2 conduc-
tion band and of the possibility of dye regeneration by means
of the electrolyte redox couple, the estimation of the HOMO
(highest occupied molecular orbital) and LUMO energy values
is mandatory. To this end, cyclic voltammetry (CV) techniques
are commonly employed. A scan of the anodic region (Fig-
ure S4 and S5) revealed that G1 and G2 exhibit three subse-
quent oxidation events. However, due to the relevant contribu-
tion of the electron-donating triarylamine group to the HOMO
electronic distribution (see theoretical calculations), only the
first oxidation event (Figure S6 and S7) is reversible for both
dyes. The evaluation of the HOMO energy levels (see the Ex-
perimental Section) led to very similar values (ꢀ5.04 and
ꢀ5.03 eV) for G1 and G2. The obtained results suggest that, in
DSSCs, an efficient regeneration of the oxidized dyes by the
iodide/triiodide couple is thermodynamically feasible, thus re-
ducing the possibility of geminated recombination. A similar
behavior is observed for the cathodic process of G1 and G2,
characterized by three reduction events, of which only the first
is fully reversible (Figure S5 and S6). The obtained LUMO
values of G1 and G2 are ꢀ3.16 and ꢀ3.18 eV, respectively. The
calculated LUMO energy values lie above the conduction band
edge of TiO2 (ꢀ4.0 eV), ensuring sufficient driving force for
electron injection from the excited dyes into the conduction
band of the inorganic semiconductor. Furthermore, the ob-
tained electrochemical band-gap of G1 and G2 (1.88 and
1.85 eV, respectively) is in good agreement with that obtained
by optical measurements.
The calculated electron density distributions of the main en-
ergetic levels along with the optimized geometry obtained for
G1 and G2 are shown in Figure S9. Their HOMOs are very simi-
lar and mainly distributed on the electron-rich triarylamine
group as well as on part of the p-bridge, suggesting that the
electron binding energy for the HOMO is not sensitive to the
structural difference between the two dyes, as observed for
the measured redox energies. Conversely, the LUMOs of G1
and G2 are mainly described by the p-A segment and, there-
fore, localized on the anchoring group, well separated from
the HOMO electronic distribution. To overcome the approxima-
tion of the Koopmans’ theorem, the ionization potential
(minus HOMO) of G1 and G2 was calculated as the difference
between the total energies of the N-1-electron and N-electron
states, while their electron affinities (minus LUMO) as the dif-
ference between the total energies of the N-electron and N+
1-electron states.[21] All energies were calculated starting from
the geometrically optimized structures of both dyes. The calcu-
lations returned the same results for the ionization potentials
Together with the photo-stability, the electrochemical stabili-
ty of an organic sensitizer is a critical factor in assessing the
potential lifetime of a DSSC, since degradation of an organic
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of G1 and G2 (ꢀ5.65 eV), while a slight difference
was observed for their electron affinities (ꢀ2.27 and
ꢀ2.15 eV for G1 and G2, respectively). Despite the
discrepancy between theoretical and experimental
values of ionization potential and electron affinities,
attributable to the fact that calculations were carried
out in vacuo, the trend observed for the calculated
results strictly followed that experimentally obtained
by electrochemistry. Furthermore, it is of interest to
gain insights into the geometric localization of the
Table 1. Excitation energies, oscillator strength (f), and configuration of the main cal-
culated transitions of G1 and G2.
Dye l [nm]
f
Configuration
G1
G2
538
416
527
395
1.93 H!L (50%); H-1!L (30%); H!L+1 (13%)
0.70 H-1!L (32%); H!L+1 (30%); H-2!L (23%); H-1!L+1 (14%)
1.66 H!L (53%); H-1!L (30%); H!L+1 (13%)
0.75 H-1!L (42%); H!L+1 (29%); H-2!L (19%)
hole of the oxidized dyes (described by the LUMO of the N-1-
electron state) since an accepting molecular orbital of the oxi-
dized sensitizer placed in proximity of the anchoring group
might promote geminate recombination during operation of
the DSSC, consequently lowering the overall performances.[14e]
As evidenced in Figure S10, the hole of the oxidized G2 is
mainly localized on the dithienyl-benzothiadiazole segment of
the p-bridge and, due to the presence of the ethynylene-phen-
ylene spacer, distinctly separated from the anchoring portion
of the sensitizer. Conversely, due to the presence of a lower
energy unit (thiophene vs benzene), the hole of the oxidized
G1 seems to be delocalized along the entire p-bridge and ex-
tending up to the anchoring moiety. These results suggest
more relevant geminate recombination effects for G1 with re-
spect to G2 in DSSC devices. The stronger intermolecular inter-
actions, indicated by the solid
non-negligible contributions of HOMO!LUMO+1 and
HOMO-2!LUMO character. The calculated natural transition
orbitals (Figure 2) show that the photo-dynamics of the two
main excitations (S0!S1 and S0!S2) implies a partial shift of
the electron density towards the acceptor unit of the dyes, fa-
voring the electron transfer at the titania/dye interface.
Photovoltaic performances and electrochemical impedance
analysis
Several favorable characteristics emerge, in terms of potential
photovoltaic properties, from the characterization of G1 and
G2 discussed so far, in particular, concerning their light har-
vesting capability as well as electrochemical and photochemi-
cal stability. Therefore, the properties of the two dyes were
state optical properties of G1 are
supported by its larger calculat-
ed dipole moment (12.5 Debye)
with respect to that calculated
for G2 (9.2 Debye).
A
deeper understanding of
the optical behavior of the two
sensitizers was provided by the
calculation of the vertical S0!Sn
excitation energies at the TD-
DFT level of theory using the
CAM-B3LYP functional. Table 1
shows that the experimental and
calculated optical data, in terms
of both excitation energies and
oscillator strength, are in good
agreement. According to the cal-
culation, for G1 and G2, the ex-
citation in the visible region cor-
responds to a S0!S1 transition,
predominantly described as
HOMO!LUMO, although with
additional
and
significant
HOMO-1!LUMO and HOMO!
LUMO+1 contributions. The
S0!S2 transition, corresponding
to the absorption experimentally
observed in the UV region, is
mainly ascribed to a HOMO-1!
LUMO transition for both G1
and G2, although with other Figure 2. Calculated natural transition orbitals defining the photo-excitation dynamics in G1 and G2.
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tested in photo-electrochemical solar cells and their perform-
ances have also been investigated by electrochemical impe-
dance spectroscopy (EIS).
of CDCA, are shown in Figure 3 and Figure 4, respectively. Also
in the case of the G2-based devices, the use of CDCA signifi-
cantly enhanced the performances: the JSC was, in fact, in-
creased from 10.4 mAcmꢀ2 to 15.9 mAcmꢀ2. This result can be
attributed to the higher electron injection efficiency in the de-
vices obtained by G2/CDCA co-deposition. As expected, an in-
Due to the high planarity of the sensitizers, resulting from
the presence of the ethynyl-thiophene (G1) and of the ethynyl-
benzene (G2) spacers, a fast intramolecular charge-transfer
should be expected as well as an unfavorable p–p stacking
during the uptake process, which may lead to intermolecular
quenching or back transfer of the injected electrons from the
TiO2 conduction band. On this basis, the use of chenodeoxy-
cholic acid (CDCA) as coadsorbent was evaluated. By compet-
ing with the dye for binding on the titania surface, the CDCA
can, in fact, fill the gaps between the bulky sensitizers, thus
minimizing detrimental dye aggregation effects.[14] Moreover,
the use of a co-adsorbent such as CDCA ensures a uniform
coverage of the inorganic semiconductor, thereby lowering the
probability of recombination between injected electrons with
I3ꢀ and other acceptor species.[22]
Bearing this in mind, we focused on the fabrication of G1-
and G2-based DSSC by the implementation of the co-adsorp-
tion strategy. We started by preparing 0.2 mm solutions of G1
in THF with CDCA concentrations spanning the range 0–
100 mm and systematically studied the influence of the
amount of additive on the photovoltaic parameters, as shown
in Table 2. Despite the decrease in dye loading with the in-
crease in CDCA concentration (Table 2), the performances (Fig-
ure S11) of the G1-sensitized device were improved. In particu-
lar, the open-circuit voltage (VOC) increased from 0.64 up to
0.69 V and the short-circuit photocurrent density (JSC) raised
from 7.4 up to 12.1 mAcmꢀ2, with the best results at a CDCA
concentration of 80 mm. This amount of CDCA allows to ach-
ieve a power conversion efficiency (PCE) of 6.0%, while that
obtained for the G1-sensitized devices without the addition of
CDCA did not exceed 3.5% due to the effect of dye aggrega-
tion. A further increase of the CDCA concentration (100 mm)
led to a decrease of both JSC and VOC, suggesting that, in these
conditions, the additive unfavorably competes with the dye
molecules for the anchoring sites.
Figure 3. J–V curves of the best devices based on G1 and G2 dyes used as
reference without or with CDCA (80 mm) under 1 sun illumination.
After the screening carried out on G1, we decided to opti-
mize the photovoltaic performances of G2 (Table 2) using
CDCA at 80 mm concentration. The J–V curves and the incident
photon-to-current conversion efficiency (IPCE) spectra of
DSSCs based on G1 and G2 dyes, in the presence and absence
Figure 4. IPCE spectra of the best devices based on G1 and G2 dyes without
or with CDCA (80 mm).
Table 2. Photovoltaic parameters for DSSCs based on G1 and G2 dyes.[a]
Dye
CDCA [mm]
PCE [%]
VOC [V]
JSC [mAcmꢀ2
]
FF[b]
Dye loading [molcmꢀ2
]
G1
0
10
60
80
100
0
80
0
80
3.5 (3.3ꢂ0.2)
4.1 (3.9ꢂ0.2)
5.2 (5.0ꢂ0.2)
6.0 (5.6ꢂ0.4)
4.9 (4.6ꢂ0.3)
5.1 (4.9ꢂ0.2)
8.1 (7.9ꢂ0.2)
8.2 (8.1ꢂ0.2)
2.7 (2.6ꢂ0.1)
0.64 (0.65ꢂ0.01)
0.66 (0.67ꢂ0.01)
0.69 (0.69ꢂ0.01)
0.69 (0.69ꢂ0.01)
0.66 (0.65ꢂ0.03)
0.68 (0.67ꢂ0.02)
0.71 (0.71ꢂ0.01)
0.75 (0.75ꢂ0.01)
0.76 (0.77ꢂ0.01)
7.4 (7.1ꢂ0.3)
0.72 (0.70ꢂ0.02)
0.73 (0.72ꢂ0.01)
0.72 (0.72ꢂ0.02)
0.71 (0.69ꢂ0.02)
0.72 (0.69ꢂ0.03)
0.73 (0.71ꢂ0.03)
0.72 (0.72ꢂ0.01)
0.74 (0.73ꢂ0.04)
0.83 (0.80ꢂ0.03)
3.0ꢂ10ꢀ7
2.4ꢂ10ꢀ7
1.8ꢂ10ꢀ7
1.2ꢂ10ꢀ7
0.9ꢂ10ꢀ7
3.4ꢂ10ꢀ7
1.3ꢂ10ꢀ7
2.1ꢂ10ꢀ7
0.2ꢂ10ꢀ7
8.4 (8.1ꢂ0.5)
10.4 (10.0ꢂ0.4)
12.2 (11.9ꢂ0.4)
10.3 (10.2ꢂ0.3)
10.4 (10.3ꢂ0.6)
15.9 (15.3ꢂ0.6)
14.8 (14.8ꢂ0.7)
4.2 (4.2ꢂ0.3)
G2
N719
[a] Average values in parentheses are based on three replicate measurements. [b] Fill factor.
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crease in VOC (from 0.68 V to 0.71 V) was also observed for the
G2/CDCA based devices, due to the attenuation of the recom-
bination processes with the electrolyte. As a consequence, the
PCE remarkably increased from 5.1% to 8.1%, an efficiency
value almost equal (99%) to that obtained for the reference
devices based on commercial N719 (8.2%), giving the highest
performance in the absence of the co-adsorbent. Remarkably,
the efficiency of G1 was slightly higher than those reported for
HKK-BTZ4, clearly pointing out the effect of the p-bridge ex-
tension.
crease the interfacial resistance of the obtained device, as indi-
cated by the higher VOC measured (Table 2), mainly due to the
blocking of recombination sites. On the other hand, a counter-
intuitive trend of the RCT was evidenced for the G2-based pho-
toanodes: the G2/CDCA devices showed lower RCT values with
respect to those in which no co-adsorbent was used. This
result can be interpreted by comparing the recombination re-
sistances at a similar number of injected electrons (i.e., at the
same energy difference between the electron Fermi level and
the TiO2 conduction band).[26] The potential scales of Figure S12
and S13 have therefore been corrected in order to take into
account the shift of the conduction band due to the electron
injection. In Figure S14 and S15, the reported potential scale is
the equivalent conduction band voltage (Vecb); the reference
dye is G2 without CDCA. Inspection of Figure S15 confirms
that CDCA-based devices show the highest RCT values, indicat-
ing a better suppression of charge recombination effects and
explaining the higher VOC observed for CDCA-based devices
despite the lower value of the conduction band edge.
The molecular structures of G1 and G2 differ only by the
ethynylene-thiophene or ethynylene-benzene spacer, respec-
tively, placed between the dithienyl-benzothiadiazole unit and
the anchoring group. Nevertheless, the small structural modifi-
cation on the p-bridge has a considerable effect on the DSSC
performances. In fact, despite the slightly higher molar extinc-
tion coefficient of G1, which should lead to a better light har-
vesting capability, the G2-based devices showed slightly better
performances. The superior JSC value for G2 also emerged from
the IPCE spectra (Figure 4), which showed a plateau of approxi-
mately 80% at 550 nm for the G2/CDCA-based device com-
pared to the plateau of approximately 60% obtained for the
G1/CDCA-based one at 550 nm. Given that, taking the absorp-
tion and reflection of the conductive glass into account,[14f] the
IPCE should not exceed the limit of ꢁ80–90%, it can be con-
cluded that G2 presents nearly unity IPCE values in the visible
region.
The electron lifetime (tn) in a DSSC is a fundamental parame-
ter to assess the recombination dynamics and is defined as the
average lifetime of free and trapped electrons in the TiO2
before recombination with the dye oxidized form or with the
oxidant in the electrolyte. It is calculated by the following
equation: tn =RCTCm. The trend of the apparent electron life-
time, shown in Figure 5, is in agreement with that revealed by
the photovoltaic performance.
To further investigate this aspect and the relationship be-
tween co-adsorption of CDCA and photovoltaic performance,
the charge-transfer resistance and the capacitance at the TiO2/
dye/electrolyte interface were measured by EIS.[23] Since all de-
vices were fabricated and tested in the same conditions, only
the differences in the molecular structure of the sensitizers in-
fluenced the electrochemical parameters. EIS spectra were ana-
lyzed through the well-known equivalent circuit.[24] All electro-
chemical parameters have been plotted as a function of the
corrected potential; the measured potential needs, in fact, to
be corrected in order to take into account the losses due to
the total series resistance, which lead to a potential drop that
is not associated with the displacement of the Fermi level.[25]
Figure S12 and Figure S13 show the measured capacitance
(Cmeas) and the charge-transfer resistance (RCT) as a function of
corrected voltage for a set of devices based on G1 and G2 ab-
sorbed on TiO2 in the absence or in the presence of CDCA
(80 mm). From the analysis of Cmeas (Figure S12), it can be no-
ticed that the G2/CDCA-based devices show the most relevant
downward shift of the TiO2 conduction band edge. As it is
well-known, the lower value of the conduction band facilitates
the electron injection from the dye to the TiO2 resulting in
higher photocurrents. Moreover, this downward shift decreases
the energy difference with the potential of the redox mediator,
leading to a decrease in VOC.
The higher electron lifetime observed for the G1/CDCA- and
G2/CDCA-based DSSC confirms that the use of the coadsorb-
ent leads to a suppression of the dark current. Comparing G1-
and G2-based devices under the same test condition, G2
shows longer electron lifetimes with respect to G1, implying
that the recombination phenomena (geminate recombination
and dark current) are considerably suppressed as also substan-
tiated by the higher VOC values. From these results, it can be
deduced that the electron transfer towards the TiO2 is more fa-
vored for G2 in comparison to G1. As suggested by the dye
loading measurements, which indicate that the amount of the
dye adsorbed onto the TiO2 surface for G1 and G2 is very simi-
The lowest RCT was found for the G1-based devices without
CDCA, which confirms that injected electrons are more likely,
under these conditions, to undergo recombination with the
oxidized form of the redox species present in the electrolyte.
The addition of CDCA to the G1 dye solution was helpful to in-
Figure 5. Apparent electron lifetime as a function of the corrected potential
for DSSCs based on G1 and G2 dyes without or with CDCA (80 mm).
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lar, it is reasonable to suppose that the CDCA presence has
a lower disaggregating effect on G1 than on G2. In fact, it is
the higher tendency of G1 towards aggregation due to the
presence of the thiophene moiety (evidenced by the photo-
physical measurements) which can promote aradiative deacti-
vation pathways for the excited states, lowering the probability
of the electron injection.
bridge extension is a viable strategy to improve dye efficiency
and suggest analogous structural engineering of known low
band-gap dyes as a promising strategy to obtain even more ef-
ficient organic sensitizers.
Experimental Section
On the other hand, the enhanced and unwanted electron re-
combination observed for the G1-based devices might be clari-
fied by taking into account the potential interactions at the
dye/electrolyte interface between iodine and an electron-rich
heteroatom,[27] which lead to the formation of iodine-dye ad-
ducts through the following displacement reaction:
General remarks: All reactants were purchased from standard
commercial sources and used without any further purification. All
solvents used were carefully dried and freshly distilled according to
standard laboratory practice. All manipulations were carried out
under inert nitrogen atmosphere. The synthesis of 4-(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)-N,N-bis(4-(2-ethyl-hexyloxy)phenyl)-
aniline and 4,7-bis-thiophen-2-yl-benzo[2,1,3]thiadiazole were car-
ried out according to a literature procedure.[14g] Flash chromatogra-
phy was performed using a silica gel of 230–400 mesh. 1H and
13C{1H} NMR spectra were recorded on a Bruker Avance 700 MHz
instrument. Melting points were measured on a Bꢃchi B-545 instru-
ment. Elemental analyses were obtained on a EuroVector CHNS
EA3000 elemental analyser on the basis of three replicates. FTIR
measurements were carried out on a JASCO FTIR 4200 spectropho-
tometer. UV/Vis spectra were recorded on a Jasco V-670 instru-
ment. UV-photodecomposition experiments were carried out by ir-
radiating the sample with a 150 W high-pressure Hg lamp for
20 min. High resolution electrospray ionization mass spectrometry
(HR ESI-MS) analyses were performed using a Bruker microTOF QII
instrument equipped with an electrospray ion source. The sample
solutions (CH2Cl2/MeOH) were introduced by continuous infusion
at a flow rate of 180 mLminꢀ1 with the aid of a syringe pump. The
instrument was operated in positive (negative) ion mode with end-
plate offset and capillary voltages set to ꢀ500 V (500 V) and
ꢀ4500 V (3500 V), respectively. The nebulizer (N2) pressure was
0.8 bar, and the drying gas (N2) flow rate was 7.0 Lminꢀ1. The capil-
lary exit and skimmer voltages were 90 V and 30 V, respectively.
The drying gas temperature was 1808C. The calibration was carried
out with sodium formate. Cyclic voltammetry was carried out on
a Metrohm Autolab PGSTAT 302-N potentiostat. The materials were
drop cast on a platinum working electrode from a 1 mgmLꢀ1 THF
solution. Measurements were carried at 258C in acetonitrile solu-
tion containing tetrabutylammonium tetrafluoroborate (0.025m) as
supporting electrolyte with a scan rate of 50 mVsꢀ1. The potentials
were measured versus Ag/Ag+ as the quasi-reference electrode.
After each experiment, the potential of the Ag/Ag+ electrode was
calibrated against the ferrocene/ferrocenium (Fc/Fc+) redox
couple. The electrochemical energy gap was determined as the dif-
ference between the onsets of the oxidation and the reduction po-
dye þ I3 ! dye-I2 þ Iꢀ:
ꢀ
In the case of G1, the presence of the thiophene unit in
proximity to the mesoporous semiconductor might favor the
onset of a dark current by electron transfer from the titania to
the coordinated iodine.
Conclusions
Aiming at evaluating the effect of the p-bridge extension on
their DSSC performances, two new low band-gap sensitizers
were designed with D–A–p–A structure containing: i) a triaryla-
mine electron-donating group as the donor part, ii) a dithienyl-
benzothiadiazole chromophore, iii) a further ethynylene-thio-
phene (G1) or ethynylene-benzene (G2) p-spacer and iv) a
cyano-acrylic moiety as acceptor and anchoring part. Regard-
ing the classical weak points of the organic sensitizers (i.e.,
light-harvesting, stability), the new configuration brings several
advantages: i) a broad absorption profile spanning the range
350–700 nm; ii) the absence of hypsochromic shifts (the origin
of which was substantiated by density functional theory) as
a consequence of deprotonation, ascribable to the uncommon
length of the p-bridge and the simultaneous presence of the
electron-withdrawing benzothiadiazole unit; iii) an appreciable
optical stability, ascertained by accelerated photo-degradation
test, indicating the preservation of the light-harvesting proper-
ties of the synthesized organic dyes in DSSC devices; iv) a con-
siderable electrochemical stability as thin films in the voltam-
metric oxidation/re-reduction cycles that suggest a remarkable
dye stability also in their oxidized form. The interpretation on
the optical and electrochemical properties of G1 and G2 were
supported by DFT calculations. Indeed, the p-bridge structural
modification on G1 and G2 seems to have a significant influ-
ence on dye aggregation and charge recombination, as point-
ed out by electrochemical impedance spectroscopy. In DSSC
devices, the higher electron injection efficiency and injected
electron lifetimes observed for G2 led to a superior power con-
version efficiency, reaching 8.1% obtained with a suitable
amount of a co-adsorbent (CDCA). Its performances are re-
markably higher than those exhibited by G1 (6.0%) and slight-
ly higher than those of literature congeners. Despite the seri-
ous problems of dye aggregation, overcome by the use of
high amounts of CDCA (80 mm), these results indicate that p-
onset
tentials (Egelc =Eox ꢀEredonset). The HOMO and LUMO energy
values were estimated from the onset potentials of the first oxida-
tion and reduction event, respectively. After calibration against Fc/
Fc+, the HOMO and LUMO energy levels were calculated according
to the following equations:[28]
onset
EHOMO ðeVÞ ¼ ꢀ½Eox
ELUMO ðeVÞ ¼ ꢀ½Ered
ꢀE1=2ðFc=FcþÞþ4:8ꢃ
ꢀE1=2ðFc=FcþÞþ4:8ꢃ
onset
where E1/2(Fc/Fc+) is the half-wave potential of the Fc/Fc+ couple
(the formal potential of which is assumed to be 4.8 eV in the Fermi
scale) against the Ag/Ag+ electrode. Analyses of the ground-state
structures for the molecules were carried out by density functional
theory (DFT) calculations. The B3LYP function was used in conjunc-
tion with the 6-31G(d,p) basis set. Time-dependent DFT (TD-DFT)
was applied for the assessment of the excited-state transition ener-
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gies. All calculations were carried out with the Gaussian 09 pro-
gram package[29] and performed on isolated molecules in vacuo.
Fabrication of DSSC and photovoltaic measurement: Fluorine-
doped tin oxide (FTO, 15 W sqꢀ1, provided by Solaronix S.A.) glass
plates were first cleaned in a detergent solution using an ultrasonic
bath for 15 min, and then rinsed with water and ethanol. Double-
layer electrodes (overall thickness 19 mm) were prepared as fol-
lows: a layer of commercial colloidal titania paste (Dyesol 18NR-T)
was deposited onto the FTO glass and gradually heated in an oven
in air to obtain a ~14 mm transparent nanocrystalline film; the tem-
perature gradient was programmed as follows: 1708C (40 min),
3508C (15 min), and 4308C (30 min). This procedure was repeated
for the scattering layer (5 mm) constituted by Solaronix D/SP colloi-
dal paste. The double-layer electrode was eventually sintered at
4508C for 30 min. The thickness and the active area (0.16 cm2) of
the sintered photoanodes was measured using a profilometer
(Veeco Dektak 150 Surface Profiler). The dye loading was per-
formed by keeping the electrodes for 14 h and under dark in
0.2 mm THF solutions of G1 and G2 containing, where needed,
a known amount of chenodeoxycholic acid (CDCA). The reference
photoanodes were prepared analogously by dyeing the electrodes
with 0.2 mm solutions of bis(tetrabutylammonium)-cis-di(thiocya-
nato)-N,N’-bis(4-carboxylato-4’-carboxylic acid-2,2’-bipyridine) ruth-
enium(II) (N719, purchased by Solaronix S.A.) in a mixture of aceto-
nitrile and tert-butyl alcohol (1:1 v/v). In order to evaluate the dye
loading capability, the sensitized photoelectrodes were placed in
a solution of tetrabutylammonium hydroxide in DMF (0.05m) for
14 h, whereas the N719-sensitized photoelectrodes were immersed
in a NaOH aqueous solution (0.01m). The complete desorption was
testified by the substrate discoloration. The evaluation of the dye
concentration in the solvent, obtained by UV/Vis measurements, al-
lowed the calculation of the amount of the adsorbed sensitizer, ex-
pressed in terms of moles of dye anchored per projected unit area
of the photoelectrode. The counter-electrodes were prepared by
sputtering a 50 nm Pt layer on a hole-drilled cleaned FTO plate. In
a typical device construction procedure, the photoanode and the
counter-electrode were faced and assembled using a suitably cut
50 mm thick Surlyn hot-melt gasket for sealing. The redox electro-
lyte (0.1m LiI, 0.02m I2, 0.6m 1-methyl-3-propylimidazolium iodide,
and 0.5m tert-butylpyridine in dry acetonitrile) was vacuum-inject-
ed into the space between the electrodes through pre-drilled
holes on the back of the counter electrode. The holes were eventu-
ally sealed using Surlyn hot melt film and a cover glass. Photocur-
rent–voltage measurements were performed using a Keithley unit
(Model 2400 Source Meter). A Newport AM 1.5 Solar Simulator
(Model 91160A equipped with a 1000 W xenon arc lamp) serving
as a light source. The light intensity (or radiant power) was calibrat-
ed to 100 mWcmꢀ2 using as reference a Si solar cell. A mask with
an aperture area of 0.25 cm2 was applied to the devices before
measurements. The IPCE was measured by the DC method using
a computer-controlled xenon arc lamp (Newport, 140 W, 67005)
coupled with a monochromator (Newport Cornerstore 260 Oriel
74125). The light intensity was measured by a calibrated silicon
UV-photodetector (Oriel 71675) and the short circuit currents of
the DSSCs were measured by using a dual channel optical power/
energy meter, (Newport 2936-C). EIS was performed by an AUTO-
LAB PGSTAT 302N (Eco Chemie B.V.) in a frequency range between
100 kHz and 10 mHz. The impedance measurements were carried
out at different voltage biases under 1.0 sun illumination. The re-
sulting impedance spectra were fitted by using ZView (Scribner As-
sociates) software.
4,7-bis-thiophen-2-yl-benzo[2,1,3]thiadiazole (1.51 g, 5.00 mmol) in
DMF (45 mL) kept at 08C. After the addition, the obtained reaction
mixture was warmed to room temperature and allowed to react
for further 2 h before quenching with water (50 mL). The com-
pound was extracted with diethyl ether (3ꢂ50 mL) and the com-
bined organic phases were washed with brine and dried over
Na2SO4. After solvent removal, the crude product was purified by
column chromatography (SiO2, petroleum ether 40–608C/CH2Cl2 =
4:1 v/v) followed by crystallization with ethanol to afford 1 (0.65 g,
1
35%) as a red solid (mp: 117.5ꢂ0.58C). H NMR (700 MHz, CDCl3):
d=8.11 (dd, J=3.7, 1.1 Hz, 1H), 7.86–7.73 (m, 3H), 7.46 (dd, J=5.0,
1.0 Hz, 1H), 7.20 (dd, J=5.0, 3.9 Hz, 1H), 7.14 ppm (d, J=3.9 Hz,
1H). 13C{1H} NMR (176 MHz, CDCl3): d=152.6, 152.2, 140.8, 139.3,
130.8, 128.1, 127.6, 127.4, 126.9, 126.4, 125.8, 125.1, 125.0,
114.6 ppm. HRMS (ESI) m/z: [M+Na]+calcd for C14H7BrN2S3Na:
402.8825; found: 402.8873. IR (KBr): n˜max =3090, 1481, 1423, 1215,
879, 833 cmꢀ1. Elemental analysis calcd for C14H7BrN2S3: C 44.33, H
1.86, N 7.39; found: C 44.17, H 1.86, N 7.36.
Synthesis of 4-(5-iodo-thiophen-2-yl)-7-(5-bromo-thiophen-2-yl)-
benzo[2,1,3]thiadiazole (2): N-iodo-succinimide (NIS, 0.56 g,
2.47 mmol) was added portionwise to a solution of 1 (0.75 g,
1.98 mmol) in CH2Cl2 (50 mL) and CH3COOH (50 mL) at room tem-
perature. The resulting mixture was allowed to react overnight
before the addition of water (50 mL). The organic phase was then
separated and dried over Na2SO4. Product 2 (0.62 g, 62%) was ob-
tained by solvent removal as a dark red solid; mp: 215.3ꢂ0.58C.
1H NMR (700 MHz, CDCl3): d=7.84–7.81 (m, 3H), 7.74 (d, J=4.0 Hz,
1H), 7.37 (d, J=4.0, 1H), 7.18 ppm (d, J=4.0 Hz, 1H). 13C{1H} NMR
(176 MHz, CDCl3): d=152.3, 152.2, 145.1, 140.6, 137.8, 130.8,
128.49, 128.47, 127.4, 125.5, 125.31, 125.27, 125.1, 114.8 ppm.
HRMS (ESI) m/z: [M+Na]+ calcd for C14H6BrIN2S3Na, 528.7791;
found: 528.7002. IR (KBr): n˜max =3081, 1480, 1412, 1214, 1074, 872,
832, 792, 509 cmꢀ1. Elemental analysis calcd for C14H6BrIN2S3: C
33.28, H 1.20, N 5.54; found: C 33.34, H 1.20, N 5.52.
Synthesis
of
4-(5-((7-(5-bromo-thiophen-2-yl)-benzo-
[2,1,3]thiadiazol-4-yl)-thiophen-2-yl)-N,N-bis(4-(2-ethyl-hexyl-
oxy)phenyl)-aniline (3): A mixture of 4-(4,4,5,5-tetramethyl-1,3,2-di-
oxaborolan-2-yl)-N,N-bis(4-(2-ethyl-hexyloxy)phenyl)-aniline (1.18 g,
1.88 mmol), 2 (0.95 g, 1.88 mmol) and Pd(PPh3)4 (0.11 g, 9.4ꢂ
10ꢀ2 mmol) in THF (24 mL) and a 1m NaHCO3 aqueous solution
(18 mL) was refluxed for 24 h. After cooling down the reaction to
room temperature, methylene chloride (50 mL) was added to the
mixture and the resulting organic phase was separated, washed
with water (50 mL) and dried over Na2SO4. After solvent removal,
the crude product was purified by column chromatography (SiO2,
petroleum ether (40–608C)/CH2Cl2 =7:3 v/v) to obtain 3 (1.23 g,
1
74%) as a purple solid, mp: 63.6ꢂ0.58C. H NMR (700 MHz, CDCl3):
d=8.12 (d, J=3.9 Hz, 1H), 7.86–7.77 (m, 4H), 7.51 (d, J=8.7 Hz,
2H), 7.16 (d, J=4.0 Hz, 1H), 7.10 (d, J=8.7 Hz, 4H), 6.96 (d, J=
8.5 Hz, 2H), 6.87 (d, J=8.7 Hz, 4H), 3.91–3.81 (m, 4H), 1.79–1.71
(m, 2H), 1.57–1.39 (m, 8H), 1.38–1.32 (m, 8H), 1.00–0.91 ppm (m,
12H). 13C{1H} NMR (176 MHz, CDCl3): d=156.1, 152.44, 152.40,
146.4, 140.9, 140.2, 137.8, 137.0, 130.74, 130.66, 129.1, 127.4, 126.9,
126.8, 126.5, 125.3, 124.8, 124.4, 122.7, 120.1, 115.4, 114.3, 70.8,
39.5, 30.6, 29.1, 23.9, 23.1, 14.1, 11.2 ppm. HRMS (ESI) m/z: [M]+
calcd for C48H52BrN3O2S3, 879.2387; found: 879.2433. IR (KBr): n˜max
=
3038, 2950, 1600, 1318, 1106, 1033, 827, 722, 511 cmꢀ1. Elemental
analysis calcd for C48H52BrN3O2S3: C 65.58, H 5.96, N 4.78; found: C
65.51, H 5.98, N 4.76.
Synthesis
of
4-(5-((7-(5-trimethylsilylethynyl-thiophen-2-yl)-
benzo[2,1,3]thiadiazol-4-yl)-thiophen-2-yl)-N,N-bis(4-(2-ethyl-
hexyloxy)phenyl)-aniline (4): A mixture of 3 (0.70 g, 0.80 mmol),
PdCl2(PPh3)2 (28.1 mg, 4.0ꢂ10ꢀ2 mmol), CuI (3.8 mg, 2.0ꢂ
10ꢀ2 mmol), trimethylsilyl-acetylene (0.10 g, 1.03 mmol) in diethyla-
Synthesis of 4-(5-bromo-thiophen-2-yl)-7-(thiophen-2-yl)-benzo-
[2,1,3]thiadiazole (1): A solution of N-bromo-succinimide (0.89 g,
5.00 mmol) in DMF (30 mL) was added dropwise to a solution of
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mine (10 mL) was refluxed overnight. After cooling down the reac-
tion to room temperature, methylene chloride (50 mL) was added
to the mixture and the resulting organic phase was washed with
water (50 mL) and dried over Na2SO4. After solvent removal, the
crude product was purified by column chromatography (SiO2, pe-
troleum ether (40–608C)/CH2Cl2 =7/3 v/v) to obtain 4 (0.39 g, 55%)
125.7, 124.7, 124.2, 122.8, 122.6, 120.0, 115.4, 91.7, 87.5, 70.8, 39.5,
30.6, 29.1, 23.9, 23.1, 14.1, 11.1 ppm. HRMS (ESI) m/z: [M]+ calcd for
C55H55N3O3S4, 933.3121; found: 933.3105. IR (KBr): n˜max =3038, 2954,
2924, 2868, 2854, 2180, 1668, 1504, 1238, 827, 798 cmꢀ1. Anal.
calcd for C55H55N3O3S4: C 70.70, H 5.93, N 4.50; found: C 70.65, H
5.91, N 4.51.
1
as a purple solid; mp: 69.3ꢂ0.58C. H NMR (700 MHz, CDCl3): d=
Synthesis of 4-((5-(7-(5-(4-(bis(4-(2-ethyl-hexyloxy)phenyl)ami-
no)phenyl)thiophen-2-yl)benzo[2,1,3]thiadiazol-4-yl)-thiophen-2-
yl)ethynyl)benzaldehyde (7): A mixture of 5 (0.17 g, 0.21 mmol),
4-bromo-benzaldehyde (41.9 mg, 0.23 mmol), Pd(PPh3)4 (11.6 mg,
1.0ꢂ10ꢀ2 mmol) and CuI (1.9 mg, 1.0ꢂ10ꢀ2 mmol) in diethylamine
(10 mL) was refluxed for 24 h. After cooling down the reaction mix-
ture to room temperature, methylene chloride (50 mL) was added
and the resulting organic phase was washed with water (50 mL)
and dried over Na2SO4. After solvent removal, the crude product
was purified by column chromatography (SiO2, CH2Cl2) yielding 7
8.14 (d, J=3.9 Hz, 1H), 7.96 (d, J=3.9 Hz, 1H), 7.88–7.85 (m, 2H),
7.51 (d, J=8.7 Hz, 2H), 7.34–7.29 (m, 2H), 7.10 (d, J=8.7 Hz, 4H),
6.96 (d, J=8.5 Hz, 2H), 6.87 (d, J=8.7 Hz, 4H), 3.89–3.82 (m, 4H),
1.76–1.72 (m, 2H), 1.57–1.39 (m, 8H), 1.38–1.33 (m, 8H), 0.98–0.92
(m, 12H), 0.30 ppm (s, 9H). 13C{1H} NMR (176 MHz, CDCl3): d=
156.0, 152.6, 152.5, 148.8, 146.5, 140.8, 140.2, 137.0, 133.4, 129.2,
126.8, 126.72, 126.69, 126.5, 125.9, 125.8, 124.8, 124.6, 124.3, 122.7,
120.1, 115.4, 100.8, 97.7, 70.8, 39.5, 30.6, 29.7, 29.1, 23.9, 23.1, 14.1,
11.2, ꢀ0.1 ppm. HRMS (ESI) m/z: [M]+ calcd for C53H61N3O2S3Si,
895.3690; found: 895.3730. IR (KBr): n˜max =2956, 2857, 2149, 1600,
1504, 1484, 1447, 1238, 1035, 855, 798 cmꢀ1. Elemental analysis
calcd for C53H61N3O2S3Si: C 71.02, H 6.86, N 6.69; found: C 71.10, H
6.84, N 6.70.
1
(0.10 g, 53%) as a violet solid; mp: 97.6ꢂ0.58C. H NMR (700 MHz,
CDCl3): d=10.05 (s, 1H), 8.17 (d, J=3.9 Hz, 1H), 8.04 (d, J=3.9 Hz,
1H), 7.94–7.88 (m, 4H), 7.71 (d, J=8.2 Hz, 2H), 7.52 (d, J=8.8 Hz,
2H), 7.45 (d, J=3.8 Hz, 1H), 7.31 (d, J=3.8 Hz, 1H), 7.11 (d, J=
8.8 Hz, 4H), 6.97 (d, J=8.8 Hz, 2H), 6.87 (d, J=8.8 Hz, 4H), 3.88–
3.83 (m, 4H), 1.77–1.71 (m, 2H), 1.57–1.39 (m, 8H), 1.38–1.34 (m,
8H), 0.98–0.92 ppm (m, 12H). 13C{1H} NMR (176 MHz, CDCl3): d=
191.3, 156.1, 152.6, 152.5, 148.9, 146.6, 142.0, 140.2, 136.9, 135.5,
133.7, 131.8, 129.6, 129.3, 129.2, 127.02, 126.97, 126.9, 126.5, 126.1,
125.7, 124.7, 124.4, 123.4, 122.7, 120.0, 115.4, 94.1, 87.2, 70.8, 39.5,
30.6, 29.1, 23.9, 23.1, 14.1, 11.1 ppm. HRMS (ESI) m/z: [M]+ calcd for
C57H57N3O3S3, 927.3557; found: 927.3620. IR (KBr): n˜max =3036, 2955,
2924, 2857, 2193, 1701, 1482, 1500, 1235, 827, 798 cmꢀ1. Elemental
analysis calcd for C57H57N3O3S3: N 4.53, C 73.75, H 6.19; found: N
4.52, C 73.72, H 6.17.
Synthesis
of
4-(5-((7-(5-ethynyl-thiophen-2-yl)-benzo-
[2,1,3]thiadiazol-4-yl)-thiophen-2-yl)-N,N-bis(4-(2-ethyl-hexyl-
oxy)phenyl)-aniline (5): A mixture of 4 (0.38 g, 0.42 mmol) and KF
(0.12 g, 2.10 mmol) in methylene chloride (6 mL) and methanol
(6 mL) was stirred for 30 min at 558C. After cooling down the reac-
tion mixture to room temperature, methylene chloride (50 mL) was
added and the resulting organic phase washed with water (50 mL)
and dried over Na2SO4. After solvent removal, the crude product
was purified by column chromatography (SiO2, petroleum ether
(40–608C)/CH2Cl2 =4/1 v/v) obtaining 5 (0.36 g, 99%) as a purple
1
solid; mp: 88.0ꢂ0.58C. H NMR (700 MHz, CDCl3): d=8.14 (d, J=
3.9 Hz, 1H), 7.97 (d, J=3.9 Hz, 1H), 7.87–7.85 (m, 2H), 7.51 (d, J=
8.7 Hz, 2H), 7.37 (d, J=3.8 Hz, 1H), 7.30 (d, J=3.8 Hz, 1H), 7.10 (d,
J=8.7 Hz, 4H), 6.96 (d, J=8.5 Hz, 2H), 6.87 (d, J=8.7 Hz, 4H),
3.88–3.81 (m, 4H), 3.49 (s, 1H), 1.78–1.71 (m, 2H), 1.57–1.39 (m,
8H), 1.38–1.31 (m, 8H), 0.98–0.90 ppm (m, 12H). 13C{1H} NMR
(176 MHz, CDCl3): d=156.0, 152.6, 152.5, 148.8, 146.5, 141.1, 140.2,
136.9, 133.9, 129.2, 126.9, 126.8, 126.72, 126.68, 126.5, 126.10,
125.8, 124.7, 124.5, 122.7, 120.1, 120.0, 115.4, 82.8, 70.8, 39.5, 30.6,
29.1, 23.9, 23.1, 14.1, 11.2 ppm. HRMS (ESI) m/z: [M]+ calcd for
C50H53N3O2S3, 823.3294; found: 823.3298. IR (KBr): n˜max =3036, 2956,
2924, 2857, 2137, 2094, 1599, 1503, 1238, 826, 799 cmꢀ1. Elemental
analysis calcd for C50H53N3O2S3: C 72.87, H 6.48, N 5.51; found: C
72.80, H 6.47, N 5.48.
Synthesis of 2-cyano-3-(5-((5-(7-(5-(4-(bis(4-(2-ethyl-hexyloxy)-
phenyl)amino)phenyl)-thiophen-2-yl)benzo[2,1,3]thiadiazol-4-yl)-
thiophen-2-yl)ethynyl)-thiophen-2-yl)acrylic acid (G1): A mixture
of 6 (0.10 g, 0.11 mmol), cyano-acetic acid (45.9 mg, 0.54 mmol)
and ammonium acetate (2.8 mg, 3.6ꢂ10ꢀ2 mmol) in acetic acid
(10 mL) was refluxed overnight. After cooling down the reaction to
room temperature, the mixture was poured into water (200 mL) to
obtain a precipitate that was collected by filtration and washed
with water. The crude product was purified by column chromatog-
raphy (SiO2, CH2Cl2/MeOH=10/1 v/v) to afford G1 (43.1 mg, 40%)
1
as a violet solid. H NMR (700 MHz, CD2Cl2/CF3COOD): d=8.58–8.40
(br, 4H), 8.36–7.89 (br, 7H), 7.55–7.04 (br, 10H), 3.89–3.82 (br, 4H),
1.76–1.70 (br, 2H), 1.60–1.32 (br, 16H), 1.03–0.90 ppm (br, 12H).
HRMS (ESI) m/z: [MꢀH]ꢀ calcd for C58H55N4O4S4, 999.3101; found:
999.3917. IR (KBr): n˜max =3432, 2960, 2920, 2876, 2850, 2214, 2173,
1617, 1505, 1378, 1237, 825, 798 cmꢀ1. Elemental analysis for
C58H56N4O4S4: C 69.57, H 5.64, N 5.60; found: C 69.45, H 5.66, N
5.59.
Synthesis of 2-cyano-3-(4-((5-(7-(5-(4-(bis(4-(2-ethyl-hexyloxy)-
phenyl)amino)phenyl)-thiophen-2-yl)benzo[2,1,3]thiadiazol-4-yl)-
thiophen-2-yl)ethynyl)-phenyl)acrylic acid (G2): A mixture of 7
(86.4 mg, 9.3ꢂ10ꢀ2 mmol), cyano-acetic acid (39.6 mg, 0.47 mmol)
and ammonium acetate (2.4 mg, 3.1ꢂ10ꢀ2 mmol) in acetic acid
(10 mL) was refluxed overnight. After cooling down the reaction to
room temperature, the mixture was poured into water (200 mL) to
obtain a precipitate that was collected by filtration and washed
with water. The crude product was purified by column chromatog-
raphy (SiO2, CH2Cl2/MeOH=10:1 v/v) to give G2 (35.4 mg, 38%) as
a violet solid. 1H NMR (700 MHz, CD2Cl2/CF3COOD): d=8.59–8.41
(br, 4H), 8.35–7.90 (br, 9H), 7.55–7.05 (br, 10H), 3.88–3.83 (br, 4H),
1.77–1.71 (br, 2H), 1.60–1.32 (br, 16H), 1.03–0.90 ppm (br, 12H).
HRMS (ESI) m/z: [MꢀH]ꢀ calcd for C60H57N4O4S3, 993.3536; found:
993.4329. IR (KBr): n˜max =3432, 2956, 2925, 2870, 2855, 2216, 2193,
Synthesis of 5-((5-(7-(5-(4-(bis(4-(2-ethyl-hexyloxy)phenyl)ami-
no)phenyl)thiophen-2-yl)benzo[2,1,3]thiadiazol-4-yl)-thiophen-2-
yl)ethynyl)thiophene-2-carbaldehyde (6): A mixture of 5 (0.17 g,
0.21 mmol),
5-bromo-thiophene-2-carbaldehyde
(43.9 mg,
0.23 mmol), Pd(PPh3)4 (11.6 mg, 1.0ꢂ10ꢀ2 mmol) and CuI (1.9 mg,
1.0ꢂ10ꢀ2 mmol) in diethylamine (10 mL) was refluxed for 24 h.
After cooling down the reaction mixture to room temperature,
methylene chloride (50 mL) was added and the resulting organic
phase was washed with water (50 mL) and dried over Na2SO4. After
solvent removal, the crude product was purified by column chro-
matography (SiO2, CH2Cl2) to yield 6 (0.11 g, 59%) as a violet solid;
mp: 95.0ꢂ0.58C. 1H NMR (700 MHz, CDCl3): d=9.90 (s, 1H), 8.16
(d, J=3.9 Hz, 1H), 8.03 (d, J=3.9 Hz, 1H), 7.91 (d, J=7.6 Hz, 1H),
7.88 (d, J=7.6 Hz, 1H), 7. 71 (d, J=3.9 Hz, 1H), 7.53–7.50 (m, 2H),
7.45 (d, J=3.9 Hz, 1H), 7.36 (d, J=3.9 Hz, 1H), 7.31 (d, J=3.9 Hz,
1H), 7.12–7.09 (m, 4H), 6.97–6.95 (m, 2H), 6.89–6.86 (m, 4H), 3.87–
3.83 (m, 4H), 1.77–1.72 (m, 2H), 1.57–1.39 (m, 8H), 1.38–1.34 (m,
8H), 0.98–0.92 ppm (m, 12H). 13C{1H} NMR (176 MHz, CDCl3): d=
182.3, 156.1, 152.54, 152.46, 148.9, 146.7, 144.1, 142.6, 140.2, 136.9,
136.0, 134.0, 132.5, 132.4, 129.4, 127.1, 127.0, 126.9, 126.5, 126.2,
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 12
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CHEMSUSCHEM
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1627, 1505, 1385, 1235, 827, 798 cmꢀ1. Elemental analysis calcd for
C60H58N4O4S3: C 72.40, H 5.87, N 5.63; found: C 72.33, H 5.88, N
5.61.
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Acknowledgements
Italian MIUR-PON project “Molecular Nanotechnologies for
Health and Environment-MAAT” (PON02_005633316357-CUP
B31C12001230005); the European Community Seventh Frame-
work Programme (FP7/2007–2013) grant agreement n8 261920
(Efficient Solar Cells based on Organic and hybrid Technology-
ESCORT); Regione Puglia (APQ-Reti di Laboratorio, Project PHOE-
BUS, cod. 31-FE1.20001) and Daunia Wind are acknowledged for
funding.
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Keywords: benzothiadiazole · dye sensitized solar cell · light-
harvesting
· structure-property relationship · p-extended
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Published online on && &&, 0000
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R. Grisorio, L. De Marco,* R. Agosta,
R. Iacobellis, R. Giannuzzi, M. Manca,
P. Mastrorilli, G. Gigli, G. P. Suranna*
&& – &&
Enhancing Dye-Sensitized Solar Cell
Performances by Molecular
Engineering: Highly Efficient p-
Extended Organic Sensitizers
Can I have an extension please? Two
new p-extended D–A–p–A dyes are syn-
thesized for application in dye-sensi-
tized solar cells (DSSCs). The introduc-
tion of an ethynylene–phenylene
ably higher efficiency in DSSCs com-
pared to the sensitizer with the ethyny-
lene–thienylene spacer due to higher
electron injection, inhibition of back
electron-transfer as well as dark current.
moiety in the p-bridge leads to remark-
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 12
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