Donor-π-acceptors containing the 10-(1,3-dithiol-2-ylidene)anthracene unit for dye-sensitized solar cells

Article abstract of DOI:10.1002/chem.201201022

Two donor-acceptor molecular tweezers incorporating the 10-(1,3-dithiol-2-ylidene)anthracene unit as donor group and two cyanoacrylic units as accepting/anchoring groups are reported as metal-free sensitizers for dye-sensitized solar cells. By changing the phenyl spacer with 3,4-ethylenedioxythiophene (EDOT) units, the absorption spectrum of the sensitizer is red-shifted with a corresponding increase in the molar absorptivity. Density functional calculations confirmed the intramolecular charge-transfer nature of the lowest-energy absorption bands. The new dyes are highly distorted from planarity and are bound to the TiO₂ surface through the two anchoring groups in a unidentate binding form. A power-conversion efficiency of 3.7 % was obtained with a volatile CH ₃CN-based electrolyte, under air mass 1.5 global sunlight. Photovoltage decay transients and ATR-FTIR measurements allowed us to understand the photovoltaic performance, as well as the surface binding, of these new sensitizers. Copyright

Full text of DOI:10.1002/chem.201201022

FULL PAPER
DOI: 10.1002/chem.201201022
Donor-p-Acceptors Containing the 10-(1,3-Dithiol-2-ylidene)anthracene
Unit for Dye-Sensitized Solar Cells
Pierre-Antoine Bouit,[a] Magdalena Marszalek,[b] Robin Humphry-Baker,[b]
Rafael Viruela,[c] Enrique Ortꢀ,*[c] Shaik M. Zakeeruddin,*[b] Michael Grꢁtzel,*[b]
Juan Luis Delgado,*[a, d] and Nazario Martꢀn*[a, d]
Dedicated to Professor Thomꢀs Torres on the occasion of his 60th birthday
Abstract: Two donor–acceptor molecu-
lar tweezers incorporating the 10-(1,3-
dithiol-2-ylidene)anthracene unit as
donor group and two cyanoacrylic units
as accepting/anchoring groups are re-
ported as metal-free sensitizers for
dye-sensitized solar cells. By changing
the phenyl spacer with 3,4-ethylene-
dioxythiophene (EDOT) units, the ab-
sorption spectrum of the sensitizer is
red-shifted with a corresponding in-
crease in the molar absorptivity. Densi-
ty functional calculations confirmed
the intramolecular charge-transfer
nature of the lowest-energy absorption
bands. The new dyes are highly distort-
ed from planarity and are bound to the
TiO2 surface through the two anchor-
ing groups in a unidentate binding
form. A power-conversion efficiency of
3.7% was obtained with a volatile
CH3CN-based electrolyte, under air
mass 1.5 global sunlight. Photovoltage
decay transients and ATR-FTIR meas-
urements allowed us to understand the
photovoltaic performance, as well as
the surface binding, of these new sensi-
tizers.
Keywords: absorption
· dyes/pig-
ments · optical properties · sensitiz-
ers · solar cells
Introduction
sensitized solar cells (DSCs) are amongst the best alterna-
tives for exploiting solar energy and, therefore, the develop-
ment of new organic dyes for the preparation of efficient
DSCs is of utmost importance, as well as being a challenging
research topic. Thus far, a few examples of DSCs that lead
to power-conversion efficiencies (PCEs) of 11% have been
reported;[1] however, further research is needed to improve
the PCE and the stability of such devices.
The increasing global energy demand has emerged as one of
the key issues that need to be addressed, because this con-
sumption enhances the depletion of fossil fuel reserves and
leads to further aggravation of environmental pollution. A
rational alternative to overcome this dependence on fossil
fuels and their inherent pollution is the use of renewable
energy sources, such as solar energy. In this context, dye-
Within the field of organic sensitizers, a large variety of
organic dyes have been reported,[2] including coumarins,[2a]
indolines,[2b] tetrahydroquinolines,[2c] hemicyanines,[2d] boron-
dipyrromethenes,[2e] oligothiophenes,[2f] triarylamines,[2g] and
anthracenes and heteroanthracenes.[2h] Recently, we intro-
duced the use of p-extended tetrathiafulvalene (exTTF, 2-[9-
(1,3-dithiol-2-ylidene)anthracen-10-(9H)-ylidene]-1,3-dithi-
[a] Dr. P.-A. Bouit, Dr. J. L. Delgado, Prof. Dr. N. Martꢀn
IMDEA-Nanociencia, Facultad de Ciencias
Ciudad Universitaria de Cantoblanco, 28049 Madrid (Spain)
E-mail: juanluis.delgado@imdea.org
[b] M. Marszalek, Dr. R. Humphry-Baker, Dr. S. M. Zakeeruddin,
Prof. Dr. M. Grꢁtzel
AHCTUNGTRENNUNG
ole)[3] as a donor unit in a sensitizer (1, Figure 1) for the
preparation of DSCs that exhibit efficient photovoltaic con-
version, as well as unusual electrochemical and kinetic prop-
Laboratory of Photonics and Interfaces
Institute of Chemical Sciences and Engineering
Ecole Polytechnique Fꢂdꢂrale de Lausanne
Station 6, CH-1015 Lausanne (Switzerland)
E-mail: shaik.zakeer@epfl.ch
michael.graetzel@epfl.ch
[c] Dr. R. Viruela, Prof. Dr. E. Ortꢀ
Instituto de Ciencia Molecular
Universidad de Valencia, 46980 Paterna (Spain)
E-mail: enrique.orti@uv.es
[d] Dr. J. L. Delgado, Prof. Dr. N. Martꢀn
Departamento de Quꢀmica Orgꢃnica
Facultad de Ciencias Quꢀmicas
Universidad Complutense, 28040 Madrid (Spain)
E-mail: nazmar@quim.ucm.es
Figure 1. A previously reported exTTF-based sensitizer for DSCs (1) and
a push–pull chromophore based on the 10-(1,3-dithiol-2-ylidene)anthra-
cene core (2).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201022.
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erties.[4] The donor ability of the exTTF unit increases the
HOMO level of the organic dye and significantly lowers the
required energy for the regeneration of the dye by the elec-
trolyte to 150 mV. Stimulated by these results, we decided to
explore the closely related 10-(1,3-dithiol-2-ylidene)anthra-
cene group as an appealing electron-donor unit for the prep-
aration of efficient DSCs.
Figure 1 shows the structure of a push–pull chromophore
(2), which incorporates the 10-(1,3-dithiol-2-ylidene)anthra-
cene electron-donor unit.[5] Interestingly, molecule 2 is
highly distorted from planarity owing to strong S···H steric
repulsions, which is crucial for preventing dye aggregation.
In addition, compound 2 shows strong UV/Vis absorption
owing to the efficient intramolecular charge transfer (ICT)
transition that takes place from the donor to the acceptor
unit upon excitation. Molecular engineering by using the 10-
(1,3-dithiol-2-ylidene)anthracene core allows the design of
dyes that contain two anchoring groups; this feature was
previously shown to enhance the performance and the sta-
bility of photovoltaic (PV) devices.[6]
Taking all of these precedents into account, we designed
two new push–pull sensitizers (6 and 7) that were based on
the 10-(1,3-dithiol-2-ylidene)anthracene core, as electron-
donor group, with two cyanoacrylic acid units, as acceptor/
anchor groups. To modulate the optical-absorption proper-
ties, each sensitizer contained a different p bridge between
the donor group and the anchoring units: benzene rings in
compound 6 and 3,4-ethylenedioxythiophene (EDOT) units
in compound 7. EDOT spacers have previously been used
for red-shifting the spectroscopic response and enhancing
the molar extinction coefficient of triphenylamine and
exTTF-based sensitizers.[2g,4,7] Theoretical calculations have
been carried out at the level of density functional theory
(DFT) to gather information about the geometrical and
electronic properties of these new sensitizers. Their photo-
voltaic potential in DSCs has been investigated by using J–V
measurements. The differences between the performances
of these two molecules were scrutinized by using the transi-
ent-photovoltage technique. This technique supported the
existence of enhanced recombination processes within devi-
ces that were functionalized by compound 7.
Scheme 1. Synthesis of new sensitizers 6 and 7; reagents and conditions:
a),b) [PdCl2A(PPh3)2], CuI, THF/Et3N 4:1, reflux overnight; c),d) cyano-
acetic acid, NH4A(CH3COO), CH3COOH, reflux overnight.
CTHUNGTRENNUNG
CTHUNGTRENNUNG
ceptor/anchor units was achieved by the twofold Knoevena-
gel condensation of aldehydes 4 and 5 with cyanoacetic acid
to afford the new sensitizers (6 and 7) in moderate yields
(40 and 64%, respectively). The structures of all of the new
compounds, were confirmed by 1H and 13C NMR spectrosco-
py.
Optical and electrochemical properties: The optical proper-
ties of sensitizers 6 and 7 were studied by UV/Vis spectros-
copy. The absorption spectra of both compounds display
two absorption bands (Figure 2). The first band, with
Results and Discussion
Synthesis: The synthesis of the two new sensitizers was ac-
complished according to a multistep synthetic procedure
(for experimental details, see the Supporting Information).
Intermediate 3, which featured two terminal alkyne groups
(Scheme 1), was synthesized in four steps in good yield ac-
cording to a modified literature procedure.[9]
The conjugated bridges were introduced by means of the
Sonogashira cross-coupling reaction with [PdCl2ACTHNUTRGEN(UGN PPh3)2] and
CuI as catalysts. Thus, the reaction of compound 3 with
either 4-iodobenzaldehyde or 7-bromo-3,4-ethylenedioxy-
thiophene-5-carbaldehyde[XXX] afforded compounds 4 and 5
in moderate yields (both 50%). The introduction of the ac-
Figure 2. UV/Vis absorption spectra of sensitizers 6 (solid line) and 7
(dotted line) in dilute DMSO solution (2ꢅ10ꢀ5 m).
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Optical Properties of Donor-p-Acceptor Sensitizers
FULL PAPER
maxima at 354 nm and 440 nm for compounds 6 and 7, re-
ꢀ
spectively, is in the region of the p p* transitions of the an-
thracene core.[5] The weaker, broad band in the visible
region, with absorption maxima at 495 and 540 nm for com-
pounds 6 and 7, respectively, is assigned to an ICT transition
from the donor to the acceptor moieties.[5] As expected, ex-
tending the conjugation with EDOT in compound 7 allowed
a red-shift of the absorption up to 700 nm. Compared to the
previously reported exTTF sensitizers, which mainly absorb
in the 400–500 nm region,[4] this extension represents a signif-
icant improvement in the optical properties of the sensitizer.
Electrochemical characterization of the new dyes was per-
formed by using cyclic voltammetry and differential pulse
voltammetry in DMF. Both molecules show reversible redox
behavior. According to the optical measurements, a decrease
of the gap is observed when the phenyl spacer in 6 is re-
placed by the EDOT in 7 (Table 1), The oxidation potential
for both compounds (Table 1), which is related to their
Figure 3. FTIR spectra of dye 6 as a powder (dotted line) and adsorbed
onto a transparent TiO2 layer (solid line); the spectra were corrected for
residual CO2, surface-adsorbed water, and TiO2 itself.
Table 1. Redox potentials and experimental/theoretical HOMO–LUMO
energy gaps (Eg) for dyes 6 and 7.
Dye Eox
[V][a]
Ered
E
(elect)
E
(opt)
EgACTHNGUTERNN(UG DFT)
[V][a]
[eV][b]
[eV][c]
[eV][d]
6
7
0.25
0.23
ꢀ1.76
ꢀ1.69
2.01
1.92
2.10
1.95
2.25
2.11
[a] Values determined by differential pulse voltammetry in DMF versus
Fe+/Fe. [b] Estimated as EoxꢀEred. [c] Taken as the intersection of the ab-
sorption and emission spectra. [d] Obtained from DFT calculations
(B3LYP/6-31G**) in DMF.
HOMO, is around 0.25 V versus the ferrocenium/ferrocene
(Fc+/Fc) couple and is sufficiently more-positive than the
ꢀ
redox potential of the Iꢀ/I3 electrolyte (ꢀ0.21 V versus Fc+/
Figure 4. FTIR spectra of dye 7 as a powder (dotted line) and adsorbed
onto a transparent TiO2 layer (solid line); the spectra were corrected for
residual CO2, surface-adsorbed water, and TiO2 itself.
Fc in an ionic liquid)[10] for efficient dye-regeneration. The
reduction potential of the dyes, which is related to their
LUMO, is considerably more-negative than the TiO2 con-
duction band, thereby providing enough of a driving force
for electron-injection.
asymmetric stretching mode can be observed at about
1600 cmꢀ1. The position of these frequencies is consistent
with the unidentate form of binding of the carboxylate to
the TiO2 surface.[11]
ATR-FTIR studies: The surface-binding mode of the sensi-
tizer on titania is an interesting aspect for investigation, be-
cause two anchoring groups are present in dyes 6 and 7,
which may allow these dyes to attach onto the semiconduc-
tor surface through either one or both anchoring groups.
ATR-FTIR measurements that were performed on the dye
powders and on the sensitized films (Figure 3 and Figure 4)
revealed that, most likely, both dyes 6 and 7 were bound to
the surface through both anchoring groups. In the ATR-
FTIR spectra of the dye powders, the peaks at 1680–
1720 cmꢀ1 are attributed to the carboxylic acid groups. The
spectra for the dyes adsorbed onto transparent TiO2 films
did not contain these peaks; this result indicates that both
anchoring groups form bonds with the surface of titania.
Moreover, the symmetric stretching modes of the carboxyl-
ate groups can be identified at 1340–1380 cmꢀ1 and the
Theoretical calculations: The molecular geometries of the
two dyes were fully relaxed, both as isolated systems and in
the presence of the solvent, by using DFT calculations that
were performed at the B3LYP/6-31G** level. Among the
various possible conformations, owing to internal rotation
and to the relative orientation of the cyanoacrylic acid
groups (see the Supporting Information, Figures S7 and S8),
Figure 5 shows the calculated minimum-energy structures
for dyes 6 and 7 that favored the unidentate anchoring of
both carboxylate groups. As shown in Figure 5b, for dye 6,
the molecule adopts the typical butterfly- or saddle-like
shape that is observed for exTTFs.[5,12,13] To relieve the
short-range interactions of the peri hydrogen atoms of the
anthracene core with both the sulfur atoms of the dithiol
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E. Ortꢀ, S. M. Zakeeruddin, M. Grꢁtzel, J. L. Delgado, N. Martꢀn et al.
lated for dye 6 in DMF. Identical MO topologies are ob-
tained for the isolated dye in the gas phase. The HOMO
(ꢀ5.14 eV) is mainly localized on the electron-donor dithiol
unit and is calculated at lower energy than the HOMO of
the tetramethylthio–exTTF molecule (ꢀ4.90 eV), which con-
tains two dithiol units.[14] The HOMOꢀ1 (ꢀ5.75 eV) spreads
over the dithiol and the diacetylene units and is separated
by 0.64 eV from HOMOꢀ2 (ꢀ6.39 eV), which is fully local-
ized over the acceptor part of the molecule. The LUMO
(ꢀ2.89 eV) and LUMO+1 (ꢀ2.63 eV) levels are close in
energy and correspond to the symmetric and antisymmetric
combination of the LUMOs of the two molecular arms that
constitute the acceptor moiety, respectively. An equivalent
MO distribution is obtained for molecule 7. Therefore, dyes
6 and 7 present small HOMO–LUMO energy gaps of 2.25
and 2.11 eV, respectively, and low-energy charge-transfer
(CT) absorption bands are expected, in agreement with that
observed in the electronic spectra (Figure 2).
Figure 5. Optimized B3LYP/6-31G** structures: a) front view and b) side
view of dye 6, which show the distortion from planarity, and c) dye 7.
ring and the carbon atoms of the acetylene units, the central
ring of the anthracene unit folds in a boat conformation
The nature of the HOMO and LUMO indicates that oxi-
dation should mainly affect the dithiole moiety, whereas re-
duction concerns the acetylene/benzene/cyanoacrylic-acid
units (Figure 6). DFT calculations support the observed ex-
perimental trends for the redox potentials (Table 1): On
passing from dye 6 to dye 7, the HOMO increases in energy
slightly from ꢀ5.14 to ꢀ5.10 eV, which is in good agreement
with the small cathodic shift of 0.02 V that is observed ex-
perimentally for the first oxidation potential. In contrast,
the LUMO decreases in energy from ꢀ2.89 eV (6) to
ꢀ2.99 eV (7), which justifies the less-negative reduction po-
tential for dye 7 (ꢀ1.69 V) compared with dye 6 (ꢀ1.76 V).
The shift of the HOMO to higher energy and of the LUMO
to lower energy leads to a narrowing of the HOMO–LUMO
gap on going from compound 6 (2.25 eV) to compound 7
(2.11 eV), which is in good accordance with the electro-
chemical (6: 2.01 eV; 7: 1.92 eV) and spectroscopic estimates
(6: 2.10 eV; 7: 1.95 eV).
ꢀ
along the C9 C10 vector by an average angle of 36.98 for
both dyes 6 and 7 in the gas phase. This value is very similar
to that determined by X-ray crystallography for tetraACHTUNTRGNEUNGmeth-
ACHTUNGTRENNUNG
ylthio–exTTF (388),[5] for which an almost identical value
(38.38) was calculated at the B3LYP/6-31G** level.[13] The
dithiol ring and the diacetylene unit are tilted by 33.3 and
31.98, respectively, with respect to the plane that is defined
ꢀ
ꢀ
ꢀ
by anthracene atoms C11 C12 C13 C14. Almost-identical
tilting angles were computed for compound 7 (34.0 and
31.28, respectively). Therefore, the molecular structures of
dyes 6 and 7 are highly distorted from planarity, which helps
to prevent molecular aggregation.
Figure 6 shows the atomic orbital (AO) composition of
the highest occupied (HOMOꢀ2 to HOMO) and lowest un-
ocupied (LUMO and LUMO+1) molecular orbitals calcu-
To investigate the nature of the electronic transitions that
give rise to the absorption bands in the electronic spectra,
the lowest-energy singlet-excited states (Sn) of dyes 6 and 7
were calculated by using the time-dependent DFT
(TDDFT) approach and the B3LYP/6-31G**-optimized
ground-state geometries. The TDDFT calculations predict
that the low-energy absorption band that is observed experi-
mentally at 495 nm (2.50 eV) for dye 6 and at 540 nm
(2.30 eV) for dye 7 (Figure 2) mainly results from the elec-
tronic transition to the first excited singlet state (S1) at
2.12 eV (6) and 1.94 eV (7) with oscillator strengths (f) of
0.34 and 0.54, respectively. The calculated S0!S1 transition
supports both the shift to the red and the higher intensity of
the low-enery absorption band for 7. The second excited sin-
glet state (S2), which is computed at slightly higher energy
(6: 2.34 eV, f=0.26; 7: 2.25 eV, f=0.16), could also contrib-
ute to that band. The S1 and S2 states originate from the
HOMO!LUMO and HOMO!LUMO+1 monoexcita-
tions, respectively, and imply an electron-density transfer
from the dithiol moiety, where the HOMO mainly resides,
to the acceptor part of the molecule, where the LUMO and
Figure 6. Electron-density contours (0.025 ebohrꢀ3) and orbital energies
for the HOMOs and LUMOs of dye 6 calculated in DMF at the B3LYP/
6-31G** level. H and L denote the HOMO and LUMO, respectively.
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Optical Properties of Donor-p-Acceptor Sensitizers
FULL PAPER
LUMO+1 are located (Figure 6). Therefore, the calculations
confirm the CT nature of the low-energy absorption band in
the electronic spectra of the new dyes (6 and 7). The higher
intensity that was experimentally observed and theoretically
calculated for this band in compound 7 can be ascribed to
the better electronic communication existing between the
donor and acceptor units through the EDOT bridge. The
high-energy absorption band that was recorded in the spec-
tra of 6 and 7 (Figure 2) results from the excited states that
are associated with the HOMOꢀ2!LUMO excitation,
which is localized on the acceptor part of the molecule, and
the HOMOꢀ1!LUMO and LUMO+1 excitations, which
also imply some intramolecular electron-density transfer
(Figure 6).
The calculations clearly show that molecules 6 and 7
become more polarized in the S1 state (see the molecular
electrostatic potentials calculated for 6 in Figure S11). The
acceptor part of dye 6 accumulates a charge of ꢀ0.54e in
the S1 state, which is extracted from the anthracene moiety
(+0.31e) and the dithiol unit (+0.23e). The charge transfer
that is associated with the S1 state is also reflected by the
computed value for the molecular dipole moment, which in-
creases from 1.41 D in S0 to 12.94 D in S1. The charge that is
transferred from the donor unit to the acceptor unit in the
S1 state is slightly smaller for dye 7 (acceptor unit: ꢀ0.43e;
anthracene moiety: +0.27e; dithiol unit: +0.16e). The po-
larization of the dye in the excited S1 state facilitates the
electron injection in the semiconductor TiO2 surface.
Figure 7. J–V plots of the DSCs that were sensitized with dyes 6 (black)
and 7 (gray) measured under 1 sun (AM 1.5G, 100 MWcmꢀ2) conditions
(solid line) and in the dark (dashed line).
Photovoltaic performance: The photovoltaic performance of
dyes 6 and 7 was tested with a volatile CH3CN-based elec-
trolyte and a solvent-free ionic-liquid electrolyte. Device-
fabrication methods and -conditions, as well as the composi-
tions of the electrolytes, are described in the Experimental
Section.
The J–V curves for the volatile systems are shown in
Figure 7. The devices that were sensitized with dye 6
showed improved photovoltaic performance, owing to
higher Jsc and Voc values. This result seems somewhat sur-
prising because the absorption spectra of the dyes, either in
solution (Figure 2) or adsorbed on a transparent TiO2 layer
(Figure 8), show a better spectroscopic response for dye 7.
The bathochromic shift of 40 nm, which is accompanied by
a significant broadening of the spectra on the film, would
suggest a better performance for the device that contained
compound 7. However, the Jsc value is 1.3–2.5 mAcmꢀ2
lower, depending on the electrolyte used, than that obtained
for the device that contained dye 6. Detailed photovoltaic
parameters of both devices are given in Table 2.
Figure 8. Comparison of the absorption spectra of dyes 6 (black) and 7
(gray), which were adsorbed onto a transparent 1.9 mm TiO2 film (dashed
line, right axis), and the IPCE spectra of the devices with a 5.5 mm TiO2
layer (solid line, left axis).
Table 2. Photovoltaic parameters of devices with volatile- and ionic-
liquid
electrolytes
under
simulated
AM 1.5G
illumination
(100 mWcmꢀ2).
Dye
Electrolyte
Jsc [mAcmꢀ2
]
Voc [V]
FF
h [%]
6
Z960
Z952
Z960
Z952
8.8
7.4
6.3
6.1
0.56
0.52
0.50
0.49
0.75
0.71
0.70
0.70
3.7
2.8
2.2
2.1
7
The absorption spectrum of dye 6 adsorbed onto the tita-
nia surface is notably broadened in the complete device
(filled with electrolyte). The incident photon-to-current-con-
version efficiency (IPCE) spectra of the complete devices
are also shown in Figure 8. The integrated current that was
obtained from the IPCE spectrum of the device that con-
tained dye 7 (35%) was significantly smaller than that mea-
sured for the device that incorporated dye 6 (60%). This
result is consistent with the lower current densities as mea-
sured from the J–V curves for the device that contained dye
7 (Figure 7 and Table 2). To calculate the amount of dye
that was adsorbed onto the surface of TiO2, we stained
transparent 2 mm-thick films with the dyes and measured
their optical density by using UV/Vis spectroscopy. By using
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the Lambert–Beer law, we calculated the concentration of
the dye within this 2 mm-thick volume, assuming that the
film porosity and the surface area were exactly the same for
all of the samples. This result enabled us to compare the cal-
culated concentrations: 150 mm for dye 6 and 158 mm for
dye 7. Because the dye-loading results show that there is no
difference between the number of molecules that effectively
sensitize the semiconductor, the differences between the
performances of devices based on dyes 6 and 7 should origi-
nate from other factors that affect the processes in the cell,
such as charge recombination.
In addition to the above-observed difference in the Jsc
value, there is also a substantial difference between the Voc
values of the two devices: at 1 sun illumination, the Voc
value for the cell that is sensitized with dye 6 is 60 mV
(Z960 electrolyte) higher than that with dye 7. This differ-
ence is even-more-pronounced when comparing the dark
currents: the logarithmic plot of the dark currents of both
devices (Figure 9) reveals that there is a tenfold difference
Figure 10. Apparent electron lifetime plotted against the density of states
*
*
for DSCs that contained dyes 6 ( ) and 7 ( ).
Conclusions
Herein, we focused on the investigation of the synthesis,
characterization, and applications of new dye molecules that
were based on the 10-(1,3-dithiol-2-ylidene)anthracene core.
These molecules were designed to improve their spectro-
scopic response, thereby making them more attractive for
incorporation into DSCs. The insertion of EDOT into their
structure led to an extension of the absorption spectrum of
the dye up to 700 nm, which, in comparison with previously
reported analogous molecules, gave an additional 200–
300 nm of red-shift. FTIR measurements revealed that both
dyes are bound to the TiO2 surface through the two anchor-
ing groups in a unidentate binding form. The preferable con-
formations that were obtained from DFT calculations sup-
port this binding and showed that the molecular geometry is
highly distorted from planarity owing to the saddle-like-
shaped exTTF-like core, with the anthracene unit being
folded into a boat-type structure. Owing to this spatial ar-
rangement, less aggregation is expected. Optical and elec-
trochemical characterization of these new materials satisfied
the criteria for their successful incorporation into photovol-
taic devices. The highest efficiency (3.7%) in which the
IPCE was stretched out beyond 700 nm, was obtained for
the device that contained dye 6. Dye 7, which is an EDOT-
containing molecule, despite exhibiting a slightly broader
UV/Vis spectrum, was inferior to the phenyl-substituted dye
in terms of overall PV performance. The main reason for
this lower overall PV performance for dye 7 is its higher re-
combination rate, as witnessed by a higher dark current and
a shorter electron-lifetime.
Figure 9. Dark currents of the DSCs that incorporated dyes 6 (solid line)
and 7 (dotted line), which show the undesired behavior (“leak”) of the
dark current in the case of the device that was sensitized with dye 7, thus
indicating the presence of additional channels for recombination.
between the currents, which can be interpreted as a sign of
enhanced charge-recombination in the device that is sensi-
tized with dye 7. To investigate this behavior, transient pho-
tovoltage-decay measurements were performed on both de-
vices. The apparent electron lifetimes, which were measured
at the same density of states level, were much shorter for
the DSCs that incorporated dye 7 (Figure 10). By assuming
that the position of TiO2 conduction band remains at the
same level in both cases, these results support the previous
suggestion that a much-faster recombination takes place in
the device that is sensitized with dye 7 and explains the
lower photovoltaic performances that are obtained when
using this dye.
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Optical Properties of Donor-p-Acceptor Sensitizers
FULL PAPER
afford 7 as a dark brown solid (75 mg, 64%). 1H NMR (300 MHz,
[D6]DMSO): d=2.42 (s, 6H), 4.45–4.50 (m, 8H), 7.44 (ddd, 3J=7.5 Hz,
3J=7.5 Hz, 2H), 7.55 (dd, 3J=7.5 Hz, 3J=7.5 Hz, 2H), 7.70 (d, 3J=
7.5 Hz, 2H), 8.17 (s, 2H), 8.27 ppm (d, 3J=7.5 Hz, 2H); 13C NMR
(75 MHz, CDCl3): d=19.3, 65.8, 66.4, 85.4, 97.7, 98.3, 100.3, 101.7, 106.0,
113.1, 117.5, 118.8, 122.4, 126.0, 127.3, 128.2, 130.0, 132.2, 134.9, 135.3,
Experimental Section
General: NMR spectra (1H, 13C) were recorded at RT on a Bruker DPX
300 MHz spectrometer. Data are listed in parts per million (ppm) and
are reported relative to tetramethylsilane; residual solvent peaks were
used as an internal standard (CDCl3; 1H: 7.26 ppm, 13C: 77.36 ppm).
High-resolution mass spectrometry measurements were performed at the
Unidad de Espectrometrꢀa de Masas of the Universidad Complutense de
Madrid. Column chromatography was performed on Merck 60 (40–
63 mm) silica gel. UV/Vis spectra were recorded on a Varian Cary 50
spectrometer at a constant temperature of 258C in dilute DMSO solution
(about 10ꢀ5 mollꢀ1).
140.5, 148.1, 144.5, 164.3 ppm; UV/Vis:
(20000); HRMS (MALDI): m/z calcd for C44H26N208S6: 902.001 [M]+;
found: 901.964; IR (KBr): n˜C O =1706; n˜CꢁN =2213; n˜COO H =3000 cmꢀ1
l (e)=495 (45000), 540 nm
.
=
ꢀ
Fluorescence measurements: Steady-state excited-state emission was
measured on a FluoroLog-322 (Horiba) that was equipped with a 450W
Xe arc lamp. Dye powders were dissolved in CH2Cl2 and diluted to a con-
centration of about 10ꢀ5 m. The samples were excited at a wavelength
that corresponded to the absorption maximum.
Synthesis: Synthesis of compound 4: 3 (240 mg, 0.55 mmol, 1 equiv) and
p-iodobenzaldehyde (385 mg, 3 equiv) were dissolved in THF/NEt3
(30 mL, 4:1). Then, the solution was degassed for 20 min and [PdCl2-
Electrochemical measurements: A standard three-electrode setup was
employed to determine the redox potentials of the dyes. Measurements
were performed on a PC-controlled Autolab system (PGSTAT-10, Eco
Chimie). A glassy carbon electrode (Metrohm) was used as a working
electrode, a Pt plate was used as an auxiliary electrode, and Pt wire was
used as a quasi-reference electrode. Ferrocene was used as an internal
standard in each case. Dye solutions were prepared by dissolving 0.5 mm
of the powder in DMF and adding 0.1m of tetrabutylammonium hexa-
fluorophosphate (TBAPF6) as a supporting electrolyte.
ACHTUNGTRENNUNG(PPh3)2] (39 mg, 0.1 equiv) and CuI (20 mg, 0.2 equiv) were added, and
the solution was refluxed overnight. After cooling down to RT, the mix-
ture was filtrated and washed with CH2Cl2. The organic layer was washed
with saturated NH4Cl solution and water, dried over MgSO4, and the sol-
vents were evaporated. The crude mixture was purified by column chro-
matography on silica gel (eluent: CH2Cl2) to afford 4 as a red solid
(170 mg, 50%). 1H NMR (300 MHz, CDCl3): d=2.41 (s, 6H), 7.37 (ddd,
3J=7.5 Hz, 3J=7.5 Hz, 4J=1.5 Hz, 2H), 7.45 (ddd, 3J=7.5 Hz, 3J=
4
3
3
4
ATR-FTIR measurements: ATR-FTIR spectra were measured on an
FTS 7000 FTIR spectrometer (Digilab) that was equipped with
a “Golden Gate” diamond anvil. The final spectra were integrated from
64 individual scans at a resolution of 2 cmꢀ1. A similar force, which led to
an intimate contact between the anvil and the sample (powder or dye ad-
sorbed onto the film), was exerted on the specimen during the experi-
ment. The films were rinsed with CH3CN and dried before the measure-
ments were recorded.
7.5 Hz, J=1.5 Hz, 2H), 7.62 (d, J=8 Hz, 4H), 7.69 (dd, J=7.5 Hz, J=
3
3
4
1.5 Hz, 2H), 7.86 (d, J=8 Hz, 4H), 8.35 ppm (dd, J=7.5 Hz, J=1.5 Hz,
2H); 13C NMR (75 MHz, CDCl3): d=19.2, 92.2, 92.8, 98.7, 122.4, 125.3,
125.8, 126.3, 127.6, 128.4, 129.3, 129.6, 132.0, 132.7, 134.1, 135.6, 135.7,
148.9, 191.3 ppm; MS (ESI): m/z calcd for C38H2402S4 +Na+: 663.1
[M+Na]+; found: 633.1; IR (KBr): n˜C O =1696 cmꢀ1
.
=
Synthesis of compound 5: 3 (350 mg, 0.81 mmol, 1 equiv) and 7-bromo-
3,4-ethylenedioxythiophene-5-carbaldehyde (605 mg, 3 equiv) were dis-
solved in THF/NEt3 (50 mL, 4:1). Then, the solution was degassed for
Theoretical calculations: All theoretical calculations were carried out by
using density functional theory (DFT) with the A.02 revision of the
Gaussian 09 program package.[15] Geometry optimizations were per-
formed with Becke’s three-parameter B3LYP exchange functional[16] and
the 6–31G** basis set.[17] The molecular conformations (Figure 5) for
dyes 6 and 7 were optimized within Cs symmetry constraints. Geometry
optimizations were performed both in the gas phase and in the presence
of the solvent (DMF and DMSO). Solvent effects were considered within
the self-consistent reaction field (SCRF) theory by using the SMD key-
word, which performs polarized continuum model (PCM)[18] calculations
by using the solvation model of Truhlar and co-workers.[19] The SMD sol-
vation model is based on the polarized continuous quantum-chemical
charge density of the solute (the “D” in the name stands for “density”).
The solvent had no relevant effect on the geometrical parameters but it
significantly affected the relative energy of the molecular conformations
that could be present in solution (see the Supporting Information). Mo-
lecular orbitals were plotted by using Molekel 4.3.[20]
20 min and [PdCl2ACHTUNGTRENNUNG(PPh3)2] (56 mg, 0.1 equiv) and CuI (28 mg, 0.2 equiv)
were added, and the solution was refluxed overnight. After cooling down
to RT, the mixture was filtrated and washed with CH2Cl2. The organic
layer was washed with saturated NH4Cl solution and water, dried over
MgSO4, and the solvents were evaporated. The crude mixture was puri-
fied by column chromatography on silica gel (eluent: CH2Cl2/ethyl ace-
tate 9:1) and then recrystallized in methanol to afford 5 as a red solid
(170 mg, 50%). 1H NMR (300 MHz, CDCl3): d=2.43 (s, 6H), 4.40 (m,
8H), 7.33 (ddd, 3J=7.5 Hz, 3J=7.5 Hz, 4J=1.5 Hz, 2H), 7.44 (ddd, 3J=
7.5 Hz, 3J=7.5 Hz, 4J=1.5 Hz, 2H), 7.66 (dd, 3J=7.5 Hz, 4J=1.5 Hz,
2H), 8.33 (dd, 3J=7.5 Hz, 4J=1.5 Hz, 2H), 9.91 ppm (s, 2H); 13C NMR
(75 MHz, CDCl3): d=19.7, 65.2, 65.5, 84.4, 98.6, 99.8, 109.3, 119.0, 123.0,
125.6, 126.4, 126.7, 128.0, 128.8, 132.8, 134.5, 135.8, 144.6, 147.8, 148.8,
180.0 ppm; MS (MALDI): m/z calcd for C38H2406S6: 767.990 [M]+; found:
767.878; IR (KBr) n˜C O =1654 cmꢀ1
.
=
Synthesis of compound 6: 4 (100 mg, 0.15 mmol, 1 equiv) was dissolved in
glacial acetic acid (5 mL), and then cyanoacetic acid (133 mg, 10 equiv)
and ammonium acetate (100 mg) were added. The solution was refluxed
for 6h and precipitated in cold water. The solid was filtrated, dissolved in
CH2Cl2, washed with water, and diluted in a solution of hydrochloric
acid. The organic layer was dried with MgSO4 and evaporated. The crude
mixture was then dissolved in a minimum quantity of CH2Cl2 and precipi-
tated in pentane to afford 6 as a dark brown solid (50 mg, 40%).
1H NMR (300 MHz, [D6]DMSO): d=2.40 (s, 6H), 7.50–7.60 (m, 4H),
7.65–7.70 (m, 6H), 8.08 (d, 3J=8 Hz, 4H), 8.30–8.40 ppm (m, 4H);
13C NMR (75 MHz, CDCl3): d=18.3, 79.1, 91.5, 92.5, 98.1, 104.6, 115.9,
118.4, 121.7, 121.8, 125.1, 125.8, 126.5, 127.3, 128.1, 128.8, 128.9, 130.8,
131.6, 131.8, 131.9, 133.4, 134.5, 147.8, 152.8, 163.0 ppm; UV/Vis: l (e)=
354 (40000), 440 nm (14000); HRMS (MALDI): m/z calcd for
Vertical electronic-transition energies were computed at the B3LYP/6–
31G** level by using the TDDFT approach[21] with the optimized
ground-state molecular geometries. The vertical excitation energies were
also calculated by using the hybrid PBE0 functional,[22] which provided
a similar description (energies and nature) of the lowest-energy excited
states. Both the B3LYP and PBE0 functionals underestimated the energy
of the HOMO!LUMO CT excited state. This shortcoming of standard
global hybrid functionals has been reported for donor–acceptor com-
pounds for which there is a small overlap between the HOMO and the
LUMO,[23] as is the case for dyes 6 and 7 (Figure 6). To solve this prob-
lem, the use of long-range corrected functionals, such as the CAM-
B3LYP functional,[24] is recommended. The CAM-B3LYP approach com-
pletely fails in reproducing the optical spectra of dyes 6 and 7 because
the CT states are calculated too high in energy and mixed with the p!
p* states of the donor and acceptor units of the molecule. A summary of
the TDDFT results for dye 6 by using the B3LYP and PBE0 functionals
is given in the Supporting Information, Table S1.
C44H26N202S4: 774.077 [M]+; found: 774.051; IR (KBr): n˜C O =1711,
=
n˜CꢁN =2183, n˜COO H =3000 cmꢀ1
.
ꢀ
Synthesis of compound 7: 5 (100 mg, 0.13 mmol, 1 equiv) was dissolved in
glacial acetic acid (5 mL), and then cyanoacetic acid (110 mg, 10 equiv)
and ammonium acetate (100 mg) were added. The solution was refluxed
for 12h and precipitated in cold water. The crude mixture was then dis-
solved in a minimum quantity of THF and precipitated in diethyl ether to
Cell fabrication: The devices were fabricated by using a transparent
5.5 mm-thick TiO2 layer (nanoparticle size: 200 nm). The layer was
screen-printed onto FTO glass (NSG-10, Nippon Sheet Glass) and sin-
Chem. Eur. J. 2012, 00, 0 – 0
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E. Ortꢀ, S. M. Zakeeruddin, M. Grꢁtzel, J. L. Delgado, N. Martꢀn et al.
tered according to a literature procedure.[25] The photoanodes that were
prepared in this way were immersed in the dye solutions (0.3 mm in
DMSO/EtOH, 1:9 v/v, with 10 mm chenodeoxycholic acid) for 2 h after
prior heating at 5008C for 30 min. TEC-15 glass (Pilkington) with a pre-
drilled hole was covered with Pt by drop-casting an 8 mm solution of hex-
achloroplatinic acid in isopropanol and heated at 4508C for 15 min. This
procedure was repeated twice. The electrodes were then sealed by using
a 25 mm-thick Surlyn gasket. Electrolyte was introduced into the space
between the electrodes by using a vacuum back-filling system. In this
study, two types of electrolyte were used: volatile electrolyte Z960 (1.0m
1,3-dimethylimidazolium iodide, 0.03m iodine, 0.1m guanidinium thiocya-
nate, 0.5m tert-butylpyridine, 0.05m lithium iodide in CH3CN/valeroni-
trile, 85:15 v/v) and ionic-liquid-based electrode Z952 (1,3-dimethylimi-
dazolium-iodide/1-ethyl-3-methylimidazolium-iodide/1-ethyl-3-methylimi-
dazolium-tetracyanoborate/iodine/N-butylbenzoimidazole/guanidinium-
thiocyanate, 12:12:16:1.67:3.33:0.67). Metal contacts were placed on the
edges of the device to ensure good conductivity during the measure-
ments. A UV-light-filtering, self-adhesive foil (AKTOP, Asahi Glass) was
placed on top of the device.
M. K. Nazeeruddin, E. W.-G. Diau, Ch.-Y. Yeh, S. M. Zakeeruddin,
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Photovoltaic characterization: A 450 W Xe light source (Oriel, USA)
was used to characterize the devices. The spectroscopic output of the
lamp was matched in the region 350–750 nm with the aid of a Schott
K113 Tempax sunlight filter (Prꢁzisions Glas & Optik GmbH, Germany)
so as to reduce the mismatch between the simulated- and actual solar
spectra to less than 2%. The current–voltage characteristics of the cell
under these conditions were obtained by applying an external potential
bias to the cell and measuring the generated photocurrent with a Keithley
model 2400 digital source meter (Keithley, USA). A similar data-acquisi-
tion system was used to control the incident photon-to-current conver-
sion efficiency (IPCE) measurements. Under computer control, light
from a 300 W Xe lamp (ILC Technology, USA) was focused through
a Gemini-180 double monochromator (Jobin Yvon Ltd., UK) onto the
photovoltaic cell. The devices were masked to attain an illuminated
active area of 0.159 cm2.
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light bias allowed us to estimate the recombination rate constant (and
hence the apparent electron lifetime) at different open-circuit potentials
by controlling the concentrations of the free charges in TiO2. Red-light
perturbation pulses were adjusted to a very low level to maintain single-
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Acknowledgements
This work was supported by the MINECO of Spain (CTQ2011–27934,
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CTQ2011–24652,
CTQ2009–08790/BQU,
and
Consolider-Ingenio
CSD2007–00010 on Molecular Nanoscience), by the Comunidad de
Madrid (MADRISOLAR-2, S2009/PPQ-1533), by the Generalitat Va-
lenciana (PROMETEO/2012/053), and by the EU project (FUNMOLS
FP7–212942–1). J.L.D. thanks the MINECO of Spain for a Ramꢆn y
Cajal Fellowship. P.-A.B. thanks IMDEA-Nanociencia for a postdoctoral
research grant. We thank Mr. Pascal Comte for providing the TiO2 films
and Dr. Carole Grꢁtzel for fruitful discussions.
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Optical Properties of Donor-p-Acceptor Sensitizers
FULL PAPER
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Received: March 26, 2012
Published online: && &&, 0000
Chem. Eur. J. 2012, 00, 0 – 0
ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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E. Ortꢀ, S. M. Zakeeruddin, M. Grꢁtzel, J. L. Delgado, N. Martꢀn et al.
Housing the rising sun: The optical
properties of metal-free sensitizers that
were composed of 10-(1,3-dithiol-2-yli-
dene)anthracene-unit-based molecular
tweezers were tuned for dye-sensitized
solar cells. With the use of 3,4-
Optical Properties
P.-A. Bouit, M. Marszalek,
R. Humphry-Baker, R. Viruela,
E. Ortꢁ,* S. M. Zakeeruddin,*
M. Grꢂtzel,* J. L. Delgado,*
ethylenedioxythiophene (EDOT)
units, the absorption spectrum of the
sensitizer was red-shifted. A power-
conversion efficiency value of 3.7%
was obtained, under air mass 1.5
global sunlight.
N. Martꢁn* ..................... &&&& — &&&&
Donor-p-Acceptors Containing the 10-
(1,3-Dithiol-2-ylidene)anthracene Unit
for Dye-Sensitized Solar Cells
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www.chemeurj.org
ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!