Bis-donor-bis-acceptor tribranched organic sensitizers for dye-sensitized solar cells

Article abstract of DOI:10.1002/ejoc.201100821

A new class of tribranched dye-sensitized solar cell (DSC) sensitizers carrying two conjugated donors and two acceptor/anchoring groups is introduced. The approach leads to significantly different optical properties and enhanced stability with respect to related di- and monobranched dyes and yields power conversion efficiencies of up to 5.05 %, which is possibly limited by the computed nonoptimal dye packing on the semiconductor surface.

Full text of DOI:10.1002/ejoc.201100821

FULL PAPER
DOI: 10.1002/ejoc.201100821
Bis-Donor–Bis-Acceptor Tribranched Organic Sensitizers for Dye-Sensitized
Solar Cells
Alessandro Abbotto,*[a,b] Valentina Leandri,[a] Norberto Manfredi,[a] Filippo De Angelis,*[b]
Mariachiara Pastore,[b] Jun-Ho Yum,[c] Mohammad K. Nazeeruddin,*[c] and
Michael Grätzel[c]
Keywords: Conformation analysis / Dyes/pigments / Donor-acceptor systems / Energy conversion / Sensitizers
A new class of tribranched dye-sensitized solar cell (DSC)
sensitizers carrying two conjugated donors and two acceptor/
anchoring groups is introduced. The approach leads to sig-
nificantly different optical properties and enhanced stability
with respect to related di- and monobranched dyes and
yields power conversion efficiencies of up to 5.05%, which
is possibly limited by the computed nonoptimal dye packing
on the semiconductor surface.
all, are associated with an extended structural variety that
can be adjusted to optimize the spectral absorption proper-
ties. Although their efficiencies are still lower than those of
ruthenium sensitizers, very recently, values approaching, or
even exceeding, 10% have been reported, showing the great
potential of the new design.[7] The design of these efficient
chromophores, in analogy with most organic sensitizers, has
been based on a dipolar D-π-A structural motif, where “D”
is an electron-rich group, “π” is a conducting bridge, and
“A” an electron-poor moiety embedding the COOH an-
choring functionality for grafting onto the TiO2 surface.
Triarylamine has been by far the most widely used D core.[8]
In the search for improved optical and structural proper-
ties, we,[9] and others,[10,11] have recently introduced a novel
model class of dibranched dianchoring organic sensitizers
composed of one donor and two anchoring groups (A-π-
D-π-A). We have shown that the dibranched dye DB-1
(Scheme 1) offers improved optical properties (higher molar
extinction coefficients and red-shifted absorption), in-
creased photocurrent, and enhanced stability compared to
its monobranched analogues.[9] Recently, a porphyrin pos-
Introduction
Solar energy has not yet found widespread use due to
the high cost of crystalline silicon panels, which are not
competitive with electricity generated from fossil fuels.[1]
Among the new approaches to solar energy conversion,
dye-sensitized solar cells (DSCs) have one of the best poten-
tial for high conversion efficiency and low-cost manufac-
ture.[2] One strategic route to improving cell efficiencies in-
volves the optimization of the dye-sensitizer, which captures
photons to generate an electron/hole pair at the interface
with the inorganic semiconductor (TiO2) and the redox
electrolyte, respectively. The RuII complex bis(2,2Ј-bi-
pyridyl-4,4Ј-dicarboxylate)ruthenium(II) (N719)[3] is the
most representative DSC sensitizer, with power conversion
efficiencies exceeding 11%.[4]
One of the main drawbacks of RuII–bipyridyl complexes
is the modest molar absorptivity of the low-energy absorp-
tion band (14200 m–1 cm–1 at 530 nm for N719),[3] which
hinders larger photocurrent densities and thus improved
efficiencies.[5] In the last years, metal-free organic dyes have
been proposed as alternative sensitizers to circumvent this
problem.[6] Organic dyes are readily available, easier to pu-
rify, are relatively inexpensive to manufacture, and, most of
[a] Department of Materials Science and Milano-Bicocca Solar
Energy Research Center-MIB-Solar,
Via Cozzi 53, 20125, Milano, Italy
Fax: +39-02-64485400
E-mail: alessandro.abbotto@mater.unimib.it
[b] Istituto CNR di Scienze e Tecnologie Molecolari (CNR-ISTM),
Department of Chemistry, University of Perugia,
Via Elce di Sotto 8, 06123, Perugia, Italy
E-mail: filippo@thch.unipg.it
[c] Laboratory for Photonics and Interfaces, School of basic
Sciences, Swiss Federal Institute of Technology,
1015 Lausanne, Switzerland
E-mail: mdkhaja.nazeeruddin@epfl.ch
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201100821.
Scheme 1. Reference photosensitizers.
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sessing two anchoring groups has been successfully applied Here, we wish to report the fabrication of DSCs with the
to DSC with a power conversion efficiency comparable to novel sensitizers, and to disclose their photoelectrochemical
that of N719 under the same conditions.[12]
properties and stability data, which were measured under
Here, we present an extension of this design, aimed at various conditions. DFT/TDDFT calculations have been
further expanding the possibility of improving the optical performed in order to highlight the main properties associ-
and energetic properties of the sensitizers. The new ap- ated with the tribranched geometry, both in solution and
proach, based on a A-π-D-πЈ-D-π-A tribranched structural on TiO2 models.
motif (where πЈ is other than π), implies the use of two
donor and two acceptor/anchoring fragments and different
Results and Discussion
π spacers, as pictorially depicted in Figure 1. Generally
speaking, the tribranched approach should allow a much
larger structural variety with respect both to the D-π-A
(Figure 1a) and A-π-D-π-A (Figure 1b) motifs due to the
presence of two donor and two acceptor groups and three
Synthesis, Spectroscopic, and Electrochemical
Characterization
TB-1 and TB-2 were synthesized according to Scheme 3.
π-spacers. All of these constituent units can be fruitfully 4-[N-(4-Methoxyphenyl)-N-phenylamino]benzaldehyde (1a)
[17]
selected and combined to finely tune the donor and ac-
was brominated in high yields to 1b using N-bromosuc-
ceptor strength, π-framework extension and, ultimately, the cinimide (NBS) in anhydrous N,N-dimethylformamide
optical, energetic, and photovoltaic response of the sensitiz- (DMF) while keeping the temperature between –5 and 0 °C.
ers.
At temperatures below –10 °C the yields were much more
modest (less than 50%) whereas at higher temperatures
(0 °C to r.t.) a complex mixture of brominated products was
recovered. After reduction to the corresponding brominated
alcohol 2b, the phosphonate 3b was directly prepared in one
step with I2, using P(OEt)3 as a solvent. Horner–Witting
condensation between diethyl methylphosphonate 3b and
aldehyde 1b afforded the electron-rich brominated trans-
4,4Ј-bis(diarylamino)stilbene intermediate 4b. Similarly p-
phenylenevinylogue derivative 5b was prepared using tetra-
ethyl 1,4-phenylenebis(methylene)diphosphonate[18] and
two equivalents of aldehyde 1b. It should be noted that aro-
matic bromination of the corresponding precursors 4a and
5a, which were obtained in a similar manner to that de-
scribed above from 1a and 3a, always lead to complex mix-
tures of mono- and polybrominated derivatives, regardless
of reaction conditions. Unfortunately, this result prevented
us from fully exploiting the high-yielding McMurry reac-
tion of 1a, which easily allowed the isolation of gram-quan-
tities of 4a in only one step. The two bromoderivatives 4b
and 5b were then submitted to the same reaction sequence,
involving the microwave-mediated Suzuki coupling reaction
with 5-formyl-2-thienylboronic acid[13] to give the corre-
sponding 2-thiophenecarbaldehydes 6 and 7. The final
Knoevenagel condensation step with cyanoacetic acid and
piperidine, under microwave irradiation (which led to im-
proved yields), afforded the corresponding piperidinium
carboxylate derivatives of TB-1 and TB-2. The correspond-
ing carboxylic acids were then isolated after treatment with
saturated aqueous NH4Cl.
Figure 1. Schematic representation of monobranched D-π-A, di-
branched A-π-D-π-A, and tribranched A-π-D-πЈ-D-π-A (this
work) structural motifs.
To design the new tribranched prototypes, we selected
some of the most common donor and π-spacer groups.
Namely, as donor cores we have adapted the common tri-
arylamino group[8] by using the trans-4,4Ј-bis(diarylamino)-
stilbene and trans-4,4Ј-bis(diarylamino)distyrylbenzene
building blocks. The simple thiophene-based π-A-COOH
arm of L1[13] and C213[14] (Scheme 1), which we previously
chose as reference monobranched dyes, and which have
been used in other previously reported sensitizers,[15,16] was
selected as a side-branch. Based on these premises, we de-
signed the tribranched dyes TB-1 and TB-2 (Scheme 2).
The absorption and emission spectra of TB-1 and TB-2
are shown in Figure 2. The main optical parameters, to-
gether with those of reference mono- and dibranched dyes,
are collected in Table 1. Both TB-1 and TB-2 present two
bands in the visible region. Comparison with the absorp-
tion peak of the corresponding 4,4Ј-bis(diarylamino)stil-
bene 4a and bis(diarylamino)distyrylbenzene 5a building
blocks (393 and 415 nm in CH2Cl2, respectively) shows that
the higher energy visible band (393 and 421 nm in CH2Cl2
for TB-1 and TB-2, respectively) is attributed to a local
Scheme 2. Investigated tribranched organic sensitizers.
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Dye-Sensitized Solar Cells
Scheme 3. Synthesis of TB-1 and TB-2. Reagents and conditions: (i) NBS, DMF, –5 to 0 °C, 3 h; (ii) NaBH4, EtOH, 0 °C to r.t.; (iii) I2,
P(OEt)3, 0 °C to r.t.; (iv) n = 0, 1a or 1b, tBuOK, THF, 0 °C to r.t.; (v) n = 1, tetraethyl 1,4-phenylenebis(methylene)diphosphonate,
tBuOK, THF, 0 °C to r.t.; (vi) TiCl4, Zn, pyridine, THF; (vii) 5-formyl-2-thienylboronic acid, [Pd(dppf)Cl2], K2CO3, MeOH/toluene,
70 °C, 10 min, microwave irradiation; (viii) cyanoacetic acid, piperidine, EtOH or toluene, 100 °C, microwave irradiation followed by
NH4Cl.
transition of the donor core (see the Supporting Infor- sition from the electron-rich bis(diarylamino) moieties to
mation, Figure S1), whereas the lower energy band is asso- the 2-cyanoacrylic acid acceptor end-groups. Indeed, the
ciated to the π–π* intramolecular charge transfer (CT) tran- absorption peaks are almost identical in the two dyes, as a
consequence of the presence of the same donor–acceptor
side arm. Upon addition of a small amount of HCl, the
spectrum remains unchanged, confirming that both dyes
are present as carboxylic acids.[9] This picture was con-
Table 1. Absorption and emission parameters of TB-1 and TB-2
compared to reference dyes.
Compound λabs / nm (ε / m–1 cm–1)
λem / nm (Φ)[a]
TB-1[b]
397 (40 900)
478 (35 800)
421 (54 000)
480 (40 200)
494 (44 500)
410 (25 800)
482 (27 500)
485 (0.058)[c]
TB-2[b]
540 (0.013)[d]
DB-1[e]
L1[f]
549
660
C213[g]
[a] Fluorescence quantum yield; Coumarin 540A was used as a
standard (0.58 in EtOH). [b] This work (DMSO). [c] λexc = 397 nm.
[d] λexc = 421 nm. [e] Ref.[9] (EtOH). [f] Ref.[13] (tert-butyl alcohol/
acetonitrile, 1:1). [g] Ref.[14] (THF).
Figure 2. Normalized absorption (solid line) and emission (dashed
line) spectra of TB-1 and TB-2 in DMSO.
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firmed by investigating the emission properties. Excitation of the C213 blocks). As expected, the interaction between
at λmax produced fluorescence spectra that were different in the virtual orbitals is less pronounced, with the quasi-de-
TB-1 and TB-2, according to their different donor cores, generate LUMO and LUMO+1, being the combinations of
and resembled those of 4a (473 nm in CH2Cl2) and 5a the LUMOs on the two monobranched units, and the
(504 nm in CH2Cl2), respectively.
LUMO+2 localized on the linking π bridge. Although a
The donor–acceptor CT band of the tribranched dyes is comparison between experimental electrochemical poten-
hypsochromically shifted with respect to the monobranched tials and calculated HOMO/LUMO energy levels is not
dye C213, although the former chromophores have a more straightforward, we notice a rather good agreement be-
extended π-conjugated system. At the same time, TB-1 and tween the calculated and experimental HOMO energy,
TB-2 present a significant hyperchromic effect, likely for the whereas the calculated LUMO is substantially less negative
same reason. Similar, albeit less pronounced, hypsochromic than the electrochemical estimate. This is due to the em-
effects are observed when the optical properties of the tri- ployed exchange-correlation functional, which contains a
branched dyes are compared with those of the dibranched large percentage of Hartree–Fock exchange (ca. 42%), but
sensitizer DB-1. In conclusion, the tribranched geometry which, however, allows us to nicely reproduce the optical
induces a negligible blueshift but a substantial increase in properties of the dyes.
absorbance with respect to mono- and dibranched dyes. In
addition, a second and more intense visible band at higher
energy is present due to local transitions of the donor core,
thus enabling more efficient sunlight harvesting in this re-
gion.
The electrochemical properties of TB-1 and TB-2 were
examined by cyclic voltammetry in dimethylformamide
(DMF). Using a glassy carbon working electrode, Pt
counter electrode, and 0.05 m tetrabutylammonium hexaflu-
orophosphate [TBA(PF6)] as a supporting electrolyte, the
potentials measured vs. Fc+/Fc were converted into normal
hydrogen electrode (NHE) potentials by addition of
+0.63 V. TB-1 and TB-2 showed half-wave potentials corre-
sponding to molecular oxidation potentials at +0.91 and
+0.98 V vs. NHE, respectively. Their reduction potentials
were detected at –0.87 and –0.97 V vs. NHE, respectively.
By using the value of –4.5 eV vs. vacuum for NHE,[19] the
HOMO/LUMO energies were calculated as –5.4/–3.6 and
–5.5/–3.5 eV for TB-1 and TB-2, respectively.
Figure 3. Molecular orbital levels of L1, C213, and TB-1. The val-
ues were computed in the gas phase at the MPW1K/6-31G* level.
Computational Investigation
To investigate the electronic structure of the representa-
tive TB-1 standalone dye and in relation to its constituent
subunits, DFT/TDDFT calculations were performed on
TB-1 and on the monobranched L1 and C213 systems. As
shown in Figure 3, in which the energy levels of the Molecu-
lar Orbitals (MOs) are reported, the introduction of the π
conjugated bridge linking the two monobranched subunits,
induces a significant perturbation (mixing) among the occu-
pied MOs. The strong mixing among the highest occupied
MOs of the isolated building blocks is also evident by look-
ing at the plots of the HOMO, HOMO–1, HOMO–2,
LUMO, LUMO+1, and LUMO+2 of TB-1 reported in Fig-
ure 4. Taking the C213 dye as a reference monobranched
unit, it can be seen that the HOMO, which is located at
–5.90 eV in C213, is split in the tribranched system into
three levels: the HOMO at –5.77 eV [mainly localized on
the donor trans-4,4Ј-bis(diarylamino)stilbene core], the
HOMO–1 at –6.18 eV (essentially extended on the two
monobranched units), and the HOMO–2 at 6.91 eV (re-
sulting from the strong mixing of the MOs of the linker and 2, LUMO, LUMO+1, and LUMO+2 of TB-1.
Figure 4. Isodensity surfaces of the HOMO, HOMO–1, HOMO–
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Dye-Sensitized Solar Cells
The effect of the geometry of the tribranched dyes on the 5.3 kcal/mol more stable than the B cisoid conformer, poss-
optical properties of the sensitizer is shown in Table 2, in ibly because of the preferential arrangement of the aromatic
which the lowest excitation energies and the corresponding moieties around the N-terminated bridge.
oscillator strengths of the three dyes in the gas phase are
compared. As could be expected on the basis of the molecu-
lar orbital energy diagram depicted in Figure 3, the lowest
absorption peak moves to longer wavelengths on going
from L1 to C213 as a consequence of HOMO destabiliza-
tion induced by the two electron-donor hexyloxy substitu-
ents, coupled to only a slight LUMO energy upshift. Com-
parison of the calculated values with the experimentally de-
termined band maxima is not straightforward because the
deprotonated dye was measured for L1, whereas the proton-
ation state of C213 is not clear. However, because proton-
ation/deprotonation can shift the absorption maxima by up
to 0.3 eV,[20] the data presented here can be considered to
be in good overall agreement with the experimental values.
An absorption maximum of 479 nm was calculated for
C213, which can be compared with the 482 nm experimen-
tal value. For the protonated dye L1, a 461 nm absorption
maximum was calculated, which can be compared to the
404 nm maximum measured for the deprotonated dye.
Table 2. Computed lowest excitation wavelengths and oscillator
strengths in the gas phase and DMSO solution for TB-1, L1, and
C213.
Compound
λabs / nm
vacuum/DMSO
Oscillator strength
vacuum/DMSO
Figure 5. Optimized molecular structures of the A and B TB-1 con-
formers.
TB-1
L1
C213
457/495
421/461
436/479
1.940/1.903
1.228/1.316
1.222/1.316
In the tribranched structure, the computed lowest ab-
sorption band is slightly red-shifted compared with C213
both in vacuo and in solution, with an oscillator strength
almost 70% larger than that of the monobranched systems.
The calculated value in solution (495 nm) is in excellent
agreement (within 0.1 eV) with the 478 nm experimental
data. The lowest energy and most intense transition of TB-
1 turns out to be a mixing of HOMO Ǟ LUMO (65%),
HOMO–1 Ǟ LUMO+1 (20%), and HOMO Ǟ LUMO+2
(5%) orbital excitations. The mixed nature of the main TB-
1 optical transition is responsible for the modest redshift
computed for this system compared with C213 [495 vs.
479 nm in dimethyl sulfoxide (DMSO) solution], despite the
sizeable reduction of the HOMO–LUMO gap computed for
TB-1 compared with C213 (3.61 vs. 3.83 eV).
To gain insight into the possible interaction between the
newly synthesized TB-1 dye and the nanostructured TiO2
semiconductor, we examined the optimized geometry of
TB-1 and docked the resulting structure (with no further
geometry optimization) onto extended TiO2 models. The
optimized structure of TB-1 shows two conformers, a more
stable conformer with a transoid arrangement of the two
branches (Figure 5A) and a less stable conformer (Fig-
ure 5B) corresponding to a cisoid arrangement of the two
Figure 6. Possible interaction of TB-1 conformers A and B with
branches. The A transoid conformer was calculated to be the TiO2 substrate.
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As can be noticed from Figure 6, the more stable A con-
former exhibits a non-optimal interaction with the TiO2
surface, which might reduce the dye molecular packing on
TiO2, which, in turn, may impact the photovoltaic perform-
ance, as discussed below. The less stable B conformer shows
better dye packing on TiO2, occupying essentially a similar
space to that expected for two single branched dyes. The
lower stability in solution, however, might imply that this
conformer is present as a minor fraction upon adsorption
onto TiO2.
Photovoltaic Investigation in DSCs
The new sensitizers, TB-1 and TB-2, were used to fabri-
cate DSCs to explore the current–voltage characteristics.
The incident monochromatic photon-to-current conversion
efficiency (IPCE) of TB-1 plotted as a function of excitation
wavelength exhibits a high value at plateau (81%) and more
than 70% from 400 to 560 nm (Figure 7). In contrast, al-
though exhibiting a photoelectrochemical response in a
Figure 8. The J/V characteristics of DSCs based on TB-1 and TB-
2 under various light intensities.
Table 3. Photovoltaic parameters of TB-1 and TB-2 compared with
similar wavelength range, the IPCE of TB-2 showed a reference dyes.
slightly lower maximum value of 75%. The integrated cur-
rent under the IPCE curve is 10.8 and 9.5 mAcm–2, for TB-
1 and TB-2, respectively, which is consistent with the photo-
current of solar cells under standard global AM 1.5 solar
conditions (see below). The main photovoltaic parameters
are shown in Figure 8 and Table 3. The TB-1 cell showed a
10.9 mAcm–2 short circuit current density (Jsc), 641 mV of
open circuit voltage (Voc), and 72.5% fill factor (ff) at stan-
dard global AM 1.5 solar conditions. Thus, the TB-1 power
conversion efficiency η, derived from the equation η = Jsc
ϫVoc ϫff, is 5.05%. Under similar conditions, the TB-2
cell performance showed a Jsc value of 9.65 mAcm–2,
662 mV Voc, and a fill factor of 70.6, resulting in a power
conversion efficiency (η) of 4.51%. The different Jsc value
is consistent with the estimate obtained by IPCE integra-
tion. All devices showed a very linear response of Jsc and a
slightly higher ff under a lower light intensity (see Table 3).
Compound I0 / sun Jsc / mAcm–2
Voc / V
FF / %
η / %
TB-1
TB-2
1
0.5
0.10
1
0.5
0.10
1
10.9
5.45
1.10
9.65
4.84
0.99
5.42
12.8
11.9
641
616
558
662
640
584
735
620
775
72.5
74.5
76.6
70.6
72.6
73.6
69
5.05
5.01
4.75
4.51
4.50
4.30
2.75
5.20
6.88
L1[13]
1
1
66
74.7
C213[14]
It is interesting to compare the photovoltaic data ob-
tained for TB-1 and TB-2 with the previously reported de-
vice data obtained for the related L1 and C213 dyes. A sur-
vey of such data is reported in Table 3, showing that the
difference between TB-1 and the better performing C213 is
mainly related to an approximate 20% increase in Voc ob-
tained for the latter, which, together with a 9% (3%) in-
crease in Jsc (ff), provides an overall 36% increase in η. The
incorporated long alkoxy chain in C213 could also justify
the higher Voc. Indeed, Hagberg et al. have recently shown
a correlation between Voc and bulky butoxy chains.[16]
A
similarly higher Voc is obtained for L1, despite the fact that
a larger amount of LiI (0.1 and 0.05 m, respectively) was
used than in the present investigation (0.05 m).[13,14] Thus,
it is quite unlikely that the reduced Voc value obtained with
TB-1 and TB-2 is due to the composition of the electrolyte,
which renders the higher Jsc. We could speculate that the
unfavorable interaction with TiO2 discussed on the basis of
the structural parameters of the dye is responsible for the
lower Voc, whereby the non-optimal dye packing on the
semiconductor surface would allow the oxidized species in
the electrolyte to approach the surface, thus increasing re-
combination between electrons injected into TiO2 and the
electrolyte. However, we did not observe increased recombi-
nation rates in TB-1 and TB-2 when compared with mono-
Figure 7. Photocurrent action spectra of TB-1 and TB-2 DSCs.
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Dye-Sensitized Solar Cells
branched dye D5 (see the Supporting Information, Fig- which is likely caused by protonation processes that take
ure S6).[21] Moreover, the measured dye loadings[22] of TB- place during light soaking, leading to a positive shift of the
1 (4.8ϫ10–8 molcm–2) and TB-2 (4.1ϫ10–8 molcm–2) on 3- TiO2 band edge.[23]
μm TiO2 films were 96 and 82% with respect to that of
monobranched dye D5 (4.1ϫ10–8 molcm–2) under same
conditions. In other words, neither overloaded dyes nor fast
recombination were observed in TB-1 and TB-2.
Conclusions
We have introduced a novel design based on tribranched
organic sensitizers, carrying two donors, two acceptors,
three π spacers, and two anchoring points. Such molecular
architecture should provide more variety in the sensitizer
chemical structure, allowing further structural optimization
that can be tailored to achieve the desired properties. Com-
parison of DSCs based on the tribranched dyes TB-1 and
TB-2 with the corresponding monobranched system C213
shows a decrease in power conversion efficiency, mainly due
to lower photovoltages in the former system. This was as-
cribed to the selected electrolyte for higher photocurrents,
and to the computed non-optimal interaction with the TiO2
surface, which might reduce the molecular packing of the
dye and favor detrimental recombination between the semi-
conductor and oxidized electrolyte. However, it should be
noted that the twisted structure of the adsorbed transoid
conformer is particularly suitable for efficient interaction
with the chemisorption sites, for which a distance of 10.2 Å
was measured.[24] Furthermore, such adsorption geometry
inhibits aggregation as a result of steric hindrance. Indeed,
the efficiency stabilities of DSCs based on TB-1 and TB-2
after light soaking are improved with respect to dibranched
DB-1, which, in turn, offers increased stability with respect
to the corresponding monobranched dyes.[9] Although the
two first prototypes did not offer improved power conver-
sion efficiencies with respect to the conventional uni-dimen-
sional design, we believe that the tribranched design could
trigger the development of optimized systems through the
incorporation of appropriate new branches.
We previously reported that dibranched dyes, having two
anchoring groups, showed good stability data when em-
ployed in DSCs.[9] It was thus interesting to test the tempo-
ral stability of DSCs fabricated with the new TB-1 and TB-
2 dyes. Figure 9 shows the photovoltaic performance
changes during a long-term accelerated ageing of TB-1 and
TB-2 sensitized solar cells using an ionic liquid electrolyte
(1,3-dimethylimidazoliumiodide/1-ethyl-3-methylimida-
zoliumiodide/1-ethyl-3-methylimidazolium tetracyanobor-
ate/iodine/N-butylbenzoimidazole/guanidinium thiocyanate
with molar ratio 12:12:16:1.67:3.33:0.67) under light soak-
ing at full intensity (100 mWcm–2) at 60 °C. Values for Jsc,
Voc, ff, and η were recorded over a period of 1000 h. The
overall efficiencies of TB-1 and TB-2 remained at 88 and
74% of the initial value, respectively, after light soaking.
These stabilities are an improvement over our previously
reported dibranched dye DB-1, which exhibited a 67% re-
sidual value. The Jsc values of both TB-1 and DB-2 showed
a drop at the initial ageing stage but recovered after 300 h
of ageing. The reason for this drop of Jsc is not clear at this
stage. The losses in η are mainly caused by the drop in Voc,
namely 68 and 55 mV for TB-1 and TB-2, respectively,
Experimental Section
General: NMR spectra were recorded with a Bruker AMX-500
spectrometer operating at 500.13 MHz (1H). Coupling constants
are given in Hz. High resolution mass spectra (HRMS) were re-
corded with a Bruker Daltonics ICR-FTMS APEX II spectrometer
equipped with an electrospray ionization (ESI) source. Flash
chromatography was performed with Merck grade 9385 silica gel
230–400 mesh (60 Å). Reactions were performed under nitrogen in
oven-dried glassware and monitored by thin layer chromatography
using UV light (254 and 365 nm) as a visualizing agent. All rea-
gents were obtained from commercial suppliers at the highest pu-
rity grade and used without further purification. Anhydrous sol-
vents were purchased from Sigma-Aldrich and used without further
purification. Toluene and MeOH were dried with CaCl2 and
MgSO4, respectively. Extracts were dried with Na2SO4 and filtered
before removal of the solvent by evaporation. Melting points are
uncorrected. Absorption spectra were recorded with a V-570 Jasco
spectrophotometer. Emission spectra were recorded with a FP6200
Jasco spectrofluorometer. Fluorescence spectra were collected by
exciting at the maximum of absorption. Fluorescence quantum
yields Φ were recorded by using the relative comparative method
with a standard sample of known Φ value and optically dilute solu-
Figure 9. Photovoltaic parameter (Jsc, Voc, ff, and η) variations with
ageing time for the devices sensitized with TB-1 (closed symbols)
and TB-2 (open symbols) based on ionic liquid electrolyte during
successive light soaking for 1000 h.
Eur. J. Org. Chem. 2011, 6195–6205
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6201

A. Abbotto, F. De Angelis, M. K. Nazeeruddin et al.
FULL PAPER
tions.[25] Coumarin 540A in EtOH (Φ = 0.58) was used to estimate
fluorescence quantum yields.[26] The absorbance at excitation wave-
lengths were kept below 0.1. Refractive index (DMSO 1.477, EtOH
1.359) corrections were included in order to account for the dif-
ferent solvents of the test and the reference solutions. Infrared spec-
tra (IR) were recorded with an ATR-FTIR Perkin–Elmer Spectrum
100 spectrometer.
in one pot. The reaction mixture was warmed to r.t. and stirred
overnight. The excess of P(OEt)3 was distilled off under reduced
pressure and the pure product was obtained after filtration through
silica gel (n-hexane to EtOH), as a clear oil (1.50 g, 2.98 mmol,
1
94%). H NMR ([D6]DMSO): δ = 7.37 (d, J = 8.9 Hz, 2 H), 7.19
(dd, J = 8.5, 2.3 Hz, 2 H), 7.03 (d, J = 8.8 Hz, 2 H), 6.94 (d, J =
8.8 Hz, 4 H), 6.69 (d, J = 8.8 Hz, 2 H), 4.01 (quintet, J = 8.2 Hz,
4 H), 3.75 (s, 3 H), 3.16 (d, J = 21.3 Hz, 2 H), 1.25 (t, J = 7.1 Hz,
6 H) ppm.
4-[N-(4-Bromophenyl)-N-(4-methoxyphenyl)amino]benzaldehyde
(1b): A solution of NBS (0.65 g, 3.63 mmol) in anhydrous DMF
(3 mL) was slowly added to a stirred solution of 4-[N-(4-meth-
oxyphenyl)-N-phenylamino]benzaldehyde (1a)[17] (1.00 mg,
3.30 mmol) in the same solvent (10 mL). The solution was kept at
–5 to 0 °C for 3 h. The reaction mixture was poured into water
(50 mL) and the formation of a yellow precipitate was observed.
The solid was filtered under reduced pressure to yield the pure
product (1.12 g, 2.93 mmol, 89 %) as a green gummy solid. 1H
NMR ([D6]DMSO): δ = 9.77 (s, 1 H), 7.72 (d, J = 8.8 Hz, 2 H),
7.56 (d, J = 8.8 Hz, 2 H), 7.18 (d, J = 8.9 Hz, 2 H), 7.13 (d, J =
8.8 Hz, 2 H), 7.02 (d, J = 8.9 Hz, 2 H), 6.87 (d, J = 8.7 Hz, 2 H),
3.78 (s, 3 H) ppm.
p,pЈ-Bis[N-(4-methoxyphenyl)-N-phenylamino]stilbene (4a)
Route A (Scheme 3, step iv): tBuOK (0.18 g, 1.60 mmol) was added
to an ice-cold and stirred solution of diethyl benzylphosphonate 3a
(0.48 g, 1.12 mmol) in THF (20 mL). After 30 min, a solution of
benzaldehyde 1a (0.34 g, 1.12 mmol) in the same solvent (10 mL)
was added dropwise and the resulting mixture was stirred at r.t. for
24 h. The solvent was partially evaporated under reduced pressure,
then AcOEt (20 mL) and water (50 mL) were added. After collect-
ing the organic phase, the aqueous layer was extracted with AcOEt
(3ϫ20 mL), and the combined organic phases were dried and the
solvent was evaporated under reduced pressure to afford an orange
4-[N-(4-Methoxyphenyl)-N-phenylamino]benzyl Alcohol (2a): A oil, which was submitted to flash chromatography (Et2O/cyclohex-
solution of NaBH4 (0.22 g, 5.81 mmol) in THF (15 mL) was added
to an ice-cold and stirred solution of 1a (1.48 g, 4.88 mmol) in the
same solvent (25 mL). The mixture was warmed to r.t. and stirred
overnight. The solvent was partially evaporated under reduced
ane, 3:7). The pure product was obtained as a yellow-orange solid
(0.14 g, 0.24 mmol, 22%), m.p. 161–162 °C. H NMR (CDCl3): δ
= 7.30 (d, J = 8.6 Hz, 4 H), 7.22 (t, J = 7.5 Hz, 4 H), 7.08 (d, J =
8.9 Hz, 4 H), 7.06 (d, J = 8.6 Hz, 4 H), 6.99 (d, J = 8.1 Hz, 4 H),
1
pressure, then Et2O (15 mL) and water (30 mL) were added. After 6.95 (t, J = 7.3 Hz, 2 H), 6.91 (s, 2 H), 6.85 (d, J = 8.9 Hz, 4 H),
separating the organic phase, the aqueous layer was extracted with
Et2O (3ϫ15 mL). The combined organic phases were dried and
the solvent evaporated to afford the product as a yellow waxy solid
3.81 (s, 6 H) ppm.
Route B (Scheme 3, step vi): Zn (2.16 g, 32.9 mmol) was suspended
in anhydrous THF (50 mL) and the suspension was cooled to
–20 °C. TiCl4 (3.13 g, 16.5 mmol) was added dropwise (formation
of a yellow precipitate was observed). The suspension was heated
to reflux for 30 min (gradual darkening was observed). The mixture
was cooled to –15 °C and a solution of benzaldehyde 1a (2.50 g,
1
(1.45 g, 4.75 mmol, 97%). H NMR (CDCl3): δ = 7.24–7.19 (m, 4
H), 7.06 (d, J = 8.9 Hz, 2 H), 7.03 (d, J = 7.5 Hz, 2 H), 7.02 (d, J
= 8.4 Hz, 2 H), 6.96 (t, J = 7.3 Hz, 1 H), 6.84 (d, J = 8.9 Hz, 2 H),
3.80 (s, 3 H) ppm.
4-[N-(4-Bromophenyl)-N-(4-methoxyphenyl)amino]benzyl Alcohol 8.2 mmol) and anhydrous pyridine (1.69 g, 21.4 mmol) in anhy-
(2b): A solution of NaBH4 (0.23 g, 6.15 mmol) in EtOH (15 mL)
was added to an ice-cold and stirred solution of 1b (1.96 g,
drous THF (20 mL) was added dropwise. The resulting suspension
was heated to reflux for 2 h and then poured into a 3:2 mixture
5.13 mmol) in the same solvent (50 mL). The mixture was warmed of water and CH2Cl2 (100 mL). The dark suspension was filtered
to r.t. and stirred overnight. Et2O (15 mL) and water (30 mL) were
then added, the organic phase was separated, and the aqueous layer
was extracted with Et2O (3ϫ15 mL). The combined organic phases
were dried and the solvent was evaporated. The product was ob-
through a pad of Celite. The organic phase was separated, extracted
with water (3ϫ30 mL), dried, and the solvent evaporated under
reduced pressure to yield the pure product as a yellow solid (2.00 g,
3.48 mmol, 84%).
1
tained as a yellow waxy solid (1.21 g, 3.14 mmol, 61%). H NMR
p,pЈ-Bis[N-(bromophenyl)-N-(4-methoxyphenyl)amino]stilbene (4b):
tBuOK (0.42 g, 3.70 mmol) was added to an ice-cold and stirred
solution of diethyl benzylphosphonate 3b (1.50 g, 2.97 mmol) in
anhydrous THF (20 mL). After 30 min, a solution of aldehyde 1a
(1.22 g, 3.20 mmol) in the same solvent (10 mL) was added drop-
wise and the resulting mixture was stirred at r.t. for 24 h. The sol-
vent was partially evaporated under reduced pressure, then Et2O
(20 mL) and water (50 mL) were added. After collecting the
organic phase, the aqueous layer was extracted with Et2O
(3ϫ20 mL). The combined organic phases were dried and the sol-
vent evaporated to afford the pure product as a yellow-orange solid
(0.99 g, 1.35 mmol, 46%). 1H NMR ([D6]DMSO): δ = 7.60 (d, J =
8.8 Hz, 4 H), 7.52 (d, J = 8.8 Hz, 4 H), 7.16 (d, J = 8.9 Hz, 4 H),
7.08 (d, J = 8.8 Hz, 4 H), 7.01 (s, 2 H), 7.00 (d, J = 8.9 Hz, 4 H),
6.88 (d, J = 8.8 Hz, 4 H), 3.78 (s, 6 H) ppm.
([D6]DMSO): δ = 7.36 (d, J = 8.9 Hz, 2 H), 7.24 (d, J = 8.3 Hz, 2
H), 7.03 (d, J = 8.9 Hz, 2 H), 6.97 (d, J = 8.4 Hz, 2 H), 6.93 (d, J
= 8.9 Hz, 2 H), 6.78 (d, J = 8.9 Hz, 2 H), 5.11 (t, J = 5.5 Hz, 1 H),
4.44 (d, J = 5.2 Hz, 2 H), 3.75 (s, 3 H) ppm.
Diethyl 4-[N-(4-Methoxyphenyl)-N-phenylamino]benzylphosphonate
(3a): Alcohol 2a (1.45 g, 4.75 mmol) was dissolved in P(OEt)3
(10 mL) at 0 °C and I2 (1.27 g, 5.00 mmol) was added in one pot.
The reaction mixture was warmed to r.t. and stirred overnight. The
excess of P(OEt)3 was distilled off under reduced pressure and the
pure product was obtained after filtration through silica gel (n-
hexane to AcOEt/petroleum ether, 8:2) as a clear oil (1.24 g,
2.91 mmol, 62%). 1H NMR (CDCl3): δ = 7.19 (t, J = 8.5 Hz, 2 H),
7.13 (dd, J = 8.6, 2.5 Hz, 2 H), 7.04 (d, J = 8.9 Hz, 2 H), 7.00 (d,
J = 7.6 Hz, 2 H), 6.97 (d, J = 8.0 Hz, 2 H), 6.92 (t, J = 7.3 Hz, 1
H), 6.82 (d, J = 8.9 Hz, 2 H), 4.06 (quintet, J = 7.1 Hz, 4 H), 3.79
(s, 3 H), 3.08 (d, J = 21.3 Hz, 2 H), 1.26 (t, J = 7.1 Hz, 6 H) ppm.
p,pЈ-Bis[N-(4-methoxyphenyl)-N-phenylamino]distyrylbenzene (5a):
tBuOK (0.11 g, 0.99 mmol) was added to an ice-cold and stirred
solution of the tetraethyl 1,4-phenylenebis(methylene)diphos-
phonate[18] (0.15 g, 0.41 mmol) in anhydrous THF (20 mL). After
Diethyl 4-[N-(4-Bromophenyl)-N-(4-methoxyphenyl)amino]benz-
ylphosphonate (3b): Alcohol 2b (1.21 g, 3.14 mmol) was dissolved
in P(OEt)3 (10 mL) at 0 °C and I2 (0.96 g, 3.78 mmol) was added 30 min, a solution of aldehyde 1a (0.25 g, 0.82 mmol) in the same
6202
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 6195–6205

Dye-Sensitized Solar Cells
solvent (5 mL) was added dropwise and the resulting mixture was
stirred at r.t. for 36 h. The solvent was partially evaporated under
reduced pressure, then AcOEt (50 mL) and water (100 mL) were
added. After collecting the organic phase, the aqueous layer was
extracted with AcOEt (3ϫ30 mL). The combined organic phases
were dried and the solvent was evaporated to afford an orange oil.
The crude product was purified by flash chromatography (Et2O/
cyclohexane, 7:3) to give the product as a yellow-orange solid
(0.15 g, 0.22 mmol, 54%). 1H NMR ([D6]DMSO): δ = 7.55 (s, 4
H), 7.48 (d, J = 8.9 Hz, 4 H), 7.29 (t, J = 8.6 Hz, 4 H), 7.20 (d, J
TB-1: Cyanoacetic acid (0.26 g, 3.02 mmol) and a catalytic amount
of piperidine were added to a solution of bis-aldehyde 6 (80 mg,
0.10 mmol) in EtOH (5 mL). The mixture was heated by microwave
irradiation at 100 °C for 1 h and then water (30 mL) was added.
The formed precipitate was collected and dissolved in CH2Cl2. Af-
ter extracting the solution with saturated aqueous NH4Cl, the or-
ganic phase was separated, dried, and the solvent removed by ro-
tary evaporation to afford the crude product, which was purified
as
a dark-red solid by recrystallization (AcOH) (0.021 g,
1
0.02 mmol, 23%); m.p. Ͼ 200 °C (dec.). H NMR ([D6]DMSO): δ
= 16.2 Hz, 2 H), 7.08 (d, J = 16.2 Hz, 2 H), 7.07 (d, J = 9.0 Hz, 4 = 8.09 (s, 2 H), 7.69 (d, J = 3.9 Hz, 2 H), 7.59 (d, J = 8.7 Hz, 4
H), 7.01 (t, J = 6.3 Hz, 2 H), 7.00 (d, J = 8.4 Hz, 4 H), 6.96 (d, J
= 9.0 Hz, 4 H), 6.89 (d, J = 8.4 Hz, 4 H), 3.90 (s, 6 H) ppm.
H), 7.50 (d, J = 8.7 Hz, 4 H), 7.48 (d, J = 3.9 Hz, 2 H), 7.10 (d, J
= 8.9 Hz, 4 H), 7.08 (s, 2 H), 7.01 (d, J = 8.6 Hz, 4 H), 6.98 (d, J
= 8.8 Hz, 4 H), 6.97 (d, J = 8.5 Hz, 4 H) 3.78 (s, 6 H) ppm. IR
p,pЈ-Bis[N-(bromophenyl)-N-(4-methoxyphenyl)amino]distyryl-
benzene (5b): tBuOK (0.49 g, 4.32 mmol) was added to an ice-cold
and stirred solution of the tetraethyl 1,4-phenylenebis(methylene)
diphosphonate[18] (0.68 g, 1.80 mmol) and 1b (1.38 g, 3.61 mmol)
in anhydrous THF (30 mL). The resulting mixture was stirred at
r.t. for 6 h. The solvent was partially evaporated under reduced
pressure, then AcOEt (30 mL) and water (50 mL) were added. The
combined organic phases were dried and the solvent removed un-
der reduced pressure to afford an orange oil, which was purified
by flash chromatography (CH2Cl2/hexane, 1:1) to give the product
as a yellow solid (0.97 g, 1.16 mmol, 65%). 1H NMR ([D6]DMSO):
δ = 7.56 (s, 4 H), 7.51 (d, J = 8.7 Hz, 4 H), 7.42 (d, J = 8.9 Hz, 4
H), 7.22 (d, J = 16.3 Hz, 2 H), 7.11 (d, J = 16.4 Hz, 2 H), 7.08 (d,
J = 8.9 Hz, 4 H), 6.97 (d, J = 9.1 Hz, 4 H), 6.95 (d, J = 9.0 Hz, 4
H), 6.89 (d, J = 8.9 Hz, 4 H) 3.77 (s, 6 H) ppm.
(neat): ν = 3500–2000 (s), 2217 (m), 1712 (s), 1571 (vs), 1502 (vs),
˜
1424 (vs), 1317 (vs), 1283 (s), 1239 (vs), 1178 (vs), 1059 (s), 1027
(s), 797 (s) cm–1. HRMS-ESI: m/z = 463.11390 [(M – 2H)/2 requires
463.11164], 419.12302 [(M – 2COOH)/2 requires 419.12181].
C56H40N4O6S2: calcd. C 72.39, H 4.34, N 6.03; found C 72.28, H
4.76, N 6.29.
TB-2: Cyanoacetic acid (0.33 g, 3.89 mmol) and a catalytic amount
of piperidine were added to a solution of bis-aldehyde 7 (0.174 g,
0.19 mmol) in toluene (5 mL). The mixture was heated by micro-
wave irradiation at 100 °C for 40 min. The reaction was quenched
by the addition of saturated aqueous NH4Cl (30 mL). The formed
solid was collected to afford, after recrystallization (AcOH), the
pure product as a dark-red solid (0.081 g, 0.08 mmol, 41%); m.p.
1
Ͼ 200 °C (dec.). H NMR ([D6]DMSO): δ = 8.43 (s, 2 H), 7.97 (d,
J = 3.9 Hz, 2 H), 7.68 (d, J = 8.7 Hz, 4 H), 7.63 (d, J = 3.9 Hz, 2
H), 7.59 (s, 4 H), 7.56 (d, J = 8.7 Hz, 4 H), 7.25 (d, J = 16.3 Hz, 2
H), 7.15 (d, J = 16.3 Hz, 2 H), 7.14 (d, J = 8.7 Hz, 4 H), 7.06 (d,
J = 8.4 Hz, 4 H), 7.00 (d, J = 8.8 Hz, 4 H), 6.99 (d, J = 8.5 Hz, 4
p,pЈ-Bis{N-[(5-formyl-thien-2-yl)phenyl]-N-(4-methoxyphenyl)-
amino}stilbene (6): A solution of 5-formyl-2-thienylboronic acid
(213 mg, 1.36 mmol) and K2CO3 (481 mg, 3.75 mmol) in anhy-
drous MeOH (3 mL) was added to a solution of dibromo derivative
4b (250 mg, 0.34 mmol) and [Pd(dppf)Cl2] (55.7 mg, 0.068 mmol)
in anhydrous toluene (2.5 mL). The mixture was heated by micro-
wave irradiation at 70 °C for 10 min. After adding water (30 mL),
the mixture was extracted with CH2Cl2 (3ϫ30 mL), the combined
organic extracts were dried, and the solvent was removed by rotary
evaporation leaving a residue that was taken up with hot EtOH.
The product was obtained as a brown solid (210 mg, 0.26 mmol,
78%). 1H NMR ([D6]DMSO): δ = 9.87 (s, 2 H), 8.01 (d, J = 4.0 Hz,
2 H), 7.68 (d, J = 8.7 Hz, 4 H), 7.61 (d, J = 4.0 Hz, 2 H), 7.53 (d,
J = 8.7 Hz, 4 H), 7.12 (d, J = 8.7 Hz, 4 H), 7.11 (s, 2 H), 7.04 (d,
J = 8.6 Hz, 4 H), 6.99 (d, J = 8.9 Hz, 4 H), 6.95 (d, J = 8.7 Hz, 4
H) 3.78 (s, 6 H) ppm.
H), 3.78 (s, 6 H) ppm. IR (neat): ν = 3260–2000 (s), 2215 (m),
˜
1685 (s), 1565 (vs), 1506 (vs), 1408 (vs), 1318 (vs), 1287 (vs), 1243
(vs), 1220 (vs), 1193 (vs), 1181 (vs), 1063 (m), 828 (w), 801 (w)
cm–1. HRMS-ESI: m/z
= 514.13783 [(M – 2H)/2 requires
514.13512], 470.14602 [(M – 2COOH)/2 requires 470.14529].
C64H46N4O6S2·4H2O: calcd. C 69.67, H 4.93, N 5.08; found C
70.03, H 4.67, N 5.07.
Computational Details: DFT and TDDFT calculations were carried
out using the Gaussian03 suite of programs.[27] Following the pre-
viously proposed successful strategy,[20] the ground state geometries
were optimized in the gas phase using the B3LYP exchange-corre-
lation functional[28] and a standard 6-31G* basis set, whereas the
hybrid MPW1K functional[29] was employed for the excited state
calculations. Finally, the solvation effects were taken into account
by applying the non-equilibrium C-PCM method[30] as im-
plemented in Gaussian03.
p,pЈ-Bis{N-[(5-formyl-thien-2-yl)phenyl]-N-(4-methoxyphenyl)-
amino}distyrylbenzene (7): A solution of 5-formyl-2-thienylboronic
acid (220 mg, 1.44 mmol) and K2CO3 (510 mg, 3.95 mmol) in an-
hydrous MeOH (3 mL) was added to a solution of dibromo deriva-
tive 5b (303 mg, 0.36 mmol) and [Pd(dppf)Cl2] (59 mg, 0.072 mmol)
in anhydrous toluene (2.5 mL). The mixture was heated by micro-
wave irradiation at 70 °C for 10 min. The reaction was quenched
by the addition of water (30 mL) and extracted with CH2Cl2
(3ϫ30 mL). The combined organic extracts were dried and the sol-
vent removed by evaporation under reduced pressure, leaving a resi-
due that was taken up with hot EtOH. The product was obtained as
Preparation and Characterization of DSCs: FTO glass plates (Nip-
pon Sheet Glass, Solar 4 mm thickness) were immersed in aqueous
TiCl4 (40 mm) at 70 °C for 30 min and washed with water and etha-
nol. A paste composed of 20 nm anatase TiO2 particles for the
transparent nanocrystalline layer was coated on the FTO glass
plates by screen printing. The coating-drying procedure was re-
peated to increase the thickness to that required. The TiO2 elec-
trodes were made of an approximate 6 μm transparent layer and
a brown solid (290 mg, 0.32 mmol, 89%). 1H NMR ([D6]DMSO): δ were gradually heated under a flow of air. After a second TiCl4
= 9.87 (s, 2 H), 8.01 (d, J = 4.0 Hz, 2 H), 7.69 (d, J = 8.7 Hz, 4
H), 7.61 (d, J = 3.9 Hz, 2 H), 7.58 (s, 4 H), 7.56 (d, J = 8.6 Hz, 4
H), 7.25 (d, J = 16.4 Hz, 2 H), 7.15 (d, J = 16.4 Hz, 2 H), 7.13 (d,
treatment, the TiO2 electrodes were immersed in a 0.3 mm solution
of TB-1 or TB-2 in tetrahydrofuran (THF) and kept at r.t. for 18 h.
The liquid electrolyte consisted of 0.6 m N-methyl-N-butyl imid-
J = 8.8 Hz, 4 H), 7.05 (d, J = 8.5 Hz, 4 H), 7.00 (d, J = 8.9 Hz, 4 azolium iodide, 0.04 m iodine, 0.05 m LiI, 0.05 m guanidinium thio-
H), 6.97 (d, J = 8.7 Hz, 4 H), 3.78 (s, 6 H) ppm. cyanate, and 0.28 m tert-butylpyridine in 15:85 (v/v) mixture of va-
Eur. J. Org. Chem. 2011, 6195–6205
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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6203

A. Abbotto, F. De Angelis, M. K. Nazeeruddin et al.
FULL PAPER
Park, C. Kim, Chem. Commun. 2010, 46, 1335; c) H. Choi, I.
Raabe, D. Kim, F. Teocoli, C. Kim, K. Song, J.-H. Yum, J. Ko,
Md. K. Nazeeruddin, M. Grätzel, Chem. Eur. J. 2010, 16, 1193;
d) G. Zhang, H. Bala, Y. Cheng, D. Shi, X. Lv, Q. Yu, P. Wang,
Chem. Commun. 2009, 2198.
leronitrile and acetonitrile. The dye-adsorbed TiO2 electrode and
counter electrode were assembled into a sealed sandwich-type cell
with a gap of a hot-melt ionomer film, Surlyn (25 μm, Du-Pont).
For stability evaluation, the TiO2 electrodes were made of an ap-
proximate 7 μm transparent layer (20 nm diameter) and approxi-
mate 5 μm scattering layer (400 nm diameter, CCIC, HPW-400).
The ionic liquid electrolyte consisted of 1,3-dimethylimidazolium-
iodide, 1-ethyl-3-methylimidazoliumiodide, 1-ethyl-3-methylimida-
zolium tetracyanoborate, iodine, N-butylbenzoimidazole, and gua-
nidinium thiocyanate (molar ratio 12:12:16:1.67:3.33:0.67).[31] The
other procedure was identical to that mentioned above. In order to
reduce scattered light from the edge of the glass electrodes of the
dyed TiO2 layer, a light shading mask was used on the DSCs, so
the active area of DSCs was fixed to 0.2 cm2. For photovoltaic
measurements of the DSCs, the irradiation source was a 450 W
xenon light (Osram XBO 450, Germany) fitted with a filter (Schott
113), the power of which was regulated to the AM 1.5 G solar
standard by using a reference Si photodiode equipped with a color-
matched filter (KG-3, Schott) in order to reduce the mismatch in
the region of 350–750 nm between the simulated light and AM
1.5 G to less than 4%. The measurement of incident photon-to-
current conversion efficiency (IPCE) was plotted as a function of
excitation wavelength by using the incident light from a 300 W xe-
non lamp (ILC Technology, USA), which was focused through a
Gemini-180 double monochromator (Jobin Yvon Ltd.). The mea-
surement settling time between applying a voltage and measuring
a current for the I-V characterization of DSCs was fixed to 40 and
200 ms for liquid and ionic liquid electrolytes, respectively.
[8]
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Received: June 7, 2011
Published Online: September 5, 2011
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