New bithiazole-based sensitizers for efficient and stable dye-sensitized solar cells

Article abstract of DOI:10.1002/chem.201103702

A series of new push-pull organic dyes (BT-I-VI), incorporating electron-withdrawing bithiazole with a thiophene, furan, benzene, or cyano moiety, as p spacer have been synthesized, characterized, and used as the sensitizers for dye-sensitized solar cells (DSSCs). In comparison with the model compound T1, these dyes containing a thiophene moiety between triphenylamine and bithiazole display enhanced spectral responses in the red portion of the solar spectrum. Electrochemical measurement data indicate that the HOMO and LUMO energy levels can be tuned by introducing different p spacers between the bithiazole moiety and cyanoacrylic acid acceptor. The incorporation of bithiazole substituted with two hexyl groups is highly beneficial to prevent close φ-φ aggregation, thus favorably suppressing charge recombination and intermolecular interaction. The overall conversion efficiencies of DSSCs based on bithiazole dyes are in the range of 3.58 to 7.51%, in which BT-I-based DSSCs showed the best photovoltaic performance: A maximum monochromatic incident photon-to-current conversion efficiency (IPCE) of 81.1%, a short-circuit photocurrent density (J_sc) of 15.69 mAcm^ae2, an open-circuit photovoltage (V_oc) of 778 mV, and a fill factor (ff) of 0.61, which correspond to an overall conversion efficiency of 7.51% under standard global AM 1.5 solar light conditions. Most importantly, long-term stability of the BT-I-IIIbased DSSCs with ionic-liquid electrolytes under 1000 h of light soaking was demonstrated and BT-II with a furan moiety exhibited better photovoltaic performance of up to 5.75% power conversion efficiency.

Full text of DOI:10.1002/chem.201103702

FULL PAPER
DOI: 10.1002/chem.201103702
New Bithiazole-Based Sensitizers for Efficient and Stable Dye-Sensitized
Solar Cells
Jinxiang He, Fuling Guo, Xin Li, Wenjun Wu,* Jiabao Yang, and Jianli Hua*[a]
Abstract: A series of new push–pull or-
ganic dyes (BT-I–VI), incorporating
electron-withdrawing bithiazole with a
thiophene, furan, benzene, or cyano
moiety, as p spacer have been synthe-
sized, characterized, and used as the
sensitizers for dye-sensitized solar cells
(DSSCs). In comparison with the
model compound T1, these dyes con-
taining a thiophene moiety between tri-
phenylamine and bithiazole display en-
hanced spectral responses in the red
portion of the solar spectrum. Electro-
chemical measurement data indicate
that the HOMO and LUMO energy
levels can be tuned by introducing dif-
ferent p spacers between the bithiazole
moiety and cyanoacrylic acid acceptor.
The incorporation of bithiazole substi-
tuted with two hexyl groups is highly
beneficial to prevent close p–p aggre-
gation, thus favorably suppressing
charge recombination and intermolecu-
lar interaction. The overall conversion
efficiencies of DSSCs based on bithia-
zole dyes are in the range of 3.58 to
7.51%, in which BT-I-based DSSCs
showed the best photovoltaic perform-
ance: a maximum monochromatic inci-
dent photon-to-current conversion effi-
ciency (IPCE) of 81.1%, a short-circuit
photocurrent
density
(Jsc)
of
15.69 mAcmÀ2, an open-circuit photo-
voltage (Voc) of 778 mV, and a fill
factor (ff) of 0.61, which correspond to
an overall conversion efficiency of
7.51% under standard global AM 1.5
solar light conditions. Most important-
ly, long-term stability of the BT-I–III-
based DSSCs with ionic-liquid electro-
lytes under 1000 h of light soaking was
demonstrated and BT-II with a furan
moiety exhibited better photovoltaic
performance of up to 5.75% power
conversion efficiency.
Keywords: bithiazoles
· dyes/pig-
ments · photophysics · sensitizers ·
solar cells
Introduction
as large molar extinction coefficient, simple synthesis, low
cost, and facile molecular design. Recently, studies have
been focused on molecular engineering of chromophores to
attain the capability of panchromatic light harvesting. Vari-
eties of organic dyes, such as cyanine,[4] merocyanine,[5] cou-
marin,[6] indoline,[7] hemicyanine,[8] phenothiazine,[9] phenox-
azine,[10] and diketopyrrolopyrrole or pyrrole dyes,[11] have
been reported, thus making organic dyes fruitful in their ap-
plication to DSSCs. Impressive photovoltaic performances
have been obtained by using organic dyes that show effi-
ciencies exceeding 10%.[12]
Converting solar energy into electricity is generally consid-
ered to be the most promising way to solve the energy crisis
in the whole world. A variety of light-harvesting devices
were developed rapidly including dye-sensitized solar cells
(DSSCs),[1] which have attracted great worldwide attention.
As potential candidates for the future modality of photovol-
taic cells, DSSCs manifest considerable efficiency and low
cost, in which the sensitizer is deemed to be the crucial com-
ponent. At present, DSSC sensitizers based on RuII-poly-
A
The organic dyes are commonly constructed with donor–
p bridge–acceptor (D-p-A)[13] configurations due to the ef-
fective photoinduced intramolecular charge-transfer proper-
ty. For efficient solar energy conversion, it is desirable to
use dyes that possess high molar extinction coefficients that
extend throughout the visible into the near-IR regions. As is
well known, extending p-conjugated bonding bridges always
brings a low stability although it might efficiently redshift
the absorption response. Recently, a few organic D-A-p-A
dyes incorporating electron-withdrawing terephthalonitrile
or electron-deficient heteroarenes, such as quinoline,[14] iso-
xazole,[15] thiazole moieties,[16] and the pyrimidine ring,[17] as
linkages to cyanoacrylic acid, showed more redshifted ab-
sorption than their parent analogues and thus enhanced the
light harvesting and stability. Meanwhile, some new D-A-p-
A configuration dyes with diketopyrrolopyrrole, bithiazole,
and benzothiadiazole moieties have been reported by our
cies of almost 12% at standard global air mass (AM) 1.5.[2]
However, the large-scale application of Ru complexes has
become a critical problem due to limited resources and the
hefty purification steps needed.[3] In recent years, the inter-
est in organic dyes as substitutes of noble metal complexes
has been increasing because of their many advantages, such
[a] J. He,+ F. Guo,+ Dr. X. Li, W. Wu, J. Yang, Prof. Dr. J. Hua
Key Laboratory for Advanced Materials
Institute of Fine Chemicals and Department of Chemistry
East China University of Science and Technology
130 Meilong Road, Shanghai 200237 (P.R. China)
Fax : (+86)21-64252758
[+] These authors contributed equally to this work.
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group.[7d,11a,18] It should be noted that bithiazole is a well-
known electron-deficient unit that acts as an acceptor. Two
long alkyl chains attached on a thiazole moiety are consid-
ered to suppress charge recombination and raise the conduc-
tion band edge and hence to acquire a high open-circuit
photovoltage (Voc) of DSSCs.[19] 2-Cyano-3-(5-{5’-[4-(diphe-
nylamino)phenyl]-4,4’-dihexyl-2,2’-bithiazol-5-yl-4,4’-dihexyl-
2,2’-bithiazol-5-yl}thiophen-2-yl)acrylic acid (T1; Figure 1)
was reported as a dye for efficient DSSCs in our previous
studies,[18] in which we identified that solar cells with bithia-
zole-based sensitizers could achieve high performance; how-
ever, the short-circuit photocurrent density (Jsc) for the sen-
sitizer still has room for improvement and optimization. As
reported, different p spacers could lead to diverse positions
of the HOMO and LUMO energy levels, thus resulting in a
different driving force for electron injection and tuning the
short-circuit photocurrent density (Jsc). To enrich the re-
search field of bithiazole dyes for DSSCs and gain more in-
formation on the structure–property relationships, we syn-
thesized six new bithiazole dyes (BT-I–VI, Figure 1).
which has a smaller resonance energy (16 kcalmolÀ1) in the
spacer than the thiophene (29 kcalmolÀ1) and benzene
(36 kcalmolÀ1), would be more efficient for the hole location
and reinforce the stability of the dye sensitizers.[20] Also,
some reports have shown that incorporation of a furan
moiety could somewhat improve the solubility of the organ-
ic dyes.[21] In addition, we also synthesized bithiazole-based
sensitizers containing a cyano moiety (BT-IV–VI). The cya-
novinyl group can improve the energy gap of dyes to in-
crease the absorption wavelength for better light harvesting.
A cyanovinyl unit can effectively stabilize the LUMO level
and shift the electronic absorption band.[22] Finally, the six
sensitizers were applied to the sensitization of nanocrystal-
line TiO2-based solar cells, and the effect of p-spacer linkers
on the photophysical and electrochemical properties of the
dyes and the solar cell performance are detailed.
Results and Discussion
For BT-I–III, we added a thiophene moiety between tri-
phenylamine and bithiazole to extend the absorption wave-
length and improve the molar extinction coefficient (e).[6a]
At present most of the metal-free dye sensitizers with high
efficiency rely on thiophene or thiophene-based heterocy-
cles as the p spacer. Furan, thiopheneꢁs oxygen analogue,
Synthesis: The synthetic route to the six dyes (BT-I–VI)
containing the bithiazole moiety is depicted in Scheme 1. As
is well known, the strong intermolecular p–p interaction
may lead to interfacial charge recombination, thus resulting
in a decrease in Voc. The two long hexyl groups on the bi-
thiazole unit can improve the solubility and reduce the inter-
molecular p–p interaction. The
Suzuki coupling reaction of
5,5’-dibromo-4,4’-dihexyl-2,2’-
bithiazole with thiophen-2-yl-
boronic acid afforded the
monosubstituted compound 1.
The reaction produced the dis-
ubstituted side products as well;
fortunately monocapped com-
pounds can be easily separated
by column chromatography. In
the next step, the Suzuki cou-
pling reaction of 1 with 5-for-
mylthiophen-2-ylboronic acid,
5-formylfuran-2-ylboronic acid,
and 4-formyl phenylboronic
acid afforded compounds 2–4,
respectively. Then, the reactions
of compounds 5–7 with 4-(di-
phenylamino)phenylboronic
acid provided aldehyde precur-
sors 8–10, respectively. As de-
picted in Scheme 1, the synthe-
ses of the important intermedi-
ates 14–16 were also started
from 5,5’-dibromo-4,4’-dihexyl-
2,2’-bithiazole. The asymmetri-
cal Suzuki coupling with 4-(di-
phenylamino)phenylboronic
acid afforded monosubstituted
Figure 1. Chemical structures of the sensitizers (BT-I–VI) and the reference dye T1.
compound 11, and then com-
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Bithiazole-Based Sensitizers for Dye-Sensitized Solar Cells
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Scheme 1. Synthetic routes to bithiazole-based dyes BT-I–VI. NBS=N-bromosuccinimide, N-FMP=N-formylmorpholine.
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J. Hua, W. Wu et al.
Table 1. Optical and electrochemical properties of bithiazole dyes BT-I–
VI and T1.
pound 12 was obtained by bromoformylation of 11. The
Knoevenagel condensation reaction of 12 with 2-(4-bromo-
phenyl)acetonitrile afforded 13, in which the Suzuki cou-
pling reaction of 13 with 5-formylthiophen-2-ylboronic acid,
5-formylfuran-2-ylboronic acid, and 4-formyl phenylboronic
acid afforded precursors 14–16, respectively. Finally, the
target products (BT-I–VI) were synthesized by the Knoeve-
nagel condensation reaction of aldehydes 8–10 and 14–16
with cyanoacetic acid in the presence of acetic acid and am-
monium acetate. All the key intermediates and six new or-
ganic bithiazole-based sensitizers (BT-I–VI) were confirmed
by 1H and 13C NMR spectroscopy and HRMS.
[a]
[b]
[d]
Dye
lmax [nm]
lmax
HOMO[c] [V] E0À0
A
LUMO[e] [V]
(vs. NHE)
(e [10À4 mÀ1 cmÀ1]) [nm]
ACHTUNGTRENNUNG
BT-I
470 (4.07)
483
512
434
540
534
517
471
1.10
1.12
1.13
1.27
1.28
1.31
1.06
2.12
2.16
2.34
2.19
2.21
2.28
2.42
À1.02
À1.04
À1.20
À0.92
À0.93
À0.97
À1.36
BT-II 475 (3.84)
BT-III 433 (3.65)
BT-IV 471 (5.02)
BT-V 480 (4.13)
BT-VI 459 (4.40)
T1
457 (3.44)
[a] Absorption maximum in CH2Cl2. [b] Absorption maximum on TiO2
film. [c] HOMOs were measured in THF with 0.1m tetrabutylammonium
hexafluorophosphate (TBAPF6) as the electrolyte (working electrode:
Pt; reference electrode: SCE; calibrated with ferrocene/ferrocenium (Fc/
Fc+) as an external reference; counter electrode: Pt wire). [d] E0À0 was
estimated from the absorption spectrum in solution. [e] LUMO is esti-
mated by subtracting E0À0 from the HOMO.
Absorption properties in solution and on TiO2 film: The
UV/Vis absorption spectra in dichloromethane and adsor-
bed onto 5 mm transparent TiO2 films of the dyes BT-I–VI
and T1 are shown in Figure 2 and the corresponding data
are summarized in Table 1. In the UV/Vis spectra, these
new dyes exhibit three major bands at 280–325, 340–360,
and 420–550 nm, respectively. The absorption band at 280–
325 nm was ascribed to a localized aromatic p–p* transition
typical of triphenylamine, and the middle weak shoulder
peak was due to the p–p* transition of triphenylamine and
the thiophene moiety for BT-I–III. The three dyes contain-
ing a cyano moiety exhibit no obvious peak in the 340–
360 nm range. The prominent bands at around 420–550 nm
can be attributed to the intramolecular charge transfer
(ICT) between the triphenylamine donor and the acceptor.
For the charge-transfer band, the absorption maxima in
CH2Cl2 are at 470, 475, and 433 nm for BT-I–III, and 471,
480, 459, and 457 nm for BT-IV–VI and T1, respectively.
Except for BT-III, the other five dyes exhibit slightly red-
shifted absorption compared with that of dye T1 due to the
extended conjugated structure.
For the three dyes containing a thiophene moiety between
triphenylamine and bithiazole, BT-I and BT-II have very
similar absorption properties. Compared with BT-I and BT-
II, BT-III exhibits about a 40 nm blueshift in absorption.
This may be caused by the decrease of coplanarity between
the bithiazole moiety and the electron acceptor due to the
introduction of the benzene unit. Meanwhile, similar phe-
nomena were observed for the dyes with a cyano moiety
(BT-IV–VI), in which BT-VI exhibits about a 12 and 21 nm
blueshift in absorption relative to BT-IV and BT-V, respec-
tively.
From the optimized ground-state geometries shown in
Table 2, the dihedral angles between the bithiazole rings and
p spacer near the anchoring moiety are predicted to be 0.6,
0.1, 34.8, 22, 0.5, 34.2, and 0.28 for BT-I–VI and T1, respec-
tively. The relatively large dihedral angles for phenylene-
bridged BT-III and BT-VI lead to a large blueshift of ab-
sorption in solution. The torsion angle for furan-bridged BT-
II is a little smaller than that for thiophene-bridged BT-I,
which results in a 5 nm redshifted absorption. Meanwhile
BT-V exhibits about a 9 nm redshift in absorption compared
to BT-VI.
The molar extinction coefficients (emax) of BT-I–VI and
T1 are 40700, 38400, 36500, 50200, 41300, 44000, and
34400mÀ1 cmÀ1, respectively, in which the BT-I–VI dyes have
a higher molar extinction coefficient than the corresponding
dye T1 and the emax of BT-IV–VI is higher than those of BT-
I–III, respectively, thus suggesting that BT-IV–VI can har-
vest sunlight more effectively. In comparison with conven-
tional ruthenium complexes (for example, 1.39 ꢂ104 mÀ1 cmÀ1
for N719),[23] the greater maximum absorption coefficients
of the organic dyes allow a correspondingly thinner nano-
Figure 2. Absorption spectra of BT-I–VI and T1, measured a) in CH2Cl2
and b) adsorbed onto 5 mm transparent TiO2 films.
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Bithiazole-Based Sensitizers for Dye-Sensitized Solar Cells
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Table 2. Optimized structures and electron distribution in HOMO and LUMO levels of the BT dyes.
Dye Optimized structure[a]
HOMO
LUMO
BT-
I
BT-
II
BT-
III
BT-
IV
BT-
V
BT-
VI
T1
[a] The dihedral angles between the bithiazole rings and p spacer near the anchoring moiety are given in the graphics.
crystalline film, thereby avoiding a decrease in the filmꢁs
mechanical strength. This also benefits the electrolyte diffu-
sion in the film and reduces the recombination possibility of
the light-induced charges during transportation.[4c,24]
onto transparent TiO2 show a markedly broad profile, which
is beneficial to light harvesting.
Electrochemical properties: To evaluate the possibility of
electron transfer from the excited dye to the conduction
band of TiO2, cyclic voltammetry was performed in tetrahy-
drofuran, with 0.1m tetrabutylammonium hexafluorophos-
phate as the supporting electrolyte, Pt as the working elec-
trode and counter electrode, and a saturated calomel elec-
trode (SCE) as the reference electrode. The examined
HOMO and LUMO levels are collected in Table 1. The
SCE reference electrode was calibrated by using a ferro-
cene/ferrocenium (Fc/Fc+) redox couple as an external
Compared to the spectrum in CH2Cl2 solution, the ab-
sorption peaks for BT-I–VI and T1 on 5 mm transparent
TiO2 film are all redshifted (Table 1). Generally, when sensi-
tizers are anchored onto a nanocrystalline TiO2 surface, they
have a strong tendency to aggregate in solution or at the
solid/liquid interface due to a strong attractive force be-
tween the molecules. Dye aggregates usually have three
forms: redshifted J-aggregates, blueshifted H-aggregates,
and both red- and blueshifted herringbone aggregates. From
the above, all of the dyes have strong J-aggregates. In partic-
ular, BT-IV–VI have much stronger aggregation than the
dyes BT-I–III without a cyano moiety (see Figure 2). Also,
the experimental results indicate that the bithiazole-based
dyes used in this work form J-aggregates on the TiO2 nano-
crystal surface. The interaction between the carboxylate
group and the surface Ti4+ ions may lead to increased de-
standard and the E = of the Fc/Fc+ redox couple was found
1
2
to be 0.55 V versus the SCE reference electrode. The poten-
tials versus the normal hydrogen electrode (NHE) were cali-
brated by addition of 0.63 V to the potentials versus
Fc/Fc+.[9a] Therefore, the potentials measured versus the
SCE were converted to NHE by addition of 0.08 V. It can
be seen from Figure 3 that the first half-wave potentials of
the dyes BT-I–VI are 1.02, 1.04, 1.05, 1.19, 1.20, and 1.23 V
(vs. SCE), respectively. Therefore, the ground-state oxida-
tion potential corresponding to the HOMO levels are 1.10,
1.12, 1.13, 1.27, 1.28, and 1.31 V (vs. NHE), respectively. The
bandgap energies (E0À0) of the six dyes were 2.12, 2.16, 2.34,
ACHTUNGTRENNUNGlocalization of the p* orbital of the conjugated framework.
The energy of the p* level is decreased by this delocaliza-
tion, which explains the redshift of the absorption spectra. It
is noteworthy that the absorption spectra of dyes adsorbed
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J. Hua, W. Wu et al.
are considerably more negative than the conduction-band-
edge energy level (Ecb) of the TiO2 electrode (À0.5 V vs.
NHE), which implies that electron injection from the excit-
ed dye into the conduction band of TiO2 is energetically per-
mitted.[25] The relatively large energy gaps between the
LUMO and Ecb provide the possibility for the addition of 4-
tert-butylpyridine (TBP) to the electrolyte, which can shift
the Ecb of TiO2 more negatively and, consequently, improve
the voltage and total efficiency.[26]
To gain better insight into the electronic properties of
these dyes, density functional theory (DFT) calculations
were performed at the B3LYP/6-31G* level. The electron
distribution of the HOMOs and LUMOs of bithiazole dyes
are shown in Table 2. The electron density of the HOMOs
of BT-I–VI and T1 sensitizers are primarily located at tri-
phenylamine, whereas those of the LUMOs are delocalized
over the aromatic rings, thiophene, furan, and benzene moi-
eties, and the anchoring group. In particular, the electron
density of the LUMOs is located at the electron trap of cya-
novinyl units for the dyes with a cyano moiety. This leads to
inefficient electron injection. The electron density on the an-
choring group also suggests that the excitation from the
HOMO to the LUMO can move the electron distribution
from the triphenylamine donor unit to the cyanoacrylic acid
anchoring moiety to realize electron injection into the con-
duction band of TiO2.
Figure 3. Cyclic voltammetry plots of bithiazole dyes BT-I–VI measured
in THF with TBAPF6 (0.1m) as the electrolyte.
2.19, 2.21, and 2.28 eV, respectively, which were estimated
from the absorption thresholds from absorption spectra in
solution. The estimated excited-state potential correspond-
ing to the LUMO levels, calculated from EHOMOÀE0À0, are
À1.02, À1.04, À1.20, À0.92, À0.93, and À0.97 V, respectively.
It is clear that the HOMO energy level of T1 is higher than
those of BT-I–VI. At the same time, introduction of a thio-
phene unit between triphenylamine and bithiazole elevates
the HOMO levels. Based on Figures 3 and 4, BT-IV–VI are
Photovoltaic device performance: Figure 5 shows the action
spectra of incident photon-to-current conversion efficiency
(IPCE) for DSSCs based on these dyes. The dye-coated
TiO2 film was used as the working electrode, platinized fluo-
rine-doped tin oxide (FTO) glass as the counter electrode,
and
0.6m
1,2-dimethyl-3-n-propylimidazolium
iodide
(DMPII), 0.05m I2, 0.10m LiI, and 0.5m TBP in acetonitrile
and methoxypropionitrile (volume ratio, 7:3) mixed solution
as the redox electrolyte. Generally, the solar cells sensitized
by BT-I–III have better performances than the dyes with a
Figure 4. Experimental energy-level diagrams of the HOMO and LUMO
for BT-I–VI and T1 versus SCE. The dashed lines show the relative posi-
tions of the energy levels of dyes. CB=conduction band.
oxidized at more positive potentials than BT-I–III. On the
other hand, the introduction of the cyano group into BT-
IV–VI lowers their LUMO energy level by increasing the
electron affinity.
From these data, we found that the HOMO levels of the
dyes are much more positive than the iodine/iodide redox
potential value (0.4 V), thus, indicating that the oxidized
dyes, formed after electron injection into the conduction
band of TiO2, could accept electrons from IÀ ions thermody-
namically (see Figure 4). The LUMO levels of these dyes
Figure 5. Photocurrent action spectra of the TiO2 electrodes sensitized by
BT-I–VI.
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Bithiazole-Based Sensitizers for Dye-Sensitized Solar Cells
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cyano moiety in the range of 370–500 nm, especially for BT-
IV–VI in the range of 370–630 nm. These results can be as-
cribed to the more negative LUMO energy level of BT-I–III
leading to higher electron-injection efficiency. The solar
cells based on BT-I and BT-III exhibit high IPCE values
above 60% in the range of 400–500 nm and with a highest
value of 81.1% at 480 nm for BT-I, and 76.9% at 440 nm
for BT-III. The BT-II- and BT-VI-sensitized cells have IPCE
values above 55% in the range of 370–520 nm, whereas the
IPCE value of BT-IV and BT-V are reduced significantly.
This leads to the poor performance of the BT-IV- and BT-V-
based DSSCs in terms of photovoltage efficiency. The
threshold wavelengths of the IPCE spectra for all of the
dyes were located below 640 nm, especially for BT-III at
about 600 nm, which is consistent with the absorption spec-
tra when dyes were adsorbed onto TiO2 films. Figure 6
The short-circuit current Jsc is related to the molar extinc-
tion coefficient of the dye molecule, for which a higher
molar extinction coefficient yields a higher short-circuit cur-
rent. The onset wavelengths of BT-I and BT-II are redshift-
ed by 50 and 40 nm with respect to T1 in solution, respec-
tively. The molar extinction coefficient of BT-I–III is higher
than that of T1, in which BT-I has the largest emax. Com-
pared with T1, BT-I–III have an increased light-harvesting
efficiency and consequently improved Jsc due to their higher
molar extinction coefficients and broader photocurrent
action spectra. In addition, the open-circuit photovoltage of
BT-III is better than those of BT-I- and BT-II-based solar
cells. This may be caused by the introduction of benzene,
which leads to reduction of electron recombination. Consis-
tent with the results of IPCE, the photovoltaic data of BT-I
and BT-II are superior to those of BT-III. Different from
BT-I–III, the dyes BT-IV–VI linked by the cyanovinyl unit
perform with relatively lower photovoltage efficiency. There
is no improvement of the device efficiency, although incor-
poration of the cyano moiety in the conjugated bridge leads
to bathochromic and hyperchromic shifts for light harvest-
ing. This outcome may be comprehended in that the charge-
trapping effect of the internal electron-withdrawing charac-
ter of the cyanovinyl unit hampers electron injection from
the excited sensitizers into TiO2, suggested by the low Jsc
values of DSSCs in this study.[22] Similar to BT-I–III, BT-VI
connected by the benzene ring for BT-IV–VI performs with
the best Voc and leads to much better performance than BT-
IV and BT-V. This phenomenon can be understood by the
fact that the BT-VI-based cell shows much higher IPCE
values between 370 and 560 nm than BT-IV- and BT-V-
based cells. On the other hand, from the absorption spectra
of dyes adsorbed onto TiO2 films, BT-IV and BT-V have
similar large-scale absorption peaks. These two dyes exhibit
greater aggregation, which is not of benefit to light-to-cur-
rent conversion.
Figure 6. Current–voltage characteristics of DSSCs sensitized by the dyes
BT-I–VI and T1 under irradiation of AM 1.5G simulated solar light
(100 mWcmÀ2) with liquid electrolyte.
Overall, the BT-I–III-sensitized cell gave a Jsc of 15.69,
15.42, and 12.47 mAcmÀ2, a Voc of 778, 748, and 789 mV,
and a ff of 0.61, 0.62, and 0.61, corresponding to an overall
conversion efficiency of 7.51, 7.11, and 6.01%, respectively.
Under the same conditions, the photovoltaic parameters (Jsc,
Voc, ff, and h) of cells sensitized by BT-IV–VI are:
8.95 mAcmÀ2, 625 mV, 0.66, and 3.69%; 8.43 mAcmÀ2,
623 mV, 0.68, and 3.58%; and 11.54 mAcmÀ2, 728 mV, 0.65,
and 5.49%, respectively. It was found that the Voc value of
BT-III is 11 mV higher than those of BT-I and BT-II, and of
BT-VI is 103 mV higher than those of BT-IV and BT-V, thus
indicating that the charge recombination process was effec-
tively retarded by introducing benzene as the p spacer.
We also measured the photovoltaic performance of BT-
IV- and BT-V-based DSSCs with co-adsorbent (chenodeoxy-
cholic acid) for comparison, and they exhibited conversion
efficiencies of 3.83 and 3.79%, respectively. Therefore, co-
adsorbents have less impact on improving the photovoltaic
performance of DSSCs based on the two dyes. To under-
stand why T1 has the largest open-circuit voltage, the effect
of BT-I–VI and T1 on dark current was studied as shown in
shows the current–voltage characteristics of DSSCs fabricat-
ed with these bithiazole dyes (BT-I–VI and T1) as sensitiz-
ers under standard global AM 1.5 solar light conditions. The
detailed parameters of short-circuit current density (Jsc),
open-circuit voltage (Voc), fill factor (ff), and photovoltaic
conversion efficiency (h) are summarized in Table 3.
Table 3. Photovoltaic performance of DSSCs based on the dyes BT-I–VI
and T1 with a liquid electrolyte.[a]
Dye
Jsc [mAcmÀ2
]
Voc [mV]
ff
h [%]
BT-I
15.69
15.42
12.47
8.95
8.43
11.54
11.78
778
748
789
625
623
728
810
0.61
0.62
0.61
0.66
0.68
0.65
0.60
7.51
7.11
6.01
3.69
3.58
5.49
5.73
BT-II
BT-III
BT-IV
BT-V
BT-VI
T1
[a] Illumination: 100 mWcmÀ2 simulated AM 1.5G solar light; electrolyte
containing: 0.1m LiI, 0.05m I2, 0.6m DMPII, and 0.5m TBP in the mixed
solvent of acetonitrile and 3-methoxypropionitrile (7:3, v/v).
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J. Hua, W. Wu et al.
Figure 6. It is clear that the T1 dark-current onset potential
shifted to a larger value than that of BT-I–VI. This dark-cur-
rent change indicates that T1 can inhibit charge recombina-
tion between injected electrons and I3 ions in the electro-
lyte.
chemical model.[27] In Figure 7c, RS, Rrec, and RCE represent
series resistance and charge-transfer resistances at the dye/
TiO2/electrolyte interface and counter electrode (CE), re-
spectively. The series resistance (RS) and electrolyte reduc-
tion resistance (RCE) corresponding to the first semicircle in
Figure 7a show almost the same value in the six dye-based
DSSCs due to the same electrode material and same elec-
trolyte, and the Rrec corresponds to the middle semicircle.
From the EIS measurements, the electron lifetime (te) ex-
pressing the electron recombination between the electrolyte
and TiO2 was calculated following a literature procedure.[28]
The reaction resistance of the DSSCs consisting of TiO2/dye/
electrolyte and Pt/electrolyte interfaces based on BT-I–VI
were analyzed by software (ZSimpWin) using an equivalent
circuit. As shown in Table 4, the Rrec of the BT-I–VI dyes
À
Electrochemical impedance spectroscopy: In addition, elec-
trochemical impedance spectroscopy (EIS) was employed to
study the electron recombination in DSSCs based on these
BT-I–VI dyes under À0.70 V bias applied voltage in the
dark. The Nyquist plot, Bode plot, and equivalent circuit are
shown in Figure 7a–c, respectively. Some important parame-
ters can be obtained by fitting the EIS data with an electro-
Table 4. Parameters obtained by fitting the impedance spectra of the
DSSCs with sensitizers by using the equivalent circuit (Figure 7c).
Dye
RS [W]
RCE [W]
Rrec [W]
te [ms]
BT-I
31.6
27.6
39.4
36.6
35.2
29.4
14.0
14.9
11.8
18.0
14.8
17.3
134.6
84.6
343.0
32.5
18.4
64.7
62.2
35.9
169.7
13.1
2.4
BT-II
BT-III
BT-IV
BT-V
BT-VI
34.5
are 134.6, 84.6, 343.0, 32.5, 18.4, and 64.7 W, respectively.
The electron lifetimes of the BT-I–VI dyes are 62.2, 35.9,
169.7, 13.1, 2.4, and 34.5 ms, respectively. Dye BT-III gives
the longest te among these dyes. A dye with a longer life-
time can avoid electron recombination efficiently and has a
better Voc. This result is in agreement with the observed
shift in the Voc value under standard global AM 1.5 illumina-
tion.
Performance of a solvent-free solar cell and its stability:
Long-term stability is a vital parameter for sustained cell op-
eration, so we substituted the liquid electrolyte with a sol-
vent-free ionic liquid. Bithiazole dyes BT-I–III were evaluat-
ed as sensitizers for the solvent-free ionic liquid electrolyte
DSSC with 0.1m iodine, 0.1m lithium iodide, and 0.45m ben-
zimidazole in 1-propyl-3-methylimidazolium iodide as redox
electrolyte, because the DSSCs employing dyes BT-I–III
showed a better photovoltaic performance than those with
BT-IV–VI. The photovoltaic performances of the BT-I–III-
sensitized TiO2 film electrodes with an ionic liquid electro-
lyte are listed in Table 5 under standard global AM 1.5 solar
light conditions (100 mWcmÀ2), and the corresponding pho-
tocurrent–voltage curves are shown in Figure 8. The BT-I–
III-sensitized cells gave Jsc values of 13.87, 13.80, and
12.00 mAcmÀ2, Voc values of 0.633, 0.641, and 0.676 V, and ff
values of 0.63, 0.65, and 0.64, corresponding to h values of
5.49, 5.75, and 5.22%, respectively. The h value of BT-II is
higher than that of BT-I in a solvent-free ionic-liquid elec-
trolyte, which is the opposite result to that in a liquid elec-
trolyte for BT-I and BT-II. This could be explained by the
Figure 7. Impedance spectra for DSSCs based on dyes BT-I–VI measured
at À0.70 V bias in the dark. a) Nyquist plots; b) Bode phase plots;
c) equivalent circuits. The lines in (a) and (b) show theoretical fits by
using the equivalent circuits in (c). CPE=constant phase element (in EIS
circuit).
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Bithiazole-Based Sensitizers for Dye-Sensitized Solar Cells
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Table 5. Performance parameters of solvent-free ionic-liquid electrolyte
solar cells sensitized by BT-I–III.[a]
Dye
Jsc [mAcmÀ2
]
Voc [mV]
ff
h [%]
BT-I
BT-II
BT-III
13.87
13.80
12.00
633
641
676
0.63
0.65
0.64
5.49
5.75
5.22
[a] Illumination: 100 mWcmÀ2 simulated AM 1.5G solar light; electrolyte
containing: 0.1m LiI, 0.05m I2, 0.6m DMPII, and 0.5m TBP in the mixed
solvent of acetonitrile and 3-methoxypropionitrile (7:3, v/v).
Figure 8. Photocurrent–voltage curves of BT-I–III-sensitized TiO2 electro-
des under standard global AM 1.5 illumination (100 mWcmÀ2) with a sol-
vent-free ionic-liquid electrolyte.
different thickness of TiO2 films used in the liquid electro-
lyte and solvent-free ionic-liquid electrolyte DSSCs. The dif-
À
fusion of IÀ and I3 ions in an ionic-liquid electrolyte is slow
because of their high viscosity, so a thin nanocrystalline
TiO2 film is necessary to reach high conversion efficiencies
in solvent-free ionic-liquid electrolyte DSSCs.[29]
Figure 9 shows the variations of the conversion efficiency
of solar cells sensitized with BT-I–III under continuous visi-
ble-light irradiation (AM 1.5G, 100 mWcmÀ2). Values for Jsc,
Voc, ff, and h were recorded over a period of 1000 h. Voc re-
mained almost constant for the first 800 h and then in-
creased gradually. The open-circuit potential for BT-I–III in-
creased by 13, 32, and 9 mV, respectively. In contrast to the
Voc increase under illumination, all three cells showed an in-
crease in Jsc during the initial 400 h, which may be caused by
filling of electrolyte into the inner mesopores of the TiO2
electrode. Upon a continuous 1000 h of light soaking, the
photocurrent of the DSSCs based on BT-I–III showed an
8.7, 9.0, and 13.3% decrease, respectively. The overall effi-
ciency remained at 90% of the initial value after 1000 h of
visible-light soaking. This demonstrates that the amount of
dye on the TiO2 surface remained intact after a long time of
light soaking.
Figure 9. Stability test of photovoltaic parameter (Jsc, Voc, ff, and h) varia-
tions with aging time for the devices based on a) BT-I-, b) BT-II-, and
c) BT-III-sensitized TiO2 film with a solvent-free ionic-liquid electrolyte
during 1 sun visible light irradiation.
Conclusion
Six metal-free organic dyes (BT-I–VI), comprising a triphe-
nylamine moiety as the electron donor, a cyanoacrylic acid
as the anchoring group, bithiazole substituted with two
hexyl groups as the core, and different p spacers, were de-
signed and synthesized for use in DSSCs. The modification
of the bithiazole dyes is an effective method to control dye
absorption properties, which determines the light-harvesting
efficiency. The introduction of a thiophene group between
triphenylamine and bithiazole can improve the light-harvest-
ing ability of the dyes. Introduction of a cyano unit can
extend the capability for light harvesting of bithiazole, but
the charge-trapping effect of the internal electron-withdraw-
ing character of the cyanovinyl unit hampers electron injec-
tion into TiO2 and decreases the device performance.
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J. Hua, W. Wu et al.
(8 mL) was injected under an argon atmosphere. After heating the reac-
tion mixture at reflux for 3 h, the product was cooled to room tempera-
ture and poured into water (20 mL). The mixture was extracted with
CH2Cl2 (3ꢂ20 mL). The combined organic layers were dried with anhy-
drous Na2SO4. The solvent was evaporated, and the residue was purified
by column chromatography on silica gel (PE/CH2Cl2 =3:1–1:1, v/v) to
give a red solid (200 mg, yield 87%). 1H NMR (CDCl3, 400 MHz): d=
9.92 (s, 1H), 7.40 (d, J=4.0 Hz, 1H), 7.28 (d, J=4.0 Hz, 1H), 7.06 (d, J=
4.0 Hz, 1H), 6.95 (d, J=3.6 Hz, 1H), 2.99 (t, J=7.8 Hz, 2H), 2.89 (t, J=
7.8 Hz, 2H), 1.82–1.75 (m, 4H), 1.46–1.28 (m, 12H), 0.91–0.88 ppm (m,
6H).
DSSCs based on BT-I, containing a thiophene moiety be-
tween triphenylamine and bithiazole, exhibit the best overall
light-to-electricity conversion efficiency of 7.51% (Jsc =
15.69 mAcmÀ2, Voc =778 mV, ff=0.61) under AM 1.5 irradi-
ation (100 mWcmÀ2). Most importantly, long-term stability
of the BT-I–III-based DSSCs with ionic-liquid electrolytes
under 1000 h of light soaking was demonstrated, and BT-II,
with a furan moiety, exhibited a better photovoltaic per-
formance of up to 5.75% power conversion efficiency. All
of the results reveal that these metal-free organic bithiazole
dyes are promising in the development of DSSCs.
5-[5’-(5-Bromothiophen-2-yl)-4,4’-dihexyl-2,2’-bithiazol-5-yl]furan-2-car-
baldehyde (6): The synthetic method resembled that of compound 5, and
the compound was purified by column chromatography on silica gel (PE/
CH2Cl2 =3:1–1:1, v/v) to give an orange solid (147 mg, yield 98%).
1H NMR (CDCl3, 400 MHz): d=9.68 (s, 1H), 7.34 (d, J=3.6 Hz, 1H),
7.06 (d, J=3.6 Hz, 1H), 6.95 (d, J=3.6 Hz, 1H), 6.71 (d, J=3.6 Hz, 1H),
3.03 (t, J=7.8 Hz, 2H), 2.89 (t, J=7.8 Hz, 2H), 1.82–1.76 (m, 4H), 1.46–
1.30 (m, 12H), 0.91–0.88 ppm (m, 6H).
Experimental Section
5-Bromo-4,4’-dihexyl-5’-(thiophen-2-yl)-2,2’-bithiazole (1): 5,5’-Dibromo-
4-[5’-(5-Bromothiophen-2-yl)-4,4’-dihexyl-2,2’-bithiazol-5-yl]benzalde-
hyde (7): The synthetic method resembled that of compound 5, and the
compound was purified by column chromatography on silica gel (PE/
4,4’-dihexyl-2,2’-bithiazole (500 mg, 1.02 mmol), [PdACTHUNTRGNE(UNG PPh3)4] (20 mg,
0.017 mmol), and K2CO3 (1.02 g, 0.01 mol) in THF (10 mL) and H2O
(5 mL) were heated to 458C under an argon atmosphere for 30 min. A
solution of thiophen-2-ylboronic acid (130 mg, 1.02 mmol) in THF
(5 mL) was added slowly, and the mixture was heated at reflux for a fur-
ther 6 h. After cooling to room temperature, THF was evaporated and
the mixture was extracted with CH2Cl2 (3ꢂ20 mL). The combined organ-
ic layers were dried with anhydrous Na2SO4. The solvent was evaporated,
and the residue was purified by column chromatography on silica gel (pe-
CH2Cl2 =2:1–1:1, v/v) to give
a yellow solid (350 mg, yield 97%).
1H NMR (CDCl3, 400 MHz): d=10.07 (s, 1H), 7.96 (d, J=8.4 Hz, 2H),
7.64 (d, J=8.0 Hz, 2H), 7.06 (d, J=4.0 Hz, 1H), 6.95 (d, J=4.0 Hz, 1H),
2.89 (t, J=7.8 Hz, 2H), 2.85 (t, J=7.8 Hz, 2H), 1.81–1.73 (m, 4H), 1.42–
1.26 (m, 12H), 0.91–0.84 ppm (m, 6H).
5-(5’-{5-[4-(Diphenylamino)phenyl]thiophen-2-yl}-4,4’-dihexyl-2,2’-bithia-
zol-5-yl)thiophene-2-carbaldehyde (8): Compound 5 (120 mg, 0.20 mmol),
[PdACHTUNGTRNE(NUG PPh3)4] (22 mg, 0.02 mmol), and K2CO3 (1.02 g, 0.01 mol) in THF
troleum ether (PE)/CH2Cl2 =10:1–4:1, v/v) to give
a yellow solid
1
(450 mg, yield 89%). H NMR (CDCl3, 400 MHz): d=7.38 (d, J=5.2 Hz,
1H), 7.18 (d, J=3.6 Hz, 1H), 7.09 (t, J=4.4 Hz, 1H), 2.91 (t, J=7.8 Hz,
2H), 2.76 (t, J=7.6 Hz, 2H), 1.80–1.68 (m, 4H), 1.43–1.29 (m, 12H),
0.91–0.86 ppm (m, 6H).
(10 mL) and H2O (5 mL) were heated to 458C under an argon atmos-
phere for 30 min. A solution of 4-(diphenylamino)phenylboronic acid
(57 mg, 0.38 mmol) in THF (3 mL) was added slowly, and the mixture
was heated at reflux for a further 2 h. After cooling to room temperature,
THF was evaporated and the mixture was extracted with CH2Cl2 (3ꢂ
20 mL). The combined organic layers were dried with anhydrous Na2SO4.
The solvent was evaporated, and the residue was purified by column
chromatography on silica gel (PE/CH2Cl2 =1:1, v/v) to give a red solid
(120 mg, yield 79%). 1H NMR (CDCl3, 400 MHz): d=9.92 (s, 1H), 7.74
(d, J=4.0 Hz, 1H), 7.47 (d, J=8.4 Hz, 2H), 7.30–7.27 (m, 5H), 7.20 (d,
J=3.6 Hz, 1H), 7.16 (d, J=4.0 Hz, 1H), 7.13 (d, J=7.6 Hz, 4H), 7.09–
7.04 (m, 4H), 3.00 (t, J=4.0 Hz, 2H), 2.98 (t, J=4.2 Hz, 2H), 1.85–1.77
(m, 4H), 1.46–1.32 (m, 12H), 0.91–0.88 ppm (m, 6H).
5-[4,4’-Dihexyl-5’-(thiophen-2-yl)-2,2’-bithiazol-5-yl]thiophene-2-carbal-
dehyde (2): Compound
1 (200 mg, 0.40 mmol), [PdACHTUNTGNRUEGN(PPh3)4] (20 mg,
0.017 mmol), and K2CO3 (1.02 g, 0.01 mol) in THF (10 mL) and H2O
(5 mL) were heated to 458C under an argon atmosphere for 30 min. A
solution of 5-formylthiophen-2-ylboronic acid (126 mg, 0.81 mmol) in
THF (5 mL) was added slowly, and the mixture was heated at reflux for
a further 2 h. After cooling to room temperature, THF was evaporated
and the mixture was extracted with CH2Cl2 (3ꢂ20 mL). The combined
organic layers were dried with anhydrous Na2SO4. The solvent was
evaporated, and the residue was purified by column chromatography on
silica gel (PE/CH2Cl2 =3:1–1:1, v/v) to give an orange solid (200 mg,
yield 94%). 1H NMR (CDCl3, 400 MHz): d=9.92 (s, 1H), 7.74 (d, J=
4.0 Hz, 1H), 7.40 (d, J=4.4 Hz, 1H), 7.28 (d, J=4.0 Hz, 1H), 7.21 (d, J=
3.6 Hz, 1H), 7.11 (t, J=4.4 Hz, 1H), 2.99 (t, J=7.8 Hz, 2H), 2.94 (t, J=
7.8 Hz, 2H), 1.84–1.75 (m, 4H), 1.45–1.32 (m, 12H), 0.90–0.84 ppm (m,
6H).
5-(5’-{5-[4-(Diphenylamino)phenyl]thiophen-2-yl}-4,4’-dihexyl-2,2’-bithia-
zol-5-yl)furan-2-carbaldehyde (9): The synthetic method resembled that
of compound 8, and the compound was purified by column chromatogra-
phy on silica gel (PE/CH2Cl2 =1:1, v/v) to give a red solid (130 mg, yield
98%). 1H NMR (CDCl3, 400 MHz): d=9.68 (s, 1H), 7.47 (d, J=8.8 Hz,
2H), 7.34 (d, J=4.0 Hz, 1H), 7.29 (d, J=8.4 Hz, 3H), 7.20 (d, J=3.6 Hz,
1H), 7.16–7.12 (m, 6H), 7.09–7.04 (m, 4H), 6.71 (d, J=3.6 Hz, 1H), 3.04
(t, J=7.8 Hz, 2H), 3.00 (t, J=7.8 Hz, 2H), 1.85–1.77 (m, 4H), 1.46–1.30
(m, 12H), 0.89 ppm (t, J=7.0 Hz, 6H).
5-[4,4’-Dihexyl-5’-(thiophen-2-yl)-2,2’-bithiazol-5-yl]furan-2-carbaldehyde
(3): The synthetic method resembled that of compound 2, and the com-
pound was purified by column chromatography on silica gel (PE/
CH2Cl2 =2:1–1:1, v/v) to give an orange solid (130 mg, yield 90%).
1H NMR (CDCl3, 400 MHz): d=9.68 (s, 1H), 7.40 (d, J=5.2 Hz, 1H),
7.34 (d, J=4.0 Hz, 1H), 7.21 (d, J=4.0 Hz, 1H), 7.11 (t, J=4.4 Hz, 1H),
6.71 (d, J=3.6 Hz, 1H), 3.04 (t, J=7.8 Hz, 2H), 2.94 (t, J=7.8 Hz, 2H),
1.84–1.75 (m, 4H), 1.48–1.28 (m, 12H), 0.91–0.88 ppm (m, 6H).
4-(5’-{5-[4-(Diphenylamino)phenyl]thiophen-2-yl}-4,4’-dihexyl-2,2’-bithia-
zol-5-yl)benzaldehyde (10): The synthetic method resembled that of com-
pound 8, and the compound was purified by column chromatography on
silica gel (PE/CH2Cl2 =1:1, v/v) to give an orange solid (200 mg, yield
97%). 1H NMR (CDCl3, 400 MHz): d=10.06 (s, 1H), 7.96 (d, J=8.4 Hz,
2H), 7.64 (d, J=8.0 Hz, 2H), 7.47 (d, J=8.8 Hz, 2H), 7.29 (d, J=8.0 Hz,
2H), 7.20 (d, J=3.6 Hz, 1H), 7.16–7.12 (m, 5H), 7.09–7.04 (m, 5H), 6.71
(d, J=3.6 Hz, 1H), 2.98 (t, J=7.8 Hz, 2H), 2.85 (t, J=7.8 Hz, 2H), 1.85–
1.75 (m, 4H), 1.37–1.26 (m, 12H), 0.90–0.85 ppm (m, 6H).
4-[4,4’-Dihexyl-5’-(thiophen-2-yl)-2,2’-bithiazol-5-yl]benzaldehyde
(4):
The synthetic method resembled that of compound 2, and the compound
was purified by column chromatography on silica gel (PE/CH2Cl2 =3:1–
1:1, v/v) to give a yellow solid (320 mg, yield 91%). 1H NMR (CDCl3,
400 MHz): d=10.07 (s, 1H), 7.96 (d, J=8.4 Hz, 2H), 7.64 (d, J=8.0 Hz,
2H), 7.39 (d, J=5.2 Hz, 1H), 7.21 (d, J=3.6 Hz, 1H), 7.11 (t, J=4.4 Hz,
1H), 3.04 (t, J=7.8 Hz, 2H), 2.94 (t, J=7.8 Hz, 2H), 1.84–1.75 (m, 4H),
1.48–1.28 (m, 12H), 0.91–0.88 ppm (m, 6H).
2-Cyano-3-[5-(5’-{5-[4-(diphenylamino)phenyl]thiophen-2-yl}-4,4’-dihex-
yl-2,2’-bithiazol-5-yl)thiophen-2-yl]acrylic acid (BT-I): Compound
8
(120 mg, 0.16 mmol), 2-cyanoacetic acid (93 mg, 1.09 mmol), ammonium
acetate (50 mg), and acetic acid (8 mL) were heated at 1208C under an
argon atmosphere for 6 h. After cooling to room temperature, the mix-
ture was added to water. The precipitate was isolated by filtration and
washed with water. The residue was purified by column chromatography
5-[5’-(5-Bromothiophen-2-yl)-4,4’-dihexyl-2,2’-bithiazol-5-yl]thiophene-2-
carbaldehyde (5): Compound 2 (200 mg, 0.38 mmol) and N-bromosuccini-
mide (68 mg, 0.38 mmol) were placed in a flask, and then chloroform
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Bithiazole-Based Sensitizers for Dye-Sensitized Solar Cells
FULL PAPER
on silica gel (CH2Cl2–CH2Cl2/EtOH=10:1, v/v) to give a dark red solid
(110 mg, yield 85%). 1H NMR ([D6]DMSO, 400 MHz): d=8.30 (s, 1H),
7.88 (d, J=4.0 Hz, 1H), 7.58 (d, J=8.0 Hz, 2H), 7.54 (d, J=4.0 Hz, 1H),
7.46 (d, J=4.0 Hz, 1H), 7.37–7.32 (m, 5H), 7.12–7.05 (m, 6H), 6.97 (d,
J=8.0 Hz, 2H), 2.98–2.92 (m, 4H), 1.76–1.72 (m, 4H), 1.41–1.28 (m,
12H), 0.87–0.84 ppm (m, 6H); 13C NMR ([D8]THF, 100 MHz): d=156.3,
153.9, 147.7, 147.4, 133.6, 132.6, 131.0, 129.2, 128.7, 127.5, 126.3, 124.6,
123.2, 123.1, 122.9, 31.7, 30.9, 30.4, 29.7, 29.2, 29.0, 28.9, 22.6, 13.5 ppm;
HRMS: m/z calcd for C48H45N4O2S4: 837.2425 [MÀH]À; found: 837.2420;
elemental analysis calcd (%) for C48H46N4O2S4: C 68.70, H 5.53, N 6.68;
found: C 68.60, H 5.62, N 6.72.
8.0 Hz, 4H), 3.02 (t, J=8.0 Hz, 2H), 2.77 (t, J=8.0 Hz, 2H), 1.78–1.66
(m, 4H), 1.27–1.19 (m, 12H), 0.83–0.78 ppm (m, 6H).
2-(4-Bromophenyl)-3-{5’-[4-(diphenylamino)phenyl]-4,4’-dihexyl-2,2’-bi-
thiazol-5-yl}acrylonitrile (13):
A mixture of compound 12 (350 mg,
0.58 mmol), 2-(4-bromophenyl)acetonitrile (117 mg, 0.60 mmol), a cata-
lytic amount of potassium tert-butoxide, and methanol (20 mL) was
placed in a three-neck round-bottomed flask (100 mL). After the mixture
was heated at reflux for 3 h, the product was isolated by filtration and
dried. The final crude product was purified by column chromatography
on silica gel (PE/CH2Cl2 =3:2, v/v) to yield an orange solid (400 mg, yield
88%). 1H NMR (CDCl3, 400 MHz): d=7.66 (s, 1H), 7.59 (d, J=8.0 Hz,
2H), 7.52 (d, J=8.0 Hz, 2H), 7.30 (t, J=8.0 Hz, 6H), 7.16 (d, J=8.0 Hz,
4H), 7.11–7.06 (m, 4H), 2.93 (t, J=8.0 Hz, 2H), 2.84 (t, J=8.0 Hz, 2H),
1.83–1.74 (m, 4H), 1.39–1.27 (m, 12H), 0.90–0.87 ppm (m, 6H).
2-Cyano-3-[5-(5’-{5-[4-(diphenylamino)phenyl]thiophen-2-yl}-4,4’-dihex-
yl-2,2’-bithiazol-5-yl)furan-2-yl]acrylic acid (BT-II): A procedure similar
to that for the dye BT-I, but with compound 9 (120 mg, 0.17 mmol) in-
stead of compound 8, was performed to give BT-II as dark red solid
(110 mg, yield 87%). 1H NMR ([D6]DMSO, 400 MHz): d=8.04 (s, 1H),
7.58 (d, J=8.0 Hz, 3H), 7.46 (d, J=4.0 Hz, 1H), 7.38–7.32 (m, 5H), 7.20
(d, J=4.0 Hz, 1H), 7.12–7.05 (m, 6H), 6.97 (d, J=8.0 Hz, 2H), 3.04 (t,
J=8.0 Hz, 2H), 2.95 (t, J=8.0 Hz, 2H), 1.77–1.68 (m, 4H), 1.38–1.26 (m,
12H), 0.85 ppm (t, J=8.0 Hz, 6H); 13C NMR ([D8]THF, 100 MHz): d=
159.0, 156.5, 154.1, 149.1, 147.8, 147.4, 145.3, 129.2, 127.5, 126.3, 124.5,
123.2, 123.0, 122.8, 111.8, 31.8, 31.7, 30.7, 30.4, 29.7, 29.2, 29.1, 28.9, 28.6,
22.6, 13.5 ppm; HRMS: m/z calcd for C48H45N4O3S3: 821.2654 [MÀH]À;
found: 821.2627; elemental analysis calcd (%) for C48H46N4O3S3: C 70.04,
H 5.63, N 6.81; found: C 70.11, H 5.59, N 6.73.
3-{5’-[4-(Diphenylamino)phenyl]-4,4’-dihexyl-2,2’-bithiazol-5-yl}-2-[4-(5-
formylthiophen-2-yl)phenyl]acrylonitrile (14): Compound 13 (160 mg,
0.20 mmol), [PdACTHNUTRGNE(UNG PPh3)4] (20 mg, 0.017 mmol), and K2CO3 (1.02 g,
0.01 mol) in THF (10 mL) and H2O (5 mL) were heated to 458C under
an argon atmosphere for 30 min. A solution of 5-formylthiophen-2-ylbor-
onic acid (34 mg, 0.22 mmol) in THF (5 mL) was added slowly, and the
mixture was heated at reflux for a further 2 h. After cooling to room tem-
perature, the mixture was extracted with CH2Cl2 (3ꢂ20 mL). The com-
bined organic layers were washed with water and brine and dried with
anhydrous Na2SO4. The solvent was evaporated, and the residue was pu-
rified by column chromatography on silica gel (PE/CH2Cl2 =1:2, v/v) to
give a red solid (130 mg, yield 78%). 1H NMR (CDCl3, 400 MHz): d=
9.92 (s, 1H), 7.81–7.72 (m, 6H), 7.53–7.47 (m, 1H), 7.33–7.28 (m, 6H),
7.17–7.07 (m, 8H), 2.96 (t, J=8.0 Hz, 2H), 2.85 (t, J=8.0 Hz, 2H), 1.81–
1.62 (m, 4H), 1.42–1.31 (m, 12H), 0.91–0.78 ppm (m, 6H); HRMS: m/z
calcd for C50H49N4SO3: 817.3069 [M+H]À; found: 817.3095.
2-Cyano-3-[4-(5’-{5-[4-(diphenylamino)phenyl]thiophen-2-yl}-4,4’-dihex-
yl-2,2’-bithiazol-5-yl)phenyl]acrylic acid (BT-III): A procedure similar to
that for the dye BT-I, but with compound 10 (120 mg, 0.17 mmol) instead
of compound 8, was performed to give BT-III as a red solid (190 mg,
yield 88%). 1H NMR ([D6]DMSO, 400 MHz): d=8.25 (s, 1H), 8.09 (d,
J=8.0 Hz, 2H), 7.70 (d, J=8.0 Hz, 2H), 7.57 (d, J=8.4 Hz, 2H), 7.43 (d,
J=3.6 Hz, 1H), 7.35–7.31 (m, 5H), 7.11–7.05 (m, 6H), 6.97 (d, J=8.8 Hz,
2H), 2.93 (t, J=7.4 Hz, 2H), 2.82 (t, J=7.4 Hz, 2H), 1.77–1.67 (m, 4H),
1.39–1.23 (m, 12H), 0.86–0.81 ppm (m, 6H); 13C NMR ([D6]DMSO,
100 MHz): d=163.1, 157.9, 156.3, 154.7, 153.6, 147.2, 146.7, 144.6, 132.9,
130.8, 129.8, 129.6, 129.5, 129.4, 127.8, 126.6, 126.5, 124.4, 123.7, 123.6,
122.7, 30.9, 29.8, 29.2, 28.8, 28.6, 28.4, 28.3, 22.0, 13.9 ppm; HRMS: m/z
calcd for C50H47N4O2S3: 831.2861 [MÀH]À; found: 831.2850; elemental
analysis calcd (%) for C50H48N4O2S3: C 72.08, H 5.81, N 6.72; found: C
72.15, H 5.76, N 6.68.
3-{5’-[4-(Diphenylamino)phenyl]-4,4’-dihexyl-2,2’-bithiazol-5-yl}-2-[4-(5-
formylfuran-2-yl)phenyl]acrylonitrile (15): Compound 13 (100 mg,
0.13 mmol), [PdACTHNUTRGNE(UNG PPh3)4] (20 mg, 0.017 mmol), and K2CO3 (1.02 g,
0.01 mol) in THF (10 mL) and H2O (5 mL) were heated to 458C under
an argon atmosphere for 30 min. A solution of 5-formylfuran-2-ylboronic
acid (21 mg, 0.15 mmol) in THF (5 mL) was added slowly, and the mix-
ture was heated at reflux for a further 2 h. After cooling to room temper-
ature, the mixture was extracted with CH2Cl2 (3ꢂ20 mL). The combined
organic layers were washed with water and brine and dried with anhy-
drous Na2SO4. The solvent was evaporated, and the residue was purified
by column chromatography on silica gel (PE/CH2Cl2 =1:3, v/v) to give a
red solid (80 mg, yield 78%). 1H NMR (CDCl3, 400 MHz): d=9.69 (s,
1H), 7.96–7.90 (m, 2H), 7.74 (d, J=12.0 Hz, 3H), 7.36–7.28 (m, 7H),
7.17–7.05 (m, 8H), 6.96–6.92 (m, 1H), 2.98–2.71 (m, 4H), 1.84–1.76 (m,
4H), 1.34–1.26 (m, 12H), 0.91–0.87 ppm (m, 6H); HRMS: m/z calcd for
C50H49N4S2O2: 801.3297 [M+H]+; found: 801.3290.
4-(5’-Bromo-4,4’-dihexyl-2,2’-bithiazol-5-yl)-N,N-diphenylaniline
5,5’-Dibromo-4,4’-dihexyl-2,2’-bithiazole (1000 mg, 2.04 mmol), [Pd-
(PPh3)4] (20 mg, 0.017 mmol), and K2CO3 (1.02 g, 0.01 mol) in THF
(11):
ACHTUNGTRENNUNG
(10 mL) and H2O (5 mL) were heated to 458C under an argon atmos-
phere for 30 min. A solution of 4-(diphenylamino)phenylboronic acid
(590 mg, 2.04 mmol) in THF (10 mL) was added slowly, and the mixture
was heated at reflux for a further 6 h. After cooling to room temperature,
the mixture was extracted with CH2Cl2 (3ꢂ20 mL). The combined organ-
ic layers were washed with water and brine and dried with anhydrous
Na2SO4. The solvent was evaporated, and the residue was purified by
column chromatography on silica gel (PE/CH2Cl2 =3:1–2:1, v/v) to give a
yellow solid (600 mg, yield 45%). 1H NMR (CDCl3, 400 MHz): d=7.31–
7.21 (m, 6H), 7.15 (d, J=8.0 Hz, 4H), 7.10–7.05 (m, 4H), 2.80 (t, J=
8.0 Hz, 2H), 2.76 (t, J=8.0 Hz, 2H), 1.78–1.68 (m, 4H), 1.36–1.27 (m,
12H), 0.91–0.85 ppm (m, 6H).
3-{5’-[4-(Diphenylamino)phenyl]-4,4’-dihexyl-2,2’-bithiazol-5-yl}-2-(4’-for-
mylbiphenyl-4-yl)acrylonitrile (16): Compound 13 (130 mg, 0.17 mmol),
[PdACHTUNGTRNE(NUG PPh3)4] (20 mg, 0.017 mmol), and K2CO3 (1.02 g, 0.01 mol) in THF
(10 mL) and H2O (5 mL) were heated to 458C under an argon atmos-
phere for 30 min. A solution of 4-formylphenylboronic acid (30 mg,
0.20 mmol) in THF (5 mL) was added slowly, and the mixture was heated
at reflux for a further 2 h. After cooling to room temperature, the mix-
ture was extracted with CH2Cl2 (3ꢂ20 mL). The combined organic layers
were washed with water and brine and dried with anhydrous Na2SO4.
The solvent was evaporated, and the residue was purified by column
chromatography on silica gel (PE/CH2Cl2 =1:2, v/v) to give a red solid
(110 mg, yield 78%). 1H NMR (CDCl3, 400 MHz): d=10.08 (s, 1H),
8.01–7.98 (m, 2H), 7.84–7.73 (m, 7H), 7.33–7.28 (m, 6H), 7.17–7.07 (m,
8H), 2.98–2.71 (m, 4H), 1.84–1.76 (m, 4H), 1.42–1.31 (m, 12H), 0.92–
0.85 ppm (m, 6H); HRMS: m/z calcd for C52H51N4SO2: 811.3504
[M+H]+; found: 811.3522.
5’-[4-(Diphenylamino)phenyl]-4,4’-dihexyl-2,2’-bithiazole-5-carbaldehyde
(12): A solution of nBuLi (2.5m, 0.48 mL, 1.2 mmol) in hexane was
added to a solution of 11 (650 mg, 1 mmol) in THF (20 mL) at À788C.
After stirring for 1 h,
a solution of N-formylmorpholine (172 mg,
1.5 mmol) in THF (5 mL) was added. After additional stirring for 1 h at
À788C, the mixture was allowed to warm to room temperature overnight.
The final solution was acidified with 1m HCl solution (10 mL) and stirred
for 45 min at room temperature. The aqueous phase was extracted with
dichloromethane, and the organic layer was dried over magnesium sul-
fate. After evaporation of the solvent, the final crude product was puri-
fied by column chromatography on silica gel (PE/CH2Cl2 =1:1, v/v) to
yield an orange solid (350 mg, yield 58%). 1H NMR (CDCl3, 400 MHz):
d=10.03 (s, 1H), 7.24–7.19 (m, 6H), 7.08 (d, J=8.0 Hz, 4H), 7.01 (t, J=
2-Cyano-3-{5-[4-(1-cyano-2-{5’-[4-(diphenylamino)phenyl]-4,4’-dihexyl-
2,2’-bithiazol-5-yl}vinyl)phenyl]thiophen-2-yl}acrylic acid (BT-IV): Com-
pound 14 (130 mg, 0.16 mmol), 2-cyanoacetic acid (93 mg, 1.09 mmol),
ammonium acetate (50 mg), and acetic acid (8 mL) were heated at 1208C
for 4 h. After cooling to room temperature, the mixture was added to
water. The precipitate was isolated by filtration and washed with water.
Chem. Eur. J. 2012, 00, 0 – 0
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J. Hua, W. Wu et al.
The residue was purified by column chromatography on silica gel
(CH2Cl2–CH2Cl2/EtOH=5:1, v/v) to give red solid (120 mg, yield
THF. The ferrocene/ferrocenium (Fc/Fc+) redox couple was used as an
internal potential reference.
a
93%). 1H NMR ([D8]THF, 400 MHz): d=8.41 (s, 1H), 7.96–7.86 (m,
3H), 7.75–7.57 (m, 4H), 7.40–7.25 (m, 6H), 7.14–7.03 (m, 8H), 3.04 (d,
J=8.0 Hz, 2H), 2.94 (d, J=8.0 Hz, 2H), 1.82–1.76 (m, 4H), 1.36–1.29 (m,
12H), 0.93–0.87 ppm (m, 6H); 13C NMR ([D8]THF, 100 MHz): d=165.2,
160.3, 153.2, 147.5, 142.8, 136.7, 134.4, 129.4, 128.5, 128.0, 127.8, 127.1,
126.4, 124.9, 123.5, 122.2, 117.2, 105.9, 88.6, 78.3, 78.0, 31.9, 30.4, 29.8,
29.6, 29.4, 28.9, 28.8, 22.8, 22.7, 14.2 ppm; HRMS: m/z calcd for
C53H48N5O2S3: 882.2970 [MÀH]+; found: 882.2976; elemental analysis
calcd (%) for C53H49N5O2S3: C 71.99, H 5.59, N 7.92; found: C 72.07, H
5.49, N 7.86.
Device fabrication: The TiO2 films were fabricated with a screen-printing
method according to the published procedure.[18b] The thickness was
about 15 mm. The trilayer TiO2 electrodes were heated under an air flow
at 4508C for 15 min and 5008C for 15 min. The sintered films were fur-
ther treated with 40 mm TiCl4 aqueous solution at room temperature for
12 h and washed with water and ethanol, which significantly increased
the short-circuit photocurrent. Aqueous TiCl4 removed the iron contami-
nation source, which can quench the dye-sensitized photocurrent (Jsc) ef-
fectively.[31] Then, the films were annealed at 4508C for 30 min. After the
TiCl4-pretreated films were cooled to around 508C, they were immersed
in a 3ꢂ10À4 m dye bath in CH2Cl2 solution and maintained in the dark for
18 h at room temperature. The electrodes were then rinsed with CH2Cl2
and ethanol and dried. The size of the TiO2 electrodes used was 0.25 cm2.
To prepare the counter electrode, a Pt catalyst was deposited on cleaned
FTO glass by spinning with a drop of H2PtCl6 solution (20 mm 2-propanol
solution) with heat treatment at 4008C for 15 min. A hole (0.6 mm diam-
eter) was drilled on the counter electrode using a drill press. The perfo-
rated sheet was cleaned with ultrasound in an ethanol bath for 10 min.
For the assembly of DSSCs, the dye-covered TiO2 electrode and Pt coun-
ter electrode were assembled into a sandwich-type cell and sealed with a
hot-melt gasket of thickness 25 mm made of the ionomer Surlyn 1702
(DuPont). The electrolyte was introduced into the cell by vacuum back-
filling from a hole in the back of the counter electrode. Finally, the hole
was sealed with a self-adhesive silver film. In this work, two kinds of elec-
trolyte were employed: liquid electrolyte comprising 0.1m LiI, 0.05m I2,
0.6m DMPII, and 0.5m TBP in a mixture of acetonitrile and methoxypro-
pionitrile (volume ratio, 7:3); and ionic-liquid electrolyte comprising
0.1m LiI, 0.1m I2, and 0.45m benzimidazole in 1-methyl-3-n-propyl-
2-Cyano-3-{5-[4-(1-cyano-2-{5’-[4-(diphenylamino)phenyl]-4,4’-dihexyl-
2,2’-bithiazol-5-yl}vinyl)phenyl]furan-2-yl}acrylic acid (BT-V): A proce-
dure similar to that for the dye BT-IV, but with compound 15 (80 mg,
0.10 mmol) instead of compound 14, was performed to give BT-V as a
red solid (80 mg, yield 92%). 1H NMR ([D8]THF, 400 MHz): d=8.35 (s,
1H), 7.91–7.80 (m, 4H), 7.63–7.47 (m, 3H), 7.22–7.14 (m, 6H), 7.06–6.92
(m, 8H), 3.05 (d, J=8.0 Hz, 2H), 2.84 (d, J=8.0 Hz, 2H), 1.83–1.78 (m,
4H), 1.40–1.26 (m, 12H), 0.92–0.86 ppm (m, 6H); 13C NMR ([D8]THF,
100 MHz): d=164.5, 160.8, 154.5, 147.9, 143.8, 137.2, 135.0, 132.4, 129.5,
128.4, 128.2, 127.6, 127.2, 126.6, 124.5, 123.4, 123.2, 117.2, 106.2, 90.6,
78.6, 78.3, 32.1, 30.9, 29.8, 29.7, 29.5, 29.4, 28.9, 28.6, 28.2, 22.9, 22.5,
13.8 ppm; HRMS: m/z calcd for C53H48N5O3S2: 866.3199 [MÀH]+; found:
866.3185; elemental analysis calcd (%) for C53H49N5O3S2: C 73.33, H
5.69, N 8.07; found: C 73.40, H 5.78, N 8.15.
2-Cyano-3-[4’-(1-cyano-2-{5’-[4-(diphenylamino)phenyl]-4,4’-dihexyl-2,2’-
bithiazol-5-yl}vinyl)biphenyl-4-yl]acrylic acid (BT-VI): A procedure simi-
lar to that for the dye BT-IV, but with compound 16 (120 mg, 0.15 mmol)
instead of compound 14, was performed to give BT-VI as a red solid
(110 mg, yield 85%). 1H NMR ([D8]THF, 400 MHz): d=8.31 (s, 1H),
8.18 (d, J=4.0 Hz, 2H), 8.02 (s, 1H), 7.96–7.89 (m, 6H), 7.39–7.25 (m,
6H), 7.14–7.03 (m, 9H), 3.05 (d, J=8.0 Hz, 2H), 2.72 (d, J=8.0 Hz, 2H),
1.86–1.78 (m, 4H), 1.44–1.28 (m, 12H), 0.93–0.87 ppm (m, 6H);
13C NMR ([D8]THF, 100 MHz): d=164.2, 161.8, 154.2, 147.3, 140.0, 135.9,
131.4, 129.9, 129.3, 128.3, 127.5, 127.2, 126.4, 124.9, 123.5, 122.2, 117.2,
109.5, 78.6, 78.3, 78.0, 31.7, 31.6, 29.7, 29.6, 29.5, 29.1, 28.9, 22.6, 22.5,
13.5 ppm; HRMS: m/z calcd for C55H50N5O2S2: 876.3406 [MÀH]+; found:
876.3406; elemental analysis calcd (%) for C55H51N5O2S2: C 75.22, H
5.85, N 7.98; found: C 75.13, H 5.93, N 7.87.
AHCTUNGTERGiNNUN midazolium iodide (MPII).
Photovoltaic performance measurements: Photovoltaic measurements
employed an AM 1.5 solar simulator equipped with a 300 W xenon lamp
(Model No. 91160, Oriel). The power of the simulated light was calibrat-
ed to 100 mWcmÀ2 by using a Newport Oriel PV reference cell system
(Model 91150 V). J–V curves were obtained by applying an external bias
to the cell and measuring the generated photocurrent with a Keithley
model 2400 digital source meter. The voltage step and delay time of the
photocurrent were 10 mV and 40 ms, respectively. Action spectra of the
incident monochromatic photon-to-electron conversion efficiency (IPCE)
for the solar cells were obtained with a Newport-74125 system (Newport
Instruments). The intensity of monochromatic light was measured with a
Si detector (Newport-71640).
Materials and reagents: Tetrahydrofuran (THF) was predried over 4 ꢃ
molecular sieves and distilled under an argon atmosphere from sodium
benzophenone ketyl immediately prior to use. Dichloromethane was dis-
tilled under normal pressure and dried over calcium hydroxide. 5,5’-Di-
bromo-4,4’-dihexyl-2,2’-bithiazole and 1-propyl-3-methylimidazolium
iodide (PMII) were prepared according to published procedures.[30] Me-
thoxypropionitrile (MPN) and 1,2-dimethyl-3-n-propylimidazolium
iodide (DMPII) were purchased from Aldrich. Benzimidazole (BI) was
from J&K. Tetra-n-butylammonium hexafluorophosphate (TBAPF6), 4-
tert-butylpyridine (TBP), and lithium iodide were from Fluka and iodine
(99.999%) was from Alfa Aesar. Transparent fluorine-doped tin oxide
(FTO) conducting glass (transmission >90% in the visible, sheet resist-
ance 15 WsquareÀ1) was obtained from the Geao Science and Education-
al Co. Ltd. of China. All other solvents and chemicals used in this work
were of reagent grade and used without further purification unless other-
wise noted.
Acknowledgements
This work was supported by NSFC/China (2116110444, 21172073, and
61006048), the National Basic Research 973 Program (2011CB808400),
the Fundamental Research Funds for the Central Universities
(WJ0913001), the Ph.D. Programs Foundation of the Ministry of Educa-
tion of China (20090074110004), and the Scientific Committee of Shang-
hai (10520709700). We are grateful to Dr. Xin Li in the Department of
Theoretical Chemistry and Biology, School of Biotechnology, KTH
Royal Institute of Technology in Sweden for computational assistance.
Characterization: 1H and 13C NMR spectra were recorded on Brꢄcker
AM 400 MHz instruments with tetramethylsilane as the internal stand-
ard. HRMS was performed with a Waters LCT Premier XE spectrome-
ter. The absorption spectra of sensitizer dyes in solution and adsorbed on
TiO2 films were measured with a Varian Cary 500 spectrophotometer.
Emission spectra of sensitized dyes in solution were measured with a
Varian Cary Eclipse spectrometer. Cyclic voltammograms were deter-
mined with a Versastat II electrochemical workstation (Princeton Ap-
plied Research) by using a three-electrode cell with a Pt working elec-
trode, a Pt wire counter electrode, and a saturated calomel reference
electrode in saturated KCl solution; 0.1m tetrabutylammonium hexa-
fluorophosphate (TBAPF6) was used as the supporting electrolyte in
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Received: November 24, 2011
Revised: March 3, 2012
Published online: && &&, 0000
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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These are not the final page numbers!

J. Hua, W. Wu et al.
Catch the sun: A series of dyes com-
prising bithiazole with a thiophene,
furan, benzene, or cyano moiety as the
p spacer act as the sensitizers for dye-
sensitized solar cells (DSSCs, see
figure). The conversion efficiencies are
3.58–7.51%, in which BT-I-based
DSSCs show the best photovoltaic per-
formance under AM 1.5 irradiation.
The BT-I–III-based DSSCs with ionic-
liquid electrolytes have long-term sta-
bility and BT-II has up to 5.75%
power conversion efficiency.
Solar Cells
J. He, F. Guo, X. Li, W. Wu,* J. Yang,
J. Hua* ......................... &&&& — &&&&
New Bithiazole-Based Sensitizers for
Efficient and Stable Dye-Sensitized
Solar Cells
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www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!