Aryl/hetero-arylethyne bridged dyes: The effect of planar π-bridge on the performance of dye-sensitized solar cells

Article abstract of DOI:10.1039/c0nj00653j

We have designed and synthesized three metal-free sensitizers consisting of dimethoxy-substituted triphenylamine as donor, cyanoacrylic acid as acceptor with planar π-bridge. The photon to current conversion efficiency of these dyes were measured as sensitizers for DSSCs analysis. The results obtained from these experiments show that the overall conversion efficiency of D3 gave the highest overall efficiency 4.98% with short circuit current density (J _sc) of 11.29 mA cm^-2, an open-circuit voltage (V _oc) of 0.637 V and a fill factor of 69.2% under standard global AM 1.5 solar conditions. In addition, the experimental results show that π-conjugated bridge can extend the absorption of dye molecular and strong planarity (close-packing) can suppress the charge recombination at TiO ₂/electrolyte interface. Hence, the donor-π bridge-acceptor architectures can generate a unidirectional current flow, to some extent, which can enhance the cell performance. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011.

Full text of DOI:10.1039/c0nj00653j

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PAPER
Aryl/hetero-arylethyne bridged dyes: the effect of planar p-bridge on the
performance of dye-sensitized solar cellsw
a
a
Jun-Ling Song,z Pitchamuthu Amaladass,z Shu-Hao Wen,b
Kalyan Kumar Pasunooti,a An Li,a Yao-Lun Yu,c Xin Wang,c Wei-Qiao Deng*b
and Xue-Wei Liu*a
Received (in Montpellier, France) 21st August 2010, Accepted 15th September 2010
DOI: 10.1039/c0nj00653j
We have designed and synthesized three metal-free sensitizers consisting of dimethoxy-substituted
triphenylamine as donor, cyanoacrylic acid as acceptor with planar p-bridge. The photon to
current conversion efficiency of these dyes were measured as sensitizers for DSSCs analysis.
The results obtained from these experiments show that the overall conversion efficiency of
D3 gave the highest overall efficiency 4.98% with short circuit current density (Jsc) of
11.29 mA cmꢀ2, an open-circuit voltage (Voc) of 0.637 V and a fill factor of 69.2% under
standard global AM 1.5 solar conditions. In addition, the experimental results show that
p-conjugated bridge can extend the absorption of dye molecular and strong planarity
(close-packing) can suppress the charge recombination at TiO2/electrolyte interface. Hence,
the donor-p bridge-acceptor architectures can generate a unidirectional current flow, to some
extent, which can enhance the cell performance.
and the heavy-metal toxicity, metal-free organic dyes have
Introduction
received vital research interest. In general, the metal-free
sensitizers for DSSCs mainly comprise a large conjugated
system (donor-p bridge-acceptor moieties) which can expand
the absorption spectrum and thereby increase the short circuit
current (Jsc).6 Furthermore, the rate of charge transfer within
molecules can also be enhanced by intramolecular donor–
acceptor delivery.8 However, introducing more p-conjugated
bridge moiety to conjugated dyes will lead to unfavorable
p-stacked dye aggregation on TiO2, which may result in a
decrease in DSSCs performance.9 Consequently, an appropriate
bridge system is of the essence to obtain the high power
conversion efficiency of organic dye sensitized solar cells. In
this regard, these two factors should be adjusted to reach a
compromise when designing novel dyes to improve the overall
efficiency of DSSCs. Recently, several metal-free organic dyes
including coumarin,10–12 merocyanine,13–15 indoline,16,17
polyene,18 hemicyanine,19,20 triphenylamine,21–23 fluorine24–26
and tetrahydroquinoline27 as the donating group have been
developed for DSSCs. Thus far, the highest photoelectric
conversion efficiency of metal-free dye sensitized solar cells
was reported to be around 9 to 9.8%.28 It is well accepted that
4-methoxy-triphenylamine moiety as a donor in metal-free
sensitizer can effectively suppress the dye-aggregation.29–31
On the other hand, cyanoacrylic acid as an acceptor is better
than other anchoring groups such as rhodanine-3-acetic acid
due to its coplanarity with respect to spacer unit and good
electron coupling with TiO2.22,33–34 In addition to the intrinsic
properties of donor and acceptor, the electron transfer can
Even though silicon and other semiconductor-based solar cells
have dominated the solar-cell market for decades, dye-sensitized
solar cells (DSSCs) have attracted much consideration due to
their low cost and relatively high power conversion efficiency (Z)
in the past few years.1–3 In DSSCs systems, the dye sensitizers
play an important role and many research groups have devoted
to exploring novel ruthenium (Ru) polypyridyl complexes,
such as cis-dithiocyanate-N,N0-bis(4-carboxylate-4-tetrabutyl-
ammonium-carboxylate-2,20-bipyridine)-Ru(II) (N719) that could
improve the cell performance and push the efficiency above
10% under air mass 1.5 global (AM 1.5G) irradiation.4–6
Promoted by continuous material innovation and the systematic
study of device fabrication in conjugation with highly volatile
electrolytes, the cells with the ruthenium polypyridyl sensitizers
have reached high efficiencies of 11.1% measured under the
AM 1.5G conditions.7 Due to the limited ruthenium resources
a Division of Chemistry and Biological Chemistry, SPMS,
Nanyang Technological University, 21 Nanyang Link, Singapore
637371, Singapore. E-mail: xuewei@ntu.edu.sg;
Fax: (+65) 6791-1961
b Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
Dalian 116023, P.R. China. E-mail: dengwq@dicp.ac.cn
c Division of Chemical and Biomolecular Engineering, SCBE,
Nanyang Technological University, Singapore 639798
w Electronic supplementary information (ESI) available: Synthesis,
electrochemistry spectra and additional figures. See DOI: 10.1039/
c0nj00653j
z J. L. Song and P. Amaladass contributed equally to this work.
c
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New J. Chem., 2011, 35, 127–136 127

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The synthetic outlines of the new metal-free organic dyes
are shown in Schemes 2–5. The essential intermediates
were achieved according to literature methods.14,38,39 The
dimethoxy-substituted triphenylamine donor group was
synthesized by Buchwald coupling and the yield was only
30% (Scheme 2). On compared to the previous report,40 the
synthetic strategy employed for arylethyne based dyes is quite
facile and the yield of these dyes obtained by this synthetic
route is also moderate. It involves only two important steps to
attain these organic dyes such as (1) Sonogashira cross-coupling
of the ethynyl unit with halogen unit to afford the corres-
ponding aldeheydes as precursors, and (2) Knoevenagel
condensation of the precursor aldehydes with cyanoacetic acid
afforded the corresponding metal-free organic dyes. The
precursor aldehydes 7 and 11 were achieved by Sonogashira
coupling method using dichlorobis(triphenylphosphine)
palladium(II) mediation afforded the yield of 51% and 50%
respectively (Scheme 3 and 4).
Scheme 1 Molecular structures of dyes (D1–D3).
also be improved when the bridge moiety can be turned on or
off on demand in donor–bridge–acceptor (D–B–A) system.
According to Geppert and Vivie-Riedle’s results,35 the bridge
molecule can suppress the electron transfer when the molecule
is in the closed form, while in the open form, the molecule can
allow the electron transfer from D via B to A. Moreover,
experimental results indicated that linear bridge moiety with
good planarity can supress the electron recombination.36
In view of the key role of bridge moiety in DSSCs systems,
triphenylamine moiety has been extensively used in DSSCs as
donor units due to slowing down the recombination reactions
in the cell,30–32 Here, we reported the design and synthesis of
three new organic dyes bearing suitable linear p-conjugated
bridge moieties with good planarity as metal-free sensitizers
for DSSCs (D1, D2 and D3 as shown in Scheme 1).
The electrochemical and photovoltaic properties of these
metal-free dyes have also been investigated.
These intermediates were converted into corresponding dyes
by treating with cyanoacetic acid using piperdine as base in a
yield of 36% (D1) and 40% (D2), respectively. The dye D3 was
obtained from building unit 14 by Sonogashira followed by
41
Knoevenagel condensation
in a yield of 36% (Scheme 5).
The chemical structures of the new four metal-free organic
1
dyes were characterized by H and 13C NMR spectroscopy,
EI mass spectroscopy, and high-resolution mass spectroscopy.
Absorption properties in solution and on TiO2 film
The UV-visible absorption spectra of the three metal-free
organic dyes in CHCl3 solution and on TiO2 films are shown
in Fig. 1. The molar extinction coefficients decrease in the
order of D1 4 D2 4 D3 in ultraviolet region, while opposite
order is observed in the visible region and the spectra exhibit
corresponding red-shift. The bands at around 350 nm are
attributed to the p–p* transition of the conjugated molecule
and intramolecular transfer between the donor and the
acceptor.29 The positions of the bands are strongly influenced
by the extended conjugation and bathochrome group. The
hyperchromic effect intensity of bands in UV region might be
attributed to introducing planar macrocyclic structure to the
bridge moiety, while the intensity of bands in visible region
can be enhanced by introducing auxochrome group. The
p-conjugation groups would facilitate the absorption spectra
by bathochromic shift towards near IR region, which
causes an improved match of the absorption bands of
sensitizer to the solar radiation. It is obvious that the
absorption bands of thiophene are at a much longer wave-
length range due to substantial resonance enhancement
Results and discussion
Synthesis
The three new metal-free organic dyes are made of three
important parts such as donor group (dimethoxy-substituted
triphenylamine system), a conjugated spacer (alkyne bridged
systems) and acceptor group (cyanoacrylic acid). The follow-
ing three important points made us to design the arylethyne
bridged metal-free organic dyes, namely (1) the donor group
ꢀ
such as triphenylamine may effectively inhibit I3 by making
denser film formation on surface of the TiO2 film, (2) the
aryl/heteroarylethyne spacer is a semirigid conjugated linker37
which should support electron injection and (3) the cyano-
acrylic acid group acts as acceptor group and also
anchoring group.
Scheme 2 Synthetic route of dimethoxy substituted triphenylamine.
c
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128 New J. Chem., 2011, 35, 127–136

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Scheme 3 Synthetic route of the D1 sensitizer.
Scheme 4 Synthetic route of the D2 sensitizer.
Scheme 5 Synthetic route of the D3 sensitizer.
effect compared with benzenoid moieties.42,43 The calculated
torsion angles with respect to the conjugated spacers in
three D–B–A dyes are shown in Scheme 1. D1 has a negligible
torsion angle compared with D2 and D3, and all the three
dyes have small torsion angle. This indicates that triple
bond can enhance the planarity compared with the
other similar dyes as reported.29,40,43,44c Compared with the
spectra in CHCl3 solution, the absorption bands of these
dyes on TiO2 films exhibit larger red-shift and significant
spectral broadening due to the interaction of the anchoring
groups with the surface of TiO2 and the J-aggregation of dye
molecules.44 At the same time, the absorption bands are
broadened due to the aggregation of dyes on TiO2. To
investigate the degree of aggregation for these three dyes, we
dipped TiO2 film into dye solution including 0.5 mM dye and
10 mM chenodeoxycholic acid (CDCA) for 12 h. It was found
that the CDCA co-adsorbent can prevent the aggregation on
TiO2 film, which is consistent with those previously
reported.44d
Compared with D1 and D3, the aggregation of D2 onto the
TiO2 surface dimished in 10 mM CDCA in sensitizer solution.
The effect of CDCA as the coadsorbent on cell performance
needs to be investigated in details. The emission spectra of
these dyes were measured in CHCl3 (Fig. S1, ESIw). The first
emission peaks of D1 (598 nm) and D2 (614 nm) are hypso-
chromically shifted by 50–60 nm relative to D3 (661 nm),
which is consistent with the results of the UV-Vis absorption
spectra in CHCl3.
c
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New J. Chem., 2011, 35, 127–136 129

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couple (E(S+
) and the 0–0 excitation energy (E(0–0))
/S)
according to eqn (1)
E(S+
= E(S+ ꢀ E(0–0)
/S)
(1)
/S*)
Transition energy E(0–0) was estimated from the interception of
the absorption and emission spectra in CHCl3 solution.
As shown in Table 1, E(0–0) energies for D1, D2 and D3 were
extracted to be 2.37 eV, 2.28 eV and 2.18 eV, respectively. The
calculated excited-state oxidation potentials for D1, D2 and
D3 are ꢀ1.37 V, ꢀ1.29 V and ꢀ1.18 V vs. NHE, respectively,
which are more negative than the equivalent potential for
N719 sensitizer (ꢀ0.98 V vs. NHE) and the conduction band
(CB) of TiO2 (ꢀ0.5 V vs. NHE). Therefore, there are sufficient
driving forces for charge injection to the conduction band of
semiconductors.46 From spectroscopic and electrochemical
measurements, driving forces, DGinj, for electron injection
from the dye excited singlet state to the CB of TiO2
(ꢀ0.5 V vs. NHE), and DGreg, the regeneration of the dye
radical cation by electrolyte redox couple (+0.5 V vs. NHE)
for the dye-sensitized solar cells are extracted and shown in
Table 1. From these data, it can be seen that both of the
processes are thermodynamically feasible.
To gain insight into the electronic structures of excited
states, we performed time-dependent density functional theory
(TD-DFT) calculations on these three organic dyes. We
calculated the contribution of singly excited state configurations
to each electronic transitions and the molecular orbital
contributions to analyze the charge-separated states of these
dyes. All the three dyes have small torsion angle as shown in
Scheme 1. The calculated molecular orbital energy diagrams
and isodensity surface plots for three dyes D1, D2 and D3 are
shown in Fig. 2. The HOMO–LUMO band gap of D1, D2 and
D3 three dyes is 2.18 eV, 2.17 eV and 2.01 eV, respectively. D3
has the smallest bandgap, causing the red-shifted absorption
spectra, which is ideally preferred for DSSC application.
Compared with D2 and D3 with two thiophene units, it has
lower a LUMO–HOMO gap due to the increased conjugation
along the dye, which is consistent with the previous reports.
Meanwhile, the LUMO of D3 is closer to the conduction band
of TiO2 than that of D1 and D2. This result shows that the
extended p-system could promote the intramolecular charge
transfer and charge separation behavior from bridge group to
acceptor group after photo-excitation.3b,c Compared with D2,
the HOMO level of D3 (vs. NHE) is more positive, indicating
that the regeneration of the oxidized dyes with Iꢀ ions is
thermodynamically possible.1c Based on the calculated
Fig. 1 UV-vis absorption spectra of the dyes (a) in CHCl3 solution
and (b) TiO2 film.
Electrochemical properties
Cyclic voltammetry was employed to determine the oxidation
potentials of the three metal-free organic dyes. The measurements
were performed in CHCl3 solution with 0.2 M tetrabutyl-
ammonium hexafluorophosphate (Bu4NPF6) using ferrocene
as an internal standard at 0.63 V vs. NHE (Fig. S2, ESIw). The
oxidation potential and the reduction potential correspond to
the HOMO and LUMO energies, respectively. The oxidation
potentials of D1, D2 and D3 are 1.00 V, 0.99 V and 1.00 V vs.
NHE, respectively, which are more positive than the that of
Iꢀ/I3 couple (0.5 V vs. NHE),44 ensuring the regeneration of
ꢀ
sensitizer radical cations by the electrolyte redox couple. By
neglecting any entropy change during the electron-injection
process,32–34 the excited-state oxidation potential (E(S+/S*)) of
the sensitiser can be derived from the ground-state oxidation
Table 1 Experimental data for the spectral and electrochemical properties of the dyes
Dye
Absmaxa/nm
e/Mꢀ1 cmꢀ1
Eoxb/V vs. NHE
E0–0c/V
Eox*d/V vs. NHE
DGinje/eV
DGregf/eV
D1
D2
D3
446
476
511
24 402
33 053
34 604
1.00
0.99
1.00
2.37
2.28
2.18
ꢀ1.37
ꢀ1.29
ꢀ1.18
ꢀ0.87
ꢀ0.79
ꢀ0.68
ꢀ0.50
ꢀ0.49
ꢀ0.50
a
b
Absorption and emission spectra of D1, D2 and D3 in chloroform solution. Measured ground-state oxidation potential of the dyes. Potentials
c
measured vs. Fc+/Fc were converted to normal hydrogen electrode (NHE) by addition of +0.63 V. 0–0 transition energy, E(0–0), estimated from
d
the intercept of the normalized absorption and emission spectra in chloroform. Estimated LUMO energies, E(LUMO) vs. NHE from the estimated
highest occupied molecular orbital (HOMO) energies obtained from the ground-state oxidation potential by adding the 0–0 transition energy,
e
f
E(0–0)
.
Driving forces for electron injection from the dye excited singlet state (Eox*) to the conduction band of TiO2 (ꢀ0.5 V vs. NHE). Driving
forces for the regeneration of the dye radical (Eox) by Iꢀ/I3 redox couple (+0.5 V vs. NHE).
ꢀ
c
130 New J. Chem., 2011, 35, 127–136
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Fig. 2 Molecular orbital energy diagrams (TD-B3LYP/6-31G*//B3LYP/6-31G* in chloroform using C-PCM framework) and isodensity surface.
molecular orbital contribution (Table S1, ESIw), we found
that, among three dyes, D3 exhibits the most obvious intra-
molecular charge seperation. This means that D3 possesses
more stronger photo-induced intramolecular charge transfer
properties capability than D1 and D2, and the major portions
of the D3 HOMO are located on the donor moiety. LUMO
contains the acceptor moiety, the triple bond and the bridge,
which strongly suggests that electrons move from donor
moiety, passing through bridge moiety, to the acceptor and
form linearly photoexcitation-induced intramolecular charge
transfer. When the p-conjugation increased, the overlapping
extension can be enhanced by the bridge which might improve
the electronic coupling between the dye and TiO2 film. The
excitation energies of D1, D2 and D3 were also compared by
TD-DFT calculation results and shown in Table S2 (ESIw).
The results clearly indicate that introduction of thiophene
group in the bridge moiety increases the oscillator strength
of the visible range, which could give rise to the enhancement
of the solar spectrum overlap. In contrast, introducing phenyl
group in the bridge moiety can obviously increase the
oscillator strength in ultraviolet region though phenyl group
which makes against the bathochromic effect. Both the two
dominating excited states of these three dyes originated from
HOMO - LUMO and HOMO-1 - LUMO contributions
and the intensities are coincident with the experimental
absorptions of these three dyes. Thus, the transitions give rise
to an intramolecular charge flow from the donor moiety to the
acceptor moiety. Potentially, the resulting excited state is
strongly coupled to the semiconductor surface, due to charge
delocalization involving the cyanoacetic acid anchoring
group.29–31,35,42–44
The incident monochromatic photon-to-current conversion
efficiency (IPCE) values for the three organic dyes are shown
in Fig. 3. The IPCEs are represented by eqn (2):
1240ðeV nmÞJphðmA cmꢀ2
Þ
IPCEð%Þ ¼
ꢁ 100
ð2Þ
lðnmÞfðmW cmꢀ2
Þ
where Jph is the short-circuit photocurrent density for mono-
chromatic irradiation, l and f are the wavelength and the
intensity of the monochromatic light, respectively, The highest
IPCE values for D1, D2 and D3 are found to be around 80%,
80% and 90%, respectively. There are some red-shift in the
IPCE compared to the absorption spectra in Fig. 1, which can
47–49
be attributed to the aggregation of the dyes on TiO2
and
the interaction of the anchoring groups with the TiO2
surface.30,43 These results also show that the absorption
intensity in 350–400 nm region can be increased by the rigid
functional group because of the high molar extinction
coefficient as well as p-p-conjugated bridge which would
extend the absorption of dye molecule as mentioned above.
The photocurrent–voltage curves of the sensitizers are
shown in Fig. 4. In this work, we selected Z907 dye as control
in comparison with our three organic dyes and all devices were
Fig. 3 IPCE values for DSSCs based on the three organic dyes.
c
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Fig. 5 Nyquist plots of three films in the dark (inset) and illumination
condition.
Fig.
4
Photocurrent density–photovoltage curve of three dyes
sensitized cells under AM 1.5 G radiation (100 mW cmꢀ2).
electronic states in the conduction band. Since the recombination
of dye can be omitted, the electron recombination between the
injected electron from the conduct band of TiO2 and the
triiodide could reduce the Voc.43b According to the similar
results reported previously, the aggregation of the dye and the
orientation of the dye on TiO2 surface can aggravate charge
recombination. The rigid conjugate molecules are easy to
aggregate due to strong dipole–dipole interaction.44b–d What’s
more, when the dye molecules are adsorbed on TiO2 surface,
which can affect the fermi level of conduction band of TiO2,
resulting in a negative shift and this shift is increased with the
increasing dipole at the dye/TiO2 interface.44e This band-edge
shift is more significant than the increase in recombination
rate. Thus, the Voc of D2 gave the highest value and the
D1 gave the lowest value and due to the different level of
p-stacked aggregation and band-edge shift.44–45
To further investigate the internal resistance, the kinetics of
charge transfer and recombination in the devices, we carried
out the electrochemical impedance spectroscopy (EIS)
measure-ments in the dark and under illumination conditions.
Fig. 5 shows the Nyquist plots for the devices in the dark and
under illumination conditions. The equivalent circuit is shown
in Scheme 6. Rs represents the series resistance consisting of all
ohmic components in the device, R1 and R2 are charge transfer
resistance components forming a parallel circuit with constant
phase elements (CPE1 and CPE2) and W1 is the Finite-Length
Warburg element for Nernst diffusion impedance. Three semi-
circles were observed at various frequency range. The first
semicircle in the high-freꢀquency region is attributed to the
redox reaction of Iꢀ/I3 at the Pt/electrolyte interface,
the central semicircle denotes the electron transfer at the
oxide/dye/electrolyte interface and the low frequency one is
caused by ion diffusion within the electrolyte.47–49 the dark
condition, the impedance spectra were obtained under forward
bias (ꢀ0.65 V), the charge-transfer resistance related to the
recombination of electron, Rk, was estimated from the
diameter of the central circle. The Rk value increases in
the order of D3 (69.6 O) o D2 (129.3 O) o D1 (175.5 O).
In the dark condition, under reverse bias, TiO2 is an insulator
and the dye molecules with different dielectric constant, which
Table 2 DSSC performance parameters of various dyesa
Dye
Jsc/mA cmꢀ2
Voc/V
ff (%)
Z (%)
D1
D2
D3
Z907
6.95
8.04
11.29
15.11
0.623
0.684
0.637
0.686
74.2
70.1
69.2
58.9
3.21
3.86
4.98
6.11
a
Performance of DSSC was measured with a working area of
0.16 cm2.
fabricated under similar conditions. The short-circuit current
densities, Jsc, open-circuit voltages, Voc, fill factors, ff, and
power conversion efficiencies of the cells, Z, are summarized in
Table 2. The D3-sensitized cell device gives the best
performance and offers Jsc of 11.39 mA cmꢀ2, Voc of 0.637 V
and ff of 69.2%, respectively. The overall conversion efficiency
Z can be calculated according to eqn (3), where Pin represents
the intensity of the incident light (mW cmꢀ2). Among the three
organic dyes, D3 shows the highest efficiency of 4.98% which
reaches around 82% of the value for Z907 under similar
conditions. D2 shows an efficiency of 3.86%, while D1 gives
the lowest overall efficiency of 3.21%. The poor efficiency of
D1-sensitized cell is attributed to the lower light-harvesting
capability in the visible range, evidenced by the UV-vis
absorption spectra (Fig. 1), while D3 shows the hightest Jsc,
which is consistent with its broad absorption spectra. The
open circuit bias (Voc) increase in the order of D1 o D3 o D2.
Z = Jsc ꢁ Voc ꢁ ff/Pin
(3)
Under illumination condition, VOC of the cell is potential
difference between the quasi-Fermi level of the electrons
(EFn*) in the nanocrystalline TiO2 film and Eredox of the
electrolyte, also VOꢀC depends logarithmically on the inverse
concentration of I3 and increases with incident photo flux
according to the following equation,44a
ꢀ
ꢁ
RT
AI
n0k1½Iꢀ3 ꢂ þ n0k2½Dþꢂ
Voc
¼
ln
;
bF
where k1, k2 are the kinetic constants for the back reaction of
injected electrons with triiodide (I3ꢀ) and recombination of
these electrons with oxidized dye (D+), respectively, RT/bF is
the thermal voltage and n0 is the concentration of accessible
Scheme 6 Equivalent circuit of used to fit Nyquist plots.
c
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are also insulating in the ground state and hence the injected
electrons through TiO2/dye were coupled to that of Iꢀ/I3ꢀ ions
in electrolyte. Then the dye coating as tightly packed
insulating monolayer leads to a barrier blocking the dark
current,44c,45 and as mentioned above, the bridge molecule
can suppress the electron transfer when the molecule is in the
closed form;36 accordingly, the recombination resistance at the
TiO2/dye/electrolyte interface decreased according to this
order of D3 o D2 o D1 may be result from those factors.
Under illumination (50 mW cmꢀ2, open-circuit voltage
(OCV))
UV-Vis-Near IR spectrophotometer and photoluminescence
analyses were performed on a Cary Eclipses fluorescence
spectrometer. Cyclic voltammetric (CV) experiments were
conducted with a computer-controlled Eco Chemie Autolab
PGSTAT 100 with an ADC fast scan generator. Working
electrodes were 1 mm diameter planar Pt disks, used in
conjunction with a Pt auxiliary electrode and an Ag/Ag+
reference electrode connected to the test solution via a salt
bridge containing 0.2 M tetrabutylammonium hexafluoro-
phosphate (Bu4NPF6) in CHCl3. Accurate potentials were
obtained using ferrocene as an internal standard.
conditions, which gives rise to a new processes at the TiO2/
dye/electrolyte interface including the electron injection from
excited dye to TiO2, recombination with the oxidised dye,
and regeneration of the sensitizer.44a The diameter of the
intermediate-frequency semicircle in the Nyquist plot increases
in the order of D3 o D2 o D1, which indicates that the
electrons generation and transport are enhanced in the order
of D3 4 D2 4 D1. From these data, it can be seen that planar
conjugation bridge can retard the charge recombination, large
conjugated bridge can increases the intramolecular charge
transfer and the charge-injection rate. The cell performance
can be improved by harmonizing these factors.
Calculation method
We performed time-dependent density functional theory
TD-DFT calculation by using the Gaussian 03 program
package.50 The geometries of the three metal-free organic dyes
in vacuo were optimized using density functional theory (DFT)
with the B3LYP density functional and the 6-31G* basis set.50
All optimized geometries were subjected to vibrational
frequency analysis, and characterized as a minimum (no
imaginary frequencies). The time-dependent DFT (TD-DFT)
at the level of B3LYP/6-31G* was carried out to study the
excited state of selected molecules. The absorption spectra
were simulated by the 20 lowest spin-allowed singlet transitions
and the C-PCM frameworks were used to describe the electro-
static solute–solvent interactions.50 The contribution of single
excited state configurations to each electronic transition of the
three metal-free organic dye analogues were calculated using
the Swizard program, version 4.2.51 Graphical molecular
orbital were generated with the Gauss View software program
(version 3.09)50 and the molecular orbital contributions from
the groups of atoms were obtained from the VMOdes A 7.1.51
Conclusions
In summary, we have designed three new linear dye molecules
with good planarity and investigated the effect of the bridge
moiety on the properties of dye-sensitized solar cells. The
results based on UV-Vis absorptions, TD-DFT calculations,
photovoltaic experiment and EIS show that p-conjugated
bridge can extend the absorption of dye molecule and strong
planarity (closed-packing) can suppress the charge recombination
at TiO2/electrolyte interface. Improving the cell performance
has been demonstrated for the dye molecules with large-
conjugated planarity and p-p conjugation as bridge due to
the retarded recombination and enhanced light harvesting,
which provides a skilful strategy to explore metal-free
sensitizers with high photo-to-current conversion efficiency.
To further improve the cell performance of DSSCs based on
metal-free dyes, we will focus on the development of novel
organic sensitizers with large planar donor-p bridge-acceptors
in the future. Currently, the study of the coadsorbent effect on
these conjugated molecules and the rate of charge transfer
between the sensitizers and TiO2 surface is in progress.
Electrochemical impedance measurement
Electrochemical impedance experiments were carried out in
the dark and under illumination condition using an Autolab
electrochemical workstation with a frequency range from 0.1 Hz
to 1 MHz and a potential modulation of 10 mV. The forward
bias is ꢀ0.65 V and open-circuit voltage in the dark and under
illumination condition, respectively. The results were fitted
with the Z-view software (ZView 3.1c, Scribner Associate Inc.)
to appropriate equivalent circuits. The electrochemical
measurements were done using a 20 W halogen lamp as light
source.
Photovoltaic characterization
Experimental details
The current–voltage measurement of DSSCs were performed
under one sun condition (air mass (AM) 1.5) using a Sun 2000
solar simulator light source (Abet-technologies, USA) and the
incident light intensity was calibrated using a radiant power
meter (Oriel, 70260). The current–voltage characteristics of the
cell under these conditions were obtained by applying external
potential bias to the cell and measuring the generated photo-
current with a Keithley model 2440 digital source meter
(Keithley, USA). The incident photon to collected electron
conversion efficiency (IPCE) measurement was collected using
Oriel QE/IPCE Kit equipment.
Materials and instrumentation
All reactions were carried out under Ar atmosphere. Solvents
were dried by standard procedures. cis-Bis(isothiocyanato)-
(2,20-bipyridyl-4,40-dicarboxylato)(2,20-bipyridyl-4,40-di-nonyl)-
ruthenium(II) (known as Z-907, dyesol) and electrolyte (EL-HPE)
were obtained from Dyesol Company. 1H and 13C NMR
spectra were recorded on JEOL 400 MHz Instrument. Mass
spectra under the conditions of electron impact (EI) ionisation
were recorded on Thermo Finnigan MAT 95XP spectro-
meter. Absorption spectra were measured on JASCO V-670
c
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New J. Chem., 2011, 35, 127–136 133

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Synthesis
N-(4-Iodophenyl)-4-methoxy-N-(4-methoxyphenyl)benzene
25 1C, TMS): d = 165.07 (COOH), 155.76 (CHQC–CN),
150.04 (Ph (C)–OCH3), 141.58, 132.01, 131.63, 131.49, 129.77,
ꢀ
ꢀ
ꢀ
129.95, 129.55, 127.70, 127.30, 125.98, 121.95, 118.77, 114.95,
amine (3). 4-Iodoanisole (5 g, 21.36 mmol), 4-iodoaniline
(2 g, 9.13 mmol) and 1,10-phenanthroline (0.30 g, 1.70 mmol)
were dissolved in toluene (50 mL). After the solution was heated to
100 1C, CuCl (0.16 g, 1.70 mmol) and KOH (3.83 g, 68.29 mmol)
were added under Ar. The mixture was refluxed for 12 h. After
cooling to room temperature, acetic acid (5 mL) and toluene
(20 mL) were added. The mixture was washed with H2O (100 mL)
three times and the organic phase was dried over Na2SO4. After
removal of the solvent, the residual was purified on a silica gel
column (ethyl acetate–hexane: 1/9) to afford the target compound
93.31 (alkynyl carbon), 80.64 (alkynyl carbon), 55.59 (OCH3);
ꢀ
MS (EI) m/z: 500.22; HRMS (ESI, M + 1): m/z cald for
C32H25 N2O4 501.1814, found 501.1814.
5-(2-(4-(Bis(4-methoxyphenyl)amino)phenyl)ethynyl)thiophene-
2-carbaldehyde (11). 5-(2-(Trimethylsilyl)ethynylthiophene-2-
carbaldehyde (0.22 g, 1.61 mole) was dissolved in THF (10 mL)
followed by addition of copper(I) iodide (10 mg, 0.053 mol),
dicholorobis (triphenylphosphine) palladium(II) (18 mg, 0.026 mol)
and triethylamine (0.30 mL). Iodo compound 3 (0.58 g, 1.34 mmol)
was added from addition funnel as a solution in THF (10 mL)
to the above reaction mixture. The reaction mixture was
heated at 30 1C with stirring for 30 min and at 25 1C for
20 h. Monitor of the above reaction mixture by TLC indicated
the completion of the reaction. After removal of the solvent,
the residue was purified on silica gel column (hexane–ethyl
acetate: 8/2) to afford the target compound as yellow solid
(0.29 g, 50%). 1H NMR (400 MHz, CDCl3, 25 1C, TMS):
1
as a yellowish-orange solid (1.18 g, 30%). H NMR (400 MHz,
CDCl3, 25 1C, TMS): d = 7.38 (d, J = 8.68 Hz, 2H, Ph), 7.00
(d, J = 9.12 Hz, 4H, Ph), 6.79 (d, J = 9.16 Hz, 4H, Ph), 6.64
(d, J = 8.68 Hz, 2H, Ph), 3.80 (s, 6H, OCH3); 13C NMR
ꢀ
(100 MHz, CDCl3, 25 1C, TMS): d = 156.10 (Ph(C)–OCH3),
148.53, 140.46, 137.77, 126.77, 122.37, 115.03, 82.03 (Ph(C)–I),
ꢀ
ꢀ
55.63 (OCH3); MS (EI) m/z: 431.
ꢀ
4-(2-(4-(Bis(4-methoxyphenyl)amino)phenyl)ethynyl benzal
dehyde (7). 4-Ethynylbenzaldehyde (90 mg, 0.69 mmol) was
dissolved in THF (10 mL) followed by addition of copper(I)-
iodide (5 mg, 0.026 mmol), dicholorobis(triphenylphosphine)
palladium(II) (8 mg, 0.012 mol) and triethylamine (0.15 mL).
Iodo compound 3 (0.27 g, 0.00062 mol) was added from
addition funnel as a solution in THF (10 mL) to the above
reaction mixture. The reaction mixture was warmed at 30 1C
with stirring for 30 min and at 25 1C for 20 h. Analysis of the
reaction mixture by TLC indicated the completion of the
reaction. After removal of the solvent, the residue was purified
on silica gel column (hexane–ethyl acetate: 8/2) to afford the
d = 9. 81 (s, 1H, CHO), 7.62 (d, J = 4.12 Hz, 1H, Th), 7.29
(d, J = 8.68 Hz, 2H, Ph), 7.22 (d, J = 4.12 Hz, 1H, Th), 7.09–7.05
ꢀ
(m, 4H), 6.86–6.80 (m, 6H), 3.78 (s, 6H, OCH3); 13C NMR
ꢀ
(100 MHz, CDCl3, 25 1C, TMS): d = 182.53 (CQO), 156.85
(Ph(C)–OCH3), 150.00, 143.15, 139.73, 136.31, 134.11, 132.64,
ꢀ
ꢀ
131.67, 127.51, 118.71, 114.80, 111.86, 99.63 (alkynyl carbon),
81.05 (alkynyl carbon), 55.37 (OCH3); MS (EI) m/z: 439.16.
ꢀ
(E)-3(4-(4-(Bis(4-methoxyphenyl)amino)phenyl)ethynyl)
thiophen-2-yl)-2-cyanoacrylic acid (D2). Push-pull aldehyde
11 (0.27 g, 0.61 mmol) dissolved in acetonitrile (30 mL)
solution was condensed with 2-cyanoacetic acid (0.052 g,
0.61 mmol) in the presence of piperidine (0.052 g, 0.61 mmol).
The mixture was refluxed for 4 h under nitrogen atmosphere.
The solvent was removed completely after cooling to room
temperature; the residue was washed with 2 M aqueous
HCl and extracted with CHCl3. The organic phase was
collected and dried over Na2SO4. After removal of the solvent,
the crude product was purified on the silica gel column
(acetic acid/dichloromethane: 1/99) to afford the target
compound as a dark red solid (0.124 g, 40%). 1H NMR
1
target compound as a yellow solid (0.15 g, 51%). H NMR
(400 MHz, CDCl3, 25 1C, TMS): d = 9. 97 (s, 1H, CHO),
7.80 (d, J = 8.24 Hz, 2H, Ph), 7.59 (d, J = 8.24 Hz, 2H, Ph),
ꢀ
7.30 (d, J = 9.16 Hz, 2H, Ph), 7.09–7.05 (m, 4H, Ph),
6.86–6.82 (m, 6H, Ph), 3.79 (s, 6H, OCH3); 13C NMR
ꢀ
(100 MHz, CDCl3, 25 1C, TMS): d = 191.56 (CQO), 156.60
(Ph(C)–CHO), 149.50, 139.94, 135.04, 133.19, 132.79, 131.83,
ꢀ
ꢀ
129.67, 127.39, 118.79, 114.94, 112.59, 94.98 (alkynyl carbon),
87.66 (alkynyl carbon), 55.57 (OCH3); MS (EI) m/z: 433.30.
ꢀ
(400 MHz, CDCl3, 25 1C, TMS): d = 8.25 (s, 1H, CHQ(C)–CN),
7.68 (d, J = 4.12 Hz, 1H, Th), 7.29-7.22 (m, 3H), 7.09 (d, J = 9.16
ꢀ
(E)-3(4-(4-(Bis(4-methoxyphenyl)amino)phenyl)ethynyl) phenyl)-
Hz, 4H, Ph), 6.88–6.79 (m, 6H), 3.79 (s, 6H, OCH3); 13C NMR
2-cyanoacetic acid (D1). Push-pull aldehyde
7 (0.10 g,
ꢀ
0.23 mmol) dissolved in acetonitrile (10 mL) solution was
condensed with 2-cyanoacetic acid (19 mg, 0.23 mmol) in the
presence of piperidine (29 mg, 0.23 mmol). The above reaction
mixture was refluxed for 4 h under nitrogen atmosphere.
After cooling to room temperature, the solvent was removed
completely. The reaction residue was washed with 2 M
aqueous HCl and extracted with CHCl3. The organic phase
was collected and dried over Na2SO4. After removal of the
solvent, the crude product was purified on the silica gel column
(acetic acid–dichloromethane: 1/99) to afford the target
compound as a dark red solid (40 mg, 36%).1H NMR
(100 MHz, CDCl3, 25 1C, TMS): d = 168.02 (COOH),
ꢀ
156.82 (CHQC–CN), 149.99 (Ph (C)-OCH3), 147.47, 144.51,
143.03, 139.64, 139.00, 135.53, 132.82, 131.76, 127.53, 126.23,
ꢀ
ꢀ
118.35, 115.00, 111.59, 102.12 (alkynyl carbon), 81.29
(alkynyl carbon), 55.62 (OCH3); MS (EI) m/z: 506.18; HRMS
ꢀ
(ESI, M + 1): m/z cald for C30H23 N2O4S 507.1379, found
507.1376.
5-(5-(2-(4-(Bis(4-methoxyphenyl)amino)ethynyl)thiophene-2-yl)-
thiophene-2-carbaldehyde
(16).
5-(5-Ethynylthiophen-2-yl)-
thiophene-2-carbaldehyde (0.25 g, 1.14 mmol) was dissolved
in THF (10 mL), followed by addition of copper(I) iodide
(10 mg, 0.05 mmol), dicholorobis(triphenylphosphine)palladium(II)
(70 mg, 0.09 mmol) and triethylamine (0.50 mL). Iodo
compound 3 (0.50 g, 1.16 mmol) was added from addition funnel
(400 MHz, CDCl3, 25 1C, TMS): d = 8.27 (s, 1H, CHQC–CN),
8.02 (d, J = 8.24 Hz, 2H, Ph), 7.85 (d, J = 8.24 Hz, 2H, Ph),
ꢀ
7.48 (d, J = 8.08 Hz, 2H, Ph), 7.11–7.06 (m, 4H), 6.82–6.88
(m, 6H), 3.79 (s, 6H, OCH3); 13C NMR (100 MHz, CDCl3,
ꢀ
c
134 New J. Chem., 2011, 35, 127–136
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011

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as a solution in THF (10 mL). The reaction was heated with
stirring at 30 1C for 30 min and then at 25 1C for 20 h. Analysis
of the reaction by TLC indicated the completion of the reaction.
After removal of the solvent, the residue was purified on silica gel
column chromatography to afford the target compound 16
(0.30 g, 51%). 1H NMR (400 MHz, CDCl3, 25 1C, TMS):
were bonding together, and the electrolyte (EL-HPE) was
introduced between the electrodes by capillary action.
Acknowledgements
Nanyang Technological University (RG54/07 and SUG41/06)
and the Ministry of Education, Singapore (ARC24/07,
no. T206B1218RS) are gratefully acknowledged.
d = 9. 84 (s, 1H, CHO), 7.63 (d, J = 4.12 Hz, 1H, Th), 7.26
(d, J = 8.75 Hz, 2H, Ph), 7.20–7.22 (m, 2H), 7.11 (d, J = 4.12
ꢀ
Hz, 1H, Th), 7.05 (d, J = 9.60 Hz, 4H, Ph), 6.80–6.86 (m, 6H),
3.78 (s, 6H, OCH3); 13C NMR (100 MHz, CDCl3, 25 1C, TMS):
ꢀ
References
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