Efficient metal-free sensitizers bearing circle chain embracing π-spacers for dye-sensitized solar cells

Article abstract of DOI:10.1039/c3ta12368e

The circle chain embracing D-π-A dyes have been demonstrated as a promising type of sensitizer for dye-sensitized solar cells (DSCs). To further develop this type of organic dye for DSCs and discover the heuristic points on energy level matching, we introduced various donor moieties (carbazole, indoline and dimethoxytriphenylamine) into a D-π-A system bearing a circle chain embracing a π-spacer and cyanoacrylic acid as an acceptor, resulting in three new organic dyes LJ-4, LJ-5 and LJ-6, respectively. Among these dyes, the energy levels of the molecular orbitals in carbazole dye LJ-4 match best with the conduction band of TiO₂ and the redox potential of the redox species. DSCs employing the sensitizer LJ-4 and the iodine redox electrolyte exhibit a high power conversion efficiency of 9.20%, measured under the 100 mW cm^-2, simulated AM1.5 sunlight. This work highlights the importance of energy level matching in the further molecular design of a circle chain embracing D-π-A dyes.

Full text of DOI:10.1039/c3ta12368e

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Efficient metal-free sensitizers bearing circle chain
embracing p-spacers for dye-sensitized solar cells†
Cite this: DOI: 10.1039/c3ta12368e
Jian Liu,a Xudong Yang,a Ashraful Islam,a Youhei Numata,a Shufang Zhang,a
a
b
ab
*
Noviana Tjitra Salim, Han Chen and Liyuan Han
The circle chain embracing D–p–A dyes have been demonstrated as a promising type of sensitizer for dye-
sensitized solar cells (DSCs). To further develop this type of organic dye for DSCs and discover the heuristic
points on energy level matching, we introduced various donor moieties (carbazole, indoline and
dimethoxytriphenylamine) into a D–p–A system bearing a circle chain embracing a p-spacer and
cyanoacrylic acid as an acceptor, resulting in three new organic dyes LJ-4, LJ-5 and LJ-6, respectively.
Among these dyes, the energy levels of the molecular orbitals in carbazole dye LJ-4 match best with the
conduction band of TiO2 and the redox potential of the redox species. DSCs employing the sensitizer
LJ-4 and the iodine redox electrolyte exhibit a high power conversion efficiency of 9.20%, measured
under the 100 mW cmꢀ2, simulated AM1.5 sunlight. This work highlights the importance of energy level
matching in the further molecular design of a circle chain embracing D–p–A dyes.
Received 18th June 2013
Accepted 3rd July 2013
DOI: 10.1039/c3ta12368e
www.rsc.org/MaterialsA
photovoltage in DSCs based on organic D–p–A dyes,7 the
introduction of steric hindrance on the dye molecule has been
Introduction
demonstrated as an effective strategy in molecular design.2c,8
Currently, two styles of substituent are adopted for steric
hindrance in organic D–p–A dyes. One is the exible bulky
groups (alkyl chains, etc.) side linked on the donor part and/or
p-conjugated backbone,2c,8a,c and another is the rigid circle
chain surrounding the p-spacer.9 The former has been dis-
cussed to be effective in suppressing the intermolecular inter-
action and charge recombination during the past decade,
resulting in successful development of various dye structures
with high efficiency.5e,f,h,8 On the other hand, the circle chain
surrounding the dye molecule not only suppresses intermolec-
ular interactions and charge recombination efficiently, but also
immobilizes the conjugated backbone in a planar congura-
tion, hence extending the absorption spectra for efficient light
harvesting. However, only LJ-3, is an example of a dye bearing a
circle chain embracing p-spacer that has been recently devel-
oped with a conversion efficiency of 8.34% recently.9 Many
fundamental issues need to be investigated for further devel-
opment of this promising kind of circle chain embracing D–p–A
sensitizers for DSCs. The most urgent current issue that needs
to be addressed is the energy level matching for efficient elec-
tron injection and dye regeneration,10 which is signicant for
the rational molecular design of this new type of organic dye.
In this context, we introduced various donor moieties
(carbazole, indoline and dimethoxytriphenylamine) into the
D–p–A system bearing a circle chain embracing p-spacer and
cyanoacrylic acid as an acceptor, resulting in three new organic
dyes LJ-4, LJ-5 and LJ-6 (Scheme 1), respectively. Essentially,
these three new dyes were designed based on the following
Dye-sensitized solar cells (DSCs), a type of innovative photo-
voltaic device with an operating principle resembling the
natural photosynthetic process, are considered to be the next
generation of photovoltaics due to their potential applications
in cost-efficient renewable energy, as well as in the production
of electronics. Since the pioneering report in 1991,1 unremitting
research effort has been devoted to this type of device, resulting
in signicant progress, in molecular engineering,2 investigation
of the mechanism,3 and the recorded efficiencies continue to
increase.4 In order to avoid the use of noble metals and having
to face the environmental issues that follow, metal-free organic
D–p–A dyes have emerged, and DSCs based on organic dyes
have yielded competitive conversion efficiencies up to 9%.5
Achieving higher efficiencies, which involves the improvement
of open-circuit voltage and short-circuit current, is required for
practical application. However, such improvements have been
considered as the most important and challenging subject in
the development of organic sensitizers.6
In order to overcome the common issues involving aggra-
vated intermolecular interactions and charge recombination,
which deteriorate photocurrent generation and decrease
aPhotovoltaic Materials Unit, National Institute for Materials Science, Tsukuba,
Ibaraki 305-0047, Japan. E-mail: HAN.Liyuan@nims.go.jp; Fax: +81 298592304;
Tel: +81 298592747
bState Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 800
Dong Chuan RD. Minhang District, Shanghai 200240, China
† Electronic supplementary information (ESI) available: Spectra characterization,
DFT calculations and HPLC analysis of dyes LJ-4, LJ-5 and LJ-6. See DOI:
10.1039/c3ta12368e
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Scheme 1 Molecular structures and synthetic routes of circle chain embracing D–p–A dyes LJ-4, LJ-5 and LJ-6.
1
considerations: (1) further expanding the absorption spectra by dyes were characterized by H and 13C NMR spectroscopy, as
introducing stronger electron donating parts for light harvest- well as mass spectrometry. The purities of the circle chain
ing; (2) tuning the energy levels of dye molecular orbitals and embracing D–p–A dyes LJ-4, LJ-5 and LJ-6 were characterized by
suggesting suitable driving forces for efficient electron injection HPLC analysis (Fig. S6–S8, ESI†).
and dye regeneration; (3) investigating the inuence of the
electron donating ability of various donor parts (indoline >
dimethoxytriphenylamine > triphenylamine > carbazole) on the
UV-visible absorption spectra
The absorption spectra of the circle chain embracing D–p–A
photovoltaic performance of DSCs based on circle chain
dyes LJ-3, LJ-4, LJ-5 and LJ-6 in dichloromethane are shown in
embracing D–p–A dyes. The synthesis, spectra, electrochemical
Fig. 1, and the data is summarized in Table 1. Being similar to
dye LJ-3, the three new dyes LJ-4, LJ-5 and LJ-6 exhibit two
and photovoltaic properties of the dyes LJ-4, LJ-5 and LJ-6 have
been investigated, and dye LJ-3 (Scheme 1) is also presented for
comparative analysis.
Results and discussion
Molecular design and synthesis
The preparation of three new circle chain embracing D–p–A
dyes LJ-4, LJ-5 and LJ-6 was achieved via bromination, Suzuki
coupling and Knoevenagel condensation reactions in
a
moderate yield by the rational synthetic routes depicted in
Scheme 1. Bromination of the circle chain embracing bithio-
phene 1 by NBS produced the resulting compound 2,11 which
was then coupled with 5-formylthiophene-2-boronic acid to
yield aldehyde 3,9 a common intermediate for the three new
dyes. Suzuki coupling between aldehyde 3 and the corre-
sponding borate reagents gave the dye precursors 4, 5 and 6,
which were converted to dyes LJ-4, LJ-5 and LJ-6 as purple solids
Fig. 1 Absorption spectra of dyes LJ-3, LJ-4, LJ-5 and LJ-6 in CH2Cl2 (2 ꢁ 10ꢀ5
M); normalized absorption spectra of LJ-3, LJ-4, LJ-5 and LJ-6 anchored on a 4 mm
via the Knoevenagel condensation, respectively. The desired transparent TiO2 film (inset).
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Table 1 Optical and redox parameters of dyes LJ-3, LJ-4, LJ-5 and LJ-6
Dye
almax/nm (3/L molꢀ1 cmꢀ1
)
blmax/nm
cS+/0/V (vs. NHE)
dS+/*/eV (vs. NHE)
eE0–0/V
LJ-3
LJ-4
LJ-5
LJ-6
551(50420), 393(23250)
550(46480), 380(20610)
570(58300), 401(32940)
556(53010), 399(29250)
496
496
510
506
0.81
0.94
0.72
0.76
1.10
ꢀ0.98
ꢀ1.21
ꢀ1.18
1.91
1.92
1.93
1.94
a
The absorption spectra were measured in CH2Cl2 solution (2 ꢁ 10ꢀ5 M). b Measured on TiO2 lm. c The S+/0 corresponding to the ground-state
oxidation potential (vs. NHE) in CH2Cl2 internally calibrated with ferrocene. S+/* ¼ S+/0 – E0–0, where E0–0 is the zero–zero transition energy.
d
e
E0–0 values were estimated from the intersection between the normalized absorption and emission spectra in CH2Cl2 (Fig. S1, ESI †).
distinct absorption bands: one absorption band is in the UV voltammetry (CV) in CH2Cl2 containing 0.1 M tetra-n-buty-
region (360–430 nm) corresponding to the p–p* electron tran- lammonium hexauorophosphate (TBAHFP) as a supporting
sitions of the conjugated molecules, as well as the overlapping electrolyte. As shown in Fig. 2a, all these dyes exhibit two
n–p* electron transitions; and the other is in the visible region reversible oxidative waves. The different electron–donor moie-
(500–600 nm) that can be assigned to intramolecular charge ties in LJ-3, LJ-4, LJ-5 and LJ-6 inuence their rst oxidation
transfer (ICT) from the donating unit to the cyanoacrylic acid potentials, respectively. The HOMO levels corresponding to the
anchoring moiety. The maxima absorption peaks (lmax) of the rst oxidation potentials (vs. NHE) increase in the order of
dyes vary: indoline dye LJ-5 (570 nm) > dimethoxytriphenyl- indoline dye LJ-5 (0.72 V) < dimethoxytriphenylamine dye LJ-6
amine dye LJ-6 (556 nm) > triphenylamine dye LJ-3 (551 nm) z (0.76 V) < triphenylamine LJ-3 (0.81 V) < carbazole dye LJ-4
carbazole dye LJ-4 (550 nm), which could be explained by the (0.94 V), which was associated with the electron-donating ability
strength of the electron donating ability of the donor group in of indoline > dimethoxytriphenylamine > triphenylamine >
each dye.12 Due to the relatively stronger electron donating carbazole. They are all more positive than the Nernst potential
ability of the indoline moiety, the ICT transitions absorption of Iꢀ/I3ꢀ redox couple (0.4 V vs. NHE), ensuring regeneration of
band in indoline dye LJ-5 is redshied ꢂ20 nm relative to those the oxidized dyes by Iꢀ aer electron injection, with the driving
of the carbazole dye LJ-4 and dimethoxytriphenylamine dye force of 0.41 V, 0.54 V, 0.32 V and 0.36 V for LJ-3, LJ-4, LJ-5 and
LJ-6, and with enhancement of molar extinction coefficient (3). LJ-6, respectively (Fig. 2b). The excited state redox potentials of
Circle chain embracing D–p–A dyes LJ-4, LJ-5 and LJ-6 exhibit these three dyes (ꢀ0.98 to ꢀ1.21 V vs. NHE) are sufficiently more
similar emission in CH2Cl2 solution but different Stokes shis negative than the conduction band of TiO2 (ꢀ0.5 V vs. NHE),
(Fig. S1, ESI†). Among these three new dyes, the carbazole dye ensuring sufficient driving force (0.48–0.71 V) for electron
LJ-4 exhibits the largest Stokes shi, which indicates signicant injection from the excited dyes to the conduction band of TiO2.
structural reorganization of the dye molecule upon
photoexcitation.13
The normalized UV-vis absorption spectra of these dyes DFT calculations
adsorbed on TiO2 lms are shown in the inset in Fig. 1. The lmax
for the dye-loaded TiO2 lms are also listed in Table 1. Aer
To gain further insight into the molecular structure and frontier
molecular orbitals of these new dyes, the geometries of dyes
being adsorbed on the TiO2 lm, LJ-3, LJ-4, LJ-5 and LJ-6 show
LJ-4, LJ-5 and LJ-6 were optimized by density functional theory
ne absorption peaks, similar to those in solution, suggesting
(DFT) calculations at the B3LYP/6-31G** level. Fig. S4 (ESI†)
that the steric hindrance of the insulating outer ring suppresses
displays the relative energies and electron distributions of the
the intermolecular interactions effectively. The maximum
HOMOs and LUMOs of these dyes. The HOMOs are mainly
delocalized from the donor parts to the oligothiophene
absorption of these dyes anchored on the TiO2 lm exhibits a
hypochromatic shi of approximately 50 nm in comparison
p-spacer, whereas the LUMOs show localized electron distri-
with that in solution. Such a blue shi could be assigned to the
butions through the cyanoacrylic acid and its adjacent p-spacer.
solvent effect and/or deprotonation of the carboxylic acid in the
Hence, both orbitals provide sufficient overlap between the
anchoring process.14 As compared to the analogue dye MK-2
donor and acceptor to guarantee a fast charge transfer transi-
containing four thiophene units linked long alkyl chains,2c,15
tion. Therefore, excitation from the HOMO to the LUMO should
carbazole dye LJ-4 exhibits obvious red-shi both in solution
lead to efficient photoinduced electron transfer from the donor
and on TiO2 lm, as well as higher 3 under the same conditions
group to the TiO2 lm via the terminal cyanoacrylic acid. The
(Fig. S2 and S3, ESI†), even though dye LJ-4 only possesses three
optimized ground-state geometries and corresponding dihedral
angles between each plane are shown in Fig. S5.† The circle
thiophene units. The results further indicate that employing a
circle chain to x the conjugated backbone was an efficient way
chain embracing p-spacers in these three new dyes perform
to improve the light harvesting efficiency.
similar planar conguration with a dihedral angle below 1ꢃ
between the corresponding thiophene units, which will facili-
tate the ICT transition, as suggested by their excellent absorp-
Electrochemical properties and energy levels
The electrochemical properties of the circle chain embracing tion in visible region described above. The geometry results
D–p–A dyes LJ-3, LJ-4, LJ-5 and LJ-6 were studied by cyclic indicate that p-conjugations in this type of circle chain
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Fig. 2 (a) Cyclic voltammograms of dyes LJ-3, LJ-4, LJ-5 and LJ-6 in CH2Cl2–TBAHFP (0.1 M), [c] ¼ 1 ꢁ 10ꢀ4 mol Lꢀ1, 293 K, scan rate ¼ 100 mV sꢀ1; (b) energy-level
diagram of dyes LJ-3, LJ-4, LJ-5 and LJ-6, the electrolyte and TiO2, ECB: energy level of conduction band of TiO2; DG1: driving force for electron injection; DG2: driving
force for regeneration of the oxidized dyes.
embracing D–p–A dyes are not altered by their donor moieties, based cells (88%), which is consistent with the driving force for
which is convenient for molecular tailoring.
the regeneration of the oxidized dye in the trend of indoline dye
LJ-5 (0.32 V) < dimethoxytriphenylamine dye LJ-6 (0.36 V) < tri-
phenylamine dye LJ-3 (0.41 V) < carbazole dye LJ-4 (0.54 V)
(Fig. 2b). The results indicate that a negative shi of the ground-
state oxidization potential might dominate the slower regener-
ation of the oxidized state of dyes LJ-5 and LJ-6 by the Iꢀ in the
redox electrolyte. Among these four circle chain embracing D–
p–A dyes, carbazole dye LJ-4 possesses the lowest driving force
(0.48 V) for electron injection, but the DSCs based on dye LJ-4
gave the highest IPCE, indicating that a driving force of 0.48 V is
sufficient for electron injection from the excited dyes to the
conduction band of TiO2. As a result, among these four dyes,
the energy levels of carbazole dye LJ-4 match best with the
conduction band of TiO2 and the redox potential of the redox
species for efficient electron injection and dye regeneration.
We plotted current–voltage curves and calculated the
photovoltaic properties of the LJ-3, LJ-4, LJ-5 and LJ-6 cells
under the same conditions (Fig. 4 and Table 2, respectively). The
short-circuit currents (JSC) vary in the order of LJ-3-based
Photovoltaic properties
The photovoltaic characteristics of the DSCs based on these
dyes under AM1.5G condition (100 mW cmꢀ2) have been
investigated,16 employing a solution of 0.6 M 1-propyl-2,3-
dimethylimidazolium iodide, 0.05 M I2, 0.1 M LiI and 0.5 M tert-
butylpyridine (TBP) in acetonitrile as the electrolyte. The onset
wavelength of the IPCE spectra shi towards the red region was
in the order LJ-4 z LJ-3 < LJ-6 < LJ-5 cells (Fig. 3), in accordance
with the absorption spectra observed for the dye-loaded TiO2
lms. Comparing with the maximum IPCE value (83%)
observed in the range of 400–600 nm for the DSCs based on
triphenylamine dye LJ-3, the maximum value of IPCE for
carbazole dye LJ-4 increases to 88%, whereas those for indoline
dye LJ-5 and dimethoxytriphenylamine dye LJ-6 decrease to 77%
and 81%, respectively. Considering the IPCE results together
with the energy levels of these four circle chain embracing D–p–
A dyes, the IPCE values vary in the order of LJ-5-based cells
(77%) < LJ-6-based cells (81%) < LJ-3-based cells (83%) < LJ-4-
cells (15.03 mA cmꢀ2) < LJ-6-based cells (15.67 mA cmꢀ2
)
Fig. 3 IPCE spectra of DSCs based on dyes LJ-3, LJ-4, LJ-5 and LJ-6.
Fig. 4 I–V curves of DSCs based on LJ-3, LJ-4, LJ-5 and LJ-6.
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Table 2 Photovoltaic performance for DSCs based on dyes LJ-3, LJ-4, LJ-5 and LJ-6a
JSC
Dye-loading amount
DSC
IPCEmax [%]
[mA cmꢀ2
]
VOC [V]
FF
h [%]
[10ꢀ7 mol cmꢀ2
]
LJ-3
LJ-4
LJ-5
LJ-6
83
88
77
81
15.03
16.34
15.68
15.67
0.737
0.747
0.710
0.732
0.752
0.754
0.667
0.723
8.34
9.20
7.42
8.29
1.74
1.85
1.44
1.65
a
Measurements were performed under AM1.5 irradiation of the DSC devices with a 0.2304 cm2 active surface. JSC, short circuit current; VOC, open
circuit voltage; FF, ll factor; h, conversion efficiency.
z LJ-5-based cells (15.68 mA cmꢀ2) < LJ-4-based cells (16.34 mA chain embracing D–p–A dyes LJ-4, LJ-5 and LJ-6 exhibit an
cmꢀ2). The slightly larger JSC of the DSCs based on LJ-5 and LJ-6, approximately linear increase in VOC as a function of the loga-
in comparison with that of DSCs based on LJ-3, demonstrates rithm of electron density. Under the same condition, the CEM
the benecial inuence of the red-shied absorption spectra of plots for DSCs based on these three dyes almost overlap, indi-
LJ-5 and LJ-6 on TiO2 lm and the broadening of the IPCE cating that DSCs based on LJ-4, LJ-5 and LJ-6 share the similar
spectra of the LJ-5 cells and LJ-6 cells. The high JSC observed for conduction band position. The results suggest that the adjust-
the LJ-4 cells is attributed to its high IPCE value in the range of ment of donor parts in this type of organic sensitizer has less
400–600 nm. The open-circuit voltages (VOC) of the DSCs based inuence on the shi of the conduction band of TiO2. There-
on circle chain embracing D–p–A dyes LJ-4, LJ-5 and LJ-6 were fore, the VOC difference of DSCs based on LJ-4, LJ-5 and LJ-6
0.747 V, 0.710 V and 0.732 V, respectively. The VOC values of all would be caused by their various charge recombination rates,
these cells are comparable to those of alkylated dyes, such as which were conrmed by the measurement of electron lifetimes
MK-2 (ꢂ0.73 V).2c,15 This implies that the circle chain (se) in the conduction band of TiO2 by means of intensity-
surrounding the dye backbone is effective in suppressing modulated photovoltage spectroscopy (IMVS). As shown in
charge recombination between the injected electron in TiO2 and Fig. 6, at a xed voltage, the se values of the DSCs based on these
I3ꢀ in electrolyte. As a result, DSCs based on the three new circle three dyes vary in the order of LJ-5 < LJ-6 < LJ-4, suggesting that
chain embracing D–p–A dyes LJ-4, LJ-5 and LJ LJ-6 show overall the higher dye-loading amount (Table 2) results in a denser dye
conversion efficiencies of 9.20%, 7.42% and 8.29%, respectively. layer formed on the TiO2 surface, which may be effective in the
ꢀ
suppression of charge recombination between I3 and the
injected electron in TiO2. The various dye loading amounts
observed for the circle chain embracing D–p–A dyes LJ-3, LJ-4,
CEM and IMVS characterization
LJ-5 and LJ-6 may be caused by their different solubility and/or
sizes of donor moieties. The different solubility is associated
with the different polarity of dye molecules resulting from the
varying donating abilities of donor moieties.
The VOC is related to the conduction band position of TiO2 and
the charge recombination rate in DSCs.17 In order to understand
the difference of the VOC observed for the DSCs based on the
circle chain embracing D–p–A dyes LJ-4, LJ-5 and LJ-6, the
relative conduction band positions and electron lifetimes in
the DSCs based on the corresponding sensitizers were investi-
gated. Firstly, we measured the relative conduction band posi-
tion of TiO2 through a charge extraction method (CEM) reported
by Frank et al.18 As shown in Fig. 5, the DSCs based on circle
EIS characterization
To obtain further insight into the important interfacial charge
transfer processes in DSCs based on circle chain embracing
Fig. 5 Charge density at open circuit as a function of VOC for the DSCs based on
dyes LJ-4, LJ-5 and LJ-6.
Fig. 6 Electron lifetime (se) as a function of VOC for the DSCs based on dyes LJ-4,
LJ-5 and LJ-6.
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photovoltages is scrutinized by evaluating the titania conduc-
tion band and the variation of interfacial charge recombination
kinetics. Among these three dyes, the energy levels of carbazole
dye LJ-4 match best with the conduction band of TiO2 and the
redox potential of the redox species for efficient electron injec-
tion and dye regeneration. The presented results further
demonstrate that employing a circle chain embracing p-spacer
is a promising strategy to construct efficient organic sensitizers
for DSCs. Our work will attract the increasing attention of
researchers in the same eld, and provide enlightenment for
further molecular design of this type of organic dye.
Fig. 7 EIS Nyquist plots for DSCs based on dyes LJ-4, LJ-5 and LJ-6 under dark
(ꢀ0.65 V).
Experimental
Materials and instruments
All chemicals and reagents were used as received from chemical
companies without further purication. Anhydrous solvents
were degassed by Ar bubbling for 20 min before use. Compound
3 was synthesized according to our previous method.10 Column
chromatography was performed with Wakogel-C300 as a
stationary phase. UV-Vis spectra were measured in CH2Cl2
solution or TiO2 lm using a UV-3600 Spectrophotometer
(SHIMADZU). Cyclic voltammetry (CV) was performed on a CH
Instruments 624D potentiostat/galvanostat system. All CV
measurements were carried out in anhydrous CH2Cl2 contain-
ing 0.1 M TBAHFP as a supporting electrolyte, purging with
argon prior to conducting the experiment. A platinum electrode
was used as a working electrode, Ag/AgNO3 in saturated
KNO3(aq.) as a reference electrode, and a platinum wire as a
counter electrode. The mass spectra were measured on a Shi-
madzu Biotech matrix-assisted laser desorption ionization
(MALDI) mass spectrometer. The 1H- and 13C-NMR measure-
ments were performed by a DRX-600 spectrometer (Bruker
BioSpin). Incident photon-to-current conversion efficiency
spectra were measured by a CEP-200BX spectrometer (Bunko
Keiki). The current–voltage (I–V) curves were obtained by a WXS-
90S-L2 Super solar simulator (WACOM).
D–p–A dyes LJ-4, LJ-5 and LJ-6, electrochemical impedance
spectroscopy (EIS) was performed in the dark under a forward
bias of ꢀ0.65 V with a frequency range of 10ꢀ1 Hz to 106 Hz.
Generally, three semicircles are observed in EIS Nyquist plots
(Fig. 7). The small and large semicircles located at the high- and
middle-frequency regions, which are attributed to the redox
reaction resistance at the platinum counter electrode, and the
electron transfer resistance at the TiO2/dye/electrolyte interface
(Rrec), respectively.19 Another small semicircle located at the low-
frequency region corresponding to the Warburg diffusion
process resistance of Iꢀ/I3ꢀ in the electrolyte oen overlap with
the middle-frequency large semicircle, probably due to the low
viscosity of the solvents used in our electrolytes, and the shorter
length for Iꢀ ion diffusion as only the thin spacer was used.20
For the same counter electrode and electrolyte, a similar
diameter semicircle was obtained at the low frequency region.
At the middle frequency region, the semicircle diameter is
responsible for the charge recombination, which is associated
with the structure of the sensitizer and interfacial engineering.
The Rrec values for the DSCs based on LJ-4, LJ-5 and LJ-6 are 40 U
cm2, 23 U cm2 and 31 U cm2, respectively. Thus it can be
concluded that the surface blocking effect of these circle chain
embracing D–p–A dyes decreases in the order of LJ-4 > LJ-6 >
LJ-5, as suggested by their dye-loading amount on TiO2 lm.
The trend is in accordance with the decreasing sequence of the
VOC of the DSCs.
Synthesis and characterization
Preparation of compound 4. To a stirred solution of
compound 3 (80 mg, 0.109 mmol) in 1,4-dioxane (20 mL) was
added 9-ethyl-9H-carbazol-3-ylboronic acid (70 mg, 0.218
mmol), potassium carbonate (60 mg, 0.436 mmol), H2O (5 mL),
Conclusion
followed by Pd(PPh3)4 (24 mg, 0.02 mmol). The resulting
ꢃ
In summary, we successfully developed three new D–p–A dyes mixture was stirred at 90 C under argon atmosphere for 12 h.
LJ-4, LJ-5 and LJ-6 bearing circle chain embracing oligothio- The solvent was evaporated under reduced pressure and the
phene as a p-spacer and cyanoacrylic acid as the acceptor residue was treated with water (30 mL), extracted twice with
moieties for high-efficiency DSCs. DSCs employing the sensi- CH2Cl2 (30 mL ꢁ 2). The organic layers were combined and
tizer LJ-4 and the iodine redox electrolyte exhibit a good power washed twice with water and once with brine, dried over
conversion efficiency of 9.20% measured under 100 mW cmꢀ2
,
anhydrous sodium sulfate. Aer removing the solvent under
simulated AM1.5 sunlight, while DSCs based on indoline dye reduced pressure, the residue was loaded onto a silica gel
LJ-5 and dimethoxytriphenylamine dye LJ-6 show a slightly column with PE–EA (5/1, v/v) as the eluent to afford the desired
lower efficiency of 7.42% and 8.29%, respectively, due to their compound 4 as an orange solid (68 mg, 74%). 1H NMR (CDCl3,
less positive ground-state oxidization potentials resulting in 600 MHz), d: 9.80 (s, 1H), 8.19 (s, 1H), 8.04 (d, J ¼ 7.8 Hz, 1H),
slower regeneration of the oxidized state of LJ-5 and LJ-6 by the 7.59 (d, J ¼ 3.6 Hz, 1H), 7.54 (d, J ¼ 9.0 Hz, 1H), 7.46–7.51 (m,
Iꢀ in the redox electrolyte. The dissimilarity of the cells’ 3H), 7.40 (d, J ¼ 8.4 Hz, 1H), 7.34 (d, J ¼ 8.4 Hz, 1H), 7.24
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(d, J ¼ 6.6 Hz, 1H), 7.14 (s, 1H), 7.13 (s, 1H), 7.02 (d, J ¼ 2.4 Hz, 0.99 (m, 4H). 13C NMR (CDCl3, 150 MHz), d: 182.27, 158.85,
1H), 6.76 (d, J ¼ 9.0 Hz, 4H), 4.36 (q, J ¼ 7.2 Hz, 2H), 4.11–4.15 158.68, 155.99, 148.63, 147.93, 141.75, 140.70, 140.36, 137.62,
(m, 4H), 3.76–3.84 (m, 4H), 1.52–1.59 (m, 8H), 1.42 (t, J ¼ 7.2 Hz, 136.21, 134.01, 132.20, 131.75, 130.82, 130.75, 130.68, 129.10,
1H), 1.25–1.32 (m, 4H), 0.94–1.03 (m, 4H). 13C NMR (CDCl3, 150 126.61, 125.75, 124.60, 122.78, 120.50, 116.47, 116.01, 114.75,
MHz), d: 182.30, 158.94, 158.72, 148.68, 143.04, 140.40, 14.35, 106.93, 106.82, 69.79, 55.52, 31.78, 27.61, 27.53. ESI (m/z): calcd
139.30, 137.63, 136.32, 134.01, 132.47, 132.13, 131.69, 130.90, for C57H53NO7S3, 959.30 (M)+; found, 959.30.
130.81, 130.72, 129.12, 125.95, 125.72, 124.71, 123.59, 123.25,
Preparation of dye LJ-4. A solution of aldehyde 4 (60 mg,
122.90, 122.77, 120.48, 119.04, 117.04, 116.65, 116.08, 108.64, 0.071 mmol), cyanoacetic acid (18 mg, 0.212 mmol), piperidine
108.59, 107.06, 106.90, 69.85, 38.74, 30.35, 27.54. ESI (m/z): (36 mg, 0.424) in CHCl3 (15 mL) was stirred at reux for 10 h.
calcd for C51H47NO5S3, 849.26 (M)+; found, 849.26.
The reaction mixture was cooled to room temperature, then
Preparation of compound 5. To a stirred solution of treated with water (30 mL) and acidied with 1 M aqueous
compound 3 (80 mg, 0.109 mmol) in 1,4-dioxane (20 mL) was hydrochloric acid (10 mL) and extracted twice with CHCl3
added 4-phenyl-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indol-7- (30 mL ꢁ 2). The organic layers were combined and washed
ylboronic acid (80 mg, 0.218 mmol), potassium carbonate (60 twice with water and once with brine and dried over anhydrous
mg, 0.436 mmol), H2O (5 mL), followed by Pd(PPh3)4 (24 mg, sodium sulfate. Aer removing the solvent under reduced
ꢃ
0.02 mmol). The resulting mixture was stirred at 90 C under pressure, the residue was loaded onto a silica gel column with
argon atmosphere for 12 h. Evaporation of the solvent under CH2Cl2–CH3OH (15/1, v/v) as eluent to afford the desired dye LJ-
reduced pressure and the residue was treated with water (30 4 as a purple solid (56 mg, 87%). 1H NMR (DMSO-d6, 600 MHz),
mL), extracted twice with CH2Cl2 (30 mL ꢁ 2). The organic d: 8.34 (d, J ¼ 1.8 Hz, 1H), 8.31 (s, 1H), 8.12 (d, J ¼ 7.8 Hz, 1H),
layers were combined and washed twice with water and once 7.84 (s, 1H), 7.61 (d, J ¼ 9.0 Hz, 2H), 7.57 (t, J ¼ 8.4 Hz, 1H), 7.46–
with brine and dried over anhydrous sodium sulfate. Aer 7.50 (m, 2H), 7.40 (dd, J1 ¼ 1.8 Hz, J2 ¼ 8.4 Hz, 1H), 7.21–7.25
removing the solvent under reduced pressure, the residue was (m, 4H), 6.89 (d, J ¼ 8.4 Hz, 2H), 6.86 (d, J ¼ 8.4 Hz, 2H), 4.44 (q,
loaded onto a silica gel column with PE–EA (5/1, v/v) as eluent to J ¼ 7.2 Hz, 2H), 4.10–4.12 (m, 4H), 3.74–3.81 (m, 4H), 1.41–1.46
afford the desired compound 5 as an orange solid (69 mg, 71%). (m, 8H), 1.30 (t, J ¼ 7.2 Hz, 3H), 1.14–1.19 (m, 4H), 0.94–0.97 (m,
1H NMR (CDCl3, 600 MHz), d: 9.79 (s, 1H), 7.62 (s, 1H), 7.45 (t, 4H). 13C NMR (DMSO-d6, 150 MHz), d: 158.84, 142.54, 140.55,
J ¼ 4.2 Hz, 1H), 7.43 (t, J ¼ 4.2 Hz, 1H), 7.31–7.36 (m, 2H), 7.03– 139.46, 134.61, 132.86, 131.55, 131.48, 130.35, 129.68, 126.67,
7.11 (m, 2H), 6.90–7.01 (m, 5H), 6.70–6.73 (m, 5H), 4.71–4.80 125.40, 125.16, 124.00, 123.23, 123.16, 122.52, 120.94, 119.55,
(m, 1H), 4.08–4.11 (m, 4H), 3.75–3.80 (m, 5H), 2.01–2.06 (m, 116.89, 116.32, 115.76, 110.21, 109.87, 107.67, 107.47, 69.60,
1H), 1.92–1.96 (m, 1H), 1.81–1.87 (m, 2H), 1.51–1.67 (m, 8H), 37.55, 30.33, 27.54, 27.49, 14.12. ESI (m/z): calcd for
1.22–1.28 (m, 4H), 0.96–0.99 (m, 4H). 13C NMR (CDCl3, 150
MHz), d: 182.23, 158.91, 158.69, 148.72, 146.62, 142.92, 142.59,
C
54H48N2O6S3, 916.27 (M+); found, 916.26.
Preparation of dye LJ-5. A solution of aldehyde 5 (60 mg,
140.30, 137.58, 136.39, 135.64, 133.92, 131.93, 131.52, 130.73, 0.067 mmol), cyanoacetic acid (17 mg, 0.2 mmol), piperidine (34
130.63, 130.05, 129.19, 129.12, 125.48, 124.58, 123.90, 122.69, mg, 0.4) in CHCl3 (15 mL) was stirred at reux for 10 h. The
121.68, 121.51, 119.17, 116.67, 116.10, 108.18, 107.01, 106.89, reaction mixture was cooled to room temperature, then treated
106.83, 69.83, 45.36, 34.69, 33.90, 31.59, 30.36, 27.52, 24.45, with water (30 mL) and acidied with 1 M aqueous hydrochloric
22.66, 14.12. ESI (m/z): calcd for C54H51NO5S3, 889.29 (M)+; acid (10 mL) and extracted twice with CHCl3 (30 mL ꢁ 2). The
found, 889.29.
organic layers were combined and washed twice with water and
Preparation of compound 6. To a stirred solution of once with brine, and dried over anhydrous sodium sulfate. Aer
compound 3 (80 mg, 0.109 mmol) in 1,4-dioxane (20 mL) was removing the solvent under reduced pressure, the residue was
added 4-dimethoxyphenylaminophenylboronic acid (94 mg, loaded onto a silica gel column with CH2Cl2–CH3OH (15/1, v/v)
0.218 mmol), potassium carbonate (60 mg, 0.436 mmol), H2O (5 as eluent to afford the desired dye LJ-5 as a purple solid (52 mg,
mL), followed by Pd(PPh3)4 (24 mg, 0.02 mmol). The resulting 81%). 1H NMR (DMSO-d6, 600 MHz), d: 8.35 (s, 1H), 7.86 (s, 1H),
ꢃ
mixture was stirred at 90 C under argon atmosphere for 12 h. 7.50 (t, J ¼ 8.4 Hz, 1H), 7.24–7.36 (m, 7H), 7.03 (s, 1H), 6.96–6.99
The solvent was evaporated under reduced pressure and the (m, 2H), 6.92 (d, J ¼ 8.4 Hz, 1H), 6.83–6.85 (m, 4H), 4.83 (t, J ¼
residue was treated with water (30 mL) and extracted twice with 7.8 Hz, 1H), 4.08–4.10 (m, 4H), 3.81 (t, J ¼ 8.4 Hz, 1H), 3.73–3.76
CH2Cl2 (30 mL ꢁ 2). The organic layers were combined and (m, 4H), 2.00–2.02 (m, 1H), 1.84–1.87 (m, 1H), 1.78–1.82 (m,
washed twice with water and once with brine and dried over 1H), 1.72–1.74 (m, 1H), 1.61–1.64 (m, 1H), 1.41–1.48 (m, 8H),
anhydrous sodium sulfate. Aer removing the solvent under 1.31–1.38 (m, 1H), 1.15–1.18 (m, 4H), 0.85–0.88 (m, 4H). 13C
reduced pressure, the residue was loaded onto a silica gel NMR (DMSO-d6, 150 MHz), d: 158.76, 146.61, 142.67, 142.26,
column with PE–EA (5/1, v/v) as the eluent to afford the desired 136.34, 134.63, 132.68, 131.44, 131.25, 129.76, 129.55, 124.88,
compound 6 as an orange solid (77 mg, 74%). 1H NMR (CDCl3, 124.59, 123.97, 121.76, 121.69, 119.08, 116.28, 115.77, 108.41,
600 MHz), d: 9.77 (s, 1H), 7.56 (d, J ¼ 4.2 Hz, 1H), 7.44 (t, J ¼ 8.4 107.61, 107.50, 107.43, 69.56, 68.60, 45.04, 34.85, 33.82, 30.31,
Hz, 1H), 7.39 (t, J ¼ 8.4 Hz, 1H), 7.21 (d, J ¼ 9.0 Hz, 2H), 7.09 (s, 27.51, 24.43. ESI (m/z): calcd for C57H52N2O6S3, 956.30 (M+);
1H), 7.03 (d, J ¼ 9.0 Hz, 4H), 6.98 (d, J ¼ 4.2 Hz, 1H), 6.94 (s, 1H), found, 956.29.
6.83 (d, J ¼ 8.4 Hz, 2H), 6.80 (d, J ¼ 9.0 Hz, 4H), 6.70 (d, J ¼ 8.4
Preparation of LJ-6. A solution of aldehyde 6 (60 mg, 0.063
Hz, 2H), 6.67 (d, J ¼ 8.4 Hz, 2H), 4.07–4.13 (m, 4H), 3.78 (s, 6H), mmol), cyanoacetic acid (17 mg, 0.2 mmol), and piperidine
3.73–3.76 (m, 4H), 1.48–1.54 (m, 8H), 1.23–1.26 (m, 4H), 0.98– (34 mg, 0.4) in CHCl3 (15 mL) was stirred at reux for 10 h. The
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reaction mixture was cooled to room temperature, then treated frequency ranging from 10ꢀ1 to 106 Hz. The CEM was per-
with water (30 mL), acidied with 1 M aqueous hydrochloric formed with the same monochromatic light source. The solar
acid (10 mL) and extracted twice with CHCl3 (30 mL ꢁ 2). The cell was illuminated at an open-circuit condition for 5 s to attain
organic layers were combined and washed twice with water and a steady state and then the light source was switched off when
once with brine, dried over anhydrous sodium sulfate. Aer the device simultaneously switched to a short-circuit condition
removing the solvent under reduced pressure, the residue was to extract the charges generated at that light intensity. The EIS
loaded onto a silica gel column with CH2Cl2–CH3OH (15/1, v/v) spectra were measured with an impedance analyzer (Solartron
as eluent to afford the desired dye LJ-6 as a purple solid (54 mg, Analytical, 1255B) connected with a potentiostat (Solartron
85%). 1H NMR (DMSO-d6, 600 MHz), d: 8.22 (s, 1H), 7.64 (s, 1H), Analytical, 1287) under illumination using a solar simulator
7.44 (t, J ¼ 8.4 Hz, 1H), 7.37 (t, J ¼ 8.4 Hz, 1H), 7.22 (d, J ¼ 8.4 Hz, (WXS-155S-10: Wacom Denso Co. Japan).
2H), 7.13 (s, 1H), 7.03 (d, J ¼ 9.0 Hz, 4H), 6.95–6.96 (m, 2H),
6.81–6.85 (m, 6H), 6.71 (d, J ¼ 8.4 Hz, 2H), 6.65 (d, J ¼ 8.4 Hz,
Acknowledgements
2H), 4.09–4.12 (m, 4H), 3.79 (s, 6H), 3.71–3.76 (m, 4H), 1.41–1.49
(m, 8H), 1.16–1.20 (m, 4H), 0.94–0.99 (m, 4H). 13C NMR (DMSO- This work was supported by the Core Research for Evolutional
d6, 150 MHz), d: not sufficiently soluble to obtain a spectrum. Science and Technology (CREST) of the Japan Science and
ESI (m/z): calcd for C60H54N2O8S3, 1026.30 (M+); found, 1026.29. Technology Agency.
Notes and references
Cell fabrication and characterization
The photoanode (thickness 19 mm; area 0.25 cm2) of the DSC
was a double-layered TiO2 lm containing a transparent layer 14
mm thick with titania particles (ꢂ20 nm) and a scattering layer 6
mm thick (400 nm nanoparticles). The double-layered lm was
heated to 520 ꢃC, sintered for 1 hour and then cooled to ca. 80
ꢃC and treated with 0.1 M HCl aqueous. Aer the HCl treatment,
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´
the lms were washed, dried and then immersed in 3 ꢁ 10ꢀ4
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¨
acetonitrile–tert-butyl alcohol (1/1, v/v) solution of the sensi-
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metal mask with an aperture area of 0.2304 cmꢀ2 under stan-
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